WO2005112259A1

WO2005112259A1 - Method of manufacturing lithium tantalate substrate for surface acoustic wave elements and lithium tantalate substrate manufactured by the same

Info

Publication number: WO2005112259A1
Application number: PCT/KR2005/001417
Authority: WO
Inventors: Soo-Byoung Chae; Young-Jae Bae; Han-Gyun Kim
Original assignee: Iljin Diamond Co Ltd
Current assignee: Iljin Diamond Co Ltd
Priority date: 2004-05-14
Filing date: 2005-05-14
Publication date: 2005-11-24
Anticipated expiration: 2006-11-14
Also published as: TW200538590A; KR100651760B1; KR20050109174A; JP2005328499A

Abstract

Disclosed herein is a method of manufacturing a lithium tantalate substrate for surface acoustic wave elements, which includes heating a lithium tantalate material to a temperature of not less than the Curie temperature of lithium tantalate to reduce it, to prepare a lithium tantalate reducing agent; laminating the lithium tantalate reducing agent and a poled lithium tantalate substrate so that the reducing agent is positioned on both surfaces of the poled substrate, to form a laminate; heating the laminate to a temperature less than the Curie temperature, to reduce the lithium tantalate substrate; and removing the lithium tantalate substrate from the laminate and processing the removed lithium tantalate substrate into a substrate for surface acoustic wave elements. According to the method of the current invention, the reduction time is drastically decreased, thus shortening the process time, and as well, electrical conductivity or volume re¬ sistivity of LT becomes more improved, compared to conventional methods. Moreover, static electricity caused by pyroelectric properties of lithium tantalate is rapidly removed, and hence, process instability due to static electricity induced upon manufacturing the surface acoustic wave element may be greatly lowered.

Description

Description METHOD OF MANUFACTURING LITHIUM TANTALATE SUBSTRATE FOR SURFACE ACOUSTIC WAVE ELEMENTS AND LITHIUM TANTALATE SUBSTRATE MANUFACTURED BY THE SAME Technical Field

[1] The present invention relates, generally, to a method of manufacturing a lithium tantalate substrate, and more particularly, to a method of manufacturing a lithium tantalate substrate for surface acoustic wave elements, which enables lithium tantalate to be rapidly reduced even at a temperature lower than the Curie temperature (Tc) of lithium tantalite, to improve the electrostatic properties of lithium tantalate. Background Art

[2] In general, lithium tantalate (LT), which has excellent piezoelectric and electro- optical properties, has been widely used in the fields of surface acoustic wave signal processing elements, infra red sensors, light switches, light memory, etc. Particularly, in order to apply LT to a substrate of a surface acoustic wave element, LT should un dergo a poling process which arranges all the poles formed in LT in the same direction.

[3] The poling process includes applying an electrode material on both surfaces of an LT substrate, heating the substrate to a temperature of not less than Tc and maintaining the heated substrate for a predetermined time to remove the randomly distributed poles from the LT, applying direct current (DC) voltage to the electrode to form an electrical field, and cooling the substrate to a temperature not more than Tc So called spontaneous poles caused by cooling the substrate to a temperature not more than Tc as mentioned above are arranged in the same direction by the DC electrical field, thus obtaining the poled substrate. Disclosure of Invention Technical Problem

[4] However, to both surfaces of the poled LT substrate, electrostatic charges corresponding to the polarity of poles are attached, so that the electrostatic charges cause many problems in a subsequent process of manufacturing the substrate for surface acoustic wave elements. That is, static electricity due to electrostatic charges attached to the LT substrate may cause incorrect operation of the device for manufacturing the surface acoustic wave element upon transporting the substrate and performing the process. Also, due to static electricity, the substrate is firmly attached to the manufacturing device and thus may break down under mechanical stress, leading to remarkably decreased process stability. As well, in the case where the temperature of the substrate drastically varies upon attaching the element to the LT substrate, spark attributed to electrical discharge of electrostatic charges attached to the substrate may degrade the substrate or the element circuit formed on the substrate. Further, the device for manufacturing the element undergoes electrical impacts due to spark, and thus, may become out of order. In particular, recently, while the operation frequency of the el ement becomes increasingly higher, the width between signal lines of the element is decreased from tens of m m to ones of m m. Thus, the signal lines having fine line width break down due to small electrical discharge caused by the partial difference of electrostatic charges on the substrate. Thereby, the lifetime of the element is shortened.

[5] The above phenomenon is based on the pyroelectric properties of LT. The Lt that is an electrical insulator requires a long period of time to achieve electrical neutrality by transferring the electrostatic charges attached to the surface of LT. However, the above electrostatic charges cause electrical discharge under drastically varying conditions in temperature as in the process of manufacturing the surface acoustic wave element, thus easily generating spark.

[6] Accordingly, with the aim of solving the above problems, techniques for increasing the electrical conductivity of the LT substrate have been proposed to improve the electrostatic properties of the substrate so that the electrostatic charges attached to the surface of the substrate are rapidly transferred.

[7] In this regard, U.S. Patent No. 6,319,430 discloses a method of heating a lithium niobate (LN) substrate or an LT substrate to a temperature less than Tc thereof in a reducing atmosphere to reduce it, to increase the electrical conductivity of the substrate (that is, decrease the volume resistivity). The reason why the heat treatment for reduction of LN or LT results in the increase in its electrical conductivity is known to be that the electron density is increased due to the change of the oxidation state. In the above patent, the reduction temperature of the LT substrate is limited to a temperature less than Tc of LT. This is because the poling state of the substrate breaks down at a temperature of not less than Tc which is the temperature at which the crystal structure of the substrate changes. However, the reduction rate of LT is drastically increased as the reaction temperature is increased. Thus, the LT substrate having a low Tc (Tc: about 605 °C) has shortcomings, such as too slow a reaction rate. That is, the above patent is disadvantageous because the LT substrate has desired electrostatic properties only when treatment for a period of time too long to be suitable for commercial use is performed using expensive equipment.

[8] Recently, thorough attempts to increase the reduction rate of LT at a temperature less than Tc of LT have been made. For example, Korean Patent Laid-open Publication No. 2004-29248, which was previously filed by the present applicant, discloses a method of heating an LT substrate to a temperature less than Tc while a vacuum atmosphere and a reducing atmosphere are alternately applied, thereby increasing the reduction rate. Also, Japanese Patent Laid-open Publication No. 2004-35396 and WO 04/002891 disclose a method of increasing the reduction rate of LT by diffusing metal (e.g., zinc) vapor serving as a reducing agent onto the surface of a substrate at a temperature not more than Tc However, the former patent is disadvantageous because expensive equipment is required to alternately apply the vacuum atmosphere and the reducing atmosphere, and also, the desired volume resistivity is difficult to obtain. The latter patents are disadvantageous because the chemical properties of LT are changed due to the diffusion of zinc onto the LT, and therefore, the properties of final products may be degraded. Technical Solution

[9] Accordingly, the present invention has been made keeping in mind the above problems occurring in the related art, and an object of the present invention is to provide a method of manufacturing an LT substrate for surface acoustic wave elements, which enables LT to be rapidly reduced even at a temperature less than Tc of LT, to improve the electrostatic properties of LT.

[10] Another object of the present invention is to provide a method of manufacturing an LT substrate having improved electrostatic properties for surface acoustic wave elements.

[11] In order to accomplish the above objects, the present invention provides a method of manufacturing an LT substrate, comprising heating an LT material to a temperature of not less than Tc of LT to reduce it, to prepare an LT reducing agent; laminating the LT reducing agent and a poled LT substrate so that the LT reducing agent is positioned on both surfaces of the poled LT substrate, to form an LT laminate; heating the LT laminate to a temperature less than Tc of LT, to reduce the LT substrate; and removing the LT substrate from the laminate, and processing the removed LT substrate into a substrate for surface acoustic wave elements.

[12] Preferably, the reduced LT substrate has the volume resistivity of 10 - 10 12 Ohm- cm, and more preferably, of 10 - 10 Ohm- cm.

[13] Also, the laminate is preferably heated in a reducing atmosphere. Mode for Invention

[14] In the present invention, a method of manufacturing an LT substrate for surface acoustic wave elements is provided, by heating an LT material to a temperature of not less than Tc of LT to reduce it, to prepare an LT reducing agent, laminating the LT reducing agent and a poled LT substrate so that the LT reducing agent is positioned on both surfaces of the poled substrate, to form an LT laminate, heating the LT laminate to a temperature less than Tc of LT to reduce the LT substrate, removing the LT substrate from the laminate, and processing the removed LT substrate into a substrate for surface acoustic wave elements.

[15] LT is a material that is considerably difficult to reduce at a temperature less than Tc (about 605 °C) thereof. This means that LT strongly tends to oxidize at a temperature less than Tc, whereas it is relatively easy to reduce at a temperature of not less than Tc

[16] Thus, in the present invention, first LT, which is difficult to reduce at a temperature less than Tc and thus has a low reduction rate at a temperature less than Tc, is reduced at a temperature less than Tc, along with second LT, which has been reduced more than the first LT. At this time, LT may be rapidly reduced while maintaining the poling state thereof, thereby improving the electrostatic properties of LT.

[17] That is, although any other component, such as metal vapor, negatively affecting the chemical properties of LT, may be used as a reducing agent in conventional techniques (Japanese Patent Laid-open Publication No. 2004-35396 and WO 04/002891), in the present invention, the same LT is used as a reducing agent, which thus may rapidly reduce LT in the poling state while not deteriorating the chemical properties of LT. Further, according to the method of the present invention, the electrostatic properties of LT are further improved, compared to conventional methods of increasing a reduction rate.

[18] Hereinafter, a detailed description will be given of the present invention.

[19] Before performing the method of the present invention, a poled LT substrate is first prepared as a target substrate which is manufactured into a substrate for surface acoustic wave elements. As such, since the poling process is typically conducted to prepare a substrate for surface acoustic wave elements, a detailed description thereof is omitted.

[20] For reduction, an LT material is heated to a temperature of not less than Tc thereof, to prepare an LT reducing agent. As such, the above material may be used as a reducing agent, regardless of crystal state, such as monocrystals or polycrystals, and shape, such as substrates, ingots, powders, or sintered bodies. However, in consideration of the reaction area with the LT substrate, it is preferable that the above material be processed in the form of a substrate, corresponding to the shape of the LT substrate, and then reduced. The heat treatment temperature for reduction to prepare the reducing agent should be not less than about 605 °C, that is, the Tc of LT. Theoretically, the above heat treatment temperature may be increased up to 1650 °C, which is the melting point of LT.

[21] Since the LT material should be applied as a reducing agent to the poled LT which is a target substrate, its reduction degree should be at least higher than that of a final target substrate. The reduction degree of LT is in proportion to the electrical con- ductivity of LT and is in inverse proportion to the volume resistivity thereof. Thus, in the present invention, the reduction degree is represented by the magnitude of volume resistivity. The LT which is not reduced has the volume resistivity of 10 - 10 Ohm- cm. In order to inhibit the generation of static electricity on the surface of the LT 12 substrate, the volume resistivity should be 10 Ohm-cm or less. If the volume re- sistivity is less than 10 Ohm-cm, the mechanical strength of LT becomes weak. Therefore, the lower limit of volume resistivity is preferably determined to 10 Ohm-cm or more. More preferably, considering the inhibiting effects of the generation of static electricity and the mechanical strength of LT, the volume resistivity of LT ranges from 10 to 10 Ohm-cm.

[22] The LT material should be reduced until it has a volume resistivity of 10 12 Ohm-cm 12 or less. To obtain a volume resistivity of 10 Ohm-cm or less at a temperature less than Tc, the reduction treatment should be typically performed for about 72 hr in a strong reducing atmosphere. However, at a temperature of not less than Tc, the volume resistivity in the above range may be obtained by performing the reduction treatment for a time period shorter than 20 hr in a weak reducing atmosphere.

[23] Then, the LT reducing agent thus prepared and the poled LT substrate serving as a target substrate are laminated so that the reducing agent is positioned on both surfaces of the target substrate, to form an LT laminate.

[24] When the laminate is heated at a temperature less than Tc, the target substrate is reduced. As such, since the reducing agent has been reduced at a high temperature of not less than Tc of LT, it is in the state of being over-reduced at a temperature less than Tc thereof. Hence, when the laminate is heated to a temperature less than Tc, the reducing agent is considered to oxidize (that is, low reduction degree) and provide excess electrons to the surface of the target substrate adjacent thereto, and thus, function as a reducing agent of the target substrate. By the function of the reducing agent, the reduction rate of the target substrate is further increased, and the target substrate may be reduced more quickly according to the method of the present invention, compared to the technique disclosed in U.S. Patent No 6,319,430, hence obtaining the desired electrical conductivity.

[25] The heat treatment for reduction of the target substrate is performed until the LT substrate has the volume resistivity of 10 - 10 Ohm-cm that is required to inhibit the generation of static electricity on the surface of the substrate. However, considering the inhibiting effects of the generation of static electricity and the mechanical strength of the LT, the volume resistivity preferably falls in the range from 10 to 10 Ohm-cm. In the following example, according to the method of the present invention, when the reduction time is shorter than 30 hr, the LT substrate is confirmed to have a lower volume resistivity. [26] The reduction for preparation of the reducing agent and the reduction of the target substrate may be conducted in a non-oxidation atmosphere, that is, a reducing atmosphere or an inert atmosphere. However, if the reduction of the target substrate is conducted in an inert atmosphere, the reducing agent is rapidly oxidized. Thereby, electron transfer from the reducing agent becomes more active, and thus, the reduction of the target substrate becomes much faster, resulting in non-uniform reduction of the target substrate. The reduction of the target substrate, that is, the heat treatment of the laminate, is preferably carried out in a reducing atmosphere.

[27] The reducing atmosphere, which is required for preparation of the reducing agent or reduction of the target substrate, may be formed using at least one gas and/or solid selected from among a general reducing gas, such as hydrogen, carbon monoxide, water vapor, etc., and a reducing solid, such as graphite, etc.

[28] After the heat treatment of the laminate, that is, the heat treatment for reduction of the target substrate, the reducing agent is removed, and the target substrate is processed into a substrate for surface acoustic wave elements. The reducing agent, the volume resistivity of which is increased by the heat treatment of the laminate, is reduced at a high temperature, whereby it may be recycled for reduction of other target substrates.

[29] A better understanding of the present invention may be obtained through the following Examples and Comparative Examples which are set forth to illustrate, but are not to be construed as the limit of the present invention.

[30] Examples 1 to 10 [31] An LT material was heated to a high temperature (Table 1, below) not less than Tc in an atmosphere using hydrogen as a reducing gas, to prepare a reducing agent having a volume resistivity of 10 Ohm-cm or less. The reducing agent thus prepared and a poled target substrate were laminated so that the reducing agent was positioned on both surfaces of the poled substrate, to form a laminate. For reduction, the laminate was heated to 595 °C less than Tc for 24 hr in a hydrogen atmosphere. The preparation conditions of the reducing agent, and the volume resistivity of each of the reducing agent and the reduced target substrate measured using a device for measuring high resistance are summarized in Table 1, below.

[32] Table 1

[33] As is apparent from Table 1, the reducing agent can be prepared to have the volume resistivity of 10 Ohm-cm or less for a very short time. In this case, it can be seen that, as the reduction temperature is increased, the reduction time is relatively shortened and the volume resistivity is decreased.

[34] Further, when the laminate composed of the reducing agent and the target substrate is heated for reduction, the target substrate having a volume resistivity of 10 Ohm-cm or less, preferably 10 Ohm-cm or less, can be obtained, thus sufficiently inhibiting the generation of static electricity on the surface of the LT substrate. Also, while the target substrate is reduced using a reducing agent having a high reduction degree, a target substrate having a lower volume resistivity can be obtained in the same time.

[35] Comparative Example I [36] As disclosed in U.S. Patent No. 6,319,430, LT was poled and then heat treated for reduction at a temperature less than Tc using hydrogen as a reducing gas, after which the volume resistivity of the LT substrate was measured using a device for measuring high resistance. The results are given in Table 2, below.

[37] Table 2

[38] As is apparent from Table 2, in Comparative Example I, even if the heat treatment for reduction is performed for a long time of 72 hr or more, it is difficult to obtain the volume resistivity of 10 Ohm-cm or less that is a level able to sufficiently remove static electricity from the LT substrate. Further, in No. 8 of Comparative Example I having the volume resistivity of 10 Ohm-cm, reduction treatment for a time period (72 hr) much longer than in Examples (24 hr) of the present invention is required, and also, problems such as partial removal of the poling state are caused.

[39] Comparative Example II [40] As disclosed in Korean Patent Laid-open Publication No. 2004-29248 previously filed by the present applicant, the reduction rate of the LT substrate was increased by alternately applying a vacuum atmosphere and a reducing atmosphere at a temperature less than Tc The poled LT substrate was reduced at a temperature less than Tc in an atmosphere where a vacuum atmosphere of 100 mmHg or less and a reducing atmosphere using hydrogen as a reducing gas were alternately maintained. Subsequently, the volume resistivity of the substrate was measured using a device for measuring high resistance. The results are given in Table 3, below, in which a repetition number of 1 means that the vacuum atmosphere and the reducing atmosphere were alternately maintained once for corresponding periods of time. In Comparative Example II, the maintenance time of the vacuum atmosphere and the total reduction time were set to 5 min and 24 hr, respectively.

[41] Table 3

[42] As is apparent from Table 3, in Comparative Example II, the reduction of the target substrate is performed for the same time period (24 hr) as in Examples of the present invention. However, the volume resistivity in Comparative Example II is much higher than those in Examples. That is, the volume resistivity is 10 Ohm-cm or 10 Ohm-cm 11 13 in the present invention, whereas the volume resistivity is 10 -10 Ohm-cm in Comparative Example II. Although Nos. 7-10 of Comparative Example II have volume resistivity similar to Examples of the present invention, it is difficult to alternately apply the vacuum state and the reduction atmosphere, because expensive equipment is undesirably required.

[43] The LT substrate manufactured by the method of the present invention is processed into a substrate for surface acoustic wave elements using a typical process. Then, the surface acoustic wave element is manufactured, and measured for electrical properties. As a result, it can be found that such an element has excellent resistivity properties and is not affected by static electricity due to poling. Also, the shortening of the lifetime of the element is drastically decreased.

[44] Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. Industrial Applicability

[45] As described hereinbefore, the present invention provides a method of manufacturing an LT substrate for surface acoustic wave elements and an LT substrate manufactured by the above method. According to the method of manufacturing an LT substrate for surface acoustic wave elements of the present invention, the reduction time is drastically decreased, and thus the process time is lessened, and as well, electrical conductivity or volume resistivity of LT becomes more improved, compared to conventional methods.

[46] Further, since static electricity generated by pyroelectric properties of LT may be quickly removed, process instability due to static electricity induced upon manufacturing the surface acoustic wave element may be greatly lowered.

Claims

[1] 1. A method of manufacturing a lithium tantalate substrate for surface acoustic wave elements, comprising: heating a lithium tantalate material to a temperature of not less than a Curie temperature of lithium tantalate to reduce it, to prepare a lithium tantalate reducing agent; laminating the lithium tantalate reducing agent and a poled lithium tantalate substrate so that the lithium tantalate reducing agent is positioned on both surfaces of the poled lithium tantalate substrate, to form a lithium tantalate laminate; heating the lithium tantalate laminate to a temperature less than the Curie temperature of lithium tantalate, to reduce the lithium tantalate substrate; and removing the lithium tantalate substrate from the laminate, and processing the removed lithium tantalate substrate into a substrate for surface acoustic wave elements.

[2] 2. The method as set forth in claim 1, wherein the reduced lithium tantalate substrate has a volume resistivity of 10 -10 Ohm-cm. [3] 3. The method as set forth in claim 2, wherein the reduced lithium tantalate substrate has a volume resistivity of 10 -10 Ohm-cm. [4] 4. The method as set forth in any one of claims 1 to 3, wherein the laminate is heated in a reducing atmosphere. [5] 5. A lithium tantalate substrate for surface acoustic wave elements, manufactured by the method of claim 4.