New MBE system install: Day 5

Faebian Bastiman

The fifth day began by connecting the hoses from the cryo pump to the single phase 2.1kW He compressor. Each hose was attached at the cryo pump end first and then at the compressor to avoid unintended He loss. The compressor’s He pressure was raised from the initial 50 psi to 125 psi with a high pressure regulator and a simple He cylinder. Once filled, the compressor was activated and the system was populated with a suitable pump for the first time. Over the course of the next hour the cryo pump’s internal temperature fell from 293K to <10K. The pressure in the growth chamber fell accordingly from 10-4 Torr to 1.2 µTorr.

After the successful installation of the cryo pump, our attention turned to testing the electrics in the system’s 3 racks. The first unit under test was the 12 port shutter controller. The 3U rack unit comprises a maximum of 12 swappable stepper motor driver cards, and RS 232 module, digital I/O interface and a manually selector button array in the front panel. The 9 stepper motors corresponding to the 8 cells shutter and single main shutter were connected in turn using D-Sub 15 cables. Manual toggling between the closed and open state verified each shutter’s successful operation.

MBE system utilise a combination of analogue, digital and serial communication. Simple RS232 is typically employed in favour of the more complicated RS422 and non-standardised RS485 protocols, though all three can be present. Serial communication is by far the most robust and reliable communication method, however the protocol requires prior knowledge of the “language” or “conversation” that the hardware utilises and acquisition or development of suitable software drivers for interfacing with the control software. Digital and analogue control of hardware is inherently less precise and is prone to errors, but is far simpler to implement. Digital “on/off” or “high/low” or “open/close” is invariably enacted by pulling a pre-defined reference single of either 5V or 24V to ground. Analogue signals are commonly created with a 10 to 14-bit digital to analogue converter (DAC) operating in the 0 – 5V or 0 – 10V range. The established rhetoric is “use serial communication where you can, and digital/analogue where you must”.

The Epimax (www.epimax.com) suite is an excellent control software package comprising a comprehensive number of user configurable serial, digital and analogue ports. The primary advantage is the logical, modular implementation that can be adapted to run all MBE systems.

In the following days, the system installation will involve acquiring and installing the cabling required to operate the Riber 32P via Epimax. With suitable preparation the software system can be installed, configured and be operating within an hour. The proceeding “New MBE system install” articles will focus on the preparatory configuration and continued evolution of the MBE chamber toward completion.

New MBE system install: Day 4

 Faebian Bastiman

The fourth day began by creating a comprehensive list of the parts required to complete the installation. After this crucial task, we removed the CF-200 blanking flange in the lower half of the growth chamber; located adjacent to the titanium sublimation pump (TSP) and ion pump. A manual VAT VACUUM gate valve was fitted on the exposed flange, with the required 45 mm engineering studs hand sawn with a hack saw from 300 mm lengths of SS A2 M8 threaded rod. A CF-200 CTI Cryogenics cyro pump was then installed on the gate valve, using 40 mm SS A2 M8 bolts and double washers to ensure they bolts did not “bottom out” on the gate valve’s tapped holes before a suitable bite was achieved on the copper gasket. The cryo pump alignment was achieved by holding the copper gasket on the gate valve and (i) pinning it with the flat of the cryo pump’s flange and (ii) sliding the cryo pump into place whilst maintaining forward pressure to ensure the gasket would not move.

The Varian 300L scroll pump was once again utilised to pump down the growth chamber. The all metal valves on the exhaust of the cryo pump were sealed and the gate valve opened, such that the scroll pump also pumped the innards of the cryo pump to a mTorr. The Pfeiffer Hi Cube then replaced the scroll and the growth chamber was evacuated to 10-4 Torr in the established manner.

New MBE system install: Day 3

Faebian Bastiman

Today began with a thorough inspection of the sample transfer system. The trolley was loaded into the FEL and transferred to the far end of the PREP. We discovered that the central length of chain was too slack, and the trolley was disassociating from the drive pins upon the chain. Upon inspection of the drive, a grub screw was located on the free-running sprocket mounting (i.e. the one at the opposite end to the handle mechanism). The grub screw was adjusted until proper tension was restored.

Sample transfer on the Riber 32P is performed by a linear trolley drive and two orthogonal transfer arms. The trolley is used to move the sample to one of three a specific location (i) load/unload in the FEL, (ii) outgas stage (iii) growth chamber. The loading/unloading is achieved by physically removing the trolley from the system. The outgas stage and growth chamber are accessed by removing the Mo sample block (aka platen) from the trolley with the transfer arm, moving the trolley out of the transfer arm’s path, and then using the transfer arm to insert the platen into its intended destination.

A total of two transfer arms are required for the operation of this system. They comprise a magnetically coupled rotating platen holder with a bayonet style holding system (similar in operation to bayonet light bulbs), a CF-63 port aligner to “aim” the transfer arm and a CF-63 to CF-100 adapter for coupling the arm onto the Riber 32P’s CF-100 ports. At present the system only has a single CF-63 to 100 adapter, and hence only the outgas stage’s transfer arm was fitted. The fitting of the growth chamber’s arm is delayed pending delivery of the adapter flange.

MBE systems typically utilise grade A2 stainless steel (SS) M8 bolts, nuts and washers. Grade A2 is a highly corrosive resistant steel suitable for all environment save submarine conditions. Smaller flanges utilise M6 and M4 thread of the same material. In order to secure the end flange on the PREP chamber from the previous day 24 x SS A2 M8x70mm bolts were used. The extra length is necessary to penetrate the 20mm thick spacer flange.

With the end flange fitted the vacuum integrity of the entire system could be once again tested. The Varian 300L scroll pump was used to obtain a base pressure in the mTorr range before switching to the Pfeiffer HI Cubes. Despite venting the system with humid air that morning, the FEL and PREP chamber swiftly reached the low uTorr range. The larger growth chamber remained in the low 10‑4Torr range. The poor vacuum is believed to be purely a result of the small pumping capacity of the Hi Cube (77 litre/s). The Hi Cube is an excellent turbo pump but it is not intended for such a large volume system. A typical turbo pump for this system would be 300-500 litre/s. Hence, a higher capacity pump needs installing in order to eliminate the possibility of a leak.

New MBE system install: Day 2

Faebian Bastiman

Sadly after obtaining a vacuum in the low uTorr range over night, the first task of the day was to open the system once more. With no N2 supply, it was necessary to vent with standard atomosphere (humid air) which is highly non-desirable but nevertheless essential at this time.

The first task was to install the load lock (aka fast entry lock (FEL)) through which samples will be later introduced. The Riber 32P uses a trolley and track system, similar to a train and hence careful alignment is necessary to ensure the trolley can move between the FEL and preparation (PREP) chamber tracks. The FEL chamber is designed to stand on a valved ion pump, which is not present on our system. A turbo pump will handle the pumping of the system, though at present the downward facing CF-150  flange is blanked. A simple wood-and-threaded-bar support system was utilised to attach and temporarily support the FEL chamber.

A generic high strength square magnet was used to manipulate the trolley upon the FEL chamber’s track. After successful transfer from the FEL track to PREP chamber’s track and drive chain, the FEL-to-PREP flange was securely tightened.

The far end of the PREP chamber is fitted with a CF-150 gate valve for serially connecting multiple transfer chambers. With no sample entry flange, we decided to move the PREP chamber’s end gate valve to operate as an entry valve on the FEL. The track on the RIber 32P extends around 10mm beyond the end of the outer chamber, and hence in order to blank the chamber end it is necessary to fit a CF-150 spacer flange of sufficient thickness. 24 x SS A2 M8x70mm bolts are required to fasten the blank-and-spacer assembly. As these were unavailable, 4 x 150mm M8 thread bars were used to lightly nip the copper gasket and hold the flange in place overnight. Since this does not provide a sufficient vacuum seal, the central PREP gate valve was closed at this time and the FEL and left half of the PREP were pumped only.

On the growth chamber, effusion cell installation was undertaken. First the 6 Knudsen cells (k-cells) were integrity tested. This comprises testing (i) the resistance of the thermocouples (~2 Ω for C-type and 3 Ω for K-type), (ii) the resistance of the heater filament (~1 – 1.5Ω), (iii) the resistance of the thermocouple to filament (ideally open circuit, minimum 1MΩ), and (iv) the resistance of the thermocouple and filament to ground (again ideally open circuit, minimum 1MΩ). The four cells that passed were installed, the remaining two were marked for future repair.

Similar integrity checks were preformed on the ADDON As cracker. It should be noted that whilst the Riber 32P possess 4 x CF-63 cell flanges, the inner bore of the LN2 cooled base plate is similar to that on the 4 x CF-38 flanges (i.e. 40mm). Thus only narrow diameter CF-63 cells can be installed. The LN2 cooled base plate negates the requirement for individual cell cooling shrouds. Hence the superfluous water cooling jacket was removed from the As cracker, after which the suitably slender cracking zone of the As cracker was installed with a CF-63-63 spacer-adapter utilised to align the orifice closely with port’s shutter blade. The growth chamber was then pumped down for several hours with the 300L scroll pump before switching to the Pfeiffer Hi cube over night.

End of day 2

New MBE system install: Day 1

Faebian Bastiman

After a Christmas time crossing of the North Sea, our new Riber 32P system arrived from Norway. The delivery comprised the growth and preparation chambers on two separate stands, 3 racks, the bake out panels and two boxes of cables and other parts. The courier placed the Riber 32P system in our new lab (see photo below).

Start of day 1

The first task was manoeuvring the growth and preparation chambers into position and physically bolting the chambers together. The growth chamber has a CF-200 port aligner that allows the angle of the flange to be altered. The actual connection was greatly assisted by a pair of wheeled prying bars (see before and after shots below).

before and after

Once the preparation and the growth chamber were connected, the valve between the two was close. First the preparation chamber was pumped down to 10-3 mBar using a scroll pump. After one hour the scroll was then exchanged for a Pfeiffer Hi-Cube and the preparation ion gauge was turned on, the reading was 1 x 10‑4 mBar and dropping.  A good start for system that had been at atmospheric pressure for weeks. The preparation chamber was down to 3.4 x 10-6 mBar the next morning.

The scroll pump was then transferred to the growth chamber, which was pumped to 10‑3 mBar through the night. The growth chamber was still populated by aluminium blanking flanges and rubber gaskets. The quality of the vacuum is therefore much poorer compared to the seal achieved with a stainless steel conflat (CF) flange and a cut copper gasket. The chamber itself is close to 200 litres capacity and so the scroll pump takes much longer to achieve a 10‑3 vacuum.

With the system on site and in one piece, it is time to turn our minds to the parts and services required to create a functioning MBE system. The following days’ posts will document our installation procedure, outlining the steps involved and problems encountered along the way.