WO2024027920A1 - Method for producing a surface acoustic wave resonator - Google Patents

Method for producing a surface acoustic wave resonator Download PDF

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Publication number
WO2024027920A1
WO2024027920A1 PCT/EP2022/072044 EP2022072044W WO2024027920A1 WO 2024027920 A1 WO2024027920 A1 WO 2024027920A1 EP 2022072044 W EP2022072044 W EP 2022072044W WO 2024027920 A1 WO2024027920 A1 WO 2024027920A1
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WIPO (PCT)
Prior art keywords
layer
yag
top surface
piezoelectric
resonator device
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PCT/EP2022/072044
Other languages
French (fr)
Inventor
Kiryl KUSTANOVICH
Ventsislav Yantchev
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Huawei Technologies Co., Ltd.
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Priority to PCT/EP2022/072044 priority Critical patent/WO2024027920A1/en
Publication of WO2024027920A1 publication Critical patent/WO2024027920A1/en

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Classifications

    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H9/00—Networks comprising electromechanical or electro-acoustic devices; Electromechanical resonators
    • H03H9/02—Details
    • H03H9/02535—Details of surface acoustic wave devices
    • H03H9/02543—Characteristics of substrate, e.g. cutting angles
    • H03H9/02574—Characteristics of substrate, e.g. cutting angles of combined substrates, multilayered substrates, piezoelectrical layers on not-piezoelectrical substrate
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H3/00—Apparatus or processes specially adapted for the manufacture of impedance networks, resonating circuits, resonators
    • H03H3/007—Apparatus or processes specially adapted for the manufacture of impedance networks, resonating circuits, resonators for the manufacture of electromechanical resonators or networks
    • H03H3/08—Apparatus or processes specially adapted for the manufacture of impedance networks, resonating circuits, resonators for the manufacture of electromechanical resonators or networks for the manufacture of resonators or networks using surface acoustic waves
    • H03H3/10—Apparatus or processes specially adapted for the manufacture of impedance networks, resonating circuits, resonators for the manufacture of electromechanical resonators or networks for the manufacture of resonators or networks using surface acoustic waves for obtaining desired frequency or temperature coefficient
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H9/00—Networks comprising electromechanical or electro-acoustic devices; Electromechanical resonators
    • H03H9/02—Details
    • H03H9/02535—Details of surface acoustic wave devices
    • H03H9/02818—Means for compensation or elimination of undesirable effects
    • H03H9/02834—Means for compensation or elimination of undesirable effects of temperature influence

Definitions

  • Embodiments of invention relate to a method for producing a surface acoustic wave resonator device.
  • a surface acoustic wave (SAW) resonator device is a type of resonator device which converts an electrical signal into an acoustic wave in a piezoelectric layer.
  • SAW resonator devices are components for signal processing, frequency generation, filtering, sensing applications, etc.
  • Wireless communication devices heavily rely on high performance band-pass transmission filters, which are used to reject any unwanted incoming RF signals, and to only keep the wanted transmitted signal.
  • the band-pass transmission filters are required to have a very good selectivity of the incoming signal, which means letting through only a very narrow strip of the incoming frequency spectrum.
  • Some technological requirements of such filters are: high frequency; high selectivity for suppression of spurious signals; large bandwidth for high data rates; low insertion loss factor for low power consumption; and miniaturized packages or integrated filter modules for small hand held devices.
  • SAW-based frequency filters are used in most of the existing mobile telephone systems.
  • SAW resonator devices To reduce the inherently high insertion loss of conventional SAW resonator devices, a variety of special low-loss techniques has been developed, each technique being optimized for a specific application.
  • An objective of embodiments of the invention is to provide a solution which mitigates or solves the drawbacks and problems of conventional solutions.
  • Another objective of embodiments of the invention is to provide a solution for producing SAW resonator devices with high performance at low cost.
  • a method for producing a surface acoustic wave, SAW, resonator device comprising: obtaining an Yttrium Aluminum Garnet, YAG, layer; forming an interleave layer on a top surface of the YAG layer; forming a piezoelectric layer on a top surface of the interleave layer; and applying a conductive layer on a top surface of the piezoelectric layer for forming an interdigital transducer, IDT, on the top surface of the piezoelectric layer, wherein the IDT comprises first conductive electrodes connected to a first busbar and second conductive electrodes connected to a second busbar.
  • An advantage of the method for producing a SAW resonator device according to the first aspect is that it may rely on standard manufacturing techniques used in semiconductor and MicroElectromechanical Systems (MEMS) industries.
  • MEMS MicroElectromechanical Systems
  • State-of-the-art layered SAW resonator devices utilize high-resistivity silicon as a high phase velocity layer in their structure, which is costly to produce.
  • TR complex “trap-rich” layers, e.g., polysilicon, at the interface with the interleave layer. This significantly decreases the manufacturing complexity, and lowers the production cost.
  • the YAG layer has a higher phase velocity than a phase velocity of the piezoelectric layer.
  • An advantage with this implementation form is that due to wave-guiding effects, a high Q factor SAW resonators can be produced. That is, either the substrate itself is a low loss YAG substrate, or the YAG layer prevents acoustic losses from the resonator region, i.e. , from the piezoelectric layer, into the high phase velocity YAG layer.
  • the piezoelectric layer is any of: a lithium tantalate (LT) layer, a lithium niobate (LN) layer, and an aluminum nitride (AIN) layer.
  • LT lithium tantalate
  • LN lithium niobate
  • AIN aluminum nitride
  • the interleave layer comprises a silicon oxide layer.
  • the silicon oxide layer may be configured for temperature compensation requirements of the SAW resonator device.
  • interleave layer may be used as a wafer bonding layer between layers above the YAG layer and the YAG layer.
  • the conductive layer comprises any of: an aluminum layer, an aluminum-copper- alloy layer, a copper layer, a copper-aluminum-alloy layer, a chromium layer, a titanium layer, a tungsten layer, a gold layer, a palladium layer, and a molybdenum layer.
  • the method comprises, previous to applying the conductive layer on the top surface of the piezoelectric layer: applying a high velocity dielectric layer on the top surface of the piezoelectric layer; and creating openings in the high velocity dielectric layer for forming the IDT on the top surface of the piezoelectric layer.
  • the high velocity dielectric layer comprises any of: silicon nitride, aluminum oxide, aluminum nitride, and beryllium oxide.
  • the method comprises: forming a dielectric layer on a top surface of the conductive layer and/or the top surface of the piezoelectric layer; or forming a dielectric layer on a top surface of the high velocity dielectric layer.
  • the conductor pattern and the piezoelectric layer can be protected from the following production steps such as packaging and assembly, as well as the dielectric layer itself can be used for adjusting resonance frequency of the SAW resonator device and improving the yield as a result.
  • the dielectric layer comprises any of: silicon nitride, and silicon oxide.
  • the YAG layer is a YAG thin film layer, and wherein the method comprises: forming the YAG layer on a top surface of a substrate.
  • An advantage with this implementation form is that potentially the production cost can be decreased by eliminating the need of YAG substrate manufacturing. Moreover, the diameter of a layered SAW wafer stack is no longer constrained by the YAG crystal boule production size limitations. Utilizing thin YAG layer might also result in improved thermal performance of the SAW resonator device.
  • the YAG layer has a thickness in the range of 1 to 3 times a pitch between the conductive electrodes of the IDT.
  • An advantage with this implementation form is that it will effectively allow to constrain the acoustic wave to the layers above the YAG layer to achieve the desired waveguiding effect.
  • the pitch defines the resonance frequency of the SAW resonator device, and wherein the pitch is dependent on a phase velocity of a primary mode of a surface acoustic wave propagating in the piezoelectric layer when in operation.
  • the substrate comprises any of: fused silica, quartz, and silicon.
  • obtaining the YAG layer comprises: growing a YAG boule; and watering the YAG boule to form the YAG layer.
  • the YAG layer has any one of: a ⁇ 111> crystal orientation, ⁇ 100 crystal orientation, a ⁇ 110> crystal orientation, or a polycrystalline structure.
  • crystal with ⁇ 111> orientation has the smallest number of defects associated with its growth (production) mechanism.
  • ⁇ 100> and ⁇ 110> crystals will have a central core, i.e., a defect channel due to crystal faceting.
  • secondary cores may also appear at the edges along the (100) and (010) axes. This will effectively compromise the desired material properties.
  • a root mean square, RMS, surface roughness of the YAG layer is less than 1 .0 nm and preferably less than 0.5 nm.
  • An advantage with this implementation form is that small surface roughness is important for minimizing the interfacial scattering of the acoustic wave in a layered SAW device structure.
  • good surface roughness is a prerequisite for the wafer bonding process used in stack fabrication.
  • a thickness of the YAG layer is in the range of 100 - 1000 pm.
  • An advantage with this implementation form is that it provides a good mechanical support for the entire structure of the SAW resonator device.
  • Fig. 1 shows a flow chart of a method for producing a SAW resonator device according to embodiments of the invention
  • Fig. 2 shows a SAW resonator device produced according to the flow chart in Fig. 1 in a cross-sectional view
  • FIG. 3 shows a SAW resonator device produced according to the flow chart in Fig. 1 ;
  • FIG. 4 shows a flow chart of a method for producing a SAW resonator device according to further embodiments of the invention
  • Fig. 5 shows a SAW resonator device produced according to the flow chart in Fig. 4 in a cross-sectional view
  • FIG. 6 shows a flow chart of a method for producing a SAW resonator device according to yet further embodiments of the invention
  • Fig. 7 shows a SAW resonator device produced according to the flow chart in Fig. 6 in a cross-sectional view
  • Fig. 8 illustrates the application of a high velocity layer and a passivation layer.
  • Fig. 1 shows a flow chart of a method for producing a SAW resonator device according to embodiments of the invention
  • Fig. 2 shows a SAW resonator device produced according to the flow chart in Fig. 1 in a cross-sectional view.
  • the method 100 comprises obtaining 110 an Yttrium Aluminum Garnet (YAG) layer 210.
  • the method 100 further comprises forming 120 an interleave layer 220 on a top surface 212 of the YAG layer 210.
  • the method 100 further comprises forming 130 a piezoelectric layer 230 on a top surface 222 of the interleave layer 220.
  • the method 100 further comprises applying 140 a conductive layer 240 on a top surface 232 of the piezoelectric layer 230 for forming an interdigital transducer (IDT) 250 on the top surface 232 of the piezoelectric layer 230.
  • the IDT 250 comprises first conductive electrodes 252 connected to a first busbar 254 and second conductive electrodes 256 connected to a second busbar 258.
  • Fig. 3 shows a SAW resonator device 100 produced according to the flow chart in Fig. 1 in which the IDT 250 is shown more in detail.
  • the first conductive electrodes 252 and the second conductive electrodes 256 are alternatingly arranged in the interdigital transducer 106.
  • the IDT 106 is configured to convert an electrical signal ES into an acoustic wave AW in the piezoelectric layer 230.
  • the IDT 250 may comprise any number of first conductive electrodes 252 and any number of second conductive electrodes 256 e.g., from a few conductive electrodes up to thousands of conductive electrodes.
  • the number of the first conductive electrodes 112 and second conductive electrodes 114 may be the same in a symmetric configuration.
  • the conductive electrodes and busbars are naturally made of any suitable conductive material, such as different metals.
  • the conductive layer 240 from which the IDT 250 is formed may comprise any suitable conductive material such as: an aluminum layer, an aluminum-copper-alloy layer, a copper layer, a copper-aluminum-alloy layer, a chromium layer, a titanium layer, a tungsten layer, a gold layer, a palladium layer, and a molybdenum layer.
  • the YAG layer 210 should have a higher phase velocity than a phase velocity of the piezoelectric layer 230 to achieve the waveguiding effect and prevent acoustic losses from the resonator region, i.e. , from the piezoelectric layer, into the substrate.
  • the piezoelectric layer 230 is any of: a lithium tantalate layer, a lithium niobate layer, and an aluminum nitride layer.
  • the YAG layer 210 may have any one of: a ⁇ 111> crystal orientation, a ⁇ 100> crystal orientation, a ⁇ 110> crystal orientation, or a polycrystalline structure.
  • the ⁇ 111 > crystal orientation is preferred, as its manufacturing process allows to minimize the number of defects associated with the material growth.
  • the surface roughness of the YAG layer 210 is also important and the root mean square (RMS) surface roughness of the YAG layer 210 is less than 1.0 nm, and preferably less than 0.5 nm in embodiments of the invention.
  • RMS root mean square
  • the thickness d of the YAG layer 210 may be in the range of 100 - 1000 pm to ensure that enough mechanical support is provided for layers above the YAG layer 210.
  • the interleave layer 220 herein used may comprises a silicon oxide layer.
  • the purpose of the interleave layer 220 is to provide an effective temperature compensation of the SAW resonator device 100, as well as to act as a wafer bonding layer between layers above the YAG layer and the YAG layer. Variation in temperature of the environment, as well as operation at high input power are affecting the SAW resonator device temperature.
  • the frequency stability as a function of temperature is described in terms of temperature coefficient of frequency (TCF).
  • the temperature drift of the SAW resonator device 100 is related to changes in piezoelectric material stiffness coefficients as a function of temperature, but also to the thermal expansion that affects both the dimensions of the SAW resonator device 100 and the density of a piezoelectric material.
  • Most materials become “softer” upon exposure to elevated temperatures resulting in decrease of the acoustic wave velocity (i.e., negative first order TCF), while other become “stiffer” and the velocity increases with increasing the temperature (i.e., positive first order TCF).
  • LN, LT and AIN have negative TCF, while silicon oxide has a positive TCF.
  • the obtaining 110 of the YAG layer 210 comprises the steps of: growing 112 a YAG boule 216, and thereafter watering 114 the YAG boule 216 to form the YAG layer 210. This is particular interesting for the case when the YAG layer 210 has a certain thickness so that the YAG layer 210 also can form a substrate for the SAW resonator device 200.
  • a YAG boule for producing a YAG base substrate growth of a single crystal YAG boule is required.
  • the single crystal YAG boule can be grown using the Czochralski method or similar method. Control over the YAG boule crystal orientation is important for minimizing the amount of defects in the YAG boule.
  • a preferred crystal orientation is ⁇ 111>, while other crystal orientations are also possible depending on performance requirements.
  • Watering process is required to prepare the YAG base substrate for the present application.
  • the watering process involves, among other things, dicing of the YAG boule to produce wafer blanks, and chemical-mechanical polishing (CMP) to achieve the desired thickness and flatness of the wafer.
  • the root mean square (RMS) surface roughness R rms of the YAG layer 210 should be less than 1 nm, i.e., R rms ⁇ 1 nm, and preferably less than 0.5 nm, i.e., R rms ⁇ 0.5.
  • Control over the watering parameters is required at each preparation step to ensure that desired specifications are met.
  • the surface roughness can be measured by means of atomic force microscopy (AFM) or various optical interferometry techniques.
  • AFM atomic force microscopy
  • the interleave layer 220 functions as a temperature compensation (TC) layer.
  • Growth/deposition methods include, but are not limited to: Plasma-Enhanced Chemical Vapor Deposition (PECVD), Reactive Sputter Deposition, and Pulsed Laser Deposition (PLD).
  • Alternative methods include Low-pressure CVD (LPCVD), and Atomic Layer Deposition (ALD). The latter can produce films with excellent thickness uniformity and does not require thickness trimming as a result, but limited by low deposition rates.
  • PECVD Plasma-Enhanced Chemical Vapor Deposition
  • PLD Pulsed Laser Deposition
  • Alternative methods include Low-pressure CVD (LPCVD), and Atomic Layer Deposition (ALD). The latter can produce films with excellent thickness uniformity and does not require thickness trimming as a result, but limited by low deposition rates.
  • the interleave layer 220 is deposited on both the YAG layer 210 and the piezoelectric layer 230 in this step.
  • Metrology by means of, e.g., optical measurements, is required to determine the thickness and surface roughness of the deposited interleave layer 220.
  • CMP and thickness trimming by means of e.g., scanning Ion Beam Milling (IBM), are required to obtain a uniform thickness (e.g., ⁇ +/- 2% thickness tolerance from nominal) and low surface roughness (i.e., R rms ⁇ 1 nm) of the interleave layer 220.
  • IBM scanning Ion Beam Milling
  • These steps of deposition/growth, metrology, trimming and CMP can be repeated several times to achieve the desired surface roughness of the interleave layer 220.
  • the low surface roughness is needed to ensure high quality bond between two layers.
  • An example of a common interleave layer 220 material includes silicon dioxide (SiO2). Consequently, the interleave layer 220 may in such case have to be activated by annealing process.
  • the piezoelectric layer 230 with preferred crystallographic orientation is bonded with the YAG layer 210 and interleave layer 220 stack in the next step.
  • the piezoelectric layer 230 may be created from a piezoelectric wafer by ion implantation process. Definition of the piezoelectric layer 230 thickness is performed prior to bonding. Implanted ions create defects in the crystal structure along a plane that defines a boundary between what will become the piezoelectric layer 230 and the donor wafer.
  • the energy of implanted ions defines the thickness of a piezoelectric layer 230.
  • Target layer thickness may be ⁇ 1 urn with thickness tolerance ⁇ +/- 2% from nominal.
  • the piezoelectric layer 230 is bonded with the stack comprising YAG layer 210 and interleave layer 220.
  • Activation of one or both of mating surfaces may be carried out by a plasma process, or chemical treatment.
  • the mating surfaces may then be pressed together with considerable force. Thermal stress separates the bonded piezoelectric layer 230 from donor piezoelectric wafer along the defect plane created by implanted ions.
  • Metrology e.g., optical measurements, is performed to determine the thickness and surface roughness of the bonded piezoelectric layer 230.
  • Forming of the IDT topology on top of the composite substrate formed by the YAG layer 210, interleave layer 220 and the piezoelectric layer 230 comprises patterning of IDT metal.
  • IDT topology formation method may include lift-off lithography and metal evaporation. Metal deposition can be also achieved with sputtering techniques. Alternative approach to IDT topology formation involves plasma, reactive ion, or wet chemical etching to remove the excess metal through patterned photoresist or other masking material.
  • a passivation layer e.g., composite or single film with thickness ⁇ 50 nm
  • ALD reactive sputter deposition
  • Radio Frequency (RF) test a RF test of the SAW resonator device 100 is performed to determine its resonance frequency.
  • Resonance frequency of the SAW resonator device 100 is adjusted by thickness trimming of the passivation layer through selective/partial etching e.g., using a scanning IBM, or etching via masking layer.
  • Fig. 4 shows a flow chart of a method for producing a SAW resonator device according to further embodiments of the invention
  • Fig. 5 shows a SAW resonator device produced according to the flow chart in Fig. 4 in a cross-sectional view.
  • This is the second method for obtaining 110 of the YAG layer 210, i.e., as a YAG thin film layer.
  • the method 100 comprises: forming 160 the YAG layer 210 on a top surface 272 of a substrate 270.
  • the steps 110 - 140 are the same as previously described with reference to Fig. 1.
  • the substrate 270 used herein is any suitable substrate such as comprising any of: fused silica, quartz, and silicon.
  • the second method differs from the first method in the following step:
  • the YAG thin film layer has any one of: a ⁇ 111>, ⁇ 100>, or ⁇ 110> crystal orientation or a polycrystalline structure.
  • Thickness trimming and CMP of YAG thin film layer 210 is performed to obtain a surface roughness in the range of R rms ⁇ 1 nm and preferably R rms ⁇ 0.5 with thickness tolerance ⁇ +/- 2% from nominal. Thereafter, steps 3 - 13 according to the first method above are applied.
  • the YAG thin film layer 210 has a thickness in the range of 1 to 3 times a pitch p between the conductive electrodes 252, 256 of the IDT 250.
  • the pitch p defines the resonance frequency of the SAW resonator device 200.
  • the pitch p is dependent on a phase velocity of a primary mode of a surface acoustic wave S A propagating in the piezoelectric layer 230 when in operation.
  • Fig. 6 shows a flow chart of a method for producing a SAW resonator device according to yet further embodiments of the invention
  • Fig. 7 shows a SAW resonator device produced according to the flow chart in Fig. 6 in a cross-sectional view.
  • a high velocity dielectric layer is applied on the top surface 232 of the piezoelectric layer 230.
  • the method 100 comprises, previous to applying 140 the conductive layer 240 on the top surface 232 of the piezoelectric layer 230 applying 132 a high velocity dielectric layer 260 on the top surface 232 of the piezoelectric layer 230.
  • the method further comprises creating 134 openings in the high velocity dielectric layer 260 for forming the IDT 250 on the top surface 232 of the piezoelectric layer 230.
  • the method 100 thereafter continues with the step 140 of applying the conductive layer 240 on the top surface 232 of the piezoelectric layer 230 as previously described.
  • Fig. 8 illustrates the formation of a high velocity (HV) layer with reference to the following steps:
  • One or more high velocity dielectric layer(s) 260 may be formed by depositing one or more layers of high velocity dielectric material on the top surface of the piezoelectric layer 230.
  • the high velocity dielectric layer 260 may comprise any of: silicon nitride, aluminum oxide, aluminum nitride, and beryllium oxide.
  • the application of high-velocity dielectric layer(s) can be carried out using conventional deposition techniques such as based on physical vapor deposition, chemical vapor deposition, or other methods.
  • one or more lithography processes may be used to create openings resembling the conductor pattern in the photoresist or other masking materials and exposing areas of the high velocity dielectric material(s) to be removed, and to protect areas to remain unmodified.
  • Exposed areas of high velocity dielectric material(s) can be removed using dry or wet etching techniques, or other suitable methods.
  • a method to form a conductor pattern involves a lift-off process.
  • the conductor pattern that forms the IDT 250 of the SAW resonator device 100 may be created by depositing one or more conductor layers into the openings of high-velocity dielectric material and photoresist, on the top surface of the piezoelectric layer 230.
  • Conductive metals such as aluminum, an aluminum alloy, copper, a copper alloy, or some other can serve as a conductor layer. Additionally, one or more layers of other materials may be disposed above and/or below the conductor layer, i.e., between the piezoelectric layer 230 and the conductor layers/film.
  • the adhesion between the piezoelectric layer 230 and conductor film can be improved by a thin film of titanium, chrome, or other metal.
  • Conductivity of a first conductor pattern can be improved by a second conductor pattern made of higher conductivity metals such as aluminum, copper, gold or other, formed over portions of the first conductor pattern, such as the IDT busbars and interconnections between the IDTs.
  • the photoresist may then be removed, which removes the excess material leaving the conductor pattern.
  • a so-called passivation layer may also be applied.
  • the purpose of the passivation layer is to protect the SAW resonator device 100 from further processing steps, e.g., packaging and assembly.
  • the passivation layer may be formed with a dielectric layer 290.
  • the method 100 may further comprise the step of forming 150 a dielectric layer 290 on a top surface 242 of the conductive layer 240; or forming 150 a dielectric layer 290 on a top surface 262 of the high velocity dielectric layer 260.
  • the dielectric layer 290 may comprises any of: silicon nitride, and silicon oxide. These materials are commonly available at fabrication facilities, can be applied by a number of methods, and have good mechanical and chemical stability.
  • One or more passivation/tuning dielectric layer(s) may be formed by depositing one or more layers of dielectric material on the top surface of the piezoelectric layer comprising the conductor pattern of the I DT.
  • the application of passivation/tuning dielectric layer(s) can be carried out using conventional deposition techniques based on physical vapor deposition, chemical vapor deposition, or other method.
  • An optical measurement tool may be used to measure the thickness of the passivation/tuning dielectric layer(s).
  • the optical measurement tool may be, e.g., ellipsometer and/or reflectometer.
  • the measurement process is repeated to determine the thickness of passivation/tuning dielectric layer(s) at multiple measurement points on the top surface of the piezoelectric layer.
  • a grid or matrix of multiple measurement points may be formed on the surface of passivation/tuning dielectric layer.
  • a map of the thickness of the passivation/tuning dielectric layer may be then created from the measurement data.
  • the frequency of the SAW resonator device 200 may then be tuned by removal of portions of passivation/tuning dielectric layer by the material removal tool.
  • the material removal tool may e.g., be a scanning ion mill, a tool utilizing Fluorine-based reactive ion etching, or some other suitable tool.

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  • Physics & Mathematics (AREA)
  • Acoustics & Sound (AREA)
  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • Surface Acoustic Wave Elements And Circuit Networks Thereof (AREA)

Abstract

The invention relates to method (100) for producing a surface acoustic wave, SAW, resonator device (200), the method (100) comprising: obtaining (110) an Yttrium Aluminum Garnet, YAG, layer (210); forming (120) an interleave layer (220) on a top surface (212) of the YAG layer (210); forming (130) a piezoelectric layer (230) on a top surface (222) of the interleave layer (220); and applying (140) a conductive layer (240) on a top surface (232) of the piezoelectric layer (230) for forming an interdigital transducer, IDT, (250) on the top surface (232) of the piezoelectric layer (230), wherein the IDT (250) comprises first conductive electrodes (252) connected to a first busbar (254) and second conductive electrodes (256) connected to a second busbar (258). Thereby, a solution for producing SAW resonator devices at low cost is provided.

Description

METHOD FOR PRODUCING A SURFACE ACOUSTIC WAVE RESONATOR
TECHNICAL FIELD
Embodiments of invention relate to a method for producing a surface acoustic wave resonator device.
BACKGROUND
A surface acoustic wave (SAW) resonator device is a type of resonator device which converts an electrical signal into an acoustic wave in a piezoelectric layer. SAW resonator devices, are components for signal processing, frequency generation, filtering, sensing applications, etc.
Wireless communication devices heavily rely on high performance band-pass transmission filters, which are used to reject any unwanted incoming RF signals, and to only keep the wanted transmitted signal. In some wireless communication applications, the band-pass transmission filters are required to have a very good selectivity of the incoming signal, which means letting through only a very narrow strip of the incoming frequency spectrum. Some technological requirements of such filters are: high frequency; high selectivity for suppression of spurious signals; large bandwidth for high data rates; low insertion loss factor for low power consumption; and miniaturized packages or integrated filter modules for small hand held devices.
SAW-based frequency filters are used in most of the existing mobile telephone systems. To reduce the inherently high insertion loss of conventional SAW resonator devices, a variety of special low-loss techniques has been developed, each technique being optimized for a specific application.
SUMMARY
An objective of embodiments of the invention is to provide a solution which mitigates or solves the drawbacks and problems of conventional solutions.
Another objective of embodiments of the invention is to provide a solution for producing SAW resonator devices with high performance at low cost.
The above and further objectives are solved by the subject matter of the independent claims.
Further embodiments of the invention can be found in the dependent claims. According to a first aspect of the invention, the above mentioned and other objectives are achieved with a method for producing a surface acoustic wave, SAW, resonator device, the method comprising: obtaining an Yttrium Aluminum Garnet, YAG, layer; forming an interleave layer on a top surface of the YAG layer; forming a piezoelectric layer on a top surface of the interleave layer; and applying a conductive layer on a top surface of the piezoelectric layer for forming an interdigital transducer, IDT, on the top surface of the piezoelectric layer, wherein the IDT comprises first conductive electrodes connected to a first busbar and second conductive electrodes connected to a second busbar.
An advantage of the method for producing a SAW resonator device according to the first aspect is that it may rely on standard manufacturing techniques used in semiconductor and MicroElectromechanical Systems (MEMS) industries. State-of-the-art layered SAW resonator devices utilize high-resistivity silicon as a high phase velocity layer in their structure, which is costly to produce. Additionally, there is no need of complex “trap-rich” (TR) layers, e.g., polysilicon, at the interface with the interleave layer. This significantly decreases the manufacturing complexity, and lowers the production cost.
In an implementation form of a method for producing a SAW resonator device according to the first aspect, the YAG layer has a higher phase velocity than a phase velocity of the piezoelectric layer.
An advantage with this implementation form is that due to wave-guiding effects, a high Q factor SAW resonators can be produced. That is, either the substrate itself is a low loss YAG substrate, or the YAG layer prevents acoustic losses from the resonator region, i.e. , from the piezoelectric layer, into the high phase velocity YAG layer.
In an implementation form of a method for producing a SAW resonator device according to the first aspect, the piezoelectric layer is any of: a lithium tantalate (LT) layer, a lithium niobate (LN) layer, and an aluminum nitride (AIN) layer.
An advantage with this implementation form is that these materials are either widely available commercially in a form of wafers with various diameters and cut angles, such as LT and LN, or can be deposited by a number of well-established techniques, such as AIN. In an implementation form of a method for producing a SAW resonator device according to the first aspect, the interleave layer comprises a silicon oxide layer.
The silicon oxide layer may be configured for temperature compensation requirements of the SAW resonator device.
An advantage with this implementation form is that the interleave layer may be used as a wafer bonding layer between layers above the YAG layer and the YAG layer.
In an implementation form of a method for producing a SAW resonator device according to the first aspect, the conductive layer comprises any of: an aluminum layer, an aluminum-copper- alloy layer, a copper layer, a copper-aluminum-alloy layer, a chromium layer, a titanium layer, a tungsten layer, a gold layer, a palladium layer, and a molybdenum layer.
In an implementation form of a method for producing a SAW resonator device according to the first aspect, the method comprises, previous to applying the conductive layer on the top surface of the piezoelectric layer: applying a high velocity dielectric layer on the top surface of the piezoelectric layer; and creating openings in the high velocity dielectric layer for forming the IDT on the top surface of the piezoelectric layer.
In an implementation form of a method for producing a SAW resonator device according to the first aspect, the high velocity dielectric layer comprises any of: silicon nitride, aluminum oxide, aluminum nitride, and beryllium oxide.
In an implementation form of a method for producing a SAW resonator device according to the first aspect, the method comprises: forming a dielectric layer on a top surface of the conductive layer and/or the top surface of the piezoelectric layer; or forming a dielectric layer on a top surface of the high velocity dielectric layer.
An advantage with this implementation form is that the conductor pattern and the piezoelectric layer can be protected from the following production steps such as packaging and assembly, as well as the dielectric layer itself can be used for adjusting resonance frequency of the SAW resonator device and improving the yield as a result. In an implementation form of a method for producing a SAW resonator device according to the first aspect, the dielectric layer comprises any of: silicon nitride, and silicon oxide.
In an implementation form of a method for producing a SAW resonator device according to the first aspect, the YAG layer is a YAG thin film layer, and wherein the method comprises: forming the YAG layer on a top surface of a substrate.
An advantage with this implementation form is that potentially the production cost can be decreased by eliminating the need of YAG substrate manufacturing. Moreover, the diameter of a layered SAW wafer stack is no longer constrained by the YAG crystal boule production size limitations. Utilizing thin YAG layer might also result in improved thermal performance of the SAW resonator device.
In an implementation form of a method for producing a SAW resonator device according to the first aspect, the YAG layer has a thickness in the range of 1 to 3 times a pitch between the conductive electrodes of the IDT.
An advantage with this implementation form is that it will effectively allow to constrain the acoustic wave to the layers above the YAG layer to achieve the desired waveguiding effect.
In an implementation form of a method for producing a SAW resonator device according to the first aspect, the pitch defines the resonance frequency of the SAW resonator device, and wherein the pitch is dependent on a phase velocity of a primary mode of a surface acoustic wave propagating in the piezoelectric layer when in operation.
In an implementation form of a method for producing a SAW resonator device according to the first aspect, the substrate comprises any of: fused silica, quartz, and silicon.
An advantage with this implementation form is that wafers made of these materials are widely available commercially, are inexpensive, have good thermal conduction and coefficient of thermal-expansion (CTE) properties, as well as provide a solid mechanical support for the whole structure of the SAW resonator device.
In an implementation form of a method for producing a SAW resonator device according to the first aspect, obtaining the YAG layer comprises: growing a YAG boule; and watering the YAG boule to form the YAG layer. An advantage with this implementation form is that it allows to obtain a monocrystalline substrate with low number of defects in its structure. This ensures the highest Q factor, thermal conductivity and Young's modulus of the material.
In an implementation form of a method for producing a SAW resonator device according to the first aspect, the YAG layer has any one of: a <111> crystal orientation, <100 crystal orientation, a <110> crystal orientation, or a polycrystalline structure.
An advantage with this implementation form is that crystal with <111> orientation has the smallest number of defects associated with its growth (production) mechanism. <100> and <110> crystals will have a central core, i.e., a defect channel due to crystal faceting. In addition, secondary cores may also appear at the edges along the (100) and (010) axes. This will effectively compromise the desired material properties.
In an implementation form of a method for producing a SAW resonator device according to the first aspect, a root mean square, RMS, surface roughness of the YAG layer is less than 1 .0 nm and preferably less than 0.5 nm.
An advantage with this implementation form is that small surface roughness is important for minimizing the interfacial scattering of the acoustic wave in a layered SAW device structure. In addition, good surface roughness is a prerequisite for the wafer bonding process used in stack fabrication.
In an implementation form of a method for producing a SAW resonator device according to the first aspect, a thickness of the YAG layer is in the range of 100 - 1000 pm.
An advantage with this implementation form is that it provides a good mechanical support for the entire structure of the SAW resonator device.
Further applications and advantages of embodiments of the invention will be apparent from the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
The appended drawings are intended to clarify and explain different embodiments of the invention, in which: - Fig. 1 shows a flow chart of a method for producing a SAW resonator device according to embodiments of the invention;
- Fig. 2 shows a SAW resonator device produced according to the flow chart in Fig. 1 in a cross-sectional view;
- Fig. 3 shows a SAW resonator device produced according to the flow chart in Fig. 1 ;
- Fig. 4 shows a flow chart of a method for producing a SAW resonator device according to further embodiments of the invention;
- Fig. 5 shows a SAW resonator device produced according to the flow chart in Fig. 4 in a cross-sectional view;
- Fig. 6 shows a flow chart of a method for producing a SAW resonator device according to yet further embodiments of the invention;
- Fig. 7 shows a SAW resonator device produced according to the flow chart in Fig. 6 in a cross-sectional view; and
- Fig. 8 illustrates the application of a high velocity layer and a passivation layer.
DETAILED DESCRIPTION
The disclosed solution relates to a layered SAW resonator device employing an Yttrium Aluminum Garnet (YAG) high phase velocity layer or substrate in its structure. Embodiments of the invention aim at a production or fabrication method of such a multi-layer SAW resonator device. Thus, Fig. 1 shows a flow chart of a method for producing a SAW resonator device according to embodiments of the invention, and Fig. 2 shows a SAW resonator device produced according to the flow chart in Fig. 1 in a cross-sectional view.
With reference to Fig. 1 and 2, the method 100 comprises obtaining 110 an Yttrium Aluminum Garnet (YAG) layer 210. The method 100 further comprises forming 120 an interleave layer 220 on a top surface 212 of the YAG layer 210. The method 100 further comprises forming 130 a piezoelectric layer 230 on a top surface 222 of the interleave layer 220. The method 100 further comprises applying 140 a conductive layer 240 on a top surface 232 of the piezoelectric layer 230 for forming an interdigital transducer (IDT) 250 on the top surface 232 of the piezoelectric layer 230. The IDT 250 comprises first conductive electrodes 252 connected to a first busbar 254 and second conductive electrodes 256 connected to a second busbar 258.
Fig. 3 shows a SAW resonator device 100 produced according to the flow chart in Fig. 1 in which the IDT 250 is shown more in detail. The first conductive electrodes 252 and the second conductive electrodes 256 are alternatingly arranged in the interdigital transducer 106. The IDT 106 is configured to convert an electrical signal ES into an acoustic wave AW in the piezoelectric layer 230. The IDT 250 may comprise any number of first conductive electrodes 252 and any number of second conductive electrodes 256 e.g., from a few conductive electrodes up to thousands of conductive electrodes. The number of the first conductive electrodes 112 and second conductive electrodes 114 may be the same in a symmetric configuration. The conductive electrodes and busbars are naturally made of any suitable conductive material, such as different metals. The conductive layer 240 from which the IDT 250 is formed may comprise any suitable conductive material such as: an aluminum layer, an aluminum-copper-alloy layer, a copper layer, a copper-aluminum-alloy layer, a chromium layer, a titanium layer, a tungsten layer, a gold layer, a palladium layer, and a molybdenum layer.
Generally, the YAG layer 210 should have a higher phase velocity than a phase velocity of the piezoelectric layer 230 to achieve the waveguiding effect and prevent acoustic losses from the resonator region, i.e. , from the piezoelectric layer, into the substrate. Thus, in embodiments of the invention, the piezoelectric layer 230 is any of: a lithium tantalate layer, a lithium niobate layer, and an aluminum nitride layer.
It has further been realized that certain properties and structures of the YAG layer 210 provide better results than others. Therefore, in embodiments of the invention, the YAG layer 210 may have any one of: a <111> crystal orientation, a <100> crystal orientation, a <110> crystal orientation, or a polycrystalline structure. However, the <111 > crystal orientation is preferred, as its manufacturing process allows to minimize the number of defects associated with the material growth. The surface roughness of the YAG layer 210 is also important and the root mean square (RMS) surface roughness of the YAG layer 210 is less than 1.0 nm, and preferably less than 0.5 nm in embodiments of the invention. Such surface roughness allows to minimize the interfacial scattering of acoustic waves and is a prerequisite for the wafer bonding process required for attaching layers of the stack to each other. The thickness d of the YAG layer 210 may be in the range of 100 - 1000 pm to ensure that enough mechanical support is provided for layers above the YAG layer 210.
Furthermore, the interleave layer 220 herein used may comprises a silicon oxide layer. The purpose of the interleave layer 220 is to provide an effective temperature compensation of the SAW resonator device 100, as well as to act as a wafer bonding layer between layers above the YAG layer and the YAG layer. Variation in temperature of the environment, as well as operation at high input power are affecting the SAW resonator device temperature. The frequency stability as a function of temperature is described in terms of temperature coefficient of frequency (TCF). The temperature drift of the SAW resonator device 100 is related to changes in piezoelectric material stiffness coefficients as a function of temperature, but also to the thermal expansion that affects both the dimensions of the SAW resonator device 100 and the density of a piezoelectric material. Most materials become “softer” upon exposure to elevated temperatures resulting in decrease of the acoustic wave velocity (i.e., negative first order TCF), while other become “stiffer” and the velocity increases with increasing the temperature (i.e., positive first order TCF). LN, LT and AIN have negative TCF, while silicon oxide has a positive TCF. By combining these materials together and carefully choosing their dimensions, a SAW resonator device with zero TCF can be constructed.
There are two major methods of obtaining the YAG layer 210 according to embodiments of the invention. In a first method the obtaining 110 of the YAG layer 210 comprises the steps of: growing 112 a YAG boule 216, and thereafter watering 114 the YAG boule 216 to form the YAG layer 210. This is particular interesting for the case when the YAG layer 210 has a certain thickness so that the YAG layer 210 also can form a substrate for the SAW resonator device 200.
The first method will be described in more detail in the following steps:
1) Growing a YAG boule: for producing a YAG base substrate growth of a single crystal YAG boule is required. The single crystal YAG boule can be grown using the Czochralski method or similar method. Control over the YAG boule crystal orientation is important for minimizing the amount of defects in the YAG boule. A preferred crystal orientation is <111>, while other crystal orientations are also possible depending on performance requirements.
2) Watering process: watering is required to prepare the YAG base substrate for the present application. The watering process involves, among other things, dicing of the YAG boule to produce wafer blanks, and chemical-mechanical polishing (CMP) to achieve the desired thickness and flatness of the wafer. The root mean square (RMS) surface roughness Rrms of the YAG layer 210 should be less than 1 nm, i.e., Rrms < 1 nm, and preferably less than 0.5 nm, i.e., Rrms < 0.5. Control over the watering parameters is required at each preparation step to ensure that desired specifications are met. For example, the surface roughness can be measured by means of atomic force microscopy (AFM) or various optical interferometry techniques.
3) Deposition/growth of the interleave layer 220 with thickness ranging from few hundred nm to one urn is carried out in the next step. The interleave layer 220 functions as a temperature compensation (TC) layer. Growth/deposition methods include, but are not limited to: Plasma-Enhanced Chemical Vapor Deposition (PECVD), Reactive Sputter Deposition, and Pulsed Laser Deposition (PLD). Alternative methods include Low-pressure CVD (LPCVD), and Atomic Layer Deposition (ALD). The latter can produce films with excellent thickness uniformity and does not require thickness trimming as a result, but limited by low deposition rates. ) The interleave layer 220 is deposited on both the YAG layer 210 and the piezoelectric layer 230 in this step. ) Metrology by means of, e.g., optical measurements, is required to determine the thickness and surface roughness of the deposited interleave layer 220. CMP and thickness trimming by means of e.g., scanning Ion Beam Milling (IBM), are required to obtain a uniform thickness (e.g., < +/- 2% thickness tolerance from nominal) and low surface roughness (i.e., Rrms < 1 nm) of the interleave layer 220. These steps of deposition/growth, metrology, trimming and CMP can be repeated several times to achieve the desired surface roughness of the interleave layer 220. The low surface roughness is needed to ensure high quality bond between two layers. An example of a common interleave layer 220 material includes silicon dioxide (SiO2). Consequently, the interleave layer 220 may in such case have to be activated by annealing process. ) The piezoelectric layer 230 with preferred crystallographic orientation is bonded with the YAG layer 210 and interleave layer 220 stack in the next step. The piezoelectric layer 230 may be created from a piezoelectric wafer by ion implantation process. Definition of the piezoelectric layer 230 thickness is performed prior to bonding. Implanted ions create defects in the crystal structure along a plane that defines a boundary between what will become the piezoelectric layer 230 and the donor wafer. The energy of implanted ions defines the thickness of a piezoelectric layer 230. Target layer thickness may be < 1 urn with thickness tolerance < +/- 2% from nominal. ) After ion implantation process, the piezoelectric layer 230 is bonded with the stack comprising YAG layer 210 and interleave layer 220. Activation of one or both of mating surfaces may be carried out by a plasma process, or chemical treatment. To establish molecular bonds between the interleave layer 220 and the piezoelectric layer 230, the mating surfaces may then be pressed together with considerable force. Thermal stress separates the bonded piezoelectric layer 230 from donor piezoelectric wafer along the defect plane created by implanted ions. 8) Metrology, e.g., optical measurements, is performed to determine the thickness and surface roughness of the bonded piezoelectric layer 230.
9) CMP and thickness trimming, by means of e.g., scanning IBM, of the piezoelectric layer 230 is performed. Required surface roughness is in the range of Rrms < 1 nm or so called “SAW-grade”.
10) Forming of the IDT topology on top of the composite substrate formed by the YAG layer 210, interleave layer 220 and the piezoelectric layer 230 comprises patterning of IDT metal. IDT topology formation method may include lift-off lithography and metal evaporation. Metal deposition can be also achieved with sputtering techniques. Alternative approach to IDT topology formation involves plasma, reactive ion, or wet chemical etching to remove the excess metal through patterned photoresist or other masking material.
11) Deposition of a passivation layer (e.g., composite or single film with thickness < 50 nm) by ALD or reactive sputter deposition.
12) Radio Frequency (RF) test: a RF test of the SAW resonator device 100 is performed to determine its resonance frequency.
13) Resonance frequency of the SAW resonator device 100 is adjusted by thickness trimming of the passivation layer through selective/partial etching e.g., using a scanning IBM, or etching via masking layer.
Fig. 4 shows a flow chart of a method for producing a SAW resonator device according to further embodiments of the invention, and Fig. 5 shows a SAW resonator device produced according to the flow chart in Fig. 4 in a cross-sectional view. This is the second method for obtaining 110 of the YAG layer 210, i.e., as a YAG thin film layer. Thus, the method 100 comprises: forming 160 the YAG layer 210 on a top surface 272 of a substrate 270. The steps 110 - 140 are the same as previously described with reference to Fig. 1. The substrate 270 used herein is any suitable substrate such as comprising any of: fused silica, quartz, and silicon.
The second method differs from the first method in the following step:
1) Obtaining YAG thin film layer 210 which is applied on a substrate 270 by PLD or sputter deposition. The YAG thin film layer has any one of: a <111>, <100>, or <110> crystal orientation or a polycrystalline structure.
2) Thickness trimming and CMP of YAG thin film layer 210 is performed to obtain a surface roughness in the range of Rrms < 1 nm and preferably Rrms < 0.5 with thickness tolerance < +/- 2% from nominal. Thereafter, steps 3 - 13 according to the first method above are applied. The YAG thin film layer 210 has a thickness in the range of 1 to 3 times a pitch p between the conductive electrodes 252, 256 of the IDT 250. The pitch p defines the resonance frequency of the SAW resonator device 200. The pitch p is dependent on a phase velocity of a primary mode of a surface acoustic wave SA propagating in the piezoelectric layer 230 when in operation.
Fig. 6 shows a flow chart of a method for producing a SAW resonator device according to yet further embodiments of the invention, and Fig. 7 shows a SAW resonator device produced according to the flow chart in Fig. 6 in a cross-sectional view. In these embodiments a high velocity dielectric layer is applied on the top surface 232 of the piezoelectric layer 230. Hence, the method 100 comprises, previous to applying 140 the conductive layer 240 on the top surface 232 of the piezoelectric layer 230 applying 132 a high velocity dielectric layer 260 on the top surface 232 of the piezoelectric layer 230. The method further comprises creating 134 openings in the high velocity dielectric layer 260 for forming the IDT 250 on the top surface 232 of the piezoelectric layer 230. The method 100 thereafter continues with the step 140 of applying the conductive layer 240 on the top surface 232 of the piezoelectric layer 230 as previously described. Such solution allows to achieve the piston mode of operation of multilayer surface acoustic wave resonator elements, which is a prerequisite for successfully suppressing the spurious transverse modes.
Fig. 8 illustrates the formation of a high velocity (HV) layer with reference to the following steps:
1) One or more high velocity dielectric layer(s) 260 may be formed by depositing one or more layers of high velocity dielectric material on the top surface of the piezoelectric layer 230. The high velocity dielectric layer 260 may comprise any of: silicon nitride, aluminum oxide, aluminum nitride, and beryllium oxide. The application of high-velocity dielectric layer(s) can be carried out using conventional deposition techniques such as based on physical vapor deposition, chemical vapor deposition, or other methods.
2) In order to remove the excess dielectric material(s) and to make openings for the consequent formation of the conductor pattern of the IDT 250, one or more lithography processes may be used to create openings resembling the conductor pattern in the photoresist or other masking materials and exposing areas of the high velocity dielectric material(s) to be removed, and to protect areas to remain unmodified.
3) Exposed areas of high velocity dielectric material(s) can be removed using dry or wet etching techniques, or other suitable methods.
4) A method to form a conductor pattern involves a lift-off process. The conductor pattern that forms the IDT 250 of the SAW resonator device 100 may be created by depositing one or more conductor layers into the openings of high-velocity dielectric material and photoresist, on the top surface of the piezoelectric layer 230. Conductive metals such as aluminum, an aluminum alloy, copper, a copper alloy, or some other can serve as a conductor layer. Additionally, one or more layers of other materials may be disposed above and/or below the conductor layer, i.e., between the piezoelectric layer 230 and the conductor layers/film. For example, the adhesion between the piezoelectric layer 230 and conductor film can be improved by a thin film of titanium, chrome, or other metal. Conductivity of a first conductor pattern can be improved by a second conductor pattern made of higher conductivity metals such as aluminum, copper, gold or other, formed over portions of the first conductor pattern, such as the IDT busbars and interconnections between the IDTs.
5) The photoresist may then be removed, which removes the excess material leaving the conductor pattern.
In embodiments of the invention, a so-called passivation layer may also be applied. The purpose of the passivation layer is to protect the SAW resonator device 100 from further processing steps, e.g., packaging and assembly. The passivation layer may be formed with a dielectric layer 290. Thus, the method 100 may further comprise the step of forming 150 a dielectric layer 290 on a top surface 242 of the conductive layer 240; or forming 150 a dielectric layer 290 on a top surface 262 of the high velocity dielectric layer 260. The dielectric layer 290 may comprises any of: silicon nitride, and silicon oxide. These materials are commonly available at fabrication facilities, can be applied by a number of methods, and have good mechanical and chemical stability.
Thus, with reference to Fig. 8 the following step may be added to the previous described steps 1 - 5:
6) One or more passivation/tuning dielectric layer(s) may be formed by depositing one or more layers of dielectric material on the top surface of the piezoelectric layer comprising the conductor pattern of the I DT. The application of passivation/tuning dielectric layer(s) can be carried out using conventional deposition techniques based on physical vapor deposition, chemical vapor deposition, or other method.
An optical measurement tool may be used to measure the thickness of the passivation/tuning dielectric layer(s). The optical measurement tool may be, e.g., ellipsometer and/or reflectometer. The measurement process is repeated to determine the thickness of passivation/tuning dielectric layer(s) at multiple measurement points on the top surface of the piezoelectric layer. A grid or matrix of multiple measurement points may be formed on the surface of passivation/tuning dielectric layer. A map of the thickness of the passivation/tuning dielectric layer may be then created from the measurement data.
The frequency of the SAW resonator device 200 may then be tuned by removal of portions of passivation/tuning dielectric layer by the material removal tool. The material removal tool may e.g., be a scanning ion mill, a tool utilizing Fluorine-based reactive ion etching, or some other suitable tool.
Finally, it should be understood that the invention is not limited to the embodiments described above, but also relates to and incorporates all embodiments within the scope of the appended independent claims.

Claims

1. A method (100) for producing a surface acoustic wave, SAW, resonator device (200), the method (100) comprising: obtaining (110) an Yttrium Aluminum Garnet, YAG, layer (210); forming (120) an interleave layer (220) on a top surface (212) of the YAG layer (210); forming (130) a piezoelectric layer (230) on a top surface (222) of the interleave layer (220); and applying (140) a conductive layer (240) on a top surface (232) of the piezoelectric layer (230) for forming an interdigital transducer, IDT, (250) on the top surface (232) of the piezoelectric layer (230), wherein the IDT (250) comprises first conductive electrodes (252) connected to a first busbar (254) and second conductive electrodes (256) connected to a second busbar (258).
2. The method (100) according to claim 1 , wherein the YAG layer (210) has a higher phase velocity than a phase velocity of the piezoelectric layer (230).
3. The method (100) according to claim 2, wherein the piezoelectric layer (230) is any of: a lithium tantalate layer, a lithium niobate layer, and an aluminum nitride layer.
4. The method (100) according to any one of the preceding claims, wherein the interleave layer (220) comprises a silicon oxide layer.
5. The method (100) according to any one of the preceding claims, wherein the conductive layer (240) comprises any of: an aluminum layer, an aluminum-copper-alloy layer, a copper layer, a copper-aluminum-alloy layer, a chromium layer, a titanium layer, a tungsten layer, a gold layer, a palladium layer, and a molybdenum layer.
6. The method (100) according to any one of the preceding claims, wherein the method (100) comprises, previous to applying (140) the conductive layer (240) on the top surface (232) of the piezoelectric layer (230): applying (132) a high velocity dielectric layer (260) on the top surface (232) of the piezoelectric layer (230); and creating (134) openings in the high velocity dielectric layer (260) forforming the IDT (250) on the top surface (232) of the piezoelectric layer (240).
7. The method (100) according to claim 6, wherein the high velocity dielectric layer (260) comprises any of: silicon nitride, aluminum oxide, aluminum nitride, and beryllium oxide.
8. The method (100) according to any one of the preceding claims, wherein the method (100) comprises: forming (150) a dielectric layer (290) on a top surface (242) of the conductive layer (240) and/or the top surface of the piezoelectric layer (230); or forming (150) a dielectric layer (290) on a top surface (262) of the high velocity dielectric layer (260).
9. The method (100) according to claim 8, wherein the dielectric layer (290) comprises any of: silicon nitride, and silicon oxide.
10. The method (100) according to any one of the preceding claims, wherein the YAG layer (210) is a YAG thin film layer, and wherein the method (100) comprises: forming (160) the YAG layer (210) on a top surface (272) of a substrate (270).
11. The method (100) according to claim 10, wherein the YAG layer (210) has a thickness in the range of 1 to 3 times a pitch (p) between the conductive electrodes (252, 256) of the IDT (250).
12. The method (100) according to claim 11 , wherein the pitch (p) defines the resonance frequency of the SAW resonator device (200), and wherein the pitch (p) is dependent on a phase velocity of a primary mode of a surface acoustic wave (SA) propagating in the piezoelectric layer (230) when in operation.
13. The method (100) according to claim any one of claims 10 to 12, wherein the substrate (270) comprises any of: fused silica, quartz, and silicon.
14. The method (100) according to any one of the preceding claims, wherein obtaining (110) the YAG layer (210) comprises: growing (112) a YAG boule (216); and watering (114) the YAG boule (216) to form the YAG layer (210).
15. The method (100) according to any one of the preceding claims, wherein the YAG layer (210) has any one of: a <111> crystal orientation, a <100> crystal orientation, a <110> crystal orientation, or a polycrystalline structure.
16. The method (100) according to any one of the preceding claims, wherein a root mean square, RMS, surface roughness of the YAG layer (210) is less than 1.0 nm and preferably less than 0.5 nm.
17. The method (100) according to any one of the preceding claims, wherein a thickness (d) of the YAG layer (210) is in the range of 100 - 1000 pm.
PCT/EP2022/072044 2022-08-05 2022-08-05 Method for producing a surface acoustic wave resonator WO2024027920A1 (en)

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