Process flow part 2
 Develop a basic-level process flow for creating a simple MEMS device
 State and explain the principles involved in attaining good mask alignment
 Identify and explain the various issues involved with designing good process flows
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
etching
die separation
packaging
release
This is where process
flow becomes
complicated.
Mask design and layout
Mask layout
• The complete design with all mask layers combined is called the layout of the device.
• Typically use software specifically designed for masks
• Program allows you to place mask layers on top of each other to ensure good
alignment
• Each mask layer shown in a different color and/or line style
• The software will separate the layers into the individual masks for fabrication.
• The software also keeps track of whether masks should be positive or negative
depending on whether the process is typically additive or subtractive
Mask design and layout
Mask alignment
Every mask must have alignment marks that will align the mask to the features on the wafer.
alignment
feature on
mask
alignment
feature on
wafer
mask aligned
with wafer
Mask design and layout
Mask alignment
Issues to think about when designing the shape and the placement of the alignment mark:
• Does the alignment mark shape give wafer orientation as well as alignment?
• Asymmetry is good  a cross is better than a “plus”
• A circular mask opening will produce a square etch in Si showing crystal directions. Align
next masks to the square
• Make sure your mask does not obscure your alignment mark!
• You must be able to see the entire alignment mark through your mask
• Dark areas on masks very dark in order to keep light from going through
• Features on a wafer tend to be gray
• It is good to leave a little “wiggle room” around alignment mark on the wafer
Mask design and layout
Mask alignment
• Use a variety of alignment marks
o Use one large alignment mark one to get a sense of where you are on the wafer
o Use smaller ones to fine tune the alignment
o Use several marks on opposite sides of the wafer. A small error in angle can propagate into a
large error across the distance of the wafer
T
• Be sure your alignment mark is in a material you can see.
o You can see edges in most structural materials and in metals
o You cannot see diffusion!
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o If your first step in the process flow is diffusion, you may need to add another mask to
create an alignment mark. Otherwise, you may place the first alignment mark in the first
mask you use
First patterning  first alignment mark
Mask design and layout
Mask alignment
• Know the process flow of your alignment marks
o The process flow of the alignment marks may be different than that of the whole device
since the alignment marks see every mask layer and most of your structural/sacrificial
layers do not
o Does a process step obscure or eliminate an alignment mark you intended to use? (E.g.,
does a deposited layer covers it up?) If so, you must create another one.
• Backside alignment
Backside alignment requires a special “backside aligner” that uses lasers and/or mirror to
find the alignment mark on the backside of the wafer.
Surface μ-machined pressure sensor
Silicon substrate
Poly-Si diaphragm forms one plate of
capacitor.
n+ diffusion layer forms other “plate”
of capacitor
Aluminum wires send capacitive
electrical signal off the chip.
Oxide layer insulates aluminum wires
from rest of chip
Nitride insulates poly-Si diaphragm
from n+ diffusion.
Notches to prevent uncompensated
stresses from breaking diaphragm
during release
Process flow, pass 1
We can go through this example a little quicker.
What are the major steps to create the device?
1.
2.
3.
4.
5.
6.
C
Diffusion of n+ dopant for bottom “plate” of
capacitor
Deposit nitride for electrical insulation
Deposit sacrificial oxide
Add poly-Si diaphragm
o Need pedestals and
o notches to form diaphragm
Sacrificial etch
Create wires
But isn’t release supposed to be last?
Detailed process flow
Mask 2
1. Diffusion of n+ dopant for bottom “plate” of
capacitor
a. Note that since we cannot see diffusion we will need to etch
alignment marks in the wafer first.
b. Mask 2 – what does it look like? (Assume positive
resist.) Mask 1 is for alignment marks
c. Breakdown of this step:
i.
Etch alignment marks into wafer
ii. Photolithography so that ion implantation
only goes where you want it to go
iii. Ion implantation
iv. Remove photoresist
v. Drive-in
2. Deposit nitride
No mask is required since it covers the entire wafer
Mask 2
Detailed process flow
3.
Deposit sacrificial oxide
a. No mask is required since it covers the entire wafer
b. Why cover the whole wafer? Why not pattern oxide to go
just under the diaphragm and nowhere else?
4.
Add poly-Si diaphragm
a. How do we produce notches and pedestals?
We will need two different etches.
b. What will our etch stop method be?
 Timed etch
b. Breakdown of this step:
i.
Photolithography notches (Mask?)
ii.
Etch notches
iii. Photolithography pedestals (Mask?)
iv. Etch pedestals
v.
Deposit poly-Si
vi. Photolithography poly-Si (Mask?)
vii. Etch poly
Mask 534
Mask
Mask 34
Mask 5
Detailed process flow
Mask 67
5.
Sacrificial etch
a. If using oxide for both sacrificial layer and insulation for
the wires, need to do sacrificial etch before laying the oxide
for the wires. Why?
b. Contact lithography requires release to be done last. Why?
How would we change our process flow if we have to do
contact lithography?
6.
Create wires
The height of the features is
a. Deposit oxide
exaggerated, but the
b. Photolith contact cuts (mask?)importance of the “depth of
c. Etch contact cuts
focus” idea is very clear. 
d. Deposit Al
e. Photolith Al (mask?)
f. Etch Al
Mask 7
Mask 6
Final process flow
Final Process Flow for Surface Micromachined Pressure Sensor
Starting material: 100mm (100) p-type silicon, 1×1015 cm-3 boron with a 10 mm n-type epilayer, 5×1016 cm-3 phosphorus
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
Clean: Standard RCA cleans with HF dip
Photolithography: Mask 1 (alignment)
Etch: Etch alignment marks into Silicon.
Strip: Strip photoresist
Photolithography: Mask 2 (n+ diffusion)
Implant: Ion implantation of phosphorous
Strip: Strip photoresist
Clean: RCA cleans, no HF dip
Drive-in: Drive in diffusion
Clean: RCA cleans, no HF dip
Nitride: Deposit insulating nitride layer
Oxide: Deposit sacrificial SiO2
Photolithography: Mask 3 (notches)
Etch: Short etch to get notches
Strip: Strip photoresist
Photolithography: Mask 4 (pedestals)
Etch: Longer etch to get pedestals
Strip: Strip photoresist
Clean: RCA cleans, no HF dip
Polysilicon: Deposit polysilicon for diaphragm
Photolithography: Mask 5 (diaphragm)
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
Etch: Etch polysilicon
Strip: Strip photoresist
Sacrificial etch: Remove oxide leaving pedestal
Clean: RCA cleans, no HF dip
Oxide: Deposit SiO2 for insulation
Photolithography: Mask 6 (vias)
Etch: Etch oxide to get vias
Strip: Strip photoresist
Clean: RCA cleans, no HF dip
Metal: Deposit aluminum for wires
Photolithography: Mask 7
Etch: Etch Aluminum
Strip: Strip photoresist
Sinter: Anneal contacts
Other issues in process flow
Other issues in designing good process flows
System partitioning:
• Whether or not to integrate the MEMS device and any necessary electronics on the same chip
• Integration limits MEMS process steps due to temperature, materials, etc.
Process partitioning:
• Material used in one process bond with and/or affect the properties of materials in another processes?
• If so, the order of the process steps may matter significantly.
Backside processing
• Makes many fabrication processes easier, but alignment is more difficult
• Also must take into account which steps affect both sides of wafer and which ones affect only one side
Thermal constraints
• E.g., photoresist cannot withstand high temperatures,
• High temperatures further drive-in dopants
Other issues in process flow
Other issues in designing good process flows
Device geometry
• Hard to visualize the 2-D and 3-D aspects of devices  Solid-modeling, CAD software
developed specifically for MEMS
• Combination of conformal deposited layers with directional etching can result in stringers
stringer
• Can use planarization to avoid stringers,
depth of focus problems, and other issues
arising from large changes in topography
An example of a “floating stringer” (Courtesy of
Sandia National Laboratory
Other issues in process flow
Other issues in designing good process flows
Mechanical stability:
Fabrication can result in the formation of different stresses
in structural layers, causing them
C
to bend or break
Coming up next!
Process accuracy:
• Expansion or shrinkage of photoresist
• Variations in the thickness of layers
• Presence of photoresist in structural layers
• Mask misalignment between layers
Design rules
Other issues in process flow
C
A MEMS wheel and hub
If using a wet etchant in step 7
isotropic
or anisotropic
C
Undercutting
A win-win process flow
The self-aligned gate transistor
metal
electrode
poly Si
electrode
use poly as electrode and as
mask for doping
n+ doping
virtually no gap increases in
switching speed significantly
p type
wafer
overlap causes unwanted
increase in capacitance,
slower switching speed