Electrical Properties of Hard Carbon Films
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ELECTRICAL PROPERTIES OF HARD CARBON FILMS WALTER VARHUE*, KIRIL PANDELISEV** and BRIAN SHINSEKI*** * Dept. of Electrical Eng., University of Vermont, Burlington, VT 05405 ** Cominco Ltd./P.O. Box 3000, Trail, British Columbia, Canada VIR 455 ***Intel Corporation, Chandler, AZ ABSTRACT The electrical resistivity, optical band gap and activation energy for electrical conduction have been determined as a function of preparation conditions. The operating conditions for the glow discharge reactor have been interpreted in terms of ion energy and reactive species production. The change in the electrical properties could not be explained as a percentage of [SP 3 ] versus [SP 2 ] bonding ratio. Rather, these two species are embedded in an amorphous medium which determines the materials electrical properties. INTRODUCTION Hard carbon or diamond-like films have generated considerable interest because of their unusual electrical, chemical and mechanical properties [1]. J. C. Angus, Et. Al. [2] have assembled an extensive bibliography on matters concerning the deposition process and physical properties of the films. The electronic properties of a-C:H have been carefully reviewed in references [3] and [4]. The later reference contains a large bibliography on the electronic nature of the films. Theoretically, because of carbon's low atomic mass, a high thermal conductivity can be expected. This, coupled with a large band gap, makes diamond films a very attractive possibility for some specialized solid state devices. An example would be a semiconductor device operating in an environment with high temperatures or ionizing radiation. Carbon films are also extremely hard and chemically resistive. These properties make it an excellent encapsulation layer for high density integrated circuits. The film would efficiently act to conduct heat away to a sink while protecting the underlying circuitry. Films have been prepared by a variety of techniques including sputtering from solid sources and by glow discharge from gaseous hydrocarbon sources. Both techniques have produced, under proper conditions, films which are classified as "dense carbon". The use of hydrocarbon sources is an attractive method and have yielded films with a large hydrogen content as high as 60% [5]. The structure, hydrogen content and electrical properties of the films are extremely sensitive to the energy with which carbon ions hit the surface. This energy is controlled by reactor configuration and operating parameters such as power and pressure. The ion energies can Ipproach 1 KeV which could result in heating a small volume of the film to 10 *K for a brief instant. These high temperatures and short times are capable of forming metastable entities such as diamond. In this investigation we have performed a systematic study of the effect of operating conditions on the electrical conductivity and chemical structure of hard carbon films. The operating conditions have been translated into effective ion energies with measurement of the plasma potential and the dc bias fou
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