JP4359978B2 - Ladder type surface acoustic wave filter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a ladder-type surface acoustic wave filter, and more particularly to a ladder-type surface acoustic wave filter with improved passband width and attenuation near the passband.
[0002]
[Prior art]
In recent years, surface acoustic wave filters have been widely used in the communication field, and are often used particularly for cellular phones because they have excellent characteristics such as high performance, small size, and mass productivity.
Among surface acoustic wave filters (hereinafter referred to as SAW filters), many ladder-type SAW filters having a steep attenuation slope and a small insertion loss are used in the RF stage of cellular mobile phones. Yes.
As is well known, a ladder-type SAW filter has a ladder-type structure in which a plurality of one-terminal-pair surface acoustic wave resonators (hereinafter referred to as SAW resonators) are alternately arranged in parallel, series, and parallel on the same piezoelectric substrate. It is the comprised resonator type SAW filter.
[0003]
FIG. 4A is a plan view showing the configuration of the SAW resonator used in the ladder-type SAW filter. The IDT electrode 22 is formed on the main surface of the piezoelectric substrate 21 along the propagation direction of the surface wave and on both sides thereof. A SAW resonator is formed by arranging grating reflectors (hereinafter referred to as reflectors) 23a and 23b.
The IDT electrode 22 is composed of a pair of comb-shaped electrodes each having a plurality of electrode fingers that are interleaved with each other, and one comb-shaped electrode and the other comb-shaped electrode constitute a one-terminal pair SAW resonator. . Here, the symbol W indicates the crossing width of the IDT electrode 22.
[0004]
In general, as shown in FIG. 4 (b), a ladder-type circuit has a ladder-type filter basic section (hereinafter, referred to as a basic section) in which the parallel arm Yp and the serial arm Zs are both formed of one resonator as impedances. N sections (4 sections in the example shown in the figure) are connected in cascade. According to the image parameter theory, by setting the anti-resonance frequency fa of the parallel arm (Yp) in the basic section and the resonance frequency fs of the series arm (Zs) to be approximately the same, a bandpass filter having the frequency as the center frequency. And attenuation poles are formed by the resonance frequency of the parallel arm (Yp) and the anti-resonance frequency of the series arm (Zs), respectively.
[0005]
As shown in FIG. 4B, in a ladder circuit in which four basic sections are cascade-connected, a circuit in which two resonators each having a series arm impedance Zs are connected in series has a single impedance of 2Zs. It is equivalent to a resonator, and two resonators having a parallel arm admittance Yp connected in parallel are equivalent to one resonator of admittance 2Yp.
Therefore, the ladder type circuit of the four basic sections shown in FIG. 4B is equivalently converted to the five element ladder type circuit shown in FIG. That is, it is converted into a ladder circuit composed of a parallel arm having admittance Yp, a serial arm having impedance 2Zs, a parallel arm having admittance 2Yp, a serial arm having impedance 2Zs, and a parallel arm having admittance Yp. Therefore, although the SAW resonators 24, 26, and 28 of the parallel arm have the same resonance frequency, the SAW resonator 26 has twice the admittance of the SAW resonators 24 and 28.
[0006]
FIG. 5A is a schematic diagram showing a cross-sectional view of a conventional ladder-type SAW filter. A ladder-type SAW filter (hereinafter referred to as a SAW filter chip when housed in a package) 32 is provided in a recessed portion of the ceramic package 31. And the bottom surface of the chip 32 and the bottom surface of the recessed portion are bonded and fixed using an adhesive 33. Further, after connecting the lead electrode disposed on the main surface of the SAW filter chip 32 and the terminal electrode 35 of the package 31 using bonding wires, a metal lid 36 is attached to the metal flange on the upper surface of the package 31 by means such as resistance welding. A ladder-type SAW filter is completed by air-tight welding using the
[0007]
Here, for example, as shown in FIG. 5B, it is known that a wide band can be realized by connecting an inductance L in series to SAW resonators 24, 26, and 28 of parallel arms. As is well known, when an inductance is connected in series to a SAW resonator, the anti-resonance frequency does not change, but the resonance frequency shifts to the low frequency side, so the interval between resonance and anti-resonance frequency is widened, and the filter has a wide passband. It will become.
[0008]
[Problems to be solved by the invention]
For example, Japanese Patent Laid-Open No. 5-183380 proposes using a bonding wire 34 for connecting the SAW filter chip 32 and an external terminal as an inductance L as means for widening a ladder-type SAW filter. However, according to Japanese Patent Laid-Open No. 5-183380, in order to broaden a ladder-type SAW filter, an inductance L of about 4 nH is required. However, in the range where wire bonding is normally used, an inductance of only about 1 nH is required. In order to obtain an inductance as high as 4 nH, a long wire is required, which is impractical from the viewpoint of miniaturization.
Further, recently, due to the demand for further downsizing of the SAW filter, the pad electrode on the SAW filter chip 42 accommodated in the recessed portion of the ceramic package 41 is shown in FIG. The flip chip bonding technique for connecting the package terminal electrodes 44, 44,... Via the metal bumps 43, 43,. Along with this, instead of forming the inductance by the bonding wire, a method for realizing a broadband ladder-type SAW filter by forming a strip line on a piezoelectric substrate is proposed as disclosed in, for example, JP-A-10-93376. Has been. However, the same SAW filter is often used for different applications, and the inductance value may need to be changed depending on the application. In the case of a stripline, the length is finely adjusted to obtain a desired inductance. There is a problem that the value cannot be obtained. In general, the stripline is formed by the same process as the SAW resonator. Therefore, when the inductance needs to be changed, not only the strip line but also the photolithography mask including the SAW resonator is remanufactured, and the cost becomes high.
The present invention has been made to solve the above problems, and an object of the present invention is to provide a ladder-type SAW filter having a wide band and improved attenuation near the passband at a low cost.
[0009]
[Means for Solving the Problems]
Engaging Lula Zehnder type surface acoustic wave filter of the present invention in order to achieve the above object, the surface acoustic wave resonator and a grating reflectors disposed on both sides of the IDT electrode on the main surface of the piezoelectric substrate In a ladder-type surface acoustic wave filter having a plurality of surface acoustic wave resonators arranged in parallel arms and series arms , the surface acoustic wave resonators are connected to the ground side of the surface acoustic wave resonators arranged in the parallel arms , and A lead electrode group composed of a series connection of the lead electrodes, a first pad electrode connecting one end of the lead electrode group and the IDT and wider than the lead electrode, and the other of the lead electrode group A second pad electrode for grounding which is provided at an end and is wider than the lead electrode; and a third pad electrode which connects the plurality of lead electrodes and is wider than the lead electrode. Specially It shall be the.
Furthermore the present invention, the first, the metal bumps formed on at least one of the second and third pad electrode, you characterized in that the electrical connection through the bumps.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings.
FIG. 1A is a plan view showing a configuration example of a ladder-type SAW filter according to the present invention, and FIG. 1B is a schematic sectional view of a ladder-type SAW filter accommodated in a ceramic package and flip-chip bonded. is there. FIG. 1A shows a SAW resonator including IDT electrodes and reflectors arranged on both sides thereof on the main surface of the piezoelectric substrate 1, and parallel arm SAW resonator 2 and series arm SAW resonance alternately from the input side IN. This is a five-element ladder-type SAW filter 11 configured such that the connection circuit is arranged in a ladder shape with the child 3, the parallel arm SAW resonator 4, the series arm SAW resonator 5, and the parallel arm SAW resonator 6.
The IDT electrodes constituting the SAW resonators 2 to 6 are each constituted by a pair of comb electrodes having a plurality of electrode fingers interleaved with each other, and one comb electrode and the other comb electrode of the IDT electrode are identical. A terminal pair SAW resonator is formed.
The SAW filter chip 11 shown in FIG. 1A is accommodated in the recessed portion of the ceramic package 12 with the pattern forming surface facing downward, and the pad electrodes Q1, Q2, and Q _E provided on the SAW filter chip 11 and the package 12 The terminal electrodes 14, 14... Hermetically penetrating from the outside to the inside are fixed via the metal bumps 13, 13.
[0011]
A feature of the present invention is that, as shown in FIG. 1A, the lead electrodes L1, L2, and L3 on the ground side of the parallel arm SAW resonators 2, 4 and 6 of the ladder-type SAW filter are padded in the vicinity of the respective resonators. with arranging the electrodes Q3, Q4 and Q5, in that it provided a plurality of pad electrodes until the pad electrodes Q _E provided at the center lower portion in the figure in the substrate 1. The example of FIG. 1A shows an example of a filter in which a metal bump is formed at one point of the pad electrode Q _E and grounded.
Each of the lead electrodes L1, L2, and L3 exhibits an inductance at a high frequency of 800 MHz band, and its magnitude is proportional to the length to the ground point. In other words, as shown in FIG. 1A, when grounded at one point of the pad electrode Q _E , the inductance value directly connected to each parallel arm is substantially the same and exhibits a large value.
On the other hand, inductance is formed also in the conductor portion between the internal terminal electrode and the external terminal electrode of the package 12, and the inductance portions Lp _I , Lp _E , and Lp _O are exhibited in the conductor portions of the input terminal, the ground terminal, and the output terminal, respectively. become. Accordingly, the sum of the inductance proportional to the length of the lead electrode to the ground point and the inductance Lp _E of the ground conductor of the package 12 is connected in series to the SAW resonators 2, 4 and 6 of each parallel arm. become.
[0012]
The lead electrodes L1, L2 and L3 on the ground side extend long, and a plurality of pad electrodes are provided in the middle of each, so that the respective inductances directly connected to the SAW resonators 2, 4 and 6 of the parallel arm are appropriately set. It becomes possible to set. That is, when the lead electrode on the ground side of the parallel arm SAW resonator is grounded at the point farthest from each resonator, that is, the pad electrode Q _E , the inductance connected in series to the SAW resonator becomes the largest, and the SAW of the parallel arm The resonance frequency of the resonator is shifted to the low frequency side, and the bandwidth of the filter is increased.
[0013]
FIG. 2 shows an N-CDMA RF filter with a center frequency of 851 MHz, a bandwidth of 38 MHz, LiTaO ₃ as a piezoelectric substrate, and 86 IDT electrode pairs of parallel arm SAW resonators 2 and 6 on both sides. The number of reflectors is 74, the crossing width is 16.8λ (λ is the electrode period of the IDT electrode), the number of IDT electrode pairs of the central parallel arm SAW resonator 4 is 146, the number of reflectors is 34, and the crossing width 20λ, the number of IDT electrode pairs of the SAW resonators 3 and 5 in series arms is 56, the number of reflectors is 106, the crossing width is 17.2λ, and the SAW resonator 2 as shown in FIG. A curve α is a characteristic obtained by simulating a filter in which the ground-side lead electrodes 4 and 6 extend to one pad electrode Q _E and grounded to the electrode via a metal bump.
On the other hand, the curve β is a filter characteristic for comparison, and the circuit configuration, the constant value of the SAW resonator, etc. are the same as the filter of the curve α, but the pad electrode Q3 in the vicinity of the parallel arm SAW resonators 2, 4, 6 is used. , Q4 and Q5 are characteristics obtained by simulating a filter in which bumps are provided and grounded.
As can be seen from FIG. 2, when the pad electrode Q _E is grounded, the lower pass band is widened and the attenuation characteristic near the pass band is improved.
[0014]
FIG. 3 shows the filter characteristics when the circuit configuration and the constant value of the SAW resonator are the same as those shown in FIG. 2, but the ground point of the ground lead electrode of the parallel arm SAW resonator is changed. That is, as shown in FIG. 1A, the curve α extends to the pad electrode Q _E provided at the lower center of the lead electrode on the ground side of the SAW resonators 2 and 6 of the parallel arms on both sides as shown in FIG. The pad is grounded via a metal bump, and the SAW resonator 4 disposed on the central parallel arm is grounded by measuring a ladder type SAW filter grounded by a pad electrode Q4 provided in the vicinity of the resonator. It is.
On the other hand, the curve β is a filter characteristic shown for comparison, and pad electrodes Q3, Q4, and Q5 in which the ground side lead electrodes of the SAW resonators 2, 4, and 6 of the parallel arm are provided in the vicinity of the respective resonators. This is a measurement example when grounded.
From the filter characteristics of FIG. 3, it has been found that the attenuation characteristics in the vicinity of the passband change depending on the ground contact position of the SAW resonator of the parallel arm. When the ground-side lead electrodes of the parallel arm SAW resonator are grounded in the vicinity of the resonator, the inductance due to the lead electrodes is reduced, so that the attenuation is improved on the low-pass side. On the other hand, if the lead electrodes for grounding of the parallel arm SAW resonators on both sides are lengthened and grounded at one point, the center SAW resonator of the parallel arm is grounded by a pad electrode provided in the vicinity thereof. It has been found that the attenuation on the high frequency side near the passband can be improved with a slight increase in bandwidth.
That is, the required characteristics can be obtained without changing the design of the electrode pattern on the piezoelectric substrate simply by selecting the pad electrode for forming the bump according to the application.
[0015]
In the above description, a five-element ladder type SAW filter has been described as an example, but it goes without saying that the present invention can be applied to filters having other numbers of elements.
Although the description has been made using lithium tantalate as the piezoelectric substrate, it is needless to say that the present invention can be applied to other piezoelectric materials such as lithium niobate, langasite, lithium tetraborate and the like.
In the above example, the case where the lead electrode is striplined and a plurality of pad electrodes are provided has been described. However, the lead electrode may be linear and a plurality of pad electrodes may be provided on the lead electrode. The lead electrode itself may have a large width and may be configured such that bumps can be formed at arbitrary positions.
[0016]
【The invention's effect】
Since the present invention is configured as described above, it contributes to widening of the pass band, and it is possible to improve the attenuation near the pass band by the number of grounding points, realizing filter characteristics according to the application. can do. If the flip-chip ladder type SAW filter according to the present invention is used as an N-CDMA RF filter, an effect that a cellular phone having excellent characteristics can be obtained is obtained.
[Brief description of the drawings]
FIG. 1A is a plan view showing a configuration of a flip chip ladder type SAW filter according to the present invention, and FIG. 1B is a schematic sectional view thereof.
FIG. 2 is a graph obtained by simulating the characteristics of the flip chip ladder type SAW filter of the present invention, and a curve β is a diagram showing characteristics of a conventional flip chip ladder type SAW filter for comparison.
3 is a characteristic of a ladder-type SAW filter in which the SAW resonators of the parallel arms on both sides are grounded at one point, and a curve β is a filter characteristic for comparison, each of which is grounded in the vicinity of the SAW resonator. It is.
4A is a plan view showing the configuration of a conventional SAW resonator, FIG. 4B is a ladder-type SAW filter in which four basic sections of a ladder-type SAW filter are cascade-connected, and FIG. 4C is a series arm, in parallel. The SAW resonator of the arm is synthesized to form a 5-element ladder type SAW filter.
5A is a cross-sectional view of a conventional ladder-type SAW filter in which a SAW filter chip is accommodated in a recessed portion of a package and is connected to a terminal electrode using a bonding wire, and FIG. 5B is an electrical equivalent thereof. Circuit.
FIG. 6 is a schematic view showing a cross section of a flip-chip bonding type ladder-type SAW filter.
[Explanation of symbols]
1 .. Piezoelectric substrate 2, 3, 4, 5, 6, .. SAW resonator 11 .. Flip chip ladder type SAW filter (SAW filter chip)
L1, L2, L3 ·· lead electrodes Q1, Q2, Q3, Q4, Q5, Q E ·· pad electrode
L _pI , L _pE , L _{pO ..Inductance} of electrode conductor of package 12 .. Package 13 ..Metal bump 14 ..Terminal electrode 15 ..Metal lid

Claims (2)

A ladder in which a plurality of surface acoustic wave resonators including IDT electrodes and grating reflectors disposed on both sides thereof are provided on the main surface of the piezoelectric substrate, and the surface acoustic wave resonators are disposed in parallel arms and serial arms. Type surface acoustic wave filter,
A lead electrode group that is connected to the ground side of the surface acoustic wave resonator disposed on the parallel arm and includes a plurality of lead electrodes connected in series;
A first pad electrode connecting between one end of the lead electrode group and the IDT and wider than the lead electrode;
A second pad electrode for grounding provided at the other end of the lead electrode group and wider than the lead electrode;
A third pad electrode connecting the plurality of lead electrodes and wider than the lead electrode;
Ladder-type SAW filter is characterized by having a. 2. The ladder type surface acoustic wave filter according to claim 1, wherein a metal bump is formed on at least one of the first, second and third pad electrodes, and electrical connection is made through the bump.

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