HOW TO CHOOSE THE RIGHT PHOTOLITHOGRAPHY HOT PLATE FOR YOUR SU-8 MICROFLUIDIC MOLD

There are two kinds of equipment commonly used in laboratory to bake SU-8 photoresists for the fabrication of your microfluidic mold: ovens or hot plates. Note that the advice given here for SU-8 photoresist baking also remains relevant for other types of photoresists involved in the fabrication of microfluidic molds (like AZ9260). The SU-8 pre-baking step is a critical photolithography step since it enables the evaporation of the SU-8 photoresist solvents enough to harden the photoresist (A fraction of 7% of solvent is a good value to succeed the exposure) and during soft-bake it allows a good cross link after exposure.

Ovens and photolithography of SU-8 photoresist

Ovens heat the air around the wafer and thus the SU-8 photoresist is heated from the outside to the center, this scheme doesn’t help for the solvent evaporation. What’s more, even if it’s possible to bake several wafers at the same time, the temperature which is not really uniform depends on the wafer and their position inside the oven. Once you know the importance of this step, it seems obvious not to use an oven to bake your SU-8 photoresist to fabricate your microfluidic mold.

Hot plates and photolithography of SU-8 photoresist

The hot plate can heat only one wafer at a time, however it heats from the back to the top. The heat goes up and thus promotes the solvent evaporation. This equipment offers different options and some are better than others but the hot plate is the best heating tool to bake SU-8 photoresists when you want to fabricate a microfluidic mold. For that reason, we will focus this tutorial on the hot plate only.

During your SU-8 photolithography process, the hot plate aim is to heat your photoresist. But the way to do so can be different according to all the equipment, each offering different options, some deeply needed and others just to make your life simpler.

We did a comparison between many hot plates. We offer the possibility to benefit from the best solutions for your Soft-Lithography processes. Do not hesitate to contact us for advice.

Real temperature display of photolithography hot plate surface

One important point is to check if the real temperature is the same as the set point temperature wanted. So to ensure a controlled baking of your SU-8 photoresist, give priority to hot plates which display in real time the measured temperature. We noticed that some photolithography hot plates have a temperature sensor which drifts overtime. Since this temperature drift could downgrade the reproducibility of your process, check the sensors technology of your hot plate. If you are not sure of the temperature drift of your hot plate over time, ideally a monthly scheduled infrared check of the real surface temperature of your hot plate would ensure you reproducible SU-8 photolithography process.

Overshoot and stabilization time of your photolithography hot plate

When you set a new temperature and according the regulation mode (PID values) the temperature of your hot plate can exceed the set point before stabilizing. This overshoot depends on the gap between the hot plate temperature and the set point. This overshoot can reach several degrees and strongly downgrade some of your SU-8 process. Ask about the temperature overshoot of your hot plate before buying it. If you cannot get the information, we advise you to measure this overshoot with a simple pyrometer for usual set points (and also to check the stabilization time). Even if it is often difficult to know the real stabilization time and temperature overshoot of your hot plate before buying it, those parameters remain important to have a good and repeatable SU-8 photolithography process.

Temperature homogeneity on the whole photolithography hot plate

It is rare that the temperature of a hot plate remains the same on the whole plate. The center is usually at the set point temperature and the temperature decreases when going to the edges of few degrees. You need to know or measure this non homogeneity to make sure to have an enough homogeneous wide space to put your wafer. Here too, even if it is often difficult to know the real homogeneity of your hot plate before buying it, these parameters remain important to have a good and repeatable SU-8 photolithography process. If you cannot be sure of the temperature homogeneity of your hot plate before buying it, we recommend you to use a pyrometer or IR-camera to check the temperature homogeneity over the hot plate when you receive it.

The ceramic plate makes it possible to heat at high temperature (>450°C). It is really resistant to a large range of solvents and chemicals products, and what’s more: it is easy to clean. But because of its quite weak thermal conductivity, the temperature will not be really homogeneous on the whole surface.

The aluminum plate cannot be heated at a temperature as high as ceramic and is less resistant against solvents and chemical products (NaOH for example). In contrast, thanks to its high thermal conductivity, the temperature is more homogeneous on the whole plate surface than with the ceramic one. For SU-8 photoresist processing, you have to give priority to the homogeneity and thus the aluminum plate appears to be the best choice.

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Photolithography hot plate with automatic timer

The bake steps have to be done during a precise time; the possibility to have an intern timer to warn the user at the end of each step without using an external timer is little a practical advantage. Some equipment offers the possibility to automatically stop the heat at the end of the pre-set time. We often recommend this kind of option since it avoids inattention mistakes when processing several wafers in parallel for example. Moreover it allows the researcher not to be dependent of the time of the step and to not have to stay close to the hot plate during the SU-8 mold baking.

Possibility to do temperature ramp for gentle SU-8 baking

The possibility to increase and/or decrease slightly the temperature is important in SU-8 photolithography process. Indeed, the SU-8 photoresist becomes really tight during baking. To decrease the mechanical stresses on your SU-8 film, it’s relevant to decrease and increase the temperature slightly. By increasing or decreasing the temperature it is possible to do ramps “by hand” regularly change the hot plate set point. However this solution is really constraining and not reproducible. Some photolithography hot plates offer to heat and cool down at different rate. Usually for the decrease, it is only possible to slow down by natural cooling because being able to effectively decrease the temperature impose to have an azote line and a more complicated control. It’s difficult to control the cool down but generally a natural cool down allows a slow enough decrease for the fabrication of optimal SU-8 molds for microfluidic applications.

Program photolithography steps

The baking processes of your SU-8 microfluidic mold often have several steps with different stages at different temperatures. Being able to save those temperatures steps and even the different ramps will make your life simpler and allow you to get reproducible and high quality photolithography process.

Temperature increment size and precision of your photolithography hot plate

Generally and depending on the equipment, the increment is by 5°C or by 1°C. Beware that the temperature precision is directly relied to this. To bake your SU-8 photoresist strongly prefer an increment by 1°C. Almost all photolithography hot-plates provide 1°C temperature precision so it should not be a problem when buying the hotplate.

Safety and user protection

Most of the equipment offers a warning light when the plate is over a particular temperature (often 50 °C). This option alerts other users on the danger of the device. Nevertheless, be careful, this light switches off with the device so check to put a warning pictogram and make the other users aware of the danger. We do not insist on this point since it does not enter into account on the quality of your final microfluidic mold.

Photolithography hot plate size

There are different kinds and sizes of photolithography hot plates, choose one, which will fit your needs the best. A too big plate will occur a great inertia during the temperature decrease. To ensure a good SU-8 photolithography process choose a plate a bit bigger than the silicon wafer to heat to be free from the edge effects that makes the temperature cooler on the edges than on the center. For a inch wafer (diameter of 10cm) you can use an 18 by 18 cm photolithography hot plate.