IMPROVED VACUUM LEVEL OF SILICON-MIGRATION-SEALED CAVITY BY HYDROGEN DIFFUSION ANNEALING FOR WAFER-LEVEL PACKAGING FOR MEMS

A sealing vacuum level better than 10 Pa was achieved by Silicon Migration Seal (SMS) technology without film deposition or using a getter for the first time. SMS utilizes silicon reflow phenomena in hydrogen at high temperature (>1000 degrees C) to close release holes. In this study, we confirmed the sealing vacuum level using silicon diaphragms fabricated in a device wafer. And the samples were placed in a vacuum chamber with a diaphragm pressure gauge to evaluate vacuum level of the sealed cavity. The vacuum level got better than the detection limit of about 10 Pa after 35 hours annealing in nitrogen, during which hydrogen diffused out. In addition, we did thermal desorption spectroscopy (TDS) to analyze residual gas in a sealed cavity, which was almost hydrogen as expected. SMS is promising for inexpensive high vacuum encapsulation of gyroscopes, timing resonators etc.