Electron transport in CF 4 under linearly gradient antiparallel magnetic fields was analysed in order to investigate the fundamental properties of magnetic neutral loop discharge plasmas used for material processing. The electron motion was simulated by a Monte Carlo method under a radio-frequency (rf) electric field applied perpendicularly to both the directions of the magnetic field and its gradient. Two typical electron motions, meandering in a weak magnetic field and gyration in a strong magnetic field, were observed with particular directionalities. The meandering electrons drifted forward on average similarly to those under a dc electric field. The gyration induced an electron drift towards the inverse direction. The direction of electron flux was dependent not only on the rf phase but also on the distance from the magnetically neutral midplane between the antiparallel magnetic fields. The electron conduction path formed along the midplane had a structure consisting of forward and inverse lanes. A peculiar result was that the direction of local electron flux was always forward in the strong magnetic field whereas the drift of gyrating electrons was towards the inverse direction. This seemingly paradoxical result can appear in the presence of the density gradient of the electron distribution.

A Monte Carlo simulation of electron transport in electric and magnetic fields was performed to analyze experimental data of the electron temperature T e and electron number density n e measured in a magnetic neutral loop discharge (NLD) plasma driven in Ar at 0.13 Pa. T e and n e in the vicinity of the substrate holder were measured with a triple probe, and their radial profiles had peaks at different radial positions.The simulation reproduced these peak positions well under the chosen boundary condition that the electron reflectivity of the side wall was lower than that of the reactor ceiling and the substrate holder. It was explained that the T e peak was formed by high-energy electrons transported from the neutral loop along a separatrix of the quadrupole magnetic field and that the n e peak consisted of electrons undergoing reciprocating motion between the reactor ceiling and the substrate.

Abstract. Electron motion in a CF 4 magnetic neutral loop discharge (NLD) plasma was simulated using a Monte Carlo method. The spatial distribution of electrons illustrated the fundamental structure of the NLD plasma, and its dynamics were depicted from the distributions of the mean electron energy, the electron energy gain and the azimuthal electron flux. The peak of mean electron energy appeared at the neutral loop (NL), which confirmed that electrons gain energy near this loop.High mean electron energy was observed not only near the NL but also along the separatrices of the quadrupole magnetic field. Energetic electrons were transported along the separatrices and induced ionization at those locations. However, the electron distribution had valleys along the separatrices, because electrons accelerated near the NL were likely to leave this region where the binding of the magnetic field is weak. The azimuthal electron flux representing the loop plasma current showed that the electron conduction path around the NL, which has conventionally been modelled as a ring conductor, has a particular directionality due to the rectification effect of antiparallel magnetic fields composing the quadrupole magnetic field. The directionality in the upper and lower regions of the quadrupole magnetic field was opposite to that in the inner and outer regions.

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-We simulated the electron motion in a quadrupole magnetic field that drives a neutral loop discharge (NLD) plasma, using a Monte Carlo method. The structure of the electron distribution in the NLD plasma was observed. Electrons underwent reciprocating motion between reflecting boundaries as they gyrated along magnetic field lines. During this motion, they deviated from the neutral loop and concentrated beside the separatrices of the quadrupole magnetic field.Index Terms -Electron number density, Monte Carlo method, neutral loop (NL) discharge (NLD) plasma, quadrupole magnetic field, separatrix.AGNETIC neutral loop (NL) discharge (NLD) plasma, used for dry etching, is a type of inductively coupled plasma. It can operate at low gas pressures (~ 1 mTorr) and high plasma densities (10 . The magnetic field induced by the coil currents is quadrupole, and a ring of zero magnetic field, i.e., the socalled NL, is formed in the vessel. A ring-shaped plasma, i.e., the NLD plasma, is generated along the NL by the electric power from an RF antenna [3]. The radius and position of the NLD plasma are controllable via the coil currents. A dynamic control utilizing this feature for uniform wide-area etching has been proposed [4]. To understand the structure of the NLD plasma, the electron motion in the quadrupole magnetic field was simulated by a Monte Carlo method. In this work, we observed the three-dimensional structure of the electron distribution in the NLD plasma. The diameter and height of the vessel were both 40 cm. Currents I 1 , I 2 , and I 3 of the top, middle, and bottom coils (60-turn coils) were set at +47.8, −55.75, and +47.8 A, respectively. The radius of the NL formed was 11 cm, and the gradient of the magnetic field was approximately 0.17 mT/cm near the NL. An RF (13.56 MHz) electric field with a sinusoidal waveform was induced by the oneturn RF antenna. Its amplitude at the NL was approximately 4.0 V/cm.The gas medium was assumed to be CF 4 at 5 mTorr.The electron collision cross sections of CF 4 were taken from [5]. The initial electrons were released in the NL region, which is the region where the magnetic field is weaker than the 0.48-mT strength of the RF-resonant magnetic field and the electric power is deposited into the plasma. The electrons were traced for 200 RF periods and sampled in the last 50 to obtain the time-averaged electron distribution after the initial relaxation process. We assumed a high electron reflectivity of 0.99 at the sidewall, ceiling, and bottom of the vessel, regarding it as the sum of the surface reactions such as electron absorption and emission of the secondary electrons [6]. Fig. 2 shows the electron number density n e . The highdensity region in which n e is higher than 20% of its peak value is extracted. n e is low near the separatrices of the quadrupole magnetic field, and electrons concentrate beside the separatrices. Electrons deviate from the separatrices, but further diffusion hardly proceeds. This is explained in the following manner. In the NL region, electr...