Conduction Mechanisms in Discontinuous Copper Films
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CONDUCTION MECHANISMS IN DISCONTINUOUS COPPER FILMS P. BIEGA9SKI AND E. DOBIERZEWSKA-MOZRZYMAS Institute of Physics, Technical University of Wroclaw, Wyspiazskiego 27, 50-370 Wroclaw, Poland
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ABSTRACT Copper films of coverage coefficients ranging between 0.2 and 1 were evaporated onto glass substrates under vacuum conditions (p w 10-8 Torr). For these films, the temperature dependences of resistance were measured in vacuo in situ. Making use of the obtained data TCR values, as a function of coverage coefficient and the activation energy for the films with different coverage coefficients were established. The resistance of films was found to change, and TCR values approaching zero -for coverage coefficients between 0.63 and 0.75. This interval (Aq0 ) can be regarded as the percolation interval for discontinuous Cu films. INTRODUCTION In the past decade, particular attention has been given to the physical properties of two- and three-dimensional inhomogeneous systems. While references to the electrical properties of discontinuous silver, gold or platinum films are frequent in literature, only few publications have been devoted to copper films (1]. In the present study, the effect of the film structure on the electrical properties of discontinuous copper films is investigated. EXPERIMENTAL Copper discontinuous films were evaporated-on quartz glass substrates at a rate varying from 5 to 10 nm s-1, a temperature of 1500 C and under ultra-high vacuum conditions (p=10- 8 Torr). After evaporation, the films were annealed at 4000 C in high vacuum for 2 hours. The thicknesses of the films were determined by the multiple beam interference method, whereas the film structure was investigated by electron microscopy, using carbon replica. Making use of the data obtained, the coverage coefficients were Mat. Res. Soc. Symp. Proc. Vol. 195. @1990 Materials Research Society
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established by the method described by McKenna and Ward [2]. The coverage coefficient was measured with an accuracy of Aq = = ±0.04. Using the system shown in Fig. 1, electrical measurements were performed in situ under vacuum conditions. The temperature dependence of resistance was measured in the temperature glass
5mm
Ag electrodes DC AMMETER
DIGITAL VOLTMETER
DC SUPPLY
Fig. 1. Electric circuit for resistance measurement
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range from 50 to 3500 C for discontinuous films with a coverage coefficient varying from 0.20 to 1. The measuring accuracy was to withinAR/R = ±5%. The measured results made it possible to calculate the temperature dependence of TCR for films with different coverage coefficients. The TCR was related to the coverage coefficients for two temperatures, T 1 = 1500 C and T 2 = 2000 C.
Microscopic examinations have revealed microstructure variations with the increasing coverage coefficient.
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Figure 2
c
b 0.5 jrm
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I II different coyfilms with Fig. 2. Electron micrographs for Cu erage coefficients: q = 0.33 (a), q = 0.48 (b), q = 0.63 Cc) II
I
gives the electron micrographs of discontinuous films w
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