Structural and Electrical Properties of Thin Metal Films Deposited at Low Temperature
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ABSTRACT Thin metal layers play an important role in the development of electronic devices. The thin metal films deposited at low temperature (LT=77K) showed some unique properties which enhanced device performance. The micro-structural properties of thin metal films formed at room temperature (RT) and LT were investigated. An insulating substrate was used for Au, Pd Al and Ag metal deposition. The metal films were deposited by vacuum evaporation with thickness ranged from 100A to 1000A. The surface morphology of the metal films was determined by transmission electron spectroscopy (TEM). The resistance of the films was insitu measured as a function of film thickness and temperature. Electrical measurement found that these films shown several orders lower resistance compared to the film obtained at room temperature at very thin thickness, which, implies potential application of these films on electronic and optoelectronic devices. It is found that the LT films showed much lower densities of grain boundaries than the RT samples. This is consistent with the resistivity measurement results.
INTRODUCTION Thin metal films formed by the vacuum evaporation have been great interest due to their application on electronic devices and circuits. The resistivity of the thin film, which plays important role for the thin film application, however, often show unsatisfactory results. According to modem quantum electronic theory, electrical conduction in metals is due to electrons, while electrical resistivity, p, defined as the reciprocal of the conductivity, is the result of electron collisions with impurities, lattice imperfections and phonons[l-2]. Electrons always undergo some scattering as they move through a solid, the average distance they travel between collisions being called the mean free path. The variation in the resistivity correspond to changes in the mean free path of the conduction electrons. Since an electron will suffer surface reflection when electron happens to reach it, the resistivity increases whenever the specimen becomes thin enough for collisions with the surface to be significant fraction of the total number of collisions. Therefore the resistivity of thin films is often much higher than that of the pure bulk materials[l]. For electronic device application, especially for connection purpose, however, it is often desired that the thin metal films to be with low resistivity. Normally, it is necessary for the electronic devices that the thin film to have low contact resistance between the metallic film and the semiconductor, then to have low electrical resistance along the film and also low contact resistance between the film and the external connections. Obviously, the resistance of the thin film itself is very important. In opto-
455 Mat. Res. Soc. Symp. Proc. Vol. 382 e 1995 Materials Research Society
electronic device applications, a metal-semiconductor (M-S or Schottky contact) structure is often used. In an M-S structure, the metal layer must be thin enough (normally semitransparent to visible light
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