JP2006321682A - Method for producing lithium tantalate crystal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To simply and securely produce uniform lithium tantalate crystals having satisfactory reproducibility and having high electric conductivity to some extent of 1×10<SP>-13</SP>Ω<SP>-1</SP>×cm<SP>-1</SP>to <1×10<SP>-12</SP>Ω<SP>-1</SP>×cm<SP>-1</SP>. <P>SOLUTION: In the method for producing lithium tantalate crystals having increased electric conductivity, lithium tantalate crystals are exposed to a reducing atmosphere together with titanium at a temperature T lower than the Curie temperature. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for producing a lithium tantalate crystal used for applications such as forming a pattern with a metal electrode on a wafer such as a surface acoustic wave element and processing an electrical signal.

  Lithium tantalate is used for applications that utilize electrical characteristics such as surface acoustic wave (SAW) signal processing. Lithium tantalate crystals suitable for this application show the piezoelectric response (piezoelectricity) required for SAW devices due to their crystal structure, but the lithium tantalate crystals available in the usual way add to the piezoelectricity. A pyroelectric response (pyroelectricity).

  The piezoelectricity of the lithium tantalate crystal is an indispensable characteristic when the lithium tantalate crystal is used as a SAW device. On the other hand, pyroelectricity gives a temperature change to the lithium tantalate crystal to change the outer surface of the crystal. It is observed as a surface charge generated in the substrate and charges the crystal. This surface charge is considered to cause a spark discharge between metal electrodes formed on a wafer made of lithium tantalate crystals when using the lithium tantalate crystals as a SAW device, causing a significant performance defect of the SAW device. ing. For this reason, in designing a SAW device using a lithium tantalate crystal, a device that does not generate surface charges, a device that releases generated surface charges, or a device that widens the interval between metal electrodes is required. In order to adopt this, there is a disadvantage that the design of the SAW device itself is restricted.

  In the manufacturing process of a SAW device using a lithium tantalate crystal, there is a process in which heat is applied to the lithium tantalate crystal in a process such as deposition of a metal film and removal of a resist layer. When applied to the lithium tantalate crystal, a charge is generated on the outer surface due to the pyroelectric property of the lithium tantalate crystal. This surface charge causes a spark discharge between the metal electrodes, resulting in destruction of the electrode pattern. Therefore, in the manufacturing process of the SAW device, devise not to change the temperature as much as possible, or to make the temperature change moderate. However, due to these contrivances, there is a disadvantage that the throughput of the manufacturing process is reduced or the margin for guaranteeing the performance of the SAW device is narrowed.

  In the lithium tantalate crystal produced by the usual method, the charge on the outer surface generated by pyroelectricity is neutralized by free charge from the surrounding environment and disappears over time, but this disappearance time is several hours or more In the SAW device manufacturing process, this spontaneous pyroelectric loss cannot be expected.

  For applications such as surface acoustic wave (SAW) devices, there is no generation of charge on the outer surface of the crystal due to the above background while maintaining the piezoelectricity required to exhibit device characteristics. There is an increasing demand for piezoelectric crystals, and there is a need for lithium tantalate crystals that do not show surface charge accumulation for such applications. One solution for achieving this problem is to improve the conductivity of the lithium tantalate crystal and eliminate the generated surface charge without accumulating it.

  As a method for producing a lithium tantalate crystal with improved conductivity, there is JP-A-11-92147 (Patent Document 1). Here, the lithium tantalate crystal is exposed to a reducing atmosphere at 500 ° C. or higher. A method is disclosed. However, when the lithium tantalate crystal is reduced by the method disclosed in Patent Document 1, the single-domain structure required for SAW device use is lost when the processing temperature in the reducing atmosphere is 610 ° C. or more, which is the Curie point. In addition, at a temperature of 610 ° C. or lower, which is the Curie point, the speed of the reduction treatment becomes extremely slow, and as a result, the method disclosed in Patent Document 1 cannot industrially improve the conductivity of the lithium tantalate crystal. all right.

  In view of the above, the present inventor makes contact with the lithium tantalate crystal at a temperature lower than the temperature T1 and the lithium tantalate crystal at a temperature lower than the temperature T1. Proposed a method for improving surface properties and eliminating the surface charge generated in the lithium tantalate crystal without accumulating it (Patent Documents 2, 3: Japanese Patent Laid-Open No. 2004-269300, International Publication No. 2004/079061 pamphlet) .

In order to prevent the accumulation of surface charges, the higher the conductivity, the better. However, the lithium tantalate crystal used in various devices including SAW devices must have a certain level of insulation. In that case, it is necessary to consider the effect of preventing the accumulation of surface charges and the insulation. Insulation required for the lithium tantalate crystal in that case varies depending on performance required for the device, device structure, design method, and the like. One guideline for the required insulation is less than 1 × 10 ⁻¹² Ω ⁻¹ · cm ⁻¹ . With the previously proposed method (see Patent Documents 2 and 3), it is possible to obtain a material with a uniform and high conductivity of 1 × 10 ⁻¹¹ Ω ⁻¹ · cm ⁻¹ or more with good reproducibility. In this case, there has been a demand for a method of obtaining a uniform, somewhat high conductivity, 1 × 10 ⁻¹³ Ω ⁻¹ · cm ⁻¹ or more and less than 1 × 10 ⁻¹¹ Ω ⁻¹ · cm ⁻¹ with good reproducibility.

As another method for producing a lithium tantalate crystal with improved conductivity, there is JP-A-2004-35396 (Patent Document 4). Here, the temperature is lower than the Curie temperature in an environment containing metal vapor. A method of heating is disclosed. Furthermore, a method is disclosed in which the metal vapor consists of zinc. However, when the lithium tantalate crystal is treated by the method disclosed in Patent Document 4, those having a ^{density of} 1 × 10 ⁻¹² Ω ⁻¹ · cm ^{−1 to} 1 × 10 ⁻¹¹ Ω ⁻¹ · cm ⁻¹ are uniform. Although it can be obtained with good reproducibility, those having a density of less than 1 × 10 ⁻¹² Ω ⁻¹ · cm ⁻¹ are uniform and cannot be obtained with good reproducibility.

JP-A-11-92147 JP 2004-269300 A International Publication No. 2004/079061 Pamphlet JP 2004-35396 A

  The present invention has been made in response to the above-mentioned demands. Tantalum that can perform uniform and reproducible conductivity improvement processing, such as SAW devices, without impairing the insulation required as a device, and prevents accumulation of surface charges. It aims at providing the manufacturing method of a lithium acid crystal.

As a result of intensive studies to achieve the above object, the present inventor has performed reduction treatment of lithium tantalate crystals together with titanium and / or titanium hydride at a temperature T lower than the Curie temperature, thereby achieving uniform and reproducibility. Highly conductive, 1 × 10 ⁻¹³ Ω ⁻¹ · cm ⁻¹ or more and less than 1 × 10 ⁻¹² Ω ⁻¹ · cm ⁻¹ , accumulation of surface charge is reliably prevented, and insulation is achieved The inventors have found that lithium tantalate crystals that are not impaired can be obtained, and have made the present invention.

  Accordingly, the present invention provides a method for producing a lithium tantalate crystal with increased conductivity, characterized in that the lithium tantalate crystal is exposed to a reducing atmosphere at a temperature T lower than the Curie temperature together with titanium or titanium hydride.

  The reduction treatment is preferably performed in a reducing gas containing one or more mixed gases selected from hydrogen, carbon monoxide, and dinitrogen monoxide. In this case, the reducing gas is further diluted with a rare gas. It may contain one or more mixed gases selected from gas, nitrogen and carbon dioxide.

  As the lithium tantalate crystal to be processed, a single-polarized crystal, particularly a wafer that has been sliced and / or lapped as a single-polarized crystal can be used. Staged crystals can also be used. Furthermore, it is preferable that the temperature T is in the range of 400 to 600 ° C. After the treatment at the temperature T, it is preferable to introduce air at a temperature of 250 ° C. or less.

According to the present invention, a lithium tantalate crystal having a uniform, reproducible, and high conductivity to some extent and having a resistance of 1 × 10 ⁻¹³ Ω ⁻¹ · cm ^{−1 to} 1 × 10 ⁻¹² Ω ⁻¹ · cm ⁻¹ is easily obtained. And it can manufacture reliably.

  The method for producing a lithium tantalate crystal with increased conductivity according to the present invention comprises subjecting a lithium tantalate crystal together with titanium and / or titanium hydride to a reducing atmosphere at a temperature T lower than the Curie temperature. The conductivity of the lithium crystal can be increased. As a result, pyroelectricity generated when a temperature change is applied to the lithium tantalate crystal can be suppressed.

  Thus, the surface charge generated by changing the temperature of the lithium tantalate crystal can be eliminated without accumulating the generated surface charge by improving the conductivity of the lithium tantalate crystal. It is.

  Here, as the lithium tantalate crystal to be treated at the temperature T, a single-polarized crystal can be used. When using a single-polarized lithium tantalate crystal as described above, The obtained lithium tantalate crystal does not require a single domain treatment after the reduction treatment at the temperature T. In this case, as a form of a single polarized crystal, a crystal before slicing, a wafer subjected to slicing processing, or a wafer subjected to lapping processing can be used.

  Note that the normal monodomain treatment is performed in the atmosphere at a high temperature above the Curie point (about 610 ° C.) of the lithium tantalate crystal. On the other hand, in the lithium tantalate crystal with improved conductivity obtained in the present invention, the improved conductivity is lost when the temperature is set to 400 ° C. or higher in the atmosphere. As a result, the treatment according to the present invention is performed. The lithium tantalate crystal subjected to the above has a property of returning to the state before the reduction heat treatment when the single domain treatment is performed thereafter. However, in the present invention, the conductivity T is lost because the temperature T for reducing heat treatment of lithium tantalate is set to a temperature lower than the Curie point (610 ° C.) of the lithium tantalate crystal and the treatment atmosphere is a reducing atmosphere. There will be no problem.

  In the present invention, the reductive heat treatment of the lithium tantalate crystal at a temperature T may be a commonly known reducing gas atmosphere, such as hydrogen, carbon monoxide, dinitrogen monoxide, A reduction-treated substance can be obtained by performing in a reducing gas composed of these mixed gases. In this case, the reduction treatment can be performed in an atmosphere in which a rare gas such as He, Ne, Ar, nitrogen, carbon dioxide, or an inert gas composed of a mixed gas thereof is added to the reducing gas. In addition, it is preferable to make these inert gas content 0-90 volume% in the reducing gas which carries out the reduction process of the said substance, especially 0-80 volume%.

  The temperature for treating the lithium tantalate crystal is preferably 400 to 600 ° C. as described above, and the time for the reduction treatment is appropriately selected, but is usually 0.5 to 20 hours, more preferably 0.5 to 10 hours. Yes, when the lithium tantalate crystal to be treated is in a state of being uniformly light black in the surface, the reduction treatment can be terminated.

  As described above, after the reduction heat treatment, the temperature may be lowered. In this case, when the temperature falls to 250 ° C. or lower, introduction of air prevents cracking due to heat shrinkage or the like at the time of removal, or in the air Recommended in view of preventing reaction with other ingredients.

  As a preferred method for obtaining a lithium tantalate crystal having a single-domain structure intended in the present invention and having improved conductivity, for example, single-domain conversion to be subjected to reduction heat treatment at the temperature T of the present invention is possible. The treated lithium tantalate crystal wrap wafer and titanium or titanium hydride or a mixture thereof are placed in a furnace, and reducing gas such as hydrogen gas is 0.1 to 50 liters per minute, especially 1 to 30 liters per minute. The temperature of the furnace is raised from room temperature at a rate of 1 to 20 ° C. per minute, particularly 1 to 10 ° C., and kept at the required temperature T for 0.5 to 20 hours, particularly 0.5 to 10 hours. Thereafter, it is preferable that the furnace is cooled at a rate of 1 to 20 ° C. per minute, particularly 1 to 10 ° C., the atmosphere is introduced into the furnace at 250 ° C. or less, and the wafer is taken out of the furnace when it becomes 30 ° C. or less.

  The lithium tantalate crystal obtained in the present invention has a characteristic that accumulation of surface charges caused by a temperature change is substantially not observed due to an improvement in crystal conductivity. As a result, the lithium tantalate crystal obtained in the present invention maintains the piezoelectricity, and no charge is accumulated on the outer surface of the crystal. At the same time, the insulating properties necessary for the device are not impaired, and the SAW It is an extremely advantageous material for device manufacturing. Further, in the method of the present invention, the above-described lithium tantalate crystal can be obtained in a very short time, and the conductivity can be improved uniformly and reproducibly, which is an industrially advantageous production method. ing.

Here, although the measurement of the electrical conductivity with respect to the lithium tantalate crystal is as described in the Examples, the electrical conductivity of the lithium tantalate crystal before the reduction heat treatment according to the present invention is usually 10 ⁻¹⁴ to 10 ⁻¹⁵ Ω ^−1. In contrast to cm ⁻¹ , the conductivity of the lithium tantalate crystal after the reduction heat treatment of the present invention is 1 × 10 ⁻¹² to 1 × 10 ⁻¹³ Ω ⁻¹ · cm ⁻¹ .

  EXAMPLES Hereinafter, although an Example and a comparative example are shown and this invention is demonstrated concretely, this invention is not restrict | limited to the following Example. In the following examples, lithium tantalate is abbreviated as LT.

[Example of LT wafer production]
LT wafer fabrication was performed as follows. A lithium tantalate crystal having a diameter of 100 mm and a length of 50 mm oriented by rotating 36 ° in the y direction with respect to the surface normal was obtained by using the Czochralski method and a conventional secondary processing method (hereinafter referred to as the “secondary processing method”). , Written as LT crystal). The LT crystal was cut and lapped to obtain a double-sided lapped wafer having a thickness of 0.4 mm (hereinafter, this wafer is referred to as a LT lapped wafer). One side of the LT wrap wafer was polished to obtain a wafer having a thickness of 0.35 mm (hereinafter, this wafer will be referred to as an LT polished wafer). This wafer was colorless and translucent.

[Examples and Comparative Examples]
The LT wrap wafer was placed in a sealed furnace in which each gas in Table 1 circulated at a rate of about 10 liters per minute. The outline of the furnace is shown in FIG.

  In the furnace 10, an alumina treatment tube 11 having a diameter of 200 mm is disposed along the horizontal direction, and three zones A, B, and C are formed in the treatment tube 11. Thermocouples a, b, and c are provided corresponding to the bands A, B, and C. Both ends of the alumina treatment tube 11 are extended from the furnace 10 to the outside, and one of the extended end faces is hermetically closed, and a gas supply port 12 is formed in the closed face, and the other extension A cap 14 is detachably attached to the extended end face, and a gas discharge port 13 is formed in the vicinity of the other extended end face. Although not shown in the drawing, both extended base ends of the alumina processing tube 11 serving as a wafer outlet are hermetically sealed by O-rings. The means for heating the inside of the furnace is based on resistance heating.

  First, the cap 14 is removed, and the alumina carrier 15 supporting the LT wrap wafer 16 and 30 g of titanium or titanium hydride 18 placed in a container 17 are placed in the alumina processing tube 11, and the gases shown in Table 1 are supplied. While the gas flow was introduced into the alumina treatment tube 11 from the port 12, heating of the furnace was started in a state where the gas flow was discharged from the gas discharge port 13. The furnace temperature was raised from room temperature to the temperature T in Table 1 at a rate of about 6.7 ° C. per minute. After holding at temperature T for 1 hour, the furnace was cooled at a rate of about 6.7 ° C. per minute. Atmosphere was introduced into the furnace at 250 ° C. or lower, and the wafer was taken out of the furnace when it became 30 ° C. or lower.

The conductivity was measured as follows. The conductivity is the reciprocal of the volume resistivity, and the volume resistivity was measured using 4329A High Resistance Meter and 16008A Resistivity Cell manufactured by Hewlett Packard. The volume resistivity can be obtained by the following formula.
ρ = (πd ² / 4t) · R
ρ: Volume resistivity (Ω · cm)
π: Circumference d: Center electrode diameter (cm)
t: LT wafer thickness (cm)
R: Resistance value (Ω)
The resistance value was measured by applying a voltage of 500 volts to the sample and measuring the resistance value one minute after the voltage was applied.

  The surface potential was measured as follows. Pyroelectricity is the amount of charge that accumulates on the surface when a temperature difference occurs. This is the same as static electricity, and surface potential measurement is known as a quantitative measurement. The LT wafer subjected to the reduction treatment is placed on a hot plate at 100 ° C. for 1 minute, and the absolute maximum or minimum value of the surface potential that changes in 1 minute is obtained by using SK-030, SK-200 manufactured by Keyence Corporation. The value was set to the maximum value, and the absolute value of the maximum or minimum value after 1 minute on the hot plate was set to 1 minute value.

Table 1 shows the conductivity and surface potential. In the table, the description of conductivity, for example, “9.3E-14” means “9.3 × 10 ⁻¹⁴ ”.

The surface charge disappearance ability is indicated by the maximum value of the surface potential and the value of 1 minute, and requires 2500 V or less and 50 V or less, respectively. In addition, the necessary insulating property is less than 1 × 10 ⁻¹² Ω ⁻¹ · cm ⁻¹ . In this embodiment, it can be seen that the above criteria are satisfied. As for the inert gas, the ratio is good when the ratio is 0 to 90%. When the wafer obtained in this example was visually inspected, it was confirmed that all of the wafers were discolored uniformly in the wafer surface.

It is the schematic of the furnace used in the Example.

Explanation of symbols

11 Alumina treatment tube 12 Gas supply port 13 Gas discharge port 14 Cap 15 Alumina carrier 16 LT wrap wafer 17 Container 18 Titanium or titanium hydride

Claims (9)

  A method for producing a lithium tantalate crystal with increased conductivity, characterized by exposing the lithium tantalate crystal together with titanium to a reducing atmosphere at a temperature T lower than the Curie temperature.   A method for producing a lithium tantalate crystal with increased conductivity, characterized in that the lithium tantalate crystal is exposed to a reducing atmosphere at a temperature T lower than the Curie temperature together with titanium hydride.   3. The lithium tantalate crystal according to claim 1, wherein the reduction treatment is performed in a reducing gas containing one or more mixed gases selected from hydrogen, carbon monoxide, and dinitrogen monoxide. Manufacturing method.   4. The method for producing a lithium tantalate crystal according to claim 3, wherein the reducing gas further contains one or more mixed gases selected from a rare gas, nitrogen, and carbon dioxide.   The method for producing a lithium tantalate crystal according to any one of claims 1 to 4, wherein a single-polarized crystal is used as the lithium tantalate crystal to be treated.   6. The method for producing a lithium tantalate crystal according to claim 5, wherein a wafer subjected to slicing and / or lapping is used as the single polarized crystal.   6. The method for producing a lithium tantalate crystal according to claim 5, wherein a crystal before slicing is used as the single polarized crystal.   The method for producing a lithium tantalate crystal according to any one of claims 1 to 7, wherein the temperature T is in a range of 400 to 600 ° C. 9. The method for producing a lithium tantalate crystal according to claim 8, wherein after treatment at temperature T, air is introduced at a temperature of 250 ° C. or lower.

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