JPH0379320B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for growing crystals such as silicon single crystals used as materials for semiconductor devices, for example, while preventing the occurrence of segregation.

[Prior art]

There are various methods to grow single crystals, but
One of them is the rotational pulling method. As shown in FIG. 7, this method involves melting all of the material inserted into a crucible 13, and then pulling the molten liquid 14 upwards with a pulling rod 17, thereby solidifying the molten liquid into a single crystal. It is a way to grow. However, in the single crystal 15 grown by this method, in order to adjust the resistivity, conductivity, etc. of the semiconductor crystal, for example, impurities added to the melt at once before pulling are added along the pulling direction. A phenomenon of segregation is occurring.

This segregation is the ratio of the impurity concentration at the start of solidification to the impurity concentration at the end of solidification at a certain point in the single crystal, that is, the impurity concentration in the single crystal that actually occurs at the melt/single crystal interface during crystal growth. The ratio of Cs to the impurity concentration Cl in the melt, Cs/Cl, that is, the effective segregation coefficient Ke
arises due to. To elaborate on this, for example
When Ke<1, as the single crystal grows, the concentration of impurities in the melt naturally increases, causing segregation in the single crystal.

There is a fused layer method as a method of growing a single crystal while suppressing the occurrence of the above-mentioned segregation. In this method, the material inserted into the crucible is melted from the top to the bottom, and regardless of the amount of single crystal grown, the amount of molten liquid in the crucible is maintained constant to suppress segregation. It is.

When using this method, regardless of the value of the effective segregation coefficient Ke, the impurity concentration is reduced by the newly generated melt as the single crystal grows, so the melt in the crucible is In order to suppress changes in impurity concentration in the liquid, segregation can generally be suppressed by continuously adding impurities to the amount of the melt in the crucible.

[Problem that the invention seeks to solve]

In all growth methods using a crucible including the above two methods, for example, when growing a silicon single crystal using a quartz (SiO ₂ ) crucible,
The quartz crucible melts and oxygen (O ₂ ) is eluted,
Oxygen is contained in the single crystal. When a silicon wafer obtained by slicing a silicon single crystal containing oxygen in this way is heat-treated in order to use it as a material for a semiconductor device, crystal defects occur due to the oxygen content.

In order to reduce oxygen, which has a negative effect on silicon single crystals, the rotation speed of the crucible, which is generally rotated around a vertical axis, is lowered.
Alternatively, a method has been adopted in which a magnetic field is applied to the molten liquid in the crucible to suppress the convection that occurs in the molten liquid in the crucible.

However, in the fused layer method, which can suppress the occurrence of segregation, impurity elements (generally powder) are added to the melt during single crystal growth, and in order to sufficiently suppress segregation, the impurity elements are added to the melt. It is necessary to diffuse the melt uniformly within the melt, and therefore it is desirable to have sufficient convection of the melt, contrary to the case where oxygen is reduced.

In this way, prevention of segregation and reduction of oxygen are contradictory requirements, and therefore, when growing single crystals by the fused layer method under convection suppression conditions for the purpose of reducing oxygen, impurity elements must be There was a problem that the impurity elements were not diffused, and therefore segregation of impurity elements occurred in the single crystal.

[Means for solving problems]

The present invention has been made in view of the above circumstances, and is intended to compensate for the concentration of impurities that occurs on the melt side based on the segregation phenomenon at the crystal/melt interface due to solidification, for example in the case of Ke<1. By newly melting the amount of molten liquid, the impurity concentration in the molten liquid is kept constant while the crystal is growing, and the impurity concentration in the crystal is maintained constant, and stirring is performed for the purpose of lowering oxygen. An object of the present invention is to provide a crystal growth method that can grow crystals while preventing the occurrence of segregation in the pulling direction even when the above steps are not performed.

The crystal growth method according to the present invention is a method of growing crystals by melting a crystal material inserted into a crucible from the upper side to the lower side, and then pulling the molten liquid upward and solidifying it. In this method, impurities are added to the melt at the stage where the material is partially melted, and then the melt is pulled up, and during the pulling, the amount of the melt in the crucible is reduced as the crystals grow. Features.

[Principle of the invention]

First, the principle of the present invention will be explained below. 1st
The figure is an explanatory diagram of the principle of the present invention, in which the upper part of a single crystal material 10 inserted into a crucible 3 is melted by a certain thickness by a heater (not shown) to add impurities, and then the material 10 is placed on the upper side. 2 is a schematic diagram showing a state in which a single crystal 5 is grown by pulling up the melt 4 upward by a pulling chuck 7 and solidifying it while melting it downward. FIG.

Regarding the mass balance of impurities in such a state, the following equation (1) holds true if diffusion of impurities within the single crystal 5 is ignored.

∫ ^gs ₀ Cs(g)dg+Cl(gs)・gl(gs)=A ……(1) However, gs: Ratio of the total weight of single crystal pulled to the total weight W of single crystal materials and impurities inserted Cs(g) : Impurity concentration Cl (gs) in the single crystal at the interface in contact with the melt when the ratio is g: Impurity concentration in the melt when the ratio is gs GL (gs): Inside the crucible for W when the ratio is gs Ratio of melt weight A: constant Differentiating equation (1) above with respect to gs, Cs(gs) + dCl/dgs・gl+Cl・dgl/dgs=0...(2) However, Cs(gs): Ratio is Impurity concentration in the single crystal when gs Cl: Impurity concentration in the melt gl: The ratio of the weight of the melt to W, but at the single crystal/melt interface (hereinafter referred to as the solid-liquid interface), Cs (gs) =Ke・Cl(gs)...(3) However, since Ke: effective segregation coefficient holds, the above equation (2) becomes the following equation.

(1+1/Ke・dgl/dgs)・Cs+gl/Ke・dCs/dgs=
0...(4) In this equation (4), dgl/dgs in the first term on the left side
If dgl/dgs=-Ke...(5), then gl/dgs in the second term on the left side is Ke does not become zero during crystal growth, and as a result, dCs/dgs=0...(6).

Therefore, from equations (5) and (6) above, gs (ratio of total weight of single crystal pulled to W) at a certain point during single crystal growth
By matching the ratio of the change in gl (the ratio of the weight of the melt in the crucible to W) to the change in -Ke (the negative value of the effective segregation coefficient), the ratio of Cs (in the single crystal) to the change in gs The amount of change (impurity concentration) is zero, and segregation can be prevented. This is the effective segregation coefficient
There is a difference in impurity concentration at the solid-liquid interface based on Ke,
If the amount of melt is constant from the start to the end of single crystal formation, the impurity concentration in the melt will gradually increase, but the ratio of the change in gl to the change in gs will be -Ke. By melting the unmelted material under the melt, the impurity concentration in the melt is always kept constant, and as a result, the impurity concentration in the single crystal is always kept constant regardless of its growth level. This is because that.

Now, in order to grow a single crystal while melting it, it is necessary to satisfy gs+gl<1...(7). Incidentally, the case where gs+gl=1 corresponds to the above-mentioned rotational pulling method.

In order to satisfy the above equation (7), that is, to prevent the occurrence of segregation and to grow all the inserted single crystal material as a single crystal, at the end of crystal growth, that is, when pulling is stopped, gs → 1 , gl→0, and the conditions for the crystal growth will be explained below.

First, when adding impurities, the ratio gl ₀ of the weight of the initial melt to W is gl ₀ =Ke (<1) (8). The reason for this is that pulling and melting are performed while maintaining dgl/dgs=-Ke in equation (5), so the relationship between gl and gs is gl=-Ke・gs+C...(9) However, C: constant linear expressed as a function. In this equation (9), gs → 1
This is because in order to make gl→0 when , it is necessary to set C as C=Ke...(10).

The amount of pulling and the amount of melt after starting the pulling are performed so that gl and gs satisfy both equations (9) and (10). In other words, line B in Figure 2 (gl=-Ke・gs+
Ke) Manage gl and gs so that they are above.

As a result, all the single crystal materials inserted into the crucible can be made into single crystals while preventing segregation even under low oxygen conditions.

〔Example〕

The present invention will be specifically explained below based on the drawings. FIG. 3 is a schematic side sectional view showing the implementation state of the present invention, and 1 in the figure indicates a chamber. The chamber 1 is a substantially cylindrical vacuum container with its axial direction perpendicular, and a rotation shaft 7' of a lifting chuck 7 that rotates at a predetermined speed in the direction of the arrow is passed through the center of the upper surface with an air shield. . Lifting chuck 7
A seed (single crystal serving as a nucleus for crystal growth) 5' is attached to the.

A support shaft 6 for a crucible 3, which rotates at a predetermined speed in the opposite direction on the same axis as the pulling chuck 7, passes through the center of the bottom surface of the chamber 1 in an air-shielded manner. A crucible 3' made of graphite is attached to the tip of the support shaft 6 with a crucible 3 made of quartz (SiO ₂ ) fitted inside the crucible 3'. A storage box (not shown) for storing impurities is provided in the chamber 1 above the crucible 3, and the impurities can be added into the crucible 3 by opening the bottom cover using an opening/closing means (not shown).

A resistance heating type heater 2 is disposed at a position slightly outside the rotation range of the crucible 3, and a heat shield 8 is disposed in a concentric cylindrical shape at a position further outside the crucible 3 between the heater 2 and the chamber 1. The heater 2 has an axial length that is appropriately shorter than that of the crucible 3, and is supported so that it can be raised and lowered by a lifting device (not shown).
can be partially heated in a length region shorter than its axial length.

The method of the present invention using the apparatus configured as described above will be explained next. After inserting and fixing the required amount of solid single crystal material 10 into the crucible 3, the upper layer is heated by the heater 2 so that the ratio of the weight of the initial melt to the total weight W of impurities and material 10 to be added later is gl ₀ . Melt so that Note that the amount of impurities added is
If it is extremely small compared to the insertion amount of 0, use material 10.
There is no problem even if the insertion amount is W. Then, when the weight of the melt 4 satisfies gl ₀ , the bottom cover of the storage box (shown) in which the required amount of impurities with Ke<1 is stored is opened and this is added to the melt 4, and the impurities are removed. After a period of time for the seeds to diffuse and be uniformly distributed in the melt 4, the seed 5' attached to the chuck mentioned above is brought into contact with the surface of the melt 4 and pulled up with or without rotation, and the melt is liquid 4
The single crystal material 10 below is melted from above. During this pulling and melting, in order to grow all of the single crystal material 10 as a single crystal as described above, it is necessary to control gl and gs so that they are on line B in FIG. GL is managed by heater 2,
For example, the power input to the heater 2 is kept constant, and the relative position of the single crystal material 10 and the heater 2 is determined by a method such as fixing the crucible and lowering the heater, fixing the heater and raising the crucible, or raising and lowering the crucible and the heater at the same time. Change as the single crystal grows.

However, if gl and gs become extremely small and it becomes difficult to manage them, as shown in Figure 4, instead of setting gl ₀ as Ke, set a slightly larger amount of melt and After that, connect gl/gs to line B in Figure 2.
When gl and gs become smaller, gl/gs is made smaller (gs/gl is made larger), that is, it is managed so that the slope is larger than the slope of line B.

Even when managed in this manner, all of the inserted single crystal material 10 can be grown into a single crystal, and there is almost no segregation in the grown single crystal. In addition, since the temperature at the bottom of the melt is lower than the temperature at the top, the convection of the melt is weaker than in the rotary pulling method, and even if a quartz crucible is used, the grown single crystal has a low oxygen level. is maintained.

In the above description, a single crystal is grown, but the present invention is of course not limited to this, and can of course be applied to, for example, growing a polycrystalline metal material.

Further, although a resistance heating type heater is used in the above embodiment, the present invention is not limited to this, and it goes without saying that an induction heating coil may be used for heating and melting.

〔effect〕

A quartz crucible with an inner diameter of 300 mm is used, and after inserting the silicon single crystal material into it, it is melted until the height of the initial melt reaches 200 mm, and an impurity, phosphorus, whose Ke value relative to silicon is 0.35 is added. Then, rotate the crucible at a speed of 0.5 rpm, and turn the lifting chuck in the opposite direction to the crucible rotation direction.
Rotate at a speed of 15 rpm, melt the material and impurities with the GL, and pull up the molten liquid with the GS until the diameter is 100 mm.
was grown into a single crystal. In the case of this crystal growth, gl and gs were controlled so that they were on the line shown in FIG. 4, and the initial melt amount gl ₀ was set to 0.4, which was larger than Ke.

And the gs of the obtained single crystal is 0, 0.1, 0.2,
The resistivity in the axial direction was measured at points at 0.3, 0.4, 0.5, and 0.6. FIG. 5 shows the resistivity measurement results (black circles), with gs plotted on the horizontal axis and ρ/ρ ₀ (resistivity distribution) plotted on the vertical axis.

For comparison, the measurement results (dashed line) for a single crystal grown by the conventional rotational pulling method are also shown, and the dashed line in the figure shows an example of the allowable range of resistivity. It shows.

As can be understood from this figure, in the case of the conventional method, the resistivity gradually decreases from 1.0 as the single crystal grows. In other words, the concentration of P, which affects resistivity, changes in the axial direction of the single crystal, causing segregation. On the other hand, in the case of the present invention, there is some variation probably because the control of the melt layer thickness by the movement of the heater is not perfect, but the resistivity is close to 1.0, that is, in the axial direction of the single crystal. The P concentration is constant, and segregation is prevented from occurring.

In addition, we investigated the resistivity distribution within the same cross section, and found that the variation in resistivity was ±
It was within 2.5%, and both single crystals were good with no radial segregation.

In addition, in the above explanation, as a condition for satisfying equation (4),
Equations (5) and (6) are obtained, but in the present invention, even if Equation (5) does not hold strictly, dgl/dgs=-Ke(1+ε)...(11) However, ε is Of course, the objective can also be achieved by using a constant representing the degree of departure from equation (5), which is the strict non-segregation condition. The reason for this will be explained next.

Integrating equation (11) with respect to gs, we obtain gl=gl ₀ −Ke(1+ε)gs...(12). Here, gl ₀ is the start of crystal pulling (gs=0)
is the value of gl (initial liquid phase ratio).

Substituting equations (11) and (12) into equation (4), dgl/
When dgs and gl are deleted, {1+1/Ke[-Ke(1+ε)]}・Cs+[
gl ₀ /Ke−(1+ε)gs]dCs/dgs=0. If we rearrange this equation, we get -εCs+[gl ₀ /Ke−(1+ε)gs] ・dCs/dgs=0 (13). Integrating this equation (13) with respect to gs gives us However, Cs ₀ : can be expressed as the impurity concentration in the initial single crystal.

On the other hand, Cs in the rotational pulling method is so-called
It is known that Pfann's formula Cs=Cs ₀ (1−gs) ^Ke-1 ...(15) is followed.

For example, under the conditions of Ke=0.35 and gl ₀ =Ke, equations (14) and (15) can be expressed as lines as shown in FIG. The solid line in the figure indicates that ε in equation (14) is ±
There are six cases: 0.1, ±0.3, and ±0.5, and the broken line indicates equation (15). As can be understood from this figure, when growing a crystal according to the present invention, even if ε is around ±0.5, dgl/dgs does not exactly match -Ke, and some fluctuations in dgl/dgs occur. Also, the grown crystals have less segregation than those grown by the rotational pulling method.

As detailed above, according to the present invention, crystals can be grown while preventing segregation even under low oxygen conditions. The impurity concentration becomes constant,
For example, no matter which part of a single crystal a material for a semiconductor device is made from, there is no variation in the resistivity of the material.
Furthermore, the present invention has excellent effects such as high material yield.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of the principle of the present invention, Fig. 2 is an explanatory diagram when all the materials in the crucible are made of single crystal according to the principle, Fig. 3 is a schematic diagram showing the implementation state of the present invention, and Fig. 4 is an explanatory diagram of the principle of the present invention. The figure is a diagram explaining the management of gs and gl in that case,
Fig. 5 is an explanatory diagram of the effects of the present invention, Fig. 6 is an explanatory diagram of the management range of GL and GS that can achieve the purpose of the present invention,
FIG. 7 is an explanatory diagram of the prior art. 2... Heater, 3... Crucible, 4... Molten liquid,
5... Single crystal, 10... Material for single crystal.

Claims (1)

[Claims] 1. A method of growing crystals by melting a crystal material inserted into a crucible from the top to the bottom, and pulling the molten liquid upward to solidify it, It is characterized by adding impurities to the melt at the stage where the material is partially melted, and then pulling up the melt, and during the pulling, the amount of the melt in the crucible is reduced as the crystals grow. crystal growth method.