Laterally Vibrating Bulk Acoustic Wave Resonator
A laterally vibrating bulk acoustic wave (LVBAW) resonator includes a piezoelectric plate sandwiched between first and second metal layers. The second metal layer is patterned into an interdigital transducer (IDT) with comb-shaped electrodes having interlocking fingers. The width and pitch of the fingers of the electrodes determine the resonant frequency. A combined thickness of the first and second metal layers and the piezoelectric layer is less than the pitch of the interlocking fingers.
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This application is a division of U.S. patent application Ser. No. 16/505,669, filed Jul. 8, 2019, which claims priority to U.S. Provisional Patent Application No. 62/696,213 filed 10 Jul. 2018, the entirety of which is incorporated herein by reference.TECHNICAL FIELD
This relates to laterally vibrating bulk acoustic wave resonators.BACKGROUND
Microelectromechanical system (MEMS) resonators are useful for multiple applications, such as low-power, low-phase noise, high stability oscillators. Usually, a challenging aspect of such devices is integrating the MEMS resonators with the integrated circuit chips in a regular semiconductor packaging. Multiple approaches have been used, such as wire-bonding, flip-chip, and CMOS-MEMS.
In at least one example, a bulk acoustic wave (BAW) resonator is a MEMS device that includes a piezoelectric thin film sandwiched between two electrodes and acoustically isolated from the surrounding medium. Some BAW resonators may use piezoelectric films with thicknesses ranging from several micrometers down to tenth of micrometers, and they may resonate in a frequency range of 100 MHz to 10 GHz. Aluminum nitride and zinc oxide are two examples of piezoelectric materials in MEMS acoustic wave resonators.SUMMARY
An example laterally vibrating bulk acoustic wave (LVBAW) resonator includes a piezoelectric plate sandwiched between metal layers. An upper metal layer is patterned into an interdigital transducer (IDT) with comb-shaped electrodes having interlocking fingers. The width and pitch of the fingers of the electrodes determine the resonant frequency. The combined thickness of the upper metal layer, a lower metal layer and the piezoelectric layer is less than the pitch of the interlocking fingers.
In the drawings, like elements are denoted by like reference numerals for consistency.
A BAW device may be formed by a piezoelectric plate sandwiched between electrodes and an acoustic Bragg mirror in an unreleased acoustic structure in which its resonance frequency (fr) is primarily defined by the acoustic phase velocity (vp) and the height of the vibrating piezoelectric stack (h) such that fr=vp/(2h). Consequently, the frequency is affected by the thickness of the piezoelectric stack, resulting in a fixed, limited frequency range for one fabrication process. Mass load splits can be added to achieve certain frequency tunability on the same wafer, but additional steps further increase manufacturing time and cost. Frequency tunability is useful for filters because series and shunt resonators have different center frequencies, and oscillators when various frequency modulators are implemented in the same RF front-end system.
Surface acoustic wave (SAW) devices, which rely on the excitation of Rayleigh waves on a piezoelectric substrate, have been employed in telecommunication, radar and broadcasting systems for years. However, these devices are usually limited to <1 GHz applications, cannot be monolithically integrated with CMOS and require large footprints. Alternatively, contour mode resonators (CMRs), which rely on the excitation of low-dispersive S0-mode Lamb waves in a piezoelectric thin film, have emerged as replacements for SAWs, because they enable the same lithographic frequency tunability, while offering better resonator performance above 1 GHz, monolithic integration with CMOS and smaller footprints. That type of MEMS device requires releasing steps to separate the active region from the substrate for energy confinement, which increases a risk of failure during vibration or shock tests, needs costly wafer-level packaging, and becomes unsuitable for high power applications. “Releasing” requires creation of a space between the active region of the piezoelectric film and the substrate on which it is mounted.
A laterally vibrating bulk acoustic wave (LVBAW) resonator architecture, which is described hereinbelow, exhibits lateral vibrations in an unreleased substrate configuration and offers the advantages of contour mode resonator technology: full lithographic tunability, CMOS integration and small footprints. The LVBAW device is based on a piezoelectric plate sandwiched between metal layers and either a bottom Bragg mirror (BM) or both bottom and top BMs. The upper metal is patterned to form an interdigital transducer (IDT) and side reflectors while the lower metal can work as a non-electrified floating plate in a 1-port configuration or a ground plate in a 2-port configuration with a single IDT or isolated IDTs. The electrode pitch (p) sets the LVBAW resonance frequency (fr) and that requires that the equivalent thickness of the laterally-vibrating film formed by the piezoelectric layer and electrode metals (h) is less than one-half the electrode pitch. Thus, p=λ/2 and h<λ/4. The acoustic BMs and side reflectors are incorporated within the resonant structure to confine the acoustic energy under the IDT and increase the Q factor. In comparison to SAW, LVBAW requires fewer number of side reflectors to achieve the same amount of acoustic attenuation, reducing the device footprint.
LVBAW resonator 100 includes a piezoelectric material with an interdigital transducer (IDT) and side reflectors on one surface with a lower metal layer that works as a floating plate. An active area of the piezoelectric material is acoustically isolated from the surrounding medium. MEMS acoustic wave resonator devices using piezoelectric films with thicknesses ranging from several micrometers down to tenths of nanometers resonate in the frequency range of roughly 100 MHz to 10 GHz. Aluminum nitride and zinc oxide are two examples of piezoelectric materials in MEMS acoustic wave resonators.
In this example, LVBAW resonator 100 is implemented on a semiconductor substrate that is separate from the semiconductor substrate on which oscillator circuit 101 is fabricated. LVBAW resonator 100 may be mounted on the CMOS oscillator chip. Bond wires may be used to connect MEMS resonator terminals 101, 102 to bond pads on the CMOS oscillator chip. In another example, a MEMS acoustic wave resonator similar to LVBAW resonator 100 may be fabricated on the same substrate that includes an oscillator circuit.
Electrode 214 is patterned into a comb-shaped structure that has several parallel “fingers” 221, 222. Electrode 216 is similarly patterned into a comb-shaped structure that has several parallel fingers 223, 224, 225 that are interlocked with the fingers of electrode 214. In this example, electrode 214 has two fingers 221, 222 and electrode 216 has three fingers 223, 224, 225. In another example, there may be fewer fingers, such as one finger interlocked with two fingers. In another example, there may be more fingers in each electrode.
Each finger has a width (w) that is selected to be less than one half of a wavelength (y) of a target resonance frequency of a signal that is propagating laterally in piezoelectric layer 205, such that w<λ/2. The spacing between the fingers is selected to have a finger to finger pitch (p) equal to one half of the resonant frequency wavelength, such that p=λ/2. The electrode width can vary in different examples and is only restricted by the electrode pitch. However, when w becomes closer to p the performance gets worse and it is not practical to have electrodes of different polarities almost touching each other.
As described in more detail hereinbelow, to achieve a low-dispersive S0-mode of operation, the thickness (h) of the piezoelectric plate 205 together with the electrode metals 204, 206 is reduced to less than one half the electrode pitch (p), such that h<λ/4. In this example, h=0.75λ/4. In another example, the thickness of piezoelectric plate 205 and the upper and lower metal layers may be selected such that h<0.75λ/4.
In this example, LVBAW resonator 200 uses an acoustic reflector 208 at the bottom to prevent acoustic energy that propagates in the vertical direction from leaking into substrate 210. In some examples, a second acoustic reflector may be located on top of upper metal layer 206 to isolate the resonator from any other material or specimen that could be in direct contact with its top surface.
In laterally vibrating BAW resonators, the resonance frequency exhibits a low-dispersive S0 mode of vibration primary dominated by a lateral displacement component that shows large acoustic propagation in the lateral direction. As a result, this lateral energy leakage can affect the resonator quality factor (Q), which is defined as the ratio of energy stored over the energy lost per cycle of vibration. In the case of small devices, the energy lost becomes dominated by acoustic losses.
Patterned side reflectors 232 placed on one side of IDT 231 and patterned side reflectors 233 placed on an opposite side of IDT 231 mitigate the amount of lateral acoustic energy leakage. For efficient energy confinement, the width and separation of the metal strip lines 212 forming the resonant confiners 232, 233 is selected to be λ/4, where λ represents the lateral wavelength of the propagating acoustic wave leaving the resonator at resonance frequency. The reflectors are placed within the area of vibration, which is delimited by the Bragg mirror 208. The metal strip lines 212 are electrically isolated from each other.
The number of strip reflectors 212 can be increased up to the point in which the lateral displacement becomes highly attenuated, minimizing the amount of energy leakage and improving Q. While only three strips 212 are illustrated on each side of IDT 231, an example LVBAW may include ten to twenty, or more, strip reflectors.
In this example, lower metal layer 204 and electrodes 214, 216 and side reflectors 212 are patterned from layers of metal, such as Al, Mo, Cu, Au, etc. Piezoelectric layer 205 is fabricated using various piezo materials, such as AlN, quartz, GaN, ZnO, lithium nobate, etc. A temperature compensating layer of oxide may be formed on top of piezoelectric layer 205, or any position in between the resonator stack. Oscillator terminals 241, 242 are coupled electrodes 214, 216 respectively and thereby provide contacts for coupling LVBAW resonator 200 to an oscillator circuit 240 to form a signal at the resonant frequency of LVBAW 200.
In this example, acoustic reflector 208 is a distributed Bragg reflector (DBR). A DBR is a structure formed from multiple layers of alternating materials with varying acoustic impedance. Each layer boundary causes a partial reflection of an acoustic wave. For bulk acoustic waves whose wavelength is close to four times the thickness of the layers, the many reflections combine with constructive interference, and the layers act as a high-quality reflector. Any known or later developed Bragg mirror, dielectric mirror, acoustic reflector, etc. may be used to implement the acoustic reflector 208. The example acoustic reflector 208 includes alternating layers of materials with different acoustic impedances. In some examples, acoustic reflector 208 includes: (a) alternating flat conductive members (e.g., layers, sheets, plates, etc. of metal), two of which are designated by reference numerals 235 and 237; and (b) flat dielectric members (e.g., layers, sheets, plates, etc. of a dielectric material), three of which are designated by reference numerals 234, 236, and 238. The thicknesses of and distances between the conductive members 235 and 237 are selected based on an intended resonance frequency of the LVBAW resonator 200. As a result, the acoustic reflector 208 reduces spurious modes, and it confines (such as by reflecting, directing, containing, etc.) acoustic energy of the main mode at that frequency in piezoelectric layer 205. In some examples, the conductive members 235, 237 are formed by tungsten (W), titanium tungsten (TiW) or copper (Cu). In some examples, the dielectric members 234, 236, 238 are formed by silicon dioxide (SiO2), or a carbon doped oxide dielectric (such as SiCOH), or aluminum nitride (AlN). In some examples, the acoustic reflector 208 may be implemented using a two-dimensional (2D) or a three-dimensional phononic crystal. Lower metal layer 204 may be formed in contact with the adjacent Bragg mirror 208.
In one example, substrate 210 may include Si and may be between 275 nm and 925 nm in thickness depending on the wafer diameter. The acoustic Bragg mirror 208 includes alternating layers of SiO2 and TiW in order to achieve high acoustic impedance mismatch, and each layer may be between 10 to 1000 nm in thickness as a result of being one quarter wavelength of the acoustic modes excited by the LVBAW at resonance. The bottom metal layer 204 may include Mo or Pt and its thickness may be between 10 to 1000 nm. The piezoelectric plate 205 may include AlN or doped-AlN and its thickness may be between 50 to 5000 nm. The top metal layer 206 (including, for example, the fingers 212) may include Mo or Pt and its thickness may be between 10 to 1000 nm. The thicknesses of the bottom metal layer 204, piezoelectric plate 205, and top metal layer 206 are designed to be less than one fourth the wavelength at the acoustic phase velocity of the LVBAW resonance frequency.
In this example, only three microstrip fingers 421 within IDT 431 are illustrated for simplicity, and only a portion of side reflectors 432, 433 is illustrated for simplicity.
Lower metal layer 504 acts as a ground plate in this example. An input port 540 is coupled to first signal terminal 541 and thereby to electrode 514 of IDT 531 and to ground plate 504. An output port 544 is coupled to second signal terminal 542 and thereby to electrode 516 of IDT 531 and to ground plate 504.
Lower metal layer 604 acts as a floating plate in this example. An input port 640 is coupled to electrode 614 of IDT 631 and to electrode 616. An output port 644 is coupled to electrode 615 of IDT 634 and to electrode 617.
In this example, lateral vibration emanating from IDT 631 couples to IDT 634 to produce an output signal at port 644.
The A0 plot corresponds to the zeroth-order antisymmetric mode. The other dotted plot lines represent other modes of vibration that are possible within the simulated regions. For example, plot line S0 represents the zeroth-order symmetrical mode, while plot line S1 represents the first-order symmetrical mode. The A1 plot corresponds to the first-order antisymmetric mode of known modes of acoustic vibration that are possible in the simulated stack-up of materials.
Each mode of vibration has a characteristic mode shape that allows the type of vibration to be identified. For a given structure, the size, shape and various constraints on the structure will cause a particular mode of vibration to dominate. Referring again to
An alternative BAW architecture with a similar interdigitated transducer is described in more detail in U.S. patent application Ser. No. 16/236,601, entitled “Highly Dispersive Bulk Acoustic Wave Resonators.” Highly-dispersive BAW (HDBAW) resonators show some lithographic tunability, but their resonance frequency (or wavelength, λ) is a function of the electrode width (w) and the equivalent thickness of the vibrating piezoelectric layer (h). Consequently, the range of frequency tunability that can be achieved with HDBAW is limited to h, resulting in a disadvantage with respect to LVBAW, whose resonance frequency is primarily set by the electrode pitch (p) and the design of BMs, as described in more detail hereinbelow. In relation to the mode of vibration, HDBAW exhibits a highly-dispersive S0 Lamb wave as indicated at 701, while LVBAW exhibits a low-dispersive S0 Lamb wave, as indicated at 702. Moreover, LVBAW resonators with the same number of IDTs, Q and driving power display a stronger lateral displacement in comparison to HDBAW devices, which makes the LVBAW resonators better candidates to be used as acoustic delay lines, strain sensors or distributed bandpass filters.
To achieve a low-dispersive S0-mode (LVBAW) instead of a highly-dispersive S0-mode (HDBAW), the thickness of the piezoelectric plate and electrode metals (h) is reduced to less than one half the electrode pitch (p). λ/4 metal strip reflectors are included besides the IDTs in order to confine the lateral propagating energy. LVBAW resonator has a wider lithographic tunability than HDBAW, but this frequency range is ultimately limited by the design of BMs, which work as λ/4 transformers of the shear and thickness modes.
For example, an LVBAW can be designed to operate at approximately 3 GHz using the Bragg mirror represented by plot line 801 and by selecting an electrode pitch corresponding to λ/2 at 3 GHz.
In another example, an LVBAW can be designed to operate at approximately 1.75 GHz using the Bragg mirror represented by plot line 802 and by selecting an electrode pitch corresponding to λ/2 at 1.75 GHz. Using the same Bragg mirror design represented by plot 802, a different LVBAW can be fabricated simultaneously using a different layout for the upper metal layer in which the electrode pitch corresponds to λ/2 at 4.25 GHz and the LVBAW would then operate at 4.25 GHz.
Using transmission line theory, reflections can be determined based on the characteristic impedance and varying frequencies to determine how the overall stack of layers performs as a Bragg mirror. Simulated layer thicknesses and type of materials can be changed to arrive at a design for a selected target frequency or range of frequencies.
Referring again to
The side reflectors described herein are separated from the top electrode and their dimensions (linewidth and separation) are equivalent to λ/4. The thickness of the side reflectors can be the same as the thickness of the electrode. The side reflectors therefore can be fabricated with the same process steps that form the electrodes of IDT 231.
To achieve the displacement decay necessary to improve Q, the width and separation of the metal strip lines forming the reflectors need to be selected as one fourth of the periodicity of the propagating lateral wave (λ/4). The λ value will change depending on the material composition of the propagation regions. Specifically, λ will be selected based on the acoustic velocity of the propagating wave in the piezoelectric stack as well as the resonance frequency value (λ=vp/fr). The periodicity will change based on the material properties of the stack where the wave will propagate through.
To determine the vp of each of the side reflectors, such as reflector 212, equivalent elastic modulus (Eeq) and mass density (ρeq) of the material stack forming each region of the side reflectors is calculated. The strip lines, such 531, will be formed by a block of material that includes a metal layer and all of the other layers that make up the substrate of MEMS device 200 and the non-metalized regions between each reflector will be formed by a second block of material that does not include a metal layer and all of the other layers that make up the substrate of MEMS device 200. The propagation velocity (vp) will be determined as shown in expression (2).
The relationship of longitudinal phase velocity (Vl) and resonance frequency (fr) is given by expression (3).
The resonator Q depends on the reflection coefficient (Γ) as shown in expression (4), where n is the number of electrodes and N is the number of pairs of reflectors. Γ is a function of the number of pairs of reflectors (N) and their acoustic impedance mismatch (r) as shown in expression (5). Acoustic impedance mismatch (r) is a function of acoustic impedance in metalized regions (Zm) and in non-metalized regions (Z), as shown in expression (6).
In this example, plot line 1001 indicates a reflection coefficient of approximately 0.8 for the LVBAW device with 20 reflectors, while plot lines 1002, 1003 indicate reflection coefficients of less than 0.35 for the SAW devices with 20 reflectors. This means that the LVBAW can be physically smaller than a SAW device because fewer reflectors are required and therefore provide a higher Q for more efficient operation.
In this example, a material 1250 that has a low modulus of elasticity may be placed over MEMS resonator 200 to prevent high modulus mold compound 1251 from touching LVBAW resonator 200. In this manner, low modulus material 1250 acts as a stress buffer and provides a stress-free structure for LVBAW resonator 200 within an encapsulated package 1200.
In another example, a protective “cavity wafer” or “hard hat” may be placed over LVBAW resonator 200 to prevent mold compound from touching LVBAW resonator 200. IC die 1240 and MEMS die 200 may then be encapsulated with mold compound using a known or later developed encapsulation process.
Two or more versions of an LVBAW design can be made in which the only difference between the two versions is the pitch and or width of the elements of the IDT and side reflectors. This means that only a single metal mask may need to be changed to create an LVBAW that has a different resonant frequency.
Referring again to
At 1302, a first version of an LVBAW is fabricated using a first metal mask to define an IDT and side reflectors that have a first element spacing and element width that will cause the LVBAW to operate at a first resonant frequency. The various layers of the substrate, Bragg mirror, piezoelectric layer, lower metal layer and other interconnect layers are fabricated using appropriate masks and fabrication techniques.
At 1304, a second version of an LVBAW is fabricated in a second batch using a second metal mask to define an IDT and side reflectors that have a second element spacing and/or a second element width that will cause the LVBAW to operate at a second resonant frequency. In this case, all of the other processing steps and masks are the same as used in 1302.
In another example, a metal mask may be created for fabrication of a wafer of LVBAWs in which a portion of the LVBAWs on the wafer are fabricated using the first element spacing and width and another portion of the LVBAWs on the wafer are fabricated using the second element spacing and width. In this manner, LVBAWs that operate at different resonance frequencies may be fabricated simultaneously on the same wafer.
In an example, masks are used in a lithographic process to expose a photo resist on a metal layer, after which an etchant removes a portion of the metal layer using known fabrication techniques. In another example, the mask may be replaced by a direct write lithographic technique.
Digital phase locked loop (DPLL) 1401 uses a reference frequency signal provided by LVBAW resonator 1430 to generate an RF signal that is provided to power amplifier 1403 for wireless transmission of data produced by on-chip processor 1420 and/or 1422. RF receiver 1404 includes a low noise amplifier (LNA) that receives wireless signals. The received signals are then down-converted and digitalized using analog to digital converters (ADC) and then provided to DSP modem 1410. DSP modem 1410 extracts digital information from the received signals and provides the digital information to processor 1420 and/or main CPU 1422.
SoC 1400 is a low-power multi-standard device supporting Zigbee, Thread, Bluetooth Low Energy, and proprietary 2.4-GHz connectivity on a single chip. Enabling more design options and flexibility in a wider range of applications and environments, SoC 1400 is designed to work in the full −40° C. to 85° C. temperature range, unlike many crystal-based solutions.
As described hereinabove with reference to
In another example, a stress force may be applied to an LVBAW resonator to cause deformation of the LVBAW resonator. A change if resonance frequency in response to the stress force can be determined. In this manner, a measure of the stress force can be determined by detecting a difference in resonance frequency between the stressed and unstressed LVBAW resonator.Other Embodiments
In described examples, a single LVBAW resonator is mounted on a substrate, but in other examples there may be two or more LVBAW resonators mounted on one or more substrates.
A target width for elements of the side reflector elements equal to λ/4 of resonance frequency in the metalized region and space between elements equal to λ/4 of resonance frequency in the non-metalized region is calculated as described herein, but acceptable operation may be obtained within a range of +/−10% of the calculated values. As used herein, the terms “λ/4” and one fourth the wavelength” includes +/−10% λ/4. Similarly, the terms “λ/2” and “one half the wavelength” includes +/−10% λ/2. Devices designed to operate at a higher or at a lower target frequency will have side reflector parameters selected to match the target frequency.
The width and pitch of the interlocking fingers of the IDT may vary due to normal manufacturing tolerance. In some examples, a variation in width and/or pitch of the interlocking fingers may be in a range of +/−10%.
The example IC package 1200 of
Many devices are encapsulated with an epoxy plastic that provides adequate protection of the semiconductor devices and mechanical strength to support the leads and handling of the package. Some integrated circuits have no-lead packages, such as quad-flat no-leads (QFN) and dual-flat no-leads (DFN) devices, which physically and electrically couple integrated circuits to printed circuit boards. Flat no-lead devices, also known as micro leadframe (MLF) and small outline no-leads (SON) devices, are based on a surface-mount technology that connects integrated circuits to the surfaces of printed circuit boards without through-holes in the printed circuit boards. Perimeter lands on the package provide electrical coupling to the printed circuit board. Another example may include packages that are entirely encased in mold compound, such as a dual inline package (DIP).
In this description, the term “couple” and derivatives thereof mean an indirect, direct, optical, and/or wireless electrical connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection, through an indirect electrical connection via other devices and connections, through an optical electrical connection, and/or through a wireless electrical connection.
Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.
1. A method of fabricating a microelectromechanical system (MEMS) device, comprising:
- forming a piezoelectric layer on a substrate;
- forming a metal layer on a portion of the piezoelectric layer; and
- patterning a portion of the metal layer into an interdigital transducer (IDT) and a reflector on a side of the IDT, in which the IDT and the reflector are part of a laterally vibrating bulk acoustic wave (LVBAW) resonator, the IDT includes a comb-shaped electrode having fingers, the reflector is spaced from the IDT by a distance based on a sum of first and second terms, the first term is an odd multiple of a quarter of a wavelength of a signal having a target resonant frequency of the LVBAW resonator, and the second term is half of a spacing between the fingers.
2. The method of claim 1, wherein the portion of the metal layer is a first portion, the LVBAW resonator is a first LVBAW resonator, the IDT is a first IDT, the reflector is a first reflector, the side is a first side, the comb-shaped electrode is a first comb-shaped electrode, the distance is a first distance, the wavelength is a first wavelength, the target resonant frequency is a first target resonant frequency, the fingers are first fingers, and the method further comprises:
- patterning a second portion of the metal layer into a second IDT and a second reflector on a second side of the second IDT, in which the second IDT and the second reflector are part of a second LVBAW resonator, the second IDT includes a second comb-shaped electrode having second fingers, the second reflector is spaced from the second IDT by a second distance based on a sum of third and fourth terms, the third term is an odd multiple of a quarter of a second wavelength of a signal having a second target resonant frequency of the second LVBAW resonator, and the fourth term is half of spacing between the second fingers.
3. The method of claim 2, wherein patterning the first portion of the metal layer and patterning the second portion of the metal layer are performed with a metal mask.
4. The method of claim 2, wherein patterning the first portion of the metal layer and patterning the second portion of the metal layer are performed simultaneously.
5. The method of claim 2, wherein patterning the first portion of the metal layer and patterning the second portion of the metal layer are performed on different batches of LVBAW resonator devices.
6. The method of claim 1, wherein patterning the portion of the metal layer is performed by a lithographic process.
7. The method of claim 1, wherein patterning the portion of the metal layer is performed by a direct write lithographic process.
8. The method of claim 1, wherein the reflector is a first reflector, the metal layer is a first metal layer, and the method further comprises:
- forming a second reflector on a portion of the substrate; and
- forming a second metal layer on a portion of the second reflector,
- wherein the piezoelectric layer is formed on the second metal layer.
9. The method of claim 8, wherein the second reflector includes a Bragg mirror.
10. The method of claim 8, wherein the second metal layer includes a non-electrified floating plate, the IDT includes first and second electrodes, and the method further comprises coupling one of the first or second electrodes to a ground terminal.
11. The method of claim 8, further comprising coupling the second metal layer to a ground terminal.
12. The method of claim 1, wherein the comb-shaped electrode has a finger to finger pitch based on half of the wavelength.
13. The method of claim 1, wherein a respective width of each finger is less than half of the wavelength of the signal.
14. The method of claim 1, wherein the wavelength is based on a propagation speed of the signal between the IDT and the reflector.
15. The method of claim 1, wherein the reflector include metal strips that are electrically isolated from each other.
16. The method of claim 1, wherein adjacent fingers are separated by a space; and
- wherein a width of each finger is wider than the space.
17. The method of claim 1, wherein adjacent fingers are separated by a space; and
- wherein a width of each finger is narrower than the space.