Ta-Mo Isomorphous Alloy Systems for Metal Gate Apllications

Abstract

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 400oC 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). Figure 1