Investigation of the performance of 248 nm excimer laser assisted photoresist removal process in gaseous media by response surface methodology and artificial neural network

Abstract

Photoresist removal by Excimer laser in different gaseous media viz., hydrogen, helium, argon, nitrogen and air is studied under different parameters viz., laser pulse energy, pulse repetition frequency (PRF) and gas flow rate. Totally 180 experiments (36 experiments for each gaseous medium) were conducted based on the full factorial design. Statistical analysis of data by 3way ANOVA (ANalysis Of VAriance) was carried out to investigate the main effects on photoresist removal process. Photoresist removal rate is observed to be significantly higher in case of hydrogen gas medium compared to other gaseous media studied. Hence, another 48 set of experiments (based on full factorial design) was carried out in hydrogen gas medium. These experimental data were used for modeling the photoresist removal process. Three key operating parameters (i.e. laser energy, pulse repetition rate and hydrogen gas flow rate) were selected as independent variables, and the amount/depth of photoresist removed was considered as the dependent variable (response). Response Surface Methodology (RSM) and Artificial Neural Network (ANN) models were developed based on the experimental design approach to predict etch depth. The predictive capacity of the two models was compared. Analysis of the results showed that the operating parameters can also influence each other’s optimum value. Higher photoresist removal rate observed in hydrogen medium may be attributed to possible Laser Assisted Chemical Reaction (LACR) of hydrogen gas with photoresist polymer. A mechanism has been proposed to explain the interaction of hydrogen molecule with photoresist during the laser irradiance, resulting in generation of hydrogen radicals, which may subsequently undergo additional bond-breaking process of olefinic functional groups (having π bond), along with laser bond breaking process. To the best of the author’s knowledge, this is the first study providing information of LACR of hydrogen gas with photoresist polymer, resulting in higher photoresist removal rate. The LACR of hydrogen gas with photoresist polymer, being a cold process, has potential application in semiconductor industry, where researchers are trying to exploit the process of photoresist removal using hydrogen radical generated at 2000 °C by tungsten hot wire.