MBE operation should be a constant cycle of growth campaigns and regular, scheduled maintenance periods. I put the word should in italics because, unless you are careful, maintenance can enter the cycle of reactionary quick fixes that inevitably lead to extended downtime. So how do we avoid this undesirable maintenance cycle? We need to put in some extra effort and employ some good operation/maintenance practices: fault logging, planning and regular servicing and keep a stock of spare parts.
Fault logging is highly underrated, but in retrospect it is one of the fundamentally most useful aspects of maintenance. It can be as simple as a quick note in a logbook at the end of each day to (hopefully) say “everything is ok” or (sadly) to note the occurrence of a new fault. I like to keep a spread sheet log that lists the problem, the date it was detected, actions taken and the date it was fixed. The log is also a useful reference during maintenance periods, since it is essentially a task list comprising all the little jobs that need to be completed.
Some problems are minor irritations, some are ex situ and can be repaired at your leisure and (again sadly) some are serious and require the premature interruption of the growth campaign. It is the latter type of problem that we would like to avoid. That is where two crucial aspects of good maintenance practice come in: planning and regular servicing.
Every part of the system has a finite lifetime. The service intervals are (i) on-going, (ii) once per campaign (which can be 3 – 12 months depending on your system, configuration and usage), (iii) annual, (iv) biennial, (v) once every 5 years and (vi) once every 10 years.
The on-going maintenance tasks are those that do not require the system to be opened up, repaired and baked, but are nevertheless essential. This includes cleaning the sample holders (see Essential Maintenance: Sample holders), regenerating the cryo pumps (see Essential Maintenance: Cryo pumps) and outgassing the cells after the weekend off (see Essential Maintenance: Cell material regeneration).
The once-per-campaign maintenance tasks should be undertaken during regular, scheduled maintenance periods before each growth campaign. The tasks are:
- Replace Al and Bi crucibles (or replacing the PBN crucible inserts)
- Replenishing effusion cell source material
- Cleaning shutters (depends on system)
- Cleaning viewports (depends on system)
It is a good idea to try to anticipate and time your material usage so everything runs out at the same time. This is easier said than done and experience (as always) plays a big part. It is a good start to put as little Al and Bi as possible, since it is best to fully deplete this sources each campaign (see Essential Maintenance: Crucible cracking for more details). Of course your other choice is to simply switch your alloy composition until all your cells are down. For example once my Ga cells are depleted I grow some InAs/InAs(100) pins diode detectors for a colleague.
Annual and biennial tasks are in some ways system dependent and hence it is up to the user to decide what good practice is for their own particular circumstances. The tasks are:
- LN2 cryo-panel cleaning/regeneration
- Cleaning the substrate heater/manipulator
- Inspect RHEED screen As coverage
- Clean MIG ion gauge head (see Essential Maintenance: MIG head)
- Replace any broken TSP filaments (as necessary)
- Replace broken ion gauge filaments (as necessary)
- Service scroll pumps
The LN2 cyro panel regeneration is a neglected service item. In an ideal world each III-V MBE machine would possess two panels, employed with a “one to wash against the other” philosophy. I personally cleaned my Omicron MBE-STM cyro-panel once a year. This is good practice because the As layer is still relatively thin and can be brushed away. If you are going to service your cyro-panel once every 5 years you will need to have the whole thing shipped and dipped (see Essential Maintenance: Cryo-panel).
Arsenic gets everywhere. A thin film starts to build up on the growth chamber interior, despite the presence of shutters, and that means it starts to grow where it is not wanted. In particular this is the RHEED screen, MIG and the manipulator metal work. The latter point can result in poor RHEED access to the sample and eventually no RHEED at all.
5 year service items are often forgotten simply because they are 5 year service items. A lot can happen in 5 years, the entire composition of a research group could well change. Here are some notes to leave for your successors:
- Inspect, clean or replace rubber seals on load lock and gate valves
- Service UHV pumps
- Service Al and Si cells
Lengthier service items are:
- Other effusion cell replacement or service
- Ion pump replacement or service
- Software upgrade
- Rack hardware service/upgrade
- Clean and service RHEED gun
The UHV pumps are probably the most neglected regular service items. Oil-and-ball-bearing based turbo pumps require a return to manufacturer and full service once every 5 years, those with magnetic bearings can be operated up to 10 years. Ion pumps too require regenerating/replacing every 8-10 years.
It is a good idea to have your Al and Si cells serviced once every 5 years, since they are high operating temperature cells. The other effusion cells will typically need servicing once every 10-15 years. The best practice for effusion cells is to buy ones with separate water cooling jackets and shutters, and hence the cell itself is simple to replace or repair. As crackers can operate 20+ years, but they will need refilling once every 5-15 years depending on your usage. The cost of a full service on an As cracker is around 65% of the price of a new one.
Finally let’s consider our stock of spare parts. Obviously the safest way to operate is to have two of every item; though for financial or space reasons this is impractical. You can buffer against some problems by entering into a service contract with the manufacturer whereby effusion cells and pumps are serviced on a rolling-replacement basis. It is however good practice to standardise your parts as much as possible and keep in stock:
- One of each kind of crucible
- One of each kind of bellows
- Numerous gaskets of various sizes
- Blanking flanges of various sizes
- Cell source material and an As charge
- Ion gauge heads and filaments
- TSP filaments
- A RHEED filament
- A RHEED screen
- A gate valve seal kit for each type of gate valve on your system
- Manipulator/outgas stage filament
- Multiple shutter blades
- Cell parts (should you decide to service them yourself)
- DC power supplies
- A water chiller pump
- An ion gauge controller
Clearly maintenance should not be taken lightly. It is a large part of MBE operation; however by establishing good maintenance practice it need not be overwhelming or full of unexpected surprises. And most importantly, good maintenance practice forms the foundation upon which one can establish very fruitful growth campaigns with minimal disruption.