Determination of residual casting solvent concentration gradients in resist films by a "halt development" technique

The resolution of the microlithography process used to manufacture semiconductor devices is a function of many variables. One interesting and important contributor to resolution is the "surface induction" phenomenon that occurs during development of many photoresist materials. This phenomenon is manifested by a change in dissolution rate as a function of thickness. The top or surface regions of these films dissolve more slowly than the bulk of the film for reasons that are not understood. One popular theory for surface induction is that the variation in rate is caused by a gradient in the concentration of residual casting solvent. However, no study has been performed that directly measured the concentration gradient in residual casting solvent and related that gradient to the extent of surface induction. A ''halt development" procedure has been developed that allows isolation and analysis of thin layers (slices) from resist films. The analysis for concentration of residual casting solvent was accomplished by radiolabeling the solvent and analyzing the layers by scintillation counting. The dissolution rate data was acquired by a multiwavelength interferometry technique. Two resists systems were studied, one with a large extent of surface inhibition and one with a nearly constant dissolution rate. Neither formulation has a significant concentration gradient of residual casting solvent over a range of bake temperatures (70-110 degreesC). Therefore, it can be concluded that concentration gradients in the residual casting solvent alone cannot account for the surface induction observed in this particular resist system. (C) 2001 American Vacuum Society.