Wavelength-Invariant Resist Composed of Bimetallic Layers

Abstract

Two layer co-sputtered Bi over In thin films (40 nm/layer) act as a microfabrication resist with many potential applications. Their physical, chemical and optical characteristics change after laser exposures that produce a rapid thermal anneal in selected areas. Unlike organic photoresists, Bi/In is a bimetallic thermal resist whose sensitivity shows a near wavelength invariance for wavelengths from Near IR to UV. The laser-induced patterns are developed by an etch that selectively removes unexposed areas and retains converted ones. The optical density (OD) of 40 nm thick Bi/In films on quartz substrates, for example, changes from 3.3 OD to 0.37 OD in the annealed area. This has enabled the creation of direct-write photomasks for standard photoresist exposures. In this paper, the composition, morphology, and nanostructure of the resist before and after laser processing were studied in order to determine the mechanism of the laser-induced material conversion. AFM, XRD, and TEM show that the as-deposited films are polycrystalline, continuous, but with a rough, island morphology. Furnace anneals in air above the eutectic temperature (150–250°C, 3 hours) result in the formation of the tetragonal phase BiIn with a small degree of oxidation. The island morphology is maintained but there is evidence of melting and recrystallization. Transparency is much lower than after laser annealing. RBS and NRA depth profile analysis show that Bi/In films exposed to laser annealing in air contain a large fraction of oxygen and suggest that the converted film may be a BiIn0.6O6 /Bi0.3InO6 bilayer.

Wavelength invariant Bi/In thermal resist as a Si anisotropic etch masking layer and direct-write photomask material

“…At laser powers above the threshold for reaction the physical and chemical properties of converted Bi/In films change and most importantly for masks it becomes almost transparent compared to the unexposed Bi/In [1]. As covered in section 7 the heat converts the Bi/In into an transparent alloy or alloy oxide [15], while the unexposed area remained bilayer structure. Figure 1 shows the two steps to make a Bi/In direct-write photomask.…”

“…This width is really only the limits of the mask creation program and the laser system, not that of the resist. Although we have not conducted any experiments to find out the smallest feature size possible with the Bi/In film, TEM (Transmission Electron Microscopy) analysis showed that the grain size of the Bi/In film is 150 nm, indicating that the smallest feature can be ~ 150 nm [15]. In order to test the direct write photomask, a Quintel 4" mask aligner with a 365 nm Hg source was used to expose a Shipley SPR2FX-1.3 photoresist coated on a chrome film.…”

“…Recent work by the authors, done in collaboration with Prof. K. Kavanagh at Simon Fraser University and Dr. W. Lennard at University of Western Ontario has explored the structural behavior of the Bi/In films before and after laser exposure [15]. Bi/In films were prepared on different substrates, such as glass and silicon wafers.…”

Creating direct-write gray-scale photomasks with bimetallic thin film thermal resists

“…At laser powers above the threshold for reaction, the physical and chemical properties of the converted Bi/In or Sn/In films change and most importantly for mask applications the films become almost transparent compared to the unexposed [8]. The heat converts the Bi/In into an transparent alloy oxide [14], while the unexposed area remains bilayer structure. Figure 1 shows the two steps to make a bimetallic directwrite gray-scale photomask.…”

“…Recent work by the authors, done in collaboration with Prof. K. Kavanagh at Simon Fraser University and Dr. W. Lennard at University of Western Ontario has explored the structural behavior of the Bi/In films before and after laser exposure [14]. Bi/In films were prepared on different substrates, such as glass and silicon wafers.…”