Bulk acoustic resonator

A bulk acoustic resonator includes a first electrode disposed on an upper side of a substrate, a piezoelectric layer disposed on an upper surface of the first electrode, and a second electrode disposed on an upper surface of the piezoelectric layer, wherein an upper surface of at least one of the first electrode and the second electrode has a recess region, wherein a depth of the recess region is D, a width of the recess region is W, and a resonance frequency is F, and ln is a natural logarithm, and wherein [{ln(D*W)}/(−0.59*F)] is [[ln{0.008 (μm)2}]/{−0.59*(3.5 GHz)}] or more and [[ln{0.022 (μm)2}]/{−0.59*(3.5 GHz)}] or less.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 USC 119(a) of Korean Patent Application No. 10-2021-0010053 filed on Jan. 25, 2021, in the Korean Intellectual Property Office, the entire disclosure of which is incorporated herein by reference for all purposes.

BACKGROUND

The present disclosure relates to a bulk acoustic resonator.

2. Description of the Background

Demand for a bulk acoustic wave (BAW) filter using a BAW resonator is gradually increasing as the performance of mobile devices increases, and such a filter can have advantages of high withstand power characteristics and high frequency, compared to a surface acoustic wave (SAW) filter.

Among performances required for mobile devices, low loss is important. To this end, a band low-loss design of a filter may be important, and use of a low-loss resonator is most effective. The loss of the BAW resonator has various causes, including dielectric loss of a piezoelectric body, incomplete crystallinity of the piezoelectric body itself, and resistance of electrode materials. In addition, spurious noise (SN), occurring in a proximity frequency region before reaching a resonance frequency, is also a major cause of loss of a resonator, a physical phenomenon accompanying a BAW resonance phenomenon.

SUMMARY

In a first general aspect, a bulk acoustic resonator includes a first electrode disposed on an upper side of a substrate, a piezoelectric layer disposed on an upper surface of the first electrode, and a second electrode disposed on an upper surface of the piezoelectric layer, wherein an upper surface of at least one of the first electrode and the second electrode has a recess region, wherein a depth of the recess region is D, a width of the recess region is W, a resonance frequency is F, and ln is a natural logarithm, and wherein [{ln(D*W)}/(−0.59*F)] is [[ln{0.008 (μm)2}]/{−0.59*(3.5 GHz)}] or more and [[ln{0.022 (μm)2}]/{−0.59*(3.5 GHz)}] or less.

D may be 5 nm or more.

D may be 1% or more of a thickness of the piezoelectric layer.

The recess region may be located on an upper surface of the second electrode, and D may be 5/174 times a thickness of the second electrode or more and less than a thickness of the second electrode.

Each of the first electrode and the second electrode may include molybdenum (Mo).

The upper surface of the piezoelectric layer may have a recess region, and a product of depth and width of the recess region of at least one of the first electrode and the second electrode may be less than a product of depth and width of the recess region of the piezoelectric layer.

The bulk acoustic resonator may further include a protective layer disposed on the upper surface of the second electrode, wherein an upper surface of the protective layer may have a recess region, and the product of depth and width of the recess region of the piezoelectric layer may be less than a product of depth and width of the recess region of the protective layer.

The bulk acoustic resonator may further include a seed layer disposed on the lower surface of the first electrode, wherein an upper surface of the seed layer may have a recess region, and the product of a depth and a width of a recess region of the protective layer may be less than a product of a depth and a width of the recess region of the seed layer.

The seed layer may include AlN, and the protective layer may include SiO2.

In another general aspect, a bulk acoustic resonator includes a first electrode disposed on an upper side of a substrate, a piezoelectric layer disposed on an upper surface of the first electrode, and a second electrode disposed on an upper surface of the piezoelectric layer, wherein the upper surface of the piezoelectric layer has a recess region, a depth of the recess region is D, a width of the recess region is W, a resonance frequency is F, and ln is a natural logarithm, wherein D is 1% or more and less than 100% of a thickness of the piezoelectric layer, and wherein [{ln(D*W)}/(−0.412*F)] is [[ln{0.015 (μm)2}]/{−0.412*(3.5 GHz)}] or more and [[ln{0.03 (μm)2}]/{−0.412*(3.5 GHz)}] or less.

The bulk acoustic resonator may further include a seed layer disposed on the lower surface of the first electrode, wherein an upper surface of the seed layer may have a recess region, and a product of depth and width of the recess region of the piezoelectric layer may be less than a product of depth and width of the recess region of the seed layer.

In another general aspect, a bulk acoustic resonator includes a seed layer, a first electrode disposed on an upper surface of the seed layer, a piezoelectric layer disposed on an upper surface of the first electrode, a second electrode disposed on an upper surface of the piezoelectric layer, and a protective layer disposed on an upper surface of the second electrode, wherein upper surfaces of at least two of the seed layer, the first electrode, the piezoelectric layer, the second electrode, and the protective layer have first and second recess regions having different products of depth and width, respectively.

The first recess region may be located on an upper surface of at least one of the first electrode and the second electrode, the second recess region may be located on an upper surface of at least one of the seed layer, the piezoelectric layer, and the protective layer, and a product of depth and width of the first recess region may be less than a product of depth and width of the second recess region.

Each of the first electrode and the second electrode may include molybdenum (Mo), and at least one of the seed layer, the piezoelectric layer, and the protective layer may include at least one of AlN, ScAlN, and SiO2.

The first recess region may be located on an upper surface of at least one of the first electrode, the piezoelectric layer, and the second electrode, the second recess region may be located on an upper surface of at least one of the seed layer and the protective layer, and a product of depth and width of the first recess region may be less than a product of depth and width of the second recess region.

The first recess region may be located on an upper surface of at least one of the first electrode, the piezoelectric layer, the second electrode, and the protective layer, the second recess region may be located on the upper surface of the seed layer, and a product of depth and width of the first recess region may be less than a product of depth and width of the second recess region.

The first and second recess regions may have different depths.

Throughout the drawings and the detailed description, the same reference numerals refer to the same elements. The drawings may not be to scale, and the relative sizes, proportions, and depictions of elements in the drawings may be exaggerated for clarity, illustration, and convenience.

DETAILED DESCRIPTION

Hereinafter, while example embodiments of the present disclosure will be described in detail with reference to the accompanying drawings, it is noted that examples are not limited to the same.

Throughout the specification, when an element, such as a layer, region, or substrate, is described as being “on,” “connected to,” or “coupled to” another element, it may be directly “on,” “connected to,” or “coupled to” the other element, or there may be one or more other elements intervening therebetween. In contrast, when an element is described as being “directly on,” “directly connected to,” or “directly coupled to” another element, there can be no other elements intervening therebetween. As used herein “portion” of an element may include the whole element or a part of the whole element less than the whole element.

An aspect of the present disclosure is to provide a bulk acoustic resonator.

FIG.1is a view illustrating a recess region of a bulk acoustic resonator (bulk acoustic wave resonator) according to an embodiment of the present disclosure.

Referring toFIG.1, a bulk acoustic resonator according to an embodiment of the present disclosure may include a resonator135, and the resonator135may include a first electrode140, a piezoelectric layer150, and a second electrode160.

The first electrode140may be disposed on an upper side of a substrate110, the piezoelectric layer150may be disposed on an upper surface of the first electrode140, and the second electrode160may be disposed on an upper surface of the piezoelectric layer150.

Each of the first and second electrodes140and160may be formed using a conductive material such as molybdenum (Mo) or an alloy thereof to improve coupling efficiency with the piezoelectric layer150, but is not limited thereto. Each of the first and second electrodes140and160may be formed of a conductive material such as ruthenium (Ru), tungsten (W), iridium (Ir), platinum (Pt), copper (Cu), titanium (Ti), tantalum (Ta), nickel (Ni), chromium (Cr), or the like, or an alloy thereof.

The piezoelectric layer150may include a piezoelectric material to generate a piezoelectric effect converting electrical energy into mechanical energy in a form of elastic waves. For example, the piezoelectric material may include one of aluminum nitride (AlN), zinc oxide (ZnO), and lead zirconate titanate (PZT; PbZrTiO), may further include rare earth metal and transition metal, and may also include magnesium (Mg), divalent metal. For example, the rare earth metal may include at least one of scandium (Sc), erbium (Er), yttrium (Y), and lanthanum (La). The transition metal may include at least one of titanium (Ti), zirconium (Zr), hafnium (Hf), tantalum (Ta), and niobium (Nb).

The resonator135may convert electrical energy of a radio frequency (RF) signal into mechanical energy through a piezoelectric characteristic of the piezoelectric layer150and inversely convert it. As a frequency of the RF signal is closer to a resonance frequency of the bulk acoustic resonator, an energy transfer rate between the first and second electrodes140and160can be greatly increased. As the frequency of the RF signal is closer to an anti-resonance frequency of the bulk acoustic resonator, the energy transfer rate between the first and second electrodes140and160can be greatly reduced. According to the piezoelectric characteristic, the anti-resonance frequency may be higher than the resonance frequency.

Referring toFIG.1, the bulk acoustic wave resonator according to an embodiment of the present disclosure may be a thin film bulk acoustic resonator (FBAR) in which an air cavity112is located between the substrate110and the resonator135, but is not limited thereto. For example, the bulk acoustic wave resonator according to an embodiment of the present disclosure may be a solidly mounted resonator (SMR) type resonator in which a support portion in which at least one insulating layer and at least one metal layer are alternately stacked is located between the substrate110and the resonator135.

Referring toFIG.1, a bulk acoustic wave resonator according to an embodiment of the present disclosure may further include at least one of a substrate110, an insulating layer120, a sacrificial layer130, a protective layer170, and metal layers181and182.

The substrate110may be composed of a conventional silicon substrate or a silicon substrate having high specific resistance, and an insulating layer120may be provided on an upper surface of the substrate110to electrically isolate the substrate110and the resonator135. The insulating layer120may be formed on the substrate110through any one process of chemical vapor deposition, RF magnetron sputtering, and evaporation of at least one of silicon dioxide (SiO2), and aluminum oxide (Al2O3).

An air cavity112may be disposed on the insulating layer120. The air cavity112may be located below the resonator135so that the resonator135can vibrate in a predetermined direction. The air cavity112may be formed by a process of forming a sacrificial layer130on the insulating layer120, forming a membrane on the sacrificial layer130, and then etching and removing a portion of the sacrificial layer130.

A seed layer for improving crystal orientation of the piezoelectric layer150may be additionally disposed below the first electrode140. For example, the seed layer may be formed of one of aluminum nitride (AlN), zinc oxide (ZnO), lead zirconium titanium oxide (PZT; PbZrTiO) having the same crystallinity as the piezoelectric layer150.

A protective layer170may be disposed on the second electrode160of the resonator135to prevent the second electrode160from being exposed externally. The protective layer170may be formed of one of a silicon oxide-based, a silicon nitride-based, and an aluminum nitride-based insulating material.

Metal layers181and182may be an electrical node between a plurality of bulk acoustic resonators or an electrical connection node between a bulk acoustic resonator and a connection port, and may be implemented with a material having relatively low specific resistance such as gold (Au), a gold-tin (Au—Sn) alloy, copper (Cu), a copper-tin (Cu—Sn) alloy, aluminum (Al), aluminum alloy, and the like, but is not limited thereto.

An upper surface of at least one of the first and second electrodes140and160of the bulk acoustic resonator according to an embodiment of the present disclosure may include at least one recess region.

FIG.2is a view illustrating a width and a depth of a recess region of a bulk acoustic resonator according to an embodiment of the present disclosure.

Referring toFIG.2, one surface (an inner surface or an outer surface) of the bulk acoustic resonator according to an embodiment of the present disclosure may include a recess region (Recessed) and an active region (Active), and may further include a raised region (Raised).

The recess region may have a depth D corresponding to a vertical direction and a width W corresponding to a horizontal direction. For example, the depth D may be an average depth of the recess region, and the width W may be an average distance between one side surface of the active region and one side surface of the raised region.

For example, the recess region may have a trench shape two-dimensionally surrounding the active region. When the recess region has a trench shape, the two recess regions of a cross-section obtained by cutting the bulk acoustic resonator in the vertical direction may be the same as one three-dimensional recess region. Here, the depth D and the width W of the recess region may be an average of depths and widths of each of the two recess regions of a cross-section obtained by cutting the bulk acoustic resonator in the vertical direction.

A side surface of the raised region may provide a portion of a vertical boundary surface of the recess region. The raised region may be the same component as the component (e.g., an electrode) in which the recess region is formed, and may correspond to the recess region and to the portion other than the active region remaining on one surface of the bulk acoustic resonator. Depending on the design, the raised region may also be a component (e.g., an electrode), different from a component (e.g., a metal layer) in which a recess region is formed.

A side surface of the active region may provide a portion of a vertical boundary surface of the recess region. A height of the active region may be lower than a height of the raised region, but is not limited thereto.

For example, a bulk acoustic resonator of a Type II may have a structure in which the recess region is adjacent to the active region and is located outwardly thereof, and the raised region is adjacent to the recess region and is located outwardly thereof, and a Type I bulk acoustic resonator may have a structure in which the raised region is adjacent to the active region and is located outwardly thereof, and the recess region is adjacent to the raised region and is located outwardly thereof.

A magnitude relationship of a resonance frequency of the recess region, the raised region, and the active region (a cutoff frequency corresponding to kx=0 in a dispersion curve) may be a relationship of [Fcutoff_recessed]>[Fcutoff_active]>[Fcutoff_raised]. Accordingly, the raised region and the recess region may be defined as a difference in physical thickness from the active region, but more fundamentally, may also be defined as a region having a difference in resonance frequency for each type. For example, although the recess region, raised region, and active regions have the same thickness in appearance, they may be distinguished from each other by having different resonance frequencies or cutoff frequencies by a method in which different types of materials are stacked.

Referring toFIG.2, a bulk acoustic resonator according to an embodiment of the present disclosure may include a seed layer138a, a first electrode140a, a piezoelectric layer150a, a second electrode160a, and a protective layer170a. The recess region may be formed in at least one of the seed layer138a, the first electrode140a, the piezoelectric layer150a, the second electrode160a, and the protective layer170a.

FIGS.3A to3Eare views illustrating a recess region of a bulk acoustic resonator according to embodiments of the present disclosure.

Referring toFIG.3A, a recess region having a depth D may only be formed on a protective layer170bamong a seed layer138b, a first electrode140b, a piezoelectric layer150b, a second electrode160b, and a protective layer170b.

Referring toFIG.3B, a recess region having a depth D may only be formed on a protective layer170cand a second electrode160camong a seed layer138c, a first electrode140c, a piezoelectric layer150c, a second electrode160c, and a protective layer170c. For example, the protective layer170cmay be deposited at a uniform thickness over the entire area on the second electrode160cin which the recess region is formed.

Referring toFIG.3C, a recess region having a depth D may only be formed on a protective layer170d, a second electrode160d, and a piezoelectric layer150damong a seed layer138d, a first electrode140d, a piezoelectric layer150d, a second electrode160d, and a protective layer170d.

Referring toFIG.3D, a recess region having a depth D may only be formed on a protective layer170e, a second electrode160e, a piezoelectric layer150e, and a first electrode140eamong a seed layer138e, a first electrode140e, a piezoelectric layer150e, a second electrode160e, and a protective layer170e.

Referring toFIG.3E, a recess region having a depth D may be formed on each of a seed layer138f, a first electrode140f, a piezoelectric layer150f, a second electrode160f, and a protective layer170f.

For example, the first electrode140f, the piezoelectric layer150f, the second electrode160f, and the protective layer170fmay be deposited at a uniform thickness over the entire area on the seed layer138fin which the recess region is formed. That is, the bulk acoustic resonator according to an embodiment of the present disclosure may have a plurality of recess regions formed in different surfaces.

FIGS.4A to4Eare views illustrating a difference in depths between a plurality of recess regions of a bulk acoustic resonator according to embodiments of the present disclosure.

Referring toFIG.4A, a recess region having a deep depth (2D) may be formed on a protective layer170gamong a seed layer138g, a first electrode140g, a piezoelectric layer150g, a second electrode160g, and the protective layer170g. The recess region having a depth D may be formed in the second electrode160g. For example, the protective layer170gmay be deposited on the second electrode160gin which a recess region having a depth D is formed with a difference in thickness of the depth D.

Referring toFIG.4B, the recess region having a deep depth (2D) may be formed on the protective layer170hamong the seed layer138h, the first electrode140h, the piezoelectric layer150h, the second electrode160h, and the protective layer170h, and the recess region having a depth D may be formed in the second electrode160hand the piezoelectric layer150h.

Referring toFIG.4C, the recess region having a deep depth (2D) may be formed in the protective layer170iand the second electrode160iamong the seed layer138i, the first electrode140i, the piezoelectric layer150i, the second electrode160i, and the protective layer170i, and the recess region having a depth D may be formed in the piezoelectric layer150i.

Referring toFIG.4D, the recess region having a deep depth (2D) may be formed in the protective layer170jand the second electrode160jamong the seed layer138j, the first electrode140j, the piezoelectric layer150j, the second electrode160j, and the protective layer170j, and the recess region having a depth (D) may be formed in the piezoelectric layer150jand the first electrode140j.

Referring toFIG.4E, the recess region having a deep depth (2D) may be formed in the protective layer170k, and the recess region having a depth D may be respectively formed in the seed layer138k, the first electrode140k, the piezoelectric layer150k, and the second electrode160k. For example, in the first electrode140k, the piezoelectric layer150k, and the second electrode160k, the recess region having a depth D may be deposited with a uniform thickness over the entire area on the seed layer138kin which the recess region having a depth D is formed, and the protective layer170kmay be deposited on the second electrode160kwith a difference in thickness of the depth D.

That is, the bulk acoustic resonator according to an embodiment of the present disclosure may have a plurality of recess regions formed in different surfaces and having different depths from each other. Accordingly, a product of depth and width of each of the plurality of recess regions formed in different surfaces and having different depths may be different from each other.

Referring back toFIG.2, an acoustic wave of the raised region of the bulk acoustic resonator according to an embodiment of the present disclosure may have a vibration displacement according to Equation 1 below according to a wave equation, and an acoustic wave of the recess region may have a vibration displacement according to Equation 2 below according to a wave equation, and an acoustic wave of the active region may have a vibration displacement according to Equation 3 below.
U10ejβ1xEquation 1
U20cos(β2x−ϕ2)  Equation 2
U30Equation 3

β1 is kx(1/μm), a propagation number (or a wave number) of the raised region, β2 is kx(1/μm), a propagation number (or a wave number) of the recess region, U is a vibration displacement constant, φ is a phase constant, and x is a coordinate in a direction corresponding to a width.

The vibration displacement and stress (including a gradient component of the vibration displacement) may be continuous at an interface between the raised and recess regions and an interface between the recessed and active regions.

Equation 4 below represents a combination of Equations 1 and 2 and wave equations in a state in which an x value at the interface between the raised and recess regions is defined as 0.

Equation 5 below represents a combination of Equations 2, 3, and wave equations in a state in which the x value at the interface between the recessed and active regions is defined as W.

According to Equations 4 and 5, the width W may be a result value of a function in which φ2 and β2 are applied as variables, and φ2 may be a result value of a function in which β1 and β2 are applied as variables.

As the width of the recess region is closer to the width according to Equations 4 and 5, the active region is in a piston mode state with substantially no surface acoustic waves, thereby suppressing spurious resonance, in the raised region, energy leakage may be suppressed by attenuating acoustic waves, and the recess region may further smoothly connect between the active region and the recess region.

For example, as the width of the recess region is closer to the width according to Equations 4 and 5, spurious noise in a frequency range, lower than the resonance frequency of the bulk acoustic resonator may decrease, the resonance frequency can be formed more sharply, and an insertion loss near the resonance frequency can be reduced. Accordingly, a skirt characteristic of a filter including a bulk acoustic resonator may be improved, and an energy loss (e.g., an insertion loss and a return loss) may be further reduced.

FIG.5is a graph illustrating an increase in an acoustic wave number as the depth of a plurality of recess regions increases.

D09ofFIG.5represents a dispersion curve, a change curve of the wave number (β1) according to the frequency (Freq) of the raised region, D10represents a dispersion curve, a change curve of the wave number according to the frequency (Freq) of the active region, D11represents a dispersion curve, a change curve of the wave number (β2) according to the frequency (Freq) when the depth (D) of the recess region is the shortest, D12represents a dispersion curve, a change curve of the wave number (β2) according to the frequency (Freq) when the depth (D) of the recess region is the second shortest, D13represents a dispersion curve, a change curve of the wave number (β2) according to the frequency (Freq) when the depth (D) of the recess region is the third shortest, D14denotes a dispersion curve, a change curve of the wave number (β2) according to the frequency (Freq) when the depth (D) of the recess region is the fourth shortest, and D15denotes a dispersion curve, a change curve of the wave number (β2) according to the frequency (Freq) when the depth (D) of the recess region is the fifth shortest.

The wave number is a physical value that can be determined by physical properties and thickness of a corresponding region, and may correspond to kx (a propagation number). When kx is negative, it means an imaginary value of a complex number (real+imaginary number). That is, the real number of kx means the vibration of the acoustic wave, the imaginary number of kx means the attenuation of the acoustic wave, and the complex number of kx means that the acoustic wave is attenuated as it vibrates.

Referring toFIG.5, the wave number of the active region at 3.5 GHz may be 0, the wave number of the raised region at 3.5 GHz may be a negative number, and the wave number of the recess region at 3.5 GHz may be a positive number, and the wave number may increase as the depth of the recess region increases.

FIGS.6A and6Bare graphs illustrating an optimal area curve of a recess region located on an upper surface of a second electrode.

When the wave number at a specific frequency (e.g., 3.5 GHz) ofFIG.5is applied to Equations 4 and 5, it can be seen that the optimal width and depth of the recess region are in inverse proportion to each other, and an optimal area curve R14such asFIG.6Amay be obtained. Here, the optimal area means the product of the width and the depth.

Referring toFIG.6B, since an optimal area (depth×width) in an optimal area curve R24may be saturated at about 0.015 (μm)2, an intermediate value of an optimal area range of the recess region may be 0.015 (μm)2, and since a maximum deviation of the optimal area range of the recess region may be about 0.007 (μm)2, the optimal area range of the recess region may be 0.008 (μm)2or more and 0.022 (μm)2or less.

FIGS.6A and6Billustrate values based on a structure in which a recess region is formed in the second electrode, but since the physical properties and thickness of the first electrode and the second electrode may be similar, so an optimal area curve in which the recess region is formed in the first electrode may also be similar to that ofFIGS.6A and6B.

Accordingly, in the bulk acoustic resonator according to an embodiment of the present disclosure, by including a structure in which a recess region having a product (D*W) of a depth (D) and a width (W) of 0.008 (μm)2or more and 0.022 (μm)2or less formed in the first and second electrodes, the bulk acoustic resonator according to an embodiment of the present disclosure may have improved performance (e.g., spurious noise reduction, sharpness of an (anti)-resonance frequency, or the like) based on the width of the recess region, close to the width according to Equation 5.

Referring toFIG.6B, when the depth D of the recess region is less than 1% of the thickness of the piezoelectric layer, the slope of the optimal area (depth×width) in the optimal area curve R24may be relatively steep.

The depth D of the recess region may be 1% or more of the thickness of the piezoelectric layer. Accordingly, since the optimal area (depth×width) can be stable, the influence received from the dispersion of the process of manufacturing the bulk acoustic resonator can be small. However, depending on the structure, shape, material, and required standard of the bulk acoustic resonator, the depth D of the recess region may be designed to be less than 1% of the thickness of the piezoelectric layer.

Alternatively, the depth of the recess region may be 5 nm or more. Accordingly, it can be prevented that the depth of the recess region is greatly influenced by the dispersion of the process of manufacturing the bulk acoustic resonator as the depth of the recess region is too thin.

FIGS.6C and6Dare graphs illustrating optimal area curves of recess regions located on upper surfaces of a seed layer, a first electrode (lower electrode), a piezoelectric layer, a second electrode (upper electrode), and a protective layer.

Referring toFIGS.6C and6D, the optimal area curves R31and R41of the seed layer, the optimal area curves R32and R42of the first electrode, the optimal area curves R33and R43of the piezoelectric layer, the optimal area curves R34and R44of the second electrode, and the optimal area curves R35and R45of the protective layer may be different from each other.

The optimal area curves R31and R41of the seed layer may be values based on the seed layer containing AlN, the optimal area curves R33and R43of the piezoelectric layer may be values based on the piezoelectric layer containing ScAlN, the optimal area curves R35and R45of the protective layer may be values based on the protective layer containing SiO2, and the optimal area curves R32and R42of the first electrode and the optimal area curves R34and R44of the second electrode may be values based on the first and/or second electrodes containing molybdenum (Mo). Depending on the design, the seed layer or the protective layer may be replaced with the same material or different materials among AlN or SiO2, and in this case, a relative position of the graph may also be changed. In addition, a scandium (Sc) concentration of the piezoelectric layer ScAlN can be increased from 0 at. % depending on the design.

The overall optimal area (depth×width) of the seed layer may be greater than the overall optimal area (depth×width) of the protective layer, the overall optimal area (depth×width) of the protective layer may be greater than the overall optimal area (depth×width) of the piezoelectric layer, and the overall optimal area (depth×width) of the piezoelectric layer may be greater than the overall optimal area (depth×width) of the first and/or second electrodes.

Accordingly, the bulk acoustic resonator according to an embodiment of the present disclosure may include a plurality of recess regions, and the optimal area (depth×width) of the recess region formed in the seed layer among the plurality of recess regions may be greater than the optimal area (depth×width) of the recess region formed in the protective layer, the optimal area (depth×width) of the recess region formed in the protective layer among the plurality of recess regions may be greater than the optimal area (depth×width) of the recess region formed in the piezoelectric layer, and the optimal area (depth×width) of the recess region formed in the piezoelectric layer among the plurality of recess regions may be greater than the optimal area (depth×width) of the recess region formed in the first and/or second electrodes. For example, one of the depth and width of each component may be the same, and the other may be different.

Accordingly, in the bulk acoustic resonator according to an embodiment of the present disclosure, since each of the plurality of recess regions may have a structure, close to the optimal area (depth×width), the bulk acoustic resonator may have improved performance (e.g., spurious noise reduction, sharpness of a resonance frequency, or the like) based on the width of the recess region, close to the width according to Equations 4 and 5.

FIGS.7A and7Bare graphs illustrating a difference in dispersion curves compared to active when a recessed region having the same depth is formed in each of a seed layer, a first electrode, a piezoelectric layer, a second electrode, and a protective layer.

Referring toFIGS.7A and7B, kx according to a frequency (Freq) of the active region (Active), kx according to the frequency (Freq) of the seed layer (Seed), kx according to the frequency (Freq) of the protective layer (Passivation), kx of the frequency (Freq) of the piezoelectric layer (PZL), and kx of the frequency (Freq) of the first and second electrodes BE and TE may be different from each other. The optimal area curves shown inFIGS.6C and6Dmay be a value based on this. An amount of displacement of dispersion curves is different depending on which layer the recess region is formed on, and accordingly, a value of kx meeting a cutoff frequency of the active region may be different from each other. Due to this effect, the optimal area of the recess region may be different for each layer.

A specific value of kx shown inFIGS.7A and7Bmay vary slightly depending on the position, material, density, stiffness, and thickness of each of the seed layer, the first electrode, the piezoelectric layer, the second electrode, and the protective layer, which is a value when the depth of the recess region is 10 nm.

FIGS.8A and8Bare graphs illustrating an optimal area curve in which a thickness variable of a piezoelectric layer is added to an optimal area curve of a recess region located on an upper surface of a seed layer, a first electrode, a piezoelectric layer, a second electrode, and a protective layer.

FIG.8Aillustrates an optimal area curve when a thickness of the piezoelectric layer is 345 nm, andFIG.8Billustrates an optimal area curve when the thickness of the piezoelectric layer is 600 nm.

Referring toFIGS.8A and8B, optimal area curves R51and R61of the seed layer, optimal area curves R52and R62of the first electrode, optimal area curves R53and R63of the piezoelectric layer, optimal area curves R54and R64of the second electrode, and optimal area curves R55and R65of the protective layer may have a steep slope when a depth D of the recess region is less than 1% of thickness of the piezoelectric layer, and may have a gentle slope, when it is 1% or more the thickness of the piezoelectric layer.

That is, regardless of the location or number of the recess regions of the bulk acoustic resonator according to an embodiment of the present disclosure, the depth D of the recess region may be 1% or more of the thickness of the piezoelectric layer. Accordingly, since the optimal area (depth×width) can be stable, an influence received from a dispersion of the process of manufacturing the bulk acoustic resonator can be small. However, depending on the structure, shape, material, and required standard of the bulk acoustic resonator, the depth D of the recess region may also be designed to be less than 1% of the thickness of the piezoelectric layer.

FIGS.9A and9Bare graphs illustrating a change in an optimal area according to a depth of a recess region located on an upper surface of a seed layer, a first electrode, a piezoelectric layer, a second electrode, and a protective layer.

FIG.9Aillustrates an optimal area curve when a thickness of the piezoelectric layer is 345 nm, andFIG.9Billustrates an optimal area curve when the thickness of the piezoelectric layer is 600 nm.

Referring toFIGS.9A and9B, optimal area curves R72and R82of the first electrode, optimal area curves R73and R83of the piezoelectric layer, and optimal area curves R74and R84of the second electrode may have characteristics of being saturated in a specific optimal area, and optimal area curves R71and R81of the seed layer and optimal area curves R75and R85of the protective layer may not have the characteristic that the optimal area is saturated, and may be saturated when the depth is deeper.

The optimal area (depth×width) in the optimal area curves R73and R83of the piezoelectric layer optimal area curves can be saturated between about 0.02 (μm)2and 0.024 (μm)2, and since a maximum deviation of an optimal area range of the recessed range may be about 0.01 (μm)2, the optimal area range of the recess region of the piezoelectric layer may be 0.012 (μm)2or more and 0.032 (μm)2or less.

The bulk acoustic resonator according to an embodiment of the present disclosure may include a product (D*W) of a depth (D) and a width (W) of the recess region of the piezoelectric layer of 0.012 (μm)2or more and 0.032 (μm)2or less formed in the piezoelectric layer, such that it can have improved performance based on the width of the recess region, close to the width according to Equations 4 and 5 (e.g., spurious noise reduction, sharpness of a resonance frequency, or the like).

Meanwhile, a value of the optimal area curve ofFIG.9Ais a value based on a protective layer having a thickness of 130 nm, a second electrode having a thickness of 174 nm, a first electrode having a thickness of 215 nm, and a seed layer having a thickness of 57 nm, and a value of the optimal area curve ofFIG.9Bis a value based on a protective layer having a thickness of 130 nm, a second electrode having a thickness of 120 nm, a first electrode having a thickness of 161 nm, and a seed layer having a thickness of 57 nm.

For example, since the thickness of the second electrode may be 174 nm or less, the depth D of the recess region formed in the upper surface of the second electrode may be 5/174 times or more and less than the thickness of the second electrode. Accordingly, since the optimal area (depth×width) can be stable, an influence received from a dispersion of the process of manufacturing the bulk acoustic resonator can be small.

FIGS.10A and10Bare graphs illustrating a difference in spurious noise in a frequency range, lower than the resonance frequency according to the presence or absence of a recess region.

Referring toFIGS.10A and10B, since an S parameter S11(SR) of the bulk acoustic resonator having a recess region may have a ripple, smaller than that of an S parameter S11(SW) of the bulk acoustic resonator having no recess region, it can have further lower spurious noise.

FIG.11Ais a graph illustrating spurious noise according to a product of a width and a depth of a recess region.

Referring toFIG.11A, spurious noise may be the lowest when a product of width and depth of the recess regions of the first and/or second electrodes is 0.010 (μm)2or more and 0.016 (μm)2or less. Values inFIG.11Aare values based on a recess region having a depth of 8 nm.

Accordingly, the bulk acoustic resonator according to an embodiment of the present disclosure may have a recess region in which the product of the width and depth of the recess regions of the first and/or second electrode is 0.010 (μm)2or more and 0.016 (μm)2or less, so that spurious noise may be further reduced.

FIG.11Bis a graph illustrating an optimal area according to a resonance frequency of a recess region of the first and/or second electrode.

Referring toFIG.11B, a curve SN of the optimal area (width×depth) of the recess region of the first and/or second electrode may be an exponential function of the resonance frequency, and may be a curve approximated based on a number of large points (data according to production). Values inFIG.11Bare values based on the resonator having a width in a horizontal direction of 4900 (μm)2.

Accordingly, [{ln(D*W)}/{−0.59*F}] of the recess region of the first and/or second electrode of the bulk acoustic resonator according to an embodiment of the present disclosure may be [[ln{0.008 (μm)2}]/{−0.59*(3.5 GHz)}] or more and [[ln{0.022 (μm)2}]/{−0.59*(3.5 GHz)}] or less. Here, F may denote the resonance frequency of the bulk acoustic resonator, and can be determined based on a dimension of the resonator of the bulk acoustic resonator. ln is a natural logarithm. Accordingly, the bulk acoustic resonator according to an embodiment of the present disclosure may have improved performance (e.g., spurious noise reduction, sharpness of a resonance frequency, etc.) over a wide frequency range.

FIG.12is a graph illustrating spurious noise according to a width of a recess region.

Referring toFIG.12A, when a depth D2of a recess region of a first and/or a second electrode is 8 nm, spurious noise may be lowest when the width of the recess region is 1.4 μm. The width of the recess region in which spurious noise is the lowest may be shorter when the recess region has a long depth D1, and may be longer when the recess region has a short depth D3.

For example, when the depth of the recess region of the first and/or second electrode is 10 nm, an optimal width of the recess region may be 3.6 μm. For example, when the depth of the recess region of the first and/or second electrode is 20 nm, the optimal width of the recess region may be 1.8 μm.

FIG.12Bis a graph illustrating an optimal area of a recess region of a piezoelectric layer according to a resonance frequency.

Referring toFIG.12B, a curve SN of an optimal area (width×depth) of a recess region of a piezoelectric layer may be an exponential function of a resonance frequency, and may be a curve approximated based on a number of large points (data according to production). A value ofFIG.12Bare values based on a resonance frequency having a width of 4900 (μm)2in a horizontal direction.

Therefore, [{ln(D*W)}/{−0.412*F}] of the recess region of the bulk acoustic resonator according to an embodiment of the present disclosure may be [[ln{0.015 (μm)2}]/{−0.412*(3.5 GHz)}] or more and [[ln{0.015 (μm)2}]/{−0.412*(3.5 GHz)}] or less. Accordingly, the bulk acoustic resonator according to an embodiment of the present disclosure may have improved performance (e.g., spurious noise reduction, sharpness of a resonance frequency, and the like) over a wide frequency range.

FIG.13is a view illustrating a specific form of a bulk acoustic resonator according to an embodiment of the present disclosure.

Referring toFIG.13, a bulk acoustic resonator according to an embodiment of the present disclosure may include at least a portion of a substrate110, a cavity112, an insulating layer120, a sacrificial layer130, an etch stop layer132, a seed layer138, a first electrode140, a piezoelectric layer150, a second electrode160, a protective layer170, an insertion layer172, and a metal pad180. At least a portion of the structure shown inFIG.13may have the same material as at least a portion of the structure of the bulk acoustic resonator shown inFIG.1, or may be formed by the same or similar process as the process of forming at least a portion of the structure of the bulk acoustic resonator shown inFIG.1.

The piezoelectric layer150, the second electrode160, and/or the protective layer170may include a recess region (Recessed), and may further include a raised region (Raised) and an active region (Active).

The insertion layer172may be formed on an upper surface of the first electrode140so that the piezoelectric layer150, the second electrode160and/or the protective layer170have a recess region. For example, the insertion layer172and the etch stop layer132may have an insulating material that is the same as or similar to the material of the insulating layer120, and may be formed in the same or similar process as the process of forming the insulating layer120.

As set forth above, according to one or more embodiments of the present disclosure, a bulk acoustic resonator may reduce spurious noise of the bulk acoustic resonator, may form a resonance frequency of the bulk acoustic resonator more sharply, and may reduce an insertion loss near the resonance frequency. Accordingly, a skirt characteristic of a filter including a bulk acoustic resonator may be improved, and an energy loss (e.g., insertion loss and return loss) may be further reduced.