Local porosity control in magnetron sputtered thin films: Layer design, argon pressure tuning and Kirkendall effect

Controlled Kirkendall voiding is a promising process to fabricate hollow nano- and micro-structures. This study expands beyond spherically and radially symmetric structures by addressing the voiding behavior of copper-gold thin films manufactured by direct current magnetron sputtering. Double and triple layered samples were fabricated using low and high argon gas pressures during the deposition. Annealing the samples at 300 °Celsius resulted in large voids located at the wafer to film interface and in the center of the film, depending on the layer structure. Additionally, small voids are present in high argon sputtering pressure samples. These are located along grain boundaries, as this is a location of preferential void nucleation. The final position of the voids is a combination of location of preexisting nuclei, the Kirkendall effect and mechanical stresses present in the film. To compare the final porosity of the films after annealing a novel combined approach of Rutherford backscattering spectrometry, atomic force microscopy and focused ion beam tomography was used. It revealed initial porosities of the as-deposited films to be between 0.5 and 5%. The porosity determined by FIB tomographies showed deviations compared to the results from the RBS/AFM measurements. Analyzing the void network by PoreSpy revealed insights into the void network structure. This work extends the use of the Kirkendall voiding as a route to fabricate voids in magnetron sputtered thin films, a process with a high degree of control over the deposited thin films and a pathway to scalable, novel applications of self organized thin film technology.