Shinya Iwashita

Control of plasma properties in capacitively coupled oxygen discharges via the electrical asymmetry effect

Using a combined experimental, numerical and analytical approach, we investigate the control of plasma properties via the electrical asymmetry effect (EAE) in a capacitively coupled oxygen discharge. In particular, we present the first experimental investigation of the EAE in electronegative discharges. A dual-frequency voltage source of 13.56 MHz and 27.12 MHz is applied to the powered electrode and the discharge symmetry is controlled by adjusting the phase angle θ between the two harmonics. It is found that the bulk position and density profiles of positive ions, negative ions, and electrons have a clear dependence on θ, while the peak densities and the electronegativity stay rather constant, largely due to the fact that the time-averaged power absorption by electrons is almost independent of θ. This indicates that the ion flux towards the powered electrode remains almost constant. Meanwhile, the dc self-bias and, consequently, the sheath widths and potential profile can be effectively tuned by varying θ. This enables a flexible control of the ion bombarding energy at the electrode. Therefore, our work proves the effectiveness of the EAE to realize separate control of ion flux and ion energy in electronegative discharges. At low pressure, the strength of resonance oscillations, which are found in the current of asymmetric discharges, can be controlled with θ.

Highly stable a-Si: H films deposited by using multi-hollow plasma chemical vapor deposition

Hydrogenated amorphous silicon (a-Si:H) films of high stability against light exposure have been deposited by using a newly developed multi-hollow plasma chemical vapor deposition (CVD) method. Films deposited in the upstream region in the multi-hollow plasma CVD reactor are a-Si:H films without incorporating a-Si:H nano-particles (clusters), while those in the downstream region are a-Si:H films with incorporating clusters. A-Si:H films without clusters have a low initial defect density of 5 ×1015 cm-3 and keep the value even after 100 h exposure of intense light intensity of 240 mW/cm2, whereas a-Si:H films with clusters show a significant increase in defect density from its initial value of 5 ×1015 cm-3 to 2 ×1016 cm-3 after 100 h light exposure. These results indicate that suppression of clusters incorporated into films is the key to realizing highly stable a-Si:H films.

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

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

Transport of nano-particles in capacitively coupled rf discharges without and with amplitude modulation of discharge voltage

Transport of nano-particles in rf discharges without and with an amplitude modulation (AM) of the discharge voltage has been examined using a two-dimensional laser-light-scattering method. During the discharging period, nano-particles are mainly generated in the plasma/sheath boundary region near the powered electrode. They move away from their generation region towards the upper grounded electrode after turning off discharges due to thermophoretic force. Using AM discharges, nano-particles can be transported, during the modulation period, from their generation region to the plasma/sheath boundary region near the upper grounded electrode. The transport time is at least six times shorter than that after turning off unmodulated discharges. Two key parameters for the fast transport using AM discharges are the discharge voltage and the period of the AM.

Transport control of dust particles via the electrical asymmetry effect: experiment, simulation and modelling

The control of the spatial distribution of micrometre-sized dust particles in capacitively coupled radio frequency discharges is relevant for research and applications. Typically, dust particles in plasmas form a layer located at the sheath edge adjacent to the bottom electrode. Here, a method of manipulating this distribution by the application of a specific excitation waveform, i.e. two consecutive harmonics, is discussed. Tuning the phase angle θ between the two harmonics allows one to adjust the discharge symmetry via the electrical asymmetry effect (EAE). An adiabatic (continuous) phase shift leaves the dust particles at an equilibrium position close to the lower sheath edge. Their levitation can be correlated with the electric field profile. By applying an abrupt phase shift the dust particles are transported between both sheaths through the plasma bulk and partially reside at an equilibrium position close to the upper sheath edge. Hence, the potential profile in the bulk region is probed by the dust particles providing indirect information on plasma properties. The respective motion is understood by an analytical model, showing both the limitations and possible ways of optimizing this sheath-to-sheath transport. A classification of the transport depending on the change in the dc self-bias is provided, and the pressure dependence is discussed.

Rapid transport of nano-particles having a fractional elementary charge on average in capacitively-coupled rf discharges by amplitude-modulating discharge voltage

We have observed transport of nano-particles having, on average, a fractional elementary charge in single pulse and double pulse capacitively-coupled rf discharges both without and with an Amplitude Modulation (AM) of the discharge voltage, using a two-dimensional laser-light scattering method. Rapid transport of nano-particles towards the grounded electrode is realized using rf discharges with AM. Two important parameters for the rapid transport of nano-particles are the discharge voltage and the period of AM. An important key of the rapid transport is fast redistribution of ion current over the whole discharge region; that is, fast change of spatial distribution of forces exerted on nano-particles. The longer period of the modulation is needed for rapid transport for the larger nano-particles. The higher discharge voltage of the modulation is needed for rapid transport of nano-particles having a smaller mean charge. Local perturbation of electric potential using a probe does not bring about global rapid transport of nano-particles, whereas it leads to their local transport near the probe.

Sheath-to-sheath transport of dust particles in a capacitively coupled discharge

Transport of micrometer-sized dust particles in a capacitively coupled radio frequency discharge at low pressures of a few Pa is realized experimentally and understood by a kinetic particle simulation combined with a model. Applying a voltage waveform, which consists of two consecutive harmonics with a variable phase angle, θ, to one electrode, control of both the spatial potential profile and the ion density distribution is obtained by adjusting θ. In this way, the electrostatic and ion drag forces, on dust particles initially located at the sheath edge adjacent to the lower electrode, are controlled. The sudden change of θ leads to an abrupt change of the sheath width. This introduces the particles instantaneously into a high potential that accelerates them to high kinetic energies. The experimental results show that a certain minimum change of the discharge symmetry, i.e. of the phase angle, is required to allow the transport of dust particles through the plasma bulk. Beyond this threshold, a part of the transported particles can even be trapped around the edge of the opposing (upper) sheath. (Some figures may appear in colour only in the online journal)

Theory for correlation between plasma fluctuation and fluctuation of nanoparticle growth in reactive plasmas

We propose a simple theoretical model that describes the correlation between plasma fluctuation and fluctuation of nanoparticle growth in reactive plasmas. The model predicts that the high density of nanoparticles brings about small mean size, narrow size dispersion, and sharp size slope on the large side of the size distribution. The model suggests some methods of tuning the size dispersion, and it also suggests that a self-limiting process is the key to markedly suppressing fluctuations in nanostructure fabrication. All predictions coincide with the experimental results reported previously. Moreover, the model suggests that plasma fluctuation induces both the linear and nonlinear responses of nanoparticle growth.

Formation of carbon nanoparticle using Ar+CH4 high pressure nanosecond discharges

We have studied formation of carbon nanoparticles using Ar+CH4 high pressure nanosecond discharge non-thermal plasmas. Transition pressure from uniform glow discharges to filamentary ones was clarified to obtain conditions under which uniform glow discharges are sustained. We have produced nanoparticles using the glow discharges, and then we have collected nanoparticles on the grounded electrode by the filtered vacuum collection method. Size distribution analysis reveals that the CH4 concentration is an important parameter in controlling nanoparticle growth. We have also studied film deposition on the powered electrode and the grounded electrode. The deposition rate on the powered electrode is 7 times higher than that on the grounded electrode. Optical emission observations suggest that radical generation rate near the powered electrode is twice higher than that near the grounded electrode, leading to high deposition rate on the powered electrode.

Temperature Dependence of Dielectric Constant of Nanoparticle Composite Porous Low-k Films Fabricated by Pulse Radio Frequency Discharge with Amplitude Modulation

Nanoparticle composite porous low-k films are deposited by pulse radio frequency (RF) discharge with the amplitude modulation (AM) of discharge voltage. The deposition rate obtained with AM is 0.65 nm/s, which is sevenfold as high as that obtained without AM, and porosity = 60–63% and dielectric constant k = 1.1–1.4 for the films obtained with AM are nearly equal to those obtained without AM. The deposition of porous low-k films by pulse RF discharge with AM is a promising method for increasing the deposition rate with a less pronounced agglomeration and without variations in the properties of the films. With decreasing substrate temperature from 403 to 368 K, the porosity of the films increases from 3.5 to 60%, leading to a reduction in their dielectric constant from 2.9 to 1.4. Substrate temperature is a key parameter that determines the porosity and dielectric constant of the porous low-k films.