Dear Ninad, >From my point of view, it is not possible to create a total circulare etch hole with KOH etching. There are some compensation stuctures, which round the corners a little bit, but you will got some rough steped side wall planes. I mean, that only the 45 degree 100-plane is usable for stress reduction on sharp 111-corners . Some examples and tests for this problem you can also find under our simulation examples for SIMODE (www.gemac-chemnitz.de/mst/mst_eng.htm) With kind regards Dirk > Dear Colleagues, > > I am designing a Si piezoresistive pressure sensor to be utilized to > measure pressures of the order of 5 MPa. I have approached the problem > from the conventional method using a Wheatstone bridge. The wafers used > will be <100> wafers and the membrane is made by anisotropically back > etching using KOH so that the <111> planes form the walls. > > My problems and corresponding questions are: > > 1) I am limited by the magnitudes of sensitivities that I can achieve. > These are between 1-10 microV/V per mm Hg while typical pressure sensors > for lower pressure measurement applications have sensitivities atleast an > order of magnitude larger. I cannot make the device more sensitive by > increasing the side (square diaphragms) or the radius (circular > diaphragms) or by reducing the thickness of the membrane since either of > these three actions would cause the maximum stress at the edge of the > membrane to exceed the rupture stress. > > How should one physically modify the sensor ? > > 2) It is commonly known (ref. Sooriakumar et all) that sharp corners in > square diaphragms cause stress concentration factors of the order of 30 or > more. What value should one use for the fracture stress of Si if one were > designing CIRCULAR DIAPHRAGMS. Petersen suggests 7000 MPa while 300 MPa is > what is used for square diaphragms. > > Any input or reference to a source would be greatly appreciated. > > Regards, > > Ninad > Purdue University > @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ > Christo! Ergo sum. > nInad sHiNde > 918 Hawkins offiCe: rm # 320 > w. lafAyeTte me blDg > iN 47906-3572 > U.S.A. > pH @ 765-495-7092 (hoMe) @ 765-494-8610 (offIce) > URL:http://widget.ecn.purdue.edu/~shinden > @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ > Dirk Zielke GEMAC mbH Zwickauer Str. 227 09116 Chemnitz Germany Tel.: +49 371 3377 131 Fax.: +49 371 3377 272 email: info@gemac-chemnitz.de http://www.gemac-chemnitz.de (Deutsch) http://www.gemac-chemnitz.de/mst/mst_eng.htm (English)