Structural Studies of Hydrogenated Amorphous Carbon Infrared Coatings
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STRUCTURAL STUDIES OF HYDROGENATED AMORPHOUS CARBON INFRARED COATINGS
C. J. Robinson, M. G. Samant, J. Stohr, V. S. Speriosu, C. R. Guarnieri* and J. J. Cuomo* IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120-6099 IBM Thomas J. Watson Research Center, Yorktown Heights, New York 10598
ABSTRACT: Hydrogenated amorphous carbon thin films are well known for their mechanical hardness and optical properties which make them useful for applications in infrared device coatings. In this work films have been prepared by plasma .decomposition of methane using an RF diode reactor operating under conditions of high self bias potential (Vb = 1 KeV). The resulting ion bombardment during film growth leads to the formation of hard, insulating carbon coatings which have a band gap of - 1.1 eV and are transparent in the IR. Nuclear reaction analysis has been used to quantify the atomic concentration of hydrogen incorporated in the films and extended x-ray absorption fine structure (EXAFS) has been used to determine local site geometry. Only first and second nearest neighbor bond lengths are observed with no evidence of further long range order or microcrystallinity. A model for atomic structure is proposed which includes both sp' and sp 3 bond configurations and direct comparisons are made with data obtained from sputtered carbon films, graphite and diamond. INTRODUCTION Research on hard carbon films has intensified in recent years as more applications are being found, particularly in the field of IR coatings [1,21. A bibliography of papers and reports covering the historical development of these films has recently been published by Woollam et al. [3]. On studying the literature it is found that hard carbon films have been produced by a wide variety of techniques including plasma deposition [4] ion beam deposition [5], sputtering [6], reactive sputtering in hydrogen [7] microwave discharge processes [8] and other variations thereof. In each case, properties of the films produced are reported for the particular preparation technique employed. Recent work, for example, has reported the formation of polycrystalline diamond thin films [8]. Many films however, are produced from a hydrocarbon gas precursor (usually CH 4 ) or involve hydrogen in some way so as to result in the formation of C-H bonding in the deposited material. Such films are reported to contain both sp 2 and sp 3 hybridized bonds [91 and are often termed "diamond like" because of their hardness, high refractive index (1.9
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