Nanofabrication of superzone solid immersion vortex lens and subwavelength antireflection surfaces for enhanced scanning laser imaging

Abstract

This paper describes the development of a superzone solid immersion diffraction vortex lens and sub-wavelength antireflection surfaces for analyzing dense integrated circuitry through the silicon substrate. The diffractive optic is used with the scanning laser microscope imaging system at a wavelength of 1064 nm. The silicon substrate consists of wafer grade silicon, 1015 cm−3 P-type, with an integrated memory array at the 90 nm technology node as the target structures. The solid immersion diffractive optic is designed using computer generated holography, and the functionality is simulated using a finite difference time domain numerical method. The blazed phase profile and sub-wavelength structures of the diffractive optic are fabricated using a two step procedure of gray scale gallium implantation with the focused ion beam, followed by the reactive ion etching using CHF3 chemistry. The developed fabrication conditions for the optical structures investigate implant doses ranging from 0 to 1000 pC μm−2 using the 1000 pA beam current, with etch times from 0 to 25 min. Experimental results demonstrate functional optical vortex lenses that achieve selective suppression of signals from target IC structures to enhance the contrast of the surrounding circuitry. To reduce reflection losses, sub-wavelength, antireflection surfaces targeting the 1064 and 3300 nm wavelengths are investigated. Spatial periods of 140 and 500 nm are achieved, with fill factors of 0.3 to 0.4 over the quarter wavelength antireflective height. For resolution enhancement, a second order superzone solid immersion lens with a numerical aperture of 3.4 is fabricated and demonstrates a 1.6× improvement in image resolution over the first order solid immersion lens with a numerical aperture of 1.7, which in turn achieves a 3.5× improvement over no immersion lens.