Flexible substrate single-port SAW resonator based on BCB bonding

Abstract

The invention discloses a flexible substrate single-port SAW resonator based on BCB bonding, and relates to the technical field of surface acoustic wave devices. The invention provides a thermal stress buffer structure, which effectively solves the problems of lattice mismatch and thermal mismatch between a piezoelectric film material and a heterogeneous substrate, especially a flexible substrate, so that the growth of a high-quality LiNbO3 piezoelectric single crystal film is realized; the flexible SAW resonator has high sensitivity and a considerable strain range, is good in repeatability and stability, and provides a new technical approach for development and application of the conformal acoustic strain sensor. The flexible substrate is adopted, the SAW resonators are integrated on the curved surface, certain bending capacity is achieved, and the problem that conformal integration or compatible wearable electronics cannot be achieved on the curved surface is solved.

Description

Technical Field

[0001] The invention relates to the technical field of surface acoustic wave devices, and in particular to a flexible substrate single-port SAW resonator based on BCB bonding. Background Art

[0002] SAW resonators are excited by interdigital electrodes, and their operating frequency is related to the speed of the excited sound waves and the width of the interdigital lines. The preparation of SAW resonators can be achieved by preparing interdigital electrodes on the surface of piezoelectric materials. The manufacturing process is simple, the design method is mature, the device yield is high, and the production cost is low. Therefore, SAW resonators have received great attention in the field of RF filters. Thanks to the fact that the SAW resonator frequency is sensitive to changes in the external environment, researchers have successfully prepared various types of sensors using this feature for measuring environmental parameters such as gas, temperature, and pressure. SAW resonant sensors have the characteristics of high sensitivity, small size, simple design, and low preparation cost. With the development of the Internet of Things, traditional SAW resonators can no longer meet the requirements of sensors.

[0003] In the field of SAW sensing, limited application scenarios and defects in sensor performance are the main reasons that hinder its development. First, SAW resonators based on piezoelectric wafers or prepared by depositing piezoelectric films on Si wafers do not have the ability to bend, so they cannot be conformally integrated on curved surfaces or compatible with wearable electronics; second, sensors prepared based on piezoelectric blocks, such as strain sensors, have a narrow measurement range and low strain sensitivity. Even if flexible SAW sensors based on ZnO or AlN polycrystalline films have a certain degree of bending ability, the polycrystalline characteristics and high surface roughness of the piezoelectric film lead to poor sensor performance, and repeated bending deformations can easily cause the deterioration of piezoelectric performance, making the sensor performance prone to fluctuations, which in turn affects the stability of the sensor. Summary of the invention

[0004] In view of the above-mentioned deficiencies in the prior art, the present invention provides a flexible substrate single-port SAW resonator based on BCB bonding, which solves the problem that the existing SAW resonator does not have the bending ability and thus cannot achieve conformal integration on a curved surface or be compatible with wearable electronics; even if it has a certain bending ability, the corresponding sensor performance is poor, which reduces the stability of the SAW resonator.

[0005] In order to achieve the above-mentioned object of the invention, the technical solution adopted by the present invention is:

[0006] A single-port SAW resonator based on a flexible substrate and BCB bonding is provided, which includes a thermal stress buffer structure; the thermal stress buffer structure includes a metal electrode layer, a piezoelectric film layer, an optional functional layer, a BCB bonding layer and a flexible substrate connected in sequence; wherein:

[0007] A metal electrode layer for providing a path for current so that the SAW resonator responds to changes in external strain and converts the changes into electrical signals;

[0008] A piezoelectric film layer is used to generate a piezoelectric effect and convert mechanical strain into an electrical signal;

[0009] Optional functional layer for preparing acoustic reflection structure and improving the performance of SAW resonator;

[0010] BCB bonding layer, used for bonding flexible substrate;

[0011] Flexible substrates are used to integrate SAW resonators on curved surfaces for common integration or wearable electronic applications.

[0012] Furthermore, the metal electrode layer includes an IDT and a short-circuit reflector; the IDT is placed in the central area of ​​the top of the piezoelectric film layer; the short-circuit reflector is placed on both sides of the top of the piezoelectric film layer, that is, on both sides of the IDT, and is arranged along the z-axis propagation direction of the SAW resonator.

[0013] Furthermore, the material of the piezoelectric film layer is lithium niobate; the material of the BCB bonding layer is styrene cyclobutene; and the material of the flexible substrate is polyimide, a high temperature resistant organic material.

[0014] Furthermore, the metal electrode layer is prepared by a lift-off process; the piezoelectric film layer is prepared by a CIS ion implantation stripping technique, and low-energy argon ion irradiation technology is used to repair implantation stripping damage; and the BCB bonding layer is prepared by a spin coating process.

[0015] Furthermore, the thickness of the BCB bonding layer is greater than 1λ, wherein λ represents the operating wavelength of the SAW resonator.

[0016] Furthermore, the thermal expansion coefficients of the metal electrode layer, the piezoelectric film layer, the optional functional layer, the BCB bonding layer and the flexible substrate increase in sequence from top to bottom.

[0017] Furthermore, the calculation formula of the resonant frequency f of the SAW resonator is:

[0018]

[0019] Wherein, v represents the phase velocity of the SAW resonator, λ represents the operating wavelength of the SAW resonator, Eeq represents the equivalent Young's modulus of the SAW resonator, and ρeq represents the equivalent density of the SAW resonator.

[0020] The beneficial effects of the present invention are:

[0021] 1. The present invention adopts CIS ion implantation and stripping technology to prepare the piezoelectric film layer, and adopts low-energy argon ion irradiation technology to repair the implantation and stripping damage, which can improve the quality of the piezoelectric film layer, and adopts a thermal stress buffer structure to effectively solve the lattice mismatch and thermal mismatch problems between the piezoelectric film material and the heterogeneous substrate, especially the flexible substrate, thereby realizing the growth of high-quality LiNbO3 piezoelectric single crystal film, and improving the reliability and stability of the oscillator composite film in the preparation process and the actual application process;

[0022] 2. The flexible SAW resonator of the present invention has high sensitivity and considerable strain range, and good repeatability and stability, which provides a new technical approach for the development and application of conformal acoustic strain sensors;

[0023] 3. The present invention uses a flexible substrate to integrate the SAW resonator on a curved surface, which has a certain bending ability, solving the problem of being unable to achieve conformal integration or compatibility with wearable electronics on a curved surface. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a structural schematic diagram of the present invention;

[0025] Figure 2 is a schematic diagram of a flexible SAW resonator;

[0026] Figure 3 Schematic diagram of the cross section of a semi-cylinder and an arched slot with different radii;

[0027] Figure 4 is the strain response test result;

[0028] Among them: 1. Metal electrode layer; 2. Piezoelectric film layer; 3. Optional functional layer; 4. BCB bonding layer; 5. Flexible substrate. DETAILED DESCRIPTION

[0029] The specific implementation modes of the present invention are described below so that those skilled in the art can understand the present invention. However, it should be clear that the present invention is not limited to the scope of the specific implementation modes. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the attached claims, these changes are obvious, and all inventions and creations utilizing the concept of the present invention are protected.

[0030] like Figure 1 As shown, a flexible substrate single-port SAW resonator based on BCB bonding includes a thermal stress buffer structure; the thermal stress buffer structure includes a metal electrode layer 1, a piezoelectric film layer 2, an optional functional layer 3, a BCB bonding layer 4 and a flexible substrate 5 connected in sequence; wherein:

[0031] The metal electrode layer 1 is used to provide a path for current so that the SAW resonator responds to changes in external strain and converts the changes into electrical signals;

[0032] The piezoelectric film layer 2 is used to generate a piezoelectric effect and convert mechanical strain into an electrical signal. The resonant frequency of the piezoelectric film layer 2 can be adjusted according to the film thickness and material selection of the piezoelectric film layer 2 .

[0033] The optional functional layer 3 is used to prepare a sound reflection structure to improve the performance of the SAW resonator;

[0034] A BCB bonding layer 4, used for bonding a flexible substrate 5;

[0035] The flexible substrate 5 is used to integrate the SAW resonator on a curved surface to achieve common integration or wearable electronic applications.

[0036] like Figure 2 As shown, SAW resonators have certain bending capabilities and can be used in wearable electronic applications.

[0037] The metal electrode layer 1 includes an IDT and a short-circuit reflector; the IDT is placed in the central area of ​​the top of the piezoelectric film layer 2; the short-circuit reflector is placed on both sides of the top of the piezoelectric film layer 2, that is, on both sides of the IDT, and is arranged along the z-axis propagation direction of the SAW resonator.

[0038] The material of the piezoelectric film layer 2 is lithium niobate, and the piezoelectric film layer 2 is a single crystal film, that is, LiNbO3 piezoelectric single crystal film; the material of the BCB bonding layer 4 is styrene cyclobutene; the material of the flexible substrate 5 is high temperature resistant organic material polyimide (PI).

[0039] The metal electrode layer 1 is prepared by a lift-off process; the piezoelectric film layer 2 is prepared by a CIS ion implantation stripping technique, and implantation stripping damage is repaired by a low-energy argon ion irradiation technique; and the BCB bonding layer 4 is prepared by a spin coating process.

[0040] The thickness of the BCB bonding layer 4 is greater than 1λ, where λ represents the operating wavelength of the SAW resonator.

[0041] The thermal expansion coefficients of the metal electrode layer 1 , the piezoelectric film layer 2 , the optional functional layer 3 , the BCB bonding layer 4 and the flexible substrate 5 increase in sequence from top to bottom.

[0042] The calculation formula for the resonant frequency f of the SAW resonator is:

[0043]

[0044] Wherein, v represents the phase velocity of the SAW resonator, λ represents the operating wavelength of the SAW resonator, Eeq represents the equivalent Young's modulus of the SAW resonator, and ρeq represents the equivalent density of the SAW resonator.

[0045] In one embodiment of the present invention, the metal electrode layer 1 is a metal aluminum electrode layer, the piezoelectric film layer 2 is a lithium niobate film layer, and the flexible substrate 5 is PI-100EN. The multilayer film structure is arranged from top to bottom according to the metal aluminum electrode layer, the lithium niobate film layer, the BCB bonding layer 4 and the flexible PI substrate to form a thermal buffer structure. Among them, the thermal expansion coefficient of PI-100EN is 35×10 -6 K -1 The thermal expansion coefficient of the cured BCB bonding layer 4 is 25×10 -6 K -1 The thermal expansion coefficient of lithium niobate piezoelectric film is 18.26×10 -6 K -1 .

[0046] The lithium niobate thin film layer is prepared by ion implantation stripping technology (CIS), and the film thickness can be precisely controlled by controlling the ion implantation energy. Different implantation energies correspond to different film thicknesses, and submicron-scale films can be prepared without complex thinning and polishing processes. The prepared film has high thickness uniformity, and the thickness uniformity difference is less than 1%. The film-to-film repeatability is high, and the thickness difference of films stripped from the same batch is less than 0.5%, so the process has mass production capabilities. The process for preparing piezoelectric single crystal films on flexible substrates is simplified, and the piezoelectric single crystal films can be directly transferred to the flexible substrate 5 without secondary transfer.

[0047] The BCB glue used in the BCB bonding process has good fluidity and can completely fill the bonding pores of the rough surface PI substrate, providing good interface contact for the uneven bonding surface.

[0048] The bonded lithium niobate film is irradiated with low-energy argon ions to remove the damaged layer on the film surface.

[0049] The metal electrode layer 1 is prepared by a lift-off process, and the prepared interdigital electrodes and reflective gate electrodes are completely peeled off and have good quality.

[0050] The SAW resonator is tested and the frequency-strain characteristics of the SAW resonator are calculated. The corresponding method steps are as follows:

[0051] S1. Prepare and print by 3D printing technology Figure 3 The semi-cylinder and the arched groove with different radii are shown. The semi-cylinder and the arched groove are complementary in shape, the semi-cylinder is used for tensile stress testing, and the arched groove is used for compressive stress testing;

[0052] S2, according to the formula:

[0053]

[0054] Calculate the distance χ between the neutral plane of the composite film (SAW resonator) and the bottom surface of the PI substrate; where hi and hj represent the film thicknesses of the i-th and j-th layers of the composite film, respectively, Ei represents the Young's modulus of the i-th layer material of the composite film, and ∑(·) represents the summation function.

[0055] S3. According to the distance χ between the neutral plane and the bottom surface of the PI substrate and the thickness of each layer of the SAW resonator, determine the distance between the LiNbO3 piezoelectric single crystal film and the neutral plane, and according to the formula:

[0056]

[0057] Calculate the strain magnitude ε of the composite film; where r represents the bending radius and z represents the distance between the LiNbO3 piezoelectric single crystal film and the neutral plane;

[0058] S4. Place the flexible SAW resonator on a semi-cylinder and an arched groove with different radii respectively. Given the thickness and bending radius of the lithium niobate film (the bending radius is equal to the radius of the semi-cylinder or the arched groove), calculate the strain size of the film under different strain states and determine the strain range of the flexible SAW resonator.

[0059] S5. According to the strain range described in step S4, the resonant frequency of the SAW resonator is tested under different strain states, and the frequency-strain characteristics of the SAW resonator are obtained by integrating different data. The corresponding results are as follows: Figure 4 As shown, under the action of tensile stress, the strain of the SAW resonator increases the distance between the interdigital electrodes, making the effective wavelength larger. At the same time, the changes in the Young's modulus and density of the thin film materials that make up the resonator, such as lithium niobate film, BCB and PI, cause a certain decrease in the SAW sound velocity, which in turn leads to a decrease in the resonant frequency. Similarly, under the action of compressive stress, the strain of the SAW resonator reduces the distance between the interdigital electrodes, making the SAW effective wavelength smaller. Similarly, the changes in the Young's modulus and density of the material increase the sound velocity, and finally increase the resonant frequency. At the same time, the conclusions obtained using this method also show that the flexible SAW resonator of the present invention has good frequency-strain response characteristics and high strain sensitivity.

[0060] In summary, the present invention adopts CIS ion implantation and stripping technology to prepare the piezoelectric film layer, and adopts low-energy argon ion irradiation technology to repair the implantation and stripping damage, which can improve the quality of the piezoelectric film layer, and adopts a thermal stress buffer structure to effectively solve the lattice mismatch and thermal mismatch problems between the piezoelectric film material and the heterogeneous substrate, especially the flexible substrate, thereby realizing the growth of high-quality LiNbO3 piezoelectric single crystal film, and improving the reliability and stability of the resonator composite film in the preparation process and the actual application process; the flexible SAW resonator has a high sensitivity and a considerable strain range, and has good repeatability and stability, which provides a new technical approach for the development and application of conformal acoustic strain sensors; the flexible substrate 5 is used to integrate the SAW resonator on a curved surface, which has a certain bending ability, and solves the problem that conformal integration or compatibility with wearable electronics cannot be achieved on a curved surface.

Claims

1. A flexible substrate single-port SAW resonator based on BCB bonding, characterized in that: It comprises a thermal stress buffer structure; the thermal stress buffer structure comprises a metal electrode layer (1), a piezoelectric film layer (2), an optional functional layer (3), a BCB bonding layer (4) and a flexible substrate (5) connected in sequence; wherein: A metal electrode layer (1) for providing a path for current so that the SAW resonator responds to changes in external strain and converts the changes into electrical signals; A piezoelectric film layer (2) is used to generate a piezoelectric effect and convert mechanical strain into an electrical signal; An optional functional layer (3) is used to prepare a sound reflection structure to improve the performance of the SAW resonator; A BCB bonding layer (4) for bonding to a flexible substrate (5); A flexible substrate (5) is used to integrate a SAW resonator on a curved surface to achieve common integration or wearable electronic applications.

2. The flexible substrate single-port SAW resonator based on BCB bonding according to claim 1, characterized in that: The metal electrode layer (1) comprises an IDT and a short-circuit reflector; the IDT is placed in the central area of ​​the top of the piezoelectric film layer (2); the short-circuit reflector is placed on both sides of the top of the piezoelectric film layer (2), that is, on both sides of the IDT, and is arranged along the z-axis propagation direction of the SAW resonator.

3. The flexible substrate single-port SAW resonator based on BCB bonding according to claim 1, characterized in that: The material of the piezoelectric film layer (2) is lithium niobate; the material of the BCB bonding layer (4) is styrene cyclobutene; and the material of the flexible substrate (5) is polyimide, a high-temperature resistant organic material.

4. The flexible substrate single-port SAW resonator based on BCB bonding according to claim 1, characterized in that: The metal electrode layer (1) is prepared by a lift-off process; the piezoelectric film layer (2) is prepared by a CIS ion implantation and stripping technique, and implantation and stripping damage repair is performed by a low-energy argon ion irradiation technique; and the BCB bonding layer (4) is prepared by a spin coating process.

5. The flexible substrate single-port SAW resonator based on BCB bonding according to claim 1, characterized in that: The thickness of the BCB bonding layer (4) is greater than 1λ, wherein λ represents the operating wavelength of the SAW resonator.

6. The flexible substrate single-port SAW resonator based on BCB bonding according to claim 1, characterized in that: The thermal expansion coefficients of the metal electrode layer (1), the piezoelectric film layer (2), the optional functional layer (3), the BCB bonding layer (4) and the flexible substrate (5) increase in sequence from top to bottom.

7. The flexible substrate single-port SAW resonator based on BCB bonding according to claim 1, characterized in that: The calculation formula of the resonant frequency f of the SAW resonator is: Wherein, v represents the phase velocity of the SAW resonator, λ represents the operating wavelength of the SAW resonator, Eeq represents the equivalent Young's modulus of the SAW resonator, and ρeq represents the equivalent density of the SAW resonator.

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