Development of an off-line silicon wafer warpage measuring tool
Linas Čapas
Master of Science Thesis TRITA-ITM-EX 2021:8
KTH Industrial Engineering and Management
Machine Design SE-100 44 STOCKHOLM
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Examensarbete TRITA-ITM-EX 2021:8
Utveckling av formmätningsverktyg för off-line mätning av vrängning hos kiselplattor
Linas Čapas
Godkänt
2021-01-18
Examinator
Ulf Sellgren
Handledare
Björn Möller
Uppdragsgivare
ASML Netherlands B.V.
Kontaktperson
Gijs Kramer
Sammanfattning Vrängda kiselplattor och de problem som uppstår på grund av det är ett känt fenomen inom
halvledarindustrin. För att kringgå dessa problem behövs god mätnoggranhet och det nuvarande
sättet att hantera vrängda kiselplattor på inom företaget är långt från idealt. En batch kiselplattor
hämtas hos kunden med antagandet att alla kiselplattor är identiskt vrängda. Ett enda exemplar
som representerar hela batchen väljs sedan ut och skickas till ett externt mätföretag. Metoden som
används för att mäta kiselplattan innehåller föroreningar och metoden repar även kiselplattan, som
därmed inte kan användas efteråt. Utöver mätmetodens brister tillkommer även en utökad logistik
och större materialspill som tillför kostnader för företaget.
Examensarbetets syfte är att förbättra mätmetoden som används för att utvärdera kiselplattornas
vrängning och målet med projektet är att utveckla en prototyp som tillåter att mätmetoden görs
internt inom företaget.
Rapporten innehåller metodiken som användes för att uppnå det slutgiltiga konceptet samt
resultatet, och innehåller planeringsmoment samt projektets delmoment som: WBS, GANNT,
funktionsnedbrytning, kravspecifikationer samt urvalsmatriser.
Det valda konceptet består av en sorteringsmaskin kombinerat med mätutrustningen och liknar en
FOUP (Front Opening Unified Pod), vilket tillåter sorteringsmaskinen att tillföra och byta ut
kiselplattorna som ska mätas. Mätutrustningen består av en roterande rörelse hos kiselplattan och
en linjär rörelse hos en konfokal sensor placerad ovanför kiselplattan. Kombinationen av de båda
rörelserna tillåter att hela kiselplattans yta mäts med ett givet vinkel- och radiellt steg. Genom att
vända kiselplattan uppochner med sorteringsmaskinen och utföra samma mätning igen kan
kiselplattans korrekta form estimeras genom att eliminera gravitationseffekten.
Konceptet utvecklades i detalj och tillverkningsunderlag och ritningar togs fram samt
komponenter avsedda för tillverkning av en prototyp beställdes. På grund av COVID-19 pandemin
uppstod dock kommunikationssvårigheter och förseningar i ledtider. Detta påverkade leveranserna
och en del komponenter kom inte fram förrän i slutet av examensarbetet och det fanns därmed
ingen tid över för montering eller tester som kan styrka konceptet, vilket får lämnas över till
företagets anställda.
Nyckelord: Mikrolitografi, Halvledare, Kisel, Kiselplatta, Vrängning.
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Master of Science Thesis TRITA-ITM-EX 2021:8
Development of an off-line silicon wafer warpage measuring tool
Linas Čapas
Approved
2021-01-18
Examiner
Ulf Sellgren
Supervisor
Björn Möller
Commissioner
ASML Netherlands B.V.
Contact person
Gijs Kramer
Abstract Warped wafers and all the issues arise with them. are known issue in semiconductor industry. To
solve those issues, the shape of the wafer needs to be known precisely. Current way of working
when it comes to warped wafers is far from ideal within the company. A batch of wafers is acquired
at customer’s site and it is assumed, that all the wafers in the batch are warped identically. A single
specimen, representing the whole batch, is then taken to external company to be measured. As the
method of measuring currently used contaminates and scratches the wafer, wafer must be scrapped
afterwards. All the logistics and scrapped wafers add unnecessary costs to the company.
To optimize the warpage measuring procedure, a graduation internship project was initiated with
a goal to develop a prototype of the tool, enabling inhouse warpage measuring.
The report contains all the methodology used to reach the final concept and results and includes
methods such as: WBS, GANTT chart, Functional breakdown, Design requirement specification,
Morphological matrix and PUGH’s matrix.
Final concept of warpage measuring tool consisted of implementing wafer sorting apparatus for
wafer handling and enclosing the measuring tool to a custom housing, resembling a FOUP (Front
Opening Unified Pod), allowing wafer sorting apparatus to load and unload test specimen for
measuring. The measuring concept consists of rotary stage, where the wafer is loaded and rotated
in addition to linear stage, that holds a confocal sensor above the wafer and moves it across the
surface of the wafer, measuring the profile of the wafer, rotated every defined number of degrees
between the measurements. Gravity induced deflection is eliminated by flipping the wafer using
same wafer sorting apparatus and measuring the wafer inverted, thus allowing to estimate the true
shape of the wafer.
The concept was developed in more detail, drawings for manufacturing the parts were created and
the parts for building a functional prototype were ordered. Because of the COVID-19 pandemic,
there were inevitable communication difficulties and delays in lead times, resulting in parts
arriving on the last days of the internship, leaving no time for assembling and testing the actual
prototype, therefore proof of concept is yet left to be tested by the employees of the company.
Keywords: Photolithography, Semiconductor, Silicon, Wafer, Warpage
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FOREWORD
I express my sincerest gratitude to ASML and all the people inside the organization for providing
me with the opportunity to write my master’s thesis at this astonishing company and always
helping me on the way even through this uncertain time of worldwide pandemic.
I would like to thank Gijs Kramer for trusting me with the assignment, welcoming me and guiding
me in every aspect during my stay at ASML.
Additionally, I would like to thank colleagues from the Service Lab, Modelshop, Wafer Processing
groups and everyone involved in general.
Finally, I am grateful for my academic supervisor Björn Möller and examiner Ulf Sellgren for all
the help while writing this thesis.
Thank you all.
Linas Čapas
Eindhoven, October 2020
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NOMENCLATURE
This chapter presents the notations and abbreviations used in the document.
Abbreviations and expressions
3D Three-dimensional
CAD Computer Aided Design
D&E Design and Engineering
DUV Deep Ultraviolet
EUV Extreme Ultraviolet
FEA Finite Element Analysis
FOUP Front Opening Unified Pod
FOSB Front Opening Shipping Box
Off-line External or separate, i.e., not within the photolithography machine
PEEK Polyether ether ketone
RFID Radio-Frequency Identification
TTV Total Thickness Variation
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TABLE OF CONTENTS
1 INTRODUCTION 12
1.1 Background 12
1.2 Purpose 12
1.3 Delimitations 13
1.4 Method 13
1.5 Sustainability and ethical considerations 14
2 FRAME OF REFERENCE 15
2.1 Introduction to chip fabrication process 15
2.2 Silicon wafers 16
2.3 Commercial wafer warpage measuring 18
2.4 Wafer handling and contamination 19
2.5 Warpage measuring 29
2.5.1 Measuring principle 29
2.5.2 Measuring sensor 32
2.6 Gravity induced deflection 37
3 IMPLEMENTATION 41
3.1 Functional breakdown 41
3.2 Product requirement specification 41
3.3 Concept generation 43
3.3.1 Concept 1 - Stationary measuring 44
3.3.2 Concept 2 - Line scanning 45
3.3.3 Concept 3 - XY vertical measuring 46
3.4 Concept evaluation 48
3.5 Concept development 49
3.5.1 Detailed design of mechanical system 49
3.5.2 Detailed design of electronics and control system 61
3.5.3 Wafer sorter integration 64
4 RESULTS 66
4.1 Manufactured parts 66
4.2 Assembled prototype 67
4.3 Measuring results 67
5 DISCUSSION AND CONCLUSIONS 68
5.1 Conclusion 68
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5.2 Discussion 69
6 RECOMMENDATIONS AND FUTURE WORK 70
6.1 Recommendations 70
6.2 Future work 70
7 REFERENCES 71
APPENDIX A: RISK ASSESSMENT TABLE 73
APPENDIX B: GANTT CHART 74
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1 INTRODUCTION
This chapter describes the background, the purpose, the limitations, and the methods used in the
presented project.
1.1 Background
This document is a documented report of a graduation internship project at ASML Netherlands
B.V. The graduation internship and the project results achieved during it will act as a master thesis
project to obtain master’s degree from KTH Royal Institute of Technology in Stockholm, Sweden
in Engineering Design study program, Machine Design study track.
The off-line warpage measuring tool is a part of an ongoing project within the company. The tool
is intended to change the way of working when it comes to issues that warped silicon wafers cause
as they are not perfectly flat and as they are used for production of semiconductor chips, even the
smallest imperfections of wafer geometry can cause issues. Current way of working is inefficient
and rather uncertain due to its nature. Currently, silicon wafer warpage is measured outside of the
company, thus many additional potential risks arise since silicon wafers are brittle and fragile to
handle. Transporting them to outside facilities and back takes multiple days and causes a risk of
wafers being damaged. Usually, a batch of warped wafers is acquired at a customer’s site. One
sample from the whole batch is then taken to external supplier for measuring. Measuring is done
by contact probe method in non-cleanroom environment, both factors contributing to
contaminating and scratching the wafer, irreversibly damaging it, which leads to scrapping the
wafer. It is assumed afterwards, that the measured warpage value is valid for the whole batch of
wafers.
The off-line warpage measuring tool would enable to measure and (or) verify every wafer
individually inhouse without having to scrap the wafer once it has been measured as the measuring
process now would-be non-contact and wafers would be handled in cleanroom environment by
automated machinery, leaving less error for human factor errors. All this combined results in wafer
warpage being measured faster and cleaner, allowing for potential financial savings as well.
1.2 Purpose
The purpose of this project is to develop a worked thorough concept of measuring the amount of
warpage present on silicon wafers in a clean, contamination free and non-contact way inhouse
cheaper, faster, while meeting the accuracy requirements, as well as to design, manufacture and
test a worked thorough prototype. To fulfil the purpose of the project, following research questions
will be addressed:
How to handle silicon wafers and measure silicon wafer warpage in a contamination free
way?
How deflection of a silicon wafer due to gravity could be eliminated during the measuring
procedure?
What methods and what hardware components could be implemented to measure the
warpage of silicon wafers to reach desired levels of measuring accuracy?
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1.3 Delimitations
Because of limited period of the Master Thesis internship as well as rather broad scope of the
project, the following delimitations apply to the project:
Due to Master Thesis project including manufacturing and testing an actual prototype, the
concept generation will be limited to an extent, that allows choosing the most feasible
concept to be developed in detail further on.
The parts will be designed with professional attitude, having economic, environmental and
performance impacts in mind, but no in depth FEA calculations will be performed to
optimize the parts once they are manufactured, unless they do not meet the performance
requirements.
Final appearance and user friendliness of the prototype will not be emphasized and will not
be considered to be a critical requirement since the project work will focus on functional
prototype that proves the concept and thus will be operated by qualified member of D&E
team.
A boundary condition is established, that silicon wafer handling equipment present in
premises of the company is capable of handling silicon wafers that are warped up to ± 1
mm, thus everything related to warped wafer handling is left outside of the scope of the
project.
No active particle measurement will be performed to estimate the cleanliness inside the
operating environment, the result is meant to be clean by design.
1.4 Method
Because of a broad project scope and a limited time available, a strong emphasis was put on project
planning and structuring the workflow. Using WBS, presented in Figure 1, following main stages
of the project were identified: Project planning, product development, product realization, project
closure and administrative. Each of the main stages were divided into smaller sub-stages.
A GANTT chart was created to plan and schedule the work throughout duration of the project. To
minimize uncertainty and risks, a risk assessment table was created. All are added as Appendices.
Since ASML has extensive knowledge in the field that is well documented, major source of
information was confidential documents of the company as well as interviews and meetings with
employees of ASML.
Because of the size and complexity of the company, variety of different departments get involved
in projects. For that reason, meetings were conducted with multiple employees from different
departments and a design requirement specification was finalized and approved by people of
multiple competences.
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Functional breakdown of expected wafer warpage measuring tool was developed and the functions
were used in morphological matrix to generate multiple possible concepts. The concepts were
presented in an organized meeting with multiple team and group leaders from different
departments of the company and the final concept to be developed further was chosen.
1.5 Sustainability and ethical considerations
Throughout entire design process, a strong emphasis was put on sustainability and environmental
impact. The primary source of materials, manufacturing facilities and components was intended
to be internal facilities and resources of the company, the reason being faster lead times, no
transportation of the parts between multiple suppliers, better communication, allowing to notice
possible design and drawing flaws, leaving less room for errors. The initial sensor of choice was
chosen to be a different one that ended up in the final design, the reason being that a relatively old
sensor, which is not being produced by the manufacturer anymore, was found in stock at the
company’s measuring facilities. The sensor met all the performance requirements, however quite
a few design changes were needed to implement the sensor successfully, due to new sensor being
older, therefore bulkier, taking more space and having different mounting interfaces. Functionality
of the prototype could be easily further improved if needed and more features could be
implemented without having to completely discard the old version.
Figure 1. Work breakdown structure
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2 FRAME OF REFERENCE
This chapter provides the information about wafer warpage in terms of the warpage phenomenon
itself, its causes, and consequences as well as how warpage influences the work of ASML.
2.1 Introduction to chip fabrication process
Fabrication of semiconductor chips is a highly complicated process, involving many steps within
multiple disciplines. To fully understand the role ASML plays in the process as well as what is a
silicon wafer, what causes it to warp and what are the issues that arise with it, a simplified flowchart
of semiconductor chip fabrication process is presented below in Figure 2, and process step 5 is
where ASML’s machines are the key contributors.
Figure 2. Simplified flowchart of semiconductor chip fabrication process (van Gerven, 2017)
The first step of the long process is fabrication of a silicon wafer, which is the fundamental element
of a semiconductor chip. An ingot of crystalline silicon is formed by melting silicon and drawing
the molten silicon upwards, allowing it to cool down and form a solid cylinder-shaped ingot. The
ingot is then sliced into thin pieces, that are known as wafers. Wafers are then further processed
by lapping, etching, polishing to achieve needed geometric and physical properties.
Once the wafer has been processed, materials are applied to the wafer, such as silicon oxide layer,
silicon nitride layer and layer of photoresist. Once the wafer has been coated, this is the stage,
where ASML comes in. ASML manufactures photolithography machines, that use Deep
Finally, the chips are
enclosed in special plastic packaging in
another plant
Wafers are sawed out of a block (ingot) of very
pure crystalline
silicon
Polishing
Material deposition or modification
The resist is applied to a
spinning wafer to achieve a
uniform layer
Lithography for semiconductor manufacturing in a nutshell:
lenses shrink a mask pattern and project it
onto a wafer
Light
Reticle
mask
Lens
The chip pattern is “burned” into the
resist in an exposure step
The print is developed
through etching and
heating
Ion implantation The resist is
removed
The wafer processing cycle is complete, and
a single chip layer has been
fabricated
After all the required cycles
have been completed, the chips can be cut out of the
wafer and
tested
Wafer
Pattern being transferred onto wafer
Repeat thirty to
forty times
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ultraviolet (DUV) or Extreme ultraviolet (EUV) light to expose reticle mask and transfer mask’s
patterns onto the wafer. This step, while briefly described in a single sentence, requires state-of-
the-art machines of enormous complexity. Exposed photoresist can then be chemically removed.
These patterns, where photoresist is removed, are then etched. Etched regions are exposed to
ionized gases, implementing ions to the features. These steps are then repeated multiple times for
multiple layers of the chip to be created.
When all layers are exposed, the wafer is cut into individual chips and then individually tested,
packaged, and proceeded for further usage.
Whole semiconductor chip fabrication process is explained in a very brief manner with plenty of
simplifications. It is done so to introduce the reader to the process, so further stages of thesis work
are understood better.
2.2 Silicon wafers
Silicon wafer is a thin slice of semiconductor, such as crystalline silicon, used for fabrication of
integrated circuits. The wafer serves as substrate for microelectronic devices built in and upon the
wafer as silicon has semiconductor material properties.
Wafers are formed of highly pure, nearly defect free single crystalline material. In the industry of
electronics, wafers are generally varying from 25 mm to 450 mm in diameter, most common being
the 300 mm ones. Wafer size has been increasing throughout the years due to the fact, that with
increasing wafer area, proportionally increasing amount of chips can be produced, while the price
of production step increases at slower rate than the area of wafer increases.
Basic properties of 200 mm and 300 diameter wafers can be found in Table 1 and multiple sized
patterned wafers can be seen in Figure 3 below:
Table 1. Silicon wafer properties
Wafer diameter, mm 200 300
Thickness, µm 725 775
Thickness variation, µm ± 10 ± 3
Weight, g 53 125
Figure 3. Patterned silicon wafers
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Warpage is a term used to define the height difference between the highest and the lowest points
of median surface of a free, unclamped wafer versus a reference plane that is globally parallel to
the wafer (ASML, 2020). Schematically warpage is depicted below in Figure 4.
Figure 4. Silicon wafer warpage (ASML, 2020)
Silicon wafers have orthotropic crystalline structure, therefore X, Y and Z axes are defined and
depicted in Figure 5 below.
Depending on the warp per axis, a few typical warpage shapes can be identified, and those terms
are commonly used within the company when referring to warped wafers:
Table 2. Terms used to define warpage shape
Shape Warp on X axis Warp on Y axis
Bowl + +
Umbrella - -
Saddle + -
Saddle - +
Taco 0 +/-
Taco +/- 0
In Figure 6 below, saddle, umbrella and taco shaped wafers are depicted.
Warp
+Y
+X
+Z
Figure 5. Axes of a silicon wafer
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Figure 6. Saddle, umbrella, and taco shaped wafers (Microchemicals GmbH, 2020)
2.3 Commercial wafer warpage measuring
Semiconductor industry being one of the most complex and technologically demanding industries,
requires cutting-edge equipment and machinery to keep pushing the limits and driving the world
further. Silicon wafer warpage is a well-known phenomenon and warpage measurement frequently
comes along with measuring additional properties of the wafer, such as thickness, total thickness
variation (TTV), bow, roughness, etc.
Commercial equipment for measuring mentioned properties exists with exceptional performance.
High performance of these instruments is valuable for wafer metrology purposes, however, the
measuring tool to be designed within this thesis is meant to be used for less accurate wafer
measuring than the commercial measuring equipment.
Several well-known tools can be seen summarized in a table below:
Table 3. Comparison of well-known commercial warpage measuring tools
Name KLA-Tencor PWG Ultratech 4G+ Sentronics
SemDex A31
FRT MicroProf
300
Picture
Warpage limit (mm) 0.5 0.7 8 0.6
Wafer handling Automatic Automatic Automatic Automatic
XY resolution (mm) 0.2 0.2 0.2 0.2
Z resolution (nm) 0.1 0.1 20 20
Warpage measuring
accuracy (µm) <0.1 <0.1 <1 1
Number of measured
points >1M >1M >1M >1M
Measurement method Optical, contactless Optical, contactless Optical, contactless Optical, contactless
Other measured
properties
Thickness, flatness, nano-
topography -
Thickness, flatness,
nano-topography -
Approximate cost €10M €4M €4M €4M
The tools mentioned above are highly complex, enabling them to be integrated into the
manufacturing and processing processes in semiconductor fabs. Less complex equipment and
methods for wafer warpage measurement can be found as well and are presented below.
E&H MX2012
Whole product series MX 20x from E+H is based on two heavy plates mounted parallel to each
other. In the plates, there are capacitive distance sensors mounted. The wafer can be loaded
automatically and manually depending on the tool’s model. MX 2012 (Figure 7.) allows measuring
thickness, TTV, warpage, stress of the wafers of 300 mm in diameter, 500-1000 µm in thickness,
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with an accuracy of 1 µm. MX2012 has an array of 69 capacitive measuring sensors, thus resulting
in 69 measured points across the wafer.
Figure 7. E&H MX2012 (E+H Metrology GmbH, 2020)
Contact probe measuring
This warpage measuring method is the most primitive one out of all mentioned. Warpage
measuring is carried out by using coordinate measuring machine (CMM), whilst the test specimen
is resting on 3 points on the bottom surface. An array of 177 points is probed, showing height
deviations across the wafer and curve fitting is performed within the points to get more detailed
results. To minimize gravity induced the deflection, the wafer is flipped, and corresponding points
are measured on a flipped wafer. This measuring method, while being simple, irreversibly
scratches and contaminates the wafer due to measurements happening outside the cleanroom,
mechanical probe touching the surface of the wafer and the wafer touching 3 metal support points
on both front and back surfaces. Another downside of this method could be added to the list as the
wafers are intended to be used further after warpage inspection, however, as the wafers are
scratched and can’t be used anymore, it is assumed that a single wafer represents a whole batch of
warped wafers. This assumption reduces the accuracy and result certainty for exact wafer
specimen. The advantages of this method over other methods are that warpage, thickness, and size
of the wafer are not limiting factors and even highly warped wafers can be measured. The setup
for contact probe measuring is depicted below in Figure 8.
Figure 8. Contact probe warpage measurement setup (ASML, 2020)
2.4 Wafer handling and contamination
Silicon wafers being such fragile items must be handled with extreme care. While blank wafers
are expensive, they become significantly more expensive once exposed and require even more care
when handling. As much as silicon wafers are vulnerable to handling, in production they are as
vulnerable to contamination. A single dust particle might ruin entire batch of chips on the wafer.
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Even though all wafers are handled in cleanroom environment, the wafer must be stored in a
special container – industry standard front opening unified pod (FOUP) and cannot even be opened
surrounding cleanroom environment if the wafer is meant to be further processed.
For those reasons, handling silicon wafers is usually done by automated equipment with enclosed
environment. One type of such machines is called wafer sorters. These machines allow multiple
FOUPs to be loaded and once loaded, wafers can be aligned, sorted, inspected, and moved from
one FOUP to another in enclosed environment with high positioning accuracy, gentle and
contamination-free handling. One of the wafer sorters is presented in Figure 9. Wafers of 300 mm
in diameter are industry standard, therefore wafers of such size can be handled by majority of the
equipment. For smaller, 200 mm diameter wafers, a different end effector of the robotic
manipulator is needed, therefore the same wafer sorter cannot be used to handle wafers of both
sizes without additional tweaks and hardware changes.
Figure 9. Wafer sorting machine by Brooks Automation (Brooks Automation, Inc, 2020)
Inside every wafer sorter, there is a robotic manipulator, which moves along the wafer sorter,
which is capable of gripping and manipulating the wafers. Such wafer handling manipulator can
be bought off-the-shelf and used for developing a custom solution for wafer handling. An example
of wafer handling robot is presented in Figure 10 below.
Figure 10. Wafer handling robot by Brooks Automation (Brooks Automation, Inc, 2020)
Looking into the scope of master thesis project, as it involved building and testing actual prototype
of the tool, the choice on wafer handling solutions becomes even more narrow. Developing a
device for wafer handling that has custom interfaces for loading and measuring the wafers would
be expensive and complicated multi-disciplinary project on its own and it would be unrealistic to
develop such machine in addition to actual warpage measuring instrument.
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Interface for wafer handling
In the semiconductor industry, silicon wafers are typically stored, carried, and transported in either
FOUP or FOSB (Front Opening Shipping Box) containers. These containers have unified top,
bottom, and frontal interfaces so they are compatible with all wafer processing equipment and
wafers are possible to load/unload for processing. The top interface consists of a flange, allowing
robotic carriers clamp and transport the container around the fab. The bottom interface
accommodates three V-shaped grooves acting as a kinematic coupling for precisely locating the
container on the tool, a rounded rectangle groove for securing and clamping the container to the
tool, and occasionally a RFID tag, so the container and stored wafers could be recognized by the
tool allowing for custom settings for a specific container. The front interface of both FOUP and
FOSB consists of a front frame with a removable door. All the standard tools in semiconductor
fabs are capable of opening and closing the door automatically after ensuring a tight connection
between the container and the tool. The plastic body of the container with removable door prevents
the stored wafers from external contamination.
Figure 11. Universal interfaces of a FOUP
Figure 12. Interface of a wafer sorting apparatus, where a FOUP is placed (ASML, 2020)
In case a custom solution is chosen to be developed to handle the wafers, it must be able to
accommodate the FOUP and open the front door automatically. In addition to that, FOUP must be
accommodated, meaning there must be three protruding pins, that allow FOUP to be accurately
located and seated every time it is loaded.
All mentioned features already exist on the standard wafer sorting equipment at premises of
ASML. This leaves no other rational option than to design the warpage measuring tool in a such
way, that usage of existing wafer sorting equipment is implemented. As adding additional
hardware to the inside of expensive commercial equipment is risky and yet results are uncertain,
the design choices are narrowed down to the warpage measuring tool resembling a FOUP.
Top interface
Frontal interface
Bottom interface
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All the reasoning above would lead to the following design choices and realization of previously
mentioned functions – wafer handling, preventing contamination and providing interface for wafer
handling in a way, which is schematically depicted from the top view in Figure 13.
Specimen of warped wafers are stored in up to three FOUPs (depending on the configuration of
the wafer sorter), whereas one of the load ports is occupied by the warpage measuring tool itself.
The robotic manipulator inside the wafer sorter can pick up warped wafer from any of the FOUPs,
move and load it to the tool for measuring. Once the measuring is completed, the robotic
manipulator unloads the warped wafer and returns it to the same slot in the same FOUP or into
different FOUP if desired. A 3D visualization of the wafer sorter with warpage measuring tool and
a FOUP loaded can be seen in a Figure 14 below.
Figure 14. 3D CAD render of warpage measuring tool (green) as an add-on to the wafer sorting
apparatus with a standard FOUP (orange) loaded
Designing warpage measuring tool, that resembles a standard FOUP, so it can be integrated with
a wafer sorter is possible in a few methods that are covered below.
First option of providing required interfaces is modifying a standard FOUP by drilling holes and
adding supplementary parts and equipment needed to measure the warpage. This ensures all the
interfaces of a FOUP are correct and the tool can be successfully integrated with the wafer sorter.
There are a couple of downsides to this method, however. First and most important of all, FOUPs
are meant to be used for transporting and storing the wafers, not to be used as a base for precise
Warpage
measuring
tool FOUP FOUP FOUP
Wafer sorter
Test wafer
pickup/return
Test wafer loaded
for measuring
Wafer sorter moves the test wafer
Figure 13. Schematic drawing of warpage measuring tool as an add-on to the wafer
sorting apparatus
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measuring equipment. The main body of a FOUP is made of polycarbonate material which is not
stiff enough to be used as a base for measuring equipment, where normally thick granite or steel
plates are being used. Secondly, using standard FOUP leaves very little to almost no space for
adding supplementary structures and parts to hold the sensor(s) and other parts or apparatus should
they be needed. A similar execution of a tool (not warpage measuring related) can be seen in one
of Estion-Technologies GmbH products below in Figure 15, where additional equipment is added
to a standard FOUP.
Figure 15. E-Wafer-Dockingstation of Estion-Technologies GmbH (Estion-Tech GmbH, 2020)
The second option is to design a custom enclosure, that resembles standard FOUP. This method
allows for designing stiff and robust structure with better utilization of the space available on the
wafer sorter between the load ports. While this method should lead to a better performance of the
warpage measuring tool, it is more challenging task to design the body of the tool, as it has to have
matching interfaces with a FOUP – the V grooves with a center hole for kinematic pins and active
clamping on the bottom surface, as well as the frontal interface, to accommodate the door from a
FOUP, ensuring a tight seal between the frame and the door, and also ensuring that door latch
correctly and can be automatically opened by the wafer sorter. Similar solutions of executing a
tool such way (not warpage measuring related) can be found in products from Brooks Automation
and are presented in Figure 16 below.
Figure 16. Custom tool for calibrating wafer sorting apparatus with interfaces of a FOUP
(MicroTool Technology, 2020)
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Throughout the procedure of measuring the wafer, it is important to have the wafer located and
supported accurately and steadily the whole time. Ideally, the wafer should be supported without
adding any additional loads and stresses that lead to distortion from the original shape of the wafer.
In case the measuring procedure involves moving the wafer itself, wafer support shall be robust
against forces, vibrations and similar disturbances caused by the wafer moving mechanism.
During the research phase and talks with employees from ASML, many potential ways of
supporting the wafer were found. The complexity varies from having the wafer to sit on three
support points, to mechanically gripping the wafer by the edges and even suspending the wafer
mid-air by implementing vacuum and compressed air in cases where the requirements are the most
demanding.
In the sections below, a few most feasible options of supporting the wafer are presented and
explained in more detail.
Vacuum chuck
One of commonly used methods to support the wafer is using a vacuum chuck and a common
example of a vacuum chuck can be seen in Figure 17. A vacuum chuck is a cylinder with milled
pocket grooves on the top surface, where a flow of vacuum is provided to the grooves. When
silicon wafer is placed on top of the chuck, a tight seal appears between the pocket and the wafer
surface, resulting in silicon wafer being clamped to the chuck.
Figure 17. Vacuum chuck (MTI Corporation, 2020)
This method of wafer clamping is already used in some of the modules within ASML machines.
Implementing this method of supporting the wafer allows rotating the wafer while it is clamped.
Since warped wafers are in interest in this project, clamping not a flat wafer results in applied
clamping force over circular area, therefore the wafer is distorted from its original shape and to
get accurate results of wafer warpage, the additional deformation must be taken in to account.
Using this method, both 200 mm and 300 mm diameter wafers can be supported with no additional
tweaks. The downside to this method is that silicon wafers can only be supported on the bottom
surface, as flipping, and clamping the wafer would result in top surface being contaminated and
this would result in failing to meet the performance requirements stated in upcoming chapters.
Three-point support on the bottom surface
A plane can be defined by three points in space. Having three support points around the
circumference of the wafer it is guaranteed that wafer will stay stable, despite the shape of the
wafer. The supports are spaced 120° apart of each other. Three-point support on the surface case
is presented in Figure 18 below.
25
Figure 18. Schematic of three-point wafer support on the bottom surface (Ito, Natsu, & Kunieda,
2010)
In case the wafer is supported using this method, the stability of the wafer depends entirely on the
weight and the friction between the surface of the wafer and the support, since there is no active
clamping mechanism present, such as vacuum or mechanical clamping. In case the wafer needs to
be moved or rotated while being supported using this method, friction between the wafer and the
support and acceleration are the key factors for a stable wafer support.
This support method, however, would not fully comply with performance requirements (stated in
upcoming chapters) in case the wafer had to be flipped, as it would result in the top wafer surface
being touched, thus limiting the number of options to compensate for gravity induced deflection.
What is more, due to natural shape of wafer warpage, which tends to be mostly symmetrical with
respect to X and Y axes of the wafer, the wafer will most likely not rest horizontally, since the
supports are not symmetric with respect to X and Y axes. Depending on the measuring principle,
this might be a factor that needs addressing when measuring or processing measured data.
Using this method, both 200 mm and 300 mm diameter wafers can be supported with no additional
tweaks.
Three-point support on the edge
Like the previous method, the only difference is that the support points are moved further away
from the center and the wafer rests on the edge rather than resting on the surface as in the previous
method.
Figure 19. Schematic of three-point wafer support on the edge (Natsu, Ito, Kunieda, Naoi, &
Iguchi, 2005)
Having the wafer to rest on three points touching the very edge of the wafer, complies with design
requirement (described in upcoming chapters), which states that the wafer cannot be touched
further away than two millimeters from the edge and thus allowing the wafer to be flipped and
supported on the other surface should it be needed.
Supports
Wafer
120°
26
To simplify the post processing of measuring data in case the wafer cross-section is scanned, a
cutout slit for the beam of the sensor can be added to the support pins if wafer is supported by the
edge. Such case is presented in Figure 20. This would result in sensor registering only the values
while reading the distance to the wafer and registering no values while outside the range of the
wafer. This is only valid, when increments of rotation are chosen in a such way, that once the
support is located underneath the sensor, the sensor beam is located within the slot, not above the
protruding edges of the support.
Figure 20. Wafer support pin with a slit.
According to W.Natsu et al. (2005), three-point support is better in terms of positioning
repeatability and anti-disturbance ability when compared to one-point support (vacuum chuck).
Using this method, both 200 mm and 300 mm diameter wafers cannot be supported without
additional tweaks. Supporting a 200 mm diameter wafer on the edge would result supports sitting
on the surface of 300 mm diameter wafer. In case this support method should be implemented for
both type of wafers, support arms, holding the support pins, would have to be made extendable,
so in both cases wafer is supported by the edges.
Four-point support on the edge
This method is identical to the three-point support on the edge method, mentioned above, the only
difference being added fourth support point and now the supports being spaced symmetrically with
respect to X and Y axes of the wafer. While having fourth support might seem counter-intuitive,
as only three points are needed to define a plane, and four points would result in over constrained
system, in ideal case silicon wafer warpage follows symmetrical shape with respect to X and Y
axes of the wafer, the wafer would rest horizontally, as both wafer warpage and the supports would
be symmetrical with respect to the X and Y axes of the wafer.
However, the ideal case scenario mentioned is highly unlikely as even the smallest inaccuracies
would contribute significantly to inaccurate end results. Even though silicon wafer would deform
and comply to the four support points resting stable, this would result in deformed wafer, therefore
unreliable warpage measurement results.
Silicon wafer
Wafer support pin
27
Figure 21. Schematic drawing of saddle shaped warped wafer supported on four points on the
edge
Like for the three-point support on the edge mentioned above, same additional adjustments would
need to be implemented to support wafers of different diameters.
Mechanical edge grip
One of the methods available to grip and securely locate the wafer, commonly used in wafer sorting
equipment is mechanical grip. Supporting the wafer in such way does not contaminate nor scratch
any of wafer’s surfaces. This method requires at least three supports spaced 120 degrees apart and
at least one support to be extendable on longitudinal direction to apply the force and mechanically
clamp the edge of the wafer. Schematically, the method is depicted below.
Figure 22. Schematic drawing of mechanical edge grip (top view)
This method requires force being applied to the silicon wafer. As silicon wafer is very thin in
comparison to its diameter, even minor forces can cause unwanted deflections of the wafer. While
these minor deflections are not that critical when it comes to wafer handling in mentioned wafer
High
zone
High
zone
Low
zone
Low
zone
Support
point
Support
point Support
point
Support
point
Fclamp
Silicon wafer
Support in clamped
position
Support in unclamped
position Stationary support
Stationary edge supports
Retractable edge support
Silicon wafer
Figure 23. Schematic drawing of mechanical edge grip (side view)
28
sorting equipment, when measuring the wafer is in interest, deflections due to external forces
ideally should be non-existing, so the wafer is in its true shape.
Vertical wafer support
In case gravity induced deflection compensation method (discussed in further chapters) is chosen
to be to measuring the wafer vertically, the wafer must be supported accordingly. Horizontally and
vertically supported wafers are schematically presented below in Figure 25.
Figure 24. Horizontally and vertically supported wafer (Jansen, 2006)
To support silicon wafer vertically, it should be done by locating the wafer on 2 stationary V-
grooves on the bottom of the wafer and gently clamp it to a third support point. Such clamping
mechanism is schematically depicted in Figure 25. Even though such clamping method would
cause additional deformation on the wafer, it has been observed by Jansen (2006), that
deformations of a 200 mm diameter wafer with a thickness of 0.6 mm can be as low as 0-3 µm.
Figure 25. Schematic drawing of supporting the wafer vertically (Jansen, 2006)
As it is most likely for the wafers to be loaded and unloaded by existing wafer sorting machine, it
means that wafers will be handled in a horizontal orientation, eventually meaning that once the
wafer is loaded onto the measuring tool, the wafer will have to be rotated 90 degrees to a vertical
position. This results in additional tilting mechanism or stage that must be implemented, adding
even more complexity to the final design.
29
2.5 Warpage measuring
2.5.1 Measuring principle
Stationary multi sensor array
One of the least complex methods, requiring no moving actuators, stages or gantries is to place the
silicon wafer stationary and have multiple sensors to measure different points of the wafer.
Illustration of such principle is presented in Figure 26 below. The XY resolution of measured
results is dependent on the number of sensors. Such method is used in E+H instruments mentioned
in Chapter 2.2. Some of the instruments have as much as 69 sensors for wafers of 300 mm in
diameter. (E+H Metrology GmbH, 2020)
Figure 26. Multiple distance sensors measuring different locations of a surface (Keyence
Corporation, 2020)
The benefits of this method are that silicon wafer is measured stationary, meaning no additional
vibrations occur and affect the results and that it requires no moving mechanisms to measure the
wafer. The downside to it is that the number of sensors limits result resolution. In case dense
measuring grid is needed, the number of sensors and peripheral equipment to process sensor data
might become unacceptably expensive. Another limiting factor for the XY resolution is the size of
the sensor – resolution of measuring grid can be as dense, as densely sensors are mounted next to
each other. As an example, if a measuring sensor is a cylinder, 20 mm in diameter, and measures
a single point, physical spacing between measured points of two sensors is 20 mm and that is
excluding any additional space for hardware required to mount the sensors.
XY scanning
Like the contact measuring probe method, mentioned in Chapter 2.2, a contact probe can be
replaced with a contactless measurement sensor and wafer can be measured non-contact way,
eliminating, or reducing contamination and wafer damaging related to contact measuring method.
XY resolution of the measured grid now is limited by the measuring spot size of the sensor in
addition to the mechanical positioning resolution.
This measuring method allows to have a grid of points measured across the surface of the wafer.
Depending on the resolution requirements, number of points measured can be easily adjusted for
the optimal ratio between the measuring time and XY resolution of the results.
Positioning of the sensor above the wafer can be achieved using cartesian positioning gantry or
also a polar coordinate gantry. In case of cartesian gantry, silicon wafer shall remain stationary,
whereas the XY gantry is located above the wafer. A cartesian gantry wafer measuring setup was
implemented in a research by H.Liu, et.al. (2013) and is presented in Figure 27 below.
30
Figure 27. XY stage for wafer warpage measuring (Liu, et al., 2013)
The other option, polar gantry, is a convenient solution, providing more design freedom and better
utilization of the space available, since silicon wafer has a circular shape. Instead of having two
linear stages, positioning the measuring sensor on X and Y coordinates accordingly, one linear
stage is replaced with a rotary stage, thus resulting in positioning the measuring sensor in polar
coordinate system. Measuring grid and schematic drawing of such measuring system is presented
in Figure 28 below.
Figure 28. Polar coordinate system and schematic drawing of polar measuring system
Having a polar coordinate positioning system, and knowing the coordinate of a linear actuator and
rotation angle of a rotary stage, the position of the sensor can be transformed into cartesian
coordinates with a following relation:
{𝑥 = 𝑟 cos 𝜃𝑦 = 𝑟 sin 𝜃
(1)
where x, y – cartesian coordinates of the sensor, r – position of a linear stage, 𝜃 – angular position
of the rotary stage.
Using either of the methods to measure the wafer warpage would result in a point grid with height
values. Knowing X and Y coordinates, data fitting could be performed to get more dense height
map of the wafer in case measured grid is too sparse. Measurement grid can be plotted, and the
visual representation of measurement results can be expected to be as below in Figure 29.
31
Figure 29. Height measurement grid (ASML, 2020)
Line scanning
Same hardware setup as mentioned in the paragraph above allows for a slightly different execution
of measuring the wafer – section scanning. Instead of creating XY grid of measured points, the
wafer is scanned across a straight line through its center, and then another line across the wafer is
measured, rotated by defined angle around the center point of the wafer. Measuring method is
depicted below in Figure 30.
Figure 30. Wafer line scanning principle (Kobelco, 2020)
The resolution of measurement results using this method would be the same as using the XY
scanning method described in previous paragraph. Using the line scanning method, however, the
line scan is performed in one continuous movement in a single direction, in comparison to multiple
bi-directional steps when the grid is scanned. Using a single continuous movement minimizes the
influence on backlash of the mechanical components in the gantries improving the accuracy and
repeatability of the results.
Alternatively, instead of scanning a straight line and then rotating the wafer by an increment, it is
possible to perform a scan while rotating the wafer at fixed linear position and then moving the
sensor by an increment on the linear direction. This would result in multiple concentric circles
measured going from outside towards the center of the wafer. This method gives no advantage, it
might be more convenient to have measured data in that circular order for ease of post-processing.
Measured cross sections can be plotted in polar coordinates, resulting in 3D height map of the
wafer.
32
Edge scanning
The last method to be discussed is measuring the edge height of the wafer around the
circumference. This can be done either having a distance sensor mounted on XY positioning
gantry, as mentioned in previous paragraphs, yet using such measuring method on XY positioning
gantry would be inefficient, or using dedicated edge measuring sensor, while rotating the wafer
around its center point. While the latter method is commonly used for inspection of wafer edges,
detecting cracks and chipped edges, the height variation of the edge can be measured as well.
Measuring method is schematically depicted below in Figure 31.
This method requires only a rotary stage with a vacuum chuck wafer support and a single sensor
while providing detailed height measurements around the edge. The downside to this method is
that using such setup, a major part of the wafer is not measured. By placing the sensor to a known
position in relation to the vacuum chuck, the height of center part, that is clamped by the vacuum
chuck can be known in addition to the height of the wafer’s edge. The situation is schematically
depicted below from the top view, where the blue zone represents known or measured area,
whereas the orange color shows the unmeasured or unknown area. Wafer diameter depicted is 300
mm, diameter of the vacuum chuck is 50 mm and measured area around the edge is 20 mm wide,
as the measuring range is taken from product’s catalog by BRS-Bright Red Systems GmbH (BRS
- Bright Red Systems GmbH, 2020).
2.5.2 Measuring sensor
There are multiple ways of measuring silicon wafer warpage depending on the accuracy and
resolution needed. Majority of commercially available tools, some of them mentioned in Chapter
2.2, use highly expensive optical interferometers, that are unmatched in terms of accuracy and
resolution. For this project, considering the accuracy requirements, that are roughly an order of
magnitude lower, compared to commercially available equipment, interferometers were omitted,
and more conventional measuring tools and methods were evaluated.
Warped wafer
Rotary stage
Edge measuring
sensor
Warped wafer
Edge measuring
sensor
Vacuum chuck
Measured edge
height
Unmeasured area
Figure 31. Schematic drawing of wafer edge scanning
Figure 32. Schematic drawing of wafer edge scanning and the area that is scanned/not scanned]
33
According to definition of the term warpage in Chapter 2.3, warpage is defined as a difference
between minimum and maximum deviation of a mid-plane points from a reference plane, parallel
to the wafer. To measure true wafer warpage, mid plane of the wafer must be precisely known and
therefore wafer’s thickness would need to be measured, for the thickness variation of the wafer
not to affect the result accuracy.
In scope of this project, according to design requirement specification, requirement P.4 (stated in
upcoming chapters), the results of measured warpage must be within ±10 µm of actual warpage.
According to the document with a list of wafers used within the company provided by the company
itself, it can be observed that all 300 mm wafers come with TTV from 0 to 0.6 µm and wafer’s
center thickness within ±3 µm. This indicates, that even without measuring wafer’s thickness and
calculating the real distance to the median plane, but only by measuring the distance to wafer’s
surface, as long as the measuring sensor’s accuracy is within a range of ±6.4 µm, the performance
requirement is still met, and the necessity of measuring wafer’s thickness can be abandoned.
As stated by the product requirement specification in further chapters, requirement P.3, the
measurable warpage must be ±1 mm, meaning the sensor must have a measuring range of at least
two millimeters in an ideal case scenario.
While there are no hard requirements on XY resolution of the measurements, the measuring spot
size of the sensor might be one of the factors limiting the XY resolution. Depending on the type
of sensor, for measuring range of at least 2 mm and resolution of ±10 µm, measuring spot size
could vary from approximately 5 mm all the way down to 3 µm (Micro-Epsilon, 2020).
A several sensors to measure wafer warpage are covered in more detail below.
Capacitive displacement sensors
Capacitive proximity sensors are non-contact measuring devices, capable of high-resolution (up
to nanometer level) distance and thickness measurements. Working principle of capacitive sensor
is based on change of capacity between electrodes of the sensor when the distance between the
measured object and the sensor changes. Fairly simple construction of the sensor provides high-
accuracy measurements for relatively low price. The downside to capacitive sensors is relatively
large measuring spot size, meaning capacitive sensor is not well suited for measuring the warpage
by scanning the wafer with a single sensor for a high-resolution measuring grid and therefore well
suited for a stationary multi sensor array measuring.
For the criteria mentioned above, a suitable capacitive displacement sensor for measuring wafer
warpage can be found with following performance specifications:
Table 4. CSH2-CAm1,4 performance specification
Sensor code, manufacturer CSH2-CAm1,4, Micro-Epsilon
Measuring range, mm 2
Extended measuring range, mm 4
Linearity, µm ±0,5
Resolution, nm 1,5
Active measuring area (spot size), mm Ø 8.1
34
35
Confocal distance sensors
Confocal sensors optical distance measuring sensors based on reflected light principle.
Polychromatic white light is focused onto target surface by a multiple lens system, that are
arranged in a such way, that white light is dispersed into multiple color light. Depending on the
distance to the measuring target, one specific wavelength is focused on the target and it is being
reflected to a light sensitive sensor element. Depending on the color, thus wavelength, of the color
reflected to the light sensitive sensor element, the distance to a target object is determined.
Confocal sensors are capable of measuring thickness of transparent objects using a single sensor;
therefore, the thickness of a silicon wafer can be measured at any point as well, increasing the
measuring accuracy by eliminating the wafer thickness error.
Figure 33. Confocal sensor working principle (Micro-Epsilon, 2020)
While confocal distance sensors are highly compact, accurate and versatile, they are significantly
more expensive than the rest. Confocal sensors typically have a measuring spot size within the
range ≥ 60µm, going down all the way even to 3 µm. Such small measuring spot size makes these
sensors ideal for measuring the warpage by scanning the wafer using a single sensor. Using such
sensor, the XY measuring resolution is more likely to be limited by positioning accuracy of the
positioning mechanism rather than the sensor itself.
A confocal sensor, matching criteria mentioned above can be found with such performance
specifications:
Table 5. IFS2405-3 performance specification
Sensor code, manufacturer IFS2405-3, Micro-Epsilon
Measuring range, mm 3
Linearity (distance), µm ±0,75
Linearity (thickness), µm ±1,5
Resolution, nm 36
Active measuring area (spot size), µm 9
While searching for a suitable measuring sensor, an existing confocal sensor was found at the
premises of the company. Even though the sensor is a relatively old one and is no longer produced
by the manufacturer, the performance specifications of the sensor match the ones needed for the
purpose. The specifications of the existing sensor can be found in the table below:
36
Table 6. IFS2401-3 performance specification
Sensor code, manufacturer IFS2401-3, Micro-Epsilon
Measuring range, mm 3
Linearity (distance), µm ±1,5
Resolution, nm 120
Active measuring area (spot size), µm 25
Laser displacement sensors
Laser displacement sensors typically use a laser light source and a CMOS (Complementary metal–
oxide–semiconductor) detector. The light beam is projected through a lens to a target object, then
it is reflected from the surface and through another lens is focused to a CMOS detector. Depending
on the change of the distance to a measured object, the angle between projected and reflected beam
will change and that change is registered by the CMOS detector which translates the reflection
angle into distance. Working principle of laser triangulation sensor can be found in Figure 34.
below:
Figure 34. Laser triangulation sensor schematic and working principle (MTI Instruments Inc,
2019)
Like confocal sensors, laser triangulation sensors also tend to have small measuring spot size,
making them ideally suited for scanning the wafer with a single sensor. Suitable laser triangulation
sensor can be found with the following performance specification:
Table 7. ILD1750-2 performance specification
Sensor code, manufacturer ILD1750-2, Micro-Epsilon
Measuring range, mm 2
Linearity (distance), µm ±1,6
Active measuring area (spot size), µm 35
37
2.6 Gravity induced deflection
Since silicon wafers have a large diameter over thickness ratio, wafers sag and deform significantly
when placed onto supports in comparison to the magnitude of wafer warpage. Gravity induced
deflection could be in order of tens of micrometers for a 200 mm diameter wafer all the way over
100 micrometers for a 300 mm diameter wafer according to Jansen (2006). As only the
deformation due to warpage is in interest, all the other causes of deformation must be eliminated
or reduced to a minimum, one of them being the weight of the wafer.
Figure 35 below illustrates the principle of eliminating gravity induced deflection. In the figure,
wafer is depicted supported by a single chuck, however, the principle stands for three-point support
as well. When wafer is horizontally resting on three support points, total deflection of the wafer
(y(x,y)) is superposed of deflection of the true shape of the wafer (s(x,y)) and also deflection due
to gravity (g(x,y)).
Figure 35. Schematic principle of wafer deflection due to gravity (Natsu, Ito, Kunieda, Naoi, &
Iguchi, 2005)
There are several methods of how gravity induced deflection could be minimized, each having
pros and cons compared to each other.
Measuring the wafer vertically
By placing silicon wafer vertically and freely supporting it, the wafer becomes substantially more
rigid and does not sag as if it was placed horizontally. Vertical wafer measurement is used in
commercial, high precision wafer inspection tools. It should yield the most accurate results out of
all methods proposed. However, such method is the most complex out of all, due to the reasons
mentioned in vertical wafer support section above.
Figure 36. Horizontally and vertically supported wafer (Jansen, 2006)
38
Inverting the wafer
This method is one of the most used methods amongst researchers and it is also the one used in
current way of measuring wafer warpage. What is more, using such method allows geometric error
of measuring equipment to be eliminated as the wafer would be measured twice, i.e., front, and
back surfaces. When the top surface of the wafer is measured, total deflection of the wafer can be
expressed as follows:
𝑦𝑓(𝑥, 𝑦) = 𝑔𝑓(𝑥, 𝑦) + 𝑠𝑓(𝑥, 𝑦) (2)
where according to Figure 35, y(x, y), g(x, y) and s(x, y) are total deflection, deflection due to
gravity and deflection of the wafer true shape accordingly, index t meaning top surface of the
wafer.
Similarly, the wafer is inverted and now the bottom surface is measured the same way and the
deflection of the bottom surface can be expressed the same way as for top surface:
𝑦𝑏(𝑥, 𝑦) = 𝑔𝑏(𝑥, 𝑦) − 𝑠𝑏(𝑥, 𝑦) (3)
where index b represents the bottom surface.
As the thickness deviation of the wafer is significantly smaller than the total deformation of the
wafer, it is assumed that deformation of the true wafer shape is the same for top and bottom
measurements:
𝑠𝑓(𝑥, 𝑦) = 𝑠𝑏(𝑥, 𝑦) = 𝑠(𝑥, 𝑦) (4)
It is also assumed, that gravity induced deflection is the same for measured top and bottom
surfaces, therefore expressed:
𝑔𝑓(𝑥, 𝑦) = 𝑔𝑏(𝑥, 𝑦) = 𝑔(𝑥, 𝑦) (5)
Subtracting two measurements at corresponding points yields the result of wafer warpage, which
can be expressed:
𝑠(𝑥, 𝑦) =𝑦𝑓(𝑥, 𝑦) − 𝑦𝑏(𝑥, 𝑦)
2 (6)
Gravity induced deflection can be acquired using the following equation:
𝑔(𝑥, 𝑦) =𝑦𝑓(𝑥, 𝑦) + 𝑦𝑏(𝑥, 𝑦)
2 (7)
Equations and description acquired from W.Natsu et al. (2005)
Implementing this method for minimizing the effect of gravity induced deflection requires no
additional hardware in case existing wafer sorting apparatus is implemented to manipulate the
wafers, as the apparatus has wafer flipping mechanism within itself already. After top surface of
the wafer is measured the data is stored. The operator would use the wafer sorting apparatus to
pick up the wafer from the tool, flip it, align the notch for precise grip and accurate placement, and
put the flipped wafer back to the measuring tool. It is worth mentioning, that once the wafer is
measured, in case it was rotated, it should be moved back to its initial position, then flipped and
39
loaded back. All the actions are executed automatically, therefore the accuracy of wafer alignment
and positioning is limited by performance of the wafer sorting apparatus, should it be used. As
finding information on existing wafer sorting apparatus was complicated, alternative industrial
solutions are found to be positioning the wafer within accuracy of ± 0.1 – 0.3 mm as well as rotary
aligning the wafer within ± 0.1 – 0.3°, depending on the model (Jel Corporation, 2020). The
performance of wafer sorting apparatus at the company’s premises is expected to match or even
outperform similar equipment with mentioned positioning and alignment accuracies, therefore the
positioning and alignment accuracy of the existing wafer sorting apparatus will be considered to
be the same. After surface of flipped wafer is scanned and data is stored, a calculation script would
be run where the data is stored to align corresponding measured points and perform the calculations
as described above. This method, however, is not possible if the measured wafer is supported on
a vacuum chuck or three-point support closer to the center, since if the wafer is flipped, wafer’s
top surface would be scratched and damaged while there is a “need” requirement, that wafer must
not be touched on the top surface.
Limitations of such method are that there are several assumptions made during the calculations
described above. For very accurate results, these assumptions should be replaced by actual and
weighted numbers, such as wafer thickness variation. Additionally, slope of wafer placement,
deflection due to stress of surface treatments should be considered. It is also assumed that stiffness
is linear throughout the warpage range, and that is something that should be investigated in more
detail for more accurate results.
Subtracting the sag of a flat silicon wafer
Another method to compensate for gravity induced deflection is to measure the sag of a flat silicon
wafer that comes with warpage ≤ 5.00 µm according to the documentation provided by the
company. After the top surface of a test wafer specimen is measured, for each corresponding point
of the measured wafer, sag value of corresponding point of flat silicon wafer, would be subtracted,
resulting in warpage of a free form wafer. This method, however, while requiring least effort and
no additional hardware is most likely to provide least accurate results as in this case it is assumed,
that the gravity induced deflection of a flat and warped wafer would be the same, even though with
higher amount of warpage on the wafer, the stiffness will change accordingly therefore
gravitational sag will change as well. The accuracy of this method could be easily tested and
compared to other methods once the prototype is assembled.
Subtracting FEA calculated sag
Last of the methods proposed to compensate for gravity induced deflection once more consists of
subtracting known sag value on corresponding measured points. Using this method, a FEA must
be performed to get gravitational sag values, which are then subtracted from measured deflection
of the wafer surface. This method requires no additional hardware mechanisms, only accurate
results of FEA. The downside of this method would be that result is dependent of quality of FEA.
Many factors contribute to the accuracy of the simulation results, one of which being mechanical
properties of the silicon wafer. Silicon wafer has orthotropic structure which results in different
material properties, depending on the orientation of the wafer, such as Young’s modulus, Poisson’s
ratio and similar. Multiple sources provide different values of mentioned properties. Depending
on the loading type and orientation of the crystalline structure of the wafer, Young’s modulus can
vary all the way from 130 GPa to 169 GPa (Hopcroft, Nix, & Kenny, 2010). What is more, while
measuring the wafer with gravitational sag, the true shape of the wafer is unknown and therefore
FEA might be inaccurate, as gravitational sag would be different for a flat wafer, bowl or saddle
warped wafers. As mentioned, such method requires no additional hardware mechanisms,
40
therefore if accurate FEA is performed, the results could be relatively easy compared to other
proposed methods.
All the proposed methods, expect measuring the wafer vertically, can be implemented without
additional hardware if wafer sorting apparatus is used for handling the wafers and the results can
be compared, leaving options for improvements.
41
3 IMPLEMENTATION
In this chapter the process leading to generated concepts is defined, including identifying the main
functions of the product, defining product requirement specification, and describing potential
solutions for executing the functions and therefore ensuring requirement criteria is met.
3.1 Functional breakdown
The first step in product implementation stage was to break down the intended functionality of the
product into sub-functions so the means to execute the functions could be found and developed.
The resulting function breakdown tree is presented in Figure 37. Resulting sub-functions were
used for product requirement specification in the upcoming stage of the project.
The functionality of the tool was divided into 3 categories: wafer handling, wafer measuring and
data acquisition, interpreting and result display.
Wafer warpage measuring
tool
1. Wafer handling 2. Wafer measuring3. Data acquisition, analysis
and result display
2.1 Measure the wafer warpage
2.2 Compensate gravity induced deflection
3.1 Process raw sensor data
3.2 Display results
1.1 Provide interface for loading specimen
1.2 Pick up the test wafer
1.4 Load the wafer
1.3 Orient/align the wafer
1.5 Support/fix the wafer
1.6 Unload the wafer
1.7 Prevent contamination of the test specimen
Figure 37. Functional breakdown of the warpage measuring tool
3.2 Product requirement specification
Since the project was initiated by the company to design and manufacture a functional prototype
so it can contribute to and enhance system testing and verification workflow, several employees
and stakeholders were invited to participate in finalizing design requirement specification that the
final product must meet. The criteria of the most critical requirements were identified as a need
whereas additional features, that would benefit the user, but are not critical, were identified as a
want. The full product requirement specification can be seen in Table 8 below.
42
Table 8. Product requirement specification
No. Requirement Criteria Verification Need/want
Performance
P.1 Measurable wafer diameter 300 mm Analysis/Testing Need
P.2 Measurable wafer diameter 200 mm Analysis/Testing Want
P.3 Measurable wafer warpage ± 1 mm Analysis/Testing Need
P.4 Measuring accuracy ± 10 µm Analysis/Testing Need
P.5 Measurable wafer thickness 775 ± (3 + coating thickness) µm Testing Need
P.6 Measurable wafer thickness 725 ± (15 + coating thickness) µm Testing Need
P.7 Measurable wafer thickness 1.2 mm ± (thickness variation + coating thickness) Testing Want
P.8 Measuring method Non-contact Analysis/Testing Need
P.9 Compensation for deflection due to
gravity
Accurate enough to satisfy measuring accuracy
requirement Analysis/Testing Need
P.10 Robust against wafer coatings,
exposed and processed features No influence on measuring performance Analysis/Testing Need
P.11 Coated and processed wafers must not
be damaged during the measuring
Sensor must not cause any damage to coated and
processed wafers (e.g., cure photosensitive resist) Analysis/Testing Need
P.12 Display magnitude of measured
warpage Absolute value of the warpage of the wafer is displayed Testing Need
P.13 Identify warpage shape Display amount of warpage on X and Y axis Analysis/Testing Need
P.14 Display 3D height map of the wafer Height map on measured points presented in a plot Testing Need
P.15 Amount of measuring points Enough to do curve fitting on X and Y axis to see how
measured points deviate from the curve Analysis/Testing Need
Operation
O.1 Step by step manual operation by the
operator
Wafer measuring sequence executed step by step by
operator input Demonstration Need
O.2 Automatic measuring via single
button push
Wafer measuring sequence fully automatic after pushing
the button Demonstration Want
O.3 Control via external computer Convenient enough for a trained member of D&E to use
it with an operation manual Testing Need
O.4 Single screen GUI Custom single screen GUI to be used alongside GUI of
wafer sorter Inspection Want
Wafer handling
W.1 Clean interface to load test specimen Accommodate FOUP of test specimen Analysis/Testing Need
W.2 Clean, closed environment handling Wafers loaded and unloaded automatically in enclosed
surrounding Analysis/Testing Need
W.3 Wafer support throughout the
measuring process Wafer supported either by edges or bottom surface only Analysis/Testing Need
W.4 No touching the wafer on the top
surface
Wafer can’t be touched or supported on the top surface
at any time Analysis/Testing Need
W.5
No touching wafer on the bottom
surface further than 2 mm away from
edge
Wafer can’t be touched or supported further away than 2
mm from the outer edge on the bottom surface Analysis/Testing Want
W.6 Wafer support must not scratch,
contaminate, or damage the wafer Non-metal wafer support Analysis/Testing Need
Spatial requirements
S.1 Weight Must be light enough to fit wafer sorter should it be
needed Inspection Need
S.2 Dimensions Must fit wafer sorter on outermost FOUP slots should it
be needed Inspection Need
Maintenance
M.1 Easily disassembled Easy to disassemble to improve the prototype, change
parts if needed Demonstration Want
43
3.3 Concept generation
To generate potential concepts of the final product in organized and structured way, a
morphological matrix was created and can be seen in Table 9. Functions that were identified in the
functional breakdown stage previously, were now listed vertically and multiple methods to execute
those functions were presented in the columns alongside in the matrix. As several functions could
be executed by the same means, therefore they were combined into the same function tabs and the
methods proposed could fulfill the combined functionality. Such case is seen in the second function
tab in the morphological matrix, where the body and housing of the tool would provide both
interfaces for wafer handling and protecting the inside of the tool and the test specimen from
outside contamination.
The concepts are generated going from top to bottom of the matrix by choosing one method per
function.
Table 9. Morphological matrix
Function Method 1 Method 2 Method 3 Method 4 Method 5 Method 6
1. Handling the
wafer
Wafer sorting apparatus Custom wafer
handling equipment
2.
Interface for
wafer
handling and
preventing
contamination
Modified FOUP Custom FOUP Custom housing
(not a FOUP type)
3. Supporting
the wafer
Vacuum chuck 3-point support on the
bottom surface
3-point support on the
edge Mechanical edge grip
4-point support on
the edge
Vertical
support
4. Measuring
sensor Capacitive sensor Confocal sensor Laser sensor
5. Measuring
the wafer
Stationary multi sensor
array XY scanning Line scanning Edge scanning
6.
Compensating
gravity
induced
deflection
Measuring the wafer
mounted vertically Inverting the wafer
Subtract FEA acquired
deflection value
Subtracting sag value
of a flat wafer
44
3.3.1 Concept 1 - Stationary measuring
Generation of the first concept is presented in morphological matrix below, going from top to
bottom for every function of the tool.
Table 10. Morphological matrix – concept 1
The first proposed concept makes use of wafer sorting apparatus for wafer handling. The main
structure of the tool resembles all the interfaces of a FOUP, so it can be easily integrated with the
wafer sorting apparatus for loading/unloading the wafers as well as protecting the wafers from
external contamination. Silicon wafers would be supported by three points on the edge of the
wafer. To measure the warpage, a stationary array of nine capacitive distance sensors should be
used. This arrangement of distance sensors would allow five points per axis measured on
orthogonal X and Y axes. Sparse array of measured points would require fitting a curve between
the points. Sensor arrangement is presented in Figure x. below. To minimize gravity induced
deflection, gravitational sag value, acquired by FEA could be subtracted, instead of inverting the
wafer, as it requires no additional mechanical hardware.
Function Method 1 Method 2 Method 3 Method 4 Method 5 Method 6
1. Handling the
wafer
Wafer sorting apparatus Custom wafer
handling equipment
2.
Interface for
wafer
handling and
preventing
contamination
Modified FOUP Custom FOUP Custom housing
(not a FOUP type)
3. Supporting
the wafer
Vacuum chuck 3-point support on the
bottom surface
3-point support on the
edge Mechanical edge grip
4-point support on
the edge
Vertical
support
4. Measuring
sensor Capacitive sensor Confocal sensor Laser sensor
5. Measuring
the wafer
Stationary multi sensor
array XY scanning Line scanning Edge scanning
6.
Compensating
gravity
induced
deflection
Measuring the wafer
mounted vertically Inverting the wafer
Subtract FEA acquired
deflection value
Subtracting sag value
of a flat wafer
45
Figure 38. Concept 1 schematic top and front views
3.3.2 Concept 2 - Line scanning Generation of the first concept is presented in morphological matrix below, going from top to
bottom for every function of the tool. Dashed line indicates an alternative possible solution.
Table 11. Morphological matrix – concept 2
Function Method 1 Method 2 Method 3 Method 4 Method 5 Method 6
1. Handling the
wafer
Wafer sorting apparatus Custom wafer
handling equipment
2.
Interface for
wafer
handling and
preventing
contamination
Modified FOUP Custom FOUP Custom housing
(not a FOUP type)
3. Supporting
the wafer
Vacuum chuck 3-point support on the
bottom surface
3-point support on the
edge Mechanical edge grip
4-point support on
the edge
Vertical
support
4. Measuring
sensor Capacitive sensor Confocal sensor Laser sensor
5. Measuring
the wafer
Stationary multi sensor
array
XY scanning Line scanning Edge scanning
6.
Compensating
gravity
induced
deflection
Measuring the wafer
mounted vertically Inverting the wafer
Subtract FEA acquired
deflection value
Subtracting sag value
of a flat wafer
46
The second proposed concept follows the first one for wafer handling and providing the
interface for that. Silicon wafer should be supported by three points on the edge. To measure
the wafer, both laser triangulation and confocal sensors are suitable options, as they both have
sufficient measuring accuracy and small measuring spot diameters. Confocal sensors are more
expensive than laser triangulation sensors, therefore laser triangulation sensor is preferred for
less expensive prototype. However, there was a possibility to use already existing confocal
sensor for this project, therefore the warpage should be measured using a confocal sensor.
Measuring the wafer itself should be done by line scanning method, i.e., wafer should be
supported on a rotary stage, and above it, a linear stage should move the sensor across the
wafer. To minimize gravity induced deflection, both wafer inversion method and subtracting
deflection value, acquired by FEA could be used as wafer sorting apparatus allows flipping the
wafer. The concept is schematically depicted below in Figure x.
Figure 39. Concept 2 schematic front view
3.3.3 Concept 3 - XY vertical measuring
Generation of the third concept is presented in morphological matrix below, going from top to
bottom for every function of the tool in Table 12.
The third proposed concept once again incorporates usage of wafer sorting apparatus and
embodiment of a custom FOUP. As the name of the concept suggests – wafer should be supported
vertically. Bottom of the wafer should be placed in V-shaped grooves and a small force should be
applied to the top of the wafer, pushing it against a support point and preventing the wafer tipping
over. Since wafer sorting apparatus can only load and unload the wafers in a horizontal orientation,
there should be a tilting mechanism added, so once the wafer is loaded to the tool by the wafer
sorting apparatus and is clamped in position, the wafer is rotated 90 degrees to a vertical
orientation. To measure the warpage, an XY positioning stage with a laser triangulation sensor
should be used, which should be located parallel to the vertical wafer. Measuring the wafer
vertically causes no significant gravity induced deflection on the shape of the wafer.
47
Table 12. Morphological matrix – concept 3
Function Method 1 Method 2 Method 3 Method 4 Method 5 Method 6
1. Handling the
wafer
Wafer sorting apparatus Custom wafer
handling equipment
2.
Interface for
wafer
handling and
preventing
contamination
Modified FOUP Custom FOUP Custom housing
(not a FOUP type)
3. Supporting
the wafer
Vacuum chuck 3-point support on the
bottom surface
3-point support on the
edge Mechanical edge grip
4-point support on
the edge
Vertical
support
4. Measuring
sensor Capacitive sensor Confocal sensor Laser sensor
5. Measuring
the wafer
Stationary multi sensor
array
XY scanning Line scanning Edge scanning
6.
Compensating
gravity
induced
deflection
Measuring the wafer
mounted vertically Inverting the wafer
Subtract FEA acquired
deflection value
Subtracting sag value
of a flat wafer
Concept generation overview
Looking into 3 generated concepts a few observations can be made. First, handling silicon wafers
is a delicate process, requiring complex machinery, it was inevitable choice for this project to use
wafer sorting apparatus, as developing a custom wafer handling solution would be too
complicated. Following that, housing of the tool must also resemble standard FOUP with all its
interfaces to successfully integrate it with a wafer sorting apparatus. An existing FOUP could have
been implemented, but as it is dedicated for transporting and storing the wafers only, the space
inside is limited as well as plastic body of the FOUP makes it not a robust choice for a measuring
instrument. Therefore, all three concepts follow same wafer sorter and custom FOUP combination.
What is more, robotic arm inside wafer sorting apparatus is designed to handle wafers of one size.
That means, that handling multiple sized wafers is not possible and for that, the concept of the tool
will be designed for handling the most common sized wafers – 300 mm ones. If concept is proven
to be working, a similar tool can be designed for handling 200 mm diameter wafers. Compensating
gravity induced deflection is also a task, which can be executed in multiple ways. In case wafer is
measured horizontally, all but the first options of compensating gravity induced deflection can be
implemented and the most convenient one, yielding best results can be chosen for the future use.
48
3.4 Concept evaluation
While multiple concepts were generated in Chapter 3.3, to choose the one for further development,
a PUGH’s evaluation matrix was implemented. The criteria for ranking and evaluation were
generated both according to design requirement specification and also recommendations and
comments from experts within the company, including the ones to be using the tool if proven to
be successful and meeting the performance requirements.
PUGH’s matrix, however, served a purpose of being a guideline or recommendation. The choice
of the final concept was made during a meeting held with the same group of experts and fellow
employees of the company, and some ratings from the matrix came second to expert knowledge.
Importance factor was added to emphasize the key performance aspects of the tool (3 being the
most important, 1 – least important). Each concept was ranked in terms of how well it performs or
how well it matches the requirement (3 being good, 1 being bad). Score for how well concepts
meet the criterion is multiplied by the importance factor and the weighted sum of points is
presented in the last row.
Table 13. Scoring matrix
No. Criterion Importance Concept 1 Concept 2 Concept 3
1. Overall feasibility 3 3 2 1
2. Overall warpage measuring result quality 3 1 2 3
3. Z measuring accuracy 3 3 3 3
4. XY resolution 2 1 3 3
5. Mechanical system complexity 2 3 2 1
6. Price 2 3 2 1
7. Differently sized wafers can be measured 1 1 1 1
8. Electronics and control system
complexity 1 3 2 1
Sum: 18 17 14
Sum weighted: 39 38 33
The weighted results show that concept 1 got the highest score rating, while concept 2 came only
1 point short. Concept 3 has fallen behind the first two, due to higher mechanical complexity,
involving mechanisms that clamp the wafer, tilt it by 90 degrees, and XY stage to move the sensor.
This order of concepts turned out as expected. The simplicity of concept 1 results in highest
feasibility of manufacturing a prototype, as there would be only simple, easily manufacturable
parts and no off the shelf components, that could have a long lead time and result in delayed
assembly of the prototype. However, concept 1 would result in a very sparse measuring grid,
providing results that are merely satisfying and very reliant on polynomial curve fitting between
the measured points.
For that reason, it was decided to proceed with a concept 2 for further design and development.
49
3.5 Concept development
3.5.1 Detailed design of mechanical system
Concept overview
To better understand detailed design process later, a final CAD model of the concept is presented
below in Figure 40. The tool is shown without front door, which keep the inside of the tool clean
and the door is meant to be opened and lowered automatically by the wafer sorting apparatus.
Figure 40. Final 3D CAD model of the warpage measuring tool
In Figure 41, warpage measuring tool (green) is shown integrated with a wafer sorting apparatus
(white) and a FOUP container with warped wafer specimen inside (orange). Operation of the
tool, such as loading and unloading the wafers is covered in further chapters.
Figure 41. Wafer sorting apparatus (white) with integrated warpage measuring tool (green) and
a FOUP (orange)
50
Baseplate
Baseplate is intended to be used as a base to mount all additional components and assemblies on.
The baseplate needs to accommodate a linear stage, rotary stage, enclosure, and the bottom
interface grooves of a FOUP. The baseplate must be stiff and robust to make the warpage
measuring tool as an instrument robust and accurate, yet also light enough to make the tool easy
to transport and set up. Aluminum 5083-H111 is an ideal candidate for that purpose. Material stock
comes already machined within acceptable flatness and thickness tolerances. By choosing right
thickness of the baseplate, the part does not need to undergo various surface machining operations
to get the stock flat for further machining. CAD model of the baseplate is presented below in
Figure 42.
Figure 42. Baseplate from top and below
Initial choice of thickness was assumed to be 5 mm. To accommodate the depth of bottom grooves
of FOUP’s interface, additional bolted parts were designed. After design of complete mechanical
system reached the point where total mass of the system could have been evaluated, a rough FEM
analysis was performed on the baseplate.
The baseplate was constrained with enforced displacement Z=0 on two vertices furthest away from
the center on the slots, as well as displacement constraints X=0 and Y=0 on the same vertices on
one of the grooves to represent the kinematic pins from the wafer sorting apparatus, where the
baseplate will be seated. Mesh elements were chosen to be a size of 1 mm, judged by absence of
high local stress concentrations and continuous smooth gradient of deflection. FEA setup is
presented in Figure 43 below.
Figure 43. FEA mesh and constrains on a 5 mm baseplate
51
Initial FEM analysis shows vertical displacement results of roughly 370 µm, on the location, where
the pillars of linear stage are attached, since linear stage, which is meant to move the sensor and
scan the wafer will be attached to pillars, that are mounted towards the edges of the baseplate,
where the deflection is the highest. Too high deflections would cause unwanted stress on the
structure supporting the linear actuator and therefore robust and repeatable performance of the tool
would be compromised. The acquired deformation was deemed to be too high as it is more than
an order of magnitude higher than the wanted measuring accuracy of the tool. This resulted in
increased thickness from 5 mm to 15 mm. New thickness was sufficient to machine the interface
grooves directly on the plate, instead of having additional parts. Running the FEA again with same
conditions, resulted in vertical deflection less than 10 µm on the same locations, which is deemed
to be acceptable, as it is on the same order of magnitude as accuracy required.
Figure 44. FEA on the 15mm thickness baseplate. Red triangles show areas of interest of
deflection.
Rotary stage
The rotary stage consists of a housing, shaft-bearing assembly, wafer support arms, and a stepper
motor driven belt-pulley system with a gearing ratio i = 4. A CAD model of the rotary stage alone
is presented below in Figure 45.
52
Figure 45. Front and side view of the rotary stage with a silicon wafer loaded
The structure of the rotary stage consists of two vertical structural members, two cover plates and
a horizontal structural plate. All the parts have dowel pin holes and grooves for precise alignment
and assembly.
Belt-pulley gear system was designed to add more torque for driving the shaft and to prevent direct
coupling of the motor to the shaft, as it would result in relatively long drivetrain compromising the
spatial limits. The belt-pulley drive system also dampens the vibration better, compared to direct
coupling. What is more, belt-pulley system reflects less inertia to the motor by the square of the
gear ratio, in this case 42 = 16 times, causing less stress for the motor. The small pulley on the
motor is press-fitted and the big pulley is located by a setscrew. A section view of the rotary stage,
showing the essential components is presented in Figure 46 below.
Figure 46. Section view of the rotary stage
Wafer support
arms
Vertical
structure plate
Stepper
motor
Motor
mount
Belt-pulley
drive
Shaft-bearing
assembly
Top structure
plate
Hard homing
column and
pin
Silicon wafer
53
Vertical shaft is supported by two deep groove ball-bearings, that are pre-loaded by tightening the
bolt on the end of the shaft, as well as spring washers, for constant and even preload at any time.
The load path of the bearing assembly is shown in dashed red lines in Figure 47 below.
Figure 47. Shaft-bearing assembly
The big pulley is designed to have a protruding pin, limiting its’ rotation. The purpose of the
protruding pin is to ensure same home position of the rotary stage once the hard limit of rotation
is reached. Current design limits the rotation of the shaft to 340°, meaning the wafer can be rotated
in equal steps at minimum of 20°, meaning that line scanning is performed every 20°. To get full
360° of rotation, hard homing option should be replaced with more elegant solution, such as
contactless sensor to register home position or a rotary encoder on the shaft.
Figure 48. Rotary stage in home and 340° positions
Shaft
Bearing
housing
Deep-groove
ball bearing
Collar
Pulley
Preload
springs
Setscrew
Tensioning
bolt
54
Since the solution for finding home position for the rotary stage is chosen to be “hard homing”, it
means that few elements will be pushed against each other and cause stress. Stiffnesses of the
components can be roughly estimated, to identify the least stiff elements, suffering the most due
to such homing method. Parts that are mostly affected by the hard homing are labeled can be seen
in Figure 49 and Figure 50 below. Labelled items are listed in a Table 14 below.
Figure 49. Rotary stage assembly with labelled parts mostly affected by hard homing
Figure 50. Rotary stage assembly with labelled parts mostly affected by hard homing
Both motor and main shafts were modelled as cylinders of 5 and 15 mm in diameter respectively,
protruding pins were modelled as cantilever beams.
Stiffness of the cantilever beams were calculated using the following equation:
𝑘 =3𝐸𝐼
𝑙3 (8)
where 𝐸 – Young’s modulus of the material, 𝐼 – area moment of inertia and 𝑙 – length of the beam.
Area moments of inertia were acquired from CAD software.
Similarly, torsional rigidities of the shafts were calculated using the following equation:
1.
2.
3.
4.
5.
2.
4. 5.
55
𝑘𝑡𝑜𝑟 =𝐺𝐼𝑝
𝐿 (9)
where 𝐺 – shear modulus of elasticity, 𝐼𝑝- area moment of inertia, 𝐿 – length of the shaft. Area
moment of inertia for a circular section can be calculated as:
𝐼𝑝 =𝜋𝐷4
32 (10)
where 𝐷- diameter of the shaft.
Stiffness of the belt was calculated using equation presented below (Gates Mectrol, 2020):
𝑘𝑏𝑒𝑙𝑡 = 𝑐𝑠𝑝 ∙𝑏
𝐿 (11)
where 𝑐𝑠𝑝 – specific stiffness of the belt, provided by manufacturer, 𝑏 – width of the belt, 𝐿 – total
length of the belt not engaged to pulleys.
Calculated stiffness values are presented in Table 14 below:
Table 14. Stiffnesses of parts affected by hard homing the most
No. Part Stiffness
1. Shaft of the motor 54.43 Nm/rad
2. Belt 2.61 ·106 N/m
3. Shaft (smallest part) 16524.15 Nm/rad
4. Pin 22.9·106 N/m
5. Column 7.42·106 N/m
Wafer support pins are designed to be made from polyether ether ketone (PEEK). PEEK is one of
a few materials, that can be in contact with silicon wafers. The wafer is meant to rest on three
supports with no additional clamping mechanism, meaning that only friction force prevents wafer
from moving once it is being rotated. Friction force between the wafer support and the wafer itself
resists the inertial torque of the rotating support mechanism. Since the torque is dependent on the
rotating inertia and angular acceleration of that inertia, the angular acceleration becomes a limiting
factor if the wafer will remain stable.
Figure 51. Silicon wafer resting on the support
Silicon
wafer
Supporting
arm
Supporting
pin
56
A rough estimation of the critical acceleration is presented below. When shaft rotates with a wafer
on it, it accelerates and creates the torque, that can be expressed as:
𝑇 = (𝐽𝑠ℎ𝑎𝑓𝑡 + 𝐽𝑤𝑎𝑓𝑒𝑟)�̈� (12)
where 𝐽𝑠ℎ𝑎𝑓𝑡 is rotational moment of inertia of the shaft assembly including pulley, wafer support
arms, mounted on the shaft, 𝐽𝑤𝑎𝑓𝑒𝑟 is rotational moment of inertia of the wafer, and �̈� is angular
acceleration.
When a silicon wafer is placed on the supporting pins, it creates a friction torque, between the
wafer and surface of supporting pins, when shaft starts to rotate. Friction torque can be expressed
as:
𝑇𝑓𝑟 = 𝐹𝑓𝑟𝑅 (13)
where 𝐹𝑓𝑟 is a friction force between the wafer and support pins and R is radius of the wafer. 𝐹𝑓𝑟
can be calculated with the formula:
𝐹𝑓𝑟 = µ𝑚𝑔 (14)
where µ is friction coefficient between silicon wafer and support pins, 𝑚 is mass of the wafer.
Slippage between the wafer and the rotating supports will occur, when 𝑇𝑓𝑟 = 𝑇. Rearranging the
equations results in formula to find the critical acceleration:
�̈� =µ𝑚𝑔𝑅
𝐽𝑠ℎ𝑎𝑓𝑡 + 𝐽𝑤𝑎𝑓𝑒𝑟 (15)
𝐽𝑠ℎ𝑎𝑓𝑡 of rotating assembly is acquired from a CAD model and is equal to 0.596*10-3 kg·m2. µ is
assumed to be equal to 0.2, based on a rule of thumb figure, commonly used within the company,
when doing rough calculations to get a ballpark range. Mass of the wafer is 127.5 grams and 𝐽𝑤𝑎𝑓𝑒𝑟
can be calculated as a rotational moment of inertia for a disk and is equal to 1.4*10-3 kg·m2.
Performing the calculation yields a critical angular acceleration of the shaft assembly to be 18.48
rad/s2. Since the shaft assembly is connected to the motor via belt drive with a gear ratio i=4,
critical angular acceleration of the motor is correspondingly 4 times higher, which corresponds to
73.92 rad/s2.
57
Linear stage
Linear stage is meant to move measuring sensor across the wafer’s surface. For that purpose, a
stepper motor driven leadscrew actuator was used. Since linear actuators are common components
in machine design industry, a broad range of choices exist. Since the wafer will be measured both
ways – normal and inverted (flipped), systematic error can be identified and corrected, therefore
the linear actuator can be rather simple and does not need to be high precision one. For that reason,
an actuator of choice was Haydon Kerk BGS 06.
The leadscrew has a pitch of 6.25 mm and the stepper motor has a step angle of 1.8°. In addition
to that, stepper motor drive enables micro stepping mode, reducing the step angle up to 64 times.
Theoretically, in ideal case scenario, the maximal actuation resolution of the drive can be
calculated as follows:
𝑟𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛𝑚𝑎𝑥 =𝑝𝑖𝑡𝑐ℎ ∗ 𝑠𝑡𝑒𝑝 𝑎𝑛𝑔𝑙𝑒
360 ∗ 𝑚𝑖𝑐𝑟𝑜𝑠𝑡𝑒𝑝𝑝𝑖𝑛𝑔 𝑣𝑎𝑙𝑢𝑒=
6.25 ∗ 1.8
360 ∗ 64= 0.000488 (𝑚𝑚) = 0.488 (µ𝑚)
This number is however unrealistic in real life scenario, as thermal expansion, precision of the
parts, backlash and similar factors will contribute to the inaccuracies. What is more, such high
positioning accuracy is not needed for measuring wafer warpage. Therefore, it is safe to assume,
that positioning accuracy for the linear stage can go as low as 0.2 mm.
To support linear actuator, a portal shaped structure was designed. The structure consists of two
vertical stainless-steel pillars and a horizontal, stainless-steel connecting plate. Mating surfaces
were machined for precise alignment and assembly.
Figure 52. Baseplate with linear actuator mounting gantry
One of the vertical pillars has a hole and a slot for alignment pins, to locate the assembly precisely
in relation to the baseplate and the rotary stage and is directly bolted to the baseplate from
underneath. To provide repeatable results in case the structure must be disassembled and
reassembled again, the structure is meant to be assembled by aligning the parts to each other, then
whole assembled structure is located using two dowel pins on one of the pillars, whereas the other
pillar rests on a small contact area on the baseplate. Milled pockets around the contact area
58
accommodate displacements, if any, instead of over constraining the assembly and causing
unpredictable deflections are internal stresses, which result in unreliable measuring results. The
pillar, resting over small contact area is attached to the baseplate via intermediate block, that has
holes with a significantly larger clearance between a hole and a bolt, therefore allowing the pillar
to be attached without over constraining and without internal stresses (Figure 53).
Figure 53. Intermediate connecting plate, attaching the gantry to the baseplate
To precisely locate and mount linear actuator to the structure, the plate has milled surface and
machined dimples, two on the bottom and one on the right side, where the actuator is accurately
positioned before attaching it to the plate. Dimple locations are shown in Figure 54 and attached
linear actuator is shown in Figure 55.
Figure 54. Locating dimples on a mounting plate
Figure 55. Linear actuator located on the mounting plate
59
Enclosure
To provide enclosed environment and prevent sensitive wafer surface from contamination, an
enclosure was designed. The enclosure consists of a frame made from Bosch-Rexroth aluminum
extrusion profiles, polycarbonate cover plates and 3D printed protruding covers.
Aluminum extrusion profiles were chosen as the structure because they are easily accessible and
quick to work with and require very little additional machining. What is more, aluminum
extrusions come with anodized surface as per standard, requiring covering only machined ends,
where bare aluminum is exposed. To prevent particle emission, bare ends of machined extrusions
were covered by thin layer of epoxy glue/masking tape. Extrusions on the front side of the
enclosure, have through holes for attaching the frontal FOUP’s interface, where bare aluminum
surface is also exposed. To cover surfaces of the holes, press-fit bushings were designed out of
POM.
Figure 56. Enclosure with covers on
Figure 57. Aluminium extrusion with a bushing, securing the frontal interface
To fully enclose the frame, polycarbonate plates were designed to be laser cut and attached to the
aluminum construction. Side and top plates have cutouts to allow protruding elements of the tool
60
to be mounted. To cover protruding elements, covers were designed to be 3D printed in-house. To
route all cables and wires outside of the enclosure, a cable entry system from company Icotek was
added to the top cover. The solution consists of a frame and square grommets with cutouts for
wires, that are clamped and provide tight seal when cables transition from the inside to the outside
of the tool.
Figure 58. Cable entry system
A standard FOUP consists of the pod itself and removable front door. Since the tool must be
integrated with the wafer sorter, the enclosure must accommodate FOUP’s door. For that reason,
a set of parts were designed to be mounted on the frontal side of the aluminum frame. For ease of
manufacturing, whole frontal piece was designed to be an assembly instead of a single part. Parts
were designed to be made from polyoxymethylene better known as POM. Top and bottom pieces
of frontal interface have milled pockets, where FOUP’s door latch to fully enclose the inside of
the tool.
Figure 59. Frontal interface of the tool
Rigid
frame
Compressible
elastic grommets
with cutouts for
cables and wires
Grooves for
latching FOUP
front door
61
3.5.2 Detailed design of electronics and control system
Operation logic flowchart
To present the operating logic of the tool, flowcharts below were implemented. Since the chosen
concept allows several ways to scan the wafer, two different flowcharts were presented. Since all
the hardware used is the same, the only difference between the two methods would be only visual
representation of the results, which is entirely preference of the user, for convenience of further
data processing.
The first flowchart in Figure 60 presents the operation of the tool, when wafer is scanned line by
line, rotating the wafer by an increment in between each scan.
Figure 60. Operation flowchart for line scanning method
62
The flowchart in Figure 61 presents operation logic of the tool, where wafer is scanned by rotating
it by 360° and moving it closer to the center every increment once it has been rotated.
Figure 61. Operation flowchart for concentric scanning method
Electronics system overview
From the previous chapters it can be observed that there are two motorized actuators that need to
be controlled. In addition to that, data from the sensor needs to be recorded, stored, and accessed
afterwards. A schematic overview of a general control system to execute the functions can be seen
in Figure 62. below. PSU in Figure 62 stands for power supply unit.
PLC
Motor driver
Motor driver
Stepper motor
Stepper motor
PSU Logic
Measuring sensor
Sensor controller
PSU Motor
Figure 62. Schematic overview of the electronic and control system.
63
The red arrows represent power signals whereas green arrows represent logic signals.
To reduce the number of hardware components, a solution from Haydon-Kerk was chosen. Since
the performance of the tool has no requirements on dynamic performance, nor needs high
computational power, a simple hardware can be implemented.
IDEA Smart stepper motor drives are easily programmable through graphic user interface (GUI)
on a computer. These drives can provide up to 2.6 A continuous current and each has 4
programable digital input and output channels allowing to trigger data recording and storing on
the sensor’s controller as well as triggering actuation of the stepper motors once the reading of the
sensor changes. Therefore, stepper motor drives serve the purpose of the programmable logic
controller (PLC)
There are few options of how to execute the actuation of the drives. For the first stage of testing
the concept, it is suggested to connect both drives and control them manually, meaning once the
line scan is complete, the operator manually sends the command to rotate the rotary stage by one
increment and then sends another command to trigger the linear stage and data logging.
To synchronize and control both linear and rotary actuator, both drives can be connected to the
same bus and same computer using RS-485 communication protocol. Using this method, a unique
identifier is assigned to each drive, and every command line from the computer must be sent to the
drive with the identifier, so it is ignored by other axis.
Since there are only two motorized actuators in the warpage measuring tool, another option is to
use master-slave principle, where only one drive is connected to the computer and both drives are
connected together. The master drive is then programmed through USB connection through IDEA
Graphic Interface Software and digital outputs of the master drive can be sent to the slave drive
and act as triggers to execute the wanted commands. To illustrate the case using warpage
measuring tool, the master drive could be the one driving linear actuator, whereas slave could be
the one driving rotary stage. A command is sent to the master drive to drive the actuator until the
end of stroke, scanning the cross section of the wafer. Once the command is executed, a triggering
signal is sent from the master drive to the slave to rotate the rotary stage with the wafer by one
increment and then steps are repeated until complete wafer is scanned.
While the first proposed method has a lot of unnecessary manual work for the operator, it is a
beneficial to run the tool this way as it reduces the risk of malfunctioning and the operator has
more control over the process in case something goes wrong.
The pros and cons of the other, more automated methods are not that significant and depends more
on the preference of the operator. The method of connecting both drives together and assigning
each drive a unique identifier should allow programming the whole sequence in the same window,
making it slightly more convenient and easier to handle.
More accurate schematic representation of the control system with the chosen components is
depicted below in Figure 63.
64
Master motor drive
Slave motor driver
Stepper motor (rotary stage)
Stepper motor (linear stage)
PSU
Measuring sensor
Sensor controller
Figure 63. Overview of the electronic and control system
3.5.3 Wafer sorter integration
As the warpage measuring tool is meant to be integrated with the wafer sorter, there are quite a
few challenges to be faced. While there are only few types of FOUPs being used, for every type,
there is a list of wafer sorter parameters stored, defining coordinates and additional parameters,
such as offsets, for smooth and successful wafer handling.
The way this warpage measuring tool is designed, is that wafer sorting apparatus is ”tricked” into
thinking, that a standard FOUP is loaded. Warpage measuring tool does not have RFID tag yet
implemented, that would allow wafer sorting apparatus to only access a single slot, where the
warped wafer specimen needs to be loaded and unloaded for measuring. All the interfaces and
wafer supports in the warpage measuring tool are designed at such height, that even without having
the ability to adjust the placement coordinates of the wafer sorting apparatus (because no RFID
implementation yet), wafers can be set to be loaded onto 11th slot of a standard FOUP (counting
from bottom towards the top). Figure 64 shows a standard FOUP and warpage measuring tool
stacked on each other in CAD environment and Figure 65 shows a close-up of wafer placement in
both FOUP and warpage measuring tool.
Figure 64. Warpage measuring tool together with a standard FOUP located on the same height
on the bottom interface
65
Figure 65. Warpage measuring tool together with a standard FOUP located on the same height
on the bottom interface. Matching slot heights
Standard wafer loading/unloading procedure consists of lifting up/lowering down the wafer
roughly by 5 mm, prior to moving it outside the FOUP. This ensures and allows current design of
wafer supports to be implemented, even though the wafer sits flush with respect to the top of the
support. In Figure 66 below, isolated rotary stage, silicon wafer and a purple robotic gripper from
wafer sorting apparatus is shown. Normally, the gripper is inside wafer sorter. When a wafer needs
to be loaded/unloaded, it comes all the way inside to a FOUP, grabs the wafer, lifts it up slightly
and takes it back to wafer sorter for further actions. Wafer unloading sequence is presented in solid
arrows, whereas unloading sequence is presented in dashed lines in Figure 66 below.
Figure 66. Top and side views of the wafer being loaded by the robotic end effector of wafer
sorting apparatus
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4 RESULTS
In the results chapter the results that are obtained with the process/methods described in the
previous chapter are compiled and analyzed and compared with the existing knowledge and/or
theory presented in the frame of reference chapter.
4.1 Manufactured parts
Majority of the parts for the prototype, as described in the previous chapters, were CNC milled
and turned. All the cover parts were laser-cut, as they were made of polycarbonate and thin
stainless steel. Protruding parts were 3D printed using a FDM technique. As per standard of the
company, all aluminum parts for tooling equipment were anodized in red color to protect the
surfaces and prevent particles of bare aluminum contaminating inside of the tool.
Some of manufactured parts can be seen in figures below:
Figure 67. Manufactured baseplate
Figure 68. Manufactured parts
Figure 69. Manufactured actuator mounting plate
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4.2 Assembled prototype
Once majority of the parts arrived, the prototype was assembled. Before graduation internship
came to an end, there were parts that had not been delivered, therefore a complete prototype could
not have been assembled. Assembled prototype, without the sensor and cable entry system can be
found below in Figure 70.
Figure 70. Assembled prototype (excluding the sensor and cable entry system)
4.3 Measuring results
Before graduation internship came to an end, some parts still had not been delivered, therefore full
measuring tests could not have been performed and the actual performance of the tool is yet to be
tested. However, it is possible to theoretically evaluate the factors contributing to measuring
accuracy and results.
The list of factors is presented in a Table 15 below.
Table 15. Factors contributing to an error of measuring results
Factor
Measuring accuracy of the sensor
Wafer placement position and the Z position difference once placed
Inverted wafer placement position and the Z position mismatch from non-inverted
wafer placement position
Mismatch of wafer support pin geometry
Gravity induced deflection estimation error
Thickness variation of the wafer
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5 DISCUSSION AND CONCLUSIONS
A discussion of the results and the conclusions that the authors have drawn during the Master of
Science thesis are presented in this chapter. The conclusions are based on the analysis with the
intention to answer the formulation of questions that is presented in Chapter 1.
5.1 Conclusion
Warped wafers are a well-known problem within the company and the way of working when it
comes to measuring wafer warpage is far from ideal. To optimize the process by measuring the
warped wafers inhouse, this graduation internship project was initiated.
As a result, a concept of off-line warpage measuring tool is proposed. The concept was developed
in a structured and organized way, implementing tools and methodologies of a new product
development. The final proposed concept could also be suitable for in-line usage. However, in-
line usage would require additional changes or improvements, such as improved contamination
protection, ensuring the components and parts can be used in machine environment.
Three concepts were proposed for a measuring tool, that should perform and execute the functions
according to the design requirement specification.
Automatically handling warped silicon wafers is a complicated task and the handling equipment
is highly complex and expensive. For that reason, measuring tool was designed as an add-on to
the wafer sorting apparatus, enabling wafers to be loaded and unloaded onto the tool. Wafer sorting
apparatus is however limited in terms of handling warped wafers to what is currently successfully
tested at ±800 µm according to the employees of the company and the tests are still ongoing.
Limitations of wafer sorting apparatus might result in performance requirement P.3 from design
requirement specification not to be fully met. The process of integrating warpage measuring tool
with a wafer sorting apparatus is a challenging task, ensuring no failures happen, therefore the
integration process must be thoroughly and carefully tested.
To measure true shape of the wafer, several methods were proposed to minimize the effects of
gravity induced deflection. Most of the methods required no additional mechanisms, besides
already existing wafer flipping module inside the wafer sorting apparatus. The methods are
described in the report, nonetheless, they are well known to the employees of the company. The
current design of the tool leaves options for testing all, but vertical wafer measuring method, so
the results can be compared and the most accurate one could be chosen.
The proposed concept is based on rotating the wafer around its’ center point and scanning the
surface in straight lines, measuring the deflection of the wafer. This method allows for simple
visualization of the results and quick testing. Both rotary and linear axes could be coupled to scan
the XY grid of the wafer, so existing scripts and methods of analyzing and plotting the results can
be retrofit if needed. The measuring tool is equipped with an existing confocal sensor, even though
the specifications of the sensor are too advanced for the required performance of the tool. The
confocal sensor, however, can be changed to a laser triangulation sensor for slightly worse
measuring results, yet still meeting design requirement specification, while costing less.
69
5.2 Discussion
The proposed concept of the warpage measuring tool has potential of becoming highly valuable
addition to the portfolio of measuring instruments of ASML and change the current way of
working when it comes to measuring warped silicon wafers. If the prototype is proven to be
performing as intended and it can be successfully integrated with the wafer sorting apparatus, the
off-line warpage measuring tool would result in low-cost method of solving wafer warpage
measuring related issues if compared to current expanses that include transporting, measuring and
then discarding the wafers.
Due to long process of budget approval for the manufacturing of the prototype, part ordering was
delayed by roughly 3 weeks in addition to summer holiday period in the Netherlands, therefore
manufactured parts started arriving only during the last week of graduation internship, meaning
no time for assembly and testing of the prototype.
Absence of assembled prototype means the performance of the tool cannot be evaluated yet. This
report should serve as foundation and documentation for actual assembly and testing.
As COVID-19 pandemic emerged during the beginning of the internship, access to the premises
and face-to-face interactions were highly compromised, resulting in less frequent and less
thorough design reviews, leaving rooms for flaws and errors in final design and drawings. In
addition, the learning curve of the author was negatively affected and thus yielding less satisfactory
outcome of the internship, both in terms of personal development and the end result of the warpage
measuring tool.
70
6 RECOMMENDATIONS AND FUTURE WORK
In this chapter, recommendations on more detailed solutions and/or future work in this field are
presented.
Since the warpage measuring tool is yet to be assembled, the functionality and performance of the
tool cannot be evaluated yet. However, several suggestions and recommendations for future work
can already be proposed in bullet points below, as due to limited time of the graduation internship,
compromises had to be made for accelerating manufacturing and assembly of a physical prototype.
6.1 Recommendations
To test, compare and choose the best method for eliminating gravity induced deflection
from the proposed ones, requiring no additional hardware.
To add more elegant solution than hard homing for both linear and rotary stages, such as
mechanical end switches or sensors and therefore enabling unlimited rotation of the rotary
stage.
To optimize the parts by reducing the mass while maintaining the stiffness to make the tool
lighter, yet still having robust performance.
To test and verify, to what magnitude of warpage wafer sorting apparatus can handle
warped wafers to prevent malfunctioning and potentially damaging the wafer sorting
apparatus.
6.2 Future work
To assemble the prototype, check if stages are moving as intended.
To set up data acquisition using the sensor, controller and the software that comes with it.
To check the functionality by manually loading the wafers and measuring them.
To check the integration of the bottom and front interface, meaning how accurately the tool
is resting on the three kinematic pins of the wafer sorting apparatus and also how well the
door of the FOUP is accommodated by the warpage measuring tool.
To add RFID tag to the tool so the warpage measuring tool can be recognized by the wafer
sorter and therefore custom settings for loading/unloading both the tool and wafers can be
programmed and stored.
71
7 REFERENCES
1. ASML. (2020). Veldhoven.
2. Brooks Automation, Inc. (2020). Spartan™ Sorters. Retrieved from
https://www.brooks.com/products/semiconductor-automation/factory-
automation/spartan-sorters
3. Brooks Automation, Inc. (2020). Wafer Handling Robotics. Retrieved from
https://www.brooks.com/products/semiconductor-automation/wafer-handling-robotics
4. BRS - Bright Red Systems GmbH. (2020). BRS - Inline Wafer Edge Inspection
Metrology. Retrieved from Bright Red Systems: https://www.bright-red-
systems.com/quality-assurance-products/inline-wafer-edge-inspection/
5. E+H Metrology GmbH. (2020). MX 2012. Retrieved from https://www.eh-
metrology.com/products/manual-tools/mx-20x-series/mx-2012.html
6. Estion-Tech GmbH. (2020). Products: E-Wafer-Dockingstation. Retrieved from
http://www.estion-tech.com/products/e-wafer/e-wafer-dockingstation
7. Gates Mectrol. (2020, 12 01). Product Sourcing and Supplier Discovery Platfrom.
Retrieved from https://www.thomasnet.com/pdf.php?prid=101106
8. Hopcroft, M. A., Nix, W. D., & Kenny, T. W. (2010). What is the Young’s Modulus of
Silicon? JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 19, NO. 2,
229.
9. Ito, Y., Natsu, W., & Kunieda, M. (2010, May). Simultaneous Measurement of Warp and
Thickness of Silicon Wafer Using Three-Point-Support Inverting Method. Journal of the
Japan Society for Precision Engineering.
10. Jansen, M. J. (2006). Development of a wafer geometry measuring system : a double
sided stitching interferometer. Technische Universiteit Eindhoven.
11. Jel Corporation. (2020). Leading Manufacturer of Clean Robot. Retrieved from
https://www.jel-robot.com/products/index.html
12. Keyence Corporation. (2020). Laser displacement sensors. Retrieved from Keyence:
https://www.keyence.com/products/measure/laser-1d/
13. Kobelco. (2020). Profile measurement system. Retrieved from Flatness profile
measurement system: https://www.kobelcokaken.co.jp/leo/en/item/sbw/
14. Liu, H., Kang, R., Gao, S., Zhou, P., Tong, Y., & Guo, D. (2013). Development of a
Measuring Equipment for Silicon Wafer Warp. Advanced Materials Research Vol. 797,
561-565.
15. Microchemicals GmbH. (2020). Wafer specification. Retrieved from Microchemicals
website: https://www.microchemicals.com/products/wafers/wafer_specification.html
16. Micro-Epsilon. (2020). Sensors for displacement, distance & position. Retrieved from
Micro-Epsilon: https://www.micro-epsilon.com/displacement-position-sensors/
17. MicroTool Technology. (2020). Product catalog. Retrieved from MicroTool technology -
Semiconductor Equipment Enhancment Products: https://microtooltech.com
18. MTI Corporation. (2020). 4" Vacuum chuck - EQ-ECO-402. Retrieved from 4" Vacuum
chuck - EQ-ECO-402: https://www.mtixtl.com/4vacuumchuck-eco402.aspx
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19. Natsu, W., Ito, Y., Kunieda, M., Naoi, K., & Iguchi, N. (2005). Effects of support method
and mechanical property of 300 mm siliconwafer on sori measurement. Precision
Engineering, 19-26.
20. van Gerven, P. (2017). ASML for beginners. Bits&Chips.
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APPENDIX A: RISK ASSESSMENT TABLE
No. Risk Probability Effect Consequence Risk mitigation
1. Violating the
NDA 1 3 Potential lawsuit
Consult the mentor and Technical
Publication Board prior to sharing any
information in any way.
2.
Failure to find
and develop a
solution that
meets the
requirements
2 3
Dissatisfaction of both
the company and the
author
Weekly meetings with the mentor.
Brainstorming sessions with more people
involved.
3.
Too much time
spent on
background
research and
concept
generation
2 2
Delayed detail design,
manufacturing and
testing as well as final
deliverables of the
project
Plan the project with great care. Stick to the
GANTT chart. Ask for guidance from people
with higher competences instead of trying to
overcome some difficulty entirely on my
own.
4.
Running short on
time for
manufacturing
and testing the
prototype
2 2 Delayed final results
Company is willing to extend the duration of
the internship/thesis project in order to have
a good end result or to reduce the scope of
the project so the thesis work can act as a
solid foundation for future work.
5.
Delayed delivery
of manufactured
parts or standard
components
2 2 Delayed assembly,
testing and final results
Check the lead times of manufactured parts
with people in charge as well as lead times
for the standard components. Add some
reserve to lead times
6. Design errors,
flaws 2 2
Need of re-design, re-
manufacturing resulting
in delayed assembly,
testing and final results
Pay high attention to detail when doing
design work. Organize design review
meetings with people of multiple
competences to minimize all potential risk or
errors.
7. Manufacturing
flaws 1 1
Re-ordering and re-
manufacturing parts
delay testing and final
results
Low chance of happening because of
company’s high standards and trusted list of
suppliers. Human errors might occur though,
that are inevitable.
8. Sickness/injury 1 1 Delayed work, as well as
results
Impossible factor to predict. Possible re-
arrangement of GANTT chart might be
needed resulting in extended period of the
internship or narrowed project scope
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APPENDIX B: GANTT CHART