Bulk acoustic wave device and method for manufacturing bulk acoustic wave device

Abstract

The invention relates to a bulk acoustic wave device (100) comprising a first electrode (1a) and a second electrode (2a) and a piezoelectric element (3a) arranged between the first electrode and the second electrode, the piezoelectric element being designed such that a first electromagnetic signal fed into the first electrode is converted into an acoustic signal in the piezoelectric element, the acoustic signal is inverted in the second electrode into a second electromagnetic signal. The piezoelectric element comprises at least two piezoelectric layers (31, 33) having polarity in the same direction and at least one intermediate layer (32a) between the at least two piezoelectric layers. The acoustic layer thickness of the piezoelectric layer and the acoustic layer thickness of the intermediate layer are respectively equivalent to odd times of the semi-acoustic wave length of an acoustic signal to be transmitted.

Description

Technical Field

[0001] The present invention relates to a bulk acoustic wave device and a method for manufacturing a bulk acoustic wave device. Background Art

[0002] In high-frequency technology, bulk acoustic wave (BAW) components are used as resonators in filters and oscillators. The operating frequency of the component is primarily determined by the layer thickness of the piezoelectric layer and by the speed of sound in the piezoelectric material. In order to achieve higher operating frequencies, the layer thickness must be reduced. Therefore, tolerances are becoming increasingly important.

[0003] The capacitance of a BAW component increases due to a lower layer thickness of the piezoelectric material. In order to maintain the wave impedance, the component area must be reduced at the same time. However, the energy of the acoustic wave is lost at the edges of the component. When the component is reduced in size, the edge losses increase with the square of the operating frequency. Therefore, conventional BAW component (BAW) technology reaches its limit at about 10 GHz.

[0004] Documents US 2018 / 085787 A1 and US 2013 / 193808 A1 disclose a BAW component in which a low capacitance can be maintained even at higher operating frequencies by stacking two different piezoelectric materials with opposite polarities, so that edge losses are reduced and higher operating frequencies can be achieved in the millimeter wave range.

[0005] The manufacture of inhomogeneous piezoelectric bilayers is difficult to control. In addition, the influence of temperature on different piezoelectric materials causes a bimorph effect, which can lead to thermal drift. Finally, piezoelectric materials have different piezoelectric properties, which makes the design of low-loss components extremely difficult. Summary of the invention

[0006] The invention provides a bulk acoustic wave device and a method for producing a bulk acoustic wave device having the features of the independent claims.

[0007] Preferred embodiments are the subject matter of the respective dependent claims.

[0008] Therefore, according to a first aspect, the present invention relates to a bulk acoustic wave device having a first electrode and a second electrode, and a piezoelectric element arranged between the first electrode and the second electrode, wherein the piezoelectric element is constructed in such a way that a first electromagnetic signal fed into the first electrode is converted into an acoustic signal in the piezoelectric element, wherein the acoustic signal is back-converted into a second electromagnetic signal in the second electrode. The piezoelectric element comprises at least two piezoelectric layers, preferably of the same material and with polarity in the same direction, and at least one intermediate layer located between the at least two piezoelectric layers. The acoustic layer thicknesses of the piezoelectric layers and the intermediate layers respectively correspond to odd multiples (1x, 3x, ...) of half the acoustic wavelength of the acoustic signal to be transmitted.

[0009] According to a second aspect, the present invention relates to a method for manufacturing a bulk acoustic wave device. A substrate is provided. In addition, a first electrode, a second electrode and a piezoelectric element arranged between the first electrode and the second electrode are arranged on the substrate, wherein the piezoelectric element is constructed in such a way that a first electromagnetic signal fed into the first electrode is converted into an acoustic signal in the piezoelectric element, wherein the acoustic signal is back-converted into a second electromagnetic signal in the second electrode. The piezoelectric element comprises at least two piezoelectric layers having polarities in the same direction and at least one intermediate layer between the at least two piezoelectric layers. The acoustic layer thicknesses of the piezoelectric layer and the intermediate layer respectively correspond to odd multiples (1x, 3x, ...) of half the acoustic wavelength of the acoustic signal to be transmitted.

[0010] Advantages of the present invention

[0011] The BAW device according to the invention enables the development of higher frequency ranges with improved behavior with respect to thermal drift. A high-frequency signal can be fed (injected) into a first electrode of the BAW device. The high-frequency signal is a first electromagnetic wave, which is converted into an acoustic wave by the piezoelectric element at the first electrode and is converted back into a second electromagnetic wave at the opposite second electrode. At a given electromagnetic or acoustic wave frequency, the sound velocity c for the corresponding layer material is obtained according to the following formula: s The wavelength of the sound wave λ a :

[0012] λ a =c s / f

[0013] As long as the thickness of the piezoelectric layer and the acoustic layer of the intermediate layer stack is d p This corresponds to a multiple (1x, 3x, ...) of half the wavelength of the acoustic wave of the converted electromagnetic wave (ie, the acoustic signal), namely:

[0014] d p =(n+1 / 2)*λ a

[0015] n = {0, 1, ...}, the input signal is transmitted, otherwise it is reflected. The BAW device can thus be used as a BAW resonator.

[0016] According to one embodiment, a plurality of such resonators can be appropriately connected together in a circuit to form a so-called conductor configuration and / or a gate configuration. Thus, a filter component can be realized, which is permeable for a limited frequency range and can be designed for each frequency band, for example, in mobile communications. With the help of such a filter, signal interference between the transmission and reception channels can be avoided not only in the communication module of the mobile terminal device but also in the base station.

[0017] Higher frequencies require smaller layer thicknesses, which leads, for example, to higher requirements in terms of layer thickness accuracy. Conventionally, the lateral dimensions of the resonator must be scaled down simultaneously with the reduction in layer thickness in order to compensate for the increase in capacitance caused by the reduction in layer thickness and to be able to maintain a target impedance value of, for example, 50Ω. However, scaling down can lead to higher acoustic energy losses, since the ratio of the periphery to the resonator area is thereby increased.

[0018] According to the invention, the capacitance is therefore reduced by inserting at least one additional intermediate layer and a further piezoelectric layer. The insertion of the additional piezoelectric layer and the additional at least one intermediate layer is equivalent to connecting additional series capacitances in series, which can each have larger values, since the reciprocals of the individual capacitances add up to the reciprocal of the total capacitance in the case of a series connection. In this way, the resonator can be larger in the lateral direction than without the intermediate layer. As a result, smaller edge losses occur and the resonator can be designed and used for higher frequencies.

[0019] In order not to destroy the resonance, the total acoustic layer thickness of the intermediate layer is adapted to the target wavelength of the sound waves in the intermediate layer. The total acoustic layer thickness of the piezoelectric element thus corresponds to an odd multiple (1x, 3x, ...) of half the sound wave length. This also includes, in particular, the half sound wave length itself.

[0020] At least one further piezoelectric layer located on the second electrode serves to efficiently convert the acoustic waves back into electromagnetic waves on the second electrode.

[0021] According to a further embodiment, the bulk acoustic wave device comprises a plurality of piezoelectric layers, wherein an intermediate layer is located between every two successive piezoelectric layers.

[0022] According to another embodiment of the BAW device, the intermediate layer is formed by a single layer whose acoustic layer thickness thus corresponds to half the acoustic wavelength of the acoustic signal to be transmitted, ie an odd multiple (1x, 3x, ...) of the target wavelength.

[0023] According to another embodiment of the BAW device, at least one intermediate layer consists of a plurality of sublayers, wherein the sum of the acoustic layer thicknesses of the sublayers corresponds to an odd multiple (1x, 3x, ...) of half the acoustic wavelength of the acoustic signal to be transmitted. For example, the intermediate layer comprises two sublayers, wherein the wavelength:

[0024] λ a / 4+λ a / 4=λ a / 2

[0025] or:

[0026] λ a / 4+5λ a / 4=3λ a / 2

[0027] or:

[0028] 3λ a / 4+3λ a / 4=3λ a / 2

[0029] Here, the first addend specifies the acoustic layer thickness of the first sublayer and the second addend specifies the acoustic layer thickness of the second sublayer. However, this principle can be transferred to more than two sublayers in an analogous manner.

[0030] According to another embodiment of the BAW device, the acoustic layer thickness of the piezoelectric layer and the intermediate layer each corresponds to half the acoustic wavelength of the acoustic signal to be transmitted. In other words, according to the invention, the total acoustic layer thickness of the piezoelectric element is then 3λ a / 2. Therefore, the fundamental resonance (ie n=0, lowest order) occurs in the intermediate layer and the piezoelectric layer. This is advantageous because in this case the highest quality factor occurs, the filter edge can be particularly steep and the insertion losses are particularly low.

[0031] According to another embodiment of the BAW device, the material of the intermediate layer includes a dielectric, such as silicon oxide, silicon nitride, silicon carbide, aluminum oxide or diamond-like carbon (DLC). Preferred materials have intrinsically low dielectric and / or acoustic damping and an adapted thermal expansion coefficient.

[0032] According to another embodiment of the BAW device, at least one intermediate layer is constructed in multiple layers, for example from a plurality of layers having λ a The acoustic wave Bragg reflector layer is composed of a layer thickness of an odd multiple (1x, 3x, ...) of 1 / 4. Suitable material pairs for the reflector layer are characterized by differences in the sound speed of the materials and minimal material damping. Possible materials are, for example, titanium (Ti), tantalum (Ta) or copper (Cu) for low sound speeds or aluminum (Al), nickel (Ni), tungsten (W) or molybdenum (Mo) for high sound speeds.

[0033] According to a further embodiment of the bulk acoustic wave device, a combination of a dielectric and a semiconducting and / or metallic layer is provided as one or more intermediate layers.

[0034] According to another embodiment of the BAW device, the different intermediate layers described above can be present in any combination with one another. For example, the piezoelectric element can have more than two piezoelectric layers and more than one intermediate layer. In this case, at least one of the intermediate layers can be composed of a single layer having an odd multiple of λ. a / 2 layer thickness and at least one further intermediate layer is formed by a plurality of layers having an odd multiple (1x, 3x, ...) λ a A sound wave Bragg reflection layer with a thickness of / 4 layers is formed.

[0035] According to a further embodiment of the BAW device, the intermediate layer described above is not only arranged between the piezoelectric layers but also additionally between one and / or two outer piezoelectric layers and the electrodes or the multi-layer acoustic Bragg reflector layer.

[0036] According to another embodiment of the bulk acoustic wave device, the material of the piezoelectric layer includes aluminum nitride (AlN) or scandium-doped aluminum nitride (ScAlN). This is advantageous due to the high sound velocity, which allows relatively large layer thicknesses. Other possible materials are, for example, zinc dioxide (ZnO2), lithium niobate (LiNbO3) or lithium tantalate (LiTaO3).

[0037] According to another embodiment of the bulk acoustic wave device, the first electrode and / or the second electrode is designed as an acoustic wave Bragg reflector, as has already been described above for the intermediate layer. In this case, the Bragg reflector layer can be inserted between the substrate and the piezoelectric element, whereby acoustic energy losses in the substrate can be avoided in order to keep insertion losses low. This is an SMR (solidly mounted resonator) architecture. Due to the good thermal coupling to the substrate, the SMR architecture can be advantageously used in applications in which high powers must be handled, for example in base stations and in the transmission path of mobile radio devices.

[0038] According to another embodiment of the bulk acoustic wave device, the first and / or the second electrode (and therefore the piezoelectric element) is released. This is an FBAR (film bulk acoustic resonator) architecture. In this case, the acoustic waves are reflected at the electrode-air surface, so that the FBAR architecture has low insertion losses, which is advantageous for a larger bandwidth and for the transmission path in mobile radio devices.

[0039] According to another embodiment, the BAW device can be used as a high-precision clock crystal in a filter element for frequencies in the GHz range (especially also greater than 10 GHz) or as a gravity harmonic sensor. The BAW device can be used in particular in high-frequency systems, for example in the mobile radio sector (20 GHz-100 GHz) or in the radar sector.

[0040] Further advantages, features and details of the invention emerge from the following description, in which various exemplary embodiments are explained in detail with reference to the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The accompanying drawings show:

[0042] Figure 1 : A schematic cross-sectional view of a bulk acoustic wave device according to an embodiment of the present invention;

[0043] Figure 2 : A schematic cross-sectional view of a bulk acoustic wave device according to another embodiment of the present invention;

[0044] Figure 3 : A schematic cross-sectional view of a bulk acoustic wave device according to another embodiment of the present invention;

[0045] Figure 4 : A schematic cross-sectional view of a bulk acoustic wave device according to another embodiment of the present invention;

[0046] Figure 5 : Flowchart of a method for manufacturing a bulk acoustic wave device according to an embodiment of the present invention.

[0047] In all figures, identical or functionally identical elements and devices are provided with the same reference symbols. The numbering of the method steps is unambiguous and does not generally imply a specific chronological sequence. In particular, a plurality of method steps can also be performed simultaneously.

[0048] The drawings described here are to be understood as exemplary embodiments of the invention. It should be expressly pointed out that the combination of different features of the individual embodiments also lies entirely within the scope of the invention. DETAILED DESCRIPTION

[0049] Figure 1 1 shows a cross-sectional view of a bulk acoustic wave device 100. The bulk acoustic wave device 100 comprises a substrate 4 on which a second acoustic wave Bragg reflector 2a is arranged. The second acoustic wave Bragg reflector comprises a plurality of sublayers 21 to 26 which alternately have high and low sound velocities or acoustic damping. In addition, a first acoustic wave Bragg reflector 1a is provided which can be constructed similarly.

[0050] The first Bragg reflector 1 a is composed of an electrically conductive material and serves as a first electrode, and the second Bragg reflector 2 a is also composed of an electrically conductive material and serves as a second electrode.

[0051] A piezoelectric element 3a is arranged between the second Bragg reflector 2a and the first Bragg reflector 1a. The first electromagnetic signal fed into the first Bragg reflector 1a or the first electrode via the first power supply line 6 is converted into an acoustic signal in the piezoelectric element 3 during operation. The acoustic signal is converted back into a second electromagnetic signal in the second Bragg reflector 2a or the second electrode. If the acoustic resonance condition is met, the second electromagnetic signal is output via the through-hole contact 9 and the second power supply line 5.

[0052] The piezoelectric element 3a comprises two substantially identical piezoelectric layers 31, 33 with polarities in the same direction and an intermediate layer 32a arranged between the two piezoelectric layers 31, 33. The acoustic layer thicknesses of the piezoelectric layers 31, 33 and the intermediate layer 32a correspond to half the acoustic wavelength of the acoustic signal to be transmitted, i.e., an odd multiple (1x, 3x, ...) of a predetermined acoustic wavelength (corresponding to a predetermined pass-through frequency of the bulk acoustic wave device). Preferably, half the acoustic wavelength of the desired pass-through frequency enters the piezoelectric layers 31, 33 and the intermediate layer 32a as the fundamental first resonance of the piezoelectric element 3a (see schematically indicated waves).

[0053] The first Bragg reflector 1a, the second Bragg reflector 2a, the piezoelectric element 3a and the via 9 are surrounded by a dielectric 7. A passivation layer 8 is located above the supply lines 5, 6 and the dielectric 7 for protection against environmental influences.

[0054] For example, the resonator can be designed as shown in Table 1 for a resonance frequency of 24 GHz in the millimeter wave frequency range.

[0055] Reference Numbers layer <![CDATA[Speed of sound c s > Acoustic layer thickness 31，33 Piezoelectric Aluminum Nitride (AIN) 11400m / s <![CDATA[238nm(λ a / 2)]]> 32a Intermediate layer: Silicon carbide (SiC) 13006m / s <![CDATA[271nm(λ a / 2)]]> 21，23，25 Bragg reflector layer Titanium (Ti) 4140m / s <![CDATA[43nm(λ a / 2) <!-- 4 -->]]> 22，24，26 Bragg reflector layer tungsten (W) 5174m / s <![CDATA[54nm(λ a / 2)]]>

[0056] Table 1

[0057] It should be explicitly pointed out here that an additional intermediate layer 32a may be arranged between the outer piezoelectric layer 31 and the first Bragg reflector 1a and / or between the outer piezoelectric layer 33 and the second Bragg reflector 2a and this is fully to be regarded as within the scope of the present invention.

[0058] Figure 2 FIG. 2 shows a cross-sectional view of another BAW device 200. Figure 1 In contrast to the bulk acoustic wave device 100 shown in FIG. 1 , the piezoelectric element 3 b comprises four parallel-polarized piezoelectric layers 31 , 33 , 35 , 37 , wherein an intermediate layer 32 a , 34 , 36 is located between each two successive piezoelectric layers 31 , 33 , 35 , 37 .

[0059] Figure 3 FIG. 4 shows a cross-sectional view of another BAW device 300. Figure 1 Unlike the bulk acoustic wave device 100 shown in FIG. 1 , the piezoelectric element 3c includes an intermediate layer 32b, which is composed of two λ a / 4 Bragg reflector layer, where λ a represents the target wavelength to be transmitted, which thus corresponds to the desired pass frequency.

[0060] The thicknesses of the piezoelectric layers 31, 33 and the intermediate layer 32b are selected such that the half acoustic wavelength of the desired pass-through frequency is respectively suitable as the basic first resonator of the overall stack. Figure 2 As described in , the device can also have more than two piezoelectric layers 31, 33 and / or more than one intermediate layer 32b.

[0061] It should be clearly pointed out here that in this case, the two λ a The / 4 Bragg reflector layer can have a different material pairing than the outer Bragg reflector layers of the Bragg reflectors 1 a and 2 a that serve as boundaries.

[0062] It should also be pointed out that Figure 2 and Figure 3 A mixed form, ie a combination of intermediate layers 32a and 32b in one device, is also within the scope of the present invention.

[0063] Figure 4 FIG. 4 is a schematic cross-sectional view of another bulk acoustic wave device 400. Figure 1 Unlike the bulk acoustic wave device 100 shown in FIG. 1 , metal electrodes 1b, 2b are provided here that are not designed as Bragg reflectors. The second electrode 2b is released here, so that a cavity 10 is formed. The thickness of the electrodes 1b, 2b is adapted to the wavelength of the passing frequency. In this case, the acoustic wave input caused by the reflection takes place at the electrode surface (or passivation) facing the ambient air.

[0064] It should also be pointed out here again that different hybrid forms with regard to the electrode form (metal electrodes 1 b / 2 b or Bragg reflectors 1 a / 2 a ) can be combined with one another in one arrangement and are within the scope of the invention.

[0065] Figure 5 A flow chart of a method for manufacturing a bulk acoustic wave device is shown. Figures 1 to 4 The bulk acoustic wave devices 100 to 400 are shown in FIG.

[0066] In a first method step S1 , a substrate 4 , for example made of silicon, is provided.

[0067] In a second method step S2, first electrodes 1a, 1b, second electrodes 2a, 2b, and piezoelectric elements 3a, 3b, 3c arranged between the first electrodes 1a, 1b and the second electrodes 2a, 2b are formed on the substrate 4. To this end, the second electrode 2a can be formed on the substrate 4 first. The piezoelectric elements 3a, 3b, 3c can then be formed on the second electrodes 2a, 2b. Finally, the first electrodes 1a, 1b are formed on the piezoelectric elements 3a, 3b, 3c. In addition, the dielectric 7 as well as the supply lines 5, 6 and the passivation layer 8 can be formed.

[0068] The piezoelectric elements 3a, 3b, 3c are constructed in such a way that the first electromagnetic signal fed into the first electrodes 1a, 1b is converted into an acoustic signal in the piezoelectric elements 3a, 3b, 3c, wherein the acoustic signal is converted back into a second electromagnetic signal in the second electrodes 2a, 2b. The piezoelectric elements 3a, 3b, 3c include at least two piezoelectric layers 31, 33 (35, 37) with polarity in the same direction and at least one intermediate layer 32a (34, 36, 32b) arranged between the at least two piezoelectric layers 31, 33, 35, 37. The acoustic layer thickness of the piezoelectric layers 31, 33, 35, 37 and the intermediate layers 32a, 34, 36; 32b respectively corresponds to an odd multiple (1x, 3x, ...) of the half acoustic wavelength of the acoustic signal to be transmitted.

Claims

1. A bulk acoustic wave device (100; 200; 300; 400) having: a first electrode (1a; 1b) and a second electrode (2a; 2b); and A piezoelectric element (3a; 3b; 3c) arranged between the first electrode (1a; 1b) and the second electrode (2a; 2b), wherein: The piezoelectric element (3a; 3b; 3c) is constructed in such a way that a first electromagnetic signal fed into the first electrode (1a; 1b) is converted into an acoustic signal in the piezoelectric element (3a; 3b; 3c), wherein the acoustic signal is converted back into a second electromagnetic signal in the second electrode (2a; 2b); The piezoelectric element (3a; 3b; 3c) comprises at least two piezoelectric layers (31, 33, 35, 37) having polarities in the same direction and at least one intermediate layer (32a, 34, 36; 32b) between the at least two piezoelectric layers (31, 33, 35, 37); and The acoustic layer thicknesses of the piezoelectric layers (31, 33, 35, 37) and the intermediate layers (32a, 34, 36; 32b) are respectively equivalent to odd-numbered multiples of half the acoustic wavelength of the acoustic signal to be transmitted.

2. The bulk acoustic wave device (200) according to claim 1, comprising a plurality of piezoelectric layers (31, 33, 35, 37), wherein: An intermediate layer (32a, 34, 36; 32b) is located between every two successive piezoelectric layers (31, 33, 35, 37).

3. The bulk acoustic wave device (300) according to claim 1 or 2, wherein: The at least one intermediate layer (32b) is composed of a plurality of sub-layers, and the sum of the acoustic layer thicknesses of the sub-layers corresponds to an odd multiple of half the acoustic wavelength of the acoustic signal to be transmitted.

4. A bulk acoustic wave device (100; 200; 300; 400) according to any one of the preceding claims, wherein: The acoustic layer thickness of the piezoelectric layer (31, 33, 35, 37) and the intermediate layer (32a, 34, 36; 32b) respectively corresponds to half the acoustic wavelength of the acoustic signal to be transmitted.

5. A bulk acoustic wave device (100; 200; 300) according to any one of the preceding claims, wherein: The first electrode (1a) and / or the second electrode (2a) are designed as acoustic Bragg reflectors.

6. The bulk acoustic wave device (400) according to any one of the preceding claims, wherein: The first electrode (1a; 1b) and / or the second electrode (2a; 2b) are released.

7. A method for manufacturing a bulk acoustic wave device (100; 200; 300; 400), comprising the following steps: providing (S1) a substrate (4); and A first electrode (1a; 1b), a second electrode (2a; 2b) and a piezoelectric element (3a; 3b; 3c) arranged between the first electrode (1a; 1b) and the second electrode (2a; 2b) are constructed (S2) on the substrate (4), wherein: The piezoelectric element (3a; 3b; 3c) is constructed in such a way that a first electromagnetic signal fed into the first electrode (1a; 1b) is converted into an acoustic signal in the piezoelectric element (3a; 3b; 3c), wherein the acoustic signal is converted back into a second electromagnetic signal in the second electrode (2a; 2b), The piezoelectric element (3a; 3b; 3c) comprises at least two piezoelectric layers (31, 33, 35, 37) having polarities in the same direction and at least one intermediate layer (32a, 34, 36; 32b) between the at least two piezoelectric layers (31, 33, 35, 37); and The acoustic layer thicknesses of the piezoelectric layers (31, 33, 35, 37) and the intermediate layers (32a, 34, 36; 32b) are respectively equivalent to odd-numbered multiples of half the acoustic wavelength of the acoustic signal to be transmitted.

8. The method according to claim 7, wherein: A plurality of piezoelectric layers (31, 33, 35, 37) are formed, wherein an intermediate layer (32a, 34, 36; 32b) is respectively provided between two successive piezoelectric layers (31, 33, 35, 37).

9. The method according to claim 7 or 8, wherein: The at least one intermediate layer (32b) is composed of a plurality of sub-layers, wherein the sum of the acoustic layer thicknesses of the sub-layers corresponds to an odd multiple of half the acoustic wavelength of the acoustic signal to be transmitted.

10. The method according to any one of claims 7 to 9, wherein: The first electrode (1a; 1b) and / or the second electrode (2a; 2b) are released.

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