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Isolated Packaging for MEMS Sensors in Harsh Shock and Vibration Environments
Keywords: Isolation, Packaging, Harsh Environment
MEMS sensors are often used in applications that are constrained by limits on size, weight, power, and cost (SWaP-C). These systems are also exposed to severe shock and vibration environments. This combination of requirements has proven difficult to meet using current sensor technology. MEMS technology has made significant strides in performance in recent years. MEMS-based devices are commonly found in consumer applications for pointing, orientation, and motion tracking. However, MEMS based sensors experience performance degradation while operating through high shock and vibration environments. In general, for high-performance MEMS sensors, the very characteristics that lead to high performance also lead to high environmental sensitivity. The sensors are typically underdamped second-order systems. Their high quality factors lead to tendencies to ring under shock loads, as well as susceptibility to multiple frequencies aligning to the multiple resonances of the device. The large masses employed in the sensors lead to large deflections that are easy to detect, but also large deflections under shock and vibration loads. As the sensor structures become stiffer to increase natural frequencies and reduce deflections, high sensitivity deflection measurement approaches are required. These approaches tend to require small gaps and tight tolerances, and tend to be highly sensitive to environmental issues and require extensive, large, and complex external components and electronics. A traditional and successful approach for mitigating these shock and vibration issues in operational system is through the application of an isolator separating the sensor from the structure onto which it is mounted. However, isolators are large in terms of both size and cost. They also require significant design compromises. This paper will present the use of MEMS technology to realize microisolators that can be employed and customized at the sensor and package level to remove unwanted shock and vibration signals without compromising the SWaP-C benefits of MEMS devices, resulting in sensor technology that will meet performance requirements through the harsh shock and vibration environment. In the micro-isolator approach, a MEMS-based silicon interposer, with customized dynamic properties, separates the MEMS sensor from its package, and hence the external environment. This interposer passes the lower frequency vibrations and accelerations while filtering out large higher frequency components. The interposer also includes damping mechanisms that provide shock absorption even in the evacuated cavity of a MEMS package. The structure consists of a micromachined spring structure, with an inner platform connected to a surrounding frame by multiple flexures and to which sensors will be attached. The microisolator is itself attached to a vacuum-compatible damper, and the composite unit is mounted in the electrical package. Flexible electrical traces can connect the package to the frame of the microisolator, while thin film surface traces can run along the beams to landings on the isolator device pad for electrical connection to the attached sensor. This micro-isolator can also provide additional benefits such as isolation from stress gradients that induce errors, as well as the ability to serve as a platform for sensor thermal control and/or ovenization. This paper will present results from first and second generation micro- isolator designs as well as characterization of a variety of materials and 3D printed damping structures that can be integrated with the microisolator to attenuate large shock and vibration signals.
Michael Kranz, CEO
Huntsville, Alabama
United States

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