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iLab Name: C - Cascade Tek TVO-2 Vacuum Oven
iLab Kiosk: BRK Lithography Core
FIC: Shared
Owner: Joon Park
Location: Cleanroom - M Bay
Maximum Wafer Size: 12"/300 mm
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Overview
Vacuum oven for PMMA and ZEP (for solvent Anisole bake-out)
Standard Operating Procedure
Questions & Troubleshooting
- Is it enough to bake PMMA at 75C for 30 min?
(answered by Justin Wirth)
PMMA “A” uses anisole as the solvent that we’re trying to remove in the soft bake, which is normally done by baking for 3 – 5 minutes at 180 C atmospheric pressure. Anisole has a vapor pressure of 3.54 Torr (3540 mTorr) @ 25 C (https://pubchem.ncbi.nlm.nih.gov/compound/Anisole#section=Vapor-Pressure) and 200 Torr at 110 C (https://webbook.nist.gov/cgi/cbook.cgi?ID=C100663&Mask=4&Type=ANTOINE&Plot=on).
Coming up with some vapor pressure values from here: https://webbook.nist.gov/cgi/cbook.cgi?ID=C100663&Mask=4&Type=ANTOINE&Plot=on
1545 Torr @ 180 C (estimated)
1180 Torr @ 170 C (estimated)
900 Torr @ 160 C
760 Torr @ 153 C
687 Torr @ 150 C
524 Torr @ 140 C
80 Torr @ 70 C (estimated)
For effective outgassing, we’d like the vapor pressure at our temperature to be double the actual temperature. For example, the vapor pressure at the softbake temperature of 180 C is 1545 Torr, which is 2x ambient pressure (760 Torr). Put another way, for efficient outgassing, we’d need to be at half of the vapor pressure of anisole at a given temperature.
At 25 C, this would be 3.54 Torr/2 = 1.77 Torr (1770 mTorr), and I’d estimate at 20 C it’s ~1530 mTorr. So if we can get the vacuum oven below 1500 mTorr (TEK vacuum oven can go down to 1000 mTorr), even at room temperature, it will be sufficient to properly outgas the anisole in PMMA.
The reason for the elevated temperature is to add a factor of safety, and to further increase the rate of outgassing. There’s about a 31% increase in outgassing effectiveness for every 10 C. Working at 75 C gives a very thorough factor of safety (4x, (100%+31%)^5).
Process Library
1. PMMA Bake: bake at 75C for 30 min