Monitoring photoresist dissolution in supercritical carbon dioxide using a quartz crystal microbalance

New lithographic techniques are being implemented to help further reduce feature sizes in microelectronics. A technique for the development of standard commercial extreme ultraviolet (EUV) photoresists in a carbon dioxide compatible salt (CCS) and supercritical carbon dioxide (scCO(2)) solution is being investigated to reduce line edge roughness and image collapse of high aspect ratio features.(1,2) To understand the kinetics and overall mechanism of photoresist dissolution into the high pressure CCS/scCO(2) solution, we use a quartz crystal microbalance (QCM). QCM measures the frequency changes of the quartz crystal when mass loadings, temperature, pressure, and solution viscosity change. In the last decade, QCM has been used to monitor dissolution of photoresist materials in liquid solutions in real time.(3) The technique has been adapted to high pressure systems, with corrections for pressure and solution viscosity effects.(4) In this paper, QCM was used in high pressure scCO(2) conditions to monitor the dissolution kinetics of the photoresist using the CCS/scCO(2) solution. The frequency changes of the quartz crystal were recorded and corrected for both pressure and solution viscosity to estimate the mass removed as a function of time. The initial photoresist dissolution rates in the CCS/scCO(2) solution at temperatures between 35 degrees C and 50 degrees C and pressures ranging from 3500 psi to 5000 psi are reported. The plots of photoresist removal with time are linear signifying a zero order overall removal rate. The activation energy for photoresist removal at a CO, density of 0.896 g/ml is 76 mJ/mol.