Structure And Composition Dependence Of The Anisotropy Of The Wet Chemical Etching Of Germanium-Selenium Films

Abstract

ABSTRACT A model for image formation in the Ag2 Se/GexSe<| _x resist systems had been proposed by Tai et al.1 The first step of the process consists of forming a thin latent image of photodoped Ag in the top layer of the GexSe<|_x film. This Ag doped layer then serves as a mask for the anisotropic wet chemical etching of the undoped GexSe-|_x structure with a bicomponent etchant. Anisotropic chemical etching results from an unusual, interlocked, phase-separated columnar structure of the GexSe-j_x film. The formation of such phase-separated columnar structures depends on the composition of the Germanium-Selenium film. We examine here the dependence of film microstructure on film composition and describe the relationship bewtween etching characteristics and the concentration ratios of a bicomponent developer.IntroductionThe Ag sensitized Ge-Se resist systems have been shown to exhibit optical lithographic characteristics that are unsurpassed by any other lithographic material reported to date, including that of the contrast enhancement material (CEM) sold by General Electric Company. A model for image formation in the Ag2 Se/GexSe ^ <j _x^ resist systems had been proposed* As illustrated in Figure 1, the model purposes that during photoexposure, a thin latent image is formed in the GexSe-|_x by photodoping of Ag into the GexSe-|_x. This Ag doped layer subsequently serves as a mask for the anisotropic wet chemical etching of the non-doped GexSe(1-xj structure with a bicomponent etchant. Phillips2 has predicted a phase separated structure in the Ge-Se chalcogenide glass, and Chen et al3 have suggested that the density variations observed in high resolution TEM of the GeSe2 film resulted from a columnar structure of the film.The phase diagram of the GexSe/<|_x) sytem (x<1/3) shows two stable phases, GeSe2 and Sen. In this paper, we will snow that the anisotropic wet chemical development of the Ge-Se resist systems results from a phase-separated columnar structure which is composition dependent. The two different phases of the GexSe(l-x) microstructure can then be selectively chemical etched with a bicomponent etchant.Experimental ResultsGexSe/1 _x% films with x<1/3 were deposited by e-beam evaporation to yield a 2000A thick layer of each cmomposition on to a silicon wafer. Each GexSe(1-x) glass film was then selectively etched with 0.1 M tetramethylammonium hydroxide (TMAHJ and 0.01 M Sodium sulfide (Na2S). The composition of each glass film was determined with X-ray fluorescence1* prior to etching. In addition, the composition of the material removed by each etchant, as well as the etch rate of each etchant, was also determined using X-ray fluorescence. Table I lists the composition of each GexSe(l-x) glass film studied, the composition of the material removed by each etchant, and the ratio of the rate at which GeSe2 was etched by the TMAH to the rate at which Se was etched by the Na2S. The results indicate that TMAH preferentially etched GeSe2 , while the Na2S etched both GeSe2 and Se.*-Present Address: Advanced Micro Devices Sunnyvale, California 94088 52 / SPIE Vol. 539 Advances in Resist Technology and Processing II (1985)