Thin films are formed by formed by atomic layer deposition, whereby the composition of the film can be varied from monolayer to monolayer during cycles including alternating pulses of self-limiting chemistries. In the illustrated embodiments, varying amounts of impurity sources are introduced during the cyclical process. A graded gate dielectric is thereby provided, even for extremely thin layers. The gate dielectric as thin as 2 nm can be varied from pure silicon oxide to oxynitride to silicon nitride. Similarly, the gate dielectric can be varied from aluminum oxide to mixtures of aluminum oxide and a higher dielectric material (e.g., ZrO.sub.2) to pure high k material and back to aluminum oxide. In another embodiment, metal nitride (e.g., WN) is first formed as a barrier for lining dual damascene trenches and vias. During the alternating deposition process, copper can be introduced, e.g., in separate pulses, and the copper source pulses can gradually increase in frequency, forming a transition region, until pure copper is formed at the upper surface. Advantageously, graded compositions in these and a variety of other contexts help to avoid such problems as etch rate control, electromigration and non-ohmic electrical contact that can occur at sharp material interfaces. In some embodiments additional seed layers or additional transition layers are provided.
REFERENCE TO RELATED APPLICATION
The present application is a continuation in part of U.S. patent application Ser. No. 10/253,859, filed Sep. 23, 2002, now U.S. Pat. No. 6,933,225 which is a continuation of application Ser. No. 09/800,757, file Mar. 6, 2001, now U.S. Pat. No. 6,534,395, issued Mar. 18, 2003, which claims priority benefit under 35 U.S.C. .sctn.119(e) to provisional application No. 60/187,423, filed Mar. 7, 2000, all hereby incorporated by reference in their entireties.
A semiconductor substrate is loaded into a reaction chamber to form a tungsten layer. A source gas including tungsten (W) is introduced into the reaction chamber to grow a crystal nucleus of the tungsten on the semiconductor substrate. A reduction gas containing boron (B) is introduced into the reaction chamber to form a tungsten layer on the semiconductor substrate by actions of the source gas and the reduction gas. A hydrogen (H.sub.2) gas is introduced into the reaction chamber to remove the boron (B) remaining in the tungsten layer.
