Czochralski growth Floating zone Ribbon or tube growth Directional solidification

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1 Journal of Crystal Growth 318 (2011) Contents lists available at ScienceDirect Journal of Crystal Growth journal homepage: Nucleation and bulk growth control for high efficiency silicon ingot casting Hui Zhang a,n, Lili Zheng b,xuma b, Bo Zhao c, Cheng Wang c, Fanghua Xu c a Department of Engineering Physics, Tsinghua University, Beijing, China b School of Aerospace, Tsinghua University, Beijing, China c Jinggong Science and Technology Co., Keqiao, Zhejiang, China article info Available online 28 October 2010 Keywords: A1. Directional solidification A2. Growth from melt A2. Industrial crystallization B3. Solar cells abstract Directional solidification and casting systems used in silicon production are reviewed first. Common and distinguished features are summarized, and the problems appearing at nucleation and bulk growth stages are discussed. The relationship between growth rate and temperature in the heat exchanger block for an industrial growth system is developed and the predicted results are compared with experimental measurements. A commercial industrial system is simulated to study fluid flow and heat transfer in the growth system, and hot zone design is optimized. Suggestions are made for further improvement of industrial silicon casting systems. & 2010 Elsevier B.V. All rights reserved. 1. Current status of silicon production facility Silicon ingot produced by directional solidification is a major technology for photovoltaic industry and it has attracted a lot of attention recently. Cost-effectiveness, ease of operation, and flexibility of feedstock materials being the benefits, the directional solidification technology shows strong capability for further expansion in market share, especially in China. Bell Laboratory built the first silicon solar cell in 1954, in which the PV wafer was produced by the Czochralski (Cz) growth technique. Since then, new and low-cost techniques emerged in wafer production, e.g. casting (then Solarex, now BP Solar), heat exchanger method (Crystal Systems and GT Equipment Technology), ribbon growth (then Ebara, now SPI and Evergreen), and tube growth (then Mobile Solar, now Schott Solar). Directional solidification was extended from the heat exchanger method. Currently, commercial 270 and kg directional solidification systems are widely used in production and the systems capable of producing kg ingots are also being developed. It is seen in Table 1 that in comparison with other methods, directional solidification or casting method can generate the highest throughput (kg/h) due to its capability to produce large cross-section ingots and low requirement on feedstock. A furnace producing kg can produce 25 ingots (156 mm 156 mm mm) during one growth run compared with only 1 ingot with Czochralski (156 mm 15 mm 1800 mm) and floating zone techniques. The furnace producing kg silicon ingot(s) can produce 36 or more ingots (156 mm 156 mm 340 mm) simultaneously. n Corresponding author. address: zhhui@tsinghua.edu.cn (H. Zhang) /$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi: /j.jcrysgro The average growth rate of mm/h and electricity cost of 7 9 kwh/kg currently in kg ingot production can be further improved in the growth rate and ingot quality through hot zone design and process optimization. Various directional solidification furnaces are used in industry for producing multi-crystalline or quasi-single crystalline silicon ingots/ wafers worldwide. Among them, GTS-DSS450, ALD 450, JingGong-JJL 500, and JYT-JZ460 are widely used in China (Fig. 1). Average solar cell efficiency in the range of % has been achieved for multicrystalline wafers. Recently, quasi-single crystal and large grain growth techniques have also been developed through seeded or nucleation control growth. For example, quasi-single crystalline silicon ingots have been produced by BP solar and produced solar cell efficiencies up to 19% [2]. A few companies in China have also produced excellent results in quasi-single silicon growth, and solar cell efficiencies of more than 18% have been achieved. It is well known that conversion efficiency up to 16.2% can be achieved through process control such as using high quality feedstock, optimizing growth receipts, and reducing process time. To further improve conversion efficiency to 17.0% or further reduce the cost of production, it is important to understand the current status of commercial furnaces and provide the feasible solutions for improvement. 2. Directional solidification or casting systems used in silicon production A directional solidification furnace generally contains a heat exchanger block made of graphite or other high thermal conductivity materials. The crucible is placed on top of the heat exchanger block to 转载

2 284 H. Zhang et al. / Journal of Crystal Growth 318 (2011) Table 1 Comparison between different growth techniques. Czochralski growth Floating zone Ribbon or tube growth Directional solidification Growth rate (cm/h) ingot ingot wafers or 25 ingots Diameter or size (cm) , Crystal structure Single Single Multi Multi or Single Feedstock requirement High/moderate High Moderate Low Solar cell efficiency Comments Mature Mature Thin/fast growth possible Growth rate/ ingot quality improvement Note: horizontal ribbon growth on silicon melt surface can reach an amazing 54 m/h with solar cell efficiency of 12% in 1990 [1]. Current status of this technology is unclear. However, high throughput together with future solar cell technique makes this method a strong candidate to compete with current directional solidification process. a Jinggong JJL500 furnace, which can produce 500 kg ingot in one run, a bottom insulation layer can move downwards to create a gap between the heat exchanger block and water cooled steel furnace wall. A single side heater and radiation cooling as well as partition block can generate suitable thermal environment for nucleation and bulk crystal growth. Partition blocks in ALD and JJL furnaces allow physical separation between the heating and cooling zones. It is beneficial to reduce unnecessary energy caused by coupling between two zones as well as provide an easy temperature control during nucleation and bulk growth. Various gas control devices are designed for ALD and JJL furnaces (not shown in the figure). Seeded growth is also possible. Seeds should be preserved during melting. The growth process is similar to that in multi-crystalline silicon growth. Silicon charge is loaded into a quartz crucible of cm 3 or cm 3 with a mass of approximately kg. Thermocouples and optical pyrometers are installed to monitor temperature at specific locations in the hot zone. The crucible remains stationary to release heat from heat exchanger block and achieve the desired cooling rate. Directional solidification for multi-crystalline silicon production can be roughly divided into two steps: nucleation and bulk growth steps. Requirements for nucleation and bulk growth are different. The commercial production systems reviewed here do not consist of nucleation control design. Modified versions, however, are capable of nucleation control to either produce large grains or preserve the seeds. After nucleation, a suitable temperature gradient will be created by adjusting the gap opening and heating powers. Fig. 1. Different configurations of industrial directional solidification furnaces. achieve heating and cooling. One or multiple heaters together with a cooling device generate suitable thermal conditions for nucleation and bulk growth. Fig. 1 shows the schematics of GTS-DSS (USA), ALD (Germany), and Jinggong-JJL (China) furnaces. Multi-crystalline silicon ingot is usually directionally solidified in a squared fused silica (quartz) crucible supported by graphite susceptor placed in the furnace. After silicon feedstock is totally melted (if it is not a seeded growth process), solidification is initiated at the bottom of the crucible when silicon melt is cooled below the melting temperature. Different cooling methods are designed. In a GTS-DSS450 furnace, the side insulation layer is moved upwards. A gap is therefore created to provide radiation between the heat exchanger block and water cooled stainless steel furnace wall. The gap and heaters can generate suitable temperature distribution in the furnace. A lid is designed in GT s 450 furnace, which is different from the previous one such as GTS-DSS240 and GTS-HEM240. In an ALD450 furnace, a shutter between the heater and cooler withdraws quickly after melting so that the heat exchanger block can be cooled by an inside cooler. A temperature gradient can be generated through the top and bottom heaters as well as the cooler. In 3. Early stage of growth: nucleation control or seed preservation The nucleation step is related to undercooling temperature, crucible coating wetability, and temperature gradients in both vertical and horizontal directions. Nucleation may start from the edge of the crucible and grow inwards, or from the cooling spots and grow outwards. Fujiwara et al. [3,4], Yeh et al. [5], and Wang et al. [6] performed extensive studies to improve the grain size, orientation, and stress during the initial stages of nucleation control. Unfortunately, most experiments were performed in small crystal growth systems. Since the horizontal dendrite growth depends strongly on crucible diameter and thickness, the experimental results in a small crucible may not be repeatable in an industrial crucible system. It is generally believed that a horizontal temperature gradient generated from spots cooling is beneficial in the industrial nucleation process. Also, undercooling and twinning controls play an important role [5]. The capability of precise control of silicon melt temperature is crucial for undercooling/nucleation control. Seed preservation is critical during quasi-single crystalline casting. Temperature gradient control during seed preservation and bulk growth is essential to maintain the quasi-single crystal structure all the way to the top.

3 H. Zhang et al. / Journal of Crystal Growth 318 (2011) Fig. 2 shows two results produced by a JJL500 furnace. The figure on the left is a side view of an 156 mm 156 mm 300 mm ingot produced by the undercooling control strategy. The bottom portion of the melt is set at a relatively low temperature and temperature at the center is lower than that at the edge. Different strategies are used for nucleation control to generate large grains, such as control cooling rate of silicon melt (undercooling), control cooling spots (patterned spots), applying different crucible materials (such as graphite), applying different coating materials, and applying different crucible bottom designs. Although excellent results can be produced, randomness of nucleation will affect the repeatability in industrial operation. Robustness is not good. The figure on the right shows a top view of a quasi-single crystalline 156 mm 156 mm wafer. The structure can be preserved all the way to the ingot top. During seeded casting, suitable temperature gradient is needed during the early stage and also during the bulk stage. Consistent temperature gradient during the entire growth process is critical for high solar cell performance. 4. Bulk crystal growth stage After nucleation or after a thin layer of silicon ingot is formed at the bottom of the crucible, bulk ingot growth starts. A suitable vertical temperature gradient should be generated by cooling of heat exchanger block and/or by adjusting heating powers. Fig. 2. Ingots with large grains: (a) side view of an ingot with the size of 156 mm 300 mm and (b) top view of a mm 2 wafer covering 95% single grain. Horizontal temperature gradient, which is needed in the nucleation stage, should be suppressed. The gap for radiation cooling will be opened to sufficient size at relatively short time so that directional solidification condition and large temperature gradient in the vertical direction can be established quickly. A convex solidification interface is preferred so that less new grains can be generated on the crucible wall. Also, a convex interface promotes the outward growth of the grains and enlarges the grain size. During seeded casting, a suitable temperature gradient is applied at the melting stage. The gap opening will be relatively slow so that the latent heat can be removed, but vertical temperature gradient can be preserved. In productions growth receipts are set. A quartz stick is occasionally used to monitor the interface location. Without knowing the interface location, closed loop feedback control is not possible. A method to predict the growth rate is therefore needed. Wei et al. [7] proposed a method based on the temperature measurement by thermocouples on the top surface of the melt and in the heat exchanger block. They proved the relationship between measured temperatures and solidification location numerically. In this paper, we will further improve the simple model proposed by Wei et al. [7]. At the solidification interface (Fig. 3a), the following equations can be written rl dx dt þhðt T m T B T B T H L T m Þ¼k s ¼ k c ¼ FesðTH 4 x d T4 1 Þ where T L and T H are the temperatures on the top surface of the melt and in the heat exchanger block, respectively, T M is the melting point of silicon, T B is the temperature at the bottom of the crucible, k s and k c are the thermal conductivity of solid and crucible, respectively, h is the heat transfer coefficient of silicon melt, and d the thickness of the bottom of the crucible. Assuming no melt undercooling, we can then write T H as follows: T H ¼ T m rl dx x dt þhðt L T m Þ þ d ð2þ k s k c If the unknown parameters T L and x are obtained in experiments, T H can be calculated. If T L and T H are measured, the growth rate can be determined numerically. T L and T H are obtained using a Jinggong JJL500 furnace, and the growth rate is also measured. Fig. 3(b) shows the predicted results compared with experimental data. The experimental growth rate has large fluctuations. The predicted growth rate agrees with the experimental results. Following parameters are used in the calculation: the density of silicon is 2530 kg/m 3, the latent heat is 1788 kj/kg, the thermal ð1þ Vertical Growth Rate (mm/h) 20 predicted results 18 experimental results time (h) Fig. 3. (a) Schematic of a growth system and (b) predicted and experimental growth rates.

4 286 H. Zhang et al. / Journal of Crystal Growth 318 (2011) conductivity of silicon is 22 W/mK, the thermal conductivity of the crucible is 3 W/mK with the thickness of 22 mm, the silicon melting point is 1685 K, and the heat transfer coefficient of the silicon melt, h, is assumed to be 30 W/(m 2 K). It can be seen that the slope of predicted growth rate is lower than the experimental one. The predicted result can fit better with the experimental one if a time variation of the heat transfer coefficient, h, is applied. Based on the analysis, the key parameter limiting growth rate is thermal resistance of the crucible. The largest temperature drop occurs across the crucible bottom due to low thermal conductivity of the crucible. Growth rate is limited even when the heat exchanger block is completely open to the cold environment. As the size of the crucible increases, the melt flow near the crucible wall may also increase. It affects the Si 3 N 4 coating layer and the Si 3 N 4 particles may dissolve in the silicon melt and eventually get trapped in the solidified silicon [8 10]. To balance the aforementioned effects, it is necessary to reduce the temperature difference in the melt, which is not beneficial for removing impurity. Considering all limitations, the best strategy Fig. 4. (a) Hot zone design of JJL500 and temperature of distribution, (b, c) gas field for a system without a lid, and (d) gas field for a system with a lid.

5 H. Zhang et al. / Journal of Crystal Growth 318 (2011) will be to improve the crucible, e.g., using a high purity quartz crucible with high thermal conductivity or using a crucible made of other materials. Another possible direction is to produce silicon on the top surface of the melt, such as horizontal ribbon growth. 5. Hot zone design and process optimization It is validated in the experiments that temperature gradient, cooling rate, solidification interface shape, and impurity distribution are important for producing high quality ingots. Numerical simulations are widely used to reveal transport phenomena in the growth system, such as temperature and impurity distributions in the furnace. Delannoy et al. [11], and Mangelinck-Noel and Duffar [12] developed a 3D global model considering relative movements of various parts in the furnace. The model was applied to study the effect of system geometry on the efficiency of energy usage by the growth system. Miyazawa et al. [13], Rajendran et al. [14], and Gao et al.[15] developed a 3D transient global model to simulate temperature and iron distributions. Conductive and radiative heat transfers in the entire furnace have been coupled with melt convection in the crucible. Fig. 4(a) presents a hot zone design of Jinggong JJL500 furnace and temperature distribution of a quarter system. The interface is almost flat in Fig. 4(a), which is beneficial for the uniformity. Silicon carbide particles formed in the molten silicon are the key defects, which may cause cascade dislocations in the grain leading to low solar cell performance. It is critical that carbon impurity can be reduced in the silicon melt. Gao et al. [15] proposed that carbon impurity is mainly caused by the back transport of CO to the melt surface. They developed a coupled model to study carbon transport in a directional solidification furnace. By introducing a gas shield made of tungsten and molybdenum to limit the back diffusion of CO, carbon level can be dramatically reduced if the distance between the cover and the melt surface is short. Fig. 4(b) (d) shows three preliminary results of gas flow in a Jinggong JJL500 furnace. Fig. 4(b) shows predominated downwards flow inside the insulation chamber since argon gas will come in from the top and go out from the bottom. The gas surrounding the side heater rises to the ceiling and moves inwards, thus enhances downwards flow in the center. A strong recirculation is formed above the silicon melt. Fig. 4(c) shows the effect of holes on the top insulation. Gas surrounding the side heater moves upwards and exits from the holes on the ceiling. However, the recirculation cell in the growth chamber is observed although the intensity is reduced. This indicates that the carbon species is difficult to be removed. Fig. 4(d) shows the effect of a molybdenum gas lid on carbon impurity reduction. It is seen that the coupling between the outside furnace and the melt surface is reduced using the gas lid. Diffusion of reacting components from the quartz crucible and the graphite susceptor will be difficult into the growth cell. However, recirculation under the gas lid may prohibit the impurity that comes out from the melt surface from being taken away. Gao et al. [15] showed a very possible solution using a gas lid in a small furnace. The result presented here does not show the same benefits. The primary reason is the presence of strong natural convection due to the large temperature difference and large size of crucible, which eventually led to strong recirculation in the system. As the size of the reactor increases, e.g., doubling the crucible size, the Grashof number will increase (8 times). It is, therefore, concluded that a different gas shield or gas partition design is needed for an industrial system. 6. Suggestions on future improvements Although multi-crystalline and mono-crystalline ingots are produced regularly commercially, further improvement on impurity control, defect reduction, energy reduction, and growth rate increase is possible. The following equipment improvements are needed: nucleation control system, growth interface monitoring and closed loop feedback control capability, hot zone design and process, optimization for slightly convex or flat interface, melt flow control, gas shield or gas partition design, etc. The coupling between the top hot zone and bottom heat exchanger block should be limited so that energy cost can be reduced and independent control is possible. A key limitation on maximum growth rate is crucible technology. Crucibles with low impurity and high thermal conductivity are needed. Acknowledgements The first three authors are partially sponsored by Jiangxi Provincial Science and Technology Agency. References [1] T.F. Ciszek, Silicon for solar cells, in: E. Kaldis (Ed.), Crystal Growth of Electronic Materials, Elsevier Science Publishers, [2] N. Stoddard, B. Wu, I. Witting, M. Wagener, Y. Park, G. Rozgonyi, Roger Clark, Casting single crystal silicon: novel defect profiles from BP solar s mono2 wafers, Solid State Phenom (2008) 1. [3] K. Fujiwara, W. Pan, N. Usami, K. Sawada, M. Tokairin, Y. Nose, A. Nomura, T. Shihido, K. Nakajima, Growth of structure-controlled polycrystalline silicon ingots for solar cells by casting, Acta Mater. 54 (2006) [4] K. Fujiwara, W. Pan, K. Sawada, M. Tokairin, N. Usami, Y. Nose, A. Nomura, T. Shishido, K. Nakajima, Directional growth method to obtain high quality polycrystalline silicon from its melt, J. Cryst. Growth 292 (2006) 282. [5] K.M. Yeh, C.K. Hseih, W.C. Hsu, C.W. 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