Selective etching of B-doped silicon: mechanism and two-dimensional delineation of concentration profiles

We have investigated the selective chemical etching of B-doped silicon in p+n and in p(+)p structures using an HF:HNO3 mixture under illumination with ultraviolet light. The etching profiles were observed by transmission electron microscopy (TEM) and compared with the boron concentration depth profiles obtained by spreading resistance measurements. In the case of n-type substrates the removed thickness of the B-doped region is a factor 2.7 larger than in the case of p-type substrates. In the former case, no correlation exists between etching and concentration profile and the etched region exhibits a step-like morphology, if crystallographic defects are present in the sample. For p-type substrates the etching rate of the B-doped region is proportional to the log of the boron concentration with a sensitivity which worsens by increasing the etching time. The experimental results are interpreted by taking into account the role of light-generated free carriers. This information is used to develop a sample preparation technique to obtain the TEM images of two-dimensional junction profiles in samples containing micrometer size features.