CN116032240A - Thin film bulk acoustic resonator based on phononic crystals to improve quality factor - Google Patents

Abstract

The invention discloses a film bulk acoustic resonator based on improving quality factor of phonon crystal, which can inhibit transverse pseudo mode caused by inconsistent orientation of C axis of material generated in processing of the existing film bulk acoustic resonator, limit longitudinal vibration of the resonator to piezoelectric film at upper and lower electrode covering parts, and improve quality factor of the resonator. According to the invention, based on the air gap structure of the FBAR, phonon crystals are introduced into the sandwich structure of the traditional upper electrode, the piezoelectric film and the lower electrode to serve as a side sound wave suppression structure, so that the pseudo mode of transverse propagation in the FBAR is suppressed, and clutter is reduced. The specific unit structure of the phonon crystal designed by the invention is as follows: 1. electrode groups surrounding the upper and lower electrodes of the conventional polygon, thereby reducing energy leakage from the electrode region; 2. the patterning holes penetrating the piezoelectric film restrict the sound wave energy to the region of the piezoelectric film covered by the upper electrode and the lower electrode, namely solve the problem of sound wave transmission of transverse pseudo mode, thereby remarkably improving the quality factor.

Description

Thin film bulk acoustic resonator based on phononic crystals to improve quality factor

technical field

The invention belongs to the technical field of radio frequency micro-electromechanical systems, and in particular relates to a thin-film bulk acoustic wave resonator based on a phononic crystal to improve the quality factor.

Background technique

With the rapid development of wireless communication technology, the frequency band of wireless signals is becoming more and more crowded, and the frequency band used is getting higher and higher, which puts forward the requirements of high integration, low power consumption and high performance for radio frequency devices. RF filters play a vital role in signal filtering in wireless communications. Thin film bulk acoustic resonator has the characteristics of high resonance frequency, CMOS process compatibility, high quality factor, low loss, low temperature coefficient and high power carrying capacity, and gradually replaces surface acoustic wave resonator to become the mainstream of the market.

The working principle of the thin film bulk acoustic resonator is to apply radio frequency electrical signals on the upper and lower electrodes, and use the piezoelectric effect of the piezoelectric material to generate vibration in the longitudinal mode, thereby generating longitudinally transmitted acoustic waves in the sandwich structure composed of the upper and lower electrodes and the piezoelectric material. The signal and the acoustic signal oscillate in the sandwich structure and then convert it into an electrical signal output through the piezoelectric effect. Only the radio frequency signal matching the resonance frequency of the piezoelectric material can be transmitted through the thin film bulk acoustic resonator, thereby realizing the filtering function. Ideally, longitudinal vibrations are generated in the resonator. In fact, due to the possible defects or incomplete C-axis orientation in the prepared piezoelectric material, the resonator also generates lateral vibrations while longitudinal vibrations, and lateral vibrations will cause the sound wave energy to dissipate. At the same time, it brings the influence of clutter, which leads to the decrease of the quality factor of the film bulk acoustic resonator.

Contents of the invention

The invention provides a thin film bulk acoustic resonator based on phononic crystals to improve the quality factor, which solves the problem of transverse clutter caused by defects in the background thin film bulk acoustic resonator due to unavoidable factors in material preparation and process, and effectively suppresses false mode, reduce energy loss, and improve the quality factor of the resonator.

In order to achieve the above object, the present invention provides a thin-film bulk acoustic resonator based on phononic crystals to improve the quality factor, which is characterized in that it includes a substrate, a cavity etched in the substrate, and a buffer layer sequentially arranged on the substrate , a seed layer, a piezoelectric stack structure, a phononic crystal formed by an air column penetrating the piezoelectric film, and an electrode group surrounding the piezoelectric stack structure;

The cavity is located on the substrate and below the seed layer and the piezoelectric stack structure;

The piezoelectric stack structure is respectively an upper electrode, a piezoelectric film and a lower electrode from top to bottom, and the shapes of the upper and lower electrodes are both pentagonal;

The seed layer is between the buffer layer and the piezoelectric stack structure, and the material of the seed layer is consistent with the piezoelectric film material;

The electrode groups surrounding the piezoelectric stack structure are evenly distributed in space around the pentagonal upper and lower electrodes, and the pattern formed by the electrode groups is any one of pentagonal, square or circular.

As a preferred solution, the shape of the cavity is circular or any polygon;

The shape of the electrode group surrounding the piezoelectric stack structure is one or more of cuboid, triangular prism, cylinder, and regular hexagonal column;

The shape of the air column that runs through the piezoelectric film is one or more of cuboid, triangular prism, cylinder, and regular hexagonal column; the pattern surrounded by the topological structure of the phononic crystal formed by the air column on the space plane is: Either pentagon, circle or square.

Further, the electrode group is closer to the piezoelectric stack structure than the phononic crystal air column, and the distance between the centers of the electrode group, the phononic crystal air column and the piezoelectric stack structure is the same.

Furthermore, the upper and lower electrodes of the piezoelectric stack structure are both metal thin films, and the material of the metal thin films is any one of gold, silver, platinum, molybdenum or chromium;

The piezoelectric film material is any one of aluminum nitride, scandium-doped aluminum nitride, lithium niobate, lithium tantalate, PZT or zinc oxide.

Furthermore, the material of the electrode group surrounding the piezoelectric stack structure is any one of gold, silver, platinum, molybdenum or chromium.

The preparation method of the film bulk acoustic resonator that the present invention adopts comprises steps as follows:

S1: Etching a cavity on the substrate;

S2: depositing a sacrificial layer on the substrate;

S3: removing the redundant sacrificial layer, so that the remaining sacrificial layer just fills the cavity;

S4: Depositing a bottom electrode on the substrate and the sacrificial layer;

S5: Etching the redundant bottom electrode to form the desired shape of the bottom electrode and the bottom electrode group;

S6: Depositing a piezoelectric material above the bottom electrode through step S5;

S7: Depositing a top electrode over the piezoelectric material layer;

S8: Etching the redundant top electrodes to form the desired shape of the top electrodes and the top electrode group;

S9: Etch the part of the piezoelectric stack structure that is not in the air gap, and form a phononic crystal air column that runs through the piezoelectric material according to a certain topology;

S10: Etching release holes on the piezoelectric stack structure;

S11: releasing the cavity by corrosive liquid or corrosive gas to form a thin film bulk acoustic resonator.

Advantage of the present invention and beneficial effect are as follows:

Different from the prior art, the present invention uses different parts of the phononic crystal structure. In the prior art, some are on the package, and some are on the substrate, that is, to reduce the acoustic wave from above and below the resonator. Scattering, reducing energy leakage. However, the present invention improves the Q value from the source of the vibration by optimizing the piezoelectric sandwich structure (upper electrode, piezoelectric material and lower electrode) of the resonator.

The invention effectively makes up for the performance reduction caused by the process in the prior art through the optimization of the structure. The technical solution of the present invention is based on the air-gap structure of FBAR, and a phononic crystal is introduced into the sandwich structure of the traditional upper electrode-piezoelectric film-lower electrode as a side acoustic wave suppression structure, so that the false mode of lateral propagation in the FBAR is suppressed , reducing clutter and improving the quality factor of the device. The specific unit structure of the phononic crystal designed in the present invention is: 1) the electrode group surrounding the traditional polygonal upper and lower electrodes, thereby reducing the energy leakage in the electrode area; 2) the patterned holes running through the piezoelectric film, thereby confining the acoustic wave energy In the area of the piezoelectric film covered by the electrode, the acoustic wave transmission of the transverse mode is solved, and the false mode is suppressed, thereby significantly improving the quality factor.

The thin film bulk acoustic resonator proposed by the present invention forms a surrounding electrode group and a phononic crystal air column by etching the upper and lower electrodes and the piezoelectric film other than the piezoelectric stack structure directly above the cavity. The cylindrical electrodes surrounding the traditional polygonal upper and lower electrodes can reduce the energy leakage in the electrode area; the patterned holes running through the piezoelectric film can confine the acoustic wave energy to the piezoelectric film area covered by the upper and lower electrodes, and resonate in the effective area of the piezoelectric material The sound waves propagating laterally in the interior are reflected by the air at the edge, which solves the sound wave transmission of the transverse mode and suppresses the false mode, thereby significantly improving the quality factor.

Description of drawings

1 is a cross-sectional view of a thin-film bulk acoustic resonator based on a phononic crystal to improve the quality factor of the present invention;

Fig. 2 is the top view of the film bulk acoustic resonator based on the phononic crystal to improve the quality factor of the present invention;

Fig. 3 is the topological structure of three kinds of phononic crystal air columns penetrating the piezoelectric film of the film bulk acoustic resonator based on the phononic crystal to improve the quality factor of the present invention;

Fig. 4 is the impedance curve comparison of the thin film bulk acoustic resonator based on the phononic crystal to improve the quality factor of the present invention and the traditional thin film bulk acoustic resonator of the same characteristic geometric size; wherein the solid line represents the resonator curve of the structure of the present invention, and the dotted line Represents the conventional resonator curve.

In the picture:

110. substrate; 120. cavity; 130. buffer layer; 140. seed layer; 150. first piezoelectric film; 160. lower electrode; 170. upper electrode; sub-crystal; 190, the first electrode group surrounding the piezoelectric stack structure;

210. The second piezoelectric film; 220. The first phononic crystal air column; 230. The second electrode group surrounding the piezoelectric stack structure; 240. The piezoelectric stack structure;

310. The topological structure of the first phononic crystal air column; 311. The second phononic crystal air column; 312. The unetched part of the first piezoelectric film; 320. The second phononic crystal air column topology; 321. The first Three phononic crystal air columns; 322, the unetched part of the second piezoelectric film; 330, the topological structure of the third phononic crystal air column; 331, the fourth phononic crystal air column; 332, the unetched part of the third piezoelectric film etched part.

Detailed ways

The technical solutions of the present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

Example 1

In this embodiment, a thin-film bulk acoustic resonator based on a phononic crystal to improve the quality factor, through the preparation of the substrate, the cavity etched in the substrate, and the sequential deposition of a buffer layer, a seed layer, and a piezoelectric stack structure on the substrate (below Electrode, piezoelectric film, upper electrode), and when depositing the lower electrode and the upper electrode, photolithography is performed to generate an electrode group surrounding the piezoelectric stack structure, and then finally photolithography produces a specific topology composed of air columns penetrating the piezoelectric film Phononic crystals.

During specific implementation, as shown in Figure 1, the structure of the thin film bulk acoustic resonator based on phononic crystals to improve the quality factor is: substrate 110, the substrate material is silicon, sapphire or SOI substrate; The cavity 120 etched out; the buffer layer 130 deposited on the substrate, the material of the buffer layer is silicon dioxide; the seed layer 140 deposited on the buffer layer, the material of the seed layer is the same as that of the first piezoelectric film 150, and the material is preferably One of aluminum nitride, scandium-doped aluminum nitride, lithium niobate, lithium tantalate, PZT, and zinc oxide; the upper structure of the seed layer is a piezoelectric oscillator stack structure (including the lower electrode 160, the piezoelectric film layer 150 and The upper electrode 170), the phononic crystal 180 formed by the air column penetrating the piezoelectric film, and the first electrode group 190 surrounding the piezoelectric stack structure; wherein, the upper electrode 170 and the lower electrode 160 of the piezoelectric stack structure are both metal thin films, The material is preferably one of gold, silver, platinum, molybdenum, and chromium; the material of the first piezoelectric film 150 is preferably aluminum nitride, scandium-doped aluminum nitride, lithium niobate, lithium tantalate, PZT, and zinc oxide. kind of. The material of the electrode group 190 surrounding the piezoelectric stack structure is preferably one of gold, silver, platinum, molybdenum and chromium.

Fig. 2 is a top view of the embodiment in Fig. 1, including the second piezoelectric film 210 in the piezoelectric stack structure, corresponding to 150 in Fig. 1; the first phononic crystal air column 220 formed by the air column penetrating the piezoelectric film , corresponding to 180 in FIG. 1; the second electrode group 230 surrounding the piezoelectric stack structure, corresponding to 190 in FIG. 1; the piezoelectric stack structure 240 above the air gap, corresponding to the lower electrode 160 in FIG. 1, the first piezoelectric The portion of the membrane 150 above the air gap 120 and the upper electrode 170 .

Figure 3 shows the topological structures of three phononic crystal air columns formed under three photolithographic conditions, including the pentagon formed by the second phononic crystal air column 311, the square formed by the third phononic crystal air column 321, and the third phononic crystal air column 321. The circle formed by four phononic crystal air columns 331, and the unetched part 312 of the first piezoelectric film that is not penetrated, the unetched part 322 of the second piezoelectric film, and the unetched part of the third piezoelectric film Section 332.

Fig. 4 is the data curve of the pentagonal FBAR under the electrode group and the phononic crystal air column with or without the present invention, the solid line represents the embodiment shown in Fig. 1, the dotted line represents the traditional FBAR, and the embodiment shown in the present invention can be found It can effectively reduce transverse clutter, reduce energy loss, limit vibration to the longitudinal direction of the piezoelectric stack structure above the air gap, and effectively improve the quality factor. That is, the resonator adopting the structure of the present invention can be clearly seen from the curve in FIG. 4 , the clutter is less, that is, the pseudo mode is suppressed and the quality factor is improved.

It should be understood that the parts not described in detail in this specification belong to the prior art.

It should be understood that the above-mentioned descriptions for the preferred embodiments are relatively detailed, and should not therefore be considered as limiting the scope of the patent protection of the present invention. Within the scope of protection, replacements or modifications can also be made, all of which fall within the protection scope of the present invention, and the scope of protection of the present invention should be based on the appended claims.

Claims (5)

1. A thin-film bulk acoustic resonator based on phononic crystals to improve the quality factor, characterized in that it includes a substrate, a cavity etched out of the substrate, a buffer layer, a seed layer, a piezoelectric The stack structure, the phononic crystal formed by the air column penetrating the piezoelectric film, and the electrode group surrounding the piezoelectric stack structure; The cavity is located on the substrate and below the seed layer and the piezoelectric stack structure; The piezoelectric stack structure is respectively an upper electrode, a piezoelectric film and a lower electrode from top to bottom, and the shapes of the upper and lower electrodes are both pentagonal; The seed layer is between the buffer layer and the piezoelectric stack structure, and the material of the seed layer is consistent with the piezoelectric film material; The electrode groups surrounding the piezoelectric stack structure are evenly distributed in space around the pentagonal upper and lower electrodes, and the pattern formed by the electrode groups is any one of pentagonal, square or circular. 2. The thin-film bulk acoustic resonator based on phononic crystals to improve quality factor according to claim 1, characterized in that: The shape of the cavity is circular or any polygon; The shape of the electrode group surrounding the piezoelectric stack structure is one or more of cuboid, triangular prism, cylinder, and regular hexagonal column; The shape of the air column that runs through the piezoelectric film is one or more of cuboid, triangular prism, cylinder, and regular hexagonal column; the pattern surrounded by the topological structure of the phononic crystal formed by the air column on the space plane is: Either pentagon, circle or square. 3. The thin film bulk acoustic resonator based on phononic crystals to improve quality factor according to claim 1 or 2, characterized in that: the electrode group is closer to the piezoelectric stack structure than the phononic crystal air column, and the electrode group , the center-to-center spacing of the phononic crystal air column is the same as that of the piezoelectric stack. 4. The thin-film bulk acoustic resonator based on phononic crystals to improve quality factor according to claim 3, characterized in that: The upper and lower electrodes of the piezoelectric stack structure are metal thin films, and the material of the metal thin films is any one of gold, silver, platinum, molybdenum or chromium; The piezoelectric film material is any one of aluminum nitride, scandium-doped aluminum nitride, lithium niobate, lithium tantalate, PZT or zinc oxide. 5. The thin-film bulk acoustic resonator based on phononic crystals to increase the quality factor according to claim 1, 2 or 4, characterized in that: the material of the electrode group surrounding the piezoelectric stack structure is gold, silver, platinum, Either molybdenum or chromium.

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