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How should one compare material fatigue strength in thin film area?
Sun, 2008-10-05 19:05 - Yunfei Ma
For several common electroplated materials: Ni, Cu, Au and Al, which one has strongest fatigue strength? We know the mechanical properties of thin films are different with their bulk counterparts due to the so called "size effect", and material properties depends largely on the microstructure and processing technique. But is there some mateiral laws or guidelines for designers to choose the "best material" as for fatigue resistant? Yield limit? ultimate tensile strength? or elongation? How could one compare material candidates without doing time-consuming test?
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comparing fatigue strengths of thin films
Hello Yunfei,
I think designing for fatigue requires a test that most closely simulates the anticipated operational conditions. Do you expect your electroplated materials to be subjected to purely mechanical cyclic loading? Or, do you expect them to be subjected to loading due to thermal cycling?
We have been developing a method for performing very rapid thermal cycling of patterned films, making use of joule heating as induced by alternating current applied with a probe station. We are able to distinguish fatigue resistance in terms of differences in lifetime (to open circuit failure) for a given temperature or strain range. Since the test is electrical, we can perform it quickly. It can effectively run at electrical frequencies of up to about 10 kHz (loading frequency of 20 kHz since the loading goes as the power) before there are concerns with whether the heat flow can keep up at such frequencies.
This test simulates thermal cycling very well, and uses electrical probes rather than a furnace. It is less clear whether the test accurately simulates purely mechanical cycling, since that depends on what is considered to be "high temperature" for a given material. Test conditions are such that electromigration does not occur.
We can distinguish lifetimes among different materials (e.g., Al vs. Cu vs. Au), different constraint (e.g., damascene Cu vs. nonpassivated Cu), line geometries (thickness, width, etc.).
Basics of the test method are covered in R. Mönig, R. R. Keller, and C. A. Volkert, "Thermal fatigue testing of thin metal films," Rev. Sci. Instrum. 75, 4997-5004 (2004). There are a few more papers out there too (search on any of the three authors), largely detailing observations of microstructural/damage evolution during the tests.
It is an option. I'd be happy to discuss it with you further if you're interested.
With regards,
Bob Keller
NIST
thermal fatigue of thin films
A similar question was asked by Xiaodong Li: http://imechanica.org/node/2880#comment-6963.
Good to see you here, Bob!
RH
Fatigue of thin films and micro/nanostructures
Thanks. I agree the issues like mechanical and thermal fatigue of thin films are of critical importance for practical applications. I have done some experiments on nanofatigue of thin films and Si nanostructures for MEMS/NEMS (see below).
Xiaodong Li and Bharat Bhushan, "Development
of a Nanoscale Fatigue Measurement Technique and Its
Application to Ultrathin Amorphous Carbon Coatings,"
Scripta Materialia, 47 (2002) 473-479.
Xiaodong Li and Bharat Bhushan, "Fatigue Studies of Nanoscale Structures for MEMS/NEMS Applications Using Nanoindentation Techniques," Surface and Coatings Technology, 163-164 (2003) 503-508.
I would like to see if there are any more papers about fatigues of thin films and interconnects. I would like to learn more about this topic. Thank you for your help.