Facility Setup: A room with a view

Faebian Bastiman

One of the questions I get asked frequently is: does an MBE system need to be in a cleanroom? My answer is: no, but it needs to be in a clean room. What do I mean? Well the room must be designed in a way that is easy to clean, and must be kept clean. The ideas are summed up very nicely in this blog post from Hutchins and Hutchins.

So what is the difference between a clean room and a cleanroom? What does an ideal MBE lab look like? Well consider the classic laboratory (and I am not talking about Dr Frankenstein’s castle’s highest tower complete with lightning rod) I am talking about the classic white walls, floors and ceilings, plethora of air vents and cold fluorescent light. Ghastly!

The white surfaces and ventilation can be maintained, but one of the first things you should consider is to flood the room with natural light. This of course means windows. Not only is sunlight a free and available mood enhancer, it also increases productivity and assists is healthy sleep patterns. Everyone, even scientist chained to their MBE systems, deserve natural light. Furthermore strong sunlight is an excellent means of detecting airborne or surface dust, and prompting you to do some cleaning.

The next thing to consider is that the dirtiest thing in the room is the human being, so one should limit the room one wishes to keep clean to human exposure. The MBE lab is also generally unpleasantly noisy due to the various pumps maintaining UHV. The best thing to do is therefore to divide the floor space into an office and a lab area. I direct your attention to figure 1 which represents an example MBE layout in a corner building location.

MBE lab layout

Figure 1: An example MBE lab

Note the light blue rectangles are windows and run the length of the upper and right edges. The corridor at the lower edge of the image has a door shown in purple that leads into the yellow office area. The office area is equipped with a desk that houses the MBE system’s control PC and has a further internal window where one can view the MBE system’s racks. The office should perhaps be air-conditioned to a desired temperature (e.g. 22 °C) but need not be filtered or humidity controlled.

The dark green space directly above the office is the dressing area. Here the user removes their external footwear and replaces them with non-shedding, clean footwear. One should also wear a knee length clean room coat (and if one is going to load/unload samples a hair net and face mask). The floor to the dressing room should be coated in sticky mats and there should be a sink for washing hands on entry and exit.

The light green lab is entered primarily through the dressing room, though the lower windowed wall should be removable to enable the MBE system to be installed in the first instance. The lab is where one should focus their intentions to create cleanliness. The room should be air-conditioned (e.g. 22 °C) to counteract the heat load from the racks. The air conditioning could also consist of low level filtering to reduce the typical class 1,000,000 or ISO 9 in a typical office room to 100,000 or ISO 8 to reduce cleaning frequency, but this is not essential for MBE research. Additionally one could consider employing humidity control (down to 40%) to reduce moisture intake into system during sample loading and maintenance. Most importantly the floors, walls and ceiling must be constructed from non-shedding material that is easy to clean with a solvent wipe. This is liable to be the highest expense, especially the ceiling tiles. The MBE system itself should be situated on the centre of the lab with at least a metre free on all sides. Ideally, the rack should be installed next to the system to avoid passing cables between the two. All cables should feed down from the ceiling above the rack and not across the floor, the same goes for the services (process gas, cooling water, cryo pump He, LN2) all should be fed down from the ceiling. The floor around the system can then be easily cleaned and there are no obstacles in the way that prevent dust being drawn into the low level extracting ventilation on the lab’s walls.

The lab requires a laminar flow fume cupboard for exchanging samples or performing maintenance. The remainder of the available wall space can be used for essential and convenient storage of bake out panels, substrates, ultra-pure metals, gaskets, flanges, spare cells, etc. Spare cells could optionally be stored on an outgas rig if there is sufficient space.

The lab should be free from paper, pencils, books, ink etc. All of that should be stored in the office that houses the MBE system’s operator. Ultimately, there are only three reasons to enter the lab space:

  1. Load or unload samples
  2. Clean
  3. Perform system maintenance

If the MBE system has automatic sample transfer and can execute batches of samples (see my Dream Machine blog post), one need only enter the room for a few minutes every other day. In doing so one need only clean the lab once per week. It should be vacuumed with a high efficiency particulate air (HEPA) filtered vacuum cleaner and all surfaces should be wiped down with appropriate solvents. In most cases the system maintenance should be of the scheduled kind and should be followed by the most exhaustive cleaning. Maintaining a slightly higher differential pressure in the lab compared to adjoining rooms will also reduce particle intake on ingress.

Note the LN2 phase separator to provide cryo cooling is not shown in the figure, but is intended to be installed in the ceiling space above the MBE system.

The final room to the left of the figure is the service corridor. This houses any of the services that create particles or dirt. Particularly one may wish to install the air compressor for pneumatic gas, the He compressor for the cryo pump, the water chiller for cell cooling and a pair of N2 cylinders for ultra clean process gas. One can also include an AC power distribution panel and a large roughing pump that augments any turbo pumps on the system. The service corridor need not be located on the same floor as the lab, and could be a full service area that includes the facility’s backup generator, UPS system and end line trap for the roughing pump’s exhaust.

The result is a very clean, easy to maintain lab space where cutting edge opto-electronic and electronic semiconductor research can take place that does not cost the earth.

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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.

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.

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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: LN2 system

Faebian Bastiman

LN2 usage can represent the major expense in LN2 operation. Designing an efficient LN2 cooling system for an MBE operation is more involved than a simple fluid transport system. At one end the system should comprise a large dedicated external storage tank and the other end consists of the cryo panel of the MBE system. There are two main loss mechanisms that must be minimised in order to transport the LN2 from one end of the system to the other.

Firstly there is the compound effect of connection loss (via heat loss) and N2 gas heating. The heat loss results in evaporation of LN2 into N2 within the pipework. The actual LN2 lost at the point of heat loss is fractional compared to the “heating” the “warm” N2 gas causes to the “cold” LN2. This may seem like an odd concept, but the N2 gas is indeed hot compared to the LN2. The poorly insulated connection that resulted in heat loss is localised at a single point on the system, however the N2 gas permeates the entire pipework system and constantly converts LN2 to N2, which in turn exacerbates the problem. In order to minimise this loss mechanism the entire transport system, including valves, pipework and connections, must be vacuum insulated. Moreover, the integrity of the vacuum insulations must be regularly checked and maintained.

The second form of loss is flash loss. Flash loss is a foreign concept to those outside the LN2 community. It is a loss induced by reducing the pressure of the LN2. The change in pressure causes a fraction of the LN2 to “flash off” as N2. The reason for the dedicated tank is now clear: The tank must store the LN2 as close to atmospheric pressure as possible. A small pressure must be maintained in order to ensure the LN2 can reach the destination with the intended flow rate. The lower the LN2 storage pressure, the lower the flash loss.

The presence of N2 and LN2 in the system is termed “two phase flow”. Two phase flow implies the cooling is inefficient since the LN2-cyro panel contact area is reduced in the presence of N2 gas. Hence a “phase separator” is typically employed in order to separate the gas and liquid phases and ensure only LN2 reaches the cryo panel. The phase separator stores the LN2 at atmospheric pressure (the most efficient pressure) and uses gravity to feed LN2 into the cryo panel. It is important to remember that N2 gas will be generated inside the cryo panel. The cryo panel is the only “intended” heat loss present on the system. Additional triaxial pipes ensure that the N2 gas generated in the cryo panel can leave without interrupting the LN2 entering the cryo panel. Thus the cryo panel is optimally cooled and achieves optimum efficiency resulting in an optimal vacuum and (hopefully) optimal samples.

The ideal LN2 system would therefore comprise:

  1. a low pressure LN2 tank
  2. a vacuum insulated tank connection valve
  3. vacuum insulated pipework
  4. a vacuum insulated phase separator connection valve
  5. an atmospheric pressure phase separator
  6. a vacuum insulated triax LN2 feed hose and a vacuum insulated return hose from the phase separator to the cryo panel
  7. the cryo panel
  8. a stainless steel N2 gas exhaust line from the phase separator to the outside world

LN2 system crop2

The volume of the tank (1) will depend on your individual usage (typically 100 – 250L /system/day) and the frequency of your deliveries. Expect to pay £10-20k for the tank. The valves (2 and 4) are generally included. The pipework (3) varies by manufacturer (~£300/m) and typically requires a small, dedicated pump to ensure vacuum integrity. The phase separator (5) is around £16k and the hoses (6) are around £5k. The entire system will therefore cost around £50k. Which may seem like a lot, but to put it in perspective it roughly equates to the amount of money you would waste in the first two years of operation of an inefficient system. All that remains is to choose your manufacturer: VBCVBSDeMaCo or PECO.