MBE Design and Build: Vacuum system

Faebian Bastiman

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.

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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:

  1. FEL: through the dedicated circular valve
  2. PREP: either through the FEL or via an optional dedicated valve
  3. 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.

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MBE Design and Build: Bake out controller

Faebian Bastiman

The bake out runs infrequently, but has a huge power demand when operating. In MBE: AC Power, we discussed the power requirements for an MBE system. I suggested a 32A 3PNE line was needed to operate the system, whereas a dedicated 20A 3PNE line was suggested for the bake out. This article provides a design of the control system for the bake out utilising a dedicated 20A 3PNE line.

The bake out can be performed in two general ways:

  1. A box or tent with a combination of 2.5kW fan heaters and 1kW ceramic heaters
  2. With heat wraps or a heater jacket

The principle of the bake out is fairly simple: heat the system to the target temperature for X hours, then cool down. The heat and cool ramp rates should not exceed 1°C /min in order to avoid thermal stress. Furthermore the heat ramp should suspend if the pressure in the chamber being heated exceeds 5.0 x 10-6 mBar. The control hardware of choice for managing the temperature and pressure requirements of a bake out is the Epimax PVCx ion gauge controller. The PVCx is a multi-purpose tool: primarily it functions as an ion gauge controller, it has units of mBar, Pascal, Torr and Amps thus it further functions as a picoammeter for beam flux measurements, it can operate a pirani, be utilised as an 8 channel normally open/normally closed (NO/NC) relay/trip switch and control the bake out. All these controls and trip conditions can be configurable via either the PVCx menu or via serial comms.

The basic components of the bake out control system are shown in Figure 1. The MBE chamber (depicted in blue) has an ion gauge monitoring the internal pressure and a k-type thermocouple monitoring the external temperature. The entire chamber is surrounded by an insulating SS box or fibreglass tent. Two heaters are mounted inside the box/tent: Firstly a 2.5kW fan heater (BesTec) and secondly a 1KW ceramic heater (VG Scienta). The actual number of heaters will depend on the size of your system. The AC heater power is simply controlled via NO contactors rated at 32A with a 24Vdc coil (RS). The PVCx then actuates the contactors with its relays by modulating the external 24Vdc supply.

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The fan contactor is on whenever the bakeout is above ambient temperature (i.e. constantly during bakeout) whereas the heating contactors make and break several times a minute to regulate the temperature. The fans can therefore simply be controlled by an AC switch and manually turned on before and off after the bake (Figure 2).

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Alternatively the bakeout can be performed with heat wraps (VG Scienta) or custom made jackets (sadly currenty no know supplier).  The jacket essentially comprises a heater element attached to a fibreglass fabric covering an inner fibregalss insulating layer (essentially loft insulation) that is custom shaped to fit the contours of the system. The bake out is therefore more efficient since the heat is injected view conduction into the chamber’s metal body and the is more readily retained. The jacket can be attached more swiftly than the wraps which need to be laboriously assembled each bake out, or left as a permanent feature for convenience at the cost of aestheticism. It is a good idea to use some insulation, even if is just Al foil to cover the heat wraps to retain the heat. In this alternative bake out the PVCx simply makes and breaks the contactor to control the power to the heat wraps/jacket rather than the heaters (Figure 3).

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The actual heater configuration, power demand and wiring will depend on your system configuration. Figure 4 shows my wiring diagram for a custom built bakeout heater for a Riber 32P system. The system comprises a dedicated 20A 3PNE line, a rack mounted miniature circuit breaker (MCB) and fuse panel, a rack mounted 5 x contactor array, a DIN rail mounted distribution enclosure (FIBOX) mounted on the Riber 32P frame, 2 x 2.5kW heaters, 4 x 1kW heaters and 4 x 1kW heat wraps. The bake out controls the temperature and pressure of each chamber: MBE, preparation (PREP) and the fast entry lock (FEL) with 3 PVCx controllers. The rack mounted switching panel utilises 5 x 3-way switches to enable the system to be set into manual/automatic/off. Note: for clarity I did not include the EARTH connections (green), but any electrician reading this can rest assured all heaters, fans, racks and the MBE system itself were suitably earthed.

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