Ryoji Nishio

Origins of charged particles in vapor generated by electron‐beam evaporation

The origins of electrons and ions in uranium vapor generated by electron‐beam evaporation have been determined. Measurements were made for the electron emission current due to high‐energy electron‐beam irradiation on a uranium surface (backscattered electrons, etc.), thermionic emission current from the melt surface, and electron current due to vapor ionization. Comparison of these currents confirmed that vapor ionization was the main electron generation process at evaporation surface temperatures above 2200 K. The ionized vapor formed a weakly ionized plasma of very low electron temperature: The degree of ionization ≤1%, electron temperature ≤0.3 eV. The electron‐impact ionization process contributed mainly to plasma formation. Beam electrons, their backscattered electrons, and secondary electrons from the beam‐irradiated uranium surface were the source electrons for this process. Thermal ionization was the next major process. In addition to the plasma formation model, plasma behavior in vapor was descri...

Wafer-Voltage Measurement in Plasma Processes by Means of a New Probe Method and an Impedance Monitor

A new probe method, which can measure wafer voltage directly from the wafer back-side without fluctuating the plasma, was developed. This method clarified the effect of the bias power, gas pressure and gas component on wafer Vpp, Vdc, and Vdc/Vpp. In addition, current, voltage, and phase shift were measured with an impedance monitor (IM) installed between the matching network and the bias electrode. Furthermore, wafer Vpp was calculated from IM data according to a devised equivalent circuit model. It was found that calculated wafer Vpp agrees with actual wafer Vpp within 3.4% when bias power is ranged from 200 W to 500 W.

Ion Transport from Laser Induced Metal Plasma to Ion Extraction Electrodes

Ion transport from a laser-induced metal plasma in a vapor to a cathode surface has been investigated experimentally using conventional parallel plate electrodes and a new conceptual electrode system, which consists of an anode and two cathodes, designated as “M-type electrodes”. Rapid ion transport is possible using the M-type electrodes compared with the parallel plate electrodes and there are two modes of ion transport. One is a mode similar to that seen for parallel plate electrodes, in which the bulk plasma upstreams with a velocity of 1,160 m/s during the ion extraction. The other is a special mode for the M-type electrodes, in which the plasma upstream stops at its origin and ions are transported to the parallel cathodes from both sides of the plasma.

Behavior of a continuous uranium plasma stream expanding across a weak magnetic field

When an electron beam generates a uranium vapor in the electron beam evaporator, a uranium plasma is formed on the evaporating surface and expands. It was confirmed experimentally that this plasma could expand with increasing its drift velocity and that the plasma possessed a Boltzmann electron distribution along the streamline, even when it propagated across a magnetic field of 40–50 G. This behavior can be understood by the cross‐field plasma expansion theory at 1/e→0 limit, where e is the dielectric constant of plasma [Phys. Fluids 25, 730(1982)]. The ion drift energy calculated from the measured polarization electric field is in good agreement with that estimated from the potential distribution and vapor drift energy, which confirms the ion acceleration along the plasma stream.

A Novel Plasma Etching Tool with RF-Biased Faraday-Shield Technology: Chamber Surface Reaction Control in the Etching of Nonvolatile Materials

A novel electro-magnetically coupled plasma (EMCP) etching tool for nonvolatile materials has been developed. The EMCP etcher is based on inductively coupled plasma generation and has a function for controlling surface reactions by supplying RF bias to a Faraday shield which covers a ceramic discharge dome. We investigated plasma controllability and chamber surface reaction controllability of the EMCP etcher and found that the RF-biased Faraday shield effectively keeps the internal surface of the dome clean in the etching processes of nonvolatile materials. Because of this feature, the EMCP etcher can be applied to the etching processes of various nonvolatile materials such as Pt, Ru, Ir, NiFe, Au, Mo, Ta, Al2O3, HfO2, ZrO2, and indium tin oxide (ITO).

Propagation Behavior of Uranium Plasma from Electron Beam Metal Evaporator

When uranium vapor is generated with an electron beam evaporator, a uranium plasma is formed on the evaporating surface. This plasma rises and expands with the vapor. Propagation behavior of this plasma was investigated by measuring plasma parameters, drift energy of ions and vapor flux along the propagation path. Over the range of 20-50 cm from the evaporation surface, the plasma density decreased from 3 × 109 cm−3 to 3 × 108 cm−3, while the electron temperature had a constant value of 0.29 eV. When the space potential was lowered from 1.48 to 0.80 V, the plasma ions were accelerated to increase the drift energy from 1.50 to 2.14 eV. Validity of the Boltzmann electron distribution was checked by comparing the space potential distribution with the plasma density distribution, and also the floating potential distribution with the ion flux distribution. These results confirm that the ambipolar diffusion governs the plasma propagation behavior. The change in the plasma density during its propagation occurred...

Plasma removal by box type electrode from electron beam generated uranium vapor

When an electron beam produces a uranium vapor in an electron beam evaporator, a uranium plasma is formed on the evaporating surface which expands with the vapor. To clarify the effect of the polarity of the applied voltage to the electrodes and also the effect of electrode areas when the plasma is removed by surrounding electrodes, the relationship between the plasma reaction to the applied voltage and the amount of plasma removed was investigated by using a box‐type electrode with inner fins which involves the plasma. When a positive bias, compared to the plasma potential, was applied to extract electrons from the plasma, the plasma was effectively removed while increasing the bias potential. The plasma became more diffusive due to increases in electron temperature, attributable increases in plasma sheath instabilities. However, when the vapor density increased, the instability resulted in ionization of the vapor to form an additional plasma. Therefore, under this condition, it became difficult to remov...

Sensitivity determination of quadrupole mass spectrometer for ion density measurement.

A sensitivity determination method for a quadrupole mass spectrometer using an electron impact ionization process was investigated to determine ion density from observed ion current. Theoretical expression of the relation among measured ion current, irradiated electron beam current and electron energy was obtained, in which direct ionization was assumed. Sensitivity was determined as a fitting factor of the calculated value to the measured ion current by the least square method.Sensitivity was measured for argon under the condition that ionization processes except direct ionization were negligible. The dependence of measured ion current on the irradiated electron energy and current agreed with the theoretical prediction and the sensitivity was determined with an uncertainty of 6%. The same measurement was performed for uranium and the sensitivity was successfully determined.