A CEMS apparatus for in situ studies of ion beam modified metals

An apparatus for CEMS studies of ion beam modified metals is described. The spectrometer can be coupled directly to the ion implanter. During ion bombardement the sample can be cooled to liquid nitrogen temperature or heated to about 500 K. CEMS measurements can be taken directly after ion beam modification between room temperature and liquid nitrogen temperature. As a first test of the performance of the apparatus CEMS spectra of boron-ion implanted iron at room temperature are presented.     

Secondary ion mass spectra of alkali halides

Mass spectra of positive and negative secondary ions from various alkali halides have been measured in the Manitoba time-of-flight mass spectrometer. The ions were produced by Cs⁺ and K⁺ bombardment at primary ion energies of 3 to 19 keV for the positive spectra, and 11 to 28 keV for the negative spectra. The ions measured were those emitted within a time interval ～ 20 ns after the primary ion impact. The secondary ion yields are strongly dependent on the sample composition and treatment; prior irradiation may change the yield by an order of magnitude or more. The secondary ion yields also depend strongly on the energy loss of the primary ion, but the ratio of yields of different cluster ions from a given target is almost independent of this parameter. The results appear to be consistent with models in which the clusters are ejected directly from the target, but do not determine whether or not they possess the original surface structure. The results may also be described by a recombination model if the recombination is essentially complete.     

Investigation of the binary model of scattering of 1 keV to 25 eV Ar+ ions from a Cu surface 1

The validity of the binary collision approximation for describing the scattering of low energy Ar ions from a polycrystalline Cu surface was investigated. The Ar ions were incident upon the target surface at an angle of 45° (with respect of the plane of the surface). The range of primary ion energy studied was 1000–25 eV. An ultra-high vacuum magnetic sector mass spectrometer was used to analyze those secondary ions emitted from the specimen surface at an angle of 90° with respect to the primary ion beam. The scattered ions were identified and their energies measured. No significant deviation from the prediction of the binary collision model was found throughout the range of energy studied.     

Relative ion sputtering yield measurements by integration of secondary ion energy distribution using a retarding-dispersive Ion energy analyzer

The application of a retarding-dispersive energy analyzer as the pre-filter of a quadrupole mass analyzer has made it possible to combine a standard high-speed Secondary Ion Mass Spectrometer (SIMS) and a high-resolution secondary-ion energy analyzer into one instrument. Data taken with this instrument indicate the presence of very significant high-energy tails in the energy distribution of all observed secondary ions, even with relatively low (2 keV) primary ion energies. The shape of the energy distribution varies widely from element to element, for atomic compared to molecular species sputtered from a clean metal surface, and depends, for a given species sputtered from a metal surface, on the degree of surface oxidation. The variations established in the present work are large enough to introduce in many cases substantial discrepancies between published values of both relative ion sputtering yields and surface elemental concentrations and values obtained by considering the complete energy distribution. Methods of obtaining accurate secondary ion yields by integrating the energy distribution are discussed. Work performed under the auspices of the Division of Physical Research of the U.S. Energy Research and Development Administration.     

Analysis of the nitrogen ion beam generated in a low-energy plasmafocus device by a Faraday cup operating in the secondary electronemission mode

The energy distribution and flux of fast nitrogen particles generated in a Mather-type plasma focus device operating at 0.4 Torr of N₂ pressure is reported. A Faraday cup operating in the secondary electron emission mode was employed. To determine the total number of beam particles, multiple scattering of the ions was taken into account. It has been possible to register the ion energy up to a lower kinetic energy threshold of ≈50 keV, which is a value much lower than that obtained with a Thomson spectrometer in a previous work     

Secondary ion emission from organic films under bombardment with a 32S beam at 3 MeV/A

Abstract

An experimental investigation of the secondary ion emission from organic films under heavy ion bombardment at high energies was carried out by using a time-of-flight mass spectrometer coupled to a Tandem heavy ion accelerator. Preliminary results obtained at 3 MeV/A are reported and discussed.     

Biomolecule desorption yields as a function of the angle of incidence of fast heavy ions

A time-of-flight mass spectrometer that allows variation of the angle of incidence of the primary ion beam in desorption experiments has been developed. It has been used with heavy-ion beams of MeV energies from the Uppsala EN-tandem accelerator in order to study the yield of molecular ions from samples of ergosterol, glycylglycine and cesium iodide. The general behaviour of the angular distributions is that the yield increases sharply when oblique incidence is approached. The shape of the angular distribution for ergosterol ions is found to depend on the stopping power of the primary ion. The results are discussed in terms of thermal spike models for the desorption process.     

Depth profiling of organic materials using improved ion beam conditions

We used the so-called dual beam mode of depth profiling to start a systematic investigation of organic depth profiling with a time of flight secondary ion mass spectrometer (TOF-SIMS) instrument. Similar to inorganic profiling, we found the dual beam mode beneficial because sample erosion and sample analysis are decoupled and can be optimised independently. We applied different primary projectiles such as C₆₀, O₂ and Cs for sputtering to a variety of organic specimens, using a wide range of impact energies. Results are discussed with respect to the feasibility of the different approaches to organic depth profiling in SIMS.     

Energy and mass dependence of isotopic enrichment in sputtering

Silver, copper, and boron (from a boron nitride target) were sputtered with xenon ions. The isotopic composition of secondary ions of silver was measured at ion energies ranging from 300 eV to 3 keV and, for copper and boron, at 2.0, 2.5, and 3.0 keV. An ion gun was used to generate the ion beam. The secondary ions were detected at a small emission angle by a quadrupole mass spectrometer. The secondary-ion flux of silver was found to be enriched in heavy isotopes at lower incident-ion energies. The heavy-isotope enrichment was observed to decrease with increasing primary-ion energy. Beyond 500 eV, light isotopes of silver were sputtered preferentially with the enrichment increasing to a constant value of 1.018. The sputtered flux of copper and boron also indicated constant enrichments (1.008 and 1.281 for copper and boron respectively) in light isotopes at high ion energies. Received: 2 August 2002 / Accepted 9 August 2002: / Published online: 4 December 2002 RID="*" ID="*"Present address: Pacific Northwest National Laboratory, Richland, WA 99 352, USA RID="**" ID="**"Present address: Philips Display Components Company, Ottawa, OH 45 875 USA RID="***" ID="***"Corresponding author. Fax: +1-334/727-8090, E-mail: pkray@tusk.edu     

Secondary ion emission from Si under nitrogen ion bombardment

The yield and energy distribution of positive secondary ions emitted from Si under N₂⁺ ion bombardment were measured. The obtained mass peaks correspond to three types of secondary ion species, that is, physically sputtered ions (Si⁺, Si₂⁺), chemically sputtered ions (SiN⁺ Si₂N⁺) and doubly charged ions (Si²⁺). The dependence of secondary ion emission on the primary ion energy was studied in a range of 2.0–20.0 keV. The yields of physically and chemically sputtered ions were almost independent of the primary ion energy. The yield of the doubly charged ion strongly depended on the primary ion energy. The energy distribution of secondary ions of the three types showed the same dependence on the primary ion energy. The most probable energy of the distribution increased with the primary ion energy. On the other hand, for the energy distribution curves of sputtered ions, the tail factors N in E^?N were constant and showed a m/e dependence.     

Atomic collision processes relevant for heavy ion beam probes

Atomic collision processes of fast Tl and Cs ions with particles in a high temperature fusion plasma are investigated. At low beam energies (<5 MeV), ion impact collisions and charge exchange processes can be neglected compared to electron ionization processes. At beam energies above 5 MeV and high plasma ion temperatures, collisions with ions start to contribute significantly to signal generation and attenuation. Also, collisions with the neutral background gas in the beamlines can attenuate the ion beam significantly and lower the signal level, if the vacuum pressure is above 10^-4 Torr. For the heavy ion beam probes operating today, only electron impact ionization processes are important and accurate predictions of the secondary signal level and electron density profile measurements are possible because of the good knowledge of electron impact ionization cross sections for Cs ⁺ and Tl⁺ ions     

ToF-SIMS analysis of a fluorocarbon-grafted PET with a gold cluster ion source

Cluster ions have been recognized as a superb primary species in time of flight secondary ion mass spectroscopy (ToF-SIMS) compared with monatomic primary ions, as they significantly enhance the secondary ion yields from bulk samples. Self-assembled monolayers provide an important system for studying the fundamental mechanism involved in the yield enhancement.We used a gold cluster ion source to analyze a new type of self-assembled monolayer: a fluorocarbon-grafted polyethylene terephthalate. In addition to the structure details, which helped to understand the grafting mechanism, ToF-SIMS analysis revealed that fluorocarbon secondary ion yield enhancements by cluster ions were due to the enhanced sputter efficiency. A larger information depth may also be expected from the enhancement. Both mathematical definitions of damage cross-section and disappearance cross-section were revisited under a new context. Another cross-section parameter, sputter cross-section, was introduced to differentiate the beam induced sputter process from damage process.     

Application of solid-state nuclear track detectors for studies of fast ion beams within PF-1000 and other plasma-focus facilities

The paper presents results of studies of the fast ion emission from two plasma facilities: PF-1000 and PF-II. The measurements were carried out mainly with solid-state nuclear track detectors (SSNTDs). To register ion mass- and energy-spectra, obtained when primary ion beams are deflected by magnetic and electric fields of a Thomson-type spectrometer, the SSNTDs plates were installed inside the analyzer. To measure ion angular distributions the SSNTDs were located within the main discharge chamber, at different angles to the electrode axis. The SSNTDs were also applied to register ion beam images formed within an ion pinhole camera, and to determine a spatial location of the fast ion sources.     

A gas cluster ion beam accelerator

An assembled CO₂ gas cluster ion beam system was assessed using a retarding field analyzer and a time-of-flight mass spectrometer. The CO₂ gas was expanded to form gas clusters at the input pressure of 1–5 bar through a quartz Laval nozzle. At 4 bar, it is confirmed that the clusters consisted of about 500 molecules. Also the dependence of the mean cluster size distribution on source temperature was examined. At the low fluence of ion beam, an isolated gas cluster ion impact on solid surfaces was investigated. CO₂ gas cluster ions were irradiated at the acceleration voltage of 40–60 kV on highly oriented pyrolytic graphite. Si with native oxide layers, and Cu film deposited on Si wafer. After very short exposure of cluster ions, induced hillocks with about 0.8–1 nm in height and 20 nm in width were outgrown from the impacted surfaces. After prolonged irradiation on Si and Cu/Si, humping was more developed and consequently the surface morphology seemed to be saturated because of gradual filling the gap between the hillocks.     

The influence of ion bombardment on emission properties of small dust grains

Dust grains that are present in many plasma and vacuum systems and in the space usually carry a non-negligible charge. Their charging significantly depends on surface properties of the grain material. In cold plasma, charging is mainly given by electron attachment, nevertheless, when plasma becomes hot, other processes (secondary electron emission, field emission, etc.) take place. Emission properties of the grain surface could be modified by grain baking or by ion bombardment. Our study is carried out at the dust charging experiment dealing with a single dust grain electro-dynamically levitated in a 3D quadrupole trap. The grain can be exposed to the ion beam in the energy range of 100 eV–5 keV and to the electron beam in the energy range of 100 eV–10 keV. We have chosen He⁺ and Ar⁺ ions for the surface treatment and the observed influence on the surface properties is discussed in terms of secondary emission. A non-negligible shift of the secondary electron emission yield, as well as a change of energy distribution of secondary electrons, were measured after Ar⁺ bombardment. A preliminary study suggests that the effects of He⁺ and Ar⁺ are comparable.     

Investigation of surface layers by SIMS and SIIMS

In the Secondary Ion Mass Spectrometry (SIMS) the sample to be analysed is bombarded with a beam of primary ions. The secondary ions sputtered away from the sample, characteristic for its composition near the surface at any time, are mass selected and detected in a mass spectrometer. The yields of several elements in a Fe-matrix and in technically pure samples bombarded with positive oxygen and argon ions have been determined to study the influence of the matrix and the primary ions on the ion yields. The properties of SIMS and of two of its special modes viz. Static Secondary Ion Mass Spectrometry (SSIMS) and Secondary Ion Imaging Mass Spectrometry (SIIMS) with respect to the analysis of surface layers are discussed.     

Influence de la chimisorption de l'oxygéne et l'azote sur l'émission ionique secondaire d'échantillons monocristallins de nickel et d'alliage nickel-chrome

The influence of an active gas adsorption on the secondary ion emission yields of a mono-crystalline target bombarded with a primary argon beam has been investigated. On the one hand, the chemisorption changes the electronic structure on the surface metal atoms and increases the positive ion emission yields, on the other hand, channeling conditions of the primary ions through the target lattice are modified according to the relation of the chemisorbed layer with the metal lattice. Two samples were examinated: pure nickel and a nickel chromium alloy in either an oxygen or a nitrogen partial pressure. Important modifications of the coverage are due to the sputtering yield variations. The experiments described here suggest that secondary ion emission could be used for chemisorption studies.     

Experimental study of the influence of the processes in the ion-beam drift space on beam current measurements

A method is proposed for separate measurements of the current produced by slow charge-exchange ions and that produced by the ions generated due to gas ionization by the beam ions and fast secondary electrons in the beam drift space. The method is based on an analysis of the current distribution over the electrodes of a modified Faraday cup with nonequipotential electrodes and allows one to determine the coefficient of ion-induced electron emission from the ion collector and the charge-exchange cross section of the accelerated ions. The method has been employed to measure the current of an argon ion beam with an ion energy from a few electronvolts to several tens of kiloelectronvolts and to study the processes in the beam drift space at pressures of 0.03–0.15 Pa.     

Surface analysis of insulating materials by Secondary Ion Mass Spectrometry (SIMS)

For a non-conducting solid sample (TiO₂ powder) the conditions for compensation of the charge build-up at the surface caused by the ion impact in SIMS are experimentally investigated. The compensation is achieved by an additional auxiliary electron beam of low energy. The resolution and the intensities of the secondary ions were measured as a function of the ratio of the current densities of the electron and the ion beams. The compensation for negative secondary ions, and especially for those with higher masses, is more critical than for positive ones. The intensities are influenced by the different values of the mean emission energies and the form of the energy distributions. Examples of mass spectra by SIMS for some insulators are given.     

Schottky mass spectrometry at the heavy ion storage ring ESR

Schottky mass spectrometry is a novel method of precision nuclear mass spectrometry based on the measurement of the revolution frequencies of cooled ions in storage rings performed by non-destructive frequency analysis of the beam noise, the well-established Schottky diagnosis technique. The method was applied for the first time at the Experimental Storage Ring ESR at GSI observing electron cooled highly charged ions up to bare nuclei at relativistic energies around several hundred MeV/u. To demonstrate the performance and feasibility of the method at the ESR, experimental tests have been carried out using beams of nuclear fragments produced in the ring itself by the interaction of different primary beams with the internal gas jet target. Futhermore, first Schottky mass measurements of secondary nuclear beams produced by projectile fragmentation of Au and Bi primary beams in a thick Be-target were carried out in order to determine the masses for numerous heavy neutron deficient nuclei which had not been measured before. Relative accuracies for the measured mass values in the order of 1×10^–6 and below can be achieved. The method is briefly discussed and some early results are presented.