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Novel Temperature and Vibration Tolerant Packaging for Inertial Sensors (MEMS)


OBJECTIVE: Develop a cost-effective"generic"vacuum package which can accommodate a wide variety of micro- and milli- scale inertial sensors, increase robustness, and provide the sensor with temperature and vibration isolation from the environment. Vacuum packaging is critical to inertial sensors performance and determines the final sensor quality factor. The lower the vacuum, the better the quality factor (Q Factor). As a result, in this case the end product must be rugged and should maintain ultra low vacuum levels to achieve maximum accuracy. Note, increase Q factor results in a higher signal to noise ratio. DESCRIPTION: MEMS based inertial sensors must reach extreme sensitivity levels in order to sense the Earth's rotation. As a result, any improvements in stabilizing the sensor environment (temperature, vibration, humidity) serve to improve sensor signal to noise ratio, thereby improving the final delivered accuracy. Additionally, by controlling the sensor environment, overall system design complexity is reduced, as feedback loops and other control schemes may be greatly simplified. Note, all MEMS based sensors require ultra low pressures to improve performance (reach maximum Q factor). Leak rates and outgassing are the two factors which affect long term reliability of a typical vacuum packaged device. Assuming a device with a 2.5x2.5x0.1mm3 (0.625 mm3) cavity: for example, to maintain less then a 2 mTorr change in pressure in 1 day requires a<2e-14 cc/s leak rate and in 1 year it requires a<5e-17 cc/s leak rate. Typical He leak rate tests only have a resolution of approximately 1e-10cc/s. Given these assumptions, microscale packages need to have virtually no leak rate for long term reliability. The second factor, outgassing, involves the desorption of trapped process gases from inside the package. Due to the very small package volumes, desorption of fabrication process gases has been shown to be the dominant mechanism for increasing the pressure over time. In the development of long term and stable vacuum packages, it is important to characterize and eliminate both of these factors to produce a high quality packaged device. PHASE I: Clearly identify micro- and milli- scale inertial sensors and their packaging needs with respect to temperature and vibration isolation and pressure. Provide a detailed package design and specify plans that can achieve and maintain extremely low vacuum levels (lower is better for Q factor) and to mitigate stress, vibration, shock and temperature effects from the environment. Identify military suppliers which may be used in this phase and provide a detailed price reduction strategy coupled to packaging innovations. PHASE II: Utilize sensors for evaluating the package performance, demonstrating lowest vacuum level possible and robustness to outside temperature fluctuations and vibration. These can be sensors fabricated specifically for package evaluation or sensors provided by military or potential military suppliers. Given the stress, temperature and vibration sensitivity of the target sensor applications, this package should enable navigation grade performance. PHASE III: Apply this technology and package to an inertial sensor from at least one supplier for military targeting and navigation applications for prototyping and pre-production runs. Provide a direct performance comparison of the environmentally robust packaging application developed to existing packaging technology and characterize all performance improvements. PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: MEMS device isolation technology is needed to protect MEMS devices from temperature fluctuations, vibration and mechanical shock, as well as, maintain an ultra low vacuum level to maximize the Q factor (increase signal to noise ratio). Development of this technology may be used in any application (military / commercial) which requires improved accuracy or control of sensor environment to achieve a set of desired accuracy objectives.
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