Method for producing single crystal ingot and method for producing single crystal wafer

Abstract

Provided are: a method for producing a single crystal ingot with which it is possible to produce a single crystal ingot having excellent crystallinity at a good yield; and a method for producing a single crystal wafer with which it is possible to produce a wafer having excellent device characteristics. A method for producing a single crystal ingot according to the present invention comprises: a step for preparing a first single crystal ingot; a step in which the evaluation substrate (102) and the seed crystal ingot (103) are cut from the first single crystal ingot, and a projection surface (104) is formed on the seed crystal ingot; a step for measuring the local sound velocity of the evaluation substrate (102), and disposing a normal point (105) at a position on the evaluation substrate at which the measured value of the measured sound velocity is within a prescribed range; a step in which a map (107) of a normal point (105) is arranged on the projection surface; a step for cutting, from the seed crystal ingot, a crystal piece (109) that includes a straight line that passes through the map (107) of the normal point (105) and is parallel to the crystal growth axis; and a step for producing a second single crystal ingot using the crystal piece 109 as a seed crystal. A method for producing a single crystal wafer according to the present invention includes a step for slicing a second single crystal ingot produced by the method for producing a single crystal ingot to produce a single crystal wafer.

Description

This application is a divisional application of the application with an application date of October 30, 2020, application number "202011188769.2", and invention name "Method for manufacturing single crystal ingots and method for manufacturing single crystal wafers". Technical Field

[0001] The present invention relates to a method for producing a single crystal ingot and a method for producing a single crystal wafer by using the single crystal ingot produced by the method for producing a single crystal ingot. Background Art

[0002] Known methods for producing single crystals by solidifying a melt include the Czochralski method and the Bridgman method. In these methods, a crystal sheet with uniform crystal alignment is used as a seed crystal, which is then used as the starting material for crystal growth while maintaining the alignment. A portion of the large single crystal ingot produced in this way is cut into a crystal sheet, which is then used as a seed crystal to produce the next single crystal ingot.

[0003] The quality of the grown single crystal ingot depends largely on the quality of the seed crystal used as the starting material. Using poor-quality seed crystals for growth can lead to abnormal growth ridges, or dislocations in the seed crystal can propagate through the growing crystal, reducing the quality of the resulting single crystal ingot. Therefore, it is desirable to use high-quality crystal sheets for the seed crystals.

[0004] Patent Document 1 describes a method for cutting a wafer from a crystal of a lanthanum gallium silicate type structural material, garnet, lithium niobate, or lithium tantalate in a plane substantially perpendicular to the Z-axis direction, which is the direction of crystal growth, and then etching one surface of the cut wafer after finishing it to a mirror finish. The method then describes a method for determining whether the quality of the seed crystal is good based on the size and distribution of the etching marks on the etched surface.

[0005] Patent Document 2 describes a method of mirror-polishing a seed crystal, examining the seed crystal under visible light or polarized light to see whether or not there is a striped pattern in the Z-axis direction, and then selecting and growing the crystal.

[0006] However, the method of Patent Document 1 is limited to observing internal defects in the crystal as etching marks in specific orientations of the material. For example, it cannot be applied to crystals made of lithium tantalate (LT) or lithium niobate (LN) pulled in the Y-axis direction at 30° to 50°.

[0007] Furthermore, the method of Patent Document 2 requires time to mirror-polish all the cut seed crystals, and requires skill to visually recognize the stripe-shaped pattern.

[0008] It should be noted that, as a means of evaluating local characteristics of a single crystal wafer, Non-Patent Document 1 describes a method of measuring the speed of sound on the wafer surface using a linear focused beam ultrasonic microscope.

[0009] Non-Patent Document 2 describes the relationship between the composition and lattice constant of a lithium tantalate single crystal and the speed of sound.

[0010] Prior art literature Patent Literature Patent Document 1: Japanese Patent Application Laid-Open No. 2002-187799 Patent Document 2: Japanese Patent Application Laid-Open No. 6-211595 Non-patent literature Non-patent document 1: J.Kushibiki, N.Chubachi, "Material Characterization by Line-Focus-Beam Acoustic Microscope", IEEE Transactions on Sonics and Ultrasonics, Vol.SU-32, No.2, pp.189-212, 1985. Non-patent document 2: J.Kushibiki; Y.Ohashi; T.Ujiie, "Standardized evaluation of chemical compositions of LiTaO3 / single crystals for SAW devices using the LFBultrasonic material characterization system", IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Volume: 49, No.4, pp.454-465, 2002. Summary of the Invention

[0011] Problems to be solved by the invention The object of the present invention is to provide a method for manufacturing a single crystal ingot by simply selecting a high-quality portion from a single crystal ingot and using a seed crystal obtained by cutting out the portion to grow a single crystal, thereby being able to manufacture a single crystal ingot with excellent crystallinity with a good yield, and further to provide a method for manufacturing a single crystal wafer by making a wafer from the single crystal ingot, thereby being able to manufacture a wafer with excellent device characteristics.

[0012] Means used to solve problems In order to achieve the above-mentioned object, the present invention provides the following method for manufacturing a single crystal ingot and a method for manufacturing a single crystal wafer. [1] A method for manufacturing a single crystal ingot, characterized in that it includes: a step of preparing a first single crystal ingot having a crystal growth axis; a step of cutting out an evaluation substrate and a seed crystal ingot from the first single crystal ingot, and forming a projection surface on the seed crystal ingot; a step of measuring a local sound velocity on the evaluation substrate, and arranging a normal point at a position where the measured sound velocity on the evaluation substrate is within a specified range; a step of arranging a mapping of the normal point on the projection surface; a step of cutting out a crystal piece including a straight line passing through the mapping of the normal point and parallel to the crystal growth axis from the seed crystal ingot; and a step of using the crystal piece as a seed crystal to manufacture a second single crystal ingot.

[0013] [2] The method for manufacturing a single crystal ingot according to the above-mentioned [1] is characterized in that the sound velocity of the evaluation substrate representing the aforementioned evaluation substrate or the evaluation substrate cut out from the first single crystal ingot different from the aforementioned first single crystal ingot is specified as the representative value of the sound velocity; and the position where the absolute value of the difference between the representative value of the sound velocity and the measured value of the sound velocity reaches below the specified value is set as the normal point.

[0014] [3] The method for manufacturing a single crystal ingot according to [2] above is characterized in that the representative value of the aforementioned sound velocity is the median of the sound velocity measured at multiple points.

[0015] [4] The method for manufacturing a single crystal ingot according to [2] or [3] above is characterized in that the position where the absolute value of the difference between the representative value of the aforementioned sound velocity and the measured value of the aforementioned sound velocity reaches less than 0.60 m / s is set as the normal point.

[0016] [5] The method for manufacturing a single crystal ingot according to any one of [1] to [4] above is characterized in that it further includes: a step of configuring an abnormal point at a position where the measured value of the sound velocity measured on the aforementioned evaluation substrate deviates from a specified range; and a step of configuring a mapping of the aforementioned abnormal point on the aforementioned projection surface; the aforementioned seed crystal is a crystal piece that does not contain a straight line mapping passing through the aforementioned abnormal point and is parallel to the aforementioned crystal growth axis.

[0017] [6] The method for manufacturing a single crystal ingot according to any one of [1] to [5] above, wherein the sound velocity is the phase velocity of LSAW (Leaky Surface Acoustic Wave) propagating on the surface of the evaluation substrate.

[0018] [7] The method for manufacturing a single crystal ingot according to any one of [1] to [6] above, characterized in that when producing the second single crystal ingot, the crystal is grown starting from the seed side of the seed crystal.

[0019] [8] The method for manufacturing a single crystal ingot according to any one of [1] to [7] above, characterized in that the material of the first single crystal ingot and the second single crystal ingot is lithium niobate single crystal or lithium tantalate single crystal.

[0020] [9] A method for manufacturing a single crystal wafer, characterized in that it includes the step of slicing the second single crystal ingot manufactured by the method for manufacturing a single crystal ingot described in any one of [1] to [8] to produce a single crystal wafer.

[0021] Effects of the Invention According to the present invention, a method for producing a single crystal ingot capable of producing a single crystal ingot having excellent crystallinity with a high yield, and a method for producing a single crystal wafer capable of producing a wafer having excellent device characteristics can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a diagram showing an example of a first single crystal ingot.

[0023] Figure 2 This is a diagram showing an example of an evaluation substrate and a seed crystal ingot.

[0024] Figure 3 This is a diagram showing an example of mapping between normal points on the evaluation substrate and normal points on the projection surface of the seed crystal ingot.

[0025] Figure 4 This is a schematic diagram showing an example of an ultrasonic interference detection unit of a linear focused beam ultrasonic microscope.

[0026] Figure 5 This is a diagram showing an example of the shape of a curve of intensity V(z) recorded while changing the focal position of the acoustic lens in the depth direction z of the evaluation substrate.

[0027] Figure 6 This is a diagram showing an example of a second single crystal ingot.

[0028] Figure 7 This is a diagram for explaining the step of slicing the second single crystal ingot to produce single crystal wafers.

[0029] Figure 8 It is a diagram showing a method for producing a seed crystal according to another embodiment of the method for producing a single crystal ingot according to the embodiment of the present invention.

[0030] Figure 9 It is a diagram showing a method for producing a seed crystal according to another embodiment of the method for producing a single crystal ingot according to the embodiment of the present invention.

[0031] Figure 10 This is a diagram illustrating a method for producing a seed crystal in another embodiment of the present invention. DETAILED DESCRIPTION

[0032] [Method for Manufacturing Single Crystal Ingot] The method for manufacturing a single crystal ingot according to an embodiment of the present invention is characterized by comprising: step A of preparing a first single crystal ingot having a crystal growth axis; step B of cutting an evaluation substrate and a seed crystal ingot from the first single crystal ingot and forming a projection surface on the seed crystal ingot; step C of measuring the local sound velocity of the evaluation substrate and arranging a normal point at a position on the evaluation substrate where the measured sound velocity value is within a predetermined range; step D of arranging a map of the normal point on the projection surface; step E of cutting a crystal slice from the seed crystal ingot containing a straight line parallel to the crystal growth axis and passing through the map of the normal point; and step F of using the crystal slice as a seed crystal to produce a second single crystal ingot. Thus, single crystal ingots with excellent crystallinity can be manufactured with a high yield. The steps of the method for manufacturing a single crystal ingot according to the embodiment of the present invention are described in detail below with reference to the figures. It should be noted that the method for manufacturing a single crystal ingot according to the embodiment of the present invention is particularly suitable for manufacturing single crystal ingots made of lithium niobate or lithium tantalate.

[0033] (Step A) Below, refer to Figure 1 Let's explain step A. Figure 1 is a diagram illustrating an example of a first single crystal ingot. In step A, a first single crystal ingot 101 having a crystal growth axis is prepared. For example, first single crystal ingot 101 can be produced by the CZ method. In this case, the first single crystal ingot is slowly pulled from a raw material melt, so that the produced single crystal ingot has a crystal growth axis parallel to the pulling direction.

[0034] (Step B) Below, refer to Figure 2 Explain step B. Figure 2 1 is a diagram showing an example of an evaluation substrate and a seed crystal ingot. In step B, an evaluation substrate 102 and a seed crystal ingot 103 are cut out from a first single crystal ingot, and a projection surface 104 is formed on the seed crystal ingot. For example, it is preferable to cut out a first single crystal ingot 101 (see Figure 1 ) is cut off from the end material of the tail portion (end portion of crystal growth by the CZ method) not shown in the figure, and then the evaluation substrate 102 and the seed crystal ingot 103 are cut out. Crystal defects propagate in the direction of crystal growth. In order to detect defective points without omission, it is preferable to cut out the evaluation substrate 102 from the tail side (end side of crystal growth) of the first single crystal ingot 101. In addition, from the first single crystal ingot 101 (refer to Figure 1When seed crystal ingot 103 is cut, the cut surface produced on seed crystal ingot 103 can be used as projection surface 104 of seed crystal ingot 103 relative to evaluation substrate 102. It should be noted that the projection surface is a surface that indicates the position of the seed crystal ingot at a predetermined position relative to the evaluation substrate, parallel to the crystal growth axis, before the evaluation substrate and seed crystal ingot are cut from the first single crystal ingot. For example, it is preferable to pre-mark projection surface 104 so that the cutting position and orientation are consistent with evaluation substrate 102. It should be noted that the first single crystal ingot can be cut using a band saw, wire saw, or the like.

[0035] (Step C) Below, refer to Figure 3 Explain step C. Figure 3 This diagram shows an example of mapping between normal points on the evaluation substrate and normal points on the projection surface of the seed crystal ingot. In step C, the local sound velocity is measured on the evaluation substrate 102, and normal points 105 are arranged at positions on the evaluation substrate where the measured sound velocity value is within a predetermined range.

[0036] <Measurement of the speed of sound> It is preferred to mirror-polish one side of the evaluation substrate 102, such as the tail side, and measure the local sound velocity on the mirror surface. The local sound velocity can be measured, for example, using a linear focused beam ultrasonic microscope described in non-patent document 1. By using a linear focused beam ultrasonic microscope, the local sound velocity (in the range of tens of μm) on the evaluation substrate 102 can be measured simply and accurately. Figure 4 , a method for measuring the local sound velocity of the evaluation substrate 102 is described. Figure 4 This is a schematic diagram showing an example of an ultrasonic interference detection unit of a linear focused beam ultrasonic microscope.

[0037] The acoustic lens (ACOUSTIC LENS) 2 of the ultrasonic microscope is positioned opposite to the measuring point 4 on the surface of the evaluation substrate 102, and ultrasound generated by the piezoelectric transducer (TRANSDUCER) 1 provided on the back of the acoustic lens 2 is irradiated onto the measuring point 4. The space between the acoustic lens 2 and the measuring point 4 is filled with "water" of the reference medium 3. The ultrasound irradiated onto the evaluation substrate 102 is reflected and returned to the acoustic lens 2, and the reflected wave is transmitted to the transducer 1, where the intensity of the reflected wave is converted into an electrical signal and recorded. The curve shape of the intensity V(z) recorded while changing the focal position 5 of the acoustic lens in the depth direction z of the evaluation substrate 102 (for example, referring to Figure 5 ), the phase velocity of the LSAW (Leaky Surface Acoustic Wave) propagating at the interface between the evaluation substrate 102 and the water 3 at the measurement point 4 is calculated by the following formula.

[0038] [Number 1] Here, V LSAW is the sound velocity (phase velocity) of LSAW at the measurement point 4 of the evaluation substrate 102, V w is the speed of sound in water. In addition, f is the ultrasonic frequency, and Δz is the vibration period of the V(z) curve (for example, see Figure 5 ).

[0039] <Normal point configuration> The normal point is arranged at a position where the measured value of the sound velocity on the evaluation substrate is within the specified range. The results of the study show that if defects are generated in the crystal, the deviation of the lattice constant exceeds 10 -5 About (0.001%), the deviation of the growth direction of the crystal locally develops and is fixed. If the lattice spacing of the crystal deviates or the composition changes, the density and elastic coefficient of the part change, and therefore the sound speed changes. Therefore, the judgment of whether the measured sound speed is normal or abnormal can be made by evaluating the extent to which the sound speed value deviates from the representative value. Specifically, the position where the absolute value of the difference between the representative value of the specified sound speed and the measured value of the sound speed is equal to or less than the specified value is recorded as a normal point. On the other hand, the position where the absolute value of the difference between the representative value of the specified sound speed and the measured value of the sound speed is greater than the specified value is recorded as an abnormal point. For example, at Figure 3 In FIG. 1 , a position on the evaluation substrate 102 where the sound velocity is determined to be normal is a normal point 105 . Also, a position on the evaluation substrate 102 where the sound velocity is determined to be abnormal is an abnormal point 106 .

[0040] The representative value of the sound velocity may be determined by specifying the sound velocity of a representative evaluation substrate or an evaluation substrate cut from a first single crystal ingot different from the first single crystal ingot. For example, if the composition of the single crystal ingot is constant along the growth direction, the representative value may be determined by pre-determining the average or median of the sound velocity values measured for evaluation substrates of single crystal ingots from which multiple high-quality seed crystals were previously obtained.

[0041] On the other hand, if the composition of a single crystal ingot may slowly change during growth, the sound velocity can be measured over a wide range within a single evaluation substrate 102, and a "representative value" for the evaluation substrate can be determined based on the measurement results. For example, when growing single crystals of lithium tantalate or lithium niobate using the CZ method, the Li2O content slowly increases from the start of crystal growth to the end of crystal growth, resulting in a gradual increase in the sound velocity. In such cases, since the composition is nearly identical within a plane perpendicular to the crystal growth direction, the evaluation substrate can be produced by cutting the substrate along a plane as perpendicular as possible to the crystal growth direction (growth axis), i.e., at an angle of 70° to 90°, preferably 80° to 90°, and even more preferably 85° to 90°, relative to the crystal axis. Furthermore, measuring the sound velocity at multiple points within a single substrate over as wide a range and as evenly (approximately at equal intervals) as possible is preferred because a representative value with high statistical reliability can be obtained. Alternatively, the average of the sound velocities at all measurement points can be used as a representative value for the sound velocity of the evaluation substrate. However, it is preferable to use the median value, which is less affected by the deviation value, as the representative value of the sound velocity in the evaluation substrate. It should be noted that the median value refers to the sound velocity value in the middle when the sound velocity values of all measurement points are arranged in order of magnitude.

[0042] Alternatively, if the plane from which the evaluation substrate was cut is not perpendicular to the growth axis, the sound velocity can be measured at as wide and evenly spaced points as possible (approximately at equal intervals) along a straight line perpendicular to the growth axis on the evaluation substrate. The average or median of these measurements can be used as the representative sound velocity value for that line. In this case, a representative sound velocity value can be derived for each straight line perpendicular to the growth axis on the evaluation substrate, and the normality / abnormality of the sound velocity can be determined for each of these straight lines.

[0043] For example, when the material of the first single crystal ingot is lithium tantalate, the sonic velocity V LSAW (Surface waves (SV-type surface waves) with variations in the propagation direction and the thickness direction of the substrate) are about 3125 m / s at 23°C. If the lattice constant of the crystal increases by about 0.001% (10 -5 ), then the speed of sound V LSAWIt only decreases by approximately 0.50 to 0.60 m / s. Therefore, in this case, when the absolute value of the difference between the representative value of the sound velocity and the measured value of the sound velocity is 0.60 m / s or less, the measured sound velocity is judged to be normal, and when the absolute value of the difference between the representative value of the sound velocity and the measured value of the sound velocity is greater than 0.60 m / s, the measured sound velocity is judged to be abnormal. The absolute value of the difference between the representative value of the sound velocity and the measured value of the sound velocity, which serves as the criterion for judging whether the measured sound velocity is normal or abnormal, is preferably 0.50 m / s or less, and more preferably 0.40 m / s or less. When the material of the first single crystal ingot is lithium niobate, the same value can also be used as the criterion for judgment.

[0044] (Step D) Below, refer to Figure 3 Step D is described. In step D, a mapping 107 of normal point 105 is arranged on the projection plane. It should be noted that the mapping refers to the position on the projection plane of the seed crystal ingot, located parallel to the crystal growth axis, relative to the predetermined position of the evaluation substrate before the evaluation substrate and the seed crystal ingot are cut from the first single crystal ingot. For example, while maintaining the arrangement of the evaluation substrate 102 and the seed crystal ingot 103 before they are cut and separated, a projection parallel to the crystal growth axis is made from normal point 105 onto the projection plane 104, and the mapping 107 of the normal point is arranged on the projection plane. Alternatively, while maintaining the arrangement of the evaluation substrate 102 and the seed crystal ingot 103 before they are cut and separated, a projection parallel to the crystal growth axis is made from abnormal point 106 onto the projection plane 104, and a mapping 108 of the abnormal point is arranged on the projection plane.

[0045] (Step E) Below, refer to Figure 3 Step E is now described. In step E, a crystal piece 109 is cut from the seed crystal ingot 103, which includes a straight line passing through the normal point map 107 and parallel to the crystal growth axis. Since the normal point 105 of the evaluation substrate 102 has few crystal defects, the normal point map 107 of the seed crystal ingot 103 also has few crystal defects. Crystal defects expand as the crystal grows, so if the normal point map 107 of the seed crystal ingot 103 has few crystal defects, then the portion cut from the seed crystal ingot 103 that includes a straight line passing through the normal point map 107 and parallel to the crystal growth axis can be expected to also have few crystal defects in the resulting crystal piece 109. Such a crystal piece 109 becomes a high-quality seed crystal 109. It should be noted that the seed crystal 109 is cut from the seed crystal ingot 103 using a band saw, wire saw, or the like.

[0046] It is more preferable that the seed crystal 109 does not include a straight line that passes through the map 108 of the outlier point and is parallel to the crystal growth axis. Furthermore, it is more preferable that the seed crystal 109 does not include a region 110 that includes a straight line that passes through the map 108 of the outlier point and is parallel to the crystal growth axis. This can further prevent the seed crystal from including a crystal with many crystal defects.

[0047] The shape of the cross section of the crystal sheet (seed) in the direction perpendicular to the length direction is not particularly limited and can be appropriately selected based on the single crystal ingot grown from the crystal sheet (seed). The cross-sectional shape of the crystal sheet (seed) includes, for example, a rectangle, a square, a triangle, a pentagon, a hexagon, a circle, an ellipse, etc. Among these, from the viewpoint of being easy to cut out from the seed ingot, the cross-sectional shape of the crystal sheet (seed) is preferably a rectangle or a square. The size of the cross section of the crystal sheet (seed) can also be appropriately selected based on the single crystal ingot grown from the crystal sheet (seed). For example, when the cross-sectional shape of the crystal sheet (seed) is a rectangle or a square, the length of one side of the rectangle or square is preferably 6 to 12 mm, more preferably 8 to 10 mm. For the region containing the straight line mapped through the above-mentioned abnormal point and parallel to the crystal growth axis, it is also the same cross-sectional shape and cross-sectional size as the crystal sheet (seed).

[0048] (Step F) Below, refer to Figure 6 Explain step F. Figure 6 : is a diagram showing an example of a second single crystal ingot. In step F, a crystal sheet is used as a seed crystal to produce a second single crystal ingot. If the high-quality seed crystal 109 produced in this way is used to produce the second single crystal ingot 111, a second single crystal ingot with excellent crystallinity can be produced with a good yield. At this time, it is preferred to grow the second single crystal ingot in such a way that the seed side of the first single crystal ingot 101 of the seed crystal 109, that is, the side close to the seed crystal (the opposite side of the growth direction of the first single crystal ingot) becomes the base point. The defects generated during the cultivation of the first single crystal ingot expand as the crystal grows. That is, the more defects there are toward the bottom of the first single crystal ingot, and the less defects there are on the seed crystal side. Therefore, the probability of defects in the second single crystal ingot is reduced by using the seed crystal side of the seed crystal as the base point.

[0049] [Method for Manufacturing Single Crystal Wafer] Below, refer to Figure 7 A method for manufacturing a single crystal wafer according to an embodiment of the present invention will be described. Figure 7This figure illustrates the steps of slicing a second single crystal ingot to produce a single crystal wafer. The single crystal wafer manufacturing method according to an embodiment of the present invention includes the steps of slicing a second single crystal ingot 111 manufactured by the single crystal ingot manufacturing method according to an embodiment of the present invention to produce a single crystal wafer 112. As described above, the second single crystal ingot 111 has excellent crystallinity, and therefore, the single crystal wafer 112 obtained by slicing the second single crystal ingot 111 has excellent device characteristics. It should be noted that the second single crystal ingot 111 can be sliced using a band saw, a wire saw, or the like.

[0050] As described above, by selecting a high-quality portion from a single crystal ingot and using the seed crystal 109 obtained by cutting this portion to produce a second single crystal ingot 111, the formation of abnormal growth ridges and the generation of cracks due to dislocations in the seed crystal propagating through the crystal are suppressed, enabling the production of single crystal ingots with excellent crystallinity at a high yield. Furthermore, by using single crystal ingots with excellent crystallinity to produce wafers, single crystal wafers with excellent device characteristics can be produced.

[0051] [Modification] The method for producing a single crystal ingot in the embodiment of the present invention can be modified as follows.

[0052] (Variation 1) In the method for producing a single crystal ingot according to the embodiment of the present invention, a seed crystal can be prepared as follows. Figure 8 and Figure 9 It is a diagram showing a method for producing a seed crystal according to another embodiment of the method for producing a single crystal ingot according to the embodiment of the present invention.

[0053] For example, in this embodiment, the first single crystal ingot is cut and separated into the evaluation substrate 202 and the seed crystal ingot 203 (see Figure 8 The seed crystal ingot 203 is cut into a grid shape with the crystal growth direction of the long side, and then cut into strips to pre-cut out a plurality of crystal slices that are candidates for seed crystals. In this way, a large number of seed crystal slices can be cut from the seed crystal ingot 203. Figure 8 The example shows the case where a crystal piece having 9 vertical rows and 9 horizontal columns is cut out. Figure 9 As shown, the sound velocity measurement points A01 to A69 are arranged so that the sound velocity measurement points and a straight line passing through the projection of the points on the projection plane 204 and parallel to the crystal growth axis intersect with each crystal piece. Figure 9 An example is shown in which 69 measurement points A01 to A69 are arranged so as to correspond to 69 crystal pieces.

[0054] In this case, the sound velocity measurement points on the evaluation substrate 202 are arranged in a grid pattern, with the spacing between them preferably being equal to or less than the sum of the short side dimensions of the produced crystal piece and the cutting margin (cutting margin). For example, if the short side of the seed crystal is 9 mm and the cutting margin is 1 mm, the measurement points can be spaced 10 mm apart (9 mm short side + 1 mm cutting margin) or less, so that the measurement points do not overlap with the cutting margin.

[0055] Since the sound velocity is measured uniformly over a wide range of the evaluation substrate 202 in this manner, it is appropriate to derive a representative value of the sound velocity of the evaluation substrate 202 by using the average value or median thereof.

[0056] Furthermore, by identifying a normal point from among the multiple sound velocity measurement points on the evaluation substrate 202, projecting this normal point onto the projection plane 204 to form a normal point map, and using only crystal slices that have a straight line passing through the map of this normal point and parallel to the crystal growth axis intersecting as seed crystals, it is possible to efficiently produce seed crystals in large quantities. Furthermore, if an abnormal point exists on the evaluation substrate 202, it is further preferred to exclude crystal slices that contain a straight line passing through the map of this abnormal point and parallel to the crystal growth axis from use as seed crystals. Specifically, step D can be performed after step E, or after crystal slices are cut from the seed crystal ingot, the normal point can be arranged on the projection plane of the seed crystal ingot within the crystal slice.

[0057] That is, the step of cutting the seed crystal ingot 203 to produce a crystal piece (seed crystal) can be performed after measuring the sound velocity of the evaluation substrate 202 and determining whether the normal point / abnormal point is present. Alternatively, the seed crystal ingot 203 can be cut into crystal pieces in advance, and then the normal point / abnormal point on the evaluation substrate 202 can be determined, and the seed crystal can be selected based on the result.

[0058] By using the crystal piece selected as a normal point in this manner as a seed crystal, a second single crystal ingot is produced, and slicing the ingot can produce high-quality single crystal wafers with excellent device characteristics.

[0059] (Variation 2) In the method for producing a single crystal ingot according to the embodiment of the present invention, a seed crystal can be prepared as follows. Figure 10 In the above step E, if the seed crystal (crystal sheet) contains a straight line that passes through the normal point and is parallel to the crystal growth axis, then Figure 10 In this way, a seed crystal 309 having a crystal growth axis not parallel to the longitudinal direction of the seed crystal 309 can be cut out from the seed crystal ingot 303. Thus, the method for manufacturing a single crystal ingot according to the embodiment of the present invention can be applied to processing single crystals in all directions.

[0060] Specifically, the local sound velocity is measured on the evaluation substrate 302, and a normal point 305 is arranged at a position on the evaluation substrate where the measured sound velocity value is within a predetermined range. Next, a normal point map 307 or an abnormal point map 308 is arranged on the projection plane 304 of the seed crystal ingot 303, while maintaining the positional relationship between the evaluation substrate 302 and the seed crystal ingot 303. Furthermore, a crystal piece (seed crystal) 309 is cut from the seed crystal ingot 303, including a straight line passing through the normal point map 307 and parallel to the crystal growth axis, and having a crystal growth axis that is not parallel to the longitudinal direction of the seed crystal. Even in such a case, a high-quality seed crystal 309 can be obtained, and this seed crystal can be used to produce a second single crystal ingot, which can then be used to produce a wafer. This allows single crystal wafers with excellent device characteristics to be produced regardless of the wafer orientation. In this case, the seed crystal is more preferably a crystal piece that further does not contain a straight line mapped 308 passing through the abnormal point and parallel to the crystal growth axis, and is even more preferably a crystal piece that does not contain a region 310 containing a straight line mapped 308 passing through the abnormal point and parallel to the crystal growth axis.

[0061] The method for manufacturing a single crystal ingot and the method for manufacturing a single crystal wafer of the present invention are not limited to the method for manufacturing a single crystal ingot and the method for manufacturing a single crystal wafer of the embodiment of the present invention described above.

[0062] The present invention is particularly suitable for use in piezoelectric single crystals used in surface acoustic wave devices, that is, lithium tantalate single crystals or lithium niobate single crystals. Example

[0063] Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

[0064] (Example) The first lithium tantalate (LT) single crystal ingot with a crystal orientation of 38°RY, a straight body length of about 90 mm, and a diameter of 4 inches was grown by the CZ method. Specifically, the raw material of LT was filled into an iridium crucible, and after the raw material was melted by high-frequency heating, the tip of a seed crystal with a crystal orientation of 38°RY was brought into contact with the melt and slowly pulled up to obtain a 1.5- ... Figure 1 The LT single crystal ingot is shaped as shown in the example. It should be noted that the pulling is performed in a mixed gas of nitrogen and oxygen at a pulling speed of 1 to 5 mm / hour and a rotation speed of 5 to 20 rpm.

[0065] Next, the tail end material (the terminal end of crystal growth) is cut off from the obtained 4-inch diameter LT single crystal ingot in a manner that the cut surface is perpendicular to the crystal growth axis, and an evaluation substrate with a thickness of about 250 μm is further cut out from the tail side to obtain an evaluation substrate and a seed crystal ingot.

[0066] The obtained evaluation substrate was mirror-polished on the rear side, and the sound velocity of the mirror side was measured by material analysis using a linear focused beam ultrasonic microscope. Figure 4 ) is measured with a line of 10 mm spacing, and the measurement line is further repeatedly moved in the direction perpendicular to the x-axis (y-axis direction) (refer to Figure 4 ) is offset by 10mm and a line measurement is performed with a pitch of 10mm in the direction parallel to the x-axis. Figure 9 The sound velocity was measured at 69 measurement positions A01 to A69 arranged as shown. The measured sound velocity values are shown in Table 1.

[0067] The median of these 69 sound velocities, 3130.57 m / s, was recorded as the representative sound velocity value for the evaluation substrate. Next, locations where the absolute difference between the sound velocity values at each measurement point and the representative sound velocity value was greater than 0.60 m / s, namely A31, A32, A33, A34, A35, and A36, were designated as abnormal points. Other locations with a sound velocity of 0.60 m / s or less were designated as normal points, and these points were marked on the evaluation substrate. Furthermore, while maintaining the positional relationship between the evaluation substrate and the seed crystal ingot, a map of the normal and abnormal points was plotted on the projection surface of the seed crystal ingot.

[0068] Next, crystal slices were cut from the seed crystal ingot. The slices were cut with a crystal orientation of 38° RY. Specifically, 69 slices were cut, each with its length parallel to the crystal growth axis, a 9×9 mm square bottom, a length of approximately 90 mm, and an X-plane on each end face. The cutting allowance during slice production was approximately 1 mm. Sixty-three mapped crystal slices with normal points marked were used as seed crystals.

[0069] A 5mm diameter hole was drilled through the X-plane on the trailing side of the resulting crystal piece, processing it into a shape that could be pinned. Using each crystal piece as a seed crystal, an LT single crystal ingot with a crystal orientation of 38° RY, a straight body length of approximately 90mm, and a diameter of 4 inches was grown.

[0070] In the growth of LT single crystal ingots, LT raw materials are filled into an iridium crucible, and after the raw materials are melted by high-frequency heating, the front end of these seed crystals, in this case, the seed side of the first LT single crystal ingot, is brought into contact with the melt and slowly pulled up to obtain Figure 6 LT single crystal ingots were shaped as shown. Pulling was performed in a mixture of nitrogen and oxygen at a pulling rate of 1 to 5 mm / hour and a rotation speed of 5 to 20 rpm. Once an LT single crystal ingot was produced, the seed crystal was cut off at the tapered portion of the ingot and used to produce the next batch of LT single crystal ingots. A total of 184 batches of LT single crystal ingots were produced in this manner.

[0071] [Table 1]

[0072] (Comparative Example) Among the crystal pieces cut from the seed crystal ingot of the embodiment, the six crystal pieces corresponding to the abnormal points A31, A32, A33, A34, A35, and A36 whose absolute value of the difference from the representative value of the sound velocity of the evaluation substrate is greater than 0.60 m / s are used as seeds for the comparative example.

[0073] A hole with a diameter of 5 mm was made on the tail side of the comparative example seed crystal so as to penetrate the X surface, and the hole was processed into a shape that could be pinned. Using each seed crystal, LT single crystal ingots with a crystal orientation of 38°RY, a straight body length of approximately 90 mm, and a diameter of 4 inches were grown. The cultivation of LT single crystal ingots was performed under the same process and conditions as in the embodiment. Once the LT single crystal ingot was produced, the seed crystal was cut off at the conical part of the single crystal ingot and used to produce the next batch of LT single crystal ingots. A total of 38 batches of LT single crystal ingots were produced in this way.

[0074] Table 2 shows the results of examining the presence or absence of cracks and abnormal growth ridges for the LT single crystal ingots produced in the examples and the LT single crystal ingots produced in the comparative examples.

[0075] [Table 2]

[0076] The LT single crystal ingot of the example had a much lower incidence of cracks and abnormal growth ridges than the LT single crystal ingot of the comparative example, and the yield of good quality crystals was high.

[0077] Furthermore, the LT single crystal ingot of the embodiment and the LT single crystal ingot of the comparative example were sliced to produce single crystal wafers, and the device characteristics were compared. After slicing and grinding the LT single crystal ingot, the single crystal wafer was made by finishing it into a mirror surface by grinding one side. In addition, the evaluation device was made by vapor-depositing aluminum (Al) with a thickness of 0.4μm on the mirror surface of the obtained wafer, leaving a resist on the electrode part by photolithography, and then removing the unnecessary Al part by dry etching. Furthermore, the resist was removed by ashing, and a 1-port resonator was made on the entire surface of the wafer. The crystal orientation of the LT single crystal wafer is 38° rotated Y-cut (38° rotation Y-cut).

[0078] Evaluation of a 1-port resonator for the maximum Q value (Qmax) and electromechanical coupling coefficient K 2 , and the average value within the wafer surface is calculated. The operating frequency of the 1-port resonator is set to approximately 850 MHz. Note that the Q value is calculated using the following formula.

[0079] [Number 2] Here, ω is the angular frequency, τ(f) is the group delay time, and Γ is the reflection coefficient measured with a network analyzer.

[0080] In addition, the electromechanical coupling coefficient (K 2 ) is obtained by the following formula.

[0081] [Number 3] Here, f r is the resonant frequency, f a is the anti-resonance frequency.

[0082] The evaluation results are shown in Table 3.

[0083] [Table 3]

[0084] The single crystal wafers of the embodiment have a higher Qmax value and a lower K value than the single crystal wafers of the comparative example. 2 High value and excellent device characteristics.

[0085] Description of Reference Numerals 1 transducer 2 Acoustic Lens 3 Reference medium 4 measuring points 5. Focal position of the acoustic lens 101 First Single Crystal Ingot 102, 202, 302 evaluation substrates 103, 203, 303 seed ingots 104, 204, 304 projection surfaces 105, 305 normal 106, 306 abnormal points 107, 307 Normal Point Mapping 108, 308 Outlier Mapping 109, 309 crystal pieces (seed crystals) 110, 310 outlier areas 111 Second single crystal ingot 112 single crystal wafer

Claims

1. A method for manufacturing a single crystal ingot, characterized in that: include: a step of preparing a first single crystal ingot having a crystal growth axis; cutting out an evaluation substrate and a seed crystal ingot from the first single crystal ingot, and forming a projection surface on the seed crystal ingot; The step of measuring a local sound velocity on the evaluation substrate, arranging a normal point at a position on the evaluation substrate where the sound velocity value is within a predetermined range, and arranging an abnormal point at a position on the evaluation substrate where the sound velocity value is outside the predetermined range; a step of arranging a mapping of the normal points and a mapping of the abnormal points on the projection surface; a step of cutting out a crystal piece from the seed crystal ingot, the crystal piece including a straight line that passes through the normal point and is parallel to the crystal growth axis; and The step of using the crystal sheet as a seed crystal to produce a second single crystal ingot, The seed crystal is a crystal piece that does not include a straight line that passes through the abnormal point and is parallel to the crystal growth axis. specifying the sound velocity of a representative evaluation substrate or an evaluation substrate cut out from a first single crystal ingot different from the first single crystal ingot as a representative value of the sound velocity; The representative value of the aforementioned sound velocity is the median of the sound velocity values measured at multiple points. The position where the absolute value of the difference between the representative value of the speed of sound and the measured value of the speed of sound becomes 0.60 m / s or less is defined as the normal point. The position where the absolute value of the difference between the representative value of the speed of sound and the measured value of the speed of sound is greater than 0.60 m / s is defined as the abnormal point. The evaluation substrate has both the normal point and the abnormal point. The sound velocity is the phase velocity of LSAW (Leaky Surface Acoustic Wave) propagating on the surface of the evaluation substrate.

2. The method for manufacturing a single crystal ingot according to claim 1, wherein: When producing the second single crystal ingot, crystal growth is caused using the seed crystal side of the seed crystal as a starting point.

3. The method for manufacturing a single crystal ingot according to claim 1 or 2, wherein: The material of the first single crystal ingot and the second single crystal ingot is lithium niobate single crystal or lithium tantalate single crystal.

4. A method for manufacturing a single crystal wafer, characterized in that: include: A step of slicing the second single crystal ingot produced by the method for producing a single crystal ingot according to any one of claims 1 to 3 to produce single crystal wafers.

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