JP2017114725A - Method for raising lithium tantalate single crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for raising a lithium tantalate (LT) single crystal that improves productivity by suppressing cracks liable to occur when raising a large-sized crystal having a crystal diameter of 4 inches or more.SOLUTION: A crucible 3 and a doughnut-plate type reflector 9 made of a noble metal at the top edge of the crucible are installed. A seed crystal is brought into contact with raw material melt in the crucible melted by high-frequency heating and drawn up while being rotated to raise a LT crystal 11 having a crystal diameter of 4 inches or more. An outside diameter (a) of the reflector and an outside diameter (b) of the crucible are set to satisfy the relation: 0 (mm)≤a-b≤5 (mm). By this setting, a gap hardly occurs between the crucible and the reflector and thus a sharp-angle part to cause intensive heating is prevented from being exposed. Heating of a trunk part of the crystal by the reflector is thus alleviated to prevent cracks.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for growing a lithium tantalate single crystal used for a surface acoustic wave device or the like, and in particular, suppresses cracks that are likely to occur when growing a large crystal having a crystal diameter of 4 inches or more, thereby improving its productivity. The present invention relates to a method for growing a lithium tantalate single crystal.

  A lithium tantalate (hereinafter sometimes abbreviated as LT) crystal is a ferroelectric having a melting point of about 1650 ° C. and a Curie temperature of about 600 ° C., and uses of the lithium tantalate substrate manufactured using this crystal Is a surface acoustic wave (SAW) filter material mainly used for transmitting and receiving devices of mobile phones.

  The lithium tantalate single crystal is generally grown by the “Czochralski method”. The growth method using the “Czochralski method” will be described below. The growth furnace used in this method is a crucible made of noble metal such as iridium, a refractory (heat insulating material) provided so as to surround the crucible, and the upper end of the crucible. A donut plate-shaped noble metal reflector installed on the reflector and a cylindrical after-heater provided on the reflector and having a heater function for properly maintaining the temperature gradient at the top of the crucible. After melting a mass of lithium tantalate crystal, which is a raw material filled in, by high-frequency induction heating, bringing the seed crystal into contact with this raw material melt, tantalum is pulled up while rotating the seed crystal at a predetermined rotational speed. A lithium acid single crystal has been grown (for example, see Patent Documents 1 and 2).

  By the way, the direction of the crystal pulling axis is the same as the orientation of the seed crystal cut in the direction of the Y-axis (hereinafter referred to as the off angle) rotated by 36 ° to 50 ° from the relationship between the propagation speed and propagation loss of surface acoustic waves. Is set to be When the crystal is grown in this way, cracks are likely to occur in the pulled single crystal when the off-angle increases, and in particular, cracks become prominent when growing large crystals with a crystal diameter of 4 inches or more, and productivity is increased. There was a problem of lowering.

Japanese Patent Laid-Open No. 10-36193 JP 09-20584 A

  The present invention has been made paying attention to such problems, and the problem is that a method for growing lithium tantalate in which generation of cracks is suppressed even when a large crystal having a crystal diameter of 4 inches or more is grown. Is to provide.

  Therefore, when the present inventors diligently investigated the cause of the occurrence of cracks in order to solve the above problems, when the straight body portion of the lithium tantalate single crystal was grown, it was positioned near the opening edge of the noble metal reflector. As a result, a large compressive stress is generated on the side of the straight body of the crystal, and as a result of dislocations occurring inside the straight body of the crystal due to the compressive stress, it is determined that this accumulation of dislocations is the origin of the crack. It came to.

  In other words, the donut-plate-shaped noble metal reflector placed at the top of the crucible suppresses rapid crystal growth at the time of crystal seeding and crystal shoulder growth, which is the pre-process of straight body growth. Therefore, it acts so as to have an appropriate temperature gradient in the radial direction of the melt surface. However, when growing the straight body of the lithium tantalate single crystal, the opening edge of the noble metal reflector is heated due to the action of high frequency, so that the crystal straight line located near the opening edge of the reflector As a result of heating the side surface of the body part by the reflector, the temperature of the outer peripheral part of the side surface becomes higher than the temperature of the center part in the crystal body part, so that the compressive stress is generated on the crystal side surface.

  Therefore, the present inventors have intensively studied under the expectation that the crack is suppressed by optimizing the shape of the reflector and reducing the compressive stress generated on the crystal side face, and as a result, the present invention has been completed. It was.

That is, the present invention
A crucible and a doughnut-shaped noble metal reflector installed at the upper end of the crucible, the seed crystal is brought into contact with the raw material melt in the crucible melted by high frequency induction heating, and the seed crystal is rotated upward In a method for growing a lithium tantalate single crystal having a crystal diameter of 4 inches or more by pulling up to
The amount of protrusion from the outer peripheral edge of the outer peripheral portion of the noble metal reflector is set to 0 mm or more and 2.5 mm or less,
In other words, the relationship between the outer diameter of the doughnut-shaped noble metal reflector and the outer diameter of the crucible,
When the outer diameter of the reflector is a (mm) and the outer diameter of the crucible is b (mm),
It is set to satisfy the condition of 0 (mm) ≦ a−b ≦ 5 (mm).

According to the method for growing a lithium tantalate single crystal according to the present invention,
When the outer diameter of the reflector is a (mm) and the outer diameter of the crucible is b (mm),
It is set to satisfy the condition of 0 (mm) ≦ ab−5 (mm),
By eliminating or reducing the amount of protrusion of the outer periphery of the noble metal reflector from the outer periphery of the crucible, the concentration of heating at the “sharp corner” due to the edge effect, which is one of the characteristics of high-frequency induction heating, is reduced. Temperature rise can be suppressed.

  As a result, side heating of the straight body of the crystal by the noble metal reflector is suppressed, and the difference between the temperature of the central portion and the temperature of the outer peripheral portion of the straight body is reduced. As a result, the side surface of the lithium tantalate single crystal straight body Therefore, it is possible to suppress cracks generated in the lithium tantalate single crystal.

1 is a schematic cross-sectional view showing an example of a growth apparatus used in a method for growing a lithium tantalate single crystal.

  Embodiments of the present invention will be described below.

  The method for growing a lithium tantalate single crystal according to the present invention is carried out in accordance with an ordinary method of the “Czochralski method” in which a seed crystal is brought into contact with a raw material melt contained in a crucible and is made of iridium or the like. The donut plate-shaped noble metal reflector installed at the upper end of the noble metal crucible is characterized in that the amount of protrusion of the outer peripheral portion from the outer periphery of the crucible is set to 0 mm or more and 2.5 mm or less. That is, when the outer diameter of the reflector is a (mm) and the outer diameter of the crucible is b (mm), the reflector is set to satisfy the condition of 0 (mm) ≦ ab−5 (mm). It is what.

  When the outer diameter “a” of the doughnut-shaped noble metal reflector is set smaller than the outer diameter “b” of the crucible (that is, a <b), even if the wall thickness of the crucible is taken into consideration, Due to the deformation, when the reflector is installed at the upper end of the crucible, a gap is formed in a part between the inner peripheral edge of the crucible and the outer peripheral edge of the reflector. In this gap portion, the outer peripheral edge of the reflector and the inner peripheral edge of the crucible become the “acute angle part”, and high-frequency induction heating is concentrated and the symmetry of the heat generation state is lost, so the convection of the melt in the crucible becomes complicated. In this case, since it becomes difficult to maintain a slightly convex solid-liquid interface shape important for crystal growth, the risk of polycrystallization increases.

  Moreover, the case where the outer diameter a of the reflector is equal to the outer diameter b of the crucible (a = b) is most ideal, but as described above, since there is a concern about the generation of a gap due to the deformation of the crucible, the outer diameter of the reflector is increased. May have some margin. In this case, the amount of protrusion from the outer peripheral edge of the outer peripheral portion of the reflector is 2.5 mm or less. If it exceeds 2.5 mm, the temperature rise at the reflector outer periphery due to high-frequency induction heating becomes too large, and heating of the crystal straight body side surface during growth due to heat transfer increases, resulting in excessive stress on the crystal side surface.

  Examples of the present invention will be specifically described below with reference to comparative examples, but the present invention is not limited to the following examples. In the examples and the like, the following growth apparatus shown in FIG. 1 was used as an apparatus for growing a lithium tantalate single crystal.

  That is, this growing apparatus surrounds the crucible 3 as shown in FIG. 1, the ceramic crucible base 2, the alumina base 4 installed at the bottom of the crucible base 2 and on which the iridium crucible 3 is disposed. A heat insulating material 5 installed in this manner, a donut-shaped iridium reflector 9 attached along the open edge of the crucible 3, and a cylindrical iridium afterheater 13 attached on the reflector 9 A lid 7 provided on the upper open edge of the after-heater 13 and provided with an opening for passing the lifting shaft 6 in the center; and a lower side of the lifting shaft 6 and holding the seed crystal 8 And a heating coil (high frequency coil) which is provided so as to surround the crucible base 2 and melts the raw material filled in the crucible 3 by induction heating. Le) 1 and its has the main part is composed of, in which a single crystal is grown 11 by pulling upward while rotating the seed crystal 8 with contacting a seed crystal 8 in the melt 10 of the material.

[Example 1]
On the open edge of an iridium crucible 3 having an outer diameter (ie, outer diameter) of 175 mmφ, an inner dimension height of 170 mm, and a wall thickness of 2.5 mm, a donut plate shape having an inner diameter of 140 mmφ, an outer diameter of 175 mmφ, and a thickness of 1.5 mm An iridium reflector 9 was attached, and a cylindrical iridium afterheater 13 having a diameter of 147 mmφ and a height of 150 mm (thickness 1 mm) was attached on the reflector 9.

  Then, the crucible 3 is filled with a confluent composition, that is, LT calcined powder prepared by harmonic melting (coincidence melting) that can easily obtain crystals having a uniform composition without composition deviation (composition fluctuation), It was melted at about 1700 ° C. Thereafter, the melt temperature was adjusted to the vicinity of the melting point (1650 ° C.) of LT, and the seed crystal was brought into contact with the melt surface while rotating at 10 rpm. At this time, the orientation of the seed crystal 8 was 42 ° rotation Y axis, and the single crystal having a crystal diameter of 105 mm and a straight body length of 90 mm was pulled up.

  In addition, in the reflector 9 which concerns on Example 1, the protrusion amount from the crucible 3 outer periphery of the outer peripheral part is 0 mm, ie, the outer diameter of the reflector 9 is a (mm), and the outer diameter of the crucible 3 is b (mm). In this case, (a−b) = 175 mm−175 mm = 0 (mm).

  And when the crystal pulling on the conditions which concern on Example 1 was implemented 10 times, the LT single crystal without a crack was obtained in all implementation.

[Example 2]
The single crystal was pulled up under the same conditions as in Example 1 except that the outer diameter of the iridium reflector 9 having a donut shape was changed to 177 mmφ.

  In the reflector 9 according to Example 2, the amount of protrusion of the outer peripheral portion from the outer peripheral edge of the crucible 3 is 1 mm, that is, the outer diameter of the reflector 9 is a (mm) and the outer diameter of the crucible 3 is b (mm). In this case, (a−b) = 177 mm−175 mm = 2 (mm).

  Then, when crystal pulling was performed 10 times under these conditions, an LT single crystal without cracks was obtained in 8 times.

[Example 3]
The single crystal was pulled up under the same conditions as in Example 1 except that the outer diameter of the iridium reflector 9 having a donut shape was changed to 179 mmφ.

  In addition, in the reflector 9 which concerns on Example 3, the protrusion amount from the crucible 3 outer periphery of the outer peripheral part is 2 mm, ie, the outer diameter of the reflector 9 is a (mm), and the outer diameter of the crucible 3 is b (mm). In this case, (a−b) = 179 mm−175 mm = 4 (mm).

  Then, when crystal pulling was performed 10 times under these conditions, an LT single crystal without cracks was obtained in 8 times.

[Example 4]
The single crystal was pulled up under the same conditions as in Example 1 except that the outer diameter of the iridium reflector 9 having a donut shape was changed to 180 mmφ.

  In the reflector 9 according to Example 4, the amount of protrusion of the outer peripheral portion from the outer periphery of the crucible 3 is 2.5 mm, that is, the outer diameter of the reflector 9 is a (mm), and the outer diameter of the crucible 3 is b ( mm), (a−b) = 180 mm−175 mm = 5 (mm).

  Then, when crystal pulling was performed 10 times under these conditions, an LT single crystal without cracks was obtained in 8 times.

[Comparative Example 1]
The single crystal was pulled up under the same conditions as in Example 1 except that the outer diameter of the iridium reflector 9 having a donut shape was changed to 181 mmφ.

  In addition, in the reflector 9 which concerns on the comparative example 1, the protrusion amount from the crucible 3 outer periphery of the outer peripheral part is 3 mm, ie, the outer diameter of the reflector 9 is a (mm), and the outer diameter of the crucible 3 is b (mm). In this case, (a−b) = 181 mm−175 mm = 6 (mm).

  Then, when crystal pulling was performed 10 times under these conditions, it was possible to obtain an LT single crystal without cracks 5 times.

[Comparative Example 2]
The single crystal was pulled up under the same conditions as in Example 1 except that the outer diameter of the iridium reflector 9 having a donut shape was changed to 174 mmφ.

In this comparative example 2, when the outer diameter of the reflector 9 is smaller than the outer diameter of the crucible 3, the outer diameter of the reflector 9 is a (mm), and the outer diameter of the crucible 3 is b (mm).
(Ab) = 174 mm-175 mm = -1 (mm).

  Since the crucible has a thickness of 2.5 mm, although the crucible was gradually deformed by repeated use, there was no gap between the crucible and the reflector until the seventh crystal pulling, and there was no crack. Crystals could be obtained. However, since a gap was generated between the crucible and the reflector thereafter, three pulling tests were performed thereafter, and polycrystallization was observed in a range of about 40 mm from the bottom of the crystal.

"Confirmation"
From the above results, it is confirmed that according to Examples 1 to 4, a lithium tantalate single crystal without cracks can be produced in a high yield.

  According to the present invention, since the crack of the lithium tantalate single crystal is suppressed and the number of wafers that can be obtained can be greatly increased, the lithium tantalate single crystal used for the surface acoustic wave device or the like can be grown. Has industrial applicability.

1 Heating coil (high frequency coil)
2 Ceramic crucible base 3 Crucible 4 Alumina base 5 Heat insulating material 6 Lifting shaft 7 Lid 8 Seed crystal 9 Reflector 10 Melt 11 Single crystal 12 Seed crystal holding jig 13 After heater

Claims (1)

A crucible and a doughnut-shaped noble metal reflector installed at the upper end of the crucible, the seed crystal is brought into contact with the raw material melt in the crucible melted by high frequency induction heating, and the seed crystal is rotated upward In a method for growing a lithium tantalate single crystal having a crystal diameter of 4 inches or more by pulling up to
When the outer diameter of the reflector is a (mm) and the outer diameter of the crucible is b (mm),
A method for producing a lithium tantalate single crystal, which is set so as to satisfy a condition of 0 (mm) ≦ ab ≦ 5 (mm).

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