- Email: [email protected]
Preparation of superhard amorphous carbon films with low internal stress
Surface & Coatings Technology 188 – 189 (2004) 268 – 273 www.elsevier.com/locate/surfcoat
Preparation of superhard amorphous carbon films with low internal stress Steffen Weissmantel*, Gqnter Reisse, Dirk Rost Hochschule Mittweida, University of Applied Sciences, Technikumplatz 17, Mittweida 09648, Germany Available online 3 October 2004
Abstract Tetrahedral amorphous carbon (ta-C) films were prepared by pulsed laser deposition using a 248-nm excimer laser wavelength and up to 45 J/cm2 laser pulse energy fluence. Fluences above 6 J/cm2 and mean kinetic energies of the ablated species above 30 eV, respectively, as well as substrate temperatures below 100 8C were found to be necessary for the formation of ta-C films with high sp3 percentage. Such films are completely amorphous and have up to 85% sp3 bonds. As-deposited films show high compressive stresses in the range of 8–10 GPa. The possibilities to reduce those stresses by means of thermal and pulsed laser annealing were investigated. We found that both methods allow the preparation of nearly stress-free diamond-like carbon films with several micrometers of thickness and good adherence to Si and WC hard metal substrates. The Vickers microhardness of those films was measured to be 55–65 GPa and the Young’s modulus was measured to be 800–900 GPa by using a dynamic indentation method. D 2004 Elsevier B.V. All rights reserved. Keywords: Tetrahedral amorphous carbon (ta-C); Pulsed laser deposition (PLD); Stress relaxation; Mechanical properties
1. Introduction Hydrogen-free tetrahedral amorphous carbon (ta-C) is the second hardest material known. This, combined with their low surface roughness and friction, makes it an interesting material for applications as wear-resistant coatings. Pulsed laser deposition (PLD) [1–3] and arc evaporation (AED) [4] are two methods used for the preparation of ta-C films that have industrial potential. Those films show, however, high compressive stresses if deposited at optimum parameters with regard to hardness, which prevents the preparation of micrometer-thick films due to delamination. A breakthrough was therefore the preparation of stress-free films by using thermal annealing, first reported by Friedmann et al. [2]. We present here a novel method to prepare superhard ta-C films with low internal stress. The method is a combination of PLD and pulsed laser annealing, and we will show that it allows the rapid preparation of ta-C films with several * Corresponding author. Tel.: +49 3727 581449; fax: +49 3727 581449. E-mail address: [email protected] (S. Weissmantel). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.08.070
micrometers of thickness on Si and WC hard metal substrates.
2. Experimental details Carbon films were deposited by PLD in a high-vacuum system illustrated previously [5,6]. A KrF excimer laser with a laser pulse energy of 1.2 J, a pulse duration of 30 ns, and a maximum repetition rate of 50 Hz was used for ablation. The target material was graphite with a purity of 99.9%. The laser beam was moved in spirals and with constant vector velocity across the target area. The residual gas pressure during deposition was 1104 Pa. Prior to deposition, the substrates were bombarded for about 1 min with an Ar+ ion beam (700 eV energy, 150 AA/ cm2 current density at the substrate) produced in a Kaufman ion source in order to remove impurities. Films were deposited on Si, SiO2, and WC hard metal substrates. The substrate temperature was measured through a thermocouple affixed to the substrate surface. Thermal annealing was accomplished by heating the samples
S. Weissmantel et al. / Surface & Coatings Technology 188 – 189 (2004) 268–273
Fig. 1. Mean kinetic energy of the carbon atoms and ions ablated from a graphite target as a function of laser pulse energy density (calorimetric measurement).
immediately after deposition using a BoralectricR resistance heating element from Advanced Ceramics. In the pulsed laser annealing experiments, the same excimer laser beam that was used for ablation was guided onto the substrate. Its cross-sectional area on the substrate surface was 2 cm2. Changing between deposition and annealing can swiftly be done by means of a tiltable mirror. All annealing experiments were made in vacuum. High-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and electron energy loss spectroscopy (EELS) were performed on a Philips CM 20 FEG 200 keV transmission electron microscope equipped with a Gatan imaging filter (GIF). The spectral dependence of the refractive index and extinction coefficient was computed from the transmission and reflection curves of the films measured with a spectrophotometer in the wavelength range of 200–800 nm. The optical band gaps were determined from those curves using the Tauc plot, which is valid for amorphous semiconductors. The dynamic indentation measurements were performed on a UMIS 2000 equipped with a Berkovich indenter. The calculation of hardness and Young’s modulus from the obtained load–depth curves was made according to the ISO
269
14577 standard using a modified Oliver–Pharr method [7]. The values given in the paper are the mean values of 10 measurements each time using maximum loads of 10 mN. Film adhesion was characterised by means of a commercial scratch testing system using a Rockwell C diamond tip with 1208 included angle and 0.2 mm tip radius. The measurements were performed with continuous loading increase of 30 N/min and at a scratching speed of 10 mm/ min. The film stress was determined, using Stoney’s equation, from the bending of the silicon substrates induced by the deposited films. The bending was measured either after deposition by means of a DEKTAK profilometer or in situ by means of laser deflection using a device, which we illustrated and described previously [8]. For the in situ measurements, 400-Am-thick Si substrates of 50 mm length and 5 mm width were clamped one-sided to the substrate holder. The film thicknesses needed for the calculation of stress were determined from the reflected intensity of the measuring laser beam (670 nm wavelength).
3. Results and discussion 3.1. Deposition of ta-C films Dependent on the laser energy fluence at the target and the substrate temperature during deposition, hydrogen-free amorphous carbon films prepared by PLD may have a content of diamond-like (sp3) bonds in the range of 0–85% (see also Section 3.3) and are correspondingly soft to superhard. For the formation of superhard carbon films with high sp3 content (ta-C), fluences of more than 6 J/ cm2 and substrate temperatures of less than 90 8C were found to be necessary. The essential parameter is thereby the mean kinetic energy of the film-forming species. Calorimetric measurements showed that it is in the range of 30– 100 eV at fluences of 6–30 J/cm2 (see Fig. 1). In this range, sp3 contents of 70–85% were measured. At lower kinetic energies and fluences, respectively, the sp 3 content decreases. Substrate temperatures of more than 90 8C result,
Fig. 2. Stress reduction by thermal annealing of 200-nm-thick ta-C films prepared at 12 J/cm2 target laser pulse energy density on 400-Am Si substrates. Dependence on annealing temperature (left) and time (right).
270
S. Weissmantel et al. / Surface & Coatings Technology 188 – 189 (2004) 268–273
Fig. 3. Development of stress during pulsed laser annealing of 150-nmthick ta-C films, each deposited at 16 J/cm2 target laser fluence on 400-Am Si substrates in dependence on substrate laser fluence.
independently of the laser fluence, also in decreasing sp3 content and, already at 120 8C, the films are completely sp2-bonded. In our experimental set-up, the maximum deposition rate of ta-C films is currently 120 nm/min, where the laser energy fluence at the target is 10 J/cm2, the laser beam cross-section on the target surface is 3.5 mm2, and the pulse repetition frequency is 50 Hz. With these parameters, the mean substrate temperature increases to about 70 8C during deposition, which is due to the energy input of the filmforming species. Therefore, ta-C films can be prepared without substrate cooling. 3.2. Stress reduction by thermal and pulsed laser annealing As-deposited ta-C films possess high internal compressive stresses of 8–10 GPa, which leads to the delamination of 200- to 400-nm-thick films. Therefore, for the preparation of thicker films, measures must be taken to avoid or reduce those stresses. It is well known for several years that the stress can be removed completely by thermal annealing for
5 min at 600 8C [2]. We have also used this method in our investigations and our results are in good agreement. As can be seen in Fig. 2, the residual stress depends on the annealing temperature and time. A nearly complete stress relaxation requires a minimum annealing temperature of 600 8C and a minimum annealing time of several minutes. Thereby, the sp3 content and, with it, the diamond-like properties of the ta-C films are maintained (see also Section 3.3). In order to grow thick ta-C films in the micrometer range, it is necessary to successively deposit and anneal sublayers of a few hundred nanometers of thickness. We prepared ta-C films with several micrometers of thickness on Si and WC hard metal substrates in this way, where the thickness of the sublayers was 200 nm. This took, however, a rather long time as the samples had always to cool down after annealing, for which the minimum time required was 30 min if argon background gas at atmospheric pressure was used to accelerate the cooling. Recently, we studied a novel method for the stress relaxation in ta-C films, for which we have a patent pending [9]. It concerns pulsed laser annealing performed either during, using a second laser, or alternately to the deposition process. So far, we performed the annealing exclusively alternately to deposition and used for it the same KrF excimer laser as for ablation. The stress relaxation in three different 150-nm-thick ta-C films measured during the irradiation of the as-deposited films with a number of laser pulses at different energy fluences can be observed in Fig. 3. Remarkably, the stresses are largely reduced after only 200 laser pulses of 30 ns duration (i.e., relaxation occurs in a very short time). The residual stress decreases with increasing laser energy fluence from some 3 GPa at 110 mJ/cm2 to 1 GPa at 165 mJ/cm2 and nearly 0 GPa at 180 mJ/cm2. The temperature fields induced in ta-C/Si systems by the laser irradiation were calculated by using a numeric computer program [10]. The temperature-dependent material constants of silicon were taken from the literature [10]. The optical constants of ta-C at 248 nm wavelength (refractive index, extinction, and absorption coefficient of
Fig. 4. Calculated temperature field (left) generated in a 1000-nm-thick ta-C film on Si by irradiation with an excimer laser pulse (248 nm wavelength, 30 ns pulse duration, 150 mJ/cm2 laser fluence) and maximum temperature during one pulse in dependence on film thickness (right). The maximum of the Gaussian time-dependent laser pulse is at 50 ns.
S. Weissmantel et al. / Surface & Coatings Technology 188 – 189 (2004) 268–273
271
Fig. 5. Development of stress and reflected intensity normalized to the uncoated substrate during the preparation of a 1-Am-thick ta-C film on a 400-Am Si substrate by successive deposition and annealing (target laser energy fluence 12 J/cm2, substrate laser energy fluence 150 mJ/cm2).
Fig. 7. Sp3 content (from EELS) and optical energy band gap of pulsed laser-deposited carbon films as a function of target laser fluence. The values of the band gap have been determined according to the method of Tauc, which is applicable to amorphous semiconductors.
3.0, 0.35, and 1.8105 cm1, respectively) and the density (3.3 g/cm2) were measured on our own films. The specific heat capacity was calculated according to the Debye theory with a Debye temperature of 1770 K, which was temperature-dependent. For heat conductivity, a value of 9.2102 W/cm/K was used, which was measured on pulsed laserdeposited ta-C films by Morath et al. [11]. Finally, a Gaussian time dependence of the laser pulse was used, where the pulse duration was 30 ns. According to those calculations, the maximum temperature during one pulse in a 150-nm-thick ta-C film on Si is 763 K at 110 mJ/cm2 laser energy fluence and 1053 K at 180 mJ/cm2—the fluence at which complete stress relaxation is achieved. Furthermore, at constant fluence, the maximum temperature during one pulse depends relatively strongly on film thickness only up to a thickness of some 300 nm (see Fig. 4, right) and remains then nearly constant due to the low heat conductivity of ta-C. The latter is also the reason for the low penetration depth of the temperature fields into the films. Only some 50 nm are heated up nearly homogeneously (see Fig. 4, left). That means, however, that the sublayers to be annealed should not be thicker. The temperature needed for complete relaxation seems to be 180 K higher than that needed for conventional annealing. We
have, however, used so far only 150-nm-thick films for the annealing experiments. As there is a temperature gradient of some 200 K between the upper and lower parts of the film and complete relaxation requires relaxation of all parts of the film, the temperature needed for complete relaxation is in the range of conventional annealing. On the other hand, the temperature exceeds 650 K only for 40 ns and 400 K only for 230 ns per pulse (i.e., the relaxation process during pulsed laser annealing takes place within several tens to several hundreds of microseconds, much faster than in conventional annealing). The results suggest that the process is also determined mainly by thermal processes and that the high heating and cooling rates on the order of 1010 K/s (compared to the maximum rates of less than 1 K/s during thermal annealing) result in an acceleration of the atomic site interchanging and reordering processes, which are necessary for stress relaxation. Further investigations concerned the preparation of thick ta-C films by successive PLD and annealing of sublayers. In Fig. 5, the variation of total stress with film thickness measured during the preparation of a 1-Am-thick ta-C film on Si is shown, for example, where eight sublayers of some 150 nm thickness have successively been deposited and annealed. It can be observed that the total residual stress in
Fig. 6. Electron energy loss spectra of various pulsed laser-deposited carbon films.
272
S. Weissmantel et al. / Surface & Coatings Technology 188 – 189 (2004) 268–273
Table 1 Mechanical properties of various ta-C films on WC hard metal substrates Sample
Parameter
Hardness [GPa]
Young’s modulus [GPa]
Critical load scratch test [N]
Friction coefficient
Density [g/cm3]
252 262 253 044 WC
18 J/cm2 18 J/cm2 12 J/cm2 12 J/cm2 uncoated
55.2 48.9 61.7 56.1 26
690 705 878 915 600
50
0.12
52 42
0.12 0.11
3.08 3.17 3.26 3.28
thermal annealing thermal annealing pulsed laser annealing pulsed laser annealing
The film thickness was in the range of 1–2 Am.
the whole film is 0.3 GPa, whereas that in the first sublayer is still 2 GPa after annealing. The reason is that the annealing fluence was kept constant at 150 mJ/cm2 for all sublayers. That fluence is sufficient for the complete stress relaxation in all sublayers except for the first, for which 180 mJ/cm2 should have been used according to the temperature calculations. Apparently, the total residual stress in the whole film is made up from that in the first sublayer, which can be concluded from the successive decrease in the total residual stress after the annealing of each sublayer. Thereby, it must be taken into account that the total stress is derived from the stresses of the sublayers according to the formula: P r tot=1/d tot (d i r i ) (i=1,. . .,n; r tot,d tot—total stress and thickness of a film consisting of n sublayers, r i ,d i —stress and thickness of the ith sublayer). The behaviour is in agreement with the results of the temperature calculations and, in particular, with the variation of maximum temperature during one pulse with film thickness. From the variation of the reflectivity, which was measured also in situ and normalized to the uncoated substrate (see Fig. 5), with film thickness, a refractive index of 2.47 and an extinction coefficient of 0.035 were calculated (670 nm wavelength, 658 angle of incidence), indicating an optical energy band gap of 2.0 eV. Apparently, the film is not influenced by the pulsed laser annealing process with respect to its optical properties, even though the maximum temperature in the film was about 1100 K. So far, we prepared several-micrometers-thick adherent ta-C films on Si and WC hard metal substrates. The preparation of a 2-Am-thick film on a 20-cm2 substrate area requires about 1 h in our experimental set-up. Thereby, it is important that we can go immediately from deposition to annealing and vice versa as the mean temperature during annealing does not exceed 90 8C at our maximum laser pulse repetition rates of 50 Hz.
carbon films with a high percentage of sp3 bonds, while the spectrum of the film deposited at 3.3 J/cm2 shows maxima at 6 and 25 eV. The sp3 percentage of the films was determined from the intensities of the K-edge peak at 286 eV, caused exclusively by the sp2–k* bonds in the films. For this, the ratio of the intensity of that peak (I k*) to the intensity of the peak at 296 eV (I r ), which is caused by the sp2–j* and sp3– j* bonds, of the films is compared with that of an amorphous carbon films with 100% sp2 bonds. Provided that the films contain exclusively sp2 and sp3 bonds, the sp3 content can then be determined according to [12]: csp3 ¼
1q 1 þ q3
with
q¼
½Ik =Ij film ½Ik =Ij 100% sp2
The sp3 content of the ta-C films deposited at a fluence of 16 J/cm2 was found to be 80.3% (unannealed film) and 82.4% (pulsed laser annealed film). This shows that the sp3 content of the films does not decrease in consequence of pulsed laser annealing, which is in accordance with the measured properties. Whether the slight increase is signifi-
3.3. Microstructural, optical, and mechanical properties As also known from literature, the ta-C films were found to be completely amorphous, independent of the target laser fluence and substrate temperature (up to 200 8C). The diamond-like character of films deposited at fluences above 6 J/cm2 can be observed in the EELS spectra taken at low energy and near the K-edge (see Fig. 6). The low-loss spectra of those films show a maximum at 30 eV, which is typical for
Fig. 8. Load–depth curves obtained during indentation measurements made on a 1-Am-thick carbon film on WC hard metal substrate. The average hardness and Young’s modulus values derived from those curves are given in Table 1 along with those of other samples.
S. Weissmantel et al. / Surface & Coatings Technology 188 – 189 (2004) 268–273
273
thereby determined by recording sound emission during the test and visually in an optical microscope afterwards. An example is shown in Fig. 9, where the onset of damage is marked by the steep increase of the friction force and the simultaneous onset of sound emission.
4. Conclusions
Fig. 9. Friction force, Friction coefficient and sound emission as a function of load measured on a 1Am thick ta-C film on WC hard metal using scratch testing.
cant must be further investigated. The sp3 content decreases continuously with decreasing fluence and was 74.5% at 6.7 J/cm2. Remarkably, even the film deposited at a relatively low fluence of 3.3 J/cm2 still had an sp3 content of 20.3%. The variation of the optical band gap of the films with laser fluence correlates very well with those findings (see Fig. 7). The maximum values were found to be 2.1 eV at 16 J/cm2 and to decrease with decreasing fluence and sp3 content, respectively. As mentioned above, the optical properties are not influenced either by thermal or pulsed laser annealing using proper parameters. The mechanical properties are preserved, too. As can be seen in Table 1, extraordinarily high values of hardness and Young’s modulus were measured on several 1–2-Am-thick films prepared on WC hard metal substrates. Thereby, the hardness even appears to be somewhat too low compared to the Young’s modulus. This is probably attributed to the presence of particulates with sizes of several hundred nanometers in our films—an assumption that is supported by a relatively large scattering of the 10 values measured on different spots of the same sample, from which the average hardness denoted in the table was derived. This large scattering can be observed in Fig. 8, where the load–depth curves obtained from different regions of the same sample are shown. The highest values of those single measurements, measured probably in a particulate-free film region, were as high as 85–95 GPa. Both thermal and pulsed annealed ta-C films show good adherence to Si and WC hard metal substrates. The high values of the critical loads (see Table 1), at which film damage occurred during the scratch test, is of great significance for industrial applications. Film damage was
Ta-C films were prepared at high growth rates up to 120 nm/min by PLD. Complete stress relaxation can be achieved by thermal as well as pulsed laser annealing. Stress relaxation occurs rapidly with pulsed laser annealing—the process itself needing only a few tens to hundreds of microseconds. By using that method, alternating with the deposition process, several-micrometers-thick stress-free and superhard t-aC films with good adhesion can be prepared on WC hard metal substrates.
Acknowledgments The authors gratefully acknowledge the financial support of the present work by the Bundesministerium fqr Bildung und Forschung (project no. 03I1702). Special thanks are due to Mrs. Baumann and Dr. S. Schulze (TU Chemnitz) for the TEM investigations and Dr. T. Chudoba (Advanced Surface Mechanics, or ASMEC) for the hardness measurements. We also thank the Roth and Rau Oberfl7chentechnik for the scratch testing measurements.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
D.L. Pappas, et al., J. Appl. Phys. 72 (1992) 3966. T.A. Friedmann, et al., Appl. Phys. Lett. 71 (26) (1997) 3820. M. Bonelli, et al., Eur. Phys. J. B 25 (2002) 269. H.-J. Scheibe, Diamond Relat. Mater. 1 (1992) 98. S. Weissmantel, G. Reisse, B. Keiper, A. Weber, U. Falke, M Rfder, Appl. Surf. Sci. 127–129 (1998) 444. S. Weissmantel, G. Reisse, U. Falke, Thin Solid Films 355–356 (1999) 256. W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. G. Reisse, S. Weissmantel, D. Rost, Diamond Relat. Mater 11 (3–6) (2002) 1276. G. Reisse, S. Weissmantel, D. Rost, Patentanmeldung, Aktenzeichen 10319206.9, Anmeldetag 25.04.03. A. Fischer, Thesis A, Hochschule Mittweida University of Applied Sciences (1984). C.J. Morath, et al., J. Appl. Phys. 76 (5) (1994) 2636. S. Deutschmann, PhD thesis 36017, TU Chemnitz (1994).
Preparation of superhard amorphous carbon films with low internal stress Steffen Weissmantel*, Gqnter Reisse, Dirk Rost Hochschule Mittweida, University of Applied Sciences, Technikumplatz 17, Mittweida 09648, Germany Available online 3 October 2004
Abstract Tetrahedral amorphous carbon (ta-C) films were prepared by pulsed laser deposition using a 248-nm excimer laser wavelength and up to 45 J/cm2 laser pulse energy fluence. Fluences above 6 J/cm2 and mean kinetic energies of the ablated species above 30 eV, respectively, as well as substrate temperatures below 100 8C were found to be necessary for the formation of ta-C films with high sp3 percentage. Such films are completely amorphous and have up to 85% sp3 bonds. As-deposited films show high compressive stresses in the range of 8–10 GPa. The possibilities to reduce those stresses by means of thermal and pulsed laser annealing were investigated. We found that both methods allow the preparation of nearly stress-free diamond-like carbon films with several micrometers of thickness and good adherence to Si and WC hard metal substrates. The Vickers microhardness of those films was measured to be 55–65 GPa and the Young’s modulus was measured to be 800–900 GPa by using a dynamic indentation method. D 2004 Elsevier B.V. All rights reserved. Keywords: Tetrahedral amorphous carbon (ta-C); Pulsed laser deposition (PLD); Stress relaxation; Mechanical properties
1. Introduction Hydrogen-free tetrahedral amorphous carbon (ta-C) is the second hardest material known. This, combined with their low surface roughness and friction, makes it an interesting material for applications as wear-resistant coatings. Pulsed laser deposition (PLD) [1–3] and arc evaporation (AED) [4] are two methods used for the preparation of ta-C films that have industrial potential. Those films show, however, high compressive stresses if deposited at optimum parameters with regard to hardness, which prevents the preparation of micrometer-thick films due to delamination. A breakthrough was therefore the preparation of stress-free films by using thermal annealing, first reported by Friedmann et al. [2]. We present here a novel method to prepare superhard ta-C films with low internal stress. The method is a combination of PLD and pulsed laser annealing, and we will show that it allows the rapid preparation of ta-C films with several * Corresponding author. Tel.: +49 3727 581449; fax: +49 3727 581449. E-mail address: [email protected] (S. Weissmantel). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.08.070
micrometers of thickness on Si and WC hard metal substrates.
2. Experimental details Carbon films were deposited by PLD in a high-vacuum system illustrated previously [5,6]. A KrF excimer laser with a laser pulse energy of 1.2 J, a pulse duration of 30 ns, and a maximum repetition rate of 50 Hz was used for ablation. The target material was graphite with a purity of 99.9%. The laser beam was moved in spirals and with constant vector velocity across the target area. The residual gas pressure during deposition was 1104 Pa. Prior to deposition, the substrates were bombarded for about 1 min with an Ar+ ion beam (700 eV energy, 150 AA/ cm2 current density at the substrate) produced in a Kaufman ion source in order to remove impurities. Films were deposited on Si, SiO2, and WC hard metal substrates. The substrate temperature was measured through a thermocouple affixed to the substrate surface. Thermal annealing was accomplished by heating the samples
S. Weissmantel et al. / Surface & Coatings Technology 188 – 189 (2004) 268–273
Fig. 1. Mean kinetic energy of the carbon atoms and ions ablated from a graphite target as a function of laser pulse energy density (calorimetric measurement).
immediately after deposition using a BoralectricR resistance heating element from Advanced Ceramics. In the pulsed laser annealing experiments, the same excimer laser beam that was used for ablation was guided onto the substrate. Its cross-sectional area on the substrate surface was 2 cm2. Changing between deposition and annealing can swiftly be done by means of a tiltable mirror. All annealing experiments were made in vacuum. High-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and electron energy loss spectroscopy (EELS) were performed on a Philips CM 20 FEG 200 keV transmission electron microscope equipped with a Gatan imaging filter (GIF). The spectral dependence of the refractive index and extinction coefficient was computed from the transmission and reflection curves of the films measured with a spectrophotometer in the wavelength range of 200–800 nm. The optical band gaps were determined from those curves using the Tauc plot, which is valid for amorphous semiconductors. The dynamic indentation measurements were performed on a UMIS 2000 equipped with a Berkovich indenter. The calculation of hardness and Young’s modulus from the obtained load–depth curves was made according to the ISO
269
14577 standard using a modified Oliver–Pharr method [7]. The values given in the paper are the mean values of 10 measurements each time using maximum loads of 10 mN. Film adhesion was characterised by means of a commercial scratch testing system using a Rockwell C diamond tip with 1208 included angle and 0.2 mm tip radius. The measurements were performed with continuous loading increase of 30 N/min and at a scratching speed of 10 mm/ min. The film stress was determined, using Stoney’s equation, from the bending of the silicon substrates induced by the deposited films. The bending was measured either after deposition by means of a DEKTAK profilometer or in situ by means of laser deflection using a device, which we illustrated and described previously [8]. For the in situ measurements, 400-Am-thick Si substrates of 50 mm length and 5 mm width were clamped one-sided to the substrate holder. The film thicknesses needed for the calculation of stress were determined from the reflected intensity of the measuring laser beam (670 nm wavelength).
3. Results and discussion 3.1. Deposition of ta-C films Dependent on the laser energy fluence at the target and the substrate temperature during deposition, hydrogen-free amorphous carbon films prepared by PLD may have a content of diamond-like (sp3) bonds in the range of 0–85% (see also Section 3.3) and are correspondingly soft to superhard. For the formation of superhard carbon films with high sp3 content (ta-C), fluences of more than 6 J/ cm2 and substrate temperatures of less than 90 8C were found to be necessary. The essential parameter is thereby the mean kinetic energy of the film-forming species. Calorimetric measurements showed that it is in the range of 30– 100 eV at fluences of 6–30 J/cm2 (see Fig. 1). In this range, sp3 contents of 70–85% were measured. At lower kinetic energies and fluences, respectively, the sp 3 content decreases. Substrate temperatures of more than 90 8C result,
Fig. 2. Stress reduction by thermal annealing of 200-nm-thick ta-C films prepared at 12 J/cm2 target laser pulse energy density on 400-Am Si substrates. Dependence on annealing temperature (left) and time (right).
270
S. Weissmantel et al. / Surface & Coatings Technology 188 – 189 (2004) 268–273
Fig. 3. Development of stress during pulsed laser annealing of 150-nmthick ta-C films, each deposited at 16 J/cm2 target laser fluence on 400-Am Si substrates in dependence on substrate laser fluence.
independently of the laser fluence, also in decreasing sp3 content and, already at 120 8C, the films are completely sp2-bonded. In our experimental set-up, the maximum deposition rate of ta-C films is currently 120 nm/min, where the laser energy fluence at the target is 10 J/cm2, the laser beam cross-section on the target surface is 3.5 mm2, and the pulse repetition frequency is 50 Hz. With these parameters, the mean substrate temperature increases to about 70 8C during deposition, which is due to the energy input of the filmforming species. Therefore, ta-C films can be prepared without substrate cooling. 3.2. Stress reduction by thermal and pulsed laser annealing As-deposited ta-C films possess high internal compressive stresses of 8–10 GPa, which leads to the delamination of 200- to 400-nm-thick films. Therefore, for the preparation of thicker films, measures must be taken to avoid or reduce those stresses. It is well known for several years that the stress can be removed completely by thermal annealing for
5 min at 600 8C [2]. We have also used this method in our investigations and our results are in good agreement. As can be seen in Fig. 2, the residual stress depends on the annealing temperature and time. A nearly complete stress relaxation requires a minimum annealing temperature of 600 8C and a minimum annealing time of several minutes. Thereby, the sp3 content and, with it, the diamond-like properties of the ta-C films are maintained (see also Section 3.3). In order to grow thick ta-C films in the micrometer range, it is necessary to successively deposit and anneal sublayers of a few hundred nanometers of thickness. We prepared ta-C films with several micrometers of thickness on Si and WC hard metal substrates in this way, where the thickness of the sublayers was 200 nm. This took, however, a rather long time as the samples had always to cool down after annealing, for which the minimum time required was 30 min if argon background gas at atmospheric pressure was used to accelerate the cooling. Recently, we studied a novel method for the stress relaxation in ta-C films, for which we have a patent pending [9]. It concerns pulsed laser annealing performed either during, using a second laser, or alternately to the deposition process. So far, we performed the annealing exclusively alternately to deposition and used for it the same KrF excimer laser as for ablation. The stress relaxation in three different 150-nm-thick ta-C films measured during the irradiation of the as-deposited films with a number of laser pulses at different energy fluences can be observed in Fig. 3. Remarkably, the stresses are largely reduced after only 200 laser pulses of 30 ns duration (i.e., relaxation occurs in a very short time). The residual stress decreases with increasing laser energy fluence from some 3 GPa at 110 mJ/cm2 to 1 GPa at 165 mJ/cm2 and nearly 0 GPa at 180 mJ/cm2. The temperature fields induced in ta-C/Si systems by the laser irradiation were calculated by using a numeric computer program [10]. The temperature-dependent material constants of silicon were taken from the literature [10]. The optical constants of ta-C at 248 nm wavelength (refractive index, extinction, and absorption coefficient of
Fig. 4. Calculated temperature field (left) generated in a 1000-nm-thick ta-C film on Si by irradiation with an excimer laser pulse (248 nm wavelength, 30 ns pulse duration, 150 mJ/cm2 laser fluence) and maximum temperature during one pulse in dependence on film thickness (right). The maximum of the Gaussian time-dependent laser pulse is at 50 ns.
S. Weissmantel et al. / Surface & Coatings Technology 188 – 189 (2004) 268–273
271
Fig. 5. Development of stress and reflected intensity normalized to the uncoated substrate during the preparation of a 1-Am-thick ta-C film on a 400-Am Si substrate by successive deposition and annealing (target laser energy fluence 12 J/cm2, substrate laser energy fluence 150 mJ/cm2).
Fig. 7. Sp3 content (from EELS) and optical energy band gap of pulsed laser-deposited carbon films as a function of target laser fluence. The values of the band gap have been determined according to the method of Tauc, which is applicable to amorphous semiconductors.
3.0, 0.35, and 1.8105 cm1, respectively) and the density (3.3 g/cm2) were measured on our own films. The specific heat capacity was calculated according to the Debye theory with a Debye temperature of 1770 K, which was temperature-dependent. For heat conductivity, a value of 9.2102 W/cm/K was used, which was measured on pulsed laserdeposited ta-C films by Morath et al. [11]. Finally, a Gaussian time dependence of the laser pulse was used, where the pulse duration was 30 ns. According to those calculations, the maximum temperature during one pulse in a 150-nm-thick ta-C film on Si is 763 K at 110 mJ/cm2 laser energy fluence and 1053 K at 180 mJ/cm2—the fluence at which complete stress relaxation is achieved. Furthermore, at constant fluence, the maximum temperature during one pulse depends relatively strongly on film thickness only up to a thickness of some 300 nm (see Fig. 4, right) and remains then nearly constant due to the low heat conductivity of ta-C. The latter is also the reason for the low penetration depth of the temperature fields into the films. Only some 50 nm are heated up nearly homogeneously (see Fig. 4, left). That means, however, that the sublayers to be annealed should not be thicker. The temperature needed for complete relaxation seems to be 180 K higher than that needed for conventional annealing. We
have, however, used so far only 150-nm-thick films for the annealing experiments. As there is a temperature gradient of some 200 K between the upper and lower parts of the film and complete relaxation requires relaxation of all parts of the film, the temperature needed for complete relaxation is in the range of conventional annealing. On the other hand, the temperature exceeds 650 K only for 40 ns and 400 K only for 230 ns per pulse (i.e., the relaxation process during pulsed laser annealing takes place within several tens to several hundreds of microseconds, much faster than in conventional annealing). The results suggest that the process is also determined mainly by thermal processes and that the high heating and cooling rates on the order of 1010 K/s (compared to the maximum rates of less than 1 K/s during thermal annealing) result in an acceleration of the atomic site interchanging and reordering processes, which are necessary for stress relaxation. Further investigations concerned the preparation of thick ta-C films by successive PLD and annealing of sublayers. In Fig. 5, the variation of total stress with film thickness measured during the preparation of a 1-Am-thick ta-C film on Si is shown, for example, where eight sublayers of some 150 nm thickness have successively been deposited and annealed. It can be observed that the total residual stress in
Fig. 6. Electron energy loss spectra of various pulsed laser-deposited carbon films.
272
S. Weissmantel et al. / Surface & Coatings Technology 188 – 189 (2004) 268–273
Table 1 Mechanical properties of various ta-C films on WC hard metal substrates Sample
Parameter
Hardness [GPa]
Young’s modulus [GPa]
Critical load scratch test [N]
Friction coefficient
Density [g/cm3]
252 262 253 044 WC
18 J/cm2 18 J/cm2 12 J/cm2 12 J/cm2 uncoated
55.2 48.9 61.7 56.1 26
690 705 878 915 600
50
0.12
52 42
0.12 0.11
3.08 3.17 3.26 3.28
thermal annealing thermal annealing pulsed laser annealing pulsed laser annealing
The film thickness was in the range of 1–2 Am.
the whole film is 0.3 GPa, whereas that in the first sublayer is still 2 GPa after annealing. The reason is that the annealing fluence was kept constant at 150 mJ/cm2 for all sublayers. That fluence is sufficient for the complete stress relaxation in all sublayers except for the first, for which 180 mJ/cm2 should have been used according to the temperature calculations. Apparently, the total residual stress in the whole film is made up from that in the first sublayer, which can be concluded from the successive decrease in the total residual stress after the annealing of each sublayer. Thereby, it must be taken into account that the total stress is derived from the stresses of the sublayers according to the formula: P r tot=1/d tot (d i r i ) (i=1,. . .,n; r tot,d tot—total stress and thickness of a film consisting of n sublayers, r i ,d i —stress and thickness of the ith sublayer). The behaviour is in agreement with the results of the temperature calculations and, in particular, with the variation of maximum temperature during one pulse with film thickness. From the variation of the reflectivity, which was measured also in situ and normalized to the uncoated substrate (see Fig. 5), with film thickness, a refractive index of 2.47 and an extinction coefficient of 0.035 were calculated (670 nm wavelength, 658 angle of incidence), indicating an optical energy band gap of 2.0 eV. Apparently, the film is not influenced by the pulsed laser annealing process with respect to its optical properties, even though the maximum temperature in the film was about 1100 K. So far, we prepared several-micrometers-thick adherent ta-C films on Si and WC hard metal substrates. The preparation of a 2-Am-thick film on a 20-cm2 substrate area requires about 1 h in our experimental set-up. Thereby, it is important that we can go immediately from deposition to annealing and vice versa as the mean temperature during annealing does not exceed 90 8C at our maximum laser pulse repetition rates of 50 Hz.
carbon films with a high percentage of sp3 bonds, while the spectrum of the film deposited at 3.3 J/cm2 shows maxima at 6 and 25 eV. The sp3 percentage of the films was determined from the intensities of the K-edge peak at 286 eV, caused exclusively by the sp2–k* bonds in the films. For this, the ratio of the intensity of that peak (I k*) to the intensity of the peak at 296 eV (I r ), which is caused by the sp2–j* and sp3– j* bonds, of the films is compared with that of an amorphous carbon films with 100% sp2 bonds. Provided that the films contain exclusively sp2 and sp3 bonds, the sp3 content can then be determined according to [12]: csp3 ¼
1q 1 þ q3
with
q¼
½Ik =Ij film ½Ik =Ij 100% sp2
The sp3 content of the ta-C films deposited at a fluence of 16 J/cm2 was found to be 80.3% (unannealed film) and 82.4% (pulsed laser annealed film). This shows that the sp3 content of the films does not decrease in consequence of pulsed laser annealing, which is in accordance with the measured properties. Whether the slight increase is signifi-
3.3. Microstructural, optical, and mechanical properties As also known from literature, the ta-C films were found to be completely amorphous, independent of the target laser fluence and substrate temperature (up to 200 8C). The diamond-like character of films deposited at fluences above 6 J/cm2 can be observed in the EELS spectra taken at low energy and near the K-edge (see Fig. 6). The low-loss spectra of those films show a maximum at 30 eV, which is typical for
Fig. 8. Load–depth curves obtained during indentation measurements made on a 1-Am-thick carbon film on WC hard metal substrate. The average hardness and Young’s modulus values derived from those curves are given in Table 1 along with those of other samples.
S. Weissmantel et al. / Surface & Coatings Technology 188 – 189 (2004) 268–273
273
thereby determined by recording sound emission during the test and visually in an optical microscope afterwards. An example is shown in Fig. 9, where the onset of damage is marked by the steep increase of the friction force and the simultaneous onset of sound emission.
4. Conclusions
Fig. 9. Friction force, Friction coefficient and sound emission as a function of load measured on a 1Am thick ta-C film on WC hard metal using scratch testing.
cant must be further investigated. The sp3 content decreases continuously with decreasing fluence and was 74.5% at 6.7 J/cm2. Remarkably, even the film deposited at a relatively low fluence of 3.3 J/cm2 still had an sp3 content of 20.3%. The variation of the optical band gap of the films with laser fluence correlates very well with those findings (see Fig. 7). The maximum values were found to be 2.1 eV at 16 J/cm2 and to decrease with decreasing fluence and sp3 content, respectively. As mentioned above, the optical properties are not influenced either by thermal or pulsed laser annealing using proper parameters. The mechanical properties are preserved, too. As can be seen in Table 1, extraordinarily high values of hardness and Young’s modulus were measured on several 1–2-Am-thick films prepared on WC hard metal substrates. Thereby, the hardness even appears to be somewhat too low compared to the Young’s modulus. This is probably attributed to the presence of particulates with sizes of several hundred nanometers in our films—an assumption that is supported by a relatively large scattering of the 10 values measured on different spots of the same sample, from which the average hardness denoted in the table was derived. This large scattering can be observed in Fig. 8, where the load–depth curves obtained from different regions of the same sample are shown. The highest values of those single measurements, measured probably in a particulate-free film region, were as high as 85–95 GPa. Both thermal and pulsed annealed ta-C films show good adherence to Si and WC hard metal substrates. The high values of the critical loads (see Table 1), at which film damage occurred during the scratch test, is of great significance for industrial applications. Film damage was
Ta-C films were prepared at high growth rates up to 120 nm/min by PLD. Complete stress relaxation can be achieved by thermal as well as pulsed laser annealing. Stress relaxation occurs rapidly with pulsed laser annealing—the process itself needing only a few tens to hundreds of microseconds. By using that method, alternating with the deposition process, several-micrometers-thick stress-free and superhard t-aC films with good adhesion can be prepared on WC hard metal substrates.
Acknowledgments The authors gratefully acknowledge the financial support of the present work by the Bundesministerium fqr Bildung und Forschung (project no. 03I1702). Special thanks are due to Mrs. Baumann and Dr. S. Schulze (TU Chemnitz) for the TEM investigations and Dr. T. Chudoba (Advanced Surface Mechanics, or ASMEC) for the hardness measurements. We also thank the Roth and Rau Oberfl7chentechnik for the scratch testing measurements.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
D.L. Pappas, et al., J. Appl. Phys. 72 (1992) 3966. T.A. Friedmann, et al., Appl. Phys. Lett. 71 (26) (1997) 3820. M. Bonelli, et al., Eur. Phys. J. B 25 (2002) 269. H.-J. Scheibe, Diamond Relat. Mater. 1 (1992) 98. S. Weissmantel, G. Reisse, B. Keiper, A. Weber, U. Falke, M Rfder, Appl. Surf. Sci. 127–129 (1998) 444. S. Weissmantel, G. Reisse, U. Falke, Thin Solid Films 355–356 (1999) 256. W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. G. Reisse, S. Weissmantel, D. Rost, Diamond Relat. Mater 11 (3–6) (2002) 1276. G. Reisse, S. Weissmantel, D. Rost, Patentanmeldung, Aktenzeichen 10319206.9, Anmeldetag 25.04.03. A. Fischer, Thesis A, Hochschule Mittweida University of Applied Sciences (1984). C.J. Morath, et al., J. Appl. Phys. 76 (5) (1994) 2636. S. Deutschmann, PhD thesis 36017, TU Chemnitz (1994).