TECHNICAL FIELD

The present invention relates generally to the field of acoustic wave devices, and in particular to a surface acoustic wave (SAW) device. The present invention presents a SAW device including an acoustic bandgap structure, and further presents a method for fabricating such a SAW device.

BACKGROUND

Acoustic wave devices, particularly SAW devices, are key components of modem electronic circuits. For instance, achieving a high frequency selectivity, while maintaining low electronic insertion loss, requires high quality factor (Q factor) mechanical SAW resonators coupled in a filter topology.

SAW devices are generally configured to couple an electrical, time varying signal to mechanical waves traveling on the surface of a piezoelectric material. While the excited waves trigger the required modes of vibration in the piezoelectric material, vibrational energy leakage is also triggered in a substrate of the SAW device, on which substrate the piezoelectric material is provided. This negatively impacts the Q factor of a SAW resonator, and therefore also the insertion loss of the filter.

Conventional SAW devices/resonators are produced on a piezoelectric layer, with alternating metal interdigital transducer (IDT) electrodes on the surface of the piezoelectric layer, which acts as a resonant cavity. In order to reduce the lateral leakage, one conventional approach is to use periodic metal reflectors as the IDT electrodes, which are added on each side of the main resonator section, to realize a Bragg mirror like structure for confining the vibrational energy.

In particular, conventional approaches include the following:

• Bragg mirrors arranged in the horizontal direction (for a lateral confinement; i.e. in the X-direction). However, such Bragg mirrors do not prevent leakage to the substrate of the SAW device (i.e. they provide energy confinement only in the lateral direction, but no control of spurious modes and leakage to the substrate is achieved). • Bragg mirrors arranged in the vertical direction (for a vertical confinement; i.e. in the Z- direction). However, such Bragg mirrors significantly increase the costs and manufacturing complexity for producing precisely-defined (i.e. in thickness) material depositions.

• The use of an advanced material stack to produce a“wave-guide” structure by increasing the internal reflection (rather than radiating to the bulk). However, this approach is not always applicable and easy to produce.

SUMMARY

In view of the above-mentioned problems and disadvantages of the conventional approaches, an object of the embodiments of the present invention is to improve the conventional SAW devices.

In particular, an objective is to provide a SAW device with an improved confinement of vibrational energy in the piezoelectric layer. That is, particularly vibrational leakage to a substrate is to be suppressed. A goal thereby is to reach the same reflection bandwidth as is achieved with Bragg mirror approaches, while requiring less repeated layers in the thickness direction of the device (Z-direction). In addition, Bragg layers disposed periodically in the thickness direction are inherently sensitive to thickness variations due to process tolerances, requiring a tight process control and deposition rate of material. Thus a further goal is to provide a solution, which reduces manufacturing costs. Further, the possibility of confining modes in the transverse direction (Y-direction) is desired. Additionally, enabling temperature control is another goal.

The objective is achieved by the embodiments provided by the enclosed independent claims. Advantageous implementations of the embodiments of the present invention are further defined in the dependent claims.

In particular, it is proposed to reduce the vibrational leakage to the substrate stack of a SAW device, by creating a periodic replication of alternating low acoustic impedance and high acoustic impedance material structures. The structures are embedded in the substrate stack, and can effectively create an acoustic mirror for any vertical bulk or shear wave. A first aspect of the invention provides a SAW device, comprising a substrate stack, a piezoelectric layer provided on the substrate stack and configured to propagate a SAW, wherein dielectric and/or semiconductor elements are embedded in the substrate stack in a periodic arrangement, the embedded elements having an acoustic impedance different than the surrounding material of the substrate stack.

In the SAW device of the first aspect, the embedded dielectric and/or semiconductor elements effectively suppress vibrational energy leakage to the substrate stack. The SAW device reaches the same reflection bandwidth as with a Bragg mirror approach, but requires less repeated layers in the thickness direction of the device. The embedded material may form an acoustic band-gap structure below the piezoelectric layer, which acts as acoustic resonator. Thus, no mass loading of the piezoelectric layer is required. Mass loading is defined as any material section provided on top of the piezoelectric layer, which will inevitably affect the SAWs in the piezoelectric layer (i.e. the acoustic resonator) and adds a new path for energy loss.

The SAW device of the first aspect also enables compensating for temperature variations through the use of distinctive embedded materials in the substrate stack in a ID or 2D pattern. The temperature compensation comes naturally, due to the different temperature coefficient of frequency (TCF) of the different materials used in the substrate stack. Materials with different TCF will expand/contract differently. The embedded material may be selected such that it produces a net zero-stress/strain in the substrate stack as the temperature varies.

The SAW device may further comprise an upper bus bar arranged on the piezoelectric layer.

In an implementation form of the first aspect, at least one of the vertical and horizontal dimensions of the embedded elements are a fraction of the acoustic wavelength propagating in the substrate stack at the working frequency of the SAW device. The vertical dimension may be a direction perpendicular to a top surface of the SAW device. The horizontal dimension may include a dimension along a longitudinal and/or transverse direction of the SAW device.

In this way, the embedded material effectively suppresses vibrational leakage, and thus the vibrational energy is better confined in the piezoelectric layer. Furthermore, periodicity in the transverse direction helps acoustic decoupling of nearby structures and gives additional control on the confinement of the vibrational energy in the wanted piezoelectric layer.

In an implementation form of the first aspect, the embedded elements form a phononic crystal in the substrate stack.

By forming a phononic crystal, phononic excitations in at least one frequency band can be most effectively suppressed, i.e. by providing at least one phononic bandgap (i.e. a region in the frequency band where there is no acoustic mode available). When considering the dispersion diagram of such structures, the phase velocity of the operating SAW mode and the acoustic bandgap formed by the phononic crystal is then designed to be located bellow the slowest bulk- wave velocity, essentially supressing any leakage of bulk waves into the substrate.

In an implementation form of the first aspect, the phononic crystal in the substrate stack has a bandgap centered around an operating frequency of the SAW device.

Thus, the phononic crystal is optimally adapted to the piezoelectric layer/resonator.

In an implementation form of the first aspect, the substrate stack is bonded via a transfer layer to the piezoelectric layer.

Such bonding is the result of a simple process of producing the SAW device.

In an implementation form of the first aspect, the dielectric and/or semiconductor elements are embedded in a substrate layer of the substrate stack.

In an implementation form of the first aspect, the substrate layer comprises a semiconductor material, and/or the embedded elements comprise a dielectric material.

In an implementation form of the first aspect, the dielectric and/or semiconductor elements are embedded in the transfer layer of the substrate stack, and the transfer layer is provided on a substrate layer of the substrate stack. In an implementation form of the first aspect, the transfer layer comprises an oxide, and/or the embedded elements comprise a dielectric material.

In an implementation form of the first aspect, the embedded elements include a plurality of vertically arranged material strips.

In particular, it is possible in this way to provide specific 2D or ID patterns of embedded material, in order to provide acoustic bandgap structures embedded in the substrate stack that are effective and tailored to the SAW resonator.

In an implementation form of the first aspect, the embedded elements include a plurality of horizontally and vertically arranged material islands.

In particular, the material islands may have unit lengths in the order/fraction of the acoustic wavelength at the frequency of the SAW device, in order to provide effective suppression of leakage.

In an implementation form of the first aspect, the embedded elements have different electrical and/or optical properties than the surrounding material of the substrate stack.

In an implementation form of the first aspect, a spatial periodicity of the periodic arrangement of the embedded elements is in the order of a wavelength of the SAW propagating in the substrate stack at the working frequency of the SAW device.

The periodicity may be about the wavelength but may also take fractional numbers thereof.

In an implementation form of the first aspect, the SAW device further comprises a plurality of interdigital transducer, IDT, electrodes provided on the piezoelectric layer and configured to couple an electrical signal to a SAW propagating in the piezoelectric layer.

A second aspect of the invention provides a method for fabricating a surface acoustic wave, SAW, device, the method comprising: fabricating a piezoelectric layer configured to propagate a SAW on a first wafer, fabricating a substrate stack on a second wafer, embedding dielectric and/or semiconductor elements in a periodic arrangement in the substrate stack, the embedded elements having an acoustic impedance different than the surrounding material of the substrate stack, and bonding the first wafer to the second wafer so that the piezoelectric layer is provided on the substrate stack.

The method of the second aspect provides a non-complex way to manufacture the SAW device, thus achieving the advantages mentioned above.

In an implementation form of the second aspect, the method comprises: selecting a width, height and/or spatial periodicity of the periodic arrangement so that the embedded elements form a phononic crystal with a bandgap tuned to a frequency of interest, particularly to an operating frequency of the piezoelectric layer.

In an implementation form of the second aspect, at least one of the vertical and horizontal dimensions of the embedded elements are a fraction of the acoustic wavelength propagating in the substrate stack at the working frequency of the SAW device.

In an implementation form of the second aspect, the embedded elements form a phononic crystal in the substrate stack.

In an implementation form of the second aspect, the phononic crystal in the substrate stack has a bandgap centered around an operating frequency of the SAW device.

In an implementation form of the second aspect, the first wafer is bonded via a transfer layer to the second wafer.

In an implementation form of the second aspect, the dielectric and/or semiconductor elements are embedded in a substrate layer of the substrate stack.

In an implementation form of the first second aspect, the substrate layer comprises a semiconductor material, and/or the embedded elements comprise a dielectric material.

In an implementation form of the second aspect, the dielectric and/or semiconductor elements are embedded in the transfer layer, and the transfer layer is provided on a substrate layer of the substrate stack. In an implementation form of the second aspect, the transfer layer comprises an oxide, and/or the embedded elements comprise a dielectric material.

In an implementation form of the second aspect, the embedded elements include a plurality of vertically arranged material strips.

In an implementation form of the second aspect, the embedded elements include a plurality of horizontally and vertically arranged material islands.

In an implementation form of the second aspect, the embedded elements have different electrical and/or optical properties than the surrounding material of the substrate stack.

In an implementation form of the second aspect, a spatial periodicity of the periodic arrangement of the embedded elements is in the order of a wavelength of the SAW propagating in the substrate stack at the working frequency of the SAW device.

In an implementation form of the second aspect, the method further comprises forming a plurality of IDT electrodes on the piezoelectric layer, which are configured to couple an electrical signal to a SAW propagating in the piezoelectric layer.

With the method of the second aspect and its implementation forms, the SAW device of the first aspect and its implementation forms with all advantages and effects may be achieved.

A further specific aspect of the invention provides a SAW device comprising: a semiconductor substrate; an acoustic resonant cavity disposed over the semiconductor substrate (e.g. a piezoelectric layer made of e.g. Quartz, Lithium niobate LNO (LiNb03), Lithium tantalite LTA (LiTa03), AIN, Sc-AIN, GaN); a 2D or ID phononic crystal structure disposed below the acoustic resonant cavity and embedded in the semiconductor substrate, wherein the phononic crystal structure comprises a plurality of unit cells disposed in a periodic arrangement; and a transfer layer arranged between the acoustic resonant cavity and the semiconductor substrate.

It has to be noted that all devices, elements, units and means described in the present application could be implemented in the software or hardware elements or any kind of combination thereof. All steps which are performed by the various entities described in the present application as well as the functionalities described to be performed by the various entities are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities. Even if, in the following description of specific embodiments, a specific functionality or step to be performed by external entities is not reflected in the description of a specific detailed element of that entity which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities can be implemented in respective software or hardware elements, or any kind of combination thereof

BRIEF DESCRIPTION OF DRAWINGS

The above described aspects and implementation forms of the present invention will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which

FIG. 1 shows a SAW device according to an embodiment of the invention.

FIG. 2 shows cross-sections of a SAW device according to an embodiment of the invention, in which the embedded elements are located in a substrate layer of the substrate stack.

FIG. 3 shows cross-sections of a SAW device according to an embodiment of the invention, in which the embedded elements are located above the substrate layer of the substrate stack, below the piezoelectric layer, and particularly within a transfer layer of the substrate stack.

FIG. 4 shows a SAW device according to an embodiment of the invention.

FIG. 5 shows a method according to an embodiment of the invention.

DETAIFED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a SAW device 100 according to an embodiment of the invention. The SAW device 100 may be a SAW resonator. The SAW device 100 may particularly include an acoustic bandgap structure provided in a substrate beneath a layer functioning as resonator. In particular, the SAW device 100 shown in FIG. 1 comprises a substrate stack 101 (substrate). The substrate stack 101 may include (see more details in FIG. 2) a substrate layer 201 and a transfer layer 200. Provided on the substrate stack 101 is a piezoelectric layer 102 (resonator), which is configured to propagate a SAW.

Further, dielectric elements and/or semiconductor elements 103 are embedded in a periodic arrangement in the substrate stack 101. That is, embedded elements 103 made of a dielectric material, embedded elements 103 made of a semiconductor material, or embedded elements made of both dielectric and semiconductor materials (e.g. both materials are separately and/or altematingly used, or a material is used that is both). In particular, the embedded elements 103 are arranged periodically so that they can form the above-mentioned acoustic bandgap structure. The embedded elements 103 have an acoustic impedance, which is different (higher or lower) than an acoustic impedance of the surrounding material of the substrate stack 101, i.e. than the embedding material.

FIG. 2 shows cross-sections of a SAW device 100 according to an embodiment of the invention, which builds on the SAW device 100 shown in FIG. 1. Same elements in FIG. 1 and FIG. 2 are labelled with the same reference signs and function likewise. That is, the SAW device 100 shown in FIG. 2 also includes the substrate stack 101, the piezoelectric layer 102 and the embedded material 103.

In the SAW device 100 of FIG. 2, in particular a thin piezoelectric layer 102 is provided over a transfer layer 200. The transfer layer 102 is arranged between a substrate layer 201 and the piezoelectric layer 102. The transfer layer 200 may form the substrate stack 101 together with the substrate layer 201. However, the transfer layer 200 can also be provided on the substrate layer 201 forming the substrate stack 101. The transfer layer 102 is, for example, made of Si02, or of any other material suitable for the bonding of two separate substrates/wafers.

In the SAW device 100 of FIG. 2, the substrate layer 201 is configured with the embedded elements 103, i.e. the dielectric and/or semiconductor elements 103 are embedded in the periodic arrangement in the substrate layer 201. The embedded elements 103 particularly may form a 2D phononic crystal (as shown in FIG. 2), in order to create a reflective band-gap structure. In the left cross-section of FIG. 2 is shown how the embedded elements 103 are arranged periodically to form a unit cell. The embedded elements 103 may in particular include a plurality of horizontally and vertically arranged material islands, for instance, with a width a, and a depth b (see right side of FIG. 2).

The SAW device 100 of FIG. 2 includes further a plurality of IDT electrodes 203 provided on the piezoelectric layer 102. The IDT electrodes 203 are configured to couple an electrical signal to a SAW propagating in the piezoelectric layer 102. The IDT electrodes 203 may be embedded in a passivation layer 202.

FIG. 3 shows cross-sections of a SAW device 100 according to an embodiment of the invention, which builds on the SAW device 100 shown in FIG. 1. Same elements in FIG. 1 and FIG. 3 are labelled with the same reference signs and function likewise. The SAW device 100 shown in FIG. 3 thus also includes the substrate stack 101, the piezoelectric layer 102 and the embedded material 103. Like the SAW device of FIG. 2, the SAW device of FIG. 3 may also include the IDT electrodes 203 and the passivation layer 202.

In the SAW device 100 of FIG. 3, the embedded elements 103, particularly forming a 2D phononic crystal (as shown), are embedded in the transfer layer 200. The transfer layer 200 is provided on the substrate layer 201, which may together form the substrate stack 101 like in FIG. 2. Notably, the transfer layer 200 is preferably made of a material, which enables the bonding of two separate substrates/wafers. This enables the manufacturing method described below with respect to FIG. 5.

FIG. 4 shows a cross-section of a SAW device 100 according to an embodiment of the invention, which builds on the SAW device 100 shown in FIG. 1 and FIG. 2. Same elements in FIG. 1, FIG. 2 and FIG. 3 are labelled with the same reference signs and function likewise. The SAW device 100 shown in FIG. 4 thus also includes the substrate stack 101 , the piezoelectric layer 102 and the embedded material 103. Like the SAW device of FIG. 2, the SAW device of FIG. 4 may also include the IDT electrodes 203 and the passivation layer 202.

In the SAW device 100 of FIG. 4, the geometrical parameters of the embedded material 103, particularly forming a 2D phononic crystal, are tuned for different reflection band-gaps (i.e. width, height, spatial periodicity are factors, which may be tuned specifically for the required frequency and reflection strength). In the SAW devices 100 described above, the bandgap of the phononic crystal may be centered around the filter operating frequency of the piezoelectric layer 102 / metal IDT electrode 203 pattern. This creates a full acoustic mirror at the frequency of interest to limit the radiation to the substrate stack 101. The geometry of the phononic crystal may depend on the material used and the acoustic properties of all the layers (i.e. acoustic impedance, etc.). In particular, in the SAW device 100 of FIG. 4, the embedded elements 103 include a plurality of vertically arranged material strips.

The embedded elements 103 in the above-mentioned SAW devices 100, particularly the 2D phononic crystal, may be created separately on a substrate wafer, using a conventional Back End of Line (BEOL) Complementary Metal Oxide Semiconductor (CMOS) material process. The piezoelectric substrate 102 may be processed on a separate manufacturing line. The two wafers may then bonded together via the transfer layer 200. This is reflected in the method shown in FIG. 5.

Possible implementations of unit cells in the SAW devices 100 can either be some long strips of deposited material (see e.g. FIG. 4) or more fine 3D geometries (e.g. some parallelepiped shape, which can easily be manufacture with conventional surface and bulk micromachining).

An important physical property is the acoustic impedance of the embedded material 103, which should be different from the surrounding (embedding) material, so that scattered waves are generated at the interfaces. The periodic arrangement of these reflections/transmissions at the interfaces then generates the appropriate band-gap structures that allow suppressing the vibrational leakage. The periodic arrangement may be replication of a unit-cell structure, here defined as a geometric region with physical properties (such as acoustic impedance) different from the surrounding material, in which it is embedded.

Using different materials with various acoustic impedances will naturally lead to materials having different optical properties. Also a 3D region having different electrical properties from the surrounding material (i.e. conductor, partial conductor or isolator) may be formed.

The spatial periodicity of the periodic arrangement of the embedded elements may be in the order of a wavelength of the SAW propagating in the substrate stack 101 at the working frequency of the SAW device 100. The spatial periodicity may be chosen such that wave components incident on these structure will generate wave components in the frequency band of interest to the filter. In a practical case, for the SAW device 100 being a SAW resonator, the periodicity will be some length in the order of the fundamental SAW/BAW wavelengths at the particular frequency of interest. This geometrical periodic length may be defined as a fractional number of the acoustic wavelengths (lambda), assuming an integer N: any value given by any combination of N/L* lambda, where N and L are positive values, for instance positive integers. In some exemplary embodiments the geometric periodic length may be chosen from any combination of N*lambda/16, N*lambda/8, N* lambda/4, N* lambda/2.

In the following, possible materials for the individual elements/layers of the SAW device 100 according to an embodiment of the invention are provided.

The substrate layer 201 may be made of a material include at least one of: silicon, glass, ceramic. In particular, the substrate layer 201 may include at least one of: silicon, a SOI technology substrate, gallium arsenide, gallium phosphide, gallium nitride, and/or indium phosphide or other example substrate, an alloy semiconductor including GaAsP, AlInAs, GalnAs, GalnP, or GalnAsP.

The piezoelectric layer 102 may include at least one of: lithium niobate, lithium tantalate.

The IDT electrodes 203 may be made of a metal and/or metal alloy layer such as: copper, titanium, or may be a highly doped silicon layer.

The embedded material 103 (acoustic impedance material) may include a dielectric material and/or a semiconductor material. For example, the embedded material 103 may include one or more of: lude SiCOH, a phosphosilicate glass, an oxide or a nitride of aluminum, silicon, germanium, gallium, indium, tin, antimony, tellurium, bismuth, titanium, vanadium, chromium, manganese, cobalt, nickel, copper, zinc, zirconium, niobium, molybdenum, palladium, cadmium, hafnium, tantalum, or tungsten.

Materials available in the BEOL CMOS process may be as follows. Available materials for making the embedded material 103 are particularly: copper metallization, tungsten, low-k dielectrics, silicon dioxides, copper capping layers, etch stop layers, anti-reflecting coatings. Further, materials for dielectric layers may include: copper capping layers (CCL), etch stop layers (ESL), diffusion barriers (DB), antireflection coating (ARC) and low-k dielectrics such as, for example, SiCOH, SiOCN, SiCN, SiOC, SiN. Further, metallization layers, for example for making IDT electrodes 203, may include: copper, aluminum, tungsten, titanium. FIG. 5 shows a method 500 according to an embodiment of the invention. The method 500 is for fabricating a SAW device 100, like the SAW device 100 of FIG. 1, 2, 3 or 4. The method 500 comprises: a step 501 of fabricating a piezoelectric layer 102 configured to propagate a SAW on a first wafer; a step 502 of fabricating a substrate stack 101 on a second wafer; a step 503 of embedding dielectric and/or semiconductor elements 103 in a periodic arrangement in the substrate stack 101, wherein the embedded elements 103 have an acoustic impedance different than the surrounding material of the substrate stack 101; and a step 504 of bonding the first wafer to the second wafer so that the piezoelectric layer 102 is provided on the substrate stack 101. The present invention has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed invention, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word “comprising” does not exclude other elements or steps and the indefinite article“a” or“an” does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.