Energy Distributions Of Negative Ions Research Articles

High-resolution grid-less retarding potential analyser and its application for sputter negative ion source

An advanced, grid-less, retarding field analyzer (RFA) operating in energy distribution and absolute energy determination modes was developed Two different experimental geometries were used in order to verify the SIMION calculated resolution of the RFA. In energy distribution operation mode, an acceptable resolution of RFA σR of a few eV was achieved by selecting the appropriate geometry responsible for the topography of the retarding field. The device was used to measure the absolute energies and energy distributions of negative ions: H−, Fe−, Ni− and Cu− sputtered from solid cathode surfaces by a 4.2–4.5 keV Cs+ beam employing a modified Source of Negative Ions by Caesium Sputtering (SNICS). All spectra were similar to that of sputtered neutrals with a FWHM of <10 eV and a high energy tail. Depending on the value of surface binding energy, ESBE, the energy of negative beams surpassed or fell behind eU cath by a fraction of eV. After considering ESBE and the work functions of emitting and retarding surfaces, the preliminary conclusion was that the negative ions under investigation were created near the cathode surface.

Phenyl Sulfate Derivatives: New Thermometer Ions for Characterization of Internal Energy of Negative Ions Produced by Electrospray Ionization.

Although positive thermometer ions are widely used for evaluating the internal energy distribution of gas-phase ions, negative thermometer ions have not yet been proposed. In this study, phenyl sulfate derivatives were tested as thermometer ions to characterize the internal energy distribution of ions produced by electrospray ionization (ESI) in the negative mode because the activation of phenyl sulfate preferentially undergoes SO3 loss, providing a phenolate anion. The dissociation threshold energies for the phenyl sulfate derivatives were determined using quantum chemistry calculations at the CCSD(T)/6-311++G(2df,p)//M06-2X-D3/6-311++G(d,p) level of theory. The values for the appearance energies of the fragment ions of the phenyl sulfate derivatives depend on the dissociation time scale in the experiment; therefore, the dissociation rate constants of the corresponding ions were estimated using the Rice-Ramsperger-Kassel-Marcus theory. The phenyl sulfate derivatives were used as thermometer ions to determine the internal energy distribution of negative ions activated by the in-source collision-induced dissociation (CID) and higher-energy collisional dissociation. Both mean and full width at half-maximum values increased with increasing ion collision energy. In the in-source CID experiments, the internal energy distributions obtained by phenyl sulfate derivatives are similar to that when all voltages are mirrored, and the traditional benzylpyridinium thermometer ions are used. The reported method will aid in determining the optimum voltage for ESI mass spectrometry and the subsequent tandem mass spectrometry of acidic analyte molecules.

Investigation of Negative Ion Energy Distribution and Extraction Mechanism With a Compact Retarding Field Energy Analyzer in a Large Filament-Arc Source for Neutral Beam Injectors

In large beam sources for neutral beam injectors (NBIs) operating at high energies, negative hydrogen ions (NI) are extracted from a caesiated plasma discharge. H− ions are generated from volume and surface processes; surface production is enhanced by cesium deposition on the source walls and in particular on the plasma-facing electrode of the accelerator. The mechanisms of negative ion production and of extraction in the magnetic field present at the extraction apertures influence the formation of the single beamlets and their optics. A newly developed compact retarding field energy analyzer (CRFEA) was constructed and installed at the upstream surface of the plasma grid (PG) of a filament-arc source, the research negative ion source (RNIS) at the National Institute of Fusion Science. It was used to characterize the negative ions in the plasma source with and without cesium. The first electrode of the analyzer has a conical shape, resembling the PG apertures from which the beam is extracted. The measured saturation current was analyzed and correlated with the source parameters and the current of the negative-ion beam. The energy distribution of collected ions depends on the origin of the negative ions, and on possible energy exchange processes; but it is found to be also strongly influenced by the curvature of the meniscus (the plasma edge where the beam is formed). This compact retarding field analyzer proved to be effective for the study of the formation of a negative ion beam. The limits of the present design, and possible improvements are discussed, in particular to minimize the dependence of the transmission on perveance and extend the dynamic range of the diagnostic.

Negative-ion bombardment increases during low-pressure sputtering deposition and their effects on the crystallinities and piezoelectric properties of scandium aluminum nitride films

Scandium aluminum nitride (ScAlN) films are being actively researched to explore their potential for use in bulk acoustic wave and surface acoustic wave resonators because of their good piezoelectric properties. Sputtering is commonly used in ScAlN film deposition. Unfortunately, it has been reported that film quality metrics such as the crystallinity and piezoelectric properties can deteriorate before the Sc concentration reaches 43% without an isostructural phase transition. One reason for this is bombardment with negative ions generated from carbon and oxygen impurities in the Sc ingots. Because the number of negative ions increases during low-pressure sputtering deposition, their effect on film quality may be considerable. In this study, we investigated negative-ion bombardment of the substrate during sputtering deposition and its effects on ScAlN crystallinity and piezoelectric properties. Negative-ion energy distribution measurements indicated that many more negative ions collide with the substrate during ScAlN film deposition than during AlN deposition. In addition, decreasing the sputtering pressure further increased the number of negative ions and their energies. It is well known that film quality improves at low pressures because increasing the mean free path reduces thermalization and scattering of sputtered particles. Although, AlN crystallinity and piezoelectric properties improved at low pressures, the properties of ScAlN films deteriorated dramatically. Therefore, the results indicated that ion bombardment increase at low pressure adversely effects ScAlN crystal growth, deteriorating crystallinity and piezoelectric properties. ScAlN films may be improved further by suppressing negative-ion bombardment of the substrate.

Enhancing surface production of negative ions using nitrogen doped diamond in a deuterium plasma

The production of negative ions is of significant interest for applications including mass spectrometry, particle acceleration, material surface processing, and neutral beam injection for magnetic confinement fusion. Methods to improve the efficiency of the surface production of negative ions, without the use of low work function metals, are of interest for mitigating the complex engineering challenges these materials introduce. In this study we investigate the production of negative ions by doping diamond with nitrogen. Negatively biased (−20 V or −130 V), nitrogen doped micro-crystalline diamond films are introduced to a low pressure deuterium plasma (helicon source operated in capacitive mode, 2 Pa, 26 W) and negative ion energy distribution functions are measured via mass spectrometry with respect to the surface temperature (30 °C to 750 °C) and dopant concentration. The results suggest that nitrogen doping has little influence on the yield when the sample is biased at −130 V, but when a relatively small bias voltage of −20 V is applied the yield is increased by a factor of 2 above that of un-doped diamond when its temperature reaches 550 °C. The doping of diamond with nitrogen is a new method for controlling the surface production of negative ions, which continues to be of significant interest for a wide variety of practical applications.

PIC simulations of capacitively coupled oxygen rf discharges

Capacitively coupled discharges with a radio-frequency operated voltage (ccrf) are important for plasma assisted material processing. Experiments with electronegative oxygen ccrf discharges show a high-energy peak in the energy distribution of negative ions arriving at the anode, depending on the cathode material used. One possible explanation is ionization at or close to the surface of the cathode for the production of negative ions. By introducing an additional surface ionization model into a Particle-In-Cell (PIC) simulation with Monte Carlo Collisions (MCC) the experimental result is reproduced qualitatively. Comparison of one dimensional and two dimensional simulation results allows an improved understanding of the microscopic processes determining the dynamics of negative ions.

Reconstruction of energy and angle distribution function of surface-emitted negative ions in hydrogen plasmas using mass spectrometry

A new method involving mass spectrometry and modeling is described in this work, which may highlight the production mechanisms of negative ions (NIs) on surface in low pressure plasmas. Positive hydrogen ions from plasma impact a sample which is biased negatively with respect to the plasma potential. NIs are produced on the surface through the ionization of sputtered and backscattered particles and detected according to their energy and mass by a mass spectrometer (MS) placed in front of the sample. The shape of the measured negative-ion energy distribution function (NIEDF) strongly differs from the NIEDF of the ions emitted by the sample because of the limited acceptance angle of the MS. The reconstruction method proposed here allows to compute the distribution function in energy and angle (NIEADF) of the NIs emitted by the sample based on the NIEDF measurements at different tilt angles of the sample. The reconstruction algorithm does not depend on the NI surface production mechanism, so it can be applied to any type of surface and/or NI. The NIEADFs for highly oriented pyrolitic graphite (HOPG) and gadolinium (low work-function metal) are presented and compared with the SRIM modeling. HOPG and Gd show comparable integrated NI yields, however the key differences in mechanisms of NI production can be identified. While for Gd the major process is backscattering of ions with the peak of NIEDF at 36 eV, in case of HOPG the sputtering contribution due to adsorbed H on the surface is also important and the NIEDF peak is found at 5 eV.

Negative-ion surface production in hydrogen plasmas: Determination of the negative-ion energy and angle distribution function using mass spectrometry

This work focuses on the understanding of the production mechanism of negative-ions on surface in low pressure plasmas of H2/D2. The negative ions are produced on a Highly Oriented Pyrolytic Graphite sample negatively biased with respect to plasma potential. The negative ions created under the positive ion bombardment are accelerated towards the plasma, self-extracted, and detected according to their energy and mass by a mass spectrometer placed in front of the sample. The shape of the measured Negative-Ion Energy Distribution Function (NIEDF) strongly differs from the NIEDF of the ions emitted by the sample because of the limited acceptance angle of the mass spectrometer. To get information on the production mechanisms, we propose a method to obtain the distribution functions in energy and angle (NIEADFs) of the negative-ions emitted by the sample. It is based on an a priori determination of the NIEADF and on an a posteriori validation of the choice by comparison of the modelled and experimental NIEDFs.

Comparison of ion energies and fluxes at the substrate during magnetron sputtering of ZnO : Al for dc and rf discharges

Energy spectra of positive and negative ions have been measured in magnetron sputtering of a ZnO : Al target. The discharges have been operated in dc or rf, the latter with a frequency of 13.56 or 27.12 MHz, at the same geometry, argon pressure, and discharge power. While the average energy of the positive ions against a grounded surface increases with increasing frequency due to increased plasma potential, the average energy of the negative ions strongly decreases due to a target voltage decrease. The flux of negative ions simultaneously decreases, whereas that of the positive ions slightly increases. Combining these quantities to the energy flux onto floating substrates shows a drastic decrease of the energy flux with increasing frequency for high-energetic negative ion bombardment of the films while the low-energetic positive ion impingement is weakly increased. From the point of the energetic bombardment rf discharges are hence favoured to obtain high-quality films. The energy distributions of negative ions have a sharp peak in the dc mode but broad distributions in the rf discharges. Additionally, the latter are characterized by an overlaid peak structure that is explained by the oscillation of target and plasma potential.

Negative-ion surface production in hydrogen plasmas: modeling of negative-ion energy distribution functions and comparison with experiments

A negatively biased graphite sample (highly oriented pyrolitic graphite) was placed in a H2 low-pressure plasma. The negative ions were produced on the graphite surface upon positive-ion bombardment. Surface-produced H− negative-ion distribution functions (NIDFs) were measured by means of an energy-resolved mass spectrometer. The shapes of the recorded NIDFs depend not only on the surface production mechanisms but also on the negative-ion trajectories in the plasma and their collection probability by the mass spectrometer. In order to gain an insight into the surface production mechanisms, NIDFs were computed using Stopping and Range of Ions in Matter simulations and calculations of the ion transmission function between the sample and the mass spectrometer detector. The calculations were based on 3D modeling of the sheath potential in the extraction region and 3D modeling of ion transport inside the mass spectrometer. The excellent agreement between experiments and computations led to a better understanding of the experimental NIDFs. The method developed in this work to study H-surface production on graphite can be generalized to any other negative ions and/or surface material.

Scattering of neutral hydrogen at energies less than 1 keV from tungsten and diamondlike carbon surfaces

Neutral atom to negative ion conversion efficiencies were studied for polished tungsten and diamondlike carbon surfaces using a beam of incident hydrogen atoms. Neutral atoms at energies below 1 keV were scattered from the surfaces using incident angles between 6° and 20° measured from the surface plane. The angular and energy distributions of negative ions backscattering from the surfaces were measured and used to calculate the fraction of the incident beam that was converted to negative ions. The highest number and the lowest overall energy loss of backscattered ions were both observed near the specular reflection angle of the incident beam for the two surface materials studied. The total conversion efficiency was calculated to be near 2% for the tungsten and diamondlike carbon surfaces. Measurements taken while the surfaces were heated show a significant reduction in conversion efficiency, which is credited to the removal of adsorbates from the top layers of the surfaces.

Probe Measurements in Electronegative Plasmas: Modeling the Perturbative Effects of the Probe‐Holder

AbstractA basic property of an electronegative plasma is its separation into two distinct regions: an ion‐ion region far from boundaries, where the densities of positive and negative ions are higher then electron density, and a near‐boundary electron‐ion region, where negative ions have practically negligible density. This is due to the influence of the ambipolar electric field, which depends on electron (not negative ion) plasma parameters. This electric field “holds off” negative ions from the boundary, as the ions have lower mobility and temperature compared to the electrons. Therefore, negative ions will be repelled by any object inserted into the plasma. This can lead to errors in measurements of negative ion and electron parameters by any invasive method. Numerical modeling of electric probes in an argon‐oxygen plasma clearly demonstrates possible errors of direct measurements of negative ion probe current. This can also affect results from the photo‐detachment method and direct measurements of negative ion energy distribution (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Effects of cross field diffusion in a low pressure high density oxygen/silane plasma

A low pressure high density oxygen/silane radio frequency (13.56 MHz) plasma coupled in a helicon reactor used for silicon dioxide deposition is characterized by using an energy selective mass spectrometer situated at the wall of the processing chamber: measurements of positive and negative ion energy distribution functions and mass spectra (1⩽amu⩽150) are obtained for various flow-rate ratios (R=O2/SiH4=1 to 10 but constant total flow rate of 30 sccm), and for a constant radio frequency power and magnetic field of 800 W and 70 G, respectively. Plasma potentials between 35 (R=10) and 60 V (R=1) are measured depending on the silane and oxygen flows showing charging of the silica-covered diffusion chamber wall. The magnitude of the wall charging most likely depends on the effective capacitance formed by the silica layer (which results from months of deposition) and on the inbalance between the positively and negatively charged particles which impinge onto the sidewalls at the discharge initiation until equilibrium of fluxes is reached. This inbalance is a result of the magnetic field configuration generated by the four coils surrounding the reactor and of the subsequent cross-field diffusion of the positively and negatively charged particles to the sidewalls. Maximum wall charging is observed when the silane flow is maximum (R&amp;lt;2), a situation where polymerization is observed in the mass spectrum of the positive ions, and where a minimum density of negative ions (O−, OH−, and H−) are detected close to the walls. Although the polymerization appeared to be a primary candidate for the increase of the plasma potential to ∼60 V, its presence does not significantly change the total positive ion density at equilibrium but is accompanied by a dramatic decrease (two orders of magnitude for the O− ions) in the negative ion density close to the walls when R is increased from 1 to 10. The change in the degree of electronegativity close to the walls affects the global plasma equilibrium and appears as an indirect factor in the magnitude of the wall charging.

Sputter process diagnostics by negative ions

We measured the energy distributions of negative ions during reactive sputtering of silicon in oxygen. Various oxygen containing negative ions are formed in the cathode sheath or directly at the sputter target, respectively. These negative ions are accelerated away from the cathode by the electrical field, and can be detected using a mass spectrometer facing the sputter magnetron. The origin of each ion can be determined from peak structures in the energy distribution. Additionally the flux of different negative ions provides information on poisoning of the target by oxide films.

High energy negative ions in a radio-frequency discharge

We measured energy distributions of negative ions at the grounded electrode of an oxygen parallel-plate rf discharge. Negative ions are generated in the plasma sheath in front of the rf electrode, are accelerated away from the driven electrode, and can be detected at the grounded electrode. The maximum energy of the negative ions corresponds to the negative self-bias voltage of the rf electrode. Structures in the energy distribution reflect sheath properties and characteristics of the ion generation processes.

The sputter-generation of negative ion beams: Analytical modeling

The secondary negative ion generation process is examined through the use of energy-dependent analytical models. Nonlinear least-squares fit parameters for the models are determined from fits to the experimental data of Yu. These models are used to illustrate the effect of the work function on the energy distributions of negative ions sputter-ejected from a polycrystalline molybdenum surface covered with fractional layers of cesium. Predictions are also made of the functional dependence of the probability for negative ion formation on cesium coverage.

Background reduction for heavy element accelerator mass spectrometry

The energy distribution of negative ions generated by low energy (keV) Cs + bombardment of solids has been studied over 11 decades of negative ion intensity. The higher energy component of this ion spectrum is characterized approximately by an E −2 distribution, terminated by the kinematic limit for two-body collisions. In the case of 27 keV Cs bombardment on carbon an excess energy of 6 keV beyond the mean emission energy was observed. A more complex, low energy ion spectrum was attributed to molecular fragmentation processes within the ion extraction field and subsequent beam transport system. The elimination of these tails and other ions prior to tandem acceleration, and hence the removal of potential sources of background in accelerator mass spectrometry (AMS) are discussed in this paper, with a view to extending the detection limits in AMS for elements other than 14C and especially for heavy elements.

Energy distribution of negative ions sputtered from caesiated surfaces

A retarding field energy analyzer has been used to measure the energy distribution of negative ions, mainly 12C −, sputtered from solid surfaces by a 20-keV Cs + beam. Two types of spectra have been observed: the first is similar to that of sputtered neutrals, with a FWHM of ca. 10 eV and a high-energy tail that decays according to d N/d E ∝ E − n with n ⋍ 2; the second type is considerably broader (ca. 70 eV), rather flat and sometimes exhibits a double peak.

Energy distribution of negative ions in hydrogen and argon positive ion bombardment of aluminium surface: J Stockel and K Jakubka, Proc 5th Czech Conf Electron Vacuum Phys, Czech Acad Sci, Brno, 1972, I a-18

Energy distribution of negative ions in hydrogen and argon positive ion bombardment of aluminium surface: J Stockel and K Jakubka, Proc 5th Czech Conf Electron Vacuum Phys, Czech Acad Sci, Brno, 1972, I a-18

Kinetic-Energy Distribution of Negative Ions Formed by Dissociative Attachment and the Measurement of the Electron Affinity of Oxygen

The kinetic-energy distribution of ions produced by a dissociative ionization process is derived, taking into account the effect of thermal motion of the target molecule. In the case of dissociative attachment of monoenergetic electrons to a diatomic molecule, the width at half-maximum of the negative-ion energy distribution is given by ${(11\ensuremath{\beta}\mathrm{kT}{E}_{0})}^{\frac{1}{2}}$, where $\ensuremath{\beta}$ is the ratio of the mass of the ion to that of the parent molecule, $T$ is the target-gas temperature, and ${E}_{0}$ is the most probable ion energy. Using a crossed-field velocity filter, ${\mathrm{O}}^{\ensuremath{-}}$ ion energy distributions arising from the attachment of essentially monoenergetic electrons to ${\mathrm{O}}_{2}$ are studied as a function of electron energy at two gas temperatures. The measured widths of the distributions are consistent with the above relationship. Measurements of ${E}_{0}$ as a function of the electron energy allow a determination of the electron affinity $A$ of atomic oxygen. The result, $A=1.5\ifmmode\pm\else\textpm\fi{}0.1$ eV, is in excellent agreement with photodetachment-threshold determinations.