Electronic device with suspended mass

Abstract

Electronic device (10) comprising: a single-crystal semiconductor substrate (100); a cavity (110) which is completely embedded in the semiconductor substrate (100); a movably suspended mass (120) defined in the semiconductor substrate (100) by one or more trenches (130) extending from one side of the semiconductor substrate (100) to the cavity (110); a first electrode layer (140) on the suspended mass (120); and a cover layer (200) covering the suspended mass (120) and including a second electrode layer (210) disposed opposite the first electrode layer (140) and separated therefrom by a gap (220), wherein the first electrode layer (140) includes a first set of comb-like electrode fingers (141, 142; 143, 144); wherein the second electrode layer (210) includes a second set of comb-like electrode fingers, wherein the first set of comb-like electrode fingers (141, 142; 143, 144) comprises first electrode fingers (141; 143) connected to a first voltage terminal (140A) and second electrode fingers (142; 144) connected to a second voltage terminal (140B), wherein the second set of comb-like electrode fingers comprises: - first electrode fingers operable to be charged according to a first polarity, and - second electrode fingers operable to be charged according to a second polarity opposite to the first polarity, and wherein the first electrode layer (140) comprises notches (150) separating electrode fingers (141, 142; 143, 144) of the first set of comb-like electrode fingers, the notches (150) extending into the movably suspended mass (120).

Description

[0001] The present invention relates to an electronic device and a method for manufacturing an electronic device.

[0002] In the field of microelectromechanical systems (MEMS), it is known to cantilever certain elements of a semiconductor material, allowing electronic functionalities to be combined with mechanical effects. Examples of such MEMS are accelerometers, in which the movement of a suspended mass is detected based on the capacitive coupling of an electrode on the suspended mass with a counter electrode. Another example of such MEMS are so-called energy harvesters (i.e., energy-harvesting devices), in which the movement of a suspended mass is converted into electrical energy by utilizing the electrostatic coupling of an electrode on the suspended mass with a counter electrode.

[0003] In some cases, however, such MEMS devices can be difficult to integrate with other circuit structures, suffer from high production costs, or be difficult to scale. This can be due, for example, to the need to prepare the suspended mass from a semiconductor using advanced etching processes, while also requiring precise positioning of electrodes or other circuit structures on or near the suspended mass. For example, in "DESIGN, MODELING, FABRICATION AND CHARACTERIZATION OF AN ELECTRET-BASED MEMS ELECTROSTATIC ENERGY HARVESTER", G. Altena et al., Transducers'11, Beijing, China, June 5-9, (2011), an electrostatic energy harvester is described which is based on bonding three different wafers together. In this structure, a seismic mass is embodied in an SOI (Silicon On Insulator) wafer.Silicon on insulator) is prepared, and a carrier wafer, which provides metallic contact layers, and an electret wafer are bonded to both sides of the SOI wafer.

[0004] Consequently, there is a need for techniques that enable an electronic device provided with a suspended mass to be efficiently provided. One object of the invention disclosed herein is to meet this need.

[0005] EP 1411024 B1 describes a MEMS actuator. US 2008 / 0122313 A1 describes an electrostatic induction generator. EP 2778692 A1 describes a MEMS acceleration sensor. Further acceleration sensors are known from DE 4032828 A1 and DE 102009055389 A1. Further micromechanical components and manufacturing methods for such micromechanical components are described in DE 102011081014 A1, DE 102014103389A1, DE 102012213313 A1, and JP 2004127871 A.

[0006] According to embodiments disclosed herein, this object is achieved by an electronic device according to claim 1 and by a method for manufacturing an electronic device according to claim 8. The appended claims define further embodiments.

[0007] According to further embodiments of this disclosure, additional devices or methods may be provided. Such embodiments will be apparent from the following detailed description taken in conjunction with the accompanying drawings. Fig. 1 schematically illustrates an electronic device according to an embodiment of this disclosure. Fig. 2 shows an electrode configuration according to an embodiment of this disclosure. Fig. 3 shows another electrode configuration according to an embodiment of this disclosure. Fig. 4 shows a charging pattern of cap layer electrodes as used in one embodiment in this disclosure. Fig. 5 schematically illustrates the preparation of a cavity according to an embodiment of this disclosure. Fig. 6 schematically illustrates the preparation of an electrode layer according to an embodiment of this disclosure. Fig. 7 schematically illustrates the definition of a suspended mass according to an embodiment of this disclosure. Fig. 8 schematically illustrates the preparation of a cover layer according to an embodiment of this disclosure. Fig. 9 schematically illustrates the removal of a sacrificial layer according to an embodiment of this disclosure. Fig. 10 shows a flowchart for schematically illustrating a method for manufacturing an electronic device according to an embodiment of this disclosure.

[0008] Various embodiments are described in detail below with reference to the accompanying drawings. It should be understood that these embodiments serve only as examples and are not to be understood as limiting. For example, while some embodiments are described with a plurality of features, other embodiments may include fewer features and / or alternative features. Furthermore, features from different embodiments may be combined with one another unless specifically stated otherwise.

[0009] The embodiments illustrated below relate to an electronic device. The electronic device is provided with a suspended mass, which is formed from a semiconductor material of the electronic device. The suspended mass is movable with respect to other structures of the electronic device, and the electronic device is designed to convert such mechanical movement into electrical signals. The electronic device can, for example, correspond to an electromechanical accelerometer, in which a movement of the suspended mass can be detected based on a capacitive coupling of an electrode on the suspended mass with a counter electrode. Furthermore, the electronic device can be an energy harvester (iean energy harvesting device) in which a movement of the suspended mass can be converted into electrical energy by utilizing an electrostatic coupling of an electrode on the suspended mass with a counter electrode.

[0010] Fig. 1 schematically illustrates exemplary structures of the electronic device 10.

[0011] As illustrated, electronic device 10 includes a semiconductor substrate 100. In the illustrated examples, semiconductor substrate 100 is a single-crystal substrate, e.g., a single-crystal silicon substrate. A cavity 110 is formed within semiconductor substrate 100. As explained in more detail below, cavity 110 may be prepared using a silicon-on-nothing (SON) process.

[0012] The cavity 110 is located below an upper surface of the semiconductor substrate 100. On this upper surface, electronic circuit structures of the electronic device 100 can be prepared, e.g., using conventional CMOS technology (CMOS: "Complementary Metal-Oxide Semiconductor"). As an example of elements of such electronic circuit structures, Fig. 1 an electrode layer 140. The electrode layer 140 can be prepared, for example, by selectively doping certain parts of the semiconductor substrate.

[0013] As further illustrated, a suspended mass 120 is formed from the semiconductor substrate 100. The suspended mass 120 corresponds to a portion of the semiconductor substrate 100 that covers the cavity 110. The electrode layer 140 is located on the portion of the semiconductor substrate 100 that corresponds to the suspended mass 120.

[0014] The suspended mass 120 is defined by one or more trenches 130 that extend from the top surface of the semiconductor substrate 100 to the cavity 110. Consequently, the suspended mass 120 is a part of the semiconductor substrate 100. Laterally, the suspended mass 120 is partially separated from the rest of the semiconductor substrate 100 by the trenches 130. Vertically, the suspended mass 120 is separated from the semiconductor substrate 100 by the cavity 110. The lateral separation of the suspended mass 120 from the rest of the semiconductor substrate 100 is not continuous, so that one or more suspension structures (in Fig. 1 not shown) through which the suspended mass 120 is connected to the rest of the semiconductor substrate 100. Consequently, the suspended mass 120 is movable by elastic deformation of the suspension structures. The suspension structures can be designed according to the desired movement characteristics of the suspended mass 120. In the illustrated example, it is assumed that the suspended mass 120 is movable in a lateral direction, as shown in Fig. 1 is illustrated by a double-sided arrow.

[0015] The electronic device 100 further includes a cap layer 200. The cap layer 200 is formed on the upper surface of the semiconductor substrate 100. The cap layer 200 may, for example, correspond to a dielectric material deposited on the upper surface of the semiconductor substrate 100.

[0016] The cap layer 200 includes a further electrode layer 210, which is located vertically above the electrode layer 140 and separated therefrom by a gap 220. The gap 220 is formed between the cap layer 200 and a part of the semiconductor substrate 100 corresponding to the suspended mass 120. The gap 220 separates the suspended mass 120 vertically from the cap layer 200, such that the movement of the suspended mass 120 is not impaired by the cap layer 200. The further electrode layer 210 can be formed, for example, as an electret layer, e.g., by embedding conductive electrode structures in the dielectric material of the cap layer 200. The conductive electrode structures can be formed, for example, from polycrystalline silicon, metal, or another suitable material.

[0017] The cover layer 200 may also be based on an ILD (inter-level dielectric) structure with multiple dielectric layers of different dielectric constants. Furthermore, the cover layer 200 may also include one or more metallization layers, e.g., for coupling the electrode layer 140 to the electrode layer 210 or to other electronic circuit structures of the electronic device 10.

[0018] The electrode layer 140 and the further electrode layer 210 may each include electrode structures in the form of finger electrodes. For example, the electrode layer 140 may include a first set of comb-like electrode fingers, and the further electrode layer 210 may include a second set of comb-like electrode fingers. The electrode fingers of both sets may extend in a lateral direction substantially perpendicular to the lateral direction of movement of the suspended mass 120. Consequently, the movement of the suspended mass 120 may have the effect of displacing the first set of comb-like electrode fingers relative to the second set of comb-like electrode fingers.

[0019] The suspended mass 120 can move between a first position in which each electrode finger of the first set is aligned with an electrode finger of the second set, and a second position in which each electrode finger of the first set is aligned with a clearance between two electrode fingers of the second set. In this disclosure, the second position is intended to encompass not only exact alignment with the clearance, but also any position different from the first position such that the electrode fingers of the first and second sets are substantially out of alignment. Fig. Figure 1 illustrates the suspended mass 120 in the first position. In the first position, a strong capacitive coupling can be achieved between the first set of comb-like electrode fingers and the second set of comb-like electrode fingers. In comparison, a lower capacitive coupling occurs in the second position. When a static electrical charge is applied to the further electrode layer 210, the variations in the capacitive coupling induced during the movement of the suspended mass 120 cause charge fluctuations in the electrode layer 140, which in turn can be converted into electrical signals. Such electrical signals can be used, for example, for the purpose of detecting accelerations or for the purpose of energy harvesting. For energy harvesting applications, the further electrode layer 210 can be charged to a static potential of, for example, 10 V to 50 V.For acceleration sensing applications, lower static potentials may be sufficient. For example, the further electrode layer 210 may be at a floating potential and can be charged by tunneling currents. This may allow the further electrode layer 210 to be efficiently precharged to a desired potential.

[0020] As further illustrated, the electrode layer 140 can be provided with notches 150 that separate the electrode fingers of the first set of comb-like electrode fingers from one another. Similarly, the further electrode layer 210 can be provided with notches 230 that separate the electrode fingers of the second set of comb-like electrode fingers from one another. The notches 150 and / or the notches 230 can help reduce lateral coupling between the first and second sets of comb-like electrode fingers and thereby achieve a higher degree of efficiency. This, in turn, can enable the use of lower static potentials for charging the further electrode layer 210, thereby reducing the risk of a so-called pull-in, in which the suspended mass 20 is pulled against the cover layer 200.

[0021] Fig. 2 shows a top view of an exemplary comb electrode structure that can be used for the electrode layer 140.

[0022] As shown, the comb electrode structure has first finger electrodes 141, which are laterally spaced along the direction of movement of the suspended mass 120, and second finger electrodes 142, which are laterally spaced along the direction of movement of the suspended mass 120. The first finger electrodes 141 and the second finger electrodes 142 have a symmetrical configuration, wherein the arrangement of the second finger electrodes 142 is rotated by 180° with respect to the arrangement of the first finger electrodes 141. The first finger electrodes 141 and the second finger electrodes 142 interlock, so that in one embodiment, exactly one of the second electrodes 142 is always located between two of the first finger electrodes 141 and exactly one of the first finger electrodes 141 is always located between two of the second finger electrodes 142.

[0023] As further illustrated, the first finger electrodes 141 are connected to a first voltage terminal 140A, and the second finger electrodes 142 are connected to a second voltage terminal 140B. The first voltage terminal 140A and the second voltage terminal 140B are located on opposite sides of the suspended mass 120 and extend beyond the suspension structures to the remainder of the semiconductor substrate 100.

[0024] In addition, Fig. 2, the trenches 130, which define the suspended mass 120, and further trenches 131, which define the elastically deformable suspension structures. Similar to the trenches 130, the further trenches 131 extend from the upper surface of the semiconductor substrate 100 to the cavity 110 (in Fig. 2 not shown). In this way, the suspension structures can be defined to exhibit desired deformation properties.

[0025] Fig. 3 shows a top view of another exemplary comb electrode structure that can be used for the electrode layer 140. The comb electrode structure of Fig. 3 is generally similar to that of Fig. 2, and elements of Fig. 3, which corresponds to that of Fig. 2 have been given the same reference numerals. The comb electrode structure of Fig. 3 has first finger electrodes 143, which are laterally spaced along the direction of movement of the suspended mass 120, and second finger electrodes 144, which are laterally spaced along the direction of movement of the suspended mass 120. However, in this case, the first finger electrodes 143 and the second finger electrodes 144 have an asymmetric configuration and differ in their number and spacing. The first finger electrodes 143 and the second finger electrodes 144 are interleaved such that an outermost pair of the first finger electrodes 143 is arranged between an outermost pair of the second finger electrodes 144. This asymmetric configuration can help avoid excessive field strengths at the sides of the suspended mass 120, thereby reducing the risk of pull-in.

[0026] Fig. Figure 4 shows an example of a charging pattern of the further electrode layer 210. As shown, the finger electrodes of the further electrode layer 210 are charged in an alternating pattern with opposite polarities. This can be achieved, for example, by providing the further electrode layer with a comb electrode structure similar to that of Fig. 2 or Fig. 3, i.e., having first finger electrodes and second finger electrodes that can be charged independently. The alternating pattern of opposite charges in the further electrode layer 210 can correspond to the alternating pattern of separately contacted first and second electrode fingers in the electrode layer 140. By providing the alternating pattern of opposite charges in the further electrode layer 210, the overall electrostatic force acting between the electrode layer 140 and the further electrode layer 210 can be reduced. This, in turn, helps reduce the risk of pull-in.

[0027] In the following, exemplary processes for manufacturing the electronic device 10 are described in more detail with reference to Fig. 5-10. This illustrates Fig. 5 Preparing a cavity, Fig. 6 Preparation of an electrode layer, Fig. 7 Defining a suspended mass, Fig. 8 Preparing a top layer and Fig. 9 Removing a sacrificial layer.

[0028] Initially, the cavity 110 may be formed in the semiconductor substrate 100. This may be achieved by a SON process as schematically shown in Fig. 5 is illustrated.

[0029] As in Fig. 5, a set of parallel trenches 510 may be prepared extending vertically from the top surface of the semiconductor substrate 100 into the semiconductor substrate 100. The depth of the trenches 510 may be substantially the same as the desired depth of the cavity 110 below the top surface. The semiconductor substrate 100 may then be subjected to a heat treatment that causes the top surface of the semiconductor substrate 100 to reform, filling the upper portions of the trenches 510 while causing the lower portions of the trenches 510 to coalesce, forming the cavity 110. As a result, the cavity 110 is formed below the top surface of the semiconductor substrate 100. At this point, the cavity 110 is completely embedded within the single-crystal material of the semiconductor substrate 100.

[0030] Details regarding the implementation of such a SON process can be found, for example, in “Fabrication of Silicon-on-Nothing Structure by Substrate Engineering ...”, Sato et al, Japanese Journal of Applied Physics, Vol. 43, No. 1, 2004, pp. 12-18.

[0031] In a next step, the electrode layer 140 may be formed on the upper surface of the semiconductor material, as shown in Fig. 6. This can be achieved, for example, by depositing conductive semiconductor material or metal on the upper surface, whereby lithographic techniques can be used to obtain the desired structure of the electrode layer 140. At this stage, the recesses 150 between the electrode fingers of the electrode layer 140 can also be formed. Furthermore, further electronic circuit structures of the electronic device 10 can also be formed on the upper surface of the semiconductor substrate 100.

[0032] Fig. Figure 7 illustrates the definition of the suspended mass 120 by the trenches 130. The trenches 130 can be formed, for example, by dry etching into the upper surface of the semiconductor substrate 100. The trenches 130 are formed with a sufficient depth to reach the cavity 110 formed below the upper surface of the semiconductor substrate 100.

[0033] In a next step, the cover layer 200 can be formed. As in Fig. 8, this may involve first forming a sacrificial layer 810 covering the suspended mass 120 and the trenches 130. As illustrated, the sacrificial layer 810 may be formed to substantially only cover the suspended mass 120. However, in some scenarios, the sacrificial layer 810 may also at least partially fill the trenches 130 and the cavity 110. The sacrificial layer 810 may, for example, be carbon-based to facilitate its later removal by a dry process. Next, the capping layer 200 may be deposited on the top surface of the semiconductor substrate 100 and on the sacrificial layer 810. As part of this process, the further electrode layer 210 may also be formed within the capping layer 210, e.g., by using lithographic techniques to apply the conductive material of the further electrode layer 210 with the desired pattern.At this stage, the incisions 230 between the electrode fingers of the electrode layer 210 can also be defined.

[0034] As in Fig. As illustrated in Figure 9, the sacrificial layer 810 may then be removed. This may be achieved by a dry process, e.g., by ashing in an oxidizing atmosphere, through vias 910 formed in the cap layer 210. After removing the sacrificial layer 810, the vias 910 may be closed. Before closing the vias 910, the empty space formed by the cavity 110, the trenches 130, and the gap 220 may be filled with a suitable gas, e.g., nitrogen.

[0035] Fig. Figure 10 shows a flowchart illustrating an exemplary method for manufacturing an electronic device. The method can be applied, for example, to manufacture the above-mentioned electronic device 10 and can be based on process steps as described in connection with Fig. 5-9. The electronic device may be, for example, an electromechanical accelerometer or an electromechanical energy harvester.

[0036] In step 1010, a cavity is formed in a single-crystal semiconductor substrate. For example, step 1010 may include forming the cavity 110 in the semiconductor substrate 100. The semiconductor substrate may be, for example, a silicon substrate. The cavity may be formed by a SON process, e.g., as described in connection with Fig. 5 explained.

[0037] In step 1020, an electrode layer is formed on one side of the semiconductor substrate. For example, step 1020 may include forming the electrode layer 140 on the top surface of the semiconductor substrate 100, e.g., as described in connection with Fig. 6. Other electronic circuit structures may also be formed on the semiconductor substrate along with the electrode layer. Consequently, the electrode layer can be efficiently integrated with such other electronic circuit structures. The electrode layer may be based on conductive semiconductor material and / or metal. The electrode layer may include a first set of comb-like electrode fingers. Step 1020 may also include providing notches that separate electrode fingers of the first set of comb-like electrode fingers, such as the aforementioned notches 150.

[0038] At step 1030, a suspended mass is defined in the semiconductor substrate. For example, step 1030 may include defining the suspended mass 120 in the semiconductor substrate 100, e.g., as described in connection with Fig. 7. The suspended mass may be defined by one or more trenches extending from the surface of the semiconductor substrate to the cavity. Examples of such trenches are the trenches 130 mentioned above. The suspended mass is defined in such a way that the electrode layer formed in step 1020 is located on the suspended mass. The suspended mass is movably suspended, typically by one or more suspension structures formed between the suspended mass and the rest of the semiconductor substrate. The suspended mass may, for example, be movable in a lateral direction extending parallel to the surface of the semiconductor substrate.

[0039] At step 1040, a cap layer is provided. For example, step 1040 may include providing the cap layer 200 on the semiconductor substrate 100, e.g., as described in connection with Fig. 8 and Fig. 9. The cover layer includes a further electrode layer, which is arranged opposite the first electrode layer formed in step 1020 and separated therefrom by a gap. An example of such a further electrode layer is the further electrode layer 210, which is separated from the electrode layer 140 by the gap 220. The further electrode layer can be formed as an electret layer with electrodes embedded in a dielectric material. The further electrode layer can include a second set of comb-like electrode fingers. Step 1040 can also include providing incisions that separate electrode fingers of the second set of comb-like electrode fingers, such as the above-mentioned incisions 230.The cover layer formed at step 1040 may also close a void formed by the cavity, the trenches defining the suspended mass, and the gap between the cover layer and the suspended mass.

[0040] If the electrode layer formed on the suspended mass includes a first set of comb-like electrode fingers and the further electrode layer formed in the cover layer includes a second set of comb-like electrode fingers, the movement of the suspended mass can extend at least between a first position in which each electrode finger of the first set is aligned with an electrode finger of the second set, and a second position in which each electrode finger of the first set is aligned with a free space between two electrode fingers of the second set.

[0041] It is noted that the steps of the method of Fig.10 need not be performed in the order shown. For example, the electrode layer on the surface of the semiconductor substrate could also be formed after defining the suspended mass.

[0042] It should be understood that the concepts and embodiments described above can be modified in various ways. For example, the concepts can be applied to various types of electronic devices based on a suspended mass. Furthermore, various types of semiconductor conductor materials and process technologies can be used.

Claims

[1] Electronic device (10) comprising: a single-crystal semiconductor substrate (100); a cavity (110) which is completely embedded in the semiconductor substrate (100); a movably suspended mass (120) defined in the semiconductor substrate (100) by one or more trenches (130) extending from one side of the semiconductor substrate (100) to the cavity (110); a first electrode layer (140) on the suspended mass (120); and a cover layer (200) covering the suspended mass (120) and including a second electrode layer (210) disposed opposite the first electrode layer (140) and separated therefrom by a gap (220), wherein the first electrode layer (140) includes a first set of comb-like electrode fingers (141, 142; 143, 144); wherein the second electrode layer (210) includes a second set of comb-like electrode fingers, wherein the first set of comb-like electrode fingers (141, 142; 143, 144) comprises first electrode fingers (141; 143) connected to a first voltage terminal (140A) and second electrode fingers (142; 144) connected to a second voltage terminal (140B), wherein the second set of comb-like electrode fingers comprises: - first electrode fingers operable to be charged according to a first polarity, and - second electrode fingers operable to be charged according to a second polarity opposite to the first polarity, and wherein the first electrode layer (140) comprises notches (150) separating electrode fingers (141, 142; 143, 144) of the first set of comb-like electrode fingers, the notches (150) extending into the movably suspended mass (120). [2] The electronic device (10) of claim 1; wherein the suspended mass (120) is movable between at least a first position in which each electrode finger (141, 142; 143, 144) of the first set is aligned with an electrode finger of the second set, and a second position in which each electrode finger (141, 142; 143, 144) of the first set is aligned with a clearance between two electrode fingers of the second set. [3] The electronic device (10) of any preceding claim, wherein the second electrode layer (210) comprises notches (230) separating electrode fingers of the second set of comb-like electrode fingers. [4] The electronic device (10) according to any one of the preceding claims, wherein an arrangement of the first electrode fingers (143) of the first set of comb-like electrode fingers (144) is asymmetrical with respect to an arrangement of the second electrode fingers of the first set of comb-like electrode fingers. [5] Electronic device (10) according to one of the preceding claims, wherein the second electrode layer (210) comprises an electret layer with electrodes embedded in a dielectric material. [6] Electronic device (10) according to one of the preceding claims, wherein the electronic device (10) is an electromechanical acceleration sensor. [7] Electronic device (10) according to any one of claims 1-5, wherein the electronic device (10) is an electromechanical energy harvesting device. [8] A method of manufacturing an electronic device (10), the method comprising: Forming a cavity (110) in a single-crystal semiconductor substrate (100), wherein the cavity (110) is completely embedded in the single-crystal semiconductor substrate (100); Defining a movably suspended mass (120) by one or more trenches (130) extending from one side of the semiconductor substrate (100) to the cavity (110); Providing a first electrode layer (140) on the suspended mass (120); and Providing a cover layer (200) covering the suspended mass (120) and including a second electrode layer (210) disposed opposite the first electrode layer (140) and separated therefrom by a gap (220), wherein the first electrode layer (140) includes a first set of comb-like electrode fingers (141, 142; 143, 144); wherein the second electrode layer (210) includes a second set of comb-like electrode fingers, wherein the first set of comb-like electrode fingers comprises first electrode fingers (141; 143) connected to a first voltage terminal (140A) and second electrode fingers (142; 144) connected to a second voltage terminal (140B), wherein the second set of comb-like electrode fingers comprises: - first electrode fingers operable to be charged according to a first polarity, and - second electrode fingers operable to be charged according to a second polarity opposite to the first polarity, and wherein the first electrode layer (140) comprises notches (150) separating electrode fingers (141, 142; 143, 144) of the first set of comb-like electrode fingers, the notches (150) extending into the movably suspended mass (120). [9] The method of claim 8, wherein the suspended mass (120) is movable at least between a first position in which each electrode finger (141, 142; 143, 144) of the first set is aligned with an electrode finger of the second set, and a second position in which each electrode finger (141, 142; 143, 144) of the first set is aligned with a space between two electrode fingers of the second set. [10] The method of claim 8 or 9, wherein the second electrode layer comprises notches (230) separating electrode fingers of the second set of comb-like electrode fingers. [11] The method of any one of claims 8-10, wherein an arrangement of the first electrode fingers (143) of the first set of comb-like electrode fingers is asymmetrical with respect to an arrangement of the second electrode fingers (144) of the first set of comb-like electrode fingers. [12] The method of any one of claims 8-11, wherein the semiconductor substrate (100) is formed of silicon and the cavity (110) is formed by a SON process.

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