CN102732948A - Method for improving ingot-casting monocrystaline silicon yield - Google Patents

Abstract

The invention relates to a method for increasing the yield of ingot single crystal silicon, which is realized by controlling the diffusion of impurities at the bottom of the crucible to the seed crystal. The present invention is based on preventing impurities from the bottom of the crucible from diffusing into the seed crystal, and a barrier layer is arranged at the bottom of the seed crystal; the barrier layer includes powder, film, block or gap partition. Compared with the ingot single crystal without a diffusion barrier layer, the minority carrier lifetime at the bottom of the ingot single crystal ingot prepared by the present invention has been greatly improved. According to the same cut-off standard, the ingot yield has increased by more than 3 percentage points ( absolute difference).

Description

A method for increasing the yield of cast monocrystalline silicon

technical field

The invention relates to the field of photovoltaic cells, in particular to a method for setting an impurity diffusion barrier layer at the bottom of an ingot crucible to block the diffusion of impurities into an ingot, thereby increasing the yield of ingot single crystal silicon.

Background technique

Inducing the growth of ingot monocrystalline silicon through seed crystals with a specific crystal orientation is a technology that has been widely used in the field of photovoltaic silicon cells in recent years. Related patents that have been applied for include:

(1) The patent US2007/0169685(A1) mentions the use of <100> monocrystalline silicon as a seed crystal to induce the growth of ingot monocrystalline silicon. It is believed that this process can significantly reduce or eliminate the defect density, and the crystal grains are at least larger than 10cm of crystal ingots.

(2) In the patent CN201010284738.7, the inventor proposes to place a quartz plate with a hole in the center at the bottom of the crucible. The quartz plate has a trapezoidal structure, which can save the amount of seed crystal. The grown out is <100> oriented single crystal.

(3) In the patent CN 201010299013.5, the inventor proposed to use <111> single crystal silicon as the seed crystal to induce the generation of ingot single crystal silicon with <111> crystal orientation. Since <111> is the preferred growth direction, a high proportion can be obtained ingot single crystal.

During the growth process of the ingot single crystal, the seed crystal is partially melted, and induced growth occurs on the solid-liquid interface between the remaining seed crystal and the melted silicon liquid, and the crystal orientation of the grown quasi-single crystal is consistent with that of the seed crystal. The unmelted part of the seed crystal has experienced four stages of silicon material heating, melting, crystal growth and annealing, and the thermal history is much longer than that of the bottom silicon crystal of ordinary ingot polycrystalline. The experimental results show that the yield (percentage) of ingot monocrystalline silicon is 5-8 percentage points lower than that of ordinary ingot polycrystalline silicon, which greatly increases the production cost of ingot monocrystalline silicon. The low minority carrier lifetime at the bottom of the ingot is the main reason for the low yield of cast single crystals. For example, if the minority carrier lifetime is 1 microsecond as the truncation standard, the truncation of ingot single crystal reaches 50~60mm, which is much higher than the 15~25mm of ordinary ingot polycrystalline. The above-mentioned truncated part is called the red zone in the industry, that is, the part where the life expectancy of the minority birth does not meet the requirements.

After analysis, the reason for the high red area of the ingot single crystal is mainly from the impurity diffusion at the bottom of the crucible. The purity of the quartz crucible for casting ingots is much lower than that of the silicon material. The impurities in the crucible will diffuse into the silicon crystal, which will greatly reduce the minority carrier life as a minority carrier recombination center. The total amount of impurities diffused into the silicon crystal is closely related to the temperature and diffusion time. , the higher the temperature and the longer the time, the greater the total amount of impurities diffused into, the more low minority carrier lifetime regions are formed, and the lower the yield of the crystal ingot. The remaining seed crystals of cast single crystals have a longer thermal history, so the total amount of impurities diffused into them is larger, and they are often cut off as low minority carrier lifetime regions.

Therefore, controlling the diffusion of crucible impurities into the seed crystal is of great significance to improving the minority carrier lifetime at the bottom of the ingot single crystal ingot, reducing the cut-off length of the red zone at the bottom, and increasing the yield of the ingot.

Contents of the invention

The technical problem to be solved by the present invention is to propose a method capable of controlling the diffusion of impurities in the crucible into the seed crystal, increasing the minority carrier lifetime at the bottom of the ingot single crystal ingot, reducing the truncated length of the red zone at the bottom, and increasing the yield of the crystal ingot.

The technical solution adopted in the present invention is: a method for improving the yield of cast monocrystalline silicon, comprising the following steps:

1) An impurity diffusion barrier is set at the bottom of the ingot crucible to prevent the diffusion of impurities into the ingot;

2) Arrange the monocrystalline silicon blocks with a thickness of 5-50 mm closely, and use them as seed crystals to cover the bottom of the crucible;

3) Then place the silicon material and the doping element raw material in the crucible in turn;

4) Vacuum is drawn and inert gas N ₂ or Ar is introduced, and inert gas is introduced while vacuuming;

5) Heating the crucible so that the part of the single crystal silicon block as the seed crystal in contact with the bottom of the crucible does not melt, and the part not in contact with the crucible, the silicon material and the doping element raw material are all melted and fully mixed with each other on the atomic scale;

6) During directional solidification, the bottom of the crucible is used as the cold end, and the unmelted part of the seed crystal induces the solidification and growth of the silicon melt to obtain ingot single crystal silicon with a specific crystal orientation.

Specifically, the impurity diffusion barrier layer in step 1) of the present invention is silicon nitride film, silicon nitride powder, silicon nitride ball, quartz plate, porous silicon, silicon powder, silicon wafer, overhead or other The form of the diffusion barrier function. The impurity diffusion barrier layer adopts thin film or silicon wafer with a thickness of 0.01-5mm; adopts plate material with a thickness of 5-20mm; adopts powder with a loose thickness of 0.5-20mm; adopts overhead, and the height of the overhead is 1 ~20mm. The above-mentioned overhead is achieved by supporting the seed crystal with a support.

The ingot single crystal mentioned in the present invention refers to the single crystal produced in the process of directional solidification by adopting the method of single crystal seed crystal induction. The crucibles used include but are not limited to commonly used quartz crucibles. The single-crystal seed crystal in the present invention refers to a crystal orientation of <100>, <110>, <111>, <112> or a hybrid splicing of the above crystal orientation seed crystals.

The beneficial effects of the present invention are: based on the barrier to the diffusion of impurities at the bottom of the crucible to the inside of the seed crystal, a barrier layer is provided at the bottom of the seed crystal; The minority carrier life at the bottom of the crystal ingot has been greatly improved, and the yield of the crystal ingot has increased by more than 3 percentage points (absolute difference) according to the same cut-off standard.

Detailed ways

Example 1

Silicon nitride spheres with a diameter of 6 mm were densely packed on the bottom of the crucible to form a layer, and then <111> seed crystal silicon blocks with a size of 330×150 mm were densely laid on the bottom of the quartz crucible, and the thickness of the seed silicon blocks was 40 mm. 210kg of silicon material is placed on the seed silicon block, mixed with 40 mg of dopant boron, and 210kg of silicon material is added to realize furnace loading. The furnace is evacuated, and Ar is used as the protective gas. The thermal field is designed so that the temperature of the silicon material in the furnace reaches above 1420°C and gradually melts, so that the <111> single crystal silicon in the furnace in contact with the silicon material is melted by 20 mm, and the 20 mm in contact with the quartz crucible remains unmelted. Finally, the insulation cover is opened to gradually solidify the silicon melt from the bottom of the crucible, and the directional growth of the silicon crystal is achieved by the induction of the <111> crystal to the seed crystal, and an ingot single crystal is obtained.

Compared with the ingot single crystal without silicon nitride balls placed on the bottom of the crucible, if the low minority carrier lifetime area at the bottom is cut off at 0.6 microseconds, the bottom cutoff in this embodiment is reduced from the original 45 mm to 30 mm, and the ingot is closed. rate increased from 61% to 65%.

Example 2

Take 16 quartz bars with a thickness of 5 mm, a width of 10 mm, and a length of 400 mm, and lay them evenly on the bottom of a crucible with a size of 800×800×480 mm, and then place <100> seeds with a size of 400×400 mm The crystalline silicon blocks are closely arranged on the quartz plate, and the thickness of the seed silicon block is 30 mm. 210kg of silicon material is placed on the seed silicon block, mixed with 40 mg of dopant boron, and 210kg of silicon material is added to realize furnace loading. The furnace is evacuated, and Ar is used as the protective gas. Design the thermal field so that the temperature of the silicon material in the furnace reaches above 1420°C and gradually melt, so that the <100> single crystal silicon in the furnace in contact with the silicon material is melted by 20 mm, and the 10 mm in contact with the quartz crucible remains unmelted. Finally, the insulation cover is opened to gradually solidify the silicon melt from the bottom of the crucible, and the directional growth of the silicon crystal is achieved by the induction of the <100> crystal to the seed crystal, and an ingot single crystal is obtained.

Compared with the ingot single crystal with no quartz strip placed on the bottom of the crucible, if the low minority carrier lifetime area at the bottom is cut off at 0.6 microseconds, the bottom cutoff in this embodiment is reduced from the original 40 mm to 20 mm, and the ingot yield is reduced from 62% increased to 66%.

Example 3

Lay high-purity silicon powder with a particle size of 0.05 mm on the bottom of the crucible. The loose thickness of the silicon powder is 3 mm. The crystalline silicon block is 30 mm thick. 210kg of silicon material is placed on the seed silicon block, mixed with 40 mg of dopant boron, and 210kg of silicon material is added to realize furnace loading. The furnace is evacuated, and Ar is used as the protective gas. The thermal field is designed so that the temperature of the silicon material in the furnace reaches above 1420°C and gradually melts, so that the <110> single crystal silicon in the furnace in contact with the silicon material melts 5 mm, and the 20 mm in contact with the silicon powder remains unmelted. Finally, the insulation cover is opened to gradually solidify the silicon melt from the bottom of the crucible, and the directional growth of the silicon crystal is achieved by the induction of the <110> crystal to the seed crystal, and an ingot single crystal is obtained.

Compared with the ingot single crystal without high-purity silicon powder placed on the bottom of the crucible, if the low minority carrier lifetime area at the bottom is cut off at 0.6 microseconds, the cut-off amount at the bottom of this embodiment is reduced from the original 45 mm to 25 mm, and the ingot is closed. The rate increased from 61% to 65.5%.

What is described in the above description is only the specific implementation of the present invention, and various illustrations do not limit the essence of the present invention. Those of ordinary skill in the art can modify or modify the previous specific implementation after reading the description. variations without departing from the spirit and scope of the invention.

Claims (4)

1. A method for improving the yield of cast monocrystalline silicon, characterized in that it may further comprise the steps: 1) An impurity diffusion barrier is set at the bottom of the ingot crucible to prevent the diffusion of impurities into the ingot; 2) Arrange the monocrystalline silicon blocks with a thickness of 5-50 mm closely, and use them as seed crystals to cover the bottom of the crucible; 3) Then place the silicon material and the doping element raw material in the crucible in turn; 4) Vacuum is drawn and inert gas N ₂ or Ar is introduced, and inert gas is introduced while vacuuming; 5) Heating the crucible so that the part of the single crystal silicon block as the seed crystal in contact with the bottom of the crucible does not melt, and the part not in contact with the crucible, the silicon material and the doping element raw material are all melted and fully mixed with each other on the atomic scale; 6) During directional solidification, the bottom of the crucible is used as the cold end, and the unmelted part of the seed crystal induces the solidification and growth of the silicon melt to obtain ingot single crystal silicon with a specific crystal orientation. 2. A method for increasing the yield of ingot single crystal silicon according to claim 1, characterized in that: the impurity diffusion barrier layer in the step 1) is silicon nitride film, silicon nitride powder, nitride Silicon spheres, quartz plates, porous silicon, silicon powder, silicon wafers, overhead or other forms with impurity diffusion barrier function. 3. A method for increasing the yield of cast monocrystalline silicon as claimed in claim 2, characterized in that: the impurity diffusion barrier layer is made of thin film or silicon wafer with a thickness of 0.01-5 mm; a plate with a thickness of 5~20mm; if powder is used, the loose thickness is 0.5~20mm; if overhead is used, the height of the overhead is 1~20mm. 4. A method for increasing the yield of cast single crystal silicon as claimed in claim 3, characterized in that: said overhead is achieved by supporting the seed crystal with a support.