Measuring and interpreting the mechanical–thermal noise spectrum in a MEMS

The meta-stability of the pull-in displacement of an electrostatically operated parallel plate micromechanical structure is used for the capacitive measurement of the mechanical–thermal noise spectrum in a MEMS. Pull-in time depends on force and is not affected by the input-referred noise of the readout circuit. Repeatedly bringing the microstructure to pull-in while measuring the pull-in time followed by FFT enables the measurement of the mechanical noise spectrum with a non-mechanical noise level set primarily by the resolution of the time measurement. The white noise level is found to be in agreement with the theory on damping. The 1/ f noise spectrum is found to be independent of ambient gas pressure with a 1/ f noise–white noise cross-over frequency at 0.007 Hz for a 1 bar gas pressure and is reproducible for devices fabricated in the same process and the same run.