Thin Solid Films, 40 (1977) 299-308 © Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands 299

VACUUM-DEPOSITED CARBON COATINGS*

N. K. AGARWAL AND A. D. HAUBOLD General Atomi~ Company, San Diego, Calif 92138 (U.S.A.)

(Received April 23, 1976; accepted July 20, 1976)

In recent years, highly favorable results have been obtained using low temperature isotropic pyrolytic carbons in prosthetic devices requiring a high degree of thromboresistance. The development of vacuum-deposited carbon coatings was undertaken to extend the application of carbon to geometries and configurations that cannot be fabricated from low temperature isotropic carbon. Vacuum-deposited coatings have been produced on a variety of metallic and polymeric substrates.

The different vacuum deposition processes which have been investigated include electron beam gun evaporation using high vacuum, gas scattering and ion- plating conditions. In addition, sputtering processes using ion beams and magneti- cally confined plasmas were studied.

The surface morphology, structure and preferred orientation of the coatings produced by the different processes were characterized by scanning and trans- mission electron microscopy. Film purity and interfacial characteristics were examined by Auger electron spectroscopy.

The scanning electron microscopy study shows that thin carbon films generally have a smooth and featureless surface morphology. However, other surface morphology features are obtained in thicker films, depending on the processing conditions. The transmission electron micrographs show the absence of structure and growth features. Electron diffraction indicates that the films consist of a turbostratic phase and a non-crystalline phase. The apparent crystallite sizes are small, and there is no three-dimensional ordering. Generally, the films are isotropic and consist of relatively pure carbon, with the degree of disorder dependent on the process conditions.

1. INTRODUCTION

Low temperature isotropic (LTI) carbons deposited in a fluidized bed have been used extensively for coatings on artificial heart valve components 1. Normally, only relatively small and refractory substrates can be coated with this recognized biomaterial since the deposition temperature is in excess of 1000°C. Vacuum

* Paper presented at the International Conference on Metallurgical Coatings, San Francisco, California, U.S.A., April 5--8, 1976.

300 N.K. AGARWAL, A. D. HAUBOLD

processes, in many cases, allow deposition at much lower temperatures. A variety of vacuum deposition techniques were investigated to extend and

increase the types of substrates and configurations. In all cases, the primary goal was to deposit carbon films at lower temperatures while maintaining the character- istics of LTI carbon, namely density, structure, preferred orientation and crystallite size. While high vacuum evaporation, gas scattering, ion beam sputtering and planar magnetron sputtering have all been used, the primary results reported in this paper are the studies on ion-plated carbon films.

In the last 15 years, one of the most significant contributions in the field of vacuum deposition technology was made by Mattox 2 when he first reported the ion- plating process. Detailed descriptions of this work have been published in a number of excellent reports and review papers 3'4. Chambers and Carmichael 5 made an important advance by using electron-beam-heated sources, thus allowing high rate refractory material coatings while using the ion-plating process. Many re- searchers 6-9 have reported successful applications of the ion-plating process in solving different coating problems. Other applications can be found in refs. 3 and 4.

The distinguishing feature of the ion-plating process is that the substrate surface is subjected to a flux of energetic ions that is sufficient to cause appreciable sputtering before and during film formation 3, Ion bombardment sputter cleans the substrate surface, provides an energy flux and influences nucleation and growth processes, all of which should result in a better adhesion of the film. The introduction of microroughness and high defect concentration in the substrate surface, enhanced diffusion and chemical reaction processes, and even physical mixing of film and substrate material during interface formation are considered to be important factors. Another feature of the ion-plating process is the gas scattering of the depositing flux, which results in relatively uniform coatings on non-planar geometries without complex and often difficult substrate rotation.

A number of investigations have been reported on the structure-property relationships in thick coatings deposited by vacuum evaporation. Most notably, the effect of substrate temperature on growth morphology and mechanical properties has been well established for a number of materials including pure metals, alloys and refractory compounds 1°, 11. The microstructure of the deposit shows columnar growth at substrate temperatures above 0.3 T~ (where Tm is the melting point of the deposit in K), which changes to an equiaxed grain morphology above 0.45T~- 0.5Tin. At substrate temperatures below 0.3Tm the structure grows as tapered crystallites containing appreciable longitudinal porosity. The above observations are explained in terms of temperature effects on surface mobility, nucleation density, grain growth and recrystallization. These temperature effects are important in both vacuum evaporation and the ion-plating process. However, the energy flux and sputtering/redeposition phenomena occurring in the ion-plating process may influence the growth morphology and properties of thick films 12. Quite often, higher density films with disrupted columnar growth and desirable mechanical properties are obtained 13

Wan e t al . 14 have studied the effect of processing conditions on the characteristics of ion-plated coatings. In addition to substrate temperature and bias power density, gaseous discharge pressure and deposition rates can have significant effects on the microstructure and properties of the deposits. The desirable features

VACUUM-DEPOSITED CARBON COATINGS 301

in most metallurgical coatings are a high density, an equiaxed grain morphology, a smooth surface and good mechanical properties. A careful study of process variables is important in order to obtain these features. Some of these process variables as related to the development of ion-plated carbon coatings are discussed.

2. EXPERIMENTAL TECHNIQUES

2.1. Synthesis An electron-beam-heated pure carbon source was used to introduce carbon

vapor into the gaseous discharge (argon) surrounding the negatively biased substrates. A differential pumping mechanism allowed high vacuum in the electron emitter region while the carbon source was placed in the high pressure ((4-20) × 10 -3 Torr) coating chamber. This was accomplished by providing a barrier plate with a small orifice for the electron beam to enter the coating chamber and to impinge upon the carbon source. The entire vacuum chamber was evacuated to a base pressure of less than 5 × 10- 7 Torr, and pumping was continued during the coating process.

The important process variables that were studied were the source-to-substrate distance, the deposition rate, the coating chamber pressure and the substrate bias voltage. The substrates included polished stainless steel (type 304) and titanium sheets approximately 0.0127 cm in thickness. The chamber pressure and the deposition rates were maintained at predetermined values by using closed-loop control devices. The substrate temperature was measured by using an IR radiometry technique.

2.2. Electron microscopy The surface morphology of carbon films was studied by scanning electron

microscopy (SEM); a Hitachi HHS-2R microscope was used. Both thin ( < 1 tam) and thick ( > 5 lxm) films were examined for growth features in the cross section and the surface. The structure of thin carbon film specimens ( < 800 A) was investigated by transmission electron microscopy (TEM); a Phillips EM-300 microscope operating at 100 kV was used. The preferred orientation in these films was examined by looking at the changes in the diffraction pattern as the specimen was tilted in the microscope. The tilting angle, i.e. the angle between the incident electron beam and the specimen normal, was varied from 0 ° to 57 °. A preferred orientation is indicated when a continuous (002) ring at 0 ° tilt is broken into fragments at a higher tilt angle15.

2.3. Auger electron spectroscopy An Auger system (Physical Electronics Industries scanning Auger microprobe

model 541) was employed to determine the film surface composition using a cylindrical mirror energy analyzer with an internal coaxial electron beam. Up to six peaks in the Auger point spectra were multiplexed and displayed on a y- t point plotter. An in situ ion beam sputtering gun was used for composit ion-depth profiling through the film and the interface. The film specimens consisted of 1000 A carbon films deposited on titanium and stainless steel (type 304) using the ion-plating process.

302 N. K. A G A R W A L , A. D. t t A U B O L D

3. RESULTS A N D DISCUSSION

3.1. Thickness uniformity One of the important considerations in the ion-plating process is to make gas

scattering effective so that relatively uniform coating thicknesses can be obtained over the entire substrate surface. An adequate measure of gas scattering effective- ness is to compare the thicknesses on the back and front sides of a flat disc placed above the source. Normally, increasing the source-to-substrate distance and the lateral shift of the substrate from the center increases the uniformity. A high value of the atomic mass of the gas chosen for scattering compared with the mass of the depositing vapor species is also important in achieving good uniformity. The above observations were made for different process conditions and materials 16 is. The gaseous pressure has a pronounced effect on the gas scattering phenomena, and an opt imum value is determined depending on the relative atomic masses of evaporant and gas and the effect of pressure on the properties of the deposit. In ion- plated carbon coatings, the low atomic mass of carbon compared with argon results in effective gas scattering. It should be noted that, for stainless steel I v ion-plated coatings, an argon pressure of about (25-30) x 10- 3 Torr was necessary for high uniformity. On the other hand, the uniformity of carbon coatings drops at pressures above 10 x 10-3 Torr. There is some improvement in uniformity when the source- to-substrate distance is varied from 15 to 30 cm. A small improvement in uniformity was noticed at lower deposition rates in the range 4 40 A s 1. It seems that the effect on film properties and the required film thickness are more important con- siderations in selecting an opt imum value for the deposition rate.

The substrate bias potential was varied from 1.5 to 5 kV. Increasing the bias increases the number of gas ions bombarding the substrate surface and the average ion energy. These, in turn, enhance the surface sputtering and heating processes. During sputter cleaning of the substrate surface, bias voltage and time are carefully selected to provide adequate preparation for the film deposition.

3.2. Surface morphology Figure l(a) shows a typical SEM of a thin carbon film ( < 1 gm) deposited on a

polished metallic sheet. The film surface is smooth and featureless, replicating the underlying substrate surface. There is little effect of bias, pressure or deposition rate on the surface morphology.

The surface morphology of thick films ( > 4 lam) depends on substrate temperature, bias voltage and film thickness. Pressure and deposition rates were kept constant. Figure 1 (b)-(e) shows the surface morphology of carbon films 5 gm thick deposited at bias voltages of 2, 3, 4 and 5 kV respectively. It can be seen that the film surfaces are featureless at biases of 2, 3 and 4 kV and that they develop pronouncekl growth features at a higher bias. The substrate temperatures were higher in the last cases, and it is thought that structural changes occurring in films deposited at higher substrate temperatures and ion bombardment energies give rise to these growth features. This is evident in Fig. l(e), where sputtering of the growing film has resulted in a surface morphology very similar to that normally seen in thick vacuum condensates 11 but not previously reported for carbon films. A fourfold increase in the deposition rate, while keeping the sputtering rate constant, resulted

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in more regular and uniform columnar features (see Fig. l ( f ) ) typical o f thick vacuum films. This means that a careful selection o f sputtering and deposit ion rates can allow one to obtain desired modifications in the growth features o f ion-plated carbon films.

3.3. Structure Figure 2(a) and (b) shows electron micrographs o f a typical ion-plated carbon

film on a stainless steel substrate. The microroughening of the substrate surface due to ion bombardmen t before film deposit ion and continued bombardmen t during film nucleation and growth may be responsible for the appearance o f these electron micrographs. Fur ther studies should be performed to examine these structural features under varying process conditions.

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(c) (d) Fig. 2. TEM micrographs and electron diffraction patterns of an ion-plated carbon film 800 A thick on stainless steel: (a), (c) at a tilt angle of 0 ° ; (b), (d) at a tilt angle of 57 °.

Figure 2(c) and (d) shows electron diffraction patterns at two different tilt angles, It can be seen that a s t rong background is superimposed on the weak and

V A C U U M - D E P O S I T E D C A R B O N C O A T I N G S 305

broad diffraction bands in both patterns. The basic diffraction pattern did not change with tilt angle in the range examined. This indicates an isotropic structure in thin ion-plated carbon films.

The weak reflections and the strong background intensity in the diffraction patterns make utilization of optical densitometric techniques difficult. Therefore the approximate position of the halos was visually determined. The first band corresponds to the angular position of (002) in the graphite crystals, the second to (100) and the third to (110). Since there were no (h, k, /)-type reflections, it is concluded that the observed bands correspond to the (002), (10) and (11) bands of the turbostratic carbon found in carbon blacks and isotropic pyrolytic carbons 19.

The weak and broad bands in the diffraction patterns indicate that the constituent crystallites are extremely small 2°. These crystallites are turbostratic, i.e.

they have a two-dimensional structure with graphite layer planes, but no order exists between the atoms in adjacent planes. The apparent crystallite height Lc, which is a measure of the crystalline perfection of the carbon, can be measured from the broadening of the (002) diffraction peak. However, the presence of the strong background intensity makes an accurate quantitative estimation of Lc very difficult. Comparison with electron diffraction patterns obtained elsewhere 21'22 indicates that the apparent crystallite size of ion-plated carbon films is of the order of from 8

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306 N. K. AGARWAL, A. D. HAUBOLD

to 15 A. These turbostratic regions consist of two to six layers and are indicative of an extremely low crystalline order in these films.

3.4. Auger electron spectroscopy Thin carbon films (1000 •) deposited on titanium and stainless steel were

examined by Auger electron spectroscopy (AES). The object was to investigate the composition of the film surface and the film-substrate interface. All film surfaces showed a major energy peak for carbon and a small minor energy peak for oxygen. No other impurities were detected. The oxygen peak disappeared soon after sputtering of the film was begun. With titanium substrates (Fig. 3), the shape of the carbon spectrum changed at the interface, resembling that seen in titanium carbide 23. This indicates that chemical reaction processes may be occurring at the interface during the ion plating of carbon on titanium substrates at temperatures below 500 °C. Further studies should be undertaken to investigate the chemical and structural nature of the t i tanium-carbon interface in relation to the deposition conditions. A small oxygen peak appeared at the interface on most samples, which is indicative of the difficulty involved in sputter cleaning reactive materials. Chemical pre-etching has been found to be effective in greatly reducing the surface oxygen level for certain materials 24 but was only partially successful with titanium

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VACUUM-DEPOSITED CARBON COATINGS 307

substrates. The thickness of the interfacial region is difficult to estimate because the sputtering rate at the interface could not be calibrated accurately. However, a rough correlation with carbon sputtering rate data shows that it is of the order of a few hundred hngstr6ms.

As shown in Fig. 4, identical results were obtained for stainless steel substrates. The change in the shape of the carbon spectrum at the interface may be due to chemical interactions between the carbon and the substrate elements. A small oxygen signal was observed on the film surface before sputter etching and at the interface. The interfacial region thickness is also probably a few hundred gmgstr6ms.

4. SUMMARIZING REMARKS

(1) Thin carbon films ( < 1 gm) deposited by using the ion-plating process show a smooth and featureless surface morphology under a range of process variables. Thick films ( > 4 ~tm) develop growth features which depend on substrate temperature and the relative values of sputtering rate and deposition rate.

(2) Thin carbon films show turbostratic structures, i.e. they have a two- dimensional structure with graphite layer planes but no order exists between the atoms in adjacent planes. There was no evidence of any preferred orientation; therefore the structure of these films is similar to the isotropic turbostratic structure of LTI pyrolytic carbons. The apparent crystallite size is of the order of 8 to 15 A, indicating an extremely low crystalline order in these films.

(3) The films consist of high purity carbon. Absorbed oxygen was present in the top few layers. An oxygen signal at the interface shows that the sputter cleaning was not totally effective in removing oxygen from the substrate surface. The interfacial thickness could not be determined accurately but was estimated to be of the order of a few hundred gmgstr6ms.

(4) While the ion-plating process is complex, carbon films have been produced which somewhat resemble LTI carbons.

ACKNOWLEDGMENTS

The authors would like to thank P. A. Salvatierra for his assistance in carrying out the film synthesis work and D. R. Wall and S. Liang for the electron microscopy.

REFERENCES

1 J .C. Bokros, Chem. Phys. Carbon, 9 (1972) 103. 2 D . M . Mattox, Sandia Rep. SC-DR281-63, 1963. 3 D . M . Mattox, J. Vac. Sci. Technol., 10 (1973) 47. 4 D . M . Mattox, Sputter Deposition and Ion Plating Technology, American Vacuum Society, New

York, 1973. 5 D .L . Chambers and D. C. Carmichael, Res. Dev., 22 (1971) 32. 6 T. Spalvins, NASA-Lewis Rep. N-70-32006, 1970. 7 T. Spalvins, NASA-Lewis Rep. TN-D3707, 1966. 8 J .L. Vossen and J. J. O'Neill, Jr., RCA Rev., 31 (1970) 276. 9 H .R . Harker and R. J. Hill, or. Vac. Sci. Technol., 9 (1972) 1395.

308 Y . K . AGARWAL, A. D. HAUBOLD

10 B.A. Movchan and A. V. Demchishin, Fiz. Met. Metalloved., 28 (1969) 653. 11 R .F . Bunshah, J. Vac. Sci. Technol., 11 (1974) 633. 12 R.D. Bland, G. J. Kominiak and D. M. Mattox, J. Vac. Sci. Technol., 11 (1974) 671. 13 D .M. Mattox and G. J. Kominiak, J. Vac. Sci. Technol., 9 (1972) 528. 14 C.T. Wan, D. L. Chambers and D. C. Carmichael, Proc. 4th Int. Conj'. on Vacuum Metallurgy,

Japan Institute of Metals, Tokyo, 1974, p. 231. 15 G.E. Bacon, J. Appl. Chem., 6 (1956) 477. 16 H . A . Beale, F. Weiler and R. F. Bunshah, Proc. 4th Int. Conf. on Vacuum Metallurgy, Japan

Institute of Metals, Tokyo, 1974, p. 238. 17 C.T. Wan, D. L. Chambers and D. C. Carmichael, J. Vac. Sci. Technol., 8 (1971) VM99. 18 K. Matsubara, Y. Enomoto, G. Yaguchi, M. Watanabe and R. Yamazaki, Jpn. J. Appl. Phys.,

Suppl. 2, Part 1 (1974) 455. 19 J. Biscoe and B. E. Warren, J. Appl. Phys., 13 (1942) 364. 20 B.E. Warren, Jr. Chem. Phys., 2 (1934) 551. 21 J. Kakinoki, Proc. 5th Carbon Conj,, Vol. 2, Pergamon, Oxford, 1963, p. 499. 22 I.S. McLintock and J. C. Orr, Chem. Phys. Carbon, 11 (1974) 243. 23 C.C. Chang, in P. F. Kane and G. R. Larrabee (eds.), Characterization of Solid Surfaces, Plenum,

New York, 1974, p. 509. 24 D .M. Mattox, Jpn. J. Appl. Phys., Suppl. 2, Part 1 (1974) 443.