TITLE: Microfluidics 2 - Mold Fabrication: UV Lithography  
AUTHORS: Serhat Sevli,  C. Yunus Sahan

[DESCRIPTION]
<div class = "text-blocks"><div class = "text-block"><div class = "justify" style = "text-align:left">Su8 is a photoresist resin used in MEMS technology for thick structures. PDMS microfluidic chips fabrication is suitable for SU8+Si wafer molds. This protocol describes the direct UV laser lithography application performed in our laboratories.</div></div></div>

[STEPS]
?. [Direct UV Laser Lithography Setup]
Lithography on photoresists is mostly performed by 2 methods; UV exposure using a mask and direct UV exposure on the material. NehirBT uses a maskless direct UV laser lithography method. It transfers drawings by CAD software to channels on mold. The important parameters arranged are; SpecsInfoTotal size100x100 mmLight sourceUV laserLaser point2 micronsVelocity 0,1 - 10 mm/secResolution2 micronsAspect ratio1:153D structuresmicrolens and microslope
SpecsInfoTotal size100x100 mmLight sourceUV laserLaser point2 micronsVelocity 0,1 - 10 mm/secResolution2 micronsAspect ratio1:153D structuresmicrolens and microslope
?. [Microfluidic Channel Design]
The designs are generated using a CAD software in DXF format and with micron as a unit.The flow simulation is performed using a multiphysics finite element analysis software, so the design optimization is performed by simulation support and correcting the errors. Kloedesign software is used for converting the design file to LWO format which is suitable for KLOE DILASE 250 instrument.Dilasesoft software controls the KOE DILASE 250 instrument by arranging the power of the laser source and velocity of stages.
?. [Post-Exposure Baking]
Post-exposure baking is for thermally catalyzing the chemical transformation of photoresist film which is exposed by UV light at a certain design.Wafer, coated with photoresist and exposed, is heated on a hot plate device. The thickness of the photoresist is important for the duration of heating steps. For example;- For 25-40 micron thickness; 1 minute of baking at 65oC and 5 minutes of baking at 95oC are applied.
- For 45-80 micron thickness; 2 minutes of baking at 65oC and 6-7 minutes of baking at 95oC are applied.
Extended durations of baking causes lift-off of SU8 mold during or after the development process.
?. [SU8 mold development process]
Development is the chemical clearing the unexposed photoresist away and cleaning of the exposed photoresist to obtain microchannel mold. The wafer is left to cool down for a few seconds after removing off the hot plate and then developed with SU-8 developer chemical. Development is performed at room temperature inside fume hood and within a suitable glass plate.Development recipe can be calculated using website:https://cleanroom.byu.edu/su-8-information-page
Please learn well about the hazards of SU8 development chemicals and perform all reactions in a fume hood.
?. [Washing and Hard Baking of the Mold]
The prepared mold is washed with isopropanol after developing and then dried with pressurized nitrogen gas.The mold is now ready for PDMS microchannel manufacturing after heating at 95oC for 10 minutes and then left to cool down for a few seconds. Put the mold in a petri dish, and label it with all appropriate information. All the above steps are performed inside the cleanroom facility. Pouring PDMS material on mold in a petri dish is performed in class2 laminar flow hood.
?. [UV Lithography]
The photoresist coated wafer is put inside KLOE DILASE 250 instrument. The objective focus is aligned by the thickness of the photoresist. The vacuum is arranged and the instrument is started by the setup of Energy Modulation and writing Velocity.