Microfluidics 2 - Mold Fabrication: UV Lithography
C. Yunus Sahan, Serhat Sevli
Abstract
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.
Before start
This protocol is derived from
[Reference: Processing guidelines for permanent epoxy negative photoresist SU8 2025, SU8 2035, SU8 2050 and SU8 2075, MicroChem company ]
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;
| A | B |
|---|---|
| Specs | Info |
| Total size | 100x100 mm |
| Light source | UV laser |
| Laser point | 2 microns |
| Velocity | 0,1 - 10 mm/sec |
| Resolution | 2 microns |
| Aspect ratio | 1:15 |
| 3D structures | microlens and microslope |
Equipment
| Value | Label |
|---|---|
| Direct UV Laser Lithography | NAME |
| Kloe | BRAND |
| Dilase 250 | SKU |
| www.nehirbt.com.tr | LINK |
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.
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.
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._ oC and 5 minutes of baking at 95oC are applied.
-
For 45-80 micron thickness; 2 minutes of baking at 6ooC and 6-7 minutes of baking at 9ooC are applied.
Equipment
| Value | Label |
|---|---|
| Hot Plate | NAME |
| Electromag | BRAND |
| LB.EM.M4060 | SKU |
| www.nehirbt.com.tr | LINK |
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
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.
