H. Kuramochi

Current, charge, and capacitance during scanning probe oxidation of silicon. I. Maximum charge density and lateral diffusion

A comprehensive analysis of the electrical current passing through the tip-substrate junction during oxidation of silicon by scanning probe microscopy (SPM) is presented. This analysis of experimental results under dc-bias conditions resolves the role of electronic and ionic contributions, especially for the initial stages of the reaction, determines the effective contact area of the tip-substrate junction, and unifies the roles of space charge and meniscus formation. In Part I of this work, we demonstrate that SPM oxidation is governed by a maximum charge density generated by electronic species within the junction at the onset of the oxidation process. Excess charge is channeled into lateral diffusion, keeping the charge density within the reaction zone constant and reducing the aspect ratio of the resulting oxide features. A uniform charge density implies that SPM oxides contain a fixed defect concentration, in accordance with the space-charge model. The effective (electrical) thickness of SPM oxides de...

A magnetic force microscope using CoFe-coated carbon nanotube probes

Ferromagnetic-film-coated carbon nanotube (CNT) probes were employed in a magnetic force microscope (MFM) observation. We succeeded in making a uniform ferromagnetic film on the CNT probes by improving the coating process and selecting materials. The performance of the CoFe-coated CNT probe was evaluated in ultra-high-density perpendicular magnetic storage media with densities from 600 to 1100?k flux changes per inch (FCI). The magnetic domain structure of the magnetic storage media was clearly observed up to 1100?kFCI. The ultimate lateral resolution of the newly developed MFM probes is down to about 10?nm, which exceeds the bit length of a magnetic recording with a density of Tbit?inch?2.

Faradaic current detection during anodic oxidation of the H-passivated p-Si(001) surface with controlled relative humidity

Faradaic current during anodic oxidation is measured over a relative humidity range of 40–70% using an atomic force microscope with humidity control. The level of detected current during the fabrication of oxide dots on H-passivated Si(001) is in the picoampere (pA) level. Current flow began immediately (within a few milliseconds) after applying an oxidation voltage above a threshold value and decreased with time according to oxide growth. The total charge resulting from the current flow was calculated by integrating the current–time curve and was found to agree well with an estimation of expected current from the volume of the fabricated oxide dots. Actual monitoring of the oxidation process by the Faradaic current is demonstrated during the fabrication of a two-dimensional lattice.

Measuring electrical current during scanning probe oxidation

Electrical current is measured during scanning probe oxidation by performing force versus distance curves under the application of a positive sample voltage. It is shown how the time dependence of the current provides information about the kinetics of oxide growth under conditions in which the tip–surface distance is known unequivocally during current acquisition. Current measurements at finite tip–sample distance, in particular, unveil how the geometry of the meniscus influences its electrical conduction properties as well as the role of space charge at very small tip–sample distances.

Current, charge, and capacitance during scanning probe oxidation of silicon. II. Electrostatic and meniscus forces acting on cantilever bending

A comprehensive analysis of the electrical current passing through the tip-substrate junction during oxidation of silicon by scanning probe microscopy (SPM) is presented. This analysis identifies the electronic and ionic contributions to the total current, especially at the initial stages of the reaction, determines the effective contact area of the tip-substrate junction, and unifies the roles of space charge and meniscus formation. In this work, we concentrate on noncontact SPM oxidation. We analyze simultaneous force-distance and current-distance curves to demonstrate that total current flow during noncontact oxidation is significantly less for noncontact mode than for contact oxidation, although the resulting oxide volume is nearly identical. Ionization of water layers and mobile charge reorganization prior to and following meniscus formation is also shown to alter the tip-substrate capacitance and, therefore, the bending of the SPM cantilever.

CoFe-Coated Carbon Nanotube Probes for Magnetic Force Microscope

CoFe-coated carbon nanotube (CNT) probes have been successfully fabricated by a radio-frequency sputtering method. The sputtering conditions for obtaining a uniform ferromagnetic film on the CNT probe were investigated. The optimal sputtering conditions were determined from the observations using a scanning electron microscope and an atomic force microscope. CNT magnetic force microscope (MFM) probes prepared under optimized conditions provided us clear images with an ultimate lateral resolution of approximately 10 nm.

In situ detection of faradaic current in probe oxidation using a dynamic force microscope

A faradaic current on the order of a sub-pico-ampere was detected while fabricating two-dimensional oxide nanostructures on H-passivated Si(001) surfaces. The detected faradaic current has been shown to faithfully reflect the degree of probe oxidation with a clear dependence on the variation of voltage and the tip speed. The faradaic current in dynamic mode can serve as a sensitive monitor of the nano-oxidation reaction for implementing precise closed-loop control of the oxide growth.

Nano-oxidation and in situ faradaic current detection using dynamic carbon nanotube probes

Carbon nanotube-attached atomic force microscope probes were successfully used without nanotube bending to make simultaneous precision nano-oxidation and faradaic current measurements in the dynamic mode. Probe oxidation on H-passivated Si(001) surfaces was carried out by two methods involving vector-scan and raster-scan with a much higher resolution and precision compared to the nanofabrication by standard cantilevers. Faradaic current of the order of a sub-picoampere was detected during nano-oxidation using a carbon nanotube probe, accurately reflecting the subtle difference in the oxidation reaction. The minute current detection through the AFM tip is sensitive enough for the detection of very thin oxides and small-sized features. The dimension of the meniscus during nano-oxidation, which is indispensable for establishing the mechanism model, was evaluated, based on the in situ faradaic current detection and edge broadening.

Why nano-oxidation with carbon nanotube probes is so stable: I. Linkage between hydrophobicity and stability

Carbon nanotube (CNT) probes enhance the stability of the nano-oxidation process under dynamic-mode operation. In this paper we investigate how the hydrophobic nature of the CNT allows oxide nanostructures to be fabricated with constant aspect ratio over a wide range of relative humidity values. In particular, we characterize oxide growth by measuring both the integrated ionic current and volume expansion. Behaviour of different CNT probes was compared to assess individual stability and performance under identical voltage and humidity conditions. While much remains to be established about the relationship between exposure conditions and dynamic-mode parameters on nanoscale oxide fabrication, hydrophobicity is a key factor in the improved reliability of CNT probes over conventional ones.

Perpendicular magnetic properties of CoCr films on GaAs

CoCr films were deposited on three types of GaAs substrates, GaAs(001), GaAs(111), and Al oxide∕GaAs(001). The perpendicular magnetic properties were investigated by magneto-optical Kerr-effect measurements. The direct deposition of the CoCr film on the GaAs substrate did not show any perpendicular magnetic properties. This fact indicates that the lattice distortion influenced by the GaAs lattice suppresses the perpendicular magnetism. The CoCr film on the Al oxide layer showed a tilted squarelike hysteresis loop. The thickness dependence of the hysteresis loop and the magnetic force microscopy showed that the onset thickness of ferromagnetism was 6.5nm. The domain size of the CoCr films monotonously decreases with the increasing thickness (6.5–75nm).