An Electrical Method for Measuring Fatigue and Tensile Properties of Thin Films on Substrates
Robert R. Keller, Nicholas Barbosa, Roy H. Geiss, David T. Read
A novel approach for measuring thermal fatigue lifetime and ultimate strength of patterned thin films on substrates is presented. The method is based on controlled application of cyclic joule heating by means of low-frequency, high-density alternating current. Such conditions preclude electromigration, but cause cyclic strains due to mismatch in coefficients of thermal expansion between film and substrate. Strain and stress are determined from measurement of temperature. Fatigue properties are a natural fit to testing by alternating current. S-N data were obtained from patterned aluminum lines, where stress amplitude was varied by changing current density, and lifetimes were defined by open circuit failure. Electron microscopy and electron backscatter diffraction observations of damage induced by a.c. testing suggested that deformation took place by dislocation mechanisms. We also observed rapid growth of grains - the mean diameter increased by more than 70 % after a cycling time of less than six minutes - which we attribute to strain-induced boundary migration. Ultimate strength was determined by extrapolating a modified Basquin relation for high cycle data to a single load reversal. A strength estimate of 250 -40 MPa was determined based on a.c. thermal fatigue data. In principle, an electrical approach allows for testing of patterned films of any dimension, provided electrical access is available. Furthermore, structures buried beneath other layers of materials can be tested.