Noninvasive optical probe for detecting electrical signals in silicon IC's

A 1.3 micrometer optical probing system was developed to detect electrical signals in silicon integrated circuits. Free carriers within integrated active devices perturbed the refractive index of the material and a Nomarski interferometer was used to sense this perturbation. Typical charge-density modulation in active devices also produced a substantial index perturbation and this led to the use of an InGaAsP semiconductor laser to observe real time 0.8V digital signals applied to a bipolar transistor. These signals were detected with a signal-to-noise ratio of 20 dB in a system detection bandwidth of over 200MHz.