Film bulk acoustic resonator having dual-interdigital electrode structure and manufacturing method therefor

Abstract

Disclosed in the present invention are a film bulk acoustic resonator having a dual-interdigital electrode structure and a manufacturing method therefor. The bulk acoustic resonator comprises a substrate, a piezoelectric layer, and interdigital electrodes which are different in height and which are offset ; the substrate is internally provided with a cavity; the piezoelectric layer is arranged on the substrate; and the interdigital electrodes are arranged on the upper surface of the piezoelectric layer and comprise an interdigital electrode A and an interdigital electrode B, the density of a metal material used by the interdigital electrode B being greater than that of the interdigital electrode A, the thickness of the interdigital electrode B being less than that of the interdigital electrode A, and the interdigital electrode A and the interdigital electrode B being offset. The bulk acoustic resonator can suppress clutter, and increase bandwidths and Q values.

Description

A thin-film bulk acoustic resonator with a double interdigitated electrode structure and its fabrication method Technical Field

[0001] This invention belongs to the fields of radio frequency filtering devices and MEMS technology, specifically relating to a bulk acoustic resonator for suppressing clutter and its fabrication method. Background Technology

[0002] Laterally-excited bulk acoustic resonators (XBARs) consist of a thin piezoelectric film and interdigitated electrodes above it. They have significant advantages in terms of high operating frequency and high electromechanical coupling coefficient, making them an ideal choice for next-generation communication technologies such as 5G, 6G cellular networks, Wi-Fi 6E, and Wi-Fi 7.

[0003] XBAR devices were proposed against the backdrop of the rapid development of mobile communications. Due to the congestion of resources in the low and mid frequency bands, people are paying more attention to the development of high frequency bands. Traditional RF filter devices cannot achieve both high frequency and large bandwidth. Therefore, the market demand for large bandwidth RF filter devices that can operate in the high frequency range has increased dramatically. XBAR devices can well meet this demand.

[0004] XBAR devices utilize the antisymmetric mode (A1 mode) of Lamb waves propagating in a piezoelectric thin film as the dominant mode. Similar to surface acoustic wave (SAW) devices, they use interdigitated electrodes (IDTs) to excite vibrations. Compared to traditional SAW devices, which are limited to operating frequencies below 3500MHz, XBARs offer higher operating frequencies, adjustable between 3 and 7 GHz depending on the piezoelectric thin film thickness. XBARs also possess high electromechanical coupling coefficients; for example, XBARs fabricated from commonly used Z-cut lithium niobate piezoelectric thin films can achieve electromechanical coupling coefficients exceeding 25%. Therefore, with their significant advantages of high frequency and wide bandwidth, XBARs have garnered considerable attention in the field of radio frequency filtering devices in recent years.

[0005] Patent application CN116470878A discloses a transversely excited thin-film bulk acoustic resonator, relating to the field of radio frequency filters. It includes a substrate, a piezoelectric functional film on the substrate, and interdigital transducer electrodes on the piezoelectric functional film. The piezoelectric functional film specifically includes a piezoelectric material region and a non-piezoelectric insulating material region, with the piezoelectric material and the non-piezoelectric insulating material having different acoustic impedances. By setting the position of the non-piezoelectric insulating material region, the distance between the first envelope of multiple second electrode fingers and the second edge of the first non-piezoelectric insulating material region, and the distance between the second envelope of multiple first electrode fingers and the fourth edge of the second non-piezoelectric insulating material region, are all no more than 3 μm. The difference between this patent and this application is that the interdigital electrodes in the patent are uniform, while the structure of this application is divided into high-density, low-thickness electrodes (A interdigital electrodes) and low-density, high-thickness electrodes (B interdigital electrodes). This design can solve the problem of excessive transverse mode clutter in existing devices. In comparison, the impedance curve of the bulk acoustic resonator provided by this patent has clutter.

[0006] Ideally, XBARs should have only one resonant mode, the A1 mode, within their operating frequency band, resulting in a clean, glitch-free impedance curve. However, in actual production, XBARs face the problem of complex spurious modes, manifesting as numerous glitches on the impedance curve. Each glitch represents a noise, which affects the resonator's Q value and electromechanical coupling coefficient, reducing the resonator's figure of merit (FOM, the product of Q value and electromechanical coupling coefficient). It also negatively impacts filter design, such as causing in-band dips and affecting the steep drop in the transition band.

[0007] Summary of the Invention

[0008] This invention provides a thin-film bulk acoustic resonator with a dual interdigitated electrode structure, which can suppress clutter and improve the Q value.

[0009] This invention provides a thin-film bulk acoustic resonator with a dual interdigitated electrode structure, comprising a substrate, a piezoelectric layer, and interdigitated electrodes;

[0010] The piezoelectric layer has substrates on both sides of its bottom, and the piezoelectric layer and the substrates form a cavity. The interdigitated electrodes are placed on the upper surface of the piezoelectric layer.

[0011] The interdigitated electrode includes an A interdigitated electrode and a B interdigitated electrode. The interdigitated electrode is made of a metallic conductive material. The metallic conductive material density of the B interdigitated electrode is greater than that of the A interdigitated electrode. The thickness of the B interdigitated electrode is less than that of the A interdigitated electrode. The A interdigitated electrode and the B interdigitated electrode are arranged alternately.

[0012] This invention utilizes staggered B-interdigital electrodes (high density, low thickness) and A-interdigital electrodes (low density, high thickness) arranged on a piezoelectric layer to disrupt the ground resonance of stray modes generated by the piezoelectric layer, thereby effectively suppressing clutter. This, in turn, improves bandwidth and Q value.

[0013] Preferably, the thickness of the piezoelectric layer is 100-1000nm, the thickness of the A interdigital electrode is 50-2000nm, and the thickness of the B interdigital electrode is 10-50nm, corresponding to a frequency band within the range of 1-7GHz. The interdigital electrodes and the piezoelectric layer vibrate together to excite the A1 mode. The dual interdigital electrode structure can effectively suppress clutter near the resonator operating mode (A1).

[0014] Preferably, the materials of the A interdigitated electrode and the B interdigitated electrode are aluminum, platinum, chromium, titanium, molybdenum, tungsten or copper, or their metal alloys, and the thickness of the A interdigitated electrode is 1.0-2.0 times the thickness of the piezoelectric layer, and the thickness of the B interdigitated electrode is 0.05-1 times the thickness of the piezoelectric layer.

[0015] Preferably, the metallization rate of the interdigitated electrodes is 5%-50%. A suitable metallization rate can avoid introducing other noise that cannot be eliminated by adjusting the height of the high and B interdigitated electrodes. The metallization rate provided by this invention is obtained by dividing the electrode width by the center-to-center distance between adjacent electrodes.

[0016] Preferably, the substrate comprises a silicon oxide layer and a polycrystalline silicon layer, wherein the piezoelectric layer is located on the silicon oxide layer and the silicon oxide layer is located on the polycrystalline silicon layer.

[0017] Preferably, the cavity is rectangular, trapezoidal, or parallelogram in shape, but is not limited to these shapes.

[0018] Preferably, the thin-film piezoelectric layer is a single-crystal lithium niobate, lithium tantalate, or aluminum nitride. This invention obtains a wide-bandwidth piezoelectric layer by Z-cutting single-crystal lithium niobate.

[0019] The present invention also provides a method for fabricating a thin-film bulk acoustic resonator with the aforementioned double interdigitated electrode structure, comprising:

[0020] Ion implantation is performed on monocrystalline lithium niobate, and the implanted monocrystalline lithium niobate forms a piezoelectric layer.

[0021] A substrate is obtained, the substrate comprising a silicon oxide layer and a polycrystalline silicon layer stacked sequentially from top to bottom;

[0022] The piezoelectric layer is bonded to the polycrystalline silicon layer, and the unimplanted single-crystal lithium niobate is removed to expose the piezoelectric layer.

[0023] A interdigitated electrode A and B interdigitated electrode B are sequentially deposited on the piezoelectric layer using photolithography and metal sputtering methods. A protective layer is then deposited on the interdigitated electrode A, the interdigitated electrode B and the piezoelectric layer.

[0024] The polysilicon layer and silicon oxide layer of the substrate are etched sequentially until the piezoelectric layer is exposed, thereby forming a cavity through the substrate;

[0025] Removing the protective layer yields a bulk acoustic resonator that suppresses noise.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] This invention uses a double interdigitated electrode structure to suppress the resonance of stray modes in the piezoelectric layer. The interdigitated electrode vibrates along with the piezoelectric layer, generating the A1 mode. From the interdigitated direction, the resonance within the electrode is disrupted by the alternating arrangement of the double interdigitated electrodes, thus suppressing the generation of clutter, widening the bandwidth of the resonator, and improving the Q value. Attached Figure Description

[0028] Figure 1 is a schematic diagram of the bulk acoustic resonator of the bifid electrode structure thin film provided in a specific embodiment of the present invention;

[0029] Figure 2 is a schematic diagram of the acoustic vibration of a thin-film bulk acoustic resonator with a double interdigital electrode structure provided in a specific embodiment of the present invention.

[0030] Figure 3 shows a top view, an AA cross-sectional view, and an enlarged view of the double interdigitated electrode structure thin film bulk acoustic resonator provided in a specific embodiment of the present invention, wherein Figure 3(a) is a top view, Figure 3(b) is an AA cross-sectional view, and Figure 3(c) is an enlarged view;

[0031] Figure 4 is a comparison of the impedance curves of the double interdigitated electrode structure thin-film bulk acoustic resonator provided in a specific embodiment of the present invention and a traditional bulk acoustic resonator.

[0032] Figure 5 shows the particle X-direction vibration displacement diagrams of the double interdigitated electrode structure thin-film bulk acoustic resonator and the conventional bulk acoustic resonator in A1 mode according to a specific embodiment of the present invention. Figure 5(a) shows the particle X-direction vibration displacement diagram of the conventional bulk acoustic resonator in A1 mode, and Figure 5(b) shows the particle X-direction vibration displacement diagram of the bulk acoustic resonator with suppressed clutter provided in a specific embodiment of the present invention.

[0033] Figure 6 is a process flow diagram of the fabrication method of the thin-film bulk acoustic resonator with a double interdigitated electrode structure provided in a specific embodiment of the present invention;

[0034] Figure 7 shows the actual object under the microscope and the impedance curve of the measured data of this patent, corresponding to curve 2 in Figure 4.

[0035] The substrate is 100, polycrystalline silicon is 110, silicon oxide is 120, cavity is 130, piezoelectric layer is 200, interdigitated electrode is 300, A interdigitated electrode is 310, B interdigitated electrode is 320, time-varying electric field is 101, shear deformation is 102, unimplanted single crystal lithium niobate is 500, first photoresist is 501, second photoresist is 502, third photoresist is 503, and fourth photoresist is 504. Detailed Implementation

[0036] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0037] In view of the fact that existing bulk acoustic resonators have a lot of noise, which affects the bandwidth and Q value, the specific embodiment of the present invention uses XBAR with alternating arrangement of bifurcation electrodes to suppress noise. This can significantly suppress noise near the resonator operating mode (A1), obtain a relatively pure impedance curve, and improve the Q value of the device.

[0038] As shown in Figure 1, a specific embodiment of the present invention provides a thin-film bulk acoustic resonator with a dual interdigital electrode structure, including a substrate 100, a piezoelectric layer 200, and interdigital electrodes 300.

[0039] In a specific embodiment of the present invention, a piezoelectric layer 200 is provided with a substrate 100 on both sides of its bottom, and the piezoelectric layer 200 and the substrate 100 form a cavity 130.

[0040] In a specific embodiment of the present invention, the piezoelectric layer 200 is placed on the substrate 100, and the interdigitated electrode 300 is placed on the upper surface of the piezoelectric layer 200;

[0041] The interdigitated electrode 300 provided in a specific embodiment of the present invention includes an A interdigitated electrode 310 and a B interdigitated electrode 320, which are arranged alternately. The density of the metal conductive material of the B interdigitated electrode 320 is greater than that of the A interdigitated electrode 310, and the thickness of the B interdigitated electrode 320 is less than that of the A interdigitated electrode 310.

[0042] As shown in Figure 2, the time-varying electric field 101 generated by the high and B interdigitated electrodes of the bulk acoustic resonator provided in the specific embodiment of the present invention does not show a wave-like displacement distribution near the shear deformation 102, that is, the arrow in Figure 2 is in the horizontal direction, and the loss to the device is small.

[0043] As shown in Figure 3(a), the interdigitated electrode 300 provided in a specific embodiment of the present invention is located on the piezoelectric layer 200. The interdigitated electrode 300 includes an A interdigitated electrode 310 and a B interdigitated electrode 320, which are arranged alternately, as shown in Figures 3(b) and 3(c). The thickness of the A interdigitated electrode 310 is greater than that of the B interdigitated electrode 320. In one embodiment, because the A interdigitated electrode and the B interdigitated electrode are arranged alternately and have different thicknesses, the stray waves of the piezoelectric layer cannot maintain the same vibration when passing through the A and B interdigitated electrodes, and cannot form resonance, thereby suppressing the formation of stray waves.

[0044] In one specific embodiment, the thickness of the A interdigital electrode 310 is 440 nm, the thickness of the B interdigital electrode 320 is 30 nm, and the thickness of the piezoelectric layer is 400 nm.

[0045] In one specific embodiment, the A interdigital electrode 310 provided in this embodiment of the invention is made of low-density aluminum, and the B interdigital electrode 320 is made of high-density platinum, chromium, titanium, or copper. By adjusting the densities of the A interdigital electrodes and the B interdigital electrodes, the piezoelectric layer 200 is subjected to uniform stress, preventing collapse and ensuring a smooth surface.

[0046] As shown in Figure 4, curve 1 is the impedance curve of a traditional bulk acoustic wave resonator. The difference between this traditional bulk acoustic wave resonator and the bulk acoustic wave resonator for suppressing clutter provided in the specific embodiment of the present invention is that the thickness of the interdigital electrodes is the same, both being 440 nm, and both being made of aluminum.

[0047] Curve 2 in Figure 4 is the impedance curve of the bulk acoustic wave resonator provided in a specific embodiment of the present invention. The piezoelectric layer thickness is 400nm, and the electrode thickness is composed of alternating 440nm and 30nm. Compared with curve 1, clutter near 3.8GHz and 4.4GHz is significantly suppressed, and clutter near 5.6GHz is also slightly suppressed. The physical diagram of the bulk acoustic wave resonator corresponding to curve 2 in Figure 4 is shown in Figure 7(a), and the impedance curve of the measured data is shown in Figure 7(b). It can be seen from the figure that clutter can be suppressed well.

[0048] As shown in Figure 5(a), the displacement is concentrated at the upper and lower boundaries of the piezoelectric layer 200, with the upper and lower boundaries exhibiting opposite displacements, similar to shear bulk waves. Significant horizontal higher harmonics (displayed as a wavy displacement distribution) exist within the A1 mode of a conventional XBAR. These higher harmonics couple with the A1 mode and are considered detrimental clutter to device performance, as shown in Figure 5(b). The bulk acoustic resonator shown for suppressing clutter has a metallization of 20%, an A interdigital electrode thickness of 440 nm, and a B interdigital electrode thickness of 30 nm. The A interdigital electrode is made of aluminum, and the B interdigital electrode is made of titanium. The amplitude of the clutter vibration is significantly reduced, and the A1 mode vibration is cleaner.

[0049] Returning to Figure 3(b), a cavity 130 is provided below the piezoelectric layer 200, and the area of ​​the cavity 130 is larger than the area of ​​the interdigitated electrode 300.

[0050] As shown in Figure 6, a specific embodiment of the present invention provides a method for fabricating a thin-film bulk acoustic resonator with a dual interdigital electrode structure, including:

[0051] S1. As shown in Figures 6a and 6b, ion implantation is performed on single-crystal lithium niobate (LN) 500, typically using hydrogen ions. The implantation depth is controlled to approximate the desired piezoelectric layer thickness, and the LN layer is divided into an implanted ion layer, namely the piezoelectric layer 200, and an unimplanted ion layer 500.

[0052] S2. As shown in Figure 6c, the piezoelectric layer 200 is bonded to the silicon substrate. 502 and 503 represent a very thin layer of silicon oxide (typically 0.5µm to 2µm) and a polycrystalline silicon substrate, respectively, produced during the process.

[0053] S3. As shown in Figure 6d, the unimplanted LN layer 500 is removed using a Smart-cut process to obtain a single-crystal piezoelectric thin film of the desired thickness. The ion implantation depth can be appropriately increased or decreased according to the actual process deviation.

[0054] S4. As shown in Figure 6e, a high-resolution thin first photoresist 501 is uniformly applied to the front side of the single-crystal piezoelectric thin film. The photoresist pattern of the A interdigitated electrode is prepared on the surface of the single-crystal lithium niobate thin film using high-precision photolithography or laser direct writing (DWL) and other methods.

[0055] S5. As shown in Figure 6f, metal is sputtered onto the photoresist pattern as the A interdigital electrode 310 using methods such as magnetron sputtering and electron beam evaporation. The material selected is aluminum. Then, the excess first photoresist 501 is removed using a lift-off process.

[0056] S6. As shown in Figure 6g and Figure 6h, metal is sputtered onto the second photoresist 502 pattern using methods such as magnetron sputtering and electron beam evaporation as the B interdigital electrode 320. The B interdigital electrode must be strictly aligned with the A interdigital electrode. The material of the B interdigital electrode can be platinum, chromium, titanium, or copper. Then, the excess second photoresist 502 is removed using a lift-off process.

[0057] S7. As shown in Figure 6I and Figure 6j, a third photoresist 503 is coated on the surface of interdigital electrode A 310, interdigital electrode B 320 and piezoelectric layer 200, and a fourth photoresist 504 is coated on the surface of polysilicon 110. The polysilicon substrate is etched using deep reactive ion etching (DRIE) until the silicon oxide layer is reached and the etching stops.

[0058] S8. As shown in k of Figure 6, silicon oxide 120 is etched by wet etching with BOE solution. Since BOE solution does not react with lithium niobate, the etching time can be slightly extended to ensure that there is no silicon oxide residue at the bottom of the piezoelectric layer.

[0059] S9. As shown in Figure 6, remove the photoresist and clean to obtain a complete bidigital electrode structure thin-film bulk acoustic resonator.

Claims

1. A thin-film bulk acoustic resonator with a double interdigitated electrode structure, characterized in that, Includes substrate, piezoelectric layer and interdigitated electrodes; The piezoelectric layer has substrates on both sides of its bottom, and the piezoelectric layer and the substrates form a cavity. The interdigitated electrodes are placed on the upper surface of the piezoelectric layer. The interdigitated electrode includes an A interdigitated electrode and a B interdigitated electrode. The interdigitated electrode is made of a metallic conductive material. The metallic conductive material density of the B interdigitated electrode is greater than that of the A interdigitated electrode. The thickness of the B interdigitated electrode is less than that of the A interdigitated electrode. The A interdigitated electrode and the B interdigitated electrode are arranged alternately.

2. The thin-film bulk acoustic resonator with a double interdigital electrode structure according to claim 1, characterized in that, The thickness of the piezoelectric layer is 100-1000 nm, the thickness of the A interdigitated electrode is 50-2000 nm, and the thickness of the B interdigitated electrode is 10-50 nm.

3. The thin-film bulk acoustic resonator with a double interdigital electrode structure according to claim 1, characterized in that, The materials of the A interdigitated electrode and the B interdigitated electrode are aluminum, platinum, chromium, titanium, molybdenum, tungsten or copper, or their metal alloys. The thickness of the A interdigitated electrode is 1.0-2.0 times the thickness of the piezoelectric layer, and the thickness of the B interdigitated electrode is 0.05-1 times the thickness of the piezoelectric layer.

4. The thin-film bulk acoustic resonator with a double interdigital electrode structure according to claim 1, characterized in that, The metallization rate of the interdigitated electrodes is the ratio of the distance between electrodes to the width of the electrodes, which is 5%-50%, and the number of interdigitated fingers is 10-500.

5. The thin-film bulk acoustic resonator with a double interdigital electrode structure according to claim 1, characterized in that, The substrate includes a silicon oxide layer and a polycrystalline silicon layer, wherein the piezoelectric layer is located on the silicon oxide layer and the silicon oxide layer is located on the polycrystalline silicon layer.

6. The thin-film bulk acoustic resonator with a double interdigital electrode structure according to claim 1, characterized in that, The piezoelectric layer is a Z-cut monocrystalline lithium niobate piezoelectric thin film.

7. The thin-film bulk acoustic resonator with a double interdigital electrode structure according to claim 1, characterized in that, The cavity is rectangular, trapezoidal, or parallelogram in shape.

8. A method for fabricating a thin-film bulk acoustic resonator with a double interdigitated electrode structure according to any one of claims 1-7, characterized in that, include: Ion implantation is performed on monocrystalline lithium niobate, and the implanted monocrystalline lithium niobate forms a piezoelectric layer. The piezoelectric layer is bonded to the substrate, and the unimplanted single-crystal lithium niobate is removed to expose the piezoelectric layer. A interdigitated electrode A and B interdigitated electrode B are sequentially deposited and photolithographically formed on the piezoelectric layer using photolithography and metal sputtering methods. Then, a protective layer is deposited on the interdigitated electrode A, the interdigitated electrode B and the piezoelectric layer. The polysilicon layer and silicon oxide layer of the substrate are etched sequentially until the piezoelectric layer is exposed, thereby forming a cavity through the substrate; Removing the protective layer yields a thin-film bulk acoustic resonator with a double interdigitated electrode structure.

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