K. Inai

Monte Carlo Simulation of Hydrocarbon Redeposition on a Graphite Surface in Hydrogen Plasma

To investigate the hydrocarbon redeposition process on the graphite tile surface, we developed a modeling code that considers a complete set of collisional reactions in the plasma, energy- and species-dependent reflection coefficients, and fragmentation at the tile surface. The model calculations revealed local redeposition characteristics. The redeposition rate, which is the number of redeposited particles on the tile surface per launched CH4 molecule, decreases with decreasing plasma temperature due to a steep decrease in the rate coefficients for electron impact ionization at temperatures of less than 10 eV. At elevated plasma temperatures, large numbers of singly or multicharged carbon ions are redeposited because of the high sticking coefficient of the multicharged ions, which is strongly accelerated by the sheath field. The hydrogen concentration of the deposited species increases because of a decrease in the number of dissociative reactions in the plasma with decreasing plasma density. Using energy- and species-dependent sticking coefficients, low-energy ion species are reflected by the tile surface where dissociation occurs and are subsequently dissociated due to transport in the plasma; accordingly, the hydrogen concentration of deposited species is low.

Modelling and observations of electron beam charging of an insulator/metal bilayer and its impact on secondary electron images in defect inspection equipment

A self-consistent simulation of secondary electron (SE) emission and charging of a SiO(2) layer with the thickness of several tens of nanometres on Si is incorporated into a trajectory simulation of emitted SEs above the surface, the centre area of which is charged by electron beams (EBs) at the energy range from 300 to 2000 eV. In order to study the influence of the charging of an insulating layer on defect inspection, a pseudo-image is reconstructed from net SE yields calculated at each point of the SiO(2) surface locally applied the positive voltage. The image contrast between charged and uncharged areas is compared with the observation of thermally oxidized layer with the thickness of 24-106 nm on a Si wafer. The image contrast is very sensitive to the thickness of the SiO(2) layer, which is verified by both observed and calculated images. The calculated changes of the images with the layer thickness and the primary electron energy reproduce the experimental observations fairly well. This confirms a highly sensitive detection mechanism for tiny defects in insulating patterns on a metal hard mask for an EB defect inspection equipment.

Simulation of secondary electron emission in helium ion microscope for overcut and undercut line-edge patterns

In order to study the topographic contrast of line-edge patterns in a scanning ion microscope (SIM) using helium (He) beam, a Monte Carlo simulation of secondary electron (SE) emission from silicon (Si) by the impact of He ions in the energy range of tens of keV is performed. The edges with overcut and undercut profiles for different sidewall angles are modeled and the patterns are scanned by using 30 keV He ion beam, so that the line profiles of the SE intensity are calculated assuming zero-sized beams. The results are compared with those of 30 keV Ga ion and 1 keV electron beams. Furthermore, the pseudo-images of critical-dimension (CD) line patterns with different widths are constructed from the SE profiles. The calculated SE yields of Si for 10-40 keV He ions increase with increasing impact energy, which become larger than that for low-energy electrons (keV or less). When scanning the line edges formed on a Si surface, there appear both large and sharp peak and small dip of the SE yield. The height of the peak is much more for the He ion beam than the Ga ion and electron beams, whereas the width is less: the FWHMs are 3.8 nm for 30 keV Heion, 7.2 nm for 30 keV Ga-ion and 8.0 nm for 1 keV electrons. This indicates that the line edge is more clearly distinguished by He ions. The change in the sidewall angle causes the change in the shape of the hump in the SE profile at the sidewall of overcut edges due to the incident angle dependence of the SE yield, which is clearly seen for all beams. However, much less change in the line profiles of undercut edges is found for Ga ion and electron beams.

Modeling of charging effects in scanning ion microscopes

Unwilling deformations of secondary electron (SE) images due to charging of an insulating layer on materials is one of important issues for semiconductor industry applications of scanning ion microscopes (SIM). This paper presents a Monte Carlo model of SE emission from SiO2 in which the charging induced by ion bombardment at the energy range of tens of keV is taken into account. A self-consistent calculation is carried out for the transport of a projectile ion, recoiled material atoms and SEs, the creation of space charges trapped in the material and the resultant electric field in/out the material. Drift motion of trapped charges is calculated as well, where the recombination with a charge of opposite sign is taken into account. Therefore, the evolution of the charging is simulated with successive arrivals of ions. Since the surface voltage is positive due to ejection of SEs and injection of positive ions, some of ejected SEs are drawn back to the surface and can rebound on it; these SEs are unable to produce a net emission. Dynamic changes in the SE yield and surface voltage are compared among He ions, Ga ions and low-energy (<1 keV) electrons, along with the space charge distributions and the in/out electric fields. The net SE yield is decreased during ion bombardment and finally it vanishes, which is different from the case of electron bombardment where the net SE yield (including BSEs) is kept to one due to a balance between coming and outgoing electrons. Even if there is not net emission of SEs, the surface voltage does not reach any steady-state condition but progressively increases due to successive injection of positive ions. The growth rate of the surface voltage depends on both the SE yield with no charging and the spatial distribution of the ions penetrating into the material.

Ultrafine Particle Removal Using Gas Cluster Ion Beam Technology

In this paper, the ultrafine particle removal using CO2 gas cluster ion beam (GCIB) technology is investigated. The CO2 GCIB is irradiated the sample at an angle of 0 ° with respect to the surface normal. The higher particle removal efficiency can be achieved at the higher kinetic energy of the gas cluster, and the inside space of line and space pattern particles can be removed. We also suggested that the CO2 GCIB process has the high particle removal uniformity without redeposition of the removed particles. It is possible to remove the ultrafine particle as small as 12 nm in diameter (which is required for 2014 by ITRS 2011). The pattern damage is not observed for 45 nm poly-Si pattern. Moreover, the molecular dynamics simulation is performed to investigate the mechanisms of the particle removal by GCIB irradiation.

Electron beam charging of a SiO2 layer on Si: a comparison between Monte Carlo-simulated and experimental results

Recently, a unique capability in highly sensitive detection of residue defects in photoresist patterns on a metal hard mask has been verified experimentally [T. Hayashi et al., Proc. SPIE, 6922 (2008) 6922-129]. In order to reveal the mechanism for the new defect inspection technique, the charging up induced by 300 eV - 2000 eV electron bombardment of thin insulating layers (SiO2, ~tens of nm) on Si is studied by using a self-consistent Monte-Carlo simulation of the transport of a primary electron and secondary electrons (SE) and the generation of an electric field due to the charges in the layer. The calculation is compared with the contrast changes in the SEM images of thermally oxidized layers (20~100 nm) on a Si wafer. Low-energy EB (or thick SiO2 layer) causes the positive charging of the layer, whereas the high-energy EB, which penetrates under thin SiO2 layer, relaxes the charging of the layer due to electron-hole recombination in Si. The thickness dependence of the SE yield for low- and high-energies is investigated, which explains the observed changes in the SEM images of the insulating layers on Si.

An EDDY/Particle-in-Cell Simulation of Erosion of Plasma Facing Walls Bombarded by a Collisional Plasma

To investigate the erosion of a plasma-facing wall intersecting an oblique magnetic field, we performed a kinetic particle-in-cell (PIC) simulation of magnetized plasma, in which collision processes between charged and neutral particles were taken into account. Sheath formation and local physical quantities, such as the incident angle and energy distributions of plasma ions at the wall, were examined at a plasma density of 1018 m-3, a temperature of 10 eV, and a magnetic field strength of 5 T. The erosion rate of a carbon wall was calculated using the ion–solid interaction code EDDY. At a high neutral density (>1020 m-3), the impact energy of the ions dropped below the threshold for physical sputtering, so that the sputtering yield was drastically decreased and wall erosion was strongly suppressed. Sputter erosion was also suppressed when the angle of the magnetic field with respect to the surface normal was sufficiently large.

Modeling of Impurity Transport in Edge Plasmas and Tritium Codeposition on Plasma Facing Walls in ITER

Abstract In order to simulate carbon deposition profile in the divertor of ITER, long-distance transport in the scrape-off-layer and divertor plasma of carbon and hydrocarbons eroded from the divertor target plates are modeled. Physically eroded carbons dominate a sharp profile on the outer target plate, whereas at the inner target plate, a very small redeposition is observed. Chemically eroded hydrocarbons produce a redeposition on the dome area as well as both inner and outer target plates. Assuming tritium content in the redeposited layers, tritium co-deposition profile on the inner and outer target plates and dome is estimated, which allows us to predict the long-term tritium retention in the divertor of ITER.

Hydrocarbon Redeposition on Plasma Facing Walls Intersecting Magnetic Field at Shallow Angles

In present nuclear fusion devices, hydrocarbons resulting from the chemical sputtering of carbon-based walls redeposit on other areas of the wall after transport in plasmas, forming hydrogen-rich carbon layers. A particle-in-cell calculation of a sheath region between the plasma and the wall is incorporated into the transport simulation of methane (CH4) and the fragments in the plasma. The effect of the magnetic field intersecting the wall surface at shallow angles on the redeposition characteristics is studied, taking the reflection and sticking on the wall into account. The redeposition rate is rather slowly increased with increasing angle between the magnetic field line and the surface normal, whereas it strongly depends on the plasma temperature and the sticking probability, S, of hydrocarbons (CHx) returning to the surface. By assuming S = 1, the redeposition of large molecular ions (CH3+ and CH4+) is suppressed at shallow angles (θ> 85°), whereas the redeposition of atomic C ions is enhanced. For zero sticking (S = 0), the redeposition is dominated by C ions at high temperature, whereas at low temperature, it is dominated by neutral C atoms.