Thin films are 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., ZrO2) 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 graded 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.