| A subsurface microscopy technique that significantly improves the light-gathering, resolving, and magnifying power of a conventional optical microscope is presented. The Numerical Aperture Increasing Lens (NAIL) is a plano-convex lens placed on the planar surface of an object to enhance the amount of light coupled from subsurface structures within the object, to a conventional optical microscope. In particular, the collection of light at angles beyond the critical angle increases the limit on numerical aperture, from unity for conventional subsurface microscopy through the planar surface of the object to the refractive index of the object for NAIL microscopy. The aberrations associated with conventional subsurface microscopy are also eliminated by the NAIL. Consequently, both the amount of light collected and diffraction-limited spatial resolution are improved beyond the limits of conventional subsurface microscopy. A theoretical optical model and experimental procedures for imaging structures below the planar surface of an object, both without and with a NAIL are presented, and the results are compared.;The NAIL technique was experimentally demonstrated in two Si integrated circuit (Si IC) failure analysis applications. Since the spatial resolution of conventional failure analysis microscopy is insufficient for current Si IC technology, the improvements using a NAIL, due to the high refractive index of Si, have significant implications for the industry. The NAIL technique was first applied to near-infrared inspection microscopy. By realizing a numerical aperture of 3.4, a lateral spatial resolution of better than 0.23 mum and a longitudinal spatial resolution of better than 1.7 mum were experimentally demonstrated at a free space wavelength of 1.05 mum. The NAIL technique was then applied to thermal emission microscopy. By realizing a numerical aperture of 2.9, a lateral spatial resolution of 1.4 mum and a longitudinal spatial resolution of 7.4 mum were experimentally demonstrated at free space wavelengths up to 5 mum. To the best of my knowledge, these results are spatial resolution records for their respective applications. |
