PowerPoint 프레젠테이션 - KOCW

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Seong J. Cho Research

초소형 기전공학 설계 #6

Micro Electro Mechaical System & Future Technology

조성진

Office: 042-821-5648 [email protected]

mems.cnu.ac.kr

기계공학부 충남대학교

2016/1

PowerPoints organized by Prof. J. Kim, G. Lim, POSTECH and Prof. Seong J. Cho, CNU

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Review

 Photolithography process

 Cleaning

 Photoresist coating

 Soft bake

 Alignment & Exposure

 Development

 Hard bake (optional)

 Stripping

 Photoresist

 Positive photoresist

 Negative photoresist

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Micromachining

 Surface Micromachining

is the process of forming movable structures by placing the structures on initially rigid platforms, then removing the platforms, usually by etching the material away.

 Bulk Micromachining

means that 3D features are etched into the bulk of crystalline and non-crystalline materials.

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Etching

 Type of Etching

 Dry etching vs. Wet etching

 Anisotropic etching vs. Isotropic etching

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Wet Etching

 Isotropic Etching

 Etchant: HNA (HF + HNO

₃

+ Acetic Acid)

 Etch masks: silicon oxide, silicon nitride, Au

 Room temperature

 Sensitive to agitation

(a) Isotropic etching w/o agitation

(b) Isotropic etching with agitation

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Wet Etching

 Anisotropic Etching

 Etching rate is dependent on the crystal orientation.

 The difference in etch rate is used creatively to generate

unique 3D structures.

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Wet Etching

 Miller Index

 A notation system in crystallography for planes in crystal lattice

 (hkl); a set of numbers which quantify the intercepts and thus may be used to uniquely identify the plane or surface.

x

y z

x

y z

x

y z

(110) plane or {110}

[110] direction or <110>

(100) plane or {100}

(111) plane or {111}

1*x+0*y+0*z=1 1*x+1*y+0*z=1 1*x+1*y+1*z=1

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Wet Etching

 Silicon Orientation

 Diamond cubic structure

 There are eight corner atoms, six face centered atoms & four more atoms

 8 × ⅛ + 6 × ½ + 4 = 8

a/4 Z

Y 2r

W a/4 a/4

X

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Wet Etching

 Silicon Lattice

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Wet Etching

 Silicon wafer identification

 Position of major and minor flats indicate:

- doping type

- crystal orientation

Crystal directions of (100) silicon wafer

x

y z

x

y z

x

y z

(110) plane [110] direction (100) plane

[100] direction (111) plane

[111] direction

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Wet Etching

 Silicon Etchant

 KOH

- Etch rate: {110} > {100} >> {111}

- Used at elevated temperature

- Selective p-type doping etch stop possible (not effective as TMAH, EDP) - Attack PR, aluminum, and slowly attack oxide

- Nitride is not attacked (best masking material)

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Wet Etching

 Silicon Etchant

 TMAH (Tetramethyl Ammonium Hydroxide)

- Etch rate: {100} : {111} on the order of 10 to 35 ( 0.5 ~ 1.5 ㎛/min at 90℃) - Used at elevated temperature

- Selective p-type doping etch stop possible (not effective as EDP) - Safer than EDP

- Gentle with many etch mask materials

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Wet Etching

 Silicon Etchant

 EDP (Ethylene Diamine Pyrocatechol) - Etch rate: {100}> {110} >> {111}

- Used at elevated temperature - Good selective with p-type doping - Very corrosive, toxic

- Attack aluminum. Nitride & oxide can be used as etch mask materials

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Wet Etching

 E

cos a b⋅ = a b 

θ

1 1

2 2 2 2 2 2

1 0 0 1

cos cos 54.74

1 0 0 1 1 1 3

θ = ⁻ ^ ^{+ +} ^= ⁻ = o

+ + + +

 



Angles between two specific planes

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Wet Etching

(100) Silicon surface (110) Silicon surface

(111) Silicon surface

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Wet Etching

 Etching rate depending on crystal orientation

 Precise control of designed dimension (V groove depth and opening pit)

 (111) planes have the slowest etch rate

 Using high selectivity between (111) and others (other planes are etched much faster)

 Etching at “ concave” corner on (100) silicon plane stops at (111) intersections.

 But, “convex” corners are attacked (undercut)

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Wet Etching

 E

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Wet Etching

 E

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Wet Etching

 E

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Wet Etching

 E

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Wet Etching

 E

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Wet Etching

 E

( )

2 cot 54.74 2

o m m

W = W − ° × = z W − z