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Microstructural and optical properties of BN films deposited by inductively coupled plasma CVD
Vacuum 82 (2008) 1296–1301
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Microstructural and optical properties of BN films deposited by inductively coupled plasma CVD S. Dalui, A.K. Pal* Department of Instrumentation Science, USIC Building, Jadavpur University, Calcuttta, West Bengal 700 032, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 August 2007 Received in revised form 20 March 2008 Accepted 25 March 2008
Boron nitride films were deposited at different substrate temperatures (400–600 K) on Si (100) substrates by RF plasma CVD technique using a mixture of borane-ammonia, argon and nitrogen as the precursor gases. No intentional bias was applied during deposition to modulate the ion energy. The surface textures of the films were obtained by Atomic Force Microscope studies (AFM). X-ray Photoelectron Spectroscopy (XPS) studies were carried out to determine the bonding environment in the coatings. Fourier Transformed Infra-Red (FTIR) spectrums indicated that the films deposited at lower substrate temperature contained h-BN phases and the percentage of c-BN content in the films increased with the substrate temperature. Films deposited at temperature w553 K were predominantly c-BN in nature. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Boron nitride RF plasma CVD FTIR
1. Introduction During the last three decades, boron nitride had attracted continuous interest due to its promising properties [1–8]. Among the different phases of BN, two most important phases for various practical applications are c-BN having sp3 bonding like diamond and h-BN having sp2 bonding like graphite. c-BN is a good insulating material, which has high values of electrical resistivity, optical transmittance and thermal conductivity along with high chemical and thermal stability. h-BN is transparent to X-rays and it has excellent tribological properties, which do not degrade in air. c-BN having hardness close to that of diamond is ideal for protecting cutting tools and optical devices while h-BN which is inert towards gas adsorption, is suitable for coating the surfaces of vacuum components. Various deposition techniques mainly based on physical and chemical vapour deposition techniques have been adopted for the deposition of c-BN [9–19] having their own merits and demerits. Main goal was to deposit c-BN and h-BN in a pure form as far as practicable. In most cases, BN films grew in a mixed phase with the presence of significant amount of h-BN. Several models have been postulated [1–4,8–16] for describing the growth process in BN films deposited by different methods cited in a review [7]. Most accepted model postulated the requirement of threshold ion bombardment energy for producing c-BN rich film [6,11–13]. There are reports depicting the effect of gas pressure, relative amount of hydrogen in the plasma and substrate bias on the growth of c-BN. Ion bombardment during the growth of BN films participating would have an * Corresponding author. E-mail address: [email protected] (A.K. Pal). 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.03.081
additional effect of re-sputtering of the deposits that may culminate in thinner coating. Thus, the understanding of growth mechanism for c-BN film is not fully satisfactory. Keeping this in mind, we have carried out the deposition of BN films without any intentional bias in an inductively coupled RF plasma system in our laboratory and the results are interesting and important enough to report. In the present work, we have prepared and characterized BN films deposited by inductively coupled RF plasma CVD technique onto silicon and fused silica substrates without any intentional bias. Keeping the precursors, RF power and deposition time invariant, only deposition condition that was varied for depositing BN films was the substrate temperature. The results are quite interesting and important to re-look the growth mechanism of BN films. 2. Experimental details BN films were deposited onto Si (100) substrates at temperatures ranging from 400 to 600 K by using inductively coupled plasma CVD system. The experimental jig for the deposition of BN films onto the substrates consisted of a quartz reaction chamber (3 inch diameter and 12 inch long) connected to a stainless steel base chamber (6 inch diameter and 8 inch high) having appropriate connectors for vacuum system, vacuum gauge heads, hot emissive probe and window for optical emission spectrometer (OES). Following Schumacher et al. [8] and Cameron et al. [14], an air-stable crystalline borane-ammonia adduct was used as boron containing precursor. Borane-ammonia vapour was introduced through a regulated needle valve. Argon and nitrogen in appropriate proportions, used as diluting gases, were introduced through mass flow meters. The substrate holder with an embedded heater and thermocouple
S. Dalui, A.K. Pal / Vacuum 82 (2008) 1296–1301
were introduced from the bottom of the base chamber. The CVD system was evacuated to a base pressure of w106 mbar and the system pressure during deposition was kept w0.4 mbar. A RF generator (13.56 MHz; 1 kW) was used to generate plasma inside the quartz tube by coupling the RF power to the copper ribbon wound around the quartz tube. The RF power was kept constant at w200 W. An impedance matching network with a blocking capacitor was used to reduce the reflected power to a minimum (nearly zero) value. The temperature of the substrate was controlled and monitored by an electronic on/off temperature controller with a thermocouple acting as the sensing element. For generating borane-ammonia vapour to be used as boron containing precursor, a Corning round-bottom flask containing commercially available borane-ammonia (from Alpha Products) was heated uniformly by immersing it in a water bath using, a magnetic stirrer-cum-heater. Dry nitrogen gas (99.95%) was used to keep dry inert atmosphere inside the flask. Appropriate care was taken to heat the source container and the feeder line to about 80–85 C to prevent condensation of the reactants on their way to the chamber. An optical emission spectrometer (OES) was utilized to monitor the emission spectrum of the plasma through the quartz window by using appropriate fiber optics assembly. The optical emission spectrometer comprised of an optical fiber to collect the optical signals from the source (here plasma) to be analyzed by using a spectrometer (UV–vis–NIR) and a charge-coupled device (CCD) array detector. The signals are analyzed by data acquisition by interfacing the OES to a PC via an analog-to-digital converter with an accuracy w0.5 nm. The optical fiber was appropriately positioned to collect the emission of the plasma located at the substrate/plasma interface through appropriate quartz port. The films were deposited without applying any intentional substrate bias at a chamber pressure w0.4 mbar and the deposition time was kept constant at w4 h to obtain films with thickness w0.5 mm. The films were characterized by optical and microstructural measurements. FTIR studies were carried out in the range 400– 4000 cm1 by using an IR spectrometer (Nicolet, Magna-IR). The film
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texture was studied by atomic force microscope (AFM). The XPS spectra were recorded using a VG Microtech XPS setup incorporated with 1256.4 eV Mg Ka X-ray source with a hemispherical analyzer and a channeltron detector with a resolution 1.3 eV. 3. Results and discussion Despite the common observation [19] that c-BN films prepared by CVD technique might contain lesser cubic phase than those prepared by physical vapour deposition technique, Yang et al. [17] recently demonstrated that it was possible to deposit BN with significant c-BN content by inductively coupled plasma CVD (ICPCVD) technique. Konyashin et al. [18] indicated that substrate bias might not be required for obtaining predominant c-BN phase when the plasma contains excess hydrogen. Atomic hydrogen is considered to evolve from borane-ammonia by temperature induced splitting and collisions with energetic plasma electrons. This was confirmed by optical emission spectroscopy (section 3.4). Hence, we did not add any hydrogen intentionally in the deposition chamber and consider the reactions as: BH3 NH3 D 2H [ BH2 NH2 D 2H2 BH3 NH3 D 4H [ BHNH D 4H2 Interaction among the BH2NH2 and BHNH radicals and atomic or ionized hydrogen might lead to the formation of c-BN as proposed by Konyashin et al. [18]. Zedlitz et al. [20] observed that hydrogen not only saturate the dangling bonds, but led to a reduction in B–N lattice bonding. Thus, evolution of c-BN with increasing substrate temperature with no intentional bias would ensure a re-look at the growth mechanism of c-BN. 3.1. Microstructural studies Fig. 1(a–d) shows the AFM pictures of four representative BN films deposited on Si substrates at four different substrate
Fig. 1. AFM micrographs of four representative BN films deposited on Si substrate at different substrate temperatures: (a) 403 K; (b) 463 K; (c) 498 K and (d) 553 K.
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temperatures during deposition. Films deposited at lower substrate temperature (Fig. 1a) had a rough surface with assorted crystallite sizes. As the substrate temperature was increased, a distinct change in morphology of the films (Fig. 1b–d) could be observed. The surface became smoother for films deposited at higher substrate temperature during deposition. The observed increase in surface smoothness may be due to increase in surface mobility of the adatoms at higher substrate temperature in conjunction to other associated effects that may arise due to structural evolution. Growth of crystallites with well faceted structure could be observed (Fig. 1d) for films deposited at 553 K. The maximum value of grain size was w0.5 mm. 3.2. FTIR studies Fig. 2(a–d) shows the FTIR spectra of four representative films deposited at different substrate temperature during deposition. The corresponding AFM pictures of these films were shown in Fig. 1(a– d). Presence of c-BN was indicated by the peak at 1063 cm1 appearing due to the transverse optical (TO) phonons while the absorption bands of h-BN at 1380 and 780 cm1 corresponded to the B–N valence vibration and B–N–B out of plane bending, respectively. The above observation is in conformity with the findings of Schumacher and Oechsner [8] who deposited BN films by plasma enhanced CVD technique using electron cyclotron wave resonance. They observed that the intensity of the peaks for h-BN reduced significantly while the intensity of the peak for c-BN became stronger as the films were deposited with increasing ion energy keeping the substrate temperature during deposition at 653 K. In our study, the only variable was substrate temperature while other parameters like gas composition, r.f. power, bias etc. were kept invariant. It may also be observed from this study that as the substrate temperature was increased, the intensity of the peak at w1063 cm1 for c-BN increased significantly while the intensities of the peaks for h-BN became low. This trend is similar to that observed by Schumacher and Oechsner [8] for their BN films deposited at 653 K at various ion energies.
The ratio of the intensities of the peaks due to c-BN and h-BN at 1060–1080 cm1 and 780–810 cm1, respectively, was considered to determine the percentage of sp3 c-BN as: % of sp3 phase ¼ Ic-BN =ðIc-BN þ Ih-BN Þ 100
(1)
Variation of c-BN content with substrate temperature for these films is shown in Fig. 3. It may be observed that film deposited at w400 K contained mostly h-BN phase and films deposited at higher temperatures had higher c-BN content. Films deposited at w553 K was seen to contain 98% c-BN. It may be noted here once again that precursor gases, system pressure during deposition and RF power were kept constant. It will thus be interesting to study the dependence of c-BN content with substrate temperature by monitoring the plasma species present near the substrate as well as at the central region of the plasma. Since the above observation was reproducible, it would necessarily imply that a critical study on the plasma species present at the substrate/plasma interface as well as at the center of the plasma column may lead one to decipher the role of plasma species along with the substrate temperature during deposition in obtaining c-BN phase in the deposits as the other deposition conditions remain invariant. 3.3. XPS studies XPS measurements were carried out on the BN films deposited at different substrate temperatures to bring out the information on the binding energies for different hybridized BN. Since core-level binding energies are closely related to the local chemical and topological environment, changes in the valance-band structure due to bonding would be reflected in distinct binding-energy shifts of core electrons for films containing predominant hexagonal and cubic BN content. Fig. 4a and b show the XPS spectra for films containing 98% h-BN and 98% c-BN, respectively. It may be observed that the binding energies for both N(1s) and B(1s) for films containing 98% h-BN (Fig. 4a) shifted to higher energies for films containing 98% c-BN (Fig. 4b). Similar observation in the shift in the position of the core-level spectra for BN films were reported
Fig. 2. FTIR spectra of four representative BN films deposited at different substrate temperature: (a) 403 K; (b) 463 K; (c) 498 K and (d) 553 K.
S. Dalui, A.K. Pal / Vacuum 82 (2008) 1296–1301
Fig. 3. Variation of c-BN and h-BN contents with deposition temperature (Ts).
by Widmayer et al. [21] and Angleraud et al. [22]. This result would reaffirm the growth of c-BN at higher substrate temperature as indicated in the FTIR studies discussed in earlier section. 3.4. Optical emission spectroscopy studies
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(SPS), being the more probable one, is easily obtained in a discharge. In case of Argon, emission from singly ionized species would be abundant in the power regime of this study. Typical OES spectra recorded at the center (figures on the left column of Fig. 5) and substrate/plasma interface (figures on the right hand column of Fig. 5) of the plasma column recorded during deposition of three representative films deposited at different substrate temperature are shown in Fig. 5. The spectra were taken during the deposition of BN. All the spectra indicated strongest peak for N2 at w337.4 nm followed by the peak at 650 nm for Ha Blamer line and at w751 nm for Ar. The most prominent lines due to the Second Positive system (SPS) of N2 are 296.2, 316.4, 337.5, 353 and 399.4 nm [23]. All of them could be detected in the spectrum. The strongest peaks for argon and nitrogen are located at 751 and 337.5 nm, respectively. Other emission lines for Arþ ions, located at 653.3, 668.1, 687.4, 736.9 and 763.2 nm, are prominent in the spectrum. There are some medium intensity peaks located at 812.2, 826.5 and 841.8 nm for singly ionized Ar species. The peaks due to boron species were difficult to detect due to overcrowding of stronger N2 plasma lines and also one must keep in mind that concentration of borane-ammonia was significantly low. The broad distinctive band for hydrogen Balmer line (Ha) is due to the transitions 3d2D / 2 p2p0, which may have originated from the electron impact processes in the plasma [24]. The broad band with
Optical emission spectroscopy was utilized to determine the nature of the ion species in the plasma, their abundance and distribution (especially near the substrate surface). The interdependence of the above parameters on the microstructure and hence on the physical properties of the c-BN films could be addressed critically. For nitrogen plasma, it is known that there are a large number of band systems attributed to the neutral nitrogen molecule. In emission mode, the First and Second Positive systems are the most readily developed ones. The Second Positive system
Fig. 4. XPS spectra for two representative films containing: (a) 98% h-BN and (b) 98% cBN.
Fig. 5. Emission spectra for three representative films deposited at different substrate temperatures (a) 403 K; (b) 463 K and (c) 553 K. Left hand spectra are recorded at the center of the plasma column and the right hand spectra are recorded at the plasma/ substrate interface.
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S. Dalui, A.K. Pal / Vacuum 82 (2008) 1296–1301
relatively lower intensity located at w600 nm may be due to molecular hydrogen. It may be interesting to note that the spectra also contained hydrogen Balmer line (Hg) at w434 nm and BHmolecular band at w433 nm as observed by Schumacher and Oechsner [8] but the intensities are quite low compared to the broad distinctive band for hydrogen Balmer line (Ha). If we examine the relative intensities of nitrogen, hydrogen and argon in the plasma during deposition of BN films at different substrate temperature, quite an interesting and informative trend be observed. Table 1 shows the intensities of different plasma species present at the center (indicated by C) and at the substrate/ plasma interface (indicated by B) of plasma column. One may observe that both the relative intensities of hydrogen and nitrogen with respect to argon increase significantly when the films are deposited at higher substrate temperature i.e. for films with more c-BN content. Increase in nitrogen species at higher substrate temperature was higher than that for hydrogen. The relative abundance of hydrogen with respect to nitrogen at the plasma/ substrate interface showed an increasing trend with deposition temperature. The above trend is more or less similar for the plasma recorded at the central portion of the plasma column with the exception that relative amount of hydrogen showed an increase compared to that at the substrate/plasma interface (Table 1). This may be due to higher thermal conductivity and smaller size of hydrogen. FTIR spectra recorded in the higher wave number region (Fig. 6) indicate small peaks of B–H and N–H vibrational modes in the films deposited at 403 K. Schumacher and Oechsner [8] reported that there was no perceptible signature of B–H and N–H bonds in their films deposited at 653 K. Cameron et al. [14] reported for films generated at lower temperatures a N–H stretching mode at 3220 cm1 which vanished at deposition temperatures above 573 K. Thus, one may assert that the peaks related to N–H and B–H bonds become imperceptible at higher deposition temperatures. Since the amounts of input gases along with borane-ammonia vapour and RF power were kept invariant for all the depositions reported here, it may be inferred that the substrate temperature facilitated ionization of nitrogen and hydrogen that were available from the boron containing precursor only. Thus, the observation points to the fact that relative abundance of the hydrogen and nitrogen species at the plasma/substrate interface would play an important role in the growth of cubic boron nitride phase in the films. The abundance of hydrogen and nitrogen species during the deposition at higher substrate temperature may lead to possible termination of p (dangling) bonds of the sp2 hybridized boron nitride under the impact of hydrogen species and formation of N–H and B–H bonds on the surface. This would result in transformation of electron orbital from the sp2 to sp3 state of boron nitride. Similar observation was reported by Konyashin et al. [25]. It may be worthwhile to mention the work of Ehrhardt et al. [26] on the transformation of sp2 to sp3 binding configuration in c-BN. They observed that for the nucleation of c-BN not only compressive stress was necessary, but also a certain mobility of the particles over a time period was also required. This time period must be long
Fig. 6. FTIR spectra of a representative BN film deposited at 403 K.
compared to the vibrational time constants to achieve crystalline ordering in the deposit. Two temperature sources were considered for imparting mobility: one due to substrate heating and the other due to thermal spike of the ion impact. In our experiment, self-bias varied within the range 10–15 V and as such one would experience quite low ion energy for originating thermal spikes. Thus, the substrate temperature during deposition and relative abundance of plasma species present at the substrate/plasma interface played an important role in the synthesis of c-BN. 4. Conclusion Boron nitride films were deposited by RF plasma CVD technique using a mixture of borane-ammonia, argon and nitrogen as the precursor gases. No intentional bias was applied during deposition to modulate the ion energy. FTIR studies indicated that the films deposited at lower substrate temperature contained h-BN phases and the percentage of c-BN content in the films increased with the substrate temperature. Films deposited at temperature above 553 K contained predominantly c-BN phases. It was observed that both the relative intensities of hydrogen and nitrogen with respect to argon increased significantly when the films are deposited at higher substrate temperature i.e. for films with more c-BN content. Increase in nitrogen species at higher substrate temperature during deposition was higher than that for hydrogen. The substrate temperature during deposition and relative abundance of plasma species present at the substrate/plasma interface played an important role in the synthesis of c-BN. Acknowledgement
Table 1 Intensities of different plasma species present at the center (indicated by C) and at the substrate/plasma interface (indicated by B) of plasma column
S Dalui wishes to thank the Council for Scientific and Industrial Research, Government of India, for extending fellowships to him.
% h-BN
N2
Ar
N2:Ar
H2:N2
H2:Ar
Reference
98% B C
2574 1480
1446 812
771 408
1.79 1.82
0.299 0.279
0.53 0.502
59% B C
3993 3974
1856 1970
1166 1290
2.15 2.01
0.313 0.324
0.67 0.65
2% B C
4405 4021
2105 1781
1602 1440
2.1 2.25
0.36 0.36
0.76 0.80
H2
[1] [2] [3] [4] [5] [6] [7]
Ye J, Oechsner H. Thin Solid Films 2006;514:138–44. Oechsner H. Thin Solid Films 2006;515:33–8. Wong SF, Ong CW, Pang GKH, Li Q, Lau WM. Thin Solid Films 2005;489:63–7. Deng J, Chen G. Appl Surf Sci 2006;252:7766–70. Yin H, Boyen H-G, Zhang X-W, Ziemann P. Diam Rel Mater 2007;16:46–9. Matsumoto S, Zhang W. Jpn J Appl Phys 2000;39:L442–4. Riedel R. Adv Mater 1994;6:549–60.
S. Dalui, A.K. Pal / Vacuum 82 (2008) 1296–1301 [8] Schumacher A, Oechsner H. Le Vide: Science, Technique et Applications, Suppl 1999;291:106–9. [9] Bello I, Chan CY, Chong YM, Leung KM, Lee ST, Lifshitz Y. Diam Rel Mater 2005; 14:1154–62. [10] Reinke S, Kuhr M, Kulisch W. Diam Rel Mater 1994;3:341–5. [11] Robertson J. Diam Rel Mater 1996;5:519–24. [12] McKenzie DR. J Vac Sci Technol B 1993;11:1928–35. [13] Kester DJ, Ailey KS, Davis RF, More KL. J Mater Res 1993;8:1213–6. [14] Cameron DC, Karim MZ, Hashmi MSJ. Thin Solid Films 1993;236:96–102. [15] Yoshida T. Diam Relat Mater 1996;5:501–7. [16] Hofsa¨ss H, Feldermann H, Merk R, Sebastian M, Ronning C. Appl Phys A 1998; 66:153–81. [17] Yang H, Iwamoto C, Yoshida T. Thin Solid Films 2002;407:67–71.
[18] [19] [20] [21] [22] [23] [24] [25] [26]
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Konyashin I, Loeffler J, Bill J, Aldinger F. Thin Solid Films 1997;308–309:101–6. Mirkarimi PB, McKarty KF, Medlin DL. Mater Sci Eng R 1997;21:47. Zedlitz R, Heintze M, Schubert MB. J Non-Cryst Solids 1996;198–200:403–6. Widmayer P, Boyen H-G, Ziemann P, Rinke P, Oelhafen P. Phys Rev B 1999;59: 5233–41. Angleraud B, Cahoreau M, Jauberteau I, Aubreton J, Catherinot A. J Appl Phys 1998;83:3398–403. Hofsa¨ss H, Feldermann H, Eyhusen S, Ronning C. Phys Rev B 2002;65:115410. Kim HT, Park DK, Choi WS. J Korean Phys Soc 2003;42:S916–9. Konyashin I, Khvostov V, Babaev V, Guseva M, Bill J, Aldinger F. Diam Rel Mater 1999;8:2053–8. Ehrhardt H, Dworschak W, Ulrich S, Jung K, Scheib M, Weiler M. In: CIMTEC Conference, Florence, France, July 1994.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Microstructural and optical properties of BN films deposited by inductively coupled plasma CVD S. Dalui, A.K. Pal* Department of Instrumentation Science, USIC Building, Jadavpur University, Calcuttta, West Bengal 700 032, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 August 2007 Received in revised form 20 March 2008 Accepted 25 March 2008
Boron nitride films were deposited at different substrate temperatures (400–600 K) on Si (100) substrates by RF plasma CVD technique using a mixture of borane-ammonia, argon and nitrogen as the precursor gases. No intentional bias was applied during deposition to modulate the ion energy. The surface textures of the films were obtained by Atomic Force Microscope studies (AFM). X-ray Photoelectron Spectroscopy (XPS) studies were carried out to determine the bonding environment in the coatings. Fourier Transformed Infra-Red (FTIR) spectrums indicated that the films deposited at lower substrate temperature contained h-BN phases and the percentage of c-BN content in the films increased with the substrate temperature. Films deposited at temperature w553 K were predominantly c-BN in nature. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Boron nitride RF plasma CVD FTIR
1. Introduction During the last three decades, boron nitride had attracted continuous interest due to its promising properties [1–8]. Among the different phases of BN, two most important phases for various practical applications are c-BN having sp3 bonding like diamond and h-BN having sp2 bonding like graphite. c-BN is a good insulating material, which has high values of electrical resistivity, optical transmittance and thermal conductivity along with high chemical and thermal stability. h-BN is transparent to X-rays and it has excellent tribological properties, which do not degrade in air. c-BN having hardness close to that of diamond is ideal for protecting cutting tools and optical devices while h-BN which is inert towards gas adsorption, is suitable for coating the surfaces of vacuum components. Various deposition techniques mainly based on physical and chemical vapour deposition techniques have been adopted for the deposition of c-BN [9–19] having their own merits and demerits. Main goal was to deposit c-BN and h-BN in a pure form as far as practicable. In most cases, BN films grew in a mixed phase with the presence of significant amount of h-BN. Several models have been postulated [1–4,8–16] for describing the growth process in BN films deposited by different methods cited in a review [7]. Most accepted model postulated the requirement of threshold ion bombardment energy for producing c-BN rich film [6,11–13]. There are reports depicting the effect of gas pressure, relative amount of hydrogen in the plasma and substrate bias on the growth of c-BN. Ion bombardment during the growth of BN films participating would have an * Corresponding author. E-mail address: [email protected] (A.K. Pal). 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.03.081
additional effect of re-sputtering of the deposits that may culminate in thinner coating. Thus, the understanding of growth mechanism for c-BN film is not fully satisfactory. Keeping this in mind, we have carried out the deposition of BN films without any intentional bias in an inductively coupled RF plasma system in our laboratory and the results are interesting and important enough to report. In the present work, we have prepared and characterized BN films deposited by inductively coupled RF plasma CVD technique onto silicon and fused silica substrates without any intentional bias. Keeping the precursors, RF power and deposition time invariant, only deposition condition that was varied for depositing BN films was the substrate temperature. The results are quite interesting and important to re-look the growth mechanism of BN films. 2. Experimental details BN films were deposited onto Si (100) substrates at temperatures ranging from 400 to 600 K by using inductively coupled plasma CVD system. The experimental jig for the deposition of BN films onto the substrates consisted of a quartz reaction chamber (3 inch diameter and 12 inch long) connected to a stainless steel base chamber (6 inch diameter and 8 inch high) having appropriate connectors for vacuum system, vacuum gauge heads, hot emissive probe and window for optical emission spectrometer (OES). Following Schumacher et al. [8] and Cameron et al. [14], an air-stable crystalline borane-ammonia adduct was used as boron containing precursor. Borane-ammonia vapour was introduced through a regulated needle valve. Argon and nitrogen in appropriate proportions, used as diluting gases, were introduced through mass flow meters. The substrate holder with an embedded heater and thermocouple
S. Dalui, A.K. Pal / Vacuum 82 (2008) 1296–1301
were introduced from the bottom of the base chamber. The CVD system was evacuated to a base pressure of w106 mbar and the system pressure during deposition was kept w0.4 mbar. A RF generator (13.56 MHz; 1 kW) was used to generate plasma inside the quartz tube by coupling the RF power to the copper ribbon wound around the quartz tube. The RF power was kept constant at w200 W. An impedance matching network with a blocking capacitor was used to reduce the reflected power to a minimum (nearly zero) value. The temperature of the substrate was controlled and monitored by an electronic on/off temperature controller with a thermocouple acting as the sensing element. For generating borane-ammonia vapour to be used as boron containing precursor, a Corning round-bottom flask containing commercially available borane-ammonia (from Alpha Products) was heated uniformly by immersing it in a water bath using, a magnetic stirrer-cum-heater. Dry nitrogen gas (99.95%) was used to keep dry inert atmosphere inside the flask. Appropriate care was taken to heat the source container and the feeder line to about 80–85 C to prevent condensation of the reactants on their way to the chamber. An optical emission spectrometer (OES) was utilized to monitor the emission spectrum of the plasma through the quartz window by using appropriate fiber optics assembly. The optical emission spectrometer comprised of an optical fiber to collect the optical signals from the source (here plasma) to be analyzed by using a spectrometer (UV–vis–NIR) and a charge-coupled device (CCD) array detector. The signals are analyzed by data acquisition by interfacing the OES to a PC via an analog-to-digital converter with an accuracy w0.5 nm. The optical fiber was appropriately positioned to collect the emission of the plasma located at the substrate/plasma interface through appropriate quartz port. The films were deposited without applying any intentional substrate bias at a chamber pressure w0.4 mbar and the deposition time was kept constant at w4 h to obtain films with thickness w0.5 mm. The films were characterized by optical and microstructural measurements. FTIR studies were carried out in the range 400– 4000 cm1 by using an IR spectrometer (Nicolet, Magna-IR). The film
1297
texture was studied by atomic force microscope (AFM). The XPS spectra were recorded using a VG Microtech XPS setup incorporated with 1256.4 eV Mg Ka X-ray source with a hemispherical analyzer and a channeltron detector with a resolution 1.3 eV. 3. Results and discussion Despite the common observation [19] that c-BN films prepared by CVD technique might contain lesser cubic phase than those prepared by physical vapour deposition technique, Yang et al. [17] recently demonstrated that it was possible to deposit BN with significant c-BN content by inductively coupled plasma CVD (ICPCVD) technique. Konyashin et al. [18] indicated that substrate bias might not be required for obtaining predominant c-BN phase when the plasma contains excess hydrogen. Atomic hydrogen is considered to evolve from borane-ammonia by temperature induced splitting and collisions with energetic plasma electrons. This was confirmed by optical emission spectroscopy (section 3.4). Hence, we did not add any hydrogen intentionally in the deposition chamber and consider the reactions as: BH3 NH3 D 2H [ BH2 NH2 D 2H2 BH3 NH3 D 4H [ BHNH D 4H2 Interaction among the BH2NH2 and BHNH radicals and atomic or ionized hydrogen might lead to the formation of c-BN as proposed by Konyashin et al. [18]. Zedlitz et al. [20] observed that hydrogen not only saturate the dangling bonds, but led to a reduction in B–N lattice bonding. Thus, evolution of c-BN with increasing substrate temperature with no intentional bias would ensure a re-look at the growth mechanism of c-BN. 3.1. Microstructural studies Fig. 1(a–d) shows the AFM pictures of four representative BN films deposited on Si substrates at four different substrate
Fig. 1. AFM micrographs of four representative BN films deposited on Si substrate at different substrate temperatures: (a) 403 K; (b) 463 K; (c) 498 K and (d) 553 K.
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temperatures during deposition. Films deposited at lower substrate temperature (Fig. 1a) had a rough surface with assorted crystallite sizes. As the substrate temperature was increased, a distinct change in morphology of the films (Fig. 1b–d) could be observed. The surface became smoother for films deposited at higher substrate temperature during deposition. The observed increase in surface smoothness may be due to increase in surface mobility of the adatoms at higher substrate temperature in conjunction to other associated effects that may arise due to structural evolution. Growth of crystallites with well faceted structure could be observed (Fig. 1d) for films deposited at 553 K. The maximum value of grain size was w0.5 mm. 3.2. FTIR studies Fig. 2(a–d) shows the FTIR spectra of four representative films deposited at different substrate temperature during deposition. The corresponding AFM pictures of these films were shown in Fig. 1(a– d). Presence of c-BN was indicated by the peak at 1063 cm1 appearing due to the transverse optical (TO) phonons while the absorption bands of h-BN at 1380 and 780 cm1 corresponded to the B–N valence vibration and B–N–B out of plane bending, respectively. The above observation is in conformity with the findings of Schumacher and Oechsner [8] who deposited BN films by plasma enhanced CVD technique using electron cyclotron wave resonance. They observed that the intensity of the peaks for h-BN reduced significantly while the intensity of the peak for c-BN became stronger as the films were deposited with increasing ion energy keeping the substrate temperature during deposition at 653 K. In our study, the only variable was substrate temperature while other parameters like gas composition, r.f. power, bias etc. were kept invariant. It may also be observed from this study that as the substrate temperature was increased, the intensity of the peak at w1063 cm1 for c-BN increased significantly while the intensities of the peaks for h-BN became low. This trend is similar to that observed by Schumacher and Oechsner [8] for their BN films deposited at 653 K at various ion energies.
The ratio of the intensities of the peaks due to c-BN and h-BN at 1060–1080 cm1 and 780–810 cm1, respectively, was considered to determine the percentage of sp3 c-BN as: % of sp3 phase ¼ Ic-BN =ðIc-BN þ Ih-BN Þ 100
(1)
Variation of c-BN content with substrate temperature for these films is shown in Fig. 3. It may be observed that film deposited at w400 K contained mostly h-BN phase and films deposited at higher temperatures had higher c-BN content. Films deposited at w553 K was seen to contain 98% c-BN. It may be noted here once again that precursor gases, system pressure during deposition and RF power were kept constant. It will thus be interesting to study the dependence of c-BN content with substrate temperature by monitoring the plasma species present near the substrate as well as at the central region of the plasma. Since the above observation was reproducible, it would necessarily imply that a critical study on the plasma species present at the substrate/plasma interface as well as at the center of the plasma column may lead one to decipher the role of plasma species along with the substrate temperature during deposition in obtaining c-BN phase in the deposits as the other deposition conditions remain invariant. 3.3. XPS studies XPS measurements were carried out on the BN films deposited at different substrate temperatures to bring out the information on the binding energies for different hybridized BN. Since core-level binding energies are closely related to the local chemical and topological environment, changes in the valance-band structure due to bonding would be reflected in distinct binding-energy shifts of core electrons for films containing predominant hexagonal and cubic BN content. Fig. 4a and b show the XPS spectra for films containing 98% h-BN and 98% c-BN, respectively. It may be observed that the binding energies for both N(1s) and B(1s) for films containing 98% h-BN (Fig. 4a) shifted to higher energies for films containing 98% c-BN (Fig. 4b). Similar observation in the shift in the position of the core-level spectra for BN films were reported
Fig. 2. FTIR spectra of four representative BN films deposited at different substrate temperature: (a) 403 K; (b) 463 K; (c) 498 K and (d) 553 K.
S. Dalui, A.K. Pal / Vacuum 82 (2008) 1296–1301
Fig. 3. Variation of c-BN and h-BN contents with deposition temperature (Ts).
by Widmayer et al. [21] and Angleraud et al. [22]. This result would reaffirm the growth of c-BN at higher substrate temperature as indicated in the FTIR studies discussed in earlier section. 3.4. Optical emission spectroscopy studies
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(SPS), being the more probable one, is easily obtained in a discharge. In case of Argon, emission from singly ionized species would be abundant in the power regime of this study. Typical OES spectra recorded at the center (figures on the left column of Fig. 5) and substrate/plasma interface (figures on the right hand column of Fig. 5) of the plasma column recorded during deposition of three representative films deposited at different substrate temperature are shown in Fig. 5. The spectra were taken during the deposition of BN. All the spectra indicated strongest peak for N2 at w337.4 nm followed by the peak at 650 nm for Ha Blamer line and at w751 nm for Ar. The most prominent lines due to the Second Positive system (SPS) of N2 are 296.2, 316.4, 337.5, 353 and 399.4 nm [23]. All of them could be detected in the spectrum. The strongest peaks for argon and nitrogen are located at 751 and 337.5 nm, respectively. Other emission lines for Arþ ions, located at 653.3, 668.1, 687.4, 736.9 and 763.2 nm, are prominent in the spectrum. There are some medium intensity peaks located at 812.2, 826.5 and 841.8 nm for singly ionized Ar species. The peaks due to boron species were difficult to detect due to overcrowding of stronger N2 plasma lines and also one must keep in mind that concentration of borane-ammonia was significantly low. The broad distinctive band for hydrogen Balmer line (Ha) is due to the transitions 3d2D / 2 p2p0, which may have originated from the electron impact processes in the plasma [24]. The broad band with
Optical emission spectroscopy was utilized to determine the nature of the ion species in the plasma, their abundance and distribution (especially near the substrate surface). The interdependence of the above parameters on the microstructure and hence on the physical properties of the c-BN films could be addressed critically. For nitrogen plasma, it is known that there are a large number of band systems attributed to the neutral nitrogen molecule. In emission mode, the First and Second Positive systems are the most readily developed ones. The Second Positive system
Fig. 4. XPS spectra for two representative films containing: (a) 98% h-BN and (b) 98% cBN.
Fig. 5. Emission spectra for three representative films deposited at different substrate temperatures (a) 403 K; (b) 463 K and (c) 553 K. Left hand spectra are recorded at the center of the plasma column and the right hand spectra are recorded at the plasma/ substrate interface.
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relatively lower intensity located at w600 nm may be due to molecular hydrogen. It may be interesting to note that the spectra also contained hydrogen Balmer line (Hg) at w434 nm and BHmolecular band at w433 nm as observed by Schumacher and Oechsner [8] but the intensities are quite low compared to the broad distinctive band for hydrogen Balmer line (Ha). If we examine the relative intensities of nitrogen, hydrogen and argon in the plasma during deposition of BN films at different substrate temperature, quite an interesting and informative trend be observed. Table 1 shows the intensities of different plasma species present at the center (indicated by C) and at the substrate/ plasma interface (indicated by B) of plasma column. One may observe that both the relative intensities of hydrogen and nitrogen with respect to argon increase significantly when the films are deposited at higher substrate temperature i.e. for films with more c-BN content. Increase in nitrogen species at higher substrate temperature was higher than that for hydrogen. The relative abundance of hydrogen with respect to nitrogen at the plasma/ substrate interface showed an increasing trend with deposition temperature. The above trend is more or less similar for the plasma recorded at the central portion of the plasma column with the exception that relative amount of hydrogen showed an increase compared to that at the substrate/plasma interface (Table 1). This may be due to higher thermal conductivity and smaller size of hydrogen. FTIR spectra recorded in the higher wave number region (Fig. 6) indicate small peaks of B–H and N–H vibrational modes in the films deposited at 403 K. Schumacher and Oechsner [8] reported that there was no perceptible signature of B–H and N–H bonds in their films deposited at 653 K. Cameron et al. [14] reported for films generated at lower temperatures a N–H stretching mode at 3220 cm1 which vanished at deposition temperatures above 573 K. Thus, one may assert that the peaks related to N–H and B–H bonds become imperceptible at higher deposition temperatures. Since the amounts of input gases along with borane-ammonia vapour and RF power were kept invariant for all the depositions reported here, it may be inferred that the substrate temperature facilitated ionization of nitrogen and hydrogen that were available from the boron containing precursor only. Thus, the observation points to the fact that relative abundance of the hydrogen and nitrogen species at the plasma/substrate interface would play an important role in the growth of cubic boron nitride phase in the films. The abundance of hydrogen and nitrogen species during the deposition at higher substrate temperature may lead to possible termination of p (dangling) bonds of the sp2 hybridized boron nitride under the impact of hydrogen species and formation of N–H and B–H bonds on the surface. This would result in transformation of electron orbital from the sp2 to sp3 state of boron nitride. Similar observation was reported by Konyashin et al. [25]. It may be worthwhile to mention the work of Ehrhardt et al. [26] on the transformation of sp2 to sp3 binding configuration in c-BN. They observed that for the nucleation of c-BN not only compressive stress was necessary, but also a certain mobility of the particles over a time period was also required. This time period must be long
Fig. 6. FTIR spectra of a representative BN film deposited at 403 K.
compared to the vibrational time constants to achieve crystalline ordering in the deposit. Two temperature sources were considered for imparting mobility: one due to substrate heating and the other due to thermal spike of the ion impact. In our experiment, self-bias varied within the range 10–15 V and as such one would experience quite low ion energy for originating thermal spikes. Thus, the substrate temperature during deposition and relative abundance of plasma species present at the substrate/plasma interface played an important role in the synthesis of c-BN. 4. Conclusion Boron nitride films were deposited by RF plasma CVD technique using a mixture of borane-ammonia, argon and nitrogen as the precursor gases. No intentional bias was applied during deposition to modulate the ion energy. FTIR studies indicated that the films deposited at lower substrate temperature contained h-BN phases and the percentage of c-BN content in the films increased with the substrate temperature. Films deposited at temperature above 553 K contained predominantly c-BN phases. It was observed that both the relative intensities of hydrogen and nitrogen with respect to argon increased significantly when the films are deposited at higher substrate temperature i.e. for films with more c-BN content. Increase in nitrogen species at higher substrate temperature during deposition was higher than that for hydrogen. The substrate temperature during deposition and relative abundance of plasma species present at the substrate/plasma interface played an important role in the synthesis of c-BN. Acknowledgement
Table 1 Intensities of different plasma species present at the center (indicated by C) and at the substrate/plasma interface (indicated by B) of plasma column
S Dalui wishes to thank the Council for Scientific and Industrial Research, Government of India, for extending fellowships to him.
% h-BN
N2
Ar
N2:Ar
H2:N2
H2:Ar
Reference
98% B C
2574 1480
1446 812
771 408
1.79 1.82
0.299 0.279
0.53 0.502
59% B C
3993 3974
1856 1970
1166 1290
2.15 2.01
0.313 0.324
0.67 0.65
2% B C
4405 4021
2105 1781
1602 1440
2.1 2.25
0.36 0.36
0.76 0.80
H2
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