Masahide Gotoh

Roughening of the Si/SiO2 interface during SC1-chemical treatment studied by scanning tunneling microscopy

The Si/SiO2 interface morphology is observed with subnanometer resolution by an ultrahigh vacuum scanning tunneling microscope (STM). We analyze the roughness of the Si/SiO2 interface for a chemical oxide film formed by a wet chemical process (NH4OH/H2O2/H2O treatment). The oxide film is selectively removed by irradiating a field emission electron beam extracted from a STM tip at a temperature of 300–350 °C. We find that during the chemical process the roughness of the Si/SiO2 interface increases with the treatment time.

Analysis of SiO2/Si(001) interface roughness for thin gate oxides by scanning tunneling microscopy

We studied the interface roughnesses of SiO2/Si(001) for gate oxides of 8 and 15 nm thicknesses together with RCA-treated samples by using scanning tunneling microscopy (STM). By STM observation and scaling analysis we made clear that the interface roughnesses of thermal oxides/Si substrates were similar to each other and to that of the chemical oxide/Si substrate prior to thermal oxidation; the correlation length was 23–26 nm and the rms roughness at length scales larger than the correlation length was 0.28–0.29 nm. The results indicate that the interface roughnesses of the oxides are determined by the processes prior to the oxidations.

Quantum Yield of Electron-Beam Induced Decomposition of SiO2 Overlay on Si in Nanolithography Using a Scanning Tunneling Microscope

We evaluate the quantum yield of SiO2 decomposition caused by electron-beam irradiation from the tip apex of a scanning tunneling microscope over an electron energy range of 10–180 eV and find onsets at 40 and 120 eV. These onsets are close to those found previously for electron-beam induced SiO2 dissociation by Auger electron spectroscopy and electron stimulated desorption. Based on the excitation function, we consider that the decomposition is activated by core level excitations like the Knotek–Feibelman mechanism.

Electron-Beam-Induced Decomposition of SiO2 Overlay on Si in STM Nanolithography

Combined with thermal annealing, a locally confined low energy electronbeam from a STM tip can be used for nanofabrication of a Si oxide film on a Si substrate: the oxide layer within the e-beam exposed area can be decomposed and then evacuated from the surface at elevated temperatures above 300°C. We evaluate the quantum yield of the SiO2 decomposition over an electron energy range of 10 to 180 eV and find onsets near 30 and 120 eV. These onsets are close to those found previously for electron beam induced SiO2 dissociation by Auger electron spectroscopy and electron stimulated desorption. The SiO2 decomposition cross-section evaluated is about 10-21 cm2 at 150 eV which also agrees with the previously reported values. These findings indicate that the Si oxide layer is decomposed by electron irradiation in a STM nanolithography process by a Knotek-Feibelman-like mechanism: primary ionization of high-lying core levels, followed by Auger processes which lead to localized valence holes, constitutes important channels for SiO2 dissociation.

Anisotropic Transformation of 4H-SiC Etching Shapes by High-Temperature Annealing and Its Enhancement by Ion Implantation

The transformation of 4H-SiC etching shapes by high-temperature annealing was investigated. Although the opening of the etching mask was circular, the resulting etched shape was a hexagon, dodecagon, or rounded polygon with more edges, depending on the diameter. A hexagon was transformed into a dodecagon following high-temperature annealing, and a dodecagon was transformed into a rounded polygon.

Interface Morphology of Thermal-Oxide/Si(001) Studied by Scanning Tunneling Microscopy.

The SiO2/Si interface morphology is observed with subnanometer resolution using an ultrahigh vacuum scanning tunneling microscope (STM). The oxide films are formed on flash-cleaned Si(001) surfaces by introducing O2 gas into an ultrahigh vacuum (UHV) chamber at 600°C, at room temperature, and at 600°C after oxidation at room temperature. The oxide overlayers are selectively removed by irradiation with a field-emission electron beam extracted from the STM tip at a substrate temperature of 300°C. We find that for the oxide overlayer formed under a saturation condition at room temperature, there are many islands and vacancies on the terraces and the terrace–step structure is hardly recognizable. When the sample is further oxidized at 600°C for 10 min at a pressure of 2.0 ×10-4 Pa, we find that the terrace–step structure becomes conspicuous, in good agreement with the previous observations by Watanabe et al. [Phys. Rev. Lett. 80 (1998) 345].

Effect of Surface Structure on Transformation of 4H-SiC by High-Temperature Annealing

The transformation of SiC etching shapes by high-temperature annealing has been investigated. Without silicon atoms on the surface, transformation of the etching shapes hardly occurred even after annealing in pure Ar at 1700 °C, where transformation should occur without the loss of silicon atoms. When SiH4 was added to Ar, the surface tended to revert to SiC, and the transformation was enhanced with increasing SiH4 addition. Therefore, the presence of silicon atoms is necessary to transform the etching shapes on SiC surfaces.