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

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Molecular Beam Epitaxy: Run costs

Faebian Bastiman

New and updated figures February 2014

The run costs of an MBE system need not be astronomical, however like any system time and effort must be spent tuning it to efficient, economical operation. The MBE system costs can be separated into set up costs (see Molecular Beam Epitaxy: Initial Outlay) and the run costs discussed here. I will discuss the prices in GBP (£) since my current system is based in the UK. The day-by-day, year-by-year operation requires a number of consumables:

  1. LN2 (or alternative cooling)
  2. Compressed N2 or air
  3. Substrates
  4. Cell materials
  5. Cell crucibles
  6. Copper gaskets
  7. Electricity
  8. Miscellaneous maintenance items

Liquid nitrogen cooling cost: £12.4k per annum

Some form of cryogenic cooling is essential to MBE, though it need not be LN2 specifically. The alternatives are discussed in Molecular Beam Epitaxy: Initial Outlay. Assuming you decided to follow the “spend more now, save more later” philosophy in Molecular Beam Epitaxy: LN2 system and bought an efficient LN2 system from VBS, decoupled your liquid and gas generation sub-systems and that your system is 2” capable MBE system with suitable heat shielding you will have a MBE system LN2 requirement of 200 L/day with an associated “heat loss” of 10%. At £0.11 per litre, LN2 is pretty cheap. Based on this you will use ~£24 a day or £8.8k per annum. However to obtain this low LN2 usage level you need to have a fully vacuum insulated line from the tank to the phase separator and from the phase separator to and from the MBE system. High integrity vacuum insulation requires constant regeneration, so you will need to run a dedicated pump all day every day, which will cost you an extra £1.6k per annum (pumping cost courtesy of Bart Limpens, VBS) . You will also probably be renting your main LN2 tank, so you need to add another £2k per annum. Note a poorly designed LN2 system can cost more than twice as much!  

Gas system: £1.4k per annum

Pressurised gas is necessary in two forms: a 5 bar pneumatic line and a 1.5 bar venting line. Again in Molecular Beam Epitaxy: Initial Outlay the pressurised gas choices were discussed. Assuming you decided to follow the example in Molecular Beam Epitaxy: N2/Gas system and divide your demand into 5 bar and 1.5 bar compressed air (from a compressor) and 1.5 bar N2 (from a convenient and efficient boil off system using a dedicated tank), you will have an electrical run cost of £1k per annum to run the compressor and 10L (£400) of LN2 per day in producing gas, sadly most of which is flash loss. The actual gas has a volume nearly x100 that of the liquid. So the actual gas used is very small compared to the cost of generating that gas. This is a strong argument for NOT using boil off. Of course boil off is very convenient, but a convenience that will cost you £1.4k per annum.

Substrates: £7k per annum

Substrates can be very cheap or very expensive depending on what you are growing. Si substrates are notoriously cheap, and whilst the industry is phasing out 2” lightly doped Si substrates, you can still buy them for around £8 each. 2” GaAs substrates are around £50 each. 2” InP or InAs substrates are decidedly more expensive: £125 each. I will use 2” GaAs from now on by way of example. Handily you can do two things to save some money. Firstly, III-V substrates can be repolished (35reclaim) for around £10 – £20 each depending on the material. My experience with these substrates is that they are equal in quality to “new” epiready ones. Secondly, you can ¼ your substrates to effectively get 4 growths out of a single substrate at the cost of reduced sample area. Unfortunately you cannot do both, i.e. you cannot repolish a quarter. You can however repolish a repolished wafer several times. The choice, in the end, is whether you want to grow on full wafers or not. But bear in mind that a ¼ of a 3” wafer is about the same area as a whole 2” wafer at a fraction of the price. Of course the annual cost depends on how many samples you grow a day. Two per day is typical for a 280 day working year, or £7 per annum assuming you decided to grow on ¼ of 2″ substrates.

Cell material: £2k per annum

Cell material is not actually as expensive as you may think; check out the latest prices SciTech Solutions has to offer. You will want 7N material (that is 99.99999% pure) as standard. Though in many research applications 6N5 (99.99995) is perfectly suitable. The table below lists some experimentally gathered usage versus cost values. All materials are 7N aside from the Al which is 6N5. 7N Al is about x10 the cost.

Usage Actual material
Material £/g g/micron £/micron microns grams cost (£)
Al (6N5) 3 0.33 1.0000 60 20 60
Ga 2.2 0.17 0.3667 6000 1000 2200
In 1 0.17 0.1667 6000 1000 1000
As 2.29 0.33 0.7639 7200 2400 5500
Bi 1.7 0.33 0.5667 3 1 1.7
Si 10 0.00 0.0007 14400 1 10
Be 3500 0.00 0.2431 14400 1 3500

To help you interpret the table: The far right column (“cost (£)”) is the cost for the amount of material listed in the adjacent “Grams” column. Note: minimum order quantities apply. The “microns” column is an estimate of the number of microns grown with that number of grams. The left-hand columns are calculations based on the values in the right-hand columns. Note that Be is very expensive but also a gram will last 20+ years. Si is cheap, since it is essentially smashed up substrates. As is a rather expensive one of cost for a 2.4 kg charge, but that single charge should last 5 ‑ 15 years depending on individual usage. This means for an initial outlay of £20k you can grow for around 10 years, or £2k per annum.

Crucibles: £3k per annum

The cell crucibles are actually only £600 each from SciTech Solutions. Handily most of them are also reusable. Once they have been soaked in AQUA REGIA for a few days to remove any material, they can be rinsed in DI water, then IPA, blown dry with N2 and baked to 100°C in a little desktop oven. Al and Bi can very easily shatter a crucible so you are better off using a crucible liner and employing “good filling policy” (see Molecular Beam Epitaxy: Crucible Cracking). All in all you may spend up to £3k per annum on crucibles/liners per year, and that is if you are very unfortunate.

Gaskets:  £250 per annum

Next we have gaskets and again SciTech Solutions have the lowest prices around. UHV systems use conflat (CF) flanges that comprise hard stainless steel (SS) blades that bite into a soft copper gasket. The gaskets are then effectively consumed once used and would need to be melted down and reforged. A standard system service involves replenishing depleted cells, cleaning/replacing shutter blades, cleaning the manipulator of excess As, and perhaps replacing an ion gauge or TSP filament. Every component requires a replacement gasket and it is very easy to get through a few kgs of copper per annum. This is around £50 of copper. However the actual gaskets cost around £250 per annum. It is good practice to recycle your copper to recuperate costs.

Electricity: £13k per annum

Electricity is something you may or may not have to pay yourself, depending on how your research is funded. The bottom line is someone, somewhere will have to pay. An MBE system typically runs off a 64A, 3 phase AC supply. You can find some more detailed calculations in Molecular Beam Epitaxy: AC power but mostly it runs at about ~50% capacity during operation, at 25% idling over-night, perhaps 70% when baking but never at 100%. Applying some calculation based on general 280 day usage your electric bill for an MBE machine is therefore: £13k per annum. Note the essential parts of the system should be backed up on a suitable uninterruptable power supply and generator team (£10k), this may seem an expensive initial outlay but I can guarantee it will save you a cell per power failure. Which means it will pay for itself after the first power cut and hence significantly reduce your “unexpected maintenance” bill (see below)

General rainy day fund: £10k per annum

Finally you have general maintenance. Sadly UHV metals are rather rare and very hard (which makes them difficult and expensive to machine). The typical metals are stainless steel in grades A2 (304) or A4 (316), molybdenum, tantalum, niobium, some aluminium (as long as it is kept away from anything approaching 660°C!), copper (which is slowly eaten by arsenic), silver and (possibly) gold. There are various parts than can fail. Commonly Ga or In can run into places where they are not wanted and cause short circuits to electrical feedthroughs or high friction to rotary feedthroughs. In addition anything electrical: cells, manipulator, ion gauge, TSP, pumps etc requires annual servicing and minor replacement of parts. It is good to keep around £10k per annum simply for unexpected repairs. Of course the cost of repairs can be reduced if you are confident and able to do it yourself.

Total: £50k

The small system I have described here would cost £50k per annum in run costs for 2 samples a day or £90 per sample (however this obviously does not include salaries). Of course if you grow 4 samples a day, your run cost goes up to £60k per annum, but your sample cost drops to £54 per sample. This probably represents the lowest annual run cost for a productive MBE system. Remember that in order to reduce your run costs you will need to buy more infrastructure during the initial outlay. It is always beneficial to do so, since the outlay is typically recuperated within 1 – 2 years.

Molecular beam epitaxy: Systems and sub-systems

Faebian Bastiman

In essence an MBE system comprises:

  1. A vacuum system
    1. Pumps
    2. Valves
    3. Gauges
    4. Mass spectroscopy
    5. LN2
    6. Pneumatics (5 bar N2)
    7. Venting (1.5 bar N2)
  2. A deposition system
    1. A substrate heating and manipulation system
    2. Cells
    3. Shutters
    4. Temperature monitors
    5. Power supplies
    6. Water cooling
  3. A control, monitoring and/or interlock system
  4. A RHEED system
  5. A bakeout system

The vacuum system is largely set at the time of manufacture and post-production customisation is only necessary during upgrading beyond the original specification. The control system is a matter of personal preference, and the author’s endorsement of the Epimax suite is borne from a desire to possess a system that can be implemented, expanded and adapted at will. Staib produce a very reliable and robust RHEED system and kSpace have developed a powerful data acquisition suite. The bakeout system also comes down to personal preference. Bakeout jackets, tents or panels (boxes) with an array of heaters are available. Jackets are highly desirable, as they can be employed with minimal decommissioning of the system, but they are notably expensive or nonexistent. Tents and panels utilising “filament-and-fan” style heaters are the standard means of baking, and aside from the lengthy pre-bake preparation are perfectly adequate.

The deposition system (and its integration into the control system) is by far the most complicated system. Like all systems, it is necessary to break it down into its sub-systems in order to understand the function and interaction of the separate elements. The substrate heater and cells are essential identical. They comprise a heater filament (Ta, W, SiC or C-track) and a thermocouple (typically C or K type). They can be powered with either AC (using a thyristor) or DC (using a suitable power supply) and are maintained stable to within <0.1°C by a proportional, intergral and derivative (PID) loop usually resident in the temperature controller. The P, I and D values need to be carefully tune in order to achieve stability and suitable dynamic response, luckily all modern controllers have suitable automatic tuning algorithms. Water cooling is used to protect and equilibrate the delicate innards of the cell/heater. The cooling can be either integral or auxiliary to the cell. A mixture of water and glycol similar to that utilised in a car’s coolant system is preferred. The shutters too are either integral to the cell or separate and hence integral to the MBE system. They are typically actuated by a pneumatic or stepper motor driven mechanism. For automatic growth recipe execution it is of course necessary to manipulate the shutter state through the computer resident control software.

The actual sample manipulation, from the epiready supplier boxes to the growth chamber’s heater stage, is an area of serious neglect in the MBE world. Manipulating a sample from the start to the end of the journey is often the most complicated task involved in MBE operation, particularly on older system. A number of mechanisms are available, and until recently only Vacuum Generators (VG) had engineered a user-friendly system. The VG system employed on V90 and larger systems utilises optical sensors to identify when the sample is in a valid transfer state. The sample transfer can be conducted manually using switches, automatically from computer designated site to site or even as part of a 6 sample batch without any operator interaction. The system was literally decades ahead of its time, though in many ways also long overdue. More recently the “cluster” tool has been engineered in favour of the “linear” MBE system. The cluster tool comprises a central distribution chamber with a computer controlled arm for sample transfer. Six to eight discrete chambers can be installed on the cluster chamber’s side flanges with the possibility for a FEL chamber, outgas stage, park/storage chamber and 3 independent growth/deposition chambers. The cluster tool employs either optical sensor or “intelligent” stepper motor control for position determination. The bells and whistles of the cluster tool, however, come with a hefty price tag.  In terms of economics and pure practicality the VG V90 is easily the best research/semi-production MBE reactor ever manufactured. Though, unfortunately, since the dissolution of Vacuum Generators the V90 systems are now discontinued and, in the author’s opinion, we are stuck with either very expensive or inferior alternatives.