Vacuum package design for a MEMS based IR detector array

Abstract

A vacuum package was designed and manufactured for the testing of a MEMS-based thermal imaging detector array. Optimum operation of the pixel array requires low pressure and stabilized temperature. The pixel array devices are under development; therefore, the package needs to be reusable as a test fixture for chip revisions. Two optical windows are required: the top window to be transmissive at the long-wave infra-red (LWIR) wavelengths and the bottom window at visible (VIS) readout wavelengths. Thermal noise is a fundamental source deteriorating the performance of the pixel array; hence, a thermocoupler, thermo electric cooler (TEC) and heatsink were included in the package to allow thermal stabilization. To minimize the thermal noise, the TEC and heatsink were located between the VIS window and the pixel array. Such a configuration for thermal stabilization resulted in the use of special TEC and heatsink designs that include holes matched with the VIS window size. Simulations on pixel array performance indicate a 1 mTorr vacuum pressure to provide the optimum performance of the pixel array. In order to meet the vacuum requirement, a Kovar package with the minimum possible size, considering the size of elements assembled in the package, was chosen. The removable ZnSe IR window was hermetically sealed with an O-ring and vacuum grease. The VIS window was glass soldered to the bottom of the package. Active pressure control was obtained by a vacuum pump.