Wafer 620: porosity vs. position
We’ve been assuming that porosity follows a linear relationship with respect to wafer position, but this turns out to be incorrect. I imaged membranes from five points on wafer 620 and found that maximum porosity does not occur at the outer most sample, as previously thought, but instead at r=3.
Does this provide a better fit between our experimental/theoretical hydraulic permeability data?
When I first looked the plot, it reminded me of the non-uniformity profile of our sputtered a-Si (or oxide) film. Is the non-uniformity in film thickness translating to a non-uniformity in the morphology? What is puzzling is that we see much better uniformity with our TEOS masking oxide. Is it possible that the TEOS process is putting down a chemical layer on the frontside that enhances film uniformity? It may be worth depositing a film on a TEOS wafer and then measuring uniformity with the ellipsometer…
How would the P5000 (PECVD) deposit a “chemical layer” on the front-side, when the tool requires a 6″ wafer carrier for 4″ wafers? The front-side of the wafer is completely isolated by the 6″ carrier during deposition so that would not make sense.
Do you know what the carrier is made of? Funny things can happen with you introduce a plasma…
You saw the carrier. It’s made of another 6″ silicon wafer that has been polished down to house the 4″ wafer. I think this is highly unlikely in explaining pore density uniformity.
I will point out that TEOS has some very strange characteristics. For one, the stress it induces (on the substrate) over time changes rapidly due to the absorbance of local moisture. TEOS is also very porous oxide that usually requires an anneal to “densify”. I think the answer lies (somewhere) in the material properties.
In vacuum, gas can move between the wafer and chuck, but the plasma itself will not. The front side is exposed to something, and experiments can be designed to test this theory.
Another issue is what happens in the sputter system when the TEOS is heated to 450C, well above its deposition temperature. Anything driven out of the film could end up on the front surface.
I would NOT think that either of these effects is significant, and certainly would never guess that they would improve uniformity. However, I don’t think curvature of the wafer is a likely explanation either.
Do we have a similar set of uniformity images across a TEOS wafer? I still would like quantitative data that shows the extent of the uniformity difference between the 2 mask types. We also see improved uniformity of the etch too, right? Are we sure about all this?
By eye, I can see that there is a slight non-uniformity in the TEOS wafers, as well; although by comparison they are much better than this. Who knows, maybe TEOS isn’t so “magical” after all.
I’m not sure if the uniformity of etch is real since we see the same “break-through” pattern for both types: clear in a ring around the center; center; then outside. I do think that the time it takes from the center clearing to the outside is shorter for the TEOS wafers.
A more detailed study is needed to be certain, though.