Carbonization Contact Angles
To supplement the work that Dave’s been doing with carbonization, I’m doing some chemical stability/discoloration tests. The first step of these tests is to measure the contact angles. The following are images of contact angle tests and quantitative data from ImageJ of the angles. These are SC141 chips and they are not rinsed or treated in any way after I receive them from Dave.
There is definitely an increase in contact angle. The RTP only surfaces have ~ the same contact angle as the Carbonized samples. There MIGHT be an increase in contact angle with an increase in temperature, but it’s only 1 trial, so it’s hard to say for sure. This is basically the same trend as I saw here, in which RTP increased the angle about the same as Carbonization (although the untreated contact angles here are much higher than before). Maybe we should look at higher temperatures/putatively thicker C films to see if there is a change in contact angles.
Untreated:
RTP only:
Carbonized (700C, 1LPM):
Carbonized (750C, 1LPM):
Angles:
| Sample | Contact Angle |
| Untreated | 59.116 |
| RTP only | 70.456 |
| Carbonized (700C, 1LPM) | 68.698 |
| Carbonized (750C, 1LPM) | 72.519 |
Why is there a darker bubble inside of each of these droplets?
I think its reflection of something on the ceiling, which looks circular because of the curvature of the bubble.
These are lit from above in that cardboard box setup. It might be a reflection from the light fixture above the samples. Also, the camera was really close to the samples, so it might a reflection from the camera.
Why is the RTP treatment alone increasing contact angle? Residual acetylene?
It’s possible that carbon is being re-deposited off the walls of the susceptor during rtp-only treatment, but I’m not seeing a wrinkling of the film post-rtp like I do post-carbonization; so if it is a deposition it’s not enough to change the stress in the film. What about some kind of surface chemisty modification at the high temperature?
I doubt there is extra acetylene. Did anyone measure contact angle following RTP in the past? Has it always caused an increase in contact angle?
I suspect that we are reorganizing the surface a little bit, changing the nanotexture or surface groups, and therefore changing the contact angle and dissolution properties. Condensation of a contaminant as the samples cool is another possibility, but with a good flow of clean Ar, I have my doubts.
The link in this post goes back to the previous study which showed an increase in contact angle after RTP-only, so this isn’t the first demonstration.
If our theory is that RTP is “densifying” the native oxide, couldn’t RTP be taking away some surface oxides (negative charges) and forming more Si-O-Si bonds or even O-O bonds? I’m guessing the atomic composition of the surface doesn’t change much due to RTP, so I’m not sure how we could confirm this.
I checked the link, and I think that was still from a time when we were carbonizing. Is there any contact angle data from before acetylene was introduced to the RTP?
I’m not sure if I like the term “densifying” anymore. Perhaps “stabilizing” is a better word for converting unstable bond structures to more stable ones. I don’t think O-O is very common, but certainly Si-O-Si is all over the literature. There are lots of FTIR studies on porous silicon that could shed some light on this. I think generally, the argument is that the surface is loosing Si-H passivation. It can be displaced by bridging Si-O-Si or Si-OH terminations or a mess of partially unbalanced valences. I have not looked at these papers in quite a while, though.