Surface acoustic wave device

A surface acoustic wave (SAW) device including a piezoelectric layer, a high acoustic velocity layer coupled to the piezoelectric layer, and at least one transducer. The SAW device may include a multi-layer graphene layer in the electrodes of the transducer and/or in a conductive layer that is coupled to the piezoelectric layer.

FIELD

The disclosure relates to surface acoustic wave (SAW) devices. In various examples, the disclosure relates to SAW filters, resonators, and/or duplexers with improved electromechanical coupling and higher power durability.

BACKGROUND

In communication systems (both terminal and base station infrastructure), surface acoustic wave (SAW) filters and resonators are widely used. For the next generation wireless communication of 5G new radio (NR), there is growing demand for higher operating frequency, lower Insertion loss, higher transmitting power, and/or wider channel bandwidth. New piezoelectric materials or structures are desirable to support the growing demand for higher frequency and wider channel bandwidth. A high electromechanical coupling coefficient for SAW filters is typically desirable. As the transmitting power increases, the durability of the SAW device deteriorates. Therefore, it may be useful to increase the durability for a higher operating frequency and higher transmitting power.

Scandium-doped aluminum nitride (ScAlN) films have been attempted due to their high piezoelectricity, high thermal conductivity, and relatively high acoustic wave velocity. One such structure is described in “High Q surface acoustic wave resonators in 2-3 GHz range using ScAlN-single crystalline diamond structure” by Hashimoto et al. (Conf.: Ultrasonics Symposium (IUS), 2012 IEEE International), herein expressly incorporated by reference in its entirety.

Similarly, another such structure is described in “Surface acoustic wave propagation characteristics of ScAlN/diamond structure with buried electrode” by Zhang et al. (Piezoelectricity Acoustic Waves and Device Applications (SPAWDA) 2014 Symposium on, pp. 271-274, 2014), herein expressly incorporated by reference in its entirety.

These structures have been explored but have not been adequate to address the specific challenges as described herein.

SUMMARY

In various examples described herein, the incorporation of a multi-layer graphene layer in a SAW device results in high effective electromechanical coupling coefficient, high operating frequency and high power durability.

In some aspects, the present disclosure describes a surface acoustic wave (SAW) device. The SAW device includes a piezoelectric layer, a high acoustic velocity layer coupled to the piezoelectric layer at a first surface of the piezoelectric layer, and at least one transducer between the piezoelectric layer and the high acoustic velocity layer. The at least one transducer includes a first multi-layer graphene layer. The at least one transducer is configured to propagate a surface acoustic wave having an operating wavelength (λ) along the piezoelectric layer.

In any of the preceding aspects/embodiments, the first multi-layer graphene layer may include 2-10 atomic layers of graphene.

In any of the preceding aspects/embodiments, the first multi-layer graphene layer may include 3-5 atomic layers of graphene.

In any of the preceding aspects/embodiments, the SAW may include a conductive layer coupled to a second surface of the piezoelectric layer, opposing the first surface of the piezoelectric layer.

In any of the preceding aspects/embodiments, the conductive layer may be a second multi-layer graphene layer coupled to a metal layer.

In any of the preceding aspects/embodiments, the second multi-layer graphene layer may be 3-10 atomic layers of graphene.

In any of the preceding aspects/embodiments, the conductive layer may be a second multi-layer graphene layer.

In any of the preceding aspects/embodiments, the second multi-layer graphene layer may be 3-10 atomic layers of graphene.

In any of the preceding aspects/embodiments, the transducer may include the first multi-layer graphene layer coupled to a metal layer.

In any of the preceding aspects/embodiments, the first multi-layer graphene layer may be 3-10 atomic layers of graphene.

In any of the preceding aspects/embodiments, the at least one transducer may be embedded in the piezoelectric layer.

In any of the preceding aspects/embodiments, the at least one transducer may be embedded in the high acoustic velocity layer.

In some aspects, the present disclosure describes a surface acoustic wave (SAW) device. The SAW device includes a piezoelectric layer, and a high acoustic velocity layer coupled to the piezoelectric layer at a first surface of the piezoelectric layer. The piezoelectric layer and the high acoustic velocity layer are coupled to each other via a conductive layer. The SAW device also includes at least one transducer coupled to a second surface of the piezoelectric layer, opposing the first surface of the piezoelectric layer. The at least one transducer includes a first multi-layer graphene layer coupled to a metal layer. The at least one transducer is configured to propagate a surface acoustic wave having an operating wavelength (λ) along the piezoelectric layer.

In any of the preceding aspects/embodiments, the conductive layer may be a second multi-layer graphene layer.

In any of the preceding aspects/embodiments, the second multi-layer graphene layer may include 3-10 atomic layers of graphene.

In any of the preceding aspects/embodiments, the first multi-layer graphene layer may include 3-10 atomic layers of graphene.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Within the 5G new radio frequency spectrum, 1-6 GHz offers a good mixture of coverage and capacity for 5G services, and the 3.3-3.8 GHz range provides a solid basis for initial commercial 5G services. Especially the 3.4-3.6 GHz range drives the economies of scale needed for low-cost devices. To obtain the wideband SAW filter at the 3.4-3.6 GHz range (f0=3.5 GHz, BWdb=200 MHz and a relative bandwidth of 5.7%) for 5G service deployment, a piezoelectric structure with high electromechanical coupling coefficient k2(e.g., k2>17%) is desired. Although some ScAlN/diamond multilayer structures may exhibit sufficient electromechanical coupling (e.g., having k2>17%), the coupling coefficient of the structure typically decreases with an increase in the thickness of the metal electrode, resulting in lower electromechanical coupling in actual application. Although a ScAlN/diamond structure improves over traditional SAW filters in terms of device power handling capability due to the superior thermal conductivity of diamond layer, further improvement of device power handling capability is needed in SAW filters for RF front-end applications in the 5G New Radio base stations.

In various examples, the present disclosure describes SAW devices that maintain high electromechanical coupling coefficient and provide power durability. Examples of the disclosed SAW devices use multi-layer graphene layers to reduce stresses in the electrodes and to maintain thinness of the electrodes.

An example of a physical implementation of a surface acoustic wave (SAW) device100, such as for example a wideband transversal filter, is shown inFIG. 1. The SAW device may be intended for use at the front-end of a radio-frequency (RF) transmitter/receiver able to handle high power, such as greater than 32 dBm. Although the description below makes reference to a particular SAW device100, the techniques described herein may equally apply to other types of SAW filters and/or resonators. For example, any SAW transducer, interdigital transducer (IDT), Inter-digitated Inter-digital transducer (IIDT), ladder-type filter, or other such devices may incorporate the multi-layer structure(s) described herein.

The SAW device100, which in this example embodiment only shows a basic filter for clarity and illustrative purposes, may include a multi-layered body102(as discussed further below) with an input transducer104and an output transducer106, each indicated by their respective dashed box. In this example, an upper conductive layer202and a piezoelectric layer204of the body102have been partially cut away to show the transducers104,106. The input transducer104and the output transducer106may be coupled to the body102. The input transducer104and the output transducer106each comprise a plurality of electrically conductive interdigital transducer (IDT) electrodes108. The IDT electrodes108may be generally parallel to each other within their respective transducer104/106and each of the fingers of each IDT electrode108may be spaced by half of an operating wavelength (λ/2). The IDT electrodes108may be electrically coupled to lead-out bonding pad110for either supplying power to the IDT electrodes108by way of input terminals112(in the case of the input transducer104) or receiving power by way of output terminals114from the IDT electrodes108(in the case of the output transducer106). The IDT electrodes108may have an IDT thickness between approximately 0λ and approximately 0.10λ, for example between about 0.01λ and about 0.10λ, for example about 0.02λ or about 0.08λ. In the present disclosure, a layer or material may be described as having a thickness of approximately 0λ to indicate that the layer or material provides electric conduction but no mechanical mass-loading effect (e.g., for simulation purposes); a thickness of approximately 0λ does not indicate that the layer or material is omitted. For example, in various cases a thickness of approximately 0λ may be achieved using a material having a thickness on the order of tens of atoms, such as a multi-layer graphene material.

When power is supplied to input terminals112, the input transducer104converts the electrical signal energy into a SAW (e.g. transduction) propagating along the body102. The SAW may be carried across the body102and the SAW may be converted back into an electrical signal by the output transducer106. The electrical signal may then be provided at the output terminals114, for example to be received by a processor (not shown) via an analog-to-digital converter (not shown). The center frequency of a SAW filter may be given by the velocity of the SAW divided by the operating wavelength.

FIG. 2illustrates a side cross-sectional view, along line A-A inFIG. 1, of an example configuration of the SAW device100. The dimensions of certain features have been exaggerated for illustration purposes. The body102of the SAW device100includes a high acoustic velocity layer206(e.g., a diamond layer), a piezoelectric layer204(e.g., a ScAlN film) and a conductive layer202(which may also be referred to as a short layer or a short surface). The input transducer104and the output transducer106each comprise electrodes108. The transducers104,106are positioned between the piezoelectric layer204and the high acoustic velocity layer206, and configured to propagate a surface acoustic wave along the piezoelectric layer204, at an operating frequency f0.

In various examples described herein, the layers of the body102may be arranged in different orders. The electrodes108may be positioned between various pairs of the layers of the body102, and may be embedded in different layers of the body102. In this example the electrodes108may be embedded in the piezoelectric layer204; in other examples, the electrodes108may be embedded in the high acoustic velocity layer206.

The electrodes108in this example may be made of a multi-layer graphene layer, for example multi-layer graphene having 2-10 atomic layers, for example 3-5 atomic layers. Simulations have found that using such a multi-layer graphene layer for the electrodes108achieves the highest electromechanical coupling coefficient, in the configuration ofFIG. 2. Generally, for a metal with thickness less than 50 nm, the thinner the material the greater the Ohmic loss, particularly at high operating frequency; this is not the case for graphene.

The conductive layer202in this example may be a metal layer202acoupled to another multi-layer graphene layer202b. The metal layer202amay be copper (Cu), and the multi-layer graphene layer202bmay have 2-10 atomic layers, for example 3-5 atomic layers. In this example, the transducers104,106are considered to be coupled to a first surface of the piezoelectric layer204and the conductive layer202is considered to be coupled to a second surface of the piezoelectric layer204, opposing the first surface.

In an example implementation, the configuration shown inFIG. 2may have an operating frequency of 2.1 GHz, and an operating wavelength of λ=3.85 μm. The thickness of the piezoelectric layer204(e.g., ScAlN layer) may be about 0.2λ (0.77 μm), and the thickness of the high acoustic velocity layer206(e.g., diamond layer) may be about 20 μm. In the conductive layer202, the metal layer202a(e.g., copper layer) may have a thickness of about 0.01λ (38.5 nm). The electrodes108made of the multi-layer graphene layer may have a thickness of essentially 0λ. The multi-layer graphene layer202bof the conductive layer202may also have a thickness of essentially 0λ.

The use of a multi-layer graphene layer for the electrodes108may provide high effective electromechanical coupling coefficient even at very high operating frequency (e.g., up to 10 GHz), because the multi-layer graphene layer is highly conductive and has very small thickness.

Using a combination of metal and multi-layer graphene for the conductive layer202may provide higher power durability and/or may help to achieve a desired high electromechanical coupling coefficient. The use of 2-10 atomic layers for the multi-layer graphene layer202bhas been found to impart high power durability to the SAW device100, with better performance than when fewer atomic layers of graphene are used.FIG. 3Ais a chart showing the von Mises stress in the conductive layer202awhen different numbers of atomic layers are used for the multi-layer graphene layer202bin the conductive layer202, and for the electrodes108.FIG. 3Bis a chart showing the von Mises stress in the electrodes108when different numbers of atomic layers are used for the multi-layer graphene layer202bin the conductive layer202, and for the electrodes108. InFIGS. 3A and 3B, the thicknesses of the multi-layer graphene layer202band the electrodes108were varied together (i.e., x-axis values reflect the number of atomic layers of graphene in the graphene layer202band the same number of atomic layers of graphene in the electrodes108).FIGS. 3A and 3Bshow results of simulations performed for the configuration shown inFIG. 2, based on the example dimensions provided above and varying the thicknesses of the multi-layer graphene layer202band of the electrodes108. Further,FIG. 3Ashows simulation results for different configurations of the conductive layer202, specifically by coupling the graphene layer202bon top of the metal layer202a(graphene/Cu); by coupling the metal layer202aon top of the graphene layer202b(Cu/graphene); and by sandwiching the metal layer202abetween two graphene layers202b(graphene/Cu/graphene).

As shown inFIG. 3A, reduction in von Mises stress, compared with the case where no graphene layer is present (i.e., # of graphene layers is 0), occurred only for the conductive layer configuration where the graphene layer202bis coupled on top of the metal layer202a(graphene/Cu), as illustrated inFIG. 2. Interestingly, simulations have found that the graphene/Cu/graphene conductive layer configuration exhibited the most degradation of electromechanical coupling coefficient with increasing thickness of graphene, among the three conductive layer configurations, and also exhibited the highest von Mises stress among the three conductive layer configurations.

Thus, the following discussion ofFIGS. 3A and 3Bwill focus only on the graphene/Cu configuration for the conductive layer202.

As shown inFIGS. 3A and 3B, using monolayer graphene for the graphene layer202band for the electrodes108was found to result in higher von Mises stress in the conductive layer202and the electrodes108, and using 2-10 atomic layers for the graphene layer202band for the electrodes108was found to result in tower von Mises stress in the conductive layer202and the electrodes108. Further, using 3-5 atomic layers for the graphene layer202band for the electrodes108was found to result in even lower von Mises stress in the conductive layer202and the electrodes108. Thus, simply using the thinnest possible graphene (e.g., monolayer graphene) may not be desirable.FIGS. 3A and 3Balso show that using the 2-10 atomic layer graphene layer202bover the metal layer202aresulted in lower von Mises stress in the conductive layer202and the electrodes108, compared to when no graphene layer202bis used (i.e., when the number of atomic layers of graphene is 0). At greater than 10 atomic layers, the multi-layer graphene begins to behave like graphite and loses the properties of graphene.

Simulations also found that the electromechanical coupling coefficient is expected to reach a maximum (k2>17.5%) when the example SAW device100ofFIG. 2is operating in the second mode (i.e., Sezawa mode) and when the metal layer202a(e.g., a copper layer) has a thickness of about 0.01λ.

Simulations have also found that, in the configuration shown inFIG. 2, the acoustic velocity increases (and hence results in higher SAW device operating frequency) when the electrode108thickness decreases, when the metal layer202athickness of the conductive layer202decreases, and when the number of atomic layers of the graphene layer202bin the conductive layer202increases.

Balancing the various considerations discussed above, including factoring in trade-offs between higher acoustic velocity and higher electromechanical coupling coefficient, for example, it has been found that using an electrode108of essentially 0λ thickness (e.g., using 2-10 atomic layer or 3-5 atomic layer graphene for the electrode108), using a metal layer202aof 0.01λ (e.g., a copper layer having thickness of 0.01λ) and using 2-10 atomic layer or 3-5 atomic layer graphene layer202bover the metal layer202a, achieved a SAW device100with the desired performance.

FIG. 4illustrates a side cross-sectional view of another example configuration of the SAW device100. The dimensions of certain features have been exaggerated for illustration purposes. The body102of the SAW device100includes the high acoustic velocity layer206(e.g., a diamond layer), the piezoelectric layer204(e.g., a ScAlN film) and the conductive layer202. Unlike the example configuration ofFIG. 2, the configuration illustrated in the example ofFIG. 4positions the conductive layer202between the high acoustic velocity layer206and the piezoelectric layer204, and positions the transducers104,106on top of the piezoelectric layer204. In this example, the conductive layer202is considered to be coupled to a first surface of the piezoelectric layer204and the transducers104,106are considered to be coupled to a second surface of the piezoelectric layer204, opposing the first surface.

In this example, the conductive layer202may be a multi-layer graphene, for example having 3-10 atomic layers. The electrodes108of the transducers104,106in this example may include a metal layer108acoupled to another multi-layer graphene layer108b. The metal layer108amay be copper (Cu), and the multi-layer graphene layer108bmay have 3-10 atomic layers.

In an example implementation, the configuration shown inFIG. 4may have an operating frequency of 2.1 GHz, and an operating wavelength of λ=2.63 μm. The thickness of the piezoelectric layer204(e.g., ScAlN layer) may be about 0.8λ (2.11 μm), and the thickness of the high acoustic velocity layer206(e.g., diamond layer) may be about 10 μm. The conductive layer202(e.g., multi-layer graphene) may have a thickness of 3-10 atomic layers (thickness of essentially 0λ). In the electrodes108, the metal layer108a(e.g., copper layer) may have a thickness of about 0.08λ (0.21 μm) and the multi-layer graphene layer108bmay have a thickness of 3-10 atomic layers (thickness of essentially 0λ).

The configuration in the example ofFIG. 4may be suitable for wave propagation in the Sezawa mode.FIG. 5Ashows simulation results, based on the example dimensions provided above and varying the thickness of the metal layer108aof the electrodes108. As illustrated inFIG. 5A, simulations have found that a maximum electromechanical coupling coefficient may be achieved when the thickness of the electrodes108is about 0.08λ, rather than being as thin as possible. Therefore, unlike the example configuration ofFIG. 2, using graphene alone for the electrodes108may not be desirable.FIG. 5Bshows simulation results, based on the example dimensions provided above and varying the number of atomic layers in the multi-layer graphene layer108bof the electrodes108and in the conductive layer202, and for different arrangement of layers for the electrodes108. Specifically,FIG. 5Bcompare the performance of electrodes108formed by coupling the graphene layer108bon top of the metal layer108a(graphene/Cu); by coupling the metal layer108aon top of the graphene layer108b(Cu/graphene); and by sandwiching the metal layer108abetween two graphene layers108b(graphene/Cu/graphene). InFIG. 5B, the thicknesses of the multi-layer graphene layer108band the conductive layer202were varied together.

As shown inFIG. 5B, reduction in von Mises stress, compared with the case where no graphene layer is present (i.e., # of graphene layers is 0), occurred only for the electrode configuration where the graphene layer108bis coupled on top of the metal layer108a(graphene/Cu). This is the configuration illustrated inFIG. 4. Interestingly, the electrode configuration with the metal layer108asandwiched between two graphene layers108b(graphene/Cu/graphene) exhibited the highest von Mises stress among the three electrode configurations.

As shown inFIG. 5B, regardless of the configuration of the electrodes108, when the number of atomic layers of graphene is only one, the von Mises stress in the electrodes108was found to be highest. Thus, simply using the thinnest possible graphene in the electrodes108and in the conductive layer202may not be desirable. Using 3-10 atomic layers for the graphene layer108band for the conductive layer202was found to result in lower von Mises stress in the electrodes108than when no graphene layer108bwas used (i.e., when the number of atomic layers of graphene is 0), thus enhancing the high power durability of the electrodes108.

These simulation results indicate that the selection of an appropriate number of atomic layers for the graphene layer108band how the graphene layer108bis coupled to the metal layer108ato form the electrode108are non-trivial considerations when designing the example SAW device100ofFIG. 4.

Simulations also found that using 3-10 atomic layer graphene for the multi-layer graphene layer108band for the conductive layer202between the piezoelectric layer204and the high velocity acoustic layer206supported achievement of the maximum electromechanical coupling coefficient.

FIG. 6illustrates a side cross-sectional view of another example configuration of the SAW device100. The dimensions of certain features have been exaggerated for illustration purposes. The body102of the SAW device100includes the high acoustic velocity layer206(e.g., a diamond layer), the piezoelectric layer204(e.g., a ScAlN film) and the conductive layer202. In this example, the electrodes108are positioned between the piezoelectric layer204and the high acoustic velocity layer206, with the conductive layer202coupled to the piezoelectric layer204, similar to the configuration ofFIG. 2. Unlike the example shown inFIG. 2, the transducers104,106in the example ofFIG. 6are embedded in the high acoustic velocity layer206, the electrodes108of the transducers104,106include a metal layer108a(e.g., copper) coupled to a multi-layer graphene layer108b(for example having 3-10 atomic layers, similar to the electrodes108in the example ofFIG. 4), and the conductive layer202includes only the multi-layer graphene, for example having 3-10 atomic layers.

In this example, the transducers104,106are considered to be coupled to a first surface of the piezoelectric layer204and the conductive layer202is considered to be coupled to a second surface of the piezoelectric layer204, opposing the first surface.

In an example implementation, the configuration shown inFIG. 6may have an operating frequency of 2.1 GHz, and an operating wavelength of λ=3.76 μm. The thickness of the piezoelectric layer204(e.g., ScAlN layer) may be about 0.2λ (0.77 μm), and the thickness of the high acoustic velocity layer206(e.g., diamond layer) may be about 10 μm. In the conductive layer202(e.g., multi-layer graphene) may have a thickness of 3-10 atomic layers (thickness of essentially 0λ). In the electrodes108, the metal layer108a(e.g., copper layer) may have a thickness of about 0.01λ (38.5 nm) and the multi-layer graphene layer108bmay have a thickness of 3-10 atomic layers (thickness of essentially 0λ).

FIG. 7Ashows simulation results, based on the example dimensions provided above and varying the number of atomic layers in the multi-layer graphene layer108bof the electrodes108and in the conductive layer202. InFIG. 7A, the thicknesses of the multi-layer graphene layer108band the conductive layer202were varied together. As shown inFIG. 7A, using single-layer or bilayer graphene for the graphene layer108band the conductive layer202was found to result in higher von Mises stress in the electrodes108. Thus, simply using the thinnest possible graphene layer108b(e.g., monolayer graphene) may not be desirable. Using 3-10 atomic layers for the graphene layer108band conductive layer202was found to result in lower von Mises stress in the electrodes108than when no graphene layer108bwas used (i.e., when the number of atomic layers of graphene is 0), thus enhancing the high power durability of the electrodes108.

FIG. 7Bshows simulation results comparing the von Mises stress in the electrodes108for different positioning of the electrodes108. As shown inFIG. 7B, when the electrodes108have only the metal layer108aand are embedded in the piezoelectric layer204, the von Mises stress in the electrodes108is higher than when the metal-only electrodes108are embedded in the high acoustic velocity layer206. Further,FIG. 7Bshows that even lower von Mises stress in the electrodes108can be achieved by coupling a multi-layer graphene layer108bto the metal layer108ato form the electrodes108. These results illustrate some possible benefits of coupling the multi-layer graphene layer108bto the metal layer108ain the electrodes108, namely reduced von Mises stress in the electrodes108(hence longer time-to-failure). This may be because the multi-layer graphene layer108bacts as a metal diffusion barrier, reducing acoustomigration (e.g., due to voids and hillocks in the metal layer108a) and hence reducing stress in the electrodes108and enhancing the power durability of the SAW device100. Embedding the electrodes108in the high acoustic velocity layer206with high thermal conductivity may also be beneficial by providing a good thermal path for heat escape from the electrodes108, which also may hence power durability of the SAW device100. Although not shown inFIG. 78, the von Mises stress in the electrodes108may also be suitably low where the electrodes108include the multi-layer graphene layer108bcoupled to the metal layer108a, and the electrodes108aare embedded in the piezoelectric layer204.

Simulations also found that the phase velocity of the SAW device100increases with increasing number of atomic layers in the multi-layer graphene layer108bof the electrodes108, and that the electromechanical coupling coefficient increases with decreasing number of atomic layers in the multi-layer graphene layer108bof the electrodes108. Thus, selection of the appropriate number of atomic layers for the multi-layer graphene layer108b, and placement of the electrodes108, involves a consideration of various trade-offs and is not a trivial or straightforward matter.

FIG. 8illustrates a side cross-sectional view of another example configuration of the SAW device100. The dimensions of certain features have been exaggerated for illustration purposes. The body102of the SAW device100includes a high acoustic velocity layer206(e.g., a diamond layer) and a piezoelectric layer204(e.g., a ScAlN film). The electrodes108of the transducers104,106are positioned between the piezoelectric layer204and the high acoustic velocity layer206. The configuration ofFIG. 8is similar to the example configuration ofFIG. 2; however, the conductive layer202is omitted. This configuration may be suitable for wave propagation in the Rayleigh mode. In this example the transducers104,106may be embedded in the piezoelectric layer204. The electrodes108in this example may be made of a multi-layer graphene layer, for example multi-layer graphene having 3-10 atomic layers.

In an example implementation, the configuration shown inFIG. 8may have an operating frequency of 10 GHz, and an operating wavelength of λ=0.46 μm. The thickness of the piezoelectric layer204(e.g., ScAlN layer) may be about 0.45λ (0.21 μm), and the thickness of the high acoustic velocity layer206(e.g., diamond layer) may be about 5 μm. The electrodes108may have 3-10 atomic layers of graphene (thickness of essentially 0λ).

Simulations have found that using the highly electrically conductive multi-layer graphene as electrodes108resulted in high effective electromechanical coupling coefficient and high acoustic velocity in the SAW device100. Compared with other SAW devices having similar layer arrangement but using metal instead of graphene for the electrodes108, the example configuration ofFIG. 8may enable higher operating frequency (e.g., 10 GHz) and at the same time keep Ohmic losses low (and hence maintaining a high electromechanical coupling coefficient) by keeping the thickness of the electrodes108low.

As noted above, the example configuration ofFIG. 8may be suitable for Rayleigh mode wave propagation, whereas the example configurations ofFIGS. 2, 4 and 6may be suitable for Sezawa mode wave propagation. Thus, different configurations of the SAW device100may be selected depending on the desired application.

The example SAW devices100disclosed herein may be fabricated using any suitable fabrication techniques.FIGS. 9 and 10are flowcharts illustrating example fabrication methods900,1000that may be used.

The example method900illustrated inFIG. 9may be used for fabricating the example SAW devices100ofFIGS. 2, 6 and 8, for example, depending on which steps are performed and which are omitted.

At902, the high acoustic velocity layer206is provided. For example, a diamond layer may be provided with suitable thickness (typically several times thicker than λ) using chemical vapor deposition (or other suitable technique). The diamond layer may also be polished and cleaned.

If the electrodes108are to be embedded in the high acoustic velocity layer206(e.g., in the example ofFIG. 6), then at904the high acoustic velocity layer206is etched with a mask for patterning the electrodes108. Otherwise,904is omitted.

If the electrodes108include a metal layer108acoupled to a multi-layer graphene layer108b(e.g., in the example ofFIG. 6), then at906the metal layer is formed to the desired thickness, for example using metal deposition techniques. If904was omitted, the metal layer108amay be patterned with a mask.

At908, the multi-layer graphene is formed, either coupled to the metal layer108a(e.g., in the example ofFIG. 6) or directly on the high acoustic velocity layer206(e.g., in the examples ofFIGS. 2 and 8). This may be by epitaxial graphene growth to the desired number of atomic layers over the high acoustic velocity layer206, followed by patterning (e.g., using electron-beam nano-etching).

At910, the piezoelectric layer204is formed. For example, this may be by sputtering or chemical vapor deposition of ScAlN to the desired thickness.

If a conductive layer202is required (e.g., in the examples ofFIGS. 2 and 6), then at912the conductive layer202is formed by growth of the multi-layer graphene layer (e.g., in the example ofFIG. 6) or by deposition of the metal layer202afollowed by growth of the multi-layer graphene layer202b(e.g., in the example ofFIG. 2).

Thus, the example ofFIG. 2may be obtained using the fabrication steps902,908,910and912; the example ofFIG. 6may be obtained using the fabrication steps902,904,906,908,910and912; and the example ofFIG. 8may be obtained using the fabrication steps902,908and910.

The example ofFIG. 4may be obtained using the example method1000illustrated inFIG. 10.

At1002, the high acoustic velocity layer206is provided. For example, a diamond layer may be provided with suitable thickness (typically several times thicker than λ) using chemical vapor deposition (or other suitable technique). The diamond layer may also be polished and cleaned.

If a conductive layer202is required (e.g., in the example ofFIG. 4), then at1004the conductive layer202is formed by growth of the multi-layer graphene layer (e.g., in the example ofFIG. 4). The conductive layer202may also be formed by deposition of a metal layer, with or without growth of a multi-layer graphene layer on top.

At1006, the piezoelectric layer204is formed. For example, this may be by sputtering or chemical vapor deposition of ScAlN to the desired thickness.

If the electrodes108include a metal layer108acoupled to a multi-layer graphene layer108b(e.g., in the example ofFIG. 4), then at1008the metal layer is formed to the desired thickness, for example using metal deposition techniques. In some examples, where the electrodes108are embedded in the piezoelectric layer204, formation of the electrodes108may be preceded by etching of the piezoelectric layer204.

At1010, the multi-layer graphene layer108bof the electrodes108is formed. This may be by epitaxial graphene growth to the desired number of atomic layers over the metal layer108aformed at1008, followed by patterning (e.g., using electron-beam nano-etching).

Thus, the example ofFIG. 4may be obtained using the fabrication steps1002,1004,1006,1008and1010.

In various examples, the metal layer used in the graphene/metal electrodes or the graphene/metal conductive layer has been described as being copper (Cu). Other metals may also be suitable, such as aluminum (Al), platinum (Pt), and/or Aluminum-copper alloys (Al/Cu/Al), molybdenum (Mo), tungsten (W), titanium (Ti), gold (Au), nickel (Ni) and titanium nitride (TiN), and silver (Ag), cobalt (Co), chromium (Cr), copper-iron alloy (Cu—Fe), niobium (Nb), nickel (Ni), zinc (Zn), zirconium (Zr), and/or alloys comprising any number of these metals.

In various examples, the piezoelectric layer has been described as being a ScAlN layer. Other materials may also be used for the piezoelectric layer, such as aluminum nitride (AlN), zinc oxide (ZnO) and other piezoelectric materials.

In various examples, the high acoustic velocity layer has been described as being a diamond layer. Other materials may also be used for the high acoustic velocity layer, such as silicon carbide (SiC), or other substrate materials.

Different configurations of the SAW device has been disclosed herein. A particular configuration may be selected for use depending on the specific application. For example, for a lower operating frequency, the electrodes may be designed as metal layer coupled with multi-layer graphene layer, whereas the electrodes may be designed as only graphene for a higher operating frequency. However, if the SAW device is intended only for generating waves and not reflecting acoustic waves, then the electrodes may be designed as only graphene even for lower operating frequencies.

In various examples disclosed herein, a SAW device is provided that may achieve a high effective electromechanical coupling coefficient (k2>17.5%) and high operating frequency (2.1 GHz or higher), at the same time providing high power durability, through the use of multi-layer graphene as the electrode and/or as the conductive layer. The use of a multi-layer graphene layer, with selected number of atomic layers (e.g., 3-10 atomic layers) may solve performance degradation issues of prior SAW devices while achieving the desired high effective electromechanical coupling coefficient and high operating frequency.

Achieving a high electromechanical coupling coefficient enables realization of wide-band SAW filters, such as a filter for E-UTRA band42with relative bandwidth of 5.7%. As well, a SAW filter with high electromechanical coupling coefficient may also be used to replace film bulk acoustic resonator (FBAR) filters at terminals, which may lead to cost savings because SAW filters tend to be lower cost than FBAR fitters.

Achieving high phase velocity in a SAW device may also enable realization of high frequency (e.g., 10 GHz) SAW filters.

The disclosed SAW devices may be useful for implementation as SAW filters in mobile terminals, base station and other infrastructure equipment.

The present disclosure may be embodied in other specific forms without departing from the subject matter of the claims. The described example embodiments are to be considered in all respects as being only illustrative and not restrictive. Selected features from one or more of the above-described embodiments may be combined to create alternative embodiments not explicitly described, features suitable for such combinations being understood within the scope of this disclosure. For examples, although specific sizes and shapes of IDT electrodes108are disclosed herein, other sizes and shapes may be used. In another example, although a particular SAW device100(e.g. filter) may be described herein, the structures described may be adapted to other SAW device configurations.

The thicknesses of each of the layers described herein are meant to be illustrative and not restrictive. The figures may exaggerate or minimize the height of these layers for illustrative purposes and/or for ease of reference.

Although the example embodiments may be described with reference to a particular orientation (e.g. top and base), this was simply used as a matter of convenience and ease of reference in describing the figures.