WO2012050410A1

WO2012050410A1 - Method of purification of silicon

Info

Publication number: WO2012050410A1
Application number: PCT/LT2011/000013
Authority: WO
Inventors: Igor Evgenjevich Maronchiuk; Tamara Fatyxovna Kuliutkina; Igor Igorjevich Maronchiuk; Rokas Virbickas; Arvydas APERAVIČIUS
Original assignee: Uab "Naujausių Technologijų Centras"; Uab "Telebaltikos Importas Ir Eksportas"
Priority date: 2010-10-14
Filing date: 2011-10-03
Publication date: 2012-04-19
Also published as: LT5856B; LT2010086A

Abstract

Method of purification of technical silicon based on dissolving -crystallization in alloys of fusible metals comprises three stages: two (preparatory and finishing) - removal of impurities and one removal of atoms of metal-solvent. In preparatory stage a load of technical silicon is added periodically into alloy of fusible metal, the alloy is mixed in a vacuum by an impulse scavenge with an inert gas, and silicon is crystallized in the shape of tabular silicon crystals. In the finishing stage loads of tabular silicon crystals are added into the other alloy of fusible metal, the alloy saturated with silicon is transported to the crystallization zone wherein silicon is grown in the shape of a block on a silicon plate. Removal of atoms of fusible metals is achieved by application of direct crystallization method, when monocrystals of silicon are grown from blocks of silicon that were obtained in the finishing stage.

Description

Method of purification of silicon

FIELD OF THE INVENTION

Purification method of metallurgical grade silicon is ascribed to the production technology of semiconductor materials, namely - obtaining of pure silicon -by purification of metallurgical grade silicon in melts of fusible metals.

BACKGROUND OF THE INVENTION

In UA patent No. 12665 U a method of purification of metallurgical silicon in the melt of gallium is disclosed. Metallurgical silicon in the form of solid lumps is lowered to the bottom of a crucible, gallium melt is heated up to 800-1000 °C, temperature gradient of 4-8 °C/cm is formed along the crucible containing the melt, crystalline silicon wafer is immersed into the upper, coldest part of the melt. Subsequently, by transfer of mass the melted silicon flows upwards from the bottom of a crucible, due to the natural convection and crystallizes on a wafer at a rate of 1 kg per 3.5 hours.

However, this method has drawbacks:

1. Due to the unstable front of crystallization that takes place in the oversaturated melt when the plate is hot, the temperature gradient along the crucible degrades the morphology of the silicon crystal structure.

2. The efficiency of the crucible is limited because the amount of silicon, purified in the said way depends on the load of silicon which is attached to the bottom of the crucible.

3. The transfer of a mass of the dissolved silicon to the plate by direct convection limits the growth rate of a silicon deposit on a wafer because it is impossible to achieve thickness of less than 1 mm of the marginal diffusion layer.

A purification method of the metallurgical silicon described in UA patent No. 84653 C was selected as the prototype. In said patent metallurgical silicon is continuously added to a melt of a fusible metal at a constant temperature, where the temperature is limited by the volatility of a fusible metal. Fusible metal is selected from the group of Ga, Sn, In, Al, Pb. Metallurgical silicon is fed into a melt of a fusible metal. The melt is forced- stirred thus inducing the transfer of mass of the melted silicon to the crystallization front. The purified silicon crystallizes on the crystalline plate when the opposite surface of said plate is cooled according to a program, which allows to achieve a steady overcooling of the main surface of the plate (the crystallization front) by not less than 5 °C. The melt is additionally stirred at a rate that ensures the formation of the marginal diffusion layer in a crystallization front. Solar grade mono-crystalline silicon is produced from crystalline silicon by the Czochralski process using standard technology.

The disadvantages of this method are:

1. Only metallurgical silicon is purified.

2. Single technological process includes crystallization of the purified silicon and purification process of the metallurgical grade silicon. .

3. Metallurgical silicon is continuously loaded into a melt of a fusible metal, thus forming on its surface a layer consisting of a solid-phase crystallized particles of the metallurgical silicon and slag, which prevents the removal of volatile impurities from the melted metallurgical silicon contained in a melt of a fusible metal.

4. Choice of a fusible metal from the group of Ga, Sn, In, Al, Pb is insufficient, because the potentialities of other fusible metals and their alloys are overlooked.

5. The melt is stirred only by rotating the crucible about its central axis.

6. Slag is not removed.

7. For purification and crystallization processes of a silicon, single fusible metal is used.

8. Constant overcooling by 5 °C and more promotes the formation of tabular crystals near the crystallization front of an ingot.

9. The technological process of obtaining silicon ends when a plate with the grown ingot of the crystalline silicon is removed, i.e. the process is cyclical.

DETAILED DESCRIPTION OF THE INVENTION

The technological features of the proposed purification method of the metallurgical grade silicon ensures that the metallurgical grade silicon will be cleared from volatile impurities and impurities composed of high-pressure gases, which forms electrically active recombinant centres (Al, B, P, C, Cr, Fe, Mn, Ni, Ti, V), of the level from 0.1 to 10 ppm, and that purification will be realized in a continuous production process. Said method ensures production of the solar grade silicon which is used in manufacturing of solar cells.

Technological features of said method are realized in the following stages:

Stage 1. Initial removal of impurities from the metallurgical grade silicon comprises melting of silicon inside a melt of a fusible metal and obtaining tabular crystals through the process of mass crystallization.

Stage 2. Final removal of impurities from the metallurgical grade silicon comprises dissolving the tabular silicon crystals (obtained in the first step) in a melt of a fusible metal Stage 3. Removal of atoms of the volatile metals from silicon: comprises growing the mono-crystals through the directional crystallization of a silicon obtained in the second step.

Method of purification of metallurgical silicon, which contains both metallurgical silicon and sludge, where during the initial treatment phase the metallurgical grade silicon is periodically introduced as loads into the surface of a melt of a fusible metal selected from gallium, tin, indium, lead, aluminium, bismuth, zinc and the group of their alloys, where loads are lowered to the bottom of the crucible preventing them from floating up. Melt is intensively stirred under vacuum by an impulse scavenging using gas, composed mainly of inert gases, the resulting slag is removed. Further on, the mass of the melted silicon in the layer of a melt of a fusible metal is transported by convection and silicon is crystalized in the shape of tabular silicon crystals. Crystals are removed. In the final purification step loads of tabular silicon crystals are periodically added into the surface of a melt of a fusible metal, followed by an intensive stirring. The silicon-saturated melt is transported from the melting zone to the crystallization zone wherein silicon is grown in the shape of a block on a silicon wafer, where the surface of said silicon plate is overcooled in the temperature range of 0.5 to 3 °C. Silicon-free melt is returned back to the melting zone where slags are removed and subsequently silicon ingots are extracted.. Continuous process is based on the periodical removal of tabular crystals of a silicon, replacement of silicon wafers with formed ingots on their surfaces and replacement of crucibles containing a melt of repeated use. Removal of atoms used fusible metals is achieved by application of direct crystallization method, when mono-crystalline silicon is grown from ingots of silicon that were obtained during the finishing stage of the removal of impurities.

From the known technological solution the proposed method differs in the following key features:

1. The proposed method can be used not only to purify the metallurgical silicon, but also to remove the slag obtained after separation of impurities which are generated during the cutting of silicon ingots into plates. This is possible because the purification processes of the metallurgical silicon in a melt of a fusible metal by the described method is split into two stages of removal of impurities: the first is initial cleaning and the second, is final cleaning. The initial treatment is used to remove volatile impurities during the treatment of metallurgical silicon in order to transfer the elemental silicon from a slag into a melt of a fusible metal, later on removing the slags containing silicon carbide, silicon dioxide and other impurities. Final cleaning can reduce the amount of impurities in obtained silicon ingot by up to 1-10 ppm. 2. Unlike the prototype, where two stages are required to remove the impurities from metallurgical silicon and to grow the purified mono-crystalline silicon, in the proposed method said two processes are conducted in three stages: during the first two stages impurities are removed from metallurgical silicon, while during the third one the atoms of used fusible metals are removed from silicon crystals. Purification process of metallurgical silicon was divided into two phases due to the fact that it is impossible to combine effective removal of volatile impurities and crystallization of treated silicon crystals in a single technological process due to the difference between the essential optimal temperatures of said processes. Evaporation temperatures of volatile impurities (Table 1) has to be as high as possible in order to: maximize the pressure of these impurities, to ensure the lowest possible temperature for crystallisation of the silicon crystal for the silicon crystals to be able to aggregate less atoms of impurities. Impurities entering with the cleansing agent into the melt are there in low concentrations and far from saturation, thus do not undergo the process of crystallization in the melt. The first stage is completed during the process of mass crystallization, that is during the growth of the tabular silicon crystals, which is conditioned by the convectional transfer of the silicon to the surface of a melt due to the essential difference between densities of a silicon and a fusible metal. The second stage is completed with the formation of a silicon ingot on the cooled silicon wafer.

3. In the proposed method, unlike the prototype, where the metallurgical silicon is continuously fed into a melt of a fusible metal, silicon is periodically loaded into the melt thus preventing a layer composed of solid crystallized metallurgical silicon and particles of a slug to form on the surface of a melt allowing the removal of volatile impurities from a melt of a fusible metal. By periodically loading loads of tabular silicon crystals (formed in the first treatment stage) into the melting zone and transferring silicon-enriched melt into the crystallization zone, subsequently transferring it back after the silicon is removed, it is possible to control the transfer of mass of molten silicon to the plate thus optimizing the ingot growth on the wafer.

4. Unlike the prototype, where the fusible metal is selected from the group of Ga, Sn, In, Al, Pb, in the presented embodiment of invention said group of used fusible metals is supplemented with Bi and Zn. By using alloys of fusible metals it is possible to control the change of solubility of a silicon at a given temperature of the cleaning process. Additionally, when Bi and Zn are used as melts they allow formation of tabular silicon crystals with small amounts of atoms of said metals.

5. Unlike the prototype, where a melt is stirred by rotating the crucible about its vertical axis, in the presented method the crucible and the stirrer (or silicon plate) are turned in the opposite directions at the time of intensive stirring of a melt, as well as scavenging the melt by a pulses of an inert gas or a mixture of gases composed mainly of inert gases, thus making it possible to reduce the concentration of volatile impurities in a melt and to create a variety of compounds with impurities, which are removed from the melt in the form of a slag.

6. Unlike the prototype, where there is no removal of slags, in the proposed method slags from a surface of a melt are removed by a vacuum suction, which allows using the fusible metal many times.

7. Unlike the prototype, wherein a single fusible metal is used in both cleaning and silicon crystallization processes, in the proposed method use of several different fusible metals in said processes are suggested. Use of several fusible metals are more favourable because in the first stage it is necessary to use a fusible metal of low volatility under high temperatures (Tl < 1500 K) during the process of purification, in order to effectively remove volatile impurities. In the second stage, during the crystallization of a silicon, it is necessary to maintain low temperatures during the growing process, which is possible when using a more volatile fusible metals compared to the first stage.

8. Unlike the prototype, where constant overcooling of the surface of the plate is no less than 5 °C, in the proposed method constant overcooling takes place on the front surface of a plate in a temperature range of 0.5 to 3 °C which helps silicon to crystallize on the plate and prevents formation of tabular silicon crystals near the plate. As only the plate is cooled the crystallization takes place only on its surface and not in the melt around the plate, where the temperature is much higher than of the plate. Due to the centrifugal forces arising in the flow of a melt when the crucible is turned, light fractions of a melt with enriched silicon are formed in the centre of a crucible at the position of the plate, which increases the efficiency of the growth of silicon ingots. The increase of crystallization rate of an ingot, compared to the prototype, is also conditioned by the turbulent motion in the marginal layer occurring duo to the rotation of a plate and crucible in opposite directions.

9. Unlike the prototype, where technological production process of a silicon is completed by removing the plate with a formed ingot of a crystalline silicon, the purification process of a technical grade silicon in the proposed method does not stop at the said point. The processes in the first and the second stages, comprising the removing of the tabular silicon crystals and ingots, periodic replacement of silicon wafers and crucibles containing reusable melts, are continuous. Table 1. Evaporation temperatures (K) of the volatile impurities, at the vapour pressure of2-10 mmHg

DESCRIPTION OF IMPLEMENTATION OF THE INVENTION

Example.

Production of solar grade silicon with p-type conductivity.

Metallurgical silicon is cleaned in three stages. In the first stage silicon, consisting of 98.5 % of mass of Si and tin of SN-0000 brand is used. In the second stage cleaned tabular silicon crystals and gallium GA-0000 are used. The third stage comprises mainly the cleaning of silicon from atoms of melts of fusible metals which were used in the first and second stages. Quartz crucible (180 mm in diameter and 250 mm high) is filled at a room temperature with 25 kg of tin. After the pressure in a chamber is decreased to 10^"2 mmHg the crucible is heated up to 1150 °C. During the heating process of a quartz crucible, a load of 1kg of metallurgical silicon is fed through the lock chamber into the tin alloy by lowering the load on a grating to the bottom of a crucible. Pulse-scavenging by argon gases takes place for several minutes during the melting process of the metallurgical silicon At the same time as the gas is supplied through a tube in the grates the crucible is rotated wherein an intensive stirring of the melt takes place. Slag which has formed on the surface of tin is removed from the crucible by vacuum suction. Dissolved elemental silicon by convective mass transfer in a melt of a tin floats up through the openings of a grille and crystallizes on the surface of a melt in the form of tabular silicon crystals, where large silicon particles are sorted out by the openings of a grille. Tabular silicon crystals of linear size of 2-5 mm obtained from a single load, are separated and extracted from the melt by lifting up the grille. In the crucible in a single melt of a tin, silicon is cleaned by periodically lowering 40-50 loads into the melt, afterwards the crucible is replaced by a new one containing fresh SN-0000 brand tin.; in this manner the technological process of removal of impurities from silicon is made continuous. The extracted tabular silicon crystals are stored for the treatment in the second stage.

In the second stage at room temperature a crucible made of quartz (diameter - 180 mm, height - 250 mm), is filled with 25 kg of gallium. Device for lowering of loads of tabular silicon is introduced into the crucible. During the process of heating of said crucible up to 1000 °C load of 100 g of tabular crystalline silicon is placed into the loading device and when lowered into a crucible is melted down in the melt of gallium which is intensively stirred. The stirring takes place in the medium of purified argon. After the gallium melt is saturated with silicon it is transferred to the crystallization zone, where at the same time the loads and a silicon plate (the front surface of which was constantly overcooled at temperature range of 0.5 to 3 °C) are lowered into the melt. Silicon is grown on a plate for 10 minutes in the stream of a continuously stirred melt which is flowing around the rotating silicon wafer. The melt, from which silicon is removed by crystallization of silicon on a plate, is moved into the loading device, removing the accumulated slags. Within one hour of periodicall lowering of 100 g loads an ingot of silicon of 200 g mass is formed, within 12 hours 4 kg of silicon are extracted from the crucible and a new plate is introduced, on which a new ingot is grown. 4-5 kg of purified silicon are obtained using a single gallium melt and 20-25 plates. After said process a crucible is replaced by a new one and is filled with gallium of GQ-0000 grade, and the process is repeated. Technological process of purification of silicon is made continuous by periodically changing the loads, plates and crucibles. Main impurities in silicon ingots were gallium of concentration of 3 x 10¹⁹ cm³. Such crystals did not show any forms of a shell neither remains of a melt and contents of residual impurities were less than 1-10 ppm. Tabular silicon crystals are removed from the replaced crucibles, wherein said tabular crystals are formed during the cooling of a melt, said tabular crystals are separated from the melt by treatment with acid and subsequently are stored together with tabular silicon crystals obtained in the first cleaning stage.

The third stage of purification of the metallurgical silicon is carried out by recrystallizing the silicon ingots which were received in the second phase by method of directional crystallization. This is done in order to remove atoms of tin and gallium and to reduce the residual concentration of impurities (Table 2). Said recrystallization is done by growing a volumetric mono-crystalline silicon ingot from the melt of previously obtained ingots. After treatment of metallurgical silicon in the third stage silicone is of p-type conductivity is obtained, its comparative resistance is up to 10 Ω/cm, the life-time of electrons er 100 ms. Such silicon is suitable for manufacturing of high-efficiency solar cells.

Table 2. Concentration of residual impurities.

Industrial Applicability

Parameters of equipment and materials used for cleaning of metalurgical grade silicon

Technical

Property description U.m.

specifications

Diameter of crucible mm 406 (300)

Mass of a load of a fusible metal kg 200 (150)

Load mass of metalurgical grade silicon kg 10 (8)

Container weight with receptacles kg no more than 20

Stroke of the mechanism of a motion of a holder mm 500

Movement speed of a holder mm/min 300

Temperature of purification process of silicon °C 1000

Capacity of the device kg/day 100

Pressure in the chamber of silicon purification mmHg 2 l0-³

Consumption of inert gas 1/h 600÷1000

Consumption of cooled water m³/h. 4

Pressure of cooled water MPa 0,3

Consumed power kW 80

Mains voltage V 380

Mains frequency Hz 50

Claims

1. Silicon purification method, involving removal of impurities from silicon by melting it in a crucible containing forcibly stirred melt of a fusible metal , selection of fusible metal from the group consisting of gallium, tin, indium, lead, aluminium, and convective mass transfer in a melt of dissolved silicon to the crystallization front and its crystallization on a plate which has an overcooled front surface, extraction of crystalline silicon from a melt and removal of the atoms of a fusible metal by using the directional crystallization method, growing a mono-crystalline silicon, characterised in that impurities from a silicon are removed in three stages:

- during the initial removal of impurities silicon in the form of loads is periodically fed into a surface of a melt of one of the fusible metals, silicon load is lowered down to the bottom of a crucible and is kept from floating up to the surface, forced stirring of a melt is carried out in a vacuum, the stirred melt is additionally treated with scavenging by a mixture of gases comprised mainly of inert gases, the resulting slag from a surface of a melt is removed by vacuum suction, silicon melted during the process of convective mass transfer and subsequently crystallized in the form of tabular crystals is removed from the surface of a melt;

- during the second stage of removal of impurities, load of tabular silicon crystals which was obtained in the initial stage of removal of impurities is fed into a melt of different fusible metal which is intensely stirred, silicon-saturated melt is transferred from the melting area to the crystallization area where silicon ingot is grown on a silicon wafer with a constantly overcooled frontal surface, slag is removed and silicon ingot is extracted together with a base- wafer;

- in the third stage atoms of the used fusible metals are removed from the silicon ingots obtained in the second stage by directional crystallization method.

2. The method of silicon purification according to Claim 1, characterised in that bismuth, zinc, as well as alloys of gallium, tin, indium, lead, aluminium, bismuth and zinc are also used in the process of cleaning of silicon.

3. The method of silicon purification according to Claims 1 or 2, characterised in that the said front surface of a silicon wafer is constantly overcooled in a temperature range of 0.5 to 3 °C.

4. The method of silicon purification according to any of the preceding Claims, characterised in that the melt of a fusible metal from which the silicon was removed, is returned to the melting area of a silicon, crucibles with reusable melts of fusible metals are periodically changed.

5. The method of silicon purification according to any of the preceding Claims, characterised in that in the second stage of removal of impurities the melt of a fusible metal is stirred by turning the crucible and the silicon wafer in opposite directions.