Physical description of lithographic processes: correlation between bake conditions and photoresist contrast

An extensive representation of lithographic processes should not be limited to the description of chemical modifications, but should also take into account the variations in physical properties of the film. The origin of these possible variations are shown to lie in the specific film formation technique used in microlithography, namely spin-coating. Deposited from the same solution, layers of different densities exhibiting different properties, can be obtained under various process conditions. The main performance criteria, i.e., absorption, energy-to- clear, contrast, and resolution are investigated as a function of these density variations. The results obtained clearly show that variations in film properties with process conditions cannot only be explained by chemical modifications of the lithographic material. This study tries to point out and quantify, in addition to chemical reactions, the importance of physical properties in lithographic processes.