Two-Dimensional Flex Sensor Exploiting Stacked Ultrathin Chips

A new concept for mechanically flexible stress and flex sensors is presented. A stack of two ultrathin chips is exploited to compensate both thermal effects and the vertical piezoresistive effect. By tailoring the thickness of the top die, the active devices of the bottom chip become stress compensated, whereas the devices of the top chip receive maximum stress. The structural arrangement enforces that the apparent vertical stresses and the temperatures of both dies are basically identical. Thus, the differential signal of both dies provides a temperature-compensated information about the 2-D stress and flexure.