Comparative evaluation of mask cleaning performance

Cleaning is one of the most important processes in photomask manufacturing, because the smallest particles may be printable on wafers. Moreover, mask cleaning requirements are stricter than that for wafers because masks are the master image from which all wafers prints will be made. We now face difficult challenges as we enter the 90nm era with 193nm DUV lithography and more prominent use of phase shifting applications. As defect sizes to be controlled in the cleaning process decrease, cleaning performance depends not only on conventional chemical treatments and megasonic hardware, but also on new cleaning methods such as UV/Ozone treatment. We investigated and compared the cleaning performance of UV/Ozone treatment + traditional chemical cleaning methods with standalone conventional wet chemical cleaning methods on glass, chrome, and MoSiON blank surfaces over pattern densities at 70% and 30% clear in the pattern area. Contact angle measurements and wettability tests were performed as well to evaluate cleaning performance results. The cleaning effectiveness with different drying methods on EAPSMs has been also investigated by controlling phase and transmission of KrF EAPSMs to within ±3'and ±0.3% respectively. Overall, it was found that the UV/Ozone pre-treatment combined with the traditional chemical cleaning process results in a better particle removal rate compared to conventional cleaning methods when it comes to removing the smallest mask particles., and it did not adversely affect EAPSM optical properties.