Diamondlike amorphous carbon films prepared by

Diamondlike amorphous carbon films prepared by magnetron sputtering of graphite N. Savvides and B. Window Citation: Journal of Vacuum Science & Technology A 3, 2386 (1985); doi: 10.1116/1.572887 View online: http://dx.doi.org/10.1116/1.572887 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/3/6?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing

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Diamondlike amorphous carbon films prepared by magnetron sputtering of graphite N. Sawides and B. Window CSIRO Division of Applied Physics, NSW, Australia 2070

(Received 7 May 1985; accepted 16 May 1985) Amorphous carbon films, a-C, were prepared by employing dc magnetron sputtering of a graphite target in an argon plasma. A series of a-C films were deposited as a function of sputtering power. The power was varied in the range 5-500 Wand the target effective sputtering area was about 20 cm 2 • The physical and optical properties of the a-C films show a dependence on the sputtering power. The films, although un hydrogenated, possess diamondlike properties such as high hardness, HV = 1200-2400 kgfmm- 2 , and are transparent in the infrared (IR) region with optical gap E oP! = 0.40-0.74 eV. We observe a systematic variation of film properties with increasing sputtering power which suggest a transition from fourfold (diamondlike) to threefold (graphitic) coordination of the carbon atoms. For films prepared at low sputtering power about 3/4 of the carbon atoms have tetrahedral coordination and 1/4 have trigonal coordination. At the highest sputtering power the a-C films consist of equal mixtures of tetrahedrally and trigonally bonded carbon atoms.

I. INTRODUCTION Hard, diamondlike amorphous carbon films have applications as optical and protective coatings. 1-7 The films are often produced from a plasma of ionized hydrocarbon species and depending on the method of preparation they can contain substantial amounts of hydrogen (up to 30 at. %). Thus the films are in fact amorphous carbon-hydrogen alloys and they are known as hydrogenated amorphous carbon, a-C:H, or as i-carbon (i-C). 1.3.7,8 The role of the hydrogen bonded into the amorphous carbon matrix is not well understood, but it is believed to help stabilize the tetrahedral coordination (Sp3 bonding) of the carbon atoms which is the origin of the diamondlike properties of the a-C:H films. We have produced thin films of amorphous carbon a-C containing no hydrogen but yet possessing diamondlike properties that are comparable to and in some instances better than those of a-C:H films. By controlling sputtering power and keeping all the other deposition conditions fixed the properties of the a-C films could be controlled precisely and reproducibly. In this paper we present the results of a systematic study of film properties. The a-C films were characterized using various techniques to probe their microstructure and chemical bonding and to carry out measurements of their physical and optical properties which relate to the use of these films as optical and protective coatings. Some of these properties are discussed further elsewhere. 8 . 9 II. PREPARATION OF a-C FILMS

The a-C films were prepared by dc planar magnetron sputtering of a graphite target in ultrapure argon gas (99.999% purity) at a pressure of 1 Pa and at a substrate temperature of about 300 K. The details of the magnetron apparatus and mode of operation are given elsewhere. 8 . 10 The use of dc magnetrons to deposit thin films offers a number of advantages over other deposition systems, including high deposition rates and the ability to scale to the larger sources and substrate areas necessary for industrial applications.

We have found that under conditions of fixed substrate temperature and sputtering plasma pressure, both the deposition process and the properties of the a-C films could be controlled precisely and reproducibly by the sputtering power. The graphite target used was a disk, 70 mm in diameter and 2 mm thick, which was metallurgically bonded onto a water-cooled copper backing plate. A permanent ring magnet with a center core provided the magnetic field confinement of electrons in a magnetic "tunnel" close to the surface of the target; the horizontal component of the magnetic field was 0.03 T. This configuration led to an effective sputtering area of about 20 cm 2 . Suitable substrates for film deposition were placed on a rotating substrate table whose axis was off center with respect to the center of the target so that the substrate to be coated was always directly under the center of the target. A mask having a rectangular aperture, and a movable shutter allowed up to eight distinct specimens to be produced in a single pumpdown by sequentially depositing onto each substrate while the others were masked. Graphite is known to absorb large quantities of atmospheric gases. In order to avoid the contamination of the growing films with these gases the target was degassed by pres puttering at 300-500 W for about 30 min immediately prior to film deposition. The a-C films were deposited at various power levels in the range 5-500 W. The deposition rate determined from thickness measurements was approximately proportional to sputtering power. Figure I shows the deposition rate measured directly below the center of the target at a target-to-substrate distance of 3-4 cm. The deposition rate per unit power is about 1.7XlO- 4 f-lmmin-I W- I; this is about ten times higher than conventional dc sputtering systems and it is comparable to the deposition rates attainable with e-beam evaporation. II III. PHYSICAL PROPERTIES AND STRUCTURE OF a-C FILMS

A systematic study of the influence of sputtering power on the mechanical, electrical and optical properties and on the

American Vacuum 2386 subject J. Vac.toSci. Technol. (6), Nov/Dec 1985 0734-2101/85/062386-05$01.00 238606:42:47 Redistribution AVS license Aor3copyright; see http://scitation.aip.org/termsconditions. Download @ to 1985 IP: 149.171.67.164 On: Society Tue, 15 Oct 2013

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N. Savvides and B. Window: Diamondlike amorphous carbon films

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microstructure of a-C films was undertaken. Recently we reported optical data which show the influence of sputtering power on the optical properties of films.8 Using these data we determined the average coordination number of the carbon atoms. This varied from 3.76 to 3.44 as the sputtering power increased from 5 to 500 W which implied that in films deposited at low sputtering power about 3/4 of the carbon atoms were tetrahedrally bonded, whereas about 1/4 were trigonally bonded. Films deposited at the highest power levels consisted of about equal mixtures of tetrahedrally and trigonally bonded carbon atoms. In this paper we discuss more general film properties which may help to establish applications of the films as optical and metallurgical coatings.

A. Density and electrical conductivity Figure 2 shows the mass density of a-C films determined using measurements of mass, surface area, and thickness. At

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the lowest sputtering power levels the films have a density in the range 2.1-2.2 g cm- 3 which is comparable to that of single crystal graphite reported to be 2.26 g cm- 3 • 12 Pyrolytic graphite has a lower density, 1.6--1.95 g cm - 3, as a result of voids and open porosity. 12 Increasing the sputtering power to 50 W causes a sharp decrease in density of the a-C films to about 1.9 g cm- 3 • Thereafter, the density decreases gradually to about 1.6 g cm -3 as the power increases to 500 W. By contrast, diamondlike hard a-C:H or i-C films (containing 5-50 at. % hydrogen) have densities in the range 1.51.8 g cm- 3 , although densities as low as 0.8 g cm- 3 and as high as 2.8 g cm -3 have been reported in literature. 5 •7 •13 Diamond has a density of3.5 g cm- 3 which is much greater than that of any of the diamondlike amorphous carbon films. Electrical measurements were carried out over the temperature range 77-500 K in order to establish the electrical transport mechanism in the a-C films. A four-probe geometry was used to determine the electrical conductivity (T of films 0.5-1 f-lm thick which were deposited onto Pyrex glass substrates. The measurements show the conductivity obeys the relation log (T ~ T - 1/ 4 over the employed temperature range. This temperature dependence of (T is consistent with Mott's variable range hopping mechanism for charge transport. 14.15 That is, the conduction in a-C is by thermally activated hopping or tunneling of electrons between localized states which are associated with the 1T bonds of threefold coordinated (graphitic) carbon atoms. 16• 17 Figure 2 shows the variation of the room temperature conductivity with sputtering power. It is apparent that a-C films prepared at low power possess insulating properties ((T~ 10- 4 ohm- I cm -I) which are similar to other extrinsic amorphous semiconductors such as a-Si, a-Ge, or a-C:H, where the hydrogen content is < 10 at. %. As the sputtering power increases to 50 W the conductivity shows an exponential rise with increasing power to I ohm - I cm - I . Thereafter, it rises asymptotically to 5 ohm - I cm - I.

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The Vickers hardness of the a-C films was determined using a hardness tester capable of applying loading forces in the range 0.01-0.1 N in a controlled manner. Films 2-3 f-lm thick were deposited on polished single crystal silicon substrates. About five indents per load were produced for two loading forces, one of 0.01 N and one of 0.04 N. The diameter d of the indents was measured using a scanning electron microscope. The hardness of the a-C films was derived using the relation HV

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where the Vickers hardness HV has units of kg force mm-2 with the loading force F in units of N, and d in mm. Several authors 3 • 18 have expressed reservations about this expression. We have been able to show that in the range of F = 0.01-0.06 N the above expression is valid for our films (see inset of Fig. 3). The hardness of the a-C films depends on sputtering power as shown in Fig. 3. Because hardness measurements are relative, we measured the hardness of a polished sapphire wafer using our procedure and obtained

Redistribution subject to AVS or copyright; see1985 http://scitation.aip.org/termsconditions. Download to IP: 149.171.67.164 On: Tue, 15 Oct 2013 06:42:47 J. Vac. Sci. Technol. A,license Vol. 3, No.6, Nov/Dec

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N. Savvides and B. Window: Diamondlike amorphous carbon films

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