Young-Kwang Lee

Annealing behaviour and crystal structure of RF-sputtered Bi-substituted dysprosium iron-garnet films having excess co-sputtered Bi-oxide content

We investigate the magneto-optic properties, crystal structure and annealing behaviour of nano-composite media with record-high magneto-optic quality exceeding the levels reported so far in sputtered iron-garnet films. Bi-substituted dysprosium–gallium iron-garnet films having excess bismuth oxide content are deposited using RF co-sputtering, and a range of garnet materials are crystallized using conventional oven-annealing processes. We report, for the first time ever, the results of optimization of thermal processing regimes for various high-performance magneto-optic iron-garnet compositions synthesized and describe the evolution of the optical and magneto-optical properties of garnet-Bi-oxide composite-material films occurring during the annealing processes. The crystallization temperature boundaries of the system (BiDy)3(FeGa)5O12 : Bi2O3 are presented. We also report the results of x-ray diffraction and energy-dispersive x-ray spectroscopy studies of this recently developed class of high-performance magneto-optic composites. Our hypothesis of iron oxides being the cause of excess optical absorption in sputtered Bi-iron-garnet films is confirmed experimentally.

Characterization of a side-type ferrite inductively coupled plasma source for large-scale processing

A new type of inductively coupled plasma (ICP) utilizing an array of auxiliary discharges enhanced with ferromagnetic cores and placed at the chamber side is developed and characterized over a wide range of discharge conditions. The ICP electrical and plasma characteristics are measured over a wide range of discharge powers and argon gas pressures. It is shown that at 400 kHz driving frequency the antenna power factor of this ICP is close to 1, so the antenna voltage and current are much lower than those in a conventional ICP at similar rf power. Due to low driving frequency and low antenna voltage, the capacitive coupling in the ICP mode is practically eliminated, while due to enhanced ferromagnetic core coupling, the power transfer efficiency is higher than 95% at an rf power larger than 0.5 kW. Langmuir probe measurements show that the radial plasma non-uniformity over 300 mm can be less than 3%. This plasma source is expected to be suitable for large-scale plasma processing.

Ionization in inductively coupled argon plasmas studied by optical emission spectroscopy

Contribution of stepwise ionization to total ionization was experimentally investigated in low-pressure inductively coupled argon plasmas. In the pressure range 3–50 mTorr, optical emission spectroscopy was employed to determine metastable fractions (metastable density relative to ground state density) by measuring the emission intensity of selected lines. The measured metastable fractions were in good agreement with the calculation, showing a dependence on the discharge pressure. The rate of stepwise ionization was estimated from the excited level densities (measurements and model predictions) and their ionization rate coefficients. It is observed that at relatively low discharge pressures (<10 mTorr) the ionization is mainly provided by the direct ionization, whereas at higher pressure the stepwise ionization is predominant with increasing absorbed power.

Effect of the dielectric layer thickness on the electromagnetic response of cut-wire-pair and combined structures

This report investigates the effect of the dielectric layer thickness on both magnetic and electric resonances of cut-wire-pair (CWP) structures in the microwave frequency regime. It was found that the resonances are sensitive to the thickness of the dielectric layer. As the thickness increases, the bandwidth of the magnetic resonance is slightly extended to a higher frequency, while the low-frequency edge of the electric-resonance band is remarkably shifted to a lower frequency. It was also found that the dependence of the magnetic resonance frequency on the dielectric layer thickness follows the trend of the closed formula based on the cavity model for the coupled metallic elements (Cai et al 2007 Opt. Express 15 3333). In addition, we also studied the effect of the dielectric layer thickness on the left-handed behaviour of a combined structure consisting of CWP and continuous wire. The actual measurements are compared with the numerical simulation values to show a good coincidence.

Measurements of the total energy lost per electron–ion pair lost in low-pressure inductive argon, helium, oxygen and nitrogen discharge

Experimental measurements of the total energy lost per electron–ion pair lost, eT, were performed in a low-pressure inductive atomic gases (Ar, He) and molecular gases (O2, N2) discharge. The value of eT was determined from a power balance based on the electropositive global (volume-averaged) model. A floating harmonic method was employed to measure ion fluxes and electron temperatures at the discharge wall. In the pressure range 5–50 mTorr, it was found that the measured eT ranged from about 70 to 150 V for atomic gases, but from about 180 to 1300 V for molecular gases. This difference between atomic and molecular discharge is caused by additional collisional energy losses of molecular gases. For argon discharge, the stepwise ionization effect on eT was observed at relatively high pressures. For different gases, the measured eT was evaluated with respect to the electron temperature, and then compared with the calculation results, which were derived from collisional and kinetic energy loss. The measured eT and their calculations showed reasonable agreement.

Determination of metastable level densities in a low-pressure inductively coupled argon plasma by the line-ratio method of optical emission spectroscopy

The line-ratio method of optical emission spectroscopy (OES) is used for the diagnosis of plasma parameters. In this work, electrostatic probe-assisted OES is employed to measure metastable level densities from spectral lines and electron energy distribution functions (EEDFs) in a low-pressure inductively coupled argon plasma. Emission spectroscopy is based on plasma modelling through a simple collisional–radiative model. The line intensities of Ar(3p54p → 3p54s) are modified due to the plasma reabsorption at relatively high pressures where the plasma becomes optically thick. To consider this effect, a pressure dependence factor αij(P) is first derived from both the measured intensity and pressure-dependent cross-section for electron excitation. It is found that the obtained metastable densities range from 1.3 × 109 to 1.2 × 1010 cm−3 and their ratios are nearly constant by a factor of about 3–5 in the investigated pressure range (3–50 mTorr). The effect of non-Maxwellian EEDF on the metastable densities is also discussed. The results measured by the line-ratio method are consistent with that of the OES-branching fraction method taking into account the photon escape factor to treat the radiation trapping.

Experimental measurement of the total energy losses in a low pressure inductively coupled argon plasma

Total energy losses per electron-ion pair lost (eT) were measured experimentally in a low pressure inductively coupled argon plasma. A floating probe working at very low bias voltage (∼1 V) was used to obtain the electron temperatures and plasma densities at the plasma-sheath boundary. eT was found from a power balance equation between the absorbed power and dissipated power by electrons and ions. At 10 mTorr, the measurement shows that the measured eT (∼100 V) gradually decreased with absorbed power, and this indicates that the ionization efficiency enhances by multistep ionizations. These eT are consistent with the theoretical results.

Measurement of the total energy losses per electron-ion lost in various mixed gas inductively coupled plasmas

The total energy loss per electron-ion lost was measured at various gases and mixtures (Ar, He, N 2 , O 2 , Ar / N 2 , Ar / O 2 , He / N 2 , and He / O 2 ) in the pressure range of 5–50 mTorr in an inductively coupled plasma. To measure electron temperatures and ion fluxes at a chamber wall, the floating harmonics method was used. The absorbed power was determined by measuring antenna resistances and currents. The total energy losses were obtained from the power balance equation of a global model. In the case of Ar mixture plasma with molecular gas, the total energy loss decreased with fractional Ar flow rate. He mixture plasma decreased more than the decrease in total energy loss of Ar mixture plasma. These experimental results were compared and were consistent with average collisional energy loss.

Spatial measurements of electron energy distribution and plasma parameters in a weakly magnetized inductive discharge

Spatial characteristics of plasma parameters such as electron temperature, plasma density, plasma potential, and electron energy distribution (EED) were studied in inductively coupled plasma with an axial dc magnetic field. With dc magnetic field, the measured EEDs in the total electron energy scale are spatially coincided except cutting of the low electron energy part indicating the conserved non-local electron kinetics in an axial direction, even though the dc magnetic field is applied. Spatial distributions of the plasma densities at axial positions have almost same trends with various magnetic field strengths. We also discuss the reduction of the ambipolar potential along the axial direction as the applied magnetic field increased.

Measurement of the sheath capacitance of a planar probe

The sheath capacitance was measured on a planar probe dc-biased with respect to the plasma potential using the phase sensitive detection method in the region separated from the rf discharge plasmas by an immersed grid. It was observed that the sheath capacitance was negative when the collecting electrode of the probe was positioned downward toward the grid and biased near the plasma potential. This indicates that a double sheath had built up near the probe electrode. This tendency can be explained by the sheath capacitance, which is calculated using Poisson’s equation with a non-zero electrical field and an ion velocity condition at the sheath edge.