20 nm lines and Smaller but Lacking Consistency

It has been a busy couple of weeks but mostly in preparation of events which come and go very quickly.  About 3 weeks ago (I have been quite busy and at one point decided to wait to see if I could improve the results prior to posting but the technician was out of town)  I saw lines that were 20 nm and less in width, which are seen in Figures .  However, I was unable to consistently achieve these line widths and in the end they seem more like happy coincidences than planned results.  They reason I say this that the smaller lines appear on a small pattern located within another pattern which does not display the same line widths.  I have been using the wheel arrays for some time to try and obtain smaller line widths and they have served their purpose very well up to this point but now I am stuck at 40-45 nm consistently and I thought it might have something to do with a curving line compared to a straight line.  The 40-45 nm results are a big improvement over where I began and with each visit I am fine tuning the procedure but I feel that a small something is missing that is necessary for making these lines smaller. I have been in contact with the manufacturer of the e-beam set up being used in Tulsa and he has given many suggestions of why we haven't been getting the finest lines possible and it is his advice which I will be following up on Friday.  With the help of a research scientist here at OU I have been using a new pattern which is a set of straight lines which I felt might give an idea if the curve in the lines was what was causing the large line widths.  As seen in Figures , the pattern worked very well, which I was a little surprised about since it was my first custom pattern where I input new current dosages and ran it from scratch, but the line widths still hover close to 40 nm at the lowest line dosages used in the dosage array.  As the dosage is increased the sharpness of the line is greater but the line widths also increase.  This is expected and what we would want to find is a "sweet spot" where the resolution of the line is greatest while also giving the smallest line widths, which is what I'm hoping will occur on Friday when I go to try out the suggestions of the manufacturer.  Until I am able to figure out the problem I have been working with AFM to get images of surfactant on the surface of my substrate, but it is more difficult that one initially thinks.  It is a very delicate process but I believe that I am slowly getting the hang of it and hope to have some images soon!
On the terraces fore front things are not going as well.  I have been trying to contact the research scientist who has been helping me to obtain the pillars via Reactive Ion Etching but I have so far been un-successful.  The main problem occurring is that some of the equipment is in need of repair and the technician helping me was gone for a couple weeks for research and had not been able to fix it the last time I spoke with him.  So in the mean time I have been working on protocols to use and trying to fine tune the parameters in order to get the cleanest etch with highest aspect ratio. I am hoping that the issues will be resolved soon and I will be able to continue with the project in it's entirety.
All in all I'd say that for a short while everything was working and moving forward and now once again I am at a veritable standstill.  However, I am seeing improved imaging of surfactants on silicon which is keeping me busy while I wait for the etching equipment to be fixed, while preparing for the time when 20 nm lines are reached, which I have a feeling is going to be very soon!!

Figure 1. Etched 15 nm lines in PMMA on Silicon 

Figure 2. Etched 20 nm lines in PMMA on Silicon  

Figure 2. Etched 20 nm lines in PMMA on Silicon 

Figure 3. Etched 20 nm lines in PMMA on Silicon

Figure 4. Etched 20 nm lines in PMMA on Silicon

Figure 5.  40 nm lines at the lowest current in a custom straight line array 

Figure 6.  90 nm lines achieved at highest dosage in straight line array 

Figure 7.  Image showing the entirety of one straight line array which ranges from 1 nC/cm to 8 nC/cm.
Etching in PMMA on Silicon using E-Beam Lithography

Polystyrene Bead Etching and E-beam Optimization

It has been some time since I have posted, mainly due to holidays and equipment malfunctions but I have new images and new plans to report!  On the pillar or "trough" side of the project we have successfully completed an etching trial which displayed some interesting results.  Five samples of 100 nm polystyrene beads on silicon were run under different conditions using CF4 and O2 etching.  From figures 1 through 5 below, it can be seen that there was some etching of the silicon and in a few instances there were cylindrical formations, but the beads are being etched by the O2 which causes them to either adhere to the silicon and melt, which causes the final structure to be malformed, or as the bead is eaten it shrinks causing a cone or rounded shape hemisphere shape.  From literature I am finding that shorter etching times with higher power (W) seems to give more anisotropic etching profiles (following performance optimization) and the use of a passivation step (usually flowing a polymer) was used in some, but not all, cases.  It seemed though that in some of the cases that a reactor had been specially ordered or built for such etching and may not be feasible for our purposes because alternative means of achieving near comparable results are possible.  Our next step may be to shorten the etching time for each pulse from 45-60 seconds to somewhere around 5-15 seconds to allow the sample to cool.  Also, in the literature the lowest power used in the RIE (reactive ion etching) step was 600 W and we use at the most 60 W and I am interested to see if we increased the power and shortened the pulse time if our etch might become more anisotropic.  We will also have to use a different plasma (perhaps argon) and remove all O2 etching steps as they degrade the beads quickly.

Figure 1. Polystyrene Bead Etching on Silica

Figure 2. Polystyrene Bead Etching on Silica

Figure 3. Polystyrene Bead Etching on Silica

Figure 4. Polystyrene Bead Etching on Silica

Figure 5. Polystyrene Bead Etching on Silica

The other side of the project is the "trench" or "trough" formation in PMMA using e-beam lithography.  After a few setbacks (new source installation and a cooling pump break down) I was able to return and etch a few samples to continue to try and achieve 20 nm lines.  So far I can get 50 nm almost every time and still have a few things that I can adjust to see if they shrink that number a bit.  Some of the most recent images can be seen in figures 6, 7 and 8.  Next I will try to reduce the scope current and see if a smaller dosage per length might help reduce the beam interaction with the sample, creating a smaller line width overall.

Figure 6. Line width of e-beam wheel on PMMA

Figure 7. Line width of e-beam wheel on PMMA

Figure 8. Wheel Array with Different dosage per wheel

"Imagine trying to hit a bullet with a smaller bullet while riding a horse..."

Since it's been so long between posts, today I thought I would give a double update!  The title of this post was chosen regarding the optimization of the SEM for creating the line widths needed for this project.  It is a quote from J.J. Abrams Star Trek, and is an explanation of the difficulty associated with beaming a person onto a ship already travelling at warp speed.  While my difficulties are not quite that bad I thought it captured how delicate it sometimes feels to optimize this process.

Since I already did one on the "terrace" portion of the project I will now go onto the "trench" portion.  I have uploaded some images below to help with my explanation of how it's going. 

First off, my last post in October included the third image of this post, which was a great step forward as before that I hadn't been able to get anything at all, but now that I'm older and wiser I realize how far away I was from getting it right.  First off, if my car had a wheel this shape it would shake itself to pieces pretty quickly.  By this I mean it is not round it it elliptical.  I thought this was just a figment of my imagination or a random mistake but the more samples I ran the more I realized that it was not going away.  This is an aspect ratio problem and could only be remedied by re-doing the painful and time consuming calibration I did back in late July.  The calibration itself is not hard, but figuring out the set-up can be a little frustrating because it is a DOS based program and therefore not the most user friendly.  However, once that was done I saw a marked improvement in the wheel as can be seen in the first and second images below!! The scope was having some beam issues when this were performed which is why things look wavy and one wheel showed up so close to another.  This did not affect the calibration and did not appear to affect the etching of individual wheels but the final product was difficult to image because of it and some wheels were moved because their locations relative to each other shifted between wheels.  The next thing that should be noted about the third image below is that the line width is up around 200 nm, which is about 10 times what we are shooting for, and the surrounding polymer is dotted where the polymer was exposed surrounding the wheel.  That dotting is because I was still naive and did not know that the beam must first be blanked and then the location to be etched is brought to the beam location using the X and Y stage controls.  Then the sample is etched and moved back and the beam can be un-blanked.  This way the area is not over exposed causing larger lines and the dotting observed in this image.  You'll notice that no such dotting is seen in the more recent images, which are the top two.  In the second image shown here you can also note that we have reached 50 nm lines!! This is a big achievement because I have been told that improving upon line width, i.e. getting smaller line widths, is a delicate art and we were stuck at 90 nm for so long I was losing hope that we were ever going to get them smaller.  The trick, I've learned, is the use of "contamination spots".  These are spots made by using the spot mode on the SEM which focuses all of the beams electrons to one spot.  If the beam is out of focus this spot will be large, and the more in focus it becomes the smaller the spot.  Also, if the stigmators are out, the spot will not be circular but elliptical.  Using these it is possible to more accurately determine if the beam is more or less in focus.  Before it was more like an eye-test, where you ask yourself "Is 1 or 2 better?", which at some point with the spot mode it is also but it is much easier to tell if a spot is bigger or smaller than it is to tell if an image is more in focus or less in focus to the degree of precision required for obtaining lines bellow 90 nm.  Overall, things are steadily moving forward, although the scope issue that was occurring during my last visit to Tulsa got worse and therefore my trips are on hold until this is fixed so I do not know when I will be able to perform more tests but with these steps forward I finally feel as though I am going to be able to start using AFM to look at surfactant in the lines!  Woo Hoo!

Comparison of Two Sphere Deposition Techniques

I apologize to my readers for the length of time that has passed between this post and my last.  It is funny how time can get away from you when you are always doing something.  I want to begin this update with a little more in depth explanation regarding some of the backtracking with the "terrace" phase of the project.  Below I have added images of spheres which have been deposited on silica using two different methods.  The pictures are slightly out of order due to a difficult image adding process but the captions for each are correct and should help with any confusion.  What you see are the Langmuir-Blodgett technique and the drop evaporation technique.  The LB technique is done using 200 nm spheres whereas the drop technique is done using 100 nm spheres.  For the LB technique, extra surfactant must be added to help drive the spheres to the silica surface during the withdrawal step, which is crucial to keeping the amount of spheres at the liquid solid interface roughly constant and facilitate the formation of the mono-layer   This extra surfactant is what you see crammed in between the spheres in the middle two images below, and as you might notice it is quite thick.  The formation of the desired pillars is dependent on etching in the spaces between the beads and if it is clogged with the extra surfactant the etching is not able to be done correctly.  To correct this a different approach had to be taken which is where the drop evaporation method comes in.  This technique does not require the use of any extra surfactant and actually allows for a cleaning step where the beads are centrifuged to try and remove as much surfactant as possible.  I made several different concentrations of 100 nm beads and deposited them onto silica in order to determine what the best concentration of beads was for forming the best mono-layer   I determined it was 0.5 wt % and made a few more samples, the results of which can be seen in the first and 4th images below.  As you can see they are still a mono-layer (with more interruptions that the LB method but a good amount of coverage comparatively  and they are much cleaner.  The only surfactant observed exists as the connections between the spheres which is essential for their forming a layer.  Etching has finally begun again and although due to an already hectic schedule I have not yet been able to properly observe the results of the etching yet I am excited to see what kind of progress this has made!   

100 nm spheres-Drop evaporation Technique

200 nm spheres-Langmuir-Blodgett Technique

200 nm spheres-Langmuir-Blodgett Technique

100 nm spheres-Drop evaporation Technique

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!!