Effect of Surfactants in Functional Water on EUV Mask Cleaning in Megasonic Cleaning System

Abstract

Due to the pattern damage problem, the applied frequency of Megasonic is constantly increasing from 1 MHz to 5 MHz in the semiconductor cleaning process. The main motivation for increasing frequency is that it will reduce the transient cavitation to avoid destroying the sensitive pattern of the mask. However, the increase of frequency will also reduce the particle removal efficiency drastically, in turn, suppresses the overall cleaning performance. Surfactant addition to the cleaning formulation may change the cavitation exceedingly to meet the requirements. Generally, three types of surfactants (anionic, cationic, nonionic) are used for the cleaning application. However, cationic are less preferred due to contamination issues from the surfactant. Usually, bubble characteristics are studied in the presence of surfactants in terms of bubble size growth through rectified diffusion. So, in this study, the surfactant effect on acoustic cavitation has been studied. Also, a suitable hypothesis is required for the performance results based on visual observation of bubble cavitation characteristics. It is expected that the nature of surfactant would play a key role in cleaning performance. To evaluate its effect, one general surfactant (SDS-anionic) and other different types of surfactants (S1-anionic, S2-nonionic, S3-nonionic, S4-cationic, BASF, Germany) were used to do the evaluation. Furthermore, to explain the phenomenon (PRE, PD) after cleaning performance, the mechanism of a bubble in the sonicated field was suggested. The hypothesis could be explained with visualized bubble dynamic result. Also, High-speed camera is one of the efficient equipment to confirm bubble visually. Therefore, in this study bubble size, dynamics and population with editing parameters was observed.