Emission microscopical investigation of the proximity effect in electron beam lithography

Abstract

In electron beam lithography the thin film of electron resist - usually polymethyl methacrylate (PMMA) - is not only exposed to the incident primary electron beam but also to electrons backscattered in the substrate. In contrast to the well focused primaries the backscattered electrons leave, for instance, a silicon substrate as far as 4 μm from the impinging point of the primary beam and therefore contribute to the exposure of the electron resist several microns away. As a consequence the optimum exposure of an array of closely spaced lines or dots is different from that for single isolated structures /1/. We have investigated this proximity effect in two ways: First, we measured the line width as a function of the linear charge density used for exposure. PMMA-films about 150 nm in thickness on a silicon substrate were exposed along parallel lines by a 23 keV electron beam with a halfwidth of about 500 nm.