SU-8 photoresist baking : pre back, soft back and hard back

The SU-8 photoresist is baked 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.

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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 remains 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 influence greatly 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.



Wafer position

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 you do the bakes.

Cleaning of the hot plate

Be careful to put your silicon wafer on a clean surface, the presence of dust will change the temperature homogeneity.

Flat and horizontal hot plate

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.

Warm your SU-8 photoresist using smooth temperature ramp

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 following some temperature ramp. Some classical values are 10°C per min to the increase and 5°C per minutes for the decrease.



The maximal temperatures of SU-8 photoresist

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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