Microsystems Technologies: Basic concepts and terminology
1. Microsystems Technologies
Basic concepts and terminology
Selected traditional micromachining photolithography
and mask design, wet and dry bulk etching, bonding, thin
film deposition and removal, metallization, sacrificial
processes, other inorganic processes, electroplating
Polymer techniques
thick-film polymers, stamping, soft lithography and
micromolding, stereolithography, LIGA
Disclaimer: Not trying to cover ENSC851 or all microfluidic
fabrication in a few weeks; just enough so you can understand
process flows.
2. Photolithography
Photolithography: Basics
Latin: “stone-writing”
Optical means for transferring
thin film
patterns onto a substrate or
Patterns are first transferred to an imageable photoresist
(polymer) layer,a film that can be easily applied then
selectively patterned using a light source
3. Photolithography
Typical photolithography process
1.
2.
3.
4.
5.
6.
Surface preparation
Spin coating of photoresist
Pre-bake (soft-bake) to remove majority of solvents
Alignment (putting mask plate pattern where you want it)
Exposure through patterned mask plate to light source
Development of photoresist layer to selectively remove
either exposed or unexposed parts
Processing using the photoresist as a masking layer
Stripping of the photoresist layer
Do it all again!
7.
8.
9.
4. Photolithography
Photoresist (resist) spinning
Wafer is held on a chuck using vacuum
Resist is spin coated to uniform thickness typically at speeds
1000-6000RPM for 15 seconds – several minutes
viscosity and speed
constant, typ. 80-100
Resulting resist thickness depends on
t = kp2/w1/2 k = spinner
p = resist solids content in %
w = spinner speed RPM/1000
typical final result
5. Photolithography
Pre-bake
Used to evaporate solvent
Typical thermal cycles:
and densify resist
90-100 C for 20 min in oven
75-85 C for 1 min on hotplate
Hot plate preferred: faster, more control, does not trap solvent
Alignment
and exposure
6. Photolithography
Photomasks
Types: photographic emulsion, transparency,
chrome on soda lime/quartz
Polarity:
light field: mostly clear, drawn features are opaque
dark field: mostly dark, drawn features are clear
Minimum feature size depends on how mask is made
transparency (e.g., Fineline Imaging): 10-25
optical pattern generator: several microns
ebeam-writer: 1 micron or even <1
Alignment
3 DOF between mask and wafer (x,y,θ)
Align marks on mask to marks already
patterned on substrate
microns
Need at least 2: one each side of
Typical layer-to-layer accuracy: 2
(micron = 1x10-6 m = 1 μm)
wafer
– 5 microns wafer
mask
7. Photolithography
Development
Different developers work better for different photoresists
Typically 1 minute for 2 μm thick resist, can be 10+ minutes for
very thick films (e.g., SU-8 photoepoxy)
Post bake (hard bake)
Removes remaining solvent and “hardens” resist
Absolutely necessary before some wet etches, vacuum
processes
Temperature and time significantly effect resulting profile
time
Important for steps such as lift-off and etching with significant mask erosion
Stripping
Acetone, commerically available stripper, O2 plasma
8. Two most common substrate materials in traditional
micromachining; historically first materials for microfluidics
Many methods developed for patterning: fast prototyping
Good/reasonable properties for many bio application of
Microsystems(glass) or active devices (silicon)
fluidic port modules
tubing
micro-channels
Bulk Micromachining of Silicon and Glass
9. Bulk Processing: Silicon
Why silicon?
•
•
readily available in ultra-pure form
mech. properties: strong (Yield Strength = 2.8-6.8 GPa)
lightweight (ρ = 2.33 g/cm2)
E = 190 GPa; 150 GPa for poly
• compatible with integrated electronics
Silicon is well understood and silicon processing is
already well established in the IC industry.
Structure of silicon – 3 types
crystalline:
completely
amorphous: no
long-range order
polycrystalline:
ordered in segments ordered
10. Bulk Processing: Silicon
Structure of silicon: diamond lattice unit cell
a/2 4 nearest
neighbors
Two interpenetrating FCC
Atypical silicon wafer is a single
crystal of silicon of a specific
orientation.
Important because some properties,
e.g., electrical and etching, are
orientation dependent.
11. Bulk Processing: Silicon
Miller Indicies:
method of identifying planes and directions in crystal
Example:
z
What plane is this?
(xyz) = (2 1 3)
Invert to form 1/intercept
(1/2 1 1/3)
Convert to smallest
possible whole numbers
(3 6 2)
3a
a y
2a
x
is considered No intercept: to be ∞, so 1/intercept
For negative
number = 0
numbers, a bar is used: x
12. Bulk Processing: Silicon
z z
a
More examples of planes:
(100) a (111)
a y y
a a a
x x
Equivalent planes arise from crystal symmetry:
(010), (001), (100), (010), (001) are called the {100} planes
The [hkl] direction is normal to the (hkl) plane.
Thus, the [100] and [111] directions are as shown in red arrows.
(xyz) crystal plane
{xyz} equivalent planes
[xyz] crystal direction
<xyz> equivalent directions
13. Bulk Processing: Silicon Crystal
ଛ Silicon is a fairly complicated crystal
and silicon substrates are always single
crystal (thin films may be amorphous or
poly). The main concept here is that when
we pattern silicon, it may etch differently
in different directions, differently
depending on whether single crystal
not, etc., and this will determine the
ultimate shape of the structure.
or
14. Silicon Bulk Processing: Wet Isotropic Etching
(etches same for all directions)
drives oxidation of Si to form SiO2
attacks oxide to form H2SiF6
prevents disassociation of HNO3
into NO3- or NO2-
Components:
(N)1.
(H)2.
(A)3.
nitric acid (HNO3)
hydrofluoric acid (HF)
acetic acid (CH3COOH)
soluable gas
Overall reaction:
18HF + 4HNO3 + 3Si
Mask materials (films):
silicon dioxide
3H2SiF6 + 4NO(g) + 8H2O
(less selective: 30-70 nm/min)
silicon nitride (very selective)
10 μm
15. Silicon Bulk Processing: Wet Isotropic Etching
Room temp.
ACETICACID DILUENT
H2O DILUENT
1
1 High HF conc.: contours
parallel to lines of constant
HNO3; etch rate controlled
by HF
High HNO3 conc.: contours
parallel to lines of
constant HF; etch rate
controlled by HNO3;
3
2
3 Etch rate falls as
diluent increases
for 1:1 HNO3:HF
2
Weight % Diluent
16. Silicon Bulk Processing: Wet Anisotropic Etching
Anisotropic wet etchants: different crystal planes etch at
different rates (ER(111) <ER(100) <ER(110) )
Etch profile: depends
sk
a e
on wafer
on orientation of ma
and orientation of w f r
ଛ Only two most used shown here
(100) Si
[100]
mask
[111]
54.74º
substrate
Mask orientation “D”
(See supplemental material on Web
Under “Part II”.)
CT Note: can make
trenches but not holes
17. Silicon Bulk Processing: Wet Anisotropic Etching
Mask undercut: consider 100 silicon
1. Stops on concave
corners
Undercut of convex
corners is severe
Can make
overhanging
structures
2.
3.
18. Silicon Bulk Processing: Wet Anisotropic Etching
Mask undercut (continued)
4.
5.
Can make pillars
Must often be very clever in
mask design
23. (Silicon Bulk Processing: Dry Etching)
Xenon difluoride etching: non-plasma dry etch
Basic reaction:
2XeF2 + Si 2Xe + SiF4
Unfortunately: EXOTHERMIC
2XeF2 + 2H2O Xe2 + 4HF + 2O2
•
•
•
•
Simple set-up
Isotropic etch
Post-processing for standard CMOS
Does not attack oxide, nitride, metals, photoresist
24. Silicon Bulk Processing: Dry Etching
Silicon RIE etching
Common etching chemistries are based on CF4, SF6, or
chlorine, with O2 or another gas (e.g., C2ClF5
Freon115) added to assist with sidewall passivation.
For CF4 and SF6, the volatile SiF4 species is produced which
is then carried away from the surface.
Example: SF6 and O2 (taken from
SxFy + SxFy* +
Bhardwaj,et.al.1998)
due to SF6 + e- + F* + e-plasma{
O2 + e- O+ + O* + e-
O* + Si Si-nO SiOn (creating an oxide
the surface)
on
This surface passivation must be removed for Si etching.
25. Silicon Bulk Processing: Dry Etching
ion energy
(critical!)
SiOn + F* SiOn-nF SiFx(ads) + SiOxFy(ads)
Where F adsorbs (ads) onto the surface and ion bombardment plays a
critical role in the passivation removal by enhancing adsorption, reaction,
and desorption ଛ removes oxide mainly
(how silicon gets etched)
from horizontal surfaces
Si + F* Si-nF
ion energy
(not critical)
Si-nF SiFx(ads)
SiFx(ads) SiFx(gas)
Basically, this amounts to a plasma etch converting Si to
volatile SiF4, with O2 added to provide surface passivation that
must be removed by RIE ion bombardment (not on sidewalls),
resulting in anisotropic etching with straight sidewalls.
26. Silicon Bulk Processing: Dry Etching
Deep RIE (DRIE) etching
Very high-aspect ratio (30:1) anisotropic etching Two
methods: Bosch process and cryogenic Both use
inductively coupled plasma (ICP) Photoresist or
silicon dioxide mask
ICP
Creates a magnetic envelope inside
the etch chamber
Reduces the loss of charge species to the
surroundings
Maintains a very high plasma density
Compared to conventional RIE:
Better etch uniformity
Higher etch rates (>3μm/min)
Better mask selectivity (100:1)
27. Silicon Bulk Processing: Dry Etching
Bosch process
SF6 and C4F8 gases are cycled for alternating isotropic and
surface passivation steps.
SF6 C4F8
time
28. Silicon DRIE Continued
Resulting etch profile: Bosch process
(scalloping)
Cryogenic
Cryogenic cooling of the wafer lowers the surface mobility of
reactive ions so that they do not migrate once they hit the
surface, but rather etch directly perpendicularly into the wafer.
ଛ no “scalloping”
29. Silicon DRIE Problems (Features?)
Rough sidwalls
result from uneven masking
material recession
Notching:
results from interface of silicon
and another material (oxide,
photoresist, metal, etc.)
DRIE lag:
smaller features etch slower
also called aspect ratio
dependent etching
30. Silicon DRIE Problems (Features?)
Profile variation
results from differences in size of surrounding etched area;
can also be controlled using etch parameters
31. Silicon DRIE Continued
Applications
Complex multilevel structures Less
restrictive geometries Combine
with wafer bonding for
sealed cavities, channels,
and through-holes
inlet 2
stage 1
inlet 1 stage 2
stage 3
inlet 2
400 μm
inlet 1
Turbine Multilevel mixer
32. Bulk Processing: Glass
Why glass?
Key for
optical
biochem
detection
methods
{Optical transparency for imaging and detection
Laser light transmission for excitation
Easily combined with silicon, polymers, and metals
Readily available substrates
crystalline quartz
amorphous glass (borosilicate, soda lime)
fused silica
Insulator (important for applications with electrodes)
Relatively easy and inexpensive to microstructure
Chemists and biologists are familiar with glass
Disadvantages
Nasty chemicals are needed to pattern it
Currently etch geometries are very limited
Being replaced by polymers in most applications
33. Bulk Processing: Glass
Wet etching of amorphous bulk glass (pyrex)
HF acid chemistries
Isotropic etch
Typical etch rate in 6:1 buffered oxide etch (BOE): 300Å/min
Useful for structure undercuts, channels, well arrays Useful
for through-wafer holes if size is not critical Masking
materials: polymer, metal (e.g., Cr/Au), silicon
HF-etched glass channels With electrodes
Micro
titer-plate
34. Anisotropic Etching of Glass
SF6 plasma
ULVAC glass DRIE system
Alcatel glass DRIE system
Nickel mask (15μm thick)
http://www.ulvac.com/mems_optic/nld-6000.asp
a-Si mask
35. Wafer Bonding
Very useful for making enclosed channels or cavities
Various methods
silicon-silicon bonding glass-glass bonding
silicon-glass bonding
Today we will only consider traditional, non-polymeric
substrates (glass and silicon).
36. Bonding:Anodic
Bonding glass to silicon
Characteristics:
Works with either glass wafers or thin film glass (ebeam,
sputtered, spin-on glass)
Will bond over small step heights (e.g., thin metal lines)
Usually performed at about 400 C using high voltage (1.2 kV), but
much less voltage needed for thin film glass
Basics:
Clean substrates to prevent voids: RCA clean, hot sulphuric
Use glass, silicon with similar thermal expansion coefficient
Place glass and silicon in close proximity
Heat glass, stabilize, apply voltage
Wait for monitored current to peak, then drop to 10% of peak
37. Bonding:Anodic
Process details
Silicon biased positive, glass negative
Positive ions in glass drift away from silicon causing high
field at interface and contact/reactivity for oxide bond
cation
_ + glass high V motion
supply glass - - - - - - - - - -
+ silicon
ipeak
0.1*ipeak
time
Cations are often sodium or boron
Often accumulate near electrode
current
silicon
chuck
++++++++++
38. Bonding: Silicon Fusion
Bonding silicon to silicon; oxides can also work
Characteristics:
High physical strength
Highly sensitive to wafer surface uniformity, roughness
Basics:
Clean substrates to prevent voids
Bonding surfaces must be very smooth, very flat
Require hydroxyl groups on surface: exposure to a strong base
such as ammonium hydroxide
Start bond front from one side (manual) or use a center pin
Must anneal at high temperature (usually 800 - 1200 C)
39. Bonding:
Process details
Silicon Fusion
Si-O-H +
Si
H-O-Si Si-O-Si
Si
+ H2O
Si Si
Si Si Si Si Si Si Si Si
H2O
Si Si Si Si
O O
Si
O
O
H H H Si Si
Si
O O O
Si Si Si
Si Si Si Si Si Si Still being
Si Si Si
investigated[.
H
H
H
40. Other Common Bonding Methods
Thermal
Used to bond glass-to-glass, ceramics, metals, and
polymers to themselves (will discuss polymers later)
Glasswafers: are cleaned, then immersed in nitric or
hydrochloric acid solution
annealed at 600 C for 8 hours
Adhesive
Glass, glue or other curable, potentially photosensitive,
polymer
