Discussion Forum

Hi everybody,

when my 2 wafers get cooled down from 400°C to room temperature, they look like this: (attached pics). The graph shows the edge at lower left. This means, that the wafer's sidewalls move upstairs during cooling down.
Fixed point for this study was (0,0,0) - the corner at the lower center of the symmetric model.

Now the membrane on top of the second wafer should get stressed. I can't use a fixed constraint for the bottom of the lower wafer since his edges are lifted up. So again I use (0,0,0) as fixed point for boundary condition. But this only works as long as the remaining lower edges reach z=0 (they move downstairs due to membrane load) . As from now I need further boundaries that fix the corners at the bottom at z=0. Otherwise these corners would move to z<0 which stands for lifting up the center of the lower wafer. That won't happen in reality!

So I need boundaries for the lower corners, that only get effective if their z-coordinate gets z<=0. Is it possible to implement this in Comsol?

Next question concerns membrane load. I want to avoid a point load in the centre of the membrane since this is not a realistic case. In a bend test a stamp would transfer a defined force into the membrane. Don't know how to model this in Comsol. Any suggestions?

Thanks a lot!
Chris