Effect of Carbon Addition and Passivation on the Mechanical Behavior of Freestanding Al Thin Films

Thin films are widely used as functional and structural elements in micro-electronic devices. Therefore, understanding the mechanical behavior of thin films at different length scales and environmental conditions is essential for the design of reliable devices. However, it is difficult to precisely measure the properties of small-scale materials with the methods that are employed for bulk materials; probing micro/nano scale samples is challenged by the inherent difficulties associated with fabricating and handling of extremely small specimens. For several decades, much work has been dedicated to characterizing the mechanical behavior of thin metal films and it has been observed that the mechanical properties of thin films can be very different to those of their bulk counterparts. In addition, passivated metal thin films showed interesting features such as higher strain hardening rate and enhanced failure strain.<br/> In this presentation, the effect of carbon addition and passivation on the microstructure and mechanical behavior of freestanding Al thin films will be discussed. Tensile specimens with sub-micron thickness were fabricated via sputter deposition followed by standard Si-based microfabrication techniques. Mechanical characterization was carried out by performing micro-tensile tests and membrane deflection tests. Supersaturated Al-C thin films containing 9.8at% C exhibit yield strength of about 400 MPa, over three times the yield strength of the Al thin films. This strengthening can be explained by the role of interstitial C atoms which block dislocations movement inside the Al matrix. In addition, an upper yield followed by drop in stress was observed in Al-C thin films which implies the formation of Cottrell atmosphere in supersaturated Al-C thin films. For passivated films, a large increase in the failure strain was observed when an ultra-thin Si₃N₄ layer (&lt; 10 nm) is deposited, which is attributed to delay in the strain localization due to constraint imposed by the passivation layer. Ongoing efforts utilizing high-throughput combinatorial experiments to identify the effect of carbon content on the mechanical behavior will also be introduced.