Light sources using energy transfer from excimer to line radiation

Abstract

Garching, GermanyD. E. Murnick, H. DahiDepartment of Physics, Rutgers University101 Warren Street, Newark, New Jersey 07102, USAABSTRACTEfficient energy transfer between neon excimer molecules and hydrogen has been found. A small, high gas density lightsource has been developed, emitting entirely on the hydrogen 2p-l s transition at I 21 .567nm (Lyman-a). Light outputdensities of lOW/cm2 are obtained. Electron beam energy conversion efficiencies of lO% have been measured.Keywords: Neon excimer, Lyman-ct radiation, excimer lamp1. INTRODUCTIONIn this article the development of a bright, efficient light source, emitting hydrogen H (2p-ls) line radiation in the vacuumultaviolet spectral region is described.Qualities important in such a light source include spectral output and power, brilliance and efficiency. The spectralcomposition is most significant for many applications. Lasers, of course, have high brilliance but for many applications suchas photolithography coherence and interference effects can be a problem. To circumvent the interference problem, bright highpower spontaneously emitting light sources are disirable. In this class of light sources, high efficiency, high specific powerdeposition, short andlor tunable wavelength, and small emitting bandwidth usually cannot be met with one device.Due to their relative high power efficiency and wide selection of the emission spectra by choosing the appropriatephosphor material, fluorescence lamps have found wide application for lighting devices. Emitting mainly the mercuryresonance lines these lamps also are used in photolithography steppers in semiconductor industry. To achieve high resolutionin photolithograpy applications, monochromatic, short wavelength light has to be used. One access to shorter wavelenghts isexciting the resonance lines of rare gases', resulting in very high efficiencies2. The disadvantage, however, is the lowpressure, required for efficient production of resonance line radiation, and thus a quite low specific output power. As bright,point like light sources high pressure arc discharges are widely used e.g. in xenon arc lamps. Arc lamps, however, have thedisadvantage of a relative low efficiency and broad spectral bandwidth, due to high temperatures and large optical densities inthe arc zone. A recent approach combining high pressure, and thus high power densities, with high efficiency and shortwavelengths of resonance lines, is using the formation and radiative decay of excimer molecules. Excimers, here mainly raregas or rare gas halide excimers, are weakly bound molecules4 formed by an excited rare gas atom together with a ground stateatom. Due to the low binding energy of typically less than one eV, and the relatively high potential energy of the rare gasresonance levels, special excitation schemes, providing high excitation energy in a low temperature, high pressure gas have tobe used. Since the first excinier laser5, excitation schemes, using high voltage, high current pulsed beam facilities have beenused to produce pure rare gas excimer laser radiation. For lower excitation power, as necessary in rare gas halide lasers orexcinier lamps, the afterglow regime of pulsed discharges is used, providing the conditions necessary for excimer formation.Recent commercial excimer lamps are barrier discharge lamps68 or devices using special glow discharges9. Pulsed x-raybeams'0, synchrotron1 ' and laser radiation'2, as well as heavy ion beams' or supersonic cooling'4 have also been used forexcimer research.Here a novel type of a small, versatile excimer lamp, using low energy ( 1 5keV) electron beams, sent through a thin, strongand vacuum tight ceramic foil into a gas target is described'. The technology is described in detail in ref. 16.For broad band vacuum ultraviolet (VUV) lamps, as well as for excirner lasers of pure rare gases, the emission on the so