Brittle-to-ductile transition in ultrathin Ta/Cu film systems

Abstract

Current semiconductor technology demands the use of compliant substrates for flexible integrated circuits. However, the maximum total strain of such devices is often limited by the extensibility of the metallic components. Although cracking in thin films is extensively studied theoretically, little experimental work has been carried out thus far.Here, we present a systematic study of the cracking behavior of 34 to 506 nm thick Cu films on polyimide with 3.5 to 19 nm-thick Ta interlayers. The film systems have been investigated by a synchrotron-based tensile testing technique and in situ tensile tests in a scanning electron microscope. By relating the energy release during cracking obtained from the stress-strain curves to the crack area, the fracture toughness of the Cu films can be obtained. It increases with Cu film thickness and decreases with increasing Ta film thickness. Films thinner than 70 nm exhibit brittle fracture, indicating an increasing inherent brittleness of the Cu films.

Temperature dependence of mechanical properties in ultrathin Au films with and without passivation

“…Cracking leads to local stress relaxation, 29,30 which results in a continuous decrease of the average film stress. 27,28 Fracture was observed for Au samples at 123 K and for Au/SiN x and SiN x /Au/SiN x samples at low and high temperatures with a decreasing fracture strain with increasing temperature. Different mechanisms may be responsible for this.…”

“…This stress drop is typical for stress relaxation by sequential cracking of a ductile metallic film on a compliant substrate. 27,28 Upon unloading, compressive stresses developed in the film because the film was plastically deformed, whereas the substrate was still elastic. No delamination or buckling was observed.…”

“…6,37 The latter is promising because the size of dislocation sources is similar to the pinning distance of moving dislocations, and the corresponding Orowan stresses are close to experimentally observed flow stresses. 28,37 In this sense, the film thickness and temperature dependence of the flow stress for our Au films may result from impeded dislocation source activation and subsequent thermally activated dislocation glide.…”

Mechanical characterization of nanostructured thin films at different scales

“…Enhanced properties can be obtained by combining different materials in form of nanostructured thin film [4]. In particular, tungsten-copper (W-Cu) composites show very important mechanical properties resulting from the compromise between the high strength of W and the ductility of Cu [5][6][7].…”

Electro-Mechanical Testing of Conductive Materials Used in Flexible Electronics

“…In situ AFM studies have been used to better compare ductile deformation with brittle fracture with the use of the deformation spacing and through thickness crack spacing (Cordill and Marx, 2013a,b) as well as buckle growth (Jin et al, 2011;Marx et al, 2015a). XRD studies have been fruitful in the investigation of microstructure effects, thickness, and even the role of interlayers (Olliges et al, 2008;Gruber et al, 2009;Marx et al, 2015b). The use of in situ CLSM imaging is a relatively new in situ method and delivers information in three dimensions similar to AFM without height or scanning area restrictions.…”