A silicon force sensor for robotics and medicine

This paper describes the development of a silicon-based force sensor packaged in a flexible polyimide-based package. The fabrication process is compatible with standard integrated circuit processes and produces a flexible package that sandwiches the metal leads between protective polyimide layers, Silicon direct bonding and bulk micromachining (both isotropic and anisotropic) are utilized to fabricate the silicon sensing element, The sensing element consists of a circular diaphragm (200 mu m thick with a 2000 mu m radius) over a 10 mu m deep sealed cavity. The shallow capacity depth provides built-in overforce protection. The diaphragm is instrumented with piezoresistors in a Wheatstone bridge configuration. Sensitivity to force is realized via the addition of a solid dome over the silicon diaphragm, The dome transmits the applied force to the diaphragm. Torlon and epoxy domes are bench tested. The epoxy dome produces significant hysteresis, while the Torlon dome shows low hysteresis (2.4% of the mean output) and low nonrepeatability (<2.8% of the mean output). The Torlon dome is subjected to a variety of loads to investigate the sensor's performance. In all cases, force accounts for at least 99.2% of the total variance in the output. Output sensitivities of 1.4 mV(-1) N-1 are typical. The response is linear for low forces (<10 N) and becomes curvilinear at higher forces when the diaphragm bottoms out, The corner point between the linear and curvilinear portions of the output response can be controlled via diaphragm radius, diaphragm thickness and cavity depth. Details of the microfabrication and micromachining processes are presented along with characterization of the force-sensor system. Preliminary finger-mounted results are presented.