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The growth kinetics of silicon nitride deposited from the SiH4–N2 reactant mixture in a remote plasma
Journal of Non-Crystalline Solids 338–340 (2004) 37–41 www.elsevier.com/locate/jnoncrysol
The growth kinetics of silicon nitride deposited from the SiH4–N2 reactant mixture in a remote plasma W.M.M. Kessels *, F.J.H. van Assche, P.J. van den Oever, M.C.M. van de Sanden Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Available online 19 March 2004
Abstract The growth mechanism of silicon nitride (a-SiNx :H) from the SiH4 –N2 reactant mixture is discussed on the basis of results obtained in a remote plasma. From the measured radical densities, it is concluded that ground-state N and SiH3 radicals dominate the a-SiNx :H growth process, as has been confirmed by the correlation between the N and SiH3 density in the plasma and the incorporation flux of N and Si atoms into the a-SiNx :H. From this correlation acceptable sticking probabilities for N and SiH3 (on the order of 0.01 and 0.1, respectively) are deduced while further support for the growth mechanism is given by the different temperature dependences of the Si and N incorporation flux. It is proposed that a-SiNx :H growth takes place by SiH3 radicals forming an a-Si:H-like surface layer that is simultaneously nitridated by the N radicals converting the surface layer into a-SiNx :H. 2004 Elsevier B.V. All rights reserved. PACS: 81.05.)t; 81.70.)q; 82.33.Pt
1. Introduction Understanding of the mechanism of amorphous silicon nitride (a-SiNx :H) growth is essential for full exploitation of the a-SiNx :H properties for present and emerging applications of the material, e.g., in nonsemiconductor applications which have requirements such as high deposition rates (>1 nm/s) and very low substrate temperatures (<150 C). Up to now, many studies of the a-SiNx :H growth kinetics during plasmabased synthesis have been reported in the literature, but the studies by Smith and co-workers have probably been the most comprehensive. For the SiH4 –NH3 reactant mixture, they have shown that gas phase reactions lead to aminosilanes and that the triaminosilane radical, Si(NH2 )3 is the key growth precursor [1,2]. For the SiH4 –N2 reactant mixture, which will be considered in this article, they observed no Si–N products and they concluded that the deposition proceeds by silane radi-
*
Corresponding author. Tel.: +31-40 247 3477; fax: +31-40 245 6442. E-mail address: [email protected] (W.M.M. Kessels). 0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.02.017
cals and N radicals at the surface [2,3]. In this article, we will validate this growth mechanism by providing some additional support for this mechanism as obtained from recent studies in our remote plasma. Moreover, from our experiments we will refine the mechanism; it will be shown that SiH3 is the key growth precursor for the Si atoms in the a-SiNx :H whereas radicals such as Si and SiH have only a very small contribution to growth. The SiH3 radicals create an a-Si:H-like layer on the surface while having a sticking probability similar in magnitude as on pure a-Si:H films (on the order of 0.1). At the same time, this a-Si:H-like surface layer is nitridated by ground-state N radicals with a sticking probability on the order of 0.01. The SiH3 and N incorporation into the a-SiNx :H show (nearly) no substrate temperature dependence. This growth mechanism is schematically summarized in Fig. 1. As mentioned, we will use experimental observations obtained from a remote plasma. This so-called ‘expanding thermal plasma’ is able to deposit good quality a-SiNx :H films at high deposition rates (so far mainly used for photovoltaics [4,5]) and the remote nature of the technique is also beneficial for fundamental plasma and surface studies [6].
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W.M.M. Kessels et al. / Journal of Non-Crystalline Solids 338–340 (2004) 37–41
3. Results 3.1. Plasma chemistry and radical densities
Fig. 1. Schematic representation of the a-SiNx :H growth model from the SiH4 –N2 reactant mixture.
2. Experimental details The expanding thermal plasma (ETP) for a-SiNx :H deposition from SiH4 –N2 has been described in detail in previous work [4,6] and here only the basics will be summarized. In the ETP, plasma creation, growth precursor formation, and deposition are spatially separated. An Ar–H2 –N2 plasma is created at high pressure (300 Torr) upstream in a plasma source and the plasma subsequently expands into a low pressure (150 mTorr) deposition reactor where SiH4 is admixed. The SiH4 is dissociated by the reactive species emanating from the plasma source and the growth precursors can deposit at a substrate holder positioned about 35 cm from the plasma source. The reactive species (electrons, ions, radicals) emanating from the plasma source have been investigated in detail and it has been concluded that the source acts mainly as an atomic N and H source while electrons and ions are basically unimportant for the SiH4 dissociation process [6]. Downstream radical densities have been determined by laser absorption spectroscopy and mass spectrometry as described in Ref. [6]. The radicals Si, SiH, and SiH3 have been investigated by cavity ringdown spectroscopy measurements at a position of 3.6 cm from the substrate holder while ground-state N radicals have been detected by threshold ionization mass spectrometry at the position of the substrate holder. From both techniques absolute densities in the plasma have been determined. Information about the growth process has also been obtained by considering the ‘incorporation flux’ of Si and N atoms into the film. This incorporation flux is the number of Si and N atoms incorporated into the film (in cm2 s1 ) and has been calculated from the product of the deposition rate and the Si and N atomic density in the film as determined by elastic recoil detection [4].
The radicals have been investigated under different plasma conditions and their typical densities in the plasma are given in Table 1. Although not all possible radicals have been measured, it will be shown below that the results on the Si, SiH, SiH3 , and N density yield enough insight to reveal the a-SiNx :H growth mechanism to a large extent. Furthermore, it has been concluded that ions have only a very small contribution to growth while no evidence has been found for a significant density of Si–N products or excited N2 molecules [6]. From Table 1 it is clear that both SiH3 and N have a very high density in the plasma which might imply that these radicals are the most important in the a-SiNx :H growth process. On the basis of the plasma source operation and our previous work on a-Si:H [7], we conclude that SiH3 is mainly created by H abstraction reactions from SiH4 by H atoms emanating from the plasma source. The N density in the plasma is very high which corroborates the fact that N radicals are very unreactive with SiH4 [8] and other species in the plasma. Consequently most N radicals that emanate from the plasma source survive in transit to the substrate holder. Our conclusion that SiH3 and N are the key precursors for a-SiNx :H growth is in agreement with the work of Smith et al. who inferred that N and SiHn radicals govern the growth process [2,3]. Moreover, our results imply that SiH3 is the dominant SiHn radical and not Si or SiH. In the next section we will discuss whether this conclusion is validated by the magnitude of the surface reactivity of the radicals.
3.2. Si and N incorporation flux and their sticking probability In case that SiH3 and N govern a-SiNx :H growth, it is expected that their plasma densities correlate with their incorporation flux into the a-SiNx :H film. Fig. 2(a) shows the SiH3 plasma density vs. the Si incorporation Table 1 The typical plasma densities of the radicals investigated and their measured or estimated sticking probabilities with their reference
N SiH3 SiH Si
Density (cm3 )
Sticking probability
Reference
5 · 1014 2 · 1013 8 · 1010 1010
0.007–0.04 0.15 0.98 1a
This work This work [11] [12]
See text for details. a The Si sticking probability is assumed unity on the basis of results for a-Si:H.
W.M.M. Kessels et al. / Journal of Non-Crystalline Solids 338–340 (2004) 37–41
Si incorporation flux (1016 cm-2s-1)
16 14 12 10 8 6 4 2 0 0
5
0
20
(a)
10 15 20 25 30 SiH3 density (1012 cm-3)
35
40
N incorporation flux (1016 cm-2s-1)
7 6 5 4 3 2 1 0 (b)
40
60
80
100
120
140
N density (1012 cm-3)
Fig. 2. (a) The Si incorporation flux into the a-SiNx :H film vs. the SiH3 density in the plasma, (b) the N incorporation flux into the a-SiNx :H film vs. the N density in the plasma. The data in the figures have been obtained by varying the SiH4 flow while keeping the N2 flow constant.
flux as obtained by varying the SiH4 flow while keeping the N2 flow constant. As expected, the figure shows a linear dependence which confirms the importance of SiH3 for growth. From the figure also the sticking probability of SiH3 can be estimated. As for a-Si:H we must to distinguish the sticking probability sSiH3 from the surface reaction probability bSiH3 because SiH3 can also have other reactions than sticking at the surface. Assuming sSiH3 ¼ 1=2bSiH3 analogous to a-Si:H [9] and a SiH3 thermal velocity of 1000 m/s [6] we find that sSiH3 0:15 (see Table 1). Although this is only a rough estimate -basically showing that sSiH3 is on the order of 0.1 for a-SiNx :H–it is exactly in the same range as for aSi:H [9]. The corresponding density and incorporation flux of N are given in Fig. 2(b). The figure shows that the N density in the plasma decreases when the incorporation flux increases. This striking behavior can perfectly be understood when realizing that this figure is based on
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results in which the SiH4 flow was varied for a constant N2 flow. This means that the supply of N radicals is constant and consequently the N density drops when N is lost from the plasma by incorporation into the film. This implies also that the ‘sticking probability’ sN 1 of the N radicals varies with the plasma condition. Basically it means that sN depends on the Si incorporation flux with a lower limit imposed by the stochiometric limit (Si3 N4 ) of the films. The range of sN estimated from Fig. 2(b) is given in Table 1. The values of sN are apparently relatively low and even lower than the recombination probability of N at stainless steel (at the N2 partial pressure of 2 Pa) [10]. This also explains the particular behavior that the N density goes up when SiH4 is admixed to our plasma starting with a clean reactor [6]. Furthermore, the relatively low sN invalidates the hypothesis that the high surface roughness and poor step coverage of a-SiNx :H deposited from SiH4 – N2 are caused by a very high surface reactivity of the N atoms [3]. Apparently another mechanism is responsible for these observations. In Table 1 also the (estimated) sticking probabilities for Si and SiH are given [11,12]. Although these radicals have a very high sticking probability (almost unity) it is clear from their plasma density that they have only a very small contribution to film growth (0.5% and 5%, respectively). In summary, it has been shown that both the SiH3 and N density are directly correlated with the Si and N incorporation flux into the a-SiNx :H, and that the sticking probabilities deduced are in good agreement with the expected values and with other experimental observations. This provides strong evidence for the growth mechanism proposed. 3.3. Temperature dependence of the Si and N incorporation flux In our remote plasma the substrate temperature has no effect on the plasma chemistry and plasma radical densities. Consequently, the substrate temperature dependence of the Si and N incorporation flux is governed by the surface reactions. In Fig. 3 the Si and N incorporation flux are shown vs. the substrate temperature for two plasma settings. Although very small, the Si and N incorporation fluxes show a significantly different temperature dependence. This is in agreement with the fact that Si and N are brought to the surface by two separate growth precursors. The Si incorporation 1 When considering the incorporation flux, a sticking probability sN has to be defined with sN 6 cN and with cN being the surface reaction probability of N. Under non-deposition conditions cN is usually referred to the recombination probability. In this work we make the assumption that sN ¼ cN under deposition conditions. This assumption has no significant consequences for the final results.
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Incorporation flux (1016 cm-2s-1)
10
8
6
4
2
0 200
250
300 350 400 450 Substrate temperature (oC)
500
Fig. 3. The N (triangles) and Si (circles) incorporation flux into the aSiNx :H film for different substrate temperatures. Data from two different plasma settings are given and the data with the open symbols (with a lower deposition rate) has been scaled by a factor 2.3 for comparison purposes.
flux shows a very slight temperature dependence (activation energy ¼ 8 ± 1 meV when applying a simple Arrhenius expression) while the N incorporation flux is basically temperature independent (activation energy ¼ 1 ± 1 meV).
4. Discussion on the surface reactions during a-SiNx :H growth Although now sufficient evidence has been provided for the fact that SiH3 and N are the key precursors during a-SiNx :H growth from SiH4 –N2 , the surface reactions of SiH3 and N are not yet understood. The temperature (in)dependence of the Si and N incorporation flux form a good starting point in this respect. The Si incorporation flux has revealed only a very weak temperature dependence similar to the behavior of the Si incorporation flux during a-Si:H growth [9]. Furthermore, we have seen that the sticking probability of SiH3 on a-SiNx :H is about equal to or at least in the same order of magnitude as for a-Si:H. These similarities suggest, therefore, that the outermost surface layer of a-SiNx :H resembles the a-Si:H surface to a large extent. Therefore we can most probably assume that the SiH3 adsorption reactions on a-SiNx :H deposited from the SiH4 –N2 mixture are basically the same as for a-Si:H [13]. For the reaction of N with this a-Si:H-like surface we make use of the results obtained by density functional theory calculations for atomic N interacting with a Si(1 0 0)-2 · 1 surface as only recently reported by Ueno and Ornellas [14] and by Widjaja et al. [15]. Both groups considered the reaction of N with the dimerized surface
with as a first step the adsorption of N on one Si atom of the dimer. From this state they considered the insertion of the N atom into the Si–Si dimer but they both found that the formation of other structures was more likely. Widjaja et al. [15] found that the insertion of N into Si– Si backbonds by the formation of a Si–N–Si bridgebond is most likely with a total reaction enthalpy of )4.05 eV. A small activation energy of 0.48 eV is present however when progressing from the N adsorbed on the Si dimer atom to the Si–N–Si bridge-bond. A slight further decrease in energy (total reaction enthalpy )4.2 eV) has been predicted by the formation of a third bond by the N atom, i.e., NSi3 with the N atom bonded to three Si atoms as for perfect Si3 N4 . This involves a relatively high transition state energy (1.5 eV) however and therefore this reaction has been considered to be less important [15]. Uena and Ornellas reported only the NSi3 structure with no evidence for the Si–N–Si bridgebond. They found a total reaction enthalpy of )3.65 eV with a transition state energy of 0.60 eV when progressing from the N adsorbed on the Si dimer atom to the final NSi3 structure. These results are difficult to translate to the a-Si:H case in which case the surface is not fully terminated by dimers and also covered by H to a large extent. However, the overall exothermicity of the sub-surface reactions – forming either Si–N–Si or NSi3 – is on the order of 4 eV with enough energy available to overcome possible transition state energies. This suggests that similar reaction schemes should be possible for the a-Si:H-like surface. Moreover, it also explains the temperature-independence of the N incorporation flux for the a-SiNx :H because the transition states provide no significant energy barriers for the insertion reactions to occur. The relatively low sticking probability of N, in spite of the favorable energetics, imply that the surface reactions by N are limited by the available reaction sites (related to the Si incorporation flux) as is also suggested by the results in Fig. 2(b). We propose therefore that the nitridation by N radicals takes place by sub-surface insertion reactions into the Si–Si backbonds while leaving the outermost a-Si:H-like surface layer virtually intact as sketched schematically in Fig. 1.
5. Conclusions The growth mechanism of a-SiNx :H from the SiH4 – N2 reactant mixture as proposed by Smith et al. [2,3] has been corroborated and further refined. It has been concluded that a-SiNx :H growth takes place mainly by SiH3 radicals, that create an a-Si:H-like outermost surface layer on the a-SiNx :H, and by N radicals that insert into Si–Si backbonds of this a-Si:H-like surface layer.
W.M.M. Kessels et al. / Journal of Non-Crystalline Solids 338–340 (2004) 37–41
Acknowledgements This work has been carried out within the ‘HR-Cel’ project which is part of the E.E.T.-program of the Netherlands Ministry of Economic Affairs, the Ministry of Education, Culture and Science and the Ministry of Public Housing, Physical Planning and Environment. The research of W.M.M.K. has been made possible by a fellowship of the Royal Netherlands Academy of Arts and Sciences (KNAW).
References [1] D.L. Smith, A.S. Alimonda, C.-C. Chen, S.E. Ready, B. Wacker, J. Electrochem. Soc. A 137 (1990) 614. [2] D.L. Smith, Mater. Res. Soc. Symp. Proc. 165 (1990) 69. [3] D.L. Smith, A.S. Alimoda, F.J. von Preisig, J. Vac. Sci. Technol. B 8 (1990) 551. [4] W.M.M. Kessels, J. Hong, F.J.H. van Assche, J.D. Moschner, T. Lauinger, W.J. Soppe, A.W. Weeber, M.C.M. van de Sanden, J. Vac. Sci. Technol. A 20 (2002) 1704.
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[5] J. Hong, W.M.M. Kessels, F.J.H. van Assche, H.C. Rieffe, W.J. Soppe, A.W. Weeber, M.C.M. vandeSanden, Prog. Photovolt.: Res. Appl. 11 (2003) 125. [6] W.M.M. Kessels, F.J.H. van Assche, J. Hong, D.C. Schram, M.C.M. van de Sanden, J. Vac. Sci. Technol. A 22 (2004) 96. [7] W.M.M. Kessels, J.P.M. Hoefnagels, M.G.H. Boogaarts, D.C. Schram, M.C.M. van de Sanden, J. Appl. Phys. 89 (2001) 2065, and references therein. [8] L.G. Piper, G.E. Caledonia, J. Phys. Chem. 95 (1991) 698. [9] W.M.M. Kessels, M.C.M. van de Sanden, R.J. Severens, D.C. Schram, J. Appl. Phys. 87 (2000) 3313. [10] H. Singh, J.W. Coburn, D.B. Graves, J. Appl. Phys. 88 (2000) 3748. [11] W.M.M. Kessels, P.R. McCurdy, K.W. Williams, G.R. Barker, V.A. Ventura, E.R. Fisher, J. Phys. Chem. B 106 (2002) 2680. [12] J.P.M. Hoefnagels, A.A.E. Stevens, M.G.H. Boogaarts, W.M.M. Kessels, M.C.M. van de Sanden, Chem. Phys. Lett. 360 (2002) 189. [13] W.M.M. Kessels, A.H.M. Smets, D.C. Marra, E.S. Aydil, D.C. Schram, M.C.M. van de Sanden, Thin Solid Films 383 (2001) 154. [14] L.T. Ueno, F.R. Ornellas, Surf. Sci. Lett. 490 (2003) L637. [15] Y. Widjaja, A. Heyman, C.B. Musgrave, J. Phys. Chem. B 106 (2002) 2643.
The growth kinetics of silicon nitride deposited from the SiH4–N2 reactant mixture in a remote plasma W.M.M. Kessels *, F.J.H. van Assche, P.J. van den Oever, M.C.M. van de Sanden Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Available online 19 March 2004
Abstract The growth mechanism of silicon nitride (a-SiNx :H) from the SiH4 –N2 reactant mixture is discussed on the basis of results obtained in a remote plasma. From the measured radical densities, it is concluded that ground-state N and SiH3 radicals dominate the a-SiNx :H growth process, as has been confirmed by the correlation between the N and SiH3 density in the plasma and the incorporation flux of N and Si atoms into the a-SiNx :H. From this correlation acceptable sticking probabilities for N and SiH3 (on the order of 0.01 and 0.1, respectively) are deduced while further support for the growth mechanism is given by the different temperature dependences of the Si and N incorporation flux. It is proposed that a-SiNx :H growth takes place by SiH3 radicals forming an a-Si:H-like surface layer that is simultaneously nitridated by the N radicals converting the surface layer into a-SiNx :H. 2004 Elsevier B.V. All rights reserved. PACS: 81.05.)t; 81.70.)q; 82.33.Pt
1. Introduction Understanding of the mechanism of amorphous silicon nitride (a-SiNx :H) growth is essential for full exploitation of the a-SiNx :H properties for present and emerging applications of the material, e.g., in nonsemiconductor applications which have requirements such as high deposition rates (>1 nm/s) and very low substrate temperatures (<150 C). Up to now, many studies of the a-SiNx :H growth kinetics during plasmabased synthesis have been reported in the literature, but the studies by Smith and co-workers have probably been the most comprehensive. For the SiH4 –NH3 reactant mixture, they have shown that gas phase reactions lead to aminosilanes and that the triaminosilane radical, Si(NH2 )3 is the key growth precursor [1,2]. For the SiH4 –N2 reactant mixture, which will be considered in this article, they observed no Si–N products and they concluded that the deposition proceeds by silane radi-
*
Corresponding author. Tel.: +31-40 247 3477; fax: +31-40 245 6442. E-mail address: [email protected] (W.M.M. Kessels). 0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.02.017
cals and N radicals at the surface [2,3]. In this article, we will validate this growth mechanism by providing some additional support for this mechanism as obtained from recent studies in our remote plasma. Moreover, from our experiments we will refine the mechanism; it will be shown that SiH3 is the key growth precursor for the Si atoms in the a-SiNx :H whereas radicals such as Si and SiH have only a very small contribution to growth. The SiH3 radicals create an a-Si:H-like layer on the surface while having a sticking probability similar in magnitude as on pure a-Si:H films (on the order of 0.1). At the same time, this a-Si:H-like surface layer is nitridated by ground-state N radicals with a sticking probability on the order of 0.01. The SiH3 and N incorporation into the a-SiNx :H show (nearly) no substrate temperature dependence. This growth mechanism is schematically summarized in Fig. 1. As mentioned, we will use experimental observations obtained from a remote plasma. This so-called ‘expanding thermal plasma’ is able to deposit good quality a-SiNx :H films at high deposition rates (so far mainly used for photovoltaics [4,5]) and the remote nature of the technique is also beneficial for fundamental plasma and surface studies [6].
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W.M.M. Kessels et al. / Journal of Non-Crystalline Solids 338–340 (2004) 37–41
3. Results 3.1. Plasma chemistry and radical densities
Fig. 1. Schematic representation of the a-SiNx :H growth model from the SiH4 –N2 reactant mixture.
2. Experimental details The expanding thermal plasma (ETP) for a-SiNx :H deposition from SiH4 –N2 has been described in detail in previous work [4,6] and here only the basics will be summarized. In the ETP, plasma creation, growth precursor formation, and deposition are spatially separated. An Ar–H2 –N2 plasma is created at high pressure (300 Torr) upstream in a plasma source and the plasma subsequently expands into a low pressure (150 mTorr) deposition reactor where SiH4 is admixed. The SiH4 is dissociated by the reactive species emanating from the plasma source and the growth precursors can deposit at a substrate holder positioned about 35 cm from the plasma source. The reactive species (electrons, ions, radicals) emanating from the plasma source have been investigated in detail and it has been concluded that the source acts mainly as an atomic N and H source while electrons and ions are basically unimportant for the SiH4 dissociation process [6]. Downstream radical densities have been determined by laser absorption spectroscopy and mass spectrometry as described in Ref. [6]. The radicals Si, SiH, and SiH3 have been investigated by cavity ringdown spectroscopy measurements at a position of 3.6 cm from the substrate holder while ground-state N radicals have been detected by threshold ionization mass spectrometry at the position of the substrate holder. From both techniques absolute densities in the plasma have been determined. Information about the growth process has also been obtained by considering the ‘incorporation flux’ of Si and N atoms into the film. This incorporation flux is the number of Si and N atoms incorporated into the film (in cm2 s1 ) and has been calculated from the product of the deposition rate and the Si and N atomic density in the film as determined by elastic recoil detection [4].
The radicals have been investigated under different plasma conditions and their typical densities in the plasma are given in Table 1. Although not all possible radicals have been measured, it will be shown below that the results on the Si, SiH, SiH3 , and N density yield enough insight to reveal the a-SiNx :H growth mechanism to a large extent. Furthermore, it has been concluded that ions have only a very small contribution to growth while no evidence has been found for a significant density of Si–N products or excited N2 molecules [6]. From Table 1 it is clear that both SiH3 and N have a very high density in the plasma which might imply that these radicals are the most important in the a-SiNx :H growth process. On the basis of the plasma source operation and our previous work on a-Si:H [7], we conclude that SiH3 is mainly created by H abstraction reactions from SiH4 by H atoms emanating from the plasma source. The N density in the plasma is very high which corroborates the fact that N radicals are very unreactive with SiH4 [8] and other species in the plasma. Consequently most N radicals that emanate from the plasma source survive in transit to the substrate holder. Our conclusion that SiH3 and N are the key precursors for a-SiNx :H growth is in agreement with the work of Smith et al. who inferred that N and SiHn radicals govern the growth process [2,3]. Moreover, our results imply that SiH3 is the dominant SiHn radical and not Si or SiH. In the next section we will discuss whether this conclusion is validated by the magnitude of the surface reactivity of the radicals.
3.2. Si and N incorporation flux and their sticking probability In case that SiH3 and N govern a-SiNx :H growth, it is expected that their plasma densities correlate with their incorporation flux into the a-SiNx :H film. Fig. 2(a) shows the SiH3 plasma density vs. the Si incorporation Table 1 The typical plasma densities of the radicals investigated and their measured or estimated sticking probabilities with their reference
N SiH3 SiH Si
Density (cm3 )
Sticking probability
Reference
5 · 1014 2 · 1013 8 · 1010 1010
0.007–0.04 0.15 0.98 1a
This work This work [11] [12]
See text for details. a The Si sticking probability is assumed unity on the basis of results for a-Si:H.
W.M.M. Kessels et al. / Journal of Non-Crystalline Solids 338–340 (2004) 37–41
Si incorporation flux (1016 cm-2s-1)
16 14 12 10 8 6 4 2 0 0
5
0
20
(a)
10 15 20 25 30 SiH3 density (1012 cm-3)
35
40
N incorporation flux (1016 cm-2s-1)
7 6 5 4 3 2 1 0 (b)
40
60
80
100
120
140
N density (1012 cm-3)
Fig. 2. (a) The Si incorporation flux into the a-SiNx :H film vs. the SiH3 density in the plasma, (b) the N incorporation flux into the a-SiNx :H film vs. the N density in the plasma. The data in the figures have been obtained by varying the SiH4 flow while keeping the N2 flow constant.
flux as obtained by varying the SiH4 flow while keeping the N2 flow constant. As expected, the figure shows a linear dependence which confirms the importance of SiH3 for growth. From the figure also the sticking probability of SiH3 can be estimated. As for a-Si:H we must to distinguish the sticking probability sSiH3 from the surface reaction probability bSiH3 because SiH3 can also have other reactions than sticking at the surface. Assuming sSiH3 ¼ 1=2bSiH3 analogous to a-Si:H [9] and a SiH3 thermal velocity of 1000 m/s [6] we find that sSiH3 0:15 (see Table 1). Although this is only a rough estimate -basically showing that sSiH3 is on the order of 0.1 for a-SiNx :H–it is exactly in the same range as for aSi:H [9]. The corresponding density and incorporation flux of N are given in Fig. 2(b). The figure shows that the N density in the plasma decreases when the incorporation flux increases. This striking behavior can perfectly be understood when realizing that this figure is based on
39
results in which the SiH4 flow was varied for a constant N2 flow. This means that the supply of N radicals is constant and consequently the N density drops when N is lost from the plasma by incorporation into the film. This implies also that the ‘sticking probability’ sN 1 of the N radicals varies with the plasma condition. Basically it means that sN depends on the Si incorporation flux with a lower limit imposed by the stochiometric limit (Si3 N4 ) of the films. The range of sN estimated from Fig. 2(b) is given in Table 1. The values of sN are apparently relatively low and even lower than the recombination probability of N at stainless steel (at the N2 partial pressure of 2 Pa) [10]. This also explains the particular behavior that the N density goes up when SiH4 is admixed to our plasma starting with a clean reactor [6]. Furthermore, the relatively low sN invalidates the hypothesis that the high surface roughness and poor step coverage of a-SiNx :H deposited from SiH4 – N2 are caused by a very high surface reactivity of the N atoms [3]. Apparently another mechanism is responsible for these observations. In Table 1 also the (estimated) sticking probabilities for Si and SiH are given [11,12]. Although these radicals have a very high sticking probability (almost unity) it is clear from their plasma density that they have only a very small contribution to film growth (0.5% and 5%, respectively). In summary, it has been shown that both the SiH3 and N density are directly correlated with the Si and N incorporation flux into the a-SiNx :H, and that the sticking probabilities deduced are in good agreement with the expected values and with other experimental observations. This provides strong evidence for the growth mechanism proposed. 3.3. Temperature dependence of the Si and N incorporation flux In our remote plasma the substrate temperature has no effect on the plasma chemistry and plasma radical densities. Consequently, the substrate temperature dependence of the Si and N incorporation flux is governed by the surface reactions. In Fig. 3 the Si and N incorporation flux are shown vs. the substrate temperature for two plasma settings. Although very small, the Si and N incorporation fluxes show a significantly different temperature dependence. This is in agreement with the fact that Si and N are brought to the surface by two separate growth precursors. The Si incorporation 1 When considering the incorporation flux, a sticking probability sN has to be defined with sN 6 cN and with cN being the surface reaction probability of N. Under non-deposition conditions cN is usually referred to the recombination probability. In this work we make the assumption that sN ¼ cN under deposition conditions. This assumption has no significant consequences for the final results.
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Incorporation flux (1016 cm-2s-1)
10
8
6
4
2
0 200
250
300 350 400 450 Substrate temperature (oC)
500
Fig. 3. The N (triangles) and Si (circles) incorporation flux into the aSiNx :H film for different substrate temperatures. Data from two different plasma settings are given and the data with the open symbols (with a lower deposition rate) has been scaled by a factor 2.3 for comparison purposes.
flux shows a very slight temperature dependence (activation energy ¼ 8 ± 1 meV when applying a simple Arrhenius expression) while the N incorporation flux is basically temperature independent (activation energy ¼ 1 ± 1 meV).
4. Discussion on the surface reactions during a-SiNx :H growth Although now sufficient evidence has been provided for the fact that SiH3 and N are the key precursors during a-SiNx :H growth from SiH4 –N2 , the surface reactions of SiH3 and N are not yet understood. The temperature (in)dependence of the Si and N incorporation flux form a good starting point in this respect. The Si incorporation flux has revealed only a very weak temperature dependence similar to the behavior of the Si incorporation flux during a-Si:H growth [9]. Furthermore, we have seen that the sticking probability of SiH3 on a-SiNx :H is about equal to or at least in the same order of magnitude as for a-Si:H. These similarities suggest, therefore, that the outermost surface layer of a-SiNx :H resembles the a-Si:H surface to a large extent. Therefore we can most probably assume that the SiH3 adsorption reactions on a-SiNx :H deposited from the SiH4 –N2 mixture are basically the same as for a-Si:H [13]. For the reaction of N with this a-Si:H-like surface we make use of the results obtained by density functional theory calculations for atomic N interacting with a Si(1 0 0)-2 · 1 surface as only recently reported by Ueno and Ornellas [14] and by Widjaja et al. [15]. Both groups considered the reaction of N with the dimerized surface
with as a first step the adsorption of N on one Si atom of the dimer. From this state they considered the insertion of the N atom into the Si–Si dimer but they both found that the formation of other structures was more likely. Widjaja et al. [15] found that the insertion of N into Si– Si backbonds by the formation of a Si–N–Si bridgebond is most likely with a total reaction enthalpy of )4.05 eV. A small activation energy of 0.48 eV is present however when progressing from the N adsorbed on the Si dimer atom to the Si–N–Si bridge-bond. A slight further decrease in energy (total reaction enthalpy )4.2 eV) has been predicted by the formation of a third bond by the N atom, i.e., NSi3 with the N atom bonded to three Si atoms as for perfect Si3 N4 . This involves a relatively high transition state energy (1.5 eV) however and therefore this reaction has been considered to be less important [15]. Uena and Ornellas reported only the NSi3 structure with no evidence for the Si–N–Si bridgebond. They found a total reaction enthalpy of )3.65 eV with a transition state energy of 0.60 eV when progressing from the N adsorbed on the Si dimer atom to the final NSi3 structure. These results are difficult to translate to the a-Si:H case in which case the surface is not fully terminated by dimers and also covered by H to a large extent. However, the overall exothermicity of the sub-surface reactions – forming either Si–N–Si or NSi3 – is on the order of 4 eV with enough energy available to overcome possible transition state energies. This suggests that similar reaction schemes should be possible for the a-Si:H-like surface. Moreover, it also explains the temperature-independence of the N incorporation flux for the a-SiNx :H because the transition states provide no significant energy barriers for the insertion reactions to occur. The relatively low sticking probability of N, in spite of the favorable energetics, imply that the surface reactions by N are limited by the available reaction sites (related to the Si incorporation flux) as is also suggested by the results in Fig. 2(b). We propose therefore that the nitridation by N radicals takes place by sub-surface insertion reactions into the Si–Si backbonds while leaving the outermost a-Si:H-like surface layer virtually intact as sketched schematically in Fig. 1.
5. Conclusions The growth mechanism of a-SiNx :H from the SiH4 – N2 reactant mixture as proposed by Smith et al. [2,3] has been corroborated and further refined. It has been concluded that a-SiNx :H growth takes place mainly by SiH3 radicals, that create an a-Si:H-like outermost surface layer on the a-SiNx :H, and by N radicals that insert into Si–Si backbonds of this a-Si:H-like surface layer.
W.M.M. Kessels et al. / Journal of Non-Crystalline Solids 338–340 (2004) 37–41
Acknowledgements This work has been carried out within the ‘HR-Cel’ project which is part of the E.E.T.-program of the Netherlands Ministry of Economic Affairs, the Ministry of Education, Culture and Science and the Ministry of Public Housing, Physical Planning and Environment. The research of W.M.M.K. has been made possible by a fellowship of the Royal Netherlands Academy of Arts and Sciences (KNAW).
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