JP2013001581A - Method for growing lithium tantalate single crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for growing a lithium tantalate single crystal which is improved in productivity by suppressing the bending of the lithium tantalate single crystal during crystal growing.SOLUTION: In the growing method, a seed crystal 8 is brought into contact with a molten raw material liquid 10 held in a crucible 3 and lifted upward while being rotated to grow the lithium tantalate single crystal 11. When the straight cylinder part of the lithium tantalate single crystal is grown, the rotating direction of the single crystal is reversed at a constant interval of 5-800 min. By this growing method, since a phenomenon in which the lithium tantalate single crystal during crystal growing is bent to the X-axis direction can be suppressed, when a wafer is obtained from a grown lithium tantalate single crystal ingot, a diameter-defective wafer can hardly be generated. Accordingly, the number of obtainable wafers can be increased, so that the productivity of the wafer is improved eminently to contribute to cost reduction.

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, a lithium tantalate single crystal that has improved productivity by suppressing the bending of the lithium tantalate single crystal during crystal growth. The present invention relates to a crystal growth method.

  The lithium tantalate crystal is a ferroelectric with a melting point of about 1650 ° C. and a Curie temperature of about 600 ° C. The use of the lithium tantalate substrate manufactured using this crystal is mainly for removing signal noise from mobile phones. It is a material for surface acoustic wave (SAW) filters.

  The lithium tantalate single crystal is generally grown by the “Czochralski method”. Hereinafter, the growth method by the “Czochralski method” will be described. A heater for surrounding a noble metal crucible such as iridium with a refractory and maintaining an appropriate temperature gradient at the top of the crucible on the crucible or the refractory. In a furnace in which a noble metal structure having a function is installed, a mass of lithium tantalate crystals as a raw material filled in the crucible is heated and melted, and a seed crystal is brought into contact with the raw material melt. The crystal is grown by pulling it upward while rotating the seed crystal at a rotational speed of (see, for example, Patent Document 1).

  By the way, the direction of the pulling axis of the crystal is set so as to be the same as the orientation of the seed crystal cut in the 36 ° to 50 ° rotation Y-axis (RY) direction because of the propagation speed of the surface acoustic wave. However, when the crystal is grown in this way, the crystal is bent by about 2 ° in the + X-axis direction, that is, in the <100> direction. When the straight body length of the ingot exceeds 100 mm, a defect in diameter occurs in a part of the wafer due to the bending of the crystal, resulting in a decrease in productivity.

Japanese Patent Laid-Open No. 10-36193

  The present invention has been made paying attention to such problems, and the problem is to suppress the above-mentioned bending of the lithium tantalate single crystal and increase the number of wafers that can be obtained, thereby increasing productivity. The object is to provide a greatly improved method for growing lithium tantalate.

  Therefore, as a result of intensive studies by the inventor in order to solve the above-mentioned problems, it has been found that the bending direction of the lithium tantalate single crystal depends on the rotation direction of crystal growth.

  That is, when the growth state of the lithium tantalate single crystal is observed from directly above, the lithium tantalate single crystal is oriented in the + X axis direction when the crystal growth is clockwise, and in the −X axis direction when the crystal growth is counterclockwise. Was found to bend, and the magnitude of the bend was confirmed to be approximately 2 °.

  For this reason, while revising the conventional growth method in which the rotation direction of crystal growth is only one direction, and predicting that the bending of the crystal can be suppressed when the rotation direction is reversed at a certain interval, the present invention. As a result of intensive experiments, the inventors have completed the present invention.

That is, the invention according to claim 1
In a method for growing a lithium tantalate single crystal by bringing a seed crystal into contact with a raw material melt contained in a crucible and pulling upward while rotating the seed crystal,
When the straight body portion of the lithium tantalate single crystal is grown, the rotation direction of the single crystal is reversed at regular intervals of 5 to 800 minutes.

Next, the invention according to claim 2
In the method for growing a lithium tantalate single crystal according to the invention of claim 1,
The inversion interval in the rotation direction is 120 to 180 minutes,
The invention according to claim 3
In the method for growing a lithium tantalate single crystal according to claim 1 or 2,
The direction of the crystal pulling axis is set to be the same as the orientation of the seed crystal cut in the Y-axis direction of 36 ° to 50 ° rotation.

  According to the method for growing a lithium tantalate single crystal according to the present invention, when the straight body portion of the lithium tantalate single crystal is grown, the rotation direction of the single crystal is reversed at regular intervals of 5 to 800 minutes. This makes it possible to suppress the phenomenon that the lithium tantalate single crystal is bent in the X-axis direction during crystal growth.

  When wafers are obtained from the grown lithium tantalate single crystal ingot, it is difficult to generate wafers with defective diameters, so the number of wafers that can be acquired can be increased, thus greatly improving wafer productivity and reducing costs. It has an effect that can contribute.

The schematic structure sectional drawing which shows an example of the manufacturing apparatus used for the growth method of a lithium tantalate single crystal.

  Hereinafter, embodiments of the present invention will be described in detail.

  First, 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 pulled up. Then, the crystal growth of the lithium tantalate single crystal is performed in the same manner as in the conventional method up to the crystal shoulder, and the rotation direction is reversed after the growth of the straight body of the crystal.

  Further, the rotation speed is performed by applying the rotation speed that is normally performed as it is, and the rotation direction is reversed at regular intervals. If the rotation direction of the seed crystal is changed at a very short interval, the convection of the melt becomes complicated and it becomes difficult to maintain the convex solid-liquid interface shape important for crystal growth. . For this reason, it is desirable to maintain the rotation direction for a long time as long as no crystal bending occurs.

  And as a result of inventor's earnest analysis of the reversal interval in the rotation direction, if it is 5 to 800 minutes, it does not significantly affect the melt convection, and therefore it can be grown without greatly changing the solid-liquid interface. It came to discover. Single crystals can be grown even if the inversion interval is outside the above range, but if the inversion interval is too short, the solid-liquid interface becomes flat or concave and dislocations occur in the crystal. The risk of polycrystallization increases due to the easy concentration of dislocations. If the inversion interval is too long, crystal bending occurs due to growth during that interval. When the inversion interval is 120 to 180 minutes, the risk of polycrystallization is suppressed, a high single crystallization rate is maintained, and the bending of the crystal is also suppressed, which is preferable because the final wafer yield is good. .

  When the crystal is pulled by the method of the present invention, since the crystal is hardly bent, the ingot is pulled in substantially the same direction as the orientation of the seed crystal. For this reason, when processing into a wafer, it becomes less likely that the diameter is insufficient, and the number of wafers that can be taken from one ingot can be maximized.

  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. Moreover, as an apparatus for growing a lithium tantalate single crystal in Examples and the like, the following manufacturing apparatus shown in FIG. 1 was used.

  That is, this manufacturing apparatus includes a ceramic crucible base 2, an alumina base 4 that is installed at the bottom of the crucible base 2 and on which the iridium crucible 3 is disposed, and heat insulation that is installed so as to surround the crucible 3. A material 5, a donut-shaped reflector 9 attached along the open edge of the crucible 3, a cylindrical afterheater 13 attached on the reflector 9, and an upper open edge of the afterheater 13 A lid 7 provided with an opening for passing the lifting shaft 6 in the center, a seed crystal holding jig 12 provided on the lower side of the lifting shaft 6 and for holding the seed crystal 8, The heating coil 1 is provided so as to surround the crucible base 2 and melts the raw material filled inside by induction heating of the crucible 3. While rotating the seed crystal 8 with contacting the two crystal 8 is intended to grow a single crystal 11 by pulling upward.

[Example 1]
On the open edge of the iridium crucible 3 with a diameter of 180 mmφ and a height of 170 mm, an iridium reflector 9 in the form of a donut plate with an outer diameter of 180 mmφ, an inner diameter of 120 mmφ and a thickness of 2 mm is attached. A cylindrical iridium after heater 13 having a height of 170 mm was attached.

  In the crucible 3, 18 kg of the firing raw material having a composition ratio of Li / Ta = 0.944 (molar ratio) is first added, 12 kg is added, and the remaining 6 kg is added and melted by additional charge. By a ski method, a lithium tantalate single crystal having a diameter of 4 inches and a straight body length of 110 mm was pulled up.

  The orientation of the seed crystal 8 is 42 ° RY (Rotated Y), and the seed crystal 8 is rotated up to the shoulder of the ingot while the rotation direction of the seed crystal 8 is fixed in the clockwise direction. The rotation direction of the seed crystal 8 was reversed every minute and pulled up 10 times.

  Then, the bending in the X-axis direction (°), the single crystallization rate (%), the processed weight yield (%) after completion of the wire saw, and the final weight yield (%) were determined. The result was as follows.

  The single crystallization rate (%) indicates the ratio of the number of ingots from which single crystals were pulled 10 times, and the processed weight yield (%) after the wire saw was finished was expressed by the following formula: The final weight yield (%) was obtained by multiplying the single crystallization rate (%) by the processed weight yield (%), and the wafer cutting pitch was 0.5 mm.

Processing weight yield (%) = [weight of non-defective wafer / weight of ingot] × 100
From the results in Table 1, it was confirmed that almost no crystal bending occurred in Example 1, and a high processing weight yield (%) of 70% could be achieved.

  However, the single crystallization rate (%) was slightly low at 75%. This is probably because the reversal interval in the rotation direction is as short as 5 minutes, so that upward convection occurs in the melt and the solid-liquid interface shape is flat or slightly concave. In other words, it is presumed that the dislocations are easily concentrated due to the solid-liquid interface shape becoming flat or slightly concave, and the occurrence rate of polycrystallization is increased.

  Therefore, the final weight yield (%) was 53%.

[Example 2]
The seed crystal 8 was pulled up under the same growth conditions as in Example 1 except that the rotation direction of the seed crystal 8 was reversed every 120 minutes.

  Then, the bending in the X-axis direction (°), the single crystallization rate (%), the processed weight yield (%) after completion of the wire saw, and the final weight yield (%) were determined. The result was as follows.

  From the results shown in Table 1, it was confirmed that in Example 2, crystal bending hardly occurred, and the processing weight yield (%) could be as high as 70%.

  Furthermore, the single crystallization rate (%) was a good result of 92%. This is considered to be because the reversal interval in the rotation direction is as long as 120 minutes, so that the rising convection in the melt decreases and the solid-liquid interface shape becomes convex. That is, it is considered that dislocations easily escape to the outside of the crystal due to the convex shape of the solid-liquid interface, thereby suppressing polycrystallization and increasing the single crystallization rate.

  For this reason, the final weight yield (%) was 64%.

[Example 3]
The seed crystal 8 was pulled up under the same growth conditions as in Example 1 except that the rotation direction of the seed crystal 8 was reversed every 150 minutes.

  Then, the bending in the X-axis direction (°), the single crystallization rate (%), the processed weight yield (%) after completion of the wire saw, and the final weight yield (%) were determined. The result was as follows.

  From the results shown in Table 1, it was confirmed that even in Example 3, crystal bending hardly occurred and a high processing weight yield (%) of 70% could be achieved.

  Furthermore, since both the single crystallization rate (%) and the final weight yield (%) were the same as those in Example 2, it is considered that the same phenomenon as in Example 2 occurred in Example 3. .

[Example 4]
The seed crystal 8 was pulled up under the same growth conditions as in Example 1 except that the rotation direction of the seed crystal 8 was reversed every 180 minutes.

  Then, the bending in the X-axis direction (°), the single crystallization rate (%), the processed weight yield (%) after completion of the wire saw, and the final weight yield (%) were determined. The result was as follows.

  From the results shown in Table 1, it was confirmed that even in Example 4, crystal bending hardly occurred, and a high processing weight yield (%) of 70% could be achieved.

  Furthermore, since both the single crystallization rate (%) and the final weight yield (%) were the same as in Example 2, it is considered that the same phenomenon as in Example 2 occurred in Example 4. .

[Example 5]
The seed crystal 8 was pulled up under the same growth conditions as in Example 1 except that the rotation direction of the seed crystal 8 was reversed every 200 minutes.

  Then, the bending in the X-axis direction (°), the single crystallization rate (%), the processed weight yield (%) after completion of the wire saw, and the final weight yield (%) were determined. The result was as follows.

  From the results in Table 1, it was confirmed that the crystal bending was slightly increased in Example 5 as compared with Examples 1 to 4, and accordingly, the processing weight yield (%) was slightly lowered to 68%. It was confirmed. This is thought to be because the effect of suppressing the crystal bending is weakened due to the longer inversion time in the rotation direction. The single crystallization rate (%) was 91%, which was a good result.

  And it was confirmed that the final weight yield (%) was slightly reduced to 62% as compared with Examples 2 to 4 due to a decrease in the processed weight yield (%) due to an increase in crystal bending.

[Example 6]
The seed crystal 8 was pulled up under the same growth conditions as in Example 1 except that the rotation direction of the seed crystal 8 was reversed every 800 minutes.

  Then, the bending in the X-axis direction (°), the single crystallization rate (%), the processed weight yield (%) after completion of the wire saw, and the final weight yield (%) were determined. The result was as follows.

  From the results of Table 1, it was confirmed that the crystal bending was increased in Example 6 as compared with Examples 1 to 5, and accordingly, the processing weight yield (%) was also reduced to 65%. confirmed. This is considered to be because the effect of suppressing the crystal bending is further weakened due to the longer inversion time in the rotational direction as in Example 5. The single crystallization rate (%) was 90%, which was a good result.

  It was confirmed that the final weight yield (%) was 59%, which was lower than in Examples 2 to 5, due to the decrease in the processed weight yield (%) due to the increase in crystal bending.

[Comparative Example 1]
The seed crystal 8 was pulled up under the same growth conditions as in Example 1 except that the rotation direction of the seed crystal 8 was fixed in the clockwise direction without being reversed.

  Then, the bending in the X-axis direction (°), the single crystallization rate (%), the processed weight yield (%) after completion of the wire saw, and the final weight yield (%) were determined. The result was as follows.

  In Examples 1 to 6, almost no crystal bending occurs, or even if it occurs, the processed weight yield (%) is not significantly reduced, and the processed weight yield (%) is also a high yield of 53 to 64%. However, in Comparative Example 1, a crystal bending of about 2 ° in the X-axis direction was generated, and the processing weight yield (%) was 45%, which was significantly lower than Examples 1-6. Confirmed to do. This is because a portion of the wafer has a short diameter due to the bending of the crystal, resulting in a decrease in the number of non-defective products.

  The single crystallization rate (%) was 90%, which was a good result, but it was confirmed that the final weight yield (%) was 41%, which was significantly lower than in Examples 1-6.

[Comparative Example 2]
The seed crystal 8 was pulled up under the same growth conditions as in Example 1 except that the rotation direction of the seed crystal 8 was reversed every 3 minutes.

  Then, the bending in the X-axis direction (°), the single crystallization rate (%), the processed weight yield (%) after completion of the wire saw, and the final weight yield (%) were determined. The result was as follows.

  From the results in Table 1, it was confirmed that in Comparative Example 2, almost no crystal bending occurred, and the processing weight yield (%) could be as high as 70%.

  However, as compared with Examples 1 to 6, it was confirmed that the single crystallization rate (%) was greatly reduced to 50%. This is thought to be because the inversion interval in the rotation direction is as short as 3 minutes, so that strong upward convection occurs in the melt and the solid-liquid interface shape becomes a strong concave shape. That is, it is considered that dislocations are concentrated due to the solid-liquid interface shape becoming a strong concave shape, and the occurrence rate of polycrystallization is significantly increased.

  For this reason, it was confirmed that the final weight yield (%) is significantly reduced compared with Examples 1-6, 35%.

[Comparative Example 3]
The seed crystal 8 was pulled up under the same growth conditions as in Example 1 except that the rotation direction of the seed crystal 8 was reversed every 1000 minutes.

  Then, the bending in the X-axis direction (°), the single crystallization rate (%), the processed weight yield (%) after completion of the wire saw, and the final weight yield (%) were determined. The result was as follows.

  From the results of Table 1, although not as much as Comparative Example 1, a crystal bending of 1.5 to 2.3 ° occurred in the X-axis direction, and accordingly, the processing weight yield (%) compared to Examples 1-6. ) Was greatly reduced to 50%. This is because, as in Comparative Example 1, a portion of the wafer has a diameter shortage due to the bending of the crystal, resulting in a decrease in the number of non-defective products.

  Therefore, it was confirmed that the final weight yield (%) was 45%, which was significantly lower than those in Examples 1-6.

  According to the present invention, since the bending 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 a surface acoustic wave device or the like can be grown. Has industrial applicability.

DESCRIPTION OF SYMBOLS 1 Heating 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 (3)

In a method for growing a lithium tantalate single crystal by bringing a seed crystal into contact with a raw material melt contained in a crucible and pulling upward while rotating the seed crystal,
A method for growing a lithium tantalate single crystal, wherein when the straight body portion of the lithium tantalate single crystal is grown, the rotation direction of the single crystal is reversed at a constant interval of 5 to 800 minutes.   2. The method for growing a lithium tantalate single crystal according to claim 1, wherein the inversion interval in the rotation direction is 120 to 180 minutes.   3. The tantalum acid according to claim 1, wherein the direction of the crystal pulling axis is set to be the same as the orientation of the seed crystal cut in the Y-axis direction of 36 ° to 50 ° rotation. 4. A method for growing lithium single crystals.

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