Electroacoustic resonator and method for its manufacture

Abstract

Method for manufacturing an electroacoustic resonator (1) comprising the following steps: Deposition of a diamond layer (2) with a first side (21) and an opposing second side (22) on a first substrate (3), wherein the second side (22) of the diamond layer (2) is in contact with a first side (31) of the substrate (3), removal of the first substrate (3), Generating a piezoelectric layer (4) on the second side (22) of the diamond layer (2), Applying the diamond layer (2) with the first side (21) onto a second substrate (6), characterized by the fact that the piezoelectric layer (4) contains or consists of AlScN.

Description

[0001] The invention relates to an electroacoustic resonator comprising a diamond layer with a first side and an opposing second side, and a piezoelectric layer arranged on the second side of the diamond layer. Electroacoustic resonators of this type can be used in high-frequency filters or electroacoustic sensors. The invention further relates to methods for manufacturing such an electroacoustic resonator.

[0002] From US patent 5,652,436 A, a diamond-based structure is known. This structure comprises a substrate, an adhesive material on a surface of the substrate, and an array of spaced-apart diamond mesas bonded to the substrate by the adhesive material. This structure can be fabricated by providing a sacrificial substrate, forming a multitude of diamond mesas on a surface of the sacrificial substrate, bonding the diamond mesas to a transfer substrate, and then removing the sacrificial substrate.

[0003] US patent 5,576,589A discloses a surface acoustic wave (SAW) filter. This filter contains an oriented diamond layer on a piezoelectric layer. Interdigital electrodes can be fabricated by electrically conductive metal conductors and / or by heavily doped surface sections of the diamond layer.

[0004] From P. Kirsch et al.: “5 GHz surface acoustic wave devices based on aluminum nitride / diamond layered structure realized using electron beam lithography”. Appl. Phys. Lett. 88 (2006) 223504, it is known to provide a heteroepitactically deposited diamond layer with a piezoelectric layer. By applying an electric field to this piezoelectric layer, surface acoustic waves can be generated in the diamond. Thus, the diamond layer can be used as a surface acoustic wave filter.

[0005] However, due to the steadily increasing carrier frequencies and data rates of wireless data transmission, there is a need for electronic filter circuits that exhibit a higher operating frequency and improved resonator quality compared to the prior art. The object of the present invention is therefore to provide an electroacoustic resonator with a higher operating frequency and higher quality factor, as well as a method for its manufacture.

[0006] The problem is solved according to the invention by a method according to claim 1, a method according to claim 2 and / or an electroacoustic resonator according to claim 7. Advantageous embodiments of the invention are found in the dependent claims.

[0007] According to the invention, an electroacoustic resonator is proposed which contains a diamond layer with a first side and an opposing second side. This diamond layer is generally deposited heteroepitactically from the gas phase in a low-pressure synthesis. For this purpose, a first substrate can be introduced into a vacuum chamber. This chamber is then flooded with a working gas that essentially contains hydrogen. In addition, the working gas contains a carbon-containing gas, for example, methane or acetylene. Finally, dopants can be introduced. The gas phase can be activated by microwave radiation or heated filaments made of refractory metals, so that… 3Hybridized carbon in a diamond structure is deposited on the first substrate. Such processes are known in numerous variations. The invention does not teach the use of a specific diamond layer deposition process as a solution principle.

[0008] In some embodiments of the invention, the first substrate can be selected from silicon, molybdenum, strontium titanate, aluminum nitride, zinc oxide, diamond, or single-crystal metals such as Ni, Mo, Pt, Ir, etc. The first substrate has a first side and an opposing second side with intervening edge surfaces. During low-pressure synthesis, a diamond layer is deposited on the first side of the first substrate. This layer also has a first side facing the gas phase and an opposing second side, the second side being in contact with the first side of the first substrate.

[0009] In some embodiments of the invention, the first substrate can be removed after the diamond layer has been produced. This exposes the second side of the diamond layer, allowing a piezoelectric layer or an AlScN layer to be heteroepitactically deposited there. This can be done in a manner known per se by MBE, MOCVD, MOVPE, sputtering, or similar processes. If the first substrate itself contains or is coated with a piezoelectric material or AlScN, the diamond layer grows directly onto the piezoelectric layer with its second side. In this case, the removal of the first substrate can be omitted, or the first substrate can simply be thinned but not completely removed.

[0010] Finally, the diamond layer is deposited with its first side onto a second substrate. In some embodiments of the invention, this can be done by a wafer bonding process. In some embodiments of the invention, the second substrate can contain or consist of silicon, a metal, or an alloy. In other embodiments of the invention, the second substrate can be a ceramic, for example, aluminum oxide, silicon oxide, a nitrite, or an oxynitride.

[0011] In some embodiments of the invention, the second substrate has an acoustic impedance that is 5, 10, 20, or 30 times lower than the acoustic impedance of diamond. For the purposes of this description, acoustic impedance is defined as the product of material density and the speed of sound within the material.

[0012] According to the invention, it has been recognized that sound energy is reflected at the interface between the second side of the diamond layer and the adjacent material, thus concentrating the sound energy onto the diamond layer. This leads to the low-loss propagation of surface and Lamb waves, allowing the resonator quality factor and / or the operating frequency to increase. The greater the difference in the acoustic impedances of the diamond layer on the one hand and the adjacent material on the other, the greater the sound energy reflected back into the diamond layer at the interface. This can reduce clamping losses.

[0013] In some embodiments of the invention, the ratio of the acoustic impedances of the piezoelectric layer and the diamond layer is less than approximately 4, less than approximately 3, less than approximately 2, or approximately 1. This results in sound energy being reflected only to a small extent at the interface between the diamond layer and the piezoelectric layer, so that the diamond layer and the piezoelectric layer oscillate essentially in phase, but are acoustically isolated from the second substrate.

[0014] In some embodiments of the invention, the diamond, the diamond layer, the diamond substrate, or a portion thereof can contain a dopant. This allows the diamond layer to be used as a buried electrode. During operation of the electroacoustic resonator, this electrode can be provided with at least one terminal contact and brought to a predetermined potential, for example, ground potential. In other embodiments of the invention, the diamond can be electrically insulated, resulting in a floating potential.

[0015] In some embodiments of the invention, the entire diamond layer can be doped. In other embodiments, it may be sufficient to dope only a portion of the diamond layer. For example, the diamond layer in the upper region can be doped to a layer depth of up to 100 or 500 nm. For example, the diamond layer can be doped at least in the region that is in contact with the piezoelectric film. These embodiments of the invention can provide sufficient doping for a functional lower electrical contact for the piezoelectric layer.

[0016] In some embodiments of the invention, the diamond can be doped with boron or phosphorus. This can create either p- or n-type doping.

[0017] In some embodiments of the invention, a polymer layer can be arranged between the first side of the diamond layer and the second substrate. Such a polymer layer can, firstly, provide the adhesive forces required for the wafer bonding process between the second substrate and the diamond layer. Furthermore, the polymer layer can have a lower acoustic impedance than the second substrate, so that a higher proportion of the sound energy is reflected at the interface. Finally, the polymer layer can fill surface roughness on the first side of the diamond layer, thus eliminating the need for complex polishing of the diamond layer. This significantly simplifies the production of the resonator according to the invention.

[0018] In some embodiments of the invention, the polymer layer can contain a thermosetting resin. In some embodiments of the invention, the polymer layer can contain or consist of benzocyclobutene. In some embodiments of the invention, the polymer layer can also contain or consist of other hydrocarbon-based polymers. Such a polymer layer can be easily produced by spin coating to the desired thickness. Furthermore, such a polymer layer exhibits the desired low acoustic impedance.

[0019] In some embodiments of the invention, the polymer layer can have an acoustic impedance between approximately 0.05·10 7 kg·s -1 ·m -2 and 0.5·10 7 kg·s -1 ·m -2Such a material exhibits an acoustic impedance that is significantly lower than the acoustic impedance of diamond, so that a sufficient portion of the sound energy is reflected at the interface between the polymer and the diamond to provide the desired resonator quality.

[0020] In some embodiments of the invention, the polymer layer can have a thickness of 1 µm to approximately 10 µm. Such a polymer layer ensures sufficient acoustic decoupling from the second substrate and enables a secure bond between even rough diamond surfaces and the second substrate.

[0021] In some embodiments of the invention, an interdigital transducer can be arranged on the piezoelectric layer. Such an interdigital transducer contains at least one, and often two, comb-shaped structures, wherein the prongs or fingers of one structure engage in the spaces between the opposing structures. Such an interdigital transducer makes it possible to generate electric fields in the piezoelectric layer. In the case of alternating electromagnetic fields, the piezoelectric layer is excited to vibrate, and these vibrations propagate as acoustic waves within the diamond. Due to the strong binding forces of the sp 3 Due to the hybridized carbon atoms in the diamond lattice, the diamond has a comparatively high speed of sound, which is why the operating frequency of such an electroacoustic resonator can be increased.

[0022] In some embodiments of the invention, a Bragg reflector can be arranged on the piezoelectric layer. Such a Bragg reflector can restrict the resulting acoustic wave to a predefinable partial volume of the resonator or the diamond layer and thus improve the resonator quality.

[0023] In some embodiments of the invention, a layer containing or consisting of SiO2 can be deposited. In this case, an SiO 2- A layer is deposited on the processed resonator. This can cover the active region of the resonator, allowing the formation of IDT or Bragg reflector structures. The thickness of the oxide layer can range from approximately 10 nm to several µm. The oxide layer can serve multiple functions, such as passivating the piezoelectric surface, tuning the resonator frequency, and / or reducing TCF, etc.

[0024] In some embodiments of the invention, the diamond layer can have a thickness of approximately 1 µm to approximately 30 µm. In other embodiments of the invention, the diamond layer can have a thickness of approximately 2 µm to approximately 20 µm. In yet other embodiments of the invention, the diamond layer can have a thickness of approximately 2 µm to approximately 10 µm. In some embodiments, the diamond layer can have a thickness of approximately 3 µm to approximately 8 µm. In other embodiments of the invention, the diamond layer can have a thickness of approximately 8 µm to approximately 25 µm. Finally, the diamond layer can have a thickness of approximately 10 µm to approximately 30 µm. Due to the mechanical stabilization provided by the second substrate, the diamond layer in some embodiments of the invention can be chosen to be thinner than in known electroacoustic resonators, so that the resonator according to the invention can be manufactured more cost-effectively and easily.In other embodiments of the invention, the diamond layer allows for improved thermal management. This can result in higher load capacity and / or a reduction in the temperature coefficient of frequency.

[0025] In some embodiments of the invention, the diamond layer can be provided to be connected to ground potential during operation of the electroacoustic resonator, or the diamond layer can be arranged in an electrically insulated manner. According to the invention, ground contact can also occur in a "floating potential" state.

[0026] In some embodiments of the invention, an acoustic layer thickness d, the speed of sound c, the operating frequency f, and the period λ of the interdigital converter can be related as follows: f∗d=cdλ<<5 MHz mm.

[0027] Under these conditions, Lamb waves can be generated, which, unlike pure surface waves, can exhibit a higher frequency and / or a higher resonator quality factor. Thus, the component according to the invention can also be used reliably at higher frequencies and enable reliable channel separation.

[0028] The invention will now be explained in more detail with reference to exemplary embodiments or limitations of the general inventive concept. This will show Fig. 1 a view of an electroacoustic resonator according to the invention. Fig. Figure 2 shows a cross-section through an electroacoustic resonator according to the invention. Fig. Figure 3 shows a first embodiment of a manufacturing process for the electroacoustic resonator. Fig. Figure 4 shows a second embodiment of the manufacturing process of the electroacoustic resonator.

[0029] Based on the Fig. 1 and Fig. Section 2 below describes an electroacoustic resonator according to the invention. It shows Fig. 1 a view and Fig. 2 a cross-section.

[0030] The electroacoustic resonator 1 has a diamond layer 2. The diamond layer 2 has a first side 21 and an opposing second side 22. The diamond layer 2 is deposited as a quasi-encrystalline, polycrystalline, or nanocrystalline diamond layer from a carbon-containing gas phase. For this purpose, a first substrate is introduced into a vacuum chamber containing an atmosphere of hydrogen and a carbon-containing gas, for example, methane, ethane, or acetylene. This gas phase is activated by microwave radiation or filaments made of a refractory metal, causing the components to dissociate and the carbon to be deposited in sp 3-hybridization is deposited on the first substrate. The second side 22 of the diamond layer 2, located on the first substrate, is comparatively smooth. The first side 21 of the diamond layer 2, facing the gas phase, can exhibit greater roughness due to lattice defects. The diamond layer 2 produced in this way can have a thickness between approximately 2 µm and approximately 20 µm.

[0031] As from Fig. 1 and Fig. As can be seen further in Figure 2, the first substrate is removed after the diamond layer 2 has been produced. A piezoelectric layer 4 with a thickness of approximately 0.1 µm to approximately 5 µm or approximately 0.5 µm to approximately 1.5 µm is deposited onto the resulting second side 22 of the diamond layer 2. The piezoelectric layer 4 contains or consists of AlScN. The piezoelectric layer 4 can be deposited using a sputtering, MOCVD, MBE, or MOVPE process.

[0032] At least one interdigital transducer 7 is generated on the piezoelectric layer 4. The interdigital transducer consists of two essentially identical, comb-like structures 7a and 7b. The individual fingers or prongs of structure 7b lie in the spaces between the fingers or prongs of structure 7a and vice versa.

[0033] The individual structures 7a and 7b of the interdigital converter contain a metal or alloy, which is either deposited on a masking layer, so that after a lift-off process the in Fig. 1 and Fig. 2 visible structure is obtained. In other embodiments of the invention, the metallization can be deposited over the entire surface of the piezoelectric layer 4 and subsequently masked, structured, and partially removed by wet or dry chemical etching using a photoresist.

[0034] To facilitate electrical contacting of the interdigital converter 7, structures 7a and 7b each have an optional connection contact 9a and 9b. This contact can also contain or consist of a metal or alloy and, due to its greater thickness, exhibits increased mechanical strength, enabling reliable contacting of the interdigital converter 7, for example by means of a soldered connection or bond wires.

[0035] Furthermore, an optional Bragg reflector 8 is shown in the figures. In the illustrated embodiment, Bragg reflectors 8a and 8b are located on both sides of the resonator in the longitudinal direction. These also consist of an approximately grid-like metallization of a partial surface of the piezoelectric layer 4. Acoustic waves in the diamond layer 2 or the piezoelectric layer 4 are reflected by the Bragg reflectors 8a and 8b, so that the vibrational energy is conserved in the electroacoustic resonator.

[0036] As especially in Fig. As can be seen in Figure 2, the diamond layer 2 is attached to a second substrate 6. This attachment can be achieved using an adhesion-mediating polymer 5 in a wafer bonding process. The polymer 5 can have a thickness of approximately 1 µm to approximately 10 µm. This allows the polymer 5 to compensate for the roughness of the first side 21 of the diamond layer 2, thus eliminating the need for complex polishing of the diamond layer 2.

[0037] The acoustic impedance, i.e., the product of density and speed of sound of the diamond layer 2 and the piezoelectric layer 4, is approximately equal, so that acoustic waves propagate uniformly within the two layers and are reflected only to a small extent at the interface between the diamond layer 2 and the piezoelectric layer 4. In contrast, the acoustic impedance of the polymer layer 5 is at least a factor of 5 lower than the acoustic impedance of the diamond layer 2. This results in acoustic waves propagating in the diamond layer 2 being reflected substantially at the first side 21 of the diamond layer and only to a small extent coupling out into the polymer layer 5 and the second substrate 6. This allows the acoustic energy density in the diamond layer 2 and the piezoelectric layer 4 to be increased, which contributes to improved properties of the electroacoustic resonator according to the invention.

[0038] Based on the Fig. 3 and Fig. Section 4 describes two different manufacturing processes. Identical components of the invention are identified by the same reference numeral, allowing the following description to be somewhat more concise.

[0039] Fig. Figure 3a shows a semi-finished product after the first manufacturing steps of an acoustic resonator according to the invention. A first substrate 3 is shown, which contains, for example, silicon, a ceramic such as aluminum oxide, silicon oxide, silicon nitride or silicon oxynitride, or a metal, in particular a refractory metal.

[0040] A piezoelectric layer 4 is deposited on one side of the substrate 3. The layer thickness can be, for example, between approximately 1 µm and approximately 5 µm or between approximately 1 µm and approximately 10 µm. The diamond layer 2 is then produced on the piezoelectric layer 4 by means of a low-pressure synthesis, as described above. The second side 22 of the diamond layer 2 is thus positioned on the piezoelectric layer 4. Since the piezoelectric layer 4 is relatively smooth, the second side 22 of the diamond layer 2 is also smooth. In contrast, the first side 21 of the diamond layer 2, which faces the gas phase, can exhibit a greater roughness.

[0041] Fig. 3b explains a second procedural step. As from Fig. As can be seen in Figure 3b, the semi-finished product obtained in the preceding process steps is applied to a second substrate 6 by means of a polymer layer 5. A wafer bonding process can be used for this purpose. In some embodiments of the invention, the polymer used for this purpose may contain or consist of benzocyclobutene.

[0042] After wafer bonding, a semi-finished product is obtained, which is then... Fig. Figure 3c shows the connection between the second side 22 of the diamond layer 2 and the second substrate 6. The polymer layer 5, which fills the roughness of the first side 21 of the diamond layer 2, eliminates the need for time-consuming polishing of the second side 22.

[0043] Fig. Figure 3d shows how the first substrate 3 was subsequently removed. A mechanical method, such as grinding, milling, or polishing, can be used for this purpose. In other embodiments of the invention, the first substrate 3 can be removed by wet or dry chemical etching. If necessary, parts of the piezoelectric layer 4 can also be removed to reduce it to its final thickness, which can be approximately 1 µm.

[0044] Fig. Figure 3e finally shows how the Bragg reflectors 8 and the interdigital structure 7 are produced by depositing a metal layer with a layer thickness of about 0.1 µm to about 1 µm and subsequent structuring, with which an electric field is later generated in the piezoelectric layer 4 when the electroacoustic resonator is operated.

[0045] Based on the Fig. Section 4 explains an alternative manufacturing process. How Fig. Figure 4a shows that the manufacturing process starts with the provision of a first substrate 3, which can contain, for example, silicon, a metal, diamond, or strontium titanate. A diamond layer 2 with the desired thickness is then deposited onto this substrate in a manner known per se.

[0046] Subsequently, the diamond layer 2 is bonded to the second substrate 6 using a wafer bonding process with the first substrate 3. This process step is described in Fig. 4b is shown. The cross-section through the semi-finished product obtained in this way is shown in Fig. 4c shown.

[0047] How Fig. Figure 4d shows that the first substrate 3 is subsequently removed, as described above, by wet or dry chemical etching and / or by mechanical processing. In this way, the second side 22 of the diamond layer 2 is exposed.

[0048] Finally, in the next process step, a piezoelectric layer 4 is deposited on the second side 22 of the diamond layer 2.

[0049] Fig. Figure 4e shows how the interdigital transducer 5 and the optional Bragg reflector 8 are created on the surface of the piezoelectric layer 4 by metallizing and subsequent structuring of the metal layer.

[0050] Naturally, the invention is not limited to the embodiments described. The foregoing description is therefore not to be considered limiting, but rather explanatory. The following claims are to be understood as meaning that a named feature is present in at least one embodiment of the invention. This does not preclude the presence of further features. Where the claims and the foregoing description define "first" and "second" embodiments, this designation serves to distinguish between two similar embodiments without establishing any hierarchy.

Claims

[1] Method for manufacturing an electroacoustic resonator (1) comprising the following steps: Deposition of a diamond layer (2) with a first side (21) and an opposing second side (22) on a first substrate (3), wherein the second side (22) of the diamond layer (2) is in contact with a first side (31) of the substrate (3), removal of the first substrate (3), Generating a piezoelectric layer (4) on the second side (22) of the diamond layer (2), Applying the diamond layer (2) with the first side (21) onto a second substrate (6), characterized by , that the piezoelectric layer (4) contains or consists of AlScN. [2] Method for manufacturing an electroacoustic resonator (1) comprising the following steps: Deposition of a piezoelectric layer (4) on a first substrate (3), Deposition of a diamond layer (2) on the piezoelectric layer (4), wherein the diamond layer (2) is applied with its first side (21) to a second substrate (6) and the first substrate (3) is removed, characterized by , that the piezoelectric layer (4) contains or consists of AlScN. [3] Method according to claim 1 or 2, characterized by , that a polymer layer (5) is arranged between the first side (21) of the diamond layer (2) and the second substrate (6). [4] Method according to claim 3, characterized by that the polymer layer (5) contains or consists of benzocyclobuthene and / or that the polymer layer (5) has an acoustic impedance between 0.05·10 7 kg·s -1 ·m -2 and 0.5·10 7 kg·s -1 ·m -2 exhibits and / or that the polymer layer (5) is produced with a thickness of 1 µm to 10 µm. [5] Method according to any one of claims 1 to 4, characterized by , that an interdigital transducer (7) is generated on the piezoelectric layer (4) and / or that a Bragg reflector (8) is generated on the piezoelectric layer (4). [6] Method according to any one of claims 1 to 5, characterized by , that a layer containing or consisting of SiO2 is further deposited on the electroacoustic resonator (1). [7] Electroacoustic resonator (1) with a diamond layer (2) with a first side (21) and an opposite second side (22) and a piezoelectric layer (4) which is arranged on the second side of the diamond layer (22), wherein the diamond layer (2) is applied with its first side (21) to a second substrate (6), characterized by , that the piezoelectric layer (4) contains or consists of AlScN. [8] Electroacoustic resonator according to claim 7, characterized by , that the diamond (2) contains a dopant or that the diamond (2) is doped with boron or phosphorus. [9] Electroacoustic resonator according to one of claims 7 or 8, characterized by , that a polymer layer (5) is arranged between the first side (21) of the diamond layer (2) and the second substrate (6). [10] Electroacoustic resonator according to claim 9, characterized by , that the polymer layer (5) contains or consists of benzocyclobuthene and / or that the polymer layer (5) has an acoustic impedance between 0.05·10 7 kg·s -1 ·m -2 and 0.5·10 7 kg·s -1 ·m -2 exhibits and / or that the polymer layer (5) has a thickness of 1 µm to 10 µm. [11] Electroacoustic resonator according to any one of claims 7 to 10, characterized by, that an interdigital transducer (7) is arranged on the piezoelectric layer (4) and / or that a Bragg reflector (8) is arranged on the piezoelectric layer (4). [12] Electroacoustic resonator according to any one of claims 7 to 11, characterized by , that a layer containing or consisting of SiO2 is deposited on the electroacoustic resonator (1). [13] Electroacoustic resonator according to any one of claims 7 to 12, characterized by , that the diamond layer (2) has a thickness of 1 µm to 30 µm or of 2 µm to 20 µm or of 2 µm to 10 µm or of 3 µm to 8 µm or of 8 µm to 25 µm or of 10 µm to 30 µm. [14] Electroacoustic resonator according to any one of claims 7 to 13, characterized by that the diamond layer is intended to be connected to an earth potential when the electroacoustic resonator is in operation, or that the diamond layer is arranged in an electrically insulated manner. [15] Electroacoustic resonator according to any one of claims 11 to 14, characterized by , that an acoustic layer thickness d, the speed of sound c, the operating frequency f and the period 1 of the interdigital converter (7) are related as follows: f∗d=cdλ<<5 MHz mm.

Images