Assessment of vacuum lifetime in nL-packages

Vacuum wafer bonding technology provides a number of very effective techniques to produce low-cost, hermetic sealed packages for micromachined sensors and actuators. Beside the protection of the device from outside environmental stresses, the package must also provide a cavity ambient compatible with the device performance and reliability. The paper reviews the shortcomings of existing fine leak tests to assess the stability of the cavity atmosphere over long time. A new ultra-fine leak test based on Q-factor monitoring was developed that has the potential to be used for in-line critical leak rate testing on wafer level before device singulation. With the example of a poly-silicon resonating microsensor, it is shown that the test is sensitive enough to predict the cavity pressure for a demanded device lifetime of 15 years. The role of an integrated thin film getter in stabilizing the cavity vacuum and extending device lifetime is discussed