Taijiro Uchida

Application of Radio-Frequency Discharged Plasma Produced in Closed Magnetic Neutral Line for Plasma Processing

A new plasma processing method is presented. Use of plasma produced in a closed magnetic neutral line is essential and the merit is easily illustrated in principle with a preliminary experimental result. This method is considered to be useful for many kinds of plasma processing.

Usefulness of Magnetic Neutral Loop Discharge Plasma in Plasma Processing

Usefulness in plasma processing is demonstrated for a plasma produced in a closed magnetic neutral loop, which consists of zero magnetic field points connected continuously. A preliminary experiment was carried out to show the advantage of magnetic neutral loop discharge (NLD) plasma in sputter etching processing of SiO2/Si wafer in the lower Ar gas pressure range. The experiment shows that a high-density plasma was obtained for Ar gas lower than 0.1 Pa and the sputter etching rate is 3 times higher than that for the usual inductively coupled plasma (ICP). In a large-loop case, the sputter etching profile obtained has a peak at a radius along the azimuthal direction. This implies that a uniform etching profile could be realized by controlling the radius of the neutral loop during process operation. The NLD plasma current induced with an rf primary one-turn antenna coil reaches up to one-third of that of the primary.

Magnetic neutral loop discharge (NLD) plasmas for surface processing

A neutral loop discharge (NLD) is a plasma generated along a magnetic neutral (i.e. null field) loop at magnetic cusps by an externally applied radio frequency (RF) electric field. Due to partial electron cyclotron resonance and good electron confinement by the magnetic cusps, the NLD plasma can efficiently absorb power from the RF field. For applications to material surface processing, NLD plasmas have two important advantages. One is the high ionization rates, which allow the production of high-density plasmas with relatively low electron temperatures at low gas pressures. The other is the spatial and temporal controllability of magnetic field configuration, which allows optimization of plasma configuration during processing. For example, highly uniform plasma processing over a large wafer can be achieved with time-varying shape control of an NLD plasma. In this paper, some basic properties of NLD plasmas as well as several examples of their applications to plasma processing are reviewed. It is shown that, due to their high plasma densities and low collisionality, NLD plasmas are especially suited for highly anisotropic high-throughput etching processes such as those for fabrication of micro-electromechanical systems/nano-electromechanical systems.

Plasma Production Using Energetic Meandering Electrons

When an electron is accelerated by an oscillating electric field in an inhomogeneous magnetic field which has a null point, it meanders along a chaotic orbit. The kinetic energy becomes larger than that of regular motion in an off-resonant homogeneous magnetic field. This effect explains the high efficiency of plasma production in the neutral line discharge (NLD).

Collisionless Heating of Electrons by Meandering Chaos and Its Application to a Low-Pressure Plasma Source

Near a magnetic neutral line, where the magnetic field vanishes, the adiabatic invariants of charged particle motion are destroyed, and hence the number of degrees of freedom increases. When a radio-frequency electric field is applied to the system, the particle motion becomes strongly chaotic. This effect can be used to produce heat in a plasma in the absence of any collisional process. The kinetic entropy of an electron orbit increases very rapidly by the mixing effect. The velocity distribution is approximated by a Gaussian function, so the thermodynamic entropy production can also be estimated. This collisionless heating mechanism enables high-efficiency plasma production at a low gas pressure.

Etching Characteristics of Organic Polymers in the Magnetic Neutral Loop Discharge Plasma.

Etchings of organic low-k materials, FLARE, SiLK and polyimide films were carried out in a N2-dominant mixed gas plasma generated by a magnetic neutral loop discharge (NLD) method at a low pressure below 1 Pa. The results showed that the uppermost layers on the top surface and sidewall surface were composed of tightly bonded C–N sp3, so anisotropic profiles were obtained in the case of the N2-dominant mixture ratio of the N2 + H2 plasma. Based on this result, we also investigated polyimide deep etching under the same condition, and successfully obtained an anisotropic etched profile with the depth of about 8 µm and the linewidth of about 0.5 µm (aspect ratio of 16).

Electron Temperature and Density Profiles in a Neutral Loop Discharge Plasma

Thomson scattering measurements of electron temperature and density in a neutral loop discharge (NLD) plasma were performed in order to reveal electron behavior around the neutral loop. The method yielded unambiguous and reliable results, enabling quantitative analyses of particle balance to be made. The results showed that the electron temperature had a peak on the neutral loop and, in contrast, the electron density had a peak at a position radially inward from the neutral loop. We suggest that charged particles are produced by electron impact ionization of argon atoms on the neutral loop and then flow radially inward. The effect of the magnetic field strength on the NLD plasma formation is also discussed.

Computer Simulation of Ion Beam Extraction from a Free Plasma Surface

A method of computer simulation of an ion beam emitted from a free boundary plasma surface is described for the case of a three-electrode ion extractor. The geometry of the beam and extractor here treated is two dimensional. The case of an asymmetric extractor is simulated and it is shown that the beam is deflected when the saturated ion current density, which can be extracted from the plasma, is nonuniform over the plasma surface and/or the electrodes of the extractor are mutually displaced. Results of the simulation are available for the optimal design of the beam extractor.

Observation of Induction Power Dependence on the Magnetic Neutral Loop Discharge Plasma Thermalization Phenomena

A few years ago, the concept of magnetic neutral line discharge plasma was presented, and a preliminary experiment demonstrated its usefulness in plasma processing. Subsequently, the electron behavior was analyzed theoretically, and the possibility of a high energy gain of electrons due to meandering motion in the magnetic neutral loop under a rf induction field along the loop was revealed. Recently, evidence has been found that the stronger the applied rf power is, the easier is the thermalization of electrons. Here, the results of studying this dependence are reported and the reason for the dependence is presented.

Computer Simulation of Ion Beam Extraction by Finite Element Method

A simulation code of the ion beam extraction from a plasma is described. It treats 2-D problems, and the finite element method (FEM) is adopted for the computation of potential distribution with good accuracy. In accel-decel type one-stage extraction, the relations among the divergence angle and extracting conditions, and the optimum extracting conditions, are obtained. By accel-accel-decel type two-stage extraction, an ion beam of higher energy is extracted with small divergence angle. Improvement with precel is also examined. But in this case a large bias voltage can not be applied because of the large heat load of the positive electrode and electrical breakdown. One method which avoids these shortcomings is to insert one more electrode between the plasma and the positive electrode. The characteristics of this configuration are examined.