4,797 research outputs found
UV photon assist ionization for low temperature plasma
No full textA new type plasma Source has been developed for the generation of low temperature plasma. The plasma generation process consists of two steps, the generation of metastable neutral gas by injecting a low energy electron beam (the thermionic Source) and the ionization of the metastable neutral gas by application of a UV light source. The key characteristic of this plasma source is the capability of producing extremely low temperature plasma. In the experiment, the filament heating current is 6.5 A and the electron acceleration voltage varies from 16 V to 25 V. Plasma parameters are measured by a single Langmuir probe. The plasma density increases 100% from 4.5 x 10(9) cm(-3) to 9.8 x 10(9) cm(-3) in Ar 30 mTorr when the neutrals excited by the e-beam are exposed to the UV light. However, the electron temperature is still low, i.e., similar to 0.5 eV. A similar result is observed in the case of Xe. (c) 2005 Elsevier B.V. All rights reserved
Experimental investigation of plasma dynamics in dc and short-pulse magnetron discharges
No full textThe spatiotemporal evolution of the electron energy distribution function (EEDF) and of plasma parameters such as the electron density, the electron temperature and the plasma and floating potentials has been investigated using spatially and temporally resolved single Langmuir probe measurements in dc and mid-frequency, short-pulse magnetron discharges with a repetition frequency of 10 kHz and a duty cycle of 10%. In the pulsed discharge of the short duty cycle, a peak electron temperature higher than 10 eV was observed near the cathode fall region during the early phase of the pulse-on, which is about three times higher than the steady-state value of the electron temperature in the dc discharge. The temporal evolution of the measured EEDFs showed the initial efficient electron heating during the early phase of the pulse-on and the subsequent relaxation of electron energy by the inelastic collisions and the diffusive loss. The high-energy electrons generated during the pulse-on phase diffused the downstream region toward the grounded substrate, resulting in a bi-Maxwellian EEDF consisting of the background low-energy electrons and the high-energy electrons. The results of the spatially and temporally resolved probe measurements will be presented and the enhanced efficiency of the electron heating in the short-pulse discharge will be explained on the basis of the global model of a pulsed discharge
Temporal evolution of electron energy distribution function and plasma parameters in the afterglow of drifting magnetron plasma
No full textThe temporal behaviour of the electron energy distribution function (EEDF) and the plasma parameters such as electron density, electron temperature and plasma and floating potentials in a mid-frequency pulsed do magnetron plasma are investigated using time-resolved probe measurements. A negative-voltage do pulse with an average power of 160 W during the pulse-on period, a repetition frequency of 20 kHz and a duty cycle of 50% is applied to the cathode of a planar unbalanced magnetron discharge with a grounded substrate. The measured electron energy distribution is found to exhibit a bi-Maxwellian distribution, which can be resolved with the low-energy electron group and the high-energy tail part during the pulse-on period, and a Maxwellian distribution only with low-energy electrons as a consequence of initially rapid decay of the high-energy tail part during the pulse-off period. This characteristic evolution of the EEDF is reflected in the decay characteristics of the electron density and temperature in the afterglow. These parameters exhibit twofold decay represented by two characteristic decay times of an initial fast decay time tau(1), and a subsequent slower decay time tau(2) in the afterglow when approximated with a bi-exponential function. While the initial fast decay times are of the order of 1 mu s (tau(T1) - 0.99 mu s and tau(N1) similar to 1.5 mu s), the slower decay times are of the order of a few tens of microseconds (tau(T2) similar to 7 mu s and tau(N2) similar to 40 mu s). The temporal evolution of the plasma parameters are qualitatively explained by considering the formation mechanism of the bi-Maxwellian electron distribution function and the electron transports of these electron groups in bulk plasma
Measurements of electron energy distribution functions and electron transport in the downstream region of an unbalanced dc magnetron discharge
No full textIn this study, electron energy distribution functions (EEDFs) are measured using a Langmuir probe in conjunction with the ac superposition method in the downstream region of a planar and unbalanced magnetron argon discharge and the effects of an anode sheath boundary on the discharge characteristics are investigated. The potential of the anode sheath can be controlled by applying a dc voltage to the substrate, and the nonlinear behaviour of the plasma potential with respect to the dc substrate voltage causes the distinctive evolution of the potential of the anode sheath. It is found that when the potential of the anode sheath reaches a specific value, which is related to the threshold energies of argon for the inelastic collisions, an outstanding EEDF transition from a bi-Maxwellian distribution to a single Maxwellian distribution occurs. We introduce the concept of the total electron bounce frequency as an indicator of how the electron collisions such as the electron-electron collision and the inelastic collisions affect the EEDF features as the potential of the anode sheath changes. This result provides the decisive clue to explaining the appearance of the bi-Maxwellian distribution in magnetron discharges. The results of the spatially resolved measurements of EEDF and plasma characteristics are also presented. From these results, we will discuss the electron transport in the downstream region in detail
Electron transport in the downstream region of planar unbalanced magnetron discharge
No full textIn this study, we will investigate the electron transport in the downstream region of a planar and unbalanced (type II) magnetron discharge. The effects of the anode sheath boundary and diverging magnetic field on the electron kinetics such as the electron loss mechanism at plasma-sheath boundary and the electron distribution function will be examined through the probe measurements. The spatially resolved probe measurements reveal the existence of an electron drift from the cathode fall region to the downstream region. It is found that this drift is caused by the axial gradient of magnetic field (the magnetic mirror force) and then derives an electron current to the grounded substrate on which the potential of the sheath is very low; so the current balance between the cathode and anode currents is kept. The experimental results show that the electron transport in the downstream region is not governed by the classical diffusion (mobility and diffusion dominated) but is dominated by the modified diffusion including the electron drift caused by the magnetic mirror force. Additionally, the mechanism and the experimental evidence on the presence of a non-Maxwellian electron energy distribution function (in particular, bi-Maxwellian distribution) in magnetron discharge will be presented showing that the non-Maxwellian electron energy distribution function is due to the combined effects of the electron drift toward the substrate and the sheath boundary condition. (C) 2004 American Institute of Physics
Silencer design by using array resonators for low-frequency band noise reduction
No full textHelmholtz resonators are often used to reduce noise. They are particularly useful when noise has a narrow frequency band. In this study we aim to broaden its narrow band characteristics by combining many resonators. Serial and parallel arrangements of resonators have been tested to obtain broader impedance mismatch characteristics in the broader band. Theoretical and experimental results explain these characteristics in the absence of mean flow. The serial arrangement mainly increases the peak of TL at the resonance frequency. But the parallel arrangement logarithmically increases the peak of TL and expands the bandwidth. The change of acoustic characteristics is explained by introducing an "equivalent impedance analysis." This shows that the transmission loss has a maximum value when the distance between resonators is lambda/4 of its wavelength. In this study we propose a novel design method that optimizes the arrangement of resonators while keeping the volume minimized. Various transmission loss characteristics are possible when we select different objective functions under constraints. (c) 2005 Acoustical Sociey of America
Anomalous behaviors of plasma parameters in unbalanced direct-current magnetron discharge
No full textThe pressure dependences of electron distribution functions and plasma parameters are investigated in an unbalanced direct-current magnetron sputtering system. The anomalous behaviors of electron density and electron temperature, and the transition of the electron energy distribution function, which are obtained from the cylindrical probe measurement, from a bi-Maxwellian distribution at low pressures to a Druyvesteyn distribution at relative high pressures with changing pressure can be observed. The planar probe measurement shows that the low-energy electron group in the electron energy distribution function consists of electrons, which are scattered back from the sheath wall formed on the substrate and the population decreases with the decreasing plasma potential as the pressure increases. It then disappears at high pressures above 20 mTorr when the plasma potential drops to almost ground level, resulting in a Druyvesteyn electron energy distribution. These observed results are explained by considering the mechanism of the electron transport in the downstream region and the effect of the sheath boundary, which is determined by the plasma potential with respect to the grounded substrate, on the electron energy distribution, especially the depletion of the low-energy part in the electron energy distribution function. (C) 2004 American Institute of Physics
Effects of a sheath boundary on electron energy distribution in Ar/He dc magnetron discharges
No full textIn this study, the effects of a sheath boundary on electron energy distribution and discharge characteristics in an unbalanced and planar-type dc magnetron sputtering system are investigated. The anode sheath potential is changed by applying dc bias voltages to the substrate. The electron energy distribution functions (EEDFs) are measured in argon and helium discharges using a single Langmuir probe in conjunction with the ac superposition method. The evolutions of the EEDFs are first observed in argon at 3 mTorr and then in helium at 30 mTorr. The results show that, as the substrate bias voltage decreases to high negative voltage, the EEDF transition from the bi-Maxwellian to the Maxwellian in the downstream region occurs at a specific bias voltage that depends on the operating gas. The major factors that affect the EEDF formation are investigated. In particular, the concept of total electron bounce frequency is introduced to represent the change of the sheath boundary condition. The observed EEDF transition is explained by comparing it with the plasma characteristic frequencies calculated from the measured EEDFs. As a result, the bi-Maxwellian distribution observed at the small substrate bias voltage is attributed to the low electron-electron collision frequency and the different loss mechanisms of two electron groups: the ambipolar diffusion loss of low-energy electron group confined by low plasma potential and the direct thermal loss of high-energy electron group, providing the electron current that compensates for the discharge current in a steady state. (C) 2004 American Institute of Physics
Electron drift and the loss balance of charged particles in planar-unbalanced dc magnetron discharge
No full textThe electron drift phenomenon is investigated in the downstream region of an unbalanced dc magnetron argon discharge. The spatially resolved measurements of the electron velocity distribution function (EVDF) using a planar probe reveal the existence of a strong on-axis electron drift parallel to magnetic field in spite of a very small axial variation less than 1 V in the plasma potential. The average drift velocities calculated from the asymmetry of the measured EVDFs show that there exists a significant electron drift from cathode to substrate with a maximum speed of about 1x10(6) m/s, which is comparable to the bulk electron temperature. The magnetic mirror force which is driven by the axial gradient of the magnetic field (i.e., the parallel delB force) is suggested as a possible source for the parallel electron drift. Carrying out a scaling of current densities with the measured data, it is found that the parallel delB force can produce the electron current enough to balance the discharge current, implying that the electron transport in the downstream region is determined not by the classical diffusion model in which electron motion toward the anode is diffusion and mobility dominated but by the modified diffusion model in which electron motion is drift dominated. (C) 2005 American Institute of Physics
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