Encapsulated submillimeter piezoresistive accelerometers

While micromachined accelerometers are widely available and used in various applications, some biomedical applications require extremely small dimensions (< mm) or mass (< mg) that cannot be fulfilled with commercially available accelerometers. In this work, we present a fully packaged piezoresistive accelerometer that has the smallest dimension (0.034 mm(3)) ever published. We achieve miniaturization by using a film encapsulation technique with a thick epitaxial polysilicon layer. This packaging technique enables the dimensions of the die to be only tens of microns larger than the micromechanical structure. We have fabricated accelerometers as small as 0.034 mm(3) (387 mu m x 387 mu m x 230 mu m) with noise floor of 0.25 mg/root Hz. These ultra-miniature motion sensors have potential opening up new frontiers in biomedical science and engineering.