The Atomic Fabrication Facility (AFF) was established in 2001 and contains 5 interlinked laboratories all dedicated to the development of atomic-scale devices in silicon and in particular a scaleable quantum computer prototype, using a combination of Scanning Tunnelling Microscopy (STM) and Molecular Beam Epitaxy (MBE). This new facility has been constructed to house both an Omicron Variable Temperature STM and a customised Omicron STM-SEM/MBE system which combines a high quality 4" SiGe MBE system with a dual STM-SEM (Scanning Electron Microscope) system. This unique ultra-high vacuum (UHV) microscope and crystal growth system will allow the fabrication of the complete qubit architecture at the atomic-scale and occupies two independent rooms of the AFF.
To date the majority of the pioneering work on the phosphorus array fabrication has been carried out on the Variable Temperature Scanning Tunnelling Microscope (VT STM), see figure 1. The first chamber of this system houses an STM that can be operated at temperatures in the range 25 K - 1500 K with a silicon EFM-3 evaporation source directly attached to the STM stage. This silicon source deposits high-quality silicon films with monolayer or sub-monolayer thickness and allows for direct STM observation of the silicon growth dynamics. Alternatively, this system can also be used as an Atomic Force Microscope (AFM). The STM/AFM tool is used both to image silicon surfaces and perform atomic-scale lithography. The second UHV preparation chamber houses a silicon sublimation source (SUSI) for high quality silicon MBE growth with a thickness ranging from sub-monolayer to several nm and Reflection High-Energy Electron Diffraction (RHEED) monitoring capability. Facilities to analyse surface structure and contaminants are provided in the third UHV analysis chamber, which incorporates both Low-Energy Electron Diffraction (LEED) and Auger Electron Spectroscopy (AES).
Figure 1: Variable Temperature STM/AFM system.
The second STM system has both SEM and MBE capability providing the necessary registration ability and high purity silicon growth required for the fabrication of complete electrical devices and the final multi-qubit device. In particular the MBE side (figure 2 (a)) can produce device quality SiGe growth on 4" wafers. The system has been designed with Si and Ge beam flux control, liquid nitrogen cryo-shrouds and a separate sample preparation chamber. It is also compatible with growth on 1 cm2 samples on specialised sample adapter plates that can also be imaged in the STM-SEM system. To minimise vibrations from the pumps on the crystal growth side affecting the atomic resolution of the STM, the MBE system is located on a separate concrete block and in a separate laboratory. The block is isolated from the main floor of the laboratory using piers drilled 10 m into bed-rock.
The STM-SEM side (figure 2 (b)) of the system is connected under UHV to the MBE chamber but housed in a different, acoustically shielded room in the AFF. A transfer tube between the two systems (penetrating the dividing wall) is attached to a 3-tonne concrete block to dampen vibrations from the MBE side reaching the STM. The STM system incorporates an SEM that allows registration markers to be easily found without damaging the STM tip. A specially designed optical position readout system is also incorporated to allow precise 10nm alignment of features during successive fabrication steps.
Figure 2a: Customised combined SiGe MBE
Figure 2b: STM-SEM system
The AFF laboratory also houses a Leica Stereoscan s440 Scanning Electron Microscope with a cryogenic stage. This system is used for training students on Electron Beam Lithography (EBL) and for studying defects in silicon by cathodoluminescence (CL). The Leica SEM is configured with an Oxford Instruments MonoCL2 Microanalysis System for CL imaging and spectroscopy allowing an image or map of the spatial distribution of any dopants or defects present in a semiconductor.