As minimum feature sizes decrease for ultra large scale integration, deleterious effects of smallest defects become increasingly important. In order to detect, measure and analyze these defects in buried structures, complementary techniques to those presently used must be developed and explored. Conventional optical microscopy techniques such as UV, confocal and laser scanning are approaching their fundamental limits of resolution. The near-field scanning optical microscope (NSOM) offers sufficiently high spatial resolution (50 nm), and an excellent signal-to-noise ratio to image buried structures inside optically transparent media. In order to investigate defects in layers below the surface of completed devices, we have developed a special sample preparation technique and have demonstrated optical resolution at the 50 nm level. In addition, we have explored the interaction in the image formation of a mixture of near and far field contributions. We show how useful buried layer information may be obtained via NSOM and demonstrate the present limitations of the technique. We compare our results to those obtained by conventional optical microscopy techniques.

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