A rutherford scattering study of the diffusion of heavy metal impurities in silicon to ion-damaged surface layers

Rutherford backscattering of 1.75 and 2 MeV ⁴He⁺ ions has been utilized to study the high temperature gettering of Fe, Co, Ni, Cu and Au from silicon by ion-damaged surface layers. In a typical experiment a metal film was evaporated onto one side of a silicon wafer (125 microm thick) which had received ion implantation damage (10¹⁶/cm² Si⁺ ions at 100 keV) on the opposite side; the wafer was then annealed at 900°C, usually for 30 min. The results of such experiments show that the metals studied may be divided into two classes, those which are gettered slowly - Fe, Co, and Au, and those gettered rapidly - Cu and Ni. Fe, Co, and Au were found at levels of 1×10¹³?1×10¹⁴/cm² in the damaged layer, whereas Cu and Ni appeared at levels of 6×10¹⁴ to 5 ×10¹⁶cm². The gettered level of Au, one of the “slow” group, was increased ten-fold by an equal increase in the anneal time to 300 min. The gettered Cu and Au exhibited double peaks in the scattered ion spectra, corresponding to metal concentrated at the most heavily damaged region (end of range for Si implant) and also at the outer surface, with a separation of ~ 1300 Å. A simple model is proposed to explain the slow and fast gettering, based on published interstitial diffusivities and solubilities of the five elements studied.Rutherford scattering has proven to be well suited for the quantitative identification of low levels of impurities on Si surfaces and for impurities gettered at damaged layers close to the surface.