Comment stsetch SNF 2010-11-03 12:15:19: grass only for > 1-2 hour etches

sorry, i wasn't perfectly clear in my grass problem report. the grass first appears at the edge of the wafer after about 1.5 hours of etching and then steadily expands until it covers the entire wafer at about the 4 hour mark. the exact time likely varies from user to user, but the grass problem is unlikely to appear for short (< 2 hour) tests. the problem suggests that the etch rate decreases over time for some reason.