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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4 thoughts on “Molecular Beam Epitaxy: N2/Gas system

  1. Pingback: Essential maintenance: System venting | Dr. Faebian Bastiman

  2. Pingback: Molecular Beam Epitaxy: Initial Outlay | Dr. Faebian Bastiman

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

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