Preparation an study of laser plasma diamond

Surface and Coatings Technology, 47 (1991) 244—251

244

Preparation and study of laser plasma diamond C. B. Collins, F. Davanloo, E. M. Juengermañ, D. R. Jander and T. J. Lee Center for Quantum Electronics, University of Texas at Dallas, P.O. Box 830688, Richardson, TX 75083-0688 (U.S.A.)

Abstract Films of diamond-like material can be deposited with a laser plasma source of carbon ions in an ultrahigh vacuum environment without involving hydrogen in the growth mechanism. These films are distinguished transparency at first visible wavelengths which is aatresult of a deposition high percentage 3 bonds. They by resemble materials quenched from ion beams very slow rates. of sp In our method an Nd:YAG laser was focused on a graphite feedstock in an ultrahigh vacuum chamber at intensities in excess of 5 x 1011 W cm2. A high current discharge confined to the path of the laser-ignited plasma provided further heat and aided processing of the ion flux. At a laser repetition rate of 10 Hz, a deposition rate of 0.5 ~m h-’ over a 100 cm2 area was attainable with no measurable substrate heating. The substrates required no special preparation or seeding and materials including silicon, fused silica, glass, gold, copper, germanium, InP, ZnS, and polycarbonate and polyimide plastics were readily coated. Complex shapes could be accommodated and spheres of 440C stainless steel were covered successfully. Over 1000 samples were prepared to a variety of specifications with thicknesses reaching 5 ~m and hardness exceeding 37 GPa.

1.

Introduction

The past few years have witnessed a renaissance in the preparation and study of thin films of carbon with diamond-like properties (see ref. 1 for an excellent review). However, while natural diamond is a well defined substance, these diamond-like films are not. In many cases different materials result from the different methods of preparation and this has contributed much complexity to the evaluation of the merits of the different techniques of growth. Because of its amorphous appearance, the form known as diamondlike carbon (DLC) has posed a special challenge to understanding, particularly when prepared without hydrogen in the growth process. Denoted as a-C films, these unhydrogenated DLC films have received relatively little attention, although they have been known about for quite some time. As early as 1971, Aisenberg and Chabot [2] reported the quenching of a beam of C~ions in the presence of Ar and Ar~onto a cold substrate to form an amorphous layer containing no hydrogen, yet having some diamond-like properties. Subsequent efforts [3—6] continued to produce material seeming to have neither order nor hydrogen. Too transparent to be graphite, their structures remained obscure and further study was hindered by the extremely slow rates of growth. 0257-8972/91/$3.50

© Elsevier Sequoia/Printed in The Netherlands

245

The preparation of thin films of diamond-like material which are hydrogen free seems best accomplished with a laser plasma source. In 1985 Nagel and coworkers [7] first reported the use of a laser ablation source of carbon ions to produce diamond-like films at relatively high rates of growth approaching 0.3 ~zmh~. He determined a critical threshold intensity of 5 x 1010 W cm2 on the carbon feedstock, above which DLC was condensed from the carbon plasma and below which only soft, graphitic layers were deposited which resembled those produced by the thermal evaporation of carbon. The introduction of the laser plasma source of carbon ions in 1988 ignited an explosion of effort in the production and characterization of unhydrogenated DLC. First used unknowingly by Wagal et al. [8] to produce diamond-like films at intensities slightly below the Nagel criterion, it was first described by Collins et al. [9] and subsequently rediscovered by Krishnaswamy and coworkers [10]. In each case the critical concept has been the use of a high current discharge confined to the path of the laser-ignited plasma to heat further and process the ion flux. Within the past few months it has been suggested [11, 12] that a-C films were actually crystalline, consisting of nanocrystals of diamond embedded in a matrix of other carbon polytypes. That surprising possibility seems to have been confirmed [13, 14] by the newest data and explains the unique advantages of a-C over a-C:H which have been reported previously [11]. In IR applications there are no C—H absorption bands with which to contend and scattering losses are minimized by the extremely small sizes of the component diamond grains. The growth of these films is continuously self-seeding at room temperature and the substrates require no preliminary scratching or nucleating. Tribological applications are equally favored by the smooth optical finishes generally displayed and by the ability of such films to conform to complex shapes being coated. The purpose of this paper is to review the properties of diamond-like films prepared without hydrogen under conditions reported to cause polycrystalline growth. Both optical and mechanical characteristics are considerably improved over those reported previously [11].

2. Preparation of diamond-like carbon without hydrogen Figure 1 shows a schematic representation of the system reported earlier [9, 11] for growing diamond-like films from laser plasma discharges. The dimensions were not particularly critical and depositions could be made either onto substrates mounted in a carrier able to rotate four disks 32 mm in diameter about two axes as shown, or onto a fixed witness plate from which the dispersion of properties with position could be determined. With the substrate carrier the dependence of deposition parameters on angular displacement from the input axis of the laser beam was compensated and films of good uniformity could be grown [9].

246

LASER INPUT Subsfrate

Wi/ness p/ate

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Fig. 1. Schematic representation of the laser plasma source used in this work. When mounted in the carriers and rotated in the assembly shown by solid lines the substrates could be uniformly coated to thicknesses exceeding 5 ~m. In an alternative arrangement shown by the dotted lines, a witness plate recorded variations of optical properties produced by plasma which had traversed different paths from the ablation plume.

Basically, the mechanism for producing the laser ablation was straightforward, as seen in Fig. 1. Passing through a window into the ultrahigh vacuum chamber with a diameter of about 8 mm, the pulsed laser beam was turned and focused with high quality optics fixed in the evacuated space. We were unable to maintain a suitably tight focus with external optics and found it to be a critical aspect of the process to place the optics in the evacuated space while protecting them from a build-up of ablated material. This was sufficient to maintain foci of diameters set to values selected in the range 30—300 pm. In operation the system was evacuated to pressures of the order of iO~Torr and the discharge current triggered by the laser ablation flowed between the graphite feedstock and a rod electrode to complete the circuit [9]. The witness plate consisted of a plate glass disk, 20 cm in diameter. It was crossed by a number of Kapton tapes so that thicknesses at various points could be measured with a surface profilometer after removal of the tape. A central hole was made to pass the laser beam to the feedstock and programmed movements of the graphite insured that each ablation occurred from a fresh surface. In both arrangements the primary ablation was excited by a 1.4 J Nd:YAG laser, Q-switched and operated at 1.06 pm with a 10 Hz repetition frequency. With this system we have produced over 1000 different films of thicknesses varying from 0.1 to 5.0 pm. Modeling studies have recently shown that the laser input alone is sufficient to insure that the resulting plasma is fully ionized. Plotted in Fig. 2(a) and 2(b) are values of ion densities and kinetic energies reported by Stevefelt and Collins [15] for these operating conditions. It can be seen that the impact of the laser plasma on a substrate is equivalent to its irradiation with a very high fluence ion beam. In general agreement with Nagel et al. [7]

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Fig. 2. Typical values of (a) ion concentrations and (b) energies calculated in ref. 15 for a model of the laser plasma ablated with a 15 ns pulse from an Nd:YAG laser at 1.06 rim. Populations of charge states not plotted were less by at least a factor of five.

3-hybridized we have found that the quenching of such energetic ionscarbon yields produces sp amorphous carbon while the condensation of neutral only graphite, perhaps defected graphite as suggested by Tamor and Wu [16].

3. Film characteristics As mentioned above, DLC is not a single type of material, even when prepared without hydrogen. Properties can be varied by changing any of the process parameters. A particularly useful quantity with which to rank the quality of a film prepared under a particular set of conditions has been found [11, 121 to be the imaginary part of the index of refraction n 1(k). A single figure-of-merit of considerable utility is n1(6328 A). The corresponding photon energy of 1.96 eV lies above the optical band gap of all a-C materials reported to date, so sensitivity is excellent. At the same time, this energy is not so far above the band gap in materials of interest to create difficulties with thick films. For convenience in this report if no wavelength is specified, it is implied that n, = n1(6328 A). The particular material used as a baseline reference work with was 2 in a in15this ns pulse that produced at current a peak and intensity of 5 onto x 1011substrates W cm rotated in the carrier 10 A of discharge deposited shown in Fig. 1, as described earlier [11]. It had n 1 = 0.3 with an apparent band gap of Eg = 1.0 eV and the primary process variable was the peak laser intensity. Savvides and Window [5, 6] convincingly argued that comparison such films 3 bonding and a close are characterized by 75% diamond-like sp between n 1(A) for the films prepared from quenched ions [5, 6] and from laser plasmas has served to transfer that conclusion to the latter. As seen in Fig. 3, the deliberate lowering of power density focused onto the feedstock effected a change to the material identified by Robertson [17] being composed of graphite islands for which n1 = 0.6.

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PHOTON ENERGY 1eV) Fig. 3. Plot of the loss coefficients for the transmission of photons of the energies shown through three carbon films deposited onto fused silica substrates. Data are shown for films prepared from carbon ablated by laser beams having the relative intensities I indicated. Losses are plotted in terms suitable for comparison with the parameterization of eqn. (1).

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Fig. 5. Graph of IR transmittance of a film deposited onto a germanium window 25mm in is diameter. Comparison with computed values shows the film to be 0.3 ~omthick with the real 1 part of the refractive index, ~r = 2.1. The absence of absorption from C—H bonds at 2940 cm clear.

Of considerable interest are the IR properties of the baseline film. Figure 5 is a plot of the transmittance measured for a 0.3 pm thick film on a germanium substrate 25 mm in diameter. Of greatest importance is the complete absence of the C—H absorption band at 3.4 pm which had revealed the presence of hydrogen in Sato et al.’s films of lesser thickness [18, 19]. Even thicker films showed no trace of absorption in that region, enabling us to place an upper limit of 1% on the hydrogen content from spectroscopic measurements.

250

The optical properties reviewed in this work agree in indicating that DLC prepared without hydrogen is an sp3 bonded material containing varying amounts of graphitic contamination accidentally collected from undesirable fringes of the laser plasma source. The benchmark material with n, = 0.3 seems well characterized as being 75% diamond. Measurements of the density of such materials as well as the clustered structure reported in a companion paper [14] strongly support this identification. The remaining discrepancy has been relatively poor hardness cited in earlier work [11]. A very limited set of measurements reported there had produced only a hardness comparable with that of silicon. As a part of the present work a more extensive examination was made at Oak Ridge National Laboratory with a Nanoindenter system. On samples prepared to benchmark specifications with n 1 = 0.3, the average of 20 separate measurements was 37 GPa, with the maximum observed being 77 GPa. This seems to be more consistent with a composition of 75% diamond and suggests the particular sample examined earlier was not representative.

4. Conclusions Films of DLC prepared without hydrogen at energy densities above the Nagel criterion show completely different properties from those deposited by lower energy plasmas. The former are dense, clear, hard and seem best described by a model of very fine diamond clusters or grains in a matrix of other carbon allotropes. The latter material may be described by the model of defected graphite, but it excites little interest because it is comparatively soft, light, and opaque. The work reported here provides additional support 3-hybridized amorphous to previous conclusions that carbon ions deposit sp carbon while more nearly neutral vapors produce the softer material. Laser plasmas appear to be ideal sources of the more diamond-like of the hydrogen-free films because of the large amount of feedstock that can be brought above the Nagel criterion and processed to ions. With this technique we have grown films over areas of 100 cm2 on substrates including silicon, fused silica, glass, silver, copper, germanium, SiC, InAs, ZnS, and polycarbonate and polyimide plastics at rates of 0.5 pm h’. Essentially a point source of ions, the laser plasma is readily masked to control the area deposited and films tend to adhere well, even to curved surfaces. Spheres of 440C stainless steel, 1.25 cm in diameter were smoothly coated by successive exposures applied after discrete rotations. Seams were invisible to our examinations.

Acknowledgments The authors gratefully acknowledge the contributions of many colleagues and friends in the characterization of this thin film material. In particular, we wish to express our sincere appreciation to Kurt Osmer at

:

251

General Dynamics for the IR measurements, to R. J. Lauf at Oak Ridge National Laboratory for the hardness measurements, and to R. K. Krause for arranging the deposition system.

References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

J. C. Angus and C. C. Hayman, Science, 241 (1988) 913. S. Aisenberg and R. Chabot, J. Appl. Phys., 42 (1971) 2953. E. G. Spencer, P. H. Schmidt, D. C. Joy and F. J. Sansalone, Appi. Phys. Lett., 29(1976) 118. T. Miyazawa, S. Misawa, S. Yoshida and S. Gonda, J. Appi. Phys., 55 (1984) 188. N. Savvides and B. Window, J. Vac. Sci. Technol. A, 3 (1985) 2386. N. Savvides, J. Appi. Phys., 58 (1985) 518; 59 (1986) 4133. C. L. Marquardt, R. T. Williams and D. J. Nagel, Mater. Res. Soc. Symp. Proc., 38(1985) 325. S. S. Wagal, E. M. Juengerman and C. B. Collins, Appi. Phys. LetI., 53 (1988) 187. C. B. Collins, F. Davanloo, E. M. Juengerman, W. R. Osborn and D. R. Jander, Appi. Phys. Leti., 54 (1989) 216. J. Krishnaswamy, A. Rengan, J. Narayan, K. Vedam and C. J. McHargue, Appi. Phys. Leit., 54 (1989) 2455. F. Davanloo, E. M. Juengerman, D. R. Jander, T. J. Lee and C. B. Collins, J. Appi. Phys., 67 (1990) 2081. F. Davanloo, E. M. Juengerman, D. R. Jander, T. J. Lee and C. B. Collins, J. Mater. Res., 5 (1990) 2398. C. B. Collins, F. Davanloo, E. M. Juengerman, D. R. Jander and T. J. Lee, J. Appi. Phys., in the press. C. B. Collins, F. Davanloo, E. M. Juengerman, D. R. Jander and T. J. Lee, Surf. Coatings Technol., SCT 1812. J. Stevefelt and C. B. Collins, J. Phys. D., in the press. M. A. Tamor and C. H. Wu, J. AppI. Phys., 67(1990) 1007. J. Robertson and E. P. O’Reilly, Phys. Rev. B, 35 (1987) 2946. T. Sato, S. Furuno, S. Iguchi and M. Hanabusa, Jpn. J. Appi. Phys., 26 (1987) L1487. T. Sato, S. Furuno, S. Iguchi and M. Hanabusa, Appi. Phys. A, 45 (1988) 355.