CMOS-based force sensor with overload protection and improved assembly tolerance

This paper reports on a novel CMOS-based force sensor with integrated overload protection and improved assembly tolerance. The device consists of an anodically bonded silicon-glass stack comprising an n-type Wheatstone bridge as the piezoresistive stress-sensing element. Narrow trenches defining cross or seesaw shaped structures are micromachined into the surface of the Si chip. Depending on the desired measurement range of the sensor, these etched structures are either suspended via two hinges over a cavity etched into the glass substrate or bonded directly to the glass chip. A two-point force bridge is used to split the load to be detected into two components acting on the free ends of the cross or seesaw structure. This sensor concept provides an overload protection and an increased assembly tolerance with regard to the force application point. Twelve low and one high force device have been fabricated and characterized for loads up to 10 N and 30 N, respectively.