Vacuum Packaging of MEMS Inertial Sensors

Navigation grade MEMS gyroscopes and accelerometers utilize resonance structures with Q’s of 30,000 or more, which are driven by electro-static comb drives. Gas pressure in excess of 10 milli-Torr degrades the Q and performance of these instruments. Another reason for vacuum packaging MEMS inertial sensors is to thermally isolate them, and thus reduce the power requirements for temperature control. Most navigation applications for MEMS inertial sensors specify operational lifetimes of twenty years, which for a typical 20 lead chip carrier, translates to a gas leakage rate of 10-13 standard cubic centimeters per second. This value is well beyond the range of commercial helium leak detectors, which measure down to 10-9 [SCCS].

These demanding requirements on package pressure and leakage rate have been met by using leadless ceramic chip carriers with integral getter elements. The packages are cleaned, degassed and sealed under high vacuum, by brazing on covers using gold-tin eutectic alloy. Several refinements to this basic packaging approach have been made to further improve instrument performance. New vacuum sealing systems have been built to achieve base pressures of 10-9 Torr and incorporate ion gun systems for in-situ cleaning of packages and sensors, prior to sealing. This level of cleanliness is necessary to achieve high stability in accelerometers with micro-g level sensitivity.

In a conventional chip carrier assembly, the sensor is brazed to the floor of the chip carrier. In this configuration, the package can apply significant stresses to the die, which alter its shape and consequently its scale factor. Changes in temperature, atmospheric pressure, or inertial loading can adversely affect the sensor in this way.

A number of compliant attachment schemes have been developed for mounting sensors within chip carriers. They range from thermo-compression bonded gold bumps to interposers formed from intricately shaped spring structures, which are fabricated in metal or silicon. Interposer structures also facilitate thermal isolation of the sensor from the package, and make it feasible to control the temperature of the sensor with a modest amount of power.

Instruments have also been successfully packaged by flip chip bonding them to gold bumps on the floor of a chip carrier or on a low temperature co-fired ceramic substrate. This latter approach is particularly attractive for packaging larger, multi-axis instruments, which have thirty or more electrical connections, many of which require careful electrical shielding.

Both multi-axis gyroscopes and accelerometers have been packaged with this approach. The full potential of this technology has yet to be realized, however, because the control ASIC chips and passive components can be assembled on the package to achieve miniaturized high performance instruments.