Elasto-Plastic Properties of Thin Gold Films Over Polymeric Substrates

Metallic thin films have been extensively used as coatings, interconnections, sensors and as part of micro and nano-electromechanical devices (MEMS and NEMS). The conventional substrates utilized to deposit those films are normally rigid, such as silicon. However, for applications where the substrate is subjected to significant mechanical strain (e.g. automotive coatings, electronic textiles, bioengineering, etc.) the film-substrate system needs to be flexible and conformable. Compliant polymeric substrates are ideal candidates for such a task. Some interesting mechanical properties not achieved with conventional rigid substrates can be transmitted to the film by the use of polymeric substrates. In this work, mechanical properties of 50 to 300 nm gold films deposited by thermal deposition over two thermoplastic substrates are investigated. A commercial thermoplastic, Polysulfone (“PSF”), and a home-synthesized isophthalic polyester based on the reaction of 4, 4′-(1-hydroxyphenylidene) phenol and isophthaloyl dichloride (“BAP”) [1] were used as raw materials for substrate production. Substrates were selected based on their good mechanical properties and flexibility. The use of two different substrates allows us to investigate the influence of the substrate mechanical properties in the bimaterial response. Substrates of 80 μm thickness were prepared by solution casting and cut to rectangular shapes of nominal dimensions of 30 mm × 5 mm. High purity (99.999%) commercial gold splatters were used for film deposition. Gold films with thickness of 50, 100, 200, and 300 nm were deposited onto PSF substrates by thermal evaporation inside a vacuum chamber at 3×10−5 Torr. Au films with 100 nm thickness were also deposited over BAP substrates. Four replicates of each type were deposited (at the same time) and used for tensile testing. Tensile testing of Au/PSF (film thickness 50–300 nm) and Au/BAP (film thickness 100 nm) specimens was conducted. Tests of the neat PSF and BAP substrates (6 replicates) were also conducted as a baseline. Tensile testing was conducted in a small universal testing machine with a load cell of 200 N and a cross head speed of 0.05 mm/min. The film mechanical properties were extracted from the tensile response of the film/substrate system, considered as a bimaterial. Based on sum of forces and strain compatibility, the film modulus (Ef) and stress (σf) can be extracted from characteristics of the bimaterial (EBim, σBim) and substrate (Es, σs), to generate a stress-strain curve for the film, see e.g. [2], Ef=1Af[ABimEBim−AsEs]=1+tstfEBim−tstfEs(1a)σf=1Af[P−Ps]=1+tstfσBim−tstfσs(1b) where P is the applied load, A = wt is the cross sectional area and sub-index “Bim” corresponds to the film-substrate bimaterial (ABim = w(ts+tf)). Figure 1 shows film stress (σ)-strain (ε) representative curves for Au films with different thicknesses extracted from the Au/PSF bimaterials. The film behavior presents only a small region of plasticity close to the ultimate strain. Thus, the numerical value of the maximum stress (strength) is close to its yield strength. The large plasticity of the substrate may hinder the plasticity of gold when acting as a bimaterial. As observed from this figure, the film modulus, strength and ultimate strain increase as the film thickness decreases, evidencing a “thickness-effect” not observed in bulk materials. Slightly different properties were obtained for the Au films deposited over the BAP substrate, which evidences some substrate-dependency of the film properties.