Discusses about photolithography, mask design, wet and dry bulk etching, bonding, thin film deposition and removal, metallization, sacrificial process and other inorganic processes.

1. Microsystems T echnologies
Basic concepts and terminology
Review of 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

2. Goals of Microfabrication Module
Fabrication could be one or more classes by itself! Thus:
• Our goal for ENSC859 is to get you to the point where you
can understand, adapt, and develop high level processes
similar to fabrication process flows found in the literature.
 Emphasis will be on high level process design,
rather than tool-specific details (e.g., “deposit 0.2 μm of Au using e-beam
evaporation”, rather than specifying the e-beam power, etc., for the 0.2 μm
ofAu).
Emphasis will be on overall process design and making sure processes are
possible in the order given (e.g., not trying to fusion bond wafers at 1000ºC with
aluminum, which has a lower melting point, already patterned).
We will cover many different processes, with those more relevant to
Biomedical Microdevices (e.g., polymer processing, metallization, etching)
in more detail.
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3. Some Basic Terminology
Substrates - What we build into (bulk) or on (surface)
silicon
glass, quartz
polymers
ceramics
Thin/thick films – go on the substrate
metals: e.g., Al, Au, Cu, Ni, Pt,Ag, Ti
insulators: e.g., silicon oxide, silicon nitride, polymers
others: e.g., silicon (crystalline, amorphous, poly-),
SiC, diamond, shape memory alloys
thin films
substrate substrate

4. More Basic Terminology
added
} materials
Additive processes
adding a material to the substrate
Photolithography
pattern transfer method of using a light
sensitive polymer to prevent subtraction
Subtractive processes
removing material, either
substrate or thin film
Bulk “substrate”processes
machining the substrate itself
Thin film “surface”processes
substrate
machining films on the surface of a substrate

5. More Basic Terminology
Additive vs. Subtractive Processes
Additive examples Subtractive examples
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oxidation
CVD/LPCVD/PECVD
epitaxy
sputtering
evaporation
spin-on methods
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wet isotropic etching
wet anisotropic etching
plasma
RIE
DRIE
lift-off techniques
drilling
bonding (anodic, fusion,
adhesive, eutectic)
electroplating
implantation, diffusion
Other processes
• molding, embossing
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6. More Basic Terminology
Planar vs. non-planar processes
Non-planar shape
Planar processes are still
more common and generally
batch processed.
(Even inherently non-planar processes,
such as injection molding, are often used
for planar-compatible shapes.)
high-aspect ratio structure
Aspect ratio
Ratio of depth to width
low-aspect ratio hole
high-aspect ratio hole
low-aspect ratio structure
Shapes based on planar
processing

7. Etching Terminology
Selectivity
how fast one thing happens (etches) compared to another
Mask
material used to cover another during etching with a much
slower etch rate (high selectivity) to material being etched
Good mask selectivity Bad mask selectivity
Undercut
etching under the mask (can be desirable)

8. More Etching T erminology
Isotropic etchants:
etch approximately at the same rate in all directions
Mask undercut: severe, same in all directions
Etch profile:
Isotropic Etching:
limited control with agitation
no agitation
with agitation

9. More Etching T erminology
Anisotropic etchants:
etch at different rates in different directions
Mask undercut: depends on etching anisotropy directions
Etch profile:
depends on type of etch
(wet, dry), crystal direction,
mask orientation
[100]
[111]
dry etching crystal direction anisotropy
(approximately vertical)

10. Wet etching
More Etching Terminology
uses wet chemicals to selectively remove substrate or film
Dry etching
uses gases (usually) to selectively remove substrate or film
Non-plasma dry chemical etching
Plasma etching:
RF power is used to drive chemical reactions
Plasma is used instead of high temperatures or very reactive
chemicals
Simple plasma etching
RIE etching
additional acceleration of ions toward surface
Deep RIE etching
RIE etching featuring high aspect ratio process

11. Plasma
Still More Basic Etching Concepts
Partially ionized gas consisting of equal numbers of +ions
and –ve electrons and a different number of neutral molecules.
Plasma etching basic process
1. Generation of reactive species
2. Diffusion to surface
3. Adsorption on surface
4. Chemical reaction
5. Desorption of by-products
6. Diffusion into bulk gas
Plasma etch specifications
Temperature
Pressure
Power
Gas flows and composition
These specs affect side wall
Profile,anisotropy, selectivity,
Mask undercut, etc.; take
ENSC851 to see!

12. Still More Etching T erminology
Reactive ion etching (RIE)
Chemical plasma etch is assisted by ionic bombardment
Bombardment opens areas for reactions
Results in:
no undercutting since side walls are not bombarded greatly
increased etch rate
lower selectivity (will happily etch everything including maskK)

13. Material Mechanical Properties
Young's modulus (E) is a measure of the stiffness. It is defined as the ratio of the
uniaxial stress over the uniaxial strain. This can be experimentally determined from the
slope of a stress-strain curve. E is constant where Hooke’s Law holds (linear relation).
Yield strength is defined as the stress at which a material begins to deform plastically.
Prior to the yield point the material will deform elastically and will return to its original
shape when the applied stress is removed. Once the yield point is passed some fraction
of the deformation will be permanent and non-reversible.
Poisson's ratio is the ratio of the relative transverse strain (normal to the applied load),
divided by the relative axial strain (in the direction of the applied load). Ex: when a
material is stretched in one direction, it tends to contract in the other two directions.
Knoop hardness test is a test for mechanical hardness for very brittle materials or thin
sheets. Apyramidal diamond point is pressed into the polished surface with a known
force, for a specified dwell time, and the resulting indentation is measured. The Knoop
hardness HK is then given by the formula: HK=P/(CpL2)
where:
L = length of indentation along its long axis
Cp = correction factor related to the shape of the indenter, ideally 0.070279
P = load
Lots of other hardness tests for other material typesK

14. Example Mechanical Material Properties
Meting temp. Youngs mod. Yield strength Poisson ratio Density
Kg/m3
Knoop hard.
°C GPa GPa GPa
Bulk silicon 1415 160-200 6.8 0.22 2330 5-13
Polysilicon 1415 150-200 10-13
Silicon dioxide 1700 70-75 8.4 0.17 2200 15-18
Pyrex glass 64 0.2 2230
Silicon nitride 1800 210-380 14 0.25 3100 8
SiC High 300-400 21 0.19 3210 24-27
CVD diamond 800-1100 53 0.07 3530
Aluminum 661 70 0.2 0.33 2700
Platinum 1772 170 0.137-0.17 0.38 21440
Gold 1065 80 0.12 0.38-0.42 19280
Stainless steel 200 2.1 0.3 7900 6.5
Polymide 7.5-15 0.045-0.345 0.35-0.45 1420
Parylene-n 410 2..4 0.042 1100
Parylene-c 290 2.7 0.055 1290
Parylene-d 380 2.6 0.062 1418
SU-8 82 4-5 0.034 0.22 1200
PDMS 0.0075 0.003-0.015

15. Other Important Material Properties
Thermal properties
Melting point
Specific heat: J/K (energy to heat)
Thermal expansion coefficient: α = ΔL/(L ΔT) (expansion of heat)
Thermal conductivity: W/cmK or W/mK (how well conducts heat)
Electrical properties
Dielectric constant
Breakdown strength: V/cm
7740 pyrex
Optical properties (λ dep.)
Transmission spectrum
Reflectivity
Refractive index
(speed of light reduction)

16. Different from bulk properties
Sufaces are uniquely reactive
Surfaces are readily contaminated
Surfaces readily alter their structure
Important surface properties
Roughness and flatness
Dangling bonds/surface chemistry
Hydrophobicity (wettability and contact angle)
Biocompatibility and fouling
Surface charge
Light and thermal reflection and scattering
Surfaces are very important for microfluidic flow and especially for samples
containing surface-reactive analytes such as DNA.

17. Thank You
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