Pattern Writing is working!!!

A big leap forward was taken yesterday while using the e-beam equipment in Tulsa!!  After weeks of not even being able to get an image using the lithographic software I was able to obtain one on my last visit.  Then yesterday I was able to etch a pattern, develop it, sputter coat it and view it!  I used a few different beam parameters so that I could being optimizing it for the smallest possible lines.  I found that high accelerating voltages work the best to get impressions in the PMMA, while low voltages actually give you raised patterns.  Not exactly sure why that is at this point but I know it's not what we want.  The second parameter that was varied was the dosage (current applied per length/area) used to etch the sample.  A pre-designed pattern which has a variety of currents programmed in already was used.  Each of 9 wheels is etched with a different dosage and the wheels can then be used as an array to compare the etching quality.

I'm convinced that high voltage is what we want to use because it leaves the best "impressions" in the polymer.  However, yesterday the SEM viewing parameters were optimized at a low kV so as not to melt the polymer while viewing it.  Therefore, when the beam was switched to 30 kV for the etching, the beam was shifted slightly by changing between the voltages.  I don't think that there is a huge difference but when I go back again we may need to choose an area where we can optimize the SEM at a higher voltage, then view at a lower voltage so as not to destroy the areas we are viewing, then for etching increase the voltage again.  I believe this will give us accurate results and from there we can determine the correct dosage to use to get the highest resolution lines, which should also mean the thinnest lines.  

I brought the sample which was etched yesterday back to OU with me to begin practicing finding such small features using the AFM liquid cell so that once the best sample is produced we can begin using surfactants on it!  

I've added some images of some of the etching done as examples.  I know they may not look great but considering what it has taken to get to this point I am very proud of them!  Figure 1 and 2 shows the capabilities of the e-beam (I thought the map of the USA was pretty cool) and that using low voltages the final product seems to be raised up.  Figures 3 and 4 show show that at higher voltages the etch is clean and sunken in, but the lines are about 10 times wider than what we need.  Optimizing the microscope at high voltage will hopefully take care of this. 

To clarify, the small dots surrounding some of the patterns are the scan lines from the microscope, indicating that shorter exposure times (immediately after etching, not viewing) may be advisable.  The patterns that look torn are because the scope shifted the etch onto a line I scraped into the PMMA as a tool to find the patterns post development.

Figure 1.  Low voltage wheel

Figure 2.  Low voltage United States

Figure 3.  High voltage wheel

Figure 4.  High voltage wheel

Working with Polystyrene nanospheres

The optimization of this parameters surrounding the application of nanospheres has been by far the most difficult part of the project so far.  I have reached a catch 22 wherein by increasing the amount of surfactant in the nanosphere diffusion I obtain better coverage and a more uniform monolayer.  However, by increasing the amount of surfactant I inhibit the etching process by creating a "junk" layer in between the spheres which creates its own etching mask.  This additional mask reduces the resolution of the etching process.  By adding no surfactant I decrease the coverage and the uniformity of the layer.  So I have started working with another group on campus that uses these spheres, albeit of larger size, and does so by first cleaning them of surfactant and using hydrophobic treating of the substrate to create an area of hydrophilicity which allows the spheres to adhere through evaporation.  Tomorrow I will test a few of my samples at their lab using bead sizes closer to what I want to be using and see if the results are comparable.  If they are then I will take samples made using each procedure for etching and see which one produces better results.  I was hoping to be finished with this part of the project months ago, but I have been constantly reminded that there are a thousand ways to not make a light bulb.  On the channels side, I will be going to Tulsa early next week to make some samples which should be of sufficient quality to be used in AFM studies.  The SEM there was still having a problem with scaling the last time I spoke with the technician but with any luck that problem will have been solved by the end of this week.

An inch forward is still forward movement

When I first came to OU to work and study I thought I had an idea of how a journal publication worthy project might work, but I was wrong.  As an undergraduate we are given experiments and procedures in the lab which have been done thousands of times and troubleshooted for years and years and we are surprised when a result that we do not understand is reached.  That is how I my naive undergraduate mind imagined research in a graduate lab, point and shoot, get an anwer, write a paper and move on.  The reality is much more convoluted and time consuming than anything that will be undertaken by a laboratory class.  This introduction leads into the update for fabricating nanoscale features.  On the "terraces" side, my contact at OU and I have had scheduling difficulties and I am hopeful that next week we will again try and etch using the clean sample.  On the "channels" side, progress has been made with using the e-beam!!  After hours of reading the manual and testing connections, computer settings and  SEM parameters I was able to get an image with the e-beam software which allowed me to calibrate the beam.  Before I had to return from Tulsa I was able to run a couple of sample patterns, develop them and sputter coat the sample.  However, I was not able to locate them on the SEM before leaving and therefore I was not able to ascertain the resolution of the  lines made or if the calibration was successful.  I did learn ways of marking the sample for next time, so that when I am looking for the location to use the beam, and find them again afterwards I will be able to with much greater precision and use less time to find them.  All in all, everything is moving forward, although much much slower than I my undergraduate self would have thought.  I will not be able to return to Tulsa for another 2 weeks as the technician in charge of the their SEM is out and I am not allowed to use the machine without her present, but once she gets back I think that one or two more trips and I will have channels!!

We seem to be experiencing more technical difficulties... thank you for your patience

I am still not able to use the e-beam equipment at the University of Tulsa due to the program not having x-y control over the beams position.  The program is able to communicate with the hardware and activate the beam but because the beam does not move anywhere during the process all that is left is a burned spot where all of the beam energy is focused during the trial.  The beam technician has been working with me on this project, trying to get the x-y control back but still has had no luck and has contacted the e-beam company for support.  We are awaiting their response and hope that this will allow us to continue with the project.
One thing that I am unsure about lately is the surface integrity once the etching has been done.  Long story short, the PMMA bonding on the silica is disrupted by the e-beam, so that when the sample is immersed in a developer, the disrupted bits lift away leaving the pattern that we are looking for.  I am not worried about the resolution because that has already been shown to have high definition.  I am concerned because the developer is an alcohol and ketone, which I am hopeful will not leave a residue.  If it does, I am not sure how it will affect the adsorption of the surfactants when we move onto that stage of the research.  I am thinking I will run a few tests with the developer on a clean silicon sample and determine how much of a film, if any, the developer leaves behind after evaporation.
The other side of the project, involving etching the silicon, is also having difficulty, as the etching machine is having a performance check and will be available again shortly.  Until then, I am going to focus on the developer residue tests and nanosphere soaking, for retaining spheres and monolayer converage.

Happy Spheres are Clean Spheres and Clean Spheres Come from Trial and Error Solvent Testing

After several attempts at optimizing the SEM for taking images of the polystyrene nanospheres, the layer of contaminant that was affecting the etching process was imaged.  It does not spread throughout the entire sphere layer, but rather is in small areas strewn throughout the monolayers.  The contaminant has been seen previously but only as a lighter (contrast) region of spheres, not the contaminant itself directly.  It was imaged by increasing the power of the beam and taking several images using the backscatter detector to first identify the regions of contamination.  Then switching to the In Lens detector and imaging the previously found areas the "gunk" between and on top of the spheres could be seen.  When looking at samples which have been subjected to solvent-testing, in an effort to remove the contaminant, progress has been seen.  The tested samples show large areas of contaminant free spheres layers.  These layers still have line interruptions throughout them, and their coverage has decreased due to the loss of some spheres during the cleaning process.  Although the decrease in coverage is not ideal, having clean spheres with which to etch was the primary concern, as imaging techniques can later be used to determine the areas where spheres are still present so that when AFM is performed following etching we will know where to probe.  Having clean spheres means that etching can now be performed, and if the results are good we can use smaller spheres and work on shrinking the size of the fabricated pillars!  Moving right along!

Technical Difficulties

Progress has been made on the front of fabricating the "trench" structures using PMMA on silica.  The layers are a suitable depth and the program to use the e-beam has been set up to run test runs to determine the power of the beam to get clean edges and correct depth etching.  However, all of this is waiting to be used while the e-beam software and the e-beam hardware are being fixed.  The problem lies in that they are not communicating with each other, so that while they are both technically functional, the beam cannot be used because it cannot be told what to do until it can communicate with the software.  The technicians at TU have been working very hard to resolve this issue themselves but have had no luck and have had to call in the SEM manufacturers for assistance.  I am confident that once they have fixed the problems I will be able to have sample made within two weeks (one week to fine tune the parameters and one week to make the samples) and then start using them with surfactants and AFM.  I am excited about this because all of the problems that have been overcome to finally reach that point will make it more rewarding.
The other half of the fabrication process, the "terraces", is also slow moving due to problems involving surfactant on the surface of the beads and silica making it difficult to etch cleanly.  The researcher I have been working with believes that we may have had success with solvent testing, and sees a possibility of etching again next week.  I am looking forward to this because school starts soon and I would like to have all of the preparation work out of the way so that I won't have to stretch myself between school and research too much, but I am mentally preparing myself to do so anyway!  Knock on wood that the AFM is working at the end of all of this.

Getting closer to more etching

Yesterday my contact at OU and I were able to perform a few different soaking tests in solvents in order to determine if they had any effect on the sample cleanliness.  First, we characterized the surface using a Dynamic Interference Contract microscope and chose an area that could be found again.  The DIC microscope is different in that it allows one to see various layers on a sample.  So we were able to see that aside from multilayers of spheres there seemed to be another layer of stuff on the monolayers of spheres, which we believe to be left over surfactant from the solution used to layer the spheres.  Then we let the samples soak in different solvents and air dry.  Then we looked at the samples again under the microscope, looking for differences in bead number and whether or not we could see and "scum" on the surface of the beads.  We saw that some of the solvents completely removed the spheres while others did very little.  We saw that one sample had less of a scum layer than before, almost non at all by what we could see, but did removed most of the beads.  However, this sample has enough on it to try etching with and see if our results improve from last time.  My contact will be away next week but once he returns we may be able to do another series of etching!!  On the other side of the project, the group at TU still has not been able to get the e-beam and the controlling computer to "talk" to each other yet.  Therefore I will not be making a trip there this week, and instead will be trying to improve the cleanliness of the samples and the PMMA layer to etched once the e-beam is up and running.