JP2014009111A - Silicon nitride powder for mold-releasing agent - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a silicon nitride powder for a mold-releasing agent, which has an improved adhesive force to the inner wall of a melting crucible and thereby, can reduce the contamination of impurities and improve yield when a polycrystalline silicon ingot is manufactured.SOLUTION: The silicon nitride powder for a mold-releasing agent has a 90% particle diameter by a laser diffraction scattering method of 3.0-10 μm, a ratio of α-phase of 20-60%, and an iron content of 100 ppm or less, and is characterized in that the particle size distribution has two maximum values; one maximum value is in the range of 0.2 μm or more and less than 1.0 μm (maximum value 1) and another maximum value is in the range of 1.0-8.0 μm (maximum value 2); the ratio of the frequency of the maximum value 2 to the frequency of the maximum value 1 {(frequency of maximum value 2)/(frequency of maximum value 1)} is 1.0-5.0, and the interval between the maximum values 1 and 2 is 0.8-7.8 μm.

Description

  The present invention relates to a silicon nitride powder suitable as a release agent.

  Solar cells are expected as a clean energy source in recent years, and a significant increase in demand is expected. Examples of solar cells include single crystal silicon solar cells, polycrystalline silicon solar cells, amorphous silicon solar cells, compound solar cells, etc., but relatively low cost polycrystalline silicon solar cells are currently available Mainstream.

  A polycrystalline silicon substrate used for a polycrystalline silicon solar cell is generally manufactured by a method called a casting method. This casting method is a method in which raw material silicon is put into a quartz crucible or a graphite crucible and heated and melted at about 1500 ° C. in an inert atmosphere to form a polycrystalline silicon ingot. The end of the silicon ingot is removed, cut into a desired size, cut out, and the cut silicon ingot is sliced to a desired thickness to obtain a polycrystalline silicon substrate for forming a solar cell.

  When producing a silicon ingot by the casting method, a release agent is applied to the inner surface of the crucible in order to improve releasability from the crucible. The mold release agent is preferably a powder containing silicon as a main component and a component that is chemically stable at a high temperature from the viewpoint of preventing impurities from being mixed.

  An example of using silicon nitride powder as a mold release agent at the time of producing a polycrystalline silicon ingot for a solar cell has already been reported (Patent Document 1). However, if silicon nitride powder is simply used, impurities contained in silicon nitride contaminate the silicon ingot, and the power generation efficiency of a solar cell manufactured using the silicon ingot deteriorates. Furthermore, the coating strength and coating properties of the release agent vary depending on the silicon nitride used.

  A high-purity silicon nitride fine powder having a 50% particle size of 0.7 μm or less and a 90% particle size of 2 μm or less by pulverizing high-purity silicon processing waste has been reported (Patent Document 2). However, Patent Document 2 does not describe preventing impurities from being mixed into a silicon ingot for a solar cell or improving applicability by using it as a mold release agent.

Patent Document 3 has been reported as a silicon nitride powder for a release agent with a small amount of impurity diffusion into a silicon ingot. However, it does not describe the adhesion and coating properties when used as a release agent.

  Patent Document 4 describes a mold release agent using a silicon nitride powder having an average particle diameter of 5 μm or less and an iron content of 20 ppm or less. By using a silicon nitride powder having an average particle diameter of 5 μm or less, a release agent layer is used. It has been reported to increase the strength. However, when the average particle size is only 5 μm or less, release agents having different strengths are produced depending on the synthesis method or the pulverization method, so this definition alone is insufficient.

Japanese Patent Application Laid-Open No. 07-206419 JP 2011-051856 A JP 2012-001385 A JP 2007-261832 A

  The present invention relates to a mold release agent used at the time of producing a polycrystalline silicon ingot, which improves the workability at the time of application and reduces the contamination of the polycrystalline silicon ingot by mixing impurities by suppressing the release of the mold release agent. A silicon nitride powder for an agent is provided.

The present invention employs the following means in order to solve the above problems.
(1) A silicon nitride powder having a 90% particle diameter of 3.0 to 10 μm, an α-phase ratio of 20 to 60%, and an iron content of 100 ppm or less by laser diffraction scattering method, and has two maximum particle size distributions. One value is 0.2 μm or more and less than 1.0 μm (maximum value 1), the other is 1.0 μm or more and 8.0 μm or less (maximum value 2), and each frequency of the maximum values 1 and 2 The ratio {(frequency of local maximum value 2) / (frequency of local maximum value 1)} is 1.0 to 5.0, and the distance between local maximum values 1 and 2 is 0.8 to 7.8 μm. Silicon nitride powder for molds.
(2) A melting crucible using the silicon nitride powder for a release agent according to (1) as a release agent.

  When the silicon nitride powder of the present invention is used as a mold release agent, it is possible to reduce the mixing of impurities from the molten crucible into the polycrystalline silicon ingot. Moreover, the applicability of the release agent is improved, and the workability during application is improved.

Hereinafter, the present invention will be described in detail.
Examples of the method for producing silicon nitride include a direct nitriding method and an imide pyrolysis method. The process is not particularly limited, but a direct nitriding method is preferable for obtaining the silicon nitride particles of the present invention. The reason is that the particle size can be easily adjusted in the pulverization step after nitriding. The reason why the particle size needs to be adjusted is because it affects the properties of the prepared slurry.

  The silicon nitride powder of the present invention is characterized by a low iron content. It is known that transition metals such as iron, chromium, nickel, copper, tungsten, and molybdenum reduce the power generation efficiency of solar cells when present in polycrystalline silicon. In particular, iron is likely to be mixed due to various factors, so the point is how to reduce the amount of iron mixed. As a method for reducing the amount of iron, (1) using a silicon raw material with a small amount of iron, (2) dissolving and removing iron mixed with an acid such as nitric acid, etc. can be considered. In view of complication, it is preferable to use the high purity silicon raw material (1) with a small amount of iron contamination.

  In general metal silicon, iron is usually mixed at a level of several hundred to several thousand ppm, so that it is not suitable as a raw material for the silicon nitride powder of the present invention. For example, silicon scrap obtained at the time of processing and polishing from silicon for semiconductor use is preferable as a raw material because of high purity. The amount of iron contained in the raw material is 100 ppm or less, preferably 10 ppm or less.

A further feature of the silicon nitride powder of the present invention is that the α phase ratio is 20 to 60%. Silicon nitride has a low temperature stable α phase and a high temperature stable β phase, and the rearrangement from the α phase to the β phase is irreversible. Further, since dislocation occurs from the α phase to the β phase by a so-called dissolution reprecipitation mechanism, it is accompanied by grain growth. That is, the particle size is generally larger in the β phase than in the α phase.
The proportion of the α phase is 20 to 60%, preferably 30 to 40%. When the ratio of the α phase exceeds 60%, the fine powder increases and the particle size distribution having the two maximum values is lost or the ratio of the maximum values is less than 1. When the α phase ratio is less than 20%, the coarse powder increases and the particle size distribution having the two maximum values is lost.

Furthermore, in the production atmosphere in which the α-phase ratio is less than 20%, β-phase particles tend to grow and become columnar. These columnar β-phase particles deteriorate the fluidity of the prepared slurry and deteriorate the coating performance such as coating unevenness.
Most of the columnar β-phase particles have a major axis of about 10 μm, an aspect ratio (for example, select columnar particles from an electron micrograph, measure the major axis and minor axis, and calculate from (major axis) / (minor axis)) of about 3 to 6 It is. Further, when the α phase ratio is less than 20%, these particles are present in an amount of 10% or more, and as described above, the coating performance of the slurry is deteriorated.

  The iron content of the silicon nitride powder of the present invention is 100 ppm or less, preferably 20 ppm or less. Outside this range, impurities may easily diffuse from the release agent in the silicon ingot, which may lead to a decrease in quality and yield of the silicon ingot.

  The feature of the silicon nitride powder of the present invention is that when used as a mold release agent, the workability at the time of application is improved and the contamination of the polycrystalline silicon ingot is reduced by suppressing the release of the mold release agent. . For this purpose, it is necessary to optimize the particle diameters of the α phase and the β phase and the ratio of the α phase and the β phase. Since these are reflected in the particle size distribution, it is important to optimize the particle size distribution.

  The silicon nitride powder of the present invention needs to have a 90% particle size of 3.0 to 10 [mu] m, preferably 3.0 to 6.0 [mu] m, as measured by a laser diffraction scattering method. When the thickness is less than 3.0 μm, the adhesion of the produced release agent is deteriorated. When the thickness exceeds 10 μm, the coating property is deteriorated and coating unevenness is likely to occur.

  The particle size distribution needs to have two maxima. The particle size distribution with a maximum value of 1 is mostly α phase or β phase, and when these silicon nitride powders are used as a release agent, the applicability is deteriorated or the adhesion is insufficient.

  Of the two maximum values, the maximum value on the low particle diameter side (maximum value 1) needs to be 0.2 μm or more and less than 1.0 μm, preferably 0.6 to 0.8 μm. If the thickness is less than 0.2 μm, the thickening of the release agent is severe and difficult to apply, and if it is 1.0 μm or more, the fine particles are insufficient, and coating unevenness is likely to occur during application. The maximum value is the particle diameter when the frequency (volume%) of particles in the particle size distribution reaches a peak (top of a mountain).

  Of the two maximum values, the maximum value on the high particle diameter side (maximum value 2) needs to be 1.0 μm or more and 8.0 μm or less, and preferably 2.0 to 4.0 μm. If the thickness is less than 1.0 μm, the thickening of the release agent is severe and application becomes difficult, and if it exceeds 8.0 μm, coarse particles become too large to form close-packed packing with fine particles, resulting in poor adhesion.

  The ratio of the frequencies of the maximum values 1 and 2 {(frequency of the maximum value 2) / (frequency of the maximum value 1)} needs to be 1.0 to 5.0, preferably 1.5 to 3.0. It is. If it is less than 1.0, the viscosity of the mold release agent is so strong that it becomes difficult to apply. If it exceeds 5.0, close-packing with coarse particles and fine particles is not formed, and adhesion is deteriorated.

  The interval between the maximum values 1 and 2 needs to be 0.8 to 7.6 μm, preferably 2.0 to 4.0 μm. If it is less than 0.8 μm, the maximum value is no longer different from the particle size distribution of 1, and as described above, the applicability is deteriorated or the adhesion is insufficient. If it exceeds 7.6 μm, close-packing with elementary particles and fine particles will not be formed, and adhesion will deteriorate.

  The silicon nitride powder of the present invention is applied to the inner wall of the crucible, for example, by preparing a water-dispersed slurry and applying it with a spatula or a brush or spraying. There is no problem even if an organic binder such as acrylic, cellulose or polyvinyl alcohol is added to the slurry, but the initial adhesion is improved by adding the organic binder.

Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples.
Example 1
The metal silicon sludge generated from the back grinding process in semiconductor production was dried and then crushed to obtain metal silicon powder having an average particle diameter of 2 to 3 μm. 5 kg of this metal silicon powder is filled into a container made of a composite sintered body of silicon carbide and silicon nitride, placed in a batch type nitriding furnace and replaced with a nitrogen atmosphere, and then heated at 100 ° C./hr in a nitrogen atmosphere. did. When the temperature reached 1150 ° C., a part of the nitrogen atmosphere gas in the furnace was replaced with argon, the temperature increase rate was changed to 10 ° C./hr, the temperature was raised to 1300 ° C., and then held at 1300 ° C. As a result, the reaction rate was gradually increased to a maximum of 2.4% / h. After the cumulative nitriding rate reached 75%, the temperature was raised to 1450 ° C. at a rate of temperature rise of 10 ° C./hr, and then held at 1450 ° C. for 3 hours.
After cooling to room temperature, the nitride ingot was taken out, coarsely pulverized with a jaw crusher and roll crusher, and finely pulverized with a jet mill to produce silicon nitride powder.
The obtained silicon nitride powder was added to water with stirring to prepare a 30% by mass slurry (viscosity 10 cP).

Example 2
Metallic silicon powder having an average particle size of 6 to 7 μm was obtained by pulverizing metallic silicon waste discharged from the ingot cutting step in semiconductor manufacturing. The subsequent steps are the same as those in the first embodiment.

Example 3
The metal silicon sludge generated from the back grinding process in semiconductor production was dried and then pulverized with a ball mill to obtain a metal silicon powder having an average particle diameter of 1 to 1.5 μm. The subsequent steps are the same as those in the first embodiment.

Example 4
The same as Example 1, except that the reaction rate was adjusted to 1.2% / h at maximum.

Example 5
The same as Example 1, except that the reaction rate was adjusted to 3.6% / h at the maximum.

Examples 6-12
The silicon nitride powder produced in Examples 1 to 5 was classified using a classifier (trade name “Aero Fine Classifier” manufactured by Nissin Engineering Co., Ltd., product name “Crusheal” manufactured by Seishin Enterprise Co., Ltd.), or different classified powders. By mixing at a predetermined ratio, silicon nitride powder satisfying the physical properties shown in Table 1 was produced.

Comparative Example 1
Metallic silicon powder having an average particle diameter of 10 to 11 μm was obtained by pulverizing metallic silicon waste discharged from the ingot cutting step in semiconductor manufacturing. The subsequent steps are the same as those in the first embodiment.

Comparative Example 2
The metal silicon sludge generated from the back grinding process in semiconductor production was dried and then pulverized with a ball mill to obtain a metal silicon powder having an average particle diameter of 1 to 1.5 μm. The subsequent steps are the same as those in the first embodiment.

Comparative Example 3
The same as Example 1, except that the reaction rate was adjusted to 0.8% / h at maximum.

Comparative Example 4
The same as Example 1, except that the reaction rate was adjusted to 4.0% / h at the maximum.

Comparative Examples 5-12
Using the silicon nitride powder produced in Examples 1 to 5 and Comparative Examples 1 to 4, using a classifier (trade name “Aero Fine Classifier” manufactured by Nissin Engineering Co., Ltd., product name “Crusheal” manufactured by Seishin Enterprise Co., Ltd.) Or the silicon nitride powder which satisfy | fills the physical property of Table 1 was produced by mixing different classification powders by a predetermined ratio.

Comparative Example 13
The silicon nitride powder produced in Example 1 was mixed with a ball mill filled with iron balls for 30 minutes to produce a silicon nitride powder satisfying the table.

<Measurement method>
90% particle size and particle size distribution: 2 ml of 20% aqueous solution of sodium hexanemetaphosphate and 60 mg of measurement sample are added to 200 ml of pure water, and dispersed for 3 minutes with an ultrasonic homogenizer (trade name “US-300T” manufactured by Nippon Seiki Seisakusho). Then, measurement was performed with a microtrack (manufactured by Nikkiso, trade name “MT3300EXII”). The 90% particle size and particle size distribution were measured based on the volume (%) of the particles.
α phase: Measured in a range of 2θ = 32 to 38 ° with a powder X-ray diffractometer (trade name “Ultima IV” manufactured by Rigaku), and calculated from the intensity of the peak appearing in the range by the following equation.
α conversion rate = {(I _a102 + I _a210 ) / (I _a102 + I _a210 + I _b101 + I _b210 )}
I _a102 : Peak intensity of α phase (102) plane I _a210 : Peak intensity of α phase (210) plane I _b101 : Peak intensity of β phase (101) plane I _b210 : Peak intensity iron content of β phase (210) plane Amount: Measured with a fluorescent X-ray analyzer (trade name “ZSX-Primus II” manufactured by Rigaku).

<Method for evaluating coating film properties>
After coating 300 mm with a width of 30 mm on a stainless steel (SUS304) plate, the coating film was dried at 120 ° C. for 1 hour.
(Applicability)
The coating unevenness state of the coating film was visually evaluated.
◎: Uneven coating is not observed ○: Uneven coating is 10% or less of the entire coating area Δ: Uneven coating is 50% or less of the entire coating area ×: Uneven coating exceeds 50% of the entire coating area (adhesiveness)
The adhesion (cracking and peeling) of the coating film was visually evaluated.
○: No cracking or peeling is observed ×: Cracking or peeling is observed <Amount of silicon ingot impurity mixed>
The above 30% by mass slurry was spray-coated on the inner wall of a quartz crucible having a bottom surface of 220 × 220 mm and a height of 300 mm so that the thickness after drying would be 0.3 mm, and dried by heating at 120 ° C. for 1 hour. .
Thereafter, 10 kg of high-purity silicon powder (purity 7N) was added, and after maintaining at 1480 ° C. for 10 hours in an argon atmosphere, it was cooled.
After the produced high-purity silicon ingot was taken out, the impurity mixing rate was calculated from the following formula, particularly for the upper surface of the ingot where impurities were mixed.
Impurity contamination rate (%) = {(area of impurity contamination portion) / (top area of silicon ingot)} × 100
It has been confirmed that most of the impurities are derived from a release agent that has fallen off the inner wall of the quartz crucible when the silicon powder is melted and contracted.
The results are shown in Table 1. From the examples and comparative examples in Table 1, when the silicon nitride powder of the present invention was used as a release agent, it was possible to reduce the contamination of impurities from the quartz crucible into the polycrystalline silicon ingot. In addition, the applicability of the release agent to the quartz crucible was improved, and the workability during application was improved.

Claims (2)

This is a silicon nitride powder with a 90% particle diameter of 3.0 to 10 μm, an α-phase ratio of 20 to 60%, and an iron content of 100 ppm or less by laser diffraction scattering method, and the particle size distribution has two maximum values. One is 0.2 μm or more and less than 1.0 μm (maximum value 1), the other is 1.0 μm or more and 8.0 μm or less (maximum value 2), and the ratio of each frequency between the maximum values 1 and 2 {( The frequency of the maximum value 2) / (the frequency of the maximum value 1)} is 1.0 to 5.0, and the interval between the maximum values 1 and 2 is 0.8 to 7.8 μm. Silicon nitride powder. A melting crucible using the silicon nitride powder for a release agent according to claim 1 as a release agent.