Classifications

• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
  • C30B29/02—Elements
  • C30B29/06—Silicon
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B11/00—Single-crystal growth by normal freezing or freezing under temperature gradient, e.g. Bridgman-Stockbarger method
  • C30B11/002—Crucibles or containers for supporting the melt
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B35/00—Apparatus not otherwise provided for, specially adapted for the growth, production or after-treatment of single crystals or of a homogeneous polycrystalline material with defined structure
  • C30B35/002—Crucibles or containers

Definitions

• the present invention relates to a crucible for the crystallization of molten silicon, to the manufacture of such crucible and to the use of such crucible for crystallization of molten silicon.
• Polycrystalline semiconductors represent the most important photovoltaic material for solar cell manufacture because of its low cost.
• Polycrystalline silicon (PCS) is generally produced using a Bridgman growth technique, wherein a pool of molten semiconductor material contained in a crucible is cooled in a controlled manner to solidify the material from the bottom of the crucible and moving up the crystal-liquid front towards the top of the crucible.
• a crucible is positioned in an oven and filled with a semiconductor feedstock. The oven is activated to melt the whole mass of feedstock. Heat is then extracted through the bottom floor with a heat sink positioned below the crucible; generally, the heat sink comprises a gas flowing in pipes.
• the resulting block of multi-crystalline silicon is generally cut into bricks having a cross- section that is the same as or close to the size of a wafer used for manufacturing a photovoltaic cell.
• the bricks are then sawed or otherwise cut into such wafers.
• the multi- crystalline silicon produced in such manner is an agglomeration of crystal grains where, within the wafers made therefrom, the orientation of the grains relative to one another is effectively random.
• the crucibles are often made of fused silica or based on other materials essentially constituted of silicon dioxide.
• the fused silica crucibles are conventionally coated with silicon nitride to prevent the molten silicon from sticking to the wall of crucible.
• An alternative method consists in depositing silicon seeds on the crucible bottom.
• This technique is largely used in Asia to grow HPM (High Performance Multi) wafers.
• HPM High Performance Multi
• This technic confers to the silicon ingots a very fine microstructure composed of randomly orientated grains with very low dislocation defects leading to high performing cells.
• a significant height of the ingot is lost in the bottom (red zone) because of remaining un-melted seeds.
• the temperature of the crucible bottom must be very well controlled during the melting step in order not to melt the seeds. If seeds are melted, there is indeed no more nucleation effect.
• the furnace hot zone temperature control is a difficult task.
• Another layer of silicon nitride coating having a thickness in the range of 30-50 ⁇ must then be sprayed on the previous coating. Care must be taken as a too low thickness induces severe sticking and oxygen dissolution while a too high thickness results in a lower nucleation effect. Although improvement of cell efficiency compared to seedless or multicrystalline assisted seeded method is observed, many steps are required among which some critical steps.
• [001 1 ] DE-B4-1 0201 0000687 proposes a simpler method by using a crucible with nuclei partially embedded in a coating which has a direct contact with silicon melt.
• the nuclei must be chosen among SiO, S1O2, S13N4, BN, BP, AIP, AIN, AI2O3 and BeO; the effective nuclei surface density is defined between 0.001 and 1 00 cm " 2 preferably between 0.03 and 5 nuclei cm ⁇ 2 while the particle size is defined between 0.01 to 50000 ⁇ and preferably between 1 and 500 ⁇ .
• AI2O3 is presented as a preferred nucleus.
• CN-A-1 05063748 also discloses a crucible with nuclei having a diameter of 5-15 mm preferably Si and polysilicon, partially embedded in a S13N4 coating. The nuclei are sprayed on the coating, then the crucible is fired. A density of 4-1 00 nuclei cm ⁇ 2 is used. Silicon and polysilicon are preferred as nuclei meaning that the control of the temperature in the bottom must be carefully controlled to avoid melting of the nuclei.
• An objective of the present invention is to provide a crucible which allows to obtain an ingot having as good or higher properties as an ingot obtained by the H PM technique which however, will show a reduced red zone in the bottom.
• An advantage of the proposed solution is that the silicon can be completely melted into the crucible without having to very precisely care for the temperature of the bottom.
• the melting points of S1O2 (1 600°C) as nuclei or of S13N4 (1 900°C) as coating are indeed much higher than the melting point of silicon (1412°C). The process is therefore less sensitive.
• the invention relates to a crucible for the crystallization of silicon comprising a base body comprising a bottom surface and side walls defining an inner volume; a silicon nitride layer having an oxygen content of at least 5 wt.% at the bottom surface, characterized in that S1O2 particles are partially embedded in the silicon nitride layer, said particles having a particle size up to 500 ⁇ , at least a portion of said S1O2 particles emerging in the inner volume and wherein the surface density of the Si02 particles is > 1 00 cm ' ⁇ .
• the silicon nitride layer must have a minimum of oxygen content (at least 5 wt.%) in order to improve the adhesion of the coating to the crucible. The crucible can then be easily transported.
• Particles having a size higher than 500 ⁇ lead to a limited nucleating effect.
• the particle size of S1O2 particles is ⁇ 300 ⁇ , more particularly ⁇ 150 ⁇ and more particularly between 80 ⁇ and 150 ⁇ .
• Manipulating very fines particles ( ⁇ 80 ⁇ ) is tricky and leads to an important loss of particles due to the dusting behavior of the particles.
• the surface density of the S1O2 particles is > 500 cm ⁇ 2 more particularly > 1 0.000 cm ⁇ 2 but ⁇ 20.000 cm ⁇ 2. It is preferred not to stack S1O2 particles in order to optimize the use of the S1O2 particles.
• a minimum thickness of 150 ⁇ is preferred. Below a thickness of 150 ⁇ of coating, problem of ingot sticking to the bottom can be observed.
• the S1O2 particles are fused quartz particles because less impurities are then able to diffuse in the silicon ingot.
• the total surface covered by the S1O2 particles is > 25 % and more specifically ⁇ 45 %. Higher than 45% will lead to possible sticking of the ingot to the crucible.
• the crucible generally comprises material suitable for high temperature, preferably fused silica.
• the present invention also relates to a process for producing such crucible.
• a silicon nitride slurry comprising silicon nitride particles having a BET specific surface area of 2-20 m ⁇ /g, preferably 5 -1 0 m ⁇ /g is applied on a crucible. Coating cracks during the firing are avoided by selecting this specific surface.
• S1O2 particles are deposited on the wet coating. The particles float on the surface of the coating. More than 80 % of the S1O2 particles are emerging from the coating. Only few particles are completely covered by the nitride coating.
• the crucible is fired at a temperature between 800 and 1200°C to partially oxidize the nitride coating and to fix the S1O2 particles in the coating. Below 800°C, the oxidation of the nitride coating is too low while above 1200°C the oxidation is difficult to control.
• a series of tests were carried out to assess the properties of the ingot of crystallized silicon obtained with crucibles according to the invention.
• First fused quartz particles were obtained by crushing commercial fused quartz tube. The purity of the fused quartz is 99.999%.
• a slurry made of 50-75 wt.% of S13N4 and 50-25 wt.% of deionized water was applied on the inner surface of fused crucibles. Three ranges of particles size were isolated by using different sieves.
• a first grade comprises particles having a size >800 ⁇
• a second grade comprises particles having a particle size between 300-500 ⁇
• a third grade comprises particles having a particle size betweenl 50-300 ⁇ .
• Comparative ingots were obtained by multicrystalline process without seeds (multi-Ref) and using H PM technique (H PM).
• Grain boundary length fractions are first determined. Grain boundary is the interface between two grains. Grain boundaries are 2D features in the crystal structure and tend to decrease the electrical property of the material. Two types of grain boundary ( ⁇ 3 and ⁇ ra ndom) are measured.
• ⁇ 3 represents a low disorientation of the interface between two grains where metallic impurities can accumulate. The consequence is an increase of the dislocation which impacts the minority carrier lifetime.
• ⁇ ran d om represents a high disorientation of the interface between two grains which prevents the spread of the dislocation in the ingot.
• the grain boundary length fraction is the length fraction of the grain boundary type in relation to the total measured grain boundary length.
• 30% ⁇ 3 grain boundary length fraction means that 30% of the total measured grain boundary is of ⁇ 3 type.
• ⁇ 3 and ⁇ ra ndom 9 ra i n boundary length fractions measured on an ingot obtained by HPM technique are taken as target values: ⁇ 3 should be below 25% and ⁇ ranc
• Fig ure 1 shows the ⁇ r a ndom 9 ⁇ ' ⁇ boundary length fraction in % (represented by a black circle) and ⁇ 3 grain boundary length fraction in % (represented by a black square) measured on different ingots produced by using different nuclei having a particle size between 300-500 ⁇ .
• I represents values ( ⁇ ran dom anc ' ⁇ 3 ) measured on ingot when using S1O2 as nuclei;
• I I represents values ( ⁇ ran dom ar
• I II represents values ( ⁇ ran dom ar
• IV represents values ( ⁇ ran dom ar
• V represents values ( ⁇ ran dom ar
• d ⁇ 3 9 ra i n boundary length fractions measured on ingot obtained by using AI2O3 are the worst: ⁇ ranc
• d ⁇ 3 9 ra i n boundary length fractions measured on ingot obtained by using S1O2 are the most promising values.
• orn grain boundary length fraction is 59 % while ⁇ 3 grain boundary length fraction is 28 %.
• Fig ure 2 shows the ⁇ random 9 ⁇ ' ⁇ boundary length fraction in % (represented by a. black circle) and ⁇ 3 grain boundary length fraction in % (represented by a black square) measured on different ingots.
• the comparative examples are ingots produced without additional seeds (X) (multi-Ref), ingot produced by H PM technique (XI) (HPM) and an ingot produced by using S1O2 nuclei having a particle size higher than 800 ⁇ (XIV).
• the present invention is represented by an ingot produced by using S1O2 having a particle size between 300-500 ⁇ (XI) or by an ingot produced by using S1O2 having a particle size between 150-300 ⁇ (XI I).
• ⁇ 3 measured on comparative ingot multi-Ref is higher than ⁇ ra ndom which confirms the lower quality of the ingot produced without additional seeds.
• HPM standard ingot gives the best results related to the quality of the ingot: ⁇ ra ndom 9 ra i n boundary length fraction is around 70 % while ⁇ 3 grain boundary length fraction is around 20 %.
• controlling the thermal parameters is crucial and very tricky. The results measured on an ingot produced by using S1O2 particles having a particle size higher than 800 ⁇ are not satisfactory while the results measured on an ingot produced by using S1O2 having a particle size between 150-300 ⁇ or between 300-500 ⁇ are close to HPM values.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Crystallography & Structural Chemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Silicon Compounds (AREA)
• Crystals, And After-Treatments Of Crystals (AREA)

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• 2017
  • 2017-08-02 TW TW106126008A patent/TW201816200A/zh unknown
  • 2017-08-03 SG SG11201900650WA patent/SG11201900650WA/en unknown
  • 2017-08-03 EP EP17746135.7A patent/EP3494247A1/de not_active Withdrawn
  • 2017-08-03 WO PCT/EP2017/069661 patent/WO2018024830A1/en unknown

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