Katsunori Ichiki

Generating high-efficiency neutral beams by using negative ions in an inductively coupled plasma source

To minimize radiation damage caused by charge buildup or ultraviolet and x-ray photons during etching, we developed a high-performance neutral-beam etching system. The neutral-beam source consists of an inductively coupled plasma (ICP) source and parallel top and bottom carbon plates. The bottom carbon plate has numerous apertures for extracting neutral beams from the plasma. When a direct current (dc) bias is applied to the top and bottom plates, the generated positive or negative ions are accelerated toward the bottom plate. Most of them are then efficiently converted into neutral atoms, either by neutralization in charge-transfer collisions with gas molecules during ion transport and with the aperture sidewalls in the bottom plate, or by recombination with low-energy electrons near the end of the bottom plate. We found that negative ions are more efficiently converted into neutral atoms than positive ions. The neutralization efficiency of negative ions was almost 100%, and the maximum neutral flux dens...

High-Efficiency Neutral-Beam Generation by Combination of Inductively Coupled Plasma and Parallel Plate DC Bias

To avoid several kinds of radiation damage caused by charge build-up and by ultraviolet and X-ray photons during etching processes, we have developed a high-performance, neutral-beam etching system. The neutral-beam source consists of an inductively coupled plasma (ICP) source and top and bottom carbon parallel plates. The bottom carbon plate includes numerous apertures for extracting neutral beams from the plasma. By supplying a direct current (DC) bias to the top plate, the generated ions are accelerated towards the bottom plate. Most of them are efficiently converted into neutral atoms, either by neutralization in charge-transfer collisions with gas molecules during the ion transport and with aperture sidewalls in the bottom plate, or by recombination with low-energy electrons near the end of the bottom plate. When the aperture diameter and aperture length were 1 mm and 10 mm, respectively, the neutralization efficiency was almost 100% and the neutral flux density was equivalent to 1.2–2.8 mA/cm2. A neutral beam could thus be produced efficiently from the ICP source and the apertures in the bottom plate.

High-Efficiency Low Energy Neutral Beam Generation Using Negative Ions in Pulsed Plasma

To prevent several kinds of radiation damage caused by charge build-up and by ultraviolet and X-ray photons during etching processes, we have developed a high-performance, neutral-beam etching system. The neutral-beam source consists of an inductively coupled plasma (ICP) source and top and bottom carbon parallel plates. The bottom carbon plate includes many apertures for extracting neutral beams from the plasma. By supplying a positive or negative direct current (DC) bias to the top and bottom carbon plates in the pulsed SF6 plasma, the generated positive or negative ions are respectively accelerated towards the bottom plate. The negative ions are more efficiently converted into neutral atoms in comparison with the positive ions, either by neutralization in charge-transfer collisions with gas molecules during the ion transport or with aperture sidewalls in the bottom plate. The neutralization efficiency of negative ions was more than 98% and the neutral flux density was equivalent to 4 mA/cm2.

50 nm gate electrode patterning using a neutral-beam etching system

A 50-nm-width metal-oxide-semiconductor (MOS) gate etching process was established using a recently-developed neutral-beam etching system by optimizing the gas chemistry and the electrode bias condition. In a comparison with poly-Si gate etching using either SF6 or Cl2 gas chemistries, opposite etching characteristics were observed in the pattern profile. Consequently, the use of a mixture of these gases was proposed in order to achieve fine control of the etching profiles. The energy of the neutral beam was increased by applying a 600 kHz rf bias to the bottom electrode. The rf bias was very effective in increasing the etch rate and the anisotropy of the poly-Si gates, with no deterioration of the neutralization efficiency. The oxide leakage current achieved for a MOS capacitor etched by the neutral beam was one order of magnitude lower than that achieved by conventional plasma etching.

Fast Atom Beam Etching of Glass Materials with Contact and Non-Contact Masks.

Fast atom beam (FAB) etching of multicomponent glass and silica glass was performed using a contact mask (electron beam resist) and two non-contact masks (typically 5-µ m-diameter particles and a copper mesh with a 5 µ m line width and 20 µ m line spacing). FAB etching of a multi component glass substrate with the micro-particle mask successfully fabricated a precisely projected, 1.0-µ m-high outline pattern on the substrate. FAB etching of a silica glass substrate with the copper-mesh mask, which was separated from the substrate by about 100 µ m, successfully produced a projected, 34-nm-high outline pattern on the substrate. A combination of electron beam lithography with FAB etching on silica glass successfully fabricated nano-scale ultrafine patterns whose aspect ratio was higher than 7 (50 nm line width and 360 nm height). In all three fabrications, the side walls and etched surfaces were very smooth and were perpendicular to each other.

Characterization of neutral beam source using dc cold cathode discharge and its application processes

Fast atom beam (FAB) sources using dc cold cathode discharge, comprising parallel plate electrodes and thick carbon plate cathodes with multiple beam extracting apertures, were developed to generate parallel and straight energetic neutral beams for precise etching of three-dimensional microstructures consisting of insulating materials. Conventional FAB sources and their applications are briefly reviewed, and the advantages of newly developed FAB sources and new applications are introduced. By using SF6 gas, a precise etching of quartz glass with an etch rate of 30 nm min−1, a uniformity of within 4% (P–V) in O76 mm, vertical etch profile, smooth etch surface and long-term etch rate stability over 250 min were realized. The neutralization coefficient and the beam current density were also measured using a secondary electron method and a pulse-counting method, making it possible to measure the neutralization coefficient without referring to databases for secondary electron yields. A neutralization coefficient of 98% was obtained at maximum, although, under practical etching conditions, the neutralization coefficient is less than 70%. By comparing the results of the simple model calculation with the experimental data, it was determined that the neutralization mechanism was dominated by charge transfer. The importance of neutralization in a process chamber is also discussed.

New-type focused fast atom beam (F-FAB) source and evaluation of emitted beam density distribution

Abstract We have developed a new type of focused fast atom beam (F-FAB) source, which we call radial-surface electrode F-FAB source (RSEFF source), and evaluated its emitted beam density distribution. The F-FAB source consists of two electrodes whose shapes and downstream cathode holes are designed to focus the beam on a point. The neutralization ratio of the beam emitted was 2–10 times higher than that of beams emitted by conventional F-FAB sources. To evaluate the emitted F-FAB density distribution, we used the RSEFF with chlorine gas to etch GaAs workpieces. The etched depth distributions were measured for three different distances between the focus point and the workpiece surface (at the focus point, and at 20 mm above and below the focus point). The results confirmed that the RSEFF source is stable and emits a focused neutral chlorine beam that has the narrowest and highest HMFW (half-maximum, full-width) of the beam density distribution when the workpiece is at the focus point. Moreover, the results showed that the etched depth distribution is effective for evaluating the beam density distribution. We then successfully used the RSEFF to produce contracted pattern etching, demonstrating the applicability of the RSEFF to contracted projection processing.