HOW TO CHOOSE THE RIGHT UV SOURCE FOR THE PHOTOLITHOGRAPHY OF SU-8 MICROFLUIDIC MOLD?
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 make 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 mask aligner for multi-layer SU-8 processing requiring photomask alignment.
WHAT MUST BE TAKEN INTO ACCOUNT WHEN BUYING A PHOTOLITHOGRAPHY UV SOURCE FOR SU-8 PROCESSING?
We can distinguish the parameters into two families, the main ones and the optional ones.
|A) Main parameters||B) Optional parameters|
Get the right UV power to get the right exposure timing
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 at 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 around 10 seconds.
UV source with integrated timer
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) 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 source offers an automatic measure, if not, make sure to take a UV power measure device.
The wavelength spectra of the UV source
Each photolithography UV source offers one or several wavelength to do 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 UV part the most used 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.
Photolithography UV source with mercury bulb or UV LED lamp
Traditionally the mercury lamp is more used in photolithography process, however since a few years some equipment offers 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 a great expertise on it. What’s more, it is a powerful lamp that emits a great surface energy and it is able to expose 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 make possible to have a cold UV source and a light 100% monochromatic with a bandwidth centered at 10nm. A LED is cheap, needs really less power than a mercury lamp and has a longer life time. 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 little surface and has to be associated in grid to expose an entire wafer (until 200 for a 4inches wafer). What’s more the surface energy is generally weaker (around few mW.cm-2) and thus leads to longer insolation time.
|A) Advantages +||B) Drawbacks -|
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 wavelenghts.
Parallelism of the UV light beam
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.
The variability in time and space of your UV source
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 of your photolithography UV source
The different contact modes between the mask and the wafer are: A proximity contact, “soft” contact, “hard” contact and “vacuum” contact.
- Proximity contact: The mask is not in real contact of the wafer but at a really tiny distance (about few µm). This mode is often used when a thick photoresist risks to stick to the mask and a process that do not need good resolution.
- Soft contact: The mask is softly placed on the substrate.
- Hard contact: The mask is holding against the wafer with a particular strength that makes the contact closer.
- Vacuum contact: The mask and the wafer are holding each other thanks to vacuum. This option, the most difficult to apply, offers better resolutions.
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 UV source with automatic contact and try to avoid to do the contact between the mask and the wafer by pressing them together with your hand (reducing strongly the quality of your SU-8 photolithography).
Is your photolithography UV source intuitive ?
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 photolithography UV source quite simple to use particularly if you have a strong PhD turnover.
The noise and the size of your photolithography UV source
Apart from the lamp that can do 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 is really changeable and can require several meter square, make sure to have enough space for that.
The protection around the photolithography UV source
According to the photolithography UV source, the exposure can be internal or external and so visible by the user. The UV at this power is 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 nothing less than 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!
GOOD QUESTIONS TO ASK YOURSELF WHEN CHOOSING UV SOURCE
Classical photolithography UV source or mask Aligner?
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…
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Tutorials : Introduction about soft lithography
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