Optically transparent, scratch-resistant, diamond-like carbon coatings

[0002] This is a continuation-in-part of application Ser. No. 09/268,096, filed on Mar. 13, 1999.

[0004] The present invention relates generally to the formation of optically transparent, scratch-resistant adherent coatings on substrates and, more particularly, to the formation of hard, hydrogenated diamond-like carbon coatings on substrates.

[0005] Hard, hydrogenated diamond-like carbon (DLC) films have received considerable attention due to their unique properties, which are close to those of diamond. Their high hardness, chemical inertness, optical transparency, and high electrical resistivity provide DLC films a strong potential for high-technological applications, such as hard, transparent optical coatings and protective scratch-resistant coatings. Currently, DLC films are prepared either from solid sources (vacuum plasma arc, ion beam sputtering, etc.) or from glow-discharge techniques using hydrocarbon arc, ion beam sputtering, etc.) or from glow-discharge techniques using hydrocarbon gases (direct current (dc)), radio frequency (rf), microwave and electron cyclotron resonance (ECR), with the substrates on the cathode). See, e.g., Y. Catherine DIAMOND AND DIAMOND-LIKE FILMS AND COATINGS, NATO ASI Series B, Vol. 266, Eds. R. E. Clausing et al. (Plenum Press, New York (1991), pages 193 ff.

[0006] Other techniques for generating DLC are mass-selected ion beam deposition, filtered vacuum arc deposition, plasma beam deposition, and plasma source ion implantation (PSII).

[0007] Unlike these methods, plasma immersion ion processing (PIIP), which combines the advantages of ion-beam-assisted deposition and plasma source ion implantation, is well-suited for producing conformal growth of DLC films. The PIIP technique is capable of producing solid films using high-density plasmas, at low substrate temperatures, and with independent control of ion energy. Non-line-of sight depositions are also possible.

[0008] In “Amorphous Diamond Films,” U.S. Pat. No. 5,763,087, which issued to Steven Falabella on Jun. 9, 1998, the generation of amorphous diamond films on cooled, negatively rf-biased substrates using a cathodic arc source for producing a carbon ion beam from a graphite cathode is described. Intrinsic stress of the amorphous diamond coatings is reported to have been significantly reduced (1-2 GPa), thereby providing a more durable coating on substrates. Amorphous diamond is an electrically insulating, inert, transparent allotrope of carbon. It lacks long-range ordered structure, contains little or no hydrogen, and is characterized by extraordinary hardness. By contrast, DLC is hydrogenated and softer than amorphous diamond.

[0009] Plastic materials such as PMMA (polymethyl methacrylate) are inexpensive, colorless and optically transparent materials. PMMA can be fabricated into optical components such for watch crystals or camera lenses, for commercial applications. The processing cost is low. However, PMMA is very soft and susceptible to scratching and thermal distortion.

[0010] Accordingly, it is an object of the present invention to provide a method for enhancing the surface hardness and wear resistance of substrates without deterioration of the optical properties thereof.

[0011] Another object of the invention is to provide a method for generating hard, well-bonded coatings on substrates having complex shapes at low temperature.

[0012] Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

[0013] To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the method for forming an adherent, optically transparent, scratch-resistant diamond-like coating on a substrate hereof includes the steps of: applying a negative-pulsed bias voltage to the substrate, and immersing the biased substrate in a plasma containing ions simultaneously bearing carbon and hydrogen, whereby the ions are projected onto the surface of the substrate and form an optically transparent, scratch-resistant diamond-like coating on the surface thereof.

[0014] Preferably, the plasma is formed in a gas mixture of acetylene and an inert gas.

[0015] It is preferred that the inert gas includes argon.

[0016] In another aspect of the present invention, in accordance with its objects and purposes, the method for forming an adherent, optically transparent, scratch-resistant diamond-like coating on a substrate hereof includes the steps of: applying a negative-pulsed bias voltage to the substrate, and immersing the biased substrate in a plasma containing ions simultaneously bearing carbon and hydrogen, and ions bearing a dopant species, whereby the ions are projected onto the surface of the substrate and form an optically transparent, scratch-resistant diamond-like coating on the surface thereof.

[0017] Preferably, the plasma is formed in a gas mixture of acetylene, a doping gas and an inert gas.

[0018] It is preferred that the inert gas includes argon.

[0019] It is also preferred that the dopant species include boron and silicon and that the doping gas includes diborane (B₂H₆) or silane (SiH₄), respectively.

[0020] Benefits and advantages of the present method include: (1) the maintenance of low substrate-processing temperatures which allows treatment of temperature-sensitive material such as plastics; (2) the use of rf inductive plasmas having high ion density which allows treatment of complicated shapes; and (3) the use of relatively low processing pressures which permits extensive plasma formation and the simultaneously processing of large areas or a large number of small components, in the production of optically transparent films having high hardness, excellent resistance to wear, and low stress.

[0021] The accompanying drawings, which are incorporated in and form a part of the specification, illustrate an embodiment of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings:

[0022] FIG. 1 is a schematic representation of an end view of the apparatus used to perform the method of the present invention showing, in particular, the location of the reentrant inductively coupled plasma generator inside of the vacuum chamber and in the vicinity of the sample.

[0023] FIG. 2 is a graph showing the rates of deposition of diamond-like carbon (DLC) films prepared according to the method of the present invention as a function of sample pulsed bias voltage for several substrates.

[0024] FIG. 3 is a graph of the hydrogen content of the DLC film and the density of DLC films as a function of bias voltage.

[0025] FIG. 4 is a graph of the stress (solid circles) and hardness (open circles) of DLC films as a function of chamber pressure, and illustrates that minimum stress and maximum hardness were obtained at the lowest gas pressure employed (0.04 Pa).

[0026] FIG. 5 is a graph of the stress and hardness of DLC films as a function of sample pulsed-bias voltage for two chamber pressures, the pulsed-bias of −150 V which gives the best hardness and stress characteristics corresponds to carbon energy of 80 eV.

[0027] FIG. 6 is a graph of the refractive index of the DLC films as a function of the sample pulse-bias voltage.

[0028] FIG. 7 shows FT-IR absorbance spectra of the DLC films as a function of wavenumber for sample pulsed-bias voltages of −300 V, −200 V, −100 V, and −70 V.

[0029] FIG. 8 shows UV (262 nm laser light)/Raman spectra of DLC films prepared at two pressures and that amorphous carbon as a function of wavenumber.

[0030] FIG. 9 is a graph of the optical band gap of the DLC films as a function of sample pulsed bias voltage.

[0031] FIGS. 10a-care graphs of the optical transmission as a function of wavelength for various DLC films synthesized under different conditions and with specified atomic dopings.

[0032] FIGS. 11a-11cshow the optical transmission of silicon-doped DLC coatings on different substrates: glass (FIG. 11a), PMMA (FIG. 11b) and polycarbonate (FIG. 11c).

[0033] FIG. 12aillustrates the variation of the hardness and stress as a function of boron doping in DLC, while FIG. 12billustrates the variation of optical band gap and refractive index as a function of boron doping in DLC.

[0034] FIG. 13ashows the variation of the hardness and stress as a function of silicon doping in DLC, while FIG. 13bshows the variation of the optical band gap and refractive index as a function of silicon doping in DLC.

[0035] Briefly, the present invention includes the use of Plasma Immersion Ion Processing (PIIP) using inductively coupled plasma generation, in a C₂H₂-Ar gas mixture to generate optically transparent, high-hardness diamond-like carbon (DLC) films on metals, plastics, such as polymethyl methacrylate (PMMA), and glass substrates which are negatively pulse biased. The maximum hardness was observed to be about 30 GPa and the compressive stress was 9 GPa at pulsed -bias voltage of −150 V(carbon energy of 80 eV) and a chamber pressure of 0.04 Pa. DLC films synthesized in the bias range of −(100˜200) V exhibited clear sp³bonding structures, low hydrogen contents, and high mass density. The DLC films were found to have high hardness (>22 GPa) and refractive indices of approximately 2. Lowering the bias to −70 V or less induces the formation of carbon films with high hydrogen content, high optical band gap, and very low hardness. A negative bias of −100 V was found to be an important low threshold energy condition for deposition of DLC films with excellent optical properties. DLC films formed at a bias value of −100 V exhibited high optical gaps (≈1.90 eV), refractive indices of approximately 1.87, and low optical absorption coefficients. Film thicknesses ranged between 100 and 400 nm.

[0036] The process uses inductively coupled rf sources to generate a plasma from a mixture of argon and acetylene gases. Negative voltage pulses are used to extract positive ions from the plasma to bombard and deposit the DLC film. This plasma-based process can be scaled-up to simultaneously treat large areas (many square meters) consisting of large single components or collections of many small components. The components can have complicated surface geometries.

[0037] Doping the DLC with boron or silicon using B₂H₆or SiH₄as part of the plasma gas improves the optical transmission response of the coatings and significantly reduces coating stress. However, coating hardness is slightly reduced. The PIIP process at low-energy ion bombardment has been demonstrated to generate B- or Si-doped DLC films having improved optical transmittance on PMMA, glass and Polycarbonate. As will be described hereinbelow, boron doping of DLC between 4 and 8 atom percent produces coatings having hardnesses of between 27 and 29 GPa, coating stress levels that are more than 40% less than those for undoped DLC, optical gap energies of 2.2 eV, and refractive indices between 2.2 and 2.4 for film thicknesses between 100 and 400 nm. Incorporating silicon into DLC at level less than 17 atom percent produces coatings having hardnesses of between 26 and 28 GPa, coating stress levels that are more than 60% less than those for undoped DLC, and optical gap energies between 2 and 2.17 eV.

[0038] In the following description, intrinsic film stress was determined using the substrate curvature method. The curvature of the substrate was measured before and after deposition by laser scanning (see, e.g., C. A. Volkert, J. Appl. Phys. 70, 3521 (1991)), and stress was calculated using the Stoney equation (see, e.g., G. G. Stoney, Proc. Roy. Soc. (London) A82, 172 (1909)): $\sigma =\frac{{\mathrm{Et}}^2}{6h}\left(\frac{1}{R_d}-\frac{1}{R_s}\right),$

[0039] where σ is the stress, E is the biaxial modulus of the substrate, t is the substrate thickness (˜380 μm for the wafers used), h is the film thickness, and R_dand R_sare the radii of curvature after and before deposition, respectively. Silicon substrates were used in the hardness, stress and refractive index measurements, while band gaps were measured on PMMA substrates.

[0040] Reference will now be made in detail to the present preferred embodiment of the invention examples of which are illustrated in the accompanying drawings. Turning now to FIG. 1, a schematic representation of an end view of the apparatus used to perform the method of the present invention is shown. The PIIP deposition system includes a stainless steel, high-vacuum cylindrical vacuum chamber (Ø01.2 m×1.8 m), 10, equipped with a reentrant inductively coupled plasma source (generates ion densities of 10¹⁰cm⁻³at 150 W rf power input), 12, which is powered by a 0.46 MHz rf source, 14. Water-cooled substrate holder, 16, was biased with a negative voltage of between 1 V and 1000 V at 1 kW maximum average power by and insulated-gate, bipolar transistor, pulsed-bias modulator, 20. The bias pulse width was typically 20 μs and the pulse frequency was 10 kHz. A base chamber pressure below 1.3×10⁻⁴Pa was achieved using a turbomolecular pump, backed by a rotary mechanical pump (not shown in FIG. 1). Uniform thin films having thicknesses between 100 and 400 nm were deposited over an area of about 840 cm²at temperatures close to ambient using this apparatus.

[0041] Typically, substrate, 22, was cleaned prior to coating using the following procedure: glass and Si(100) wafers employed were first cleaned in an acetone, then with PMMA in an ethanol bath, and finally ultrasonically rinsed with distilled water. Prior to film growth substrates were subjected to an Ar⁺ sputter cleaning using a −600 V pulsed bias and the incident dose of 5×10¹⁶cm²to remove any residual contaminants or surface oxides. During the DLC generation, the reactant stream was a mixture of pure C₂H₂and pure Ar used in a flow ratio of C₂H₂to Ar (or F_C2H2/F_AR) equal to 0.2. The reactant pressure was varied between 0.04 and 0.93 Pa. The films were synthesized with bias voltages between −70 V and −300 V. The film thickness during the growth was monitored in-situ using a laser interferometer, not shown in FIG. 1. Plasma densities, electron temperatures, plasma potentials, and electron energy distribution functions were measured using a cylindrical Langmuir probe, 24. An aluminum liner, 26, is used to protect the chamber walls from contamination.

[0042] Having generally described the invention, the following EXAMPLES provides additional details thereof.

[0043] The thickness of undoped DLC films generated in accordance with the teachings of the present invention was between 100 and 300 nm, and the films were visually observed to be smooth and flat. No grain features were observed under investigation using a scanning electron microscope (SEM). FIG. 2 shows the dependence of the film growth rate on the substrate pulse bias voltage for three different substrates. From FIG. 2 it may be observed that the deposition rate increases with an increase in negative bias voltage, reaching a maximum at −100 V, and then decreases slowly at higher negative biases. In addition, the growth rates of DLC films on different substrates are different. FIG. 2 also shows that the growth rates of DLC films on PMMA and glass are higher than that on Si (100) wafers. It is believed by the present inventors that the different insulating properties of the substrates may affect the growth rate of DLC films in PIIP.

[0044] The hydrogen content and density of DLC films prepared on Si (100) wafers at various biases are shown in FIG. 3. By increasing the negative bias voltage, from −70 V to −300 V, the hydrogen content was observed to diminish from 33 atom percent to 22 atom percent, while the film density increases from 1.6 g cm⁻³to 2.5 g cm⁻³. FIG. 3 also shows that films synthesized at −70 V have a high hydrogen content of 33.5 atom percent and a low density of 1.60 g cm⁻³. These data indicate that when the bias is kept between −100 V and −200 V, the resultant DLC films exhibit a relatively stable hydrogen content of 27.0±0.5 atom percent and a density of 2.05±0.10 g cm⁻³.

[0045] FIG. 4 shows how DLC film stress and hardness change with gas pressure. The maximum magnitude of the compressive stress and maximum hardness were observed at the lowest pressure of 0.04 Pa. Higher gas pressure during processing resulted in greater ion scattering and broadening of the ion energy distribution (IED). Ions in the low-energy portion of the IED generate graphitic carbon bonds having poor surface adhesion. In FIG. 4, the argon-to-acetylene ratio was 5:1 and the pulsed-bias voltage was −200 V. Compressive stress values were between 4 GPa and 7.5 GPa which corresponds to nanoindentation hardness values of between 24 GPa and 28 GPa. Nanoindentation hardness values were obtained at a depth of 50 nm from the surface, and film thicknesses ranged from about 100 nm to 300 nm.

[0046] FIG. 5 shows the affect of bias voltage on film hardness and stress at two pressures. At higher gas pressure, the film adhesion was poor, the stress was less compressive and the hardness lower than those properties for DLC films deposited at lower gas pressures.

[0047] FIG. 6 shows the refractive index of DLC films as a function of negative bias voltage. The refractive index is shown to increase from 1.71 at −70 V bias to 2.16 at −300 V. The refractive index also reaches the approximately constant value of between 1.87 and 1.95 for films prepared in the bias range of between −100 V and −200 V. From FIG. 3 and FIG. 6, it can be observed that as the negative bias is increased, the refractive index and film density both increase. A reduction of the bias to −70 V would deteriorate the quality of the films which exhibit high hydrogen content, low density and low refractive index.

[0048] FIG. 7 shows Fourier transform infrared (FTIR) spectra of DLC films deposited on glass at various substrate biases. The absorption peaks at 2864 cm⁻¹, 2925 cm⁻¹and 2960 cm⁻¹correspond to Sp³CH₃(symmetrical), sp³CH₂(asymmetrical), and Sp³CH₃(asymmetrical) vibration modes, respectively. No Sp²CH₂absorption peak is observed in the FTIR spectra. These data show that under constant gas flow ratio (F_C2H2/F_AR=0.2), negative bias voltages higher than −70 V lead to the formation of DLC films with common sp³bonding features, as indicated by the three IR peaks as 2864 cm⁻¹, 2925 cm⁻¹, and 2960 cm⁻¹in FIG. 7. The data also show that films produced at negative bias voltages less than or equal −70 V contain lower concentrations of C—H sp³bonds as is indicated by the substantially lower absorption intensity at 2864 cm⁻¹, 2925 cm⁻¹, and 2960 cm⁻¹. The IR spectra in FIG. 7 indicate that by decreasing the negative bias voltage from −300V to −100 V, the peaks at 2864 cm⁻¹, and 2960 cm⁻¹all become smaller with respect to a principal peak at 2925 cm⁻¹, possibly indicating a higher sp³bonding fraction at a negative pulse-bias of −100 V.

[0049] Film characterization by Raman spectroscopy and shown in the spectra of FIG. 8 indicate that films deposited between '100V and −300V bias voltage have the typical structure characteristics of DLC. The films are identified by their deposition pressures. For comparison, the spectrum of an amorphous carbon film deposited by pulsed laser ablation under conditions that favor formation of sp²(three-fold-coordinated) carbon is shown (“a−C”). The peak shift from 1570 cm⁻¹(P=0.53 Pa) to 1590 cm⁻¹(P=0.04 Pa) implies a higher content of sp³-carbon bonds. With UV-excitation and in the context of the growth conditions and the physical properties of these films, the appearance of a relatively low-frequency shoulder (˜1250 cm⁻¹) in the P=0.04 Pa films suggests an enhanced fraction of sp³-carbon bonding, rather than an sp²-bonded structure. The higher frequency shoulders (1350-1400 cm⁻¹) in the other two films indicate the presence of extended sp²-bonded structures.

[0050] For the optical investigation of generated films, absorption coefficients calculated from the optical absorbency and thickness of the DLC films, were used to evaluate the optical transparency of the films. The optical band gap of DLC films formed at various bias voltages is shown in FIG. 9. These data were obtained by fitting the absorption coeffecients to the Tauc relation and extrapolating the linear portion of the Tauc curves towards the abscissa. See, e.g., J. Robertson, Surface and Coatings Technology 50, 185 (1992). FIG. 9 shows that the optical gap energy decreases monotonically with increasing negative substrate bias. However, for DLC films synthesized at biases of between −100 V and −200 V, the optical band gaps on PMMA and glass are very similar with values between 1.95 to 1.86 eV. This implies that the films formed in bias the range of between −100 V and −200 V possess stable optical properties despite their formation on different substrates. The films formed at higher biases, −250 to −300 V, exhibit relatively small band gaps, and high absorbance.

[0051] Hydrogen content, density and optical properties of DLC films were found to exhibit a substantial dependence on the pulsed-bias. A bias voltage of −70 V was too low to generate ion impingement with sufficient energy to deposit sp³-bonded carbon atoms and to effectively re-sputter hydrogen atoms out of the film. The resultant films possessed optical band gaps greater than 2.1 eV, a low density of 1.6 and a low hardness of 8 GPa. When the applied bias was larger than −250 V, the resultant DLC films exhibited high absorption coefficients and a relatively low optical gap energy of between 1.68 eV and 1.84 eV. Thus, the PIIP synthesis of DLC films with optimal optical properties requires a bias voltage in the region between −100 V and −150 V. DLC films formed under these conditions exhibit low hydrogen content, high density, and good optical properties. Therefore, a bias voltage of −100 V is the low-energy threshold for ion bombardment for PIIP synthesis of an optical DLC coating, since bias voltage equal to or larger than −100 V produce films exhibiting a significant reduction of hydrogen content, from about 27 at. %, and concurrent increase of film density, from 1.98 g cm⁻³, and hardness above 22 GPa. In addition, the films deposited at bias voltages of −100 V exhibited the lowest absorption coefficients and a high optical band gap of around 1.90 eV.

[0052] FIGS. 10a-10care graphs of the optical transmission as a function of wavelength for various DLC films synthesized under different conditions and different doping levels on a PMMA substrate. The approximately 300 nm thick DLC coating illustrated FIG. 10awas formed at 1 mT gas pressure with C₂H₂and Ar as the plasma gases, an rf power level of 400 W and a substrate bias of −150 V, and was determined to have a hydrogen concentration of between 25 and 33 atom percent. Curve 1 shows the transmission of the PMMA substrate, while curves 2-4 show transmissions of the resulting coatings when the ratio of C₂H₂to Ar in the plasma gas is 0.25, 0.33 and 0.5, respectively. The approximately 300 nm thick DLC coating illustrated FIG. 10bwas formed at 1 mT gas pressure with C₂H₂, B₂H₆and Ar as the plasma gases, an rf power level of 400 W and a substrate bias of −200 V, and was determined to have a hydrogen concentration of between 28 and 38 atom percent. Curve 1 shows the transmission of the PMMA substrate, while curves 2-4 show transmissions of the resulting coatings when the boron atom weight percent is 12.1, 9.2 and 4, respectively. The approximately 320 nm thick DLC coating illustrated FIG. 10cwas formed at 1 mT gas pressure with C₂H₂, SiH₄and Ar as the plasma gases, an rf power level of 400 W and a substrate bias of −150 V, and was determined to have a hydrogen concentration of between 31 and 40 atom percent. Curve 1 shows the transmission of the PMMA substrate, while curves 2-5 show transmissions of the resulting coatings when silicon atom weight percent is 10.6, 13.7, 28, and 31, respectively. Wavelengths between 200 and 900 nm were investigated. It may be observed from the data that doping improves transmission in the wavelength range between 250 and 550 nm, with boron doping providing more improvement than silicon doping.

[0053] FIGS. 11a-11cshow the optical transmission of silicon-doped DLC coatings on different substrates: glass (FIG. 11a), PMMA (FIG. 11b) and polycarbonate (FIG. 11c). Approximately 320 nm coatings were generated at 0.1 Pa gas pressure where the SiH₄and C₂H₂were mixed together at a ratio of 0.5 SiH₄to C₂H₂and introduced into the plasma at a flow rate of 6 sccm, while the Ar was introduced into the plasma at a flow rate of 12 sccm. In FIGS. 11a-c, curve 1 represents the transmission of the substrate, while curves 2-5 represent the transmissions of the coatings formed when the substrates were biased at −40 V, −80 V, −150 V, and −250 V, respectively.

[0054] FIG. 12aillustrates the variation of the hardness and stress as a function of boron doping in DLC, while FIG. 12billustrates the variation of optical band gap and refractive index as a function of boron doping in DLC. For B/C ratios in the range of 0.06-0.16 (4-8 atom percent B), the stress was reduced by greater than 40% (from −7 to −4.2 GPa), the hardness reduced by 6% (29 to above 27 GPa), the optical gap energy was about 2.2 eV, and the refractive index was between 2.2 and 2.4. Coatings were prepared with the substrate biased at −200 V and with a B₂H₆and C₂H₂mixture introduced into the plasma at a flow rate of 6 sccm. It may be observed from FIGS. 12aand 12bthat boron doping of up to 8 atom percent improves the properties of the DLC.

[0055] FIG. 13ashows the variation of the hardness and stress as a function of silicon doping in DLC, while FIG. 13bshows the variation of the optical band gap and refractive index as a function of silicon doping in DLC. For Si/C ratios between 0.17-0.32 (10-17 atom percent Si), the coating stress was reduced by a factor of 2 (from −7 to less than −3 GPa), the hardness was reduced by 9% (from 29 to about 26 GPa), the optical gap energy was about 2-2.17 eV, and the mass density was between 2.7 and 2.9 g/cm³. Coatings were prepared with the substrate biased at −200 V and with a SiH₄and C₂H₂mixture introduced into the plasma at a flow rate of 6 sccm. It may be observed from FIGS. 13aand 13bthat silicon dopings of less than 17 atom percent improves the properties of the DLC. For the data illustrated in FIGS. 12a, 12b, 13aand 13b, hardness, stress and refractive index measurements were made for coatings deposited on silicon substrates, while the band gap measurements were made using PMMA substrates.

[0056] The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. For example, hydrocarbons other than acetylene can be used to produce acceptable DLC films according to the teachings of the present invention, as can other inert gases.

[0057] The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.