MicroSystems & MicroFluidics Lab National Tsing Hua University ESS ESS 5810 Lecture 10 體型微加工製程技術 曾繁根 助理教授 國立清華大學工程與系統科學系 MicroSystems & MicroFluidics Lab Prof. Fan-Gang Tseng

矽體型微加工技術 Introduction Isotropic Wet Etching Silicon Crystal Orientation Anisotropic wet etching Etching Stop Possible Shapes

1. 矽體型微加工技術概論 Pressure sensor l  Definition of Bulk Micromachining:Based on the device shaping by etching a bulk substrate. l  Materials used for bulk micromachining: single crystal silicon, gallium arsenide, quartz, etc… Pressure sensor

1. 矽體型微加工技術概論 Silicon as A Mechanical Material: Major difference: Silicon yields by fracturing (at room temp) while metals usually yield by deforming inelastically—chipping, cleave along crystallographic planes.

2. Silicon Isotropic Wet Etching-1 HNA system (HNO3+HF), etching rate can be 50 μm/min: a. Hole injection: b. Oxide formation: c. Oxide etching: Total reaction:

2. Isotropic Wet Etching-2 Effects: High HF Low HNO3: oxidation limit, rough surface High HNO3 Low HF: etching limit, smooth surface SZE

2. Isotropic Wet Etching-3 Mask materials

3. Silicon Crystal Orientation

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)

Wafer flat <111>

4-1. Anisotropic wet etching (on <100> wafer) Concentration ↑: etching rate↓ selectivity ↑ surface roughness↓ Temperature ↑: etching rate ↑ selectivity ↓ surface roughness↑

4-1. Anisotropic wet etching (on <100> wafer)

Comparison (100) wafer <110> direction (110) wafer <110>

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

Corner compensation-1 KOH B Minimum=1.6 B For EDP For KOH

Corner compensation-2 EDP KOH (100)>(110)>(111) (110)>(100)>(111)

Method to get 90° and 45° wall on (100) silicon wafer by bulk micromachining EDP (100)>(110)>(111) KOH (110)>(100)>(111)

Crystal orientation finding on (100) wafer <110>Wafer flat provide 2-5° accuracy Long slot give around 1 ° accuracy Circular squares along <110> side give 0.1 ° accuracy (80 m pitch) Tan-1(80/45000) =0.1 ° Steckenborn, A et al, Micro System Technologies 91

Crystal orientation finding on (100) wafer (<110> direction) F. G. Tseng, unreleased figures <110> direction

Possible shapes on (100) silicon wafer

4-2. Anisotropic wet etching (on <110> wafer) (110) <111>

4-2. Anisotropic wet etching (on <110> wafer)

4-2. Anisotropic wet etching (on <110> wafer)

4-2. Anisotropic wet etching (on <110> wafer)

Crystal orientation finding on (110) wafer <111>Wafer flat provide 2-5° accuracy Long slot give around 1 ° accuracy Circular squares along <100> side give 0.06 ° accuracy (35 m pitch) F. G. Tseng, K. C. Chang, ASME IMECE MEMS’01, JMM. <111> <110> <100> R=48.9mm (110) wafer 55 Alignment Circles r R  x 109.5°

Crystal orientation finding on (110) wafer (<100> direction) -1 1 30 μm 2 4 5 <100> direction Center hexagon 30 μm -1 1 2 3 4 5

Possible Shape on (110) silicon wafer

4-3. Anisotropic wet etching (on <111> wafer)

5. Etching Stop-1 High Boron Concentration:

5. Etching Stop-2 Electrochemical etch stop:

6. Bulk Micromachining by Dry Etching Deep silicon RIE:

7. 正八面體晶體輔助工具 Fan-Gang Tseng ESS 5850 <110> <111> <100> 7. 正八面體晶體輔助工具