(A) study on the mechanism of amorphous phase formation by interdiffusion in multilayer Ni/Zr thin films

Abstract

Amorphous interlayer is formed between two elemental crystalline layers when some binary diffusion couples are heated to an appropriate reaction temperature. In this temperature range, named as the "temperature window", the formation of equilibrium compounds with a still lower free energy than the amorphous phase has been considered to be frustrated by the kinetic constraints: one constituent diffuses very slowly, there is a diffusional asymmetry, and apparently both constituents must diffuse reasonably fast if crystalline nuclei of an intermetallic phase are to be formed. However, the transformation mechanism producing the amorphous phase is still a matter of speculation. To understand the evolution of the solid-state amorphization, therefore, the author prepared the Ni/Zr multilayers with different structures, and developed a newly designed reactor for the heat treatment. Ni/Zr multilayers were prepared by DC magnetron sputtering with a base pressure below $5 \times 10^{-7}$ Torr, and a rotating substrate holder for preparing multilayer samples. The sputter pressures were 3 or 8 mTorr of argon gas. Multilayered thin films with alternating layers of Ni and Zr were deposited upon oxidized (100) silicon wafer substrates. For the heat treatment with sufficiently fast heating and cooling rates, we designed a quartz tube reactor that has two closed ends and one open end connected to a argon gas source: the initial heating rate was about $1500\,^\circ\!C$/min and the annealed sample was quenched into the liquid argon after the heat treatment. Sample preparation for cross-sectional transmission electron microscopy was extremely difficult and tedious job. It is important to note that each sample was mechanically thinned to approximately $2 \sim 3 \mu m$ and was finally ion milled less than an hour. XRD, DSC, AES, and ESCA were also performed to analyze the reaction in the diffusion couples. The microstructure of the sputter deposited film is strongly affected by such ...