A STUDY OF BACKSIDE LASER-PROBE SIGNALS IN MOSFETS

The performance of a laser-probing system due to analyzing signals in MOSFETs is studied. A heterodyne laser interferometer is used for the detection of changes in the free carrier concentrations induced by ter minal biases in n-channel and p-channel MOSFETs. Measurements of the p hase shift and intensity changes are carried out in accumulation, inve rsion and depletion and compared to the results of a rigorous numerica l modeling of the wave propagation. In the depletion region, the effec t of the standing wave resulting from the reflection from the gate is observed in experiment. As the sensitivity of the phase shift measurem ents is in the order of 10(-5) rad, doping profiles could be derived f rom optical experiment. We found that a strong impact of the reflectio n properties of the polysilicon gate on the intensity signal poses a r estriction to the applicability of the intensity modulation measuremen ts for the inversion and accumulation conditions.