KR20040029247A - Method of producing lithium tantalate substrate for surface acoustic wave element - Google Patents

Abstract

PURPOSE: A method of fabricating a LiTaO3 single crystalline substrate for a surface acoustic wave substrate is provided to reduce the lead-time and improve the electric conductivity by performing a reduction process within a short period of time. CONSTITUTION: A reduction process for a LiTaO3 single crystalline ingot or a LiTaO3 single crystalline substrate is performed above the Curie temperature under the reductive atmosphere. A single polarization process for the reduced LiTaO3 single crystalline ingot or the reduced LiTaO3 single crystalline substrate is performed under the reductive atmosphere or the inactive atmosphere. A surface acoustic wave substrate is formed by processing the LiTaO3 single crystalline ingot or the LiTaO3 single crystalline substrate.

Description

METHODS OF PRODUCING LITHIUM TANTALATE SUBSTRATE FOR SURFACE ACOUSTIC WAVE ELEMENT}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a lithium tantalate single crystal substrate, and more particularly, to a surface acoustic wave device in which the single crystal substrate is reduced at or above the Curie temperature of lithium tantalate, thereby increasing the electrical conductivity of the single crystal substrate to remove static electricity from the surface of the substrate. A method for producing a lithium tantalate single crystal substrate.

Lithium tantalate single crystal is widely used in the fields of surface acoustic wave signal processing device, infrared sensor, optical switch, and optical memory because of its excellent piezoelectricity and electro-optic properties.

However, when monopolarization treatment is performed to use the lithium tantalate single crystal as the substrate of the surface acoustic wave device, sparks are generated due to the discharge of the electrostatic charge attached to both surfaces of the lithium tantalate single crystal substrate, thereby producing the surface acoustic wave device. It causes a lot of problems in the process. This phenomenon is due to the pyroelectric properties of lithium tantalate, and in the case of lithium tantalate, which is basically an electrical insulator, it takes a lot of time to move the electrostatic charge attached to the surface of lithium tantalate to achieve electrical neutrality. Under conditions where the temperature changes abruptly as in the step, the electrostatic charge causes discharge and sparks are likely to occur. As such, the static electricity due to the electrostatic charge attached to the lithium tantalate substrate may cause an operation error in the manufacturing apparatus of the surface acoustic wave device during substrate transfer or operation, and the substrate is strongly attached to the manufacturing apparatus, There is a problem that the work stability is remarkably inferior, such as breakage of the substrate.

In addition, when the temperature of the substrate is suddenly changed during the process of attaching the device to the lithium tantalate substrate, the device circuit manufactured on the substrate or the substrate may be damaged by the spark generated by the discharge of the electrostatic charge attached to the substrate, or further, It may cause electric shock to the device manufacturing apparatus and cause failure.

In recent years, as the frequency band of the device becomes higher and higher, the width of the signal line of the device decreases from several tens of micrometers to several micrometers. As a result, signal lines having a fine line width are destroyed, thereby shortening the lifespan of the device.

In order to solve the above problems, a method of manufacturing a device on a substrate after generating a conductive metal film on the back side of the device manufacturing surface of the substrate and grounding it to remove static electricity or by electrically connecting both sides of the substrate for a long time discharge Has been. However, according to the above method, additional processes such as the generation of the conductive metal film and the removal of the conductive metal film after the final device fabrication are required, or the cost increases due to the increase in the processing time. In addition, even if the above methods are used, there is a problem in that the shortening of the device life due to the micro discharge generated due to the local electrostatic charge difference on the surface of the substrate cannot be effectively suppressed.

In order to solve this problem, recently, a technique for increasing the electrical conductivity of a monopolarized lithium tantalate single crystal substrate and rapidly moving the electrostatic charge attached to the surface of the substrate has been proposed to improve the electrostatic characteristics of the substrate. U.S. Patent No. 6319430, which is representative of the patent, discloses that the lithium niobate or lithium tantalate substrate is heated to a temperature below its Curie temperature so as to maintain a single polarization to reduce the electrical conductivity of the single crystal substrate. The thermal conductivity of lithium niobate or lithium tantalate in a reducing atmosphere is known to increase the electron density due to the change in the oxidation state.

However, the lithium tantalate reduction reaction increases rapidly as the treatment temperature increases, so in the case of lithium tantalate substrates having a low Curie temperature (Curi temperature; about 605 ° C), the reaction rate is too slow for commercial application. There is this. That is, according to the U.S. patent, there is a problem that satisfactory electrostatic properties of the lithium tantalate single crystal substrate can be obtained only when using a long time treatment and expensive equipment that are difficult to apply commercially.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and after reducing a lithium tantalate single crystal substrate above the Curie temperature of lithium tantalate, and then monopolarizing it in a reducing atmosphere or an inert atmosphere, tantalic acid for surface acoustic wave device manufacturing It aims at improving the electrostatic characteristic of a lithium single crystal substrate.

In order to achieve the above object, the method for producing a lithium tantalate single crystal substrate of the present invention,

Reducing the lithium tantalate single crystal ingot or the substrate by heating the lithium tantalate single crystal ingot or the substrate to a temperature above the Curie temperature (Tc) of lithium tantalate under a reducing atmosphere;

Monopolarizing the reduced lithium tantalate single crystal ingot or substrate under a reducing atmosphere or an inert atmosphere;

Processing the lithium tantalate single crystal ingot or substrate to produce a lithium tantalate single crystal substrate for a surface acoustic wave device;

Characterized in that comprises a.

The volume resistivity of the reduced tantalum single crystal ingot or lithium tantalate single crystal substrate is preferably 10 ⁸ to 10 ¹¹ Ω · cm.

In the reduction reaction of lithium tantalate, the reaction rate increases rapidly as the reaction treatment temperature increases. By the way, as mentioned above, in the past, in order to maintain the monopolar state of the lithium tantalate single crystal, the temperature at the time of a reduction reaction was made below curie temperature.

Monopolarization refers to arranging all the polarizations formed in a single crystal in one direction. In the monopolarization treatment step, electrodes are coated on both sides of a single crystal along the direction of polarization, then heated above the Curie temperature and maintained for a certain time to remove polarization, and a DC voltage is applied to the electrodes to form an electric field. Cooling to below the Curie temperature. The so-called spontaneous polarization generated while cooling below the Curie temperature by the above process is arranged in one direction by the direct current applied to the single crystal.

However, such a monopolar state cannot be maintained above the Curie temperature at which the crystal structure of the single crystal is changed. As described above, the temperature at the time of the reduction reaction has been limited to below the Curie temperature. In other words, conventionally, reduction treatment has been performed under the premise of monopolarization treatment forever. As a result, the lithium niobate single crystal (Tc = about 1140 ℃), which had a high Curie temperature and had a sufficiently fast reduction reaction in the state of maintaining a single polarization, was actively used in various fields. In the case of the slow lithium tantalate single crystal, the same reduction treatment method has been rarely used. Moreover, the conventional method was not only slow in the reduction reaction rate, but also very difficult to achieve a volume resistivity of 10 ¹¹ Ω · cm or less, which is a level capable of removing surface static electricity of the lithium tantalate substrate.

However, the present inventors have diligently researched the lithium tantalate single crystals, and as a result, they drastically overturn the existing common sense on the premise of a monopolarization state, and perform a reduction reaction in a high-temperature reducing atmosphere capable of securing a sufficient reduction reaction rate. When the monopolarization treatment is carried out in a reducing atmosphere or an inert atmosphere to maintain the reduced state, it has been found that the monopolar state can be maintained while significantly increasing the reduction reaction rate of the lithium tantalate single crystal.

In addition, the general monopolarization process is performed by heating up to a temperature above the Curie temperature in an air atmosphere and applying an electric field to both ends of the crystal. However, in the present invention, the lithium tantalate single crystal heat-treated in a reducing atmosphere is reduced in a single air atmosphere. When the polarization treatment removes the reduced state by the reoxidation reaction, the monopolarization process is also made into a reducing atmosphere or at least an inert atmosphere, thereby increasing the reduction reaction rate and maintaining the reduced state and the monopolar state.

The temperature during the reduction reaction heat treatment is 605 ° C. or more, which is the Curie temperature of lithium tantalate, and the upper limit can theoretically be up to 1650 ° C., the melting point of lithium tantalate.

The reducing atmosphere during the reduction reaction heat treatment or monopolarization treatment may be formed using at least one gas and / or solid selected from general reducing gases such as hydrogen, carbon monoxide and water vapor, and reducing solids such as graphite.

In addition, the inert atmosphere at the time of monopolarization process can be comprised by the inert gas containing nitrogen and argon, or vacuum.

The change in electrical conductivity of the reduced heat treated lithium tantalate single crystal can be known by measuring its volume resistivity with a high resistance measuring instrument. The volume resistivity of the unreduced lithium tantalate single crystal is about 10 ¹⁴ to 10 ¹⁵ Ω · cm, and the volume resistivity decreases as the degree of reduction is increased. In order to suppress the generation of static electricity on the surface of the lithium tantalate substrate, a volume resistivity of at least 10 ¹¹ Ω · cm or less should be achieved, and preferably 10 ¹⁰ Ω · cm or less. However, when the volume resistivity is less than 10 ⁸ Ω · cm, the mechanical strength of the single crystal is weakened. Therefore, the lower limit of the volume resistivity is preferably 10 ⁸ Ω · cm or more.

Meanwhile, as the reduction degree of the lithium tantalate single crystal increases, the color of the single crystal changes to dark gray or black, and the higher the reduction degree, the darker the black color becomes.

Comparative Example I:

Table 1 shows the measurement of the volume resistivity of a lithium tantalate substrate subjected to a monopolarization treatment and reduced using hydrogen as a reducing gas at a temperature below the Curie temperature using a high resistance measuring instrument.

Sample Number Reduction Treatment Temperature Reduction Treatment Time Volume resistivity Whether to maintain single polarization One 550 ℃ 10 hours 3 x 10 ¹³ cm maintain 2 550 ℃ 20 hours 9 × 10 ¹² cmcm maintain 3 550 ℃ 72 hours 7 × 10 ¹² cmcm maintain 4 570 ℃ 12 hours 2 x 10 ¹³ cm maintain 5 570 ℃ 27 hours 6 x 10 ¹² cm maintain 6 570 ℃ 72 hours 4 × 10 ¹² mm maintain 7 600 ℃ 50 hours 3 x 10 ¹² cm Keep some 8 600 ℃ 72 hours 9 x 10 ¹¹ cm Keep some

As can be seen from Table 1, in the comparative example, it is difficult to obtain a volume resistivity of 10 ¹¹ Ω · cm or less, which is a level capable of removing static electricity of the lithium tantalate substrate even after reducing heat treatment for a long time of 72 hours or more. Further, when a reducing treatment at a temperature not higher than 550 ℃ there, and the reduction of volume resistivity about insignificant even increase the processing time, 10 ¹¹ Ω and the adjacent levels with the volume specific resistance of cm Comparative Example 8, a single polarization state portion removed for There is a problem such as being.

Example I:

The unpolarized lithium tantalate single crystal ingot or substrate was reduced using hydrogen, a reducing gas, at a temperature above the Curie temperature, and its volume resistivity was measured using a high resistance measuring instrument.

Example I shape Reduction Treatment Temperature Reduction Treatment Time Volume resistivity One Board 700 ℃ 5 hours 9 × 10 ¹⁰ cm 2 Board 700 ℃ 10 hours 5 x 10 ¹⁰ cm 3 Board 700 ℃ 20 hours 3 x 10 ¹⁰ cm 4 Ingot 700 ℃ 5 hours 2 x 10 ¹¹ cm 5 Ingot 700 ℃ 10 hours 7 × 10 ¹⁰ cm 6 Ingot 700 ℃ 20 hours 3 x 10 ¹⁰ cm 7 Board 800 ℃ 5 hours 4 × 10 ¹⁰ cm 8 Board 800 ℃ 10 hours 2 x 10 ¹⁰ cm 9 Board 800 ℃ 15 hours 2 x 10 ¹⁰ cm 10 Ingot 800 ℃ 5 hours 2 x 10 ¹⁰ cm 11 Ingot 800 ℃ 10 hours 2 x 10 ¹⁰ cm 12 Ingot 800 ℃ 15 hours 2 x 10 ¹⁰ cm 13 Board 900 ℃ 5 hours 1 × 10 ¹⁰ cm 14 Board 900 ℃ 10 hours 1 × 10 ¹⁰ cm 15 Ingot 900 ℃ 5 hours 1 × 10 ¹⁰ cm 16 Ingot 900 ℃ 10 hours 1 × 10 ¹⁰ cm 17 Board 1000 ℃ 3 hours 6 x 10 ⁹ cm 18 Board 1000 ℃ 5 hours 6 x 10 ⁹ cm 19 Ingot 1000 ℃ 3 hours 6 x 10 ⁹ cm 20 Ingot 1000 ℃ 5 hours 6 x 10 ⁹ cm

As can be seen from Table 2, in Example I of the present invention, a volume resistivity of 10 ¹¹ Ω · cm or less capable of removing surface static electricity of a lithium tantalate substrate can be achieved within a much shorter time than the comparative example. . In addition, it can be seen that as the reduction treatment temperature increases, the reduction treatment time becomes shorter and the volume resistivity becomes smaller.

Comparative Example II and Example II:

When the reduced ingot or the substrate is monopolarized in an air atmosphere as in a general monopolarization process, reoxidation occurs in a short time, thereby rapidly increasing the volume resistivity again. As can be seen from Table 3, even if the reduction treatment at the Curie temperature or higher as in the present invention conditions, as in Comparative Example II, it can be seen that the volume resistivity increases again when monopolarized in the air atmosphere. On the other hand, as in Example II of the present invention, when the monopolarization treatment in a reducing atmosphere such as hydrogen or an inert gas atmosphere such as nitrogen, it can be seen that the volume resistivity before monopolarization is maintained as it is.

Reduction heat treatment condition Monopolarization conditions Temperature time Volume resistivity atmosphere Temperature time Volume resistivity Comparative Example II 800 ℃ 5 hours 5 x 10 ¹⁰ cm air 650 ℃ 3 hours 4 x 10 ¹⁴ cm 900 ℃ 5 hours 1 × 10 ¹⁰ cm air 650 ℃ 3 hours 4 x 10 ¹⁴ cm Example II 800 ℃ 5 hours 5 x 10 ¹⁰ cm Hydrogen 650 ℃ 3 hours 5 x 10 ¹⁰ cm 900 ℃ 5 hours 1 × 10 ¹⁰ cm Hydrogen 650 ℃ 3 hours 1 × 10 ¹⁰ cm 800 ℃ 5 hours 5 x 10 ¹⁰ cm nitrogen 650 ℃ 3 hours 5 x 10 ¹⁰ cm 900 ℃ 5 hours 1 × 10 ¹⁰ cm nitrogen 650 ℃ 3 hours 1 × 10 ¹⁰ cm

The lithium tantalate ingot or substrate manufactured by the method of the present invention is processed by a conventional method to make a substrate for a surface acoustic wave device, and the surface acoustic wave device is manufactured thereon, and the electrical characteristics of the device are examined, and thus the resistivity characteristics are excellent. In addition, it was found that it is not affected by the static electricity due to the monopolarization, and it was also confirmed that the life shortening of the device is significantly reduced.

The method of the present invention may be performed independently on the already prepared lithium tantalate single crystal or substrate, but may be performed during the manufacturing process of the lithium tantalate single crystal, in particular, during the growth of the lithium tantalate single crystal and cooling to room temperature. . That is, after growing the lithium tantalate single crystal near the melting point temperature of lithium tantalate, and forming a reducing atmosphere inside the crystal growth furnace during the cooling process to room temperature and maintaining the same temperature and time as in the above embodiment of the present invention A reduced lithium tantalate single crystal ingot can be obtained.

As described above, according to the manufacturing method of the lithium tantalate single crystal substrate for the surface acoustic wave device of the present invention, the reduction treatment time is significantly reduced compared to the conventional manufacturing method, thereby reducing the working time and more excellent lithium tantalate single crystal. There is an effect that the electrical conductivity or volume resistivity of can be obtained.

As a result, since static electricity generated by the pyroelectric properties of lithium tantalate can be quickly removed, there is an advantage that work instability due to static electricity generated in the surface acoustic wave device manufacturing process can be significantly improved.

Claims (4)

Reducing the lithium tantalate single crystal ingot or the substrate by heating the lithium tantalate single crystal ingot or the substrate to a temperature above the Curie temperature (Tc) of lithium tantalate under a reducing atmosphere; Monopolarizing the reduced lithium tantalate single crystal ingot or substrate under a reducing atmosphere or an inert atmosphere; Processing the lithium tantalate single crystal ingot or substrate to produce a surface acoustic wave device substrate; Method for producing a lithium tantalate single crystal substrate for a surface acoustic wave device comprising a. The method of claim 1, A volume resistivity of the reduced tantalum single crystal ingot or lithium tantalate single crystal substrate is 10 ⁸ to 10 ¹¹ Ω · cm. The method of claim 1, The reducing atmosphere is a method for producing a lithium tantalate single crystal substrate for a surface acoustic wave device, characterized in that it comprises a reducing gas such as hydrogen, carbon monoxide, water vapor, or at least one gas and / or a solid selected from a reducing solid containing graphite. . The method of claim 1, The inert atmosphere is a method of manufacturing a lithium tantalate single crystal substrate for a surface acoustic wave device, characterized in that the inert gas containing a nitrogen, argon or vacuum.