MEMS_18-20 gw chat 4_chpt 9

MEMS_18-20 gw chat 4_chpt 9 - Chapter 4 Wet etching Wet...

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Chapter 4 Wet etching
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Wet isotropic etching and anisotropic etching Introduction Use pool of liquid as tool Principle of Si micromachining tool set: Wet bulk micromachining + surface micromachining Micromolding: from a lithography defined master Reaction product has to be removed away from the surface To etch uniformly, the etchant need to be homogeneously distributed -> mixing (stirring) Etchant: Acid: isotropic Base : anisotropic
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Wet isotropic etching and anisotropic etching Introduction Application: Cleaning, shaping 3D structures, removing surface change characterizing structural and compositional feature Parameters: Bias (undercut), tolerance, etch rate, anisotropy, selectivity, overetch, feature size control and loading effect Higher degree selectivity than dry etching Fast etching rate: ~ μm/min
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Wet isotropic etching and anisotropic etching Introduction Materials to be etched: Semiconductor, conductor and insulator Mainly focus on Si wet etching It is a chemical reaction and transport process Etching process: Diffusion (mixing) -> surface reaction (T, C materials) -> diffusion (mixing) The slowest one determine the entire rate
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Wet isotropic etching and anisotropic etching Isotropic etchant Etch all crystallographic directions at the same rte Acidic Round feature Room T Diffusion control Anisotropic etchant Alkaline Depends on the orientation of the crystalline Reaction rate control
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Wet isotropic etching and anisotropic etching Common: They are chemical process the two principle reactions are oxidation of Si followed by dissolution of the hydrated silicate
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Can etch SiO 2 , but hardly attack Si itself Most photoresists are especially resistant towards HF -> photolithography patterns SiO 2 on the top of Si wafer -> the patterns can then be developed by using HF etching The overall etching reaction for pure HF is SiO 2 + 6 HF -> H 2 SiF 6 + 2 H 2 O In the solution the complex HF 2 - is also present, which etch 4.5 times fast than HF in following reaction SiO 2 + 3 HF 2 - + H + -> SiF 6 2- + 2H 2 O HF etching
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HF concentration 1-50%: High C of HF and HF 2 - -> high etching rate Linear increase up to 10 M Non-lineareffect: further higher C -> higher order complecs , such as H 2 F 3 -, which etch even faster than HF 2 - C change with time -> Rate change with C of HF Buffer system consisting of ammonium fluoride (NH 4 F) together with HF is often used to prevent large change in C of HF and HF 2 - and maintain a constant pH The reaction is SiO 2 + 4 HF + 2 NH 4 F -> (NH 4 ) 2 SiF 6 + 2 H 2 O NH 4 F is often termed BOE (buffered oxide etch) or BHF (buffered HF) HF is toxic and dangerous chemical HF etching
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Wet isotropic etching Usage of isotropic etchants Remove of work-damage surface Rounding sharp edge anisotropically etched corner (to avoid stress concentration)
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This note was uploaded on 10/03/2011 for the course BMEN 589 taught by Professor Wang during the Spring '11 term at South Carolina.

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MEMS_18-20 gw chat 4_chpt 9 - Chapter 4 Wet etching Wet...

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