The historic trend toward smaller working feature size within the semiconductor industry is making increased demands on the exposure tools. More and more, exposure tools are expected to provide lithography at and below the 1 pm level. To meet this requirement the technology of optical instruments must stretch its limits to stave off such competing technologies as Electron Beam and X-ray. The theoretical resolution for an optical system is proportional to the numerical aperture of the projection optics and inversely to the wavelength of the illumination used. Reducing either quantity will lead to increased resolution of the exposing system. The use of exposure tools which are mainly composed of reflecting elements provides the opportunity to utilize short wavelength UV, and thus to gain increased resolution, without redesigning the projection optics system. Simultaneously, the option of designing for high numerical apertures is not precluded by the use of reflective systems. This paper describes how submicron resolution is achievable with currently available projection equipment by a simple filtering of the illumination source and utilizing deep UV resists. Data is provided on resolution, resist profiles, step coverage, and wafer throughput.
A. W. McCullough,
"Deep Ultraviolet High Resolution Lithography", Proc. SPIE 0394, Optical Microlithography II: Technology for the 1980s, (7 November 1983); doi: 10.1117/12.935128; https://doi.org/10.1117/12.935128