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Laser crystallized multicrystalline silicon thin films on glass
Thin Solid Films 487 (2005) 77 – 80 www.elsevier.com/locate/tsf
Laser crystallized multicrystalline silicon thin films on glass G. Andr7, J. Bergmann, F. Falk* Institut fu¨r Physikalische Hochtechnolgie, Jena, Germany Available online 26 February 2005
Abstract By zone melting using a scanned cw frequency doubled Nd:YAG laser beam multicrystalline silicon films on glass have been produced with grains of 100 Am in size. These films are useful for solar cells and for thin film transistors (TFTs). In the paper the possibilities and limitations of cw laser crystallization of amorphous silicon thin films on glass are discussed. Particularly an industrial relevant high throughput process using a high power diode laser is presented. D 2005 Elsevier B.V. All rights reserved. Keywords: Silicon; Crystallization; Laser irradiation
1. Introduction Crystalline silicon thin films on glass substrates with as large crystallites as possible are useful for thin film solar cells and for thin film transistors TFTs in active matrix flat panel displays. Crystallites exceeding several micrometers in size can be generated via a melting and solidification process only, requiring a temperature above 1412 8C, whereas glass endures 600 8C only. This contradiction can be solved by using laser irradiation to produce the high temperature for a time short enough not to damage the glass substrate but long enough to allow for silicon crystal growth. For TFTs the crystallization of amorphous silicon layers by ultra violet excimer laser pulses of several 10 ns in duration is state of the art [1,2]. In this way silicon layers 10 to 100 nm thick can by crystallized. Due to the high cooling rate after the short pulse, the size of the produced crystallites is limited to about 1 Am which reduces the carrier mobility to values below 300 cm2/Vs [2]. Since the grain boundaries act as recombination centers these films are not very useful for solar cells [3]. For solar cells, grains larger than the film thickness are required, that is grains above 10 Am in size. This can be achieved by a cw laser zone melting process operating at a wavelength in the visible range [4]. By
* Corresponding author. E-mail address: [email protected] (F. Falk). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.01.082
optimizing the process parameters such as beam cross section and scanning rate areas with crystallites even larger than 100 Am have been prepared [5]. Films prepared in this way are of interest not only for solar cells but also for TFTs. Mobilities as large as 690 cm2/Vs have been reached [6]. For an industrial use of the laser zone melting process in both applications mentioned, several problems must be solved. Firstly, for TFTs a film thickness of 100 nm or below is appropriate. The same applies for seed layers in solar cells which are inactive optoelectronically and therefore should be as thin as possible. We observed, however, that in the cw laser process the damage of the layers increases dramatically in films thinner than about 400 nm. Secondly, crystallization without producing cracks is necessary. Finally, a reduction of the process time is needed for economic reasons. In the present paper solutions for the mentioned challenges are discussed.
2. Experimental Amorphous silicon films 100 to 700 nm thick were deposited by electron beam evaporation or by PECVD onto borosilicate glass substrates (Corning 7059 or Schott Boro 33). These glass substrates coincide with respect to the thermal expansion coefficient with crystalline silicon in the temperature range up to 600 8C. The PECVD films were
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deposited at a substrate temperature of 500 8C so that the hydrogen content was about 3 at.%. Laser crystallization was performed on a heating stage under ambient air using a frequency doubled Nd:YAG laser emitting a wavelength of 532 nm at a power of up to 10 W. Alternatively a diode laser emitting 806 nm at a power of up to 750 W was used. The films were characterized by optical microscopy. Grains sizes were determined by transmission electron microscopy.
3. Results and discussion Fig. 1 shows a 2.52.5 cm2 area of a film crystallized by scanning the beam of the Nd:YAG laser. This demonstrates that large area crystallization is feasible which in contrast to TFT applications is required for solar cells. The crystal structure of a 400 to 700 nm thick film crystallized using a laser power of 4.5 W consists of grains several 10 Am in size (Fig. 2). The diameter of the Gaussian beam was 100 Am and the heater temperature 620 8C. The laser beam was scanned at a speed of 5 cm/s in rows overlapping by 50%. Under these conditions the melting time is about 1 ms. The large grains form by lateral epitaxial solidification. In the first row, epitaxy starts at the fine grains of the cold rim, in the following rows this occurs at the already large grains of the previous row. The solidification front moves opposite to the direction of the temperature gradient, i.e. in a direction to the center following the running laser spot. The large grains are oriented statistically with respect to the surface normal [7] since already the fine grains at the rim do not show any preferred orientation. Texture could be achieved if one starts not from amorphous but from textured microcrystalline silicon produced by excimer laser crystallization [8]. Problems arise under the mentioned crystallization conditions if the film thickness is reduced to below 400 nm when film damage occurs as shown in Fig. 3. In order to
Fig. 1. Multicrystalline area (light) produced from amorphous silicon (dark surroundings) on glass produced by frequency doubled cw Nd:YAG laser crystallization.
Fig. 2. Optical micrograph of a 400 nm thick silicon film crystallized by scanning a frequency doubled cw Nd:YAG laser beam in rows (power 4.5 W, Gaussian beam 100 Am in diameter, scanning rate 5 cm/s, 50% row overlap, heater temperature 620 8C). First row on top.
investigate this problem more thoroughly, single spot irradiations were performed on films 100 or 400 nm thick. Varied were the laser power and the irradiation time. Fig. 4 shows the result for parameters optimized with respect to large grains. The large grains grow from the cold rim to the center, i.e. again opposite to the direction of the temperature gradient. If the laser power or the irradiation time is reduced, fine grains as in the rim of Fig. 4 form throughout the spot. Higher laser power or longer irradiation time leads to film damage. Fig. 5 shows diagrammatically the results of the variation of the laser power and of the irradiation time for two values of the film thickness. The lower curves give the power necessary for generating large grains as shown in Fig. 4. The upper curves show the damage threshold so that the region in between is the useful range. The laser power for optimum crystal growth has to be slightly increased for thinner films whereas the damage threshold decreases. So the useful parameter range decreases for thinner films but a sufficiently large window remains. However, even in the useful parameter range, occasionally, we find damage which occurs more often in thinner films. We explain this behavior
Fig. 3. Nd:YAG laser crystallized film 200 nm thick with damage. Irradiation parameters as in Fig. 2.
G. Andra¨ et al. / Thin Solid Films 487 (2005) 77–80
Fig. 4. 400 nm thick silicon film crystallized by Nd:YAG laser spot irradiation using optimized parameters for large grains.
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Fig. 6. Typical cracks in films after fast cooling after laser crystallization.
by dewetting starting at film irregularities such as powder or dust particles if the relevant particle is thicker than the film and therefore protrudes through the surface. The number of those active particles is higher in thinner films. In the case of scanned rows this local damage is perpetuated since dewetting further continues with the running liquid spot. To avoid dewetting a cover layer or an interface layer which is better wetted by liquid silicon would be useful. Adding a silica or silicon nitride cover layer was not successful because of cracking after solidification due to different thermal expansion coefficients. Experiments with an amorphous silicon nitride wetting layer deposited by PECVD onto the substrate led to film damage already far below the crystallization threshold. The reason was laser crystallization of the nitride and the release of hydrogen during laser heating. Further investigations concerning wetting and cover layers stable against laser irradiation are necessary. The same applies to diffusion barrier layers required for TFTs on glass. Another way to prevent dewetting is to reduce the time of melting by faster scans. This was performed by Sasaki [9] who used a scanning rate of 20 cm/s and a beam diameter of
20 Am in scan direction leading to a melting time of about 0.1 ms. This approach, however, yields grains of only 3 to 5 Am in size. Problems with glass cracking during crystallization, as shown in Fig. 6, cannot be avoided if the substrate is held at room temperature or up to the annealing point of the glass defined by a viscosity of 1013 dPa. If the glass substrate is heated above the annealing point (560 8C in our case) immediate cracking can be avoided. However, there is a tendency of cracking after several hours. The probability of cracks increases with the size of the crystals produced. This effect can be eliminated by keeping the system for 30 min after crystallization at elevated temperature. According to glass manufacturers [10] internal stress in the glass is eliminated at the annealing temperature within 15 min. At lower temperature no reduction of internal stress is to be expected. Up to now we did not check if this procedure is successful for films covered by silica layers for avoiding dewetting. A fact is, however, that covered layers are more prone to cracks. If for specific applications such as TFTs, grains have to be placed at predetermined positions, this can be achieved by periodically switching the scanned laser beam on and off as shown in Fig. 7. In these islands containing in the center a
Fig. 5. Diagram of crystallization and damage thresholds for films 100 or 400 nm thick using spot irradiation.
Fig. 7. Single crystalline patches produced by switching the scanned Nd:YAG laser beam on and off periodically. Sickle shaped beam.
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4. Summary
Fig. 8. 800 Am wide multicrystalline silicon trace crystallized by scanning a diode laser beam [beam cross section 200 Am800 Am, laser power 455 W (see text), scanning rate 10 cm/s].
perfect single crystal 50 Am wide and 250 mm long cracks do not occur even if the substrate is cooled down fast. Due to the amorphous surroundings the internal stress in the crystalline patches are able to relax. For a later industrial application economic aspects become important. At an optimum scan speed of 5 cm/s, a beam diameter of 100 Am in scanning direction a laser power of 5 W is sufficient to crystallize a 100 Am wide row yielding grains several 10 Am in size. Crystallizing a 3030 cm2 area needs 5 to 10 h depending on the overlap of the rows which is not acceptable. There are two possibilities for time reduction: firstly one can use a higher scanning speed which, however, leads to smaller crystallites [9]. The second more attractive possibility is to use high power lasers. The only choice are diode lasers available with a power of up to several kilowatts. Then the time required for a 3030 cm2 area to crystallize reduces to minutes. However, the available laser wavelength of 806 nm is only weakly absorbed by silicon and the beam quality is rather bad. In Fig. 8 it is shown that laser crystallization by a diode laser is possible. At scan rate of 10 cm/s, a beam cross section of 200800 Am2 and a power of 455 W, of which only a small part was used due to blinds reducing the original ray cross section of 1.30.9 mm2, large grains 100 Am wide and more than 1 mm long were produced. Further work is required concerning beam shaping and optimizing the crystallization process in order to use the whole amount of power offered.
By cw laser crystallization silicon films on glass substrates can be prepared consisting of grains exceeding 100 Am in size. These films are suitable for solar cells and for TFTs in flat panel displays. An industrial solar cell production using this technology is possible. Films without cracks and without damage can be fabricated in a reasonable time by using diode lasers. For TFTs in flat panel displays further work is necessary concerning films with a thickness of 100 nm or below. To this end cover layers for avoiding dewetting and diffusion barrier layers on the glass substrate have to be investigated. These layers have to endure the laser crystallization process without damage or cracking. One route to avoid cracks is to use crystalline patches instead of continuous layers. To reach an acceptable process time diode lasers must be used. It has to be tested if an appropriate beam shaping is possible. Acknowledgment This work was partially funded by the German Ministries of Economics and of Environment (contract 0329613), and by DAAD. References [1] W. Staudt, S. Borneis, K.-D. Pippert, Phys. Status Solidi A 166 (1998) 743. [2] G. Fortunato, L. Mariucci, R. Carluccio, A. Pecora, V. Foglietti, Appl. Surf. Sci. 154–155 (2000) 95. [3] R.B. Bergmann, J.H. Werner, Thin Solid Films 403–404 (2002) 162. [4] S. Kawamura, J. Sakurai, M. Nakano, M. Takagi, Appl. Phys. Lett. 40 (1982) 394. [5] G. Andr7, J. Bergmann, F. Falk, E. Ose, Appl. Surf. Sci. 154–155 (2000) 123. [6] A. Saboundji, T. Mohammed-Brahim, G. Andr7, J. Bergmann, F. Falk, J. Non-Cryst. Solids 338–340 (2004) 758. [7] S. Christiansen, M. Nerding, C. Eder, G. Andr7, F. Falk, J. Bergmann, E. Ose, H.P. Strunk, Mater. Res. Soc. Symp. Proc. 621 (2000) Q7.5.1. [8] D.P. Gosain, A. Machida, T. Fujino, Y. Hitsuda, K. Nakano, J. Sato, Jpn. J. Appl. Phys. 42 (2003) L135. [9] F. Takeuchi, A. Hara, N. Sasaki, Proc. Int. Disp. Manuf. Conf. (2002) 73. [10] Schott Technische Gl7ser, Schott Glaswerke, Mainz, 1990.
Laser crystallized multicrystalline silicon thin films on glass G. Andr7, J. Bergmann, F. Falk* Institut fu¨r Physikalische Hochtechnolgie, Jena, Germany Available online 26 February 2005
Abstract By zone melting using a scanned cw frequency doubled Nd:YAG laser beam multicrystalline silicon films on glass have been produced with grains of 100 Am in size. These films are useful for solar cells and for thin film transistors (TFTs). In the paper the possibilities and limitations of cw laser crystallization of amorphous silicon thin films on glass are discussed. Particularly an industrial relevant high throughput process using a high power diode laser is presented. D 2005 Elsevier B.V. All rights reserved. Keywords: Silicon; Crystallization; Laser irradiation
1. Introduction Crystalline silicon thin films on glass substrates with as large crystallites as possible are useful for thin film solar cells and for thin film transistors TFTs in active matrix flat panel displays. Crystallites exceeding several micrometers in size can be generated via a melting and solidification process only, requiring a temperature above 1412 8C, whereas glass endures 600 8C only. This contradiction can be solved by using laser irradiation to produce the high temperature for a time short enough not to damage the glass substrate but long enough to allow for silicon crystal growth. For TFTs the crystallization of amorphous silicon layers by ultra violet excimer laser pulses of several 10 ns in duration is state of the art [1,2]. In this way silicon layers 10 to 100 nm thick can by crystallized. Due to the high cooling rate after the short pulse, the size of the produced crystallites is limited to about 1 Am which reduces the carrier mobility to values below 300 cm2/Vs [2]. Since the grain boundaries act as recombination centers these films are not very useful for solar cells [3]. For solar cells, grains larger than the film thickness are required, that is grains above 10 Am in size. This can be achieved by a cw laser zone melting process operating at a wavelength in the visible range [4]. By
* Corresponding author. E-mail address: [email protected] (F. Falk). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.01.082
optimizing the process parameters such as beam cross section and scanning rate areas with crystallites even larger than 100 Am have been prepared [5]. Films prepared in this way are of interest not only for solar cells but also for TFTs. Mobilities as large as 690 cm2/Vs have been reached [6]. For an industrial use of the laser zone melting process in both applications mentioned, several problems must be solved. Firstly, for TFTs a film thickness of 100 nm or below is appropriate. The same applies for seed layers in solar cells which are inactive optoelectronically and therefore should be as thin as possible. We observed, however, that in the cw laser process the damage of the layers increases dramatically in films thinner than about 400 nm. Secondly, crystallization without producing cracks is necessary. Finally, a reduction of the process time is needed for economic reasons. In the present paper solutions for the mentioned challenges are discussed.
2. Experimental Amorphous silicon films 100 to 700 nm thick were deposited by electron beam evaporation or by PECVD onto borosilicate glass substrates (Corning 7059 or Schott Boro 33). These glass substrates coincide with respect to the thermal expansion coefficient with crystalline silicon in the temperature range up to 600 8C. The PECVD films were
78
G. Andra¨ et al. / Thin Solid Films 487 (2005) 77–80
deposited at a substrate temperature of 500 8C so that the hydrogen content was about 3 at.%. Laser crystallization was performed on a heating stage under ambient air using a frequency doubled Nd:YAG laser emitting a wavelength of 532 nm at a power of up to 10 W. Alternatively a diode laser emitting 806 nm at a power of up to 750 W was used. The films were characterized by optical microscopy. Grains sizes were determined by transmission electron microscopy.
3. Results and discussion Fig. 1 shows a 2.52.5 cm2 area of a film crystallized by scanning the beam of the Nd:YAG laser. This demonstrates that large area crystallization is feasible which in contrast to TFT applications is required for solar cells. The crystal structure of a 400 to 700 nm thick film crystallized using a laser power of 4.5 W consists of grains several 10 Am in size (Fig. 2). The diameter of the Gaussian beam was 100 Am and the heater temperature 620 8C. The laser beam was scanned at a speed of 5 cm/s in rows overlapping by 50%. Under these conditions the melting time is about 1 ms. The large grains form by lateral epitaxial solidification. In the first row, epitaxy starts at the fine grains of the cold rim, in the following rows this occurs at the already large grains of the previous row. The solidification front moves opposite to the direction of the temperature gradient, i.e. in a direction to the center following the running laser spot. The large grains are oriented statistically with respect to the surface normal [7] since already the fine grains at the rim do not show any preferred orientation. Texture could be achieved if one starts not from amorphous but from textured microcrystalline silicon produced by excimer laser crystallization [8]. Problems arise under the mentioned crystallization conditions if the film thickness is reduced to below 400 nm when film damage occurs as shown in Fig. 3. In order to
Fig. 1. Multicrystalline area (light) produced from amorphous silicon (dark surroundings) on glass produced by frequency doubled cw Nd:YAG laser crystallization.
Fig. 2. Optical micrograph of a 400 nm thick silicon film crystallized by scanning a frequency doubled cw Nd:YAG laser beam in rows (power 4.5 W, Gaussian beam 100 Am in diameter, scanning rate 5 cm/s, 50% row overlap, heater temperature 620 8C). First row on top.
investigate this problem more thoroughly, single spot irradiations were performed on films 100 or 400 nm thick. Varied were the laser power and the irradiation time. Fig. 4 shows the result for parameters optimized with respect to large grains. The large grains grow from the cold rim to the center, i.e. again opposite to the direction of the temperature gradient. If the laser power or the irradiation time is reduced, fine grains as in the rim of Fig. 4 form throughout the spot. Higher laser power or longer irradiation time leads to film damage. Fig. 5 shows diagrammatically the results of the variation of the laser power and of the irradiation time for two values of the film thickness. The lower curves give the power necessary for generating large grains as shown in Fig. 4. The upper curves show the damage threshold so that the region in between is the useful range. The laser power for optimum crystal growth has to be slightly increased for thinner films whereas the damage threshold decreases. So the useful parameter range decreases for thinner films but a sufficiently large window remains. However, even in the useful parameter range, occasionally, we find damage which occurs more often in thinner films. We explain this behavior
Fig. 3. Nd:YAG laser crystallized film 200 nm thick with damage. Irradiation parameters as in Fig. 2.
G. Andra¨ et al. / Thin Solid Films 487 (2005) 77–80
Fig. 4. 400 nm thick silicon film crystallized by Nd:YAG laser spot irradiation using optimized parameters for large grains.
79
Fig. 6. Typical cracks in films after fast cooling after laser crystallization.
by dewetting starting at film irregularities such as powder or dust particles if the relevant particle is thicker than the film and therefore protrudes through the surface. The number of those active particles is higher in thinner films. In the case of scanned rows this local damage is perpetuated since dewetting further continues with the running liquid spot. To avoid dewetting a cover layer or an interface layer which is better wetted by liquid silicon would be useful. Adding a silica or silicon nitride cover layer was not successful because of cracking after solidification due to different thermal expansion coefficients. Experiments with an amorphous silicon nitride wetting layer deposited by PECVD onto the substrate led to film damage already far below the crystallization threshold. The reason was laser crystallization of the nitride and the release of hydrogen during laser heating. Further investigations concerning wetting and cover layers stable against laser irradiation are necessary. The same applies to diffusion barrier layers required for TFTs on glass. Another way to prevent dewetting is to reduce the time of melting by faster scans. This was performed by Sasaki [9] who used a scanning rate of 20 cm/s and a beam diameter of
20 Am in scan direction leading to a melting time of about 0.1 ms. This approach, however, yields grains of only 3 to 5 Am in size. Problems with glass cracking during crystallization, as shown in Fig. 6, cannot be avoided if the substrate is held at room temperature or up to the annealing point of the glass defined by a viscosity of 1013 dPa. If the glass substrate is heated above the annealing point (560 8C in our case) immediate cracking can be avoided. However, there is a tendency of cracking after several hours. The probability of cracks increases with the size of the crystals produced. This effect can be eliminated by keeping the system for 30 min after crystallization at elevated temperature. According to glass manufacturers [10] internal stress in the glass is eliminated at the annealing temperature within 15 min. At lower temperature no reduction of internal stress is to be expected. Up to now we did not check if this procedure is successful for films covered by silica layers for avoiding dewetting. A fact is, however, that covered layers are more prone to cracks. If for specific applications such as TFTs, grains have to be placed at predetermined positions, this can be achieved by periodically switching the scanned laser beam on and off as shown in Fig. 7. In these islands containing in the center a
Fig. 5. Diagram of crystallization and damage thresholds for films 100 or 400 nm thick using spot irradiation.
Fig. 7. Single crystalline patches produced by switching the scanned Nd:YAG laser beam on and off periodically. Sickle shaped beam.
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4. Summary
Fig. 8. 800 Am wide multicrystalline silicon trace crystallized by scanning a diode laser beam [beam cross section 200 Am800 Am, laser power 455 W (see text), scanning rate 10 cm/s].
perfect single crystal 50 Am wide and 250 mm long cracks do not occur even if the substrate is cooled down fast. Due to the amorphous surroundings the internal stress in the crystalline patches are able to relax. For a later industrial application economic aspects become important. At an optimum scan speed of 5 cm/s, a beam diameter of 100 Am in scanning direction a laser power of 5 W is sufficient to crystallize a 100 Am wide row yielding grains several 10 Am in size. Crystallizing a 3030 cm2 area needs 5 to 10 h depending on the overlap of the rows which is not acceptable. There are two possibilities for time reduction: firstly one can use a higher scanning speed which, however, leads to smaller crystallites [9]. The second more attractive possibility is to use high power lasers. The only choice are diode lasers available with a power of up to several kilowatts. Then the time required for a 3030 cm2 area to crystallize reduces to minutes. However, the available laser wavelength of 806 nm is only weakly absorbed by silicon and the beam quality is rather bad. In Fig. 8 it is shown that laser crystallization by a diode laser is possible. At scan rate of 10 cm/s, a beam cross section of 200800 Am2 and a power of 455 W, of which only a small part was used due to blinds reducing the original ray cross section of 1.30.9 mm2, large grains 100 Am wide and more than 1 mm long were produced. Further work is required concerning beam shaping and optimizing the crystallization process in order to use the whole amount of power offered.
By cw laser crystallization silicon films on glass substrates can be prepared consisting of grains exceeding 100 Am in size. These films are suitable for solar cells and for TFTs in flat panel displays. An industrial solar cell production using this technology is possible. Films without cracks and without damage can be fabricated in a reasonable time by using diode lasers. For TFTs in flat panel displays further work is necessary concerning films with a thickness of 100 nm or below. To this end cover layers for avoiding dewetting and diffusion barrier layers on the glass substrate have to be investigated. These layers have to endure the laser crystallization process without damage or cracking. One route to avoid cracks is to use crystalline patches instead of continuous layers. To reach an acceptable process time diode lasers must be used. It has to be tested if an appropriate beam shaping is possible. Acknowledgment This work was partially funded by the German Ministries of Economics and of Environment (contract 0329613), and by DAAD. References [1] W. Staudt, S. Borneis, K.-D. Pippert, Phys. Status Solidi A 166 (1998) 743. [2] G. Fortunato, L. Mariucci, R. Carluccio, A. Pecora, V. Foglietti, Appl. Surf. Sci. 154–155 (2000) 95. [3] R.B. Bergmann, J.H. Werner, Thin Solid Films 403–404 (2002) 162. [4] S. Kawamura, J. Sakurai, M. Nakano, M. Takagi, Appl. Phys. Lett. 40 (1982) 394. [5] G. Andr7, J. Bergmann, F. Falk, E. Ose, Appl. Surf. Sci. 154–155 (2000) 123. [6] A. Saboundji, T. Mohammed-Brahim, G. Andr7, J. Bergmann, F. Falk, J. Non-Cryst. Solids 338–340 (2004) 758. [7] S. Christiansen, M. Nerding, C. Eder, G. Andr7, F. Falk, J. Bergmann, E. Ose, H.P. Strunk, Mater. Res. Soc. Symp. Proc. 621 (2000) Q7.5.1. [8] D.P. Gosain, A. Machida, T. Fujino, Y. Hitsuda, K. Nakano, J. Sato, Jpn. J. Appl. Phys. 42 (2003) L135. [9] F. Takeuchi, A. Hara, N. Sasaki, Proc. Int. Disp. Manuf. Conf. (2002) 73. [10] Schott Technische Gl7ser, Schott Glaswerke, Mainz, 1990.