FTIR spectroscopy for the hermeticity assessment of micro-cavities

This paper presents an alternative method for the hermeticity assessment of micro-cavities. The method consists in monitoring the internal gas pressure of a cavity by Fourier Transform Infra Red (FTIR) spectroscopy, and then deducing the cavity's leak rate from the pressure variations. In order to carry out the IR transmission measurements, nitrous oxide was chosen as bombing gas due to its excellent absorption properties for mid-IR radiations. First FTIR spectroscopy measurements performed on test vehicles have demonstrated the method feasibility on silicon-silicon micro-cavities with internal volumes of 5mm3 or more. These test vehicles were then bombed with nitrous oxide, and standard leak rates of 3.4 × 10−7 atm.cm3.s−1 were assessed by FTIR spectroscopy. These leak rates have been confirmed by N2O pressure variation monitoring for a 20 mm3 micro-cavity.