Cavity pressure determination and leakage testing for sealed surface micromachined membranes: a novel on-wafer test method

The knowledge of the cavity pressure and the tightness of sealed surface micromachined membranes is essential for the description of their behavior and for the evaluation of their long-term stability. We are presenting a novel procedure to determine the cavity pressure and the tightness of sealed surface micromachined structures. The basic idea for cavity pressure determination is to compare two pressure-dependent resonance frequency measurements of a sealed and subsequently opened structure. The pressure dependent resonance frequency of the sealed structure depends on the clamping conditions, while the resonance frequency of the opened structure strongly depends on the compression of the gas-film between the plates. To characterize the structures with high accuracy a setup containing a temperature and pressure controlled chamber was installed. To equalize the ambient and the cavity pressure after the first characterization the sealed structures have to be opened by means of with an focused ion beam (FIB) and again measured pressure-dependent. An important application of the developed method is on-wafer process monitoring of surface micromachining processes.