Search

We report on the formation of anodic aluminum oxide (AAO) film using phytic acid (C6H18O24P6), a naturally obtainable and non-toxic organic acid electrolyte. When the temperature of the phytic acid electrolytes changes from 0 °C to 20 °C, the pore size increases, and the AAO film becomes less dense because the growth rate of the AAO film increases. In particular, when the temperature of the phytic acid electrolyte is above 15 °C, the AAO film changes dramatically. According to XRD analysis, the alpha-alumina intensity was relatively higher in the AAO film grown at low-temperatures (0, 5, 10 °C) than high-temperatures (15, 20 °C). It was possible to obtain a result that the properties were better. The microstructural change of the AAO film according to the temperature of the phytic acid electrolytes affected the breakdown voltage and the number of contamination particles that are generated, which are important properties for plasma corrosion protection coating materials. The AAO film grown in the low-temperature has the most suitable properties for use as a plasma corrosion protection coating material. Notably, the breakdown voltage of the AAO film is 0.45 kV at 14.9 μm thickness, with no observable crack on its surface after the plasma corrosion test. These findings provide significant evidence to support the application of phytic acid as an electrolyte to grow AAOfilms, and the AAOfilm grown in a phytic acid bath can be applied as a plasma corrosion protection coating material.

In general, as the radio frequency (RF) power increases in a capacitively coupled plasma (CCP), the power transfer efficiency decreases because the resistance of the CCP decreases. In this work, a parallel resonance circuit is applied to improve the power transfer efficiency at high RF power, and the effect of the parallel resonance on the electron energy distribution function (EEDF) is investigated in a 60 MHz CCP. The CCP consists of a power feed line, the electrodes, and plasma. The reactance of the CCP is positive at 60 MHz and acts like an inductive load. A vacuum variable capacitor (VVC) is connected in parallel with the inductive load, and then the parallel resonance between the VVC and the inductive load can be achieved. As the capacitance of the VVC approaches the parallel resonance condition, the equivalent resistance of the parallel circuit is considerably larger than that without the VVC, and the current flowing through the matching network is greatly reduced. Therefore, the power transfer efficiency of the discharge is improved from 76%, 70%, and 68% to 81%, 77%, and 76% at RF powers of 100 W, 150 W, and 200 W, respectively. At parallel resonance conditions, the electron heating in bulk plasma is enhanced, which cannot be achieved without the VVC even at the higher RF powers. This enhancement of electron heating results in the evolution of the shape of the EEDF from a bi-Maxwellian distribution to a distribution with the smaller temperature difference between high-energy electrons and low-energy electrons. Due to the parallel resonance effect, the electron density increases by approximately 4%, 18%, and 21% at RF powers of 100 W, 150 W, and 200 W, respectively.

In this study, electron-assisted photoresist (PR) etching is conducted using oxygen inductively coupled plasma at a pressure of 3 mTorr. During the PR etching, a low-energy electron beam is generated and is controlled by varying the acceleration voltage (0–40 V) on the grid to assist with the PR etching. When a low acceleration voltage (25 V), the etch rate increases, and the ion bombardment energy decreases with increasing electron beam energy. The electron energy probability function is measured to verify the relation between the etch rate and acceleration voltage with respect to the sheath thickness on the grid. Furthermore, low contribution of the [Formula: see text] radical to the etch rate increment is observed via optical emission spectroscopy measurements.

A highly efficient plasma source is developed in inductively coupled plasmas (ICPs) using a parallel capacitor, which is connected to an antenna in parallel. The power absorbed by the ICP is proportional to the equivalent resistance of the ICP. In order to improve the plasma generation, a parallel resonance is used between the parallel capacitor and the equivalent inductance by the plasma and the antenna. In all experiments conducted under an H-mode regime where the inductive heating is dominant, the resistance of a load involving the plasma increases about ten times near the resonance, and the power consumed by the plasma is greatly increased. Consequently, the electron density is greatly increased up to about 350% in the argon plasma and is significantly increased up to about 1000% in the oxygen plasma. For analysis, the transformer model of the ICP and the power balance equation of the global model are introduced, and they show good agreement with the experimental results.

The effect of the electron energy distribution function (EEDF) on the behavior of the electron density (n e) is investigated under various gas pressures of nitrogen (N2) in inductively coupled plasma (ICP) operated at low and high input powers. A Langmuir probe is used to measure the EEDFs and electron densities, and the antenna coil current is measured to obtain the absorbed power in the plasma (P abs). At gas pressures above 2.67 Pa (20 mTorr) and 2500 W, P abs increases continually with increasing the gas pressure, but the electron density slightly decreases. In this case, the EEDF has a Maxwellian distribution with a high-energy tail. On the other hand, at 300 W, P abs decreases slightly with increasing gas pressure, but the electron density dramatically decreases, and the EEDF evolves from a bi-Maxwellian to a non-Maxwellian distribution with substantially highly depleted high-energetic part (high-energy tail). To analyze the difference in the behavior of the decrease rate in electron density, the total energy loss per electron–ion pair lost (ε T) is measured through the probe diagnostics, and the measured electron density is compared with the calculated electron density from the global model. An additional experiment is performed in Ar plasma under the same discharge conditions as N2 plasma to compare the EEDF effect. This study provides experimental evidence that the EEDF has a decisive effect on the behavior of the electron density in plasmas.

A wafer-type monitoring apparatus that can simultaneously measure the two-dimensional (2D) distributions of substrate temperature and plasma parameters is developed. To measure the temperature of the substrate, a platinum resistance temperature detector is used. The plasma density and electron temperature are obtained using the floating harmonics method, and incoming heat fluxes from the plasma to the substrate are obtained from the plasma density and electron temperature. In this paper, 2D distributions of the substrate temperature, plasma density, and electron temperature are obtained simultaneously for the first time in inductively coupled plasma. The shapes of the 2D distributions of the substrate temperature and incoming heat flux are similar to each other, but some differences are found. To understand that, an energy balance equation for the substrate is established, which shows good agreement with the experimental results. This apparatus will promote the understanding of surface reactions, which are very sensitive to the temperatures and plasma densities in plasma processing.

A reliability assessment was performed to predict the lifetime of an anodic aluminum oxide film coated on equipment used in plasma etching processes. An anodic aluminum-oxide-coated part was exposed to an argon plasma, and its weight loss was monitored. This plasma test was conducted at four different pressure levels (125, 150, 200, and 250 mTorr). The failure distribution data obtained for each pressure level were found to indicate the same failure mechanism. Subsequently, an inverse power model was the best fit model for describing the relationship between the applied pressure and the lifetime of a part. The failure of the anodic aluminum oxide film caused by long-term exposure to plasma is, thus, shown to be closely linked to a decrease in the weight of the film.

A hysteresis phenomenon observed in neon inductive discharge at low gas pressure is investigated in terms of the evolution of the electron energy distribution function (EEDF). Generally, the hysteresis phenomenon has been reported at high-pressure Ramsauer gas discharges. However, in neon plasma, we found that the hysteresis phenomenon occurs even at low gas pressure (5 mTorr). Furthermore, the hysteresis vanishes with an increase in the gas pressure (10 and 25 mTorr). To analyze this hysteresis, the EEDF is measured depending on the radio frequency power. The EEDF at 10 mTorr sustains the bi-Maxwellian distribution during an E–H transition. On the other hand, the EEDF at 5 mTorr changes dramatically between discharge modes. At 5 mTorr, the measured EEDF for the E mode has the Maxwellian distribution due to high collisional heating in the bulk plasma. The EEDF for the H mode has the bi-Maxwellian distribution because collisionless heating in the skin depth is dominant. This apparent evolution of the EEDF causes a nonlinear energy loss due to collisions during the discharge mode transition. Therefore, the plasma can maintain the H mode discharge with high ionization efficiency, even at a lower applied power, which results in the hysteresis.

The plasma etch process has become more difficult and longer than other processes and the etch process engineers have tried to confirm whether the results of etch process were normal by monitoring the equipment. However, it is difficult for the engineers unfamiliar with plasma to discover the parameter correlated to the real etch results, so the intuitive parameter to easily estimate the etch results is required. In this study, we focused on analyzing the correlation of the etch rates of SiO2 in CF4 plasma with electrical circuit parameters closely related to genuine plasma, which were obtained by chamber modeling and VI probe. We also introduced the intuitive parameter by combining several electrical circuit parameters to estimate the etch rate more precisely. The proposed parameter was strongly correlated to the etch rates and the coefficient of determination between the etch rates and the proposed parameter was over 0.94. We expect that using the proposed parameter can contribute to maintaining the stability of etch process.

The effect of low-frequency power and high-frequency power on the electron energy probability function (EEPF) and the physical and electrical characteristics of plasma are experimentally investigated in a dual-frequency capacitively coupled plasma. RF powers of 2 MHz (low-frequency) and 13.56 MHz (high-frequency) are simultaneously applied to an electrode. EEPFs and DC self-bias voltages (V DC) are measured as one of the two RF powers is increased while the other is fixed. When the 2 MHz power increases at a fixed 13.56 MHz power, the electron density decreases, and the electron temperature increases with the decrease in the population of low-energy (below 5 V) electrons in the EEPF. Note that the increase in the low-frequency power is accompanied by a large decrease in V DC, which is related to the ion energy. On the other hand, when 13.56 MHz power increases at a fixed 2 MHz power, the electron density and the electron temperature significantly increase with the increase in the population of high-energy (above 5 V) electrons in the EEPF, while V DC decreases slightly. Experimental results show that the increase in the low-frequency power enhances the ion energy, and the increase in the high-frequency power enhances electron heating and ionization efficiency.

Plasma generation efficiency in a capacitively coupled plasma (CCP) at high densities or high conductivity tends to be lower due to low plasma resistance. An inductor is installed to a powered electrode in parallel to improve plasma generation efficiency at higher density in the CCP. To reduce the power loss in a system, a parallel resonance is used between the capacitance of the CCP and the inductance of the parallel inductor. When parallel resonance occurs, the impedance of the chamber, including the plasma, increases. Therefore, the current flowing in the system is expected to decrease. At the resonance, the current in the system significantly decreases, and the voltages and currents at the powered electrode significantly increase. This phenomenon indicates that the system power loss is decreased, and the power absorbed by the plasma is increased. As a result, the ion density and the voltage at the powered electrode are increased up to 66% and 25% at the parallel resonance condition, respectively. To understand these increases, a circuit model for the plasma and the parallel inductor is suggested which shows good agreement with the experimental results. This method can be applied to the CCP for improving plasma generation.

The parts of equipment in a process chamber for semiconductors are protected with an anodic aluminum-oxide (AAO) film to prevent plasma corrosion. We added cerium(Ⅳ) ions to sulfuric acid in the anodizing of an AAO film to improve the plasma corrosion resistance, and confirmed that the AAO film thickness increased by up to ~20% when using 3 mM cerium(Ⅳ) ions compared with general anodizing. The &alpha, Al2O3 phase increased with increasing cerium(Ⅳ) ion concentration. The breakdown voltage and etching rate improved to ~35% and 40%, respectively. The film&rsquo, s performance regarding the generation of contamination particles reduced by ~50%.

Low-energy electron beam generation using a DC biased grid was investigated in an inductively coupled plasma (ICP). The electron beam was measured in argon gas at various pressures, ICP source powers, and substrate voltages (V sub). At a low ICP source power (50 W), an electron beam was generated even at small values of V sub (10 V), however at a high ICP source power (200 W), an electron beam was only generated when a higher voltage (30 V) was applied due to the short sheath thickness on the grid surface. The sheath on the grid surface is an important factor for generating electron beams because low-energy electrons are blocked. If the sheath thickness to small, a high voltage should be applied to generate an electron beam, as accelerate regions cannot exist without the sheath. At high pressure, since electrons experience numerous neutral collisions, a high substrate voltage is needed to generate an electron beam. However, if the applied substrate voltage becomes too high (40 V) at high pressure, high-energy electrons result in secondary plasma under the grid. Therefore, maintaining a low pressure and low ICP source power is important for generating electron beams.

We investigated the plasma and electrical characteristics depending on the antenna position in an inductively coupled plasma with a passive resonant antenna. When the powered antenna and passive resonant antenna are installed near the top plate and in the middle of the cylindrical reactor (setup A), respectively, the ion density at the resonance is about 2.4 times to 9 times higher than that at non-resonance. This is explained by the reduction in power loss in the powered antenna (including the matching circuits) and the increase in power absorbed by the plasma discharge. However, when the powered antenna and passive resonant antenna are interchanged (setup B), the ion density at the resonance is not significantly different from that at the non-resonance. When RF power is changed from 50 to 200 W, the ion density at the resonance of setup B is 1.6 times to 5.4 times higher than at the non-resonance of setup A. To analyse this difference, the profile of the z-axis ion density is measured and the electric and magnetic field simulations are investigated. The results are discussed along with the electron kinetics effect and the coupling loss between the antenna and the metal plate.

We developed a hybrid probe comprised of an electrical probe and optical probe to investigate the properties of the far scrape-off layer (SOL) plasma and the blob in Korea Superconducting Tokamak Advanced Research (KSTAR). The hybrid probe is attached to a horizontal manipulator, and the radial profiles of plasma parameters such as electron temperature, ion flux, and the Dα signals are measured with time resolution (100 μs) by varying the radial position (0.15–0.4 m away from the last closed flux surface (LCFS)). In ELMy H-mode discharge, the blob radial velocity is obtained from the time delay between the start of the peak Dα signal and the peak electrical probe current and the corresponding radial position. The obtained blob radial velocity is in good agreement with the radial velocity of the filament obtained using the two radially separated electric probes at the far-SOL in the KSTAR. The profiles of the blob radial velocity for five discharges are obtained depending on the radial position in similar experimental parameters. The blob radial velocity is estimated as 44.1–250 m/s and 25.4–135 m/s at the radial position of 0.15 m (near the LCFS) and 0.4 m (near the outer wall), respectively. It is observed that the blobs are propagated to the outer wall with various radial velocities, and the radial velocity slows down as they get closer to the outer wall. The hybrid probe measurement found that the blob properties are about 2 to 10 times larger than the background plasma, and these show the same trend with the blob radial velocity. The hybrid probe is expected to contribute to the analysis and understanding of far-SOL plasma research in fusion reactors.

Argon plasma jet (Ar-PJ) has been widely used in clinical medicine; however, the cellular effects of Ar-PJ therapy applying to living tissues have not been clarified yet. It is necessary to investigate cellular responses to Ar-PJ in establishing guidelines on the therapeutic use of Ar-PJ. Interestingly, in the Ar-PJ-treated cells, the fragmented mitochondria, a typical cellular stress indicator, were discovered even in the cells located in the live zones (1∼3 zones). Using microscopic measurements of the mitochondrial length, we found that the fragmented mitochondria were mainly in the zones 1 and 2, the closest to the direct exposure point of Ar-PJ. Whereas, the mitochondria in the zone 4 retained their lengths to normal. This quantitative measurement of mitochondrial morphology was combined with the color scores of the polymerizable supramolecular (PS) sensor in diagnostic categories. The results demonstrate that the mitochondrial length (0.98∼3.94 μm) is inversely proportional to the PS sensor color scores (87∼0) in the zones 1∼4. On the combination of these three diagnostic parameters, the effective range of Ar-PJ for cellular responses was determined: the zones 1∼3, the color scores 87∼12 and the mitochondrial lengths 0.98∼2.57 μm. Our study is the first demonstration of mitochondrial fragmentation in response to Ar-PJ and the first attempt to establish the diagnostic guideline for Ar-PJ therapies by combinations with biological, physical and chemical aspects. Thus, this study will make great advances in the field of bioplasma applications.

The electron temperature Te is a key plasma parameter in both industrial plasma processes and fundamental laboratory research. A noninvasive method to measure Te in current and next-generation semiconductor plasma processes is urgently required for fine-tuning the processing result and virtual metrology based on the plasma monitoring. In this Letter, we propose a noninvasive method for obtaining Te in radio frequency capacitively coupled plasma (CCP) by measuring the floating potential and substrate potential. The Te obtained using this simple method was compared with the results measured by a floating Langmuir probe in both pure Ar and Ar/He mixture CCPs, and they showed good agreement.

We developed a high-efficiency source in a capacitively coupled plasma using magnetic resonance wireless power transfer (MRWPT) systems, which has the advantage that the matcher efficiency is very high at low gas pressures (2 mTorr to 20 mTorr) and high density plasmas (about 1 × 1010 cm−3 to 5 × 1010 cm−3). At the non-resonance conditions, most of the RF power is dissipated by the transmitter coil and the plasma is not discharged. However, at the resonance condition, the plasma is discharged as the current flowing through the transmitter coil rapidly decreases, and the matcher efficiency is higher than 90% in all experiments. For analysis, the transformer model with MRWPT systems is developed. The experimental result is consistent with the model, and the results are discussed with the relevant physical mechanisms.

Total energy loss per ion–electron pair lost ( e T) is investigated to optimize the plasma generation at various RF powers and gas pressures in an argon inductively coupled plasma (ICP). The ion densities and electron temperatures are measured to obtain e T at the plasma–sheath edge. At a fixed RF power, the obtained e T has a minimum at a certain electron temperature, and at this condition, an optimal plasma generation is achieved according to a global model. Since the electron temperature is a function of the gas pressure, at that certain gas pressure the energy loss in the plasma is minimized and plasma is generated most efficiently. Interestingly, the electron temperature at which e T becomes the minimum decreases as the RF power increases. This is explained by multistep ionization and the electron density dependence of the density of the excited states. Measured e T is compared with the calculated result from the global model that includes multistep ionization, and these are consistent with each other.

The effects of external discharge parameters (applied RF power, operating pressure, and gas flow rate) on ion flux uniformity are investigated in a dual inductively coupled plasma. The ion fluxes and electron temperatures are measured using a Langmuir probe floated by a DC blocking capacitor based on the floating harmonic method. The RF power applied to the top antenna focuses on the total ion flux control throughout the chamber, while the power introduced to the bottom antenna can control the ion flux uniformity in the diffusion chamber. At high pressures, the local electron heating in the vicinity of the bottom antennas results in local maximum ionization, thereby increasing the ion flux near the chamber wall. Furthermore, the ion flux uniformity also can be affected by the gas flow rate and pressure because they are associated with the gas residence time and the electron heating region. A physical understanding of the effects of external discharge parameters on plasma uniformity is useful for optimizing plasma processes.

Control of local electron and ion density using passive resonant coils is experimentally investigated in an inductive argon discharge. Four passive resonant coils are installed under a powered coil; each coil has a fan shape and good azimuthal symmetry. Electron energy probability functions and two-dimensional ion density profiles were measured under both resonant and non-resonant regimes. At non-resonance, almost all of the current flows through the powered coil located in the center of the reactor, and the profiles of the electron and ion density are convex. However, at resonance, a large current flows through the passive resonant coil, and dramatic changes are observed in the electron and ion density profiles. At resonance, the electron and ion densities near the passive resonant coil are increased by 300% compared to the non-resonant condition, and radial distributions become almost flat. Experimental results show that the electron and ion density profiles can be effectively controlled by a passive resonant coil at both low pressure (5 mTorr) and high pressure (50 mTorr). These changes in electron and ion density profiles can be understood by the changes of the electron heating and ionization regions.

To observe the evolution of electron temperature and electron density when varying gas mixing ratio and pressure, the measurement of electron energy probability functions (EEPFs) and a kinetic simulation are performed in an O2/Ar inductively coupled plasma. When O2 gas is diluted to pure Ar plasma, the electron density significantly drops regardless of gas pressures and the reduction of electron density increases with gas pressure. This sudden drop of plasma density is due to various inelastic collisions between electrons and O2 molecules. However, the trend of electron temperature variation with respect to the ratio of O2 and Ar is opposite at low and high pressures. At low pressure (5 mTorr), the electron temperature decreases with the addition of O2 up to a 10% O2 ratio. With further addition of O2, the electron temperature increases gradually. On the other hand, at high pressures (25 and 80 mTorr), the electron temperature rises with addition of O2 up to a 10%–20% O2 ratio and then falls gradually with further addition of O2. Simulations show a similar variation of EEPFs to the EEPFs from experiments. In addition, we measured the EEPFs along axial positions and the variation of electron temperature with respect to the ratio of O2 and Ar is opposite at low and high pressures, regardless of spatial positions. This result indicates that the opposite trend of electron temperature variation is mainly affected by the gas mixing ratio and gas pressure.

Electron energy distribution functions (EEDFs) are measured with varying radio-frequency (RF) power in an oxygen inductive discharge. At a pressure of 10 mTorr, the EEDF has a Maxwellian distribution, and the low-energy (1–5 eV) electron temperature, T e , low, monotonically decreases with RF power. However, at a pressure of 100 mTorr, T e , low increases at low RF power (80–150 W) and decreases at high RF power (150–300 W), and the EEDF has a Druyvesteyn-like distribution. These changes in T e , low are attributed to electron attachment and detachment reactions, which are the main creation and loss reactions of negative ions. To investigate this relationship between T e , low and collisional reactions, the reaction coefficients of several reactions are calculated, and the electronegativity ( α) is measured with varying RF power. The EEDF is modified by electron attachment and detachment reactions, which lead to a change in T e , low; this modification of EEDF is supported by calculated reaction coefficients and measured α.

We developed a hybrid plasma source combined with an inductively coupled plasma (ICP) antenna and a capacitively coupled plasma (CCP) electrode. The ICP antenna and the CCP electrode are connected to a single RF power generator in parallel and a variable capacitor Cv is connected to the ICP antenna in series. The currents flowing through each source and the CCP electrode voltage are measured for analysis of the electrical characteristics, and the ion densities are measured while adjusting the capacitance of the Cv. Interestingly, when a series LC resonance occurs between the inductance of the ICP antenna and the capacitance of the Cv, different trends are observed depending on the discharge mode. In capacitive mode (E-mode), the ion density is minimized and is controlled by the CCP current. On the other hand, in inductive mode (H-mode), the ion density is maximized and is affected by the ICP current. The change of the ion density can be explained by the balance between the total power absorption and power dissipation. It is also in good qualitative agreement with the calculated plasma density from the power balance equation. By adjusting the Cv, linear control of the ion density can be achieved. To evaluate the proposed source in terms of O atom generation, the number density ratio of O atom nO to Ar nAr is obtained by using the optical emission spectroscopy actinometry method. These results show that nO/nAr is controlled. Our source can be applied to plasma processing, in which ion density and O atom generation controls are important factors.

Changing the RF bias is widely used to control the ion energy in inductively coupled plasma (ICP). Here, the plasma densities were measured using the floating harmonic method at various ICP powers and RF bias power frequencies. It is observed that there is an RF bias power (PB,min) that minimizes the plasma density. With increasing ICP power, PB,min is increased. When the frequency is changed from 12.5 MHz to 2 MHz, PB,min is decreased. To understand this phenomenon, the relative variation of the plasma density (δn) with the RF bias power is considered based on a power balance equation. PB,min is determined by δn, and δn changes based on the self-bias voltage caused by the RF bias power. Because the self-bias voltages change depending on the ICP power and frequency of the RF bias power, PB,min is shifted by altering the ICP power and the RF bias power frequency. The results are in good agreement with the experimental results.

This work investigates the negative ion density via the floating harmonic method (FHM) in oxygen inductively coupled plasma (ICP). When a small sinusoidal voltage is applied to the specific potentials of a planar Langmuir probe, harmonic currents will be generated by the sheath nonlinearity. From the harmonic current analysis, it is possible to obtain the positive ion currents, electron currents, and electron temperatures. One probe potential is the floating potential, and the other is in the range of floating potential to the plasma potential. From the current ratios of positive ions to electrons, electronegativity and modified pre-sheath potential can then be deduced and, thereby obtaining the negative ion density from quasi-neutrality. The variations in negative ion density and electronegativity distributions at various gas pressures and applied powers are compared with those from a 2D fluid model with electron heating kinetics and a two-probe method in Ref. [Chabert et al. 1999 Plasma Source Sci. Technol. 8 561].

We investigated the profiles of electron and ion densities dependence on the gas pressure in the inductively coupled plasma with a passive resonant antenna. The inner powered antenna and the passive resonant antenna are independently installed on the top of the chamber, and a power of 13.56 MHz is applied to the powered antenna. A significant change in the plasma density profile is observed when the capacitance of the passive resonant antenna is adjusted from non-resonance to resonance. At a high pressure of 50 mTorr, the point of maximum electron density and temperature shift from near the powered antenna to near the passive resonant antenna. However, at a low pressure of 1 mTorr, the profile of the electron density and temperature hardly change, and the maximum electron density at resonance increases about 1.4 times (at 150 W) to 4 times (at 50 W) higher than at non-resonance. The controllability of the electron density profile, which depends on the gas pressure, can be understood by the electron kinetics effect, and the increase in the electron density can be explained by the increase in power transfer efficiency calculated from the circuit model. The electron density profile at bulk is compared to the ion density distribution at the wafer-level, and the results are discussed with the relevant physical mechanisms.

A hysteresis loop has been observed during the E–H mode transition in an inductively coupled plasma at high pressures. The cause of the hysteresis has been reported as a nonlinearity of the transferred and dissipated powers due to capacitive coupling, multi-step ionization, the change in the electron energy probability function (EEPF), and so on [M. M. Turner and M. A. Lieberman, Plasma Sources Sci. Technol. 8(2), 313–324 (1999) and H. C. Lee and C. W. Chung, Sci. Rep. 5, 15254 (2015)]. However, when a coil current reduction is considered, the previous interpretation of hysteresis cannot explain the observations in the intermediate pressure region, where the coil current reduction occurs, but hysteresis is not observed. In this work, the E–H mode transition and its hysteresis are discussed in three pressure regions, the low, intermediate, and high pressure regions, whether or not the coil current and the hysteresis are observed. The power transfer efficiency, transferred power, EEPF, and total energy loss are obtained at 10, 100, and 300 mTorr. Hysteresis is only observed at 300 mTorr, and the coil current reduces at 100 and 300 mTorr during the E–H mode transition. The mechanism of hysteresis is explained in a power balance diagram that includes the transferred power and the dissipated power by considering the power transfer efficiency in the E and H modes. In addition, the conditions of hysteresis and coil current reduction are revealed and classified in this work.

In plasma processing and application, the electron energy distribution function (EEDF) is of fundamental interest because the ion and radical densities related to physical and chemical reactions on the substrate are predominantly governed by the EEDF or electron temperature. In this paper, the effect of low frequency power on the EEDF is investigated when 2 MHz power is added to the plasma originally driven at 13.56 MHz. In a 13.56 MHz operation, the EEDF shows a Maxwellian-like distribution, and as the RF power increases, the electron density increases and the electron temperature decreases. However, when a small amount of 2 MHz power is applied to the 13.56 MHz discharge, the electron density slightly increases and the electron temperature significantly increases. In dual-frequency operation, EEDFs have a low slope of low-energy region and evolve into a Druyvesteyn-like distribution. It turns out that the dual-frequency operation can significantly change the electron temperature. This is consistent with the results calculated using the analytical electron heating model, and the relevant heating mechanism is also presented.

When dual frequency ω α , ω β voltages with small amplitudes are biased toward a floating Langmuir probe, intermodulation currents of i ω β ± ω α , i ω β ± 2 ω α are generated due to the nonlinearity of the sheath near the probe tip. Electron temperatures and plasma densities can be obtained from the intermodulation currents using the sideband method. If the probe has a capacitive load such as a contaminated probe tip, harmonic and intermodulation voltages of the probe bias can be applied to the sheath. The sideband method can be modified using a harmonic balance technique that analyzes the nonlinear circuit. The intermodulation currents from the modified sideband were compared with the measured intermodulation currents. The electron temperatures and plasma densities obtained from the intermodulation currents are in close agreement with those of the electron energy probability functions in argon inductively coupled plasmas.

The E to H transition is investigated in 2 MHz, 13.56 MHz, and dual frequency (2 MHz + 13.56 MHz) inductively coupled plasmas (ICPs). At argon gas pressures of 1 mTorr and 10 mTorr, the E to H transition power of 2 MHz ICP is the lowest and the transition power of 13.56 MHz ICP is the highest. However, at pressures of 25 mTorr and 50 mTorr, the transition power of 13.56 MHz ICP decreases and the transition power of 2 MHz ICP significantly increases. Interestingly, despite these changes, the E to H transition in the dual frequency operation occurs at the power between the 2 MHz and the 13.56 MHz transition power. The measurement shows that the plasma density in dual frequency operation is from about 1.1 to 11.7 times higher than that in single frequency operation, but this is changed with the pressure. For quantitative analysis, a global model including the capacitive coupling and the inductive coupling in the solenoidal ICP was developed and the E to H transition powers were calculated. It turns out that the results at various pressures and frequencies agree well with the experimental measurements and the relevant physical mechanism is also presented.

As antenna length increases for the large-area plasma source, problems occur, which cause non-uniform distribution of the plasma density due to the increase of the antenna inductance. Uniform plasmas are essential for plasma processing, because the distribution of the plasma density is directly related to the processing uniformity. In order to reduce the inductance of an antenna and generate uniform plasma, a parallel antenna is used. However, even if the parallel is used, high uniformity of the plasma density cannot be achieved. This is because larger currents flow to the inner coil than to the outer coil. In this study, a LC parallel circuit is added to the inner coil to control the current flowing through the inner coil. By adjusting the capacitance of the LC parallel circuit, the radial distribution of the plasma density can be controlled.

Plasma parameters are obtained by applying square voltages at a floating potential. To measure plasma density and electron temperature, circuit model with nonlinear sheath and a capacitor connected to a probe is suggested. When discharge power changes, magnitude of the sheath resistance varies due to the change of plasma density. This variation of sheath resistance changes the time constant. To in vestigate the relation of time constant and plasma parameters, probe currents are measured. Due to the nonlinearity of the sheath, measured probe currents have harmonic components. The harmonic components of the probe current is obtained using the fast Fourier transform and then plasma parameters are obtained by the relation of these harmonic components and time constant. The results are compared to those of a single Langmuir probe.

Argon plasma jet (Ar-PJ) has been widely used in clinical medicine; however, the cellular effects of Ar-PJ therapy applying to living tissues have not been clarified yet. It is necessary to investigate cellular responses to Ar-PJ in establishing guidelines on the therapeutic use of Ar-PJ. Interestingly, in the Ar-PJ-treated cells, the fragmented mitochondria, a typical cellular stress indicator, were discovered even in the cells located in the live zones (1∼3 zones). Using microscopic measurements of the mitochondrial length, we found that the fragmented mitochondria were mainly in the zones 1 and 2, the closest to the direct exposure point of Ar-PJ. Whereas, the mitochondria in the zone 4 retained their lengths to normal. This quantitative measurement of mitochondrial morphology was combined with the color scores of the polymerizable supramolecular (PS) sensor in diagnostic categories. The results demonstrate that the mitochondrial length (0.98∼3.94 μm) is inversely proportional to the PS sensor color scores (87∼0) in the zones 1∼4. On the combination of these three diagnostic parameters, the effective range of Ar-PJ for cellular responses was determined: the zones 1∼3, the color scores 87∼12 and the mitochondrial lengths 0.98∼2.57 μm. Our study is the first demonstration of mitochondrial fragmentation in response to Ar-PJ and the first attempt to establish the diagnostic guideline for Ar-PJ therapies by combinations with biological, physical and chemical aspects. Thus, this study will make great advances in the field of bioplasma applications.

Spatial distributions of the effective electron temperature (Teff) and plasma potential were studied from the measurement of an electron energy probability function in a side type ferrite-core inductively coupled plasma with an argon–helium mixture. As the helium gas was diluted at the fixed total gas pressure of 5 mTorr in an argon discharge, the distribution of the plasma density and plasma potential changed from a concave to a flat profile, and finally became a convex profile, while all spatial profiles of Teff were hollow shapes with helium dilution in the argon discharge. This evolution of the plasma potential with helium gas could be explained by the increased energy relaxation length (λe), indicating the transition of electron kinetics from local to non-local kinetics.

The effects of plasma parameters such as plasma density, electron temperature, and sheath voltage on the uniformity of Cu nanoparticle arrays were investigated. These parameters were controlled by varying the pressure, RF power, and substrate bias voltage. A floating harmonic method was used to monitor the plasma parameters. Uniform nanoparticle arrays were produced when hole generation was increased by using a high ion.bombardment energy. As oppose to a low energy flux condition, where small and large nanoparticles coexisted due to a small number of holes, a larger number of holes was generated and distributed more uniformly during a high energy flux condition.

It is generally recognized that the electron temperature T_e either remains constant or decreases slightly with plasma power (plasma density). This trend can be simply verified using a single-step or multi-step fluid global model. In this work, however, we experimentally observed that T_e evolved with plasma power in radio frequency (RF) inductively coupled plasmas. In this experiment, the measured electron energy distributions were nearly Maxwellian distribution. In the low RF power regime, T_e decreased with increasing plasma power, while it increased with plasma power in the high RF power regime. This evolution of T_e could be understood by considering the coupling effect between neutral gas heating and stepwise ionization. Measurement of gas temperature via laser Rayleigh scattering and calculation of T_e using the kinetic model, considering both multi-step ionization and gas heating, were in good agreement with the measured value of T_e. This result shows that T_e is in a stronger dependence on the plasma power.