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Isolated Interposers for MEMS Sensors
Keywords: MEMS Packaging, Shock, Vibration
Sensors based on MicroElectroMechanical System (MEMS) technologies are beneficial for volume-constrained applications. However, MEMS devices 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. They also 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. From the perspective of random vibrations, a common issue for MEMS sensors occurs when that vibration is at or near the natural resonant frequency of the structure. Being underdamped second-order systems, MEMS devices often exhibit a large response when excited at their natural frequencies. This response must be accommodated by the mechanics of the structure as well as the control systems used. From the perspective of shock, the performance issue for MEMS sensors is again for frequencies near the resonance of the device. In addition, the mechanical structures of a MEMS device and its packaging can be vulnerable to high frequency components of the shock, as well as large random vibrations. In some cases, the structure can displace to its extreme limits, leading to nonlinear operation or mechanical failure if there are tiny manufacturing defects or if the deflection of the structure exceeds the designed range. A traditional and successful approach for mitigating these shock and vibration issues in IMU systems is through the application of an isolator separating the IMU 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. MEMS technology, however, can structures that can be employed and customized at the sensor and package level to isolate the sensors from unwanted shock and vibration signals without compromising the benefits of MEMS devices. In an isolated packaging approach, a MEMS-based silicon interposer, with customized dynamic properties, separates the MEMS sensor from its package, and hence the external environment. This interposer filters out large high frequency components of shock and vibration. 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 three-fold symmetric spring arrangement suspending a symmetric platform. The interposer is itself attached to a vacuum-compatible damper, and the unit is mounted in the electrical package. Flexible electrical traces connect the package to the frame of the microisolator, while thin film surface traces can run along the springs to landings on the isolator device pad for electrical connection to the attached sensor. This interposer 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 the design, fabrication, and test results of a three-fold symmetric isolating silicon interposer incorporating vacuum-compatible damping that allows its integration in an evacuated MEMS package.
Michael Kranz, CEO
Elkmont, AL
United States

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