Table of contents

A low-power microwave-excited argon microstrip plasma source operated at 2.45 GHz is studied by a three-dimensional fluid model. The electrodeless microwave-excited plasmas are produced in the gas channel with the gas pressures of 50 and 100 Torr at the input power of 2 W. Simulations are performed by the plasma module of COMSOL Multiphysics^@. Results show that the electric field induced by the electromagnetic wave is concentrated in the neighborhood of the inner surface of gas channel under the microstrip line. The electromagnetic wave is restricted to transit from being propagating to evanescent in a very thin zone at which the electron density is equal to the critical density. The resonance zone is solved by adding an effective collision frequency to the momentum collision frequency. The governed ions are found to be atomic argon ions (Ar⁺) and molecular argon ions (Ar₂⁺) and the latter has a wider distribution. The three-body reactions to produce Ar₂⁺ ions become important at high gas pressures.

A VHF H₂ plasma was produced by a narrow-gap discharge at high pressures, and the plasma parameters were examined with the Langmuir probe. A bi-Maxwellian electron distribution was observed near the discharge electrode at a discharge gap of 10 mm, while a Maxwellian distribution was seen near the center. When the discharge gap was 15 mm, electrons had a Maxwellian distribution independent of the position. It was found that there must be a threshold in the discharge gap for stochastic heating to occur. The plasma potential near the discharge electrode was higher than that near the center of the interelectrode gap, suggesting the existence of negative ions. The simulation using the plasma hybrid code was carried out. The spatial profiles of the density and temperature of electrons were similar to the experimental results. The plasma potential had a hill-like profile that was quite different from the measured one. The negative ion density was negligible.

A reactive gas cluster injection system with a scanning function was developed in order to increase the processing area. High-precision anisotropic etching with an aspect ratio of 7 was achieved for ClF₃ cluster etching without scanning. However, with scanning, the aspect ratio for etching decreased to 1.5 because the side walls were etched by the gas retained in the trench. By reducing the source gas pressure, increasing the target distance, and mixing He in the source gas, anisotropic etching with an aspect ratio of about 6.3 was achieved with this apparatus.

The etching of polycrystalline silicon (poly-Si)/SiO₂ stacks by using VHF plasma was studied for three-dimensional NAND fabrication. One critical goal is achieving both a vertical profile and high throughput for multiple-stack etching. While the conventional process consists of multiple steps for each stacked layer, in this study, HBr/fluorocarbon-based gas chemistry was investigated to achieve a single-step etching process to reduce process time. By analyzing the dependence on wafer temperature, we improved both the etching profile and rate at a low temperature. The etching mechanism is examined considering the composition of the surface reaction layer. X-ray photoelectron spectroscopy (XPS) analysis revealed that the adsorption of N–H and Br was enhanced at a low temperature, resulting in a reduced carbon-based-polymer thickness and enhanced Si etching. Finally, a vertical profile was obtained as a result of the formation of a thin and reactive surface-reaction layer at a low wafer temperature.

A new method of generating obliquely incident ions has been investigated. A plasma system with a cathode consisting of a repetition of a group of four electrode rods connected to their respective RF power supplies is proposed. The ion angular distribution (IAD) is controlled by modulating the phase shift of the four RF powers. The IAD of an argon high-density plasma was analyzed on the basis of transient plasma simulation. When the RF voltages are controlled so that the phase shift is π/2, a convex-shaped plasma sheath corresponding to each group of four rods appears and propagates parallel to the wafer with time. By propagating this "wavy" sheath, a bimodal IAD consisting of ions obliquely incident mainly from two directions are obtained nearly uniformly across the wafer. This method is capable of generating obliquely incident ions, which is expected to be effective as an additional knob for precise profile control in fine-pattern reactive-ion etching (RIE).

We propose an electrical method, named capacitance–voltage (C–V) monitoring, for quantifying plasma-induced damage (PID) to interlayer dielectrics. By this method, we measure the C–V hysteresis loops to assign carrier trap sites created by PID, and simultaneously obtain the change in the dielectric constant and thickness. We optimized the bias-sweep configuration for measuring the hysteresis curves. It is found that the C–V curve shifted in the negative direction during the optimized voltage sweep from accumulation to inversion in a pseudo-metal–oxide–semiconductor (MOS) structure. This implies the appearance of net positively charged sites owing to PID, presumably near the surface of the SiOC film. We estimate the density of defects created near the surface by monitoring the obtained C–V hysteresis curve shift. Since the degradation of interlayer dielectrics affects the circuit performance, the proposed quantitative method should be used for plasma process designs.

Hydrogenated amorphous silicon carbide films have been fabricated by the decomposition of hexamethyldisilane with a microwave discharge flow of Ar. Mechanically hard films were obtained by applying radio-frequency (RF) bias voltages to the substrate. The atomic compositions of the films were analyzed by a combination of Rutherford backscattering and elastic recoil detection, X-ray photoelectron spectroscopy (XPS), and glow discharge optical emission spectroscopy. The chemical structure was analyzed by carbon-K near-edge X-ray absorption fine structure spectroscopy, high-resolution XPS, and Fourier transform infrared absorption spectroscopy. The structural changes upon the application of RF bias were investigated, and the concentration of O atoms near the film surface was found to play a key role in the mechanical hardness of the present films.

Semiconducting iron disilicide (β-FeSi₂) island grains of a few hundred nanometers in diameter were formed on the surface of Si powder by metal–organic chemical vapor deposition. On Au-coated Si powder, the Au–Si liquidus phase was obtained by melting the Si surface via the Au–Si eutectic reaction, which contributed to the formation of island grains. The dramatic decrease in the defect density in β-FeSi₂, which was due to this growth mechanism, was confirmed by the photoluminescence properties. The β-FeSi₂/Si composite powder could evolve hydrogen from formaldehyde aqueous solution under irradiation of visible light with wavelengths of 420–650 nm.

β-FeSi₂ thin films were epitaxially grown on p-type Si(111) substrates at a substrate temperature of 560 °C and Ar pressure of 2.66 × 10⁻¹ Pa by radio-frequency magnetron sputtering (RFMS) using a sintered FeSi₂ target, without postannealing. The resultant n-type β-FeSi₂/p-type Si heterojunctions were evaluated as near-infrared photodiodes. Three epitaxial variants of β-FeSi₂ were confirmed by X-ray diffraction analysis. The heterojunctions exhibited typical rectifying action at room temperature. At 300 K, the heterojunctions showed a substantial leakage current and minimal response for irradiation of near-infrared light. At 50 K, the leakage current was markedly reduced and the ratio of the photocurrent to dark current was considerably enhanced. The detectivity at 50 K was estimated to be 3.0 × 10¹¹ cm Hz^1/2/W at a zero bias voltage. Their photodetection was inferior to those of similar heterojunctions prepared using facing-target direct-current sputtering (FTDCS) in our previous study. This inferiority is likely because β-FeSi₂ films prepared using RFMS are located in plasma and are damaged by it.

In situ ellipsometry was carried out as well as ex situ measurements by scanning electron microscopy and Raman spectroscopy for the analyses of substrate surface in the first stage of graphene growth in plasma-enhanced chemical vapor deposition. Evolutions of the ellipsometric parameters Ψ and Δ were precisely measured during the growth of graphene with the sensitivity far less than 1 nm in film thickness. By the fitting of the experimentally obtained trajectory of ellipsometric parameters on the Ψ–Δ coordinate plane to that of the calculated ones, we confirmed that the graphite volume fraction decreased with growth after a dense graphite material initially formed. This suggests that carbon nanowalls grew on a thin graphitic layer.

We report a novel low-power nitriding technique by utilizing a 2.45 GHz microwave-excited nitrogen radical flow system. Nitrogen plasma was produced at the nozzle with dimensions of 50 × 0.5 mm² and blown onto the surface of a target substrate. A titanium substrate has been used as a target plate since it is easy to visualize a nitriding effect. The titanium substrate was treated under the conditions of 60 W microwave power, 20 Torr of nitrogen gas pressure, and a plate temperature of ∼800 °C. As a result, we have succeeded in nitriding of the titanium substrate in a quasi-atmospheric region of 20 Torr and of a very low power of 60 W with the hardness kept high, which is almost the same as the hardness processed by conventional nitriding methods.

The plasma jet generation of reactive oxygen and nitrogen species (RONS) in solution is important in biology, medicine, and disinfection. Studies using a wide variety of plasma jet devices have been carried out for this purpose, making it difficult to compare the performance between devices. In this study, we compared the efficiency of RONS generation in deionized (DI) water between 3.7-mm- and 800-µm-sized helium (He) plasma jets (hereafter mm-jet and µm-jet, respectively) at different treatment distances and times. The efficiency of RONS generation was determined by considering the total amount of RONS generated in DI water with respect to the input energy and gas consumption. We found that the mm-jet generated 20% more RONS in the DI water than the µm-jet at the optimized distance. However, when the input power and He gas consumption were taken into account, we discovered that the µm-jet was 5 times more efficient in generating RONS in the DI water. Under the parameters investigated in this study, the concentration of RONS continued to increase as a function of treatment time (up to 30 min). However treatment distance had a marked effect on the efficiency of RONS generation: treatment distances of 25 and 30 mm were optimal for the mm-jet and µm-jet, respectively. Our method of comparing the efficiency of RONS generation in solution between plasma jets could be used as a reference protocol for the development of efficient plasma jet sources for use in medicine, biology, and agriculture.