Focusing anomalies in the vicinity of dielectric interfaces

Abstract

Abstract-We investigate the interface effects on high numerical aperture focusing of linearly polarized illumination. Theoretical and experimental demonstration is conducted in subsurface backside microscopy of silicon integrated circuits. I. INTRODUCTIONThe need for high resolution imaging necessitates the use of high numerical aperture (NA) optical systems. Solid immersion lens (SIL) microscopy is a diffraction limited imaging technique that provides high NAs taking advantage of the large optical index n of the immersion medium as well as the increased excitation and collection angles [1]. Numerical aperture increasing lens (NAIL) microscopy is an application of the SIL technique to integrated circuit (IC) imaging [2]. A silicon NAIL placed on the backside of a silicon substrate effectively transforms the NAIL and the planar sample into an integrated SIL. This imaging scheme allows for high resolution backside imaging through the substrate of an IC which is often necessary because opaque interconnect metal layers hinder frontside optical microscopy [2], [3], [4]. Focusing under the high NA conditions provided by the NAIL enables us to observe the effects of polarization and dielectric interfaces in the focal region. Enhanced contrast and ellipticity of the focal spot induced by the linearly polarized illumination are common results for this geometry [5], [6], [7]. In this study, we demonstrate that the longitudinal focusing parameters are also different for the directions parallel and perpendicular to the polarization direction in the vicinity of a dielectric interface. We theoretically show that the tightest spot on the interface occurs at two different focusing depths for two orthogonal directions. Experimental results validating the theoretical predictions are exhibited employing confocal microscopy of ICs.