BIBLIOGRAPHY
A.1 Polystyrene Nanofilm Preparation
The quality of the solid nanofilms which were then liquefied at the start of the molten nanofilm experiments detailed above is of critical importance. In the case of the flat plate experiments described in Ch. 3, Ch. 4, and Ch. 5, the presence of defects can lead to nonlinear growth or dewetting, which obscures the desired instability and corrupts the measured wavelengths and growth rates. Defects will also destroy the optical properties of the micro-optical devices described in Ch. 6 and Ch. 7 because they are scattering centers which induce optical loss. The preparation of high quality nanofilms starts with pure solvents to make sure that additional contaminants are not introduced. Specifically, the polystyrene (PS) solvent was toluene and it dissolves both standard washbottles and plastic filters. This causes contamination in the final films, so any materials used during nanofilm preparation were either glass or metal. To reduce all sources of contamination precise filtering was performed during preparation and while dispensing the solution for spin coating. The nanofilm preparation procedure has been divided into two sections: one devoted to the initial creation of the solution and the other devoted to the spin coating process. Note that all of these steps were accomplished in labs which were not a cleanroom, although this would have been helpful. Instead, polyester filters were placed in the air registers of the room and changed annually. To further reduce the level of airborne contaminants a HEPA filter (Alen BreatheSmart) was run continuously on its highest setting near the fume hoods.
A.1.1 Dissolving and Filtering Polystyrene in Toluene
1. Begin preparations by donning a lab coat, nitrile gloves, safety glasses, a hair net, and a surgical mask. The use of a hair net and surgical mask are to reduce the amount of contamination that is introduced from external sources into the solution.
2. Rinse a glass bottle and a glass beaker with toluene and dry with nitrogen.
DO NOT USE TOLUENE FROM A WASHBOTTLE.Toluene dissolves the bottle and leads to contaminants in the resulting nanofilms. Instead, pour
the toluene from the large 4 L glass bottle into a smaller beaker and use it directly from the beaker. Refill this secondary beaker as needed throughout the process. The cleansed beaker will be the initial mixing vessel and the bottle will house the final solution. The toluene in the washbottle is only for cleaning the spin coater.
3. Rinse a small metal scoopula with toluene and dry with nitrogen. Tear off a small section (approximately 2” x 2”) of weighing paper and put it onto the balance. Tare the balance.
4. Weigh out the appropriate amount of polymer onto the weighing paper for your desired solution concentration. Several different weight percent solutions and typically make at least 10 mL of solution at a time. In 10 mL of toluene, the amount of PS that is needed for the corresponding weight percent is listed:
1% = 87.9 mg; 2% = 177.6 mg; 4% = 362.5 mg; 8% = 756.5 mg.
5. Fill the initial mixing beaker with 10 mL of toluene and carefully pour the polymer on the weighing paper into this solution. Cover and leave solution to dissolve while performing the next three steps.
6. Clean the glass syringe with stainless steel Leur lock tip and glass plunger (Cadence Science) by rinsing in pure toluene from a glass beaker. Separate the two pieces and clean them individually. Then dry them using nitrogen gas.
7. Disassemble and clean each part of the stainless steel filter holder (Whatman) with toluene individually. Dry each component with nitrogen and then re- assemble in the same order (large donut gasket beneath the steel mesh, flat spacer above steel mesh) with the Anodisc filter (0.02 micron pore size) in- serted directly above the steel mesh and below the flat spacer. Attach the filter holder assembly to the end of the 5 mL glass syringe. The holder will screw into the metal end of the syringe. Both of these seals should be at least finger tight and should not leak during normal filtering operations.
8. Clean the metal scoopula with toluene, dry it with nitrogen and put it away. If the polymer solution is not completely mixed at this point, it can be swirled by hand to promote mixing. In cases of high PS weight percentage, the solution can also be sonicated for one to two minutes. The solution can also be heated
on a hotplate to promote mixing, but care must be taken so that toluene does not evaporate during this process and so generally sonication is preferred.
9. Once the solution has mixed completely, remove the plunger from the 5 mL glass syringe and pour about half the 10 mL solution into the syringe.
Replace the plunger and apply just enough pressure so that the solution passes through the filter at a rate of approximately 1 drop per second. Do not exert too much pressure to prevent the Anodisc filter from cracking which will cause solution to leak out of the connection between the filter holder and the syringe. Refill the syringe and repeat this process until the entire solution has been filtered. After filtering approximately two full syringes, the filter will tend to be saturated with material and the pressure required to filter the solution will increase markedly. This is more of a problem at high weight percentages (e.g. at 8% this typically occurs after one full syringe) and can be remedied by cleaning the filter holder and inserting a new Anodisc filter, as in Step 7.
10. Label the solution with the date, the preparation parameters such as the type of polymer and weight percentage, and any other relevant information.
11. Clean up the rest of the materials by disassembling the filter holder, disposing of the Anodisc filter in the glass disposal container, and rinsing each piece of the filter holder with toluene. Dry the pieces and then reassemble the holder.
Wrap the filter holder in aluminum foil to prevent contamination from dust and store in the cabinet. Rinse the plunger and glass syringe separately with toluene and then dry with toluene. Store in the box and return it to the cabinet.
Dispose of any remaining toluene that is not in the large 4L bottle by cleaning the initial mixing beaker and then disposing of any other toluene in the waste container.
A.1.2 Spin Coating Nanofilms
1. Begin preparations by donning a lab coat, nitrile gloves, safety glasses, a hair net, and a surgical mask. Clean the glass syringe and the filter holder as in Steps 6 and 7 in Sec. A.1.1.
2. Rinse a glass bottle and a glass beaker with toluene and dry with nitrogen.
DO NOT USE TOLUENE FROM A WASHBOTTLE.Toluene dissolves the bottle and leads to contaminants in the resulting nanofilms. Instead, pour
the toluene from the large 4 L glass bottle into a smaller beaker and use it directly from the beaker. Refill this secondary beaker as needed throughout the process. The cleansed beaker will be the initial mixing vessel and the bottle will house the final solution. The toluene in the washbottle is only for cleaning the spin coater.
3. Grab a clean 2” silicon wafer from the pack by grasping the flat edge and place it onto the spin coater. Try to stay as close to the edge as possible while maintaining mechanical stability because anywhere the tweezers touch becomes contaminated with defects in the resulting nanofilm. Silicon wafers are generally cleanest as they come from the manufacturer and that more defects are introduced by attempting to clean them (with solvents, piranha, HF, etc.).
4. Turn on the spin coater (Cee-100, Brewer Science) and select the program by pressing ‘Run’ and then the program number. There is an initial centering step that should be completed before the solution is applied to the wafer. This step can be completed by pressing ‘Start’ once. If the wafer is not centered, it can be adjusted and tested again by pressing ‘0’. All of the programs for the spin coater should be listed on the blackboard next to the fume hoods. They can all be modified and the interested user should consult the manual for a full listing of the possible options. If a program is changed, then the list on the blackboard should be updated.
5. Once the wafer is centered, draw solution slowly into the 5 mL glass syringe without the filter holder attached. Take care to have a constant pull speed to avoid introducing extraneous bubbles into the solution. Dispense three to five drops of solution back into the container to fill the dead space in the filter holder. Dispense the solution slowly onto the wafer opposite the flat edge to avoid spreading defects. Dispense close to the wafer so that a meniscus forms and the solution can be dispensed continuously without the formation of drops. To improve reproducibility, dispense solution until the point when the entire wafer is covered by solution. An alternative technique to this is to always draw the same amount of solution into the syringe (1 mL, for example) and then dispense the entire syringe. When utilizing this approach, care must be taken to fill the dead space of the filter holder before measuring the correct amount of solution in the syringe.
6. Close the spin coater and start the spin coating process which is typically a 30 second spin at 3000 RPM with an acceleration of 1000 RPM/s. Once the chuck has stopped rotating, visually inspect the quality of the film, looking for dust and comets in the film. After inspection, immediately place the wafer into a 2” wafer carrier. Label the carrier with the date, your initials, the type of solution, and any other information relevant to this wafer. The sample is now ready for film height measurement with the ellipsometer or deformation in the experimental setup.