Over the past two months I was ill and had finals for school and was not able to go to Tulsa for EBL as often as I wanted to but I will update with what I have. When going on some of the images given to me by the group who was able to reach ~20 nm lines I noticed that their trenches were not totally straight up and down, as seen in Figure 1. I was able to travel to travel to Tulsa to perform some exposures and noticed that even though I do not have cross sectional images there does seem to be a wide opening of the trench at the top (PMMA surface) and a smaller line width at the bottom of the trench (substrate surface). Figures 2 and 3 show the phenomenon that I am discussing, and upon correspondence with Mr. Joe Nabity, the creator of the EBL system we are using, he agreed with my conclusion that there seems to be a V shape to the trench. The tops of the trenches appear to be between 40 nm and 70 nm on average while the trench bottoms appear to be anywhere from 10nm to 20 nm (although it is difficult to confirm this without first performing a metal liftoff which we will do next visit to TU). The reason for using EBL in PMMA was that straight up and down walled trenches have been obtained using it and a "V" shape may introduce prevent adequate confinement of the surfactant molecules which we are attempting to observe using AFM. The time spent on this project is becoming lengthy and therefore in the interest of moving forward we will be making a sample to use with AFM. The sample will most likely display the small trench bottom and have a large trench opening but will provide a way of working on a method for imaging surfactant aggregates at the bottom of this trench while also working on fixing the problem of the "V" shape. I have contacted several corresponding authors of publications which have been able to obtain the types of line widths we are looking for while also keeping the trench wall vertical. A few of the papers are
Systematic study of the interdependence of exposure and development conditions and kinetic modelling for optimizing low-energy electron beam nanolithography (Mohammad) and
Fundamentals of Electron Beam Exposure and Development (Stepanova).
I have also been doing AFM on some of the previous samples I have made using EBL. They have a layer of gold roughly 20 nm thick with a pattern called Line Dose Test which had 56 sets of lines with line widths between 40 and 60 nm. Figures 4 and 5 show an AFM and SEM image of one of the lines with an unknown location in the array of lines, due to the nature of AFM. Being able to find these lines is a feat in itself but I've been able to do it several times now on the same sample, even in liquid! Because the AFM tip is roughly 90 nm we have not been able to obtain data on the trench bottom but high-resolution tips (smaller tip radius) have been ordered and will be used in further experiments. Another observation made is that the AFM showed a line width of roughly 300 nm, seen in Figure 7, which according to the SEM micrograph of the same sample in Figure 6 is not accurate. One possible explanation is that the PMMA has contracted over time leading to a wider trench opening that previous seen, and this possibility will be checked using SEM once again. Another possibility is that the PMMA is elastic enough that the AFM tip is dragging the top edge of the trench as it scans along and it snaps back to its original width as the AFM tip moves past. I feel that this is unlikely as the PMMA is layered in gold which should be thick enough to hold it PMMA in place, but this would able contradict the explanation of the PMMA contracting. We will perform AFM with the new tips to see if the same phenomenon occurs and explore it further if it continues.
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| Figure 1. V shape seen by previous group |
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| Figure 2. Plan view image of exposures showing wide trench opening and narrow trench bottom |
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| Figure 3. Plan view image of exposure showing wide trench opening and a slant to narrow trench bottom |
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| Figure 4. AFM image of Line Dose Test Exposure |
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| Figure 5. SEM micrograph of Line Dose Test exposure |
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| Figure 6. Line Dose Test Line Width via SEM micrograph |
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| Figure 7. Line width of Line Dose Test Line via AFM metrology |
