Lateral and vertical dopant profiling in semiconductors by atomic force microscopy using conducting tips

Abstract

The determination of the spatial distribution of charge carriers in semiconducting structures with an atomic force microscope (AFM) is presented. This new technique is based on the measurement of the spreading resistance of a conducting AFM tip on a sample as the tip is scanned over the surface. The high spatial resolution of this method allows for its application on the cross section of a device providing one-dimensional (vertical) and two-dimensional (lateral) profile information. First, a detailed study is presented regarding the properties of the tip (tip material and preparation, lifetime) and the evolution of I–V characteristics with pressure on uniformly doped silicon. From this work a model for the electric properties of the microcontact is deduced. It includes an ohmic contribution to the overall resistance that is related to the plastically deformed area, and contributions from a barrier as well as from tunneling. Through the determination of a calibration curve, whereby the resistance was measured on homogeneously doped substrates with varying resistivities, the quantification properties of the method are determined. Finally, this method has been applied for junction delineation on the cross section of a device. The results show a 50 nm resolution and a sufficient sensitivity to encompass a dynamic range of concentrations between 1014 and 1019 cm – 3. These measurements were found to be in good agreement with those performed by the classical spreading resistance profiling technique and by secondary ion mass spectroscopy.