DESC:FIELD OF INVENTION
[0001]
The present subject matter relates, in general, to devices comprising hybrid stacks of ferroelectric and piezoelectric materials and, in particular but not exclusively, to microelectromechanical systems (MEMS) resonators incorporating laterally stacked multiple ferroelectric and piezoelectric materials. 5
BACKGROUND
[0002]
Piezoelectric materials exhibit various inherent properties that make the piezoelectric materials suitable for various applications in microelectromechanical systems (MEMS), devices. The piezoelectric materials may be broadly categorized 10 based on their ferroelectric or non-ferroelectric nature, each offering distinct advantages in different operational contexts. For example, ferroelectric materials, characterized by high piezoelectric coefficients, are often employed in actuator applications. Conversely, non-ferroelectric piezoelectric materials, which exhibit lower permittivity, find utility in sensing applications where reduced parasitic and 15 feedthrough capacitance is desirable. The performance of the piezoelectric resonators is frequently evaluated using a figure of merit (FoM), expressed as a ratio ??2/??, where ?? is the piezoelectric coefficient, and ?? is the permittivity.
[0003] Generally, piezoelectric MEMS resonators employ a multilayer structure comprising electrodes for actuation and sensing, with an intervening piezoelectric 20 film. This configuration may also include an optional device layer or substrate. The selection of piezoelectric materials for MEMS resonators involves considerations of their respective piezoelectric coefficients and permittivity values. Materials such as lead zirconate titanate (PZT) and aluminium nitride (AlN) represent two distinct classes of piezoelectric materials, each with its own set of characteristics suitable 25 for different aspects of resonator operation. SUMMARY
[0004]
This summary is provided to introduce concepts related to MEMS resonators incorporating laterally stacked multiple ferroelectric and piezoelectric 30 materials. This summary is not intended to identify essential features of the claimed
2
subject matter nor is it intended for use in determining or limiting the scope of the claimed subject matter.
[0005]
In an aspect of the present subject matter, a laterally stacked micro-electro-mechanical system (MEMS) resonator is disclosed. The MEMS resonator includes a first set of electrodes. Further, the MEMS resonator comprises one or 5 more actuation layers and one or more sensing layers deposited laterally on the first set of electrodes. The one or more actuation layers and the one or more sensing layers are made of combinations of distinct ferroelectric-piezoelectric material, piezoelectric-ferroelectric material, ferroelectric-ferroelectric material, and piezoelectric-piezoelectric material. In addition, the MEMS resonator includes a 10 second set of electrodes deposited on the one or more actuation layers and the one or more sensing layers. The one or more actuation layers and the one or more sensing layers are interposed between the first set of electrodes and the second set of electrodes.
[0006] In another aspect of the present subject matter, a method for fabricating 15 a laterally stacked micro-electro-mechanical system (MEMS) resonator is disclosed. The method includes depositing a first set of electrodes on a substrate. Further, the method includes depositing one or more actuation layers and one or more sensing layers on the first set of electrodes. The one or more actuation layers are deposited laterally on the one or more sensing layers. In an example, the one or 20 more actuation layers and the one or more sensing layers are made of combinations of distinct ferroelectric-piezoelectric material, piezoelectric-ferroelectric material, ferroelectric-ferroelectric material, and piezoelectric-piezoelectric material. The method also includes depositing a second set of electrodes on the one or more actuation layers and the one or more sensing layers. The one or more actuation 25 layers and the one or more sensing layers are interposed between the first set of electrodes and the second set of electrodes.
BRIEF DESCRIPTION OF THE DRAWINGS 3
[0007] Systems and/or methods are now described, in accordance with examples of the present subject matter and with reference to the accompanying figures, in which:
[0008] FIG. 1 illustrates a cross-sectional view of a MEMS resonator with laterally stacked ferroelectric and piezoelectric materials, according to an example 5 of the present subject matter;
[0009] FIG. 2A illustrates a perspective view of a circular MEMS resonator with laterally stacked ferroelectric and piezoelectric materials, according to another example of the present subject matter;
[0010] FIG. 2B illustrates a cross-section view of the circular piezoelectric 10 MEMS resonator, according to an example of the present subject matter;
[0011] FIG. 2C illustrates another cross-section view of the circular piezoelectric MEMS resonator, according to an example of the present subject matter;
[0012] FIG. 2D illustrates an optical micrograph of the circular piezoelectric 15 MEMS resonator with lateral stacked ferroelectric and piezoelectric materials, according to an example of the present subject matter;
[0013] FIG. 3A illustrates a mechanical frequency response of a circular MEMS resonator, according to an example of the present subject matter;
[0014] FIG. 3B illustrates a second flexure mode of the circular MEMS 20 resonator, according to an example of the present subject matter;
[0015] FIG. 4A depicts an electrical frequency response graph of a circular MEMS resonator when a piezoelectric material is used for actuation and a ferroelectric material is used for sensing, according to an example of the present subject matter; 25
[0016] FIG. 4B depicts an electrical frequency response graph of a circular MEMS resonator when a ferroelectric material is used for actuation and a piezoelectric material is used for sensing, according to an example of the present subject matter; 4
[0017] FIG. 5A depicts frequency response plots of a laterally stacked MEMS resonator when a piezoelectric thin-film is used for actuation, according to an example of the present subject matter;
[0018] FIG. 5B depicts frequency response plots of a laterally stacked MEMS resonator when a ferroelectric thin-film is used for actuation, according to an 5 example of the present subject matter;
[0019] FIG. 6A illustrates a cross-sectional view of a laterally stacked MEMS resonator, according to another example of the present subject matter;
[0020] FIG. 6B illustrates a cross-sectional view of a laterally stacked MEMS resonator, according to another example of the present subject matter; 10
[0021] FIG. 7 illustrates a schematic diagram illustrating another circular configuration of regions in a MEMS resonator, according to an example of the present subject matter;
[0022] FIG. 8 illustrates another schematic diagram illustrating another circular configuration of regions in a MEMS resonator, according to an example of the 15 present subject matter;
[0023] FIG. 9A depicts a frequency response graph comparing performance of laterally stacked ferroelectric and piezoelectric materials in MEMS resonators, according to an example of the present subject matter;
[0024] FIG. 9B depicts additional frequency response graphs comparing 20 performance of laterally stacked ferroelectric and piezoelectric materials in MEMS resonators, according to an example of the present subject matter;
[0025] FIG. 10A illustrates a top view of an array of circular resonator devices with hybrid materials, according to an example of the present subject matter;
[0026] FIG. 10B illustrates another top view of an array of circular resonator 25 devices with hybrid materials, according to an example of the present subject matter; and
[0027] FIG. 11 illustrates a method for fabricating a laterally stacked MEMS resonator, according to an example of the present subject matter. 30 5
DETAILED DESCRIPTION
[0028]
Existing piezoelectric MEMS resonators face significant limitations in their design and performance. Traditionally, these resonators rely on a single piezoelectric material for both actuation and sensing functions. This approach inherently forces a compromise between optimal actuation and sensing capabilities. 5 Materials with high transduction coefficients, which are ideal for actuation, often exhibit high permittivity, leading to reduced sensing efficiency due to increased parasitic capacitance. Conversely, materials better suited for sensing, with lower permittivity, typically provide suboptimal actuation performance. For example, if the resonator uses a material like lead zirconate titanate (PZT), it performs well in 10 actuation but less so in sensing, whereas a material like aluminum nitride (AlN) is effective for sensing but not for actuation. This trade-off severely constrains the overall figure of merit (FoM) achievable in conventional resonator designs.
[0029]
Furthermore, the use of a single material limits the resonator's versatility in terms of vibration modes and frequency ranges. It also poses challenges in 15 temperature compensation and control over nonlinear behavior, which are critical for many applications. Attempts to address these issues through complex structural modifications or specialized fabrication techniques often result in increased device footprint, higher manufacturing costs, or compromised performance in other areas, which is undesirable in the context of miniaturization efforts in MEMS technology. 20 This fundamental trade-off between performance and size has limited the advancement of piezoelectric MEMS resonators.
[0030]
The present subject matter discloses a laterally stacked micro-electro-mechanical system (MEMS) resonator (hereinafter referred to as MEMS resonator). The MEMS resonator includes a first set of electrodes, one or more actuation layers 25 and one or more sensing layers deposited laterally on the first set of electrodes. The MEMS resonator further includes a second set of electrodes deposited on top of the one or more actuation layers and one or more sensing layers. In an example, the actuation and sensing layers are made of combinations of distinct ferroelectric and
6
piezoelectric materials, allowing for optimized performance in both actuation and sensing functions.
[0031]
This performance enhancement is especially evident when comparing the transduction and sensing coefficients of two widely used materials, PZT and AlN. PZT, a ferroelectric material, offers a high transduction coefficient, making it ideal 5 for actuation by producing significant mechanical stress/strain per unit of applied field/charge. This attribute allows PZT to function effectively in applications requiring robust actuation performance. On the other hand, AlN, a non-ferroelectric material, excels in sensing applications due to its ability to generate a relatively high electrical output per unit of applied mechanical stress even in the presence of its 10 own feedthrough/parasitic capacitances for a wide range of frequency. This sensitivity to mechanical input makes AlN advantageous for high-precision sensing. Therefore, the integration of both ferroelectric and piezoelectric thin-film materials within a single resonator maximizes the unique advantages of each material, achieving a balanced optimization of actuation and sensing performance that would 15 be challenging to attain with a single-material approach.
[0032]
In an example, the actuation and sensing layers may be made of distinct combinations of ferroelectric-piezoelectric material, piezoelectric-ferroelectric material, ferroelectric-ferroelectric material, and piezoelectric-piezoelectric material, including but not limited to lead zirconate titanate (PZT), bismuth ferrite 20 (BiFeO3), lithium niobate (LiNbO3), potassium niobate (KNbO3), sodium niobate (NaNbO3), potassium sodium niobate ((K, Na)NbO3), lithium tantalate (LiTaO3), barium titanate (BaTiO3), lead titanate (PbTiO3), strontium titanate (SrTiO3), barium strontium titanate ((Ba, Sr)TiO3), polyvinylidene fluoride (PVDF), aluminium nitride (AlN), gallium nitride (GaN), indium nitride (InN), scandium 25 aluminum nitride (ScAlN), boron aluminum nitride (BAlN), and zinc oxide (ZnO). It may be evident to a person skilled in the art that the above is only a list of some of the commonly used piezoelectric and ferroelectric materials, but the usage of present subject matter may be expanded beyond these materials (bulk and thin-film) as long as piezoelectric or ferroelectric characteristics are exhibited. 30 7
[0033]
In an example, the lateral stacking facilitates the MEMS resonator to be customized to any lithographically defined shape and is capable of operating across different acoustic modes, including higher order vibrational modes. Further, the actuation layers may be designed to have a high transduction coefficient, while the sensing layers may have low permittivity, addressing the trade-offs typically 5 encountered in single-material resonators. In addition, the actuation layers and the sensing layers are deposited according to the strain distribution of the resonator to minimize asymmetry effects. Further, the MEMS resonator may incorporate vias to separate the actuation and sensing layers.
[0034]
In various embodiments of the present invention, the MEMS resonator's 10 electrode configuration may be flexible, with options for ground electrodes and input/output electrodes in either the first or second set. For example, the first and second electrodes may serve as ground electrodes in conjunction with the set of top electrodes. In this configuration, one electrode in the set may receive an input electrical signal while another may collect the output signal. Alternatively, the roles 15 may be reversed, with the first and second electrodes acting as input and output electrical ports and two electrodes from the top functioning as ground electrodes. The multiple electrodes architecture may also be used for various mixed-mode driving and sensing configurations. This versatility in electrode configuration allows for adaptability to meet the optimal actuation and sensing requirements of 20 the resonator.
[0035]
Accordingly, the present subject matter describes a resonator topology that leverages advantages of multiple distinct ferroelectric and piezoelectric materials within a single resonator unit. By using a hybrid lateral stack of ferroelectric and piezoelectric materials, the present subject matter may enhance the 25 resonator's performance by the unique properties of each material. While all ferroelectric materials are piezoelectric, not all piezoelectric materials are ferroelectric. Ferroelectric materials like PZT have high piezoelectric coefficients, while non-ferroelectric piezoelectric materials like AlN have relatively low permittivity. This approach utilizes various high-quality thin films, with some 30 8
selected for their high piezoelectric coefficient to optimize actuation, while others are chosen for their low permittivity to enable efficient sensing.
[0036]
The present subject matter is further described with reference to FIG. 1 to FIG. 11. It should be noted that the description and figures merely illustrate principles of the present subject matter. Various arrangements may be devised that, 5 although not explicitly described or shown herein, encompass the principles of the present subject matter. Moreover, all statements herein reciting principles, aspects, and examples of the present subject matter, as well as specific examples thereof, are intended to encompass equivalents thereof.
[0037]
FIG. 1 illustrates a cross-sectional view of a microelectromechanical 10 systems (MEMS) resonator 100 with laterally stacked ferroelectric and piezoelectric materials, according to an example of the present subject matter. In an example, a shape of the MEMS resonator 100 may be customizable according to any lithographically defined shape. For example, the MEMS resonator 100 may have a square shape, a rectangular shape, a circular shape, a hexagonal shape, and 15 so on. In an implementation, the MEMS resonator 100 may be configured to operate across different acoustic modes. For example, the MEMS resonator 100 may be configured as a platform adaptable to use various categories of acoustic modes, such as, but not limited to, a flexure mode, a torsion mode, a bulk mode, a shear mode, a surface acoustic mode, and a plate wave mode. 20
[0038]
In an implementation, the MEMS resonator 100 includes a substrate 102 that may serve as a handle layer. In an example, the substrate 102 may be made of a silicon material, a glass material, or any other suitable material. The MEMS resonator 100 may be defined by selectively etching and releasing portions of the substrate 102 through a micromachining process, with the remaining portions of the 25 substrate 102 functioning as a support layer. It will be evident to a person skilled in the art that while FIG. 1 depicts the use of a silicon wafer as the substrate 102, any suitable semiconductor substrate (or stack of heterogeneous substrate and thin-films) may be employed. 9
[0039]
Further, the MEMS resonator 100 may include a device or structural layer 104 formed by etching the substrate 102. In an example, the etching may be limited to backside etching, depending on a configuration of the MEMS resonator 100. In another example, both sides of the substrate 102 may be etched to form the MEMS resonator 100. The device layer 104 may provide additional thickness, thereby 5 positioning a neutral axis further from the ferroelectric and piezoelectric thin films, which may help to generate optimum bending moment upon applying the electrical field. The device or structural layer 104 may have a thickness ranging from approximately 0.1 microns to 100 microns, depending on the design of the MEMS resonator 100 and the required operational frequency for the intended application; 10 however, this range is not intended to be limiting. In another implementation, the device or structural layer 104 is optional for the MEMS resonator 100.
[0040]
The MEMS resonator 100 further includes a first set of electrodes. The first set of electrodes may include a first electrode106a and a second electrode 106b (collectively referred to as the first set of electrodes 106). The first electrode 106a 15 may include a single layer of any suitable metallic material with a selected thickness, although, in certain examples, the first electrode 106a may consist of multiple metallic layers to achieve specific characteristics suited to the growth requirements of the ferroelectric or piezoelectric thin film. Further, the second electrode 106b is positioned adjacent to the first electrode 106a and may consist of 20 one or more metallic layers with selected thicknesses, carefully chosen for compatibility with the ferroelectric and piezoelectric thin films being deposited.
[0041]
In addition, the MEMS resonator 100 includes one or more actuation layers 108 and one or more sensing layers 110 deposited laterally on the first set of electrodes 106. For example, the one or more actuation layers 108 may be deposited 25 onto the first electrodes 106a. Further, the one or more sensing layers 110 may be deposited onto the second electrode 106b. In an example, the one or more actuation layers 108 and the one or more sensing layers 110 are made of combinations of distinct ferroelectric-piezoelectric materials, piezoelectric-ferroelectric materials, ferroelectric-ferroelectric materials, and piezoelectric-piezoelectric materials. 30 10
[0042]
Examples of the material of the one or more actuation layers 108 and the one or more sensing layers 110 may include, but are not limited to, lead zirconate titanate (PZT), bismuth ferrite (BiFeO), lithium niobate (LiNbO3), potassium niobate (KNbO3), sodium niobate (NaNbO3), lithium tantalate (LiTaO3), barium titanate (BaTiO3), lead titanate (PbTiO3), strontium titanate (SrTiO3), 5 Polyvinylidene fluoride (PVDF), aluminum nitride (AlN), gallium nitride (GaN), lead magnesium niobate-lead titanate (PMNPT), zinc oxide (ZnO), or a combination thereof.
[0043]
It may be evident to a person skilled in the art that the above is only a list of some of the commonly used piezoelectric and ferroelectric materials, but the 10 usage of present subject matter may be expanded beyond these materials (bulk and thin-film) as long as piezoelectric or ferroelectric characteristics are exhibited.
[0044]
In an example, where both the actuation layers 108 and the sensing layers 110 are made of materials selected from a group of ferroelectric materials or piezoelectric materials, a transduction coefficient of the one or more actuation 15 layers 108 should be above a first range of values, and a permittivity of the one or more sensing layers 110 should be below a second range of values. The first range of values and the second range of values indicate thresholds that help ensure optimal performance of the MEMS resonator 100. For example, the optimal performance of the MEMS resonator 100 may indicate that the one or more actuation layers 108 20 provide sufficient mechanical displacement or force in response to electrical input, and the one or more sensing layers 110 maintain high sensitivity and signal clarity by minimizing dielectric loading or electrical noise. Selecting materials according to these criteria helps to decouple the actuation and sensing functions, reducing cross-interference and improving overall transducer efficiency of the MEMS 25 resonator 100.
[0045]
The thickness and composition of the actuation layers 108 and sensing layers 110 may be tailored to meet specific performance requirements of the MEMS resonator 100. These parameters may be adjusted based on factors such as target 11
resonance frequency, electromechanical coupling efficiency, dielectric properties, and thermal or mechanical stability.
[0046]
In an example, the one or more actuation layers 108 and the one or more sensing layers 110 are deposited according to strain distribution of the MEMS resonator 100 and to minimize asymmetry effect. The strain distribution refers to 5 the spatial pattern of mechanical deformation experienced by the resonator structure during vibration. In case of the MEMS resonator 100, such distribution is not uniform but varies across the geometry, with certain regions experiencing higher strain amplitudes than others.
[0047]
The actuation layers 108 and the sensing layers 110 are arranged 10 according to the strain distribution of the MEMS resonator 100 to facilitate actuation and sensing of higher-order modes. The higher-order modes refer to vibration patterns of increased complexity that occur at frequencies above the fundamental resonance frequency. These modes are characterized by multiple nodes (points of zero displacement) and antinodes (points of maximum displacement) 15 across the resonator structure. By aligning the actuation layers 108 and the sensing layers 110 with the strain maxima of these higher-order modes, the ability of the MEMS resonator 100 to excite and detect such complex vibration patterns is significantly enhanced. This arrangement allows for more efficient energy transfer during actuation and improved signal transduction during sensing. 20
[0048]
Referring back to FIG. 1, the MEMS resonator 100 includes a second set of electrodes deposited on the one or more actuation layers 108 and the one or more sensing layers 110. This arrangement creates a sandwich-like configuration where the one or more actuation layers 108 and the one or more sensing layers 110 are interposed between the first set of electrodes and the second set of electrodes. Such 25 a structure enables precise control over the electric field distribution within the active layers, optimizing the conversion between electrical and mechanical energy. FIG. 1 depicts an arrangement comprising four electrodes in the second set of electrodes, designated as 112a, 112b, 112c, and 112d, (collectively referred to as the second set of electrodes 112) positioned atop the ferroelectric thin film 108 and 30 12
piezoelectric thin film 110. In an example, the second set of electrodes 112 may be formed from one or more metallic compositions, configured to achieve the desired thickness and performance characteristics.
[0049]
In certain embodiments, the second set of electrodes 112 may include a dual-layered structure, incorporating a 30 nm layer of chromium (Cr) and a 100 nm 5 layer of gold (Au). In alternative embodiments, other metal combinations may be employed, such as a 20 nm layer of titanium (Ti) overlaid with a 100 nm layer of platinum (Pt). Preferably, the materials chosen for the second set of electrodes 112 may provide optimal adhesion to the ferroelectric and piezoelectric thin films, ensuring reliable bonding during subsequent wire bonding processes and 10 facilitating effective electrical contact to generate a suitable electric field. Additional embodiments may involve other metallic configurations that are similarly compatible with both ferroelectric and piezoelectric layers, allowing for flexible adaptation of electrode materials based on specific application requirements and performance considerations. 15
[0050]
The second set of electrodes 112 may be formed through any suitable deposition technique. In various embodiments, the second set of electrodes 112 may be deposited either through a lift-off process or by etching, using any of the listed deposition techniques, among others. FIG. 1 illustrates the MEMS resonator 100 with four electrodes 112a, 112b, 112, and 112d; however, the number of electrodes 20 may be adjusted as necessary, with configurations ranging from a reduced two-electrode arrangement to configurations incorporating more than four electrodes. This flexibility allows for optimization of actuation and sensing capabilities based on the specific operational requirements of the designed resonators.
[0051]
In one embodiment, the first set of electrodes 106 acts as ground 25 electrodes and the second set of electrodes 112 acts as input/output electrodes. In another embodiment, the second set of electrodes 112 acts as ground electrodes and the first set of electrodes 106 acts as input/output electrodes. In addition, the ground electrodes for the one or more actuation layers 108 may differ from the ground electrodes associated with the one or more sensing layers 110. Such deliberate 30 13
separation of ground paths helps to electrically isolate the actuation and sensing sections of the MEMS resonator 100, thereby minimizing electrical cross-talk and reducing feed-through capacitance. As may be evident, the feed-through capacitance, which can arise due to capacitive coupling between layers or shared electrode paths, is a significant source of signal interference that can degrade the 5 accuracy and resolution of the sensing function. By employing separate ground electrodes for the actuation layers 108 and the sensing layers 110, an overall signal integrity is improved. Moreover, this configuration enhances the performance of the MEMS resonator 100 across a broad frequency spectrum, allowing for more stable operation and higher signal-to-noise ratios in various applications, such as 10 filters, oscillators, and frequency-selective components.
[0052]
Further, a material of the ground electrodes has material characteristics for optimal growth/deposition of the one or more actuation layers 108 and one or more sensing layers 110, respectively. The material characteristics may include, but are not limited to, crystal structure compatibility, lattice matching, thermal 15 expansion coefficient, electrical conductivity, and chemical stability, all of which contribute to the formation of high-quality, defect-free functional layers during fabrication. For instance, the ground electrode material may be chosen to promote epitaxial or highly oriented growth of the piezoelectric or ferroelectric films, which in turn can enhance electromechanical coupling efficiency and improve 20 performance of the MEMS resonator 100.
[0053]
In an example, the ground electrodes may include one or more layers of a material selected from, but not limited to, Titanium, Platinum, Molybdenum, or a combination thereof. In another example, the input/output electrodes may include at least two layers of a material selected from, but not limited to, Chromium, Gold, 25 Titanium, Platinum, or a combination thereof.
[0054]
In an example, ground electrodes and the input/output electrodes may be deposited using any suitable deposition technique. For instance, the first set of electrodes 106 and the second set of electrodes 112 are deposited by using one of a chemical vapour deposition (CVD) technique, and a physical vapour deposition 30 14
(PVD) technique. The PVD techniques may include, but are not limited to, sputtering, evaporation, and pulsed laser deposition (PLD), while the CVD techniques may include low-pressure CVD, ultra-high vacuum CVD, plasma-enhanced CVD (PECVD), metalorganic CVD (MOCVD), and vapor-phase epitaxy (VPE), among others. Such techniques may be implemented to achieve the desired 5 metal deposition in forming the first set of electrodes 106 and the second set of electrodes 112. The selection of materials for the first set of electrodes 106 may be dictated by the combination of ferroelectric and piezoelectric materials used for actuation and sensing functions.
[0055]
Further, the one or more actuation layers 108 and the one or more sensing 10 layers 110 may be deposited on the ground electrodes by any suitable deposition technique. For example, the one or more actuation layers 108 may be deposited using PVD, CVD, or chemical solution deposition (CSD), while the one or more sensing layers 110 may be deposited using PVD, Molecular Beam Epitaxy (MBE), Metal-Organic Chemical Vapor Deposition (MOCVD), and so on. The selection of 15 deposition techniques for the actuation layers 108 and the sensing layers 110 is not limited to above mentioned techniques but is based on compatibility with the crystal structure of the underlying electrode or conductive material.
[0056]
In an implementation, the one or more actuation layers 108 and the one or more sensing layers 110 may be separated by at least one via. For example, via 20 114a may separate the first electrode 106a and the second electrode 106b. The via 114a may also separate the actuation layers 108 and the sensing layers 110, which are formed on the first electrode 106a and the second electrode 106b, respectively. In addition, the second set of electrodes 112 may be separated by vias 114a, 114b, and 114c, as depicted in FIG. 1. 25
[0057]
Further, vias 116a and 116b may be etched through the device layer 104, the first set of electrodes 106, and actuation layers 108, and the sensing layers 110 to define the resonator structure, as illustrated in FIG. 1. To fully release the MEMS resonator 100, backside etching may be employed to etch both the substrate 102 and any buried oxide layers. In some embodiments, the etching process may include 30 15
both wet and dry etching techniques, depending on the material composition, layer thickness, and dimensional requirements of the MEMS resonator 100. As may be understood, the vias may be increased or decreased by incorporating additional or fewer combinations of multiple ferroelectric and piezoelectric materials, as well as by patterning additional or fewer sets of electrodes to suit the specific vibration 5 mode architecture.
[0058]
In an example, during actuation and sensing operations, the first set of electrodes 106 may serve as ground electrodes in conjunction with the second set of electrodes 112. In this configuration, one electrode from the second set of electrodes 112 may receive an input electrical signal, while another electrode within 10 the second set of electrodes 112 may function to collect the output electrical signal. Alternatively, the configuration may be reversed, wherein the first set of electrodes 106 may function as input and output electrical ports, respectively, while two electrodes from the second set of electrodes 112 may operate as ground electrodes. This versatility in electrode configuration provides adaptability to meet general 15 actuation and sensing requirements for the resonator.
[0059]
Accordingly, the lateral stacking of ferroelectric and piezoelectric materials within the MEMS resonator 100 allows for the optimization of both actuation and sensing capabilities within a single device by leveraging the unique properties of different materials. The high transduction coefficient of ferroelectric 20 materials may be utilized for efficient actuation, while the low permittivity of certain piezoelectric materials enhances sensing performance. This arrangement minimizes the trade-offs typically encountered when using a single material for both functions. Furthermore, the lateral stacking approach may reduce mass loading effects and strain gradients compared to vertical stacking, preserving the optimal 25 properties of both films. The ability to pattern distinct electrodes for each material layer enables reduced feed-through capacitance, potentially enhancing the resonator's performance across a wide frequency range. Additionally, the present subject matter provides flexibility in designing resonators for various acoustic modes, including flexural, torsional, and contour modes, among others. The lateral 30 16
stacking technique also allows for the integration of materials with different thermal expansion coefficients, offering potential for temperature compensation and improved stability. Overall, this approach enables the creation of high-performance, multi-functional MEMS resonators with a compact footprint, opening up new possibilities for advanced sensing, actuation, and signal processing applications. 5
[0060] FIG. 2A illustrates a perspective view of a circular MEMS resonator 200 with laterally stacked ferroelectric and piezoelectric materials, according to an example of the present subject matter. The circular MEMS resonator 200 is formed on a silicon-on-insulator wafer 202, which may serve as both a substrate and a supporting layer for various layers. Further, the MEMS resonator 200 includes a 10 first set of electrodes directly patterned on the substrate 202. The first set of electrodes comprises a first electrode 204a and a second electrode 204b (collectively referred to as the first set of electrodes 204). In an example, the selection of the first set of electrodes 204 may be based on a choice of ferro/ piezoelectric materials deposited on top of the first electrode 204a and the second 15 electrode 204b. In an example, a number of electrodes (n-number) may be chosen according to the selection of n-number combinations of ferroelectric and piezoelectric materials within a single resonator.
[0061] Further, an actuation layer 206 and a sensing layer 208 is deposited on the first electrode 204a and the second electrode 204b from the first set of electrodes 20 204. For example, ferroelectric material, such as lead zirconate titanate (PZT) may be selected as the actuation layer 206. Further, piezoelectric material, such as Aluminium Nitride (AlN) may be selected as the sensing layer 208. It would be evident to a person skilled in the art that the materials of the actuation layer 206 and the sensing layer 208 is not limited to the above examples, and any combination of 25 materials that exhibit optimized transduction and sensing coefficients may be laterally stacked in place of these materials.
[0062] Further, in an implementation, a sequence of deposition of the actuation layer 206 and the sensing layer 208 is not limited to a specific order. In certain examples, the actuation layer 206 and the sensing layer 208 may be deposited in a 30 17
reversed or alternative sequence relative to one another. In such cases, the material composition or configuration of the first electrode 204a and the second electrode 204b may be correspondingly adjusted to ensure compatibility with the respective overlying ferroelectric or piezoelectric layer, thereby maintaining or enhancing the desired transduction and sensing performance of the circular MEMS resonator 200. 5
[0063] In addition, the circular MEMS resonator 200 includes a second set of electrodes comprising of electrode 210 and electrode 212. In an example, the second set of electrodes may be patterned on an upper surface of the actuation layer 206 and the sensing layer 208, thereby forming a laterally stacked electrode-material-electrode configuration. Further, the lateral arrangement of the first electrode 204a, 10 the second electrode 204b, the actuation layer 206, the sensing layer 208, the electrode 210, and the electrode 212 is separated by vias 214. The vias 214 may serve as electrical and/or structural separation features, ensuring electrical isolation between neighbouring structures while maintaining mechanical integrity of the circular MEMS resonator 200. 15
[0064] As described with reference to FIG. 1, the first set of electrodes 204 and the second set of electrodes may be patterned based on the strain profile of a flexural mode of the circular MEMS resonator 200. By aligning the electrode positions with regions of maximum or optimal strain within the resonating structure, the electromechanical coupling efficiency of the circular MEMS resonator 200 may be 20 improved. This strain-engineered electrode placement may enable enhanced energy transfer between the electrical and mechanical domains of the circular MEMS resonator 200, thereby improving performance metrics such as resonance quality factor (Q), sensitivity, and transduction efficiency.
[0065]
In some embodiments, fabrication of the circular MEMS resonator 200 25 may involve a substrate release process that includes dry etching the substrate 202, such as the Silicon-on-insulator (SOI) wafer substrate 202 from a side opposite to the side on which the first set of electrodes 204 is patterned. Such back-side etching operation may be carried out using anisotropic or isotropic etching techniques, such as reactive ion etching (RIE) or deep reactive ion etching (DRIE), to selectively 30 18
remove the bulk silicon layer and thereby releasing the circular resonator structure. In an example, dimensions of the released structure, including radius, may be predetermined based on desired resonant frequency characteristics for the target application.
[0066]
In an implementation, one or more seed layers may be deposited prior to 5 the deposition of the actuation layer 206 and the sensing layer 208 to promote proper nucleation, crystalline orientation, surface morphology, and adhesion characteristics during the deposition process. In an example, the one or more seed layers may be deposited directly over the first set of electrodes 204. In another example, the one or more seed layers may be deposited directly on the substrate 10 202 before patterning the first set of electrodes 204, particularly if a specific electrode microstructure or orientation is desired. Optionally, a seed layer may also be formed between a top surface of the actuation layer 206 and/or the sensing layer 208 and the second set of electrodes 210, 212 to ensure robust adhesion or interface conductivity, especially when thin-film stack integrity is critical for resonator 15 reliability.
[0067]
The specific composition, thickness, and deposition conditions of the seed layers may be tailored based on the selected deposition technique used for forming the actuation layer 206 and/or the sensing layer 208. These techniques may include, but are not limited to, sputtering, chemical solution deposition (e.g., sol-20 gel), metal-organic chemical vapor deposition (MOCVD), atomic layer deposition (ALD), or pulsed laser deposition (PLD).
[0068] FIG. 2B illustrates a cross-sectional view of the circular MEMS resonator 200, according to another example of the present subject matter. As already described with reference to FIG. 2A, the circular MEMS resonator 200 includes a 25 first set of electrodes having a first electrode 204a and a second electrode 204b (collectively referred to as the first set of electrodes 204). Further, an actuation layer 206 and a sensing layer 208 is deposited on the first electrode and the second electrode from the first set of electrodes 204. In addition, the circular MEMS resonator 200 includes a second set of electrodes comprising of electrode 210 and 30 19
electrode 212. The lateral arrangement of the first electrode 204a, the second electrode 204b, the actuation layer 206, the sensing layer 208, the electrode 210, and the electrode 212 is separated by vias 214.
[0069] In addition, the circular MEMS resonator 200 includes a device layer 216 positioned above the substrate 202. The device layer 216 provides structural support 5 for the various components of the MEMS resonator 200, including the electrodes, actuation layer 206, and the sensing layer 208. The device layer 216 may provide a stable platform upon which other functional components may be built. The device layer 216 may be composed of a material such as silicon, which offers both mechanical stability and compatibility with standard semiconductor fabrication 10 processes. The thickness and properties of the device layer 216 may be tailored to optimize the resonator's performance characteristics, including its resonant frequency and quality factor. Additionally, the device layer 216 may incorporate features, such as anchors or tethers that help define the resonator's mode of vibration and isolate the circular MEMS resonator 200 from the surrounding substrate 202, 15 thereby enhancing overall efficiency and sensitivity of the circular MEMS resonator 200.
[0070] Further, the circular MEMS resonator 200 defines a backside cavity 218 formed beneath the device layer 216 to enable the circular MEMS resonator 200 to vibrate in the transverse direction, thereby establishing a clamped boundary at the 20 periphery of the circular structure. In an example, the backside cavity 218 may create a suspended structure with desired dimensions, and it also allows the resonator to vibrate more freely in the transverse direction.
[0071] The presence of the backside cavity 218 may establish a clamped boundary condition at the periphery of the circular structure. This clamping effect 25 occurs where the device layer 216 may meet the substrate 202, creating a fixed edge around the circumference of the circular MEMS resonator 200. The clamped boundary serves to confine the vibrational energy within the active region of the circular MEMS resonator 200, thereby leading to higher quality factors and improved frequency selectivity. The combination of the laterally stacked 30 20
ferroelectric and piezoelectric materials with the backside cavity 218 may provide a unique device configuration. This arrangement may enable the exploitation of both ferroelectric and piezoelectric properties within a single resonator structure, while the backside cavity 218 provides the necessary freedom of movement for efficient electromechanical coupling. 5
[0072] FIG. 2C illustrates another cross-sectional view of the circular piezoelectric MEMS resonator 200, according to an example of the present subject matter. As illustrated in FIG. 2C, the circular MEMS resonator 200 includes second set of electrodes having four electrodes 210a, 210b, 212a, 212b that may be patterned on top of each actuation layer 206 (ferroelectric layer) and each sensing 10 layer 208 (piezoelectric layer) in multiple configurations to improve actuation and sensing for other vibration modes of the circular MEMS resonator 200.
[0073] Further, the circular MEMS resonator 200 includes vias 214a, 214b, and 214c. For example, the lateral arrangement of the first electrode 204a, the second electrode 204b, the actuation layer 206, the sensing layer 208, the electrode 210b, 15 and the electrode 212a is separated by vias 214a. In addition, vias 214b may laterally separate the electrodes 210a and 210b on the actuation layer 206. Likewise, the vias 214c may laterally separate the electrodes 212a and 212b on the sensing layer 208. The strategic placement of the vias 214b and 214c may allow for more precise control over the electric field distribution and the resulting mechanical 20 deformation of the resonator structure.
[0074] As may be noted, the electrodes 210a and 212b are positioned on the opposite strain side of the electrodes 210b and 212a to optimize the functionality of the circular MEMS resonator 200. These electrodes are strategically positioned to interact with different regions of the underlying materials. This arrangement may 25 enable the circular MEMS resonator 200 to excite and detect a wider range of vibration modes compared to simpler electrode configurations. By placing the second set of electrodes in regions of opposing strain, the circular MEMS resonator 200 may achieve improved electromechanical coupling, thereby leading to
21
enhanced sensitivity, better signal-to-noise ratio, and the ability to selectively excite or detect specific vibration modes.
[0075] FIG. 2D illustrates an optical micrograph of the circular piezoelectric MEMS resonator 200 with lateral stacked ferroelectric and piezoelectric materials, according to an example of the present subject matter. In an example, the circular 5 piezoelectric MEMS resonator, with a radius of 300 microns is fabricated using a 5-micron silicon-on-insulator wafer providing a robust foundation for the MEMS resonator. The circular geometry of the resonator may offer advantages in terms of symmetry, mode shape, and frequency allocation.
[0076] Further, the MEMS resonator comprises four electrodes in the second set 10 of electrodes, depicted by 210a, 210b, 212a, 212b. These electrodes are strategically positioned to interact with both the ferroelectric and piezoelectric regions of the device. The separate ground electrodes 204a and 204b are laterally patterned for both ferroelectric and piezoelectric thin films. As a non-limiting example, the ferroelectric material used is PZT, and the piezoelectric material is AlN. The use of 15 PZT as the ferroelectric material and AlN as the piezoelectric material represents a combination of materials with distinct properties. PZT, known for its strong piezoelectric response and ferroelectric behavior, may provide enhanced actuation capabilities and tunability. AlN, on the other hand, offers high acoustic velocity and good thermal stability. The lateral integration of these materials within a single 20 resonator structure may allow for the exploitation of their complementary properties, potentially enabling novel functionalities or improved performance in sensing, frequency control, or signal processing applications.
[0077] Preliminary characterization of the fabricated circular piezoelectric MEMS resonator with laterally stacked ferroelectric and piezoelectric thin films can 25 be conducted by measuring the large-span frequency response and corresponding mode shapes.
[0078] FIG. 3A illustrates a mechanical frequency response 300 of a circular MEMS resonator, such as the MEMS resonator 200, according to an example of the present subject matter. In an example, FIG. 3A depicts the frequency response 300 30 22
of first four flexure modes of the circular MEMS resonator 200 and their respective mode shapes, as measured using a laser Doppler vibrometer (LDV). Flexural modes, also known as bending modes, refer to the vibrational patterns of a structure where it bends or flexes perpendicular to its plane. In the context of a circular MEMS resonator, these modes involve a circular plate deforming out of its resting 5 plane, with different mode shapes corresponding to various patterns of nodal lines and antinodes across the resonator's surface.
[0079] To observe the mechanical frequency response 300, the circular MEMS resonator 200 may be actuated by a piezoelectric AlN thin film, such as the actuation layer 206, with a driving signal being inputted through the second set of 10 electrodes. Further, the first set of electrodes 204 may be connected to a ground. As may be understood, the second set of electrodes, used for inputting the driving signal is positioned atop the piezoelectric AlN thin film. The second set of electrodes, therefore allow the LDV to track the single-point transverse motion and capture the average spectrum for mode shapes. 15
[0080] As depicted in FIG. 3A, distinct resonance peaks corresponding to the first four flexural modes of the circular structure, with each peak associated with a unique mode shape visualized alongside the frequency response curve. Each flexural mode of the circular MEMS resonator 200 may indicate a distinct pattern of vibration at a specific resonant frequency. These modes may be characterized by 20 their unique spatial distributions of displacement across the resonator's surface, with alternating regions of maximum (antinodes) and minimum (nodes) displacement.
[0081] For example, the first mode 302 may correspond to a lowest frequency peak, occurring around 100 kHz. The first flexural mode 302 typically exhibits a 25 simple dome-like shape with maximum displacement at the center and minimum at the edges. Further, the second mode 304 may be associated with the second peak, appearing at approximately 300 kHz. For example, the second mode 304 may have a nodal line across the diameter of the circular MEMS resonator 200, creating two regions vibrating out of phase with each other. The third mode 306 may be 30 23
represented by the third peak, manifesting at roughly 600 kHz. Finally, the fourth mode 308 may correspond to the highest frequency peak, emerging near 900 kHz. The third mode 306 and fourth mode 308 may introduce additional nodal lines and regions, resulting in more intricate vibrational patterns. Each successive mode occurs at a higher frequency and represents a different way the circular MEMS 5 resonator 200 may store and transfer energy between its kinetic and potential forms. These distinct modes are helpful in various applications, as these modes offer different sensitivity distributions and frequency options for sensing, actuation, and signal processing tasks.
[0082] FIG. 3B illustrates a second flexure mode, such as the second mode 304 10 of the circular MEMS resonator 200, according to an example of the present subject matter. In an example, the second mode 304 may be characterized by two distinct regions of displacement separated by a nodal line 310 that bisects the circular resonator 200. The rainbow colormap representation of the second flexural mode 304 shows areas of positive displacement (represented by lighter shades 312) on 15 one side of the nodal line 310 and negative displacement (represented by darker shades 314) on the opposite side.
[0083] It would be evident to a person skilled in the art that while the second mode 304 is used here as an example for demonstrating the present subject matter, other modes may also be utilized to achieve similar performance, provided they 20 activate both ferroelectric and piezoelectric materials in an equivalent manner for efficient actuation and sensing. This flexibility allows for the validation of performance of the circular MEMS resonator 200 across different modes, enhancing the overall applicability and versatility of the laterally stacked material configuration in the circular MEMS resonator 200. 25
[0084] FIG. 4A depicts an electrical frequency response graph 400A of a circular MEMS resonator, such as the MEMS resonator 200, when a piezoelectric material is used for actuation and a ferroelectric material is used for sensing, according to an example of the present subject matter. Specifically, FIG. 4A illustrates the frequency response performance of the second mode of the circular MEMS resonator 200. A 30 24
system for measuring the electrical frequency response may include a lock-in amplifier, employed to provide a driving signal with a frequency sweep range and various AC actuation voltages ranging from 0.1 V to 0.7 V to the actuation layer 206 made of the piezoelectric thin film (AlN).
[0085] Thereafter, an output signal may be received from the sensing layer 208 5 made of ferroelectric film (PZT), with no DC bias voltage applied. The frequency response reveals that the laterally stacked piezoelectric and ferroelectric thin films effectively function for both actuation and sensing. As the driving AC actuation voltage increases, the frequency response exhibits nonlinear behavior.
[0086] The electrical frequency response graph 400A depicts a series of overlaid 10 frequency response curves, each corresponding to a different AC actuation voltage. For example, the resonant peaks are centered around 280 kHz, with the magnitude of the response increasing as the actuation voltage increases. The maximum peak magnitude reaches approximately 1.6 mV at the highest actuation voltage. The electrical frequency response graph 400A clearly illustrates the resonator's behavior 15 across a frequency range of 270-290 kHz, showing well-defined resonant peaks and consistent baseline responses away from the resonance. The increasing peak magnitudes and slight frequency shifts with higher actuation voltages demonstrate the voltage-dependent response of the circular MEMS resonator 200, highlighting the effectiveness of the piezoelectric actuation and ferroelectric sensing. 20
[0087] FIG. 4B depicts an electrical frequency response graph 400B of a circular MEMS resonator, such as the circular MEMS resonator 200 when a ferroelectric material is used for actuation and a piezoelectric material is used for sensing, according to an example of the present subject matter. A system for measuring the electrical frequency response may include a lock-in amplifier, employed to provide 25 a driving signal with a frequency sweep range and various AC actuation voltages ranging from 0.1 V to 0.7 V to the actuation layer 206 made of the ferroelectric thin film (PZT), with no DC bias voltage applied.
[0088] Thereafter, an output signal may be received from the sensing layer 208 made from the piezoelectric thin film (AlN). The resulting response demonstrates a 30 25
higher output electrical magnitude without any bias voltage compared to the case when piezoelectric materials is used for actuation and ferroelectric material is used for sensing. This finding aligns with one embodiment of the present disclosure, wherein the PZT material, due to its higher transduction coefficient, is more efficient for actuation, whereas the AlN material, with its higher sensing coefficient, 5 is better suited for sensing. As the driving AC actuation voltage increases, the frequency response exhibited by the PZT actuation layer 206 and the AlN sensing layer 208 is a linear behavior. The increasing peak magnitudes and slight frequency shifts with higher actuation voltages demonstrate the voltage-dependent response of the circular MEMS resonator 200. 10
[0089] The electrical frequency response graph 400B shows a series of overlaid frequency response curves, each corresponding to a different AC actuation voltage applied to the actuation layer 206 made of the ferroelectric material. The resonant peaks are centered around 280 kHz, similar to FIG. 4A, but with higher magnitude responses. The maximum peak magnitude reaches approximately 3.0 mV at the 15 highest actuation voltage, nearly double the magnitude observed in FIG. 4A. The graph 400B clearly illustrates the behavior of the circular MEMS resonator 200 across a frequency range of 270-290 kHz, showing well-defined resonant peaks and consistent baseline responses away from the resonance.
[0090] The comparison between FIG. 4A and FIG. 4B illustrates the impact of 20 swapping the actuation and sensing roles of the piezoelectric and ferroelectric materials, demonstrating the flexibility and potential advantages of this laterally stacked configuration in MEMS resonator design.
[0091] FIG. 5A depicts frequency response plots 500A of a laterally stacked MEMS resonator when a piezoelectric thin film is used for actuation, according to 25 an example of the present subject matter. Specifically, FIG. 5A depicts the frequency response plot 500A of the second mode of a circular MEMS resonator, such as the MEMS resonator 200, upon application of a DC bias. In an example, a 500 mV AC actuation voltage may be applied to the actuation layer 206 made of the piezoelectric thin film (AlN), and the electrical output signals may be collected 30 26
from the second set of electrodes of the sensing layer 208 made of the ferroelectric thin film (PZT).
[0092] In the present example, a 5 V DC bias voltage is applied to the sensing layer 208 made of the PZT film during sensing. The application of the 5 V DC bias voltage to the PZT film may result in an enhanced overall FoM of the circular 5 MEMS resonator 200. The frequency response plot 500A may show the magnitude of response of the circular MEMS resonator 200 versus frequency in the range of approximately 270-290 kHz. The plot 502 with the DC bias applied may exhibit a higher peak magnitude of about 4.5 mV at around 279 kHz, while the plot 504 without DC bias may display a lower peak of about 1.3 mV at the same frequency. 10
[0093] The enhanced response with DC bias may be attributed to several factors. In some cases, the application of the DC bias to the sensing layer 208 may lead to domain alignment, potentially increasing the overall piezoelectric response of the material. This alignment may result in a stronger electromechanical coupling, which in turn may improve the overall capabilities of the device. 15
[0094] FIG. 5B depicts frequency response plot 500B of a laterally stacked MEMS resonator, such as the circular MEMS resonator 200, when a ferroelectric thin film is used for actuation, according to an example of the present subject matter. Specifically, FIG. 5B depicts the frequency response of the second mode of the circular MEMS resonator 200 when the drive signal is applied to the actuation layer 20 206 made of the ferroelectric thin film (PZT), upon application of a DC bias. In an example, a 500 mV AC actuation voltage and the 5 V DC bias voltage may be applied to the ferroelectric thin film (PZT). Further, the electrical output signals may be collected from the second set of electrodes of the sensing layer 208 made of the piezoelectric thin film (AlN). 25
[0095] The frequency response plot 500B displays two frequency response curves, each representing different actuation conditions applied to the ferroelectric thin film (PZT) while sensing from the piezoelectric thin film (AlN). The x-axis of the frequency response plot 500B represents a frequency range, centered around 27
279 kHz, which corresponds to the resonant frequency of the second mode. The y-axis shows the magnitude of the output signal in millivolts (mV).
[0096] The first curve 506 may represent the frequency response when a 500 mV AC actuation voltage is applied to the actuation layer 208 (PZT film) in combination with a 5 V DC bias voltage. The curve 506 may exhibit a pronounced 5 peak with a maximum magnitude of approximately 20 mV at the resonant frequency. The second curve 508 may illustrate the frequency response under the same 500 mV AC actuation voltage applied to the actuation layer 208 (PZT film), but without the DC bias voltage. The curve 508 also displays a resonant peak, but with a significantly lower magnitude of about 500 mV at the resonant frequency. 10
[0097] The difference in peak magnitudes between the first curve 506 and the second curve 508 may demonstrate the impact of applying a DC bias voltage to the ferroelectric thin film. The addition of the 5 V DC bias may result in an approximately four-fold increase in the output electrical voltage compared to the case without bias. This enhancement in output voltage may be attributed to several 15 factors, including improved domain alignment, enhanced piezoelectric coefficients, reduced hysteresis effects, and optimized operating point of the ferroelectric material. This increase in output voltage further enhances the overall FoM of the circular MEMS resonator 200.
[0098] The comparison between FIG. 5A and FIG. 5B illustrates the impact of 20 swapping the actuation and sensing roles of the piezoelectric and ferroelectric materials with biasing, demonstrating the flexibility, tuning, and potential advantages of this laterally stacked configuration in MEMS resonator design.
[0099] In an implementation, the transduction coefficient may be enhanced by performing poling of the ferroelectric thin film. Poling involves applying a strong 25 electric field to the ferroelectric material, typically at elevated temperatures, to align the ferroelectric domains permanently. Poling may enhance the alignment of the dipoles within the ferroelectric material, thereby optimizing actuation and sensing capabilities of the MEMS resonator 200. 28
[00100] FIG. 6A illustrates a cross-sectional view of a laterally stacked MEMS resonator 600A, according to another example of the present subject matter. The MEMS resonator 600A may be built on a substrate 602 with a device layer 604 positioned above the substrate 602. The MEMS resonator 600A may incorporate laterally arranged electrodes and materials, including a first set of electrodes having 5 a first electrode 606a and a second electrode 606b (collectively referred to as the first set of electrodes 606) deposited on the device layer 604. As would be evident from FIG. 6B, the first electrode 606a and the second electrode 606b are same. Further, an actuation layer 608 made of a ferroelectric thin film may be positioned above the first electrode 606a, while a sensing layer 610 made of a piezoelectric 10 thin film may be is located above the second electrode 606b, creating a lateral arrangement of these functional materials.
[00101] Further, a top portion of the MEMS resonator 600A features a second set of electrodes. For example, the second set of electrodes includes multiple electrodes arranged laterally, i.e., a first top electrode 612a, a second top electrode 612b, a 15 third top electrode 612c, and a fourth top electrode 612d (collectively referred to as the second set of electrodes 612). These top electrodes are positioned above the actuation layer 608 and the sensing layer 610, allowing for various actuation and sensing configurations. In addition, a separation via 614a provides electrical isolation between different sections of the MEMS resonator 600. 20
[00102] In the present embodiment, the MEMS resonator 600 includes a cavity 616 that may be created through the substrate 602 by etching the substrate 602 from a backside using a dry etch. For example, as shown in FIG. 6A, the cavity 616 is created using surface micromachining techniques rather than through-wafer etching. In the surface micromachining approach, a sacrificial layer (not shown) 25 may be initially deposited on the substrate 602 in the region where the cavity 616 is desired. This sacrificial layer may be composed of materials, such as single-crystal silicon, polycrystalline silicon, silicon nitride, or silicon oxide. The choice of material depends on factors including etch selectivity, deposition methods, and compatibility with subsequent processing steps. The sacrificial layer can be 30 29
deposited using various methods such as Physical Vapor Deposition (PVD), Chemical Vapor Deposition (CVD), or thermal oxidation in the case of silicon oxide.
[00103] After the MEMS resonator 600A is built on top of the sacrificial layer, the sacrificial layer may be selectively removed through etching processes, creating 5 the cavity 616. The surface micromachining may allow for precise control over dimensions and shape of the cavity 616, potentially enabling more complex resonator designs and improved performance characteristics.
[00104] FIG. 6B illustrates a cross-sectional view of a laterally stacked MEMS resonator 600B, according to another example of the present subject matter. In the 10 present embodiment, the MEMS resonator 600B may be built directly on a substrate 602 and a device layer is entirely omitted. In an example, the first set of electrodes comprising the first electrode 606a and the second electrode 606b (collectively referred to as the first set of electrodes 606) may be deposited from a backside of the MEMS resonator 600B. Further, a structural layer 607 may be deposited on a 15 top side of the substrate 602 using a suitable low-temperature deposition technique. The structural layer 607 may serve as the foundation for the subsequent functional layers and electrodes. In an example, a deposition method for the structural layer 607 may depend on factors such as material compatibility, desired thickness, and thermal budget constraints. Example deposition techniques may include plasma-20 enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD), or sputtering, among others.
[00105] In addition, the MEMS resonator 600B may incorporate laterally arranged functional materials, with an actuation layer 608 made of a ferroelectric material is positioned adjacent to a sensing layer 610 made of a piezoelectric 25 material. The actuation layer 608 and the sensing layer 610 may be deposited above the structural layer 607 and may be tailored to optimize the actuation and sensing capabilities of the MEMS resonator 600B. Further, a top portion of the MEMS resonator 600B may feature a second set of electrodes. In an example, the second set of electrodes include multiple electrodes arranged laterally, such as a first top 30 30
electrode 612a, a second top electrode 612b, a third top electrode 612c, and a fourth top electrode 612d (collectively referred to as the second set of electrodes 612). The second set of electrodes 612 are positioned above the actuation layer 608 and the sensing layer 610, allowing for various actuation and sensing configurations.
[00106] Further, the MEMS resonator 600B includes vias etched through the 5 actuation and sensing layers to allow for electrical contact with the first set of electrodes 606, enabling the actuation and sensing functions of the MEMS resonator 600B. For example, the vias may include separation via 614a and lateral vias 614b and 614c to provide electrical isolation between different sections of the MEMS resonator 600B and allow for connections to the underlying electrodes. 10 Further, the MEMS resonator 600B may include a cavity 616 formed beneath the structural layer 607, allowing for mechanical movement of the MEMS resonator 600B. The formation of the cavity 616 may involve selective etching of a sacrificial layer or other micromachining techniques, depending on the specific fabrication process employed. 15
[00107] It would be evident to a person skilled in the art that the order of the first set of electrodes 606, the second set of electrodes 612, the actuation layer 608, and the sensing layer 610 may be inverted based on design requirements. This flexibility in the fabrication process allows for various configurations to be utilized, tailoring the resonator design to specific application needs. For example, the positions of the 20 ferroelectric and piezoelectric layers may be swapped, or the arrangement of second set of electrodes may be modified to achieve different actuation and sensing characteristics. This adaptability in design and fabrication enables the optimization of the resonator's performance for a wide range of applications in MEMS devices.
[00108] FIG. 7 illustrates a top view of a circular configuration of a MEMS 25 resonator 700, according to an example of the present subject matter. In an example, the MEMS resonator 700 may include an outer region 702 and a central region 704. The outer region 702 may form an annular ring surrounding the central region 704, which may be a circular area. For example, the central region 704 may be covered with a ferroelectric material, while the outer region 702 may be covered with a 30 31
piezoelectric material. In another example, the MEMS resonator 700 may include an outer region 706 and a central region 708. The outer region 706 may form an annular ring surrounding the central region 708, which may be a circular area. For example, the central region 708 may be covered with a piezoelectric material, while the outer region 706 may be covered with a ferroelectric material. 5
[00109] Such arrangement of the outer region and the central region may allow for selective actuation and sensing in different areas of the MEMS resonator 700, thereby enabling complex mode shapes or improved control over the resonator's behavior. This approach contrasts with configurations where the circular region is divided into two halves, each covered by a different piezoelectric material, 10 highlighting the versatility of laterally depositing multiple ferroelectric and piezoelectric materials to suit specific functional requirements.
[00110] Referring back to FIG. 7, the central regions 704, 708 and the outer regions 702, 706 may be associated with different electrodes. For instance, the central regions 704, 708 may correspond to a central circular electrode, while the 15 outer regions 702, 706 may correspond to an outer annular electrode. This electrode configuration, combined with the distinct material regions, may provide flexibility in how the MEMS resonator 700 is driven and sensed.
[00111] The concentric arrangement of materials in FIG. 7 may also facilitate the excitation of radial modes or other symmetrical vibration patterns that may be 20 particularly suited to circular resonator geometries. By carefully selecting the materials and their placement in the outer regions 702, 706 and the central regions 704, 708, the MEMS resonator 700 may be optimized for specific frequency ranges or mode shapes.
[00112] In various implementations, the actuation and sensing layers may serve 25 multiple functions as driving, sensing, floating, or ground electrodes. It may not be necessary for the first set of electrodes to be used exclusively as a ground electrode; it may alternatively act as an actuation or driving electrode, or even a floating electrode, depending on the specific design requirements, since each electrode is individually patterned. Furthermore, rather than utilizing just two piezoelectric 30 32
materials, multiple distinct piezoelectric and ferroelectric materials may be incorporated within a single MEMS resonator, allowing for a variety of configurations. For example, a material with a higher piezoelectric coefficient may be used in one region, while a material with a different Young's modulus or Poisson's ratio may be used in the other region. These materials may differ in their 5 properties, such as piezoelectric coefficients, Young's modulus, Poisson's ratio, and permittivity, among others.
[00113] When these distinct materials are stacked laterally, they enable excitation in both in-plane and out-of-plane modes, thus facilitating higher-order vibrational modes. This arrangement allows for the use of different driving and sensing 10 combinations tailored to the specific material properties.
[00114] FIG. 8 illustrates different configurations of a circular MEMS resonator 800, according to an example of the present subject matter. For example, FIG. 8 illustrates configurations with multiple combinations of first set of electrodes and the second set of electrodes enabling the utilization of two or more piezoelectric or 15 ferroelectric materials within a single device. Such an arrangement may permit the application of DC bias voltages to each lateral ferroelectric layer, thereby tuning and enhancing the transduction and sensing parameters for different combinations of ferroelectric and piezoelectric materials. Additionally, the ferroelectric layer may be poled in one or multiple directions, which may further augment the device's 20 performance characteristics.
[00115] In various embodiments, the lateral stacking of ferroelectric and piezoelectric materials in the MEMS resonator 800 may also be used to enhance transduction and sensing performance of the MEMS resonator 800 by poling (one or multi-direction) the ferroelectric layer. Additionally, lateral stacking of multiple 25 combinations of ferroelectric and piezoelectric materials with low and high thermal expansion coefficients (TCF) may also be used to control the overall TCF of the MEMS resonator 800. Furthermore, the nonlinear behavior of the MEMS resonator 800 may be modulated by strategically depositing and driving calculated combinations of laterally stacked ferroelectric and piezoelectric materials. These 30 33
materials may possess different intrinsic and extrinsic material properties, as well as mechanical nonlinearities, allowing for precise control over the resonator's nonlinear response characteristics.
[00116] FIGS. 9A and 9B depict frequency response graphs 900A and 900B, respectively, comparing performance of laterally stacked ferroelectric and 5 piezoelectric materials in MEMS resonators, according to examples of the present subject matter. The graphs 900A and 900B illustrate the results of finite element analysis simulations conducted using COMSOL to demonstrate the working combination of ferroelectric and piezoelectric materials in MEMS resonators beyond the circular configuration. The simulations may focus on the frequency 10 response of torsional and contour vibration modes in two distinct MEMS resonator designs.
[00117] In these simulations, PZT may be used as the ferroelectric material and AlN as the piezoelectric material. The frequency responses shown in FIG. 9A may be obtained by simultaneously actuating and sensing through both PZT and AlN 15 layers in the laterally stacked configuration. This approach may allow for a direct comparison of the performance characteristics when using different combinations of drive and sense configurations, such as AlN Drive & PZT Sense versus PZT Drive & AlN Sense. The resulting graphs may provide insights into how the lateral stacking of ferroelectric and piezoelectric materials can influence the resonator's 20 frequency response and overall performance in various vibration modes. Specifically, the graph 900A shows how a cantilever MEMS resonator responds to different frequencies when the cantilever MEMS resonator twists. The graph 900A displays behavior of the cantilever MEMS resonator upon vibration in a torsional (twisting) motion at various frequencies. 25
[00118] As depicted in FIG. 9A, the graph 900A displays a magnitude response versus frequency for a cantilever MEMS resonator operating in a torsional mode, within the range of 300-320 kHz. The graph 900A indicates a comparison between two configurations: curve 902 depicting AlN Drive & PZT Sense versus curve 904 depicting PZT Drive & AlN Sense, with magnitude plotted on a logarithmic scale. 30 34
In graph 900A, distinct resonant peaks are visible for each configuration, showing differences in resonant frequency, peak magnitude, and overall response shape. These variations may indicate how the choice of drive and sense materials (AlN or PZT) affects the resonator's performance in the torsional mode. The comparison depicted in 900A provides an assessment of how the laterally stacked ferroelectric 5 (PZT) and piezoelectric (AlN) materials perform when used in different drive and sense combinations within the same resonator structure.
[00119] On the other hand, the graph 900B displays how a support transducer with side-by-side layers of PZT and AlN materials responds to different frequencies. The graph 900B shows behavior of the support transducer upon 10 vibration in a contour mode, where the edges of the structure expand and contract. Specifically, FIG. 9B shows the magnitude response versus frequency in the range of 16-17 MHz, comparing the performance of AlN Drive & PZT Sense (curve 906) versus PZT Drive & AlN Sense (curve 908) configurations. The graph 900B demonstrates distinct resonant peaks for these different drive-sense combinations, 15 with the magnitude plotted on a logarithmic scale on the y-axis.
[00120] The comparison between AlN Drive & PZT Sense and PZT Drive & AlN Sense configurations reveal differences in performance characteristics, such as resonant frequency, peak magnitude, and quality factor. These differences could be attributed to the unique properties of the ferroelectric (PZT) and piezoelectric (AlN) 20 materials when used for driving or sensing in the laterally stacked arrangement. The frequency responses shown in FIGS. 9A and 9B collectively demonstrate a wide range of operational frequencies, with FIG. 9A illustrating the torsional mode of the cantilever resonator and FIG. 9B representing the contour mode in the support transducer. 25
[00121] In an implementation of the present subject matter, multiple resonators of the type described herein may be fabricated on a single substrate as an array. FIG. 10A illustrates a top view of an array 1000A of circular resonator devices with hybrid materials, according to an example of the present subject matter. Specifically, FIG. 10A depicts an example of the array 1000A containing multiple 30 35
circular piezoelectric MEMS resonators with laterally stacked ferroelectric and piezoelectric materials integrated on a common substrate. Each resonator in the array 1000A may comprise two distinct semicircular regions, one containing ferroelectric material 1002 and the other containing piezoelectric material 1004. The array 1000A may allow for the integration of both ferroelectric and 5 piezoelectric properties within a single resonator unit, while maintaining a uniform circular shape across the array. The resonators may be arranged in a regular grid pattern, which may facilitate efficient use of substrate area and potentially simplify fabrication processes.
[00122] FIG. 10B illustrates a top view of an array 1000B of circular resonator 10 devices with hybrid materials, according to an example of the present subject matter. In an example, the array 1000B may also be fabricated by arranging multiple MEMS resonators, each containing different ferroelectric and piezoelectric materials and integrated in a specific order on a single substrate, as shown in FIG. 10B. This configuration may present an alternating checkerboard pattern of circular 15 resonators, where individual resonators may contain either ferroelectric 1006 or piezoelectric 1008 materials. The alternating arrangement may allow for a diverse set of resonator properties within the array, potentially enabling a wider range of functionalities or improved overall performance. The uniform spacing and consistent circular shape of the resonators may be maintained in this configuration, 20 which may provide advantages in terms of predictable behavior and ease of integration with other components or systems.
[00123] FIG. 11 illustrates a method 1100 for fabricating a laterally stacked MEMS resonator, according to an example of the present subject matter. The order in which the method is described is not intended to be construed as a limitation, and 25 any number of the described method blocks may be combined in any order to implement the methods, or an alternative method. Additionally, implementation of the method is not limited to such examples.
[00124] Referring to FIG. 11, at block 1102, the method 1100 may include depositing a first set of electrodes on a substrate. This step involves the formation 30 36
of the bottom electrodes that will serve as the foundation for the resonator structure. The substrate may be a semiconductor material such as silicon, and the electrodes may be deposited using techniques such as physical vapor deposition, sputtering, or electron beam evaporation. The electrode material may be selected based on its conductivity, compatibility with subsequent layers, and ability to withstand the 5 fabrication process. The pattern and spacing of these electrodes are crucial as they define the active areas of the resonator and influence its overall performance.
[00125] In an example, the first set of electrodes may be deposited using common thin film deposition techniques such as physical vapor deposition (PVD), sputtering, or electron beam evaporation. These methods allow for precise control 10 of electrode thickness and composition, with sputtering often preferred for its ability to produce uniform, high-quality conductive layers on various substrate materials.
[00126] At block 1104, the method 1100 may include depositing one or more actuation layers and one or more sensing layers on the first set of electrodes. As 15 may be evident, the one or more actuation layers may be composed of materials that can convert electrical energy into mechanical motion, such as piezoelectric or ferroelectric materials. The actuation layers are responsible for generating the vibrations or displacements necessary for the resonator's operation when an electrical signal is applied. Further, the one or more sensing layers are designed to 20 convert mechanical energy into electrical signals. The sensing layers detect the mechanical vibrations or displacements of the resonator and transform them into measurable electrical outputs, allowing the device to function as a sensor or signal processing element.
[00127] The one or more actuation layers are deposited laterally to the one or 25 more sensing layers. The lateral stacking allows for the integration of materials with different properties within the same device, enabling separate actuation and sensing functionalities. In an example, the one or more actuation layers and the one or more sensing layers are made of combinations of distinct ferroelectric-piezoelectric material, piezoelectric-ferroelectric material, ferroelectric-ferroelectric material, 30 37
and piezoelectric-piezoelectric material. The deposition process may involve techniques such as sol-gel deposition, sputtering, or chemical vapor deposition, depending on the specific materials used and the desired layer properties.
[00128] At block 1106, the method 1100 may include depositing a second set of electrodes on the one or more actuation layers and the one or more sensing layers. 5 The one or more actuation layers and the one or more sensing layers are interposed between the first set of electrodes and the second set of electrodes. The deposition process for the second set of electrodes, i.e., top electrodes is similar to that used for the first set of electrodes, i.e., bottom electrodes. The configuration of the top electrodes may be designed to allow for specific excitation and detection modes in 10 the resonator.
[00129] At block 1108, the method 1100 may include forming at least one via to separate electrodes in the first set of electrodes and the second set of electrodes, and to separate the one or more actuation layers and the one or more sensing layers. These vias serve multiple purposes: they provide electrical isolation between 15 different electrodes and functional regions of the resonator, they define the boundaries of the active resonator areas, and they may also contribute to the mechanical properties of the device. The via formation process may involve techniques such as reactive ion etching, wet etching, or laser ablation, depending on the materials used and the desired via characteristics. The placement and 20 dimensions of these vias are critical for the proper functioning of the resonator, as they influence the electrical and mechanical behavior of the device.
[00130] The present subject matter provides several advantages in the design and performance of various MEMS resonators. By utilizing laterally stacked ferroelectric and piezoelectric materials, the resonator architecture as described 25 here allows for the optimization of actuation and sensing through distinct material combinations with varying mechanical and electrical properties, such as piezoelectric coefficient, Young’s modulus, Poisson’s ratio, and permittivity. Further, the configuration of electrodes may be tailored to achieve desired actuation and sensing arrangements, enabling fundamental and higher-order mode excitation 30 38
and both in-plane and out-of-plane vibration modes. Furthermore, the arrangement of electrodes is flexible, allowing for the use of multiple electrodes for driving, sensing, and floating functions, which enhances the versatility of the device.
[00131] By applying DC bias voltage to the ferroelectric material, the actuation and sensing performance may be significantly improved, resulting in a 5 higher overall figure of merit for the resonator. Additionally, the temperature compensation capability may be realized by carefully selecting distinct material combinations with appropriate thicknesses, ensuring stable performance across varying environmental conditions.
[00132] The present subject matter outlines various examples and configurations 10 to illustrate the versatility and adaptability of the laterally stacked MEMS resonator. Although examples for the present disclosure have been described in language specific to structural features, it is to be understood that these examples are not necessarily limited to the specific features described. Rather, the specific features are disclosed and explained as examples of the present description. Any equivalent 15 implementations that align with the concepts described are also included within this scope.
39
I/We claim:
1. A laterally stacked micro-electro-mechanical system (MEMS) resonator (100, 200, 600A, 600B, 700, 800) comprising:
a first set of electrodes (106, 204, 606);
one or more actuation layers (108, 206, 608) and one or more sensing layers 5 (110, 208, 610) deposited laterally on the first set of electrodes (106, 204, 606), wherein the one or more actuation layers (108, 206, 608) and the one or more sensing layers (110, 208, 610) are made of combinations of distinct ferroelectric-piezoelectric material, piezoelectric-ferroelectric material, ferroelectric-ferroelectric material, and piezoelectric-piezoelectric material; and 10
a second set of electrodes (112, 210, 212, 612) deposited on the one or more actuation layers (108, 206, 608) and the one or more sensing layers (110, 208, 610), wherein the one or more actuation layers (108, 206, 608) and the one or more sensing layers (110, 208, 610) are interposed between the first set of electrodes (106, 204, 606) and the second set of electrodes (112, 210, 212, 612). 15
2. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 1, wherein a shape of the MEMS resonator (100, 200, 600A, 600B, 700, 800) is customizable according to any lithographically defined shape.
3. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 1, wherein the MEMS resonator (100, 200, 600A, 600B, 700, 800) 20 is configured to operate across different acoustic modes.
4. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 1, wherein the one or more actuation layers (108, 206, 608) have a transduction coefficient above a first range of values, and the one or more sensing layers (110, 208, 610) have a permittivity below a second range of values. 25
5. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 1, wherein the one or more actuation layers (108, 206, 608) and the one or more sensing layers (110, 208, 610) are deposited according to the strain 40
distribution of the desired modes of the MEMS resonator (100, 200, 600A, 600B, 700, 800) and to minimise signal loss or maximise actuation efficiency.
6. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 1, wherein the one or more actuation layers (108, 206, 608) are configured to actuate higher order vibrational modes, and the one or more sensing 5 layers (110, 208, 610) are configured to sense the higher order vibrational modes.
7. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 1, wherein one of the first set of electrodes (106, 204, 606) and the second set of electrodes (112, 210, 212, 612) act as ground electrodes.
8. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as 10 claimed in claim 7, wherein the ground electrodes are different for the one or more actuation layers (108, 206, 608) and the one or more sensing layers (110, 208, 610).
9. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 7, wherein a material of the ground electrode has material characteristics for optimal growth/deposition of the one or more actuation layers 15 (108, 206, 608) and one or more sensing layers (110, 208, 610), respectively.
10. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 7, wherein the ground electrodes comprise one or more layers of a material selected from but not limited to Titanium, Platinum, Molybdenum, or a combination thereof. 20
11. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 7, wherein the ground electrodes are deposited by using one of a chemical vapour deposition (CVD) technique, and a physical vapour deposition (PVD) technique.
12. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as 25 claimed in claim 1, wherein one of the first set of electrodes (106, 204, 606) and the second set of electrodes (112, 210, 212, 612) acts as input/output electrodes. 41
13. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 12, wherein the input/output electrodes comprise at least two layers of a material selected but not limited to Chromium, Gold, Titanium, Platinum, or a combination thereof.
14. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as 5 claimed in claim 12, wherein the input/output electrodes are deposited by using one of a chemical vapour deposition (CVD) technique, and a physical vapour deposition (PVD) technique.
15. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 1, wherein the one or more actuation layers (108, 206, 608) and 10 the one or more sensing layers (110, 208, 610) are separated by at least one via (114a, 614a).
16. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 1, wherein the ferroelectric material is composed of at least lead zirconate titanate (PZT), bismuth ferrite (BiFeO), lithium niobate (LiNbO3), 15 potassium niobate (KNbO3), sodium niobate (NaNbO3), lithium tantalate (LiTaO3), barium titanate (BaTiO3), lead titanate (PbTiO3), strontium titanate (SrTiO3), Polyvinylidene fluoride (PVDF), aluminum nitride (AlN), gallium nitride (GaN), lead magnesium niobate-lead titanate (PMNPT), zinc oxide (ZnO), or a combination thereof. 20
17. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 1, wherein the laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) comprises a substrate (102, 202, 602) on which the first set of electrodes (106, 204, 606) is deposited.
18. A method for fabricating a laterally stacked micro-electro-mechanical system 25 (MEMS) resonator (100, 200, 600A, 600B, 700, 800), the method comprising:
depositing a first set of electrodes (106, 204, 606) on a substrate (102, 202, 602); 42
depositing one or more actuation layers (108, 206, 608) and one or more sensing layers (110, 208, 610) on the first set of electrodes (106, 204, 606), the one or more actuation layers (108, 206, 608) are deposited laterally to the one or more sensing layers (110, 208, 610), wherein the one or more actuation layers (108, 206, 608) and the one or more sensing layers (110, 208, 610) are made of combinations 5 of distinct ferroelectric-piezoelectric material, piezoelectric-ferroelectric material, ferroelectric-ferroelectric material, and piezoelectric-piezoelectric material; and
depositing a second set of electrodes (112, 210, 212, 612) on the one or more actuation layers (108, 206, 608) and the one or more sensing layers (110, 208, 610), wherein the one or more actuation layers (108, 206, 608) and the one or more 10 sensing layers (110, 208, 610) are interposed between the first set of electrodes (106, 204, 606) and the second set of electrodes (112, 210, 212, 612).
19. The method as claimed in claim 18, the method comprising forming at least one via (114a, 614a) to separate electrodes in the first set of electrodes (106, 204, 606) and the second set of electrodes (112, 210, 212, 612), and to separate the one or 15 more actuation layers (108, 206, 608) and the one or more sensing layers (110, 208, 610).
43
ABSTRACT
RESONATORS HAVING LATERALLY STACKED FERROELECTRIC AND PIEZOELECTRIC MATERIALS
Example microelectromechanical systems (MEMS) resonators incorporating laterally stacked multiple ferroelectric and piezoelectric materials and 5 methods for fabricating the same are disclosed. A MEMS resonator (100, 200, 600A, 600B, 700, 800) comprises one or more actuation layers (108, 206, 608) and one or more sensing layers (110, 208, 610) deposited laterally on a first set of electrodes (106, 204, 606). The actuation layers and the sensing layers are made of combinations of distinct ferroelectric-piezoelectric material, piezoelectric-10 ferroelectric material, ferroelectric-ferroelectric material, and piezoelectric-piezoelectric material. In addition, the MEMS resonator includes a second set of electrodes (112, 210, 212, 612) deposited on the actuation layers and the sensing layers such that the actuation layers and the sensing layers are interposed between the first set of electrodes and the second set of electrodes. 15
<> 44 ,CLAIMS:I/We claim:
1. A laterally stacked micro-electro-mechanical system (MEMS) resonator (100, 200, 600A, 600B, 700, 800) comprising:
a first set of electrodes (106, 204, 606);
one or more actuation layers (108, 206, 608) and one or more sensing layers 5 (110, 208, 610) deposited laterally on the first set of electrodes (106, 204, 606), wherein the one or more actuation layers (108, 206, 608) and the one or more sensing layers (110, 208, 610) are made of combinations of distinct ferroelectric-piezoelectric material, piezoelectric-ferroelectric material, ferroelectric-ferroelectric material, and piezoelectric-piezoelectric material; and 10
a second set of electrodes (112, 210, 212, 612) deposited on the one or more actuation layers (108, 206, 608) and the one or more sensing layers (110, 208, 610), wherein the one or more actuation layers (108, 206, 608) and the one or more sensing layers (110, 208, 610) are interposed between the first set of electrodes (106, 204, 606) and the second set of electrodes (112, 210, 212, 612). 15
2. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 1, wherein a shape of the MEMS resonator (100, 200, 600A, 600B, 700, 800) is customizable according to any lithographically defined shape.
3. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 1, wherein the MEMS resonator (100, 200, 600A, 600B, 700, 800) 20 is configured to operate across different acoustic modes.
4. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 1, wherein the one or more actuation layers (108, 206, 608) have a transduction coefficient above a first range of values, and the one or more sensing layers (110, 208, 610) have a permittivity below a second range of values. 25
5. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 1, wherein the one or more actuation layers (108, 206, 608) and the one or more sensing layers (110, 208, 610) are deposited according to the strain 40
distribution of the desired modes of the MEMS resonator (100, 200, 600A, 600B, 700, 800) and to minimise signal loss or maximise actuation efficiency.
6. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 1, wherein the one or more actuation layers (108, 206, 608) are configured to actuate higher order vibrational modes, and the one or more sensing 5 layers (110, 208, 610) are configured to sense the higher order vibrational modes.
7. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 1, wherein one of the first set of electrodes (106, 204, 606) and the second set of electrodes (112, 210, 212, 612) act as ground electrodes.
8. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as 10 claimed in claim 7, wherein the ground electrodes are different for the one or more actuation layers (108, 206, 608) and the one or more sensing layers (110, 208, 610).
9. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 7, wherein a material of the ground electrode has material characteristics for optimal growth/deposition of the one or more actuation layers 15 (108, 206, 608) and one or more sensing layers (110, 208, 610), respectively.
10. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 7, wherein the ground electrodes comprise one or more layers of a material selected from but not limited to Titanium, Platinum, Molybdenum, or a combination thereof. 20
11. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 7, wherein the ground electrodes are deposited by using one of a chemical vapour deposition (CVD) technique, and a physical vapour deposition (PVD) technique.
12. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as 25 claimed in claim 1, wherein one of the first set of electrodes (106, 204, 606) and the second set of electrodes (112, 210, 212, 612) acts as input/output electrodes. 41
13. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 12, wherein the input/output electrodes comprise at least two layers of a material selected but not limited to Chromium, Gold, Titanium, Platinum, or a combination thereof.
14. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as 5 claimed in claim 12, wherein the input/output electrodes are deposited by using one of a chemical vapour deposition (CVD) technique, and a physical vapour deposition (PVD) technique.
15. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 1, wherein the one or more actuation layers (108, 206, 608) and 10 the one or more sensing layers (110, 208, 610) are separated by at least one via (114a, 614a).
16. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 1, wherein the ferroelectric material is composed of at least lead zirconate titanate (PZT), bismuth ferrite (BiFeO), lithium niobate (LiNbO3), 15 potassium niobate (KNbO3), sodium niobate (NaNbO3), lithium tantalate (LiTaO3), barium titanate (BaTiO3), lead titanate (PbTiO3), strontium titanate (SrTiO3), Polyvinylidene fluoride (PVDF), aluminum nitride (AlN), gallium nitride (GaN), lead magnesium niobate-lead titanate (PMNPT), zinc oxide (ZnO), or a combination thereof. 20
17. The laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) as claimed in claim 1, wherein the laterally stacked MEMS resonator (100, 200, 600A, 600B, 700, 800) comprises a substrate (102, 202, 602) on which the first set of electrodes (106, 204, 606) is deposited.
18. A method for fabricating a laterally stacked micro-electro-mechanical system 25 (MEMS) resonator (100, 200, 600A, 600B, 700, 800), the method comprising:
depositing a first set of electrodes (106, 204, 606) on a substrate (102, 202, 602); 42
depositing one or more actuation layers (108, 206, 608) and one or more sensing layers (110, 208, 610) on the first set of electrodes (106, 204, 606), the one or more actuation layers (108, 206, 608) are deposited laterally to the one or more sensing layers (110, 208, 610), wherein the one or more actuation layers (108, 206, 608) and the one or more sensing layers (110, 208, 610) are made of combinations 5 of distinct ferroelectric-piezoelectric material, piezoelectric-ferroelectric material, ferroelectric-ferroelectric material, and piezoelectric-piezoelectric material; and
depositing a second set of electrodes (112, 210, 212, 612) on the one or more actuation layers (108, 206, 608) and the one or more sensing layers (110, 208, 610), wherein the one or more actuation layers (108, 206, 608) and the one or more 10 sensing layers (110, 208, 610) are interposed between the first set of electrodes (106, 204, 606) and the second set of electrodes (112, 210, 212, 612).
19. The method as claimed in claim 18, the method comprising forming at least one via (114a, 614a) to separate electrodes in the first set of electrodes (106, 204, 606) and the second set of electrodes (112, 210, 212, 612), and to separate the one or 15 more actuation layers (108, 206, 608) and the one or more sensing layers (110, 208, 610).