Contact angles on EA treated pncSi

I etched 12 pncSi chips and gave it to Chris for EA treatment. I wanted to test 3 storage conditions: gel pack, vacuum container, and water, for total of 4 days including the day of coating as day zero. Unfortunately I got the chips 3 days after the EA treatment, so the time points begin from day 4 onwards till day 7.

Initial measurement of the contact angles revealed that the surfaces were pretty hydrophobic (CA ~ 60 ish), and this value remained more or less the same for all the chips stored in gel pack and vacuum container for all the next 3 days. Surprisingly, the samples kept in water became positively hydrophilic (CA ~ 38), after keeping it immersed in DI water for 1 day. At first I thought it might be due to the water monolayer sitting on the top of pncSi, which is making it hydrophilic. Hence I dried it using nitrogen and re-measured the CA. Still hydrophilic (CA ~ 36)! Still skeptical, I kept it open in atmosphere and allowed it to dry once again for about 2 hours, and remeasured CA. Again CA ~ 36. So I kept it exposed to atmosphere for another 2 days, and measured CA on each of those two days. 24 hours after removing it from water, I found CA was 41 ish, while 48 hours later it was 45 ish. Note that although these values are higher than what is expected from hydrophilic coatings, these values are still much impressive that those samples kept in dry conditions, or in ambient humid conditions. Implying that EA coating, when exposed to water, may be more hydrophilic than dry.

In order to see if this is a phenomenon exhibited by pure pncSi alone, I etched another 4 chips and am repeating the above experiment. These chips haven’t seen any EA or PEG and are perfectly hydrophobic due to BOE. The data from this experiment will tell us if the above observations are virtue of 4 day old EA coatings or bare pncSi itself.