Katsuaki Aoki

Inductively Coupled Plasma Source with Internal Straight Antenna

An inductively coupled plasma source with an internal straight antenna was developed. By inserting an antenna into plasma, the induction of a strong electric field in the plasma and the efficient transmissions of power to plasma is enabled. However, there was a practical problem in that antenna sputtering occurred. Suppression of antenna sputtering and methods of insulating the antenna were studied. Consequently, it was found that sputtering impurities were reduced by covering the straight antenna with a quartz pipe. Furthermore, the amount of quartz pipe etching could be reduced to as little as 1/10th the original value. As a result of fabricating and evaluating the plasma source in which four straight antennas were arranged in parallel, electron density was determined to be as high as 1011 cm-3 even at a pressure as low as 4 mTorr. When the processing performance of the plasma source was evaluated, the ashing rate of the photoresist and the etching rate of the poly-Si were, respectively, 4.8 µm/min and 450 nm/min. These values are at practically applicable levels.

Microwave-Excited Large-Area Plasma Source Using a Slot Antenna

We developed a stable and uniform large-area plasma source using surface waves generated from a slot antenna. The propagation of microwaves radiating from the slot antenna was studied and the optimal shape of the slot antenna for uniform plasma generation was determined. The characteristics of a dry etching process using the plasma were also evaluated. It was confirmed that some of the microwaves which radiated from the slot antenna propagate as surface waves in the dielectric window. By adjusting the angle of the slot antenna, the plasma was generated in the direction perpendicular to the axis of the slot antenna. An Ar plasma electron density of the order of 1011/cm3, which exceeds the electron density required for blocking the propagation of microwaves (cutoff frequency), was obtained. On the basis of these observations, it was considered that the microwaves radiating from the slot antenna propagate in the form of surface waves in the direction perpendicular to the axis of the slot antenna, and generate surface-wave plasma. The results of the measurements of the etching rate and ashing rate yielded a poly-Si etching rate of 470 nm/min or greater with a uniformity of 5% or better, and an ashing rate of 3000 nm/min or greater with a uniformity of 7% or better. The poly-Si etching rate was approximately twofold and the ashing rate was approximately fivefold the corresponding values obtained by the conventional chemical dry etching method.

Measurement of Electron Density of Reactive Plasma Using a Plasma Oscillation Method

We measured the electron density of reactive plasma using a plasma oscillation probe and analyzed the mode change of power coupling during etching. The unique behavior of plasmas containing reactive negative ions is clarified. The change in the plasma reactor mode during silicon dioxide etching, from the inductive mode to the capacitive mode with increasing pressure, is investigated.

Power generation characteristics of Schottky-type solar cells fabricated using barium silicide

We studied a novel method of increasing the efficiency of solar cells using BaSi2 as a semiconductor. BaSi2 could be deposited by RF magnetron sputtering using a polycrystalline BaSi2 target, followed by annealing at 500 °C for 30 min in N2 ambient. Furthermore, Schottky-type solar cells using BaSi2 were fabricated. The crucial point is that Al–Nd was used to form the Schottky junction between the BaSi2 film and the Al–Nd electrode. Additionally, Si3N4 (3–5 nm) was used as an oxidation prevention layer. Under irradiation at 90 K, resulting in a short-circuit current density (Jsc) of 3.19 mA/cm2, an open-circuit voltage (Voc) of 0.76 V, and a fill factor (FF) of 0.28 were obtained.

Disaggregation of polyribosomes in the spinal anterior horn cells in a patient with X-linked spinal and bulbar muscular atrophy.

Abstract The spinal anterior horn cells (AHCs) in a patient with X-linked spinal and bulbar muscular atrophy (SBMA) were examined by light and electron microscopy, giving special attention to alterations in the rough endoplasmic reticulum (ER). Seven age-matched subjects were used as controls. The patient with SBMA showed a severe decrease of AHCs, but the Nissl substance in the remaining AHCs appeared well preserved on light microscopy. Electron microscopy revealed a relatively well preserved parallel lamellar pattern of ER and marked disaggregation of the polyribosomes surrounding the ER in the remaining AHCs. These findings indicate that the Nissl substance was affected in spite of its light microscopic appearance in SBMA, and that the AHCs degenerate through disaggregation of the polyribosomes of the ER.

Evaluation of Oxygen-Plasma Damage in GaAs Exposed to a Surface-Wave Plasma Source Developed for the Ashing Process

O2 surface-wave plasma radiation of GaAs substrates and photoresist films were performed as a function of the plasma-substrate distance D and the chamber pressure P in order to obtain the optimum ashing condition for achieving lower damage, higher ashing rate (RA) and higher uniformity than currently possible. The photoluminescence (PL) peak intensity observed from GaAs at 1.514 eV (free exciton) increased with increasing D; in particular, the PL intensity obtained from the samples with D112 mm and 70 Pa was stronger than that from the untreated GaAs. This result demonstrates that plasma damage can be negligible above D=112 mm even though oxides are created. On the other hand, by increasing D from 97 mm to 157 mm, RA decreased but its uniformity improved. In the range of D157 mm and P=70 Pa, RA and uniformity were saturated, while the ion energy measured in plasma decreased with increasing D, finally becoming almost 0 above D=157 mm at 70 Pa. From these results, we concluded that the optimum ashing process can be achieved at D157 mm and P=70 Pa, by which RA of 3.1 µm/min and its uniformity of ±3% were obtained on an 8 inch wafer.

Plasma ashing using microwaves via slot antenna for 300-mm wafers

We developed a downflow asher which incorporates a large-sized microwave excited plasma source with a slot antennas, for 300 mm wafers. An ashing rate of 4.5 micrometer/min and uniformity of plus or minus 5.1% were obtained at a wafer temperature of 250 degrees Celsius. The ashing rate was approximately fourfold and the uniformity level was similar to those obtained with conventional downflow asher. The newly developed asher incorporates: (1) a high-density plasma source with slot antennas, (2) a processing chamber the shape of which is optimized by gas flow simulations and (3) a compact, high- speed wafer transportation system with an originally developed vacuum robot which is primarily responsible for the high ashing rate. The maximum overall throughput, including that of the transportation system, is 160 wafers/h. Application of this system to the ashing of 300 mm wafers is expected.