Nonlinear photoresist films fabricated by coevaporation of As ₂S₃ glass and silver

Silver metal evaporated below or above a layer of AS₂S ₃ glass are known to propagate under action of light within the glass layer and strongly modify its photochemical properties. We demonstrate that when silver is co-evaporated together with the glass, and within a certain range of silver concentration, this property is conserved locally. This leads to a new kind of photosensitive effect in the chalcogenide glass with completely new photoresist properties such as giant contrast and superlinear dissolution. These new properties make silver doped AS₂S₃ glass an ideal material for the fabrication of three-dimensional structures with sub-micron feature size. The effect is explained within a percolation model. The advantages of superlinear photoresists, especially for the case of maskless photolithography, are shortly discussed.