|Numéro de publication||USRE41813 E1|
|Type de publication||Octroi|
|Numéro de demande||US 12/276,318|
|Date de publication||12 oct. 2010|
|Date de dépôt||22 nov. 2008|
|Date de priorité||20 oct. 2003|
|État de paiement des frais||Payé|
|Autre référence de publication||CN1610254A, CN100474766C, DE102004050507A1, DE102004050507B4, US7211931, US20050099094|
|Numéro de publication||12276318, 276318, US RE41813 E1, US RE41813E1, US-E1-RE41813, USRE41813 E1, USRE41813E1|
|Inventeurs||Tokihiro Nishihara, Tsuyoshi Yokoyama, Takeshi Sakashita, Masafumi Iwaki, Tsutomu Miyashita|
|Cessionnaire d'origine||Taiyo Yuden Co., Ltd., Fujitsu Media Devices Limited|
|Exporter la citation||BiBTeX, EndNote, RefMan|
|Citations de brevets (20), Classifications (22), Événements juridiques (9)|
|Liens externes: USPTO, Cession USPTO, Espacenet|
1. Field of the Invention
This invention generally relates to piezoelectric thin-film resonator and a filter using the same.
2. Description of the Related Art
Wireless devices as represented by mobile telephones have spread rapidly, and there has been an increasing demand for a downsized and lightweight resonator and a filter equipped with the same. A dielectric substance and a surface acoustic wave have been used extensively so far; however, the piezoelectric thin-film resonator and the filter equipped with the same have excellent high frequency characteristics, can be downsized, and can be incorporated into a monolithic circuit. Therefore, the piezoelectric thin-film resonator and the filter using the same are attracting attention.
The piezoelectric thin-film resonator may be categorized into FBAR (Film Bulk Acoustic Resonator) type and SMR (Solidly Mounted Resonator) type. The FBAR type includes main components on a substrate from the top, namely, an upper electrode, a piezoelectric film, and a lower electrode. There is a cavity below the lower electrode that is overlapped with the upper electrode through the piezoelectric film. The cavity is defined by wet etching a sacrifice layer on the surface of the silicon substrate, wet or dry etching from the backside of the silicon substrate, or the like. In the present description, a membrane is defined as a film-laminated structure that is located above the cavity and a main component composed of the lower electrode, piezoelectric film and the upper electrode. The SMR type employs an acoustic reflector instead of the cavity, the acoustic reflector being composed of films having high and low acoustic impedances alternately laminated with a film thickness of λ/4 where λ is a wavelength of an elastic wave. When a high-frequency electric signal is applied across the upper electrode and the lower electrode, an elastic wave is excited inside the piezoelectric film sandwiched between the upper electrode and the lower electrode, due to the inverse piezoelectric effect. Meanwhile, a distortion generated by the elastic wave is converted into an electric signal due to piezoelectric effect. The elastic wave is totally reflected by the surfaces of the upper and lower electrodes that respectively interface with air, and it is thus converted into a thickness-extensional wave having a main displacement in the thickness direction. In the above-mentioned structure, a resonance occurs at frequencies at which the total thickness H of the membrane is equal to integer multiples (n times) of half the wavelength of the elastic wave. When the propagation velocity, which depends on materials, is denoted as V, the resonance frequency F is described as F=nV/2H. The resonator and the filter having desired frequency characteristics can be produced by utilizing the resonance and controlling the resonance frequency with the film thickness.
Materials for the electrodes may, for example, be aluminum (Al), copper (Cu), molybdic (Mo), tungsten (W), tantalum (Ta), platinum (Pt), rhodium (Ru), or iridium (Ir). Materials for piezoelectric films may, for example, be aluminum nitride (AlN), zinc oxide (ZnO), lead zirconate titanate (PZT), or lead titanate (PbTiO3). The substrate may be made of silicon, glass, or the like.
However, in addition to the thickness-extensional wave, the above-mentioned piezoelectric thin-film resonator has undesired waves of the lateral mode that are propagated in parallel with the electrode surface, and are reflected by the interfaces or an edge of the cavity. This adversely generates an unnecessary spurious component in the impedance characteristics of the resonator, or a ripple in the passband of the filter. This causes a problem in an application. In order to suppress such adverse affects caused by the lateral mode wave, U.S. Pat. No. 6,150,703 (hereinafter referred to as Document 1) and U.S. Pat. No. 6,215,375 (hereinafter referred to as Document 2) disclose piezoelectric thin-film resonators having electrodes including non-square and irregular polygons in which any two sides are not parallel. In the proposed piezoelectric thin-film resonators, the lateral mode waves reflected by any points are reflected and travel in different directions from the previous directions. Thus, the lateral mode waves do not resonate, so that the above-mentioned problem can be solved effectively. In addition, in order to solve a similar problem, Japanese Patent Application Publication No. 2003-133892 (hereinafter referred to as Document 3) discloses a piezoelectric thin-film resonator having an upper electrode of elliptical shape. The upper electrode satisfies 1.9<a/b<5.0, where a is the main axis of the elliptical shape, and b is the sub axis thereof.
The structures and configurations of Documents 1, 2 and 3 are certainly effective in solving the above-mentioned problems. However, the proposed structures and configurations degrade the strength of the membrane or the productivity of the cavity to the contrary. This will be described below. The thickness of the membrane, which depends on the sound speed of the material, is as very thin as approximately 0.5 to 3 μm in a wireless system having a frequency range of 900 MHz to 5 GHz. An unexpected external force easily damages the membrane, and it is thus important to consider the technique to improve the strength.
One solution is to reduce the damage of the membrane caused by internal stress by reducing the internal stress of each film at the time of forming the film. However, the inventors' study shows that piezoelectricity is improved when compression stress is exerted on the piezoelectric film, and a resonance characteristic having a large electromechanical coupling coefficient (K2) is obtainable. From this viewpoint, the membrane having compression stress is very effective if a technique to achieve a desired strength of the membrane is available. One of the effective methods is to design the membrane so that stress is evenly applied to the membrane or the membrane is not damaged easily by the same internal stress. Unfortunately, any one of Documents 1, 2, and 3 has a structurally unbalanced symmetry, and the force applied to the membrane is not equal. Thus, the membrane is easily distorted and damaged. This results in a serious problem that resonance characteristics and filter characteristics show large irregularity.
Preferably, the cavity has the same shape as that of the region in which the upper electrode overlaps with the lower electrode, and has a similar size to that of the region. If the size of the cavity is much bigger than that of the overlapping region, the membrane will be easily damaged. Thus, it is not recommended. In addition, the productivity of the cavities disclosed in Documents 1 through 3 is not good. The cavities described in Documents 1 and 2 have corners. The cavity described in Document 3 has an elliptical shape with a ratio a/b as large as 1.9<a/b<5.0 where the length of the main axis is denoted as a and that of the sub axis is denoted as b. That is, the desired shape of the cavity is not obtainable because the etching velocity is low at the corners of the cavity. The lower electrode disclosed in Document 3 has a considerably large size, as compared to that of the upper electrode. This results in stray capacitance between the overlapping extensions of the upper electrode and the lower electrode, and degrades the electromechanical coupling coefficient (K2).
The present invention has been made in view of the above circumstances and provides a piezoelectric thin-film resonator and a filter using the same.
More specifically, the present invention provides a piezoelectric thin-film resonator and a filter equipped with the same that show little irregularity in characteristics, by employing a structure that makes it possible to suppress the adverse affects caused by the lateral mode waves and to achieve a sufficient strength of the membrane and excellent productivity of the cavity.
Another object of the present invention is to provide a piezoelectric thin-film resonator and a filter equipped with the same having a large electromechanical coupling coefficient (K2) by the use of a film having a desired compression stress.
According to an aspect of the present invention, there is provided a piezoelectric thin-film resonator including a substrate, a lower electrode arranged on the substrate, a piezoelectric film arranged on the lower electrode, and an upper electrode arranged on the piezoelectric film, in which a region in which the upper electrode overlaps with the lower electrode through the piezoelectric film has an elliptical shape, and 1<a/b<1.9 is satisfied, where a is a main axis of the elliptical shape, and b is a sub axis thereof.
A cavity may be formed in the substrate and located below the region having the elliptical shape.
According to another aspect of the present invention, there is provided a piezoelectric thin-film resonator comprising a substrate, a lower electrode arranged on the substrate, a piezoelectric film arranged on the lower electrode, and an upper electrode arranged on the piezoelectric film. A cavity is provided in the substrate and is located under the lower electrode in a region in which the upper electrode overlaps with the lower electrode through the piezoelectric film. A membrane that includes the upper electrode and the lower electrode is formed above the cavity and is curved outwards. The membrane has a maximum height that is measured from a surface of the substrate and is at least 1.5 times the thickness of the membrane.
According to a further aspect of the present invention, there is provided a filter with any of the above-mentioned piezoelectric thin-film resonators.
Preferred embodiments of the present invention will be described in detail with reference to the following figures, wherein:
A description will now be given, with reference to the accompanying drawings, of embodiments of the present invention.
The technical merits of the present invention may be obtained by materials other than the above-mentioned materials of the substrate 10, the upper and lower electrodes 11 and 13, and the piezoelectric film 12. For example, the materials disclosed in Documents 1, 2, and 3 may be used. In addition, the cavity 15 as shown in
As shown in
The inventors found out a problem caused during the process of forming the cavity 15 when the length ratio of a/b is large and the elliptical shape is greatly curved.
In the above-mentioned range of 1<a/b<1.9, it is essential that a ripple caused by a lateral mode is suppressed to the level that does not pose a problem. Then, four different filters equipped with piezoelectric thin-film resonators are made to evaluate the ripple in a passband. The four filters have the ratios of 1.0 (a circle), 1.2, 1.9, and 4.0 in the region in which the upper electrode 13 overlaps the lower electrode 11 through the piezoelectric film 12. Table 1 shows sizes of the elliptical shape in series-arm and parallel-arm resonators.
A description will now be given of a second embodiment of the present invention. The second embodiment has a specific relationship between the shape of elliptical shape and the size of the membrane 14 used in the first embodiment. The inventors evaluated any influence on characteristics, when altering the ratio of b′/b where b is the length of the sub axis of the elliptical shape in the membrane 14 where the upper electrode 13 overlaps with the lower electrode 11, and b′ is the length of the sub axis of the cavity 15, as shown in
A description will now be given of a third embodiment of the present invention. The third embodiment has a specific relationship between the direction of the current flowing through a piezoelectric thin-film resonator and the axis direction of the elliptical shape in the membrane 14 where the upper electrode 13 overlaps with the lower electrode 11 through the dielectric film 12. The inventors studied the relationship for three piezoelectric thin-film resonators shown in
Type C is larger in both lowest insertion loss and irregularity than those of the types A and B. In terms of warping of the membrane, the types A and B are finely curved in the shape of a dome, while the type C is irregularly distorted like a potato chip. The types A and B are highly symmetric, and get finely curved when compression stress is applied in parallel with the current direction through the membrane. However, the type C is not finely symmetric with respect to the current direction in which stress is applied, and is irregularly curved. This results in affects on the above-mentioned characteristics.
Thus, as shown in
Referring back to
A fourth embodiment of the present invention is focused on the structure of the extensions of the upper electrode 13 and the lower electrode 11 in which the extensions extend outwardly from the elliptical shape in which the upper electrode 13 overlaps with the lower electrode 11 through the dielectric film 12.
The inventors produced the piezoelectric thin-film resonators as shown in
The sub axis of the elliptical shape is substantially parallel to the current direction shown in FIG. 8A. However, even if the main axis of the elliptical shape is substantially parallel to the current direction, the same function and effect as mentioned above are obtainable by arranging the extensions of the upper and lower electrodes so as to have an increasing width outwards from the center of the elliptical shape. Preferably, the extension 21 of the upper electrode 13 and the extension 22 of the lower electrode 11 are formed as shown in FIG. 8A. However, only one of the extensions 21 and 22 may be arranged so that the width becomes larger outwards from the center of the elliptical shape.
A fifth embodiment of the present invention has a structure defined by taking internal stress and resonance characteristic of a film laminate into consideration. The film laminate is composed of the lower electrode 11, the piezoelectric film 12, and the upper electrode 13. The inventors conducted an experiment directed to investigating the affect of the internal stress on the resonance characteristic. The piezoelectric thin-film resonator used in the experiment had the following laminate structure. The lower electrode 13 had a double layer structure of Ru (100 nm)/Cr (50 nm). The piezoelectric film 12 was AlN and 400 nm thick. The upper electrode 13 was made of Ru and 100 nm thick. The elliptical shape of the region defined by overlapping the upper electrode 13 overlaps with the lower electrode 11 had a size such that a=60.2 μm and b=50.2 μm (a/b=1.2). The cavity 15 was a size such that a=66.2 μm and b=55.2 μm (a/b=1.2).
In the case where the film with stress is employed, the membrane is warped after the cavity is provided. Especially, in the case where the film with compression stress is employed, the membrane 14 is curved outwards in the opposite side to the cavity 15 after the cavity 15 is provided, as shown in FIG. 10.
A sixth embodiment of the present invention is a piezoelectric thin-film resonator and a filter device that employs an acoustic reflector substituted for the cavity 15 located below the membrane 14. The acoustic reflector is composed of high and low acoustic impedance films that are alternately laminated by the thickness of λ/4, where λ is a wavelength of an elastic wave.
In accordance with the present invention, even with the structure intended to suppress adverse affects caused by the lateral mode waves, the piezoelectric thin-film resonator is configured so as to obtain a sufficient strength and excellent productivity of the cavity. Thus, the piezoelectric thin-film resonator with less irregularity in characteristics and the filter thereof are obtainable. In addition, by utilizing the film having the desired compression stress, the piezoelectric thin-film resonator with a large electromechanical coupling coefficient (K2) and the filter thereof are obtainable.
The present invention is not limited to the above-mentioned first embodiment, and other embodiments and modifications may be made without departing from the scope of the present invention.
The present invention is based on Japanese Patent Application No. 2003-360054 filed on Oct. 20, 2003, the entire disclosure of which is hereby incorporated by reference.
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|Classification aux États-Unis||310/324, 310/366|
|Classification internationale||H01L41/09, H03H9/13, H01L29/82, H03H9/54, H03H9/17, H03H9/56, H01L41/187, H03H9/58, H03H9/15, H01L41/08|
|Classification coopérative||H03H9/568, H03H9/174, H03H9/02133, H03H9/132, H03H9/564|
|Classification européenne||H03H9/13S, H03H9/56F, H03H9/17A1B, H03H9/02B8J, H03H9/56P1|
|13 mai 2010||AS||Assignment|
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|30 oct. 2012||CC||Certificate of correction|
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