Speaker
Description
Vacuum sealing devices with a small inner volume like micro-electro-mechanical system (MEMS) require a strict helium leak test to ensure the lifetime because the inner pressure rapidly increase even if a fine leak is occurred. However, it is difficult to measure the helium leak rate less than 10-11 Pa m3/s reliably because the range of commercially available helium standard leak is limited down to 10-11 Pa m3/s in typical. In this study, we have developed a helium leak testing devise including a calibration system whose calibration range is less than 10-11 Pa m3/s. The principle of the calibration is a constant conductance method where the leak rate is obtained by the product of the molecular conductance of a leak element and its upstream pressure. A porous plug made of a sintered stainless-steel with a molecular conductance of 3.04×10-9 m3/s for He was used as the leak element. The upstream pressure was set by using a static expansion method down to 10-5 Pa. The generated helium leak down to 10-14 Pa m3/s was measured by an accumulation method with a quadrupole mass spectrometer (QMS) and non-evaporated getter (NEG) pump. The increase of partial pressure of helium was measured by QMS. The NEG pump was used to maintain the operational pressure of QMS by evacuating outgases from the inner wall of the vacuum chamber and QMS except for helium. The increase ratio of helium partial pressure was proportional to the generated helium leak rate down to 10-14 Pa m3/s. `