JP2001156586A - Ladder type surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a means that enhances the increase in the bandwidth and the attenuation in the vicinity of the pass band f a flip chip ladder type surface acoustic wave filter. SOLUTION: In the flip chip ladder type surface acoustic wave filer where surface acoustic wave resonators are alternately arranged as a parallel arm, a series arm and a parallel arm on a major side of a piezoelectric substrate, a lead electrode of the parallel arm toward the ground side is connected to the ground via one pad electrode.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field 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 having improved pass band width and attenuation near the pass band.

[0002]

2. Description of the Related Art In recent years, surface acoustic wave filters have been widely used in the field of communications, and have been used particularly in portable telephones and the like because of their excellent characteristics such as high performance, small size, and mass productivity.
Among surface acoustic wave filters (hereinafter, referred to as SAW filters), a ladder-type SAW filter having a feature of a steep attenuation gradient and a small insertion loss has been widely used in an RF stage of a cellular type mobile phone. I have. As is well known, a ladder-type SAW filter is a one-port pair surface acoustic wave resonator (hereinafter, referred to as a SAW resonator) on the same piezoelectric substrate.
This is a resonator type SAW filter in which a plurality is arranged alternately in parallel, series, and parallel to form a ladder structure.

FIG. 4A is a plan view showing a configuration of a SAW resonator used in a ladder type SAW filter.
ID along the propagation direction of the surface wave on the main surface of the piezoelectric substrate 21
A grating reflector (hereinafter, referred to as a T electrode 22)
(Reflectors) 23a and 23b are arranged to form a SAW resonator. The IDT electrode 22 is composed of a pair of comb-shaped electrodes each having a plurality of electrode fingers interposed between each other, and one comb-shaped electrode and the other comb-shaped electrode constitute a one-terminal SAW resonator. . Here, the symbol W indicates the intersection width of the IDT electrodes 22.

In general, as shown in FIG. 4B, a ladder-type circuit has a ladder-type filter basic section (hereinafter, referred to as a basic section) in which a parallel arm Yp and a series arm Zs are both composed of one resonator. , So that the impedances match each other
Sections (four sections in the example in the figure) are cascade-connected. According to the image parameter theory, by setting the anti-resonance frequency fa of the parallel arm (Yp) of the basic section and the resonance frequency fs of the series arm (Zs) to be substantially equal to each other, a bandpass filter having the center frequency as the center frequency is set. Are formed, and an attenuation pole is formed by the resonance frequency of the parallel arm (Yp) and the anti-resonance frequency of the series arm (Zs).

As shown in FIG. 4B, in a ladder-type circuit in which four basic sections are connected in cascade, a circuit in which two resonators having an impedance Zs of a series arm are connected in series has a known impedance of 2Zs. A resonator in which two resonators having the admittance Yp of the parallel arm are connected in parallel is equivalent to one resonator, and one resonator having the admittance 2Yp is connected in parallel.
It is equivalent to three resonators. 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. 4C. That is, admittance
Parallel arm with Yp, series arm with impedance 2Zs, parallel arm with admittance 2Yp, impedance
It is converted into a ladder-type circuit consisting of a 2Zs series arm 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 an admittance twice that of the SAW resonators 24 and 28.

FIG. 5A is a schematic view 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) is formed in a concave portion of a ceramic package 31. Name)
The chip 32 is accommodated, and the bottom surface of the chip 32 and the bottom surface of the concave portion are bonded and fixed using an adhesive 33. Furthermore, SA
After connecting the lead electrodes provided on the main surface of the W filter chip 32 and the terminal electrodes 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. To complete a ladder-type SAW filter.

Here, for example, as shown in FIG. 5 (b), it is known that a wider band can be realized by connecting an inductance L to each of the SAW resonators 24, 26, 28 in parallel arms in series. 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 a lower frequency side, so that the interval between the resonance and the anti-resonance frequency is widened, and the pass band of the filter is wide. Will be transformed.

[0008]

For example, Japanese Patent Laid-Open No. 5-1
83380 proposes a means for increasing the band of a ladder-type SAW filter by using a bonding wire 34 for connecting the SAW filter chip 32 to an external terminal as an inductance L. However, according to Japanese Patent Application Laid-Open No. 5-183380, a ladder type SA
To increase the bandwidth of the W filter, the inductance L needs to be about 4 nH, but in the range where wire bonding is normally used, only about 1 nH is obtained at most. Since such a wire is required, it has been considered unrealistic from the viewpoint of miniaturization. Recently, in order to further reduce the size of the SAW filter, as shown in a schematic cross-sectional view of the ladder-type SAW filter shown in FIG.
The upper pad electrode and the package terminal electrodes 44, 44,.
Are connected to the SAW filter by using a flip-chip bonding technique for connecting the SAW filters via the metal bumps 43, 43,... In connection with this, instead of forming an inductance by a bonding wire, for example,
As disclosed in Japanese Patent Application No. 0-93376, there has been proposed a method of forming a strip line on a piezoelectric substrate to realize a wideband ladder-type SAW filter. However, it is not rare that the same SAW filter is used for different applications, and the inductance value may need to be changed depending on the application. In the case of a strip line, however, the length of the strip line is finely adjusted to obtain a desired inductance. There is a problem that the value cannot be obtained. This strip line is generally formed by the same process as the SAW resonator. Therefore, if the inductance needs to be changed, a photolithography mask including not only the strip line but also the SAW resonator is re-manufactured, and the cost becomes high. SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problem, and has as its object to provide a ladder-type SAW filter having a wider band and an improved attenuation near a passband at a low cost.

[0009]

In order to achieve the above object, a flip-chip ladder type surface acoustic wave filter according to the present invention comprises an IDT electrode and grating reflectors disposed on both sides thereof. In a ladder type surface acoustic wave filter in which the provided surface acoustic wave resonators are sequentially arranged on the main surface of the piezoelectric substrate as a parallel arm, a serial arm, and a parallel arm, the grounding of the surface acoustic wave resonators arranged in the parallel arms is performed. A ladder-type surface acoustic wave filter characterized in that the lead electrodes on the side extend long and several ground pad electrodes are provided on each of the lead electrodes. The invention according to claim 2 is characterized in that a metal bump is formed on at least one of the ground pad electrodes, and an electrical connection is made via the bump.
It is a ladder type surface acoustic wave filter of the above.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on an embodiment 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,
FIG. 2B is a schematic cross-sectional view of a ladder-type SAW filter housed in a ceramic package and subjected to flip chip bonding. FIG. 1A shows a SAW resonator having an IDT electrode on the main surface of a piezoelectric substrate 1 and reflectors arranged on both sides thereof, and a parallel arm SAW resonator 2 and a series arm SAW resonator are alternately arranged from an input side IN. This is a five-element ladder-type SAW filter 11 that is configured by arranging a connection circuit with a child 3, a parallel arm SAW resonator 4, a series arm SAW resonator 5, a parallel arm SAW resonator 6, and a connection circuit. SAW resonators 2-6
Is constituted by a pair of comb-shaped electrodes each having a plurality of electrode fingers interposed between each other,
One comb electrode of the IDT electrode and the other comb electrode constitute a one-port SAW resonator. The SAW filter chip 11 shown in FIG. 1A is housed in the recess of the ceramic package 12 with the pattern forming surface facing down,
Pad electrodes Q1, Q provided on SAW filter chip 11
2 and Q _E and terminal electrodes 14, 14...
3 and are electrically connected.

As shown in FIG. 1A, the characteristic of the present invention is that the grounded lead electrodes L1, L2 and L3 of the parallel arm SAW resonators 2, 4 and 6 of the ladder type SAW filter are connected to the respective resonators. with arranging the pad electrodes Q3, Q4 and Q5 in the vicinity, 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. In the example of FIG. 1 (a) forming a metal bump on a point of the pad electrode Q _E shows an example of a filter subjected to ground. The lead electrodes L1, L2, and L3 each exhibit inductance at a high frequency of 800 MHz band, and the magnitude is proportional to the length to the ground point. That is, FIG.
If grounded at one point of the pad electrode Q _E as shown in (a) becomes to exhibit a large value with the value of inductance is directly connected to the parallel arm is approximately the same. on the other hand,
Inductance is also formed in the conductor between the internal terminal electrode and the external terminal electrode of the package 12, and each of the conductors of the input terminal, the ground terminal, and the output terminal has an inductance.
Lp _I , Lp _E and Lp _O will be exhibited. Therefore, 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.

Since the ground-side lead electrodes L1, L2 and L3 extend long and a plurality of pad electrodes are provided in the middle thereof, each of the lead electrodes L1, L2 and L3 is directly connected to the SAW resonators 2, 4 and 6 of the parallel arm. The inductance can be set appropriately. That is, the point furthest most ground side lead electrode of the parallel arm SAW resonators from the resonators, that is, when grounded pad electrodes Q _E, inductance to be connected in series becomes greatest SAW resonator, the parallel arm SAW The resonance frequency of the resonator is shifted to a lower frequency side, and the bandwidth of the filter is increased.

FIG. 2 shows a center frequency of 851 MHz and a bandwidth of 38 MHz for an N-CDMA RF filter.
IDT of parallel arm SAW resonators 2 and 6 on both sides using aO ₃
The number of electrode pairs is 86, the number of reflectors is 74 each, the intersection width is 16.8 λ (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, and the number of reflectors is As shown in FIG. 1A, the number of the IDT electrodes is 34, the number of the IDT electrodes of the SAW resonators 3 and 5 is 56, the number of the reflectors is 106, and the width of the intersection is 17.2λ. on the ground side lead electrode of the SAW resonator 2,4,6 extend to one pad electrodes Q _E, characteristic of simulating filters grounded via the metal bump to the electrode is curved alpha. On the other hand, the curve β is a filter characteristic for comparison. The circuit configuration, the constant value of the SAW resonator, etc. are the same as those of the curve α, but the parallel arm SA
Pad electrodes Q3, Q4, Q near W resonators 2, 4, 6
5 is a characteristic obtained by simulating a filter in which bumps are provided on the filter No. 5 and grounded. As apparent from FIG. 2, towards the case of a ground pad electrode Q _E together with passband low frequency side is widened, it can be seen that the damping characteristics of the passband vicinity is improved.

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-side lead electrode of the parallel arm SAW resonator is changed. . That is, the curve α extends to the pad electrode Q _E having a ground side lead electrode on both sides of the parallel arm of the SAW resonator 2,6 the bottom center in the figure, as shown in FIG. 1 (a), the The pad is grounded via a metal bump and the ground of the SAW resonator 4 arranged in the central parallel arm is grounded by a pad electrode Q4 provided near the resonator.
This is a filter characteristic obtained by measuring a filter. On the other hand, the curve β
Is the filter characteristic shown for comparison, and the SAW of the parallel arm
This is a measurement example when the ground-side lead electrodes of the resonators 2, 4, and 6 are grounded by pad electrodes Q3, Q4, and Q5 provided near the respective resonators. From the filter characteristics of FIG.
It has been found that the attenuation characteristic near the passband changes depending on the ground position of the SAW resonator of the parallel arm. When the ground-side lead electrodes of the parallel arm SAW resonators are grounded in the vicinity of the resonators, respectively, the inductance of the lead electrodes is reduced, so that the amount of attenuation is improved on the lower passband side. On the other hand, if the ground lead electrodes of the parallel arm SAW resonators on both sides are lengthened, they are grounded at one point, and the SAW resonator at the center of the parallel arm is grounded by a pad electrode provided in the vicinity thereof. It was found that the bandwidth was slightly widened and the attenuation on the high frequency side near the passband could be improved. In other words, the required characteristics can be obtained without changing the design of the electrode pattern on the piezoelectric substrate only by selecting the pad electrode on which the bump is formed in accordance with the application.

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. In addition, 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, and lithium tetraborate. In the above example, the case where a plurality of pad electrodes are provided with a lead electrode in a stripline shape has been described, but 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 so that bumps can be formed at arbitrary positions.

[0016]

According to the present invention, as described above, the present invention contributes to broadening the pass band and improves the attenuation near the pass band by the number of ground points. Filter characteristics can be realized. When the flip-chip ladder type SAW filter according to the present invention is used as an N-CDMA RF filter, a mobile phone having excellent characteristics can be 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 shows a curve α of a flip chip ladder type SA of the present invention.
FIG. 4 is a diagram showing characteristics of a W filter obtained by simulation;
Curve β is a conventional flip chip ladder type S for comparison.
FIG. 4 is a diagram illustrating characteristics of an AW filter.

FIG. 3 shows the characteristics of a ladder-type SAW filter in which the SAW resonators of the parallel arms on both sides are grounded at one point, and the curve β shows the filter characteristics for comparison, and the filter characteristics are grounded in the vicinity of the SAW resonators. It is.

4A is a plan view showing a 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 cascaded, FIG. 4C is a series arm, and FIG. Combining the SAW resonator of the arm, 5 element ladder type S
This is an AW filter.

FIG. 5A is a cross-sectional view of a conventional ladder-type SAW filter in which a SAW filter chip is housed in a recess of a package and is electrically connected to a terminal electrode by using a bonding wire, and FIG. Circuit.

FIG. 6 shows a ladder type S of a flip chip bonding method.
It is a schematic diagram which shows the cross section of an AW filter.

[Explanation of symbols]

1. Piezoelectric substrate 2, 3, 4, 5, 6, SAW resonator 11. Flip chip ladder type SAW filter (SA
W filter chip) L1, L2, L3 ·· lead electrodes Q1, Q2, Q3, Q4, Q5, Q E ·· pad electrodes L _pI, L _pE, the inductance of the electrode conductor of L _pO · Package 12 ... package 13 ..Metal bumps 14.Terminal electrodes 15.Metal covers

Claims (2)

[Claims] 1. A ladder-type elastic surface in which a surface acoustic wave resonator having an IDT electrode and grating reflectors arranged on both sides thereof is alternately arranged on a main surface of a piezoelectric substrate in parallel arms, series arms, and parallel arms. A ladder-type surface acoustic wave, wherein a ground-side lead electrode of the surface acoustic wave resonator disposed in the parallel arm extends long and a plurality of grounding pad electrodes are provided in each of the wave filters. filter. 2. A ladder type surface acoustic wave filter according to claim 1, wherein a metal bump is formed on at least one of said ground pad electrodes, and electrical connection is made via said bump.