Tuesday, July 23, 2013

False Positives

A few weeks ago I was able to travel to Tulsa to try some new E-beam lithography development procedures and the results were very promising.  The new procedures, both of which used much shorter development times, were different in that one was at room temperature and the other was at -10 degrees Celsius.  The results showed 20 nm trenches and even sub-20 nm trenches in some areas.  At first there was some trouble viewing the samples because when they were sputtered with gold and viewed the PMMA shifted heavily and the movement could be seen to happen in real time under the SEM.  This lead to the theory that if the V shape, which I have discussed in previous posts, was occurring before the sputter coating then perhaps vertical walled trenches were not having the side walls coated effectively.  The samples were brought back to OU and placed in a rotating thermal evaporator and then viewed.  The PMMA was motionless, otherwise moved very slowly, and the trenches were small in width as can be seen in Figures 1, 2 and 3 .  One side of the wheels were larger than the other which lead me to believe that there may have been a stigmation issue.  
However, when trying to replicate the results just recently I saw the larger line widths between 30 and 50 nm that I have gotten in the past, seen in Figure 4.
                          When discussing with Mr. Nabity he thinks that the wedges of the wheels may have shifted causing one side to be smaller in width than the other and not just a stigmation as I originally thought which is supported by the left side of the wheel being wider than the right, shown in Figure 2.  However, because all wheels in the exposure are shifted is seems that it is more than just shifting as I thought if shifting had occurred it would have only been one or two wheels but not all of them.  Attempts will continue to improve the resolution and straight wall profile of the trenches but much more focus will be put on imaging micelles using AFM, which is still moving along slowly.
           I received some free samples from an AFM probe company and have been trying to coax high resolution images out of them but have been having some difficulty.  Part of the problem, I believe, is that all of the literature I have read states that they used cantilevers with very low resonant frequencies and force constants, which makes sense because I am trying to see "soft" things (micelles).  However, some of the probes are so flexible is seems that even after centering the laser off of the cantilever onto the photodetector they are prone to drift heavily.  Also, the autotune function of the AFM has trouble picking up the low resonant frequencies so manual tuning procedures are going to be done in order to work around this.  
           Another issue that I have found is that apparently the gel packs in which the AFM tips are stored and shipped cause contamination.  I am trying to arrange a way of cleaning the tips in an ozone chamber on campus here at OU and depending on scheduling I am hopeful that I can get this done sometime in the next week!
All in all, still working on the trenches but I believe that they are at a good place where I can start using them for initial results once I can image micelles on the silica surface.  
Figure 1.  Trenches developed June 27th using Cold development with shortened time 


Figure 2. Trenches developed June 27th using Cold development with shortened time 
Figure 3. Trenches developed June 27th using Cold development with shortened time

Figure 4. Trenches developed July 18th using Cold development with shortened time

Monday, July 1, 2013

Minor Successes Can Lead to Great Victories...(Metal Lift-Off and Hi-Res AFM Probes)

In the world of science I have come to learn that with every small step forward you gain a tool or skill that will make research easier and more efficient in the future.  At times I feel like I'm not really moving forward, but then I remember that a marathon is not completed in a single event but many steps.  Lately I have had two small victories in the areas of E-beam lithography and AFM.  The first is that upon completion of a metal life-off procedure the lines created in the PMMA are actually 20 nm or less!  Images of these lift-offs can be seen in Figures 1 and 3.   The problem that we continue to battle is that the side walls are not straight up and down vertical but at a slant.  At TU last week I used a much shorter development time which lead to the very small metal lift-off lines.  When the PMMA was viewed in the SEM, however, it was seen to be continuously pulling away (trenches were widening) due to beam damage. 
This raises the question of whether or not we have actually been reaching the small line widths before and the SEM viewing was distorting the PMMA or if they were wide and then viewed.  I believe the latter to be true because this was the first instance where the PMMA was seen to be actively moving under the electron beam, whereas before it had already moved and was then viewed.  I think that the longer development times overdeveloped the walls, causing the slant, which were then viewed under SEM and did not move because they had already reached their "equilibrium" position.  So when the development time was shortened only the exposed area was developed and upon viewing the PMMA began shifting to its equilibrium position and we were able to see it. 
Another question that arises is how it the beam damage occurring if we are sputtering with gold?  Two possibilities is that the gold layer is not thick enough (roughly 2.5 nm on the surface may not be enough to negate the effects of the beam) or perhaps the side walls of the trench were not coated because they were straight up and down.  If this were the case if could also explain why the overdeveloped slanted walls did not move when viewed.  Because they were already slanted the gold formed a layer there which could have helped protect against beam damage.  The straight vertical walls may not be coated as well and therefore when viewed under SEM they are not protected and experience the beam damage.  This theory will be checked using a rotating thermal evaporator in an attempt to coat the side walls of an un-viewed sample, and it will then be viewed to see if the same damaged and extreme movement occurs again. 
The second small victory is the use of high-resolution AFM probes in order to see the bottom of one of our trenches and make a more accurate measurement.  The cross sectional view of such a trench can be seen in Figure 4.  As compared with the last post in which the trench was roughly measured as 300 nm across and "V" shaped, this new measurement shows a trench with a square bottom.  The slanted walls are still present, and show some tip-sample interaction on the left side of the cross section which is still being investigated, but the sample bottom appears to be roughly 58 nm across.  The sample used was an older one with many doses being tested and it is difficult to know which dosage this was but due to the recent success with the metal lift-off procedure a usable dosage is much more easily identified.
The next goal is to obtain micelle images on a silicon surface using high-resolution AFM probes, along with the work being done to check how vertical the walls of the trenches are and perhaps obtain an image of them using SEM if possible.
Figure 1.  Metal Lift-Off using Gold with a PMMA mask.  Line widths are sub 20 nm
Figure 2.  Metal Lift-off Using Gold with PMMA Mask showing Trench Widths Sub 20 nm
Figure 3. AFM Scan of PMMA trench made by E-Beam Exposure

Figure 4.  Cross Section of PMMA Trench Obtained using High-Resolution AFM Probes