Determination of yielding and debonding in Al-Cu thin films from residual stress measurements via diffraction

The yield strength and interfacial bonding are properties of interest for understanding void formation in thin film interconnect and subsequent failure of VLSI devices. A method is presented to examine the mechanical properties of thin polycrystalline films attached to substrates by measuring the change in thermal residual stress, due to the difference in coefficient of expansion between the film and substrate, as a function of decreasing temperature of the sample. The yield strengths of passivated 0.5, 1.0, and 2.0 μm thin films of Al-2% Cu on oxidized Si wafer substrates have been determined with this method to be 325, 170, and 120 MPa, respectively. Unpassivated films of the same thicknesses were also examined, but yielding did not occur for these films even though the residual stress reached a value of over 400 MPa. The lack of yielding in the unpassivated samples and the thickness dependence of the passivated samples is attributed to the grain size of these materials, which is less than the film thickness for the unpassivated case and greater than the film thickness after passivation. Debonding occurred in the 2 μm unpassivated sample but in none of the others, indicating a thickness dependence of the energy for delamination.