JP4590549B2 - Raw material composition determination method for manufacturing ferroelectric single crystal, acoustic related physical constant calibration method of ferroelectric single crystal, and design parameter determination method of surface acoustic wave device - Google Patents

Description

The present invention relates to a method for determining a raw material composition for producing a ferroelectric single crystal such as LiNbO3 or LiTaO3, a method for calibrating acoustic-related physical constants of the ferroelectric single crystal, and a surface acoustic wave device. The present invention relates to a method for determining design parameters with high accuracy.

Ferroelectric LiNbO ₃ and LiTaO ₃ single crystals have not only piezoelectric effects but also excellent optical properties such as electro-optic effects and nonlinear optical effects, so not only surface acoustic wave (SAW) devices but also optoelectronic devices Applications are thriving. The device performance ultimately depends on the quality of the single crystal substrate. Therefore, improvement of the homogeneity of the single crystal substrate (mainly uniform chemical composition ratio) is an indispensable element for improving the performance of the device. However, measurement of the Curie temperature T _C which has been widely used conventionally for chemical composition evaluation differs depending on the measurement conditions and apparatus of each crystal manufacturer, also not sufficient even measurement reproducibility Therefore, reliable crystal evaluation could not be performed ([Non-Patent Document 1], [Non-Patent Document 2]).

On the other hand, as a highly reliable evaluation technique, a linearly focused beam (LFB) ultrasonic material characteristic analyzer ([Non-Patent Document 3], [Non-Patent Document 4]) has been proposed. This apparatus can measure the phase velocity V _LSAW of a leaky surface acoustic wave (LSAW), which is a sound wave propagating through the boundary between water and the sample substrate, with high accuracy. Evaluation is performed by grasping the change in chemical composition ratio, which is the main factor affecting the uniformity of crystals, as the change in V _LSAW . In [Patent Document 1], in order to keep the chemical composition distribution in mass-produced crystals and between crystals within an allowable range, a true congruent composition for LiTaO ₃ single crystal (a composition in which the composition of the raw material melt and the grown crystal coincides) In addition, an allowable range of deviation of the raw material melt composition is given.

However, their values for LiNbO ₃ single crystals are not required. Also, low T _C measurements because it was estimated chemical composition from conventional sought-LiNbO _3, LiTaO ₃ single crystal with respect to chemical composition and acoustical physical constants (elastic constant reliable as described above, The relationship between the piezoelectric constant, dielectric constant, and density) is also low in reliability [Non-patent Document 5]. Furthermore, if a SAW device or the like is designed based on parameters with low reliability (chemical composition ratio, acoustic-related physical constants), the device performance and yield are reduced.
Japanese Patent Application No. 2004-103891 J. Kushibiki, Y. Ohashi, and N. Mishima, “Calibration of Curie temperatures for LiTaO3 single crystals by the LFB ultrasonic material characterization system,” IEEE Trans. Ultrason., Ferroelect., Freq. Contr., Vol. 50, pp. 544-552, May 2003. J. Kushibiki, Y. Ohashi, and M. Mochizuki, "Evaluation of mass-producedcommercial LiTaO3 single crystals using the LFB ultrasonic material characterization system," IEEE Trans. Ultrason., Ferroelect., Freq. Contr., Vol. 51, pp. 748-755, June2004. J. Kushibiki and N. Chubachi, "Material characterization by line-focus-beam acoustic microscope," IEEE Trans. Sonics Ultrason., Vol. SU-32, pp.189-212, Mar. 1985. J. Kushibiki, Y. Ono, Y. Ohashi, and M. Arakawa, "Development of the line-focus-beam ultrasonic material characterization system," IEEETrans. Ultrason., Ferroelect., Freq. Contr., Vol. 49, pp. 99-113, Jan. 2002. J. Kushibiki, I. Takanaga, S. Komatsuzaki, and T. Ujiie, "Chemical composition dependences of theacoustical physical constants of LiNbO3 and LiTaO3 singlecrystals," J. Appl. Phys., Vol.91, pp.6341-6349, May 2002.

For strength measurement of the Curie temperature T _C which are conventionally used for the evaluation of the single-crystal dielectric is varied between measurement conditions and apparatus, also the measurement reproducibility is also not sufficient, homogeneous crystals It was difficult to obtain a true congruent composition that can be grown.

In addition, when designing a SAW device or the like using acoustic-related physical constants obtained as a function of the chemical composition ratio estimated from a low-reliable _TC measurement value, it impedes high-performance device design.

  Therefore, the present invention provides a method for determining a true congruent composition in which a chemical composition in a crystal becomes uniform using an LFB ultrasonic material characteristic analyzer in order to grow a uniform ferroelectric single crystal.

Further, the present invention provides a method for calibrating a chemical composition ratio that has been estimated from the Curie temperature with a more reliable V _LSAW . In addition, a method for high precision SAW device design is provided.

The raw material composition determination method for manufacturing a ferroelectric single crystal according to the first invention is:
(a) producing n single crystals having different chemical compositions in the vicinity of the congruent composition, and n is an integer of 2 or more,
(b) in each single crystal, cutting a plurality of substrates having different cut surfaces, measuring the leaky surface acoustic wave velocity while changing the propagation direction of the leaky surface acoustic wave within each substrate surface;
(c) Based on the result obtained in the step (b), a step of selecting at least one leakage surface acoustic wave propagation direction of the cut surface having a large rate of change of the leakage surface acoustic wave velocity with respect to the chemical composition ratio change. When,
(d) In the propagation direction of the cut surface selected in step (c), the step of measuring the leaky surface acoustic wave velocity at the top and bottom of each single crystal and the top and bottom of the single crystal are the seeds at the time of crystal growth. The crystal side is the top, the opposite side is the bottom,
(e) From the relationship between the upper and lower leaky surface acoustic wave velocities obtained in step (d) and the raw material composition at the time of charging, the raw material composition in which the leaky surface acoustic wave velocities match at the upper and lower parts of the crystal. With a true congruent composition C _C ,
(f) From the relationship between the leaky surface acoustic wave velocity, the leaky surface acoustic wave velocity gradient, the raw material composition, the crystal composition, and the density, an allowable width ± ΔC _C of deviation from the congruent composition was obtained, and a ferroelectric single crystal was manufactured. Determining the raw material composition for
Including.

A composition determination method according to a second invention is the material composition determination method according to the first invention, wherein the allowable range ± ΔC _C is ± 0.017 Li ₂ O-mol%.

A composition determining method according to a third invention is the composition determining method according to the first or second invention, wherein the ferroelectric single crystal is LiNbO ₃ and the congruent composition is 48.481-Li ₂ Omol%.

A fourth invention is a method for calibrating acoustic-related physical constants of a ferroelectric single crystal,
(a) measuring a leaky surface acoustic wave velocity V _LSAW for a plurality of samples having different chemical composition ratios;
(b) from a measured value such as a Curie temperature for the sample, a step of obtaining a chemical composition ratio using a relationship between the known Curie temperature and the chemical composition ratio;
(c) The relationship between the measurement rate V _LSAW obtained in the step (a) and the chemical composition ratio obtained in the step (b) (approximate straight line) is shown in (c) and (d) of the first invention. ) The chemical composition ratio in the congruent obtained in the process of step) and the chemical composition ratio are calibrated to match the value of V _LSAW , and the relationship between the two (calibrated approximate straight line) is obtained,
(d) Leaky surface acoustic wave calculated using the acoustic-related physical constants (elastic constant, piezoelectric constant, dielectric constant, density) for the n ferroelectric single crystals in step (a) of the first invention. Using the velocity and the calibrated approximate straight line obtained in step (c) to determine the chemical composition ratio of each of the n crystals, and determining an acoustic related physical constant related to the chemical composition ratio;
A method for calibrating acoustic-related physical constants , comprising:

5th invention is the design parameter determination method of a surface acoustic wave device,
( 1 ) measuring a leaky surface acoustic wave velocity with respect to the grown crystal, and using the relationship obtained in step (c) of the fourth invention to determine the chemical composition ratio of the grown crystal;
( 2 ) Using the chemical composition ratio obtained in the step ( 1 ) and the acoustic-related physical constant relating to the chemical composition ratio obtained in the step (d) of the fourth invention, the grown crystal of the step ( 1 ) Obtaining acoustic related physical constants for
( 3 ) Using the acoustic-related physical constant obtained in the above step ( 2 ), calculating a surface acoustic wave velocity with respect to the propagation direction of the cut surface used as the surface acoustic wave device;
( 4 ) determining the design parameters (electrode period, electrode width, electrode thickness) for the surface acoustic wave device design using the surface acoustic wave velocity obtained in the above step ( 3 ),
A method for determining a design parameter of a surface acoustic wave device, comprising:

The present invention can determine a true congruent composition for uniform crystal growth. Therefore, if a ferroelectric crystal having such a composition is used, a device with little variation in characteristics can be manufactured. In addition, highly reliable chemical composition evaluation using _VLSAW measurement is possible for single crystals that are mass-produced. Furthermore, since the absolute value of the chemical composition ratio of the grown crystal, which has been unclear until now, becomes clear, the chemical composition ratio, acoustic-related physical constants, and other chemical and physical properties (V _LSAW , V _SAW , T _C , lattice constant, etc.) can be obtained, and absolute crystal evaluation by various chemical and physical characteristics and high-accuracy device design are possible.

  The congruent composition determination method and the chemical composition ratio calibration method will be described with reference to the flowchart of FIG.

Step S1: Prepare n crystal ingots (n is an integer of 2 or more) having different chemical compositions near the congruent composition.
Step S2: In order to investigate the LSAW propagation direction with high sensitivity to changes in the chemical composition ratio, substrate samples of several types of cut surfaces are cut out from each crystal ingot in Step S1 (see FIG. 2). However, a substrate surface parallel to the growth axis is prepared so that the distribution in the growth axis direction of the crystal can be examined, or in the case of a substrate surface perpendicular to the growth axis, two or more substrates are prepared from the growth axis direction ( [See Reference 1]).
Step S3: The Curie temperature for each crystal ingot is measured, and the chemical composition ratio of each crystal ingot is obtained using the known relationship between the chemical composition ratio and the Curie temperature.
Step S4: For each substrate sample of each crystal ingot cut out in step S2, the propagation direction dependence of the LSAW velocity is measured at the position indicated by x in FIG.
Step S5: From the results obtained in steps S3 and S4, the chemical composition ratio dependence of the LSAW velocity in each propagation direction on each substrate surface is obtained, and the LSAW propagation direction having high sensitivity to the chemical composition ratio change is determined.
Step S6: The LSAW velocity distribution in the growth axis direction of each crystal ingot is measured in the propagation direction determined in step S5.
Step S7: From the relationship between the charged raw material composition of each crystal ingot and the LSAW velocity distribution in the growth axis direction, the composition at which the LSAW velocity distribution in the growth axis direction becomes 0 is determined as the true congruent composition, and the LSAW velocity at that time is obtained.
Step S8: The chemical composition ratio is calibrated so that the relationship between the chemical composition ratio obtained in step S5 and the LSAW velocity matches the relationship between the chemical composition ratio in the true congruent composition obtained in step S7 and the LSAW velocity. Get the correct relationship between composition ratio and LSAW speed.
Step S9-1: According to the procedure of [Patent Document 1], an allowable width (deviation from the true congruent composition) ± ΔC _C of the raw material melt composition that can be regarded as a congruent composition is obtained.
Step S9-2: The correct relationship between the chemical composition ratio and the acoustic related physical constant is obtained from the relationship between the chemical composition ratio obtained in step S8 and the LSAW speed.
Step S10-2: A chemical composition ratio is obtained from the LSAW velocity or Curie temperature measured for the grown crystal using the relationship obtained in Step S8, and an acoustic related physical constant corresponding to this chemical composition ratio is obtained in Step S9-. The SAW device design parameters (electrode width, electrode thickness, electrode spacing, etc.) optimum for the grown crystal are obtained using the acoustic-related physical constants, and the SAW device is produced.

LiNbO ₃ single crystals grown at a chemical composition ratio of 48.0, 48.5, 49.0 Li ₂ O-mol% at the time of raw material preparation were prepared. The growth conditions and size of each crystal are shown in FIG. Both are grown in the Z-axis direction by the Czochralski method. In order to evaluate the homogeneity of each crystal in the growth axis direction and the diameter direction, as shown in FIG. 2, one X- and Y-cut substrate and three Z-cut substrates were cut and double-sided optically polished. A sample was used. The shape of each substrate and the measurement position of the LSAW speed are indicated by crosses and broken lines in FIG.

In order to investigate the chemical composition of each crystal, the Curie temperature was measured by a differential thermal analysis (DTA) method on another Z-cut substrate adjacent to the Z2 substrate shown in FIG. The results are shown in FIG. Furthermore, the result of obtaining the actual Li ₂ O concentration of each crystal using the relationship between the Curie temperature and the chemical composition ratio shown in [Reference 2] is also shown as “analyzed Li ₂ O concentration” in FIG.

The density was measured on the Z2 substrate based on Archimedes' principle. In addition, the lattice constants a and c were measured by X-ray diffraction using the Bond method at the positions indicated by x in FIG. 3 for the X-cut and Z2 samples, respectively. Each result is shown in FIG. As the Li ₂ O concentration increases, the Curie temperature increases monotonically and the density and lattice constant decrease. From the linear approximation to the result of FIG. 5, the rate of change is 43.9 ° C / mol%, -5.93
(kg / m ³ ) / mol%, −1.3 × 10 ⁻⁴ nm / mol% (a axis), and −5.7 × 10 ⁻⁴ nm / mol% (c axis).

In order to investigate the dependence of acoustic characteristics on the chemical composition ratio, the propagation direction dependence of the LSAW velocity was measured for the X, Y, and Z2 substrates of crystal 1, 2, and 3 (at the x position in Fig. 3) at 225 MHz. did. The results are shown in FIG. At each crystal plane, the LSAW velocity changes to reflect the crystal symmetry. In the Z-cut substrate, the LSAW speed increases with increasing Li ₂ O in all propagation directions, but between 70 ° and 160 ° of X-cut and between 40 ° and 140 ° of Y-cut. The direction of propagation is reversed.

FIG. 7 shows the LSAW velocity and the analysis Li ₂ O concentration with respect to the propagation direction in which the ratio of the speed change (difference) to the change (difference) in chemical composition ratio (difference) between the substrates is maximum and the propagation direction of the basic axis. Show the relationship. The rate of velocity change obtained by linear approximation using the least squares method varies depending on the propagation direction, but it can be seen that there is a substantially linear relationship between the chemical composition ratio and each LSAW velocity near the congruent composition. The maximum rate of velocity change on each substrate surface is -24.9 (m / s) / Li ₂ O-mol% in the 123.4 ° Y-axis direction for the X-cut surface, and X for the Y-cut surface. It is +21.5 (m / s) / Li ₂ O-mol% in the axial direction, and +39.4 (m / s) / Li ₂ O-mol% in the Y-axis direction with respect to the Z-cut plane.

  In crystal evaluation, the direction of LSAW propagation should be chosen so that the basic axis or power flow angle is zero. Also, the sensitivity to the chemical composition ratio must be high in the selected propagation direction. From these viewpoints, the propagation in the Y-axis direction of the Z-cut plane is considered optimal. However, in order to investigate the detailed distribution in the growth axis direction (Z-axis), where the chemical composition ratio distribution appears to appear greatly, the Y-cut substrate is more advantageous than the Z-cut substrate, as can be seen from FIG. is there. Therefore, the LSAW velocity distribution in the crystal is measured here by taking the LSAW of the Y-cut substrate along with the X-axis propagation along with the Y-axis propagation of the Z-cut plane.

FIG. 8 shows the sensitivity and measurement resolution of this evaluation method (LSY speed of ZY-LiNbO ₃ and YX-LiNbO ₃ ) for various chemical and physical properties. The measurement resolution is calculated as the measurement reproducibility of LSAW speed ± 0.0013% (equivalent to ± 0.05 m / s near 3800 m / s). This ultrasonic method has a very high resolution compared to the resolution of the Curie temperature (about ± 1 ° C.), and is very advantageous in evaluating the local composition distribution. In addition, when using the LSAW speed of Z-cut Y-axis propagation, the resolution is about twice as high as when using the LSAW speed of Y-cut X-axis propagation.

  Next, the result of measuring the LSAW velocity distribution in each crystal ingot is shown. First, as a result of measuring the LSAW velocity of Y-axis propagation every 1 mm in the radial direction (dotted line in Fig. 3) of the Z-cut substrate (upper Z1, middle Z2, lower Z3) cut from crystals 1, 2, and 3. Is shown in FIG. In the radial direction of the upper part of the crystal, a speed reduction of about 0.5 to 0.8 m / s is observed in the peripheral part.

  The change in the distribution in the growth axis direction is larger than that in the radial direction with respect to the change in the raw material composition. FIG. 10 shows the result of obtaining the growth axis direction distribution from the relationship between the measured LSAW velocity at the center of each Z-cut substrate (position indicated by x in FIG. 3) and the distance from the top of the crystal. In each crystal, the LSAW speed changes almost linearly from the top to the bottom of the crystal.

The solid line in FIG. 10 is an approximate straight line by the least square method. The gradient is -0.037 (m / s) / mm for crystal 1, -0.001 (m / s) / mm for crystal 2, and +0.042 (m / s) / mm for crystal 3. mm. These values are converted to the change rate of the Li ₂ O concentration using the relationship shown in FIG.
mol% / mm, -0.00002 mol% / mm for crystal 2 and +0.00107 mol% / mm for crystal3.

  Similarly, in the radial direction of the Y-cut substrate cut from the crystals 1, 2, and 3 (dotted lines T, M, and B in FIG. 3), the LSAW speed of X-axis propagation was measured every 1 mm. Shown in As in the case of FIG. 9, in the radial direction of the upper part of the crystal (line T), the peripheral part tends to decrease in speed by about 0.3 to 0.6 m / s. The radial distribution is small.

  Next, the result of measuring the LSAW speed of the X-axis propagation at every 1 mm in the growth axis direction of the Y-cut substrate (dotted line P in FIG. 3) is shown by a solid line in FIG. In each crystal, the LSAW speed changes almost linearly from the upper part of the crystal to the lower part. However, in the vicinity of the upper part of the crystal 3, a unique change deviating from the linear change is observed.

  Since bubbles were observed in the upper part of the crystal 3, it is considered that a problem related to the growth condition was detected. This is a change that is difficult to capture with a Z-cut substrate, and can be said to be an advantage when using a Y-cut substrate. The dotted line in FIG. 12 is an approximate straight line by the least square method. The gradient is -0.022 (m / s) / mm for crystal 1, +0.001 (m / s) / mm for crystal 2, and +0.024 (m / s) / mm for crystal 3. mm.

These are converted to the rate of change of Li ₂ O concentration using the relationship shown in FIG. 7, -0.00102 mol% / mm for crystal 1, +0.00006 mol% / mm for crystal 2, and On the other hand, it is +0.00112 mol% / mm. In any crystal, the Li ₂ O concentration gradient in the growth axis direction almost coincides with that obtained from the measurement results for ZY-LiNbO ₃ .

In order to grow more homogeneous crystals, the congruent composition is determined from the obtained results. FIG. 13 shows the relationship between the LSAW velocity at the upper (0 mm) and lower (80 mm) positions of each crystal determined from the approximate straight lines in FIGS. 10 and 12, and the Li ₂ O concentration at the time of charging each crystal. The result of having been obtained is shown.

The solid line in the figure is an approximate straight line by the least square method. In FIG. 13, the intersection of two solid lines is considered to be a congruent composition in which the LSAW velocity distribution in the growth axis direction is zero. When obtaining an intersection, congruent composition is 48.477 Li ₂ O-mol% when calculated from 48.481 Li ₂ O-mol%, the result of YX-LiNbO ₃ when determined from the results of ZY-LiNbO _3, LSAW at that time The velocities were estimated to be 3875.0 m / s (ZY-LiNbO ₃ ) and 3711.6 m / s (YX-LiNbO ₃ ), respectively. The composition obtained from the result of ZY-LiNbO ₃ having the highest resolution is considered to have the highest reliability as the congruent composition. However, even when obtained from the result of YX-LiNbO ₃ , only a slight difference (0.004 Li ₂ O-mol%) as high as the measurement resolution occurs.

The analytical Li ₂ O concentration in FIGS. 5 and 7 is a value converted from the measured value of the Curie temperature. From our previous studies ([Non-Patent Document 1], [Non-Patent Document 2]), the measured value of the Curie temperature varies depending on the measurement conditions and equipment, so the analysis Li shown in FIGS. The reliability of the absolute value of ₂ O concentration is low.

Therefore, with reference to the LSAW speed in FIG. 7, the analytical Li ₂ O concentration is calibrated so as to match the Li ₂ O concentration and the LSAW speed in the true congruent composition obtained in FIG. The results for ZY-LiNbO ₃ and YX-LiNbO ₃ are shown in FIG. 14A and 14B, the calibration amounts of the analytical Li ₂ O concentration are 0.057 Li ₂ O-mol% and 0.056 Li ₂ O-mol, respectively, which are almost equal calibration amounts. This corresponds to a calibration amount of about 2.5 ° C. when converted to the Curie temperature. The result after calibration in FIG. 14A is expressed as follows.

C (Li ₂ O) = 0.0254 × (V _LSAW -3875.0)
+ 48.481 (1)

Here, C (Li ₂ O) represents the analytical Li ₂ O concentration, and V _LSAW represents the LSAW rate for ZY-LiNbO ₃ . From the formula (1), the analytical Li ₂ O concentration for the crystals 1, 2, and 3 was found, and in order, 48.324, 48.489, 48.641
Li ₂ O-mol%. Therefore, it can be seen that the crystal 2 had a composition very close to the congruent composition (within ± 0.01 Li ₂ O-mol%).

By using the relationship of Formula (1), the correct relationship between the acoustic-related physical constants (elastic constant, piezoelectric constant, dielectric constant, density) reported in [Non-Patent Document 5] and the chemical composition ratio can be obtained. In [Non-Patent Document 5], acoustic-related physical constants are determined for the crystals 1, 2, and 3 shown in FIG. Each constant is used to calculate the LSAW velocity for ZY-LiNbO ₃ [Reference 3], and by substituting it in equation (1), the chemical composition ratio corresponding to each constant is given. The results are shown in FIG.

FIG. 15 also shows the gradient of each constant with respect to the chemical composition ratio and the constant corresponding to the congruent composition 48.481 Li ₂ O-mol% estimated in FIG. 13A. The LSAW speed shown in the equation (1) is for ZY-LiNbO ₃ , but can be converted to the LSAW speed for the desired cut surface and propagation direction by calculation using the constants of FIG. Also for LiTaO ₃ single crystal, the correct relationship between the acoustic-related physical constants and the chemical composition ratio can be obtained by the same procedure based on the result obtained in [Patent Document 1] as shown in FIG.

  By using the relationship between the chemical composition ratio and the acoustic-related physical constants shown in FIG. 15, highly accurate SAW device design parameters can be obtained. First, a Z-cut substrate is cut from the top and bottom of the grown crystal, and the LSAW velocity of propagation in the Y-axis direction is measured.

Using the relationship of equation (1), the chemical composition ratio of the upper and lower grown crystals is determined from the measured LSAW rate. Using the obtained chemical composition ratio, the chemical composition ratio is obtained as a function of the distance from the upper part of the crystal by linear approximation. When the substrate to be cut out from the grown crystal is other than Z-cut, it is converted into the LSAW velocity for ZY-LiNbO ₃ by the calculation using the acoustic related physical constants of FIG. 15 according to the method shown in [Patent Document 1]. After that, substitute it into equation (1). An acoustic related physical constant corresponding to the obtained chemical composition ratio is obtained from the data of FIG. Using the obtained acoustic-related physical constants, the SAW velocity with respect to the propagation direction of the desired substrate surface used as the SAW device is numerically calculated.

  From the obtained SAW velocity, the SAW velocity is obtained as a function of the distance from the top of the crystal by linear approximation. The SAW device design parameters (electrode width, electrode thickness, electrode cycle, etc.) are determined using the SAW speed corresponding to the cut-out position of the SAW device substrate. When the difference in the chemical composition ratio between the upper part and the lower part of the crystal is within an allowable range, it is easier to perform the above calculation using the intermediate value of the chemical composition ratio between the upper part and the lower part of the crystal as the chemical composition ratio of the grown crystal.

In the step of determining the chemical composition ratio of the grown crystal, the chemical composition ratio of the crystal is obtained by obtaining in advance a correct relationship between the _TC and the LSAW speed with respect to LiNbO ₃ by the method shown in [Patent Document 1]. Can be obtained from T _C instead of LSAW speed. In addition, if the correct relationship between optical characteristics (refractive index, nonlinear optical constant, etc.) and chemical composition ratio is obtained in advance instead of acoustic related physical constants, high accuracy in optoelectronic device design can be achieved in the same way. . The above example can be similarly applied to LiTaO ₃ .

In order to estimate how much composition change is allowed from the true congruent composition determined in FIG. 13, a simulation was performed in the case where repeated crystal growth was performed 100 times in accordance with the procedure of [Patent Document 1] The relationship between the allowable width of SAW velocity (V _SAW ) distribution for the substrate for SAW devices and the allowable width of melt composition fluctuation was obtained.

In the simulation, assuming that the total weight of the charge material (melt) is 5000 g, the crystal solidification rate is 50% (2500 g), and a columnar crystal with a diameter of 77 mm is grown, the deviation of the charge material composition is true. The congruent composition of 48.481 Li ₂ O-mol% is 0 to ± 1 Li ₂ O-mol%.

Assuming 127.86 ° Y-cut X-axis propagation LiNbO ₃ as the substrate for SAW devices, the allowable width of V _SAW is ± 0.1%, ± 0.05%, ± 0.02%, ± 0.01%, ± 0.005%, ± 0.002% corresponding, V _LSAW, the crystal composition, the charge material composition, seeking the variation of T _C. The results are shown in FIG.

If the allowable width of the required V _SAW distribution is ± 0.02% [Reference 4], the range of the raw material composition that can be regarded as a confluent composition for mass production can be considered as 48.481 ± 0.017 Li ₂ O-mol%. In this case, the allowable range ± 3 ° C. from T _C which is defined as a standard wafer for SAW devices in current with respect to [Reference 5], the crystal mass is performed which produces only a slight distribution of ± 1.0 ° C..

As described above, according to the present invention, by providing a true congruent composition, the distribution within and between crystals to be mass-produced can be made extremely small. Further, in view of the chemical composition evaluation method, than T _C is indicative of the conventional crystal evaluation allows evaluation by V _LSAW can guarantee absolute accuracy. Furthermore, in congruent composition crystals, the absolute value of the chemical composition ratio of the grown crystal, which has been unclear until now, becomes clear, so the chemical composition ratio and acoustic-related physical constants and other chemical and physical properties (V _LSAW , V _SAW , T _C , lattice constant, etc.) can be obtained, and absolute crystal evaluation based on various chemical and physical characteristics can be performed.

This invention, for example, the determination of the raw material composition when mass-producing ferroelectric single crystals for producing SAW devices used as filters in mobile communication devices and the like, or optoelectronic devices such as optical modulators and frequency multipliers, It can be used for single crystal evaluation and highly accurate device design.
[References]
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for surface acoustic wave device
-Specification and Measuring Method-, "IEICE / Std-0002, April 17, 2001.

The flowchart which shows the method of calculating | requiring a true congruent composition, and the method of calibrating a chemical composition. The figure which shows the position which extracts the sample board | substrate of various cut surfaces so that LSAW speed distribution measurement of the growth axis direction can be performed from a crystal ingot. The figure which shows the shape of the sample board | substrate of each cut surface, and the LSAW speed measurement position. The figure which shows the growth conditions and size of the used crystal ingot. Curie temperatures for each crystal ingot, density, shows the measured values of the lattice constants, the results of the analysis Li ₂ O concentration determined from the known relationship between the chemical composition and the Curie temperature. Is a diagram showing the propagation direction dependence of the LSAW velocity for LiNbO ₃ different compositions, result A is for X-cut substrate, B is the result for the Y-cut substrate, C represents the results for Z-cut substrate. Shows the rate of change with respect to Li ₂ O concentration in the LSAW velocity of each propagation direction with respect to the LiNbO _3. Li ₂ O concentration by LSAW velocity measurement, the Curie temperature, density, shows the sensitivity and resolution for the lattice constant. It shows the results of measuring the radial distribution of LSAW velocity in the Y-axis direction propagation relative to Z-cut substrate sample cut from the LiNbO ₃ single crystal ingot having a composition different. It shows the development axial distribution of the LiNbO ₃ single crystal ingot obtained from the results of FIG. It shows the results of measuring the radial distribution of LSAW velocity in the X-axis direction propagation relative to Y-cut substrate sample cut from the LiNbO ₃ single crystal ingot having a composition different. It shows the results of measuring the development axial distribution of LSAW velocity in the X-axis direction propagation relative to Y-cut substrate sample cut from the LiNbO ₃ single crystal ingot having a composition different. Is a diagram showing a method of obtaining a congruent composition from the relationship between the Li ₂ O concentration at the time of LSAW velocity and charge of the top and bottom of each LiNbO ₃ single crystal ingot, A is results for ZY-LiNbO _3, B is YX-LiNbO ₃ The result for is shown. Is a diagram showing the relationship between the Li ₂ O concentration after calibration before and calibration and LSAW velocity, A is the result for the ZY-LiNbO _3, B shows the results for YX-LiNbO _3. Diagram showing the relationship between Li ₂ O concentration and the acoustical physical constants after calibration against LiNbO _3. Diagram showing the relationship between Li ₂ O concentration and the acoustical physical constants after calibration against LiTaO _3. LSAW speed corresponding to each allowable width of the SAW velocity for the 127.86 ° YX-LiNbO _3, the Curie temperature, shows a tolerance of the crystal composition, and a charge feed composition.

Claims (11)

In the raw material composition determination method for a ferroelectric single crystal manufacturing, n pieces having different compositions from each other in the vicinity of the congruent composition (n is an integer on 2 or more) cut at a predetermined cut plane of a ferroelectric single crystal of The propagation direction that takes the maximum value of the difference ratio (difference ratio) of the leaky surface acoustic wave velocity to the difference in chemical composition ratio between the first substrates is defined as the maximum value propagation direction, and each of the ferroelectric single crystals The leaky surface acoustic wave velocity of the second substrate cut out from the upper and lower parts in the growth axis direction is measured in the maximum value propagation direction, and the distribution of the leaky surface acoustic wave velocity in the crystal growth axis direction becomes zero from the measured value. obtains a true congruent composition Cc, the ferroelectric tolerance required for the surface acoustic wave velocity of the single crystal, or a leaky surface acoustic wave velocity corresponding thereto, the Curie temperature, one of the allowable range of the density or crystalline composition above using Tolerance ± △ seek Cc, ferroelectric possible material composition determining method comprising determining the material composition for producing a single crystal of deviation from congruent composition Cc.   2. The raw material composition determination method according to claim 1, wherein the first substrate cut out from the ferroelectric single crystal is a plurality of substrates having different cut surfaces.   The measurement of the leaky surface acoustic wave velocity is performed in a plurality of propagation directions in the plane of the first substrate, and the difference in leaky surface acoustic wave velocity for obtaining the difference ratio of the leaky surface acoustic wave velocity is the n In a pair of first substrates obtained from a single ferroelectric single crystal with the same cut surface, the surface acoustic wave velocity measured in the same leakage surface acoustic wave propagation direction is compared, and the difference is the difference. The raw material composition determination method according to claim 1, wherein the difference is determined as a propagation velocity difference. The difference ratio between the leaky surface acoustic wave velocities is the difference between the propagation speed difference and the chemical composition ratio difference between a pair of first substrates obtained from the n ferroelectric single crystals on the same cut surface . 2. The ratio is obtained by combining each pair of first substrates obtained from the n ferroelectric single crystals, and one or more maximum values are obtained. Or the raw material composition determination method of 2. The cut surface has three directions of X, Y, and Z, with the growth direction of the ferroelectric single crystal as the Z axis, and the cut surface of the surface passes through the central axis of the Z direction and is the diameter of the ferroelectric single crystal. A Y plane extending over the Y plane, an X plane perpendicular to the Y plane and passing through the central axis in the Z direction and extending over the radius of the ferroelectric single crystal, and a plurality of Z planes positioned opposite to the X plane with respect to the Y plane. The raw material composition determination method according to any one of claims 1 to 4 . A raw material composition determination method for manufacturing a ferroelectric single crystal,
(A) In a ferroelectric single crystal, a step of producing n (n is an integer of 2 or more) ferroelectric single crystals having different chemical compositions in the vicinity of the congruent composition;
(B) a step of cutting a plurality of substrates with different cut surfaces from each of the ferroelectric single crystals;
(C) in each of the substrates, measuring a first leaky surface acoustic wave velocity V _LSAW in the substrate plane while changing the propagation direction;
(D) obtaining a chemical composition ratio of each ferroelectric single crystal from the Curie temperature of each substrate using a known relationship between the chemical composition ratio and the Curie temperature;
(E) selecting a pair of the substrates cut out from the ferroelectric single crystal at the same cut surface;
(F) In the pair of substrates, a step of obtaining a difference in each chemical composition ratio obtained in the step (d) as a chemical composition ratio change;
(G) Leaky surface acoustic wave velocity change, which is a difference in velocity in the same propagation direction, among the first leaky surface acoustic wave velocities obtained in step (c) on the pair of substrates determined in step (f). The process of seeking
(H) A step of obtaining a value of change in leakage surface acoustic wave velocity / chemical composition ratio, which is a rate of change of the change in leakage surface acoustic wave velocity obtained in step (g) with respect to the change in chemical composition obtained in step (f). When,
(I) performing the steps (f), (g), and (h) for each combination of the substrates cut out from the ferroelectric single crystal;
(J) selecting one of the values of the leaky surface acoustic wave velocity change / chemical composition ratio change obtained in the step (i) and specifying the cut surface indicating the value and the leaky surface acoustic wave propagation direction; ,
(K) The ferroelectric single crystal in which the above steps (e) to (j) were measured with the upper and lower portions of the ferroelectric single crystal being the upper portion on the seed crystal side during crystal growth and the opposite side being the lower portion. Obtaining a substrate from the top and bottom from the crystal;
(L) measuring a second leaky surface acoustic wave velocity in the leaky surface acoustic wave propagation direction specified in step (j) on the upper substrate and the lower substrate obtained in step (k);
(M) performing the steps (e) to (l) on n ferroelectric single crystals produced in the step (a);
(N) Each raw material composition value of the ferroelectric single crystal measured in the step (m), and each ferroelectric single crystal upper substrate and lower portion obtained in the step (l) obtained in the step (m) The true congruent composition Cc in which the distribution of the leaky surface acoustic wave velocity in the crystal growth axis direction is zero from the second leaky surface acoustic wave velocity of the substrate, and the corresponding third leaky surface acoustic wave velocity V _LSAW (Cc) The process of seeking
(P) Using the allowable width of any one of the leaky surface acoustic wave velocity, the leaky surface acoustic wave velocity gradient, the raw material composition, the crystal composition, and the density of the n ferroelectric single crystals produced in the step (a). Obtaining a tolerance ±± Cc of deviation of the true congruent composition Cc;
(Q) determining a ferroelectric single crystal raw material composition for manufacturing a ferroelectric single crystal from the true congruent composition Cc and the allowable width ± ΔCc;
The raw material composition determination method characterized by including these. Each raw material composition value of the ferroelectric single crystal measured in the step (m), and the upper and lower substrates of the ferroelectric single crystal obtained in the step (l) obtained in the step (m). The step (n) for determining the true congruent composition Cc and the corresponding third leakage surface acoustic wave velocity V _LSAW (Cc) from the leakage acoustic surface wave velocity of 2 Construct a graph with the surface wave velocity as the vertical axis, and draw the leaky surface acoustic wave velocity curve in the upper substrate and the leaky surface acoustic wave velocity curve in the lower substrate, and the raw material composition value and the leaky surface acoustic wave where both curves intersect Velocity is obtained, the raw material composition value is set as a true congruent composition Cc, and the leaky surface acoustic wave velocity is set as a third leaky surface acoustic wave velocity V _LSAW (Cc) corresponding to the true congruent composition. Determination of raw material composition according to claim 6 Method. In the raw material composition determination method for a ferroelectric single crystal manufacturing, the ferroelectric single crystal is LiNbO _3, claims 1 to 7 above congruent composition is characterized by a 48.481Li 2 _O-mol% The raw material composition determination method according to any one of the above. 9. The raw material composition determination method according to claim 8 , wherein the permissible width ± ΔCc is ± 0.017 Li ₂ O-mol% in the raw material composition determination method for manufacturing a ferroelectric single crystal. It is a calibration method for acoustic-related physical constants of ferroelectric single crystals,
(A) in a ferroelectric single crystal, producing n (n is an integer of 2 or more) ferroelectric single crystals having different chemical compositions in the vicinity of the congruent composition;
(B) a step of cutting a plurality of substrates with different cut surfaces from each of the ferroelectric single crystals;
(C) in each of the substrates, measuring a first leaky surface acoustic wave velocity V _LSAW in the substrate plane while changing the propagation direction;
(D) obtaining a first chemical composition ratio of each ferroelectric single crystal from a Curie temperature of each substrate using a known relationship between the chemical composition ratio and the Curie temperature;
(E) selecting a pair of the substrates cut out from the ferroelectric single crystal at the same cut surface;
(F) In the pair of substrates, a step of obtaining a difference between the first chemical composition ratios obtained in the step (d) as a chemical composition ratio change;
(G) Leaky surface acoustic wave velocity change, which is a difference in velocity in the same propagation direction, among the first leaky surface acoustic wave velocities obtained in step (c) on the pair of substrates determined in step (f). The process of seeking
(H) A step of obtaining a value of a change in the leaky surface acoustic wave velocity / chemical composition ratio, which is a rate of change of the change in the leaky surface acoustic wave velocity obtained in the step (g) with respect to the chemical composition change obtained in the step (f). When,
(I) performing the steps (f), (g), and (h) for each combination of the substrates cut out from the ferroelectric single crystal;
(J) selecting one of the values of the leaky surface acoustic wave velocity change / chemical composition ratio change obtained in the step (i) and specifying the cut surface indicating the value and the leaky surface acoustic wave propagation direction; ,
(K) The ferroelectric single crystal in which the above steps (e) to (j) were measured with the upper and lower portions of the ferroelectric single crystal being the upper portion on the seed crystal side during crystal growth and the opposite side being the lower portion. Obtaining a substrate from the top and bottom from the crystal;
(L) measuring a second leaky surface acoustic wave velocity in the leaky surface acoustic wave propagation direction specified in step (j) on the upper substrate and the lower substrate obtained in step (k);
(M) performing the steps (e) to (l) on n ferroelectric single crystals produced in the step (a);
(N) Each raw material composition value of the ferroelectric single crystal measured in the step (m), and each ferroelectric single crystal upper substrate and lower portion obtained in the step (l) obtained in the step (m) The true congruent composition Cc in which the distribution of the leaky surface acoustic wave velocity in the crystal growth axis direction is zero from the second leaky surface acoustic wave velocity of the substrate, and the corresponding third leaky surface acoustic wave velocity V _LSAW (Cc) The process of seeking
(O) The relationship between the first leaky surface acoustic wave velocity V _LSAW obtained in the step (c) and the first chemical composition ratio obtained in the step (d) is represented by an approximate line, and the approximate line is represented by The first chemical composition ratio is calibrated to match the values of the true congruent composition Cc and the third leaky surface acoustic wave velocity V _LSAW (Cc) obtained in the step (n), and the relationship between the two Obtaining a calibrated approximate straight line representing
(P) Fourth leaky surface acoustic wave calculated by using any acoustic-related physical constant of elastic constant, piezoelectric constant, dielectric constant, or density of the n ferroelectric single crystals of (a) in the step Using the velocity and the calibrated approximate straight line obtained in the step (o), the calibrated second chemical composition ratio of the n ferroelectric single crystals is obtained, and the calibrated second Obtaining the acoustic-related physical constant in the chemical composition ratio of
And a method for calibrating acoustic-related physical constants. A method for determining design parameters of a surface acoustic wave device using the calibration method for acoustic-related physical constants of claim 10 ,
(1) measuring a leaky surface acoustic wave velocity with respect to the grown crystal and determining a second chemical composition ratio of the grown crystal using the calibrated approximate straight line obtained in the step (o);
(2) Using the second chemical composition ratio obtained in the step (1) and the acoustic related physical constant in the calibrated second chemical composition ratio obtained in the step (p), Obtaining the acoustic related physical constant of the grown crystal in step (1);
(3) calculating the surface acoustic wave velocity V _SAW in the propagation direction on the cut surface for cutting out as a surface acoustic wave device using the acoustic-related physical constant obtained in the step (2); and (4) Determining design parameters (electrode period, electrode width, electrode thickness) for surface acoustic wave device design using the surface acoustic wave velocity V _SAW obtained in the step (3);
A method for determining a design parameter of a surface acoustic wave device, comprising:

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