nmems-7a [compatibility mode]- etching
TRANSCRIPT
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INTRODUCTION TO MEMS
EA C415
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ETCHING WET
DRY
WET ETCHING:
substrate in a suitable liquid chemical (etchant) that
attacks the exposed region
DRY ETCHING:
Removal of material by chemically reactive vapors
& ionic species acts as etchant and removes material
from substrate
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ISOTROPIC ETCHING : Rate of material
removal does not depend on the orientation of thesubstrate
ANISOTROPIC ETCHING: Etching single
crystal substrate with certain etchants results in
orientation dependent etching.
ISOTROPIC ETCH
Masking Layer
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Curved Undercut
HEAVY ETCH OF SUBSTRATE/BULK MICROMACHINING
ISOTROPIC ANISOTROPIC COMPLETELY
ANISOTROPIC
direction
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ETCH SELECTIVITY: etch rate of layer to be etched should be fast
compared to etch rate of mask
x
y
xy >>
#EX. HF etches SiO2 100 nm/min. and mask Si3N4 0.04 nm/min.
x
1. CVD SiO2
2. Si3N4
3. Polysilicon
4. Aluminium
5. Copper
6. Gold
Material ETCHANT
1. Buffered HF (5:1 NH4F: Conc. HF)
2. Hot phosphoric acid
3. KOH/Ethyl diamine pyrochatel
4. PAN (phosphoric, acetic, nitric acid)
5. Ferric chloride
6. Ammonium Iodide
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More physical an etch process more directional and less selective
More chemical an etch process lessdirectional and more selective
Strong bases like KOH, EDP, Tetramethyl ammonium hydroxide (TmAH)
exhibit orientation dependent etchcharacteristics
* Suitable mask for strong base etchant are SiO2, Si3N4
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DRY ETCHING Vapor etching
#Ex: XeF2 etch Si very fast incomparison to other materials)
Plasma Assisted etching
Low pressure ionic species of gases (etchants) produced in plasma
removes material i) by direct bombardment and ii) chemical reaction by
converting surface atoms to volatile species
1. SiO2
2. Si3N4
3. Silicon/Polysilicon
4. Aluminum
5. Organic
Material ETCHANT
1. CF4, H2
2. CF4, O2
3. CF4, SF6
4. BCl3
5. O2, CF4, SF6
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Parameters affecting rate of etching and
shape of etch
1. Type of substrate
2. Specific chemistry of etchant
3. Masking layer
4. Temperature (controls rate of reaction, directionality)
5. Solution stirring
6. Pressure (in dry etching)High pressure Higher rate, more isotropic etch
Low pressure Low rate, more directional etch
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At limits of low pressure (high directionality),the process is called REACTIVE ION ETCHING
(RIE)
DEEP REACTIVE ION ETCHING
Used for creation of deep vertical wells on substrate
working principle is based on use of polymeric species
most plasma process are critical race between deposition of
polymeric material and material removal from surface
Plasma chemistries can be controlled and under right temp.
and pressure condition, removal dominates
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ETCH PASSIVATE ETCH
SF6 C4F8 SF6
Etch proceeds in alternating steps of reactive ion
etching in SF6 plasma and polymer deposition from C4F8
plasma
During etch, the polymer is rapidly removed from the
bottom but lingers on the sidewall
DEEP REACTIVE ION ETCHING www-samlab.unine.ch/activities/process.htm
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DEEP REACTIVE ION ETCHING
SEM image of a microscale tensile test specimen in theprocess of fabrication using SF6 and Ar plasma. Single-
crystal SiC has been etched to a depth of 80 m.
www.grc.nasa.gov/
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Time-multiplexed
etch-passivate
(TMEP) process is
widely used in the
DRIE of silicon informing structures
with aspect ratios
reater than 30.
DEEP REACTIVE ION ETCHING
SF6 is used as the etching gas, and octafluorocyclobutane(C4F8) is used to deposit a fluorocarbon polymer film that
protects the sidewalls from lateral etching.
www.grc.nasa.gov/
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LINEAR ETCH MODEL
fluxIonflux;Chemical
constantrate&t;coefficiensticking
==
==
+
==
ic
ifc
iicfc
e
FF
kkSN
FkFkSrrateetch
#Ex Silicon is plasma etched for 5 min. and the process follows the linear etchmodel. The flux of chemical species, Fc, on an unobstructed flat surface is
equal to . The sticking coefficient is 0.01 and kf is 0.2. The
flux of ionic species on flat surface is equal to and ki is 1. The
s/atom/cm105.2 218
s/atom/cm101 216322 .
trench is etched in silicon. How far is the trench etched in vertical and lateraldirection under these conditions
mrr 9.0300depthetchand105
1011105.22.001.022
1618
==
+
=
Solution: In vertical direction (both chemical & ionic fluxes are present)
In Lateral direction (virtually no ionic fluxes)
mrr 3.0300depthetchand105
0105.22.001.0
22
18
==
+=
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Synergistic Model/ Ion Enhanced Etching Model
11
+
=
cci FSF
Nr
1
k
1
i
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ETCH STOPS
NOTE: While it is always possible to use a rate and a specified
time to determine the depth of etched features, it is always
desirable to use a well defined structural feature to stop an etch
Diffused n-type region
p- type s con
Electrode EtchantFinal Structure
Working Principle: reverse bias p-n junction, no current flows
p-silicon etched
on reaching n-type (junction exposed), current flows
n-silicon oxidizes, gets passivated, no more etching
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NEXT LECTURE
WAFER LEVEL PROCESSES CONTINUED
MICROMACHINING