Electroless cobalt and nickel films can serve as both barriers and capping layers for copper metallization of high density integrated circuits applications. The top barrier layer, also called capping layer, is required to have additional properties, such as inhibiting Cu oxidation and corrosion and withstanding various cleaning steps as compared to the bottom barrier layers. In this work, we present a study of thin electroless cobalt-tungsten-boron layers that can serve as both bottom barriers (liners) and top barriers (capping) layers. The study is focused on the role of the boron and tungsten on the material barrier and oxidation resistance properties. In this study, we use cobalt-tungsten-boron (CoWB) films that had been made from cobalt-citrate based plating baths with amino-borane reducing agents. Two different bath types were studied; one bath contains alkali ions and another one is alkali ions free. In this paper, we present results of various cobalt-tungsten-boron compositions, including one sample with cobalt and boron only and another one with cobalt and tungsten only. We present those CoWB layer properties, such as composition, morphology, structure and resistivity. We demonstrate that boron and tungsten has different effect on the barrier and capping properties and define guidelines to optimize film properties for getting the best oxidation protection. Finally, we discuss the oxidation enhanced diffusion of Cu in barrier layers, its occurrence and possible mechanism.