Written Nov 2015 by NTS

The Bake Out System (BOS) is critical for the preparation of many parts before use on the growth chambers.
Such parts include: new cells, crucibles, nipples, flanges, etc.
Note that the heater tape is made of fiberglass, so be careful. I'd suggest double gloving and just being mindful, as one always must be in the MBE lab.

Steps

Prep

Attach part to BOS
Pump BOS down and let it pump on part for a half-day or overnight
Wrap relevant part of BOS and part in a layer of doubled-over normal Al foil
Wrap relevant part of BOS and part in heater tape
Attach thermocouple to bottom foil between heater tape and on part being baked
Wrap over heater tape and bottom foil with a layer of doubled-over top foil, and be sure to let heater tape power leads and thermocouple escape from the foil
Plug Variac (at 0%) into power strip, and plug heater tape power leads into variac

Ramp up power to heater tape, via variac, in steps, while monitoring the temperature and pressure
First step will likely be 10-20% depending on the size of the part and the surface density of the heater tape.
Next step will likely be 8-15%
Next steps will likely be 5-10%, til the temperature gets close to the target temperature
Target temperature is 190-195 C, but as low as 180C is OK
Do not exceed 200 C, as that is the heater tapes top rating-- give yourself a buffer too
Did I mention monitoring the temperature and pressure?
Each step will likely take 15-60 minutes
Smaller steps should be taken once your're near the target temp
Adjust the power level so that the temp settles at a nice stable temp in the acceptable range
Continue to monitor the temperature and pressure
Bake for the desired time, such as half a day or overnight

Lower the variac power by half while monitoring the temperature and pressure
Wait for the decrease to taper off
Lower to 0%, assuming the above went well, while monitoring the temperature and pressure
Once the heater tape is under 35 C, which takes ~5 hours, it is safe to begin removing the foil and heater tape (and TC)

We vented the BOS with UHV nitrogen (no glovebag) with a positive nitrogen overpressure as usual, and attached the new nipple onto one of the arms.
- The new nipple has the normal atmospheric/water contamination from being shipped to us, so attaching it in a glovebag just makes it a bit harder to put on the BOS. Thus we did it in open air.

The nipple has a static flange and a rotatable flange. We attached the static flange to the BOS and have the rotatable flange on the other end.
After attaching the nipple to the BOS, we had to attach a blank to the rotatable flange end of the nipple.
- This is slightly different in procedure and hardware.
- You have to be aware of the flange rotating, and displacing slightly along the length of the nipple.
- The flange and blank both had through holes for bolts, so we used bolts and self-gripping nuts to seal them together (as opposed to normal bolts going into a tapped flange).
The rotatable end was chosen to be away from the BOS and blanked now so that blanking the nipple in the glovebag tomorrow (BOS end) is easier-- we won't be worried about the flange to-be-blanked rotating.

After attachment, we pumped the BOS down, performed a leak check, and then set up the bake, which I'll return to.
We did not start the bake right away, so that the BOS could pump out overnight.

To setup the bake, once the nipple is attached, blanked, and the BOS has been pumped down and leaked check, is relatively simple.
We need to cover the relevant part of the BOS and part being baked in doubled-over normal aluminum foil (from Costco).
- We want it to be nice and tight, but not ripping on bolts and corners.
- In this case, we covered the flange we attached the nipple to (not the BOS arm), the nipple, and the nipple blank.
- Two layers of foil everywhere with some overlap and extra foil to make sure everything is covered takes a few minutes.
Next we have to wrap heater tape around the foiled part of the BOS and baking part.
- We found a nice long piece of heater tape and started at the BOS flange. We wrapped it along the flange, nipple, rotatable flange, and ended on the end blank.
- Spacing of 0.5-3" inches between wraps should be sufficient, though you want to be careful to get heat to most surfaces (e.g. flange, flange face, part, blanks).
- The heater tape can be held in place with kapton tape as necessary.
Next you need to attach the thermocouple sensor to the first layer of double-foil between two wraps of heater tape.
Finally, wrap it all in double-layer normal Al foil again, making it tight but not ripping, while also not wrapping the power leads for the heater tape and thermocouple wire inside this outer foil.
We now have the bake foil-heater tape-foil sandwich setup.

The bake was started this morning, ramping up in steps, until we got to the target temperature range of ~185-195 C.
It is critical to exceed 100 C (make sure the water is boiling off) and not pass 200 C (that is the temperature to which the heater tape is rated).

Now that the new nipple will be baked out, we do not want it to see normal air.
- - Thus tomorrow we will be using a glovebag to pull it off, blanking the currently not blanked end (the one on the BOS), and moving it over to the growth chamber RGA head glovebag, setup on top of Echo.
- If the new nipple were to see air, it would defeat the purpose of the bake. It would have to be re-baked on the BOS.