Insertion Force Sensor by Sidewall-Doping with Rapid Thermal Diffusion

We propose an insertion force sensor with piezoresistors fabricated by sidewall-doping with rapid thermal diffusion. The sensor consists of a main body, to which a pointed tip is installed, and supporting beams whose sidewalls are made piezoresistive. When an insertion force is applied to the tip and the silicon structure deforms, the change of the strain distribution condition on the piezoresistive sidewall varies the electrical resistance of the sensor, which is in turn transduced into output voltage signal via an outside circuitry. The beams are 250 µ m in length, 20 µ m in thickness, 8 µ m in width. By etching an appropriate pattern onto the wafer, additional exposed surfaces for doping can be created where vertical dope-sites are needed. With rapid thermal diffusion, we are able to fabricate dope-sites which are perpendicular to the surface of the wafer. Fabricated piezoresistive sensor has 20 pairs of supporting beams. The spring constant is 2500 N/m. The electrical resistance is 1.6 kΩ. The displacement sensitivity is 1.03 % per µ m. This displacement sensitivity on the insertion direction is as high as that on the vertical direction of reported sensors, which means that our proposal is an effective doping method for sidewall.