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
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                              Christo! Ergo sum.
nInad sHiNde
918 Hawkins                                             offiCe: rm # 320
w. lafAyeTte                                                    me  blDg
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pH @ 765-495-7092 (hoMe)                           @ 765-494-8610 (offIce)
URL:http://widget.ecn.purdue.edu/~shinden
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