Strengths and Limitations of the Vacancy Engineering Approach for the Control of Dopant Diffusion and Activation in Silicon

Abstract

The fabrication of highly doped and ultra-shallow junctions in silicon is a very challenging problem for the materials scientist. The activation levels which are targeted are well beyond the solubility limit of current dopants in Si and, ideally, they should not diffuse during the activation annealing. In practice, the situation is even worse and when boron is implanted into silicon excess Si interstitial atoms are generated which enhance boron diffusion and favor the formation of Boron-Silicon Interstitials Clusters (BICs). An elegant approach to overcome these difficulties is to enrich the Si layers where boron will be implanted with vacancies before or during the activation annealing. Spectacular results have been recently brought to the community showing both a significant control over dopant diffusion and an increased activation of boron in such layers. In general, the enrichment of the Si layers with vacancies is obtained by Si⁺ implantation at high energy. We have recently developed an alternative approach in which the vacancies are injected from populations of empty voids undergoing Ostwald ripening during annealing. While different, the effects are also spectacular. The goal of this work is to establish a fair evaluation of these different approaches under technologically relevant conditions. The application domains of both techniques are discussed and future directions for their development/improvement are indicated.