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Tensile Properties of Thin Gold Film
Figure 1. A gold tensile specimen.
The specimen is one micron (one millionth meter) thick and 150 micron wide by two millimeters long in the straight section. The film is deposited on a silicon wafer and photolithography creates a pattern around the entire specimen to allow etching away of the wafer underneath the gold. The left end remains attached to the film on the wafer, and four tiny straps, which are broken before testing, support the wide right end. A thin silicon fiber fiber (a bit bigger than a human hair) is glued to the right end; it is connected to a force transducer and piezoelectric actuator to pull the specimen until it breaks.
Figure 2. Gold stress-strain curves.
The results of an extensive series of tests on gold film of three different thicknesses – 0.15, 0.5, and 1.0 micron – are shown in Figure 2. All three show a strength that is very much higher than larger specimens of pure gold; this is a common observation and related to the microscopic mechanisms of ductile material behavior. However, the thinnest material is much more brittle, i.e. cannot elongate to a very large strain. This film – only 150 nanometers thick – contains tiny pores that cause it to fail at smaller strains.
Gold film is widely used in the microelectronics industry to connect transistors and other electronic devices on circuit boards. One must know its mechanical properties to properly design systems that can withstand thermal expansion/contraction and shock /vibration loading. The Sensors and Electronic Devices Directorate of the Army Research Laboratory sponsor this work.
References
Sharpe, W. N., Jr., J. Pulskamp, B. G. Mendis, C. Eberl, D. S. Gianola, R. Polcawich, K. J. Hemker, “Tensile Stress-Strain Curves of Gold Film”, Proceedings of IMECE 2006, 2006 ASME International Mechanical Engineering Congress and Exposition, imece2006-13290, 2006