Atomic Fabrication Facility

The Atomic Fabrication Facility (AFF) was established in 2001 and is a unique laboratory world-wide, dedicated to the development of atomically precise devices in silicon and germanium. The ultimate goal of this facility is to develop a scalable quantum computer prototype using a combination of Scanning Tunneling Microscopy (STM), Scanning Electron Microscopy (SEM) and Molecular Beam Epitaxy (MBE). This facility has been constructed to house six state of the art scanning tunneling microscopes, including 3 Variable Temperature STMs (VT STM) from Omicron Nanotechnology GmbH, a combined Multi-scan STM-SEM/MBE system, an Omicron Nanoprobe (4 point probe STM) and a low temperature STM. Each of these systems has been designed in collaboration with Omicron and MBE Komponenten GmbH in Germany to combine high quality silicon growth with high resolution STM.


The majority of work on understanding the phosphorus in silicon surface chemistry has been carried out on Variable Temperature Scanning Tunneling Microscopes. An initial instrument was installed in 1998 and consists of a customconfigured, triple-chamber UHV STM/MBE system. The STM can be operated at temperatures ranging from 25 K to 1100 K and is used to image the silicon surface and perform atom-scale lithography. The second UHV chamber houses a SUSI™ silicon source for the MBE growth of thin epitaxial silicon films with thicknesses ranging from sub monolayer to several tens of nanometers. Facilities to analyze surface structure and contaminants are provided in the third UHV chamber which incorporates both Low-Energy Electron Diffraction (LEED) and Auger Electron Spectroscopy (AES). Two further VT-STMs were installed in 2011.


A multi-chamber STM-SEM/MBE system provides the necessary registration and high purity silicon growth capabilities required for multi-qubit fabrication. Specifically, the MBE system is capable of device quality Si and SiGe growth onto 4” wafers. Using liquid nitrogen cryo-shrouds, this instrument achieves very low base pressures and low background doping levels. A liquid nitrogen gravity feed tank, provides a continuous flow of liquid nitrogen at a constant pressure and a constant fill level in the MBE. The MBE system has been designed with silicon and germanium beam flux control and a separate sample preparation chamber for outgassing of samples before introduction into the MBE system. The MBE system is also compatible with growth on 1cm2 samples on small sample plates. To minimize vibrations from the crystal growth system affecting the atomic resolution of the STM, the MBE system is located on a separate concrete base. This is isolated from the main floor of the laboratory using piers drilled 10 m down into the foundation bed-rock. In addition, the two main chambers of the STM-SEM/MBE system are housed in separate rooms to reduce acoustic interference between them. A low temperature oxide chamber, for the development of high quality silicon dioxide barrier layers, is equipped with a RHEED system, resistive silicon sublimation source (SUSI) and a neutral atomic oxygen source extracted from an RF plasma. The oxide chamber also includes liquid nitrogen cryo-shrouds to achieve very low base pressures. The instrument is routinely used to deposit silicon -dioxide at low temperatures for gating atomically precise devices in silicon.


Operating under UHV conditions, the STM-SEM and MBE chambers are physically connected even though they are housed in different, acoustically shielded laboratories in the AFF. A transfer line between the two systems (penetrating a dividing wall) is attached to a 3-tonne concrete block to prevent vibrations from the MBE 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 alignment of features during successive fabrication steps.


In 2006, an Omicron Nanoprobe system was installed. This new four probe STM system is designed for in-situ electrical characterization of nano- and atomic-scale devices.


In 2011 a combined Variable Temperature (VT) STM and a Low Temperature (LT) STM system was installed following the new academic hire of Prof Sven Rogge from Delft. Since then this system has been extended with a phosphine chemical vapor deposition source for atomically precise dopant engineering. In 2012, the LT STM was upgraded with a Nanonis™ STM controller from SPECS Zurich for state of the art spectroscopy which allows the determination of the level spectrum of a sub-surface dopant or multi-dopant arrangement.