2 Miller Index (hkl) (1 00) X {hkl} {100} (100),(010),(001),(100),(0 10),(00 1) [hkl] (hkl) [100] (100) <hkl> 3 Characteristics of Etching Techniques

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1 1 National Kaohsiung First University of Sci. & Tech. Silicon Anisotropic Wet Etching Department of Mechanical and Automation Engineering National Kaohsiung First University of Science and Technology MicroSystem Fabrication Lab. Miller Indices 2

2 Miller Index (hkl) (1 00) X {hkl} {100} (100),(010),(001),(100),(0 10),(00 1) [hkl] (hkl) [100] (100) <hkl> 3 Characteristics of Etching Techniques Selectivity Etching method removes a specific

2 2 Miller Index (hkl) (1 00) X {hkl} {100} (100),(010),(001),(100),(0 10),(00 1) [hkl] (hkl) [100] (100) <hkl> 3 Characteristics of Etching Techniques Selectivity Etching method removes a specific material but does not (or only slightly) remove other materials Directional Property Isotropic: etching speed is the same in every direction. Anisotropic: etching speed is NOT the same in every direction. 4

3 3 Bulk Micromachining Structuring of silicon in three dimensions using anisotropic etching High aspect ratio is possible. Crystal orientation decides the structure geometry 5 Etching Mechanisms Chemical Wet Etching Dipping the substrate into an etching bath or spraying it with the etching solution. Etching solution is acidic or alkaline to dissolve the material to be removed. Dry Etching Expose the substrate to an ionized gas. Chemical or physical interaction occur between the ions in the gas and the atoms of the substrate. 6

4 4 MEMS Si (Single-Crystal Silicon, Polysilicon) Anisotropic etching: KOH (Potassium Hydroxide), EDP (Ethylene- Diamine Pyrocatecol), TMAH Isotropic etching: HNA etching system, hydrofluoric (HF), nitric (HNO 3 ), acetic acid (CH 3 COOH) SiO 2 (Thermal SiO 2, LTO, PSG) Release process using 49% HF Oxide patterning using buffered HF (28 ml 49% HF, 170 ml H 2 O, 113 g NH 4 F), also know as Buffered Oxide Etch (BOE) Quartz (Crystalline SiO 2 ): Anisotropic etching in heated HF and ammonium fluoride (NH 4 F) solution SOG: dissolved in HCl:HF:H 2 O 7 Isotropic Etching Chemical wet etching for amorphous or polycrystalline (ex. polysilicon) is always isotropic. Cavities with rounded off edges (undesired in design). Resist is undercut. Limited in deep forming. Dimension control is difficult. Useful in surface micromachining. 8

5 5 Anisotropic Etching Etching speed depends on the crystal s orientation in single-crystal silicon. Typical etching rate : {110}>{100}>>{111} Precisely structured in 3D. 9 TMAH (Hydrazine)EDP (Hydrazine)EDP TMAHIC (etching mask) KOH, NaOH, LiOH, CsOH, NH 4 OH, (IPA) KOH IC PH 10

6 6 Note: etch rate of SiO 2 =435 nm/hr at 80 C 30% KOH 11 Si 2 4H O + 4e 4OH OH Si(OH) + 4e 2 2H 2 ++ Si(OH) 2 + 4OH SiO2(OH) 2 + 2H 2O - Si + 2OH + 2H 2O SiO2(OH) 2 + 2H 2 12

7 7 Silicon Lattice Structure 13 {100}{111} KOH - {111} {100} Si atoms on a {100} plane R{111}/R{100} = 1:80~1:120 Si atoms on a {111} plane 14

8 8 {110} (Channeling effect) (Hesketh, 1993) 15 Bulk Micromachining (111) 16

9 9 Basic Structures in Silicon Anisotropic Etching of Silicon (110) 17 18

10 {100} {111}{110} α = tan 1 (( 2 / 2) /1) = 35.

10 10 {100} {111}{110} α = tan 1 (( 2 / 2) /1) = <111><110> = cosα 19 {100} {100} <110> 20

11 11 {100} {111} 21 {100} {100}2d d 22

12 12 {100} <111><100> = cosθ, θ = <100> <111> 23 Undercut of {100} Wafer Undercut due to finite selectivity between the etch rates of {111} and {100}. tanθ = 1/3 R{111}/ R{100} 24

13 13 {100} V-Groove Depth Ruler {100} V 2 :1 Source: 25 Anisotropic Etching in {110} Wafer 26

14 14 {110} {110} {111} 27 {110} 28

15 15 Mask Patterning and Etching in {110} Wafer {110} [111] º {110} [111] 29 Anisotropic Etching in {110} Wafer {110} <111> 30

16 16 Long Narrow Grooves in a <110> Wafer Rough side wall due to misalignment Vertical side wall due to perfect alignment 31 Selection of Silicon Wafer 32

17 17 underetch undercut Si 3 N 4 Si 33 KOH 34

18 18 Z l Mb <100> <110> h M X l Mt h Y 35 Square Compensation W s =etching depth <100> Band Compensation W r =2(etching depth) L r =1.6W r L r 36

19 19 {100} KOH (110)>(100)>(111) {100} EDP (100)>(110)>(111) 37 20%KOH µm µm 2 {100} 300 µm square beam triangle band 38

21 (Doping) Diffusion Furnace heating. The doping atoms from the surrounding gas or a thin preapplied surface layer. Main difficulty Determination of the absolute concentration of the doping.

21 21 (Doping) Diffusion Furnace heating. The doping atoms from the surrounding gas or a thin preapplied surface layer. Main difficulty Determination of the absolute concentration of the doping. Only create a doping profile on the surface of a wafer. Ion implantation Shooting charged doping ions, which are externally accelerated in a vacuum, into the silicon wafer. The ions can penetrate up to a few micrometers below the surface. The doping concentration gets an improved homogeneity, and the doping profile under the wafer's surface can be controlled more exactly. 41 Comparison of Two Doping Methods Source: Fundamentals of Microfabrication, Madou 42

22 22 P+ Etch-Stop Doping the silicon substrate with germanium, phosphorus or boron atoms P + (Boron) concentration of about cm -3 or higher will drastically reduce etching rate Doping Diffusion method Difficult to get a uniform membrane thickness Ion implantation Good concentration and depth control but slow 2 µm with a concentration of cm -3, a 150 µa implanter needs up to an hour per wafer 43 44

etching 45 Electrochemical Etch-Stop Technique Etching process is

23 23 Boron doping Membrane using P+ Etch-Stop Silicon layer by epitaxy Both sides oxidized Lithography of the silicon dioxide Anisotropic etching 45 Electrochemical Etch-Stop Technique Etching process is interrupted when an electric voltage is applied to an np or pn silicon substrate 46

24 24 47 Source: MEMS handbook p

25 25 Membrane Nozzle 49 Piezoresistive Pressure Sensor Process steps of piezoresistive sensor using bulk micromachining Source: Fundamentals of Microfabrication, Madou, Fig

26 26 Reference Fundamentals of Microfabrication, Marc Madou, 2nd Ed., CRC Press (2002) Chapter Microsystem Technology and Microrobotics, Fatikow and Rembold,Springer, (1997) Chapter

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untitled 理論 Lecture 8 (Etching) 1 Outline Introduction How to Control Etching Process? Isotropic Wet Etching Anisotropic Wet Etching Dry Etching 2 Lecture 4 Lecture 5&8 3 類路理兩留路兩類 4 Types of Etching Processes