Intermetallic phase formation and breakdown of Mo diffusion barriers in Ni-Mo-Cu layers
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I.
INTRODUCTION
D I F F U S I O N barriers are sometimes introduced into systems in which interdiffusion between two dissimilar materials would otherwise result in the degradation of mechanical or electrical properties of the materials in service. For example, a diffusion barrier may be placed between a substrate and a high-temperature protective coating, m Thin film diffusion barriers t2-51 have been introduced during metallization processes in integrated circuit technology. The thickness of a diffusion barrier, depending on its specific application, may range from nanometers to micrometers. In general, an effective diffusion barrier has to meet the following criteria: (1) The rate of transport of the adjoining materials through the barrier material should be slow. (2) The barrier material should adhere strongly with the adjoining materials so that the composite is resistant to thermal and mechanical stresses. (3) The barrier should be thermodynamically stable in contact with the adjoining materials. Owing to the importance of the different applications of diffusion barriers, extensive investigations have been undertaken, especially for thin films. However, most investigations emphasized the electrical and mechanical properties of the diffusion products, and diffusion-induced morphological changes were rarely studied except for a few systems, t6'71 In these studies, Cu/(Sn, N i ) / A u and C u / A g / N i triads were studied. In both cases, the breakdown of the barrier involved diffusive penetration and phase instability at the barrier interface. Another class of barrier is the sacrificial type, in which the barrier layer reacts to form a compound with one of the surrounding phases. A system of this type, Ni-Mo-Cu, was selected to extend our previous work tT] on mechanisms of breakdown of diffusion barriers. The system YINSHENG SHUEH, Associate Scientist, is with the New Materials Research and Development Department, China Steel Corporation, Kaohsiung 81233, Taiwan, Republic of China. JOHN P. HIRTH, Professor, is with the Mechanical and Materials Engineering Department, Washington State University, Pullman, WA 99164. ROBERT A. RAPP, Professor, is with the Department of Materials Science and Engineering, The Ohio State University, 116 West 19th Avenue, Columbus, OH 43210. Manuscript submitted December 14, 1989. METALLURGICAL TRANSACTIONS A
satisfies the above requirements initially, since Mo is insoluble in Cu at all temperatures and forms a single intermetallic phase, NiMo, at temperatures above 910 ~ Since diffusion data for the Ni-Mo system were limited, baseline measurements of NiMo formation kinetics were also performed for Ni-Mo couples. II.
EXPERIMENTAL PROCEDURE
A. Sample Preparation and Heat Treatments Nickel and copper rods of 99.997 and 99.999 pct purity, respectively, as well as 99.8 pct molybdenum foils of various thicknesses were used. The as-received Ni and Cu rods were annealed at 920 ~ and 700 ~ respectively, under a gas mixture of 5 pct H2/N2 for 48 hours. The diffusion couples and the end me
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