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.

Little known MBE facts: High Purity Material

Faebian Bastiman

I was asked recently what purity of material a user should use in their MBE system. Here the grower was referring to the number of “N’s”. The N of the material is a reference to its purity and is actually the number of nines: Material that is 99% pure and 1% impure is termed 2N. 99.5% is termed 2N5, the “2N5” tells you it is at least half way to being 3N. Of course this does not tell you what the impurities are specifically, however you can assume they are made up of all the things you do not want in your III-V thin film layers (Zn, Cu, Fe, Sn, Hg, Ca, Te, etc) and a few III-V elements that you do not need to worry too much about (you are, after all, growing III-Vs anyway).The purity of material will ultimately determine the quality of your thin films, since any impurities in your cell material will likely incorporate as unintentional dopants in your layers.

If you are growing metals (for example MnAs) you are not too worried about unintentional doping and you may use a maximum of 5N5 material (99.9995%). If you are growing electronic or opto-electronic grade semiconductors (for example GaAs) you will want higher purity. Ultimately the highest you can get commercially is 9N (99.9999999%), however I would not recommend you all rush to the shops and buy such expensive material for general research. 9N is exponentially more expensive than 8N, 8N is exponentially more expensive than 7N. Similarly the ultimate background doping you can achieve with 9N is an order of magnitude lower than with 8N, and the same applies when comparing 8N to 7N. Before I suggest the appropriate material quality, let’s do a thought exercise with GaAs…

GaAs has a lattice constant of 0.565338 nm, and therefore an atomic density of 4.42 x 1022 cm-3. When opto-electronics people (specifically detector people) talk about background doping requirements they say that 1015 cm-3 is already good.  This means they would like the unintentional doping level to be 5×107 times lower than the atomic density. In this case 7N5 would be the appropriate choice for you group III and V material. One could argue that you should use 7N5 for all materials and that replacing a specific material with 8N is a waste of money. However this “all or nothing” philosophy is not really justified since unintentional doping is accumulative, and hence replacing your group IIIs with 8N might reduce your background doping from high 1015 to low 1015. Thus 7N5 to 8N is a good choice for opto-electronic research, and 9N is only for exceptional high mobility cases or world record attempts. Moreover, if you are simply experimenting with a new opto-electronic alloy and not so interested in device quality at this stage 6N5 is acceptable. Note that the source material is only one factor in your ultimate achievable material quality, for more information read my Optimum Quality post.

Finally consider the dopant material. When we dope a semiconductor we typically dope in the 1016 to 1019 cm-3 range depending on the application. At 1019 cm-3 the dopant is around 104 times lower in atomic density than the III or V species. This means the doping fluxes are around 104 times lower that the group III and V fluxes and hence any impurities introduced into your system from the dopant source will also be 104 times lower. Add to this that when you dope the alloy you are “trying” to add impurities and you can say that 6N is already a very good grade for dopant material and 5N may even be suitable.

MBE Dreams: Dream Software

Faebian Bastiman

Returning to the theme of “Dream MBE”, I would like to spend a moment discussing molecular beam epitaxy software. Off the shelf MBE software is passable at best. Ok, so it can open and close shutters, perform temperature ramp ramps and execute a sequence of commands in some form of recipe: This is, when all is said and done, the bare minimum. When you spend some time actually considering the features you would like in MBE software, you quickly realise nothing on the market really hits the spot. So… I have spent the past six months writing my own MBE control software: EPIC™.

You cannot create the world’s greatest MBE software in six months, however you can implement an exciting list of features and lay the foundation for fantastic things to follow. Beyond the bare minimum functionality, the features of EPIC™ are:

  1. Comprehensive, optimised software-hardware interface (SHI)
  2. <1 millisecond to process a shutter request
  3. Full device parameter explorer and editor
  4. Ability to monitor PID loops, pyrometers, plasma sources, pressure gauges, pumps, valves, motors, services (LN2, water flow, gas pressures) status and other hardware (e.g. reflectivity data) all from a central, grouped, tabulated status screen
  5. Associate any number of physical devices with an individual control loop
  6. Ramp to temperature set point or output power for PID controllers
  7. Flexible, simple script recipe with full control flow, local and global variables, unrestricted parallel editing and execution including “edit on the fly” and access to any parameter from any device
  8. A colour co-ordinated message box
  9. Wake-up and sleep scheduling
  10. Substrate location and status tracking
  11. Automatic flux scheduling, gathering and Arrhenius/polynomial fitting
  12. Atomic flux, growth rate, doping and composition calculation tools
  13. Material usage tracking and cell fill level estimation
  14. Two data logging modes: SPAM (log at fixed interval) and SMART (log only on change)
  15. Watchdogs warnings and actions
  16. Integrated simulation mode with full hardware emulation

Ok let’s analyse that numbered list. (1) SHI is a fantastic interface with every device possessing its own read and write thread. SHI enables (2) and (3) by its very nature. Not every process requires less than 1 ms shutter response, though since it exists by default I thought I might as well include it in my list. The device explorer is also a nice feature, since the front panel menu on most devices can be a pain.

The status screen (4) is very comprehensive and the layout facilitates ease of reading. A quick glance can reassure you that every part of the MBE system is ok (or equally quickly warn you when it isn’t!). All necessary items are interactive, giving the ability to perform ramps and open/close single/multiple shutters. The ability to associate any number of devices with a control loop (5) is very useful for plasma sources when they may have: a mass flow controller, a RF power controller, a main shutter, several valves and an optical monitor. So too is performing power ramps (6) for certain control loops, like carbon sources.

The recipe script (7) is essentially its own programming language complete with interpreter. Each recipe executes on its own thread and all commands run to 1 ms precision. The control flow allows you to step through each line individually one-by-one or allows the recipe to run freely, you can pause any line, go to any line from any line, skip lines, stop the recipe at any point and edit the recipe on the fly (only when paused for safety) and the recipe checks every line for errors on edit. Local and global variables allow parallel recipes to control complicated processes. Whereas access to any device parameter gives the ability to do almost anything, including in situ reflectivity control (see Dream Machine).

The message box (9) simply keeps you informed with a variety of colour co-ordinated, time stamped status updates including “Recipe X completed”, “Ramp X started”, “Sample X moved to position Y” or the less pleasant but equally useful “device X failed to respond”.

Wake up and sleep is fantastic (9). One can sleep much better knowing the machine will wake up at 06:30, do all the flux checks for the day and wait for you to stroll in at 09:00. Equally, the sleep mode prevents that troublesome “did I ramp down the Gallium before I left?” since it sets the system to a predefined standby state.

When you have more than a handful of substrates and/or more than a single user it is very useful to have a substrate tracking screen (10). Each substrate can be individually tagged with a name, a colour co-ordinated status (new / degassed / grown) and its current location in the system. The substrate tracking paves the way for extended functionality such as automatic run logging, automatic growth sheet creation, automatic sample transfer and batch sample processing (all of which are on my to do list).

Any software that purports to convert effusion cell temperature into growth rate is just plain lying! Beam equivalent pressure (BEP) is typically what you measure and atomic flux in atoms/nm2/s is what you get. Growth rate is simply something you induce depending on the atomic density of your substrate. Software that gathers flux data for you (11) and plots it on a graph is a welcome start. A one click fitting option ranging from simple Arrhenius to 12th order polynomial fitting is sufficient to handle all source and opens the door for much, much more. The first step is enabling conversion from BEP to atomic flux. Once done you can use a growth rate and doping calculator tool that updates to the correct doping density and growth rate for any substrate (12). You can then “ramp to doping density” or “ramp to growth rate” rather than “ramp to set point”. You can estimate the real time composition during growth and calculate the composition ahead of time for a given cell temperature combination. Perhaps my personal favourite is that you can integrate the flux with time every single second and track the cell material usage, display it on a simply bar chart and estimate the fill level of each of your sources to 1%. All of these flux related features are present in the current version.

Of course you want to log all relevant data values (14) and the option to log only on change prevents large data files being created when the system is idle (over a weekend). You also want to monitor the instantaneous data values and be warned when they are outside of an “OK” range. Moreover, for certain values you would like the software to take an action on your behalf to prevent damage (15). Finally you may want to run the software in “off line” mode to “try before you buy” or even to “test a recipe before you run it for real” and in these circumstances the simulation mode is very handy.

Q: What could possibly be missing?

A: Access to the source code.

The reasons for open source MBE software are almost as numerous as the feature list itself, so let’s stick to the two most important ones:

  1. Add device drivers for any hardware
  2. Add/edit/remove content to the user interface

Adding device drivers is somewhat essential. The system hardware changes over time as the system is upgraded or re-commissioned for a new purpose. The current EPIC™ has a generic driver template that can be used to create a new device driver in around 30 minutes. The next step is to make that driver template available to all and to use device driver DLLs. Fairly simple.

The user interface is a very personal thing. The software-hardware interface (SHI) is rather universal once done right, but the user interface is a matter of taste. Script recipes are very powerful but perhaps not for everyone. Tabulated data makes for fast referencing but perhaps diagrams are more your thing. Etc. Etc. Etc. There are three solutions beyond making the software open source:

  1. Create a software development kit (SDK) for EPIC™ to allow everyone to customise it with their own desired features
  2. Allow SHI to be a stand-alone data acquisition tool that can be used either via windows API or via a socket interface to allow the user to create their own graphic user interface or web application
  3. Provide customisation on demand and provide a tailor made solution for each user

Happily EPIC™ will have all three.

One final question: Where can I try the latest EPIC™ beta demo version?

Here.

Little known MBE facts: RHEED oscillations (3)

Faebian Bastiman

I was recently reading a nanowire publication and I was reminded of another means of calibrating the group V flux that I used in my III-Sb days. This is the preferred method for III-Sb epitaxy, however  it is also applicable to general III-V growth. In the following I will use GaAs as an example.

The first step is to establish your Ga growth rate using the method in my RHEED oscillations (1) post. Then convert this into atoms/nm2/s using the method in my flux and growth rate post. Finally, you can then calculate the atomic flux of the Ga cell versus temperature using the method in my Arrhenius plot post.

Then set the As cracker to a value that you wish to determine in atomic flux, for example set it to 25 % open. Set the Ga cell temperature to give you a flux of 0.069 atoms/nm2/s [i.e. 0.1 ML/s growth rate on GaAs(100)]. Open the Ga shutter and record the RHEED oscillations in the usual manner. As long as the As flux is larger than the Ga flux you should obtain a growth rate of 0.1 ML/s ± an error of up to 5% depending on how well the RHEED intensity oscillated. Note the error gets larger the fewer oscillations you obtain. Hopefully you can get at least 10 oscillations.

Next double the Ga cell’s flux to obtain a growth rate to 0.2 ML/s and (importantly) leave the As flux set to the original value. You will need to leave the Ga cell to settle for 10 minutes after changing its temperature.  As long as the As flux is larger than the Ga flux you should be able to obtain a growth rate of 0.2 ML/s ± 5%. The magnitude of the As flux compared to the Ga flux is key to this method. Keep increasing your Ga flux until (eventually) the RHEED oscillations no longer yield the growth rate determined by the Ga cell. When this happens the growth rate is no longer dictated by the Ga flux, it is dictated by the As flux. You can check this by increasing the As flux and repeating the measurement that gave the lower than expected growth rate.

If you plot out all your data points you should obtain a graph like the one shown in Figure 1. You can see that starting at small Ga flux, the growth rate initially increases linearly until eventually it becomes As poor (Ga rich) and the growth rate is limited by the As flux. The growth rate you obtain under these As poor (Ga  rich) conditions indicates the As growth rate in ML/s. 0.5 ML/s in this example. You can then convert the As growth rate into an As flux using my flux and growth rate post once more.

Rheed 3 fig

How to grow your first sample: As capping

Faebian Bastiman

Having a metallic As source offers us the ability to create a very useful protective epilayer: an As cap. Once you have grown a layer you may want to remove the sample from your system and for example transfer it into a neighbouring system or send it to a collaborator for analysis. As soon as you remove the sample from vacuum the surface will of course oxidise. Oxidation cannot be avoided, however if you deposit an As cap of sufficient thickness before you remove the sample only the As will oxidise and your III-V epilayer surface will remain perfectly intact. Once the sample is safely bask inside a vacuum system, one need only heat it to ~300 °C and the As sublimes. A quick look with RHEED will show the original reconstruction is even preserved.

Amazing.

How does one deposit an As cap?

You can deposit an As cap on any surface. In this example I will discuss depositing an As cap on GaAs(100). At the end of your epilayer growth you will typically observe an As-rich (2×4) reconstruction. If you want to know what a reconstruction is see my What is a reconstruction? post. Essentially to deposit a cap all you need to do is fully open the As and turn off the sample heating. However there are a few things to consider:

Firstly, As2 tends to create a better cap than As4, so wherever possible you should use As2.

Secondly, the As cap grows very slowing whilst the substrate is above 100°C and it is preferable to cool the substrate down to 0°C in order to rapidly form a cap.

Thirdly, not only As but every other background species will readily condense onto the substrate at 0 °C including the hydrocarbon, water vapour, oxides and other undesirables lurking inside your vacuum.

In order to ensure the highest purity of your surface, it is a good idea to pre-protect the GaAs by creating an even more As-rich reconstruction than the (2×4). The reconstruction of choice is called a c(4×4). The c(4×4) is simply created by holding the substrate at 500 – 540 °C  in a moderate As flux for several minutes.

Test it yourself. Align the RHEED along the [-110] azimuth so you can see the 4x of the (2×4) shown in Figure 1a and then lower your substrate temperature until the 2x pattern of the c(4×4) appear as shown in Figure 1b. You may be asking yourself why a c(4×4) reconstruction has a 2x pattern. Well if you look closely at the reconstruction in Figure 1b you will see it is not simply a 2x, it is a pair of 2xs. The 2x at the top is out of phase with the 2x at the bottom (i.e. the lines on the RHEED screen do not line up). The 2x at the bottom is in fact exactly half way between the 2x at the top.

As cap v2

Rotate the sample to the [110] azimuth. You will see the exact same pair of 2x reconstructions there too. The c(4×4) looks the same along both [110] and [-110] azimuths. What is going on? Well the reconstruction’s unit cell is not aligned to the same directions as the (2×4). It is in fact centred on the original [100] and [010] directions of the (001) surface, which means it is at 45° to the (2×4) reconstruction.

The c(4×4) is composed of 1.75 ML of As on Ga. That  is a full ML of As plus another ¾ of a ML of As on top of that. This is significantly more As than the ¾ of a ML of the (2×4) reconstruction. The As of the upper ¾ ML of the c(4×4) are back bonded onto the As of the lower full ML, meaning that the As on the upper layer “sees” no Ga at all. Which more importantly means that the Ga in the buried layer is protected from contamination. The As dimers of the upper ¾ ML  arrange  themselves 3 abreast in a “brickwork pattern” and it is this arrangement that gives the pair of 2xs on the RHEED screen.

The c(4×4) is therefore perfect to pre-protect the epilayer. Once it has formed, simply ramp the substrate down to 0 °C  (or as low as you can go) and watch the RHEED. To get a more even As cap you can start the substrate rotation again now.

Once the substrate gets down below 100 °C you should find that the 2x is replaced by the 1x pattern shown in Figure 1c. This is because a thin amorphous As layer now exists on top of the c(4×4) and all you can see are the bulk lattice rods. As the amorphous As layer gets thicker, the RHEED beam can no longer penetrate through to the substrate and the RHEED pattern becomes the amorphous haze shown in figure 1d. This is probably somewhere around 5 – 10 nm thick. It may take >30 minutes from the moment the ramp down was started to the time the amorphous pattern is completed, depending on the background sample heating and the rate of cooling you can create inside your particular MBE chamber.

The As growth rate is highly temperature dependent. You want a final thickness of about 15 nm and from experience this takes about an additional 5 minutes after the amorphous RHEED has appeared. In order to get a more accurate estimate of the As cap growth rate, you can first create the c(4×4), then ramp down to 400°C, then turn off the As flux and leave the sample to equilibriate at 0°C  for half an hour, then apply the As flux and monitor the time it takes to create the amorphous pattern, then triple it. If you want to be really accurate with your cap thickness I suggest you perform cross sectional SEM on the sample and extract the cap thickness.

The As cap is not only useful to protect your epilayer ex situ, but can be used to perform two temperature calibrations at ~300 and ~400 °C  (in order to do that see my Making a static reconstruction map post.

How to grow your first sample: (2×4)/(4×2) transition 

Faebian Bastiman

The (2×4)/(4×2) RHEED transition is a very useful flux calibration point, since it tells you when your As and Ga fluxes are equal. I described what a (2×4) and a (4×2) reconstruction are in my What is a reconstruction post. In this post I will explain how to spot the transition with RHEED.

First of all you need to produce a good starting (2×4) surface. In order to do that follow the steps in my Oxide remove post. With the Ga shutter closed and an As overpressure incident on the sample you should see the As-rich (2×4) reconstruction on your RHEED pattern (Figure 1a). How can you be sure this is a (2×4) and not a (4×2) in disguise?

You can be 99% certain that if you are annealing at a temperature around 15 °C below the oxide remove temperature and the As flux that you are supplying is sufficient to grow GaAs that this is a (2×4) reconstruction. You can test very simply by stopping the rotation so you can see the 4x on the [‑110] azimuth and reducing the temperature. Reduce the temperature until a 2x appears. Rotate to 90° to the [110] azimuth. Still 2x?

If yes, then this a c(4×4) and the early reconstruction was a (2×4). Return to the original temperature and retrieve your (2×4). Note if you want to know what a c(4×4) is read my As cap post.

If no, and you are looking at a 4x, then this is now a (2×4). Remain at this temperature.

If no, and you are also not looking at a 4x, then something is very, very wrong with your sample. Are you sure it is GaAs(100)?

Assuming you have found your (2×4) make sure it looks like the one in Figure 1. If not change your temperature by 5 – 10 °C and your As flux by 10-20% and try to get the best (2×4) you can. You will note that in Figure 1a on the [‑110] azimuth the 4x rods are fairly short and bright even if the 2nd order rods are not very clear. The 2x on the [110] azimuth is simply a 2x and cannot give you any quality information at this point.

Try opening the Ga shutter. Does your 4x change to look like Figure 1b? It should. The Ga flux breaks up the static (2×4) and creates some disorder. This disorder results in an elongation of the 4x rods and an overall decrease in the intensity. The 2x on the other hand does not really change. You can consider Figure 1b as the dynamic (2×4), where “dynamic” means “growing”.

RHEED Ga to As

We are about to search for the Ga-rich (4×2) pattern. In your first few attempts do not rotate the sample, but rather keep the RHEED pointing along the [110] azimuth and looking at the 2x pattern. During the search the 2x will change into a 4x on [110] and the 4x will change into a 2x on [-110]. You can see the Ga-rich reconstructions in Figure 1d.

The problem with performing the transition without rotation is that you are only seeing the transition at the point the RHEED beam is hitting with the Ga and As flux pattern present at that point. This As/Ga ratio can be very different from the one created whilst rotating, since when you rotate you create a more even flux across the wafer. On the other hand since both the starting and ending reconstructions have both a 2x and a 4x azimuth it is not so obvious when the transition has happened until you have some experience looking at it.

So with the rotation off, the Ga shutter open and the 2x on the [110] azimuth on the screen close the As shutter (or close your cracker valve if you do not have a shutter).

Am I serious?

Yes.

Close the As shutter for a few seconds and watch the RHEED, it should quite rapidly turn into a 4x like Figure 1d. Once it does open the As shutter again and make sure the 2x returns. When the As flux is greater than the Ga flux, you see the 2x of the (2×4) when the Ga flux is greater than the As flux you see the 4x of the (4×2). In the extreme case we just witnessed the Ga was of course practically infinitely bigger than the As flux.

Ok step 2 is a little less brutal… with the rotation off, the Ga shutter open and the 2x on the [110] azimuth on the screen… half the As flux. Wait 15s. Did the 4x appear? If yes return to the original As flux, if no half the As flux again. What you are doing is performing a binary search. You take the lowest As flux you know gives you a 2x and the highest As flux you know gives you a 4x and you apply the average of the two and see if you get a 2x or a 4x. Always wait the same 15s between flux changes for consistency.  

You may notice on your search that the 2x creates a rather dim 3x rather than the desired 4x (Figure 1c). This is a mixed reconstruction between the (2×4) and (4×2) that many people overlook. It is weak on both [110] and [-110] azimuths and hence probably lacks the long range order of the (2×4) and (4×2) reconstructions. The low intensity makes it difficult to say whether it is a (3×1) or a (3×4) or a (3×6) and indeed the physical surface of the GaAs could be made up of domains of each.

Once you have found the maximum As flux that gives you a 4x on the [110] in 15s, return to the original As flux that gave you the 2x on the [110]. In the third step gradually reduce the As flux from the start value toward the value you found and watch the RHEED pattern evolve. Double check the As flux you believe gives you a 4x on the [110] is reproducible and get familiar with the process.

Finally set the substrate rotation to around 0.2 revolutions per second and perform the third step again. The As flux required to create the 4x on the [110] azimuth may be a little different this time. With the rotation on you will see the 2x on the [-110] azimuth for the first time. Take a closer look at the reconstructions in Figure 1d.

The dynamic 4x you get when you are Ga rich is much sharper than the dynamic 4x you get in Figure 1b. This is because that whilst the Ga breaks up the As-rich (2×4), the Ga only ever improves the Ga-rich (4×2). Hence the dynamic 4x of Figure 1d is similar to the static 4x in Figure 1a. Note too that the 2x in Figure 1d is very different. There exist some extra bright spots at the bottom of the image that are much dimmer in the 2x of Figure 1a and cannot be seen in Figure 1b.

Hopefully you are now familiar with the process and can readily ascertain the maximum As flux required to create a (4×2) reconstruction in 15s. This is called the “1 to 1” point. The point at which the As flux is approximately equal the Ga flux. You could argue that the Ga flux is slightly higher than the As flux, since the surface turns Ga rich; however they are approximately equal. More importantly this transition is very repeatable. You need to make sure you always do the test in the same way. i.e. you must always wait 15s and always do the test at this temperature. Otherwise the flux will vary from check to check. It is worthwhile observing that variation yourself.

Once you have found this point you can calculate your fluxes in the system independent units of atoms/nm2/s (see Flux determination post) and state the atomic fluxes in any journal articles you write.

MBE Dreams: Efficiency of Effusion Cells

Faebian Bastiman

Have you ever wondered what the material delivery efficiency of standard MBE effusion cells actually is? We typically spend on average £2k a year on source materials and £1.5k a year on substrates. We slice our substrates into 11.4 x 11.8 mm2 pieces and get 12 from a 2” wafer. This means our substrate usage efficiency is around 80%. This is the price we pay for cleaving square pieces out of circular substrates. This means we are using £1.2k out of our £1.5k of wafers and losing some £300 a year directly into the recycling bin.

But what about our £2k of cell material?

Well we buy a 240g charge of As every 3 years. This means we load 1.93 x 1024 atoms into our system every 3 years. We grow on average 1 µm a day, on a ~10 x 10 mm2 active area, 280 days a year for 3 years. This is 84 mm3 of GaAs or 1.86 x 1021 As atoms. That means out of 1.93 x 1024 As atoms we put into our system only 1.86 x 1021 actually end where we want them i.e. in our epilayers. That is 0.1%. Ouch, 99.9% material wasted!

Ok, just breathe. We know As is pretty wasteful, it is so gaseous that it goes everywhere. What about our group III’s?

Well we buy a 30g charge of Ga every 6 months. This means we place 2.6 x 1023 Ga atoms in our small 10cc effusion cells every 6 months. We still grow the same 1 µm a day, 87% of which is Ga. We grow this for 140 days in 6 months on our same ~10 x 10 mm2 active area. This is 12 mm3 of GaAs or 2.7 x 1020 Ga atoms. That is also an efficiency of 0.1%.

So out of our £2k a year on material we are only using £2 usefully. The other £1.998k ends up plastered all over the chamber walls, caking shutter blades and gathering in a large pool at the lowest point on the system.

Can we do anything to improve the situation? Sadly for standard effusion cells the answers is: very little.

We could reduce the cell-substrate distance. In the extreme case you could have the sample at the cell orifice, however the temperature rise when opening the shutter would be horrendous and group III cells tend to spit material too. The cell-sample distance is related to the number of sources, and assuming you want 10 cells and a pyrometer pointed towards your sample the current distances are already the lowest possible.

We could reduce the cell’s orifice with a Ta aperture. Effusion cells are designed to create a uniform flux across a certain sample area however they also spray material non-uniformly in almost every direction. In our case the sample area is 4x less than intended, so we could potentially achieve 4x the efficiency (a jaw dropping 0.4%) by reducing the orifice. The minimum aperture is related to the required flux uniformity, and in most cases this is optimized at the time of manufacture too.

We could also lower the deposition rate. There are some fixed time intervals for MBE, like the time the shutters are closed whilst the oxide is being removed and whilst samples are being transferred. When you grow at 0.1 ML/s you are losing 10x less material than when you grow at 1 ML/s in these fixed times. Of course some times lowering the deposition rate does not help. When you are growing a 5 nm / 5 nm GaAs/AlAs superlattice, for example, it does matter what the deposition rate is since (assuming  the growth rates of the two cells are identical) the atomic equivalent of 5nm of Ga will be deposited on the Ga shutter whilst it is closed during the 5nm AlAs layer, and vice versa. The efficiency increase depends on the ratio of growth time to preparation time. If we grow a 100 period 5 nm / 5 nm superlattice at 0.11 ML/s we waste 0.37x less material than if we grow at 0.55 ML/s but the total sample growth is increased by 1.9x. This comes down to throughput, the data in this article is gathered for a deposition rate of 0.55 ML/s, 0.000001 ML/s is very efficient but… 

With the orifice and the 0.11 ML/s growth rate the efficiency of a Ga cell would be 1%. This is certainly better, but still extremely wasteful.  

The final option is to used valved sources. Whilst valved sources are not particularly effective for high vapour pressure elements like As and P, Sb can be delivered with much higher efficiency because it behaves more like an Architypal molecular beam than a gas source. Valved sources are pretty standard for group Vs but what about group IIIs? Well e-Science are currently introducing their Valved Titan effusion cells for Ga and In. A valved source has two distinct advantages over a standard cell. On the one hand the material wastage is significantly reduced, since the cell can be idled hot with the valve closed. On the other hand, varying the valve position can enable instantaneous changes in deposition rate. A fully valve sourced III-V MBE system is certainly amongst my MBE dreams.

How to growth your first sample: What is a reconstruction?

Faebian Bastiman

In this article I will explain what a reconstruction is and how it appears in a RHEED pattern without using terms like “Ewald sphere” and “reciprocal lattice rods”. To answer the question of what a reconstruction is, we must first consider why a surface reconstructs. Let’s use zinc blende GaAs(100) as an example.

The simplest lattice to imagine is a cubic lattice. The unit cell of a lattice is the smallest collection of atoms, that when repeated over and over again, can recreate the entire lattice. For a cubic lattice this is simply a cube with sides of length a and an atom on each corner (vertex). With 8 vertices it is easy to make the mistake that the unit cell comprises 8 atoms. Instead consider that each of those atoms overlaps into 7 neighbouring unit cells within a full lattice. Thus each vertex atom contributes 1/8 of an atom to each unit cell, and therefore each cubic unit cell comprises (on average) 1 atom.

The next two simplest lattices to image are the body centred cubic (bcc) and the face centred cubic (fcc). When comparing a bcc lattice to a cubic lattice, there exists an extra atom right at the centre of the cube. In this case a bcc unit cell comprises 2 atoms. Similarly a fcc unit cell has an extra atom on each face. In this case each of the 6 atoms on the faces are shared between 2 neighbouring unit cells. The fcc unit cell then comprises of 3 + 1 = 4 atoms.

Imagine a blue fcc lattice of As atoms and a green fcc lattice of Ga atoms. Now take the unit cells of each and place them side by side. Pick up the Ga fcc unit cell and push it into the As fcc unit it overlaps by 75% of its longest diagonal. You would have two cubes that would look like Figure 1a. If you remove any of the green atoms that are not inside the As fcc unit cell, and connect any of the nearest neighbouring As and Ga atoms with a red line: you have just created zinc blende GaAs!

 

 interleaved fcc  

 a

 b

 Figure 1: (a) Two interleaved fcc unit cells (b) zinc blende unit cell

 

Why the long winded explanation? Well on the one hand it is useful to know that a zinc blende unit cell can be considered as two interleaved fcc unit cells, since it saves us trying to image zinc blende without a framework. On the other hand, when you consider the {100} (i.e. 100 family of planes) you can simply consider the As (or Ga) fcc face.

N.B. A quick aside on parentheses: (100) is the 100 plane, {100} are family of identical planes e.g. (100), (010), (001). Similarly [100] is the 100 direction, <100> are the family of directions.

So consider the {100} planes, each plane is made up of multiple fcc faces tiled to make a pattern. During MBE growth GaAs is typically As terminated, so we will consider a surface made of As fcc faces repeated over and over. When you look at the {100} face of an fcc unit cell you see a 2D square consisting of 5 atoms (Figure 2a). However you can see that (in 2D) each of the corner atom overlap with the 3 neighbouring squares, and so each corner atoms only contributes ¼ of an atom to the square. When you add 4 x ¼ to the one in the centre of the face this gives 2 atoms per square. Since  the length of the sides of the square is the lattice constant, a, you can say that the {100} plane of a zinc blende semiconductor has 2 atoms per a2 and in doing so can work out the atomic surface density. Similarly you can add up the atoms inside a zinc blende unit cell and say there are 8 x 1/8 + 6 x ½ + 4 = 8 atoms per a3 and work out the atomic volume density.

 Slide1_crop  Slide2_crop

 a

 b

 Figure 2 (a) As atoms of {100} plane aligned to <100> (b) As atoms of {100} plane aligned to <110>

Consider the {100} plane of As atoms. The surface represents an abrupt end to the lattice. All the As atoms are bound to Ga atoms below, and would like to bind to Ga atoms above. However they cannot because there are no Ga atoms: the crystal is terminated at this plane. This means that every other As atom within the lattice is bonded to 4 Ga atoms, except the ones on the exposed surface. Or to put it a different way, all the As atoms in the crystal have 8 electrons in their outer shell except the surface atoms. With nothing else to bind to, the As atoms choose to bind to each other. The surface is the only point in the lattice where an As atom bonds to another As atom. The As atoms are then said to dimerise (i.e. create a dimer) and in so doing the surface is said to have reconstructed.

In fact not only do they always dimerise, they always dimerise in the same direction. They dimerise in the <-110> direction. They dimerise in this direction because their motion is restricted in the <110> direction by the Ga atoms below them and in order to dimerise the As atoms need to move closer to each other. If you take the plane described in Figure 2a and rotate it 45° you can identify a smaller square with 1 atom on each corner and sides with length a/√2 (Figure 2b).  The square’s sides run in the <110> and <-110> directions. The row of blue As atoms at the top of Figure 2b also includes the position of the green Ga atoms in the plane below. Note the Ga atoms appear in the <110> direction. In this way the As atoms motion is restricted in the <110> direction and they can only move in the <-110> direction. Conversely when the surface is Ga terminated, the As atoms lay in the <-110> direction so Ga always dimerises in the <110> direction. Thus the two species create reconstructions that are at right angles to each other.

When the As atoms dimerise they create a feature on the surface that reapeats with half the periodicity (or occupies twice the space) of a single As atom. A reconstruction is therefore similar to a unit cell, in that it is the smallest feature that when repeated can recreate the entire surface. If one could freeze the {100} plane before it reconstructed it would look like Figure 3a. One can then draw a square around the smallest repeating pattern and note that the sides of the square are equal to a/√2. To save us using the term (a/√2 x a/√2) we call this (1 x 1). Once the surface has dimerised it looks like Figure 3b. Now because the As atoms have moved together  the repeating pattern is a rectangle with lengths (2a/√2 x a/√2) or (2 x 1). This is then a (2 x 1) reconstruction.

 

 Slide3_crop 2  Slide4_crop 2

 a

 b

 Figure 3 (a) (1 x 1) reconstruction (b) (2 x 1) reconstruction

 

The fact is GaAs  does not  form a (2 x 1) reconstruction. The force of coulomb repulsion prevents several As dimers aligning side-by-side in the <110> direction. In fact it is energetically favourable for only 2 As dimers to exist side-by-side and the next two are missing. This creates a pattern with a repeat period of (2 x 4). Since the Ga reconstruction appears at right angles to the As reconstruction, the Ga reconstruction is a (4 x 2). These are the main two reconstructions and along with c(4 x 4) they are the most commonly created and most useful reconstructions. They depend on both the As/Ga flux ratio and substrate temperature and can be used to create a static reconstruction map and to define when the As/Ga ratio = 1.

In order to do this you use RHEED. RHEED is a very useful tool to an MBE grower. It allows you to see a reciprocal space representation of the real space surface reconstruction. Though the fact is you do not need to know you are creating a reciprocal space reconstruction or even that RHEED pattern is in reciprocal space. In order to achieve a practical working knowledge of RHEED you simply need to know three things:

  1. the substrate surface acts like a diffraction grating to electrons
  2. when something gets bigger in real space, it gets smaller in reciprocal space
  3. when something is n times as far apart in real space, it is 1/n times as far apart in reciprocal space

The RHEED pattern can then be considered a diffraction pattern. Typically you would align the RHEED beam with the <110> and <-110> directions and you would observe the diffraction pattern created by each. First consider the surface of Figure 3a. Regardless of whether  you view the surface in the <110> or the <-110> direction As atoms are evenly spaced. This would create a diffraction pattern consisting of only the primary (zero order) rods in both directions: a so called 1x (one-by) reconstruction. The RHEED pattern you would observe in each direction is shown diagrammatically in the figure. Since the real space (1 x 1) reconstruction spacing is proportional to a, the reciprocal space RHEED pattern spacing is proportional to 1/a. So when you look at a substrate with a bigger lattice constant, like InAs, the zero order RHEED pattern spacing is smaller.

Now consider Figure 3b. When you align the RHEED beam in the <-110> direction the pattern would be the same 1x, since the As atoms still possess the same spacing. However when you look in the <110> direction the pattern would appear as a 2x pattern. That is doubling the pattern in real space leads to a halving of the spacing between the RHEED pattern streaks in reciprocal space. Another way to think of it is a doubling of the number of streaks in the RHEED pattern. Note that the 2x occurs when you align the RHEED beam in the <110> direction (termed the <110> azimuth) and this indicates the atoms have rearranged (or moved) in the <-110> direction. This is because you are using the sample to create a diffraction grating at right angles to the electron beam.

So for the As rich (2 x 4) surface, the <110> azimuth is 2x, and the <-110> azimuth is 4x. Similarly for the Ga rich (4 x 2) surface, the <110> azimuth is 4x, and the <-110> azimuth is 2x.

Essential maintenance: Leak Detection

Faebian Bastiman

After performing routine maintenance (Essential Maintenance: The Maintenance Cycle) and pumping down your system to the mid-high 10-8 mBar range (Essential Maintenance: Pump down) you are almost ready to bake. Almost!

First of all you will need to fire up your trusty quadrupole mass spectrometer (QMS) or residual gas analyser (RGA) and sample the vacuum. Here I recommend the (Pfeiffer Prisma) though in this post I will use its predecessor the trail blazing Balzers Prisma. Set up your analyser as per the user manual and start a 100 atomic mass unit (amu) scan. You will (hopefully) obtain a scan similar to that shown in Figure 1.

 RGA spectra

Figure 1: Pre-bake out RGA scan of a III-As system

What are all those little peaks telling us? Well they represent the chemical fingerprint of the vacuum. Each element is easily recognisable by its individual mass number; however compounds require some knowledge of the prominent peaks and the relative intensity. To do that you can use the values in Table 1.

QMS/RGA cracking patterns
Name Formula Mass peak 1 peak 2 peak 3 peak 4 rel sens
m/z % m/z % m/z % m/z %
1 acetone C3H6O 58 43 100 58 33 15 20 42 6 3.6
2 Acetylene C2H2 26 26 100 25 20 24 6 13 6 1
3 air 28 100 32 27 14 6 16 3 1
4 ammonia NH3 17 17 100 16 80 15 8 14 2 1.3
5 argon Ar 40 40 100 20 16 36 1 38 1 1.2
6 benzene C6H6 78 78 100 77 19 52 16 51 15 5.9
7 boron trichloride BCl3 117 81 100 83 64.7 35 28.7 1
8 carbon dioxide CO2 44 44 100 16 9 28 8 12 7 1.4
9 carbon monoxide CO 28 28 100 12 5 16 2 14 1 1.05
10 carbon tetrafluoride CF4 88 69 100 50 12 19 7 31 5 1
11 diborane B2H6 27 26 100 27 97.4 24 89.6 1
12 DP oil DC 705 78 100 76 83 39 73 43 58 1
13 DP oil FOMBLIN 69 100 20 28 16 16 31 9 1
14 DP oil PPE 446 446 100 77 79 51 29 39 10 1
15 ethane C2H6 30 28 100 27 33 30 26 26 23 2.6
16 ethanol C2H5OH 46 31 100 45 39 27 24 29 24 3.6
17 ethylene C2H4 28 28 100 27 63 26 61 25 11 3.6
18 Fomblin oil 69 100 20 28 16 16 1
19 Freon 12 CCl2F2 104 85 100 87 32 50 16 2.7
20 Freon 13 CClF3 104 69 100 85 28 35 15 50 15 1
21 helium He 4 4 100 0.14
22 hydrogen H2 2 2 100 1 10 0.44
23 hydrogen chloride HCl 36 36 100 38 32 35 17 37 5 1.6
24 hydrogen sulphide H2S 34 34 100 32 44 33 42 1 5 2.2
25 isopropyl alcohol C3H7OH 60 45 100 43 17 27 16 29 10 1
26 krypton Kr 84 84 100 86 31 82 21 83 21 1.7
27 methane CH4 16 16 100 15 85.8 14 15.6 13 8 1.6
28 methanol CH3OH 32 31 100 32 66.7 29 64.7 28 6 1.8
29 neon Ne 20 20 100 22 9.9 21 0.3 0.23
30 nitrogen N2 28 28 100 14 5 29 1 1
31 oxygen O2 32 32 100 16 11.4  34 0.4 1
32 phosphine PH3 34 34 100 33 33.1 31 32.1 2 1 2.6
33 pump oil 500 57 100 43 73.3 55 72.7 41 33 1
34 silane SiH4 32 30 100 31 78 29 29 28 27 1
35 silicon tetrafluoride SiF4 104 85 100 86 5.2 28 4 33 4 1
36 sulphur dioxide SO2 64 64 100 48 50 32 10 16 5 2.1
37 Trichloroethane C2H3Cl3 132 97 100 99 64 61 58 26 31 1
38 Trichloroethylene C2HCl3 130 95 100 130 89 132 85 60 65 1
39 water H2O 18 18 100 17 21 16 2 1
40 xenon Xe 132 132 100 129 98 131 79 134 39 3

Table 1: QMS/RGA cracking patterns for common MBE vacuum species

Before we go into detail analysing the spectra, the first thing to do is to check the most prominent peak. The peak at 18 is almost an order of magnitude higher than the peaks around 2 and 28 and only the peaks around 16, 17 and 91 come close. From table 1 we can deduce that the family of peaks at 16, 17 and 18 are in fact water. Since we have just opened our system to atmosphere we would naturally expect water to be the most abundant species in the system. What about the other peaks?

Analysing the spectra in figure 1 from left to right, we can first of all see the prominent H and H2 peaks that are caused by the stainless steel (SS) outgassing into the vacuum. The exact height of these hydrogen peaks varies for system to system and largely relate to how the MBE system was heat treated when it came off the production line. Often this involves heating the system to 250°C for 400 hours then 350°C for 120 hours causing the SS to take on a “golden” tinge. After the water peaks discussed above, the next peak is at 28 and corresponds to N2, as is to be expected from the multiple N2 flushes we used during the pump down. Notice that the O2 peak at 32 amu is very low, but together with the peaks at 28 and 14 we can say there is some air in the vacuum chamber. But how much air do we have in the chamber? Do we have a leak? Well let me finish the spectra analysis first and then I will answer those questions… the peaks at 37.5 and 45.5. are probably double ionised As and AsO (thanks to Scott for his comment), and the peaks at 75 and 91 are the single ionised As and AsO (as to be expected on a III-As system). Some of the As has also formed arsine-type compounds AsH and AsH3 with masses of 76 and 78 respectively. The peak at 44 is CO2 which was soluable in the water vapour at atomspheric pressure and is now dissociating. CO2 has a corresponding peak at 12 and adds to the peaks at 16 and 28. The other small peaks at 15, 19, 20, 25, 26 and 30 are various simple hydrocarbons formed from the interaction of the numerous C and H ions. All in all a very clean pre-bake vacuum.

Back to the oxide: Well from table 1 we can see that air has an O2 peak that is 27% of the N2 peak in the “air” spectrum. Remember figure 1 is on a log scale so we would expect the O2 peak to be much higher (~2.4 x 10-10 A) for a N2 peak at 9 x 10-10 A to represent purely air. Clearly the O2 peak is at around 5 x 10-11 A and thus the N2 peak is formed predominantly from pure N2 (and 8% CO2) gas. From the relative peak heights we can estimate there is in fact roughly 5x as much N2 as there is air. The air is actually two orders of magnitude lower than the water vapour, so we can conclude that there is no significant leak, at least no leak above 10-10 mbar.

What if the situation were reversed? If the air peak had been dominant and the O2 peak had been in the mid 10-10 A range, we could only conclude we had a small leak in the 10-9 mbar range. If the background pressure had been in the low 10-7 mbar range and the “air” spectra was an order of magnitude higher, we could only conclude we had a more significant leak in the 10-8 mbar range. What to do?

Well first of all do some simple checks and work through a process of elimination. Check the pump exhaust fittings, particularly the turbo pump to roughing pump connections and the TSP ceramics. Close the various gate valves and try to determine on which chamber the leak is present. Spray some IPA onto any suspicious looking flanges (where the gasket looks unevenly compressed) and look for pressure and current spikes in the ion gauge and RGA spectra. You can find a guide that details the flange nut sequence required for leak free flange in MBE maintenance: how to tighten a CF flange. If none of these work you will need to utilise He leak detection. To do this set the RGA to 4 amu for He detection only, turn off your air conditioning so it does not waft the He around the MBE system and with a narrow aperture pipe and very low flow rate of He systematically spray every area of the machine. Once you have identified a likely origin of the leak wrap a plastic bag around it and test both inside and outside the bag to see if you can “enhance” and “supress” the He signature.

In summary, because the water vapour pressure is higher than the N2, and the N2 is higher than the “air” and because our pressure is in the high 10-8 mBar range we can conclude that this system has no significant leak. After baking to 140°C for 48 hours (plus 8 hour ramp up and ramp down times) we get the RGA spectra shown in Figure 2. Note this was immediately after baking, whilst the system was at around 40°C with no water or LN2 cooling attached. The ion gauge pressure was in the high 10-10 mBar range.

post bake crop

During the baking the water vapour rapidly evaporated and was swept out through the pumps with the other temperature assisted highly mobile species. The result is a spectra that is almost identical to Figure 1, except that each peak is reduced by 1-2 orders of magnitude. So we cannot conclude that  the hydro-carbons and oxygen are “gone”, they are simply beneath the sensitivity of the detector (that is to say they are now in very dilute quantities). What do we have left in the vacuum? Well the most dominant peak is now the H2 (2) closely followed by As (75, 37.5) and water (18, 17, 16) then N2 (28),  CO2 (44, 28, 16, 12) and AsO (91). On the one hand there is an argument for baking the system for longer, since every additional day at high temperature would see more of the contaminants removed from the system.  On the other hand since the rate of removal of contaminants falls exponentially with time the “time-purity trade off” results in a most efficient bake out time between 16 to 72 hours. The system will continue to “clean up” through the first week of operation and the contaminants will experience another 1-2 orders of magnitude reduction once the LN2 cooling is turned on. And so now it is time to perform the many little post-bake tasks that get the system ready to grow.

Essential maintenance: how to tighten a CF flange

Faebian Bastiman

Earlier this week I was asked how to tighten a CF-40 flange. The implied but unvoiced condition being: and ensure that the Cu gasket is evenly clamped thus creating a leak-free seal. I happily replied with the adage from my early MBE days: “do one miss one” in regard to the order to tighten the nuts, however realised as I said it that when taken literally one would only ever tighten 3 of the 6 nuts. The more correct but slightly longer adage would seem to be: “Do one miss one, until you land back where you started…when you do, do not do that one but instead the next one, then do one miss one in the opposite direction and try to ensure even loading by only increasing the tightness by ¼ a turn on each nut” Needless to say this is expressed more simply in a diagram, and so I direct you attention to figure 1. Once you reach nut 6, restart again from nut 1 and continue until the copper of the gasket is just visible between the flanges (~1.5 mm) or for those of you with a torque wrench set it to 20 Nm.

Slide2

Figure 1: CF-40 flange nut tightening sequence

However that made me think: what is the correct sequence to use on other flanges? Well both a CF-16 and CF-40 flanges have 6 nuts so the sequence for both is shown in figure 1. Generalising the sequence you would come up with a do one miss (n/3) -1, where n is the total number of nuts on the flange. Or put a different way, tighten every (n/3)th nut. Great (!) but what about when the number of nuts is not divisible by 3? Like on a CF-63, 100 or 160 flange? Hmm. Well the whole point of the sequence is to evenly bite the gasket. So for any flange the “triangular-reverse-direction” loading would seem logical. Hence for a CF-63 flange you would use the sequence in figure 2 and for a CF-100 the sequence in figure 3. This same selection rules can be applied to CF-160 flanges.

Slide3

Figure 2: CF-63 flange nut tightening sequence

 Slide4

Figure 3: CF-100 flange nut tightening sequence

Once you reach CF-200 you have 24 holes (n/3 = 8) and the sequence is a multi-pass version of the original CF-40 flange shown in figure 4. Here I have made the first half of the sequence green and the second half black so you can see the pattern is still essentially “do one miss one” at the halfway point.

flange tighten order

Figure 4: CF-100 flange nut tightening sequence

Armed with these sequences and a good set of spanners I wish you a leak free maintenance cycle.