Suppression of Fluorine Impurity in Blanket Chemical Vapor Deposited Tungsten Film for Via Fills with A Novel Two-step Deposition Technique

Blanket chemical vapor deposited tungsten (CVD-W) offers the potential to fabricate reliable contacts for submicron multilevel metallization. In via filling application, various aluminum fluorides were formed by the reduction of WF 6 with aluminum underlayer. These compounds will stay at CVD-W/Al interface and act as insulating layers which cause electrical degradation. In addition, impurities like fluorine or oxygen induce the formation of β-W lattices as well as high film resistivity. In this work, a two-step chemical vapor deposition of tungsten was developed to suppress the fluorine impurities in tungsten films and at CVD-W/Al interface for blanket CVD-W application. The first step involves a gas phase nucleation with high SiH 4 /WF 6 flow ratio (i.e., 2.5) to deposit a thin tungsten film as the glue layer. It was found that the probability of the WF 6 reduction with underlying aluminum was suppressed because the WF 6 was completely consumed by SiH 4 before arriving onto aluminum surface. Meanwhile, this gas phase nucleated tungsten exhibits blanket deposition capability and an amorphous structure. The second step includes typical CVD-W process (i.e., SiH 4 /WF 6 flow ratio <1) to grow thick tungsten film. Secondary Ion Mass Spectroscopy (SIMS) measurements indicate that the fluorine impurities in tungsten film and at CVD-W/Al interface are drastically reduced. Also, the gas phase nucleated tungsten can be reproducibly deposited without attacking the aluminum underlayer. Moreover, a lower tungsten resistivity, lower via resistance and longer electromigration lifetime are achieved in the Al/W/Al Kelvin structures produced by two-step deposition technique than those of the typical CVD-W films deposited directly on aluminum.