Influence of Si content on the phase formation of W_1-xSi _x (0.30 ≤ x ≤ 0.74) gate electrodes and their work function

Tungsten silicide (W_1-xSi_x, continuous increase in Si content ranging from 30 to 74 atom %) films are deposited on SiO₂Si substrates by cosputtering from W and Si targets. After deposition, the samples are subject to a thermal annealing at 900°C to form crystalline silicide phases. With their almost stoichiometric compositions, 900°C annealed W _0.64Si_0.36 and W_0.31Si_0.69 exhibit W₅Si₃ and WSi₂ phases, respectively, while W_0.44Si_0.56 displays mixed phases of W₅Si ₃ and WSi₂. With excess W, W_0.70Si _0.30 film exhibits W₅Si₃ phase with a trace of body-centered cubic W. W_0.26Si_0.74 contains excess Si but shows a WSi₂ phase of similar crystallinity to that of W _0.31Si_0.69. Work functions of 900°C annealed W _0.70Si_0.30, W_0.64Si_0.36, W _0.44Si_0.56, W_0.31Si_0.69, and W _0.26Si_0.74 are extracted from the plot of flatband voltage vs SiO₂ thickness of the related metal-oxide-semiconductor capacitors, and the values are 4.34, 4.31, 4.61, 4.76, and 4.72 eV, respectively. It is deduced that the work function of W_1-xSi _x is mainly related with the phases, and their connection is discussed.