WO2022242869A1

WO2022242869A1 - Surface acoustic wave device with improved topologies for reduction of transverse spurious modes

Info

Publication number: WO2022242869A1
Application number: PCT/EP2021/063574
Authority: WO
Inventors: Ventsislav Yantchev; Yuancheng JI; David GARCIA PASTOR
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2021-05-21
Filing date: 2021-05-21
Publication date: 2022-11-24
Also published as: CN117223215A

Abstract

A surface acoustic wave, SAW, apparatus (100) includes a piezoelectric substrate (132) with a surface (130) configured to support propagation of SAW. An interdigital transducer (102), IDT, disposed on the surface of the substrate is configured to induce piston mode SAW. The IDT includes a first bus (104) running parallel to a second bus (106), a first electrode (120) configured to conductively connect to the first bus, and a second electrode (122) parallel to the first electrode and configured to conductively connect to the second bus. A first piston mass (124) adjoins a base region (134) of the first electrode, and a second piston mass (126) adjoins a tip region (136) of the second electrode. The first piston mass is disposed between the first electrode and the surface and the second piston mass is disposed between the second electrode and the surface. Piston masses may be arranged in a two-dimensional pattern alongside a main SAW track.

Description

SURFACE ACOUSTIC WAVE DEVICE WITH IMPROVED TOPOLOGIES FOR REDUCTION OF TRANSVERSE SPURIOUS MODES

TECHNICAL FIELD

[0001] The aspects of the disclosed embodiments relate generally to acoustic wave devices and more particularly to piston mode surface acoustic wave devices.

BACKGROUND

[0002] Surface acoustic wave (SAW) devices, such as temperature compensated surface acoustic wave (TC-SAW) resonators, enjoy increased use in areas such as mobile communication devices. A problem suffered by TC-SAW devices is performance degradation caused by transversal mode spurs. This problem may be mitigated by inducing a piston mode wave shape in the surface acoustic waves to decouple higher order transversal wave modes from the electric field.

[0003] Piston mode is achieved by matching the interdigital transducer (IDT) to the outer media with various “piston region” topologies in the vicinity of the IDT electrode ends. The piston region is a region running alongside the IDT aperture with a SAW velocity slower than the SAW velocities in the surrounding media. Conventional piston region topologies, such as forming a local widening of the IDT electrodes, depositing a thin layer of metal overtop of the IDT electrodes, and forming a dielectric strip line along the resonator, can provide useful reduction of the transversal mode spurs. However, with the proliferation of applications it is beneficial to provide new piston region topologies that can provide improvements in performance, manufacturability, and cost.

[0004] Accordingly, it would be desirable to provide an apparatus and methods that address at least some of the problems described above. SUMMARY

[0005] The aspects of the disclosed embodiments are directed to a surface acoustic wave

(SAW) apparatus employing piston masses disposed between the electrodes of an interdigital transducer (IDT) and the surface of a piezoelectric substrate and arranged in a linear or two- dimensional pattern along an aperture of the SAW apparatus. The aspects of the SAW apparatus supress transversal mode spurs by decoupling higher order transverse modes from the IDT electric field, provide design flexibility, and improve manufacturability of the apparatus.

[0006] According to a first aspect, the above and further advantages are obtained by an apparatus. In one embodiment, the apparatus includes a piezoelectric substrate having a surface configured to support propagation of surface acoustic waves, and an IDT disposed on the surface. The IDT includes a first bus and a second bus disposed on the surface, where the first bus is parallel to the second bus. The IDT further includes a first electrode configured to conductively connect to the first bus and a second electrode configured to conductively connect to the second bus. The second electrode is disposed parallel to the first electrode. A first piston mass adjoins a base region of the first electrode and a second piston mass adjoins a tip region of the second electrode. The first piston mass includes a first portion of a piston layer, with the first portion of the piston layer being disposed between the surface and the first electrode. The second piston mass comprises a second portion of the piston layer, with the second portion of the piston layer being disposed between the surface and the second electrode.

[0007] In a possible implementation form of the apparatus, the piston layer comprises one or more layers of metal disposed underneath one or more of the first electrode and the second electrode. Disposing the piston mass underneath the electrode allows for larger piston masses and can improve manufacturability of the apparatus.

[0008] In a possible implementation form of the apparatus, a mass width of the first piston mass is greater than or equal to one half of an electrode width of the first electrode and less than or equal to one and a half times the electrode width. These dimensions produce effective piston mode SAW to supress transversal mode spurs.

[0009] In a possible implementation form of the apparatus, a mass width of the second piston mass is equal to the mass width of the first piston mass. Using the same or similar dimensions in the first and second piston mass improves propagation of SAW. [0010] In a possible implementation form of the apparatus, a mass length of the first piston mass is equal to a mass length of the second piston mass. Keeping the same or similar dimensions in all of the piston masses improves propagation of SAW.

[0011] In a possible implementation form of the apparatus, the mass length is greater than or equal to a minimum feature size and less than or equal to a wavelength of the surface acoustic waves. Using a mass length in this range improves manufacturability and promotes a beneficial piston mode SAW.

[0012] In a possible implementation form of the apparatus, the surface comprises at least one recess, and at least one of the first piston mass and the second piston mass is disposed within the at least one recess. A depth of the at least one recess is less than or equal to a thickness of the piston layer. Embedding the piston masses into the surface improves manufacturability and produces improved piston mode SAW.

[0013] In a possible implementation form of the apparatus, the first piston mass is aligned with the second piston mass within a slower track, where a first width of the slower track is equal to the mass length. Aligning the piston masses in a track along the aperture reduces the SAW velocity in this slower track and induces a piston mode in the SAW.

[0014] In a possible implementation form of the apparatus, one of the first piston mass and the second piston mass includes a hammerhead, where the hammerhead is formed as a local widening of the electrode width. Hammerheads are formed by the electrode layer thereby eliminating the need for an additional piston layer.

[0015] In a possible implementation form of the apparatus, a second width of the slower track is equal to the piston mass length plus an offset distance, and the first piston mass is offset from the second piston mass by the offset distance. Offsetting the piston masses in the slower track spreads the mass over a larger area of the surface creating a wider slower track which can improve suppression of transversal modes.

[0016] In a possible implementation form of the apparatus, the second width of the slower track is greater than the mass length and less than or equal to three times the mass length. Selecting the offset distance to keep the slower track width in this range promotes piston mode SAW. [0017] In a possible implementation form of the apparatus, the offset distance is greater than or equal to the minimum feature size and less than or equal to the wavelength. An offset distance in this range improves manufacturability and produces improved piston mode SAW.

[0018] In a possible implementation form of the apparatus, the apparatus includes a third piston mass disposed beneath a gap portion of the first electrode, wherein the third piston mass comprises a piston layer disposed between the surface and the first electrode. Disposing additional mass in the gap improves the piston mode of the SAW.

[0019] In a possible implementation form of the apparatus, the first piston mass and the second piston mass each comprise a hammerhead. Use of hammerhead type piston masses can reduce manufacturing costs.

[0020] In a possible implementation form of the apparatus, a width of the hammerhead is greater than the electrode width and less than one half the wavelength. A hammerhead width in this range improves the piston mode of the SAW.

[0021] In a possible implementation form of the apparatus, the offset distance is based on the width of the hammerhead, the minimum feature size, and the wavelength of the surface acoustic waves. Using an offset distance based on these factors promotes improved piston mode SAW.

[0022] In a possible implementation form of the apparatus, the third piston mass comprises a hammerhead. Using hammerheads for all of the piston masses can reduce manufacturing costs.

[0023] In a possible implementation form of the apparatus, the apparatus includes a dielectric layer overlaying the surface and the interdigital transducer, wherein the dielectric layer has a positive coefficient of elasticity. The dielectric layer provides temperature compensation.

[0024] According to a second aspect, the above and further advantages are obtained by a method for fabricating a surface acoustic wave device (SAW). In one embodiment, the method includes forming one or more piston masses on a surface of a piezoelectric substrate and forming an interdigital transducer (IDT) over the one or more piston masses. The one or more piston masses include one or more layers of metal and the interdigital transducer comprises one or more electrodes. The one or more electrodes are disposed over the one or more piston masses. The method produces a surface acoustic wave (SAW) device configured to supress transversal mode spurs by decoupling higher order transverse modes from the IDT electric field.

[0025] According to a possible implementation form of the method, the method includes forming one or more recesses in the surface, and disposing each of the one or more piston masses in a respective one of the one or more recesses. Embedding the piston masses into the surface improves manufacturability and produces an improved piston mode SAW.

[0026] According to a possible implementation form of the method, the method further includes forming a dielectric layer over the interdigital transducer and the surface, wherein the dielectric layer has a positive coefficient of elasticity. The dielectric layer provides temperature compensation.

[0027] According to a possible implementation form of the method, the method includes aligning the piston masses within the slower tracks. Aligning the piston masses in a track along the aperture reduces the SAW velocity in this slower track and induces a piston mode in the SAW. [0028] According to a possible implementation form of the method, the method includes offsetting the piston masses within the slower tracks. Offsetting the piston masses in the slower track spreads the piston mass over a larger area of the surface creating a wider slower track which can improve a piston mode of the SAW.

[0029] These and other aspects, implementation forms, and advantages of the exemplary embodiments will become apparent from the embodiments described herein considered in conjunction with the accompanying drawings. It is to be understood, however, that the description and drawings are designed solely for purposes of illustration and not as a definition of the limits of the disclosed invention, for which reference should be made to the appended claims. Additional aspects and advantages of the invention will be set forth in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention. Moreover, the aspects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In the following detailed portion of the disclosure, the invention will be explained in more detail with reference to the example embodiments shown in the drawings, in which like references indicate like elements and: [0031] Figure 1 illustrates a diagram showing an exemplary surface acoustic wave apparatus incorporating aspects of the disclosed embodiments.

[0032] Figure 2 illustrates a detail view showing two interdigital transducer (IDT) electrodes incorporating aspects of the disclosed embodiments.

[0033] Figure 3 illustrates a cross sectional view of two piston masses formed adjacent to IDT electrodes in accordance with the aspects of the disclosed embodiments.

[0034] Figure 4 illustrates a cross sectional view of a multi-layer IDT electrode incorporating aspects of the disclosed embodiments.

[0035] Figure 5 illustrates a cross sectional view of a multi-layer IDT electrode with a recessed piston mass incorporating aspects of the disclosed embodiments. [0036] Figure 6 illustrates a graph showing normalized deflection of a surface acoustic wave in an apparatus incorporating aspects of the disclosed embodiments.

[0037] Figure 7 illustrates a diagram showing an exemplary SAW apparatus incorporating aspects of the disclosed embodiments.

[0038] Figure 8 illustrates a detail view of two IDT electrodes incorporating aspects of the disclosed embodiments.

[0039] Figure 9 illustrates a cross sectional view of two piston masses formed adjacent to IDT electrodes in an apparatus incorporating aspects of the disclosed embodiments.

[0040] Figure 10 illustrates a diagram showing an exemplary SAW apparatus employing a two-dimensional pattern of piston layer masses incorporating aspects of the disclosed embodiments.

[0041] Figure 11 illustrates a detail view showing two IDT electrodes with offset piston masses incorporating aspects of the disclosed embodiments. [0042] Figure 12 illustrates a diagram showing an exemplary SAW apparatus employing a two-dimensional pattern of offset hammerheads incorporating aspects of the disclosed embodiments.

[0043] Figure 13 illustrates a graph showing surface deflection of a piston mode SAW in an apparatus incorporating aspects of the disclosed embodiments.

[0044] Figure 14 illustrates a flow diagram of an exemplary process appropriate for fabricating a SAW apparatus incorporating aspects of the disclosed embodiments.

DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENTS [0045] Referring to Figure 1, there can be seen a simplified drawing of an exemplary surface acoustic wave (SAW) apparatus 100 incorporating aspects of the disclosed embodiments. The apparatus 100 of the disclosed embodiments is directed to a SAW apparatus 100 incorporating improved piston region topologies configured to supress transversal mode spurs by decoupling higher order transverse modes from the interdigital transducer (IDT) electric field.

[0046] As shown in Figure 1, the apparatus 100 includes a piezoelectric substrate 132.

In one embodiment, the piezoelectric substrate 132 includes a surface that is configured to support propagation of surface acoustic waves. An interdigital transducer (IDT) 102 is disposed on the surface. In the example of Figure 1, the interdigital transducer 102 comprises a first bus 104 and a second bus 106 disposed on the surface 130 of the piezoelectric substrate 132. The first bus 104 is disposed in an orientation that is substantially parallel to an orientation of the second bus 106 on the surface 130 of the piezoelectric substrate 132.

[0047] The interdigital transducer 102 includes a first electrode 120 and a second electrode 122. The first electrode 120 is configured to conductively connect to the first bus 104. The second electrode 122 is configured to conductively connect to the second bus 106. The second electrode 122 is disposed in an orientation that is substantially parallel to an orientation of the first electrode 120 on the surface 130 of the piezoelectric substrate 132. [0048] The interdigital transducer 102 also includes a first piston mass 124 and a second piston mass 126. The first piston mass adjoins a base region 134 of the first electrode 120. A second piston mass 126 adjoins a tip region 136 of the second electrode 122.

[0049] The first piston mass 124 shown in Figure 1 comprises a first portion of a piston layer 302 that is disposed between the surface 130 of the piezoelectric substrate 132 and the first electrode 120. The second piston mass 126 comprises a second portion of the piston layer 302 that is disposed between the surface 130 of the piezoelectric substrate 132 and the second electrode 122.

[0050] In one embodiment, the piezoelectric substrate 132 can comprise lithium niobate

(LiNb03), with a crystal cut adapted to support highly electromechanically coupled Rayleigh surface acoustic waves (SAW). In embodiments employing lithium niobate for the piezoelectric substrate 132, typical crystal cuts are in the rage of about 115 degrees to and including 135 degrees (QU-C LiNb03; Q: 115 deg. - 135 deg.). Lithium niobate is provided as an example of an appropriate piezoelectric substrate 132, however any piezoelectric substrate having a surface 130 configured to support propagation of electromechanically coupled SAW may be advantageously employed in the apparatus 100 without straying from the spirit and scope of the disclosed embodiments.

[0051] An IDT 102 is disposed on the surface 130 of the piezoelectric substrate 132 and is configured to convert electric signals into surface acoustic waves or to convert surface acoustic waves into electric signals. The term "disposed" as used herein indicates that a feature or element is positioned in a particular place. As will be discussed in more detail below, the IDT 102 is configured to induce a piston mode SAW. A piston mode SAW is characterized by having a deflection profile that is relatively flat over a majority of a main SAW track 112 and decreases sharply within the adjacent slower tracks 110, 114. Examples of such a deflection profile are illustrated in Figures 6 and 13, described further herein.

[0052] Referring again to Figure 1, the IDT 102 includes a first bus 104 and a second bus 106 disposed on the surface 130 of the piezoelectric substrate 132. An orientation of the first bus 104 in Figure 1 is substantially parallel to an orientation of the second bus 106 on the surface 130 of the piezoelectric substrate 132. The pair of busses 104, 106, also referred to as bus bars, are configured to receive an electric signal and distribute that signal to one or more electrodes, such as electrodes 120 and 122, conductively coupled to the busses 104, 106. [0053] The portion or area of the surface 130 located between the first bus 104 and the second bus 106 may be referred to as the aperture and is divided into several stripes or tracks 108, 110, 112, 114, 116 running in parallel between the busses 104, 106 as illustrated in Figure 1. Each track defines a different portion or area of the surface 130. In the example of Figure 1, the tracks include a Faster Track 108, a Slower Track 110, a Main Saw Track 112, a Slower Track 114 and a Faster Track 116. In alternate embodiments, the tracks can include any suitable number and types of tracks.

[0054] As shown in Figure 1, the tracks 108, 110, 112, 114 and 116 are physical areas on the surface 130 of the piezoelectric substrate 132 configured to have different SAW velocities, as will be further described below. The main saw track 112 is substantially centered between the two busses 104, 106. The first slower track 110 and the second slower track 114 run along each side of the main saw track 112 and are characterized by a SAW velocity that is slower than the SAW velocity of the main SAW track 112.

[0055] It should be understood that the drawings are intended an aid to understanding only and do not accurately depict relative sizes of the elements. For example, in the exemplary apparatus 100, the main SAW track 112 may be much wider than the adjacent slower tracks 110, 114. To avoid reducing clarity of the drawings the respect to the width of the Main Saw track 112, the length of the plurality of electrodes 138, 140 has been reduced as depicted by gaps 128 in the plurality of electrodes 138, 140, further outlined by a pair of curved lines.

[0056] The first faster track 108 runs or is disposed between the first slower track 110 and the first bus 104. The second faster track 116 runs or is disposed between the second slower track 114 and the second bus 106. The faster tracks 108, 116, which are also sometimes referred to as a gap or gap regions, have a SAW velocity that is greater than the SAW velocity of the slower tracks 110, 114.

[0057] A first plurality of electrodes 138 is disposed on the surfacel30 extending transversely from the first bus 104 towards the second bus 106. The individual electrodes in the first plurality of electrodes 138 are oriented substantially parallel to one another and are also conductively coupled to the first bus 104. Each electrode, such as the first electrode 120, is conductively connected to the first bus 104. The first electrode 120 extends over top of the first faster track 108, the first slower track 110, the main SAW track 112, and the second slower track 114. The first electrode 120 terminates at an edge of the second slower track 114 where the second slower track 114 meets the second faster track 116.

[0058] In the example of Figure 1, a second plurality of electrodes 140 is disposed on the surfacel30 of the piezoelectric substrate 132 extending transversely from the second bus 106 towards the first bus 104. The individual electrodes in the second plurality of electrodes 140 are oriented substantially parallel to one another and are also conductively coupled to the second bus 106.

[0059] Each electrode, such as the second electrode 122 in the second plurality of electrodes 140, is conductively connected to the second bus 106. The second electrode 122 extends over top of the second faster track 116, the second slower track 114, the main SAW track 112, and the first slower track 110. The second electrode 122 terminates at an edge of the first slower track 110 where the first slower track 110 meets the first faster track 108.

[0060] As illustrated in Figure 1, electrodes in the first plurality of electrodes 138 are interleaved with electrodes in the second plurality of electrodes 140 in a zipper style fashion. An orientation of each electrode in the first plurality of electrodes 138 and the second plurality of electrodes is substantially parallel relative to one another.

[0061] All the electrodes 138, 140 are spaced evenly apart with a constant pitch P between adjacent electrodes, where the pitch P is the distance from one electrode, such as electrode 120, to the next adjacent electrode, such as electrode 122. This even spacing yields a wavelength of the induced surface acoustic waves of twice the pitch 2P.

[0062] In one embodiment all electrodes in the first plurality of electrodes 138 and the second plurality of electrodes 140 are formed with the same width M where a width M of one electrode is equal to about half the pitch P between adjacent electrodes. When desired, the width M of an electrode may be more or less than one half the pitch P.

[0063] All electrodes in the first plurality of electrode 138 and second plurality of electrodes 140 may be formed from a layer of metal. In one embodiment, the first bus 104 and second bus 106 may both be formed from the same layer of metal. The metal layer used to form the electrodes is referred to herein as the electrode layer 304 and is described in more detail below and with reference to Figure 3. [0064] Depositing additional material on an area of the surface 130 of the piezoelectric substrate 132 affects the velocity of surface acoustic waves induced thereon by the plurality of electrodes 138 and the plurality of electrodes 140. Additional mass may be deposited by increasing the electrode width or increasing the electrode thickness, such as by depositing additional material adjacent desired portions of the electrodes. The additional material may be deposited alongside an electrode, such as by a local widening of the electrode itself. Alternatively, the additional material may be formed by depositing an additional layer of material underneath the electrode. The shape and location of the IDT 102 is configured to generate a desired SAW velocity within each track 108, 110, 112, 114, 116 on the surface 130 of the piezoelectric substrate 132.

[0065] A bare Lithium Niobate substrate, one which has no layers of material deposited on its surface, has a Rayleigh surface acoustic wave (RSAW) velocity of about 4000 meters per second (m/s). In one exemplary embodiment an appropriate SAW velocity of the main SAW track 112 may be about 3650 m/s. In most embodiments the SAW velocity of the main SAW track 112 should be below the RSAW velocity of the bare substrate 132. In certain embodiments, the faster tracks 108, 116, also referred to as the gap regions, may have a SAW velocity that is about 5 percent greater than the SAW velocity of the main SAW track 112, and the slower tracks 110, 114, also sometimes referred to as the piston regions, may have a SAW velocity that is about 5 percent lower than the SAW velocity of the main SAW track 112.

[0066] Appropriately adjusting the SAW velocity of each track 108, 110, 112, 114, 116 induces a piston mode in the surface acoustic waves induced in the main SAW track 112 by the IDT 102. As described above, the orientation of the various SAW tracks 108, 110, 112, 114, 116 relative to one other, run in parallel between the first bus 104 and the second bus 106. The main SAW track 112 runs through a center portion of the surface 130 located between the first bus 104 and the second bus 106. All the electrodes in both the first plurality of electrodes 138 and the second plurality of electrodes 140 cross or run over top of the main SAW track 112.

[0067] Piston masses, such as piston masses 124, 126 shown in Figure 1, are aligned within the first slower track 110 to create a SAW velocity within the first slower track 110 that is slower than the SAW velocity of the main SAW track 112. The second slower track 114 is similarly configured with piston masses to have a slower SAW velocity than the main SAW track 112. [0068] As shown in Figure 1, the first faster track 108 runs between the first slower track 110 and the first bus 104. The second faster track 116 runs between the second slower track 114 and the second bus 116. Each electrode in the first plurality of electrodes 138 is connected to the first bus 104 with a strip of electrode material extending from the first slower track 110 over the first faster track 108 and connecting to the first bus 104. Similarly, the electrodes in the second plurality of electrodes 140 are connected to the second bus 106 with a strip of electrode material extending over the second faster track 116.

[0069] The end 134 of the electrode 120 and the end 136 of electrode 122 that is lying within a portion of one or more of the slower track 110 or slower track 114 is referred to herein as the end of an electrode or an electrode end. As shown in Figure 1, the end 134 of the electrode 120 adjacent to the faster track 108 is conductively connected to the bus 104 by a strip of material extending over the adjoining gap or faster track 108. This end 134 of electrode 120 is conductively connected to the bus 104, and is also referred to herein as the base or base region of the electrode 120.

[0070] In the example of Figure 1, an end 136 of electrode 122 adjacent to the faster track 108 is not connected to the bus 104. This unconnected end 136 of the electrode 122 is referred to herein as the tip or tip region 136 of the electrode 122.

[0071] Piston mode is induced in the exemplary apparatus 100 by aligning the piston masses within the slower tracks 110, 114. This alignment is illustrated in Figure 1, where the exemplary first piston mass 124 of the piston masses is aligned with the exemplary second piston mass 126 of the piston masses within the first slower track 110. The ends of each piston mass 124, 126 are coincident with the sides of the slower track 110 yielding a slower track 110 having a width Wst equal to the length of the piston masses Lp. Aligning the piston masses within a slower track can be viewed as a linear or one-dimensional alignment. This is illustrated by the line 146 which runs through the center of all piston masses aligned within the second slower track 114.

[0072] As shown in the example of Figure 1, each piston mass 124, 126 is centered beneath a respective electrode, such as first and second electrodes 120, 122. The first and second electrodes 120, 122 are shaded with stippling to aid clarity and show their relationship to the following detailed views. [0073] Figure 2 illustrates a detail view 200 showing the first and second electrodes

120, 122 of the IDT 102 incorporating aspects of the disclosed embodiments. The detail view 200 corresponds to the area 118 indicated in Figure 1, and shows a first piston mass 124 disposed in the base portion 134 of the first electrode 120 aligned with a second piston mass 126 disposed in the tip portion 136 of the second electrode 122. The first piston mass 124 is aligned with the second piston mass 126 within the first slower track 110.

[0074] The first piston mass 124 is disposed underneath the first electrode 120 and a second piston mass 126 is disposed underneath the second electrode 122. Cross hatched rectangles are used in the figures to more clearly point out locations on the surface of the piston layer masses 124, 126. However, it is important to note that the piston layer masses 124, 126 are disposed underneath the respective electrodes 120, 122 and are also disposed between an adjacent electrode and the surface 130 of the piezoelectric substrate 132. As will be discussed further below and with reference to Figure 3, the piston masses 124, 126, are formed with a layer of metal, referred to herein as the piston layer 302, disposed below the electrode layer 304.

[0075] As shown in the detail view 200 of Figure 2, the first piston mass 124 has a mass width Wp, and the second piston mass 126 has a second mass width Wp2. In certain embodiments, the second mass width Wp2 of the second piston mass 126 is equal to the mass width Wp of the first piston mass 124. In the exemplary apparatus 100 of Figure 1, the mass width Wp of the first piston mass 124 is greater than or equal to one half of a width M of the first electrode 120 and less than or equal to one and a half times the width M of electrode 120, as shown in equation 1 :

0.5 M < Wp < 1.5 M, Eq. 1 where M is the width of the electrode 120.

[0076] In the exemplary apparatus 100, a length Lp of the first piston mass 124 is the same as the length Lp2 of the second piston mass 126. In certain embodiments it is advantageous to have the length Lp of the first piston mass 124 and the length Lp2 of the second piston mass 126 be greater than or equal to a minimum feature size and less than or equal to a wavelength (2P) of the surface acoustic waves as shown in equation 2: f_min < Lp < 2P, Eq. 2 where f _t is the minimum feature size. The minimum feature size f _t corresponds to the minimum feature size or size of the smallest feature that can be reliably produced by the process being used to manufacture the apparatus 100. In certain embodiments the minimum feature size f_min may be about 300 nanometres (nm). Conventional photolithographic process such as an optical lithographic process or a UV lithographic process may have a minimum feature size of about 125 to 300 nanometres (nm). Alternatively, other processes with smaller or larger minimum feature sizes may be advantageously employed to manufacture the apparatus 100.

[0077] As illustrated in the detail view 200 of Figure 2, the first piston mass 124 is aligned with the second piston mass 126 within the slower track 110. As used herein, aligning piston masses within the slower track 110 describes the linear arrangement of piston masses 124, 126 as illustrated in Figure 1 and Figure 2 where the center 204, 206 of each piston mass 124, 126, respectively, falls along a centerline 202 of the slower track 110. The width Wp of the piston mass 124 is centered below a corresponding electrode with the center 204 of the first piston mass 124 falling on a centerline 210 of the electrode 122.

[0078] Figure 3 illustrates a cross sectional view 300 of two piston masses 124, 126 formed adjacent to electrodes 120, 122, respectively in accordance with the aspects of the disclosed embodiments. The cross section 300 illustrates an endwise view of the first electrode 124 and the second electrode 126 corresponding to the section indicated by line A- A shown in the detail view 200 of Figure 2. Cross section 300 illustrates how the piston layer 302 is deposited directly on the surface 130 of the piezoelectric substrate 132 and below, the electrode layer 304, resulting in each piston mass 124, 126 being between the surface 130 of the piezoelectric substrate 132 and the respective electrode 120, 122. Disposing the piston masses 124, 126 between the respective electrodes 120, 122 and the surface 130 as illustrated in the cross section 300, allows the piston mass width Wp to exceed the electrode width M without adversely affecting the manufacturing process.

[0079] Changes in temperature of a SAW device may cause changes in the velocity of surface acoustic waves thereby changing the frequency response of the apparatus 100 shown in Figure 1. This thermal sensitivity of a SAW apparatus 100 may be measured using a value referred to as a temperature coefficient of frequency (TCF) with units of parts per million per degree Celsius (ppm/°C). Many materials used in a SAW apparatus 100, such as lithium niobate, experience a lowering of their frequency characteristics as temperature increases thereby yielding a negative TCF. Certain dielectric materials, such as silicon dioxide Si02 , germanium oxide GeO, or tellurium oxide Te02, have a positive TCF. Disposing a layer 306 of dielectric material having a positive TCF over the surface 130 and IDT 102 of a SAW apparatus 100 having a piezoelectric substrate 132 with a negative TCF can reduce the overall temperature sensitivity of the SAW apparatus 100. A SAW apparatus that includes temperature compensation, such as a layer of Silicon Dioxide, may be referred to as a temperature compensated SAW (TC-SAW) device.

[0080] Temperature compensation in the exemplary apparatus 100 is provided by a dielectric layer 306 overlaying the surface 130 of the piezoelectric substrate 132, the piston layer 302, and the electrode layer 304 as illustrated in Figure 3. In certain embodiments suitable temperature compensation is achieved by a silicon dioxide dielectric layer 306 having a significant thickness, such as about 0.3 to about 0.8 times the pitch P, where the pitch is the distance from the center of one electrode to the center of an adjacent electrode.

[0081] As used herein the term "piston mass" refers to an additional amount of metal added to the IDT 102 and configured to induce a piston mode within the surface acoustic waves. The exemplary piston masses of the disclosed embodiments may be formed either by depositing an additional layer of material below an electrode of the first and second plurality of electrodes 138, 140, or as will be described in more detail below, by a local widening of an electrode of the first and second plurality of electrodes 138, 140. As used herein the term "piston layer" mass refers to a piston mass that is formed by depositing an additional layer of metal or other suitable material between the electrode layer 304 and the surface 130 of the piezoelectric substrate 132. The piston masses 124 and 126 described above and with respect to Figures 1, 2, and 3 are examples of piston layer masses.

[0082] As shown in Figure 3, the electrode layer 304 creates electrodes 120, 122 having a constant thickness He. In the exemplary apparatus 100, the thickness Hp of each piston mass 124, 126 is beneficially greater than 0.05 times the electrode thickness He, as shown in equation 3:

Hp > 0.05He. Eq. 3

[0083] The electrode layer 304 may be deposited on the surface 130 of the piezoelectric substrate 132 using any suitable microfabrication process and may be formed from any suitable metal such as copper, Cu or aluminum Al. Alternatively, the electrode layer 304 may be formed with a multi-layer combination of low-density metal such as aluminum Al, beryllium Be, etc., and high-density metal such as tungsten W, molybdenum Mo, platinum Pt, iridium Ir, silver Ag, etc. In embodiments employing a multi-layer combination in the electrode layer 304, it may be advantageous to include thin adhesive layers of for example, chromium Cr, or titanium Ti, to provide better adhesion between the layers of the multi-layer combination.

[0084] Figure 4 illustrates a cross sectional view 400 of the detail view 200 of Figure

2. The cross section 400 illustrates a side view of the second electrode 122 corresponding to the section indicated by line B-B shown in the detail view 200. The cross section 400 illustrates the piston mass structure formed by disposing the piston mass 126 between the surface 130 and the electrode 122.

[0085] As shown in the cross section 400 the electrode 122 is formed overtop of the piston mass 126 causing the electrode 122 to be elevated above the piston mass 126. This can lead to manufacturing defects in the electrode 122 near the piston mass 126.

[0086] Figure 5 illustrates a cross-sectional view 500 of the detail view 200 of Figure

2. The cross section 500 illustrates a side view of the second electrode 122 corresponding to the section indicated by the line B-B shown in Figure 2. The cross-sectional view 500 illustrates an alternative embodiment where the piston mass 126 is embedded in the surface 130 of the piezoelectric substrate 132 thereby mitigating manufacturing problems that may be created by elevating the electrode 122 over the piston mass 126. Embedding of the piston mass 126 is achieved by forming a recess 502 in the surface 130 of the piezoelectric substrate 132 configured to receive the piston mass 126. The recess 502 may be formed prior to deposition of the piston layer 302 and may be of a size and location corresponding to the size and location of the piston mass 126.

[0087] The piston mass 126 may be fully or partially embedded into the piezoelectric substrate 132 as desired. Fully embedding the piston mass 126 into the piezoelectric substrate 132 is achieved by forming a recess 502 having a depth dHp equal to the piston mass thickness Hp. Alternatively, the piston mass 126 may be partially embedded into the substrate by forming a recess 502 with a depth dHp less than the thickness of the piston mass Hp. In general, the depth dHp of the recess 502 should be less than or equal to the thickness Hp of the piston mass 126 and greater than or equal to zero. [0088] Figure 6 illustrates a graph 600 showing surface deflection of a piston mode

SAW apparatus incorporating aspects of the disclosed embodiments. The graph 600 illustrates a deflection profile induced by aligning piston layer masses within each slower track 110, 114 as shown in the exemplary apparatus 100 shown in Figure 1. The graph 600 corresponds to an embodiment where the piston masses 138, 140 are fully embedded in the piezoelectric substrate 132. In the graph 600, normalized surface displacement is depicted along the vertical axis 602, and distance along the aperture is depicted along the horizontal axis 604 in micrometres (xlO ⁶ meters) where zero corresponds to a centre of the aperture. The aperture loosely corresponds to the area between the first bus 104 and the second bus 106 and includes the main SAW track 112, the adjacent slower tracks 110, 114, and the faster tracks or gap regions 108, 116. Vertical dashed lines are imposed on the graph 600 to show relative locations of the SAW tracks 108, 110, 112. 114, 116 along the horizontal axis 604. A piston mode SAW is characterized by a relatively constant deflection or flat top across the main track 112 portion of the aperture. As can be seen in the graph 600 aligning piston layer masses within the slower tracks 110, 114 induces piston mode SAW.

[0089] Referring to Figure 7, in one embodiment, the apparatus 100 of Figure 1, referred to in Figure 7 as SAW apparatus 700, incorporates multiple types of piston masses disposed within the slower tracks 110, 114. In this example, the apparatus 700 is configured to suppress transversal mode spurs by decoupling higher order transverse modes from the electric field of the IDT 102 of Figure 1, shown in this example as IDT 702. The exemplary apparatus 700 includes a combination of piston layer masses, such as the piston layer mass 124, and hammerhead masses, such as the piston mass 726, similar to piston mass 126 of Figure 1, aligned within the slower tracks 110, 114 running along-side and adjacent to the main SAW track 112.

[0090] In addition to the piston layer mass described above, the exemplary apparatus

700 employs an additional type of piston mass referred to herein as a hammerhead mass or hammerhead. A hammerhead is a type of piston mass is formed by a local widening of the electrode layer 304. For example, the piston mass 726 is a hammerhead formed by a widening of the electrode 722 in the tip region 136 of the electrode 722.

[0091] The exemplary IDT 702 includes an alternating pattern of piston layer masses and hammerhead masses disposed within the slower tracks 110, 114. A piston layer mass, such as piston layer mass 124, is formed beneath the base region 134 of each electrode, and a hammerhead, such as hammerhead 726, is formed by widening the tip region, such as the tip region 136, of each electrode.

[0092] Figure 8 illustrates a detail view 800 of a portion 718 of the exemplary IDT 702 incorporating aspects of the disclose embodiments. The detail view 800 shows a piston layer mass 124 disposed below the base region of a first electrode 120, aligned with a hammerhead 726 formed at the tip of the adjacent electrode 722. In the illustrated embodiment, the piston layer mass 124 has a mass length Lp which is the same as the mass length Lp2 of the hammerhead 726. The mass length Lp is beneficially within the range described above and shown in Equation 2.

[0093] Width Wp of the piston layer mass 124 is in the range described above as shown in Equation 1. The width Wh of the hammerhead 726 may be greater than the electrode width M and less than the electrode spacing or pitch P as shown in Equation 4:

M < Wh < P. Eq. 4

[0094] In the apparatus 700 the first piston mass 124 is aligned with the second piston mass 726 within a slower track, such as for example the first slower track 110. In an alternate embodiment, the first piston mass 124 could be aligned with the second piston mass 726 within the slower track 114. As discussed above, aligning piston masses within a slower track, refers to the illustrated arrangement where the center 804 of the first piston mass 124 and the center 806 of the second piston mass 726 fall along the centerline 202 of the slower track 110.

[0095] Figure 9 illustrates a cross sectional view 900 of two piston masses 124, 726 formed adj acent to IDT electrodes in accordance with the aspects of the disclosed embodiments. The cross section 900 illustrates an endwise view corresponding to the section indicated by the line C-C shown in the detail view 800 of Figure 8. As can be seen in the cross-sectional view 900, the exemplary apparatus 700 employs both piston layer masses and hammerheads. The first piston mass 124 is formed as a piston layer mass having a layer of material 902 disposed between the first electrode 120 and the surface 130 of the piezoelectric substrate 132. The piston layer 902 is similar to the piston layer 302 described above and may be formed with any of the piston layer configurations described above and with reference to the piston layer 302. A second piston mass 726 is formed as a hammerhead by increasing the width of the electrode 722 in the tip region from the base electrode. The hammerhead width Wh is wider than the electrode width M. [0096] Referring to Figure 10, in one embodiment, the apparatus 100 of Figure 1, referred to as SAW apparatus 1000 in Figure 10, employs a two-dimensional pattern of piston layer masses. The two-dimensional piston mass pattern of the apparatus 1000 is configured to induce piston mode SAWs on the surface 130 of a piezoelectric substrate 132. The exemplary IDT 1002 in this example, similar to the IDT of the apparatus 100 of Figure 1, is configured with an arrangement of piston masses, such as piston masses 124, 126. To more clearly distinguish the two-dimensional piston mass arrangement of the apparatus 1000 from the linear alignment described above with respect to the apparatus 100, the piston masses 124, 126 are referred to in the present apparatus 1000 as piston masses 1024, 1026 respectively.

[0097] For example, adjacent piston masses 1024, 1026 are offset by an offset distance

Op. In certain embodiments, it is beneficial to dispose additional piston masses 1028 in the faster track 108 where each additional piston mass 1028 is formed as a piston layer mass disposed adjacent the gap region 1030 of a corresponding electrode 1020.

[0098] In the exemplary apparatus 1000 the length of each piston mass Lp is greater than or equal to a minimum feature size and less than or equal to a wavelength 2P of the surface acoustic waves as described above and shown in equation 2. The thickness Hp of each piston mass 1024, 1026, 1028 is greater than 0.05 times the electrode thickness He, as shown in equation 3 above.

[0099] Figure 11 illustrates a detail view 1100 of a portion 1018 of the exemplary apparatus 1000. The detail view 1100 illustrates an offset arrangement of two exemplary piston masses 1024, 1026. In the exemplary offset arrangement, each of the piston masses 1024, 1026 is formed as a piston layer mass disposed between the surface 130 of the piezoelectric substrate 132 and a respective electrode 1020, 1022. As described above, the mass width Wp of each piston mass 1024, 1026 is greater than or equal to one half of an electrode width M and less than or equal to one and a half times the electrode width M as shown in equation 1.

[00100] Offsetting the piston masses 1024, 1026 by an offset distance Op increases the width Wst2 of the slower track 110 producing a slower track 110 having a width equal to the mass length Lp plus the offset distance Op. In the exemplary apparatus 1000, the offset distance Op may be in the range shown in equation 5 : fmin < Op < 2P. Eq. 5 where fmin is the minimum feature size as described above, and 2P is twice the electrode pitch. The offset distance Op is shown in the detail view 1100 as the distance between a center of the fist piston mass 1024 and a middle 1106 of the second piston mas 1026.

[00101] When desired, an additional piston mass 1028 may be disposed in the gap region 1030 of an electrode 1020. The additional piston mass 1028 is formed as a piston layer mass with one end 1108 coincident with an outer side 1108 of the slower track 110.

[00102] Referring to Figure 12, in one embodiment, the exemplary SAW apparatus 100 of Figure 1 is shown employing a two-dimensional pattern of offset hammerheads. In this example, the apparatus 100 is referred to as apparatus 1200 for descriptive purposes.. The exemplary apparatus 1200 of the disclosed embodiments is similar to the exemplary apparatus 100 and 1000 described above where like references indicate like elements. The exemplary IDT 1202 in this example, similar to the IDT 102 of Figure 1, is configured with an arrangement of piston masses, such as piston masses 124, 126. However, in this example, the piston masses 124, 126 of Figure 1, shown in Figure 12 as first piston mass 1224 and second piston mass 1226, are hammerheads disposed in an offset arrangement.

[00103] As shown in Figure 12, a first piston mass 1224, disposed in the base region of a first electrode 1220, is offset from a second piston mass 1226, disposed in the tip region of a second electrode, by an offset distance Op. In contrast with the exemplary IDT 1002 described above, the two piston masses 1224, 1226 of the exemplary IDT 1202 are formed as hammerheads or hammerhead type piston masses. In certain embodiment it is advantageous to form a third hammerhead type piston mass 1228 in the gap portion 1030 of first electrode 1220.

[00104] Figure 13 illustrates a graph 1300 showing surface deflection of a piston mode SAW incorporating aspects of the disclosed embodiments. The graph 1300 illustrates a deflection profile induced by the exemplary IDT 1202 described above and with reference to Figure 12. The graph 1300 corresponds to an embodiment where the piston masses 1224, 1226, 1228, are formed as hammerheads from a local widening of the electrodes 1220, 1222. In the graph 1300, normalized surface displacement is depicted along the vertical axis 1302, and distance across the aperture, is depicted along the horizontal axis 1304 in micrometres (xlO ⁶ meters) where zero corresponds to a centre of the aperture. Vertical dashed lines are imposed on the graph 1300 to show relative locations of the SAW tracks 108, 110, 112, 114, 116 along the horizontal axis 604. As can be seen in the graph 1300, arranging hammerheads in the two- dimensional pattern shown in the IDT 1202 induces a surface acoustic wave having a relatively constant deflection across the main SAW track 112.

[00105] Figure 14 illustrates a flow diagram of an exemplary process 1400 appropriate for fabricating SAW apparatus incorporating aspects of the disclosed embodiments. The exemplary process 1400 of the disclosed embodiments may be a microfabrication process used to fabricate any of the exemplary SAW apparatus described above and with reference to Figures 1 through 13.

[00106] Fabrication of an exemplary SAW apparatus begins by preparing 1402 a piezoelectric substrate with a surface configured to support propagation of surface acoustic waves. For example, in certain embodiments the piezoelectric substrate, such as that shown with respect to Figure 1, may be formed from lithium niobate, LiNb03, with a crystal cut adapted to support highly electromechanically coupled Rayleigh surface acoustic waves (SAW).

[00107] One or more recesses are formed 1404 in the surface of the piezoelectric substrate in preparation for deposition of piston masses. The recesses may be advantageously located along slower tracks running along each side of a main saw track on the surface of the substrate. When desired, additional recesses may be formed within the gap region between each slower track and a bus portion the IDT which is deposited in a later step in the process 1400. The recesses are configured to partially or fully embed piston layer masses in the surface of the substrate.

[00108] Piston masses are deposited 1406 on the surface and within each recess. The piston masses may be deposited 1406 for example through the use of a photolithographic process adapted to shape a layer of material, referred to as a piston layer, such that a piston mass is deposited 1406 within each recess formed in step 1404. The piston layer may be formed from any suitable metal such as copper, Cu or aluminum Al. Alternatively, the piston layer may be formed with a multi-layer combination of low-density metal such as aluminum Al, beryllium Be, etc., and high-density metal such as tungsten W, molybdenum Mo, platinum Pt, iridium Ir, silver Ag, etc. In embodiments employing a multi-layer combination in the electrode layer 304, it may be advantageous to include thin adhesive layers of for example chromium Cr, or titanium Ti, to provide better adhesion between the layers of the multi-layer combination. [00109] In one embodiment it may be advantageous to skip the recess formation step 1404 and deposit 1406 the piston masses directly on the surface of the piezoelectric substrate.

[00110] In certain embodiments the deposited piston masses may be aligned within the slower tracks which run along each side of a main SAW track as is described above and with reference to apparatus 100. Alternatively, the piston masses may be deposited in an offset arrangement within the slower tracks as is described above and with reference to apparatus 1000

[00111] An IDT is formed 1408 over the piston masses and the surface of the substrate. The IDT may be formed with a process and materials similar to the photolithographic process and materials used to deposit the piston masses as described above. The IDT includes a plurality of electrodes configured to cover each of the one or more piston masses thereby sandwiching each piston mass between the surface of the piezoelectric substrate and the electrode.

[00112] In certain embodiments a dielectric layer by be formed 1410 over the IDT and surface of the piezoelectric substrate to provide temperature compensation and reduce changes in frequency characteristics of the SAW apparatus being produced by the process 1400. When lithium niobate is used for the piezoelectric substrate, forming 1410 the dielectric layer from a material such as silicon dioxide can provide beneficial temperature compensation. As discussed above silicon dioxide has a positive TCF that can mitigate frequency variation caused by the negative TCF of lithium niobate.

[00113] In the embodiments described above, all piston masses in an IDT have the same mass length Lp, however those skilled in the art will readily recognize that piston masses having differing lengths may be advantageously employed to form an IDT without straying from the spirit and scope of the present disclosure.

[00114] Thus, while there have been shown, described and pointed out, fundamental novel features of the invention as applied to the exemplary embodiments thereof, it will be understood that various omissions, substitutions and changes in the form and details of devices and methods illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit and scope of the presently disclosed invention. Further, it is expressly intended that all combinations of those elements, which perform substantially the same function in substantially the same way to achieve the same results, are within the scope of the invention. Moreover, it should be recognized that structures and/or elements shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.

Claims

What is claimed is:

1. An apparatus (100) comprising: a piezoelectric substrate (132) comprising a surface (130) configured to support propagation of surface acoustic waves; and an interdigital transducer (102) disposed on the surface (130), the interdigital transducer (102) comprising: a first bus (104) and a second bus (106) disposed on the surface (130), wherein the first bus (104) is parallel to the second bus (106); a first electrode (120) configured to conductively connect to the first bus (104); a second electrode (122) configured to conductively connect to the second bus (106), wherein the second electrode (122) is parallel to the first electrode (120); a first piston mass (124) adjoining a base region (134) of the first electrode (120); and a second piston mass (126) adjoining a tip region (136) of the second electrode

(122), wherein the first piston mass (124) comprises a first portion of a piston layer (302), the first portion of the piston layer (302) being disposed between the surface (130) and the first electrode (120), and the second piston mass (126) comprises a second portion of the piston layer (302), the second portion of the piston layer being disposed between the surface (130) and the second electrode (122).

2. The apparatus (100) according to claim 1 wherein the piston layer (302) comprises one or more layers of metal disposed underneath one or more of the first electrode (120) and the second electrode (122).

3. The apparatus (100) according to any one of the preceding claims wherein a mass width (Wp) of the first piston mass (124) is greater than or equal to one half of an electrode width (M) of the first electrode (120) and less than or equal to one and a half times the electrode width (M).

4. The apparatus (100) according to any one of the preceding claims wherein a mass width

(Wp2) of the second piston mass (126) is equal to the mass width (Wp) of the first piston mass (124).

5. The apparatus (100) according to any one of the preceding claims wherein a mass length (Lp) of the first piston mass (124) is equal to a mass length (Lp2) of the second piston mass (126).

6. The apparatus (100) according to any one of the preceding claims wherein the mass length

(Lp) is greater than or equal to a minimum feature size and less than or equal to a wavelength (2P) of the surface acoustic waves.

7. The apparatus (100) according to any one of the preceding claims wherein the surface (130) comprises at least one recess (502), and wherein at least one of the first piston mass (124) and the second piston mass (126) is disposed within the at least one recess (502), wherein a recess depth (Dr) is less than or equal to a thickness of the piston layer (dHp).

8. The apparatus (100) according to any one of the preceding claims wherein the first piston mass (124) is aligned with the second piston mass (126) within a slower track (110), wherein a first width of the slower track (Wst) is equal to the mass length (Lp).

9. The apparatus (100) according to any one of the preceding claims wherein one of the first piston mass (124) and the second piston mass (126) comprises a hammerhead piston mass (726), the hammerhead piston mass (726) comprising a local widening (Wh) of the electrode width (M).

10. The apparatus (100) according to any one of the preceding claims 1 through 8, wherein the apparatus (100) comprises a two-dimensional pattern of piston layer masses and a second width (Wst2) of the slower track (110)is equal to the piston mass length (Lp) plus an offset distance (Op), and wherein the first piston mass (124) is offset from the second piston mass (126) by the offset distance (Op).

11. The apparatus (100) according to claim 10 wherein the second width (Wst2) of the slower track (110) is greater than the mass length (Lp) and less than or equal to three times the mass length (3Lp).

12. The apparatus (100) according to any one of claims 10 or 11 wherein the offset distance (Op) is greater than or equal to the minimum feature size and less than or equal to the wavelength (2P).

13. The apparatus (100) according to any one of claims 10 through 12 further comprising a third piston mass (1028) disposed beneath a gap portion (1030) of the first electrode (120), wherein the third piston mass (1028) comprises a piston layer (304) disposed between the surface (130) of the piezoelectric substrate (132) and the first electrode (120).

14. The apparatus (100) according to any one of claims 10 through 12 wherein the first piston mass (1244) and the second piston mass (126) each comprise a hammerhead.

15. The apparatus (100) according to claim 14 wherein a width of the hammerhead (Wh) is greater than the electrode width (M) and less than one half the wavelength (P).

16. The apparatus (100) according to any one of claims 14 or 15 wherein the offset distance (Op) is based on the width of the hammerhead (Wh), the minimum feature size, and the wavelength (2P) of the surface acoustic waves.

17. The apparatus (100) according to any one of claims 14 through 16 wherein the third piston mass (1228) comprises a hammerhead.

18. The apparatus (100) according to any one of the preceding claims further comprising a dielectric layer (306) overlaying the surface (130) of the piezoelectric substrate (132) and the interdigital transducer (102), wherein the dielectric layer (306) has a positive coefficient of elasticity.

19. A method (1400) for fabricating a surface acoustic wave device comprising: forming (1206) one or more piston masses on a surface of a piezoelectric substrate, wherein the one or more piston masses comprises one or more layers of metal; forming (1408) an interdigital transducer (IDT) over the one or more piston masses, wherein the interdigital transducer comprises one or more electrodes, and wherein the one or more electrodes are disposed over the one or more piston masses;.

20. The method (1400) of claim 19 further comprising: forming (1404) one or more recesses in the surface; and disposing (1406) each of the one or more piston masses in a respective one of the one or more recesses. 21. The method (1400) of any one of claims 19 or 20 further comprising forming (1410) a dielectric layer over the interdigital transducer and the surface, wherein the dielectric layer has a positive coefficient of elasticity.