Synchrotron White Beam Topography Studies of Residual Stress in SiC Single Crystal Wafers with Epitaxial Thin Films
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Also, the determination of stress as a function of temperature and time is a valuable part of the evaluation of a measurement technique. Measurement of the curvature induced in a wafer by the stress in a thin film has long been used as a convenient and accurate technique for the determination of the deformation field[I-5]. Curvature may be measured by optical means or by x-ray diffraction techniques. Since the wavelength of x-rays is shorter than that of visible light, X-rays provide higher precision. In this work, synchrotron white beam x-ray topography (SWBXT), which is commonly used to image individual defects and defect distributions in single crystals, is used to determine the deformation in 6H-SiC single crystals with 3C-SiC thin films grown on them. By using the highly collimated beam, synchrotron white beam which is available at the Stony Brook topography facility at National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL), accurate measurements can be obtained. EXPERIMENTAL The crystals used in this study, consisting 6H-SiC substrates with 3C-SiC thin film overlayers, were provided by the Oregon Graduate Institute. 3C-SiC has good electronic properties such as high low-field electron mobility. However, since 3C-SiC substrates of sufficient quality are not yet routinely available, efforts have focused on hetero-epitaxial growth 425
Mat. Res. Soc. Symp. Proc. Vol. 399 0 1996 Materials Research Society
of 3C-SiC layers on 6H-SiC substrates, which is commercially available. The synchrotron white beam X-ray diffraction topography experiments were carried out at the Stony Brook Synchrotron Topography Facility, Beamline X-19C, at the NSLS in BNL. The transmission, or Laue geometry, and the reflection, or Bragg geometry, were employed to image the wafers. In transmission geometry, the convex side of the bent specimen was set as the x-ray exit surface in order to minimize the focusing resulting from the curvature of the single-crystal sample. The film was on the convex side in all the wafers tested, which suggested the presence of tensile stress in the substrate. The orientation of the specimen was controlled by a Huber four-circle goniometer. A minimum rotation step of 0.0025', controlled by a stepper motor, could be obtained. Absorption-edge-contour (AEC) mapping [6,7] was used to determine the curvature of the substrate. This technique is analogous to contour mapping using a conventional x-ray source. However, the contour resolution and the efficiency of data collection is much higher with the synchrotron source. Due to the fact that SiC is a relatively low-absorbing material, a Mo filter, with an absorption edge at 0.6198A, was used so as to impose an abrupt change in spectrum intensity near the peak of the spectrum, as shown in figure 1. When the specimen is properly oriented such that the range of diffraction wavelengths spans the absorption edge of the filter, an absorption edge contour forms. The contour line moves as the crystal is rotated. The line position as a function of crys
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