Piezoelectric micromachined ultrasonic transducers having differential transmit and receive circuitry

Abstract

An apparatus comprises an ultrasonic transducer having a first and second electrode and switches which configured to selectively connect the first and second electrodes to a transmit voltage source or to a receive amplifier. The switches are configured to selectively connect a first input of the amplifier to the first electrode of the transducer and to selectively connect a second input of the amplifier to the second electrode of the transducer. The switches are also configured to selectively connect the voltage source to the first and second electrodes of the transducer. The transducer may include a piezoelectric layer attached to and sandwiched between the first electrode and the second electrode, and a flexible membrane attached to the first electrode. The piezoelectric layer may be patterned to form an annular ring at the outer diameter of the flexible membrane.

Claims

1. An apparatus comprising: a piezoelectric micromachined ultrasonic transducer having a first and second electrode; a unipolar charge pump; and a first set of one or more switches configured to selectively connect the first and second electrodes to unipolar charge pump and a second set of switches configured to selectively couple the first and second electrodes to a receive amplifier; and wherein the first set of one or more switches is configured to selectively connect a first input of the receive amplifier to the first electrode of the ultrasonic transducer and to selectively connect a second input of the receive amplifier to the second electrode of the ultrasonic transducer; and wherein the second set of one or more switches is configured to selectively connect the unipolar charge pump to the first and second electrodes of the ultrasonic transducer.

2. The apparatus of claim 1, wherein the switches are configured to produce a bipolar transmit signal from the unipolar charge pump by sequentially reversing the connections between the voltage source terminals and the first and second electrodes of the ultrasonic transducer.

3. The apparatus of claim 1, wherein the second set of switches is configured to produce a bipolar transmit signal from the unipolar charge pump by sequentially reversing the connections between the voltage source terminals and the first and second electrodes of the ultrasonic transducer, wherein in a first state the first electrode is connected to a positive terminal of the unipolar charge pump and the second electrode is connected to a negative terminal of the unipolar charge pump and wherein in a second state the first electrode is connected to a negative terminal of the unipolar charge pump and the second electrode is connected to a positive terminal of the unipolar charge pump.

4. The apparatus of claim 1, further comprising the receive amplifier.

5. The apparatus of claim 4, wherein the receive amplifier is a differential receive amplifier.

6. The apparatus of claim 1, wherein the ultrasonic transducer includes a piezoelectric layer attached to and sandwiched between the first electrode and the second electrode, and a flexible membrane attached to the first electrode.

7. The apparatus of claim 6, wherein the piezoelectric layer is patterned to form an annular ring at the outer diameter of the flexible membrane.

8. The apparatus of claim 7, wherein the first and second electrodes are designed such that an electrical impedance between each of the first and second electrodes and a transducer substrate are substantially identical.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be more fully understood by reference to the following drawings which are for illustrative purposes only:

(2) FIG. 1 shows a simplified equivalent circuit diagram of a transducer design known from prior art.

(3) FIG. 2A shows a top view of a piezoelectric transducer design known from prior art.

(4) FIG. 2B shows a cross-section view of a piezoelectric transducer design known from prior art.

(5) FIG. 3 shows a simplified receiving circuit diagram corresponding to an embodiment of the invention.

(6) FIG. 4 shows a simplified transmitting circuit diagram corresponding to an embodiment of the invention.

(7) FIG. 5A shows a top view of a piezoelectric transducer design corresponding to a representative embodiment.

(8) FIG. 5B shows a cross-section view of a piezoelectric transducer design corresponding to a representative embodiment.

(9) FIG. 6 shows the simulated resonant frequency of a representative embodiment compared to a prior-art piezoelectric transducer.

DETAILED DESCRIPTION

(10) Although the description herein contains many details, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Therefore, it will be appreciated that the scope of the present invention fully encompasses other embodiments which may become obvious to those skilled in the art.

(11) This application discloses a piezoelectric micromachined ultrasonic transducer (PMUT) device and associated circuitry used to transmit and receive ultrasound signals. It will be appreciated that the following embodiments are provided by way of example only, and that numerous variations and modifications are possible. For example, while circular embodiments are shown, the PMUT may have many different shapes such as square, rectangular, hexagonal, octagonal, and so on. Furthermore, while PMUTs are shown having a unimorph construction, consisting of a single piezoelectric layer on a passive layer, bimorph and multimorph PMUTs having multiple piezoelectric layers and various electrode patterns are possible. All such variations that would be apparent to one of ordinary skill in the art are intended to fall within the scope of this disclosure. It will also be appreciated that the drawings are not necessarily to scale, with emphasis being instead on the distinguishing features of the PMUT device disclosed herein.

(12) FIG. 3 shows a simplified diagram of the PMUT receiver circuit 300 in accordance with a representative embodiment. The circuit comprises a piezoelectric transducer 301 connected to a differential amplifier 302 through a pair of receive (RCV) switches 303. Switches 303 are closed when the transducer is used to receive ultrasound signals. Similarly, to transmit ultrasound, RCV switches 303 are opened and a pair of transmit (XMT) switches 304 are closed, connecting the transducer electrodes to a transmit amplifier (omitted for clarity). Another pair of reset (RST) switches, 305 and 306, are used to periodically reset the inputs of amplifier 302 to a common potential during a reset phase. In an operational mode where the PMUT is used to transmit and receive, the reset phase can be timed to precede the receive cycle. In an operational mode where the PMUT is only used to receive, the reset phase can be periodic with a reset phase which is much shorter in time duration than the time constant of the PMUT. Parasitic capacitances C.sub.P1 307 and C.sub.P2 308 represent the capacitance from the non-inverting (+) and inverting () inputs of amplifier 302 to ground. The layouts of the PMUT and integrated circuit are designed such that parasitic capacitances C.sub.P1 307 and C.sub.P2 308 are approximately equal, thereby ensuring that common-mode noise on the non-inverting and inverting terminals of amplifier 302 is substantially cancelled.

(13) FIG. 4 shows a simplified diagram of the PMUT transmitter circuit 400 in accordance with a representative embodiment. For clarity, the receiving circuitry is omitted from this figure. The circuit comprises a piezoelectric transducer 301, a voltage source V.sub.DD 401, and several switches. The voltage source may be a unipolar voltage source. By way of example, and not by way of limitation, voltage source 401 may be a charge pump that converts a low voltage power supply to a higher voltage to increase the amplitude of the transmit signal. In such an implementation, the charge pump may be a unipolar charge pump. The switches allow the voltage polarity across piezoelectric transducer 301 to be alternated from positive to negative in order to double the voltage swing across the piezoelectric transducer, thereby doubling the transmitted ultrasound pressure. Positive polarity is applied by closing switches 402 and 403, connecting the positive terminal of source 401 to electrode 1a of piezoelectric transducer 301 and the negative terminal to electrode 1b. Negative polarity is applied by closing switches 404 and 405, connecting the negative terminal of source 401 to electrode 1a of piezoelectric transducer 301 and the positive terminal to electrode 1b.

(14) FIG. 5A shows a top view and FIG. 5B shows a cross-section view of a PMUT in accordance with an embodiment. The PMUT is composed of a flexible membrane 501 that is supported by a substrate 500. Acoustic signals are transmitted and received using a ring-shaped piezoelectric transducer 301 surrounding the perimeter of membrane 501. Ring-shaped transducer 301 is composed of a piezoelectric layer 502 and top electrode 503 and bottom electrode 504. The top electrode and bottom electrode are designed to be substantially the same size so that the parasitic capacitance to ground is nearly identical at top electrode contact 1a and bottom electrode contact 1b. For example, the first and second electrodes may be designed such that electrical impedances between each of the first and second electrodes and the transducer substrate 500 are substantially identical.

(15) PMUTs are resonant devices wherein ultrasound is transmitted and received in a frequency band centered at the PMUTs flexural resonance frequency. For manufacturability, it is desirable for a PMUT design to have a resonant frequency that is insensitive to fabrication variations, such as residual stress. In prior art, such as the transducer design shown in FIG. 2A and FIG. 2B, variations in the residual stress of the piezoelectric layer 201 and metal layers 202 and 203 create large changes in the resonant frequency of the PMUT. The embodiment shown in FIG. 5A and FIG. 5B, the piezoelectric layer 502 and metal layers 503 and 504 are removed from the majority of the surface of flexible membrane 501, the resonant frequency of the PMUT is determined primarily by the geometry (diameter, thickness) and material properties (Young's modulus, Poisson's ratio, residual stress) of the flexible membrane 501.

(16) FIG. 6 shows the results of a finite element method (FEM) simulation of the resonant frequency of a PMUT using the new ring transducer design presented here compared to the resonant frequency of a prior art PMUT where the piezoelectric layer extends over the entire surface of the flexible membrane. The resonant frequency of the ring transducer design is >20 less sensitive to stress variations in the piezoelectric layer than the prior art design. For a similar layer stack, and for a stress condition that is nominally near zero, the new ring transducer design has a stress sensitivity S=0.01 kHz/MPa, while the prior art PMUT has S=0.21 kHz/MPa.

(17) The preferred new ring PMUT also shows better operational frequency bandwidth, when compared with the prior art PMUT. The bandwidth .sub.0 is a function of the membrane damping b and mass m: .sub.0b/m. The damping b is the same for both designs, since it is a function of the membrane area. However the mass m is lower for the new ring pMUT, since the piezoelectric material is removed in the center of the membrane. The ring PMUT will then have higher bandwidth, inversely proportional to its reduced mass.

(18) All cited references are incorporated herein by reference in their entirety. In addition to any other claims, the applicant(s)/inventor(s) claim each and every embodiment of the invention described herein, as well as any aspect, component, or element of any embodiment described herein, and any combination of aspects, components or elements of any embodiment described herein.

(19) The appended claims are not to be interpreted as including means-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase means for. Any element in a claim that does not explicitly state means for performing a specified function, is not to be interpreted as a means or step clause as specified in 35 USC 112, 6. In particular, the use of step of in the claims herein is not intended to invoke the provisions of 35 USC 112, 6.