Detection of Metal Segregation at the Oxide-Silicon Interface

Various techniques for the detection of metal segregation at wafer surface were compared by using nickel and cobalt implanted wafers as the test vehicles. Generation lifetime gave the maximum sensitivity. Noncontact interface state density measurements have limited sensitivity and require an integration of the spectrum over the measurement energy range. The Elymat surface recombination velocity is a compromise between sensitivity and mapping capabilities. Nickel is a fast diffuser and completely segregates at wafer surface, as reported in literature. So, nickel cannot be detected by measurements of bulk diffusion length, neither by surface photovoltage (SPV) nor by Elymat techniques. Cobalt does segregate at wafer surface, but in spite of what is reported, some cobalt is left in the bulk even after a thermal treatment with a slow ramp-down (furnace treatment). Cobalt surface segregation is negligible after thermal treatment with a fast ramp-down so standard bulk lifetime techniques such as Elymat or SPV can be used for monitoring purposes.

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