Low temperature metal-organic chemical vapor deposition of tungsten nitride as diffusion barrier for copper metallization

A metal-organic chemical vapor deposition process has been developed for the growth of amorphous tungsten nitride thin films for barrier layer applications in ultralarge scale integration copper interconnect schemes. The process employs tungsten hexacarbonyl, $[\mathrm{W(CO}{)}_6]$ and ammonia $({\mathrm{NH}}_3)$ as, respectively, the tungsten and nitrogen sources. Tungsten nitride films were produced within a wide process window, including a substrate temperature of 200–350 °C, $\mathrm{W(CO}{)}_6$ flow rate of 1–20 sccm, reactor pressure of 0.2–0.5 Torr, and ${\mathrm{NH}}_3$ flow rates of 100–500 sccm. The films were analyzed by x-ray photoelectron spectroscopy, cross-section scanning electron microscopy, x-ray diffraction, transmission electron microscopy, four-point resistivity probe, and Rutherford backscattering spectrometry. These studies indicated that the films consisted predominantly of a ${\mathrm{W}}_{\mathrm{2}}\mathrm{N}$ phase. Films were grown with carbon and oxygen concentrations ⩽5 at. %, even at the lowest processing temperature investigated, where precursor dissociation would be expected to be the least efficient given the reduced thermal budget available to the decomposition reaction. Films deposited below 275 °C were amorphous, while those deposited between 275 and 350 °C were polycrystalline. Resistivities as low as 123 μΩ cm were achieved for 50-nm-thick films, with corresponding step coverage better than 90% in nominal 0.25 μm trench structures with aspect ratio of 4:1.