Molecular Beam Epitaxy: Which Pump?

Faebian Bastiman

Invented in 1915 the diffusion pump is the king of UHV pumps. The principle is rather simple, a unidirectional jet of oil (yes oil, yes oil!) is sprayed inside the vacuum trapping particulates and sweeping them through the exhaust. Possessing low cost, no moving parts, a high pump rate for all gasses (up to 50kL/s) and capable of operating in the 10-2 to 10-10 mBar range they offer superior characteristics to other UHV pumps. Sadly they have two major disadvantages. Firstly, diffusion pumps passively back stream small quantities of oil (yes oil, yes oil!) into the vacuum chamber they are pumping. The amount of back stream oil is minimised by optimal pump design and employing traps, however for many users the idea of any oil vapour entering their vacuum system is tantamount to blasphemy. Secondly, diffusion pumps rely on fore pumping in the form of a rotary vane pump to create an outlet pressure of around 0.1 mBar. Failure of the fore pumping causes significant back streaming of oil (yes oil, yes oil!) into the vacuum system. Failure of water cooling results in a similar back streaming of oil. The absolute system wide contamination of several MBE reactors (due to failure of one of these systems) saw the popularity of the diffusion pump rapidly diminish in the early 1990s.

The cryogenic pump (cyro pump) is arguably the ultimate evolution of earlier sorption pumps. Both employ a cryogenically cooled interior coated with highly absorbing material to trap gasses. The sorption pump is cooled by submerging the pump’s housing in LN2. They were often employed as a roughing pump on earlier MBE system. Whilst cheap and reliable, sorption pumps are notoriously slow with a low pumping volume. Cyro pumps possess the same broad pumping range as the diffusion pump (10-2 to 10-10 mBar) however at severely reduced capacity (1-2k L/s depending on the material). They also possess significant mechanical displacement (usually by the means of a piston) that can induce vibrations within the MBE system. The cyro pumps cooling is achieved with a He compressor that doubles the cost of the cryo pumps. In multi system facilities, three individual cyro pumps can be run off a single compressor. Both cyro pump and compressor require a full service every 5-8 years. Furthermore, sorption and cyro pumps require regular regeneration owing to saturation of their absorbing material and hence neither can operate continuously. Compressor failure leads to a rapid increase in internal temperature and pressure, resulting in back streaming of molecules into the chamber. This is prevented via a dedicated gate valve, however with no exit the rapidly building internal pressure can cause a fracture of the pump housing and thoughtful implementation is needed to avoid damage.

The ion pump is perhaps the most iconic of all MBE pumps. Older ion pumps and their controllers required hefty initialisation currents (~18A) and lower but equally significant operation currents (~5A). Modern ion pumps and controllers require around 1A for continuous operation, meaning (i) they are very efficient and (ii) they can be backed up with a small, desktop uninterruptable power supply (UPS). Ion pumps can therefore safely hold a system’s vacuum in the event of a power failure. Uniquely ion pumps also operate as a pressure gauge (see MBE: Which Gauge?). Their pump capacity is however smaller than other UHV pumps (~200-500 L/s depending on the pump dimensions) and they require a start pressure around 10-5 mBar however they can create an ultimate pressure close to 10-11 mBar. Note that the low pump capacity means that this low pressure is not possible in the growth chamber, however can readily be achieved in the preparation or buffer chambers. Ion pump have the advantage that they are clean, need little maintenance, produce no vibrations and unlike the cryo and turbo pump they can effectively pump H2 and He. Hence they are perfect for standalone pumping of a preparation or buffer chamber, however they must be combined with a cyro pump and/or a turbo pump to achieve the UHV condition necessary for low background doping material.

Turbo molecular pumps (or simply turbo pumps) come in all shapes and sizes and can be employed for a variety of applications. Like the diffusion pump they require fore pumping in the form of a roughing pump on their exhaust in order to operate. The roughing pump can take various forms (rotary vane, diaphragm or dry scroll) though in the interest of maximum pump capacity and low contamination the scroll pump is the roughing pump of choice. The turbo and scroll combination handily spans the atmospheric pressure to HV to UHV range. Turbo pump capacities range from 70 L/s to 4.5k L/s depending on the pump dimensions and the bore of the flange they are connected through. Smaller capacity pumps are employed to regenerate cryo pumps and pump down fast entry locks (FELs), whereas the large capacity pumps help the ion and/or cyro pump on an MBE’s growth chamber. Turbo pumps are “through” pumps with similarities in operation to a jet engine, unlike ion and cryo pumps which are “trap” pumps. Some applications, like phosphorus, require the through pump capabilities of a turbo pump along with additional phosphorus trapping capabilities. Since the turbo pump operates by accelerating molecules the pump’s efficiency is directly proportional to the molecules mass.

Titanium sublimation pumps (TSPs) operate in a similar manner to cyro and sorption pumps. In the case of TSPs the adsorbing material is a thin film of highly reactive titanium that is evaporated by supplying 40-50A through a titanium filament and the condensing surface is the MBE reactors chamber wall. Once saturated, the film can be re applied with a further current heating cycle. Pumping effectiveness is also increased by cooling the chamber walls with LN2 or positioning the TSP near the main LN2 cyro panel. Since titanium is particularly reactive with CO, H2O and O2, it can significantly reduce background doping and hence it is recommended to fire the TSP particularly prior to oxide removal and during growth of high mobility materials.

The cyro panel is a compulsory constituent of all high quality MBE systems. LN2 (77K) is by far the most common cooling medium and MBE systems typically consume 150 – 200 L/day when the LN2 is delivered from an efficient combination of high vacuum lines and a phase separator (see MBE: LN2 system). However some users argue that a custom chiller until pumping silicone oil (193K) or antifreeze (233K) is perfectly viable in certain situations.

So which pump? The cyropanel and TSP form a very effective secondary pumping combination and should always be present. As for the main pump… honestly I would like to see some new development go into the design and auxiliary services of the diffusion pump. Redundant fore pump configurations employing a team of pumps and triple backing up the water cooling (see MBE: water cooling system) along with suitable interlocks could make diffusion pumps fool proof. Resurgence, of course, relies on consumer confidence. Sadly the alternative is to employ all 3 of the remaining pumps (cyro, ion and turbo) in order to achieve even close to the same pumping capacity. This triple combo easily increases the pumping cost by a factor of 5x and of course occupies a great footprint and imposes other run cost and maintenance overheads. Of course the more pumps the better the vacuum, however most R&D reactors can be adequately pumped with a combination of 2 of the 3 pumps on the growth chamber. The exact combination depends on what you are pumping: for As systems an ion pump and cryo pump make a good combination, for N plasma systems two cyropumps make a good combination, for P systems an ion pump and a turbo (with a P trap) are needed. For further information, a simple R&D pump configuration can be found in MBE Design and Build: Vacuum System.

Molecular Beam Epitaxy: Which Gauge?

Faebian Bastiman

We MBE growers are interested in measuring nothing. We have various pieces of equipment specifically purchased to measure the degree of “nothing” inside our vacuum system. There are a number of vacuum measuring gauges available. Each with a specific purpose and each suited to a specific job. The gauges are:

  1. Pirani (including the Super Bee from Instrutech)
  2. Capacitance Manometer (Baratron from MKS)
  3. Wide range gauges (Edwards / Pfeiffer)
  4. Ion gauges and controllers (Epimax)
  5. Ion pumps
  6. Quadrupole mass analyser (Pfeiffer Prisma)

Interestingly only Manometers are “true” pressure gauges, since they work with the force per unit area exerted by the gas and are therefore independent of the gas species. Unfortunately the manometer is rather delicate and can be damaged by exposing it to greater than atmospheric pressure. The manometer can be used in the FEL automatic pump down system (see Design and Build: FEL Automatic pump down and vent), so long as you take precautions to protect against over pressure. However for MBE purposes a manometer is actually surplus to requirements.

All the other gauges are “relative” pressure gauges, i.e. they are not independent of the gas species. This means that the pressure reading is not the true reading (even when it is displayed to 3 d.p. of accuracy!), it could be the true pressure if the species being detected were the same species used during calibration, however due to humidity, temperature and composition variation of the species this is unlikely to be the case. Thus with an ion gauge you are in fact not measuring the amount of the species in the vacuum, you are measuring the ionisation potential of the species. For example, this is why the same number of atoms/nm2/s flux register with very different beam equivalent pressures (mBar) for Ga and In. However for MBE purposes this relative pressure measurement is perfectly adequate.

The Pirani gauges was actually invented in 1906 by a German physicist called Marcello Pirani. The gauge relies on thermal conductivity (in the form of heat loss) from a heated filament into the carrier gas. Hence a better vacuum results in less heat loss. A similar principle is used in MOVPE and MOCVD to monitor the gas flows. The Pirani needs careful calibration and the reading is affected by the thermal properties of the gas being monitored. Standard Pirani gauges can be operated in the 1000 to 0.01 mBar (1 to 10-5 Bar) range and are therefore useful to monitor the pressure of backing pumps. A Pirani’s sensitivity, response time and accuracy are strongly affected by both their hardware design and software control. The Super Bee from Instrutech is the best Pirani gauge I have used. It includes an integrated controller with digital display and configurable relay trips. The Pirani filament has a lifespan of decades.

Wide Range Gauges (WRGs) are actually two gauges in one. They utilize a Pirani down to 0.1 mBar (10-4 Bar) and then “seamlessly” switch to an inverted magnetron from 0.1 to 10-9 mbar. In truth there is a small “grey” area when switching, but since we are not particularly interested in pressures around 0.1 mBar, we can live with it. The inverted magnetron is a cold cathode filament-free ion collector utilizing a high voltage (~4kV) that is automatically deactivated at high pressure. The WRG is a variation of the Penning Gauge invented by the Dutch physicist Frans Michel Penning in 1937. Unfortunately the gauge’s reluctance to start at UHV means that they are not useful to measure the PREP and GROWTH chamber pressures. They are however very useful to monitor the pressure of the FEL which is constantly cycled between atmosphere and UHV. Whilst a WRG can be operated for a decade or more, it is a good idea to service and recalibrate it every 2-3 years.

Ion Gauges (IGs) are hot cathode gauges. They are composed of 3 electrodes. Namely, a filament that acts as an electron source, a grid that accelerates the electrons and a collector to detect ionized gases generated by collisions between the electrons and atoms inside the vacuum (specifically inside the grid). All MBE ion gauge heads are similar, but not identical. Two electrode configurations exists, each with reflectional symmetry to the other, and each with a specific cable set. It is therefore worth clarifying which layout you have before reordering. The vacuum feed-through typically consists of an 11 pin circular plug, though again certain manufacturer’s support different controllers with different cables and different feed-through pin outs. The three electrodes consume 5 of the 11 pins, with 3 for dual filament and the central pin reserved for the collection. It is a good idea therefore to buy the ion gauge head from the same manufacturer as the ion gauge controller. Here I recommend Epimax throughout.

Tungsten (W) creates a very robust ion gauge filament that can be operated up to 100W (much like a light bulb) and in so doing it can actually be used as a light source inside the vacuum system! Unfortunately In and to a lesser extend Ga modify the work function of gauges fitted with W filaments, and so either Yttrium (Y) or Thorium (Th) coated  Iridium (Ir) filaments are more reliable/accurate in III-V systems (and in particular in beam flux monitors (BFMs)  (aka monitoring ion gauges (MIGs)). Sadly the Ir-filament cannot be run at such high power, and so does not make a practical light source. The Ir-filament can however be operated with a DC ion gauge controller (Epimax PVCx) to make them less sensitive to AC noise and hence give better stability. A DC operated ion gauge can detect from 0.01 to 10-12 mBar. This broad range is only permissible utilizing very small currents within the noise floor of AC units. The IG is therefore the preferred gauge for the PREP chamber, GROWTH chamber and the BFM. The lifespan of an IG depends on its usage. If you are measuring background pressure with a W filament it may last 10+ years, however its life is significant;y reduced if you do not shut it off and cool it down to room temperature before venting the system. If you are using the IG as a BFM/MIG then the filament will be constantly bombarded and eroded by various metal from the effusion cells and it may not last more than 6 months. Furthermore the entire IG head will become coated in III-V material creating a high impedance short circuit between the filament and grid. The rate of build up of material is of course proportional to your flux rates. Any III-V material condensed on the PTFE of the IG head can be etched away in 5 – 10 s  with a few drops of Aqua Regia. If you are careful, and rinse (with di-water then IPA ) and bake the IG head thoroughly you can reuse it for 10+ years.

Ion pumps are in fact vacuum gauges! Admittedly they only monitor the vacuum inside the pump itself, but once they have stabilized the ion impacts that are registered in the form of a ion current  can be readily converted into a pressure with some fitting factors that utilize the pump dimensions. This conversion is available as an option in modern ion pump controllers. Thus the ion pump is similar in operation to an ion gauge and can be used (as a backup) to monitor the pressure inside the PREP and GROWTH chambers.

Finally we have the Quadrupole mass analyser which is a form of quadrupole mass spectrometer (QMS). It consists of four parallel cylindrical rods (hence quadrupole) each making an electrical pair powered with radio frequency (RF) voltage. The rods then “filter” the ions based on modification of their trajectory due to selectivity of their mass-charge ratio. QMS typically have a detection range from 1 to 100 mass units and therefore can detect the concentrations of a broad range of useful species including: He (4), Water (18), N2 (28) and O2 (32) making them very useful for leak detection in the GROWTH chamber. The hardware design has a significant effect on sensitivity and resolution. The Prisma range from Pfeiffer are excellent QMS for MBE purposes.