Dc Self-bias Voltage Research Articles

On the in-situ determination of the effective secondary electron emission coefficient in low pressure capacitively coupled radio frequency discharges based on the electrical asymmetry effect

Abstract We present a method for the in-situ determination of the effective secondary electron emission coefficient (SEEC, γ) in a capacitively coupled plasma (CCP) source based on the γ-dependence of the DC self-bias voltage that develops over the plasma due to the electrical asymmetry effect (EAE). The EAE is established via the simultaneous application of two consecutive radio-frequency harmonics (with a varied phase angle) for the excitation of the discharge. Following the measurement of the DC self-bias voltage experimentally, particle-in-cell/Monte Carlo collision simulations coupled with a diffusion-reaction-radiation code to compute the argon atomic excited level dynamics are conducted with a sequence of SEEC values. The actual γ for the given discharge operating conditions is found by searching for the best match between the experimental and computed values of the DC self-bias voltage. The γ ≈ 0.07 values obtained this way are in agreement with typical literature data for the working gas of argon and the electrode material of stainless steel in the CCP source. The method can be applied for a wider range of conditions, as well as for different electrode materials and gases to reveal the effective SEEC for various physical settings and discharge operating conditions.

Hybrid simulation of a capacitive Ar/SiH4 discharge driven by electrically asymmetric voltage waveforms

Voltage waveforms associated with the electrical asymmetry effect (EAE) have the potential to be used in the deposition of the silicon-based film, since they are expected to decouple ion energy and flux at the wafer surface, and further facilitate control of the process. In this study, a one-dimensional fluid/electron Monte Carlo hybrid model is employed to examine the EAE in a capacitively coupled argon-silane discharge, encompassing both amplitude asymmetry effect (AAE) and slope asymmetry effect (SAE). In the case of AAE, with the increasing pressure, the discharge electronegativity gradually intensifies, in conjunction with a transition of the electron heating mode from α to drift-ambipolar, a reduction of the absolute value of the DC self-bias voltage, and a decrease in Ar+ content, with an increase in SiH3 + content. For SAE, the trend in the discharge characteristics with the increasing pressure is similar to that for AAE, but the details are different. In SAE, the electronegativity and bulk electric field are much enhanced, resulting in higher content of high-energy electrons and Ar+ in the bulk. In addition, the absolute value of the self-bias is lower, but shows a fewer decline with the increasing pressure. The deposition rate is lower in SAE, due to the lower electron heating efficiency. However, larger voltage drop difference between two sheaths leads to a wider range of ion energy modulation at higher pressures. This study systematically investigates and compares Ar/SiH4 discharges driven by two electrically asymmetric voltage waveforms across various parameters including electron dynamics, ion and neutral transport properties, and deposition rates, with the aim of providing valuable insights and a reference for industrial applications.

Kinetic simulations of capacitively coupled plasmas driven by tailored voltage waveforms with multi-frequency matching

Impedance matching is crucial for optimizing plasma generation and reducing power reflection in capacitively coupled plasmas (CCP). Designing these matchings is challenging due to the varying and typically unknown impedance of the plasma, especially in the presence of multiple driving frequencies. Here, a computational design method for impedance matching networks (IMNs) for CCPs is proposed and applied to discharges driven by tailored voltage waveforms (TVW). This method is based on a self-consistent combination of particle in cell/Monte Carlo collision simulations of the plasma with Kirchhoff’s equations to describe the external electrical circuit. Two Foster second-form networks with the same structure are used to constitute an L-type matching network, and the matching capability is optimized by iteratively updating the values of variable capacitors inside the IMN. The results show that the plasma density and the power absorbed by the plasma continuously increase in the frame of this iterative process of adjusting the matching parameters until an excellent impedance matching capability is finally achieved. Impedance matching is found to affect the DC self-bias voltage, whose absolute value is maximized when the best matching is achieved. Additionally, a change in the quality of the impedance matching is found to cause an electron heating mode transition. Poor impedance matching results in a heating mode where electron power absorption in the plasma bulk by drift electric fields plays an important role, while good matching results in the classical α-mode operation, where electron power absorption by ambipolar electric fields at the sheath edges dominates. The method proposed in this work is expected to be of great significance in promoting TVW plasma sources from theory to industrial application, since it allows designing the required complex multi-frequency IMNs.

Magnetic enhancement of the electrical asymmetry effect in capacitively coupled plasmas

The development of real-time control strategies for key discharge parameters, such as densities, fluxes, and energy distributions, is of fundamental interest to many plasma sources. Over the last decade, multi-harmonic ‘tailored’ voltage waveforms have been successfully employed to achieve enhanced control of key parameters in a wide range of radio-frequency (RF) plasma sources through application of the electrical asymmetry effect (EAE). More recently, the analogous magnetic asymmetry effect (MAE) has been numerically and experimentally demonstrated to achieve a notable degree of control in parallel plate RF plasma sources. The MAE is achieved via selectively magnetising the charged species adjacent to one electrode, altering the charge flux to the surface and enforcing a DC self-bias to maintain quasineutrality. This study addresses the degree of control achieved by the MAE in a non-planar geometry via 2D fluid/kinetic simulations of a magnetised RF capacitively coupled plasma source employing two different magnetic topologies. The simultaneous application of the EAE and MAE is then presented for the same geometry, demonstrating a degree of non-linear behaviour dependant upon the applied magnetic topology. Control of the DC self-bias voltage ηdc is demonstrated for a single 600 Vpp , 13.56 MHz discharge in both ‘convergent’ (maximum on-axis field strength) and ‘divergent’ (minimum on-axis field strength) magnetic topolgies. MAE induced modulations of ηdc = 0.13 Vpp and ηdc = 0.03 Vpp are achieved for each magnetic topology, respectively, for magnetic field strengths between 50 and 1000 G. Simultaneous application of an EAE and MAE is achieved through a multi-harmonic ‘peak’-type tailored voltage waveform employing varying harmonic phase offsets between 0∘ ⩽ θ ⩽ 360∘ . The degree to which the DC self-bias voltage is modulated by the applied EAE is mediated by the orientation and magnitude of the applied magnetic field. The EAE induced DC self-bias modulations exhibit non-linear behaviour in response to a superimposed MAE, such that the resulting DC self-bias differs from an additive combination of the two effects alone Simultaneous application of the electrical and MAEs offers the possibility of further decoupling ion and electron dynamics in RF plasma sources, and represents an improvement over each approach in isolation.

Investigation of the dual-frequency bias effect on inductively coupled Cl2 plasmas by hybrid simulation

In the etching process, a bias source is usually applied to the bottom electrode in inductively coupled plasmas (ICPs) to achieve independent control of the ion flux and ion energy. In this work, a hybrid model, which consists of a global model combined bi-directionally with a fluid sheath model, is employed to investigate the dual-frequency (DF) bias effect on the inductively coupled Cl2 plasmas under different pressures. The results indicate that the DC self-bias voltage developed on the biased electrode is approximately a linear function of the phase shift between the fundamental frequency and its second harmonic, and the value only varies slightly with pressure. Therefore, the ion energy on the bottom electrode can be modulated efficiently by the bias voltage waveform, i.e. the fluctuation of the ion energy with phase shift is about 40% for all pressures investigated. Besides, the ion energy and angular distribution functions (IEADFs) in DF biased inductive discharges is complicated, i.e. the IEADFs exhibits a four-peak structure under certain phase shift values. Although the species densities and ion fluxes also evolve with phase shift, the fluctuations are less obvious, especially for Cl2 + ions at low pressure. In conclusion, the independent control of the ion energy and ion flux are realized in DF biased ICPs, and the results obtained in this work are of significant importance for improving the etching process.

Influence of magnetic field gradient on the capacitive argon discharge at 8 MHz and 40 MHz

A one-dimensional implicit particle-in-cell/Monte Carlo collision model is used to study the effects of magnetic field gradients on the capacitively coupled argon plasma at 8 MHz and 40 MHz. The magnetic field strength at the powered electrode is fixed at 10 G, while varies from 30 to 100 G at the grounded electrode. The simulations show that the magnetic field with variable gradient can produce controllable asymmetry in the plasma density and ion flux profiles to each electrode. Increasing the magnetic field gradient will generate a significant dc self-bias, which results in a large ion bombardment energy at the powered electrode. The magnetic field gradients have been demonstrated to be an approach to create the dc self-bias and also effectively improve the plasma density. It is also found that at a higher frequency of 40 MHz, the dc self-bias voltage decreases, due to the fact that high collision rate of electrons with background gas will disturb the cyclotron motion of electrons, so the effect of the magnetic field is weakened. As a result, the ability to independently control ion energy and flux is weakened.

Electron density and temperature of dual-frequency capacitively coupled argon plasma in two-dimensional distribution obtained and studied in experiment

Behaviors of plasma in spatial distributions are vital for understanding physical mechanisms of low pressure capacitively coupled discharges. However, due to the lack of simple and reliable diagnostic techniques, few experimental studies have focused on two-dimensional distributions of plasma. Recently, we have developed a new method for rapid two-dimensional plasma diagnosis. With this method, the dual-frequency capacitively coupled plasma, an important plasma commonly used in etching, is systematically studied. Three circumstances are presented here: In case I, the plasma discharge was driven by a radio-frequency source; in case II, two radio-frequency sources were applied to one electrode; and in case III, such sources were applied to two separate electrodes. Results show that both the emission intensity and electron density in case III are higher than those in case II, and because the absolute value of the DC self-bias voltage is smaller, the structure of axial discharge is more symmetric in case III. For the asymmetrically capacitive discharges (cases I and II), the position of the maximum axial electron density is close to the powered electrode due to the presence of the DC self-bias voltage. As the low-frequency power increases or gas pressure decreases, the position of the maximum axial electron density moves gradually toward the grounded electrode because of the enlarged thickness of the sheath. However, high-frequency power has a limited influence on the position of the maximum axial electron density. The radial distribution of plasma is more uniform when plasma density is relatively low. For a higher plasma density situation, the center-peaked density profile results from the higher power deposition at the electrode center and/or more local charged particle dynamics. In addition, under the same discharge conditions, a lower electrode gap is favorable for the occurrence of the α-γ mode transition.

Effects of magnetic field on the secondary electron asymmetry effect in capacitively coupled plasmas

The secondary electron asymmetry effect (SEAE) provides the opportunity to generate the dc self-bias voltage and asymmetric plasma response, where secondary electron emission coefficients at both electrodes are unequal. In this work, we use one-dimensional implicit particle-in-cell/Monte Carlo collision simulation to investigate the effects of the homogeneous and inhomogeneous magnetic field on the SEAE. The magnetic field is applied parallel to electrodes, and the discharge is operated in a geometrically and electrically symmetric capacitively coupled argon plasma. By comparing the simulation results of the effects of the homogeneous and inhomogeneous magnetic field on the SEAE, the homogeneous magnetic field can increase the dc self-bias voltage to a certain extent and has little effect on the plasma density distribution. The inhomogeneous magnetic field is more advantageous in generating the dc self-bias but at the cost of uneven plasma density distribution. In addition, by comparing the results of inhomogeneous magnetic fields with opposite gradients, we found that the value of the self-bias voltage can be changed by adjusting the magnetic field gradient. Aside from that the roles of two electrodes can be easily reversed by changing the slopes of the magnetic field gradient. The results suggest that such a configuration could be desired in the microelectronics industry, since the controllable self-bias voltage is needed for deposition and etching processes.

Self-bias voltage formation and charged particle dynamics in multi-frequency capacitively coupled plasmas

In this work, we analyze the creation of the discharge asymmetry and the concomitant formation of the DC self-bias voltage in capacitively coupled radio frequency plasmas driven by multi-frequency waveforms as a function of the electrode surface characteristics. For the latter, we consider and vary the coefficients that characterize the elastic reflection of electrons from the surfaces and the ion-induced secondary electron yield. Our investigations are based on particle-in-cell/Monte Carlo collision simulations of the plasma and on a model that aids the understanding of the computational results. Electron reflection from the electrodes is found to slightly affect the discharge asymmetry in the presence of multi-frequency excitation, whereas secondary electrons cause distinct changes to the asymmetry of the plasma as a function of the phase angle between the harmonics of the driving voltage waveform and as a function the number of these harmonics.

The magnetic asymmetry effect in geometrically asymmetric capacitively coupled radio frequency discharges operated in Ar/O2

Previous studies in low pressure magnetized capacitively coupled radio frequency (RF) plasmas operated in argon with optimized geometric reactor symmetry have shown that the magnetic asymmetry effect (MAE) allows to control the particle flux energy distributions at the electrodes, the plasma symmetry, and the DC self-bias voltage by tuning the magnetron-like magnetic field adjacent to one electrode (Oberberg et al 2019 Plasma Sources Sci. Technol. 28 115021; Oberberg et al 2018 Plasma Sources Sci. Technol. 27 105018). In this way non-linear electron resonance heating (NERH) induced via the self-excitation of the plasma series resonance (PSR) was also found to be controllable. Such plasma sources are frequently used for reactive RF magnetron sputtering, but the discharge conditions used for such applications are significantly different compared to those studied previously. A high DC self-bias voltage (generated via a geometric reactor asymmetry) is required to realize a sufficiently high ion bombardment energy at the target electrode and a reactive gas must be added to deposit ceramic compound layers. Thus in this work, the MAE is investigated experimentally in a geometrically asymmetric capacitively coupled RF discharge driven at 13.56 MHz and operated in mixtures of argon and oxygen. The DC self-bias, the symmetry parameter, the time resolved RF current, the plasma density, and the mean ion energy at the grounded electrode are measured as a function of the driving voltage amplitude and the magnetic field at the powered electrode. Results obtained in pure argon discharges are compared to measurements performed in argon with reactive gas admixture. The results reveal a dominance of the geometrical over the magnetic asymmetry. The DC self-bias voltage as well as the symmetry parameter are found to be only weakly influenced by a change of the magnetic field compared to previous results obtained in a geometrically more symmetric reactor. Nevertheless, the magnetic field is found to provide the opportunity to control NERH magnetically also in geometrically asymmetric reactors. Adding oxygen does not alter these discharge properties significantly compared to a pure argon discharge.

Plasma chemistry and dust-particle generation in pure methane plasma: Influence of the RF electrode cleanliness

Hydrocarbon dust-particles are formed and grown to a large size (around 1 μm) in a low pressure capacitively coupled radio frequency discharge in methane. The methane decomposition leads to the formation of both spherical dust-particles in the plasma bulk and coating on the electrodes. Under ion bombardment, the coating on the biased top electrode peels off, leading to the fall of flakes. To better understand the role played by the electrode surface state on the plasma chemistry and on the dust-particle growth, experiments are carried out in two different initial conditions for the electrodes: (i) without any coating and (ii) with a homogeneous hydrocarbon coating. Spherical dust-particle growth is followed using the temporal evolution of the DC self-bias voltage. At the end of the experiment, dust-particles and flakes are collected and observed by scanning electron microscopy. Using mass spectrometry, temporal evolutions of neutrals, ionic species, and positive ion energies are investigated. Between the two experimental conditions, no strong differences were observed on the plasma chemistry. However, the self-bias voltage is strongly modified as well as the dust-particle growth and dynamics.

Control of electron, ion and neutral heating in a radio-frequency electrothermal microthruster via dual-frequency voltage waveforms

The development of low power micro-propulsion sources is of recent interest for application on miniature satellite platforms. Radio-frequency (rf) plasma electrothermal microthrusters can operate without a space-charge neutralizer and provide increased control of spatiotemporal power deposition. Further understanding of how the phase-resolved rf plasma heating mechanisms affect the phase-averaged bulk plasma properties, e.g. neutral gas temperature, could allow for in-flight tailoring of plasma thrusters. In this work, experimentally validated two-dimensional fluid-kinetic simulations were employed to study the spatially resolved electron and ion power deposition and neutral gas heating in a dual-frequency rf electrothermal microthruster operating at 1.5 Torr plenum pressure in argon. Experimental validation was performed through a comparison of the measured and simulated phase resolved Ar(2p1) excitation rates, showing close agreement. Two types of dual-frequency voltage waveforms were investigated, and comprise the combination of a 13.56 MHz voltage waveform with 27.12 MHz and 40.68 MHz waveforms, respectively. Varying the phase offset of the higher harmonic relative to the fundamental 13.56 MHz voltage waveform was found to modulate the dc self-bias voltage by 11% and 3% of the maximum applied peak-to-peak voltage, respectively. The 13.56 MHz, 27.12 MHz dual-frequency voltage waveform provided the highest degree of control, where the fraction of total rf power deposited into Ar+ ions was found to vary from 57% to 77%, modulating the on-axis neutral gas temperature by 35%. This control is attributed to the variation in the fraction of the rf phase cycle for which the sheath is collapsed, altering the phase-averaged electric field strength adjacent to the radial wall. The application of dual-frequency waveforms provides the ability to optimize the particle heating mechanisms with application to electrothermal propulsion.

The electrical asymmetry effect in a multi frequency geometrically asymmetric capacitively coupled plasma: A study by a nonlinear global model

We investigate the electrical asymmetry effect (EAE) and the current dynamics in a geometrically asymmetric capacitively coupled radio frequency plasma driven by multiple consecutive harmonics based on a nonlinear global model. The discharge symmetry is controlled via the EAE, i.e., by varying the total number of harmonics and tuning the phase shifts (θk) between them. Here, we systematically study the EAE in a low pressure (4 Pa) argon discharge with different geometrical asymmetries driven by a multifrequency rf source consisting of 13.56 MHz and its harmonics. We find that the geometrical asymmetry strongly affects the absolute value of the DC self-bias voltage, but its functional dependence on θk is similar at different values of the geometrical asymmetry. Also, the values of the DC self-bias are enhanced by adding more consecutive harmonics. The voltage drop across the sheath at the powered and grounded electrode is found to increase/decrease, respectively, with the increase in the number of harmonics of the fundamental frequency. For the purpose of validating the model, its outputs are compared with the results obtained in a geometrically and electrically asymmetric 2f capacitively coupled plasmas experiment conducted by Schuengel et al. [J. Appl. Phys. 112, 053302 (2012)]. Finally, we study the self-excitation of nonlinear plasma series resonance oscillations and its dependence on the geometrical asymmetry as well as the phase angles between the driving frequencies.

Enhanced control of the ionization rate in radio-frequency plasmas with structured electrodes via tailored voltage waveforms

Radio-frequency capacitively coupled plasmas that incorporate structured electrodes enable increases in the electron density within spatially localized regions through the hollow cathode effect (HCE). This enables enhanced control over the spatial profile of the plasma density, which is useful for several applications including materials processing, lighting and spacecraft propulsion. However, asymmetries in the powered and grounded electrode areas inherent to the hollow cathode geometry lead to the formation of a time averaged dc self-bias voltage at the powered electrode. This bias alters the energy and flux of secondary electrons leaving the surface of the cathode and consequentially can moderate the increased localized ionization afforded by the hollow cathode discharge. In this work, two-dimensional fluid-kinetic simulations are used to demonstrate control of the dc self-bias voltage in a dual-frequency driven (13.56, 27.12 MHz), hollow cathode enhanced, capacitively coupled argon plasma over the 66.6–200 Pa (0.5–1.5 Torr) pressure range. By varying the phase offset of the 27.12 MHz voltage waveform, the dc self-bias voltage varies by 10%–15% over an applied peak-to-peak voltage range of 600–1000 V, with lower voltages showing higher modulation. Resulting ionization rates due to secondary electrons within the hollow cathode cavity vary by a factor of 3 at constant voltage amplitude, demonstrating the ability to control plasma properties relevant for maintaining and enhancing the HCE.

Time-dependent dielectric breakdown characterizations of interlayer dielectric damage induced during plasma processing

We proposed an electrical evaluation method on the basis of leakage current–stress time (I–t) measurement for plasma-induced damage (PID) in interlayer dielectric films. The I–t measurement, which is widely used for the time-dependent dielectric breakdown (TDDB) test is applied to the evaluation of PID in thick SiO2 films. We focus on the electron trapping process during the TDDB test. The initial stage of time evolution of leakage current is monitored, and compared among the samples damaged under various plasma conditions. It was found that the initial electron trapping rate depends on the amount of damage, i.e., the rate increases with increasing in process time (tpr) and dc self-bias voltage (Vdc). The proposed technique can be used to assess the reliability degradation of interlayer dielectric films caused by PID.

Indium oxide-based transparent conductive films deposited by reactive sputtering using alloy targets

High-quality transparent conductive oxide (TCO) films, Sn-doped In2O3 (ITO) and In2O3–ZnO (IZO), were successfully deposited on either synthetic silica or polyethylene terephthalate (PET) substrates in the “transition region” by reactive dc magnetron sputtering using In–Zn and In–Sn alloy targets, respectively, with a specially designed plasma emission feedback system. The composition, crystallinity, surface morphology, and electrical and optical properties of the films were analyzed. All of the IZO films were amorphous, whereas the ITO films were polycrystalline over a wide range of deposition conditions. The minimum resistivities of the IZO and ITO films deposited on the heated PET substrates at 150 °C were 3.3 × 10−4 and 5.4 × 10−4 Ω·cm, respectively. By applying rf bias to unheated PET substrates, ITO films with a resistivity of 4.4 × 10−4 Ω·cm were deposited at a dc self-bias voltage of −60 V.

Reactive-ion etching of nylon fabric meshes using oxygen plasma for creating surface nanostructures

A facile one-step oxygen plasma irradiation in reactive ion etching (RIE) configuration is employed to nylon 6,6 fabrics with different mesh sizes to achieve surface nanostructures and improved wettability for textile and filtration applications. To observe the effects of power and irradiation time on the samples, the experiments were performed using constant irradiation time in varying power and using constant power in varying irradiation times. Results showed improved wettability after the plasma treatment. The FTIR spectra of all the treated samples exhibited decreased transmittance of the amide and carboxylic acid groups due to surface etching. The changes in the surface chemistry are supported by the SEM data wherein etching and surface nanostructures were observed for the plasma-treated samples. The etching of the surfaces is enhanced for higher power plasma treatments. The thermal analysis showed that the plasma treatment resulted in decreased crystallinity. Surface chemistry showed that the effects of the plasma treatment on the samples have no significant difference for all the mesh sizes. However, surface morphology showed that the sizes of the surface cracks are the same for all the mesh sizes but samples with larger mesh sizes exhibited enhanced etching as compared to the samples with smaller mesh sizes. Higher power induced higher negative DC self-bias voltage on the samples that favored anisotropic and aggressive etching.

Kinetic simulation of capacitively coupled plasmas driven by trapezoidal asymmetric voltage pulses

A kinetic Particle-In-Cell simulation with Monte Carlo Collisions was performed of a geometrically symmetric capacitively coupled, parallel-plate discharge in argon, driven by trapezoidal asymmetric voltage pulses with a period of 200 ns. The discharge was electrically asymmetric, making the ion energy distributions at the two electrodes different from one another. The fraction of the period (α), during which the voltage was kept at a constant (top-flat) positive value, was a critical control parameter. For the parameter range investigated, as α increased, the mean ion energy on the grounded electrode increased and the ions became more directional, whereas the opposite was found for the ions striking the powered electrode. The absolute value of the DC self-bias voltage decreased as α increased. Plasma instabilities, promoted by local double layers and electric field reversals during the time of the positive voltage excursion, were characterized by electron plasma waves launched from the sheath edge.

Chemical vapor deposition of a-CNx:H films for electron field emission using band supermagnetron plasma

A supermagnetron plasma apparatus with two band magnetron cathodes, which can form uniform magnetron plasma under a stationary magnetic field (about 160 G), was used to deposit a-CNx:H films at N2 gas concentrations of 0 and 70%. A high deposition rate of about 85±5 nm/min was obtained at a low dc self-bias voltage, between -20 and -62 V. The optical band gap could be controlled between 1.1 and 3.8 eV. The double-layer a-CNx:H films with optical band gaps of 2.2 eV (upper layer; N2 0%) and 1.5 eV (lower layer; N2 70%) formed on p-Si substrate showed a low-threshold-emission electric field of 9 V/μm.

Funnelling of rf current via a plasmoid through a grid hole in an rf capacitive plasma reactor

The boundaries of capacitively coupled radio-frequency (rf) plasma reactors generally include at least one grounded metal grid or perforated plate for purposes of gas flow or diagnostic access. When increasing the rf power, an intense localized plasma (a plasmoid) can spontaneously ignite in a hole of a grounded surface. Experiments described here show that the plasmoid funnels rf current through the hole to the other side of the grounded plate, thereby increasing the effective grounded area in contact with the plasma. Hence, plasmoid ignition is always accompanied by a drop in the dc self-bias voltage of the rf electrode.The small area of the plasmoid aperture means that the rf current density passing through the plasmoid is very high, causing intense optical emission and strong local heating. Plasmoid ignition can therefore cause a loss of process reproducibility and potentially lead to melting and eventual destruction of reactor components.