Discusses about Microsystems Technologies,micromachining,polymer techniques,photolithography and mask design, wet and dry bulk etching, bonding, thin film deposition and removal, metallization, sacrificial processes, other inorganic processes, electroplating,polymer materials and basics, thick-film polymers, soft lithography and multiple methods of micromolding, stereolithography, LIGA

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 (not neccesarilly exact order()
polymer materials and basics, thick-film polymers, soft
lithography and multiple methods of micromolding,
stereolithography, LIGA
*** Please note posted article by Holger Becker ***

2. Polymer Microfabrication Methods
Introduction: polymer materials and basics Various polymers and their
characteristics General polymer definitions
Thick film mask patternable polymers
Including SU-8 photo-epoxy, polysiloxane, polyimide, and PMMA
Electroforming using polymer thick films; LIGA
Stereolithography
Micromolding
Soft lithography
Hot embossing
Other micromolding processes
(injection-molding, etc.)
Polymer bonding
Other polymer microfabrication
Laser methods (photoablation, polymerization)
Micromilling
CVD
Plasma etching
Others…

3. Polymer Materials and Basics
Why micromachine polymers?
Inexpensive, especially for mass production
Flexible substrates may be needed
Many polymers have good transmission spectrums for visible
and uv wavelengths
Easily molded/mass produced
Surface properties easily modified
Improved biocompatibility and bioreactivity, nontoxic
Chemically inert
Thermal and electrical isolation (insulating substrate)
High aspect ratio microstructures easily realized (some processes)
Problems with micromachining polymers
Combining processes with other micromachining substrates/processes
(e.g., metal interconnect/electrodes)
Still “working the bugs out” (as opposed to mature silicon industry supported by
microelectronics)

4. Polymer Materials and Basics
Polymers
Consist of long chains of smaller molecules called monomers
Carbon allows formation of long chains
example: ethylene (monomer) and polyethylene
CH2 (ethylene) …CH2=CH2=CH2( (polyethylene)
Monomers must undergo polymerization to create polymers
addition polymerization:
monomer units directly attach (e.g., polyethylene)
condensation polymerization:
monomer units are linked by intermolecular reaction that
gives off another species and/or use another species to
react, e.g., water or simple alcohols
examples: polyimides, polyesters, silicones

5. Polymer Materials and Basics
Polymer structure
Linear, branched, or cross-linked/networked
Linear polymers are composed of a linear
arrangement of identical units.
Branched polymers have side chains that
branch off the main chain. The size of
these side chains can vary.
Cross-linked polymers form covalent bonds
between adjacent chains at various points
using branches.
linear
branched
cross-linked
network
Network polymers form complex 3D structures.
Network
example:
SU-8
epoxy
Linear example: polyethylene

6. Polymer Materials and Basics
Polymer structure (contiuned)
Homopolymer (same monomer repeated, eg . , polyethylene
Copolymer (more than one monomer,
e.g., ethylene and propylene)
Copolymers can be further classified
based on the arrangements of
Their different monomers
monomer #1
monomer #2

7. Polymer Materials and Basics
Polymer types
Thermosplastics (e.g., PS, PE, PVC, PC, PMMA, parylene, PETG)
linear or branched polymers that can be melted/softened upon
application of heat; harden when cooled
soft, plastic behavior at low temp; high viscosity at high temp
melting point typically around 120 – 180 C
usually soluble in common solvents to make liquid
Thermosets (e.g., SU-8, other epoxies)
highly cross-linked, often network polymers, but not always (PDMS)
some can also be processed using melting
require “curing” (heat, radiation) to polymerize (“set”)
often come in two parts that when mixed will cure
Elastomers (e.g., PDMS - thermoset, PU - thermoplastic)
weakly cross-linked polymers; usually large molecular weights
called “elastomers” because they are generally elastic (stretchy)
can be thermosets or thermoplastics (with associated fabrication)

8. Polymer Materials and Basics
Polymer examples
Polymer example
(abbrieviation)
Example (typical) micromachining method(s)
Polystyrene (PS) Hot embossing, laser ablation
Polyvinylchloride(PVC) Hot embossing, laser ablation
Polycarbonate (PC) Hot embossing, injection molding, laser ablation
Polymethylmethacrylate
(PMMA)
Hot embossing, injection molding, x-ray lithography, laser ablation
Parylene CVD
SU-8 Photolithography
Polyimide Photolithography, laser ablation, RIE etching
Polydimethylsiloxane
(PDMS)
Soft lithography
Photosensitive silicone Photolithography
Polyethylene tere-phthalate
glycol (PETG)
Hot embossing, laser ablation
Cyclic olefin polymer
(COP)
Hot embossing, laser ablation, injection molding

9. Polymer Materials and Basics
Curing methods for thermosets and elastomers
Room temperature curing
Heat curing
Radiation curing (including photopolymerization)
Room temperature curing (RTV, room-temp-vulcanization) One-part
formulations usually moisture cure (from surrounding air) In two-part RTV, one
part is “curing agent”; cure at room
temperature by reacting with eachother and/or moisture
Heat curing
Initiates or speeds up (two-part) polymerization reactions with thermal
energy
Radiation curing
Initiates polymerization reactions with radiant energy, such as uv light
Areas radiated cross-link and remain, while uncured polymer is
washed away; hence a negative process (negative photoresist)

10. Polymer Materials and Basics
Radiation (UV, x-ray, etc.) curing
versus heat or room temp curing
Some advantages of radiation curing
1. High turn-around (fast processing)
2. Low heat generation (heat-sensitive substrates can be used)
3. Very eco-friendly due to low organic emissions
4. UV cure uses conventional microelectronics equipment
Some disadvantages of radiation curing
1. Limited to planar geometries
2. High capital equipment costs, although not too bad for uv cure
3. May not be cost effective for wide applications
4. May have post-cure instability when exposed to direct sunlight
5. Some components are skin irritants

11. Polymer Materials and Basics
Radiation curing
Bond breaking
Photons have a fixed energy, ε, given by: ε = hν
where h is Planck’s constant (9534 x 10-14 cal-s/mol)
and v = c/λ where c is the speed of light and λ is the wavelength
Energy and light
wavelengths required
to break bonds
Radiation spectrum
Once bonds are broken, they can rearrange into a polymer aided
by photoinitiators (PI) in the material that utilize the light energy

12. Thick Film Polymers
Radiation curing
X-ray patterning of PMMAfor LIGA
UV-light patterned polymers
SU-8 photoepoxy
Photopattenable silicone elastomer
Photosensitive polyimide

13. LIGA
LIGAis a method of producing high aspect ratio microstructures
consisting of three major steps (the acronym is German):
LI– Lithographie (lithography) X-ray lithography produces
high aspect ratio primary structures in PMMA(polymethyl-methacrylate)
“photoresist”; a thick-film polymer process
G– Galvanoformung (electroforming) metal is electroformed
into the PMMAstructures, possibly planarized, and released
A– Abformung (molding) released metal molds are used to
fabricate secondary structures in polymers, metals, or ceramics

14. LIGA
Process flow
Shadow printing
using x-rays
Special mask
blocks xrays
overview
Exposed areas
removed in
solution (GG
(PMMA) developer*)
(Cu-Ti stack)
Resist and metal
on substrate (Ti)
sacrificially
etched leaving
metal mold free
of substrate
Electroplating of
metal into
resist
structure
(e.g., Ni)
Covered with polymer micromolding lectures
*Available commercially

15. LIGA
PMMAthick films
PMMAdisc bonded to si substrate
1.
2.
PMMAcasting onto substrate
Twowidelyusedmethods:
solvent bonding of PMMA
disc (cut from a sheet)
onto silicon substrate
Casting of PMMA onto
substrate; PMMA/MMA
plus an polymerization
initiator
Results in a stressy film

16. LIGAmasks and x-ray lithography
X-ray masks must be fabricated using micromachining
techniques; they are often a thin membrane of x-ray-transparent
material (e.g., 5 μm titanium , 500 μm beryllium,
125 μm polyimide) with an x-ray opaque pattern (e.g., gold)
Be “wafer”
15 μm thick Au
Opaque metal is then
electroformed through the
photoresist
Pattern is often copied from
an optical mask
LIGA

17. LIGA
LIGA masks and x-ray lithography
PMMAis exposed using synchrotron x-rays through this mask
that hasAu areas opaque to x-rays
LIGAphotolithography, from mask materials to the
synchrotron x-ray source, is an expensive process

18. LIGA
Results in breakdown of
PMMAbonds
bond broken
under x-rays
Does not reorganize
Stripped away in solvent
(methyl methacrylate)
Thus, is a positive (like
photoresist) process
PMMAchanges
MW when
exposed to x-rays
PMMAexposure

19. LIGA
Complex structures
1. Mutiple PMMAlevels for multi-level molds
2. Substrate tilting
Multi-level patterning

20. LIGA
Note: bottom
picture is SU-8
not PMMA
Complex structures
1. Mutiple PMMAlevels for multi-level molds
2. Substrate tilting
Substrate tilting

21. LIGA
ExampleApplications
PMMA microneedles
LIGA microtweezers
LIGAmicromotor

22. SU-8 Thick Film Photopolymer
What is SU-8?
Negative working epoxy-based uv-patternable photopolymer originally
developed by IBM
What is so great about SU-8?
Patterning using standard uv mask aligner (not x-ray!)
Thicknesses 2 μm - 2mm with straight side walls Aspect
ratios >25:1 (10:1 more typical)
Structures can be used directly, or used to mold others (“poor
person’s LIGA” or “uv-LIGA” w/PDMS soft lithography or e-forming) Large
number of different formulations/viscosities (thicknesses) Compatible with
silicon devices, glass, metal, and other polymers
Can be patterned on top of circuits, photodiodes, etc. Chemically
inert once cured (also makes it difficult to remove) Good thermal
stability
Appears to be biocompatible

23. SU-8 Thick Film Photopolymer
What is in SU-8 resin?
Epoxy resin: a bisphenol/formaldehyde novolac co-polymer
Solvent: gamma-butyrolactone (GBL)
Photoinitiator (PI): triarylium salt
In the presence of uv-light, the PI converts to “Lewis acid”
which in turn is what drives the polymerization through the opening of
the epoxy rings. Example Lewis acid: H+SbF6
- from uv effect on
Ar+SbF6
- salt
Epoxies are thermosetting polymers containing epoxy rings:
O
epoxy ring
CH2CH CH2

24. SU-8 Thick Film Photopolymer
SU-8 resin structure O
epoxy ring
CH2CH CH2
(this figure: no H shown)
H
Glycidyl ether of bisphenolA(SU-8)
bisphenolA:
[(CH3)2C(C6H4OH)2]
glycidyl ether: overall structure;
“novolac”
formaldehyde

25. SU-8 Thick Film Photopolymer
Typical process steps for SU-8 (similar to PR)
1.
2.
3.
4.
5.
6.
7.
8.
Substrate preparation
Spin coating
Soft bake to remove solvents (hot plate)
Exposure to uv light (350nm and higher, optimally 365nm)
Post exposure bake (PEB) to fully cross-link (hot plate)
Develop
(hard bake)
(removal)
(sometimes optional steps in parentheses)

26. SU-8 Thick Film Photopolymer
Spin-coating
Resulting film thickness depends mainly on
SU-8 formulation and viscosity
Spin speed
Manufacturer spin speed curves (often not very accurate)
SU-8-2 is Viscosity: the least (4.3x10-5 m2/s)
SU-8-100 is the most (5150x10-5 m2/s)

27. SU-8 Thick Film Photopolymer
Soft-baking
Removes solvent
Hot plate recommended as oven
baking may form a top “skin”
and trap solvent
Ramping bake avoids thermal
shocks
These are Microchem’s
Soft bake times (hot plate)
suggested times; actual times
depend on hot plate
conditions
Hotplate should be level, otherwise
your film may look
like this:
SU-8
wafer

28. SU-8 Thick Film Photopolymer
Exposure dose at 365nm for SU-8
1000
Exposure
Converts salt into Lewis acid
for polymerization
Dosage depends on
film thickness
Explosure for i-line (365 nm)
uv source (example)

29. SU-8 Thick Film Photopolymer
PEB times (hot plate)
Post-exposure bake
Polymerization step drive by Lewis
acid created during exposure
Can be performed at lower
temperatures (even room
temperature), but takes longer
Similar pe-cautions as soft-bake
These are Microchem’s suggested
times; must characterize

30. SU-8 Thick Film Photopolymer
Development Development times
Immersion development at room temp.
Special developer contains:
1-methoxy-2-propyl acetate
After development, rinse in isopropyl
alcohol (IPA); water is death
Strong agitation is needed to get into
high aspect ratio trenches

31. SU-8 Thick Film Photopolymer
Hard Bake
Not necessary, but if you do: 150-200 ºC , avoid thermal shocks
Removal
Cross-linked SU-8 is very difficult to remove

can easily ruin fabrication equipment if gets inside
If an adhesion promoter was used (Omnicoat) can sacrificially
etch (e.g., MF319 developer) to lift SU-8 from substrate; can also use
other sacrificial polymers
Microchem offers a stripper (Nanoremover PG) Other
stripping methods:
fuming nitric acid
O2 plus CF4 (3-25%) using RIE and heated chuck laser
ablation

32. SU-8 Thick Film Photopolymer
Properties of SU-8
Physical
Mechanical
Electromagnetic and optical
Physical

33. SU-8 Thick Film Photopolymer
Properties of SU-8
Mechanical

34. SU-8 Thick Film Photopolymer
Dielectric constant: 4.5 at 10MHz
4.2 at 10GHz
1.1x105 V/m
1.67 at 408 nm
Breakdown voltage:
Refractive index:
1.575 at 1550 nm
SU-8 fluorescence
SU-8
deep well
(60 μm thick) analyte
Excitation: 350nm
Detection: 450nm
Properties of SU-8
Electromagnetic and optical

35. SU-8 Thick Film Photopolymer
Mutli-level SU-8
Very easy provided
top structure does not
overhang (e.g.,
closed
microchannels)
Methods of fabricating closed SU-8
microfluidic channels
ed SU 8
(a)
(b)
Use of a sacrificial material
Embedded metal mask to block uv-light for multiple layer
exposures
Use of thin-film dry resist materials
Exposure using selective proton writing
(c)
(d)
(e) Partial exposure of SU-8 film through careful dosage or
wavelength control of conventional uv light

36. SU-8 Thick Film Photopolymer
Compatibility with photodiodes
well
SU8
photodiode
SU-8 microgripper silicon
2nd SU8
level 1st
SU-8 microfluidic valve
substrate
SU8 level
Direct SU-8 applications
High-aspect ratio structures

37. SU-8 Thick Film Photopolymer
SU-8 as a mold
Electroforming with SU-8 (uv-LIGA)
Surpassing PMMA-based LIGAas it uses conventional equipment
and is much, much cheaper
Ni structures fabricated
with uv-LIGA

38. SU-8 Thick Film Photopolymer
SU-8 as a mold for other materials
PDMS mold form
most common mold used in soft lithography (covered
with molding notes)
SU-8 general web resources
(http://mems.gatech.edu/msmawebsite/members/processes/processes_files/SU8/SU-8.htm
appears to be dead ®)
SU-8 supplier: http://www.microchem.com/ Newer site:
http://memscyclopedia.org/su8.html

39. Photopatternable Silicone
Commercially available (e.g., Dow Corning WL-5000 series)
Spin-on, photopatternable silicone
Aspect ratio 1.3:1 with 15 μm minimum features
Sloping sidewalls on most formualtions (approx. 60º slope)
Transparent solution (presumably at least to visible) Patternable with
standard uv aligner (i-line or broadband) Thick films (e.g., 50 μm thick)
Easily removed
DIY concoctions
Uncured PDMS mixed with xylenes (solvent) and dimethoxy phenyl
acetophenone (DMAP) the photoinitator
Spin-on, photopatternable at 420nm
Aspect ratio 1:1
Films up to 100 μm thick
Also transparent and easily removed

40. Photopatternable Silicone
Applications
Microelectronics: elastic contacts
Microfluidics: gaskets
Photopatterned PDMS gasket
Elastic wafer level
interconnects

41. Photosensitive Polyimide
Commercially available for the microelectronics industry
Single spin film thicknesses up to 40 μm
Uv-patternable using standard equipment (negative working) Can be
used directly or as an electroforming mold
Compatible with silicon devices, glass, metal, and other polymers
Can be patterned on top of circuits, photodiodes, etc.
Chemically inert
Good thermal stability
Good biocompatibility
Complaint polyimide hinges for
rigid silicon structures
Copper electroformed gear
using polyimide mold

42. Thank You
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