Optical probing of internal signals in silicon ICs

After a short discussion about the current trends in internal probing techniques for chip verification or single device characterisation, a charge-sensing optical probing system using a 0.8 mW, 1.55 /spl mu/m continuous wave He-Ne laser, is described. The method is based on the local refractive index modulation by the electrical charge density variation induced by external signals. The measurement difficulties are essentially due to the set up alignment and to noise. In particular the sensitivity of the measurement system depends mainly upon the noise level in the detector system and on the transfer function of the probing system. To overcome the difficulties connected to noise, a digital filtering was adopted to process the signal at the output of the detection system.