EP4078191A1

EP4078191A1 - A method and system for performing characterization measurements on an elongated substrate

Info

Publication number: EP4078191A1
Application number: EP20828160.0A
Authority: EP
Inventors: Paul ZABBAL
Original assignee: Nearfield Instruments BV
Current assignee: Nearfield Instruments BV
Priority date: 2019-12-16
Filing date: 2020-12-15
Publication date: 2022-10-26
Also published as: NL2024470B1; WO2021125944A1

Abstract

The invention relates to performing characterization measurements on an elongated substrate using a scanning probe microscopy system, the system comprising a plurality of cantilevers, each cantilever having a probe tip arranged thereon. The cantilever is moved towards a surface of the substrate for enabling contact between the probe tips of the plurality of cantilevers and the surface of the substrate. Further, at least one first cantilever of the plurality of cantilevers is vibrated for emitting ultrasonic waves in the substrate at one or more first locations on the surface of the substrate, wherein the ultrasonic waves are adapted so as to propagate one or more guided waves into the substrate. Then, the generated guided waves at a plurality of second locations on the surface of the substrate are detected by means of at least one second cantilever of the plurality of cantilevers, the plurality of second locations being distanced from the one or more first locations. Substrate characterization is performed based on the detected guided waves at the plurality of second locations.

Description

Title: A method and system for performing characterization measurements on an elongated substrate

FIELD OF THE INVENTION

The invention relates to a method for performing characterization measurements on an elongated substrate using a scanning probe microscopy system. The invention further relates to a scanning probe microscopy system for performing sub-surface measurements on a substrate. Further, the invention relates to a computer program product.

BACKGROUND TO THE INVENTION

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 substrate 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 substrate, including sub-surface dimensions or features of the substrate. It can be challenging to accurately detect sub-surface features in the semiconductor industry. For example, in a substrate 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.

SUMMARY OF THE INVENTION

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.

Additionally or alternatively, it is an object of the invention to provide for an improved ultrasound substrate characterization method, using a scanning probe microscope.

Thereto, the invention provides for a method for performing characterization measurements on an elongated substrate using a scanning probe microscopy system, the system comprising a plurality of cantilevers, each cantilever having a probe tip arranged thereon, the method comprising the steps of: moving the cantilevers towards a surface of the substrate for enabling contact between the probe tips of the plurality of cantilevers and the surface of the substrate; vibrating at least one first cantilever of the plurality of cantilevers for emitting ultrasonic waves in the substrate at one or more first locations on the surface of the substrate, wherein the ultrasonic waves are adapted so as to propagate one or more guided waves into the substrate; detecting the generated guided waves at a plurality of second locations on the surface of the substrate by means of at least one second cantilever of the plurality of cantilevers, the plurality of second locations being distanced from the one or more first locations; and performing substrate characterization based on the detected guided waves at the plurality of second locations.

Advantageously, the elongated substrate can be characterized with ultrasonic guided waves. The method can be used for effective characterization of subsurface features and/or other properties of the substrate. A quantitative ultrasound approach can be provided exploiting a multimode waveguide response of elongated substrates (e.g. semiconductor device) for assessing properties such as subsurface features, sublayer thickness, mechanical properties (e.g. stiffness), etc. Guided waves are capable of propagating long distances, thus making them an ideal candidate to interrogate large plate and shell structures, such as the elongated substrate.

Guided waves which are propagated within the elongated substrate are dispersive and the relationship between the frequency and the wave number, which is specific to each guided mode, is determined by the geometric and elastic properties of the waveguide. For example, changes in a sublayer thickness or elastic properties (e.g. due to a presence of a subsurface feature) can change the propagation characteristics of the guided waves. Consequently, guided waves measurements on long/elongated substrates have the potential for yielding properties of the elongated substrate which can be used as markers for identifying subsurface characteristics of the elongated substrate or judge whether the elongated substrate has the desired or expected properties.

The method can be used for evaluating and/or quantifying the mechanical integrity of the elongated substrate and/or other small-scale structures. For example, the method can be used for detection of defects in adhesion and deviation interfacial stiffness.

The guided waves measurements are performed with the plurality of cantilevers. In some examples, wideband ultrasonic pulses at a central frequency are transmitted, and the received signals are recorded. The experimental dispersion curves which represent the frequency-dependent wave numbers, i.e. k(f), of guided modes propagating in the waveguide, can be determined in various ways. Optionally, the detected generated guided waves are used in dispersion imaging in which dispersion curves are identified, wherein mechanical properties of the substrate are determined based on the identified dispersion curves.

In this guided wave propagation approach, the elongated substrate can be described by distributed mass and stiffness. For a finite frequency range, a finite number of modes can be analyzed using modal decomposition. The elongated substrate can be represented as guided waves, since the substrate is relatively thin in one direction. The substrate can have higher dimensions in the other two directions. The waves can travel long distances within the substrate before they decay. The wave modes can be represented through dispersion curves, which provide a relationship between the wave number and the frequency. The dispersion curves can be seen as separate lines that each represent an individual mode.

When the elongated substrate is excited with a displacement and/or a force at one end, a number of waves start to propagate to another side of the substrate. Each wave travels with a velocity described as the phase velocity and group velocity. All the waves can travel together under an envelope, having the group velocity (speed at which the energy is transported). Dispersion curves can effectively explain the dynamics of the elongated substrate (which can be seen as a coupled system). Dispersion curves also provide insight into what happens inside the system at different frequencies.

Optionally, the elongated substrate is modelled, guided waves propagating along it are computed, and such waves are represented by means of dispersion curves.

Optionally, the substrate characterization is performed based on one or more guided wave variations, wherein the detected generated guided waves at the plurality of second locations are used for determining the one or more guided wave variations.

Guided waves or lamb waves can be dispersive and change shape as they propagate, meaning that the higher harmonics may travel at different group velocities than the fundamental portion. Lamb waves are multimode by nature, which can make it potentially difficult to separate the individual mode contributions from a complicated time-domain wave form. The characteristics of the inspection waves after interacting with subsurface features can be determined. One or more of the plurality of cantilevers can generate ultrasonic-guided waves which propagate into the structure of the elongated substrate, interacting with subsurface features, and carrying the subsurface information with them. These ultrasonic-guided waves having interacted with the internal structure of the elongated substrate can then be picked up again by cantilevers, e.g. at a remote location on the surface of the sample with respect to the locations of the one or more cantilevers that emitted waves into the elongated substrate. The subsurface features within the elongated substrate (e.g. void, defect, feature, subsurface structure, etc.) can be considered as a new wave source at a location of the subsurface feature. Similarly, the method can also be employed for determining thickness variations (e.g. of a sublayer of the elongated substrate). An accurate and reliable procedure can be provided for measuring an harmonic of a guided wave (e.g. lamb wave) propagating in the elongated plate.

The elongated plate can be sufficiently long in order to ensure that the received signals are not influenced by boundary reflections.

Optionally, the plurality of cantilevers are arranged in a cantilever array, the array including a first subset and a second subset, the first subset including the at least one first cantilever, and the second subset including the at least one second cantilever, wherein the second subset includes multiple cantilevers regularly spaced with respect to each other enabling detection of the generated guided waves at the plurality of second locations on the surface of the substrate.

The guided waves measurements can be performed with a plurality of cantilevers arranged in one or more arrays. The one or more arrays each having multiple cantilevers can be brought in contact with the substrate for performing measurements.

Optionally, the first subset includes multiple cantilevers arranged for successively emitting ultrasonic waves in the substrate so as to generate successive guided waves propagating into the substrate, and wherein the cantilevers of the second subset are used for retrieving each successively generated guided wave in response to the emitted ultrasonic waves. The probe tips of the array of cantilevers can touch the sample surface and one or more of the probe tips of the first subset can coordinately emit a guided 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 a wave in the sample, and then all the cantilevers are used for measuring the return signal. Then a different subsequent cantilever emits a 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 dispersion curves.

Optionally, the array and the substrate are movable with respect to each other, wherein the array is scanned over an area of the surface of the substrate for performing multiple measurements.

Optionally, two or more cantilevers of the first subset are used for concurrently emitting ultrasonic waves in the substrate, wherein the concurrently emitted ultrasonic waves are coordinated in order to select a guided mode.

A guided mode selection can be performed based on the AFM/SPM emission reception. The mode can be selected using the cantilevers (e.g. multilayer case). The right mode that is sensitive to a layer for example for thickness extraction, or sensitive to a subsurface feature can be selected. A line of cantilevers with an arbitrary waveform generator can be employed, for performing a guided mode selection in order to excite the specific desired mode.

Optionally, the selected guided mode has a higher sensitivity to at least one of: a subsurface feature of the substrate, or one layer of the substrate having multiple layers. For example, if a specific mode is found sensitive to a layer that causes issue in the semiconductor device, this technique can be used. The guided wave selection can be done using the cantilevers as emitters. For each selected mode, different signals can be emitted.

Optionally, a plurality of guided modes are selected, wherein substrate characterization is performed by emitting the plurality of selected guided modes into the substrate.

Optionally, the at least one second cantilever is scanned over the surface of the substrate for detecting at the plurality of locations the emitted guided waves. Optionally, a multi-layer substrate, preferably a multi-layer semiconductor substrate, is characterized.

Optionally, the substrate characterization includes at least one of: characterization of subsurface features in the substrate, characterization mechanical properties of the substrate, characterization of one or more layers or thickness variations of a multi-layer substrate, characterization of heterogeneities or voids in the substrate, or non-destructive testing of the substrate.

Optionally, more than three cantilevers are provided, more preferably more than five cantilevers, even more preferably more than ten cantilevers.

Optionally, an inverse method using an elongated plate model is applied for retrieving properties (e.g. thickness, stiffness) of the elongated plate from experimental data obtained by means of the cantilevers.

Optionally, a singular value decomposition is applied to a response matrix at each frequency. Signal-to-noise ratio (SNR) enhancement can be achieved by removing the singular vectors corresponding to the lowest singular values.

According to an aspect, the invention provides for a scanning probe microscopy system for performing sub-surface measurements on a substrate, the system including a sensor head including a plurality of cantilevers, each cantilever having a probe tip arranged thereon, 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 cantilevers towards a surface of the substrate for enabling contact between the probe tips of the plurality of cantilevers and the surface of the substrate; vibrating at least one first cantilever of the plurality of cantilevers for emitting ultrasonic waves in the substrate at one or more first locations on the surface of the substrate, wherein the ultrasonic waves are adapted so as to propagate one or more guided waves into the substrate; detecting the generated guided waves at a plurality of second locations on the surface of the substrate by means of at least one second cantilever of the plurality of cantilevers, the plurality of second locations being distanced from the one or more first locations; and performing substrate characterization based on the detected guided waves at the plurality of second locations.

Different dispersion curves imaging techniques can be used for the detection of subsurface features in a semiconductor device. The system can be operated in different modes in order to obtain high quality imaging. In some examples, a single emitter is used (e.g. 2D-FT, Linear Radon Transform etc.). In some examples, more advanced techniques can be used, such as for instance SVD- based method.

Dispersion curves are highly sensitive to the mechanical properties of the sample and therefore can be used for acoustic inspection: detection of subsurface features but also the determination of thickness variation in a multilayer, or even for non-destructive testing with the possibility to detect heterogeneities or voids.

In some examples, it is possible to perform guided mode selection by combining multiple cantilevers to an arbitrary waveform generator. With this setup, it is possible to emit a unique mode with higher energy, which facilitates the post-processing and a scan can be performed on the device to check all variations (amplitude, phase) of this mode. With guided wave it is possible to scan a large surface as there is almost no attenuation and they are sensitive to all the mechanical properties of the structure.

The mechanical properties of a device can be determined based on dispersion curves. If the mechanical properties are determined, it is possible to accurately characterize the subsurface feature.

For example, a subsurface feature in the elongated substrate can act as a new wave source (cf. transmission + mode conversion and reflection + mode conversion).

The system can be configured to launch a lamb mode into the substrate. A lamb wave propagating into the elongated substrate can then be measured. The generated lamb wave(s) can be detected by one or more cantilevers of the array, which can effectively measure the out-of-plane displacement and/or velocity of a point location on the surface of the substrate. By means of the array, the measurements can be performed over a range of propagation distances.

It will be appreciated that the elongated substrate can be a plate-like sample. The substrate can be sufficiently long in order to ensure that the received signals are not influenced by boundary reflections. The wave propagating into the elongated substrate can interact with one or more subsurface features. After the interaction with the subsurface substrate feature(s), information can be carried with them (subsurface feature can act as a new wave source), and this additional information, relating to the subsurface feature(s) can be picked up by one or more cantilevers of the array (cf. different from the emitter cantilever(s)).

According to an aspect, 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.

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 and device. It will also be clear that any one or more of the above aspects, features and options can be combined.

BRIEF DESCRIPTION OF THE DRAWING

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.

In the drawing:

Fig. 1 shows a schematic diagram of an embodiment of a system;

Fig. 2 shows a schematic diagram of an embodiment of a system;

Fig. 3 shows a schematic diagram of an embodiment of a system;

Fig. 4 shows a schematic diagram of dispersion curves;

Fig. 5 shows a schematic diagram of dispersion curves;

Fig. 6 shows a schematic diagram of plots;

Fig. 7 shows a schematic diagram of a system; and

Fig. 8 shows a schematic diagram of a method.

DETAILED DESCRIPTION

Fig. 1 shows a schematic diagram of an embodiment of a system 1. The system 1 is a scanning probe microscopy system for performing sub -surface measurements on an elongated substrate 3. The system 1 includes a sensor head 5 including a plurality of cantilevers 9, each cantilever 9 having a probe tip 11 arranged thereon. The system 1 further comprises a processor configured to perform the steps of: moving the cantilevers 9 towards a surface 3a of the substrate 3 for enabling contact between the probe tips 11 of the plurality of cantilevers 9 and the surface 3a of the substrate 3; vibrating at least one first cantilever of the plurality of cantilevers 9 for emitting ultrasonic waves in the substrate at one or more first locations on the surface 3a of the substrate 3, wherein the ultrasonic waves are adapted so as to propagate one or more guided waves into the substrate 3; detecting the generated guided waves at a plurality of second locations on the surface of the substrate by means of at least one second cantilever of the plurality of cantilevers 9, the plurality of second locations being distanced from the one or more first locations; and performing substrate characterization based on the detected guided waves at the plurality of second locations.

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 3 can be used. It is also possible to use a sample 3 with a larger number of layers.

The sensor head 5 and the sample 3 can be moveable with respect to each other as indicated by arrows 19. The array of probes 7 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 9 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 32 up to 256 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. In some examples, a plurality of cantilevers are employed for generating guided waves and determining dispersion curves, wherein imaging of the sample is performed based on the dispersion curves. An ultrasound wave can be emitted in a substrate by means a first cantilever, the substrate can be scanned by a second cantilever for detecting one or more guided waves variations, and a subsurface location can be determined based on the detected one or more guided waves variation. Different types of dispersion imaging methods can be employed, such as for example 2D Fourier transfer, linear radon transform, etc. By means of selection of a guide mode the characterization of the sample can be further enhanced. In some examples, multiple cantilevers are arranged in an array and brought in contact with a surface of the elongated substrate, providing different contact points. The array of cantilevers may have a subset of emitters (e.g. one or more cantilevers) and a subset of receivers (e.g. one or more cantilevers) in the array. Different configurations are possible, for example 5 emitters, 20 receivers; 1 emitter, 100 receivers; etc. In some examples, the subset of receivers includes multiple cantilevers, providing a plurality of measuring locations on the surface of the substrate by the cantilevers being regularly spaced from each other. An emitter cantilever can send one or more pulses at a frequency in GHz.

Based on dispersion curves measurements, it is possible to retrieve the mechanical properties of the sample and therefore detect possible material properties variations (thickness, Young’s modulus, etc.), subsurface features, and also possible defects such as voids.

Fig. 2 shows a schematic diagram of an embodiment of a system 1. The system includes an array of probes 7 for performing (subsurface) characterization of the substrate 3. For this purpose the array of probes 7 is brought in contact to the surface 3a of the substrate 3. In this example, the substrate is a semiconductor device includes a subsurface feature 15 (Si02) and two layers (a Si bottom layer attached to an upper layer). The upper layer may for instance have a thickness around 200 nm, and the lower layer may for instance have a thickness around 150 nm. The subsurface feature may for instance have a thickness of 100 nm. These are merely exemplary dimensions and other dimensions of the substrate are also possible. Furthermore, other substrates can also be characterized using the method/system according to the invention. At least one first cantilever of the plurality of cantilevers can be vibrated for emitting ultrasonic waves in the substrate at one or more first locations on the surface of the substrate. These ultrasonic waves are adapted so as to propagate one or more guided waves into the substrate. An exemplary excitation signal in time domain and frequency domain is shown. Subsequently, the generated guided waves at a plurality of second locations on the surface of the substrate can be detected by means of at least one second cantilever of the plurality of cantilevers, the plurality of second locations being distanced from the one or more first locations. This process may be optionally repeated a coupled of times.

The substrate characterization can be carried out based on the detected guided waves at the plurality of second locations.

Fig. 3 shows a schematic diagram of an embodiment of a system 1 with a plurality of probes arranged in an array of probes 7. The probes 7i are schematically represented as boxes next to each other. This figure illustrates a measurement technique employed using the array of probes 7, namely different successive steps (a)-(i) are illustrated. In a first step (a), a first probe 7-1 is actuated. The cantilever of the first probe 7-1 is vibrated in order to emit an ultrasonic wave in the sample. This emitting cantilever is also indicated by “E”.

The ultrasonic wave is adapted so as to propagate one or more guided waves into the substrate. In a subsequent second step (b), a generated guided wave is determined by each of the cantilevers 7-R (also indicated with letter “R”. In a subsequent third step (c), a second probe 7-2 is vibrated in order to emit an ultrasonic wave in the sample. In a subsequent fourth step (d), the induced guided wave is again determined by each of the cantilevers 7-R of the receiving subset of cantilever. In a subsequent fifth step (e), a next probe, third probe 7-3, is vibrated in order to emit an ultrasonic wave in the sample, again propagating one or more guided waves in the elongated substrate. In a subsequent sixth step (f), the generated guided wave(s) is again retrieved by each of the cantilevers 7-R. This process can be repeated until all probes 7-E of a first subset of emitting probes have been actuated individually, and a resulting return signal is measured by all the probes of a receiving subset 7-R of probes. 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 a subset of cantilevers are concurrently vibrated in order to emit an acoustic wave in the sample. Then for each successive guided wave generated within the sample, by means of the subset of probes, a return signal can be retrieved by each of the cantilevers of the second subset of probes 7-R.

In this example, the array of probes 7 includes a total number of twenty probes 7-1 to 7-20 arranged next to each other. However, a different number of probes may also be arranged in accordance with the invention.

Fig. 4 shows a schematic diagram of dispersion curves 20. Dispersion curves are a representation in frequency and wavenumber, or frequency and phase velocity of the guided waves in a structure and are depending on the parameters of the structure (cf. elastic Modulus, thickness, density, Poisson’s ratio). Lamb modes can be subdivided in 2 categories, namely a symmetric and an asymmetric mode. The cantilevers can be used for guided waves dispersion curves imaging and for selection of a guide mode. The elongated substrate may for instance be a semiconductor device. At least one cantilever can emit an ultrasound wave (preferably large band in order to obtain the higher number of modes) in a semiconductor device. At least one second cantilever can detects the guided waves variation to determine the subsurface feature location for example. Example of possible dispersion curves imaging method based on this configuration are 2D- Fourier Transform or Linear Radon Transform.

The plurality of probes can be used as multiple emitters. One cantilever can be used at different locations, increasing the number of sources, or multiple cantilevers are used in a configuration as presented here. Increasing the number of sources can be useful and permit the use of better imaging methods such as: SVD- based methods, or Multichannel-MUSIC. A scan of the surface under the multicantilever element can be scanned.

Dispersion curves can be used for detecting subsurface features. The mechanical properties of the sample can be measured based on dispersion curves measurements. Therefore, possible material properties variations (thickness, Young’s modulus etc.), subsurface features, and also possible defects such as voids in the elongated substrate can be detected.

In plot (a), dispersion curves are depicted for a two cases. In a first case ON’, the elongated substrate has a subsurface feature, and in the second case OFF’, the elongated substrate is free of the subsurface feature. As can be seen, the presence of the subsurface feature has significant impact on the dispersion curves. In plot (b) and (c), analytical theoretical dispersion curves are compared to results coming from finite element method (FEM) simulations, for the ON’ case (subsurface feature present) and the OFF’ case (subsurface feature not present), respectively. In this example, the results are obtained with one cantilever acting as an emitter, and the other cantilevers in the array acting as receivers. For this example one cantilever was used as emitter, and 100 other cantilevers in the array were used as receivers. It is also possible to use a different number of cantilevers.

It is also envisaged that a plurality of cantilevers are used as emitters, for instance operating concurrently.

A plurality of modes are depicted in the dispersion curves of plots (a),

(b), and (c). A specific lamb mode can be launched into the elongated plate, and the generated lamb waves can be detected with contact SPM probes/cantilevers of the array. In this way, the out-of-plane particle velocity of a point on the surface of the substrate can for instance be measured. The measurements can be performed over a range of propagation distances by means of the multiple arrays of the array of probes.

Fig. 5 shows a schematic diagram of dispersion curves for the ON’ case (subsurface feature present) and the OFF’ case (subsurface feature not present). In a case that the elongated substrate has a plurality of sublayers (multilayer substrate), it can be difficult to extract the right mode which is sensitive to a layer for example for thickness extraction, or sensitive to a subsurface feature. A guided mode selection can be achieved by using the plurality of arrays in order to excite the specific desired node. The array may for instance have one or more lines of cantilevers providing an arbitrary waveform generator. The guided mode selection can be based on AFM emission/reception. If a specific mode is found sensitive to a layer that causes issues (e.g. to be detected), this technique can be employed for obtaining improved results.

Fig. 6 shows a schematic diagram of plots relating to performing a mode selection. A signal (e.g. Gaussian signal) can be emitted from the cantilevers of the array. The response can then be measured in order to obtain the measured response. This measured response can be fitted to a simulated response (e.g. least squares algorithm). One or more relevant modes can be identified.

Plots (a), (b) and (c) relate to selecting mode in an OFF’ case (subsurface feature(s) not present). Plots (d), (e) and (f) relate to selecting mode in an ON’ case (subsurface feature(s) present). In the example shown in plots (a), (b), and (c), a fourth mode is select and excited. A gain of almost 15dB is obtained after the mode selection in comparison to the other modes. For the mode selection, the plurality of cantilevers can be concurrently employed for shaping a wavefront, by applying different signals to different cantilevers. The wavefront can be shaped based on the selected mode.

Fig. 7 shows a schematic diagram of a system 1. The system 1 includes a probe head 5 with an array 7 of probes. Each probe has a cantilever 9 with a probe tip 11. In this example, two or more cantilevers of the first subset are used for concurrently emitting ultrasonic waves in the substrate, wherein the concurrently emitted ultrasonic waves are coordinated in order to select a guided mode. The selected guided mode can have a higher sensitivity to at least one of: a subsurface feature of the substrate, or one layer of the substrate having multiple layers. For each selected mode, different signals can be emitted. The signal to emit by the cantilevers can be determined by the following equation: in which s_n(t) corresponds to a signal to emit by the cantilever n, G_n(t) corresponds to a signal corresponding to the n^th cantilever, / corresponds to the frequency, V_φ(f) corresponds to a phase velocity of selected mode, d_n corresponds to a distance of n^th cantilever from the first one, and FT corresponds to a Fourier Transform.

Fig. 8 shows a schematic diagram of a method 100 for performing characterization measurements on an elongated substrate using a scanning probe microscopy system, the system comprising a plurality of cantilevers, each cantilever having a probe tip arranged thereon. In a first step 101, the cantilevers are moved towards a surface of the substrate for enabling contact between the probe tips of the plurality of cantilevers and the surface of the substrate. In a second step 102, at least one first cantilever of the plurality of cantilevers are vibrated for emitting ultrasonic waves in the substrate at one or more first locations on the surface of the substrate, wherein the ultrasonic waves are adapted so as to propagate one or more guided waves into the substrate. In a third step 103, the generated guided waves are detected at a plurality of second locations on the surface of the substrate by means of at least one second cantilever of the plurality of cantilevers, the plurality of second locations being distanced from the one or more first locations. In a fourth step 104, substrate characterization is performed based on the detected guided waves at the plurality of second locations.

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.

Various embodiments may be implemented using hardware elements, software elements, or a combination of both. 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. It will, however, be evident that various modifications, variations, alternatives and changes may be made therein, without departing from the essence of the invention. For the purpose of clarity and a concise description features are described herein as part of the same or separate embodiments, however, alternative embodiments having combinations of all or some of the features described in these separate embodiments are also envisaged and understood to fall within the framework of the invention as outlined by the claims. 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 spirit and 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.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word ‘comprising’ does not exclude the presence of other features or steps than those listed in a claim. Furthermore, the words ‘a’ and ‘an’ shall not be construed as limited to ‘only one’, but instead are used to mean ‘at least one’, and do not exclude a plurality. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to an advantage.

Claims

1. A method for performing characterization measurements on an elongated substrate using a scanning probe microscopy system, the system comprising a plurality of cantilevers, each cantilever having a probe tip arranged thereon, the method comprising the steps of: moving the cantilevers towards a surface of the substrate for enabling contact between the probe tips of the plurality of cantilevers and the surface of the substrate, vibrating at least one first cantilever of the plurality of cantilevers for emitting ultrasonic waves in the substrate at one or more first locations on the surface of the substrate, wherein the ultrasonic waves are adapted so as to propagate one or more guided waves into the substrate, detecting the generated guided waves at a plurality of second locations on the surface of the substrate by means of at least one second cantilever of the plurality of cantilevers, the plurality of second locations being distanced from the one or more first locations, and performing substrate characterization based on the detected guided waves at the plurality of second locations.

2. The method according to claim 1, wherein the detected generated guided waves are used in dispersion imaging in which dispersion curves are identified, wherein mechanical properties of the substrate are determined based on the identified dispersion curves.

3. The method according to claim 1 or 2, wherein the substrate characterization is performed based on one or more guided wave variations, wherein the detected generated guided waves at the plurality of second locations are used for determining the one or more guided wave variations.

4. The method according to any one of the preceding claims, wherein the plurality of cantilevers are arranged in a cantilever array, the array including a first subset and a second subset, the first subset including the at least one first cantilever, and the second subset including the at least one second cantilever, wherein the second subset includes multiple cantilevers regularly spaced with respect to each other enabling detection of the generated guided waves at the plurality of second locations on the surface of the substrate.

5. The method according to claim 4, wherein the first subset includes multiple cantilevers arranged for successively emitting ultrasonic waves in the substrate so as to generate successive guided waves propagating into the substrate, and wherein the cantilevers of the second subset are used for retrieving each successively generated guided wave in response to the emitted ultrasonic waves.

6. The method according to claim 4 or 5, wherein the array and the substrate are movable with respect to each other, wherein the array is scanned over an area of the surface of the substrate for performing multiple measurements.

7. The method according to any one of the claims 4-6, wherein two or more cantilevers of the first subset are used for concurrently emitting ultrasonic waves in the substrate, wherein the concurrently emitted ultrasonic waves are coordinated in order to select a guided mode.

8. The method according to claim 7, wherein the selected guided mode has a higher sensitivity to at least one of: a subsurface feature of the substrate, or one layer of the substrate having multiple layers.

9. The method according to any one of the preceding claims, wherein a plurality of guided modes are selected, wherein substrate characterization is performed by emitting the plurality of selected guided modes into the substrate.

10. The method according to any one of the preceding claims, wherein the at least one second cantilever is scanned over the surface of the substrate for detecting at the plurality of locations the emitted guided waves.

11. The method according to any one of the preceding claims, wherein a multi-layer substrate, preferably a multi-layer semiconductor substrate, is characterized.

12. Method according to any one of the preceding claims, wherein the substrate characterization includes at least one of: characterization of subsurface features in the substrate, characterization mechanical properties of the substrate, characterization of one or more layers or thickness variations of a multi-layer substrate, characterization of heterogeneities or voids in the substrate, or nondestructive testing of the substrate.

13. Method according to any one of the preceding claims, wherein more than three cantilevers are provided, more preferably more than five cantilevers, even more preferably more than ten cantilevers.

14. A scanning probe microscopy system for performing sub -surface measurements on a substrate, the system including a sensor head including a plurality of cantilevers, each cantilever having a probe tip arranged thereon, 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 cantilevers towards a surface of the substrate for enabling contact between the probe tips of the plurality of cantilevers and the surface of the substrate, vibrating at least one first cantilever of the plurality of cantilevers for emitting ultrasonic waves in the substrate at one or more first locations on the surface of the substrate, wherein the ultrasonic waves are adapted so as to propagate one or more guided waves into the substrate, detecting the generated guided waves at a plurality of second locations on the surface of the substrate by means of at least one second cantilever of the plurality of cantilevers, the plurality of second locations being distanced from the one or more first locations, and performing substrate characterization based on the detected guided waves at the plurality of second locations.

15. 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 any one of the preceding claims 1-13.