Atsushi Kikukawa

Silicon pn junction imaging and characterizations using sensitivity enhanced Kelvin probe force microscopy

We observed the surface potential of silicon pn junctions using a Kelvin probe force microscope whose sensitivity was about five times better than that of a conventional one. It was achieved by three major improvements: electrostatic force detection using the second cantilever resonance, cantilever Q‐value enhancement by operating in a vacuum, and direct cantilever resonance frequency detection using the frequency modulation technique. It was demonstrated that the surface potential of the pn junctions made by thermal diffusion varies gradually compared to those made by ion implantation, possibly reflecting their gradual dopant concentration profile.

Vacuum compatible high‐sensitive Kelvin probe force microscopy

A vacuum compatible Kelvin probe force microscopy (KPFM) is presented. Difficulties in operating KPFM in a vacuum were overcome by utilizing the direct cantilever resonance frequency detection in the tip height control whereas the indirect resonance frequency detection scheme was used in primordial KPFM. The potential measurement sensitivity was improved by 14 dB compared to that in air. It is due to the increased cantilever Q value and the reduction in the interference from the tip height detection signal because potential measurement is conducted using the cantilever’s second resonance while tip height control was conducted using the first resonance. A silicon wafer whose surface is partially doped with arsenic by ion implantation was observed, and surface potential difference at the junctions were clearly imaged.

Phase change recording using a scanning near‐field optical microscope

The formation and observation, with reflected light, of 60‐nm‐diam phase‐changed domains in a thin GeSbTe film using a scanning near‐field optical microscope with a 785 nm wavelength laser diode is demonstrated. The dependence of the domain size on incident laser power was obtained, and the size changed from 150 to 60 nm in diameter with incident power of 8.4–7.3 mW in the probe. At the threshold power of 7.3 mW, the film temperature rose to around 180 °C to partially phase change the local area of the film from amorphous to crystalline. A detected reflectivity increase due to phase change in the formed domain was 8%–2%. The observing (reading) was performed with an incident laser power of 0.2 mW, which corresponds to 10−2–10−3 times less than in a magneto‐optical recording. The incident laser power shows that the phase change reading using the reflection scanning near‐field optical microscope has the potential to read the recorded bit at a speed over 10 MHz.

SPM-based data storage for ultrahigh density recording

The possibility of SPM-based data storage is described regarding both its recording density and readout speed for ultrahigh density data storage. We consider their gap control to achieve high-speed readout. Suitable SPM-based storages are selected and their details are studied. As a result, scanning near-field optical microscope (SNOM)- and atomic force microscope (AFM)-based storages are expected to be candidates for future storage. SNOM-based storage is for . AFM-based storage is for . Using new force modulation AFM pit recording, an ultrahigh recording density of and a readout speed of are demonstrated.

Noise characteristics of double-layered perpendicular media using novel soft magnetic underlayer materials

The read/write characteristics of perpendicular media with different soft magnetic underlayer (SMU) materials (Co-Nb-Zr, Co-Ta-Zr, Fe-Ta-C, and Fe-Al-Si), and identical recording layers were investigated using a spin-stand and a merged-type MR head. It was observed that there are strong correlations between the SMU materials and noise characteristics, and found that Co-Ta-Zr and Fe-Ta-C underlayers yield the lowest noise when combined with a Co-Cr-Pt-Ta magnetic recording layer. An advanced medium, made based on those findings, was tested and a maximum D/sub 50/ of 275 kFCl was observed by using a single pole-type writer and a GMR reader with a shield gap length of 80 nm.

Megahertz silicon atomic force microscopy (AFM) cantilever and high-speed readout in AFM-based recording

Small atomic force microscopy (AFM) cantilever with a maximum resonant frequency of about 6.6 MHz was developed using a Si microprocess with silicon-on-insulator wafer. The cantilever was triangular in shape with a length of 7–20 μm and a thickness of <0.3 μm and had a tip on the lever. These structures were precisely fabricated by buffer step and prestep and using dual-side aligner. Furthermore, we developed a new optical lever deflection detection system for the small cantilever with a magnifying power of 0.2 μm/A and a spot size of 2 μm. We demonstrate a high resonant frequency of the cantilever and a high speed readout at higher than 5 Mb/s using the prototype of rotation AFM recording system with a new detection system.

Nanometer Recording on Graphite and Si Substrate Using an Atomic Force Microscope in Air

Nanometer recording has been demonstrated with tens nanometer diameter pits and gold mounds formed on graphite and Si substrate using an atomic force microscope at atmospheric pressure. The probe is prepared by means of coating thin gold film on an SiO2 birdbeak-type cantilever probe, fabricated by a Si microprocess. Applications of voltage pulses between the probe and the graphite make about 10-nm diameter pits and Au mounds. Furthermore, about 50-nm to 30-nm diameter Au mound formations on Si wafer covered with natural silicon oxide are also demonstrated. The results indicate that the technique has potentials to achieve Tera-bit/in2 highly packed storage in air, and directly to write nanometer sized patterns on an insulating thin film.

Control of Aperture Size of Optical Probes for Scanning Near-Field Optical Microscopy Using Focused Ion Beam Technology

We propose a fabrication technique for apertures of optical probes for scanning near-field optical microscopy (SNOM) using a focused ion beam (FIB) process. We tried two FIB processes, FIB drilling and FIB slicing. The FIB slicing technique is very useful for fabrication of nm-sized SNOM apertures of less than 50 nm. The problem with the FIB drilling process is that it is difficult to identify the apex of the tip and to control the beam onto the apex. The FIB slicing technique can easily fabricate an aperture at an apex and control aperture size by cut-off-depth. It is easy for a sharp tip to obtain accurate size of aperture. It can be considered to obtain accurate size of aperture with fabricated error of 35 nm in a sharp tip with cone angle of 30 deg.

Magnetic force microscope using a direct resonance frequency sensor operating in air

A magnetic force microscope (MFM) using a direct resonance frequency sensor which can be operated in the air has been developed. This instrument is simple and easy to handle because a vacuum system is not needed. The cantilever is used as a resonator in the oscillator, and its oscillating frequency is detected by a frequency modulation (FM) demodulator. A phase locked loop (PLL) FM demodulator is used which is less affected by noise. The cantilever is fabricated by Si microprocessing and its tip formed by precise focused ion beam (FIB) milling. MFM images demonstrate that this instrument has a performance almost the same as that which can be achieved in vacuum. It also has a scanning speed which is approximately 10 times as fast as that of an instrument using conventional slope detection techniques, and its signal‐to‐noise ratio is comparable to that of conventional systems.

Reduction of spike noise in perpendicular recording media by using MnIr antiferromagnetic films

We introduced a NiFe/antiferromagnetic–MnIr bilayer or a NiFe/MnIr/NiFe trilayer below a CoTaZr soft magnetic underlayer in perpendicular recording media as a way of controlling the magnetic domain structure of the soft magnetic underlayer, and we investigated the effect of exchange biasing on the spike noise. Samples consisting of a layer structure—NiFe (5 nm thick)/MnIr (2.5–50 nm)/NiFe (5 nm)/CoTaZr (50–200 nm)—were sputter deposited on precoated glass disks. The samples were heated with a lamp heater and cooled in a magnetic field along the radial direction of the disk. Both uniaxial and unidirectional anisotropies were induced along the magnetic field when the thickness of the MnIr layer was more than 5 nm. The first NiFe layer promoted a fcc–MnIr (111) crystalline texture, while the second NiFe layer enhanced the value of exchange-bias field by about 20%. The exchange-bias field increased from 6 to 24 Oe as the CoTaZr-layer thickness decreased from 200 to 50 nm. Many spikes along the radial direction were observed for a 100-nm-thick CoTaZr single-layer film, while no remarkable spikes were observed for a NiFe/MnIr/NiFe/CoTaZr (100 nm) film. It was found that the NiFe/MnIr/NiFe trilayer restrained the formation of domain walls in the CoTaZr layer, thereby reducing the spike noise.