Piezoresistive accelerometer fabricated by selective formation and etching of porous silicon

This is a report on the selective formation and etching of porous silicon, and the fabrication of a piezoresistive silicon accelerometer with eight beams on a selectively diffused (111)-oriented n/n⁺/n substrate using a porous silicon micromachining technique. This technique has many of the advantages of both bulk and surface micromachining without their disadvantages, and is capable of precisely constructing a microstructure by defining the n⁺-diffusion region and patterning the n-epitaxial layer, thereby, avoiding any lateral etching or undercutting. Through the use of an eight-beam structure, the mechanical strength of the accelerometer could be greatly improved due to a smaller shear stress. The lower sensitivity of the sensor was easily adjusted by combining the four output signals of the four half-bridges. The effectiveness of the sensor was confirmed through an experiment in which the accelerometer was characterized.