The Mechanical Properties of Electroplated Cu Thin Films Measured by means of the Bulge Test Technique
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The Mechanical Properties of Electroplated Cu Thin Films Measured by means of the Bulge Test Technique Yong Xiang, Xi Chen and Joost J. Vlassak Division of Engineering and Applied Sciences, Harvard University Cambridge, MA 02138, U.S.A. ABSTRACT The mechanical properties of freestanding electroplated Cu films were determined by measuring the deflection of Si-framed, pressurized membranes. The films were deformed under plane-strain conditions. The pressure-deflection data are converted into stress-strain curves by means of simple analytical formulae. The microstructure of the Cu films was characterized using scanning electron microscopy and x-ray diffraction. The yield stress, Young's modulus, and residual stress were determined as a function of film thickness and microstructure. Both yield stress and Young's modulus increase with decreasing film thickness and correlate well with changes in the microstructure and texture of the films. INTRODUCTION The mechanical properties of thin films are of great importance in many engineering applications such as integrated circuits and microelectromechanical systems, since they are directly related to device reliability [1]. In the past, much attention was focused on studying the mechanical properties of thin films of aluminum and its alloys as the material of choice for interconnects in integrated circuits. Recently, Cu has replaced Al in interconnects because it has higher electrical and thermal conductivity, and it has lower rates of electromigration [2]. Thus, the advent of Cu as an interconnect material makes it necessary to achieve the same level of understanding of its mechanical behavior as for Al thin films. During the past decade, several experimental techniques have been developed for measuring thin film mechanical properties. There are two categories of experimental techniques for testing thin films. The first category focuses on films on substrates and comprises techniques such as nanoindentation and the substrate curvature technique. These techniques require minimal sample preparation, but film and substrate properties need to be decoupled. The other category concentrates on freestanding thin films. Techniques in this category require some sample preparation but it is usually straightforward to extract intrinsic film properties. The bulge test belongs to this category [3,4,5]. In the bulge test, a freestanding thin film is deflected by applying a uniform pressure to the film (Figure 1). The thin film membrane is initially flat. As the membrane deflects, the film Figure 1. Schematic of the bulge test experiences an in-plane strain. By measuring the pressure (P) and the deflection (h) at the center of the membrane, the in-plane stress and strain can be determined. Elastic, plastic, and timedependent properties of the film can thus be obtained. One of the advantages of the technique is that strain can be changed quickly and isothermally, and that large strains can be imposed. For a L4.9.1
long rectangular membrane, the deformation can be taken as plane strain in the
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