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Investigation of interdiffusion in copper/nickel bilayer thin films

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 احمد محمود عبد اللطيف الخفاجي 6/10/2011 4:42:40 PM
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          Introduction  The importance of understanding diffusion processes in thin films for controlling the quality of microelectronic devices, in terms of both efficiency and stability, dose not need to be emphasized. Copper is widely used in microelectronics and silicon solar cells, but copper is known to be a fast diffuser into silicon substrate and this could deteriorate the p-n junction [1]. Also, the poor adhesion of copper with some kinds of substrates in chip-packaging devices could be a problem. Therefore, an intermediate metal such as nickel is deposited first to reduce the diffusion and to improve the adhesion between copper and its substrateSince the fabrication of integrated circuits involves thermal annealing of thin  film systems at temperatures ranging from 473 K to 773 K, these systems can undergo diffusion processes, which result in a degradation of their performance. Diffusion in thin films can cause serious problems in microelectronic packing such as loss of bond strength, loss of solderability and loss of conductivity [2]. Therefore, the investigation of diffusion in thin film systems is of great importance. Diffusion in thin film is much faster than equilibrium diffusion in bulk materials since these films are characterized by a high density of defects such as dislocations, vacancies, and grain boundaries which act as paths of easy diffusion. So diffusion in thin films cannot be described by the extrapolation of data obtained for bulk materials at higher temperatures.In thin films the change in concentration profiles due to interdiffusion can be studied by Auger electron depth profiling. The interdiffused samples are sputtered by argon ions and simultaneously the composition of the surface is measured by Auger electron spectroscopy [3]. The degree of interdiffusion in copper/nickel thin films is of considerable interest in the microelectronics industry, so it has been extensively investigated [4-8], and it is still a subject of current researches [9-12] using a variety of analytical models and techniques. The objective of the present work is to study the interdiffusion in copper/nickel bilayer thin films emulating the fabrication processing conditions of typical chip-packaging module, using a modified Whipple model and employing Auger depth profiling (ADP) technique, X-ray diffraction (XRD) analysis, and scanning electron microscopy (SEM).             Experimental details    Nickel-copper bilayer thin films were deposited on highly cleaned [13] silicon (111) wafers by sequential evaporation, in single pump-down cycle, of pure (99.99%) nickel and pure (99.99%) copper in a vacuum of 2×10-6 mbar. Sequential evaporation in a single pump-down cycle is necessary to minimize interfacial oxides formation, which severely inhibit interdiffusion. The thickness of nickel layer was 150 nm and the thickness of copper layer was 300 nm. The deposition rate was 15 nm/min for nickel layer and 30 nm/min for copper layer. Since for thin films couples the grain structure is an important factor in controlling the rate of mass transport, the matrix metals were evaporated onto heated silicon (111) wafers at 383 K in order to stabilize the grain structure during diffusion. The substrate temperature was not raised above 383 K to prevent premature interdiffusion during deposition. After preparation, diffusion annealing was performed in a vacuum furnace of 4×10-6 mbar pressure at constant temperatures (473, 573, 673, and 773 K) for an annealing time between 5 min and 190 min.Composition-depth profiles were obtained by Auger electron spectroscopy in combination with in situ argon ion sputtering. The Auger system used was a SAM 660 scanning Auger electron microprobe manufactured by Perkin Elmer and was operated at the following conditions and specifications. The basic vacuum in the analysis chamber was 3×10-10 mbar and during sputtering it was 2×10-8 mbar. The analysis area for the Auger signal was (100×100) m2. The electron energy was 5 keV with a spot size of 1 m in a diameter and a current ranging between 500 and 600 nA. The samples were sputtered with 3 keV argon ions. The ion current was 800 nA with a spot size of 800 m in a diameter. The ions beam was scanned during sputtering in an area of (3×3) mm2.X-ray diffraction analysis was carried out employing a Philips PW 1710 automated diffractometer using monochromated Cu  radiation of 1.5406  in wavelength. The X-ray tube was operated at a voltage of 40 kV and a current of 30 mA. Surface morphology and microstructure of the investigated films were tested using a JEOL scanning electron microscope model JSM-6400. The probe current  was in the range 10-10-10-7A. The accelerating voltage of electron gun was 20 kV. 

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  • Auger depth profiling

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