Polymer ultrathin films by spin-coating

2.1 Materials and Methods

• Solvent: for spin coating, a good solvent for the specific polymer has to be used, otherwise the polymer will not be completely dissolved pro-ducing a turbid solution, which normally can be observed by the naked eye. These concentration fluctuations normally have dimensions in the meso-scale range, deposited on the solid surface leading to extremely rough surfaces. This can be observed on PMMA thin films spin coated from a PMMA-toluene solutions. It is also evident that ultra high pu-rity solvents have to be used for the solution preparation. The solvent chosen has to show a considerably high boiling point (normally higher than 100 ^○C). If this is not the case, a skin will form on the surface during the application of the solution on the solid substrate, not al-lowing a homogeneous drying during spinning. This leads to extremely rough films. An example is when films are produced by spin-coating PMMA dissolved in acetone. Acetone has a low boiling point (≈57^○C), whereas MEK (methyl ethyl ketone) another good solvent for PMMA, has a higher but still low boiling point of 79.64 ^○C. A good solvent for PMMA having a quite high boiling point (155.65^○C) is cyclohexanone, which is recommended for spin coating of PMMA. However, this sol-vent can suffer oxidation, what brings a yellow color to the solutions, if kept in ambient air atmosphere and exposure to light. For polystyrene a good choice is toluene, which has a boiling point of 110.6^○C. All PS films in this work were prepared by spin coating using a PS-toluene solution.

• Spinning time: as good solvents are necessary for a high film quality, it also means that the interactions between polymer and solvent are strong. Consequently, the solvent will hardly leave the polymer film completely (even under high temperature and ultra high vacuum). If the spinning time is short (less than 2 minutes), a considerable amount of solvent will remain on the film. Handling the coated surface makes this remaining solvent to flow across the film, leading to waviness in the film. This phenomenon can be observed by the naked eye if one watches thicker films under white light right after a short spinning time:

Newton rings are formed and different colors appear, creating a very inhomogeneous coloration. In fact, when observed by the naked eye, a high quality film should appear to have a homogeneous color over the whole surface. Drying during spinning happens mostly due to diffusion of the solvent to the adjacent air layers and this slows down the drying process in ultrathin films. For these reasons all polymer films prepared in the present work were spun for 10 minutes at constant speed.

• Spinning speed: moderate spinning rates should be used in order not to have chain scission and further increase the polydispersity (speeds higher than 6000 rpm should be avoided). A general accepted number is around 3000 rpm [65]. If thinner films are needed, one should play with the concentration of polymer in the solution, rather than increasing the spinning rate.

• Application of the solution: the surface to be coated should be completely covered with the solution before spinning, otherwise the lateral inhomogeneity caused in the borders of the solution droplet can affect the smoothness several millimeters into the central part of the film. Therefore, applying a single droplet to the center of the substrate expecting to achieve a complete surface coverage by spinning is a common mistake.

• Deposition time: the solution should be carefully applied on the complete area of the solid surface (this procedure should not take more than 5 seconds), and afterwards one should wait around 5 seconds be-fore start spinning. If a longer interval is taken between the deposition of the solution and spinning, a skin can be formed, resulting in a rough film. This systematic procedure also helps to achieve a reproducible way of preparing thin films.

• Filtration of the solution: dust is normally present in the polymer, solvent and in the laboratory environment. For these reasons, filtration is a necessary procedure. Some polymers form globules hard to be dissolved even under mixing for several minutes in a good solvent (as very high molecular weight polymers). Therefore, the filtration of the solution also helps to remove these clusters. In case the solutions are not dust free or contain undissolved polymer, these inhomogeneities will cause roughness and will work as centers of concentrated tension finally nucleating dewetting.

• Filter cleaning: another important procedure to be observed is the cleaning of the filters before filtering the solution. Filters are normally not dust free and can contaminate the solution with particles (as PTFE particles from the filter itself). For this reason the filters should be rinsed with about 10 ml of solvent and dried under clean atmosphere in an ideal case (to protect from new contamination by dust particles).

In this work 0.2µm pore size PTFE filters from Millipore were used.

• Syringe cleanness: plastic syringes must not be used. These syringes are normally made of semi-crystalline polymers as polypropylene, which will not easily dissolve and contaminate the solution in a considerably way, but they may contain plasticizers that can be released to the solu-tion. Polymeric syringes may contain lubricants as PDMS oil or anal-ogous in order to facilitate the sliding of the piston. These substances will lead to catastrophic results as phase separation and consequently to film dewetting. Of course,T_g reductions will also take place, just as it happens in bulk materials by the addition of plasticizers. In many cases the common needles for medical applications are believed to be ideal for this use due to their cleanness. In fact, those needles are sterile but often externally and internally coated with anti-coagulant materi-als as Heparin, which again will work as a plasticizer. Ideally, for this purpose special metallic needles and glass syringes selected for DLS should be used. The cleaning procedure of these needles and syringes is the same as the one for DLS cuvettes in hot acetone fountains.

• Solution concentration: high polymer concentrations (> 5 % w./w.) can make the filtering procedure impossible due to the high solution viscosity and, the formation of a skin on the film surface, leading to rough films. In general dilute solutions are used (1-2 % w./w.), with low enough intrinsic viscosity [103].

• Surface cleanness: impurities on the glass or metal surface (as dust or even imperfections) will lead to the formation of the so-called island effect, i.e., the local induced inhomogeneity will extend its effect from the vicinities of the impurity to the range of millimeters, what can be easily observed by the naked eye. Therefore, it is clear that clean and smooth metal surfaces are crucial for obtaining high quality polymer thin films. Before depositing the metal film by thermal evaporation, the glass surface also has to be extremely smooth and clean.

• Affinity between surface and polymer solution: it is necessary that the polymer solution wets the surface to be coated. Otherwise a high quality polymer film will never be achieved.

• Size of the substrate: very small substrates are not recommended for spin coating when the here described spin coated method is applied. If the surface to be coated has lateral dimensions<<20 mm, border effects will be expanded towards the central part of the substrate so that a homogeneous film across the whole substrate will hardly be obtained.