JP4741309B2 - Surface acoustic wave device and manufacturing method thereof - Google Patents

Description

  The present invention relates to a surface acoustic wave element such as a surface acoustic wave filter used in a mobile communication device such as a mobile phone, and relates to a surface acoustic wave element in which an IDT (Inter Digital Transducer) electrode is formed on a piezoelectric substrate.

  A problem in manufacturing a SAW (Surface Acoustic Wave) device (surface acoustic wave element) on a piezoelectric substrate is that during the manufacturing process, the piezoelectric substrate is subjected to a rapid temperature change as a result of each step of the manufacturing process. In other words, polarization occurs in the piezoelectric substrate, and charges are separated. The metallized region on the one main surface side of the piezoelectric substrate forming each SAW device tends to be a voltage related to the total charge on the other main surface side of the piezoelectric substrate. However, due to the high resistivity of the piezoelectric substrate, the accumulated charge cannot be quickly leaked, thus creating a potential difference between adjacent metallized regions of this SAW device, particularly in the adjacent metallized regions. Charges are neutralized by arc discharge at narrow intervals. In addition, the electric field generated between the metallized regions due to the accumulated electric charges mechanically damages the piezoelectric substrate even if arcing does not occur. Inspection using an electron microscope has confirmed that the electric field causes the piezoelectric substrate to crack and break at the edges of the metallized region, and that arcing can damage the metallized region and possibly the piezoelectric substrate. Yes. Such damage will undesirably alter the interception of sound waves or surface acoustic waves, leading to SAW device failure or performance degradation.

  Further, when a rapid thermal history is applied in the manufacturing process as described above, the piezoelectric substrate wafer may be attracted to the stage of the manufacturing apparatus, the transfer jig, or the like by the generated pyroelectricity. In addition, stress may occur in the piezoelectric substrate, and warpage and cracks may occur. Moreover, dust may adhere to the surface of the piezoelectric substrate due to the generated pyroelectricity.

  In recent years, many surface acoustic wave elements have been used as constituent elements such as filters, delay lines, and oscillators of electronic devices that use radio waves. In particular, a surface acoustic wave filter that is small and lightweight and has high steep cut-off performance as a filter has been widely used in the mobile communication field, particularly as an RF stage filter of a portable terminal device. There is a demand for a loss and a cutoff characteristic outside the passband having a high attenuation characteristic and a wide bandwidth.

A single crystal such as lithium tantalate (LiTaO ₃ ) or lithium niobate (LiNbO ₃ ) is generally used for the IDT electrode forming piezoelectric substrate used for the surface acoustic wave filter used in the RF stage of a cellular phone or the like. It has been. Since these single crystal materials have a larger electromechanical coupling coefficient than lithium tetraborate (Li ₂ B ₄ O ₇ ) single crystals, quartz substrates, and the like, high attenuation characteristics can be obtained in a wide band.

  However, a commonly used single crystal substrate of lithium tantalate or lithium niobate has a so-called pyroelectric effect in which an electrostatic charge is generated on the substrate surface due to a temperature change as described above. For this reason, if a temperature change is applied to the wafer of the piezoelectric substrate made of these single crystals used in the manufacturing process, the fine electrode such as the IDT electrode produced on the wafer is easily discharged and destroyed by static electricity. Many bugs occurred.

  Thus, various process devices have been proposed in the past to prevent such discharge breakdown.

<Conventional technology 1>
For example, a structure has been proposed in which a ground electrode of a surface acoustic wave element and an input / output electrode of an IDT electrode are connected to a dicing line so that all conductor patterns on a piezoelectric substrate have the same potential (for example, Patent Documents). 1).

<Conventional technology 2>
In addition, all the electrode patterns that become floating electrodes (electrodes that do not conduct with other conductors) are connected to the ground (ground) electrode using a high resistance pattern, so that the frequency characteristics are not degraded. Static electricity generated by the electric effect is released to the ground.

  As a specific example, as shown in a plan view of the electrode structure of the surface acoustic wave element in FIG. 13, a high resistance pattern 9 made of a high resistance thin film formed of a silicon (Si) pattern doped with impurities is used as a floating electrode. It is connected to all of the electrode patterns, and the static electricity is released to the ground electrode (see, for example, Patent Document 2). In FIG. 13, reference numeral 8 denotes an IDT electrode and a reflector constituting the surface acoustic wave resonator.

  As another example, as shown in the plan view of the electrode structure of the surface acoustic wave element in the same manner as in FIG. 14, the high resistance pattern 10 is a finely bent pattern (meander line) having a line width of about 1 μm. Some use (for example, refer to Patent Document 3).

<Conventional technology 3>
Recently, a method for increasing bulk conductivity by heating a lithium niobate single crystal in an atmosphere containing a reducing gas has been proposed. According to this, oxygen ions are released from the surface of the lithium niobate material by the reducing gas, leaving excess electrons and increasing the bulk conductivity. The increase in bulk conductivity prevents charge accumulation in the surface acoustic wave device manufacturing process (see, for example, Patent Document 4).
JP-A-3-293808 JP 2000-183680 A Japanese Patent Laid-Open No. 10-200363 JP 2004-35396 A

  However, in the case of the surface acoustic wave device as in <Prior Art 1>, the input / output electrode of the IDT electrode and the ground electrode are short-circuited to the dicing line, so the characteristic inspection cannot be performed in the process. The frequency characteristics in each process could not be grasped and the frequency characteristics could not be adjusted. As the frequency band used in the portable terminal device becomes higher, the line width of the IDT electrode of the resonator constituting the surface acoustic wave element is reduced in inverse proportion to the frequency used in principle. Conventionally, the electrode line width of the surface acoustic wave resonator is about 1 μm in the 800 MHz band, but is further reduced to about 0.5 μm in the 1.9 GHz band. For this reason, the distance between the electrode fingers is further narrowed, and with the above method, it is difficult to completely prevent the discharge breakdown of the IDT electrode due to the temperature gradient in the manufacturing process.

  In addition, in the method for manufacturing a surface acoustic wave device as in <Prior Art 2>, there is a problem in that the number of electrode pattern forming steps increases and the manufacturing becomes complicated. For example, in a surface acoustic wave device that releases static electricity by using a high resistance pattern 9 made of Si as shown in FIG. 13, the steps shown in the flowchart of FIG. 12 are required.

  First, a piezoelectric substrate made of, for example, a lithium tantalate single crystal is washed with pure water (step J1), and then an Al electrode is formed on the entire surface of the piezoelectric substrate (step J2). Thereafter, a photoresist is applied and formed on a predetermined region of the Al electrode (step J3) and then exposed (step J4). Next, after developing with an organic solvent (step J5), the Al electrode formed in step J2 is etched by RIE (reactive ion etching) to form an IDT electrode (step J6).

  Next, a back electrode is formed to neutralize charges generated on the back surface of the piezoelectric substrate (step J7), and a protective film is formed on the IDT electrode (step J8). Next, a photoresist is applied and formed on the protective film formed in Step J8 (Step J9), a rectangular window is formed in the region for forming the Si film, and exposure is performed in the same manner as in Step J4. (Step J10). Next, the same development processing as in step J5 is performed (step J11). Next, the protective film formed in Step J8 is etched (Step J12), and Si is formed by sputtering in the place where the window is formed (Step J13). Next, excess Si is removed by a lift-off method (step J14). Then, the steps J9 to J12 are repeated (steps J15 to J18).

  Next, a thick film of a conductor for connection with the package is formed (step J19), and lift-off similar to step J14 is performed (step J20). Then, a probe inspection for confirming the electrical characteristics of the IDT electrode is performed on the wafer (step J21).

  As described above, steps J9 to J14 and the like are required as a process for forming the Si pattern, and the manufacturing becomes complicated, and it takes a lot of time to complete the surface acoustic wave device.

  Further, in the surface acoustic wave element shown in FIG. 14, the electric resistance is increased by reducing the line width of the meander line in the high resistance pattern 10 (several MΩ), and similarly, the electrode is destroyed by the pyroelectric effect. Was preventing. The reason why the high resistance wiring is used is that only static electricity can be removed without changing characteristics in the high frequency region.

  However, even in this case, after the wafer process (a series of processes from substrate cleaning to probe inspection) is completed and before the high-resistance pattern 10 of the meander line is cut in the dicing process, wafer probe inspection is performed on the wafer. In this wafer probe inspection process, if the high resistance pattern 10 is connected, the frequency characteristics are usually distorted. A process for removing only 10 is required, which also increases the number of processes.

  As described above, even in the manufacturing process of the surface acoustic wave device according to the conventional technique 2, an increase in the number of processes is inevitable in order to prevent electrostatic breakdown of the fine electrode due to the pyroelectric effect.

  In <Prior Art 3>, the reduction treatment of the piezoelectric substrate for the purpose of preventing the accumulation of electric charges in the manufacturing process of the surface acoustic wave device causes the oxygen ions on the surface of the piezoelectric substrate to escape and the conductivity of the piezoelectric substrate to increase. If it is lowered too much, the composition of the surface of the piezoelectric substrate may fluctuate greatly and the SAW speed may fluctuate. As a result, the piezoelectric characteristics of the piezoelectric substrate deteriorate, and the propagation characteristics of the surface acoustic wave are affected. Therefore, the frequency characteristics such as the insertion loss and the pass bandwidth of the surface acoustic wave element may be deteriorated. Some characteristics could not be obtained.

  The present invention has been proposed in order to solve the above-described problems in the prior art, and the object thereof is that there is no electrostatic breakdown of the microelectrode due to the pyroelectric effect of the piezoelectric substrate, and the surface acoustic wave device. It is an object of the present invention to provide a surface acoustic wave element that can be easily manufactured and a method of manufacturing the same without deteriorating frequency characteristics such as insertion loss and pass bandwidth.

  The surface acoustic wave device of the present invention is formed by 1) forming an IDT electrode on a non-pyroelectric piezoelectric substrate comprising a lithium tantalate single crystal or a lithium niobate single crystal and having an oxygen content lower than the stoichiometric composition. The IDT electrode is formed on a region of the surface where the oxygen content of the piezoelectric substrate is higher than the surroundings, and the piezoelectric substrate between electrode fingers of the IDT electrode The oxygen content of the surface of the IDT electrode is larger than the region under the electrode finger of the IDT electrode.

  Here, the non-pyroelectric piezoelectric substrate refers to a piezoelectric substrate having a charging voltage of less than 1 kV as shown in FIG. FIG. 15 is a diagram showing the results of measuring the rate of occurrence of sparks generated in an IDT electrode produced under the condition of 0.5 μmL / S (both line width and space (interval) are 0.5 μm). It shows the relationship with the incidence of sparks. In FIG. 15, the horizontal axis represents temperature (unit: ° C.), the vertical axis represents the charging voltage (unit: kV) of the piezoelectric substrate due to static electricity generated by the pyroelectric effect, and the black circle and the characteristic curve represent the charging voltage at which sparks are generated. The temperature characteristics are shown. As shown in FIG. 15, according to the non-pyroelectric piezoelectric substrate, when the charging voltage of the piezoelectric substrate is less than 1 kV, the occurrence of spark hardly occurs (when it is less than 0.7 kV, there is no occurrence of spark). This is a result in the case of a SAW filter having an electrode line width of 0.5 μmL / S, which is the current mainstream, and it seems that the lower limit voltage at which sparks are generated is lower in a SAW filter having an electrode line width of less than this. Therefore, a suitable non-pyroelectric piezoelectric substrate can be said to have a charging voltage of less than 0.7 kV.

  In addition, the surface acoustic wave device of the present invention comprises 2) an IDT electrode on a non-pyroelectric piezoelectric substrate made of a lithium tantalate single crystal or a lithium niobate single crystal and having an oxygen content less than the stoichiometric composition. A surface acoustic wave device formed by forming a surface of the piezoelectric substrate between the electrode fingers of the IDT electrode, wherein the oxygen content of the surface of the piezoelectric substrate is greater than that of the region under the electrode finger of the IDT electrode. Is.

  The surface acoustic wave device of the present invention is a surface acoustic wave device comprising 3) an IDT electrode for generating a surface acoustic wave on the piezoelectric substrate and a reflector in the surface acoustic wave device having the configuration 1) or 2). A plurality of resonators are formed, and an extraction electrode connected to the surface acoustic wave resonator is formed, and a region between electrode fingers of the IDT electrode and a grating electrode of the reflector are formed in the piezoelectric substrate. A groove is formed in each of the regions between them.

Further, the surface acoustic wave device of the present invention, 4) 1) above to the surface acoustic wave device of any one of the above 3), made of lithium tantalate single crystal or lithium niobate single crystal, 1 × 10 ^{- The} IDT electrode is formed on a piezoelectric substrate having a high conductivity of ¹² Ω ⁻¹ · cm ^{−1 to} 1 × 10 ⁻¹⁰ Ω ⁻¹ · cm ⁻¹ .

  The surface acoustic wave device according to the present invention is characterized in that 5) the surface acoustic wave device according to any one of 1) to 4) above, wherein the piezoelectric substrate has a charging voltage of less than 1 kV. It is.

  The surface acoustic wave device according to the present invention is characterized in that 6) In the surface acoustic wave device having the structure 5), the piezoelectric substrate has a charging voltage of less than 0.7 kV.

  The surface acoustic wave device according to the present invention is characterized in that 7) The surface acoustic wave device having any one of the constitutions 1) to 6) above, wherein the piezoelectric substrate contains a trace amount of a transition metal element. The surface acoustic wave device according to claim 1.

  The surface acoustic wave device according to the present invention is characterized in that 8) In the surface acoustic wave device having the structure 7), the piezoelectric substrate contains 2-3% by mass of iron as the transition metal element. Is.

  The surface acoustic wave device manufacturing method according to the present invention includes: 9) a non-pyroelectric piezoelectric substrate comprising a lithium tantalate single crystal or a lithium niobate single crystal and having an oxygen content less than the stoichiometric composition. A step of oxidizing the surface, a step of forming an IDT electrode on the oxidized surface of the piezoelectric substrate, and then removing an oxidized surface excluding a region of the surface of the piezoelectric substrate where the IDT electrode is formed And a step of further oxidizing the oxidized surface between the electrode fingers of the IDT electrodes of the piezoelectric substrate.

  In addition, the method for producing a surface acoustic wave device according to the present invention includes: 10) a non-pyroelectric piezoelectric substrate comprising a lithium tantalate single crystal or a lithium niobate single crystal and having an oxygen content less than the stoichiometric composition. A step of oxidizing the surface, a step of forming an IDT electrode on the oxidized surface of the piezoelectric substrate, and a step of removing the oxidized surface excluding a region of the surface of the piezoelectric substrate where the IDT electrode is formed. And a process.

  In addition, the communication device of the present invention includes 11) at least one of a reception circuit and a transmission circuit including the surface acoustic wave element having any one of the above-described configurations 1) to 8). .

  According to the surface acoustic wave device of 1) of the present invention, on a non-pyroelectric piezoelectric substrate comprising a lithium tantalate single crystal or a lithium niobate single crystal and having an oxygen content lower than the stoichiometric composition, A surface acoustic wave element formed by forming an IDT electrode, wherein the IDT electrode is formed on a region of the surface of the non-pyroelectric piezoelectric substrate where the oxygen content is higher than the surroundings, and the electrode of the IDT electrode Since the oxygen content of the surface of the piezoelectric substrate between the fingers is higher than the region under the electrode fingers of the IDT electrode, the conductivity of the surface of the piezoelectric substrate between the IDT electrodes is reduced without deteriorating the piezoelectricity. Since it can be kept higher than the electric piezoelectric substrate, the non-uniformity of the charge due to the spontaneous polarization of the piezoelectric substrate due to the temperature change is improved, and the generated charge is applied to the surface of the piezoelectric substrate between the IDT electrodes. By moving , The potential difference between the IDT electrodes is relaxed instantaneously, because the charge that is accumulated in the IDT electrode itself is suppressed, it is possible to prevent destruction of the IDT electrode due to static electricity generated by the pyroelectric effect. For this reason, since the process for producing the structure for preventing electrostatic breakdown of the IDT electrode is not required as in the prior art, the manufacturing is simplified. In addition, since the wafer, which is a piezoelectric substrate, is not charged by static electricity, it is not attracted to the wafer stage, jig, etc., and handling performance is marked in the process for discriminating between good and defective wafers. improves. Furthermore, since the oxygen content of the surface of the piezoelectric substrate between the electrode fingers of the IDT electrode is larger than the region under the electrode finger of the IDT electrode, the surface acoustic wave is excited and propagated on the surface of the non-pyroelectric piezoelectric substrate. By increasing the oxygen content at the location that affects the surface and recovering it, without causing deterioration of the piezoelectricity of the piezoelectric substrate, while preventing the discharge breakdown of the IDT electrode, the insertion loss of the surface acoustic wave element, the pass bandwidth, etc. It becomes possible to maintain the characteristics of the above.

  According to the surface acoustic wave device of 2) of the present invention, on the non-pyroelectric piezoelectric substrate comprising a lithium tantalate single crystal or a lithium niobate single crystal and having an oxygen content less than the stoichiometric composition. In addition, the surface acoustic wave element is formed by forming IDT electrodes, and the piezoelectric content is higher than the area under the electrode fingers of the IDT electrode because the oxygen content on the surface of the piezoelectric substrate between the electrode fingers of the IDT electrode is larger. Without degrading the electrical conductivity, the electrical conductivity of the surface of the piezoelectric substrate between the IDT electrodes can be kept higher than that of the conventional pyroelectric piezoelectric substrate. The uniformity is improved and the generated charge moves on the surface of the piezoelectric substrate between the IDT electrodes, so that the potential difference between the IDT electrodes is instantly relaxed and the accumulation of charges on the IDT electrode itself is suppressed. Ru Because, 1 similarly to the surface acoustic wave device), it is possible to prevent destruction of the IDT electrode due to the discharge of the charge due to the pyroelectric effect.

According to the surface acoustic wave element of 3) of the present invention, in the configuration of 1) or 2) above, in the piezoelectric substrate, the region between the electrode fingers of the IDT electrode and the region between the grating electrodes of the reflector In addition, since each groove is formed, the piezoelectric substrate surface is bombarded with Ar ions or O ₂ ions by dry etching or the like, and the groove is formed on the piezoelectric substrate surface. The content can be controlled. As a result, similar to the surface acoustic wave device of 1), it is possible to prevent the IDT electrode from being destroyed by the discharge of electric charge due to the pyroelectric effect, and the surface acoustic wave is generated by the bulk wave by the groove formed in the piezoelectric substrate. Mode conversion can be suppressed, and the insertion loss characteristic of the surface acoustic wave element can be kept good.

According to the surface acoustic wave device of 4) of the present invention, any one of the structures 1) to 3) is made of a lithium tantalate single crystal or a lithium niobate single crystal, and 1 × 10 ^−12. By forming an IDT electrode on a piezoelectric substrate having a high conductivity of Ω ⁻¹ · cm ^{−1 to} 1 × 10 ⁻¹⁰ Ω ⁻¹ · cm ⁻¹ , the conventional oxygen content is stoichiometric. Since the electrical conductivity of the piezoelectric substrate is higher than that of the piezoelectric substrate having a specific composition, it is possible to prevent discharge breakdown of the IDT electrode due to temperature change in the manufacturing process, and the electrical conductivity is too high to deteriorate the piezoelectricity of the piezoelectric substrate. Therefore, it is possible to reduce the propagation loss of the surface acoustic wave, so that the characteristics such as the insertion loss and the pass bandwidth of the surface acoustic wave element can be kept good. Therefore, desired frequency characteristics of the surface acoustic wave element can be obtained, and the yield rate in manufacturing can be improved.

  According to the surface acoustic wave element of 5) of the present invention, in any of the configurations of 1) to 4), the piezoelectric substrate has an electrode finger of the IDT electrode because the charging voltage is less than 1 kV. It is possible to prevent the occurrence of sparks in the.

  According to the surface acoustic wave element of 6) of the present invention, in the surface acoustic wave element having the structure of 5), the piezoelectric substrate has an electrode finger of the IDT electrode because the charging voltage is less than 0.7 kV. The occurrence of sparks can be further prevented. In the case of a SAW filter having an electrode line width of 0.5 μmL / S, which is the current mainstream, particularly in a SAW filter having an electrode line width of less than this, as the electrode line width becomes narrower, pyroelectric resistance is further required. By making the charging voltage less than 0.7 kV, the occurrence of sparks between the electrode fingers can be further prevented.

  In addition, according to the surface acoustic wave device of the above 7) of the present invention, the transition metal element is distributed almost uniformly over the entire crystal of the piezoelectric substrate, thereby increasing the conductivity as in the case of the reduced piezoelectric substrate. Can do. In order to uniformly distribute the transition metal element, for example, a method of adding a small amount to the raw material powder at the time of crystal growth may be used. Further, similarly to 1), since the oxygen content of the surface of the piezoelectric substrate between the electrode fingers of the IDT electrode is larger than the region under the electrode finger of the IDT electrode, the surface of the non-pyroelectric piezoelectric substrate In this example, the oxygen content of the portion that affects the excitation of the surface acoustic wave is increased, so that the discharge loss of the surface acoustic wave element is prevented and the IDT electrode is prevented from being destroyed without deteriorating the piezoelectricity of the piezoelectric substrate. It is possible to maintain good characteristics such as bandwidth.

  According to the surface acoustic wave element of 8) of the present invention, the piezoelectric substrate contains 2-3% by mass of iron as the transition metal element. As in the case of, the conductivity can be increased. Further, the magnitude of the insertion loss depends on the mass of the transition metal element to be added, and the heavier element has a larger surface acoustic wave propagation loss, which increases the insertion loss. In addition, by adding iron as a transition metal element, the conductivity is higher than other elements and the mobility of electrons is large, so that the polarization of the piezoelectric substrate is easily relaxed, and the deterioration of insertion loss is suppressed. it can.

  In addition, according to the method for producing a surface acoustic wave device according to 9) of the present invention, non-pyroelectricity comprising a lithium tantalate single crystal or a lithium niobate single crystal and having an oxygen content less than the stoichiometric composition. A step of oxidizing the surface of the piezoelectric substrate, a step of forming an IDT electrode on the oxidized surface of the piezoelectric substrate, and an oxidized surface excluding one region of the surface on which the IDT electrode of the piezoelectric substrate is formed. Including a step of removing and a step of further oxidizing the oxidized surface between the electrode fingers of the IDT electrodes of the piezoelectric substrate to form an IDT electrode that has been previously oxidized on the surface of the non-pyroelectric piezoelectric substrate. After creating the region, it is possible to create a region in which the oxygen content is increased by further oxidation between the electrode fingers of the IDT electrode, which is a part that affects the excitation of the surface acoustic wave. Further, in the case where, for example, dry etching is used to remove the oxidized surface excluding a region of the surface of the piezoelectric substrate where the IDT electrode is formed, dry etching is performed on the comb-like electrode portion constituting the IDT electrode. In combination with the fact that the etching rate in the fine electrode portion is slow due to the microloading effect of the substrate, a reduced substrate portion or a transition metal element is added to a region of the surface of the piezoelectric substrate where the IDT electrode is formed. As a result, the oxide layer remains without exposing the substrate portion. For this reason, the propagation loss of the surface acoustic wave is reduced, and a surface acoustic wave element and a surface acoustic wave element having a low loss can be realized as compared with the case where no oxide layer is formed in the region where the IDT electrode is formed. Furthermore, it is possible to prevent the discharge breakdown of the IDT electrode due to a temperature change in the manufacturing process.

  Here, by reducing the wafer of a piezoelectric substrate made of a normal single crystal of lithium tantalate or lithium niobate, a non-pyroelectric piezoelectric substrate having a lower oxygen content than the stoichiometric composition, that is, pyroelectricity. A mechanism capable of producing a piezoelectric substrate with the effect disappearing will be described. For example, a piezoelectric substrate made of a normal lithium tantalate single crystal generates charges due to spontaneous polarization on the surface, which is considered to be mainly due to oxygen molecules. For this reason, the amount of spontaneous polarization of the piezoelectric substrate can be reduced by making the surface of the piezoelectric substrate oxygen deficient, for example, by performing annealing (heat treatment) on the piezoelectric substrate in a hydrogen atmosphere. By performing this annealing for a long time, the amount of spontaneous polarization can be reduced uniformly over the entire thickness direction of the wafer of the piezoelectric substrate. Thereby, generation | occurrence | production of the spark by the temperature change resulting from a pyroelectric effect can be completely eliminated.

  Similarly, the piezoelectric substrate is annealed in an oxygen atmosphere before the electrode is formed, that is, oxygen is applied to the oxygen deficient region on the very surface of the piezoelectric substrate (for example, a depth of about several μm) in a short time. It is preferable to compensate. This is because by supplying oxygen to the IDT electrode formation region, distortion of the crystal lattice of the piezoelectric substrate is reduced when the IDT electrode is formed, and an increase in insertion loss can be prevented as much as possible. is there.

  In addition, according to the method for producing a surface acoustic wave device of 10) of the present invention, non-pyroelectricity comprising a lithium tantalate single crystal or a lithium niobate single crystal and having an oxygen content less than the stoichiometric composition. A step of oxidizing the surface of the piezoelectric substrate, a step of forming an IDT electrode on the oxidized surface of the piezoelectric substrate, and an oxidation excluding a region of the surface of the piezoelectric substrate on which the IDT electrode is formed. By including the step of removing the surface, the insertion loss of the surface acoustic wave element and the deterioration of the pass bandwidth do not occur while preventing the discharge breakdown of the IDT electrode as in the manufacturing method of 9). Furthermore, since the last oxidation step is omitted as compared with the manufacturing method 9), the manufacturing step can be simplified.

  According to the communication device of 11) of the present invention, since it has at least one of the reception circuit and the transmission circuit having the surface acoustic wave element having any one of the configurations of 1) to 8), it is elastic. Since the desired filter characteristics of the surface acoustic wave element can be obtained with high reliability, the sensitivity as the communication device is remarkably improved, and the reliability of the communication device can be kept high.

  Hereinafter, an example of an embodiment of a surface acoustic wave device according to the present invention will be described in detail with reference to the drawings schematically shown.

FIG. 1 is a cross-sectional view showing a principal cross-sectional structure of a portion where an IDT electrode of a surface acoustic wave element of the present invention is formed on a piezoelectric substrate. In FIG. 1, reference numeral 1 denotes a piezoelectric substrate, which is obtained by subjecting a normal lithium tantalate single crystal or lithium niobate single crystal piezoelectric substrate grown by the Czochralski method to a reduction in oxygen content. Fewer non-pyroelectric piezoelectric substrates (wafers) 1 can be obtained. The oxygen content is, for example, 0 <x <0.3 when lithium tantalate is represented by the chemical formula LiTaO _3-x and lithium niobate is represented by the chemical formula LiNbO _3-x .

  An oxide layer 2 rich in oxygen is formed on the surface of the piezoelectric substrate 1 by annealing under conditions of high temperature and oxygen atmosphere, and then a thin film forming method and a photolithography process are used on the oxide layer 2. Thus, for example, the IDT electrode 3 which is a fine electrode made of an Al—Cu alloy is formed. At this time, due to the difference in the area where the IDT electrode 3 is etched between the region where the IDT electrode 3 is formed (one region of the surface of the piezoelectric substrate 1 which is the IDT electrode forming portion) and the other region, microloading is performed. Due to the effect (the etching rate varies depending on the thickness of the electrode finger of the IDT electrode 3, that is, the thinner the electrode finger, the harder it is to etch), a difference appears in the removal rate of the oxide layer 2. Next, by performing an annealing process under conditions of a high temperature and oxygen atmosphere, a region 4 having a higher oxygen content is formed in a region between the electrode fingers of the IDT electrode 3 on the surface of the piezoelectric substrate 1.

  Thereby, as shown in FIG. 1, the oxide layer 2 remains in the region where the IDT electrode 3 is formed, and the oxide layer 2 is removed in the other regions. The generation of static electricity due to the pyroelectric effect is related to the removal of the oxide layer 2, and the remainder of the oxide layer 2 in the region where the IDT electrode 2 is formed does not alleviate the pyroelectric effect, and a certain amount of static electricity is generated. Although generated, the crystal lattice distortion is small in the remaining portion, so that loss of surface acoustic waves due to oxygen deficiency can be prevented. Further, in a region where the microloading effect does not occur other than the region where the IDT electrode 3 is formed, the oxide layer 2 (having a thickness of several hundred mm) on the surface can be sufficiently removed by dry etching, and the pyroelectric effect. It is possible to satisfactorily suppress the generation of static electricity due to. It should be noted that there is no problem because the loss of the surface acoustic wave in this region does not affect the characteristics of the surface acoustic wave element.

At this time, by controlling the oxygen concentration, the annealing temperature, and the annealing time, the conductivity in the oxide layer 2 of the piezoelectric substrate 1 is not increased too much, and 1 × 10 ⁻¹² Ω ⁻¹ · cm ⁻¹ to 1 × 10 6. It is sufficiently possible to control in the range of ⁻¹⁰ Ω ⁻¹ · cm ⁻¹ .

Here, in the surface acoustic wave device of the present invention, the piezoelectric substrate is made of a lithium tantalate single crystal or a lithium niobate single crystal and has an electric conductivity of 1 × 10 ⁻¹² Ω ⁻¹ · cm ⁻¹ to 1 × 10 ^{− The} reason why the range of ¹⁰ Ω ⁻¹ · cm ⁻¹ is suitable will be described.

FIG. 9 shows a surface acoustic wave device according to the present invention in which the piezoelectric substrate is made of a lithium tantalate single crystal and manufactured in the same process and has a conductivity of 1 × 10 ⁻¹¹ Ω ⁻¹ · cm ^−1. The frequency dependence of insertion loss representing the transmission characteristics in the vicinity of the passband with the surface acoustic wave device of the reference example having a substrate conductivity of 1 × 10 ⁻⁸ Ω ⁻¹ · cm ⁻¹ is shown by a diagram. In FIG. 9, the horizontal axis represents the frequency (unit: MHz), the vertical axis represents the insertion loss (unit: dB), and the transmission characteristics of the surface acoustic wave device of the present invention having a low conductivity of the piezoelectric substrate are represented by solid lines. The transmission characteristics of the surface acoustic wave device of the reference example having high substrate conductivity are indicated by broken lines. As is clear from the results shown in FIG. 9, in the surface acoustic wave device of the reference example having a high conductivity shown by a broken line, compared with the surface acoustic wave device of the present invention having a low conductivity of the piezoelectric substrate shown by a solid line, The steepness of the transmission characteristics in the pass band is deteriorated, and the shoulder characteristics on the high frequency side are particularly deteriorated. Therefore, in the surface acoustic wave element of the reference example, the pass band width is narrowed and the insertion loss is also deteriorated.

  This is because if the electrical conductivity of the piezoelectric substrate is increased by reducing the oxygen content on the surface of the piezoelectric substrate, it becomes difficult to store charges on the surface of the piezoelectric substrate, and as a result, the tan δ of the piezoelectric substrate increases and the Q value increases. Since (the amount representing the sharpness of resonance) tends to be low, the passband width is narrowed, and the insertion loss tends to deteriorate.

Here, the relationship between the electrical conductivity of the piezoelectric substrate, the pass bandwidth of the surface acoustic wave element, and the insertion loss is shown in FIGS. 10 and 11, respectively. In general, when used for PCS (Personal communication service) communication, it is desirable that the passband width is 72 MHz or more in consideration of changes in frequency temperature characteristics, variations in characteristics, and the like. Further, it is desirable that the value of the insertion loss is 3.1 dB or less. FIG. 10 is a diagram showing the dependence of the pass bandwidth of the surface acoustic wave device on the conductivity of the piezoelectric substrate. In FIG. 10, the horizontal axis represents the electrical conductivity σ (unit: Ω ⁻¹ · cm ⁻¹ ) of the piezoelectric substrate, the vertical axis represents the pass bandwidth (unit: MHz), and the black square and the characteristic curve represent the pass bandwidth. The result of conductivity dependence is shown. FIG. 11 is a similar diagram showing the dependence of the insertion loss of the surface acoustic wave device on the conductivity of the piezoelectric substrate. In FIG. 11, the horizontal axis represents the conductivity σ (unit: Ω ⁻¹ · cm ⁻¹ ) of the piezoelectric substrate, the vertical axis represents the insertion loss (unit: dB), and the black square and the characteristic curve represent the conductivity of the insertion loss. The dependency results are shown.

As is apparent from the results shown in FIG. 10, in the range where the passband width is 72 MHz or more, the conductivity of the piezoelectric substrate is 1 × 10 ⁻¹⁰ Ω ⁻¹ · cm ⁻¹ or less, preferably the conductivity of the piezoelectric substrate. The rate ranges from 1 × 10 ⁻¹² Ω ⁻¹ · cm ⁻¹ to 1 × 10 ⁻¹⁰ Ω ⁻¹ · cm ⁻¹ . As is clear from FIG. 11, the range in which the insertion loss is not extremely deteriorated is that the conductivity of the piezoelectric substrate is 1 × 10 ⁻¹⁰ Ω ⁻¹ · cm ⁻¹ or less, and more preferably the insertion loss is 3.1 dB. The following range is a range of 1 × 10 ⁻¹² Ω ⁻¹ · cm ⁻¹ to 1 × 10 ⁻¹⁰ Ω ⁻¹ · cm ⁻¹ . That is, the electrical conductivity of the piezoelectric substrate is preferably in the range of 1 × 10 ⁻¹² Ω ⁻¹ · cm ⁻¹ to 1 × 10 ⁻¹⁰ Ω ⁻¹ · cm ⁻¹ , thereby reducing the tan δ of the piezoelectric substrate. Elasticity that provides a surface acoustic wave filter that has a high Q value, has a sufficient passband of 72 MHz or more required for PCS communication, and can sufficiently reduce the insertion loss to 3.1 dB or less required for the PCS method. It turns out that it becomes a surface wave element.

  2 is a cross-sectional view of the piezoelectric substrate 1 showing a state in which the oxide layer 2 is formed on the surface of the piezoelectric substrate 1 before forming the surface acoustic wave element of FIG. 1, that is, before forming the IDT electrode 3. It is. Since the piezoelectric substrate 1 is non-pyroelectric, as described above, the charging voltage is less than 1 kV (more preferably less than 0.7 kV).

  FIG. 3 is a cross-sectional view of the principal part showing another example of the piezoelectric substrate in the surface acoustic wave element of the present invention, before the IDT electrode 3 is formed and before the periphery of the IDT electrode 3 is removed by etching. It is principal part sectional drawing which shows the mode of, Comprising: It is principal part sectional drawing which shows a mode that oxide layer 2 'was formed in the surface of piezoelectric substrate 1' which added the transition metal element. This transition metal element (especially iron (Fe)) is added to the piezoelectric substrate 1 ′ by mixing a small amount (several mass%) into the raw material from the stage of crystal growth. Thus, the entire crystal of the piezoelectric substrate 1 ′ is thus added. By distributing the transition metal element almost uniformly over the entire area, there is an effect of increasing the conductivity as in the case of the piezoelectric substrate 1 subjected to the reduction treatment. In order to uniformly distribute the transition metal element, for example, a method of adding a small amount to the raw material powder at the time of crystal growth may be used.

  The optimum value of the addition amount varies depending on the type of transition metal element to be added. For example, when Fe element is selected as the transition metal element, it is about 1% by mass in the lithium tantalate single crystal or lithium niobate single crystal. It is desirable to add. This is because the magnitude of the insertion loss depends on the mass of the added transition metal element, and heavier elements have more surface acoustic wave propagation loss and increase the insertion loss. The reason why the transition metal element is used as the additive element is that the polarization of the piezoelectric substrate 1 is easily relaxed because the conductivity is higher than the other elements and the movement of electrons is large.

  FIG. 4 is a cross-sectional view of an essential part showing another example of the embodiment of the surface acoustic wave device of the present invention. Compared with the surface acoustic wave element shown in FIG. 1, the first oxidation treatment is omitted, the oxide layer 2 is not provided, and the IDT electrode 3 is used as a mask between the electrode fingers of the IDT electrode 3 on the surface of the piezoelectric substrate 1. A region 4 having a high oxygen content is formed.

  Such an example of the surface acoustic wave device of the present invention is a surface acoustic wave device in which an IDT electrode is formed on a non-pyroelectric piezoelectric substrate having an oxygen content lower than the stoichiometric composition. Since the oxygen content on the surface of the piezoelectric substrate between the electrode fingers of the IDT electrode is greater than the region under the electrode finger of the IDT electrode, the surface of the piezoelectric substrate between the IDT electrodes is not degraded without deteriorating the piezoelectricity. Since the conductivity can be kept higher than that of the conventional pyroelectric piezoelectric substrate, the non-uniformity of the charge due to the spontaneous polarization of the piezoelectric substrate due to the temperature change is improved, and the generated charge is generated between the IDT electrodes. By moving the surface of the piezoelectric substrate, the potential difference between the IDT electrodes is instantly relaxed and the accumulation of charges in the IDT electrodes themselves is suppressed, so that the IDT electrodes due to the discharge of charges caused by the pyroelectric effect Prevent the destruction of It is possible.

FIG. 5 is a cross-sectional view of the main part showing another example of the embodiment of the surface acoustic wave device of the present invention. Compared with the surface acoustic wave element shown in FIG. 1, in the region 4 having a large oxygen content between the electrode fingers of the IDT electrode 3 on the surface of the piezoelectric substrate 1, the piezoelectric substrate 1 is subjected to Ar ions or O ₂ ions by dry etching or the like. The surface is bombarded to form a groove on the surface of the piezoelectric substrate 1. As a result, the oxygen content on the surface of the piezoelectric substrate 1 can be controlled, and the destruction of the IDT electrode 3 due to the discharge of electric charge due to the pyroelectric effect can be prevented, as in the surface acoustic wave device of 1) above. In addition, it is possible to suppress the surface conversion of the surface acoustic wave into a bulk wave by the groove formed in the piezoelectric substrate 1, and it is possible to keep the insertion loss characteristics of the surface acoustic wave element favorable.

  Next, an example of an embodiment of a method for manufacturing a surface acoustic wave device according to the present invention will be described below with reference to a flowchart showing a manufacturing process shown in FIG. Here, the manufacturing process of the surface acoustic wave device of FIG. 1 will be described as an example.

  As shown in FIG. 6, first, a wafer which is a piezoelectric substrate made of a lithium tantalate single crystal is washed with an organic solvent, and then dried using a spin dryer (step S1). Next, this wafer is annealed for a short time in an oxygen annealing furnace, and a region of the surface of the non-pyroelectric piezoelectric substrate having an oxygen content lower than the stoichiometric composition (necessary for forming an IDT electrode). The oxide layer is formed by oxidizing the surface layer portion of the region (step S2).

  Next, an Al—Cu alloy film is formed on the wafer by a sputtering apparatus (step S3). Next, a photoresist is coated on the Al—Cu alloy with a uniform thickness using a spin coater (step S4). Next, the reticle pattern is baked by exposing the surface of the photoresist on the wafer with ultraviolet rays that have passed through the reticle in a reduction projection exposure machine (step S5). Next, a developing solution is dropped on the exposed wafer, and after standing still for several seconds, it is developed by shaking it off to reveal a desired pattern (step S6). Next, the surface of the wafer is subjected to a plasma treatment using an RIE apparatus with a reactive gas, so that only the Al—Cu alloy that is not covered with the photoresist is removed by etching to form an Al—Cu electrode. The oxidized surface excluding one region of the surface where the IDT electrode of the piezoelectric substrate is formed is removed by etching (step S7).

  Next, an Al—Cu conductor layer is formed on the entire back surface of the wafer by sputtering (step S8). Next, the wafer is annealed again in an oxygen annealing furnace for a short time, and the region between the electrode fingers of the IDT electrode on the surface of the non-pyroelectric piezoelectric substrate whose oxygen content is less than the stoichiometric composition Is oxidized again (step S9). Thereby, oxygen can be supplemented to the oxygen deficient region on the surface of the piezoelectric substrate, the piezoelectricity on the surface of the piezoelectric substrate is not deteriorated, the propagation loss of the surface acoustic wave is reduced, and the region where the IDT electrode is formed Therefore, a surface acoustic wave element with a low loss can be realized as compared with the case without an oxide layer. Furthermore, since the electrical conductivity of the surface of the piezoelectric substrate between the IDT electrodes can be kept higher than that of the conventional pyroelectric piezoelectric substrate, non-uniform charge due to spontaneous polarization of the piezoelectric substrate due to temperature changes in the manufacturing process In addition to improving the performance, the generated charge moves on the surface of the piezoelectric substrate between the IDT electrodes, so that the potential difference between the IDT electrodes is instantly relaxed and the accumulation of charges on the IDT electrode itself is suppressed. Therefore, destruction of the IDT electrode due to static electricity generated by the pyroelectric effect can be prevented. At this time, since the Al—Cu conductor layer is formed on the entire back surface of the wafer, even if the wafer has pyroelectricity, the charging voltage is increased and the Al—Cu electrode is destroyed. There is nothing.

Next, a protective film (SiO ₂ film) is formed on the surface of the Al—Cu electrode on the surface of the wafer by a CVD apparatus (step S10). Next, in order to open the window of the protective film, a photoresist is coated with a uniform film thickness by a spin coater (step S11). Next, in order to open the window of the protective film, the reticle surface is baked by exposing the wafer surface with ultraviolet rays transmitted through the reticle by a reduction projection exposure machine (step S12). Next, a developer is dropped on the exposed wafer, and after standing for a few seconds, the developer is shaken off to reveal a desired pattern (step S13). Next, an unnecessary SiO ₂ protective film exposed from the photoresist pattern is removed by etching using an RIE apparatus (step S14). Next, a thick film of a conductor for connection to the package is formed on the portion from which the SiO ₂ protective film has been removed (step S15). Next, the wafer is immersed in a photoresist stripping solution to remove an unnecessary thick film together with the photoresist (step S16). Next, an RF probe is set on the completed SAW device, and a characteristic selection inspection is performed (step S17).

  Thus, the manufacturing process of the surface acoustic wave device of the present invention requires the annealing process of the piezoelectric substrate (wafer) in step S2, but the pyroelectric process in steps J9 to J14 of the conventional manufacturing process shown in FIG. The steps necessary for the countermeasure can be omitted, and the surface acoustic wave device can be manufactured very simply.

  Then, the surface acoustic wave device of the present invention is obtained by separating the wafer-level surface acoustic wave devices manufactured as described above into individual surface acoustic devices by a dicing process. The surface acoustic wave device according to the present invention thus obtained has an IDT electrode 304 having a plurality of electrode fingers disposed on a piezoelectric substrate 302 as shown in a plan view of an example of the electrode structure in FIG. These electrodes are composed of a pair of comb-like electrodes engaged with each other, and an electric field is applied to the pair of comb-like electrodes to generate a surface acoustic wave. By inputting an electric signal from an input terminal 315 connected to one comb-like electrode of the IDT electrode 304, the excited surface acoustic wave is propagated to the IDT electrodes 303 and 305 arranged on both sides of the IDT electrode 304. The In addition, an electric signal is output from one comb-like electrode constituting each of the IDT electrodes 303 and 305 to the output terminals 316 and 317 through the IDT electrodes 306 and 309. In the figure, reference numerals 310, 311, 312 and 313 denote reflector electrodes. Thus, a surface acoustic wave element of a so-called longitudinally coupled resonator type surface acoustic wave filter in which electrode patterns are cascaded in two stages can be obtained. The surface acoustic wave element of the present invention is not limited to a longitudinally coupled resonator type surface acoustic wave filter, and can be applied to, for example, a ladder type surface acoustic wave filter.

  Thus, according to the method for manufacturing a surface acoustic wave device of the present invention, a non-pyroelectric piezoelectric substrate comprising a lithium tantalate single crystal or a lithium niobate single crystal and having an oxygen content lower than the stoichiometric composition. A step of oxidizing the surface; a step of forming an IDT electrode on the oxidized surface of the piezoelectric substrate; and a step of removing the oxidized surface excluding a region of the surface of the piezoelectric substrate where the IDT electrode is formed. A step of further oxidizing the oxidized surface between the electrode fingers of the IDT electrodes of the piezoelectric substrate, so that a region for forming an IDT electrode that has been previously oxidized is formed on the surface of the non-pyroelectric piezoelectric substrate. In the above, since it is possible to create a region where the oxygen content is increased by further oxidizing between the electrode fingers of the IDT electrode, which is a part that affects the excitation of the surface acoustic wave, the conventional pyroelectric piezoelectric When using a substrate Base, it is possible to prevent destruction of the IDT electrode due to static electricity generated by the pyroelectric effect while ensuring satisfactory piezoelectricity. For this reason, since the process for producing the structure for preventing electrostatic breakdown of the IDT electrode as in the prior art is not required, the manufacture becomes simple. In addition, since the wafer, which is a piezoelectric substrate, is not charged by static electricity, it is not attracted to the stage or jig of the wafer, and handling performance is particularly high in a probe inspection process for discriminating between good and defective wafers. To improve. Furthermore, by controlling the oxygen concentration, annealing temperature, and annealing time in the step of oxidizing the surface of the piezoelectric substrate and the step of further oxidizing the oxidized surface between the electrode fingers of the IDT electrodes of the piezoelectric substrate, the surface of the piezoelectric substrate is controlled. To provide a surface acoustic wave device that can be controlled so that the conductivity does not become too high, and does not deteriorate the piezoelectricity of the piezoelectric substrate, and as a result, the insertion loss and pass bandwidth of the filter do not deteriorate. Can do.

  Further, in the comb-like electrode portion constituting the IDT electrode, for example, due to the property that the etching rate in the fine electrode portion is slow due to the microloading effect of etching, the substrate portion to which the reduction treatment or the transition metal element is added is provided. Are not exposed and an oxide layer remains on the surface of the piezoelectric substrate (step S7). For this reason, the deterioration of the propagation loss of the surface acoustic wave is reduced, and a surface acoustic wave element having a low loss can be realized as compared with the case where there is no oxide layer.

  In the example of the embodiment of the surface acoustic wave device manufacturing method of the present invention described above, the manufacturing process for forming the IDT electrode after oxidizing at least one region of the surface of the piezoelectric substrate (wafer) has been described. For example, the surface of the piezoelectric substrate is oxidized first, and then only the periphery is removed by etching, and an IDT electrode is formed in the remaining oxide layer region. In addition, after the IDT electrode is manufactured in advance, only the IDT electrode forming part may be oxidized, or the periphery thereof may be subjected to a reduction treatment, which is appropriately changed without departing from the gist of the present invention. Can do.

  Further, another example of the embodiment of the method for manufacturing the surface acoustic wave device of the present invention is shown in FIG. 7 in the same flowchart as FIG. According to this example, the step of oxidizing the surface between the electrode fingers of the IDT electrodes of the piezoelectric substrate (step S2) and the oxygen content is stoichiometrically composed of a lithium tantalate single crystal or a lithium niobate single crystal. A step of forming IDT electrodes on a non-pyroelectric piezoelectric substrate having a smaller specific composition (steps S3 to S7), and a step of removing an oxidized surface excluding a region of the surface of the piezoelectric substrate where the IDT electrodes are formed (steps). By including S7), the insertion loss of the surface acoustic wave element and the deterioration of the pass bandwidth do not occur while preventing the discharge breakdown of the IDT electrode as in the example shown in FIG. Furthermore, since the last annealing process (step S9 shown in FIG. 6) is omitted, the manufacturing process can be simplified.

  According to the communication apparatus of the present invention, the desired filter characteristic of the surface acoustic wave element is provided by including at least one of the receiving circuit and the transmitting circuit having the surface acoustic wave element of the present invention as described above. Can be obtained with high reliability, so that the sensitivity as a communication device becomes remarkably good, and the reliability of the communication device can be kept high.

  In the communication apparatus of the present invention, the surface acoustic wave element of the present invention is used as a transmission or reception band-pass filter included in a reception circuit or a transmission circuit of the communication apparatus, for example. For example, the transmission signal output from the transmission circuit is placed on the carrier frequency with a mixer, the unnecessary signal is attenuated with a transmission band-pass filter composed of a transmission IDT electrode, and then the transmission signal is amplified with a power amplifier. A communication device equipped with a transmission circuit that can transmit from an antenna through a duplexer, or a reception signal received by an antenna, a reception signal that has passed through the duplexer is amplified by a low-noise amplifier, and then configured by an IDT electrode for reception Applicable to a communication device having a receiving circuit that attenuates an unnecessary signal with a band-pass filter for reception, separates the signal from a carrier frequency with a mixer, and transmits the signal to a receiving circuit that extracts the signal. Or, by providing the surface acoustic wave element of the present invention in the transmission circuit, excellent communication with improved sensitivity is achieved. It is possible to provide a device.

  In order to fully exhibit the effects of the surface acoustic wave element of the present invention in the communication apparatus of the present invention, the surface acoustic wave element of the present invention is preferably provided in both the receiving circuit and the transmitting circuit. Therefore, both the reception signal and the transmission signal can be reduced sufficiently, the communication quality can be improved, and the reception filter and the transmission filter can be made into small filters. A quality communication device can be obtained.

Next, an embodiment in which the surface acoustic wave element according to the present invention is embodied will be described. First, in order to reduce a 36 ° Y-cut lithium tantalate single crystal wafer to be a piezoelectric substrate, annealing was performed at 300 ° C. for 10 hours or more in a hydrogen annealing furnace. The hydrogen flow rate was 80 sccm, and the gas pressure was 7.5 × 10 ⁻³ Torr. The wafer processed under these conditions was reduced almost uniformly over the entire thickness direction of the wafer. Next, an electrode layer made of an Al (98 mass%)-Cu (2 mass%) alloy was formed on the wafer by a DC sputtering apparatus. Next, in an oxygen annealing furnace, annealing was performed for 10 minutes under the same conditions as hydrogen annealing to form an oxide layer having a thickness of 200 to 300 mm on the surface of the wafer.

  Next, a photoresist was spin-coated on the wafer to a thickness of about 0.5 μm, and a desired portion was exposed by a reduction projection exposure apparatus (stepper). Next, an unnecessary portion of the photoresist was dissolved with an alkaline developer using a developing device to reveal a desired resist pattern. The minimum line width of the resist pattern was about 0.5 μm.

  Next, the exposed portion of the electrode was dry-etched by an RIE (Reactive Ion Etching) apparatus, and the electrode was patterned to form an IDT electrode in one region of the wafer surface. At this time, the fine electrode portion of the IDT electrode is less likely to exchange the reaction gas due to the microloading effect, and the etching is difficult to proceed. For this reason, since the electrode portions other than the fine electrode portions are etched excessively due to the relative etching, the oxide layer on the wafer surface was scraped. Next, annealing was performed in an oxygen atmosphere for 10 minutes under the same conditions as described above to further form an oxide layer on the wafer surface.

Next, a SiO ₂ protective film is formed on this wafer by a CVD (Chemical Vapor Deposition) apparatus, and then the electrode pad portion is patterned on the SiO ₂ protective film again by photolithography, and bonding by bump is performed by the RIE apparatus. The unnecessary SiO ₂ film was etched in the electrode pad portion for use. Subsequently, a thick film (thickness of 1 μm or more) of Al or the like was formed from this upper portion by a sputtering method, and then unnecessary portions were dissolved and peeled together with the photoresist with a resist stripping solution.

  Finally, non-defective / defective products were inspected by wafer probe inspection, and the wafer process was completed. In addition, since the wafer becomes a black color by using the wafer to be the surface acoustic wave element of the present invention, the manufacturing apparatus in the process (for example, a wafer prober (recognizes whether there is a wafer). In the pre-alignment method), since the conventional piezoelectric substrate was a transparent substrate, the wafer recognition rate was improved from approximately 0% to almost 100%. In addition, when using a piezoelectric substrate which is a conventional transparent substrate, special adjustment was necessary for wafer recognition.

  Next, Au bumps were formed on the piezoelectric substrate by a bump bonder. Thereafter, the piezoelectric substrate was cut along dicing lines and divided for each surface acoustic wave element. Then, the surface acoustic wave elements separated by dicing were picked up by a die mount apparatus, and adhered and fixed in an SMD (surface mount type) package at a temperature of about 150 ° C.

  Thereafter, the lid was placed on the SMD package and welded and sealed with a sealing machine to complete the surface acoustic wave device. At this time, in the case of a surface acoustic wave device using a conventional lithium tantalate single crystal substrate, a rapid temperature change of about 150 ° C. caused discharge breakdown, which was about 90% defective. However, the use of the piezoelectric substrate serving as the surface acoustic wave device of the present invention eliminated any defects due to discharge breakdown.

  In this example, a longitudinally coupled resonator type surface acoustic wave filter was manufactured using the surface acoustic wave element of the present invention having the electrode structure shown in FIG. For comparison, a comparative example of a longitudinally coupled resonator type surface acoustic wave filter was also produced by a conventional manufacturing method using a conventional piezoelectric substrate. The electrode structure of these longitudinally coupled resonator type surface acoustic wave filters is such that all floating electrodes on the surface acoustic wave element are connected to the ground electrode by a high resistance pattern 9 such as Si as shown in FIG. In order to form the high resistance pattern 9, seven extra steps are required. As already described, FIG. 11 shows a flowchart of a manufacturing process for forming the high resistance Si pattern 9. Here, the high resistance Si pattern 9 was formed by RF sputtering using a sputtering target to which a small amount of boron (B) was added. The resistance value of Si to which B was added was 8 to 13 MΩ with a pad of 40 × 40 μm and a thickness of 5000 mm.

  Next, the characteristics of the surface acoustic wave filter of this example were measured. Here, a signal of 0 dBm was input, and measurement was performed under the conditions of a frequency of 780 MHz to 960 MHz and a measurement point of 800 points. The number of samples is 30, and the measuring instrument is a multi-port network analyzer E5071A manufactured by Agilent Technologies.

When the relationship between the conductivity and the passband width of the examples of the surface acoustic wave elements obtained by reducing the piezoelectric substrates of the present invention and the comparative example of the conventional structure was examined, According to the piezoelectric substrate having a high conductivity in the range of 1 × 10 ⁻¹² Ω ⁻¹ · cm ⁻¹ to 1 × 10 ⁻¹⁰ Ω ⁻¹ · cm ⁻¹ as compared with the conventional structure of the comparative example. As a result, it was confirmed that the passband characteristics were improved by the surface acoustic wave device of the present invention at a size of 1.1 dB.

Similarly, when the relationship between the conductivity and the minimum insertion loss of the surface acoustic wave element of the present invention obtained by reducing these piezoelectric substrates and the comparative example of the conventional structure was examined, According to the example, it has a high conductivity in the range of 1 × 10 ⁻¹² Ω ⁻¹ · cm ⁻¹ to 1 × 10 ⁻¹⁰ Ω ⁻¹ · cm ⁻¹ as compared with the comparative example of the conventional structure. It was confirmed that the minimum insertion loss characteristic was improved by the surface acoustic wave device of the present invention at a size of 0.05 dB by using the piezoelectric substrate.

According to the surface acoustic wave element in which the surface acoustic wave resonator of the present invention is formed, the amount of charge on the piezoelectric substrate due to the pyroelectric effect can be reduced. FIG. 16 is a diagram showing experimental values of changes in surface charge amount due to temperature changes when a LiTaO ₃ single crystal substrate of a comparative example is used as in the diagram shown in FIG. As the temperature rises, the charging voltage gradually increases, and when it exceeds 120 ° C., it becomes a value of −3 kV or more, which is a voltage sufficient to easily destroy the fine electrode. In contrast, in the case of the reduction-treated lithium tantalate single crystal piezoelectric substrate of the present invention shown in FIG. 15, the charging voltage was less than -0.5 kV at the maximum.

  Similar results were obtained in the case of a piezoelectric substrate of lithium tantalate single crystal added with 2-3% by mass of Fe as a transition metal element.

  As described above, also in the present example, according to the present invention, it was confirmed that the surface acoustic wave device excellent in reliability that prevents charging due to the pyroelectric effect and does not cause electrode breakdown can be provided.

It is principal part sectional drawing which shows an example of embodiment of the surface acoustic wave element of this invention. It is principal part sectional drawing which shows a mode that the oxide layer was formed in the surface of the piezoelectric substrate in the surface acoustic wave element of this invention. It is principal part sectional drawing which shows a mode that the oxide layer was formed in the surface of the piezoelectric substrate which added the transition metal element in the surface acoustic wave element of this invention. It is principal part sectional drawing which shows the other example of embodiment of the surface acoustic wave element of this invention. It is principal part sectional drawing which shows the other example of embodiment of the surface acoustic wave element of this invention. It is a flowchart explaining the example of the process of the manufacturing method of the surface acoustic wave element of this invention. It is a flowchart explaining the other example of the process of the manufacturing method of the surface acoustic wave element of this invention. It is a top view explaining an example of the electrode structure of the surface acoustic wave element of this invention. It is a characteristic view which shows the pass bandwidth characteristic of a surface acoustic wave element. It is a diagram showing the frequency characteristics of insertion loss in the passband of the surface acoustic wave element and its vicinity. FIG. 6 is a characteristic diagram showing minimum insertion loss characteristics of a surface acoustic wave element. It is a flowchart explaining the manufacturing process of the conventional surface acoustic wave element. It is a top view explaining an example of the electrode structure of the conventional surface acoustic wave element. It is a top view explaining the other example of the electrode structure of the conventional surface acoustic wave element. It is a diagram which shows the charging voltage change by the temperature change of the lithium tantalate single crystal substrate which carried out the reduction process. It is a diagram which shows the charging voltage change by the temperature change of the lithium tantalate single crystal substrate which is not reducing.

Explanation of symbols

1, 1 ': Piezoelectric substrate (wafer)
2, 2 ': Oxide layer 3: IDT electrode 4: Region with high oxygen content 8: Surface acoustic wave resonator

Claims (8)

A surface acoustic wave device comprising an IDT electrode on a non-pyroelectric piezoelectric substrate comprising a lithium tantalate single crystal or a lithium niobate single crystal and having an oxygen content less than the stoichiometric composition,
The IDT electrode is formed on a region of the surface where the oxygen content of the piezoelectric substrate is higher than the surroundings, and the oxygen content of the surface of the piezoelectric substrate between electrode fingers of the IDT electrode is More than the area under the electrode fingers;
Of the region on the surface of the piezoelectric substrate, the conductivity in the region between the electrode fingers having an oxygen content greater than the region under the electrode finger of the IDT electrode is 1 × 10 ⁻¹² Ω ⁻¹ · cm ^{−. 1 to} 1 × 1
A surface acoustic wave device characterized by being set in a range of 0 ⁻¹⁰ Ω ⁻¹ · cm ⁻¹ .   2. The surface acoustic wave device according to claim 1, wherein the electrode fingers of the IDT electrode formed on the one region have both a line width and an interval between adjacent electrode fingers of 0.5 μm.   A plurality of surface acoustic wave resonators including IDT electrodes and reflectors for generating surface acoustic waves are formed on the piezoelectric substrate, and lead electrodes connected to the surface acoustic wave resonators are formed. 3. The surface acoustic wave according to claim 1, wherein grooves are formed in a region between electrode fingers of the IDT electrode and a region between grating electrodes of the reflector in the piezoelectric substrate. element. The piezoelectric substrate, surface acoustic wave device according to any one of claims 1 to 3, characterized in that it contains 2 to 3 wt% iron as transition metal elements. A step of oxidizing the surface of a non-pyroelectric piezoelectric substrate comprising a lithium tantalate single crystal or a lithium niobate single crystal and having an oxygen content lower than the stoichiometric composition;
Forming an IDT electrode on the oxidized surface of the piezoelectric substrate;
Removing an oxidized surface excluding a region of the surface on which the IDT electrode of the piezoelectric substrate is formed;
The oxidized surface between the electrode fingers of the IDT electrode of the piezoelectric substrate is further oxidized, and the oxygen content of the surface of the piezoelectric substrate between the electrode fingers of the IDT electrode is changed to a region under the electrode finger of the IDT electrode. And a method of manufacturing the surface acoustic wave device. 6. The surface acoustic wave according to claim 5, wherein the electrode fingers of the IDT electrode formed on the one region are both set to have a line width and an interval between adjacent electrode fingers of 0.5 μm. Device manufacturing method.   The method for manufacturing a surface acoustic wave device according to claim 6, wherein the step of removing the oxidized surface is performed by dry etching. Of the region on the surface of the piezoelectric substrate, the conductivity in the region between the electrode fingers having an oxygen content greater than the region under the electrode finger of the IDT electrode is 1 × 10 ⁻¹² Ω ⁻¹ · cm ^{−. 1 to} 1 × 1
The method for manufacturing a surface acoustic wave device according to claim 7, wherein the surface acoustic wave element is set in a range of 0 ⁻¹⁰ Ω ⁻¹ · cm ⁻¹ .

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