Neutral Atom Fluxes Research Articles

Vacuum beam studies of photoresist etching kinetics

One factor limiting the development of reliable models of high density, low pressure oxide etch plasmas is the relatively poor understanding of the plasma-photoresist surface interactions. In particular, the relatively high rates of photoresist (PR) loss experienced in high density fluorocarbon plasmas is a significant problem. It has long been accepted that fluorine plays a key role in controlling the oxide to PR etch rate selectivity. The addition of hydrogen has been shown to improve this selectivity, presumably by scavenging fluorine from the tool by forming HF. By reducing the fluorine to carbon ratio in the plasma and more specifically at the PR surface itself, the rate of polymer deposition increases causing the net PR etch rate to decrease. In this work, the complex surface chemistry of fluorocarbon plasmas is simplified to facilitate the study of the interaction of fluorine atoms and hydrogen atoms on the PR surface. This chemistry is modeled in vacuum beam experiments with argon ions and independent fluxes of neutral deuterium and fluorine atoms intersecting at the surface of photoresist samples. We present experimental evidence that the etch yield of photoresist (carbon atoms removed per incident argon ion) under these conditions is high compared to that of silicon and silicon dioxide. The presence of a simultaneous flux of deuterium atoms on the photoresist surface does not affect the etch yield despite the fact that DF is formed during the etching process.

Collisional interactions of precipitating energetic neutral atoms with upper‐atmospheric particles in the low‐latitude region

Altitude profiles of energetic neutral atom (ENA) fluxes precipitating into the upper atmosphere are simulated numerically by using a Monte Carlo method with source magnetospheric protons distributed between L shells 4 and 7, taking account of collisional interactions with atmospheric particles. Numerical results are in reasonable agreement with the profile of ∼10keV neutral atom fluxes recently measured under a geomagnetically quiet condition. Through collisional interactions, ENAs can be converted to ions and then the ionized ENAs can be reneutralized and repeat this cycle. Therefore the precipitating ENAs lose their original directionality because of gyromotions of the ionized ENAs. However, the source information is still seen in ENA data measured at low altitudes where collisions are significant, because their pitch angles are almost conserved with the forward scattering approximation for each collision in the case of parallel geomagnetic fields at low altitudes. On the other hand, a part of ENAs precipitating into the atmosphere returns to the magnetosphere. These reflected ENAs act as a secondary source that cannot be neglected in ENA data measured in the magnetosphere.

Simulation of ENA fluxes along the Geotail spacecraft orbit

Simulations of quiet time terrestrial H-ENA fluxes are shown. The simulations are obtained by using average proton differential fluxes from AMPTE-CCE/CHEM. A functional form which provides the ring current quiet time proton fluxes as a function of energy, L-shell and magnetic local time is also used. The energy neutral atom (ENA) fluxes are simulated at a specific location along the Geotail spacecraft orbit, where ENA data have been collected by the HEP-LD spectrometer. A detailed analysis of the ENA generation along the instrument look directions is presented, in order to evaluate the significance of the ENA signal with respect to the locations of ion sources and to the instrument energy resolution. At energies above 70 keV, we show that the ENA energy is directly related to the ion source location.

Temperature dependence of inductively coupled plasma assisted growth of TiN thin films

Abstract The use of low pressure high density plasmas to assist the synthesis of ceramic thin film materials is in its infancy. Using an inductively coupled plasma assisted magnetron sputtering system, we examine the dependence of plasma-assisted growth of TiN thin films on growth temperature at different ratios of ion flux to neutral atom flux. Our results indicate that a temperature independent densification of TiN films occurs above a certain ion to neutral atom flux ratio. As an example of this temperature independent densification, we demonstrate the formation of dense B1 TiN crystalline thin films at growth temperatures down to ∼100 K.

Can the direction of interstellar magnetic field be determined from the heliotail ENA flux?

The effect of pressure of the (arbitrarily directed) interstellar magnetic field on the shape of the heliotail and consequently on space distribution of the pick‐up and anomalous cosmic ray ions convected into the heliotail region is estimated in a simple model based on a Newtonian approximation. The ion distribution in the heliotail produces (via transcharging on background gas) a flux of energetic neutral atoms which can be detected from the vicinity of the Earth. Directional dependence of this flux can be used to determine the direction and possibly strength of the interstellar field.

Neutral Atom Modeling of the TFTR First Wall, Pump Ducts, and Neutral Beams

The DEGAS neutral transport code is used in two separate cases to simulate the neutral beam box and vessel of the Tokamak Fusion Test Reactor (TFTR). For the neutral beam box simulation, known input parameters include the ion density at the source exit and the proportion of input gas that is converted to the high-energy atomic beam. The T0 current to the torus is (1.61 ± 0.03) × 1020 s-1, with the high-energy beam having a median energy above 95 keV. Corresponding results are found for the D0 current. In addition, the amount of gas reaching the torus, the pressure, and the flux and energy distributions of the ions and neutrals to the walls are found. For the tritium case, it is calculated that 92.4 ± 0.2% of the input tritium reaches the cryopanels, 6.64 ± 0.05% reaches the torus, and 1.0 ± 0.2% reaches the ion dump. In the second run, DEGAS was used to calculate the neutral atom flux and energy of particles incident on the walls of the vacuum vessel and the neutral pressure in the pump duct of TFTR during a typical supershot with a 50/50 mixture of deuterium-tritium. Output quantities are the current and energy to the bumper limiter and first wall. The total amount of tritium implanted in the vacuum vessel after 150 shots of 1-s duration is estimated to be 0.5 ± 0.1 g in the bumper limiter and 0.042 ± 0.023 g in the outer wall and pumping duct, which is well within the 5-g on-site inventory and the 2-g in-vessel inventory. The implications of these results are discussed.

Diagnostic measurements and modelling of an ion-based PVD-process with high deposition rates

Abstract The controlled plasma-PVD deposition of metal ions provides a tool to deposit metal coatings or interlayers of high quality. Another advantage is the possibility to confine the depositing ionic species by using externally applied magnetic fields. Using adequate field configurations, very homogeneous films can be grown on large substrates. A critical issue to be solved in such ion-based deposition processes is to efficiently ionize large fluxes of metal atoms and transport the ions to the substrate. In the PVD-process described in this paper, a flux of neutral metal atoms (Al) was produced by evaporation from an effusion cell or, alternatively, by magnetron sputtering. The Al atoms entered a dense, magnetically confined plasma of k T e =10 eV and n e =10 11 –10 12 cm −3 which served as a source to ionize the Al-neutrals. Subsequently the Al + -ions were guided along a magnetic field to the substrate. In order to understand and to optimize these process, experimental measurements and numerical analyses were carried out. Electric probes were used to determine local plasma parameters and quantitative optical emission spectroscopy (OES) measurements gave information on the local neutral Al-atom and of the Al + -ion population. After the experiments the deposition rate was derived from weight gain and film thickness measurements. These results were compared with a numerical model of the plasma based processes. Good agreement was found for the measured and calculated Al-population in the plasma and also for the measured and calculated film growth rates.

Energetic neutral atom imaging of space plasmas

Experimental techniques and instrumentation for space plasma imaging in fluxes of energetic neutral atoms (ENAs) are reviewed. ENAs are born in charge exchange collisions between space plasma energetic ions and background neutral gas. ENAs are ubiquitous in the space environment and their energies are in the range from a few eV up to >100 keV.Contrary to charged particles, ENAs can travel large distances through space with minimal disturbance, and by recording ENA fluxes as a function of observational direction, one can reconstruct a global image of a planetary magnetosphere or the heliosphere. Plasma ion energy distribution and ion composition can be remotely established by measuring ENA energies and masses. ENA imaging opens a new window on various phenomena in space plasmas with a promise to qualitatively improve our understanding of global magnetospheric and heliospheric processes. At first we review ENA fluxes in space and their properties, and present a brief history of ENA experimental studies and the evolution of experimental approaches. The concepts of ENA imaging and particle identification are considered and followed by comparison with corpuscular diagnostics of fusion plasmas. Basic ENA techniques and instrument components are then described in detail and critically evaluated; performance characteristics, limitations, and requirements to key instrumental elements are discussed. And finally, representative ENA instruments are shown, and promising instrumental approaches are identified.

Mechanisms of intrinsic stress generation in amorphous carbon thin films prepared by magnetron sputtering

Abstract Amorphous carbon (a-C) films have been deposited on biased silicon substrates at room temperature by conventional and unbalanced magnetron sputtering from a graphite target in pure argon discharges. The flux and energy of positive ions striking the surface of growing films were determined as functions of the substrate bias voltage by grid probe measurements. The flux of neutral carbon atoms condensed on Si substrates was deduced from Rutherford backscattering spectroscopy measurements. The mass density of a-C films, the argon concentration and intrinsic stress in these films were investigated as functions of the substrate bias voltage, i.e. the energy of positive ions impinging on the film surface. The similarity between the variations of the mass density and argon concentration as the substrate bias voltage was increased suggests that the increase in mass density was likely caused by the argon incorporation in the films. The intrinsic stress level was essentially dependent on the energy of positive ions striking the film surface. The experimental values of the intrinsic stress were compared with the values predicted by models of intrinsic stress generation in thin films reported in the literature. The good agreement between experimental and theoretical values of the intrinsic stress in a-C films sputter-deposited using conventional and unbalanced magnetron modes contributes to assess the validity of the model proposed by Davis.

A Monte Carlo model of the ring current decay

A Monte Carlo type calculation has been used to model the decay of the magnetospheric ring current during the severe geomagnetic storm of February 1986. It is found that in principle our model is capable of reproducing the Dst variation observed during this event. We also show that the fluxes of energetic neutral atoms generated in charge-exchange collisions are quite large (up to 10 5 cm −2 s −1 sr −1 keV −1) and therefore well-suited to study the decay of the ring current.

A mass resolving neutral atom imager

Neutral atom imaging of space plasmas, the measurement of neutral atom flux as a function of angle and energy, can provide qualitative information about the global dynamics of a space plasma. In addition, mass and energy resolved neutral atom images can be used to remotely measure convective electric fields in planetary magnetospheres. Direct detection of neutral atoms using ultraviolet-rejecting waveguides and microchannel plate sensors has been proposed as a method of imaging low-energy neutral atoms. We discuss a new type of direct detection neutral atom imager that is capable of producing mass and energy resolved images. The imager has a large geometric factor (G≈0.2 cm2 sr eV/eV), an energy range of 1–100 keV, and single coincidence noise suppression. The estimated mass of the imager is less than 1.5 kG.

Corpuscular diagnostics and processing methods applied in investigations of laser-produced plasma as a source of highly ionized ions

This paper presents a set of complementary corpuscular diagnostics applied in experiments for investigation of laser-produced plasma as a source of ions. The measuring possibilities and methods for processing experimental data of a cylindrical electrostatic ion energy analyzer, a Thomson parabola ion analyzer, various types of electrostatic probes, a detector of neutral atom fluxes, as well as methods for visualization of ion emission areas are discussed. Special attention was focused on the ion-induced secondary electron emission problem and its influence on the accuracy of the measurements.

Multiple charge exchange and ionization collisions within the ring current‐geocorona‐plasmasphere system: Generation of a secondary ring current on inner L shells

Charge exchange (CE) collisions between ring current ions and geocoronal atoms erode the ring current and give rise to a pervasive flux of energetic neutral atoms (ENAs). ENAs generated in the main ring current traversing the inner magnetosphere can be reionized in several ways, converting ENAs back into ring current ions albeit on new L shells (provided the angle between the direction of ENA travel and the local magnetic field direction at the point of collision corresponds to a pitch angle outside the local loss cone). These displaced ring current ions in turn undergo subsequent CE collisions with geocoronal atoms, generating secondary ENA fluxes that can participate in further ionizing collisions. A modeling scheme for exploring the effectiveness of this mechanism in establishing and maintaining a secondary ring current at L ≲ 3 has been developed taking full account of multiple CE and ionizing collisions. It is based on a matrix solution technique for an integral form of the Boltzmann transport equation and relies on two simplifications: (1) Since gyroperiods ≪ bounce periods, gyroradii ≪ scale lengths for variations in geocoronal and plasmaspheric densities, and single‐bounce collisional depths along ion gyropaths ≪ 1, it is adequate to work in terms of quantities integrated along bounce paths and averaged in gyrophase. (2) The thermal populations are not directly perturbed by the energetic populations, while the sources of the main ring current are external to the charge‐exchange‐coupled systems, so that the transport equation for the secondary flux is linear. The results are steady state fluxes of trapped ions as functions of L shell, equatorial pitch angle, and energy. This model is described in full, and an example of steady state secondary H+ ring current fluxes for quiet conditions is presented using the quiet time empirical model of Sheldon and Hamilton [1993] to prescribe the main proton ring current.

Effect of energetic particles on the residual stresses in nonhydrogenated amorphous carbon films deposited on grounded substrates by dc magnetron sputtering

Amorphous carbon (a-C) films have been deposited on grounded silicon substrates by magnetron sputtering from a graphite target in pure argon discharges. The characteristics of the magnetron discharge were determined by Langmuir probe measurements as functions of the argon pressure and sputtering power. The flux and energy of argon ions impinging on the grounded substrates were deduced from Langmuir grid probe measurements. The flux of carbon atoms condensed on Si substrates was obtained from Rutherford backscattering spectroscopy measurements. The residual stresses in a-C films were found to be dependent on the argon pressure. The average kinetic energy of carbon atoms ejected from the target was determined from the energy distribution calculated from the model proposed by Thompson. The kinetic energy of carbon atoms condensed on the film surface was determined on the basis of calculation developed from the kinetic gas theory. The intensity of the intrinsic stress developed in a-C films was correlated to the flux and energy of neutral carbon atoms and argon ions impinging on the surface of growing films. The dependence of the intrinsic stress on the energetics of the deposition process matched the prediction deduced from the model of Windischmann.

Process control of magnetron sputtering of TiN coatings studied by in situ AES and plasma diagnostics

The understanding of reaction mechanisms during reactive sputtering requires the knowledge of the flux and energy of the various particles impinging on the substrate and their effects on the growth rate and composition of the coating. Experiments have been performed in an ultrahigh vacuum receiver equipped with an in situ Auger electron spectroscopy system for chemical analysis of the substrate composition and instruments for plasma diagnostics. TiN x films were deposited on high speed steel substrates in an ArN 2 atmosphere by planar magnetron sputtering in the d.c. mode. The composition of the TiN x coatings was determined as a function of the Ar and N 2 partial pressure, bias voltage, d.c. power and substrate temperature. Mass spectrometer measurements were used to determine the ion fluxes to the substrate and their energies during film deposition. The flux of neutral titanium atoms was measured by optical spectrometry. Most of the experiments were carried out in a transition region for the composition of TiN x , with 0 < x < 1, in order to get a sensitive response of the composition to changes in sputter parameters. Relevant reaction mechanisms occurring in the plasma, on the target and on the substrate surface are discussed.

Isotopic mass effects in low-energy sputtering of copper and molybdenum

Spherical and planar targets of copper and molybdenum were sputtered with Ar + ions in the energy range from 40 to 440 eV. The isotopic composition of the flux of neutral atoms under steady-state conditions was determined by means of a novel secondary-neutral mass spectrometer with electron-gas post-ionization. In combination with a retarding-field energy analyzer, the ejected particle flux was investigated with respect to an emission-energy dependence of the natural isotopic abundances. It was found that the isotopic composition in the near-threshold regime of sputtering changes drastically with the Ar + impact energy. Mass-dependent variations of the angular distributions lead to an enrichment of the lighter isotopes of several percent. Most prominent is the influence of the different emission energy distributions. The lighter isotopes are preferably ejected with higher kinetic energies; this effect exhibits a pronounced enhancement at very low bombarding energies. Depending on the retardation potential of the energy analyzer and thus the energy of the transmitted atoms, deviations from the natural isotopic composition of several 10% were observed.

Neutral atom imaging mass spectrograph

We describe a concept for an instrument to measure 2-D space plasma distributions by remote sensing of neutral atoms. The instrument measures in one dimension, and from a spinning spacecraft one obtains 2-D (line-of-sight) maps of the neutral flux. Because we want to employ this instrument for measurements in the magnetosphere, the main species of interest are neutral H and O atoms with kinetic energies ranging from about 10 eV to 1 keV. The instrument makes use of a low-work-function surface to convert neutral atoms efficiently to negative ions. The ions are then accelerated away from the surface and brought to an intermediate focus by a large aperture lens. After further acceleration, the ions are deflected by a spherical electrostatic analyzer into a time-of-flight mass spectrometer. Mass resolution of the device is sufficient to resolve H, D, He, and O. Energy and azimuth angle information are obtained by position imaging of the secondary electrons produced at the carbon foil. The large geometric factor combined with simultaneous angle-energy-mass measurement eliminates the need for cycling and provides the necessary high sensitivity for imaging at short time intervals. On a spinning spacecraft this instrument is capable of producing 2-D maps of low-energy neutral atom fluxes.

Potential merits for substorm research from imaging of charge-exchange neutral atoms

Abstract. The in situ observations of the Earth magnetosphere performed over the past decades of space research have provided a rather good understanding of many partial localized processes of the magnetospheric substorm. The continuing lack of global observations inhibits the construction of a coherent picture of the substorm as a whole, which is actually determined by the coupling of the partial processes. In this context the importance of global observations for the advancement of magnetospheric substorm studies is critical. This paper presents briefly a promising technique of global observations, namely the imaging of charge exchange neutral atoms, or neutral atom imaging (NAI) of the magnetosphere. Model and theoretical estimates of charge-exchange neutral atom fluxes, as well as appropriate spacecraft orbit and instrumentation requirements are presented and discussed for specific regions of interest and vantage points. The potential merits of NAI for substorm research are presented along with possible combinations with other types of observational methods. Substorm issues that would benefit from NAI should include among others the assessment of the ionospheric contribution to the hot magnetospheric plasma, the relative importance of various ionospheric ion source regions, the resolution of spatial and temporal characteristics of substorm ion injections. NAI observations can be precious complements to local observations and lead to the understanding of how local processes, many of which are resolved quite well today, combine to form the global process of the magnetospheric substorm.

Potential merits for substorm research from imaging of charge-exchange neutral atoms

The in situ observations of the Earth magnetosphere performed over the past decades of space research have provided a rather good understanding of many partial localized processes of the magnetospheric substorm. The continuing lack of global observations inhibits the construction of a coherent picture of the substorm as a whole, which is actually determined by the coupling of the partial processes. In this context the importance of global observations for the advancement of magnetospheric substorm studies is critical. This paper presents briefly a promising technique of global observations, namely the imaging of charge exchange neutral atoms, or neutral atom imaging (NAI) of the magnetosphere. Model and theoretical estimates of charge-exchange neutral atom fluxes, as well as appropriate spacecraft orbit and instrumentation requirements are presented and discussed for specific regions of interest and vantage points. The potential merits of NAI for substorm research are presented along with possible combinations with other types of observational methods. Substorm issues that would benefit from NAI should include among others the assessment of the ionospheric contribution to the hot magnetospheric plasma, the relative importance of various ionospheric ion source regions, the resolution of spatial and temporal characteristics of substorm ion injections. NAI observations can be precious complements to local observations and lead to the understanding of how local processes, many of which are resolved quite well today, combine to form the global process of the magnetospheric substorm.

Parametric modeling and measurement of silicon etching in a high density chlorine plasma

The plasma parameter scaling of silicon etching in a high density chlorine discharge has been measured and described by an ion assisted etch model. The measurements were performed in an electron cyclotron resonance chlorine plasma at pressures between 0.5 and 4.0 mTorr. When the Cl neutral atom flux to the wafer is sufficiently high, the etch rate is controlled by the ion power flux to the substrate, JiVs, where Ji is the ion current density and Vs is the plasma sheath potential. There is a threshold value of the ion power flux below which no etching occurs. When the Cl flux is low, the etch rate is controlled by the limited supply of Cl atoms and is approximately independent of the ion power flux.