Spin Coating: Complete Guide to Theory and Techniques

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Spin coating is a common technique for applying thin films to substrates. When a solution of a material and a solvent is spun at high speeds using a spin coater, the centripetal force and the surface tension of the liquid together create an even covering. After any remaining solvent has evaporated, spin coating results in a thin film ranging from a few nanometres to a few microns in thickness.

Spin coating is used in a wide variety of industries and technology sectors. Its primary advantage of spin coating over other methods is its ability to quickly and easily produce very uniform films.

The use of spin coating in organic electronics and nanotechnology is widespread and has built upon many of the techniques used in other semiconductor industries. The relatively thin films and high uniformity required for effective device preparation, as well as the need for self-assembly and organisation to occur during the casting process, do however necessitate some differences in method.

This guide aims to introduce general spin coating concepts, cover spin coating equations and theory, and describe some of the specific techniques useful in organic electronics and nanotechnology.

Introduction to Spin Coating

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Spin coating generally involves the application of a thin film (a few nm to a few um) evenly across the surface of a substrate by coating (casting) a solution of the desired material in a solvent (an "ink") while it is rotating. Put simply, a liquid solution is deposited onto a spinning substrate in order to produce a thin film of solid material, such as a polymer.

The rotation of the substrate at high speed (usually >10 rotations per second = 600 rpm) means that the centripetal force combined with the surface tension of the solution pulls the liquid coating into an even covering. During this time the solvent then evaporates to leave the desired material on the substrate in an even covering.

This process can be broadly divided into 4 main steps:

1. Deposition
2. Spin up.
3. Spin off
4. Evaporation

In the initial step, the solution is cast onto the substrate, typically using a pipette. Whether the substrate is already spinning (dynamic spin coating) or is spun after deposition (static spin coating), the centrifugal motion will spread the solution across the substrate.

The substrate then reaches the desired rotation speed – either immediately or following a lower-speed spreading step. At this stage, most of the solution is expelled from the substrate. Initially, the fluid may be spinning at a different rate than the substrate, but eventually the rotation speeds will match up when drag balances rotational accelerations – leading to the fluid becoming level.

The fluid now begins to thin, as it is dominated by viscous forces. As the fluid is flung off, often the film will change colour due to interference effects (see video below). When the colour stops changing, this will indicate that the film is mostly dry. Edge effects are sometimes seen because the fluid must form droplets at the edge to be thrown off.

Finally, fluid outflow stops and thinning is dominated by evaporation of the solvent. The rate of solvent evaporation will depend the solvent volatility, vapour pressure, and ambient conditions. Non-uniformities in evaporation rate, such at the edge of a substrate, will cause corresponding non-uniformities in the film.

Ambient Conditions and Changes in Drying Time

For many organic electronics and nanotech applications the drying time and film properties are intimately linked and for this reason the ambient conditions can sometimes have significant effects. A professional cleanroom will usually have a relatively well controlled temperature and humidity and a glove box is usually a pure nitrogen atmosphere. However, research labs are often not so well controlled and while this may not make much of a difference in most cases there are some extreme examples that can affect consistency and result in spin coating defects.

Here in the UK the ambient humidity in summer can swing quite dramatically from less than 20% to nearly 100% depending upon the weather. On times of very high humidity, typically during a rainstorm, films spun from aqueous solutions may still be wet after the normal 30 second spin duration and this can have a significant effect on device performance. As such, although the majority of work is done in temperature/humidity controlled environments it is advisable to keep a close eye on the ambient conditions and place a thermometer/hygrometer next to the spin coater.

If coating multiple large substrates with large volumes of solvent (100 µl or more) then it is also possible for the solvent to start pooling in the basin of the spin coater, which can also have the same effect as to increase the ambient vapour pressure and increase drying times. A simple solution to this is to wipe dry the spin coater basin after every spin if large volumes of solvent are used.

Spin Cleaning and Wash Steps

Spin coating can also be used to clean substrates, however experience of this is that in general it produces relatively poor results compared to sonic bath based cleaning and is slow compared to cleaning a batch of substrates at a time. It also often results in large volumes of solvents being used in the spin coater, which can be a problem if these steps are performed in an environment where that solvent needs to be removed (for example a laminar flow).

However, there are times when spin cleaning and wash steps can be very useful, for example HMDS and certain other surface modification treatments benefit from a spin-wash. Meanwhile, spin coating a semi-orthogonal solvent can also be used to remove certain additives.

When spin washing, researchers generally use a dynamic dispense of a medium volume of solvent (50 µl for the standard 20 x 15 mm substrates) and may wash several times. Beware of the fact though that this will create significant solvent vapour that can affect the drying dynamics of subsequent films if the solvent is not removed first (usually by wiping excess solvent from the inside of the spin coater with a clean-room tissue).

Preventing Common Defects

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Two Step Spin Coating and Edge / Corner Bead Removal

When spin coating with viscous or high boiling point solvents such as trichlorobenzene at very low spin speeds (below 500 rpm), the middle of the substrate often dries significantly quicker than the edge of the substrate. While the middle might be dry within a few seconds, in some cases the edge of the substrate may take minutes to dry. While the edges of the substrate are often designed not to contain any active/critical components, if the spin coater stops then the edge/corner bead may spread back towards the middle which can ruin the film quality.

There are essentially two ways to remove the edge/corner beads. The first and more preferable way is to use a two-step spin, with the first step programmed to give the desired film thickness and sufficient time for the ink to dry over the majority of the substrate, and a second step at maximum rpm to fling off the corner beads and dry the remainder of the substrate.

In most cases a two stage spin is the easier and preferable way to remove edge beads to improve spin coating uniformity. However there are some occasions where the second stage (high speed) spin is not desirable, for example if you want to remove the substrate from the spin coater while it is still slightly wet for further processing such as a methanol wash, vacuum dry or even slower drying in a solvent saturated atmosphere. In these cases a second but more delicate technique can be used to remove edge beads where by a fine cotton bud is used to absorb the excess solution while it is still spinning.

The key is to avoid knocking the substrate off the chuck and also not to damage the active area on the substrate. For effective corner bead removal via cotton bud follow the below points:

• Approach the substrate very slowly.
• Steady your hand by resting it on the lid and use the centre-hole in the lid to steady and guide the cotton bud in.
• Try not to touch the actual substrate; hold the tip of the cotton bud a fraction above the substrate so that it is in contact with only the edge bead but not the substrate.
• Pay attention to the movement of the cotton bud; even the most gentle touch of the cotton bud onto the substrate will produce a vibration which you should be able to feel.
• Watch the cotton bud closely; many organic and nanotech inks are coloured so you should be able to see when the cotton bud is close enough to absorb the ink.
• Try to keep the cotton bud as close to the edge of the substrate as possible.

Avoiding a Hole in Middle of Film

A common spin coating defect for beginners is to see a hole in the middle of the substrate with no coating on it such as in the image below.

This is typically caused by the ink not being dispensed in the middle of the substrate. Since the centrifugal force will always make the ink flow to the edge of the substrate the middle will not get coated. By dispensing the ink closer to the centre of the substrate this should be eliminated. Note that by using the edge of the centre-hole in the lid to guide the pipette it can be positioned more accurately.

Avoiding Vacuum Warping of Substrate

Many spin coaters use vacuums to hold the substrates in place. Not only does this often cause problems for spin coater maintenance (the vacuum is often trying to suck in the ink and solvents) but it can also warp the substrates slightly which causes uniformity problems. The extent of the problem will depend upon the thickness of the substrate, the strength of the vacuum and the size of the vacuum aperture. The image below demonstrates the effects of vacuum warpage on film uniformity.

The mechanical stiffness of a substrate is proportional to the cube of its thickness so a 1.1 mm thick substrate will be eight times stiffer than the equivalent 0.55 mm substrate. In general, substrate warping becomes an issue for substrates of less than around 1 mm in thickness and the effects are worse at low speed (where there is less centripetal force).

The effects of vacuum warpage are one of the primary reasons why Ossila developed the vacuum free spin coater. Firstly, in order to improve the uniformity of the films, the vacuum had to be eliminated. This led to the design of specialist chucks to place on top of older spin coaters to hold the substrates in place mechanically. This technique worked initially, but the subsequent design of a new spin coater eliminated the demanding servicing requirements that come with having a vacuum pump.

Although we have worked with flexible PET substrates extensively in the past to make real devices such as OLEDs and OPVs, here we take the the technique to the extreme and demonstrate spin coating parafilm (yes, really, parafilm). Even we didn't think this would be possible until we tried it but it's a great demonstration that spin coating onto flexible substrates is really easy using a vacuum free chuck.

The below points will also help to alleviate the problem of vacuum warpage:

• Use thicker substrates
• Use higher spin speeds
• Mount the substrate on a thick carrier plate before spin coating

Spin Coating Low Viscosity Solvents

When spin coating viscosity of a solvent can have a major impact on film quality. Certain solvents which are prevalent in research fields for historical or other reasons have very low viscosities which can provide significant issues. Examples of this kind of solvent include chloroform and acetone and there are two primary issues here:

1. The solution dripping out of the pipette before it is supposed to
2. The film drying before an even wet-layer has been produced, which creates swirls in the substrate

With regards to the ink dropping out of the pipette before it is supposed to, the first thing to do is to use the smallest size pipette tip that is available as it will have a smaller diameter aperture at the end and reduce dripping. It also helps to pipette the minimum amount of ink required for coating as that will reduce the effect of the weight compared to the surface tension (10 µl of solution will weigh half as much as 20 µl but the surface tension remains the same).

If the solvent is halogenated (such as chloroform) then it can also help to leave the pipette tip in the ink for a few seconds before removing it. The reason for this is that most pipette tips are made of polypropylene which will not dissolve in halogenated solvents but will swell slightly which will reduce the effective diameter of the aperture.

This technique does not work for non-halogenated solvents such as acetone as they will not swell the polypropylene. As such an alternative is to tilt the pipettor at an angle to reduce the weight to surface tension ratio, returning the pipettor to vertical only just before dispensing the solvent. Care must be taken not to tilt the pipettor to the extent that solvent will enter the pipettor and damage it.

Incomplete Coating of Substrate

The wetting of an ink onto a substrate is generally characterised by the contact angle of the liquid onto the surface. A low contact angle means good wettability (the ink likes to spread across the substrate) while a high contact angle means poor wettability (the ink likes to ball up).

The contact angle will depend upon the surface tension of the liquid and the surface energy of the substrate. A liquid with a high surface tension wants to ball up more than a liquid with a low surface tension. Meanwhile, a substrate with a high surface energy is more likely to attract the ink as it will result in an overall lower energy state.

Some ink / substrate combinations will coat very nicely with the ink wanting to spread across the surface while in other cases it will want to ball up. In extreme cases this can mean that it is simply not possible to coat an ink onto a surface. More commonly it is simply difficult to wet a surface and this often results in partial coating of the substrate.

The first and simplest solution to this problem is simply to deposit more ink - for example for with standard 20 x 15 mm substrates you might increase the dispense volume on the pipettor from 20 to 50 or even 100 µl. Increasing the temperature of the ink and dispensing warm or hot can also help by reducing the surface tension and increase the evaporation rate so that there is less time for the ink to dewet from the surface.

However, if the above options don't work then it is usually necessary to either change the solvent in the ink to something with a lower surface tension or to treat the substrate in some way such as by a UV/ozone or oxygen plasma to increase surface energy (note that this is often not desirable for organic based transistors where a low surface energy is key to high performance).

Pin Hole Defects & Comet Streaks

The physical and chemical cleanliness of a substrate is critical for high quality films regardless of the application method. For spin coating the effect of dust and particulate matter is usually to cause pin holes and comet streaks, however this can also be caused by particles in the ink described in the below section.

To remove dust and particulate matter the use of an electronic grade detergent such as Hellmanex III is generally recommended. However, it's also important to remove any residues on the surface for which a semi-polar solvent such as acetone/IPA is helpful. Finally, it is usually worth chemically preparing a substrate before applying the first layer by using either a basic NaOH solution or an oxygen plasma/UV Ozone to produce "-OH" terminations which are excellent for most coating processes. As such, the below standard cleaning routine should be used for most substrates. Where possible, use a solution-based NaOH process for providing "-OH" surface terminations for ease and simplicity but in the case of silicon this will wreck the surface, so use an oxygen plasma treatment instead.

Once a substrate is clean, store it in a clean environment (often in DI water) to avoid dust and other contaminants. This is important as even though dust can be blown off a substrate it often leads to a change in surface energy where the dust particle was placed which can lead to pin-holes on the surface such as the below example.

Aggregation and Filtration

For many OE/nanotech inks there is the possibility of aggregates or crystallites forming in solution prior to deposition of even during the spin coating process. This can lead to comet streaks or large lumps of aggregation left on the surface.

Generally, heating and stirring a solution will help to dissolve the active materials and then cooling and filtering will remove any aggregates and undissolved material and this can often have significant effects on device performance. In the below examples, improvement can be seen in device performance for both OFETs and OPVs after a solution has been filtered.

However, for many materials an ink solution may not be stable and will re-form aggregates or crystallites over time, such as the below examples of P3HT films, PCBM crystallites and F8BT aggregates, all of which will form if an ink is left standing for long times (hours to days).

Aggregates of different materials and the approximate timescales required. In all cases the solutions were filtered prior to storage.

In some cases it's possible to re-dissolve these particles by re-heating/stirring and often it's worth filtering a second time after the solution has cooled. However, in some cases, such as for PCBM, the energy of crystallisation is significant and therefore it's very difficult, if not impossible (depending on the solvent) to re-dissolve them, therefore fresh solutions should be used each time.

However, before filtering any solution it is always worth considering the size of any solutes relative to the filter pore size; while polymers, PCBM, and small nanoparticles (<20 nm or so) can all be filtered without problems, larger nanoparticles or graphene flakes have a large chance of being caught in the filter and totally removed from the solution (leaving just solvent).

Coating Difficult Solutions

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While the spin coating 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 poor solution solubility, highly-volatile solutions, poorly-wetting solutions, and viscous or non-Newtonian solutions.

This section discusses 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.

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.⁵

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,⁶ 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, 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.

Poorly-Wetting Solutions

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.⁷ 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 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 the 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. 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.

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Keep Reading...

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For more information and details on film thickness equations and mathematical models, see:

• Journal Article: Dynamic, crystallization and structures in colloid spin coating, M. Pichumani et al., Soft Matter (9), 3220-3229 (2013); 10.1039/C3SM27455A.
• Book: Liquid Film Coating. R. G. Larson and T. J. Rehg, ed. S. F. Kitsler and P. M. Schweizer, Chapman & Hall, 1^st edn, 1997, ch. 14, pp. 709-734.

For further information on spin coating difficult solutions, see:

• Hansen Solubility Parameters: Organic photovoltaic devices with enhanced efficiency processed from non-halogenated binary solvent blends, J. Griffin et al., Org. Electron. (21), 216-222 (2015).
• Marangoni Instabilities: Liquid Film Coating. R. G. Larson and T. J. Rehg, ed. S. F. Kitsler and P. M. Schweizer, Chapman & Hall, 1^st edn, 1997, ch. 14, pp. 709-734. 

References

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1. Flow of a Viscous Liquid on a Rotating Disk, E. G. Alfred et al., J. Appl. Phys. (29), 858–862 (1958).
2. Characteristics of resist films produced by spinning, D. Meyerhofer, J. Appl. Phys. (49), 3993–3997 (1978).
3. An Investigation of the Thickness Variation of Spun-on Thin Films Commonly Associated wtih the Semiconductor Industry, J. W. Daughton, J. Electrochem. Soc. (129), 173–179 (1982).
4. Dynamics of polymer film formation during spin coating, Y. Mouhamad, J. Appl. Phys. (116), 123513 (2014).
5. The Three Dimensional Solubility Parameter and Solvent Diffusion Coefficient, C. M. Hansen, Danish Technical Press (1967).
6. Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells, H. Hu et al., Nat. Commun. (5), 1–8 (2014).
7. Flexible organic transistors and circuits with extreme bending stability, T. Sekitani et al., Nat. Mater. (9), 1015–1022 (2010).