Surface micromachining
and Process flow part 1
 Identify the basic steps of a generic surface micromachining  Define the terms
process
 Structural layer/material
 Identify the critical requirements needed to create a MEMS
 Sacrificial layer/material,
using surface micromachining
 Release, and
 List common structural material/sacrificial material/etchant
 Die separation
combinations used in surface micromachining
 Develop a basic-level process flow for creating a simple
 Compare and contrast the relative merits of wet
MEMS device
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
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.
+
= Surface micromachining
Review of surface micromachining process
Surface micromachining example –
Creating a cantilever
Deposit poly-Si (structural layer—
the Jenga pieces that remain)
Deposit SiO2 (sacrificial layer—the
Jenga pieces that are removed)
Remove sacrificial
layer (release)
Etch part of
the layer.
Often the most critical
Silicon wafer (Green
Lego® plate)
Reminder of the surface micromachining process
side view
top view
silicon
oxide
metal
Process flow for surface μ-machined cantilever
Mask 1 (negative resist)
Mask 1 (positive resist)
1. .
2. .
Top view (4)
3. .
Top view (5)
4. .
5. .
6. .
Top view (6)
Process flow for surface μ-machined cantilever
Top view (7)
7. .
8. .
Mask 2 (negative resist)
Mask 2 (positive resist)
9. .
10. .
11. .
12. .
Top view (9)
Top view (10)
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 include
o 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)
or
Physical vapor deposition (PVD)
If PVD
Sputtering
or
Evaporation
Common material/etchant combinations for surface μ-machining
Structural material
Sacrificial Material
Si/Polysilicon
SiO2
Al
Photoresist
Phosphosilicate glass
(PSG)
Polysilicon
Polyimide
Si3N4
Etchant
Buffered oxide
etch (BOE) (HFNH4F ~ 1:5)
Oxygen plasma
HF
XeF2
Problems and issues
Wet etching
Dry etching
• 40 years of experience and data in the
semiconductor industry
• Ability to remove surface contaminants
• Better resolution than wet etching
• Very high selectivities
• Lower selectivities
• No undercutting
• Usually isotropic  always involve
undercutting
• More directionality (High aspect ratios )
Stiction
moisture
Stiction
Stiction = static + friction
An example of a portmanteau
Stiction = stick + friction
An example of an unfavorable scaling
surface tension 
L
 ~ 2
restoritive force F L
1
~
L
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-offC bumps” on the underside of moving
parts. Pillars prop up (soportar) movable parts
Problems and issues
“Dimple” resulting from a
stand-off bump on the
underside of the cold arm
Polysilicon hotarm actuator created using surface μ-machining
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
(+) or (-)
C resist?
1. Photoresist is spun on a wafer and exposed to create
pattern
Resist 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 material
3. 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
Typical process steps for surface micromachining
•
•
•
modeling and simulation
design a layout
design a mask set
thin film formation (by
growth or deposition)
1
2
3
4
mask
set
lithography
C
etching
die separation
C
packaging
release
This is where process
flow becomes
complicated.
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)
die separation
• Packaging a difficult engineering problem
C
• Largest cost of producing many (most) devices
packaging
packaging
More on packaging
Wafer-level packaging
Die-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)
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?
• Nature of crystalline silicon
• Adding material
o Doping
o Oxidation
o Deposition
 PVD
 CVD
• Photolithography
• Bulk Micromachining
• Surface Micromachining
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)
Hemos pasado mucho tiempo acá.
http://juliaec.files.wordpress.com/2011/04/blooms_taxonomy.jpg
http://ictintegration.wikispaces.com/Bloom
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.
Temperature
High 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 stress
If 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 chemicals
If 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 resistors
4.
Deposit nitride
Do 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.
8.
6
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..
Podemos
combinarlos, ¿no?
Mask 3
Metallization: Add aluminum for vias and pads
Mask 2
Pass 2, Detailed process flow
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:
1.
2.
3.
All steps in the proper order, including when to clean the wafer
Any chemicals necessary
Thicknesses of materials
•
•
4.
These choices come for modeling.
The “process people” can turn chemicals and thicknesses into times necessary for etches,
depositions, etc.
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
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. implantation
ii. drive-in
2. Oxide
a. 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 wires
a. 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 implantation
iii. Remove photoresist (Must be done before
drive-in. Why?)
iv. Drive-in
4. Deposit nitride
a. No mask is required since it covers the entire wafer
b. 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 etch
a. 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 steps
i. Photolithography to determine where you want
the backside etch to start
ii. Etch nitride
iii. 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
Mask 3
6. Contact Cuts/Diaphragm cut
a. 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 steps
i. Photolithography to determine where you
want material removed for the metal
ii. Etch the nitride and oxide
iii. Remove the photoresist
Mask 3
Detailed process flow
Mask 4
7. Metallization
a. 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 steps
i. Deposit the Aluminum
ii. Photolithography to determine which Al you
want to remove
iii. Etch unwanted Al
iv. Remove the photoresist
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 Sensor
Starting material: 100mm (100) p-type silicon, 1×1015 cm-3 boron
1. Clean: Standard RCA clean with HF dip
2. Oxide: Grow SiO2 on both sides of wafer
3. Photolithography: Mask 1 (alignment)
C
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-in
8. Strip: Strip photoresist
Note: following step is a high temp step, so must clean wafer before.
9. Clean: RCA cleans, no HF dip
10. Drive-in: Drive in diffusion to achieve 0.2 μm junction depth
Note: following step is a material deposition, so must clean wafer before.
11. Clean: RCA cleans, no HF dip
12. Nitride: Deposit 50 nm silicon nitride using LPCVD
13. Photolithography: Mask 3 (backside photolithography for the
diaphragm)
14. Etch: Remove nitride and oxide from back of wafer
15. Backside etch: Etch backside with KOH using electrochemical
etch stop
Note: 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 photoresist
19. Clean: RCA cleans, no HF dip
20. Metal: Deposit 1 μm of aluminum
21. Photolithography: Mask 5 (aluminum)
C
22. Etch: Remove Al with PAN etch
23. Strip: Strip photoresist
24. Sinter: Anneal contacts at 425°C, 30 minutes