Volume acoustic element, acoustic filter and method for producing a volume acoustic element

Abstract

A volume acoustic element comprises a plurality of resonator devices arranged side by side. Each resonator device has at least three electrodes arranged parallel and offset from one another. The electrodes form at least two electrode pairs. At least one piezoelectric layer is formed between the electrodes of each electrode pair. A voltage source can apply voltage to the electrode pairs. In this process, an electric field is formed between the electrodes of the electrode pairs, with the field directions of the electric fields being opposite for adjacent electrode pairs of the same resonator device.

Description

[0001] The present invention relates to a volume acoustic element, an acoustic filter and a method for producing a volume acoustic element. State of the art

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

[0003] As an alternative to reducing the layer thickness, resonators can be fabricated in which higher modes (overtones) are excited. Such resonators are called overtoned bulk acoustic resonators (OBARs). However, such resonators are susceptible to the oscillation of additional modes and are inefficient in their excitation.

[0004] As a further alternative, it is possible to use thick cavities with alternating material polarization, as described in Wang et al., "A Film Bulk Acoustic Resonator Based on Ferroelectric Aluminum Scandium Nitride Films," Journal of Microelectromechanical Systems, Vol. 29, No. 5, pp. 741-747, 2020. In such approaches, an exciting electric field can directly excite an overtone, since the exciting force also changes direction with the changing material polarization. However, the fabrication of such alternating polarization layers is challenging. Disclosure of the invention

[0005] The invention provides a volume acoustic element, an acoustic filter and a method for producing a volume acoustic element having the features of the independent patent claims.

[0006] Preferred embodiments are the subject of the respective subclaims.

[0007] According to a first aspect, the invention relates to a volume acoustic element comprising a plurality of resonator devices arranged side by side. Each resonator device has at least three electrodes arranged spatially parallel and offset from one another. The electrodes form at least two electrode pairs. At least one piezoelectric layer is formed between the electrodes of each electrode pair. Furthermore, a voltage source is provided, which can apply voltage to the electrode pairs. In this case, an electric field is formed between the electrodes of the electrode pairs, with the field directions of the electric fields for adjacent electrode pairs of the same resonator device being opposite.

[0008] According to a second aspect, the invention relates to an acoustic filter with a volume acoustic element according to the first aspect.

[0009] According to a third aspect, the invention relates to a method for producing a volume acoustic element. In one step, a plurality of resonator devices arranged next to one another are provided, each resonator device having at least three electrodes arranged parallel and offset from one another, the electrodes forming at least two electrode pairs, and at least one piezoelectric layer being formed between the electrodes of each electrode pair. In one step, a voltage source is provided which is designed to apply voltage to the electrode pairs in such a way that an electric field is formed between the electrodes of the electrode pairs, the field directions of the electric fields for adjacent electrode pairs of the same resonator device being opposite. Advantages of the invention

[0010] The invention provides a multilayer volume acoustic element, in particular a bulk acoustic wave (BAW) overtone resonator. Appropriate electrical interconnection of the electrodes enables a reversal of the electric field direction, so that the acoustic standing wave, which represents an overtone of the acoustic resonator, is amplified in each of its half-waves. This makes it possible to achieve higher resonance frequencies in a relatively thick mechanical resonator, which, due to its large overall thickness, exhibits low power losses.

[0011] In contrast to volume acoustic elements with alternating material polarization, no complex process technology is required here. Instead, the piezoelectric material can be grown with the same polarization direction across all layers, and thus across all half-waves of the mechanical vibration. This eliminates the need for material polarization reversal, for example, through high DC voltages. Instead of alternating material polarization, the polarization of the exciting electric field is rotated for each mechanical half-wave, which is achieved by appropriately interconnecting the electrodes.

[0012] The invention thus enables the development of higher operating frequencies for volume acoustic elements. The alternating orientation of the electric field enables the efficient excitation of overtones.

[0013] Furthermore, the series connection of electrodes reduces the capacitance. In addition to the efficient excitation of high frequencies up to f = 20 GHz, preferably up to f = 40 GHz, a moderate, not too high capacitance C of the volume acoustic element can be achieved. This avoids low component impedances Z = 1 / jωC, which would otherwise lead to strong reflections at the component boundaries, since the surrounding system is typically designed for impedances of 50 Ω. The circular wave number ω = 2πf is given by the operating frequency f. The capacitance is approximately C=ε0εrA / d corresponding to a plate capacitor with the area A, the plate spacing d, as well as the electric field constant ε0 and the relative permittivity ε rof the dielectric. Accordingly, large electrode surfaces and small electrode spacings result in high capacitances and correspondingly low impedances. Furthermore, the capacitance of capacitors connected in parallel adds up, while capacitances connected in series add up reciprocally, so the effective capacitance of a series circuit is always lower than the smallest individual capacitance.

[0014] The volume acoustic element according to the invention features multiple layers or pairs of electrodes, each with alternating polarity. This results in the resonators being connected in parallel, thus increasing the capacitance. In addition, instead of large, continuous electrodes, smaller electrodes are used, which are connected in series. This reduces the capacitance again without compromising the ability to efficiently excite overtones.

[0015] Finally, established materials and volume-capable manufacturing technologies can be used.

[0016] According to a further development of the volume acoustic element, the voltage source is designed to apply voltage to the electrode pairs in such a way that harmonics develop between the electrodes of the electrode pairs. This allows the application range to be expanded.

[0017] According to a further development of the volume acoustic element, field directions of the electric fields for adjacent electrode pairs of different resonator devices are parallel.

[0018] According to a further development of the volume acoustic element, the voltage source is designed to apply voltage to the electrode pairs in such a way that an acoustic wave forms between two outer reflector components of the volume acoustic element, with at least one of the outer reflector components being designed as a Bragg reflector. The volume acoustic element is thus defined by acoustic reflectors, with interference-induced frequency-dependent Bragg reflection occurring through several layers, each with a thickness of one-quarter of a mechanical wavelength.

[0019] According to a further development of the volume acoustic element, the voltage source is designed to apply voltage to the electrode pairs in such a way that an acoustic wave forms between two outer reflector components of the volume acoustic element, with at least one of the outer reflector components being designed as a material interface. The stiffness changes at the material interfaces (e.g., electrode-air, electrode-vacuum, piezoelectric-air).

[0020] According to a further development of the volume acoustic element, one of the outer reflector components is designed as a Bragg reflector and a second of the outer reflector components is designed as a material interface.

[0021] According to a further development, the volume acoustic element comprises at least three resonator devices arranged next to one another, each resonator device having at least three pairs of electrodes.

[0022] According to a further development of the volume acoustic element, the capacitance of the volume acoustic element is less than 100 femtofarads. The capacitance is thus comparatively small.

[0023] The volume acoustic element can be used, for example, for volume acoustic resonator components such as resonators, oscillators, filters or gravimetric sensors.

[0024] In particular, the volume acoustic element can be used in high-frequency systems in the mobile communications sector (e.g. as filters) or in the radar sector (e.g. as oscillators).

[0025] Further advantages, features and details of the invention will become apparent from the following description, in which various embodiments are described in detail with reference to the drawings. Short description of the drawings

[0026] They show: Fig. 1 a schematic representation of a volume acoustic element according to an embodiment of the invention; Fig. 2 a schematic representation of a circuit of electrodes of a volume acoustic element according to an embodiment of the invention; and Fig. 3 a flowchart of a method for producing a volume acoustic element according to an embodiment of the invention.

[0027] In all figures, identical or functionally equivalent elements and devices are provided with the same reference numerals. The numbering of process steps serves the purpose of clarity and is generally not intended to imply a specific chronological order. In particular, several process steps can be performed simultaneously. Description of the embodiments

[0028] Fig. Figure 1 shows a schematic representation of a volume acoustic element 1 with three resonator devices 2a-2c arranged side by side. The volume acoustic element 1 can be used, for example, in an acoustic filter.

[0029] Each resonator device 2a-2c has four parallel to each other with a distance d sub staggered electrodes 31a-34a, 31b-34b, 31c-34c. The electrodes 31a-34a, 31b-34b, 31c-34c are made of an electrically conductive material, such as a metal, a metal alloy, or a heavily doped semiconductor.

[0030] The electrodes 31a-34a, 31b-34b, 31c-34c form three electrode pairs for each resonator device 2a-2c. For example, a first electrode 31a (an upper electrode) and a second electrode 32a (a lower electrode) form a first electrode pair of the first resonator device 2a, the second electrode 32a and a third electrode 33a form a second electrode pair of the first resonator device 2a, and the third electrode 33a and a fourth electrode 34a form a third electrode pair of the first resonator device 2a.

[0031] Between the electrodes 31 a-34a, 31 b-34b, 31 c-34c of each electrode pair there is at least one piezoelectric layer with a total thickness d sub formed, e.g. with or from aluminum nitride.

[0032] Furthermore, a voltage source 6 is provided, which can apply a high-frequency voltage with amplitude U0 to the electrode pairs. For this purpose, the voltage source 6 can comprise a control device that can adjust the frequency of an alternating voltage applied to the electrodes 31a-34a, 31b-34b, 31c-34c. The voltage source 6 can also be provided externally, for example, as a component of a larger system and serves for external excitation.

[0033] In this case, an electric field is formed between the electrodes 31a-34a, 31b-34b, 31c-34c of the electrode pairs. The electrodes 31a-34a, 31b-34b, 31c-34c are interconnected in such a way that the field directions of the electric fields for vertically adjacent electrode pairs of the same resonator device 2a-2c are opposite. For example, the field between the first electrode 31a and the second electrode 32a of the first resonator device 2a is opposite (i.e., antiparallel) to the field between the second electrode 32a and the third electrode 33a of the first resonator device 2a, which in turn is opposite to the field between the third electrode 33a and the fourth electrode 34a of the first resonator device 2a.

[0034] The electrodes 31 a-34a, 31 b-34b, 31 c-34c are further interconnected in such a way that field directions of the electric fields for horizontally adjacent electrode pairs of different resonator devices 2a-2c are parallel. For example, the field between the first electrode 31a and the second electrode 32a of the first resonator device 2a is oriented in the same way as the field between a first electrode 31b and a second electrode 32b of the second resonator device 2b, the field between the second electrode 32a and the third electrode 33a of the first resonator device 2a is oriented in the same way as the field between the second electrode 32b and a third electrode 33b of the second resonator device 2b, and the field between the third electrode 33a and the fourth electrode 34a of the first resonator device 2a is oriented in the same way as the field between the third electrode 33b and a fourth electrode 34b of the second resonator device 2b.The same applies to the second resonator device 2b compared to the third resonator device 2c.

[0035] When a voltage is applied to electrodes 31a-34a, 31b-34b, and 31c-34c, an acoustic wave forms between two outer reflector components 4, 5 of the volume acoustic element. The acoustic wave oscillates in a resonator formed between the two outer reflector components 4, 5. To enable multiplication of the resonance frequency, the volume acoustic element 1 is operated as an overtone resonator. This results in the deflection of the thickness vibration mode shown at a reference time.

[0036] For efficient overtone operation, phase-correct excitation of the acoustic wave is enabled. To achieve this, the electric field is rotated layer by layer due to the interconnection of electrodes 31a-34a, 31b-34b, and 31c-34c. This means that a positive acoustic half-wave is excited with an upward-directed (in a first direction) electric field, while a negative acoustic half-wave is excited with a downward-directed (in a second direction opposite the first direction) electric field.

[0037] In the volume acoustic element 1, a frequency three times higher than a fundamental frequency f0 can be achieved, whereby the individual electrode distances d sub = d0 / 3, where d0 is the thickness of a pair of electrodes corresponding to the fundamental frequency with plate size A0 and capacitance C0.

[0038] The outer reflector components 4, 5 can both be designed either as Bragg reflectors or as material interfaces. It is also possible for one outer reflector component 4, 5 to be designed as a Bragg reflector and the other outer reflector component 4, 5 to be designed as a material interface.

[0039] According to a further embodiment, the voltage source 6 can be configured to apply voltage to the electrode pairs in such a way that harmonics develop between the electrodes 31a-34a, 31b-34b, and 31c-34c of the electrode pairs. For this purpose, an alternating voltage with a higher frequency is applied.

[0040] As in Fig. 1, the volume acoustic element 1 comprises three resonator devices 2a, 2b, 2c arranged laterally next to one another, each with three vertically stacked electrode pairs, ie three layers of the piezoelectric material which are surrounded and separated by four electrode layers.

[0041] The volume acoustic element 1 thus has a plurality of electrode pairs arranged in an array, whereby the number is not limited to the 3x3 array shown but can be any nx m array with integers n > 1 and m > 1.

[0042] Fig. 2 shows a schematic representation of a circuit of electrodes of a volume acoustic element, for example the volume acoustic element 1 according to Fig. 1.

[0043] A first output of the voltage source 6 is electrically connected to an upper electrode of a first electrode pair 71a of a first resonator device, a lower electrode of a second electrode pair 72a of the first resonator device and an upper electrode of a third electrode pair 73a of the first resonator device.

[0044] A lower electrode of the first electrode pair 71a of the first resonator device is electrically connected to an upper electrode of the second electrode pair 72a of the first resonator device and an upper electrode of a first electrode pair 71b of a second resonator device.

[0045] A lower electrode of the third electrode pair 73a of the first resonator device is electrically connected to a lower electrode of a second electrode pair 72b of the second resonator device and an upper electrode of a third electrode pair 73b of the second resonator device.

[0046] A lower electrode of the first electrode pair 71b of the second resonator device is electrically connected to an upper electrode of the second electrode pair 72b of the second resonator device and an upper electrode of a first electrode pair 71c of a third resonator device.

[0047] A lower electrode of the third electrode pair 73b of the second resonator device is electrically connected to a lower electrode of a second electrode pair 72c of the third resonator device and an upper electrode of a third electrode pair 73c of the third resonator device.

[0048] A second output of the voltage source 6 is electrically connected to a lower electrode of the first electrode pair 71c of the third resonator device, an upper electrode of the second electrode pair 72c of the third resonator device and a lower electrode of the third electrode pair 73c of the second resonator device.

[0049] The resonator devices can be those in Fig. 1 shown resonator devices 2a-2c.

[0050] The effective total capacity of the volume acoustic element 1 is Ctot≈0.8 Ci, where C ithe capacitance of the electrode pairs 71 a-73 c.

[0051] Since the area A sub of a single resonator, neglecting the lateral electrode distance, is one third of the original area A0, the single resonator capacity is Ci=3 / 3×ε0εrA0 / d0=C0.

[0052] The total capacity is given as Ctot≈0.8 C0.

[0053] This enables an increase in the component resonance frequency without any significant change in the capacitance, in particular without increasing the capacitance and without reducing the effective area and the effective component volume.

[0054] Fig. Figure 3 shows a flow diagram of a method for manufacturing a volume acoustic element. In particular, the Fig. 1 shown volume acoustic element 1 can be manufactured.

[0055] In a step S1, a plurality of resonator devices 2a-2c arranged next to one another are provided, wherein each resonator device 2a-2c has at least three electrodes 31a-34a, 31b-34b, 31c-34c arranged parallel to one another and offset, wherein the electrodes 31a-34a, 31b-34b, 31c-34c form at least two electrode pairs, and wherein at least one piezoelectric layer is formed between the electrodes 31a-34a, 31b-34b, 31c-34c of each electrode pair.

[0056] In a step S2, a voltage source 6 is provided which is designed to apply voltage to the electrode pairs in such a way that an electric field is formed between the electrodes 31 a-34a, 31 b-34b, 31 c-34c of the electrode pairs, wherein field directions of the electric fields for adjacent electrode pairs of the same resonator device 2a-2c are opposite.

[0057] Furthermore, two outer reflector components 4, 5 of the volume acoustic element 1 are formed, wherein the outer reflector components 4, 5 are designed as Bragg reflectors and / or as material interfaces. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited non-patent literature

[0000] Wang et al., “A Film Bulk Acoustic Resonator Based on Ferroelectric Aluminum Scandium Nitride Films,” Journal of Microelectromechanical Systems, Vol. 29, No. 5, pp. 741-747, 2020

[0004]

Claims

[1] Volume acoustic element (1), with: a plurality of resonator devices (2a-2c) arranged side by side, each resonator device (2a-2c) having at least three electrodes (31a-34a, 31b-34b, 31c-34c) arranged parallel to one another and offset from one another, the electrodes (31a-34a, 31b-34b, 31c-34c) forming at least two electrode pairs, and at least one piezoelectric layer being formed between the electrodes (31a-34a, 31b-34b, 31c-34c) of each electrode pair; and a voltage source (6) which is designed to apply voltage to the electrode pairs in such a way that an electric field is formed between the electrodes (31a-34a, 31b-34b, 31c-34c) of the electrode pairs, wherein field directions of the electric fields for adjacent electrode pairs of the same resonator device (2a-2c) are opposite. [2] Volume acoustic element (1) according to claim 1, wherein the voltage source (6) is designed to apply voltage to the electrode pairs in such a way that harmonics are formed between the electrodes (31a-34a, 31b-34b, 31c-34c) of the electrode pairs. [3] Volume acoustic element (1) according to claim 1 or 2, wherein the voltage source (6) is designed to apply voltage to the electrode pairs in such a way that field directions of the electric fields for adjacent electrode pairs of different resonator devices (2a-2c) are parallel. [4] Volume acoustic element (1) according to one of the preceding claims, wherein the voltage source (6) is designed to apply voltage to the electrode pairs in such a way that an acoustic wave is formed between two outer reflector components (4, 5) of the volume acoustic element (1), wherein at least one of the outer reflector components is designed as a Bragg reflector. [5] Volume acoustic element (1) according to one of the preceding claims, wherein the voltage source (6) is designed to apply voltage to the electrode pairs in such a way that an acoustic wave is formed between two outer reflector components (4, 5) of the volume acoustic element (1), wherein at least one of the outer reflector components (4, 5) is designed as a material interface. [6] Volume acoustic element (1) according to one of the preceding claims, with at least three resonator devices (2a-2c) arranged next to one another. [7] Volume acoustic element (1) according to claim 6, wherein each resonator device (2a-2c) has at least three pairs of electrodes. [8] Volume acoustic element (1) according to one of the preceding claims, wherein a capacitance of the volume acoustic element (1) is less than 100 femtofarads. [9] Acoustic filter with a volume acoustic element (1) according to one of the preceding claims. [10] Method for producing a volume acoustic element, comprising the steps: Providing (S1) a plurality of resonator devices (2a-2c) arranged side by side, each resonator device (2a-2c) having at least three electrodes (31a-34a, 31b-34b, 31c-34c) arranged parallel to one another and offset from one another, the electrodes (31a-34a, 31b-34b, 31c-34c) forming at least two electrode pairs, and at least one piezoelectric layer being formed between the electrodes (31a-34a, 31b-34b, 31c-34c) of each electrode pair; and Providing (S2) a voltage source (6) which is designed to apply voltage to the electrode pairs in such a way that an electric field is formed between the electrodes (31 a-34a, 31 b-34b, 31 c-34c) of the electrode pairs, wherein field directions of the electric fields for adjacent electrode pairs of the same resonator device (2a-2c) are opposite.

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