Thermal Stability Limits of Thin TiSi₂: Effect on Submicron Line Resistance and Shallow Junction Leakage

TiSi², the silicide most commonly used for a low resistivity self-aligned salicide process, must become thinner as the junction depth and poly-Si gate height decrease so as not to affect junction leakage and gate work function. The thermal stability of the thinner TiSi² during the back-end thermal process cycles, is an important concern. We report on the thermal stability of 300 to 700 A thin TiSi² on As, P, or BF² doped poly-Si to annealing at 750 to 850°C for 10 to 30 min determined by the increase in the resistance of long 0.3 to 1.5 μm wide poly-Si meander lines. The increase in line resistance is correlated with changes in the TiSi² microstructure. Poly-Si lines ≤0.5 μm wide with 500 A TiSi² increase their resistance after annealing at 750°C, 30 min. 500 A TiSi² is stable on 0.5 μm wide poly-Si lines after annealing at 800°C, 15 min. Silicide instability increases the reverse bias diode leakage measured for —1500 A shallow n⁺(As + P)/p-well junctions whereas it does not increase diode leakage for —2000 A shallow p⁺(BF²)/n-well junctions. Increasing TiSi² thickness improves thermal stability. Dopant type and concentration affect the TiSi² thermal stability through their effect on the TiSi² thickness (thinner on As doped Si) sintered with a particular Ti sintering process. We use Rutherford backscattering spectroscopy, transmission electron microscopy, and scanning electron microscopy to correlate the increase in effective sheet resistance of submicron wide poly-Si lines and the increase in ultra shallow junction leakage with an increase in roughness for 700 A thick TiSi² and agglomeration for ≤500 A thin TiSi².