In-situ TEM observations of abnormal grain growth, coarsening, and substrate de-wetting in nanocrystalline Ag thin films

Abnormal grain growth is studied in nanocrystalline sputtered Ag films. Eig hty nanometer thick Ag films are DC sputter deposited onto back-etched amor phous silicon nitride membranes. Specimens are annealed in a heating stage in an in-situ TEM for various temperatures and hold times. With the same sp ecimen, we proceed to higher temperatures after the apparent halt of growth for sufficiently long hold times. The grain size distribution of the as-de posited films is bi-modal, with large abnormal grains with 100 nm diameters , embedded in a matrix of smaller grains of 15 nm diameters. Coarsening beg ins at temperatures of approximately 100 degrees C, and quickly reaches a p lateau. The growth process restarts only after sufficient temperature incre ases, and plateaus at each succeeding temperature. Using a variation of the Mullins-Von Neumann law, the activation energy for the abnormal growth is found to be 0.274 eV, consistent with the value reported for pore formation during electromigration via surface diffusion in Ag. Grain growth appears to stop above temperatures of 350 degrees C, eventually leading to triple j unction pore formation at 350 degrees C and de-wetting of the film from the substrate at 600 degrees C. The de-wetting is the high temperature limit o f the thermal grooving which cancels the driving force for grain growth at the lower temperatures. TEM images as evidence of this effect are presented , along with observations on the pore formation that support surface diffus ion as the mass transport mechanism for grooving, Fore formation, and as th e limiting mass transport mechanism for the grain growth. (C) 2000 Elsevier Science S.A. All rights reserved.