Interfacial reactions of cobalt thin films on BF+2 ion-implanted (001) silicon

Abstract

Interfacial reactions of cobalt thin films on BF+2 -implanted (001)Si have been studied by both cross-sectional and plan-view transmission electron microscopy as well as by sheet resistance measurements. The implantation-amorphous samples were found to favor the formation of CoSi2 at 400°C and laterally uniform growth of the phase at higher temperatures. Two discrete layers of fluorine bubbles were observed in cobalt silicides in all BF+2 -implanted samples annealed at 400-800°C. In 800-900°C annealed samples, large bubbles were often observed to distribute near the CoSi2 grain boundaries. The results indicated that (1) fluorine atoms diffuse rapidly at a temperature as low as 400°C, and (2) appreciable amounts of fluorine atoms are present with low solubility in cobalt silicides after annealing at 400-900°C. The residual interstitial defects in BF+2 -implanted samples were completely annihilated by CoSi2 formation at 800-900°C. The elimination of all interstitial defects in these samples is attributed to the injection of a high density of vacancies which were generated during silicide formation. The electrical resistivity of CoSi on blank silicon was measured to be about 350 μΩcm. The presently measured electrical resistivity value is believed to be one of the most accurate values obtained for CoSi to date. Factors influencing the formation of cobalt silicides and defect structure are discussed.