Tantalum-molybdenum (Ta-Mo) thin films with various compositions are deposited on Si and HfO₂/Si substrates by rf (radio frequency) magnetron sputtering. The isomorphous Ta-Mo thin film shows an amorphous structure with Mo/Ta rf power at 20/80 W. It is reported that an amorphous structure of gate electrodes can reduce threshold voltage variability among different devices, caused by different orientations of polycrystalline grains [1]. This amorphous layer may eliminate the effect for the fabrication of next generation CMOS devices.

Different thicknesses of HfO₂ dielectrics (20, 40, 60, 80, and 100 Å) are obtained by metal-organic chemical vapor deposition. The Ta-Mo thin films are deposited on HfO₂/Si by cosputtering Ta and Mo targets. The composition of Ta-Mo alloys are controlled by varying the rf power (0/100, 20/80, 40/60, 60/40, 80/20, 100/0 W) on Ta and Mo targets. After that, a TaN layer is deposited on Ta-Mo layers to form a TaN/Ta-Mo/HfO₂/Si MOS capacitor. The capacitor is further annealed at 400^oC in a N₂-H₂ mixture for 30 min to study the thermal stability of the gate structure. The crystal structure is identified by grazing incident angle X-ray diffraction. Chemical bonding of Ta-Mo films is characterized by X-ray photoelectron spectroscopy. The resistivity is measured by using a four-point probe. Electrical properties of the MOS capacitors are characterized by capacitance–voltage (C-V) measurements. The work function is extracted from the C-V curves with different thicknesses of HfO₂.

The GIAXRD result suggests that the Ta and Mo phases appear for Ta-Mo films deposited with Mo/Ta rf power at 0/100 W and 100/0 W, respectively. Both of them are body-centered cubic (BCC) structures. For Ta-Mo thin films deposited with different Mo/Ta rf powers, the structures are also BCC structures, except for film deposited with Mo/Ta rf powers at 20/80 W. It exhibits an amorphous structure. In addition, the lattice constant decreases but the grain size increases as the rf power on Mo targets increases. The physical properties of Ta-Mo films and the effective work functions of the bilayer TaN/Ta-Mo gate electrodes will be addressed.

[1] M. E. Grubbs, X. Zhang, M. Deal, Y. Nishi, and B. M. Clemens, Appl. Phys. Lett. 97, 223505 (2010).