Analysis of thin chrome on photomasks

Abstract

"Thin chrome" is a major cause for the rejection of photomasks during their fabrication process. This paper will characterize this phenomenon and identify a working solution to the problem. The thinned regions in chrome are characterized by -100 |xm wide isolated depressions which are detected by visual inspection under an optical microscope. Analysis of the chromium surface using AES and EDXA showed no chemical contamination or inclusions which could be responsible for the thinned chrome. Thin chrome is observed more frequently on plates from Vendor one than from Vendor two. Observations of the removal of the chromium from the glass substrate suggest that the stress gradient within the Vendor one chromium is greater than the Vendor two chromium. ESC A revealed two distinct chemical state of nitrogen present within Vendor l’s chromium film. However, using TEM, the grain structure of chromium from both Vendor one and Vendor two was found to be fine and uniform. The occurrence of thin chrome has been associated with the presence of chromic acid in the resist stripping solution. An alternative sulfuric acid: hydrogen peroxide solution can successfully strip e-beam resist without attacking the chromium mask.