Stresses in Sputtered Tungsten Thin Films

Internal stresses in 0.6 μm thick tungsten RF sputtered films, deposited on silicon wafers, were investigated in connection with microstructural aspects as a function of low positive and negative DC substrate bias voltage, in the range from +30 to −60 V. The argon pressure was fixed at 1.8 Pa and the substrate temperature measured at zero bias was varied so that the microstructure of the film could be typical of zone I or zone T in Thornton's model. No variation was observed in the structural aspects or in the stress value of the film when a low positive voltage greater than +5 V was applied. With increasing bias voltage from −10 to −60 V, both stress and microstructures properties were demonstrated to follow the same evolution as with a decreasing argon pressure: (a) for initially zone I films, presenting a porous structure, stress increased from zero towards tension as voids disappeared and then abruptly decreased when the structure became dense. The more porous the structure at zero bias, the more the zone I/zone T transition shifted towards increasing negative bias; (b) for dense zone T films, stress monotonically decreased towards compression. It was confirmed that oxygen absorption in the pores of the zone I structures produced a shift of stress with time towards compression.