SUBMICRON OPTICAL SECTIONING MICROSCOPY - A PARTICULAR INVERSE PROBLEM SOLUTION ADAPTED TO EPILAYER DEFECT ANALYSIS

Previous work devoted to infrared transmission microscopy or laser scattering tomography showed that bulk semiconductor materials as well as epilayers contain imperfections of various origins, which are often microprecipitates. The role of these small clusters on the device properties is obviously important and their study is essential for improving the specifications and integration scale. Laser scanning tomography is a powerful tool for analysing the volume distribution of microprecipitates in bulk material. Sensitivity, contrast and resolution of such dark field experiments are of a very high level; nevertheless selecting very thin layers (below the micron range) close to the surface is outside the ability of the method due to the laws of optics. Optical shallow sectioning and reconstruction or deep field deconvolution microscopy are typical problems of modern optical microscopy: hardware solutions (confocal microscopes) or digital processing have classically been proposed; the result is usually not straightforward because of the unknown 3D structures of the observed object. The experimental and theoretical knowledge of the point spread function of the optical transformation allows us to precisely calculate the geometrical position of the microprecipitates and to transfer the information into a synthesized image. In this communication we propose a new approach of imaging based on such a computer processing. It is a typical "inverse problem" solution which can be valuably and unambiguously derived in the present situation of point images. © 1990.