Electrical characterization of ultra-shallow junctions formed bydiffusion from a CoSi2 layer

Ultra-shallow p⁺/n and n⁺/p junctions were fabricated using a Silicide-As-Diffusion-Source (SADS) process and a low thermal budget (800-900°C). A thin layer (50 nm) of CoSi₂ was implanted with As or with BF₂ and subsequently annealed at different temperatures and times to form two ultra-shallow junctions with a distance between the silicide/silicon interface and the junction of 14 and 20 nm, respectively. These diodes were investigated by I-V and C-V measurements in the range of temperature between 80 and 500 K. The reverse leakage currents for the SADS diodes were as low as 9×10 ^-10 A/cm² for p⁺/n and 2.7×10^-9 A/cm² for n⁺/p, respectively. The temperature dependence of the reverse current in the p ⁺/n diode is characterized by a unique activation energy (1.1 eV) over all the investigated range, while in the n⁺/p diode an activation energy of about 0.42 eV is obtained at 330 K. The analysis of the forward characteristic of the diodes indicate that the p⁺ /n junctions have an ideal behavior, while the n⁺/p junctions have an ideality factor greater than one for all the temperature range of the measurements. TEM delineation results confirm that, in the case of As diffusion from CoSi₂, the junction depth is not uniform and in some regions a Schottky diode is observed in parallel to the n⁺/p junction. Finally, from the C-V measurements, an increase of the diodes area of about a factor two is measured, and it is associated with the silicide/silicon interface roughness