MicroSystems & MicroFluidics Lab

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National Tsing Hua University ESS
ESS 5810 Lecture 10
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MicroSystems MicroFluidics Lab
Prof. Fan-Gang Tseng
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• Introduction
• Isotropic Wet Etching
• Silicon Crystal Orientation
• Anisotropic wet etching
• Etching Stop
• Possible Shapes

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l  Definition of Bulk MicromachiningBased on the
device shaping by etching a bulk
substrate. l  Materials used for bulk
micromachining single crystal silicon,
gallium arsenide, quartz, etc
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Silicon as A Mechanical Material
Major difference Silicon yields by fracturing
(at room temp) while metals usually yield by
deforming inelasticallychipping, cleave along
crystallographic planes.
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2. Silicon Isotropic Wet Etching-1
HNA system (HNO3HF), etching rate can be
50 µm/min
a. Hole injection
b. Oxide formation
c. Oxide etching
Total reaction
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2. Isotropic Wet Etching-2
Effects High HF Low HNO3 oxidation limit, rough
surface High HNO3 Low HF etching limit, smooth
surface
SZE
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2. Isotropic Wet Etching-3
Mask materials
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3. Silicon Crystal Orientation
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Miller index
1. Take the intercepts with three axes, say a, b,
c 2. Take the reciprocal of these three integers,
multiplied by smallest common denominator, get
miller indices (d,e,f)
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Wafer flat
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4-1. Anisotropic wet etching (on lt100gt wafer)
Concentration ? etching rate? selectivity ?
surface roughness? Temperature ? etching rate ?
selectivity ? surface roughness?
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4-1. Anisotropic wet etching (on lt100gt wafer)
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Comparison
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Undercut on convex corner
Concave and convex corner The surface revealed
in concave corner is the slowest one, however, it
is the fastest one in convex cases
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Corner compensation-1
KOH
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Corner compensation-2
EDP (100)gt(110)gt(111)
KOH (110)gt(100)gt(111)
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Method to get 90 and 45 wall on (100) silicon
wafer by bulk micromachining
EDP (100)gt(110)gt(111)
KOH (110)gt(100)gt(111)
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Crystal orientation finding on (100) wafer

• lt110gtWafer flat provide 2-5 accuracy
• Long slot give around 1 accuracy
• Circular squares along lt110gt side give 0.1
  accuracy (80 ?m pitch)

Tan-1(80/45000) 0.1
Steckenborn, A et al, Micro System Technologies 91
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Crystal orientation finding on (100) wafer
(lt110gt direction)
F. G. Tseng, unreleased figures
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Possible shapes on (100) silicon wafer
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4-2. Anisotropic wet etching (on lt110gt wafer)
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4-2. Anisotropic wet etching (on lt110gt wafer)
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4-2. Anisotropic wet etching (on lt110gt wafer)
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4-2. Anisotropic wet etching (on lt110gt wafer)
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Crystal orientation finding on (110) wafer

• lt111gtWafer flat provide 2-5 accuracy
• Long slot give around 1 accuracy
• Circular squares along lt100gt side give 0.06
  accuracy (35 ?m pitch)

F. G. Tseng, K. C. Chang, ASME IMECE MEMS01,
JMM.
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Crystal orientation finding on (110) wafer
(lt100gt direction)
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Possible Shape on (110) silicon wafer
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4-3. Anisotropic wet etching (on lt111gt wafer)
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5. Etching Stop-1
High Boron Concentration
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5. Etching Stop-2
Electrochemical etch stop
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6. Bulk Micromachining by Dry Etching
Deep silicon RIE
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