Effect of substrate bias on SE, XPS and XAES studies of diamond-like carbon films deposited by saddle field fast atom beam source
Introduction
Hydrogenated amorphous carbon (a-C:H) or so called diamond-like carbon (DLC) films are of considerable interest due to their unique combination of mechanical and optical properties [1], [2]. In recent times, it has been agreed upon that the technique employing a Saddle field fast atom beam (FAB) source for deposition of DLC films can eliminate many of the problems like incorporation of relatively high amounts of unbound hydrogen and charging of the substrates posed by traditional techniques of deposition like plasma enhanced chemical vapor deposition (PECVD). The FAB source operates on a dc power supply and the beam coming out from the source is almost neutral so that insulating substrates can easily be coated without any charging effect. The DLC films deposited using this technique have been studied by a few groups world wide [3], [4], [5]. We have made extensive characterization study of DLC films deposited using FAB source by aspirating various hydrocarbon gases [6], [7], [8] and have also reported a novel method of filtering the beam coming out of a FAB source using deflector arrangement [9], [10].
The bonding of carbon atoms in DLC films can hybridize in three different ways: sp3 hybridization which is a typical structure of diamond and silicon, sp2 hybridization which is a two dimensional graphite layered structure and sp1 hybridization which is a one dimensional structure. DLC films have got a mixture of sp2 and sp3 hybridization and the properties of these films depend very strongly on the ratio of sp3/sp2. Hence, while characterizing the structure of these films, the knowledge of the relative amounts of bonding is an important property to be looked into. Determination of sp3/sp2 ratio can be performed using bulk techniques, such as electron energy loss spectroscopy (EELS) [11], nuclear magnetic resonance spectroscopy (NMR) [12] and inelastic neutron spectroscopy [13]. EELS has got its limitation for hydrogenated DLC films due to their sensitivity toward irradiation with an electron beam [12]. Grill [14] have supported the NMR spectroscopy technique most suitable for this purpose. Lascovich et al. [18] Lascovich and Seaglione [19] have used XPS and XAES techniques for interpretation of this ratio. But, till now the researchers have not agreed upon a single technique, which can be used for the interpretation of this very important parameter accurately.
Given this backdrop, we in this paper, report our results regarding the effect of positive substrate bias (Vs) ranging from 0 to 180 V on the spectroscopic ellipsometry (SE), X-ray photoelectron spectroscopy (XPS) and X-ray auger electron spectroscopy (XAES) studies of diamond-like carbon (DLC) films deposited by FAB source using CH4 gas as source precursor.
Section snippets
Experimental details
DLC films of thickness about 0.2–0.3 μm were deposited on well cleaned mirror polished p-type silicon substrates with 〈100〉 orientation of about 0.3–0.5 Ω cm resistivity and cover glass of 0.15 mm thickness by saddle field fast atom beam source (ION TECH LTD., presently Atom Tech Ltd. FAB 110-2) using CH4 gas as a source precursor. Other details of the experimental set up and the saddle field FAB source have already been discussed earlier [6]. Besides applying the required potential to the source,
Spectroscopic ellipsometry (SE) studies
Fig. 1a and b show the typical variation of refractive index (n), extinction coefficient (k) for the DLC films deposited by a FAB source using CH4 as the source gas at different substrate biases over the photon energy range from 1.5 to 5.0 eV. Fig. 2a and b show the corresponding typical variation of real and imaginary parts of dielectric constants ε1 and ε2 for DLC films deposited this way. The values of n were found to fall in the range from 1.505 to 1.720, k range from 0.03 to 0.125, ε1 range
Conclusion
A point of inflection in all the properties of DLC films studied under present investigation at around 90 V of applied substrate bias has been observed. Exceptionally, high values of sp3 content, maximum of 97% in DLC films deposited using FAB source have also been observed. This has been attributed to the typical characteristic of DLC films deposited using a saddle field fast atom beam source.
Acknowledgements
The authors are grateful to Director, National Physical Laboratory, New Delhi (India) for his kind permission to publish this paper and to the learned referee for detecting an important mistake in the original version of the manuscript. Rajnish Sharma is thankful to Ministry of Science and Technology for providing the financial assistance during the course of this work and also to Director, BITS, Pilani for providing all the necessary facilities for making possible the communication of this
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