Equilibrium sorption and rate of diffusion of water into photoresist thin films

The equilibrium resist film water content at various water partial pressures and the rate of diffusion of water into photoresist thin films was investigated for three model resist polymers: (1) a traditional novolac matrix, (2) poly(p-hydroxystyrene), and (3) bis-trifluoromethyl carbinol substituted polynorbornene. A quartz crystal microbalance (QCM) was used to measure the mass of water added to resist films exposed to environments of differing relative humidity. All three polymer systems absorbed significant quanitities of water at 100% relative humidity with PHOST absorbing the most (9.8 wt.%) followed by the polynorbornenes (5 - 8 wt%) and the novolacs (2 - 3 wt%). The diffusion of water into the polymer films was observed to follow Fickian diffusion behavior initially (Mt/Ms < 0.6) followed by behavior indicative of concentration dependent diffusion at large water uptake values. Finally, interdigitated electrodes were utilized to estimate the impact of varying humidity upon the net dielectric constant of the resist films. A linear relationship was observed for measured capacitance versus water uptake for all polymers measured and a power law dielectric mixing rule was found to properly describe the effective dielectric constant of the water/polymer mixtures.