Abstract
We demonstrate a through the substrate, numerical aperture increasing lens (NAIL) technique for high-resolution inspection of silicon devices. We experimentally demonstrate a resolution of 0.2 micrometers , with the ultimate diffraction limit of 0.14 micrometers . Absorption limits inspection in silicon to wavelengths greater than 1 micrometers , placing an ultimate limit of 0.5 micrometers resolution on standard subsurface microscopy techniques. Our numerical aperture increasing lens reduces this limit to 0.14 micrometers , a significant improvement for device visual inspection (patent pending). The NAIL technique yields a resolution improvement over standard optical microscopy of at least a factor of n, the refractive index of the substrate material, and up to a factor of n 2. In silicon, this constitutes a resolution improvement between 3.6 and 13. This is accomplished by increasing the numerical aperture of the imaging system, without introducing any spherical aberration to the collected light. A specialized lens made of the same material as the substrate is placed on the back surface of the substrate. The convex surface of this lens is spherical with a radius of curvature, R. The vertical thickness of the lens, D, should be selected according to D equals $ (1 + 1/n)-X and the substrate thickness X.
