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Optimization of phase-modulated excimer-laser annealing method for growing highly-packed large-grains in Si thin-films
Applied Surface Science 154–155 Ž2000. 105–111 www.elsevier.nlrlocaterapsusc
Optimization of phase-modulated excimer-laser annealing method for growing highly-packed large-grains in Si thin-films Chang-Ho Oh ) , Mitsuru Nakata, Masakiyo Matsumura Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8550, Japan Received 1 June 1999; accepted 29 July 1999
Abstract Optimization has been done theoretically for the phase-modulated excimer-laser annealing method to grow highly packed large grains in the Si film, where the divergence of the laser light beam plays an important role. Generalized optimum annealing conditions were given graphically as a function of the maximum-to-minimum light intensity ratio. Theoretical results were verified also experimentally by growing grains as large as 7 mm with 10 mm-pitch using a single shot of excimer-laser light pulse. It is pointed out that there is a room for improving the packing density to almost 100% by simply shortening the pitch of the phase shifters. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Excimer-laser crystallization; Polycrystalline silicon; Grain growth; Phase-shift mask; Phase-modulation; Thin-film transistors; Lateral growth
1. Introduction For high-performance active-matrix liquid-crystal displays ŽAMLCDs., preparation of single-crystal Si Žc-Si. thin film transistors ŽTFTs. at low temperatures is a core technology w1x. Also, only c-Si TFT seems to have today the capability of high drivingcurrents, low leakage and high uniformity of the current–voltage characteristics. Aiming at AMLCDs with high definition and built-in peripheral circuits, such c-Si TFTs should be formed with high packing density. Thus, a new technique that is capable of growing grains larger than the TFT feature size with high packing density over a large area of glass ) Corresponding author. Tel.: q81-3-57342696; fax: q81-357342559; http:rrsilicon.pe.titech.ac.jp. E-mail address: [email protected] ŽC.-H. Oh..
substrates and with reasonable process cost is desired. It is well known that the excimer-laser annealing ŽELA. method is one of the most promising among various technologies that can form the poly-Si film at low temperatures w2x, although various problems remain in the current ELA method. In particular, AMLCDs using small-grain poly-Si TFTs formed by the ELA method is used in production recently. We have invented a novel ELA method, called ‘‘phase-modulated ELA ŽPMELA. method’’ w3–5x, which was advanced from ‘‘energy gradient ELA method’’ w6x for a satisfactory control of a laser light-intensity distribution on the sample surface. This method is an application of the well-known technology based on the phase-shift concept in photolithography. That is, a phase-shift mask is placed between the sample and the laser to modulate the spatial phase of excimer-laser light waves, so that
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 4 7 7 - 8
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C.-H. Oh et al.r Applied Surface Science 154–155 (2000) 105–111
mutual interference of light waves results in spatially modulated light intensity on the sample surface. Since thermal energy given from the light to the Si thin film on the sample surface is proportional to the intensity, time-dependent crystallization kinetics of molten Si varies along the sample surface. Then, grains start to grow laterally in the ‘‘completely molten’’ Si film from the low intensity region to the high intensity region. Grains as large as 7 mm in length or 3 mm in diameter were successfully grown by a single shot irradiation Ži.e., with low process cost. of KrF excimer-laser light at 5008C. However, spacing of periodically grown grains, i.e., the pitch of phase-shifters formed on the phase-shift mask, had to be sufficiently wide since interference effects result in a strong spurious intensity oscillation even at far from the shifter w4x. In this paper, we present the optimizing procedure of the PMELA method to produce ultra large grains with high packing density by localizing the interference effects.
2.1. The effects of beam diÕergence
from mutual interference as shown in Fig. 1, as an example, where the calculated distribution is plotted. Fig. 1a shows the individual patterns on the sample surface formed by isolated shifters, i.e., under the assumption that p is infinitely long. There are also strong interference effects at positions far from the isolated shifter itself, resulting in large spurious intensity oscillation also near a position where the next shifter will be. Fig. 1b shows the total distribution resulting from all the shifters aligned with 10 mmpitch, as an example. The smooth individual pattern is distorted seriously, resulting in a ‘‘canyon-like’’ shaped distribution form. Since grain growth stops at each peak, it seems impossible to produce large grains with a short spacing such as 10 mm. There are two different approaches for solving this complicated interference problem. The first one is taking the interference from all shifters aligned with short pitch into account from the beginning, and then selecting the best shifter parameters for the desirable distribution. This straightforward optimization method will need a long computation time but will give no generalized results. The other approach is localizing the interference effects from one isolated shifter near the shifter itself. If the interference effects are well localized, then summing up the
In a previous paper w4x, we have described that the lateral growth can begin at a critical light intensity IC for complete melting of the Si film and can end at another larger critical intensity IA for ablation of the film. Thus, the ideal intensity distribution for large grains is equilateral triangle-like shaped Žin case of linear shifter. or cone-like shaped Žin case of circular shifter. one with the minimum intensity IC and the maximum intensity IA . Thus, a ratio g Žs IC rIA . is an important ELA parameter since its values can be changed by the average intensity IO of the laser light. Another important parameter is the distance L between the maximum and minimum intensity positions. L should be equal to the maximum attainable grain size, which is determined by the lateral crystallization rate and solidification duration. The pitch p of periodically aligned shifters on the mask should be as short as 2 L for highly packed grain growth, because grains can be grown on both sides from the minimum intensity point w3x. There is, however, a serious problem in the intensity distribution resulting
Fig. 1. The effects of narrow pitch of phase-shifters, Ža. individual distributions Žb. resulting distribution by mutual interference.
2. Optimization procedures
C.-H. Oh et al.r Applied Surface Science 154–155 (2000) 105–111
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100 mrad, although the intensity at x s 0 is increased. This increased intensity is, however, desirable to grow grains bi-directionally from x s 0. It can be then concluded that the beam divergence has the ability to suppress the strong spurious oscillations and keep the local pattern near the shifter desirable. Thus, by optimizing the beam divergence ultra-large grains can be grown with high packing density.
2.2. Optimization method Fig. 2. Calculated intensity distribution Ž Ir IO . for several values of the beam divergence for the case of a linear phase-shifter.
individual patterns from the shifter can approximate well the real distribution generated by the shifter array. The optimization is then necessary only for the isolated shifter to generate the desirable local pattern. In this work, we concentrate our efforts on the latter approach since it can give generalized optimum conditions with straightforward understanding. We have pointed out w4x that good interference patterns with slight damping can be obtained on the sample surface only when the beam divergence is very small and that the beam divergence modifies the pattern especially on the outside. Fig. 2 shows clearly the effects of beam divergence D P in the case of the isolated linear shifter having a phase retardation u s p. For D P s 0 mrad, the intensity oscillates strongly, with only gradual damping, at the positions far from the shifter position, i.e., x s 0. For D P s 50 mrad, the spurious oscillation is suppressed dramatically and the intensity at x ) 15 mm becomes almost constant, although the intensity near x s 0 is changed a little. This tendency becomes more clear for D P s
Optimization was done for the linear shifters whose parameters are the phase retardation u , mask-sample spacing d and beam divergence D P . Fig. 3 shows the intensity distribution model used in the optimization. First, we assume values of grain growth parameters, L and g . Next, we calculate the representative amplitude g of the spurious oscillation, i.e. the characteristic intensity difference between the first maximum and the second minimum, as a function of u , d, and D P . Third, we find the set of parameters Ž u , d, and D P . that minimize the g value Žunder fixed g and L conditions.. Finally, the total intensity distribution is calculated, for checking purpose, for shifter arrays with various p values, by taking the interference effects from all shifters into account. For the case of a sample structured with a-Si Ž200 nm-thick.rSiO 2 substrate, we obtained experimentally; g s 2 Ž IC s 550 mJrcm2 , IA s 1100 mJrcm2 . and L s 3.5 mm. The optimum conditions in the case of an isolated linear shifter, were u s p, d s 41 mm and D P s 167 mrad, respectively. This distribution is given by the solid curve shown in Fig. 4. The
Fig. 3. The intensity distribution model used for optimization.
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Fig. 4. Optimized intensity distribution for a linear phase-shifter with u s p, ds 41 mm and D P s167 mrad.
intensity near x s 0 is raised; corresponding with the bi-directional lateral growth from x s 0. The beam divergence of D P s 167 mrad can suppress the strong spurious oscillations appearing in the original pattern for D P s 0 mrad, and the intensity is almost constant for x ) 6 mm. The real intensity distribution for the shifter array with p s 10 mm is then calculated and shown by a dot-dashed curve in Fig. 4. The distribution near x s 0 is almost the same as that for the isolated shifter. A uniform intensity region around x s 5 mm still remains, where lateral grain growth is not expected. Thus, the packing density will be about 70%. It is found, however, that the distribution takes a sinusoidal form, which is very near the ideal one Žequilateral triangle-like shape. by shortening p to 7 mm, i.e., as long as 2 L, as shown by a sharp curve in the figure. The intensity near x s 0 is about the same for p s 10 mm and p s 7 mm. But, the peak value around x s 3.5 mm is about 20% higher for p s 7 mm than for p s 10 mm. Thus, we expect that almost all the irradiated area can be crystallized into large grains by a single shot irradiation by shortening p to 7 mm Žwith a little modification of D P and d values for compensating the 20% higher peak intensity as discussed in the following paragraph.. The interference pattern along the sample surface is a function of the phase difference along optical paths, and thus its characteristic distances can be normalized by d lrL2 , where l is the wavelength of the excimer-laser light. Thus the effects of the beam divergence along the sample surface can be normal-
Fig. 5. Generalized optimum conditions in case of linear phaseshifters where l is the wavelength of the excimer-laser light.
ized by L D Prl. The optimum conditions derived above are normalized to the parameter set Ž d lrL2 s 0.85, L D Prl s 2.3 and u s p . under the condition that g is fixed at 2. By similar calculations for various g values, the generalized optimum conditions can be given, as a function of g , as shown in Fig. 5. It is interesting that u is always p. The optimum conditions are calculated also for a circular shifter. Fig. 6 shows the intensity distribution for the circular shifter array under the optimum conditions of u s pr3, d s 27 mm, r s 1.5 mm Žthe radius of circular shifter. and D P s 202 mrad while p was assumed 10 mm. The distribution along the
Fig. 6. Optimized intensity distribution for a circular phase-shifter with u s p r3, r s1.5 mm, ds 27 mm and D P s 202 mrad.
C.-H. Oh et al.r Applied Surface Science 154–155 (2000) 105–111
center of shifters is very similar to that for the linear shifter arrays with the same pitch Žsee the dotted curve in Fig. 4.. Thus we expect that p can be reduced also to 7 mm for the circular shifter array.
3. Experimental results and discussion Samples consist of a capping SiO 2 Ž120 nmthick.ra-Si film Ž200 nm-thick. structure on a Si substrate with 850 nm-thick SiO 2 . The starting a-Si film was deposited by a low-pressure chemical vapor deposition ŽCVD. system at 5008C using Si 2 H 6 gas. The shifters on the quartz substrate were fabricated as linear Žone-dimensional. and circular Žtwo-dimensional. forms. We used a diffusive plate made of quartz for controlling the beam divergence. The experimental equipment consisted of an excimer-laser operating at 248 nm ŽKrF., optics, and a high-vacuum chamber. A single shot of laser-light was irradiated onto the sample heated at 5008C through the diffusion plate and phase-shift mask which in turn were placed in front of the sample with spacing d. Microstructural analysis of the crystallized Si film was
109
performed by means of a scanning electron microscope ŽSEM., after grain boundary dislocation etch ŽSecco etch.. Fig. 7 shows a top view of the crystallized Si film together with the calculated intensity distribution for a linear shifter array with p s 10 mm. Fig. 7a is the case of a nearly perfect plane wave without a diffusion plate. Corresponding to the ‘‘canyon-shaped’’ intensity distribution Žby calculation., a complicated morphology with grains as small as 2 mm in length was observed, although it was periodic. Two dark regions with 2 mm-wide also appeared. They were formed in etched away regions with in the original a-Si structure. Fig. 7b is the case of D P s 150 mrad, which is close to the optimum condition as discussed previously. Here large grains appeared, extending to left and right, as long as 3.5 mm. Also there were small grain regions 3 mm wide. These results match well the calculated results. A few large grains as long as 7 mm could be observed. These grains were grown along both directions from only one nucleus at the center in the molten Si. Packing density of large grains was about 70%. Since this value was about the same as the theoretically expected one
Fig. 7. Top view of the crystallized Si film after Secco etching for the case of linear phase-shifters with u s p, d s 45 mm, Ža. D P s 0 mrad Žwithout diffusion plate., Žb. D P s 150 mrad Žwith diffusion plate..
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Fig. 8. Top view of the crystallized Si film after Secco etching for the case of a circular phase-shifter with u s pr3, r s 1.5 mm and d s 200 mm, Ža. D P s 0 mrad Žwithout diffusion plate., Žb. D P s 50 mrad Žwith diffusion plate..
under the same crystallization conditions, we believe that the density can be improved to nearly 100% by shortening the pitch to 7 mm. In the same manner, we examined the case of the circular shifter. The calculated intensity distribution and a top view of the crystallized Si film are shown in Fig. 8 for an isolated shifter. In the case without
diffusive plate, there were small grains with coaxial rings and the laterally grown grains, corresponding to the oscillating intensity distribution. A small gain region exists at the center, where the intensity is locally uniform, although more than IC . Using a diffusive plate, large grains grew with a circular shape along the radial direction from the center. We note that there were no coaxial rings outside the circular grains due to the sufficient suppression of the interference effects by beam divergence. Finally, we present a top eye view of the film crystallized morphology by using the circular shifter array with p s 10 mm in Fig. 9. Quasi single grains as large as 7 mm were grown with 10 mm-pitch with the cone-like shaped distribution shown in Fig. 7.
4. Conclusions
Fig. 9. Top eye view of the quasi Si grains after Secco etching for the case of 10 mm pitched circular phase-shifters with u s p r3, r s1.5 mm, ds 30 mm and D P s 200 mrad.
We have proposed a method of growing large grains with high packing density by the phase-modulated ELA method. Generalized optimum ELA conditions were presented where the beam divergence of the laser light played an important role. This optimization procedure was verified theoretically and
C.-H. Oh et al.r Applied Surface Science 154–155 (2000) 105–111
experimentally for linear and circular shifter arrays. Quasi single grains as large as 7 mm were grown successfully with a pitch as narrow as 10 mm for both arrays by single shot excimer-laser light pulse. Nearly 100% packing density with large grains was expected by shortening the pitch to 7 mm. This optimization method will be useful not only for high-performance AMLCDs, but also for high-efficiency poly-Si solar cells.
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References w1x M. Matsumura, Phys. Status Solidi A 166 Ž1998. 715. w2x M.A. Crowder, P.G. Carey, P.M. Smith, R.S. Sposili, H.S. Cho, J.S. Im, IEEE Electron Device Lett. 19 Ž8. Ž1998. 306. w3x C.H. Oh, M. Ozawa, M. Matsumura, Jpn. J. Appl. Phys. 37 Ž1998. L492. w4x C.H. Oh, M. Matsumura, Jpn. J. Appl. Phys. 37 Ž1998. 5474. w5x M. Matsumura, C.H. Oh, Thin Solid Films 337 Ž1999. 123. w6x K. Ishikawa, M. Ozawa, C.H. Oh, M. Matsumura, Jpn. J. Appl. Phys. 37 Ž1998. 731.
Optimization of phase-modulated excimer-laser annealing method for growing highly-packed large-grains in Si thin-films Chang-Ho Oh ) , Mitsuru Nakata, Masakiyo Matsumura Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8550, Japan Received 1 June 1999; accepted 29 July 1999
Abstract Optimization has been done theoretically for the phase-modulated excimer-laser annealing method to grow highly packed large grains in the Si film, where the divergence of the laser light beam plays an important role. Generalized optimum annealing conditions were given graphically as a function of the maximum-to-minimum light intensity ratio. Theoretical results were verified also experimentally by growing grains as large as 7 mm with 10 mm-pitch using a single shot of excimer-laser light pulse. It is pointed out that there is a room for improving the packing density to almost 100% by simply shortening the pitch of the phase shifters. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Excimer-laser crystallization; Polycrystalline silicon; Grain growth; Phase-shift mask; Phase-modulation; Thin-film transistors; Lateral growth
1. Introduction For high-performance active-matrix liquid-crystal displays ŽAMLCDs., preparation of single-crystal Si Žc-Si. thin film transistors ŽTFTs. at low temperatures is a core technology w1x. Also, only c-Si TFT seems to have today the capability of high drivingcurrents, low leakage and high uniformity of the current–voltage characteristics. Aiming at AMLCDs with high definition and built-in peripheral circuits, such c-Si TFTs should be formed with high packing density. Thus, a new technique that is capable of growing grains larger than the TFT feature size with high packing density over a large area of glass ) Corresponding author. Tel.: q81-3-57342696; fax: q81-357342559; http:rrsilicon.pe.titech.ac.jp. E-mail address: [email protected] ŽC.-H. Oh..
substrates and with reasonable process cost is desired. It is well known that the excimer-laser annealing ŽELA. method is one of the most promising among various technologies that can form the poly-Si film at low temperatures w2x, although various problems remain in the current ELA method. In particular, AMLCDs using small-grain poly-Si TFTs formed by the ELA method is used in production recently. We have invented a novel ELA method, called ‘‘phase-modulated ELA ŽPMELA. method’’ w3–5x, which was advanced from ‘‘energy gradient ELA method’’ w6x for a satisfactory control of a laser light-intensity distribution on the sample surface. This method is an application of the well-known technology based on the phase-shift concept in photolithography. That is, a phase-shift mask is placed between the sample and the laser to modulate the spatial phase of excimer-laser light waves, so that
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 4 7 7 - 8
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C.-H. Oh et al.r Applied Surface Science 154–155 (2000) 105–111
mutual interference of light waves results in spatially modulated light intensity on the sample surface. Since thermal energy given from the light to the Si thin film on the sample surface is proportional to the intensity, time-dependent crystallization kinetics of molten Si varies along the sample surface. Then, grains start to grow laterally in the ‘‘completely molten’’ Si film from the low intensity region to the high intensity region. Grains as large as 7 mm in length or 3 mm in diameter were successfully grown by a single shot irradiation Ži.e., with low process cost. of KrF excimer-laser light at 5008C. However, spacing of periodically grown grains, i.e., the pitch of phase-shifters formed on the phase-shift mask, had to be sufficiently wide since interference effects result in a strong spurious intensity oscillation even at far from the shifter w4x. In this paper, we present the optimizing procedure of the PMELA method to produce ultra large grains with high packing density by localizing the interference effects.
2.1. The effects of beam diÕergence
from mutual interference as shown in Fig. 1, as an example, where the calculated distribution is plotted. Fig. 1a shows the individual patterns on the sample surface formed by isolated shifters, i.e., under the assumption that p is infinitely long. There are also strong interference effects at positions far from the isolated shifter itself, resulting in large spurious intensity oscillation also near a position where the next shifter will be. Fig. 1b shows the total distribution resulting from all the shifters aligned with 10 mmpitch, as an example. The smooth individual pattern is distorted seriously, resulting in a ‘‘canyon-like’’ shaped distribution form. Since grain growth stops at each peak, it seems impossible to produce large grains with a short spacing such as 10 mm. There are two different approaches for solving this complicated interference problem. The first one is taking the interference from all shifters aligned with short pitch into account from the beginning, and then selecting the best shifter parameters for the desirable distribution. This straightforward optimization method will need a long computation time but will give no generalized results. The other approach is localizing the interference effects from one isolated shifter near the shifter itself. If the interference effects are well localized, then summing up the
In a previous paper w4x, we have described that the lateral growth can begin at a critical light intensity IC for complete melting of the Si film and can end at another larger critical intensity IA for ablation of the film. Thus, the ideal intensity distribution for large grains is equilateral triangle-like shaped Žin case of linear shifter. or cone-like shaped Žin case of circular shifter. one with the minimum intensity IC and the maximum intensity IA . Thus, a ratio g Žs IC rIA . is an important ELA parameter since its values can be changed by the average intensity IO of the laser light. Another important parameter is the distance L between the maximum and minimum intensity positions. L should be equal to the maximum attainable grain size, which is determined by the lateral crystallization rate and solidification duration. The pitch p of periodically aligned shifters on the mask should be as short as 2 L for highly packed grain growth, because grains can be grown on both sides from the minimum intensity point w3x. There is, however, a serious problem in the intensity distribution resulting
Fig. 1. The effects of narrow pitch of phase-shifters, Ža. individual distributions Žb. resulting distribution by mutual interference.
2. Optimization procedures
C.-H. Oh et al.r Applied Surface Science 154–155 (2000) 105–111
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100 mrad, although the intensity at x s 0 is increased. This increased intensity is, however, desirable to grow grains bi-directionally from x s 0. It can be then concluded that the beam divergence has the ability to suppress the strong spurious oscillations and keep the local pattern near the shifter desirable. Thus, by optimizing the beam divergence ultra-large grains can be grown with high packing density.
2.2. Optimization method Fig. 2. Calculated intensity distribution Ž Ir IO . for several values of the beam divergence for the case of a linear phase-shifter.
individual patterns from the shifter can approximate well the real distribution generated by the shifter array. The optimization is then necessary only for the isolated shifter to generate the desirable local pattern. In this work, we concentrate our efforts on the latter approach since it can give generalized optimum conditions with straightforward understanding. We have pointed out w4x that good interference patterns with slight damping can be obtained on the sample surface only when the beam divergence is very small and that the beam divergence modifies the pattern especially on the outside. Fig. 2 shows clearly the effects of beam divergence D P in the case of the isolated linear shifter having a phase retardation u s p. For D P s 0 mrad, the intensity oscillates strongly, with only gradual damping, at the positions far from the shifter position, i.e., x s 0. For D P s 50 mrad, the spurious oscillation is suppressed dramatically and the intensity at x ) 15 mm becomes almost constant, although the intensity near x s 0 is changed a little. This tendency becomes more clear for D P s
Optimization was done for the linear shifters whose parameters are the phase retardation u , mask-sample spacing d and beam divergence D P . Fig. 3 shows the intensity distribution model used in the optimization. First, we assume values of grain growth parameters, L and g . Next, we calculate the representative amplitude g of the spurious oscillation, i.e. the characteristic intensity difference between the first maximum and the second minimum, as a function of u , d, and D P . Third, we find the set of parameters Ž u , d, and D P . that minimize the g value Žunder fixed g and L conditions.. Finally, the total intensity distribution is calculated, for checking purpose, for shifter arrays with various p values, by taking the interference effects from all shifters into account. For the case of a sample structured with a-Si Ž200 nm-thick.rSiO 2 substrate, we obtained experimentally; g s 2 Ž IC s 550 mJrcm2 , IA s 1100 mJrcm2 . and L s 3.5 mm. The optimum conditions in the case of an isolated linear shifter, were u s p, d s 41 mm and D P s 167 mrad, respectively. This distribution is given by the solid curve shown in Fig. 4. The
Fig. 3. The intensity distribution model used for optimization.
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C.-H. Oh et al.r Applied Surface Science 154–155 (2000) 105–111
Fig. 4. Optimized intensity distribution for a linear phase-shifter with u s p, ds 41 mm and D P s167 mrad.
intensity near x s 0 is raised; corresponding with the bi-directional lateral growth from x s 0. The beam divergence of D P s 167 mrad can suppress the strong spurious oscillations appearing in the original pattern for D P s 0 mrad, and the intensity is almost constant for x ) 6 mm. The real intensity distribution for the shifter array with p s 10 mm is then calculated and shown by a dot-dashed curve in Fig. 4. The distribution near x s 0 is almost the same as that for the isolated shifter. A uniform intensity region around x s 5 mm still remains, where lateral grain growth is not expected. Thus, the packing density will be about 70%. It is found, however, that the distribution takes a sinusoidal form, which is very near the ideal one Žequilateral triangle-like shape. by shortening p to 7 mm, i.e., as long as 2 L, as shown by a sharp curve in the figure. The intensity near x s 0 is about the same for p s 10 mm and p s 7 mm. But, the peak value around x s 3.5 mm is about 20% higher for p s 7 mm than for p s 10 mm. Thus, we expect that almost all the irradiated area can be crystallized into large grains by a single shot irradiation by shortening p to 7 mm Žwith a little modification of D P and d values for compensating the 20% higher peak intensity as discussed in the following paragraph.. The interference pattern along the sample surface is a function of the phase difference along optical paths, and thus its characteristic distances can be normalized by d lrL2 , where l is the wavelength of the excimer-laser light. Thus the effects of the beam divergence along the sample surface can be normal-
Fig. 5. Generalized optimum conditions in case of linear phaseshifters where l is the wavelength of the excimer-laser light.
ized by L D Prl. The optimum conditions derived above are normalized to the parameter set Ž d lrL2 s 0.85, L D Prl s 2.3 and u s p . under the condition that g is fixed at 2. By similar calculations for various g values, the generalized optimum conditions can be given, as a function of g , as shown in Fig. 5. It is interesting that u is always p. The optimum conditions are calculated also for a circular shifter. Fig. 6 shows the intensity distribution for the circular shifter array under the optimum conditions of u s pr3, d s 27 mm, r s 1.5 mm Žthe radius of circular shifter. and D P s 202 mrad while p was assumed 10 mm. The distribution along the
Fig. 6. Optimized intensity distribution for a circular phase-shifter with u s p r3, r s1.5 mm, ds 27 mm and D P s 202 mrad.
C.-H. Oh et al.r Applied Surface Science 154–155 (2000) 105–111
center of shifters is very similar to that for the linear shifter arrays with the same pitch Žsee the dotted curve in Fig. 4.. Thus we expect that p can be reduced also to 7 mm for the circular shifter array.
3. Experimental results and discussion Samples consist of a capping SiO 2 Ž120 nmthick.ra-Si film Ž200 nm-thick. structure on a Si substrate with 850 nm-thick SiO 2 . The starting a-Si film was deposited by a low-pressure chemical vapor deposition ŽCVD. system at 5008C using Si 2 H 6 gas. The shifters on the quartz substrate were fabricated as linear Žone-dimensional. and circular Žtwo-dimensional. forms. We used a diffusive plate made of quartz for controlling the beam divergence. The experimental equipment consisted of an excimer-laser operating at 248 nm ŽKrF., optics, and a high-vacuum chamber. A single shot of laser-light was irradiated onto the sample heated at 5008C through the diffusion plate and phase-shift mask which in turn were placed in front of the sample with spacing d. Microstructural analysis of the crystallized Si film was
109
performed by means of a scanning electron microscope ŽSEM., after grain boundary dislocation etch ŽSecco etch.. Fig. 7 shows a top view of the crystallized Si film together with the calculated intensity distribution for a linear shifter array with p s 10 mm. Fig. 7a is the case of a nearly perfect plane wave without a diffusion plate. Corresponding to the ‘‘canyon-shaped’’ intensity distribution Žby calculation., a complicated morphology with grains as small as 2 mm in length was observed, although it was periodic. Two dark regions with 2 mm-wide also appeared. They were formed in etched away regions with in the original a-Si structure. Fig. 7b is the case of D P s 150 mrad, which is close to the optimum condition as discussed previously. Here large grains appeared, extending to left and right, as long as 3.5 mm. Also there were small grain regions 3 mm wide. These results match well the calculated results. A few large grains as long as 7 mm could be observed. These grains were grown along both directions from only one nucleus at the center in the molten Si. Packing density of large grains was about 70%. Since this value was about the same as the theoretically expected one
Fig. 7. Top view of the crystallized Si film after Secco etching for the case of linear phase-shifters with u s p, d s 45 mm, Ža. D P s 0 mrad Žwithout diffusion plate., Žb. D P s 150 mrad Žwith diffusion plate..
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Fig. 8. Top view of the crystallized Si film after Secco etching for the case of a circular phase-shifter with u s pr3, r s 1.5 mm and d s 200 mm, Ža. D P s 0 mrad Žwithout diffusion plate., Žb. D P s 50 mrad Žwith diffusion plate..
under the same crystallization conditions, we believe that the density can be improved to nearly 100% by shortening the pitch to 7 mm. In the same manner, we examined the case of the circular shifter. The calculated intensity distribution and a top view of the crystallized Si film are shown in Fig. 8 for an isolated shifter. In the case without
diffusive plate, there were small grains with coaxial rings and the laterally grown grains, corresponding to the oscillating intensity distribution. A small gain region exists at the center, where the intensity is locally uniform, although more than IC . Using a diffusive plate, large grains grew with a circular shape along the radial direction from the center. We note that there were no coaxial rings outside the circular grains due to the sufficient suppression of the interference effects by beam divergence. Finally, we present a top eye view of the film crystallized morphology by using the circular shifter array with p s 10 mm in Fig. 9. Quasi single grains as large as 7 mm were grown with 10 mm-pitch with the cone-like shaped distribution shown in Fig. 7.
4. Conclusions
Fig. 9. Top eye view of the quasi Si grains after Secco etching for the case of 10 mm pitched circular phase-shifters with u s p r3, r s1.5 mm, ds 30 mm and D P s 200 mrad.
We have proposed a method of growing large grains with high packing density by the phase-modulated ELA method. Generalized optimum ELA conditions were presented where the beam divergence of the laser light played an important role. This optimization procedure was verified theoretically and
C.-H. Oh et al.r Applied Surface Science 154–155 (2000) 105–111
experimentally for linear and circular shifter arrays. Quasi single grains as large as 7 mm were grown successfully with a pitch as narrow as 10 mm for both arrays by single shot excimer-laser light pulse. Nearly 100% packing density with large grains was expected by shortening the pitch to 7 mm. This optimization method will be useful not only for high-performance AMLCDs, but also for high-efficiency poly-Si solar cells.
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