Film bulk acoustic resonator with suspended interdigital electrode and preparation method thereof

Abstract

The invention provides a film bulk acoustic resonator with a suspended interdigital electrode and a preparation method of the film bulk acoustic resonator, and belongs to the technical field of acoustic resonators. The film bulk acoustic resonator comprises a substrate, a piezoelectric layer located on the substrate and an interdigital transducer located on the piezoelectric layer. A cavity is formed between the substrate and the piezoelectric layer; through holes communicated with the cavity are formed in the positions, located on the two sides of the interdigital transducer, of the piezoelectric layer. A dielectric layer is arranged on the lower surface of an interdigital electrode of the interdigital transducer, and a gap exists between the dielectric layer and the piezoelectric layer. The interdigital electrode is suspended above the piezoelectric layer, so that stray modes caused by acoustic impedance mismatching and mechanical load of the interdigital electrode and the piezoelectric layer are remarkably reduced, and the quality factor of the device is increased.

Description

Technical Field

[0001] The invention belongs to the technical field of acoustic wave resonators, and in particular relates to a thin film bulk acoustic wave resonator with suspended interdigital electrodes and a preparation method thereof. Background Art

[0002] The acoustic wave resonator is the basic unit of the acoustic wave filter. The transversely excited film bulk acoustic wave resonator (XBAR) can utilize the high acoustic velocity and large electromechanical coupling coefficient (k) in the piezoelectric thin plate. eff 2 >20%), making it an ideal solution for broadband filters above 6 GHz to solve the insertion loss (IL) caused by increased electrode resistance, reduced power handling capability, and increased lithography costs. XBAR can be used in various radio frequency (RF) filters, including band-stop filters, bandpass filters, duplexers, and multiplexers. However, the XBAR devices in the prior art have the problem of large spurious mode response, which in turn affects the main mode (e.g., first-order antisymmetric Lamb wave A1), resulting in the inability to better achieve the target acoustic mode. Therefore, the existence of spurious modes remains the main bottleneck for further promoting XBAR devices into practical large-scale applications, because it will generate unnecessary ripples when forming the filter, increasing the insertion loss of the device.

[0003] In order to reduce lateral stray modes, the U.S. patent application publication US20230111410A1 discloses a lateral excitation thin film bulk acoustic wave resonator with a curved finger-shaped end or a relative busbar, having a substrate with a surface and a single crystal piezoelectric plate, a portion of the piezoelectric plate forming a diaphragm across the cavity in the substrate. An interdigital transducer (IDT) is formed on the surface of the single crystal piezoelectric plate, so that the staggered fingers of the interdigital transducer are arranged on the diaphragm. The ends of the staggered fingers of the IDT or the inner surface of the relative busbar have a curved shape. In this scheme, the distance between the busbar and the end of the IDT is increased by setting the edge between the busbar and the IDT to an arc shape or a parabola shape, so as to reduce the influence of the propagation of lateral acoustic waves (vibrations excited by the electric field Ey in a direction parallel to the IDT fingers and perpendicular to the busbar), thereby reducing the lateral stray modes. However, the arc-shaped boundary between the IDT and the bus bar will focus and enhance the acoustic waves of other stray modes (except the stray modes caused by the electric field Ey) transmitted to the end of the IDT and reflect them to the IDT.

[0004] The Chinese patent application publication CN116529557A discloses a transversely excited thin film bulk acoustic resonator with spiral interdigital transducer fingers, including a substrate and a piezoelectric plate, a portion of which is a diaphragm spanning a cavity in the substrate. The conductor pattern on the front surface of the piezoelectric plate includes interlaced interdigital transducer (IDT) fingers alternately connected to a first busbar and a second busbar. The interlaced IDT fingers are on the diaphragm, and the interlaced IDT fingers include at least a first pair of interlaced spiral IDT fingers. In this scheme, since the electrode edges of the spirally structured IDT are not parallel, the shear wave has a different reflection path at each point on the electrode edge, and the propagation path becomes longer. When the shear wave propagation path becomes longer, the energy of the standing wave is attenuated very low, so the stray mode amplitude on the impedance curve is greatly reduced. However, in this solution, the spiral structure of the electrode leads to a longer electrode length, greater resistance and thermal resistance, which in turn increases the RF current in the IDT, increases the TCF (Temperature Coefficient of Frequency) of the device, reduces the quality factor (Q value), increases power consumption, and exacerbates the heat dissipation and temperature drift problems of the device.

[0005] US patent application US17133498 discloses a transversely excited thin film bulk acoustic resonator with two layers of electrodes having a narrower top layer, comprising a substrate having a surface and a single crystal piezoelectric plate having a front surface and a rear surface, wherein the rear surface is attached to the surface of the substrate in the substrate except for a portion of the piezoelectric plate forming a diaphragm spanning the cavity. The device also includes an interdigital transducer formed on the front surface of the piezoelectric plate, wherein the staggered fingers of the IDT arranged on the diaphragm are configured so that the radio frequency signal applied to the IDT excites the main shear acoustic mode in the diaphragm. The staggered fingers include a first metal layer adjacent to the diaphragm and a second metal layer located above the first layer, the second metal layer having a narrower width than the first metal layer. The second metal layer uses a metal with high transverse acoustic impedance, and the first metal layer uses a metal with low acoustic impedance. Through this structural setting, the transition from low transverse acoustic impedance to high transverse acoustic impedance can limit some parasitic modes to the first metal layer, reducing acoustic losses. However, this solution requires the deposition of two layers of IDT finger electrodes and the control of the sidewall angle of the IDT, thus increasing the difficulty and complexity of the process. Summary of the invention

[0006] Based on the above technical problems, the present invention provides a thin film bulk acoustic wave resonator with suspended interdigital electrodes and a manufacturing method thereof. The acoustic wave resonator significantly reduces the stray modes caused by the acoustic impedance mismatch between the interdigital electrodes and the piezoelectric layer and the mechanical load by suspending the interdigital electrodes above the piezoelectric layer, thereby increasing the quality factor of the device.

[0007] Specifically, the present invention adopts the following technical solutions to achieve the above objectives:

[0008] A thin film bulk acoustic wave resonator with suspended interdigital electrodes comprises a substrate, a piezoelectric layer located on the substrate and an interdigital transducer located on the piezoelectric layer; a cavity is formed between the substrate and the piezoelectric layer; the interdigital transducer comprises interdigital electrodes; through holes are provided on the piezoelectric layer at positions on both sides of the interdigital transducer along a first direction; the through holes are connected to the cavity; a dielectric layer is provided on the surface of the interdigital electrodes on one side close to the piezoelectric layer; a gap is provided between the dielectric layer and the piezoelectric layer; the first direction is a direction perpendicular to the direction in which the interdigital electrodes extend and parallel to the piezoelectric layer.

[0009] In a preferred solution, the film bulk acoustic wave resonator with suspended interdigital electrodes further comprises a supporting structure; the supporting structure is located between the piezoelectric layer and the dielectric layer.

[0010] In a further preferred embodiment, the material of the support structure includes at least one of silicon dioxide, silicon nitride, polysilicon, aluminum, aluminum alloy, copper, copper alloy, chromium, titanium, platinum, beryllium, gold, molybdenum, and tungsten.

[0011] In a preferred embodiment, the height of the gap in the direction from the substrate to the piezoelectric layer is 20-200 nm.

[0012] In a preferred embodiment, the thickness of the dielectric layer in the direction from the substrate to the piezoelectric layer is 10-250 nm.

[0013] In a preferred embodiment, the material of the dielectric layer is at least one of silicon dioxide, aluminum oxide, and silicon nitride.

[0014] In a preferred embodiment, the thickness of the piezoelectric layer along the direction from the substrate to the piezoelectric layer is 100-1000 nm.

[0015] In a preferred embodiment, the material of the piezoelectric layer is a single crystal material of at least one of lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, aluminum nitride, and scandium-doped aluminum nitride; wherein the scandium content in the scandium-doped aluminum nitride is: 0<Sc≤45wt%.

[0016] In a preferred embodiment, the material of the IDT is at least one of aluminum, aluminum alloy, copper, copper alloy, chromium, titanium, platinum, beryllium, gold, molybdenum and tungsten.

[0017] In a preferred embodiment, the substrate includes a base and an intermediate layer, and the material of the intermediate layer is at least one of silicon dioxide, silicon nitride, and polysilicon.

[0018] In a further preferred embodiment, along the direction from the substrate to the piezoelectric layer, the thickness of the intermediate layer is 10 nm to 50 μm.

[0019] The present invention also provides a method for preparing the thin film bulk acoustic wave resonator with suspended interdigital electrodes as described in any of the above solutions, comprising the following steps:

[0020] S1. providing a substrate, and forming a piezoelectric layer on the substrate;

[0021] S2, forming a first bus bar and a second bus bar on the upper surface of the piezoelectric layer;

[0022] S3, forming a sacrificial layer on the upper surface of the piezoelectric layer except for the first bus bar and the second bus bar and patterning the layer;

[0023] S4, forming a dielectric layer on the upper surface of the sacrificial layer and patterning it;

[0024] S5, forming a first electrode and a second electrode on the upper surface of the dielectric layer, the upper surface of the first bus bar, and the upper surface of the second bus bar to obtain an interdigital transducer;

[0025] S6, etching through holes on the piezoelectric layer on both sides of the IDT along the first direction;

[0026] S7. Etching the sacrificial layer and / or the middle portion of the substrate through the through hole, so that the first electrode and the second electrode are suspended above the piezoelectric layer, and a cavity is formed between the piezoelectric layer and the substrate.

[0027] In a preferred embodiment, step S2 includes the step of forming a support structure on the upper surface of the piezoelectric layer.

[0028] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0029] (1) In the present invention, by partially or completely suspending the interdigital electrodes above the upper surface of the piezoelectric layer, the contact area between the two can be reduced, thereby reducing the interface acoustic wave loss caused by the mismatch of the acoustic impedance of the two materials, thereby reducing the stray modes of the acoustic wave resonator.

[0030] (2) The present invention further decouples the interdigital electrodes from the piezoelectric layer by adding a dielectric layer between the piezoelectric layer and the interdigital electrodes, thereby reducing the intensity of the stray modes generated by the overlapping portion of the piezoelectric layer and the electrodes, thereby achieving the effect of suppressing the stray modes.

[0031] (3) Since the spurious modes are significantly suppressed, the quality factor of the acoustic wave resonator is significantly improved and the energy loss is significantly reduced.

[0032] (4) In the acoustic wave resonator structure of the present invention, as the distance between the dielectric layer and the piezoelectric layer increases, the electromechanical coupling coefficient of the acoustic wave resonator gradually decreases, which indicates that the acoustic wave resonator structure of the present invention can adjust the bandwidth of the filter and improve the adjacent band suppression performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 A top view of a thin film acoustic wave resonator with suspended interdigital electrodes according to the present invention;

[0034] Figure 2 for Figure 1 A cross-sectional view of a thin film acoustic wave resonator with suspended interdigitated electrodes along section AA;

[0035] Figure 3 for Figure 1 A cross-sectional view of a thin film acoustic wave resonator with suspended interdigitated electrodes along section BB;

[0036] Figure 4 Another thin film acoustic wave resonator with suspended interdigital electrodes according to the present invention is shown along section AA (reference Figure 1 ) cross-sectional view;

[0037] Figure 5 for Figure 4 The interdigitated electrodes of the film acoustic wave resonator are suspended along the cross section BB (reference Figure 1 ) cross-sectional view;

[0038] Figure 6 A top view of another thin film acoustic wave resonator with suspended interdigital electrodes according to the present invention;

[0039] Figure 7 for Figure 6 A cross-sectional view of a thin film acoustic wave resonator with suspended interdigitated electrodes along section AA;

[0040] Figure 8 for Figure 6 A cross-sectional view of a thin film acoustic wave resonator with suspended interdigitated electrodes along section BB;

[0041] Fig. 9 A comparison diagram of the admittance curves of a thin film acoustic wave resonator with suspended interdigital electrodes and a thin film acoustic wave resonator with non-suspended interdigital electrodes;

[0042] Fig.10 This is a trend diagram of the change in the gap height of the admittance curve of the thin film acoustic wave resonator with suspended interdigital electrodes provided in Example 2 of the present invention.

[0043] In the figure: 1, substrate; 11, base; 12, intermediate layer; 2, piezoelectric layer; 21, through hole; 3, interdigital transducer; 31, first bus bar; 32, second bus bar; 311, first electrode; 321, second electrode; 4, dielectric layer; 5, gap; 6, cavity; 7, supporting structure; AA, cross section; BB, cross section. DETAILED DESCRIPTION

[0044] The following content is combined with the embodiments to clearly and completely describe the technical solution of the present application so that those skilled in the art can fully understand the present application. Obviously, the described embodiments are only some preferred embodiments of the present application, rather than all embodiments. Any equivalent transformation or substitution made to the following implementation modes by those of ordinary skill in the art without creative work belongs to the protection scope of the present application.

[0045] Directional terms described in this application, such as "upper", "lower", "inner", "outer", "bottom", "upper surface", etc., indicate directions or positional relationships based on directions or positional relationships in the drawings of the specification, or directions or positional relationships in which the products of this application are usually placed when in use, and are only for the convenience of describing and understanding the product structure of this application. Therefore, directional terms cannot be understood as limiting this application. In this application, unless otherwise clearly defined, expressions such as "upper", "above", "above", and "upper surface" of a first feature on a second feature indicate that the first feature and the second feature may be in direct contact or indirect contact through an intermediate medium; the first feature may be directly above or obliquely above the second feature, or simply indicate that the first feature is higher in level than the second feature. Expressions such as "lower", "below", "below", and "lower surface" of a first feature on a second feature indicate that the first feature and the second feature may be in direct contact or indirect contact through an intermediate medium; the first feature may be directly below or obliquely below the second feature, or simply indicate that the first feature is lower in level than the second feature. Ordinal numbers used in this application, such as "first", "second", etc., are only used for descriptive purposes to distinguish similar objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. High-resistance silicon in this application refers to a silicon substrate with a resistivity greater than 10000Ω·cm. The methods not described in detail in the following embodiments are all conventional methods well known to those skilled in the art.

[0046] Example 1

[0047] Reference Figure 1 to Figure 3, a thin film bulk acoustic wave resonator with suspended interdigital electrodes, comprising a substrate 1, a piezoelectric layer 2 located on the substrate 1, and an interdigital transducer 3 located on the piezoelectric layer 2. A groove is provided on the substrate 1; the opening of the groove is on the surface of the substrate 1 close to the piezoelectric layer 2, and the bottom is located in the substrate 1. The groove and the piezoelectric layer 2 together form a cavity 6. The interdigital transducer 3 comprises a first bus bar 31, a second bus bar 32, a first electrode 311, and a second electrode 321. The direction perpendicular to the direction in which the first electrode 311 extends (or the direction in which the second electrode 321 extends) and parallel to the piezoelectric layer 2 is defined as a first direction, the direction from the substrate 1 to the piezoelectric layer 2 or from the piezoelectric layer 2 to the substrate 1 is defined as a second direction, and the direction in which the first electrode 311 extends is defined as a third direction. The first direction, the second direction, and the third direction are perpendicular to each other. The first electrode 311 extends from the surface of the first bus bar 31 away from the piezoelectric layer 2 along the third direction toward the second bus bar 32. The second electrode 321 extends from the surface of the second bus bar 32 away from the piezoelectric layer 2 along the third direction toward the first bus bar 31. The first electrode 311 and the second electrode 321 are arranged alternately along the first direction. Along the first direction, two through holes 21 are opened on the piezoelectric layer 2 on both sides of the interdigital transducer, and the through holes 21 are connected to the cavity 6. The piezoelectric layer 2 and the interdigital transducer 3 are configured so that the radio frequency signal applied to the interdigital transducer 3 excites the main shear acoustic mode in the piezoelectric layer 2. A dielectric layer 4 is provided on the surface of the first electrode 311 and the second electrode 321 on one side close to the piezoelectric layer 2. There is a gap 5 between the surface of the dielectric layer 4 away from the first electrode 311 and the second electrode 321 and the surface of the piezoelectric layer 2 away from the substrate 1, that is, the dielectric layer 4 is suspended above the piezoelectric layer 2, so that one end of the first electrode 311 and the second electrode 321 are attached to the first bus bar 31 and the second bus bar 32 respectively, and the other end is suspended above the piezoelectric layer 2.

[0048] As an example, the substrate 1 includes a base 11 and an intermediate layer 12, wherein the intermediate layer 12 is located between the base 11 and the piezoelectric layer 2 and at both ends of the base 11, so that a groove is formed between the intermediate layer 12 and the surface of the base 11 close to the piezoelectric layer, and the groove and the piezoelectric layer 2 together form a cavity 6. Alternatively, a groove is provided on the base 11, the opening of the groove faces the piezoelectric layer 2, and the bottom is located inside the base 11; the groove and the intermediate layer 12 and the piezoelectric layer 2 together form a cavity 6. Alternatively, the substrate only includes the base 11 (such as Figure 4 , Figure 5 As shown), a groove is provided on the substrate 11, the opening of the groove faces the piezoelectric layer 2, and the bottom is located inside the substrate 11; the groove and the piezoelectric layer 2 together form a cavity 6.

[0049] As an example, the material of the substrate 11 is any one of high-resistance silicon, silicon carbide, diamond, and sapphire, or a combination of several of them.

[0050] As an example, the material of the intermediate layer 12 is any one of silicon dioxide, silicon nitride, and polysilicon, or a combination of several thereof. For example, the material of the intermediate layer 12 is silicon dioxide.

[0051] As an example, the dimension of the intermediate layer 12 along the second direction (i.e., the thickness of the intermediate layer 12) is 10nm to 50μm, for example, 10nm, 11nm, 12nm, 15nm, 20nm, 30nm, 50nm, 100nm...1000nm, 1001nm, 1005nm, 1100nm...2μm, 2.1μm, 2.2μm...10μm, 10.1μm, 10.2μm, 10.5μm, 11μm, 12μm...50μm.

[0052] As an example, the material of the piezoelectric layer 2 is any single crystal material or a combination of any single crystal materials selected from lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, aluminum nitride, and scandium-doped aluminum nitride (scandium content is 0<Sc≤45wt%, for example, 4wt%, 10wt%, 15wt%, 20wt%, 25wt%, 30wt%, 35wt%, 40wt%, and 45wt%). Anisotropic piezoelectric materials have rich piezoelectric coefficients (e), relative dielectric constants (ε), and elastic coefficients (c). Adjustment of the crystal tangent, Euler angle, and propagation direction will further change the above physical parameters. For example, the material of the piezoelectric layer 2 can be set to single crystal lithium niobate, and the cut type can be Z-cut, YX128°cut, or other preset cut types. Wherein X represents the direction parallel to the first direction, Y represents the direction parallel to the third direction, and Z represents the direction parallel to the second direction.

[0053] As an example, the dimension of the piezoelectric layer 2 along the second direction (i.e., the thickness of the piezoelectric layer 2) is 100 to 1000 nm, for example, 100 nm, 101 nm, 102 nm, 105 nm, 110 nm...500 nm, 501 nm, 502 nm, 503 nm...1000 nm; typically, 100 nm, 200 nm, 210 nm, 220 nm, 240 nm, 250 nm, 270 nm, 300 nm, 350 nm, 600 nm or 800 nm.

[0054] As an example, the dimension of the through hole 21 along the first direction (the width of the through hole 21) is 5 to 30 μm, for example, 5 μm, 5.5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 15 μm, 16 μm, 18 μm ... 30 μm, and the dimension along the third direction (the length of the through hole 21) is 60 to 150 μm, for example, 60 μm, 61 μm, 62 μm, 63 μm, 65 μm, 70 μm, 75 μm, 80 μm ... 130 μm, 135 μm, 140 μm, 142 μm, 146 μm, 149 μm, 150 μm. The through hole 21 is used to release the substrate 1 so that a cavity 6 is formed between the substrate 1 and the piezoelectric layer 2.

[0055] As an example, the structures of the first bus bar 31 , the second bus bar 32 , the first electrode 311 , and the second electrode 321 are all one or multiple conductive material layers.

[0056] As an example, a portion of the first bus bar 31, the second bus bar 32, the first electrode 311, and the second electrode 321 has a structure of a single conductive material layer, and another portion has a structure of multiple conductive material layers. For example, the first bus bar 31 and the second bus bar 32 are both multiple conductive material layers, and the first electrode 311 and the second electrode 321 are both single conductive material layers; or the first bus bar 31 and the first electrode 311 are both multiple conductive materials, and the second bus bar 32 and the second electrode 321 are both single conductive material layers.

[0057] As an example, the dimensions of the first bus bar 31, the second bus bar 32, the first electrode 311, and the second electrode 321 along the second direction (i.e., the thickness of the first bus bar 31, the second bus bar 32, the first electrode 311, and the second electrode 321) may be the same or different. For example, the thickness of the first bus bar 31 and the thickness of the second bus bar 32 are the same, and the thickness of the first electrode 311 and the second electrode 321 are the same; or, the thickness of the first bus bar 31 and the second bus bar 32 are the same, and the thickness of the first electrode 311 and the second electrode 321 are different; or, the thickness of the first bus bar 31 and the thickness of the second bus bar 32 are different, and the thickness of the first electrode 311 and the second electrode 321 are the same.

[0058] As an example, the materials of the first bus bar 31 , the second bus bar 32 , the first electrode 311 , and the second electrode 321 are any one or a combination of aluminum, aluminum alloy, copper, copper alloy, chromium, titanium, platinum, beryllium, gold, molybdenum, and tungsten.

[0059] As an example, a first electrode 311 and a second electrode 321 form a pair of interdigital electrodes, and the number of pairs of interdigital electrodes is 1 to 50. Figure 1The number of interdigital electrode pairs of the thin film bulk acoustic wave resonator with the interdigital electrodes suspended in the air is one pair. Figure 2 to Figure 4 The number of pairs of interdigital electrodes in the FBAW resonator with suspended interdigital electrodes is 2.

[0060] As an example, the size of the gap 5 along the second direction (ie, the height h of the gap 5) g ) is 20 to 200 nm, for example 20 nm, 21 nm, 22 nm, 25 nm, 28 nm ... 100 nm, 101 nm, 110 nm, 120 nm ... 200 nm; preferably 30 to 100 nm, for example 30 nm, 31 nm, 32 nm ... 55 nm, 56 nm, 60 nm ... 100 nm.

[0061] As an example, a specific structure of the interdigital transducer 3 is as follows: the first electrode 311 and the second electrode 321 are both 70 nm thick gold layers. The size of the first electrode 311 and the second electrode along the first direction (i.e., the width of the electrode) is 3 μm. The distance between adjacent first electrodes 311 and second electrodes 321 is 18 μm. The first electrode 311 and the second electrode 321 are in section BB (see FIG. Figure 1 or Figure 6 ) has an overlapping length of 115 μm, and the length of the non-overlapping parts of the two projections is 55 μm, that is, the lengths of the first electrode 311 and the second electrode 321 are both 170 μm. The first bus bar 31 and the second bus bar 32 are both 40 nm thick gold layers. The dimensions of the first bus bar 31 and the second bus bar 32 along the third direction (that is, the width of the bus bar) are both 40 μm. The overlapping width (dimension along the third direction) of the projections of the first electrode 311 and the first bus bar 31 on the surface of the piezoelectric layer is equal to the width of the first bus bar 31. The overlapping width (dimension along the third direction) of the projections of the second electrode 321 and the second bus bar 32 on the surface of the piezoelectric layer is the same as the width of the second bus bar 32. There are two pairs of interdigitated electrodes. The height h of the gap 5 g It is 60nm.

[0062] As an example, another specific structure of the IDT 3 is as follows: the first electrode 311 and the second electrode 321 are both 150 nm thick aluminum layers. The dimensions of the first electrode 311 and the second electrode 321 along the first direction (i.e., the width of the electrodes) are both 5 μm. The distance between adjacent first electrodes 311 and second electrodes 321 is 20 μm. The first electrode 311 and the second electrode 321 are in section BB (see FIG. Figure 1 or Figure 6) overlap by 120 μm, and the length of the non-overlapping part of the two projections is 72 μm, that is, the length of the first electrode 311 and the second electrode 321 are both 192 μm. The first bus bar 31 and the second bus bar 32 are both 30 nm thick aluminum layers. The dimensions of the first bus bar 31 and the second bus bar 32 along the third direction (i.e., the width of the bus bar) are both 60 μm. There are 10 pairs of interdigitated electrodes. The height h of the gap 5 is g is 30nm.

[0063] As an example, the first bus bar 31 and the second bus bar 32 are both two-step structures. The dielectric layer 4 may be located only on the lower surface of the first electrode 311 and the second electrode 321, and there is no dielectric layer 4 in the area between the first electrode 311 and the second electrode 321 (eg, Figures 1 to 5 The dielectric layer 4 may also be an integral structure (as shown in FIG. Figures 6 to 8 As shown), one end of the dielectric layer 4 is located on the step of the first bus bar 31 close to the piezoelectric layer 2, and the other end is located on the step of the second bus bar 32 close to the piezoelectric layer 2; the upper surface of the dielectric layer 4 is in direct contact with the first electrode 31 and the second electrode 32, and a gap 5 is formed between the lower surface and the piezoelectric layer 2.

[0064] As an example, the material of the dielectric layer 4 is one or a combination of silicon dioxide, aluminum oxide, and silicon nitride; for example, the material of the dielectric layer 4 is silicon nitride.

[0065] As an example, the dimension of the dielectric layer 4 along the second direction (i.e., the thickness of the dielectric layer 4) is 10 to 250 nm, for example, 10 nm, 11 nm, 12 nm, 13 nm, 15 nm, 20 nm...100 nm, 101 nm, 102 nm...200 nm, 210 nm, 220 nm, 230 nm, 240 nm, 250 nm; typically 20 nm.

[0066] Reference Figure 4 and Figure 5 In a preferred embodiment, the film bulk acoustic wave resonator with suspended interdigitated electrodes further includes at least one supporting structure 7 , and the supporting structure 7 is located between the piezoelectric layer 2 and the dielectric layer 4 .

[0067] As an example, the material of the support structure 7 is a dielectric material or a conductive material; preferred materials include any one or a combination of silicon dioxide, silicon nitride, polysilicon, aluminum, aluminum alloy, copper, copper alloy, chromium, titanium, platinum, beryllium, gold, molybdenum, and tungsten.

[0068] As an example, the dimension of the support structure 7 along the first direction (i.e., the width of the support structure) is the same as the width of the interdigitated electrode, for example, the width of the support structure is 5 μm; the dimension along the third direction (i.e., the length of the support structure) is 1 to 15 μm, for example, 1 μm, 1.5 μm, 2 μm, 3 μm, 4 μm, 5 μm, 5.5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm...15 μm; the dimension along the second direction (i.e., the height of the support structure) is the same as the height of the gap 5.

[0069] Example 2

[0070] A specific method for preparing the thin film bulk acoustic wave resonator with suspended interdigital electrodes of Example 1 comprises the following steps:

[0071] S1. Provide a substrate, the substrate comprising a base with a thickness of 675 μm and an intermediate layer with a thickness of 2 μm. The material of the base is high-resistance silicon. The material of the intermediate layer is silicon dioxide. The intermediate layer is located on the base. A piezoelectric layer with a thickness of 270 nm is formed on the upper surface of the intermediate layer by Smart-Cut intelligent peeling technology. The material of the piezoelectric layer is single crystal Z-cut lithium niobate.

[0072] S2. Form a first bus bar and a second bus bar with a thickness of 60 nm on the upper surface of the piezoelectric layer by a lift-off process. The first bus bar and the second bus bar are both made of gold (Au).

[0073] S3. Form a sacrificial layer on the upper surface of the piezoelectric layer except for the first bus bar and the second bus bar by plasma enhanced chemical vapor deposition (PECVD) or magnetron sputtering, and pattern the sacrificial layer by photolithography and etching, wherein the dimension of the sacrificial layer along the second direction (i.e., the thickness of the sacrificial layer) is 60 nm. The material of the sacrificial layer is silicon dioxide.

[0074] S4. A dielectric layer with a thickness of 20 nm is formed on the upper surface of the sacrificial layer by low pressure chemical vapor deposition (LPCVD) or plasma enhanced chemical vapor deposition (PECVD) and patterned. The material of the dielectric layer is silicon nitride (Si3N4).

[0075] S5. Form a first electrode and a second electrode each having a thickness of 65 nm on the upper surface of the dielectric layer, the upper surface of the first bus bar, and the upper surface of the second bus bar by a lift-off stripping process. One end of the first electrode is located on the upper surface of the first bus bar, and the rest is located on the upper surface of the dielectric layer. One end of the second electrode is located on the upper surface of the second bus bar, and the rest is located on the upper surface of the dielectric layer. The materials of the first electrode and the second electrode are both gold (Au). The first electrode, the second electrode, the first bus bar, and the second bus bar constitute an interdigital transducer.

[0076] S6. Etching through holes on the piezoelectric layer on both sides of the IDT along the first direction.

[0077] S7. Etching the sacrificial layer and the intermediate layer through the through hole so that the first electrode, the second electrode and the dielectric layer are suspended above the piezoelectric layer, and a cavity is formed between the lower surface of the piezoelectric layer and the substrate to form a resonator.

[0078] Example 3

[0079] Another specific method for preparing the thin film bulk acoustic wave resonator with suspended interdigital electrodes of Example 1 comprises the following steps:

[0080] S1. Provide a substrate, the substrate comprising a base body with a thickness of 675 μm. The material of the base body is high-resistance silicon. A piezoelectric layer with a thickness of 600 nm is formed on the upper surface of the base body. The material of the piezoelectric layer is single crystal YX128° cut lithium niobate.

[0081] S2. A dielectric layer with a thickness of 20 nm is formed on the upper surface of the piezoelectric layer by low pressure chemical vapor deposition (LPCVD) or plasma enhanced chemical vapor deposition (PECVD) and a support structure is formed by patterning. The material of the support structure is silicon nitride (Si3N4). The width of the support structure is 5 μm, and the length of the support structure is 10 μm. A first bus bar and a second bus bar with a thickness of 30 nm are formed on the upper surface of the piezoelectric layer. The materials of the first bus bar and the second bus bar are both aluminum (Al).

[0082] S3, forming a sacrificial layer on the upper surface of the piezoelectric layer except for the first bus bar and the second bus bar and patterning the sacrificial layer, the dimension of the sacrificial layer along the second direction (ie, the thickness of the sacrificial layer) is 30 nm, and the material of the sacrificial layer is polycrystalline silicon.

[0083] S4, forming a dielectric layer with a thickness of 30 nm on the upper surface of the support structure on the upper surface of the sacrificial layer and patterning the dielectric layer, wherein the material of the dielectric layer is silicon dioxide.

[0084] S5. Form a first electrode and a second electrode on the upper surface of the dielectric layer, the upper surface of the first bus bar, and the upper surface of the second bus bar. One end of the first electrode is located on the upper surface of the first bus bar, and the rest is located on the upper surface of the dielectric layer. One end of the second electrode is located on the upper surface of the second bus bar, and the rest is located on the upper surface of the dielectric layer. The materials of the first electrode and the second electrode are both aluminum (Al). The first electrode, the second electrode, the first bus bar, and the second bus bar constitute an interdigital transducer.

[0085] S6. Etching through holes on the piezoelectric layer on both sides of the IDT along the first direction.

[0086] S7. Etching the sacrificial layer and the middle of the substrate through the through hole, so that the first electrode, the second electrode and the dielectric layer are suspended above the piezoelectric layer, and a cavity is formed between the lower surface of the piezoelectric layer and the substrate to form a resonator.

[0087] Performance Test of Film Bulk Acoustic Wave Resonator

[0088] The simulation experiment was conducted on the thin film bulk acoustic wave resonator with suspended interdigital electrodes and the thin film bulk acoustic wave resonator with non-suspended interdigital electrodes, and the results were as follows: Fig. 9 The admittance curve comparison diagram shown in the figure has an offset of 40 dB between the two curves for easy observation. The only structural difference between the FBAR with non-suspended interdigitated electrodes and the FBAR of the present invention is that both the first electrode and the second electrode are in direct contact with the upper surface of the piezoelectric layer, and there is no dielectric layer or gap.

[0089] In a thin film bulk acoustic wave resonator where the interdigitated electrodes are not suspended, the lower surface of the interdigitated electrodes is in direct contact with the upper surface of the piezoelectric layer. Since the materials of the interdigitated electrodes and the piezoelectric layer are different, the acoustic impedances of the two are also different. When sound waves propagate in materials with different acoustic impedances, they will produce reflection, scattering, refraction and other phenomena, and may also excite some non-main mode resonance peaks, also called stray modes. Fig. 9 It can be seen from the figure that in addition to the main modes (with the highest peaks) of the resonant frequency (fr) and anti-resonant frequency (fa) A1 formed near 6770MHz and 7900MHz respectively, there are also multiple spurious modes with weaker intensity. The existence of spurious modes will seriously damage the quality factor of the resonator and increase the energy loss of the device. At the same time, when the resonator is cascaded into a filter, the in-band ripple and insertion loss of the filter will increase, reducing the filtering effect of the device. The thin film bulk acoustic wave resonator with suspended forked finger electrodes of the present invention can reduce the contact area between the forked finger electrodes and the piezoelectric layer by suspending part or all of the forked finger electrodes on the upper surface of the piezoelectric layer, thereby reducing the interface acoustic wave loss caused by the mismatch of acoustic impedance, which is reflected in the admittance curve, that is, reducing the spurious modes. At the same time, by adding a dielectric layer between the piezoelectric layer and the forked finger electrodes, the forked finger electrodes and the piezoelectric layer are further decoupled, and the intensity of the spurious modes generated by the overlapping part of the piezoelectric layer and the electrode is reduced, thereby achieving the effect of suppressing the spurious modes. Fig. 9 As shown in the figure, there are almost no spurious modes except the A1 main resonant mode near the resonant frequency and anti-resonant frequency. This also shows that by suspending the interdigitated electrodes and dielectric layer, the resonance intensity of the spurious modes in the piezoelectric layer overlapping with the electrodes can be reduced, and the effect of suppressing the spurious modes can be achieved. Since the spurious modes are significantly reduced, the quality factor (Q value) of the device will also be significantly increased, and the energy loss of the device will also be significantly reduced.

[0090] Fig.10The admittance curve obtained when the gap of the FBAW resonator with suspended interdigital electrodes provided in Example 2 changes from 0 to 280 nm, and the offset between two adjacent curves in the figure is 40 dB for observation. It can be seen from the figure that as the height h of the gap changes, g As the gap height increases from 0 to 30 nm, the amplitude of the stray peak near the antiresonance frequency of 7.9 GHz gradually decreases, and the admittance curve of the entire resonator becomes smooth, with only the resonant peak and antiresonant peak of the main mode. g When the distance between the interdigital electrodes of the resonator and the piezoelectric layer increases, the resonant frequency reflected in the admittance curve gradually increases, while the antiresonant frequency remains basically unchanged, which also shows that the electromechanical coupling coefficient of the resonator is Gradually reduce This indicates that the bandwidth of the corresponding filter can be adjusted by suspending the interdigital electrodes in the present invention, thereby improving the adjacent band suppression performance.

[0091] The above-described embodiments are only preferred embodiments of the present application and are not intended to limit the scope of protection of the present application. For any person skilled in the art, the present application may have various changes and modifications. Any simple equivalent changes and modifications made based on the scope of protection of the present application and the contents of the specification should be included in the scope of protection of the present application.

Claims

1. A thin film bulk acoustic resonator with suspended interdigital electrodes, characterized in that: It includes a substrate, a piezoelectric layer located on the substrate and an interdigital transducer located on the piezoelectric layer; a cavity is formed between the substrate and the piezoelectric layer; the interdigital transducer includes interdigital electrodes; along a first direction, through holes are opened on the piezoelectric layer at positions on both sides of the interdigital transducer; the through holes are connected to the cavity; a dielectric layer is provided on the surface of the interdigital electrodes on one side close to the piezoelectric layer; a gap is provided between the dielectric layer and the piezoelectric layer; the first direction is perpendicular to the direction in which the interdigital electrodes extend and parallel to the direction of the piezoelectric layer.

2. The thin film bulk acoustic wave resonator with suspended interdigital electrodes according to claim 1, characterized in that: A support structure is included; the support structure is located between the piezoelectric layer and the dielectric layer.

3. The thin film bulk acoustic wave resonator with suspended interdigital electrodes according to claim 2, characterized in that: The material of the support structure includes at least one of silicon dioxide, silicon nitride, polysilicon, aluminum, aluminum alloy, copper, copper alloy, chromium, titanium, platinum, beryllium, gold, molybdenum, and tungsten.

4. The thin film bulk acoustic wave resonator with suspended interdigital electrodes according to claim 1, characterized in that: The height of the gap along the direction from the substrate to the piezoelectric layer is 20 to 200 nm; or / and the thickness of the dielectric layer along the direction from the substrate to the piezoelectric layer is 10 to 250 nm; or / and the thickness of the piezoelectric layer along the direction from the substrate to the piezoelectric layer is 100 to 1000 nm.

5. The thin film bulk acoustic wave resonator with suspended interdigital electrodes according to claim 1, characterized in that: The material of the piezoelectric layer is at least one of lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, aluminum nitride, and scandium-doped aluminum nitride; wherein the scandium content in the scandium-doped aluminum nitride is: 0<Sc≤45wt%.

6. The thin film bulk acoustic wave resonator with suspended interdigital electrodes according to claim 1, characterized in that: The material of the dielectric layer is at least one of silicon dioxide, aluminum oxide, and silicon nitride; or / and the material of the interdigital transducer is at least one of aluminum, aluminum alloy, copper, copper alloy, chromium, titanium, platinum, beryllium, gold, molybdenum, and tungsten.

7. The thin film bulk acoustic wave resonator with suspended interdigital electrodes according to claim 1, characterized in that: The substrate comprises a base and an intermediate layer, and the material of the intermediate layer is at least one of silicon dioxide, silicon nitride and polysilicon.

8. The film bulk acoustic wave resonator with suspended interdigital electrodes according to claim 7, characterized in that: Along the direction from the substrate to the piezoelectric layer, the thickness of the intermediate layer is 10 nm to 50 μm.

9. The method for preparing a thin film bulk acoustic wave resonator with suspended interdigital electrodes according to any one of claims 1 to 8, characterized in that: The following steps are involved: S1. providing a substrate, and forming a piezoelectric layer on the substrate; S2, forming a first bus bar and a second bus bar on the upper surface of the piezoelectric layer; S3, forming a sacrificial layer on the upper surface of the piezoelectric layer except for the first bus bar and the second bus bar and patterning the layer; S4, forming a dielectric layer on the upper surface of the sacrificial layer and patterning it; S5, forming a first electrode and a second electrode on the upper surface of the dielectric layer, the upper surface of the first bus bar, and the upper surface of the second bus bar to obtain an interdigital transducer; S6, etching through holes on the piezoelectric layer on both sides of the IDT along the first direction; S7. Etching the sacrificial layer and / or the middle portion of the substrate through the through hole, so that the first electrode and the second electrode are suspended above the piezoelectric layer, and a cavity is formed between the piezoelectric layer and the substrate.

10. The preparation method according to claim 9, characterized in that: Step S2 includes the step of forming a support structure on the upper surface of the piezoelectric layer.

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