Patent Publication Number: US-2023138355-A1

Title: Microelectromechanical systems (mems) rectifier and storage element for energy harvesting

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of the filing date of U.S. Provisional Application No. 63/273,525, filed Oct. 29, 2021, the entire disclosure of which is incorporated by reference herein. 
    
    
     BACKGROUND 
     The Internet of Things (IoT) is the inter-networking of physical objects, such as products, packages, vehicles, buildings, etc., that are embedded with electronic components for network connectivity. The e1mbedded components enable objects to detect others, be detected by others, collect data and/or transmit data. In some examples, the embedded components may include tags or labels attached to the physical objects. These tags or labels may be passive or active. The inter-networking capabilities may be leveraged for tracking locations of physical objects. 
     BRIEF SUMMARY 
     Aspects of the disclosure provide for an electronic device that includes a microelectromechanical system (MEMS) rectifier. The MEMS rectifier includes a mainboard having one or more radiofrequency (RF) inputs configured to receive an RF signal; and a first electrical contact. The MEMS rectifier also includes a sub-board positioned parallel to the mainboard with a gap in-between. The sub-board has a thin film piezoelectric layer; a second electrical contact positioned opposite the first electrical contact; and a ground plane. The sub-board is configured to vibrate as the RF signal is received at the one or more RF inputs, and the thin film piezoelectric layer is configured to generate energy due to the vibration and piezoelectric properties of the thin film piezoelectric layer. 
     In one example, the electronic device also includes an energy storage device that is configured to receive and store the generated energy. In this example, the energy storage device optionally includes one or more capacitors. In another example, the generated energy is configured to flow from the second electrical contact to the first electrical contact when the vibration of the sub-board causes the second electrical contact to come into contact with the first electrical contact. In this example, a separate electronic device is optionally connected to the first electrical contact and is configured to receive electric charge from the MEMS rectifier via the first electrical contact. 
     In a further example, the sub-board is configured to vibrate at a resonant frequency within a particular frequency band, and the RF signal is also within the particular frequency band. In yet another example, the gap is packaged in vacuum. In a still further example, the MEMS rectifier is in a chip with monolithic integration of piezoelectric materials, electrostatic MEMS, and solid state components. 
     Other aspects of the disclosure provide for a method of harvesting radiofrequency (RF) energy. The method includes receiving, by one or more RF inputs of a microelectromechanical system (MEMS) rectifier, a RF signal; vibrating a sub-board of the MEMS rectifier as the RF signal is received by the one or more RF inputs, the sub-board including piezoelectric materials; generating energy due to the vibration of the sub-board and properties of the piezoelectric materials; and storing the generated energy. 
     In one example, the storing of the generated energy is in a mechanical domain. In another example, the storing of the generated energy is in an energy storage device. In this example, the energy storage device optionally includes one or more capacitors. 
     In a further example, the method also includes causing a first electrical contact on the sub-board to come into contact with a second electrical contact on a mainboard of the MEMS rectifier using the vibration of the sub-board, wherein the mainboard is positioned parallel to the sub-board with a gap in-between; and upon contact of the first electrical contact with the second electrical contact, generating a current using the generated energy. In this example, the causing the first electrical contact to come into contact with the second electrical contact optionally includes increasing an amplitude of the vibration of the sub-board. Also in this example, the method also optionally includes powering an electronic device using the generated current. Further in this example, the method also optionally includes processing the generated current to accumulate charge before powering the electronic device. Also in this example, the method also optionally includes processing the generated current to meet requirements for powering the electronic device. 
     Further aspects of the disclosure provide for a method of manufacturing a microelectromechanical system (MEMS) rectifier. The method includes mounting a first electrical contact and one or more radiofrequency (RF) inputs onto a mainboard; mounting a second electrical contact to a sub-board, the sub-board including a ground plane; attaching the sub-board to the mainboard with a gap in-between in a position where the second electrical contact is opposite the first electrical contact across the gap and the sub-board is parallel to the mainboard. 
     In one example, the attaching of the sub-board to the mainboard includes configuring the sub-board to vibrate at a resonant frequency within a frequency band. In another example, the method also includes fabricating at least a portion of the sub-board or the mainboard using an integrated circuit manufacturing process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a functional diagram of a microelectromechanical system rectifier in accordance with aspects of the disclosure. 
         FIG.  1 B  is a pictorial diagram of a microelectromechanical system rectifier in accordance with aspects of the disclosure. 
         FIG.  2    is a functional diagram of an example system including a rectifier in accordance with aspects of the disclosure. 
         FIG.  3    is a pictorial diagram of an example network in accordance with aspects of the disclosure. 
         FIG.  4    is a functional diagram of the example network in  FIG.  2    in accordance with aspects of the disclosure. 
         FIG.  5    is a flow diagram of an example method in accordance with aspects of the disclosure. 
         FIG.  6    is a flow diagram of another example method in accordance with aspects of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Overview 
     The technology relates to a rectifier for radiofrequency (RF) energy. The rectifier may include a microelectromechanical system (MEMS) resonator configured to vibrate in response to RF modulation frequencies. Using piezoelectric materials, the non-linearity of electrostatic force may be used to rectify captured RF signals. This mechanical capture of energy may be more sensitive than using diodes or other solid-state technologies. As a result, using piezoelectric RF energy harvesting in addition to or instead of using solid-state technologies, lower levels of RF energy in the environment may be captured and stored than using solid-state electronics alone. 
     The rectifier may include a mainboard including one or more RF inputs and a first electrical contact. The one or more RF inputs may be configured to receive an RF signal, such as from an antenna. The RF signal may be in a particular frequency band. For example, the RF signal may include 750, 850, or 1900 MHz cellular bands, 900 MHz, 2.4 or 5 GHz ISM bands, and/or 3.5 or 6 GHz AFC bands. The rectifier may also include a sub-board positioned parallel to the mainboard with a small gap in between. The sub-board may be secured in position using a first anchor at a first end and a second anchor at a second end. The sub-board may include a thin film piezoelectric layer, a second electrical contact, and a ground plane. The thin film piezoelectric layer may include crystalline aluminum nitride (AlN) and/or other materials that have piezoelectric properties. Dimensions of the sub-board may be configured based on the particular frequency band to be captured. In some examples, the sub-board may include a different material than AN that has piezoelectric properties. The second electrical contact may be positioned opposite the first electrical contact. The small gap between the mainboard and the sub-board may be a vacuum or filled with a finite amount of air. 
     As the RF signal is received by the one or more RF inputs, the sub-board is configured to vibrate at a resonant frequency. In particular, the dimensions, materials, and other features of the sub-board may be configured to allow the sub-board to vibrate at a resonant frequency in the particular frequency band. The vibration of the sub-board can increase in amplitude over time as the RF signal is received. 
     Energy may be generated due to the vibration and the piezoelectric properties of AlN. The generated energy may be stored and accumulated in a mechanical domain, in the form of vibration of the board (e.g., in an amplitude of the vibration). Alternatively, the generated energy may be stored and accumulated in an energy storage device, such as one or more capacitors. 
     Energy may be accumulated until an amplitude for the vibration allows the second electrical contact to contact the first electrical contact. Upon contact between the first and second electrical contacts, electric charge may flow from the second electrical contact to the first electrical contact, generating a current. 
     The first electrical contact may be connected to an electronic device. The generated current may therefore flow from the first electrical contact to the electronic device to power the electronic device. In some cases, the connection between the electrical contact and the electronic device may include an energy storage device, such as one or more capacitors, to accumulate charge before powering the electronic device. Also in some cases, the connection may include a transformer or a filter configured to prepare the current to meet the requirements for powering the electronic device. In some examples, the rectifier may be integrated with the transformer or filter in a single component. 
     The MEMS rectifier described herein is able to capture very low levels of RF energy in an environment. The increased sensitivity provided by the MEMS resonator may allow for capturing more energy at a more efficient rate. When used in connection with a tracking system, this MEMS rectifier allows for more consistent powering of tracking system components in areas that have lower levels of energy in an environment. In addition, as the energy harvesting function may be relegated at least partially to the MEMS rectifier, the form of the components of the tracking system becomes more flexible. The MEMS rectifier may provide access to minute amounts of energy available, which can also be used in parallel with semiconducting rectifiers that only work at higher incident power levels to form an overall system that captures a wider amount of energy at a range of efficiencies. 
     Example Systems 
       FIGS.  1 A and  1 B  are functional and pictorial diagrams of a MEMS rectifier  100 . The rectifier  100  includes a mainboard  102  including one or more RF inputs  104  and a first electrical contact  106 . The one or more RF inputs  104  may be configured to receive an RF signal, such as from an antenna. The one or more RF inputs  104  may be configured to receive the RF signal having a particular frequency band. For example, the particular frequency band may include 750, 800-1000, or 1900, MHz cellular bands, 2.4 or 5 GHz ISM bands, and/or 3.5 or 6 GHz AFC bands. When the RF signal has a frequency in the particular frequency band, the rectifier  100  may vibrate to generate an electrical current as described herein. The first electrical contact  106  may be connected to an electronic device  150  and may be configured to output an electrical current from the rectifier  100  to the electronic device  150 . 
     The rectifier  100  also includes a sub-board  112  positioned parallel to the mainboard  102  with a small gap in between. The sub-board  112  is secured in position using a first anchor  110   a  at a first end and a second anchor  110   b  at a second end. The sub-board  112  includes thin film crystalline aluminum nitride (AlN)  114 , which has piezoelectric properties, a second electrical contact  116 , and a ground plane  118 . Dimensions of the sub-board  112  may be configured based on the particular frequency band to be captured. In some examples, the sub-board may include a different material than AlN that has piezoelectric properties. The second electrical contact  116  is positioned opposite the first electrical contact  106 . 
     The first electrical contact  106  and the second electrical contact  116  may be a raised portion on the mainboard  102  and sub-board  112 , respectively. The raised portion may be a sharp contact point. The raised portion may be formed using a combination of self-terminating etch into a small opening in a mask layer, followed by the top layer being deposited. Other known methods of forming a raised electrical contact on a board may be used in addition or in the alternative to this combination. 
     The gap between the second electrical contact  116  and the first electrical contact  106  may be minimized such that a lower threshold of RF energy is needed for actuation that results in contact between the electrical contacts. For example, the gap between the contacts or between the sub-board and the mainboard may be 5 nm, 100 nm, or any measurement in between. The size of the gap may be formed or adjusted using thin film technologies such as LPCVD films, or ALD films. The small gap between the mainboard and the sub-board may be a vacuum or air-filled. When the small gap is a vacuum, the rectifier  100  may be able to achieve a higher quality factor resonance, which increases the efficiency of rectification. 
     The combination of the mainboard  102  and the sub-board  112  comprises a resonator portion of the rectifier  100 . For example, the sub-board  112  is configured to vibrate at a resonant frequency in response to the RF signal received by the one or more RF inputs. In particular, the sub-board  112  may have the dimensions, materials, and other features that allow the sub-board  112  to vibrate at a resonant frequency in the particular frequency band. The vibration of the sub-board  112  can increase in amplitude over time as the RF signal is received. Due to the piezoelectric properties of the material in the sub-board  112 , energy may be generated from the vibration of the sub-board  112 . The generated energy may be stored and accumulated in the mechanical domain, such as in an amplitude of the vibration. Alternatively, the rectifier  100  may include an energy storage device, such as one or more capacitors, that is configured to receive and store the generated energy from the vibration of the sub-board  112 . In some implementations, the rectifier  100  described above may be included in a monolithic chip. In other implementations, a multi-chip module may be created using a plurality of rectifiers. 
     The sub-board  112  may be configured to accumulate energy in the mechanical domain, increasing in amplitude up to a point in which the second electrical contact  116  comes into contact with the first electrical contact  106 . Upon contact between the first and second electrical contacts, electric charge may flow from the second electrical contact  116  to the first electrical contact  106 , generating a current. The generated current may therefore flow from the first electrical contact  106  to an electronic device  150  connected to the first electrical contact and power the electronic device. In some cases, the connection between the first electrical contact and the electronic device may include an energy storage device, such as one or more capacitors, to accumulate charge before powering the electronic device. Also in some cases, the connection may include a transformer or a filter configured to prepare the current to meet the requirements for powering the electronic device. In some examples, the rectifier may be integrated with the transformer or filter in a single component. 
     The lateral size of the rectifier  100  may be in the range of 5 μm-50 μm, inclusive, but may vary from this range as needed for different frequencies and situations. The rectifiers having larger widths will have lower spring constants than smaller widths owing to the lower flexural stiffness (or lower rigidity). The wider rectifiers may enable higher motion generated by the piezoelectric forces, but may be more susceptible to vibration-induced contact events between the electrical contacts due to the lower rigidity. Hence, the dimensions of the rectifier may be selected to provide a target flexural stiffness or rigidity that provides a maximum energy output, while also preventing environmental trigger events. 
     In some examples, the electronic device  150  connected to the rectifier  100  may be one or more components in a tracking system, such as tracking system  200  shown in  FIG.  2   . The tracking system may include electronic components such as a plurality of tracking devices, such as identifier tags or sensors, and a reader. As shown in  FIGS.  2 A and  2 B , the tracking system  200  may include a plurality of identifier tags  204  (such as identifier chips), and a reader  206 . Each identifier tag may be attached to an item to be tracked, like a package. The rectifier  100  may therefore capture RF energy  210  in the environments where the tracking system  200  is implemented and power one or more of the electronic components of the system. 
     After capturing energy  210  from the environment, a given rectifier  100  may transmit an electrical current to one or more identifier tags  204 . As shown in  FIG.  2   , an electrical current is transmitted from the MEMS rectifier  100   a  to identifier tag  204   a , from the MEMS rectifier  100   b  to identifier tag  204   b , from MEMS rectifier  100   c  to identifier  204   c , from MEMS rectifier  100   d  to identifier  204   d , and from MEMS rectifier  100   e  to identifier  204   e . When powered, the plurality of passive tags  204  may emit a signal to indicate a respective location. 
     The reader  206  may be a computing device configured to detect the signal emitted by the plurality of identifier tags  204 , then store and/or transmit data related to the locations of the detected tags. In some implementations, the reader  206  may be connected to a MEMS rectifier to receive power. The reader  206  may include one or more processors  214 , memory  216  and other components typically present in general purpose computing devices. 
     The one or more processors  214  may be any conventional processors, such as commercially available CPUs. Alternatively, the one or more processors may be a dedicated device such as an ASIC or other hardware-based processor, such as a field programmable gate array (FPGA). Although  FIG.  2    functionally illustrates the processor(s), memory, and other elements of the reader  206  as being within the same block, it will be understood by those of ordinary skill in the art that the processor, computing device, or memory may actually include multiple processors, computing devices, or memories that may or may not be stored within the same physical housing. For example, memory may be a hard drive or other storage media located in a housing different from that of the reader  206 . Accordingly, references to a processor or computing device will be understood to include references to a collection of processors or computing devices or memories that may or may not operate in parallel. 
     The memory  216  stores information accessible by the one or more processors  214 , including data  217  and instructions  218  that may be executed or otherwise used by the processor(s)  214 . The memory  216  may be of any type capable of storing information accessible by the processor(s), including a computing device-readable medium, or other medium that stores data that may be read with the aid of an electronic device, such as a hard-drive, memory card, ROM, RAM, DVD or other optical disks, as well as other write-capable and read-only memories. Systems and methods may include different combinations of the foregoing, whereby different portions of the instructions and data are stored on different types of media. 
     The data  217  may be retrieved, stored or modified by processor(s)  214  in accordance with the instructions  218 . For instance, although the claimed subject matter is not limited by any particular data structure, the data may be stored in computing device registers, in a relational database as a table having a plurality of different fields and records, XML documents or flat files. The data may also be formatted in any computing device-readable format. 
     The instructions  218  may be any set of instructions to be executed directly (such as machine code) or indirectly (such as scripts) by the processor. For example, the instructions may be stored as computing device code on the computing device-readable medium. In that regard, the terms “instructions” and “programs” may be used interchangeably herein. The instructions may be stored in object code format for direct processing by the processor, or in any other computing device language including scripts or collections of independent source code modules that are interpreted on demand or compiled in advance. Functions, methods and routines of the instructions are explained in more detail below. 
       FIGS.  3  and  4    are pictorial and functional diagrams, respectively, of an example system  300  that includes a plurality of computing devices  310 ,  320 ,  330 ,  340  and a storage system  350  connected via a network  360 . System  300  also includes passive tags  204   a ,  204   b  and reader  206 . Although only a few tags and computing devices are depicted for simplicity, a typical system may include significantly more. 
     Using the client computing devices, users, such as user  322 ,  332 ,  342 , may view the location data on a display, such as displays  324 ,  334 ,  344  of computing devices  320 ,  330 ,  340 . As shown in  FIG.  4   , each client computing device  320 ,  330 ,  340  may be a personal computing device intended for use by a user  322 ,  332 ,  342 , and have all of the components normally used in connection with a personal computing device including a one or more processors (e.g., a central processing unit (CPU)), memory (e.g., RAM and internal hard drives) storing data and instructions, a display such as displays  324 ,  334 ,  344  (e.g., a monitor having a screen, a touch-screen, a projector, a television, or other device that is operable to display information), and user input devices  326 ,  336 ,  346  (e.g., a mouse, keyboard, touch screen or microphone). The client computing devices may also include speakers, a network interface device, and all of the components used for connecting these elements to one another. 
     Although the client computing devices  320 ,  330 , and  340  may each comprise a full-sized personal computing device, they may alternatively comprise mobile computing devices capable of wirelessly exchanging data with a server over a network such as the Internet. By way of example only, client computing device  320  may be a mobile phone or a device such as a wireless-enabled PDA, a tablet PC, a wearable computing device or system, or a netbook that is capable of obtaining information via the Internet or other networks. In another example, client computing device  330  may be a wearable computing system, shown as a wristwatch in  FIG.  3   . As an example, the user may input information using a small keyboard, a keypad, microphone, using visual signals with a camera, or a touch screen. 
     Example Methods 
     In addition to the operations described above and illustrated in the figures, various operations will now be described. It should be understood that the following operations do not have to be performed in the precise order described below. 
       FIG.  5    is an example flow diagram  500  including a method of operation for a MEMS rectifier in accordance with some of the aspects described above. While  FIG.  5    shows blocks in a particular order, the order may be varied and that multiple operations may be performed simultaneously. Also, operations may be added or omitted. 
     At block  502 , a RF signal is received by one or more RF inputs  104  of the MEMS rectifier  100 . At block  504 , as the RF signal is received by the one or more RF inputs, a sub-board  112  of the MEMS rectifier  100  vibrates. In some cases, the RF signal is in a particular frequency band that causes the sub-board  112  to vibrate at a particular resonant frequency. The vibration of the sub-board  112  may increase in amplitude over time as the RF signal is received. At block  506 , energy is generated due to the vibration and the piezoelectric properties of the sub-board  112 . At block  508 , the generated energy is stored. The energy may be stored in a mechanical domain, such as in an amplitude of the vibration, or in an energy storage device, such as one or more capacitors. 
     At block  510 , an electrical contact on the sub-board  112  comes into contact with an electrical contact on a mainboard  102  due to the vibration of the sub-board  112 . The mainboard  102  is positioned substantially parallel to the sub-board  112  with a gap between the mainboard  102  and the sub-board  112 . The contact between the two electrical contacts may occur when an amplitude of the vibrating sub-board  112  is able to span the gap between the mainboard  102  and the sub-board  112 . At block  512 , once the contact between the electrical contacts occurs, electric charge may flow from the sub-board to the mainboard, generating a current. 
     At block  514 , the generated current may flow from the mainboard  102  to an electronic device  150  to power the electronic device  150 . As the generated current flows from the mainboard  102  to the electronic device  150 , the current may flow through one or more electronic components to prepare the current to meet the requirements for powering the device. For example, the one or more electronic components may include an energy storage device, such as one or more capacitors, to accumulate charge before powering the electronic device, or a transformer or a filter to modify the current. In some examples, the rectifier may be integrated with the transformer or filter in a single component. Once powered, the electronic device  150  may perform a function; for example, an identifier tag, when powered, may emit a signal. 
       FIG.  6    is an example flow diagram  600  including a method for manufacturing a MEMS rectifier in accordance with some of the aspects described above. The method may be performed by one or more computing devices controlling machinery that is customized for the steps of the method. While  FIG.  6    shows blocks in a particular order, the order may be varied and that multiple operations may be performed simultaneously. Also, operations may be added or omitted. 
     At block  602 , a first electrical contact  106  and one or more RF inputs  104  may be mounted onto a mainboard  102 . At block  604 , a second electrical contact  116  may be mounted to a sub-board  112 . The sub-board  112  may include a ground plane  118 . At block  606 , the sub-board  112  may be attached to the mainboard  102  with a gap between the sub-board  112  and the mainboard  102 , thereby forming a resonating unit. The position of the attached sub-board may include the second electrical contact  116  being directly across the gap from the first electrical contact  106 , such that at least a portion of the second contact is located at the point on the sub-board that is the shortest distance from the first contact  106 . When attached, the sub-board  112  may be substantially parallel to the mainboard  102 . The sub-board  112  may be attached using one or more anchors  110 . In some implementations, at least a portion of the mainboard  102  or the sub-board  112  is fabricated using the integrated circuit manufacturing portion. Alternatively, the MEMS rectifier described herein, such as MEMS rectifier  100 , may be manufactured as an integrated circuit or monolithic chip using steps included in the integrated circuit manufacturing process. The monolithic chip may include an integration of the piezoelectric components, electrostatic components, and solid state components. For example, piezoelectric materials, such as AlN, electrostatic circuitry, such as traces or other parts of the MEMS, and CMOS circuitry or electronics, such as semiconductors, may be integrated in a chip to form the rectifier  100 . 
     The MEMS rectifier described herein is able to capture very low levels of RE energy in an environment. The increased sensitivity provided by the MEMS resonator may allow for capturing more energy at a more efficient rate. When used in connection with a tracking system, this MEMS rectifier allows for more consistent powering of tracking system components in areas that have lower levels of energy in an environment. In addition, as the energy harvesting function may be relegated at least partially to the MEMS rectifier, the form of the components of the tracking system becomes more flexible. 
     Unless otherwise stated, the foregoing alternative examples are not mutually exclusive, but may be implemented in various combinations to achieve unique advantages. As these and other variations and combinations of the features discussed above can be utilized without departing from the subject matter defined by the claims, the foregoing description of the embodiments should be taken by way of illustration rather than by way of limitation of the subject matter defined by the claims. In addition, the provision of the examples described herein, as well as clauses phrased as “such as,” “including” and the like, should not be interpreted as limiting the subject matter of the claims to the specific examples; rather, the examples are intended to illustrate only one of many possible embodiments. Further, the same reference numbers in different drawings can identify the same or similar elements.