ULTRASOUND SUB-SURFACE PROBE MICROSCOPY DEVICE AND CORRESPONDING METHOD

Abstract

An ultrasound sub-surface probe microscopy device (1) is provided comprising a stage (10), a signal generator (20), a scanning head (30), a signal processor (50) and a scanning mechanism (16). In use, the stage (10) carries a sample (11) and the scanning M mechanism (16) provides for a relative displacement between the sample (11) and the scanning head (30), along the surface of the sample. The scanning head (30) comprises an actuator (31) configured to generate in response to a drive signal (S.sub.dr) from the signal generator (20) an ultrasound acoustic input signal (I.sub.ac). The generated ultrasound acoustic input signal (I.sub.ac) has at least one acoustic input signal component (I.sub.ac1) with a first angular frequency (ω1). The scanning head (30) further comprises a tip (32) to transmit the acoustic input signal (I.sub.ac) through a tip-sample interface (12) as an acoustic wave (W.sub.ac) into the sample. Due to a non-linear interaction in the tip-sample interface (12) at least one up mixed acoustic signal component (W.sub.ac2) in said acoustic wave that has a second angular frequency (ω2) higher than the first angular frequency (ω1) Contrary to known approaches, the sensor signal (S.sub.sense) provided by the sensor facility is indicative for a contribution (W′.sub.ac2) of the at least one up mixed acoustic signal component in reflections (W′.sub.ac) of the acoustic wave within the sample (11). Therewith a relatively high resolution can be achieved with which subsurface features can be detected.

Claims

1. An ultrasound sub-surface probe microscopy device comprising: a stage for carrying a sample; a signal generator to generate a drive signal; a scanning head comprising: an actuator configured to generate, in response to the drive signal, an ultrasound acoustic input signal having at least one acoustic input signal component with a first angular frequency, and a tip to transmit the acoustic input signal through a tip-sample interface as an acoustic wave into the sample, wherein the tip-sample interface provides for a non-linear interaction resulting in at least one upmixed acoustic signal component in the acoustic wave having a second angular frequency higher than the first angular frequency; a signal processor to generate an image signal in response to the sensor signal; and a scanning mechanism to provide for a relative displacement between the sample and the scanning head, along the surface of the sample, wherein the scanning head further comprises a sensor facility to provide a sensor signal that is indicative for a contribution of the at least one upmixed acoustic signal component in reflections of the acoustic wave within the sample.

2. The ultrasound sub-surface probe microscopy device according to claim 1, wherein the signal processor includes a lock-in amplifier to lock in at a specific sensor signal component in the sensor signal as an indicator for the contribution of the at least one upmixed acoustic signal component.

3. The ultrasound sub-surface probe microscopy device according to claim 1, wherein the sensor facility of the scanning head has a sensor configured to directly sense the contribution of the at least one upmixed acoustic signal component in the reflections.

4. The ultrasound sub-surface probe microscopy device according to claim 1, wherein the sensor facility of the scanning head is configured to indirectly sense the contribution of the at least one upmixed acoustic signal component in the reflections of the acoustic wave within the sample by downmixing the at least one upmixed acoustic signal component in a non-linear tip-sample interaction to at least one downmixed acoustic signal component with a third angular frequency lower than the second angular frequency.

5. The ultrasound acoustic microscopy device according to claim 4, wherein the sensor facility comprises an acoustic sensor to sense the downmixed signal component.

6. The ultrasound sub-surface probe microscopy device according to claim 4, wherein the angular frequency of the downmixed signal component corresponds to a contact resonance frequency of a flexible carrier used to carry a tip for sensing, wherein the sensor facility comprises a sensor configured to sense a magnitude of resonance of the flexible carrier.

7. The ultrasound sub-surface probe microscopy device according to claim 1, wherein the acoustic input signal is a continuous signal.

8. The ultrasound sub-surface probe microscopy device according to claim 1, wherein the acoustic input signal is a pulse signal.

9. The ultrasound sub-surface probe microscopy device according to claim 1, comprising: a feedback unit to control the signal generator, and/or a force-distance analytic module to control a property of the non-linear transmission by controlling a set point force exerted by the tip to the sample.

10. The ultrasound sub-surface probe microscopy device according to claim 9, wherein the signal generator is configured to generate the drive signal with a number of signal components, and wherein the feedback unit is configured to: control the number of signal components, and/or properties of one or more of the components, the properties including one or more of the group consisting of: an amplitude, an angular frequency and a phase.

11. The ultrasound sub-surface probe microscopy device according to claim 10, wherein the feedback unit has one or more operational modes taken from the group consisting of: a first operational mode to optimize imaging resolution; a second operational mode to optimize a signal to noise ratio (SNR); and a third operational mode to minimize an attenuation.

12. The ultrasound sub-surface probe microscopy device according to claim 9, wherein the feedback unit is configured to operate dependent on an input signal indicative of a property of the sample.

13. A method for operating an ultrasound sub-surface probe microscopy device comprising: carrying a sample; generating a drive signal; generating, in response to the drive signal, an ultrasound acoustic input signal having at least one acoustic component with a first angular frequency and using a non-linear tip-sample interaction to transmit the acoustic input signal as an acoustic wave into the sample at a contact position on the sample, wherein the acoustic wave has at least one upmixed acoustic signal component of a second angular frequency higher than the first angular frequency as a result of the non-linear interaction; generating an image signal in response to the sensor signal; displacing the contact position along the surface of the sample; and providing a sensor signal indicative for a contribution of the at least one upmixed acoustic signal component in reflections of the acoustic wave within the sample.

14. The method according to claim 13, further comprising the step of post processing the image signal, the post-processing including one or more of the group consisting of: extracting critical dimensions, computing an accuracy with which specified dimensions are complied with, and computing an accuracy with which elements in mutually different layers are overlaid.

15. The method according to claim 13, wherein prior to displacing the contact position along the surface of the sample, an operation is performed to configure, a property of the drive signal for improving a property of the sensor signal.

16. The ultrasound sub-surface probe microscopy device according to claim 6, comprising a piezo-electric sensor to sense the magnitude of resonance of the flexible carrier.

17. The ultrasound sub-surface probe microscopy device according to claim 6, comprising an optical arrangement configured to derive the magnitude of resonance of the flexible carrier by detecting an angle of reflection of an optical beam.

18. The ultrasound sub-surface probe microscopy device according to claim 2, wherein the sensor facility of the scanning head has a sensor configured to directly sense the contribution of the at least one upmixed acoustic signal component in the reflections.

19. The method according to claim 13, comprising indirectly sensing the contribution of the at least one upmixed acoustic signal component in the reflections by downmixing the at least one upmixed acoustic signal component in a non-linear tip-sample interaction to at least one downmixed acoustic signal component with a third angular frequency lower than the second angular frequency.

20. The method according to claim 19, wherein the third angular frequency of the downmixed acoustic signal component corresponds to a contact resonance frequency of a flexible carrier.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] These and other aspects are illustrated in more detail with reference to the drawings. Therein:

[0039] FIG. 1 schematically shows an embodiment of an ultrasound sub-surface probe microscopy device according to the present disclosure;

[0040] FIG. 1A, shows an embodiment of a scanning head for use in an ultrasound sub-surface probe microscopy device according to the present disclosure;

[0041] FIG. 1B, shows another embodiment of a scanning head for use in an ultrasound sub-surface probe microscopy device according to the present disclosure;

[0042] FIG. 2 illustrates a non-linearity at a tip-sample interface;

[0043] FIGS. 3A, 3B and 3C respectively show components of an acoustic input signal, components of an acoustic wave formed in a quadratic type tip-sample interface, and components of an acoustic wave formed in a Hertzian type tip-sample interface;

[0044] FIG. 4 schematically shows another embodiment of an ultrasound sub-surface probe microscopy device according to the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

[0045] Like reference symbols in the various drawings indicate like elements unless otherwise indicated.

[0046] FIG. 1 schematically shows an ultrasound sub-surface probe microscopy device 1 according to the present disclosure. The improved ultrasound sub-surface probe microscopy device comprises a stage 10, a signal generator 20, a scanning head 30, a signal processor 50 and a scanning mechanism 16. In use, the stage 10 carries a sample 11 and a scanning mechanism 16 provides for a relative displacement between the sample 11 and the scanning head 30, along the surface of the sample. As shown in FIG. 1, the scanning mechanism 16 may therewith hold the scanning head 30 at a fixed position with respect to the reference frame, while displacing the stage 10 with the sample 11. Alternatively, the scanning mechanism 16 may therewith hold the stage 10 with the sample 11 at a fixed position with respect to a reference frame, while displacing the scanning head 30. As a further alternative, the scanning mechanism 16 may displace the stage 10 with the sample 11, for example in the x-direction, as well as the scanning head 30, for example in the y-direction, with respect to the reference frame.

[0047] As shown in more detail in FIG. 1A, the scanning head 30 comprises an actuator 31 arranged at flexible carrier 34, e.g. a cantilever, with tip 32, that is configured to generate in response to a drive signal S.sub.dr from the signal generator 20 an ultrasound acoustic input signal L.sub.ac. The generated ultrasound acoustic input signal I.sub.ac has at least one acoustic input signal component I.sub.ac1, with a first angular frequency ω1. The scanning head 30 further comprises a tip 32 to transmit the acoustic input signal I.sub.ac through a tip-sample interface 12 as an acoustic wave W.sub.ac into the sample. As shown in FIG. 2, the force (vertical axis) exerted by the tip 32 on the sample 11 in the tip-sample interface 12 is non-linearly related to the distance (horizontal axis) between the tip and the sample.

[0048] This non-linear interaction results in at least one upmixed acoustic signal component W.sub.ac2 in said acoustic wave that has a second angular frequency ω2 higher than the first angular frequency ω1.

[0049] In operation, the acoustic wave W.sub.ac propagates through the sample 11, and may reflect at sub-surface features 13 to be detected within the sample. The sub-surface features 13 may for example be structures in a lower arranged layer in a multilayer semiconductor device, with which features in a next semiconductor layer are to be aligned. A sub-surface feature in the lower arranged layer may for example be an electronic functional feature, e.g. a conductor or it may be a separate alignment mark. The scanning head 30 further comprises a sensor facility 35 to provide a sensor signal S.sub.sense. Contrary to known approaches, the sensor signal S.sub.sense provided by the sensor facility is indicative for a contribution of the at least one upmixed acoustic signal component in reflections W′.sub.ac of the acoustic wave within the sample 11. The contribution of the at least one upmixed acoustic signal component, having a second angular frequency ω2 higher than the first angular frequency ω1 enables detection with a higher resolution than the resolution that could be achieved when sensing an acoustic signal component at the first angular frequency ω1, corresponding to the frequency of the acoustic input signal I.sub.ac generated with the actuator 31. The signal processor 50 is to generate an image signal Sim in response to the sensor signal S.sub.sense. It is noted that the wording “image” used in “image signal” is to be interpreted according to the mathematical definition, i.e. the image signal is a function of the sensor signal. The image signal is indicative of observed features in the sample. The image signal may correspond to a two-dimensional image of those features, but may alternatively represent another type of image, e.g. a 1 dimensional image showing a height profile of those features along a scanning direction, or a higher dimensional image, e.g. a moving image having the time as a coordinate in addition to one or more spatial coordinates.

[0050] In practice, electronic components used in the transmission and reception chain may be different. For example, currently, state of the art arbitrary waveform generators cannot achieve the highest frequencies that theoretically could be used with piezo electric transducers. Therewith the electronic components used in the transmission and reception chain would form a bottleneck. Alternatively, in case of a substantial improvement in waveform generator technology without a corresponding improvement in transducer technology, the transducer would be a bottleneck. The present disclosure provides solutions to such restrictions.

[0051] In the embodiment of FIG. 1A, the sensor facility 35 of the scanning head 30 has a sensor 36 arranged at cantilever 39 with sensing tip 33. The sensor 36 in this embodiment is configured to directly sense the contribution W′.sub.ac2 of the at least one upmixed acoustic signal component in the reflections. The signal processor 50 may include a lock-in amplifier to lock in at the specific sensor signal component in the sensor signal S.sub.sense as the indicator for the contribution W′.sub.ac2 of the at least one upmixed acoustic signal component, i.e. a lock-in amplifier tuned to the second angular frequency ω2 of the at least one upmixed acoustic signal component.

[0052] In the embodiment of FIG. 1B, the sensor facility 35 provides for a further non-linear tip-sample interaction in the further tip-sample interface 14 between tip 33 and sample 11. The further non-linear tip-sample interaction in the further tip-sample interface 14 provides for a downmixing of the at least one upmixed acoustic signal component with another upmixed acoustic signal component originating from the non-linear tip-sample interaction in the tip-sample interface 12. This downmixing results in at least one downmixed acoustic signal component O.sub.ac3 with a third angular frequency ω3 lower than the second angular frequency ω2. Therewith the sensor facility 35 of the scanning head 30 is configured to indirectly sense the contribution W′.sub.ac2 of the at least one upmixed acoustic signal component by sensing the at least one downmixed acoustic signal component O.sub.ac3. In this embodiment the third angular frequency ω3 of the downmixed acoustic signal component O.sub.ac3 corresponds to a contact resonance frequency of a flexible carrier, here a cantilever 39 used to carry the tip 33 for sensing. This can be achieved by measuring the contact resonance frequency and then tuning the input signal S.sub.dr so that the downmixed signal matches this value.

[0053] Furthermore, the sensor facility 35 comprises a sensor 36B configured to sense a magnitude of resonance of the cantilever 39. In the embodiment shown, the sensor 36B is an optical sensor that measures a deflection of a laser beam BM generated by a laser 36A and reflected at the cantilever 39. Alternatively an acoustic sensor, e.g. formed by a piezo-electric element may be coupled to the cantilever 39 and may be used to sense the downmixed acoustic signal component O.sub.ac3 as the component indicative for a contribution W′.sub.ac2 of the at least one upmixed acoustic signal component in reflections W′ac of the acoustic wave within the sample.

[0054] Alternatively, the sensor facility 35 of the scanning head 30 may be configured to sense a downmixed signal component at a frequency differing from the contact resonance frequency of the flexible carrier. However, by selecting a frequency of the signal to be sensed that corresponds to the contact resonance frequency of the flexible carrier, the flexible carrier serves as a filter that selectively improves the sensitivity for the downmixed component to be sensed.

[0055] Whereas the improved microscopy device may be used to locate deliberately formed subsurface features, e.g. to enable alignment of other features therewith, it may also be used to detect any undesired subsurface features, like dust particles, e.g. for a product quality check.

[0056] In the embodiments shown in FIGS. 1A and 1B, separate facilities are provided for acoustic signal generation and sensing. This has the advantage that the elements used in these facilities can be optimized separately for each of these functions. For example, the shape of the tip 32 and the cantilever 34 as well as the contact pressure exerted by the tip 32 can be selected to maximize the amplitude of the at least one upmixed acoustic signal component W.sub.ac2 in the acoustic wave Way. Likewise the shape of the tip 33 and the cantilever 39 as well as the contact pressure exerted by the tip 33 can be selected to optimize sensing. Alternatively, however it may be contemplated to use one or more elements in common, for example a scanning head may be provided with a single flexible tipped carrier for transmitting the input acoustic signal and for sensing acoustic signal component(s) indicative for the contributions of W′.sub.ac2 of the at least one upmixed acoustic signal component in reflections W′.sub.ac of the acoustic wave within the sample. The acoustic input signal S.sub.ac generated with the actuator 31 may be either a continuous signal or a pulse signal. An advantage of using a continuous input signal is that it enables a higher throughput. An advantage of a pulse-wise operation is that it facilitates signal analysis. This is particularly the case for scanning arrangements using shared means for transmitting and receiving acoustic signals. In that case sensing can take place between subsequent pulses of the acoustic input signal S.sub.ac.

[0057] Several options are available to control the generation of the at least one upmixed acoustic signal component W.sub.ac2 in the acoustic wave W.sub.ac. This depends for example on the shape of the tip 32. The nature of the non-linear relationship can further be controlled dynamically by setting the average contact pressure. In the embodiment of FIG. 1, the average contact-pressure is maintained at a predetermined value by a servo-system, comprising a driver 51 and a z-actuator (not shown) that positions the scanning head 30 in the z-direction with respect to the surface 15 of the sample 11.

[0058] In the embodiment shown in FIG. 1, the microscopy device 1 comprises a force-distance analytic module 55 to perform a measurement for determining a relationship between a force F exerted to the tip and a resulting sample-tip distance d. Near a setpoint do for the distance d, selected for operation, the relationship can be approximated by

[00003]$F\left(d\right)=a_0+a_1*\left(d-d_0\right)+a_3*{\left(d-d_0\right)}^3$

[0059] Wherein a.sub.0>0, a.sub.1<0, a.sub.3>0. Or

[00004]$F\left(d\right)=a_0+a_2*{\left(d-d_0\right)}^2$

[0060] Wherein: a.sub.0<0, a.sub.2>0.

[0061] Therewith the nature of the non-linearity can be controlled by a proper selection of the set-point d.sub.0.

[0062] Subsequent to a determination of the force-distance relationship, the operator provide a control signal C.sub.H/Q to the force-distance analytic module 55 to specify a desired type of non-linearity, for example Hertzian or Quadratic and the force-distance analytic module 55 determines which set-point for the distance d is required to achieve the required non-linearity. For example, if a Hertzian non-linearity is specified it may determine d.sub.0=d.sub.01 as the set-point, and if a Quadratic non-linearity is required, it may determine d.sub.0=d.sub.02 as the set-point.

[0063] The signal processor 50 issues an input signal S.sub.z to the driver 51 indicative of a measured contact pressure and the driver 51 provides a control signal C.sub.z to the z-actuator to maintain the measured contact pressure close to a value required for a particular non-linear behavior in the tip-sample interface 12 in accordance with the specification by the force-distance analytic module 55.

[0064] In an exemplary mode of operation, the non-linear relationship between distance and force is of a quadratic nature. The acoustic input signal I.sub.ac may comprise a first acoustic input signal component I.sub.ac1 having an angular frequency ω1. Due to the quadratic relationship, the acoustic wave W.sub.ac can be written as

[00005]$\mathrm{Wac}=\alpha .Math.({\sin \left({\omega }_1.Math.t\right)}^2=1/2(1-\cos \left(2{\omega }_1.Math.t\right),$

wherein α is a constant.

[0065] Accordingly, the acoustic wave W.sub.ac includes an upmixed acoustic signal component W.sub.ac2 with an angular frequency double that of the component provided as the acoustic input signal.

[0066] The acoustic input signal I.sub.ac may include more components to therewith further change the properties of the acoustic wave. For example, as shown in FIG. 3A, the acoustic input signal I.sub.ac may comprise a first acoustic input signal component I.sub.ac1 having an angular frequency ω1 and a second acoustic input signal component I.sub.ac2 having an angular frequency ω.sub.2 with ω.sub.2<<ω1. The input signal is defined as the multiplication of the two input components signals I.sub.ac1 and I.sub.ac=I.sub.ac1×I.sub.ac2. In that case, as shown in FIG. 3B, the input signal I.sub.ac consists of the two input angular frequencies ω.sub.1−ω.sub.2 and ω.sub.1+ω.sub.2, the acoustic wave W.sub.ac from the tip-sample interface with quadratic behavior comprises upmixed components with an angular frequency of 2ω.sub.1, 2(ω.sub.1−ω.sub.2), 2(ω.sub.1+ω.sub.2), as well as aDC component (ω=0) and a downmixed component 2 ω.sub.2.

[0067] In another exemplary mode of operation, the non-linear relationship between distance d and force F is characterized as Hertzian, wherein variations ΔF in the force F are approximately related to variations Δd in the distance d as

ΔF=β(Δd).sup.1.5, wherein β is a constant.

In this case acoustic wave W.sub.ac with a rich spectrum is achieved with also includes acoustic components having substantially higher frequencies, as is shown in FIG. 3C.

[0068] Accordingly, by generating the acoustic input signal I.sub.ac with one, two or more acoustic signal components, and controlling the nature of the non-linear interaction in the tip-sample interface, the spectrum of the acoustic wave W.sub.ac can be shaped as desired. Moreover properties of the acoustic signal components can be controlled, for example including their amplitude, their angular frequency and their relative phases.

[0069] According to one approach, a simulation can be performed. For example such a simulation may be performed as a brute-force approach to determine which from all possible combinations maximizes the desired upmixed component (performance metrics can be amplitude, SNR, amplitude of other components in a given bandwidth). Known optimization techniques may be used to converge faster. Also certain boundary requirements may be specified, for example the requirement that the frequencies of the involved components are at least 1/10.sup.th the angular frequency of the desired component.

[0070] In the embodiment of the microcopy device shown in FIG. 4, a feedback unit 70 is provided. The feedback unit 70 receives a feedback signal F.sub.spectr, that is indicative of a property of the sense signal S.sub.sense. The sensed property as indicated with the feedback signal F.sub.spectr is for example a magnitude of a contribution of the at least one upmixed acoustic signal component W.sub.ac2 that is sensed directly or indirectly with the sense signal S.sub.sense. Based on the a feedback signal F.sub.spectr, the feedback unit 70 controls the signal generator 20 with a control signal C.sub.s. The feedback unit 70 may therewith control the signal generator to generate the drive signal S.sub.dr with a particular number of signal components, and/or control properties of one or more of these components, such as their amplitude, their angular frequency and their mutual phase relationship. In the embodiment shown, the feedback unit 70 further controls a property of the non-linear transmission at the tip-sample interface 12 by controlling with signal C.sub.zz a set point force exerted by the transmitter tip 32 to the sample 11.

[0071] The feedback unit 70 may for example operate as follows: [0072] Prepare a first input pulse based simulation, given tip, sample parameters [0073] Quickly iterate close to the optimum pulse in the feedback unit to ensure that the signal amplitude is always maximal.
The iteration preferably continues during operation of the microscope, preferably for each scanning point to adapt to changes in the surface of the sample along the x/y scanning trajectory.

[0074] The feedback unit 70 may have one or more of the following potential operational modes: [0075] A first operational mode to optimize imaging resolution; [0076] A second operational mode to optimize a signal to noise ratio (SNR); [0077] A third operational mode to minimize an attenuation;

[0078] The feedback unit 70 may further be configured to operate dependending on an input signal indicative for a property of the sample. The input signal may for example be provided by an operator or may be obtained from measurements.

[0079] A method for operating an ultrasound sub-surface probe microscopy device 1, for example as shown in FIG. 1, 1A, 1B and/or FIG. 4 may comprise the following steps: [0080] carrying a sample 11; [0081] generating a drive signal S.sub.dr; [0082] generating in response to said drive signal S.sub.dr an ultrasound acoustic input signal I.sub.ac having at least one acoustic component I.sub.ac1, with a first angular frequency and using a non-linear tip-sample interaction to transmit the acoustic input signal I.sub.ac as an acoustic wave W.sub.ac into the sample 11 at a contact position on the sample, the acoustic wave W.sub.ac having at least one upmixed acoustic signal component W.sub.ac2 of a second angular frequency higher than said first angular frequency as a result of said non-linear interaction; [0083] providing a sensor signal S.sub.sense indicative for a contribution W′.sub.ac2 of the at least one upmixed acoustic signal component in reflections of the acoustic wave within the sample; [0084] generating an image signal Sim in response to the sensor signal S.sub.sense and; [0085] displacing the contact position along the surface of the sample.

[0086] The exemplary method may further comprise the step of post processing the image signal. For example, said post-processing may include one or more of extracting critical dimensions, computing an accuracy with which specified dimensions are complied with, computing an accuracy with which elements in mutually different layers are overlaid.

[0087] The method may comprise a preliminary step, wherein a property of the drive signal S.sub.dr is configured for optimizing a property of the sensor signal at a fixed contact position of the sample, for example at a location of predetermined marking provided as a subsurface feature in the sample. Also an optimal reference value for the contact pressure may be determined in this preliminary step. A feedback unit 70 may be used for this purpose. After this preliminary step, the actual scanning process may start with the settings determined in the preliminary step.

[0088] While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom within the scope of this present invention as determined by the appended claims. Therein the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single component or other unit may fulfill the functions of several items recited in the claims. 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 advantage. Any reference signs in the claims should not be construed as limiting the scope.