Time-averaged Ion Energy Distribution Research Articles

Influence of plasma ionization on the elastic modulus and tribology behavior of carbon films deposited by the HiPIMS technique

This work reveals the influence of discharge current on carbon-ion energies of plasma, elastic modulus, and friction coefficient at the nano- and macroscale of carbon films deposited via high-power impulse magnetron sputtering. Three applied discharge current conditions in deposition processes were employed to obtain three-carbon films of interest. The number of carbon ions with their energies was obtained via mimic tests of the deposition process using three similar discharge currents through a quadrupole mass spectrometer detector based on the time-averaged ion energy distribution function. The bonding structure of the films was evaluated using Raman spectroscopy, fitting the Diamond and Graphite peaks to obtain a semiquantitative analysis. The elastic modulus of the carbon films was determined from atomic force acoustic microscopy measurements avoiding the influence of the substrates. The friction coefficient was analyzed at the nanoscale via atomic force microscopy and at the macroscale via tribometry. Significant alterations were observed in the number and energy level of the carbon ions with the variation of discharge current. These alterations significantly influenced the bonding properties, elastic modulus, and tribology behavior. A higher elastic modulus and higher sp3 bond content were observed for the film with a lower number of carbon ions and less energy.

Experimental characterization of ID-Hall, a double stage Hall thruster with an inductive ionization stage

We present experimental results on the characterization of ID-Hall, a double stage Hall thruster with an inductively coupled magnetized ionization stage. This first experimental prototype of ID-Hall operates in a low power regime, typically below 400 W. The purpose of this work is not only to study the properties of ID-Hall and provide directions for optimization but also, and more generally, to study the relevance of the double stage concept with respect to important physical issues such as current oscillations and anomalous electron transport. We present experimental measurements of the current–voltage characteristics, extracted ion beam current (Faraday cup measurements), and ion energy and angular distribution functions (RPA measurements) in single and double stage regimes of operation, as a function of DC voltage and RF power. The results demonstrate that ions can be extracted from the inductive RF ionization stage even under conditions where the applied DC voltage is not sufficient to ionize the gas flow, i.e., ID-Hall can indeed operate in a double stage regime. At intermediate voltages, the overshoot observed in the current–voltage characteristics of single stage thrusters disappears in double stage operation, indicating a decrease in anomalous electron transport. Time-resolved and time-averaged ion energy distribution measurements show a coupling between ion energy, breathing mode oscillations, and the magnetic configuration of ID-Hall.

Phase shift effects of radio-frequency bias on ion energy distribution in continuous wave and pulse modulated inductively coupled plasmas**Project supported by the Important National Science and Technology Specific Project, China (Grant No. 2011ZX02403-001), the National Natural Science Foundation of China (Grand No. 11675039), and the Fundamental Research Funds for the Central Universities, China (Grand No. DUT16LK06).

A retarding field energy analyzer (RFEA) is used to measure the time-averaged ion energy distributions (IEDs) on the substrate in both continuous wave (CW) and synchronous pulse modulated radio-frequency (RF) inductively coupled Ar plasmas (ICPs). The effects of the phase shift θ between the RF bias voltage and the RF source on the IED is investigated under various discharge conditions. It is found that as θ increases from 0 to π, the IED moves towards the low-energy side, and its energy width becomes narrower. In order to figure out the physical mechanism, the voltage waveforms on the substrate are also measured. The results show that as θ increases from 0 to π, the amplitude of the voltage waveform decreases and, meanwhile, the average sheath potential decreases as well. Specifically, the potential drop in the sheath on the substrate exhibits a maximum value at the same phase (i.e., θ = 0) and a minimum value at the opposite phase (i.e., θ = π). Therefore, when ions traverse across the sheath region above the substrate, they obtain less energies at lower sheath potential drop, leading to lower ion energy. Besides, as θ increases from π to 2π, the IEDs and their energy widths change reversely.

A comparative study of thick TiSiCN nanocomposite coatings deposited by dcMS and HiPIMS with and without PEMS assistance

As one version of high power impulse magnetron sputtering (HiPIMS) technique, deep oscillation magnetron sputtering (DOMS) generates large oscillation high power pulses to achieve a high fraction of metallic ions of the target species for arc-free reactive HiPIMS sputtering. Plasma enhanced magnetron sputtering (PEMS), an advanced version of conventional dc magnetron sputtering (dcMS), generates an extremely high flux of low energy gas ions with hot filament thermionic emission to produce an extra global plasma in the deposition system. As compared to conventional dcMS, both DOMS and PEMS aim at utilizing a highly ionized plasma to improve the structure and properties of the coatings. To gain a better understanding of the advantages and limits of these techniques, we present a comparative study for the thick TiSiCN nanocomposite coatings (15 μm) deposited using dcMS and DOMS with and without the PEMS assistance. The time averaged ion energy distributions of the plasmas generated by these techniques for TiSiCN coating depositions were characterized using an electrostatic quadrupole plasma mass spectrometer (EQP). The microstructure and mechanical properties of the coatings were characterized using X-ray diffraction, scanning electron microscopy, scratch test and nanoindentation. The microstructure, residual stress, and mechanical properties of the coatings were linked to the plasma properties and discussed in details. It is clear that the PEMS assistance significantly improves the microstructure and mechanical properties of the coatings produced by either dcMS or DOMS along with residual stress reduction.

Ion energy distributions in a pulsed dual frequency inductively coupled discharge of Ar/CF4 and effect of duty ratio

Controlling time averaged ion energy distribution (IED) is becoming increasingly important in many plasma material processing applications for plasma etching and deposition. The present study reports the evolution of ion energy distributions with radio frequency (RF) powers in a pulsed dual frequency inductively discharge and also investigates the effect of duty ratio. The discharge has been sustained using two radio frequency, low (P2 MHz = 2 MHz) and high (P13.56 MHz = 13.56 MHz) at a pressure of 10 mTorr in argon (90%) and CF4 (10%) environment. The low frequency RF powers have been varied from 100 to 600 W, whereas the high frequency powers from 200 to 1200 W. Typically, IEDs show bimodal structure and energy width (energy separation between the high and low energy peaks) increases with increasing P13.56 MHz; however, it shows opposite trends with P2 MHz. It has been observed that IEDs bimodal structure tends to mono-modal structure and energy peaks shift towards low energy side as duty ratio increases, keeping pulse power owing to mode transition (capacitive to inductive) constant.

An analytical model for time-averaged ion energy distributions in collisional rf sheaths

An analytical model is proposed for time-averaged ion energy distributions (IEDs) in collisional rf sheaths (λi < sm, where λi is the ion mean free path and sm is the sheath thickness), in which charge transfer is the dominant ion-neutral collision mechanism. Our model is different from the model in Israel et al 2006 J. Appl. Phys. 99 093303 in two aspects. Firstly, to calculate the ion flux, we consider ions created in both the space charge region and the quasi-neutral region instead of only the quasi-neutral region. Secondly, to calculate the ion energy, we use an rf-modulated transit time instead of only the average transit time. Consequently, our model is valid over a wider pressure range (from a weakly collisional sheath to a strongly collisional sheath) compared with the model in Israel et al 2006 J. Appl. Phys. 99 093303. Our model shows that, in a collisional rf sheath (argon, 5 Pa, 27.12 MHz and 100 W), 65% of the ion flux in the IED comes from secondary ions created in the space charge region. Results of our model are compared with those obtained by measurement, direct integration method and particle-in-cell/Monte Carlo collision simulation.

Phase modulation in pulsed dual-frequency capacitively coupled plasmas

Particle-in-cell/Monte Carlo collision simulations, coupled with an external circuit, are used to investigate the behavior of pulsed dual-frequency (DF) capacitively coupled plasmas (CCPs). It is found that the phase shift θ between the high (or low) frequency source and the pulse modulation has a great influence on the ion density and the ionization rate. By pulsing the high frequency source, the time-averaged ion density shows a maximum when θ = 90∘. The time-averaged ion energy distribution functions (IEDFs) at the driven electrode, however, keep almost unchanged, illustrating the potential of pulsed DF-CCP for independent control of ion density (and flux) and ion energy. A detailed investigation of the temporal evolution of the plasma characteristics indicates that several high frequency harmonics can be excited at the initial stage of a pulse period by tuning the phase shift θ, and this gives rise to strong sheath oscillations, and therefore high ionization rates. For comparison, the pulsing of the low frequency source is also studied. In this case, the ion density changes slightly as a function of time, and the time-averaged ion density shows the same trend as in the HF modulation for different phase shifts θ. Moreover, the time-averaged IEDFs at the driven electrode can be modulated, showing the potential to reduce the maximum ion bombardment energy.

2-dimensional ion velocity distributions measured by laser-induced fluorescence above a radio-frequency biased silicon wafer

The dynamics of ions traversing sheaths in low temperature plasmas are important to the formation of the ion energy distribution incident onto surfaces during microelectronics fabrication. Ion dynamics have been measured using laser-induced fluorescence (LIF) in the sheath above a 30 cm diameter, 2.2 MHz-biased silicon wafer in a commercial inductively coupled plasma processing reactor. The velocity distribution of argon ions was measured at thousands of positions above and radially along the surface of the wafer by utilizing a planar laser sheet from a pulsed, tunable dye laser. Velocities were measured both parallel and perpendicular to the wafer over an energy range of 0.4–600 eV. The resulting fluorescence was recorded using a fast CCD camera, which provided resolution of 0.4 mm in space and 30 ns in time. Data were taken at eight different phases during the 2.2 MHz cycle. The ion velocity distributions (IVDs) in the sheath were found to be spatially non-uniform near the edge of the wafer and phase-dependent as a function of height. Several cm above the wafer the IVD is Maxwellian and independent of phase. Experimental results were compared with simulations. The experimental time-averaged ion energy distribution function as a function of height compare favorably with results from the computer model.

Ion energy distributions in inductively coupled plasmas having a biased boundary electrode

In many plasma materials processing applications requiring energetic ion bombardment such as plasma etching, control of the time-averaged ion energy distributions (IEDs) to surfaces is becoming increasingly important to discriminate between surface processes having different threshold energies. Inductively coupled plasmas (ICPs) are attractive in this regard since the plasma potential is low and so the energy of ion fluxes can be more finely tuned with externally applied biases. In these situations, pulsed plasmas provide another level of control as the IEDs from different times during the pulse power period can be combined to create the desired time-averaged IED. A recent development in controlling of IEDs in ICPs is the use of a boundary electrode (BE) in which a continuous or pulsed dc bias is applied to shift the plasma potential and modify the IEDs to surfaces without significant changes in the bulk plasma properties. Combinations of pulsing the ICP power and the BE bias provide additional flexibility to craft IEDs. In this paper we discuss results from a computational investigation of IEDs to a grounded substrate in low-pressure (a few to 50 mTorr) ICPs sustained in argon. Results are compared with experimental measurements of plasma properties and IEDs. We demonstrate the ability to customize IEDs consisting of three energy peaks corresponding to the plasma potential during the plasma active glow, plasma afterglow and the plasma potential with the applied boundary voltage.

Mass spectrometric study of discharges produced by a large-area dual-frequency–dual-antenna inductively coupled plasma source

An energy-resolved quadrupole mass spectrometer is used to investigate the time-averaged ion energy distribution (IED) of positive ionic species in an Ar/CF4 (90%/10%) discharge produced by dual-frequency–dual-antenna, next-generation large-area inductively coupled plasma source. The operating pressure is 10 mTorr. Two radio frequencies of 2 MHz (low frequency) and 13.56 MHz (high frequency) are used to initiate and sustain the discharge. The orifice of the mass spectrometer was 100 µm in diameter and placed at 30 mm below the ICP source and 20 mm outside the discharge volume.It is observed that both of the frequencies have significant effect on IEDs of all prominent discharge species. The evolution of IEDs with power shows that the discharge undergoes a mode transition (E to H) as the applied power is increased. At a fixed value of P13.56 MHz (250 and 500 W), the energy spread and the energy separation between two peaks of IEDs increase illustrating enhanced E-mode. Above P13.56 MHz = 500 W, the IEDs show opposite trends, i.e. decreasing energy spread and energy separation between two peaks, showing the strengthening of H-mode. Increasing P13.56 MHz at a fixed value of P2 MHz has similar effects. A comparison of IEDs sampled at a fixed total power (P13.56 MHz + P2 MHz) demonstrates that an IED can be tailored by changing the power ratio (P13.56 MHz/P2 MHz).

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).

Ion energy distribution measurements in rf and pulsed dc plasma discharges

A commercial retarding field analyzer is used to measure the time-averaged ion energy distributions of impacting ions at the powered electrode in a 13.56 MHz driven, capacitively coupled, parallel plate discharge operated at low pressure. The study is carried out in argon discharges at 10 mTorr where the sheaths are assumed to be collisionless. The analyzer is mounted flush with the powered electrode surface where the impacting ion and electron energy distributions are measured for a range of discharge powers. A circuit model of the discharge, in combination with analytical solutions for the ion energy distribution in radio-frequency sheaths, is used to calculate other important plasma parameters from the measured energy distributions. Radio-frequency compensated Langmuir probe measurements provide a comparison with the retarding field analyzer data. The time-resolved capability of the retarding field analyzer is also demonstrated in a separate pulsed dc magnetron reactor. The analyzer is mounted on the floating substrate holder and ion energy distributions of the impinging ions on a growing film, with 100 ns time resolution, are measured through a pulse period of applied magnetron power, which are crucial for the control of the microstructure and properties of the deposited films.

Metal Ionization in a High-Power Pulsed Sputtering Penning Discharge

Droplet-free metal plasma sources are a promising technology for enhancing the adhesion of deposited films to a smooth surface. High-power pulsed sputtering (HPPS) plasma is an arc-free glow discharge plasma with an instantaneous power consumption of several tens of kilowatts, although the average power consumed is the same as that in conventional sputtering discharge systems. Sputtered metallic species are significantly ionized. The fractions of singly and doubly charged ions of argon and titanium in an HPPS plasma are studied using a time-averaged ion energy distribution function in the power-density region from 0.5 to 2.5 kW/cm2. Facile production of metallic ions is confirmed. Singly charged ions are dominant in the plasma. Argon and titanium ions together constitute more than 90% of the charged particles in the plasma. The metal ions are produced with increasing plasma power consumption. The instantaneous power contributes to the ionization of the plasma species. The gas ion energies is in a narrow range of a few electronvolts. Conversely, metallic ions is in a wider range of energy up to approximately 10 eV. It is seen from an optical emission spectrum of the HPPS plasma that metallic species are highly ionized.

The structure and properties of chromium nitride coatings deposited using dc, pulsed dc and modulated pulse power magnetron sputtering

The time averaged ion energy distributions and ion fluxes of continuous dc magnetron sputtering (dcMS), middle frequency pulsed dc magnetron sputtering (PMS), and modulated pulse power (MPP) magnetron sputtering plasmas were compared during sputtering of a Cr target in an Ar/N 2 atmosphere in a closed field unbalanced magnetron sputtering system. The results showed that the dcMS plasma exhibited a low ion energy and ion flux; the PMS plasma generated a moderate ion flux of multiple high ion energy regions; while the MPP plasma exhibited a significantly increased number of target Cr + and gas ions with a low ion energy as compared to the dcMS and PMS plasmas. Cubic CrN coatings were deposited using these three techniques with a floating substrate bias. The structure and properties of the coatings were characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, nanoindentation, microscratch and ball-on-disk wear tests. It was found that the deposition rate of the MPP CrN depositions was slightly lower than those of the dcMS depositions, but higher than in the PMS depositions at similar average target powers. The coatings deposited in the dcMS and PMS conditions without the aid of the substrate bias exhibited large columnar grains with clear grain boundaries. On the other hand, the interruption of the large columnar grain growth accompanied with the renucleation and growth of the grains was revealed in the MPP CrN coatings. The MPP CrN coatings exhibited a dense microstructure, fine grain size and smooth surface with high hardness (24.5 and 26 GPa), improved wear resistance (COF = 0.33 and 0.36) and adhesion, which are the results of the low ion energy and high ion flux bombardment from the MPP plasma.

Ion composition produced by high power impulse magnetron sputtering discharges near the substrate

Plasma composition near the substrate was investigated in a high power impulse magnetron sputtering (HIPIMS) discharge using Langmuir probe analysis, mass spectroscopy, and atomic absorption spectroscopy. The HIPIMS discharge was operated in nonreactive Ar atmosphere at a pressure of 2.66 Pa and the magnetron cathode was furnished with Ti target. Plasma density, metal ion-to-neutral ratio, and gas ion-to-metal ion ratio were studied as a function of discharge current. At peak discharge current densities of ∼1 A cm−2, the results show that a dense plasma (ne∼1018 m−3) expanded from the target toward the substrate and lasted more than 330 μs after the supplied power was turned off. The shape of the time-averaged ion energy distribution function of sputtered material exhibited a transition from Thompson to Maxwellian distribution, indicating efficient energy transfer in the discharge. The metal content in the plasma monotonically increased with discharge current and the metal ion-to-neutral ratio reached approximately 1:1 in the postdischarge plasma at peak current density of 5 A cm−2.

The ion energy distributions and ion flux composition from a high power impulse magnetron sputtering discharge

The energy distribution of sputtered and ionized metal atoms as well as ions from the sputtering gas is reported for a high power impulse magnetron sputtering (HIPIMS) discharge. High power pulses were applied to a conventional planar circular magnetron Ti target. The peak power on the target surface was 1–2 kW/cm2 with a duty factor of about 0.5%. Time resolved, and time averaged ion energy distributions were recorded with an energy resolving quadrupole mass spectrometer. The ion energy distributions recorded for the HIPIMS discharge are broader with maximum detected energy of 100 eV and contain a larger fraction of highly energetic ions (about 50% with Ei>20 eV) as compared to a conventional direct current magnetron sputtering discharge. The composition of the ion flux was also determined, and reveals a high metal fraction. During the most intense moment of the discharge, the ionic flux consisted of approximately 50% Ti1+, 24% Ti2+, 23% Ar1+, and 3% Ar2+ ions.

Optical emission spectra and ion energy distribution functions in TiN deposition process by reactive pulsed magnetron sputtering

In this study, a pulsed magnetron sputtering was employed to synthesize TiN films. We deal with pulse parameters such as pulse frequency and duty cycle in argon–nitrogen plasmas. Also, optical emission spectroscopy and energy resolved mass spectrometry as an integrated plasma diagnostics was employed during synthesis of thin films.The major emission lines and time-averaged ion energy distribution functions (IEDFs) corresponding to various titanium and gas discharge species are in situ surveyed in terms of frequency and duty cycles during deposition. The corresponding surface morphology was analyzed by atomic force microscopy (AFM).

Stable and unstable behavior of inductively coupled electronegative discharges

Plasma instabilities have been observed in low-pressure inductive discharges, in the transition between low density capacitively driven and high density inductively driven discharges when attaching gases such as SF6 and Ar/SF6 mixtures are used. Oscillations of charged particles, plasma potential, and light, with frequencies from a few hertz to tens of kilohertz, are seen for gas pressures between 1 and 100 mTorr and discharge powers in the range of 75–1200 W. The region of instability increases as the plasma becomes more electronegative and the frequency of plasma oscillations increases as the power, pressure, and gas flow rate increase. Time-resolved measurements of electron temperature, and time averaged ion energy distribution at the wall have been made. A volume-averaged (global) model of the instability has been developed, for a discharge containing time varying densities of electrons, positive ions, and negative ions, and time invariant excited states and neutral densities. The particle and energy balance equations are integrated to produce the dynamical behavior. The model agrees well with experimental observations, and also shows a significant influence of the matching network. Values of plasma parameters and of chemical composition are measured to set the values used in the model, as required to obtain the good agreement with experiment. Capacitive coupling plays a crucial role in the instability process. A variable electrostatic (Faraday) shield has been used to control the capacitive coupling from the excitation coil to the plasma. The plasma instability disappears when the shielded area exceeds 65% of the total area of the coil. The model gives a slightly higher value of 85% for instability suppression with the same discharge conditions (Ar/SF6 1:1, 5 mTorr).

Wall effects on the chemistry in a pulsed oxygen/silane radiofrequency helicon plasma

A pulsed oxygen/silane radiofrequency (13.56 MHz) plasma is created in a helicon diffusion reactor used for silicon dioxide deposition. An energy selective mass spectrometer is used to measure the time-averaged ion energy distribution function of the four predominant positive ions (H3+, H3O+, SiH3+, and SiOH+) and the positive ion mass spectra during the breakdown phase and the steady-state phase of the pulse. Charging of the silica-covered chamber wall by ∼55 V is observed both for continuous and pulsed excitations. All positive ions with mass greater than 62 (as well as H2O+ and SiOH+) are mostly detected during the plasma breakdown phase. This could result from some physical sputtering of the chamber walls at the beginning of each pulse (when the plasma potential and the ion bombardment energy reach their maximum) or from an increase in the density of heavy radicals during the post-discharge (as a result of the electron density and temperature decrease) which can be ionized during plasma breakdown of the oncoming pulse. During the plasma breakdown an ion bombardment energy of ∼70 V is measured which is larger than the physical sputtering threshold (25 V) of thermally grown SiO2 (P-etch ∼2 Å s−1) in an oxygen plasma. Consequently, for inadequate pulsing conditions, physical sputtering of the soft and porous suboxides (P-etch >30 Å s−1) deposited on the chamber sidewalls will occur, thereby allowing hydrogen-rich radicals and positive ions to reenter the plasma chemistry.

Electrostatic quadrupole plasma mass spectrometer and Langmuir probe measurements of mid-frequency pulsed DC magnetron discharges

Time-averaged ion energy distributions (IEDs) and time-resolved floating potentials were measured for a range of reverse times at electrodes immersed in a 60 kHz pulsed magnetron discharge, and compared to the DC condition. The time-averaged IEDs correlate well to the time-resolved target voltage waveforms. From the measured data, electron temperatures for both pulsed and DC discharges were calculated to be approximately 2.5–3.0 eV, even at the ‘positive spike’ on the cathode voltage waveform observed during pulsing. Titanium oxide thin films were deposited in pulsed and DC discharges. Structure and properties of the deposited films were measured using glancing incidence X-ray diffraction and nanoindentation. Changes in crystallographic texture and hardness were observed as pulsing conditions were varied.