1. 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

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

3. 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

4. 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.

5. 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:

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

7. 2. Isotropic Wet Etching-3
Mask materials

8. 3. Silicon Crystal Orientation

9. 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)

10. Wafer flat
<111>

11. 4-1. Anisotropic wet etching
(on <100> wafer)
Concentration ↑: etching rate↓ selectivity ↑ surface roughness↓
Temperature ↑: etching rate ↑ selectivity ↓ surface roughness↑

12. 4-1. Anisotropic wet etching
(on <100> wafer)

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

14. 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

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

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

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

18. 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

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

20. Possible shapes on (100) silicon wafer

21. 4-2. Anisotropic wet etching
(on <110> wafer)
(110)
<111>

22. 4-2. Anisotropic wet etching
(on <110> wafer)

23. 4-2. Anisotropic wet etching
(on <110> wafer)

24. 4-2. Anisotropic wet etching
(on <110> wafer)

25. 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°

26. 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

27. Possible Shape on (110) silicon wafer

28. 4-3. Anisotropic wet etching
(on <111> wafer)

29. 5. Etching Stop-1
High Boron Concentration:

30. 5. Etching Stop-2
Electrochemical etch stop:

31. 6. Bulk Micromachining by Dry Etching
Deep silicon RIE:

32. 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