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Diffusion in diamond-like carbon
Diamond and Related Materials 12 (2003) 2042–2050
Diffusion in diamond-like carbon b ¨ a,*, C. Ronninga, H. Hofsass ¨ a, P. Neumaierb, A. Bergmaierb, L. Gorgens ¨ H. Kroger , G. Dollingerb
b
a ¨ Gottingen, ¨ II. Physikalisches Institut, Universitat Bunsenstr. 7-9, D-37073 Gottingen, Germany ¨ Munchen, ¨ Technische Universitat Physik Department E12, James-Franck-Str., D-85748 Garching, Germany
Abstract The diffusion of carbon and five other elements in amorphous carbon (a-C) films was studied. One set of samples were sp2 dominated a-C and the other set of samples were sp3 dominated tetrahedral amorphous carbon (ta-C). The films were deposited using mass separated ion beam deposition under UHV conditions. The diffusion of 13 C as well as that of hydrogen and deuterium was studied using high resolution elastic recoil detection analysis. No apparent self-diffusion could be detected using this technique. The diffusion of hydrogen was found to start at temperatures between 600 and 800 8C. For deuterium, activation energies of 3.34(5) and 3.39(5) eV were found for diffusion in ta-C and a-C, respectively. Tungsten, copper and silver were used to study the diffusion of metals in ta-C. Up to annealing temperatures of 1000 8C no diffusion took place in the samples. During annealing at 1200 8C the ta-C is converted into graphite, making diffusion into the carbon matrix possible. The fact that there is no diffusion of copper in ta-C at temperatures below 1200 8C shows that ta-C is a possible diffusion barrier between copper and silicon. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Tetrahedral amorphous carbon; Diffusion; Impurities; Ion beam deposition
1. Introduction Diamond or diamond-like coatings are useful for many applications. In many cases, a polycrystalline material is not desirable, because it lacks the necessary smoothness. In these cases, amorphous carbon (a-C) films can be used. These films are sp2 as well as sp3 bonded. Depending on the percentage of each bonding type, different film structures can be prepared, on the one hand graphite-like films (with a high percentage of sp2 bonding), on the other hand, diamond-like films (with a high percentage of sp3 bonding) w1x. For 70% sp3 bondings and more, the films are known as tetrahedral amorphous carbon (ta-C). In contrast, diamond-like carbon (dlc) films prepared by CVD methods contain a high percentage of hydrogen. These films are still very hard, but not as hard as ta-C w2x. There is an interest in diamond-like thin films in many areas. In medical science, they are being used to coat prostheseses, making them last longer w3,4x. Computer hard disks as well as write and read heads are being coated with ta-C w5,6x. *Corresponding author. E-mail address: [email protected] ¨ (H. Kroger).
In the semiconductor industry, ta-C is being used for many purposes, for example in the manufacture of LEDs w7 x . As these examples of applications show, there is a big interest in the properties of ta-C. The properties of foreign atoms incorporated into ta-C are of particular interest, because they can be used to modify its properties. To date, only the thermal stability of hydrogen and deuterium in dlc has been studied w8x. The temperaturedependent behaviour of foreign atoms in ta-C could cause wanted or unwanted structural changes of the taC, modifying its properties, such as its hardness or its optical properties w9,10x. In the present study, the self-diffusion of 13C in 12C has been investigated using elastic recoil detection analysis (ERDA). Furthermore, the diffusion of nitrogen has been studied by resonant nuclear reaction analysis (RNRA). In addition, the thermal stability of three different metals (tungsten, copper and silver) in ta-C has been studied using Rutherford backscattering (RBS). If the elements investigated show a high thermal stability in ta-C, it could be used as a diffusion barrier for those elements.
0925-9635/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-9635Ž03.00218-8
¨ H. Kroger et al. / Diamond and Related Materials 12 (2003) 2042–2050
2. Experimental 2.1. Mass selected ion beam deposition The films investigated in this study were grown on single crystalline n-type silicon (1 0 0) substrates by direct deposition of low energy ions at room temperature using mass selected ion beam deposition (MSIBD). Two types of films were prepared. The first type of film was prepared with a carbon matrix deposited at 100 eV, which produces a ta-C matrix. The second type of sample was deposited at a carbon ion-energy of 30 eV, resulting in an a-C matrix which is mainly sp2 bonded. The films are grown under UHV conditions at a base pressure of approximately 2=10y8 mbar, and are therefore, almost free of contaminants like hydrogen, which is most important for the investigation of low dose hydrogen diffusion. The details of the deposition system are described elsewhere w11,12x. The samples were cleaned using acetone and in situ sputtering with 1 keV 40 Arq ions immediately before deposition. For each diffusion process different samples were prepared. The hydrogen isotopes were implanted with ion energies from 100 eV up to 1 keV, the metals and the nitrogen were deposited with energies of 60 eV (metals, except for silver, which was implanted with 1 keV) and 100 eV (nitrogen) between two carbon depositions, creating a d-profile of approximately 1 nm within a ta-C or a-C matrix. The samples will be referred to by the foreign element included as a d-profile in the carbon matrix, i.e. a hydrogen sample would be a ta-C or a-C matrix on silicon into which a d-profile of hydrogen has been implanted. 2.2. Characterisation Due to their small mass, the hydrogen isotopes were measured with ERDA using the Q3D magnetic spectrograph at the 15 MV tandem accelerator at the TU ¨ Munchen at Garching. Using this equipment depth resolutions of approximately 1 nm are possible w13,14x. The measurements were taken using a 40 MeV 197 Au7q beam, allowing for high angular resolution and minimized radiation damage. The samples studied using ERDA include the 13C samples for measuring selfdiffusion under annealing and the hydrogen and deuterium samples. The annealing took place in a vacuum of ¨ ;10y6 mbar at the TU Munchen. Each sample was annealed at a temperature varied between 600 and 950 8C for 10 min between each ERDA measurement. RBS measurements were conducted using a 900 keV 4 ¨ He2q ion beam supplied by the Gottinger heavy ion implanter IONAS w15x. The spectra were analyzed using the computer code RUMP w16x. RBS spectra were taken for the metal samples, which were cut in smaller pieces.
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Each piece was annealed in a vacuum oven with a base pressure during annealing of ;10y6 mbar, at temperatures ranging from 600 to 1200 8C. RNRA spectra for the nitrogen sample were obtained ¨ using the Gottinger 3 MV Pelletron accelerator MaRPel w17x. The reaction used for the nitrogen analysis takes place for protons with an energy of 898 keV hitting a nitrogen nucleus. It has a cross-section of 800 mb with a resonance width of 2.2 keV w18x, which limits the depth resolution for the RNRA spectra. In this case, it is approximately 30 nm, which was calculated using TRIM w19x. 3. Results and discussion 3.1. Self-diffusion of
13
C in
12
C
To effectively study the diffusion of foreign atoms in an a-C matrix a first step is to investigate the effect of annealing on the matrix used. Two samples were prepared, one a-C and one ta-C, each with a 13C d-profile, and were analyzed with ERD. The d-profile had a width of approximately 1 nm, resulting from the broadening of the 13C-layer caused by the subsequent deposition of 12 C. In the deposition process, the carbon atoms are implanted just beneath the surface, which causes a small amount of the most recently deposited material to float on top of the sample. This results in the layer being thinned out as the deposition continues. This is the origin of the small surface peak of 13C shown in Fig. 1. Due to the strong sp3 bonding no diffusion is expected to take place at room temperature or during annealing up to at least 600 8C. The obtained spectra for the ta-C sample show no significant diffusion of the 13C atoms within the a-C matrix at annealing temperatures of 900 8C, as shown in Fig. 1. Although the spectra appear to show a different concentration of 13C, it is not significant within error limits. This is in agreement with other measurements of the changes in the ta-C matrix by Ferrari et al. w20x, which show only slight changes in the bonding structure up to annealing temperatures of approximately 1000 8C. These measurements also show that the ta-C graphitizes at temperatures of approximately 1100 8C. The annealing experiments for the a-C sample show that there is no difference in behaviour between the carbon in the a-C sample and in the ta-C sample. Therefore, annealing experiments for foreign atoms in both carbon matrices would appear to not be affected by any carbon diffusion up to temperatures of 900 8C for our samples. The experiments also show that ta-C is thermally stable under vacuum up to the aforementioned temperature.
¨ H. Kroger et al. / Diamond and Related Materials 12 (2003) 2042–2050
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Fig. 1. ERD-spectra of a
13
C d-profile in ta-C. The surface-peak is due to the sample preparation with MSIBD.
3.2. Light elements 3.2.1. Hydrogen The first of the light elements studied was hydrogen. A ta-C sample was prepared into which hydrogen was implanted with an ion energy of 1 keV. The sample was analyzed using high resolution ERDA. In Fig. 2 spectra over the whole depth of the sample are shown. In the left part of the spectra a large surface peak corresponding to a concentration of well over 30% hydrogen can be clearly seen. This peak occurs between annealing and
measurement and is due to water contamination of the surface once the sample is taken out of the vacuum. Due to this contamination a relatively high implantation energy of 1 keV was necessary, as the hydrogen profile for lower energies was completely masked by the surface contamination. The right part of Fig. 2 shows an enlarged part of the spectra which contains only the implanted profile. The profile shows an outdiffusion of hydrogen at temperatures of 800 8C and above. Furthermore, it can be seen that the hydrogen contained in the substrate can diffuse even at lower temperatures. The
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Fig. 2. ERD-spectra of hydrogen implanted with 1 keV into ta-C for as-implanted and annealing up to 950 8C. The left part of the spectrum shows a large surface peak, which is present in every spectrum. The right part of the figure shows an enlarged section of the spectra containing the implanted hydrogen.
reason why the integrated amount of hydrogen in the bulk peak increases during annealing at 600 8C is not clear. It could be due to ERDA measurements being performed on two different parts of the sample, showing an inhomogeneity of the hydrogen within the sample.
3.2.2. Deuterium Due to the surface contamination with water, hydrogen measurements were rather difficult. To study the diffusion of hydrogen at low deposition energies it is easier to use the stable isotope deuterium, which, due
Fig. 3. ERD-spectra for the diffusion of deuterium implanted with 100 eV in ta-C. The profile changes for annealing temperatures of 800 8C and above, showing an out-diffusion of deuterium from the carbon matrix. After annealing at 950 8C the sample contains less than 20% of the implanted deuterium.
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can then be used to calculate the activation energy. For this the following equation was used: B tv
C
Phonon
E
F
(1) x D lB G where kB is Boltzmann’s constant, t is the annealing time in seconds, vPhonon is the phonon frequency of the bulk material in which the diffusion takes place, x is the diffusion length and lB is the minimum distance the atom has to cross, i.e. the bond length in the bulk material. With the values for annealing time (600(10) s), vPhonon (diamond: (1) 1013 Hz w21x, a-C: (4.4"1.0)=1013 Hz), x (6(2) nm) and lB (sp3: 0.15(1) nm; sp2: 0.14(1) nm) the resulting activation energy, EA for deuterium in ta-C is 3.34(5) and 3.39(5) eV in a-C, respectively.
EAskBTA ln
Fig. 4. A plot of the integrated content of deuterium divided by the implanted dose against annealing time for both deuterium in ta-C (top) and a-C (bottom). The interpolated temperature for a content of 50% is the activation temperature, TA, from which the activation energy, EA, can be calculated.
to its natural isotopic occurrence of 0.015%, is not likely to be present as a contaminant. In Fig. 3 the implantation profile of 100 eV deuterium in ta-C is shown. The profile does not change significantly due to annealing at temperatures of up to 600 8C. After annealing the sample at 800 8C the amount of deuterium found in the sample decreased to approximately 80% of the implanted dose. In the final annealing step at 950 8C the deuterium content dropped to 16%. The same annealing steps were performed with an a-C sample, which showed exactly the same behaviour as the ta-C sample. From these measurements an activation energy for the diffusion can be derived. This is done by a simple model based on the assumption of Arrhenius-like diffusion. The activation energy can be obtained by plotting the integral content of deuterium against annealing temperature as shown in Fig. 4. By interpolating the temperature for which the deuterium amount would be 50%, the activation temperature can be derived. This temperature
3.2.3. Nitrogen Another light element studied is nitrogen. In this case, only one ta-C sample has been prepared to study the diffusion. Using the stable isotope 15N, which is suitable for RNRA, a sample containing two d-profiles was prepared. Fig. 5 shows the obtained spectra. The surface peak has the expected minimal linewidth of approximately 30 nm due to the large full width at half maximum (fwhm) (2.2 keV) of the reaction. The bulk signal shows a broader distribution, which is due to the deposition process. The nitrogen d-profile was broadened by the subplantation of carbon atoms, which were deposited afterwards in order to create a ta-C matrix for the diffusion. The newly deposited ions become mixed with the implanted nitrogen atoms that are being transported along with the new surface. This process broadens the originally sharp nitrogen profile. After annealing the sample at 700 8C for 1 h, 30% of the integral 15N content is gone. 15N is mainly lost from the surface, as the bulk peak does not decrease much. Further annealing at 900 8C leads to the disappearance of the surface peak, whereas the bulk peak still shows more than half the implanted content. The loss is partially due to the irradiation of the sample during analysis, but the reduction of the surface peak is mainly caused by outdiffusion of the 15N at this temperature. The bulk peak reduction could be caused simply by the irradiation of the sample, confirming the 15N to be stable in the ta-C matrix up to temperatures of 900 8C. Another fact supporting this assumption is that the bulk peak does not broaden in either of the two spectra taken after annealing. If the nitrogen were diffusing instead of being annihilated, the profile should broaden which is not the case. 3.3. Heavy elements 3.3.1. Tungsten The first of the heavy elements studied was tungsten. Tungsten, when incorporated into carbon, is known to
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Fig. 5. RNRA spectra and Gaussian fit of two 15N d-profiles measured as implanted and after annealing for 1 h at 700 and 900 8C. The peak close to the surface almost completely disappears, whereas the bulk peak is more stable.
form tungsten-carbide. The sample investigated was prepared with a d-profile of 5=1015 W atoms, inserted between two ta-C layers of approximately 25 nm thickness. The sample was then cut into four pieces which were measured with RBS, after being annealed under vacuum for 10 min. The spectra obtained were analysed and deconvoluted using the RUMP computer code w16x. The results are presented in the left part of Fig. 6. The amount of tungsten integrated in the ta-C films varies in the spectra from 3 to 4=1015, showing that most of the deposited amount of tungsten ions, which was 5=1015, is present in the sample. In the spectra, a slight shift in the position of the peak can be seen. This is due to small inhomogeneities in layer thickness over the whole of the sample. The thickness of the top carbon layer, which is responsible for the shift of the tungsten peak in the obtained spectra, varies in a range of approximately 5 nm, which is approximately 10% of the total film thickness. This is a reasonable value for the error of the film thickness. The peak in the original data has been fitted using a Gaussian function. The width of the peak does not change significantly in any of the spectra obtained after annealing at temperatures of up to 1000 8C. Therefore, the tungsten is enclosed stable in the ta-C matrix and does not appear to diffuse into the carbon matrix. After annealing for 10 min at 1200 8C, the peak broadens and shows a slight double peak structure, which could be explained by tungsten
diffusing to the top of the carbon film, parallel to graphitization. 3.3.2. Copper Copper does not form carbides and, therefore, is expected to behave differently from tungsten. There are indications that copper forms small clusters within the carbon matrix w22x. The sample was again cut into four small pieces. RBS measurements are presented in the middle column of Fig. 6. Again, as for tungsten, the copper peak in the spectra shows a slight shift in position corresponding to a variation in film thickness of approximately 7 nm. The integrated amount of copper does not change significantly due to annealing up to 1000 8C. It is between 4 and 7=1015 atoms, showing that a maximum of 35% of the 2=1016 copper ions deposited has been incorporated into the ta-C. This is due to sputtering, either by copper ions during deposition andy or carbon ions which are deposited afterwards. In the spectrum taken after annealing at 1000 8C, a second peak appears next to the initial copper peak. This second peak corresponds to copper accumulating at the interface of carbon and silicon, which has been simulated using the RUMP computer code w16x. This new copper originates from the sample holder on which the sample was grown and diffuses to the interface at these high temperatures, but it still does not enter the ta-C. The original peak does not show a decrease in copper content or
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¨ H. Kroger et al. / Diamond and Related Materials 12 (2003) 2042–2050
Fig. 6. RBS spectra of tungsten (left), copper (middle) and silver (right) d-profiles in ta-C. Presented are the counts measured as a function of energy for each metal and after each annealing step.
broadening of the fwhm. The spectrum after annealing at 1200 8C is shown in the top part of Fig. 7 in a logarithmic scale, along with the fit calculated with the RUMP computer code. The signal on the right of the spectrum is the copper signal, which consists partially of the originally incorporated copper and partially of the copper coming from the interface, as has been shown in Fig. 6 in the spectrum taken after annealing at 1000 8C. The peak in the middle of the spectrum shown in Fig. 7 is due to argon, which was used to sputter-clean the surface before deposition of the sample. The ta-C is converted into graphite due to annealing at a temperature of 1200 8C. The copper can then diffuse into the graphite. The schematic of the fit is presented in the bottom part of Fig. 7. The fit shows a large mixing area
of carbon and silicon, in which the copper is situated, showing that there appears to be higher solubility for copper in silicon-carbide, as there is no copper left at the center of the originally deposited d-profile. There appears to be a high mobility of silicon and copper in the graphite at this high temperature. 3.3.3. Silver Silver was used as an example of a noble metal. It had to be implanted, because during normal growth processes the silver was sputtered almost completely w22x. The reason for this is not entirely certain. One possible explanation could be the insolubility of silver in carbon. The deposited silver possibly forms weakly bonded clusters, which are broken up by further ion
¨ H. Kroger et al. / Diamond and Related Materials 12 (2003) 2042–2050
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enclosed metal to be stable in the ta-C matrix for annealing up to temperatures of 1000 8C. After annealing the sample at 1200 8C, the silver disappeared completely, which again is due to the conversion of ta-C into graphite at this temperature. 4. Conclusions
Fig. 7. Top: Cutout of the RBS-spectrum of Cu in ta-C after annealing at 1200 8C in logarithmic scale. The line is the fit to the data calculated using the RUMP computer code w16x. Bottom: Layer structure of the fit with RUMP w16x. The Argon, which has been fitted aswell in the top part of this figure, has been left out to get a clearer picture of the fit. It is situated in a depth of 52.5 nm with a concentration of 9%. The copper is located in a broad mixing layer of carbon and silicon.
bombardment. The silver atoms created by the breakup of the clusters may then accumulate on the surface, where they are being sputtered by the carbon ions used for depositing the ta-C matrix. To study the diffusion of silver, the ions had to be implanted far enough beneath the surface to be out of reach of the 100 eV carbon ions. This was achieved by implanting the silver ions with an energy of 1 keV. The sample was then treated in the same manner as the other metal samples described above. The RBS spectra are presented in the right part of Fig. 6. The integrated amount of silver was calculated from the deconvoluted spectra, showing that only 1.5=1015 silver atoms are present in the sample. This is approximately 10% of the originally implanted silver ions. The silver signal in the RBS spectra only changed position slightly, accounting for a thickness variation of the sample of ;2 nm. The spectra again show the
The self-diffusion of 13C in a-C and ta-C was investigated using high resolution ERDA. For annealing at temperatures of up to 900 8C no significant diffusion of the carbon atoms could be detected. For the diffusion of deuterium in a-C and ta-C, activation energies were calculated, resulting in EAs 3.39(5) eV for deuterium in a-C and EAs3.34(5) eV for deuterium in ta-C. This is in agreement with the results of Ahlgren w8x, who found an activation energy of EAs2.9(1) eV for deuterium in dlc. The diffusion of hydrogen in ta-C starts at a temperature between 600 and 800 8C, which is significantly lower than known temperatures for diffusion of hydrogen in natural diamond (1200 8C) or CVD-diamonds (1400 8C). The diffusion of nitrogen in ta-C was investigated using RNRA. The nitrogen starts to diffuse out of the surface at a temperature of 700 8C. The nitrogen incorporated in the bulk of the sample was much more stable, retaining half of its initial content even after annealing under vacuum for 1 h at 900 8C. The three metals investigated all show a similar behaviour. Tungsten, copper and silver remain stable within the ta-C matrix even after annealing at a temperature of 1000 8C. At higher annealing temperatures the ta-C is converted into graphite, allowing the foreign atoms to diffuse into the carbon matrix. In the case of copper, a tendency for accumulation in the siliconcarbide region of the sample was found. The stability of the copper within the ta-C matrix suggests its usefulness as a diffusion barrier for copper to protect silicon substrates. Acknowledgments The author would like to thank M. Schwickert for performing the RNRA measurements, Petra Reinke and Emily Hooker for helpful discussions. References w1x P.J. Fallon, V.S. Veerasamy, C.A. Davis, et al., Phys. Rev. B 48 (1993) 4777. w2x J.C. Knight, A.J. Whitehead, T.F. Page, J. Mater. Sci. 27 (1992) 3939. w3x M. Allen, B. Myer, N. Rushton, J. Biomed. Mater. Res. 58 (3) (2001) 319.
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w4x V.-M. Tiainen, Diamond Relat. Mater. 10 (2001) 153. w5x P.R. Goglia, J. Berkowitz, J. Hoehn, A. Xidis, L. Stover, Diamond Relat. Mater. 10 (2001) 271. w6x W. Scott, B. Bhushan, A.V. Lakshmikumaran, J. Appl. Phys. 87 (9) (2001) 6182. w7x K. Lmimouni, C. Legrand, C. Dufour, A. Chapoton, C. Belouet, Appl. Phys. Lett. 78 (17) (2001) 2437. w8x T. Ahlgren, Diffusion of Impurities in Compound Semiconductors and Diamond-Like Carbon Films. Ph.D. thesis, University of Helsinki, 1998. w9x H.B. Liao, R.F. Xiao, J.S. Fu, P. Yu, G.K.L. Wong, P. Sheng, Appl. Phys. Lett. 70 (1997) 1. w10x Z. Liu, H. Wang, H. Li, X. Wang, J. Appl. Phys. 72 (1998) 1823. w11x J. Singh, S.M. Copley, J. Mazumder (Eds.), Beam Processing of Advanced Materials, ASM International, Cleveland, 1996, p. 29. w12x H. Hofsass, ¨ C. Ronning, Appl. Phys. A 66 (1998) 153.
w13x G. Dollinger, T. Faestermann, P. Maier-Komor, Nucl. Instrum. Meth. B 64 (1992) 422. w14x G. Dollinger, C.M. Frey, A. Bergmaier, T. Faestermann, Europhys. Lett. 42 (1) (1998) 25. w15x M. Uhrmacher, K. Pampus, F.J. Bergmeister, D. Purschke, K.P. Lieb, Nucl. Instrum. Meth. B 9 (1985) 234. w16x L.R. Doolittle, Nucl. Instrum. Meth. B 15 (1986) 227. w17x M. Schwickert, F. Harbsmeier, H. Schebela, et al., Surf. Coat. Technol. 151–152 (2002) 222. w18x J.R. Tesmer, M. Nastasi (Eds.), Handbook of Modern Ion Beam Materials Analysis, Materials Research Society, 1995. w19x J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. w20x A.C. Ferrari, B. Kleinsorge, N.A. Morrison, A. Hart, V. Stolojan, J. Robertson, J. Appl. Phys. 85 (1999) 7191. w21x A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (20) (2000) 14095. w22x I. Gerhards, C. Ronning, U. Vetter, et al., Surf. Coat. Technol.158–159 (2002) 114.
Diffusion in diamond-like carbon b ¨ a,*, C. Ronninga, H. Hofsass ¨ a, P. Neumaierb, A. Bergmaierb, L. Gorgens ¨ H. Kroger , G. Dollingerb
b
a ¨ Gottingen, ¨ II. Physikalisches Institut, Universitat Bunsenstr. 7-9, D-37073 Gottingen, Germany ¨ Munchen, ¨ Technische Universitat Physik Department E12, James-Franck-Str., D-85748 Garching, Germany
Abstract The diffusion of carbon and five other elements in amorphous carbon (a-C) films was studied. One set of samples were sp2 dominated a-C and the other set of samples were sp3 dominated tetrahedral amorphous carbon (ta-C). The films were deposited using mass separated ion beam deposition under UHV conditions. The diffusion of 13 C as well as that of hydrogen and deuterium was studied using high resolution elastic recoil detection analysis. No apparent self-diffusion could be detected using this technique. The diffusion of hydrogen was found to start at temperatures between 600 and 800 8C. For deuterium, activation energies of 3.34(5) and 3.39(5) eV were found for diffusion in ta-C and a-C, respectively. Tungsten, copper and silver were used to study the diffusion of metals in ta-C. Up to annealing temperatures of 1000 8C no diffusion took place in the samples. During annealing at 1200 8C the ta-C is converted into graphite, making diffusion into the carbon matrix possible. The fact that there is no diffusion of copper in ta-C at temperatures below 1200 8C shows that ta-C is a possible diffusion barrier between copper and silicon. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Tetrahedral amorphous carbon; Diffusion; Impurities; Ion beam deposition
1. Introduction Diamond or diamond-like coatings are useful for many applications. In many cases, a polycrystalline material is not desirable, because it lacks the necessary smoothness. In these cases, amorphous carbon (a-C) films can be used. These films are sp2 as well as sp3 bonded. Depending on the percentage of each bonding type, different film structures can be prepared, on the one hand graphite-like films (with a high percentage of sp2 bonding), on the other hand, diamond-like films (with a high percentage of sp3 bonding) w1x. For 70% sp3 bondings and more, the films are known as tetrahedral amorphous carbon (ta-C). In contrast, diamond-like carbon (dlc) films prepared by CVD methods contain a high percentage of hydrogen. These films are still very hard, but not as hard as ta-C w2x. There is an interest in diamond-like thin films in many areas. In medical science, they are being used to coat prostheseses, making them last longer w3,4x. Computer hard disks as well as write and read heads are being coated with ta-C w5,6x. *Corresponding author. E-mail address: [email protected] ¨ (H. Kroger).
In the semiconductor industry, ta-C is being used for many purposes, for example in the manufacture of LEDs w7 x . As these examples of applications show, there is a big interest in the properties of ta-C. The properties of foreign atoms incorporated into ta-C are of particular interest, because they can be used to modify its properties. To date, only the thermal stability of hydrogen and deuterium in dlc has been studied w8x. The temperaturedependent behaviour of foreign atoms in ta-C could cause wanted or unwanted structural changes of the taC, modifying its properties, such as its hardness or its optical properties w9,10x. In the present study, the self-diffusion of 13C in 12C has been investigated using elastic recoil detection analysis (ERDA). Furthermore, the diffusion of nitrogen has been studied by resonant nuclear reaction analysis (RNRA). In addition, the thermal stability of three different metals (tungsten, copper and silver) in ta-C has been studied using Rutherford backscattering (RBS). If the elements investigated show a high thermal stability in ta-C, it could be used as a diffusion barrier for those elements.
0925-9635/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-9635Ž03.00218-8
¨ H. Kroger et al. / Diamond and Related Materials 12 (2003) 2042–2050
2. Experimental 2.1. Mass selected ion beam deposition The films investigated in this study were grown on single crystalline n-type silicon (1 0 0) substrates by direct deposition of low energy ions at room temperature using mass selected ion beam deposition (MSIBD). Two types of films were prepared. The first type of film was prepared with a carbon matrix deposited at 100 eV, which produces a ta-C matrix. The second type of sample was deposited at a carbon ion-energy of 30 eV, resulting in an a-C matrix which is mainly sp2 bonded. The films are grown under UHV conditions at a base pressure of approximately 2=10y8 mbar, and are therefore, almost free of contaminants like hydrogen, which is most important for the investigation of low dose hydrogen diffusion. The details of the deposition system are described elsewhere w11,12x. The samples were cleaned using acetone and in situ sputtering with 1 keV 40 Arq ions immediately before deposition. For each diffusion process different samples were prepared. The hydrogen isotopes were implanted with ion energies from 100 eV up to 1 keV, the metals and the nitrogen were deposited with energies of 60 eV (metals, except for silver, which was implanted with 1 keV) and 100 eV (nitrogen) between two carbon depositions, creating a d-profile of approximately 1 nm within a ta-C or a-C matrix. The samples will be referred to by the foreign element included as a d-profile in the carbon matrix, i.e. a hydrogen sample would be a ta-C or a-C matrix on silicon into which a d-profile of hydrogen has been implanted. 2.2. Characterisation Due to their small mass, the hydrogen isotopes were measured with ERDA using the Q3D magnetic spectrograph at the 15 MV tandem accelerator at the TU ¨ Munchen at Garching. Using this equipment depth resolutions of approximately 1 nm are possible w13,14x. The measurements were taken using a 40 MeV 197 Au7q beam, allowing for high angular resolution and minimized radiation damage. The samples studied using ERDA include the 13C samples for measuring selfdiffusion under annealing and the hydrogen and deuterium samples. The annealing took place in a vacuum of ¨ ;10y6 mbar at the TU Munchen. Each sample was annealed at a temperature varied between 600 and 950 8C for 10 min between each ERDA measurement. RBS measurements were conducted using a 900 keV 4 ¨ He2q ion beam supplied by the Gottinger heavy ion implanter IONAS w15x. The spectra were analyzed using the computer code RUMP w16x. RBS spectra were taken for the metal samples, which were cut in smaller pieces.
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Each piece was annealed in a vacuum oven with a base pressure during annealing of ;10y6 mbar, at temperatures ranging from 600 to 1200 8C. RNRA spectra for the nitrogen sample were obtained ¨ using the Gottinger 3 MV Pelletron accelerator MaRPel w17x. The reaction used for the nitrogen analysis takes place for protons with an energy of 898 keV hitting a nitrogen nucleus. It has a cross-section of 800 mb with a resonance width of 2.2 keV w18x, which limits the depth resolution for the RNRA spectra. In this case, it is approximately 30 nm, which was calculated using TRIM w19x. 3. Results and discussion 3.1. Self-diffusion of
13
C in
12
C
To effectively study the diffusion of foreign atoms in an a-C matrix a first step is to investigate the effect of annealing on the matrix used. Two samples were prepared, one a-C and one ta-C, each with a 13C d-profile, and were analyzed with ERD. The d-profile had a width of approximately 1 nm, resulting from the broadening of the 13C-layer caused by the subsequent deposition of 12 C. In the deposition process, the carbon atoms are implanted just beneath the surface, which causes a small amount of the most recently deposited material to float on top of the sample. This results in the layer being thinned out as the deposition continues. This is the origin of the small surface peak of 13C shown in Fig. 1. Due to the strong sp3 bonding no diffusion is expected to take place at room temperature or during annealing up to at least 600 8C. The obtained spectra for the ta-C sample show no significant diffusion of the 13C atoms within the a-C matrix at annealing temperatures of 900 8C, as shown in Fig. 1. Although the spectra appear to show a different concentration of 13C, it is not significant within error limits. This is in agreement with other measurements of the changes in the ta-C matrix by Ferrari et al. w20x, which show only slight changes in the bonding structure up to annealing temperatures of approximately 1000 8C. These measurements also show that the ta-C graphitizes at temperatures of approximately 1100 8C. The annealing experiments for the a-C sample show that there is no difference in behaviour between the carbon in the a-C sample and in the ta-C sample. Therefore, annealing experiments for foreign atoms in both carbon matrices would appear to not be affected by any carbon diffusion up to temperatures of 900 8C for our samples. The experiments also show that ta-C is thermally stable under vacuum up to the aforementioned temperature.
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Fig. 1. ERD-spectra of a
13
C d-profile in ta-C. The surface-peak is due to the sample preparation with MSIBD.
3.2. Light elements 3.2.1. Hydrogen The first of the light elements studied was hydrogen. A ta-C sample was prepared into which hydrogen was implanted with an ion energy of 1 keV. The sample was analyzed using high resolution ERDA. In Fig. 2 spectra over the whole depth of the sample are shown. In the left part of the spectra a large surface peak corresponding to a concentration of well over 30% hydrogen can be clearly seen. This peak occurs between annealing and
measurement and is due to water contamination of the surface once the sample is taken out of the vacuum. Due to this contamination a relatively high implantation energy of 1 keV was necessary, as the hydrogen profile for lower energies was completely masked by the surface contamination. The right part of Fig. 2 shows an enlarged part of the spectra which contains only the implanted profile. The profile shows an outdiffusion of hydrogen at temperatures of 800 8C and above. Furthermore, it can be seen that the hydrogen contained in the substrate can diffuse even at lower temperatures. The
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Fig. 2. ERD-spectra of hydrogen implanted with 1 keV into ta-C for as-implanted and annealing up to 950 8C. The left part of the spectrum shows a large surface peak, which is present in every spectrum. The right part of the figure shows an enlarged section of the spectra containing the implanted hydrogen.
reason why the integrated amount of hydrogen in the bulk peak increases during annealing at 600 8C is not clear. It could be due to ERDA measurements being performed on two different parts of the sample, showing an inhomogeneity of the hydrogen within the sample.
3.2.2. Deuterium Due to the surface contamination with water, hydrogen measurements were rather difficult. To study the diffusion of hydrogen at low deposition energies it is easier to use the stable isotope deuterium, which, due
Fig. 3. ERD-spectra for the diffusion of deuterium implanted with 100 eV in ta-C. The profile changes for annealing temperatures of 800 8C and above, showing an out-diffusion of deuterium from the carbon matrix. After annealing at 950 8C the sample contains less than 20% of the implanted deuterium.
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can then be used to calculate the activation energy. For this the following equation was used: B tv
C
Phonon
E
F
(1) x D lB G where kB is Boltzmann’s constant, t is the annealing time in seconds, vPhonon is the phonon frequency of the bulk material in which the diffusion takes place, x is the diffusion length and lB is the minimum distance the atom has to cross, i.e. the bond length in the bulk material. With the values for annealing time (600(10) s), vPhonon (diamond: (1) 1013 Hz w21x, a-C: (4.4"1.0)=1013 Hz), x (6(2) nm) and lB (sp3: 0.15(1) nm; sp2: 0.14(1) nm) the resulting activation energy, EA for deuterium in ta-C is 3.34(5) and 3.39(5) eV in a-C, respectively.
EAskBTA ln
Fig. 4. A plot of the integrated content of deuterium divided by the implanted dose against annealing time for both deuterium in ta-C (top) and a-C (bottom). The interpolated temperature for a content of 50% is the activation temperature, TA, from which the activation energy, EA, can be calculated.
to its natural isotopic occurrence of 0.015%, is not likely to be present as a contaminant. In Fig. 3 the implantation profile of 100 eV deuterium in ta-C is shown. The profile does not change significantly due to annealing at temperatures of up to 600 8C. After annealing the sample at 800 8C the amount of deuterium found in the sample decreased to approximately 80% of the implanted dose. In the final annealing step at 950 8C the deuterium content dropped to 16%. The same annealing steps were performed with an a-C sample, which showed exactly the same behaviour as the ta-C sample. From these measurements an activation energy for the diffusion can be derived. This is done by a simple model based on the assumption of Arrhenius-like diffusion. The activation energy can be obtained by plotting the integral content of deuterium against annealing temperature as shown in Fig. 4. By interpolating the temperature for which the deuterium amount would be 50%, the activation temperature can be derived. This temperature
3.2.3. Nitrogen Another light element studied is nitrogen. In this case, only one ta-C sample has been prepared to study the diffusion. Using the stable isotope 15N, which is suitable for RNRA, a sample containing two d-profiles was prepared. Fig. 5 shows the obtained spectra. The surface peak has the expected minimal linewidth of approximately 30 nm due to the large full width at half maximum (fwhm) (2.2 keV) of the reaction. The bulk signal shows a broader distribution, which is due to the deposition process. The nitrogen d-profile was broadened by the subplantation of carbon atoms, which were deposited afterwards in order to create a ta-C matrix for the diffusion. The newly deposited ions become mixed with the implanted nitrogen atoms that are being transported along with the new surface. This process broadens the originally sharp nitrogen profile. After annealing the sample at 700 8C for 1 h, 30% of the integral 15N content is gone. 15N is mainly lost from the surface, as the bulk peak does not decrease much. Further annealing at 900 8C leads to the disappearance of the surface peak, whereas the bulk peak still shows more than half the implanted content. The loss is partially due to the irradiation of the sample during analysis, but the reduction of the surface peak is mainly caused by outdiffusion of the 15N at this temperature. The bulk peak reduction could be caused simply by the irradiation of the sample, confirming the 15N to be stable in the ta-C matrix up to temperatures of 900 8C. Another fact supporting this assumption is that the bulk peak does not broaden in either of the two spectra taken after annealing. If the nitrogen were diffusing instead of being annihilated, the profile should broaden which is not the case. 3.3. Heavy elements 3.3.1. Tungsten The first of the heavy elements studied was tungsten. Tungsten, when incorporated into carbon, is known to
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Fig. 5. RNRA spectra and Gaussian fit of two 15N d-profiles measured as implanted and after annealing for 1 h at 700 and 900 8C. The peak close to the surface almost completely disappears, whereas the bulk peak is more stable.
form tungsten-carbide. The sample investigated was prepared with a d-profile of 5=1015 W atoms, inserted between two ta-C layers of approximately 25 nm thickness. The sample was then cut into four pieces which were measured with RBS, after being annealed under vacuum for 10 min. The spectra obtained were analysed and deconvoluted using the RUMP computer code w16x. The results are presented in the left part of Fig. 6. The amount of tungsten integrated in the ta-C films varies in the spectra from 3 to 4=1015, showing that most of the deposited amount of tungsten ions, which was 5=1015, is present in the sample. In the spectra, a slight shift in the position of the peak can be seen. This is due to small inhomogeneities in layer thickness over the whole of the sample. The thickness of the top carbon layer, which is responsible for the shift of the tungsten peak in the obtained spectra, varies in a range of approximately 5 nm, which is approximately 10% of the total film thickness. This is a reasonable value for the error of the film thickness. The peak in the original data has been fitted using a Gaussian function. The width of the peak does not change significantly in any of the spectra obtained after annealing at temperatures of up to 1000 8C. Therefore, the tungsten is enclosed stable in the ta-C matrix and does not appear to diffuse into the carbon matrix. After annealing for 10 min at 1200 8C, the peak broadens and shows a slight double peak structure, which could be explained by tungsten
diffusing to the top of the carbon film, parallel to graphitization. 3.3.2. Copper Copper does not form carbides and, therefore, is expected to behave differently from tungsten. There are indications that copper forms small clusters within the carbon matrix w22x. The sample was again cut into four small pieces. RBS measurements are presented in the middle column of Fig. 6. Again, as for tungsten, the copper peak in the spectra shows a slight shift in position corresponding to a variation in film thickness of approximately 7 nm. The integrated amount of copper does not change significantly due to annealing up to 1000 8C. It is between 4 and 7=1015 atoms, showing that a maximum of 35% of the 2=1016 copper ions deposited has been incorporated into the ta-C. This is due to sputtering, either by copper ions during deposition andy or carbon ions which are deposited afterwards. In the spectrum taken after annealing at 1000 8C, a second peak appears next to the initial copper peak. This second peak corresponds to copper accumulating at the interface of carbon and silicon, which has been simulated using the RUMP computer code w16x. This new copper originates from the sample holder on which the sample was grown and diffuses to the interface at these high temperatures, but it still does not enter the ta-C. The original peak does not show a decrease in copper content or
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Fig. 6. RBS spectra of tungsten (left), copper (middle) and silver (right) d-profiles in ta-C. Presented are the counts measured as a function of energy for each metal and after each annealing step.
broadening of the fwhm. The spectrum after annealing at 1200 8C is shown in the top part of Fig. 7 in a logarithmic scale, along with the fit calculated with the RUMP computer code. The signal on the right of the spectrum is the copper signal, which consists partially of the originally incorporated copper and partially of the copper coming from the interface, as has been shown in Fig. 6 in the spectrum taken after annealing at 1000 8C. The peak in the middle of the spectrum shown in Fig. 7 is due to argon, which was used to sputter-clean the surface before deposition of the sample. The ta-C is converted into graphite due to annealing at a temperature of 1200 8C. The copper can then diffuse into the graphite. The schematic of the fit is presented in the bottom part of Fig. 7. The fit shows a large mixing area
of carbon and silicon, in which the copper is situated, showing that there appears to be higher solubility for copper in silicon-carbide, as there is no copper left at the center of the originally deposited d-profile. There appears to be a high mobility of silicon and copper in the graphite at this high temperature. 3.3.3. Silver Silver was used as an example of a noble metal. It had to be implanted, because during normal growth processes the silver was sputtered almost completely w22x. The reason for this is not entirely certain. One possible explanation could be the insolubility of silver in carbon. The deposited silver possibly forms weakly bonded clusters, which are broken up by further ion
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enclosed metal to be stable in the ta-C matrix for annealing up to temperatures of 1000 8C. After annealing the sample at 1200 8C, the silver disappeared completely, which again is due to the conversion of ta-C into graphite at this temperature. 4. Conclusions
Fig. 7. Top: Cutout of the RBS-spectrum of Cu in ta-C after annealing at 1200 8C in logarithmic scale. The line is the fit to the data calculated using the RUMP computer code w16x. Bottom: Layer structure of the fit with RUMP w16x. The Argon, which has been fitted aswell in the top part of this figure, has been left out to get a clearer picture of the fit. It is situated in a depth of 52.5 nm with a concentration of 9%. The copper is located in a broad mixing layer of carbon and silicon.
bombardment. The silver atoms created by the breakup of the clusters may then accumulate on the surface, where they are being sputtered by the carbon ions used for depositing the ta-C matrix. To study the diffusion of silver, the ions had to be implanted far enough beneath the surface to be out of reach of the 100 eV carbon ions. This was achieved by implanting the silver ions with an energy of 1 keV. The sample was then treated in the same manner as the other metal samples described above. The RBS spectra are presented in the right part of Fig. 6. The integrated amount of silver was calculated from the deconvoluted spectra, showing that only 1.5=1015 silver atoms are present in the sample. This is approximately 10% of the originally implanted silver ions. The silver signal in the RBS spectra only changed position slightly, accounting for a thickness variation of the sample of ;2 nm. The spectra again show the
The self-diffusion of 13C in a-C and ta-C was investigated using high resolution ERDA. For annealing at temperatures of up to 900 8C no significant diffusion of the carbon atoms could be detected. For the diffusion of deuterium in a-C and ta-C, activation energies were calculated, resulting in EAs 3.39(5) eV for deuterium in a-C and EAs3.34(5) eV for deuterium in ta-C. This is in agreement with the results of Ahlgren w8x, who found an activation energy of EAs2.9(1) eV for deuterium in dlc. The diffusion of hydrogen in ta-C starts at a temperature between 600 and 800 8C, which is significantly lower than known temperatures for diffusion of hydrogen in natural diamond (1200 8C) or CVD-diamonds (1400 8C). The diffusion of nitrogen in ta-C was investigated using RNRA. The nitrogen starts to diffuse out of the surface at a temperature of 700 8C. The nitrogen incorporated in the bulk of the sample was much more stable, retaining half of its initial content even after annealing under vacuum for 1 h at 900 8C. The three metals investigated all show a similar behaviour. Tungsten, copper and silver remain stable within the ta-C matrix even after annealing at a temperature of 1000 8C. At higher annealing temperatures the ta-C is converted into graphite, allowing the foreign atoms to diffuse into the carbon matrix. In the case of copper, a tendency for accumulation in the siliconcarbide region of the sample was found. The stability of the copper within the ta-C matrix suggests its usefulness as a diffusion barrier for copper to protect silicon substrates. Acknowledgments The author would like to thank M. Schwickert for performing the RNRA measurements, Petra Reinke and Emily Hooker for helpful discussions. References w1x P.J. Fallon, V.S. Veerasamy, C.A. Davis, et al., Phys. Rev. B 48 (1993) 4777. w2x J.C. Knight, A.J. Whitehead, T.F. Page, J. Mater. Sci. 27 (1992) 3939. w3x M. Allen, B. Myer, N. Rushton, J. Biomed. Mater. Res. 58 (3) (2001) 319.
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w4x V.-M. Tiainen, Diamond Relat. Mater. 10 (2001) 153. w5x P.R. Goglia, J. Berkowitz, J. Hoehn, A. Xidis, L. Stover, Diamond Relat. Mater. 10 (2001) 271. w6x W. Scott, B. Bhushan, A.V. Lakshmikumaran, J. Appl. Phys. 87 (9) (2001) 6182. w7x K. Lmimouni, C. Legrand, C. Dufour, A. Chapoton, C. Belouet, Appl. Phys. Lett. 78 (17) (2001) 2437. w8x T. Ahlgren, Diffusion of Impurities in Compound Semiconductors and Diamond-Like Carbon Films. Ph.D. thesis, University of Helsinki, 1998. w9x H.B. Liao, R.F. Xiao, J.S. Fu, P. Yu, G.K.L. Wong, P. Sheng, Appl. Phys. Lett. 70 (1997) 1. w10x Z. Liu, H. Wang, H. Li, X. Wang, J. Appl. Phys. 72 (1998) 1823. w11x J. Singh, S.M. Copley, J. Mazumder (Eds.), Beam Processing of Advanced Materials, ASM International, Cleveland, 1996, p. 29. w12x H. Hofsass, ¨ C. Ronning, Appl. Phys. A 66 (1998) 153.
w13x G. Dollinger, T. Faestermann, P. Maier-Komor, Nucl. Instrum. Meth. B 64 (1992) 422. w14x G. Dollinger, C.M. Frey, A. Bergmaier, T. Faestermann, Europhys. Lett. 42 (1) (1998) 25. w15x M. Uhrmacher, K. Pampus, F.J. Bergmeister, D. Purschke, K.P. Lieb, Nucl. Instrum. Meth. B 9 (1985) 234. w16x L.R. Doolittle, Nucl. Instrum. Meth. B 15 (1986) 227. w17x M. Schwickert, F. Harbsmeier, H. Schebela, et al., Surf. Coat. Technol. 151–152 (2002) 222. w18x J.R. Tesmer, M. Nastasi (Eds.), Handbook of Modern Ion Beam Materials Analysis, Materials Research Society, 1995. w19x J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. w20x A.C. Ferrari, B. Kleinsorge, N.A. Morrison, A. Hart, V. Stolojan, J. Robertson, J. Appl. Phys. 85 (1999) 7191. w21x A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (20) (2000) 14095. w22x I. Gerhards, C. Ronning, U. Vetter, et al., Surf. Coat. Technol.158–159 (2002) 114.