The laser probe of the scanning laser microscope generates electron-hole pairs in the IC semiconductor material which are separated at inversely-biased pn-junctions and can then be detected as photocurrent at external terminals. This OBIC (Optical Beam Induced Current) effect provides the physical basis for the localization and sensitivity measurements of the undesired latch-up in CMOS devices. The laser probe can also be used to measure logic-level time diagrams at the internal nodes of an IC. In addition, the scanning laser probe can be used to stimulate and localize malfunctions when the IC is operated at its marginal conditions.
"Circuit Analysis In ICS Using The Scanning Laser Microscope", Proc. SPIE 1028, Scanning Imaging, (9 February 1989); doi: 10.1117/12.950347; https://doi.org/10.1117/12.950347