In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

Ordered magnetic nanostructures: fabrication and properties

The phase-shifting mask consists of a normal transmission mask that has been coated with a transparent layer patterned to ensure that the optical phases of nearest apertures are opposite. Destructive interference between waves from adjacent apertures cancels some diffraction effects and increases the spatial resolution with which such patterns can be projected. A simple theory predicts a near doubling of resolution for illumination with partial incoherence σ < 0.3, and substantial improvements in resolution for σ < 0.7. Initial results obtained with a phase-shifting mask patterned with typical device structures by electron-beam lithography and exposed using a Mann 4800 10× tool reveals a 40-percent increase in usuable resolution with some structures printed at a resolution of 1000 lines/mm. Phase-shifting mask structures can be used to facilitate proximity printing with larger gaps between mask and wafer. Theory indicates that the increase in resolution is accompanied by a minimal decrease in depth of focus. Thus the phase-shifting mask may be the most desirable device for enhancing optical lithography resolution in the VLSI/VHSIC era.

Improving resolution in photolithography with a phase-shifting mask

A direct, accurate and convenient procedure for calibrating the spring constants of probes used for force microscopy/spectroscopy is described. It amounts to deflecting an 'unknown' cantilever with a 'standard' lever, where the standard lever has been precalibrated. The absolute and relative accuracies of the procedure are ±20-30% and ±10-20%, respectively; the former is limited by uncertainties in the determination of the spring constant for the 'standard' lever, as well as that for the 'unknown'. The method differs from others of the static deflection variety by its exploitation of the routine features of current instruments. The technique is compared with other static and dynamic methods currently being used.

https://iopscience.iop.org/article/10.1088/0957-4484/7/3/014/pdf

Determination of the spring constants of probes for force microscopy/spectroscopy

An antireflection surface with sub-wavelength structure has been successfully fabricated on a fused silica substrate. The fabricated antireflection structured surface consists of a microcone array of fused silica with a period shorter than the wavelengths of visible light. The microcone array is made by a reactive ion etching method using fluorocarbon plasma. A microdisk array of chromium thin film, formed by an electron-beam lithography and lift-off process, is used as the etching mask. Since an electric field induced near the substrate was focused on the edges of the metal disks, these disks gradually shrank. Consequently, a conical shape was formed. The fabricated cone array has a period of 250 nm and a height of 750 nm. Measured reflectivity of the antireflection structured surface is less than 0.5% in the wavelength range of 400–800 nm for normal incidence.

Fabrication of Microcone Array for Antireflection Structured Surface Using Metal Dotted Pattern : Optics and Quantum Electronics

We have investigated the emission of the terahertz electromagnetic wave from an undoped GaAs (200nm)∕n-type GaAs (3μm) epitaxial layer structure (i-GaAs∕n-GaAs structure), where the doping concentration of the n-GaAs layer is 3×1018cm−3. It is found that the first-burst amplitude of terahertz wave of the i-GaAs∕n-GaAs sample is remarkably larger than that of a n-GaAs crystal, which means that the i-GaAs layer enhances the terahertz emission intensity. The first-burst amplitude of the i-GaAs∕n-GaAs sample, by tuning the pump-beam energy to the higher energy side, exceeds that of an i-InAs crystal that is known as one of the most intense terahertz emitters. We, therefore, conclude that the i-GaAs∕n-GaAs structure is useful to obtain intense terahertz emission.

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Enhancement of terahertz electromagnetic wave emission from an undoped GaAs∕n-type GaAs epitaxial layer structure

The formation of porous structures of nanometre size (nanoporous structures) on germanium (Ge) surfaces by focused ion beam (FIB) irradiations was investigated using various FIB conditions such as ion species, irradiation energies, total fluences, fluence rates, and incident angles. FIB-irradiated regions were observed using a scanning electron microscope and an atomic force microscope. It is found that, using a focused Ga ion beam (Ga FIB) at an energy of 100 keV, the irradiated Ge surface swelled up to ion fluence of 2 × 1017 cm−2 with nanoporous structures and then was etched for larger fluences. The shape of swollen nanoporous structures depended on the fluence rate and the incident angle of the Ga FIB. However, such porous structures were observed neither for low-energy (15–30 keV) FIB irradiations using Si and Au ions nor for high-energy (200 keV), heavy ion (Au) irradiation. These observations might be helpful in discussing the formation mechanisms of the nanoporous structures on Ge surfaces by ion beam irradiations. Fabrication of patterned structures at selected regions on the Ge surface was demonstrated without using any masks.

http://iopscience.iop.org/article/10.1088/0953-8984/19/44/445002/pdf

Nanoporous structure formations on germanium surfaces by focused ion beam irradiations

We have measured the ballistic length lbFIB of a GaAs/AlGaAs sample using the electron focusing effect and the mean free path leFIB of the narrow channel, both formed by focused ion beam (FIB) irradiation, to estimate the damage induced by FIB irradiation. It is observed that scattering centers are induced by FIB irradiation, which exhibit dependence on the electron density, unlike scattering centers due to grown-in defects. The FIB-induced scattering centers distribute far beyond the distance of the FIB spot size. This may be due to the exponential tail distribution of FIB.

Estimation of Damage Induced by Focused Ga Ion Beam Irradiation

Maskless patterning of spin-on glass (SOG) by focused ion beam (FIB) irradiation was investigated. Silanol-type SOG and 100 keV Ga FIB were employed as SOG material and ion beam, respectively. Heat treatment up to 850°C was also performed for comparison with ion beam irradiation effect. The spectra of Fourier transform infrared (FTIR) transmission and X-ray photoelectron spectroscopy (XPS) for SOG material showed that the crosslinking reaction was induced in SOG by ion beam irradiation, which is almost the same behavior as that induced by the heat treatment. As a result, SOG can be used as a negative-type ion beam resist and patterned masklessly by FIB irradiation. The threshold dose for the exposure of SOG film using FIB was about 1.5 µC/cm2 and almost 2-3 orders of magnitude less than that for an electron beam (EB). A 0.15-µm line-and-space (L/S) pattern was delineated by 100 keV Ga FIB irradiation.

Direct Patterning of Spin-on Glass by Focused Ion Beam Irradiation

A small conductive island which is connected with source and drain via small tunnel junctions is a fundamental for single electron tunneling (SET) devices. An example of such structures consists of metal/insulator/metal double tunnel junctions and a metal oxide can be used as an insulator. Control of SET by the spin of tunneling electrons is possible by using magnetic metals as the island material, source and drain electrodes. However, microfabrication processes of magnetic metals for SET structures are not well established yet. We have proposed a new in situ process to fabricate small double tunnel junctions using a focused ion beam (FIB). This process is made up of following steps: after deposited metal films (Ni) on an insulator substrate are patterned to have a narrow channel (50 /spl mu/m in the present study) by photolithography was formed, a resist layer is spin-coated for the next lift-off step. The resist used is a double layer resist consisting of nitrocellulose and germanium films. Nitrocellulose can be used as a high-sensitive, selfdeveloping ion beam resist. 100 keV Ga FIB is irradiated across the channel to separate the metal channel and to form source and drain electrodes. To minimize the sputtered groove width which determines the island size, the resistance of the metal channel is monitored during the sputtering step and the FIB irradiation is ended just when the resistance become sharply infinite.

https://iopscience.iop.org/article/10.1143/JJAP.38.7151/pdf

Junichi Yanagisawa

Papers

Enhancement of terahertz electromagnetic wave emission from an undoped GaAs∕n-type GaAs epitaxial layer structure

Nanoporous structure formations on germanium surfaces by focused ion beam irradiations

Estimation of Damage Induced by Focused Ga Ion Beam Irradiation

Direct Patterning of Spin-on Glass by Focused Ion Beam Irradiation

Focused ion beam process for a formation of metal/insulator/metal double tunnel junctions