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Economic feeder for recharging and “topping off”
Journal of Crystal Growth 211 (2000) 372}377
Economic feeder for recharging and `topping o!a Bryan Fickett*, G. Mihalik Siemens Solar Industries, 12016 NE 95th Street, Suite 720, Vancouver, WA 98682, USA
Abstract Increasing the size of the melt charge signi"cantly increases yield and reduces costs. Siemens Solar Industries is optimizing a method to charge additional material after meltdown (top-o!) using an external feeder system. A prototype feeder system was fabricated consisting of a hopper and feed delivery system. The low-cost feeder is designed for simple operation and maintenance. The system is capable of introducing up to 60 kg of granular silicon while under vacuum. An isolation valve permits re"lling of the hopper while maintaining vacuum in the growth furnace. Using the feeder system in conjunction with Siemens Solar Industries' energy e$cient hot zone dramatically reduces power and argon consumption. Throughput is also improved as faster pull speeds can be attained. The increased pull speeds have an even greater impact when the charge size is increased. Further cost reduction can be achieved by re"lling the crucible after crystal growth and pulling a second ingot run. Siemens Solar Industries is presently testing the feeder in production. ( 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Past e!orts to develop charge replenishment technologies are well documented. The primary objective was usually to minimize the segregation e!ects for N-type ingot growth. Many researchers also recognized potential cost reduction bene"ts, mostly the result of spreading the high cost of quartz crucibles over several crystal pulls [1]. A variety of di!erent recharging techniques were developed in an attempt to realize these bene"ts. The various techniques can be grouped into one of two categories: semi-continuous (batch) or continu-
ous [2]. In semi-continuous recharging, material is added between ingot pulls. Semi-continuous techniques include lowering feed material either in rod form or by a hopper suspended from the seed cable. Another semi-continuous technique introduces feed stock by means of an external hopper/delivery system. Continuous replenishment typically employs a similar external recharge system that delivers pre-melted feedstock continuously during ingot growth. Although some success has been achieved using these approaches, there are a number of disadvantages:
* Corresponding author. Tel.: #1-360-944-9766; fax: #1360-944-9189. E-mail address: bryan."[email protected] (B. Fickett)
f Complicated and costly modi"cations to the growth equipment are often required [1]. f Delicate quartz components frequently fracture causing run aborts.
0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 8 0 1 - 5
B. Fickett, G. Mihalik / Journal of Crystal Growth 211 (2000) 372}377
373
f Some techniques require additional furnace isolations that extend cycle time and increase the possibility of leaks/contamination. f Impurities build up in the residual melt due to segregation [3]. In most cases, these disadvantages have prevented use of charge replenishment in manufacturing environments. The SSI approach is unique in that the main goal is to reduce manufacturing costs of P-type ingot growth through yield and throughput improvements. The greatest advantages are obtained by maximizing the initial charge. Subsequent recharges also impact manufacturing costs, but to a diminishing degree. For this reason, primary system development is geared towards re"ning a highly dependable method for topping o! the initial charge. A prototype feeder system was constructed to demonstrate feasibility. The prototype incorporated the following design considerations: f Low cost f Simple operation and maintenance * no electrical components/minimal moving parts. f Robust * able to withstand high volume production environment. f Vacuum integrity * vacuum tight to 50 mTorr with full isolation capability. f Adaptable * system compatible with various furnace models. f Purity * construction materials selected to prevent contamination. f Volume * capable of feeding up to 60 kg. At controlled feed rates up to 2 kg/min. Fig. 1. HTO system general assembly.
2. Apparatus The prototype system consists of a feed hopper and delivery system (Fig. 1). The transparent hopper allows material level to be checked at a glance. The top cap of the hopper assembly includes a "ll port and connections for vacuum and argon. A combination pressure/vacuum gauge is mounted in the top cap as well. A funnel is installed inside the bottom of the feed hopper to ensure complete discharge of feed material.
An angle of repose valve (AOR valve) is used for starting/stopping the media #ow. A vacuum isolation valve separates the hopper assembly from the furnace. Inside the crystal growth furnace, a charge tube extension directs the feed material to within 2A of the melt surface. The size of the ori"ce at the end of the charge tube extension determines the #ow rate. A reduced ori"ce minimizes kinetic energy and
374
B. Fickett, G. Mihalik / Journal of Crystal Growth 211 (2000) 372}377
helps to prevent feed material from bouncing out of the crucible. The control panel includes valves for controlling hopper vacuum and argon.
3. Procedure The feed hopper is "lled with a pre-determined quantity of silicon granules or sifted "nes. Dopant, if required, is crushed and placed in the hopper as well. After "lling, the system is pumped down to 50 mTorr, then back-"lled to atmospheric pressure with argon. This procedure is repeated three times to remove oxygen. When delivery of feed material is required, the crucible position is adjusted so that the melt level is approximately 2A below the charge tube outlet. Crucible rotation is set at three RPM. The isolation and the AOR valves are then opened. Material begins to #ow immediately. The process is monitored during feeding, and the crucible position adjusted as needed to keep the growing mound of unmelted silicon from contacting the charge tube extension. Once charging is complete, the AOR and isolation valves are closed. The material is melted and the temperature stabilized prior to seeding. The procedure can be repeated multiple times during a run.
4. Results and discussion A number of developmental challenges hindered progress early in this project. Vacuum leaks caused crystals to lose structure and deteriorated hot-zone components. Maintaining seal integrity proved dif"cult due to the e!ects of a pneumatic vibrator on the sealing hardware. Additionally, the vacuum isolation valve was easily abraded by silicon residue. Material spillage was another challenge. Spilled feed material interfered with crucible rotation and lift mechanisms. It also contributed to premature failure of hot-zone components and shields. Crucible deterioration also presented some di$culties. We found that our crucibles have an e!ective lifetime of about 100 h. To combat these challenges, vacuum seals were replaced with welded seams and the vibratory feed delivery system was eliminated. An abrasive resis-
tant isolation valve was also installed. Installation of a charge tube extension and introduction of dehydrogenated feedstock e!ectively solved the spillage problem. Reducing cycle time proved to be an e!ective way to minimize the e!ects of crucible degradation. Cycle time was drastically reduced with the introduction of SSIs energy e$cient hot zone, which permitted faster pull speeds. Overcoming these obstacles was necessary before any meaningful experimentation could be carried out. It was important to determine if the HTO system had any adverse a!ects on our process. A series of experiments were conducted to obtain comparative data. For these experiments, a starting charge size of 30 kg was used. An additional 10 kg were added with the HTO system after meltdown. Data from these runs was compared with data from control runs that had been grown using a 40-kg starting size and no HTO. Data analysis indicated that product yield was essentially the same. A slight reduction in throughput was evident, the result of additional time taken to perform the HTO. This increase in cycle time was expected and will be absorbed as charge size is increased. After gaining con"dence that the HTO system was not harmful to our process, e!orts shifted to maximizing yield and productivity. Yield is determined by dividing the length of usable product grown at target diameter by the theoretical maximum length for a given charge weight and target diameter. The potential yield bene"ts of increased charge capacity are indicated in Fig. 2. Yield is improved because the residual melt (potscrap), becomes a smaller portion of the total charge. Even greater improvements are expected in productivity as seen in Fig. 3. Productivity is calculated by dividing the total length (mm) grown by the total cycle time. Productivity is increased because the larger charge size permits faster pull speeds for a greater percentage of the run. Increased throughput dramatically reduces the amount of energy and argon required for each run. Actual run data supports the theoretical expectations. SSI is presently running experiments designed to determine the optimum starting charge size and pull speed con"guration. As Figs. 4 and 5
B. Fickett, G. Mihalik / Journal of Crystal Growth 211 (2000) 372}377
375
Fig. 2. Theoretical yield versus initial charge size.
Fig. 3. Theoretical throughput versus initial charge size.
indicate, yield and throughput improve as charge size is increased. Record breaking performance was demonstrated on several of the experimental runs.
One run achieved 88% yield and productivity over 41 mm/h on a 6A diameter ingot. This compares to an average for non-HTO runs of 76.2% yield and
376
B. Fickett, G. Mihalik / Journal of Crystal Growth 211 (2000) 372}377
Fig. 4. Yield Run Data * charge size and pull speed study.
Fig. 5. Throughput run data * charge size and pull speed study.
31 mm/h. Further re"nements are still necessary to realize the full potential of HTO.
5. Conclusion This project has demonstrated that yield and throughput can be signi"cantly improved by topping o! the initial charge. Further optimization of charge size and pull speed will continue with more emphasis on repeatability. Ingot characterization work will also be performed to ensure that electrical characteristics of HTO runs are equivalent to non-HTO runs. HTO will be released to pilot production when process consistency is statistically proven.
While some feasibility recharge tests have been performed, a detailed study to determine bene"ts of recharging and multi-ingot growth is still needed. Since the HTO system is capable of recharging without any hardware modi"cations, the development of recharging can move more quickly after the HTO process is released to production.
Acknowledgements The authors would like to express their sincere appreciation to the Northwest Energy E$ciency Alliance (NEEA) for their continued "nancial support and project guidance.
B. Fickett, G. Mihalik / Journal of Crystal Growth 211 (2000) 372}377
References [1] F. Shimura, Semiconductor Silicon Crystal Technology, Academic Press, New York, 1989, p. 178.
377
[2] W. Zulehner, D. Huber, Czochralski-Grown Silicon, Springer, Berlin, 1982, pp. 94, 99}103. [3] W. O'Mara, B. Herring, L. Hunt, Handbook of Semiconductor Silicon Technology, Noyes Publications, 1990, pp. 157}164.
Economic feeder for recharging and `topping o!a Bryan Fickett*, G. Mihalik Siemens Solar Industries, 12016 NE 95th Street, Suite 720, Vancouver, WA 98682, USA
Abstract Increasing the size of the melt charge signi"cantly increases yield and reduces costs. Siemens Solar Industries is optimizing a method to charge additional material after meltdown (top-o!) using an external feeder system. A prototype feeder system was fabricated consisting of a hopper and feed delivery system. The low-cost feeder is designed for simple operation and maintenance. The system is capable of introducing up to 60 kg of granular silicon while under vacuum. An isolation valve permits re"lling of the hopper while maintaining vacuum in the growth furnace. Using the feeder system in conjunction with Siemens Solar Industries' energy e$cient hot zone dramatically reduces power and argon consumption. Throughput is also improved as faster pull speeds can be attained. The increased pull speeds have an even greater impact when the charge size is increased. Further cost reduction can be achieved by re"lling the crucible after crystal growth and pulling a second ingot run. Siemens Solar Industries is presently testing the feeder in production. ( 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Past e!orts to develop charge replenishment technologies are well documented. The primary objective was usually to minimize the segregation e!ects for N-type ingot growth. Many researchers also recognized potential cost reduction bene"ts, mostly the result of spreading the high cost of quartz crucibles over several crystal pulls [1]. A variety of di!erent recharging techniques were developed in an attempt to realize these bene"ts. The various techniques can be grouped into one of two categories: semi-continuous (batch) or continu-
ous [2]. In semi-continuous recharging, material is added between ingot pulls. Semi-continuous techniques include lowering feed material either in rod form or by a hopper suspended from the seed cable. Another semi-continuous technique introduces feed stock by means of an external hopper/delivery system. Continuous replenishment typically employs a similar external recharge system that delivers pre-melted feedstock continuously during ingot growth. Although some success has been achieved using these approaches, there are a number of disadvantages:
* Corresponding author. Tel.: #1-360-944-9766; fax: #1360-944-9189. E-mail address: bryan."[email protected] (B. Fickett)
f Complicated and costly modi"cations to the growth equipment are often required [1]. f Delicate quartz components frequently fracture causing run aborts.
0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 8 0 1 - 5
B. Fickett, G. Mihalik / Journal of Crystal Growth 211 (2000) 372}377
373
f Some techniques require additional furnace isolations that extend cycle time and increase the possibility of leaks/contamination. f Impurities build up in the residual melt due to segregation [3]. In most cases, these disadvantages have prevented use of charge replenishment in manufacturing environments. The SSI approach is unique in that the main goal is to reduce manufacturing costs of P-type ingot growth through yield and throughput improvements. The greatest advantages are obtained by maximizing the initial charge. Subsequent recharges also impact manufacturing costs, but to a diminishing degree. For this reason, primary system development is geared towards re"ning a highly dependable method for topping o! the initial charge. A prototype feeder system was constructed to demonstrate feasibility. The prototype incorporated the following design considerations: f Low cost f Simple operation and maintenance * no electrical components/minimal moving parts. f Robust * able to withstand high volume production environment. f Vacuum integrity * vacuum tight to 50 mTorr with full isolation capability. f Adaptable * system compatible with various furnace models. f Purity * construction materials selected to prevent contamination. f Volume * capable of feeding up to 60 kg. At controlled feed rates up to 2 kg/min. Fig. 1. HTO system general assembly.
2. Apparatus The prototype system consists of a feed hopper and delivery system (Fig. 1). The transparent hopper allows material level to be checked at a glance. The top cap of the hopper assembly includes a "ll port and connections for vacuum and argon. A combination pressure/vacuum gauge is mounted in the top cap as well. A funnel is installed inside the bottom of the feed hopper to ensure complete discharge of feed material.
An angle of repose valve (AOR valve) is used for starting/stopping the media #ow. A vacuum isolation valve separates the hopper assembly from the furnace. Inside the crystal growth furnace, a charge tube extension directs the feed material to within 2A of the melt surface. The size of the ori"ce at the end of the charge tube extension determines the #ow rate. A reduced ori"ce minimizes kinetic energy and
374
B. Fickett, G. Mihalik / Journal of Crystal Growth 211 (2000) 372}377
helps to prevent feed material from bouncing out of the crucible. The control panel includes valves for controlling hopper vacuum and argon.
3. Procedure The feed hopper is "lled with a pre-determined quantity of silicon granules or sifted "nes. Dopant, if required, is crushed and placed in the hopper as well. After "lling, the system is pumped down to 50 mTorr, then back-"lled to atmospheric pressure with argon. This procedure is repeated three times to remove oxygen. When delivery of feed material is required, the crucible position is adjusted so that the melt level is approximately 2A below the charge tube outlet. Crucible rotation is set at three RPM. The isolation and the AOR valves are then opened. Material begins to #ow immediately. The process is monitored during feeding, and the crucible position adjusted as needed to keep the growing mound of unmelted silicon from contacting the charge tube extension. Once charging is complete, the AOR and isolation valves are closed. The material is melted and the temperature stabilized prior to seeding. The procedure can be repeated multiple times during a run.
4. Results and discussion A number of developmental challenges hindered progress early in this project. Vacuum leaks caused crystals to lose structure and deteriorated hot-zone components. Maintaining seal integrity proved dif"cult due to the e!ects of a pneumatic vibrator on the sealing hardware. Additionally, the vacuum isolation valve was easily abraded by silicon residue. Material spillage was another challenge. Spilled feed material interfered with crucible rotation and lift mechanisms. It also contributed to premature failure of hot-zone components and shields. Crucible deterioration also presented some di$culties. We found that our crucibles have an e!ective lifetime of about 100 h. To combat these challenges, vacuum seals were replaced with welded seams and the vibratory feed delivery system was eliminated. An abrasive resis-
tant isolation valve was also installed. Installation of a charge tube extension and introduction of dehydrogenated feedstock e!ectively solved the spillage problem. Reducing cycle time proved to be an e!ective way to minimize the e!ects of crucible degradation. Cycle time was drastically reduced with the introduction of SSIs energy e$cient hot zone, which permitted faster pull speeds. Overcoming these obstacles was necessary before any meaningful experimentation could be carried out. It was important to determine if the HTO system had any adverse a!ects on our process. A series of experiments were conducted to obtain comparative data. For these experiments, a starting charge size of 30 kg was used. An additional 10 kg were added with the HTO system after meltdown. Data from these runs was compared with data from control runs that had been grown using a 40-kg starting size and no HTO. Data analysis indicated that product yield was essentially the same. A slight reduction in throughput was evident, the result of additional time taken to perform the HTO. This increase in cycle time was expected and will be absorbed as charge size is increased. After gaining con"dence that the HTO system was not harmful to our process, e!orts shifted to maximizing yield and productivity. Yield is determined by dividing the length of usable product grown at target diameter by the theoretical maximum length for a given charge weight and target diameter. The potential yield bene"ts of increased charge capacity are indicated in Fig. 2. Yield is improved because the residual melt (potscrap), becomes a smaller portion of the total charge. Even greater improvements are expected in productivity as seen in Fig. 3. Productivity is calculated by dividing the total length (mm) grown by the total cycle time. Productivity is increased because the larger charge size permits faster pull speeds for a greater percentage of the run. Increased throughput dramatically reduces the amount of energy and argon required for each run. Actual run data supports the theoretical expectations. SSI is presently running experiments designed to determine the optimum starting charge size and pull speed con"guration. As Figs. 4 and 5
B. Fickett, G. Mihalik / Journal of Crystal Growth 211 (2000) 372}377
375
Fig. 2. Theoretical yield versus initial charge size.
Fig. 3. Theoretical throughput versus initial charge size.
indicate, yield and throughput improve as charge size is increased. Record breaking performance was demonstrated on several of the experimental runs.
One run achieved 88% yield and productivity over 41 mm/h on a 6A diameter ingot. This compares to an average for non-HTO runs of 76.2% yield and
376
B. Fickett, G. Mihalik / Journal of Crystal Growth 211 (2000) 372}377
Fig. 4. Yield Run Data * charge size and pull speed study.
Fig. 5. Throughput run data * charge size and pull speed study.
31 mm/h. Further re"nements are still necessary to realize the full potential of HTO.
5. Conclusion This project has demonstrated that yield and throughput can be signi"cantly improved by topping o! the initial charge. Further optimization of charge size and pull speed will continue with more emphasis on repeatability. Ingot characterization work will also be performed to ensure that electrical characteristics of HTO runs are equivalent to non-HTO runs. HTO will be released to pilot production when process consistency is statistically proven.
While some feasibility recharge tests have been performed, a detailed study to determine bene"ts of recharging and multi-ingot growth is still needed. Since the HTO system is capable of recharging without any hardware modi"cations, the development of recharging can move more quickly after the HTO process is released to production.
Acknowledgements The authors would like to express their sincere appreciation to the Northwest Energy E$ciency Alliance (NEEA) for their continued "nancial support and project guidance.
B. Fickett, G. Mihalik / Journal of Crystal Growth 211 (2000) 372}377
References [1] F. Shimura, Semiconductor Silicon Crystal Technology, Academic Press, New York, 1989, p. 178.
377
[2] W. Zulehner, D. Huber, Czochralski-Grown Silicon, Springer, Berlin, 1982, pp. 94, 99}103. [3] W. O'Mara, B. Herring, L. Hunt, Handbook of Semiconductor Silicon Technology, Noyes Publications, 1990, pp. 157}164.