J K Lee

Ion energy distribution control in single and dual frequency capacitive plasma sources

Particle-in-cell/Monte Carlo simulations are used to study the possibility of ion energy distribution function (IEDF) control on the powered electrode in asymmetric single and double frequency capacitive discharges in argon. The possibility of IEDF control is demonstrated. It is shown that the IEDF shape and spread on the cathode can be controlled by the driven voltage in a single frequency discharge and by the low frequency voltage in a dual frequency capacitive discharge. It is shown that the IEDF shape on the powered electrode can be controlled by the driven frequency in single frequency capacitively coupled plasmas. It is shown that the density of plasma decreases, and the sheath width, the plasma potential and the self-bias voltage increase, with growth of the low frequency voltage in the dual frequency capacitive discharge.

Three-dimensional fluid simulation of a plasma display panel cell

In order to understand the discharge characteristics in an alternating current plasma display panel (ac PDP) and optimize it further, a three-dimensional fluid code (FL3P) has been developed. Using this simulator, various three-dimensional features of discharges are investigated in the sustain mode of PDP. First, the striations of wall charge are observed at both the anode and cathode side. Second, the local efficiency is obtained as a function of position. It is mainly divided into the anode region and the cathode region and highest near the anode center. Finally, the effects of various three-dimensional parameters are studied. As one of the examples showing the effect of electrode shaping, the discharge characteristics of a T-shaped electrode cell are compared with those of a conventional cell. The phosphor on barrier ribs contributes to over 44% of the total luminance, but barrier ribs themselves do not play an important role in the overall discharge efficiency. Address electrode width is not always pr...

Slit shaped microwave induced atmospheric pressure plasma based on a parallel plate transmission line resonator

A new type of microwave-excited atmospheric pressure plasma source, based on the principle of parallel plate transmission line resonator, is developed for the treatment of large areas in biomedical applications such as skin treatment and wound healing. A stable plasma of 20u2009mm width is sustained by a small microwave power source operated at a frequency of 700u2009MHz and a gas flow rate of 0.9u2009slm. Plasma impedance and plasma density of this plasma source are estimated by fitting the calculated reflection coefficient to the measured one. The estimated plasma impedance shows a decreasing trend while estimated plasma density shows an increasing trend with the increase in the input power. Plasma uniformity is confirmed by temperature and optical emission distribution measurements. Plasma temperature is sustained at less than 40u2009°C and abundant amounts of reactive species, which are important agents for bacteria inactivation, are detected over the entire plasma region. Large area treatment ability of this newly developed device is verified through bacteria inactivation experiment using E. coli. Sterilization experiment shows a large bacterial killing mark of 25u2009mm for a plasma treatment time of 10u2009s.

Non-stationary charging of a dust grain in decaying streamer-channel plasma

The process of a dust grain charging in non-stationary (decaying) air plasma in external electric fields typical of a streamer channel at atmospheric air pressure is investigated numerically using two-dimensional spherical coordinates. The characteristic timescale on which the grain acquires maximal electric charge is found to be determined by the rate of electron attachment. For zero external field, the process of charging in decaying air plasma is compared with the analogous process in nitrogen (non-attaching gas). For non-zero external electric fields in air plasma, the correlation between the charging time and the field dependence of the attachment rate is revealed. For high values of external electric fields, a streamer discharge starting from the dust grain is observed, analogous to Trichel pulses in a negative corona.

Capacitively coupled plasma source operating in Xe/Ar mixtures

The parameters of a capacitively coupled plasma source operating in Xe/Ar mixtures have been studied using particle-in-cell/Monte Carlo methods. The plasma density, ion energy distributions (IEDs) for different sorts of ions, ion current densities, energy delivered by ions to the electrodes and other parameters of a Xe/Ar plasma are systematically investigated as a function of the Xe/(Ar + Xe) mixing ratio. It is shown that the addition of xenon to an argon discharge increases the plasma density, changes the electron energy distribution and lowers the electron temperature. Both single and dual frequency plasma issues are addressed. The dependence of the IED on the observation position in the sheath is also investigated. Computational results are compared with available experimental data.

On the physical mechanisms of liquid-assisted laser cleaning

This communication addresses two questions regarding the physical mechanisms of the liquid-assisted laser cleaning: (1) What is the correlation between the cleaning force and bubble dynamics in the explosive vaporization of the liquid film? (2) How does the liquid-film thickness affect the cleaning process? Transient displacement of the cleaning target is measured by optical interferometry for in situ detection of the cleaning force. Time-resolved visualization and optical reflectance probing are conducted for monitoring the behavior of the liquid film in the cleaning process, with emphasis on the effect of liquid-film thickness. The results prove that cleaning force is generated when the bubble nuclei initially grow in the strongly superheated liquid. The liquid-film thickness is shown to be irrelevant to the origin of the force though it affects the dynamics of liquid plume and possibly changes the cleaning efficiency.

Oxygen ion energy distribution: Role of ionization, resonant, and nonresonant charge-exchange collisions

Capacitively coupled rf discharge operating in oxygen/argon mixtures has been investigated using the particle-in-cell/Monte Carlo method. The roles of ionization, resonant and nonresonant charge-exchange collisions in the formation of ion energy distribution (IED) at the electrodes is investigated. By turning off/on the corresponding collisional processes, it is shown that ionization can play an essential role in formation of the IED structure. The relative contribution of ionization to IED is estimated.

A particle-in-cell simulation of the breakdown and period doubling thresholds for a RF discharge Ar plasma

RF glow-discharge plasmas provide mild energetic ion bombardment of exposed surfaces. The characteristics of a 10 MHz RF glow-discharge Ar plasma are studied by particle-in-cell simulation. The model simulates a spherical plasma device using a one-dimensional plasma model. The code is used to determine the breakdown voltage, , for a given pd, where p is the gas pressure and d is the plasma discharge length. These values are then compared with Paschens law for the DC case. Also, the breakdown voltage as a function of driver frequency is investigated. Due to the nonlinear capacitance of the plasma, the current and voltage waveforms are found to contain higher harmonic components and sub-harmonic components. Above a certain power level of the driver, sub-harmonics which are indicative of a period-doubling (PD) bifurcation are found to become dominant. The PD thresholds for varying pd, the magnitude of the current and the voltage of the Fourier-analysed components as a function of applied RF voltages are calculated. The PD threshold is always higher than the breakdown threshold. Finally, the pressure-dependence of the sheath width is discussed.

Scaling laws verification for capacitive rf-discharge Ar plasma using particle-in-cell simulations

The characteristics of a 13.56 MHz capacitively coupled rf glow‐discharge Ar plasma are studied by particle‐in‐cell simulation. The model simulates a planar plasma device which can be approximated using a one‐dimensional plasma model. The model has proven to be useful to investigate the effect of varying control parameters such as neutral gas pressure, driver frequency, applied rf voltage on the characteristics of the discharge. A set of equations describing the dynamics of the system are presented and used to give analytic scaling laws. The simulation code is used to calculate the pressure dependence of plasma density, sheath width, peak position of the ionization event, and absorbed rf power. Scaling laws relating the control parameters to other operating functions such as average plasma potential, central electron density, absorbed rf power are examined and these functions are compared with simple analytical scaling formula.

The bounce resonance heating of low-energy electrons in capacitively coupled discharges

Low-energy electrons are confined in the bulk plasma by the ambipolar potential and are typically heated inefficiently due to their low collisionality and the weak rf electric field present in the bulk when their mean-free path is much larger than the system length in capacitively coupled discharges. It is shown in this study, however, that electrons in the bulk that bounce inside the electrostatic potential well with a frequency equal to the rf excitation frequency are efficiently heated by coherent interaction with the rf field. This resonant heating manifests itself as a plateau in the electron energy probability function and is observed in a wide range of pressures from 25 mTorr to 1 Torr while decreasing the gap distance. The weak transverse magnetic field significantly influences the bounce frequency of the low-energy electrons. As a result, the electrons not in the resonant condition in the absence of a magnetic field can be led to satisfy the resonant condition, becoming effectively heated in the presence of a weak transverse magnetic field.