High energy beam sensitive glasses, HEBS-glasses herein after, used to generate the gray level mask, consist of a low expansion zinc-borosilicate glass, a white crown glass. The base glass can be produced from glass melting just like the conventional white crown optical glasses. The base glass contains alkali to facilitate the following ion-exchange reactions which achieve the sensitivity of the HEBS-glass towards high energy beams, e-beam in particular. After ion-exchange, HEBS-glass is essentially alkali free as a result of the ion-exchange process and the concurrent leaching process carried out in an acidic aqueous solution at temperatures above 320oC. The base glass composition consists of silica, metal oxides, halides and photo inhibitors. Typically TiO2, Nb2O5 or Y2O3 are used as photo inhibitors. The photo inhibitors are used to dope the silver ion containing complex crystals, silver-alkali-halide. These (AgX)m (MX)n complex crystals are the beam sensitive material and the doping of the photo inhibitors increases the energy band gap of the otherwise photosensitive glass.
After the glass is melted, drawn, ground and polished, the base glass plates are ion-exchanged in an acidic aqueous solution containing soluble ionic silver. The ion-exchange process is carried out at temperatures in excess of 320oC for a duration sufficient to cause silver ions to diffuse into the glass plates 3µm, i.e.,(x2 - x1) in the thickness dimension of Fig. 1. As a result, silver ions are present in the form of silver-alkali-halide (AgX)m(MX)n complex crystals that are about 10nm or less in each dimension within the cavity of the SiO4 tetrahedron network
Doping of the glass with the photo inhibitors causes an increased energy band gap, making the ion exchanged glasses inert to UV and actinic radiation of shorter wavelengths as the concentration of the doping with photo inhibitors increases. Nevertheless, the chemical reduction of silver ions in the silver-alkali-halide containing complex crystals to produce coloring specks of silver atoms can be accomplished by exposing the HEBS-glass to high energy beams, i.e., > 10kv electron beams. This property of the material can be utilized to generate the necessary change in transmission for a gray level mask.
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