Photolithography UV sources are used in micro fabrication to expose a layer of photoresist. Depending on the different applications and photoresist, the choice for a specific UV source is different. That’s why, in the case of the soft-lithography, it’s important to check carefully some parameters to make sure that your equipment is well tuned and allows you to easily perform your SU-8 exposure. This guide is here to give you the points to keep in mind for those who want to buy a photolithography UV source to make SU-8 microfluidic mold. We do not consider here the case of a mask aligner for multi-layer SU-8 processing requiring photomask alignment.
We can distinguish the parameters into two types, the main ones and the optional ones.
The real relevant parameter of a photolithography UV source is not really the power but more its surface energy created, in W.m-2 often given in mW.cm-2. This parameter is deeply linked to the exposure time, which is the time when the photoresist is lighted by the lamp. Indeed, these two parameters define the exposure degree of the photoresist, which is nothing more than the multiplication of these two parameters. It’s a value that has to be precise because an under-exposed SU-8 photoresist (an exposure degree too low) or an over-exposed SU-8 photoresist (an exposure degree too high) will lead to a resolution loss. The exposure degree is also called the exposure dose, it has to be sufficient to initiate the photoactive component of the photoresist in all the depth of the layer but without being to high to prevent any diffusion in the width. In function of the depth, each photoresist needs a particular exposure degree, given by the provider.
The surface energy is a proper parameter to the photolithography UV source, and thus the UV exposure degree is obtained by choosing the exposure time. For a same exposure degree, the value of the exposure time is relevant. Indeed, a really short time has to be really precise because a little uncertainty on it will lead to real consequences in the resolution. On the other hand, a long exposure time will lead to great diffusion phenomena. And a diffusion of the photoactive components leads to an enlargement of the exposed zones and thus a resolution loss. More than the resolution, the diffusion changes also the side aspect. We advise you a photolithography UV source around the 20mW.cm-2 for the exposure degree, which is well fitted to expose the SU8 photoresist with an exposure time of about 10 seconds.
The photoresist is sensitive to over and under exposure. And thus, the UV exposure time has to be defined precisely to make sure to have a resolution as best as possible. All the photolithography UV sources don’t offer the same precision (within one second or one-tenth of a second) so make sure to set the time with at least a precision of 10% of the global exposure time (For example within one second for an exposure time of 10 seconds).
Photolithography UV source with integrated UV-meter
In the same way as the time, being able to control the power or more the surface energy of a photolithography UV source is a real asset. It is possible to change and handle the power; either by changing the input power or by changing the vertical position of the lamp to the wafer. At last, it’s essential to have a way to measure the power, some photolithography UV sources offer an automatic measure, if not, make sure to take a UV power measure device.
Each photolithography UV source offers one or several wavelengths for exposure. The UV range is between 100nm and 400nm. We can divide it into three parts. The UVC or Deep UV, for wavelengths less than 280nm, the UVB or MidUV for wavelengths between 280nm and 315nm and finally the UVA or Near UV for wavelengths more than 315nm. The most used UV part is definitely the UVA. For each photoresist, one wavelength is particularly well fitted to initiate the cross-linkage, for example is 365nm for the SU-8. See in the chapter below difference between Hg bulb and UV LED spectra.
Traditionally, the mercury lamp is more used in photolithography process, however for a few years some equipment has been offering the possibility to use UV LED.
SU-8-photolithography-UV-source-tutorial-mercury bulbA mercury lamp has the advantage to generate several wavelengths at the same time and it’s some filters that select one specific wavelength. The mercury light spectrum has three particular wavelengths with a great emission power at 365, 405 and 436nm. The mercury lamp is used for a long time and so there is great expertise about it. What’s more, it is a powerful lamp that emits a great surface energy and it is able to expose a wide surface. However, the lamp heats and has to be cooled down and thus needs a great power (several hundreds of watt).
SU-8 photolithography UV source tutorial – UV ledUsing UV LED as photolithography UV source makes it possible to have a cold UV source and a light 100% monochromatic with a bandwidth centered at 10nm. A LED is cheap, needs much less power than a mercury lamp and has a longer lifetime. The wavelength depends on the kind of LED, the choice was limited at the beginning but know you can find quite all the wavelength you want. On the other hand, one UV LED lights only a small surface and has to be associated in grid to expose an entire wafer (until 200 for a 4inches wafer). What’s more is that the surface energy is generally weaker (around few mW.cm-2) and thus leads to longer insolation time.
Regarding last advances in UV LED technologies we now generally recommend UV-LED source for SU-8 photolithography. We can now find photolithography LED UV source which integrate uniform high power grid of UV LED while keeping advantage of strongly monochromatic spectra and ease of use/maintenance. Mercury UV source remains obligatory if you plan to use other kinds of photoresist at different wavelengths.
It’s important that the UV light beam arrives parallel and vertical to the photoresist surface. For that, make sure to have a collimator in the optical path length. Make sure your light is also monochromatic, the need to have a real monochromatic light comes from the fact that the refractive index of a material is different according to the wavelength.
And thus a multichromatique beam will light a bigger zone after crossing the mask. In that case a filter is essential to have better resolutions for SU-8 processing when dealing with mercury photolithography lamp. With some mercury photolithography UV sources it is possible to add filter on the optic way. These filters make possible to select one wavelength really precisely for example. That’s why a filter at 365nm is highly recommended to expose the SU-8 with Mercury UV lamp.
A turnkey offer to fabricate your su-8 mold and pdms chips
The light has to be uniform on the entire surface of the wafer to guarantee the same UV exposure dose everywhere and thus the same exposure for the whole layer. The light also has to be uniform between the mask and the photoresist, in order to have a good resolution, indeed, the light has to be well collimated and arrived as more vertical as possible on the photoresist surface. The UV light has to be uniform in time, because variability in the surface energy makes the routine process obsolete. It’s really advised to measure regularly the lamp power to detect all changes or to choose an UV-lamp with an integrated UV meter.
The different contact modes between the mask and the wafer are: A proximity contact, “soft” contact, “hard” contact and “vacuum” contact.
In case of a real contact, it has to be kept in mind that the contamination risk is greater between the mask and the photoresist and it can be unacceptable for some applications. The contact mode is relevant because the more the contact is strong, the less space between the mask and the photoresist remains. This gap has a big influence on the resolution and on the quotation respect. Indeed, the bigger is the gap the more width will be the pattern on the photoresist because of the light diffraction. If the mask must not be in contact with the photoresist, it is undeniable to choose the proximity contact but for all other cases, at least, a hard contact exposure is advised. For most SU-8 processing dedicated to microfluidic applications we generally recommend hard/ vacuum contact. For reproducible process try to choose a UV source with automatic contact and try to avoid to perform the contact between the mask and the wafer by pressing them together with your hand (reducing strongly the quality of your SU-8 photolithography).
According to the equipment, possible tuning are more or less numerous and more or less intuitive. Think of the use and remember that it is the exposure time and the mask position that are the most critical parameters. Try to choose a quite easy to use photolithography UV source, particularly if you have a strong PhD turnover.
Apart from the lamp that can make some noise, there is the cooling system that can be really noisy. Don’t forget to watch the photolithography UV source working to evaluate this parameter, after all you are going to work closely, it is better if the noise is not bothering. With all the differences and options (Mercury lamp or UV LED, cooling system, alignment system…) the size of a photolithography UV source can really vary and can require several square meters, make sure to have enough space for that.
According to the photolithography UV source, the exposure can be internal or external and so visible by the user. UV rays of such intensity are dangerous for the eyes. Make sure to protect the user (Anti-UV glass) and the people around (anti-UV cover around the lamp). The mercury lamps have to be cooled down and can reach 900°C. At this temperature it is really imperative to be careful of burn risks. The lamp must not be reachable by the user and the potentially hot surfaces have to be pointed out. In most cases, the cooling system is with the lamp but not always and thus you need to anticipate a fan system to cool down your equipment. Think about it!
Depending on the process, it is sometimes needed to superimpose several photoresist layers and to expose them one by one. In that case, you can’t do that without a mask aligner to put the layers exactly under the one before. The mask aligner is generally big equipment, expensive (close to hundreds of thousand euros), and needs a lot of maintenance. If you have to realize mold with several photoresist levels, it is often better to buy them to another laboratory specialized for few hundreds of euros than put your money into a mask aligner.
Now that we have seen the different parameters influencing the UV exposure, and the different options of a UV lamp, we are going to see how to make a choice. What are the questions to wonder and in which rank? There are basically, three main questions that will define your choice. The first one is to know with which photoresist you want to process and if you want to use only one or several. The answers will define the wavelength that you need and if you are going to use a mercury lamp or a UV LED lamp. The second and third questions are certainly: Which kind of resolution do I need? And for which aspect ration? These questions gather the issue of the photolithography and the answers will define the kind of contact you will need, the power and the precision of your lamp…
Email* I hereby agree than Elveflow uses my personal data
Do you want tips on how to best set up your microfluidic experiment? Do you need inspiration or a different angle to take on your specific problem? Well, we probably have an application note just for you, feel free to check them out!
Microfabrication techniques for a circular channel
In soft lithography, the fabrication of a mold, often made in SU-8, is required for replicating PDMS microfluidic structures.
Replicating PDMS-based structures first requires the fabrication of a SU-8 master mold that will serve as a patterned template for PDMS casting
How do you perform a successful SU-8 exposure? Here you will find the tips and tricks to do it.
How do you perform a successful photoresist baking? Here you will find the tips and tricks to do it.
How do you perform a successful spin coating? Here you will find the tips and tricks to do it.
The final PDMS layer thickness mainly depends of spin-coating speed and duration.
Here you can find a complete overview of a SU-8 mold fabrication process.
Here you can find a complete overview of a PDMS chip replication.
Unlike photolithography, soft lithography can process a wide range of elastomeric materials, i.e. mechanically soft materials.
A plasma cleaner to bond your PDMS chip, you will find here the relevant points to think about.
A spin coater creates a thin layer of photoresist or PDMS, you will find here the relevant information about how to choose one.
You have the choice between glass or plastic photolithography mask, but how do you choose? here is some information to help you with the decision
A hot plate to bake your SU-8 photoresist, you will find here the relevant points to think about.
Every following technology is based on the same system of additive process, every object is built layer by layer after being sliced by an informatic system.
Get a quote
Name*
Email*
Message
Newsletter subscription
We will answer within 24 hours
By filling in your info you accept that we use your data.
Collaborations
Need customer support?
Serial Number of your product
Support Type AdviceHardware SupportSoftware Support
Subject*
I hereby agree that Elveflow uses my personal data Newsletter subscription
How can we help you?
Message I hereby agree that Elveflow uses my personal data Newsletter subscription