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Microstructure characterization of location-controlled Si-islands crystallized by excimer laser in the μ-Czochralski (grain filter) process
ARTICLE IN PRESS
Journal of Crystal Growth 299 (2007) 316–321 www.elsevier.com/locate/jcrysgro
Microstructure characterization of location-controlled Si-islands crystallized by excimer laser in the m-Czochralski (grain filter) process R. Ishiharaa,, D. Danciua, F. Tichelaara, M. Hea, Y. Hiroshimab, S. Inoueb, T. Shimodab, J.W. Metselaara,b, C.I.M. Beenakkera,b a
Delft Institute of Microelectonics and Submicrontechnology (DIMES), Delft University of Technology, Delft, The Netherlands b Technology Platform Research Center, Seiko-Epson Corp., Nagano, Japan Received 4 April 2006; received in revised form 5 December 2006; accepted 6 December 2006 Communicated by P. Rudolph Available online 17 December 2006
Abstract Microstructure of location-controlled grains by m-Czochralski process was characterized with electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM). We confirmed that defects in the location-controlled grain are mainly S3 twin boundary generating from near the rim of the grain filter, while random grain boundaries hardly exist. Dependence of the population was investigated on process parameters. We found that most of the S3 twin boundaries have {1 1 1} facet plane, which, in same case, are massed with a nano-meter spacing. S3 twin boundaries having facet planes {1 1 2} and {1 1 1}/{1 1 5} were also found to exist. r 2007 Elsevier B.V. All rights reserved. PACS: 81.10.Fq; 81.05.Cy; 61.72.y; 61.72.Mm Keywords: A1. Crystal structure; A2. Czochralski method; A2. Growth from melt; B2. Semiconducting silicon
1. Introduction Excimer laser crystallization (ELC) is a well-established method for producing polycrystalline silicon films on nonrefractory substrates. 2D location control of Si grains in ELC [1,2] enables formation of single-grain thin-film transistors (TFTs) [3], which perform like SOI-FETs. In m-Czochralski process [2,4], 7 mm large Si grain can be grown from ‘‘grain filter’’, which refers hole created in the underlying SiO2 and filled with the a-Si. Upon excimerlaser irradiation, the grain filter melts non-completely, whereas the surrounding melts completely. During vertical growth of the pre-existing seeds in the grain filter, occlusion of grains occurs reducing the number of growing grains. Therefore, the process could alternatively be called ‘‘microCorresponding author. Delft Institute of Microelectonics and submicrontechnology (DIMES), Delft University of Technology, Feldmannweg 17, P.O. Box 5053, 2600 GB Delft, The Netherlands. Tel.:+31(0) 15 2788498; fax: +31(0) 15 2622163. E-mail address: [email protected] (R. Ishihara).
0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.12.010
Bridgeman process’’. When growing phase reaches about the rims level, vertical growth is followed by the lateral growth. The growth continues until the growth interface collides with the one growing from opposite direction which is created either by nucleation in the melt or by grain growth from other grain filters. Single-grain TFTs fabricated inside the grain showed an average field effect mobility of more than 600 cm2/Vs [3]. Investigation of microstructure of the location-controlled grain is important, since it was suggested that the deviation of the transistor characteristics is related with planar defects in the grain [4]. It is known, that some types of grain boundaries can be electrically active reducing the mobility of carriers [5]. The presence of the grain boundaries can be correlated with discontinuity of the crystalline phase of each grain on either side of the boundaries. For a given misorientation between two grains, a lattice consisting of sites common to each grain is referred to as coincidence site lattice (CSL) [6]. The coincidence index S is the reciprocal density of CSLs with respect to the original lattice. The periodicity of the boundary structure can be analyzed by
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the periodicity in the plane in the CSL. CSL boundaries with relatively small S values contain short periods. It is the purpose of this paper to identify by the mean of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) the influence of processing parameters (grain filter diameter, laser energy density) on defects generation and their density in crystallized Siislands. Using the TEM, detailed information concerning the type of boundaries and their location is also presented. Crystallographic relationship of adjacent grains gives qualitative information about the faceting of some identified twinned grains; hence, the coherency of the boundary between them.
specimens were prepared using a FEI–FIB microscope. Images obtained by bright field (BF)/dark field (DF) techniques and diffraction patterns (selected area diffraction patterns [SADP] and convergent beam electron diffraction [CBED]) were performed with a Philips 30 T microscope operated at 300 kV. The twin boundaries types between the grains were evaluated as follows: in SEM– EBSD as the misorientation angle and rotation axes from recorded Kikuchi maps in TEM as the misorientation angle between two zone axes and common planes from recorded SADP and CBED patterns.
2. Experiment
3.1. EBSD analysis
Since a detailed process description can be found in elsewhere [2,4], here we describe it shortly. Cavities having various initial diameter d (Fig. 1) in a 750 nm-thick SiO2 were narrowed down by an additional SiO2 deposition. A 250 or 150 nm thick a-Si film was deposited by LPCVD at 545 1C and crystallized by 1 shot of XeCl excimer-laser irradiation with various energy densities (DE) at a substrate temperature of 450 1C. The laser beam has top-hat shape and a size of approximately 3 5 mm, i.e., uniform light exposure for the grain filter structure. SEM–electron backscattering diffraction (SEM–EBSD) analyses were performed using a Philips scanning microscope XL 30S. For TEM investigations both planar and cross section specimens were prepared. The planar TEM specimens were prepared by conventional grinding and polishing technique up to a thickness of 10 mm. They were finally thinned to electron transparency using an ion mill with Ar+ beam at 61 and 5 kV. The cross-section TEM
Fig. 2 shows a SEM image after Secco’s etching and surface crystallographic orientation map of Si grains characterized by EBSD overlapped with SEM image. Here the location-controlled grain has a square shaped with 6 mm length, which is the spacing between the grain filters. It can be seen that random grain boundaries (black lines) inside the location-controlled grains are nearly absent, and that CSL grain boundaries often extend radially from vicinity of the center of the location-controlled grain at where the grain filter underlies. The CSL boundaries are predominantly S3 (dark gray lines), followed by S9 (light gray lines). S3 and S9 twin boundaries are associated with a rather low interfacial energy as compared with other
Fig. 1. Schematic cross-sectional drawing of the sample for m-Czochralski process. The d denotes the initial diameter of the grain filter.
3. Results and discussion
Fig. 2. EBSD orientation mapping of a 3 6 grid of grains by mCzochralski process. The curved lines indicate boundaries S3 (dark gray lines), S9 (light gray lines) CSL boundaries and random grain boundaries (black lines).
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types of twin boundaries or random high-angle boundaries [7]. Fig. 3 shows the influence of the laser energy density and initial diameter d of the filter on the population of highangle grain boundaries. It should be noted that d of 1 and 1.4 mm resulted in the final filter diameter of approximately 100 and 500 nm, respectively. Amount of high-angle (random) grain boundary decreases as either decreasing the d-value or increasing the DE value. This was explained by an increase of the grain selection capacity during the vertical growth phase in the grain filter [2,4], which was confirmed by cross-sectional TEM analyses in this study. Decreasing the d-value decreases the final diameter of the grain filter, hence, increases the grain selection capability in the vertical growth phase. Increasing DE value increases melt depth, hence, vertical growth distance inside the grain filter, which also enhances the grain selection. Fig. 4 shows volume fraction of different types of the CSL boundaries for various energy density and initial diameters of the grain filters. For all conditions, the CSL boundaries are predominantly S3, followed by S9 and S27. The result is consistent with the well-known fact that S9
Fig. 3. Amount of random grain boundaries in the location-controlled Si islands for various filter diameter d and energy density DE.
Fig. 4. Volume fraction of CSL boundaries vs. initial filter diameter d and energy density DE.
and S27 CSL boundaries are the second- and third-order twins. The second-order twin, for example, is generated by combination of two S3 boundaries [8–10]. It seems that for high-energy densities (1300 mJ/cm2) and for narrow cavities (d ¼ 1 mm), the contribution of twin boundaries is proportionally decreasing with their order. 3.2. Planar TEM analysis Fig. 5 shows a planar TEM image of the Si grain near the grain filter. An indexed SADP diffraction pattern of the crystals indicated a diffraction pattern representing a S3 relationship between two grains. A trial was made in order to identify the faceting behavior along the boundary (schematic representation in Fig. 6) between grains M and T.
Fig. 5. Planar TEM image of Si grain near the rim (dotted circle) of the grain filter (d ¼ 1.2 mm, DE ¼ 700 mJ/cm2).
Fig. 6. Schematic representation of the S3 boundary shown in Fig. 5 (a) ¯ ¯ is between grains M and T with marked segments A–J. ð1 1 1ÞM/ð1 1¯ 1ÞT ¯ is seen at BC and ð1¯ 5¯ 1ÞM/ð1 1¯ 1ÞT at seen at AB, GF, HJ, ð2 1¯ 1ÞM/ð1 2 1ÞT DE.
ARTICLE IN PRESS R. Ishihara et al. / Journal of Crystal Growth 299 (2007) 316–321
¯ M Boundaries AB, FG, HJ are coherent with ð1 1¯ 1Þ ¯ ¯ plane parallel to ð1 1 1Þ T. The curved morphology of the boundary indicates a pronounced change in the faceting behavior. Not all segments of the boundary were identified. ¯ Segment BC represents a faceting ð2 1¯ 1ÞM/ð1 2 1ÞT while ¯ ¯ ¯ segment DE a faceting ð1 5 1ÞM/ð1 1 1ÞT. Coherent twin ¯ ¯ boundaries ð1 1 1ÞM/ð1 1¯ 1ÞT are very flat as can be seen along AB segment. Massed straight boundaries parallel each other with a nano-meter distance, shown in bottom right of Fig. 5, were found to be also S3 twin boundaries having a facet of {1 1 1} plane. These characteristic features allow concluding about the high density of coherent S3 boundaries generated near the rim of the grain filter. {1 1 1}, {1 1 2} and {1 1 5}/{1 1 1} have been reported to be the planes of first, second and third highest planar density of coincidence sites among S3 boundary showing a corresponding increasing discontinuity. Considering the approach described above, identification of other boundaries inside the islands was performed. In Fig. 7, 7 grains are marked and the corresponding boundaries between them were analyzed. Based on common observations of the boundaries behavior in BF mode and corresponding diffraction patterns, boundaries between grains 1 and 2, 2 and 3 and 5 and 7 were identified as being low-angle boundaries (LABs). Previous SEM–EBSD investigations revealed possible LABs up to 21 and no other boundaries with misorientations between 21 and 101 were detected. This suggests about a very low misorientation degree between the neighboring grains 1 and 2, 2 and 3 and 4 and 6. Moreover, the grains 4 and 5 (and following, grains 5 and 6) as well as grains 5 and 7 are S3-related. However, the difference (which can be noticed in the morphology of the grain boundary) is that between grains 5–7 there is a faceting of the type (111)5/(111)7 suggesting about a higher coherency degree of this boundary.
Fig. 7. Bright-field image of Si-islands marked 1–7 considered for the identification of the boundaries between them. The position of the grain filter is marked by dotted circle. A better visualization of the grains 5–7 is shown in the right hand corner.
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3.3. Cross-sectional TEM analysis Fig. 8 shows a cross sectional TEM image of the location-controlled grain. It can be seen that a single grain is successfully selected at around the midway of the grain filter. The boundary between crystals 1 and 2 seems to be S3 boundary, corresponding with the result from planar TEM that S3-boundaries tend to generate in the vicinity of the rims during crystallization process. For cross sectional TEM in Fig. 9, the grain filter is completely melted due to a large filter diameter and a high laser energy density. From diffraction pattern a S3-relationship between grains 1 and 2 is observed by obtaining the overlap of two /1 1 0S-type of zone axes of the two grains. From the CBED analysis, grains 2 and 3 might be S3-related as well. This suggests that, in case that Si-film in the grain filter is completely melted, the tendency for S3 twinning is more pronounced and it occurs in earlier stages of the crystallization.
3.4. Size of the Si islands Fig. 10 shows the island size as a function of the energy density for various initial diameter d of the grain-filter. For d ¼ 1.0 mm, the island size increases with the energy density, and takes the maximum at 1300 mJ/cm2. Island size is limited by nucleation in the melt, which collides with the growing crystals. The island size increases with the DE because onset of the nucleation in the melt is delayed by giving more energy inside the Si with the higher DE. Similarly, for d ¼ 1.2 mm, the island size takes the maximum at 1100 mJ/cm2. The island size then decreases with d. This is because when DE is higher than 1100 mJ/ cm2, as shown in the Fig. 9, the Si inside the grain filter is completely melted. It has been simulated that the larger the
Fig. 8. Cross-sectional TEM bright field view of a location-controlled grain with a grain filter (d ¼ 1.0 mm, DE ¼ 1.25 J/cm2). The left upper portion was damaged during TEM specimen preparation.
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as there was the delay due to the cooling down to the nucleation temperature in the grain filter. The sooner nucleation outside the grain filter caused the smaller island size. It was also reported that the solidification after the nucleation in the melt is more rapid as the molten-Si is already severely undercooled. The rapid growth would be the reason why the number of the CSL grain boundaries increased and S3 twinning is more pronounced for the larger d-values. 4. Conclusion
Fig. 9. Cross-sectional TEM bright field view of a location-controlled grain with a grain filter (d ¼ 1.2 mm, DE ¼ 1.30 J/cm2).
Microstructure of location-controlled grains by mCzochralski process was characterized with EBSD and TEM. We confirmed that defects in the location-controlled grain are mainly S3 twin boundary generating from near the rim of the grain filter, while random grain boundaries hardly exist. Dependence of the population was investigated on process parameters. An increase in the filter diameter or decrease in the energy density reduces the chances for the selection of only one seed; as a result, more seeds can grow independently resulting in high-angle boundaries between them. We found that most of the S3 twin boundaries have {1 1 1} facet plane, which, in same case, are massed with a nano-meter spacing. Some other S3 grain boundaries sometimes indicated, in the short segments, faceting of the type {1 1 2} and {1 1 1}/{1 1 5}. These boundaries have a curved morphology. {1 1 1}, {1 1 2} and {1 1 1}/{1 1 5} are the planes having the first, second and third highest planar density of coincidence sites showing a corresponding decreasing continuity in the CSL. TEM investigations on the cross section specimens revealed a pronounced tendency for S3 twinning at the rims level. In case of complete melt of the Si in the grain filter, the tendency for the S3 twinning was more pronounced and it occurred in the very early stages of the crystallization in the grain filter. This is probably due to the rapid crystal growth in the severely undercooled Si. The information will be useful for further process improvement in the 2D-location control of the Si islands for single-grain TFT application. Acknowledgments
Fig. 10. Island size as a function of the energy density for various initial diameters d of the grain-filter.
d is, the more heat conductive the grain filter is. Namely the grain filter is more effectively heated up and eventually the Si inside is melted more easily with the larger d. When the Si is melted, nucleation must occur in the melt after being a severely undercooled. Solidification will then take place after the nucleation. In this case, the molten Si outside the grain filter approaches the nucleation temperature sooner
This work is part of the research program of the ‘‘Technologiestichting STW’’, Project number DEL.4542 and the ‘‘Stichting voor Fundamenteel Onderzoek der Materie (FOM)’’, Project number 97TF05, which are financially supported by the ‘‘Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)’’. References [1] R. Ishihara, A. Burtsev, P.F.A. Alkemade, Jpn. J. Appl. Phys. 39 (2000) 3872. [2] R. Ishihara, P.C. van der Wilt, B.D. van Dijk, J.W. Metselaar, C.I.M. Beenakker, in: A.T. Voutsas (Ed.), Proceedings of SPIE 5004, SPIE, Bellingham, 2003, p. 10.
ARTICLE IN PRESS R. Ishihara et al. / Journal of Crystal Growth 299 (2007) 316–321 [3] V. Rana, R. Ishihara, Y. Hiroshima, D. Abe, S. Higashi, S. Inoue, T. Shimoda, J.W. Metselaar, C.I.M. Beenakker, IEEE Trans. Electron Devices 54 (2007) 124. [4] P.Ch. van der Wilt, B.D. van Dijk, G.J. Bertens, R. Ishihara, C.I.M. Beenakker, Appl. Phys. Lett. 79 (2001) 1819. [5] A.V. Andreeva, A.V. Bazhenov, N.A. Bulenkov, A.A. Firsova, Phys. Stat. Sol. (a) 106 (1988) 351.
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[6] W. Bollmann, Crystal Defects and Crystalline Interfaces, Springer, Berlin, 1970, p. 49. [7] M. Kohyama, R. Yamamoto, Phys. Rev. B 49 (1994) 17102. [8] G.L. Bleris, Th. Karakostas, Acta Crystallogr. A 45 (1989) 297. [9] G.D. Sukhomlin, A.V. Andreeva, Phys. Stat. Sol. (a) 78 (1983) 333. [10] Ch.V. Kopezky, A.V. Andreeva, G.D. Sukhomilin, Acta Metall. Mater. 39 (1991) 1603.
Journal of Crystal Growth 299 (2007) 316–321 www.elsevier.com/locate/jcrysgro
Microstructure characterization of location-controlled Si-islands crystallized by excimer laser in the m-Czochralski (grain filter) process R. Ishiharaa,, D. Danciua, F. Tichelaara, M. Hea, Y. Hiroshimab, S. Inoueb, T. Shimodab, J.W. Metselaara,b, C.I.M. Beenakkera,b a
Delft Institute of Microelectonics and Submicrontechnology (DIMES), Delft University of Technology, Delft, The Netherlands b Technology Platform Research Center, Seiko-Epson Corp., Nagano, Japan Received 4 April 2006; received in revised form 5 December 2006; accepted 6 December 2006 Communicated by P. Rudolph Available online 17 December 2006
Abstract Microstructure of location-controlled grains by m-Czochralski process was characterized with electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM). We confirmed that defects in the location-controlled grain are mainly S3 twin boundary generating from near the rim of the grain filter, while random grain boundaries hardly exist. Dependence of the population was investigated on process parameters. We found that most of the S3 twin boundaries have {1 1 1} facet plane, which, in same case, are massed with a nano-meter spacing. S3 twin boundaries having facet planes {1 1 2} and {1 1 1}/{1 1 5} were also found to exist. r 2007 Elsevier B.V. All rights reserved. PACS: 81.10.Fq; 81.05.Cy; 61.72.y; 61.72.Mm Keywords: A1. Crystal structure; A2. Czochralski method; A2. Growth from melt; B2. Semiconducting silicon
1. Introduction Excimer laser crystallization (ELC) is a well-established method for producing polycrystalline silicon films on nonrefractory substrates. 2D location control of Si grains in ELC [1,2] enables formation of single-grain thin-film transistors (TFTs) [3], which perform like SOI-FETs. In m-Czochralski process [2,4], 7 mm large Si grain can be grown from ‘‘grain filter’’, which refers hole created in the underlying SiO2 and filled with the a-Si. Upon excimerlaser irradiation, the grain filter melts non-completely, whereas the surrounding melts completely. During vertical growth of the pre-existing seeds in the grain filter, occlusion of grains occurs reducing the number of growing grains. Therefore, the process could alternatively be called ‘‘microCorresponding author. Delft Institute of Microelectonics and submicrontechnology (DIMES), Delft University of Technology, Feldmannweg 17, P.O. Box 5053, 2600 GB Delft, The Netherlands. Tel.:+31(0) 15 2788498; fax: +31(0) 15 2622163. E-mail address: [email protected] (R. Ishihara).
0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.12.010
Bridgeman process’’. When growing phase reaches about the rims level, vertical growth is followed by the lateral growth. The growth continues until the growth interface collides with the one growing from opposite direction which is created either by nucleation in the melt or by grain growth from other grain filters. Single-grain TFTs fabricated inside the grain showed an average field effect mobility of more than 600 cm2/Vs [3]. Investigation of microstructure of the location-controlled grain is important, since it was suggested that the deviation of the transistor characteristics is related with planar defects in the grain [4]. It is known, that some types of grain boundaries can be electrically active reducing the mobility of carriers [5]. The presence of the grain boundaries can be correlated with discontinuity of the crystalline phase of each grain on either side of the boundaries. For a given misorientation between two grains, a lattice consisting of sites common to each grain is referred to as coincidence site lattice (CSL) [6]. The coincidence index S is the reciprocal density of CSLs with respect to the original lattice. The periodicity of the boundary structure can be analyzed by
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the periodicity in the plane in the CSL. CSL boundaries with relatively small S values contain short periods. It is the purpose of this paper to identify by the mean of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) the influence of processing parameters (grain filter diameter, laser energy density) on defects generation and their density in crystallized Siislands. Using the TEM, detailed information concerning the type of boundaries and their location is also presented. Crystallographic relationship of adjacent grains gives qualitative information about the faceting of some identified twinned grains; hence, the coherency of the boundary between them.
specimens were prepared using a FEI–FIB microscope. Images obtained by bright field (BF)/dark field (DF) techniques and diffraction patterns (selected area diffraction patterns [SADP] and convergent beam electron diffraction [CBED]) were performed with a Philips 30 T microscope operated at 300 kV. The twin boundaries types between the grains were evaluated as follows: in SEM– EBSD as the misorientation angle and rotation axes from recorded Kikuchi maps in TEM as the misorientation angle between two zone axes and common planes from recorded SADP and CBED patterns.
2. Experiment
3.1. EBSD analysis
Since a detailed process description can be found in elsewhere [2,4], here we describe it shortly. Cavities having various initial diameter d (Fig. 1) in a 750 nm-thick SiO2 were narrowed down by an additional SiO2 deposition. A 250 or 150 nm thick a-Si film was deposited by LPCVD at 545 1C and crystallized by 1 shot of XeCl excimer-laser irradiation with various energy densities (DE) at a substrate temperature of 450 1C. The laser beam has top-hat shape and a size of approximately 3 5 mm, i.e., uniform light exposure for the grain filter structure. SEM–electron backscattering diffraction (SEM–EBSD) analyses were performed using a Philips scanning microscope XL 30S. For TEM investigations both planar and cross section specimens were prepared. The planar TEM specimens were prepared by conventional grinding and polishing technique up to a thickness of 10 mm. They were finally thinned to electron transparency using an ion mill with Ar+ beam at 61 and 5 kV. The cross-section TEM
Fig. 2 shows a SEM image after Secco’s etching and surface crystallographic orientation map of Si grains characterized by EBSD overlapped with SEM image. Here the location-controlled grain has a square shaped with 6 mm length, which is the spacing between the grain filters. It can be seen that random grain boundaries (black lines) inside the location-controlled grains are nearly absent, and that CSL grain boundaries often extend radially from vicinity of the center of the location-controlled grain at where the grain filter underlies. The CSL boundaries are predominantly S3 (dark gray lines), followed by S9 (light gray lines). S3 and S9 twin boundaries are associated with a rather low interfacial energy as compared with other
Fig. 1. Schematic cross-sectional drawing of the sample for m-Czochralski process. The d denotes the initial diameter of the grain filter.
3. Results and discussion
Fig. 2. EBSD orientation mapping of a 3 6 grid of grains by mCzochralski process. The curved lines indicate boundaries S3 (dark gray lines), S9 (light gray lines) CSL boundaries and random grain boundaries (black lines).
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types of twin boundaries or random high-angle boundaries [7]. Fig. 3 shows the influence of the laser energy density and initial diameter d of the filter on the population of highangle grain boundaries. It should be noted that d of 1 and 1.4 mm resulted in the final filter diameter of approximately 100 and 500 nm, respectively. Amount of high-angle (random) grain boundary decreases as either decreasing the d-value or increasing the DE value. This was explained by an increase of the grain selection capacity during the vertical growth phase in the grain filter [2,4], which was confirmed by cross-sectional TEM analyses in this study. Decreasing the d-value decreases the final diameter of the grain filter, hence, increases the grain selection capability in the vertical growth phase. Increasing DE value increases melt depth, hence, vertical growth distance inside the grain filter, which also enhances the grain selection. Fig. 4 shows volume fraction of different types of the CSL boundaries for various energy density and initial diameters of the grain filters. For all conditions, the CSL boundaries are predominantly S3, followed by S9 and S27. The result is consistent with the well-known fact that S9
Fig. 3. Amount of random grain boundaries in the location-controlled Si islands for various filter diameter d and energy density DE.
Fig. 4. Volume fraction of CSL boundaries vs. initial filter diameter d and energy density DE.
and S27 CSL boundaries are the second- and third-order twins. The second-order twin, for example, is generated by combination of two S3 boundaries [8–10]. It seems that for high-energy densities (1300 mJ/cm2) and for narrow cavities (d ¼ 1 mm), the contribution of twin boundaries is proportionally decreasing with their order. 3.2. Planar TEM analysis Fig. 5 shows a planar TEM image of the Si grain near the grain filter. An indexed SADP diffraction pattern of the crystals indicated a diffraction pattern representing a S3 relationship between two grains. A trial was made in order to identify the faceting behavior along the boundary (schematic representation in Fig. 6) between grains M and T.
Fig. 5. Planar TEM image of Si grain near the rim (dotted circle) of the grain filter (d ¼ 1.2 mm, DE ¼ 700 mJ/cm2).
Fig. 6. Schematic representation of the S3 boundary shown in Fig. 5 (a) ¯ ¯ is between grains M and T with marked segments A–J. ð1 1 1ÞM/ð1 1¯ 1ÞT ¯ is seen at BC and ð1¯ 5¯ 1ÞM/ð1 1¯ 1ÞT at seen at AB, GF, HJ, ð2 1¯ 1ÞM/ð1 2 1ÞT DE.
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¯ M Boundaries AB, FG, HJ are coherent with ð1 1¯ 1Þ ¯ ¯ plane parallel to ð1 1 1Þ T. The curved morphology of the boundary indicates a pronounced change in the faceting behavior. Not all segments of the boundary were identified. ¯ Segment BC represents a faceting ð2 1¯ 1ÞM/ð1 2 1ÞT while ¯ ¯ ¯ segment DE a faceting ð1 5 1ÞM/ð1 1 1ÞT. Coherent twin ¯ ¯ boundaries ð1 1 1ÞM/ð1 1¯ 1ÞT are very flat as can be seen along AB segment. Massed straight boundaries parallel each other with a nano-meter distance, shown in bottom right of Fig. 5, were found to be also S3 twin boundaries having a facet of {1 1 1} plane. These characteristic features allow concluding about the high density of coherent S3 boundaries generated near the rim of the grain filter. {1 1 1}, {1 1 2} and {1 1 5}/{1 1 1} have been reported to be the planes of first, second and third highest planar density of coincidence sites among S3 boundary showing a corresponding increasing discontinuity. Considering the approach described above, identification of other boundaries inside the islands was performed. In Fig. 7, 7 grains are marked and the corresponding boundaries between them were analyzed. Based on common observations of the boundaries behavior in BF mode and corresponding diffraction patterns, boundaries between grains 1 and 2, 2 and 3 and 5 and 7 were identified as being low-angle boundaries (LABs). Previous SEM–EBSD investigations revealed possible LABs up to 21 and no other boundaries with misorientations between 21 and 101 were detected. This suggests about a very low misorientation degree between the neighboring grains 1 and 2, 2 and 3 and 4 and 6. Moreover, the grains 4 and 5 (and following, grains 5 and 6) as well as grains 5 and 7 are S3-related. However, the difference (which can be noticed in the morphology of the grain boundary) is that between grains 5–7 there is a faceting of the type (111)5/(111)7 suggesting about a higher coherency degree of this boundary.
Fig. 7. Bright-field image of Si-islands marked 1–7 considered for the identification of the boundaries between them. The position of the grain filter is marked by dotted circle. A better visualization of the grains 5–7 is shown in the right hand corner.
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3.3. Cross-sectional TEM analysis Fig. 8 shows a cross sectional TEM image of the location-controlled grain. It can be seen that a single grain is successfully selected at around the midway of the grain filter. The boundary between crystals 1 and 2 seems to be S3 boundary, corresponding with the result from planar TEM that S3-boundaries tend to generate in the vicinity of the rims during crystallization process. For cross sectional TEM in Fig. 9, the grain filter is completely melted due to a large filter diameter and a high laser energy density. From diffraction pattern a S3-relationship between grains 1 and 2 is observed by obtaining the overlap of two /1 1 0S-type of zone axes of the two grains. From the CBED analysis, grains 2 and 3 might be S3-related as well. This suggests that, in case that Si-film in the grain filter is completely melted, the tendency for S3 twinning is more pronounced and it occurs in earlier stages of the crystallization.
3.4. Size of the Si islands Fig. 10 shows the island size as a function of the energy density for various initial diameter d of the grain-filter. For d ¼ 1.0 mm, the island size increases with the energy density, and takes the maximum at 1300 mJ/cm2. Island size is limited by nucleation in the melt, which collides with the growing crystals. The island size increases with the DE because onset of the nucleation in the melt is delayed by giving more energy inside the Si with the higher DE. Similarly, for d ¼ 1.2 mm, the island size takes the maximum at 1100 mJ/cm2. The island size then decreases with d. This is because when DE is higher than 1100 mJ/ cm2, as shown in the Fig. 9, the Si inside the grain filter is completely melted. It has been simulated that the larger the
Fig. 8. Cross-sectional TEM bright field view of a location-controlled grain with a grain filter (d ¼ 1.0 mm, DE ¼ 1.25 J/cm2). The left upper portion was damaged during TEM specimen preparation.
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as there was the delay due to the cooling down to the nucleation temperature in the grain filter. The sooner nucleation outside the grain filter caused the smaller island size. It was also reported that the solidification after the nucleation in the melt is more rapid as the molten-Si is already severely undercooled. The rapid growth would be the reason why the number of the CSL grain boundaries increased and S3 twinning is more pronounced for the larger d-values. 4. Conclusion
Fig. 9. Cross-sectional TEM bright field view of a location-controlled grain with a grain filter (d ¼ 1.2 mm, DE ¼ 1.30 J/cm2).
Microstructure of location-controlled grains by mCzochralski process was characterized with EBSD and TEM. We confirmed that defects in the location-controlled grain are mainly S3 twin boundary generating from near the rim of the grain filter, while random grain boundaries hardly exist. Dependence of the population was investigated on process parameters. An increase in the filter diameter or decrease in the energy density reduces the chances for the selection of only one seed; as a result, more seeds can grow independently resulting in high-angle boundaries between them. We found that most of the S3 twin boundaries have {1 1 1} facet plane, which, in same case, are massed with a nano-meter spacing. Some other S3 grain boundaries sometimes indicated, in the short segments, faceting of the type {1 1 2} and {1 1 1}/{1 1 5}. These boundaries have a curved morphology. {1 1 1}, {1 1 2} and {1 1 1}/{1 1 5} are the planes having the first, second and third highest planar density of coincidence sites showing a corresponding decreasing continuity in the CSL. TEM investigations on the cross section specimens revealed a pronounced tendency for S3 twinning at the rims level. In case of complete melt of the Si in the grain filter, the tendency for the S3 twinning was more pronounced and it occurred in the very early stages of the crystallization in the grain filter. This is probably due to the rapid crystal growth in the severely undercooled Si. The information will be useful for further process improvement in the 2D-location control of the Si islands for single-grain TFT application. Acknowledgments
Fig. 10. Island size as a function of the energy density for various initial diameters d of the grain-filter.
d is, the more heat conductive the grain filter is. Namely the grain filter is more effectively heated up and eventually the Si inside is melted more easily with the larger d. When the Si is melted, nucleation must occur in the melt after being a severely undercooled. Solidification will then take place after the nucleation. In this case, the molten Si outside the grain filter approaches the nucleation temperature sooner
This work is part of the research program of the ‘‘Technologiestichting STW’’, Project number DEL.4542 and the ‘‘Stichting voor Fundamenteel Onderzoek der Materie (FOM)’’, Project number 97TF05, which are financially supported by the ‘‘Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)’’. References [1] R. Ishihara, A. Burtsev, P.F.A. Alkemade, Jpn. J. Appl. Phys. 39 (2000) 3872. [2] R. Ishihara, P.C. van der Wilt, B.D. van Dijk, J.W. Metselaar, C.I.M. Beenakker, in: A.T. Voutsas (Ed.), Proceedings of SPIE 5004, SPIE, Bellingham, 2003, p. 10.
ARTICLE IN PRESS R. Ishihara et al. / Journal of Crystal Growth 299 (2007) 316–321 [3] V. Rana, R. Ishihara, Y. Hiroshima, D. Abe, S. Higashi, S. Inoue, T. Shimoda, J.W. Metselaar, C.I.M. Beenakker, IEEE Trans. Electron Devices 54 (2007) 124. [4] P.Ch. van der Wilt, B.D. van Dijk, G.J. Bertens, R. Ishihara, C.I.M. Beenakker, Appl. Phys. Lett. 79 (2001) 1819. [5] A.V. Andreeva, A.V. Bazhenov, N.A. Bulenkov, A.A. Firsova, Phys. Stat. Sol. (a) 106 (1988) 351.
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[6] W. Bollmann, Crystal Defects and Crystalline Interfaces, Springer, Berlin, 1970, p. 49. [7] M. Kohyama, R. Yamamoto, Phys. Rev. B 49 (1994) 17102. [8] G.L. Bleris, Th. Karakostas, Acta Crystallogr. A 45 (1989) 297. [9] G.D. Sukhomlin, A.V. Andreeva, Phys. Stat. Sol. (a) 78 (1983) 333. [10] Ch.V. Kopezky, A.V. Andreeva, G.D. Sukhomilin, Acta Metall. Mater. 39 (1991) 1603.