Paper PS-TuP14
Improvement of Sputter Deposited Mo-based Barrier Films by Insertion of a Thin Al Interlayer for Copper Metallization

Tuesday, October 16, 2007, 6:00 pm, Room 4C

Copper is used as interconnects in advanced ultra large scale integration microelectronic devices due to its low electrical resistivity and superior resistance to electromigration compare to Al. However, Cu diffuses easily into Si and SiO₂, and forms copper silicide compounds at temperatures as low as 200 °C, resulting in degradation of Si devices at low temperature. Therefore, the use of diffusion barriers between Cu and Si becomes essential in order to successfully implement copper as an interconnecting metal. Sputter-deposited refractory metals, like W, Ta, Mo, and Ti and their nitrides have been recognized as diffusion barriers due to their high thermal stability, low resistivity and excellent capability of suppressing reactions between Cu and Si. In recent years, Mo-based diffusion barriers have been investigated for copper metallization. Many studies show that sputtered deposited Mo and MoN_x barrier layers are polycrystalline in nature and thus failed after annealing at relatively lower temperatures due to the diffusion of copper through the grain boundaries of the polycrystalline films. In this work, we investigate the barrier performance of sputtered deposited Mo and MoN_x due to the insertion of ultrathin Al interlayer. Al is used to stuff the grain boundaries of Mo and MoN_x thereby increasing the breakdown temperature of the barrier films. Mo and MoN_x films are sputtered deposited using Ar and Ar/N₂ mixture, respectively, under a 4.5 mtorr total sputtering pressure. The formation of crystallites takes place on the surface of the copper layer at the barrier failure temperature. The quantitative analysis of these crystallites is done using energy-dispersive spectroscopy. The thermal stability of Mo-based barrier layers are evaluated after annealing at wide range of temperatures in the presence of N₂ using four probe measurements for sheet resistance, X-ray diffraction analysis for phase identification and scanning electron microscopy for surface morphology. The interaction of different layers due to high temperature annealing is evaluated by depth profiling using X-ray photoelectron spectroscopy.