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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One thought on “Molecular Beam Epitaxy: Initial Outlay

  1. Pingback: Molecular Beam Epitaxy: Run costs | Dr. Faebian Bastiman

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