Isolation of MEMS Devices from Package Stresses by Use of Compliant Metal Interposers

Abstract

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].

Hybrid approach to MEMS reliability assessment

“…Any deformations of the folded springs that are not in direct-response to functional operation of a sensor will cause an incorrect (i.e., erroneous) output. For example, thermomechanical (TM) distortions of a package affect shape of posts and these, in turn, lead to undesired deformations of the springs and erroneous results produced by a sensor supported by these springs 34,35 . Because of the nanoscale of these deformations and microsize objects over which they take place, it was not until the Functional operation of MEMS accelerometers, just like operation of microgyroscopes, depends on motion of a proof mass in response to an applied acceleration.…”

Recent advances in microdevices for transportation industry

“…38 To mitigate adverse effects of packaging, new design/fabrication approaches are being developed. 39 Figure 18 depicts OELIM interferograms of a microaccelerometer subjected to loading Case A. Characterization of MEMS devices under this condition is equivalent to determining their shape without any loading.…”

A Temperature-Insensitive Resonant Pressure Micro Sensor Based on Silicon-on-Glass Vacuum Packaging

“…Various approaches focusing on packaging materials [8], assembling styles [9], and stress isolation structures [10,11] were proposed to reduce temperature influences. In general, packaging materials with CTE close of that of silicon, such as Pyrex7740 and BOROFLOAT®33, were used to seal the dies [8].…”

“…Moreover, several designs of stress isolation structures were proposed to address the issue of temperature shifts. For instance, an anisotropically etched intermediate layer [10] and a metal interposer structure [11] were designed to relax contacting stresses. Previously, we presented a stress isolation structure for resonant pressure sensors, which was realized by mounting the sensor die to the metal substrate, which was fixed at a corner of the sensor through stacks of small silicon dies with silicone adhesive [12].…”