Micro/Nanosystems Technology Wagner / Meyners 1
Micro/Nanosystems Technology
Prof. Dr. Bernhard Wagner
Dr. Dirk Meyners
Surface micromachining
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Outline
Introduction to surface micromachining (SMM)
SMM process sequence
Structural and sacrificial layers
Sticking issue
Anti-stiction release technologies
Summary
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Surface micromachining (SMM)
Suspended microstructured device layer
pinned to wafer surface by one or several anchor points
released by sacrificial layer etching
Only front side waferprocessing (as in IC technology)
- wafer remains rigid
- no need for double side aligning
anchor
suspended
structure
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Surface micromachining process
Deposit sacrificial (sac) spacer layer
Pattern sac layer (mask 1)
Deposit device (structural) layer
anchored on substrate
Pattern device layer (mask 2)
Release etch:
complete removal of sac layer
selective etching
to structural layer and substrate
device layer
sac layer
anchor
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Fedder
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SMM process with anchor hole filling
Deposit and pattern sac layer (mask 1)
Via filling: use of CMP rigid anchoring
Deposit and pattern device layer (mask 2)
Release etch
Process leads to rigid anchoring
avoids problems of anchor step coverage
in case of thin device layers or thick sac layers
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Single mask SMM process
Deposit sac layer
deposit device layer
(or use SOI wafer)
pattern device layer (mask 1)
Release etch
time-controlled underetching of device
sac layer is not removed completely
sac layer anchor is remaining
Top view:
complete underetching of beam
partial underetching of anchor point anchor
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SMM without sacrificial layer
SCREAM: Single-Crystal Reactive Etching And Metallization
Industrialized for high volume manufacturing of acceleration sensors (Kionix)
Isotropic structure underetch
(Si substrate etch) using RIE with SF6
Single mask process
no sacrificial layer
device layer thickness not well
controlled
Fedder
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Device layer (structural layer)
mechanical material properties determine device performance
stress control is of paramount importance to produce flat device structure
thickness: 0.2 µm - 10 µm
poly-Si is most common structural material: thermal match to substrate
but also metallic and dielectric layers
device layer is perforated
formation of multiple etch holes
allows underetching of wide structures
Poly-Si structural layer
ISIT
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Sacrificial layer (spacer layer)
uniform deposition well-defined gap between device and substrate
high selectivity of etch process
layer thickness: 0.5 – 5 µm
underetching etch rates << bulk etch rates
(diffusion limited process)
Ristic 4.13
Mechanism of
phosphosilicate glass
sacrificial layer etching
through etch hole
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Combinations of device layers and sac layers
Device layer Sacrificial layer
Si, Poly-Si SiO2 (PSG, TEOS)
Al resist, amorphous Si
Ni Cu
SiO2/ Si/ SiO2 Al
SiO2, Si3N4, Si3N4 / Si / Si3N4
Poly-Si
Polyimide Cu
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SMM process issue: Stiction during wet release process
Sticking of microstructure during drying step:
capillary force of rinsing liquid is greater than restoring spring force
microstructure is pulled down to substrate
microstruture remains stuck due to adhesion forces:
solid bridging, van der Waals force, hydrogen bonding, electrostatic forces
Note:
stiction can also happen in operation: in-use stiction
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Drying process of etched cantilever
Abe JMM 1996
Sticked cantilever tip
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Stiction test structure
Cantilever beam array: long beams are sticking
Mastrangelo 1999
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Surface tension forces
g
AF Cla cos2
A wetted area
la surface tension at liquid–air interface
C contact angle between liquid and solid
g gap (liquid layer thickness)
Attractive force between plates for C < 90°
Tas JMM 1996
Capillary force by thin liquid layer between two plates
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Anti-sticking release (1): physical countermeasures
Reduce contact area:
- introduce rough surfaces
- introduce dimples: local spikes by design
(additional mask layer)
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Anti-sticking release (2): chemical countermeasures
Low surface tension rinsing liquids:
Replace DI water by methanol, IPA, penthane, …
Chemical surface modification:
deposition of hydrophobic layers with low intersolid adhesion energy
wetting angle C > 90° water repellend
e.g. coating with self-assembled monolayers (SAM) during rinse
hydrocarbon or fluorcarbon chlorosilane based SAMs
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Anti-sticking release (3): avoid direct liquid-gas transition
Avoid drying by evaporation step: a) Sublimation drying
b) Supercritical drying (critical point drying, CPD)
C-J Kim, S&A 1998, 17
CO2 critical point drying:
rinse with liquid CO2
drying via supercritical phase
at
T > 31 °C and p > 73 bar !
CPD Equipment: e.g. Tousimis
supercritical
phase region
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Anti-sticking release (4): dry release etch
HF gas phase etch: SiO2 sac layer Equipment: e.g. Primaxx
XeF2 gas etch (no plasma): Si sac layer Equipment: e.g. Memsstar
SF6 RIE isotropic etch: Si sac layer or underetch into Si substrate
O2 plasma etch: organic sac layer, e.g. polyimide
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Surface micromachining with multiple device and sac layers
Poly-MUMPS process: www.memsrus.com
two poly-Si device layers
two SiO2 sac layers
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Summary
Surface micromachining is most important for MEMS structure fabrication
SMM uses sacrifical layer technology with front-side wafer processing only
MEMS standard uses Si device layer and SiO2 sacrifical layer
Specic equipment needed for release etching in order to avoid sticking,
e.g HF gas etch, XeF2 gas etch, critical point drying
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Literature
Madou Ch. 5