Wednesday, November 28, 2012

"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

Wednesday, October 17, 2012

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

Tuesday, October 2, 2012

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.

Friday, September 14, 2012

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!!

Monday, August 27, 2012

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.

Monday, August 13, 2012

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!

Wednesday, August 8, 2012

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.

Thursday, July 26, 2012

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.


Tuesday, July 24, 2012

Getting the Software Down

Last Thursday on my trip to Tulsa I was able to begin designing a template to be used with the e-beam hardware.  Unfortunately the e-beam and the SEM are not yet "talking" to each other, but the software itself is still operational.  Therefore, I was able to read the manual and make some decisions that will make using the e-beam much easier, as soon as it is operational.  One concern that the professor I've been working with at TU is that because the e-beam is different than the one they used previously (with the group before me) is that the counts/dose ( the energy delivered to the sample per specified area, line or spot) will be different.  This means that we can't just use the settings that were used prior and expect the same results right off the bat.
In order to determine the correct dose, I made a template the features several lines which I made different colors on the program.  Then I can make each differently colored line a different dosage and examine the results to determine which lines didn't go deep enough, which ones gave poor resolution and which give the best results.  There are still other optimization problems that go along with the system, such as the SEM optimization itself (stigmators, apertures, etc.) but I have quite a bit of practice with this and there is a very skilled technician available to assist if needed.
If the e-beam and software are not communicating by Thursday this week then I will forego a trip there this week as I feel that there is not much I can really do until that is completed, at least not that would warrant my driving there.
On the other side of the project (terraces) I am also moving forward.  I have made 5 200 nm sphere samples and today or tomorrow I am working with a research scientist here at OU to determine if there is anyway to clean the surfactant of the surface of the sphere layers using different solvents.  We will immerse the samples in 4 different solvents and keep one a control in order to determine what effect they have on their cleaning capabilities.  The reason for this detour is that using the etching system will be better optimized is the samples are as free of impurities (surfactant and other junk besides the spheres) as possible because it will create inconsistencies in the etching depth and performance.  Once we have determined that we have obtained as clean a sample as possible we are going to etch again at a length of time longer than 30 seconds but shorter than 5 min.  This should yield pillars and hopefully those pillars have a fine enough relief (edge on the top) that we can start using AFM to view surfactants on them!
I'll admit that we are moving slower than I had hoped, but at this point moving slowly is better than not moving at all! 

Friday, July 13, 2012

Tulsa Trenches and OU Terraces

Yesterday in Tulsa went very well.  Despite the e-beam technician being on vacation until next week I have still been able to make some good strides in sample preparation.  The AFM tech at TU had some trouble getting a good PMMA layer depth reading on the samples from last week. He said they were in the neighborhood of 50 nm, which is about a third of what we were expecting based on the notes of the students previously working on this type of set up.  I have found some other protocols online that suggest that 50 nm is actually right on the money.  It was stated in the proposal that as long as the wall height was 20 nm or more it should not affect the surfactant adsorption, and the lower the wall height the less it will interfere with the AFM tip when gathering images after surfactant adsorption so 50 nm is actually really good.  I'm a tad nervous because the PMMA solution they have had me using at TU was the same that the group before me was using, which has an expiration date about 4 years ago.  However, it should not cause problems for the purposes we need it for.
I have some trouble making sure that the silicon pieces are microscopically smooth (using the right cleaning solution as well as a plasma cleaner) and this may be what's causing the small imperfection that can be seen in the film.  The debris causing a wake pattern on the surface during the spin coating and I'm not sure it's a problem at this point as we can choose which area to use for the formation of trenches.
I will be returning next Thursday to continue the work and the e-beam tech should be back and we can see if we can get some trenches made!
On the other side of the project, I will be meeting with the OU researcher who has been helping me with nanosphere lithography and discuss the SEM micrographs and hopefully the week after we can try another run of etching with smaller spheres!

Tuesday, July 10, 2012

Channel Formation

Last week I was able to complete a preliminary sample of a layer of PMMA on silica.  I am still awaiting the results as to how thick the layer was deposited, but hopefully somewhere in the neighborhood of 150 nm (according to the protocol I was attempting to replicate).  I really enjoyed working with the graduate assistants and professors at TU because they were very friendly and helpful.  They could have just have easily left me to my own devices in a foreign laboratory but they helped me find equipment and supplies early one and I was able to figure out the rest as the day went on.  All in all,so far, it has been a pleasant and productive experience and I look forward to continuing my collaboration with them. I will be returning to Tulsa this Thursday to continue the preparation work and when the e-beam technician returns the week afterwards we will be able to form the channels!

Tuesday, July 3, 2012

Getting somewhere

Going to the University of Tulsa on Thursday to begin preparation on the samples to be used for the trenches samples!!  I'm slightly nervous to be working in another lab, just as I was for coming into the University of Oklahoma I suppose, only this time it's for short spurts.  Every lab has their own house rules that they expect everyone to follow and depending on who is working in the that lab and what else is being performed it can be very different and make beginning work slow.  Because I want to be done with this phase of the research by the end of the summer I am excited that it is moving along but nervous that it may take longer (months) than I want.  I am very confident in the supervisor who will be showing me the ropes there and am looking forward to learning from him and others at TU.  Happy Independence Day!

Monday, June 25, 2012

Channel sample preparation

The e-beam is ready to begin use for creating channels to observe the effects of sidewall confinement on adsorption!  Making plans to go to University of Tulsa this week to begin preparing samples so that next week (or the week after depending on scheduling) the e-beam can be used in order to create the channels.
Work to create the nanopillars is still continuing and I am anticipating seeing results after the next etching session which is set tentatively for the week after the fourth of July.  It will be easier, I believe, to move on having both of these samples completed.  I think this because the step afterwards will be to use liquid cell AFM to observe surfactant on the surface of these substrates and focusing on that area, particularly the use of the AFM and developing my skill with it, and not trying to complete the fabrication of another sample will simplify things.  Just happy to know that things are still moving along!  

Monday, June 18, 2012

Taking a small step back can be a good thing

After conferring with one of the research scientists helping me with the etching, the methanol soak may not have removed anything as we had hoped.  It is still looking promising that a sample of nanopillars, usable or not, will be completed before the end of June.  So either way I am to be moving forward, which is good.  On the other hand I am still feeling as though if these don't work or are not up to the specifications that we need them that a lot of time has been ill spent.  Looking on the bright side, it has given me opportunities to learn about what surface we are looking for and look for different approaches at getting it.  In other words, if it does not work then I have a few other options that I can switch to without having to spend more time researching.
I still have not heard back from the contact that has offered to help us with the channels, but with any luck that end of the project will also be up and running by the end of June as well.
As my summer class also come to an end, I'm feeling the pressure from several different sides to produce results and the feeling of being overwhelmed has crept up more than a few times.  However, taking one thing at a time and making sure to manage my time efficiently is definitely keeping me sane.  There is something to be said for a well used day planner.

Thursday, June 14, 2012

The Waiting Game

Have been awaiting responses from contacts regarding e-beam lithography and the etching for quite a while now.  I have said it before, but one of the most frustrating things about research is coordinating schedules with more or equally busy people.  To use try and use my time wisely I have continued reading papers about self assembled monolayers and looked for ways to improve the sphere layering techniques I have come up with so far.  The 50 nm beads, while a monolayer, are still the hardest to perfect.  I feel as though there may be some limitations with my set up that prevent the further improvement upon the smallest beads.  However, oxygen etching can be used to shrink spheres that are already on the smaller end of the spectrum (100 nm) to get spheres are 50 or smaller.  The pillars will be spaced further apart if this route is used but that may even work to our advantage in limiting the amount of interaction between sphere monophologies which will be forming on the pillar surfaces.  
I am going to continue trying to reach my contacts in order to move forward as quickly as possible.  In previous posts I have stated my goals for June.  While there is still time to complete these goals, I am currently at the mercy of others and I am not enjoying this goals being at the mercy of the schedules of others.  I will continue working to those goals anyway and make new ones when necessary to work towards getting our questions answered.  

Friday, June 8, 2012

Two birds with One Stone

Having trouble getting responses for the last couple of days.  Have been working on getting 50 nm spheres to create more continuous layers on the silica by changing the solvent ratios some more but to the naked eye there doesn't seem to be any apparent improvement in sample coverage overall.  This does not mean that it is not improving on a micro scale, just that I will need to use an SEM to tell.  This is in the mean time while I have been waiting to hear back from contacts who are graciously helping us with creating the channels (trough) for the other half of the project.  So far though I have not been able to get responses from them or the people who are helping us with the etching to create nanopillars either.  It is a difficult thing to try and schedule something else into an already packed summer for them I'm sure, but it makes me nervous because I don't want to get to a point where I've got more to accomplish than time to do it all.  
As already mentioned the plan is for the troughs to be fabricated in PMMA using e-beam lithography which the group who have been contacted to help us are adept at.  I'm excited to start this because it appears that this technique will be more precise and require less leg work to get a sample ready for surfactant trials.  Once I've gotten to a place where I can get nanopillars done it will essentially be the same deal, where to create a new sample I already have a protocol done, but this is the part where I have to perfect the protocol, and that's where the bulk of the work has gone so far.  As a added benefit from this though I have increased my knowledge of the substrate and its properties, which means that creating this sample will have served a dual purpose, which I look at as an accomplishment. 

Wednesday, June 6, 2012

Some challenges faced so far

I have been trying to get things started with the e-beam lithography to begin creating the trough structures to look at how adsorption in a channel will occur.  One thing I like about this project is that we are given the opportunity for symmetry.  That is, we are looking at the adsorption of surfactants on the top of small area open surfaces (the terraces) as well as adsorption where the surface is still small but confined.  The difficulty with this is that surfactants are, of course, very small.  Therefore, in order for the effects of the "confinement" to be noticed, or different than any other surface, the surface must be on the order of a few surfactant molecules long or smaller.  That is why this project is so challenging so far is that creating such structures isn't like picking up an x-acto knife and cutting a small piece of substrate.  It must also be smooth and, for the most part, flat so that the size of the surface is the variable causing changes.
Using the smallest size spheres is how these surfaces will be achieved, but the smaller the spheres the harder it is to obtain self assembled monolayers of the beads.  So what we tried with the etching last time is using a small amount of O2 etching along with the primary etchant.  The O2 will shrink the beads linearly with etch time, so the structure formed will (should) have a terrace smaller than the original bead size!
I am really looking forward to working with the e-beam lithography because aside from just being plain cool it will be more precise and repeatable.  Exciting things are happening!! Now if I could only get over this cold....

Monday, June 4, 2012

Sick and Graduate Research Schedule

Was sick for the first part of the weekend.  One thing that is both a blessing a curse about graduate research is that for the most part you dictate your own schedule.  Being sick means that you take off the days you can but you come in an extra day somewhere else to make up the slack.  It is very much reliant on weekly goals to keep on track and make sure things are moving forward.  Therefore I come to work in the morning and leave when I feel that I am no longer productive, no set come in or leave time.  Just try to accomplish what I set out to do for the day and then make a goal for the next day.  My current goal is to try and set up the RIE using PMMA for creating trenches, the opposite of what I have been working on lately.  Will continue to update on the progress of this endeavor.

Friday, June 1, 2012

Methanol Soaking Step

SEM was used to view the state and orientation of the 200 nm spheres on a silica surface.  There was a good amount of monolayer coverage (more than multi-layer), although those monolayers had line interruptions through out it.  A grid was used to find a spot which after soaking was performed could be found again for comparing the sphere positions.
After soaking the silica sample in methanol for 15 hours (overnight) the surface had a much deeper purple color.  When SEM was used to determine whether or not the soaking moved the spheres around significantly, it could be seen that the soaking had little to no effect on the position of the spheres, however not much could be said about the SDS on the sample surface.  From this information we can say that the soaking step can only be helpful, because the sphere positions did not change so if the SDS wasn't removed no harm was done to the sample.  If the SDS film was loosened or removed then when RIE is done there should be less of a mask from the SDS and the spheres should be able to create a better mask for creating the nanopillars.  All this is good news and hopefully next week or the week after, depending on the RIE availability, we will be able to try etching again with a shorter etch time and see what kind of shapes we can get!!  Exciting!!

Thursday, May 31, 2012

Effect of pH Conclusion

The pH of the 50 nm sphere solution was tested and found to be roughly 4.5.  From the literature, at 8.5 pH silica has a negative charge which hinders the adsorption of anionic surfactants (1).  Because our pH is much lower than this value we can say that the surface of the silica has a slightly positive charge which would increase the adsorption of the SDS (anionic) in the sphere solution.  The theory that this would increase the sphere adsorption as well would require further experimentation which is not the primary focus of this project. Therefore, we will conclude that the pH effect on the sphere adsorption is not a factor to be explored in enhancing the monolayer coverage for this project. 
However, the SDS adsorption to the silica as a result of the low pH may be a factor is creating a sharp relief.  It has been recommended that the samples be soaked in methanol to loosen the SDS on the surface of the silica.  To make sure that this soaking step does not affect the sphere monolayer, SEM will be performed prior to the soak and then again after the soak to determine the effect, if any, on the already deposited spheres.  

Tuesday, May 29, 2012

Post Memorial Day update

Hope everyone had a good Memorial Day Weekend! Today will be a short post.  Due to complications with the SEM I am not able to use it until later this week at the earliest.  Something important came up this weekend I was not able being the pH testing of the sphere diffusions as I had hoped but I should be able to do that tomorrow. On the personal side, following this weekend it is easier to understand the concept that the world continues to turn even when you want to take a moment to stand still.

Friday, May 25, 2012

Short term goals for the next 3 weeks

Next week I will be trying to remove some of the stuck on SDS from the surface of a deposited sample using a methanol soaking.  This step was recommended by the research scientist helping me with the etching process.  The idea would be to loosen and help the spreading out of the SDS in order that it does not create an etch mask on the surface of the silica.  I am not convinced that the spheres will remain in place during this process, but the worst that could happen is that the spheres became disordered and I know that it may make remove some of the SDS but at the cost of compromising the sphere locations.  I am hoping to etch again in the beginning of June using a different etch time and perhaps with smaller spheres.  We are getting close to the point where we will determine if this process will give us the surface we are looking for.  I anticipate that the answer to this question will be answered mid-June.  Memorial day weekend will be relaxing, only in the fact that I can spread out the work I hope to get done this weekend over three days instead of two, but still I am thankful for the holiday.  Everyone have a safe and enjoyable weekend!!

Thursday, May 24, 2012

Adsorption variation due to pH effects

Depending on the pH of the solution in which the substrate(silica) is immersed the surface charge of the substrate tends to vary.  A higher pH will lead to a more negative charge on the silica surface.  The polystyrene sphere solution has SDS, an anionic surfactant, and surface sulfate groups from the synthesis of the of the spheres.  These sulfate groups lead to an overall anionic charge in the sphere solution prior to the addition of the SDS surfatant.  Yesterday I was able to discuss with a colleague  the possibility of the pH of the sphere solution causing a slight repulsion between the spheres and the substrate, due to the negative charge attached by the sulfate groups and the hydrophilic nature of the silica.  It seems a simple task to check the pH of a sample solution and test the effect of it's variation on the surface coverage of the substrate.  I will note that I feel as though speaking with a peer who is extraordinarily familiar with the effect of pH on adsorption, although the conditions are different for their project, was extremely helpful in making assumptions as to the nature of it's effect and whether or not considering the pH a factor was a major necessity.  
On a personal not, the frustration of not knowing if all of this work and produce the surface we need and whether or not what these small considerations are going to make any difference in the end is a little dis-hearting at times.  However, what worries me most is the time spent on this part of the project if we have to change methods.  I do not feel that it is time wasted because I am definitely learning a lot about how all of the little things affect adsorption.  That being said, I feel that research is exciting once you get results that tell you something new and I am excited and anxious for that part of the project to be here!  It's even more exciting because you know that you didn't cut corners and all that trial and error and work made a difference!  It's like a Christmas that comes early!


Monday, May 21, 2012

Surface Chemistry Alteration

I am looking into the possibility of altering the surface chemistry of the silica.  I am wondering that if by making the silica (hydrophilic) a hydrophobic surface if the monolayer coverage would not increase?  A few concerns I have are how the beads would react to a change in pH (my guess is they won't change very much) and whether or not the increased hydrophobicity would just create more areas of multilayers instead of increased monolayer coverage.  My reasoning behind this idea in the first place is the formation of layers of surfactants on hydrophobic graphite, which promotes the formation of surfactant morphologies which cover the surface to a greater extent than they would on a hydrophilic surface.  Therefore, because of the hydrophilicity of our silica, perhaps by altering the surface chemistry enough to create a hydrophobic layer we might be able to promote enhanced layering.  Another limitation is that any alteration of the surface must not interfere with the final etching process.  Another option that I will be investigating is how a change in pH of the sphere solution might increase (or decrease) sphere layering by increasing or decreasing the binding sites on the substrate with counter-ions.

Thursday, May 17, 2012

Follow up of SEM of etched samples

The sample which was etched with the "diving board" structure masking part of the sample was viewed using SEM today.  First, using a top down view to look at the effect of the 5 min of etching on the spheres, it could be seen that the spheres had deformed and melted together in some areas.  It was not possible to tell about the depth of etching from this angle, so the sample was placed in an ultra-sonic methanol bath for 2 intervals of 5 minutes and cleaved down the middle. When viewing the sample on its edge we were able to view what had happened in different areas of the sample from the masking and etching areas.  In the ares that were not masked there was only a rough surface, nothing that would suggest spheres were ever attached to the surface at all.  There were areas that appeared noticeably higher than others though, suggesting that multi-layers may have masked those areas slightly.  As one viewed closer to the diving board masked area the surface because less rough and more uniformly etched until up near the top of the sample there were very small misshapen pillar formations.  These pillars are not flat enough for our purposes, but they do tell us that pillars were forming and that the polystyrene spheres did create pillar structures.  The length of etching will need to become shorter in order to tell if we will be able to create pillars with a flat enough surface that they can be used for our surfactant adsorption experiments.  Therefore the next step will be to try another round of etching with a shorter interval and view the results.  Right now we have been using one interval of etching, but I am wondering if having the a greater number of intervals that equal the same time might not serve our purposes better.  By increasing the interval number the heat that was generated and melted the spheres may be reduced.  So conclusions from today are that this method did create pillars, but that we need to see if shortening the etching time and/or increasing the etching intervals will provide a cleaner result.

Wednesday, May 16, 2012

SEM of etched layers

Performed SEM today on the etched samples.  The sample with 30 seconds of etching and the spheres removed showed a difference in height from the surrounding surface and where the spheres were.  With a longer etching time pillars might be clearer.  The SEM with the 50 nm spheres showed that there were monolayers but that their coverage was about even with that of multi-layers.  A conclusion that has been reached from this is that when using the Langmuir-Blodgett technique, at least with the parameters that we are using (solvent ratio, barrier speed and withdrawal speed) that the smaller the spheres the more difficult it is to control the monolayer coverage.  I'm a little nervous that after all this that the surface of the pillars might be insufficiently smooth for surfactant studies with AFM.  We will continue with this until we have pillars that show that separation in between the spheres is possible, which will come from etching for at least longer than 30 seconds before we think about trying another masking technique to create the nanostructures.

Tuesday, May 15, 2012

New Cleaning steps added

Have been using new pre-deposition cleaning procedure.  First the silica sample, which is already cut and has had an oxide layer grown on it thermally, are placed in a plastic beaker which contains methanol and is then put in an ultrasonic bath for 5 minutes.  This step is to loosen any debris which may be strongly adhered to the surface.  Next it is cleaned in a plasma cleaner for 10 min at 18 W.  Then it is placed in a solution of 1:1:5 hydrogen peroxide, ammonium hydroxide and DI water for 10 minutes at 75 degrees Celsius.  They are then removed and rinsed with DI water and dried with N2 and placed back in the plasma cleaner for 5 minutes at 10 W.  Then the sample is attached to the arm of the stepper motor and lowered into the sphere solution (2400 microliters sphere diffusion and 3600 microliters water which has been sonicated for 10 min) and the Langmuir-Blodgett barriers are advanced for 3 minutes while the sample is stationary.  The stationary step is to allow the spheres to build up a monolayer concentration near the surface of the substrate.  Then the substrate is withdrawn at 5 micrometers per second and while the barriers are advanced at 3 ticks above the slow mark.  A plastic ziplock bag is lowered around the set up to prevent any turbulent air currents from disrupting the deposition or bring contaminants into the solution.  Every 4 minutes the barriers are backed away from the substrate and restarted.  Once the process is complete, the substrate is placed in a sterile container horizontally.  The polystyrene sphere solution is pipetted back into its container and stored.  SEM was to be done today but due to scheduling conflicts was not able to secure a spot.  Looking forward to seeing how the etching from last week turned out.  Nice to finally see some results, promising or not, because at least it will point us in a direction for the next step.

Monday, May 14, 2012

First day of summer classes and Research

Today was the first day of summer classes and research.  Doing research with only one class feels like you have more time, but it can be deceiving because of the amount of time you will spend for that one class due to it being so condensed.  Implemented a few new cleaning procedures today including a methanol soaking of cleaned silica pieces to remove any dust that may have settled in between bead deposition trials.  Once the beads have dried sufficiently we will try and soak the sample in methanol to try and loosen the SDS that also adsorbed to the sample.  This will make the etching process easier as well as remove any contaminants that stuck to the surface during the deposition process.

Thursday, May 10, 2012

Big Update

Although results from previous trials had shown that 1:1 EtOH/Water solvent gave promising results, more recent trials lead us to believe that a 2:1 ratio of ethanol to water gives more continuous coverage of nanospheres on the surface of the silica film, much of it being continuous monolayer with sporadic interruptions.  A concern which has been previously discussed is the evaporation of the alcohol in the solvent during the sonication and sphere deposition steps.  The greater amount of ethanol in the solvent makes for faster evaporation from the surface of the substrate as the spheres adsorb to the surface.  This leads to the conclusion that faster solvent evaporation leads to better monolayer formation, while at the same time making keeps the volume of the solvent constant.
Evaporation being primarily a mass transport matter has lead me to question the effects of air currents on the layer deposition phase.  To try and reduce the effects of these air currents, as well as any contamination in the air that may be settings on the sample or in the solution during the bead layering, a plastic cover has been fitted for the modified Langmuir-Blodgett trough apparatus.  Since using the cover the streakiness of the sample to the naked eye has been slightly reduced and monolayers have been forming consistently, which sporadic interruptions and multi-layers still, but the results are promising.  While the formation of monolayers is crucial, the formation of multi-layers may not cause problems for the final nano-pillar formation.
It is hypothesized that during the Reactive Ion Etching (RIE) step, only the areas covered in monolayer will allow ions to the surface on the substrate through the gaps between the spheres.  Areas where the beads are absent will be etched as well but will not have nanopillars and should be distinct from areas with nanopillar formation.  Where multi-layers have formed should have enough coverage with spheres that the areas between spheres will be masked by the above layers of spheres, leading to an overall masking of the area and prevention of etching.  We think that these areas will also be distinct from nanopillar regions under AFM and we will be able to test this hypothesis using SEM to see if the overall masking takes place.
Another method of sphere layering was tested for better results.  The method is referred to as the scooping technique (1).  The theory behind it is that by adding the sphere diffusion dropwise to a solution of water and ethanol that a monolayer of the spheres will form on the surface of the solution.  Then   a solution of SDS (Sodium Dodecyl Sulfate) is added to the solution with the floating monolayer to help the crystallization of the monolayer.  Next, the silica substrate that we are using would be dipped horizontally through the surface of the solution and allowed to soak.  Then the substrate is withdrawn at a constant speed and pick up the monolayer which has formed on the surface of the solution and it will adsorb as is to the surface of the substrate.
A few difference between the Langmuir-Blodgett technique and the scooping technique, to clear up any confusion, is that only a few drops of sphere diffusion is added to the solvent solution and the concentration of SDS is much lower because only a few drops of a dilute solution are added.  Then in the LB method the substrate is withdrawn vertically and the monolayer adsorbs at a constant rate and in the scooping method it is supposed to adsorb all at once. 
The technique did produce some samples that had intermittent layer formation but not in the quantity that the LB method produced and therefore the LB technique was used still.
The velocity of the barriers of the LB trough was increased to test their effect on the monolayer formation.  The increased speed of the barriers increased the coverage of spheres, in roughly the same ratio of monolayer to multi-layer, which is good because it meant that we had increased the amount of monolayers with which to create nanopillars without creating an excess of multilayers in the process. 
Four samples of 200 nm spheres on silica were produced, one for SEM prior to RIE and three for use with RIE, using these parameters.
Etching was performed with CF4 and O2. Prior to etching an unlayered sample was cleaned using ultrasonic methanol baths in 5 minute intervals.  This piece was used to determine the etch rate.  The clean sample was etched with an area of the sample covered to prevent etching in this area and the difference in height between the etched and unetched areas were determined using a profilometer and from this the etch rate was determined to be roughly 100 nm/min etching.  The rates of gas were 10 sccm CF4 and 5 sccm O2 at a pressure of 15 mTorr.  The RIE apparatus was a Trion Minilock II.  Power settings were 30 W for RIE and 350 W for inductively coupled plasma.    Images of the pre and post etching were taken on a Nikon Optical Microscope with above incident light.
The images showed that steps to create a cleaner surface prior to sphere deposition need to be taken in order to create a layered sample clear of debris.  A possible source of this debris is the oven in which oxide layer formation was done, as well as the scribing of the silicon wafer. Both will be investigated to determine if either can be modified to provide a cleaner sample.
 One of the layered samples was etched with the same parameters as above for 30 seconds.  The spheres were then removed in a ultrasonic methanol bath.  A second layered sample was etched under the same conditions, but a metal plate was attached that that it hung over half of the sample and it was etched for 5 minutes.  The metal plate blocked the path of direct etching ions but not ones coming from an angle.  This allowed for an etching gradient along the sample.  The idea was that the area of the sample fully exposed would be fully etched and the farther under the metal plate the spheres would be etched less and less, thus providing areas of different etching environments that could help lead us to the conditions that would be optimal for the etching  that we need.  When this sample was removed, it appeared that the spheres that were under the metal plate might have been removed and/or melted from naked eye inspection.  A concern brought up is that using a solution with too much surfactant may prevent even etching due to any etching prevention being caused by a surfactant film being present.  We will be able to do SEM next week on both samples in order to gain a better idea of what may have happened, as well as what conditions we will need and if anything about the process of sphere deposition may need to altered.

1. Y.J. Zhang, Journal of Alloys and Compounds, 450 (2008), 512-516

Friday, March 9, 2012

Effects of changing solvent on monolayer formation

Three different samples were made using two different ratios of ethanol as solvent.  Two samples were made using a 1:1 ratio of water/ethanol and the 100 nm sphere diffusion, the first being the normal 1 cm squared and the other being about .25 cm squared.  The second was to make sure that the movement of the barrier which increase the concentration of spheres near the surface of the sample was not effecting the film formation.  The third sample was done with 200 spheres and pure ethanol as the solvent.  The first two samples showed the same coverage compared to each other but greater than when methanol was used.  The third sample showed very good coverage, but drops of liquid formed and remained in place until the drops were farther away from the liquid-air interface and then slid down the surface of the sample. creating chaotic multilayers in the areas where the drop slid.  The other areas shows little to no streakiness with the naked eye.  The only problem I can find with using methanol and ethanol as a solvent is that they evaporate from the solution very quickly, reducing the volume of the solution in the trough.  Also, because the SDS is decreasing only when dipping is performed and the solvent is evaporating much faster than the SDS is leaving, I believe the SDS concentration is increasing over time.  The SDS is used as a spreading agent and the optimal concentration has been found around 34.7 mM (1).  As the concentration increases above this the spheres begin to form multilayers or clusters of spheres.
This increase in SDS concentration will not occur appreciably over the course of dipping in the trough but over the course of a few days and with the use of sonication the amount may become enough to cause a difference in monolayer formation.  That said, results show that a ratio in begin 100% ethanol and a 1:1 water/ethanol mixture may give the best results we can hope for without further study in the formation of monlayers of Poly-styrene microbeads on silica.  Will proceed from here and make as many samples in as short amount of time as possible to try and counteract the evaporation of ethanol from solution and move.  On a personal not, I've learned over the past few weeks that try as you might to make every little part of the project go perfectly, it probably won't.  However, I have learned new concepts through trying to perfect the films for my samples and can apply them later or come back and try again if they still are not sufficient.  For now, they will work for what we are trying to investigate and that is what is important.  Don't lose the forest for the trees.

Thursday, March 8, 2012

Monolayers update

A variety of silica samples were made using the modified Langmuir-Blodgett dipping method (1), varying the conditions  for each and SEM was performed.  A long range hexagonal monolayer was not formed but a few displayed streaks of monolayer spheres.  Spin coating was also performed however this also failed to create even streaky monolayer coverage of the sample.  One more method will be attempted before moving on to the next stage and working with the streaky monolayers and that is using ethanol as the solvent as well as increasing the immersion time before withdrawing the sample from the Langmuir trough.  The amount of coverage of the monolayers is not as crucial to this project as having some coverage.  The next phase is to create nanopillars on the surface, which will occur in the areas with nanosphers.  

(1) 

The Use of Surface Tension to Predict the Formation of 2D Arrays of Latex Spheres Formed via the Langmuir−Blodgett-Like Technique

Maricel Marquez and and Brian P. Grady*
Langmuir 2004 20 (25), 10998-11004

Thursday, March 1, 2012

Update of monolayer formation attempts

SEM images have been taken of the previous samples and have shown that monolayers were indeed formed but not nearly close enough to the surface areas that we are looking for.  After doing some reading there are methods done using a spin coater that are worth looking into trying.  I have also contacted the previous operator of the Langmuir-Blodgett trough being used and she has given me a few tips which may also prove fruitful.  Over the next week I hope to try and employ both methods and next Friday possibly take SEM images to check for whether or not one of the techniques works for our project.

Thursday, February 23, 2012

Monolayer formation update

Work has continued in attempting to create monolayers.  SEM was performed and showed that monolayers have indeed been produced, but they are slightly patchy.  The Langmuir-Blodgett trough is still being used but have slightly altered the method for withdrawal and hoping this will form full monolayers.  Another option to be tried in this coming week is to make new solutions of spheres using a 1:1 mixture of H20 and MeOH to try and affect the patchiness of the layers.

Wednesday, February 15, 2012

Working with silica update

Silicon wafers were placed in an oven at 900 degrees C for 3 hours in order to apply a thermal oxide layer for SiO2.  They were then cut using a diamond tipped etching tool and have been running tests using the modified Langmuir-trough setup to observe the film on the surface.  So far have used 100 and 200 nm sphere solutions with films that are almost fully complete after examination with the naked eye.  However, there are still a few spots which should not be bare and therefore we will be working on changing the speed of the trough "floaters", which maintain the surfactant concentration near the surface of the substrate as it's being withdrawn.  My hope is that by increasing the speed and making sure that the substrate does not come into contact with the floaters, the increased surfactant concentration near the substrate surface will allow for a more even coating of the spheres.  If this does not increase the uniformity of the film then another option would be to increase the SDS concentration in an attempt to increase the concentration of individual spheres (non-aggregated) near the liquid-air interface.

Thursday, January 26, 2012

Two week update

So I have made progress in creating monolayers of polystyrene microspheres on glass.  Because I have been using glass slides as practice I am a little nervous about how changing to the silica substrate will affect the monolayer formation.  Using previous literature I have performed 7 trials with various sphere sizes in either water or a water/methanol mixture to improve formation and film continuity. Using an optical microscope I can see the formation of these layers with 2 micron sized spheres but am unable to say with certainty about the 500 and 200 nm sized spheres.  The glass coverage is greater with decreasing sphere size and when using methanol and 2 micron spheres the coverage was greater but from what I can tell using a microscope it appears that a multilayer may have formed over most of the surface with small patches of monolayers scattered throughout.  I have repeated the trial and the sample is being dried overnight and observations will be made again tomorrow.  I will begin using silica next week and hopefully will be able to begin taking SEM images of these in order to determine with certainty the formation of this monolayer.  The literature that I'm using was performing these trials on highly ordered pyrolytic graphite and saw the best film coverage on this so I may have to see what I with silica and consider other options.

Tuesday, January 10, 2012

Multiple Day Update

Have been working in the lab but forgot to update the blog on my progress.
Last week I mixed a 6 mL solution of 2 micron latex spheres (1% wt, 2.4 mL latex solution, 3.6 mL nanopure water) and added SDS as a spreading agent (31.093 mM).  The solution was then sonicated fo 10 min and a piece of glass (microscope slide cover) was lowered about 4 mm into the solution and withdrawn at roughly 1 micron/second for 1 hour and 12.5 minutes.  The glass was allowed to dry for 140 min and then viewed under an optical microscope.  The film appeared to be a monolayer of latex spheres but was was patchy.  To help fix this I will be performing this again with a modified version of a Langmuir-Blodgett trough to keep an effective concentration of spheres near the surface of the glass as it's withdrawn.  Will add an update of this shortly.
I have also cleaned a silicon wafer using a plasma cleaner and ammonium hydroxide/hydrogen peroxide solution and cleaved a small area for AFM.  The RMS roughness of the silicon piece was determined to be 0.268 nm.