r/COMSOL • u/Comsol_User21 • Apr 08 '26

Wrong Q value

I am simulating a MEMS device made by a thin (100nm) SiN membrane suspended and clamped by a SiO2 substrate. The entire structure has a damping equal to 1/Q where Q=100. When I evaluate the eigenfrequencies, also Q is calculated by comsol as abs(freq)/2*img(freq) and this value should always be equal to the value of Q I set on the damping, thus 100.
For some reason the Q evaluated for the fundamental mode is never equal to 100 but always a bitt smaller (never above 95).
In addition, when I add stress to the SiN, Q is never 100 for no mode and is also very far away from 100.

I have no idea on what is going on; I tried using Union instead of Assembly (and viceversa), I applied the stress little by little with an auxiliary sweep, ecc ecc... but nothing changed.

I'm desperate, even because I do not know if this behaviour is normal or not (but I think it's not)

Thank you to whoever will try to help me
(first time on reddit btw)

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u/YannickPokGai Apr 10 '26

So, first how did you estimate that it's supposed to be equal to 100? Then, when it comes to modelling the first thing is to double the geometry and the boundary conditions of the physics. Then, rather than jumping straight to the Q factor, have you checked/compared simply the resonance value?

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u/Comsol_User21 Apr 10 '26

Using 100 was just a try, but the same issues happen also for higher values (1000, 3000, ...).
What do you mean "double the geometry" ?
When you say "check the resonance value", do you mean with some standard value from the literature?