Detection of reactions and changes in thin film morphology using stress measurements

Abstract

Mechanical stresses in thin films arise from thermal expansion differences Mechanical stresses in thin films arise from thermal expansion differences between the film and substrate, structural changes within the films, and intrinsic stresses from the deposition process. These stresses can lead to open circuit failures from voiding in interconnections, short circuits from hillocks, or delamination. Measurement of this stress in situ as a function of temperature have proven to be an easy and powerful technique for obtaining information on the temperature, time, rate, and extent of formation of both solid-state reactions and changes in morphology and structure. This information is helpful in obtaining an understanding and finding solutions for stress-induced voiding. A theoretical model is developed for the stress change when precipitation occurs from elements in solid solution. The stress during thermal cycling was measured for aluminum and copper on oxidized silicon substrates and for several aluminum alloys and layered films consisting of Si, Cu, Ti, W, Ta, V, Cu–Cr, and/or TiSi2. Correlating these changes using hot-stage transmission electron microscopy and x-ray diffraction has demonstrated that some stress changes do correspond to reactions and structural changes. In most multielement films, there were changes in stress that correlated to phase changes. Morphological changes such as during secondary grain growth of ultralarge grains (50–100 μm) in Al/Cu/Cr films can also be detected using stress measurements. Finally, pure copper and Mo/Cu films exhibited several differences in stress behavior as compared to aluminum.