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 Then the substrate is withdrawn at 5 micrometers per second and while the barriers are advanced at 3 ticks above the slow mark.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
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.
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.
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