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EP-4200981-B1 - SOLIDLY MOUNTED BULK ACOUSTIC WAVE RESONATOR WITH FREQUENCY TUNING BY MASS LOADING IN ACOUSTIC REFLECTOR AND METHOD OF MANUFACTURING THEREOF

EP4200981B1EP 4200981 B1EP4200981 B1EP 4200981B1EP-4200981-B1

Inventors

  • ERBES, Andreja

Dates

Publication Date
20260506
Application Date
20200914

Claims (14)

  1. A bulk acoustic wave, BAW, resonator (100, 300A-C, 402A, 402B) on a substrate (102, 408), comprising a piezoelectric element (104), a bottom electrode (106, 414A, 414B) on a first face (104A) of the piezoelectric element (104) and a top electrode (108) on a second face (104B) of the piezoelectric element (104) facing away from the first face (104A), the BAW resonator (100, 300A-C, 402A, 402B) further comprising a reflective element (112, 412) between the bottom electrode (106, 414A, 414B) and the substrate (102, 408), said reflective element (112, 412) comprising at least a first layer (114A, 416A, 416B) of a first material having a first acoustic impedance and a second layer (114B, 418A, 418B) of a second material having a second acoustic impedance different from the first acoustic impedance, wherein the first or the second layer (114A, 416A, 416B ,114B, 418A, 418B) comprises one or more structures (116A-D, 302A-F, 406A-B) of a third material, having a third acoustic impedance different from the first and second impedances, said structures (116A-D, 302A-F, 406A-B) forming an acoustic impedance modulation layer (116, 302, 404A, 404B) embedded in the first and/or the second layer (114B, 418A, 418B), characterized in that the one or more structures (116A-D, 302A-F, 406A-B) comprise a semiconductor material with a dopant concentration sufficiently large to enable a change in mass density and /or acoustic phase velocity.
  2. The BAW resonator (100, 300A-C, 402A, 402B) according to claim 1, wherein the reflective element (112, 412) is a Bragg layer comprising a plurality of interleaved first and second layers (114B, 418A, 418B).
  3. The BAW resonator (100, 300A-C, 402A, 402B) according to claim 1 or 2 wherein the reflective element (112, 412) is composed of alternating layers of the first and second material in such a way that the reflective element (112, 412) is arranged to operate in the operating frequency of the BAW resonator (100, 300A-C, 402A, 402B).
  4. The BAW resonator (100, 300A-C, 402A, 402B) according to any one of the preceding claims, wherein the acoustic impedance modulation layer (116, 302, 404A, 404B) is embedded in a layer of the reflective element (112, 412) adjacent the bottom electrode (106, 414A, 414B).
  5. The BAW resonator (100, 300A-C, 402A, 402B) according to any one of the preceding claims, wherein the one or more structures (116A-D, 302A-F, 406A-B) have a thickness in the order of the acoustic wavelength or a fraction of the acoustic wavelength, at the fundamental operating frequency of the BAW resonator (100, 300A-C, 402A, 402B).
  6. The BAW resonator (100, 300A-C, 402A, 402B) according to any one of the preceding claims, wherein the acoustic impedance modulation layer (116, 302, 404A, 404B) is arranged so that it extends into two layers of the reflective element (112, 412).
  7. The BAW resonator (100, 300A-C, 402A, 402B) according to any one of the preceding claims, wherein the structures (116A-D, 302A-F, 406A-B) are distributed asymmetrically to form the acoustic impedance modulation layer (116, 302, 404A, 404B).
  8. The BAW resonator (100, 300A-C, 402A, 402B) according to any one of the preceding claims, further comprising at least one mass load layer (110) on the top electrode (108)
  9. The BAW resonator (100, 300A-C, 402A, 402B) according to claim 8, wherein the mass load layer (110) is embedded, or partially embedded in the top electrode (108).
  10. An integrated circuit package comprising at least a first and a second BAW resonator (402A; 402B) according to any one of the preceding claims, the first and second BAW resonators (402A; 402B) having different acoustic impedance modulation layers (404A, 404B).
  11. The integrated circuit package according to claim 10, comprising a plurality of BAW resonators (100, 300A-C, 402A, 402B) according to any one of the claims 1 - 9 configured in a ladder structure or a lattice structure.
  12. A method (500) of manufacturing BAW resonator (100, 300A-C, 402A, 402B) according to any one of the claims 1 - 9, comprising the step of depositing a reflective element (112, 412) on a substrate (102, 408), said reflective element (112, 412) comprising at least a first and a second layer (114B, 418A, 418B) from a first and a second material having a first and second acoustic impedance, respectively, with the second acoustic impedance being different from the first acoustic impedance, placing one or more structures (116A-D, 302A-F, 406A-B) of a third material having a third acoustic impedance different from the first and the second acoustic impedance, in the first and/or the second layer (114B, 418A, 418B), said structures (116A-D, 302A-F, 406A-B) forming an acoustic impedance modulation layer (116, 302, 404A, 404B) within the reflective element (112, 412), characterized in that the one or more structures (116A-D, 302A-F, 406A-B) comprise a semiconductor material with a dopant concentration sufficiently large to enable a change in mass density and /or acoustic phase velocity.
  13. The method (500) according to claim 12, wherein the step of placing the one or more structures comprises depositing a third layer of the third material on a first or a second layer (114B, 418A, 418B) and etching the third layer to form the structures (116A-D, 302A-F, 406A-B), the method (500) further comprising covering the third layer with a covering layer of the first or the second material and planarizing the covering layer.
  14. The method (500) according to claim 12 or 13, wherein the step of placing the one or more structures comprises etching a first or second layer (114B, 418A, 418B) to create spaces for the structures (116A-D, 302A-F, 406A-B), depositing a third layer of the third material and planarizing the third layer.

Description

TECHNICAL FIELD The present disclosure relates generally to the field of semiconductor technologies; and more specifically, to resonators, such as Bulk Acoustic Wave (BAW) resonators. Further, the present disclosure relates to integrated circuit packages comprising a plurality of BAW resonators. Furthermore, the present disclosure relates to a method of manufacturing BAW resonators. BACKGROUND Acoustic wave devices, such as Micro-mechanical (MEMS) resonators, are key components used in modern electronic circuits as building blocks of front-end modules (FEM) due to their exceptional mechanical and acoustic quality, and considerably lower energy losses relative to their purely electrical counterparts (e.g. capacitors, inductors, etc.). For example, acoustic wave devices are used as filters to improve reception and transmission of signals in mobile phones and Wi-Fi receivers. There are two main categories of acoustic wave devices that are currently used by the industry to produce micro-mechanical resonators (filters). The first category of acoustic wave devices are based on surface traveling waves or Surface Acoustic Wave (SAW) resonators. These are traditionally fabricated on low loss piezoelectric material, and operated by coupling an external electric-field to the acoustic fields, creating a propagation of electro-acoustic modes into the piezoelectric material. The second category of acoustic wave devices include Bulk Acoustic Wave (BAW) resonators which use the external electric field to trigger bulk waves in the piezoelectric material. Whereas the SAW resonators tend to localize the acoustic energy into surface of the piezoelectric material, the BAW resonators tend to produce waves in the whole bulk of the piezoelectric material. BAW resonators have been widely adapted to be used in high-frequency, communication applications, as they are usually quite compatible with state of the art micro-fabrication techniques which enables their use in a high yield, high volume and integration schemes for wireless chipset products. BAW resonators include thin film bulk acoustic resonators (FBARs), which include resonator stacks formed over a substrate cavity, and solidly mounted resonators (SMRs), which include resonator stacks formed over an acoustic reflector (e.g., Bragg mirror). Traditionally, the BAW resonator comprises a piezoelectric layer, and the thickness of the piezoelectric layer generally determines operating frequency of the BAW resonator. For instance, in case of SMR-BAW resonators operating in longitudinal modes, the operating frequency is mostly defined by the core resonator thickness of the piezoelectric material. If further frequencies are required on one same die, it is necessary to employ distinct mass loading elements, which are local variations in the electrode thickness and /or additional deposited materials (e.g., dielectrics) for a given resonator. Generally, the mass loading elements are deposited either over a top electrode, or below a bottom electrode in a Bragg layer structure. It is to be noted that in case of multiple frequencies are required on one same die, the actual thickness variation of the different layers due to non-perfect manufacturing processes can in some cases be significantly larger than the fine thickness difference required to tune the operating frequency of different micro-resonators. In other words, it is challenging to implement a great number of frequency elements on the same die replicated across a full wafer, and thus it makes it more difficult in terms of actual processing to achieve many distinct operating frequencies. To mitigate this, additional highly controlled deposition steps are mandatory (i.e., require extremely high precision in the thickness deposition), but that increases manufacturing complexity, putting a trade-off between performance gains/ process cost. Further, it is to be considered that when mass loading elements are deposited below the bottom electrode in the Bragg layer structure, there is a need to ensure correct planarization of the subsequent deposition of thin-films. Therefore, in light of the foregoing discussion, there exists a need to overcome the aforementioned drawbacks associated with conventional acoustic wave devices, and particularly BAW resonators, for facilitating the use of multiple frequencies all within one same die. US 2013/038408 A1 discloses a bulk acoustic wave (BAW) resonator device that includes an acoustic reflector formed over a substrate and a resonator stack formed over the acoustic reflector. The acoustic reflector includes multiple acoustic impedance layers. The resonator stack includes a first electrode formed over the acoustic reflector, a piezoelectric layer formed over the first electrode, and a second electrode formed over the piezoelectric layer. A bridge is formed within one of the acoustic reflector and the resonator stack. US 2017/272053 A1 discloses a BAW resonator with reduced heat build-up. The heat bui