Electroless cobalt films have been demonstrated to produce high quality barrier and capping layers for Cu. The best barriers so far include, in addition to the cobalt, refractory metals ions (e.g., W, Mo, or Re), and either phosphorus or boron. Electroless cobalt and its alloys can be used in micro- and nano-technologies, e.g. for microelectro-mechanical systems (MEMS), as well as for ultra large-scale integration (ULSI) technology of integrated cicuits.

In this work we present novel deposition solutions that is used to form cobalt-tungsten-boron films used morpholine borane as reducing agents. The films of cobalt that cointain small amounts of tungsten and boron were deposited by the electroless process. The cobalt-tungsten-boron coatings were deposited on the copper surface using a solutions containing (mol l^-1): CoSO₄ - 0.1, NH₂CH₂COOH (glycine) – 0.2, C₄H₈ONH·BH₃ (morpholine borane) – 0.2, Na₂WO₄ – (0-0.02), C₆H₈O₇ (citric acid) – 0.175. The bath operated at pH 7 and 60 ºC. The thickness of the compact cobalt-tungsten-boron coatings obtained under optimal operating conditions were ca. 0.5 µm. The morphology and strukture of the cobalt alloys were characterized by means of Field Emission Scanning Electron Microscopy. The composition of the cobalt-tungsten-boron films obtained was determined by means of X-ray Photoelectron Spectroscopy using an ESCALAB MKII spektrometer. To obtain depht profiles, the sample were etched in the preparation chamber by ionized argon at a vacuum of 5·10^-4 Pa.

The mikrostructure of the coatings changes with the intercorporation of tungsten into the coatings. The particles of cobalt-tungsten-boron films are larger and almost equilateral as compared with the cobalt-boron particles. XPS depth profile investigation does not show any significant diffusion of Cu atoms into cobalt after heat-treatment of the cobalt-tungsten-boron film. This fact confirms that the cobalt-tungsten-boron thin layer, deposited from glycine containing solutions, serves as a perfect diffucion barrier to prevent Cu diffusion. 

This research was funded by a Grant (No. TEC-06/2015) from the Research Council of Lithuania.