Self-diffusion of silicon in thin films of cobalt, nickel, palladium and platinum silicides

Abstract

Radioactive 31Si was used as a tracer to study silicon self-diffusion in thin film silicides of cobalt, nickel, palladium and platinum. The specimens were prepared by sequential electron beam evaporation of radioactive 31Si and of the metal onto cleaned silicon wafers. By vacuum annealing at the appropriate formation temperature a silicide about 250 nm thick containing a sharp radioactive band about 50 nm thick was generally formed. Subsequent heating above the formation temperature resulted in a spreading of the activity owing to silicon self-diffusion. Activity profiles in the silicides were measured by a combination of ion beam sputtering, radioactivity counting and Rutherford backscattering of charged nuclear particles. The diffusion coefficient of silicon in NiSi in the temperature range 650–750 °C was found to be D = 8.95 × 10 −9 exp ( -1.9 eV kT ) m 2 s −1 while in PtSi in the temperature range 480–570 °C it is known to be D = 3.59 × 10 −5 exp ( -2.1 eV kT ) m 2 s −1 Although the activation energies for self-diffusion are very similar, silicon mobility is much greater in PtSi than in NiSi. Complete intermixing of the radioactive marker and non-radioactive silicon atoms was found to take place within approximately 20 min at temperatures of about 570°C and 780°C for PtSi and NiSi respectively. Silicon atoms in Pd 2Si were also found to be very mobile at temperatures of about 570°C, but diffusion coefficients could not be measured since the initial band of radioactive silicon tracer was not sharp. Silicon was found to be very immobile in all the cobalt silicide phases and only started to spread out completely in CoSi 2 at temperatures above 800°C.