Nitride bonded silicon nitride as a reusable crucible material for directional solidification of silicon

Nitride bonded silicon nitride (NBSN) has the potential of a reusable crucible material for directional solidification of silicon. This is demonstrated in this work by reusing a NBSN crucible six times for the directional solidification of undoped multicrystalline (mc) silicon ingots. The progress of the ingot contamination at subsequent use of the NBSN crucible was studied systematically. Minority carrier lifetime, electrical resistivity as well as impurity content were analyzed after each solidification run. The results were compared to those obtained from ingots which were crystallized by using identical directional solidification process parameters in standard fused silica crucibles with silicon nitride coating. The impurity content of the ingots can be clearly correlated to the impurity content of the NBSN crucible. The main impurity is the acceptor B. Its concentration in the ingots decreases from about 10¹⁷ atoms/cm³ to 10¹⁶ atoms/cm³ with continued reuse. The contamination mechanism is most likely due to outdiffusion from the crucible wall into the Si melt.     

Estimation of growth rate in unidirectionally solidified multicrystalline silicon by the growth-induced striation method

Multicrystalline silicon was grown by unidirectional solidification method using the accelerated crucible rotation technique. The application of the accelerated crucible rotation technique in unidirectional solidification method induced growth striations across the axial direction of the grown crystal. This striation pattern was observed from carbon concentration distribution, obtained by using Fourier transform infrared spectroscopy. The generated striation pattern was found to be weak and discontinuous. Some striations were absent, probably due to back melting, caused during each crucible rotation. From the growth striations and applied time period in crucible rotation, the growth rate was estimated by using Fourier transformation analysis.     

Ageing of spherulites of an industrial relevant aromatic amine derivative

Polycrystalline spherulites of an aromatic amine derivative have been precipitated in a batch process by pH‐shift with hydrochloric acid from stirred aqueous solutions. The time dependent behaviour of the spherulites has been studied during crystallization in the temperature range from 5 °C to 60 °C. Cake resistance values have been obtained from batch filtration tests performed at 2 bar pressure difference at different stages of the crystallization process. The FBRM mean chord length of the crystals decreases with time as a result of crystal ageing into plate‐like crystals. The rate of the ageing process increases with temperature. XRD‐studies show no significant differences in the crystal structure during the ageing process, and the mechanism of the transformation was not established. The filterability of aromatic amine crystals deteriorates as the crystallization progresses. The decrease in the filterability is attributed to the appearance of small plate‐like crystals and a change in the interaction between the crystal surface and the solution, during the ageing of the particles. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Three-dimensional global analysis of thermal stress and dislocations in a silicon ingot during a unidirectional solidification process with a square crucible

A three-dimensional global model was used to obtain the solution of a thermal field within the entire furnace during a unidirectional solidification process of multicrystalline silicon with a square crucible. Then the thermal stress distribution in the silicon ingot was solved. Based on the solution of thermal stress, relaxation of stress and multiplication of dislocations were performed by using the Haasen–Alexander–Sumino model (HAS model). The influence of crucible constraint on stress levels and dislocations was investigated. It was found that the crucible constraint had significant influence on the thermal stresses and dislocations in the ingot. The results indicated that it is important to reduce the crucible constraint in order to relax thermal stresses and reduce dislocations in a silicon ingot during the solidification process.     

Validation of a 3D multi-physics model for unidirectional silicon solidification

A model for transient movements of solidification fronts has been added to X-stream, an existing multi-physics simulation program for high temperature processes with flow and chemical reactions. The implementation uses an enthalpy formulation and works on fixed grids. First we show the results of a 2D tin solidification benchmark case, which allows a comparison of X-stream to two other codes and to measurements. Second, a complete 3D solar silicon Heat Exchange Method (HEM) furnace, as built by PVA TePla is modeled. Here, it was necessary to model the complete geometry including the quartz crucible, radiative heaters, bottom cooling, inert flushing gas, etc. For one specific recipe of the transient heater power steering, PVA TePla conducted dip-rod measurements of the silicon solidification front position as function of time. This yields a validation of the model when applied to a real life industrial crystallization process. The results indicate that melt convection does influence the energy distribution up to the start of crystallization at the crucible bottom. But from that point on, the release of latent heat seems to dominate the solidification process, and convection in the melt does not significantly influence the transient front shape.     

Oxygen and carbon transfer during solidification of semiconductor grade silicon in different processes

A model is established for comparing the solute distribution resulting from four solidification processes currently applied to semiconductor grade silicon: Czochralski pulling (CZ), floating zone (FZ), 1D solidification and electromagnetic continuous pulling (EMCP). This model takes into account solid–liquid interface exchange, evaporation to or contamination by the gas phase, container dissolution, during steady-state solidification, and in the preliminary preparation of the melt. For simplicity, the transfers are treated in the crude approximation of perfectly mixed liquid and boundary layers. As a consequence, only the axial (z) distribution can be represented. Published data on oxygen and carbon transfer give a set of acceptable values for the thickness of the boundary layers. In the FZ and EMCP processes, oxygen evaporation can change the asymptotic behaviour of the reference Pfann law. In CZ and in 1D-solidification, a large variety of solute profile curves can be obtained, because they are very sensitive to the balance between crucible dissolution and evaporation. The CZ process clearly brings supplementary degrees of freedom via the geometry of the crucible, important for the dissolution phenomena, and via the rotation rate of the crystal and of the crucible, important for acting on transfer kinetics.     

Influence of furnace design on the thermal stress during directional solidification of multicrystalline silicon

Directional solidification is one of the most popular techniques for massive production of multicrystalline silicon (mc-Si). Dislocation is one of the major defects that significantly affect the photovoltaic performance. For the analysis and optimization of stress-induced dislocation, a computational tool has been developed to investigate thermal stress distribution during directional solidification process of multicrystalline silicon. Temperature distribution in the furnace, S/L interface shape and melt flow are simulated. Parametric studies are further conducted to evaluate the effect of furnace design on the interface shape and on the maximum von Mises stress in the growing ingot. To consider the effects of the crucible geometry qualitatively, three-dimensional modeling of the thermal stress is performed with or without the constraint of the crucible. The regions of dislocation multiplication are evaluated by comparing von Mises stress to critical resolved shear stress (CRSS). The results imply that the dislocation in the growing ingot can be reduced by optimizing the design of the directional solidification furnace.     

Movable partition designed for the seed‐assisted silicon ingot casting in directional solidification process

For the seed‐assisted casting process for silicon ingots, different partition blocks were designed in the directional solidification (DS) furnaces to preserve the seed crystals and optimize the thermal field in the hot‐zone. A transient global model was established to investigate the effects of different partition blocks during the solidification process. The simulation results showed that the partition blocks can significantly influence the temperature distributions and the melt flow fields. From the designed partition blocks, the movable partition block was more favorable for the seed‐assisted DS process. A suitable temperature gradient and a flat seed‐melt (s‐m) interface were obtained, which facilitated the preservation of seed crystals effectively, and an optimized crystal‐melt (c‐m) interface was achieved as well. One of the designs of the movable partition blocks was implemented in quasi‐mono crystalline silicon casting experiments and it has been confirmed that the designed movable partition block was helpful for the improvement of the single crystal area.     

Growth of semiconductor silicon crystals

This paper focuses on the recent developments in Czochralski (CZ) crystal growth of silicon for large-scale integrated circuits (LSIs) and multi-crystalline silicon growth using a directional solidification method for solar cells. Growth of silicon crystals by the CZ method currently allows the growth of high-quality crystals that satisfy the device requirements of LSIs or power devices for electric cars. This paper covers how to obtain high-quality crystals with low impurity content and few point defects. It also covers the directional solidification method, which yields crystals with medium conversion efficiency for photovoltaic applications. We discuss the defects and impurities that degrade the efficiency and the steps to overcome these problems.     

Formation and ageing of L‐glutamic acid spherulites

Polycrystalline spherulites of L‐glutamic acid have been crystallized by pH‐shift precipitation from stirred aqueous solutions. The time dependent behaviour of the spherulites has been studied during the crystallization process and batch filtration tests have been performed. It has been shown that the FBRM mean chord length of the investigated spherulites decreases in the course of time. The fact that the size reduction progresses faster at higher temperature and the solubility of resuspended polycrystalline particles decreasing with time, implies an ageing mechanism to be responsible for the observed changes in the particle size. It has been shown that the surface area decreases with time, ruling out particle breakage as a possible explanation for the decrease in particle size. XRD and Raman studies of L‐glutamic acid, however, show only marginal differences in the crystalline structure of particles obtained from different time stages. The ageing may occur due to several different mechanisms like phase transformation and Ostwald ripening. L‐glutamic acid spherulites after 3 h exhibit a 3‐fold higher value for the cake resistance as compared to particles after 0.5 h. However, particles obtained after 22 h exhibit an 8‐fold lower cake resistance as compared to the initially obtained spherulites, The increase in the cake resistance is attributed to the appearance of small plate‐like crystals and a change in the interaction between the crystal surface and the solution. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mono-crystalline growth in directional solidification of silicon with different orientation and splitting of seed crystals

This work presents the results of a systematic study of mono- and poly-crystalline grain growth in directional solidification of silicon using different kinds of seed crystals. The seed orientation was varied between 〈100〉, 〈111〉 and 〈110〉. In some experiments the seeds were split into several seed pieces. The results show that the growth of misoriented grains at the crystal periphery as well as in the gaps between split seeds depends strongly on the crystallographic orientation of the seeds. It is shown that this problem can be minimized if certain seed orientations and combinations are chosen. Generally the 〈100〉 seed orientation turns out to be most difficult with respect to mono-crystalline growth. Heterogeneous nucleation originating from the crucible walls seems to be a minor problem.     

Role of marangoni tension effects on the melt convection in directional solidification process for multi-crystalline silicon ingots

We carried out global simulations to investigate the marangoni tension effect on the thermal and flow fields in the silicon melt of the directional solidification process for multi-crystalline silicon ingots. The argon flow rate was varied to provide different solidification conditions and to change the relative values between the argon shear stress and the marangoni tension at the melt free surface. We found that the marangoni tension together with the shear stress mainly influences the upper layer melt convection while the thermal buoyancy force dominates the bulk flow of the melt. At low argon flow rates, the argon shear stress can be neglected and the marangoni tension alone enhances the melt convection intensity near the gas–melt–crucible triple junction point. The marangoni tension is so weak that it cannot modify the melt flow pattern in this case. For medium flow rate, the marangoni tension can significantly weaken the shear stress effect at the outer part of the melt free surface, leading to a distinctive flow pattern in the silicon melt. With further increase in argon flow rate, the shear stress sharply increases and dominates the upper layer melt flow, limiting the marangoni tension effect to the triple point. The numerical results are helpful for better understanding and controlling of the directional solidification process for high quality multi-crystalline silicon ingots.     

The melt temperature change influence on the crystallization rate in czochralski growth system

This paper opens a series of publications about growth system parameters change influence on the crystallization rate. The influence of crucible melt temperature fluctuations and mechanical instability will be investigated. In approximation of one-dimensional heat transfer the behaviour of crystallization rate in case of crucible melt temperature small change is investigated in this paper. As an example characteristic time values and system sensitivity are given for fluorite and silicon crystals.     

Transient analysis of the cooling process and influence of bottom insulation on the stress in the multicrystalline silicon ingot

Directional solidification is one of the most popular techniques for massive production of multicrystalline silicon. After growth and annealing, the ingot is cooled down by a designed cooling process, which is initialized by descending bottom insulation, and then is controlled both by power ramp‐down rate and by motion of the bottom insulation. Thermal stress is piled up during cooling, and associated crystal defects, such as dislocation and micro‐cracks, may generate and propagate in the ingot. In the paper, transient modeling is applied to study the effect of bottom insulation on the cooling process. Temperature, velocity and thermal stress fields are obtained with linear, parabolic and sinusoidal motions of the insulation. The measured and predicted temperatures at two points of the ingot are found consistent. Distributions of von Mises stress in the ingot at different cooling time are obtained, and the maximum von Mises stresses are presented as a function of the cooling time. Specifically, dislocation‐free regions, evaluated by the critical resolved shear stress model, at certain cooling time, and area fractions of the regions as a function of the cooling time are proposed. The linear motion is further discussed with different moving rates, considering its wide applications in the current industry and convenient realization in control.     

Investigation of the thermal conditions during silicon carbide crystal growth

In the present work an analysis of the thermal conditions during silicon carbide crystal growth from the vapour phase by the sublimation method is carried out. On the basis of the obtained results from the calculation of the temperature distribution along the length of the growing crystal it was drawn a conclusion that in order to decrease the density of dislocations in the growing crystals it is necessary to decrease the temperature gradients in the crucible for growing.     

An enhanced cooling design in directional solidification for high quality multi-crystalline solar silicon

An enhanced cooling design for nucleation was proposed for directional solidification based on the enhanced heat transfer through gas flow, and the effects of initial cooling conditions during directional solidification on the quality of multi-crystalline silicon for solar cells were studied. The properties of the grown grains under different initial cooling conditions were measured. The grain size, grain orientations, and the percentage of twin boundaries, as well as minority lifetime and defect density, were affected significantly by the initial cooling. The implementation of this design to a commercial furnace was also discussed, and promising results were obtained.     

Role of flux on synthesis of poly single crystals of Ce3+ doped yttrium aluminum garnet

Yttrium aluminum garnet powders of good morphology are composed of tiny single crystals of regular faces. However, the growth mechanism of those ploy single crystals remains unknown. This paper provides morphology images of particles uncrushed, and it is found that those tiny crystals grow from large clusters in a pomegranate‐like manner. The morphology change occurs after phase transition, and is driven by flux, which provides a semi‐liquid environment around the crystals. This paper is beneficial for understanding growth mechanism of poly tiny single crystals of yttrium aluminum garnet powders.     

降温速率对升级冶金硅定向凝固生长多晶硅少子寿命的影响

对经过前期提纯的冶金级硅料进行一次性定向凝固生长多晶硅铸锭,研究了长晶阶段降温速率对多晶硅少子寿命的影响。结果显示降温速率越低,获得多晶硅少子寿命越高,但降温速率低到一定程度时,少子寿命反而会降低。通过测试生长多晶硅硅锭曲率半径、晶体结构等数据,分析了该现象的产生原因。这将有助于升级冶金硅一次性定向凝固生长多晶硅铸锭的生产应用。     

Electromagnetic stirring and retention to improve segregation in silicon for photovoltaics

The segregation of impurities was investigated during the crystallization of upgraded metallurgical grade silicon (UMG Si). An experimental solidification system has been developed, including a strong electromagnetic stirring in order to mix the melt, to increase the effective segregation coefficient.Our crucible has a slit-like opening at the bottom of a face, to be adapted to ribbon pulling for future developments of a crystallization system for the photovoltaic industry. The electromagnetic system allowed retaining the liquid silicon inside the crucible during the crystallization of an ingot (thickness ～5 cm), or make it flow out after the partial crystallization of a bottom layer (thickness ～2 mm).Starting from UMG Si of known composition, the chemical analysis of the impurity concentrations in that multicrystalline material shows an efficient purification, despite the rather high crystallization speed (estimated between 9 and 20 cm/h). Phosphorous concentration was reduced from 6 to 1.7 ppm during the segregation, which is difficult to achieve for this impurity, which has a segregation coefficient close to unity (k₀=0.3).