Adjustable low NA Quasi-Confocal reflectometry for ultra-high-aspect-ratio micro-structures

Silicon based high-aspect-ratio micro-structures, grooves and holes, are essential for MEMS devices and 3D-ICs. The nondestructive depth measurement is especially critical for the process and even the yield on the final product. However, the increase of aspect-ratio leads to the sharp attenuation of the effective optical signal returned from the bottom, which has become a challenge to the existing reflectance spectroscopy. We propose an adjustable numerical aperture (NA) Quasi-Confocal reflectometry to enhance the collection efficiency of the light reflected from the narrow bottom of high-aspect-ratio microstructure. The aperture diaphragm and pinhole are used for Quasi-Confocal regulation to realize the precise reflection spectrum measurement with adjustable micro-spot and spatial angular frequency. For demonstration, the micro-structures with aspect-ratio more than 56:1 is detected while the linewidth is smaller than 2 μm. And the maximum depth measurable of the homemade optical system is deeper than 300 μm.