Microbridge Nanoindentation Testing of Plasma-Enhanced Chemical Vapor Deposited Silicon Oxide Films

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R12.4.1

Microbridge Nanoindentation Testing of Plasma-Enhanced Chemical Vapor Deposited Silicon Oxide Films Zhiqiang Cao1, Tong-Yi Zhang2, and Xin Zhang1 1

Department of Manufacturing Engineering, Boston University, Boston, MA 02215, USA

2

Department of Mechanical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China ABSTRACT Plasma-enhanced chemical vapor deposited (PECVD) silane-based oxides (SiOx) have been widely used in both microelectronics and MEMS (MicroElectroMechanical Systems) to form electrical and/or mechanical components. In this paper, a novel nanoindentation-based microbridge testing method is developed to measure both the residual stresses and Young’s modulus of PECVD SiOx films. Our theoretical model employed a closed formula of deflection vs. load, considering both substrate deformation and the residual stresses in the thin films. In particular, the non-negligible residual deflection caused by excessive compressive stresses was taken into account. Freestanding microbridges made of PECVD SiOx films were fabricated using bulk micromachining techniques. To simulate the thermal processing in device fabrication, these microbridges were subjected to rapid thermal annealing (RTA) up to 800ºC. A microstructurebased mechanism was applied to explain the experimental results of the residual stress changes in PECVD SiOx films after thermal annealing. INTRODUCTION The characterization of mechanical properties of thin films has been an active area of research, along with the development of Microelectronic devices and Microelectromechanical Systems (MEMS) [1, 2]. Using a conventional testing method, such as uniaxial tensile testing of a free-standing film, is often a frustrating task because the residual stresses in the thin film cause it to wrinkle or curve severely, rendering it useless for testing [3]. Microcantilever bending is also used for its simplicity in analysis [4, 5]. However, such a method does not have the capability to measure the residual stress in thin films because if there is any, it would be released at the free end of the cantilever. Yet another common technique, bulge testing, is limited by the stress concentration in the corners of the membranes, making it difficult to characterize the fracture strength of the thin films [6]. Zhang et al. have recently developed a novel nanoindentation-based microbridge testing method for mechanical characterization of thin film materials for microelectronics and MEMS applications [7-9]. Freestanding microbridges made of thin films were fabricated using conventional photolithography and bulk micromachining techniques. Nanoindentation tests were performed at the center of the microbridges and the load-deflection curves were obtained and further fit into theoretical models. Previously, this method was applied to study the tensile residual stresses in a single-layered silicon nitride thin film with negligible, if any, residual deflections [7]. In this paper, we present both theoretical analysis and experim