8 - Measurement of Stresses in Thin Films and Their Relaxation

Abstract

This chapter discusses techniques for measuring stresses in thin metal films attached to substrates and their relaxation by plastic deformation and diffusional creep. In most technological applications, thin films are rigidly attached to a substrate that is typically much thicker than the film itself. As a result of this configuration, the thin films are commonly subject to high internal stresses, which may endanger the operation of small-scale devices. Failure mechanisms discussed in this chapter include the following: First, thermal-stress-induced voiding has been a problem for many years for Al or Cu conductor lines. This phenomenon is driven by high hydrostatic tensile stresses within interconnect lines under a capping layer during cooling. Even in blanket films, where two-dimensional in-plane stresses are observed, a few cases of stress-induced voiding have been reported. Second, stresses may also cause the delamination or cracking of thin films. It has been shown that the plasticity in thin metal layers affects the interface fracture resistance in thin-film interconnect structures. Specifically, the TaN/SiO2 interface fracture energy was measured in thin-film Cu/TaN/SiO2 structures in which the Cu layer is varied over a wide range of thickness. It is found that in a regime of 0.25 to 2.5 μm, the delaminating resistance is dominated by the contribution of plastic deformation in the metallic layer. Third, the behavior of thin metal films under cyclic loading conditions is given experimental as well as theoretical consideration. In particular, cracking of a brittle layer caused by ratcheting in an adjacent ductile layer is observed in a thin-film system under cyclic thermal loading.