Shuichi Noda

Mechanism of Radical Control in Capacitive RF Plasma for ULSI Processing

The radicals of capacitive plasmas actually used in mass production were analyzed using various measurement systems. The composition of radicals in bulk plasma depends on the gas chemistry, the dissociation process, and interaction with the wall. It is revealed that parent gas (C4F8) is dissociated by multiple collision with electrons according to τne , where τ is the residence time, ne is the electron density, σ is the dissociation collision cross section and v is the electron velocity. A high-performance etching process, which can realize 0.09 µm contact holes with aspect ratio of 11, was achieved using a short residence time to suppress the excess dissociation and the control of deposition species through the addition of O2 to C4F8/Ar plasma as well as the reduction of the density of F radicals through the reaction with the Si wall.

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.

Mechanism of C4F8 dissociation in parallel-plate-type plasma

To investigate the mechanism of C4F8 dissociation in parallel-plate-type plasma, we used several of the latest diagnostic tools and made extensive measurements of electrons, radicals, and ions under conditions that greatly suppressed the effects of plasma-surface interaction. These measurements showed that the amount of light fluorocarbon radicals and ions increased with increasing electron density. The dissociation of C4F8 was analyzed by using rate equations, after confirming the stability and uniformity of the plasma. The total dissociation rate coefficient of C4F8 was 1×10−8 cm3/s, and CF2 radicals were mainly generated from products of C4F8 dissociation. F was mainly generated from CF2 by electron-impact dissociation and lost by pumping. We could estimate that the C2F4 density was roughly comparable to the densities of CF and CF3, and that the surface loss probability of C2F4 increased with increasing electron density. C2F4 might play an important role in the etching because of its rich polymerizatio...

Fabrication of FinFETs by Damage-Free Neutral-Beam Etching Technology

A high aspect ratio and damage-free vertical ultrathin channel for a vertical-type double-gate MOSFET was fabricated by using low-energy neutral-beam etching (NBE). NBE can completely eliminate the charge build-up and photon-radiation damages caused by the plasma. The fabricated FinFETs realize a higher device performance (i.e., higher electron mobility) than that obtained by using a conventional reactive-ion etching. The improved mobility is well explained by the NB-etched atomically flat surface. These results strongly support the effectiveness of the NB technology for nanoscale CMOS fabrication

Damage-free neutral beam etching technology for high mobility FinFETs

Our newly developed neutral beam (NB) etching accomplished the damage-free (defect-free and smooth surface) fabrication of high aspect rectangular Si-fins for the first time. The fabricated FinFETs realized higher device performance (higher electron mobility) than that using a conventional reactive ion etching. The improved mobility is well explained by the NB etched atomically-flat surface. Our new results strongly support the effectiveness of the NB technology for the nano-scale CMOS fabrication

Investigation of ion transportation in high-aspect-ratio holes from fluorocarbon plasma for SiO2 etching

Abstract We investigated the behavior of ions in deep contact holes under actual dry etching conditions by comparing results between experimental and numerical simulations. In the experiments, we directly measured the ions that penetrated a real-contact-hole-sized (0.2 μm o ) micro-capillary plate, which was a membrane with many throughholes fabricated on a Si wafer with a structure customized to simulate the actual electric field profiles in the contact holes. As expected, both results suggested that the ion flux and the ion energy were depressed at the bottom surface and strongly depended on the hole aspect ratio. However, we found some differences between the experiments and the calculations. We believe these were caused by variations in the experimental results and by unknown parameters in the calculation model of the ion kinetics in the deep contact holes. Quantitative analysis will be possible in the future after the results conform to each other and the unknown parameters are clarified.

Plasma-Wall Interactions in Dual Frequency Narrow-Gap Reactive Ion Etching System

The effects of plasma-wall interactions on the plasma chemistry in a dual frequency narrow-gap reactive ion etching (RIE) system is investigated as a function of electron density, residence time, and partial pressure of additive O2 gas. It is found that there is a critical point in the residence time, where the dissociation dominant region and the wall-interaction dominant region are separated. The net flux of chemically reactive species is estimated. The net flux of carbon-inclusive ions is of the order of 1016 cm-2s-1, and is larger than that of CFx radicals. Carbon-inclusive and silicon-inclusive ions dominate the total flux of chemically reactive species. The deposition rate of the fluorocarbon film is strongly dependent on the O2 partial pressure, and is controlled by the chemical etching with oxygen as well as the reduction of fluorocarbon radical density in the gas phase. Based on the estimation of the net flux of chemically reactive species and the effect of added oxygen, dominant chemical species that control the etching reaction in the RIE system are discussed.

Fabrication of a Vertical-Channel Double-Gate Metal–Oxide–Semiconductor Field-Effect Transistor Using a Neutral Beam Etching

A vertical ultrathin-channel (UTC) formation process using a low-energy neutral beam etching (NBE) for a double-gate (DG) metal–oxide–semiconductor field-effect transistor (MOSFET) is proposed for the first time. The NBE can perfectly eliminate the charge build-up and photon radiation damages from the plasma. By utilizing the NBE, fin-type vertical MOSFETs with damage-less smooth sidewalls were successfully fabricated. The fabricated FinFETs realized higher electron mobility than that using a conventional reactive ion etching. The improved mobility is well explained by the atomically-flat surface utilizing by the NBE.

Characterization of 100 MHz inductively coupled plasma (ICP) by comparison with 13.56 MHz ICP

The effect of the excitation frequency on the dissociative process of the C4F8 gas was investigated by comparing a 100 MHz [very high-frequency (VHF)] inductively coupled plasma (ICP) with a 13.56 MHz [radio frequency (rf)] ICP. The same apparatus except for the wave generator and matching network was used for both ICPs in order to investigate the frequency effect as precisely as possible. The electron density and electron temperature in an Ar plasma were measured by using a Langmuir probe. From the dependence of the radial distribution of the Ne on the ICP source power, it was found that the rf ICP was produced in a cylindrical space under the coil area, while the VHF ICP was generated throughout the reactor. In C4F8/Ar plasma, the CFx (x=1, 2, 3) radical densities near the reactor wall were measured by using appearance mass spectrometry, and the F radical density was measured by using actinometry through the optical emission spectroscopy of Ar (750.4 nm) and F (703.7 nm). The degree of dissociation of t...

Highly anisotropic gate electrode patterning in neutral beam etching using F2 gas chemistry

Pure F2 gas chemistry was evaluated in comparison with SF6 and Cl2 gas chemistries for etching poly-Si gates of metal-oxide-semiconductor field effect transistors in our neutral beam source. In the case of SF6 gas chemistry, the polycrystalline silicon (poly-Si) etch rate was high enough, whereas a large side etching was observed. In the case of Cl2 gas chemistry, the pattern profile was anisotropic, but the etch rate was very low. The tradeoff was caused by differences in the etching reactivity of F and Cl radicals with the poly-Si. Though the SF6-based neutral beam caused a large side etching due to diffused F radicals, an anisotropic profile was obtained by using the F2-based neutral beam, and the etch rate was much larger than that in the Cl2 gas chemistry. These notable characteristics were accomplished by suppressing excessive dissociation of F radicals and by generating large amount of negative F ions in a pulse-time-modulated F2 plasma in the neutral beam source.