Hydrogen and helium bubbles in silicon

Hydrogen is a quite common impurity in semiconductor-silicon technology: it is unintentionally but unavoidably added to the silicon after crystal growth during wafer processing, and continues to be present during wet oxidation, film depositions, etching and annealing steps. The effects of hydrogen in single crystal silicon at low concentration have been the subject of many papers, books and conference proceedings. Much less considered is the case of hydrogen at massive concentration. One final effect of heavy hydrogen loading is the formation of cavities and bubbles, with size up to 100 nm. Cavities and bubbles are also observed after helium loading by high-fluence ion implantation. This article reviews the basic mechanisms responsible for the formation and growth of such structures in single-crystalline silicon. In particular, starting from the loading (ion implantation) and having in mind the formation of the cavities, this paper will cover: the effects of substrate temperature, the interaction of vacancies and self-interstitials with the impurity, the mechanisms of gas segregation inside the cavities, the pressure which arises because of the segregation and the subsequent displacement field in the crystal, the stability against heat treatments of the gas in the cavities and of the cavities themselves. The understanding of the physical processes should lead to gain more insight in the processes of cleavage of the Si–Si bond and vacancy agglomeration which can induce not only the formation of cavities and bubbles, but also planar cutting or explosion.