METHOD OF PRODUCING LITHIUM NIOBATE SINGLE CRYSTAL SUBSTRATE

Abstract

To provide a method of producing a lithium niobate (LN) substrate which allows treatment conditions regarding a temperature, a time, and the like to be easily managed and in which an in-plane distribution of a volume resistance value is very small, and also variations in volume resistivity are small among substrates machined from the same ingot.

A method of producing an LN substrate by using an LN single crystal grown by the Czochralski process, in which a lithium niobate single crystal having a Fe concentration of 50 mass ppm or more and 2000 mass ppm or less in the single crystal and being in a form of an ingot is buried in an Al powder or a mixed powder of Al and Al.sub.2O.sub.3, and heat-treated at a temperature of 450 C. or more and less than 660 C., which is a melting point of aluminum, to produce a lithium niobate single crystal substrate having a volume resistivity controlled to be within a range of 110.sup.8 .Math.cm or more to 210.sup.12 .Math.cm or less.

Claims

1. A method of producing a lithium niobate single crystal substrate by using a lithium niobate single crystal grown by the Czochralski process, wherein a lithium niobate single crystal having a Fe concentration of 50 mass ppm or more and 2000 mass ppm or less in the single crystal and being in a form of an ingot is buried in an Al powder or a mixed powder of Al and Al.sub.2O.sub.3, and heat-treated at a temperature of 450 C. or more and less than 660 C., which is a melting point of aluminum, to produce a lithium niobate single crystal substrate having a volume resistivity controlled to be within a range of 110.sup.8 .Math.cm or more to 210.sup.12 .Math.cm or less.

2. The method of producing a lithium niobate single crystal substrate according to claim 1, wherein the lithium niobate single crystal being in the form of an ingot to be heat-treated is a lithium niobate single crystal ingot after growth of the single crystal and until after cylindrical abrading process.

3. The method of producing a lithium niobate single crystal substrate according to claim 2, wherein a surface roughness of the lithium niobate single crystal ingot after the cylindrical abrading process is 0.2 m or more and 2 m or less in arithmetic average roughness Ra.

4. The method of producing a lithium niobate single crystal substrate according to claim 1, wherein the heat treatment is conducted in a vacuum atmosphere or in a reduced-pressure atmosphere of an inert gas.

5. The method of producing a lithium niobate single crystal substrate according to claim 1, wherein the heat treatment is conducted for 10 hours or more.

6. The method of producing a lithium niobate single crystal substrate according to claim 2, wherein the heat treatment is conducted in a vacuum atmosphere or in a reduced-pressure atmosphere of an inert gas.

7. The method of producing a lithium niobate single crystal substrate according to claim 3, wherein the heat treatment is conducted in a vacuum atmosphere or in a reduced-pressure atmosphere of an inert gas.

8. The method of producing a lithium niobate single crystal substrate according to claim 2, wherein the heat treatment is conducted for 10 hours or more.

9. The method of producing a lithium niobate single crystal substrate according to claim 3, wherein the heat treatment is conducted for 10 hours or more.

10. The method of producing a lithium niobate single crystal substrate according to claim 4, wherein the heat treatment is conducted for 10 hours or more.

11. The method of producing a lithium niobate single crystal substrate according to claim 6, wherein the heat treatment is conducted for 10 hours or more.

12. The method of producing a lithium niobate single crystal substrate according to claim 7, wherein the heat treatment is conducted for 10 hours or more.

Description

EXAMPLES

[0040] Next, Examples of the present invention are specifically described also by giving Comparative Examples.

Example 1

[0041] A Fe-doped LN single crystal having a diameter of 4 inches was grown by the Czochralski process using a raw material having a congruent composition. The growth atmosphere was a nitrogen-oxygen mixed gas having an oxygen concentration of approximately 20%. The concentration of Fe doped in the crystal was set at 50 ppm. The crystal thus obtained was red in color.

[0042] This crystal was subjected to the heat treatment for removing the residual thermal strain under soaking and the poling treatment for making it single-polarized. Thereafter, the crystal was subjected to abrading process on its peripheral surface (cylindrical abrading) in order to adjust the external shape of the crystal.

[0043] The LN crystal ingot thus obtained was buried in an Al powder, and was then heat-treated at 600 C. for 20 hours in a vacuum atmosphere.

[0044] The ingot after the heat treatment was sliced into a wafer-shaped LN substrate. The LN substrate thus obtained was dark green brown in color, had a volume resistivity of approximately 1.210.sup.9 .Math.cm, and the variation (/Ave) in volume resistivity in the plane of the substrate was less than 3%. In addition, it was also visually observed that color non-uniformity did not occur. Here, Ave is an average value of volume resistivities measured at one point in the center portion and four points on the outer peripheral portion, five points in the surface in total, of the substrate, and is a standard deviation of these. Note that the volume resistivity was measured by the three-terminal method according to JIS K-6911.

[0045] Next, a heat cycle test was conducted in which the LN substrate kept at room temperature was placed on an 80 C. hot plate. As a result, the surface potential generated immediately after the substrate was placed on the hot plate was 60 V or less, and the phenomenon of sparking on the surface of the LN substrate was not observed.

[0046] In addition, the LN substrate thus obtained had a Curie temperature of 1140 C., and values of the physical properties that affect the properties of SAW filters were not different from those of conventional products that have not been subjected to the blackening treatment.

Example 2

[0047] The heat treatment and the slicing process after the heat treatment were conducted under substantially the same conditions as those in Example 1 except that the concentration of Fe doped in the LN crystal was changed to 2000 ppm.

[0048] The LN substrate thus obtained was dark green brown in color, had a volume resistivity of approximately 5.310.sup.9 .Math.cm, and the variation (/Ave) in volume resistivity in the plane of the substrate was less than 3%. It was also visually observed that color non-uniformity did not occur.

[0049] In addition, in the heat cycle test, the surface potential generated immediately after the LN substrate was placed on the hot plate was 100 V or less, and the phenomenon of sparking on the surface of the LN substrate was not observed.

Example 3

[0050] The heat treatment and the slicing process after the heat treatment were conducted under substantially the same conditions as those in Example 1 except that the LN crystal ingot was buried in a mixed powder of 10% by mass of Al and 90% by mass of Al.sub.2O.sub.3.

[0051] The LN substrate thus obtained was dark green brown in color, had a volume resistivity of approximately 3.910.sup.9 .Math.cm, and the variation (/Ave) in volume resistivity in the plane of the substrate was less than 3%. It was also visually observed that color non-uniformity did not occur.

[0052] In addition, in the heat cycle test, the surface potential generated immediately after the LN substrate was placed on the hot plate was 90 V or less, and the phenomenon of sparking on the surface of the LN substrate was not observed.

Example 4

[0053] The heat treatment and the slicing process after the heat treatment were conducted under substantially the same conditions as those in Example 1 except that the concentration of Fe doped in the LN crystal was changed to 2000 ppm and the LN crystal ingot was buried in a mixed powder of 10% by mass of Al and 90% by mass of Al.sub.2O.sub.3.

[0054] The LN substrate thus obtained was brown in color, had a volume resistivity of approximately 1.910.sup.10 .Math.cm, and the variation (/Ave) in volume resistivity in the plane of the substrate was less than 3%. It was also visually observed that color non-uniformity did not occur.

[0055] In addition, in the heat cycle test, the surface potential generated immediately after the LN substrate was placed on the hot plate was 120 V or less, and the phenomenon of sparking on the surface of the LN substrate was not observed.

Example 5

[0056] The heat treatment and the slicing process after the heat treatment were conducted under substantially the same conditions as those in Example 1 except that the treatment temperature was changed to 450 C. and the treatment time was changed to 80 hours.

[0057] The LN substrate thus obtained was dark green brown in color, had a volume resistivity of approximately 1.110.sup.11 .Math.cm, and the variation (/Ave) in volume resistivity in the plane of the substrate was less than 3%. It was also visually observed that color non-uniformity did not occur.

[0058] In addition, in the heat cycle test, the surface potential generated immediately after the LN substrate was placed on the hot plate was 150 V or less, and the phenomenon of sparking on the surface of the LN substrate was not observed.

Example 6

[0059] The heat treatment and the slicing process after the heat treatment were conducted under substantially the same conditions as those in Example 2 except that the treatment temperature was changed to 450 C. and the treatment time was changed to 80 hours.

[0060] The LN substrate thus obtained was dark green brown in color, had a volume resistivity of approximately 4.810.sup.11 .Math.cm, and the variation (/Ave) in volume resistivity in the plane of the substrate was less than 3%. It was also visually observed that color non-uniformity did not occur.

[0061] In addition, in the heat cycle test, the surface potential generated immediately after the LN substrate was placed on the hot plate was 190 V or less, and the phenomenon of sparking on the surface of the LN substrate was not observed.

Example 7

[0062] The heat treatment and the slicing process after the heat treatment were conducted under substantially the same conditions as those in Example 3 except that the atmosphere was changed to a nitrogen gas atmosphere as well as the treatment temperature was changed to 450 C. and the treatment time was changed to 80 hours.

[0063] The LN substrate thus obtained was dark green brown in color, had a volume resistivity of approximately 4.510.sup.11 .Math.cm, and the variation (/Ave) in volume resistivity in the plane of the substrate was less than 3%. It was also visually observed that color non-uniformity did not occur.

[0064] In addition, in the heat cycle test, the surface potential generated immediately after the LN substrate was placed on the hot plate was 190 V or less, and the phenomenon of sparking on the surface of the LN substrate was not observed.

Example 8

[0065] The heat treatment and the slicing process after the heat treatment were conducted under substantially the same conditions as those in Example 4 except that the treatment temperature was changed to 450 C. and the treatment time was changed to 80 hours.

[0066] The LN substrate thus obtained was dark green brown in color, had a volume resistivity of approximately 1.610.sup.12 .Math.cm, and the variation (/Ave) in volume resistivity in the plane of the substrate was less than 3%. It was also visually observed that color non-uniformity did not occur.

[0067] In addition, in the heat cycle test, the surface potential generated immediately after the LN substrate was placed on the hot plate was 220 V or less, and the phenomenon of sparking on the surface of the LN substrate was not observed.

Example 9

[0068] The heat treatment and the slicing process after the heat treatment were conducted under substantially the same conditions as those in Example 1 except that the treatment temperature was changed to 650 C. and the treatment time was changed to 10 hours.

[0069] The LN substrate thus obtained was dark green brown in color, had a volume resistivity of approximately 1.010.sup.8 .Math.cm, and the variation (/Ave) in volume resistivity in the plane of the substrate was less than 3%. It was also visually observed that color non-uniformity did not occur.

[0070] In addition, in the heat cycle test, the surface potential generated immediately after the LN substrate was placed on the hot plate was 10 V or less, and the phenomenon of sparking on the surface of the LN substrate was not observed.

Example 10

[0071] The heat treatment and the slicing process after the heat treatment were conducted under substantially the same conditions as those in Example 2 except that the treatment temperature was changed to 650 C. and the treatment time was changed to 10 hours.

[0072] The LN substrate thus obtained was dark green brown in color, had a volume resistivity of approximately 4.510.sup.8 .Math.cm, and the variation (/Ave) in volume resistivity in the plane of the substrate was less than 3%. It was also visually observed that color non-uniformity did not occur.

[0073] In addition, in the heat cycle test, the surface potential generated immediately after the LN substrate was placed on the hot plate was 40 V or less, and the phenomenon of sparking on the surface of the LN substrate was not observed.

Example 11

[0074] The heat treatment and the slicing process after the heat treatment were conducted under substantially the same conditions as those in Example 3 except that the treatment temperature was changed to 650 C. and the treatment time was changed to 10 hours.

[0075] The LN substrate thus obtained was dark green brown in color, had a volume resistivity of approximately 3.710.sup.8 .Math.cm, and the variation (/Ave) in volume resistivity in the plane of the substrate was less than 3%. It was also visually observed that color non-uniformity did not occur.

[0076] In addition, in the heat cycle test, the surface potential generated immediately after the LN substrate was placed on the hot plate was 40 V or less, and the phenomenon of sparking on the surface of the LN substrate was not observed.

Example 12

[0077] The heat treatment and the slicing process after the heat treatment were conducted under substantially the same conditions as those in Example 4 except that the treatment temperature was changed to 650 C. and the treatment time was changed to 10 hours.

[0078] The LN substrate thus obtained was dark green brown in color, had a volume resistivity of approximately 1.710.sup.9 .Math.cm, and the variation (/Ave) in volume resistivity in the plane of the substrate was less than 3%. It was also visually observed that color non-uniformity did not occur.

[0079] In addition, in the heat cycle test, the surface potential generated immediately after the LN substrate was placed on the hot plate was 70 V or less, and the phenomenon of sparking on the surface of the LN substrate was not observed.

Example 13

[0080] The heat treatment was conducted under substantially the same conditions as those in Example 8 except that the atmosphere was changed to a nitrogen gas atmosphere.

[0081] The LN substrate thus obtained was dark green brown in color, had a volume resistivity of approximately 2.010.sup.12 .Math.cm, and the variation (/Ave) in volume resistivity in the plane of the substrate was less than 3%. It was also visually observed that color non-uniformity did not occur.

[0082] In addition, in the heat cycle test, the surface potential generated immediately after the LN substrate was placed on the hot plate was 230 V or less, and the phenomenon of sparking on the surface of the LN substrate was not observed.

Example 14

[0083] The heat treatment was conducted under substantially the same conditions as those in Example 9 except that the atmosphere was changed to a nitrogen gas atmosphere.

[0084] The LN substrate thus obtained was dark green brown in color, had a volume resistivity of approximately 1.510.sup.8 .Math.cm, and the variation (/Ave) in volume resistivity in the plane of the substrate was less than 3%. It was also visually observed that color non-uniformity did not occur.

[0085] In addition, in the heat cycle test, the surface potential generated immediately after the LN substrate was placed on the hot plate was 20 V or less, and the phenomenon of sparking on the surface of the LN substrate was not observed.

Comparative Example 1

[0086] A LN single crystal having a diameter of 4 inches was grown by the Czochralski process using a raw material having a congruent composition. The growth atmosphere was a nitrogen-oxygen mixed gas having an oxygen concentration of approximately 20%. The crystal thus obtained was pale yellow in color.

[0087] This crystal was subjected to the heat treatment for removing the residual strain under soaking and the poling treatment for making it single-polarized. Thereafter, the crystal was abraded on its peripheral surface in order to adjust the external shape of the crystal, and then sliced into a substrate.

[0088] The LN substrate thus obtained was heat-treated at 600 C. for 1 minute in nitrogen.

[0089] The LN substrate after the heat treatment was brown in color, but it was visually observed that color non-uniformity occurred.

[0090] As inferred from the fact that color non-uniformity occurred, although the average of measured values of volume resistivity in the plane of the LN substrate was approximately 110.sup.11 .Math.cm, there were variations (/Ave) of approximately 30% at some measured spots.

[0091] In addition, in the heat cycle test, the surface potential generated immediately after the LN substrate was placed on the hot plate was 150 V or less, and the phenomenon of sparking on the surface of the LN substrate was not observed.

Comparative Example 2

[0092] A transparent pale yellow LN single crystal having a diameter of 4 inches was grown in the same manner as in Comparative Example 1, and an LN substrate was produced in the same manner as in Comparative Example 1.

[0093] The LN substrate thus obtained was buried in an aluminum (Al) powder, and was then heat-treated at 480 C. for 1 hour in a vacuum atmosphere.

[0094] The LN substrate after the heat treatment was brown in color, had a volume resistivity of approximately 110.sup.12 .Math.cm, and the variation (/Ave) in volume resistivity in the plane of the substrate was less than 3% as in the case of Examples. It was also visually observed that color non-uniformity did not occur.

[0095] Next, a heat cycle test was conducted in which the substrate kept at room temperature was placed on an 80 C. hot plate. As a result, the surface potential generated immediately after the substrate was placed on the hot plate was 200 V or less, and the phenomenon of sparking on the surface of the substrate was not observed.

[0096] In the meantime, when an LN substrate is heat-treated, conditions regarding the temperature (480 C.) and the time (1 hour) are strictly managed. When this management was neglected, there was a case where slight color non-uniformity was observed in the LN substrate after the treatment and the variation (/Ave) in volume resistivity in the surface was approximately 10%.

TABLE-US-00001 TABLE 1 Fe Reducing Temperature (ppm) Agent Atmosphere ( C.) Example 1 50 Al Vacuum 600 Example 2 2000 Al Vacuum 600 Example 3 50 Al + Al.sub.2O.sub.3 Vacuum 600 Example 4 2000 Al + Al.sub.2O.sub.3 Vacuum 600 Example 5 50 Al Vacuum 450 Example 6 2000 Al Vacuum 450 Example 7 50 Al + Al.sub.2O.sub.3 Nitrogen 450 Example 8 2000 Al + Al.sub.2O.sub.3 Vacuum 450 Example 9 50 Al Vacuum 650 Example 10 2000 Al Vacuum 650 Example 11 50 Al + Al.sub.2O.sub.3 Vacuum 650 Example 12 2000 Al + Al.sub.2O.sub.3 Vacuum 650 Example 13 2000 Al + Al.sub.2O.sub.3 Nitrogen 450 Example 14 50 Al Nitrogen 650 Comparative Nitrogen 600 Example 1 Comparative Al Vacuum 480 Example 2 Volume Surface Time Resistivity Potential (Hour) ( .Math. cm) (V) Example 1 20 1.2 10.sup.9 60 Example 2 20 5.3 10.sup.9 100 Example 3 20 3.9 10.sup.9 90 Example 4 20 1.9 10.sup.10 120 Example 5 80 1.1 10.sup.11 150 Example 6 80 4.8 10.sup.11 190 Example 7 80 4.5 10.sup.11 190 Example 8 80 1.6 10.sup.12 220 Example 9 10 1.0 10.sup.8 10 Example 10 10 4.5 10.sup.8 40 Example 11 10 3.7 10.sup.8 40 Example 12 10 1.7 10.sup.9 70 Example 13 80 2.0 10.sup.12 230 Example 14 10 1.5 10.sup.8 20 Comparative 1 min. 1.0 10.sup.11 150 Example 1 Comparative 1 1.0 10.sup.12 200 Example 2

Evaluation

[0097] (1) As acknowledged from the comparison between the volume resistivity (1.510.sup.8 .Math.cm) of the LN substrate according to Example 14 which was heat-treated under substantially the same conditions as those in Example 9 (vacuum atmosphere) except that the atmosphere was changed to a nitrogen gas atmosphere and the volume resistivity (1.010.sup.8 .Math.cm) of the LN substrate according to Example 9 described above, it can be understood that difference in treatment atmosphere, for example, does not greatly affect the volume resistivity of an LN substrate containing Fe.

[0098] Actually, as compared with the treatment method described in Patent Document 2 (Comparative Example 2), no great change was observed in the variations (/Ave) in volume resistivity in the surfaces of the LN substrates after the treatment even though the heat treatment conditions were not so strictly managed in Examples 1 to 14.

[0099] (2) On the other hand, in Comparative Example 2, it was acknowledged that when the management of the above-described heat treatment conditions was neglected, there was a case where the variation (/Ave) in volume resistivity in the plane of the LN substrate after the treatment was approximately 10% as described above.

[0100] (3) That is, it is acknowledged that Examples 1 to 14 according to the present invention are superior to the treatment method described in Patent Document 2 (Comparative Example 2).

POSSIBILITY OF INDUSTRIAL APPLICATION

[0101] The present invention makes it possible to stably obtain an LN substrate in which a volume resistivity after reduction treatment is controlled to be within a range of 110.sup.8 .Math.cm or more to 210.sup.12 .Math.cm or less, and an in-plane distribution of the volume resistivity is small. Therefore, the present invention has a possibility of industrial application for use as a material for a surface acoustic wave device (SAW filter).