The use of silicon substrate preamorphization in ultrashallow junction formation has increased in recent years. The reduction of channeling during impurity implantation, coupled with higher-than-equilibrium metastable solubility levels, produces scaled junctions with low resistances. However, a number of physical phenomena arise that must be considered for proper impurity profile and device optimization.
With respect to impurity solubility advanced annealing techniques such as solid-phase-epitaxial-regrowth (SPER), flash, and laser annealing, can place impurity atoms on substitutional sites in the silicon lattice to extremely high concentrations when combined with preamorphization. In this context there is a relationship between the equilibrium distribution coefficient and metastable solubility. The long-established equilibrium distribution coefficient of an impurity, extracted in the liquid to solid phase transformation, can make a prediction of metastable solubility after transformation of amorphous silicon into crystalline silicon during SPER, flash, and laser annealing.
With respect to impurity redistribution the significant effects can be split into 3 categories, namely before, during, and after recrystallization. Before recrystallization impurity diffusion in the amorphous region may occur. Boron is particularly susceptible to this effect, which is very significant for the formation of p-type junctions. During recrystallization many impurities move ahead of the amorphous-crystalline (a/c) interface and relocate closer to the surface. In general redistribution is more likely at high impurity concentrations. For low-temperature SPER there is a direct correlation between the magnitude of this redistribution effect and the impurity metastable solubility. After recrystallization, with SPER, flash, and laser annealing commonly leaving residual damage in the silicon substrate, interstitial-diffusers are especially vulnerable to preferential diffusion toward the surface, where impurity atoms may be trapped, ultimately leading to a more shallow profile.