Novel Photoimaging Schemes at Polymer Surfaces: Their Role in High Resolution Lithography

Abstract

ABSTRACTPolymer surfaces play essential roles in many technical arenas, but their importance in imaging science and technology has only recently been established. As optical lithography for integrated circuit applications pushes below 0.5μm feature sizes, ever shorter wavelengths and lenses with larger numerical aperatures are required. These narrow the depth-of-focus to such a degree that it eventually becomes less than the substrate topography, resist film thickness and wafer nonuniformity contributions to focus placement. Thick single-layer, surface-conforming resists will not be able to record the aerial image accurately.Surface imaging of planarized single-layer or bilayer organic films offers a means for minimizing the depth-of-focus constraints. This paper outlines this general concept which includes radiation-induced chemical changes in the surface and near-surface regions, amplification of these events by gas-solid reactions with inorganic and organometallic agents and plasma development using an oxygen plasma. Two approaches to surface imaging are discussed. The first employs Plasmask® resist, a very absorbing diazonaphthoquinone-functionalized novolac resin which is exposed and functionalized in the topmost several hundred nm of film. According to Coopmans, Roland and coworkers [14] and Pierrat, et al., [22] two tones are possible depending upon the processing. The present study reports results obtained for both processing modes using 248.4 nm lithography.The second approach involves imaging at the surface and utilizes one of the oldest polymer photoreactions, photo-oxidation, in the imaging step. Hydrophobic aromatic polymers are first irradiated in air to give hydrophilic groups. These sorb water selectively on the hydrophilic areas. The water is reacted in a separate step with an inorganic or organometallic compound such as TiC14 to give a metal oxide film (TiO2) on the exposed areas. Development with an oxygen plasma gives negative tone patterns because TiO2 reduces the etching rate by a factor of ∼ 500 in the exposed regions. Imaging is dependent upon the polymer structure, among other things, and is optimized at shorter wavelengths likely to be used in future exposure systems. From the present results we feel that surface imaging resists may realize 0.25 μm resolution at 193 nm and that <0.10 μm resolution may be achieved using x-ray radiation.