TW201816200A - Crucible for crystallization of molten silicon, process for its manufacture and use thereof - Google Patents

Abstract

Crucible for the crystallization of molten silicon coated with silicon nitride where embedded SiO2 particles having a particle size up to 500 [mu]m 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

 Bismuth for melting cerium crystallization, method for producing the same, and use thereof  

The present invention relates to a crucible for melting cerium, the manufacture of the crucible, and the use of the crucible for the melting of rhodium.

Polycrystalline semiconductors have become the most important photovoltaic materials for solar cell manufacturing due to their low cost. Polycrystalline germanium (PCS) is typically fabricated using the Bridgman crystal growth technique; the pool of molten semiconducting material contained in the crucible is cooled in a controlled manner to solidify the material from the crucible bottom and move the front end of the crystal liquid toward the top of the crucible . For this treatment, the crucible is placed in an oven and filled with semiconductor material. Start the oven and melt all the ingredients. Heat is then extracted through the bottom plate through a heat sink located below the crucible; typically the heat sink contains gas flowing in the duct. The heat extraction rate from the feedstock can be controlled by varying the gas flow rate. As the temperature in the stock layer in contact with the substrate reaches the crystallization temperature, as the crystallization front proceeds, the crystal will grow from the bottom layer and extend upward.

The resulting polycrystalline germanium is typically cut into cross-sections of the same or close to the size of the wafer used to fabricate the photovoltaic cell. The tile is then sawed or otherwise cut into such a wafer. The polycrystalline germanium produced in this manner is a grain cohesive body in which the orientation of the grains opposite each other is very irregular in the wafer made therefrom.

A major concern during polycrystalline germanium curing is to avoid impurities from entering the crucible and to minimize stresses (and thus misalignment in the material) generated during curing and subsequent cooling. All of these problems reduce battery efficiency compared to battery efficiency made with expensive single crystals.

The crucible is often made of molten ceria or based on other materials consisting essentially of ceria. The molten cerium oxide is usually coated with tantalum nitride to prevent the molten yt from adhering to the ruthenium wall.

In order to improve battery efficiency, seed crystal assisted crystallization has been developed.

In the patent US-A1-2013008371, it is stated that the continuous ceria layer facing the melt has a very good nucleation effect. However, this layer causes significant oxygen transfer to the melt due to the reaction between the metal ruthenium and the ruthenium dioxide, and causes oxygen contamination to cause photoinduced degradation.

An alternative method is to deposit the strontium seed crystals at the bottom of the crucible. This technology is mostly used in Asia to manufacture HPM (High Performance Polysilicon) wafers. This technology gives the bismuth ingot a very fine microstructure, a randomly oriented grain composition with very low misalignment defects, resulting in a high performance battery. However, at the bottom (red zone), higher bismuth ingots are lost due to residual unmelted seed crystals. Furthermore, in order not to melt the seed crystals, the temperature at the bottom of the crucible must be very well controlled during the melting step. If the seed crystal melts, there is really no more nucleation effect. In addition, boiler hot zone temperature control is a daunting task.

It is therefore desirable to provide a crucible that can produce very fine microstructured ingots that are composed of randomly oriented particles and have minimal bottom height loss.

"Journal of Crystal Growth, "Growth of multicrystalline silicon ingot with both enhanced quality and yield through quartz seeded method") (435 (2016) The publication of 91-97) discusses the improvement of cell efficiency. This method uses tantalum nitride coated tantalum. In the second step, a 5 mm mesh and 1 mm lamp screen cover is placed on the tantalum nitride coating. On the layer, a slurry made of quartz powder having a particle size of 200 mesh (<74 μm) and an organic binder is then sprayed on the mesh cover. After drying, the mesh module is removed, and a mesh pattern divided into a plurality of quartz spots is obtained. The coating must then be sprayed on the previous coating with another layer of tantalum nitride coating having a thickness in the range of 30-50 μm. Care must be taken because too low a thickness can cause severe adhesion and oxygen dissolution, The high thickness results in a lower nucleation effect. Although battery efficiency is observed to be improved compared to seedless or polycrystalline auxiliary seeding, many steps are required, some of which are critical steps.

The DE-B4-102010000687 patent proposes a relatively simple method which uses a crucible having a portion directly in contact with the crucible melt embedded in the crystal nucleus of the coating. However, it reveals a wide range of features. The crystal nucleus must be selected from the group consisting of SiO, SiO ₂ , Si ₃ N ₄ , BN, BP, AlP, AlN, Al ₂ O ₃ , and BeO; the effective nucleus surface density is defined to be between 0.001 and 100 cm ^-2 , preferably is between 0.03 to 5 cm ^-2 nuclei, and defined between a particle size from 0.01 to 50000 microns, and preferably between 1 and 500 microns. Al ₂ O ₃ is a preferred crystal nucleus.

The patent CN-A-105063748 also discloses a crucible having a crystal nucleus having a diameter of 5-15 mm, the core of which is a germanium and a polycrystalline germanium partially embedded in a tantalum nitride coating. The crystal nucleus is sprayed onto the coating and then fired. Use a density of 4-100 crystal cores ^-2 . Tantalum and polycrystalline germanium are preferred nuclei, which means that the bottom temperature must be carefully controlled to avoid melting of the nuclei.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a crucible which can obtain a crucible having the same or higher performance and having a reduced bottom red zone as compared to a crucible obtained by HPM technology.

This purpose is achieved by the shackles described in claim 1.

The advantage of the proposed solution is that the crucible can be completely melted into the crucible without having to care very much about the temperature of the bottom. The melting point of cerium oxide (1600 ° C) as a crystal nucleus or tantalum nitride (1900 ° C) as a coating layer is indeed much higher than the melting point of cerium (1412 ° C). Therefore, the production process is less sensitive.

The present invention relates to a crucible for the crystallization of silicon comprising a base body and an inner bottom surface of the sidewalls defining a volume; having an oxygen content of at least 5 weight percent of the silicon nitride layer on the bottom surface, wherein the SiO ₂ particles are partially embedded in a nitrogen In the ruthenium layer, the particle has a particle size of up to 500 microns, and at least a portion of the SiO ₂ particles are present in the internal volume, wherein the surface density of the SiO ₂ particles is 100 cm ^-2 . In order to improve the adhesion of the coating to the crucible, the tantalum nitride layer must have a minimum oxygen content (at least 5 weight percent). Then the crucible can be easily transported.

In the prior art, several types of particles can be used as crystal nuclei or seed crystals (Si, SiC, Si ₃ N ₄ , Al ₂ O ₃ , SiO ₂ ...). Surprisingly, in order to obtain powdered SiO ₂ particles having a surface density of at least 100 cm ^{-2 and} a particle size of up to 500 μm resulted in significantly improved results.

Particles with a particle size greater than 500 microns result in a limited nucleation effect. Advantageously, the particle size of the SiO ₂ particles is 300 microns, more specifically 150 microns, and more particularly between 80 microns and 150 microns. The operation of very fine particles (<80 μm) is tricky and results in severe particle loss due to the dusting behavior of the particles.

Advantageously, the surface density of the SiO ₂ particles is 500 cm ^-2 , more especially 10,000 cm ^-2 but 20,000 cm ^-2 . In order to optimize the use of the SiO ₂ particles, it is preferred not to stack the SiO ₂ particles.

The minimum thickness is preferably 150 microns. A coating thickness of less than 150 microns allows for the problem of the indole sticking to the bottom.

Advantageously, the SiO ₂ particles are fused silica particles because less impurities can diffuse in the bismuth ingot.

Advantageously, the total area covered by the SiO ₂ particles is 25%, more specifically 45%. Above 45% will cause the ingot to stick to the crucible.

The crucible typically comprises a material suitable for high temperatures, preferably fused ceria.

The invention also relates to a method of producing such a crucible. A tantalum nitride slurry comprising tantalum nitride particles having a BET specific surface area of from 2 to 20 m 2 /g, preferably from 5 to 10 m 2 /g, is coated on the crucible. Avoid cracking of the coating during firing by selecting this specific surface value. The SiO ₂ particles are deposited on the wet coating prior to the coating drying step. The particles float on the surface of the coating. More than 80% of the SiO ₂ particles emerge from the coating. Very few particles are completely covered by the tantalum nitride coating.

Once the tantalum nitride coating has dried, the crucible is sintered at a temperature between 800 and 1200 ° C to partially oxidize the nitride coating and to fix the SiO ₂ particles in the coating. Below 800 ° C, the nitride coating is too low in oxidation, while above 1200 ° C it is difficult to control the degree of oxidation.

A series of tests were conducted to evaluate the properties of the crystalline bismuth ingot obtained in accordance with the present invention. The commercial fused silica tube powder is first crushed to obtain fused silica particles. The purity of fused silica is 99.999%. A slurry made of 50-75 weight percent Si ₃ N ₄ and 50-25 weight percent deionized water is coated on the inner surface of the molten crucible. The three ranges of particle sizes were isolated by using different screens. The first stage comprises particles having a particle size of >800 microns, the second stage comprises particles having a particle size between 300 and 500 microns, and the third stage comprises particles having a particle size between 150 and 300 microns. The bismuth ingots used for comparison are obtained by a poly-crystal method of multi-Ref and using HPM technology (HPM).

Two different analyses were performed: grain boundary length ratio and minority carrier lifetime mapping.

First, the ratio of the grain boundary length is measured. The grain boundary is the interface between two grains. Grain boundaries are 2D features in crystalline structures and tend to reduce the electrical properties of the material. Two types of grain boundaries (Σ ₃ and Σ _random ) were measured.

Σ ₃ indicates a low chaos of the interface between the two crystal grains, in which metal impurities may accumulate. The result is an increase in misalignment that affects the life of a few carriers.

Σ _Randomly indicates a highly chaotic interface between the two grains, which prevents the diffusion of misalignment in the bismuth ingot.

The grain boundary length ratio is the length ratio of the grain boundary type to the total measured grain boundary length. For example, a 30% Σ ₃ grain boundary length ratio indicates that 30% of the total measured grain boundaries are Σ ₃ type.

Silicon ingot will be obtained by the measuring technique HPM Σ ₃ ratio of the crystal grain boundary length of the target value [Sigma _random as: Σ ₃ should be less than 25% and should be greater than 60% [Sigma _random.

Figure 1 shows the percentage of the _random grain boundary length ratio (indicated by a black circle) and the ratio of the Σ ₃ grain boundary length measured for different bismuth ingots produced using different crystal nuclei with a particle size of 300-500 μm. The percentage (in black squares).

Figure 2 shows the percentage of the _random grain boundary length ratio (expressed in black circles) and the ratio of the Σ ₃ grain boundary length ratio (in black squares) measured for different bismuth ingots.

Figure 3 shows the cross-section of different tantalum ingots as measured by microwave photoconductivity attenuation (μWPCD).

Figure 1 shows the percentage of the _random grain boundary length ratio (indicated by a black circle) and the ratio of the Σ ₃ grain boundary length measured for different bismuth ingots produced using different crystal nuclei with a particle size of 300-500 μm. The percentage (in black squares). I represents the value measured on the bismuth ingot when SiO _{2 is} used as the crystal nucleus (Σ ₃ and Σ _random ); II represents the value measured on the bismuth ingot when Si ₃ N _{4 is} used as the nucleus (Σ ₃ and Σ _(random ); III indicates the value measured on the bismuth ingot when SiC is used as the nucleus (Σ ₃ and Σ _random ); IV indicates the value measured on the bismuth ingot when Si is used as the nucleus (Σ ₃ and Σ _random); and V represents a value (Σ ₃ and _{a random} [Sigma) Al is used as a measure of the nuclei in the silicon ingot ₂ O _3. The ratio of Σ _random to Σ ₃ grain boundary length measured by using bismuth ingot obtained using Al ₂ O ₃ was the worst: Σ _random grain boundary length ratio was 28% and Σ ₃ grain boundary length ratio was 44%. SiO _{2 is} used for the silicon ingot obtained in the measurement of the [Sigma _random grain boundary length ratio of Σ ₃ to an optimum value: Σ _random grain boundary length ratio is 59% and the grain boundary length ratio of Σ ₃ is 28%.

Figure 2 shows the percentage of the _random grain boundary length ratio (indicated by a black circle) measured for different bismuth ingots, and the percentage of the 晶₃ grain boundary length ratio (indicated by a black square). Comparative examples are tantalum ingot (X) (multi-Ref) made without additional seed crystals, tantalum ingot (XI) (HPM) manufactured by HPM technology, and tantalum made of SiO ₂ core having a particle size larger than 800 μm. Ingot (XIV). The present invention consists of a particle size of SiO ₂ is produced between the core of 300-500 micron silicon ingot (XI), or a particle size of silicon ingots (XII) produced between the core of SiO ₂ as a representative of 150-300 microns. The Σ ₃ measured in comparison with the bismuth ingot multi-Ref is larger than the Σ _random , which confirms that the quality of the bismuth ingot produced without the additional seed crystal is low. HPM standard silicon ingot ingot having the best quality results related to: Σ _random grain boundary length ratio of about 70% of the grain boundary length ratio of Σ ₃ is about 20%. However, as mentioned above, controlling thermal parameters is critical and very tricky. The results of the silicon ingot manufactured using a particle size greater than 800 micron SiO ₂ particles measuring not satisfactory, while the silicon ingot manufactured using SiO particle size of between 150-300 microns or 300-500 microns of particles ₂ The measured result is close to the HPM value.

Further production properties are similar to those made by HPM technology, and the goal is to manufacture the smallest bismuth ingots in the red zone. In order to check this parameter, an antimony ingot (HPM) manufactured by HPM technology (Fig. 3a), and a crucible ingot (Fig. 3b) manufactured using SiO ₂ particles having a particle size of between 300 and 500 μm according to the present invention. , performs a minority carrier lifetime mapping of the passivated surface.

Figure 3 shows the cross-section of different bismuth ingots measured by microwave photoconductivity attenuation (μWPCD). The outer zone (red zone - black in the drawing) represents the material loss that causes the high manufacturing cost of the solar cell. The minority carrier lifetime is shown in grayscale, from 0 to 35 microseconds as shown. The darker the color (on the right side of the legend), the longer the life.

The results obtained by the HPM-made bismuth ingot show a larger red zone (outer zone), and the minority carrier life in the center of the bismuth ingot can reach 20-25 microseconds (Fig. 3a).

The results obtained in accordance with the present invention using a bismuth ingot produced from SiO ₂ particles having a particle size of between 300 and 500 microns (Fig. 3b) show a decrease in the red area at the center of the ingot and an increase in the minority carrier life in the center of the ingot. (30-35 microseconds). It has also been observed that bismuth ingots made using SiO ₂ particles having a particle size between 150 and 300 microns are the best choice.

Claims (11)

A crucible for crystallization of a crucible, comprising: a substrate comprising a bottom portion and a sidewall defining an inner volume; and a tantalum nitride layer having an oxygen content of at least 5 weight percent at the bottom surface, characterized in that the SiO ₂ particle is partially embedded in In the tantalum nitride layer, the particle has a particle size of at most 500 μm, and at least a portion of the SiO ₂ particles are present in the inner volume, wherein the surface density of the SiO ₂ particles is 100 cm ^-2 . As claimed in claim 1, wherein the particle size of the SiO ₂ particles is 300 microns, more specifically 150 microns. The requested item 1 or any one of the crucible 2, the SiO ₂ particles wherein the surface density of 500 cm ^-2 , more especially 10,000 cm ^-2 but 20,000 cm ^-2 . The enthalpy of any one of claims 1 to 3, wherein the SiO ₂ particles are fused silica particles. The enthalpy of any one of claims 1 to 4, wherein the SiO ₂ particles form a discontinuous layer. As claimed in any one of claims 1 to 5, more than 80% of the SiO ₂ particles are present from the coating.  The enthalpy of any one of claims 1 to 6, wherein the tantalum nitride layer has a thickness of at least 150 microns.   The method of any one of claims 1 to 7, wherein the total area covered by the SiO ₂ particles is 25%, more specifically 45%.  The item of any one of claims 1 to 7, wherein the crucible comprises fused silica.   A method of producing a crucible according to claim 1, wherein the step comprises: a. providing a crucible, b. coating comprising a specific surface area (BET) of from 2 to 20 m 2 /g, preferably from 5 to 10 m 2 / a cerium nitride slurry of cerium nitride particles, c. providing SiO ₂ particles having a particle diameter of 500 μm, d. drying the cerium, and e. sintering the cerium at a temperature between 800 and 1200 °C.  A use according to any one of claims 1 to 8, which is grown by a Bridgman crystal growth technique.