Highly reliable CVD-WSi metal gate electrode for nMOSFETs

In this paper, we first propose an improved chemical vapor deposition (CVD) WSi_x metal gate suitable for use in nMOSFETs. We studied the relationship between the Si/W ratio of CVD-WSi_x film and electrical properties of MOSFETs. As a result, it was found that the Si/W ratio strongly affects carrier mobility and the reliability of gate oxide. In the case of higher Si/W ratio, both electron and hole mobility can be improved. For CVD-WSi_3.9 electrode, electron mobility and hole mobility at 1.2 V of |V_g-V_th| are 331 and 78 cm²/V·s, respectively. These values are almost the same as those for n⁺-poly-Si electrode. The improvement of carrier mobility by controlling the Si/W ratio is due to suppression of fluorine contamination in gate oxide. F contamination at the Si/W ratio of 3.9 is found to be less than that at the Si/W ratio of 2.4 from XPS analysis. Workfunction of CVD-WSi_3.9 gate estimated from C-V measurements is 4.3 eV. In CVD-WSi_3.9 gate MOSFETs with gate length of 50 nm, a drive current of 636 μA/μm was achieved for off-state leakage current of 35 nA/μm at power supply voltage of 1.0 V. By using CVD-WSi_3.9 gate electrode, highly reliable metal gate nMOSFETs can be realized.