Patent Description:
Scanning probe microscopy (SPM) devices, such as atomic force microscopy (AFM) devices are for example applied in the semiconductor industry for scanning of semiconductor topologies on a surface, for characterization of the semiconductor. Other uses of this technology are found in biomedical industry, nanotechnology, and scientific applications. In particular, AFM may be used for critical dimension metrology (CD-metrology), particle scanning, stress- and roughness measurements. AFM microscopy allows visualization of surfaces at very high accuracy, enabling visualization of surface elements at sub-nanometer resolution. Other surface scanning measurement devices for example include optical near field scanning devices.

The probe in a SPM system comprises a cantilever and a probe tip. On one end of the cantilever, the probe is attached to a sensor head, for example (but not necessarily) through an actuator that allows to bring the probe in motion. Probe tip is usually located on the other end of the cantilever. In SPM, the probe tip can be scanned over the surface of a sample or substrate to measure the topography and mechanical properties thereof. A sensor, in many cases an optical sensor, monitors the position of the probe tip. For example, the sensor may monitor a reflected laser beam that is reflected by the cantilever or the back of the probe tip, and which changes angle when the probe tip moves up or down.

Often, there is a desire to reliably quantify dimensions of a sample, including sub-surface dimensions or features of the sample. It can be challenging to accurately detect sub-surface features in the semiconductor industry. For example, in a sample or substrate with a multi-layer stack (e.g. semiconductor), the signal-to-noise ratio (SNR) may be low as the frequency used is often from GHz up to THz, which can make ultrasound imaging rather difficult. Moreover, the stack mechanical properties are usually unknown. As there could be an opaque layer, optical methods are often limited.

There is a need for detecting sub-surface features using non-destructive evaluation method. The sub-surface features may be buried deep in a structure, for example from a few hundreds of nanometers to micrometers deep. It is desired to accurately be able to extract relevant dimension, which can be used for defect inspection and/or process control.

In this connection it is noted that <CIT>) discloses an AFM probe array integrated with ultrasonic energy transducer. The AFM probe array includes a base which is rectangular in shape; a plurality of micro-cantilevers which are connected with one side edge of the base, and a plurality of ultrasonic energy transducers, wherein each micro-cantilever is provided with a needle tip, the ultrasonic energy transducers are arranged on the back sides of the micro-cantilevers or the back side of the base, and the micro-cantilevers and the ultrasonic energy transducers form the probe array. The probe integrated with the ultrasonic energy transducers renders possible inside imaging of samples.

Also <CIT>) discloses a method of performing subsurface imaging of embedded structures underneath a substrate surface, using an atomic force microscopy system. The system comprises a probe with a probe tip, and a sensor for sensing a position of the probe tip. The method comprises the steps of: positioning the probe tip relative to the substrate; applying a first acoustic input signal to the substrate; applying a second acoustic input signal to the substrate; detecting an output signal from the substrate in response to the first and second acoustic input signal; and analyzing the output signal. The first acoustic input signal comprises a first signal component and a second signal component, the first signal component comprising a frequency below <NUM> megahertz and the second signal component either including a frequency below <NUM> megahertz or a frequency such as to provide a difference frequency of at most <NUM> megahertz with the first signal component, such as to enable analysis of an induced stress field in the substrate; and wherein the second acoustic input signal comprises a third signal component having a frequency above <NUM> gigahertz, such that the return signal includes a scattered fraction of the second acoustic input signal scattered from the embedded structures.

It is an object of the invention to provide for a method and a system that obviates at least one of the above mentioned drawbacks.

Additionally or alternatively, it is an object of the invention to provide for a method and system able to more accurately perform sub-surface characterization.

It is an object of the invention to provide for an improved ultrasound sample characterization method, using a scanning probe microscope.

Thereto, the invention according to claim <NUM> provides for a method for performing sub-surface measurements on a sample using a scanning probe microscopy system, the system comprising a sensor head including an array of probes with a plurality of probes each comprising a cantilever and a probe tip arranged on the cantilever, the method comprising the steps of: moving the array of probes towards a surface of the sample for enabling contact between the probe tips of the array of probes and the surface, performing successive measurements wherein consecutively subsets of cantilevers are vibrated in order to emit a acoustic wave in the sample, and then for each successive emitted acoustic wave a return signal is retrieved by each of the cantilevers, determining a transfer function of the whole array based on the successive measurements, and performing a measurement wherein acoustic signals are concurrently emitted by the cantilevers, wherein the acoustic signals are coordinated based on said transfer function in order to direct a focused non-planar wave towards the one or more identified sub-surface features, and then the return signal retrieved by each of the cantilevers is measured for sub-surface characterization.

By means of the plurality of cantilevers with probe tips, a waveforming technique can be employed. The plurality of cantilevers may be coordinatively vibrated in order to obtain a desired waveform directed towards sub-surface features. In this way, more accurate measurements of sub-surface characteristics of the sample can be performed. Advantageously, the method enables sub-surface characterization with improved SNR.

The probe tips of the array of cantilevers can touch the sample surface and one or more of the probe tips can coordinately emit an acoustic wave within the sample. The return signal can then be measured by the probe tips. The cantilevers of the array can both act as an emitter and receiver. In an example, one cantilever emits an acoustic wave in the sample, and then all the cantilevers are used for measuring the return signal. Then a different subsequent cantilever emits an acoustic wave in the sample, and then again all the cantilevers are used for measuring the return signal. This subsequent firing with each of the cantilevers and measuring the response signal by all the cantilevers can enable determining the transfer function of the whole array. A coupling signal from a sub-surface feature can be measured. The cantilevers can be coordinately actuated based on the transfer function, in order to shape a wavefront which is focused on a sub-surface feature. In this way, the energy used for characterization of the sample can be more focused, providing significant improvements in the SNR. The sub-surface features can thus be better imaged.

In some examples, a source/actuation transducer (e.g. PZT, EMAT, CMUT etc.) or laser can used as an emitter. A first acoustic signal at kHz or MHz level, and a different second acoustic signal at higher frequencies (MHz to THz) may be emitted such that the return signal includes a scattered fraction of the second acoustic input signal scattered from the embedded structures.

The transducer may emit an acoustic wave to the sample, such that the sub-surface features scatter this incoming wave. The cantilever will measure the out-of-plane displacement (movement in the vertical direction) and therefore will be able to obtain the scattering pattern. The tip of the cantilever can act as a point source, and different beamforming techniques can be used to image the sub-surface features.

Optionally, successive measurements are performed wherein consecutively subsets of cantilevers are vibrated in order to emit a plane acoustic wave in the sample.

Optionally, the subset of cantilevers are coordinatively vibrated in order to emit plane acoustic waves in the sample for performing the measurements.

Optionally, the concurrently emitted acoustic signals correspond to the signals reflected by the sub-surface feature.

Optionally, the focused non planar wave is directed towards a single sub-surface feature.

The focusing of the acoustic non-planar wave towards the single sub-surface feature can significantly increase the sub-surface imaging/characterization of said sub-surface feature.

Optionally, the focused non-planar wave is directed towards a plurality of sub-surface features simultaneously.

Even if the wavefront is focused on a plurality of sub-surface features, the SNR can still be improved compared to the case without a focused wavefront.

Optionally, the focused non-planar wave is directed towards a plurality of sub-surface features consecutively.

For each of the plurality of sub-surface features in the sample, a focused non planar wave can be emitted. In this way, each of the plurality of sub-surface features can be better characterized, as the measurements can be separated from each other for each of the plurality of sub-surface features.

Optionally, the data indicative of the acoustic waves emitted during the successive measurements are stored in an emitter matrix, and data indicative of the received return signal by each of the cantilevers is stored in a reception matrix, wherein a transfer matrix is determined based on the emitter matrix and the reception matrix.

Optionally, a singular value decomposition is performed on the transfer matrix for obtaining a set of singular value vectors, wherein the set of singular value vectors are used for increasing the signal-to-noise ratio of the measurement.

For example, dominant proper orthogonal decomposition (POD) modes can be determined to obtain the dominant measurement characteristics.

Optionally, the focused non-planar wave directed towards the one or more identified sub-surface features is determined based on one or more singular value vectors.

Optionally, a conjugated measured signal is emitted back when performing the measurement in order to focus the non-planar wave towards the one or more identified sub-surface features.

Optionally, based on material properties of the sample and the transfer matrix, the focused non-planar wave towards the one or more identified sub-surface features is calculated numerically, preferably based on a cylindrical equation of acoustic wave propagation.

Optionally, one or more singular values are selected for focusing the non-planar wave towards the one or more identified sub-surface features.

Optionally, the array of probes has a larger number of probes than a number of sub-surface features towards which the non-planar wave is focusable during the measurement.

A number of cantilevers of the array can be equal to or higher than a number of sub-surface reflectors within the sample. The sub-surface features within the sample can be seen as reflectors.

Optionally, the array of probes has more than two probes, more preferably more than three, even more preferably more than five probes.

A larger number of probes can result in a more accurate focusing of the wave towards identified sub-surface features. Hence, the characterization of the sample can be significantly improved in this way.

Optionally, the signals used for emitting waves into the sample are coded for increasing the signal-to-noise ratio of the measurement.

Different types of coded signals can be used, for example, chirp linear, exponential, Hadamard, maximum length sequence (MLS), etc., in order to increase the SNR. This can be used for example instead of a Gaussian pulse.

According to claim <NUM> of the invention, the invention provides for a scanning probe microscopy system for performing sub-surface measurements on a sample, the system including: a sensor head including an array of probes with a plurality of probes each comprising a cantilever and a probe tip arranged on the cantilever, wherein the system further comprises a processor configured for applying a method defined in any one of the previous claims, in particular arranged for: moving the array of probes towards a surface of the sample for enabling contact between the probe tips of the array of probes and the surface, performing successive measurements wherein consecutively subsets of cantilevers are vibrated in order to emit an acoustic wave in the sample, and then for each successive emitted acoustic wave a return signal is retrieved by each of the cantilevers, determining a transfer function of the whole array based on the successive measurements, and performing a measurement wherein acoustic signals are concurrently emitted by the cantilevers, wherein the acoustic signals are coordinated based on said transfer function in order to direct a focused non-planar wave towards the one or more identified sub-surface features, and then the return signal retrieved by each of the cantilevers is measured for sub-surface characterization.

Sub-surface features and/or sublayers can be better detected. Furthermore, the dimensions of sub-surface features or sub-layers of the sample can be better characterized. Even sub-surface features resulting in a low SNR can be characterized effectively using the system according to the invention.

Optionally, the cantilever is actuated with a signal including a first frequency and a second frequency. The first frequency component may be chosen such as to match to the cantilever resonance frequency for probing. The second frequency component may be configured to emit an acoustic wave in the sample, as the tip of the cantilever touches the sample. The reflected wave(s) can be measured by means of the cantilever. The return signal retrieved/received by array of probes <NUM> can be used for beamforming and directing an ultrasonic wave towards identified sub-surface features (e.g. void, sublayer, etc.).

According to claim <NUM> of the invention, the invention provides for a computer program product downloadable from a communication network and/or stored on a computer-readable and/or microprocessor-executable medium, comprising program code instructions that, when executed by a scanning probe microscopy system including a processor, causes the system to perform a method according to the invention, the scanning probe microscopy system including a sensor head including an array of probes with a plurality of probes each comprising a cantilever and a probe tip arranged on the cantilever, wherein the method includes the steps of: moving the array of probes towards a surface of the sample for enabling contact between the probe tips of the array of probes and the surface, performing successive measurements wherein consecutively subsets of cantilevers are vibrated in order to emit an acoustic wave in the sample, and then for each successive emitted acoustic wave a return signal is retrieved by each of the cantilevers, determining a transfer function of the whole array based on the successive measurements, and performing a measurement wherein acoustic signals are concurrently emitted by the cantilevers, wherein the acoustic signals are coordinated based on said transfer function in order to direct a focused non-planar wave towards the one or more identified sub-surface features, and then the return signal retrieved by each of the cantilevers is measured for sub-surface characterization.

It will be appreciated that any of the aspects, features and options described in view of the method apply equally to the system and the described computer program product.

The invention will further be elucidated on the basis of exemplary embodiments which are represented in a drawing. The exemplary embodiments are given by way of non-limitative illustration. It is noted that the figures are only schematic representations of embodiments of the invention that are given by way of non-limiting example.

<FIG> shows a schematic diagram of an embodiment of a system <NUM>. The system <NUM> is a scanning probe microscopy system configured to perform sub-surface measurements on a sample <NUM>. The system <NUM> includes a sensor head <NUM> including an array of probes <NUM> with a plurality of probes each comprising a cantilever <NUM> and a probe tip <NUM> arranged on the cantilever <NUM>. The system <NUM> further comprises a processor configured to operate the system <NUM> for moving the array of probes <NUM> towards a surface 3a of the sample <NUM> for enabling contact between the probe tips <NUM> of the array of probes <NUM> and the surface 3a; performing successive measurements wherein consecutively subsets of cantilevers are vibrated in order to emit an acoustic wave in the sample, and then for each successive emitted acoustic wave a return signal is retrieved by each of the cantilevers <NUM>; determining a transfer function of the whole array based on the successive measurements; and performing a measurement wherein acoustic signals <NUM> are concurrently emitted by the cantilevers <NUM>, wherein the acoustic signals <NUM> are coordinated based on said transfer function in order to direct a focused non-planar wave towards one or more identified sub-surface features <NUM>, and then the return signal retrieved by each of the cantilevers <NUM> is measured for sub-surface characterization.

In the shown example, the sample is a multi-layer sample including a first layer 17a and a second layer 17b. It will be appreciated that also a single layer sample <NUM> can be used. It is also possible to use a sample <NUM> with a larger number of layers.

The sensor head <NUM> and the sample <NUM> can be moveable with respect to each other as indicated by arrows <NUM>. The array of probes <NUM> may be moveable with respect to the sample along a surface 3a therefor for performing multiple measurements at different locations on said surface 3a. It will be appreciated that other relative moving directions are also possible.

In some examples, a set of cantilevers <NUM> can be placed in a line regularly spaced and separated by a pitch. Optionally, the cantilevers or probes can be arranged in a matrix arrangement (e.g. multi-array). In some examples, the arbitrary waveform is generated with as many channels as the number of cantilevers in the array of probes. For example, an ultrasound piezo/PZT phased arrays may have between <NUM> up to <NUM> transducers or probes. However other numbers are also possible, for instance up to thousands of probes (e.g. piezo/PZT transducers). Different excitation mechanisms can be employed for the cantilevers, such as at least one of piezo, photo-thermal, etc. Also many variants of reception systems for the cantilevers can be employed. Examples are piezo-electric (PZT), optical beam deflection (OBD), etc..

<FIG> shows a schematic diagram of an embodiment of a system <NUM> with a plurality of probes arranged in an array of probes <NUM>. The probes are schematically represented as numbered boxes next to each other. This figure illustrates a measurement technique employed using the array of probes <NUM>, namely different successive steps (a)(-f) are illustrated. In a first step (a), a first probe <NUM>-<NUM> is actuated. The cantilever of the first probe <NUM>-<NUM> is vibrated in order to emit an acoustic wave in the sample. In a subsequent second step (b), a return signal is retrieved by each of the cantilevers <NUM>-<NUM> to <NUM>-<NUM>. In a subsequent third step (c), a second probe <NUM>-<NUM> is vibrated in order to emit an acoustic wave in the sample. In a subsequent fourth step (d), a return signal is again retrieved by each of the cantilevers <NUM>-<NUM> to <NUM>-<NUM>. In a subsequent fifth step (e), a next probe, third probe <NUM>-<NUM>, is vibrated in order to emit an acoustic wave in the sample. In a subsequent sixth step (f), a return signal is again retrieved by each of the cantilevers <NUM>-<NUM> to <NUM>-<NUM>. This process can be repeated until all probes <NUM>-<NUM> to <NUM>-<NUM> of the total number of probes have been actuated individually, and a resulting return signal is measured by all the probes <NUM>-<NUM> to <NUM>-<NUM>. Although in this example, only a single probe is vibrated at a time during actuation, it is also possible that successive measurements are performed wherein consecutively subsets of cantilevers are vibrated in order to emit an acoustic wave in the sample. Then for each successive emitted acoustic wave emitted by the subset of probes, a return signal can be retrieved by each of the cantilevers <NUM>.

A transfer function of the whole array can be determined based on the successive measurements. Further, a measurement can be performed based on the transfer function. Acoustic signals can be concurrently emitted by the cantilevers <NUM>, wherein the acoustic signals <NUM> are coordinated based on said transfer function in order to direct a focused non-planar wave towards one or more identified sub-surface features. Then, the return signal retrieved by each of the cantilevers <NUM> can be measured for sub-surface characterization.

The sub-surface features can be detected by means of a scanning probe microscopy or atomic force microscopy system, based on a time-reversal operator decomposition. The sub-surface features may act as reflectors. A matrix K(m,n,t) can be measured in 3D (m-Emitters*n-Receivers*t-Time). A singular vector decomposition can be performed (after a Fourier Transform): K(m,n,f) = U(f)Σ(f)V(f)*. In this way an increase in the SNR can be obtained, such that it is enabled to better see features in the noise. Furthermore, it may not be necessary to know material properties of the sample being characterized.

In this example, the array of probes <NUM> includes a total number of twenty probes <NUM>-<NUM> to <NUM>-<NUM> arranged next to each other. However, a different number of probes may also be arranged in accordance with the invention.

<FIG> shows a schematic diagram of a system <NUM> not according to the presently claimed invention, which includes an array of probes <NUM>. Different steps (a)-(c) are shown. In a first step (a), a plane wave E<NUM>(f) is emitted by means of the array of probes <NUM>. The plane wave propagates within the sample. In a second step (b), a return signal is measured, S<NUM>(f) = K(f)E°(f), wherein E corresponds to an Emitter Matrix, S corresponds to a reception matrix, and K corresponds to a transfer matrix. In a third step (c), retro-propagation is performed E<NUM>(f) = K*(f)E<NUM>*(f). As the time-reversal operator corresponds to a phase conjugation, it is possible to emit back the conjugated measured signal in order to focus on the reflector. A rank of K corresponds to a number of reflectors (sub-surface features), in the above case only <NUM>. Meaning the first singular vector (λ) corresponds to the signals coming from the sub-surface feature, the other singular vectors correspond to the noise sub-space.

In the third step (c), the acoustic signals are coordinated based on said transfer function in order to direct a focused non-planar wave towards the one or more identified sub-surface features, and then the return signal retrieved by each of the cantilevers is measured for sub-surface characterization. The sub-surface characterization of the sub-surface feature <NUM> can be significantly enhanced in this way. The resulting image has an improved SNR.

<FIG> shows a schematic diagram of an embodiment of a system <NUM>. In this example, the measurement is performed in <NUM> steps (a)-(c), based on the study of the time-reversal operator K(f).

Other signals can be used for the measurement of the transfer matrix K. For example, this can be performed similar to a synthetic transmit aperture (STA) method. In this method, multiple sequences of sources are used to image a reflector. An 'intensity map' can be built which roughly consists to the sum of the difference signals/position. However, the main issue of this conventional method is that all signals are added together, so if the SNR is small, the noise contribution is added and can compromise the detection of the sub-surface feature. Overall a low SNR can be the result, and features close to each other can be hard to distinguish. Advantageously, according to the invention, it is possible to focus on the singular values for imaging and/or for refocusing.

<FIG>, <FIG>, and <FIG> show a schematic diagram of measured sub-surface characterizations, more particularly amplitude A intensity maps measured by the plurality of the cantilevers of the array of probes <NUM>. <FIG> shows an image obtained by employing the conventional synthetic transmit aperture method. <FIG> shows a first singular vector with the highest amount of energy. <FIG> shows an image obtained by re-emitting the first singular vector for imaging the sub-surface feature. For this purpose, the acoustic signals are concurrently emitted by the cantilevers of the array of probes <NUM>, wherein the acoustic signals are coordinated based on the transfer function in order to direct a focused non-planar wave towards the one or more identified sub-surface features. The return signal retrieved by each of the cantilevers is then measured for the improved sub-surface characterization.

<FIG> shows a schematic diagram of an embodiment of performing sub-surface measurements. In some examples, coded signals (chirp linear, exponential, Hadamard, MLS, etc.) are used to increase the SNR, for example instead of Gaussian Pulse. In some examples, a number of cantilevers is chosen to be higher than a number of reflectors. The number of reflectors can for instance be based on a prediction, a worst-case scenario, etc. In the shown example, two singular vectors are obtained as there are two reflectors. An eigenvalue distribution with two sub-surface features is shown in graph <NUM>.

In some examples, an iterative time-reversal mode can be employed. In a first step (a), a first transmitted wave can illuminate a sector containing for example two targets within the sample, i.e. sub-surface features. In subsequent step (b), reflected waves can be recorded on the array of probes <NUM>. Further, in step (c), the retrieved data can be time-reversed and reemitted back. The time-reversed wave fronts enable refocusing on the two targets. The highest amplitude wavefront can illuminate the most reflective target, while the weakest wavefront can illuminate the second target. In step (d), the new reflected wave fronts are recorded before another time reversal is carried out. The weakest target can be illuminated more weakly and reflect a wavefront much fainter than the one coming from the strongest target. After some iterations the process can converge and produce a wavefront focused on the most reflective target. In this way, sub-surface characterization using the scanning probe microscope with the array of probes <NUM> can be significantly enhanced.

<FIG> shows a schematic diagram of a method <NUM> for performing sub-surface measurements on a sample using a scanning probe microscopy system. The system comprises a sensor head including an array of probes with a plurality of probes each comprising a cantilever and a probe tip arranged on the cantilever. In a first step <NUM> of the method, the array of probes is moved towards a surface of the sample for enabling contact between the probe tips of the array of probes and the surface. In a second step <NUM>, successive measurements are performed wherein consecutively subsets of cantilevers are vibrated in order to emit an acoustic wave in the sample. Then, in a third step <NUM>, for each successive emitted acoustic wave a return signal is retrieved by each of the cantilevers. In a fourth step <NUM>, a transfer function of the whole array is determined based on the successive measurements. In a fifth step <NUM>, a measurement is performed wherein acoustic signals are concurrently emitted by the cantilevers, wherein the acoustic signals are coordinated based on said transfer function in order to direct a focused non-planar wave towards the one or more identified sub-surface features. Then, in a sixth step <NUM>, the return signal retrieved by each of the cantilevers is measured for sub-surface characterization.

It will be appreciated that the method may include computer implemented steps. All above mentioned steps can be computer implemented steps. Embodiments may comprise computer apparatus, wherein processes performed in computer apparatus. The invention also extends to computer programs, particularly computer programs on or in a carrier, adapted for putting the invention into practice. The program may be in the form of source or object code or in any other form suitable for use in the implementation of the processes according to the invention. The carrier may be any entity or device capable of carrying the program. For example, the carrier may comprise a storage medium, such as a ROM, for example a semiconductor ROM or hard disk. Further, the carrier may be a transmissible carrier such as an electrical or optical signal which may be conveyed via electrical or optical cable or by radio or other means, e.g. via the internet or cloud.

Some embodiments may be implemented, for example, using a machine or tangible computer-readable medium or article which may store an instruction or a set of instructions that, if executed by a machine, may cause the machine to perform a method and/or operations in accordance with the embodiments.

Examples of hardware elements may include processors, microprocessors, circuits, application specific integrated circuits (ASIC), programmable logic devices (PLD), digital signal processors (DSP), field programmable gate array (FPGA), logic gates, registers, semiconductor device, microchips, chip sets, et cetera. Examples of software may include software components, programs, applications, computer programs, application programs, system programs, machine programs, operating system software, mobile apps, middleware, firmware, software modules, routines, subroutines, functions, computer implemented methods, procedures, software interfaces, application program interfaces (API), methods, instruction sets, computing code, computer code, et cetera.

Herein, the invention is described with reference to specific examples of embodiments of the invention. The specifications, figures and examples are, accordingly, to be regarded in an illustrative sense rather than in a restrictive sense. The invention is intended to embrace all alternatives, modifications and variations which fall within the scope of the appended claims. Further, many of the elements that are described are functional entities that may be implemented as discrete or distributed components or in conjunction with other components, in any suitable combination and location.

Claim 1:
A method (<NUM>) for performing sub-surface measurements on a sample (<NUM>) using a scanning probe microscopy system (<NUM>), the system comprising
a sensor head (<NUM>) including an array of probes (<NUM>) with a plurality of probes each comprising a cantilever (<NUM>) and a probe tip (<NUM>) arranged on the cantilever, the method comprising the steps of:
moving (<NUM>) the array of probes towards a surface (3a) of the sample for enabling contact between the probe tips of the array of probes and the surface, characterized in that the method further comprises the steps of:
performing (<NUM>) successive measurements wherein consecutively subsets of cantilevers are vibrated in order to emit an acoustic wave in the sample, and then for each successive emitted acoustic wave a return signal is retrieved (<NUM>) by each of the cantilevers,
determining (<NUM>) a transfer function of the whole array based on the successive measurements, and
performing (<NUM>) a measurement wherein acoustic signals (<NUM>) are concurrently emitted by the cantilevers, wherein the acoustic signals are coordinated based on said transfer function in order to direct a focused non-planar wave towards the one or more identified sub-surface features (<NUM>), and then the return signal retrieved by each of the cantilevers is measured for sub-surface characterization.