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Transmission electron microscopy study of silicon nitride amorphous films obtained by reactive pulsed laser deposition
Thin Solid Films 397 (2001) 12–16
Transmission electron microscopy study of silicon nitride amorphous films obtained by reactive pulsed laser deposition V.S. Teodorescua, L.C. Nistora, M. Popescua, I.N. Mihailescub,*, E. Gyorgyb, J. Van Landuytc, A. Perroned a National Institute for Materials Physics, P.O. Box MG-7, RO-76900 Bucharest, Romania Lasers Department, National Institute for Laser, Plasma and Radiation Physics, P.O. Box MG-54, RO-76900, Bucharest V, Romania c University of Antwerpen, RUCA-EMAT, Groenenborgenlaan 171, B-2020, Antwerp, Belgium d University of Lecce and INFM, Department of Physics, 73100 Lecce, Italy
b
Received 15 March 2000; received in revised form 16 May 2001; accepted 21 July 2001
Abstract In situ decomposition of silicon nitride films was observed by high-resolution electron microscopy. The films, which were produced by reactive pulsed laser deposition from Si wafer targets at 50–100 Pa ammonia pressure, had a prevalent content of hydrogen-doped, amorphous, non-stoichiometric silicon nitride. A layered morphology of the film, consisting of a density variation in the amorphous structure, developed under electron beam irradiation. This morphology became evident only in cross-sectional observation and was related to the stress-relaxation effect, based on a rearrangement of the bonds. We presume that the stress field anisotropy in the amorphous structure must be related to the presence of some bond texture in the as-grown, amorphous film. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Amorphous material; Electron microscopy; Radiation damage; Silicon nitride
1. Introduction Amorphous silicon nitride has become an interesting material for various optoelectronic applications w1,2x. Such films are grown by either reactive sputtering w3x or reactive pulsed-laser deposition (RPLD) w4x. Amorphous non-stoichiometric silicon nitride, silicon oxynitride and silicon carbonitride also show interesting properties for high technology applications w5,6x. The structure of the amorphous non-stoichiometric silicon nitride SiNx is unstable at high temperatures. Such instability is higher in the case of hydrogenated silicon nitride SiNx:H phases w7–9x. We have already conducted investigations on silicon nitride thin films deposited by RPLD from Si wafer targets in low-pressure NH3 w10,11x. In this particular * Corresponding author. Tel.: q40-17805385; fax: q40-14231791. E-mail address: [email protected] (I.N. Mihailescu).
case, it was still unclear exactly when and where the silicon nitride synthesis took place. The main contribution to the process occurred on the collector surface, but the laser–target interaction and plasma plume stages were also involved w12,13x. We noted, however, that SiNx thin films obtained under otherwise identical conditions exhibited slight composition changes after different electron microscope investigations. Some islands with lower N2 content were also found. We therefore linked these changes to the peculiarities of electron beam interaction with the structures obtained during electron microscopy observation. We then conducted careful in situ studies of the decomposition of the SiNx:H amorphous films as a result of electron beam irradiation during observation by transmission electron microscopy (TEM). The results of these studies are reported here, together with a discussion concerning the physical phenomena involved.
0040-6090/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 4 0 8 - 0
V.S. Teodorescu et al. / Thin Solid Films 397 (2001) 12–16
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2. Experimental The SiNx thin films were produced by pulsed laser deposition using an excimer XeCl* laser source w10,11x (ls308 nm, tFWHMF30 ns). Prior to each irradiation series, the reaction chamber was evacuated down to 10y5 Pa and then filled with ammonia at a dynamic pressure within the range 50–100 Pa. In order to eliminate any contamination of the collector by the impurities still present on the target surface after chemical cleaning, a disk shield was introduced between the target and the collector and a pre-irradiation treatment was applied. We directed 1000 consecutive laser pulses while the target was rotated. In this way, the collector was shielded from the initial ablated flux, which possibly contained impurities (usually oxygen, carbon and other adsorbates). A total of 104 laser pulses were applied in succession to a Si target for the deposition of one film. They succeeded one another at a 10 Hz frequency repetition rate. The incident laser fluence was set at 5 J cmy2. The laser beam was incident at an angle of approximately 458 on the rotating w100xSi wafer target. The ablated substance was collected on a w100xSi wafer parallel to the target at a distance of 21 mm from it. The thickness of the deposited film measured by profilometry was approximately 400 nm. Specimens for cross-sectional observation (XTEM) were prepared by the conventional method. The sample was cut into small 2=0.5=0.3 mm3 pieces, which were glued from the film side with M-bond. After mechanical polishing, the final ion milling was carried out in a Baltec installation. The ion milling preparation of the plane-view specimens revealed the presence of a very high stress field in the silicon nitride films. Specimens often broke close to the transparency thickness. However, some transparent fragments were successfully used for TEM observation. Transmission electron microscopes Jeol 200CX and CM 20 were used for the investigations. The electron current density on the sample ranged from 10 to 50 A cmy2. This was estimated from the characteristic parameters in the case of the CM 20 microscope, and measured with a Faraday gauge in the case of the Jeol 200CX microscope. 3. Results As mentioned in our previous papers w10,11x and confirmed by our latest investigations, the films deposited in a 50–100 Pa ammonia atmosphere predominantly consisted of amorphous non-stoichiometric silicon nitride. These films were uniform and strongly adherent to the substrate. Our previous IR spectrometry and Xray photoelectron spectroscopy (XPS) studies revealed a significant hydrogen content in all of the structures deposited w10x.
Fig. 1. XTEM images of the SiNx film deposited by RPLD from a Si target in 100 Pa NH3. It shows the lamellar morphology as revealed by the decomposition of the film under electron beam action. (a) Image recorded during the first min of observation in the microscope; (b) the same image taken after 2 min of observation at an electron current density of approximately 50 A cmy2. The recording time for each image was 2 s.
A new important feature revealed by the XTEM studies were the formation of a lamellar morphology of the amorphous film as a result of electron beam irradiation in the electron microscope. During TEM observations, lamellae of several nm in thickness parallel to the film surface were formed as a result of local decomposition of the amorphous films. Fig. 1 shows a cross-sectional (XTEM) image of the film, where the lamellar morphology evolution during electron beam irradiation can be observed. The lamellae arising in the film are in fact zones with different densities of the amorphous structure. At a given depth inside the film (corresponding to a certain deposition sequence), the decomposition proceeds identically, producing the same local structure density, and eventually the lamellar morphology. The interface between the SiNx deposited film and the Si substrate is visible in the XTEM high-resolution image given in Fig. 2. A low-density layer, approximately 4 nm thick, always develops at the collector y film interface. The next layer (approx. 8 nm thick) always has a higher density. In spite of the low-density
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Fig. 3. (a) Nanovoid formation (arrowed) in the low-density zones of the amorphous structure after prolonged irradiation; and (b) blistering of these zones by growth and percolation of the voids.
Fig. 2. High-resolution XTEM image showing the filmysubstrate interface. The SAED pattern inserted shows the amorphous halo of the film and the reflection spots of the w110x orientation of the silicon substrate. Images are taken after approximately 4 min of observation at 50 A cmy2 of electron current density.
structure at the interface, the contact of the film to the Si substrate remains rather strong. This is also demonstrated by the stress field developing inside the Si substrate near the interface (indicated by small arrows in Fig. 1b and Fig. 2). With further electron beam irradiation, the decomposition continues, with the formation of nanovoids followed by blistering, indicating loss of gas from the film. These features are visible in Fig. 3. We note that in plane-view TEM observation, the lamellar morphology was not apparent, even if electron irradiation was applied under similar conditions. However, an interesting feature was revealed by comparing the selected-area electron diffraction (SAED) patterns taken on the same sample in plane view and crosssectional observations (Fig. 4). Even though these SAED patterns were recorded under similar conditions of focalization and specimen thickness, they were not identical in the two cases. Indeed, the plane-view SAED pattern exhibited halo reflections centered around 0.32 and 0.205 nm (Fig. 4a), while that analyzed by XTEM showed a very large first maximum extending from 0.4 to 0.25 nm and a second one located at 0.135 nm (Fig. 4b). Both SAED patterns were taken during the 1st min of observation. After prolonged (longer than 3 min) electron beam irradiation, the SAED patterns slowly changed to patterns corresponding to amorphous silicon. This is clearly evident in the SAED pattern inserted in Fig. 2, which is similar to the SAED in Fig. 4b, but was recorded after several min of electron beam irradiation. In this pattern, the amorphous maxima are located in the positions of (111) and (220) lattice reflections of silicon. This demonstrates that during decomposition, the struc-
ture deposited evolves into amorphous silicon clusters. Other authors w14x who studied similar PLD films by XPS reported that such Si clusters might be present inside the film structure right from the start. 4. Discussion We first noted that the lamellar morphology of the amorphous films, caused by the electron beam irradiation decomposition, revealed an interesting behavior characteristic of the SiNx films deposited by RPLD from Si targets in a low-pressure NH3 atmosphere. This can be related to the peculiarities of the nitridation process. The thermal decomposition of non-stoichiometric SiNx:H films is known to start upwards of 1000 K w7,8x. Other experiments showed that the Si–H bond within SiNx:H films breaks at approximately 600 K w9x. In our opinion, such a temperature cannot actually be achieved within the irradiated zone of the specimen during TEM observation. Our studies indicated that on reducing the electron beam intensity, the decomposition only slowed
Fig. 4. SAED patterns of the amorphous structures recorded during the 1st min of TEM observation: (a) plane-view SAED pattern; and (b) cross-section SAED pattern taken near the Siyfilm interface.
V.S. Teodorescu et al. / Thin Solid Films 397 (2001) 12–16
Fig. 5. Distribution of the widths of the high-density layers in the initial film decomposition stage (see Fig. 1a). The Gaussian fitting curve is superimposed. The average width value is 12.4 nm.
down and no irradiation threshold behavior could be defined. This effect was, in our opinion, essentially an irradiation-assisted thermal decomposition process, similar to that observed in the case of polymers w15x. As has been pointed out before, plane-view TEM observation does not show the layered morphology of the irradiated amorphous structure. This means there is no relation between the direction of the electron beam irradiation and the orientation of the lamellar morphology. We note that the lamellar morphology of the film could not have been revealed by electron irradiation unless some anisotropy was present in the as-grown film structure. Such a feature can be based on the nature and texture of the atoms bonded in the network. This leads to the initial stress anisotropy distribution in the asgrown film structure. We consider that this anisotropy can result from the action of two random factors: a fluctuation in the deposition parameters during the RPLD process, and a random intensity of the localized stress in the film thickness. Fig. 5 shows the distribution of widths of the high-density layers observed in the decomposition patterns (see Fig. 1a) during the initial stage of irradiation. The Gaussian shape of this distribution sustains the notion that the lamellar decomposition originates in a random relaxation process of the anisotropic stress that can be found in the as-grown amorphous films. Nevertheless, a statistical bonding texture has to be present in the film beforehand to support the development of such a high-stress anisotropy. Since the collector temperature is rather low (only 500 K w8x), the surface diffusion and bond species rearrangement take place within a very short time lapse after the plasma flux species arrive on the collector
15
surface. Unlike RF sputtering deposition, which is a continuous process, in the case of PLD and RPLD, the flux species density is very high, but only for a short time. The surface diffusion and bond formation start sometime after the laser pulse ends. This ‘hot’ period seems to play an essential part in the film formation process. The bonds existing in the film are of the types: Si–Si; Si–N; Si–NHx (xs1,2,3); and Si–H. We mention that impurities, such as SiO2, play a role in the film synthesis as well. We further suggest the stress anisotropy and bond texture formation in the film using a simple model of the film construction sequence. Thus, every laser pulse provides a species flux pulse of approximately 1014–1015 species cmy2 in a time window determined by the laser pulse duration (30 ns), contributing to the film growth by approximately 0.04 nm. In the time lapse between two sequential laser pulses, the surface of the film is thermalised and a fresh layer of NH3 molecules is chemisorbed. This chemisorbed ammonia layer is partly removed and partly used for film growth by the next species flux pulse. Consequently, a large amount of NHx species are incorporated in the SiNx:H film. However, there seems to be no reason why this scenario should create any texture in the spatial distribution of the film bonding. We note that approximately 10 pulses are necessary to cover the film surface with a new layer having a thickness of the size of a Si4 or SiNx tetrahedron. For this low rate of growth, we also cannot expect the formation of any bond composition anisotropy. According to a second scenario, each species flux pulse contributing to the film growth is involved in a certain rearrangement of the already existing film surface. In this case, the film will grow in thickness only after a maximum density bonding is obtained within the film plane. In this way, a texture of the bond system and a stress anisotropy can be created in the film. The difference between the experimental SAED patterns obtained in plane view and in cross-section is qualitative proof of the existence of a bond system texture in the film. These models are not specifically connected with the SiNx:H system for amorphous structure formation. However, the electron beam irradiation has no similar effects in the case of amorphous CNx w16x thin films obtained by RPLD. This cannot prove that the amorphous structure of these films is different from that of SiNx films. It only shows the particular sensitivity to electron irradiation of the SiNx:H film structure, thus revealing the anisotropy of the initial structure. It is interesting to note that amorphous SiCxNy films prepared by reactive sputtering of SiC in N2 and Ar are stable in the electron beam during TEM observation w6x. This is certainly a consequence of the fact that CNx thin films have very strong bonds and that the activation energy for the C–N, C–H and N–H bonds differs very much from the
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activation energy for SiNx:H films. We also observed that there are probably no efficient mechanisms to induce stress anisotropy during film growth by sputtering; this also suggests that in our case, the presence of hydrogen in the amorphous structure is the main factor inducing sensitivity to electron irradiation.
participate in this work. Eniko¨ Gyorgy acknowledges with thanks the financial support of CNR in the framework of the bilateral agreement with the Romanian Ministry of Research and Technology and to the Istituto Nazionale per la Fisica della Materia for a research contract which ensured her participation in the work.
5. Conclusions
References
We have demonstrated the existence of a high-stress anisotropy in the amorphous structure of the SiNx:H thin films obtained by RPLD from Si targets in low-pressure NH3. This anisotropy is supported by the presence of a bond texture in the film that was revealed by the differences observed between the SAED patterns obtained in plane-view and in cross-sections of the film. The electron beam irradiation in the microscope induces a lamellar decomposition morphology through random relaxation of the stress and bond rearrangement. This feature is characteristic of the SiNx:H system and has not been observed in the TEM studies of similar amorphous films (CNx) deposited under similar conditions; this is most probably due to the fact that SiNx:H films are more sensitive to electron irradiation. We consider that the evidence observed is of practical significance for the application of the SiNx structures obtained by PLD, RPLD after prolonged exposure to electron beam irradiation — e.g. in electron microscopes. Acknowledgements Valentin S. Teodorescu is grateful to the Belgian Government Prime Minister’s Office of Sciences policy programming for a fellowship at the University of Antwerpen RUCA-EMAT, where this research was partly conducted. Ion N. Mihailescu thanks the Consiglio Nationale della Ricerca (CNR) for financial support through a CNR-NATO fellowship which enabled him to
w1x M.A. Green, Silicon Solar Cells, Advanced Principles and Practice, Prentice-Hall, Englewood Cliffs, 1995. w2x S.V. Deshpande, E. Gulari, S.W. Brown, S.C. Rand, J. Appl. Phys. 77 (1995) 6534. w3x M. Vetter, Thin Solid Films 337 (1999) 118. w4x D.G. Chrisey, G.K. Hubler (Eds.), Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994. w5x K. Maruyama, Y.A. Oki, M. Matsumoto, Y. Hiroshima, H. Ohta, Appl. Surf. Sci. 96–98 (1996) 764. w6x X.C. Xiao, Y.W. Li, L.X. Song, X.F. Peng, X.F. Hu, Appl. Surf. Sci. 156 (2000) 155. w7x L.V. Chramova, T.P. Chusova, G.A. Kokovin, Thin Solid Films 147 (1987) 267. w8x G.V. Gadiyak, V.G. Gadiyak, M.L. Kosinova, E.G. Salman, Thin Solid Films 335 (1998) 19. w9x W.L. Warren, C.H. Singer, J. Robertson, J. Kanicki, E.H. Poindexter, J. Electrochem. Soc. 143 (1996) 3685. w10x I.N. Mihailescu, A. Lita, V.S. Teodorescu, E. Gyorgy, R. Alexandrescu, A. Luches, M. Martino, A. Barborica, J. Vac. Sci. Technol. A 14 (1996) 1986. w11x I.N. Mihailescu, A. Lita, V.S. Teodorescu, A. Luches, M. Martino, A. Perrone, M. Gartner, J. Mater. Sci. 31 (1996) 2839. w12x V.S. Teodorescu, L.C. Nistor, J. Van Landuyt, I.N. Mihailescu, M. Dinescu, A. Luches, A. Perrone, M. Martino, Proceedings of EUREM 96, the 11th European Congress on Electron Microscopy, Dublin, Ireland, 26–30 August 1996, II, 1996, p. 724. w13x I.N. Mihailescu, E. Gyorgy, V.S. Teodorescu, G. Steinbrecker, J. Neamtu, A. Perrone, A. Luches, J. Appl. Phys. 86 (1999) 7123. w14x C.K. Choo, T. Sakamoto, K. Tanaka, R. Nakata, Appl. Surf. Sci. 148 (1999) 116. w15x L. Reimer, Transmission Electron Microscopy, Springer Verlag, 1984, p. 21. w16x I.N. Mihailescu, E. Gyorgy, R. Alexandrescu, A. Luches, A. Perrone, C. Ghica, J. Werckmann, I. Cojocaru, V. Chumash, Thin Solid Films 323 (1998) 72.
Transmission electron microscopy study of silicon nitride amorphous films obtained by reactive pulsed laser deposition V.S. Teodorescua, L.C. Nistora, M. Popescua, I.N. Mihailescub,*, E. Gyorgyb, J. Van Landuytc, A. Perroned a National Institute for Materials Physics, P.O. Box MG-7, RO-76900 Bucharest, Romania Lasers Department, National Institute for Laser, Plasma and Radiation Physics, P.O. Box MG-54, RO-76900, Bucharest V, Romania c University of Antwerpen, RUCA-EMAT, Groenenborgenlaan 171, B-2020, Antwerp, Belgium d University of Lecce and INFM, Department of Physics, 73100 Lecce, Italy
b
Received 15 March 2000; received in revised form 16 May 2001; accepted 21 July 2001
Abstract In situ decomposition of silicon nitride films was observed by high-resolution electron microscopy. The films, which were produced by reactive pulsed laser deposition from Si wafer targets at 50–100 Pa ammonia pressure, had a prevalent content of hydrogen-doped, amorphous, non-stoichiometric silicon nitride. A layered morphology of the film, consisting of a density variation in the amorphous structure, developed under electron beam irradiation. This morphology became evident only in cross-sectional observation and was related to the stress-relaxation effect, based on a rearrangement of the bonds. We presume that the stress field anisotropy in the amorphous structure must be related to the presence of some bond texture in the as-grown, amorphous film. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Amorphous material; Electron microscopy; Radiation damage; Silicon nitride
1. Introduction Amorphous silicon nitride has become an interesting material for various optoelectronic applications w1,2x. Such films are grown by either reactive sputtering w3x or reactive pulsed-laser deposition (RPLD) w4x. Amorphous non-stoichiometric silicon nitride, silicon oxynitride and silicon carbonitride also show interesting properties for high technology applications w5,6x. The structure of the amorphous non-stoichiometric silicon nitride SiNx is unstable at high temperatures. Such instability is higher in the case of hydrogenated silicon nitride SiNx:H phases w7–9x. We have already conducted investigations on silicon nitride thin films deposited by RPLD from Si wafer targets in low-pressure NH3 w10,11x. In this particular * Corresponding author. Tel.: q40-17805385; fax: q40-14231791. E-mail address: [email protected] (I.N. Mihailescu).
case, it was still unclear exactly when and where the silicon nitride synthesis took place. The main contribution to the process occurred on the collector surface, but the laser–target interaction and plasma plume stages were also involved w12,13x. We noted, however, that SiNx thin films obtained under otherwise identical conditions exhibited slight composition changes after different electron microscope investigations. Some islands with lower N2 content were also found. We therefore linked these changes to the peculiarities of electron beam interaction with the structures obtained during electron microscopy observation. We then conducted careful in situ studies of the decomposition of the SiNx:H amorphous films as a result of electron beam irradiation during observation by transmission electron microscopy (TEM). The results of these studies are reported here, together with a discussion concerning the physical phenomena involved.
0040-6090/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 4 0 8 - 0
V.S. Teodorescu et al. / Thin Solid Films 397 (2001) 12–16
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2. Experimental The SiNx thin films were produced by pulsed laser deposition using an excimer XeCl* laser source w10,11x (ls308 nm, tFWHMF30 ns). Prior to each irradiation series, the reaction chamber was evacuated down to 10y5 Pa and then filled with ammonia at a dynamic pressure within the range 50–100 Pa. In order to eliminate any contamination of the collector by the impurities still present on the target surface after chemical cleaning, a disk shield was introduced between the target and the collector and a pre-irradiation treatment was applied. We directed 1000 consecutive laser pulses while the target was rotated. In this way, the collector was shielded from the initial ablated flux, which possibly contained impurities (usually oxygen, carbon and other adsorbates). A total of 104 laser pulses were applied in succession to a Si target for the deposition of one film. They succeeded one another at a 10 Hz frequency repetition rate. The incident laser fluence was set at 5 J cmy2. The laser beam was incident at an angle of approximately 458 on the rotating w100xSi wafer target. The ablated substance was collected on a w100xSi wafer parallel to the target at a distance of 21 mm from it. The thickness of the deposited film measured by profilometry was approximately 400 nm. Specimens for cross-sectional observation (XTEM) were prepared by the conventional method. The sample was cut into small 2=0.5=0.3 mm3 pieces, which were glued from the film side with M-bond. After mechanical polishing, the final ion milling was carried out in a Baltec installation. The ion milling preparation of the plane-view specimens revealed the presence of a very high stress field in the silicon nitride films. Specimens often broke close to the transparency thickness. However, some transparent fragments were successfully used for TEM observation. Transmission electron microscopes Jeol 200CX and CM 20 were used for the investigations. The electron current density on the sample ranged from 10 to 50 A cmy2. This was estimated from the characteristic parameters in the case of the CM 20 microscope, and measured with a Faraday gauge in the case of the Jeol 200CX microscope. 3. Results As mentioned in our previous papers w10,11x and confirmed by our latest investigations, the films deposited in a 50–100 Pa ammonia atmosphere predominantly consisted of amorphous non-stoichiometric silicon nitride. These films were uniform and strongly adherent to the substrate. Our previous IR spectrometry and Xray photoelectron spectroscopy (XPS) studies revealed a significant hydrogen content in all of the structures deposited w10x.
Fig. 1. XTEM images of the SiNx film deposited by RPLD from a Si target in 100 Pa NH3. It shows the lamellar morphology as revealed by the decomposition of the film under electron beam action. (a) Image recorded during the first min of observation in the microscope; (b) the same image taken after 2 min of observation at an electron current density of approximately 50 A cmy2. The recording time for each image was 2 s.
A new important feature revealed by the XTEM studies were the formation of a lamellar morphology of the amorphous film as a result of electron beam irradiation in the electron microscope. During TEM observations, lamellae of several nm in thickness parallel to the film surface were formed as a result of local decomposition of the amorphous films. Fig. 1 shows a cross-sectional (XTEM) image of the film, where the lamellar morphology evolution during electron beam irradiation can be observed. The lamellae arising in the film are in fact zones with different densities of the amorphous structure. At a given depth inside the film (corresponding to a certain deposition sequence), the decomposition proceeds identically, producing the same local structure density, and eventually the lamellar morphology. The interface between the SiNx deposited film and the Si substrate is visible in the XTEM high-resolution image given in Fig. 2. A low-density layer, approximately 4 nm thick, always develops at the collector y film interface. The next layer (approx. 8 nm thick) always has a higher density. In spite of the low-density
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Fig. 3. (a) Nanovoid formation (arrowed) in the low-density zones of the amorphous structure after prolonged irradiation; and (b) blistering of these zones by growth and percolation of the voids.
Fig. 2. High-resolution XTEM image showing the filmysubstrate interface. The SAED pattern inserted shows the amorphous halo of the film and the reflection spots of the w110x orientation of the silicon substrate. Images are taken after approximately 4 min of observation at 50 A cmy2 of electron current density.
structure at the interface, the contact of the film to the Si substrate remains rather strong. This is also demonstrated by the stress field developing inside the Si substrate near the interface (indicated by small arrows in Fig. 1b and Fig. 2). With further electron beam irradiation, the decomposition continues, with the formation of nanovoids followed by blistering, indicating loss of gas from the film. These features are visible in Fig. 3. We note that in plane-view TEM observation, the lamellar morphology was not apparent, even if electron irradiation was applied under similar conditions. However, an interesting feature was revealed by comparing the selected-area electron diffraction (SAED) patterns taken on the same sample in plane view and crosssectional observations (Fig. 4). Even though these SAED patterns were recorded under similar conditions of focalization and specimen thickness, they were not identical in the two cases. Indeed, the plane-view SAED pattern exhibited halo reflections centered around 0.32 and 0.205 nm (Fig. 4a), while that analyzed by XTEM showed a very large first maximum extending from 0.4 to 0.25 nm and a second one located at 0.135 nm (Fig. 4b). Both SAED patterns were taken during the 1st min of observation. After prolonged (longer than 3 min) electron beam irradiation, the SAED patterns slowly changed to patterns corresponding to amorphous silicon. This is clearly evident in the SAED pattern inserted in Fig. 2, which is similar to the SAED in Fig. 4b, but was recorded after several min of electron beam irradiation. In this pattern, the amorphous maxima are located in the positions of (111) and (220) lattice reflections of silicon. This demonstrates that during decomposition, the struc-
ture deposited evolves into amorphous silicon clusters. Other authors w14x who studied similar PLD films by XPS reported that such Si clusters might be present inside the film structure right from the start. 4. Discussion We first noted that the lamellar morphology of the amorphous films, caused by the electron beam irradiation decomposition, revealed an interesting behavior characteristic of the SiNx films deposited by RPLD from Si targets in a low-pressure NH3 atmosphere. This can be related to the peculiarities of the nitridation process. The thermal decomposition of non-stoichiometric SiNx:H films is known to start upwards of 1000 K w7,8x. Other experiments showed that the Si–H bond within SiNx:H films breaks at approximately 600 K w9x. In our opinion, such a temperature cannot actually be achieved within the irradiated zone of the specimen during TEM observation. Our studies indicated that on reducing the electron beam intensity, the decomposition only slowed
Fig. 4. SAED patterns of the amorphous structures recorded during the 1st min of TEM observation: (a) plane-view SAED pattern; and (b) cross-section SAED pattern taken near the Siyfilm interface.
V.S. Teodorescu et al. / Thin Solid Films 397 (2001) 12–16
Fig. 5. Distribution of the widths of the high-density layers in the initial film decomposition stage (see Fig. 1a). The Gaussian fitting curve is superimposed. The average width value is 12.4 nm.
down and no irradiation threshold behavior could be defined. This effect was, in our opinion, essentially an irradiation-assisted thermal decomposition process, similar to that observed in the case of polymers w15x. As has been pointed out before, plane-view TEM observation does not show the layered morphology of the irradiated amorphous structure. This means there is no relation between the direction of the electron beam irradiation and the orientation of the lamellar morphology. We note that the lamellar morphology of the film could not have been revealed by electron irradiation unless some anisotropy was present in the as-grown film structure. Such a feature can be based on the nature and texture of the atoms bonded in the network. This leads to the initial stress anisotropy distribution in the asgrown film structure. We consider that this anisotropy can result from the action of two random factors: a fluctuation in the deposition parameters during the RPLD process, and a random intensity of the localized stress in the film thickness. Fig. 5 shows the distribution of widths of the high-density layers observed in the decomposition patterns (see Fig. 1a) during the initial stage of irradiation. The Gaussian shape of this distribution sustains the notion that the lamellar decomposition originates in a random relaxation process of the anisotropic stress that can be found in the as-grown amorphous films. Nevertheless, a statistical bonding texture has to be present in the film beforehand to support the development of such a high-stress anisotropy. Since the collector temperature is rather low (only 500 K w8x), the surface diffusion and bond species rearrangement take place within a very short time lapse after the plasma flux species arrive on the collector
15
surface. Unlike RF sputtering deposition, which is a continuous process, in the case of PLD and RPLD, the flux species density is very high, but only for a short time. The surface diffusion and bond formation start sometime after the laser pulse ends. This ‘hot’ period seems to play an essential part in the film formation process. The bonds existing in the film are of the types: Si–Si; Si–N; Si–NHx (xs1,2,3); and Si–H. We mention that impurities, such as SiO2, play a role in the film synthesis as well. We further suggest the stress anisotropy and bond texture formation in the film using a simple model of the film construction sequence. Thus, every laser pulse provides a species flux pulse of approximately 1014–1015 species cmy2 in a time window determined by the laser pulse duration (30 ns), contributing to the film growth by approximately 0.04 nm. In the time lapse between two sequential laser pulses, the surface of the film is thermalised and a fresh layer of NH3 molecules is chemisorbed. This chemisorbed ammonia layer is partly removed and partly used for film growth by the next species flux pulse. Consequently, a large amount of NHx species are incorporated in the SiNx:H film. However, there seems to be no reason why this scenario should create any texture in the spatial distribution of the film bonding. We note that approximately 10 pulses are necessary to cover the film surface with a new layer having a thickness of the size of a Si4 or SiNx tetrahedron. For this low rate of growth, we also cannot expect the formation of any bond composition anisotropy. According to a second scenario, each species flux pulse contributing to the film growth is involved in a certain rearrangement of the already existing film surface. In this case, the film will grow in thickness only after a maximum density bonding is obtained within the film plane. In this way, a texture of the bond system and a stress anisotropy can be created in the film. The difference between the experimental SAED patterns obtained in plane view and in cross-section is qualitative proof of the existence of a bond system texture in the film. These models are not specifically connected with the SiNx:H system for amorphous structure formation. However, the electron beam irradiation has no similar effects in the case of amorphous CNx w16x thin films obtained by RPLD. This cannot prove that the amorphous structure of these films is different from that of SiNx films. It only shows the particular sensitivity to electron irradiation of the SiNx:H film structure, thus revealing the anisotropy of the initial structure. It is interesting to note that amorphous SiCxNy films prepared by reactive sputtering of SiC in N2 and Ar are stable in the electron beam during TEM observation w6x. This is certainly a consequence of the fact that CNx thin films have very strong bonds and that the activation energy for the C–N, C–H and N–H bonds differs very much from the
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activation energy for SiNx:H films. We also observed that there are probably no efficient mechanisms to induce stress anisotropy during film growth by sputtering; this also suggests that in our case, the presence of hydrogen in the amorphous structure is the main factor inducing sensitivity to electron irradiation.
participate in this work. Eniko¨ Gyorgy acknowledges with thanks the financial support of CNR in the framework of the bilateral agreement with the Romanian Ministry of Research and Technology and to the Istituto Nazionale per la Fisica della Materia for a research contract which ensured her participation in the work.
5. Conclusions
References
We have demonstrated the existence of a high-stress anisotropy in the amorphous structure of the SiNx:H thin films obtained by RPLD from Si targets in low-pressure NH3. This anisotropy is supported by the presence of a bond texture in the film that was revealed by the differences observed between the SAED patterns obtained in plane-view and in cross-sections of the film. The electron beam irradiation in the microscope induces a lamellar decomposition morphology through random relaxation of the stress and bond rearrangement. This feature is characteristic of the SiNx:H system and has not been observed in the TEM studies of similar amorphous films (CNx) deposited under similar conditions; this is most probably due to the fact that SiNx:H films are more sensitive to electron irradiation. We consider that the evidence observed is of practical significance for the application of the SiNx structures obtained by PLD, RPLD after prolonged exposure to electron beam irradiation — e.g. in electron microscopes. Acknowledgements Valentin S. Teodorescu is grateful to the Belgian Government Prime Minister’s Office of Sciences policy programming for a fellowship at the University of Antwerpen RUCA-EMAT, where this research was partly conducted. Ion N. Mihailescu thanks the Consiglio Nationale della Ricerca (CNR) for financial support through a CNR-NATO fellowship which enabled him to
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