JP7730768B2 - Manufacturing method and evaluation method for single crystal silicon - Google Patents

Description

The present invention relates to a method for producing and evaluating single crystal silicon.

Polycrystalline silicon is the raw material for single-crystal silicon used in semiconductors and silicon for solar cells. The Siemens process is a well-known method for producing polycrystalline silicon. The Siemens process generally involves bringing silane source gas into contact with a heated silicon core wire, and then depositing polycrystalline silicon on the surface of the silicon core wire using CVD (Chemical Vapor Deposition).

In the Siemens method, silicon core wires are assembled into a torii-gate shape, with two vertical and one horizontal core wire, and both ends of the torii-gate-shaped silicon core wires are connected to core wire holders and fixed to a pair of metal electrodes placed on a bottom plate. Generally, multiple sets of torii-gate-shaped silicon core wires are arranged inside a reactor.

A torii-shaped silicon core wire is heated to the deposition temperature by passing electricity through it, and a raw material gas, such as a mixture of trichlorosilane and hydrogen, is brought into contact with the silicon core wire, causing silicon to grow in the vapor phase, and polycrystalline silicon rods of the desired diameter are formed in an inverted U shape.

As mentioned above, polycrystalline silicon produced by the Siemens process is used as a raw material for single-crystal silicon for semiconductors or silicon for solar cells. These require high purity with low impurity concentrations. Therefore, it is also important to evaluate the quality of the polycrystalline silicon produced.

In this context, methods for evaluating the quality of polycrystalline silicon have been standardized in various standards, including JIS, JEITA, and ASTM.

For example, Section 3.2 of JIS H 0615 (Non-Patent Document 1) describes FZ samples, stating that a seed crystal with a (111) crystal orientation is used to create a dislocation-free crystal for FZ dislocation-free single crystal production.

Seed crystals are usually cut to a certain size from a single crystal block so that they can be easily grown into single crystals, and then chemically treated before use.

In addition, in both the FZ and CZ methods, a single-crystal silicon seed crystal with a crystal orientation is used, and after contact with the silicon melt, a process called drawing is performed to reduce the diameter, forming a drawn portion and then gradually widening the diameter, known as the Dash necking method.

With this method, the minimum diameter of the necked portion needs to be narrowed to 2-3 mm, and because the seed crystal is pulled upward in the CZ method, the necked portion cannot withstand the increased weight of a large-diameter crystal. Therefore, Patent Documents 1 and 2 propose making the tip of the seed crystal pointed or cut off, in order to produce a single crystal without necking.

JIS H 0615 JP 10-203898 WO2003/091483A1

However, when chemically treating seed crystals, reacted gases can adhere to the surface of the seed crystal, causing a phenomenon known as acid burn.

Sometimes a certain number of seed crystals are chemically treated together, which further increases the risk of acid burns.

When using a seed crystal with a poor surface and squeezing it to produce a single crystal, crystal habit lines may not form, and squeezing may have to be repeated, resulting in extremely poor work efficiency.

Repeated squeezing can cause the molten silicon to overflow and come into contact with mechanical parts (metal), which can cause impurities to turn into vapor and become absorbed into the molten silicon.

Impurities introduced into molten silicon affect the evaluation value of polycrystalline silicon, so the sample processing described in JIS H 0615 must be repeated.

Seed crystals are cut to a certain size from a block of single crystals, but since they cannot be obtained from low-quality single crystals or crystals that could not be made into single crystals, part of the normal product is cut into a block to obtain the seed crystals, which increases costs and reduces yields.

In Patent Documents 1 and 2, to achieve the objectives, a cylindrical or prismatic single-crystal silicon seed crystal must be processed so that the end shape is sharp or has a sharp tip that has been cut off.

In recent years, requirements for impurity concentrations have become stricter. As a result, more evaluations than before are required to determine the optimal growth conditions for single crystals. This means that the conventional method of using a seed crystal only once incurs significant costs.

The present invention was made in consideration of these problems, and provides a method for producing and evaluating single crystal silicon that can reduce the cost required for seed crystals.

The method for producing single crystal silicon according to the present invention includes the steps of:
producing first single crystal silicon by a FZ method using a seed crystal;
separating the seed crystal after producing the first single crystal silicon;
producing second single crystal silicon by a FZ method using the seed crystal;
may also be provided.

The method for producing single crystal silicon according to the present invention includes the steps of:
After producing the first single crystal silicon, removing the first single crystal silicon from the FZ apparatus;
The seed crystal may be separated from the produced first single crystal silicon by crushing an end of the removed first single crystal silicon.

In the method for producing single crystal silicon according to the present invention,
The seed crystal may have a (111) orientation.

In the method for producing single crystal silicon according to the present invention,
The seed crystal may be separated by cutting it at a sloped portion where the diameter of the seed crystal is continuously reduced.

The method for producing single crystal silicon according to the present invention may also be a method for producing single crystal silicon for evaluation.

The evaluation items for the evaluation single crystal silicon may be one or more of resistivity, carbon concentration, and impurity measurement using a photoluminescence method.

The method for producing single crystal silicon according to the present invention may also include a step of evaluating contamination of the separated seed crystal.

The present invention can reduce the costs required for seed crystals. In particular, when single-crystal silicon is used as the silicon for evaluation, the quality of the raw material, polycrystalline silicon, can be evaluated by evaluating the single-crystal silicon produced, thereby reducing the costs required for evaluating the quality of polycrystalline silicon.

1 is a schematic side view showing an FZ single crystal manufacturing apparatus used in an embodiment of the present invention. 3A and 3B are schematic side views showing examples of cutting positions of a seed crystal and a necked portion in an embodiment of the present invention. 1 is a schematic side view showing a process of producing a first single crystal silicon rod from a first polycrystalline silicon rod using an FZ single crystal manufacturing apparatus according to an embodiment of the present invention. FIG. 4 is a schematic side view showing a state that has progressed from FIG. 3 in the process of producing first single-crystal silicon rods from first polycrystalline silicon rods; FIG. 5 is a schematic side view showing a state that has progressed from FIG. 4 in the process of producing first single-crystal silicon rods from first polycrystalline silicon rods. FIG. 5 is a schematic side view showing a process of producing second single crystal silicon rods from second polycrystalline silicon rods using an FZ single crystal manufacturing apparatus in an embodiment of the present invention, and corresponds to the state of FIG. 4. FIG. 2 is a flow chart showing an example of a process for reusing a seed crystal in an embodiment of the present invention.

In this embodiment, a single crystal silicon rod 20 is used as the single crystal silicon, and a polycrystalline silicon rod 10 is used as the polycrystalline silicon. In this embodiment, a seed crystal 5 is repeatedly used when a cylindrical core extracted from a means for evaluating the quality of high-purity polycrystalline silicon produced by the CVD method is converted into a single crystal by the FZ method.

In recent years, requirements for impurity concentrations have become stricter. As a result, it is necessary to conduct more evaluations than before to determine the preferred growth conditions for single crystal silicon. As a result, the conventional method of using a seed crystal 5 only once incurs significant costs. In this regard, using a seed crystal 5 repeatedly, as in the present embodiment, is advantageous in that it reduces the costs required for the seed crystal 5.

In other words, by repeatedly using seed crystals 5 as in this embodiment, it is possible to effectively evaluate the quality of multiple lots of polycrystalline silicon, and it is also possible to provide a method for evaluating polycrystalline silicon that allows for easy quality evaluation in terms of quality, cost, and work efficiency.

As shown in FIG. 1, the FZ single crystal manufacturing apparatus 100 used in this embodiment includes, for example, a chamber 90 and an upper shaft 71 and a lower shaft 76 that are provided within the chamber 90 and are movable up and down and rotatable.

An upper holding jig 70 is attached to the upper shaft 71, and the upper holding jig 70 holds the polycrystalline silicon rod 10. A seed crystal 5 is attached to a lower holding jig 75 attached to the lower shaft 76. The seed crystal 5 passes through a narrowed portion 4, and the polycrystalline silicon rod 10 is welded to it. The seed crystal 5 has a large diameter portion 1, a small diameter portion 3, and a sloped portion 2 located between the large diameter portion 1 and the small diameter portion 3, where the diameter continuously decreases. The narrowed portion 4 is composed of the small diameter portion 3 and the sloped portion 2.

An induction heating coil 60 connected to a high-frequency oscillator 61 is installed within the chamber 90. The induction heating coil 60 heats and melts the polycrystalline silicon rods 10, producing single-crystal silicon. A floating zone 30 is formed between the polycrystalline silicon rods 10 and the single-crystal silicon rods 20. The upper shaft 71 and the lower shaft 76 can be moved vertically by a moving means. By moving the upper shaft 71 and the lower shaft 76, the floating zone 30 is moved to above the polycrystalline silicon rods 10, producing single-crystal silicon rods 20 from the polycrystalline silicon rods 10 (see Figures 3 to 5).

As mentioned above, in this embodiment, the FZ method is performed by repeatedly using the same seed crystal 5. It is beneficial to use a seed crystal 5 with a (111) orientation to crystallize the sample into a single crystal.

Next, we will explain an example of a method for performing the FZ method using the same seed crystal 5 repeatedly according to this embodiment.

First, the seed crystal 5 is attached to the FZ single crystal manufacturing apparatus 100 (see S1 in FIG. 7). The upper end of the seed crystal 5 is welded to the lower end of the first polycrystalline silicon rod 10a (see S2 in FIG. 7). While the first polycrystalline silicon rod 10a is moved downward, the first polycrystalline silicon rod 10a is melted in the floating zone 30 by the induction heating coil 60, producing single crystal silicon. The floating zone 30 is then moved to above the first polycrystalline silicon rod 10a, ultimately producing the first single crystal silicon rod 20a (see S3 in FIGS. 3 to 5 and 7).

After the first single crystal silicon rods 20a are produced, the first single crystal silicon rods 20a are removed from the FZ single crystal manufacturing apparatus 100 (see S4 in Figure 7).

The seed crystal 5 is separated from the first single crystal silicon rod 20a by splitting (crushing) the lower end of the removed first single crystal silicon rod 20a in a direction perpendicular to the extension direction of the first single crystal silicon rod 20a (left-right direction in Figure 5) (see S5 in Figure 7). In this case, the seed crystal 5 may be separated by cutting at the part of the drawn portion 4 where the diameter continuously tapers upward (inclined portion 2) (see Figure 2). Cutting at the drawn portion 4 with a (111) orientation is preferable because it allows the cut surface to be horizontal. The diameter of the large diameter portion 1 may be approximately 7 to 8 mm, and the diameter of the small diameter portion 3 may be approximately 1 to 3 mm.

It is desirable to cut the drawn portion 4 at a distance of at least 15 mm from the point where the seed crystal 5 is welded to the first single crystal silicon rod 20a. Dislocation density is very high near the welded portion, but drawing removes the dislocations. At a distance of at least 15 mm from the welded portion, dislocations are significantly reduced, and even with the dislocations that occur during re-welding, single crystallization can be easily achieved by adding a small amount of drawing.

As shown in Figure 2, by cutting the necked portion 4 at the inclined portion 2, where the diameter continuously tapers upward, the contact area with the molten silicon is extremely small. This significantly reduces the occurrence of slip dislocations caused by sudden temperature changes within the crystal when the seed crystal 5 comes into contact with the melt, compared to a normal seed crystal 5, and provides the same effect as forming a pointed tip or a truncated pointed end in a rectangular or cubic seed crystal. While adding a step to form a pointed tip or a truncated pointed end in a rectangular or cubic seed crystal is highly disadvantageous in terms of increased costs and reduced work efficiency, leaving the necked portion 4 at the cutting stage, as in this embodiment, is advantageous in these respects as well.

In some cases, to maintain the cleanliness of the cut seed crystal 5, it can be ultrasonically cleaned for 2 to 5 minutes in an ultrasonic cleaner using ultrapure water (see S6 in Figure 7), dried on a clean bench (see S7 in Figure 7), and then packaged and stored (see S8 in Figure 7).

The seed crystal 5 separated in this manner is attached to the FZ single crystal manufacturing apparatus 100. The lower end of the second polycrystalline silicon rod 10b is then welded to the seed crystal 5. Second single crystal silicon rods 20b are then produced from the second polycrystalline silicon rods 10b by the FZ method (see Figure 6).

Third single crystal silicon rods, fourth single crystal silicon rods, ..., nth single crystal silicon rods are then produced in the same manner from third polycrystalline silicon rods, fourth polycrystalline silicon rods, ..., nth polycrystalline silicon rods (where "n" is an integer equal to or greater than three).

The single crystal silicon rods 20 produced in the manner described above may be single crystal silicon rods 20 for evaluation. The evaluation items for the single crystal silicon rods 20 for evaluation may be one or more of resistivity, carbon concentration, and impurity amount measurement using a photoluminescence method.

The evaluation single crystal silicon rods 20 may have a small diameter, for example, a diameter of 10 to 50 mm. By using a small diameter, the amount of material required for evaluation can be reduced, thereby reducing evaluation costs. Furthermore, while such evaluation single crystal silicon rods 20 do not generate revenue such as sales profits and are an expense, in recent years, requirements regarding impurity concentrations have become stricter, and they are mass-produced under different conditions. For this reason, repeatedly using a seed crystal 5 in evaluation single crystal silicon rods 20, as in this embodiment, is extremely beneficial in terms of cost.

The photoluminescence method is generally used to evaluate donors and acceptors such as P, As, B, and Al. In this case, it is necessary to measure single-crystal silicon, so it is beneficial to prepare a small single-crystal silicon rod 20 for evaluation, as in this embodiment.

When evaluating C, it is common to use Fourier transform infrared spectroscopy to measure Cs, which represents substitutional carbon. In this case, it is also necessary to measure single crystal silicon, so it is beneficial to prepare a small single crystal silicon rod 20 for evaluation, as in this embodiment.

When measuring resistivity, the four-probe method may be used. Since single-crystal silicon must also be measured when measuring resistivity using the four-probe method, it is beneficial to prepare a small single-crystal silicon rod 20 for evaluation, as in this embodiment.

By performing FZ evaluation on multiple polycrystalline silicon samples produced in the same reactor using the CVD method, using the same seed crystal 5, it is possible to reduce variations in quality and make it easier to determine the quality of each. Furthermore, by using the seed crystal 5 after forming it into a single crystal at least once using the FZ single crystal production apparatus 100, the reused seed crystal 5 can have an orientation such as (111), which is beneficial in that it is less likely to cause disturbances in single crystal growth. Furthermore, while newly prepared seed crystals 5 are sometimes subjected to etching before use, which can cause contamination, this embodiment is also beneficial in that the seed crystal 5 can be used without such etching.

Contamination limits may be set for each evaluation item. Contamination evaluation may also be performed on the seed crystal 5, and if the seed crystal 5 is contaminated above a predetermined level, the seed crystal 5 may be replaced with a new one. Evaluating the contamination of the seed crystal 5 is beneficial in that it prevents contamination from the seed crystal 5 or the produced polycrystalline silicon from affecting the subsequently produced single crystal silicon.

To evaluate contamination of the seed crystal 5, one or more of resistivity, carbon concentration, and impurity amount measurement using photoluminescence method may be used.

The number of times the seed crystal 5 is reused may be predetermined, or it may be reused when contamination above a predetermined threshold is found based on the results of evaluation of the seed crystal 5.

The present invention makes it possible to reduce costs and improve work efficiency when evaluating the quality of polycrystalline silicon.

1 seed crystal 2 necked portion 10 polycrystalline silicon rod 10a first polycrystalline silicon rod 10b second polycrystalline silicon rod 20 single crystal silicon rod 20a first single crystal silicon rod 20b second single crystal silicon rod 100 FZ single crystal manufacturing apparatus

Claims (7)

Producing first single crystal silicon rods by an FZ method using seed crystals;
After producing the first single crystal silicon rod, removing the first single crystal silicon rod from the FZ single crystal manufacturing apparatus;
Separating the seed crystal from the first single crystal silicon rod removed from the FZ single crystal manufacturing apparatus ;
a step of attaching the separated seed crystal to an FZ single crystal manufacturing apparatus;
Producing a second single crystal silicon rod by an FZ method using a seed crystal attached to an FZ single crystal manufacturing apparatus ;
A method for producing a single crystal silicon rod, comprising: The method for producing a single crystal silicon rod according to claim 1, wherein the seed crystal is separated from the first single crystal silicon rod produced by crushing the end of the removed first single crystal silicon rod. The method for producing a single crystal silicon rod according to claim 1 or 2 , wherein the orientation of the seed crystal attached to the FZ single crystal production apparatus is (111). The seed crystal is separated from the first single crystal silicon rod removed from the FZ single crystal manufacturing apparatus by cutting the seed crystal at an inclined portion where the diameter is continuously narrowed. The method for manufacturing a single crystal silicon rod according to any one of claims 1 to 3 . The method for producing a single crystal silicon rod according to any one of claims 1 to 4, which is a method for producing a single crystal silicon rod for evaluation . The evaluation method according to claim 5, wherein the evaluation items for the evaluation single crystal silicon rod are one or more of resistivity, carbon concentration, and impurity amount measurement by a photoluminescence method. The method for producing a single crystal silicon rod according to any one of claims 1 to 5, further comprising a step of evaluating a seed crystal separated from the first single crystal silicon rod removed from the FZ single crystal production apparatus .