RF FEFLECTOMETER ULTRASONIC IMPEDANCE AND TIME-OF-FLIGHT SENSOR

A system and/or method for RF interrogation to read surface properties such as ultrasonic impedance and temperature in the field of measuring signals at a distance. The system includes a substrate with one or more piezoelectric transducers, at least one antenna connected to the substrate or formed onto the substrate, and one or more antenna terminals extending from the antenna and connected to terminals of at least one piezoelectric transducer. The antenna receives a radio frequency pulse and actuates at least one piezoelectric transducer. The piezoelectric transducer generates an ultrasonic pulse that reflects off a back side of the substrate. The reflected ultrasonic pulse is received at the piezoelectric transducer and drives the antenna that initially received the radio frequency pulse.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure is directed generally to measuring signals at a distance and, more particularly, to RF interrogation to read surface properties of an object.

2. Background of the Invention

Sensing surfaces directly using RF (radio frequency) pulses incident on the sensor is important to eliminate the wiring from the sensors. For example, a RF signal generated from a smartphone to a sensor, and the sensor reflecting the signal eliminates the need for wires. A wireless readout allows the sensor to be placed in locations with significant physical barriers between the reader and the sensor. For example, a sensor can be placed inside a bottle made of plastic or glass elements that do not allow any direct wires to the device. Another example consists of placing the sensor inside a body, or inside building walls, where wires are not possible. Different solutions to implementing the wireless sensor nodes have been implemented in the past. A battery-powered sensor can have on-board batteries and power sources to communicate with the RF receiver/transmitter. However, the presence of power sources often leads to excessive sensor size. A sensor-node without a power source is passive and needs to be powered directly by the interrogating RF fields. In this RF powered sensor category, the RF signal can be transduced into a DC voltage using a voltage rectifier, and the recovered energy, stored on a capacitor, can then be used to power the sensor. A second approach is to transduce the RF signal on the chip such that it directly generates an ultrasonic pulse. The ultrasonic pulse is transmitted through the device, and is reflected from a surface, back into the antenna that received the Rf signal. The signal is then transmitted out as a RF signal, read out by the receiver. The different sensor areas can be sensitized by coatings such that the reflected ultrasonic pulse and the RF pulse transmitted out contain information regarding the quantity being sensed.

Therefore, there is a need for a system and/or method for RF interrogation to read surface properties such as ultrasonic impedance and temperature in the field of measuring signals at a distance.

Description of the Related Art Section Disclaimer: To the extent that specific patents/publications/products are discussed above in this Description of the Related Art Section or elsewhere in this disclosure, these discussions should not be taken as an admission that the discussed patents/publications/products are prior art for patent law purposes. For example, some or all of the discussed patents/publications/products may not be sufficiently early in time, may not reflect subject matter developed early enough in time and/or may not be sufficiently enabling so as to amount to prior art for patent law purposes. To the extent that specific patents/publications/products are discussed above in this Description of the Related Art Section and/or throughout the application, the descriptions/disclosures of which are all hereby incorporated by reference into this document in their respective entirety(ies).

BRIEF SUMMARY OF THE INVENTION

The present disclosure is directed to a method and system for RF interrogation to read surface properties such as ultrasonic impedance and temperature in the field of measuring signals at a distance.

According to an aspect, the present invention is a system for RF interrogation. The system includes a substrate with one or more piezoelectric transducers, at least one antenna connected to the substrate or formed onto the substrate, and one or more antenna terminals extending from the antenna and connected to terminals of at least one piezoelectric transducer. The antenna receives a radio frequency pulse and actuates at least one piezoelectric transducer. The piezoelectric transducer generates an ultrasonic pulse that reflects off a back side of the substrate. The reflected ultrasonic pulse is received at the piezoelectric transducer and drives the antenna that initially received the radio frequency pulse.

According to an aspect, the present invention is a method for RF interrogation. The method includes the steps of: (i) providing an RF interrogation system comprising a substrate having a top surface and a back side, a plurality of piezoelectric transducers connected to the top surface of the substrate, and an antenna attached to each of the plurality of piezoelectric transducers; (ii) generating, by at least one of the plurality of piezoelectric transducers, ultrasonic pulses; (iii) reflecting the ultrasonic pulses off the bottom surface of the substrate as reflected ultrasonic pulses; (iv) receiving the reflected ultrasonic pulses at piezoelectric transducers; and (v) picking up reflected ultrasonic pulses by the antenna.

This and other aspects of the invention will be apparent from the embodiments described below.

DETAILED DESCRIPTION OF THE INVENTION

The present disclosure describes a system and method for RF interrogation to read surface properties such as ultrasonic impedance and temperature. For example, the ultrasonic impedance can correspond to the wetness of the surface. There are existing modalities where RF pulses are interfaced to an acoustic resonator such as a SAW (Surface Acoustic Wave) device to form a passive RFID where the SAW can be used to sense a number of variables depending on the coatings or other physical boundary conditions.

Referring now toFIG.1, there is shown an isometric view of an antenna10integrated onto a substrate chip12, which can be a CMOS (Complementary Metal Oxide Semiconductor) integrated circuit chip, according to an embodiment. The substrate chip12can be attached to a substrate14. The substrate14can have orifices (not shown) to allow access to the back side of the chip. Alternatively, the chip12can be mounted such that the antenna10is facing downwards into the substrate14. In the depicted embodiment, the substrate14is composed of flexible polymer, printed circuit board substrates, or silicon wafers; however, other materials can be used.

In the depicted embodiment ofFIG.1, the antenna10is composed of metal, such as copper or aluminum. InFIG.1, the piezoelectric transducer11is formed of materials such as aluminum nitride or PVDF (Poly Vinyl DiFluoride). The transducer11is placed between two metal electrodes13A,13B. The traces from the antenna10are connected to the electrodes13A,13B via thin film wires15. In order to connect the inner part of the spiral antenna10to the piezoelectric transducer11, conductive vias16are needed to connect the two different metal layers connecting to the electrodes13A and13B.

Turning now toFIG.2, there is shown a schematic representation of an RF interrogation system100operating at high frequencies, according to an alternative embodiment. To create the system100, an antenna102is integrated with a thin film, piezoelectric transducer104(also referred to as “ultrasonic transducer”). The piezoelectric transducer104can be an AlN transducer. The piezoelectric transducer104is connected to a substrate106. The substrate106can be silicon or any other material suitable for the purposes described below. The piezoelectric transducer104is connected to a top surface108of the substrate106, as shown inFIG.2.

In another embodiment, the CMOS chip12and substrate14ofFIG.1are integrated with the piezoelectric transducer104. In such an embodiment, the piezoelectric transducers104are positioned on the CMOS chip12and the antenna10is incorporated within the CMOS chip12and substrate14, as shown inFIG.1. The antenna10is a coil antenna integrated parallel to the piezoelectric transducer104. The coil antenna10can have portions of different inductance to achieve the resonance frequency corresponds to different standing wave resonance of the AlN piezoelectric transducer104. The coil antenna10can be distributed such that ultrasonic pulses112(FIG.2) add constructively at a certain point on the backside of the CMOS chip12.

The use of thin-film piezoelectric transducers104that generate ultrasonic pulses into a substrate14,106(and, in some cases, a CMOS chip12) are described in detail in PCT/US20/35537 assigned to the assignee hereof and incorporated herein in its entirety by reference. The following description of the use of the piezoelectric transducers104applies to both the antenna10,102embodiments shown inFIGS.1and2.

In the embodiment of the system100shown inFIG.2, a plurality of piezoelectric transducers104are connected to a proximalmost, top surface108of the substrate106. In the depicted embodiment, the piezoelectric transducers104are spaced and/or placed at predetermined locations along the top surface108. An antenna102is connected to each of the piezoelectric transducers104and are external to the substrate106.

In use, the system100is placed adjacent to or on an object200to be imaged. Specifically, the distalmost, bottom surface110of the substrate106is placed adjacent to or on an object200to be imaged. The piezoelectric transducers104emit ultrasonic pulses112toward the bottom surface110of the substrate106. The ultrasonic pulses112are reflected from the bottom surface110as incident RF pulses114(also referred to as “reflected ultrasonic pulses”), generating a voltage when received at the piezoelectric transducers104again.

Still referring toFIG.2, the piezoelectric transducers104are arranged in array and used to scan the ultrasonic impedance of the substrate106touching the object200. The transduction physics leads to generating diffraction patterns of ultrasonic pulses114(waves) that lead to higher order beams at different angles from the piezoelectric transducer104. The diffracted ultrasonic pulses114(waves) arrive to the top of the substrate106at a different location on the substrate106, as shown inFIG.2.

The incident RF pulses114are received by the piezoelectric transducers104and are picked up by the integrated RF antenna102and drive the piezoelectric transducers104. Once the ultrasonic pulse112comes back as the reflected ultrasonic pulses114after traversing the bulk substrate106, it can radiate a signal116back out of the antenna102to be picked up on a reader118. In the depicted embodiment, the reader118is a RF reader spaced from the substrate106but close enough to receive the signal116.

As shown inFIG.2, the antenna102can be connected to an initial piezoelectric transducer104A or to some (or all) of the piezoelectric transducers104at locations that pick up the diffracted orders. If the piezoelectric transducers104are spaced properly, the reflected ultrasonic pulses114will comprise RF waves emanating at different phases such that interference of the reflected ultrasonic pulses114, i.e., waves, is possible. The time-of-flight of the ultrasonic pulses112,114can be decoded by reading the phases of the reflected ultrasonic pulses114and can be measured. The time-of-flight of the ultrasonic pulses112,114has been shown to be proportional to the temperature of the substrate106.

In order to verify the feasibility of this approach, an initial calculation of the reflected signal using simulations tools was conducted. A typical CMOS integrated RF antenna impedance is approximately 60+175i ohms at 2.4 GHz, as seen a paper titled “A small OCA on a 1×0.5 mm2 2.45 GHz RFID Tag-design and integration based on a CMOS-compatible manufacturing technology” by Kwong et al. The power that can be obtained from the source is 617 uW, for a perfectly matched load. It is desired to choose a transducer size that to maximize power transfer to the transducer. The circuit diagram shown inFIG.4can be used to represent this setup, where the voltage source Vsourceand the source impedance Zantare from the antenna and the clamped capacitance C0and the radiation resistance RAare from the transducer, which is assumed to be at resonance.

For simplicity, it is assumed that the piezoelectric transducer104comprises an AlN thin film directly on top of a silicon substrate106. The radiation resistance RAcan therefore be calculated by the following formula:

where ktis the piezoelectric coupling factor, f0is the resonance frequency of the transducer, C0is the clamped capacitance of the transducer, Zpiezois the acoustic impedance of the piezoelectric layer, ZBis the acoustic impedance of the backing layer (assumed to be air) and ZTis the acoustic impedance of the transmission medium (assumed to be silicon).

For a 2.4 GHz resonance, for the particular set of film parameters we use resulted in a 2.7 um AlN thin film. Maximum power transfer is achieved for piezoelectric transducer104dimensions approximately 100 um×100 um.

Using the Redwood model to model the piezoelectric transducer104, the schematic inFIG.5was simulated in Cadence to determine the transient response—to measure the received voltage of the piezoelectric transducer104across the real part of the impedance of the antenna102. The impedance of the antenna102at 2.4 GHz is represented by a series resistor and series inductor. An additional “gain” of 0.8 is applied to the transducer response to account for expected diffraction loss.

It can be seen that for the maximum power that can be obtained from the antenna102, the received voltage across the antenna102, resistance can reach ˜0.5 Vpp at 2.4 GHz for the first acoustic echo. While this initial result shows that a large acoustic signal can be obtained on chip from a pulse116transmitted from an integrated antenna102, more modeling can be done to determine what the receive voltage on a receive antenna102will be.

The system100, i.e., the antenna24,102integrated on a CMOS chip10and non-CMOS substrate106, enables an ultra-miniature device (e.g., less than or equal to 200 um×200 um×500 um). The size and cost of the system100can be so low that they, looking like grains of sand, can be dispersed in the soil to measure soil moisture by RF interrogation from the air. The system100is small enough that the systems100can be embedded in the surfaces by adhesive attachment. A particular use of the system100can be within an adhesive bandage (e.g., Band-Aid®) and enable the measurement of dry or fluidic condition of the wound. The tiny systems100can be embedded inside objects such as wood or metal to measure the stress or temperature inside the structure. The system100may also have a sensitization coating, such as a hygroscopic film, on a top surface or bottom surface of the CMOS chip12to detect moisture.

Turning now briefly toFIG.3, there is shown a schematic representation of the RF interrogation system100, according to an alternative embodiment. In the alternative embodiment, the system100has a first piezoelectric layer104A and a second piezoelectric layer104B connected to a proximal most, top surface108of the substrate106. The substrate106can include a CMOS chip12or it can be a non-CMOS substrate, such as silicon. As shown inFIG.3, each of the piezoelectric layers104A,104B contacts the substrate106. This is possible because the second piezoelectric layer104B (proximal-most layer) extends around the first piezoelectric layer104A to the substrate106.

Still referring toFIG.3, the piezoelectric layers104A,104B are surrounded or sandwiched by electrodes. As shown, there is a bottom electrode120that is between the first piezoelectric layer104A and the top surface108of the substrate106. A common drive electrode122is between the first and second piezoelectric layers104A,104B. A top electrode124is the proximal-most part of the system100inFIG.3and is positioned on top of the second piezoelectric layer104B. A via126connects the top electrode124with a connector electrode128, which connects to a spiral inductor (not shown).

In previous implementations of GHz ultrasonic transducers104, one piezoelectric film104A is placed on top of a substrate106to launch ultrasonic waves112(pulses) into the substrate106. The substrate106can be a CMOS wafer (e.g., CMOS chip12) or other commonly used planar substrates such as a silicon wafer, or potentially flexible substrates. In the embodiment of the system100shown inFIG.3, a second piezoelectric film104B is added that shares one electrode (i.e., common drive electrode122) with the bottom, first piezoelectric layer104A. This enables the two piezoelectric transducers104A,104B formed to operate in parallel using the common drive electrode122. The electrodes120,128at the bottom can be used to implement the inductor (not shown) to receive RF energy that can now excite both piezoelectric104A,104B together. Due to the thickness of the piezoelectric devices104A,104B, and as the speed of sounds determines the frequencies of maximum coupling, the RF pulses from the transmitter can excite one or both piezoelectric transducers104A,104B.

In one implementation, the top, second piezoelectric layer104B can be a soft polymer PVDF material. Because the speed of sound in PVDF is low (˜2200 m/s), and it can be made into thicker films. For example, there are numerous examples of PVDF transducers with 10-1000 micrometer thickness, and one can achieve 10-500 MHz thickness mode resonance transducers. However, since PVDF is a polymer, it has higher internal mechanical losses at higher frequencies, and hence is more appropriate for lower frequency ultrasonic transducers. Hence, the waves launched into the substrate106or the medium above the top, second piezoelectric layer104B can now be at two different resonance frequencies. The PVDF can launch waves in the 10-200 MHz range, while the bottom piezoelectric film can be the AlN thin film transduce, and it can launch waves in the 500 MHz to several GHz range. This broad range of resonance frequency has the advantage that the lower frequency ultrasonic waves can penetrate deeper into a medium on the top or bottom of the chip and/or non-CMOS substrate. The lower frequency leads to deeper penetration of waves, at reduced lateral resolution. The ability to image and sense volumes both deeper into a material at lower spatial resolution, and sense volumes that are smaller near the interface, but at high special resolution can enable a more complete interrogation with the RF transduced pulses. The transducers formed by the two piezoelectric layers can also be actively driven with integrated CMOS transistors or external electronics to excite both transducers at simultaneously. The sharing of the common electrode is important to minimize the need for further processing to create electrodes for both piezoelectric layers.

The above-described embodiments of the described subject matter can be implemented in any of numerous ways. For example, some embodiments may be implemented using hardware, software or a combination thereof. When any aspect of an embodiment is implemented at least in part in software, the software code can be executed on any suitable processor or collection of processors, whether provided in a single device or computer or distributed among multiple devices/computers.