Micromachined ultrasonic transducer arrays with multiple harmonic modes
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.
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, wherein, for each transducer element of the plurality of transducer elements:
the membrane of the transducer element is dimensioned to enter a respective first mode of oscillation of a first plurality of modes of oscillation each by a respective membrane of the plurality of transducer elements, the first plurality of modes of oscillation each corresponding to a solution of a respective Bessel function, wherein the first plurality of modes of oscillation each correspond to a respective frequency of first resonant frequencies in a first frequency band; and
the membrane of the transducer element is dimensioned to enter a respective second mode of oscillation of a second plurality of modes of oscillation each by a respective membrane of the plurality of transducer elements, the second plurality of modes of oscillation each corresponding to a solution of a respective Bessel function, wherein the second plurality of modes of oscillation each correspond to a respective frequency of second resonant frequencies in a second frequency band, wherein the first plurality of modes of oscillation and the second plurality of modes of oscillation include different respective modes of oscillation, and wherein one or more of the second resonant frequencies is greater than any of the first resonant frequencies; and
drive or sense circuitry coupled the MUT array, the drive or sense circuitry configured to induce any of at least two modes of the MUT array, the at least two modes comprising:
a first mode including the first plurality of modes of oscillation; and
a second mode including the second plurality of modes of oscillation.
2. The apparatus of claim 1 , wherein a center of the second frequency band is approximately twice a center of the first frequency band, and wherein one or more of the first frequency band and the second frequency band includes resonant frequencies corresponding to a third, or higher, mode of the MUT.
3. The apparatus of claim 2 , wherein a resonant frequency corresponding to one of the first plurality of modes of oscillation is higher than a lowest resonant frequency corresponding to one of the second plurality of modes of oscillation.
4. The apparatus of claim 3 , wherein the first plurality of modes of oscillation and the second plurality of modes of oscillation each correspond to a respective frequency in an at least 120%-6 dB fractional bandwidth of the MUT array.
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 frequency band is a function of the range of piezoelectric 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 from at least some of the transducer elements a second electrical signal that includes a component from the second frequency band.
7. The apparatus of claim 6 , wherein the first electrical signal induces both the first plurality of modes of oscillation and the second plurality of modes of oscillation; and wherein the second electrical signal includes components from both the first frequency band and the second frequency band.
8. The apparatus of claim 7 , wherein the first plurality of modes of oscillation is induced on a first channel of the array and the second plurality of modes of oscillation 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, of the first plurality of modes of oscillation over the second plurality of modes of oscillation, induces only the first plurality of modes of oscillation; and
wherein the signal receiver is to filter a component of the second electrical signal associated with the second frequency band from another component of the second electrical signal associated with the first frequency 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 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, the plurality of transducer elements including a first transducer element and a second transducer element;
a reference electrode coupled to each of the elements; and
a drive/sense electrode, wherein subsets of the plurality of transducer elements are coupled to the drive/sense electrode in parallel with one another to form channels of the array, the channels including a first channel comprising the first transducer element and the second transducer element, wherein the first transducer 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 a membrane of the second transducer element, the second mode of oscillation other than the first mode of oscillation, wherein the first mode of oscillation and the second mode of oscillation each correspond to a solution of a respective Bessel function, and wherein dimensions of membranes between the first transducer element and the second transducer element span a range larger than a dimensional difference between membranes of any two adjacent elements within the first channel.
16. The MUT array of claim 15 , wherein respective sizes of transducer elements within the first channel vary cyclically over a first dimension, wherein, for each transducer element of the first channel, the transducer element is adjacent, along a second dimension, to a respective other transducer element of the first channel, wherein with respect to a size of the transducer element, the other transducer element is of a same size, of a next smallest size of a plurality of element sizes of the first channel or of a next largest size of the plurality of element sizes.
17. The apparatus of claim 16 , wherein the respective sizes of transducer elements within the first channel vary according to a cyclic function of at least one dimension of the array, wherein the channel comprises multiple repeating units each including:
a first series of at least three membranes, wherein successive membranes of the first series increase incrementally in size along a first direction; and
a second series of at least three membranes each adjacent to a respective other membrane of the second series, wherein successive membranes of the second series decrease in size incrementally along the first direction.