DESC:FORM 2
THE PATENTS ACT 1970
[39 OF 1970]
&
THE PATENTS RULES, 2003

COMPLETE SPECIFICATION
[See section 10; rule 13]

Title: “ULTRA-LOW POWER PIEZOELECTRIC MICROMACHINED
ULTRASONIC TRANSDUCERS (PMUTs) WITH OPTIMIZED TOP
ELECTRODE”

Name and Address of the Applicant:
INDIAN INSTITUTE OF SCIENCE; CV Raman Rd, Bengaluru, Karnataka 560012.

Nationality: India

The following specification particularly describes the invention and the manner in which it is performed.

TECHNICAL FIELD

The present disclosure generally relates to ultrasound transducers and more particularly, to ultra-low power Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) with an optimized top electrode.

BACKGROUND

Generally, Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) are Micro-Electro-Mechanical System (MEMS) devices which operate in response to flexural motion of a thin membrane coupled with a thin piezoelectric film. In comparison with bulk piezoelectric ultrasound transducers, PMUTs can offer advantages such as increased bandwidth, flexible geometries, reduced voltage requirements, mixing of different resonant frequencies and potential for integration with supporting electronic circuits especially for miniaturized high frequency applications. The PMUTs may be used in applications such as proximity sensing, biomedical imaging, photoacoustic imaging, photoacoustic spectroscopy, gesture recognition, and several other applications including Data over Sound (DoS).
Conventionally, PMUTs are designed for different mechanical configurations and layers for obtaining different goals/objectives such as maximizing acoustic power output, increasing the on-axis output pressure, increasing linear operating range, increasing resonant frequency control, bandwidth, and other enhanced operating characteristics. FIG. 1 illustrates a material layer cross-section of a conventional PMUT having a piezoelectric layer 104 shown with a continuous layer of top electrode 102 and bottom electrode 106. The top electrode 102 and the continuous layer (i.e., blanket metal layer) of the bottom electrode 106, may form a parallel plate capacitor which can influence the electrical power draw of the PMUTs.
In recent times, intensive studies have been performed to design PMUTs with optimal dimensions of a top electrode to maximize diaphragm deflection, for a given voltage and output sound pressure level. However, such conventional PMUTs consume more electrical power, which may not be desirable in low powered devices using the PMUTs. Further, such conventional PMUTs may not be suitable for applications such as DoS or proximity sensing that require PMUTs to produce a certain level of acoustic pressure with a minimum power consumption, so that the PMUTs can operate for a long time on a small battery. In an example scenario, the least drawing current is approximately 2 milli Ampere (mA) using conventional PMUTs. At this rate of discharge current, the capacity of a small coin battery may be less than 100mA hour (mAh), which translates to a battery life of about two days, which may be still less for any long-term applications.
In view of the above, there is a need for minimizing electrical power consumption of the PMUTs and to maximize efficiency of the PMUT for the sound power emitted.

SUMMARY
In an embodiment, a Piezoelectric Micromachined Ultrasonic Transducer (PMUT) device is disclosed. The PMUT device includes a structural layer and a diaphragm. The diaphragm of radius ‘a’ is supported by the structural layer. The diaphragm includes a layer stack. The layer stack includes a piezoelectric layer, a bottom electrode layer and a top electrode layer. The piezoelectric layer is sandwiched between the bottom electrode layer and the top electrode layer. The top electrode layer includes a central electrode and a peripheral electrode. The central electrode has a radius ‘rp1’. The peripheral electrode encircles the central electrode and has an inner radius of ‘rp2’ and an outer radius of ‘a’. The peripheral electrode (204) has a width ‘wp’ which is at most 20% of the radius of the diaphragm ‘a’ and such that a radius ratio (rp2/a) ranges between 0.8 to 1 for a given residual stress factor (K2) of the diaphragm.
In another embodiment, a method for fabricating a Piezoelectric Micromachined Ultrasonic Transducer (PMUT) device is disclosed. The method includes preparing a wafer substrate. The method includes forming a diaphragm of radius ‘a’ on the wafer substrate by performing at least the following steps: (1) forming a bottom electrode layer on the wafer substrate, (2) depositing a piezoelectric layer on the bottom electrode layer, and (3) forming a top electrode layer including a central electrode and a peripheral electrode on the piezoelectric layer by (a) patterning the central electrode with a radius ‘rp1’, and (b) patterning a peripheral electrode encircling the central electrode with an inner radius of rp2 and an outer radius of ‘a’, wherein a width of the peripheral electrode is at most 20% of the radius of the diaphragm ‘a’ and such that a radius ratio (rp2/a) ranges between 0.8 to 1 for a given residual stress factor (K2) of the diaphragm. The method also includes etching the wafer substrate to form a structural layer, wherein the structural layer supports the diaphragm.
The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of this disclosure, illustrate exemplary embodiments and, together with the description, serve to explain the disclosed principles. The same numbers are used throughout the figures to reference like features and components. Some embodiments of device and/or methods in accordance with embodiments of the present subject matter are now described, by way of example only, and with reference to the accompanying figures, in which:

FIG. 1 illustrates a material layer cross-section of a conventional Piezoelectric Micromachined Ultrasonic Transducer (PMUT) having a piezoelectric layer shown with a continuous layer of top electrode and bottom electrode;
FIG. 2A illustrates material layer cross-section of an ultra-low power Piezoelectric Micromachined Ultrasonic Transducer (PMUT) device with an optimized top electrode, in accordance with some embodiments of the present disclosure;
FIG. 2B illustrates a top view of the ultra-low power Piezoelectric Micromachined Ultrasonic Transducer (PMUT) device of FIG. 2A, in accordance with some embodiments of the present disclosure;
FIGS. 3A and 3B illustrate a top view of a central electrode and a peripheral electrode, respectively, of the PMUT device of FIG. 2A, in accordance with some embodiments of the present disclosure;
FIGS. 4A and 4B illustrate variation of normalized output sound power of the PMUT device with respect to various radius ratios rp/a (x-axis) for the central electrode and the peripheral electrode for increasing K2 which forms a rationale for optimizing the central electrode and the peripheral electrode of the PMUT device;
FIG. 4C illustrates a graph plot for determining optimal electrode ratio for the first top electrode and the second top electrode, with increasing residual stress factor (Kappa (K2)) on diaphragm, in accordance with some embodiments of the present disclosure;
FIGS. 5A, 5B and 5C illustrate variation of efficiency with respect to radius ratio (rp/a) for the central electrode and the peripheral electrode for (a) a residual stress factor K2 = 10 on diaphragm in FIG. 5A, (b) a residual stress factor K2 = 100 in FIG. 5B and, (c) a residual stress factor K2 = 1000 in FIG. 5C, in accordance with some embodiments of the present disclosure;
FIG. 6A illustrates variation in efficiency of the central electrode with respect to radius ratio (rp1/a) for different residual stress factors (K2) in the diaphragm, in accordance with some embodiments of the present disclosure;
FIG. 6B illustrates variation in efficiency of the peripheral electrode with respect to radius ratio (rp2/a) for different residual stress factors (K2) in the diaphragm, in accordance with some embodiments of the present disclosure; and
FIG. 7 is a flowchart illustrating a method of fabricating a piezoelectric micromachined ultrasonic transducer (PMUT) device, in accordance with an embodiment of the present disclosure.

It should be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative systems embodying the principles of the present subject matter. Similarly, it will be appreciated that any flow charts, flow diagrams, state transition diagrams, pseudo code, and the like represent various processes which may be substantially represented in computer readable medium and executed by a computer or processor, whether or not such computer or processor is explicitly shown.

DETAILED DESCRIPTION

In the present document, the word “exemplary” is used herein to mean “serving as an example, instance, or illustration”. Any embodiment or implementation of the present subject matter described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments.
While the disclosure is susceptible to various modifications and alternative forms, specific embodiment thereof has been shown by way of example in the drawings and will be described in detail below. It should be understood, however that it is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternative falling within the spirit and the scope of the disclosure.
The terms “comprises”, “comprising”, or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a setup, device, or method that comprises a list of components or steps does not include only those components or steps but may include other components or steps not expressly listed or inherent to such setup or device or method. In other words, one or more elements in a device or system or apparatus proceeded by “comprises… a” does not, without more constraints, preclude the existence of other elements or additional elements in the device or system or apparatus.
In the following detailed description of the embodiments of the disclosure, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration specific embodiments in which the disclosure may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the disclosure, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the present disclosure. The following description is, therefore, not to be taken in a limiting sense.
FIGS. 2A and 2B illustrate material layer cross-section and top view, respectively, of an exemplary ultra-low power Piezoelectric Micromachined Ultrasonic Transducer (PMUT) device 100 with an optimized top electrode, in accordance with some embodiments of the present disclosure.
The PMUT device 200 includes a structural layer 210 and a layer stack. In general, the layer stack is deposited on a substrate wafer and a diaphragm of radius ‘a’ is formed when a defined area of the substrate is etched from the back side (a side opposite to a side on which the layer stack is deposited) to form a cavity 212. In other words, the diaphragm with the radius ‘a’ is micromachined in the back side of the substrate wafer to release the diaphragm and create a suspended planar structure. The structural layer 210 provides support for diaphragm. The layer stack formed on the structural layer acts as a main mechanical diaphragm for producing varying acoustic signal levels. In some instances, the diaphragm may be deposited separately on the structural layer 210. In general, the diaphragm, may be an elastic layer, which may provide diaphragm deflection and outputs an acoustic signal level for a given input voltage. Further, the layer stack is fabricated on the structural layer 210.
The layer stack includes at least a piezoelectric layer 206, a bottom electrode layer 208 and a top electrode layer. The bottom electrode layer 208 may be a continuous layer electrode. The top electrode layer of the PMUT device 200 includes a central electrode 202 and a peripheral electrode 204, fabricated on the piezoelectric layer 206. The peripheral electrode 204 encircles the central electrode 202. In an embodiment, the central electrode 202 and the peripheral electrode 204 may be closed shapes. For example, the central electrode 202 and the peripheral electrode 204 may be arranged in concentric circles. The central electrode 202 and the peripheral electrode 204 shown as circular structures in FIGS. 2A and 2B may also be referred to herein as a central circular electrode (i.e., CCE) and a peripheral ring electrode (i.e., PRE), respectively. Moreover, the top electrode layer (i.e., the central electrode 202 and the peripheral electrode 204) may be continuous closed loop structures fabricated on the piezoelectric layer 206 or open loop structures, for example, a small separation or space between the central electrode 202 and the peripheral electrode 204 (see, FIG. 3A-3B). It shall be noted that the circular shape of the central electrode 202 and the peripheral electrode 204 are shown for exemplary purposes and the central electrode 202 and the peripheral electrode 204 may be patterned to form other shapes, for example, hexagon, square, oval and the like. Accordingly, a shape of the diaphragm may also vary based on the shape of the top electrodes (i.e., the central electrode 202 and the peripheral electrode 204). The layer stack (i.e., the top electrode layer, the piezoelectric layer 206 and the bottom electrode layer 208) forms a parallel plate capacitor which can influence the electrical power draw of the PMUT device 200. The input voltage is applied to the top electrode layer (i.e., the central electrode 202 and the peripheral electrode 204) and the bottom electrode layer 208. As such, the piezoelectric layer 206 sandwiched between the bottom electrode layer 208 and the top electrode layer may act as the active transduction material.
The material of the central electrode 202, the peripheral electrode 204, and the bottom electrode layer 208, may be, but not limited to a metal. More specifically, the bottom electrode layer 208 may be a continuous layer of metal. Further, a material of the structural layer 210 may be, but not limited to, Silicon (Si). Similarly, various materials may be used for the structural layer 210, include, but not limited to, silicon, silicon nitride, silicon carbide and silicon dioxide, and the like. The material of the piezoelectric layer 206 may be, but not limited to, Lead Zirconate Titanate (PZT). However, it will be appreciated that numerous materials exhibiting piezoelectric behaviour may be alternatively utilized without departing from the scope of the present disclosure. For example, the material may be selected for use from the group of materials exhibiting piezoelectric behaviour comprising Aluminum Nitride (AlN), Apatite, Barium Titanate (BaTiO3), Berlinite (AlPO4), various Ceramic materials, Allium Phosphate, Gallium Nitride (GaN), Gallium Orthophosphate, Lanthanum Gallium Silicate, Lead Scandium Tantalate, Lead Magnesium Niobate (PMN), Lead Zirconate Titanate (PZT), Lithium Tantalate, Polyvinylidene Fluoride (PVDF), Potassium Sodium Tartrate, Quartz (SiO2), Zinc Oxide (ZnO), and the like. It shall be noted that the layer stack may include other thin film materials such as, but not limited to, buried oxide layers, device silicon layers, thermal oxide layers and the like for forming flexural transducer on silicon substrates and addition of such layers is not explained herein for the sake of brevity.
Conventionally, the PMUT device 200 is designed with optimal dimensions of the top electrode layer to maximize diaphragm deflection and the acoustic signal level, for a given input voltage. However, the PMUT device 200 consumes more electrical power, which may not be desirable in low powered devices using the PMUT device 200. Further, for applications such as DoS or proximity sensing that require the PMUT device 200 to produce a certain level of acoustic pressure with a minimum power consumption, the PMUT device 200 may not be able to operate for a long time on a small battery.
Various embodiments of the present disclosure disclose a method for maximizing the efficiency of the PMUT device 200 by minimizing electrical power consumption. Accordingly, the PMUT device 200 is designed using a size-optimized top electrode (i.e., the central electrode 202 and the peripheral electrode 204) which is potentially more efficient than any PMUTs currently fabricated. The patterned top electrode may include the central electrode 202 and/or the peripheral electrode 204, which has a lower and optimized coverage area over the piezoelectric layer 206. More specifically, a radius of the central electrode 202 and a width (wp) of the peripheral electrode 204 as shown in the FIG. 2B are optimized to reduce the electrode coverage area. This coverage area may in turn help in determining the electromechanical coupling and hence the maximum deflection of the diaphragm and the acoustic signal level (i.e., sound) generated thereby. As such, the power consumed by the PMUT device 200 may be potentially reduced by an order of magnitude, due to reduced area of the top electrode (i.e., the central electrode 202 and/or the peripheral electrode 204). More specifically, the reduced area of the top electrode reduces a capacitance between the piezoelectric layer 206 of the PMUT device 200 due to less area coverage of the top electrodes. Such an optimized top electrode may enable the development of a PMUT device with lower capacitance and power draw. Using the patterned top electrode which includes the central electrode 202 and the peripheral electrode 204 which is optimized, the PMUT device 200 may enable ultrasound and acoustic devices, such as proximity sensors and data-over-sound beacons to be developed with compact, battery-powered form factors. This allows for the development of devices which are battery-powered, portable, fit-and-forget internet of things devices such as beacons and sensor nodes. Optimization of the top electrode and the rationale for optimizing the top electrode is explained in detail with reference to FIGS. 3A-3B to FIG. 4A-4B.
FIGS. 3A and 3B illustrate a top view of a central electrode 202 and a peripheral electrode 204, respectively, of the PMUT device 200, in accordance with some embodiments of the present disclosure. As shown in FIGS. 3A and 3B, the top electrode includes circular electrodes (i.e., the central electrode 202 and the peripheral electrode 204) as closed shapes. The central electrode 202 has a radius “rp1”. The peripheral electrode 204 is an annular ring with an inner radius of “rp2” and an outer radius equivalent to the radius of the diaphragm which is represented as “a” and hence, diaphragm diameter may be “2a” as shown in FIG. 2B. It shall be noted that the central electrode 202 and the peripheral electrode 204 may be interchangeably referred to herein as a central circular electrode (i.e., CCE) and a peripheral ring electrode (i.e., PRE), respectively based on the shapes depicted in FIGS. 3A-3B.
Accordingly, maximizing efficiency in the PMUT device 200 is translated to an electrode optimization problem in which optimal electrode coverage (also referred to herein as ‘radius ratio’) is determined as a function of the thickness of a PMUT diaphragm, a size of the diaphragm and a residual stress present in the diaphragm (i.e., layer stack). Therefore, electrode optimization is a function of extensive finite element simulations to obtain an electrode geometry of the top electrodes with the lowest possible area for producing a given sound pressure level. More specifically, the optimized coverage area of the top electrode layer (i.e., central electrode 202 and the peripheral electrode 204) leads to lower capacitance in the PMUT device 200 and in turn lower power consumption during transmission of the acoustic signal. Moreover, this coverage area may in turn help in determining the electromechanical coupling and hence the maximum deflection of the diaphragm and the acoustic level (i.e., sound) generated thereby.
The optimal dimensions of the top electrode, that is the central electrode 202 and the peripheral electrode 204, is to optimize the electrical power consumption of the PMUT device 200. As already explained, the optimum electrode dimensions of the central electrode 202 and the peripheral electrode 204 may depend on the geometry of the diaphragm and the residual stress in the layer stack of the diaphragm. Accordingly, a residual stress factor in the layer stack of the diaphragm may be determined using term Kappa (K2). The residual stress factor “K2” is expressed as the ratio of the diaphragm's in-plane residual tension “Te” to the diaphragm's total flexural rigidity “De”, for a given diaphragm radius “a” (shown in FIGS. 3A and 3B) and is a non-dimensional factor. The equation for estimating the residual stress factor (K2) in the layer stack of the diaphragm is shown below
K^2=(T_e a^2)/D_e ------------Equation 1
The equation for calculating the flexural rigidity “De” is as shown in below Equation
D_e= ?_i¦(E_i/(1-v_i^2 ))((h_i^3)/12+z_i^2 h_i ) --------Equation 2
In the above Equation 2, the terms “Ei”, “?i”, “hi” and “zi” may refer to Young's modulus, Poisson's ratio, i-th layer thickness (i.e., thickness of the bottom electrode layer 208, thickness of the piezoelectric layer 206, thickness of the top electrode layer, thickness of buried oxide layer, thickness of device silicon layer, thickness of thermal oxide layer, etc. that form part of the diaphragm) and the distance from the mid-plane of the i-th layer from the neutral axis of the PMUT device 200, respectively. The rationale for optimizing the central electrode 202 and the peripheral electrode 204 is explained next with reference to FIGS. 4A- 4C.
FIGS. 4A, and 4B illustrate variation of normalized output sound power of the PMUT device 200 with respect to various radius ratios rp/a (x-axis) for the central electrode 202 and the peripheral electrode 204, respectively for increasing residual tension in the diaphragm such that K2 = 10, K2 = 100, K2 = 1000, respectively, which forms a rationale for optimizing the central electrode 202 and the peripheral electrode 204 of the PMUT device 200. More specifically, the graph plot 400 depicts the variation of the normalized output sound power (y-axis) of the PMUT device 200 with respect to radius ratio rp/a (x-axis) for the central electrode 202. Similarly, a graph plot 420 depicts the variation of the normalized output sound power (y-axis) of the PMUT device 200 with respect to radius ratio rp/a (x-axis) for the peripheral electrode 204.
It is apparent from the graph plots 400 and 420, that for increasing values of K2, the radius ratio (rp/a) of the top electrodes (i.e., the central electrode 202 and the peripheral electrode 204) increase for producing a maximum output sound power. As such, the required top electrode area (i.e., electrode coverage area on the piezoelectric layer 206) for maximum electromechanical coupling (per unit voltage) increases with increasing radius ratio rp/a of the central electrode 202 and the peripheral electrode 204.
The values of optimal radius ratio rp/a of the central electrode 202 and the peripheral electrode 204 for achieving maximum output sound power in the graph plots 400 and 420 range from 0.7 for diaphragms with very low stress, to 0.88 for diaphragms with extremely high stresses, for increasing values of K2. As such, when the electrode coverage area (rp/a) increases, capacitance in the PMUT device 200 increases, thereby increasing power consumption of the PMUT device 200. It is understood that the capacitance of the PMUT device 200 depends on the area of the top electrode (i.e., the central electrode 202 and the peripheral electrode 204). Accordingly, in an embodiment, the power consumption of such PMUT device 200 may be reduced by optimizing the top electrode and more specifically, reducing a width ‘wp’ of the peripheral electrode 204. In an example scenario, if a diaphragm has a radius ‘a’ of 2mm, then for a radius ratio rp/a = 0.7, a radius ‘rp1’ of the central electrode 202 is 1.4 mm and the width of the peripheral ring 204 is 0.6mm if the inner radius ‘rp2’ of the peripheral ring 204 is equal to the radius ‘rp1’ of the central electrode 202 (i.e., rp2 = rp1). In general, the electrode area of the central electrode 202 increases with rp/a and the electrode area of the peripheral electrode 204 decreases with increasing rp/a, since the width (wp) of the peripheral electrode 204 gets smaller.
Such reduction in dimensions of the central electrode 202 and the peripheral electrode 204 leads to a lower capacitance in the PMUT device 200 and hence, minimizes the electrical power draw. Accordingly, a radius (rp1) of the central electrode 202 and a width (wp) the peripheral electrode 204 (i.e., an inner radius ‘rp2’ of the peripheral electrode 204) are determined based on an optimal electrode radius ratios (rp1/a) and (rp2/a) of the central electrode 202 and the peripheral electrode 204, respectively. The choice of optimal electrode ratios (rp1/a) and (rp2/a) to determine the radius (rp1) of the central electrode 202 and the width ‘wp’ of the peripheral electrode 204, respectively is explained next with reference to FIG. 4B.
FIG. 4C illustrates a graph plot 430 depicting the optimal electrode ratio for the central electrode 202 and the peripheral electrode 204 to maximize efficiency and to maximize deflection of the PMUT device 200. The graph plot 430 depicts a plot of the radius ratio rp/a (shown on the y-axis) with respect to increasing Kappa (K2) (shown on the x-axis) on the diaphragm for the different top electrodes (i.e., the central electrode 202 and the peripheral electrode 204) for maximizing efficiency and maximizing deflection.
The optimal electrode ratio (rp1/a) for the central electrode 202 and the optimal electrode ratio (rp2/a) of the peripheral electrode 204 are determined from the radius ratio rp/a shown on the y-axis with respect to increasing Kappa (K2). As already explained, the term “rp” in the graph plot may refer to the radius (rp1) of the central electrode 202 and/or inner radius ‘rp2’ of the peripheral electrode 204 and the term “a” in the graph plot 430 may refer to the radius of diaphragm or outer radius of the ring electrode 204. It shall be noted that plots 432, 434, 436 and 438 shown in FIG. 4B are based on experimental results obtained from design simulations.
As shown in FIG. 4B, plots 432 and 434 depict an optimal electrode ratio (rp1/a) and (rp2/a) for the central electrode 202 and the peripheral electrode 204, respectively to obtain maximum efficiency in the PMUT device 200 for different values of K2 in the diaphragm. The optimal electrode ratio (rp1/a) for the central electrode 202 to obtain maximum efficiency ranges between 0.55 to 0.75 for increasing residual tension such that K2 values ranging between 10-1 to 104. In one illustrative example, the optimal electrode ratio (rp/a)1 of the central electrode 202 for a residual stress factor of 103 in the diaphragm is 0.74 to obtain maximum efficiency in the PMUT device 200. As such, a radius rp1 of the central electrode 202 is determined such that the radius ratio (rp1/a) ranges between 0.55 to 0.75 for a given K2 of the diaphragm. In an example scenario, if a diaphragm has a radius ‘a’ of 2 mm, then for an optimal radius ratio rp1/a = 0.7, a radius ‘rp1’ of the central electrode 202 is 1.4 mm.
Similarly, optimal electrode ratio (rp2/a) for the peripheral electrode 204 to obtain maximum efficiency ranges between 0.8 to 1 for K2 values ranging between 10-1 to 104, for example, an optimal electrode ratio (rp2/a) of the peripheral electrode 204 is 0.9 for a residual stress factor (K2) of 102 in the diaphragm to obtain maximum efficiency in the PMUT device 200. Accordingly, a width ‘wp’ of the peripheral electrode 204 is at most 20% of the radius ‘a’ of the diaphragm. More specifically, the width of the peripheral electrode 204 is determined such that a second radius ratio (rp2/a) ranges between 0.8 to 1 for the given residual stress factor (K2). In general, the peripheral electrode 204 will have an outer radius equal to the radius of the diaphragm ‘a’ and an inner radius rp2 is obtained by a product of the radius of the diaphragm ‘a’ multiplied by the radius ratio (rp2/a). The width wp of the peripheral electrode is determined by the equation shown below:
w_p=(1-r_p2/a)*a --------Equation 3
The plots 436 and 438 depict an optimal electrode ratio (rp/a) for the central electrode 202 and the peripheral electrode 204 to obtain a maximum deflection in the PMUT device 200 for varying K2 in the diaphragm. In an instance, the optimization of PMUT device 200 towards achieving an operational band of frequencies suitable for Data-over-Sound (DoS), may include K2 value as 100, for which the optimum radius ratio rp1/a and rp2/a for maximum deflection is 0.77 for both the central electrode 202 and the peripheral electrode 204. For maximizing the deflection of the PMUT device 200, the values of optimum radius ratio may be, for instance, rp/a vales between 0.65 to 0.95 for the central electrode 202 and the peripheral electrode 204 which is similar to the behavior exhibited by the central electrode 202 and the peripheral electrode 204 for maximizing output sound power (see, identical rp/a ratios and curves in FIGS. 4A-4B).
Further, it is apparent from the plots 436 and 438 that there is no clear distinction between the design of the two top electrodes (i.e., the central electrode 202 and the peripheral electrode 204) for obtaining maximum possible deflection in the fundamental mode of vibration for a particular driving voltage. This also translates to a maximum possible on-axis sound pressure output and as such, a PMUT device such as, the PMUT device 200 may typically operate in their fundamental modes when the centre diaphragm deflections are maximum. However, this observation carries strong implications for the power consumption by PMUTS. From experimental results, the PMUTs Electrical Power (PE) draw may be calculated by taking the absolute value of the PMUTs terminal current and terminal voltage, respectively. The Acoustic Power (PA) may be estimated by taking the integral of the simulated outward-normal intensity over a hemispherical surface located at a finite distance “d”, in front of the PMUT placed on an infinite baffle. The “d” can be, for instance, 10 centimeters (cm). The efficiency of the PMUT may be obtained by taking the ratio of Acoustic Power (PA) to Electrical Power (PE) draw.
FIGS. 5A, 5B and 5C illustrate variation of efficiency (y-axis) of the PMUT device 200 with respect to various radius ratios rp/a (x-axis) for the central electrode 202 and the peripheral electrode 204 for K2 = 10, K2 = 100, K2 = 1000, respectively. More specifically, the graph plot 500 depicts the variation of efficiency of the PMUT device 200 with respect to radius ratio rp/a (x-axis) for the central electrode 202 and the peripheral electrode 204 for K2 = 10, the graph plot 530 depicts the variation of efficiency of the PMUT device 200 with respect to radius ratio rp/a (x-axis) for the central electrode 202 and the peripheral electrode 204 for K2 = 100 and the graph plot 550 depicts the variation of efficiency of the PMUT device 200 with respect to radius ratio rp/a (x-axis) for the central electrode 202 and the peripheral electrode 204 for K2 = 1000.
For diaphragms with very low values of “Te”, i.e., with “K2” less than 100, the central electrode 202 may have a higher efficiency as shown in the graph plot 500. However, for higher values “K2” (i.e., K2 =100) the efficiency of the peripheral electrode 204 is higher as shown in the graph plots 530 and 550. Such increase in efficiency of the peripheral electrode 204 is due to the reduced area of the peripheral electrode 204 (i.e., the width wp of the peripheral electrode 204) which drastically reduces the capacitance and thereby reducing the power consumption. The optimal radius ratios (rp1/a) and (rp2/a) for both the central electrode 202 and the peripheral electrode 204, respectively may increase with the residual stress, as indicated in FIGS. 4C, 5A - 5C. For instance, the optimal radius ratio rp1/a of the central electrode 202 for maximizing efficiency with a residual tension in the diaphragm such that K2=10 is 0.56 whereas the optimal radius ratio rp1/a increases to 0.74 for a K2=1000. However, values of a residual tensile stress for the piezoelectric layer 206 such as piezoelectric thin films, which may be deposited using the sol-gel method may vary between, for instance, 300 Megapascal (MPa) to 1.5 Gigapascal (GPa), for piezoelectric layer 206 such as PZT thin-films.
Using the patterned top electrode which includes the central electrode 202 and/or the peripheral electrode 204, the efficiency of the PMUT device 200 may be significantly improved. For instance, referring to FIGS. 4C and 5B, for K2 = 100, the efficiency of the central electrode 202 can be increased from 0.12% at rp1/a of 0.77 (which is optimized for deflection) to 0.21% at rp/a of 0.90 (which is optimized for efficiency). This may translate to an increase of approximately 42% in transduction efficiency of the PMUT device 200. The gain in efficiency is larger for highly stressed diaphragms (i.e., higher K2). Similarly, the capacitance in the PMUT device 200 may also be reduced by more than or approximately 50%. The optimization of the top electrode may enable the development of piezoelectric PMUT devices with lower capacitance and power draw. Using the patterned top electrode which may include the central electrode 202 or the peripheral electrode 204, the PMUTs may enable the ultrasound and acoustic devices, such as proximity sensors and data-over-sound beacons to be developed with compact, battery-powered form factors. For instance, the power consumption by PMUTs may be reduced such that the battery can last for the rated 1000 hours, or more than one month on a smaller battery.
The optimization of the top electrode (i.e., the central electrode 202 and the peripheral electrode 204) for maximizing efficiency by using a peripheral electrode with a reduced width (i.e., wp) the efficiency is normalized with the efficiency at the baseline radius ratio (rp1/a) of 0.7 for the central electrode 202 and radius ratio (rp2/a) of 0.82, 0.9, 0.96 of the peripheral electrode 204 for K2 = 10, 100 and 1000, respectively. The current at the baseline case is assumed to be 3 mA at an input voltage of 2 V to the PMUT device 200, and an equivalent current with the improved efficiency is calculated using the optimum radius ratio (rp/a)2 for each residual stress factor from FIG. 4B. The efficiency values at each K2 were obtained using graphs similar to FIGS. 5A-5C.

Normalized Efficiency Current Draw @ 2V Battery life with CR2032 Battery Life Improvement
K2 CCE at rp1/a = 0.7 Optimized PRE CCE at rp1/a = 0.7 Optimized PRE at rp2/a= CCE at rp1/a = 0.7 Optimized PRE
10 1 1.11 3 mA 2.7 mA 50 hrs 56 hrs 12 %
100 1 1.97 3 mA 1.5 mA 50 hrs 120 hrs 140 %
1000 1 5.34 3 mA 0.56 mA 50 hrs 428 hrs 750 %
Table 1
It is apparent from the table (i.e., Table 1), that as the residual stress factor (K2) of the diaphragm increases, the efficiency of the peripheral electrode 204 significantly improves for an optimal radius ratio rp2/a of 0.82 for K2 = 10, rp2/a of 0.9 for K2 = 100 and rp2/a of 0.96 for K2 = 1000 whereas the efficiency of the central electrode remains the same for an optimal radius ratio rp1/a of 0.7. As such, the current drawn by the peripheral electrode 204 also shows a significant decrease from 2.7mA for K2 = 10 to 0.56mA for K2 = 1000 thereby significantly increasing battery life from 12% to 750% for optimized top electrodes.
Referring now to FIGS. 6A-6B, variation in efficiency of the central electrode 202 and the peripheral electrode 204 with respect to radius ratio (rp1/a) and (rp2/a) for different K2 in the diaphragm is illustrated in accordance with some embodiments of the present disclosure. More specifically, the graph plot 600 depicts variation in efficiency of the central electrode 202 with respect to radius ratio (rp1/a) for different residual stress factors such as, plot 602 for K2 = 10, plot 604 for K2 = 100, and plot 606 for K2 = 1000. Similarly, the graph plot 650 depicts variation in efficiency of the peripheral electrode 204 with respect to radius ratio (rp2/a) for different residual stress factors such as, plot 652 for K2 = 10, plot 654 for K2 = 100, and plot 656 for K2 = 1000.
The optimal radius ratio rp1/a of the central electrode 202 for achieving a maximum efficiency of 1 at K2 = 10 is 0.56 and this optimal radius ratio rp1/a increases to 0.74 to achieve maximum efficiency with an increased residual stress factor in the diaphragm (i.e., for K2 = 1000) as shown by the plot 606. As shown in FIG. 6B, the peripheral electrode 204 achieves a maximum efficiency when the optimal radius ratio rp2/a is 0.82 for a residual tension such that K2 = 10 of the diaphragm and efficiency of the peripheral electrode is maximized with an optimal electrode ratio rp2/a of 0.96 for a residual stress such that K2 = 1000 in the diaphragm (see, plot 656). In general, the optimal radius ratios (rp1/a) and (rp2/a) increase with increase in K2 of the diaphragm and the radius ratio (rp1/a) ranges between 0.55 to 0.75 for obtaining maximum efficiency in the central electrode 202 whereas as the optimal radius ratio (rp2/a) ranges between 0.8 to 1 for achieving maximum efficiency in the peripheral electrode 204. A fabrication method for manufacturing a PMUT device, such as, the PMUT device 200 with optimized top electrodes is explained next with reference to FIG. 7.
FIG. 7 is a flowchart illustrating a method 700 of fabricating a piezoelectric micromachined ultrasonic transducer (PMUT) device, in accordance with an embodiment of the present disclosure.
At operation 702 of the method 700, a wafer substrate is prepared.
At operation 704 of the method 700, a diaphragm of radius ‘a’ is formed on the wafer substrate by performing at least the following operations 706 – 716. At operation 706 of the method 700, a bottom electrode layer is formed on the wafer substrate. The bottom electrode layer is a continuous layer of metal deposited on the structural layer.
At operation 708 of the method 700, a piezoelectric layer is deposited on the bottom electrode layer. More specifically, piezoelectric thin films may be deposited using sol-gel method. For example, piezoelectric thin films with a residual tensile stress such as, 300 Megapascal (MPa) to 1.5 Gigapascal (GPa) may be deposited on the bottom electrode layer for active transduction.
At operation 710 of the method 700, a top electrode layer comprising a central electrode and a peripheral electrode are formed on the piezoelectric layer by performing operations 712-714.
At operation 712 of the method 700, the central electrode is patterned with a radius ‘rp1’. In an embodiment, the radius ‘rp1’ of the central electrode (202) is determined such that a radius ratio (rp1/a) ranges between 0.55 to 0.75 for the given residual stress factor (K2) of the diaphragm.
At operation 714 of the method 700, the peripheral electrode is patterned to encircle the central electrode with an inner radius of rp2 and an outer radius of ‘a’. In an embodiment, a width ‘wp’ of the peripheral electrode (204) is at most 20% of the radius of the diaphragm ‘a’ and such that a radius ratio (rp2/a) ranges between 0.8 to 1 for a given residual stress factor (K2) of the diaphragm. In general, the inner radius rp2 is obtained as a product of the radius of the diaphragm ‘a’ multiplied by the radius ratio (rp2/a) and the width (wp) is obtained using Equation (3). It shall be noted that the diaphragm may include other layers such as, but not limited to, buried oxide layer, device silicon layer, thermal oxide layer and the like for forming flexural transducer on silicon substrates and addition of such layers is not explained herein for the sake of brevity.
At operation 716 of the method 700, the wafer substrate is etched to form a structural layer. More specifically, a predefined area of the wafer substrate is etched from a back side (i.e., a side opposite to a side on which the layer stack is deposited) to form a cavity (212). The structural layer (210) so formed by etching supports the diaphragm.
This patterned top electrode such as the peripheral electrode 204 surrounding the central electrode 202 may be optimized for an entire array of PMUT devices such as, the PMUT device 200. The dimensions of the patterned top electrode and more specifically, the peripheral electrode 204 optimized to maximize efficiency may determine the electromechanical coupling of the PMUT device 200. It should be appreciated that PMUT devices of any desired geometry may be utilized, while the array of PMUT devices itself may comprise any desired PMUT arrangement (array pattern), with any desired spacing. It should be appreciated that an array of PMUT devices such as, the PMUT device 200 may be desirable for generating sufficient pressure and/or for sensing applications. As already explained, the top electrode for each PMUT device in the array of PMUT devices may be designed as squares, hexagons, ovals and the like based on the design requirements and application. In some embodiments, patterning of the top electrode is to substantially cut away, such as to remove the piezo and conductive layers, and in a specific example even a major thickness portion of the structural layer 210. It should be appreciated that the patterning of the PMUT device 200 may be fabricated using a subtractive material removal process (e.g., cutting, etching, ablation), or the pattern formed from an additive process, or a combination of subtractive and additive processes utilized to create the patterned PMUT device 200. The circular shape of the top electrodes (i.e., the central electrode 202 and the peripheral electrode 204) of the PMUT device 200 seen in FIGS. 2A and FIG. 2B are shown by way of example, while the present disclosure may be implemented in a range of geometries without departing from the present disclosure. PMUT devices such as, the PMUT device 200 with the central electrode 202 and the peripheral electrode 204 may be used for unimorph PMUTs and/or bimorph PMUT devices, with width less than, for instance, 20% of the radius ‘a’ of the diaphragm.
Implementations of the present disclosure includes PMUTs, or other vibrating diaphragm structures, for different applications, such as proximity sensing, biomedical imaging, photoacoustic imaging, photoacoustic spectroscopy, gesture recognition, Data over Sound (DoS), range and angle detection, flow measurements, beacons, and the like. In some implementation, the present disclosure may also be used for non-ultrasonic applications which requires, but not limited to, buzzer, beep, feedback sound, and the like.
The present disclosure may maximize the efficiency of the PMUT devices by minimizing electrical power consumption for the sound power emitted, due to the optimized top electrode pattern. The present disclosure uses size-optimized top electrodes as the PMUTs top electrodes which are potentially more efficient than any PMUTs currently fabricated. The patterned top electrode may include a first top electrode or a second top electrode, which has a lower and optimized coverage area over the piezoelectric layer due to reduced thickness of the second top electrode. This coverage area may in turn help in determining the electromechanical coupling and hence the maximum deflection of the membrane and the sound generated thereby. The power consumed by the PMUT devices may be potentially reduced by an order of magnitude, due to reduced area of the top electrode and reduced capacitance between the layers of the PMUT devices due to less area coverage of the top electrode. The optimized top electrode may enable the development of piezoelectric PMUT devices with lower capacitance and power draw. Using the patterned top electrode which includes the central electrode and the peripheral electrode, the PMUT devices may enable the ultrasound and acoustic devices, such as proximity sensors and data-over-sound beacons to be developed with compact, battery-powered form factors. This allows for the development of devices which are battery-powered, portable, fit-and-forget internet of things devices such as beacons and sensor nodes.
It will be understood by those within the art that, in general, terms used herein, and are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). For example, as an aid to understanding, the detail description may contain usage of the introductory phrases “at least one” and “one or more” to introduce recitations. However, the use of such phrases should not be construed to imply that the introduction of a recitation by the indefinite articles “a” or “an” limits any particular part of description containing such introduced recitation to inventions containing only one such recitation, even when the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should typically be interpreted to mean “at least one” or “one or more”) are included in the recitations; the same holds true for the use of definite articles used to introduce such recitations. In addition, even if a specific part of the introduced description recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, typically means at least two recitations or two or more recitations).
While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following detailed description.
,CLAIMS:WE CLAIM:
1. A Piezoelectric Micromachined Ultrasonic Transducer (PMUT) device (200), the PMUT device (200) comprising:
a structural layer (210); and
a diaphragm of radius ‘a’ supported by the structural layer (210), the diaphragm comprising:
a layer stack, the layer stack comprising a piezoelectric layer (206), a bottom electrode layer (208) and a top electrode layer, wherein the piezoelectric layer (208) is sandwiched between the bottom electrode layer (208) and the top electrode layer, wherein the top electrode layer comprises:
a central electrode (202) with a radius ‘rp1’, and
a peripheral electrode (204) encircling the central electrode (202) with an inner radius of rp2 and an outer radius of ‘a’, wherein a width (wp) of the peripheral electrode (204) is at most 20% of the radius of the diaphragm ‘a’ and such that a radius ratio (rp2/a) ranges between 0.8 to 1 for a given residual stress factor (K2) of the diaphragm.

2. The PMUT device (200) as claimed in claim 1, wherein the radius ‘rp1’ of the central electrode (202) is determined such that a radius ratio (rp1/a) ranges between 0.55 to 0.75 for the given residual stress factor (K2) of the diaphragm.

3. The PMUT device (200) as claimed in claim 1, wherein the residual stress factor (K2) in the diaphragm is determined using a ratio of the diaphragm's in-plane residual tension to the diaphragm's total flexural rigidity for the diaphragm of radius ‘a’.

4. The PMUT device (200) as claimed in claim 1, wherein the bottom electrode (208) is a continuous layer of metal.

5. The PMUT device (200) as claimed in claim 1, wherein the central electrode (202) and the peripheral electrode (204) are formed using metal.

6. The PMUT device (200) as claimed in claim 1, wherein the piezoelectric layer (206) is formed of an active transduction material.

7. A method (500) of fabricating a piezoelectric micromachined ultrasonic transducer (PMUT) device (200), the method (500) comprising:
preparing a wafer substrate; and
forming a diaphragm of radius ‘a’ on the wafer substrate by performing at least the following steps:
forming a bottom electrode layer (208) on the wafer substrate,
depositing a piezoelectric layer (206) on the bottom electrode layer (208), and
forming a top electrode layer comprising a central electrode (202) and a peripheral electrode (204) on the piezoelectric layer (206) by
patterning the central electrode (202) with a radius ‘rp1’, and
patterning a peripheral electrode (204) encircling the central electrode (202) with an inner radius of rp2 and an outer radius of ‘a’, wherein a width of the peripheral electrode (204) is at most 20% of the radius of the diaphragm ‘a’ and such that a radius ratio (rp2/a) ranges between 0.8 to 1 for a given a residual stress factor (K2) of the diaphragm, and
etching the wafer substrate to form a structural layer (210), wherein the structural layer (210) supports the diaphragm.

8. The method as claimed in claim 1, wherein the radius ‘rp1’ of the central electrode (202) is determined such that a radius ratio (rp1/a) ranges between 0.55 to 0.75 for the given residual stress factor (K2) of the diaphragm.

9. The PMUT device (200) as claimed in claim 1, wherein the bottom electrode (208) is a continuous layer of metal.

10. The PMUT device (200) as claimed in claim 1, wherein the central electrode (202) and the peripheral electrode (204) are formed using metal.
Dated this 20th day of June 2022
Thanking you,

Madhusudan S. T
OF K&S PARTNERS
AGENT FOR THE APPLICANT
IN/PA-1297