Silicon wafers, coated with a silicon nitride layer and subjected to high temperature Rapid Thermal Annealing (RTA) in Ar, show—upon a subsequent two-step precipitation anneal cycle (such as 800 °C + 1000 °C)—peculiar depth profiles of oxygen precipitate densities. Some profiles are sharply peaked near the wafer surface, sometimes with a zero bulk density. Other profiles are uniform in depth. The maximum density is always the same. These profiles are well reproduced by simulations assuming that precipitation starts from a uniformly distributed small oxide plates originated from RTA step and composed of oxygen atoms and vacancies (“VO2 plates”). During the first step of the precipitation anneal, an oxide layer propagates around this core plate by a process of oxygen attachment, meaning that an oxygen-only ring-shaped plate emerges around the original plate. These rings, depending on their size, then either dissolve or grow during the second part of the anneal leading to a rich variety of density profiles.

A “2-step Rapid Thermal Annealing (RTA)” treatment in NH3 and Ar gas is capable of producing a high density (6-8x109/cm3) of precipitates in a remarkably uniform profile in the 300mm silicon wafers, independently of thermal cycle, oxygen content and pre-thermal history. The profile sharply transitions from a shallow denuded zone to the fixed and uniform density in the bulk. Such a feature would guarantee a sufficient amount of gettering sinks to be retained in the extremely thinned wafers. The shape of the depth profile evolves in proportion to the thermal budget, following well the model of VO2 nucleation and growth reported previously.