Chemically Enhanced Physical Vapor Deposition (CEPVD) of TaN Diffusion Barrier Layers for ULSI Devices

J. Vac. Sci. Technol., B 22(6), 2734-2742, Nov/Dec 2004.

Li, N., Ruzic, D. N., Powell, R. A.

Physical vapor deposition (PVD) using ionized metal plasmas (ionized PVD or IPVD) is widely used to deposit conducting diffusion barriers and liners such as Ta and TaN for use in ultra-large-scale integrated (ULSI) interconnect stacks. Ionized PVD films exhibit the low resistivity, high density, and good adhesion to underlying dielectric desired for this application. On the other hand, extending PVD beyond the 45 nm technology node is problematic since IPVD may not provide sufficient step coverage to reliably coat features having high aspect ratio and sub-100 nm dimensions. Alternatively, chemical vapor deposition (CVD) and atomic layer deposition (ALD) can be used to deposit highly conformal metal films, but the electrical performance and interfacial quality may not equal that of PVD. To address future ULSI barrier/liner deposition needs, a method providing PVD-like film quality and CVD-like step coverage would be highly attractive. We have recently reported a hybrid approach to film deposition, referred to as chemically enhanced physical vapor deposition (CEPVD), in which a chemical precursor is introduced at the substrate during IPVD to provide a CVD component to the overall deposition process. The isotropic precursor flux is intended to provide film deposition on surfaces that are not impacted by the directional ions, such as the lower sidewall of a narrow via or trench. Conversely, the kinetic energy delivered to the surface by the flux of ionized metal may serve to enhance the desorption of CVD byproducts, reduce incorporation of impurities, and increase film density. In order to investigate the potential of CEPVD to deposit barrier/liner films, we have focused on the Ta-N material system since Ta/TaN is widely used as a diffusion barrier in Cu damascene processing. IPVD TaN films were deposited by reactive sputtering of a Ta target in Ar/N₂ using a planar magnetron and internal rf coils to provide a secondary ionization plasma for the sputtered neutrals. CEPVD was carried out by introducing a Ta-containing, organometallic precursor [tert-butylimino tris(diethylamino) tantalum] in the vicinity of the substrate surface during IPVD. Film thickness and step coverage were determined by cross-sectional scanning electron microscopy (SEM). Film composition, chemical state, and crystal structure were characterized using Auger electron spectroscopy, x-ray photoelectron spectroscopy, and x-ray diffraction, respectively. Resistivity was measured by four-point probe. Cross-sectional SEM showed improved step coverage over IPVD TaN. CEPVD film properties were highly process dependent; however, unlike IPVD TaN_x films that vary in stoichiometry but not purity, CEPVD “TaN” films contained relatively large amounts of carbon (~30%–60%) and could best be described as TaC_xN_y. Resistivity as low as ~370  µ cm was obtained for planar films of approximately 90 nm in thickness.