Thin films of nanometer length scales are central components for many technological applications, especially in the electronics industry. As the devices based on thin films shrink in dimension, the density of materials interfaces and grain boundaries increase. The large interfacial energy and fast grain boundary diffusion in thin films of such small length scales can profoundly affect the thermodynamics and kinetics of reactions between two thin film components. Non-equilibrium phases and unique kinetics of reaction may be observed.

This investigation is a calorimetric study of thermodynamic processes occurring at interfaces and in the bulk of thin films. Three binary systems were chosen, Cu/a-Si, Au/a-Si and Cu/Co, with thin film multilayers being fabricated by sputter deposition. The heat flow associated with thermal processes, such as nucleation and growth reactions and recovery processes, was measured by differential scanning calorimetry as the thin films were annealed at elevated temperatures. Sample characterization before and after annealing consisted of xray diffraction, x-ray reflectivity and transmission electron microscopy.

With a large driving force to form equilibrium alloys, solid state reactions were studied in the Cu/a-Si multilayers. The bulk enthalpy of formation of Cu3Si forming from Cu and a-Si was measured from a plot of the measured enthalpy o f formation versus inverse sample modulation and found to be -13.6±0.3 kJ/mol. The reaction rate to form Cu3Si was measured in the temperature range of 455 K to 495 K and found to be orders of magnitude slower than previous researchers had observed for bulk diffusion couples. This slower reaction was attributed to the defective, nanocrystalline Cu₃Si grown at the interfaces during sputter deposition.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Annealing experiments on Au/a-Si multilayers resulted in the formation of metastable Au silicides that form by different reaction mechanisms. The first phase, a gamma brass structure (yi, a= 0.96 nm), grew at 375 K by Si diffusion into the Au grain boundaries (Harrison type C kinetics) and lateral growth in the Au layers. The second phase, also a gamma brass structure (7 2 , a= 1.95 nm), grew at 475 K during metal induced crystallization of a-Si, where Au diffused into the a-Si layers. Heating above 600 K resulted in the decomposition of the metastable silicides and final crystallization of the a-Si. The enthalpy o f crystallization of a-Si was measured to be —12.1±1.2 kJ/mol. The resulting phase mixture of Au and Si present above 600 K was heated through the eutectic temperature of 636 K and depressed melting and a depressed enthalpy of fusion were observed.

The effects of annealing on the giant magnetoresistance (GMR) of Cu/Co magnetic multilayers were investigated. A trend towards a greater GMR signal for as prepared samples was observed for system base pressures below 1×10⁻⁷ torr. Multilayers with Cu layers of 0.9 and 2.0 nm, corresponding to the first two peaks in the magnetoresistance, were annealed at temperatures ranging from 368 K to 518 K. Annealing below 400 K was found to not affect the GMR. Annealing in the range of 518 K resulted in a decrease in the GMR at 10 K from 21% to 9%. X-ray diffraction was unable to reveal precisely the structural change that occurred in multilayer, but entropy driven mixing at the interfaces is proposed as the mechanism. Traditional differential scanning calorimetry was found to lack the sensitivity necessary to observe the heat flow associated with such a small-scale reaction.