Adsorption and Segregation of Small Molecules in Polymer Thin Films

Olga Sanchez-Galofre and Erin Jablonski. Chemical Engineering, Bucknell University, 701 Moore Avenue, mail box C-3840, Lewisburg, PA 17813

Photolithography is the current method by which integrated circuit patterns are created. Typically, a chemically amplified photoresist, which contains an acid-labile polymer, a photo-acid generator (PAG) and a base quencher, is used as the imaging layer. A silicon wafer coated with this formulation is exposed to UV light through a photomask, in order to decompose the PAG according to the circuit pattern. A postexposure bake is used to alter the solubility and further catalyze deprotection reaction and improve acid diffusion. By immersing the coated wafer in an aqueous base developer, where the exposed regions are soluble, the circuit image is patterned on the photoresist. Finally, an etching process is used to transfer the pattern from the photoresist material to the underlying substrate; therefore, any defect in the photoresist pattern is also transferred.

Electronic industry progress strongly depends on the feasibility to pattern smaller feature sizes. This can be accomplished by decreasing the exposure wavelength or increasing the numerical aperture. The latter one appears to be the most promising technique to meet the semiconductor industry demand, since current photoresists are too absorbing at the lower wavelengths necessary to achieve nanoscale critical dimensions. Immersion allows the refractive index of the medium between the lenses and the imaged wafer to increase, thus amplifying the numerical aperture. Hence, researchers and the semiconductor industry are currently focused on immersion photolithography; next generation efforts will involve both immersion and decreased exposure wavelength.

One of the major drawbacks of immersion photolithography that has to be overcome is the leaching of some small molecules out of the film, causing some changes in the film formulation. The purpose of the proposed work is to gain a deeper understanding of the leaching and segregation processes during immersion photolithography. Previous studies looked into the influence of small molecule segregation on the surface chemistry using near Edge X-ray Adsorption Fine Structure (NEXAFS). This project aims to relate these results to the surface morphology of the polymer thin film, as determined using Atomic Force Microscopy (AFM).