Average Ion Energy Research Articles

Full particle orbit tracing with the RIO code in the presence of broad-spectrum MHD activity in a reversed-field pinch

In order to better understand the behaviour of both neutral beam injected and spontaneously generated fast ions in the Madison Symmetric Torus reversed-field pinch, we have developed the full orbit-following code random ion orbits (RIO). The low magnetic field and relatively large level of MHD activity present in MST require a full orbit code as the guiding centre assumptions are violated even for ions with modest energy. Furthermore, quasi-periodic bursts of MHD activity (sawteeth) generate large transient electric fields and significant modifications to the equilibrium magnetic fields. Understanding the full effect of these sawteeth on the spatial and velocity distribution of the fast ions is of great interest. To this end, RIO now has the ability to take the full 3D, time evolving, magnetic and electric fields produced by the visco-resistive MHD code DEBS as input. In static cases, where broad-spectrum magnetic perturbations from DEBS are input, but fixed in time, beam injected ions are found to be generally well confined with the core fast ion density profile largely unaffected by the magnetic modes while the fast ion density in the mid-radius is substantially reduced. In the dynamic case, the large amplitude magnetic fluctuations that occur at the sawtooth crash produce substantial fast ion loss. Those fast ions that are not lost are accelerated by a large, transient, parallel electric field in the co-current direction. This causes the average energy of the beam ions to increase by ∼20%, consistent with recent experimental measurements.

Plasma–surface interaction during low pressure microcrystalline silicon thin film growth

The role of plasma–surface interactions during microcrystalline silicon thin film growth has been studied under low power–low pressure conditions, and a correlation between the plasma–surface interactions and material properties has been provided. The atomic hydrogen flux, inferred by optical emission spectroscopy measurements, has been investigated. The hydrogen-to-silicon growth flux resulted in a ratio of 25 : 140 for the amorphous-to-mixed phase transition, whereas it increased to 40 : 170 for the mixed phase-to-microcrystalline phase transition. The ion contribution to the plasma–surface interaction has also been investigated. For this purpose, a capacitive probe has been implemented locally for direct measurement of the ion flux. For the amorphous-to-microcrystalline phase transition the ion-to-silicon growth flux ratio has been determined to increase from 0.15 to 1, two orders of magnitude smaller than the atomic hydrogen-to-silicon growth flux ratio. The ion energy has been measured with a retarding field energy analyser, which allowed determination of the ion-energy distribution as a function of the silane flow rate. Average ion energies of 15–20 eV have been found for a decreasing silane flow rate, thereby indicating the limited energy transfer to the surface. The main effect caused by the ion arrival at the surface is a locally induced thermal spike enhancing the radical surface diffusion. No prominent detrimental effect, i.e. Si surface or bulk displacement requiring energies greater than 18 eV and 40 eV respectively, or sputtering occurring above 50 eV, has been observed.

A spatially resolved retarding field energy analyzer design suitable for uniformity analysis across the surface of a semiconductor wafer.

A novel retarding field energy analyzer design capable of measuring the spatial uniformity of the ion energy and ion flux across the surface of a semiconductor wafer is presented. The design consists of 13 individual, compact-sized, analyzers, all of which are multiplexed and controlled by a single acquisition unit. The analyzers were tested to have less than 2% variability from unit to unit due to tight manufacturing tolerances. The main sensor assembly consists of a 300 mm disk to mimic a semiconductor wafer and the plasma sampling orifices of each sensor are flush with disk surface. This device is placed directly on top of the rf biased electrode, at the wafer location, in an industrial capacitively coupled plasma reactor without the need for any modification to the electrode structure. The ion energy distribution, average ion energy, and average ion flux were measured at the 13 locations over the surface of the powered electrode to determine the degree of spatial nonuniformity. The ion energy and ion flux are shown to vary by approximately 20% and 5%, respectively, across the surface of the electrode for the range of conditions investigated in this study.

A current driven capacitively coupled chlorine discharge

The effect of driving current, driving frequency and secondary electrons on capacitively coupled chlorine discharge is systematically investigated using a hybrid approach consisting of a particle-in-cell/Monte Carlo simulation and a volume-averaged global model. The driving current is varied from 20 to 80 A m−2, the driving frequency is varied from 13.56 to 60 MHz and the secondary electron emission coefficient is varied from 0.0 to 0.4. Key plasma parameters including electron energy probability function, electron heating rate, ion energy and angular distributions are explored and their variations with control parameters are analyzed and compared with other discharges. Furthermore, we extend our study to dual-frequency (DF) capacitively coupled chlorine discharge by adding a low-frequency current source and explore the effect of the low-frequency source on the discharge. The low-frequency current density is increased from 0 to 4 A m−2. The flux of ions to the surface increases only slightly while the average energy of ions to the surface increases almost linearly with increasing low-frequency current, which shows possible independent control of the flux and energy of ions by varying the low-frequency current in a DF capacitively coupled chlorine discharge. However, the increase in the flux of Cl+ ions with increasing low-frequency current, which is mainly due to the increased dissociation fraction of the background gas caused by extra power supplied by the low-frequency source, is undesirable.

Effect of Ti-Al cathode composition on plasma generation and plasma transport in direct current vacuum arc

DC arc plasma from Ti, Al, and Ti1-xAlx (x = 0.16, 0.25, 0.50, and 0.70) compound cathodes was characterized with respect to plasma chemistry and charge-state-resolved ion energy. Scanning electron microscopy, X-ray diffraction, and Energy-dispersive X-ray spectroscopy of the deposited films and the cathode surfaces were used for exploring the correlation between cathode-, plasma-, and film composition. Experimental work was performed at a base pressure of 10−6 Torr, to exclude plasma-gas interaction. The plasma ion composition showed a reduction of Al of approximately 5 at. % compared to the cathode composition, while deposited films were in accordance with the cathode stoichiometry. This may be explained by presence of neutrals in the plasma/vapour phase. The average ion charge states (Ti = 2.2, Al = 1.65) were consistent with reference data for elemental cathodes, and approximately independent on the cathode composition. On the contrary, the width of the ion energy distributions (IEDs) were drastically reduced when comparing the elemental Ti and Al cathodes with Ti0.5Al0.5, going from ∼150 and ∼175 eV to ∼100 and ∼75 eV for Ti and Al ions, respectively. This may be explained by a reduction in electron temperature, commonly associated with the high energy tail of the IED. The average Ti and Al ion energies ranged between ∼50 and ∼61 eV, and ∼30 and ∼50 eV, respectively, for different cathode compositions. The attained energy trends were explained by the velocity rule for compound cathodes, which states that the most likely velocities of ions of different mass are equal. Hence, compared to elemental cathodes, the faster Al ions will be decelerated, and the slower Ti ions will be accelerated when originating from compound cathodes. The intensity of the macroparticle generation and thickness of the deposited films were also found to be dependent on the cathode composition. The presented results may be of importance for choice of cathodes for thin film depositions involving compound cathodes.

Characteristics of reactive ion etching lag in HBr/O2 plasma etching of silicon trench for nanoscale device

In this study, we investigated the etching parameter dependence of the reactive ion etch (RIE) lag of nanometer silicon trenches using HBr/O2 plasma in an inductively coupled plasma etcher. As the O2 flow rate, pressure, and source power decreased and the substrate temperature increased, the RIE lag improved. The RIE lag dependence on the O2 flow rate correlated with surface oxidation which gives rise to charging up of positive ions and reduction in silicon etching rate. Increased oxidation, rate resulted in severer RIE lag. These were verified by actinometrical optical emission spectroscopy measurements. On the other hand, the decrease in substrate temperature worsened the RIE lag owing to the remaining etching by-products deposited on the substrate. When the pressure and source power decreased, the RIE lag improved owing to the increase in average ion energy. As the bias power increased, the RIE lag improved, but for excessively high power, the RIE lag deteriorated, as the positive ions could not reach the bottom of the trench due to charging. However, the RIE lag improved at high bias powers when the RF power was pulse-modulated. There was almost no frequency dependence of the RIE lag, but the RIE lag improved when the duty ratio was reduced. The improvement of the RIE lag in the pulsed plasma is thought to be due to the relaxation of the charging up of positive ions by the negative ions generated during the power-off period.

Ion velocities in direct current arc plasma generated from compound cathodes

Arc plasma from Ti-C, Ti-Al, and Ti-Si cathodes was characterized with respect to charge-state-resolved ion energy. The evaluated peak velocities of different ion species in plasma generated from a compound cathode were found to be equal and independent on ion mass. Therefore, measured difference in kinetic energies can be inferred from the difference in ion mass, with no dependence on ion charge state. The latter is consistent with previous work. These findings can be explained by plasma quasineutrality, ion acceleration by pressure gradients, and electron-ion coupling. Increasing the C concentration in Ti-C cathodes resulted in increasing average and peak ion energies for all ion species. This effect can be explained by the “cohesive energy rule,” where material and phases of higher cohesive energy generally result in increasing energies (velocities). This is also consistent with the here obtained peak velocities around 1.37, 1.42, and 1.55 (104 m/s) for ions from Ti0.84Al0.16, Ti0.90Si0.10, and Ti0.90C0.10 cathodes, respectively.

High reflectance ta-C coatings in the extreme ultraviolet

The extreme ultraviolet (EUV) reflectance of amorphous tetrahedrally coordinated carbon films (ta-C) prepared by filtered cathodic vacuum arc was measured in the 30-188-nm range at near normal incidence. The measured reflectance of films grown with average ion energies in the ~70-140-eV range was significantly larger than the reflectance of a C film grown with average ion energy of ~20 eV and of C films deposited by sputtering or evaporation. The difference is attributed to a large proportion of sp3 atom bonding in the ta-C film. This high reflectance is obtained for films deposited onto room-temperature substrates. The reflectance of ta-C films is higher than the standard single-layer coating materials in the EUV spectral range below 130 nm. A self-consistent set of optical constants of ta-C films was obtained with the Kramers-Krönig analysis using ellipsometry measurements in the 190-950 nm range and the EUV reflectance measurements. These optical constants allowed calculating the EUV reflectance of ta-C films at grazing incidence for applications such as free electron laser mirrors.

Optimum laser intensity for the production of energetic deuterium ions from laser-cluster interaction

We measured, using Petawatt-level pulses, the average ion energy and neutron yield in high-intensity laser interactions with molecular clusters as a function of laser intensity. The interaction volume over which fusion occurred (1–10 mm3) was larger than previous investigations, owing to the high laser power. Possible effects of prepulses were examined by implementing a pair of plasma mirrors. Our results show an optimum laser intensity for the production of energetic deuterium ions both with and without the use of the plasma mirrors. We measured deuterium plasmas with 14 keV average ion energies, which produced 7.2 × 106 and 1.6 × 107 neutrons in a single shot with and without plasma mirrors, respectively. The measured neutron yields qualitatively matched the expected yields calculated using a cylindrical plasma model.

Investigation of Ion Energy Distributions with Two Positive Ions in a Dual-Frequency Sheath of Plasma Etching

A one-dimensional fluid and Monte Carlo model is developed to study plasma sheath in dual ratio frequency plasma etching. Electrons and two positive ions are considerated. The influence of low frequency, ions mass diversity on IEDs and temperature uniformity of wafer is discussed. The results show that the IEDs are greatly modulated by the low frequency and ion mass, and the maximum and minimum ions energy can be predicted by using damped potential. The two ions with different ion mass affect each other little in IEDs but the total ion flux. The lower ion flux has higher averaged ion energy and the higher ion flux has lower averaged ion energy when keeping the total power fixed. It results in a similar temperature uniformity of wafer.

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.

Particle‐in‐cell simulation of electron and ion energy distributions in dc/rf hybrid capacitively‐coupled plasmas

A Particle‐in‐Cell simulation with Monte Carlo collisions was used to study electron and ion energy distributions (IEDs) in low‐pressure (2.67 Pa) direct‐current (dc)/radio‐frequency (rf) hybrid capacitively‐coupled Ar plasmas. One electrode (dc/rf electrode) of the parallel plate diode was powered by a 13.56 MHz source, and a negative dc bias voltage, whereas the opposite (substrate) electrode was grounded. Secondary electrons emitted from the dc/rf electrode accelerated in the adjacent sheath and entered the plasma, yielding a high‐energy tail of the electron energy distribution. For given dc bias voltage, the plasma density increased as the secondary electron emission yield due to ion bombardment increased. A fraction of the secondary electrons were energetic enough to overcome the sheath potential barrier on the substrate electrode and bombard the substrate. The electron angular distribution on the substrate electrode had a peak of directional electrons superimposed on a typical cosine distribution. The mean energy and angular spread of directional electrons could be controlled by varying the dc bias voltage. However, as the dc bias became more negative, the dc/rf sheath expanded at the expense of the bulk plasma, reducing the plasma density, in agreement with published data. The IED on the substrate electrode exhibited a dominant bimodal feature with multiple shoulder peaks due to ion‐neutral charge exchange collisions. The average ion energy decreased as the dc voltage became more negative, also in agreement with data. Pulsing the plasma power enhanced the tail of the electron energy distribution in the early activeglow (power ON), and yielded a distinct ballistic electron flux on the substrate with energy equal to the applied dc bias. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3214–3222, 2013

Optimization of the neutron yield in fusion plasmas produced by Coulomb explosions of deuterium clusters irradiated by a petawatt laser

The kinetic energy of hot (multi-keV) ions from the laser-driven Coulomb explosion of deuterium clusters and the resulting fusion yield in plasmas formed from these exploding clusters has been investigated under a variety of conditions using the Texas Petawatt laser. An optimum laser intensity was found for producing neutrons in these cluster fusion plasmas with corresponding average ion energies of 14 keV. The substantial volume (1-10 mm(3)) of the laser-cluster interaction produced by the petawatt peak power laser pulse led to a fusion yield of 1.6×10(7) neutrons in a single shot with a 120 J, 170 fs laser pulse. Possible effects of prepulses are discussed.

Ion dynamics in ultrafast laser ablation of copper target

We investigate the angular distribution and average kinetic energy of ions produced during ultrafast laser ablation (ULA) of a copper target in high vacuum. Laser produced plasma (LPP) is induced by irradiating the target with Ti:Sapphire laser pulses of ～50 fs and 800 nm at an angle of incidence of 45o. An ion probe is moved along a circular path around the ablation spot, thereby allowing characterization of the time-of-flight (TOF) of ions at different angles relative to the normal target. The angular distribution of the ion flux is well-described by an adiabatic and isentropic expansion model of a plume produced by solid-target laser ablation (LA). The angular width of the ion flux becomes narrower with increasing laser fluence. Moreover, the ion average kinetic energy is forward-peaked and shows a stronger dependence on the laser pulse fluence than on the ion flux. Such results can be ascribed to space charge effects that occur during the early stages of LPP formation.

Effect of Ion Energies on the Surface Interactions of NO Formed in Nitrogen Oxide Plasma Systems

The contributions of various gas-phase species in surface reactions are of significant value to assess and improve catalytic substrates for abatement of vehicular emissions. The impact of ions on surface scatter of NO radicals is investigated with an aim toward improving and tailoring surfaces for the reduction or removal of nitrogen oxide (N(x)O(y)) species via inductively coupled plasmas (ICPs). Nascent ions are monitored via mass spectrometry and energy analysis for a variety of N(x)O(y) precursor gases. The total average ion energy (<E(i)>(total)) determined for all ions within each respective plasma system shows a strong positive correlation with applied rf power and a negative correlation with system pressure. The imaging of radicals interacting with surfaces (IRIS) technique was used to determine the role ions play in the surface scatter of NO radicals. The net effect of ions on substrate processing is largely dependent upon <E(i)>(total). Scatter coefficients (S), determined for ion-limited and ion-rich plasma systems were used to correlate <E(i)>(total) and scatter. The resultant effect is that ions play a substantial role in scatter of NO only when <E(i)>(total) > ~50 eV. The majority of systems studied contained ions below this energy threshold, suggesting knowledge of ion energies is integral to appropriately controlling the chemistry occurring between the gas-phase and surface.

Time dependent behaviors of ion-ion plasmas exposed to various voltage waveforms in the kilohertz to megahertz frequency range

An ion-ion plasma, situated between two parallel electrodes, is studied with the use of a time dependent one-dimensional particle-in-cell Monte-Carlo collisions model. This plasma consists of only positively and negatively singly charged ions with the same order of mass and temperature (the electron density is zero). The right electrode is grounded, and the left electrode is biased with a voltage waveform varying from sinusoidal to square with the frequency in the kHz to MHz range. The sheath evolution and the particle flux towards the electrodes, as a function of both space and time, are investigated for the various waveforms and frequencies. The sheath evolution has a strong influence on the time averaged ion energy distribution function (IEDF). The IEDF is broad with a low energy tail for low frequency sinusoidal biases (25 kHz) while peaked at low energy for higher frequencies (2 MHz). For square waveforms, the IEDF is mono-energetic with some broadening when the rise time is faster than the typical time to establish the steady state sheath formation (&amp;lt;0.3 μs).

Separate control of the ion flux and ion energy in capacitively coupled radio-frequency discharges using voltage waveform tailoring

We experimentally characterize an argon plasma in a geometrically symmetric, capacitively coupled rf discharge, excited by pulse-type tailored waveforms (generated using multiple voltage harmonics). The results confirm a number of predictions made by recent particle-in-cell simulations of a similar system and demonstrate a unique form of control over the ion flux and ion energy in capacitively coupled plasmas; by increasing the number of applied harmonics (equivalent to decreasing the pulse width), it is possible to increase the plasma density and ion flux (together with the power deposition) while keeping the average ion energy on one of the electrodes low and constant.

Control of the ion flux and ion energy in CCP discharges using non-sinusoidal voltage waveforms

Using particle-in-cell simulations we perform a characterization of the ion flux and ion energy in a capacitively coupled rf plasma reactor excited with non-sinusoidal voltage waveforms. The waveforms used are positive Gaussian type pulses (with a repetition frequency of 13.56 MHz), and as the pulse width is decreased, three main effects are identified that are not present in typical symmetric sinusoidal discharges: (1) the ion flux (and plasma density) rapidly increases, (2) as the pressure increases a significant asymmetry in the ion fluxes to the powered and grounded electrodes develops and (3) the average ion energy on the grounded electrode cannot be made arbitrarily small, but in fact remains essentially constant (together with the bias voltage) for the pressures investigated (20–500 mTorr). Effects (1) and (3) potentially offer a new form of control in these types of rf discharges, where the ion flux can be increased while keeping the average ion energy on the grounded electrode constant. This is in contrast with the opposite control mechanism recently identified in Donkó et al (2009 J. Phys. D: Appl. Phys. 42 025205), where by changing the phase angle between applied voltage harmonics the ion flux can be kept constant while the ion energy (and bias voltage) varies.

Dynamics of Coulomb explosion of Xe clusters in an ultrafast high-intensity laser field

Under the classical particle-dynamics simulation, Coulomb explosion of small Xe clusters in an intense femtosecond laser field and energies of the generated particles are studied. Our results show that the average kinetic energy of the generated ions increases with the cluster size. The dependence of the average kinetic energy of the product ions on the number of atoms inside Xe clusters and the change of average ions charge states from different shells of Xe561 within the interactional time are studied. The oscillation of the mass center of electrons along the direction of the laser electric field is calculated and the time evolutions of the total potential energy and the total kinetic energy of all the product particles are presented here. The total kinetic energy of Xe ions and free electrons are also calculated as functions of the interactional time.

The energy distribution function of ions impinging on nanoparticles in a collisional low-pressure plasma

A self-consistent particle-in-cell/Monte Carlo collision (PIC/MCC) simulation is used to calculate the ion energy distribution (IED) function of ions impinging on the surface of nanoparticles in low-pressure argon plasmas. The computation includes the effects of resonant charge-exchange and elastic collisions between ions and neutrals through a Monte Carlo null-collision method and considers both Maxwellian and non-Maxwellian electron energy distributions. Results show a strong dependence of the IED on pressure. Intermediate pressures yield a remarkable reduction in the average ion energy and an enhancement in the ion flux to the surface of the nanoparticles.