Numerical analyses of the roles of gas phase and liquid phase UV photochemistry in conventional and immersion 193 nm lithography

Abstract

We examine the consequences of 193 nm photochemistry of air and water on resist compositions during exposure. The analysis uses a detailed quantitative kinetic model based on available literature mechanistic data and constructed with in-house simulation tools. In conventional 193 nm lithography, both oxidation of the resist polymer due to the UV photolysis of molecular oxygen, and film interaction with strong acids, formed by photo-oxidation of nitrogen and sulfur species in ambient air, have been proposed to lead to degraded resist imaging. We assess the extent to which such reactions can occur under typical lithographic process conditions, and find that while oxidation is minimal, acid deposition into the top of the resist film is significant and can spread over distances of millimeters. Immersion lithography using 193 nm radiation utilizes a layer of highly purified, degassed water as an index-matching fluid. When water is exposed to 193 nm light, short-lived chemical intermediates are produced by two pathways, neutral and ionic. A quantitative evaluation of this photochemistry during lithographic immersion exposure shows that neither type of intermediate nor photolyzed, leached photoacid generator molecules lead to significant water composition changes, so resist impacts are not likely to be marked. Organic immersion fluids may undergo significant photolysis, however there are insufficient experimental data to assess any potential impacts at this time.