Anomalous Scattering from Single Crystal Substrate

The nature of the scattering pattern for silicon 004 wafer is described. At incidence angles (ω) set close to the expected diffraction condition of ω = θh`k`l` αhkl,h`k`l` (hkl are the planes parallel to the wafer surface and αhkl,h`k`l` is the interfacial angle between hkl and another set of diffracting planes h’k’l’), an anomalous kind of scattering has been observed. There is a broad weak feature at about the expected diffraction angle of 2θh`k`l` and a narrow feature whose position depends solely on the incidence angle. The broad feature is the Bragg diffraction of the X-ray tube spectral components while the narrow feature is a non-Bragg scattering peak. Both features can be eliminated in some cases with knowledge of the orientation of the wafer to the diffractometer axes. INTRODUCTION Silicon single crystal wafer and other single crystal materials are used as thin film substrate. The advantage of a single crystal wafer substrate is that while the wafer may diffract strongly in some directions, it scatters weakly in most (non-Bragg) directions. Glancing incidence (or shallow angle asymmetric parallel beam) X-ray diffraction is the prevalent geometry used when analysing thin film samples. The low incidence angle keeps the X-ray beam nearly parallel to the surface region and the diffraction intensity does not suffer greatly at higher diffraction angles as is the case for a symmetric geometry. The scattering pattern of the substrate is important in the determination of the nature of the thin film X-ray diffraction pattern. A wafer is usually cut such that one set of planes (hkl) is parallel to the wafer surface. Since in glancing incidence the incidence angle (ω) is set to a low value (a few degrees), the incidence angle would be well away from ω = θhkl and the strong diffraction peak which would be expected at 2θhkl. There are, however, other sets of planes present which are at defined (interfacial) angles (αhkl,h`k`l`). If αhkl,h`k`l` + ω = θh`k`l`, then a strong diffraction peak would be expected at 2θh`k`l`. At regions around the angles 2θh`k`l` other effects may come into play depending on the relationship between αhkl,h`k`l` (interfacial angle), ω (incidence angle), 2θ, φ (rotation angle), and χ (orientation angle). The diffractometer angles are defined in Figure 1. 177 Copyright ©JCPDS-International Centre for Diffraction Data 2007 ISSN 1097-0002