For MEMS problems I use the same stress tensor as for silicon.  The FEA inputs I
use to input to Ansys are listed below:

/COM, Anisontropic silicon material properties
caa     = 165 e9                        ! c11
cab     = 64 e9                         ! c12
cdd     = 79.2 e9                       ! c44

/COM, Polysilicon material properties
mat,poly
et,poly,64                              ! Degree of freedom: UX,UY,UZ
keyopt,poly,4,0
type,poly
tb,anel,poly
tbdata,1,caa,cab,cab,0,0,0
tbdata,,caa,cab,0,0,0,caa
tbdata,,0,0,0,cdd,0,0
tbdata,,cdd,0,cdd
mp,alpx,poly,2.3e-6
mp,dens,poly,2.33e3

Good luck

Trevor Niblock;
MEMS group, Rm. 2043, Building 53, ECS, Southampton University,
Highfield, Southampton, SO17 1BJ, UK.
e-mail: t.niblock@soton.ac.uk
URL:    http://www.mems.ecs.soton.ac.uk/users/trevor/tn_hp.htm
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-----Original Message-----
From:   Chua Bee Lee [SMTP:eng50250@leonis.nus.edu.sg]
Sent:   24 January 1999 10:59
To:     MEMS@ISI.EDU
Subject:        Non-elastic deformation of polysilicon

Hi !
        Can anyone give me some pointers on the non-elastic deformation of
polysilicon ? Does humidity comes into the picture ?

        Thanks!
        CHUA Bee Lee
        National University of Singapore
        Department of Mechanical and Production Engineering