Hiromichi Isogai

Electrical Characterization of Wafer-Bonded Germanium-on-Insulator Substrates Using a Four-Point-Probe Pseudo-Metal--Oxide--Semiconductor Field-Effect Transistor

The electrical characteristics of wafer-bonded non-doped germanium-on-insulator (GOI) substrates were investigated using a four-point-probe pseudo-metal–oxide–semiconductor field-effect transistor. Annealing the wafer-bonded GOI substrates in vacuum strongly influenced their electrical characteristics. GOI samples annealed at temperatures below 500 °C exhibited n-channel depletion transistor operation, whereas GOI samples annealed at temperatures between 550 and 600 °C exhibited p-channel depletion transistor operation. The carrier mobility strongly depended on the sweep direction of the gate voltage; this characteristic disappeared after annealing at temperatures above 550 °C. The dependence of the electrical characteristics on the annealing temperature is explained in terms of the influence of the defect states on energy band bending near the interface.

Annealing Effects on Ge/SiO2 Interface Structure in Wafer-Bonded Germanium-on-Insulator Substrates

We have investigated annealing effects on Ge/SiO2 interfaces in wafer-bonded germanium-on-insulator substrates using transmission electron microscopy and electron energy loss spectroscopy. A number of nanometer-sized hollows were observed at the Ge/SiO2 interfaces after annealing at 500 and 600 °C, while the density of these hollows was very small after annealing at 700 and 800 °C. The hollows are attributed to the formation of amorphous oxides of Si-rich Si1-xGexO2. The mechanism for the formation and disappearance of these amorphous hollows on the Ge substrates is discussed.

Recovery of microstructure and surface topography of grinding-damaged silicon wafers by nanosecond-pulsed laser irradiation

Single crystalline silicon wafers whose surfaces were machined by diamond grinding were irradiated by a nanosecond-pulsed Nd:YAG laser. Changes in the subsurface crystallinity and surface topography were investigated by transmission electron microscopy and atomic force microscopy. It was found that the grinding process gave rise to amorphous layers, dislocations and micro cracks. However, all of this damage could be eliminated by a single laser pulse of suitable energy density, which also led to a remarkable amount of smoothing of the wafer surface. When excessively high energy densities were used, tiny particles were found to form on the wafer surface. It is speculated that these particles are produced by the recondensation of silicon boiled away from the wafer surface during the laser pulse. The temperature rise during laser irradiation was estimated using a simplified model. The results obtained in this study suggest that nanosecond-pulsed laser irradiation may be an effective approach for processing grinding-damaged silicon wafers.

Effect of Hydrogen Termination on Surface Roughness Variation of Si(110) by Reflow Oxidation during High-Temperature Ar Annealing

It is well known that a smooth surface can be realized for silicon (Si) wafers by Si surface reconstruction using high-temperature annealing. We previously reported that it is crucial to maintain a smooth reconstructed surface to restrict accidental oxidation during the unloading process (i.e., reflow oxidation) in high-temperature annealing. The surface roughnesses of both Si(100) and Si(110) were proved by suppressing the reflow oxidation. Furthermore, for suppressing the reflow oxidation, we evaluated the thickness of the reflow oxidation layer and the surface structure of the Si(110) wafer by replacing the injected Ar gas with H2 in the cooling process during high-temperature Ar annealing. The H2 atmosphere condition induced a change by etching the reconstructed surface, and the H-terminated surface on Si(110) formed SiH2, which effectively suppressed the reflow and characteristic line oxidations, resulting in a smooth terrace-and-step structure.

Effect of Reflow Oxidation on Si Surface Roughness during High-Temperature Annealing

It is well known that a smooth surface of silicon (Si) wafers can be obtained by Si surface reconstruction using high-temperature annealing. However, there is a possibility that smooth Si surfaces are deteriorated by oxidation (called reflow oxidation) during unloading after the high-temperature annealing. Therefore, it is important to investigate the effect of oxidation on the surface steps and terraces on Si wafers at the atomic level during unloading. We have examined the effect of unloading temperature after Ar annealing on oxide formation on the surfaces of Si(100) and Si(110) substrates. The change in the surface roughness was also measured. Our results indicate a significant improvement with rms values of 0.01 nm for both Si(100) and Si(110) wafer surfaces upon low-temperature unloading. A very flat surface with an rms value of 0.046 nm was achieved for a Si(100) wafer.

Mechanical Properties and Chemical Reactions at the Directly Bonded Si–Si Interface

Directly bonded interfaces of hydrophilic and hydrophobic Si(100) wafers were studied from the viewpoint of bonding energy and chemical products as a function of the annealing temperature. The experimental results indicated that for both hydrophilic and hydrophobic Si/Si bonded wafer pairs, the behavior of the bubbles at the bonding interface and the bonding energy were closely related to the behavior of the hydrogen and oxygen atoms at the bonding interface. The bonding mechanisms for both cases have been discussed on the basis of the chemical reactions induced by the annealing temperature.

Characterization and Analyses of Interface Structures in Directly Bonded Si(011)/Si(001) Substrates

We investigated the interface structure of directly bonded Si(011)/Si(001) substrates prepared by conventional bonding and grind-back. The interfacial structure was analyzed by transmission electron microscopy (TEM) and in-plane X-ray diffraction (XRD). The plan-view and cross-sectional TEM observations provided evidence that screw dislocation lines were localized to the interfacial plane and that threading dislocations were absent. Grazing-incidence in-plane XRD analyses confirmed the existence of mosaic structures at the interface. These structures were formed because of the deformation field produced by the screw dislocations. This allowed a high level of crystallinity to be maintained in regions away from the interface in both the Si(011) layer and the Si(001) wafer.

Characterization of microparticles and oxide layers generated by laser irradiation of diamond-machined silicon wafers

Nanosecond-pulsed laser irradiation is a potential method for removing machining-induced subsurface damage from silicon wafers. In this study, the material compositions and microstructures of microparticles and oxide layers generated during laser irradiation were investigated by atomic force microscopy, energy-dispersive x-ray spectroscopy, cross-sectional transmission electron microscopy, electron energy-loss spectroscopy and Auger electron spectroscopy. The oxide layer was found to be approximately 5 nm thick, which is significantly thicker than the native oxide layer of silicon at room temperature in air (~1 nm). The microparticles have a low-density amorphous structure and are mainly composed of silicon oxide, while a few particles contain silicon. The particles are attached to the substrate, but are distinct from it. The results indicate that silicon boiled during the laser pulse and that the particles are recondensed and oxidized liquid silicon boiled away from the wafer surface. The microparticles can be completely removed from the wafer surface by hydrofluoric acid etching.

Microscopic Structure of Directly Bonded Silicon Substrates

Using X-ray microdiffraction (XRMD) and transmission electron microscopy (TEM) techniques, we have investigated the microscopic structure of Si(011)/Si(001) direct silicon bonding (DSB) substrates. XRMD was performed to measure the local lattice spacing and tilting in the samples before and after oxide out-diffusion annealing. Diffraction analyses for (022) lattice planes with two orthogonal in-plane directions of X-ray incidence revealed anisotropic domain textures in the Si(011) layer. Such anisotropy was also confirmed by TEM in the morphology at the Si(011)/Si(001) bonded interface. The anisotropic crystallinity is discussed on the basis of interfacial defect structures which are proper to the DSB substrate.

Cross‐sectional TEM Observations of Si Wafers Irradiated With Gas Cluster Ion Beams

Irradiation by a Gas Cluster Ion Beam (GCIB) is a promising technique for precise surface etching and planarization of Si wafers. However, it is very important to understand the crystalline structure of Si wafers after GCIB irradiation. In this study, the near surface structure of a Si (100) wafer was analyzed after GCIB irradiation, using a cross‐sectional transmission electron microscope (XTEM). Ar‐GCIB, that physically sputters Si atoms, and SF6‐GCIB, that chemically etches the Si surface, were both used. After GCIB irradiation, high temperature annealing was performed in a hydrogen atmosphere. From XTEM observations, the surface of a virgin Si wafer exhibited completely crystalline structures, but the existence of an amorphous Si and a transition layer was confirmed after GCIB irradiation. The thickness of amorphous layer was about 30 nm after Ar‐GCIB irradiation at 30 keV. However, a very thin (< 5 nm) layer was observed when 30 keV SF6‐GCIB was used. The thickness of the transition layer was the sam...