In-situ Mechanical Testing of Metallic Thin Films at Elevated Temperatures
Testing of small-scale materials has historically been challenging due to the inherent difficulties associated with handling and gripping of the samples. It becomes even challenging when we try to test small-scale materials in extreme conditions. For example, the basic requirement for accurate elevated temperature mechanical testing, including a uniform temperature distribution throughout the sample, accurate measurement of temperature, and a controlled environment to prevent sample oxidation, are further complicated by small sample dimensions. As a result, only few studies report mechanical testing of freestanding thin films at elevated temperatures.
For testing sub-micron thick thin films at elevated temperature, we built a tensile testier with 9.7µN load resolution and 10nm displacement resolution that can be used inside a scanning electron microscope (SEM). Measurements at elevated temperatures were performed through use of two silicon-based micromachined heaters that support the sample. Each heater consists of a tungsten heating element that also serves as a temperature gauge. To demonstrate the testing capabilities, tensile tests were performed on sub-micron Cu and Au films at various temperatures up to 430℃. Stress-strain curves show a significant decrease in yield strength and initial slope for the samples tested at elevated temperature, which we attribute to diffusion facilitated grain boundary sliding and dislocation climb.
Testing of small-scale materials has historically been challenging due to the inherent difficulties associated with handling and gripping of the samples. It becomes even challenging when we try to test small-scale materials in extreme conditions. For example, the basic requirement for accurate elevated temperature mechanical testing, including a uniform temperature distribution throughout the sample, accurate measurement of temperature, and a controlled environment to prevent sample oxidation, are further complicated by small sample dimensions. As a result, only few studies report mechanical testing of freestanding thin films at elevated temperatures.
For testing sub-micron thick thin films at elevated temperature, we built a tensile testier with 9.7µN load resolution and 10nm displacement resolution that can be used inside a scanning electron microscope (SEM). Measurements at elevated temperatures were performed through use of two silicon-based micromachined heaters that support the sample. Each heater consists of a tungsten heating element that also serves as a temperature gauge. To demonstrate the testing capabilities, tensile tests were performed on sub-micron Cu and Au films at various temperatures up to 430℃. Stress-strain curves show a significant decrease in yield strength and initial slope for the samples tested at elevated temperature, which we attribute to diffusion facilitated grain boundary sliding and dislocation climb.
Image of (a) the fabricated microheater with magnified view of (b) the tungsten heating layer and (c) the thin film gauge length; (d) true stress-strain curve of 960nm thick Au films, showing both temperature and strain rate effect.