Measurement and Visualization of Doping Profile in Silicon Using Kelvin Probe Force Microscopy (KPFM)

This paper demonstrates that Kelvin Probe Force Microscopy (KPFM) is applicable to the measurement of 2-D dopant profiles in silicon. By measuring contact potential differences, to the extent that this work-function difference is a consequence of dopant concentration at or near the sample surface, doping profiles are inferred from the measurement. Recently, scanning resistive probe microscopy (SRPM), a variant of SPM-based techniques, where a semiconductor resistor is located at the apex of the probe tip and surface charges can be observed directly, was proposed and developed. The spatial resolution of SRPM is dependent upon the size of the prepared resistor at the apex. The size of the resistor can be determined by the width (or channel) of the SiO2 implant mask, where both sides of the mask in a p-type silicon substrate were opened, implanted with As+ ions, and diffused by an activation process at 1000 degrees C for 10 to 16 hours. Using KPFM, we investigated doping profiles of the area of the resistor or, equivalently, underneath the mask. As annealing time was increased from 10 to 16 hours, shrinkage of the width (or channel) due to out-diffusion of implanted ions occurred. As a result, the contact potential difference between implanted n(+) and p (Si substrate) regions, which is equal to the resistive region, was decreased by increasing diffusion time. In conclusion, we showed that barrier height is lowered in the resisitive region, similar to a punch-through effect in a bipolar junction. It was also demonstrated that KPFM is a useful tool for measuring two-dimensional dopant profiles with nanometer spatial resolution.