Effect of Particle Contamination on Extreme Ultraviolet (EUV) Mask and Megasonic Cleaning Process for Its Removal

Particle contaminants on EUV mask surface can cause CD error during EUV lithography process. These contaminants should be removed from EUV mask. The critical size of defect which can cause 10 % CD error is about 23~38 nm with various materials. Among them, critical defect size of SiO2 is about 30 nm. To remove this 30 nm fine silica particle, megasonic cleaning was adapted with various megasonic frequencies and with various cleaning solutions such as dilute alkali solution and gas dissolved water. At high megasonic frequency (2 MHz), though bubble cavitation effect is decreased, cavitation threshold value is higher even with the addition of gases into cleaning solution. As a result, higher particle removal efficiency of 30 nm silica particles and lower damages on PR pattern is achieved at 2 MHz megasonic cleaning in a cleaning solution of H2 gas and dilute NH4OH as compared with 1 MHz megasonic cleaning.