Tungsten metallization for LSI applications

Abstract

It is shown that MOS–LSI devices can be satisfactorily metallized with tungsten thin films using the rf-diode sputtering technique. The apparatus used consisted of a roots-blower and a Vacion pumped belljar system equipped with an 8-in. diam CVD W-cathode and a W-substrate table (anode) to which a dc bias could be applied. The control substrates were oxidized silicon wafers in which the back-oxide was removed to facilitate proper application of the bias. The effect of sputtering power (200–400 W) and of substrate bias (−200−+250 V) was studied on the deposition rate, substrate temperature, and various properties of the film such as electrical resistivity, stress, impurity concentration, constitution, and microstructure. The present process of W-film depostition has been found to offer several advantages. Low-power and low-voltage operation results in high quality MOS-compatible W-films. Substrate heating during the sputter deposition is kept to a minimum, thereby eliminating any undesirable metallurgical interactions in the W to Si Ohmic contact areas. The deposition rates can be controlled by adjusting the rf power. Use of a dc substrate bias offers further flexibility in control of film properties. Film resistivities are typically in the range 12–20 μΩ/cm upon sputtering at 250-W power with −125-V substrate bias. The film stresses are compressive (<2×1010 dyn cm−2) under negative bias; this has been found to lead to good step coverage. The films possess excellent high-resolution etchability. These factors make the present metallization process very suitable for CCD (single-level metallization) as well as LSI-IGFET (two-level metallization) applications.