Mostly Broken Equipment

Hello again everyone! I am delighted to be back with my first post since 2015.

Molecular Beam Epitaxy (MBE) has inherited the pseudonym Mostly Broken Equipment and in this post I intend to explore why.

On the one hand, yes, this makes sense since an MBE reactor is an incredibly complex assembly of discrete devices designed to achieve a coherent whole. I can still remember that feeling of quiet awe and sinking dread when I was first introduced to the shiny beast upon which hinged my hopes of gaining a PhD. Such a mass of tangled wires and hoses, blinking LEDs, and weird noises from pumps I had never before seen.

On the other hand, no, it doesn’t make sense. This incredibly complex system is incredibly expensive and almost indestructible, to the point that even the 25+ year old second hand systems can belt out world class samples. Right, right, I hear you say, I heard the story of an indestructible hammer: The owner replaced the handle 3 times, and the head only once. Regular maintenance and repairs are the topic of an older post, the focus of this post is how to keep the system running “almost every day” during its 25+ year life time.

When I visit an MBE lab, I often get the basic tour showing off the fancy ISO 3 cleanroom and the recirculating LN2 system. I grudgingly don my cleanroom (space suit) and continue onto the lab floor noting the fume cupboards and the ranks of blinking hardware racks. On the surface everything looks great. In those racks, there are usually 10+ PID controllers and their corresponding DC power supplies, 2-3 ion gauge controllers, a medley of pump controllers, a shutter controller and maybe a RHEED gun power supply.

“Are the racks backed up?” I hear myself ask.

“Oh, yes, we have an uninterruptable power supply running in the basement,” comes the proud answer.

I nod, and smile, “Does the chiller run on the same UPS?” I ask.

Hesitation follows. Sometimes even wringing of hands. “Well…erm…”

My smile vanishes, but I offer a chance to save face: “Well at least the water flow meter can protect the cells?”

My proud host starts silently mouthing the words “flow meter” over and over with a perplexed look on his face.

Here starts the problem. Here is a nice shiny (sometimes new) MBE reactor costing upwards of €0.5M but there are some gaping holes in the interlocks and watchdogs. There is often quite literally a gaping hole in the rack where the PLC controller should be. A PLC controller constantly monitors a set of inputs (like water flow rate, LN2 temperature, vacuum level, etc) and then makes decisions based on a custom program to control certain outputs (like ramping cells to standby, opening the gate valve to the turbo pump, etc). The PLC acts as a nexus and watchdog protecting the system on a second by second basis.

When I describe this functionality many people glance over at their PhD student as if to say “my PLC controller is sitting over there”. Well sorry no. Not good enough. Even a washing machine prevents you opening the door when the drum is filled with water.

In fact, whether you physically have a PLC controller or not, your implicitly have one. You might even say a PLC system represents your values toward your MBE system. So the in absence of a physical PLC controller your implicit PLC controller control flow looks like this:

IF “the water chiller fails” DO NOTHING

IF “the thermocouple breaks” IGNORE IT

IF “the vacuum integrity is lost” CARRY ON REGARDLESS

With these implicit values, is it any wonder the MBE reactor is Mostly Broken Equipment? That is simply karmic action. You may say you do not believe in karma. Well, karma believes in you, and believe me, it can be a bitch.

If you are reading this and staring at your own gaping whole in your rack whilst simultaneous suppressing your feelings of smouldering rage because the system is “down again” today, I urge you to fill that space with a PLC controller.

Thank you for reading. See you again soon. Oh and…

if you want to buy a PLC controller designed and built to protect MBE systems please contract us as Scitech Solution Ltd, we are in the processes of building one.