JP3971604B2 - Surface acoustic wave filter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to a surface acoustic wave filter, and more particularly to a surface acoustic wave filter having excellent passband characteristics in a high frequency band including a GHz band.
[0002]
BACKGROUND OF THE INVENTION
  In recent years, surface acoustic wave filters have been used in various communication devices, and have played a role in reducing the size of devices and adjusting the passband frequency. With the progress of higher frequency and higher functionality of communication devices, there is an increasing demand for filters having variations in bandwidth and attenuation.
[0003]
  For example, a filter for a cellular phone of 900 MHz band has a pass bandwidth of 25 MHz (specific bandwidth 2.8%), and a filter for a cellular phone of 1.9 GHz band has a bandwidth of 60 MHz (specific bandwidth 2.8%). ) High performance filters are required. Furthermore, a PDC (Personal Digital Cellular Phone System) filter for mobile phones in Japan is also required to have a passband bandwidth of 16 MHz (specific bandwidth 1.9%) in the 800 MHz band. If the specific bandwidth is BR, it is defined by BR = BW / fc (BW is a pass band width when the in-band insertion loss is 3 dB, and fc is a center frequency of the pass band when the in-band insertion loss is 3 dB).
[0004]
  As described above, various telephone specifications require a specific bandwidth of about 1.9 to 2.8%. However, as a filter, a frequency deviation due to a manufacturing deviation in a high frequency band and a temperature fluctuation is taken into consideration. A 4.5% specific bandwidth is required.
[0005]
  Such a surface acoustic wave filter is generally formed by arranging excitation electrodes on a piezoelectric single crystal or polycrystal substrate, but has a large electromechanical coupling coefficient k2 (= high excitation efficiency of surface waves) and high frequency. Lithium tantalate (LiTaO) as a substrate material having a small surface acoustic wave propagation loss in the band_Three) Piezoelectric crystal substrates made of single crystals are well known, especially LiTaO._ThreeSingle crystal 36 ° rotation YX substrate (Y cut surface has a cut surface rotated 36 ° around the X axis, and crystal orientation for propagating surface acoustic waves in the X axis direction: right-handed Euler angle display (0, 126, 0)) has been used as a material having a small propagation loss (hereinafter also simply referred to as Euler angle display).
[0006]
  Recently, the 36 ° rotated YX substrate has an optimum crystal orientation when the additional mass effect of the electrode formed on the piezoelectric crystal substrate can be ignored, and is excited in a frequency band of several hundred MHz or less. Even if it is effective when the wavelength of the surface acoustic wave is long, in the operation near the GHz band required for the current mobile phone, etc., the electrode thickness is ignored with respect to the excited elastic wave wavelength. It has been reported that it cannot be done and is not necessarily optimal._ThreeA single crystal 42 ° rotated YX substrate (Euler angle display: (0, 132, 0)) has been proposed as a new optimum crystal orientation (see Japanese Patent Laid-Open No. 9-167936).
[0007]
  However, as long as a substrate (rotated Y-X substrate) that simply rotates the Y-cut surface around the X axis by a predetermined angle and propagates in the X-axis direction is used, the rotation angle reduces the bulk radiation and propagation loss at an appropriate electrode film thickness. Even if there is, the effective electromechanical coupling coefficient that contributes to the bandwidth of the filter electrical characteristics is determined by the substrate material used and the specific bandwidth is determined, so the desired ratio is limited to a very narrow range. Cannot design a filter with bandwidth.
[0008]
  Furthermore, in order to obtain electrical characteristics suitable for current digital circuits, a filter having flatness more than ever is required. The present inventors have described the above-described LiTaO. ₃ It has been found that a single crystal 42 ° rotated YX substrate is not necessarily optimal in terms of insertion loss and in-band deviation of passband characteristics.
[0009]
  Note that in a so-called end-surface reflection type piezoelectric shear wave resonator in which a surface shear wave reflects both end surfaces of a substrate to give a desired resonance frequency, the rotating Y-X substrate and a substrate whose crystal orientation is slightly shifted from the substrate An acoustic wave element that optimizes the above is known (see Japanese Patent Publication No. 6-14608). However, this surface acoustic wave device uses a surface slip wave excited by an IDT electrode, and this surface slip wave is reflected by a crystal surface such as an end face of a piezoelectric crystal substrate, so that it is excited by one IDT electrode. A crystal plane must be formed on the propagation path of the generated surface slip wave. Therefore, the crystal end face of the piezoelectric crystal substrate cannot be suitably used in order to allow a plurality of resonators composed of one IDT electrode to be arranged and connected to, for example, a ladder circuit to function as a filter. For this reason, it is a problem because a filter manufacturing process becomes extremely complicated to form a special groove structure or the like on both sides of the IDT electrode.
[0010]
  Accordingly, the present invention provides a surface acoustic wave filter having an optimum insertion loss and in-band deviation of passband characteristics using a lithium tantalate single crystal substrate having a new and useful crystal orientation, and a desired ratio. An object of the present invention is to provide a surface acoustic wave filter having a bandwidth and an optimally oriented piezoelectric crystal substrate.
[0011]
[Means for Solving the Problems]
  In order to achieve the above-described object, on the main surface of the piezoelectric crystal substrate made of a lithium tantalate single crystal,By a high frequency signal with a passband of 1850 MHz to 1910 MHzIn a surface acoustic wave filter in which an excitation electrode for exciting a surface acoustic wave and at least one reflector electrode in the propagation direction of the surface acoustic wave are arranged, a right-handed Euler angle display indicating a crystal orientation of the piezoelectric crystal substrate The variables (φ, θ, ψ) are 0.5 ≦ φ ≦ 10, θ =132,-(Tan^-1(Tan φ / cos θ)) − 6 ≦ ψ ≦ − (tan^-1(Tan φ / cos θ)) + 6 is satisfied.
[0012]
  In particular, φ and θ in the Euler angle display (φ, θ, ψ) are 0.5 ≦ φ ≦ 10, θ =132Or φ and θ of the Euler angle display (φ, θ, ψ) satisfy 0.5 ≦ φ ≦ 4, θ = 132.
[0013]
  Moreover, on the main surface of the piezoelectric crystal substrate made of lithium tantalate single crystal,By a high frequency signal with a passband of 1850 MHz to 1910 MHzIn a surface acoustic wave filter provided with an excitation electrode for exciting a surface acoustic wave and at least one reflector electrode for reflecting the surface acoustic wave to the excitation electrode, a right-handed Euler indicating a crystal orientation of the piezoelectric crystal substrate Each variable of the angle display (φ, θ, ψ) satisfies 2 ≦ φ ≦ 4, θ = 132, 3.5 ≦ ψ ≦ 6. Preferably, φ = 3 and θ = 132 are satisfied. Preferably, the excitation electrode is made of an Al—Cu alloy.
[0014]
  (Delete)
[0015]
  (Delete)
[0016]
DETAILED DESCRIPTION OF THE INVENTION
  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0017]
  FIG. 1 (a) shows coordinate axes for explaining coordinate conversion by a right-handed Euler angle display (φ, θ, ψ) representing the crystal orientation (cut angle and surface acoustic wave propagation direction) of the piezoelectric crystal substrate. FIG. 1B is a perspective view for explaining the relationship between the orthogonal coordinate system (x1-x2-x3) obtained by this coordinate transformation and the piezoelectric crystal substrate 1.
[0018]
  The Euler angle display that represents the crystal orientation (cut angle and surface acoustic wave propagation direction) of the principal surface that is the device region of the piezoelectric crystal substrate is defined as follows. First, as shown in FIG. 1A, in a piezoelectric single crystal having crystal axes X, Y, and Z, the X and Y axes are rotated counterclockwise by φ degrees (°) with the Z axis as a rotation axis, Next, the rotated X-axis is used as a θ-rotating axis, the Z-axis is rotated counterclockwise by θ degrees (°), and the rotated X-axis (θ-rotating axis) and the rotated Y-axis are Rotate counterclockwise by ψ degrees (°), and let the final orthogonal coordinate axes obtained by this rotation be x1, x2, and x3.
[0019]
  Then, as shown in FIG. 1B, the crystal orientation of the piezoelectric crystal substrate 1 that propagates the surface acoustic wave in the x1 axis direction on the principal surface 1a of the piezoelectric substrate 1 with the x3 axis as a normal line is displayed as an Euler angle display. (Φ, θ, ψ).
[0020]
  A single crystal wafer for cutting out a piezoelectric crystal substrate having a predetermined crystal orientation is manufactured as follows. First, the surface of the seed crystal cut out in a direction substantially perpendicular to the Euler angle display (0.5 to 20, 115 to 147, ψ = arbitrary) is above the melting point in a crucible made of refractory metal such as iridium. The molten lithium tantalate is brought into contact with the raw material melt, and then the seed crystal is rotated simultaneously with the rotation of the seed crystal while being lowered (rotary pulling method (= Czochralski method)). For example, a single crystal having an outer diameter of about 110 mm To train.
[0021]
  Next, the single crystal grown in this way is cut while measuring the cut angle by using, for example, an inner peripheral diamond cutting machine and an X-ray diffractometer, and displayed as Euler angles (0.5 to 20, 115). ˜147, ψ = arbitrary) A wafer whose surface is the main surface is obtained.
[0022]
  Furthermore, after the main surface, which is the device fabrication surface of the wafer, is mirror-finished by polishing, a fine electrode pattern of a metal mainly composed of Al, such as Al or Al-Cu alloy, is formed on the main surface. A plurality of filter element regions are formed in which IDT electrodes and reflector electrodes serving as excitation electrodes are arranged in, for example, a ladder circuit. These electrode patterns are produced by photolithography using a reduction projection exposure machine (stepper), an RIE (Reactive Ion Etching) apparatus, or the like.
[0023]
  And in order to protect fine electrodes such as excitation electrodes, for example, thin film SiO₂After laminating the film on the main surface of the wafer, the wafer is diced by a cutting device to produce individual filter elements. Thereafter, the individual filter elements are placed in the casing, and the electrodes of the casing and the electrodes of the filter elements are electrically connected to complete the surface acoustic wave filter.
[0024]
  FIG. 2 schematically shows an example of an electrode structure formed on the piezoelectric crystal substrate 1 in the surface acoustic wave filter S1 thus manufactured. In the figure, 2 is an IDT (interdigital transducer) electrode for exciting a surface acoustic wave, 3 is a grating-like reflection formed on both sides (surface acoustic wave propagation direction) and formed in the same manner as the IDT electrode 2. Electrode. Further, 4 is a series resonator constituting a ladder circuit, and 5 is a parallel resonator.
[0025]
  According to the surface acoustic wave filter S1, a surface acoustic wave is excited by the fine IDT electrode 2 having a comb-like shape and having a predetermined electrode finger interval, and the reflection is arranged so as to sandwich the IDT electrode 2 from both sides. The surface acoustic wave is reflected to the IDT electrode 2 side by the electrode 3 and is effectively confined to cause electrical resonance. Then, as shown in the figure, a desired circuit electrical characteristic is obtained by configuring a ladder circuit by connecting resonators composed of the IDT electrode 2 and the reflector electrode 3 in series or in parallel.
[0026]
  Here, the thickness of the piezoelectric crystal substrate is preferably about 0.1 mm to 0.5 mm. If the thickness is less than 0.1 mm, the piezoelectric crystal substrate becomes brittle, and if it exceeds 0.5 mm, the material cost and component dimensions become large and cannot be used.
[0027]
  The IDT electrode is made of, for example, Al or an Al alloy (Al—Cu type, Al—Ti type, Al—Mg type), and is formed by a thin film forming method such as a vapor deposition method, a sputtering method, or a CVD method. In addition, many of the above-described metal materials may have a laminated structure in order to improve power handling performance, which is suitable for obtaining characteristics as a surface acoustic wave filter.
[0028]
  Furthermore, the electrode according to the present invention and the surface acoustic wave propagation part on the piezoelectric crystal substrate have Si, SiO₂, SiNx, Al₂O_ThreeAlternatively, a dielectric film such as a protective film may be formed to prevent energization due to conductive foreign substances and to improve power durability.
[0029]
  In the ladder-type surface acoustic wave filter S1 configured as described above, the main surface 1a of the piezoelectric crystal substrate 1 is optimal for obtaining a surface acoustic wave filter having a good insertion loss and in-band deviation of the passband characteristics. As the crystal orientation of the piezoelectric crystal substrate, each variable of Euler angle display (φ, θ, ψ)
0.5 ≦ φ ≦ 10,
θ =132,
-(Tan^-1(Tan φ / cos θ)) − 6 ≦ ψ ≦ − (tan^-1(Tanφ / cosθ)) + 6,
Is satisfied. The reason for the crystal orientation will be described in detail below. LiTaO_ThreeWith regard to the Euler angle display of a single crystal, an equivalent crystal orientation can exist due to the symmetry of the crystal. Therefore, a substrate having an equivalent crystal orientation is included in the above range.
[0030]
  3 to 11 are results showing the electrical characteristics of the surface acoustic wave filter using piezoelectric crystal substrates having various crystal orientations as average values obtained under the following configuration and measurement conditions. Symbols (Δ, ♦) in the figure are plotted for typical angles.
[0031]
  Since the resonance state changes depending on the electrode pattern structure, the optimum film thickness ratio must be changed in order to obtain the optimum filter characteristics. However, the optimum film thickness ratio of the IDT electrode and the reflector electrode used in the present invention (standard) In order to obtain an optimum resonance state, it is preferable to set 0.0 / 0.2 to H / λ (H: film thickness, λ: period length of IDT electrode, or surface acoustic wave wavelength). is there.
[0032]
  Further, the total number of IDT electrodes is 3 to 150 pairs in which a good resonance state is obtained, and the number of reflector electrodes is 1 to 200. Thereby, the surface acoustic wave excited by the IDT electrode can be favorably reflected. In addition, the normalized cross width of the IDT electrode is 3 to 200λ (λ: the period length of the IDT electrode or the wavelength of the surface acoustic wave), whereby a good resonance state is obtained. Each plot shows an average of 30 measured items.
[0033]
  The pass characteristics of the surface acoustic wave filter were measured using a vector network analyzer (manufactured by Agilent Technologies: model number HP8753ES) under the condition that the termination resistance value was 50Ω. Note that the surface acoustic wave filter is a ladder type filter shown in FIG. 2 that can be used in the 1.9 GHz band.
[0034]
  Here, in the surface acoustic wave filter of the present invention, a piezoelectric substrate is formed using a crystal material having a symmetrical structure. In order to clarify the significant difference in the filter characteristics due to the difference between the propagation direction and the crystal X axis by the symmetry of the piezoelectric crystal substrate from the crystal X axis, the crystal X axis before Euler rotation is rotated after Euler rotation. The Euler rotation is performed on an axis projected onto the crystal Z axis side on the piezoelectric crystal substrate (x1-x2 plane) (projection X axis: intersection line of crystal XZ plane and plane having x3 axis as normal). After that, the angle that extends around the x1 axis (where the rotation direction of ψ is +) is the projection axis angle ψ ′, and ψ ′ = 0 is the best in terms of characteristics. It can be an angle.
[0035]
  When the projected X-axis coincides with the x1-axis (ψ ′ = 0), the first angle φ ° and the second angle θ ° of the Euler angle display are used and expressed by the following geometric formula I by the geometrical relationship. be able to.
ψ =-(tan^-1(Tanφ / cosθ)) (Formula I)
FIG. 3 shows LiTaO._ThreeA variable of Euler angle display (φ, θ, ψ) of a piezoelectric crystal substrate made of a single crystal, the first angle φ is 0 and 3, and the projection axis angle ψ ′ is 0 (for example, φ = 0 when θ = 132) Using a surface acoustic wave filter with ψ = 0 and θ = 132 and φ = 3 when ψ = 3), and the maximum insertion loss within the passband (1850 MHz to 1910 MHz). As for the relationship, the passing transmission characteristic is measured with the vector network analyzer under the condition that the termination resistance is 50Ω. Here, the solid line L1 indicates the result when φ = 3 and ψ ′ = 0, and the broken line L2 indicates the result when φ = 0 and ψ ′ = 0.
[0036]
  FIG. 4 uses a surface acoustic wave filter formed in the same manner as in FIG. 3, and the relationship between the second angle θ and the relative bandwidth (same as BR) within 3 dB of the maximum insertion loss within the passband. Shows the results of measurement as described above.
[0037]
  FIG. 5 shows the relationship between the second angle θ, the crystal material defined by the first angle φ, and the in-passband deviation using the surface acoustic wave filter formed in the same manner as in FIG. The result measured similarly is shown. Here, the solid line L1 indicates the result when φ = 3 and ψ ′ = 0, and the broken line L2 indicates the result when φ = 0 and ψ ′ = 0.
[0038]
  The following was found from the results of FIGS.
[0039]
  When the second angle θ in the Euler angle display is around 132, there is a maximum or minimum in the characteristics of each figure. When comparing the piezoelectric crystal substrate with the first angle φ = 0 and the piezoelectric crystal substrate with φ = 3, φ It was found that the filter using the piezoelectric crystal substrate of = 3 has less change in the insertion loss and the relative bandwidth in the wide angle range of the second angle θ, and the insertion loss is small and the in-band deviation is good.
[0040]
  In particular, as shown in FIG. 5, when θ is in the range of 115 to 147, the in-band deviation is within 3 dB, which is favorable. Further, when θ is in the range of 123 to 138, as shown in FIG. 3, it has been found that the insertion loss is within 3 dB and is optimal.
[0041]
  FIG. 6 shows the relationship between the first angle φ and the maximum insertion loss in the passband when the second angle θ of the Euler angle display is 132 and 129 and the projection axis angle ψ ′ is 0. The result measured about is shown. Measurement conditions and the like are the same as in FIG. The reason for selecting θ = 129 is that the second angle in the Euler angle display of the 36 ° rotated YX substrate (θ = 126) and the 42 ° rotated YX substrate (θ = 132), which are conventionally optimal, is selected. This is because the average value of θ.
[0042]
  FIG. 7 shows the results of measuring the relationship between the first angle φ and the specific bandwidth within 3 dB of the maximum insertion loss in the pass band under the same conditions as in FIGS. 4 and 6. The solid line L1 indicates the result when θ = 129 and ψ ′ = 0, and the broken line L2 indicates the result when θ = 132 and ψ ′ = 0. A broken line L3 indicates a straight line drawn by approximation according to a plot in the case where θ = 132 and ψ ′ = 0 and the first angle φ is small.
[0043]
  FIG. 8 shows the result of measuring the relationship between the first angle φ and the deviation in the passband under the same conditions as in FIGS. 5 and 6. The solid line L1 indicates the result when θ = 129 and ψ ′ = 0, and the broken line L2 indicates the result when θ = 132 and ψ ′ = 0.
[0044]
  The following was found from the results of FIGS.
[0045]
  As is clear from FIG. 6, no significant change is observed even when the first angle φ is changed, and good characteristics can be obtained.
[0046]
  In particular, as can be seen from the straight line L3 shown in FIG. 7, the specific bandwidth tends to decrease linearly in the range where the first angle φ is small (0.5 to 10). As the angle φ increases, the in-band deviation tends to decrease. In particular, when φ is 0.5 to 20, the crystal orientation changes, so that the propagation loss is small and the effective electromechanical coupling coefficient k2 is a desired ratio. Since the region approaches the optimum region for the bandwidth BR, a good in-band deviation can be obtained.
[0047]
  The broken line L3 shown in FIG. 7 is a result of linear approximation when the second angle θ is 132 in the range of 0.5 to 10 where the first angle φ is small, and this straight line is expressed by the following formula II.
[0048]
  Therefore, when trying to obtain a filter having a desired specific bandwidth BR, the crystal angle of the piezoelectric crystal substrate used for the filter can be derived from the first angle φ according to the following formula II, which is an empirical formula.
[0049]
  φ = −2480.0 × BR + 97.0 (formula II)
Similarly, from the experimental data (♦, Δ in the figure) where θ is 132 or less in FIG. 4, the above experimental formula II is obtained, and when trying to obtain a filter having a desired specific bandwidth BR, As for the crystal orientation of the piezoelectric crystal substrate to be used, the second angle θ can be derived from the following formula III.
[0050]
  θ = 1140 × BR + 87.0 (formula III)
Further, from the experimental data (Δ in the figure) when the second angle θ in FIG. 7 is 132, the standard deviation σ of the experimental formula II is determined to be about 0.5, and the tolerance of the experimental formula is 4σ ( Here, σ is a standard deviation) and is assumed to be ± 2.
[0051]
  Further, the first angle φ and the second angle θ obtained by the above formula can be used to calculate the third angle ψ from the formula I.
[0052]
  Therefore, when the specific bandwidth BR is set to a desired value, each variable of the Euler angle display (φ, θ, ψ) indicating the crystal orientation of the piezoelectric crystal substrate satisfies the following formula. Crystal orientation can be determined.
[0053]
  F1 (BR) ≦ φ ≦ F2 (BR),
G1 (BR) ≦ θ ≦ G2 (BR),
-(Tan^-1(Tan φ / cos θ)) − 6 ≦ ψ ≦ − (tan^-1(Tanφ / cosθ)) + 6
(However, F1 (BR) = − 2480 × BR + 95, F2 (BR) = − 2480 × BR + 99, G1 (BR) = 1140 × BR + 85, G2 (BR) = 1140 × BR + 89, BR = BW / fc (BW is a band The internal insertion loss is the passband width at 3 dB, and fc is the center frequency of the passband at the inband insertion loss of 3 dB).
[0054]
  Next, a more preferable crystal orientation of the piezoelectric crystal substrate according to the present invention will be described.
[0055]
  FIG. 9 shows the maximum insertion loss in the passband (1850 MHz to 1910 MHz) when the second angle θ of the Euler angle display is 132 and the second angle φ is 0, 1, 3, 5, or 7. The measurement results are shown.
[0056]
  FIG. 10 shows the result of measurement in the same manner as in FIG. 9 with respect to the relationship between the projection axis angle ψ ′ and the specific bandwidth when the maximum insertion loss in the passband is within 3 dB. FIG. 11 shows the results of measurement of the relationship between the projection axis angle ψ ′ and the in-passband deviation in the same manner as in FIG.
[0057]
  LiTaO ₃ Since the crystal structure of the single crystal has a strong symmetry with respect to the crystal XZ plane, the obtained filter has a peak when the projection axis angle ψ ′ is near 0 from the results of FIGS. It is clear that the characteristics are also symmetric about the projection axis angle ψ ′ because the crystal material is symmetric about the crystal X axis.
[0058]
  Therefore, in particular, the projection axis angle ψ ′ where the in-band deviation is within 3 dB is good within the range of ± 6 tolerance, and within the insertion loss of 3 dB, the projection axis angle ψ ′ is within the range of ± 4. It turned out to be optimal within the scope of the lance. The reason for this is that when the crystal orientation changes, the effective electromechanical coupling coefficient k2 approaches the optimum region for the desired specific bandwidth BR, so that the in-band deviation is improved and the insertion loss is also improved. Conceivable.
[0059]
  In particular, when the specific passband width is increased to more than 4%, the piezoelectric crystal substrate capable of obtaining good electrical characteristics is expressed by Euler angles (φ, θ, ψ) (2-4, 132, 3.5-6). It turned out to be.
[0060]
  Thus, according to the surface acoustic wave filter using the piezoelectric crystal substrate of (0.5 to 20, 115 to 147, ψ ′ = − 6 to 6 (ψ = −6 to 47)) in the Euler angle display, the band The internal deviation can be within 3 dB.
[0061]
  Further, in the Euler angle display, according to the surface acoustic wave filter using the piezoelectric crystal substrate of (0.5 to 20, 123 to 138, ψ ′ = − 4 to 4 (ψ = −4 to 38)), it is inserted. A good filter characteristic with a loss within 3 dB is provided.
[0062]
  In the Euler angle display, (0.5 to 10,132According to the surface acoustic wave filter using the piezoelectric crystal substrate of ψ ′ = − 6 to 6 (ψ = −6 to 22)), the in-band deviation is surely within 3 dB with the desired specific bandwidth BR. And the insertion loss is optimum filter characteristics within 3 dB.
[0063]
  Furthermore, according to the surface acoustic wave filter using the piezoelectric crystal substrate of (0.5 to 4, 132, ψ ′ = − 4 to 4 (ψ = −4 to 10)) in the Euler angle display, the maximum band A very good filter characteristic having a width, a minimum insertion loss, a good in-band deviation, and a good attenuation characteristic in the vicinity of the pass band can be obtained.
[0064]
  In addition, according to the method for determining the crystal orientation of the piezoelectric crystal substrate according to the present invention, each variable of the Euler angle display of the piezoelectric crystal substrate can be expressed as a function of a desired specific bandwidth BR. Good LiTaO_ThreeA single-crystal piezoelectric crystal substrate material can be selected, and it is easy and simple to design and manufacture an excellent surface acoustic wave filter with good insertion loss, in-band deviation, and good attenuation characteristics near the passband Can be done.
[0065]
  In the present embodiment, the surface acoustic wave filter has been described by taking a ladder type circuit as an example. However, the surface acoustic wave filter is not limited to the ladder type circuit, and at least an excitation electrode and a reflector electrode are provided in order to efficiently excite the surface acoustic wave. For example, a surface acoustic wave filter S2 having a so-called resonator structure electrode configuration in which a plurality of IDT electrodes 2 as shown in FIG. 12 are arranged in the propagation direction x1 is applicable. It is also applicable to. In FIG. 12, the same components as those in FIG. The present invention can also be applied to a surface acoustic wave filter having various filter circuits such as a lattice-type circuit or a combination of a ladder-type circuit and a lattice-type circuit. It is possible to make changes.
[0066]
【Example】
  An example in which the surface acoustic wave filter according to the present invention is specifically manufactured will be described.
[0067]
  For each variable of Euler angle display, LiTaO with φ = 3, θ = 132, and ψ = 4.5_ThreeA fine electrode pattern of Al (98 wt%)-Cu (2 wt%) was formed on a piezoelectric crystal substrate made of a single crystal. This pattern was produced by a reduction projection exposure machine (Stepper, Nikon Corporation, model number: NSR2205i12D) and RIE (Reactive Ion Etching) apparatus (Matsushita Electric Industrial Co., model number: E646S-ICP).
[0068]
  First, the substrate material was subjected to ultrasonic cleaning with acetone / IPA or the like to remove organic components. Next, the substrate was sufficiently dried by a clean oven, and then an electrode was formed. For electrode deposition, an Al—Cu material was deposited using a sputtering apparatus. The electrode film thickness was about 0.2 μm.
[0069]
  Next, a photoresist was spin-coated to a thickness of about 0.5 μm, and desired patterning was performed by the stepper.
[0070]
  Next, an unnecessary portion of the photoresist was dissolved with an alkaline developer by a developing device to reveal a desired pattern, and then the Al—Cu electrode was etched by the RIE device to complete the patterning.
[0071]
  Thereafter, a protective film was formed on a predetermined region of the electrode. That is, SiO₂Then, a photoresist was patterned by photolithography, and a wire bonding window opening portion was etched by an RIE apparatus or the like to complete a protective film pattern.
[0072]
  Next, the substrate was diced along dicing lines and divided into chips. Then, each chip was picked up by a die bonding apparatus and adhered to the package using a resin mainly composed of a silicone resin. Then, it was dried and cured at a temperature of about 160 ° C. As the package, a ceramic package having a laminated structure of 3 mm square was used.
[0073]
  Next, an Au wire having a diameter of 30 μm was ball-bonded on the electrode part of the package and the Al electrode pad on the chip, and then the lid was placed on the package and completed by welding and sealing with a sealing machine. The ground electrodes on the chip were wired separately and bonded to the ground electrode on the package by Au ball bonding.
[0074]
  As a comparative example, for each variable of Euler angle display, LiTaO with φ = 0, θ = 132, and ψ = 0._ThreeA surface acoustic wave filter in which electrodes and the like were produced on a piezoelectric crystal substrate made of a single crystal in the same manner as in the example of the present invention was obtained.
[0075]
  The electrical characteristics of the surface acoustic wave filter thus completed were measured with a network analyzer (manufactured by Agilent Technologies: model number HP8753ES).
[0076]
  13 and 14 show characteristics when the input signal is input to each of the surface acoustic wave filters of the embodiment of the present invention and the comparative example, and the transmission amount of the output signal is plotted on the vertical axis and the frequency is plotted on the horizontal axis. The figure is shown. Here, FIG. 13 shows the characteristics in the range of 0 to −5 dB for the transmission amount to show the in-band deviation, and FIG. 14 shows the characteristics in the range of 0 to −70 dB for the transmission quantity to show the steepness. Show. Moreover, (A) in each figure is a characteristic of the surface acoustic wave filter of the present invention, and (B) in each figure is a characteristic of the surface acoustic wave filter of the comparative example.
[0077]
  As is apparent from FIGS. 13 and 14, according to this embodiment, the pass characteristic is good, and in particular, the in-band deviation is 1.2 dB with respect to 1.5 dB of the comparative example, and 0.3 dB is also good. Results were obtained. In this embodiment, the attenuation characteristic on the low frequency side of the pass band is also good. For example, in the comparative example, the attenuation is 32.5 dB with respect to the attenuation of 25.5 dB at a frequency of 1815 MHz, and the attenuation is large and excellent at 7 dB. Showed steepness.
[0078]
【The invention's effect】
  According to the surface acoustic wave filter of the present invention, on the main surface of the piezoelectric crystal substrate made of a lithium tantalate single crystal,By a high frequency signal with a passband of 1850 MHz to 1910 MHzEach variable of the right-handed Euler angle display (φ, θ, ψ) indicating the crystal orientation of the piezoelectric crystal substrate is formed by arranging an excitation electrode for exciting a surface acoustic wave and a reflector electrode. φ ≦ 10, θ =132,-(Tan^-1(Tan φ / cos θ)) − 6 ≦ ψ ≦ − (tan^-1(Tanφ / cos θ)) + 6 is satisfied, and an excellent surface acoustic wave filter having an excellent in-band deviation of 3 dB or less and capable of greatly changing the specific bandwidth without deteriorating insertion loss. Can be provided.
[0079]
  (Delete)
[0080]
  (Delete)
[0081]
  Further, when φ and θ of the Euler angle display (φ, θ, ψ) satisfy 0.5 ≦ φ ≦ 4 and θ = 132, the maximum bandwidth, the minimum insertion loss, and the good in-band It is possible to provide an optimum surface acoustic wave filter in which a deviation is obtained and such a good filter characteristic is obtained, and further, the characteristic improvement can be performed not only in the band but also in the attenuation outside the band.
[0082]
  In particular, when the specific bandwidth is increased to more than 4%, a surface acoustic wave filter using a piezoelectric crystal substrate having Euler angle display (φ, θ, ψ) of (2-4, 132, 3.5-6) Can provide very good electrical properties.
[0083]
  (Delete)
[Brief description of the drawings]
FIG. 1A is a coordinate axis for explaining coordinate conversion by a right-handed Euler angle (φ, θ, ψ) indicating a crystal orientation (cut angle and surface acoustic wave propagation direction) of a piezoelectric crystal substrate. FIG. 6B is a perspective view for explaining the relationship between the orthogonal coordinate system obtained by the coordinate conversion by the Euler angle and the piezoelectric crystal substrate.
FIG. 2 is a plan view schematically illustrating an example of an electrode configuration of a surface acoustic wave filter according to the present invention.
FIG. 3 is a graph showing a relationship between a second angle θ in Euler angle display of a piezoelectric crystal substrate having various crystal orientations and a maximum insertion loss in a pass band.
FIG. 4 is a graph showing a relationship between a second angle θ in Euler angle display and a 3 dB ratio bandwidth of piezoelectric crystal substrates having various crystal orientations.
FIG. 5 is a graph showing a relationship between a second angle θ of Euler angle display of a piezoelectric crystal substrate having various crystal orientations and a deviation in a passband.
FIG. 6 is a graph showing a relationship between a first angle φ of Euler angle display of a piezoelectric crystal substrate having various crystal orientations and a maximum insertion loss in a pass band.
FIG. 7 is a first angle φ of Euler angle display of a piezoelectric crystal substrate having various crystal orientations.WhenIt is a graph which shows the relationship.
FIG. 8 is a graph showing a relationship between a first angle φ of Euler angle display of a piezoelectric crystal substrate having various crystal orientations and a deviation in a passband.
FIG. 9 is a graph showing a relationship between an angle ψ ′ with respect to a projected X axis of a piezoelectric crystal substrate having various crystal orientations and a maximum insertion loss in the passband.
FIG. 10 is a graph showing a relationship between an angle ψ ′ with a projected X axis of a piezoelectric crystal substrate having various crystal orientations and a 3 dB ratio bandwidth.
FIG. 11 is a graph showing a relationship between an angle ψ ′ with respect to a projected X axis of a piezoelectric crystal substrate having various crystal orientations and a deviation in a passband.
FIG. 12 is a plan view schematically illustrating another electrode structure of the surface acoustic wave filter according to the present invention.
FIG. 13 is a characteristic diagram showing the relationship between the frequency of the surface acoustic wave filter and the transmission amount (0 to −5 dB), in which the passband portion is enlarged, and (A) is a surface acoustic wave filter according to an embodiment of the present invention. (B) is an electrical characteristic diagram of a conventional surface acoustic wave filter (comparative example).
FIG. 14 is a characteristic diagram showing the relationship between the frequency of the surface acoustic wave filter and the transmission amount (0 to −70 dB), (A) is an electrical characteristic diagram of the surface acoustic wave filter according to the embodiment of the present invention, ) Is an electrical characteristic diagram of a conventional surface acoustic wave filter (comparative example).
[Explanation of symbols]
S1, S2: surface acoustic wave filters
1: Piezoelectric crystal substrate
2: IDT electrode (excitation electrode)
3: Reflector electrode
4: Series resonator
5: Parallel resonator

Claims (5)

An excitation electrode that excites a surface acoustic wave by a high-frequency signal having a passband of 1850 MHz to 1910 MHz, and at least one reflection that reflects the surface acoustic wave to the excitation electrode on a main surface of a piezoelectric crystal substrate made of a lithium tantalate single crystal. In the surface acoustic wave filter provided with the ceramic electrode, each variable of the right-handed Euler angle display (φ, θ, ψ) indicating the crystal orientation of the piezoelectric crystal substrate satisfies the following formula: Surface acoustic wave filter.
0.5 ≦ φ ≦ 10
θ = 132
− (Tan ⁻¹ (tan φ / cos θ)) − 6 ≦ ψ ≦ − (tan ⁻¹ (tan φ / cos θ)) + 6 The surface acoustic wave filter according to claim 1, wherein φ and θ of the Euler angle display (φ, θ, ψ) satisfy the following expression.
0.5 ≦ φ ≦ 4
θ = 132 An excitation electrode that excites a surface acoustic wave by a high-frequency signal having a passband of 1850 MHz to 1910 MHz, and at least one reflection that reflects the surface acoustic wave to the excitation electrode on a main surface of a piezoelectric crystal substrate made of a lithium tantalate single crystal. In the surface acoustic wave filter provided with the ceramic electrode, each variable of the right-handed Euler angle display (φ, θ, ψ) indicating the crystal orientation of the piezoelectric crystal substrate satisfies the following formula: Surface acoustic wave filter.
2 ≦ φ ≦ 4
θ = 132
3.5 ≦ ψ ≦ 6 4. The surface acoustic wave filter according to claim 1, wherein φ and θ of the Euler angle display (φ, θ, ψ) satisfy the following expression. 5.
φ = 3
θ = 132   The surface acoustic wave filter according to claim 1, wherein the excitation electrode is made of an Al—Cu alloy.

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