METHOD AND SYSTEM FOR AT LEAST SUBSURFACE CHARACTERIZATION OF A SAMPLE

Abstract

Method and system for performing characterization of a sample using an atomic force microscopy system. An actuation signal is provided to a photo-thermal actuator which is configured to excite the probe by means of an optical excitation beam incident on the cantilever. The probe is configured to be bendable by means of the optical excitation beam impinging on it. The actuation signal is configured to include at least one modulation frequency. The probe tip motion is monitored for determining at least a subsurface characterization data.

Claims

1. A method of performing at least subsurface characterization of a sample using an atomic force microscopy system comprising a probe comprising a cantilever and a probe tip arranged on the cantilever, and a detector for sensing a probe tip position, and wherein the system is configured for positioning the probe tip relative to the sample, wherein the method comprises: providing an actuation signal to the probe using an actuator for inducing movement between the probe tip and the sample in a direction towards and away from the sample for enabling contact between the probe tip and a surface of the sample, wherein at least during a portion of contact between the probe tip and the surface of the sample the actuation signal is modulated by at least one frequency to vibrate the probe tip at at least one modulation frequency for making a subsurface measurement of the sample, and monitoring the probe tip position for obtaining an output signal indicative of a probe tip motion, for determining, using the output signal, a subsurface characterization data, wherein the actuator is a photo-thermal actuator configured to excite the probe by using an optical excitation beam incident on the cantilever, wherein the probe is configured to deform as a function of heating caused by the optical excitation beam impinging on the probe, wherein the movement in a direction towards and away from the sample for enabling contact between the probe tip and the surface of the sample, and vibration of the probe tip at the modulation frequency are both carried out by the photo-thermal actuator.

2. The method according to claim 1, wherein, during contact of the probe tip with the surface of the sample, a first time interval is used for performing topography characterization and a second time interval is used for performing subsurface characterization, wherein the first time interval differs from the second time interval, and wherein during the second time interval the actuation signal includes at least one modulation frequency.

3. The method according to claim 1, wherein during contact more than two time intervals are defined, wherein, during at least two time intervals, modulations of different frequencies are applied to the actuation signal.

4. The method according to claim 2, wherein at least a third interval is used for performing subsurface characterization, wherein the third interval differs from the first and second time interval, and wherein during the third time interval the actuation signal includes a frequency differing from at least one modulation frequency in the second time interval.

5. The method according to claim 1, wherein during contact between the probe tip and the sample surface, the actuation signal is configured for changing a contact force between the probe tip and the sample for enabling subsurface characterization at a plurality of depths underneath the surface.

6. The method according to claim 1, wherein, during at least one interval, during contact of the probe tip with the surface of the sample, a plurality of modulation frequencies are applied simultaneously.

7. The method according to claim 4, wherein, during the third interval, a depth of subsurface characterization is adjusted to a desired value by changing at least one modulation frequency and/or an oscillation amplitude of the vibration at at least one modulation frequency.

8. The method according to claim 1, wherein at least one of the at least one modulation frequency is varied in time to perform frequency tracking of the contact resonance frequency.

9. The method according to claim 1, wherein the photo-thermal actuator comprises an adjustment unit that adjusts the optical excitation beam incident on the cantilever.

10. The method according to claim 9, wherein the adjustment unit is configured to adjust the level of focus of the optical excitation beam incident on the cantilever.

11. The method according to claim 9, wherein the adjustment unit is configured to adjust an impinging position of the optical excitation beam incident on the cantilever.

12. The method according to claim 1, wherein the probe is made of at least one material shaped for inducing a directional deformation upon thermal expansion, and/or wherein the probe is made of at least two materials with different thermal expansion coefficients.

13. The method according to claim 1, wherein at least during approach of the probe tip towards the sample surface, a resonant frequency is applied to the probe by means of the actuator, wherein an amplitude, a phase and/or an absolute vibration frequency of the probe tip is measured to determine whether the probe tip is in contact with the surface of the sample.

14. The method according to claim 1, wherein a change of a contact resonance frequency is determined based on the output signal, wherein the actuation signal is adjusted based on the determined change of the contact resonance to provide excitation at the contact resonance frequency.

15. An atomic force microscopy system for performing at least subsurface characterization of a sample, the system comprising a probe comprising a cantilever and a probe tip arranged on the cantilever, and wherein the system is configured for positioning the probe tip relative to the sample, the system comprising: an actuator configured to actuate the probe for causing movement of the probe tip, a controller configured to provide an actuation signal to the probe using the actuator for inducing movement between the probe tip and the sample in a direction towards and away from the sample for enabling contact between the probe tip and a surface of the sample, wherein at least during a portion of contact between the probe tip and the surface of the sample the actuation signal is adapted to vibrate the probe tip at at least one modulation frequency for making a subsurface measurement of the sample, and a detector configured to detect a deflection of the probe tip, wherein an output signal indicative of a probe tip motion is obtained by monitoring the probe tip position, wherein the controller is arranged for determining, using the output signal, at least a subsurface characterization data, wherein the actuator is a photo-thermal actuator configured to excite the probe by using an optical excitation beam incident on the cantilever, wherein the probe is configured to deform as a function of heating caused by the optical excitation beam impinging on the probe, wherein the movement in a direction towards and away from the sample for enabling contact between the probe tip and the surface of the sample, and vibration of the probe tip at the at least one modulation frequency are both carried out by the photo-thermal actuator.

16. A lithographic system for manufacturing of a multilayer semiconductor device, wherein the system comprises an atomic force microscopy system for performing at least subsurface characterization of a sample, the system comprising a probe comprising a cantilever and a probe tip arranged on the cantilever, and wherein the system is configured for positioning the probe tip relative to the sample, the atomic force microscopy system comprising: an actuator configured to actuate the probe for causing movement of the probe tip, a controller configured to provide an actuation signal to the probe using the actuator for inducing movement between the probe tip and the sample in a direction towards and away from the sample for enabling contact between the probe tip and a surface of the sample, wherein at least during a portion of contact between the probe tip and the surface of the sample the actuation signal is adapted to vibrate the probe tip at at least one modulation frequency for making a subsurface measurement of the sample, and a detector configured to detect a deflection of the probe tip, wherein an output signal indicative of a probe tip motion is obtained by monitoring the probe tip position, wherein the controller is arranged for determining, using the output signal, at least a subsurface characterization data, wherein the actuator is a photo-thermal actuator configured to excite the probe by using an optical excitation beam incident on the cantilever, wherein the probe is configured to deform as a function of heating caused by the optical excitation beam impinging on the probe, wherein the movement in a direction towards and away from the sample for enabling contact between the probe tip and the surface of the sample, and vibration of the probe tip at the at least one modulation frequency are both carried out by the photo-thermal actuator.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0072] 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.

[0073] In the drawing:

[0074] FIG. 1 shows a schematic diagram of an embodiment of a system;

[0075] FIG. 2 shows frequency response plots;

[0076] FIG. 3 shows a schematic diagram of embodiments of a system;

[0077] FIG. 4 shows an actuation signal and a resulting movement of the probe tip;

[0078] FIG. 5 shows a cross-sectional schematic view of a probe tip in contact with a sample; and

[0079] FIG. 6 shows a schematic diagram of a method.

DETAILED DESCRIPTION

[0080] FIG. 1 shows an atomic force microscopy system 1 for performing characterization of a sample 2. The system 1 comprises a probe 4 including a cantilever 6 and a probe tip 8 arranged on the cantilever 6. The system 1 is configured for positioning the probe tip 8 relative to the sample 2. Further, the system 1 comprises an actuator 14 configured to actuate the probe 4 for causing motion of the cantilever 6 and thus the probe tip 8. Further, the system 1 comprises a controller 16 configured to provide a actuation signal to the actuator 14 to induce movement between the probe tip 8 and the sample 2 for enabling contact between the probe tip 8 and a surface 12 of the sample 2. At least during a portion of contact between the probe tip and the surface of the sample the actuation signal is adapted to vibrate the probe tip at at least one modulation frequency. The system 1 further includes a detector 18 configured to detect a position of the probe tip 8, wherein an output signal indicative of a probe tip motion is obtained by monitoring the probe tip position. The controller 16 is arranged for determining, using the output signal, at least a subsurface characterization data. The actuator 14 is a photo-thermal actuator 14 configured to excite the probe 4 by means of an optical excitation beam 20 incident on the cantilever 6. The probe 4 is configured to deform as a function of heating caused by the optical excitation beam 20 impinging on the probe 4. The shown sample 2 includes a number of sub-surface structures 3, which can be of any arbitrary shape, structure, material, or size.

[0081] Shear stresses can be reduced or eliminated when the probe 4 is not operated in continuous contact mode. By means of the photo-thermal actuator 14, an improved actuation can be obtained improving the SNR and/or avoiding or reducing unwanted resonances in the frequency response. Also a larger excitation frequency range can be obtained.

[0082] A light source, such as a laser, may be used as the photo-thermal actuator 14. Optionally, the actuation signal has a sinusoidal waveform at the at least one modulation frequency. The probe 4 can be configured to bend as function of heating. For instance, the probe 4 can be a bi-metal, which can be configured to deflect when actuated by means of a photo-thermal actuator 14, e.g. a laser focusing the optical excitation beam 20 on a surface of the bi-metal probe 4. Other probes 4 which are sensitive for photo-thermal excitation can also be used. For example, the cantilever can be shaped for inducing significant directed deformation upon thermal expansion. A combination of shape and material selection may also be employed.

[0083] The probe 4 is actuated by means of the photo-thermal actuator 14 for allowing the probe tip 8 to approach the sample surface 12. In this example, the photo-thermal actuator 14 can warm a portion of the probe 4 resulting in a bending movement towards the sample surface 12. For this purpose, the photo-thermal actuator 14 may provide a bias for bringing the probe tip 8 in contact with the sample surface 12, enabling interaction. The bias may for example form a non alternating component (e.g. DC) in the actuation signal. An alternating component at at least one modulation frequency may be provided during at least a portion of contact between the probe tip 8 and the sample surface 12 for enabling at least subsurface characterization of the sample. By taking away the bias, the probe tip 8 may move away from the sample surface 12.

[0084] It will be appreciated that the heating induced by means of the optical excitation beam from the photo-thermal actuator can also result in a retracting movement.

[0085] A bias provided by the photo-thermal actuator 14 configured to bend the probe such that the probe tip comes in contact with the sample surface 12 may also be applied at different times, for example per pixel during characterization measurement. The bending frequency of the probe can be determined by the excitation provided by the photo-thermal actuator 14. This can be controlled by means of the actuation signal. Hence, the bias may be provided in the form of a step function in the actuation signal, wherein at least a portion of the step function includes at least one portion including at least one modulation frequency.

[0086] In an example, movement of probe tip 8 is induced by means of an optical excitation beam 20 having a time varying optical power incident on the cantilever 6. By means of a constant component in the actuation signal, the cantilever may be bent such that the probe tip 8 is brought in contact with the sample surface 12. The optical excitation beam 20 having the time varying optical power can enable a photo thermal excitation of the probe 4 for inducing vibrations at at least one modulation frequency. In an example the probe tip is intermittently or periodically brought in contact with the sample surface at different locations of the surface of the sample (per pixel) for performing characterization or imaging. An optical excitation beam 20 may consist of a laser (or other) optical beam having an adjustable/controllable intensity. The intensity or the power of the beam may be adjusted/varied based on the actuation signal provided to the actuator 14. Thermal effects in the probe 4 can cause the probe tip to start vibrating with the frequency applied via the optical excitation beam 20.

[0087] The output signal can be sensed in different ways. In the shown example of FIG. 1, the position of the probe tip 8 is monitored using an optical detector 18, which is configured to provide an optical sensing beam 22a incident on or near the probe tip 8 and sensing a reflected beam 22b of the optical sensing beam 22a using an optical sensor 24. Hence, the probe tip 8 movements can be monitored using the incident optical beam 22a that is reflected at the probe tip 8 and detected by the optical sensor 24. The motion of the probe tip 8 results in a variation of the reflection angle of the reflected beam 22b, which results in a variation of the location of the reflected beam 22b on the optical sensor 24. This variation on the optical sensor 24 can be detected and analyzed as being the output signal of the system 1. In this example, the optical sensing beam 22a is used independent of the optical excitation beam 20. In an example (not shown), the optical excitation beam 20 which is incident on or near the probe tip 8 may also be used as the optical sensing beam 22a, by sensing the reflection of the optical excitation beam 20 by the optical sensor 24 of the system 1.

[0088] The actuation signal can be free of at least one modulation frequency (e.g. ultrasound frequency) when the probe tip is not contacting the sample.

[0089] FIG. 2 shows frequency response plots in which the amplitude and phase of the probe are plotted in function of the excitation frequency. The frequency response plots for the amplitude of the cantilever at the probe tip 8 excited by means of a photo-thermal actuator 14 and by means of a piezoelectric actuation are depicted by A, B, respectively. The resulting phases of the response of the cantilever 6 at the probe tip 8 when excited by means of a photo-thermal actuator 14 and by means of a piezoelectric actuation are depicted by A, B, respectively. The excitation energy put in the cantilever might differ between the two cases. As can be observed, a more smooth frequency response is obtained when the photo-thermal actuator 14 is employed for exciting the cantilever 6 of the probe 4. The frequency spectra obtained with photo-thermal actuation are free of spurious peaks. Excitation of unwanted vibration modes of the probe 4 can be significantly reduced or avoided by using the photo-thermal actuator 14. Advantageously, in this way, compared to i.a. a piezoelectric actuation (cf. plots B, B), a cleaner and smoother frequency response spectrum and phase measurement of the excited probe can be obtained. Moreover, a higher resolution subsurface imaging may be obtained by means of the photo-thermal actuator 14. The photo-thermal actuator 14 can have a larger excitation frequency range (e.g. up to GHz and above) than other types of actuators, including a piezoelectric actuator (typically can reach GHz). The photo-thermal actuator 14 provides a clean, stable and a frequency independent drive for the cantilever 6 of the probe 4.

[0090] Piezoelectric excitation clearly suffers from a plurality of unwanted peaks in the transfer function between the excitation voltage and the mechanical motion. These peaks are caused by spurious resonances in the coupled mechanical system. As a result of a mechanical coupling between piezoelectric actuator and the cantilever, the mounted piezoelectric actuator becomes part of a larger mechanical system having a more complex frequency response. The peaks may for example be not reproducible making the measurement result unreliable. Moreover, piezoelectric actuation can be strongly frequency-dependent. The excitation provided by the photo-thermal actuator 14 can provide a transfer function from excitation voltage to mechanical motion that is substantially independent of frequency and substantially free from spurious resonances. The photo-thermal actuator 14 may directly excite only the probe. The photo-thermal actuator 14 drives no other system resonances, so there are reduced or no unwanted peaks in the frequency response.

[0091] As can be seen in the frequency response employing the piezoelectric actuator, the response can be highly variable and not sufficiently flat so that the response of the cantilever changes at different frequencies; not only as a function of sample properties but also as a function of the response. Such distortions can negatively impact accuracy and stability among other things. In the frequency response, the cantilever can have a plurality of peaks at different eigen frequencies of torsional and flexural modes. In the frequency response employing the photo-thermal actuator 14, instead of a plurality of (unwanted) peaks, there is a single resonance peak 30 observed for the identified single resonance frequency of the cantilever (e.g. first resonance frequency). Hence, advantageously, by means of the photo-thermal actuation there are no spurious peaks coming from resonances resulting from the excited cantilever chip/holder assembly. The cantilever amplitude can remain more stable over time and tuning the cantilever resonance can be facilitated.

[0092] The actuation signal provided to the photo-thermal actuator 14 results in a modulated focused light, i.e. optical excitation beam 20, used for actuating the probe 4. The modulated light may for instance be focused on the probe, wherein the probe is configured to bend as a function of the light (e.g. as a result of heating). The modulated light may have an oscillation amplitude which can be controlled by means of changing a light intensity, a focus level (e.g. focus point for switching between in-focus and out-of-focus), and/or focus position on the probe. A technical advantage is that subsurface imaging can be obtained by direct probe excitation including bringing the probe tip in contact with the sample surface and at least during a time interval during contact between the probe tip and the sample surface provide at least one modulation frequency f1 in the actuation signal.

[0093] The photo-thermal actuator 14 may comprise an optical beam positioning unit which can be employed for heating certain parts of the cantilever 6 of the probe 4 to a varying degree. The photo-thermal actuator 14 may be configured to adjust the total optical power of the optical excitation beam 20 and the beam position relative to the probe 4 to control the temperature of a certain part of the cantilever 6 of the probe 4, such as the probe tip 8. Furthermore, the location of the optical excitation beam 20 on the probe used for photo-thermal excitation affects the drive amplitude of the probe 4. Since the probe 4 has a frequency response which can include of a plurality of normal and torsional eigenmodes (depending on the boundaries, geometrical parameters and material properties), the relationship between location of the optical excitation beam 20 on the probe and the drive amplitude is also frequency dependent.

[0094] FIG. 3 shows a schematic diagram of two embodiments of a system 1. The probe tip 8 is positioned relative to the sample 2 for enabling contact between the probe tip 8 and a surface of the sample 2. The position of the probe tip 8 is monitored by means of the optical detector 18 which transmits the optical sensing beam 22a to a target area on or near the probe tip 8 and senses the reflected beam 22b using the optical sensor 24. The variation of the reflection angle of the reflected beam 22b on the optical sensor 24 is detected and analyzed as being the output signal of the system 1 indicative of the motion of the probe 4. The system 1 further comprises a photo-thermal actuator 14 comprising an adjustment unit 50 configured to adjust the optical excitation beam 20 incident on the cantilever 6. The adjustment unit 50 is configured to adjust the level of focus of the optical excitation beam 20 incident on the cantilever 6. By controlling the level of focus of the optical excitation beam 20 on the cantilever 6, the excitation of the cantilever 6 can be adjusted.

[0095] In the shown embodiment of FIG. 3(a), the adjustment unit 50 comprises a lens 52 which is configured to be movable in at least one adjustment direction X1 substantially along a path of the optical excitation beam 20 transmitted to the cantilever 6. The lens 52 can act as a focusing lens which is arranged to change the level of focus. At a predetermined position of the lens 52, the optical excitation beam 20 may be in-focus. Movement of the lens 52 along the adjustment direction X1 (shown as lens 52) can bring the optical excitation beam 20 incident on the cantilever 6 substantially out of focus, changing the level of excitation of the probe 4. Many variants are possible. For instance, in an example, a plurality of lenses may be arranged, wherein at least a first lens and a second lens are configured to be movable with respect to each other in at least a direction substantially along the optical excitation beam 20 transmitted to the cantilever 6. Also in this way, the level of focus of the optical excitation beam 20 can be adjusted, enabling accurate excitation of the probe 4.

[0096] In the shown embodiment of FIG. 3(b), the adjustment unit 50 is configured to adjust the level of focus of the optical excitation beam 20 by changing the position of a lens 54 in at least one adjustment direction Y such as to bring the lens in and out of the path of the optical excitation beam 20 directed towards the cantilever 6 of the probe 4. The lens 54 may be a focusing lens configured to bring the optical excitation beam 20 in focus on the cantilever 4 when present in the path of said optical excitation beam 20. Removing the lens 54 from the optical path of the optical excitation beam 20 (shown as lens 54), can bring it out of focus on the cantilever 4.

[0097] Additionally or alternatively, the impinging position of the optical excitation beam 20 incident on the cantilever can be adjustable, for instance by means of the adjustment unit 50. By changing the (focus) position of the optical excitation beam impinging on the cantilever, the bending of the cantilever can be controlled. Also the intensity of the optical excitation beam 20 may be adjustable. A combination of adjustment mechanisms may also be employed for enabling accurate excitation of the probe 4. Many variants are possible. In an example (not shown), the excitation and sensing of the probe tip position/motion is combined in a unitary optical unit having means to sense a reflected beam of the optical sensing beam using an optical sensor and simultaneously actuate the probe by adjusting the optical excitation beam 20 (e.g. focus, intensity, position on cantilever, or combination).

[0098] By inducing the vibrations resulting from the photo-thermal actuator 14 solely in the probe 4 (cf. top side actuation), it can be avoided that a sample handling system needs to be modified. Therewith, a risk of backside contamination of the sample can be avoided, which can be important for example for wafers in the semiconductor industry. In this way, use of a coupling medium, which is required in case of bottom excitation to couple the sample 2 to the (sample) transducer, is obviated. Furthermore, the photo-thermal actuator 14 used for excitation of the probe can generally be smaller than one to be used for excitation of the sample (typically acoustic or piezoelectric). Also, the actuation efficiency of the probe tip 8 can be higher as the vibrations are directly applied to the cantilever 6 of the probe 4.

[0099] In the shown embodiments of FIG. 3, the sample 2 is wafer or a semiconductor device comprising a stack of device layers including at least a first layer and a second layer. The sample is a semi-finished multilayer semiconductor device that comprises a patterned device layer and a resist layer covering one or more layers including the patterned device layer.

[0100] The photo-thermal actuator 14 can result in a more accurate or smoother actuation which is needed for performing subsurface characterization of the sample, for instance employing subsurface ultrasonic resonance force microscopy.

[0101] FIG. 4 shows an actuation signal and a resulting movement of the probe tip 8 resulting from said actuation signal. In the shown graph in FIG. 4(a), the power of the optical excitation beam 20 (vertical axis) is depicted in function of time (horizontal axis). In this example, the excitation is controlled by means of varying the power output from the photo-thermal actuator 14, i.e. cf. intensity of the optical excitation beam 20. The power of the optical excitation beam 20 may be controlled by means of the actuation signal provided to the photo-thermal actuator 14. The power is first increased to a first power P1, resulting in a first amplitude of the probe tip 8. At the first amplitude, the probe tip 8 is in contact with the sample surface 12. At least during contact, the actuation signal is configured to provide the at least one modulation frequency f1. The at least one modulation frequency f1 may correspond to a contact resonance frequency. The at least one modulation frequency f1 is used for determining subsurface characterization data of the sample 2. In the shown graph in FIG. 4(b), a position of the probe tip 8 (vertical axis) is depicted in function of time (horizontal axis), resulting from the photo-thermal excitation using an optical excitation beam having a power as shown in FIG. 4(a). A position 0 depicts the surface of the sample 2. As a result of an increasing power, the cantilever 6 of the probe 4 is bent towards the sample 2. The cantilever 6 is moved down until a setpoint is reached. A first time interval T1 is used for performing surface measurement and a second time interval T2 is used for performing a subsurface measurement, wherein during the second time interval T2, the probe 4 is excited at the at least one modulation frequency f1. It is appreciated that in an example, surface measurements using the first time interval T1 are not carried out (e.g. only including second time interval T2 for performing measurements).

[0102] Approach (i.e. moving the cantilever 6 towards the sample surface) and contact modulation at the at least one modulation frequency f1, are both performed using the photo-thermal actuator 14. The approach may involve an arbitrary movement and does not have to be periodic or in resonance with the free air resonance of the cantilever. Hence, in an example, the movement of the probe tip 8 with respect to the sample 2 can be slower than the free air resonance (cf. non-resonant measurement technique). When in contact, the optical excitation beam 20 can be modulated at contact resonance frequency as at least one modulation frequency f1 to obtain subsurface image. Next to the fact that both movement and modulation can be done with one actuator 14, in an advantageous way, the photo-thermal actuator 14 can yield clean contact resonance spectra and/or allow for arbitrary waveforms to move the cantilever 6 up and down with respect to the sample 2.

[0103] Advantageously, by moving the probe tip 8 up and down with respect to the sample 2 and performing intermittent measurements (cf. per pixel) during at least a portion of contact between the probe tip 8 and the sample 2, shear stress can be reduced. In this way, the risk of damaging the sample 2 during measurements can be reduced.

[0104] FIG. 5 shows a schematic diagram of a probe tip 8 in contact with a sample 2. During contact, depth information from the subsurface can be determined. As a result of the actuation of the probe tip 8, a tip-sample contact force is obtained resulting in a stress field 70 induced in the sample. The induced stress field 70 determines a probing depth 72. In this way, subsurface features to a certain desired probing depth 72 can be characterized. The probing depth 72 can i.a. depend on the material hardness of the sample. Different types of samples can be measured in a non-destructive manner. For example, in case a resist sample is measured, a lower force may be applied (thus applying a lower probe tip amplitude) than compared to a harder sample. In this way, a wide variety of samples can be characterized, wherein the interaction between the probe tip 8 and the sample 2 is tuned to the material of the sample which is to be characterized in order to avoid or reduce the risk of damage. The photo-thermal actuator 14 provides a top excitation for direct actuation of the probe by means of light. Advantageously, the sensitivity, measurement accuracy and/or spatial resolution for at least subsurface characterization can be improved in this manner.

[0105] As already indicated above, there are different ways to influence the oscillation amplitude of the cantilever, for example, by varying the intensity of the laser by varying a spot location of the optical excitation beam on the cantilever or varying the focus point of the optical excitation beam. In this way, the stress field can be adjusted enabling non-destructive characterization of different samples having different material properties (e.g. elastic properties).

[0106] All movements, except large Z-stroke, may be carried out by means of the photo-thermal actuator 14. The probe tip 8 can be moved towards and away from the sample surface 12, but also modulations/vibrations are performed with photo-thermal stimulation of the probe 4. In this way, a much better control can be obtained. For instance, it may no longer be needed to move a holder of the cantilever 4 for bringing the probe tip 8 in contact with the sample surface 12. Instead, the probe tip 8 can be brought in contact with the sample surface 12 by means of deforming the cantilever 6 using the photo-thermal actuator 14. As only the cantilever 6 is deformed for moving it towards and away from the sample surface, for enabling contact and non-contact, instead of moving the probe holder and the probe (Z-stroke), a better control can be obtained and/or a measured response signal obtained by means of the detector can become a lot sharper (no or less spurious peaks).

[0107] FIG. 6 shows a schematic diagram of a method 1000 for performing characterization of a sample 2 using an atomic force microscopy system 1 comprising a probe 4 including a cantilever 6 and a probe tip 8 arranged on the cantilever 6, and a detector 18 for sensing a probe tip position and wherein the system is configured for positioning the probe tip 8 relative to the sample 2 for enabling contact between the probe tip 8 and a surface of the sample 12. In a first step 1001, an actuation signal is generated for operating the probe by means of an actuator. The actuation signal is provided to the probe using an actuator for inducing movement between the probe tip and the sample in a direction towards and away from the sample for enabling contact between the probe tip and a surface of the sample. At least during a portion of contact between the probe tip and the surface of the sample the actuation signal is adapted to vibrate the probe tip at at least one modulation frequency. In a second step 1002, the probe tip position is monitored for obtaining an output signal indicative of a probe tip motion, for determining, using the output signal, at least a subsurface characterization data. The actuator is a photo-thermal actuator configured to excite the probe by means of an optical excitation beam incident on the cantilever. The probe is configured to deform as a function of heating caused by the optical excitation beam impinging on the probe, wherein the movement in a direction towards and away from the sample for enabling contact between the probe tip and the surface of the sample, and vibration of the probe tip at at least one modulation frequency are both carried out by means of the photo-thermal actuator.

[0108] The approach/retract movement of the probe tip towards and away from the sample surface (cf. moving up and down with respect to the sample surface per pixel), and additionally the modulation/vibration of the probe tip can be carried out by a same photo-thermal actuator.

[0109] Hence, by means of the photo-thermal actuator 14 provided with a actuation signal including at least one modulation frequency f1 (modulation) a subsurface characterization can be performed. By means of the photo-thermal actuation, a clean frequency spectrum can be obtained enabling quantitative subsurface measurements. Furthermore, since a top actuation is employed, there is no longer a need for a wafer stage modification. Also an improved resolution and SNR can be obtained by means of the photo-thermal actuator 14.

[0110] The probe tip 8 can approach the sample and reach a set point at which surface topography of a pixel is recorded. After the surface topography is recorded, a modulation is employed, for instance in the form of an acoustic wave signal, for recording amplitude or phase of subsurface. After a subsurface measurement, the probe tip can be retracted to a predefined distance. The probe or sample can then be moved to perform the same steps on a next pixel.

[0111] It is appreciated that photo-thermal excitation of the probe may also be used in conjunction with other actuation methods.

[0112] 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.

[0113] 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.

[0114] 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.

[0115] 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.

[0116] 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.