X-Ray topography of semiconductor silicon

Abstract

This thesis describes the examination and characterisation of semiconductor silicon by the various methods of X-Ray Diffraction Topography. A brief introduction is given to the dynamical theory of X-ray diffraction and its relevance to the formation of contrast in X-ray topographs. The experimental methods used and contrast formation mechanisms are introduced. The design and construction of an inexpensive Automated Bragg Angle Controller (ABAC) is described, based around a microcomputer and using many of the existing features of the Lang camera. This enables Lang topographs of the whole of distorted crystals to be taken. Using the ABAC, the contrast of defects in Lang topographs of cylindrically bent silicon wafers is explored. A comparison is made between this data and images in Hirst topographs and contrast differences between the techniques are attributed to the presence of an inhomogeneous bending moment. The change in contrast in section and Lang topographs upon homogeneous bending for asymmetric reflections is also investigated and mechanisms for the contrast changes are suggested. A bipolar device wafer is examined with double crystal topography using synchrotron radiation and a highly asymmetric reflection with a glancing angle of incidence. By exploiting the wavelength tuneability of the synchrotron radiation, the depth penetration of the X-rays is varied and the optimum experimental conditions for observing both defects and devices determined. Using this technique it is possible to image both devices and process related defects to a high resolution and contrast. The Lang, section and glancing angle double crystal topography techniques are compared for the examination of a CMOS device wafer. The relative strengths and weaknesses of each technique are highlighted and many defects are imaged and characterised. Finally, results showing the appearance of fringes in the double crystal topographs for low angles of incidence are presented. These are attributed to the' presence of along range strain, and the dependence of the fringes upon curvature is explored for moderate bending conditions (R ~35m) .