Patent Description:
The invention is further directed at an atomic force microscopy system configured for performing subsurface imaging of one or more embedded structures in a substrate underneath a substrate surface, wherein the atomic force microscopy system comprises a probe with at least one probe tip, and a sensor for sensing a position of the probe tip for detecting probe tip motion.

Developments in the semiconductor industry are typically governed by Moore's law which predicts that the number of transistors in a dense integrated circuit doubles every two years. As will be appreciated, this poses significant technological challenges whenever technological boundaries constituted by physical laws are encountered and need to be overcome to meet the industry demands for even smaller and smaller integrated circuits.

A new type of structures that is presently on the rise are the three dimensional NAND or 3D NAND type memory structures. The term NAND, in this context, is not an abbreviation or acronym, but refers to the logical operation NAND or in other words NOT(AND(. 3D NAND devices consist of hundreds of stacked device layers having a total thickness of a few micrometers. In order to enable monitoring and inspection of alignment, overlay and/or product metrology during and after manufacturing of such devices, a subsurface imaging technology is to be applied that allows to visualize even nanometer structures buried deeply (several micrometers) below the surface of the device.

Acoustic type atomic force microscopy (AFM) has been proposed as a suitable technology to perform subsurface imaging on semiconductor structures. These methods typically apply an ultrasound signal to a sample or sometimes to the probe, while scanning the sample in contact mode (ultrasonic atomic force microscopy (UAFM)). As a result of the applied ultrasound signal, the interaction between the probe and the surface contains a component that is dependent on the elastic properties of the sample. Due to the fact that buried structures underneath a sample alter the local contact elasticity between the probe tip and the surface, the application of the ultrasound signal and proper analysis of the probes deflection (i.e. via the output signal) allows to visualize subsurface structures. In particular, the detection of subsurface structures by means of ultrasonic AFM is based on resonance frequency shifts of a cantilever that are due to contact stiffness changes. Such resonance shift is detected by measuring the amplitude or phase change at a single frequency.

A disadvantage of the abovementioned ultrasonic atomic force microscopy methods for subsurface imaging, is that although they work well for performing all kinds of imaging, monitoring and inspection at shallow depths, they are unable to do so at greater depths. In particular during manufacturing of semiconductor elements, there is a desire to monitor or check for overlay error between layers at various depths of the device manufactured.

<CIT> discloses an AFM system for imaging subsurface structures wherein a first modulated acoustic signal is applied to the probe and a second acoustic signal is applied to the sample.

<CIT> discloses an AFM system comprising a plurality of probes.

<CIT> discloses an AFM system for imaging subsurface structures wherein a first acoustic signal is applied to the probe and a second acoustic signal is applied to the sample.

It is an object of the present invention to provide a method of and system for performing subsurface imaging, in particular to perform imaging of layers at various depths.

To this end, there is provided herewith a method of performing subsurface imaging of one or more embedded structures in a substrate underneath a substrate surface according to claim <NUM>, the method being performed using an atomic force microscopy (AFM) system, wherein the atomic force microscopy system comprises a probe with at least one probe tip, and a sensor for sensing a position of the probe tip for detecting probe tip motion, the method comprising the steps of: positioning the probe tip relative to the substrate for establishing contact between the probe tip and the substrate surface; applying, using at least one first signal application actuator, a first acoustic input signal to the substrate; applying, using at least one second signal application actuator, a second acoustic input signal to the substrate; detecting, using the sensor, an output signal from the substrate in response to the first and second acoustic input signal; and analyzing the output signal for obtaining information on the embedded structures, for enabling imaging thereof;.

The present invention enables to perform imaging a various depths in one pass, across a large range of depths. This may, for example, be used for monitor of overlay error between layers at shallow and at larger depths at the same time. For examples, the exact and correct positioning of features at shallow depths (e.g. up to a few hundreds (for example <NUM>) of nanometers below the surface) may be checked against the presence of other features at deeper layers (e.g. up to a few micrometers below the surface or deeper (e.g.< <NUM> micrometers).

The first acoustic input signal, comprising a first signal component having a frequency below <NUM> megahertz and a second signal component having a frequency below <NUM> megahertz, enables to perform subsurface imaging using stiffness elasticity as contrast medium. This typically enables subsurface imaging with large signal-to-noise ratio at shallow depths, e.g. up to <NUM> micrometers below the surface, enabling accurate imaging of the top few layer of a sample (e.g. a device being manufactured or after manufacturing) below the surface.

The second acoustic input signal, comprising a third signal component having a frequency above <NUM> gigahertz, enables to perform subsurface imaging using ultrasound scattering as contrast medium. At these frequencies, the system is diffraction limited and thus the wavelength of the signal determines the dimensions that are still visible in the output signal. Most important though, due to the fact that the contrast mechanism is based on scattering of the waves (rather than an elastic stress field), measuring at these frequencies may be performed at much larger penetration depths as compared to the measurements at lower frequencies. Thus, the second acoustic input signal applied to the substrate or sample enables the imaging of deeply buried layers.

Together, the first acoustic input signal enables visualization through elastic deformation at shallow depths below the surface (i.e. the elastic stress field), whereas the second acoustic input signal is sent into the sample providing a return signal including a scattered fraction (an echo) or - in transmission mode - the second acoustic input signal whose wave-front is distorted due to the said scattered fraction. The combination thereby enables visualization in one pass at different depths across a wide range of depths. This enables to integrate this imaging technique in semiconductor manufacturing methods and to thereby enable device inspection across layers that may be remotely apart with one pass of the AFM. For example, the identification of overlay errors in complex device designs is possible in this manner. This improves the quality of the semiconductor devices manufactured, and enables complicated device designs with accurately positioned elements that may be collocated or overlapping at various depths to be manufactured within specs.

In accordance with some embodiments, the at least one second signal application actuator is attached to the at least one probe tip, wherein at least the second acoustic input signal is applied via the at least one probe tip. Application via the probe tip provides the second acoustic input signal as a point source emerging from the location of the probe tip into the sample. The second signal application actuator can then simultaneously be used as sensor for receiving the scattered return signal.

The first acoustic input signal may in some embodiments be applied by means of two or more piezo type actuators, one of which is located below the sample to apply the first signal component of high frequency (smaller than or equal to <NUM> megahertz). The other piezo, which may be located either on or near the probe or attached to the sample, applies the second signal component of low frequency (smaller than <NUM> megahertz). Alternatively, a low frequency component may be provided using a heterodyne type method, by mixing two slightly different frequencies (e.g. both up to <NUM> megahertz) having a difference frequency that provides the low frequency component. Typically, this low frequency component, in accordance with this and other embodiments, is selected to be close to (e.g. within <NUM>%) of a resonance mode frequency of the cantilever of the probe or of the probe tip.

To overcome or eliminate any inaccuracy caused by surface roughness or unevenness of the surface, in accordance with the method of the invention, the at least one probe comprises a plurality of probe tips forming a probe tip array, wherein during the step of positioning the probe is positioned such as to establish contact between the substrate surface and each of the probe tips. In this event, the multiple probe tips take measurements at multiple locations, enabling elimination or averaging out of differences.

The above described embodiments may be combined in the following preferred embodiments, wherein a plurality of second signal application actuators is attached to the plurality of probe tips, such that each probe tip having associated therewith at least one of the second signal application actuators, for applying a plurality of second acoustic input signals through the plurality of probe tips, wherein the method comprises: controlling, using a controller, operation of the second signal application actuators such as to control a phase difference between each two signals of the plurality of second acoustic input signals, such as to provide a combined wave front of the plurality of second acoustic input signals having a controllable shape. These embodiments provide the advantageous possibility of creating a wave front by proper control over the operation of the second signal application actuators located above each probe tip of the probe tip array. In particular, providing each of the second acoustic input signal with a well selected and controlled phase shift, enables to shape the wave front suitably to provide a number of different advantageous effects. The phase shifts may be achieved using controllable delays for each of the second signal application actuators, enabling any shaped wave front to be designed. These embodiments allow a phased array imaging concept to be combined with the abovementioned advantages of multiple probe tips.

Using this phased array imaging concept, various further advantages may be obtained in some embodiments of the method of the invention. In these further embodiments, the controlling of the second signal application actuators is performed such as to generate at least one of: a focused wave front focused at a focal point relative to the probe tips; a defocused wave front; a plane wave front, wherein the plane wave front is at least one of parallel to the substrate surface or under an angle with the substrate surface; and a curved wave front.

The advantages of phased array imaging may be achieved in a different manner wherein phase differences are provided differently. In these embodiments, the probe comprises a cantilever having a front end where the at least one probe tip is located and a back end forming a remote opposite end of the cantilever relative to the front end, wherein the at least one second signal application actuator is attached to the cantilever at the back end, the method comprising applying the second acoustic input signal as a guided wave through the cantilever, and wherein a phase of the second acoustic input signal applied via each probe tip is dependent on a relative position of the respective probe tip, wherein the positions of the probe tips is such as to provide a combined wave front having a shape determined by the phases of the second acoustic input signal applied via each probe tip. The guided wave type input signal propagates through the cantilever, having a propagation velocity dependent on the dimensions of the cross section of the cantilever (transverse to the longitudinal direction), primarily on the thickness. The propagation velocity is further dependent on the frequency of the applied signal. The dimensions of the cantilever being fixed for that cantilever, the frequency spectrum of the applied signal may be designed properly to obtain a desired velocity profile. Even without this, the guided waves travelling through the cantilever will reach the most nearby probe tip of the plurality of probe tips first, and subsequently the other probe tips are reached dependent on their distance to the back end of the cantilever. This, amongst the different probe tips, also provides a phase difference that may be used to shape the wave front of the combined second acoustic input signal fractions applied via the probe tips. In particular, in some embodiments, the positions of the plurality of probe tips is such as to provide at least one of a non-plane wave front, or a plane wave front which is at least one of parallel to the substrate surface or under an angle with the substrate surface.

In yet other embodiments of the present invention, the probe tip comprises a contact surface for being in contact with the substrate surface, wherein, for providing the second acoustic input signal as a point source signal, the contact surface has contact surface area smaller than <NUM>*<NUM><NUM> square nanometer, such as a contact surface radius smaller than <NUM> nanometer; or wherein, for providing the second acoustic input signal as a sound beam, the contact surface has contact surface area larger than <NUM>*<NUM><NUM> square nanometer such as a contact surface radius smaller than <NUM> nanometer, preferable larger than <NUM>*<NUM><NUM> square nanometer such as a contact surface radius larger than <NUM> nanometer, more preferable larger than <NUM>*<NUM><NUM> square nanometer such as a contact surface radius smaller than <NUM> nanometer. The large contact area in the latter case of the contact surface of the probe tip changes the applied second acoustic input signal from being a point source to becoming an excitation field associated with such larger apertures. This causes the second acoustic input signal to be applied as a sound beam. The advantage of this is that, dependent on the size and shape of the contact area and the frequency of the signal in combination with the speed of sound in the respective material of the sample, the sound beam may be focused within the sample. In particular, the sound intensity within the beam will depend on the location within the beam, the intensity having a maximum within a small area in the middle of the beam around a certain depth. Any distortion caused by local material differences of elements embedded in the sample that coincide with the focal area of the beam, will provide a strong return signal providing access to information regarding the properties of such embedded elements. Because the focal area is dependent not only on the size of the contact area, but also on the shape thereof, in accordance with some embodiments, the probe tip comprises a contact surface for being in contact with the substrate surface, wherein the contact surface has shape selected from a group comprising: square, rectangle, circular, oval, square or rectangular with rounded corners, triangular, or polygonal.

In a second aspect of the invention according to claim <NUM>, there is provided an atomic force microscopy system configured for performing subsurface imaging of one or more embedded structures in a substrate underneath a substrate surface, wherein the atomic force microscopy system comprises a probe with at least one probe tip, and a sensor for sensing a position of the probe tip for detecting probe tip motion, the system further comprising: an actuator stage for positioning the probe tip relative to the substrate for establishing contact between the probe tip and the substrate surface; at least one first signal application actuator for applying a first acoustic input signal to the substrate; at least one second signal application actuator for applying a second acoustic input signal to the substrate; wherein the at least one probe comprises a plurality of probe tips forming a probe tip array, wherein during the step of positioning the probe is positioned such as to establish contact between the substrate surface and each of the probe tips; wherein the sensor is configured for detecting an output signal from the substrate in response to the first and second acoustic input signal; wherein the system further comprises an analyzer configured for analyzing the output signal for obtaining information on the embedded structures for enabling imaging thereof; and wherein the first signal application actuator is configured for applying the first acoustic input signal comprising 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 signal application actuator is configured for applying the second acoustic input signal comprising a third signal component having a frequency above <NUM> gigahertz, such as to provide, in the output signal, one of: a return signal including a scattered fraction of the second acoustic input signal scattered from the embedded structures; or a transmitted second acoustic input signal whose wave-front is distorted due to scattering from the embedded structures.

The actuator stage may for example be a scan head of the atomic force microscope, enabling scanning of the probe relative to the surface of the sample to perform imaging of an area in accordance with the method of the first aspect of the invention.

<FIG> schematically illustrates a propagation velocity dependency graph for guided waves.

<FIG> schematically illustrates a measurement system <NUM>. In the system <NUM>, a probe <NUM> is attached to a actuator stage or scan head <NUM>. The scan head <NUM> enables scanning of the probe <NUM> relative to the surface <NUM> of a sample <NUM>. The probe <NUM> consists of a cantilever <NUM> and a probe tip <NUM>. During scanning, the probe tip <NUM> is brought in contact with the surface <NUM> of the sample <NUM>. For example the probe tip <NUM> may be scanned across the surface <NUM> of the sample <NUM> in contact mode (continuous contact between the probe tip <NUM> and the surface <NUM> of the sample <NUM>). A laser unit <NUM> provides a collimated laser beam <NUM> that impinges on a (specular reflective) section at the back of the probe <NUM> and reflects towards an optical detector <NUM> (e.g. photo diode). Using the optical detector <NUM>, vibrations in the cantilever <NUM> can be sensed due to small deflections of the reflected beam <NUM> under influence of such vibrations. This provides an output signal <NUM> for further analysis. As may be appreciated, although in <FIG> a single scan head <NUM> is depicted, the method may equally be applied in systems including multiple scan heads.

The present system enables to apply a very high frequency acoustic input signal <NUM> (e.g. see <FIG>) to the sample <NUM>. This very high frequency acoustic input signal, having a frequency in a range above <NUM> gigahertz, is the second acoustic input signal referred to herein before (the first acoustic input signal will be described further down below). The acoustic input signal <NUM> in <FIG> is applied using a piezo type actuator <NUM> mounted on the non-contact side (i.e. back side) of the probe tip <NUM>. The acoustic input signal <NUM> propagates via the probe tip <NUM> into the sample <NUM>. Within sample <NUM>, provided that a probe tip is used with a sharp tip (providing a small enough contact area with the sample surface <NUM>), the acoustic input signal <NUM> propagates as a point source signal <NUM> as illustrated in the figure. Alternative manners of stimulating the probe tip <NUM> such as to apply the very high frequency acoustic input signal <NUM> via the tip <NUM> to the sample <NUM>, may for example include stimulation by a pulsed or intensity varied laser (not shown) making use of the thermal effects of expansion and contraction to generate the acoustic signal. This may be achieved using laser <NUM> and laser beam <NUM>, or a separate laser beam (not shown), impinging on a material layer on the probe <NUM> for example.

The acoustic input signal <NUM> will propagate through the material of sample <NUM>, and any structures <NUM> or density variations encountered will cause a fraction <NUM> of the input signal <NUM> to be scattered back to surface <NUM>.

The acoustic input signal <NUM> may be of short duration, e.g. a pulse signal such as a delta pulse, or the acoustic input signal <NUM> may be of long duration or even continuous. If a short duration signal <NUM> is applied, the pulse signal or pulse train applied must be short enough to ensure that forward and backward propagation of the discontinuous signal in the sample do not mix. The spatial pulse length may thus be dependent on the material (as the sound velocity for different materials is different) and may be shorter for softer materials while being longer for harder materials. In some embodiments, the discontinuous signal has a signal time duration shorter than the intended detection depth divided by the speed of sound in the primary material of the sample. Applying a short duration acoustic input signal <NUM> into the sample will cause a scattered return signal <NUM>, scattered from any present embedded elements <NUM> in the sample <NUM>, to be received.

If on the other hand a long signal or continuous signal is applied, any elements <NUM> will likewise scatter a fraction <NUM> back. However, the scattered return signal <NUM> will then be received while the input signal <NUM> is still being applied. Additional analysis steps in the analysis of an output signal may thus be required to separate the applied input signal <NUM> from the output signal to find the return signal <NUM>. For example, using phase dependent filtering, signal fractions of equal phase as the input signal <NUM> may be ignored in the output signal such as to yield only those fractions that originate from a return signal <NUM> scattered by an embedded element <NUM> in the sample. Other techniques may likewise be applied to perform such filtering. The application of a continuous acoustic input signal <NUM> on the other hand reduces complexity at the input side, and speeds up the measurement greatly by not having to perform an alternating send-receive sequence of applying the signal <NUM> and listening to the return signal.

The sensing of the return signal <NUM> may be performed in various different manners. The return signal <NUM> may for example be picked up using the probe <NUM>. The return signal <NUM> may be obtained using the probe <NUM>, by scanning the probe tip <NUM> across the surface <NUM> in contact mode. The output signal <NUM> is obtained using a laser beam <NUM> specular reflected off the back of the probe tip <NUM> or the cantilever <NUM> or off the back of actuator <NUM>, and incident onto a photo diode <NUM>, e.g. a quadrant type photo diode. The probe tip <NUM> in contact with surface <NUM> will receive the induced acoustic vibrations of return signal <NUM>. Any sub-surface structures <NUM> may be visualized by analysis of return signal <NUM> received via the vibration response of the cantilever <NUM> and the probe tip <NUM>. The return signal <NUM> may be analyzed by analysis of the output signal <NUM> from photo sensor <NUM>. Proper analysis of output signal <NUM> allows to isolate the signal components corresponding to the return signal <NUM>. This output signal <NUM> is provided to the analysis system <NUM> to perform such analysis. In the analysis system <NUM>, a hardware or software module <NUM> isolates the scattered fraction <NUM> from the output signal <NUM> to provide an image of the subsurface structures <NUM>.

Alternatively, the return signal <NUM> may also be received using piezo type actuator <NUM> as a sensor. As may be appreciated, actuator <NUM> is particularly responsive to acoustic signals within the frequency range of the input signal <NUM>, because it is also used to apply the acoustic input signal <NUM>. Thus, an output signal <NUM> from the actuator/sensor <NUM> may be provided directly to the analyzer system <NUM> to perform such an analysis.

System <NUM> further includes a transducer <NUM> mounted underneath the sample <NUM>, and a further transducer <NUM> mounted on cantilever <NUM>. The transducers <NUM> and <NUM> enable the system <NUM> to simultaneously apply additional ultrasonic force microscopy (UFM) at lower frequencies. In system <NUM>, simultaneous to the application of the very high frequency acoustic input signal <NUM> applied to sample <NUM>, the transducer <NUM> may for example apply a further acoustic input signal at a frequency f1 within a frequency range of <NUM> to <NUM> megahertz (MHz). An additional low frequency signal at frequency f2 in the same frequency range as f1 is applied via transducer <NUM> on the cantilever <NUM>, such that the difference f1-f2 is near one of the resonance frequency of the cantilever (e.g. between <NUM> kilohertz (kHz) and <NUM>; say <NUM> as an example). The transducers <NUM> and <NUM> may be piezo type transducers or other suitable transducers. The transducer <NUM> may be mounted on the cantilever <NUM> as indicated, or on the back side of the probe tip <NUM> (i.e. above the probe tip <NUM>), or near the mounting of the probe <NUM> to scan head <NUM>. Any location where vibrations from the transducer <NUM> may be effectively fed into the probe <NUM> may be suitable for mounting the transducer <NUM>. The signal at frequency f2 may alternatively even be applied directly to the sample <NUM>, or even via transducer <NUM>. Transducer <NUM> does not have to be located underneath the sample <NUM>, but may be located on the surface <NUM> or even on a side of the sample <NUM>. Moreover, both signals from transducers <NUM> and <NUM> may be applied by a single transducer, making any or both of these two transducers obsolete in that case. The signals at frequencies f1 and f2 may also both be applied via the transducer <NUM> on the cantilever <NUM>. Optionally, a coupling medium <NUM> (e.g. a liquid, an oil or grease (e.g. vaseline)) may be applied to provide a low resistance coupling between the acoustic transducer <NUM> and the sample <NUM>.

Notwithstanding the abovementioned alternatives, in <FIG> the analyzer system <NUM> of system <NUM> comprises a signal generator <NUM> that enables to generate a first ultrasonic input signal <NUM> including frequency f1 and a second ultrasonic input signal <NUM> including frequency f2. In <FIG>, the first ultrasonic input signal <NUM> of frequency f1 is applied via transducer <NUM> as an acoustic signal <NUM> during scanning of the probe <NUM> across the surface <NUM>. The transducer <NUM> in contact with the cantilever <NUM> receives the second ultrasonic input signal <NUM> including frequency f2. The high frequency ultrasound signal at frequency f1 causes indentation of the probe tip <NUM> against the surface <NUM>. Tip-sample interaction between the probe tip <NUM> and the surface <NUM> causes the resonance frequency of the probe to shift. This is dependent on the local elastic properties of the sample, which in turn depends on the subsurface structure. Hence, structures within the elastic stress field (Hertzian field) can be measured by analysis of the output signal <NUM> around the resonance frequency (i.e. near f2, or f2-f1 in heterodyne excitation). The effect may be compared with feeling an object through a pillow, i.e. the changes in the output signal <NUM> caused by resonance frequency shifts allow to visualize subsurface structures.

As explained above, the penetration depth of the stress field is limited (up to e.g. <NUM> nanometer below the surface), and deeper structures may be detected using the abovementioned very high frequency discontinuous signal applied to the sample <NUM>. However, the additional information on shallow structures, obtained from the latter type of UFM measurement provides additional information useable to increase accuracy, as well as structural information e.g. of the integrity of various layers within the shallow stress field. In and industrial setting, such additional measurements may be highly valuable in a manufacturing process.

In addition to mapping sub-surface structures, the system <NUM> may further be arranged for performing regular atomic force microscopy such as to map on-surface structures on the surface <NUM>. In <FIG>, to this end, the output signal <NUM> after pre-amplification in pre-amplifier <NUM> and after pre-analysis in analyzer <NUM>, is provided both to the lockin amplifier analysis system <NUM> and to a low pass filter <NUM>. The low pass filter removes the high frequency components relating to the sub-surface measurements from the output signal and provides the signal to a comparator <NUM>. The comparator compares the output signal with the set-point that is received at input <NUM> (e.g. from a controller system), and yields a differential signal that is provided to the feedback controller <NUM>. The feedback controller provides a control signal to amplifier <NUM> for driving the piezo-electric actuators <NUM> for adjusting the z-level of the probe <NUM>, i.e. the distance in height of the probe <NUM> above the surface <NUM>. The corrections, which may be obtained from the feedback controller <NUM> by analyzing the control signal, may be more accurately determined with z-level sensor <NUM>. The determined z-level corrections are mapped to provide a surface topography map of the surface <NUM>.

Herein below, a plurality of different embodiments of the method of the present invention will be briefly discussed, with reference to the figures. These embodiments include various different implementations of both the application of the acoustic input signal <NUM> to the sample <NUM>, as well as the detection of the return signal <NUM>. The various embodiments further include the additional application of ultrasonic AFM (UAFM) at frequencies wherein tip-sample interaction is governed by local elastic properties, applying additional subsurface imaging at shallow depths up to <NUM> nanometer below the surface <NUM>.

In the embodiment of <FIG>, the probe <NUM> comprises a cantilever <NUM>. At the front end of the cantilever <NUM>, a probe tip array <NUM> comprises a probe tip head with a plurality of probe tips <NUM>-<NUM>, <NUM>-<NUM> and <NUM>-<NUM>. Above each of the probe tips <NUM>-<NUM> through <NUM>-<NUM>, an associated piezo actuator/transducer <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> is attached to the probe tip array <NUM>. Acoustic input signals <NUM> from each of the respective piezo actuators <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM>, propagate through the probe tip array <NUM> towards the respective probe tips <NUM>-<NUM> through <NUM>-<NUM>. At the surface <NUM> of the sample <NUM>, the very high frequency acoustic input signals <NUM> are transferred into the sample <NUM>. The coupling into the sample <NUM> takes place via the sharp end of each of the probe tips <NUM>-<NUM> through <NUM>-<NUM>. Therefore, within the samples <NUM>, each of the very high frequency input signals <NUM> acts as a point source signal, as is illustrated by the respective input signals <NUM>-<NUM>, <NUM>-<NUM> and <NUM>-<NUM> corresponding to each probe tip.

The signals to be applied by each of the actuators <NUM>-<NUM>, <NUM>-<NUM> and <NUM>-<NUM> are received from generator <NUM> where these are generated as described above, although alternatively of course these signals could be generated in a myriad of different manners. However, in the present embodiment, controller <NUM> (amongst other things) may control a plurality of delays <NUM>-<NUM>, <NUM>-<NUM> and <NUM>-<NUM> such that the phase of the acoustic input signals <NUM>-<NUM>, <NUM>-<NUM> and <NUM>-<NUM> is different dependent on the setting of each one of the delays <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>. Each delay <NUM>-<NUM>, <NUM>-<NUM> and <NUM>-<NUM> is associated with a single one of the actuators <NUM>-<NUM>, <NUM>-<NUM> and <NUM>-<NUM> respectively. In this manner, the phases of the input signal <NUM>-<NUM>, <NUM>-<NUM> and <NUM>-<NUM> may be controlled in such a manner that the combined wave in the sample <NUM> comprises the wave front <NUM> which is suitably shaped as desired for the respective measurement. For example, as illustrated in <FIG>, the wave front <NUM> may be focused onto a certain focal point (e.g. a certain depth within the sample <NUM> below the surface <NUM>). Alternatively, a defocused wave front may be generated, or the faces may be controlled to provide a linear wave front being angled with respect to the surface <NUM>.

The probe tip array <NUM> in this manner provides the possibility to apply a phased array type of measurement. For example using the focused wave front <NUM>, by focusing the wave front <NUM> onto a certain focal point the intensity of the very high frequency acoustic input signal <NUM> will be the highest in the focal point. In case embedded element <NUM> is located at this respective depth that coincides with the focal point, it will provide a very strong return signal <NUM> that may be picked up at the surface <NUM>.

The embodiment illustrated in <FIG> provides a different manner of applying the phased array concept for imaging. Here, the actuator <NUM> for providing the very high frequency acoustic input signal <NUM> is attached to the back end of the cantilever <NUM> of the probe <NUM>. The back end, for the purpose of explanation, is the remote end of the cantilever with respect to the probe tip. The very high frequency acoustic input signal <NUM> is applied via the transducer <NUM> into the cantilever <NUM>, the cantilever <NUM> thereby serving as a waveguide. In this manner, the acoustic input signal <NUM> is applied as a guided wave which propagates through the cantilever <NUM>. As follows from the graph in <FIG>, the phase velocity (in kilometers per second) depends on both the frequency of the input signal applied by the actuator <NUM> and on the thickness of the cantilever <NUM>. The phase velocity further depends on whether the guided wave induced by the acoustic input signal <NUM> is a symmetric wave (S-modes S0, S1 and S2) or an anti-symmetric wave (A-modes A0, A1, A2). Considering at first a single frequency acoustic input signal <NUM>, the thickness of the cantilever <NUM> is fixed during the measurement, and the input signal <NUM> propagates through the cantilever towards the probe tips <NUM>-<NUM>, <NUM>-<NUM> and <NUM>-<NUM>. The acoustic input signal subsequently reaches each of the probe tips <NUM>-<NUM> through <NUM>-<NUM>, and is coupled via each of the probe tips into the sample <NUM>. Assuming linear propagation through the system illustrated in <FIG>, the point sources <NUM>-<NUM>, <NUM>-<NUM> and <NUM>-<NUM> will form a wave front <NUM> that is plane but angled with respect to the sample surface <NUM>.

Given the dependencies of the propagation velocity with respect to the frequency, it is also possible to design the very high frequency acoustic input signal <NUM> by including various frequency components such as to obtain a desired wave front <NUM>. In this manner a different plane wave front or even a non-plane wave front (e.g. focused) may be obtained without requiring a controller or delays as in the embodiment of <FIG>. As may be appreciated, in absence of a controller or delays, the possibilities of shaping the wave front are somewhat more limited, however this embodiment only requires a single piezo actuator <NUM> for the very high frequency acoustic input signal <NUM> and no controllers or delays for this purpose.

A further advantageous example, not covered by the claims of this patent, is illustrated in <FIG>. In <FIG>, the probe tip <NUM> comprises a contact area <NUM> with which contact is made with the surface <NUM> of the sample <NUM>. The contact area <NUM> is not sharp, but deliberately a relatively large contact area has been devised such as to apply the very high frequency acoustic input signal <NUM> across the full contact area <NUM>. This creates an acoustic beam <NUM> in the sample where the acoustic signal intensity is very high. Outside the beam <NUM>, much lower acoustic intensities of the very high frequency component are present. As may be appreciated, if an embedded element <NUM> coincides with the sound beam <NUM>, a strong return signal will be scattered which can be picked up with a high signal-to-noise ratio at the surface <NUM>.

Not only the size of the contact area <NUM> is important to obtain a sound beam, providing the contact area <NUM> with a certain cross sectional shape has some further advantages. For example, the shape of the contact area <NUM> may be square, round, oval, polygon, or any of these or other shapes. The intensity profile of the acoustic signal within the beam <NUM> is dependent on the shape of the contact area <NUM>. Using for example a round shape as is done in the embodiment of <FIG>, results in the sound beam <NUM> to comprise a certain area where the intensity of the acoustic input signal <NUM> is very high. This focal area is schematically and generally indicated within the oval <NUM> in <FIG> as a dark area within the beam <NUM>. This can be used to focus the beam at a certain depth and to obtain a large amount of information from this depth.

The present invention has been described in terms of some specific embodiments thereof. It will be appreciated that the embodiments shown in the drawings and described herein are intended for illustrated purposes only and are not by any manner or means intended to be restrictive on the invention. It is believed that the operation and construction of the present invention will be apparent from the foregoing description and drawings appended thereto. It will be clear to the skilled person that the invention is not limited to any embodiment herein described and that modifications are possible which should be considered within the scope of the appended claims. Also kinematic inversions are considered inherently disclosed and to be within the scope of the invention. Moreover, any of the components and elements of the various embodiments disclosed may be combined or may be incorporated in other embodiments where considered necessary, desired or preferred, without departing from the scope of the invention as defined in the claims.

Claim 1:
Method of performing subsurface imaging of one or more embedded structures in a substrate underneath a substrate surface (<NUM>), the method being performed using an atomic force microscopy system, wherein the atomic force microscopy system comprises a probe (<NUM>) with at least one probe tip (<NUM>), and a sensor (<NUM>) for sensing a position of the probe tip (<NUM>) for detecting probe tip (<NUM>) motion, the method comprising the steps of:
positioning the probe tip (<NUM>) relative to the substrate for establishing contact between the probe tip (<NUM>) and the substrate surface (<NUM>);
applying, using at least one first signal application actuator (<NUM>, <NUM>), a first acoustic input signal to the substrate;
applying, using at least one second signal application actuator (<NUM>), a second acoustic input signal to the substrate;
detecting, using the sensor (<NUM>), an output signal from the substrate in response to the first and second acoustic input signal; and
analyzing the output signal for obtaining information on the embedded structures, for enabling imaging thereof;
wherein the at least one probe (<NUM>) comprises a plurality of probe tips (<NUM>) forming a probe tip array (<NUM>), wherein during the step of positioning the probe (<NUM>) is positioned such as to establish contact between the substrate surface (<NUM>) and each of the probe tips (<NUM>);
wherein 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 as to provide, in the output signal, one of:
a return signal including a scattered fraction of the second acoustic input signal scattered from the embedded structures; or
a transmitted second acoustic input signal whose wave-front is distorted due to scattering from the embedded structures.