Optical non-destructive tests for the evaluation of microprecipitates in semiconductors and devices

As semiconductor technologies evolve from micrometre to submicrometre scale the devices become more sensitive to built-in defects which have not generally been considered previously because of their very small size. In particular, the metallurgy of the materials and the various fabrication steps give rise to small aggregates of foreign or excess atoms of nanometre size and a density in the range of 1010 cm-3. There are actually very few observation techniques available for revealing these flaws inside the materials non-destructively. Of course photon-based techniques fit more consistently with the need for non-invasive and non-destructive inspection. Laser scanning tomography (LST) was the first solution providing three-dimensional information on scattering particles as small as 1 nm in bulk semi-conductors. Numerous investigations have been performed in III-V compounds as well as in silicon where this method, to date, is the only realistic way of studying the generation of oxide-related internal gettering centres. Recent results in the field are summarized. Nevertheless, LST images cannot easily be obtained from regions just below the surface because of excessive scattering from all surface artefacts (defects as well as structures). It follows the specific modifications have to be introduced in the technique, though the basic argument for light-scattering images is still preserved. Ogawa recently showed that silica particles in SIMOX layers can be investigated using a Brewster illumination whereas Fillard and Montgomery have revealed grown-in particles in MOCVD GaAs layers. These techniques are still progress, along with a new method of particle submicrometre ranging called Nanolidar which is intended to evaluate the position of particles to a very high precision. This makes it possible to explore layered structures and to identify the exact location of the disturbing defects introduced by the epitaxial or annealing processes. The present state of the art is detailed. New calculation methods are tested competitively in a computer simulation of three-dimensional diffraction patterns. It turns out that the Fourier phase shift method can be used profitably to provide three-dimensional ranging of the particles with a largely submicrometre accuracy.