High Frequency Capacitive Discharge Research Articles

On the Impedance of the High-Frequency Capacitive Discharge at Different Excitation Methods

On the Impedance of the High-Frequency Capacitive Discharge at Different Excitation Methods

Impact of the plasma flow of a low-pressure high-frequency capacitive discharge on the adhesive and physical and mechanical characteristics of aramid and carbon fibers

The changes in the adhesive and physico-mechanical characteristics of aramid and carbon fibers treated with plasma of a highfrequency capacitive gas discharge of reduced pressure are considered. An increase of ~ 30% in the wettability of the surface of aramid and carbon fibers is shown. It was found that the strength of plastics made on the basis of aramid and carbon fibers treated with a plasma flow increases by ~ 26%.

Ion energy distribution function in very high frequency capacitive discharges excited by saw-tooth waveform

Tailoring the ion energy distribution function (IEDF) is vital for advanced plasma processing applications. Capacitively coupled plasma (CCP) discharges excited using a non-sinusoidal waveform have shown its capability to control IEDF through the generation of plasma asymmetry and DC self-bias. In this paper, we performed a particle-in-cell simulation study to investigate the IEDF in a symmetric capacitive discharge excited by a saw-tooth-like current waveform at a very high frequency. At a constant driving frequency of 27.12 MHz, the simulation results predict that the ion energy asymmetry in the discharge scales with the discharge current amplitude. A transition from a single narrow ion energy peak to a bi-modal type IEDF is observed with an increase in the current density amplitude. Further studies at a constant current density and varying the fundamental excitation frequency show that the ion energy asymmetry enhances with a reduction in the driving frequency. Increase in the plasma asymmetry and significant DC self-bias at a lower driving frequency is observed to be one of the principal factors responsible for the observed asymmetry in the ion energy peaks. An investigation of DC self-bias and plasma potential confirms that the powered electrode energy peak corresponds to the DC self-bias with respect to the plasma potential, and the grounded electrode peak corresponds to the plasma potential. These results suggest that although lower driving frequency is beneficial for generating the discharge asymmetry and large DC self-bias, a narrow low energy IEDF is plausible in very high frequency driven CCP systems.

Nonlinear harmonic excitations in collisional, asymmetrically-driven capacitive discharges

The standing wave effect, which may lead to center-high density profiles in high frequency capacitive discharges, can be enhanced by nonlinearly excited harmonics. In this work, a nonlinear transmission line model, which solves for the electromagnetic fields in the time domain, is coupled to a two-dimensional bulk plasma fluid model to study nonlinear effects in asymmetric cylindrical capacitive argon discharges. An analytical collisional or collisionless (ion) sheath model is used to determine the stochastic and ohmic sheath heating and the nonlinear dependence of sheath voltage on sheath charge. We first examine a base case of a 20 mTorr argon discharge driven with an electron power P e = 40 W at a frequency f = 60 MHz, using collisionless and collisional sheath models. For the collisionless sheath model, the nonlinearly excited harmonics near the series and spatial resonance frequencies significantly enhance the on-axis power deposition and lead to a sharp peak of electron density at the discharge center. The collisional sheath model gives a smaller sheath width, leading to lower series and spatial resonance frequencies and a smaller source voltage for the fixed electron power. As a result, lower harmonics with broader spatial profiles and decreased magnitude are excited, reducing the center-high plasma nonuniformity. Then, we examine the discharge in a pressure range of 20–100 mTorr at fixed P e = 40 W and f = 60 MHz, using the collisional sheath model. As pressure increases, the harmonics gradually damp out, and the enhancement of on-axis power deposition becomes less significant. At the same time, more power is localized near the powered electrode edge due to decreased skin depth and smaller energy diffusion. As a result, the density peak shifts from the radial center to the powered electrode edge.

Electric field nonlinearity in very high frequency capacitive discharges at constant electron plasma frequency

A self-consistent particle-in-cell simulation study is performed to investigate the effect of driving frequency on the electric field nonlinearity, electron heating mechanism, and electron energy distribution function (EEDF) in a low pressure symmetric capacitively coupled plasma (CCP) discharge at a constant electron plasma frequency maintained by adjusting the discharge voltage. The driving frequency is varied from 27.12 MHz to 100 MHz for a fixed discharge gap of 3.2 cm and at a gas pressure of 1 Pa. The simulation results provide insight into higher harmonic generations in a CCP system for a constant electron response time. The spatio-temporal evolution and spatial time-averaged electron heating are presented for different driving frequencies. The simulation results predict that the electric field nonlinearity increases with a rise in driving frequency along with a concurrent increase in higher harmonic contents. In addition to the electron heating and cooling near to the sheath edge, a positive <J.E> is observed into the bulk plasma at higher driving frequencies. The EEDF illustrates enhancement in the population of mid-energy range electrons as driving frequency increases thereby changing the shape of EEDF from bi-Maxwellian to nearly Maxwellian. For the constant ion flux on the electrode surface, a decrease in the ion energy by more than half is observed with an increase in driving frequency.

Effect of copper nanoparticles on plasma glow intensity of high frequency capacitive discharge

The paper presents the results of an experimental investigation of the effect of copper nanoparticles on plasma glow intensity of high-frequency capacitive discharge. Nanoparticles are composed of atoms, molecules and nanoclusters of copper, which are agglomerated, involving into the plasma volume. Magnetron sputtering system was used to obtain flux of copper atoms and molecules into the plasma volume. It is known that only the size and density of nanoparticles affects the optical properties of a plasma. In this case, almost any magnetron cathode material can be used as a source of nanoparticles. High-frequency plasma and magnetron discharge are ignited in the same medium with highly pure argon gas. To analyze the plasma glow intensity, the emission spectral lines with wavelengths of 737 and 750 nm were used. Thereby, it was revealed, that the density of nanoparticles in the plasma medium depends on the duration of the magnetron sputtering process and saturates with time. As the experimental results show, the plasma glow intensity with the duration of the magnetron sputtering process for 7, 14, 20 and 40 seconds, accordingly, increases in relative units ​​by 4451, 5280, 10725, 13545. That is, in this case, the plasma glow intensity with nanoparticles is three times higher than in case where nanoparticles are absent. If the sputtering process continues more than one minute, then the plasma glow intensity of does not significantly increase, this evidence the beginning of saturation of nanoparticles density in the plasma volume due to the escape of nanoparticles from the plasma volume. We believe that the experimental results find application in areas related with problems of increasing the gas-discharge glow intensity for light sources without increasing their input power.

Electric field filamentation and higher harmonic generation in very high frequency capacitive discharges

The effects of the discharge voltage on the formation and nature of electric field transients in a symmetric, collisionless, very high frequency, capacitively coupled plasma are studied using a self-consistent particle-in-cell (PIC) simulation code. At a driving frequency of 60 MHz and 5 mTorr of argon gas pressure, the discharge voltage is varied from 10 V to 150 V for a fixed discharge gap. It is observed that an increase in the discharge voltage causes filamentation in the electric field transients and to create multiple higher harmonics in the bulk plasma. Correspondingly, higher harmonics, up to 7th harmonic, in the discharge current are also observed. The power in the higher harmonics increases with a rise in the discharge voltage. The plasma density continues to increase with the discharge voltage but in a non-linear manner, whereas, the bulk electron temperature decreases. Meanwhile, the electron energy distribution function evolves from a Maxwellian at lower discharge voltages to a bi-Maxwellian at higher discharge voltages.

Formation of Films of Tungsten and its Oxides in a High-Frequency Capacitive Discharge in a D2-O2 Mixture

A technique for tungsten-film deposition on different substrates in asymmetrical high-frequency (1.76 MHz) capacitive discharge in a D2−6.5 mol % O2 mixture under a total pressure of 15 Pa and at 60–130°C is considered. A circular W strip near the upper inner edge of a cylindrical hollow cathode with a radius of 4.2 cm and a height of 10 cm is the source of W particles. The smooth transition from sputtering of the inner surface to deposition occurs at a distance of about 4 cm from the upper boundary of the open part of the cathode. W, Mo, ZrO2, Si, and Cu substrates are placed in the lower closed end (bottom) and on the inner lateral cathode surface. At the upper cathode edge the sputtering yield is (4–5) × 10−2 at/ion. The mass rate of W deposition on the cathode bottom does not depend on the substrate type and is 40 μg/(cm2 h). The peculiarities of the composition, morphology, and structure of W films obtained on the lateral surface and bottom of the hollow cathode are discussed.

Electrical discharges with liquid electrodes in optical materials processing

In this work the experimental research of high frequency capacitive electric discharge in the processing of semiconductor material surface is presented.

Nonlinear series resonance and standing waves in dual-frequency capacitive discharges

It is well-known that the nonlinear series resonance in a high frequency capacitive discharge enhances the electron power deposition and also creates standing waves which produce radially center-high rf voltage profiles. In this work, the dynamics of series resonance and wave effects are examined in a dual-frequency driven discharge, using an asymmetric radial transmission line model incorporating a Child law sheath. We consider a cylindrical argon discharge with a conducting electrode radius of 15 cm, gap length of 3 cm, with a base case having a 60 MHz high frequency voltage of 250 V and a 10 MHz low frequency voltage of 1000 V, with a high frequency phase shift between the two frequencies. For this phase shift there is only one sheath collapse, and the time-averaged spectral peaks of the normalized current density at the center are mainly centered on harmonic numbers 30 and 50 of the low frequency, corresponding to the first standing wave resonance frequency and the series resonance frequency, respectively. The effects of the waves on the series resonance dynamics near the discharge center give rise to significant enhancements in the electron power deposition, compared to that near the discharge edge. Adjusting the phase shift from π to 0, or decreasing the low frequency from 10 to 2 MHz, results in two or more sheath collapses, respectively, making the dynamics more complex. The sudden excitation of the perturbed series resonance current after the sheath collapse results in a current oscillation amplitude that is estimated from analytical and numerical calculations. Self-consistently determining the dc bias and including the conduction current is found to be important. The subsequent slow time variation of the high frequency oscillation is analyzed using an adiabatic theory.

Linear electromagnetic excitation of an asymmetric low pressure capacitive discharge with unequal sheath widths

It is well-known that standing waves having radially center-high radio frequency (rf) voltage profiles exist in high frequency capacitive discharges. In this work, we determine the symmetric and antisymmetric radially propagating waves in a cylindrical capacitive discharge that is asymmetrically driven at the lower electrode by an rf voltage source. The discharge is modeled as a uniform bulk plasma which at lower frequencies has a thicker sheath at the smaller area powered electrode and a thinner sheath at the larger area grounded electrode. These are self-consistently determined at a specified density using the Child law to calculate sheath widths and the electron power balance to calculate the rf voltage. The fields and the system resonant frequencies are determined. The center-to-edge voltage ratio on the powered electrode is calculated versus frequency, and central highs are found near the resonances. The results are compared with simulations in a similar geometry using a two-dimensional hybrid fluid-analytical code, giving mainly a reasonable agreement. The analytic model may be useful for finding good operating frequencies for a given discharge geometry and power.

Nonlinear standing wave excitation by series resonance-enhanced harmonics in low pressure capacitive discharges

It is well-known that standing waves having radially center-high rf voltage profiles exist in high frequency capacitive discharges. It is also known that in radially uniform discharges, the capacitive sheath nonlinearities excite strong nonlinear series resonance harmonics that enhance the electron power deposition. In this work, we consider the coupling of the series resonance-enhanced harmonics to the standing waves. A one-dimensional, asymmetric radial transmission line model is developed incorporating the wave and nonlinear sheath physics and a self-consistent dc potential, for both conducting and insulating electrode surfaces. The resulting coupled pde equation set is solved numerically to determine the discharge voltages and currents. A 10 mTorr argon plasma is chosen with density m−3, gap width 2 cm and conducting electrode radius 15 cm, driven by a 500 V rf source with resistance 0.5 . We examine a set of frequencies from near 30 MHz up to frequencies more than three times as high. For most frequencies, no harmonics correspond exactly with the series or spatial resonances, which is the generic situation. Nevertheless, nearby resonances lead to a significantly enhanced ratio of the electron power per unit area on axis, compared to the average. Nearly similar results are found for insulating electrodes. Strong effects are seen for varying source resistance: high (50 ) resistance damps out most of the harmonic activity, while zero source resistance leads to a non-steady discharge with bias voltage relaxation oscillations. Stronger harmonic effects are seen for an increased radius of 30 cm, as lower harmonics become spatially resonant at lower frequencies. The radial dependence of electron power with frequency showed significant variations, with the central enhancement and sharpness of the spatial resonances depending in a complicated way on the amplitudes of the nearby series resonance current harmonics and the phase relations among the voltage harmonics driving these current harmonics. Significant center/average electron power per unit area enhancement is found even at the lowest frequencies for both high and low densities: 4.5 at 30 MHz and m−3, and 2.2 at m−3.

Some particularities of development of high frequency capacitive discharge between a drop-jet electrolytic electrode and a flow-type electolytic cell

Particular results of the development and burning of the high frequency capacitive discharge (HFCD) between a drop-jet electrolytic electrode and a flow-type electric cell are presented. Significant current and voltage pulsations taking place after the electric breakdown in the process of the HFCD burning confirm the multichannel shape of the discharge burning at atmospheric pressure. Three modes of HFCD burning between the combinations of the drop-jet electrolyte and the surface of the flow-type electrolyte have been specified. An HFCD with glowing luminescence and “streamer discharges” at the copper tube surface when the electrolyte jet is fed under the pressure of 3 × 103 Pa has been discovered.

Electron energy distribution functions measured by Langmuir probe with optical emission spectroscopy in very high frequency capacitive discharge in nitrogen

By using a rf compensated Langmuir probe and optical emission spectroscopy, the effects of driving frequency (13.56–50 MHz) on the electron energy probability function (EEPF), electron density, electron temperature, and the vibrational and rotational temperatures in capacitively coupled nitrogen discharge were investigated. Measurements were performed in the pressure range 60–200 mTorr, and at a fixed voltage of 140 V (peak-to-peak). With increasing the driving frequency, the dissipated power and electron density markedly increased along with the intensity of the optical emission lines belonging to the 2nd positive (337.1 nm) and 1st negative systems (391.4 nm) of N2. The EEPF at low pressure 60 mTorr is two-temperature (bi-Maxwellian) distribution, irrespective of the driving frequency, in contrast with argon and helium discharges in the similar conditions. The mechanism forming such bi-Maxwellian shape was explained by two combined effects: one is the collisionless sheath-heating effect enhancing the tail electron population, and the other is the collision-induced reduction of electrons at the energy 2–4 eV where the collision cross-section for the vibrational excitation has a resonantly large peak. The two-temperature EEPF structure was basically retained at moderate pressure 120 mTorr and high pressure 200 mTorr. The vibrational temperature Tvib and rotational temperature Trot are measured for the sequence (Δν=−2) of N2 second positive system (C3Πu→B3Πg) using the method of comparing the measured and calculated spectra with a chi-squared minimization procedure. It was found that, both of Tvib and Trot are a weakly dependent on driving frequency at low pressure 60 mTorr. At higher pressure (120 and 200 mTorr), Tvib rises monotonically with the driving frequency, whereas the Trot slightly decreases with frequency below 37 MHz, beyond which it relatively increases or saturated.

Experimental study of plasma non-uniformities and the effect of phase-shift control in a very high frequency capacitive discharge

Plasma spatial non-uniformities were studied experimentally in a very high frequency (100 MHz) capacitive triode-type reactor used for etching of 300 mm wafers. It has been shown that in the traditional mode of operation there is considerable plasma non-uniformity due to the electromagnetic effects, namely at a low power, the plasma density profile is determined by the standing-wave effect, while at a high power the skin effect dominates. The influence of phase-shift control on plasma uniformity was examined. Phase-shift control means applying to the top and the bottom electrodes very high frequency voltages with a controlled phase shift between them. The experiments were carried out in process (C4F8/O2/Ar) plasma in a wide range of pressures and powers. It has been shown that the phase-shift control can considerably improve the plasma uniformity under a wide range of experimental conditions.

Plasma uniformity and phase-controlled etching in a very high frequency capacitive discharge

The influence of controlled phase shift between very high frequency (100MHz) voltages applied to the powered electrodes on the plasma uniformity and etch rate was studied in a capacitive triode-type reactor. Radial profiles of plasma optical emission were measured as a function of the phase shift in process (C4F8∕O2∕Ar) plasma with the low frequency bias power both turned off and on. Radial profiles of KrF photoresist etch rate over a 300mm wafer were obtained in the same conditions (with the bias power turned on). Besides, plasma density at the wafer center and edge was measured versus the phase shift in pure Ar plasma. It occurred that all measured characteristics strongly depend on the phase shift and correlate with each other. It has been shown that the phase-shift control can considerably improve both the plasma and etch-rate uniformity in very high-frequency capacitive reactors.

Experimental observation of the inductive electric field and related plasma nonuniformity in high frequency capacitive discharge

To elucidate plasma nonuniformity in high frequency capacitive discharges, Langmuir probe and B-dot probe measurements were carried out in the radial direction in a cylindrical capacitive discharge driven at 90MHz with argon pressures of 50 and 400mTorr. Through the measurements, a significant inductive electric field (i.e., time-varying magnetic field) was observed at the radial edge, and it was found that the inductive electric field creates strong plasma nonuniformity at high pressure operation. The plasma nonuniformity at high pressure operation is physically similar to the E-H mode transition typically observed in inductive discharges. This result agrees well with the theories of electromagnetic effects in large area and/or high frequency capacitive discharges.

Measured radial dependence of the peak sheath voltages present in very high frequency capacitive discharges

The radial distribution of the measured voltage drop across a sheath formed between a 300mm electrode and an argon plasma discharge is shown to depend on the excitation radio frequency, under constant power and pressure conditions. At a lower frequency of 13.56MHz, the voltage drop across the sheath is uniform across the 300mm electrode, while at higher frequencies of 60 and 162MHz the voltage drop becomes radially nonuniform. The magnitude and spatial extent of the nonuniformity become greater with increasing frequency.

Inductive heating and E to H transitions in high frequency capacitive discharges

Capacitive discharges have classically been operated in the electrostatic regime, for which the excitation wavelength λ is much greater than the electrode radius and the plasma skin depth δ is much greater than the electrode spacing. However, contemporary reactors are larger and excited at a higher frequency so that electromagnetic effects become significant. A self-consistent transmission line model valid in the entire range of λ and δ of practical interest is solved. The model is the electromagnetic generalization of the lumped-element circuit model classically used for capacitive discharges. We find that the plasma may either be sustained by the usual capacitive (E) field or by an inductive (H) field and that the discharge experiences E to H transitions as the voltage between the electrodes is raised. The transitions are global at low pressure and local at high pressure. In the latter case, the plasma parameters (e.g. the ion flux to the electrodes) are radially non-uniform, due to the non-uniformities of the rf voltage and current, leading to serious industrial issues.

Suppression of the standing wave effect in high frequency capacitive discharges using a shaped electrode and a dielectric lens: Self-consistent approach

The standing wave effect causes nonuniform plasma excitation in high frequencies capacitive discharges when the electrode size is not considerably smaller than the excitation wavelength. A shaped electrode was proposed by Sansonnens and Schmitt [Appl. Phys. Lett. 82, 182 (2003)] to suppress this unwanted effect. The shape of the electrode was calculated in the vacuum approximation (no plasma was present between the electrodes), and was found to be Gaussian. The authors postulated that the presence of plasma would not significantly modify the solution. However, it was shown [Chabert et al., Phys. Plasmas 11, 1775 (2004)] using a self-consistent nonlinear transmission line model that the presence of plasma significantly shortens the wavelength for a system composed of two parallel plate electrodes. It was therefore legitimate to expect the optimized shape of the electrode and lens to be different when a plasma is present. Here it is shown that to suppress the standing wave effect the current flowing in the electrodes must be proportional to the discharge radius. This condition is independent of the medium present between the plates and indeed requires a Gaussian electrode.