Two-dimensional dopant profiling of patterned Si wafers using phase imaging tapping mode atomic force microscopy with applied biases

Abstract
Tapping mode atomic force microscopy with applied bias was used to spatially resolve areas of different doping type on Si wafers patterned with photolithography and subsequent ion implantation. The application of a direct current bias between cantilever and sample during the measurement produces Coulomb (electrostatic) forces, whose magnitude depends on the spatial variation of the doping density. This effect was utilized to detect areas of different doping type by monitoring the phase angle between the driving frequency and the cantilever response while scanning areas of different doping density. In this article we present a series of measurements at various bias voltages demonstrating that the observed phase contrast between differently doped areas is directly connected to the bias induced surface potential (band bending) present on these areas. To investigate the contrast mechanism quantitatively, we also measured deflection (force), amplitude and phase versus distance curves for a typical cantilever with an applied bias on a gold thin film. This allowed correlation between phase contrast observed and the actual Coulomb force measured.

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