Simulation-based formulation of a nonchemically amplified resist for 257-nm laser mask fabrication

The demand for smaller and more uniform features on photomasks has inspired consideration of a DUV (257 nm) resist process for optical pattern generation. Chemically amplified resists require storage and exposure in carbon filtered environments, and they require post-exposure bakes. Few mask facilities are set up to handle chemically amplified resists commonly used in deep UV wafer fabrication process. Hence, it is appropriate to explore the lithographic performance of non-chemically amplified resist materials for 257 nm laser photomask lithography. Resist characterization and lithography simulation were used to formulate a 257 nm resist from DNQ/novolak materials provided by a commercial resist supplier. Diazonaphthoquinone (DNQ)/novolak resists have not been used for DUV Integrated Circuit (IC) applications mainly due to the low sensitivity and the strong absorbance of the DNQ photoactive compound (PAC) at 248 nm. However, a 2,1,4 DNQ based resist has been characterized that bleaches at 257 nm and inhibits novolak. The photoproduct of the 2,1,4 DNQ PAC is much more transparent at 257 nm than 248 nm. Novolak resin also has an absorbance minimum in the DUV at 257 nm that provides transparency similar to poly (hydroxystyrene). Traditional photoresist formulation requires tedious, iterative, and expensive manufacturing trials. Resist characterization and lithography simulation can be used to relate lithographic performance (resolution, sidewall and process latitude) to resist formulation parameters (PAC concentration, developer concentration, etc.), thereby supporting the formulation optimization. An exposure system using a 257 nm frequency doubled argon laser system has been constructed to study the resist photokinetics. Dill exposure parameters (A, B and C) have been extracted for a 2,1,4 DNQ/novolak based resist. Dissolution rate measurements have been made with a development rate monitor developed at the University of Texas at Austin. Simulation using the exposure and development rate models were used to determine the resist formulation that maximizes the sidewall angle and exposure latitude for isolated resist spaces. Preliminary experiments reveal that a DNQ/novolak resist is capable of resolving 0.30 micrometer linewidths using a 257 nm optical pattern generator.