Yoshihiro Okuno

Observation of radio‐frequency discharges at various driving frequencies

Radio‐frequency discharges at various driving frequencies f in the range of 1.9–28 MHz are observed, especially from the viewpoint of the sheath structure. The results reveal that for f≥10 MHz, the sheath thickness Zs measured by an emissive probe varies directly as f−1, while for f≤10 MHz, Zs∝f−1/2. The electron density estimated from the ion saturation current of a needle probe is almost proportional to f2 in the former case. High‐energy electrons accelerated by the expanding rf sheath can contribute to sustaining rf plasmas in the high‐frequency region, whereas in the low‐frequency region, secondary electrons emitted from an electrode can contribute to it in a similar manner to dc and ac discharges.

Potential formation near powered electrode in radio‐frequency‐driven discharge

Potential formation at the crest and trough phases in a radio‐frequency (rf) oscillation (13.56 MHz) near a powered electrode has been observed for the first time in a rf‐driven discharge plasma on the basis of a novel method of an emissive probe. The spatial profile at the crest phase was found to form an electron sheath extremely different from the conventional one, whereas the profile at the trough phase forms apparently a conventional ion sheath. The sheath thickness estimated from the time‐averaged potential profile was also discussed by comparing with the spatial profiles of plasma density and optical emission.

Production of large diameter microwave plasma using an annular slot antenna

A new production method for a large diameter microwave plasma is proposed without magnetic coils. An annular slot antenna and two ring‐typed permanent magnets are used for the generation of high density plasma in the circumference of a chamber with the plasma confinement and diffusing to the central region. The optimum arrangement of components in the device is examined for the production of the large diameter uniform plasma. The almost uniform electron density of about 4×1010 cm−3 is realized, and the plasma with two electron temperatures is observed. The measurement with a directional ion energy analyzer reveals that ions are almost isotropic.

Measurements of electron energy distribution function in an asymmetric radio‐frequency discharge plasma

Electron energy distribution functions in an asymmetric radio‐frequency (rf) discharge helium plasma were measured using an electrostatic energy analyzer looking opposite ways toward both the powered electrode and the bulk plasma. The grid and collector in the analyzer were connected to a series of parallel inductance‐capacitance (L‐C) filters to minimize rf interference. Electrons with high energies (15–30 eV) flowing from the electrode toward the bulk plasma were observed near the electrode. The energy is caused by the acceleration mechanism due to the rf sheath expansion, which should be the rf discharge sustaining mechanism. The electrons with high energies decreased in number with the distance from the electrode and became Maxwellian far from it. Electrons directed toward the electrode from the plasma were almost Maxwellian, and this situation was almost independent of the distance from the electrode.

Production of sheet plasma with dc magnetron discharge source

A production technique for sheet plasma with a dc magnetron discharge source is proposed. The magnetron discharge was realized by a cross field of permanent magnets and a rectangular‐typed hollow cathode, which enhances the production of high‐density plasma. The plasma produced was introduced into a low‐pressure region where an axial magnetic field was applied through a slit anode to form the sheet plasma, of high electron density of 4.5×1010 cm−3, electron temperature of 2.8 eV for low discharge current (0.3 A) and low magnetic field (0.18 kG). The thickness of the sheet plasma is found to be almost the same as the ion cyclotron diameter. This plasma production technique would be useful for thin‐film preparation on large‐area substrates.

Two-dimensional ion velocity distribution functions in electron cyclotron resonance plasma under a divergent magnetic field

Two‐dimensional ion velocity distribution functions are measured in electron cyclotron resonance (ECR) argon plasmas under a divergent magnetic field configuration using a directional analyzer. Ions produced in the ECR source grow to a beam in the downstream region inside an allowed angle, while the distribution function is almost isotropic outside the angle. The beam energy corresponds to the potential difference between the ECR source and downstream locations. The ion beam is almost parallel to an applied magnetic field, the beam being deflected along the divergent magnetic field lines. The energy spread and the temperature of the ion beam, and the temperature of the bulk ions are also shown as a function of the distance from the ECR source or of the gas pressure. The validity of the directional analyzer measurements is discussed by considering the motion of an ion in a sheath in front of the analyzer.

Effects of downstream magnetic field collimation on ion behavior in electron cyclotron resonance microwave plasma

Effects of a magnetic held collimation on ion behavior in the downstream region of an electron cyclotron resonance (ECR) plasma device are clarified experimentally using a directional ion energy analyzer. The drift energy and its spread of the ion beam observed in the downstream region decrease and the beam temperature increases with the collimating magnetic flux density. Then, the ion temperature measured perpendicular to the axis of the plasma stream slightly decreases. The ion beam is found to be almost parallel to the magnetic field lines, that is, the beam tends to be collimated. >

Sheet plasma production by means of rf magnetron discharges

A production technique for sheet plasma with a radio frequency (rf, 13.56 MHz) magnetron discharge source is proposed. The magnetron discharge was realized inside the rf‐excited electrode of rectangular hollow type with a magnetic field which also maintained the plasma sheet geometry. The typical density and temperature of electrons in the sheet plasma was 4.3×1010 cm−3 and 1.8 eV, respectively, at the gas pressure of 5 mTorr and the magnetic flux density of 0.3 kG. A comparison with the sheet plasma produced by a dc magnetron discharge source, which we proposed previously, is also given.

Potential structures in asymmetrical radio frequency discharges containing negative ions

Abstract Potential structures in radio frequency (rf, 13.56 MHz) discharges containing negative ions (SF 6 in He gas) have been clarified experimentally in an asymmetrical electrode system using an emissive probe. A double layer type potential profile is picked up near an instantaneous anode unlike the almost flat profile in He plasma. A SF 6 addition provides a spatial shift of the double layer position toward the electrode, lowers the plasma potential, and decreases the sheath thickness. These results are ascribed to the negative ion production which is confirmed by an electrostatic analyzer measurement.

Modulation of electron velocity distribution function by moving cathode sheath in a low-pressure rf discharge

Interaction between a moving collisionless cathode sheath and bulk electrons in a completely asymmetrical rf glow discharge is studied numerically. The incident electron velocity distribution function, assumed to be Maxwellian, is found to be modulated markedly by the interaction with the rf sheath expanding and contracting symmetrically in time. An electron beam toward the bulk plasma with sufficiently high velocity for ionization of atoms is generated during the sheath expansion. The modulated distribution function and the beam velocity are almost independent of the voltage applied on the cathode, although a slight extension of the tail and a slight increase in the beam velocity are observed with increases in voltage.