HOW TO CHOOSE THE RIGHT SPIN COATER FOR THE PHOTOLITHOGRAPHY OF THE SU-8 MICROFLUIDIC MOLD?
Spin coaters are used in the photolithography of a microfluidic mold to coat a photoresist layer (such as SU-8) on a substrate. However, they are made to be versatile for all kinds of photoresists, that’s why for a SU-8 photolithography application it is important to verify some parameters to make sure that your device is fine-tuned to your needs. The aim of this guide is to give the relevant points for a researcher who would like to buy a spin coater for SU-8 mold fabrication.
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Short introduction on photoresist coating
The coating of a photoresist on a substrate can be made in several ways, the most used is clearly the spin coating that coats a liquid photoresist on a rotated substrate. But it also exists the spray coating, that consists in throwing a liquid photoresist on the substrate from a nozzle. At last, there is also the lamination technique that works with a dry film of resin.
Spin coating of photoresist
The photoresist is put at the center of the substrate; the speed, the acceleration and the viscosity of the photoresist will define the thickness of the layer. This method, contrary to the previous ones, uses a lot more photoresist than necessary, because 95% of the photoresist is lost, ejected during the rotation. However, it is the most used method in laboratory, because spin coating of photoresist is easy to use and really repeatable.
Spray coating of photoresist
For the spray coating, the photoresist has to be really liquid and it’s often necessary to dilute it with solvent. This mix makes the process quite hardly repeatable in the thickness of the photoresist layer coated. This kind of coating, usually requires big equipment and a certain number of passing of the nozzle over the substrate makes possible to have an homogeneous layer. The best advantage of this technique is to be able to deposit a uniform photoresist layer on a high topography. The choice in the thickness of the photoresist layer is limited in the thin value to make sure to have a homogeneous layer as much as in the thick value because of the high viscosity of the photoresist. It also exists some aerosol to do a manual coating but this kind of coating presents a lot of irregularities according to the user and the layer is far from being homogeneous.
Lamination of resin
With the lamination it is possible to coat a solid resin as a dry film with a given thickness. The main advantage is to get the possibility to create suspended structures. The film can be bought directly with different providers or can be done by spin coating on a plastic substrate.
HOW TO CHOOSE A SPIN COATER FOR SU-8 PHOTOLITHOGRAPHY?
What will really make the difference between spin coaters are the different options, some will be more detailed and others will have a real impact on the process. Make sure that some options will really make your life easier and if some are missing you could regret your purchase.
We did sort out unnecessary features and real needs. We seriously did the comparison between all spin-coaters. We wanted to provide the best solution to our customers. Do you want to make the right choice?
Maximum speed and acceleration of the spin coater
In order to have a controlled thickness, the rotated speed and the acceleration have to be precisely adjusted. These two parameters are perhaps the most important in the choice of a spin coater. The rotate speed of the substrate almost definse by itself the thickness of the resin coated. According to the resin provider abacus, it is possible to forecast the thickness. Be careful, the variation of the thickness is not linear with the speed. Most of the spin coaters you will find on the market will reach the required speed for the photolithography of SU-8 microfluidic molds (SU-8 spin coating speed are often below 6000 rpm). Take care of the acceleration setting capabilities since some models do not enable to set different accelerations for the different steps of your photoresist spin coating.
The wafer holding system of the spin coater
The substrate can be held in your spin coater with different methods. The name of the tool that holds the wafer is called a chuck. It can offer a vacuum hold or/and a mechanical hold. A mechanical chuck enables to put the wafer really centered on the rotated axe. However the pins used to hold the wafer can disturb the resin ejection during the rotation and thus create more defects on the edge of the wafer. Using a vacuum chuck resolves this problem and enables to have a really horizontal surface (at the condition that the seal is in good shape). It’s an option that remains more difficult to realize because it requires to add a vacuum pump to your spin coater. What’s more it has to be check that no liquid (resin or solvent) goes inside the vacuum canalization and breaks your spin coater.
Spin coater with automation of the SU-8 photoresist spin coating
We have seen in the introduction that the spin coater was first used for microelectronics. It’s thus normal to find automated device that can coat the photoresist on several substrates with programmable parameters. However this equipment remains voluminous, expensive and dedicated to industry or very large clean rooms.
Number of programmable spin coating steps
Generally, the spin coating process of SU-8 photoresist follows the same steps, a first step to spread the photoresist over the wafer surface followed by a second step at a higher speed to control the final thickness for a time of around to 30-60 seconds and then a deceleration step. Having said that, it’s important to have a spin coater able to program several cycles with different speeds and accelerations.
Spin coater which is able to work with the lid open or closed
According to the spin coater, it is possible or not to let the lid open during the rotation of the substrate. A closed lid enables a better control of the experimental conditions. Besides it is usually closed for classical process. However for some exceptions the possibility to keep the lid open could be useful, particularly for the coating of viscous photoresists (SU-8 with thickness upper than 20µm) and edge bead removal.
The coating of viscous photoresists
In case of really viscous photoresist, the first step where the photoresist is spread at low speed on the substrate is not enough. It can be done a longer time but the best solution is to do a dynamic coating. The dynamic coating consists in putting the photoresist on the wafer while it is already rotating at low speed.
The edge bead removal
Choose a spin coater for SU-8 photolithography – edge bead from Dr Kwang W ohThe edge bead removal consists in removing the photoresist on the edge of the wafer thanks to a developer jetted with a syringe when the wafer is rotating. This step, if done manually, has to be done with the lid open. The edge bead removal step is not an obligation however during the spin coating, the photoresist is ejected out of the wafer and a bugle appears on the edge because of the surface tension of the photoresist. This bugle depends on the kind of photoresist and mainly on its viscosity. If the photoresist viscous and the layer thick, the bugle can measure several µm. This thicker part will take away the mask during the exposure step and lead to a resolution loss (you can find more information on that with our tutorial on the UV Lamp).
Spin coater which enable dynamic coating
We have already seen that with the open lid option, for the viscous resin, it is interesting to do a dynamic coating of the resin. The coating during the rotation of the wafer is also possible with a closed lid but with a centered hole on the top of the spin coater lid. This configuration enables to have the same advantages than a closed lid (great control of the environment) and to do a dynamic coat.
Set of several spin coating accelerations
We have seen that the speed and the acceleration define the thickness of the layer, that is why there are really relevant parameters. However, all equipment don’t offer the possibility to set the speed and acceleration of one step independently from the other one. Surprisingly some famous spin coaters often used in photolithography enable to set different speeds but do not enable to set several accelerations during the same spin coating process.
Non-clogging system for spin coater
The chuck is a rotated piece and thus not linked to the base, in other words there is a space between the chuck and the base. Moreover if your sample is not well positioned on your spin coater chuck, your photoresist might enter in the vacuum system used to maintain your sample on the chuck. These two situations can create some problems because the solvent or the resin can leak inside and damage the device. Some spin coaters offer a non-clogging system, injecting some pressured azote or CDA in the specific space. This option is highly recommended and will increase the lifetime of your spin coater.
The use of a venturi system for your spin coater
As we have seen before, some devices offer a non-clogging system based on pressured air. So it is possible to use a venturi system to create the vacuum needed to maintain the wafer on the spin coater chuck thanks to the pressured air. Of course the vacuum created will be weaker than with a vacuum line or a pump but can be enough to hold the wafer. This scheme can allow you to save some place and price of a vacuum pump.
Tool to center the wafer on the spin coater chuck
It is important to have the possibility to place the wafer on the rotation axe and thus as centered as possible on the spin coater chuck. Automated spin coaters will do it automatically, same with a mechanical chuck the wafer will be automatically in the center but with a vacuum chuck, it is highly recommended to use some alignment tools usually given by the spin coater providers.
Cleaning of the spin coater
The resin, solvent, PDMS… that are coated on the substrate are dirty, the spin coater has to be easily cleanable. Several parameters can be observed here. First the material of the spin coater, msure to use solvent to clean it. What’s more some providers offer some plastic liners to protect the wall of the device that can easily be changed. At last, it is always possible to recover the device with some aluminum foil for example (be careful it can’t be tedious).
The size and portability of the spin coater
The spin coater can be really voluminous and some has to be fitted in a working surface (and this way no more movable). It is a parameter to take into account, thinking about the evolution and flexibility of your facility.
The vibration generated by the spin coater
Depending on the spin coating speed and the centering of the sample, spin coater can vibrate which could results in the sample destruction when those vibrations will disconnect the sample from the chuck. To minimize the vibrations, before buying, test the spin coater at high speed (10000 rpm), follow the advice of the providers about the place to put in the spin coater and carefully center your sample.
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