The SU-8 photoresist baking is carried out two or three times during the whole process and each bake has different meanings.
The first photoresist bake is called soft bake; it’s done just after the spin coating of the SU-8 photoresist. Its aim is to evaporate the solvent to make the SU-8 photoresist more solid. The evaporation will change a little bit the thickness of the layer and prepare the SU-8 photoresist to be exposed to the UV. Indeed a quantity around 7% of solvent enables a good exposure.
The second photoresist bake is called PEB (Post Exposure Bake). It’s done, as its name said, just after the UV exposure. The UV exposure enables the activation of the photoactive component in the SU-8 photoresist but needs energy to continue the reaction; the heat brings this energy.
The third photoresist bake is called “Hard bake”, it is the last step of the process but it is optional. A lot of strengths remain inside the SU-8 photoresist at the end of the process that can create cracks on the surface, delamination of the layer… The hard bake heats the SU-8 photoresist at high temperature (more than 120°C) to suppress these strengths. Thanks to it, some cracks disappear and the SU-8 photoresist becomes harder.
The bakes are really important and greatly influence the success of the entire process. Having a good equipment is important but there also are some tricks to know to get the best chance to succeed your SU-8 photolithography.
One of the critical parameters for your SU-8 photoresist bakes is the uniformity of the heat for the entire wafer surface. For that you have to be sure that the hot plate temperature where you are going to put the wafer is homogenous. Generally, there is a little difference between the center and the edges (cooler) of the plate, if your photolithography hot plate is big enough, the center part where the temperature is homogenous will be sufficiently wide to put your wafer. So make sure to put your wafer as centered as possible when baking.
Be careful to put your silicon wafer on a clean surface, the presence of dust will change the temperature homogeneity.
A turnkey offer to fabricate your su-8 mold and pdms chips
Especially for the first bake, the soft bake, the SU-8 photoresist is really soft and can always flow, so it is really relevant to check the horizontality of the hot plate and make sure it is flat. Do not hesitate to use a spirit level to adjust your photolithography hot plate.
The SU-8 photoresists are sensitive to temperature change. A too fast change of the temperature set will tense the SU-8 photoresist and can create cracks on the surface. So it is highly advised to increase and decrease the temperature slowly during the SU-8 photoresist baking process, following some temperature ramp. Some classical values are 10°C per min to the increase and 5°C per minutes for the decrease.
SU-8 photoresists can burn so be careful not to heat too high your substrate. The SU-8 after exposure and so cross-linkage has a transition temperature at 200°C and the polymer will be destroyed after 380°C.
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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 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 UV Lamp to expose your SU-8 photoresist. You will find here the relevant points to think about.
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.
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