When we leave the lab we make sure that everything is in its standby state and we set up an automated outgas (or “morning initialization” as we call it). This usually means that the cells are clean and ready to use for the next day. However, one Friday morning we came in to find that the water cooling system had broken; water was not being pumped through the cells and was boiling in the pipes, causing the cells to heat up and outgas into the chamber. Having failed to restart the water cooler we crashed the cell temperatures to prevent them from being damaged. When we took the machine apart (it needed restocking with gallium and indium anyway) we found the aluminium shutter looking like this.
The sizable blob of aluminium which had accumulated on the shutter had clearly reacted with the outgassed material from the cells and now looked pretty disgusting!
The first step in the cleaning process was to remove as much of the aluminium as possible with a craft knife. After this the shutter looked significantly better but still not quite good enough to be used again. The remaining aluminium had to be etched away, so the shutter was placed in a beaker of room temperature HCl. The reaction was very quick and after ten minutes the shutter looked like this. Nice and clean and ready to be put back into the system!
To improve the vacuum in the main chamber after essential maintenance a bake-out is traditionally performed on an MBE machine. The contaminants and moisture adsorbed on the side walls of the main chamber will take longer to desorb and pumped out at room temperature. After using the repeated N2 purge technique outlined in Essential Maintenance: Pump down, it is entirely possible to recover the vacuum with normal, sustained operation of an MBE system. The process is however fast-tracked by heating the machine for a period of time in excess of 100°C. The result is a lower background pressure and good vacuum within the system. Instructions on how to build your own bake out system can be found in MBE Design and build : Bake out controller.
Our Omicron MBE-STM has been baked in two stages: a mini bake using heater coils, coiled around the main chamber (Fig 1) and a more comprehensive bake by enclosing in a custom built box composed of thermally insulated stainless steel panels (Fig 2). It is important to short RHEED gun power pins with Al foil static build during the bake out. It is best to regulate the bake out ramp from room temperature to bake out temperature with a PID controller set to 1°C/min. You can use a faster ramp (2°C/min max) and cover the view ports with Al foil in order to buffer against the additional thermal stresses, but an unregulated ramp is not recommended.
The mini bake shown in Fig 1 is more flexible but less comprehensive that the full bake (Fig 2). Heater coils can be wrapped around the chambers that need baking out without the need of unplugging electrical parts or plastic tubing which are used for water cooling the effusion cells. A custom designed bake out jacket with built in heater filaments would be a MBE wardrobe worth investing in 🙂
A full bake out using thermally insulated stainless steel panels (Fig 2) is both temperature and pressure dependent. It is possible to achieve the same vacuum by baking a machine at 200°C for 24 hours, 125°C for 48 hours or 100°C for 60 hours. The maximum bake out temperature is limited by the temperature sensitive parts in the machine. The lowest pressure achievable within a “used” III-As MBE chamber during the bake-out is of the order of 10-7 mBar which can be achieved in ~50 hours at 125°C and typically consists of As vapour pressure.
The full bake out provides a better vacuum as the heat is more uniformly distributed. However, these are more cumbersome to put together and require removal of all electrical and thermally sensitive connections. It is essential to bake the chamber long enough till the moisture and contaminants adsorbed onto the side walls have sufficient thermal energy to be desorbed and eventually pumped out. Since the vapour pressure within the system has been reduced it is possible to reach ultra-high vacuum and good growth conditions which is essential for good epitaxy !