In essence an MBE system comprises:
- A vacuum system
- Mass spectroscopy
- Pneumatics (5 bar N2)
- Venting (1.5 bar N2)
- A deposition system
- A substrate heating and manipulation system
- Temperature monitors
- Power supplies
- Water cooling
- A control, monitoring and/or interlock system
- A RHEED system
- A bakeout system
The vacuum system is largely set at the time of manufacture and post-production customisation is only necessary during upgrading beyond the original specification. The control system is a matter of personal preference, and the author’s endorsement of the Epimax suite is borne from a desire to possess a system that can be implemented, expanded and adapted at will. Staib produce a very reliable and robust RHEED system and kSpace have developed a powerful data acquisition suite. The bakeout system also comes down to personal preference. Bakeout jackets, tents or panels (boxes) with an array of heaters are available. Jackets are highly desirable, as they can be employed with minimal decommissioning of the system, but they are notably expensive or nonexistent. Tents and panels utilising “filament-and-fan” style heaters are the standard means of baking, and aside from the lengthy pre-bake preparation are perfectly adequate.
The deposition system (and its integration into the control system) is by far the most complicated system. Like all systems, it is necessary to break it down into its sub-systems in order to understand the function and interaction of the separate elements. The substrate heater and cells are essential identical. They comprise a heater filament (Ta, W, SiC or C-track) and a thermocouple (typically C or K type). They can be powered with either AC (using a thyristor) or DC (using a suitable power supply) and are maintained stable to within <0.1°C by a proportional, intergral and derivative (PID) loop usually resident in the temperature controller. The P, I and D values need to be carefully tune in order to achieve stability and suitable dynamic response, luckily all modern controllers have suitable automatic tuning algorithms. Water cooling is used to protect and equilibrate the delicate innards of the cell/heater. The cooling can be either integral or auxiliary to the cell. A mixture of water and glycol similar to that utilised in a car’s coolant system is preferred. The shutters too are either integral to the cell or separate and hence integral to the MBE system. They are typically actuated by a pneumatic or stepper motor driven mechanism. For automatic growth recipe execution it is of course necessary to manipulate the shutter state through the computer resident control software.
The actual sample manipulation, from the epiready supplier boxes to the growth chamber’s heater stage, is an area of serious neglect in the MBE world. Manipulating a sample from the start to the end of the journey is often the most complicated task involved in MBE operation, particularly on older system. A number of mechanisms are available, and until recently only Vacuum Generators (VG) had engineered a user-friendly system. The VG system employed on V90 and larger systems utilises optical sensors to identify when the sample is in a valid transfer state. The sample transfer can be conducted manually using switches, automatically from computer designated site to site or even as part of a 6 sample batch without any operator interaction. The system was literally decades ahead of its time, though in many ways also long overdue. More recently the “cluster” tool has been engineered in favour of the “linear” MBE system. The cluster tool comprises a central distribution chamber with a computer controlled arm for sample transfer. Six to eight discrete chambers can be installed on the cluster chamber’s side flanges with the possibility for a FEL chamber, outgas stage, park/storage chamber and 3 independent growth/deposition chambers. The cluster tool employs either optical sensor or “intelligent” stepper motor control for position determination. The bells and whistles of the cluster tool, however, come with a hefty price tag. In terms of economics and pure practicality the VG V90 is easily the best research/semi-production MBE reactor ever manufactured. Though, unfortunately, since the dissolution of Vacuum Generators the V90 systems are now discontinued and, in the author’s opinion, we are stuck with either very expensive or inferior alternatives.