Spin Coating: Guide to Coating Difficult Solutions
|Written by Emma Spooner, a PhD student in Fullerene-Free Photovoltaic Devices at the University of Sheffield in collaboration with Ossila Ltd.|
Spin coating is a technique used to coat thin films from solution. Whilst the process is very simple, it is not always easy to produce a perfect film. Many spin coating problems can result from the properties of the solution being used, rather than from problems with the spin coating technique itself. Poor film uniformity can result from:
This article will discuss why these types of solutions can cause issues, what these issues are, and what techniques can be used in order to improve the quality of the film. A more comprehensive overview of the techniques to use whilst spin coating can be found in Ossila’s Spin Coating: A Guide to Theory and Techniques.
Solutions With Poor Solubility
Many researchers will have faced issues coating films where the solute has poor or marginal solubility in the solvent used. This is especially common when using high molecular weight materials, or when trying to replace the solvent in an established system. Solutions with poor solubility can produce films with unwanted precipitation, especially as solvent evaporation drives the concentration above the solubility limit, leading to comets or particulates in the final film. Precipitation can be avoided by keeping solutions below the solubility limit of the material, but this may lead to deposited films being too thin.
Insufficiently-thick films are best tackled with low spin speeds. As such, they may be more suited to static deposition, because dynamic deposition at low speeds can result in incomplete surface coverage. Solubility itself can be improved through additional components, such as by generating a solvent blend or using a solvent additive - where the main component gives the best performance, and the additive improves solubility. Mixing solvents can be informed by using Hansen solubility parameters to match the solvent and solute, and produce as ideal a solubility as possible.1
Solubility can also be improved through hot casting, where the solution is heated up and cast whilst at an elevated temperature. For most solutions, solubility increases as a function of temperature. Therefore, heating can be an effective way of increasing the solubility limit of the material. This is common whilst using some polymers with a tendency to aggregate at room temperature,2 such as PffBT4T-2OD (PCE11).
Another method used to help improve film quality is the sonication of solutions with poor solubility. Although sonication itself does not increase the solubility limit of materials, it can improve the rate of dissolution, which can be a slow process for materials like high molecular weight polymers. In some cases, filtering a solution will improve film quality. However, this is not always recommended, as important solution components can sometimes be accidentally filtered out.
Solutions with Extreme Volatility
Solutions with either very high or very low volatility can cause issues in spin coating.
When highly-volatile solutions are used, they can easily drip out of pipettes during spin coating, thus requiring an adjustment of technique. This is due to the evaporation of solvent inside the pipette, which consequently increases the pressure within the tip. This is a problem as the solution will not be deposited in one continuous motion - resulting in uneven coating. The resulting issue can appear as swirls, where each droplet only partially covers an area of the substrate. For the most volatile solvents - typically those with boiling points below 50°C - swirls can be seen even when using a continuous dispense technique. To counter this, either a static dispense technique or a larger volume of solution should be used.
Other evaporation defects that can occur are due to Marangoni instabilities (more information in the "Further Reading" section below), where a secondary flow is introduced in the wet film due to surface-tension gradients. This is a consequence of convection currents within the solution, caused by a temperature or concentration gradient. Temperature gradients will occur due to evaporative cooling, and concentration gradients will occur when evaporation is faster than diffusion through the film - meaning that the more volatile a solution is, both are more likely to occur. Marangoni defects normally manifest as a flower-like pattern, with a dense set of cell-like defects in the centre. Further from the middle, these ‘cells’ become stretched out - forming striations, or wavy films.
By slowing the rate of evaporation - and subsequently decreasing the temperature and concentration gradients in the film, this defect can be overcome. One such way of decreasing the evaporation rate is to use a less volatile solvent as the main solution. If this is not possible, even small amounts of a lower-volatility solution will slow down the evaporation process - usually without having a significant impact on solubility or performance. This can also be achieved by slowing down the rotation or acceleration to prolong the spinning time, therefore allowing viscous flow to stabilise the film. In some cases, very slow spin times can lead to non-uniform films. In these scenarios, it is best to move to static dispense spin coating.
Finally, saturating the spin coating atmosphere with the main solvent will also slow the rate of solvent evaporation. However, this may not be practical due to the amount of solvent waste, as well as the potential health and safety issues involved.
Marangoni defects are not always seen when using high-volatility solutions. This is because where the solution also has particularly low or high viscosity, viscous forces dominate over the Marangoni flow. However, even if uniform films have been formed, fast evaporation can lead to unfavourably amorphous films. Thus, slowing the rate of evaporation is still important to yield better crystallinity.
Solutions with low volatility can also cause issues, mainly through long drying times. Using very long spin times to dry the solvent can lead to very thin films, as evaporation takes place so slowly - which means that more of the fluid is removed via flow thinning. To tackle this without impacting film thickness, a slower drying step can be introduced as a second stage - typically at a spin speed of approximately a quarter of the main speed. Low-volatility solvents can also cause more pronounced edge effects. This is because it takes a longer time for the solution to be thrown off from the edge of the substrate, therefore slow solvent evaporation leads to a thicker film around the edges compared to the centre.
Low volatility solutions may also have poorer wetting due to higher surface tension. In the case of very viscous solutions, a static dispense may be required.
In some cases, solutions may be perfectly dissolved and of ideal volatility, but will still produce films with poor uniformity due to incomplete wetting. This issue often manifests as incomplete coverage of the substrate and can sometimes be improved by simply depositing more ink, or by improving fluid flow through an initial ‘spreading step’. This 'spreading step' is often short and at a slow speed, and works by spreading the solution across the substrate before the main high-speed step immobilises it. This is similar to the method used to spread photoresists.
If poor wetting is due to a rough or uneven substrate, this can be improved by planarising the surface, as sometimes used in organic thin-film transistors.3 Here, an inert (and often thick) layer provides a smoother surface - potentially with more favourable surface energy - for better solution wetting.
Most poor wetting can be improved by increasing the surface energy of the substrate through treatment such as UV Ozone Cleaning or argon plasma. Alternatively, the surface tension of the solution can be modified via the use of a surfactant. These generally will decrease the surface tension at the solution-air interface, improving wetting on the substrate.
Viscous and Non-Newtonian Solutions
Highly-viscous solutions can also present challenges, as they will be more resistant to deformation from shear forces during the spin coating process. This means that the outflow of solution from the substrate (as it reaches the desired spin speed) will be slower, and thinning of the solution during spinning will be reduced. This can lead to incomplete spreading of the solution across the surface of the substrate, which can sometimes be counteracted by static spin coating with a large amount of solution. Reduced thinning may also lead to undesirably thick films, thus requiring the use of lower solution concentrations.
For some solutions (e.g. colloidal solutions, polymer solutions, or solutions close to gelation), their behaviour will be significantly non-Newtonian. Newtonian solutions have a viscosity that does not change with force applied, meaning that shear stress and shear rate will scale linearly. In contrast, non-Newtonian solutions can will change viscosity depending on the force applied, meaning shear rate responds to shear stress in a different way. These are known as 'shear-thinning' or 'shear-thickening' solutions, depending on whether or not the force applied decreases or increases viscosity. Some solutions may also exhibit thixotropic or rheopectic behaviour, where the viscosity depends both on i) force applied, and ii) on how long it is applied for.
For these types of materials, final film thickness will not always be proportional to the inverse square of the spin speed - so film thickness can be difficult to predict, and final films are not always level. For a more in-depth explanation of the reasons behind this, please refer to Ossila’s Spin Coating: A Guide to Film Thickness. Due to their diverse range of behaviours, non-Newtonian solutions can present a significant challenge when it comes to the deposition of highly-uniform films.
For more information on the different aspects of spin coating, please refer to Ossila’s Spin Coating: A Guide to Theory and Techniques.
Further information can be found here:
Marangoni Instabilities: Liquid Film Coating. R. G. Larson and T. J. Rehg, ed. S. F. Kitsler and P. M. Schweizer, Chapman & Hall, 1st edn, 1997, ch. 14, pp. 709-734.
- The Three Dimensional Solubility Parameter and Solvent Diffusion Coefficient, C. M. Hansen, Danish Technical Press (1967).
- Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells, H. Hu et al., Nat. Commun. (5), 1–8 (2014).
- Flexible organic transistors and circuits with extreme bending stability, T. Sekitani et al., Nat. Mater. (9), 1015–1022 (2010).