JP2019218255A - Method for manufacturing lithium niobate single crystal - Google Patents

Abstract

To provide a method for manufacturing a lithium niobate single crystal using a Czochralski method, capable of suppressing contamination of impurities derived from a platinum crucible to a straight body part.SOLUTION: A method for manufacturing a lithium niobate single crystal using a Czochralski method comprises: a melting step of heating a platinum crucible 40 and melting lithium niobate in the platinum crucible 40 to form a melt 50; a seeding step of bringing a seed crystal 20 into contact with the melt 50 in the platinum crucible 40; a shoulder formation step of pulling the seed crystal 20 to form a shoulder 31 having a diameter of 60 mm or more by a lithium niobate single crystal; and a straight body part formation step of pulling a shoulder 31 to form a straight body part 32 by the lithium niobate single crystal. In the shoulder formation step, time of pulling the seed crystal 20 from a start of growing the shoulder 31 until a diameter of the shoulder 31 reaches a diameter of 60 mm is 4.5 hours or more.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for producing a lithium niobate single crystal by the Czochralski method, and in particular, suppresses contamination of impurities caused by a platinum crucible into a straight body of a single crystal during growth of the single crystal. It relates to a possible manufacturing method.

  Lithium niobate (hereinafter sometimes referred to as “LN”) single crystal is mainly used as a nonlinear optical material or a material for a surface acoustic wave device.

  LN single crystals are industrially grown mainly by the Czochralski method (hereinafter sometimes referred to as “Cz method”). In the Cz method, for example, a crucible filled with a single crystal raw material is placed in a single crystal growing furnace of a single crystal manufacturing apparatus, and the raw material is melted by heating the crucible to form a raw material melt. Then, the seed crystal is brought into contact with the raw material melt in the crucible (seeding), and thereafter, the seed crystal is pulled up in the vertical direction while rotating in the horizontal direction, whereby a single crystal having the same orientation as the seed crystal can be obtained. .

  When growing an oxide single crystal by the Cz method, a raw material is melted in a crucible installed in a heating furnace constituting a single crystal manufacturing apparatus to generate a raw material melt. For this reason, the material of the crucible is required to be stable even at a temperature exceeding the melting point of the oxide single crystal and not to react with the raw material melt. As a material for such a crucible, platinum (Pt), iridium (Ir), molybdenum (Mo), tungsten (W), or a molybdenum-tungsten alloy (Mo-W alloy) is a candidate. In some cases, impurities resulting from the above are included in the growing single crystal.

  For example, as described in Patent Document 1, when growing a sapphire single crystal, Mo is used as the metal material of the crucible, but depending on the growing conditions, there is a problem that Mo is included in the grown sapphire single crystal. Was. This occurs because Mo sublimated at a high temperature becomes fine particles and enters the melt.

  When growing an LN single crystal by the Cz method, for example, Pt is selected as the material of the crucible as described in Patent Document 2. The reason for this is that Pt is hardly oxidized even in a high temperature state and is easy to grow in the atmosphere, the reactivity of the raw material melt of LN and Pt is poor, and the melting point of Pt (1768 ° C.) is the melting point of the LN raw material. (1257 ° C.).

JP-A-2006-150877 JP 2008-260663 A

  However, although Pt is chemically inert and stable, it is only slightly mixed with the LN raw material melt by continuously exposing it to the atmosphere at a high temperature in order to melt LN. There was a problem that Pt was included in the crystal. As a mixing route of Pt, a route in which platinum sublimated from a crucible is mixed into the LN melt can be considered as in Patent Document 1.

  It was also found that the concentration of Pt included in the LN raw material melt had a distribution due to the balance of convection in the raw material melt generated during the growth. Due to the Pt concentration distribution, appearance defects were generated in the blackening process in the subsequent step. That is, since the portion of the LN single crystal containing Pt discolors blacker during the blackening process than the portion containing no Pt, the single crystal portion causes poor color shading, resulting in poor quality. It became.

  The present invention has been made in view of such a problem, and an object of the present invention is to produce a lithium niobate single crystal by using the Cz method, in which a straight crucible of impurities derived from a platinum crucible is used. It is an object of the present invention to provide a production method capable of suppressing the contamination of the resin.

  The present inventors have conducted intensive research to solve the above-mentioned problems. As a result, during the growth of the shoulder in the production process of the lithium niobate single crystal using the Cz method, the shoulder was grown from the start of the growth of the shoulder. By controlling the time until the diameter of the portion reaches 60 mm, Pt can be recovered at the shoulder portion, and it has been found that the incorporation of Pt into the straight body of the LN single crystal used as a product can be suppressed. .

  That is, in order to solve the above problems, a method for producing a single crystal of lithium niobate by the Czochralski method which is one embodiment of the present invention includes heating a platinum crucible to melt lithium niobate in the platinum crucible. A melting step to be a melt, a seeding step of bringing a seed crystal into contact with the melt in the platinum crucible, and after the seeding step, the seed crystal is pulled up and has a diameter of 60 mm or more by lithium niobate single crystal. A shoulder forming step of forming a shoulder, and after the shoulder forming step, a straight body forming step of pulling up the shoulder and forming a straight body with the lithium niobate single crystal, In the part forming step, the pulling time of the seed crystal from the start of the formation of the shoulder to the diameter of the shoulder reaching 60 mm is set to 4.5 hours or more.

  ADVANTAGE OF THE INVENTION According to this invention, when manufacturing a lithium niobate single crystal using Cz method, it can suppress mixing of the impurity derived from a platinum crucible into a straight body part. As a result, it is possible to manufacture a lithium niobate single crystal substrate having no appearance defect.

FIG. 2 is a cross-sectional view illustrating a schematic configuration of a single crystal manufacturing apparatus that can be used in the manufacturing method of the present invention. It is a schematic sectional drawing which shows the flow of LN raw material melt. It is a figure which shows the result of having investigated the influence which the time until the shoulder part reaches 60 mm in diameter from the start of growth | cultivation of a shoulder part to the appearance failure occurrence.

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

  The manufacturing method of the present invention is a method for manufacturing an LN single crystal by the Cz method. For example, a single crystal manufacturing apparatus 100 whose schematic sectional view is shown in FIG. 1 can be used.

[Single crystal manufacturing equipment]
The single crystal manufacturing apparatus 100 includes a pulling shaft 10, a platinum crucible 40, a resistance heater 70, and a support 60.

  The pulling shaft 10 can be provided with a seed crystal 20 at its tip. The seed crystal 20 is brought into contact with the surface of the raw material melt 50 that has been put into the platinum crucible 40 and melted (seeding), and the LN single crystal 30 is Used to pull up while rotating. For example, a shaft made of platinum can be used as the pulling shaft 10, and a rod-shaped member having a holding portion 11 that holds the seed crystal 20 at the lower end of the pulling shaft 10 can be used. The pulling shaft 10 rotates in the horizontal direction about the shaft and moves up and down so that the seed crystal 20 can be brought into contact with the surface of the raw material melt 50, and the conical shoulder portion 31 and the columnar shape The straight body 32 can be grown. Further, as the LN single crystal 30 grows, the LN single crystal 30 can be suspended and held together with the seed crystal 20.

  Further, in the single crystal manufacturing apparatus 100, for example, a pull-up shaft driving unit having a motor (not shown) may be provided in order to move the pull-up shaft 10 up and down and rotate. Note that the rotation speed and the pulling speed of the pulling shaft 10 can be appropriately set according to the size of the diameter of the LN single crystal 30 to be formed, the length of the straight body portion 32, and the like.

  The platinum crucible 40 is for holding a raw material melt 50 in which a raw material of an LN single crystal is put and is in a molten state. For example, a cup-shaped one having a circular bottom part 41 and a cylindrical side wall part 42 erected from the outer edge of the bottom part 41 and having an upper part 43 opened can be used. Note that, as the platinum crucible 40, a platinum crucible having excellent heat resistance is used to hold the raw material melt 50 in a molten state.

  The resistance heater 70 is a heater that directly heats the platinum crucible 40 and the LN single crystal 30 by radiant heat or the like by a resistance heating method and keeps the temperature, and is connected to, for example, an external power supply (not shown). The resistance heater 70 can be disposed so as to surround the outer periphery of the platinum crucible 40, and a cylindrical heater can be used so that the platinum crucible 40 and the LN single crystal 30 can be efficiently heated.

  As the resistance heater 70, a heater using carbon, nichrome (an alloy of nickel and chromium), molybdenum disilicide, or the like that generates heat by electric resistance can be used as appropriate. When manufacturing the LN single crystal 30, it is preferable to use a heater made of nichrome or molybdenum disilicide which is particularly useful in an oxygen atmosphere, and to use a heating element made of molybdenum disilicide having higher heat resistance as the heater. Is more preferred.

  The support table 60 is a table on which the platinum crucible 40 is placed, and can hold the platinum crucible 40 at an optimum position in consideration of the heating efficiency of the resistance heater 70. The support base 60 can be provided with a hole at the center so that a temperature sensor (not shown) that measures the temperature of the bottom 41 of the platinum crucible 40 can contact the platinum crucible 40. The support 60 is made of a heat-resistant ceramic such as zirconia or alumina, for example, and may be combined with a drive mechanism for vertically moving the platinum crucible 40 placed on the support 60.

  In addition to the above, the single crystal manufacturing apparatus 100 can be provided with a heat insulating material surrounding the resistance heater 70 in the radial direction and serving as a wall of the growth furnace, and a heat insulating material forming the ceiling and bottom of the furnace. These heat insulating materials are members that suppress the release of heat to the outside. For example, heat insulating ceramics such as zirconia and alumina or felt are used. Further, an outer wall that covers the outer periphery of these heat insulating materials can be provided.

  Further, the single crystal manufacturing apparatus 100 can include a control unit for controlling the entire single crystal manufacturing apparatus 100 including the process of growing the LN single crystal. The control unit includes, for example, a CPU (Central Processing Unit, central processing unit), and a memory such as a ROM (Read Only Memory) and a RAM (Random Access Memory). Further, the control means may be constituted by a microcomputer operated by a program, or may be constituted by an electronic circuit such as an ASIC (Application Specified Integrated Circuit) developed for a specific application.

[Method for producing lithium niobate single crystal]
Next, as a method of manufacturing an LN single crystal by the Cz method, a manufacturing method using the single crystal manufacturing apparatus 100 will be described as an example.

(Melting process)
In the melting step, the platinum crucible 40 is heated to melt the LN in the platinum crucible 40 to obtain a melt (raw material melt 50). For example, by heating the resistance heating heater 70 from a power supply unit (not shown) and appropriately heating the platinum crucible 40, the LN raw material in the platinum crucible 40 is heated to a melting point or higher and melted. A liquid 50 is obtained.

(Seeding process)
In the seeding step, the seed crystal 20 is brought into contact with the raw material melt 50 in the platinum crucible 40. Specifically, the pulling shaft 10 is lowered, and the seed crystal 20 installed at the lower end of the pulling shaft 10 is brought into contact with the upper surface of the raw material melt 50.

(Shoulder forming step)
The shoulder forming step is a step in which after the seeding step, the seed crystal 20 is pulled up to form a shoulder 31 having a diameter of 60 mm or more from an LN single crystal. For example, the temperature in the growth furnace is gradually lowered by the resistance heater 70, and at the same time, the seed crystal 20 is gradually pulled up while rotating the pulling shaft 10. Single crystallize. In addition, the power supplied to the resistance heater 70 is appropriately adjusted. These operations make it possible to manufacture the conical shoulder 31 until the desired diameter is reached.

  During the formation of the shoulder 31, the heating temperature of the resistance heater 70, the rotation speed of the lifting shaft 10, the lifting speed, and the like are appropriately controlled. In particular, in the shoulder forming step, the time for pulling the seed crystal 20 from the start of the formation of the shoulder 31 until the diameter of the shoulder 31 reaches 60 mm is set to 4.5 hours or more. For example, when the pulling time of the seed crystal 20 and the rotation speed of the pulling shaft 10 are kept constant and the pulling time of the seed crystal 20 is set to 4.5 hours to form the shoulder 31, the diameter of the shoulder 31 The formation speed is 13.3 mm / hour, which is obtained by dividing 4.5 hours into 60 mm.

  The reason why impurities are prevented from being mixed into the straight body 32 will be described with reference to FIG. FIG. 2 is a schematic cross-sectional view showing the flow of the LN raw material melt. FIG. 2A is a vertical cross-sectional view of the platinum crucible 40 and the raw material melt 50 before the seeding step, and FIG. FIG. 2C is a vertical sectional view of the platinum crucible 40 and the raw material melt 50 at the time, FIG. 2C is a vertical sectional view of the platinum crucible 40, the raw material melt 50 and the shoulder 31 during formation of the shoulder 31, and FIG. FIG. 3 is a cross-sectional view of the platinum crucible 40 and the raw material melt 50 taken from above, taken along the line AA ′ in FIG.

  Although Pt is chemically inert and stable, by increasing the output of the resistance heater 70 and continuously exposing the platinum crucible 40 to the atmosphere at a high temperature to melt LN, It is mixed into the raw material melt 50 of LN. After the LN raw material is melted, the output of the resistance heater 70 is lowered and the temperature of the platinum crucible 40 is also lowered, so that melting to the raw material melt 50 is not recognized.

  The raw material melt 50 generates a thermal convection X due to a partial temperature difference, and before the seeding step, a thermal convection X in which the outside of the raw material melt 50 rises and the inside descends (FIG. 2A )). The impurities in the raw material melt 50 also rise and fall by multiplying the thermal convection X.

  The lifting shaft 10 descends while rotating in the horizontal direction (FIG. 2A). When the seed crystal 20 comes into contact with the surface of the raw material melt 50 by seeding, this rotation is transmitted to the raw material melt 50. Then, a forced convection Y directed outward is generated near the surface of the raw material melt 50 (FIG. 2B). That is, the forced convection Y is generated as a flow of the melt caused by the rotation of the crystal, and the impurities also move to the outside of the raw material melt 50 by the forced convection Y.

  Due to the seeding, the direction in which the raw material melt 50 flows due to the thermal convection X and the direction in which the raw material melt 50 flows due to the forced convection Y collide, and the flow of the thermal convection X and the flow of the forced convection Y stagnate in the region A ( FIG. 2 (b)). The impurities also stagnate, and a large amount of impurities gather in the region A.

  When the seed crystal 20 is pulled up in such a state where the impurities are stagnant, the raw material melt 50 is solidified into a single crystal in the region A where a large amount of impurities are collected, and the shoulder portion 31 is formed (FIG. 2C). (D)).

  By slowly forming the shoulder 31, the impurities collected in the region A are taken into the shoulder 31 more and more, and the concentration of the impurity in the raw material melt 50 decreases. Therefore, after the shoulder portion forming step, the impurities in the raw material melt are almost eliminated, so that the contamination of the straight body portion 32 with the impurities is suppressed.

  By setting the pulling time of the seed crystal 20 from the start of the formation of the shoulder portion 31 to the diameter of the shoulder portion 31 to reach 60 mm to 4.5 hours or more, impurities derived from the platinum crucible 40 are mixed into the straight body portion 32. Can be suppressed. As a result, it is possible to eliminate color unevenness defects during the blackening process, and to manufacture a lithium niobate single crystal substrate having no external appearance defects.

  If the pulling time of the seed crystal 20 is less than 2 hours, the shoulder portion 31 may be polycrystallized, which may result in poor growth of the single crystal. When the pulling time of the seed crystal 20 is less than 2 hours to less than 4.5 hours, a single crystal can be grown, but the incorporation of impurities into the shoulder portion 31 becomes insufficient, and impurities are mixed into the straight body portion 32. There are cases.

  The upper limit of the pulling time of the seed crystal 20 from the start of the formation of the shoulder portion 31 until the diameter of the shoulder portion 31 reaches 60 mm is not particularly limited. It is possible to prevent impurities from being mixed into the straight body portion 32. Note that assuming that the shoulder forming step is performed manually, the upper limit of the pulling time of the seed crystal 20 is preferably 12 hours. For example, when the pulling speed of the seed crystal 20 and the rotation speed of the pulling shaft 10 are kept constant and the pulling time of the seed crystal 20 is set to 12 hours to form the shoulder 31, the diameter of the shoulder 31 is formed. The speed is 5 mm / hour, which is obtained by dividing 12 hours from 60 mm.

  In the shoulder forming step, it is difficult to grow a single crystal from the start of formation until the diameter of the shoulder 31 reaches 60 mm. Therefore, the operator of the single crystal manufacturing apparatus 100 manually controls the growing conditions and controls the shoulder. It is preferable to form the part 31. After the diameter reaches 60 mm, the growth may be continued manually as it is, or the growth may be continued by automatic control.

(Direct body forming step)
The straight body forming step is a step of forming the straight body 32 from the LN single crystal 30 by pulling up the shoulder 31 after the shoulder forming step. For example, by appropriately controlling the heating temperature of the resistance heater 70, the number of rotations of the pull-up shaft 10, the pull-up speed, and the like, the columnar straight body 32 having a predetermined height can be formed.

(Other processes)
The method for producing an LN single crystal of the present invention can include further steps in addition to the above steps. For example, before the melting step, a filling step of filling the powder material of the LN single crystal into the platinum crucible 40 placed on the support base 60 may be mentioned. Further, when the straight body portion 31 has a predetermined length, the pulling speed of the pulling shaft 10 is controlled to separate the upper surface of the raw material melt 50 from the lower end of the grown LN single crystal 30 (separating step). And a step of cooling the LN single crystal 30 after the separating step (cooling step).

  Hereinafter, the present invention will be described more specifically based on examples. However, the present invention is not limited to the contents of the following examples.

[Example 1]
The LN single crystal was grown using the single crystal manufacturing apparatus 100. First, the LN powder raw material was charged into a platinum crucible 40 having an outer diameter of 230 mm, an outer height of 75 mm, and a thickness of 0.75 mm. Then, by heating the resistance heating heater 70 to heat the platinum crucible 40, the raw material in the platinum crucible 40 was heated to a melting point or higher and melted to obtain a raw material melt 50. After the raw material is melted, the pulling shaft 10 is lowered under the condition that the temperature of the platinum crucible 40 becomes stable, and the seed crystal 20 placed on the holding portion 11 at the lower end of the pulling shaft 10 is brought into contact with the upper surface of the raw material melt 50. Seeding was performed. Then, the temperature in the growth furnace is gradually lowered by the resistance heating heater 70, and at the same time, the raw material melt 50 is sequentially single-crystallized on the lower side of the seed crystal 20 by gradually raising the rotation while rotating the lifting shaft 10. A shoulder 31 was formed. At this time, while monitoring the crystal diameter calculated from a weight detector (not shown) installed on the upper part of the pulling shaft 10, the output of the resistance heater 70 is appropriately adjusted, and The time required for the single crystal diameter of No. 31 to reach 60 mm was 4 hours and 30 minutes. Thereafter, the formation of the shoulder portion 31 was continued. When the shoulder portion diameter reached 150 mm, the crystal diameter was stopped from expanding, and the growth of the straight body portion 32 serving as the product portion was started. When the straight body portion 32 has a predetermined length, the pulling speed of the pulling shaft 10 is controlled to separate the upper surface of the raw material melt 50 from the lower end of the grown LN single crystal 30 and cool the LN single crystal 30. As a result, an LN single crystal was obtained.

  In the subsequent step, the obtained LN single crystal was processed into a wafer, blackened, and visually inspected for appearance. As a result, the appearance defect occurrence rate due to platinum mixed from the platinum crucible 40 was 0%. there were.

[Example 2]
An LN single crystal was obtained under the same conditions as in Example 1 except that the time from the start of the formation of the shoulder 31 to the diameter of the single crystal of the shoulder 31 reaching 60 mm was 4 hours and 45 minutes.

  In the subsequent step, the obtained LN single crystal was processed into a wafer and blackened, and the appearance was similarly inspected. As a result, the appearance defect occurrence rate due to platinum mixed in from the platinum crucible 40 was 0%. Was.

[Comparative Example 1]
An LN single crystal was obtained under the same conditions as in Example 1 except that the time from the start of the formation of the shoulder 31 to the diameter of the single crystal of the shoulder 31 reaching 60 mm was 3 hours and 45 minutes.

  In the subsequent step, the obtained LN single crystal was processed into a wafer shape and subjected to blackening treatment, and a similar appearance inspection was performed. became. The appearance defect occurrence rate due to platinum mixed in from the platinum crucible 40 was 15%.

[Comparative Example 2]
An LN single crystal was obtained under the same conditions as in Example 1 except that the time from the start of the formation of the shoulder portion 31 until the diameter of the single crystal of the shoulder portion 31 reached 60 mm was 2 hours and 12 minutes.

  In the subsequent step, the obtained LN single crystal was processed into a wafer shape and subjected to blackening treatment, and a similar appearance inspection was performed. became. The appearance defect occurrence rate due to platinum mixed in from the platinum crucible 40 was 21%.

  FIG. 3 shows the result of investigating the effect of the time from the start of growing the shoulder 31 to the time when the shoulder 31 reaches the diameter of 60 mm on the occurrence of poor appearance. The plot in FIG. 3 shows the appearance defect occurrence rate due to platinum mixed in from the platinum crucible 40 on the wafer. In FIG. 3, it is plotted that Examples 1 and 2 have no appearance defect, Comparative Example 1 has an appearance defect occurrence rate of 15%, and Comparative Example 2 has an appearance defect occurrence rate of 21%.

  In addition to Comparative Examples 1 and 2, the time from the start of the formation of the shoulder 31 to the time when the single crystal diameter of the shoulder 31 reaches 60 mm is set to some time between 2 hours and 4 hours. An LN single crystal was manufactured under the same conditions as in Example 1. Similarly, in these cases, the appearance defect occurrence rate on the wafer is plotted in FIG.

  As a result, when the time from the start of the formation of the shoulder portion 31 until the single crystal diameter of the shoulder portion 31 reaches 60 mm is between 2 hours and 3 hours, in any case, poor appearance occurs. As a result, the appearance defect occurrence rate was 50% at the maximum (FIG. 3).

  In addition, when the time from the start of the formation of the shoulder portion 31 until the single crystal diameter of the shoulder portion 31 reached 60 mm was between 3 hours and 4 hours, appearance defects occurred in any case. The appearance defect occurrence rate was a maximum of 24% (FIG. 3).

[Summary]
As described above, according to the present invention, impurities can be collected in the shoulder 31 until the diameter of the shoulder 31 reaches 60 mm. Can be suppressed. As a result, it is possible to manufacture a lithium niobate single crystal substrate without causing poor quality.

DESCRIPTION OF SYMBOLS 10 Pull-up shaft 11 Holding part 20 Seed crystal 30 LN single crystal 31 Shoulder part 32 Straight body part 40 Platinum crucible 41 Bottom part 42 Side wall part 43 Top 50 Raw material melt 60 Support base 70 Resistance heater 100 Single crystal manufacturing apparatus A area X heat Convection Y Forced convection

Claims (1)

A method for producing a lithium niobate single crystal by the Czochralski method,
A melting step of heating the platinum crucible to melt the lithium niobate in the platinum crucible to obtain a melt;
A seeding step of bringing a seed crystal into contact with the melt in the platinum crucible,
After the seeding step, the seed crystal is pulled up, and a shoulder forming step of forming a shoulder having a diameter of 60 mm or more from a lithium niobate single crystal,
After the shoulder forming step, the shoulder is pulled up, a straight body forming step of forming a straight body by the lithium niobate single crystal,
The method for producing a single crystal of lithium niobate, wherein in the shoulder forming step, a pulling time of the seed crystal from a start of the formation of the shoulder to a diameter of the shoulder reaching 60 mm is 4.5 hours or more.

Images