Development of vertical integration of MEMS inertial sensors and devices

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Development of vertical integration of MEMS inertial sensors and devices

Reiner, Philip / Elliott, Robert

There is a growing need for micro-miniature sensors for use in detecting and measuring low rotation rate and low acceleration environments. For instance, safe and arm systems for rockets and missile systems must be able to detect rotation rates as low as 1 to 100 revolutions per second and acceleration events as low as a few G's. Inertial guidance systems can have even more stringent requirements that often preclude the use of MEMS sensors simply because the devices can not readily achieve the sensitivity needed. The root cause for difficulty in achieving the sensitivity required for both of these cases is that large inertial masses are required in order to achieve detectable forces that can be converted into useful signals. Normally this is achieved by increasing the area of the moving element (swing weight) in the device layer in order to increase its mass. The drawback is that for low spin, low-G applications, the required mass is so large that the device grows in size and is no longer miniature. In this investigation, the authors report on the use of the thick handle of the SOI wafer to define the swing weights instead of the relatively thin device layer. In this way, up to 5 times as much mass could be realized for the swing weights without increasing the footprint of the device. All of the subsequent processing that was developed centered on the techniques required to fabricate devices with large swing weights in the silicon on insulator (SOI) wafer handles. In particular, they report on the application of these techniques to the development of a MEMS safe and arm device that requires simultaneous sensing of vertical accelerations and rotations about the normal to the device plane.