Design considerations for a silicon capacitive tactile cell

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Abstract This paper describes the concept of a new miniature silicon capacitive force sensor. The small size of the device and its realization on a silicon wafer make it potentially useful as a cell in a tactile sensor with a readout circuit integrated near every sensing element. The basic structure of the force sensor is a polysilicon bridge which is fabricated on top of a silicon wafer using surface micromachining. Using this basic structure, two possible realizations of the force sensor are considered. The first design consists of a single bridge. In this case, the detection of a force of a few mN requires a bridge with a thickness of 4 μm. The cell has a maximum size of 300 μm × 200 μm and, theoretically, a typical sensitivity of 2–12 fF/mN and a nominal capacitance of 140 fF. The thickness can be reduced to less than 1 μm with the second design, in which several bridges are placed in a matrix. Every bridge now has to support only a part of the force that acts on the sensor. This offers special freedom in optimizing the beam dimensions because of the ability to select the number of bridges within one force sensor. Some calculations are carried out concerning a cell composed of nine bridges, each of which has a typical size of 60 μm × 60 μm. The sensitivity of a cell in the second design is 16 fF/mN, and the nominal capacitance is 300 fF.