Advanced Bulge Test System
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ABSTRACT The bulge test is an established method of measuring the residual stress and elastic stiffness of thin-film materials. We present an advanced experimental bulge-testing system that provides advantages in speed, accuracy, and flexibility. Key innovations include the use of a Laser Doppler Displacement Meter to measure deflection, an actuated air-cylinder mechanism to vary pressure, and sophisticated computer handling of data acquisition and analysis. A test case demonstrates that repeatability of results is extremely good. INTRODUCTION The experimental technique often referred to as the "bulge test" [1] has proven to be very useful for characterizing mechanical properties of thin-film materials. In this technique, the measured deflection of a membrane as a function of applied differential pressure is used to determine the residual stress and elastic modulus parameters of the membrane. The bulge test has been implemented in various forms during the past decade by several different research groups [1-5]. Here we describe our innovative bulge test system, which we believe to have the best combination of speed, flexibility, and accuracy of any system yet reported. The capabilities of the system are especially suited to the demands of characterizing materials used in microelectromechanical systems (MEMS). In many kinds of MEMS devices, thin-film residual (processing) stresses and elastic properties are extremely important and may have to be engineered through extensive empirical work involving processing conditions. Hence it is advantageous to be able to obtain bulge test results from numerous samples rapidly. The samples may have a wide range of properties, so flexibility in testing is required in order to obtain good results for all samples. High accuracy is needed particularly for determining single-layer properties from multilayer-membrane tests and for determining Poisson's ratio. If the frame-fixed membrane is rectangular with a length-to-width ratio of three or more, it deforms elastically, in its central region, under pressure approximately as an infinitely long rectangle would [1]. For a long rectangular multilayer (composite) membrane, initially flat, the equation that relates center deflection h to applied pressure p during elastic deformation is p=4-t
E h3 +2tToh, 3a 4 1- v 2 a
(1)
where 2a is the width of the rectangle, t is the total thickness, and To is the through-thickness average in-plane residual stress. E/(l-v2 ) is also the thickness average, with E being in-plane Young's modulus and v being in-plane Poisson's ratio. Layers are assumed to be uniform and (here) isotropic in-plane. We will present a long-rectangle bulge test experiment that demonstrates the use of our system. The system can also be used for other variants of the elastic bulge test [1] as well as for tests of yielding [6] and adhesion [7] behavior of thin films. 229
Mat. Res. Soc. Symp. Proc. Vol. 505 01998 Materials Research Society
DESIGN
A schematic illustration of the entire system is shown in Fig. 1. The control of
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