Tube-shaped Pirani gauge for<i>in situ</i>hermeticity monitoring of SiN thin-film encapsulation

Abstract

Here the integration of a tube-shaped Pirani gauge with a SiN thin-film encapsulation process is presented. The tube geometry gives the sensor a very low detection limit with a small footprint, since the tube is buried under the silicon surface. The Pirani tube is very suitable for in situ evaluation of MEMS vacuum packaging. Moreover, since the Pirani gap is all around the tube and deep below the silicon surface, the deflection of the encapsulation shell is not a concern and wafer-level measurement of the device is possible. Pirani tubes with different lengths are encapsulated inside SiN micropackages in order to measure the resulting vacuum level achieved after the encapsulation step. The longest tube shows a detection limit of 0.1 Pa for a noise level of 50 μV and it has a footprint of only 0.006 mm2. The pressure inside the sealed micropackage was extracted to be 0.7 kPa. Furthermore, the pressure is monitored over time to evaluate the hermeticity of the packages. A leak rate of 8× 10−18 Pa m3 s−1 was measured over four months time.