This dissertation is intended as a guidebook for processing crystalline silicon by anisotropic potassium hydroxide (KOH) wet etching. In low cost bulk micromachining of silicon for MEMS (Micro Electro Mechanical Systems), anisotropic wet etching of single crystalline silicon in aqueous KOH solutions is a technically important process. Anisotropic etching of silicon is used to create mechanical microstructures in silicon. Differences in etch rates between the different crystal planes are utilized in the process.
The present dissertation explores six different aspects of silicon anisotropic etching using an accelerometer as a test device. The first part of the thesis concentrates on liquid phase phenomena, and the latter part with the interactions of the silicon crystal with the etching process. It is shown that etch rates and anisotropy are extremely sensitive to the process conditions and equipment, for example rotational flow in the etch bath is shown to be superior to laminar flow. It is also shown that the etch rate and the resulting surface quality are sensitive to very small changes in Pb concentrations in the range of 200 to 300 ppb.
The effects of wafer oxygen levels have been studied from etching point of view and were found to have a marked effect especially on etching the (111) crystal plane. A novel method for eliminating the effects of grinding damage on etching has been introduced. Ultra Poligrind wafers were used as substrate material for MEMS accelerometer structures. These wafers eliminate the need for double side polished wafers and are expected to reduce the cost of starting material considerably.
Taken together, the thesis provides an extensive set of guidelines for silicon anisotropic KOH etching. By careful optimization of both the etchant composition and equipment, and matching the design to wafer specifications, it is shown that a very high degree of control can be achieved by KOH wet etching. These guidelines will be useful for further MEMS and NEMS (Nano Electro Mechanical Systems) development in cases where critical structures are defined by KOH etching.; Tama tyo on tarkoitettu kaiken kattavaksi oppaaksi kiteisen piin anisotrooppiseen syovyttamiseen kaliumhydroksidilla eli KOH:lla. Piikiekkojen anisotrooppinen KOH-syovytys on edelleen teknisesti tarkea prosessi, jolla valmistetaan halpoja mikroelektromekaanisia antureita ja systeemeja, joiden lyhenne on MEMS. Anisotrooppisella KOH-syovytyksella valmistetaan MEMS-antureiden/-systeemien mekaaniset ja fysikaaliset rakenteet kayttamalla hyvaksi piikiekon kidetasojen 100 ja 111 syopymisnopeuseroja. Myos muiden kidetasojen syopymisnopeuseroja voidaan hyodyntaa.
Tassa tyossa esitellaan kuusi eri ilmiota, jotka vaikuttavat piin anisotrooppiseen syopymiseen KOH-vesiliuoksessa.Kahdessa ensimmaisessa osoitetaan, etta syopymisnopeudet ja anisotropiat ovat erityisen herkkia syovytysprosessin olosuhteille seka laitekonfiguraatiolle. Esimerkiksi KOH-vesiliuoksen virtauksessa pyorteinen virtaus on parempi kuin laminaarinen virtaus. Kolmannessa ilmiossa…

KOH anisotropic silicon etching for MEMS accelerometer fabrication

Auto monitoring of cable-stayed tension on bridge by a novel RS485 acceleration sensor

Analysis of slanted beam MEMS capacitive accelerometer structures has been presented in this paper. A new structure has been proposed to be used for high g applications up to plusmn30 g. The proposed new structure has been compared with three other different structures. All the four structures were simulated using CoventorWare TM 2005 and their performance characteristics were compared. It has been found that other three structures are found suitable only for low g applications up to plusmn2 g. One among the four designs known as single beam structure was successfully fabricated in a single step anisotropic wet chemical etching in aqueous KOH solution using plane as physical etch stop.

A.R. Sankar

Papers

Structural sensitivity analysis of slanted beam MEMS capacitive accelerometers