Electrical Methods for Mechanical Testing of Thin Films and Interconnects. The primary goal of this project is the development of electrical test methods for the mechanical properties and reliability of dimensionally-constrained materials such as films and interconnects, with present emphases on measurements of thermal fatigue and strength; a secondary goal is the development of microstructure-based lifetime prediction models using our measurements. To develop such methods and models, we also further the necessary materials science understanding of mechanical behavior in these materials.

Specific project objectives during FY06:

• Develop an electrical test to measure thermal fatigue lifetime:

  • Measure and control temperature;

  • Detect onset of open circuit failure;

  • Demonstrate influence of overlayers on lifetime;

  • Compare to fatigue data obtained by microtensile testing;

  • Devise predictive model for lifetime (based on grain size, texture).

• Develop electrical test to measure s_ys and s_UTS:

  • Detect and monitor onset of plasticity electrically;

  • Extrapolate fatigue data to single cycle (by way of low cycle fatigue rules);

  • Compare to properties measured by microtensile testing.

• Conduct end user-oriented round-robin test of nanoindentation-determined hardness and modulus:

  • Secure industry and academic participation;

  • Summarize and quantify uncertainties and their sources;

  • Compare to properties measured by microtensile testing.

This test method is based on the application of controlled joule heating to films and interconnects. Conditions are such that electromigration does not take place. Rather, heat is generated and dissipated within each power cycle. Cyclic thermal strain (De) is caused by the mismatch in coefficient of thermal expansion (Da) between the film/interconnect and the surrounding materials, due to a temperature change (DT):

For further details about the test method, please see reference 3 below.

The cyclic thermal strain causes plastic deformation within each power cycle. This is manifested as damage to an unprotected film surface (figure 1 - left) as well as crystallographic orientation changes of certain grains (figure 1 - right). We have also observed the growth of some grains during stressing. Quantitative measurements of orientation changes are shown in figure 2. Grains that deform tend to rotate towards a <112> or <113> orientation, which is consistent with slip asymmetries in tension-compression loading of fcc crystals. Lifetime plots such as that shown in figure 3 are consistent with what might be expected for behavior of bulk copper or aluminum. However, the thermal nature of this type of straining leads to somewhat different slopes from those seen in purely mechanical tests. 

Concurrent efforts are underway to develop both the quantitative measurement methodologies and the underlying materials science to explain both the measurements and the unusual nature of deformation of dimensionally-constrained materials.

Contact: Bob Keller, (303) 497-7651, email

Publications associated with this work: