MICROMACHINED ULTRASONIC TRANSDUCER ARRAYS WITH MULTIPLE HARMONIC MODES

• US 20140117812A1
• Filed: 03/14/2013
• Published: 05/01/2014
• Est. Priority Date: 10/26/2012
• Status: Active Grant

• Current Assignee: Fujifilm Dimatix, Inc. (Fujifilm Holdings Corporation)
• Original Assignee: Fujifilm Dimatix, Inc. (Fujifilm Holdings Corporation)
• Inventors: HAJATI, Arman
• Granted Patent: US 9,660,170 B2
• Time in Patent Office: Days
• US Class Current: 310/314
• CPC Class Codes: A61B 18/082 Probes or electrodes therefor
• CPC Class Codes: B06B 1/0269 for generating multiple frequencies
• CPC Class Codes: B06B 1/0276 with simultaneous generation, e.g. with modulation, harmonics
• CPC Class Codes: B06B 1/0622 on one surface
• CPC Class Codes: B06B 1/0629 Square array
• CPC Class Codes: H10N 30/081 by coating or depositing using masks, e.g. lift-off
• CPC Class Codes: H10N 30/1071 with electrical and mechanical input and output, e.g. having combined actuator and sensor parts
• CPC Class Codes: H10N 30/802 Drive or control circuitry or methods for piezoelectric or electrostrictive devices not otherwise provided for

Abstract

Micromachined ultrasonic transducer (MUT) arrays capable of multiple resonant modes and techniques for operating them are described, for example to achieve both high frequency and low frequency operation in a same device. In embodiments, various sizes of piezoelectric membranes are fabricated for tuning resonance frequency across the membranes. The variously sized piezoelectric membranes are gradually transitioned across a length of the substrate to mitigate destructive interference between membranes oscillating in different modes and frequencies.

Claims

1. An apparatus for generating and sensing pressure waves in a medium, the apparatus comprising:

a micromachined ultrasonic transducer (MUT) array comprising a plurality of transducer elements, each including a membrane, a single drive/sense electrode, and a reference electrode; and

drive or sense circuitry to operate the MUT array with at least two modes of oscillation, wherein the at least two modes comprise a first mode associated with a first resonant frequency band, and a second mode associated with a second resonant frequency band that includes one or more frequencies greater than one or more frequencies within the first resonant frequency band.

2. The apparatus of claim 1, wherein a center of the second resonant frequency band is approximately twice a center of the first resonant frequency band, and wherein one or more of the first and second resonant frequency bands includes a third, or higher, mode of oscillation.

3. The apparatus of claim 2, wherein a highest resonance frequency of the first resonance frequency band is higher than a lowest resonance frequency of the second resonance frequency band.

4. The apparatus of claim 3, wherein a total bandwidth of the first frequency band and the second frequency band is at least 120% −

6 dB factional bandwidth.

5. The apparatus of claim 1, wherein the MUT array is piezoelectric MUT (pMUT) with transducer elements of the array having a range of piezoelectric membrane sizes, and wherein the first resonant frequency band is a function of the range of membrane sizes.

6. The apparatus of claim 1, wherein the drive circuitry includes a signal generator coupled to the MUT array to drive at least some of the transducer elements with a first electrical signal that induces at least the first frequency band;

and

wherein the sense circuitry includes a signal receiver coupled to the MUT array to receive a second electrical signal from at least some of the transducer elements that includes a component from the second frequency band.

7. The apparatus of claim 6, wherein the first electrical signal induces both the first frequency band and the second frequency band;

and wherein the second electrical signal includes components from both the first and second frequency bands.

8. The apparatus of claim 7, wherein the first frequency band is induced on a first channel of the array and the second frequency band is induced on a second channel of the array;

and

wherein components from the first frequency band are collected from the first channel and the components from the second frequency band are collected from the second channel.

9. The apparatus of claim 6, wherein the first electrical signal has a pulse width and shape that preferentially induces the first band over the second band;

and

wherein the signal receiver is to filter a component of the second electrical signal associated with the second band from that associated with the first band.

10. The apparatus of claim 1, wherein membranes of differing size are spatially arranged on a substrate to avoid destructive interaction between membranes in a same channel, and to avoid cross-talk between membranes of neighboring channels.

11. The apparatus of claim 10, wherein at least one dimension of the membrane varies between a minimum and a maximum value that spans a range larger than a difference in the dimension between any two adjacent membranes.

12. The apparatus of claim 11, wherein the a size of the membranes increases incrementally over a first series of at least three adjacent membranes and decreases incrementally over a second series of at least three adjacent membranes, and wherein the first and second series both include one membrane of maximum dimension and one membrane of minimum dimension.

13. The apparatus of claim 12, wherein the element size variation is a cyclic function of at least one dimension of the array to provide two of n different membrane types within a repeating unit of the channel population.

14. The apparatus of claim 13, wherein only a first semi-axis length of an ellipsoidal membrane is changed over a first dimension of a substrate and only a second semi-axis length of the ellipsoidal membrane is changed over a second dimension of the substrate within a channel.

15. A micromachined ultrasonic transducer (MUT) array, comprising:

a plurality of transducer elements arrayed over an area of a substrate;

a reference electrode coupled to each of the elements; and

a drive/sense electrode coupled to subsets of the elements in parallel to form channels of the array, wherein a first element has a membrane dimensioned to enter a first mode of oscillation when driven by an electrical signal that induces a second mode of oscillation in membrane of a second element, and wherein the membrane dimensions between the first and second elements spans a range larger than a dimensional difference between membranes of any two adjacent elements within the channel.

16. The MUT array of claim 15, wherein element size within a channel varies cyclically over a first dimension with each transducer of a given size adjacent to another element in a second dimension of the substrate that is of the same size or of a next smallest or next largest size.

17. The apparatus of claim 16, wherein the element size variation is a cyclic function of at least one dimension of the array to provide two of n different membrane types within a repeating unit of the channel population that includes two membranes of each size, and wherein the a size of the membranes increases incrementally over a first series of at least three adjacent membranes within each repeating unit and decreases incrementally over a second series of at least three adjacent membranes within each repeating unit.

18. A method of generating and sensing pressure waves in a medium, the method comprising:

driving an electrode of at least some transducer elements of a micromachined ultrasonic transducer (MUT) array with a first electrical signal that induces at least a first mode of oscillation associated with a first resonant frequency band; and

receiving a second electrical signal from the electrode that includes at least a component corresponding to a second mode of oscillation associated with a second resonant frequency band having one or more frequencies that are greater than one or more frequencies within the first frequency band.

19. The method of claim 18, wherein the driving induces both the first frequency band and the second frequency band;

and wherein the second electrical signal includes components from both the first and second frequency bands.

20. The method of claim 19, wherein the first frequency band is induced on a first channel of the array and the second frequency band is induced on a second channel of the array;

and

wherein components from the first frequency band are collected from the first channel and the components from the second frequency band are collected from the second channel.

21. The method of claim 18, wherein the first electrical signal has a pulse width and shape that preferentially induces the first band more than the second band;

and

wherein the signal receiver is to filter a component of the second electrical signal associated with the second band from that associated with the first band.

22. A method of optimizing a frequency response of a piezoelectric micromachined ultrasonic transducer (pMUT) array, the method comprising:

nominally dimensioning features in a plurality of pMUT element patterning mask levels to achieve one or more nominal frequency response;

varying dimensions of a feature in one or more mask level over an area of the array, wherein the feature defines an area of contact between an electrode and a piezoelectric material of the transducer membrane;

generating a sensitivity matrix from measured responses associated with the varied dimensions; and

determining the area of contact between an electrode and a piezoelectric material of the transducer membrane needed to achieve a target response from the array using plurality of nominally dimensioned pMUT element patterning mask layers.

23. The method of claim 22, wherein the feature is a window in a dielectric layer disposed between the piezoelectric material and the electrode, and wherein the one or more mask levels is a single mask level defining a critical dimension (CD) of the window.

24. The method of claim 23, wherein the window CD is varied across different channels of the pMUT array.

25. The method of claim 24, wherein each channel includes plurality of nominal membrane dimensions and wherein the window CD of only one nominal membrane size is varied within a channel.

26. The method of claim 22, further comprising fabricating a first pMUT array with the plurality of nominally dimensioned pMUT element patterning mask levels and the single mask level defining a plurality of window sizes.