Growth of tungsten carbide thin films as diffusion barriers for Cu metallization

The growth of W1-xCx thin films by reactive sputtering and thermal chemical vapor deposition, and their effectiveness against Cu diffusion was examined. Films were grown in a sputtering chamber using a W target and CH4 as a carbon source. Composition control was accomplished with changes in metal sputtering power, and C was incorporated by cracking the CH4 with a substrate bias plasma. Films between 1000 Angstrom and 2000 Angstrom thick were analyzed for composition by in situ X-ray photoelectron spectroscopy (XPS). Further analysis of morphology and film thickness was accomplished with scanning electron microscopy and transmission electron microscopy (SEM/TEM). Film resistivity was determined with the use of a 4-point probe measurement. Compositions for WC films ranged from W0.5C0.5 to W0.7C0.3, and resistivities ranged between 1000 and 200 muOmega-cm. XPS of the W 4f and C 1s peaks indicated the formation of a carbide chemical state as well as graphitic C incorporation. C-peak deconvolution showed the formation of more carbide as W-content increased. Scotch tape and 4-point flexure tests of film adhesion were performed on three low-kappa substrates, and SiO2 With successful results. Diffusion analysis was performed by annealing film stacks of Cu/W2C/SiO2 and W2C/CU/SiO2 for up to 8h at 670 K. These structures were analyzed by XPS, AFM, SEM, energy dispersive X-ray spectroscopy (EDX), secondary ion mass spectroscopy (SIMS) and TEM to gather chemical state and composition, morphology, and crystallinity information. The data suggest that tungsten carbide is chemically resistant and prevents interdiffusion of copper. The TEM data show formation of W2C nano crystal lites in the barrier layer. CVD films were grown using W(CO)(6) and C2H4 at temperatures ranging from 530 to 780 K. W2C is realized below 770 K and oxygen content is less than 3%. Films grown on the low-kappa dielectrics passed the scotch tape test; the films on SiO2 failed this test. Films grown at 560 K have a resistivity of 250 muOmega-cm. TEM analysis reveals the presence of W2C nanocrystallites. TEM was also used to determine that no Cu diffused through a 7 nm barrier film after annealing a CU/W2C/SiO2 structure at 670 K for 8h.