X-Ray Reflectivity of Ultra-Thin Diamond-Like Carbon Films
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X-RAY REFLECTIVITY OF ULTRA-THIN DIAMOND-LIKE CARBON FILMS B. K. Tanner1, A. LiBassi1*, A. C. Ferrari2, and J. Robertson2 1
Physics Department, University of Durham, Durham, DH1 3LE, UK Engineering Department, University of Cambridge, Cambridge, CB2 1PZ, UK *Now at: Dipartmento di Ingegneria Nucleare, Politecnico di Milano, 20133 Milano, Italy 2
ABSTRACT Grazing incidence x-ray reflectivity has been employed to investigate ultra-thin films of tetrahedral amorphous carbon (ta-C) grown with an S-bend filtered cathodic vacuum arc. The results indicate that x-ray reflectivity can be used as a metrological tool for thickness measurements on films as thin as 0.5 nm, which is lower than the range required for carbon overcoats for magnetic hard disks and sliders if they are to reach storage densities of 100 Gbits/in2. The density of the films was derived from the best-fit to simulated reflectivity profiles from models for the structural parameters. In such thin films, the x-rays are reflected mainly at the film substrate interface, rather than the outer surface, so that the film density is derived from analysis of the oscillations of the post-critical angle reflectivity. INTRODUCTION Hard amorphous diamond-like carbon (DLC) coatings exhibit mechanical, thermal and optical properties close to those of diamond. They can be deposited over a wide range of thickness by different deposition processes, on a variety of substrates at or near room temperature. This versatility, combined with high hardness, good wear and corrosion resistance properties has resulted in their extensive use as ultra-thin overcoats for magnetic media. In the past, electron energy loss spectroscopy (EELS) has been used to obtain the sp3 content (from the size of the π* peak in the carbon K edge absorption spectrum), and the mass density (deduced from the valence plasmon energy in the low energy loss spectrum). However, this analysis is dependent on choice of an electron effective mass and there has been extensive argument in the literature as to the correct value [1-3]. Grazing incidence x-ray reflectivity measurements permit the near surface mass density to be determined independently of the plasmon energy [4]. We have recently shown that the combination of electron energy loss spectroscopy and grazing incidence reflectivity resulted in the identification of a unique effective mass for all amorphous carbons and diamond to be used to convert the plasmon energy in mass density [5]. The correct general relation between density and coordination for carbons could then be determined [5]. In Ref. [5] we also showed, from the grazing incidence x-ray reflectivity data, that layering was not intrinsic to DLC films, but a function of deposition conditions. Most of the data were from films typically 20-100nm thick. As the thickness of films for application in magnetic recording media is typically only a few nanometers, we have examined the limits to the sensitivity of the x-ray technique for thickness and density measurement. In this paper we show that the techniqu
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