Analysis of 'Thin Chrome' on photomasks
30 July 1985
"Thin chrome" is a major cause for the rejection of photomasks in the Murray Hill Mask Shop. This memorandum characterizes this phenomenon and identifies a working solution to the problem. The occurrence of thin chrome is associated with the presence of chromic acid in the resist stripping solution, SS-3. The thinned regions are characterized by ~100microns 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 thin chrome.
Thin chrome occurs more frequently on Toppan than on Hoya substrates. However, using TEM, the grain structure of chromium from both Toppan and Hoya was found to be fine and uniform. Observations of the removal of the chromium from the glass substrate suggest that the stress gradient within the Toppan chromium is greater than the Hoya chromium. ESCA revealed two district chemical state of nitrogen present within a Toppan chromium film. A 90C sulfuric acid hydrogen peroxide solution successfully strips e-beam resist without attacking the chrome masks.