JPH0224797B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a crucible used when producing a single crystal by pulling molten silicon. (Conventional technology and problems) Single crystal pulling methods for mass production of silicon wafers can be broadly divided into FZ (floating zone)
There are two methods: the CZ method and the CZ (Cyochralski) method. The CZ method is
A single crystal is pulled from molten silicon in a quartz glass crucible while being solidified. At this time, quartz glass (SiO ₂ ) melts into the molten silicon,
As a result, oxygen is included in the single crystal.
(ref, W. Zulehner and D. Huber; "Crystals"
Vol.8 “Czockralski−Grown Silicon”
p.1 Springer-Verlag (1982)). This oxygen is not an impurity that should be avoided, but plays an important role, such as increasing the heat resistance of the wafer. Therefore, when manufacturing single crystals using the CZ method, it is widely necessary to control so-called crystal pulling conditions such as the crucible rotation speed, crystal rotation speed, and atmosphere so that the required concentration of oxygen is introduced into the silicon wafer. (W. Zulehner et al., supra). On the other hand, the first characteristic required of a silica glass crucible is heat resistance, since its operating temperature reaches approximately 1500°C. In quartz glass for the semiconductor industry, the heat resistance can be approximated by a one-to-one correspondence with the OH group concentration in the glass, so creating a crucible with a low OH group content has been a challenge for crucible manufacturers. Such a crucible with a low OH group content has high viscosity, good shape stability, and slow diffusion of impurities, so it is excellent in that the molten silicon inside the crucible is kept clean, but on the other hand, it is not optimal. Even if the operating conditions are set, high-precision control is required to maintain them, and as the operating time increases, small pieces of quartz will suddenly peel off from the inner surface of the crucible, and this will reach the growth interface of the single crystal and cause the crystal to grow. Because it disturbs
The disadvantage was that repeated use (so-called multiplexing) was extremely difficult. On the other hand,
If a conventional transparent quartz glass crucible with a high OH group content is used, it is difficult to obtain a high-quality single crystal due to the diffusion and permeation of impurities from the heater and graphite susceptor. The reality was that it was impossible and could not be used repeatedly. Multi-pulling is predicted to be a technology that not only saves money on crucibles through repeated use, but also has dramatic effects in improving the uniformity of single crystal quality and increasing operating rates. However, there was no crucible that could do this, so the creation of one had been awaited. (Objective and Structure of the Invention) The present inventors have conducted various studies to solve the conventional problems when using a crucible for pulling silicon single crystals for multiple pulling. However, if you use a crucible with a lower outside surface and a distribution of OH group concentration, the quartz glass on the inner surface of the crucible will change in quality and the quartzized (crystallized) layer will react moderately with molten silicon after long-term use. However, the surface of the crucible is always kept clean, and the fact that impurities from the heater and graphite susceptor that promote crystallization are less likely to diffuse and permeate also contributes to keeping the inner surface of the crucible clean.
The combined effect is that the quartz particles no longer peel off from the inner surface of the crucible and disturb the single crystal.
As a result, we came up with the idea that the number of times of multiplexing can be dramatically increased. Since the rate of deterioration of the inner surface depends on the rate of penetration of impurities, it is necessary to adapt the elution rate so that the deterioration layer is sequentially eluted. The present inventors newly discovered that optimization of this adaptation can be achieved by controlling the OH group concentration distribution, and completed the present invention after repeated experiments regarding the concentration and layer thickness. That is, the present invention provides a high OH concentration region with an OH group content of 60 ppm or more and 195 ppm or less, which is in contact with the inner surface and has a thickness of 0.2 mm or more, and a region with a thickness of 0.5 mm that is not in contact with the inner surface.
This relates to a crucible for pulling silicon single crystals having a structure in which the above portion has a low OH concentration region in which the OH group content is 20 ppm or more lower than the average concentration in the inner surface layer with a thickness of 0.3 mm. As described above, the crucible of the present invention makes it easy to appropriately control the reaction between the molten silicon in the crucible and the crucible wall (SiO ₂ ) by providing a region with a high concentration of OH groups inside the crucible. At the same time, it has excellent shape stability and the ability to prevent impurities from diffusing and penetrating at the operating temperature of around 1480°C, making it possible to maintain a clean crucible inner surface at all times. Therefore, in the present invention, 0.2
OH group content is 60ppm or more in the range of mm or more
The high concentration region is 195 ppm or less, preferably 80 to 170 ppm, and the optimal thickness of the region is 0.6 to 1.5 mm. Such conditions mean that the OH group distribution curve O shown in FIG. 1 does not pass through part A, and preferably does not pass through parts B and C either. This will be specifically explained with reference to FIG. 1 as follows. The distribution curve O ₁ is for a normal low OH group concentration crucible, and is for an electric melting crucible called a so-called translucent crucible. In this case, the crucible passes through region A and the inner surface also has a low concentration of OH groups, so it is different from the crucible of the present invention. The distribution curve O ₂ is
This is for a transparent natural silica glass crucible.
Although the OH group concentration on the inner surface is within the above-mentioned optimum range, it is low enough to satisfy another condition of the present invention described later.
Since it does not have an OH region, it is outside the scope of the present invention. Distribution curve O ₃ is an example of the invention and is the most preferred crucible. The distribution curve O ₄ is similar to the optimal example of O ₃ , but since the thickness of the inner surface layer with an OH group content of 60 ppm or more is 0.2 mm or less, the effect of the present invention is not sufficient and is not satisfactory. isn't it. At the same time as the above conditions, in the present invention, the inner surface layer is 0.3 mm
It is necessary that a layer containing less OH groups by 20 ppm or more than the average OH group concentration exists outside the high concentration region, and this region may be in contact with the outer surface of the crucible. As shown in Figure 2, the average OH group concentration C ₁ in the inner surface layer 0.3 mm and the OH group concentration C ₂ in the layer 20 ppm lower than this.
Let the distances from the inner surface to points A and B where and intersect with the OH distribution curve O be X _A and X _B, respectively. In the crucible of the present invention, the second condition is that X _B −X _A ≧0.5 (mm). In this case, there is no intersection B,
If all points outside A are lower than C ₂ , point B can be considered to be on the outer surface. Note that C ₂
It is desirable that _C1 be at least 40 ppm smaller. X _B −
It is desirable that X _A is 1.5 mm or more. A configuration that satisfies the above two conditions occupies 1/4 or more of the inner surface of the entire crucible or 1/3 or more of the inner surface that comes into contact with molten silicon during silicon melting (usually 75% of the inner surface comes into contact with molten silicon) ) is also a necessary condition for the present invention. Methods for providing a region with a high concentration of OH groups inside the crucible include: (i) welding a quartz glass plate 2 containing a large amount of OH groups to the inside of a quartz glass crucible 1 molded in a rotary mold (inner lining); (Fig. 3); ), (ii) When manufacturing the crucible 1 by molding powder in a rotary mold 3 and melting the powder layer with an electric arc, quartz glass powder 4 with many OH groups is molded inside and double powder is formed. Body layer (double powder molding) (4th
(Fig.), (iii) When electrically melting the powder layer in a rotary mold, there are methods such as maintaining the molten state for a long time in a moisture-containing atmosphere (normal atmosphere). Method (ii) is not practical because the manufacturing process is complicated and quartz glass powder is more expensive than quartz powder. The present inventors conducted an experiment to change the OH group content of the inner layer by changing the conventional melting method using rotary molding, and found that method (iii) results in unexpected melting in normal atmosphere for a long time. In particular, it was found that the OH group content in the inner layer increased. If the melting time is extended using the conventional method, the crucible will become too thick, but by changing the cooling conditions, the thickness can be controlled to a predetermined value. The crucible of the present invention obtained by the above method has an OH group content of 30 ppm in conventional crucibles, but in the optimal example, the dough has a content of about 60 ppm, and the inner surface has an OH group content of about 60 ppm.
It is 110ppm, and the following effects can be cited when comparing the two. (Effects of the Invention) (1) Conventionally, SiO ₂ (glass) was difficult to dissolve in molten silicon, and therefore operating conditions were set such that relatively fast convection occurred. When the crucible of the present invention is used, SiO ₂ (glass) is easily melted, so the gentle movement of the molten metal allows the desired oxygen concentration to be achieved, and the altered layer, that is, the crystallized layer, that forms on the inner wall of the crucible as the usage time increases is eliminated. Moderate elution is carried out to offset the growth, and there is little intrusion of alkaline elements that enter from the outside of the crucible and promote crystallization on the inner surface, so crystallization on the inner surface exceeds elution and leads to the formation of quartz particles. There's nothing to put away. As a result, multiplexing can be performed stably without causing unexpected crystal disturbances. Since alkali atoms diffuse while substituting H atoms in OH groups, it is thought that the dough layer with a low OH group content selectively blocks the diffusion of alkali elements. The most harmful elements in terms of promoting internal crystallization are alkali elements, so
It is reasonable to use the OH group concentration layer as the diffusion prevention layer in light of the purpose of the present invention. Even if the effects of impurities were reduced as much as possible in this way, crystallization at the interface between molten silicon and silica crucible could not be completely eliminated;
By adjusting the OH group concentration, the crystals could be effectively eluted. (2) As a result, the crucible can be multiplied only 6 to 8 times using the reduced pressure CZ method, whereas in the past, the limit was 2 to 3 times, and this results in a significant increase in the operating rate. Ta. At the same time, quality uniformity improved. Next, a crucible according to the present invention and a crucible according to a conventional method were prototyped, and the OH group concentration distribution in the direction perpendicular to the inner surface was compared. Figure 5 and Table 1 show the results, and the crucibles were 25 cm in diameter and 20 cm in height.
cm. 1 is a crucible made by the conventional method of molding quartz powder in a rotary mold and heating it from the inside with an arc, and 1 is a crucible according to the present invention. That is, these are based on the methods of (ii) double powder molding, (iii) long-term melting, and (i) lining, respectively. In the conventional crucible, the OH group is distributed almost uniformly at 30 to 40 ppm throughout the crucible fabric and inner layer, but the distribution in ~ is markedly high concentration reaching 190 ppm on the inner surface. .
In this case, the average value of the 0.3 mm inner surface layer was 110 ppm. Although the lining method requires a lot of man-hours and is not suitable for mass production, it is a good sample to know what percentage of the inner surface needs to have an OH group distribution structure, so in addition to the sample with a lining ratio of 30%, A 15% sample was prototyped. Furthermore, we added a sample of double powder molding with an inner layer of synthetic quartz glass and a sample of a completely transparent quartz glass crucible made with glasswork, and we actually used these crucibles.
We compared the number of multiples and ease of use. The experimental method will be explained below. For this experiment, a prototype was made with a diameter of 25 cm and a height of 20 cm.
Set up various quartz crucibles of cm as shown in Figure 6,
Approximately 20 kg of high-purity polycrystalline silicon is filled and melted in this, and argon gas is supplied from inlet 5 at a flow rate of 30/min.
A silicon single crystal 7 having a diameter of 110 mmφ was pulled while maintaining the pressure inside the pulling chamber 6 at 10 mb. The other parts shown in FIG. 6 consist of a pulling shaft 9 that rotates upward and rises via a seed crystal 8 at the upper end of the silicon single crystal 7 to be pulled, a quartz crucible, and a carbon material inscribed therein. The crucible protector 10 heats and melts the high-purity polycrystalline silicon block filled in the quartz crucible 1 using a heater 11 surrounding the protector. , an AC or DC power supply and control device (not shown) are included to provide appropriate heating energy. 12 is a high-purity silicon melt, and 13 is an outlet from the drawing chamber 6 for the gas that has flowed in. When the single crystal silicon 7 is pulled from the silicon melt 12, it is necessary to appropriately control the rotation speed of the seed crystal 8 and the rotation speed of the quartz crucible 1 depending on the crucible used, but a conventional quartz crucible is used. If the directions are opposite to each other and the seed crystal is at 15 rpm,
A quartz crucible rotated at 2 rpm is used. but,
When using long-term melting products or lined products,
Alternatively, when a double molded product is used, the rotational speed of the seed crystal can be reduced to 10 to 5 rpm, and as a result, the stirring of the silicon melt becomes gentler, resulting in damage to the inner wall of the crucible or damage to the crucible wall that occurs as a result. It is possible to significantly prevent single crystal disorder due to contact of exfoliating quartz pieces with the silicon single crystal growth interface. In pulling a silicon single crystal, the silicon melt in the quartz crucible 1 is heated to control the concentration of phosphorus or boron, which is intentionally added to the silicon single crystal to control the conductivity type and resistivity, within a certain range. It is rare that the entire amount of silicon is converted into a single crystal in one pulling process, and after a certain amount, for example, half of the total amount of silicon, or 10 kg, has been pulled, a maximum of 10 kg is added again and a second pulling process is performed. Theoretically, it is preferable that the pulling process be carried out infinitely, but as the number of pulling processes increases, impurities remaining in the silicon melt accumulate, so it seems that the limit is 10 times at most. The results of these experiments are shown in Table 1.

[Table] From the above results, it is clear that the OH group distribution of the long-time melting type of the sample is the most desirable. It is also estimated that the inner layer region with a higher OH group concentration than the sample will not be effective unless it is 25% or more. In the example of the sample, the reason why the results are poor even though the OH group on the inner surface is appropriate at 170 ppm is because there is no fabric in the low OH group content area, so the molten silicon gets contaminated in a short time. It is presumed that the inner wall of the crucible deteriorates or the number of quartz pieces peels off from the crucible wall, making it impossible to withstand subsequent use.

[Brief explanation of drawings]

Figures 1 and 2 are diagrams showing the distribution of OH group content in the depth direction from the inner surface of the crucible.
Figure 4 is an explanatory diagram of crucible manufacturing, and Figure 5 shows the depth direction from the inner surface of various types of crucibles.
A curve diagram showing the OH group content distribution, and FIG. 6 is a cross-sectional view of a single crystal pulling apparatus using the CZ method. DESCRIPTION OF SYMBOLS 1... Crucible, 2... Glass plate, 3... Rotating mold, 4... Glass powder with high OH group content, 5... Inlet, 6... Pulling chamber, 7... Single crystal, 8... Seed crystal, 9...
Pulling shaft, 10... Crucible protector, 11... Heater,
12...Melted body, 13...Outlet.

Claims (1)

[Claims] 1. OH in a high OH concentration region with an OH group content of 60 ppm or more and 195 ppm or less, which is in contact with the inner surface and has a thickness of 0.2 mm or more, and in a region with a thickness of 0.5 mm or more that is not in contact with the inner surface.
A crucible for pulling silicon single crystals configured to have a low OH concentration region in which the group content is 20 ppm or more lower than the average concentration in the inner surface layer with a thickness of 0.3 mm. 2. The single crystal according to claim 1, wherein the structure occupies one-fourth or more of the inner surface of the entire crucible, or one-third or more of the inner surface that comes into contact with the melt when melting the raw material. Crucible for pulling.