Capacitively Coupled Plasma Reactor Research Articles

Capacitively coupled nonthermal plasma synthesis of aluminum nanocrystals for enhanced yield and size control

Uniform-size, non-native oxide-passivated metallic aluminum nanoparticles (Al NPs) have desirable properties for fuel applications, battery components, plasmonics, and hydrogen catalysis. Nonthermal plasma-assisted synthesis of Al NPs was previously achieved with an inductively coupled plasma (ICP) reactor, but the low production rate and limited tunability of particle size were key barriers to the applications of this material. This work focuses on the application of capacitively coupled plasma (CCP) to achieve improved control over Al NP size and a ten-fold increase in yield. In contrast with many other materials, where NP size is controlled via the gas residence time in the reactor, the Al NP size appeared to depend on the power input to the CCP system. The results indicate that the CCP reactor assembly, with a hydrogen-rich argon/hydrogen plasma, was able to produce Al NPs with diameters that were tunable between 8 and 21 nm at a rate up ∼ 100 mg h−1. X-ray diffraction indicates that a hydrogen-rich environment results in crystalline metal Al particles. The improved synthesis control of the CCP system compared to the ICP system is interpreted in terms of the CCP’s lower plasma density, as determined by double Langmuir probe measurements, leading to reduced NP heating in the CCP that is more amenable to NP nucleation and growth.

Two-dimensional simulation of non-equilibrium silicon nitride plasma discharge for thin film solar cell deposition

In this work, we model a Two-dimensional (2D) radiofrequency (RF) plasma discharge of hydrogenated silicon nitride (SiH4/NH3/H2) in a capacitive coupled plasma reactor (CCP), using non-Maxwellian electron energy distribution function (EEDF). The purpose is to investigate whether there is a real advantage and a significant contribution in 2D simulation using non-Maxwellian EEDF rather than Maxwellian one for determining the fundamental characteristics of radiofrequency plasma discharge. The results show a comparative study of the 2D evolution of the fundamental characteristics of the RF plasma using Maxwellian and non-Maxwellian EEDF. By considering the criterion of the density and the electronic temperature, the analysis of the results shows that the adoption of the non-equilibrium 2D EEDF makes it possible to obtain an approximation closer to the real conditions and thus a better understanding of the plasma discharge.

Effect of gas flow rate and discharge volume on CO2 methanation with plasma catalysis

CO2 methanation can be a key technology for realizing a sustainable society. CH4 is used as an energy carrier and raw material for chemical products, thereby contributing to the reduction of CO2 emissions. Methanation with plasma catalysis lowers the process temperature, which can improve the throughput and stability. In this study, we investigated the effect of the gas flow rate and the discharge volume on CO2 methanation, using a low-pressure capacitively coupled plasma reactor. Higher gas flow rates can increase the rate of CO2 throughput, but the CH4 selectivity decreases owing to the reduced transportation rate of the reactants to the catalyst surface. Increasing the discharge volume is effective in improving the transportation rate. This study suggested that the structure of the reactor significantly affects the CH4 generation rate.

Computational Fluid Dynamics Analysis of Particle Deposition Induced by a Showerhead Electrode in a Capacitively Coupled Plasma Reactor

Defect formation in the deposition of thin films for semiconductors is not yet sufficiently understood. In a showerhead-type capacitively coupled plasma (CCP) deposition reactor, the showerhead acts as both the gas distributor and the electrode. We used computational fluid dynamics to investigate ways to enhance cleanliness by analyzing the particle deposition induced by the showerhead electrode in a CCP reactor. We analyzed particle transport phenomena using a three-dimensional complex geometry, whereas SiH4/He discharges were simulated in a two-dimensional simplified geometry. The process volume was located between the RF-powered showerhead and the grounded heater. We demonstrated that the efficient transportation of particles with a radius exceeding 1 μm onto the heater is facilitated by acceleration inside the showerhead holes. Because the available space in which to flow inside the showerhead is constricted, high gas velocities within the showerhead holes can accelerate particles and lead to inertia-enhanced particle deposition. The effect of the electrode spacing on the deposition of particles generated in plasma discharges was also investigated. Smaller electrode spacing promoted the deposition of particles fed from the showerhead on the heater, whereas larger electrode spacing facilitated the deposition of particles generated in plasma discharges on the heater.

Enhancement of Cleanliness and Deposition Rate by Understanding the Multiple Roles of the Showerhead Electrode in a Capacitively Coupled Plasma Reactor

Increasing the productivity of a showerhead-type capacitively coupled plasma (CCP) reactor requires an in-depth understanding of various physical phenomena related to the showerhead, which is not only responsible for gas distribution, but also acts as the electrode. Thus, we investigated how to enhance the cleanliness and deposition rate by studying the multiple roles of the showerhead electrode in a CCP reactor. We analyzed the gas transport in a three-dimensional complex geometry, and the SiH4/He discharges were simulated in a two-dimensional simplified geometry. The process volume was installed between the showerhead electrode (radio frequency powered) and the heater electrode (grounded). Our aim of research was to determine the extent to which the heated showerhead contributed to increasing the deposition rate and to reducing the size of the large particles generated during processing. The temperature of the showerhead was increased to experimentally measure the number of particles transported onto the heater to demonstrate the effects thereof on the decrease in contamination. The number of particles larger than 45 nm decreased by approximately 93% when the showerhead temperature increased from 373 to 553 K.

Comparison between Ar+CH4 cathode and anode coupling chemical vapor depositions of hydrogenated amorphous carbon films

This research focuses on the comparison between cathode- and anode coupling deposition characteristics of hydrogenated amorphous carbon (a-C:H) using the capacitively coupled plasma (CCP) method. a-C:H films were fabricated by Ar+CH4 CCP chemical vapor deposition. The cathode and anode coupling depositions were simultaneously performed in a single CCP reactor. Compared to the anode coupling process, the cathode coupling process provides high-kinetic-energy ions impinging on the film surface. The cathode coupling process produced a-C:H films with a mass density of 1.75 g/cm3 and a hydrogen content of 36% at a deposition rate of 22.9 nm/min. By contrast, the anode coupling process resulted in a-C:H films with a mass density of 1.4 g/cm3 and a hydrogen content of 50% at a deposition rate of 10 nm/min. Raman spectroscopy shows that a-C:H films in the cathode coupling process have a highly disordered bonding, whereas a-C:H films in the anode coupling process have highly strained graphite.

Experimentally validated computations of simultaneous ion and fast neutral energy and angular distributions in a capacitively coupled plasma reactor

Accurate predictions of the ion energy and angular distribution functions (IEADFs) and fast neutral energy and angular distribution functions (FNEADFs), are essential for a range of critical applications in the plasma processing of thin film etching and deposition. Computationally efficient methods that can be applied to industrial reactor systems that are also validated across a range of operating conditions are essential for prediction of IEADFs and FNEADFs. In this work, we present a hybrid model where a capacitively coupled plasma (CCP) solution was computed using a fluid model and the IEADF/FNEADF was generated using a particle transport model with a multi-species Monte Carlo Collision model. We first compare the computed IEADFs across a range of pressures with experimental measurements for a CCP reactor. We predict FNEADFs that are typically difficult to measure experimentally but are believed to have a significant effect on etch/deposition process phenomena. Across a large pressure range, we observe significant fast neutral bombardment of the electrode surface with at least half the energy of the most energetic ions as well as an angular spread that is larger than that of the ions.

Numerical Modeling of Non-equilibrium Plasma Discharge of Hydrogenated Silicon Nitride (SiH4/NH3/H2)

In this work, we model a radiofrequency discharge of hydrogenated silicon nitride in a capacitive coupled plasma reactor using Maxwellian and non-Maxwellian electron energy distribution function. The purpose is to investigate whether there is a real advantage and a significant contribution using non-Maxwellian electron energy distribution function rather than Maxwellian one for determining the fundamental characteristics of a radiofrequency plasma discharge. The results show the evolution of the non-Maxwellian electron energy distribution function, the mobility and the diffusion coefficient required to determine the fundamental characteristics of the radiofrequency plasma discharge of a hydrogenated silicon nitride deposit at low pressure and low temperature, between the two electrodes of the capacitive coupled plasma reactor. By comparing these results using non-Maxwellian electron energy distribution function with those calculated using the Maxwellian one, we conclude that the use of non-Maxwellian electronic energy distribution function is more efficient for describing the evolution of a radiofrequency plasma discharge in a capacitive reactor, which will improve the quality of the deposition of thin films.

How to address the issue of uniformity in large area capacitively coupled plasmas? A modeling investigation

Recently, the electrical asymmetry effect (EAE) has been proven effective not only in separate control of the ion energy and ion flux, but also in modulation of the plasma radial uniformity. Since no major change in reactor constructions is required, this method is promising and of significant importance in the plasma industry. In this work, a two-dimensional electromagnetic fluid model is employed to investigate the influence of the EAE on the plasma density distribution under various voltages (i.e., the peak-to-peak voltage V0 equals to 33 V, 56 V and 100 V) and pressures (i.e., 50 mTorr, 200 mTorr and 750 mTorr), in H2 capacitively coupled plasma (CCP) discharges sustained by multiple consecutive harmonics. The results indicate that at V0 = 33 V or 750 mTorr, the plasma density is characterized by a broad maximum at the radial center, and the effectiveness of the EAE on the modulation of the plasma radial uniformity is limited. When V0 becomes higher, the plasma distribution varies obviously by adjusting the phase shift of the fundamental frequency θ1. For instance, when θ1 increases from 0 to 2π at V0 = 56 V, the plasma density shifts from uniform over center-high to uniform. Moreover, at V0 = 100 V, the best uniformity is observed at θ1 = π. When the pressure decreases to 50 mTorr, although the uniformity varies significantly with θ1, the plasma density stays low, which may not be welcomed in the plasma processing. Different influences of the EAE on the plasma radial uniformity can be understood by examining the electron heating induced by the sheath expansion and the electric field reversal under various discharge conditions. The results obtained in this work could help us to have a better understanding of the EAE on the plasma radial uniformity, which would be useful to improve the plasma processing in large area CCP reactors.

Numerical analysis for optimization of the sidewall conditions in a capacitively coupled plasma deposition reactor

Capacitively coupled plasma (CCP) is mainly being used in the semiconductor industry for plasma-enhanced chemical vapor deposition of uniform thin films. Because a discharge volume in the standard configuration of a CCP reactor is surrounded not only with electrode surfaces but also with a sidewall, the sidewall conditions affect the deposition rate profiles noticeably. By toggling the boundary condition from a grounded conductor to dielectrics with the variations of the relative permittivity and the thickness, we compare the spatial profiles for the species densities, ionization rate, power absorption, and particle fluxes in a SiH4/He CCP. Through the SiH4/He CCP fluid model, it is found that a thick and low-permittivity insulator achieves the most uniform plasma density distribution in the interelectrode region and, consequently, the best uniformity in the deposition rate profile of an a-Si:H film. As a validation, experimental results are compared with fluid modeling results, and they match well. For additional validation, a particle-in-cell simulation of pure Ar discharge is also performed. Although simulation conditions are totally different from those of the SiH4/He fluid model, it consistently demonstrates that the dielectric sidewall brings about more uniform distributions of the plasma parameters than the grounded sidewall.

Numerical Modeling of Plasma Silicon Discharge for Photovoltaic Application

Abstract In this work, we perform a numerical modeling of silicon thin film deposition for a photovoltaic application in a capacitive coupled plasma reactor (CCP) using RF excitation. Plasma equations are solved by finite element numerical method. The deposition is done by plasma of SiH4/Ar/H2 in low pressure and low temperature. The results provide the fundamental features of plasma discharge such as density, temperature and electrical potential. It is shown that the gas mixture used covers a large energy range, which entails a high deposition rate. Furthermore, the use of CCP reactor yields a uniform deposition without surface deterioration.

Investigation of RF Originated ‘Cold’ Plasma Parameters in Capacitive Coupled Reactor

We describe a capacitive coupled plasma reactor, where the inert gas molecules are ionized by the radio frequency radiation. The control and measurement system of electrical and optical parameters of the plasma are considered. A qualitative assessment is given of the energy distribution of charge carriers.

Symmetry breaking in high frequency, symmetric capacitively coupled plasmas

Two radially propagating surface wave modes, “symmetric,” in which the upper and lower axial sheath fields (Ez) are aligned, and “anti-symmetric,” in which they are opposed, can exist in capacitively coupled plasma (CCP) discharges. For a symmetric (equal electrode areas) CCP driven symmetrically, we expected to observe only the symmetric mode. Instead, we find that when the applied rf frequency f is above or near an anti-symmetric spatial resonance, both modes can exist in combination and lead to unexpected non-symmetric equilibria. We use a fast 2D axisymmetric fluid-analytical code to study a symmetric CCP reactor at low pressure (7.5 mTorr argon) and low density (∼3 × 1015 m−3) in the frequency range of f = 55 to 100 MHz which encompasses the first anti-symmetric spatial resonance frequency fa but is far below the first symmetric spatial resonance fs. For lower frequencies such that f is well below fa, the symmetric CCP is in a stable symmetric equilibrium, as expected, but at higher frequencies such that f is near or greater than fa, a non-symmetric equilibrium appears which may be stable or unstable. We develop a nonlinear lumped circuit model of the symmetric CCP to better understand these unexpected results, indicating that the proximity to the anti-symmetric spatial resonance allows self-exciting of the anti-symmetric mode even in a symmetric system. The circuit model results agree well with the fluid simulations. A linear stability analysis of the symmetric equilibrium describes a transition with increasing frequency from stable to unstable.

Low temperature plasma as a means to transform nanoparticle atomic structure

Low temperature plasma (LTP) is a highly nonequilibrium substance capable of increasing the specific free energy of mass that flows through it. Despite this attractive feature, there are few examples of the transformation of solid material with an equilibrium atomic structure into a material with a nonequilibrium atomic structure. As a proposed example of such a transformation, in this work, it is argued that the transformation of crystalline metal nanoparticles into amorphous metal nanoparticles is feasible using LTP. To inform the feasibility calculations, detailed characterization was performed to determine the electron temperature, ion density, and background gas temperature as a function of axial position in a typical flow-through, radiofrequency, capacitively-coupled plasma reactor. Measurements revealed the existence of an intense zone with sharply elevated ion density and gas temperature in the vicinity of the powered electrode. The high intensity zone, amidst an otherwise low-intensity plasma, provides a means by which to transform the atomic structure of nanoparticles while maintaining unipolar negative charge to suppress coagulation. Theory suggests that such an intense zone would provide intense heating of nanoparticles, and subsequent rapid cooling. Calculations for copper–zirconium (CuZr) alloy show that the temperature history of a nanoparticle depends primarily on the intensity of the zone in the vicinity of the powered electrode, and on particle size. If one considers melting CuZr nanoparticles in the intense zone and then rapidly cooling them in the low-intensity plasma downstream, then the quenching rates are found to be high, on the order of 105 K s−1. Since quenching rates of this magnitude are sufficient to arrest an amorphous atomic structure, LTP reactors can be used to transform crystalline metal nanoparticles into amorphous metal nanoparticles via a highly nonequilibrium quenching process.

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.

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.

2D fluid model analysis for the effect of 3D gas flow on a capacitively coupled plasma deposition reactor

The wide applicability of capacitively coupled plasma (CCP) deposition has increased the interest in developing comprehensive numerical models, but CCP imposes a tremendous computational cost when conducting a transient analysis in a three-dimensional (3D) model which reflects the real geometry of reactors. In particular, the detailed flow features of reactive gases induced by 3D geometric effects need to be considered for the precise calculation of radical distribution of reactive species. Thus, an alternative inclusive method for the numerical simulation of CCP deposition is proposed to simulate a two-dimensional (2D) CCP model based on the 3D gas flow results by simulating flow, temperature, and species fields in a 3D space at first without calculating the plasma chemistry. A numerical study of a cylindrical showerhead-electrode CCP reactor was conducted for particular cases of SiH4/NH3/N2/He gas mixture to deposit a hydrogenated silicon nitride (SiNxHy) film. The proposed methodology produces numerical results for a 300 mm wafer deposition reactor which agree very well with the deposition rate profile measured experimentally along the wafer radius.

Particle formation and its control in dual frequency plasma etching reactors

The behavior of a particle cloud in plasma etching reactors at the moment when radio frequency (RF) power changes, that is, turning off and transition steps, was observed using the laser-light-scattering method. Two types of reactors, dual-frequency capacitively coupled plasma (CCP) and the hybrid CCP/inductively coupled plasma (ICP), were set up for experiments. In the hybrid CCP/ICP reactor (hereafter ICP reactor), the position and shape of the cloud were strongly dependent on the RF frequency. The particle cloud becomes larger and approaches the electrode as the RF frequency increases. By turning the lower frequency power off later with a small delay time, the particle cloud is made to move away from the electrode. Maintaining lower frequency RF power only was also helpful to reduce the particle cloud size during this transition step. In the ICP reactor, a sufficient bias power is necessary to make a particle trap appear. A similar particle cloud to that in the CCP reactor was observed around the sheath region of the lower electrode. The authors can also use the low-frequency effect to move the particle cloud away from the substrate holder if two or more bias powers are applied to the substrate holder. The dependence of the particle behavior on the RF frequencies suggests that choosing the proper frequency at the right moment during RF power changes can reduce particle contamination effectively.

Fast 2D fluid-analytical simulation of ion energy distributions and electromagnetic effects in multi-frequency capacitive discharges

A fast 2D axisymmetric fluid-analytical plasma reactor model using the finite elements simulation tool COMSOL is interfaced with a 1D particle-in-cell (PIC) code to study ion energy distributions (IEDs) in multi-frequency capacitive argon discharges. A bulk fluid plasma model, which solves the time-dependent plasma fluid equations for the ion continuity and electron energy balance, is coupled with an analytical sheath model, which solves for the sheath parameters. The time-independent Helmholtz equation is used to solve for the fields and a gas flow model solves for the steady-state pressure, temperature and velocity of the neutrals. The results of the fluid-analytical model are used as inputs to a PIC simulation of the sheath region of the discharge to obtain the IEDs at the target electrode. Each 2D fluid-analytical-PIC simulation on a moderate 2.2 GHz CPU workstation with 8 GB of memory took about 15–20 min. The multi-frequency 2D fluid-analytical model was compared to 1D PIC simulations of a symmetric parallel-plate discharge, showing good agreement. We also conducted fluid-analytical simulations of a multi-frequency argon capacitively coupled plasma (CCP) with a typical asymmetric reactor geometry at 2/60/162 MHz. The low frequency 2 MHz power controlled the sheath width and sheath voltage while the high frequencies controlled the plasma production. A standing wave was observable at the highest frequency of 162 MHz. We noticed that adding 2 MHz power to a 60 MHz discharge or 162 MHz to a dual frequency 2 MHz/60 MHz discharge can enhance the plasma uniformity. We found that multiple frequencies were not only useful for controlling IEDs but also plasma uniformity in CCP reactors.