Optical charge modulation as an internal voltage probe for CMOS ICs

The use of optical charge modulation of refractive index as a probe for internal voltages in CMOS integrated circuits is examined. The Kramers-Kronig relations are used to calculate the index of refraction from absorption coefficient measurements. Both free-carrier depletion and the internal electric field of the p-n junction modulator are shown to affect the measurement sensitivity of this technique. The sensitivity is calculated from experimental results and is dependent on which device, NMOS or PMOS, is under test. These results contradict the predictions of a previous model which was developed using the Drude theory of free-carrier modulation of the refractive index. >