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

Abstract

Temperature and film thickness are expected to have an influence on the mechanical properties of thin films. However, mechanical testing of ultrathin metallic films at elevated temperatures is difficult, and few experiments have been conducted to date. Here, we present a systematic study of the mechanical properties of 80-500-nm-thick polycrystalline Au films with and without SiN x passivation layers in the temperature range from 123 to 473 K. The films were tested by a novel synchrotron-based tensile testing technique. Pure Au films showed strong temperature dependence above 373 K, which may be explained by diffusional creep. In contrast, passivated samples appeared to deform by thermally activated dislocation glide. The observed activation energies for both mechanisms are considerably lower than those for the bulk material, indicating that concomitant stress relaxation mechanisms are more pronounced in the thin film geometry.

High-temperature tensile behavior of freestanding Au thin films

“…Understanding the mechanical behavior of metal thin films at elevated temperature is essential for the design of reliable devices, which often operate above room temperature. Several studies have been performed on the high-temperature behavior of films on substrates [2,[16][17][18][19], but studies on freestanding films have been limited to temperatures below 200ºC [20][21][22][23] due to difficulties associated with sample handling, oxidation, and temperature uniformity in the sample. The few studies performed at higher temperatures focused on films that were several microns thick [24][25][26].…”

Thermo mechanical properties and plastic deformation of gold nanolines and gold thin films

“…For temperatures T between 223 K and 393 K, again a linear but temperature dependent behavior with a slope of about 70 meV is observed corresponding to diffusion controlled plasticity along free (1 1 1) surfaces (120 meV for self-diffusion on a free (1 1 1) surface [38]). However, for the unpassivated nanolines, rather an exponential behavior is observed with activation energies of about 20 meV in the temperature range of 173-223 K, 90 meV around RT and about 300 meV for temperatures in the range of 343 K and 393 K. While a value of about 90 meV indicates again plasticity controlled by diffusion along free (1 1 1) surfaces, an activation energy E act of about 300 meV would indicate deformation controlled by diffusion along grain boundaries (Coble-creep) [8,39] and/or along free surfaces other than (1 1 1). The latter might be plausible since this sample set has a significant amount of free non-(1 1 1)-oriented surfaces at the side walls of the nanolines, which are not present in homogeneous thin films.…”

“…However, the (i) low applied strain rates, (ii) small grain sizes and (iii) free surfaces of the present nano-shaped samples might allow for diffusion based deformation mechanisms even well below 50% of the homologous temperature of gold. Let us assume a diffusional creep equation [8] (…”

“…Indeed, thickness/dimension and temperature dependent yield strength has been observed for as deposited homogeneous gold thin films with thicknesses in the range of 80-500 nm on Kapton TM substrate, where the yield strength varies (i) for 80 nm thick gold thin films with mean grain sizes of about 125 nm between 700 MPa (123 K) and 100 MPa (423 K) and (ii) for measurements at RT between 450 MPa (80 nm -mean grain size of 125 nm) and 300 MPa (500 nm -mean grain size of 295 nm) [8]. It was also shown, that passivation of the 80 nm gold thin films by SiN x raises the yield strength from 450 MPa to 900 MPa for testing at RT.…”

Microscale Experiments at Elevated Temperatures Evaluated with Digital Image Correlation

“…Along these lines, in the past, thermal expansion coefficients have been measured from thin microbeams and thin film structures by using both resistive (Joule) and uniform specimen heating [8][9][10][11]. Gruber et al [12] performed high temperature tension tests on metallic thin films deposited on thick polyimide substrate carriers using a custom heating assembly and temperatures −150 to 200°C. They computed 2D strain fields from optical measurements by digital image correlation (DIC) while the stresses were computed from diffraction data.…”