A LITHIUM TANTALATE SINGLE CRYSTAL SUBSTRATE FOR A SURFACE ACOUSTIC WAVE DEVICE AND A DEVICE USING THE SAME, AND A MANUFACTURING METHOD THEREOF AND AN INSPECTION METHOD THEREOF

Abstract

A lithium tantalate single crystal substrate for a surface acoustic wave device that is a rotated Y-cut LiTaO3 substrate whose crystal orientation has a Y-cut angle of not smaller than 36° and not larger than 49° and which has such a Li concentration profile after diffusion of Li into the substrate from the surface thereof that the Li concentration at the surface of the substrate differs from that inside the substrate. A shear vertical type elastic wave whose main components are vibrations in the thickness direction and in the propagation direction and which is among those elastic waves which propagate in the X axis direction within the surface of this LiTaO3 substrate has an acoustic velocity of not lower than 3140 m/s and not higher than 3200 m/s.

Claims

1. A lithium tantalate single crystal substrate for a surface acoustic wave device that is a rotated Y-cut LiTaO.sub.3 substrate whose crystal orientation has a Y-cut angle of not smaller than 36° and not larger than 49° and which has such a Li concentration profile after diffusion of Li into the substrate from the surface thereof that the Li concentration at the surface of the substrate differs from that inside the substrate, characterized by that a shear vertical type elastic wave whose main components are vibrations in the thickness direction and in the propagation direction and which is among those elastic waves which propagate in the X axis direction within the surface of this LiTaO.sub.3 substrate has an acoustic velocity of not lower than 3140 m/s and not higher than 3200 m/s.

2. The lithium tantalite single crystal substrate as claimed in claim 1, characterized by that the said concentration profile is such that the Li concentration is higher in areas closer to the substrate surface and the Li concentration is lower in areas closer to the middle of the substrate.

3. The lithium tantalate single crystal substrate as claimed in claim 1, characterized by that the acoustic velocity of said shear vertical type elastic wave is not lower than 3160 m/s and not higher than 3170 m/s.

4. A surface acoustic wave device characterized by using the lithium tantalate single crystal substrate as claimed in claim 1.

5. A method for manufacturing a lithium tantalate single crystal substrate that is a rotated Y-cut LiTaO.sub.3 substrate whose crystal orientation has a Y-cut angle of not smaller than 36° and not larger than 49° and which has such a Li concentration profile after diffusion of Li into the substrate from the surface thereof that the Li concentration at the surface of the substrate differs from that inside the substrate, characterized by that an annealing treatment at 800-1000° C. is applied to the LiTaO.sub.3 substrate after said Li diffusion treatment to thereby adjust a shear vertical type elastic wave whose main components are vibrations in the thickness direction and in the propagation direction and which is among those elastic waves which propagate in the X axis direction in a manner such that said shear vertical type elastic wave comes to have an acoustic velocity of not lower than 3140 m/s and not higher than 3200 m/s.

6. The method for manufacturing a lithium tantalate single crystal substrate as claimed in claim 5 characterized by that said shear vertical type elastic wave is adjusted to have an elastic wave acoustic velocity of not lower than 3160 m/s and not higher than 3170 m/s.

7. A method for inspecting a lithium tantalate single crystal substrate that is a rotated Y-cut LiTaO.sub.3 substrate whose crystal orientation has a Y-cut angle of not smaller than 36° and not larger than 49° and which has such a Li concentration profile after diffusion of Li into the substrate from the surface thereof that the Li concentration at the surface of the substrate differs from that inside the substrate, characterized by comprising a step of measuring an acoustic velocity of a shear vertical type elastic wave whose main components are vibrations in the thickness direction and in the propagation direction and which is among those elastic waves that propagate in the X axis direction within the surface of said rotated Y-cut LiTaO.sub.3 substrate, and a step of determining whether or not the substrate can be used as a surface acoustic wave device based on a result of said measurement.

8. The method for inspecting a lithium tantalate single crystal substrate as claimed in claim 7 characterized by that if the result of said measurement is such that the acoustic velocity of said shear vertical type elastic wave is not lower than 3140 m/s and not higher than 3200 m/s, then said substrate is determined to be usable as a surface acoustic wave device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG. 1 is a schematic diagram showing a Raman profile.

[0025] FIG. 2 is a schematic diagram showing insertion loss waveforms of the SAW filter of Example 1.

[0026] FIG. 3 is a schematic diagram showing SAW resonator waveforms of Example

EMBODIMENTS FOR CARRYING OUT THE INVENTION

[0027] Hereinafter, an embodiment of the present invention will be described in detail, with reference to examples and comparative examples.

EXAMPLES

Example 1

[0028] In Example 1, first, a singly polarized 4-inch diameter lithium tantalate single crystal ingot having a roughly congruent composition and having a Li:Ta ratio of 48.5:51.5 was sliced to obtain a number of 300-μm-thick 42° rotated Y-cut lithium tantalate substrates. Thereafter, in view of a circumstance, the surface roughness of each sliced wafer was adjusted to 0.15 μm in arithmetic average roughness Ra value by a lapping procedure, and the finished thickness was set to 250 μm.

[0029] After one side surface of the resulting substrate was finished into a quasi-mirror surface having a Ra value of 0.01 μm by planar polishing, the substrate was buried in a powder containing Li, Ta and O with Li.sub.3TaO.sub.4 as a main component. In this example, the powder in which Li.sub.3TaO.sub.4 was a main component was prepared by mixing Li.sub.2CO.sub.3 powder and TaO.sub.5 powder at a molar ratio of 7:3, followed by baking at 1300° C. for 12 hours. Then, this powder containing Li.sub.3TaO.sub.4 as a main component was laid in a small container, and a plurality of said sliced wafer were buried in this mostly Li.sub.3TaO.sub.4 powder.

[0030] Then, this small container was set in an electric furnace, which was subsequently filled with an N.sub.2 atmosphere and heated at 950° C. for 60 hours, to thereby cause Li to diffuse into the sliced wafer from the surface toward the middle thereof. Thereafter, the thus treated slice substrate was subjected to an anneal treatment in an N.sub.2 atmosphere at 800° C., which is higher than the Curie temperature, for 12 hours, and when the wafer was allowed to cool, an electric field of 2000 V/m was applied in roughly +Z axis direction during the period when the temperature was between 770° C. and 500°, and thereafter the temperature was caused to fall to the room temperature. Also after this treatment, the rough side face of the wafer was subjected to a sandblasting to finish it to an Ra value of about 0.15 μm, and the quasi-mirror side face thereof was polished 3 μm deep and thus a plurality of lithium tantalate single crystal substrates were obtained.

[0031] With respect of one of these lithium tantalate single crystal substrates, a laser Raman spectrometer (LabRam HR series manufactured by HORIBA Scientific Inc., Ar ion laser, spot size 1 μm, room temperature) was used to measure the half-value width of the Raman shift peak around 600 cm.sup.−1, which is an indicator of the Li diffusion amount, through a depth-wise distance from the surface at an arbitrarily chosen site which was 1 cm or more away from the outer circumference of the circular substrate; and as the result a Raman profile as shown in FIG. 1 was obtained.

[0032] According to the result in FIG. 1, in this lithium tantalate single crystal substrate, the value of the Raman half width (half-value width) measured at the surface was found different from that measured inside the substrate, and, in particular, there exists a range, namely, the range between the depths of about 0 μm and about 80 μm where the Raman half-value width value decreases as the measurement approaches the surface of the substrate and the Raman half-value width value increases as the measurement approaches the middle of the substrate.

[0033] Also, the Raman half-value width at the surface of the lithium tantalate single crystal substrate was 6.3 cm.sup.−1, and the Raman half-width at the middle portion in the thickness direction of the substrate was 9.3 cm.sup.−1. Incidentally, according to the result of FIG. 1, the Li concentration is seen to pick up from the depth of 40 μm from the surface and increases as the depth decreases, so that in Example 1 the position where the depth from the surface is 80 μm is considered the middle part of the substrate in thickness direction. Hence, the difference between the Raman half-value width at the substrate surface and the Raman half-value width at the middle part of the substrate in thickness direction was 3.0 cm.sup.−1.

[0034] From the result of FIG. 1, it was confirmed that the lithium tantalate single crystal substrate of Example 1 has a Li concentration profile wherein the Li concentration at the surface differs from that inside the substrate, and in particular there exists a range, namely, the range between the depths of about 0 μm and about 80 μm where the Li concentration increases as the measurement approaches the surface of the substrate and the Li concentration decreases as the measurement approaches the middle portion of the substrate. Incidentally it would have been preferable if this characteristic range of the concentration profile had extended to a depth of about 100 μm from the surface of the substrate.

[0035] Also, since FIG. 1 shows that the Raman half-value width of the lithium tantalate single crystal substrate was about 6.3 cm.sup.−1 in the range between the surface and the depth of 8 μm, it follows that, from the Equation (1) below, the composition in that range becomes roughly Li/(Li+Ta)=0.50, and hence it was confirmed that pseudo-stoichiometry composition had been established there.

Li/(Li+Ta)=(53.15−0.5FWHM1)/100  (1)

[0036] Furthermore, since the Raman half-value width of the lithium tantalate single crystal substrate at its middle part in thickness direction was about 9.3 cm.sup.−1, if follows from Equation (1) that the value of Li/(Li+Ta) becomes 0.485 so that it was confirmed that a roughly congruent composition had occurred there.

[0037] Hence, in the case of the rotated Y-cut LiTaO.sub.3 substrate of Example 1, a pseudo-stoichiometry composition was established in the range between the substrate surface and a depth at which the Li concentration starts decreasing or the range between the substrate surface and a depth at which the Li concentration stops increasing, and a roughly congruent composition occurred at the middle position of the substrate in the thickness direction. And it is preferable that the position at which the Li concentration starts increasing or at which the Li concentration stops decreasing is deeper than a depth of 5-10 μm from the substrate surface in the thickness direction.

[0038] Next, the warping of the 4-inch lithium tantalate single crystal substrate after being subjected to the Li diffusion was measured by an interference method using a laser beam, and the value obtained was as small as 80 μm, and cracks and chipping were not observed

[0039] Then, a small piece was cut out from the thus obtained Li-diffused 4-inch 42° rotated Y cut lithium tantalate single crystal substrate, and, in a Piezo d33/d15 meter (model ZJ-3BN) manufactured by The Institute of Acoustics of the Chinese Academy of Sciences, the small piece was given a vertical vibration in the thickness direction to the principal face and also to the back face respectively to observe the voltage waveform thereby induced, and a waveform was observed which indicated a presence of piezoelectric response.

[0040] Hence it was confirmed that the lithium tantalate single crystal substrate of Example 1 has piezoelectricity, and thus can be used as a surface acoustic wave device.

[0041] Next, among the elastic waves that propagate in the X-axis direction on the polished surface of the 4-inch 42° Y-cut lithium tantalate single crystal substrate, which had gone through the Li diffusion treatment, the annealing treatment and the polishing treatment, those shear vertical type elastic wave (SV wave or leaky wave) whose main components are a vibration in the thickness direction and that in the propagation direction was measured for its acoustic velocity by the ultrasonic microscope described in the above Non-IP Publication 2, and the acoustic velocity of the SV wave was found to be 3166.9 (m/s) at a temperature of 23.0° C.

[0042] Incidentally, in Example 1, although the acoustic velocity of the shear vertical type elastic wave was measured with respect to the X axis direction, it is possible to adjust the acoustic velocity of a shear vertical type elastic wave that propagates in any of the in-plane directions of the substrate surface. For example, an LiTaO.sub.3 substrate subjected to the Li diffusion treatment may be annealed in a temperature range of 800 to 1000° C., and the acoustic velocity of the elastic waves of the shear vertical type whose main components are the vibrations in the thickness direction and the propagation direction among those elastic waves which propagate in the direction perpendicular to the X axis within the plane of this substrate may be adjusted to a range of from 3170 m/s through 3250 m/s, or preferably from 3195 m/s through 3205 m/s.

[0043] For comparison, the acoustic velocity of those shear vertical type elastic waves (SV wave or leaky wave) whose main components were vibrations in the thickness direction and the propagation direction among all the elastic waves which propagate in the X-axis direction of the polished surface of the 42° Y-cut lithium tantalate single crystal substrate not subjected to the Li diffusion treatment, was measured and found to be 3126.5 (m/s) at a temperature of 23.0° C. The acoustic velocity of the SV wave in the direction perpendicular to the X axis within the same plane was 3161.0 (m/s) at a temperature of 23.0° C.

[0044] Next, a sputtering treatment was performed on the 42° Y-cut lithium tantalate single crystal substrate after having been subjected to the Li diffusion treatment in Example 1, to form an Al film having a thickness of 0.2 μm thereon; thereafter, a resist was applied to the substrate to which the Al film had been formed, and an electrode pattern of a single-stage ladder type filter was exposed and developed with a stepper, and an electrode for SAW characteristic evaluation was provided by RIE. Incidentally, one wavelength of the thus patterned SAW electrode was set to 2.33 μm in the case of the series resonator and that in the case of the parallel resonator was set to 2.42 μm.

[0045] With respect to this one-stage ladder type filter, its SAW waveform characteristic was explored by an RF prober, and the result shown in FIG. 2 was obtained. In FIG. 2, for comparison, the same pattern as described above was formed on a 42° Y-cut lithium tantalate single crystal substrate not having been subjected to Li diffusion treatment, and the result of measurement of the SAW waveform is also shown in FIG. 2.

[0046] From the results in FIG. 2, the frequency width at which the insertion loss of the SAW filter made of a 42° Y-cut lithium tantalate single crystal substrate subjected to Li diffusion treatment becomes 3 dB or less is 37 MHz, and the center frequency of the filter was confirmed to be 1745 MHz.

[0047] Further, when the temperature coefficients of the anti-resonance frequency and the resonance frequency were inspected by varying the stage temperature from about 16° C. to 70° C., the temperature coefficient of the resonance frequency was −20.3 ppm/° C. and the temperature coefficient of the anti-resonance frequency was −41.7 ppm/° C., and thus it was confirmed that the average temperature coefficient of the frequency was −31 ppm/° C.

[0048] For comparison, when the temperature coefficient of the 42° Y-cut lithium tantalate single crystal substrate having not been subjected to the Li diffusion treatment was inspected, the temperature coefficient of the resonance frequency was −32 ppm/° C. and the temperature coefficient of the anti-resonance frequency was −42 ppm/° C., so that it was confirmed that the average frequency temperature coefficient was −37 ppm/° C.

[0049] Therefore, it was confirmed that, in the lithium tantalate single crystal substrate of Example 1, the band where the insertion loss of the filter is 3 dB or less is 1.3 times wider than that of the substrate without Li diffusion treatment, and that even when the same electrode was used, the center frequency became 1.03 times higher, indicating a phenomenon of frequency increase. Also regarding the temperature characteristics, the average frequency temperature coefficient of the substrate with Li diffusion treatment was 6.0 ppm/° C. lower than that of the substrate without the Li diffusion treatment, and hence the former substrate showed smaller property change with temperature so that the former is superior to the latter in temperature characteristic.

[0050] Next, in addition to providing the SAW electrode to the 42° C. Y-cut lithium tantalate single crystal substrate with the Li diffusion treatment obtained in Example 1, a 1-port SAW resonator with a wavelength of 4.8 μm was also provided to the substrate in the similar manner as described above, and a SAW waveform shown in FIG. 3 was obtained. For comparison, the same pattern as described above was formed on a 42° Y-cut lithium tantalate single crystal substrate not subjected to Li diffusion treatment, and the result of the measurement of the SAW waveform therewith is also shown in FIG. 3.

[0051] Then, on one hand, from the result of the SAW waveforms of FIG. 3 the values of the anti-resonance frequency and the resonance frequency were obtained and the difference therebetween was obtained, and on the other hand, the same procedure was taken to obtain the said difference in the case of the 42° Y-cut lithium tantalate single crystal substrate not subjected to the Li diffusion treatment, and the ratio between these differences was calculated.

[0052] This ratio represents a relative electromechanical coupling coefficient of the 42° Y-cut lithium tantalate single crystal substrate subjected to the Li diffusion treatment of Example 1, when the electromechanical coupling coefficient of the 42° Y-cut lithium tantalate single crystal substrate not subjected to the Li diffusion treatment is deemed to be 1. In Example 1, it was confirmed that the relative electromechanical coupling coefficient was 1.3. Also, the SAW acoustic velocities of the shear horizontal type as obtained by dividing the values of the anti-resonant frequency and the resonance frequency by the wavelength were averaged and the results are shown in Table 1.

Example 2

[0053] In Example 2, first, a singly polarized 4-inch diameter lithium tantalate single crystal ingot having a roughly congruent composition and having a Li:Ta ratio of 48.5:51.5 was sliced to obtain a number of 42° rotated Y-cut lithium tantalate. Thereafter, in view of a circumstance, the surface roughness of each sliced wafer was adjusted to 0.15 μm in arithmetic average roughness Ra value by a lapping procedure, and the finished thickness was set to 250 μm.

[0054] Also, after one side surface of the resulting substrate was finished into a quasi-mirror surface having a Ra value of 0.01 μm by planar polishing, the substrate was buried in a powder containing Li, Ta and O with Li.sub.3TaO.sub.4 as a main component. In this example, the powder in which Li.sub.3TaO.sub.4 was a main component was prepared by mixing Li.sub.2CO.sub.3 powder and TaO.sub.5 powder at a molar ratio of 7:3, followed by baking at 1300° C. for 12 hours. Then, this powder containing Li.sub.3TaO.sub.4 as a main component was laid in a small container, and a plurality of the said sliced wafer were buried in this mostly Li.sub.3TaO.sub.4 powder.

[0055] Then, this small container was set in an electric furnace, which was subsequently filled with an N.sub.2 atmosphere and heated at 950° C. for 60 hours to thereby cause Li to diffuse into the sliced wafer from the surface toward the middle thereof. Thereafter, the thus treated slice substrate was subjected to an anneal treatment in an N.sub.2 atmosphere at 800° C., which is higher than the Curie temperature, for 12 hours, and the temperature was caused to fall to the room temperature. Also after this treatment, the rough side face of the wafer was subjected to a sandblasting to finish it to an Ra value of about 0.15 μm, and the quasi-mirror side face of the wafer was polished 3 μm deep and thus a plurality of lithium tantalate single crystal substrates were obtained.

[0056] In this Example 2, with respect to five samples consisting of 42° Y-cut lithium tantalate single crystal substrates to which annealing treatment of 800° C.-1000° C. had been applied, the Raman half-value width at the surface, the Raman half-value width at the middle portion in the substrate thickness direction, the relative electromechanical coupling coefficient, the average frequency temperature coefficient, the SV wave acoustic velocity at a temperature of 23° C. of shear vertical type elastic wave whose main components are vibrations in the thickness direction and the propagation direction out of the elastic waves that propagate in the X-axis direction, the SV wave acoustic velocities in the X-axis direction and in the direction vertical to the X-axis at a temperature of 23° C., and the SH wave average acoustic velocity were obtained in the similar manners as describe in Example 1, and the results were as shown in Table 1.

[0057] In Table 1, the results obtained with a 42° Y-cut lithium tantalate single crystal substrate not subjected to the Li diffusion treatment as well as the results of Example 1 are shown together for comparison.

TABLE-US-00001 TABLE 1 Annealing temperature and various characteristics SV wave acoustic velocity in Raman direction half-value SV wave vertical to width at acoustic X-axis Raman thickness- average SH velocity in within half-value wise relative frequency wave X-axis water annealing width middle electro- temper- average direction surface temper- within part of mechanical ature acoustic at 23° C. at 23° C. ature surface substrate coupling coefficient velocity (m/s) (m/s) (° C.) (cm.sup.−1) (cm.sup.−1) coefficient (ppm/° C.) (m/s) Example 1 3166.9 3201.2 800 6.3 9.3 1.3 −31 4250 Example 2-1 3163.5 3197.6 800 6.4 9.3 1.3 −32 4240 Example 2-2 3159.0 3186.4 825 6.9 9.3 1.01 −27 4195 Example 2-3 3156.3 3192.1 900 7.1 9.3 0.8 −18 4080 Example 2-4 3155.4 3190.2 950 7.5 9.3 0.6 −19 4062 Example 2-5 3140.3 3173.7 1000 7.5 9.3 0.4 −16 4040 no treatment; 3126.5 3161.0 — 9.3 9.3 1 −37 4120 42° Ycut-LT

[0058] From the results shown in Table 1, it is confirmed that in all of the five samples which were subjected to annealing treatment at a temperature between 800° C. and 1000° C., the SV wave acoustic velocity in X-axis direction at 23° C. turned out to be 3140 m/s or higher, and that the absolute value of their respective average frequency temperature coefficient was smaller than that of the conventional sample which had not been subjected to the annealing treatment.

Example 3

[0059] In Example 3, using 38.5° Y-cut lithium tantalate single crystal substrates the same experiments were conducted as in Example 1 and Example 2, and it was confirmed that the resulting properties obtained in Example 3 were similar to those listed in Table 1.

Example 4

[0060] In Example 4, ten batches of the same material as in Example 1 were produced as once, and the same Li diffusion treatment and annealing treatment were applied to the 10 batches of the material as in Example 1, and, after conducting the polishing treatment, with respect to each 4-inch 42° Y-cut lithium tantalate single crystal substrate, among the elastic waves that propagate in X-axis direction within the polished surface those shear vertical type elastic waves (SV wave or leaky wave) whose main components were vibrations in the thickness direction and propagation direction were measured for their acoustic velocity using the same ultrasonic microscope as used in Example 1, and as the result it was confirmed that the SV wave acoustic velocities thus obtained in all of the ten cases were in the range of 3166±1 (m/s) at a temperature of 23.0° C. With respect to these wafers, the average SAW acoustic velocity of the shear horizontal type was determined in the same manner as in Example 1, and the resultant values were within a range of 4249±1.5 m/s at 23.0° C.

[0061] As is clear from the results of the Examples, if a LiTaO.sub.3 substrate having a roughly congruent composition is subjected to such a Li diffusion whereby Li is diffused into the substrate from the surface thereof with a result that a concentration profile is established such that the Li concentration differs between the surface and the inside of the substrate, and if the thus modified lithium tantalate single crystal substrate with 36° Y-cut through 49° Y-cut is adjusted by such an annealing treatment that the resultant acoustic velocity of the shear vertical type elastic wave (SV wave or leaky wave) whose main components are the vibrations in the thickness direction and propagation direction out of those elastic waves that propagate in X-axis direction within the substrate surface becomes from 3140 m/s through 3200 m/s, then a lithium tatalate single crystal substrate for surface acoustic wave device is obtained whose temperature characteristic is smaller than that of the conventional rotated Y-cut LiTaO.sub.3 substrate and, furthermore, which is capable of adjusting its electromechanical coupling coefficient to meet mobile phone band appropriately; and also it is possible to obtain a SAW device wherein such a substrate as described above is used.

[0062] In particular, if the acoustic velocity of the SV wave is adjusted to be not less than 3160 m/s and not more than 3170 m/s, it is possible to obtain a lithium tantalate single crystal substrate with such a broadband that is required for a smartphone and with a temperature characteristic which is smaller than that of the conventional rotated Y-cut LiTaO.sub.3 substrate, and further a SAW device using this substrate can be obtained at a low cost.

[0063] Moreover, because the SAW acoustic velocity of the shear vertical type (SV wave) can be measured without configuring a device, it is possible to non-destructively predict the SAW acoustic velocity of the shear horizontal type required for a SAW device; consequently it is possible to provide, at low cost, a material with low tendency of undergoing property changes with temperature changes.