Handbook of Silicon Based MEMS Materials and Technologies

42.5. Vacuum Lifetime Model

In a simple model a hermetic encapsulated device can be described as a cavity with vacuum inside and normal air pressure outside. First, let us assume a leak channel with a diameter of some microns, which is much larger than the mean free path of the molecules at air pressure which is about 68 nm. Therefore, we can expect a viscose gas flow at the entrance of the leak channel. Assuming a vacuum of 1 mbar or better, the mean free path inside the cavity will be much larger than the channel diameter, resulting in a molecular flow behavior. Under these conditions we have to assume a continuously changing gas flow behavior ...