A Novel Photooxidative Scheme for Imaging at Polymer Surfaces

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Abstract
A novel dry-developed single-layer resist scheme has been used to form submicrometer patterns in thick (>1 μm) polymer films exposed with deep-UV light. Hydrophobic polymers such as halogenated or chloromethylated polystyrenes or analogous poly(vinylbiphenyls) undergo oxidation to form hydroxyl and carbonyl groups when exposed in air with 248.4 or 193 nm light. The hydrophilic hydroxyl and carbonyl groups in the exposed regions bind water to the polymer surface via hydrogen bonding. In contrast, the unexposed hydrophobic regions that lack equivalent hydrogen bonding functionalities do not bind water at the polymer surface. Treatment of the patterned (exposed) films with TiCI4 converts the adsorbed water on the exposed regions into TiO2 which protects the underlying organic film from oxidative removal during a subsequent oxygen reactive ion etching step, thus affording negative tone images. The sensitivity of the polymer depends on the type, amount and the position of halogen on the aromatic group. Optimum results were obtained for chlorinated polystyrenes exposed at 193 nm where absorbance is very high (sensitivity 6-32 mJ/cm2, γ > 2.0). Patterns having submicrometer features were made with 193 and 248.4 nm light, but results obtained from 193 nm exposure were somewhat inferior than expected because of the exposure tool limitations. Exposure of the same polymers at 248.4 nm using a deep-UV stepper has provided patterns with ~0.4 μm resolution in 1.2 μm thick films (sensitivity 200 - 300 mJ/cm2, γ > 1.5) in spite of their very weak absorption at 248 nm (ε <200 I/mole-cm). Patterns made this way presently have some line-edge roughness and background residue.

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