Modeling and Measurements of Backside Laser-Probe Signals in MOSFETs

The purpose of this paper is to study the quantitative applicability of a laser-probe measurement system to conventional MOSFETs. Variations in the intensity and phase of the testing laser beam caused by the local changes in the refractive index of silicon due to variation in the free carrier concentrations are interferometrically detected and numerically modeled. The wave propagation through the substrate and the resulting standing wave due to reflection at the polysilicon gate are treated rigorously by using results of transmission line theory. The numerical results show a significant dependence of the expectable laser-probe signals on the model for the refractive index as well as on the reflection properties of the gate. We obtained a good agreement of the measured data and the numerical results for n- and p-MOSFETs, both for phase and intensity modulation on the accumulation side, and a fairly good agreement on the inversion side.