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 WF6 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 beta-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 SiH4/WF6 flow ratio (i.e., 2.5) to deposit a thin tungsten film as the glue layer. It was found that the probability of the WF6 reduction with underlying aluminum was suppressed because the WF6 was completely consumed by SiH4 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., SiH4/WF6 flow ratio <1) to grow thick tungsten film. Secondary Ion Mass Spectroscopy (SIMS) measurements indicate that the fluorine impurities in tungsten aim 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.