Calibration of ultra-fine helium leak rates for leak testing of microelectromechanical systems

Abstract

A novel method was developed to calibrate helium leak rates lower than 10−10 Pa m3/s. This method involves two essential points. One is the use of a leak artifact with known molecular conductance and known pressure at which molecular flow is realized. The other is that the upstream pressure of the leak artifact is determined by the static expansion method under a pressure in the range in which molecular flow is realized. In this study, a porous plug with a molecular conductance of 3.04 × 10−9 m3/s and a molecular flow realizing the pressure of 104 Pa was used as the leak artifact. The upstream helium pressure of the porous plug was determined from 133 to 10−5 Pa by repeated expansion up to five times, from which reference helium leak rates from 3 × 10−8 to 4 × 10−14 Pa m3/s were generated. The reliability of the helium leak rates so generated was examined using a cumulative helium leak detection method with a nonevaporated getter pump. The peak-to-peak data scatter including nonlinearity and repeatability was less than ±26%.