Post-bake tasks: Cell outgas

Faebian Bastiman

After a standard 48 hour bake out, an MBE system needs a thorough outgas. Once the system has cooled to around 75°C it is a good idea to fire up the titanium sublimation pumps (TSP) and turn the ion pumps to maximum voltage. The background pressure is liable to soar into the 10-6 mBar range, but should return to low 10-10 mBar in a few hours. Once the system is at room temperature again, the cells can be outgassed.

To begin you do not need water cooling or LN2. Simply heat the cells as outlined in Phase 1 in the table below.

outgas table crop

There will be a pressure spike as the cells’ outer bodies outgas. Ga will already have melted during the bakeout and In will join it at 150°C. Note that Bi should not be melted (271°C) in this stage since we are about to return to 90°C on all cells to apply water cooling and avoid having the water boil in the cooling shrouds.

Once the cells have reached their Phase 2 values water cooling can be applied. Check for leaks and ensure an adequate flow is reaching the cells and then swiftly proceed to the values in Phase 3. The LN2 cooling can now be applied to the cooing shroud and after an hour the background pressure should reach a low 10-10 mBar value.  Now the cell material can be outgassed.

It is a good idea to do each cell in turn, since this allows an individual cell’s effect on the background pressure to be monitored. There is no particular order, save Bi should be the penultimate cell and Al the last. This is because once these cells are “hot” they cannot be returned to room temperature without changing the crucible. Of course we hope the other cells will not fail during the outgas and we will not need to open the system to fix the problem, but we must prepare for the worst.

Approach the ultimate outgas temperature slowly whilst monitoring the background pressure. The pressure should not exceed the 10-7 mBar range at any time. Each cell typically takes several hours to reach the final temperature and should be held for an hour before being cooled to operation or standby values. The cracker part of a group V cell must be heated before the bulk, it is a good idea to leave the cracker at the outgas temperature whilst outgassing the bulk, then drop the bulk to operation values, and finally lower the outgas temperature of the cracker to operation values. The actual values depend on the exact source and vary by manufacturer. As a good rule of thumb you should outgas the cells 25 to 50°C hotter than they will ultimately be operated.  The nitrogen and phosphorus cells are speciality cells and even a general outgas procedure cannot be stated.

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4 thoughts on “Post-bake tasks: Cell outgas

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