Three important topics regarding the use of silicon crystals for applications in the electronics industry are reviewed in this paper. These are: (1) the perfection of silicon crystals required for ULSI device processing, (2) issues related specifically to large diameter crystal growth, and (3) the use of bonded wafers with SOI structures. There is no single silicon crystal specification that is ideal for all device processes. This is due to the facts that different processes have different thermal cycles and that contamination levels can vary from one facility to another. In Japan, lower oxygen concentrations are being chosen for specification of 200 mm silicon wafers compared to what is being used presently in 150mm specifications. This indicates that technology for cleaner device processing is considered more important than the use of internal gettering (IG) methods. It is believed that it is necessary to further reduce the concentrations of carbon and heavy metals in silicon crystals since they can serve as nucleation centers for harmful defects. In particular, control of point defects in large diameter crystals will be an important subject in the near future. After the 64M-bit DRAM generation of device processing, it is expected that an SOI structure may be used due to the high speed and high density requirements. A wafer-bonding process technology for such an SOI need is in-troduced. An understanding of the bonding mechanism is also described.
Taken up examples of crystal growth and evaluation of functional device materials are Gd_2(MoO_4)_3 and LiNbO_3 crystals. The Gd_2 (MoO_4)_3 crystal was grown as a high-quality crystal by measuring its unknown crystal structure, phase transition and phase relation and solving problems. In consequence, the crystal was found to be stoichiometric in composition and agree in congruent melt composition. However, the temperatuer difference from the eutictic temperature of that side of the composition rich in Gd_2O_3 is merely 7℃ and has an inclusion due to temperature variation. Further, because the three phases of α, β And β' are produced by phase transition, high-quality β'-Gd_2(MoO_4)_3 crystal cannot be obtained unless the cooling speed is accelerated. The LiNbO_3 crystal was developed as a substrate crystal for surface acoustic wave devices. To find congruent melt composition, sound speed was determined from the wafer whose upper part was made from the lower part of the crystal. Both sound speeds agree in congruent melt composition. Sound speed control is important, because the growth of large quantities of crystal unavoidably causes composition displacement.
Recent work on the single crystal growth of SiC ingots and epitaxial techniques for producing blue light emitting diodes (LEDs) are reviewed. 6H-SiC ingots have been reproducibly obtained using a sublimation method. Their quality is suitable for the substrate of a SiC blue LED. The degradation phenomenon of SiC blue LED under operation was eliminated by liquid phase epitaxy using an off-oriented substrate. The uniformity of and crystalinity of the epitaxial layer has been improved with an off-oriented substrate. The characteristics of blue LEDs and their applications are also reported. The typical luminous intensity of the blue LED is 12-15 mcd at 20mA. The peak wavelength of the LED is about 470 nm. SiC blue LEDs have been put on the market due to recent progress in crystal growth.
Cubic BN pn-junction crystals are briefly reviewed. A functionable pn-junction diode of cubic BN was fabricated by the temperature difference solvent method at high pressure. Rectification characteristics and injection luminescence of UV light were observed. These results demonstrated that the cubic BN can be a good potential candidate as an electronic and optoelectronic material.
Recent development of single crystal growth of TiC and LaB_6 are presented. High quality TiC single crystals are grown from self-combustion rods using zone-leveling floating zone technique. LaB_6 single crystals are automatically grown using a newly developed technique. Field emission properties of TiC and NbC emitters and thermionic properties of LaB_6 emitter are also reviwed.
Oxide single crystals applied to surface acoustic wave (SAW) device field are briefly reviewed. For SAW device applications, X cut-112°Y LiTaO_3, 128°Y cut-X LiNbO_3 substrates have been mainly used. The wafer size of theses single crystal SAW substrates have been scaled up to 5 inches. In scaling up oxide single crystals for SAW device applications, there are two main problems; (l) Suppression of macroscopic crystal defects such as crystal cracks occurring during and after the crystal growth, (2) Improvement on compositional variation. This paper described the development of the wafer diameters and quality improvements for application to SAW substrates. The origins of various crystal defects in pulled crystals and quality factors are discussed for application to SAW devices in mass-production level.
Bi_<12>SiO_<20> (BSO) single crystal is a piezoelectric, electro-optic, magneto-optic, photoconductive, and photorefractive crystal. High quality BSO single crystals with diameter of 80 mm are grown by means of the Czochralski methods. They are applied to optically active devices such as optical-fiber voltage and current sensors for automatic control system of electric power line and optical spatial light modulators. In this article, the BSO crystal growth techniques and interesting applications are presented.
Ever since low-loss optical fibers and laser diodes were developed, rapid progress has been made in the fields of optical systems and devices, such as long-distance oprical transmission, optical sensors, optical compact disks, and so on. However, future applications of optical technology requires more functional devices than have already developed at present, like optical memories, wavelength converters, optical amplifiers, etc. Single-crysal fibers offer great capabilities in a variety of such functional devices because of an unique combination of material properties and waveguide geometry unavailable in either glass optical fibers or bulk single crystals. This paper desctibes the growth and applications of single-crystal fibers.
A molybdenum single crystal with a large-scale and optional shape was easily obtained by the secondary recrystallization method. There are two fundamental factors for producing single crystal. The one is the presence of dis- persed second phases such as CaO and MgO, the other is the presensed of strong primary texture with fine-grained structure. A certain amount of CaO and MgO is effective as inhibitors on the grain growth behavior of worked molybdenum : initially restraining the normal grain growth by precipitation of CaO or MgO at primary recrystal- lized grain boundaries and conversely permitting the abnormal grain growth by decomposing them to single element at higher temperatures. The effect of initial grain size on the secondary grain growth behavior is important. In the case where the initial grain size was smaller than 50urn, a uniform and fine-grained recrystallized structure was obtained after annealed at 1500℃, whilst an abnormal grain growth occurred after annealed at 2000℃.
Greater jet enjine power and fuel economy have been achieved by incrased turbine firing temperature and adoption of efficient cooling system. The means for attaining reduced cooling flow while increasing life and reliability include refinement of new high temperature materials and fabrication process. Advanced engines employ single crystal turbine blades made through a directional solidification of Ni-based superalloys. This paper summarized methods and equipments for single crystal blade production, characterization of the materials and the future prospects.
The study of superconductor-applied devices has been and is being made by way of thin film processing on single crystal substrates. In accordance with developments of positive research for more sophisticated devices, the substrates of better quality come to be required among superconductor researchers. This eventually drives the substrate manufacturers to developing ever-new substrates and further improving chemical/physical qualities of conventional substrate. This paper introduces each single crystal substrate being applied to oxide superconductor thin film processings, followed by general remarks of oxide superoconductors and growth methods of single crystals for use as substrates.
Growth of organic second-order nonlinear optical crystals is critically reviwed in the light to the efforts for realization of high-performance nonlinear optical devices. Problems in such efforts are on fabrication of crystal waveguides such as planar waveguide and channel waveguide, which currently are formed by in-molding crystal growth method. The method leads to the crystal growth with involuntary orintation, so that optical nonlinearity of the crystal is often not usable in an efficient manner. Therefore, it is pointed out that; (1) a way to control orientation of crystal growth, or, (2) a processing technology applicable to crystals obainable, should somehow be developed. Photolithographic process specially developed by the present author and his group members is introduced as the only existing way for the fabricarion of waveguides with desired orientation of the optical nonlinearity of the crystal.
The (111) surface of Cu-0.01〜0.2 at% Au alloy crystals was etched for 10s at 280 K in Young's solution (1 kmol・m^<-3> (NH_4)_2S_2O_8> 6kmol・m^<-3> NH_4OH, 0.3 kmol・m^<-3> NH_4Br) using a rotating disc method. The dissolved depth of matrix surface S was measured by twobeam interferometry and the width h and depth d of triangular pyramid of dislocation etch pits were measured by replica electron microscopy. The horizontal dissolved length and the vertical dissolved depth at the dislocation site, Hand D, were determined from S, h and d, respectively. Both H and D increased to a maximum value at Au concentration as small as 0.01-0.05 at% and then decreased gradually, while S increased slightly with a small increase of Au concentration and attained a constant value at 0.03-0.2 at% Au. It was shown that these results could be explained qualitatively on the basis of the two-dimensional nucleation theory of crystal dissolution by considering the variations of the mobility of step, the activation energy for dissolution and the difference of chemical potentials when Au concentration was increased.
Crystallization kinetics of two polymorphs, tetragonal and orthorhombic forms, of hen egg white lysozyme was examined with optical method. An induction time for crystallization of each crystal was measured at different crystallization conditions (temperature, pH, concentration of three precipitants of NaCl, (NH_4)_2SO_4 and NH_4 Cl). The occurrence behavior of the two polymorphs was primarily influenced by temperature and pH, irrespective of the precipitants employed. The rate of crystallization, however, was remarkably affected by the precipitants. After examining the moderate rate of crystallization, we obtained large-size single crystals; 〜0.5 mm for tetragonal form and 〜5 mm for orthorhombic form.