Materials Science Reports最新文献

A new thin film deposition technique has emerged, which is based on various photoinduced effects. It turned out that light can be used in a variety of ways as a unique energy source required to induce reactions leading to deposition. In this article, I describe the present status of the photoinduced deposition technique with emphasis on so-called photochemical vapor deposition (photo-CVD), which has attracted the greatest attention among various photoinduced methods. In this scheme, ultraviolet light is often used to photolyze source gases either in the gas phase or on substrate surfaces. In this case films can be deposited at low temperatures. If lasers are used in photo-CVD, a high degree of area selectivity can be obtained. In this case, pyrolytic reactions induced by substrate heating can be utilized, in addition to photolysis. Low-temperature deposition is possible with plasma processes, but photoexcited processes are characterized by selective excitation with monochromatic light. There are other related methods in photo-CVD, such as Hg photosensitization or vibrational excitation. As a completely different method pulsed-laser evaporation or laser sputtering of solid targets is available. In addition to deposition, the photoinduced effects are utilized to modify solid surfaces. These topics are briefly covered in this article, but photoinduced etching is not mentioned. Semiconductor, metal, and dielectric thin films deposited by various photoinduced methods are explained. This technique has been applied also to device fabrications. The choice of a proper light source is crucially important for successful deposition, and both traditional lamps and modem lasers have been used. In particular, lasers can be used to study reaction processes through spectroscopies, such as laser-induced fluorescence (LIF) or coherent anti-Stokes Raman spectroscopy (CARS).

The origins of the sensitivities of photoresists as well as electron-beam, X-ray and ion-beam resists are elucidated by means of the method of quantum chemistry which is independent of the experimental apparatus and technological errors. The origin of the dry-etch resistance of resist materials is also clarified by the same method. Utilizing the newly obtained concepts on the sensitivity and the dry-etch resistance, one has performed successfully a molecular design of dry-developable resists for photo-, deep-UV, e-beam and SR-X-ray lithographies. The dry-developable resists gave high sensitivity and highly remaining resist patterns in photo- and deep-UV lithographies, high resolution in e-beam lithography and steep-profiled resist patterns in SR lithography. Applications of photo- and radiation-sensitive materials in microlithography as well as techniques for the high resolution and the steep profile of resist patterns are reviewed. The method of quantum chemistry used in the research is also presented.

Craze formation and breakdown in polymers is reviewed with particular emphasis on the study of crazes by small angle scattering techniques. Small angle scattering, particularly when combined with information obtainable from transmission electron microscopy, has proved to be a powerful technique for the study of crazes. The existence of the three different radiations, X-rays, neutrons and electrons, has helped both in the interpretation of the basic form of craze scattering patterns and in permitting the study of a broad range of problems. X-ray scattering is particularly useful for the acquisition of information on crazes in bulk samples, this information being valuable to test and refine the models of craze formation and growth. The intensity of X-rays from synchrotron sources has permitted the study of such failure processes as mechanical fatigue and impact in real-time. Neutron radiation has proved useful in the study of environmentally-induced crazes while electron radiation is used for the examination of crazes in thin films. The majority of the work on crazing has been done in single phase amorphous polymers, particularly polystyrene and polycarbonate, but the rubber toughened styrenics, in which crazing is an important toughening mechanism, offer a fruitful field of study. It is in these latter systems, where intense crazing is seen, that impact processes can be studied by small angle scattering. Crazing and microcracking in semicrystalline polymers has also been examined by this technique but the lack of a clear picture of the morphology of the scattering entities has made small angle scattering less fruitful here than in amorphous polymers.

This review discusses the development of materials for fibres from early work in the 1960s on oxide glass, chalcogenide glass and crystalline halides which was way ahead of its time; through the huge development on silicate fibres which has led to our present-day new IR optical communications capability; to the frontiers of current research on mid IR fluoride glass fibres and far IR glass, hollow core and crystalline fibres. Fibre fabrication technology was rapidly developed during the 1970s allowing losses of a fraction of a dB/km to be achieved at near infrared wavelengths. In the 1980s optical communications have become possible using multimode silicate glass fibres in the near IR operating at 0.8-0.9 μm, at 1.3 μm and at 1.55 μm and using monomode fibres operating at 1.3 μm and 1.55 μm. The loss in these fibres at 1.55 μm is about 0.2 dB/km which is very nearly the intrinsic loss limit of GeO₂-SiO₂ glass. To achieve repeaterless transmittance over longer distances than possible with silicate fibres it is necessary to fabricate fibre from a lower loss medium. In the infrared spectral region attenuation in a material is dominated by Rayleigh scattering and multiphonon absorption. Thus if materials transmitting farther into the infrared can be utilised then potentially much lower losses can be achieved since Rayleigh scatter has a λ⁻⁴ dependence. It may be possible to achieve of the order of 10⁻² dB/km loss in a fibre at 2.55 μm and perhaps lower losses at longer wavelengths of 3 to 4.5 μm. For these reasons researchers are now addressing the problems of making mid IR fibres from amongst the fluorozirconate and fluorohafnate glasses. There are very different applications for fibres in the far infrared spectral region mainly requiring path lengths of a few centimeters to a few meters for sensor and power delivery devices. In spite of the fact that only short lengths of fibre are required, there are problems in finding sufficiently low loss materials which also demonstrate mechanical and environmental integrity. Research is being carried out amongst the chalcogenide glasses, hollow core oxide glasses, monocrystalline halides and polycrystalline halides since there is no obvious front running material which is able to satisfy all requirements.

A question of fundamental interest in ion implantation metallurgy concerns the lattice site which the implanted ions will occupy at the end of their trajectories. This review describes the results of a systematic study on the basic mechanisms which determine the lattice site occupation of impurities implanted in metals. Current models on the prediction of the substitutionality are reviewed and the mechanisms of impurity-point-defect interactions on the lattice site occupation are outlined. Recent experimental results are reviewed which demonstrate that implanted ions will preferentially occupy substitutional lattice sites within the relaxation phase of the collision cascade. Their displacements from the substitutional sites are due to the interaction with point defects which leads to the formation of defect-impurity complexes. These processes occur during the cooling phase of the cascade and at temperatures at which point defects are mobile. The probability of the complex formation increases as a function of the heat of solution and the size-mismatch energy.

Modern permanent magnet devices require the presence of large coercive forces and in turn this requires the presence of magnetocrystalline anisotropies. Favourable candidates for sufficiently large anisotropies are rare-earth-base-materials, where this property originates from a combination of the crystal-field interaction of the 4f electrons with electrostatic charge of the surrounding ions and the relatively strong spin-orbit interaction of the 4f electrons. A sufficiently high magnetization and magnetic ordering temperature is guaranteed by combining rare-earth elements with 3d transition metals. More than a decade ago high-performance permanent magnets were based on Sm and Co, (SmCo₅). Recently and even more powerful permanent magnet material was discovered which is based primarily on the ternary intermetallic compound Nd₂Fe₁₄B and which has procreated considerable scientific and technological interest. In this review a description will be given of the basic properties of rare-earth compounds of the type R₂Fe₁₄B,R₂Co₁₄B and several related intermetallic compounds. This description comprises crystal structure, phase relationships, magnetization, magnetic structure and magnetic anisotropy. The properties of all these materials will be compared and discussed in terms of magnetic exchange interaction and crystal-field theory. A substantial part of the paper will be devoted to permanent magnet fabrication and includes a discussion of the various coercivity mechanisms and their relation to the microstructure.

The motivation for understanding the physics and chemistry of the SiO₂/Si interface lies in the pivotal role it plays in current metal-oxide-semiconductor (MOS) technology. In this paper, we are concerned with the chemical structure of this interface and its relationship to both MOS device processing chemistry and, ultimately, the resultant electrical device properties. Emphasis is placed on the use of X-ray photoemission to probe the structure of the SiO₂ near the SiO₂/Si interface as well as the composition of the SiO₂/Si chemical transition boundary itself. Complementary data from a wide range of other techniques such as ²⁹Si NMR, ellipsometry, SEXAFS, and a variety of electrical probes are also considered. The topics discussed include the presence of a structurally-distinct region of SiO₂ near the interface and its effect on the SiO₂ band gap, the distribution and crystallographic dependence of suboxide states at the monolayer SiO₂/Si transition boundary, the effect of electron irradiation on the SiO₂ network structure, the influence of hydrogen on the SiO₂ valence band discontinuity between SiO₂ and Si, and the influence of processing chemistry on the chemical and electronic structure of the interface.