ROLE OF STRESS RELIEF IN THE HEXAGONAL-CLOSE-PACKED TO FACE-CENTERED-CUBIC PHASE-TRANSFORMATION IN COBALT THIN-FILMS

Stresses in thin films can affect phase transformation temperatures. We show, however, that stress relief and grain growth in Co thin films occur at temperatures below the hcp to fcc phase transformation. Temperature ramped (10-degrees-C/min) in situ stress, differential scanning calorimetry, and resistivity were measured on evaporated 56-275 nm Co films, and microstructure was examined with transmission electron microscopy. As-deposited films are under tensile stress, almost-equal-to 1 X 10(9) Pa, which is relieved between 120 and 275-degrees-C. The as-deposited small grains (15-20 nm) undergo secondary grain growth at 230-350-degrees-C, giving a mixed phase bimodal grain size distribution with the smallest grains (both hcp and fcc) 20-25 nm in diameter and the largest grains ([0001] hcp) 150-200 nm in diameter. The hcp grain growth is signalled by an exothermic calorimetric peak at 282-degrees-C with an enthalpy of 0.15 +/- 0.04 kJ/mol and a resistivity decrease. The hcp to fcc phase transformation is signaled by an endothermic calorimetric peak with an enthalpy of 0.27 +/- 0.07 kJ/mol at 380-degrees-C although transmission electron microscopy reveals that the transformation is incomplete. There is no indication of a reverse transformation on cooling. Additional heat treatment to 736-degrees-C, on the thinner 56 nm Co film on SiO2 substrates, results in agglomeration and an accompanying decomposition of SiO2 catalyzed by Co. The results indicate that as-deposited tensile stress does not control the phase transformation temperature in Co thin films.