Nicole Deschamps - McMaster University

Placement: University of Ottawa
Supervisor: Dr. Tito Scaiano

Measurement of Acid Diffusion in Thin Polymer Films Using Laser Flash Photolysis

With the miniaturization of computer chips, the diffusion of acid in photoresist films becomes a limiting factor in the fabrication of chips with patterns containing submicron details. Two time-resolved methods to measure acid diffusion in thin polymer films were developed. In both methods, pH sensitive dyes suitable for non-aqueous systems (such as polymers) were used as acid sensors. These dyes are ideal acid sensors as they exhibit a large bathochromic shift upon protonation.

The first method employed steady state absorbance spectroscopy to measure the rate at which acid leaves a thin polymer film. The change in absorbance of the monoprotonated form of a pH sensitive dye (starting from the diprotonated form) in PVP (polyvinylphenol) films containing varying concentrations of an organosulfonic acid was monitored during a 'soft bake' (90 ^oC for 10 minutes) as acid left the film. Calibration curves relating the absorbance of the monoprotonated form of the dye to the acid concentration in the film were obtained. This allowed for the determintion of the rate at which acid left the film. The method and variations of this method are currently being used by members of Professor Scaiano's group.

The objective of the second method was to measure the diffusion of acid within a thin polymer film by monitoring the growth of protonated dye subsequent to the photo-induced generation of acid. The compounds used which generate acid upon irradiation are called photoacid generators (PAGs). In order to gain any significant information regarding the diffusion of acid in polymer films using this method, the growth of the protonated dye must be diffusion-controlled. Preliminary measurements of the protonation of two dyes in acetonitrile were performed and the rate constants obtained (~10⁹ M^-1s^-1) strongly suggest that the diffusion of acid is the rate limiting step. The condition to measure the protonation of dyes in PMMA (polymethylmethacrylate) films (such as the solubility of available PAGs and dye, excitation wavelength, monitoring wavelength, appropriate PAG and dye concentrations, and many other conditions) were optimized. Accurate rate constants for the diffusion-controlled protonation of dyes in PMMA were not obtained due to the unavoidable pre-exposure of the films to radiation. The pre-exposure of the film caused the generation of acid and hence the formation of protonated dye, which absorbs at the excitation laser wavelength. The observed kinetics using this method was therefore the superposition of two processes: 1) the protonation of neutral dye subsequent to acid generation, and 2) growth of ground state protonated dye (by the deactivation of excited state protonated dye).