Development of tungsten nitride film as barrier layer for copper metallization

Abstract

A process has been developed to deposit tungsten nitride (WxN) film below 350 °C to be used as a barrier layer in copper metallization. The film is deposited using tungsten hexa-fluoride (WF6) as a precursor in a hydrogen (H2)–nitrogen (N2)–argon (Ar) radio-frequency plasma. The tungsten-to-nitrogen ratio (x) in the tungsten nitride film determines its effectiveness as a barrier film. The process described in this article allows one to vary x from 1.2 to 3.7 by a single parameter: H2/N2 ratio. The article describes the characteristics of the film such as resistivity, stress, and deposition rate for different processing conditions. Film properties and surface conditions that cause WxN film to fail adhesion on oxide are indicated, and methods to improve adhesion are also discussed. Thickness measurements by alpha-step and scanning electron microscopy were correlated with measurements by the Rudolph Metapulse™ equipment to develop a methodology of nondestructively measuring film thickness, uniformity, and obtaining a first-order approximation of the W/N ratio of the film.