Faebian Bastiman When the time has come to perform maintenance on your MBE system you will need to raise the internal pressure to atmospheric pressure. You can vent the system at several points:
- Through the fast entry lock (FEL)
- Through a turbo molecular pump
- Through a cryopump
- Through a dedicated venting valve
Before you start to vent, take a moment to consider where you need to vent. Do you need to vent the growth chamber and the preparation? Or do you need to only vent the growth chamber? When venting through the FEL you need to vent the entire system in series with the FEL-preparation and preparation-growth chamber valves open? Alternatively you can vent through a pump or dedicated value on the growth chamber. Most modern turbos support a venting feature. Pfeiffer’s certainly possess this feature. You need to selected the correct menu options, turn off the backing line scroll pump and once the speed drops below a pre-described value the vent process starts automatically. The cyro pump vent requires a little more effort: First you need to turned it off and allow it to reach room temperature, all the while pumping it down with a scroll pump (regeneration), and after that a small turbo or Pfeiffer Hi-cube to get a good base pressure. Only then can you open the gate valve that connects the pump to the main chamber and finally flush the cryo and chamber with N2 gas from the cryo’s exhaust valve. The third alternative is simply using a spare flange as a deidicated venting flange. It only needs to be a small flange, and you would have an adapter that drops to a ¼” Swagelok fitting. This is the simplest method. The next question is with what to vent. I would recommend hi purity N2 from a cylinder that is routed through a baked SS line (see MBE: N2/Gas system). Alternatively you can use LN2 boil off N2, but in that case it is more difficult to bake down the whole line to ensure the purity of the gas. Either way the N2 venting line needs regulating down to 1.2bar. Never use dry filtered air from a compressor to vent your system! It simply not pure enough! Venting then involves a generic, multi-step procedure:
- First ensure all your cells are at room temperature, except the Ga which should be at 50°C . Remember the As and P cracker valves need to be fractionally open whilst heating/cooling the cracker head. Always cool the bulk down first, then the cracker heat to avoid condensing material on the valve.
- Next current limit the Ga PSU to 2.00A and turn all the other PSUs off. This both saves power and gives peace of mind.
- Seal the As and P needle valves manually now to avoid contaminating the bulk.
- Next turn off the ion gauges and allow the filaments to cool: 30 minutes.
- Close the valves to the pumps.
- The ion pump can be left running (ion pumps should in fact never be turned off)
- Take the opportunity to regenerate your cryo
- Switch off the turbo pump and the backing scroll pump
- Close all valves to the pumps (unless you are venting through a pump)
- Watch the Ga cell’s temperature and make sure it stays over 45°C
Venting can be dangerous. An uncontrolled vent can expose your system to dangerous pressures that cause damage to glass viewports, bellows and delicate internal items (like RHEED filaments). Which is why we regulate the N2 line to 1.2 bar. How many bars of pressure can an MBE system safely take? Well it is predominately a vacuum system so anywhere between 0 and 1. On the other hand you do not want to only partially vent it and then break a vent seal only to have atmosphere rush in at an alarming pace. The best way to vent is to monitor the internal pressure with a vacuum gauge. The Super Bee from Instrutech Inc can monitor pressures from 0.001 to 1500 mbar. You can place one atop each of your backing scroll pumps in place of the more common pirani gauges. Then you can simply valve off the scroll pump, but leave the Super Bee open to the internal pressure via the Turbo’s gate valve. Let in the N2 and watch the Super Bee’s pressure reading slowly climb. Stop when you reach 1050 mBar, either manually or automatically using the Super Bee’s pressure trip digital output signal (the configuration is shown in MBE: Auto-FEL vent/pump down). What to do once the chamber is vented? There are 2 schools of thought:
- Keep the N2 flowing at all times to prevent excess moisture entering the system whilst you are conducting maintenance
- Turn off the N2 gas it has done its job
Both are valid, but remember you cannot stop moisture entering your system with 1.2 Bar N2. You would need to have a much greater rate of flow, particularly when opening a flange greater than a CF-38. That is a lot of N2 waste gas. It is therefore more economical and practical to simply turn off the N2 and leave it off. You can quickly get rid of the moisture with the pump down technique described in Essential maintenance: Pump down. Now the system is at atmosphere you can get started on all those maintenance jobs in your maintenance log. The general dos and don’ts are outlined in Essential Maintenance: A how to guide. Once done pump the system down using: Essential maintenance: Pump down and prepare for the bake out (see Essential Maintenance: Bakeout).