Surface Micromachining
Surface Micromachining
Surface Micromachining
Chang Liu
Micro Actuators, Sensors, Systems Group
University of Illinois at Urbana-Champaign
Chang Liu
MASS
UIUC
Outline
Basic surface micromachining process
Most common surface micromachining materials - polysilicon
and silicon oxide
Chang Liu
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Sacrificial
wet etching
Chang Liu
drying
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Hinges
Used in micro optics component assembly.
Chang Liu
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UIUC
Hinge Fabrication
Step 1: deposition of
sacrificial layer.
Step 2: deposition of
structural layer.
Step 3: deposition of second
sacrificial layer.
Step 4: etching anchor to the
substrate.
Step 5: deposition of second
structural layer.
Step 6: patterning of second
structural layer
Step 7: Etch away all
sacrificial layer to release
the first structural layer.
Chang Liu
MASS
UIUC
Chang Liu
UIUC
Chang Liu
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Chang Liu
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Chang Liu
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Aluminum
(0.3m)
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Permalloy
(>20m)
PR 4620
(10m each)
copper (9m each)
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Design
(physics,
Principle)
Materials
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Fabrication
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Bulk machining
Surface machining
Three dimensional assembly
Wafer bonding (low temperature or high temperature)
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Bulk etching
Long etching time involved to etch through the wafer
Anisotropic etching: 1-2 micrometere/min
DRIE: 1 micrometere/min, high costs of equipment and consumables
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Chang Liu
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Chang Liu
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Chang Liu
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Chang Liu
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Chang Liu
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Chang Liu
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Example: Solenoid
One way of realizing surface micromachined solenoid
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A New Method
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Chang Liu
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LPCVD Process
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Chang Liu
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Silicon nitride
Silicon nitride is nonconducting and has tensile intrinsic stress on top of
silicon substrates. It is deposited at around 800 oC by reacting silane (SiH4)
or dichlorosilane (SiCl2H2) with ammonia (NH3) - SiH4+NH3 -> SixNy+ H.
Silicon oxide
The PSG is knows to reflow under high temperature (e.g. above 900 oC); it
is deposited under relatively low temperature, e.g. 500 oC by reacting silane
with oxygen (SiH4+O2-> SiO2+2H2). PSG can be deposited on top of Al
metallization.
Silicon oxide is used for sealing IC circuits after processing.
The etch rate of HF on oxide is a function of doping concentration.
Chang Liu
MASS
UIUC
Sacrificial layers
photoresist, polyimide, and other organic materials
copper
copper can be electroplated or evaporated, and is relatively inexpensive.
Silicon or polysilicon
removed by gas phase silicon etching
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A PECVD Machine
Processing
gases
Reaction
chamber
RF
plasma
generator
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Electroplating
Electroplating
process
description
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BrF3
solid phase under regular pressure and room temperature
vapor phase (sublimation) under low pressure
BrF3 when reacted with water turns into HF at room temperature.
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MASS
UIUC
Polyimide
etching by organic solvents
Advantage
extremely low temperature process
easy to find structural solutions with good selectivity
Disadvantage
many structural layers such as LPCVD are not compatible.
Structure material must be deposited under low temperatures.
Metal evaporation is also associated with high temperature metal
particles, so it is not completely compatible and caution must be
used.
Chang Liu
MASS
UIUC
Etch rate
rapid etching rate on sacrificial layer to reduce etching time
Deposition temperature
in certain applications, it is required that the overall processing
temperature be low (e.g. integration with CMOS, integration with
biological materials)
Surface smoothness
important for optical applications
Chang Liu
MASS
UIUC
Chang Liu
MASS
UIUC
Origin of Stiction
As the liquid
solution gradually
vaporizes, the
trapped liquid exert
surface tension
force on the
microstructure,
pulling the device
down.
Surfaces can form
permanent bond by
molecule forces
when they are close.
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MASS
UIUC
Limitations
only works for
structures with
magnetic material.
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Chang Liu
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Chang Liu
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CO2
critical temperature 31.1 oC
critical pressure 72.8 atm.(or
1073 psi)
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Chang Liu
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Chang Liu
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Structural-Sacrificial Compatibility
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Chang Liu
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Chang Liu
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Foundry Process
Why:
Reduce the cost of development by providing standard and
unusual processes at reasonable cost.
How:
Wafer sharing: many processes are performed on one wafer with
many users sharing the mask.
Drawback: limited process materials and steps
Machine sharing: a users wafer is dedicated and ships back-andforth among several vendors.
Drawback: long development and transport time
Chang Liu
MASS
UIUC
Chang Liu
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UIUC
Chang Liu
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Chang Liu
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Compatibility Table
Polysilicon
Metal
Yes, slower
speed
Yes, slow
No. Sputtering is
possible
HF wet
etching
No
Yes
No, avoid
long soak
Uncured
photoresist
No
No
Yes
No
Photoresist No
developer
No
Yes
No
Organic
rinse
No
No
Yes
No
Baking
No
No
No
No
Metal
etchant
No
No
No
Yes
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MASS
UIUC
Hinders:
Capacitance calculation
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Materials
Relatively difficult material
Exotic wafer
Processes
Difficulties:
Cantilever release using web silicon etchant may be a problem
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MASS
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Chang Liu
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UIUC
Material:
Simple
Readily available
Fabrication process
Does not require exotic materials or processes
Sacrificial release may be a problem, like the previous case
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MASS
UIUC
Chang Liu
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UIUC
Evaluation
Design:
Results in hermetically sealed structures
Result in large gap distance to reduce damping
Materials:
Silicon materials
Doped silicon
Fabrication:
Length steps
Delicate bonding and handling
Process development is lengthy
Chang Liu
MASS
UIUC