The authors have fabricated thin films of alpha tantalum (alpha-Ta) with crystalline and amorphous structures by nonequilibrium pulsed laser deposition techniques, and compared their electrical properties and diffusion characteristics with those of polycrystalline beta tantalum (beta-Ta) films produced by magnetron sputtering. The microstructure and atomic structure of these films were studied by x-ray diffraction and high-resolution electron microscopy, while elemental analysis was performed using electron energy-loss spectroscopy and x-ray dispersive analysis. The a-Ta with body-centered-cubic structure was formed only under clean, impurity-free conditions of laser molecular beam epitaxy. The resistivity measurements in the temperature range of 10-300 K showed room-temperature values to be 15-30 mu Omega cm for alpha-Ta, 180-200 mu Omega cm for beta-Ta, and 250-275 mu Omega cm for amorphous tentalum (a-Ta). The temperature coefficients of resistivity (TCRs) for alpha-Ta and beta-Ta were found to be positive with characteristic metallic behavior, while TCR for a-Ta was negative, characteristic of high-resistivity disordered metals. The authors discuss the mechanism of formation of a-Ta and show that it is stable in the temperature range of 650-700 degrees C. Electron energy-loss spectroscopy (EELS) and Rutherford backscattering measurements showed oxygen content in a-Ta films to be less than 0.1%. The secondary ion mass spectrometry, scanning transmission electron microscope Z-contrast imaging, and EELS studies show that, after 650 degrees C annealing for 1 h, a-Ta films have less than 10 nm Cu diffusion distance while polycrystalline Ta films have substantial Cu diffusion. The superior diffusion barrier properties of a-Ta for Cu metallization have been attributed to the lack of grain boundaries which usually lead to enhanced diffusion in the case of polycrystalline alpha-Ta and beta-Ta films. Thus, superior diffusion properties of a-Ta provide an optimum solution for copper metallization in next-generation silicon microelectronic devices. (c) 2006 American Vacuum Society.