Stabilization of MEMS accelerometer properties using stretchable beam structure

Abstract

A high-reliability design method for a MEMS accelerometer using a stretchable beam structure was proposed to prevent sensor property variations due to stresses from device fabrication and packaging. To achieve a compact piezoresistive three-axis accelerometer using a low-cost resin-mold packaging, beam buckling due to compression forces originating from residual stresses of thin films and shrinkage of mold resin must be overcome. The proposed stretchable beam structure, which gives axial flexibility by adding a ring-shape part at the center of the beam, has advantages in which it is effective against not only packaging stress but also inner stress such as thin film stress, and it does not sacrifice compactness. Formulations with a simple accelerometer model revealed that large and nonlinear variations in sensitivity occur due to beam compression. Such large variations have to be avoided to stabilize sensor performances. Besides, it is expected to enhance sensitivity up to about 1.5 times while maintaining linearity by arranging an appropriate compression force using the stretchable beam. The effects of the stretchable beam were verified using simulations and experiments on test samples. The designed stretchable beam, which has two rings connected serially, has five times the axial flexibility compared with a conventional straight beam without significant change in bending stiffness. A preferable increase of about 1.4 times and small deviations in measured sensitivity were achieved with the developed stretchable beam, compared with the straight beam with an unstable increase of about seven times and extremely large deviations. These large deviations seem to come from shape variation due to non-uniformity of dry etching technologies commonly used for MEMS devices. Thus, the effects of the proposed stretchable beam to the MEMS accelerometer design were confirmed to prevent large variations and deviations in sensor properties and to enhance sensitivity.