Energy Distributions Of Positive Ions Research Articles

Influence of positive ions on the beamlet optics for negative-ion neutral beam injectors

Neutral beam injectors are based on the neutralization of ion beams accelerated at the desired energy. In the case of the ITER heating and diagnostic neutral beams, the target heating power translates into stringent requirements on the acceptable beamlet divergence and aiming to allow the beam to reach the fusion plasma. The beamlets composing the accelerated beam are experimentally found to feature a transverse velocity distribution exhibiting two Gaussian components: the well-focused one is referred to as the core component while the rest of the beam, the halo, describes beam particles with much worse optics. The codes that simulate beam extraction and acceleration usually assume that the negative ions move towards the plasma meniscus with a laminar flow (no transverse velocity) or that the transverse velocity distribution can be modelled as a Maxwellian and that the current density is uniformly illuminating the meniscus; under such approximations, the presence of highly divergent components cannot be explained. In this work, we develop a simple test-particle tracing code with Monte Carlo collisions, named ICARO (for Ions Coming Around), to study the transport of negative ions in the extraction region and derive the spatial and velocity distribution of the negative ions at the meniscus (i.e. the plasma boundary where a beamlet is extracted). In particular, the origin of the beamlet halo and its dependence on the source parameters are discussed, highlighting as a key parameter the energy distribution of positive ions in the source plasma.

Mass Spectrometry of Atmospheric Pressure Surface Wave Discharges

By applying mass spectrometry techniques, we carried out measurements of ionic mass spectrum and their energy distribution in order to investigate an atmospheric argon discharge by using a surfatron surface-wave device. The mass and energy distribution measurements were performed with fixed flow rate (2.5 SLM) of pure argon gas (99.999%) and different Ar-O2 gas mixture compositions (99-1, 98-2 and 97-3). The mass spectra and energy distributions were recorded for Ar+, O+, O+2, N+ and N2+. The axial distribution profiles of ionic mass and their energy were obtained for different experimental conditions as a function of the plasma length. The results showed that the peak of the positive ion energy distributions shifted to higher energies and also that the distribution width increased as the distance between the sampling orifice and the launcher gap was increased. It was also found that under certain experimental conditions the ion flux of atomic species were higher than the ion flux of their diatomic counterpart. The motivation of this study was to obtain a better understanding of a surface wave discharge in atmospheric pressure that may play a key role on new second generation biofuel technologies.

Langmuir probe measurements and mass spectrometry of plasma plumes generated by laser ablation of La0.4Ca0.6MnO3

The plasma formed in vacuum by UV nanosecond laser ablation of La0.4Ca0.6MnO3 in the fluence range of 0.8 to 1.9 J cm−2 using both Langmuir probe analysis and energy-resolved mass spectrometry has been studied. Mass spectrometry shows that the main positive ion species are Ca+, Mn+, La+, and LaO+. The Ca+ and Mn+ energy distributions are quite broad and lie in the 0–100 eV region, with the average energies increasing with laser fluence. In contrast, the La+ and LaO+ distributions are strongly peaked around 10 eV. The net time-of-arrival signal derived from the measured positive ion energy distributions is broadly consistent with the positive ion signal measured by the Langmuir probe. We also detected a significant number of O− ions with energies in the range of 0 to 10 eV. The Langmuir probe was also used to measure the temporal variation of the electron density and temperature at 6 cm from the ablation target. In the period when O− ions are found at this position, the plasma conditions are consistent with those required for significant negative oxygen ion formation, as revealed by studies on radio frequency excited oxygen plasma.

Ion distribution measurements to probe target and plasma processes in electronegative magnetron discharges. II. Positive ions

Spectra of the ion mass and energy distributions of positive ions in reactive (Ar/O2) and nonreactive (Ar) dc magnetron sputtering discharges have been investigated by energy-resolved mass spectrometry. The results of three sputter target materials, i.e., Cu, In, and W are compared to each other. Besides the main gas constituents, mass spectra reveal a variety of molecular ions which are dependent on the target material. In reactive mode, ArO+ is always observed in Ar/O2 but molecules containing Ar and the metal were exclusively found for the Cu target. The occurrence of the different ions is explained in the context of their bond strengths obtained from density functional theory calculations. The energy spectra generally contain the known low-energy peak corresponding to the plasma potential. Differently extended high-energy tails due to sputtered material were observed for the different targets. Besides these, high-energetic ions were detected with up to several 100 eV. Their energies are significantly different for Ar+ and O+ with Ar+ strongly depending on the target material. The spectra are discussed together with results from transport of ions in matter (TRIM) calculation to elucidate the origin of these energetic ions.

Etch Properties of Hf-Based High- k Dielectrics Using Inductively Coupled Plasma

In this work, we investigated etching characteristics of HfO 2 thin film and Si using inductive coupled plasma (ICP) system. The maximum etch rate of HfO 2 is 85.5 nm/min at a BCl 3 /(Ar+BCl 3 ) of 20% and decreased with further addition of BCl 3 gas. From the QMS measurements, the most dominant positive ion energy distributions (IEDs) showed a maximum at 20% of BCl 3 . These tendency was very similar to the etch characteristics. This result agreed with the universal energy dependency of ion enhanced chemical etching yields. From the mass spectra, the Hf chloride and bromine oxide must be included in the etching by-products as volatile species.

Effects of the substrate to chamber wall distance on the structure and properties of CrAlN films deposited by pulsed-closed field unbalanced magnetron sputtering (P-CFUBMS)

The aim of the research was to determine the dependence of the structure and properties of CrAlN films deposited by pulsed closed field unbalanced magnetron sputtering (P-CFUBMS) on the substrate to chamber wall distance. A Hiden Electrostatic Quadrupole Plasma Analyser (EQP) has been used to investigate the positive ion energy distributions (IED) in the plasma. It was found that increasing the substrate to chamber wall distance from 127 mm to 203 mm can significantly increase the ion flux in front of the substrate. The improvement of the ion flux in front of the substrate region facilitates ion bombardment on the substrate, increases the mobility of the adatoms on the substrate surface, thereby resulting in a denser film with reduced grain size, which exhibited high hardness (∼ 30 GPa) and good wear resistance (0.35 COF, and a wear rate of 3.36 × 10 − 6 mm 3 N − 1 m − 1 ) at the substrate to chamber wall distance of 203 mm. The CrAlN film deposited at a relatively short substrate to chamber wall distance with the rotation system has comparable structure and properties with those films deposited at large substrate to chamber wall distance at fixed substrate positions.

Ion energy distributions and the density of CH3 radicals in a low pressure inductively coupled CH4/H2 plasma used for nanocrystalline diamond deposition

Ion energy distributions (IEDs) and the density of CH3 radicals (n) in a 13.56 MHz radio frequency (rf) low pressure inductively coupled CH4/H2 plasma used for nanocrystalline diamond deposition have been investigated with a quadrupole mass spectrometer. The energy distributions of positive ions were measured in a CH4/H2 plasma with 50 mTorr of the gas pressure at 500 W of the plasma input power, and were compared with those of an Ar plasma. We have found that the IEDs of Ar+, CH4+, and C2H5+ have a nearly monoenergetic peak, and a hump due to a small degree of capacitive coupling. The plasma potentials obtained from the peaks are consistent with the previously reported values measured with a Langmuir probe. On the other hand, the IEDs of H+, H2+, and H3+ have a clear asymmetric double peak due to the modulation of rf driven glow discharge. The n monotonously increases with increasing pressure. The n indicates that CH3 radicals are main precursors for the growth of nanocrystalline diamond. The estimated sticking coefficient of the CH3 radical is comparable with the reported value.

Intensity dependence of cation kinetic energies from 2,5-dihydroxybenzoic acid near the infrared matrix-assisted laser desorption/ionization threshold.

The mechanisms responsible for matrix-assisted laser desorption/ionization (MALDI) are far from being well understood, particularly where infrared laser irradiation is used to initiate the process. We measured the emission yields and kinetic energy distributions of positive ions emitted from 2,5-dihydroxybenzoic acid loaded with angiotensin II in a standard MALDI preparation during irradiation with an infrared free-electron laser tuned to 2.94 microm. As the laser intensity is scanned through the MALDI threshold, we see a marked change in the energy distributions of the matrix ion. Above threshold, the energy distributions of both analyte and matrix cations are constant over a broad range of laser intensities. This behavior does not appear to be consistent with any extant model of the MALDI mechanism.

Desorption of positive ions from ionic crystals accompanying 248 nm laser irradiation

We present a study of the energy distributions of positive ions (Na+, Li+, Ca+, and Mg+) photodesorbed from cleaved NaCl, LiF, MgO, NaNO3, and CaCO3 surfaces during 248 nm excimer laser irradiation at fluences well below the damage thresholds. The observed ion energies are significantly higher than those predicted by already existing models that allow ion rearrangement and relaxation during the electrostatic ion repulsion by the nearby photoionized sites. In contrast to what one would expect, we find that treating the ions as fixed charges and neglecting any ion rearrangement during the emission of the adions describes best the experimentally observed ion energies from all five ionic crystals.

Energy and mass spectroscopy studies during triode ion plating of TiN films in two different layouts

Abstract Plasma properties are compared in two triode ion-plating systems using a low-voltage arc discharge. The systems differ in the configuration of the chamber and magnetic field. Mass and energy distributions of positive ions during deposition of TiN films were measured by an energy-resolved mass spectrometer. Plasma parameters were determined also by Langmuir probe diagnostics. In both systems, single and multiple charged ions of Ar and N2 gas and the evaporated Ti were detected. The system with a strong magnetic field confinement exhibits a higher plasma potential (Up=55 V) under standard deposition conditions, the peak ion energy is higher and the energy distribution is narrower than in the system without magnetic confinement (Up=12 to 35 V). The probability for the multiple charged ions in this system is much higher than in the system without the magnetic confinement, so the population of Ti++ in this system is even higher than that of Ti+. The presence of high-energy neutral Ti is also observed.

Ion energy distribution functions in inductively coupled radio-frequency discharges—Mixtures of Cl2/BCl3/Ar

We have used a gridded energy analyzer to measure positive ion energy distributions and fluxes at the grounded electrode of a high-density rf discharge. We present details of ion energies and fluxes in discharges containing mixtures of chlorine, boron trichloride and argon. These feedstock mixtures have been used extensively in the patterning of metal films for semiconductor interconnects. Our experiments were carried out in a Gaseous Electronics Conference Reference Cell which had been modified to produce inductively coupled discharges. The 15-cm-diam bottom electrode was grounded for these experiments. Stainless steel, anodized aluminum, and silicon were used as bottom electrode materials to understand the effects of various chamber boundary conditions on the ion energy parameters. In most cases, the ion energy distributions had a single peak, well separated from zero energy with a 1.0–3.0 eV full width at half maximum. These peaks were typically centered at 12–16 eV, with ion energy increasing as pressure decreased. Addition of BCl3 to a Cl2 discharge resulted in a reduction of total ion flux of more than a factor of 2, e.g., from 5 to 2.5 mA/cm2 (20 mTorr, 200 W). The ion fluxes decreased with increasing pressures in BCl3/Cl2 mixtures, while increasing with pressure in pure Cl2 discharges. Addition of Ar to several mixtures of BCl3/Cl2 changed the ion fluxes and mean ion energies by less than 15%. At high pressures or low inductive rf powers, the distributions can split into two overlapping peaks separated by as much as 8 eV. This splitting is indicative of capacitive coupling between the rf coil and the discharge. Insertion of a Faraday shield between the coil and the discharge removed most of the splitting.

Experimental investigation and numerical simulation of the energy distribution of positive ions in the negative glow of an oxygen discharge

Using a two-electrode probe, we measured the energy distribution functions of positive ions in the cathode regions of an oxygen glow discharge. The ion distribution functions have two maxima, the magnitude and position of which depend on the gas pressure and the voltage applied to the probe. The proposed approximation of the observed distribution functions is based on the assumption that two groups of ions exist with different mean energies and well described experimental curves. We numerically simulated the measuring process and found an approach for reproducing the ion energy distribution in the plasma unperturbed by the probe.

Ion bombardment diagnostics in a nitrogen RF magnetron sputtering discharge

Abstract The identity, fluxes and kinetic energy distributions of positive ions bombarding the substrate during radio frequency (RF) magnetron sputtering of graphite in a balanced nitrogen plasma have been investigated for various settings of discharge power and pressure by using energy-mass spectrometry analysis and probe measurements. Three prominent groups of ionic species were identified to form the ion bombardment. The first is due to the process gas ions, of which the primary ions of nitrogen are the dominant ones. The two further ionic groups are clearly related to sputtered carbon and its reaction products with nitrogen. The net fluxes of these ionic species respond very sensitively to changes in the discharge conditions. The ratios of the net ion fluxes of C + and N + to their atom deposition rates indicate that nearly all carbon species for film growth arise from the arriving flux of sputtered C atoms, whereas the bombarding N + flux acts as an intense nitrogen source for the film chemistry. The ion kinetic energy distributions measured for predominant ions, such as C + , N + , N + 2 and (CN) + 2 , were observed to be qualitatively similar in their shapes and trends, clearly reflecting the ions' sensitivity to RF modulations and to collision processes while passing the near-substrate sheath. Over the range of power and pressure, the mean kinetic energy of the bombarding ions was from about 25 eV to 45 eV. With the found data, the energy flux density of the ion bombardment and the averaged bombardment energy per condensing carbon and nitrogen atom were calculated, these being fundamental internal process parameters for judging the effect of the bombarding ion flux on carbon nitride formation.

Pressure and Electrode Distance Effects on Ion Energy Distributions in RF Discharges

We measured quasi-simultaneously the energy distributions of positive ions at the powered RF and grounded electrode of a parallel plate 13.56 MHz discharge using an energy selective mass spectrometer. The resulting ion energy distributions show the discharge potential conditions expected from a capacitive sheath model. Effects of the discharge pressure on the collision probability in the sheath, on the tendency of the sheath thickness, and on the effective discharge geometry are clearly demonstrated. The main ionization region is located in front of the powered electrode sheath, and the probability of ions reaching the opposite grounded electrode is characteristically influenced by discharge pressure and electrode distance.

Comparison between the energies of ejected ions in the cases of KrF laser ablation and Ar ion sputtering

The kinetic energy distributions of positive ions ejected from Si, Co, Ni and Pb-Zr-Ti-O targets subjected to KrF excimer laser or Ar ion irradiation were measured. It was found that the average energies of the ejected ions originating from laser ablation were in excess of 100 eV, while those from Ar ion sputtering were a few tens of eV. The energy distributions of the ejected ions by laser ablation were peculiar ones, characterized by having a lot of peaks, which are throught to show the plasma states on the target surface, for example generation of high electrical field. On the other hand, the energy distributions in the case of Ar ion sputtering showed the ones based on the collision cascade process.

Ion energy distributions in a dc biased rf discharge

We measured quasisimultaneously the energy distributions of positive ions at the powered rf and grounded electrode of a parallel plate 13.56 MHz discharge using an energy selective mass spectrometer. The resulting ion energy distributions reflect the discharge potential conditions expected from a capacitive plasma sheath model. By means of an externally supplied dc bias of the powered electrode we are able to influence the potential structure and to control ion energy and ion flux independently. The ratio between mean ion energy and mean sheath thickness reflects the effect of collisions on the ion energy distributions and enables estimates of sheath thickness and bulk plasma parameters to be made which are compared with values obtained by Langmuir probe measurements. We are able to demonstrate that changes in sheath potential also affect, via secondary electrons, the ionization regime in the discharge and this can be utilized to control the species composition in the discharge.

Heteroepitaxial nucleation of diamond by bias pretreatment in an electron cyclotron resonance microwave chemical vapor deposition system

Bias-enhanced nucleation (BEN) pretreatment was used for the nucleation of diamond particles on β-silicon carbide substrates in an electron cyclotron resonance (ECR) microwave chemical vapor deposition (CVD) system. Consequently, experimental conditions enabled the pretreatment of large-area substrates. After BEN and diamond growth, the density of diamond particles was strongly dependent on the BEN pretreatment parameters, such as the negative bias voltage, the total pressure, the duration of pretreatment and the methane concentration. Moreover, under appropriate BEN conditions, heteroepitaxial nucleation of the diamond particles was obtained and the proportion of particles oriented with the single-crystal substrates was also determined as a function of the BEN conditions. The maximum rate of oriented diamond particles was around 30% and typical particle densities were in the range 5 × 10 7 cm −2 to 5 × 10 8 cm −2. Heteroepitaxial nucleation was also obtained on substrates over a 40 mm length. For pressures increasing from 13 Pa (0.3 Torr) to 270 Pa (2.0 Torr), the maximum rate of oriented particles was obtained with an optimum bias voltage increasing from −30 V to −90 V. Epitaxial nucleation of diamond could then be strongly dependent on the energy distribution of positive ions involved in the BEN process. Although particle densities and rates of oriented particles deteriorated with the decrease in the substrate temperature from 900 °C to 500 °C, temperatures around 400 °C were suitable for obtaining particle densities close to those obtained at high temperatures.

The role of defects in laser induced positive ion emission from ionic crystals

The energy distributions of positive ions produced by exposing single-crystal MgO to pulsed 248 nm excimer laser light at fluences of 200–1100 mJ/cm2 were determined by combined quadrupole mass spectrometry and time-of-flight techniques. The dominant ionic species is Mg+, although small amounts of Mg2+, MgO+, and Mg2O+ are also observed. In particular, the Mg+ and Mg2+ energies distributions each show two peaks, with the energies of the Mg2+ peaks at significantly higher energies. Ion trajectory simulations (accounting for Coulomb forces only and assuming no surface relaxation) suggest that Mg2+ and Mg+ adsorbed at sites directly atop surface F-centers (oxygen vacancies with two trapped electrons) would be ejected upon photo-ionization of the F-center. The experimentally observed kinetic energies and angular distributions agree well with the energies and angles predicted by these simulations.

Mechanisms of Excimer Laser induced Positive Ion Emission From Ionic Crystals

AbstractThe energy distributions of positive ions produced by exposing single-crystal MgO to pulsed 248 nm excimer laser light at fluences of 200-1200 mJ/cm2 were determined by combined quadrupole mass spectrometry and time-of-flight techniques. the dominant ionic species is Mg+, although small amounts of Mg2+, MgO+, and Mg2O+ are also observed. IN particular, theMg+ and Mg2+ energy distributions each show two broad peaks, with the energies of the Mg2+ peaks at significantly higher energies. Ion trajectory simulations (accounting for Coulomb forces only and assuming no surface relaxation) suggest that Mg2+ adsorbed at sites directly atop surface F-centers (oxygen vacancies with two trapped electrons) would be ejected upon photo-ionization of the F-center. the experimentally observed Mg2+ kinetic energies agree well with the energies predicted by the simulation.

Fragmentation processes in reactive molecular ion beam etching

In a reactive ion beam etching system, gas phase collision processes in the reaction chamber were identified from the energy distributions of positive ions originating from source plasmas with O2, SF6, and CF4 as feed gases. The ion energy distributions are determined by a quadrupole mass spectrometer for main beam energies below 500 eV at typical working pressures in the reaction chamber of 1–10×10−2 Pa. Besides near thermal ions a considerable amount of high energy fragmentation products were detected for a number of primary molecular ions. The relative intensities of these products compared to the parent ions suggest a non-negligible influence of gas phase dissociation processes on the etch or deposition characteristics of molecular ion beams and the resulting properties of surfaces treated under elevated working pressure conditions.