An MBE system can appear rather complicated when viewed as a single entity, however it is much simpler to break the system into separate sub-systems (see MBE: System and Subsystems for a full description). This article discusses the basic structure of the vacuum system. Specific information on the specifications and features of each pump can be found in MBE: Which Pump?
An MBE system typically comprises 3 chambers: FEL, PREP and MBE. The FEL (fast entry lock) is regularly vented, unloaded, loaded and then pumped down. The PREP (preparation chamber) is typically maintained at UHV and only experiences contamination from outgassing wafers or from an adjacent chamber during sample transfer. The MBE (growth chamber) is constantly bombarded with elemental/molecular beams, along with O2 from the surface oxide. It also undergoes massive temperature changes when a cell is ramped from standby to operating temperature or when the LN2 is turned off or on.
Unsurprisingly the MBE chamber has a large pump demand. Some people employ a policy of “the more pumps the better”, however for a small III-V research reactor the pumping requirements can be met with an ion pump and a cryo pump team. The PREP chamber has the lowest pumping demand, and can be operated with a single ion pump only. The FEL is best operated with two stages of pumping. Firstly a dry scroll pump to pull the chamber from atmosphere (1000 mBar) down to ~0.005 mBar fairly rapidly, then secondly a turbo pump to reach UHV conditions. This basic MBE pump configuration is shown in Figure 1.
The system is shown in an idle state. The rectangles with an “X” through them are UHV gate valves that are either red (closed) or green (open). Similarly the circles with an “X” though them are small UHV valves. The valves with a grey outline to the left of the image form the FEL’s pump down and vent sequence, which is discussed in more detail in MBE Design and build: Auto Pump Down. The “IG” refer to ion gauges used to measure UHV vacuum conditions. THE FEL pressure is monitored with a wide range gauge (WRG) capable of monitoring from atmospheric pressure to 1E-9 mBar and hence conveniently does not need turning off during venting. The scroll’s inlet pressure is monitored by a Super Bee pressure gauge from InstuTech. MBE pressure monitoring options are discussed more fully in MBE: Which gauge?
In this configuration the scroll acts as a HV pump for the entire system. In can bring any chamber from atmosphere down to 0.005 mBar via:
- FEL: through the dedicated circular valve
- PREP: either through the FEL or via an optional dedicated valve
- MBE: through the cyro pump’s exhaust valve.
The turbo pump is used to create a UHV in the FEL. When venting the FEL, it is isolated on its inlet and exhaust and set to standby speed. The N2 valve is then opened and the FEL is vented to atmospheric pressure with ultrapure N2. Once the samples have been exchanged, the FEL is first drawn down to 0.005 mbar on the dedicated bypass valve, then the turbo is reengaged. In this way the turbo is effectively idled with no backing up during the sample change, which is ok for 5-10 minutes.
The PREP chamber is pumped via “Ion pump 1”. Ion pumps should never be turned off, and so when venting the PREP it is necessary to isolate the ion pump from the chamber via a gate valve. The PREP is typically vented to atmosphere and pumped down via the FEL’s valves, however an additional bypass to the scroll line can be added if desired.
In most configurations an ion pump and cryo pump team is perfectly adequate for the MBE chamber. The cyro pump, like the turbo pump, can be used to bring the base pressure down from 0.005 mBar after venting. The N2 on the cryo pump’s exhaust can be used to regenerate the cryo pump or vent the entire system depending on whether the cryo pump’s gate valve is open or closed. Again the ion pump on the MBE chamber should never be turned off, merely isolated via its gate valve during venting. Remember the LN2 cyro-cooling shroud is also effectively a “pump”, though for the sake of clarity I have omitted from this article.
If you are growing with P then you can include an additional turbo pump on the MBE chamber (Figure 2). Providing each turbo pump with its own scroll greatly simplifies the system pumping logistics. In this example the P trap is “in line” between the turbo pump exhaust and the scroll inlet.