Fabrication of channeling masks for Ion Projection Lithogrphy

Abstract

For sub-0.5 $mu$m feature sizes in VLSI circuits, Ion Projection Lithography shows good promise. One of the key components for this type of lithography is the availability of a suitable mask. Present work involved the fabrication of channeling mask using silicon, where very thin silicon allows ions to go through and thick silicon or metal deposited on silicon would block the ions. For transmission, advantage is taken of channeling of ions through preferred orientations in single crystal substrates. For making the masks, two different routes were taken. In all-silicon mask, the patterns were etched by reactive ion etching and then the other side of silicon was thinned by electrochemical etching using n/n$sp+$ epi-silicon. Using a different approach for making mask, nickel patterns were deposited on silicon by lift-off technique and the back side was then etched by electrochemical method by applying a reverse bias at the p-n junction built into the wafer. A brief study was also done of a system made up of MBE grown epi-layers of GaAs/Al$sb{rm x}$Ga$sb{rm 1-x}$As system. Film making and pattern generating techniques were studied separately. The techniques were then integrated to make the masks. These masks were compared with respect to film qualities and pattern resolution. Some ion exposures were done with these masks using PMMA as resist and protons as exposing radiation. The results of these studies are also included here.