JPH0822797B2 - Crystal growth method - Google Patents

Description

The present invention relates to a method for growing a crystal such as a silicon single crystal,
More specifically, the present invention provides a method in which a longitudinal magnetic field is applied to the melt during crystal growth by the Czochralski method to improve the effective segregation coefficient and to grow low oxygen crystals.

[Prior art]

As a method of growing a crystal such as a silicon single crystal, there is a method of applying a static magnetic field to the melt during the crystal growth by the Czochralski method (Nikkei Electronics 1980.9.1.
5, ULSI1985.8). This method uses a transverse magnetic field method for applying a magnetic field whose main direction is a horizontal direction to a melt in a crucible under a vacuum atmosphere, and a vertical magnetic field method for applying a magnetic field whose main direction is a vertical direction, that is, a crystal pulling direction. It is roughly divided into.

FIG. 10 is a vertical cross-sectional view of the crystal growth apparatus used in the latter method.
The melt 104 in the quartz crucible 103 heated by the, while the vertical magnetic field 163 is applied by the coil 106 provided with the axial center as the vertical direction outside the vacuum vessel 101, the melt 104 In this method, the crystal 105 is grown by pulling it up and solidifying it.

The longitudinal magnetic field method has the effect of increasing the effective segregation coefficient due to the vertical magnetic field as compared with the case without it.

FIG. 11 shows the magnetic field strength Bz / T in the growth direction (Z direction) of a single crystal produced by changing the strength of the longitudinal magnetic field.
It is a graph showing the relationship between the (horizontal axis) and the effective segregation coefficient Ke [P] of the dopant (phosphorus) added to the single crystal (vertical axis). When a single crystal that is [100] is grown at a crystal rotation speed of 10 rpm and 15 rpm,
△ indicates a single crystal with a crystal orientation of [111] at a crystal rotation speed of 15 r
Shown when grown in pm.

As can be seen from this figure, Ke [P] is as small as 0.35 when Bz / T = 0, that is, when no longitudinal magnetic field is applied, but Ke [P] increases as Bz / T increases. , Bz / T = 0.1
Then it becomes 0.6.

This is because the forced convection of the melt just below the crystal caused by the rotation of the crystal is suppressed by the longitudinal magnetic field, so that the main driving force that contributes to the movement of solute atoms released into the melt as the crystal grows. It is thought that this is because it is only diffusion.

[Problems to be solved by the invention]

Fig. 12 shows Bz / T (horizontal axis) when producing a silicon single crystal by the longitudinal magnetic field method, and the oxygen content [O] in the silicon single crystal.
i] / 10 ¹⁸ cm ^-3 (vertical axis) is a graph showing the relationship,
The broken line and the alternate long and short dash line in the figure show the upper limit values of the oxygen amount in the case of not applying a magnetic field and in the case of manufacturing by the transverse magnetic field method, respectively. The symbols ●, ▲, and △ in the figure are the same as those in Fig. 11, and the symbols ■ indicate the symbols ○ and □ when a single crystal with a crystal orientation of [100] was grown at a crystal rotation speed of 25 rpm. The case where single crystals each having a crystal orientation of [111] are grown at crystal rotation speeds of 10 rpm and 25 rpm is shown. The crucible rotation speed nc is 0.5 rpm.
It was performed at 2 rpm and 2 ways.

From this figure, it can be said that there is a phenomenon that the amount of oxygen in the crystal is higher in the case of the longitudinal magnetic field method than in the other methods.

An object of the present invention is to provide a crystal growth method by a longitudinal magnetic field method, which can reduce the oxygen concentration in the crystal by elucidating the reason why this phenomenon occurs.

[Means for solving problems]

The present invention suppresses convection just below the crystal. That is, the crystal growth method according to the present invention, in the crystal growth method by the Czochralski method of pulling up the crystal while applying a DC magnetic field along the crystal pulling direction, convection in the melt portion immediately below the crystal In order to suppress only the above, a plurality of magnets are arranged in the direction of the main magnetic field in the direction of the crystal pulling direction, and at least one of the magnets is arranged in the opposite polarity to the other, and the strength of the pulling direction magnetic field at the melt surface is set. Is decreased from immediately below the crystal toward the side wall of the melt container, and is 0 near the side wall or slightly inside thereof.

[Principle of Invention]

First, the principle of the present invention will be described. As is clear from FIG. 12, the oxygen concentration in the crystal is lower in the case of the transverse magnetic field method than in the case of not applying a magnetic field. The reason why the oxygen concentration is low when a transverse magnetic field is applied has been conventionally explained as follows. That is, the source of oxygen in the crystal is the crucible, and oxygen or SiO eluted from the crucible enters into the crystal directly below the crystal through the surface of the melt by convection and is taken into the crystal. When a transverse magnetic field is applied, convection on the surface of the melt is suppressed, the time on the surface of the melt becomes longer, and during this time oxygen or SiO evaporates, and as a result, the amount of oxygen flowing directly under the crystal decreases. Of.

However, even if a longitudinal magnetic field is applied, the convection on the melt surface is suppressed, so the oxygen concentration should be low even when the longitudinal magnetic field method is used. Is high. That is, for the above reason, the phenomenon in the case of the longitudinal magnetic field method is impossible. Therefore, we tried to elucidate the cause of the high oxygen concentration by the longitudinal magnetic field method by using the numerical analysis method. Fig. 1 shows the temperature coefficient K of the surface tension, which is the cause of the Marangoni flow, as K = 0.1 m, with the melt stationary as the initial condition.
It is a graph showing the flow velocity of the melt (broken line) and the oxygen concentration in the crystal (solid line) calculated as J / m ^-2 K ^-1 , with the horizontal axis representing time (t / sec) and the vertical axis. Oxygen concentration and flow rate (ms ^-1 )
And is shown. The flow velocity of the melt is 100 mm from the crystal axis as shown in Fig. 2 (a), below the melt surface.
The flow velocity (Vr) in the radial direction (axial direction) at the position of 05 mm,
The oxygen concentration is a value on the crystal axis and is normalized by setting the saturation concentration to 1.0.

As understood from this figure, at time t = 0, that is, when the melt is at rest (point a), the oxygen concentration in the melt reaches the saturation level, and the oxygen concentration decreases as the flow velocity increases. After reaching the minimum value of about 0.001 (point b), it increases, and becomes about 0.02 at 0.95 (point c), for example.

The reason for exhibiting such characteristics is estimated as follows.

(A), (b), and (c) of FIG. 3 are a, b, and c of FIG.
It is a schematic diagram which shows the state of melt convection at this time. When the melt convection speed is slow a, as shown in FIG. 3 (a), oxygen in the melt 4 evaporates from the melt surface layer except immediately below the crystal 5, and the melt surface layer is formed. A low oxygen region h (hatched portion) is formed, but the low oxygen region h is due to the slow Marangoni flow velocity.
It is difficult for the melt to flow into the portion just below the crystal, and in the portion directly below the crystal, a high oxygen region is formed due to convection in a portion other than the low oxygen region h,
As a result, the produced single crystal has a high oxygen concentration.

Further, when the convection of the melt is fast c, FIG.
As shown in (4), since the melt in the high oxygen region near the crucible wall flows into the portion directly below the crystal without sufficient evaporation of oxygen, the oxygen concentration in the crystal does not reach a low level.

When the convection velocity of the melt is appropriate b, the formation of the low oxygen region due to oxygen evaporation and the inflow of the melt into the region just below the crystal in a well-balanced manner simultaneously proceed as shown in FIG. Therefore, the low oxygen region h is formed immediately below the crystal, and the oxygen concentration of the manufactured crystal can be reduced. That is, the oxygen concentration in the crystal can be reduced by setting the horizontal convection velocity on the surface of the melt to an appropriate value. In the case of the transverse magnetic field method, horizontal flow in the melt surface layer parallel to the direction of the magnetic field lines cannot be suppressed, so some flow remains and convection occurs near the velocity value at which the oxygen concentration has a minimum value as at point b. It is presumed that

On the other hand, in the case of the longitudinal magnetic field method, the direction of the magnetic field lines and the driving direction of the Marangoni flow are orthogonal to each other, and the horizontal flow is suppressed. Therefore, it is considered that the convection is significantly suppressed as the magnetic field increases. That is, it is considered that a slow convection such as the point a was generated.

Figure 13 shows the measurement results of the melt surface temperature in the case of the Czochralski method without applying a magnetic field (broken line) and in the case of the longitudinal magnetic field method (solid line). The latter is the one with the crucible axis and the crucible side wall. This proves that the temperature difference is large and the surface convection is slower in the longitudinal magnetic field method.

Therefore, in order to lower the oxygen content of a single crystal by the longitudinal magnetic field method, convection in the melt portion immediately below the crystal is suppressed to improve the effective segregation coefficient, which is the original purpose of the longitudinal magnetic field method. It is necessary to generate horizontal convection at an appropriate flow velocity higher than this in the surface layer of the melt other than the above in the melt.

Next, a method for realizing such convection will be described.

FIG. 4 illustrates the strength of the magnetic field at the radial position (r) in the coil at different crucible positions in the axial direction. As a parameter, the position d at the same height as the coil is used as shown in FIG. And a position e higher than that and a position f higher than that, and the magnetic field strength B at each position is shown normalized by dividing the magnetic field strength on the coil axis by B ₀ .

As can be seen from this figure, the magnetic field strength becomes stronger as it moves away from the coil axis at position d, and at a position a little away from the magnetic field strength on the coil axis at position e. It becomes a little higher, and the magnetic field becomes weaker further away.

Further, at the position f, the magnetic field on the axial center of the coil tends to be the strongest.

Therefore, for example, as shown in FIG. 6, a coil that produces a negative magnetic field is arranged so as to show the characteristic of d, and a coil that produces a positive magnetic field is arranged so as to show the characteristic of e or f, and an appropriate magnetomotive force is provided for each. By setting, it is possible to obtain a magnetic field in which the composite magnetic field is strong at the center of the crucible on the melt surface, and the position r _{0 at} which the magnetic field is zero is near the crucible side wall or slightly closer to the center. This magnetic field serves as a longitudinal magnetic field just under the crucible axis, that is, immediately below the crystal to suppress convection and contributes to the improvement of the effective segregation coefficient, while at other parts of the melt surface it becomes zero or a small value to suppress convection. do not do.

〔Example〕

The present invention will be specifically described below with reference to the drawings. Seventh
FIG. 1 is a schematic side sectional view showing an embodiment of the present invention, in which 1 indicates a chamber. The chamber 1 is a substantially cylindrical vacuum container whose axial direction is vertical, and the rotation axis of the pulling chuck 10 that rotates at a predetermined speed in the arrow direction at the center of the upper surface.
10 'is air shielded and penetrated.

At the center of the bottom of the chamber 1, the pulling chuck
A supporting shaft 9 of the crucible 3 which has the same axis center as that of 10 and rotates at a predetermined speed in the same direction or in the opposite direction is penetrated by being air-shielded.
A graphite crucible 3 ′ is attached to the tip of the support shaft 9 with the quartz (SiO ₂ ) crucible 3 fitted therein. A storage box (not shown) for storing impurities is provided in the chamber 1 above the crucible 3, and when the bottom lid is opened by an opening / closing means (not shown), the required amount of impurities can be added into the crucible 3 several times. It has become.

A resistance heating type heater 2 is arranged at a position slightly outside the rotational range of the crucible 3, and a heat shield 21 is concentrically arranged at a position between the crucible 3 and the chamber 1 further outside thereof.

A raw material for a single crystal is charged into the crucible 3, and the raw material is melted by heating the heater 2 to form a melt 4
Becomes

A seed (a single crystal that serves as a nucleus for crystal growth) is attached to the pulling chuck 10, and the seed is lowered by the chuck 10 to come into contact with the melt 4 and then pulled to grow the crystal 5. ing.

On the outer circumference of the chamber 1, three coils 71, 6, 72 are provided concentrically with the chamber 1 in this order with the height appropriately changed from the upper side. Each coil 71, 6, 72 is provided with a power source (not shown). DC current is supplied.

Upper and lower coils 71, 72 and intermediate coil 6
The coils 71, 6, 72 are energized so that the magnetic poles of and are in opposite directions. The current value is set to a level for generating the magnetic field, and the generated magnetic field 63 has a weak magnetic field in the melt portion around the crucible side wall and a strong magnetic field distribution in the crucible axial center portion.

By growing the crystal in such a magnetic field atmosphere, the convection indicated by (B) in FIG. 2 is suppressed by the longitudinal magnetic field in the melt portion directly below the crystal as shown in FIG. 6, and In the other melt portion, horizontal convection toward the crucible axis is generated in the surface layer portion without being excessively disturbed by the magnetic field.

As a result, a low oxygen region is formed in the melt as shown in FIG. 3 (C), and the melt in that region flows into the region immediately below the crystal, so that a low oxygen region is formed in the region directly below the crystal. The formed crystals have a low oxygen concentration. Further, since the convection is suppressed in the portion directly below the crystal, the effective segregation coefficient is improved.

Although the three coils 71, 6, 72 are used in the above embodiment, the present invention is not limited to this, and may be a case of two coils in which one of the coils 71, 72 is omitted. ,
Further, four or more coils may be used, and at least one coil has the opposite polarity to the others. The diameters of the coils used may be the same or different. FIG. 8 shows an apparatus using two coils 6 and 7 having different diameters. This apparatus also suppresses the convection of the melt just below the crystal, and the other convections at the surface layer of the melt. It can be born.

Further, it goes without saying that the present invention can be implemented not only by using electromagnets but also by using permanent magnets.

〔effect〕

Next, the effect of the present invention will be described. A 5 ″ φ diameter P-doped silicon single crystal having a crystal orientation of [100] and a resistivity of around 10 Ω · cm was produced by the present invention using the apparatus shown in FIG. The conditions are as follows: inner diameter of crucible: 16 ″ φ, weight of raw material charged in crucible: 30 kg, pulling speed: 1 mm ・ min ⁻¹ , crystal rotation speed: 25 rpm, crucible rotation speed (direction opposite to crystal rotation direction) ): 0.5 rpm.

FIG. 9 shows a sample of the silicon single crystal manufactured according to the present invention in the length direction of the crystal at a pitch of 100 mm, and the effective segregation coefficient calculated from the electrical resistivity of each sample and the oxygen content measurement result by the infrared absorption method. Example 1 shows the case where 200 mT immediately below the crystal and 50 mT near the crucible wall, and Example 2 the case where 200 mT immediately below the crystal and zero near the crucible wall. For comparison, the results obtained by the conventional longitudinal magnetic field method with 200 mT just below the crystal and 220 mT near the crucible wall (conventional example 1) and the Czochralski method without applying a magnetic field to the melt (conventional example 2) ) Is also shown together with the results for three charges, and the four results also show the average value and the variation range for three charges.

The above-mentioned effective segregation coefficient (Ke) is measured by measuring the electrical resistivity ρ of the sample by the four-terminal method, and assuming that the measured value is inversely proportional to the P concentration in the single crystal, Pfann shown in the following equation
Ρ = ρ ₀ (1-g) ^1-Ke where ρ ₀ : initial electric resistivity of single crystal g: pulling rate of single crystal was adapted to the measured value.

As can be understood from this figure, Conventional Example 1 can improve the effective segregation coefficient as compared with Conventional Example 2, but the oxygen level rises and both measured values have large variations. Although the oxygen level in Example 1 is similar to that in Conventional Example 2, the effective segregation coefficient can be improved, and in Example 2, the effective segregation coefficient can be improved as in Example 1 and Conventional Example 1, and the oxygen level can be improved. This was also reduced.

As described above in detail, in the case of the present invention, both improvement of the effective segregation coefficient, which is the original purpose of the longitudinal magnetic field method, and reduction of the oxygen concentration, which has been impossible in the past, can both be achieved, and, of course, The present invention has excellent effects such that the electric resistivity in the crystal growth direction becomes constant due to the improvement of the effective segregation coefficient, which improves the yield when cutting semiconductor products from the crystal.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between flow velocity and oxygen concentration and time, and FIG. 2 is an explanatory diagram of the calculation position of the flow velocity and oxygen concentration,
FIG. 3 is an explanatory view of the principle of the present invention, FIGS. 4 and 5 are explanatory views of the relationship between the coil and the magnetic field direction around the coil, and FIG. 6 is an explanatory view of magnetic field distribution that can realize the principle of the present invention. 7 is a schematic vertical sectional view showing an embodiment of the present invention, FIG. 8 is a schematic vertical sectional view showing another embodiment of the present invention, FIG. 9 is an explanatory view of the effect of the present invention, and FIG. FIG. 11 is a schematic vertical cross-sectional view of a conventional device, FIG. 11 is a relationship diagram between magnetic field strength and effective segregation coefficient, and FIGS. 12 and 13 are explanatory diagrams of problems of the conventional technique. 4 ... Melt, 5 ... Crystal, 6,7,71,72 ... Coil

Claims (1)

[Claims] 1. A method for growing a crystal by the Czochralski method in which a direct current magnetic field along the crystal pulling direction is applied to the melt while pulling the crystal, in order to suppress only convection in the melt directly below the crystal, A plurality of magnets are arranged so that the direction of the main magnetic field is along the pulling direction of the crystal, and at least one of the magnets is arranged with the opposite polarity to the others, and the strength of the pulling-up magnetic field on the melt surface is from directly below the crystal to the melt. A method for growing a crystal characterized by decreasing toward a side wall of a container and becoming 0 near or slightly inside the side wall.