ACOUSTIC WAVE RESONATOR USING MULTILAYER TRANSDUCTION MATERIALS WITH LOW/ZERO COUPLING BORDER REGION

The present disclosure relates to a Bulk Acoustic Wave (BAW) resonator, which includes a bottom electrode, a top electrode structure, and a multilayer transduction structure sandwiched therebetween. Herein, the multilayer transduction structure is composed of multiple transduction layers, at least one of which is formed of a ferroelectric material with a box-shape polarization-electric field curve. Each transduction layer includes a transduction border (BO) portion positioned at a periphery of a corresponding transduction layer and a transduction central portion surrounded by the transduction BO portion. A combination of all transduction BO portions forms a transduction BO section of the multilayer transduction structure, and a combination of all transduction central portions forms a transduction central section of the multilayer transduction structure. An electromechanical coupling coefficient of the transduction BO section is less than an electromechanical coupling coefficient of the transduction central section.

FIELD OF THE DISCLOSURE

The present disclosure relates to a Bulk Acoustic Wave (BAW) Resonator, which uses multilayer transduction materials, having low or zero electromechanical coupling at a border region of the BAW resonator, and a process for making the same.

BACKGROUND

Due to their small size, high Q values, and very low insertion losses at microwave frequencies, particularly those above 1.5 Gigahertz (GHz), Bulk Acoustic Wave (BAW) filters have been widely used in many modern wireless applications. For instance, the BAW filters incorporating BAW resonators are the filter of choice for many 3rdGeneration (3G) and 4thGeneration (4G) wireless devices, and are destined to dominate filter applications for 5thGeneration (5G) wireless devices.

One example of a conventional BAW resonator10is illustrated inFIG.1. The BAW resonator10includes a bottom electrode12, a top electrode14, and a piezoelectric layer16(which is sometimes referred to as a transduction layer) sandwiched between the bottom electrode12and the top electrode14. Because of a finite lateral dimension of the BAW resonator10, lateral wave spurious modes may be excited in the BAW resonator10, which results in degradation of the quality factor (Q) of the BAW resonator10. In this regard, a border (BO) ring18is introduced to be included in the BAW resonator10to confine the energy inside the BAW resonator10and prevent the excitation of undesired lateral wave spurious modes. The BO ring18is over a top surface of the top electrode14around a periphery of the top electrode14within what is referred to herein as a BO region20of the BAW resonator10.

Although the BO ring18effectively eliminates the lateral wave spurious modes, the BO ring18will cause an undesired BO spurious resonance mode near the main resonance of the device. The main cause for the excitation of the BO spurious resonance mode is a nonzero electromechanical coupling coefficient Ke20f the piezoelectric layer16within the BO region20(i.e., piezoelectric BO portions16_BO).FIG.2shows a typical 1−|S11|2response (1−|S11|2is equal to the power ratio lost in a resonator) of the BAW resonator10with the BO ring18. The frequency difference between the main resonance and the BO spurious resonance mode depends on a thickness and width of the BO ring18and the frequency of the main resonance (e.g., less than 100 MHz). The undesired BO spurious resonance mode increases the transmission loss of filters that incorporate BAW resonator10.

Accordingly, there remains a need for improved BAW resonator designs to reduce or eliminate the BO spurious resonance mode in the BAW resonator, while retaining high Q and low/no lateral wave spurious mode. Further, there is also a need to keep the final product cost effective.

SUMMARY

The present disclosure relates to a Bulk Acoustic Wave (BAW) resonator, which uses multilayer transduction materials, having zero or low electromechanical coupling at a border region of the BAW resonator, and a process for making the same. The disclosed BAW resonator includes a bottom electrode, a top electrode structure, and a multilayer transduction structure sandwiched between the bottom electrode and the top electrode structure. Herein, the multilayer transduction structure is composed of multiple transduction layers, at least one of which is formed of a first ferroelectric material. Polarization of the first ferroelectric material varies with an electric field across the first ferroelectric material. Each transduction layer includes a transduction border portion positioned at a periphery of a corresponding transduction layer and a transduction central portion surrounded by the transduction BO portion. A combination of all transduction BO portions forms a transduction BO section of the multilayer transduction structure, and a combination of all transduction central portions forms a transduction central section of the multilayer transduction structure. A first electromechanical coupling coefficient of the transduction BO section is smaller than a second (non-zero) electromechanical coupling coefficient of the transduction central section. The transduction central section is configured to provide a resonance of the BAW resonator.

In one embodiment of the disclosed BAW resonator, the first electromechanical coupling coefficient of the transduction BO section is a zero electromechanical coupling coefficient.

In one embodiment of the disclosed BAW resonator, the first ferroelectric material used to form the at least one of the transduction layers has a box-shaped polarization-electric field (P-E) curve.

In one embodiment of the disclosed BAW resonator, the first ferroelectric material is scandium aluminum nitride (ScxAl1-xN) and the P-E curve of ScxAl1-xN is dependent on a scandium concentration x.

In one embodiment of the disclosed BAW resonator, the first ferroelectric material is one of a group consisting of Zirconate Titanate (PZT), Lead titanate (PTO), Hafnium oxide (HfO2), Barium titanate (BTO), and Lithium niobate (LiNbO3).

In one embodiment of the disclosed BAW resonator, at least one of the transduction layers is formed of a second ferroelectric material, which has a different P-E curve compared to the first ferroelectric material.

In one embodiment of the disclosed BAW resonator, at least one of the transduction layers is formed of a piezoelectric material, whose polarization does not vary with an electric field across the piezoelectric material.

In one embodiment of the disclosed BAW resonator, each transduction layer is formed of a different ferroelectric material.

In one embodiment of the disclosed BAW resonator, the top electrode structure includes a top electrode base over the multilayer transduction structure and a BO ring protruding from a periphery of the top electrode base. Herein, a region of the BAW resonator, within which the BO ring is located, is a BO region. The transduction BO section is confined within the BO region and aligned underneath the BO ring, while the transduction central section is not covered by the BO ring.

In one embodiment of the disclosed BAW resonator, each transduction layer has a different thickness.

In one embodiment of the disclosed BAW resonator, each transduction layer has a same thickness.

According to one embodiment, the disclosed BAW resonator further includes a bottom Brag reflector formed underneath the bottom electrode, and a top Brag reflector formed over the top electrode structure.

In one embodiment of the disclosed BAW resonator, the multilayer transduction structure further includes a number of internal electrodes, which are alternated with the transduction layers.

According to an exemplary process, an initial precursor, which includes a bottom electrode, an initial multilayer transduction structure over the bottom electrode, and a bias electrode ring over a periphery of a top surface of the initial multilayer transduction structure, is firstly provided. Herein, a region of the initial precursor within which the bias electrode ring is located is a border (BO) region. The initial multilayer transduction structure is composed of a number of initial transduction layers. At least one of the initial transduction layers is formed of a first ferroelectric material, which has a box-shaped polarization-electric field (P-E) curve. Each of the initial transduction layers includes an initial transduction BO portion, which is confined within the BO region and aligned underneath the bias electrode ring, and a transduction central portion, which is surrounded by the initial transduction BO portion and not covered by the bias electrode ring. A combination of the initial transduction BO portion of each initial transduction layer forms an initial transduction BO section of the initial multilayer transduction structure, and a combination of the transduction central portion of each initial transduction layer forms a transduction central section of the initial multilayer transduction structure. Both the initial transduction BO section and the transduction central section have nonzero electromechanical coupling coefficients. Next, a direct current (DC) bias voltage is applied to the bias electrode ring to convert the initial multilayer transduction structure to a multilayer transduction structure, which includes a transduction BO section converted from the initial transduction BO section and the transduction central section surrounded by the transduction BO section. Herein, the transduction central section remains the nonzero electromechanical coupling coefficient and is configured to provide a resonance of the BAW resonator. The DC bias voltage is selected, such that an overall electromechanical coupling coefficient of the transduction BO section achieves a value less than the nonzero electromechanical coupling coefficient of the transduction central section.

According to one embodiment, the process further includes removing the DC bias voltage, and forming one or more electrode layers over the top surface of the multilayer transduction structure. Herein, the one or more electrode layers extend over the bias electrode ring, and a combination of the one or more electrode layers and the bias electrode ring composes a top electrode structure over the top surface of the multilayer transduction structure.

According to one embodiment, the process further includes removing the DC bias voltage, removing the bias electrode ring to completely expose the top surface of the multilayer transduction structure, and forming a top electrode structure over the top surface of the multilayer transduction structure. The top electrode structure has a flat shape.

In one embodiment of the method, at least one of the transduction layers is formed of a second ferroelectric material, which has a different P-E curve compared to the first ferroelectric material.

In one embodiment of the method, each transduction layer is formed of a different ferroelectric material.

In one embodiment of the method, at least one of the transduction layers is formed of a piezoelectric material, whose polarization does not vary with an electric field across the piezoelectric material.

In one embodiment of the method, the first ferroelectric material is ScxAl1-xN and the P-E curve of ScxAl1-xN is dependent on a scandium concentration x.

In one embodiment of the method, the first ferroelectric material is one of a group consisting of PZT, PTO, HfO2, BTO, and LiNbO3.

According to an exemplary system with at least one BAW resonator, the system includes a radio-frequency (RF) input circuitry, a RF output circuitry, and a filter circuitry that includes the at least one BAW resonator. Herein, the filter circuitry is connected between the RF input circuitry and the RF output circuitry. The at least one BAW resonator includes a bottom electrode, a top electrode structure, and a multilayer transduction structure sandwiched between the bottom electrode and the top electrode structure. The multilayer transduction structure is composed of multiple transduction layers, at least one of which is formed of a ferroelectric material with a box-shaped P-E curve. Each transduction layer includes a transduction border portion positioned at a periphery of a corresponding transduction layer and a transduction central portion surrounded by the transduction BO portion. A combination of all transduction BO portions forms a transduction BO section of the multilayer transduction structure, and a combination of all transduction central portions forms a transduction central section of the multilayer transduction structure. A first electromechanical coupling coefficient of the transduction BO section is smaller than a second (non-zero) electromechanical coupling coefficient of the transduction central section. The transduction central section is configured to provide a resonance of the BAW resonator.

It will be understood that for clear illustrations,FIGS.1-12may not be drawn to scale.

DETAILED DESCRIPTION

An electromechanical coupling coefficient Ke2of a Bulk Acoustic Wave (BAW) resonator, such as a thin film bulk acoustic resonator (FBAR) or a solidly mounted resonator (SMR), is a function of a piezoelectric coefficient d of a transduction layer of the BAW resonator, and the piezoelectric coefficient d is proportional to a polarization P of the transduction layer of the BAW resonator. Therefore, once the polarization P of the transduction layer varies, the piezoelectric coefficient d will change accordingly, and consequently, the electromechanical coupling coefficient Ke2of the BAW resonator will change as well.

FIG.3Aillustrates a simplified polarization-electric field (P-E) curve (i.e., hysteresis loop) of a ferroelectric material (e.g., scandium aluminum nitride). It is clear that the polarization P of the ferroelectric material can be adjusted by changing the electric field E across the ferroelectric material. A variation amount of the polarization P is determined according to a variation amount of the electric field across the ferroelectric material. Herein, changing the electric field across the ferroelectric material may be implemented by applying different direct current (DC) bias voltages to the ferroelectric material. By applying a particular DC bias voltage to the ferroelectric material, the polarization P of the ferroelectric material can achieve a particular value. Each DC bias voltage corresponds to one polarization, depending on a previously applied electric field (i.e., previously applied DC bias voltage). For instance (e.g., moving on a counterclockwise direction), changing the electric field across the ferroelectric material from 0 to E1 (i.e., by applying a particular DC bias voltage to the ferroelectric material), the polarization P of the ferroelectric material can be moved from P=PRto P=0. As such, the piezoelectric coefficient d of the ferroelectric material can be zero, and the electromechanical coupling coefficient Ke2of the BAW resonator that utilizes the ferroelectric material in the transduction layer can be zero. In another instance (e.g., moving in a counterclockwise direction), changing the electric field across the ferroelectric material from E1 to 0, the polarization P of the ferroelectric material can be moved from P=0 to P=−PR.

Notice that, once the polarization P of the ferroelectric material archives a desired value (i.e., the electromechanical coupling coefficient Ke2achieves a desired value), there is no need to remain the DC bias voltage applied to the ferroelectric material. After removing the DC bias voltage, the polarization P of the ferroelectric material (i.e., the electromechanical coupling coefficient Ke2of the ferroelectric material) will remain at that desired value, until another DC bias voltage is applied to the ferroelectric material.

Scandium aluminum nitride (ScxAl1-xN) is an exemplary ferroelectric material.FIG.3Billustrates exemplary different P-E curves of ScxAl1-xN dependent on a scandium concentration x. When the scandium concentration x=0.27, the electric field E across ScxAl1-xN requires −4.5/4 MV/cm to achieve the polarization P of ScxAl1-xN equal to zero; when the scandium concentration x=0.32, the electric field E across ScxAl1-xN requires −3.8/3.3 MV/cm to achieve the polarization P of ScxAl1-xN equal to zero; when the scandium concentration x=0.36, the electric field E across ScxAl1-xN requires −3/2.5 MV/cm to achieve the polarization P of ScxAl1-xN equal to zero; when the scandium concentration x=0.40, the electric field E across ScxAl1-xN requires −2.6/2.1 MV/cm to achieve the polarization P of ScxAl1-xN equal to zero; and when the scandium concentration x=0.43, the electric field E across ScxAl1-xN requires −2.1/1.8 MV/cm to achieve the polarization P of ScxAl1-xN equal to zero (moving on a counterclockwise direction in the P-E curves). These electric field E values can change and depend on the deposition condition of the ferroelectric material (e.g., ScxAl1-xN).

With a same electric field E (applying a same DC bias voltage), the polarization P of ScxAl1-xN will have different values and/or different directions due to the scandium concentration x. For instance, when the electric filed E is −1.5 MV/cm, the polarization P of Sc0.27Al0.73N is 105 μC/cm2, while the polarization P of Sc0.43Al0.57N is 70 μC/cm2. As such, the electromechanical coupling coefficient Ke2of the ferroelectric material, which is dependent on the polarization P of ferroelectric material, may also have different values due to the scandium concentration x.

FIG.4illustrates an exemplary BAW resonator30with zero or low electromechanical coupling at a border region according to some embodiments of the present disclosure. The exemplary BAW resonator30includes a bottom electrode32, a top electrode structure34, and a multilayer transduction structure36sandwiched between the bottom electrode32and the top electrode structure34.

In detail, the bottom electrode32may be composed of one or more electrode layers (not shown) and includes at least one metal or alloy, such as Tungsten, Aluminum Copper, and the like. The top electrode structure34includes a top electrode base37over the multilayer transduction structure36and a border (BO) ring38protruding from a periphery of the top electrode base37. Notably, a region of the exemplary BAW resonator30, within which the BO ring38is located, is referred to herein as a BO region40. The top electrode base37may be composed of one or more electrode layers (not shown) and includes at least one metal or alloy, such as Tungsten, Aluminum Copper, and the like. The BO ring38may be composed of one or more ring layers (not shown) and includes at least one metal or alloy, such as Tungsten, Aluminum Copper, and the like. The bottom electrode32and the top electrode base37each has a thickness depending on the main resonant frequency of the ferroelectric-based BAW resonator30. The BO ring38is configured to confine the energy inside the exemplary BAW resonator30(i.e., inside the multilayer transduction structure36) and to prevent laterally propagating standing waves.

The multilayer transduction structure36includes multiple transduction layers41(e.g., as shown inFIG.4, a first transduction layer41_1over the bottom electrode32, a second transduction layer41_2over the first transduction layer41_1, . . . a (N−1)th transduction layer41_N−1 over the second transduction layer41_2, a Nth transduction layer41_N over the (N−1)th transduction layer41_N−1, and the top electrode structure34over the Nth transduction layer41_N). In some cases, the multilayer transduction structure36may only include two transduction layers41(i.e., the (N−1)th transduction layer41_N−1 and the Nth transduction layer41_N are omitted, and the top electrode structure34is over the second transduction layer41_2). In some cases, the multilayer transduction structure36may only include three transduction layers41(i.e., N=3, and the (N−1)th transduction layer41_N−1 and the second transduction layer41_2represent a same layer).

Each transduction layer41may have a same or different thickness. At least one of the transduction layers41is formed of a ferroelectric material, whose polarization will vary with an electric field across the ferroelectric material. For instance, the ferroelectric material used for the at least one of the transduction layers41has a box-shaped P-E curve (e.g., as shown inFIG.3A), such as ScxAl1-xN, Lead Zirconate Titanate (PZT), Lead titanate (PTO), Hafnium oxide (HfO2), Barium titanate (BTO), Lithium niobate (LiNbO3), or the like.

In one embodiment, some of the transduction layers41may be formed of a piezoelectric material (e.g., Aluminum nitride (AlN) and/or Zinc oxide (ZnO)), whose polarization will not vary with an electric field across the piezoelectric material; while the remaining transduction layers41may be formed of a ferroelectric material (e.g., ScxAl1-xN, PTO, PZT, HfO2, or BTO), whose polarization will vary with an electric field across the ferroelectric material. Herein, by applying a particular electric field, the polarization of the ferroelectric material may have an opposite direction to the polarization of the piezoelectric material. In one embodiment, the transduction layers41formed of the piezoelectric material (e.g., the first transduction layer41_1. . . , and the (N−1)th transduction layer41_N−1) may be alternated with the transduction layers41formed of the ferroelectric material (e.g., the second transduction layer41_2. . . , and the Nth transduction layer41_N). In one embodiment, the transduction layers41formed of the piezoelectric material may be located in a top region of the multilayer transduction structure36and the transduction layers41formed of the ferroelectric material may be located in a bottom region of the multilayer transduction structure36(not shown). In one embodiment, the transduction layers41formed of the piezoelectric material may be located in the bottom region of the multilayer transduction structure36and the transduction layers41formed of the ferroelectric material may be located in the top region of the multilayer transduction structure36(not shown). In one embodiment, the transduction layers41formed of the piezoelectric material and the transduction layers41formed of the ferroelectric material may be positioned arbitrarily within the multilayer transduction structure36. The number of the transduction layers41formed of the piezoelectric material and the number of the transduction layers41formed of the ferroelectric material may be different or the same.

In one embodiment, some of the transduction layers41may be formed of one or more piezoelectric materials (e.g., AlN and/or ZnO), while the remaining transduction layers41may be formed of two or more ferroelectric materials (e.g., two or more of ScxAl1-xN, PTO, PZT, HfO2, LiNbO3, and BTO), wherein each ferroelectric material has a different P-E curve. When applying a same electric field, a polarization of each different ferroelectric material may have different values and/or directions. In a simplified instance, some of the transduction layers41may be formed of a piezoelectric material, some of the transduction layers41may be formed of a first ferroelectric material, and some of the transduction layers41may be formed of a second ferroelectric material. The first ferroelectric material and the second ferroelectric material have different P-E curves. In one embodiment, upon applying a particular electric field, at least one of the first ferroelectric material and the second ferroelectric material may have a polarization with an opposite direction to the polarization of the piezoelectric material. In one embodiment, upon applying a particular electric field, the first ferroelectric material may have a polarization with an opposite direction to the polarization of the piezoelectric material, while the second ferroelectric material may have a zero polarization. In one embodiment, upon applying a particular electric field, each of the first ferroelectric material and the second ferroelectric material may have a polarization with an opposite direction to the polarization of the piezoelectric material. In one embodiment, the transduction layers41formed of different materials may be positioned alternatively. In one embodiment, the transduction layers41formed of different materials may be positioned in groups, wherein each group utilizes one material. In one embodiment, the transduction layers41formed of different materials may be positioned arbitrarily within the multilayer transduction structure36. The number of the transduction layers41formed of each material may be different or the same.

In one embodiment, none of the transduction layers41includes a piezoelectric material, whose polarization will not vary with an electric field across the piezoelectric material. Herein, the transduction layers41may be formed of two or more ferroelectric materials, wherein each ferroelectric material has a different P-E curve. When applying a same electric field, a polarization of each different ferroelectric material may have different values and/or directions. In a simplified case, some of the transduction layers41may be formed of a first ferroelectric material, while the remaining transduction layers41may be formed of a second ferroelectric material. The first ferroelectric material and the second ferroelectric material have different P-E curves. When applying a same electric field, a polarization of the first ferroelectric material may have an opposite direction to a polarization of the second ferroelectric material. For example, the first ferroelectric material is ScxAl1-xN and the second ferroelectric material is ScyAl1-yN, where x is different from y and each of x and y is between 0.1 and 0.8. When the first ferroelectric material is Sc0.27Al0.73N and the second ferroelectric material is Sc0.43Al0.57N, upon changing the electrical field from 0 MV/cm to −2.7 MV/cm (seeFIG.3B), the polarization of the first ferroelectric material (positive) has an opposite direction to the polarization of the second ferroelectric material (negative).

In one embodiment, the transduction layers41formed of different ferroelectric materials may be positioned alternatively. In one embodiment, the transduction layers41formed of different ferroelectric materials may be positioned in groups, wherein each group utilizes one ferroelectric material. In one embodiment, the transduction layers41formed of different ferroelectric materials may be positioned arbitrarily within the multilayer transduction structure36. The number of the transduction layers41formed of each ferroelectric material may be different or the same.

In addition, each transduction layer41includes a transduction BO portion41_BO, which is confined within the BO region40and is aligned underneath the BO ring38, and a transduction central portion41_C surrounded by the corresponding transduction BO portion41_BO. A combination of all transduction BO portions41_BO (e.g., a first transduction BO portion41_BO_1, a second transduction BO portion41_BO_2. . . a (N−1)th transduction BO portion41_BO_N−1, and a Nth transduction BO portion41_BO_N) forms a transduction BO section36_BO of the multilayer transduction structure36, wherein the entire transduction BO section36_BO is confined within the BO region40and aligned underneath the BO ring38. A combination of all transduction central portions41C (e.g., a first transduction central portion41_C_1, a second transduction central portion41_C_2, . . . a (N−1)th transduction central portion41_C_N−1, and a Nth transduction central portion41_C_N) forms a transduction central section36_C of the multilayer transduction structure36, which is surrounded by the ferroelectric BO portion36_BO and not covered by the BO ring38.

Herein, a polarization of each transduction central portion41_C has a same direction (e.g., the polarization of each transduction central portion41_C is positive, represented by upward arrows), and thus the transduction central section36_C has a nonzero electromechanical coupling coefficient Ke2NONand is configured to provide a main resonance of the exemplary BAW resonator30(when an alternating current voltage is applied between the top electrode structure34and the bottom electrode32).

On the other hand, at least one transduction BO portion41_BO has a positive polarization (e.g., the first transduction BO portion41_BO_1of the first transduction layer41_1and the Nth transduction BO portion41_BO_N of the Nth transduction layer41_N are positive polarizations, represented by upward arrows), and at least one transduction BO portion41_BO has a negative polarization (e.g., the second transduction BO portion41_BO_2of the second transduction layer41_2and the (N−1)th transduction BO portion41_BO_N−1 of the (N−1)th transduction layer41_N−1 are negative polarization, represented by downward arrows). Optionally, one or more transduction BO portions41_BO may have a zero polarization (not shown). The polarization of each transduction BO portion41_BO is configured such that an electromechanical coupling coefficient Ke2BO of the entire transduction BO section36_BO is capable of being zero or a value smaller than the nonzero electromechanical coupling coefficient Ke2NONof the transduction central section36_C. In consequence, BO spurious resonance mode does not exist near the desired main resonance of the exemplary BAW resonator30. The zero/small electromechanical coupling coefficient Ke2BOof the transduction BO section36_BO will also help confine the energy inside the exemplary BAW resonator30and reduce laterally propagating standing waves.

In one embodiment, when some of the transduction layers41are formed of the piezoelectric material and the remaining transduction layers41are formed of the ferroelectric material, the transduction BO portions41_BO of these transduction layers41formed of the piezoelectric material have an opposite-direction polarization to the transduction BO portions41_BO of these transduction layers41formed of the ferroelectric material. As such, the electromechanical coupling coefficient Ke2BOof the entire transduction BO section36_BO may achieve zero/a small value (Ke2BO<Ke2NON).

In one embodiment, when some of the transduction layers41are formed of the first ferroelectric material and the remaining transduction layers41are formed of the second ferroelectric material, the transduction BO portions41_BO of these transduction layers41formed of the first ferroelectric material have an opposite-direction polarization to the transduction BO portions41_BO of these transduction layers41formed of the second ferroelectric material. As such, the electromechanical coupling coefficient Ke2BO of the entire transduction BO section36_BO may achieve zero/a small value (Ke2BO<Ke2NON).

In one embodiment, when some of the transduction layers41are formed of one or more piezoelectric materials, some of the transduction layers41are formed of the first ferroelectric material, and some of the transduction layers41are formed of the second ferroelectric material, the transduction BO portions41_BO of these transduction layers41formed of the piezoelectric material have an opposite-direction polarization to the transduction BO portions41_BO of these transduction layers41formed of the first ferroelectric material and/or the second ferroelectric material. As such, the electromechanical coupling coefficient Ke2BOof the entire transduction BO section36_BO may achieve zero/a small value (Ke2BO<Ke2NON).

FIGS.5A through5Cgraphically illustrate an exemplary process for implementing the exemplary BAW resonator30shown inFIG.4according to some embodiments of the present disclosure. Although the process steps are illustrated in a series, the process steps are not necessarily order dependent. Some steps may be done in a different order than that presented, without deviating from the scope of the present disclosure. Further, processes within the scope of this disclosure may include fewer or more steps than those illustrated inFIGS.5A-5C.

As illustrated inFIG.5A, the process begins with an initial precursor421N, which includes the bottom electrode32, an initial multilayer transduction structure361N over the bottom electrode32, and a bias electrode ring44over a periphery of a top surface of the initial multilayer transduction structure361N. Herein, a region of the initial precursor421N, within which the bias electrode ring44is located, may be referred to the BO region40. The bias electrode ring44may be formed by one or more ring layers (not shown) and includes at least one metal or alloy, such as Tungsten, Aluminum Copper, and the like.

The initial multilayer transduction structure361N includes multiple initial transduction layers41IN (e.g., a first initial transduction layer41IN_1over the bottom electrode32, a second initial transduction layer41IN_2over the first initial transduction layer41IN_1, . . . a (N−1)th initial transduction layer41IN_N−1 over the second initial transduction layer41IN_2, a Nth initial transduction layer41IN_N over the (N−1)th initial transduction layer41IN_N−1, and the bias electrode ring44over the Nth initial transduction layer41IN_N). In some cases, the initial multilayer transduction structure361N may only include two initial transduction layers41IN (i.e., the (N−1)th initial transduction layer41IN_N−1 and the Nth initial transduction layer41IN_N are omitted, and the bias electrode ring44is over the second initial transduction layer41IN_2). In some cases, the initial multilayer transduction structure361N may only include three initial transduction layers41IN (i.e., N=3, and the (N−1)th initial transduction layer41IN_N−1 and the second initial transduction layer41IN_2represent a same layer).

Each initial transduction layer41IN may have a same or different thickness. At least one of the initial transduction layers41IN is formed of a ferroelectric material, whose polarization will vary with an electric field across the ferroelectric material. For instance, the ferroelectric material used for the at least one of the initial transduction layers41IN has a box-shape P-E curve (e.g., as shown inFIG.3A), such as ScxAl1-xN, PTO, PZT, HfO2, BTO, LiNbO3, or the like. In the initial multilayer transduction structure361N, each initial transduction layer41IN may have a same direction polarization (e.g., a positive polarization, represented by upward arrows).

In one embodiment, some of the initial transduction layers41IN may be formed of a piezoelectric material (e.g., AlN and/or ZnO), whose polarization will not vary with an electric field across the piezoelectric material; while the remaining initial transduction layers41IN may be formed of a ferroelectric material (e.g., ScxAl1-xN, PTO, PZT, HfO2, LiNbO3, or BTO), whose polarization will vary with an electric field across the ferroelectric material. Herein, by applying a particular electric field, the polarization of the ferroelectric material may have an opposite direction to the polarization of the piezoelectric material. In one embodiment, the initial transduction layers41IN formed of the piezoelectric material (e.g., the first initial transduction layer41IN_1. . . , and the (N−1)th initial transduction layer41IN_N−1) may be alternated with the initial transduction layers41IN formed of the ferroelectric material (e.g., the second initial transduction layer41IN_2. . . , and the Nth initial transduction layer41_N). In one embodiment, the initial transduction layers41IN formed of the piezoelectric material may be located in a top region of the multilayer transduction structure36(e.g., the Nth initial transduction layer41IN_N, the (N−1)th initial transduction layer41IN_N−1, and etc.), and the initial transduction layers41IN formed of the ferroelectric material may be located in a bottom region of the multilayer transduction structure36(e.g., the first initial transduction layer41IN_1, the second initial transduction layer41IN_2, and etc.). In one embodiment, the initial transduction layers41IN formed of the piezoelectric material may be located in the bottom region of the multilayer transduction structure36(e.g., the first initial transduction layer41IN_1, the second initial transduction layer41IN_2, and etc.), and the initial transduction layers41IN formed of the ferroelectric material may be located in the top region of the multilayer transduction structure36(e.g., the Nth initial transduction layer41IN_N, the (N−1)th initial transduction layer41IN_N−1, and etc.). In one embodiment, the initial transduction layers41IN formed of the piezoelectric material and the initial transduction layers41IN formed of the ferroelectric material may be positioned arbitrarily within the initial multilayer transduction structure361N. The number of the initial transduction layers41IN formed of the piezoelectric material and the number of the initial transduction layers41IN formed of the ferroelectric material may be different or the same.

In one embodiment, none of the initial transduction layers41IN includes a piezoelectric material, whose polarization will not vary with an electric field across the piezoelectric material. Herein, the initial transduction layers41IN may be formed of two or more ferroelectric materials (e.g., ScxAl1-xN, PTO, PZT, HfO2, LiNbO3, and/or BTO), wherein each ferroelectric material has a different P-E curve. When applying a same electric field, a polarization of each different ferroelectric material may have different values and/or directions. In a simplified case, some of the initial transduction layers41IN may be formed of a first ferroelectric material, while the remaining initial transduction layers41IN may be formed of a second ferroelectric material. The first ferroelectric material and the second ferroelectric material have different P-E curves, where upon applying a same electric field, a polarization of the first ferroelectric material may have an opposite direction to a polarization of the second ferroelectric material. In one embodiment, the initial transduction layers41IN formed of different ferroelectric materials may be positioned in an alternative configuration, a grouped configuration, or an arbitrary configuration. The number of the initial transduction layers41IN formed of each ferroelectric material may be different or the same.

In one embodiment, some of the initial transduction layers41IN may be formed of one or more piezoelectric materials (e.g., AlN and/or ZnO), while the remaining initial transduction layers41IN may be formed of two or more ferroelectric materials (e.g., ScxAl1-xN, PTO, PZT, HfO2, LiNbO3and/or BTO), wherein each ferroelectric material has a different P-E curve. In a simplified instance, some of the initial transduction layers41IN may be formed of a piezoelectric material, some of the initial transduction layers41IN may be formed of a first ferroelectric material, and some of the initial transduction layers41IN may be formed of a second ferroelectric material. The first ferroelectric material and the second ferroelectric material have different P-E curves. In one embodiment, upon applying a particular electric field, at least one of the first ferroelectric material and the second ferroelectric material may have a polarization with an opposite direction to the polarization of the piezoelectric material. In one embodiment, upon applying a particular electric field, the first ferroelectric material may have a polarization with an opposite direction to the polarization of the piezoelectric material, while the second ferroelectric material may have a zero polarization. In one embodiment, upon applying a particular electric field, each of the first ferroelectric material and the second ferroelectric material may have a polarization with an opposite direction to the polarization of the piezoelectric material. The initial transduction layers41IN formed of different materials may be positioned in an alternative configuration, a grouped configuration, or an arbitrary configuration. The number of the initial transduction layers41IN formed of each material may be different or the same.

In addition, each initial transduction layer41IN includes an initial transduction BO portion41IN_BO (e.g., a first initial transduction BO portion41IN_BO_1, a second initial transduction BO portion41IN_BO_2, . . . a (N−1)th initial transduction BO portion41IN_BO_N−1, and a Nth initial transduction BO portion41IN_BO_N, respectively), which is confined within the BO region40and aligned underneath the bias electrode ring44. A combination of all initial transduction BO portions41IN_BO forms an initial transduction BO section361N_BO of the initial multilayer transduction structure361N, where the entire initial transduction BO section361N_BO is confined within the BO region40and aligned underneath the bias electrode ring44.

Besides the initial transduction BO section361N_BO, the initial multilayer transduction structure361N also includes the transduction central section36_C, which is surrounded by the initial transduction BO section361N_BO and not covered by the bias electrode ring44. The transduction central section36_C is a combination of a transduction central portion41_C of each initial transduction layer41IN, where the transduction central portion41_C of each initial transduction layer41IN is surrounded by a corresponding initial transduction BO portion41IN_BO. Notice that each entire initial transduction layer41IN, including the corresponding initial transduction BO portion41IN_BO and the corresponding transduction central portion41_C, has a non-zero polarization (e.g., with a same positive direction, represented by upward arrows). As such, both the transduction central section36_C and the initial transduction BO section361N_BO have non-zero electromechanical coupling coefficients, and consequently, a main resonance and a BO spurious resonance (near a main resonance) will exist in the initial precursor421N.

Next, a DC bias voltage Vo is applied to the bias electrode ring44, and the bottom electrode32is electrically coupled to ground, such that the initial multilayer transduction structure361N converts to the multilayer transduction structure36with the zero/small-electromechanical-coupling transduction BO section36_BO (the initial precursor421N converts to a precursor42), as illustrated inFIG.5B. The multilayer transduction structure36is composed of the first transduction layer41_1, the second transduction layer41_2, . . . the (N−1)th transduction layer41_N−1, and the Nth transduction layer41_N. Each transduction layer41is transformed from a corresponding initial transduction layer41IN and has a same central portion41_C as the corresponding initial transduction layer41IN.

The DC bias voltage Vo changes the electric field between the bias electrode ring44and the bottom electrode32(i.e., the electric field across the initial transduction BO section361N_BO), but barely changes the electric field across the transduction central section36_C. When some initial transduction layers41IN are formed of one or more piezoelectric material (e.g., AlN and/or ZnO), whose polarization will not vary with the electric field across the piezoelectric material, the corresponding transduction layers41will have no change from these initial transduction layer41IN. For instance, the first initial transduction layer41IN_1and the Nth initial transduction layer41IN_N are formed of one piezoelectric material. After applying the DC bias voltage Vo, the first transduction layer41_1and the Nth transduction layer41_N have no change from the first initial transduction layer41IN_1and the Nth initial transduction layer41IN_N, respectively.

When some initial transduction layers41IN are formed of one ferroelectric material (e.g., ScxAl1-xN, PTO, PZT, HfO2, LiNbO3, or BTO), whose polarization will vary with an electric field across the ferroelectric material, after applying the DC bias voltage Vo, the polarization of the transduction BO portions41_BO (underneath the bias electrode ring44, where the DC bias voltage Vo is applied) of the corresponding transduction layers41may be different from the polarization of the initial transduction BO portions41IN_BO of these initial transduction layers41IN. For instance, the second initial transduction layer41IN_2and the (N−1)th initial transduction layer41IN_N−1 are formed of one ferroelectric material. After applying the DC bias voltage Vo, the polarization of the second transduction BO portion41_BO_2of the second transduction layer412may have an opposite direction (e.g., negative, represented by a downward arrow) to the polarization of the second initial transduction BO portion41IN_BO_2of the second initial transduction layer41IN_2and the polarization of the (N−1)th transduction BO portion41_BO_N−1 of the (N−1)th transduction layer41_N−1 may have an opposite direction (e.g., negative, represented by a downward arrow) to the polarization of the (N−1)th initial transduction BO portion41IN_BO_N−1 of the (N−1)th initial transduction layer41IN_N−1. Notice that the polarization of each transduction central portion41_C does not change, because the DC bias voltage Vo is not applied across the transduction central section36_C.

When some initial transduction layers41IN are formed of two or more ferroelectric materials with different P-E curves (e.g., two or more of ScxAl1-xN, PTO, PZT, HfO2, LiNbO3, and BTO), after applying the DC bias voltage Vo, the transduction BO portions41_BO of the corresponding transduction layers41may have different polarization variations. For instance, the second initial transduction layer41IN_2is formed of a first ferroelectric material, and the (N−1)th initial transduction layer41IN_N−1 is formed of a second ferroelectric material. After applying the DC bias voltage Vo, the polarization of the second transduction BO portion41_BO_2of the second transduction layer41_2and the polarization of the (N−1)th transduction BO portion41_BO_N−1 of the (N−1)th transduction layer41_N−1 are reversed in direction, but with different values. In another case, after applying the DC bias voltage Vo, the polarization of the second transduction BO portion41_BO_2of the second transduction layer41_2is reversed in direction, while the polarization of the (N−1)th transduction BO portion41_BO_N−1 of the (N−1)th transduction layer41_N−1 is zero (not shown).

Herein, the material used to form each initial transduction layer41IN and the DC bias voltage Vo are carefully selected, such that after applying the DC bias voltage Vo to the bias electrode ring44, the polarizations of some transduction BO portions41_BO remain the initial direction and the polarizations of some other transduction BO portions41_BO are reversed in direction. Consequently, an overall electromechanical coupling coefficient Ke2BOof the transduction BO section36_BO can achieve zero or a value smaller than a nonzero electromechanical coupling coefficient Ke2NONof the transduction central section36_C. The DC bias voltage Vo is selected based on the material used for each initial transduction layer41IN and the thickness of each initial transduction layer411N.

Notice that, once the polarization P of each transduction BO portion41_BO achieves a desired value, there is no need to retain the DC bias voltage Vo applied across the transduction BO section36_BO. After removing the DC bias voltage Vo, the polarization P of each transduction BO portion41_BO will remain the same, until another DC bias voltage is applied across the transduction BO section36_BO.

After removing the DC bias voltage Vo, one or more electrode layers46are formed over the top surface of the multilayer transduction structure36, as illustrated inFIG.5C. The one or more electrode layers46extend over the bias electrode ring44, and a combination of the one or more electrode layers46and the bias electrode ring44composes the top electrode structure34. From shape aspects, the top electrode structure34includes the top electrode base37over the multilayer transduction structure36and the BO ring38protruding from the periphery of the top electrode base37. The BO region40is the region within which the BO ring38is located. The one or more electrode layers46includes at least one metal or alloy, such as Tungsten, Aluminum Copper, and the like.

Notice that a thickness of the bias electrode ring44and a thickness of the BO ring38may be different or the same. In a desired case, a width of the bias electrode ring44and a width of the BO ring38may be the same. However, in some applications, the width of the bias electrode ring44and the width of the BO ring38may be different.

FIGS.6A and6Bshow a 1−|S11|2response (1−|S11|2is equal to the power ratio lost in a resonator) of the exemplary BAW resonator30. Depending on the materials used in the multilayer transduction structure36, the undesired BO spurious resonance (which is caused by the nonzero electromechanical coupling coefficient Ke2in the initial transduction BO section361N_BO) near the main resonance of the exemplary BAW resonator30is eliminated/significantly compressed (seeFIG.6A), or shifts far away from the main resonance of the exemplary BAW resonator30(e.g., at twice the frequency of the main resonance, seeFIG.6B). The main resonance of the exemplary BAW resonator30is barely affected by the nonzero electromechanical coupling transduction BO section36_BO.

FIGS.7A-7Billustrate another exemplary procedure to implement an alternative BAW resonator according to some embodiments of the present disclosure. After removing the DC bias voltage Vo, the bias electrode ring44may be removed to form an alternative precursor48, as illustrated inFIG.7A. Herein, the electromechanical coupling coefficient Ke2BOin the transduction BO section36_BO remains at zero or the small value. Next, an alternative top electrode structure34A is applied over the top surface of the multilayer transduction structure36to complete an alternative BAW resonator30A, as illustrated inFIG.7B. The alternative top electrode structure34A has a flat shape. The alternative top electrode structure34A may be composed of one or more electrode layers (not shown) and includes at least one metal or alloy, such as Tungsten, Aluminum Copper, and the like.

Due to the zero/small electromechanical coupling coefficient Ke2BOof the transduction BO section36_BO, the undesired BO spurious resonance mode does not exist near the main resonance of the alternative BAW resonator30A. Furthermore, even without the BO ring structure, the zero/small electromechanical coupling coefficient Ke2BOof the transduction BO section36_BO will help confine the energy inside the alternative BAW resonator30A and reduce laterally propagating waves. The transduction central portion36_C in the alternative BAW resonator30A still has a nonzero electromechanical coupling coefficient Ke2NON(larger than the electromechanical coupling coefficient Ke2BOof the entire transduction BO section36_BO), and is configured to provide the main resonance of the alternative BAW resonator30A (when an alternating current voltage is applied between the alternative top electrode structure34A and the bottom electrode32).

In some applications, a BAW resonator may further include one or two reflectors.FIG.8illustrates a second alternative BAW resonator30B, wherein in addition to the bottom electrode32, the alternative top electrode structure34A, and the multilayer transduction structure36, the second alternative BAW resonator30B also includes a bottom reflector50underneath the bottom electrode32. The bottom reflector50may be a Brag reflector. In some applications, the second alternative BAW resonator30B may include the top electrode structure34(as shown inFIG.4or6C) instead of the alternative top electrode structure34A over the multilayer transduction structure36(not shown).

As illustrated inFIG.9, a third alternative BAW resonator30C, in addition to the bottom electrode32, the alternative top electrode structure34A, and the multilayer transduction structure36, also includes the bottom reflector50underneath the bottom electrode32and a top reflector52over the alternative top electrode structure34A. The bottom reflector50and the top reflector52may be Brag reflectors. In some applications, the third alternative BAW resonator30C may include the top electrode structure34(as shown inFIG.4or6C) instead of the alternative top electrode structure34A sandwiched between the multilayer transduction structure36and the top reflector52(not shown).

In some applications, in order to adjust the electric field across each transduction layer, internal electrodes may be used. As illustrated inFIG.10, a fourth alternative BAW resonator30D includes an alternative multilayer transduction structure36A between the bottom electrode32and the alternative top electrode structure34A. Herein, the alternative multilayer transduction structure36A, in addition to the multiple transduction layers41, also includes multiple internal electrodes54, which are alternated with the transduction layers41(e.g., a first internal electrode54_1is over the first transduction layer41_1, the second transduction layer41_2is over the first internal electrode54_1, a second internal electrode54_2is over the second transduction layer41_2. . . the (N−1)th transduction layer41_N−1 is over a (N−2)th internal electrode54_N−2, a (N−1)th internal electrode54_N−1 is over the (N−1)th transduction layer41_N−1, and the Nth transduction layer41_N is over the (N−1)th internal electrode54_N−1). Each internal electrode54may be composed of one or more electrode layers (not shown) and includes at least one metal or alloy, such as Tungsten, Aluminum Copper, and the like.

From functionality aspects, the alternative multilayer transduction structure36A is composed of an alternative transduction BO section36A_BO at the periphery of the alternative multilayer transduction structure36A and an alternative transduction central section36A_C surrounded by the alternative transduction BO section36A_BO. The alternative transduction central section36A_C has a nonzero electromechanical coupling coefficient, and is configured to provide a main resonance of the fourth alternative BAW resonator30D (when an alternating current voltage is applied between the top electrode structure34and the bottom electrode32). The alternative transduction BO section36A_BO has a zero/small electromechanical coupling coefficient (smaller than the nonzero electromechanical coupling coefficient of the alternative transduction central section36A_C), and thus the undesired BO spurious resonance does not exist or exists far away from the main resonance of the fourth alternative BAW resonator30D.

In some applications, the fourth alternative BAW resonator30D may include the top electrode structure34(as shown inFIG.4or6C) instead of the alternative top electrode structure34A (not shown). In some applications, the fourth alternative BAW resonator30D may further include the bottom reflector50and/or the top reflector52.

FIG.11illustrates a block diagram of an example system1100that includes at least one BAW filter, which is implemented by one or more BAW resonators30/30A/30B/30C/30D as shown inFIGS.4,7B,8,9, and10, respectively. The system1100includes RF input circuitry1102connected to filter circuitry1104. In certain embodiments, the RF input circuitry1102includes a transceiver.

For the purpose of this illustration, the filter circuitry1104includes three filters1106A,1106B, and1106C. Herein, one or more of the filters1106A,1106B, and1106C may be BAW filters, which are implemented by one or more BAW resonators30/30A/30B/30C/30D. In different applications, the filter circuitry1104may include more or fewer filters. In one embodiment, each of the filters1106A,1106B, and1106C may be a lowpass filter or a bandpass filter, and the filters1106A,1106B, and1106C may be connected in a cascaded arrangement. The filter types that are included in the filter circuitry1104may be based at least on the rejection requirements of the system1100.

The filter circuitry1104is connected to an RF output circuitry1108. In certain embodiments, the RF output circuitry1108includes an antenna. The RF input circuitry1102and/or the RF output circuitry1108may include additional or different components in other embodiments.

FIG.12illustrates a block diagram of example user elements that include at least one BAW filter, which is implemented by one or more BAW resonators30/30A/30B/30C/30D shown inFIGS.4,7B,8,9, and10, respectively. The concepts described above may be implemented in various types of user elements1200, such as mobile terminals, smart watches, tablets, computers, navigation devices, access points, and like wireless communication devices that support wireless communications, such as cellular, wireless local area network (WLAN), BLUETOOTH, and near field communications. The user elements1200will generally include a control system1202, a baseband processor1204, transmit circuitry1206, receive circuitry1208, antenna switching circuitry1210, multiple antennas1212, and user interface circuitry1214. In a non-limiting example, the control system1202can be a field-programmable gate array (FPGA) or an application-specific integrated circuit (ASIC), as an example. In this regard, the control system1202can include at least a microprocessor(s), an embedded memory circuit(s), and a communication bus interface(s). The receive circuitry1208receives radio frequency signals via the antennas1212and through the antenna switching circuitry1210from one or more base stations. A low noise amplifier and a filter of the receive circuitry1208cooperate to amplify and remove broadband interference from the received signal for processing. Down conversion and digitization circuitry (not shown) will then down convert the filtered, received signal to an intermediate or baseband frequency signal, which is then digitized into one or more digital streams using analog-to-digital converter(s) (ADC).

The baseband processor1204processes the digitized received signal to extract the information or data bits conveyed in the received signal. This processing typically comprises demodulation, decoding, and error correction operations, as will be discussed in greater detail below. The baseband processor1204is generally implemented in one or more digital signal processors (DSPs) and ASICs.

For transmission, the baseband processor1204receives digitized data, which may represent voice, data, or control information, from the control system1202, which it encodes for transmission. The encoded data is output to the transmit circuitry1206, where a digital-to-analog converter(s) (DAC) converts the digitally encoded data into an analog signal and a modulator modulates the analog signal onto a carrier signal that is at a desired transmit frequency or frequencies. A power amplifier will amplify the modulated carrier signal to a level appropriate for transmission and deliver the modulated carrier signal to the antennas1212through the antenna switching circuitry1210to the antennas1212. The multiple antennas1212and the replicated transmit and receive circuitries1206,1208may provide spatial diversity. Modulation and processing details will be understood by those skilled in the art.