JP2021031342A - Production method of lithium tantalate single crystal - Google Patents

Abstract

To provide a production method of a lithium tantalate (LT) single crystal capable of growing a LT single crystal of 4 inch φ having generation of a lineage (aggregate of dislocation, small tilt angle grain boundary) suppressed even when a large furnace of 800 mm φ inner diameter for growing a single crystal is used.SOLUTION: The rotating speed of a seed crystal 1 is set 7.5 rpm at the start of crystal growing and 4 rpm or more and 6 rpm or less at the completion of shoulder part growth using a large furnace 10 of 800 mm φ inner diameter for growing a single crystal and a crucible 12 of 170 mm φ inner diameter and 170 mm inner height to grow a LT single crystal of 4 inch φ barrel part diameter by a Czochralski method.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for producing a lithium tantalate single crystal by the Czochralski (hereinafter, may be abbreviated as Cz) method, and in particular, a lineage (aggregate of dislocations, small grain boundaries) during crystal growth. The present invention relates to a method for producing a lithium tantalate single crystal whose generation can be suppressed.

Lithium tantalate (hereinafter, sometimes abbreviated as LT) single crystal is mainly used as a material for surface acoustic wave elements (SAW filters) mounted on mobile communication devices such as smartphones and tablets.

The LT single crystal is industrially produced by the Cz method. As a flow of crystal growth by the Cz method, a high melting point iridium crucible is used, _{and a mixed powder of tantalum pentoxide (Ta 2} O ₅ ) powder and lithium carbonate (Li ₂ CO ₃ ) powder is reacted to obtain an LT powder. The baked powder is used as a raw material, and the calcined powder is filled in the crucible. Then, the calcined powder is melted in a single crystal growing furnace having a nitrogen-oxygen mixed gas atmosphere to obtain an LT raw material melt, and a seed crystal (LT single crystal) is brought into contact with the LT raw material melt to obtain a temperature gradient. The seed crystal is rotated and pulled up in a tight atmosphere to grow the shoulder and the straight body that follows it.

In order to obtain a single crystal having a desired crystal diameter, crystal growth is carried out using a highly accurate and controlled system. For example, Patent Document 1 describes a method of measuring the weight of a crystal to be pulled up and growing the crystal while automatically controlling the heating of the crucible so that the crystal has a desired shape from the amount of change per unit time. Has been done.

FIG. 1 shows an example of a single crystal growing furnace for growing an LT single crystal. A metal crucible 12 is arranged in the center of a single crystal growing furnace 10 having a cylindrical structure, and a refractory material 14 that forms an appropriate temperature gradient for growing an LT single crystal on the outer periphery and above the crucible 12. , 19 are arranged. After growing the LT single crystal, it is cooled in the single crystal growing furnace 10 at a predetermined cooling rate, and is taken out from the single crystal growing furnace 10 to obtain an LT single crystal as shown in FIG.

The grown LT single crystal is colorless and transparent or has a highly transparent pale yellow color. After the growth, in order to remove the residual strain due to the thermal stress of the crystal, heat treatment is performed under uniform heat close to the melting point, and further polling treatment is performed to obtain a single polarization. After the polling process, the LT single crystal ingot whose outer circumference is ground to adjust the outer shape of the crystal is processed into a substrate state through machining such as slicing, lapping, and polishing steps to obtain an LT substrate.

By the above method, using an iridium crucible having an inner diameter of 170 mmφ and an inner height of 170 mm, an LT single crystal having a straight body diameter of 4 inches φ and a length of 90 mm shown in FIG. 2 is grown, which is the current mainstream. We are stably producing 4-inch φLT substrates. When growing an LT single crystal having a straight body diameter of 4 inches φ, a single crystal growing furnace having an inner diameter of 600 mmφ has been conventionally used.

In recent years, the size of the LT substrate has increased, and the demand for the 6-inch φLT substrate has increased. However, in order to grow a large-diameter LT single crystal for obtaining a 6-inch φ substrate, an iridium crucible or a work coil is used. It was necessary to increase the size of the single crystal growth furnace by adding refractories and the like, and a large growth furnace having an inner diameter of 800 mmφ was used as the single crystal growth furnace.

By the way, rather than preparing single crystal growth furnaces having different inner diameters for growing an LT single crystal for a 4-inch φLT substrate and an LT single crystal for a 6-inch φLT substrate, a LT single crystal for a 6-inch φLT substrate is used. From the viewpoint of production efficiency, it is desirable to be able to grow an LT single crystal for a 4-inch φLT substrate using a large single crystal growing furnace having an inner diameter of 800 mmφ.

However, there have been few reports of growing an LT single crystal for a 4-inch φLT substrate using a large single crystal growing furnace having an inner diameter of 800 mmφ.

Therefore, we tried to grow an LT single crystal for a 4-inch φLT substrate using a large single crystal growth furnace with an inner diameter of 800 mmφ. The phenomenon of polycrystallization has come to be seen in the middle of the process. This phenomenon is considered to be due to a change in the solid-liquid interface shape during crystal growth. Fig. 3 shows the solid-liquid interface shape that is presumed to be formed at the end of shoulder growth when an LT single crystal with a straight body diameter of 4 inches φ is grown using a large single crystal growth furnace with an inner diameter of 800 mmφ. .. It is considered that the solid-liquid interface shape at this time is a so-called M-shaped solid-liquid interface shape in which the outer peripheral portion of the crystal is concave with respect to the melt and the central portion of the crystal is convex with respect to the melt. Be done. Then, lineage (aggregates of dislocations, small tilt angle grain boundaries) is likely to be concentrated on the portion where the concave shape is formed with respect to the melt, and cracks due to such crystal defects may be generated. ..

Japanese Unexamined Patent Publication No. 58-145692

The present invention has been made by paying attention to such a problem, and the subject thereof is to use a large single crystal growing furnace having an inner diameter of 800 mmφ in order to grow an LT single crystal having a straight body diameter of 4 inches φ. It is an object of the present invention to provide a method for producing an LT single crystal capable of suppressing the generation of lineage by adjusting the crystal growth conditions even in such a case.

Therefore, as a result of diligent studies by the present inventor in order to solve the above problems, an LT single crystal in which the generation of lineage is suppressed by controlling the temperature distribution inside the melt and changing the solid-liquid interface shape is grown. As a method to realize this, it was discovered that it is effective to lower the number of rotations of the seed crystal during crystal growth compared to the conventional conditions. The present invention has been completed by such technical ideas and discoveries.

That is, the first invention according to the present invention is
A metal crucible arranged inside a single crystal growing furnace having a cylindrical structure is filled with a lithium tartrate raw material powder, and the seed crystal is brought into contact with a lithium tantalate raw material melt obtained by heating the metal crucible. In the method for producing a lithium tantalate single crystal by the Czochralski method, the seed crystal is pulled up while rotating to grow a shoulder portion and a straight body portion following the seed crystal.
A large single crystal growth furnace with an inner diameter of 800 mmφ is used, a metal rut with an inner diameter of 170 mmφ and an inner height of 170 mm is used, and the number of rotations of the seed crystal at the start of crystal growth is 7.5 rpm, and the seed crystal at the end of shoulder growth. It is characterized in that the lithium tantalate single crystal having a straight body diameter of 4 inches φ is grown by setting the rotation speed of the above to 4 rpm or more and 6 rpm or less.

The second invention according to the present invention is
In the method for producing a lithium tantalate single crystal according to the first invention,
The metal crucible is characterized by being an iridium crucible.
The third invention is
In the method for producing a lithium tantalate single crystal according to the first invention or the second invention.
The straight body portion has a length of 150 mm or less.

According to the method for producing a lithium tantalate single crystal according to the present invention.
A large single crystal growth furnace with an inner diameter of 800 mmφ is used, a metal rut with an inner diameter of 170 mmφ and an inner height of 170 mm is used, and the number of rotations of the seed crystal at the start of crystal growth is 7.5 rpm, and the seed crystal at the end of shoulder growth. Since the rotation speed of the above is set to be 4 rpm or more and 6 rpm or less, it is possible to grow a lithium tartrate single crystal having a straight body portion diameter of 4 inches φ in which the generation of lineage is suppressed.

The configuration explanatory drawing which shows an example of the single crystal growth furnace which grows an LT single crystal. Explanatory drawing which shows typically the shoulder part and the straight body part of the LT single crystal grown by the Cz method. An explanatory diagram showing an M-shaped solid-liquid interface shape (a solid-liquid interface shape in which the outer peripheral portion of the crystal is concave with respect to the melt and the central portion of the crystal is convex with respect to the melt) in the process of growing an LT single crystal. _{Reference numeral L 1 in} FIG. 3 represents crystal convexity. In illustration showing the LT single crystal V-shaped solid-liquid interface shape in the process of growing the (solid-liquid interface shape a convex shape only crystal center portion with respect to the melt), sign L ₂ in FIG. 4 crystals convexity Represents. Explanatory drawing schematically showing how the propagation of dislocations introduced into an LT single crystal differs depending on whether the solid-liquid interface shape is concave or convex with respect to the melt. Explanatory drawing schematically showing an example of a solid-liquid interface shape when the number of rotations of a seed crystal is large and forced convection is strong. Explanatory drawing schematically showing an example of a solid-liquid interface shape when the rotation speed of a seed crystal is small and forced convection is weak. The graph which compared the conventional condition (control of the seed crystal rotation speed) and the control of the seed crystal rotation speed from the start of crystal growth to the end of shoulder growth which concerns on this invention. The graph which compared the solid-liquid interface shape at the end of shoulder growth under the conventional conditions, and the solid-liquid interface shape at the end of shoulder growth according to the present invention.

Hereinafter, embodiments according to the present invention will be described in detail with reference to the drawings.

[Overview of single crystal growth furnace and LT single crystal growth method]
First, a configuration example of a single crystal growth furnace by the Czochralski (Cz) method and an outline of the single crystal growth method will be described.

FIG. 1 is a configuration explanatory view schematically showing a schematic configuration of a high-frequency induction heating type single crystal growth furnace 10. In the case of high-frequency induction heating type single crystal growth, an eddy current is generated on the side wall of the metal crucible 12 installed in the work coil 15 by the high-frequency magnetic field formed by the work coil 15, and the eddy current itself causes the crucible 12 itself. Becomes a heating element and forms a temperature environment necessary for melting the raw material filled in the crucible 12 and growing crystals.

As shown in FIG. 1, in the high-frequency induction heating type single crystal growth furnace 10, the crucible 12 is arranged in the chamber 11. The crucible 12 is placed on the crucible stand 13. In the chamber 11, refractory materials 14 and 19 are arranged on the outer periphery and above the crucible 12. The work coil 15 is arranged so as to surround the crucible 12, and an eddy current flows through the wall of the crucible 12 due to the high frequency magnetic field formed by the work coil 15, and the crucible 12 itself becomes a heating element. A seed rod 16 is provided above the chamber 11 so as to be rotatable and vertically movable. A seed holder 17 for holding the seed crystal 1 is attached to the tip of the lower end of the seed rod 16.

In the Cz method, a single crystal piece to be the seed crystal 1 is brought into contact with the melt surface of the single crystal raw material 18 in the rutsubo 12, and the seed crystal 1 is pulled upward while being rotated by the seed rod 16 to pull the seed crystal 1 upward. Grow a cylindrical single crystal in the same orientation as.

The rotation speed and pulling speed of the seed crystal 1 depend on the type of crystal to be grown and the temperature environment at the time of growth, and need to be appropriately selected according to these conditions. In addition, when growing crystals, it is necessary to release the latent heat of solidification generated by the crystallization of the melt at the growth interface upward through the seed crystal, so it is necessary to carry out under a temperature gradient in which the temperature decreases upward from the growth interface. is there. In addition, in order to prevent the shape of the grown crystal from bending or twisting, even in the raw material melt, the horizontal direction from the growth interface toward the rutsubo wall and the vertical direction from the growth interface toward the bottom of the rutsubo. It is necessary to carry out under a temperature gradient where the temperature becomes high.

When growing an LT single crystal, since the melting point of the LT crystal is as high as 1650 ° C., an iridium crucible (iridium crucible) having a melting point of about 2460 ° C. and chemically stable is used. Oxygen is required for the growing atmosphere, but if the oxygen concentration is high, the iridium crucible may be worn out due to oxidation, so it is common to create an inert atmosphere containing a few percent of oxygen. The pulling speed at the time of growing is generally about several mm / H, and the rotation speed is about several rpm. Under such conditions, after growing the crystal to the desired size, the grown crystal is separated from the melt by performing operations such as changing the pulling speed and gradually increasing the melt temperature, and then simply. The crystal growth furnace is slowly cooled by reducing the output at a predetermined rate, and the crystals are taken out from the single crystal growth furnace after the temperature inside the furnace is close to room temperature.

[Solid-liquid interface shape and dislocation propagation]
In crystal growth, dislocations are likely to be introduced into the crystal due to thermal strain caused by a temperature difference between the vicinity of the solid-liquid interface and the inside of the crystal when the melt is solidified. The introduced dislocations have the property of propagating toward a new solid-liquid interface as the crystal growth progresses. It is known that the propagation direction is perpendicular to the solid-liquid interface. FIG. 5 shows how the state of dislocation propagation differs depending on whether the solid-liquid interface shape is concave or convex with respect to the melt. It can be seen that when the solid-liquid interface shape is concave with respect to the melt, the dislocations propagate so as to remain inside the crystal. On the other hand, when the solid-liquid interface shape is convex with respect to the melt, the dislocations propagate toward the outer peripheral side of the crystal. Can be obtained.

[When growing a 4-inch φLT single crystal using a single crystal growth furnace with an inner diameter of 600 mmφ]
When growing an LT single crystal for a 4-inch φLT substrate, which is the current mainstream, an iridium crucible having an inner diameter of 170 mmφ and an inner height of 170 mm is used as described above, and a single crystal growing furnace having an inner diameter of 600 mmφ is used. There is.

The inner diameter of the work coil 15 shown in FIG. 1 is set to 310 mmφ with respect to the iridium crucible 12 having an inner diameter of 170 mmφ, and the input output of the work coil 15 at the end of shoulder growth is set to 15.9 kW. It has not been confirmed that the obtained LT single crystal is polycrystallized during the growth of the straight body after the shoulder growth is completed.

[When growing a 4-inch φLT single crystal using a large single crystal growth furnace with an inner diameter of 800 mmφ]
When using an iridium rut with an inner diameter of 170 mmφ and an inner height of 170 mm and growing an LT single crystal for a 4-inch φLT substrate using a large single crystal growth furnace with an inner diameter of 800 mmφ, a single crystal growth furnace with an inner diameter of 600 mmφ is used. Since the gap space in the chamber 11 is larger than in the case, it is necessary to increase the input output of the work coil 15 to 20.2 kW at the end of shoulder growth, and the obtained LT single crystal has shoulder growth. After the completion, the above-mentioned phenomenon of polycrystallization has been confirmed during the growth of the straight body. It is considered that this phenomenon is because the solid-liquid interface shape at the end of shoulder growth has changed to the “M-shaped solid-liquid interface shape” shown in FIG.

The inner diameter of the work coil 15 is set to 310 mmφ with respect to the iridium crucible 12 having an inner diameter of 170 mmφ, as in the case of using a single crystal growing furnace having an inner diameter of 600 mmφ.

[Adjustment of crystal rotation speed during crystal growth]
Therefore, from the relationship between the solid-liquid interface shape and the propagation direction of dislocations, the present inventor examined the crystal growth conditions so that the solid-liquid interface shape during crystal growth changes from the "M-shaped solid-liquid interface shape" to a convex shape. The problem is solved by setting the crystal rotation speed during crystal growth to a lower rotation speed than the conventional conditions.

Two types of convection are roughly divided in the raw material melt for crystal growth by the Cz method. One is "natural convection" due to the temperature difference in the melt. "Natural convection" rises from the outer periphery of the bottom of the crucible along the wall of the crucible, reaches the surface of the melt, flows toward the center of the surface of the melt, and sinks toward the bottom of the crucible at the center. The other is "forced convection" caused by the rotation of the crystal during growth (rotation of the seed crystal), which is caused by the centrifugal force applied to the melt in contact with the solid-liquid interface that rotates relative to the melt. .. "Forced convection" is a flow that flows from the center of the bottom of the crucible toward the solid-liquid interface of the crystal being grown, and from the solid-liquid interface toward the outer periphery of the crucible. That is, "forced convection" is a flow in the opposite direction to "natural convection". Therefore, the strength of "natural convection" and "forced convection" strongly affects the melt convection as a whole near the solid-liquid interface.

6 and 7 show how the convex shape of the solid-liquid interface changes depending on whether the number of rotations of the crystal is high or low during crystal growth, respectively. When the number of rotations of the crystal is high, "forced convection" is also strong, and the melt flowing from the center of the crucible bottom toward the solid-liquid interface changes the temperature distribution near the solid-liquid interface to a flat state, and the solid-liquid interface becomes flat. It has a flat shape. In some cases, the melt may have a concave shape (see "M-shaped solid-liquid interface shape" shown in FIG. 3). On the contrary, when the number of rotations of the crystal is small, the "forced convection" also weakens, and the "natural convection" that sinks from the outer periphery of the crucible toward the center and toward the bottom of the crucible becomes predominant. The temperature distribution is along the "natural convection", and the solid-liquid interface easily forms a convex shape (see "V-shaped solid-liquid interface shape" shown in FIG. 4) with respect to the melt.

As an example, in the growth of a large single crystal growth furnace having an inner diameter of 800 mmφ and a 4-inch φLT single crystal using a crucible having an inner diameter of 170 mmφ and an inner height of 170 mm, a crystal growth example when the crystal rotation speed is set to a low speed condition. Is shown. FIG. 8 shows a comparison in controlling the crystal rotation speed from the start of crystal growth to the end of shoulder growth under the conventional conditions and the low-speed conditions according to the present invention. Under low-speed conditions, improvement in crystal convexity is confirmed by setting the crystal rotation speed at the start of crystal growth to 7.5 rpm and the crystal rotation speed at the end of shoulder growth to 4 rpm or more and 6 rpm or less. However, if the crystal rotation speed at the end of shoulder growth is less than 4 rpm, asymmetry of crystal growth will be observed, and a crystal having a distorted shape may be obtained. On the other hand, when the crystal rotation speed exceeds 6 rpm, the desired crystal convexity cannot be obtained, so that the crystallinity cannot be improved. FIG. 9 shows a comparison of shape data in the crystal convex portion when the crystal growth is performed until the shoulder growth is completed under the conventional condition and the low speed condition. The crystal convexity L ₁ (see FIG. 3) under the conventional conditions is about 10 mm, but the crystal convexity L ₂ (see FIG. 4) is extended to about 15 mm under the low speed condition, and the effect of the present invention is effective. I was able to confirm that it was appearing.

Hereinafter, examples of the present invention will be specifically described with reference to comparative examples.

[Example 1]
An iridium crucible with an inner diameter of 170 mmφ and an inner height of 170 mm is filled with 20.6 kg of pre-mixed and calcined LT raw material powder, and the circumference of the iridium crucible is covered with a refractory material. The raw material powder was heated and melted by heating the crucible using a single crystal growing furnace to obtain an LT raw material melt.

The height of the raw material melt at the start of crystal growth is set to 158 mm, which corresponds to 93% of the height inside the rutsubo, the rotation speed of the crystal (seed crystal) at the start of crystal growth is 7.5 rpm, and shoulder growth is performed. The rotation speed of the crystal (seed crystal) at the end was set to 4.0 rpm.

Then, crystal growth was carried out 11 times under the above conditions, and 10 LT single crystals having a straight body portion having a diameter of 4 inches φ and a straight body portion having a length of 150 mm could be obtained.

The inner diameter of the work coil is set to 310 mmφ with respect to the iridium crucible having an inner diameter of 170 mmφ, and the input output of the work coil at the end of shoulder growth is set to 20.2 kW.

[Example 2]
Under the same conditions as in Example 1 except that the rotation speed of the crystal (seed crystal) at the start of crystal growth was 7.5 rpm and the rotation speed of the crystal (seed crystal) at the end of shoulder growth was 6.0 rpm. Crystal growth was performed.

Crystal growth was carried out 10 times under the above conditions, and 10 LT single crystals having a straight body portion having a diameter of 4 inches φ and a straight body portion having a length of 150 mm could be obtained.

[Comparative Example 1]
Under the same conditions as in Example 1 except that the rotation speed of the crystal (seed crystal) at the start of crystal growth was 15.0 rpm and the rotation speed of the crystal (seed crystal) at the end of shoulder growth was 8.5 rpm. Crystal growth was performed.

When crystal growth was carried out 19 times under the above conditions, 6 LT single crystals having a straight body diameter of 4 inches φ and a straight body length of 120 mm could be obtained, but the remaining 13 crystals were polycrystals. Met.

[Comparative Example 2]
Under the same conditions as in Example 1 except that the rotation speed of the crystal (seed crystal) at the start of crystal growth was 15.0 rpm and the rotation speed of the crystal (seed crystal) at the end of shoulder growth was 4.0 rpm. Crystal growth was performed.

When crystal growth was carried out 5 times under the above conditions, all polycrystals were obtained.

According to the method of the present invention, even when a large single crystal growing furnace having an inner diameter of 800 mmφ is used, it is possible to grow a lithium tantalate single crystal having a straight body diameter of 4 inches φ in which generation of lineage is suppressed. Has industrial applicability applied to substrates.

1 Seed crystal 10 Single crystal growth furnace 11 Chamber 12 Crucible 13 Crucible stand 14, 19 Refractory 15 Work coil 16 Seed rod 17 Seed holder 18 Single crystal raw material

Claims (3)

A metal crucible arranged inside a single crystal growing furnace having a cylindrical structure is filled with a lithium tartrate raw material powder, and the seed crystal is brought into contact with a lithium tantalate raw material melt obtained by heating the metal crucible. In the method for producing a lithium tantalate single crystal by the Czochralski method, the seed crystal is pulled up while rotating to grow a shoulder portion and a straight body portion following the seed crystal.
A large single crystal growth furnace with an inner diameter of 800 mmφ is used, a metal rut with an inner diameter of 170 mmφ and an inner height of 170 mm is used, and the number of rotations of the seed crystal at the start of crystal growth is 7.5 rpm, and the seed crystal at the end of shoulder growth. A method for producing a lithium tantalate single crystal, which comprises growing a lithium tantalate single crystal having a straight body diameter of 4 inches φ by setting the number of rotations of the above to 4 rpm or more and 6 rpm or less. The method for producing a lithium tantalate single crystal according to claim 1, wherein the metal crucible is an iridium crucible. The method for producing a lithium tantalate single crystal according to claim 1 or 2, wherein the length of the straight body portion is 150 mm or less.

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