Phase Resolved Optical Emission Spectroscopy Research Articles

Mode transition in a planar-coil inductively driven discharge caused by an external magnetic field

A hydrogen discharge inductively driven by a planar coil is studied by employing the phase resolved optical emission spectroscopy method, which permits observations not only on the stationary discharge structure but also of its time evolution over the cycle of the rf signal producing the discharge. Since the discharge is considered as a single element of a matrix source of negative hydrogen ions, it is equipped with an extraction device forming an additional grounded metal wall on the side opposite to that where the coil is positioned. Regarding use of a magnetic filter (a localized external magnetic field), the modifications in the discharge caused by the magnetic field have been studied. The results show: (i) transition of the discharge from a capacitive mode to an inductive one with the shift of the magnetic filter from the extraction device towards the coil, (ii) asymmetry both of the stationary and time-varying discharge structure of the inductive mode caused, respectively, by a diamagnetic drift and an -drift in the rf field, (iii) formation in the capacitive mode of the discharge of two electron beams, starting from the position of the magnetic filter, in addition to the beams well known as electron acceleration at the wall sheath expansion and (iv) asymmetry in the structure of the capacitive mode due to -drifts in the dc and rf fields.

Fast Time Resolved Techniques as Key to the Understanding of Energy and Particle Transport in HPPMS-Plasmas

High-power pulsed magnetron sputtering (HPPMS) plasmas are pulsed discharges, where the plasma composition as well as the fluxes and energies of ions are changing during the pulse. The time resolved energy distribution for Ar <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1+</sup> ions was measured and phase resolved optical emission spectroscopy for the Ar I line at 760 nm was done to get more insight in the transport properties of the plasma forming noble gas. These measurements were performed during HPPMS of titanium with argon at 0.5 Pa. The peak power density during the 50- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">\(\mu \) </tex-math></inline-formula> s pulses was 1.8 kW/cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">\(^{2}\) </tex-math></inline-formula> . In this contribution, we demonstrate how time resolved mass spectrometry and Intensified Charge-Coupled Device, Intensified CCD cameras can be used to shed more light on energy and particle transport in HPPMS-plasmas.

Excitation Patterns and Heating Mechanisms During E-H-Mode Transition in Inductively Coupled RF Oxygen Discharges

Optical excitation patterns and electron heating mechanisms were investigated in inductively coupled radio frequency oxygen discharges by phase and space resolved optical emission spectroscopy. Four excitation patterns due to different electron heating mechanisms were found during the E-H-mode transition.

Observations of a mode transition in a hydrogen hollow cathode discharge using phase resolved optical emission spectroscopy

Two distinct operational modes are observed in a radio frequency (rf) low pressure hydrogen hollow cathode discharge. The mode transition is characterised by a change in total light emission and differing expansion structures. An intensified CCD camera is used to make phase resolved images of Balmer α emission from the discharge. The low emission mode is consistent with a typical γ discharge, and appears to be driven by secondary electrons ejected from the cathode surface. The bright mode displays characteristics common to an inductive discharge, including increased optical emission, power factor, and temperature of the H2 gas. The bright mode precipitates the formation of a stationary shock in the expansion, observed as a dark region adjacent to the source-chamber interface.

Origin of the energetic ions at the substrate generated during high power pulsed magnetron sputtering of titanium

High power impulse magnetron sputtering (HiPIMS) plasmas generate energetic metal ions at the substrate as a major difference to conventional direct current magnetron sputtering (dcMS). The origin of these very energetic ions in HiPIMS is still an open issue, which is unravelled using two fast diagnostics: time-resolved mass spectrometry with a temporal resolution of 2 µs and phase resolved optical emission spectroscopy with a temporal resolution of 1 µs. A power scan from dcMS-like to HiPIMS plasmas was performed, with a 2 inch magnetron and a titanium target as sputter source and argon as working gas. Clear differences in the transport as well as the energetic properties of Ar+, Ar2+, Ti+ and Ti2+ were observed. For discharges with highest peak power densities a high energetic group of Ti+ and Ti2+ could be identified with energies of approximately 25 eV and of 50 eV, respectively. A cold group of ions was always present. It is found that hot ions are observed only when the plasma enters the spokes regime, which can be monitored by oscillations in the IV characteristics in the MHz range that are picked up by the used VI probes. These oscillations are correlated with the spokes phenomenon and are explained as an amplification of the Hall current inside the spokes as hot ionization zones. To explain the presence of energetic ions, we propose a double layer (DL) confining the hot plasma inside a spoke: if an atom becomes ionized inside the spokes region it is accelerated because of the DL to higher energies whereas its energy remains unchanged if it is ionized outside. In applying this DL model to our measurements the observed phenomena as well as several measurements from other groups can be explained. Only if spokes and a DL are present can the confined particles gain enough energy to leave the magnetic trap. We conclude from our findings that the spoke phenomenon represents the essence of HiPIMS plasmas, explaining their good performance for material synthesis applications.

The influence of surface properties on the plasma dynamics in radio-frequency driven oxygen plasmas: Measurements and simulations

Plasma parameters and dynamics in capacitively coupled oxygen plasmas are investigated for different surface conditions. Metastable species concentration,electronegativity, spatial distribution of particle densities as well as the ionization dynamics are significantly influenced by the surface loss probability of metastable singlet delta oxygen (SDO). Simulated surface conditions are compared to experiments in the plasma-surface interface region using phase resolved optical emission spectroscopy. It is demonstrated how in-situ measurements of excitation features can be used to determine SDO surface loss probabilities for different surface materials.

Characterization of atmospheric-pressure ac micro-discharge in He–N2 mixture using time- and space-resolved optical emission spectroscopy

An ac discharge is ignited with the frequency of 170 kHz at the tip of a sharpened electrode in He–N2 gas mixture under atmospheric pressure. Plasma parameters (electron density and reduced electric field) are determined using phase-resolved optical emission spectroscopy. An absolutely calibrated ICCD camera with an appropriate filter is used for the time- and space-resolved measurement of N2(C–B,0–2) as well as (B–X,0–0) emissions. From the temporal and spatial distributions of these emission bands, time- and space-resolved plasma parameters are determined. Limits of time and space resolutions of this diagnostic method are discussed.

On the spatial distribution of the electromagnetic field in small-radius planar coil inductive discharges

In this paper, the distribution of the electromagnetic field in planar coil inductive discharges in the case of small radius of the plasma vessel is studied. The role of the radius on the skin depth and the structure of the field is investigated analytically in a homogeneous plasma. The results show that in the H-mode at moderate and low plasma densities the skin depth is determined by the radius of the tube and is almost independent of the electron concentration. The importance of the E-mode excited by the radial component of the current in the coil increases with a decrease in radius. The results are confirmed by numerical models in the more complicated cases of a plasma–vacuum–metal structure and an inhomogeneous plasma. Experimental results from the phase-resolved optical emission spectroscopy method confirming the conclusion from the analytical and numerical calculations are also presented.

Observation of Ω mode electron heating in dusty argon radio frequency discharges

The time-resolved emission of argon atoms in a dusty plasma has been measured with phase-resolved optical emission spectroscopy using an intensified charge-coupled device camera. For that purpose, three-dimensional dust clouds have been confined in a capacitively coupled rf argon discharge with the help of thermophoretic levitation. While electrons are exclusively heated by the expanding sheath (α mode) in the dust-free case, electron heating takes place in the entire plasma bulk when the discharge volume is filled with dust particles. Such a behavior is known as Ω mode, first observed in electronegative plasmas. Furthermore, particle-in-cell simulations have been carried out, which reproduce the trends of the experimental findings. These simulations support previous numerical models showing that the enhanced atomic emission in the plasma can be attributed to a bulk electric field, which is mainly caused by the reduced electrical conductivity due to electron depletion.

Phase-resolved optical emission spectroscopy for an electron cyclotron resonance etcher

Phase-resolved optical emission spectroscopy (PROES) is used for the measurement of plasma products in a typical industrial electron cyclotron resonance (ECR) plasma etcher. In this paper, the PROES of oxygen and argon atoms spectral lines are investigated over a wide range of process parameters. The PROES shows a discrimination between the plasma species from gas phase and those which come from the solid phase due to surface etching. The relationship between the micro-wave and radio-frequency generators for plasma creation in the ECR can be better understood by the use of PROES.

The effect of dust on electron heating and dc self-bias in hydrogen diluted silane discharges

In capacitive hydrogen diluted silane discharges the formation of dust affects plasma processes used, e.g. for thin film solar cell manufacturing. Thus, a basic understanding of the interaction between plasma and dust is required to optimize such processes. We investigate a highly diluted silane discharge experimentally using phase-resolved optical emission spectroscopy to study the electron dynamics, laser light scattering on the dust particles to relate the electron dynamics with the spatial distribution of dust, and current and voltage measurements to characterize the electrical symmetry of the discharge via the dc self-bias. The measurements are performed in single and dual frequency discharges. A mode transition from the α-mode to a bulk drift mode (Ω-mode) is found, if the amount of silane and, thereby, the amount of dust and negative ions is increased. By controlling the electrode temperatures, the dust can be distributed asymmetrically between the electrodes via the thermophoretic force. This affects both the electron heating and the discharge symmetry, i.e. a dc self-bias develops in a single frequency discharge. Using the Electrical Asymmetry Effect (EAE), the dc self-bias can be controlled in dual frequency discharges via the phase angle between the two applied frequencies. The Ω-mode is observed for all phase angles and is explained by a simple model of the electron power dissipation. The model shows that the mode transition is characterized by a phase shift between the applied voltage and the electron conduction current, and that the plasma density profile can be estimated using the measured phase shift. The control interval of the dc self-bias obtained using the EAE will be shifted, if an asymmetric dust distribution is present. However, the width of the interval remains unchanged, because the dust distribution is hardly affected by the phase angle.

On the two modes of operation of planar-coil-driven inductive discharges in hydrogen

Langmuir probe and laser photodetachment technique diagnostics as well as optical emission spectroscopy and phase-resolved optical emission spectroscopy are used to study a planar-coil-driven inductive discharge in hydrogen sustained in the first chamber of a two-chamber plasma source. The discharge is operated in the power range 50–400 W at 27 MHz. The gas-pressure range studied is p = 20–60 mTorr. The results obtained over the first half of the discharge length, that is starting from the position of the coil, outline discharge maintenance in the two modes of the inductive discharge. The inductive mode sustained at a high rf power appears with rf power deposition by ring-shaped rf electric field intensity close to the coil and high electron density there, followed by its strong drop in the remote plasma region. The capacitive mode sustaining low-density plasmas also appears with two regions specified by different mechanisms of rf power deposition: plasma heating by an electron beam acceleration in the wall sheath during its expansion and Joule heating in the plasma bulk. The similarity—at high and low rf power—in the axial structure of the discharge over the second half of its length, that is touching the transition between the two chambers of the source, and the appearance of a second maximum of the dc potential there are related to the configuration of the source. The obtained axial variation of the negative ion density obeys that of the potential of the dc electric field in the discharge.

Phase resolved optical emission spectroscopy of dual frequency capacitively coupled plasma

In this article we use phase resolved optical emission spectroscopy to study emission pattern in plasma sheath of dual frequency capacitively coupled plasma in Ar and Ar-O2 discharge. Two emission patterns are found in sheath region of radio frequency coupled powered electrode. The first pattern is related to electron impact excitation because of the sheath expansion. The second pattern is caused by electron impact excitation of secondary electrons. Two emission patterns are also highly modulated with the low frequency cycle. Under the condition of argon discharge, the emission intensities of the two excitation processes are very similar. The emission structure by secondary electrons becomes weak with the increase of O2 content in the gas mixture. In addition, we also use phase resolved optical emission spectroscopy to study low frequency cycle averaged axial emission profile of excited atomic argon at 751 nm in Ar-O2 mixture gas. Distance from the powered electrode (about 3.8 mm) is defined as the boundary sheath of dual frequency capacitively coupled plasma.

Electron and negative ion dynamics in electronegative cc-rf plasmas

The line-integrated density of electrons and negative ions in asymmetric capacitively coupled oxygen rf plasmas are measured by 160.28 GHz microwave interferometry combined with simultaneous laser photodetachment. The high temporal resolution of the interferometer of 200 ns as well as an optimized phase shift resolution of about 0.016° allows the investigation of discharge regimes with rather low electron densities down to 5 × 1014 m−3, fast fluctuations in the electron density due to plasma instabilities, pulsed mode operation as well as laser photodetachment. Additionally phase resolved optical emission spectroscopy is applied to investigate the fundamental physics of the excitation dynamics in the electronegative plasmas oxygen and tetrafluoromethane due to electron impact excitation. Electrons are heated by the rf sheath expansion (α-mode operation) and field reversal during sheath collapse. Secondary electrons are produced and heated during the expanded rf sheath (γ-mode operation) due to collisional detachment of negative ions and heavy particle bombardment of the electrode surface. Furthermore, the excitation due to fast heavy particle collisions in front of the electrode is discussed.

Dynamics and Electronegativity of Oxygen RF Plasmas

AbstractThe capacitively coupled radio frequency plasma at 13.56 MHz in oxygen was systematically studied by 160 GHz Gaussian beam microwave interferometry at high temporal resolution (200 ns) and simultaneous laser photodetachment for electron and negative ion density analysis. Additionally, spatio‐temporally resolved electric probe measurements were performed for comparison with microwave interferometry. A high and low electronegative operation mode was found in the asymmetric rf discharge. In the high electronegative mode it was shown the significant role of the metastable excited oxygen molecules in electron attachment and detachment processes. In particular, a temporary electron density increase is observed in the early afterglow of a pulsed rf plasma. The transition between both modes is driven by the rf power and the self‐bias voltage, respectively. In connection with the phase resolved optical emission spectroscopy and the study of the electron heating mechanisms the transition into the low electronegative mode at higher rf power shows a relation to the alpha to gamma mode transition. Furthermore, electron density fluctuations are measured over a wide field of processing parameters, e.g. due to the attachment‐induced ionization instability. PIC‐MCC simulation and fluid model calculation of a symmetric oxygen rf discharge confirm the different electron heating mechanisms and the dominance of negative atomic oxygen ions (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Evidence of weak plasma series resonance heating in the H-mode of neon and neon/argon inductively coupled plasmas

Phase-resolved optical emission spectroscopy measurements in argon and neon inductively coupled plasmas (ICPs) have revealed a surplus of high-energy electrons in neon-containing plasmas. Differences between results of emission model analyses using neon and argon lines (as well as probe measurements) also indicate a high-energy enhancement in neon-containing plasmas. The abundance of these extra high-energy electrons is correlated with the sheath thickness near the rf antenna and can be reduced by either adding a Faraday shield (external shielding) or increasing the plasma density. A comparison of modelled and experimental values of the 13.56 MHz time modulation of select neon emission lines strongly suggests plasma series resonance heating adjacent to the ICP antenna as the source of the extra heating.

Coupling effects in inductive discharges with radio frequency substrate biasing

Low pressure inductively coupled plasmas (ICP) operated in neon at 27.12 MHz with capacitive substrate biasing (CCP) at 13.56 MHz are investigated by phase resolved optical emission spectroscopy, voltage, and current measurements. Three coupling mechanisms are found potentially limiting the separate control of ion energy and flux: (i) Sheath heating due to the substrate biasing affects the electron dynamics even at high ratios of ICP to CCP power. At fixed CCP power, (ii) the substrate sheath voltage and (iii) the amplitude as well as frequency of plasma series resonance oscillations of the RF current are affected by the ICP power.

Ionization by Drift and Ambipolar Electric Fields in Electronegative Capacitive Radio Frequency Plasmas

Unlike α- and γ-mode operation, electrons accelerated by strong drift and ambipolar electric fields in the plasma bulk and at the sheath edges are found to dominate the ionization in strongly electronegative discharges. These fields are caused by a low bulk conductivity and local maxima of the electron density at the sheath edges, respectively. This drift-ambipolar mode is investigated by kinetic particle simulations, experimental phase-resolved optical emission spectroscopy, and an analytical model in CF(4). Mode transitions induced by voltage and pressure variations are studied.

Experimental and theoretical study of dynamic effects in low-frequency capacitively coupled discharges

A low-frequency capacitively coupled radio-frequency (rf) discharge in Ar excited at 1.76 MHz is studied both experimentally and theoretically. Experimental measurements of electron concentration, discharge voltage and current are presented for a wide range of rf input powers. The rf current shape is nonsinusoidal, close to the triangle one. The evolution of Ar(2p1) emission excitation function in the interelectrode gap during an rf cycle is measured using the phase-resolved optical emission spectroscopy technique. Theoretical study is based on the particle-in-cell Monte Carlo collision numerical simulation. Specific dynamic features of the low-frequency discharge are discussed. The important role of secondary electrons in discharge maintenance and power balance is shown. This study is crucial for understanding dual-frequency discharges with a corresponding value of low frequency.

Electron Dynamics in a Radio-Frequency-Driven Microatmospheric Pressure Plasma Jet

Space and phase resolved optical emission spectroscopy and numerical simulations are employed to investigate the excitation dynamics of a radio-frequency-driven microatmospheric pressure plasma jet. Different power coupling modes can be identified by characteristic structures obtained from the spectroscopic measurements. Numerical results from a 1-D fluid code with semikinetic treatment of the electrons show very good agreement with the experimental results.