Quantitative Characterization of True Leak Rate of Micro to Nanoliter Packages Using a Helium Mass Spectrometer

The helium mass spectrometer has been used extensively for qualitative analysis of fine leaks. However, qualitative analysis for the small volumes (typically < 10⁻³ cc) of MEMS packages can produce erroneous results. In addition, the qualitative use of the helium fine leak test does not allow comparison of two different packages. Nor can the results be correlated with data from accelerated tests such as 85°C/85%RH test. These limitations warrant the development of a method to assess hermeticity quantitatively. This paper proposes a procedure to quantitatively measure the true leak rate of micro to nanoliter packages using the helium mass spectrometer. The proposed method is based on the fact that the profile of the measured leak rate signal depends only on the true leak rate and the volume of the package. Prior knowledge of the volume of the package enables determination of the true leak rate by performing a weighted non-linear regression analysis of the data. The method was implemented to measure the true leak rate of a MEMS RF filter package. The package was tested under three different test conditions yielding three different signal profiles. The method yielded a consistent value for the true leak rate, which verified the robustness of the method. The proposed method provides a true leak rate as a quantitative measure of hermeticity. The accurate true leak rates will help evaluate new bonding materials/processes and package designs fast and effectively.