Resonator filter having a recess in insulating material of a multi-layered coupling structure

A resonator filter includes a substrate, a bottom electrode formed on the substrate, a multi-layered coupling structure formed on the bottom electrode, a top electrode formed on the multi-layered coupling structure, a first piezoelectric layer sandwiched in between the bottom electrode and the multi-layered coupling structure, and a second piezoelectric layer sandwiched in between the multi-layered coupling structure and the top electrode. The multi-layered coupling structure includes at least an insulating material.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a resonator filter, and more particularly, to a resonator filter in a stacked crystal filter (hereinafter abbreviated as SCF) arrangement.

2. Description of the Prior Art

In recent years, piezoelectric thin film process is prevalently employed for forming filters and duplexer used in radio frequency (RF) communication systems. The conventional piezoelectric thin film acoustic component can be classified as a thin film bulk acoustic resonator (FBAR) and a solidly mounted resonator (SMR).

A resonator filter in SCF arrangement conventionally includes a top electrode, a middle electrode, a bottom electrode, an upper piezoelectric layer sandwiched in between the top and middle electrodes, and a lower piezoelectric layer sandwiched in between the middle and bottom electrodes. For example, when the top electrode serves as the input electrode, the middle electrode serves as the ground electrode, and the bottom electrode serves as the output electrode, the input electrode receives a signal from an input terminal, and the upper piezoelectric layer then generates a bulk acoustic wave to the lower piezoelectric layer in response to the signal excitation. A resonance is therefore generated between the input electrode and the output electrode, and the output electrode outputs the signal to an output terminal.

For such as a RF filter, insertion loss and bandwidth are the key factors to judge filter performance. Therefore, it is always in need to reduce insertion loss and increase bandwidth.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, a resonator filter is provided. The resonator includes a substrate, a bottom electrode formed on the substrate, a multi-layered coupling structure formed on the bottom electrode, a top electrode formed on the multi-layered coupling structure, a first piezoelectric layer sandwiched in between the bottom electrode and the multi-layered coupling structure, and a second piezoelectric layer sandwiched in between the multi-layered coupling structure and the top electrode. The multi-layered coupling structure includes at least an insulating material.

According to the resonator filter provide by the present invention, the middle ground electrode conventionally formed in between the top electrode and the bottom electrode is replaced by the multi-layered coupling structure including the insulating material. Consequently, waves are confined in the transmission region by the multi-layered coupling structure, and thus insertion loss and transmission loss are efficaciously reduced while bandwidth is increased.

DETAILED DESCRIPTION

Please refer toFIG. 1, which is a schematic drawing illustrating a resonator filter provided by a first preferred embodiment of the present invention. As shown inFIG. 1, the resonator filter10provided by preferred embodiment includes a substrate100, a bottom electrode120formed on the substrate100, a multi-layered coupling structure140formed on the bottom electrode120, a top electrode160formed on the multi-layered coupling structure140. In other words, the multi-layered coupling structure140is arranged in between the bottom electrode120and the top electrode160. More important, a first piezoelectric layer130is sandwiched in between the bottom electrode120and the multi-layered coupling structure140, and a second piezoelectric layer150is sandwiched in between the multi-layered coupling structure140and the top electrode160. More particularly, the multi-layered coupling structure140provided by preferred embodiment is sandwiched in between the first piezoelectric layer130and the second piezoelectric layer150. In accordance with preferred embodiment, the top electrode160further includes an input terminal IN and an output terminal OUT. As shown inFIG. 1, the input terminal IN and the output terminal OUT are physically and electrically isolated from each other. Furthermore, a reflector110is formed in between the bottom electrode120and the substrate100. The reflector110can include aluminum nitride (AlN), aluminum oxide (Al2O3), tungsten (W), or silicon oxide (SiO2), but not limited to this.

The substrate100can be a silicon substrate or a gallium arsenide (GaAs), but not limited to this. The bottom electrode120and the top electrode160include any proper conductive material. The first piezoelectric layer130and the second piezoelectric layer150include zinc oxide (ZnO), aluminum nitride (AlN), zinc sulfide (ZnS) or any other piezo-electric material that can be fabricated as a thin film. As a further example, also ferroelectric ceramics can be used as the piezoelectric material. For example, plumbum titanate (PbTiO3) or plumbum zirconate-titanate (Pb(ZrxTi1-x)O3) and other member of so called lead lanthanum zirconate titanate family can be used.

Please refer toFIG. 1again. A first cavity152and a second cavity154are formed in the second piezoelectric layer150, and the top electrode160(including the input terminal IN and the output terminal OUT) is arranged in between the first cavity152and the second cavity154. The first cavity152and the second cavity154are spaced apart from each other by the second piezoelectric layer150while the second piezoelectric layer150is exposed at a bottom of the first cavity152and a bottom of the second cavity154, respectively, as shown inFIG. 1. More important, the resonator filter10provided by the preferred embodiment further includes a ground terminal170formed in the first cavity152and the second cavity154, respectively. It is noteworthy that a size of the reflector110can be defined by a distance between the first cavity152and the second cavity154according to the preferred embodiment.

Please still refer toFIG. 1. The multi-layered coupling structure140sandwiched in between the first piezoelectric layer130and the second piezoelectric layer150includes at least an insulating material. According to the preferred embodiment, the multi-layered coupling structure140upwardly and sequentially includes at least a first low-impedance layer142a, a second low-impedance layer142b, and a third low-impedance layer142c. A first high-impedance layer144ais sandwiched in between the first low-impedance layer142aand the second low-impedance layer142b, and a second high-impedance layer144bis sandwiched in between the second low-impedance layer142band the third low-impedance layer142c. At least one of the first low-impedance layer142a, the second low-impedance layer142b, and the third low-impedance layer142cincludes the insulating material. In the preferred embodiment, the first low-impedance layer142a, the second low-impedance layer142band the third low-impedance layer142call include the insulating material. For example, the first low-impedance layer142c, the second low-impedance layer142band the third low-impedance layer142ccan include silicon oxide, but not limited to this. The first high-impedance layer144aand the second high-impedance layer144bcan include amorphous silicon, platinum (Pt), molybdenum (Mo), or tantalum pentoxide (Ta2O5), but not limited to this.

Furthermore, the first low-impedance layer142aand the second low-impedance layer142binclude a thickness d1, the third low-impedance layer142cinclude a thickness d2, and the first high-impedance layer144aand the second high-impedance layer144binclude a thickness d3. It is noteworthy that the thickness d1of the first low-impedance layer142aand the second low-impedance layer142bis a half of the thickness d2of the third low-impedance layer142c. For example, when the first low-impedance layer142a, the second low-impedance layer142b, and the third low-impedance layer142cinclude silicon oxide (SiOx), of which a refractive index is 1.45, and the first high-impedance layer144aand the second high-impedance layer144binclude amorphous silicon, of which a refractive index is 3.7, the thickness d1of the first low-impedance layer142aand the second low-impedance layer142bis about 1 micrometer (μm), the thickness d2of the third low-impedance layer142cis about 2 μm, and thickness d3of the first high-impedance layer144aand the second high-impedance layer144bis about 0.12 μm, but not limited to this.

Please still refer toFIG. 1. The resonator filter10provided by the preferred embodiment further includes a first recess146formed in the insulating material of the multi-layered coupling structure140. Particularly, the first recess146is formed in the first low-impedance layer142aor the second low-impedance layer142b. As shown inFIG. 1, the first recess146is preferably formed in the second low-impedance layer142band is filled up with the second high-impedance layer144b. Additionally, the first recess146is preferably formed correspondingly to the first cavity152or the second cavity154, as shown inFIG. 1. Because of the first recess146, the multi-layered coupling structure140obtains an asymmetric configuration.

According to the resonator filter10provided by the preferred embodiment, the multi-layered coupling structure140including the insulating material is sandwiched in between the first piezoelectric layer130and the second piezoelectric layer150. The multi-layered coupling structure140includes at least a low impedance material comprising the aforementioned insulating material and a high-impedance conductive material. It is observed that the waves are confined in transmission region by the multi-layered coupling structure140. Secondly, transmission loss is reduced and distortion is alleviated by the first cavity152and the second cavity154formed in the second piezoelectric layer150. Furthermore, the multi-layered coupling structure140obtains the asymmetric configuration due to the first recess146, which is formed in the second low-impedance layer142bof the multi-layered coupling structure140and correspondingly to the first cavity152or the second cavity154. The asymmetric multi-layered coupling structure140improves coupling efficiency, and thus the resonator filter10of the preferred embodiment can be used in high bandwidth application.

Please refer toFIG. 2, which is a schematic drawing illustrating a resonator filter provided by a second preferred embodiment of the present invention. It is noteworthy that elements the same in both of the first and second preferred embodiments are designated by the same numerals and material for forming elements the same in both of the first and second preferred embodiments can be identical. Therefore such details are omitted hereinafter in the interest of brevity. Furthermore, elements the same in the first and second preferred embodiments can be formed in identical arrangement, and thus those details are also omitted for simplicity. The difference between the first and second preferred embodiments is: the resonator filter10provided by the second preferred embodiment includes a first recess146and a second recess148formed in the insulating material of the multi-layered coupling structure140. The first recess146is formed correspondingly to the first cavity152, and the second recess148is formed correspondingly to the second cavity154. It is noteworthy that though both of the first recess146and the second recess148are formed in the insulating material of the multi-layered coupling structure140, the first recess146and the second recess148are formed indifferent low-impedance layers. As shown inFIG. 2, the first recess146is formed in the second low-impedance layer142band filled up with the second high-impedance layer144bwhile the second recess148is formed in the first low-impedance layer142aand filled up with the first high-impedance layer144aaccording to the preferred embodiment. In other words, the first recess146and the second recess148are asymmetrically formed in the multi-layered coupling structure140. Because the first recess146and the second recess148are formed in different low-impedance layers, the multi-layered coupling structure140of the preferred embodiment still obtains an asymmetric configuration.

Additionally, according to a modification to the preferred embodiment, the first recess146and the second recess148can be formed in the same low-impedance layer in the multi-layered coupling structure140, but includes different depths. Therefore the multi-layered coupling structure140still obtains the asymmetric configuration. According to another modification to the preferred embodiment, the resonator filter10can include more than three recesses and those recesses can be asymmetrically formed in the same or different layers in the multi-layered coupling structure140as long as the multi-layered coupling structure140obtains the asymmetric configuration.

According to the resonator filter10provided by the preferred embodiment, the multi-layered coupling structure140including the insulating material is sandwiched in between the first piezoelectric layer130and the second piezoelectric layer150. The multi-layered coupling structure140includes at least a low impedance material comprising the aforementioned insulating material and a high-impedance conductive material. It is observed that the waves are confined in the transmission region by the multi-layered coupling structure140. Secondly, transmission loss is reduced and distortion is alleviated by the first cavity152and the second cavity154formed in the second piezoelectric layer150. Furthermore, the multi-layered coupling structure140obtains the asymmetric configuration due to the first recess146, which is formed in the second low-impedance layer142band correspondingly to the first cavity152, and the second recess148, which is formed in the first low-impedance layer142aand correspondingly to the second cavity154. The asymmetric multi-layered coupling structure140improves coupling efficiency, and thus the resonator filter10of the preferred embodiment can be used in high bandwidth application.

Please refer to Table 1, which summarizes the propagation loss of the resonator filter10provided by the first and the second preferred embodiments in three transverse electric modes (TE):

According to Table 1, it is observed that when the resonator filter includes two asymmetrically formed recesses, propagation loss of this resonator filter is reduced to be lower than one percent of the propagation loss of a resonator filer including only single one recess. In other words, the resonator filter including two asymmetrically formed recesses efficaciously reduces propagation loss and thus is more preferred in higher bandwidth application.

According to the resonator filter provide by the present invention, the middle ground electrode formed between the top electrode and the bottom electrode according to conventional SCF arrangement is replaced by the multi-layered coupling structure including the insulating material. The multi-layered coupling structure can include one recess, two recesses, or even more than three recesses asymmetrically formed in the insulating material. Consequently, waves are confined in the transmission region by the multi-layered coupling structure, and thus insertion loss and transmission loss are efficaciously reduced. Therefore the resonator filter provided by the present invention can be used in high bandwidth application when the requested bandwidth is higher than 2.5 GHz.