Anomalous stress effects in ultra-thin silicon chips on foil

Apparently anomalous and transient piezoresistive effects are observed from CMOS transistors and diffused resistors fabricated on 20 µm thin, flexible chips for system-in-foil (SiF) applications. We show that this effect results from a transformation of uniaxial to biaxial stress according to the Poisson ratio of the visco-elastic epoxy glue used for chip attachment. This effect is further analyzed through current mirror circuits composed of orthogonally oriented transistors.