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Surface exfoliation and defect structures in Si induced by 160 keV He and 110 keV H ion implantation
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NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 250–255 www.elsevier.com/locate/nimb
Surface exfoliation and defect structures in Si induced by 160 keV He and 110 keV H ion implantation Zhuo Wang a, Changlong Liu a,b,*, Tianyu Liu a, Xiaodong Zhang a, Wenrun Li a, Wenxia Li a,b, Bing Yuan a, Pei Wu a, Mengkai Li a b
a School of Science, Tianjin University, Tianjin 300072, PR China Tianjin Key Laboratory of Low Dimension Materials Physics and Preparing Technology, Institute of Advanced Materials Physics, Faculty of Science, Tianjin 300072, PR China
Received 28 August 2007; received in revised form 9 November 2007 Available online 19 November 2007
Abstract Cz n-type Si(100) wafers were implanted at room temperature with 160 keV He ions at a fluence of 5 1016/cm2 and 110 keV H ions at a fluence of 1 1016/cm2, singly or in combination. Surface phenomena and defect microstructures have been studied by various techniques, including scanning electron microscopy (SEM), atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (XTEM). Surface exfoliation and flaking phenomena were only observed on silicon by successive implantation of He and H ions after subsequent annealing at temperatures above 400 °C. The surface phenomena show strong dependence on the thermal budget. At annealing temperatures ranging from 500 to 700 °C, craters with size of about 10 lm were produced throughout the silicon surface. As increasing temperature to 800 °C, most of the implanted layer was sheared, leaving structures like islands on the surface. AFM observations have demonstrated that the implanted layer is mainly transfered at the depth around 960 nm, which is quite consistent with the range of the ions. XTEM observations have revealed that the additional low fluence H ion implantation could significantly influence thermal growth of He-cavities, which gives rise to a monolayer of cavities surrounded by a large amount of dislocations and strain. The surface exfoliation effects have been tentatively interpreted in combination of AFM and XTEM results. Ó 2007 Elsevier B.V. All rights reserved. PACS: 61.72.Ff; 61.72.Qq Keywords: Silicon; He and H ion implantation; Surface exfoliation; Cavities; XTEM
1. Introduction Recently, surface blistering, exfoliation together with bubble growth in silicon by light gas ion implantation have been paid more and more attention due to their proven capability for high quality heterogeneous materials integration in the fabrication of micro- and nanoscale devices [1]. Hydrogen implantation has been often used to shear and transfer a thin silicon layer in implanted
*
Corresponding author. Address: School of Science, Tianjin University, Tianjin 300072, PR China. Tel./fax: +86 022 27403425. E-mail address: [email protected] (C. Liu). 0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.11.005
wafer onto a supporting wafer after bonding the two wafers together and subsequent annealing [1,2]. The technology of smart-cut based on it has been built up to produce high quality silicon-on-insulator (SOI) wafers more economically than the competing processes such as separation by implantation of oxygen (SIMOX) and bond-andetch-back silicon-on-insulator (BESOI) [3]. Generally, it has been considered that the thin film separation process by H implantation mainly proceeds through both chemical interaction (bond breaking, internal surface passivation) and physical interaction (gas coalescence, pressure and fracture) in the silicon substrate [4]. However, it is much difficult to elucidate the contribution of each component to the overall process.
Z. Wang et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 250–255
High fluence He implantation into semiconductor or metal materials also gives rise to bubble formation in the bulk followed by high temperature annealing [5,6]. However, splitting by He implantation singly requires a quite high fluence (2 1017/cm2) [7]. As compared with H implantation, helium does not have chemical interaction in silicon substrate, and thus only physical interaction similar to hydrogen occurs. Therefore, coimplantation of He and H ions into Si should be a good way to study the different role of the implanted gas ions in the film transfer process, the interaction between the two kinds of atoms, and the correlation between the defects created by H and/or He ions. Furthermore, recent studies [7–9] have revealed that He and H sequential implantation, i.e. successive implantation one species after the other, could lead to reduction of the splitting thermal budget as compared to H-only case at equivalent fluence. Meanwhile, the combination of He and H implantation also results in the significant reduction of the total fluence necessary to obtain layer transfer. In addition, change of the implantation sequence of He and H may affect the behavior of the implantation induced point defect complexes [10]. Although many phenomena together with the possible mechanisms on silicon surface exfoliation and blistering have been studied by implantation of H, He singly and in combination, most of the researches are mainly focused on implantation at low energy range, i.e. several keV to a few tens keV. Moreover, the implantation fluence of H ions in most coimplantation cases is either overwhelming or comparable with that of He ions. In this article, by using sequential implantation of 160 keV He ions at higher fluence and 110 keV H ion at much lower fluence, we have studied surface phenomena and evolution of defect microstructures in Si after subsequent annealing. Synergistic surface exfoliation with thickness of about 1 lm has been achieved.
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Fig. 1. TRIM simulation results showing the He and H profiles in Si implanted by 160 keV He ions at a fluence of 5 1016/cm2 and 110 keV H ions at a fluence of 1 1016/cm2.
tion about the microstructures of defects produced by He and/or H implantation followed by high temperature annealing. For the specimen preparation, two smaller pieces of a single sample were glued together face to face, polished to around 50 lm thickness and subsequently thinned by 6 keV Ar ions at an incident angle of 12°. TEM images were obtained with a JEOL-2010 microscope operated at 200 kV. In addition, positron annihilation spectroscopy (PAS) was also used to study H implantation induced vacancy-like defects in silicon by using a monoenergetic positron beam with energies tunable between 100 eV and 25 keV. The two-photon annihilation events are detected at room temperature with a Ge detector. Approximately 106 events were collected in the peak at each positron energy value. The various annihilation processes can be distinguished by analyzing the width of the photo-peak. The width is defined by the S(hape) parameter, i.e. the relative contribution of the central part of the peak 511 [ 0.66; +0.66] keV.
2. Experimental Cz n-type, 1–10 ohm cm Si (100) wafers were implanted at room temperature with He ions at a fluence of 5 1016/ cm2 and H ions at a fluence of 1 1016/cm2, singly or in combination. The energy of He ions is 160 keV, which gives the projected range (RP) of about 1 lm according to TRIM simulations [11]. The energy of H ions (110 keV) was chosen in such a way that the H profile is nearly overlapped with He profile, as shown in Fig. 1. After implantation, Si wafers were cut into smaller pieces and subjected to furnace annealing for the purpose of various technical analyses. The subsequent annealing was carried out at temperature range up to 800 °C for 1 h at a flow of nitrogen gas. A JEOL model JSM 6700F SEM with a field-emission gun, operating at 10 kV, was used to determine the surface morphologies. AFM was performed to evaluate the thickness and the surface roughness of the exfoliated layer. Moreover, XTEM was carried out to get detailed informa-
3. Results SEM observations have not shown any surface blistering or exfoliation on silicon by single implantation of 160 keV, 5 1016/cm2 He ions or 110 keV, 1 1016/cm2 H ions even after high temperature annealing up to 800 °C. However, by sequential implantation of He and H ions at the equivalent fluence, silicon surface achieves significant modifications, which strongly depend on the applied thermal budget. For the as-implanted sample together with the implanted samples after annealing at low temperature range up to 400 °C, only flat and smooth surface has been observed by SEM. As an example, Fig. 2(a) gives the typical SEM micrograph recorded on silicon surface after successive implantation of 160 keV He and 110 keV H ions and followed by an annealing of 400 °C for 1 h. However, by increasing the annealing temperature to 500 °C, serious surface exfoliation occurs, which leaves a large amount of
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Fig. 2. Typical SEM micrographs of the silicon surfaces coimplanted with 160 keV, 5 1016/cm2 He ions and 110 keV, 1 1016/cm2 H ions after annealing for 1 h at different temperatures of (a) 400 °C, and (b) 700 °C.
Fig. 3. Typical SEM micrographs of the silicon surfaces after successive implantation of 160 keV, 5 1016/cm2 He and 110 keV, 1 1016/cm2 H ions and followed by an annealing at 800 °C for 1 h. (a) the overall morphology, and (b) close view of the remained island-like structures.
irregular craters on the sample surface. Meanwhile, the obtained SEM images show less change in crater morphology and exfoliated area in the annealing temperature range of 500–700 °C. Fig. 2(b) illustrates the typical SEM images showing craters formed on silicon surface by coimplantation of He and H ions after an annealing at 700 °C for 1 h. One can clearly see that the craters of size around 10 lm are inhomogeneously distributed on the surface. At 800 °C annealing, however, quite different surface exfoliated morphologies are observed, as shown in Fig. 3. It is obvious that such thermal budget has induced exfoliation on the most surface of silicon, leaving some irregular silicon surface regions like islands. Such island-like structures actually provide the opportunities to evaluate the thickness of the exfoliated layer as well as the microstructures of defects created under them, as presented in the following. Fig. 4 shows the main AFM results obtained on silicon surface by sequential implantation of He and H ions after 800 °C annealing for 1 h. The same surface morphologies (island-like structures) as that obtained by SEM
observations have also been observed, as presented in Fig. 4(a). Through these remained island-like surface structures, the thickness of the exfoliated layer can be measured. The typical results are given in Fig. 4(b). It is obvious that the thickness of the transferred layer is mainly distributed in the range of 900–960 nm. Such thickness is quite consistent with the projected range of the ions [11]. In addition, AFM measurements were also performed to check the surface roughness on the sheared layer. The obtained results are given in Fig. 4(c) and (d). Depending on the chosen regions, the root mean square (RMS) roughness of the sheared layer shows a bit changes. However, the ˚. maximum RMS roughness is around 500 A Fig. 5(a) gives the typical XTEM image taken in silicon implanted with 160 keV 5 1016/cm2 He ions alone after subsequent annealing at 800 °C for 1 h. It is clear that He implantation followed by annealing creates a well-defined defect band which consists of cavities as well as dislocation-like defects. The cavity band mainly distributes at depth of 820–960 nm from the sample surface. Meanwhile, one can also see that small cavities are mainly in the central
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Fig. 5. Typical XTEM images showing ion implantation induced defects in Si after annealing at 800 °C for 1 h, (a) 160 keV, 5 1016/cm2 He alone, (b) and (c) successive implantation of 160 keV, 5 1016/cm2 He and 110 keV, 1 1016/cm2 H ions.
Fig. 4. AFM analyses of silicon surfaces coimplanted with He and H ions after 800 °C annealing for 1 h. (a) the overall surface morphology, (b) analysis to show the thickness of exfoliated layer, (c) and (d) images showing the surface roughness on the sheared area.
of defect band while large cavities are usually located on both sides. Detailed measurements have demonstrated that the cavity size is in the range of 4.0–26.0 nm, with the average size of about 11.0 nm.
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As mentioned above, since most of the implanted surface has been split off in He and H coimplanted silicon followed by 800 °C annealing, it is a little difficult to obtain the detailed information in microstructures of defects around the ion range by XTEM measurements. Nevertheless, the remained island-like surface structures provide good opportunities to reveal the involved defects under them. Actually, such defect microstructures have been obtained, as shown in Fig. 5(b) and (c). From these figures, one can clearly see that He and H coimplantation followed by 800 °C annealing also creates a defect band around the ion range. However, this defect band is quite different from that observed in He-only implanted silicon (Fig. 5(a)). In such case, the defect band is mainly made up of a monolayer of cavities in large size (30 nm) and of a large amount of dislocations. The results suggest that the additional H implantation has strongly affected the thermal evolution of He bubbles. It seems to shrink the He cavity band accompanied by effective growth of the cavities. Although the exact defect microstructures in the exfoliated region cannot be obtained, one can guess that they should undergo the same processes and the effects may be even more remarkable. Such effects of H implantation on the evolution of He bubbles play an important role in the observed surface exfoliation or flaking. 4. Discussion XTEM and SEM observations have clearly shown that sequential implantation of 160 keV He ions and 110 keV H ions into the same Si region directly leads to serious surface exfoliation during subsequent high temperature annealing. Meanwhile, the total fluence for exfoliation is much lower as compared with He-only implantation case (2 1017/cm2) [7]. The observed surface exfoliation and fluence reduction should be related to the effects of the additional low fluence H implantation on the thermal evolution of He implantation induced defects, as discussed in the following. High fluence He implantation into silicon could induce formation of a band of bubbles around the ion range. Subsequent annealing leads to thermal growth of bubbles into cavities by exchanging both vacancies and He atoms, accompanied by effusion of He gas [4,12,13]. The creation of cavity band around ion range has been actually confirmed in silicon by implantation of 160 keV, 5 1016/ cm2 He ions alone after 800 °C annealing for 1 h (see Fig. 5(a)). In our experimental conditions, although He ion implantation followed by high temperature annealing produces a well-defined cavity band around the He ion range, most cavities are in smaller size and dispersedly distributed. Owing to the lack of large intersection between the cavities, surface exfoliation could not be achieved. However, in the case of He and H coimplantation, subsequent H ion implantation could introduce additional vacancy-like defects (i.e. the broken silicon bonds) as well as H atoms in the same He bubble region. In fact, the
vacancy-like defects created by 110 keV H ion implantation in the region around the ion range have been confirmed by PAS measurements, as shown in Fig. 6. Owing to its strong chemical interaction with the broken silicon bonds, hydrogen will segregate into bubbles and evolve into several states upon annealing, such as formation of H2 gas. The chemical interactions between H and the broken silicon bonds or bubbles, and also their evolution during annealing have been studied and reported in details by many authors [14–16]. Such strong chemical interaction between H atoms and the broken silicon bonds in Si could assist in surviving of more vacancy-like defects from recombination with interstitial-like defects during subsequent annealing. Since the growth of cavities in Si may undergo both Ostwald ripening and migration and coalescence processes [17,18], the introduction of additional vacancies by H implantation should give rise to the increase in growth rate of the cavities. As confirmed by XTEM observations (see Fig. 5(b)), the additional H implantation followed by an annealing at 800 °C for 1 h actually promotes the growth of He bubbles, which leads to formation of a monolayer of cavities in larger size. It is worth mentioning that similar results have also been obtained in our earlier research, in which silicon samples were implanted with 160 keV, 5 1016/cm2 He ions and then followed by high density of hydrogen plasma immersion [19]. It has been well recognized that surface exfoliation requires a gas pressure as well as a narrowly confined layer of cracks or microvoids into which the gas can segregate and expand during annealing, leading to growth and an eventual intersection of the microvoids to form two continuous internal surfaces [7,8]. We expect that the monolayer of cavities at the end of the ion range upon annealing should act as this confined layer of cracks as well as increase of gas pressure. AFM measurements have shown that the thickness of the sheared layer is around 900– 960 nm, which is quite consistent with the ion projected range. In addition, although XTEM observations have not been carried out for low temperature annealed silicon
Fig. 6. PAS measurements showing the S parameter as a function of positron energy in reference Si sample and in 110 keV, 1 1016/cm2 H ion implanted Si sample.
Z. Wang et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 250–255
samples, we can guess that the effect of H implantation on thermal evolution of He bubbles should present throughout all the annealing temperature range, and shows dependence on the annealing temperature. The higher the annealing temperature, the more significant effect should be achieved. At lower annealing temperature range, the effects of additional H implantation on He bubbles are weak. The bubbles are in smaller size that intersection of bubbles can not be expected. Therefore, no surface exfoliation or blistering occurs. At intermediate annealing temperature range (above 500 °C), more He, H and vacancies will segregate into He bubbles, which promotes the growth of the bubbles. Intersection of bubbles (or cavities) will occur in some of the implanted region accompanied with a large strain field, which leads to partial surface exfoliation. If the annealing temperature is sufficiently high (i.e. 800 °C), owing to the strong effect of hydrogen on cavities, a monolayer of cavities with larger size (larger than 30 nm) can be created (Fig. 5(c)). The intersection of these cavities can be easily achieved, which results in forming two new continuous internal surfaces with lower surface free energy density in most of the implanted area. This thus promotes the large area of surface exfoliation or flaking. Compared with similar researches, in our case the exfoliated surface seems quite rough. The exact reasons are not clear. It may be attributed to several factors, such as the higher implantation energy, the lower fluence, the proportion between He and H ions and etc. Nevertheless, it is worth pointing out that the ratio of RMS roughness to the thickness of transferred layer is still low (0.05). 5. Conclusions In summary, Cz n-type Si(100) samples were successively implanted with 160 keV, 5 1016/cm2 He ions and 110 keV, 1 1016/cm2 H ions. A thick layer of Si surface (1 lm) has been split off after subsequent furnace annealing at temperature of 800 °C. The surface exfoliation could be attributed to the effects of additional low fluence H implantation on the evolution of defect microstructures created by high fluence He implantation. He implantation first creates a large amount of bubbles with a high density of the broken bonds. The additional H implantation significantly promotes the bubble growth, leading to formation of a monolayer of cavities at sufficiently high annealing temperature. Such monolayer of cavities provides the well narrowly confined layer of cracks, from which surface exfo-
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liation, or even flaking occurs due to gas segregation, layer expanding and intersection of cavities. Acknowledgements This work was supported by Foundation of National Natural Sciences of China (Grant No. 10675089) and Natural Science Foundation of Tianjin (Grant No. 06YFJMJC01100). Authors would also like to thank Key Laboratory of Nuclear Analysis Techniques, Chinese Academy of Sciences for financial support (Grant No. K130). References [1] M. Bruel, Electron. Lett. 31 (1995) 1201. [2] M. Bruel, Nucl. Instr. and Meth. B 108 (1996) 313. [3] U. Go¨sele, H. Stenzel, T. Martini, J. Steinkirchner, D. Conrad, K. Scheerschmidt, Appl. Phys. Lett. 67 (1995) 3614. [4] M.K. Weldon, V.E. Marsico, Y.J. Chabal, A. Agarwal, D.J. Eaglesham, J. Sapjeta, W.L. Brown, D.C. Jacobson, Y. Caudano, S.B. Christman, E.E. Chaban, J. Vac. Sci. Technol. B 15 (1997) 1065. [5] C.C. Griffioen, J.H. Evans, P.C. de Jong, A. van Veen, Nucl. Instr. and Meth. B 27 (1987) 417. [6] W.K. Chu, R.H. Kastl, R.F. Lever, S. Mader, B.J. Masters, Phys. Rev. B 16 (1977) 3851. [7] A. Agarwal, T.E. Haynes, V.C. Venezia, O.W. Holland, D.J. Eaglesham, Appl. Phys. Lett. 72 (1998) 1086. [8] O. Moutanabbir, B. Terreault, Appl. Phys. Lett. 86 (2005) 051906. [9] P. Nguyen, K.K. Bourdelle, T. Maurice, N. Sousbie, A. Boussagol, X. Hebras, L. Portigliatti, F. Letertre, A. Tauzin, N. Rochat, J. Appl. Phys. 101 (2007) 033506. [10] X.Z. Duo, W.L. Liu, M. Zhang, L.W. Wang, C.L. Lin, M. Okuyama, M. Noda, W.Y. Cheung, P.K. Chu, P.G. Hu, S.X. Wang, L.M. Wang, J. Phys. D: Appl. Phys. 34 (2001) 477. [11] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. [12] V. Raineri, M. Saggio, E. Rimini, J. Mater. Res. 15 (2000) 1449. [13] G.F. Cerofolini, F. Corni, S. Frabboni, C. Nobili, G. Ottaviani, R. Tonini, Mater. Sci. Eng. R 27 (2000) 1. [14] A. Nurmela, K. Henttinen, T. Suni, A. Tolkki, I. Suni, Nucl. Instr. and Meth. B 219–220 (2004) 747. [15] Y.J. Chabal, M.K. Weldon, Y. Caudano, B.B. Stefanov, K. Raghavachari, Physica. B 273 (1999) 152. [16] M.K. Weldon, M. Collot, Y.J. Chabal, V.C. Venezia, A. Agarwal, T.E. Haynes, D.J. Eaglesham, S.B. Christman, E.E. Chaban, Appl. Phys. Lett. 73 (1998) 3721. [17] S. Frabboni, F. Corni, C. Nobili, R. Tonini, G. Ottaviani, Phys. Rev. B 69 (2004) 165209. [18] J.H. Evans, Nucl. Instr. and Meth. B 196 (2002) 125. [19] C.L. Liu, E. Ntsoenzok, A. Vengurlekar, S. Ashok, D. Alquier, M.O. Ruault, C. Dubois, J. Vac. Sci. Technol. B 23 (2005) 990.
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 250–255 www.elsevier.com/locate/nimb
Surface exfoliation and defect structures in Si induced by 160 keV He and 110 keV H ion implantation Zhuo Wang a, Changlong Liu a,b,*, Tianyu Liu a, Xiaodong Zhang a, Wenrun Li a, Wenxia Li a,b, Bing Yuan a, Pei Wu a, Mengkai Li a b
a School of Science, Tianjin University, Tianjin 300072, PR China Tianjin Key Laboratory of Low Dimension Materials Physics and Preparing Technology, Institute of Advanced Materials Physics, Faculty of Science, Tianjin 300072, PR China
Received 28 August 2007; received in revised form 9 November 2007 Available online 19 November 2007
Abstract Cz n-type Si(100) wafers were implanted at room temperature with 160 keV He ions at a fluence of 5 1016/cm2 and 110 keV H ions at a fluence of 1 1016/cm2, singly or in combination. Surface phenomena and defect microstructures have been studied by various techniques, including scanning electron microscopy (SEM), atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (XTEM). Surface exfoliation and flaking phenomena were only observed on silicon by successive implantation of He and H ions after subsequent annealing at temperatures above 400 °C. The surface phenomena show strong dependence on the thermal budget. At annealing temperatures ranging from 500 to 700 °C, craters with size of about 10 lm were produced throughout the silicon surface. As increasing temperature to 800 °C, most of the implanted layer was sheared, leaving structures like islands on the surface. AFM observations have demonstrated that the implanted layer is mainly transfered at the depth around 960 nm, which is quite consistent with the range of the ions. XTEM observations have revealed that the additional low fluence H ion implantation could significantly influence thermal growth of He-cavities, which gives rise to a monolayer of cavities surrounded by a large amount of dislocations and strain. The surface exfoliation effects have been tentatively interpreted in combination of AFM and XTEM results. Ó 2007 Elsevier B.V. All rights reserved. PACS: 61.72.Ff; 61.72.Qq Keywords: Silicon; He and H ion implantation; Surface exfoliation; Cavities; XTEM
1. Introduction Recently, surface blistering, exfoliation together with bubble growth in silicon by light gas ion implantation have been paid more and more attention due to their proven capability for high quality heterogeneous materials integration in the fabrication of micro- and nanoscale devices [1]. Hydrogen implantation has been often used to shear and transfer a thin silicon layer in implanted
*
Corresponding author. Address: School of Science, Tianjin University, Tianjin 300072, PR China. Tel./fax: +86 022 27403425. E-mail address: [email protected] (C. Liu). 0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.11.005
wafer onto a supporting wafer after bonding the two wafers together and subsequent annealing [1,2]. The technology of smart-cut based on it has been built up to produce high quality silicon-on-insulator (SOI) wafers more economically than the competing processes such as separation by implantation of oxygen (SIMOX) and bond-andetch-back silicon-on-insulator (BESOI) [3]. Generally, it has been considered that the thin film separation process by H implantation mainly proceeds through both chemical interaction (bond breaking, internal surface passivation) and physical interaction (gas coalescence, pressure and fracture) in the silicon substrate [4]. However, it is much difficult to elucidate the contribution of each component to the overall process.
Z. Wang et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 250–255
High fluence He implantation into semiconductor or metal materials also gives rise to bubble formation in the bulk followed by high temperature annealing [5,6]. However, splitting by He implantation singly requires a quite high fluence (2 1017/cm2) [7]. As compared with H implantation, helium does not have chemical interaction in silicon substrate, and thus only physical interaction similar to hydrogen occurs. Therefore, coimplantation of He and H ions into Si should be a good way to study the different role of the implanted gas ions in the film transfer process, the interaction between the two kinds of atoms, and the correlation between the defects created by H and/or He ions. Furthermore, recent studies [7–9] have revealed that He and H sequential implantation, i.e. successive implantation one species after the other, could lead to reduction of the splitting thermal budget as compared to H-only case at equivalent fluence. Meanwhile, the combination of He and H implantation also results in the significant reduction of the total fluence necessary to obtain layer transfer. In addition, change of the implantation sequence of He and H may affect the behavior of the implantation induced point defect complexes [10]. Although many phenomena together with the possible mechanisms on silicon surface exfoliation and blistering have been studied by implantation of H, He singly and in combination, most of the researches are mainly focused on implantation at low energy range, i.e. several keV to a few tens keV. Moreover, the implantation fluence of H ions in most coimplantation cases is either overwhelming or comparable with that of He ions. In this article, by using sequential implantation of 160 keV He ions at higher fluence and 110 keV H ion at much lower fluence, we have studied surface phenomena and evolution of defect microstructures in Si after subsequent annealing. Synergistic surface exfoliation with thickness of about 1 lm has been achieved.
251
Fig. 1. TRIM simulation results showing the He and H profiles in Si implanted by 160 keV He ions at a fluence of 5 1016/cm2 and 110 keV H ions at a fluence of 1 1016/cm2.
tion about the microstructures of defects produced by He and/or H implantation followed by high temperature annealing. For the specimen preparation, two smaller pieces of a single sample were glued together face to face, polished to around 50 lm thickness and subsequently thinned by 6 keV Ar ions at an incident angle of 12°. TEM images were obtained with a JEOL-2010 microscope operated at 200 kV. In addition, positron annihilation spectroscopy (PAS) was also used to study H implantation induced vacancy-like defects in silicon by using a monoenergetic positron beam with energies tunable between 100 eV and 25 keV. The two-photon annihilation events are detected at room temperature with a Ge detector. Approximately 106 events were collected in the peak at each positron energy value. The various annihilation processes can be distinguished by analyzing the width of the photo-peak. The width is defined by the S(hape) parameter, i.e. the relative contribution of the central part of the peak 511 [ 0.66; +0.66] keV.
2. Experimental Cz n-type, 1–10 ohm cm Si (100) wafers were implanted at room temperature with He ions at a fluence of 5 1016/ cm2 and H ions at a fluence of 1 1016/cm2, singly or in combination. The energy of He ions is 160 keV, which gives the projected range (RP) of about 1 lm according to TRIM simulations [11]. The energy of H ions (110 keV) was chosen in such a way that the H profile is nearly overlapped with He profile, as shown in Fig. 1. After implantation, Si wafers were cut into smaller pieces and subjected to furnace annealing for the purpose of various technical analyses. The subsequent annealing was carried out at temperature range up to 800 °C for 1 h at a flow of nitrogen gas. A JEOL model JSM 6700F SEM with a field-emission gun, operating at 10 kV, was used to determine the surface morphologies. AFM was performed to evaluate the thickness and the surface roughness of the exfoliated layer. Moreover, XTEM was carried out to get detailed informa-
3. Results SEM observations have not shown any surface blistering or exfoliation on silicon by single implantation of 160 keV, 5 1016/cm2 He ions or 110 keV, 1 1016/cm2 H ions even after high temperature annealing up to 800 °C. However, by sequential implantation of He and H ions at the equivalent fluence, silicon surface achieves significant modifications, which strongly depend on the applied thermal budget. For the as-implanted sample together with the implanted samples after annealing at low temperature range up to 400 °C, only flat and smooth surface has been observed by SEM. As an example, Fig. 2(a) gives the typical SEM micrograph recorded on silicon surface after successive implantation of 160 keV He and 110 keV H ions and followed by an annealing of 400 °C for 1 h. However, by increasing the annealing temperature to 500 °C, serious surface exfoliation occurs, which leaves a large amount of
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Fig. 2. Typical SEM micrographs of the silicon surfaces coimplanted with 160 keV, 5 1016/cm2 He ions and 110 keV, 1 1016/cm2 H ions after annealing for 1 h at different temperatures of (a) 400 °C, and (b) 700 °C.
Fig. 3. Typical SEM micrographs of the silicon surfaces after successive implantation of 160 keV, 5 1016/cm2 He and 110 keV, 1 1016/cm2 H ions and followed by an annealing at 800 °C for 1 h. (a) the overall morphology, and (b) close view of the remained island-like structures.
irregular craters on the sample surface. Meanwhile, the obtained SEM images show less change in crater morphology and exfoliated area in the annealing temperature range of 500–700 °C. Fig. 2(b) illustrates the typical SEM images showing craters formed on silicon surface by coimplantation of He and H ions after an annealing at 700 °C for 1 h. One can clearly see that the craters of size around 10 lm are inhomogeneously distributed on the surface. At 800 °C annealing, however, quite different surface exfoliated morphologies are observed, as shown in Fig. 3. It is obvious that such thermal budget has induced exfoliation on the most surface of silicon, leaving some irregular silicon surface regions like islands. Such island-like structures actually provide the opportunities to evaluate the thickness of the exfoliated layer as well as the microstructures of defects created under them, as presented in the following. Fig. 4 shows the main AFM results obtained on silicon surface by sequential implantation of He and H ions after 800 °C annealing for 1 h. The same surface morphologies (island-like structures) as that obtained by SEM
observations have also been observed, as presented in Fig. 4(a). Through these remained island-like surface structures, the thickness of the exfoliated layer can be measured. The typical results are given in Fig. 4(b). It is obvious that the thickness of the transferred layer is mainly distributed in the range of 900–960 nm. Such thickness is quite consistent with the projected range of the ions [11]. In addition, AFM measurements were also performed to check the surface roughness on the sheared layer. The obtained results are given in Fig. 4(c) and (d). Depending on the chosen regions, the root mean square (RMS) roughness of the sheared layer shows a bit changes. However, the ˚. maximum RMS roughness is around 500 A Fig. 5(a) gives the typical XTEM image taken in silicon implanted with 160 keV 5 1016/cm2 He ions alone after subsequent annealing at 800 °C for 1 h. It is clear that He implantation followed by annealing creates a well-defined defect band which consists of cavities as well as dislocation-like defects. The cavity band mainly distributes at depth of 820–960 nm from the sample surface. Meanwhile, one can also see that small cavities are mainly in the central
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Fig. 5. Typical XTEM images showing ion implantation induced defects in Si after annealing at 800 °C for 1 h, (a) 160 keV, 5 1016/cm2 He alone, (b) and (c) successive implantation of 160 keV, 5 1016/cm2 He and 110 keV, 1 1016/cm2 H ions.
Fig. 4. AFM analyses of silicon surfaces coimplanted with He and H ions after 800 °C annealing for 1 h. (a) the overall surface morphology, (b) analysis to show the thickness of exfoliated layer, (c) and (d) images showing the surface roughness on the sheared area.
of defect band while large cavities are usually located on both sides. Detailed measurements have demonstrated that the cavity size is in the range of 4.0–26.0 nm, with the average size of about 11.0 nm.
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As mentioned above, since most of the implanted surface has been split off in He and H coimplanted silicon followed by 800 °C annealing, it is a little difficult to obtain the detailed information in microstructures of defects around the ion range by XTEM measurements. Nevertheless, the remained island-like surface structures provide good opportunities to reveal the involved defects under them. Actually, such defect microstructures have been obtained, as shown in Fig. 5(b) and (c). From these figures, one can clearly see that He and H coimplantation followed by 800 °C annealing also creates a defect band around the ion range. However, this defect band is quite different from that observed in He-only implanted silicon (Fig. 5(a)). In such case, the defect band is mainly made up of a monolayer of cavities in large size (30 nm) and of a large amount of dislocations. The results suggest that the additional H implantation has strongly affected the thermal evolution of He bubbles. It seems to shrink the He cavity band accompanied by effective growth of the cavities. Although the exact defect microstructures in the exfoliated region cannot be obtained, one can guess that they should undergo the same processes and the effects may be even more remarkable. Such effects of H implantation on the evolution of He bubbles play an important role in the observed surface exfoliation or flaking. 4. Discussion XTEM and SEM observations have clearly shown that sequential implantation of 160 keV He ions and 110 keV H ions into the same Si region directly leads to serious surface exfoliation during subsequent high temperature annealing. Meanwhile, the total fluence for exfoliation is much lower as compared with He-only implantation case (2 1017/cm2) [7]. The observed surface exfoliation and fluence reduction should be related to the effects of the additional low fluence H implantation on the thermal evolution of He implantation induced defects, as discussed in the following. High fluence He implantation into silicon could induce formation of a band of bubbles around the ion range. Subsequent annealing leads to thermal growth of bubbles into cavities by exchanging both vacancies and He atoms, accompanied by effusion of He gas [4,12,13]. The creation of cavity band around ion range has been actually confirmed in silicon by implantation of 160 keV, 5 1016/ cm2 He ions alone after 800 °C annealing for 1 h (see Fig. 5(a)). In our experimental conditions, although He ion implantation followed by high temperature annealing produces a well-defined cavity band around the He ion range, most cavities are in smaller size and dispersedly distributed. Owing to the lack of large intersection between the cavities, surface exfoliation could not be achieved. However, in the case of He and H coimplantation, subsequent H ion implantation could introduce additional vacancy-like defects (i.e. the broken silicon bonds) as well as H atoms in the same He bubble region. In fact, the
vacancy-like defects created by 110 keV H ion implantation in the region around the ion range have been confirmed by PAS measurements, as shown in Fig. 6. Owing to its strong chemical interaction with the broken silicon bonds, hydrogen will segregate into bubbles and evolve into several states upon annealing, such as formation of H2 gas. The chemical interactions between H and the broken silicon bonds or bubbles, and also their evolution during annealing have been studied and reported in details by many authors [14–16]. Such strong chemical interaction between H atoms and the broken silicon bonds in Si could assist in surviving of more vacancy-like defects from recombination with interstitial-like defects during subsequent annealing. Since the growth of cavities in Si may undergo both Ostwald ripening and migration and coalescence processes [17,18], the introduction of additional vacancies by H implantation should give rise to the increase in growth rate of the cavities. As confirmed by XTEM observations (see Fig. 5(b)), the additional H implantation followed by an annealing at 800 °C for 1 h actually promotes the growth of He bubbles, which leads to formation of a monolayer of cavities in larger size. It is worth mentioning that similar results have also been obtained in our earlier research, in which silicon samples were implanted with 160 keV, 5 1016/cm2 He ions and then followed by high density of hydrogen plasma immersion [19]. It has been well recognized that surface exfoliation requires a gas pressure as well as a narrowly confined layer of cracks or microvoids into which the gas can segregate and expand during annealing, leading to growth and an eventual intersection of the microvoids to form two continuous internal surfaces [7,8]. We expect that the monolayer of cavities at the end of the ion range upon annealing should act as this confined layer of cracks as well as increase of gas pressure. AFM measurements have shown that the thickness of the sheared layer is around 900– 960 nm, which is quite consistent with the ion projected range. In addition, although XTEM observations have not been carried out for low temperature annealed silicon
Fig. 6. PAS measurements showing the S parameter as a function of positron energy in reference Si sample and in 110 keV, 1 1016/cm2 H ion implanted Si sample.
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samples, we can guess that the effect of H implantation on thermal evolution of He bubbles should present throughout all the annealing temperature range, and shows dependence on the annealing temperature. The higher the annealing temperature, the more significant effect should be achieved. At lower annealing temperature range, the effects of additional H implantation on He bubbles are weak. The bubbles are in smaller size that intersection of bubbles can not be expected. Therefore, no surface exfoliation or blistering occurs. At intermediate annealing temperature range (above 500 °C), more He, H and vacancies will segregate into He bubbles, which promotes the growth of the bubbles. Intersection of bubbles (or cavities) will occur in some of the implanted region accompanied with a large strain field, which leads to partial surface exfoliation. If the annealing temperature is sufficiently high (i.e. 800 °C), owing to the strong effect of hydrogen on cavities, a monolayer of cavities with larger size (larger than 30 nm) can be created (Fig. 5(c)). The intersection of these cavities can be easily achieved, which results in forming two new continuous internal surfaces with lower surface free energy density in most of the implanted area. This thus promotes the large area of surface exfoliation or flaking. Compared with similar researches, in our case the exfoliated surface seems quite rough. The exact reasons are not clear. It may be attributed to several factors, such as the higher implantation energy, the lower fluence, the proportion between He and H ions and etc. Nevertheless, it is worth pointing out that the ratio of RMS roughness to the thickness of transferred layer is still low (0.05). 5. Conclusions In summary, Cz n-type Si(100) samples were successively implanted with 160 keV, 5 1016/cm2 He ions and 110 keV, 1 1016/cm2 H ions. A thick layer of Si surface (1 lm) has been split off after subsequent furnace annealing at temperature of 800 °C. The surface exfoliation could be attributed to the effects of additional low fluence H implantation on the evolution of defect microstructures created by high fluence He implantation. He implantation first creates a large amount of bubbles with a high density of the broken bonds. The additional H implantation significantly promotes the bubble growth, leading to formation of a monolayer of cavities at sufficiently high annealing temperature. Such monolayer of cavities provides the well narrowly confined layer of cracks, from which surface exfo-
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