Pressure sensing in vacuum hermetic micropackaging for MOEMS-MEMS

Abstract

Packaging constitutes one of the most costly steps of MEMS/MOEMS manufacturing. Uncooled IR bolometers require a vacuum atmosphere of <10 mTorr to operate at their highest sensitivity. The bolometer response is also dependent on the package temperature. In order to minimize cost, real estate, and power consumption, temperature stabilization is typically not provided to the package. Hence, long-term high-sensitivity operation of IR bolometric radiometers requires a calibration as function of in-package pressure and temperature. A low-cost and accurate means of measuring the pressure in the package without being affected by the operating temperature is therefore needed. The Institut National d'Optique (INO) has developed a low-cost, low-temperature hybrid vacuum micropackaging technology. An equivalent flow rate of 4 × 10−14 Torr L /s for storage at 80°C has been obtained without getter. Even with such low flow, the long-term stabilization of residual pressure variations affects the sensitivity and calibration of the IR bolometers. INO has developed microelectromechanical systems pressure sensors that allow for real-time measurement of package pressure of >1 mTorr and can be integrated with the IR bolometers in a die-level packaging process or microfabricated simultaneously on the same die. We present the typical performance and measurement uncertainty of these pressure sensors along with a reading method that provides a pressure measurement with a dependence on the package temperature as low as 0.7%/°C. A complex reading circuit or temperature control of the packages are not required, making the pressure sensor well adapted for low-cost high-volume production and integration with IR bolometer arrays.