Mechanical behavior of nanocrystalline Au films as a function of strain rate and film thickness

Gold thin films are widely employed as structural and electrical contact elements in RF-MEMS. A comprehensive experimental investigation was carried out by uniaxial tension experiments to extract the strain rate dependent mechanical behavior of nanocrystalline Au films in the range of 10^-6 - 10 ^-1 s^-1 full-field strain measurements were obtained with the aid of a fine speckle pattern (1 urn particle size) that assisted the use of digital image correlation. Our microscale tension experiments on Au films at different strain rates and for 0.83 μm and 1-76 μm thick specimens showed a clear monotonic increase in the elastic limit (≤ 640 MPa), yield stress (≤ 901 MPa), and ultimate tensile strength (≤ 938 MPa) with increasing loading rate. The nanocrystalline films demonstrated very large strengths and ductility: at the slowest strain rate, the ultimate failure strain was as high as 7%. Finally, the elastic modulus was not affected by the strain rate, E= 69.1 ± 2.1 GPa, and demonstrated small scatter across all strain rates.