While hillocks usually form to relieve compressive stresses in thin films resulting from electromigration or a difference in thermal expansion in the film and substrate, we have observed hillock formation in tensile-loaded films. We have used transmission electron microscopy to study hillocks which formed in free-standing gold thin films of 25nm nominal thickness. Grain growth during 150°C anneals reduced the grain boundary area and associated free volume, placing the films under tensile stress. While hillock formation could only increase this stress, large single crystal or polycrystalline hillocks with thicknesses up to three times the film thickness are observed after 400°C annealing, after long room temperature anneals, and during TEM observation. These observations suggest the operation of a hillock formation mechanism not explained by any existing hillock formation theories.
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Harris, K.E., King, A.H. Hillock Formation in Tensile Loaded Films. MRS Online Proceedings Library 391, 73 (1995). https://doi.org/10.1557/PROC-391-73