Kohei Ono

Numerical simulation on neutral beam generation mechanism by collision of positive and negative chlorine ions with graphite surface

We investigated ion neutralization by collision with graphite by numerical simulation based on time-dependent density functional theory. It is known that the neutral beam source developed by Samukawa (2001 Japan. J. Appl. Phys. Part 2 40 L779), where neutral particles are generated by the collision of ions from plasma with a graphite electrode with numerous high-aspect-ratio apertures, can achieve very high neutralization efficiency of over 90% when negative ions (Cl−) are used compared with about 60% when positive ions are used. To understand the neutralization theoretically, we developed a numerical simulator and calculated the dynamic process of electron transfer between an ion and graphite during the whole collision process. Multiple collisions were considered in the calculation. We found that Cl− had higher neutralization efficiency (more than 90%) than (about 34%), which is in excellent agreement with the experimental result, so our simulator could successfully simulate the neutralization process. The difference in neutralization efficiency between and Cl− may be due to the relationship between the ion and graphite orbital energy levels.

Numerical study on electron transfer mechanism by collision of ions at graphite surface in highly efficient neutral beam generation

We investigated the neutralization mechanism of ions created by collisions with a graphite surface by numerical simulations using an efficient and stable simulator developed by us based on time-dependent density functional theory (TD-DFT) to clarify the mechanism responsible for generating neutral beams in a highly efficient neutral beam source developed by Samukawa et al (2001 Japan. J. Appl. Phys. 40 L779). The results from the simulations revealed that negative ions (Cl?) have higher neutralization efficiency than positive ions , which was consistent with previous experimental results. The origin of this difference was investigated in terms of the energy alignment between electronic states participating in the charge transfer process. We found that the electronic states of Cl? have similar energies with those of graphite, while those of and graphite have large differences in energies. This could be interpreted as resonant charge transfer occurring in the neutralization process of negative ions, while Auger charge transfer is dominant in that of positive ions. This interpretation was also strengthened by results where electron transfer probability to the excited states was much larger for collisions of graphite with than with Cl?. This suggested that the different mechanisms are the reason for the difference in neutralization efficiency between negative and positive ions.

Improved numerical calculation of the generation of a neutral beam by charge transfer between chlorine ions/neutrals and a graphite surface

The charge transfer process between chlorine particles (ions or neutrals) and a graphite surface on collision was investigated by using a highly stable numerical simulator based on time-dependent density functional theory to understand the generation mechanism of a high-efficiency neutral beam developed by Samukawa et al (2001 Japan. J. Appl. Phys. 40 L779). A straightforward calculation was achieved by adopting a large enough unit cell. The dependence of the neutralization efficiency on the incident energy of the particle was investigated, and the trend of the experimental result was reproduced. It was also found that doping the electrons and holes into graphite could change the charge transfer process and neutralization probability. This result suggests that it is possible to develop a neutral beam source that has high neutralization efficiency for both positive and negative ions.

Prediction of etching-shape anomaly due to distortion of ion sheath around a large-scale three-dimensional structure by means of on-wafer monitoring technique and computer simulation

A system for predicting distortion of a profile during plasma etching was developed. The system consists of a combination of measurement and simulation. An ?on-wafer sheath-shape sensor? for measuring the plasma-sheath parameters (sheath potential and thickness) on the stage of the plasma etcher was developed. The sensor has numerous small electrodes for measuring sheath potential and saturation ion-current density, from which sheath thickness can be calculated. The results of the measurement show reasonable dependence on source power, bias power and pressure. Based on self-consistent calculation of potential distribution and ion- and electron-density distributions, simulation of the sheath potential distribution around an arbitrary 3D structure and the trajectory of incident ions from the plasma to the structure was developed. To confirm the validity of the distortion prediction by comparing it with experimentally measured distortion, silicon trench etching under chlorine inductively coupled plasma (ICP) was performed using a sample with a vertical step. It was found that the etched trench was distorted when the distance from the step was several millimetres or less. The distortion angle was about 20? at maximum. Measurement was performed using the on-wafer sheath-shape sensor in the same plasma condition as the etching. The ion incident angle, calculated as a function of distance from the step, successfully reproduced the experimentally measured angle, indicating that the combination of measurement by the on-wafer sheath-shape sensor and simulation can predict distortion of an etched structure. This prediction system will be useful for designing devices with large-scale 3D structures (such as those in MEMS) and determining the optimum etching conditions to obtain the desired profiles.