Structural Changes During Transient Post-Annealing of Preannealed and Arsenic Implanted Polycrystalline Silicon Films

Abstract

Polycrystalline silicon films were transient preannealed, As implanted, and transient post-annealed at peak temperatures up to 1250°C for times up to 17.5 seconds. Structural changes occurring during post-annealing were examined by transmission electron microscopy. These results were correlated to Rutherford Backscattering and sheet resistance results. The grain size, which increased from 5–20 to 150–300 nm during preannealing, did not increase during post-annealing. During early stages of post-annealing, As diffused along grain boundaries and generated dislocation sources at grain boundary surfaces. Subsequently, as annealing progressed, a fine, As-rich cellular network structure propagated into the grains until the structure of an entire grain was transformed into a fine cellular network at the longest annealing times. Residual stresses in the film were relieved during formation of the network structure. The sheet resistance of preannealed samples, in comparison with non-preannealed samples with similar implantation and final transient anneals, was lower at shorter annealing times due to the larger grain size, which increased mobility, and the reduced grain boundary area, which trapped less As. It was also lower at longer annealing times due to the formation of the cellular network structure. In subsequent furnace stability tests for 30 minutes at 700–900°C, the sheet resistance increased less for preannealed than for non-preannealed samples.