Wafer-level packaging for environment-resistant microinstruments

A generic wafer-level packaging technology for high-performance MEMS devices, operating under harsh external conditions is developed. This technology not only provides physical protection from the surroundings, but also provides thermal and mechanical isolation to enhance device performance. The wafer-level encapsulation and generic assembly approach accommodate a wide range of MEMS devices with minimal process lead-time and manufacturing cost. To realize this environment-resistant package, thermal isolation, mechanical isolation, generic device transfer/integration, wafer-level vacuum packaging, and feedthroughs have been developed.;The environment-resistant package consists of two substrates: a platform substrate providing thermal and mechanical isolation, and a package cap wafer providing vacuum encapsulation. Thermal stabilization is provided by oven-controlling the device at a temperature higher than the maximum environment temperature utilizing a heater and a temperature sensor located on the platform or the MEMS device. The heated structure is thermally isolated from the environment by isolation suspensions, anti-radiation shield, and vacuum encapsulation to minimize heat loss. The isolation suspensions are designed with high thermal resistance for minimal heat loss, sufficient stiffness for mechanical support, and flexibility for rejecting environmental vibrations. The package cap seals the MEMS device in vacuum. Vertical feedthroughs for a signal delivery are formed on the platform substrate or the cap wafer. These vertical feedthroughs save area and allow direct attachment to circuit boards. Shock absorption layers, and a getter layer for achieving and maintaining high vacuum are deposited on the inside wall of the package.;Performance is evaluated by packaging Pirani gauges and mode-matched tuning fork gyroscopes. The package size is 1.2x1.2x0.17mm 3, and the packaged device size is 4.5x4.5x0.5mm 3. The package has maintained vacuum pressure of ∼6 mTorr for ∼1 year. A packaged gyroscope shows a high-Q mode-matched operation (Q∼65,000) at a constant temperature of -5°C. Allan variance analysis displays an estimated angle random walk (ARW) of 0.012°/√hr and a bias instability value of 0.55°/hr at a constant -5°C. Drive frequency stability of 0.22ppm/°C is obtained using a compensated oven-control approach. Low power consumption of 33mW for oven-control at 80°C is demonstrated when the environment temperature is -30°C. The temperature control accuracy is +/- 0.2°C.