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The mechanism of developing an extreme ultraviolet (EUV) commercial photoresist with supercritical carbon dioxide (scCO2) and a CO2 compatible salt (CCS) solution was studied. The cloud point of CCS in CO2 and the pressure at which the photoresist dissolves in CCS/scCO2 were determined for temperatures between 35 and 50 degrees C. For this temperature range, it was found that the CCS cloud point ranges between 11.2 and 16.1 MPa, while the photoresist dissolution point ranges from 15.5 to 21.3 MPa. The kinetics of the CCS/scCO2 development was modeled using a simplified rate equation, where the rate-limiting steps were photoresist dissolution and mass transfer. The effects of temperature, mass transfer, pressure, and CCS concentration on photoresist removal rate were further explored experimentally using a high-pressure quartz crystal microbalance (QCM). Increasing temperature (35-50 degrees C) at a constant fluid density of 0.896 g/mL was found to increase the removal rate following an Arrhenius behavior with a photoresist dissolution energy of activation, Ea, equal to 79.0 kJ/mol. The removal was zero order in CCS concentration, signifying photoresist phase transfer, photoresist mass transfer, or both were rate limiting. Mass transfer studies showed that circulation enhanced the photoresist removal rate, but that the mass transfer coefficient was independent of temperature from 35 degrees C to 50 degrees C. In pressure studies, increasing pressure (27.6-34.5 MPa) at a constant temperature of 40 degrees C increased the removal rate by enhancing the fluid density, but at 50 degrees C increasing pressure had little effect on the removal rate. When the total CCS concentration was in large global excess over the number of Bronsted acid groups in the polymer (2400:1 at 5 mM CCS concentration), the mass of photoresist removed varied linearly with time. At lower CCS concentrations but still in global excess of the number of Bronsted acid groups, the photoresist removal slowed (0.5 mm CCS, approximately 240:1) or was prevented (0.03 Mm CCS, approximately 15:1) due to partitioning of the CCS between the CO(2)-rich phase and the film. The CCS partitioning into the resist was found to decrease with increasing temperature, revealing an enthalpy-driven CCS absorption.

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 o C and 50 o 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 2 density of 0.896 g/ml is 76 mJ/mol.

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