Atsushi Kinomura

Transition-metal profiles in a multicrystalline silicon ingot

The concentrations of transition-metal impurities in a photovoltaic-grade multicrystalline silicon ingot have been measured by neutron activation analysis. The results show that the concentrations of Fe, Co, and Cu are determined by segregation from the liquid-to-solid phase in the central regions of the ingot. This produces high concentrations near the top of the ingot, which subsequently diffuse back into the ingot during cooling. The extent of this back diffusion is shown to correlate to the diffusivity of the impurities. Near the bottom, the concentrations are higher again due to solid-state diffusion from the crucible after crystallization has occurred. Measurement of the interstitial Fe concentration along the ingot shows that the vast majority of the Fe is precipitated during ingot growth. Further analysis suggests that this precipitation occurs mostly through segregation to extrinsic defects at high temperature rather than through solubility-limit-driven precipitation during ingot cooling.

Direct measurement and improvement of local soft error susceptibility in dynamic random access memories

Abstract Soft errors induced in a dynamic random access memory (DRAM) have been measured using a nuclear microprobe. Soft error susceptibility of the local position and structure to the soft error has been clarified. Collection efficiency of charge carriers, induced by incident ions on reverse biased p-n junctions with barrier well structures, has been verified for various implantation doses for well formation.

Well Structure by High-Energy Boron Implantation for Soft-Error Reduction in Dynamic Randam Access Memories (DRAMs)

The susceptibility against soft-errors in dynamic random access memories (DRAMs) has been evaluated using nuclear microprobes by monitoring various addresses of a memory cell array to determine the influence of upper wiring layers such as word lines, bit lines and other patterns. The correlations between irradiated positions of microprobes and monitored cell positions were discussed. The effect of buried implanted layers against carrier collection has also been investigated using ion-beam-induced-current (IBIC) measurement. IBIC measurement revealed that the retrograde well structure was more effective in suppressing soft errors than conventional well structures in bulk or epitaxial substrates.

Estimation of the Charge Collection for the Soft-Error Immunity by the 3D-Device Simulation and the Quantitative Investigation

The charge collection induced by incident particles was estimated by the 3-dimensional device simulation and the quantitative evaluation method using the nuclear microprobe. The role of the buried p +layer was well analyzed in terms of the soft-error immunity of DRAMs. The methods developed here are applicable to optimize the well structure for the soft-error immunity of advanced DRAMs.

Brightness enhancement method for a high-intensity positron beam produced by an electron accelerator

A method for enhancing the brightness of an intense slow positron beam produced by an electron linear accelerator (LINAC) in order to obtain an intense positron microbeam was developed. The developed brightness enhancement system is simple and consists only of a few beam optics and a transmission remoderator. The slow positron beam produced by the LINAC is magnetically guided from the positron source to an experimental room. The beam is extracted from the magnetic field and is focused by a lens on the remoderator to enhance its brightness. The brightness-enhanced beam is extracted from the remoderator and focused on a sample by a lens. The beam size at the sample was 90 μm, which was two orders of magnitude smaller than that in the magnetic transport system that was about 10 mm. The efficiency of the transmission remoderator was about 5%. Adiabatic rules in the magnetic transport and the paraxial-ray equation were used to estimate the beam size that could be produced using this method.

Interaction of defects and metals with nanocavities in silicon

Abstract Ion implantation of H or He into silicon, followed by annealing can create a band of nanocavities. Such nanocavities can exhibit a range of interesting and often non-equilibrium interactions with defects and metals during subsequent implantation and annealing. This paper gives an overview of such interactions, concentrating on cavities produced by H-implantation. The evolution of cavities during annealing is briefly treated, followed by illustrations of the very efficient gettering ability of cavities for fast diffusing metals. For low metal concentrations introduced into the near-surface by implantation, the metal atoms decorate the cavity walls during annealing but can be displaced by oxygen under certain conditions. At high metal concentrations, precipitation and second phase (silicide) formation can occur at cavities but silicide formation and dissolution are found to be controlled by the availability or removal of silicon interstitials, leading to non-equilibrium behaviour. When silicon that contains cavities is irradiated with silicon ions, irradiation-induced defects interact with cavities, leading to preferential amorphisation at certain temperatures. Continued irradiation leads to cavity shrinkage during bombardment, which is most efficient when the region around the cavities is amorphised.

Rapid three-dimensional imaging of defect distributions using a high-intensity positron microbeam

An intense positron microbeam generated by an electron accelerator has been developed for obtaining three-dimensional positron lifetime mappings in a sample to permit visual evaluation of defect distributions. The beam diameter at the sample was 80–100 μm. The counting rate of the positron annihilation γ rays used to measure positron lifetime was as large as 3×103 s−1. Three-dimensional imaging was demonstrated of positron lifetimes in a SiO2 sample, which was irradiated with ion beams through a mesh mask. The time to obtain a single image (3500 pixels for an area of 2.5×3.5 mm2) was 0.5–1 h.

Positronium annihilation and pore surface chemistry in mesoporous silica films

Lifetimes of ortho-positronium in mesoporous silica films were measured before and after surface trimethylsilylation of –OH groups. Variations of positronium lifetimes in the mesopores upon the surface modification indicate that the interaction between positronium and the pore surface is weakened in the pores, whose surface is covered with –CH3 groups, in comparison with those covered with –OH groups. This is consistent with the authors’ previous observation that positronium slowing down is less efficient in the pores covered with –CH3 groups. The present work demonstrates that in the porosimetric application of positron annihilation lifetime spectroscopy, the interaction between positronium and the pore surface has to be properly taken into consideration.

Phosphorus gettering in multicrystalline silicon studied by neutron activation analysis

Neutron activation analysis (NAA) is a powerful and sensitive technique for measuring trace amounts of impurities. In this work, we have applied NAA to the problem of identifying metallic impurities within the bulk of solar-grade cast multicrystalline silicon (mc-Si) wafers. In particular, the change in concentrations of these contaminants after phosphorus gettering is monitored, revealing a marked reduction in some metal species, but not in others.

Formation of Crystalline SiC Buried Layer by High-Dose Implantation of MeV Carbon Ions at High Temperature

A buried layer of crystalline SiC in silicon wafer is synthesized by 1.5 MeV C+ implantation at a dose of 1.5×1018 ions/cm2 at a high temperature of 880°C. The infrared absorption spectrum and the X-ray diffraction pattern of this sample show formation of 3C-type SiC crystal. The pole figures of X-ray diffraction show that crystallographic orientation of the SiC buried layer is aligned along the lattice of the Si substrate, that is, topotaxial internal growth of crystalline SiC occurs in a single crystal of Si during the high-temperature ion implantation.