I stripped and viewed samples from wafers 186, 185, 184 and 182. None of these samples had cigarette burns.
Follow this link to the updated summary: cb_report
Previously I had noticed cigarette burns (small ragged pinholes) on samples from wafers between numbers 202 and 216. All of these wafers had been stripped using a full wafer etch, but all of the samples I looked at today were etched one by one.
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Jess – please keep an eye on these samples for a few days, as the formation of the burns may no be immediate. We should also look at these in the SEM to see if there is any copper in the wells.
As long as there is contamination in the wells, single sample stripping will be cleaner, as dipping the entire wafer releases a ton of particles that can settle on the membranes. The best thing we can do is eliminate the contamination, although finding a work-around using single-sample stripping is also a reasonable approach for now.
Can someone check the older samples that were full-wafer stripped to see if any cigarette burns were present? It would be nice to know if this contamination has gotten worse or was just made more obvious by the full wafer strip. We may not have noticed the problem previously, as we were focused on burst strength and flow rates – CBs would only have been noticed as random variations.
Also, I did see a sample from w186 that a cigarette burn defect on it (right at the edge) a week or two ago. It was much less obvious than our recent samples, though. Make sure you comb the edge at high magnification when looking for these defects – they may not be the obvious black CB morphology seen recently…