Kentaro Kajiwara

Construction of topography stations at SPring-8 and first observations

Two topography experimental stations are presently available at SPring-8. The first, constructed at the short-length bending magnet beam line BL28, is designed to perform white- and selected wide-energy-range x-ray topography. The other is a high-resolution diffraction topography station located on the medium-length bending magnet beam line BL20, where the incoming beam displays a large cross section and high degree of parallelism. This allows us to observe fine structures of three-dimensionally large crystals used in industry if high energy is employed. The construction concepts, as well as first, selected experimental results are presented.

Three-dimensional structure of dislocations in silicon determined by synchrotron white x-ray topography combined with a topo-tomographic technique

We have studied the propagation and elimination of dislocations generated at the early stage of Czochralski silicon crystal growth using synchrotron white x-ray topography combined with a topo-tomographic technique. Two silicon crystals with [001] growth-axes were examined. One was intentionally grown without enlarging its diameter to easily observe the features of the dislocation propagation, and the other was grown with Dash necking, followed by a 2 inch enlargement of its diameter in order to observe the elimination of the dislocations. The three-dimensional structure of the individual dislocation, i.e. the direction of the dislocation line, its Burgers vector and the glide plane, were determined. These investigations revealed that dislocation half loops, which were generated from tangled dislocations, were expanded on their glide planes and were often deformed by their interaction, cross slip and collision with the crystal surface, followed by a gradual decrease in their density. The dislocation-elimination effect of the Dash necking was caused by the expansion of the dislocation half loops being terminated within the crystal and by their pinning on the crystal surface.

Synchrotron-radiation X-ray topography of surface strain in large-diameter silicon wafers

Using a 300 mm-wide monochromatic X-ray beam obtained at beamline BL20B2 of SPring-8, the difference in surface-strain distribution caused by various steps of silicon-wafer manufacturing, i.e. slicing, lapping, etching, grinding and polishing, was studied. The asymmetric 511 reflection of 21.45 keV X-rays, incident at a glancing angle of 0.26 degrees, was used to obtain topographs over the whole surface of a 200 mm-diameter (100) CZ silicon wafer. Differences in crystallinity and in warp between the surfaces at different steps of the manufacturing process (firstly after the lapping following the slicing, and then after successive etching, grinding and polishing) were clearly observed. The former gave a topographic image over the whole area with a one-shot exposure because of their wide rocking curves (50-70 arcsec FWHM), which indicate poor crystallinity. The latter, on the other hand, showed sharper curves (4-5 arcsec FWHM), which indicate good crystallinity in local areas, and the existence of warp, and therefore required step scanning of omega-rotation to cover the whole surface in topography measurements. The effect of each step in the process is also discussed.

Plane-wave synchrotron x-ray topography observation of grown-in microdefects in a slowly pulled CZ-silicon crystal

Plane-wave x-ray topography was used to characterize grown-in defects in silicon crystals grown by the Czochralski method at a slow speed. The 220 reflection rocking curves of the sample crystal were measured in a symmetric Laue case. Oscillatory profiles were clearly observed in the rocking curves and were in good agreement with those calculated from the dynamical theory of x-ray diffraction. Images of the grown-in defects were observed in the plane-wave x-ray topographs of the 220 reflection. The density of the defects was determined as 300 cm -3 . The lattice strain around the defects was evaluated as 10 -6 from the dependence of the defect image contrast on the x-ray incident angles. The defect images are discussed using a grown-in defect model of entangled dislocation loop clusters.

Formation of high energy white microbeam and its application to spectro-scattering topography

Scanning type X-ray scattering topography system using high energy white and parallel microbeam which was successfully formed by a very small diaphragm (microhole). The observation of a germanium single crystal clearly revealed dislocations with better contrast and resolution than in a conventional white beam topograph. The present research has shown advantages of a high energy white and parallel microbeam in materials imaging by X-ray scattering topography. Besides the imaging system equipped with an energy sensitive multi-channel detecting system has provided a function of local spectroscopy simultaneously.

Dislocation Elimination in Czochralski Silicon Crystal Growth Revealed by White X-ray Topography Combined with Topo-tomographic Technique

We have examined the neck of a large‐diameter [001]‐oriented Czochralski silicon crystal by synchrotron white X‐ray topography combined with a topo‐tomographic technique in order to explain the mechanism of dislocation elimination due to Dash necking in industrial‐scale crystal growth. In the portion where the grown crystal was transformed from a dislocated region to a dislocation‐free region, dislocation half loops were first generated at the dislocation tangles. These loops then expanded on the {111} glide planes and then terminated inside the crystal. In some cases, they reached the side of the crystal. A new mechanism for the elimination of dislocations is proposed based on the fact that dislocations in the neck are not accompanied by the solid‐melt interface during the crystal growth, and they proceed in the crystal after the movement of the interface.

Observation of silicon front surface topographs of an ultralarge-scale-integrated wafer by synchrotron x-ray plane wave

Surface roughness and undulation of unpatterned silicon wafers are serious issues for ultralarge-scale-integrated circuit devices, even after fine mechanochemical polishing. It has never been clarified whether the undulations exist only on the surface or also exist inside the bulk crystal. We produced grazing incident diffraction topographs at three x-ray photon energies, with penetration depths estimated to be 3.85 nm, 4.78 nm, and 1.28 μm. All the topographs contained striation. We also obtained clear total reflection images using synchrotron x-ray plane waves, which also showed striation patterns at penetration depths from 3.85 nm to 1.28 μm. These results indicate that the origin of the patterns is not at the surface but is inside the Si wafer. The origin of striation patterns, observed in the topographs, was found not to be due to mechanochemical polishing processes but to crystal growth.

High resolution X-ray Laue topography of thick quartz crystals at SPring-8

Abstract High resolution X-ray Laue topography was conducted on the SPring-8 BL-28B2 beamline, operating at a 5–100 keV energy range. The use of this beamline was particularly suitable for characterization of thick samples of giant blocks of natural quartz used as seeds for growing synthetic quartz; and also to understand the propagation mechanism of dislocation lines in synthetic quartz. Area enlargement of Laue topographs by scanning the sample-film stage in the vertical direction was particularly useful for high energy diffraction images, due to their relatively small vertical dimensions. The methodology applied to characterize thick crystals used in the present research, is an effective contribution to develop nearly perfect large dimension seeds and synthetic quartz bars for the next generation of “surface acoustic waves” device substrates.