Shallow boron-doped junctions in silicon

Shallow boron-doped junctions in silicon have been investigated by means of secondary ion mass spectrometry, scanning electron microscopy, transmission electron microscopy, spreading resistance profiling, and four-point probe techniques. The junctions were formed by implanting BF⁺₂ ions into n-type Si at the dose range of 1-5×10¹⁵ ions/cm², through a thin (25-nm) screen layer of silicon dioxide. We have emphasized the higher dose range (3-5×10¹⁵ ions/cm²) as it is more relevant to processes in the current level of device integration. The use of BF⁺₂ species and the screen oxide layer is necessary in order to form junctions whose depths x _j≤0.4 μm, when conventional annealing techniques are employed. We have also examined junctions that were activated in a rapid thermal annealing system that utilizes an incoherent light source. One of the main objectives of this study is to compare conventional and rapid thermal annealing techniques. We thus evaluate the results obtained by these two methods of annealing for both the junction depth x_j and the sheet resistivity R_s. Other relevant variables such as a low-temperature (77-K) implantation, surface amorphization by Si implantation, and preactivation annealing have also been examined and their effects are discussed. Based on the results of the present study, a mechanism for boron activation in silicon is discussed.