Impact of Bond Pad Thickness on Polysilicon Resistance Change Due to Probing

Polysilicon is an integral part of many devices in all CMOS process. Very consistent and accurate electrical performance of such material is a need of those devices used in Circuit Under Pad (CUP) applications. This paper presents an investigation on stress impact of probe insertions on two bond pad metal options i.e. METMID and METTHK on a polysilicon resistor placed under the bond pad. Probing results in residual stress on both Back End Of Line (BEOL) as well as Front End Of Line (FEOL) structures. This residual stress would impact the electrical properties of the polysilicon material used in such devices. In this study, such electrical impact is measured in terms of change in resistance of a polysilicon resistor which was placed underneath the bond pads.

A commercial finite element analysis software (Comsol Multiphysics) was used to predict the stress distribution in the polysilicon device prior to silicon validation. In this paper, it was observed that the bond pad thickness can influence the residual stress which in turn causes a change in resistance after wafer level probing. But the number of such probe insertions were not showing much significant impact on the polysilicon resistor placed under the pad.