Materials Growth Services

The Materials Growth Research Facility

The Materials Growth Research Facility (MGRF) in the Cavendish Laboratory of the University of Cambridge is a specialist research centre for the production of metal films and compounds. As part of the Thin Film Magnetism group, (MGRF) is recognized worldwide for its ability to prepare high-quality epitaxial or polycrystalline films and mesoscopic structures in the Ångstrom to micrometre range. The facility enables University, industrial or government groups to have access to novel materials and technologies on a collaborative or commercial basis. A wide range of equipment is also available to characterize the physical properties of the materials grown. Detailed information about facility is presented below. Those wishing to use the MGF should contact its manager.

Dr. Adrian Ionescu
Material Growth Research Facility
Cavendish Laboratory

Roy Dolby Centre
Cambridge University
J J Thomson Avenue
CB3 0US, Cambridge

E-mail: ai222@cam.ac.uk.
Tel: +44 (0)1223-767140
 

Introduction

Many of the samples and devices which we study in the Thin Film Magnetism Group at the Cavendish Laboratory are thin films or mesoscopic structures. A thin film is typically described as being a micron (1 μm) or less in thickness and many of the ultra-thin films which we are currently studying are only a few nanometers thick.

Growing very pure, high quality films, either epitaxial or in polycrystalline form, requires some specialised techniques. In our group we mainly use two techniques for this purpose: molecular beam epitaxy (MBE) and DC/RF magnetron sputtering. Both of these techniques are physcial vapour deposition processes in which individual atoms or molecules of the source material are deposited onto the surface of a desired substrate. In order to keep the materials very pure and the interfaces abrupt, these processes require the use of ultra high vacuum (<1x10-9 mbar).

Molecular Beam Epitaxy

Molecular beam epitaxy, developed in the 1960's at the Bell laboratories, is a technique commonly used in semi-conductor research and industry to grow very high-quality, single-crystalline layers of semiconductors. However, our MBE systems are mainly dedicated to the growth of metal, either magnetic or non-magnetic, and various oxide (insulating) thin films with exception of the Topological Insulator chamber..

The basic principle of MBE growth is either the direct sublimation of solid source material or evaporation of a molten source from a crucible into a gas phase at very low pressures (2-3x10-10 mbar). Due to the ultra-high vacuum (UHV) environment the individual molecules and atoms are able to travel many centimeters before colliding with other molecules. This means that the substrates and devices in the MBE chamber gets coated with an even, high-purity and very thin layer of the desired material. Because the deposition rates are very low, the atoms which reach the substrate have plenty of time to move around the surface and settle into a single crystal structure before they get buried underneath the next layer.

Our MBE systems can grow different materials, up to eight in a single chamber, in one deposition cycle, either from e-beam evaporators for metals or from the Knudsen effusion cells for oxides (insulators). During growth, the film structure can be characterised and studied by a variety of in-situ techniques which are listed below. The substrates can be cleaned by annealing (heating) to over 600°C and Ar+ -sputtered prior to deposition. The deposition rates are calibrated and monitored to almost 0.1 nm accuracy by X-ray reflectivity. Thin films and devices in conjunction with patterning techniques, such as e-beam and photo lithography, can also be grown for subsequent ex-situ measurements or other purposes.

In-situ Structural Characterisation Techniques:

    1) Reflection High Energy Electron Diffraction (RHEED)
    2) Low Energy Electron Diffraction (LEED)
    3) Auger Electron Spectroscopy (AES)
    4) UHV Scanning Tunneling Microscopy (STM)
    5) Residual Gas Analyzer for Gas dosing purposes

In-situ Magnetic Characterisation Techniques:

    1) Longitudinal DC Magneto-Optical Kerr Microscopy (MOKE) with 2,5 kOe Magnets
    2) Brillouin Light Scattering (BLS)
    3) Mott Polarimetry
 

The MBE chambers and Associated Techniques

•     Spin Detector (Mott) Chamber:  Base pressure: 3x10-10 mbar maintained by a Varian ion pump and tow Edwards turbo-molecular pump combined with a Titanium sublimation pump (TSP), dual detector (Faraday cup/electron multiplier) quadrupole mass spectrometer, 3 MBE evaporators (one of them is an OMICRON EFM3), Ar+ sputter gun, ultra sensitive transverse in-situ MOKE, SPECS 4 grid ErLEED 150 LEED/AES system, thickness monitor, residual gas analyser (RGA), VG Microtech electron gun with high spatial resolution (200 nm), in-situ measurement of the spin polarisation of electrons elastically or inelastically scattered from magnetic thin films by means of a retarding potential 25 kV UHV compatible Mott polarimeter with an efficiency of 1.6x10-4.
•     Oxide Chamber:  Base pressure: 3x10-10 mbar maintained solely by means of a Varian diode ion pump, AES using a cylinder mirror energy analyser (CMA), VG Microtech reflection high energy electron diffraction (RHEED) system, four e-beam sources suitable for the growth of transition metals (such as Au, Cu, Fe, Ni, Co, NiFe, etc.), leak valve and in-situ magnet, sample transfer system for quick substrate input and sample retrieval after growth, thickness monitor.
•  Topological Insulators MBE chamber: Base pressure: 5x10-10 mbar with seven e-beam evaporators, variable sample temperature (77 - 900 K), RHEED and residual gas analysis tools, 800 l/s ion pumps, Turbo-pump and Nitrogen shrouf. 140 l/s turbo-molecular pump and TSP, thickness monitor. Full load lock and sample transfer system.
•     Multiple Technique Chamber:  Base pressure: 3x10-10 mbar, 7 e-beam evaporators (Au, Co, Fe, Cr, Si, Cu, NiFe), variable sample temperature (77 - 900 K), Physical Electronics 11-085 Ar+ sputter gun, Spectra Vacscan RGA, VG Microtech LEG 110 e--gun RHEED, VG Microtech RVL 900 LEED/AES system, Burleigh STM, in-situ MOKE, in-situ 2.5 kOe magnet , thickness monitor.
•    "M"etal Chamber:  Base pressure: 5x10-10 mbar. This is a commercially purchased metal MBE chamber for mass production of up to 2" samples. Load lock and main chamber are pumped down by cryo pumps (CTI-Cryogenics) which operate at 12 K and are able to reach ultimate pressures of 3x10-10 mbar followed by three Varian Ion-pumps, backed by dry pumps, which bring the chambers down to the base pressure. The load lock also contains a heat gun for degassing the substrates prior to transport into the growth chamber. Two e-beam evaporators each holding four sources (variable metal, semiconductor or insulating materials) on carrousels, RHEED, VG Smart IQ+ RGA, thickness monitor and fully computer controlled deposition. Can hold 30 3" wafers at any time in the load lock.

Dc/Rf Magnetron Sputtering

Sputtering, discovered in the 1850's, is an older deposition technique but with the advent of modern UHV technology it is now possible to sputter deposit films of extremely high quality and purity.

The system is a CEVP (now Surrey Nanosystems) magnetron sputtering chamber along with a load lock and an oxidation/ ion milling chamber. The load lock is capable of holding four 3" sample holders and is used to both speed up pump down times and to keep the main chamber under UHV condition. The load lock also contains a heat gun for degassing the samples and the lock itself, again reducing pump down times. The load lock is pumped by an ATP150 turbo pump backed by a dry pump and can reach an ultimate pressure of 5 x 10-9 mbar.

The manipulators at either end of the system can move the sample holders between the three chambers when the gate valves are open.

The sputtering chamber has six interchangeable targets which are capable of depositing from both DC sputtering and by radio frequency (Rf) enabling both conducting and non conducting materials to be deposited respectively. The sample stage can be rotated and magnetic fields up to 450 Oe are able to be applied during deposition to align magnetic grains inducing thereby an uniaxial magnetic anisotropy. The chamber has a base pressure of ~5 x 10-9 mbar and deposition occurs at an Ar+ partial pressure of around 1x10-3 mbar. The sample stage can be rotated during growth to ensure even deposition of the films; this is needed mainly for fast growth as the targets are slightly angled to the substrate causing otherwise preferential deposition on one side. The sample stage can be heated to temperatures in excess of 600 °C either during growth or in vacuum to anneal or de-cap substrates. Reverse sputtering for surface cleaning is also possible. The chamber is able to grow around 100 nm in an hour. The main chamber is pumped by a cryo-pump which operates at 12 K.

The oxidation chamber has two uses, it is used for oxidizing films, i.e. for growing oxides, and for milling materials. The ion miller works in almost exactly the same way as the sputtering chamber except with the sample replacing the target material. The plasma is usually comprised of argon, although oxygen or nitrogen are also possible, and is created at the top of the chamber where there is a RF gun. For oxidizing films an oxygen plasma is used and a choice of accelerator grids can either accelerate ionised oxygen at the sample or allows only neutral oxygen to bombard it, thus eliminating any unwanted reactive ion etching. The milling chamber is pumped by an ATP400 turbo pump backed by a dry pump and is able to reach ultimate pressures of ~5 x 10-8 mbar.

The system is designed to grow complex multilayers, consisting of magnetic and non-magnetic metals as well as insulators (oxides) either by single evaporation or co-evaporation for alloys. The deposition targets are at the top, facing downwards. The substrate is positioned at the bottom on a rotating stage and between an electro-magnet. The deposition rates are calibrated and computer controlled to almost 0.1 nm accuracy. Thin films and devices in conjunction with patterning techniques, such as e-beam and photo lithography, are strictly grown for subsequent ex-situ measurements or other purposes. In addition, the oxidation/ion milling chamber makes use of different gases for oxidation and/or ion/atom source milling of heterostructures for device production.