TWI452182B - Method of casting ingot - Google Patents

Description

Method of casting crystal enamel

The present invention relates to the casting of Ingot, and more particularly to a method of seeding a wafer using a plurality of seed crystals.

According to solar energy, solar energy is the cleanest and inexhaustible energy source. In order to meet the increasing demand for solar cell use and reduce the production cost of solar cells, most of the wafers used in the production of solar cells are obtained from mass production. A crystal ray produced by a directional solidification method. The directional solidification method places a large-area single crystal seed crystal having a large end face (the current common size is about 156 × 156 mm ² ) at the bottom of a crucible, and then the crucible raw material is placed on the single crystal seed crystal. Under the premise that the single crystal seed crystal is not completely melted, the crucible raw material is heated to melt the crucible raw material. Then, the temperature gradient at the time of cooling is controlled, so that the molten tantalum raw material starts to solidify and crystallize from the position of the single crystal seed crystal, and the solidified tantalum raw material and the single crystal seed crystal together form a crystal. Thereafter, the wafer is sliced to obtain a wafer for use in solar cells.

The portion of the above-mentioned solidified cerium raw material which is closer to the single crystal seed crystal has a mono-like structure, and the farther away from the single crystal seed crystal, the structure tends to be polycrystalline. In other words, the crystal at the bottom of the wafer has fewer defects, and conversely, the number of defects closer to the top of the wafer is greatly increased. The greater the number of defects, the lower the conversion efficiency of the fabricated solar cells. In addition, in the case where the number of defects at the bottom and top of the wafer is much different, it will result in the bottom of the wafer and the top wafer. The conversion efficiencies of the fabricated solar cells vary too much. Therefore, the method of directional solidification using large-area single crystal seed crystal growth has yet to be improved.

In view of this, the main object of the present invention is to provide a method for casting a crystal crucible, which can reduce the disadvantage that the number of defects from the bottom to the top of the wafer is excessively different, and effectively reduce the number of defects on the top of the wafer.

In order to achieve the above object, the present invention provides a method for casting a wafer, comprising the steps of: providing a plurality of small single crystal seeds, and closely aligning the single crystal seeds in the same crystal orientation after alignment Arranging in a bottom portion of a crucible to form a seed layer; each of the single crystal seed crystals has an end face area of between 25 and 900 mm ² , and the height of each of the single crystal seed crystals is between 5 and 40 mm Putting a solid raw material into the crucible, stacking it on the seed layer; heating the crucible to melt the crucible material and the top of the seed layer into a liquid state; and cooling the crucible from bottom to top The solidification crystallization is carried out from the top of the seed layer until all the bismuth raw materials are solidified and crystallized; after solidification, the seed layer and the bismuth raw material together form a crystal enthalpy.

Thereby, the method of casting the wafer can effectively reduce the defects of the polycrystalline portion of the wafer, thereby improving the conversion efficiency of the polycrystalline wafer into a solar cell.

In order that the present invention may be more clearly described, the preferred embodiments are illustrated in the accompanying drawings.

1 shows a crystal growth apparatus for use in a method of casting a wafer according to a preferred embodiment of the present invention. The crystal growth apparatus includes a crucible 10, an insulating cage 20 and a plurality of heaters 30. The crucible 10 is provided for placing a plurality of small single crystal seeds 40 and a solid crucible material 50. The insulating cage 20 is disposed on the periphery of the crucible 10 for maintaining the temperature of the crucible 10, and the insulating cage 20 is movable upward or downward relative to the crucible 10, thereby controlling the change of the temperature gradient of the crucible 10. The heaters 30 are disposed on the inner wall of the insulating cage 20 for heating the crucible 10.

Next, the method for casting a crystal wafer of the present invention is carried out by using the above-mentioned crystal growth apparatus (refer to FIG. 2), which comprises the steps of providing the small single crystal seed crystals 40, and the single crystal seed crystals 40 have The end faces of the same crystal orientation face upward so that they are closely arranged in the same crystal orientation after alignment and fill the bottom of the crucible 10 to form a seed layer 42. The end surface area of each of the single crystal seed crystals 40 is between 25 and 900 mm ² , and the height of each of the single crystal seed crystals 40 is between 5 and 40 mm.

In the present embodiment, the crystal orientation of the single crystal seed crystal 40 used is (110), but not limited thereto, and a single crystal seed crystal 40 having a crystal orientation of (100) may be used. In order to be closely arranged, the shape of the single crystal seed crystal 40 is preferably a rectangular cylinder, but not limited thereto, and may be a triangular cylinder, a hexagonal cylinder, a cylinder or the foregoing. A column of various shapes is used in combination, and it is important to closely arrange each of the single crystal seeds 40. The purpose of the close alignment is to prevent the solid material 50 from falling into the gap between the adjacent single crystal seeds 40, resulting in the formation of a polycrystalline structure at the slit in the subsequent growth step after the subsequent heating step is melted. In addition, the larger the gap of the single crystal seed crystals 40, the more the defects of the produced crystal grains will be relatively increased.

The end face area of each of the single crystal seed crystals 40 is too small or too low, which means that the single crystal seed crystal 40 used is small in size, and in the subsequent processing steps, the single crystal seed crystal 40 is easily completely prevented due to improper temperature control. Melting, it is impossible to perform seeding; if the size of the single crystal seed 40 is too large, the cost required is relatively high. The preferred end face area of each of the single crystal seeds 40 is between 25 and 100 mm ² , and the preferred height is between 30 and 40 mm, taking into account the trade-off between ease of processing and cost.

After the single crystal seed crystals 40 are arranged to form the seed layer 42, the tantalum raw material 50 is placed in the crucible 10 and stacked on the seed layer 42.

A heating step is performed to heat the crucible 10 to melt the crucible material 50 and the top of the seed layer 42 into a liquid state.

In this embodiment, after the insulating cage 20 completely seals the crucible 10, the heaters 30 are controlled to be heated, and the heating time is controlled so that the crucible raw material 50 in the crucible 10 starts to melt until all the crucible raw materials 50 are obtained. Melting into a liquid state, and the top of the seed layer 42 begins to melt, and the seed layer 42 is maintained at a height of at least 5 mm without being melted to prevent the single crystal seed crystal 40 from completely melting. If the height of the unmelted seed layer 42 is less than 5 mm, it is easy to control the temperature improperly. It is completely melted and cannot be seeded.

The crystal growth step is carried out by cooling the crucible 10 from the bottom to the top to solidify crystals from the top of the seed layer 42 until all of the crucible material 50 has solidified and crystallized.

In this embodiment, after the insulating cage 20 is opened, the internal heat is dissipated from the opened gap. The opening degree of the insulating cage 20 is controlled and gradually raised upward at a predetermined speed to lower the temperature of the seed layer 42 at the bottom of the crucible 10 below the melting point temperature. The portion of the crucible 10 covered by the insulating cage 20 is maintained at a temperature above the melting point of the crucible material 50, and the crucible material 50 is in a liquid state; the temperature of the crucible 10 not covered by the insulating cage 20 is lowered, so that the crucible is lowered. The temperature of the raw material 50 drops below the melting point temperature to solidify and crystallize. As the insulating cage 20 gradually rises, the solid/liquid interface of the tantalum raw material 50 in the crucible 10 is gradually increased upward until the crucible raw material 50 in the crucible 10 is completely solidified and crystallized. Thereby, after the solidification, the seed layer 42 and the tantalum raw material 50 together form a crystal.

The crystal enamel prepared by the above method for casting crystal ruthenium has a single crystal-like structure at the bottom, that is, the area of the single crystal is large, and as the height is increased, the area of the single crystal is reduced, and the crystal yttrium is gradually transformed from the single crystal. It is a polycrystalline structure.

Figure 3 is a comparison of the defect area (curve 1) of the crystal crucible produced by the method for casting a crystal crucible according to the present invention and the defect area (curve 2) of the crystal crucible produced by the conventional large-area single crystal seed crystal growth method. . As is apparent from Fig. 3, the crystal grains produced by the large-area single crystal seed crystal growth method have relatively few defects at the bottom, but as the crystal height increases to about 80 mm or more, Area of single crystal The defect of the wafer produced by the seed crystal growth method is relatively fast. In contrast, the crystal ytterbium produced by the method of casting a crystal enamel according to the present invention has a relatively slow rate of increase in defects as the height of the crystal raft increases. Therefore, in the crystal crucible produced by the method for casting a crystal crucible according to the present invention, the difference in the number of bottom-to-top defects is significantly reduced, and the defect area at the top of the crystal crucible is larger than that of the large-area single crystal seed crystal growth method. The defect area of tantalum is reduced by about 15%, effectively reducing the number of defects in the polycrystalline portion of the top of the wafer.

Figure 4 shows the wafers cut at the bottom, middle and top of the wafers produced by the method of casting wafers according to the present invention, using a scanning electron microscope (SEM) and an electron backscattering diffractometer ( Electron Backscatter Diffraction, EBSD) analysis results. The crystal orientation of the single crystal at the bottom of the wafer is (101), which is the equivalent crystal orientation of (110). As the height of the wafer increases, the polycrystalline grains in the middle to the top gradually increase. Due to the phenomenon of grain competition, the (112) crystal grains having less defects become the dominant crystal orientation in the polycrystal. Effectively reduce defects in the polycrystalline portion. After the wafer is sliced, a high-quality polycrystalline wafer can be obtained, thereby improving the conversion efficiency of the polycrystalline wafer into a solar cell.

It should be noted that the focus of the present invention is a method of casting a wafer, and the application of the crystal growth apparatus is only an application. Further, the above description is only a preferred embodiment of the present invention, and variations of the equivalent methods of the present invention and the scope of the patent application are intended to be included in the scope of the present invention.

10‧‧‧坩埚

20‧‧‧Insulation cage

30‧‧‧heater

40‧‧‧Single crystal seeds

42‧‧‧ seed layer

50‧‧‧矽 Raw materials

1 is a flow chart of a preferred embodiment of the present invention; FIG. 2 is a flow chart of a method for casting a wafer according to a preferred embodiment of the present invention; FIG. 3 is a relationship between a defect area and a crystal height of a preferred embodiment of the present invention; Figure 4 and Figure 4 are SEM and EBSD analysis views of a wafer according to a preferred embodiment of the present invention.

Claims (6)

A method for casting a wafer, comprising the steps of: providing a plurality of single crystal seeds of the same crystal orientation, and closely arranging the single crystal seeds in the bottom of a crucible in a manner of having the same crystal orientation after alignment a single seed layer to form a single crystal orientation; the end surface area of each of the single crystal seed crystals is between 25 and 900 mm ² , and the height of each of the single crystal seed crystals is between 5 and 40 mm; Putting the raw material into the crucible, stacking it on the seed layer; heating the crucible to melt the crucible material and the top of the seed layer into a liquid state; and cooling the crucible from bottom to top, from the crystal The top of the seed layer solidifies and crystallizes until all the cerium raw materials are solidified and crystallized; thereby, the crystal seed layer and the cerium raw material together form a crystal enthalpy after solidification. A method of casting a wafer according to claim 1, wherein the crystal orientation of the single crystal seeds is (110). A method of casting a wafer according to claim 1, wherein each of the single crystal seeds preferably has an end face area of 25 to 100 mm ² . A method of casting a wafer according to claim 1, wherein each of the single crystal seeds preferably has a height of 30 to 40 mm. A method of casting a wafer according to claim 1, wherein each of the single crystal seeds has a rectangular cylinder shape, a triangular cylinder shape, a hexagonal cylinder shape or a cylinder shape. A method of casting a wafer as claimed in claim 1, wherein the seed layer is maintained at a height of at least 5 mm when the top of the seed layer is melted.