WO2018024830A1

WO2018024830A1 - Crucible for crystallization of molten silicon, process for its manufacture and use thereof

Info

Publication number: WO2018024830A1
Application number: PCT/EP2017/069661
Authority: WO
Inventors: Julien Laurent; Etienne DRODE; Christian Martin
Original assignee: Vesuvius France S.A.
Priority date: 2016-08-03
Filing date: 2017-08-03
Publication date: 2018-02-08
Also published as: SG11201900650WA; TW201816200A; EP3494247A1

Abstract

Crucible for the crystallization of molten silicon coated with silicon nitride where embedded SiO₂ particles having a particle size up to 500 µm and a surface density ≥ 100 cm^-2, emerge in the inner volume of the crucible enabling the production of silicon ingot with improved electrical properties.

Description

Crucible for crystallization of molten silicon, process for its manufacture and use thereof.

[0001 ] 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.

[0002] 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. To carry out such process, 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. By varying the gas flow rate, it is possible to control the heat extraction rate from the feedstock. As the temperature within the feedstock layer in contact with the floor reaches the crystallization temperature, crystals will start growing from the bottom floor and extend upwards, as the crystallization front proceeds.

[0003] 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.

[0004] The main concerns during solidification of multicrystalline silicon are to avoid impurities entering the silicon and to minimise the stresses that are created during solidification and subsequent cooling which generate dislocations in the material. All these problems reduce the cell efficiency which is lower than that made with expensive single crystal.

[0005] 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.

[0006] In order to improve the cell efficiency, seed-assisted crystallization methods have been developed.

[0007] In US-A1 -2013008371 , a continuous silica layer facing the melt is reported to have a very good nucleation effect. However, such layer generates significant oxygen transfer to the melt due to the reaction between metallic silicon and silicon oxide, leading to oxygen contamination which in turn leads to light induced degradation.

[0008] 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. 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. However, a significant height of the ingot is lost in the bottom (red zone) because of remaining un-melted seeds. Moreover, 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. In addition, the furnace hot zone temperature control is a difficult task.

[0009] There is thus a need to provide a crucible allowing to produce silicon ingot having a very fine microstructure composed of randomly oriented grains with minimized bottom height losses.

[0010] Increased cell efficiencies were discussed in a publication of the Journal of Crystal Growth (Growth of multicrystalline silicon ingot with both enhanced quality and yield through quartz seeded method; 435(201 6)91 -97). The method uses a silicon nitride coated crucible. In a second step, a mask of net with 5 mm mesh and 1 mm filament is placed on the silicon nitride coating. Then a slurry made of quartz powder with a particle size of 200 mesh (<74 μιη) and organic binder is sprayed on the mask. After drying, the net module is removed and a grid textured coating divided into many quartz spots is obtained. Another layer of silicon nitride coating having a thickness in the range of 30-50 μιη 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. However broad ranges of features are disclosed. 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 μιη and preferably between 1 and 500 μιη. AI2O3 is presented as a preferred nucleus.

[0012] 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.

[0013] 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.

[0014] This objective is reached with a crucible as defined in claim 1 .

[0015] 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.

[0016] 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 μιη, 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.

[0017] In the prior art, several types of particles can be used as nuclei or seeds (Si, SiC, S13N4, AI2O3, S1O2...). Surprisingly, pulverulent S1O2 particles having a particle size up to 500 μιη so as to obtain a surface density of at least 100 cm^~2 lead to significantly improved results.

[0018] Particles having a size higher than 500 μιη lead to a limited nucleating effect.

Advantageously, the particle size of S1O2 particles is < 300 μιη, more particularly < 150 μιη and more particularly between 80 μιη and 150 μιη. Manipulating very fines particles (<80 μιη) is tricky and leads to an important loss of particles due to the dusting behavior of the particles.

[0019] Advantageously, 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.

[0020] A minimum thickness of 150 μιη is preferred. Below a thickness of 150 μιη of coating, problem of ingot sticking to the bottom can be observed.

[0021 ] Advantageously, the S1O2 particles are fused quartz particles because less impurities are then able to diffuse in the silicon ingot.

[0022] Advantageously, 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.

[0023] The crucible generally comprises material suitable for high temperature, preferably fused silica.

[0024] 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. Before the drying step of the coating, 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.

[0025] Once the silicon nitride layer has dried, 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.

[0026] 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 μιη, a second grade comprises particles having a particle size between 300-500 μιη and a third grade comprises particles having a particle size betweenl 50-300 μιη. Comparative ingots were obtained by multicrystalline process without seeds (multi-Ref) and using H PM technique (H PM).

[0027] Two different kinds of analysis are carried out: grain boundary length fraction and minority carrier lifetime mappings.

[0028] 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 (Σ3 and∑_random) are measured.

[0029] ∑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.

[0030] ∑_rand_om represents a high disorientation of the interface between two grains which prevents the spread of the dislocation in the ingot.

[0031 ] The grain boundary length fraction is the length fraction of the grain boundary type in relation to the total measured grain boundary length. For example, 30% Σ3 grain boundary length fraction means that 30% of the total measured grain boundary is of Σ3 type.

[0032] ∑3 and∑_random 9^raiⁿ boundary length fractions measured on an ingot obtained by HPM technique are taken as target values: Σ3 should be below 25% and∑_ranc|_om should be higher than 60 %.

BRIEF DESCRIPTION OF THE FIGU RES

[0033] Fig ure 1 shows the∑r_andom 9^'^ boundary length fraction in % (represented by a black circle) and Σ3 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 μιη.

I represents values (∑_random ^anc'∑3 ) measured on ingot when using S1O2 as nuclei;

I I represents values (∑_random ^ar|d∑3 ) measured on ingot when using S13N4 as nuclei;

I II represents values (∑_random ^ar|d ¾ ) measured on ingot when using SiC as nuclei;

IV represents values (∑_random ^ar|d∑3 ) measured on ingot when using Si as nuclei and

V represents values (∑_random ^ar|d ¾ ) measured on ingot when using AI2O3 as nuclei.

The∑_random ^ar|d∑3 9^raiⁿ boundary length fractions measured on ingot obtained by using AI2O3 are the worst:∑_ranc|_om grain boundary length fraction is 28 % while Σ3 grain boundary length fraction is 44 %. The∑_random ^ar|d∑3 9^raiⁿ boundary length fractions measured on ingot obtained by using S1O2 are the most promising values.∑_ranc|_orn grain boundary length fraction is 59 % while Σ3 grain boundary length fraction is 28 %.

[0034] Fig ure 2 shows the ^random 9^'^ boundary length fraction in % (represented by a. black circle) and Σ3 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 μιη (XIV). The present invention is represented by an ingot produced by using S1O2 having a particle size between 300-500 μιη (XI) or by an ingot produced by using S1O2 having a particle size between 150-300 μιη (XI I). Σ3 measured on comparative ingot multi-Ref is higher than∑_random 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:∑_random 9^raiⁿ boundary length fraction is around 70 % while Σ3 grain boundary length fraction is around 20 %. However, as discussed above, 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 μιη are not satisfactory while the results measured on an ingot produced by using S1O2 having a particle size between 150-300 μιη or between 300-500 μιη are close to HPM values.

[0035] Further to producing an ingot having properties similar to those of an ingot produced by HPM technique, the production of an ingot with a minimum of red zone is also targeted. In order to check this parameter, minority carrier lifetime mappings on passivated surface were carried out on an ingot produced by H PM technique (HPM) (Fig.3 a) and an ingot produced according to the invention by using S1O2 particles having a particle size between 300-500 μιη (Fig.3 b). Figure 3 shows the cross-section of the different silicon ingots measured by Microwave Photoconductivity Decay ^WPCD). The outer zone (red zone- in black on the drawing) represents the loss of material that contributes to the high manufacturing costs for solar cells. The minority carrier lifetime is illustrated in grey scales as shown on the legend from 0 to 35 με. The darker colours (right side of the legend) represent higher lifetime.

The results obtained for an ingot produced by HPM show a larger red zone (outer zone) and the minority carrier lifetime in the centre of the ingot can reach 20-25 με (Fig.3 a).

The results obtained for an ingot produced according to the invention by using S1O2 particles having a particle size between 300-500 μιη (Fig.3 b) show a reduced red zone and an increase of the minority carrier lifetime in the centre of the ingot (30-35 με) (Fig.3 c). It was also observed that an ingot produced by using S1O2 particles having a particle size between 150-300 μιη is an optimised choice.

Claims

Claims.

I . 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 μιη, at least a portion of said S1O2 particles emerging in the inner volume and wherein the surface density of the S1O2 particles is > 1 00 cm^"2.

2. Crucible according to claim 1 , wherein the size of the S1O2 particles is < 300 μιη, more particularly < 150 μιη.

3. Crucible according to any one of the claims 1 or 2 wherein the surface density of the Si0₂ particles is > 500 cm^"2, more particularly > 1 0.000 cm^"2 but < 20.000 cm^"2.

4. Crucible according to any one of the claims 1 to 3, wherein the S1O2 particles are fused quartz particles.

5. Crucible according to any one of the claims 1 to 4, wherein the S1O2 particles form a discontinuous layer.

6. Crucible according to any one of the any one of the claims 1 to 5, wherein more than 80 % of the S1O2 particles are emerging from the coating.

7. Crucible according to any one of the any one of the claims 1 to 6, wherein the thickness of the silicon nitride layer is at least 150 μιη.

8. Crucible according to any one of the claims 1 to 7, wherein the total surface covered by the SiC>2 particles is > 25 % and more specifically < 45 %.

9. Crucible according to any one of the claims 1 to 7, wherein the crucible comprises fused silica.

1 0. Process for producing a crucible according to claim 1 comprising the following steps: a. providing a crucible,

b. applying a silicon nitride slurry comprising silicon nitride particles having a specific surface Area (BET) of 2-20 m²/g, preferably 5 -10 m²/g,

c. providing S1O2 particles having a particle size up to 500 μιη,

d. drying the crucible,

d. firing the crucible at a temperature between 800-1200 °C.

I I . Use of the crucible according to any one of claims 1 to 8 for the crystallization of silicon by the Bridgman growth technique.