Characterization of thin films using heavy ion beams

Thin films are used in many technological applications. The characterization of thin films requires compositional information as a function of sample depth. Ion beam analysis techniques, such as Rutherford backscattering spectrometry and elastic recoil detection (ERD), can provide this information in a uniform way for all elements and as absolute concentrations without relying on standards. These techniques can fully be exploited when projectile beams of heavy ions such as Si or Au are used. This improves the elemental resolution and the depth resolution when compared with standard He ion beam analysis. The use of gas ionization detectors increases detection efficiency and minimizes the beam exposure of the samples, so that the analysis is essentially nondestructive. The sampling depth of a few micrometers makes these techniques ideal for the stoichiometric analysis of the surface region of homogeneous materials and, in particular, thin surface films.     

Double scattering measurement applied for heavy ion elastic scattering at intermediate energies

The measurement of the left-right asymmetry in double scattering was applied to deduce the vector analyzing power for elastic scattering of 150 MeV ⁶Li on ¹²C. In order to reduce the instrumental asymmetry, the measurement was done at θ_lab = 7.7° and 12.3° corresponding to the second and third maxima of the angular distribution of the differential cross section for elastic scattering. The magnetic spectrograph DUMAS blocked reaction particles other than elastic ones impinging on the second scatterer. Position sensitive SSDs used for the left-right detectors in the second scattering reduced the instrumental false asymmetry. This enables us to perform the asymmetry measurement with a precision better than 10⁻².     

Accurate measurement of target thickness by heavy ion elastic scattering

The potentiality of heavy ion elastic scattering for the measurement of target thickness has been investigated considering a number of target-projectile combinations. From such studies it is inferred that the target thickness can be measured with an uncertainty of ±5% in most cases.     

Improvements in the angular current density of inductively coupled plasma ion source for focused heavy ion beams

A compact inductively coupled plasma ion source (ICPIS) is developed for producing high current micron size beams for high speed micromachining applications. Angular current density (J_Ω) of the beam extracted from ICPIS is measured and found to be three orders higher than that of the conventional liquid metal ion sources. An improvement in J_Ω by >30% is achieved through the increase of RF power density in the plasma by reducing the plasma volume instead of operating ion source at high RF power. Studies on J_Ω show that heavier ions have maximum J_Ω at lower power and vice versa for the lighter ions. Ion beams of Neon, Argon, Krypton and Xenon extracted at 5 kV, have J_Ω of 57, 51, 37 and 30 mA/Sr respectively at RF power in the range of 75 W–200 W. Measurements on proton beam which is very important for imaging applications show J_Ω of 45 mA/Sr at 200 W.     

A beam profile monitor for heavy ion beams at high impact energies

A beam profile monitor for heavy ion beams has been developed for the use in experiments at the Heavy Ion Synchrotron SIS at Gesellschaft für Schwerionenforschung Darmstadt (GSI). Four thin scintillation fibres are mounted on one wheel and scan the ion beam sequentially in two linearly independent directions. They are read out via one single photomultiplier common to all four fibres into one time spectrum, which provides all information about beam position, beam extension, time structure and lateral homogeneity of the beam. The system operates in a wide dynamic range of beam intensities.     

Absolute energy measurement of heavy ion beams using a resonant time-of-flight system

A resonant time-of-flight measurement system has been put into operation at the ATLAS facility for the determination of the energy of heavy ion beams. The system provides continuous, nondestructive monitoring of the beam energy. The system provides relative energy determination with a precision of . Absolute energy is determined to an accuracy of 10⁻³. A variety of beam tests have been performed to study the properties of the system.     

Contribution of secondary particles to the dose in 12C radiotherapy and other heavy ion beams

The results of experimental studies performed in a radiotherapy (12)C ion beam with a nominal energy of 500 MeV/amu and in (16)O and (56)Fe ion beams with a nominal energy of 1 GeV/amu have been described. Linear energy transfer (LET) spectra have been established by means of an LET spectrometer based on a chemically etched track detector, and the measured results were also compared with theoretical calculations obtained using the program Stopping and Range of Ions in Matter (SRIM). It was observed that with increasing depth in a beam, the LET spectra are shifted towards higher values of LET; one can also observe an important widening of the spectra along the range, as well as an increasing amount of nuclear reaction products and/or of fragments in the spectra. The relative contribution of these secondary particles to the total absorbed dose was assessed.     

Electronic recombination in a nonequilibrium plasma: Application to electronic cooling of heavy ion beams

We study the recombination in a neutral, fully ionized, anisotropic nonequilibrium plasma with applications to electronic cooling of heavy ion beams in a storage ring. These plasmas are characterized by low particle densities and extreme temperature differences between the free electrons and ions. Starting from a microscopic master equation for the coefficient of recombination, collective effects and three body recombination processes are considered. Because of the small density only photon-like collective modes are important for the induced recombination. As the ion temperature is high initially, it is necessary to include the movement of the ions in the calculation of the recombination coefficient. In a cooler plasma recombination into the high-lying states is cut off by the Stark effect due to the transient electric fields which appear after a Lorentz transformation of the external transversal magnetic fields to the rest frame of the beam.     

Surface analysis using proton beams

Characteristic X-rays, back-scattered protons and Auger electrons are emitted from the surface region of solids under bombardment by protons. Energy analysis of these emitted particles yields data which permit identification of the atoms present in the surface region; in addition, quantitative surface composition information has been obtained using appropriate calibrations. Results from analyses using the techniques of (1) proton-induced X-rays (PIX), (2) Rutherford back-scattering (RBS) and (3) Auger electron spectroscopy (AES) for bombardments by mono-energetic 100–400 keV protons are collated and discussed. These previously published results are reviewed here to illustrate the types of compositional information obtainable and the advantages and disadvantages of each of the techniques with regard to elemental sensitivities, depth resolutions, reaction cross sections and complex targets. Finally, the complementary nature of the three techniques is discussed with emphasis on the possibilities for simultaneous analyses.     

Longitudinally polarized proton beams at TRIUMF

Beam line 4B at TRIUMF has been modified to allow production of polarized proton beams of all polarization components (normal, sideways and longitudinal). This was achieved by the installation of two 2.4 T m superconducting solenoids in the cyclotron vault section of the beam line. It was also required to be able to rotate one vault quadrupole. It was found that for longitudinal polarization in beam line 4B the twister could no longer be operated as a unit section. The standard achromatic and dispersed modes of operation of beam lines 4A and 4B are still available.     

The intercomparison of cosmic rays with heavy ion beams at NIRS (ICCHIBAN) project

The ICCHIBAN-2 experiment, the first dedicated to the ground-based intercomparison of passive space dosemeters, was carried out between 23 May and 28 May 2002 at the National Institute of Radiological Sciences in Chiba, Japan. The primary objective of the ICCHIBAN-2 experiment was to intercompare the response of passive dosemeters used in space crew dosimetry to monoenergetic heavy ions of charge and energy spanning a significant portion of the galactic cosmic ray (GCR) spectrum. During the ICCHIBAN-2 experiment, dosemeters from 12 different laboratories in 9 countries were irradiated under identical conditions to heavy ion beams of 150 MeV n(-1) (4)He, 400 MeV n(-1) (12)C, 490 MeV n(-1) (28)Si and 500 MeV n(-1) (56)Fe at the NIRS Heavy Ion Medical Accelerator.     

Transport of intense ion beams

The maximum transportable current for an ion beam is determined by considerations of focal strength, space charge equilibrium and stability, structural practicality, and emittance. These factors are described within the context of a heavy ion driver for inertial confinement fusion. Recent supporting results from particle-in-cell simulations and transport experiments will be described.     

Focused ion beams for microfabrication

Focused ion beam (FIB) techniques are commonly used in microelectronics for prototyping, failure analysis and process control. With the growing interest in MEMS (micro-electro-mechanical systems) the importance of FIB techniques in this field has to be established. For the FIB to be considered for production, the milling time is crucial. Using as an example the fabrication of a microaccelerometer structure the authors show there is scope for the FIB in the prototyping and production of micromechanical structures, leading to novel sensors     

Oxidation processes using ion beams

Recent work demonstrates that reactive ion beams can be used to grow compound layers on metal and semiconductor surfaces. Ion beams are characterized by easily controlled ion flux and energy, with a narrow energy spread. Also, the ion species are delivered to the sample in a low pressure environment. These advantages allow greater control and simpler analysis of compound formation processes than other techniques do. Ion beam oxidation is reviewed and compared with thermal, plasma and r.f. oxidation and with oxidation by ion implantation. The ion energy range of several electronvolts to hundreds of electronvolts is suitable for depositing the oxidizing species in the metal in an active state, and simultaneous sputtering can produce a self-limiting oxide thickness when the sputtering yield and oxidation rate are in proper balance. The process of ion beam oxidation is also discussed in light of the etching behavior of metals under combined inert gas and oxygen ion bombardment and of the related technique of reactive ion beam sputter deposition using ion bombardment of a growing film. Additional examples are drawn from the use of other reactive ion beam species, including nitrogen and hydrogen.     

Low energy proton beams from laser-generated plasma

Low energy proton beams are produced by ns pulsed Nd:Yag laser operating in repetition rate at intensities of the order of 10¹⁰ W/cm². Laser pulses interacting with thick and thin solid hydrogenated targets, placed in high vacuum, produce non-equilibrium plasmas with ion emission in backward and forward directions along the normal to the target surface.The ion emission is analyzed with time-of-flight techniques by using ion collectors and ion deflecting spectrometers. The spectra analysis permits the evaluation of the plasma temperature, density, proton energy and current, ion energy and charge state distributions.Special targets, based on thin polymers coupled to metals or to conducting nanostructures, induce high electric field in the plasma and confer high kinetic energy to the protons, up to about 200 eV.By using a post-acceleration system, placed along the target normal direction, it is possible to accelerate further the protons up to the energy depending on the applied acceleration voltage. The maximum proton energy of 30 keV, the current density of about10 nA/cm² and the beam quality can be improved, as discussed.     

Integration of scanning probes and ion beams

We report the integration of a scanning force microscope with ion beams. The scanning probe images surface structures non-invasively and aligns the ion beam to regions of interest. The ion beam is transported through a hole in the scanning probe tip. Piezoresistive force sensors allow placement of micromachined cantilevers close to the ion beam lens. Scanning probe imaging and alignment is demonstrated in a vacuum chamber coupled to the ion beam line. Dot arrays are formed by ion implantation in resist layers on silicon samples with dot diameters limited by the hole size in the probe tips of a few hundred nm.     

Surface modification using lasers and ion beams

Abstract

The influences of ion beam and laser treatment on the thermal oxidation and sulphidation of metallic materials and their particular features, advantages, and disadvantages are separately described and compared with conventional alloy addition. Examples of the various mechanisms involved and the associated problems are also presented. The ion beam technique considerably improves high temperature resistance, despite the shallow depths involved and ion beam deposited coatings are already in commercial production. Laser beams produce thicker coatings and have considerable industrial potential.

MST/1073     

High-sensitivity and high-resolution low-energy ion scattering

Low-Energy Ion Scattering (LEIS or ISS) is used to selectively analyze the atomic composition of the outer atomic layer of surfaces. In addition, the spectrum gives (non-destructively) the in-depth distribution. Using a double toroidal energy analyzer with parallel energy detection and time-of-flight filtering a high sensitivity and mass resolution of LEIS is achieved. This is demonstrated for a highly dispersed catalyst of Pt/Au on γ-alumina. The improved depth resolution is illustrated for self-assembled monolayers of alkanethiols (12-20 carbon atoms) on gold. Even for these low Z carbon atoms a clear shift of 8 eV/carbon atom is observed (using 1.5 keV ⁴He⁺ ion scattering). This opens many new possibilities for studies of ultra-thin diffusion barriers, high-k dielectrics and biosensors.     

Recent developments in nanofabrication using focused ion beams

Focused ion beam (FIB) technology has become increasingly popular in the fabrication of nanoscale structures. In this paper, the recent developments of the FIB technology are examined with emphasis on its ability to fabricate a wide variety of nanostructures. FIB-based nanofabrication involves four major approaches: milling, implantation, ion-induced deposition, and ion-assisted etching of materials; all these approaches are reviewed separately. Following an introduction of the uniqueness and strength of the technology, the ion source and systems used for FIB are presented. The principle and specific techniques underlying each of the four approaches are subsequently studied with emphasis on their abilities of writing structures with nanoscale accuracy. The differences and uniqueness among these techniques are also discussed. Finally, concluding remarks are provided where the strength and weakness of the techniques studied are summarized and the scopes for technological improvement and future research are recommended.     

Preparation of laser-accelerated proton beams for radiobiological applications

This paper presents the concept of transport and filtering of laser-accelerated proton pulses used for the first cell irradiation experiments performed with the Dresden 150 TW laser DRACO. Based on a simple non-focusing magnetic dipole equipped with two apertures the concept makes use of an energy dependent angular asymmetry of the proton spectra. For micron thin target foils protons of interest with energies above 7 MeV are observed to be significantly offset from target normal where low energy emission is dominantly centered. As the effect can be controlled via the target rotation with respect to the incoming light, it can be used to optimize the transport efficiency for high energy protons while simultaneously suppressing background radiation.