Electroless deposited Co0.9W0.02P0.08 thin films have been proposed as diffusion barriers and encapsulation layers for Cu metallization in ultralarge-scale integrated microelectronic devices. In this article, we present a study of the structure of these films and their evolution with thermal anneal up to 700 °C. The as-deposited microstructure is comprised of an amorphous CoWP component and nanocrystallites of hexagonal-close-packed (hcp) Co, approximately 5 nm in size. The amorphous CoWP component crystallizes to hcp Co at approximately 290 °C with an apparent activation energy of 1.6±0.1 eV, according to the Kissinger analysis. Isothermal anneals show that the rate of nucleation with time of the hcp Co grains is constant, and grain growth is controlled by diffusion. This diffusion is most probably of the P and W elements, which enrich the grain boundaries. At approximately 420 °C, the orthorhombic (o-) Co2P phase nucleates. The apparent activation energy of this phase transformation is 4.6±0.1 eV, according to the Kissinger analysis. We suggest that the nucleation of the o-Co2P phase occurs when the grain boundaries are saturated with P. At higher temperatures, the main structural changes observed in the films are the grain growth of the predominately hcp Co grains and a delayed transformation to face-centered-cubic Co compared to bulk Co. The chemical binding state of P after high temperature anneals is covalent as in the o-Co2P phase. I- n the as-deposited film, the chemical binding state of P is significantly different, attributed to the amorphous CoWP component and the P in a solid solution in the hcp Co grains. © 2003 American Institute of Physics.