FORM 2
THE PATENTS ACT 1970
(39 of 1970)
&
The Patents Rules  2003
COMPLETE SPECIFICATION
(See section 10 and rule 13)

1. "" PROCESS FOR PRODUCING OF MULTICRYSTALLINE SILICON INGOTS BY INDUCTION METHOD ""

2.

1. (A) PILLAR Ltd.
(B) Ukraine
(C) 1  Magnitogorska Str.  office 404 Kyiv  02660 (Ukraine)
2. (A) TESYS LIMITED
(B) Ukraine
(C) 3  Pivnichno-Syretska.  Kyiv  04136 (Ukraine)
3. (A) SILICIO SOLAR  S.A.U.
(B) Spain
(C) Poligono Industrial "LA NAVA I" Avenida Roma  1 
E-13500 Puertollano (Ciudad Real) (SPAIN)

The following specification particularly describes the invention and the manner in which it is to be performed.

Technical field

The present invention relates to the production of polycrystalline silicon  particularly to the production of multicrystalline silicon by the induction method  and can be used in manufacturing solar cells from multicrystalline silicon.

Crystal silicon is used for producing solar cells to convert solar energy into electrical energy. Much attention has been recently given to the production of polycrystalline silicon formed by large crystals  typically referred to as multi-crystal silicon  which provides for the efficiency of converting solar energy into electric energy close to that of single-crystal silicon.

Background Art

Processes for the production of polycrystalline silicon ingots are disclosed in the following documents: US Pat. No. 4 572 812 (Int. Cl. B29D 7/02  B22D 27/02 [I])  EP Pat. No. 1254861 (publ. 06.11.2002  (Int. Cl. COlB 33/02 [2])  EP Pat. No. 1754806 (publ. 21.02.2007  (Int. Cl. C30B 11/00 [3]) and consist in charging a silicon raw material into the melting chamber of a cooled crucible enveloped by an inductor  forming a melt surface  melting and pulling the multicrystalline silicon ingot. None of the processes  however  describes melting conditions and ingot pulling conditions  which provide for sustained conditions for melt crystallization.

A process for the production of multicrystalline silicon ingots by the induction method  the process bearing closely on the invention  comprises charging a silicon raw material into the melting chamber of a cooled crucible enveloped by an inductor  forming a melt surface  melting while monitoring the output parameters of the inductor feed  and pulling the multicrystalline silicon ingot under controlled cooling conditions (EP Pat. No. 1930483  Int. Cl. C30B 35/00  C30B 29/06  COlB 33/02  publ. 22.02.2007  [4]). In the prior art process  melting is controlled by monitoring the output power of the inductor feed  wherein the measured frequency of an inverter is compared with the preset frequency thereof  and the output power of the heating means feed is simultaneously monitored  wherein the measured temperature

on the ingot surface is compared with the preset temperature on the ingot surface.

Under such conditions  however  the crystallization of silicon in the ingot is unstable  because a constant changing of the output power of the inductor feed in the prior art process leads to a constant change in the rate of ingot crystallization to thereby unfavorably affect its quality.

Also  according to the prior art process  an increase in the depth of the melt requires a decrease in the output power of the inductor feed. In case of an increase in the depth of the melt by raising the melt surface  the operating frequency is increased and the output power of the inductor feed is decreased. On the one hand  these dependencies result in an increase in the rate of melt crystallization and  on the other  in a decrease in the rate of melting the charged raw material  and it can result in a complete filling up of the melt surface with the raw material and its sticking to the walls of the cooled crucible. In consequence  the pulling of the ingot will be forced to stop to melt down the raw material bridging the crucible  the regular melting process disrupted  the rate of melting slowed  and the production efficiency reduced.

The present invention is aimed at an improvement in the process for the production of multicrystalline silicon ingots by the induction method  wherein silicon crystallization would become stable  ingot quality higher  and production efficiency increased due to suggested process steps.

Summary of the invention

To the accomplishment of the foregoing objective  there is provided a process for the production of multicrystalline silicon ingots by the induction method  the process comprising charging a silicon raw material into the melting chamber of a cooled crucible enveloped by an inductor  forming a melt surface  melting while monitoring the output parameters of the inductor feed  and pulling the multicrystalline silicon ingot under controlled cooling conditions  wherein  in the course of melting  the mass rate of charging the silicon raw material and the speed of pulling the ingot are set such that provide for the melt surface position below the upper plane of the inductor but not lower than 1/3 of the height thereof and the melt surface is kept at the same level. In doing this the melt surface position is kept at the same level by maintaining one of the output parameters of the inductor feed within a predetermined range  notably  operating frequency  voltage  current.

In casting multicrystalline silicon ingots by the induction method  it was experimentally established that with the melt surface position below the upper plane of the inductor but not lower than 1/3 of the height thereof  the maximum rate of melting was achieved  and with the melt surface position maintained at this level by setting the mass rate of charging the silicon raw material  the speed of pulling the ingot  and the output parameters of the inductor feed  such as operating frequency  voltage or current of the inductor  silicon stably crystallized.

Best Mode for Carrying Out the Invention

To melt the raw material  heat is consumed as the enthalpy of the raw material and of the melting heat that is absorbed at the interface of solid and liquid phases. Since heating mainly involves the liquid phase  i.e.  silicon melt  the electromagnetic energy release is limited where heat is absorbed at such position of the inductor with respect to the melt surface. As a result  the rate of melting is increased due to the melt being mixed and due to overheated melt flow coming from the zone of induced currents to the zone of melting the silicon raw material  and melting is stable and fast to provide further stability to silicon crystallization. Grain cross-sectional sizes of thus produced ingots meet the specifications of solar-cell producers for wafer grain sizes  and the ingots thus produced are suitable for the manufacture of solar cells. Also  ingot production efficiency is enhanced  specific energy consumption is decreased.

The invention operates as follows.

In a chamber  under a controlled argon atmosphere  a movable bottom is moved to delimit a melting chamber and a silicon raw material is charged into the melting chamber. A high-frequency electromagnetic field is created by an inductor that envelopes a cooled crucible. A start-up heating device is inserted into the melting chamber that is inside the high-frequency electromagnetic field created by the inductor. The start-up heating device gets heated up  and the silicon raw material warms up and melts under the influence of radiated heat from the start-up heating device and the electromagnetic field created by the inductor. The start-up heating device is removed from the electromagnetic field  while in the melting chamber  a melt pool is produced in the form of the cross section of the melting chamber. As a result of the heat transfer along the periphery of the melt pool  the melt is crystallized and a skull is formed to prevent the pool from spilling from the melting chamber. After the melt pool is formed  the silicon raw material is continuously supplied onto the surface of the melt. In the course of melting  the mass rate of charging the silicon raw material and the speed of pulling the ingot are set such that provide for the melt surface position below the upper plane of the inductor but not lower than 1/3 of the height thereof and the melt surface is kept at the same level  for example  by maintaining operating frequency  voltage or current of the inductor within a predetermined range  or otherwise.

The invention is further described by way of examples.

Example 1

Multicrystalline silicon ingots were obtained by the induction melting technique using an apparatus with a melting chamber of a square cross-section and a side length of 350 mm. In a chamber  under an argon atmosphere  a movable bottom is moved to delimit a melting chamber of a cooled crucible enveloped by an inductor 120 mm high. A lump silicon raw material is charged into the melting chamber. A high-frequency electromagnetic field is created. A start-up heating device is inserted into the melting chamber  the lump silicon raw material is warmed up and melted  the start-up heating device is removed from the electromagnetic field  and a melt pool is produced in the form of the cross section of the melting chamber. The melt is crystallized and a skull is formed along the periphery of the melt pool. The silicon raw material of a particle size in the range of 15-20 mm is continuously supplied onto the surface of the melt. The output power of the inductor feed is set at 300 kW  the mass rate of charging the silicon raw material was set at about 0.4 kg per minute  the speed of pulling the ingot was set at 1.5 mm per minute  and the melt surface position was set at 25 mm below the upper plane of the inductor. The operating frequency of the inductor feed was 16.7 kHz. In the course of melting  the melt surface was kept at the same level with the operating frequency of the inductor feed maintained in the range of 16.7 ± 0.05 kHz. The frequency was maintained within the range by way of adjusting the mass rate of charging the silicon raw material  the speed of pulling the ingot being constant. In the course of melting  the mass rate of charging the silicon raw material was adjusted within the range of 0.40-0.45 kg per minute depending on accidental variations in the raw material variables  particularly particle sizes and feeder accuracy as well. To relieve thermal stresses as the ingots grow  it is annealed in an annealing chamber and cooled under controlled conditions. Due to a constant output power of the inductor feed and to a constant speed of pulling the ingot  the crystallization front becomes stable at a single level. As a result  optimal conditions of crystal growth in a multicrystalline silicon ingot are created. Also  the position of the melt surface 25 mm lower than the upper plane of the inductor allows the maximum speed of pulling the ingot produced from the raw material of the given particle size. This is achieved by the electromagnetic coupling of the inductor with the melt surface zone. Following annealing and controlled cooling  the multicrystalline silicon ingot is taken out from the annealing chamber and cut into blocks from which wafers are subsequently cut for use in the production of solar cells.

The efficiency of the process for the production of multicrystalline silicon ingots is 25.7 kg per hour. Grain cross-sectional sizes of thus produced ingots meet the specifications of solar-cell producers for wafer grain sizes.

Example 2

Multicrystalline silicon ingots were obtained by the induction melting technique in a way similar to that described in Example 1. The output power of the inductor feed and the particle size of the silicon raw material were similar to those of Example 1. The mass rate of charging the silicon raw material was set at 0.3 kg per minute  the speed of pulling the ingot was set at 1.2 mm per minute  and the melt surface position was set at 5 mm below the upper plane of the inductor. The operating frequency of the inductor feed was 16.9 kHz. In the course of melting  the melt surface was kept at the same level with the operating frequency of the inductor feed maintained in the range of 16.9 ± 0.05 kHz. The frequency was maintained within the range by way of adjusting the mass rate of charging the silicon raw material  the speed of pulling the ingot being constant. In the course of melting  the mass rate of charging the silicon raw material was adjusted within the range of 0.32-0.37 kg per minute depending on accidental variations in the raw material variables  particularly particle sizes and feeder accuracy as well.

The efficiency of the process for the production of multicrystalline silicon ingots is 20.6 kg per hour. Grain cross-sectional sizes of thus produced ingots meet the specifications of solar-cell producers for wafer grain sizes.

Example 3

Multicrystalline silicon ingots were obtained by the induction melting technique in a way similar to that described in Example 1. The output power of the inductor feed and the particle size of the silicon raw material were similar to those of Example 1. The mass rate of charging the silicon raw material was set at 0.4 kg per minute  the speed of pulling the ingot was set at 1.3 mm per minute  and the melt surface position was set at 10 mm below the upper plane of the inductor. The operating current of the inductor feed was 4650 A. In the course of melting  the melt surface was kept at the same level with the current of the inductor feed maintained in the range of 4650 ± 5 A. The current was maintained within the range by way of adjusting the mass rate of charging the silicon raw material  the speed of pulling the ingot being constant. In the course of melting  the mass rate of charging the silicon raw material was adjusted within the range of 0.35-0.40 kg per minute depending on accidental variations in the raw material variables  particularly particle sizes and feeder accuracy as well.

The efficiency of the process for the production of multicrystalline silicon ingots is 22.3 kg per hour. Grain cross-sectional sizes of thus produced ingots meet the specifications of solar-cell producers for wafer grain sizes.

The proposed invention ensures an increased output of multicrystalline silicon and casting of multicrystalline silicon ingots of higher quality  which are suitable for solar cell fabrication.
We claim:-

1. A process for the production of multicrystalline silicon ingots by the induction method  the process comprising charging a silicon raw material into the melting chamber of a cooled crucible enveloped by an inductor  forming a melt surface  melting while monitoring the output parameters of the inductor feed  and pulling the multicrystalline silicon ingot under controlled cooling conditions characterized in that  in the course of melting  the mass rate of charging the silicon raw material and the speed of pulling the ingot are set such that provide for the melt surface position below the upper plane of the inductor but not lower than 1/3 of the height thereof and the melt surface is kept at the same level.

2. The process according to Claim 1  characterized in that the melt surface position is kept at the same level by maintaining one of the output parameters of the inductor feed within a predetermined range  notably  operating frequency  voltage  current.

Dated this 17th day of February 2012