Comparison of Two and One-Photon Optical Beam-Induced Current Imaging Through the Backside of Integrated Circuits

Optical beam induced current (OBIC) imaging through the back sides of integrated circuits is investigated in the wavelength (lambda) region from 1.15 to 1.26 microns. With a sub-picosecond excitation source and approximately 1 mW at the sample, the two-photon contribution to the generated photocurrent dominates at lambda=1.25 microns and becomes negligible for lambda1.18 microns. One -photon-(1P-) and two-photon-(2P-) OBIC images are very different. A strong contribution of scattered light to the carrier generation in the one-photon case leads to an edge enhancement effect that is entirely missing in the 2P-OBIC counterpart. 2P-OBIC images also show supply-voltage depend intensity steps that are much sharper than the optical resolution permits. The advantages of 2P-OBIC lie in the spatial confinement of the free carrier generation and a more relevant contrast mechanism. Finally, 2P-OBIC promises a substantial increase in spatial resolution because of the quadratic intensity dependence and the ability to use silicon solid immersion lenses, which could provide resolution sufficient for circuits made by deep UV-lithography.