Vacuum sealed MEMS package with an optical window

A vacuum sealed package with an optical window is a useful diagnostic tool for MEMS devices as well as a critical component of optical devices, such as imaging bolometers, scanning mirrors and variable wavelength filters. In either of these applications, the package must meet a number of stringent requirements. It cannot contaminate devices by either outgassing or shedding particulates. The window must be optically flat to allow devices to be observed or measured with interferometric tools, when the package is used as a diagnostic tool. When it serves as an integral part of an optical MEMS device, the window must also have the requisite transmissibility over the device´s operating wavelength range. The vacuum level in many applications can also be quite challenging to achieve. Typically, pressures less than a few millitorr are necessary to prevent gas damping from limiting attainable Q values. Packages utilized for diagnostic purposes are often subjected to harsh environmental testing to evaluate how MEMS devices respond to mechanical shock, vibration, or thermal shock. Consequently, package robustness, particularly the glass to package seal integrity, is an important design element. We have successfully used a sputtered composite structure of gold over platinum over titanium to fabricate a seal ring on the window. The window is attached to a leadless ceramic chip carrier package by soldering with a 50 microns thick eutectic gold-tin preform. The sealing process is to load package assemblies, preforms and windows into a high vacuum system, degas them, raise the temperature of all components to 325degC, bring them into contact, and cool. We have used finite element analysis to optimize the seal geometry as a function of CTE mismatch, solder material, and window material to meet environmental requirements and optical flatness specifications. We have validated these FEM calculations by subjecting sealed packages to mechanical shock and helium leak testing. The optical- flatness of windows was evaluated by direct optical interferometry measurements and high resolution measurements on sealed MEMS devices. The gas permeability of sealed packages was evaluated by measuring the Q of resonant devices over a period of several months. This fundamental understanding of window design, validated by experimental testing, extends our MEMS packaging capability to support the needs of both diagnostic investigations and optical device packaging.