TiW and Al Contacts to Shallow p´ Junctions - A Comparison between Furnace and Rapid Thermal Annealing (RTA)

The use of sputter deposited TiW (15/85 w.t.%) as a direct contact material to p⁺ silicon is evaluated. The effect on the contact resistance of different post-implant activation conditions, e.g. rapid thermal processing and conventional furnace annealing in dry or wet ambients, has been studied. For comparison parallell measurements were done on identically processed wafers metallized with aluminium. The resulting dopant profiLes were determined by spreading resistance profiling (SRP). Schottky barrier height measurements were performed on n-type wafers metallized with TiW or Al. The respective values were ¿Bn= 0.55 eV and ¿Bn = 0.8 eV. The results from contact resistance and SRP measurements show that a high surface carrier concentration is necessary in order to achieve a low contact resistance. This effect was more pronounced for the material with the higher barrier height to p⁺ silicon i.e. TiW was found to be sore sensitive to a small variation in the boron surface carrier concentration. Despite of this it was observed that given a high surface carrier concentration, after RTP the performance of TiW contacts to p⁺ silicon surpassed that of Al. in case of a direct Al-Si contact, silicon precipitation occurs in the contact hole thereby reducing the effective contact area. This will cause an increase in the resistance of contacts which becomes more pronounced for small dimensions. With TiW, silicon precipitation is avoided.