Isolation of MEMS devices from package stresses by use of compliant metal interposers

Many classes of MEMS devices, such as those with resonant structures, capacitive readouts, and diaphragm elements, are sensitive to stresses that are exerted by their surrounding package structure. Such stresses can arise as a result of changes in temperature, ambient pressure, or relative humidity. We have demonstrated a dramatic reduction in scale factor bias over temperature for a tuning fork gyroscope, by mounting it on an interposer structure within a conventional ceramic chip carrier. Holographic interferometry measurements confirmed that the deformation imposed on a sensor die directly brazed to the package was more than 5 times that of die mounted with an interposer. We have developed several configurations of metal interposer structures for mounting MEMS inertial sensors in standard ceramic chip carrier packages. The interposers are made by first precision chemically etching preforms in metal foil. These preforms are then electroplated with a wire bondable surface finish of gold over nickel. Next, they are excised from the multi-up foil panel and formed to hold the sensor within the package. The interposers are configured with either three or four tabs for holding the MEMS sensors. Gold bumps are applied to these tabs and then the sensors are attached with thermocompression bonding. This assembly is attached to the ceramic package by thermocompression bonding to gold bumps on lands of the wirebond shelf. Wirebonds are made to the I/O pads of the sensor to complete its installation