Surface micromachining and Process flow part 1

Identify the basic steps of a generic surface micromachining process

Identify the critical requirements needed to create a MEMS using surface micromachining

List common structural material/sacrificial material/etchant combinations used in surface micromachining

Compare and contrast the relative merits of wet micromachining versus dry micromachining

Explain the phenomenon of stiction, why it occurs, and methods for avoiding it

Describe the process of lift-off Explain what is meant by packaging and

describe the ways in which it present major

challenges in MEMS

Define the terms Structural layer/material Sacrificial layer/material, Release, and Die separation

Develop a basic-level process flow for creating a simple MEMS device

Surface micromachining

= Surface micromachining

+

The Si wafer functions like the big green flat

plate.

Some Jenga pieces are removed. The ones that remain form the MEMS

structure.

Review of surface micromachining process

Surface micromachining example –

Creating a cantilever

Silicon wafer (Green Lego®

plate)

Deposit poly-Si (structural layer—the Jenga pieces that

remain)

Remove sacrificial layer (release)Etch part

of the layer.

Deposit SiO2 (sacrificial layer—the Jenga pieces that are removed)

Often the most critical

Reminder of the surface micromachining process

side view top view

oxide

silicon

metal

Process flow for surface μ-machined cantilever

1. .

2. .

3. .

4. .

5. .

6. .

Top view (4)

Mask 1 (positive resist)

Mask 1 (negative resist)

Top view (5)

Top view (6)

Process flow for surface μ-machined cantilever

7. .

8. .

9. .

10..

11..

12..

Mask 2 (negative resist)

Mask 2 (positive resist)

Top view (9) Top view (10)

Top view (7)

Top view (11)

History and processes

• Surface micro-machining (SMM)• Developed in the early 1980s at

the University of California at Berkeley

• Originally for polysilicon mechanical structures

• Other processes includeo Sandia National Lab’s

SUMMIT (Sandia’s Ultra-planar Multi-level MEMS Technology) five levels possible with four poly layers

o MEMS CAP’s polyMUMPs (Multi User MEMS Processes) three layers of poly with a layer of metal

Photo of a PolyMUMPs surface-micromachined micro-mirror. The hinge design allows for out-of-

plane motion of the mirror.

Requirements and advantages

• Three to four different materials required in addition to the substrate o Sacrificial material (etch rate Rs)o Structural mechanical material

(etch rate Rm)o Sometimes electrical isolators

and/or insulation materials (etch rate Ri)

• Many SMM processes are compatible with CMOS (complementary metal oxide silicon) technology used in microelectronics fabrication.

• Can more easily integrate with their control electronics on the same chip

• Many SMM processes have developed their own sets of standards

efficient and inexpensive

Rs >> Rm > Ri

Best results are obtained when structural materials

are deposited with good step coverage.

Chemical vapor deposition (CVD)

orPhysical vapor deposition

(PVD)

If PVDSputtering

orEvaporation

Common material/etchant combinations for surface μ-machining

Structural material

Sacrificial Material

Etchant

Si/Polysilicon SiO2

Buffered oxide etch (BOE) (HF-NH4F ~ 1:5)

Al Photoresist Oxygen plasma

Polyimide Phosphosilicate glass (PSG) HF

Si3N4 Polysilicon XeF2

Problems and issues

Wet etching• 40 years of experience and data in

the semiconductor industry• Ability to remove surface

contaminants

• Very high selectivities• Usually isotropic always involve

undercutting

Dry etching• Better resolution than wet

etching

• More directionality (High aspect ratios )

• Lower selectivities • No undercutting

moisture

Stiction

Stiction

Stiction = static + friction

Stiction = stick + friction

An example of an unfavorable scaling

Fforce erestoritivtension surface

2~

LL

L1~

An example of a portmanteau

Ways to reduce stiction• Coat (cubrir) surface with a thin

hydrophobic layer in order to repel liquid• Dry surfaces using supercritical CO2.

Removes fluids without allowing surface tension to form.

• Use “stand-off bumps” on the underside of moving parts. Pillars prop up (soportar) movable parts

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Problems and issues

Polysilicon hotarm actuator created using surface μ-machining

“Dimple” resulting from a stand-off bump on the underside of the cold arm

Te toca a ti

Explain (with words, drawings, or both) how standoff bumps might be created.

Lift-off

Usually included as an “additive technique” by most authors

1. Photoresist is spun on a wafer and exposed to create patternResist has either straight side walls, or better, a reentrant shape.

2. Material deposited through the photoresist mask using a line-of-sight method, such as evaporation• Shadowing takes place,• Part of the photoresist sidewalls must

be free of deposited material3. Photoresist stripped leaving behind only

material deposited through the opening. Unwanted material is lifted off.Thickness of the deposited material must be thin compared to the resist thickness.

Most often used to deposit metals, especially those that are hard to etch using plasmas

(+) or (-) resist?C

Typical process steps for surface micromachining

This is where process flow becomes complicated.

• modeling and simulation• design a layout• design a mask set

thin film formation (by growth or deposition)

lithography

etching

release

packaging

die separation

12

34mask

set

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Die separation and packaging

• Must separate the individual devices• Often saw or scribe the wafer

• Provide MEMS device with electrical connections

• Protect MEMS from the environment• Sometimes must also provide limited

access to environment (e.g., pressure sensor, inkjet print heads)

• Packaging a difficult engineering problem

• Largest cost of producing many (most) devices

packaging

die separation

C packaging

More on packaging

Die-level packaging Wafer-level packaging

packaging

More on packaging

Schematic of a packaged MEMS pressure detector showing some of the requirements

unique to MEMS

Process integration (Process flow)

• Nature of crystalline silicon

• Adding materialo Dopingo Oxidationo Deposition

PVD CVD

• Photolithography• Bulk Micromachining• Surface

Micromachining

We have learned much about the many materials and techniques for used processing materials to create devices, including

How do we put these things together to create a device? Specifically:• How do we choose which steps we

need?• How do we choose the order of the

steps?• How do we communicate this

order of steps in the field?Process

integration (Process flow)

List of process steps in the correct order with the

accompanying lithography masks.

Process integration (Process flow)

http://juliaec.files.wordpress.com/2011/04/blooms_taxonomy.jpg http://ictintegration.wikispaces.com/BloomHemos pasado mucho tiempo acá.

“Pro

cess

flow

” es

tá

acá.

Bulk μ-machined pressure sensor

Thin Si diaphragm changes shape when pressure changes on one side relative to the other.  Piezoresistors (implemented using p+ diffusion) sense the deformation.

 Aluminum wires send resistive electrical signal off the chip.

 n+ diffusion is used as an etch stop for the backside etch.

 Oxide + Nitride provides wafer protection for backside etch and insulator between Al wires and wafer.

Process flow, pass 1

The first pass for determining the process flow is to decide which steps we need.

What are the basic steps necessary to build the diaphragm?  • Etch backside

(Need to protect front of wafer during backside etch)

• Add SiO2 and nitride layers• Etch area above diaphragm to give

diaphragm ability to move easily• Create an “etch stop” layer

o Reverse bias p-n junction will stop etch

o Start with p-type wafer o Dope n-type layer or grow n-type

epilayer (layer produces with epitaxy)

Process flow, pass 1

The first pass for determining the process flow is to decide which steps we need.

What are the basic steps necessary to build the sensor?  • Add diffusion to get piezoresistor• Add wires so that piezoresistor can

be connected to external world• Note that wires must be metal

(Could use diffusion if the distance is short)

Process flow, pass 1

The first pass for determining the process flow is to decide which steps we need.

What processing steps are required to produce entire device?  • Deposit/pattern oxide and nitride• Deposit/pattern Al for pads• Backside etch• n-type diffusion for etch stop• p-type diffusion for resistors/wires

Each of these steps results in more steps in the detailed process flow. But to begin, let’s

determine the order in which the steps must be placed.

Process flow, pass 1

Order of steps

What impacts our decisions on choosing an order?  1. Geometry

The oxide must be deposited before the nitride.

2. TemperatureHigh T processes must go first. High T processes can cause dopants to further diffuse and metals to melt and flow.

Which processes are high T?• Oxidation • CVD (unless PECVD)• Drive-in for diffusion

3. Mechanical stressIf a following step can cause a device to break, you may want to rethink the order if you can. This is why release steps are often (though not always) done last.

4. Interaction of chemicalsIf an etch will attack another material, you must either place is earlier in the process flow or protect the material.

Process flow, pass 1

Order of steps

Let’s choose an order  1. n-type doping 2. Oxide: Can be done before doping of

resistors if oxide is thin. (Boron will implant through thin oxide but not if oxide is thick!)

3. Dope resistors4. Deposit nitrideDo we do backside etch or metallization next?A long backside etch will attack metal, and so we must do backside etch first.

Can we pattern nitride and oxide on both front and back at the same time?Yes, but etching both sides at the same time will etch all the way through the silicon and you will not have a diaphragm! And so we do them at different times because need to protect the front side during backside etch.

Mask 1

Process flow, pass 1

Order of steps

Let’s choose an order  5. Backside etch:

Before etching backside, we must cut the nitride and SiO2 using Mask 2. Nitride and SiO2 on topside protects topside of wafer.

6. Front side etch:Etch nitride and oxide on topside of wafer

7. Metallization:How does the metal connect to the doping? Must cut through the nitride and oxide first. Holes are called “vias” or “contact cuts”. Must pattern oxide and nitride on topside of wafer to create contact cuts..

8. Metallization: Add aluminum for vias and pads

Mask 2

Mask 3

Podemos combinarlos, ¿no?

7

6

Pass 2, Detailed process flow

1. All steps in the proper order, including when to clean the wafer2. Any chemicals necessary3. Thicknesses of materials

• These choices come for modeling.• The “process people” can turn chemicals and thicknesses into times

necessary for etches, depositions, etc.

4. Equipment necessary

It is the responsibility of the process flow person to think about which equipment is necessary for each step. Why? Because if you need a high temperature deposition to follow a metallization, you need a PECVD to do it or your metal will flow. The process flow person knows the entire process and makes design decisions.

5. MASKS for photoligthography

A detailed process flow is the list of all steps necessary for the process people to implement the device. It should include each of the following:

Detailed process flow

Let’s revisit each of the basic steps that we came up with and see what is really involved. You will notice that many of the steps actually turn into several steps when coming up with the detailed process flow. For this exercise, we will ignore dimensions and chemicals. However, note that these are also important components of the design flow.1. n-type doping

a. No mask is required since it covers the entire wafer

b. This could be done by purchasing a wafer with an epilayer or it requires 2 steps

i. implantationii.drive-in

2. Oxidea. No mask is required since it covers the

entire wafer.b. Note that oxide will grow on both sides of

the wafer. If you do not want it on the backside of the wafer, you must protect the backside of the wafer.

c. In this case, we do want oxide on both sides of the wafer.

Detailed process flow

Mask 1

3. Dope resistors and wiresa. Mask 1 – what does it look like?

(Assume positive resist.)b. This step requires 4 total steps

i. Photolithography so that ion implantation only goes where you want it to go

ii. Ion implantationiii.Remove photoresist (Must be done

before drive-in. Why?)iv.Drive-in

4. Deposit nitridea. No mask is required since it covers the

entire waferb. Depending on the process, you may

need to process both sides of the wafer.i. PVD often only deposits on one side

of the wafer.ii. CVD often deposits on both sides of

the wafer

Mask 1

Detailed process flow

Mask 2

5. Backside etcha. Mask 2 – what does it look like?

(Assume positive resist.)b. Must align Mask 2 with Mask 1 so that

the resistors are on the edge of the diaphragm. Alignment marks

c. This step requires 5 stepsi. Photolithography to determine

where you want the backside etch to start

ii.Etch nitrideiii.Etch SiO2 iv.Etch Si (Nitride and the SiO2 used

as a “hard mask” for the long Si etch.)

v. Remove photoresist

Mask 2

Detailed process flow

6. Contact Cuts/Diaphragm cuta. Mask 3 – what does it look like?

(Assume positive resist.)b. Must align Mask 3 with Mask 1 so that

wires connect to resistors. Alignment marks

c. This step requires 3 total stepsi. Photolithography to determine

where you want material removed for the metal

ii. Etch the nitride and oxideiii. Remove the photoresist

Mask 3

Mask 3

Detailed process flow

7. Metallizationa. Mask 4 – what does it look like?

(Assume positive resist.)b. Must align Mask 4 with Mask 1 so that

metal does not etch away. Alignment marks

c. This step requires 4 total stepsi. Deposit the Aluminumii. Photolithography to determine

which Al you want to removeiii. Etch unwanted Aliv. Remove the photoresist

Mask 4

Mask 4

Pass 3, Final process flow

These steps can be combined to create a final process flow.

One additional requirement in process flows is to include information about when to clean the wafer. Some general guidelines are:

• Always start with an RCA clean and an HF dip to get rid of every possible

• All future cleans are usually RCA cleans without an HF dip. HF may etch away your MEMS structures.

• Always strip photoresist and clean before high temperature processes.

• Always clean before depositing a new layer.

Final process flow

Final Process Flow for Bulk Micromachined Pressure SensorStarting material: 100mm (100) p-type silicon, 1×1015 cm-3 boron

1. Clean: Standard RCA clean with HF dip2. Oxide: Grow SiO2 on both sides of wafer3. Photolithography: Mask 1 (alignment)

Note: since the first patterned material is diffusion, which you cannot see, you must add alignment marks in the wafer or the first material you can see. If the first patterned material is something you can see, you do not need a separate alignment mark mask.

4. Etch: Etch alignment marks into SiO2.5. Strip: Strip photoresist

Note: since the next step is not a material deposition or a high temp step, a clean is not necessary.

6. Photolithography: Mask 2 (piezoresistors)7. Implant: Ion implantation of boron to achieve

1×1019 cm-3 at surface after drive-in8. Strip: Strip photoresist

Note: following step is a high temp step, so must clean wafer before.

9. Clean: RCA cleans, no HF dip10.Drive-in: Drive in diffusion to achieve 0.2 μm

junction depthNote: following step is a material deposition, so

must clean wafer before.11.Clean: RCA cleans, no HF dip12.Nitride: Deposit 50 nm silicon nitride using

LPCVD13.Photolithography: Mask 3 (backside

photolithography for the diaphragm)14.Etch: Remove nitride and oxide from back of

wafer15.Backside etch: Etch backside with KOH using

electrochemical etch stopNote: photoresist strip not necessary since returning to topside of wafer and strip will be done later for topside processing.

16.Photolithography: Mask 4 (vias/diaphragm opening)

17.Etch: Plasma etch nitride and oxide for vias and diaphragm opening

18.Strip: Strip photoresist19.Clean: RCA cleans, no HF dip20.Metal: Deposit 1 μm of aluminum21.Photolithography: Mask 5 (aluminum)22.Etch: Remove Al with PAN etch 23.Strip: Strip photoresist24.Sinter: Anneal contacts at 425°C, 30 minutes

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