In situ study of the formation of silicide phases in amorphous Ni–Si mixed layers

Abstract

In this paper, we investigated Ni silicide phase formation when Si is added within an as deposited 50 nm Ni film. A series of 22 samples with a Si content varying from 0 to 50 at. % was prepared and systematically investigated with in situ x-ray diffraction. The inert oxide substrate was used to identify the phases which first crystallize in an amorphous Ni–Si mixture of a given concentration. The noncongruent silicides Ni3Si and Ni3Si2 are never observed to crystallize readily out of the mixture. A remarkable observation is the initial crystallization at low temperature of a hexagonal Ni-silicide, observed over a broad mixed layer composition [35–49%Si]; this hexagonal phase nucleates readily as a single phase [39–47%Si] or together with Ni2Si [35–38%Si] or NiSi [49%Si]. This low-temperature phase is related to the high temperature θ-phase, but covers a wide composition range up to 47%Si. For the same Ni–Si films deposited on Si(100), the initial nucleation of the Ni(Si) mixture is similar as for the samples deposited on SiO2, such that the complex sequence of metal-rich Ni-silicide phases typically observed during Ni/Si reactions is modified. For samples containing more than 21%Si, a simpler sequential phase formation was observed upon annealing. From pole figures, the phase formation sequence was observed to have a significant influence on the texture of the technologically relevant NiSi phase. For mixture composition ranging from 38% to 43%Si, the initial transient θ-phase appears extremely textured on Si(100). The observed transient appearance of a hexagonal phase is of importance in understanding the phase formation mechanisms in the Ni–Si system.