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1. Clean the wafer with acetone, isopropanol, and blow dry with filtered N₂
2. Bake the wafer at 200°C on a hotplate for 5 minutes
3. Center the wafer on the spin coater
4. Apply enough photoresist to cover the entire wafer
5. Spin at 500RPM for 15 seconds (acceleration at 100RPM/sec) then at speed for 30 seconds (acceleration at 300RPM/sec) to reach final thickness
6. Soft bake at 95°C for a specific time based on layer thickness (see table below) on a hotplate.
   • Note: While some thin layers might have soft bake times of 1 minute (see table below), better results come from a minimum soft bake time of 2 minutes.
7. Let the wafer cool to room temperature (~2-3 minutes)
8. Mount the correct mask to the mask aligner, ensuring that the mask is oriented correctly (emulsion down, top of the mask away from you)Perform a 3 second flood exposure to check the exposure energy of the lamp (in mW/cm²). This energy will be needed to calculate the exposure time in step~\ref{dose}.
9. Mount the wafer to the chuck of the mask aligner
10. Set the WEC setting. This establishes the force with which the mask contacts the wafer. Usually, a pressure of 2 bar is sufficient.
11. Set the exposure type. Usually, exposure is done under hard contact, which utilizes both physical force and pressurized nitrogen to keep the mask and wafer in contact. The more complete the contact, the less space between the mask and the photoresist, which in turn prevents the broadening of features. Usually, a 5 second hard contact is sufficient.
12. Set the exposure dose. Each thickness of photoresist has its own required exposure dose to ensure complete cross-linking of the features (see table below). The exposure dose is given in mJ/cm², and dividing the exposure dose by the lamp exposure intensity (in mW/cm²) gives the exposure time in secondswhich is programmed into the mask aligner.
13. Align the wafer using the microscope on the mask aligner. Once aligned, initiate the exposure.
14. Remove the wafer after exposure
15. To complete the cross-linking reaction, a post-exposure bake is performed at 95°C on a hotplate for a specific time based on layer thickness (see table below). If the exposure dose is correct, a `ghost' of the desired structure is visible in the photoresist after 15-30 seconds at 95°C.
    • Note: While some thin layers might have post-exposure bake times of 1 minute (see table below), better results come from a minimum post exposure time of 2 minutes.
16. Let the wafer cool to room temperature (2-3 minutes)
17. Immerse the wafer in SU-8 developer for 45-60 seconds to remove uncross-linked photoresist
18. Spray the wafer with SU-8 developer, followed with isopropanol, and blow dry with filtered N₂
19. Check the height of the features with the profilometer
20. Repeat the above steps for all required layers (beginning with step 2)
    
    • Note: For thin layers (~1µm), it is beneficial to hard bake these layers before applying the next layers. In this case, bake the wafer at 200°C on a hotplate for 5 minutes (the same as in step 21), and let cool before repeating steps 2-18.
    • Note: In constructing certain devices, a smaller layer must be applied after a taller layer, such as the addition of chaotic mixers on DAW devices. In these cases, the development step is omitted after the taller layer is constructed, and the shorter layer is applied directly to the taller layer. After construction of the shorter layer, the unlinked resist from both layers is removed by developing as in steps 17 and 18.
21. After all layers have been constructed, the device should be hard-baked to solidify the features. The hard bake is performed at 200°C on a hotplate for 5 minutes for all devices, regardless of the number of layers..
22. Let the wafer cool to room temperature (4-6 minutes)
23. To protect the wafer and make PDMS removal possible, a release agent is applied ((tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane). The wafer is placed in a desiccator, and 3 drops of release agent are dispensed into a weigh boat in the desiccator. The desiccator is placed under vacuum for 5 minutes to evenly distribute the release agent.
    • Note: To remove excess release agent, PDMS is applied to the wafer and removed in three rounds before the monoliths are used to construct devices.

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