The SU-8 photoresist UV exposure

The aim of the SU-8 photoresist UV exposure is to initiate the cross linkage by the activation of the PAC (PhotoActiv Component) in some parts of the SU-8 photoresist. This activation will change the local properties of the SU-8 which after baking will be soluble or not into a solvent.

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The contact between the photomask and the SU-8 during UV exposure

To succeed the exposure and according to the wanted resolution, the photomask has to be placed as close as possible from the SU-8 photoresist with nothing interfering. Several things have to be checked to do so.
First, the presence of dust or any other element between the photomask and the wafer will lead to an imperfect contact. This problem can be easily avoided by a cleaning of the photomask and a careful manipulation during the exposure.
But there is another unavoidable phenomena that might have huge consequences during the SU-8 photoresist exposure. Indeed, if the SU-8 photoresist is coated by spin coating, there will be a bugle around the wafer. The bugle appears because of the surface tension of the photoresist. During the exposure, the photomask will first be in contact with the bugle and won’t be able to be sticked to the entire wafer surface. This phenomena leads to an inevitable loss of resolution.
The size of the bugle is not fixed but principally depends on the SU-8 photoresist viscosity and the kind of SU-8 photoresist. It can measure around several micrometers for a layer of 100µm of SU-8 for example. It can’t be avoided but can be supressed by a step called the cut out. The cut out consists in removing the resin at the edge of the wafer (on a half centimetre generally) thanks to a thin acetone jet (with a syringe for example).
Commonly, an indicator for a good contact between the mask and the wafer is the apparition of interference fringes at the interface. If you do the contact manually, press until the apparition of the Moire patterns.

The moisture

Photoresists can be divided into two families according to their crosslinking process: Cationic or with free radical. For our application and the use of SU-8 photoresist, the reaction is cationic. The photoinitiators are some acid so sensitive to humidity.
It’s not recommended to expose SU-8 photoresist with a relative humidity more than 70%. On the contrary, a weak humidity leads to a faster cross linkage.

The SU-8 photoresist bakes

The bake steps influence the fabrication process result but also the other steps including the exposure. Indeed, a first bake too short of the SU-8 photoresist will make it soft and sticky and there is a chance that the mask stays stuck during the exposure. What’s more, if the soft bake is too long, too much solvent will be evaporated and the optimal exposure time will be different.

The time before and after the SU-8 exposure

Once the first bake of the SU-8 resin (soft bake) is done, the SU-8 photoresist can be kept in a box and in the dark during a long time (at least one month) and can always be exposed, baked and developed. So, the time before exposure is not a critical parameter.
On the contrary, after exposure, the time has to be checked before doing the second SU-8 bake (the Post Exposure Bake). Indeed, during the SU-8 exposure, the photoinitiators are activated but another energetic input is needed to continue the reaction.


The alignment of the mask before the SU-8 UV exposure

Some processes require an alignment between the mask and the wafer. But really few little UV lamp offer this option, if you have to use it make sure your device is able to do so. To align, you also need some alignment sight to know where you are on the wafer and when you are well aligned.

The exposure wavelength

The wavelength of the exposure is relevant because it is proper to each photoresist. To expose the SU-8 photoresist, it has to be used a UV light with a wavelength at 365nm. A change in the exposure wavelength can modify the energy needed by the photoresist layer to cross-link, it can be adjusted sometimes by modifying the time but not always.

The exposure time

One of the most relevant parameter is clearly the exposure time. The exposure time for the SU-8 is the time when the photoresist is lighted by the UV. The time is linked to the power of the UV light and together they will define the energy given to the photoresist. A time too short or too long will under or over expose the photoresist and will lead to a resolution loss. More precisely the width of the design and the look of the walls can be changed notably by changing the exposure time.
If the time is too short the photoresist may not be cross linked over all the depth and so will be lift off during the development step.
If the time is too long, the width of the channels can be increased.


The apparition of the pattern into the SU-8 during the PEB

During the PEB, the pattern will become visible on the layer surface.

Development in time and without SU-8 white marks

From the end of the PEB, the unexposed resin is developed and then the wafer rinsed with isopropanol. During the drying, the apparition of white marks indicates a non-complete development and consequently the wafer has to be developed for a longer time. If the developing time appears really longer than indicated in the process sheet or really shorter, it means a problem appeared during the SU-8 spin coating or during the exposure.

Control the shape and the size of the SU-8 pattern

An observation under a microscope after the development enables to:

  • Know if the SU-8 photoresist is well developed
  • Give a first indication on the pattern profile
  • Know the width of the pattern

Measure the thickness of the SU-8 pattern

In order to check the conformity of the spin coating, the last test is to measure the thickness of the layer. Several ways can be conceivable; usually the thickness is measured thanks to an optic or a mechanical profilometer.


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