Abstract
This study designs and implements a stress isolation guard-ring structure to improve the performances of the existing accelerometers. Thus, the environment disturbances, such as temperature variation and force/deflection transmittance, for a packaged accelerometer are significantly reduced. In application, the 3-axis piezoresistive accelerometer has been fabricated using bulk micromachining process on the SOI wafer. Experiment results show the out of plane deformation of the suspended spring-mass on packaged accelerometer is reduced from 0.72 um to 0.10 um at a 150 ℃ temperature elevation. The temperature coefficient of zero-g offset for the presented sensor is reduced, and the temperature-induced sensitivity variation is minimized as well. Measurements also demonstrate the guard-ring design successfully reduces the false signals induced by the force and displacement transmittance disturbances for one order of magnitude. The accelerometers with guard-ring structure is further capped with glass substrate to form the glass/Si/glass sandwich and then encapsulated in plastic package. The testing results on these packaged accelerometers have shown that the guard ring structure successfully suppresses the performance shift caused by plastic packaging process for one order of magnitude. Thus, the inexpensive plastic encapsulated package for accelerometers can be implemented on the real products. Moreover, the 3-axis acceleration sensing for the presented accelerometer with guard-ring has also been demonstrated with sensitivities of 0.12 ~ 0.17 mV/V/g and non-linearity < 1.02 %. Finally, this study further reports an optimum design to shrink the size of guard-ring, yet maintain the performance of accelerometer. Under the assistant of FEM and Taguchi method, the performance of the accelerometer is improved both in offset shift and sensitivity shift for one order of magnitude, and the size of the stress isolation structure (the sum of guard-ring length/width and connection bridge length) has been shrunk for 29 % (from 138 um to 98 um). Moreover, the unwanted higher vibration modes are far away from the first three modes. The proposed accelerometer design keeps the advantages of the original 3-axis accelerometer design.