The Elastic Moduli of Silver Thin Films Measured with a New Microtensile Tester
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THE ELASTIC MODULI OF SILVER THIN FILMS MEASURED WITH A NEW MICROTENSILE TESTER J. RUUD*, D. JOSELL, A.L. GREER** and F. SPAEPEN* "Division of Applied Sciences, Harvard University, Cambridge, MA 02138 "-Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, U.K.
ABSTRACT A new design for a thin film microtensile tester is presented. The strain is measured directly on the free-standing thin film from the displacement of laser spots diffracted from a thin grating applied to its surface by photolithography. The diffraction grating is two-dimensional, allowing strain measurement both along and transverse to the tensile direction. In principle, both Young's modulus and Poisson's ratio of a thin film can be determined. Ag thin films with strong texture were tested. The measured Young moduli agreed with those measured on bulk crystals, but the measured Poisson ratios were low, most likely due to slight transverse folding of the film that developed during the test. INTRODUCTION Experimental techniques for measuring elastic moduli of free-standing thin films include microtensile tests [1][2113], the bulge test [4], the vibrating reed method [4], and tests based on the speed of elastic waves [5]. Of these techniques, the microtensile test is the most familiar one, and its analysis is the most straightforward. It is also the only one, besides the bulge test, that can be used for large strains. Accurate strain determination is the most difficult part of thin film tensile testing since the sample elongation is usually small and methods based on measuring the distance between the specimen grips require that those grips be non-compliant and allow no slipping. In this paper we describe a microtensile tester that uses a new method for determining strain. The strain is measured directly on the sample by monitoring the position of laser spots diffracted from a grating on the surface of the specimen. Elastic modulus measurements on a Ag film are given as an example. EXPERIMENTAL PROCEDURE The apparatus is shown schematically in Fig. 1. The sample is held on the two slides, bridging the gap between them, by polished aluminum grips. The film is elongated by a computer-controlled motor-driven micrometer attached to one slide. The force on the film is measured with a load cell in series, so that the stress can be calculated from the cross-sectional area of the film. A diffraction grating is created on the surface of the film by spinning a I p.tm thick photoresist layer on the specimen, exposing it to ultraviolet light through a mask, and dissolving the exposed part with photoresist developer. To increase its reflectivity, 600 A of the silver is evaporated on the remaining photoresist. Since the pattern consists of small, unconnected islands of low modulus photoresist, its effect on the force measurement is very small. A laser beam is directed on the grating, and the diffracted spots are observed on a screen a distance R from the specimen. When the film is strained, the repeat lengths of the diffractio
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