Metastable activation of dopants by solid phase epitaxial recrystallisation

The ideal ultrashallow junction relies on (i) high dopant solubility in the crystalline substrate, in order to boost activation and reduce sheet resistance, and (ii) low dopant diffusivity, to facilitate device scaling. Equilibrium solubility is not sufficient to meet the aggressive access resistance targets at advanced device dimensions, thus above-equilibrium metastable solubility must be generated. A technique to generate such metastable solubilities involves amorphisation of the target silicon substrate, followed by recrystallisation via thermal annealing thereafter. The recrystallisation process is very efficient in placing impurity atoms onto substitutional positions within the semiconductor crystal lattice. The formation of metastable solubility requires care during subsequent processing because further supply of thermal energy, e.g. by back-end processing, causes the metastable condition to revert back to the lower equilibrium state. An approach to control deactivation is by co-implantation of non-dopant species, such as carbon, fluorine, or nitrogen. These species can sink point defects that cause metastable-activation deactivation. Implanting at cryogenic temperatures has also proved successful at reducing defect populations. Control of diffusion, to facilitate junction and device scaling, can be achieved by reducing the thermal budget of the annealing process. In silicon applications high-temperature millisecond anneals (laser and flash) are popular. Reduced thermal budget via a low-temperature process such as solid-phase-epitaxial-recrystallisation appears to achieve similar results in many regards. Note, anomalous diffusion effects prior, during, and after recrystallisation can be detrimental and cannot be ignored. In summary, impurity solubilities of group III and V elements in silicon resulting from solid-phase-epitaxial-recrystallisation can beat the maximum equilibrium values by approximately one to two orders of magnitude. This can help reduce parasitic resistances significantly and be of great benefit to the electrical performance of advanced silicon devices.