Property of nickel silicides with hydrogenated amorphous silicon thickness prepared by low temperature process

Hydrogenated amorphous silicon(a-Si: H) layers, 120 nm and 50 nm in thickness, were deposited on 200 nm SiO₂/single-Si substrates by inductively coupled plasma chemical vapor deposition(ICP-CVD). Subsequently, 30 nm-Ni layers were deposited by E-beam evaporation. Finally, 30 nm-Ni/120 nm a-Si: H/200 nm-SiO₂/single-Si and 30 nm-Ni/50 nm a-Si:H/200 nm-SiO₂/single-Si were prepared. The prepared samples were annealed by rapid thermal annealing(RTA) from 200°C to 500°C in 50°C increments for 30 minute. A four-point tester, high resolution X-ray diffraction(HRXRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and scanning probe microscopy(SPM) were used to examine the sheet resistance, phase transformation, in-plane microstructure, cross-sectional microstructure, and surface roughness, respectively. The nickel silicide on the 120 nm a-Si:H substrate showed high sheet resistance(470 Ω/□) at T(temperature) < 450°C and low sheet resistance (70 Ω/□) at T > 450°C. The high and low resistive regions contained ζ-Ni₂Si and NiSi, respectively. In case of microstructure showed mixed phase of nickel silicide and a-Si:H on the residual a-Si:H layer at T < 450°C but no mixed phase and a residual a-Si:H layer at T > 450°C. The surface roughness matched the phase transformation according to the silicidation temperature. The nickel silicide on the 50 nni a-Si:H substrate had high sheet resistance(∼1 kΩ/□) at T < 400°C and low sheet resistance (100 Ω/□) at T > 400°C. This was attributed to the formation of δ-Ni₂Si at T > 400°C regardless of the siliciation temperature. An examination of the microstructure showed a region of nickel silicide at T < 400°C that consisted of a mixed phase of nickel silicide and a-Si:H without a residual a-Si:H layer. The region at T > 400°C showed crystalline nickel silicide without a mixed phase. The surface roughness remained constant regardless of the silicidation temperature. Our results suggest that a 50 nm a-Si:H nickel silicide layer is advantageous of the active layer of a thin film transistor(TFT) when applying a nano-thick layer with a constant sheet resistance, surface roughness, and δ-Ni₂Si temperatures > 400°C.