Mechanical Testing of Thin Films
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MECHANICAL TESTING OF THIN FILMS C. T. ROSENMAYER*, F. R. BROTZEN*, AND R. J. GALE** *Rice University, Houston, Texas, 77251 "**Texas Instruments Inc., Dallas, Texas, 75265 ABSTRACT Metal films used as conductors in integrated circuits must be able to withstand stresses created during their deposition, caused by mismatches in thermal expansion coefficients and by bending which occurs during manufacture and installation. These stresses may be biaxial or uniaxial in nature. This paper discusses testing techniques and gives examples of results of mechanical testing of thin metallic films of aluminum, aluminum-copper(2%), copper, and tungsten in uniaxial and biaxial tension. The film specimens were deposited on and removed from production silicon wafers in order to simulate as much as possible actual device conditions. Basic considerations for the comparison of the two testing methods are also presented. INTRODUCTION It is well known that a substantial fraction of microcircuit failures originates in the thin metallic films that serve as interconnects. The presence of defects brought about by electromigration is a frequent source for these failures. An important additional cause for the deterioration of the films, however, is their inability to withstand the high stresses created partly in the course of the deposition and processing and partly by the large difference in thermal expansion between the metallic films and the nonmetallic substrates. As an example, the thermal expansion coefficient of aluminum is about eight times larger than that of silicon. As a result, thermal stresses generated in the metal at usual service temperatures are inordinately high. In view of this, the reliability of VLSI devices depends to a great degree on the mechanical properties of the metallic interconnects, the determination of which presents an interesting and challenging problem. In the present work, the discussion will be limited to two methods for the determination of stress-strain relations in thin metallic films: (a) the biaxial bulge-testing technique and (b) the uniaxial tensile-testing technique. Examples of film testing by both of these methods and attempts to correlate the results obtained by them will be presented. A background discussion on the techniques and the equipment used in the above methods by earlier investigators will be dispensed with here 2because it is well covered in the comprehensive reviews of this topic by Hoffman'. and, more recently, by Hardwick 3. EXPERIMENTAL PROCEDURES AND EQUIPMENT Specimen Preparation The first step in the preparation of specimens for biaxial and uniaxial testing is the ablation of the films from their substrates. In this study, Al, Al-alloys, Cu and W films of about I gim thickness of the type used in VLSI devices were used. In the case of Al Mat.Res. Soc. Symp. Proc. Vol.130. @1989 Materials Research Society
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and Al alloys, the films had been sputter deposited on previously oxidized Si-crystal surfaces of {001) orientation, while inthe case of Cu an intermediate layer of Mo of
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