After your maintenance cycle the time has come to pump the system down and start recovering the ultra-high vacuum (UHV) conditions necessary for MBE operation. In principle this simply involves turning on the pumps, but you can perform a few specific steps to greatly improve your pre-bake base pressure.
After maintenance your system has likely been exposed to atmospheric pressure for several hours. Water vapour contamination is unavoidable, though of course can be reduced by situating your MBE system in an air conditioned laboratory. Once the last flange is sealed (see Essential Maintenance: A how to guide for tips on MBE maintenance) the system interior is completely isolated with an air tight seal. Completely isolated from atmosphere, but sadly still full of atmosphere.
The first step is to pump that out. Dry scroll pumps should be used throughout on an MBE system. They have an excellent pumping rate and virtually zero possibility of contamination. The Edward’s nXDS is a good choice. Enable the ballast valve to enhance the pump’s handling of moist atmosphere and pump through until the Super Bee pressure gauge on the scroll pump’s inlet reaches 0.05 mBar. Now turn off the pump and vent the MBE system with N2.
What?! We have just pumped the system down and now you want me to vent it again? Are you sure? Yes I do. You just pumped out moist atmosphere from an air tight system, and now you are going to vent with dry, ultra pure N2. What will happen? Evaporation.
Evaporation is a wonderfully useful natural process for an MBE operator. Evaporation will transport moisture from the chamber moist inner surfaces into the dry N2 gas. Not all, but a significant fraction. The evaporation rate will depend on:
- The concentration of water in the N2: which is zero at the beginning
- Flow rate: initially high as the N2 penetrates the vacuum, but constatly dropping
- Pressure: evaporation happens faster at lower pressure
- Temperature: evaporation happens faster at higher temperature
Simply let the N2 in until the Super Bee reads 500 mBar (remember the process happens faster at lower pressure). Wait a minute. Then pump the chamber down with the scroll pump again. Once it has reached 0.05 mBar, turn the pump off and flush it with dry N2 again. On the second pump down, open the As and P cracker’s needle valve manually a whole turn. The bulk should have been protected from atmosphere when you shut the valve during Essential Maintenance: System Venting. It should therefore be free from moisture, but there is always a chance the valve has a little leak. To be safe, we will pump down the bulk with the scroll pump now. To increase the effectiveness of the N2 flush you may consider heating the N2 gas to ~50°C. This can be highly regulated with an inline gas heater or poorly regulated by baking the SS N2 pipework to 50-70°C with heat wraps during the flushing phase.
Repeat whole the pump down and vent process 5 times.
Finally once the system has been pumped down for the last time, start up your turbo pump. After 15 minutes, turn on your ion gauge to 0.1mA emission and check your pressure. It should be in the 10-6 mBar range. Having used the maintenance cycle as an opportunity to regenerate your cyro pump, it should now be operating and ready to take over too. Open the valve to the cryo and let it assist the turbo for a further hour. With the turbo and cyro pumping the growth chamber the pressure will quickly drop into the 10-7 mBar range. Then open the gate valve to the ion pump (which was never turned off) and leave the system over night.
What you should find is that in the morning the background pressure is ~5.0 x 10-8 mBar. That is a very impressive pressure before a bakeout. This is in fact at least 10x lower than without the repeated N2 purge step. If your system is not in the mid-high 10-8 mBar range you will need to leak test. Leak test either with an ex situ external leak tester or an in situ quadrupole mass analyser. Follow the instructions in Essential Maintenance: Leak Detection to identify the cause of the leak.
Once the pressure is <10-7 mBar you are ready to start powering up all the sub-systems you turned off in order to vent. First, turn on all your ion gauges to 0.1mA emission current. Next, double check all the PID controllers for your effusion sources are set to lower than ambient temperature, then turn all the PSUs on. Current limit all the PSUs to 2A to protect them during the effusion cell filament’s initial ramp up from cold. With a ramp rate of 0.1 °C /s ramp the sources (with the exception of the As bulk) to 100°C. This will drive off moisture. The pressure will inevitably rise for a few hours until the moisture is purged through the pumps. At the same time you can heat your substrate heater to 100°C too.
You are now ready to bake the system (see Essential Maintenance: Bakeout).