The effect of film thickness on the dissolution rate and hydrogen bonding behavior of photoresist polymer thin films

Previously we have reported on the film-thickness-dependent nature of the dissolution rate of a series of model photoresist polymer resins including: novolac, poly(hydroxystyrene) (PHOST), and bis-trifluoromethyl carbinol substituted polynorbornene (i.e. HFAPNB or hexafluoroalcohol substituted polynorbornene). It was found that the dissolution rate of novolac films displayed very little dependence on the initial polymer film thickness, while the dissolution rates of both PHOST and HFAPNB films displayed a strong dependence on the initial polymer film thicknesses below a critical initial film thickness. This paper presents the results of additional experimental work that was directed at determining the underlying physical cause for the observed variation in dissolution rates with initial film thickness. Fourier transform infrared spectroscopy studies have revealed that the extent of hydrogen bonding in both PHOST and HFAPNB films exhibits a strong dependence on initial film thickness below a critical film thickness. This critical film thickness below which changes in the extent of hydrogen bonding in the film are observed correlates well with the critical film thicknesses below which changes in the film dissolution behavior are observed. In the case of novolac, no substantial dependence in the extent of hydrogen bonding in the films as a function of film thickness is observed. This new experimental evidence suggests that it is the changes in the extent of hydrogen bonding which occur as a function of the initial polymer film thickness which are responsible for the observed dissolution behavior in these materials. Further work is in progress to understand the underlying causes for these differences in the hydrogen bonding behavior of the polymer films.