capacitive transducer, Harmonic Perturbation problem, output pressure

Hello,everyone
I'm trying to simulate a capacitive transducer to get the ultrasound with the vibratng membrane. In the begining I use 2D axial symmetry, MEMS module, Pressure Acoustic,Frequency Domain module to get the result.
But when I use 3D, MEMS and Pressure Acoustic,Frequency Domain module with the same boundaries conditions(because of the long time to run,I design a half model),I think it's right but Comsol told me that there is error just as bellow:

Failed to find a solution for the initial parameter.
Maximum number of segregated iterations reached.
Returned solution is not converged.
-Feature: Stationary Solver2(sol1/s2)
-Error:Failed to find a solution for the initial parameter.

BTW: for drive it, I set a Harmonic Perturbation on the terminal with the DC voltage. I think maybe there is something wrong with it. But It's OK in 2D,I don't know why.
So could anybody help me to solve it? Please help me if you have any ideas, thanks a lot in advance!
（my email: ascxw2008@gmail.com , hope to get any suggestions, email is OK !）