Molecular Beam Epitaxy: AC power

Faebian Bastiman

With a peak demand of 18kVA (240V x 25A x 3 phases) an MBE machine has the power requirement of a small industrial facility. The actual power demand involves a peak demand, operating remand and a base demand. The peak demand corresponds to when the system is baking, the operating remand is when the DC power supplies for the cells are at operating power and the base demand is essentially the demand in an idle state. You can work out your demand with a power meter or a loop meter and some systematic data gathering on each piece of hardware. Some typical values are shown in the table in Figure 1. Note the power demand is per unit and the total is the sum-product of all units.

What this shows is that the operation demand is roughly 200% the idle demand, and in turn the peak demand is roughly 110% the operation demand. What this means is that machine’s power demand can be met with one 240V x 32A x 3phase line for normal operation, and one 240V x 20A x 3 phase line for the bakeout. That will leave a safety margin with plenty of room for expansion. Decoupling the power supply can be of great advantage when you come to backing up your AC power. Note: You may find your own demand is significantly higher if you are running old (pre-2000) hardware. Also your peak demand will be significantly higher if you do not employ some eco-friendly practices like turning off the water chiller during bakeout.

AC power v4

Should you back up your AC power? That is a good question. Imagine a power cut. Everything goes off. You are in darkness. Pumps start winding down, cells crash toward room temperature, the vacuum rapid deteriorates, the cyro pump pressure rises rapidly, and worst of all you cannot see what is happening, because all the power is off! If it is just a brown out the power may come back on after a few seconds or a minute. It may switch on an off several times in rapid succession. It may stay off for several hours.

In an absolute worst case the power cut and/or surge will have damaged some of your hardware and you will not be able to restart the whole system. However, assuming all hardware survives, the worst case of prolonged power outage is that you will need to restart your system from “cold”. This means you Ga, Bi and Al will be frozen solid and your vacuum may be anywhere from 10-3 to 10-9 mBar depending on how well you leak detected. You have two options:

  1. Simply restarting/reheating everything is a viable option. If you have an Al insert or a double-walled crucible you can be confident the cell will survive.
    • Best case: The system will need a day or so to outgas
    • Worst case: You lose a cell or two (Ga/Al/Bi). Repair cost: £10k, time: 4 weeks
  2. You vent the system. Replace the crucibles and rebake. Repair cost £750 – £2250, time: 2 weeks.
  3. Back everything up on a 3 phase UPS and a LPG/diesel generator and simply keep growing

Option 3 is perhaps something you will not have considered. You do not need to back up the bake out 20A-3Phase line since it is non-critical, but the 32A-3Phase line is definetly worth backing up. The backup comes in two phases:

Firstly the UPS: A 32A-3Phase UPS costs a mere £2.5k, about 25% of that cell you may lose. The UPS is a stop gap solution, it is not intended for continuous operation. It is intended to hold for a number of minutes, 15 minutes max, until power is restored. UPS’ also buffer against power surges and fluctuations and instability, protecting your delicate equipment.

Secondly the generator: A 16kVA generator is capable of sustained operation. You will of course be consuming fuel, and a backup generator is less efficient than a power station, but you can operate indefinitely in the absence of all external power. You can in fact carry on growing if you wish. A 16kVA generator costs £7.5k, about 75% of that cell you may lose.

And so, for the price of a single effusion cell you can protect your system against the potentially very damaging and destructive effects of a mains power failure. Should you back up your AC power? I will leave you to decide.

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101 questions to ask when buying a new MBE system

Faebian Bastiman

Ordering (or even expressing an interest in) a new MBE system bears a tremendous responsibility. In a radical way you are in fact influencing the future development of MBE technology. Great care and consideration is needed in order to specify the system you want, rather than the system that is currently available. Here are a few points to consider:

  1. What do you want to grow?
  2. Which manufacturer will you choose?
    1. What is the overall system cost?
    2. What are the features?
    3. How much of the system can be customized to your needs? If not all why not!?
    4. What is the manufacturer’s customer service like?
    5. How much will the maintenance items cost?
  3. How big is the system? i.e. What is the system foot print?
  4. How much does the system weight?
  5. What system layout will you use?
    1. Where will the racks go?
    2. Where will the desk go?
    3. How long do the interconnecting cables and pipes need to be?
  6. Where should the system be installed?
    1. Will a lab be sufficient? (Loaded question, answer: Yes)
    2. Do you really need a cleanroom? (Loaded question, answer: No)
    3. Will you allow the MBE operator to experience natural light!!!
    4. Can you position the system to make guided tours easily?
    5. Will you open your facility to the public annually?
    6. What are the advantages of a “showcase” system?
      1. For you and your facility?
      2. For funding applications?
      3. For gaining PhD students?
      4. For supporting local industry?
      5. For the local community
      6. For the manufacturer?
    7. What clothing do you need to wear whilst operating the system?
    8. How will the cleanliness of the room be maintained?
  7. What other costs (initial outlay) are associated with the system install?
    1. How much is the water cooling loop?
    2. Will the water cooling be triple backed up against failure?
    3. Do you know what “triple backed up” is?
    4. How much is the LN2 loop?
    5. How much is the LN2 daily usage?
    6. How much is the pressurized gas loop?
    7. Will you use dry, compressed air as much as possible? And if not why not!?
    8. How much is the 3 phase electric?
    9. What is the current consumption?
    10. How much is the UPS?
    11. How much is a back up generator?
  8. What pumps do you need?
    1. What type of turbo do you want? Oil bearings or magnetic bearing?
    2. Are you getting dry scroll pumps? And if not why not!?
    3. Are you getting a cyropump for the growth/preparation/FEL chambers?
    4. What type of compressor are you installing? Air or water cooled?
    5. Are you getting an ion pump for the growth/preparation/FEL chamber?
    6. What type ion pump controller are you getting? A 1/2/4 pump controller?
    7. Do you need a phosphorus trap?
    8. Will you neutralize the phosphorus with water?
    9. Where will you send the waste phosphorus / phosphoric acid?
  9. What is the most economical means of running the system (day to day)?
    1. Where will you buy your samples?
    2. Will you buy 2/3/4”?
    3. Will you ¼ your samples?
    4. Will you repolish your samples?
    5. Where will you buy your Cu gaskets?
    6. Will you support local industry by buying your Cu gaskets from a local manufacturer?
    7. Where will you recycle your Cu gaskets?
    8. Where will you buy your crucibles?
    9. How will you regenerate your crucibles?
    10. Will you be using inserts for Al and Bi?
    11. Where will you buy your cell material?
    12. Will you use 6N5 or 7N material?
    13. How can you recycle energy (heat and cold)?
    14. Can you gain publicity and press by installing a “green” facility?
  10. What is the warranty of the system?
  11. What service plan does the manufacturer offer for maintainence and repair?
    1. How will you regenerate the LN2 cooling panel every 5 years?
    2. How will you regenerate the cell dividers and shutters?
    3. How will you regenerate the sample holders (platens)?
    4. How expensive are the sample holders (platens)?
    5. How will you maintain the cells?
  12. What ancillary hardware do you need?
    1. Whose PSU will you take?
    2. Whose PID controller will you take?
    3. Whose Ion Gauge controller?
    4. What makes a “good” Ion Gauge Contoller?
    5. What digital inputs and outputs do you need?
    6. What analogue inputs and outputs do you need?
  13. What software will you buy?
    1. Does the software enable automatic sample transfer?
    2. Does the software enable batch processing of multiple recipes?
    3. Does the software automatically pump down and vent the load lock (FEL)?
    4. Does the software automatically protect against water failure?
    5. Does the software allow in situ ellipsometry/reflectivity control?
    6. Does the software automatically tune to the pyrometer and/or thermocouple temperature?
    7. Will the software monitor the vacuum and valves?
    8. Will the software data log the ion gauges and PID loops?
    9. Is the software customizable/adaptable/expandable?
    10. What do you actually want from the software?
    11. What do you actually need from the software?
    12. How will you contribute to MBE development by adapting your specification and enabling the evolution of this technology?
  14. Is the sample transfer automatic? And if not why not!?
  15. Which RHEED system will you buy?
    1. Will it have bakeable cables? And if not why not!?
    2. Will it have software control?
    3. Will it have µ-Metal shielding?
    4. Does the system require µ-Metal shielding along the beam path?
    5. Can the manufacturer supply µ-Metal shielding?
    6. Is the RHEED well aligned?
    7. Will you require a port aligner or magnets to enable 360° access?
    8. Will the RHEED image capture be automatic?
    9. Can you record videos of the RHEED during growth?
    10. Can you control the azimuth/rotation from the software? What is the resolution?
  16. Is the monitoring ion gauge (MIG) integrated into the manipulator?
  17. How is the MIG head replaced?
  18. How easy is it to perform maintenance?
    1. Do you need to recover your phosphorus and double bake the system?
    2. How much of the infrastructure needs removing before bakeout?
    3. Is the bakeout a box-and-fan or a jacket system?
  19. How are the samples loaded?
    1. What is the position and orientation of the load lock (FEL)?
    2. Is it degassed to 125°C? And if not why not!?
    3. How many samples can be loaded concurrently?
    4. Is there sample storage available in the preparation chamber or elsewhere?
    5. How many sample holders (platens) do you need?
    6. What sizes and shape of cut out will you need in you sample holders (platens)?
    7. How are the samples actually physically mounted?
    8. How long does it take to exchange a sample on a sample plate (platen)?
  20. How are samples outgassed?
    1. What is the maximum temperature of the outgas stage?
    2. Can you oxide remove there?
    3. Can you have Ga/H cleaning?
    4. Can you have Auger/RHEED on the outgas?
  21. What is the orientation of the growth chamber?
    1. Is the manipulator vertically or horizontally aligned?
    2. How many cells do you need? 10? 12? 118?
    3. How many projects do you need to run simultaneously?
    4. Will there be problems with Ga spillage?
    5. How will liquid Ga spillage be trapped and removed from the system?
    6. Can you take this opportunity to improve the existing MBE technology?
  22. Who will run the system?
  23. How many samples do you want per day?
  24. What is the temperature gradient across the substrate?
  25. What is the temperature difference on different sample holders (platens)?
  26. How can you ensure the results will be systematic?
  27. How can you develop the software and hardware to eliminate systematic errors?
  28. What are the “bottle necks” to MBE throughput?
  29. What are the current limitations of MBE design?
  30. What have I missed?
  31. Where can I go to gain more information?

Molecular Beam Epitaxy: N2/Gas system

Faebian Bastiman

I am going to start this post by giving you the answer to a number of frequently asked N2 pneumatic and process gas questions. The answer to them all is: No. Just plain, simple, straight forward: No. Here are the questions:

  1. Is N2 gas creation from LN2 free?
  2. No you misunderstood, I mean N2 boil off from LN2. That is free right?
  3. Ok then, is N2 creation/boil off from LN2 at least cheap?
  4. Are you joking?
  5. Do I need ultrapure N2 gas to operate my pneumatics?
  6. Will the oil from a compressor damage my pneumatics?
  7. Should I just vent my LN2 phase separator exhaust N2 gas to atmosphere?

A MBE facility generally needs two types of pressurised gas: pneumatic and process. Pneumatic gas is there to do work. It drives valves and shutters. It is typically regulated to 3 – 5 bar. Process gas on the other hand is typically 1.1 – 1.5bar. Process gas is used to vent the fast entry lock (FEL) daily and the entire system for maintenance. Process gas is used to purge water from pipes and regenerate cryo-pumps. Process gas is used in fume hoods to blow dry delicate equipment and substrates. Process gas is converted into N-plasma for III-V semiconductor growth.

N2 gas creation from LN2 is not free, not by a long shot. It is CONVENIENT and it is SECURE and it is ULTRAPURE but do not think for a moment that it is CHEAP. The expense involved in generating N2 gas can easily double your LN2 bill versus using LN2 to cryogenically cool your MBE alone. You can design out some of the expense, but then you have to deal with the one off, initial infrastructure cost. The best thing to do to start with is to divide your gas demand into:

  1. Dry, filtered 5 bar air
  2. Dry, filtered 1.5 bar air
  3. Ultrapure N2 gas

Unlike boil-off N2 gas, dry filtered air is CHEAP. Dry, filtered air can be easily generated by a suitable compressor. Oil free compressors are also available. Two compressors working in a team can give redundancy and security of supply. A header tank can even be used as a buffer. Dry, filtered air is perfectly suited to a number of MBE requirements. Pneumatic gas can certainly be dry, filtered air. You will not want to risk contaminating your FEL or MBE system with dry, filtered air. Nor will you want to use it to blow dry substrates or regenerate cryopumps. But you can use it to purge water from cells. One (or two) compressors, dedicated pipework and several regulators are needed to produce 1.5 and 5 bar.

Next we have ultrapure N2. Ultrapure N2 is needed to vent the FEL and MBE system, blow dry delicate equipment and substrates and regenerate cryo-pumps and for making N-plasma. A single cylinder of compressed N2 gas regulated down to 1.5 bar on the outlet can last months. Again a team of cylinders enables redundancy and security of supply, but even then changing them can be tedious and presents a safety hazard. Therefore, for the sake of convenience you can create 1.5 bar N2 gas from a dedicated LN2 tank. DO NOT generate it from your LN2 cryogenic cooling tank, the work done on the liquid by the gas causes flash loss. To simplify the system you can have one moderately sized tank for LN2 liquid and two smaller tanks working in tandem to produce uninterrupted ultrapure process N2.

N2 gas may start off pure or ultra-pure, but the condition it reaches your machine depends on how clean the pipe work is. Only ever route the gas  through stainless steel (SS) piping. Swagelok provide an entire piping solution including valves and fittings. To ensure the pipe work is clean you will first want to leak test it and then bake it. Bake the entire line to 90°C from source to destination with heat tape/wrap. You can bake it in sections or all in one go, depending on the length of the run. If you are baking it in sections start at the source and work your way to the destination. Whilst baking, provide a small flow to atmosphere through a bypass valve to ensure contaminants are purged from the line.

The phase separator of your LN2 cryogenic system also produces N2 gas: Cold N2 gas. This gas is very useful. Very few process produce COLD, most of them produce HEAT. Waste heat is also useful, particularly in damp and cold British winters where a heat exchanger can be used to augment your buildings’ heating system. Similarly the waste cold is useful to augment your air conditioning and water cooling systems. You can save a lot of money by moving heat around.

Figure 1 shows an example of a LN2/N2/Gas system for a small MBE research facility. This general system is not intended to be emulated: your own specific facility needs will need to be evaluated and addressed on an ad hoc basis. It is, however, intended to make you think of other possibilities and to enable you to speak to facility designers with a fresh perspective.

Facility services

Molecular Beam Epitaxy: Water cooling system

Faebian Bastiman

MBE systems typically utilise water cooling to regulate and stabilise effusion sources. This may be integral to the cell, external though individual cooling shrouds or via a water cooled base plate. Water cooling may also be required for the outgas stage, ion pumps, heated view ports and power feedthroughs. Since you have multiple zones that require cooling you must first consider how to configure your water system. You have two choices: series or parallel.

A series system is very simple. You run the inlet of each cell from the outlet of the preceding cell and serially link all parts of the system in turn. This has the advantage that it is cheap and simple. The disadvantage is that the water gradually gets hotter and hotter from cell to cell, hence the final cell in the line will receive much hotter water than the first. Also, effusion cells tend to utilise water cooling shrouds made from 3mm (1/8”) pipe, hence serially connecting all the cells accumulates to a large resistance that the pump must overcome to create a suitable flow. By way of an explanation consider an analogy with a serial electrical circuit (figure 1), here the pressure is switched for potential difference and the litre/minute is current. Thus to achieve the required 0.5 litre/minute (0.5 A) to each cell, a potential difference of 11V must be applied to the serial 22 ohm resistance.

Slide1

A parallel system is naturally more complicated to implement, but ultimately yields a superior cooling system. Consider the parallel arrangement in figure 2, now the required cooling of 0.5 litre/minute (0.5A) per part is obtained by having larger overall flow (1.5 A) at a lower pressure (3.4 V) due to the lower overall resistance. However, when you take a closer look at the circuit it is obvious there is a serious flaw! Whilst the total flow is 1.5 litre/minute each cell is not receiving the required 0.5 litre/minute. Due to the unique resistance of each cell, the cooling flow is not divided evenly. In this case the 5 ohm cell is receiving 0.68 litre/minute, the 7 ohm cell is receiving 0.48 litre/minute and the 10 ohm cell is receiving an inadequate 0.33 litre/minute.

Slide2

The first thing we need to do on a parallel system is regulate the flow through each parallel branch. This is done by adding a tap (a variable resistor in our analogy) in line with each cell to provide additional resistance to specific loops. Figure 3 shows a balanced system. Note the pressure penalty incurred in adding the additional resistance.

Slide3

A parallel loop will typically consist of a dual chambered PTFE/brass manifold to split the water and a series of valves on the outlet side to regulate each loop. An example system is shown in figure 4. To regulate the flow along each branch you will need a flow meter with a 0-10 litre/minute span on the main chiller outlet line and a little patience: (i) close all but one of the outlet vales and turn on the chiller, (ii) make a note of the flow, (iii) open another valve and adjust until the flow is now double the initial flow, (iv) repeat with the other valves. Placing valves on inlet and outlet can be useful, since then an individual cell can be isolated from the rest of the system during unscheduled maintenance.  The manifold and valves will increase the initial outlay by around £1000. In addition a parallel system needs significantly more tubing to distribute the water.

Slide4

The demands of each individual cooling zone vary. Most effusion cells require 0.5 litre/minute and the same goes for cracker zones. Cells typically have a similar resistance, especially if they are roughly the same size. Consider buying cells with separate water cooling shrouds rather than integral ones, you can then ensure the water cooling shrouds are identical. The initial outlay is higher, but the maintenance cost is lower since the cells are simpler to repair and cheaper to replace. Bulk zones and outgassing zones may need 1 litre/minute and they offer much lower resistance to flow than a normal effusion cell. The bulk zone can be connected in series with the outgas loop and other low resistance loops to increase the overall resistance of that cooling branch.

All in all the MBE system will require 5-10 litres/minute of cooling water depending on the configuration. The cooling water is usually supplied at 8-12°C at a pressure of ~3 bar. Hence a standalone, single phase, direct, vapour-compression water chiller filled with a water-glycol mixture is perfectly suitable for a single MBE system. Suitable chillers include single phase and compact Thermo Scientific Flex1400 (Thermo Scientific) or for larger systems the 3-phase Tricool S2 are an excellent range with customable options (Tricool). A more comprehensive discussion of the types of chillers available can be found in MBE: Water cooling system types.

It is a good idea to back up your chiller on an UPS to ensure security of supply and to have a spare pump available since this is the most common part to fail. Should the system fail the damage wrought can be very expensive, and as such is highly recommended that a backup cooling loop is installed. Tap water is a perfectly viable temporary cooling solution. A flow meter, suitable array of valves and control circuitary can be used to switch to the backup system in the event of a chiller failure. The system can also be configured to automatically switch back to the chiller once flow there is restored. The backup tap water line will utilise ~7,000 litres per day however, so it is only a temporary solution to avoid multi-£10k of damage. The backup loop can also be utilised as a water purging loop with the addition of a N2 line, simplifying purging during routine maintenance. A simple system incorporating these parts is shown in figure 5. The bypass valve is normally closed and the green 3-way valves are normally straight through. The light blue 3-way valve is set to water. If the flow meter detects an interruption to flow (or insufficient flow) the bypass valve is opened and the green 3-way vales are operated to allow the tap water to flow through the cells and into the drain – and hence maintain cooling. Once the chiller is repaired, adequate flow will be detected through the bypass valve and the system can switch back into normal mode. If the chiller is turned off and the blue 3-way valve is set to N2, then the system will enter a purge mode whereby N2 will push all the water from the cells and into the drain. Very useful prior to maintenance. The control flow can be simply set up inside the EPIMAX MBE control software.

Slide5

Molecular Beam Epitaxy: Water cooling system types

Faebian Bastiman

This article introduces and discusses the types of cooling system available. Specific water cooling considerations and recommendations for discrete MBE systems can be found in MBE: Water cooling system.

Single pass

A single pass system is by far the most simple and by equal measures the most wasteful. The standard 3-5 bar mains water is perfectly suitable for achieving 5-10 litres per minute of flow through an MBE system. The system comprises connecting a tap to the inlet of the MBE manifold and piping the outlet to the nearest drain (figure 1). Sadly this means around 7000 litres of water per day are pouring down the drain. Hence this is not a permanent cooling solution, but is a perfectly viable backup to maintain cooling temporarily whilst the primary cooling system is down.

Slide1

Multi-pass/Recycling

Clearly the only sane solution is to recycling the cooling water.  There are 2 main types of recycling water cooling system: open and closed systems. An open system is by definition a direct system (whereby the cooling action is performed directly on the cooling water) in contrast a closed system can be either direct or indirect (via a heat exchanger).

Each type of system is considered in detail below, though first let’s imagine a basic cooling system: The simplest recycling system comprises cooling to counteract the MBE system’s heating and a pump to maintain suitable water flow (Figure 2). This system is arguably similar to an automotive cooling system. It relies purely on convective-and-conductive cooling and hence is very energy intensive and grossly inefficient without a suitably large airflow to hand.

Slide2

Refrigerant chiller types

In static systems it is therefore common to use a high vapour pressure fluid as a cooling medium and take advantage of the heat consumption in the process of evaporation to achieve local cooling. The two primary variants are absorption and vapour compression cooling, and they differ primarily in the method the refrigerant vapour is converted back into liquid to continue the cooling cycle.

Absorption cooling is more efficient than vapour-compression, particularly if you have abundant, high grade waste “heat” and “cold” that can be utilised.  The heat is used to evaporate the refrigerant which is then absorbed into a nearby absorber (creating a low pressure inside the system). Heat is utilised again in another chamber where the absorber is heated to release the refrigerant, which is subsequently condensed with coolant and the aid of a heat exchanger. The whole process is shown in figure 3. Handily MBE facilities have both waste heat (racks, cooling water, air conditioning) and waste cold (LN2 phase separator exhaust gas) that can be utilised to reduce power consumption.

Figure 3: Absorber (Pending: Please check back later)

Vapour-compression cooling utilises a compressor, a condenser, an evaporator and an expansion valve. The system (figure 4) is strikingly similar to figure 2 at first glance, however here the phase change of the refrigerant (from liquid to vapour) is providing a more efficient cooling action than pure convection can achieve. The excess heat and cold of an MBE facility cannot be directly coupled with commercially available vapour-compression chiller system, however a hybrid system is proposed at the end of the article.

Slide4

Heat exchange (Direct and indirect)

The systems described thus far have been closed loop systems. A closed loop system is one where the cooling water never “sees” atmosphere and therefore never has the opportunity to evaporate. Closed loops are therefore immune to external contamination (though internal contamination via corrosion of the system interior can still occur) and importantly should not require refilling. The water cooling system can be through of as a series of heat exchangers between a number of temperature zones. The terms direct and indirect refer to how the heat exchange is performed upon the cooling water itself. A direct system (figure 5) exposes the cooling water to the refrigerant pipes, whereas an indirect system keeps the cooling water contained in its own pipework (figure 6). Indirect systems are therefore less likely to introduce contamination into the cooling water. There is no particular advantage of one system over the other for MBE cooling applications.

Slide5

Slide6

Reservoir (buffer) tank

A reservoir tank can provide additional stability, aid modularity and enable multiple systems to be run from a single chiller. They can also enable the waste “cold” from the LN2 phase separator exhaust to be put to good use (see Hybrid System below). The chiller can be either direct or indirect. Note when connecting multiple system in parallel care must be taken to ensure the “resistance” of each system is approximately equal to ensure all receive equal flow (discussed in MBE: Water cooling system). This can be achieved by individual flow control valves and flow meters. A system with a reservoir tank and suitable pumps and valves serving two MBE systems and an XRD is shown in figure 7.

Slide7

Closed and Open loop

Open loop systems are a perfectly viable alternative to closed loop systems, however they are generally more suited to larger facilities and they require extensive maintenance and suffer from certain hidden costs that will be discussed herein. The cooling in an open loop system is provided by direct action via evaporation of the cooling water in a cooling tower. Banish the image from your mind of an industrial cooling tower belching out plumes of white water vapour; a cooling tower can be a much smaller, simpler, aestheticly pleasing affair. They of course need some space, either on a rooftop or adjacent to the facility. Water cooling towers can be both open (figure 8) and closed (figure 9) loop systems. Particular attention needs to be paid to the water in the tower. The evaporation action within the tower creates a “micro-sea” with a constantly increasing salt content. Hence the water requires a regular “blowdown” cycle where saltwater is remove and new tap water is used to top the system up. The removed water can still be useful (e.g. for irrigation) so long as it is done before the salt content gets too high. Hence it must be constantly monitored. Furthermore, since the water is constantly evaporating it must be constantly topped up. It must also be “treated” and “conditioned” to prevent the build up of algae. The additional environmental costs associated with open loop systems can be offset against their lower run cost, however because of these costs and complications (I will reiterate that) they are normally only applicable to large facilities.

Slide8

Slide9

Hybrid systems

As the name suggest, hybrid systems combine two or more aspects of the individual systems. The gain in reduced cooling cost is offset by an initial higher installation cost. More specifically a hybrid system should utilise the waste “heat” and waste “cold” of an MBE facility in order to reduce the cooling power requirements. The final figure is not intended to be an ultimate cooling system, nor is it intended to be perfect, it is simply intended to provoke you to consider how the discrete sub-systems of an MBE facility can be coupled together to save energy and cost (figure 10).

Water loop 2

Molecular Beam Epitaxy: Dream Machine

Faebian Bastiman

I want to start this article with a question. What do you not like about your MBE system? Take a moment and think. I am certain at least one problem will immediately materialise within your mind. Indeed, after teaching MBE operation to a number of PhD students, I constantly meet the question “Why?” “Why is it like that?”, “Why is that so slow?”, “Why is that not automatic?”, “Why is that not controlled?” and possibly my favourite: “Why do I have to come in on a Sunday evening to do that?” The reason MBE is “like that” is that we, the users, are “putting up with it”.

There has been tremendous advancement in MOCVD/MOVPE in recent decades because there has been a demand from industry. I think it is high time we, the researchers, made similar demands of MBE. Industry and academia are very different environments, but both adhere to a basic principle that I term “money-time” duality.

Consider a typical R&D MBE system. Sample transfer is an art form. Growth rate calibration is tedious and laborious. Instability hinders the systematic. Maintenance is complicated, laborious and hinders productivity. Throughput is manpower not machine limited. One skilled operator can probably grow two good samples a day. Research progress is, inevitably, slow. Bear in mind one system is probably facilitating several projects, collaborations and multiple characterisation studies.

Let’s analyse the above paragraph. I have actually operated a few MBE systems where I drew a great sense of achievement from actually successfully transferring a sample to its destination. Should sample transfer be the most challenging and rewarding activity in MBE operation? No! No it should not. So let’s automate that straight away. Automatic sample transfer is a must. It also opens up the possibility for batch processing: the execution of 12 samples without user interaction. Detailed systematic studies take days rather than months. Productivity has increased 1000%. Excellent, what next?

Growth rate calibrations are time consuming. I have operated a system where it was basically 90% of my job to monitor the cell fluxes, recipe writing the other 10%. So let’s automate that. Automatic cell flux tuning before every recipe negates the need to have user intervention and reduces instability. The growth rate can be calibrated in several ways. Until recently I considered RHEED the most valuable, now I believe in situ reflectivity to be even more powerful. With reflectivity feedback, sample temperate, layer thickness and composition can be controlled to <1% deviation. Suddenly MBE is highly systematic and stable. So let’s have automatic in situ reflectivity controlling the sample recipes. How about a dream software suite to go with out dream MBE system? Yes please, next?

Maintenance will always be a problem in MBE operation, but it need not be a crippling blow. Part of the maintenance responsibility falls to the user establishing good MBE practice but equally part of the responsibility falls to the manufacturer. There are several items that fall into the category “ease of maintenance”. Ten minutes (or perhaps an hour) talking to an experienced technician would highlight hundreds of issues that could be designed out. Maintenance should simply be regular, straightforward and swift. Let’s pay attention to the people who have spent their entire lives maintaining and fixing MBE system and incorporate their ideas, ok, what else?

One of the hidden problems is unit cost and this certainly limits the number of MBE research groups. Skill is not a limiting factor. A good teacher can teach the basics of MBE operation in about two weeks. A good student can become autonomous in about 3 months. Of course here we have a dangerous situation: if the system is fully manual, the process will take longer but the student will gain a deep knowledge of MBE. On the contrary, if the system is fully automatic the process will be much swifter, however the MBE knowledge gained will be more superficial. It is therefore always best to learn on a fully manual system. The automatic system is however essential for any serious research. This “dream machine” is not a wild fantasy. Everything mentioned in this article already exists. It is simply a matter of putting the pieces together.

So the final and most important question: How much will this system cost? Well before I answer, I will ask you: what is the cost of not having it? The answer is fewer research groups, less samples, less science, less progress, less understanding, more downtime, more maintenance, more frustration, more money wasted maintaining out of date machinery. The problem with the dream machine is not the £100k of parts; it is the £400k we pay to have those parts assembled. Much money can be saved by stripping the useful bits off old systems: the pumps, valves, cells, flanges, nuts and bolts and the racks. What we need is to invest a little time and effort into creating a compact, inexpensive stainless steel chamber and supporting frame integrated into automation software. A nice RHEED, reflectivity system and an As cracker. The substrate heater need only accommodate 2” or ¼” of 3” wafers maximum, we are performing R&D after all. 10 cell ports are plenty, by the time you have grown 360 samples in 1 month you will have thoroughly exhausted the cells, thoroughly explored one material system and can load a different combination of cells for the next month’s work.

How much does the system cost?

£500,000 new

Growing 4320 world class samples a year with less than 15 minutes work a day?

Priceless

Molecular Beam Epitaxy: Initial Outlay

Faebian Bastiman

Creating a new MBE growth facility or augmenting existing activity involves two major budgets. Firstly you have the initial outlay for the system, and secondly you have the continual, annual run costs. The annual costs were discussed in MBE: Run Costs, however the two are not mutually exclusive. Decisions you make during the install have a significant impact on your annual run costs. Ultimately the more you optimise and refine your installation, the less you will pay year on year to run your facility. Essentially spend more now, or you will spend a lot more later.

Whether you have bought a brand new system fresh off the production line or a second hand system you will still be faced with the same requirements of services and systems:

  1. 3 phase electric
  2. LN2 cooling
  3. N2 gas
  4. Water cooling
  5. Software control
  6. PID loops
  7. Cells
  8. Pumps
  9. Bake out
  10. Analysis: RHEED / Reflectivity / Pyrometry / Band edge thermography

 

Recommended 3 phase AC system cost: £10k

Right at the beginning you need to consider your power demand. Certainly new equipment is more efficient and therefore less energy intensive. Ultimately you need to check your equipment’s current demand by adding up all the individual items and apply some correction for diversity (see MBE: AC power) . You can expect to require 1 x 20A and 1 x 32A 3 Phase as a minimum, some of the older system need  an additional 32A 3 Phase (a good reason to update your pre-2000 era hardware). The cost of this of course depends on whether you have 3 Phase available in your facility or whether your supplier will need to install a new feed. Then you will need to consider back up. Uninterruptable power supplies are certainly mandatory for ion pumps and aluminium cells, and are recommended for Ga cells. The best solution is to get a 32A 3 Phase UPS system (£2.5k) backed up on a 18kVA backup generator (£7k). Hence the UPS only needs to hold the system until the backup generator kicks in. Again the solution is system specific and depends on how prone you are to power cuts and how important it is for you to protect your system. The cheaper and perfectly viable solution is to buy a few desktop UPS systems to backup the key items and simply allow the other parts of the system to lose power (£1k). Adequately protecting your system will save you £10ks per year in maintenance costs and cell repairs. 

 

Recommended LN2 system cost: ~£50k (depending on configuration).

After the electricity, the most critical system to optimise is the LN2 cooling and N2 generation. The alternatives for the former are discussed in MBE: LN2 systems, but here the optimal LN2 system is proposed. Firstly anyone working in the LN2 service and supply industry (VBSDeMaCo or PECO for example) will tell you your LN2 system must be as low pressure as possible to prevent flash loss, preferably around 1 bar. On the other hand your N2 gas system needs to be at perhaps 10 bar. Hence you need to fully uncouple these systems at the start. Fully uncoupling the systems will save you £50k a year instantly. The easiest way to uncouple the systems is to extract your N2 gas from a cylinder regulated down to 10 bar on the output valve and split and regulated down to 4 bar and 1 bar for pneumatics and process, respectively. Should you desire uninterrupted N2 or you have a facility that consumes a large amount of gas (with MOCVD on site for example) you can simply have two LN2 tanks: one at 1 bar for LN2 and one at 10-12 bar for N2 creation. The most important point is that N2 creation from LN2 is not free. It is an expensive luxury and a poorly designed system can cost you £500 a day.

Firstly consider your LN2 usage. Ideally you will have a demand (i.e. your MBE systems) located somewhere reasonably close to your supply (i.e. your tank). This will keep down the piping costs. The piping is typically vacuum insulated, bayonet coupled line at perhaps £250 per metre. After piping comes distribution. The most efficient means of distributing LN2 is with a phase separator (£16k) and special drop lines (£5k per (sub-) system). The best way to calculate the LN2 demand of your system is to fill a phase separator of known volume, turn off the supply to the phase separator and see how long it takes to empty. Most systems demand 100 – 250 L of LN2 per day depending on their size. All in all the LN2 outlay may be ~£100k, however the costs of a well designed system are typically recuperated with the first 3 years of operation. 

 

Recommended gas system cost: £3 – 5k (depending on configuration)

Next consider your N2/gas gas usage (see MBE: N2/Gas system). It is split between pneumatic (4 – 5 bar) and process (1 – 1.5bar). The demand varies by system. The pneumatics may drive the gate valves solenoids, shutters and possibly the LN2 flow valve. All of these systems can be motorised, and in such systems there is no pneumatic demand. If there is a requirement for pneumatics, you may consider running the solenoids from compressed, dry air rather than pure N2. A simple compressor and oil filter is all that is required, and aside for the extra noise pollution, this is an elegant solution. Furthermore, the pneumatic and process gas systems are now also uncoupled. In contrast the process system needs to be ultra pure N2, which is used to vent both the system for maintenance and vent the FEL to facilitate sample loading, it is also employed to regenerate cryo pumps, regulate LN2 dewar pressure and purge water cooling systems.  

 

 Recommended water system cost: £3k

Which brings us conveniently to the water system itself; and again demand varies from system to system. It is typically to enhance the stability of your cell temperature with a water cooling loop, and you may also cool the base flange and outgas stage. An overall system flow of 5-10L per minute is typical and hence a small, standalone water chiller is sufficient (£3k). A mix of water and glycol (or indeed simple automotive antifreeze) will keep the fluid clean and provide a more efficient cooling medium. After experiencing the damage wrought by a water chiller failure and the ensuing localised boiling of water in an Al cell’s cooling shroud it is recommended to back up the chiller on a UPS. As a secondary measure a N2 purge system can be created as described in MBE: water cooling system.

 

Recommended software system cost: £10k

It is a pleasantly growing practise to integrate more sub-system into the control software. Honestly MBE is falling far behind the times with regard to the ubiquitous “automation” permeating modern life in the 21st century. At the recent EURO MBE 2013 conference we joked about an MBE app, but as the laughter faded a few of us began thinking: “hey, why not?” A fully manual system is a valuable learning tool. My first system was an Omicron MBE-STM and was perfectly serviceable, BUT (and it is a big BUT) after you have mastered your trade, become an “expert” grower and want to engage in serious research, the sample transfer is not the most complicated puzzle you want to wrestle with. My ideal R&D MBE system is discussed in MBE: Dream Machine, and certainly the industry needs to move in that direction to satisfy the requirements of a new (younger) generation of growers. In the mean time we content ourselves with a conveniently interconnected MBE system that partially regulates and protects itself. Here we encounter the powerful flexibility a system like EPIMAX presents. Bakeout, water cooing, LN2 cooing, FEL vent/pump down, cell temperature control, sample recipes, vacuum pressure control (gauges and pumps) are all monitored, controllable and even user configurable.  

 

Recommended PID/PSU system cost: £30k

Consider the software control of the cells. PID loops are employed to regulate the cell temperature and a digital interface executes recipes by opening and closing shutters. These two tasks are the bread and butter of MBE system control. Typically the PID loops are rack mountable controllers operated though a serial comms interface, but in the future the entire system will certainly migrate to a virtual “PID loop” under software control; which will save you £500 per cell heater zone. For a long time the cell heater power was thyristor regulated AC, but recently standalone DC power supplies are employed. A range of I-V specifications are available, and prices vary, though £1000 per cell heater zone is typical. Why the migration to DC?  Well certainly the DC systems are slightly bulkier but far simpler to implement and maintain. Also AC systems generate AC fields, and the ensuing varying magnetic fields can interfere with sensitive sub-systems (like RHEED) for example.

 

Recommended bakeout system cost: £3k

Bakeout systems are a recurring topic for discussion in the MBE community. Groups rarely reach a consensus on their bakeout procedure, and a recent conversation with Prof Tom Foxon FRS concluded with his practice of negating the requirement for bakeout all together. The particulars of that mode of operation and bakeout system alternatives are discussed in MBE: Bakeout and MBE D&B: Bakeout controller) The most simple system would include an EPIMAX PVCx to regulate the bakeout temperature and duration (£1k), a 2A-24Vdc power supply (£50), some DIN terminals, wires, 3 phase contactors (£500) and the heater of your choice. I personally prefer fan-and-filament style heaters (£580 each) with the system situated in a steel frame enclosure, and the Omicron system features the lightest panels and finest construction I have seen.

Recommended RHEED system cost: ~£30k

Finally we come to analysis. RHEED is considered by many to be the most powerful analytical tool you possess in MBE. Indeed it is invaluable in many situations, especially during early system calibrations. STAIB are currently responsible for 80% of the market, and for good reason, their RHEED system is simply excellent. However, the recent new developments of Dr. Gassler Electron Devices have constructed a filament-free, fully bakeable RHEED system that may well represent the next generation of RHEED.  RHEED can be used to extract and infer a tremendous amount of information regarding growth rate, surface quality, flux ratio and temperature, however the technique is time consuming and laborious.

Pyrometer system cost: ~£10k

A simple pyrometer operating in the 300 – 800 °C range is a very useful tool for establishing relative temperatures; relative because the absolute temperature depends on the emissivity, which changes with temperature. However, relative temperature is useful, particularly when combined with RHEED. You can identify a good temperature to grow an alloy, like InGaAs, and it may occur at 530°C with an emissivity of 65.0%. Indeed it may actually be anywhere from 510-550°C (absolute) but that does not matter for when you tune the pyrometer to 530°C the next day you will be at exactly the same temperature you were at the day before. In this way you can return to the perfect growth temperature without knowing exactly what the temperature is in absolute terms, you only need to know that your pyrometer set to 65% emissivity will say “530” when you are at the right temperature. The value will vary when you open a cell shutter due to radient heating and reflection of light from the cell’s heater, but you can use a pyrometer to quickly find the starting temperature. This is essential for performing a systematic study. In order to use a pyrometer during growth it is a excellent practice to utilse a heated viewport on the flange (CreaTec do a fantastic heated viewport/window) to prevent the build up of As on the glass. Focusing optics and an optical fibre also allow you to easily “mount” the pyrometer and focus on the substrate.

Recommended growth control system cost: ~£35k

Two powerful alternatives that give automatic real-time temperature AND growth rate control, with the potential for software feedback and recipe control are in situ reflectivity (view link) and emissivity corrected pyrometry (ECP). An example product is the Laytec EpiTT. Do you need in situ reflectivity and ECP? That is a question only you can answer? What are you growing? If you are growing QCLs or DBRs then either one technique or the other is absolutely mandatory. If you are growing simple test structures, performing fundamental research or engaged in creating bulk thin film layers at a rate of one sample per day then perhaps you are happy to calibrate everything manually and these systems are expensive toys that fall into the “nice to have” but “not strictly necessary” category. However first consider what you can do with an in situ reflectivity system… Basic in situ reflectivity can enable you to automatically calibrate (at growth time):

  1. growth temperature
  2. growth rate
  3. film thickness
  4. morphology (roughness)

So when we combine automatic sample transfer and batch processing with in situ refelctivity we begin to realise an MBE dream machine: A system that can growth 24/7 and can calibrate itself so the sample you grow is always exactly the one you wanted, first time, every time. I think that is worth it. What do you think?

 Total initial outlay: ~£150k