Understanding fundamental mechanisms of photoresist dissolution



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Fabrication of microelectronic devices relies upon the photolithographic process for patterning devices. A key step in this process is the selective, aqueous base dissolution of exposed regions of a photoresist polymer film, yet this step is not completely understood at a fundamental level. The most successful model for photoresist dissolution has been the critical ionization dissolution model. The basic premise of this model is that a critical fraction of monomer units of a given polymer chain must be deprotonated (ionized) in order to render that chain soluble. This work is an extension of the CI model, in which subtle improvements (such as the inclusion of Coulombic forces) have been made to the lattice based CI dissolution model. The model has been used to investigate formation of surface roughness and surface inhibition during dissolution of photoresists. Surface inhibition has been investigated experimentally to determine the fundamental mechanisms of this phenomenon. A “Halt Development” technique was used to measure concentration gradients in resist films. It was determined that no significant concentration gradients of residual casting solvent, low molecular weight chains, photoactive compound, or polymer density were present, and thus were not responsible for surface inhibition. Several other theories were tested, including interfacial gel layer formation, surface oxidation, and roughness effects. The best explanation for surface inhibition (for the novolac polymer of interest) was derived from the CI model as a combination of roughness and pKa effects. Real time spectroscopic interferometry and ellipsometry were used to characterize photoresist dissolution, with a focus on interfacial gel layer formation. Within the resolution limits of the techniques (10-20 nm), interfacial gel layer formation was not observed in a series of phenolic polymers, suggesting that this assumption of the CI model is valid for most phenolic polymers. Formation of surface roughness during dissolution was characterized by AFM and compared to model predictions.