Impurity concentration mapping in multicrystalline silicon wafers

Electrical properties of multicrystalline silicon wafers depend on the interaction of crystallographic defects and impurities. Vacancies have by themselves a high recombination strength for minority carriers, conversely dislocations and grain boundaries (GBs) are poorly recombining when they are not decorated by impurities. Such impurities are oxygen or/and carbon and fast diffusers like iron, nickel, chromium etc. Such impurities are introduced during the crystal growth due essentially to the use of silicon charges constituted by wastes of ingots devoted to electronic device fabrication. It is neeful to determine the local impurity concentrations although their concentration is frequently below the limit of detection of chemical analysis techniques. Mapping of minority carrier lifetime obtained by means of the phase-shift technique allows to determine the recombining impurity concentration and in some cases to identify the nature of the impurity, (with the eventual help of the deep level transient spectroscopy), provided the surfaces of the wafers have been sufficiently passivated. It is shown that impurity concentration of iron in the range 10(14) to 10(11) cm(-3) can be measured with a lateral resolution of 50 mum. Recombining iron atoms are accumulated at dislocations lineages and clusters while at GBs such atoms may be trapped in a non electrically active form (like neutral clusters and precipitates). A similar behaviour is found with gold atoms. mprovements of the material result from the external gettering of impurities or from their passivation by hydrogenation.