WO2020162744A1

WO2020162744A1 - Method of and arrangement for mapping structural features on a surface of a sample by scanning probe microscopy

Info

Publication number: WO2020162744A1
Application number: PCT/NL2020/050056
Authority: WO
Inventors: Hamed Sadeghian Marnani
Original assignee: Nearfield Instruments B.V.
Priority date: 2019-02-05
Filing date: 2020-02-03
Publication date: 2020-08-13
Also published as: EP3921654A1; TW202032129A; TWI862547B; NL2022516B1; NL2022516A

Abstract

The present document relates to a method of mapping structural features on a surface of a sample using a scanning probe microscopy device comprising a sample support structure for supporting the sample, a scan head including a probe comprising a cantilever and a probe tip arranged on the cantilever, and an actuator for moving the probe tip relative to the sample surface for performing the mapping. The method comprises scanning the probe tip relative to the sample surface in a measurement area, wherein the probing tip is brought in contact with the sample; and determining, at least one of: a position of the probe tip relative to the surface in a direction perpendicular thereto, or an orientation of the probe tip relative to the surface. Prior to scanning, the method comprises applying a cover material to at least the measurement area to form a cover layer having a flat surface, and hardening the cover layer. The document further relates to a scanning probe microscopy arrangement.

Description

Title: Method of and arrangement for mapping structural features on a surface of a sample by scanning probe microscopy.

Field of the invention

The present invention is directed at a method of mapping structural features on a surface of a sample by performing a scanning probe microscopy technique using a scanning probe microscopy device, the scanning probe

microscopy device comprising a sample support structure for supporting the sample, a scan head including a probe comprising a cantilever and a probe tip arranged on the cantilever, and an actuator for moving the probe tip relative to the sample surface for performing the mapping, the method comprising: scanning, by the actuator, the probe tip relative to the sample surface for performing said mapping in a measurement area on the surface, wherein for performing the mapping the probing tip is brought in contact with the sample for performing said scanning probe microscopy technique; determining, using a detector, for obtaining an output signal from the scanning probe microscopy device at least one of: a position of the probe tip relative to the surface in a direction perpendicular thereto, or an orientation of the probe tip relative to the surface. The invention is further directed at a scanning probe microscopy arrangement.

Background

Scanning probe microscopy (SPM) is a widespread class of microscopy methods that is based upon the scanning of a surface by means a probe tip in continuous or periodic, i.e. intermittent, contact with the surface. The method enables the detection and mapping of surface features - e.g. trenches, dimples, edges, roughness, etcetera - on the surface of a sample with great accuracy and at high resolution. The high resolution enables the detection of even nanometer sized structures, and as a result of this high resolution has become very popular for example as a tool in the production of semiconductor elements. However, SPM is used in many other applications as well, for example the imaging and analysis of soft tissue or biological samples. Although being highly valued as a microscopy technique in view of its accuracy, disadvantages of most SPM methods is that the performance is limited and that the methods are limited in their ability to deal with high aspect ratio features on the sample surface. For example, a deep trench or a sudden steep edge or step-up such as FinFETS, cannot be measured with a same level of accuracy in the conventional manner with high scanning speed.

Work arounds have been developed such as the tilting of the probe to measure a step-up with the probe under a larger tilting angle with respect to the surface. However, a disadvantage of this is that the scanning must then at least be repeated once in a counter direction in order to measure a possible step-down or trench behind the step-up. Alternatives are known, such as the application of a hammer head shaped probe tip to overcome some of the disadvantages, however the performance remains limited. With a hammer head shaped probe it is not possible to enter narrow trenches because the width of the hammer side is relatively wider than the dimensions of the narrow trenches.

Another known solution is the use of high aspect ratio whiskers on the probe tip. The whiskers provide an extremely thin extension of the tip, like the whisker of a cat, to sense side walls and trenches at some distance from the tip. Contact between the structure and the whisker influences the interaction between the probe tip and the structure, and thus becomes measurable.

The known solutions to detecting high aspect ratio features all come with their disadvantages. Most of these solutions simply slow down the process considerably, e.g. by requiring multiple passes or due to the fact that movements have to be performed at lower speeds. Moreover, most of the solutions are only to a limited extent effective in accurately mapping or visualizing high aspect ratio (HAR) features. For example, the bottoms of very deep trenches may be

measurable by tilting the probe and neither may there be probe designs with whiskers long enough to reach the bottom of such a trench. Moreover, the length of the whiskers is limited by the bending stiffness, and whiskers that are too long will render the measurement inaccurate.

Important to realize, the problem of limited performance is not tied to high aspect ratio features only. Also in standard SPM methods the probe must follow the structures accurately. This limits the performance of scanning. In industrial settings and other applications, performance of the method in terms of both accuracy and speed are important factors that determine the suitability of the method to be used as a tool in a process, e.g. a production process.

Summary of the invention

It is an object of the present invention to provide to overcome the abovementioned disadvantages and to provide a scanning probe microscopy method that allows to accurately map structural features, e.g. both high aspect ratio features and other structural features, at high speed.

To this end, there is provided herewith a method of mapping structural features on a surface of a sample by performing a scanning probe microscopy technique using a scanning probe microscopy device, the scanning probe

microscopy device comprising a sample support structure for supporting the sample, a scan head including a probe comprising a cantilever and a probe tip arranged on the cantilever, and an actuator configured for providing a relative motion between the probe tip and the sample surface for performing the mapping, the method comprising: scanning, by the actuator, the probe tip relative to the sample surface for performing said mapping in a measurement area on the surface, wherein for performing the mapping the probing tip is brought in contact with the sample for performing said scanning probe microscopy technique; determining, using a detector, for obtaining an output signal from the scanning probe microscopy device at least one of: a position of the probe tip relative to the surface in a direction perpendicular thereto, or an orientation of the probe tip relative to the surface; wherein prior to said scanning, the method comprises the steps of:

applying, by an applicator, a cover material to at least a part of the sample surface such as to form a cover layer having a flat surface, the cover layer thereby covering said structural features in the at least part of the sample surface, wherein the at least a part of the sample surface at least includes the measurement area; wherein the step of applying is performed such that the flat surface provides a measuring surface for receiving the probe tip when brought in contact with the sample.

Instead of modifying the probe or the SPM device to measure HAE features, the method of the present invention advantageously temporarily modifies the sample to enable the application of a different type of SPM technique to be performed. By covering the measuring area on the sample surface with a cover layer having a flat surface, the probe no longer encounters any structural features on the measuring surface. Hence the scanning can be performed very fast. Instead of having to deal with the structural features on the surface, the method may now apply any other suitable technique of a myriad of available SPM techniques to determine the shapes, sizes (e.g. height, length, width), orientations, positions, and all kinds of other properties of the now covered structural features. A further advantage is that due to the cover layer, any damage to the surface structures that may be caused by the probe tip being in contact with the surface, is effectively prevented.

The cover material is applied in such a manner that it provides a flat surface that provides for a suitable measuring surface that is capable of supporting a probe tip contact therewith. This may for example be a solid surface. However, to some extend, soft surfaces may also be suitable. Moreover, some application methods enable to apply a cover material such that it immediately after application thereof leaves a solid measurement surface that may be flat or may occasionally require some flattening. For example, sputtering a metal cover layer will yield a suitable surface. Some application methods apply a liquid cover material that may require hardening. Therefore, in accordance with some embodiments, the step of applying the cover material comprises a further step of hardening the cover layer for enabling the flat surface to provide a measuring surface for receiving the probe tip when brought in contact with the sample. Dependent on the cover material of use, such hardening may be performed by any of the following processing steps: exposing the cover material to radiation, such as optical radiation (e.g. ultraviolet, infra red, or visible spectrum); heating of the sample; cooling of the sample (e.g. for freezing or otherwise hardening); or a drying step, e.g. by waiting.

For example, the method may include applying an ultrasonic force microscopy method (UFM), i.e. a subsurface measuring technique that allows to accurately map features that are below the surface of the cover layer - such as the covered structural features. Alternatively, a heterodyne force microscopy method at high frequency may be performed, enabling the visualization of both shallow and deeply buried structural features. The high spatial resolution of this technique enables to map the complete shape, size, position and orientation of otherwise difficult to measure HAR features. There are no limitations with respect to the dimensions of the HAR features. As may be appreciated, the additional cover layer may attenuate the output signal, but this is dependent on the cover material of choice and the thickness of the layer. Generally, this attenuation does not weigh up against the advantages achieved in terms of performance and accuracy.

As a further alternative, subsurface ultrasonic resonance force microscopy (SSURFM) may be applied. SSURFM can be used to obtain images of nanoscale subsurface features, e.g. when covered by optically non-transparent layers, such as metals are imaged. SSURFM enables imaging of subsurface features of various material combinations and capping layers, and can also be implemented on the cantilever side to perform measurements on large samples.

For example, SSURFM may be used for metrology applications in the

semiconductor industry, especially samples that cannot be optically imaged because the metal layer is opaque to light of the relevant wavelengths.

Various embodiments of the invention apply different types of SPM. For example, in some embodiments, the scanning probe microscopy technique includes at least one element of a group comprising: atomic force acoustic microscopy, such as ultrasonic force microscopy, contact-resonance atomic force microscopy, subsurface ultrasonic resonance force microscopy, Kelvin probe microscopy, scanning near-field ultrasound holography, heterodyne force microscopy; phase contrast atomic force microscopy; multifrequency atomic force microscopy, such as bimodal atomic force microscopy, trimodal atomic force microscopy, or multimodal atomic force microscopy; or force-distance curve based atomic force microscopy. Various other SPM techniques available may be applied advantageously, without departing from the invention.

The application of some of the abovementioned SPM technologies enable the characterization of other properties than spatial features and dimensions. For example, the method in some embodiments may further comprise analyzing the output signal for determining at least one of: material properties of the structural features or an internal structural configuration of the structural features. For example, such information may be obtained by applying force-distance curve based atomic force microscopy. In force-distance curve based AFM, a force-distance curve is determined in intermittent contact mode at each location where the probe tip impacts the measurement surface. Impact response forces and surface adhesive forces determine the deviation of the probe tip over time during each impact of the probe tip and its release again from the surface, from which a force distance curve may be obtained. From analyzing the force-distance curve, information such as sample deformation, elasticity, energy dissipation and adhesion may be obtained, which are indicative of material properties. Similarly, multifrequency AFM may provide additional information on the sample, including e.g. magnetic properties, subsurface features, etcetera.

In accordance with some embodiments, the method further comprises applying, using an acoustic actuator, an acoustic signal to at least one of the sample, the cantilever or the probe tip; and analyzing, using an analyzer, the output signal for mapping of the structural features as covered by the cover layer. This type of SPM includes many of the subsurface SFM methods, some of which have been mentioned above already and allow for accurate characterization of properties of the structural features covered by the cover layer. Some of these SPM methods also enable the mapping of structural features in deeper layers underneath the sample surface that comprises the structural features. This therefor enables the analysis of an internal structure of such features, or the examination of deeper layers and their orientation with respect to more shallow features.

In accordance with some embodiments, after the steps of scanning and determining, the method further comprises a step of removing of the cover material. The primary objective for the application of the cover material in accordance with the invention, is to enable the use of one or more of many types of SPM techniques to enable the accurate characterization of the structural features and to do this in a manner that enables scanning at full speed. The additional removal step allows for subsequent processing steps to be carried out on the sample. For example, if the sample is a wafer, dicing of the wafer into individual semiconductor elements may be a subsequent processing step.

In some of these embodiments, the cover material is dissolvable in a solvent and the step of removing of the cover material comprises dissolving of the cover material. In other embodiments, the step of hardening of the cover material comprises a step of cooling of the cover material such as to solidify the cover material by freezing, and the step of removing the cover material comprises a step of heating the cover material such as to liquefy the cover material for removal thereof. The freezing and melting may be performed very quickly and conveniently at moderate temperatures dependent on a suable choice of cover material. For example, the cover material may be a layer of water which is frozen to form a layer of ice such as to enable measurement; the step of removing in that case requires heating of the ice such as to melt the layer and remove to remove the water. In accordance with some embodiments, the cover material is a light sensitive material, such as a photoresist material, and wherein the step of hardening comprises a step of exposing the cover material to radiation. Photoresists are widely used in lithographic processes and exhibit favorable properties to perform a method as described herewith. In accordance with some embodiments, the cover material may be a metal - metal layers are a suitable candidate since they are stiffer and better match with the acoustic impedance, thereby allowing for higher sensitivity.

In accordance with a second aspect of the invention, there is provided a scanning probe microscopy arrangement configured for performing a method according to any one or more of the preceding claims, for mapping structural features on a surface of a sample, the arrangement comprising a scanning probe microscopy device a sample support structure for supporting the sample, a scan head including a probe comprising a cantilever and a probe tip arranged on the cantilever, and an actuator for moving the probe tip relative to the sample surface for performing the mapping, wherein the actuator is arranged for scanning the probe tip relative to the sample surface for performing said mapping in a measurement area on the surface, wherein the scanning probe microscopy device comprises a Z-motion actuator for bringing the probing tip in contact with the sample for performing the mapping using a scanning probe microscopy technique; the scanning probe microscopy device further comprising a detector or detector arrangement for obtaining an output signal for determining at least one of: a position of the probe tip relative to the surface in a direction perpendicular thereto, or an orientation of the probe tip relative to the surface; wherein the scanning probe microscopy arrangement further comprises an applicator for applying, prior to said scanning and determining, a cover material to at least the measurement area such as to form a cover layer having a flat surface; and a curing unit for hardening the cover layer for enabling the flat surface to provide a measuring surface for receiving the probe tip when brought in contact with the sample.

In some embodiments of the second aspect, the curing unit includes at least one of a radiation unit, a heater or a cooling unit, for performing the hardening. In some embodiments of the second aspect, the scanning probe microscopy device is configured for performing at least one element of a group comprising: atomic force acoustic microscopy, such as ultrasonic force microscopy, scanning near-field ultrasound holography, heterodyne force microscopy; phase contrast atomic force microscopy; multifrequency atomic force microscopy, such as bimodal atomic force microscopy, trimodal atomic force microscopy, or multimodal atomic force microscopy; or force-distance curve based atomic force microscopy.

The scanning probe microscopy device may in certain embodiments further comprise an acoustic actuator for applying an acoustic signal to at least one of the sample, the cantilever or the probe tip; and an analyzer for analyzing the output signal for mapping of the structural features as covered by the cover layer.

Brief description of the drawings

The invention will further be elucidated by description of some specific embodiments thereof, making reference to the attached drawings. The detailed description provides examples of possible implementations of the invention, but is not to be regarded as describing the only embodiments falling under the scope. The scope of the invention is defined in the claims, and the description is to be regarded as illustrative without being restrictive on the invention. In the drawings:

Figure 1 schematically illustrates an scanning probe microscopy arrangement in accordance with an embodiment of the present invention;

Figures 2A to 2D schematically illustrate method steps performed in a method in accordance with an embodiment of the invention;

Figure 3 schematically illustrates a method in accordance with an embodiment of the invention;

Figure 4 schematically illustrates an scanning probe microscopy arrangement in accordance with an embodiment of the present invention; Figure 5 schematically illustrates an scanning probe microscopy arrangement in accordance with an embodiment of the present invention.

Detailed descriotion

Figure 1 schematically illustrates an scanning probe microscopy arrangement in accordance with an embodiment of the present invention, suitable for applying a method in accordance with the invention. The scanning probe microscopy system 1 comprises a substrate carrier 3 and a scan head 5. The scan head 5 comprises a probe chip holder 8 onto which a probe chip 9 may be attached. The probe chip 9 comprises a probe 10 having a probe tip 11 at the end of a cantilever 12. A probe 10 is illustrated having a regular sharp tip 11 at the end thereof, however the probe 10 being used may be any type of probe suitable for performing a desired kind of imaging or measuring, and therefore is not necessarily a probe 10 as illustrated. The chip holder 8 may be a clamp, an electrostatic fixing member, a vacuum clamp, a magnetic element or any other suitable element that enables to hold and fix a probe chip 9 to the scan head 5. As will be explained further below, the chip holder 8 may be configured for applying a vibration to the chip 9, i.e. an acoustic vibration, as input signal. The scan head enables to scan the probe chip 9 relative to the sample 25 in a direction parallel to the surface 26 thereof. In figure 1, as will be explained further down below, the surface 26 of sample 25 is covered with a cover layer 29. By the wording“parallel to surface 26” it is meant that in the illustration of figure 1, the probe chip 9 can be moved relative to the sample 25 in horizontal direction in the figure, i.e. in the X and Y directions which are also parallel with the surface of the sample carrier 3. The X and Z direction are indicated by coordinate system 33 in figure 1. The Y direction is the horizontal direction which is virtually going into the paper, as indicated by the symbol in figure 1 representing an arrow going into the paper.

The scan head 5 further comprises a Z-direction actuator 14. This Z- direction actuator 14 is configured for moving the probe chip 10 relative to the substrate surface 26 in the vertical direction, bringing the probe tip 11 towards and away from the sample 25. This Z-direction actuator may comprise a piezo type actuator enabling small stroke motion of the probe tip 11 in the Z direction. It optionally may further comprise a large stroke actuator, this likewise may comprise a piezo type actuator or a stepper motor or any other type of actuation that allows to perform a large stroke movement. The Z-direction actuator, like the X and Y direction motion for scanning, may be controlled by the control and analysis system 2.

In figure 1, only Z-actuator 14 is illustrated, which is in the illustrated embodiment located on the scan head 5 to act on the probe 10, such as to enable relative movement of the probe tip 11 and the substrate surface 29 with respect to each other. In alternative embodiments, actuators may also act on any of the other parts of the system 1 to enable such relative motion. For example, in figure 5, a Z- direction actuator 14 and an X- and Y- direction actuator 15 act on a sample stage to move the sample 25, such as to achieve relative movement of the probe tip 11 and the substrate surface 29 with respect to each other. In this figure, control and analysis system 2 includes the detector electronics 2-1 and the AFM controller electronics 2-2. The AFM controller electronics 2-2 provides control signals to the respective actuators 14 and 15 to perform the relative movement of the probe tip 11 and the substrate surface 29 with respect to each other. In figure 4, the X-, Y- and Z-direction actuators 14 and 15 are integrated in an XYZ scanner 60 that acts on the probe 10.

Back to figure 1, to monitor the position of the probe tip 11, the scanning probe microscopy system 1 further comprises an optical beam deflection (OBD) arrangement. The OBD arrangement comprises a laser 16, which in use provides a laser beam 20. The beam 20 consists of an incident beam part 20-1 which is incident on the back of probe tip 11. The backside of the probe tip 11 may comprise a specular reflective surface and reflects the incident beam 20-1 such as to provide a reflected beam part 20-2. The reflected beam part 20-2 in turn is incident on a photodiode 18. The photodiode 18 may be a segmented photodiode that allows to accurately determine any change of position and change of orientation of the probe. The output signal from the photodiode is received by the control and analysis system 2. Alternative embodiments of the invention may apply different types of sensors and sensor arrangements for monitoring the position of the probe tip 11. For example, other embodiments may be based on piezoresistive, thermal, magnetic, or electrostatic type sensors, or even other sensor types. The system 1 further comprises piezo type actuators for applying an acoustic vibration signal to the probe 10 and/or to the sample 25. In figure 1, a piezo type actuator 30 is illustrated to be located below the sample 25 such as to apply an acoustic signal to the sample 25. Alternatively or additionally, an acoustic signal may also be applied to the probe tip 11, e.g. via the chip 9, the cantilever 12 or directly to the tip 11. To this end, a piezo type actuator may be present on or adjacent to chip holder 8, to vibrate the chip 9 and thereby the probe tip 11.

Alternatively, the laser beam 20-1 may be a pulsed laser beam that is pulsed at a desired frequency to drive the probe tip 11 into vibration accordingly. Various implementations are possible in this respect, and the invention is not limited to a specific manner of applying the acoustic signal. For example, for performing Kelvin probe microscopy, a variable electrical signal may be applied.

The sample 25 on it’s surface 26 (i.e. it’s own surface 26, not the cover layer 29) comprises all kinds of structural features 27. These structural features 27 may be features of a semi-manufactured semiconductor device for example. Some of the features 27 may comprise deep trenches or steep slopes or step-ups, which are referred to as high aspect ratio features 27. Using a conventional scanning probe microscopy system, to allow correct mapping of such high aspect ratio features, one would need to make use of special probe designs and/or SPM systems that allow the probes to be tilted in an exaggerated manner to ensure that the tip 11 correctly follows the surface 26 in these features 27. This, however, slows down the process and renders it more expensive in view of the reduced performance and required specialized tools.

In accordance with the present invention, the surface 26 of sample 25 has been covered with a cover material 29 that may have been applied using an applicator integrated with or external to the system 1. The cover material 29 has been applied prior to the step of scanning that is illustrated in figure 1. For example, the cover layer may be a liquid or resin that has been applied to the surface 26 and that has been hardened such as to yield a hard cover surface that may be scanned with probe 11. A standard photoresist material may be applied, that has been hardened by exposure to optical radiation. Alternatively, a liquid may have been applied which has been given a thermal treatment to temporarily or permanently harden. In some embodiments, such a thermal treatment may be heating or annealing. Yet in other embodiments, a liquid may temporarily be frozen to provide a hard cover layer 29, for example by temporary cooling of the substrate. This allows the use of liquids having a suitable freezing temperature and suitable properties to enable subsurface measurement by acoustic AFM methods for example. One may think of water, a grease or a suitable resin for example.

During scanning, e.g. as illustrated in figure 1, an acoustic signal may be applied to at least one of the sample, the cantilever or the probe tip using an acoustic actuator such as actuator 3. The analyzer 2 may be applied to analyze the output signal for mapping of the structural features as covered by the cover layer. In this case, that measurement method applied may be any of atomic force acoustic microscopy (such as ultrasonic force microscopy, scanning near-field ultrasound holography, or heterodyne force microscopy); phase contrast atomic force microscopy; multifrequency atomic force microscopy (such as bimodal atomic force microscopy, trimodal atomic force microscopy, or multimodal atomic force microscopy); or force- distance curve based atomic force microscopy. The invention is based on the insight that by covering the surface 26 with a cover layer, the surface of cover layer 29 is flat and no longer comprises structural features 27. Hence, the scanning speed is no longer limited by the structural features 27 and the speed of scanning can be much higher than with a conventional AFM scan for example. The structural features 27 will be mapped using a different measuring method that is based on SPM methods. Here, any of the abovementioned acoustic measurement methods may be applied to map the structures 27 as subsurface structures. This likewise allows accurate mapping of high aspect ratio features, such as deep trenches or sudden sharp step-ups. Also, the cover material can be selected to provide good contrast at sufficiently large depths, while at the same time allowing easy application and removal.

Subsequent to the step of applying the cover layer 29, the cover layer 29 is removed again. This brings back the sample in it’s original state and for example allows for the application of a subsequent device layer, e.g. in a lithographic process.

Figures 2A to 2D schematically illustrate method steps performed in a method in accordance with an embodiment of the invention. Figure 2A illustrates a sample 25 having a surface 26 with structural features 27. These features 27 consist of a series of sharp edges (step-up and step-down) that form deep trenches on the surface 26, thereby providing high aspect ratio features. In step 2B, a cover material layer 29 is applied that covers the structural features 27 beneath the surface. The surface of layer 29 however is not flat. Although in principle, scanning may be performed on a surface as illustrated in figure 2B which is not flat, to achieve a desired amount of accuracy it is desirable to flatten the surface of layer 29. This is illustrated in figure 2C where the surface 32 of cover layer 29 is now flat. Next, in figure 2D, the surface may be scanned at high speed with a regular probe 10 and probe tip 11 in contact or intermittent contact with the surface 32 of cover layer 29. An acoustic signal is applied to at least one of the probe or the sample to enable subsurface measurement of the structures 27.

Figure 3 schematically illustrates a method in accordance with the present invention, for mapping high aspect ratio features 27 on a surface 26 of a sample 25. In step 40, a cover layer 29 is applied to the surface 26 of sample 25 to cover the structural surface features 27 are to be mapped with a cover material.

For example, a photoresist material (these materials are known to the skilled person) may be applied as cover material for layer 29, as it may be easily removed again later. Alternatively, a material may be applied that may later on be temporarily frozen to the sample surface 26. As a further alternative, a material may be applied that may later be easily removed with a solvent (provided that the solvent will not damage the sample surface 26).

In step 42, the cover layer 29 is hardened such as to provide a solid surface. Dependent on the cover material of use, such hardening may be performed by any of the following processing steps: exposing the cover material 29 to radiation, such as optical radiation (e.g. ultraviolet, infra red, or visible spectrum); heating of the sample 25; cooling of the sample 25; a drying step, e.g. by waiting. Optionally, the layer 29 is thereafter flattened to remove any height differences.

A first scan position on the surface of layer 29 is obtained by the system (e.g. by setting or in response to input from an operator). In step 44, the probe tip

11 is moved to the scan position on the surface of layer 29. During scanning of the surface, contact mode or intermittent contact mode may be applied. Hence, the probe tip 11 in the desired scan position contacts the surface of layer 29. In step 46, an acoustic signal is applied to at least one of the probe tip 11 or the sample 25 to perform subsurface measurement. For example, a first acoustic signal at frequency f₁ may be applied to the sample 25 and a second acoustic signal at a frequency f₂ may be applied to the probe tip 11. There frequencies f₁ and f₂ may be slightly different, such that the mixed signal also contains frequency components at the frequency difference I f₂-f₁ I and at the sum of frequencies (f₁+f₂). Alternatively, only a single frequency acoustic signal may be applied to perform the subsurface sensing, e.g. applied to the sample 25 or the probe tip 11. The acoustic signal may be applied via an actuator, such as a piezo in contact with the probe chip 9 or in contact with the sample 25 (as in figure 1). Also, laser beam 20-1 in figure 1 may be a pulsed beam and thermal expansion and contraction of the probe tip 11 results in a vibration thereof at a desired acoustic frequency,

In step 48, an output signal is obtained using a photodiode arrangement 18 as described above. Steps 46 and 48 are applied simultaneously while the probe tip 11 is in contact with the sample 25 at the desired scan position. The photodiode arrangement may comprise an array of adjacent and contiguously arranged diodes. For example, four photodiodes may be contiguously arranged such that their contiguous sides form a cross. The returning laser beam 20-2 may be incident on the center of the cross forming a spot, and any slight movement in the position of the probe tip 11 may result in a disposition of the spot of the incident beam 20-2. The spot may be dispositioned such that the area illuminated by the spot is slightly more located on one of the four photodiodes. This causes one of the diodes to receive more optical energy than the other photodiodes. Not only does this allow

determination of the location of the spot and thereby an accurate estimate of the position of the probe tip, but also the acoustic vibrations may directly be obtained from the signal of the photodiodes. Hence, the signal received from the photodiode arrangement 18 will be the output signal provided to analyzer system 2.

In step 49, which may or may not be performed simultaneously with steps 46 and 48 as well, the output signal is analyzed an image or other

measurement result may be provided by analyzer system 2. From this, the high aspect ratio features 27 will become visible/detectable as subsurface features. In step 50, it is determined whether or not to perform a next measurement at a next scan position. Thus, the system 1 determines whether to continue the scan towards a new scan position, for example based on an earlier input or determination of a scan area. If a next position is to be scanned, in branch 52 the method continues in step 44 for the next scan position. If the last scan position has been reached, the method continues in branch 51 to proceed to step 54.

In step 54, the cover layer 29 may be removed to bring back the sample 25 in the original state and the process is ended for that scan area. Note, though, that step 54 is optional dependent on how the process continues after scanning of the scan area is finalized. For example, the method may be continued at will or may be ended. If a further scan is to be performed, this may be for example a measurement in the same of a different scan area but at a different depth. In that case, step 54 would be absent and the method continues in step 44 with different settings for the acoustic signal. Alternatively, it is also possible that a next sample is fed into the system and the process starts again in step 40.

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.

In the claims, any reference signs shall not be construed as limiting the claim. The term 'comprising' and‘including’ when used in this description or the appended claims should not be construed in an exclusive or exhaustive sense but rather in an inclusive sense. Thus the expression‘comprising’ as used herein does not exclude the presence of other elements or steps in addition to those listed in any 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. Features that are not specifically or explicitly described or claimed may be additionally included in the structure of the invention within its scope.

Expressions such as: "means for ..." should be read as: "component configured for ..." or "member constructed to ..." and should be construed to include equivalents for the structures disclosed. The use of expressions like: "critical", "preferred", "especially preferred" etc. is not intended to limit the invention. Additions, deletions, and modifications within the purview of the skilled person may generally be made without departing from the spirit and scope of the invention, as is determined by the claims. The invention may be practiced otherwise then as specifically described herein, and is only limited by the appended claims.

Claims

1. Method of mapping structural features on a surface of a sample by performing a scanning probe microscopy technique using a scanning probe microscopy device, the scanning probe microscopy device comprising a sample support structure for supporting the sample, a scan head including a probe comprising a cantilever and a probe tip arranged on the cantilever, and an actuator configured for providing a relative motion between the probe tip and the sample surface for performing the mapping, the method comprising:

scanning, by the actuator, the probe tip relative to the sample surface for performing said mapping in a measurement area on the surface, wherein for performing the mapping the probing tip is brought in contact with the sample for performing said scanning probe microscopy technique;

determining, using a detector, for obtaining an output signal from the scanning probe microscopy device at least one of: a position of the probe tip relative to the surface in a direction perpendicular thereto, or an orientation of the probe tip relative to the surface;

wherein prior to said scanning, the method comprises the steps of:

applying, by an applicator, a cover material to at least a part of the sample surface such as to form a cover layer having a flat surface, the cover layer thereby covering said structural features in the at least part of the sample surface, wherein the at least a part of the sample surface at least includes the

measurement area;

wherein the step of applying is performed such that the flat surface provides a measuring surface for receiving the probe tip when brought in contact with the sample.

2. Method according to claim 1, wherein the step of applying the cover material comprises a further step of hardening the cover layer for enabling the flat surface to provide a measuring surface for receiving the probe tip when brought in contact with the sample.

3. Method according to claim 1 or 2, wherein the scanning probe microscopy technique includes at least one element of a group comprising: atomic force acoustic microscopy, such as ultrasonic force microscopy, contact-resonance atomic force microscopy, subsurface ultrasonic resonance force microscopy, Kelvin probe microscopy, scanning near-field ultrasound holography, heterodyne force microscopy; phase contrast atomic force microscopy; multifrequency atomic force microscopy, such as bimodal atomic force microscopy, trimodal atomic force microscopy, or multimodal atomic force microscopy; scanning microwave imaging or force-distance curve based atomic force microscopy.

4. Method according any of preceding claims, further comprising:

applying, using an acoustic actuator, an acoustic signal to at least one of the sample, the cantilever or the probe tip; and

analyzing, using an analyzer, the output signal for mapping of the structural features as covered by the cover layer.

5. Method according to any one or more of the preceding claims, wherein after the steps of scanning and determining the method further comprises a step of removing of the cover material.

6. Method according to claim 5, wherein the cover material is dissolvable in a solvent and wherein the step of removing of the cover material comprises dissolving of the cover material.

7. Method according to claim 3, wherein the method further comprises analyzing the output signal for determining at least one of: material properties of the structural features or an internal structural configuration of the structural features.

8. Method according to claim 5, wherein the step of hardening of the cover material comprises a step of cooling of the cover material such as to solidify the cover material by freezing, and wherein the step of removing the cover material comprises a step of heating the cover material such as to liquefy the cover material for removal thereof.

9. Method according to any of the claims 1-7, wherein the cover material is a light sensitive material, such as a photoresist material, and wherein the step of hardening comprises a step of exposing the cover material to radiation.

10. Scanning probe microscopy arrangement configured for performing a method according to any one or more of the preceding claims, for mapping structural features on a surface of a sample, the arrangement comprising a scanning probe microscopy device a sample support structure for supporting the sample, a scan head including a probe comprising a cantilever and a probe tip arranged on the cantilever, and an actuator for moving the probe tip relative to the sample surface for performing the mapping, wherein the actuator is arranged for scanning the probe tip relative to the sample surface for performing said mapping in a measurement area on the surface, wherein the scanning probe microscopy device comprises a Z-motion actuator for bringing the probing tip in contact with the sample for performing the mapping using a scanning probe microscopy technique;

the scanning probe microscopy device further comprising a detector or detector arrangement for obtaining an output signal for determining at least one of:

a position of the probe tip relative to the surface in a direction perpendicular thereto, or

an orientation of the probe tip relative to the surface; wherein the scanning probe microscopy arrangement further comprises an applicator for applying, prior to said scanning and determining, a cover material to at least the measurement area such as to form a cover layer having a flat surface, and wherein the cover material is applied such that the flat surface provides a measuring surface for receiving the probe tip when brought in contact with the sample.

11. Scanning probe microscopy arrangement according to claim 10, further comprising a curing unit for hardening the cover layer for enabling the flat surface to provide a measuring surface for receiving the probe tip when brought in contact with the sample.

12. Scanning probe microscopy arrangement according to claim 11, wherein the curing unit includes at least one of a radiation unit, a heater or a cooling unit, for performing the hardening.

13. Scanning probe microscopy arrangement according to any one or more of the claims 10-12, wherein the scanning probe microscopy device is configured for performing at least one element of a group comprising: atomic force acoustic microscopy, such as ultrasonic force microscopy, scanning near-field ultrasound holography, heterodyne force microscopy; phase contrast atomic force microscopy; multifrequency atomic force microscopy, such as bimodal atomic force microscopy, trimodal atomic force microscopy, or multimodal atomic force microscopy; or force- distance curve based atomic force microscopy.

14. Scanning probe microscopy arrangement according to any one or more of the claims 10-13, wherein the scanning probe microscopy device further comprises an acoustic actuator for applying an acoustic signal to at least one of the sample, the cantilever or the probe tip; and an analyzer for analyzing the output signal for mapping of the structural features as covered by the cover layer.