Capacitively Coupled Plasma Research Articles

Novel high-frequency energy-efficient pulsed-dc generator for capacitively coupled plasma discharge.

The circuit design, assembly, and operating tests of a high-frequency and high-voltage (HV) pulsed dc generator (PDG) for capacitively coupled plasma (CCP) discharge inside a vacuum chamber are reported. For capacitive loads, it is challenging to obtain sharp rectangular pulses with fast rising and falling edges, requiring intense current for quick charging and discharging. The requirement of intense current generally limits the pulse operation frequency. In this study, we present a new type of PDG consisting of a pair of half-resonant converters and a constant current-controller circuit connected with HV solid-state power switches that can deliver almost rectangular high voltage pulses with fast rising and falling edges for CCP discharge. A prototype of the PDG is assembled to modulate from a high-voltage direct current (HVdc) input into a pulsed HVdc output, while following an input pulse signal and a set current level. The pulse rise time and fall time are less than 500 ns and 800 ns, respectively, and the minimum pulse width is 1 µs. The maximum voltage for a negative pulse is 1000 V, and the maximum repetition frequency is 500 kHz. During the pulse on time, the plasma discharge current is controlled steadily at the set value. The half-resonant converters in the PDG perform recovery of the remaining energy from the capacitive load at every termination of pulse discharge. The PDG performed with a high energy efficiency of 85% from the HVdc input to the pulsed dc output at a repetition rate of 1 kHz and with stable plasma operation in various discharge conditions. The results suggest that the developed PDG can be considered to be more efficient for plasma processing by CCP.

A Fast Solution Method for the Boundary Value Problem of Capacitively Coupled Plasmas

Solution methods of capacitively coupled plasmas (CCPs) have been facing a monumental challenge because the solution for the overwhelming number of radio frequency (RF) cycles is in demand before the plasma evolves to a time periodic steady state. It is known that the conventional solution methods require an extremely long calculation time, hence these methods would be limited in industrial applications that need a fast development period. In this paper, a new numerical simulation method for CCPs by COMSOL Multiphysics® is presented. The solution of CCPs in the time domain is converted to a boundary value problem in the additional dimension space to the computational space dimension of the CCP reactor. The fast solutions of the axisymmetric two-dimensional CCPs in Ar and Ar/N2 are performed and the computational efficiency is examined.

The role of electron induced secondary electron emission from SiO2 surfaces in capacitively coupled radio frequency plasmas operated at low pressures

The effects of electron induced secondary electron (SE) emission from SiO2 electrodes in single-frequency capacitively coupled plasmas (CCPs) are studied by particle-in-cell/Monte Carlo collisions (PIC/MCC) simulations in argon gas at 0.5 Pa for different voltage amplitudes. Unlike conventional simulations, we use a realistic model for the description of electron-surface interactions, which takes into account the elastic reflection and the inelastic backscattering of electrons, as well as the emission of electron induced SEs (δ-electrons). The emission coefficients corresponding to these elementary processes are determined as a function of the electron energy and angle of incidence, taking the properties of the surface into account. Compared to the results obtained by using a simplified model for the electron-surface interaction, widely used in PIC/MCC simulations of CCPs, which includes only elastic electron reflection at a constant probability of 0.2, strongly different electron power absorption and ionization dynamics are observed. We find that ion induced SEs (γ-electrons) emitted at one electrode and accelerated to high energies by the local sheath electric field propagate through the plasma almost collisionlessly and impinge on the opposing sheath within a few nanoseconds. Depending on the instantaneous local sheath voltage these energetic electrons are either reflected by the sheath electric field or they hit the electrode surface, where each γ-electron can generate multiple δ-electrons upon impact. These electron induced SEs are accelerated back into the plasma by the momentary sheath electric field and can again generate δ-electrons at the opposite electrode after propagating through the plasma bulk. Overall, a complex dynamics of γ- and δ-electrons is observed including multiple reflections between the boundary sheaths. At high voltages, the electron induced SE emission is found to strongly affect the plasma density and the ionization dynamics and, thus, it represents an important plasma-surface interaction that should be included in PIC/MCC simulations of CCPs under such conditions.

Modeling of low pressure plasma sources for microelectronics fabrication

Chemically reactive plasmas operating in the 1 mTorr–10 Torr pressure range are widely used for thin film processing in the semiconductor industry. Plasma modeling has come to play an important role in the design of these plasma processing systems. A number of 3-dimensional (3D) fluid and hybrid plasma modeling examples are used to illustrate the role of computational investigations in design of plasma processing hardware for applications such as ion implantation, deposition, and etching. A model for a rectangular inductively coupled plasma (ICP) source is described, which is employed as an ion source for ion implantation. It is shown that gas pressure strongly influences ion flux uniformity, which is determined by the balance between the location of plasma production and diffusion. The effect of chamber dimensions on plasma uniformity in a rectangular capacitively coupled plasma (CCP) is examined using an electromagnetic plasma model. Due to high pressure and small gap in this system, plasma uniformity is found to be primarily determined by the electric field profile in the sheath/pre-sheath region. A 3D model is utilized to investigate the confinement properties of a mesh in a cylindrical CCP. Results highlight the role of hole topology and size on the formation of localized hot-spots. A 3D electromagnetic plasma model for a cylindrical ICP is used to study inductive versus capacitive power coupling and how placement of ground return wires influences it. Finally, a 3D hybrid plasma model for an electron beam generated magnetized plasma is used to understand the role of reactor geometry on plasma uniformity in the presence of E × B drift.

Effects of the wall boundary conditions of a showerhead plasma reactor on the uniformity control of RF plasma deposition

Technical difficulties hinder the formation of uniform deposition profiles near the electrode edge during a deposition in a showerhead capacitively coupled plasma (CCP) reactor. The discharge structure, gas flow, and radial distribution of the source gas outward from the electrode edge are subject to change significantly, which in turn affects the spatial distributions of the radical fluxes toward the electrode. To control the local non-uniformity of deposition profiles in the SiH4/NH3/N2/He CCP discharges for a hydrogenated silicon nitride (SiNxHy) film, the effects of the reactor components―including the sidewall boundary condition, electrode spacing, and showerhead design―were investigated using an axisymmetric fluid model. When the sidewall is electrically grounded, the deposition rate profiles of the SiNxHy film remain consistently convex (in which the deposition rate at the reactor center is locally much higher than that near the electrode edge), regardless of electrode spacing. However, when the sidewall surface is dielectric, the deposition rate profile can be transformed between a convex and a concave shape (in which the deposition rate at the reactor center is locally much lower than that near the electrode edge) by varying electrode spacing. The showerhead design also enables the modification of edge deposition profiles by redistribution of the local depletion rate of radicals. The simulation results agree very well with the experimental measurement.

Broadband microwave absorption and standing wave effect in helium capacitively coupled plasma

The broadband microwave absorption of a large volume helium plasma, which is generated by two parallel rectangular plates based on the principle of capacitively coupled plasma (CCP) is developed in this paper. The transmission attenuation is simulated by the combination of the time-dependent fluid model and dispersion equation, and measured by a high dynamic range measurement system in a frequency range of 1–12 GHz. The results show that the plasma can absorb microwave energy efficiently, especially in the frequency range of 1–5 GHz, where the transmission attenuation is more than −5 dB. The attenuation increases with gas pressures and applied voltages. Besides, the standing wave effect in plasma can increase the microwave absorption effectively. The numerical and experimental results have a qualitative agreement, and these characteristics suggest that the helium CCP has tremendous potential to be applied in plasma stealth.

The effect of realistic heavy particle induced secondary electron emission coefficients on the electron power absorption dynamics in single- and dual-frequency capacitively coupled plasmas

In particle-in-cell/Monte Carlo collisions (PIC/MCC) simulations of capacitively coupled plasmas (CCPs), the plasma-surface interaction is generally described by a simple model in which a constant secondary electron emission coefficient (SEEC) is assumed for ions bombarding the electrodes. In most PIC/MCC studies of CCPs, this coefficient is set to γ = 0.1, independent of the energy of the incident particle, the electrode material, and the surface conditions. Here, the effects of implementing energy-dependent secondary electron yields for ions, fast neutrals, and taking surface conditions into account in PIC/MCC simulations is investigated. Simulations are performed using self-consistently calculated effective SEECs, , for ‘clean’ (e.g., heavily sputtered) and ‘dirty’ (e.g., oxidized) metal surfaces in single- and dual-frequency discharges in argon and the results are compared to those obtained by assuming a constant secondary electron yield of for ions. In single-frequency (13.56 MHz) discharges operated under conditions of low heavy particle energies at the electrodes, the pressure and voltage at which the transition between the α- and γ-mode electron power absorption occurs are found to strongly depend on the surface conditions. For ‘dirty’ surfaces, the discharge operates in α-mode for all conditions investigated due to a low effective SEEC. In classical dual-frequency (1.937 MHz + 27.12 MHz) discharges significantly increases with increasing low-frequency voltage amplitude, , for dirty surfaces. This is due to the effect of on the heavy particle energies at the electrodes, which negatively influences the quality of the separate control of ion properties at the electrodes. The new results on the separate control of ion properties in such discharges indicate significant differences compared to previous results obtained with different constant values of γ.

Room-temperature plasma-enhanced atomic layer deposition of ZnO: Film growth dependence on the PEALD reactor configuration

Room-temperature plasma-enhanced atomic layer deposition (PEALD) of ZnO was studied by depositing the films using diethylzinc and O2 plasma from inductively-coupled plasma (ICP) and capacitively-coupled plasma (CCP) plasma source configurations. The CCP-PEALD was operated using both remote and direct plasma. It was observed that the films deposited by means of remote ICP and CCP were all highly oxygen rich, independently on plasma operation parameters, but impurity (H, C) contents could be reduced by increasing plasma pulse time and applied power. With the direct CCP-PEALD the film composition was closer to stoichiometric, and film crystallinity was enhanced. The ZnO film growth was observed to be similar on silicon, polycarbonate and poly(methyl methacrylate) substrates, but changes in polymer surface morphology indicate plasma-induced damage during the deposition due to exposure to ion bombardment when direct plasma was applied.

Analysis of intermediate pressure SiH4/He capacitively coupled plasma for deposition of an amorphous hydrogenated silicon film in consideration of thermal diffusion effects

To achieve rapid, uniform deposition of an amorphous hydrogenated silicon (a-Si:H) film, a capacitively coupled plasma (CCP) is often used at an intermediate pressure (>100 Pa), with a silane (SiH4)-based mixture. At these pressures, heavy particle interactions (such as ion–ion, ion–neutral, and neutral–neutral reactions) contribute significantly to the formation of precursor radicals. By adding a consideration of the thermal diffusion effects to the neutral transport equation, the chemical processes have been numerically analyzed with variation in the number fraction of SiH4 and electrode spacing using a two-dimensional fluid model of radio frequency discharges in a cylindrically symmetric CCP reactor. The non-uniformity of the deposition rate profiles increases consistently as electrode spacing increases, although the non-uniformity of the plasma parameters decreases with the increase of electrode spacing. The simulated deposition rate profiles match well with the experimental data for the change of electrode spacing. Based on the validation of our model, we propose predictive designs to potentially improve the reactor and process by modifying the thermal and electrical surface conditions.

Effect of a dielectric layer on plasma uniformity in high frequency electronegative capacitive discharges

The authors use a fast 2D axisymmetric fluid-analytical code to study the effect of adding a dielectric layer over the wafer electrode of a high frequency capacitively coupled plasma (CCP) reactor. At higher frequencies and larger areas, the wavelengths of the radially propagating surface waves in the plasma can become significantly shorter than the reactor dimensions, leading to center-high plasma nonuniformities. These wavelengths increase with increasing sheath widths, suggesting that a method to suppress wave effects in a high frequency CCP is to increase the effective sheath width by adding a dielectric layer over the wafer electrode. The authors conducted simulations with and without a dielectric layer and found that the dielectric layer improved plasma uniformity. The authors also studied the effect of adding a thin conducting or resistive silicon wafer above the dielectric layer and found that a conducting silicon wafer shorts out the fields and shields the discharge from the dielectric layer, while the resistive silicon wafer allows the fields to pass through to the dielectric layer.

Silicon Nitride Spacer Etching with Nearly Atomic Precision for 2D and 3D Devices

With transistors size scaling down, device processing requirements become more and more stringent. For technology node beyond 14 nm, one of the most critical step is the spacer etching. It requires a perfect anisotropy (no CD loss) without damaging nor consumption of the exposed material like silicon, silicon germanium and oxide [1,2]. In planar transistor, the silicon or silicon germanium consumption is limited by the short over-etch process (30-50%). However for 3D devices, the silicon fin is directly exposed during the removal of the silicon nitride on the active area sidewalls. This is the major issue since in this case, important over etch is required (>200%) to fully remove the residues at the bottom of the fin. Therefore, the spacer etch is considered today as one of the most challenging etch process for 2D but more especially 3D devices. In this paper we propose to compare conventional etch process to new approaches to reach the stringent etch requirements for 2D and 3D devices. The first part of this study focuses on the characterization of CH2F2/O2/He/CH4 plasma performed in an Inductively-Coupled Plasma (ICP) etch tool. It was shown that playing on different parameters (bias, reagent, bias pulsing…) the maximum selectivity to silicon reached 13.5. In our experimental conditions, the lower Si consumption is estimated to 0.6 nm and 3 nm for 2D and 3D device applications. Whatever the operating plasma conditions, XPS analyses shown that selectivity to silicon has been explained by an oxide formation at the top of the silicon layer. A second part of the study will focus on an original approach for planar transistor application [3]. This approach consists in light ion implantation of hydrogen in the silicon nitride layer (in an inductively or capacitively-coupled plasma reactor), and then in a second step, in a highly selective removal process based on gaseous hydrofluoric acid (HF). Film modification and reaction with removal process have been understood thanks to XPS and infrared spectroscopy. In this part, the implanted material has been characterized and the chemical reactions with gaseous HF have been investigated. It has been pointed out that implantation proceeds by increasing the H bonding with N and Si atoms which allowed the selective reaction of gaseous HF with the damaged thickness by forming an hexafluorosilicate salt. This salt was then easily removable in an aqueous solution showing a clean surface after treatment. It has also been demonstrated that playing on the gaseous HF concentration inside the chamber improved the selectivity to non-implanted SiN and silicon dioxide up to 31 and 39 respectively. This method has been turned out to be an accurate method to etch silicon nitride with a nearly atomistic precision. A third part discusses about the spacer etching for 3D devices like FinFET or stacked nanowires. In this part, due to the complex architecture of patterns, the implantation has been performed thanks to ion implantation and a tilted angle has been used to target specific silicon nitride areas to remove. It will be shown that argon ion is the most suitable implant element for a fast etch of implanted SiN on blanket wafers and test are ongoing on pattern samples to validate the technique. [1] B. E. E. Kastenmeier, P. J. Matsuo, and G. S. Oehrlein, J. Vac. Sci. Technol. A 17 (6), 3179 (1999). [2] K. Eriguchi, Y. Nakakubo, A. Matsuda, Y. Katao, K. Ono, IEEE Electr. Device L. 30 (7), 712 (2009). [3] N. Posseme, O. Pollet, and S. Barnola, J. Appl. Phys. 105, 051605 (2014).

Modeling of Resonant Surface Wave Excitation in a Large CCP Reactor

Low-pressure plasmas produced in capacitively coupled plasma (CCP) reactors are used extensively in the plasma processing industry. Although the physics of small scale CCP reactors operated under low pressure is fairly well understood, larger scale CCP reactors constructed to meet the industrial needs often exhibit surprising behavior in experiments, which affects discharge parameters of importance to the industrial applications. This can be explained by the resonant excitation of surface modes that a CCP reactor can support. Limitations of the drift-diffusion approximation in description of the surface mode phenomena in low-pressure plasmas are discussed. It is shown that a particular class of the surface modes can be adequately modeled using the electrostatic approximation if the finite electron inertia is taken into account. Continuing a previous work [1] , the present study demonstrates how such modes are triggered and how they affect the radial plasma density profile uniformity. This is done by using a self-consistent 2d3v electrostatic implicit PIC/MCC code. It is shown that in highly collisional plasmas, the nonuniformities caused by the modes disappear due to the large mode damping.

Two-dimensional fluid simulation on transient behavior and plasma uniformity in pulsed RF CCP sustained in SiH4 /N2/O2

Improving plasma uniformity during plasma processing in the microelectronics industry is of critical importance to the quality of etching or deposition. Compared to continuous wave (CW) plasmas, pulsed plasmas have drawn much attention with the introduction of additional pulse parameters, which would be helpful to improve the plasma properties. In this paper, a two-dimensional fluid model is developed to investigate a pulsed radio-frequency capacitively coupled plasma (CCP) sustained in SiH4/N2/O2 mixture at fixed operating conditions of 70V rf power, 300 mTorr (40 Pa) gas pressure and an SiH4/N2/O2 gas ratio of 2.5/92.5/5. First, we study the temporal dynamics of densities of the electron, positive ion and negative ion, at different positions in the pulsed CCP. Under the operation conditions, charged particles, instead of neutral particles, may basically respond to the applied modulated power. The electron density in the bulk could approach a quasi-steady value by the end of the activeglow. However, the achievement of a quasi-steady state of plasma like that in the CW condition not only depends on enough activeglow time of the pulse discharge but also relies on the observed position in the discharge. In addition, we investigate the impact of pulse parameters on plasma characteristics, showing that the radial inhomogeneity of plasma caused by the edge effect can be effectively suppressed by controlling the duty cycle (DC) rather than the pulse repetition frequency (PRF). Improvement of the plasma uniformity in pulsed discharge is due to the competition between the edge effects during the activeglow and diffusion of charged species during the afterglow. Moreover, the electron density undergoes a local minimum value in the temporal profile before it rises sharply beyond that of CW discharge, since production of electrons is less than loss by the spatial movement at the very beginning of one pulse. Also, there appears to be a peak value of ion bombardment energy at the beginning of power, which may damage the wafer in actual industrial processes. At a certain DC, this peak may be flattened to a certain degree by adjusting the PRF.

Numerical characterization of magnetized capacitively coupled argon plasmas driven by combined dc/rf sources

The characteristics of magnetized capacitively coupled plasmas (CCPs) driven by combined dc/rf sources in argon have been investigated by a one-dimensional implicit Particle-in-cell/Monte Carlo collision model. Discharges operating at 13.56 MHz with a fixed rf voltage of 300 V are simulated at the pressure of 50 mTorr in argon. Four cases, i.e., CCP driven by rf source, rf + dc sources, rf source with magnetic field, and rf + dc sources with magnetic field, are presented and compared at the Vdc = −100 V, B = 50 Gs, and γi = 0.2. It is found that, with the influence of dc voltage and magnetic field, the plasma density has been greatly enhanced by over one order of magnitude over the rf-only case. This is due to the fact that the mean free path of electrons decreases by the cyclotron motion and the energetic secondary electrons are trapped by the magnetic field, leading to a significant increase in heating and ionization rates. Moreover, transition of the stochastic to Ohmic electron heating mechanism takes place as the magnetic field increases because electron kinetics can be strongly affected by the magnetic field. In general, we have demonstrated that such a configuration will enhance the discharge and thus enable CCPs work under extremely high energy density stably that can never be operated by any other configurations. We expect that such a configuration can promote many related applications, like etching, sputtering, and deposition.

The α and γ plasma modes in plasma-enhanced atomic layer deposition with O2–N2 capacitive discharges

Two distinguishable plasma modes in the O2–N2 radio frequency capacitively coupled plasma (CCP) used in remote plasma-enhanced atomic layer deposition (PEALD) were observed. Optical emission spectroscopy and spectra interpretation with rate coefficient analysis of the relevant processes were used to connect the detected modes to the α and γ modes of the CCP discharge. To investigate the effect of the plasma modes on the PEALD film growth, ZnO and TiO2 films were deposited using both modes and compared to the films deposited using direct plasma. The growth rate, thickness uniformity, elemental composition, and crystallinity of the films were found to correlate with the deposition mode. In remote CCP operations the transition to the γ mode can result in a parasitic discharge leading to uncontrollable film growth and thus limit the operation parameters of the capacitive discharge in the PEALD applications.

Intensification of separation of oil-in-water emulsions using polysulfonamide membranes modified with low-pressure radiofrequency plasma

Effect of low-pressure radiofrequency capacitively-coupled plasma parameters on the separation of model “oil-in-water” emulsions using UPM-20 polysulfonamide membranes with a pore size of 0.01 μm has been studied. Atmospheric air has been used as a plasma gas. The plasma treatment time is 1, 5, 4, and 7 min at a plasma torch anode voltage of U a = 1.5–7.5 kV. The separated medium is a 3% emulsion based on the industrial oil I-20A; the stabilizer is the surfactant Kosintol-242. The highest performance is observed in the case of membranes treated with plasma for 7 min. It is found that plasma treatment leads to an increase in the emulsion separation efficiency from 90 to 99%. It has been shown that the plasma treatment contributes to the surface hydrophilization of the membranes owing to the formation of oxygen-containing functional groups. The membrane exhibits a contact angle of water of α = 59.6° before treatment and α = 19.5° after a 4-min plasma treatment at U a = 7.5 kV. Atomic force microscopy reveals that the plasma treatment leads to a change in the surface structure of the membranes, namely, to a decrease in their roughness. The internal structure of the membranes also undergoes changes which result in an increase in their crystallinity.

A time-dependent model of pulse-driven radio frequency capacitively coupled collisional plasma sheath

The time-dependent model of ion motion is used to propose an analytical model for dual frequency (DF) capacitively coupled plasma (CCP) sheath driven by a pulsed source and a radio-frequency source. In this model, the sheath is considered to be collisional. In this model, the time dependent terms of ion fluid equations are ignored, but the electric field, ion motion and ion density remain time dependent. Electron profile is assumed to be step-like. Analytical expressions for electron sheath width and sheath potential have been developed. The calculated sheath width and potential are compared with the dual radio frequency driven time dependent models of capacitively coupled plasma sheath. From the temporal evaluation of sheath motion and potential, it has been found that pulse driven sheath has higher sheath potential and sheath width than that of conventional radio frequency driven DF CCP. Moreover, it is also found that ion energy spread can be reduced using pulsed power. From the temporal investigation of sheath motion and potential, it has been found that the duty cycle of the pulse power significantly affects sheath width and sheath potential.

The effect of intermediate frequency on sheath dynamics in collisionless current driven triple frequency capacitive plasmas

The Capacitively Coupled Plasma discharge featuring operation in current driven triple frequency configuration has analytically been investigated, and the outcome is verified by utilising the 1D3V particle-in-cell (PIC) simulation code. In this analysis, the role of middle frequency component of the applied signal has precisely been explored. The discharge parameters are seen to be sensitive to the ratio of the chosen middle frequency to lower and higher frequencies for fixed amplitudes of the three frequency components. On the basis of analysis and PIC simulation results, the middle frequency component is demonstrated to act as additional control over sheath potential, electron sheath heating, and ion energy distribution function (iedf) of the plasma discharge. For the electron sheath heating, effect of the middle frequency is seen to be pronounced as it approaches to the lower frequency component. On the other hand, for the iedf, the control is more sensitive as the middle frequency approaches towards the higher frequency. The PIC estimate for the electron sheath heating is found to be in reasonably good agreement with the analytical prediction based on the Kaganovich formulation.

Optical diagnostics of dusty plasmas during nanoparticle growth

Carbon-based thin films deposited on surfaces exposed to a typical capacitively-coupled RF plasma are sources of molecular precursors at the origin of nanoparticle growth. This growth leads to drastic changes of the plasma characteristics. Thus, a precise understanding of the dusty plasma structure and dynamics is required to control the plasma evolution and the nanoparticle growth. Optical diagnostics can reveal some particular features occurring in these kinds of plasmas. High-speed imaging of the plasma glow shows that instabilities induced by nanoparticle growth can be constituted of small brighter plasma regions (plasmoids) that rotate around the electrodes. A single bigger region of enhanced emission is also of particular interest: the void, a main central dust-free region, has very distinct plasma properties than the surrounding dusty region. This particularity is emphasized using optical emission spectroscopy with spatiotemporal resolution. Emission profiles are obtained for the buffer gas and the carbonaceous molecules giving insights on the changes of the electron energy distribution function during dust particle growth. Dense clouds of nanoparticles are shown to be easily formed from two different thin films, one constituted of polymer and the other one created by the plasma decomposition of ethanol.

Etch Characteristics of SiO2 Using Pulsed Triple-Frequency for Ar/C4F6/O2 Capacitive Coupled Plasmas

Etch Characteristics of SiO2 Using Pulsed Triple-Frequency for Ar/C4F6/O2 Capacitive Coupled Plasmas