Mechanical Characterization of Thin-Film Composites using the Load-Deflection Response of Multilayer Membranes - Elastic
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Mechanical Characterization of Thin-Film Composites using the Load-Deflection Response of Multilayer Membranes - Elastic and Fracture Properties Joao Gaspar, Patrick Ruther, and Oliver Paul Department of Microsystems Engineering (IMTEK), Microsystems Materials Laboratory, University of Freiburg, Georges-Koehler-Allee 103, Freiburg, 79110, Germany
ABSTRACT This paper reports on the refinement of a mechanical model for the load-deflection of multilayer membranes under uniform differential pressure and on its application to the experimental extraction of material parameters. Going beyond previous results, the analytical model takes into account the mechanics of multilayers and elastic supports covering all cases between rigidly clamped to simply supported structures and enables the straightforward assessment of stress profiles within the deformed structures. A comprehensive set of long membranes made of various multilayers of silicon nitride and oxide films are fabricated and characterized. The out-of-plane deflection profile under pressure load is monitored by means of a laser profilometer. The pressure is stepped up until fracture occurs. From the stress profiles in the membrane at fracture, the brittle material strength is analyzed using Weibull statistics. The bulge setup has been fully automated for the measurement of 80 membranes per wafer. This realizes, for the first time, the high throughput-acquisition of mechanical thin film data with convincing statistical control. INTRODUCTION The mechanical characterization of the mechanical properties of thin films is of great importance for their use in microelectromechanical systems (MEMS) and integrated circuits, since device functionality and reliability depend strongly on mechanical parameters. A way of characterizing mechanical parameters such as Young’s modulus E or residual stress σ0 of thin films is to measure the pressure-deflection response of membranes composed of those films. This is the so-called bulge test [1,2]. Membranes with square, rectangular and circular geometries have been measured and a wide variety of materials have been characterized [3]. This work extends the bulge test to the case of multilayers with compliant supports. This development is motivated by the fact that devices are usually composed of stacks of layers of different films rather than of a single thin film. Furthermore the bulge characterization can be quite difficult for membranes with significant levels of compressive residual stress since those structures often present complex buckled shapes [4,5]. Nevertheless, by combining such films with tensile layers, stress compensation is achieved and the analysis is simplified. A plane strain model taking into account the mechanics of long multilayered membranes with compliant supports has been developed before [6]. It is here expanded by the inclusion of pressure loads.
A bulge setup, automated for the measurement of up to 80 membranes on a silicon wafer, enables the extraction of mechanical data with statistical signific
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