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Influence of Ar–N2 gas composition on the magnetron-sputter deposition of cubic boron nitride films
Surface & Coatings Technology 205 (2010) S96–S98
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Influence of Ar–N2 gas composition on the magnetron-sputter deposition of cubic boron nitride films Sven Ulrich ⁎, Jian Ye, Michael Stüber Karlsruhe Institute of Technology, Institute for Materials Research I, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
a r t i c l e
i n f o
Available online 23 May 2010 Keywords: Cubic boron nitride Magnetron sputtering Plasma diagnostic Ion-assisted deposition
a b s t r a c t Thin films of cubic boron nitride (c-BN) can be produced by various PVD and PECVD techniques under several prerequisites referring reportedly to the growth temperature, stoichiometry of the deposited films, and energy impact of the growing surface during film deposition. For reactive magnetron-sputtering in Ar/N2 gas mixtures and using a hexagonal boron nitride (h-BN) target, we examined the values of substrate bias necessitated for the deposition of c-BN in correlation with the composition of sputter gas at a fixed radiofrequency (r.f.) target power of 500 W and a substrate temperature of 400 °C. The threshold substrate bias was found to minimize at the [N2]:[Ar] ratio of about 1:16. At low nitrogen concentrations i.e. [N2]:[Ar] b 1:32, c-BN growth was not achieved. The results are correlated with the variation of plasma parameters for different [N2]:[Ar] gas concentration ratios as evaluated from Langmuir double-probe measurements. Influence from the film stoichiometry is also shortly commented. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Cubic boron nitride (c-BN) has many fascinating material characteristics, such as the second highest hardness, chemical stability against ferrous metals at high temperatures, highest optical bandgap within III–V semiconductors, and very large thermal conductivity. It is doubtlessly an excellent material candidate of multifunctional surface coatings for many traditional as well as emerging industrial applications [1]. The synthesis of c-BN films has been already widely addressed in the past decades involving numerous growth techniques by physical vapor deposition (PVD) and plasma-enhanced chemical vapor deposition (PECVD) [2–5]. The abundant experimental results acquired so far from these studies allow for detailed assessment of a few fundamental parameters that are decisive for the nucleation and growth of the c-BN phase, referring for example to the energy, mass, incident angle of bombarding ions, flux ratio of ions and film-forming species at the growing film surface, and the growth temperature. The resulting BN phase was consequently wellcategorized in terms of those growth parameters [6,7]. More comprehensive classification was depicted in Ref. [8] where the parameter regions assigned for the cubic-phase nucleation and for c-BN growth after complete nucleation were further distinguished respectively. Recent research activities tend to be focused on the realization of low stress c-BN films and, in particular, of thick and adhered films demanded by practical applications through novel growth techniques and strategies [4,5,9,10]. However, general optimization of the deposition process is still of great
⁎ Corresponding author. Tel.: + 49 7247 82 3398; fax: + 49 7247 82 4567. E-mail address: [email protected] (S. Ulrich). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.05.032
significance in many situations, regarding, for example, the energy of bombarding ions that is of main concern for electronic properties owing to ion-induced crystalline defects, for film homogeneity on geometrically complex substrate surfaces, etc. Film deposition at lower ion energies is mostly desired in these cases. In the present study, c-BN films were deposited by reactive magnetron sputtering using Ar/N2 gas mixtures on to r.f.-biased silicon substrates. Influence of sputter-gas composition will be illustrated on the threshold value of substrate bias that has to be gratified for successful c-BN deposition. 2. Experimental Boron nitride films were deposited in a hybrid PVD/CVD facility by unbalanced r.f. (13.56 MHz) magnetron sputtering using a hot-pressed hexagonal boron nitride (h-BN) target of 99.99% in purity, 75 mm in diameter, and 3 mm in thickness. Double-sided polished Si(100) (Bdoped, 1–30 Ω cm, 350 μm thick) was used as the substrate material. The distance between the target and the substrates was 150 mm. All Si substrates were cleaned in acetone and then in isopropanol before loaded into the deposition chamber. The base pressure of the deposition chamber prior to the film growth was below 1.0× 10− 4 Pa. The substrates and h-BN target were sputter-cleaned for 15 min in an Ar discharge at 0.4 Pa pressure, 200 W target power, and −300 V substrate self-bias. A shutter was positioned between the target and substrates during this cleaning process. For film deposition, the substrate temperature was kept at 400 °C, and the r.f. power applied to the target was fixed at 500 W. In order to study the influence of sputter-gas composition on the phase components of resulting films, Ar/N2 mixtures of different N2 concentrations were used. The flow rate of Ar (99.9999%)
S. Ulrich et al. / Surface & Coatings Technology 205 (2010) S96–S98
was always 80 sccm, and the flow rate of N2 (99.999%) was chosen between 0 and 10 sccm. Corresponding deposition pressure was between 0.4 Pa and 0.5 Pa depending on the flow rate of N2. For each given gas composition, several deposition processes were conducted at diverse r.f. substrate bias until the minimum self-bias d.c. voltage required for c-BN formation was identified within an accuracy of ±5 V. The d.c. voltage on the substrate electrode was measured after eliminating the r.f. component by a high-frequency filter. All deposited films were 100 nm thick, their phase composition was determined by Fourier transformed infrared spectroscopy FTIR using a Perkin Elmer Spectrum GX FTIR spectrometer in the transmission mode. The cubic phase was recognized as usual by its characteristic transverse optical (TO) absorption mode near 1100 cm− 1, and h-BN by its in-plane stretching mode at 1375 cm− 1 and out-of-plane bending mode at 817 cm− 1, respectively [11,12]. The fundamental plasma parameters under different gas compositions were measured by Langmuir electrical double-probe at the substrate position. The two probes were separated by a distance of 35 mm, each having a plasma contact area of 3.14 mm2. Due to its extremely high electrical resistivity, a thin BN layer inevitably deposited on to the probe surface during the measurements often led to unreliable experimental results. For this work, however, we have taken special caution such that the probes were quasi-clean for each measurement, and the measured I–V curves were well confirmed for their reproducibility. The plasma parameters like electron temperature Te and ion saturation current density jion at the substrate position were then evaluated from the measured current–voltage curves. Further calculations resulted in the plasma potential Upl, plasma density ne, and electron saturation current density je. See ref. [13] for the evaluation details. 3. Results and discussion For the sputter gas of 0.45 Pa and at a composition of [N2]:[Ar] = 1:16, the content of the cubic phase appearing in the resulting 100 nm thick films was illustrated in Fig. 1 as a function of substrate bias during deposition. The minimum substrate bias for the onset of c-BN formation situates at approx. −55 V. Notice that the c-BN contents displayed in this figure were evaluated from the FTIR spectroscopy according to the intensity ratio of the c-BN TO absorption band at 1100 cm− 1 and h-BN in-plane stretching absorption at 1375 cm− 1. The value roughly represents the integral or average c-BN content in the deposited films. As a matter of fact, the c-BN-containing films discussed in this work
Fig. 1. c-BN content of sputter-deposited films (100 nm thick) in dependence of substrate bias for two different Ar/N2 gas mixtures with [N2]:[Ar] = 1:16 at 0.45 Pa and [N2]/[Ar] = 1:4.5 at 0.25 Pa, respectively. The inset shows the FTIR spectrum of c-BN film grown with [N2]:[Ar] = 1:16 and − 100 V substrate bias.
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contain a layer of sp2-bonded structure being developed before the nucleation of c-BN started. The top layer comprises on the other hand almost pure c-BN phase. (See ref. [9] for a review of typical development of complex phases in the vicinity of substrate surface at the beginning of BN growth process.) For substrate bias noticeably larger than −55 V, the deposited films show high c-BN contents above 80 vol.%, and the noncubic interfacial BN layer is usually only a few nm thick. However, the thickness of this interfacial layer depends on many deposition parameters including the substrate bias. At substrate bias near the threshold for c-BN nucleation, the non-cubic interfacial layer may become appreciably thicker [14]. We identify the substrate bias as the threshold for c-BN nucleation if below this bias value no c-BN related signal can be detected by FTIR in the resulting 100 nm thick films. Also shown in Fig. 1 is our previous result [15] corresponding to a higher N2 concentration at [N2]:[Ar] = 1:4.5, but at a lower working pressure of 0.25 Pa. In that case the deposition of c-BN requires substantially higher substrate bias showing a threshold value in the vicinity of −180 V. However, as illustrated in Fig. 2, c-BN growth becomes possible at a somewhat reduced substrate bias below the above-mentioned threshold value once a complete nucleation is accomplished. For such postnucleation growth a new bias threshold can be detected, once again depending on the N2 concentration of sputter gas, appearing at −40 V for [N2]:[Ar] = 1:16, and −80 V for [N2]:[Ar] = 1:4.5, respectively. Since the above two situations related to rather different gas pressures, the exact influence of N2 concentration cannot be obviously seen. For that reason a series of deposition experiments was further conducted at different gas compositions but with the total gas pressure controlled within 0.4–0.5 Pa. (The flow rate of Ar was always kept at 80 sccm, while that of N2 was 0–10 sccm.) Fig. 2 illustrates the bias thresholds for c-BN deposition as determined under several selected gas compositions. As can be seen from the figure, the bias threshold is in fact minimized at a N2 flow rate of around 5 sccm. With increasing or decreasing N2 flow rate, the bias threshold increases: (1) An increment of bias threshold of more than 20 V is shown when the flow rate of N2 is increased from 5 to 10 sccm. Corresponding to this concentration variation of N2, the results of Langmuir double-probe measurements (Fig. 3) show a slight reduction of plasma potential from approx. 27 V to 23 V (Fig. 3a). More considerable influence is however seen on the ion saturation current density (from 0.33 to 0.26 mA/cm2, see Fig. 3e). These results are qualitatively coincident with the
Fig. 2. Influence of N2 concentration of Ar/N2 gas mixtures on the threshold of substrate bias for c-BN nucleation. After full c-BN nucleation, a new and somewhat reduced bias threshold will come into play for continued c-BN growth, depending again on the N2 concentration of the sputter gas. Two examples are shown respectively for [N2]:[Ar]= 1:16 and 1:4.5.
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S. Ulrich et al. / Surface & Coatings Technology 205 (2010) S96–S98
from a h-BN target in a pure Ar discharge was shown to result in N-deficient films [17] due possibly to transport losses and/or a lower incorporation efficiency of N into the growing film, and that coincides with the analytical results of our films ([B]/[N] = 1.4). On the other side, various reported analysis results [17–19] indicate that c-BN films are usually nearly stoichiometric, with 0.9 b [B]/[N] b 1.1. Recent studies indicate that the establishment of a stoichiometric BN surface must be a key condition or prerequisite for ion-assisted c-BN growth [17,20,21]. The minimum substrate threshold is then expected to occur at a low concentration level of N2 as far as the stoichiometry condition for c-BN formation is still fulfilled at that low N2 concentration. Low bias c-BN deposition may be useful and important in some situations. For c-BN film deposition at substrate surfaces of complex geometries, for example, a low substrate bias could result in more homogeneous film characteristics thanks to reduced resputtering and preferential sputtering of deposited atoms. 4. Conclusions
Fig. 3. Fundamental parameters of magnetron plasma evaluated from Langmuir doubleprobe measurements at the substrate position and under various different sputter-gas compositions with the target r.f. power keeping at 400 W, including (a) plasma potential, (b) plasma density, (c) electron temperature, (d) electron saturation current density, and (e) ion saturation current density.
Φion/ΦBN − Eion parameter map reported in Refs. [8] and [16]. (Here, Φion/ΦBN is the flux ratio of bombarding ions to neutral B and N atoms. Eion is the energy of ions. Both refer to the values at the growing film surface.) It is known that a certain stress level must be built up in the h-BN base layer before the bombarding ions may act as a trigger for the c-BN nucleation process. Hence an increase of substrate bias threshold has to be demanded to generate the mentioned stress under reduced ion flux densities at the growing surface at higher N2 flow rates, and compensate for the slight reduction of ion energy at any given substrate bias because of reduced plasma potential. However, other effects cannot be excluded such as molecular N2 formation and desorption in the growing BN at higher N2 flow rate. According to Ref. [17], energetic ion bombardment could result in N enrichment. That is, an increase of substrate bias may be necessitated to promote the stoichiometry condition of c-BN nucleation as mentioned below. (2) Even more rapid increase of substrate threshold was found by decreasing N2 flow rate below 5 sccm (Fig. 2). For N2 flow rates smaller than 2.5 sccm, we were eventually unable to successfully grow any c-BN-containing films. These results obviously do not agree with the prediction of the Φion/ΦBN − Eion parameter map since both plasma potential and ion current density increase at the low flow rates of N2. A possible reason can be the difficulty for the formation of stoichiometric BN when very low concentrations of N2 are delivered to the plasma. For example, sputter deposition
For reactive magnetron sputter deposition of c-BN in various Ar/N2 mixtures, the minimum of substrate bias required for the formation of c-BN phase can be appreciably influenced by the composition (i.e. [N2]/ [Ar] ratio) of the sputter gas due to the variation of plasma parameters (plasma potential, ion current density at the growing surface, etc.), and by the chemical composition (atomic concentration ratio [B]/[N]) of the deposited films. When the total working pressure of sputter gas stays constant or only least varied, higher substrate bias is demanded at high N2 concentrations for successful c-BN growth because of low plasma potentials and decreased ion flux densities at the growing surface. At low N2 concentrations, on the other side, the film stoichiometry could become the dominant factor such that c-BN growth becomes even unrealizable due to excessive B atoms built into the films. Minimization of substrate bias occurs therefore at a moderate N2 concentration, which is [N2]/[Ar] = 1:16 under the deposition configurations exemplified in the present study. The results indicate that compositional optimization of sputter gas can enable c-BN deposition at lower substrate bias, which may be favored in many situations such as film growth on substrates with complex surface geometries and for increased deposition rates. References [1] P.B. Mirkarimi, K.F. McCarty, D.L. Medlin, Mater. Sci. Eng. R 21 (1997) 47. [2] D. Litvinov, C.A. Taylor II, R. Clarke, Diamond Relat. Mater. 7 (1998) 360. [3] T. Usamia, T. Asaji, S. Matsumoto, H. Kanda, K. Nakamura, Surf. Coat. Technol. 203 (2009) 929. [4] M. Keunecke, E. Wiemann, K. Weigel, S.T. Park, K. Bewilogua, Thin Solid Films 515 (2006) 967. [5] S. Ulrich, E. Nold, K. Sell, M. Stüber, J. Ye, C. Ziebert, in: R. Kassing, P. Petkov, W. Kulisch, C. Popov (Eds.), NATO Science Series, Functional Properties of Nanostructured Materials, vol. 223, Springer, Dordrecht, 2006, p. 275. [6] S. Reinke, M. Kuhr, W. Kulisch, Diamond Relat. Mater. 3 (1994) 341. [7] S. Reinke, M. Kuhr, W. Kulisch, R. Kassing, Diamond Relat. Mater. 4 (1995) 272. [8] W. Kulisch, S. Ulrich, Thin Solid Films 423 (2003) 183. [9] W. Kulisch, R. Freudenstein, Thin Solid Films 516 (2007) 216. [10] I. Bello, C.Y. Chan, W.J. Zhang, Y.M. Chong, K.M. Leung, S.T. Lee, Y. Lifshitz, Diamond Relat. Mater. 14 (2005) 1154. [11] P.J. Gielisse, S.S. Mitra, J.N. Plendl, R.D. Griffis, L.C. Mansur, R. Marshall, E.A. Pascoe, Phys. Rev. 155 (1967) 1039. [12] R. Geick, C.H. Perry, Phys. Rev. 146 (1966) 543. [13] T. Nguyen, S. Ulrich, J. Bsul, S. Beauvais, W. Burger, A. Albers, M. Stüber, J. Ye, Diamond Relat. Mater. 18 (2009) 995. [14] Q. Li, L.D. Marks, Y. Lifshitz, S.T. Lee, I. Bello, Phys. Rev. B65 (2002) 045415. [15] K. Sell, H. Holleck, H. Leiste, M. Stüber, S. Ulrich, J. Ye, Diamond Relat. Mater. 11 (2002) 1272. [16] S. Ulrich, J. Ye, M. Stüber, C. Ziebert, Thin Solid Films 518 (2009) 1443. [17] Y.K. Le, H. Oechsner, Appl. Phys. A 78 (2004) 681. [18] W. Dworschak, K. Jung, H. Ehrhardt, Thin Solid Films 254 (1995) 65. [19] G.P. Lamaze, R.G. Dowing, L.B. Hackenberger, L.J. Pilione, R. Messier, Diamond Relat. Mater. 3 (1994) 728. [20] J. Ye, H. Oechsner, S. Westermeyr, J. Vac. Sci. Technol. A 19 (2001) 2294. [21] Y.K. Le, H. Oechsner, Thin Solid Films 437 (2003) 83.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Influence of Ar–N2 gas composition on the magnetron-sputter deposition of cubic boron nitride films Sven Ulrich ⁎, Jian Ye, Michael Stüber Karlsruhe Institute of Technology, Institute for Materials Research I, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
a r t i c l e
i n f o
Available online 23 May 2010 Keywords: Cubic boron nitride Magnetron sputtering Plasma diagnostic Ion-assisted deposition
a b s t r a c t Thin films of cubic boron nitride (c-BN) can be produced by various PVD and PECVD techniques under several prerequisites referring reportedly to the growth temperature, stoichiometry of the deposited films, and energy impact of the growing surface during film deposition. For reactive magnetron-sputtering in Ar/N2 gas mixtures and using a hexagonal boron nitride (h-BN) target, we examined the values of substrate bias necessitated for the deposition of c-BN in correlation with the composition of sputter gas at a fixed radiofrequency (r.f.) target power of 500 W and a substrate temperature of 400 °C. The threshold substrate bias was found to minimize at the [N2]:[Ar] ratio of about 1:16. At low nitrogen concentrations i.e. [N2]:[Ar] b 1:32, c-BN growth was not achieved. The results are correlated with the variation of plasma parameters for different [N2]:[Ar] gas concentration ratios as evaluated from Langmuir double-probe measurements. Influence from the film stoichiometry is also shortly commented. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Cubic boron nitride (c-BN) has many fascinating material characteristics, such as the second highest hardness, chemical stability against ferrous metals at high temperatures, highest optical bandgap within III–V semiconductors, and very large thermal conductivity. It is doubtlessly an excellent material candidate of multifunctional surface coatings for many traditional as well as emerging industrial applications [1]. The synthesis of c-BN films has been already widely addressed in the past decades involving numerous growth techniques by physical vapor deposition (PVD) and plasma-enhanced chemical vapor deposition (PECVD) [2–5]. The abundant experimental results acquired so far from these studies allow for detailed assessment of a few fundamental parameters that are decisive for the nucleation and growth of the c-BN phase, referring for example to the energy, mass, incident angle of bombarding ions, flux ratio of ions and film-forming species at the growing film surface, and the growth temperature. The resulting BN phase was consequently wellcategorized in terms of those growth parameters [6,7]. More comprehensive classification was depicted in Ref. [8] where the parameter regions assigned for the cubic-phase nucleation and for c-BN growth after complete nucleation were further distinguished respectively. Recent research activities tend to be focused on the realization of low stress c-BN films and, in particular, of thick and adhered films demanded by practical applications through novel growth techniques and strategies [4,5,9,10]. However, general optimization of the deposition process is still of great
⁎ Corresponding author. Tel.: + 49 7247 82 3398; fax: + 49 7247 82 4567. E-mail address: [email protected] (S. Ulrich). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.05.032
significance in many situations, regarding, for example, the energy of bombarding ions that is of main concern for electronic properties owing to ion-induced crystalline defects, for film homogeneity on geometrically complex substrate surfaces, etc. Film deposition at lower ion energies is mostly desired in these cases. In the present study, c-BN films were deposited by reactive magnetron sputtering using Ar/N2 gas mixtures on to r.f.-biased silicon substrates. Influence of sputter-gas composition will be illustrated on the threshold value of substrate bias that has to be gratified for successful c-BN deposition. 2. Experimental Boron nitride films were deposited in a hybrid PVD/CVD facility by unbalanced r.f. (13.56 MHz) magnetron sputtering using a hot-pressed hexagonal boron nitride (h-BN) target of 99.99% in purity, 75 mm in diameter, and 3 mm in thickness. Double-sided polished Si(100) (Bdoped, 1–30 Ω cm, 350 μm thick) was used as the substrate material. The distance between the target and the substrates was 150 mm. All Si substrates were cleaned in acetone and then in isopropanol before loaded into the deposition chamber. The base pressure of the deposition chamber prior to the film growth was below 1.0× 10− 4 Pa. The substrates and h-BN target were sputter-cleaned for 15 min in an Ar discharge at 0.4 Pa pressure, 200 W target power, and −300 V substrate self-bias. A shutter was positioned between the target and substrates during this cleaning process. For film deposition, the substrate temperature was kept at 400 °C, and the r.f. power applied to the target was fixed at 500 W. In order to study the influence of sputter-gas composition on the phase components of resulting films, Ar/N2 mixtures of different N2 concentrations were used. The flow rate of Ar (99.9999%)
S. Ulrich et al. / Surface & Coatings Technology 205 (2010) S96–S98
was always 80 sccm, and the flow rate of N2 (99.999%) was chosen between 0 and 10 sccm. Corresponding deposition pressure was between 0.4 Pa and 0.5 Pa depending on the flow rate of N2. For each given gas composition, several deposition processes were conducted at diverse r.f. substrate bias until the minimum self-bias d.c. voltage required for c-BN formation was identified within an accuracy of ±5 V. The d.c. voltage on the substrate electrode was measured after eliminating the r.f. component by a high-frequency filter. All deposited films were 100 nm thick, their phase composition was determined by Fourier transformed infrared spectroscopy FTIR using a Perkin Elmer Spectrum GX FTIR spectrometer in the transmission mode. The cubic phase was recognized as usual by its characteristic transverse optical (TO) absorption mode near 1100 cm− 1, and h-BN by its in-plane stretching mode at 1375 cm− 1 and out-of-plane bending mode at 817 cm− 1, respectively [11,12]. The fundamental plasma parameters under different gas compositions were measured by Langmuir electrical double-probe at the substrate position. The two probes were separated by a distance of 35 mm, each having a plasma contact area of 3.14 mm2. Due to its extremely high electrical resistivity, a thin BN layer inevitably deposited on to the probe surface during the measurements often led to unreliable experimental results. For this work, however, we have taken special caution such that the probes were quasi-clean for each measurement, and the measured I–V curves were well confirmed for their reproducibility. The plasma parameters like electron temperature Te and ion saturation current density jion at the substrate position were then evaluated from the measured current–voltage curves. Further calculations resulted in the plasma potential Upl, plasma density ne, and electron saturation current density je. See ref. [13] for the evaluation details. 3. Results and discussion For the sputter gas of 0.45 Pa and at a composition of [N2]:[Ar] = 1:16, the content of the cubic phase appearing in the resulting 100 nm thick films was illustrated in Fig. 1 as a function of substrate bias during deposition. The minimum substrate bias for the onset of c-BN formation situates at approx. −55 V. Notice that the c-BN contents displayed in this figure were evaluated from the FTIR spectroscopy according to the intensity ratio of the c-BN TO absorption band at 1100 cm− 1 and h-BN in-plane stretching absorption at 1375 cm− 1. The value roughly represents the integral or average c-BN content in the deposited films. As a matter of fact, the c-BN-containing films discussed in this work
Fig. 1. c-BN content of sputter-deposited films (100 nm thick) in dependence of substrate bias for two different Ar/N2 gas mixtures with [N2]:[Ar] = 1:16 at 0.45 Pa and [N2]/[Ar] = 1:4.5 at 0.25 Pa, respectively. The inset shows the FTIR spectrum of c-BN film grown with [N2]:[Ar] = 1:16 and − 100 V substrate bias.
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contain a layer of sp2-bonded structure being developed before the nucleation of c-BN started. The top layer comprises on the other hand almost pure c-BN phase. (See ref. [9] for a review of typical development of complex phases in the vicinity of substrate surface at the beginning of BN growth process.) For substrate bias noticeably larger than −55 V, the deposited films show high c-BN contents above 80 vol.%, and the noncubic interfacial BN layer is usually only a few nm thick. However, the thickness of this interfacial layer depends on many deposition parameters including the substrate bias. At substrate bias near the threshold for c-BN nucleation, the non-cubic interfacial layer may become appreciably thicker [14]. We identify the substrate bias as the threshold for c-BN nucleation if below this bias value no c-BN related signal can be detected by FTIR in the resulting 100 nm thick films. Also shown in Fig. 1 is our previous result [15] corresponding to a higher N2 concentration at [N2]:[Ar] = 1:4.5, but at a lower working pressure of 0.25 Pa. In that case the deposition of c-BN requires substantially higher substrate bias showing a threshold value in the vicinity of −180 V. However, as illustrated in Fig. 2, c-BN growth becomes possible at a somewhat reduced substrate bias below the above-mentioned threshold value once a complete nucleation is accomplished. For such postnucleation growth a new bias threshold can be detected, once again depending on the N2 concentration of sputter gas, appearing at −40 V for [N2]:[Ar] = 1:16, and −80 V for [N2]:[Ar] = 1:4.5, respectively. Since the above two situations related to rather different gas pressures, the exact influence of N2 concentration cannot be obviously seen. For that reason a series of deposition experiments was further conducted at different gas compositions but with the total gas pressure controlled within 0.4–0.5 Pa. (The flow rate of Ar was always kept at 80 sccm, while that of N2 was 0–10 sccm.) Fig. 2 illustrates the bias thresholds for c-BN deposition as determined under several selected gas compositions. As can be seen from the figure, the bias threshold is in fact minimized at a N2 flow rate of around 5 sccm. With increasing or decreasing N2 flow rate, the bias threshold increases: (1) An increment of bias threshold of more than 20 V is shown when the flow rate of N2 is increased from 5 to 10 sccm. Corresponding to this concentration variation of N2, the results of Langmuir double-probe measurements (Fig. 3) show a slight reduction of plasma potential from approx. 27 V to 23 V (Fig. 3a). More considerable influence is however seen on the ion saturation current density (from 0.33 to 0.26 mA/cm2, see Fig. 3e). These results are qualitatively coincident with the
Fig. 2. Influence of N2 concentration of Ar/N2 gas mixtures on the threshold of substrate bias for c-BN nucleation. After full c-BN nucleation, a new and somewhat reduced bias threshold will come into play for continued c-BN growth, depending again on the N2 concentration of the sputter gas. Two examples are shown respectively for [N2]:[Ar]= 1:16 and 1:4.5.
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S. Ulrich et al. / Surface & Coatings Technology 205 (2010) S96–S98
from a h-BN target in a pure Ar discharge was shown to result in N-deficient films [17] due possibly to transport losses and/or a lower incorporation efficiency of N into the growing film, and that coincides with the analytical results of our films ([B]/[N] = 1.4). On the other side, various reported analysis results [17–19] indicate that c-BN films are usually nearly stoichiometric, with 0.9 b [B]/[N] b 1.1. Recent studies indicate that the establishment of a stoichiometric BN surface must be a key condition or prerequisite for ion-assisted c-BN growth [17,20,21]. The minimum substrate threshold is then expected to occur at a low concentration level of N2 as far as the stoichiometry condition for c-BN formation is still fulfilled at that low N2 concentration. Low bias c-BN deposition may be useful and important in some situations. For c-BN film deposition at substrate surfaces of complex geometries, for example, a low substrate bias could result in more homogeneous film characteristics thanks to reduced resputtering and preferential sputtering of deposited atoms. 4. Conclusions
Fig. 3. Fundamental parameters of magnetron plasma evaluated from Langmuir doubleprobe measurements at the substrate position and under various different sputter-gas compositions with the target r.f. power keeping at 400 W, including (a) plasma potential, (b) plasma density, (c) electron temperature, (d) electron saturation current density, and (e) ion saturation current density.
Φion/ΦBN − Eion parameter map reported in Refs. [8] and [16]. (Here, Φion/ΦBN is the flux ratio of bombarding ions to neutral B and N atoms. Eion is the energy of ions. Both refer to the values at the growing film surface.) It is known that a certain stress level must be built up in the h-BN base layer before the bombarding ions may act as a trigger for the c-BN nucleation process. Hence an increase of substrate bias threshold has to be demanded to generate the mentioned stress under reduced ion flux densities at the growing surface at higher N2 flow rates, and compensate for the slight reduction of ion energy at any given substrate bias because of reduced plasma potential. However, other effects cannot be excluded such as molecular N2 formation and desorption in the growing BN at higher N2 flow rate. According to Ref. [17], energetic ion bombardment could result in N enrichment. That is, an increase of substrate bias may be necessitated to promote the stoichiometry condition of c-BN nucleation as mentioned below. (2) Even more rapid increase of substrate threshold was found by decreasing N2 flow rate below 5 sccm (Fig. 2). For N2 flow rates smaller than 2.5 sccm, we were eventually unable to successfully grow any c-BN-containing films. These results obviously do not agree with the prediction of the Φion/ΦBN − Eion parameter map since both plasma potential and ion current density increase at the low flow rates of N2. A possible reason can be the difficulty for the formation of stoichiometric BN when very low concentrations of N2 are delivered to the plasma. For example, sputter deposition
For reactive magnetron sputter deposition of c-BN in various Ar/N2 mixtures, the minimum of substrate bias required for the formation of c-BN phase can be appreciably influenced by the composition (i.e. [N2]/ [Ar] ratio) of the sputter gas due to the variation of plasma parameters (plasma potential, ion current density at the growing surface, etc.), and by the chemical composition (atomic concentration ratio [B]/[N]) of the deposited films. When the total working pressure of sputter gas stays constant or only least varied, higher substrate bias is demanded at high N2 concentrations for successful c-BN growth because of low plasma potentials and decreased ion flux densities at the growing surface. At low N2 concentrations, on the other side, the film stoichiometry could become the dominant factor such that c-BN growth becomes even unrealizable due to excessive B atoms built into the films. Minimization of substrate bias occurs therefore at a moderate N2 concentration, which is [N2]/[Ar] = 1:16 under the deposition configurations exemplified in the present study. The results indicate that compositional optimization of sputter gas can enable c-BN deposition at lower substrate bias, which may be favored in many situations such as film growth on substrates with complex surface geometries and for increased deposition rates. References [1] P.B. Mirkarimi, K.F. McCarty, D.L. Medlin, Mater. Sci. Eng. R 21 (1997) 47. [2] D. Litvinov, C.A. Taylor II, R. Clarke, Diamond Relat. Mater. 7 (1998) 360. [3] T. Usamia, T. Asaji, S. Matsumoto, H. Kanda, K. Nakamura, Surf. Coat. Technol. 203 (2009) 929. [4] M. Keunecke, E. Wiemann, K. Weigel, S.T. Park, K. Bewilogua, Thin Solid Films 515 (2006) 967. [5] S. Ulrich, E. Nold, K. Sell, M. Stüber, J. Ye, C. Ziebert, in: R. Kassing, P. Petkov, W. Kulisch, C. Popov (Eds.), NATO Science Series, Functional Properties of Nanostructured Materials, vol. 223, Springer, Dordrecht, 2006, p. 275. [6] S. Reinke, M. Kuhr, W. Kulisch, Diamond Relat. Mater. 3 (1994) 341. [7] S. Reinke, M. Kuhr, W. Kulisch, R. Kassing, Diamond Relat. Mater. 4 (1995) 272. [8] W. Kulisch, S. Ulrich, Thin Solid Films 423 (2003) 183. [9] W. Kulisch, R. Freudenstein, Thin Solid Films 516 (2007) 216. [10] I. Bello, C.Y. Chan, W.J. Zhang, Y.M. Chong, K.M. Leung, S.T. Lee, Y. Lifshitz, Diamond Relat. Mater. 14 (2005) 1154. [11] P.J. Gielisse, S.S. Mitra, J.N. Plendl, R.D. Griffis, L.C. Mansur, R. Marshall, E.A. Pascoe, Phys. Rev. 155 (1967) 1039. [12] R. Geick, C.H. Perry, Phys. Rev. 146 (1966) 543. [13] T. Nguyen, S. Ulrich, J. Bsul, S. Beauvais, W. Burger, A. Albers, M. Stüber, J. Ye, Diamond Relat. Mater. 18 (2009) 995. [14] Q. Li, L.D. Marks, Y. Lifshitz, S.T. Lee, I. Bello, Phys. Rev. B65 (2002) 045415. [15] K. Sell, H. Holleck, H. Leiste, M. Stüber, S. Ulrich, J. Ye, Diamond Relat. Mater. 11 (2002) 1272. [16] S. Ulrich, J. Ye, M. Stüber, C. Ziebert, Thin Solid Films 518 (2009) 1443. [17] Y.K. Le, H. Oechsner, Appl. Phys. A 78 (2004) 681. [18] W. Dworschak, K. Jung, H. Ehrhardt, Thin Solid Films 254 (1995) 65. [19] G.P. Lamaze, R.G. Dowing, L.B. Hackenberger, L.J. Pilione, R. Messier, Diamond Relat. Mater. 3 (1994) 728. [20] J. Ye, H. Oechsner, S. Westermeyr, J. Vac. Sci. Technol. A 19 (2001) 2294. [21] Y.K. Le, H. Oechsner, Thin Solid Films 437 (2003) 83.