Study of grain growth and defect formation in Ag thin films by planar TEM with in-situ heating
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Study of grain growth and defect formation in Ag thin films by planar TEM with in-situ heating Brossard S.1, Martin F.1and Bourgon J.2 1 Saint Gobain Recherche, 39 quai Lucien Lefranc, 93303 Aubervilliers, France. 2 Institut de Chimie des Matériaux de Paris-Est, 2/8 rue Henri Dunant, 94320 Thiais, France. ABSTRACT Silver thin films are used as a functional layer in many applications such as lowemissivity and solar control coatings on glass for insulating windows, as well as transparent conducting electrodes for OLEDs and PV. For these applications, the conductivity of the film is critical; it is linked to the crystallinity and the grain size of silver layers which thickness ranges from 5 to 15nm. Such coatings often undergo thermal treatments up to 700°C aimed at toughening the glass substrate or improving the coating itself by promoting grain growth and curing point defects. This treatment can however dramatically damage the silver layer by inducing the formation of defects in the layer, such as holes or silver domes, decreasing both conductivity and light transmission of the coatings. Because of the extreme thinness of the films (less than 15 nm), the investigation of these phenomena requires in situ imaging at the nanoscale. In this study, grain growth and defects formation were observed in 15 nm-thick Ag films encapsulated with zinc oxide and silicon nitride using Transmission Electron Microscopy with in-situ heating from ambient temperature to 600°C. Significant grain growth was found to occur only from 400°C, and from 500°C holes in the silver layer started to form and grow, as well as thick silver domes formed by dewetting. Irradiation by the electron beam was also found to cause grain growth. INTRODUCTION Metallic thin films are used in a variety of applications where conductivity is a key property. A critical requirement to improve performance of metallic thin films is the quantification and modeling of physical effects that limit their conductivity. Electron scattering can occur at the interface with adjacent layers or be caused by grain boundaries, impurities, dislocations, twins, vacancies or holes due to de-wetting or non-complete coalescence [1]. The ability to measure grain size in metallic thin films is a requirement to assess the impact of grain boundaries on conductivity. A method of choice has been to use transmission electron microscopy in bright or dark field to study thin films on electron transparent membranes due to its superior resolution compared to scanning electron microscopy based techniques. Dannenberg et al used bright field TEM imaging to follow the evolution of grain size as a function of annealing temperature for 80nm thick Ag films on silicon nitride membranes and observed a stagnation of grain growth at each temperature step [2]. Using grain size measurement based on automatic analysis of dark field images of annealed Cu thin films 27 to 150nm thick, Sun et al were able to conclude that the contribution of grain boundary scattering to resistivity dominates that of interface scatte
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