Discussion Forum

Hi everyone,

I really could use some help for my masters project I am designing a strained Germanium membrane for applications in Photonics which introduces bi-axial strain in the center of the structure by pulling on each on the y and x-axis.

Each structure has an initial bi-axial tensile strain of 0.15% (introduced by the growth of the germanium crystal on silicon) and then by controlling the geometry the tensile strain can be enhanced in the constricted region and it relaxes (compressive strain) in the pads. I defined the initial stress corresponding as follows which corresponds to an initial bi-axial strain of 0.15% for Germanium (see attachment 1) and I defined the elasticity matrix using Voigt notation (see attachment 2) as the an-isotropic effects are key.

I first tried to calibrate and recreate the results for a uni-axial 'micro-bridge' system in which I used a fixed constraint along the boundaries perpendicular to the bridge (see attachment 3) and the rest of the boundaries are free which gives the correct stress / strain distribution (I am not too concerned about the exact accuracy of the values at the moment but the strain distribution for both structures) and I got the correct distribution (see attachment 4)

However when I try and transfer this method to the bi-axial structure which I should get tensile strain in the middle and compressive strain on the pads however this isn't what happens. I defined a fixed constraint on the boundaries framing the structure (see attachment 5). However when I use the same settings I get two compressive strain spots and two tensile in the middle which is wrong it should be all tensile in the middle (see attachment 6).

So then I tried to redefine my initial stress values to try and make it more accurate (see attachment 7) and I get two tensile spots which seem correct but the other two (which should exist with a diagonal symmetry to these two) don't appear! (see attachment 8)


So I have a suspicion it has to do with my boundary settings or my initial stress settings which I tried modify but I am at my wits end! It could be something to do with the mesh but I don't think so.

I feel like I am so close but I come from nanotechnology background not a mechanical engineering background so I am very confused. Thanks in advanced guys!