JP2018145078A - Method for growing lithium tantalate single crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for growing a lithium tantalate (LT) single crystal by the Czochralski method, capable of simply and efficiently obtaining a RY cut substrate formed by rotating the Y-axis of an orthogonal coordinate system by α° (36≤α≤46) in the clockwise direction (clockwise rotation) around the X-axis.SOLUTION: The LT single crystal is grown using a seed crystal having a plane direction obtained by rotating the Y-axis by α° (36≤α≤46) 4°-±0.5° in the clockwise direction (clockwise rotation) around the X-axis, rotating the X-axis by β° (2°±0.5°) in the clockwise direction (clockwise rotation) around the Z-axis when rotating a seed crystal in the clockwise direction (the rotational direction of the seed crystal is clockwise when observing the growth state of the LT single crystal from right above) to grow it and rotating the X-axis by β°(2°±0.5°) in the counterclockwise direction (counterclockwise rotation) around the Z-axis when rotating the seed crystal in the counterclockwise direction (the rotational direction of the seed crystal is counterclockwise when observing the growth state of the LT single crystal from right above) to grow it.SELECTED DRAWING: Figure 8

Description

  The present invention relates to a method for growing a lithium tantalate single crystal by the Czochralski method (hereinafter sometimes abbreviated as CZ method), and in particular, a desired RY cut (Rotated) from the grown lithium tantalate single crystal. Y cut) relates to a method for growing a lithium tantalate single crystal from which a substrate can be efficiently obtained.

  Lithium tantalate (hereinafter sometimes abbreviated as LT) single crystals have piezoelectricity, and there is a great demand for SAW (Surface Acoustic Wave) devices using piezoelectricity, and the market has shown explosive growth. ing. In particular, mobile communication applications represented by mobile phones are large.

  An LT single crystal that is the basis of a SAW device is often grown by a pulling method called a CZ method. A device substrate is cut out from the LT single crystal, polished, electrodes are formed, cut into chips, and the SAW device is completed through packaging. Unless otherwise specified, the crystal in this specification means a single crystal.

  An outline of the single crystal growth method by the CZ method is shown in FIG. In order to hold the high melting point raw material melt 6, the noble metal crucible 4 is often made of iridium, and there are many induction heating types using the work coil 8. First, the raw material is melted, the surface of the raw material melt is adjusted to the melting point, and the seed crystal 3 is brought into contact with the surface of the raw material melt 6 (referred to as seeding). Then, when the raw material melt is supercooled and the seed crystal 3 is pulled upward, crystal growth starts from the seed crystal 3. The seed crystal 3 is fixed to the seed crystal support rod 5 with a seed crystal fixing pin 7 or the like, but the method is not limited as long as the seed crystal 3 can be fixed. There is also a case where it is simply fixed with a precious metal wire.

  The seed crystal 3 is rotated to promote crystal growth. The seed crystal support rod 5 is often suspended by an upper load cell (not shown), so that the weight of the crystal can be known, the diameter is calculated from the weight, the temperature of the melt is adjusted, and the desired diameter is obtained. And a columnar crystal is grown.

  The grown crystal is subjected to an annealing process for removing strain and a single polarization process, and then delivered to the processing step. In processing, it is often processed into a cylinder slightly thicker than the final product by cylindrical grinding and cut into a substrate by a multi-wire saw. The cut substrate is finally finished to a product diameter by peripheral beveling and chamfered. The LT substrate has a diameter of 4 to 6 inches.

  By the way, as for the requirement of the SAW device for the grown crystal, the crystal orientation is an important requirement item as well as the crystal quality. In the case of an LT single crystal, as shown in FIG. 2, the a-axis of the crystal is called the X-axis, the c-axis is called the Z-axis, and the axis perpendicular to the X-axis and the Z-axis is called the Y-axis. 13 plane orientation, that is, the direction in which the perpendicular from the surface of the substrate 13 faces.

  In general, as shown in FIG. 3, the plane orientation of the LT substrate suitable for the SAW device is such that the surface of the substrate 13 is parallel to the X axis and the Y axis of the Cartesian coordinate system is clockwise around the X axis (clockwise). Rotation) is a range rotated by α ° (36 ° ≦ α ° ≦ 46 °). This substrate 13 is generally called a RY cut (Rotated Y cut) substrate. For example, when the rotation angle is 42 °, the substrate 13 is called a 42 ° RY cut substrate and expressed as 42 ° RY.

  3 and 4, the rotation angle 15 indicated by α ° is called the RY direction, and the direction perpendicular to the RY direction, that is, the parallel difference between the surface of the substrate 13 (wafer 13 in FIG. 4) and the X axis is X. Sometimes referred to as a deviation in direction.

  In this way, LT single crystals are required to have various substrates. For example, it is not uncommon to aim at subtle characteristic differences by making them in increments of 0.5 °, for example. Therefore, in order to efficiently obtain substrates for SAW devices having various orientations, it is desirable to accurately pull up the crystals in a desired plane orientation at the stage of crystal growth. Extremely speaking, if a 42 ° RY cut substrate is produced from an LT single crystal grown at 36 ° RY, a corresponding processing loss will occur. For this reason, the crystal is conventionally pulled up to a desired plane orientation.

  FIG. 4 is an explanatory view showing the orientation of an LT or LN (lithium niobate) single crystal. For example, when a RY cut substrate of 42 ° RY is desired, the crystal growth axis 14 on which the crystal 10 is pulled up and grown. The angle α ° (ie, the rotation angle 15) formed by the Y axis and the Y axis is 42 °.

  Since the CZ method is a method of growing a single crystal following the orientation of the seed crystal 16, the seed crystal 16 is also required for each orientation corresponding to the desired plane orientation of the RY cut substrate.

  The seed crystal is also produced from the grown crystal. However, since the seed crystal is a consumable product and in many cases has a lifetime after one growth, it is parallel to the crystal growth axis 14 as shown in FIG. It is preferable in terms of efficiency to cut out the seed crystal 16. In FIG. 5, reference numeral 16a denotes a crystal top side, reference numeral 16b denotes a crystal bottom side, and reference numeral 16c denotes a fixing hole.

JP 2013-001581 A

  By the way, when a 36-46 ° RY LT single crystal is grown by the CZ method, the crystal 10 grown as shown in FIG. 6 is bent, and the following problems exist due to this bending.

  For example, when cylindrically grinding a crystal 10 with a bend, considering the yield, the crystal 10 is processed as closely as possible to the shape of the crystal 10 rather than being processed along the pulling direction of FIG. It is desirable to process the central shaft 100). However, when the two-dot chain line in FIG. 6 is processed into the central axis 100, as a result, the angle formed between the side surface of the cylindrical grinding and the orientation of the seed crystal 16, that is, the desired growth orientation is not 90 °. The amount of bending is a level of 1 to 5 degrees in several degrees. In this state (that is, a state in which the two-dot chain line in FIG. 6 is cylindrically ground on the central axis 100), the substrate obtained by cutting the crystal 10 with a multi-wire saw so that the orientation of the substrate becomes a desired orientation is an ellipse. Become. That is, if the part shown with the dashed-dotted line of FIG. 6 is cut | disconnected, a board | substrate (it is not a desired azimuth | direction) will become circular (because the said cylindrical grinding process is made by making the two-dot chain line of FIG. 6 into the central axis 100). However, if a portion indicated by a broken line in FIG. 6 is cut with a multi-wire saw so that the orientation of the substrate becomes a desired orientation, the substrate becomes an ellipse.

  However, in order to make an elliptical substrate into a circle by beveling processing, the load in the beveling processing is increased, the processing time is lengthened, and defects such as cracks and chips are induced in the substrate. In addition, the amount of orientation adjustment on the multi-wire saw machine is increased, and the cutting width by the wire changes depending on the crystal position, which causes an increase in accuracy, particularly warpage.

  The above-mentioned bending of crystal growth originates from the fact that the orientation of 36 to 46 ° RY is away from the (012) plane called the so-called facet plane, but it is influenced by the temperature gradient due to the thermal insulation material configuration of the growth furnace. The amount of bending itself should vary slightly depending on the heat insulating material composition and furnace. In general, the temperature gradient of the furnace often counters the crystal that is about to bend, but by reversing the rotation direction of the crystal (see Patent Document 1), an attempt to obtain a crystal that is not bent as much as possible. It has also been made.

  In addition, when the above-mentioned bending of crystal growth is decomposed into the RY direction and the X-axis direction that is the orthogonal direction, bending is also confirmed in the X-axis direction of the ridge that is perpendicular to the (012) plane and is not affected by the growth interface. The cause was unknown.

  The present invention has been made paying attention to such problems, and the problem is to solve the problem caused by the bending of the grown crystal and to obtain a desired RY cut substrate efficiently. The object is to provide a method for growing a lithium acid single crystal.

  In the present invention, the seed crystal has an off-angle in advance with respect to a desired growth orientation, and the off-angle is managed in two directions of the RY direction and its orthogonal direction, and the above-mentioned seed having an off-angle managed in two directions. It is characterized by growing a lithium tantalate single crystal using the crystal.

That is, the present invention
A method for growing a lithium tantalate single crystal by the Czochralski method of bringing a seed crystal into contact with a raw material melt and pulling the seed crystal while rotating the seed crystal, wherein the a-axis of the lithium tantalate single crystal is The Y axis of the Cartesian coordinate system with the X axis, the c axis as the Z axis, and the axis orthogonal to the X axis and the Z axis as the Y axis is α ° clockwise (clockwise) around the X axis (36 ° ≦ α ° ≦ 46 °) In a method for growing a rotated lithium tantalate single crystal for an RY cut substrate,
The Y axis is rotated clockwise around the X axis by α ° (36 ° ≦ α ° ≦ 46 °) −4 ° ± 0.5 °, and the seed crystal is rotated clockwise (lithium tantalate) When the growth state of the single crystal is observed when viewed from directly above, the rotation direction of the seed crystal is clockwise.) When the crystal is grown by rotating the X axis clockwise around the Z axis (clockwise), β Rotate ° (2 ° ± 0.5 °) and rotate the seed crystal counterclockwise (the rotation direction of the seed crystal when the growth state of the lithium tantalate single crystal is observed from directly above is counterclockwise) In the case of growing, the lithium tantalate single crystal using a seed crystal having a plane orientation obtained by rotating the X-axis counterclockwise (counterclockwise) around the Z-axis by β ° (2 ° ± 0.5 °) It is characterized by nurturing.

  When a 36 to 46 ° RY LT single crystal obtained by using the growth method according to the present invention is subjected to cylindrical grinding following the crystal shape, the side surface of the cylindrical grinding is formed between the side surface and a desired growth orientation. The ideal shape is adjusted so that the angle is approximately 90 °. That is, it is possible to obtain an LT single crystal whose growth direction is parallel to a desired growth orientation.

  For this reason, a desired RY cut substrate can be obtained easily and efficiently from the grown 36-46 ° RY LT single crystal.

Explanatory drawing which shows the growth method of the single crystal by CZ method. The schematic perspective view which concerns on a basic Y cut board | substrate. The schematic perspective view which concerns on a basic RY cut board | substrate. Explanatory drawing which shows the orientation of LT and LN single crystal suitable for a SAW device. Explanatory drawing which shows the seed crystal cut out from the grown single crystal. Explanatory drawing which shows the curvature of the crystal | crystallization seen when the LT single crystal of 36-46 degrees RY is grown by CZ method. FIG. 7 (A) shows the growth state of the LT single crystal from directly above, showing the RY direction and X direction bending of the crystal observed when a 36-46 ° RY LT single crystal is grown by the CZ method. 7B is an explanatory diagram showing the bending of the LT single crystal in the Z direction and the X direction when the seed crystal is grown clockwise (clockwise), and FIG. Explanatory drawing which shows the bending of the Z direction of an LT single crystal at the time of growing by rotating a seed crystal counterclockwise (counterclockwise) when observing from. FIG. 8A is a schematic perspective view showing the plane orientation of the seed crystal obtained by rotating the Y-axis clockwise around the X-axis by α ° −4 ° ± 0.5 °, and FIG. 8B. Is a side view of FIG. 8 (A), and FIG. 8 (C) is a case where the Y axis is rotated clockwise around the X axis (clockwise) by α ° −4 ° ± 0.5 ° and the X axis is Z The schematic perspective view which shows the surface orientation of the seed crystal based on this invention rotated by (beta) degrees counterclockwise (counterclockwise) around the axis | shaft.

  Embodiments according to the present invention will be described below in detail with reference to the drawings.

(1) Bending of crystal observed when LT single crystal is grown by CZ method FIG. 7 shows Z direction and X direction of crystal observed when LT single crystal of 36-46 ° RY is grown by CZ method. FIG. 7A shows the bending in the RY direction and the X direction of the LT single crystal when grown by rotating clockwise (clockwise) when the seed crystal is observed from directly above. FIG. 7B shows the bending in the RY direction and the X direction of the LT single crystal when the seed crystal is grown by rotating counterclockwise (counterclockwise) when observed from directly above.

  7A and 7B, the plane of the paper is perpendicular to the growth direction.

(1-1) Bending in the RY direction The bending in the RY direction is simply caused by the force to grow in the 33 ° RY direction, which is the growth interface, as described above. In the heat insulating material configuration, the RY-direction bends 20 and 20 ′ were about 4 ° with respect to the seed crystal.

(1-2) X-direction bending The X-direction bending, whose cause was unknown, was about 2 ° in the X-direction bendings 21 and 21 ′ with respect to the seed crystal. Furthermore, as a result of intensive studies by the present inventors, the X-direction bends 21 and 21 'are phenomena determined by the force to grow in the 33 ° RY direction, which is the growth interface, and the direction in which the seed crystal rotates. As a result, it was confirmed that the direction of the crystal growth curve is reversed depending on the rotation direction of the seed crystal. 7A shows a case where the seed crystal is grown by rotating clockwise (clockwise) when the seed crystal is observed from directly above, and FIG. 7B is counterclockwise (counterclockwise) when the seed crystal is observed from directly above. The direction of the bend in the case of growing by rotating in a clockwise direction is shown.

(1-3) Rotation speed of seed crystal When the rotation speed of the seed crystal was changed in a common sense range (1 to 15 RPM) and the growth curve was examined, it was confirmed that the amount did not change. Since there is a temperature gradient due to the heat insulating material, the maximum value of bending is considered to be regulated to some extent. In the case of extreme bending, cylindrical grinding and cutting for obtaining a substrate cannot be performed. Therefore, in a general growth furnace, it is assumed that the amount of growth bending is naturally equal. Of course, there are some differences, and it goes without saying that fine adjustment is required.

(2) Method for Growing LT Single Crystal According to the Present Invention Since the inventor has specified the amount and direction of bending of the grown crystal, it is possible to solve the above-described problems caused by the bent crystal.

That is, the growing method of the present invention by the CZ method is:
It is premised on a method of growing a lithium tantalate single crystal for an RY cut substrate in which the Y axis of the Cartesian coordinate system is rotated α ° (36 ° ≦ α ° ≦ 46 °) clockwise around the X axis. age,
The Y axis is rotated clockwise around the X axis by α ° (36 ° ≦ α ° ≦ 46 °) −4 ° ± 0.5 °, and the seed crystal is rotated clockwise (lithium tantalate) When the growth state of the single crystal is observed when viewed from directly above, the rotation direction of the seed crystal is clockwise.) When the crystal is grown by rotating the X axis clockwise around the Z axis (clockwise), β Rotate ° (2 ° ± 0.5 °) and rotate the seed crystal counterclockwise (the rotation direction of the seed crystal when the growth state of the lithium tantalate single crystal is observed from directly above is counterclockwise) In the case of growing, the lithium tantalate single crystal using a seed crystal having a plane orientation obtained by rotating the X-axis counterclockwise (counterclockwise) around the Z-axis by β ° (2 ° ± 0.5 °) It is characterized by nurturing.

  For example, if a 42 ° RY crystal is desired, the orientation of the seed crystal is 42 ° -4 ° ± 0.5 ° = 38 ° ± 0.5 ° in the RY direction and 2 ° ± 0.5 ° in the X direction. It is sufficient to prepare a seed crystal with an off-angle given in advance.

  In the present invention, there is no change in the bending of the crystal, but the direction of bending of the crystal is adjusted. In other words, it may be directed to turn toward a desired direction.

  And there are many cases where the bending in the RY direction is generally known. However, by understanding the relationship between the behavior in the X direction and the rotation direction of the seed crystal, it is possible to easily and efficiently manufacture a desired RY cut substrate by managing the off angle related to the seed crystal. Is the greatest feature of the growing method according to the present invention.

  Examples of the present invention will be specifically described below with reference to comparative examples.

[Example 1]
An LT single crystal having a diameter of 4 inches and 36 ° RY was grown by the Czochralski method. The crystal length is about 80 mm.

  The orientation of the applied seed crystal was preliminarily provided with an off angle of 32 ° RY in the RY direction and 2 ° in the X direction. The direction of rotation of the seed crystal was clockwise and the number of rotations was 5 RPM.

  The obtained LT single crystal was subjected to cylindrical grinding, and the angle between the side surface of the cylindrical grinding and the principal surface orientation, the warp value with a multi-wire saw, the beveling time and the yield are shown in Table 1 below.

  In Example 1, since the angle between the side surface of the LT single crystal subjected to cylindrical grinding and the principal surface orientation of the desired 36 ° RY LT single crystal is adjusted to 90.5 ° to an ideal shape, A desired 36 ° RY cut substrate can be easily and efficiently manufactured from the grown 36 ° RY LT single crystal.

[Comparative Example 1]
An LT single crystal having a diameter of 4 inches and 36 ° RY was grown by the Czochralski method. The crystal length is about 80 mm.

  The orientation of the applied seed crystal was 36 ° RY in the RY direction and 0 ° in the X direction, and had no off-angle in advance. The direction of rotation of the seed crystal was clockwise and the number of rotations was 5 RPM.

  The obtained LT single crystal was subjected to cylindrical grinding, and the angle between the side surface of the cylindrical grinding and the main surface orientation, the warp value in the multi-wire saw, the beveling time and the yield are shown in Table 2 below.

  In Comparative Example 1, the angle between the side surface of the cylindrically ground LT single crystal and the principal plane orientation of the desired 36 ° RY LT single crystal is 94.0 ° (deviation from 90 ° to 4.0 °). Therefore, the shape of the RY cut substrate obtained by cutting with a multi-wire saw was elliptical.

  For this reason, the beveling time was as long as 2.5 minutes / sheet, and the yield was also deteriorated.

[Example 2]
An LT single crystal having a diameter of 4 inches and 38 ° RY was grown by the Czochralski method. The crystal length is about 100 mm.

  The orientation of the applied seed crystal was preliminarily provided with an off angle of 34 ° RY in the RY direction and 2 ° in the X direction. The direction of rotation of the seed crystal was clockwise and the number of rotations was 5 RPM.

  The obtained LT single crystal was subjected to cylindrical grinding, and the angle between the side surface of the cylindrical grinding and the principal surface orientation, the warp value in the multi-wire saw, the beveling time and the yield are shown in Table 3 below.

  In Example 2, since the angle between the side surface of the LT single crystal subjected to cylindrical grinding and the main surface orientation of the desired LT single crystal of 38 ° RY is adjusted to 90.2 ° to an ideal shape, A desired 38 ° RY cut substrate can be easily and efficiently manufactured from the grown 38 ° RY LT single crystal.

[Comparative Example 2]
An LT single crystal having a diameter of 4 inches and 38 ° RY was grown by the Czochralski method. The crystal length is about 100 mm.

  The orientation of the applied seed crystal was 38 ° RY in the RY direction and 0 ° in the X direction, and had no off-angle in advance. The direction of rotation of the seed crystal was clockwise and the number of rotations was 5 RPM.

  The obtained LT single crystal was subjected to cylindrical grinding, and the angle between the side surface of the cylindrical grinding and the principal surface orientation, the warp value in the multi-wire saw, the beveling time and the yield are shown in Table 4 below.

  In Comparative Example 2, the angle between the side surface of the cylindrically ground LT single crystal and the principal plane orientation of the desired 38 ° RY LT single crystal is 94.7 ° (shifted from 90 ° to 4.7 °). Therefore, the shape of the RY cut substrate obtained by cutting with a multi-wire saw was elliptical.

  For this reason, the beveling time was as long as 15 minutes / sheet, and the yield was also deteriorated.

[Example 3]
An LT single crystal having a diameter of 6 inches and 42 ° RY was grown by the Czochralski method. The crystal length is about 50 mm.

  The orientation of the applied seed crystal was preliminarily provided with an off angle of 38 ° RY in the RY direction and 2 ° in the X direction. The direction of rotation of the seed crystal was clockwise and the number of rotations was 5 RPM.

  The obtained LT single crystal was subjected to cylindrical grinding, and the angle between the side surface of the cylindrical grinding and the main surface orientation, the warp value in the multi-wire saw, the beveling time and the yield are shown in Table 5 below.

  In Example 3, the angle between the side surface of the LT single crystal subjected to cylindrical grinding and the principal surface orientation of the desired LT single crystal of 42 ° RY is adjusted to an ideal shape of 90.0 °. A desired 42 ° RY cut substrate can be easily and efficiently produced from the grown 42 ° RY LT single crystal.

[Comparative Example 3]
An LT single crystal having a diameter of 6 inches and 42 ° RY was grown by the Czochralski method. The crystal length is about 50 mm.

  The orientation of the applied seed crystal was 42 ° RY in the RY direction and 0 ° in the X direction, and had no off-angle in advance. The direction of rotation of the seed crystal was clockwise and the number of rotations was 5 RPM.

  The obtained LT single crystal was subjected to cylindrical grinding, and the angle between the side surface of the cylindrical grinding and the main surface orientation, the warp value in the multi-wire saw, the beveling time and the yield are shown in Table 6 below.

  In Comparative Example 3, the angle between the side surface of the LT single crystal subjected to cylindrical grinding and the principal plane orientation of the desired 42 ° RY LT single crystal is 95.0 ° (deviation from 90 ° to 5.0 °). Therefore, the shape of the RY cut substrate obtained by cutting with a multi-wire saw was elliptical.

  For this reason, the beveling time was as long as 24 minutes / sheet, and the yield was also deteriorated.

  According to the method for growing an LT single crystal according to the present invention, a desired RY cut substrate can be obtained easily and efficiently from the grown LT single crystal of 36 to 46 ° RY, and therefore it is applied as a substrate for a SAW device. Has industrial applicability.

3 seed crystal 4 precious metal crucible 5 seed crystal support rod 6 raw material melt 16a crystal top side 16b crystal bottom side 16c fixing hole 7 seed crystal fixing pin 8 work coil 10 crystal 13 substrate (wafer)
14 Crystal growth axis 16 Seed crystal 20, 20 ′ RY direction bend 21, 21 ′ X direction bend 100 Central axis

Claims (1)

A method for growing a lithium tantalate single crystal by the Czochralski method of bringing a seed crystal into contact with a raw material melt and pulling the seed crystal while rotating the seed crystal, wherein the a-axis of the lithium tantalate single crystal is The Y axis of the Cartesian coordinate system with the X axis, the c axis as the Z axis, and the axis orthogonal to the X axis and the Z axis as the Y axis is α ° clockwise (clockwise) around the X axis (36 ° ≦ α ° ≦ 46 °) In a method for growing a rotated lithium tantalate single crystal for an RY cut substrate,
The Y axis is rotated clockwise around the X axis by α ° (36 ° ≦ α ° ≦ 46 °) −4 ° ± 0.5 °, and the seed crystal is rotated clockwise (lithium tantalate) When the growth state of the single crystal is observed when viewed from directly above, the rotation direction of the seed crystal is clockwise.) When the crystal is grown by rotating the X axis clockwise around the Z axis (clockwise), β Rotate ° (2 ° ± 0.5 °) and rotate the seed crystal counterclockwise (the rotation direction of the seed crystal when the growth state of the lithium tantalate single crystal is observed from directly above is counterclockwise) In the case of growing, the lithium tantalate single crystal using a seed crystal having a plane orientation obtained by rotating the X-axis counterclockwise (counterclockwise) around the Z-axis by β ° (2 ° ± 0.5 °) A method for growing a lithium tantalate single crystal, characterized in that:

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