JP4377525B2 - Surface acoustic wave filter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electrode structure of a surface acoustic wave filter used in a mobile communication device such as a mobile phone.
[0002]
[Prior art and its problems]
In recent years, many surface acoustic wave devices have been used as components such as filters, delay lines, and oscillators of electronic devices that use radio waves. In particular, surface acoustic wave filters that are small, lightweight, and have high steep cut-off performance as filters have come to be widely used as RF stage and IF stage filters in mobile terminal devices. There is a demand for a surface acoustic wave filter having a high attenuation characteristic and a wide bandwidth with excellent loss and out-of-band cutoff characteristics.
[0003]
So far, various types of surface acoustic wave filters such as ladder type (ladder type), transversal type, and longitudinal mode coupled resonator type have been put into practical use from the viewpoint of electrode configuration. A surface acoustic wave filter is a highly promising surface acoustic wave filter that has low loss and good cutoff characteristics in the vicinity of the passband, is highly reliable in terms of power resistance due to electrode miniaturization due to higher frequencies. is there.
[0004]
In the case of such a ladder type filter, the specific bandwidth BW / fo (BW; pass bandwidth, fo: center frequency) is the difference between the resonance frequency fr of the surface acoustic wave resonator constituting the filter and the antiresonance frequency fa. Δf (= fa−fr) that is normalized by the resonance frequency is almost determined.
[0005]
In recent years, with the drastic changes in mobile phone systems, the required specifications on the system side have become stricter, and surface acoustic waves with a passband characteristic that is closer to a rectangle with a wider band and excellent in steepness than before. Filters are anxious.
[0006]
The steepness of the passband of the surface acoustic wave filter is also determined by Δf (= fa−fr). By using a surface acoustic wave resonator having a small Δf, a passband characteristic with excellent steepness can be obtained. I know it. Δf that determines the steepness of the above-mentioned specific bandwidth and passband largely depends on the electromechanical coupling coefficient, which is a material constant of the piezoelectric substrate, and the film thickness of the electrode pattern, and is optimal for obtaining a desired specific bandwidth. It is necessary to produce a filter by selecting a combination of a piezoelectric substrate having an electromechanical coupling coefficient and an electrode film thickness.
[0007]
However, it is possible to find a substrate material having an electromechanical coupling coefficient optimum for the above-mentioned specific bandwidth BW / fo and having a high material Q value (the reciprocal of the loss coefficient accompanying electromechanical conversion) reflected in the filter steepness It was a difficult problem.
[0008]
Therefore, conventionally, a method for improving the electrode structure by adopting a general substrate / orientation for the piezoelectric substrate has been studied.
[0009]
The IDT electrode formed on the piezoelectric substrate is an abbreviation for Inter Digital Transducer, and has an electrode structure in which comb-like electrodes facing each other are combined. As described above, the ladder-type electrode is considered promising for the IDT electrode connection structure. As shown in FIG. 4, a conventional ladder-type surface acoustic wave device includes a surface acoustic wave resonator R1 constituting a series arm and a surface acoustic wave resonator R2 constituting a parallel arm alternately connected on a signal line. A so-called ladder-like structure is generally used.
[0010]
However, the above-described method for improving electrical characteristics by the conventional electrode structure generally involves changing the number of IDT electrode pairs, electrode period, intersection width, number of reflectors, etc. of each resonator, although slight improvement is seen, There is no improvement that satisfies the required specification of the filter, and there is a problem that the SAW device has a quality problem.
[0011]
Accordingly, an object of the present invention is to provide an electrode for a ladder-type surface acoustic wave filter having a good quality and having a passband characteristic excellent in steepness in a state where a conventional general substrate is used for a piezoelectric substrate and a cut orientation. A structure shall be provided.
[0012]
[Means for Solving the Problems]
To solve the above problems, the present invention provides a 42 ° Y-cut X-propagation LiTaO ₃ SAW filter connected on a piezoelectric substrate made of single crystal a plurality of IDT electrodes on the ladder circuits of at least 1 Additional IDT electrodes are connected in parallel to one IDT electrode, reflector electrodes are arranged on both sides of the additional IDT electrode, the electrode film thickness of the additional IDT electrode and the reflector electrode is H (μm), and an elastic surface When the wave wavelength is λ (μm), the relationship of H / λ = 0.07 to 0.11 is satisfied, the number of the reflector electrodes is 10 or more, and the additional IDT electrode is added to the additional IDT electrode. The surface acoustic wave generated from the IDT electrode connected in parallel with the IDT electrode is arranged on the extension line of the propagation path .
[0013]
DETAILED DESCRIPTION OF THE INVENTION
DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments according to the present invention will be described in detail with reference to the drawings.
[0014]
FIG. 1 is a diagram showing the structure of a surface acoustic wave device according to the present invention. In the figure, reference numeral 1 denotes a piezoelectric substrate, on which an electrode made of a metal indicated by thin films 2 to 6 is disposed. Reference numeral 2 denotes an IDT electrode for controlling frequency characteristics. Reference numeral 3 denotes an IDT electrode constituting an ordinary ladder type surface acoustic wave filter. Reference numeral 4 denotes a reflector, which is disposed on both sides of the IDT electrode. The reflector 4 is for changing the resonance state of the IDT electrodes 2 and 3, and may not be present. Reference numeral 5 denotes a ground terminal of the filter, 6a denotes an input terminal of the filter, and 6b denotes an output terminal of the filter. As shown in FIG. 1, the present invention is characterized by having a structure in which an IDT electrode 2 for controlling frequency characteristics is connected in parallel to a series resonator R1.
[0015]
FIG. 1 is a filter in which five IDT electrodes 3 are connected, and an example of a surface acoustic wave filter when a capacitor using the surface acoustic wave resonator 2 of the present invention is added to the center of three series resonators R1. It is. FIG. 2 shows a filter in which five IDT electrodes 3 are connected. The surface acoustic wave resonator 2 of the present invention is used for the first and third series resonators R1 of the three series resonators R1. It is an example of a surface acoustic wave filter when a capacitor is added. FIG. 3 is an example of a surface acoustic wave filter in which a capacitor using the surface acoustic wave resonator 2 of the present invention is added to the parallel resonator R2 by a filter in which five IDT electrodes 3 are connected. Reference numeral 5 denotes a ground electrode connected to the ground pad of the package by wire bonding. Reference numerals 6a and 6b denote input / output electrodes connected to the input / output pads of the package by wire bonding.
[0016]
The reason why the IDT electrode 2 for frequency characteristic control is connected in this way will be described below.
[0017]
In order to realize a filter characteristic having a steep shoulder characteristic, the surface acoustic wave device according to the present invention does not change the propagation characteristic of the surface acoustic wave, and changes the elasticity by the capacitance of the IDT electrodes added in parallel. The Δf of the surface wave resonator can be arbitrarily controlled. Thereby, the pass band characteristic of the filter can be made steeper. FIG. 12 is an electrical equivalent circuit diagram in the case where an additional capacitor Cp composed of a surface acoustic wave resonator is connected in parallel to the surface acoustic wave resonator. Here, the resonance frequency fr = 1 / √ (L1 · C1) and the anti-resonance frequency fa = fr / √ (1 + C1 · (Co + Cp)) can be expressed. By increasing the additional capacitance Cp, fa decreases, but fr does not change. This is illustrated in FIG. It can be seen that, due to Cp, the resonance frequency 8 does not change, but only the antiresonance frequency 7 changes.
[0018]
Therefore, as described above, by increasing the additional capacitance Cp, that is, by increasing the capacitance of the frequency adjusting IDT electrode 2, the difference Δf between the resonance frequency and the anti-resonance frequency of the impedance characteristic is reduced. If a ladder type filter in which the surface acoustic wave resonator 2 having this configuration is connected to at least one of the series-side or parallel-side resonators R1 and R2 is formed, Δf is small. The steepness of both sides or one side is improved.
[0019]
Examples of improving the characteristics according to the present invention are shown in FIGS. FIG. 5 shows the case where the surface acoustic wave resonator of the present invention is used as a part of the series resonator R1, and it can be seen that the passband on the high frequency side and the slope of attenuation can be improved. FIG. 6 shows the case where the surface acoustic wave resonator according to the present invention is used as a part of the parallel resonator, and it can be seen that the passband on the low frequency side and the slope of attenuation can be improved. FIG. 7 shows the case where the surface acoustic wave resonator of the present invention is used for both the series resonator and the parallel resonator. In this case, it can be seen that both the high frequency band and the attenuation gradient, and the low frequency pass band and the attenuation gradient can be improved.
[0020]
In addition, the frequency characteristic control IDT electrode according to the present invention may be added to all the resonators connected in a ladder shape, but an additional capacitor is attached only to at least one resonator as shown in FIGS. Thus, the frequency of the attenuation pole generated by the surface acoustic wave resonator can be set differently, and the attenuation band can be efficiently increased.
[0021]
Further, by providing reflector electrodes on both sides of the frequency characteristic control IDT electrode, the surface acoustic wave generated by the frequency characteristic control IDT electrode is converted into a surface acoustic wave resonator (series resonator or parallel resonator) that is the original of the ladder filter. Unnecessary waves that adversely affect the resonator) can be substantially cut.
[0022]
Further, lattice type filters (so-called filters having a structure in which resonators are connected in a lattice pattern) can be applied in the same manner as described above. That is, as in the present invention, any structure can be used as long as the filter characteristics can be obtained by changing the frequency characteristics of the electrical impedance of the surface acoustic wave resonator to an arbitrary characteristic, such as a ladder filter and a lattice filter. The passband and out-of-band attenuation can be improved.
[0023]
Here, the preferred number of reflectors is H / λ = 0.07 to 0.11 (H: electrode film thickness, λ: elasticity on a LiTaO 3 single crystal 36 ° and 42 ° Y-cut X-propagation piezoelectric substrate. In the range of the surface wave wavelength), as shown in FIG. 13, surface acoustic waves are reflected by nearly 90% between 0 and 10, so 10 or more are preferable. If such a reflector is used, as shown in claim 3, it has ten or more reflector electrodes on the extension line of the surface acoustic wave propagation path of the original surface acoustic wave resonator of the ladder filter. In addition, since the frequency characteristic control IDT electrodes can be arranged close to each other and no extra space is required, the piezoelectric substrate can be miniaturized.
[0024]
In addition, according to the present invention, the surface acoustic wave resonator is added in parallel to the IDT electrode, so that the applied power is dispersed, and it takes more per ID than the IDT electrode when no additional capacitor is connected. Power is reduced. As a result, a surface acoustic wave filter having excellent power durability can be produced.
[0025]
Furthermore, according to the present invention, electrode breakdown due to static electricity generated by the pyroelectric effect or the like during electrode patterning is less likely to occur because power is dispersed.
Also, as piezoelectric substrates for SAW devices, 36 ° ± 3 ° Y-cut X-propagating lithium tantalate single crystal, 42 ° ± 3 ° Y-cut X-propagating lithium tantalate single crystal, 64 ° ± 3 ° Y-cut X-propagating niobium Lithium oxide single crystal, 41 ° ± 3 ° Y-cut X-propagation lithium single crystal, 45 ° ± 3 ° X-cut Z-propagation lithium tetraborate single crystal has a large electromechanical coupling coefficient and a small frequency temperature coefficient. Preferred as a substrate. The thickness of the piezoelectric substrate is preferably about 0.1 mm to 0.5 mm. If the thickness is less than 0.1 mm, the piezoelectric substrate becomes brittle, and if it exceeds 0.5 mm, the material cost and component dimensions increase, and the piezoelectric substrate cannot be used.
[0026]
The IDT electrode 3 and the reflector are made of Al or an Al alloy (Al—Cu system, Al—Ti system), and are formed by a thin film forming method such as a vapor deposition method, a sputtering method, or a CVD method. An electrode thickness of about 0.1 μm to 0.5 μm is suitable for obtaining characteristics as a SAW device.
[0027]
In addition, the electrode of the SAW device according to the present invention and the SAW propagation part on the piezoelectric substrate are formed with a film thickness of 150 μm or more with Si, SiO2, SiN, and Al2O3 as a protective film, thereby preventing current from flowing due to conductive foreign matter and improving power resistance. You can go.
[0028]
In addition, this invention is not limited to said embodiment, A various change does not interfere in the range which does not deviate from the summary of this invention.
[0029]
【Example】
Next, an example in which a ladder-type surface acoustic wave filter according to the present invention is prototyped will be described.
A fine electrode pattern made of Al (98 wt%)-Cu (2 wt%) was formed on a 42 ° Y-cut LiTaO ₃ single crystal substrate. For pattern production, photolithography was performed using a reduction projection exposure machine (stepper) and a RIE (Reactive Ion Etching) apparatus. First, the substrate material was subjected to ultrasonic cleaning with acetone / IPA or the like to remove organic components.
[0030]
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.
[0031]
Next, a resist was spin-coated to a thickness of about 0.5 μm, and desired patterning was performed by a reduction projection exposure apparatus (stepper). The stepper requires a reticle as an original patterning plate. This is because the image is reduced to 1/5 by the optical system of the stepper itself, and may be 5 times the size of the actual pattern. For this reason, on the contrary, a resolution five times higher than that of the conventional contact aligner can be obtained.
[0032]
Next, an unnecessary portion of the resist 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.
[0033]
Thereafter, a protective film is produced. A film of SiO 2 was formed by a sputtering apparatus, and then a resist pattern was formed by photolithography, and a wire bonding window opening was etched by an RIE apparatus or the like to complete a protective film pattern.
[0034]
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 SMD package with a resin mainly composed of Si resin. Thereafter, a temperature of about 160 ° C. was applied to dry and cure. The SMD package used was a 3 mm square laminate structure.
[0035]
Next, a 30 μmφ Au wire was ball-bonded on the pad portion of the SMD package and the Al 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 electrode patterns on the chip were separated and wired, and bonded to the ground pads on the package by Au ball bonding.
[0036]
The surface acoustic wave resonator constituting the ladder type surface acoustic wave filter has 40 to 120 pairs of comb-shaped electrodes (1/2 of the number), and an intersection width of 10 to 30λ (λ is the wavelength of the surface acoustic wave). The wavelength λ of the surface acoustic wave is different between the series and the parallel, but is approximately 2 μm. Here, the number of reflective electrodes is 20 for both the series resonator and the parallel resonator.
[0037]
Examples of filter configurations are as shown in FIGS. FIG. 1 is an example of using a 2.5-stage T type composed of three series resonators and two parallel resonators, and using an IDT in which the capacity of the present invention is added to the central series resonator of three stages. is there. FIG. 2 shows an example in which an IDT electrode having a capacitance of the present invention added to the first and third stages of a three-stage series resonator is used. FIG. 3 shows an example using a comb-like electrode in which a capacitor is added to the parallel resonator.
[0038]
5, 6, and 7 are examples of frequency characteristics when a filter is manufactured using the present invention. FIG. 5 shows the case where the surface acoustic wave resonator of the present invention is used as a part of the series resonator, and FIG. 6 shows the case where the surface acoustic wave resonator of the present invention is used as a part of the parallel resonator. FIG. 7 shows a case where the surface acoustic wave resonator of the present invention is used as part of the series resonator and the parallel resonator. 10 and 11 show actual measurement results, FIG. 10 is a graph of the frequency characteristic measurement range of 1680 MHz to 2080 MHz, and FIG. 11 is a graph of the frequency characteristic measurement range of 0 MHz to 6 GHz. The measurement was performed using a network analyzer to measure the S21 parameter, which is a pass characteristic.
[0039]
In the surface acoustic wave filter of the present invention, Δf of the resonator characteristics can be changed relatively easily by adding an additional capacitor composed of the surface acoustic wave resonator in parallel to the surface acoustic wave resonator.
[0040]
FIG. 8 is a graph plotting the resonator characteristics when the additional capacitance is changed in the surface acoustic wave resonator of the present invention. The horizontal axis indicates the frequency, and the vertical axis indicates the absolute value of the impedance. When the size of the additional capacitance is increased, the antiresonance frequency 7 is decreased, and the difference Δf between the resonance frequency 8 and the antiresonance frequency 7 is small and steep. FIG. 9 is a graph showing a change in the additional capacitance depending on the number of comb-shaped electrode pairs.
[0041]
In this case, the electrode line width Lt is set to 0.7 μm so that unnecessary spurious due to the resonance frequency of the surface acoustic wave resonator added in the pass band does not appear. In this case, spurious appears around 1600 MHz that does not affect the passband. The line width of the IDT electrode constituting the ladder type surface acoustic wave filter was approximately 0.5 μm.
[0042]
Further, the crossing width of the IDT electrode of the additional capacitor was set to W = 10λ (λ is the wavelength of the surface acoustic wave: approximately 2 μm). As shown in the graph of FIG. 9, the value of the additional capacitance Cp increases substantially linearly with the number of IDT electrode pairs Nt of the added surface acoustic wave resonator.
[0043]
For this reason, for example, if an additional capacitance of 0.3 pF is required by calculation, an IDT for frequency characteristic control with Nt = 25 pairs, W = 10λ, and 10 or more reflectors is used as an original ladder-type surface acoustic wave filter. It may be arranged in parallel with the surface acoustic wave resonator constituting the.
[0044]
10 and 11 show the filter characteristics produced using the surface acoustic wave filter with additional capacitance of the present invention. In this case, as shown in FIG. 1, when an additional capacitor is placed in parallel in the second-stage series resonator and designed so that the wirings do not cross each other, the passband is as shown in the frequency characteristics of FIGS. The shoulder on the high-frequency side becomes steep.
[0045]
【The invention's effect】
As described above, according to the surface acoustic wave filter of the present invention, a ladder-type or lattice-type elastic surface having passband characteristics excellent in steepness without changing the piezoelectric substrate, the cut orientation and the electrode film thickness. A wave filter can be provided.
[0046]
In addition, by adding IDT electrodes for frequency control in parallel, the applied power is dispersed, resulting in excellent power durability, and in addition, electrode destruction due to static electricity generated by the pyroelectric effect during electrode patterning, Since the applied power is dispersed, a highly reliable surface acoustic wave filter that can be prevented as much as possible can be provided.
[0047]
Further, since the resonance frequency of the frequency characteristic control IDT electrode is outside the pass band, the out-of-band attenuation can be improved.
[0048]
Furthermore, by arranging the IDT electrode for frequency characteristic control on the extension line of the surface acoustic wave propagation path of the surface acoustic wave resonator constituting the main circuit, the electrode area on the substrate can be kept small. A surface acoustic wave filter suitable for mass production can be provided.
[Brief description of the drawings]
FIG. 1 is a plan view schematically showing an embodiment of an electrode structure of a surface acoustic wave filter having additional capacitance according to the present invention.
FIG. 2 is a plan view schematically showing an embodiment of an electrode structure of a surface acoustic wave filter having additional capacitance according to the present invention.
FIG. 3 is a plan view schematically showing an embodiment of an electrode structure of a surface acoustic wave filter having additional capacitance according to the present invention.
FIG. 4 is a plan view schematically showing an electrode structure of a conventional surface acoustic wave filter.
FIG. 5 is a diagram illustrating characteristics when a surface acoustic wave resonator having an additional capacitance according to the present invention is used on the series resonator side.
FIG. 6 is a diagram illustrating characteristics when a surface acoustic wave resonator having an additional capacitance according to the present invention is used on the parallel resonator side.
FIG. 7 is a diagram illustrating characteristics when the surface acoustic wave resonator having an additional capacitance according to the present invention is used on both the series resonator side and the parallel resonator side.
FIG. 8 is a frequency characteristic diagram of impedance when the additional capacitor of the present invention is added in parallel to the surface acoustic wave resonator.
FIG. 9 is a graph showing a change in additional capacitance value according to the IDT logarithm when the additional capacitance of the present invention is added in parallel to the surface acoustic wave resonator.
FIG. 10 is a diagram (1680 MHz to 2080 MHz) showing frequency characteristics of the surface acoustic wave filter when the additional capacitor of the present invention is added in parallel to the surface acoustic wave resonator.
FIG. 11 is a diagram showing the frequency characteristics of a surface acoustic wave filter when an additional capacitor of the present invention is added in parallel to a surface acoustic wave resonator (0 MHz to 6000 MHz).
FIG. 12 is an electrical equivalent circuit diagram when the additional capacitor of the present invention is added in parallel to the surface acoustic wave resonator.
FIG. 13 is a graph for explaining the number of reflectors disposed at both ends of the surface acoustic wave resonator of the present invention and the amount of surface acoustic wave leakage.
[Explanation of symbols]
1: piezoelectric substrate 2: frequency characteristic control IDT electrode 3: IDT electrode 4: reflection electrode 5: ground electrode 6a of surface acoustic wave filter: input electrode 6b of surface acoustic wave filter: output electrode 7 of surface acoustic wave filter : Antiresonance point 8: Resonance point C1, Co, Cp: Capacitor L1: Inductor R1: Series resonator R2: Parallel resonator

Claims (2)

42 ° a Y-cut X-propagation LiTaO ₃ SAW filter connected on a piezoelectric substrate made of single crystal a plurality of IDT electrodes on the ladder circuits of,
An additional IDT electrode is connected in parallel to at least one IDT electrode, and reflector electrodes are disposed on both sides of the additional IDT electrode,
When the electrode thickness of the additional IDT electrode and the reflector electrode is H (μm) and the wavelength of the surface acoustic wave is λ (μm), the relationship of H / λ = 0.07 to 0.11 is satisfied.
The number of the reflector electrodes is 10 or more,
A surface acoustic wave filter , wherein the additional IDT electrode is disposed on an extension line of a propagation path of the surface acoustic wave generated from the IDT electrode connected in parallel with the additional IDT electrode . The surface acoustic wave filter according to claim 1, wherein a resonance frequency of the additional IDT electrode is outside a pass band.

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