Facility Setup: A room with a view

Faebian Bastiman

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

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

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

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

MBE lab layout

Figure 1: An example MBE lab

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

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

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

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

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

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

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

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

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

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

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MBE Design and Build: Bake out controller

Faebian Bastiman

The bake out runs infrequently, but has a huge power demand when operating. In MBE: AC Power, we discussed the power requirements for an MBE system. I suggested a 32A 3PNE line was needed to operate the system, whereas a dedicated 20A 3PNE line was suggested for the bake out. This article provides a design of the control system for the bake out utilising a dedicated 20A 3PNE line.

The bake out can be performed in two general ways:

  1. A box or tent with a combination of 2.5kW fan heaters and 1kW ceramic heaters
  2. With heat wraps or a heater jacket

The principle of the bake out is fairly simple: heat the system to the target temperature for X hours, then cool down. The heat and cool ramp rates should not exceed 1°C /min in order to avoid thermal stress. Furthermore the heat ramp should suspend if the pressure in the chamber being heated exceeds 5.0 x 10-6 mBar. The control hardware of choice for managing the temperature and pressure requirements of a bake out is the Epimax PVCx ion gauge controller. The PVCx is a multi-purpose tool: primarily it functions as an ion gauge controller, it has units of mBar, Pascal, Torr and Amps thus it further functions as a picoammeter for beam flux measurements, it can operate a pirani, be utilised as an 8 channel normally open/normally closed (NO/NC) relay/trip switch and control the bake out. All these controls and trip conditions can be configurable via either the PVCx menu or via serial comms.

The basic components of the bake out control system are shown in Figure 1. The MBE chamber (depicted in blue) has an ion gauge monitoring the internal pressure and a k-type thermocouple monitoring the external temperature. The entire chamber is surrounded by an insulating SS box or fibreglass tent. Two heaters are mounted inside the box/tent: Firstly a 2.5kW fan heater (BesTec) and secondly a 1KW ceramic heater (VG Scienta). The actual number of heaters will depend on the size of your system. The AC heater power is simply controlled via NO contactors rated at 32A with a 24Vdc coil (RS). The PVCx then actuates the contactors with its relays by modulating the external 24Vdc supply.

Slide1

The fan contactor is on whenever the bakeout is above ambient temperature (i.e. constantly during bakeout) whereas the heating contactors make and break several times a minute to regulate the temperature. The fans can therefore simply be controlled by an AC switch and manually turned on before and off after the bake (Figure 2).

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Alternatively the bakeout can be performed with heat wraps (VG Scienta) or custom made jackets (sadly currenty no know supplier).  The jacket essentially comprises a heater element attached to a fibreglass fabric covering an inner fibregalss insulating layer (essentially loft insulation) that is custom shaped to fit the contours of the system. The bake out is therefore more efficient since the heat is injected view conduction into the chamber’s metal body and the is more readily retained. The jacket can be attached more swiftly than the wraps which need to be laboriously assembled each bake out, or left as a permanent feature for convenience at the cost of aestheticism. It is a good idea to use some insulation, even if is just Al foil to cover the heat wraps to retain the heat. In this alternative bake out the PVCx simply makes and breaks the contactor to control the power to the heat wraps/jacket rather than the heaters (Figure 3).

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The actual heater configuration, power demand and wiring will depend on your system configuration. Figure 4 shows my wiring diagram for a custom built bakeout heater for a Riber 32P system. The system comprises a dedicated 20A 3PNE line, a rack mounted miniature circuit breaker (MCB) and fuse panel, a rack mounted 5 x contactor array, a DIN rail mounted distribution enclosure (FIBOX) mounted on the Riber 32P frame, 2 x 2.5kW heaters, 4 x 1kW heaters and 4 x 1kW heat wraps. The bake out controls the temperature and pressure of each chamber: MBE, preparation (PREP) and the fast entry lock (FEL) with 3 PVCx controllers. The rack mounted switching panel utilises 5 x 3-way switches to enable the system to be set into manual/automatic/off. Note: for clarity I did not include the EARTH connections (green), but any electrician reading this can rest assured all heaters, fans, racks and the MBE system itself were suitably earthed.

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