A new method for measuring the strength and ductility of thin films
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James W. Dally Mechanical Engineering Department, University of Maryland, College Park, Maryland 20742 (Received 1 June 1992; accepted 10 February 1993)
A new method of measuring the mechanical strength of thin films is described. We prepare miniature arrays of four tensile specimens, each 0.25 mm wide, 1 mm long, and 2.2 /xm thick, using deposition, patterning, and etching processes common to the semiconductor industry. Each array of four specimens is carried on and protected by a rectangular silicon frame. Thirty-six such specimens are produced on a single wafer. After a specimen frame is mounted, its vertical sides are severed without damaging the specimens. The load is applied by micrometers through a special tension spring. Tensile properties of a 2.2 /tm thick T i - A l - T i film were determined.
I. INTRODUCTION A. Background In spite of the widespread application of thin films, we know very little about their constitutive properties. Considering the vapor deposition process used in preparing thin films, we anticipate that some of the mechanical properties of the thin films may differ significantly from those of bulk materials of the same chemical composition. Measurements of the constitutive properties of thin films, including elastic modulus, yield strength, ultimate strength, ductility, strain-hardening coefficient, creep and stress relaxation constants, fatigue strength, and crack initiation toughness, are necessary to develop models that predict the behavior of films when subjected to mechanical and thermal stresses. The experimental challenges encountered in measuring the constitutive properties are due to difficulties in fabricating the thin film tensile specimens, in handling these thin and fragile specimens with cross-sectional areas of the order of 10~3 mm 2 , and in measuring small forces and displacements. Experimental techniques applied to thin-film testing1"9 can be divided into four different categories. In the first approach,1 a thin film is deposited on a suitable substrate, and then the specimen array is patterned in the film. The specimens are then stripped from the substrate and manually mounted in the grips of a miniature tensile testing device. The second approach is identical to the first, except that the grips are attached to the film specimen before it is stripped from the substrate.2"4 Mounting the grips on the specimen before stripping facilitates handling and improves alignment in the tensile loading device. A third group of investigators5'6 uses nanoindentation to measure yield strength and elastic modu1542
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lus. In nano-indentation, the force on the indenter is determined as a function of the penetration depth, with the film bonded to the substrate. The fourth approach7'8 involves tensile testing of small thin-film specimens, where the substrate is removed from under the specimen by etching. The advantage of separating the film and the substrate by etching is the relief of the residual stress produced in the vapor deposition process. B. Features
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