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Manufacturing analysis Polishing of Sapphire Substrate
FINAL PROJECT
POLISHING OF SAPPHIRE SUBSTRATES
Contents
I Abstract (2)
1 Introduction (3)
1.1 Sapphire (3)
1.2 Sapphire Substrate (5)
1.3 Why do we polish (6)
1.4 Methods to polish sapphire substrate (6)
1.5 Process to manufacture sapphire substrate (6)
2 Application Sapphire substrate polishing process (8)
3 Polishing Methods and model (9)
A. Polishing methods (9)
3.1 Mechanical Polishing (9)
3.2 Wet chemical–mechanical polishing (10)
3.3 Dry chemical–mechanical polishing (11)
3.4 Colloidal silica polishing (11)
3.5 Contactless polishing (12)
3.6 Two- step Chemical Mechanical Polishing (CMP) (13)
B. Process model of sapphire substrates polishing (18)
3.7 CMP model (18)
3.7.1 Dry-CMP (18)
3.7.2 Wet-CMP process mode (19)
3.8 Model of process variation for CMP of sapphire substrate (19)
4. Improvement of Sapphire Substrates Polishing Process (20)
4.1. Development of PAD (20)
4.2. Development of slurry (21)
III. Conclusions (22)
IV.Discussion (22)
IV. References (23)
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Manufacturing analysis Polishing of Sapphire Substrate
I. Abstract:
Single Crystal Sapphire is playing an ever-increasingly important role as a material
for, high reliability Electronics today due to its excellent mechanical characteristics, chemical
stability and light transmission. Products of sapphire have many applications in there
substrate application for growth of another material on sapphire is very important. In fact the
substrate material demands stringent surface quality requirements, that is, surface finish and
flatness, are required. The use of CMP technique can produce high quality surface finishes at
low cost and with fast material removal rates. In final project, we have mentioned
specification as well as processes to manufacture the sapphire substrate. After that process
model for CMP process was presented. And finally we have also mentioned improvement of
polishing process. Nowadays we have studied methods to improve polishing process such as
using ultrasonic flexural vibration to assist chemical mechanical polishing for sapphire
substrate, chemical etching after CMP. However this project stated improvements of slurry
and pad in CMP process.
II. Contents
1. Introduction:
1.1. Sapphire:
Definition
Sapphire is a gemstone variety of the mineral corundum, an aluminium oxide (α-
Al2O3). Sapphires are commonly worn as jewellery. Sapphires can be found naturally, by
searching through certain sediments or rock formations, or they can be manufactured for
industrial or decorative purposes in large crystal boules.
Structure
Sapphire includes two types, nature sapphire (fig. 1b) and synthetic sapphire (fig.1c).
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Manufacturing analysis Polishing of Sapphire Substrate
Properties and applications
Properties:
Sapphire is the material of choice for engineers faced with the design challenges of
extreme conditions such as those found in high-temperature, high-pressure or harsh chemical
& physical environments. Its unique properties make it a cost-effective solution for those
applications where long life and high performance are critical.
One of the hardest and durable materials in existence, sapphire is virtually scratch
proof which helps it maintain its integrity in demanding physical environments. It has low
friction coefficient, excellent optical and dielectrical characteristics and a melting point of
over 2000 °C, making it ideal for high-temperature applications. It is chemically inert. It has
the potential of delivering very high laser energies (>1 J/pulse), and easily withstands harsh
chemicals such as fluorine plasma and other industrial gasses and fluids, with no particle
generation. In addition sapphire can transmit ultraviolet, visible and infrared light was well as
microwaves, a range broader than most materials.
Tables 1, 2, 3 clearly show properties of sapphire.
Table1: Physical properties of Sapphire
Name Metric English
Chemical formula
Crystal Structure
Unit Cell Dimension
Al2O3
Hexagonal System
a = 4.758A0, c = 12.991A0,
Density
Hardness
3.98g/cc
1525-2000 Knoop, 9 mhos
0.144lb./in3
Tensile Strength
At 200
275 MPa to 400 MPa
400 MPa
40,000 to 58,000 psi
58,000 psi
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Fig. 1.1a: Unit Cell of Sapphire Fig. 1.1b: Crystal structure of sapphire
Fig. 1.1c: Synthetic sapphire
Manufacturing analysis Polishing of Sapphire Substrate
At 5000
At 10000
Flexural Strength
Compression Strength
275 MPa
355 MPa
450 MPa to 895 MPa
2.0 GPa (ultimate)
40,000 psi
52,000 psi
70,000 to 130,000 psi
300,000 psi
Young’s Modulus, E
Bulk Modulus, k
Shear Modulus, G
MOR
Poisson’s Ratio
345 GPa
250 GPa
145 GPa
350 MPa to 690 MPa
Sapphire is anisotropic
50 x 106 psi
36 x 106 psi
21 x 106 psi
50,000 to 100,000 psi
It is orientation dependent
Table 2: Thermal properties of Sapphire
Melting point
Thermal Conductivity
At 00
At 1000
At 4000
Specific Heat
At 200
Heat Capacity
At 200
At 10000
Thermal Expansion Coefficient
200 to 500
200 to 5000
2310 K (20400C)
46.06 W/(m.K)
25.12 W/(m.K)
12.56 W/(m.K)
0.187 cal/(g.0C)
0.187 cal/(mole.0C)
0.187 cal/(mole.0C)
5.8x10-6/0C
7.7x10-6/0C
37000F
319.4 BTU in/hr ft2 0F
174.2 BTU in/hr ft2 0F
87.1 BTU in/hr ft2 0F
0.1827 BTU/lb 0F
18.6 BTU/lb mole 0F
29.9 BTU /lb mole 0F
3.2 x 10-6/0F
4.3 x 10-6/0F
Table3: Electrical properties of Sapphire
(frequency)
1.0MHz
3.0GHz
8.5GHz
Volume Resistivity
Dielectric Loss
Constant tangent
9.39 0.0001
9.39 < 0.0001
9.39 < 0.00002
1014 ohm.cm
Dielectric Loss
Constant tangent
11.58 0.0001
11.58 < 0.0001
11.58 < 0.00005
Applications:
Due to these unique properties and wide optical transmission range (0.17 - 5.5 m)
sapphire is used as the material for production of UV, visible and NIR optics for operation
under critical conditions like high temperature, high pressure, chemically aggressive or
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Manufacturing analysis Polishing of Sapphire Substrate
abrasive environment. In general speaking, Sapphire material has many applications such as
in Jewelry Industry, in Engineering, in optics, in Medicine. In there, engineering application
includes field of sapphire substrate that will be found in this final project.
1.2. Sapphire Substrates:
Definition
So what is sapphire substrate? When manufacturer uses the sapphire as a substrate to
fabricate other products, that means the sapphire used in the bottom of another materials.
That sapphire is called sapphire substrate, for example Silicon on Sapphire Technology, and
Silicon on Sapphire transistor.
Properties
Properties: Sapphire substrate are available in all orientations with the more common
ones being R-plane (1-102), A-plane (11-20) also referred to as 90-degree Sapphire and C-
plane (0001) referred to as 0-degree or basal plane Sapphire, (please refers to the above figure
1a). Table 4 states typical specification of sapphire substrate. As you can see that depending
on types of sapphire substrate (dimension) we have different specification. However, only
having some change parameters such as Outside Diameter, thickness, Orientation Flat, and
Bow/Warp.
Table 4:
Typical specification of sapphire substrate
Name Kyropoulos sapphire (Super sapphire)
Material High Purity and Monocrystalline Al2O3
Purity Alumina purity 99.997%
Outside Diameter 50.8±0.2 mm and 76.2±0.2 mm
Surface orientation C-, A- and M-planes
Off angle Plane, from 0.2 to 0.5” at 0.1 step
Thickness 420μm±10μm (Typical)
No. 1 orientation flat Length=22.0±2 mm (according to supply specs.)
No. 2 orientation flat Length=11.0±1 mm (for double-side polished)
GBIR (TTV) <10μm
SOIR < 7μm
Surface Roughness (Top side) Ra<0.15nm
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1
2
3
4
5
6
7
8
Slicing
Cropping and Cylindrical Grinding
Edge Beveling /Rounding
Double-side lapping
Mirror polishing
Final cleaning
Final inspection
Manufacturing analysis Polishing of Sapphire Substrate
Surface Roughness (Rear side)
Bow/warp
Ra<1μm (Same Ra for top side applies for double-sided
products.)
0~ -10μm (for 2’’ sapphire substrate), 0~ -15μm (for 4’’)
Edge chamfering Rounded / Chamfering Angle (C) = 450
Laser Mark 8 characters, (TYMxxxxx)
(T=TXT; Y=Year; M=Month; XXXXX=serial number ) marked
in lapped surface, center aligned at OF, 1.6x0.8x0.6x1 mm
(HxWxSxD)
1.3. Why do we polish sapphire substrates?
In many of these applications critical surface quality demands of sapphire are
required, that is, surface finish and flatness are required. The generation of high-quality
surfaces with fine surface finish and low surface and subsurface damage is of critical
importance. It has been established that the crystal structure of epitaxial films is strongly
influenced not only by the substrate material and its orientation, but to a great extent also by
the surface preparation of the substrate. Therefore, it is essential to use polishing techniques
for polishing sapphire substrate. Reality shows that the polishing techniques may produce
high quality surface finishes at low cost and with fast material-removal rates.
1.4. Methods to polish sapphire substrates
There are many methods to polish the sapphire substrate including, Mechanical
polishing, Wet chemical–mechanical polishing, Dry chemical–mechanical polishing,
Colloidal silica polishing, Contactless polishing CMP has been proved to be available method
to produce high quality surface for sapphire substrate.
1.5. Processes to manufacture sapphire substrates
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Manufacturing analysis Polishing of Sapphire Substrate
Fig 2: Processes to manufacture sapphire substrates
Steps to fabricate the sapphire substrate as above, detail of processes are presented as
follow:
0. Orientation: The orientation for sapphire substrates is determined by two angles displaying
a degree of difference at the surface or the direction of the ingot axis to the crystal lattice.
1. Crystal growth: By using Kyropoulos Method to grow sapphire ingot, crystal pulling
2. Cropping and Cylindrical Grinding: Both end of the grown crystal are cut in the
direction of the intended angle to establish the ingot's surface orientation. The accuracy of the
surface orientation is verified using an X-ray equipment. The diameter of the crystal ingot is
adjusted to determine the wafer diameter by a grinding process. Through cylindrical grinding
the ingot is made into a perfect cylindrical form ready for the following slicing process.
3. Slicing: The slicing procedure is critical because the way that the wafer is cut affects
important qualities of the wafers such as thickness, taper and bow. To achieve maximum
slicing efficiency and quality, the crystal ingot is sliced into wafers using the latest multi-wire
saw technology. In a wire saw, a single strand of steel thin wire moves from a feed to a take-
up reel. In between, the wire wraps around three wire-guides containing hundred of guiding
grooves that create a web of parallel wires. The ingot is fed together with abrasive slurry
through the web to produce a concise cut with minimum kerf loss.
4. Edge Beveling /Rounding: Prior to processing the wafer surface, the edge of the
substrate is beveled using the edge grinder. This process not only adjusts the wafer diameter
to the precise specification, but also prevents edge chipping which may cause surface damage
from loss fragments in the remaining manufacturing process.
5. Double-side lapping: Lapping is performed on both sides of the wafer. The primary
purpose of lapping is to remove any irregularities on the wafers that may have occurred
during slicing. The as-cut wafers are processed between two lapping plates using an abrasive
slurry mixture to achieve first level of surface quality. Defects such as surface saw marks or
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Manufacturing analysis Polishing of Sapphire Substrate
wafer thickness variations are removed and corrected in this exercise. Once treated, the
lapped wafers are ready for more precision.
6. Mirror polishing: In this polishing process wafer characteristics such as flatness, warp
and thickness are fine toned to the precise measurements. For substrate used for SAW
applications, the wafer flatness is of utmost importance. The polished surface must maintain
the properties of single crystal and be free of scratch and digs and mechanical stress. Any
crystal or processing defects remain on the wafer surface will impair the performance of the
SAW device.
7. Final cleaning: Any remaining particles and residues on the wafer surface are removed in
a wet chemical cleaning process, after which the wafers are dried in a spin dryer before final
inspection.
8. Final inspection: Various types of inspections occur throughout our entire manufacturing
process to achieve highest product quality. During the final inspection, wafer characteristics
are checked using the most advanced inspection equipment such optical microscope. At this
point we make certain wafer quality in terms of its TTV, LTV, sori and bow either meet or
exceed our customer's requirements.
2. Applications of Sapphire Substrates Polishing Process
Sapphire substrates belong to one of the most significant aspects of the constructional
application of sapphire material. They are used for epitaxy of semiconductor films such as Si,
GaN, AlGaN, and for making integrated circuits. Sapphire substrates are inert, work at high
temperatures and mechanical loads, and can be obtained in large size. Therefore, they are
used even in those cases when the lattice parameters do not completely coincide with the
parameters of heteroepitaxial structures.
In fact Sapphire product is serving mainly two applications: GaN-based LED and RF
devices, both for mobile phones (Silicon-on-Sapphire “SoS” technology and Silicon on
Sapphire transistor).
Sapphire single crystal combines many good mechanical and optical properties that
make it become the choice of materials in a variety of modern High-Tech applications from
commercial and military optical systems to high-power laser optics and high-pressure
components, blue emitting diodes, laser diode devices, visible-infrared windows. In addition,
the (0001) sapphire crystal wafers are an important substrate materials widely used in a range
of applications such as optics, electrics, semiconductor devices, integrated circuits industry
and other applications.
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Manufacturing analysis Polishing of Sapphire Substrate
For convenience of positioning, the substrates are supplied with one or two additional
profile planes: The C-plane is used for the coating of sapphire with CdS, CdTe, CdSe, GaN,
SiC, InAlGaN, and LiNbO3 as well as for the epitaxial growth of some oxide (e.g ZnO) and
metal films. The A-plane is used for making hybrid microcircuits, devices that possess high-
temperature superconductivity, and for coating the crystal with Co, Fe (110), W(110), Au(111),
V(011).
The R plane is suitable for coating sapphire with MgO, α-ZnO, and Si by the method
of heteroepitaxy. Sapphire substrates are also employed in sensors measuring pressure, mass,
and humidity, as well as in IR-radiation detectors (HgCdTe films on sapphire) and other
devices.
Among the promising trends in the use of sapphire substrates, one should mention the
technology of carbon nanotube growth on sapphire. This new material seems to be promising
for nanotransistors and sensors. Researchers have found that a-plane (1120) sapphire surfaces
spontaneously arrange single-walled carbon nanotubes into useful patterns. No template has
to be provided to guide this structuring; it is formed automatically
3. Polishing methods and model
A. Polishing methods
3.1 Mechanical polishing
It means this process only includes mechanical factors. The polishing material surfaces
usually used are soft metals such as cast iron, tin, lead, or copper, and occasionally even
various resins and plastics. These materials favor penetration of abrasive materials into the
polishing surface. Therefore, only a part of the abrasive grains work, and reduce the load on
the treated surface and removing the material (Fig 3.1).
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Manufacturing analysis Polishing of Sapphire Substrate
Fig 3.1: Processing mechanism of mechanical polishing
3.2 Wet chemical–mechanical polishing
Its characteristic is using special substance to add to the polishing suspension, which activate
the polishing process through chemical interactions with the workpiece surface. Normally,
alkaline SiO2 solution (colloidal silica) is used for this purpose.As well as analogous
polishing methods, proceed from the assumption that the chemical interaction of aluminum
oxide with silica is followed by the formation of an aluminosilicate:
Al 2O3 + 2SiO2 + 2H2 O→ Al2 Si2O7 ·2H2O
The product of this reaction is disappered by friction force between the tool and workpice,
this chemical reaction occurs at relative low temperatures. After this polishing process, they
use an abrasive paste with a grain size of 1 μm to get microasperities with average height
about 0.03 μm. Meanwhile with CMP process using an Aerosil of SiO2 micro-asperities do
not exceed 0.01μm. The earlier considered mechanisms of chemical–mechanical polishing
are complemented by a mechanism of chemical etching of the surface, which diminishes the
incumbent defective layer. Figure 3.2 demonstrates the polishing factors essential for wet
chemical–mechanical polishing. For all referenced crystal the removal rate of material
increases as specific pressure rises; note that sapphire polishing generally requires
particularly high pressures (Fig. 3.2).
Fig 3.2Wet-type chemical-mechanical polishing
3.3 Dry chemical–mechanical polishing
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Manufacturing analysis Polishing of Sapphire Substrate
In this method, mechanical polishing is implemented through the action of chemical solid-
phase reactions between the abrasive and the sapphire workpiece. Surface porosity develops,
that enables removal of the outer layer of the crystal by soft polishers, and consequently
diminution of the surface-adjacent defective layer. A typical example of this situation is
polishing of sapphire by fine SiO 2 abrasive. When the SiO2 particles interact with the
sapphire surface, the area of contact undergoes local reactions of high pressure and
temperature. Therefore, solid-phase reactions proceed between the SiO2 and the Al 2O3; the
sapphire surface becomes porous and the reaction products are carried away by a soft polisher
easilly. The efficiency of sapphire polishing by colloidal silica is not high in the presence of
water. Dry polishing using SiO2 gel is more effective, it requires high temperatures and
protection of the working zone from sputtering of the gel. The polishing tool used has a
specially designed working surface and to constantly kept in silocozole suspension. This
arrangement provides continuous supply of the reagents to the polishing zone. Into the range
of 0.02 mg/min, the efficiency of such a polishing progresses ≈40 times higher than the
procedure of polishing on a quartz faceplate with a colloidal silica water solution (0.0005
mg/min). Using these methods, we can get a sufficiently high surface quality to be obtained,
but the problems of stress and crystal lattice distortions are remain.
3.4 Colloidal silica polishing
Colloidal silaca polishing method is based on from colloidal phenomenon that arises
when atomically small particles of silica (10–100 Å) are used in a slurry of preset alkalinity
(example pH 4-12). Very fine colloidal silica is supplied to a soft polisher, and pressure is
applied to the workpiece, a gelling phenomenon peculiar to colloidal solutions starts working.
(Fig. 3.3)
Fig 3.3 Constitutional diagram of colloidal silica polishing
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Manufacturing analysis Polishing of Sapphire Substrate
In wet and dry CMP method which we arready have studied before a certain extent
depend on the chemical properties of the workpiece surfaces, thereby can be effectively
applied to a limited set of materials.But colloidal silica polishing method is applicable to
practically all materials and provides achievement of mirror like surfaces free from stresses
(no residual stress).
3.5 Contactless polishing
This method also can be called elastic emission machining (EEM) and results in reduction
of material removal by a factor of 10–100 in comparison with mechanical polishing. Particles
with a diameter of about 100 Å interact with the workpiece surface, removing only several
tens of atoms.
Fig 3.4 Principle of elastic emission machining
Fig 3.4 shows the “polishing float” phenomenon, when the distance between the polishing
facility and the workpiece surface approaches values on the atomic or molecular order, the
slurry particles and the surface atoms will join together. By this mechanism way, separation
of the adjoined particles from the workpiece surface does not cause plastic flow. Hence, the
surface did not was damaged as these particles are removed.
Note that the treatment efficiency (MRR), the surface quality, the thickness or absence of
the defective layer, and the precision and accuracy of surface shape are interrelated.
Obviously, CMP is characterized by a relatively high treatment efficiency and insignificant
resultant defective layer.However, more important role of chemical reactions, more difficulty
to controlling process precision. The utilization of soft polishing surface materials raises the
surface quality, but deteriorates surface shape control.
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Manufacturing analysis Polishing of Sapphire Substrate
3.6 Two- step Chemical Mechanical Polishing (CMP)
In generally, CMP process requires high removal rate and low surface roughness.
However it is so difficult to meet these requirements by a single step polishing process. To
obtain an ultrasmooth surface of the sapphire substrate, we have to investigate two-step CMP
of the sapphire substrate. First step, we used ultrafine α-alumina-based slurry and nanoscale
silica-based slurry for second step.
Preparation of α -alumina-based slurry:
α -alumina-based slurry includes: Calcined α –alumina abrasives have different particle
shapes with an average diameter of 500 nm (as shown in Fig 3.5) and a bulk density of 0.8
g/cm3. The 5% wt alumina powder and 0.5% wt sodium hexametaphosphate, which act as a
dispersant, were added to deionized (DI) water in a container under stirring. Then using 0.1
M potassium hydroxide solutions to adjust the solution pH =12. Finally, the mixture was
filtrated with a 20 µm pore strainer.
Preparation of silica-based slurry:
Include Macrogol 6000 (0.5%wt) as a surfactant, 5 % wt silica gel self-made with an
average diameter of 50 nm (as shown in Fig 3.6) in a container under stirring. and using
triethanolamine to adjust the solution pH=12. Finally, the mixture was filtrated with a 1 μm
pore strainer.
Figure 3.5 SEM image of alumina particles. Figure 3.6 SEM image of
silica
Polishing tests:
Using a CMP tester (CETR, CP-4) to polish sapphire wafers [(0001) oriented] .The
parameters of polishing process are given in table 5.
Process conditions The first step The second step
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Manufacturing analysis Polishing of Sapphire Substrate
Pad rotation speed (rpm) 100 100
Wafer rotation speed (rpm) 100 100
Down force (psi) 5 5
Slurry feed rate (mL/min) 100 100
Polishing time (min) 60 30
Polishing pads Polyuretanes pad Politex pad
Table 5: Process parameters of CMP process
Characterization methods
We use Hitachi S-4700 field-emission-scanning electron microscope to investigate the
morphology of the abrasive particles. Using HCl/KOH to adjust the desired value pH (we
need pH 12 because at this value material removal rate (MRR) is highest).
MRR =107 × ∆ m
ρ× 2.542 × π ×t( nm/min)
∆m (g) is the mass variation in sapphire before and after polishing
t (min) is the polishing time
ρ is the density of sapphire
The surface topography and root-mean-square (rms) roughness was measured by a Quesant
Q-Scope 250 atomic force microscopy (AFM). The AFM operating mode was the contacting
mode, and the scan area was 10 10 μm2. The MRR and rms roughness is the average of 3
individual polishing tests.
Results
Optimization of process parameters:
The polishing parameters such as down pressure and rotation speed have an important
influence on the CMP performance. For the second step, any little change in polishing
parameters may have a strong effect on polished surface quality.The influence of polishing
pressure and rotation speed (both wafer and pad) on the MRR and rms roughness in the
second step using a silica-based slurry, and the results are shown in Fig. 3.7 and 3.8.
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Manufacturing analysis Polishing of Sapphire Substrate
COF (coefficient of friction) analysis
The friction force is dependent on interfacial electrostatic interactions, dynamic surface
conditions, properties of the opposing surfaces, and the abrasive size, which all influence the
contact area between the opposing surfaces.The COF in the second step is larger than that in
the first step. Because the polishing pad used in the second step is the soft pad that make
increase contact area between the pad and surface of the sapphire substrate.
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Fig 3.7: Effect of polishing pressure on the MRR and rms roughness
Fig3.8: Effect of rotation speed on the MRR and rms roughness
Manufacturing analysis Polishing of Sapphire Substrate
Fig 3.9: COF as a functional of polishing time for two steps
In this reasearch, the sapphire supstrates polished were ground wafers and has many rough
peak (fig 3.10) .These rough peaks were first removed during the polishing process. When
polishing time increase, the number of rough peaks decreased. Therefore, we have result like
fig3.10.When the rough peaks are completely removed, the contact area tends to be constant
and COF is stable.
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Manufacturing analysis Polishing of Sapphire Substrate
Fig 3.10 Representative AFM images from center, middle, and edge of the sapphire in
different CMP stages: (a) Before CMP, (b) after the first-step CMP, and (c) after the second-
step CMP.
MRR and rms analysis
Before polishing After the first step After second step
RMS (R0) 968.9 21.98 6.83
MRR (nm/min) 42.3 7.1
Table 6: RMS and MRR of sapphire substrates in different CMP stages
From table we see that the rms roughness value of the sapphire surface is very high before
polishing. After the first-step CMP using Al2O3 slurry, the rms value decreased from 968.9 to
21.98 Å. Less than to 6.83 Å after second step using SiO2 slurry with the optimized process
parameters. It means that the subnanometer precision sapphire surface obtained by using the
two-step CMP. The first step gives higher MRR but poor surface quality, meanwhile the
second step gives good surface quality but lower MRR. The first-step CMP is suitable for
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b
c
a
Polishing machine & other equipmentsPad/Slurry/ Sapphire substrate Sapphire substrate
Grits & material
Pad & Grits
Force friction model & Passivation Model Abrasive Abrasion Model
Manufacturing analysis Polishing of Sapphire Substrate
preliminary polishing to provide high polishing rate as well as full surface planarization,
whereas the second-step is suitable for final polishing to provide a fine local planarization.
CMP mechanism
The two kinds of slurry used in this method were composed of different abrasive particles
with different shapes, hardness, and size, which result in separate removal rate and surface
roughness. The Mohs hardness of α-alumina and silica are 7 and 9, and the sapphire has the
same hardness as α-alumina.
B. Process model of sapphire substrates polishing
3.7 CMP model
In general, CMP process can be illustrated like this schematic diagram.
Fig 3.11 schematic diagram of CMP process3.7.1 Dry-CMP
Parallel process with chemical passivation and mechanical abrasion
Fig 3.12 process model of dry-CMP
3.7.2 Wet-CMP process mode
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Polishing machine & other equipmentsPad/Slurry/ Sapphire substrate Sapphire substrate
Grits & material
Pad & Grits
Hydrodynamic model & Passivation ModelAbrasive Abrasion Model
Polishing machine & other equipments Polishing machine & other Sapphire substrate
Manufacturing analysis Polishing of Sapphire Substrate
Fig 3.13 Process model of Wet-CMP
This CMP process has lot of parameter, but it can be sumarized in this table
inputs outputs
Pad: Fiber structure conditioning, compressibility
modulus
Material removal
Wafer geometry and material Surface quality : roughness, scratching
Slurry: pH, oxidizes, buffering agents, abrasive
concentration, abrasive geometry and size distribution
Within - wafer non-uniform material
removal
Process: Pressure, velocity, temperature, slurry flow,
polishing time
Within- die non-uniform material
removal
Table 7: Parameters of CMP process
3.8 Model of process variation for CMP of sapphire substrate
Ei(t )
In CMP process of sapphire subtrate, we have these parameters:
es: Spindle velocity, down force, tool path, chemical reaction, temperature, passion layer
thickness.
e p: Pad stiffness, grit hardness chemical slurry.
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Ei(t )
es , ep ms , mp
CMP
Manufacturing analysis Polishing of Sapphire Substrate
ms: Passivation, wafer surface, stiffness, temperature material, total thickness variation.
m p: Material chemical, material hardness.
From this model we can have variation equation. This equation shows affection of
disturbances, input into out put(Y).
Resulting of output depends on α
4. Improvement of Sapphire Substrates Polishing Process
Introduction:
To gain smooth surface sapphire wafer, there are many ways to do it, but I concentrate
on improvement PAD and Slurry.
4.1. Development of PAD:
The latest VISIONPAD polishing pads are VISION PAD 6000 and VISION PAD
5200[1], which are enormous improvements not only increasing removal rate but also
reducing consumable cost.
1) VISION PADTM 5200 polishing pads are presented next-generation technology which
reaches a high removal rate for Tungsten (W), ILD and Cu bulk processes. Thanks to
implement unique polymer chemistry and more pad porosity, it gains from 10 percent to 30
percent rise in removal rate for W, ILD and Cu Bulk applications. As a result of growth
removal rate, customer can save polishing times and slurry consumption to dramatically
lower CMP consumable cost. In addition, VISIONPAD 5200 polishing pads achieve from 10
to 20 percent cut in shortcoming of W and Cu applications, also improve dishing and
corrosion in W applications over Dow’s standard IC 1000TM polishing pads.
2) VISION PADTM 6000 polishing pads have some features: low-defect, low-hardness
polymer chemistry and optimized pore size, so it can exceed the removal rates of the IC
1000TM polishing pads. As a result of testing customer, VISIONPADTM 6000 can cut down a
50 to 60 percent scratch defects, while the disc defects (Dishing) decreased by 35%, and the
wafer non-uniformity and the IC1000 pad level is very.
4.2. Development of slurry:
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Manufacturing analysis Polishing of Sapphire Substrate
a) Thanks to two-Step Chemical Mechanical Polishing of Sapphire Substrate [2],
researchers have an ultra smooth surface of sapphire wafer. The first state, when they use
alumina-based slurry, the first time the coefficient of friction declines and then it tends to be a
constant. At the first step, the root-mean-square (rms) roughness value of the polished surface
can reduce from 968.9 to 21.98 Å. In the second state, As a result using the nanoscale silicas
slurry, at the first minute the coefficient of friction grew after that became unchanged and the
rms roughness can drop at about 6.83 Å.
b) With using mixing abrasive slurries [3], which contain boron carbide and ceria
abrasive, has removal rate about 180 nm/min and gain a root mean square (rms) about 2.1
nm. More Over, reactive ion etching process, we can improve the quality sapphire wafer with
rms to 0.7nm.
This plot shows AFM images of the sapphire substrate before and after CMP. In fig (a) and
(b), with only boron carbide abrasives, the rms reduces from 297.2 to 19.9 nm after CMP, but
deep scratches remain obvious because of the direct fierce physical tear of the hard boron
carbide particles. In Fig. (c), using typical MAS for CMP, the surface quality greatly rise with
an rms of 2.1 nm due to ceria particles’ interaction with the sapphire hydration layer and a
fierce indirect physical tear of the hard B4C core. However, the surface is still smooth. From
fig (c), we can see shallow scratches and inescapable particle and dust bond existence. To
improve the surface quality, we have an RIE process. We have used a established Oxford
Plasma lab 80 Plus RIE system. With a radio frequency (13.56 MHz) glow discharge creates
this plasma. Electronic grade CF4 and Ar gases were injected into the chamber through mass
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Manufacturing analysis Polishing of Sapphire Substrate
flow controllers with the flow rates of 10 standard cubic centimeters per minute (sccm) and
40 sccm, respectively. After 20 min etching, the rms has dramatically dropped to 0.7 nm.
From Fig. d, a finely global planarization of the wafer has been gained. In this RIE process,
we have a formula:
2Al2O3(s) + 3CF4(g) → 4AlF3(s) + 3CO2(g) (4)
The product of AlF3 is not evaporative (bp 1276°C, 1 mm at 1238°C), but the ability of
reactive ions to etch a given compound greatly depends on the boiling points and vapor
pressures of potential products during etching. Low boiling points and high vapor pressures
of potential products will push the etching process, so the RIE performance can be attributed
to a weak chemical reaction between CF4 and sapphire surface and a gentle physical
sputtering by Ar gas, for which impurities adherence and the bulge of the sapphire surface
can be etched. Because of these reasons, a super surface of sapphire was obtained after the
RIE process.
III. Conclusions
Thanks to model process of CMP sapphire substrate, we can control variations to have desire
output product. Also, the improvement of Sapphire wafer plays an important role in reducing
cost of sapphire substrate and improving quality it.
IV.Discussion
Depend on the aim of different applications; we have the different requirement of
sapphire wafers quality. Therefore, we can choose the suitable polishing process. The limit
aspect of sapphire substrate is that we can not know exactly disturbances and experments.
IV. References
[1] http://content.yudu.com/A1pft8/Nanotimes09-2010/resources/26.htm?
skipFlashCheck=true
[2
]
Liu, Weili; Song, Zhitang; Hu, Xiaokai, two-step chemical mechanical polishing of
sapphire substrate, Published May 3, 2010
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Manufacturing analysis Polishing of Sapphire Substrate
[3]Liangyong Wang,z Kailiang Zhang, Zhitang Song, and Songlin Feng, chemical
Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Wafer,
(2007)
[4] Elena R. Dobrovinskaya, Leonid A. Lytvynov, Valerian Pishchik Gavish, Sapphire,
(2009)
[5] Wenhu Xu, Xinchun Lu *, Guoshun Pan, Yuanzhong Lei, Jianbin Luo, Ultrasonic
flexural vibration assisted chemical mechanical polishing for sapphire substrate, 25 January
2010
[6] Kurlov V.N. , Kiiko V.M. , Kolchin A.A. , Mileiko S.T.J. Cryst. Growth. 204 , 1999 , 499
[7] Zhukov L.F., Litvinov L.A., Chugunnyi E.G. Patent USSR. 766237, 1980
[8] Zhukov L.F., Litvinov L.A., Shumikhin V.S. Patent USSR. 1256572, 1984
[9] Ivanina B.M., Litvinov L.A., Priimach B.S., Globus M.E. Patent USSR 1316467, 1985.
[10] Schewe P. , Riordon J. , Stein B. Phys. News Update . 1 , 2003 , 619 .
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