Dip Coating: Guide to Troubleshooting Defects
|Written by Mary O'Kane, who is pursuing a PhD in Perovskite Solar Cells at the University of Sheffield in collaboration with Ossila Ltd. As part of this, she shares her knowledge with Ossila customers by creating helpful guides on solution processing methods.|
Dip coating is a simple and effective technique used across many different industries. Within research and development, it has become an important coating technique for the fabrication of thin films. When the process is optimised, it can be used to produce highly uniform films, and factors such as film thickness can be easily controlled. One advantage of dip coating is the simplicity of its design. It is low-cost to set up and maintain, and can produce films with extremely high uniformity and roughness of nanometres.
To make high-quality films using this process, parameters (such as withdrawal speed) must be optimised. Atmospheric factors (such as temperature, airflow, and cleanliness) also play a big part in film quality. All of these factors can result in variations in film quality, and must be closely monitored during the dip coating process.
This guide explains why variations in the form of defects arise, how these defects can be identified, and how they can be rectified.
Changing Withdrawal Speed
The meniscus is defined by two main forces:
- Gravitational-based viscous drag
- Surface forces between the substrate and the solution
During withdrawal of the substrate, the solution (hereby referred to as ink) in the meniscus will either fall back into the reservoir or be pulled up with the substrate to make a film, as shown in Figure 1. The meniscus terminates at the drying line. This is the point at which all the solvent has evaporated or drained, leaving a solid film. The drying line moves at the same rate as the withdrawal speed. As the substrate is withdrawn, the meniscus stays near the substrate-reservoir boundary, effectively “moving down” the substrate. With dip coating, there are two main regions defined by withdrawal speed: the capillary region (low speed) and the draining region (high speed).
In the draining region, withdrawal speeds are higher than 1mm/s. The drying line is moving faster than the solvent is evaporating. Here, film thickness is governed by the ink properties and the withdrawal speed, according to the Landau-Levich model. Evaporation rate does not factor heavily here.
In the capillary region, withdrawal speeds tend to be lower than 0.1mm/s. The key factor here is that the evaporation rate is faster than the "movement" of the drying line. Here, as soon as the ink is fed into the upper part of the meniscus, the solvent is evaporated. It is then replaced as surface forces pull more ink up the substrate - and again the solvent is evaporated. This is capillary feeding. Therefore, in this region, the lower the withdrawal speed is, the thicker the film.
There is a region in between these which can be accurately modelled by a combination of the models above. This region produces the thinnest film and defines a V-shaped dependence of thickness on withdrawal speed, shown in Figure 2. Working in the capillary region can cause some issues outlined in this guide - so if possible, it is better to work with higher withdrawal speeds. However, if the coating ink is very diluted, it may be necessary to use low withdrawal speeds to achieve a uniform coating.
For further information on the theory behind film thickness, you can refer to our page, Dip Coating Theory: Film Thickness.
Dip Coating Issues
Dip coating is a relatively simple technique. However, in order to achieve maximum control when coating a substrate, it is important to be aware of what can affect your results. There are two main things that dip coating is vulnerable to:
Defects due to instabilities in the dip coater or variation of withdrawal speed.
"External" factors. This includes the atmosphere the substrate is coated in, and the viscoelastic and chemical properties of the ink.
The following section looks at the two categories of defects and discusses commonly-occurring problems, the characteristic features of these defects, where they arise from, and the methods that can be used to remove these defects.
This section discusses "stripes" across the film, orientated perpendicularly to a substrates withdrawal as shown in Figure 3. This defect appears at regular intervals along the coated substrate.
The characteristics of stripes are:
Bands of thicker film (forming horizontal stripes), perpendicular to withdrawal direction.
Can appear as colour variation or structured inhomogeneity in the film thickness.
The defect will occur at roughly evenly-spaced intervals or frequencies.
The origin of this dip coating issue is usually due to low withdrawal speed. An example of this can be seen below.
Low Withdrawal Speeds
Films produced in the capillary region can exhibit specific defects due to the "coffee-ring effect". If ambient temperatures are high, evaporation rates are also high. Solvent at the edge of the meniscus evaporates first, leaving the solute deposited. Due to capillary feeding, as soon as the solvent has been evaporated, more ink replaces it. The edge is therefore “pinned” here (see Figure 5).
As a result of this, more solute is deposited at the edge of the meniscus, so the film will be thicker in these regions. As the substrate is moved upward, this pinned edge is separated from the meniscus. The meniscus moves down until the edge is pinned again. These “stripes” will be periodic, as this process will keep repeating.
There are two ways to reduce this effect: increase the withdrawal speed, or reduce the ambient temperature. Thick films can be produced in both regions, but these “coffee-ring” effects are only seen in the capillary region. By increasing the withdrawal speed, you negate this problem. To enter the draining region, withdrawal speeds of over 1mm/s are recommended. If you are using a particularly diluted ink, this may also involve increasing the concentration to achieve a uniform coverage on your film. It must be noted that there is an upper limit in regards to withdrawal speed – see running and curtaining for more information on this.
Alternatively, if you cannot change the concentration, this effect can be lessened by reducing temperature. Reducing temperature reduces evaporation rate, therefore lessening the capillary feeding. However, this could still lead to inhomogeneity in the film, evident by a colour variation. Another alternative is to use an alternative solvent (that the solute is dissolved in) with a higher boiling point, in order to further reduce the evaporation rate of the solvent.
Visible Particles, Pinholes, & Craters
This section discusses visible defects that can be seen in the film's microstructure. There are three main types of defects: visible marks, pinholes, and craters. An example of these effects can be seen in Figuure Whatever. The difference between these is shown in Figure 6.
There are several signs that there could be defects on a film, such as:
Visible particles on the film's surface.
Optical properties of the film could be affected (e.g. transparent coating appearing ”hazy”).
These defects can be caused by many things, including:
Dust or contamination on the substrate before coating.
Aggregation or crystallisation of the solute
Evaporation cooling effects
Dust or contamination on the substrate before coating
The presence of small particles (often visible) can be the cause of defects in the film. Solution can form comet-like trails behind the particles, or the particles can act as sites of aggregation. Even if the small particles are removed before the film is deposited, this contamination could still lead to pinholes. Previous contamination can change the surface energy of a substrate, leading to inefficient wetting in this area - creating a pin hole, where the film is thinner.
Therefore, it is important to thoroughly clean the substrate before deposition. Cleaning substrates for dip coating requires a similar process to the one used in spin coating. Initially, it is best to clean the substrate with an electronic grade cleaner, (such as Hellmanex III), and a semi-polar solvent (such as acetone/IPA). This ensures that there are no dust particles or other residues left on the substrate. Secondly, when coating a substrate, it is useful to chemically prepare it, exposing active “-OH” terminations to aid effective wetting. This involves cleaning it with an NaOH solution or an oxygen plasma/UV Ozone cleaner. The substrate must then be stored in a clean environment to reduce further contamination.
Aggregation in Solution
Depending on the inks used, aggregation or crystallisation of the solute may have occurred. This happens for materials that are weakly soluble in the solvent used. Additionally, solutes may aggregate or crystallise during the dip coating process, instead of forming an even film.
If crystallisation has occurred before deposition, it may be possible to redissolve by heating the ink. Additionally, inks should always be filtered before use to remove any contamination. The pore size used in the filtration should be roughly the same as the thickness of the film, to reduce any visible contamination. Pore sizes of around 2µm are recommended, but smaller can be used if necessary. If the inks are being stored, they should be regularly filtered.
Evaporation Cooling Effects
The evaporation of solvents during the drying phase cools both the substrate and the film. This cooling could lead to several problems during subsequent film formation, leaving crater-like marks in the fine structure of the film. On a macroscale, this can create a hazy coating where there should be a transparent one (as shown in Figure 8).
This effect can be reduced by heating the ink before deposition takes place. Even heating from room temperature to 25°C can increase film uniformity drastically. Also, it is important to leave the substrate in the ink for a period of 30-60 seconds during immersion. This allows it to reach thermal equilibrium with the ink. During withdrawal, the substrate can then store this heat, further reducing the effect of evaporation cooling.
Partial or Inhomogeneous Coating of the Substrate
An advantage of using dip coating is the level of uniformity that can be achieved. This section discusses the factors that can lead to inhomogeneity in a film when dip coating.
Characteristics of an inhomogeneous film include:
Colour variation appear across the coated substrate
Thickness variation occurs across the film
The origin of inhomogeneity could be due to:
Turbulent Airflow During Drying
Inconsistent Withdrawal Speed
Meniscus Height Issues
One way to characterise a ink's wettability (whether it will spread across a substrate) is by using contact angle analysis. A small contact angle means a liquid has high wettability with a substrate. In other words, it will spread well (see Figure 10).
Contact angle depends on two things: the surface tension of the liquid and the surface energy of the substrate. If the surface tension of the liquid is high, the molecules will have a strong attraction to each other, this results in dewetting. While if the surface energy is high, the liquid molecules will have a greater affinity towards the substrate resulting in wetting.
If either the surface energy of the substrate is too low, or the surface tension of the ink is too high, the ink will not sufficiently coat the substrate. The meniscus formed in dip coating comes from a balance between gravitational-based “draining” forces and surface-tension-based “capillary” forces. If the ink is more attracted to itself than it is to the substrate to begin with, then it will be challenging to achieve uniform coating of the substrate.
In this situation, it is often best to change the solvent to one with a lower surface tension, use a surfactant, or treat the substrate to increase its surface energy (i.e. with an oxygen plasma). The latter is not recommended for organic-based transistors where a low surface energy is needed for optimal performance.
For more information on surface wetting and contact angles, please see our guide on contact angle theory and measurement.
Turbulent Airflow During Drying
During the drying phase, the wet film is extremely sensitive to external factors, especially airflow. Turbulent airflow can affect evaporation rates, and subsequently drying rates, leading to inhomogeneities. No airflow over the substrates can also cause problems during drying. A tell tale sign of this is the inhomogenity will not show an obvious pattern, but may form "streaks" of thicker or thinner film.
It is best if the substrate is exposed to constant laminar flow during drying, so that factors like evaporation rates can be kept consistent. However, caution must be taken, as with airflow can come contamination - which the film is also vulnerable to during the drying phase. It is important that this stage takes place in a clean environment, such as a clean room.
Inconsistent Withdrawal Speed
Thicknesses of films created in dip coating are dependent on withdrawal speed. Therefore, if the withdrawal speed is not consistent, this will lead to thickness variations in the film. If this happens, the thicknesses would only vary parallel to the withdrawal direction. Thicknesses across the substrate (perpendicular to withdrawal direction) would be consistent. This is illustrated in Figure 11.
Meniscus Height Issues
For low withdrawal speeds, film thickness is dependent on evaporation rate. This, in turn, is dependent on the ambient conditions surrounding the film. As the substrate is submerged, and if the reservoir is too full, the meniscus may go higher than the reservoir (see Figure 12).
Evaporation happens at the meniscus edge for very low withdrawal speeds. This meniscus is effectively exposed to two different atmospheres: one inside the reservoir and one above the reservoir. This could lead to different evaporation rates, and thus different film thicknesses. If a thickness profile shows a peak in thickness at the initial point of film formation (see Figure 13), this is probably the error. To reduce this effect, it is important to ensure that the depth of the solution remains relatively constant. Therefore, the volume of solution must be significantly greater than the volume of the substrate.
Cracks can often be seen in a thin film after post-deposition heat treament. This section discusses cracking in thin films. This characteristics of cracking are:
Long, straight cracks on microscale of film structure
The amount of cracking will increase with film thickness
The causes of this defect could be:
Small particles on the film surface
Loss of water/organics during deposition
Thermal expansion mismatch
Small Particles on the Film Surface
A main cause of cracking in thin film formation comes from contamination during the “wet” stages. These will create structural weak points, which may put additional stress on the film during heat treatment. As mentioned before, films should be prepared in a clean environment, ideally while exposed to laminar, clean airflow. It should be noted that the cracks caused are far more significant if film thickness is above the critical thickness.
Loss of Water/Organics in Film
During dip coating, there is a substantial volume change from the initial liquid layer to a solid layer. This can lead to significant cracks developing, due to the viscoelastic relaxation of the film during heat treatment. These effects are observed if the film is above a certain thickness – the critical thickness. For example, if small particles have contaminated the film during the initial drying stage and the film is thicker than the critical thickness, significant cracks will form. The critical thickness is different for each ink and can be found experimentally.
Thermal Expansion Mismatch
Cracks may form if the substrate and ink have largely different thermal expansion coefficients. Since the substrate will not expand/contract at the same rate as the film, this places stress on the bonds linking them. This may cause these bonds to break. It is therefore important to use a substrate with a similar thermal expansion coefficient to the ink (where possible).
All these problems are dependent on film thickness. In general, the thicker the film, the more cracks will appear. Where possible, it is advantageous to have thinner films. However, if thicker layers are needed, it may be beneficial to apply multiple thin layers with annealing after each one. It must be noted that applying multiple layers may affect the microstructure of the overall film.
During the drying stages of deposition, the ink can be seen to run. This running is also sometimes referred to as "curtaining", and is in part due to long drying times caused by large wet film thicknesses. In the drainage regime, the film thickness increases with withdrawal speed. Due to the greater volume of solvent, thicker films will have longer drying times. The increased drying time increases the chance that deposited ink could begin to run before it dries, leading to an uneven film distribution.
This would normally happen over speeds of roughly 15mm/s, and with solutions with low viscosity. By modifying the viscosity of the solution, it is possible to reduce the chance of running of the wet film during the drying process. Another method of reducing running is to increase the drying rate of the film - this can be done by using a form of annealing chamber which rapidly dries the film after coating.
By controlling the parameters mentioned in this guide, it is possible to produce high-quality thin films with the dip coating method. By understanding how defects can occur, it is easier to find the root of the problem and take actions to reduce it. This guide has discussed the most common defects found in dip coating, explained the underlying causes of them - and where possible, recommended methods to remove them.
The most important things to observe when troubleshooting defects are:
- Where the defects occur
- Frequency of the defects
- Defect size & shape
- When in the coating process they occur
Using this information, a defect can easily be spotted, identified, and eliminated. Once all parameters are noted and optimised, users can begin making uniform, high-quality films using the dip coating method.
To the best of our knowledge, the technical information provided here is accurate. However, Ossila assume no liability for the accuracy of this information. The values provided here are typical at the time of manufacture and may vary over time and from batch to batch.