The Materials Growth Small Research Facility
The Materials Growth Facility (MGF) in the Cavendish Laboratory of the University of Cambridge is a specialist research center for the production of metal films and compounds. As part of the Thin Film Magnetism group, (MGF) is recognized worldwide for its ability to prepare high quality epitaxial or polycrystalline films and mesoscopic structures in the Ångstrom to micrometer 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 Facility
J J Thomson Avenue
CB3 0HE, UK
Tel: +44 (0)1223-764096
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
Figure 1: Multiple Technique MBE chamber.
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 dedicated to the growth of metal, either magnetic or non-magnetic, and various oxide (insulating) thin films.
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 a very even 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. 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/Ac Magneto-Optical Kerr Microscopy (MOKE) with 2,5 kOe Magnets
2) Brillouin Light Scattering (BLS)
3) Magneto-Resistance (MR) Measurements
4) Photo-Excitation Measurements
5) Mott Polarimetry
The MBE chambers and Associated Techniques
- In-situ Magneto-resistance (MR) Chamber: Base pressure: 4x10-10 mbar, 4 e-beam evaporators (various materials e.g. Fe, NiFe, Au, etc.), variable sample temperature 77 - 900 K, in-situ MR, magneto-optical Kerr effect (MOKE) and photo-excitation apparatus (2.5 kOe magnet and multi-pin probe), Physical Electronics 04-162 Sputter Ion Gun, SPECS 4 grid ErLEED 150 low energy electron diffraction (LEED)/Auger electron spectrometer (AES) system, thickness monitor.
- Spin Detector (Mott) Chamber: Base pressure: 1x10-10 mbar maintained by a Varian ion pump, a diffusion pump and a Varian 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.
- Spin Valve Chamber: Base pressure: 1x10-10 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.), sample transfer system for quick substrate input and sample retrieval after growth, thickness monitor.
- Dynamic Kerr (AC-MOKE) Chamber: Base pressure: 5x10-10 mbar. Operates as two separate sub-chambers: (1) scanning tunnelling microscopy (STM) chamber with 150 l/s ion pump and 50 l/s turbo-molecular pump, (2) thin film growth chamber with four e-beam evaporators (Fe and Cr), variable sample temperature (77 - 900 K), Physical Electronics 04-162 sputter ion gun, LEED and MOKE characterisation tools, 500 l/s ion pump, 140 l/s turbo-molecular pump and TSP, thickness monitor. Full load lock and sample transfer system.
- Multiple Technique Chamber: Base pressure: 2x10-10 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 Brillouin light spectroscopy, 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 samples. Load lock and main chamber are pumped down by cryo pumps (CTI-Cryogenics) which operate at 10 K and are able to reach ultimate pressures of 1x10-9 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
Figure 2: Dc/Rf Magnetron Sputtering Chamber (left) with adjunct Oxidation/Ion milling Chamber.
Sputtering, discovered apparently 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 10-8 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 ~1 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 10 K and is able to reach ultimate pressures of 1 x 10-9 mbar.
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 (Fig. 2). 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.
Publications on electrochemical deposition as far as 1797. However, it was not until 1921 that multilayered structures were developed and only in the 1980's peoples film qualities suitable for nanoscale multilayers. Since then the technique has been optimized to the extend that electrochemically deposited films can be used in the manufacture of hard disk drives, spin-valves, microactuators and copper nanotubes. Clearly a powerful and inexpensive technique it enables us the growth of nano to micron sized layers, nanowires and eliminates side wall growth.
The electrolyte is an aqueous form of the material to be deposited, usually arising from a metal sulphate (Fig. 3). By applying a voltage from the potentiostat the charge passes into the working electrode (cathode), through the electrolyte and into the platinum mesh (anode). The potential between the two is ascertained via the reference electrode.
Figure 3: Electrochemical deposition set-up showing the five major components required.