Energy Spread Of Ions Research Articles

Influence of the rf excitation on the low energy broad ion beam characteristics

Influence of the rf excitation in a capacitively coupled filamentless broad ion beam source with radial electrode configuration on the ion beam properties has been examined. Ion energy distribution was measured by use of a retarding field energy analyzer connected to a computer controlled data acquisition system. Influence of working conditions on the average ion energy and on the ion energy spread in the beam is described. Energy spread has typical values between 8 and 25 eV. Average ion energy can change due to variations of ion source working conditions (at constant extraction voltages) up to 60 eV. Increase of both the average ion energy and ion energy spread is observed with increasing rf power coupled to the plasma (20–100 W). This correlates to the increasing amplitude of plasma floating potential oscillation. Average ion energy decreases with increasing magnetic induction (0–75 mT) and with increasing pressure in the vacuum chamber (2×10−4–1.8×10−3 mbar) due to decreasing cathode oscillation amplitudes. Average ion energy decreases with increasing distance from the broad beam axis because of chromatical and spherical aberrations of the source extraction optics. Multiple peaks in the ion energy spectrum are observed. Their occurrence can be explained by charge exchange effects in the rf oscillating plasma sheath.

Influence of the rf excitation on the low energy broad ion beam characteristics (abstract)a)

Influence of the rf excitation in a capacitively coupled filamentless broad ion beam source with radial electrode configuration on the ion beam properties has been examined. Ion energy distribution was measured by use of a retarding field energy analyzer connected to a computer controlled data acquisition system. Influence of working conditions on the average ion energy and on the ion energy spread in the beam is described. Energy spread has typical values between 8 and 25 eV. Average ion energy can change due to variations of ion source working conditions (at constant extraction voltages) up to 60 eV. Increase of both the average ion energy and ion energy spread is observed with increasing rf power coupled to the plasma (20–100 W). This correlates to the increasing amplitude of plasma floating potential oscillation. Average ion energy decreases with increasing magnetic induction (0–75 mT) and with increasing pressure in the vacuum chamber (2×10−4–1.8×10−3 mbar) due to decreasing cathode oscillation amplitudes. Average ion energy decreases with increasing distance from the broad beam axis because of chromatical and spherical aberrations of the source extraction optics. Multiple peaks in the ion energy spectrum are observed. Their occurrence can be explained by charge exchange effects in the rf oscillating plasma sheath.

Surface-induced dissociation of protonated peptides: implications of initial kinetic energy spread.

Surface-induced dissociation (SID) has been used to produce daughter ion spectra of small protonated peptides generated by fast atom bombardment (FAB). The relative abundances of daughter ions depends critically upon the energy of the ion/surface collision. A wide array of decomposition processes may be observed using ELAB collision energies in the range 10-20 eV. At approximately 13-eV collision energy, the variety of decomposition processes is maximized for the small peptides studied; hence, maximum structural information may be deduced. Collisionally-activated dissociations (CAD) using argon gas and the identical protonated peptides could not produce as large an array of daughter ions in a constant condition experiment. An apparent contradiction is thereby posed because SID is known to produce a narrow distribution of ion internal energies relative to CAD. This apparent contradiction is resolved by considering the rather large kinetic energy spread of ions leaving the FAB source. For the SID process, this large initial kinetic energy distribution is converted into a significantly wider spread in center-of-mass collision energy, leading to a broader variety of decomposition processes (high and low energy) compared to CAD.

Measurement of energy spread and angular intensity of a cesium liquid metal ion source

This paper determines the optimum range of total ion current for a cesium liquid metal ion source for use as a focused ion beam (FIB). This range is determined from a figure of merit calculated from measurements of the angular intensity and energy spread of emitted ions. Judging from both the figure of merit and the tail of the energy distribution curves, the total emission current should be set near 1 μA for a cesium FIB with a high current density. Assuming that both Cs- and Ga-FIBs have the same diameter, the relative current density of a Cs-FIB is expected to be approximately 80% that of a Ga-FIB.

Aluminum cluster ions generated by a liquid metal ion source

Aluminum cluster ions Al+n are generated by a liquid metal ion source and observed up to n=10 using a magnetic mass analyzer. Relative integrated intensities of cluster ions as a function of n show a distinct step between n=7 and 8, which is explained by the electronic shell model. Cluster ions have lower kinetic energy compared with monomer ions which are accelerated to the full energy. Moreover, energy spread of cluster ions is larger than that of monomer ions. These results are attributed to the difference of ion formation mechanism. In addition to the fully accelerated monomer ions, at a high ion current region there also appear monomer ions with energy deficit which are formed in the same process as cluster ions.

Ion kinetics in low-pressure, electropositive, RF glow discharge sheaths

Ion kinetics in low-pressure (e.g., 1 mtorr), electropositive, RF glow discharge sheaths are studied using a Monte-Carlo-based computer simulation. The numerical model integrates particle trajectories using a spatially nonlinear, time-varying model of the electric field in the RF sheath. A scaling relationship is then discussed, relating the normalized ion energy spread to the ratio of ion sheath transit time to the RF period. The scaled numerical data shows good agreement with existing numerical and experimental data.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Achromatic quadrupole focusing systems for use with liquid metal ion sources

We discuss the use of achromatic quadrupole lenses to overcome the limitation due to the energy spread of ions from liquid metal ion sources as they are currently focused with axially symmetric electrostatic lenses. Achromatic quadrupoles require both magnetic and electrostatic focusing elements. Each lens acts to focus in one plane and defocus in the other so that multiplet focusing systems are required to form an image of the source. Lens acceptance angles are limited by the bore of the electrostatic elements which in turn is limited by the necessary magnetic gradients required of the magnetic elements. The coefficients relating image distance to lens excitation of some of the elements of the quadrupole multiplets are found to be much larger than those in an axially symmetric electrostatic system. These factors are discussed in terms of the stability requirements of lens power supplies for a particular achromatic triplet system which is undergoing initial test.

Droplet and Cluster Ion Emission from Ga and In Liquid Metal Ion Sources

Although several authors have used the emission of droplets from liquid metal ion sources (LMIS) to explain other phenomena such as cluster ion emission, no model has so far been developed to describe the formation of these droplets. In particular, knowledge of the initial charges and sizes of the droplets is needed in order to predict cluster ion voltage deficits. The present paper sets out a simple model whereby droplets are considered to form from the break-off of the tip sphere from the LMIS apical jet. It is shown that this simple description can be used to derive qualitative variations of LMIS mass loss, Ga and In dimer ion voltage deficits and energy spreads (absolute agreement within a factor of about 2 is found) with current.

High-field ion sources

High electric fields may be used to create ions either by field evaporation, as in the case of liquid metal ion sources (LMISs) or by field ionization, as in the case of gas-phase field ionization sources (GFISs). LMISs have now reached a mature state of development and have found an increasing number of focused beam applications, ranging from submicron mask repair to maskless ion implantation. They may be fabricated from either pure elements, or from alloys, provided that the element or alloy vapor pressure is low enough to avoid excessive evaporation. In the case of alloys, it is necessary to employ a mass filter, typically a Wien filter, to remove unwanted ion species from the ion beam. Angular intensities, at the threshold of source operation, range from 1 μA/cm2 for alloy LMIS ion species to 20 μA/cm2 for elemental LMISs; under the same operating conditions, the ion energy spreads are ∼5 eV. Recent advances in the development of GFISs raise the possibility that GFISs may be nearing the stage when they also might be used in fine focus applications. By special shaping of the metal emitter in the apex region and by cooling the emitter to ∼20 K; it has been found possible to cause confinement of the ion emission to ∼0.5° half-angle, thereby increasing angular intensities of beams of He+ or H+2 to values similar to those obtained from LMISs.

Single stage ECR ion source for the mass separation of xenon radioisotopes

The main features of ECR ion sources (high efficiency, almost unlimited lifetime, stable running conditions, low ion energy spread) make them, when optimized for a high yield at charge state 1+, ideally suited for on-line mass separation. For this reason a single stage 6.4 GHz test source was built for the Paul Scherrer Institute medical radioisotope production facility. Source geometry and operation conditions were optimized for the mass separation of xenon isotopes. Different support gases were tested and an ionization efficiency of 60% for Xe+ was achieved with H2 as support gas. The best results were found at a source pressure of 5×10−4 mbar and at a microwave power level of about 100 W. At an axial magnetic field of about 3 kG the mirror ratio showed no strong effect on the general performance of the source. Our tests showed the advantages of the ECR source type for the mass separation of gaseous isotopes in terms of efficiency and reliability.

Sequential SIMS-ISS measurements using a double focusing mass spectrometer

A new technique for sequential SIMS-ISS measurements was developed using the double focusing mass spectrometer of a SIMS instrument operated with a B 2 / E linked scanning method and a modulation technique. Formulations that give the scattered ion energy and the mass resolution were also derived in the biased sample condition. The energy resolution of this system was assessed at nearly 40 with an ISS spectrum obtained from a GaAs monocrystal surface.

Energy spread of ions from Ga liquid metal ion sources at low emission currents

Measurements of the energy distribution of Ga liquid metal ion sources (LMIS) have been carried out using different emitters and tip currents between 0.5 and 3 μA. For LMIS operating at low emission voltages and with low tip currents, distributions as narrow as 2.3 eV full width at half-maximum have been observed. These narrow distributions can only be observed if the emitter tip is placed very close (0.5 mm) to the extraction electrode.

The ORNL ECR multicharged ion source

A multicharged ion source based on electron cyclotron resonance (ECR) heating has been designed and built at ORNL. The ECR ion source, which is completely dedicated for atomic physics collisions studies, produces higher charge states and higher beam intensities than the present ORNL PIG multicharged ion source, and will thus permit study of collision processes involving ions of higher charge states in experiments requiring higher beam intensities than could be previously obtained in our laboratory. The source has already produced up to fully stripped C and O beams, as well as up to He-like Ar beams. Measurements of the energy spread of ions extracted from the ion source operating in both single-stage and two-stage mode are described. In addition, initial results of total cross section measurements for fully stripped light ions incident on atomic hydrogen in the energy range 0.2–10 keV are presented.

A theoretical model of a liquid metal ion source

We have developed a model of liquid metal ion source (LMIS) operation which gives a consistent picture of three different aspects of LMI sources: (i) the shape and size of the ion emitting region; (ii) the mechanism of ion formation; (iii) properties of the ion beam such as angular intensity and energy spread. We find that the emitting region takes the shape of a jet-like protrusion on the end of a Taylor cone with ion emission from an area only a few tens of Å across, in agreement with recent TEM pictures by Sudraud. This is consistent with ion formation predominantly by field evaporation. Calculated angular intensities and current-voltage characteristics based on our fluid dynamic jet-like protrusion model agree well with experiment. The formation of doubly charged ions is attributed to post-ionization of field evaporated singly charged ions and we calculate an apex field strength of about 2.0 V Å −1 for a Ga source. The ion energy spread is mainly due to space charge effects, it is known to be reduced for doubly charged ions in agreement with this post-ionization mechanism.

Laser desorption mass spectrometry: Mechanisms and applications

A conventional time-of-flight mass spectrometer has been adapted for pulsed laser desorption. This instrumental configuration enables investigation of “time resolved” mass spectra, i.e. delayed mass analysis following the laser pulse. The energy spread of ions formed after the laser pulse is time dependent. To date this has enabled focussing of ions produced by laser desorption up to 3628 amu. Mechanisms of laser desorption and applications of this instrumental configuration are discussed.

The hydrodynamics of liquid metal ion sources

Factors affecting the current-voltage characteristics of liquid metal ion sources are reviewed. The results demonstrate that ion emission is not controlled by space-charge effects but rather is determined by liquid flow limitations along the needle emitter. A hydrodynamic model is presented which predicts the experimental I-V characteristics. The model suggests that a reduction in the energy spread (Boersch Effect) of the emitted ions may be achieved by increasing the needle flow impedance.

Threshold sputtering effects in a Cu–Au alloy

Threshold sputtering effects in a dilute copper–gold alloy resulting from low‐energy, light ion bombardment have been studied using ion scattering spectroscopy. Preferential removal of Cu occurs at 25 °C when the alloy is bombarded at normal incidence with 150 eV 3He+. The development of a Au enriched surface layer with increasing ion fluence was followed by monitoring the scattered ion energy spectrum during bombardment. The data agree with a model for preferential sputtering in which a Au enriched layer forms with a thickness that is comparable to the range of 150 eV 3He+ in Cu. It therefore appears that preferential sputtering at the alloy surface is accompanied by atomic mixing throughout the volume penetrated by the incident ions.

Photodissociation of doubly-charged ions

Photodissociation of doubly-charged ion has been induced by radiation from an argonion laser and investigated using a sensitive double-focusing mass spectrometer of reversed geometry. Of the molecular ions studied, only those of benzene and benzonitrile were found to photodissociate. The kinetic energy release distribution f( T) for both ions was found to be independent of photoexcitation energy in the range 2.41–3.47 eV and with the same respective values of f( T) as for unimolecular dissociation. These results confirm that dissociation of doubly-charged ions is a process involving a predissociation mechanism with the major part of T being due to Coulombic repulsion of the charges on the fragment ions. However, the degree of dishing, which depends on several instrumental factors as well as on the translational energy spread of the fragment ions, was more marked in the case of photodissociation than for unimolecular dissociation. This is probably due to the good collimation of the laser beam and the fact that photodissociation occurs only for ions photoexcited in the volume of this beam. Thus, there is an effective pre-collimation of the photoexcited ions.

Real energy spread of ions produced in vacuum spark-discharge plasma

The initial energy spread of ions formed in vacuum spark-discharge plasma is studied, and is found to be ∼100 eV (at 50% of maximum intensity). Of three possible mechanisms for ion acceleration, the hydrodynamic mechanism and the action of the self-consistent electric field have been shown to be the basic mechanisms. The mechanism of ion acceleration in the inter-electrode field does not play an important role. The energy spread of ions can increase considerably during the formation of an ion beam, and this is usually the case in double-focusing mass spectrometers.

Fast high-resolution ion energy and momentum spectrometer

A novel energy/momentum spectrometer consisting of a multigrid ion energy analyzer in tandem with an inflexion focusing magnetic sector momentum analyzer is described. The entire analyzer ion flight path may be negatively biased relative to the outer (grounded) instrument housing, enabling post acceleration (up to a few keV) of grid extracted ions from laser blow-off plasmas or laser-initiated vacuum arcs. Used as a time-of-flight momentum spectrometer, the instrument is shown capable of unraveling velocity distributions and relative abundances of ions of different charge in expanding laser blow-off plasmas, even for a few keV initial ion energy spread. Used as a discharge ion analyzer, the high mass resolution of the instrument, (m/Δm=110), enables ion isotopic species of carbon, iron and copper to be readily separated and energy analyzed. A channeltron collector provides wide dynamic range (∼105) and high time resolution (∼1.0 μs), enabling detection of low abundance species such as polymer ions in carbon discharges.