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 325°C, 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. IntroductionA 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 near optically flat to allow devices to be observed or measured with interferometric tools, when the package is used as a diagnostic tool, i.e. measuring MEMS Gyro frequency modes. ...
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. IntroductionThere are two key considerations in the design of an isolation interposer [1]. It is desirable to make the interposer as compliant as possible, to achieve the highest degree of isolation, without degrading sensor operation. For inertial instruments, this requires maintaining accurate alignment of the input axis. The other design requirement is to avoid introducing potential mechanical resonance conditions, which could adversely affect sensor performance in vibration environments. We have utilized finite element analysis to determine the resonant modes of interposer-sensor assembly and to predict its stress isolating performance.Interposers could also be utilized as part of a vibration isolation scheme, by incorporating damping material into their structure [2,3]. We have begun developing finite element and analytical models to evaluate the degree of isolation that could be achieved with various combinations of materials and mechanical designs [4,5].
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