ULTRASONIC TRANSDUCERS

Abstract

A piezoelectric micro-machined ultrasonic transducer (PMUT) is provided, comprising a dedicated ultrasonic transmitter and at least one separate dedicated ultrasonic receiver on a single common semiconductor die. A plurality of PMUTs may be arranged in a tessellated array. Also disclosed is a system comprising at least one PMUT on a single common semiconductor die, a dedicated ultrasonic transmitter arranged to transmit a first ultrasonic signal and at least one separate dedicated ultrasonic receiver arranged to receive a second ultrasonic signal is also provided. The system further comprises a signal processing subsystem which comprises an analogue domain; a digital domain; a digital to analogue converter; and an analogue to digital converter. The signal processing subsystem is arranged to generate an estimated direct path signal in said digital domain, convert said estimated direct path signal to an analogue estimated direct path signal using said digital to analogue converter, subtract said analogue estimated direct path signal from said second signal to produce a modified received signal and convert said modified received signal to a digital modified received signal using said analogue to digital converter.

Claims

1. A piezoelectric micro-machined ultrasonic transducer (PMUT) comprising a dedicated ultrasonic transmitter and at least one separate dedicated ultrasonic receiver on a single common semiconductor die.

2. A PMUT as claimed in claim 1 wherein the die is square or rectangular.

3. A PMUT as claimed in claim 2 wherein the ultrasonic transmitter is located substantially at the centre of the die and the ultrasonic receiver(s) is/are located substantially in a corner or in respective corners of the die.

4. A PMUT as claimed in claim 3 comprising one ultrasonic receiver in each of the corners of said die.

5. A PMUT as claimed in claim 1 wherein the ultrasonic transmitter has a width that is at least twice as large as a width of the ultrasonic receiver.

6. A PMUT as claimed in claim 1 wherein the ultrasonic transmitter is configured to transmit signals having a main wavelength and said semiconductor die has a width substantially equal to half of said main wavelength.

7. A PMUT as claimed in claim 1 comprising one or more acoustic path barriers arranged between the ultrasonic transmitter and the ultrasonic receiver.

8. An arrangement comprising a plurality of PMUTs as claimed in claim 1 arranged in a tessellated array.

9. An arrangement as claimed in claim 8 wherein said array is a rectangular array.

10. A system for transmitting and receiving ultrasonic signals comprising at least one PMUT as claimed in claim 1, a transmitter circuit arranged to drive said ultrasonic transmitter and a receiver circuit arranged to detect signals from said ultrasonic receiver.

11. A system as claimed in claim 10 arranged to subtract a direct path signal from a received signal to produce a modified received signal.

12. A system as claimed in claim 11 arranged to subtract the direct path signal from an analogue received signal prior to conversion to digital to produce a modified analogue received signal.

13. A system as claimed in claim 10 arranged to transmit a first ultrasonic signal from the dedicated ultrasonic transmitter and arranged to receive a second ultrasonic signal from the dedicated ultrasonic receiver, the system further comprising a signal processing subsystem comprising: an analogue domain; a digital domain; a digital to analogue converter; and an analogue to digital converter, wherein the signal processing subsystem is arranged to generate an estimated direct path signal in said digital domain, convert said estimated direct path signal to an analogue estimated direct path signal using said digital to analogue converter, subtract said analogue estimated direct path signal from said second signal to produce a modified received signal and convert said modified received signal to a digital modified received signal using said analogue to digital converter.

14. A system comprising at least one piezoelectric micro-machine ultrasonic transducer (PMUT), the PMUT comprising, on a single common semiconductor die, a dedicated ultrasonic transmitter arranged to transmit a first ultrasonic signal and at least one separate dedicated ultrasonic receiver arranged to receive a second ultrasonic signal, the system further comprising a signal processing subsystem comprising: an analogue domain; a digital domain; a digital to analogue converter; and an analogue to digital converter, wherein the signal processing subsystem is arranged to generate an estimated direct path signal in said digital domain, convert said estimated direct path signal to an analogue estimated direct path signal using said digital to analogue converter, subtract said analogue estimated direct path signal from said second signal to produce a modified received signal and convert said modified received signal to a digital modified received signal using said analogue to digital converter.

15. The system as claimed in claim 14 arranged to record the direct path signal from the transmitter to the receiver to create a database of direct path signals.

16. The system as claimed in claim 15 arranged to choose the estimated direct path signal from the database.

17. The system as claimed in claim 16 arranged to monitor a quality parameter of the digital modified received signal and, based on the quality parameter, to carry out one of: using the estimated direct path signal; modifying the estimated direct path signal; choosing a new estimated direct path signal from the database; or recording one or more new direct path signals from the ultrasonic transmitter to the ultrasonic receiver.

18. A method of operating a system for transmitting and receiving ultrasonic signals as claimed in claim 14, the method comprising transmitting signals from said ultrasonic transmitter and receiving signals using said ultrasonic receiver at the same time for at least part of a period of operation.

19. A method of operating a system for transmitting and receiving ultrasonic signals as claimed in claim 14, the method comprising transmitting signals from said ultrasonic transmitter having a main wavelength which is substantially twice a width of said semiconductor die.

20. A method of operating a system for transmitting and receiving ultrasonic signals as claimed in claim 14, the method comprising periodically transmitting signals from said ultrasonic transmitter wherein each transmission period is longer than 0.1 millisecond.

21. A method of operating a system for transmitting and receiving ultrasonic signals comprising a non-planar array of piezoelectric micro-machined ultrasonic transducers (PMUTs), each comprising a dedicated ultrasonic transmitter and at least one separate dedicated ultrasonic receiver on a single common semiconductor die, the method comprising transmitting one or more signals from the transmitter of at a first one of said PMUTs in said non-planar array, receiving said signal(s) using at least one receiver of a second one of said PMUTs of said non-planar array and using said received signals to determine a mutual relative position of said first and second PMUTs.

22. A method as claimed in claim 21 comprising using the mutual relative position in subsequent signal processing of signals received by one or more receivers on said first and second PMUTs.

23. A system for transmitting and receiving ultrasonic signals comprising a non-planar array of piezoelectric micro-machined ultrasonic transducers (PMUTs), each comprising a dedicated ultrasonic transmitter and at least one separate dedicated ultrasonic receiver on a single common semiconductor die, the system being configured to carry out the method of claim 21.

Description

[0059] Certain embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0060] FIG. 1 is a view of a PMUT in accordance with a first embodiment of the invention;

[0061] FIG. 2 is a view of a PMUT in accordance with a second embodiment of the invention;

[0062] FIG. 3 is a cross-section of the PMUT of FIG. 1;

[0063] FIG. 4 is a block diagram of a system for transmitting and receiving ultrasonic signals;

[0064] FIG. 5 is a view of a rectangular array of the PMUTs as shown in FIG. 2;

[0065] FIG. 6 is a view of an array of the PMUTs as shown in FIG. 2 attached to a flexible substrate;

[0066] FIG. 7 is a view of an unmanned aerial vehicle with the array of FIG. 6 attached thereto;

[0067] FIG. 8 is a schematic diagram of a PMUT and associated system for reducing direct path signals;

[0068] FIG. 9 is a flowchart illustrating a method of generating an estimate of the direct path signals of the system shown in FIGS. 8 and 9;

[0069] FIG. 10 is a further schematic diagram of a PMUT and associated system for reducing direct path signals; and

[0070] FIG. 11 is a view of a PMUT using optical receivers.

[0071] FIG. 1 is a simplified view of a piezoelectric micro-machined ultrasonic transducer (PMUT) 2 in accordance with an embodiment of the invention. The PMUT 2 comprises a square silicon die 4 onto which an ultrasonic transmitter 6 and an ultrasonic receiver 8 are formed. Further details of the fabrication process are given below and with reference to FIG. 3.

[0072] As will be seen, the transmitter 6 is circular and located in the centre of the die. The receiver 6 is much smaller than the transmitter 6 and is located in the unused space in one corner of the die. FIG. 2 shows a variant embodiment in which respective receivers 8 are located in each corner of the die 4. Of course other numbers of receivers could be provided—e.g. two, three or more. They could also be located elsewhere or more than one could be located in a given corner. The transmitter could be differently shaped or located and/or multiple transmitters could be provided.

[0073] The transmitter 6 might be designed, for example, to transmit signals at a frequency of 40 kHz or higher. The die 4 has a width of approximately 4 mm which is half of the wavelength of these signals in air. The transmitter 6 has a diameter of approximately 3 mm whereas the receiver(s) has a diameter of approximately 0.1 mm.

[0074] FIG. 3 is a schematic diagonal cross-section which shows in more detail the layers of the PMUT 2 shown in FIG. 2. This comprises a silicon substrate 100 having an aperture 106 at its centre corresponding to the transmitter and smaller apertures 108 in the corners corresponding to the receivers. Laid on the silicon substrate 100 is a silicon membrane 102.

[0075] Above the transmitter and receiver apertures 106, 108 are respective piezoelectric stacks comprising a piezoelectric thin film material layer 104—e.g. of AlN, AlScN or PZT—sandwiched between two electrodes 110.

[0076] The device can be fabricated by using typical microfabrication technologies. The structures for the transmitters and microphones can be typically thin membranes, (one or two dimensional) cantilever structures or bridges. The main part of these mechanical structures typically comprises silicon. These structures can be manufactured by e.g. silicon bulk micromachining—i.e. removal of a major part of the silicon when starting with a silicon wafer, which leaves the intended mechanical (thin) structure or silicon surface micromachining—i.e. depositing a (structured) sacrificial layer and a silicon thin film leaving the mechanical structure after structuring the silicon film and removing the sacrificial layer.

[0077] Besides the main mechanical part of the transmitter or microphone elements, these elements include thin film metal electrodes and the piezoelectric thin film. This might be the same piezoelectric thin film material for the transmitter and microphone part of the device or different piezoelectric thin film materials with optimized properties for transmitting and sensing. The thin-film electrode materials and piezoelectric thin film material(s) are typically structured prior to the structuring of the silicon part of the mechanical structure. Depending on the actuation and read-out concept either two electrodes—one layer below and one on the top of the piezoelectric layer using the 31-mode—or one electrode—on top of the piezoelectric layer using the 33-mode—can be used.

[0078] The electrode materials are typically deposited by a sputtering process. The piezoelectric thin-film materials can—dependent on the material—also be deposited by physical methods such as sputtering or with a pulsed-laser deposition process or using chemical methods such as chemical vapor deposition (CVD) or chemical solution deposition (CSD).

[0079] FIG. 4 shows a highly simplified schematic block diagram of the typical components of an ultrasound transmission and reception system using the PMUTs 6, 8 described herein. The system includes a CPU 20 having a memory 22 and a battery 24 which will typically power all components of the system. The CPU 20 is connected to a signal generator 26 and a signal sampler 28. These could be provided in practice by a suitable digital signal processor (DSP). The signal generator 26 is connected to a transmit amplifier 30 which drives the ultrasonic transmitter 6.

[0080] On the other side the receivers 8 are connected to a receive amplifier 32 which passes signals to the sampler 28 and onto the CPU. It will be noted that because the transmitter 6 is separate from the receivers 8 and the path for driving it is independent of the path for receiving signals, there is no need for complicated switching electronics and transmission and reception can be carried out simultaneously.

[0081] In use the transmitter 6 can be driven with relatively long, low power signals—e.g. more than 0.1 or 0.2 milliseconds long rather than needing to be driven with a sharp burst signal.

[0082] FIG. 5 shows a rectangular array of PMUTs 2 of the type shown in FIG. 2. Here it will be seen that the individual dies 4 are tessellated together in a mutually abutting relationship on a common substrate (not shown) to form the array. Since the dies 4 are a half wavelength wide, the centre-centre spacings 10 of the transmitters 6 in both X and Y directions are also half a wavelength. It will also be seen that receivers 8 in respective corners of adjacent dies form respective 2×2 mini arrays 12. Due to the size of the dies 4, these mini arrays 12 are also separated by half a wavelength.

[0083] Although in FIG. 5 only six dies 4 are shown, in exemplary embodiments there might be many dies in one or both dimensions of the array.

[0084] The wavelength λ of sound depends on the velocity of sound c and its frequency f: λ=c/f

[0085] For technical usable ultrasound in air (above 40 kHz to ensure it is above the audible range for dogs) the wavelength is below 8.6 mm and half the wavelength, which is an important parameter for ultrasound arrays, is therefore below 4.3 mm. This is a typical dimension of a MEMS (microelectromechanical system) type device such as those described herein.

[0086] For typical MEMS type structures such as cantilevers and membranes, the frequency of the fundamental vibration modes can be expressed by the following equations:

[0087] Cantilever:

[00001]$f=\frac{1.015}{2\pi }\frac{t}{L^2}\sqrt{\frac{E}{\rho }}$

[0088] Circular membrane/diaphragm:

[00002]$f=\frac{40.8}{2\pi }\frac{t}{d^2}\sqrt{\frac{E}{12\left(1-v\right)\rho }}$

[0089] Here t is the thickness of the mechanical structure, E the Young's modulus, ρ is the density, L the length of a cantilever and d the diameter of a circular membrane. These equations are for a single material, but can quite easily be modified for a multi-layered structure.

[0090] These equations exemplify the feasibility of MEMS ultrasound structures. The eigenfrequency of a 8 μm thick silicon membrane with 1250 μm diameter, which are typical dimensions for MEMS structures, has an eigenfrequency of about 80 kHz.

[0091] Most standard beamforming algorithms benefit from λ/2 spacing because it means that each incoming wave front can be discerned from other incoming wavefronts with a different angle or wavenumber, which in turn means that the problem of so-called ‘grating lobes’ is prevented. Classical beamforming methods that benefit from λ/2 (or tighter) spacing include (weighted) delay-and-sum beamformers, adaptive beamformers such as MVDR/Capon, direction-finding methods like MUSIC and ESPRIT and Blind Source Estimation approaches like DUET, as well as wireless communication method, ultrasonic imaging methods with additional constraint such as entropy or information maximization.

[0092] FIG. 6 shows a further array 14 made up of a number of dies 4 of the type shown in FIG. 2 attached to a flexible substrate in the form of a ribbon 16 made, for example, of polyurethane. This array 14 can be attached to any number of objects or devices or could form part of a wearable device. FIG. 7 shows one example where the array 14 is attached to the body of an unmanned aerial vehicle or drone 18. In such an arrangement a processor (not shown) driving the transmitters and receivers thereof can be programmed to operate in a calibration phase whereby individual transmitters 6 in the array 14 transmit different signals, or signals at different times, which are them received by receivers 8 on other dies in the array. Using a suitable algorithm, such as transmitting a coded signal (CDMA type) or a chirp signal, followed by matched filtering or deconvolution, and signal peak detection such as i.e. a CFAR filter, the times of flight of such transmissions can be used to establish the relative mutual positions of the individual dies 4. In some situations one is more interested in computing relative time-differences of arrival (TDOA) between two receivers and one transmitted/reflected signal. There is a range of popular methods for this, including Generalized Cross Correlation PHAse Transform (GCC-PHAT) and Steered Response PHAse Transform (SRP-PHAT).

[0093] These can then be used during operation to apply appropriate phase differences to the signals of respective receivers to allow them to act as a coherent array—e.g. for beamforming. Such an approach is beneficial in allowing the array to be attached to any number of irregularly shaped objects so that the precise attachment is not critical.

[0094] The drone 18 can use the array 14 for echolocation, collision avoidance etc.

[0095] FIG. 8 is a schematic diagram of a PMUT 302 and associated system which is able to compensate for direct path signals. The system includes a PMUT 302 which comprises a square silicon die 304 on which a transmitter element 306, and a receiver element 308 are formed.

[0096] An ASIC (application-specific integrated circuit) or DSP (digital signal processor) 42 is connected to a primary digital to analogue (D/A) converter 34. This primary D/A converter 34 is connected to an amplifier 132 which drives the ultrasonic transmitter 306. The ultrasonic transmitter 306 thus emits an ultrasonic signal 48.

[0097] The ultrasonic receiver 308 receives reflected echoes 50 which are reflected from an object of interest. The ultrasonic receiver 308 also receives acoustic direct path signals 44, 46. One of the direct path signals 44 is an in-air direct acoustic path signal. The other direct path signal 46 is transmitted through the body of the die 304 from the transmitter 306 to the receiver 308. Other transmission mechanisms may contribute to the overall direct path signal received by the receiver 308.

[0098] The ASIC/DSP 42 further generates an estimate of the effect of the direct path signals 44, 46 on the received ultrasonic signals as will be described in more detail below with reference to FIG. 9. The ASIC/DSP 42 comprises a signal modifier 52 which may modify the estimate produced. The signal modifier 52 may for example incorporate a filter that applies a convolution to the output signal from the ASIC/DSP 42. The estimated direct path signal passes to a D/A converter 54 which converts it to an analogue signal. This analogue signal passes through an amplifier 36 to a mixer 38. The mixer subtracts the analogue estimated direct path signal from the analogue signal produced by the receiver 308, and the resultant signal is passed to an analogue to digital (A/D) converter 40 to produce a digital signal which may be further analysed e.g. for echolocation, stored etc.

[0099] Typically, the direct path signals 44, 46 are much stronger than the received echoes 50. The described embodiment advantageously removes the direct path signals 44, 46 prior to sampling for conversion to digital signals. If the direct path signals 44, 46 were not removed, the A/D converter 40 would require a high dynamic range in order to convert both the received echoes 50 to digital signals, as well as the direct path signals 44, 46. A high dynamic range A/D converter is more complex and thus more expensive and power consuming.

[0100] FIG. 9 is a flowchart illustrating a method generating the estimate of the direct path signals 44, 46 in the system shown in FIG. 8. At step 58, the system starts recording the direct path signals 44, 46 from the transmitter element 306 to an individual receiver 308. If there are multiple receivers, as shown in FIG. 2, then the process may be repeated for each individual receiver. The signal recorded does not include reflections from the environment because time-gating is used to exclude these (since they have a longer time of flight than the direct path signals).

[0101] Since the direct path signals 44, 46 can vary with conditions, such as temperature, it may be desirable to record several direct path signals 44, 46 over a longer period of time, or over multiple time instances during a day (when the system is not in use) to obtain a sufficient database in step 60. Optionally, the recordings may be used to estimate the direct path signals 44, 46 at different temperatures and pressure levels by resampling at slightly higher or lower frequencies.

[0102] At step 62, a criterion for whether a sufficient database of direct path signals has been created is tested. This criterion could be related to any suitable quality parameter such as the degree of self-repetition of the pre-recorded direct path signals i.e. whether the past signals are repeating themselves, or the criterion could be tied to a temperature sensor which requires direct path signals for a certain range of temperatures to have been collected for the database to be “complete”. The database may be updated from time to time as the physical surroundings around the elements may change. For example, the transmitter 306, or receiver 308 may be moved to a different housing, or dust may have fallen on or close to the sensor and affect the direct acoustic paths. If the database quality is not adequate, then further recording of the direct path signals is carried out.

[0103] Once the database quality is determined to be adequate, in step 64, a recording session for reflected signals begins. An initial estimate of direct path signals is provided in step 66, either as a random guess, or taking into account input from a temperature sensor (not shown) used in the direct path signal database creation steps 58-62. The D/A converter 54 then converts the estimated direct path signal from the ASIC/DSP 42 so that it can be subtracted in the mixer 38.

[0104] In step 70, the transmitter 306 transmits an ultrasound signal, and the receiver 308 receives the reflected echoes 50, and direct path signals 44, 46. In steps 72 and 74, the quality of the received data is monitored to identify whether the selected direct path signal from step 66 was a good selection. An example of a parameter for quality is minimal energy which signals that the strongest component in the received signal (the direct path 44, 46) has been successfully been removed. Alternatively, maximum sparsity may be used as a parameter, as this signals that a “clear echo” is being received. Generally, mixes of echoes 50 and direct path signals 44, 46 tend to be more complex than any one of them separately. Other parameters such as reflecting entropy or self-similarity over time could also be used.

[0105] If the quality in step 74 is good, in step 76, the received signal from the mixer 38 passes to the A/D converter 40, and may be used for further analysis such as proximity, presence or gesture sensing.

[0106] If the quality in step 74 is poor, and the quality is not below a first threshold in step 78, only minor modifications to the estimate of the direct path signal are necessary. These minor modifications may be incorporated by a filter 52 which applies a convolution to the estimated direct path signal in order to attempt to rectify the estimated direct path signal in step 80.

[0107] If the quality parameter is below a second critical threshold, in step 82, the system starts to record direct path signals again, in order to build up a new database. This may be necessary when there is a substantial change in the behaviour or surroundings of the transmitter element 306.

[0108] If the quality parameter is below the first threshold, but not below the second threshold, another candidate may be selected for the estimated direct path signal, as shown in step 84.

[0109] FIG. 10 is a schematic diagram of another embodiment of a PMUT 302′ and associated system for compensating for direct path signals. This embodiment is almost identical to that of FIG. 8 and similar parts are indicated with similar reference numerals with the addition of a prime symbol. However in this embodiment the PMUT 302′ further includes acoustic path barriers 56. These acoustic path barriers 56 may for example, be a cylinder around the transmitter 306′, a cylinder around the receiver 308′, or a cylinder around both the transmitter 306′ and receiver 308′. The acoustic path barriers 56 act to physically reduce the strength of the in-air direct acoustic path signal 44′ by reducing air transmission of the signal 44′ between the transmitter 306′ and the receiver 308′.

[0110] FIG. 11 is a view of a PMUT 402 using optical receivers 408. These could, for example comprises MEMS structures where movement of a membrane by acoustic signals is read out using light reflected from the membrane, e.g. using a diffraction grating. The optical receivers 408 may be much more closely spaced than the receivers 8 shown in FIG. 2, as optical receivers have much lower self-noise and thus much better SNR than conventional receivers. The optical receivers 408 may therefore be much more closely spaced than λ/2, with images still obtained with high resolution. As such, through use of closely spaced optical receivers 408, a compact ultrasound imaging component is formed on a single die 404.