Measuring probe for examining surface areas

Description

[0001] The invention relates to a measuring probe for examining surface areas of an object, in particular by scanning surface areas of the object, comprising a substrate, a tip for interacting with the object and a transducer for emitting acoustic signals into the object, wherein the substrate carries the tip and the transducer.

[0002] Furthermore, the invention relates to an atomic force microscopy method for examining surface areas of an object, in particular with a device of the above type, wherein a measuring probe, which comprises a substrate, a transducer and a tip, interacts with the surface of the object through the substrate attached to one side and the tip arranged thereon, while an excitation source is directed onto the transducer.

[0003] In addition to devices for imaging examination, a large number of other devices are known from the state of the art with which surface areas of an object can be examined.

[0004] In this context, devices are also used that either interact with the surface for characterization purposes, such as atomic force microscopy for examining the topography of an object, or enable the measurement of near-surface bulk properties by emitting acoustic signals, as is known from acoustic microscopy. Acoustic atomic force microscopy combines these microscopy techniques and enables the measurement of both topography and near-surface bulk properties. The measurement is applicable to a wide range of objects and is non-destructive.For this purpose, measuring probes with a cantilever and a small tip attached to it, which has a radius of curvature of only a few nanometers, are used. The bending of the cantilever allows direct conclusions to be drawn about the physical forces acting between the tip and the object. The bending modes of the cantilever of a measuring probe are used to excite the cantilever itself into oscillation, so that its natural resonant frequency, which for a cantilever less than one millimeter long is usually in the ultrasonic range, is transmitted via the tip of the measuring probe to the object under investigation. vice versa. Typically, this results in a transmission of frequencies in the range of a few kilohertz (kHz) to a few megahertz (MHz). Ultrasonic force microscopy focuses precisely on this frequency range and considers not only the bending modes of the cantilever and the associated natural resonance frequency, but also external ultrasound sources.

[0005] For example, electrically excitable transducers made of piezoelectric materials can be installed to cause the measuring probe to vibrate. This further increases the frequency range of the transmitted vibrations, so that frequencies between several MHz and a few gigahertz (GHz) can be transmitted from the measuring probe to the object under investigation. As described in DE 103 21 931 A1, such a transducer can be installed either on the mounting of the measuring probe or on the object holder, thereby transmitting the vibrations from the measuring probe to the object on the one hand, and from the object to the measuring probe on the other. Although the method disclosed in this document is used for the qualitative analysis of cantilevers, it clearly demonstrates that vibrations can be transmitted from the object to the measuring probe both in fluids and contactlessly via a gaseous medium.

[0006] In the opposite case, as described in EP 3 349 018 A1, the tip of the measuring probe comes into contact with the object surface, exerting additional repulsive forces on it and allowing surface information to be collected in parallel with the vibration transmission. Vibrations of the measuring probe are detected by irradiating the back of the cantilever, approximately at the level of the attached tip, with a laser, and the reflection of this laser is evaluated by an optical detector. Such a device also comprises a piezoelectric transducer that transmits acoustic signals to the object via the measuring probe. Signals reflected back from the object influence the vibrations of the measuring probe in such a way that structures located beneath the surface can be analyzed. Alternatively, this can also be used to detect the vibrations of the measuring probe, whereby the device would operate without a laser source.

[0007] EP 3 543 712 A1 describes a device that utilizes the photothermal effect and provides an excitation source, in particular a laser, to cause a spring bar of a measuring probe to oscillate. A tip of the measuring probe can operate both contactlessly and in contact with the object surface. An optical detector is provided to detect the oscillations of the measuring probe. This detector detects laser pulses from another laser source reflected by the measuring probe. The excitation source can also perform the function of the laser source.

[0008] WO 2021 / 028174 publishes an AFM measuring probe that is used with a transducer on a top surface, whereby the photoacoustic effect is to be used by a laser beam and the transducer is to be excited to emit ultrasonic signals.

[0009] US 5,319,977 discloses a microscopy setup with a measuring probe and a thin-film transducer mounted thereon.

[0010] US 9,482,691 B1 describes the use of multiple quantum wells to emit laser light.

[0011] The invention is based on the object of specifying a measuring probe of the type mentioned at the outset, with which acoustic signals with high frequency and high quality can be emitted, transmitted and detected with small changes in the measuring probe.

[0012] Furthermore, it is an object of the invention to provide a method of the type mentioned at the outset with which a measurement with high lateral and axial resolution is possible even with a simple apparatus design.

[0013] The first object is achieved according to claim 1 in that, in a measuring probe of the type mentioned at the outset, the transducer is constructed from several layers, wherein at least one layer can be excited to emit acoustic signals and the tip is designed to transmit acoustic signals to the object.

[0014] One advantage achieved in this way is that such a multilayer transducer can be applied directly to the substrate and / or the tip. Furthermore, through physical or chemical vapor deposition, particularly molecular beam epitaxy, it can be made so thin that the original function of the measuring probe is retained. Accordingly, the application of the transducer layers does not, or only slightly, impair the flexibility of the measuring probe, and thus does not significantly impair its vibration behavior, which is why it can be used with the usual precision to measure the topography of an object.

[0015] Furthermore, the layer structure of the transducer offers the possibility of providing not just one excitable layer for emitting acoustic signals, but several such layers, which can interact with each other during signal transmission. Alternatively, these layers can also be designed to emit acoustic signals with different frequencies and polarizations, so that they provide information about different surface areas, especially at different depths within the object, and thus different properties of the object can be detected with one and the same measuring probe. In general, however, the acoustic signals must primarily be transported from the emitting transducer layer to the object.This task is performed by the tip, where the acoustic signals are not only transmitted but also focused on a contact surface with the object, and the high frequency ensures that they remain focused within the object. Accordingly, the tip ensures high spatial resolution when measuring the surface and near-surface areas of the object.

[0016] The transducer can also be arranged or deposited above the tip, thus extending, in particular flatly, perpendicular to a longitudinal axis defined by the tip. The tip can be designed as a cone, a downward-facing pyramid, a truncated cone, or the like. The tip can be supported directly or indirectly by the substrate. If the transducer is located above the tip or its extension, short paths are created for the conduction of acoustic signals for excitation. Furthermore, the substrate or cantilever remains largely unaffected in terms of functionality.

[0017] Ideally, the substrate has a thickness between 0.2 µm and 3 µm. If the transducer is mounted on the substrate and not directly on or in the tip, the emitted acoustic signals must penetrate the substrate and tip to reach the object. Accordingly, thick substrates and high frequencies can lead to attenuation of the emitted acoustic signals, which would affect the intensity of both emitted and reflected acoustic signals.

[0018] It is also expedient to make the substrate from an elastic material so that the measuring probe can be used for all operating modes of a commercially available atomic force microscope. The substrate is therefore preferably made of silicon and / or silicon nitride (Si 3 N 4 ), which makes it particularly resistant to mechanical wear despite its high flexibility. In addition, silicon offers the possibility of absorbing and / or reflecting large portions of electromagnetic radiation with a wavelength <1 µm, while at the same time maintaining high transmittance for radiation with a wavelength >1 µm. Using Si 3 N 4 opens up a wider transmittance range, as it also has high transmittance in the optical wavelength range from 400 nm to 1 µm, thereby expanding the range of applications.Consequently, the use of this material also opens up the possibility of using lasers with shorter wavelengths. Alternatively, semiconductors with large band gaps, such as aluminum nitride (AlN), gallium nitride (GaN), and silicon carbide (SiC), can be used to manufacture the substrate, as they have similar transmittance and attenuate acoustic waves only to a small extent. In order to be able to align a radiation source as an excitation source with sufficient precision onto the substrate, the substrate is preferably between 10 µm and 80 µm wide and has an essentially rectangular, triangular, or trapezoidal basic shape. This allows for particularly simple alignment and thus focusing of the radiation source.In general, the substrate, and thus also the measuring probe, is attached to a distal end in an atomic force microscope, with the tip usually attached to an opposite end of the substrate and a radiation source such as a laser aimed at a point on the substrate near the tip.

[0019] It is advantageous if the tip has a height between 1.0 µm and 15 µm, preferably between 1.5 µm and 10 µm, in particular between 2.0 µm and 8.0 µm. In this case, the tip is usually conical with a circular, square, rectangular or polygonal base and is attached to the underside of the substrate. In an alternative variant, the tip can also represent part of the substrate and thus be formed as a single piece. Accordingly, the tip is preferably made of silicon, Si 3 N 4 , AIN, GaN and / or SiC, although it can also be constructed in multiple layers. This offers the possibility of integrating the transducer into part of the tip, preferably in the half closer to the object. The special shape of the tip allows acoustic signals emitted by the transducer to be focused towards the object.In particular, there is an interaction between the focusing of the radiation source, the height of the tip, and the opening angle of the tip, whereby a change in one parameter can lead to an adjustment of the other two parameters. If a small opening angle is selected for a comparatively high tip, the conversion of the signals passing through the tip into other mode forms at the walls of the tip can be successfully reduced. Alternatively, the focusing area of ​​the radiation source can be selected to be smaller, thus reducing the height of the tip while maintaining the same opening angle. This interaction ensures total internal reflection of the radiation within the tip, so that any loss of intensity of the incoming radiation can be kept particularly low.In addition, the focusing properties of the tip can be used not only for the radiation exciting the transducer, but also for the acoustic signals emitted by the transducer. Consequently, the path traveled by the acoustic signals in the tip is reduced, thus increasing their maximum penetration depth into the object.

[0020] It has proven useful to have a flattened tip at the pointed end with an opening angle of between 5° and 40°, preferably between 15° and 35°. The opening angle results from the imaginary extension of the side surface(s). Depending on the opening angle and / or a different size of the contact area between tip and object, the acoustic waves generated by the transducer interact differently with the object. It was found that contact areas that are many times larger than the wavelength of the emitted acoustic signals lead to a quasi-one-dimensional propagation of these signals into the object. Accordingly, the acoustic signals in the tip are not only focused in such a way that a high signal-to-noise ratio is achieved when measuring structures hidden deep below the surface, but also remain focused in the object during propagation.This setup therefore also results in high intensity of the reflected signals and excellent spatial resolution of the structures located beneath the surface. If the contact area is selected with a size that is smaller than or equal to the wavelength of the emitted acoustic signals, the signals will propagate spherically within the object. While this allows near-surface properties and / or structures of the object to be detected with high accuracy, this method is only suitable to a limited extent for detecting structures hidden deep beneath the surface. Consequently, if the contact area is selected too small, the penetration depth of the signals is reduced, resulting in a loss of both the spatial resolution of the deeper structures and the intensity of the signals transmitted into the object.The latter is due to the spherical propagation of the signals within the object, which prevents them from being reflected back to the tip. Accordingly, the wavelength of the acoustic signals can be selected to selectively detect different structures buried deep within the object. Alternatively, while maintaining the same contact area, the wavelength of the acoustic signals can be adapted to detect structures at different depths.

[0021] According to the invention, at least one layer is formed as a structure that is periodic in the propagation direction, in particular as a multiple quantum well. The periodic structures are designed such that, upon excitation, they emit acoustic signals at very high frequencies, particularly in the GHz to terahertz (THz) range. Furthermore, these periodic structures are also very well suited for generating high-frequency acoustic signals, since the width of the wells and barriers present in the structure can determine a frequency spectrum of the emitted signals. For example, a lifetime of -140 picoseconds was found for coherent acoustic phonons with a frequency of 1 THz in GaN, during which an average path length of ~1 µm is traveled. Furthermore, a low coherence loss was also demonstrated for these path lengths, so that a low attenuation effect can be assumed when transmitting such signals.

[0022] It is advantageous if the multiple quantum well is formed from a superstructure of at least two materials, preferably GaN and indium nitride (InN), so that it emits a wave packet of coherent sound quanta upon excitation. The superstructure utilizes the inverse piezoelectric effect to emit wave packets of coherent sound quanta. An excitation source, such as a laser, with photon energies greater than the electronic band gap in the quantum wells is used to excite charge carriers that shield an intrinsic piezoelectric field. The latter field is created by intrinsic strain of the not entirely perfectly fitting lattice structures of the two piezoelectric materials grown in the superstructure. Thus, the excitation duration and the period of the superstructure determine the wavelength of the wave packets of coherent sound quanta and, subsequently, their frequency.To increase the frequency of the emitted wave packets of coherent sound quanta, excitation sources with particularly short excitation pulses, particularly femtosecond lasers, are used. In the present measuring probe, InGaN / GaN superstructures with a period of, for example, 8 nm are preferably used as multiple quantum wells, as these can be excited by femtosecond lasers to emit wave packets of coherent sound quanta with a central frequency of 1 THz. If the period is adjusted so that wave packets of coherent sound quanta are emitted with a frequency of 0.1 THz, a lifetime of more than 10 ns results, which, over a travel distance of a few micrometers, results in a sufficiently low loss of coherence of the wave packets reflected from the object. However, the loss of amplitude and coherence of the wave packets depends strongly on the object itself, so it can be arbitrarily large.

[0023] It has proven useful for a device for examining surface areas of an object, in particular by scanning surface areas of the object, to comprise at least one excitation source and a measuring probe according to the invention. The measuring probe has a transducer for emitting acoustic signals, which is constructed from multiple layers, wherein at least one layer can be excited by the excitation source to emit an acoustic signal. The excitation source is preferably designed to emit electromagnetic radiation, which is not only reflected by the surface of the substrate or the transducer and detected by a photodetector, but also excites at least one layer of the transducer such that it emits acoustic signals that are transmitted to the object via the tip.The excitation source excites the layer's multi-quantum wells, emitting wave packets of coherent sound quanta. These are focused via the tip and used to detect hidden structures beneath the object's surface. In such a device, the object is placed on a sample stage and scanned by the measuring probe above it. The scanning process requires adjustment in the x, y, and z directions and can occur in two ways: Either the entire measuring probe assembly, comprising the measuring probe holder, measuring probe, excitation source, and detector, is moved and scans the object's surface, or the sample stage moves the object beneath the stationary measuring probe assembly. The first variant has the advantage that the objects to be examined only need to be insertable into the device, and there are no further requirements regarding their shape and size.The latter variant requires only a piezoelectric motion unit for the sample stage, simplifies the setup of the device compared to the former, and is also less prone to errors. Although the device can be implemented in either version, the latter is preferred.

[0024] The excitation source is expediently designed to emit laser pulses, preferably femtosecond-long. This results in particularly high-frequency wave packets of coherent sound quanta, particularly in the THz frequency range, being emitted from the at least one excitable layer of the transducer. Furthermore, the wave packets of coherent sound quanta emitted by the multiple quantum wells can be adapted to the material under investigation by varying the period length of the superlattice of the multiple quantum wells, with particular emphasis on the penetration depth of the wave packets of coherent sound quanta into the object.

[0025] It is advantageous if the excitation source is designed as a pulsed solid-state laser, preferably as a titanium:sapphire laser, particularly preferably as an erbium (Er), ytterbium (Yb), or neodymium (Nd)-doped fiber laser. Preferred solid-state laser variants are designed to generate laser pulses in a wavelength range that is transmitted virtually unaffected by the probe parts made of silicon, Si3N4, AlN, GaN, and / or SiC. In these materials, radiation is transmitted at wavelengths above 1 µm, which is why the laser types mentioned above are considered particularly suitable. Furthermore, the frequency doubling phenomenon can also emit laser pulses that have only half the wavelength of the original laser pulse.For this purpose, a partial beam of the laser is usually directed at a doubling material, usually birefringent crystals, so that the oscillating polarization in these generates secondary radiation of twice the frequency. If the refractive indices of the original laser pulse and the generated secondary radiation are the same, both interact via constructive interference and are emitted in the same direction. Crystals made of bismuth borate (BiB 3 O 6 ,(BBO)), lithium iodate (LiIO 3 ) and periodically poled lithium niobate (LiNbO 3 ,(PPLN)) are preferred as doubling materials for ultrafast lasers with particularly short pulse lengths. The partial beam of doubled frequency generated in this way can be used to sample reflected sound waves, whereas the other, unchanged partial beam is used to excite the transducer.

[0026] The excitation source is preferably designed to conduct a pump-probe experiment. This allows the multiple quantum wells in at least one layer of the transducer to be excited by the pump laser pulse emitted by the excitation source, and the oscillation behavior of the measuring probe to be detected with a time delay by a probe laser pulse. The wavelength of the excitation source is selected such that a pump laser pulse lies in a wavelength range >1 µm and the probe laser pulse, through frequency doubling, in a wavelength range <1 µm. Preferably, an Er-, Yb-, or Nd-doped fiber laser is used, which emits femtosecond-long pump laser pulses with a frequency of -1.5 µm. By beam splitting, a portion of the pump laser pulse is then used and guided to the measuring probe with a time delay via a delay stage.To achieve a variable delay between the pump and probe laser pulses, a shaker can be additionally installed along the pump laser pulse path, which changes its path length incrementally or periodically. The probe laser pulse reflected by the measuring probe is analyzed using a detector, particularly a photodetector. Consequently, the temporal resolution of the measurement apparatus is largely determined by the time difference between the pump and probe laser pulses, which, when using femtosecond lasers, is typically between a few femtoseconds and a few picoseconds.

[0027] The second task is solved by the excitation source stimulating at least one layer of the multi-layered transducer to emit acoustic signals in order to transmit them to the object via the tip.

[0028] The tip comes into contact with the object, whereby the acoustic signals are directly picked up by the object and interact with the structures below the object's surface. A laser source designed to emit ultrashort laser pulses is used as the excitation source. To optimize the resolution of the process, a femtosecond laser can be used as the excitation source, which selectively excites a layer of the transducer to emit acoustic signals. The emitted signals are transmitted directly to the tip and from there to the object. While some of the acoustic signals penetrate further from the surface into the object, the other part is reflected by the structures mentioned above and finds its way back to the tip. These reflected signals cause the tip, and thus also the measuring probe, to vibrate and can therefore be detected.Typically, a radiation source and a detector unit, usually a photodetector, are used to detect these vibrations. The radiation is reflected from the surface of the substrate, allowing even the slightest vibrations of the measuring probe to be detected. Combined with the tip's particularly small contact area on the object, this results in a high spatial resolution of the measurement. Consequently, the tip comes into contact with the surface and not only transmits information about acoustically detected structures located a few nanometers to a few micrometers below the surface, but also simultaneously provides information about the surface structure of the object.In addition to volume and area defects, which are measured directly via reflection, the subsurface structures also include line and point defects, which can be spatially resolved due to their influence on the coherence and thus the attenuation of the signal. Furthermore, using a method according to the invention, an image of the elastic and plastic properties of a near-surface region, particularly with a layer thickness of up to 100 nm, can be created with an inaccuracy of only a few percent.

[0029] Advantageously, laser pulses emitted by the excitation source are separated into test and pump laser pulses. This allows a single excitation source to be used, with the previously described frequency doubling being applied. This simplifies the process in that no additional radiation source is required for detection. The excitation source, preferably a femtosecond laser, emits a pump laser beam with a high wavelength, preferably greater than 1 µm. This beam is subsequently split by a beam splitter such that one part can be passed on as a pump laser pulse and the second part as a test laser pulse (or corresponding partial beams). While the pump laser pulse is directed directly to the substrate and / or the transducer of the measuring probe, the test laser pulse is redirected via a delay stage.The probe laser pulse is delayed so that a time window is created between excitation and detection. This time window can be adjusted by the delay stage to suit the type of object, the properties being examined, or the type of transducer layer emitting acoustic signals. Alternatively, a shaker can be installed in the path of the pump laser pulse to periodically or stepwise vary the time interval between the pump and test laser pulses. The time window between excitation and detection is crucial for observing the propagation of acoustic signals into the object and the reflection of acoustic signals from the object into the probe.Assuming that the time interval between the laser pulses is preferably adapted to the path length traveled by the signals, a delay in the range of a few nanoseconds is usually chosen for an average path length of a few micrometers.

[0030] It has proven effective to emit wave packets of coherent sound quanta by exciting a multiple quantum well contained in the layer. This allows both the surface and regions a few micrometers below the surface to be analyzed. A correspondingly high vertical resolution results from the wavelength of the wave packets, which depends significantly on the period of the multiple quantum wells and their excitation time by the laser pulse. Wave packets of coherent sound quanta transmitted to the object interact with the subsurface structures in such a way that they can be reflected and detected in a modified form, thus allowing concrete statements to be made about a multitude of the object's properties.These include surface and bulk defects, as well as regions with distributions of point and line defects larger than the wavelength of the wave packets of coherent acoustic quanta. Compared to other methods, little to no sample preparation is required to analyze the object using a method according to the invention. Furthermore, unlike other analysis methods, no special atmosphere, such as a vacuum or inert gas, is required to perform the measurement efficiently.

[0031] It is advisable to emit wave packets of coherent sound quanta with a frequency between 0.1 THz and 2 THz, preferably between 0.1 THz and 1.5 THz, and particularly preferably between 0.1 THz and 0.5 THz. This results in wave packets of coherent sound quanta with wavelengths of a few nanometers. Wave packets of coherent sound quanta with wavelengths in this range enable a temporal resolution of a few picoseconds, allowing for the precise analysis of thin layers in the micrometer to nanometer range. This allows for the determination of properties such as elastic constants, density, defects, phonon dispersion, and phonon surface and interface scattering of a material.Surface and interface scattering of phonons plays a crucial role, particularly with regard to temperature development in micro- and nanometer-sized structures, which are used, for example, in the semiconductor industry. The combination of high-frequency wave packets of coherent sound quanta with excellent vertical resolution and a probe tip that focuses the wave packets of coherent sound quanta also achieves excellent spatial resolution of the measurement method, allowing for spatially resolved imaging of the object's properties on a length scale in the micrometer to nanometer range.

[0032] It has proven effective to determine the penetration depth of the wave packets of coherent sound quanta into the object by the tip's contact area, which, for a given length and width, results in the attachment or connection area of ​​the substrate, particularly from its aperture angle. This allows the signal transmission from the tip to the object to be modified in such a way that overlaps in the signal caused by reflected and emitted wave packets of coherent sound quanta are reduced. In addition, by tuning the frequency of the wave packets of coherent sound quanta to the tip's contact area, a choice can be made between quasi-1D and spherical propagation of the wave packets of coherent sound quanta in the object. In this way, structures lying deeper below the surface can be focused with the help of quasi-1D propagating wave packets of coherent sound quanta, thus achieving a high signal-to-noise ratio.Such propagation is achieved by ensuring that the contact area is many times larger than the wavelength of the wave packets of coherent sound quanta. If the contact area and the wavelength of the wave packets of coherent sound quanta differ only slightly, spherical propagation of the wave packets of coherent sound quanta can be expected within the object, which results in higher resolution of near-surface structures.

[0033] Further features, advantages, and effects of the invention will become apparent from the following exemplary embodiment. The drawings, to which reference is made, show: Fig. 1 a perspective view of a measuring probe; Fig. 2 a sectional view of a measuring probe according to Fig. 1 ; Fig. 3 a perspective view of a device; Fig. 4 a sectional view of a device according to Fig. 3 ; Fig. 5 a perspective view of a device with a further embodiment of a measuring probe; Fig. 6 a sectional view of a device according to Fig. 5 ; Fig. 7 and Fig. 8 Diagrams of measurements.

[0034] Fig. 1 shows a perspective view of a measuring probe 1 for examining surface areas of an object 2, in particular by scanning surface areas of the object 2. The structure of a measuring probe 1, which comprises a substrate 3, a tip 4 and a transducer 5, can be seen here. The substrate 3 and the tip 4 are preferably made of silicon, Si 3 N 4 , AlN, GaN and / or SiC, which ensures high abrasion resistance of the tip 4 while at the same time ensuring high flexibility of the substrate 3. It is also clearly visible that the transducer 5 is made up of several layers, of which one layer 6 can be excited to emit acoustic signals and the same or a further one serves to detect the acoustic signals. Alternatively, several layers 6 can be provided for emitting acoustic signals and the transducer 5 can also be designed to be significantly flatter, for example using layers with a thickness of a few nanometers.The transducer 5 is therefore preferably applied to the substrate 3 by physical or chemical vapor deposition, in particular by molecular beam epitaxy. Piezoelectric materials such as GaN or InN or mixtures of these two materials are preferably used here. The layer 6 is expediently designed as a superstructure of the two aforementioned materials and forms a multiple quantum well 7, which can be excited to emit wave packets of coherent sound quanta 8. Alternatively, the transducer 5 can also be arranged on the tip 4 or within it. The tip 4 is also designed such that it focuses acoustic signals emitted by the layer 6. The geometry of the tip 4 is used here, which in the present embodiment corresponds to a pyramid with a square base.Alternatively, conical tips 4 with a circular, rectangular, or rounded base can also be attached to the substrate 3. Furthermore, a one-piece construction of substrate 3 and tip 4 is also conceivable, whereby this would provide the same starting material for substrate 3 and tip 4. In the present illustration, substrate 3 and tip 4 can be formed from different materials.

[0035] Fig. 2 shows a sectional view of a measuring probe 1 according to Fig. 1 along line II-II. The layer structure of the transducer 5 is clearly visible, with layer 6 being excitable to emit acoustic signals. The structural design of layer 6 is shown in the circular inset. It can be seen that layer 6 consists of multiple quantum wells 7, which are formed to emit wave packets of coherent sound quanta 8. Furthermore, the schematic representation of the multiple quantum wells 7 within the circle shows exactly one period of the superstructure contained in layer 6, whereby by changing the superstructure, the period of the multiple quantum wells 7 and thus the wavelength and thus the frequency of the wave packets of coherent sound quanta 8 can be modified.This illustration also shows that the tip 4 has a flattened tip with a larger contact surface 11, which is defined by the length of the tip 4 on the one hand and the shown opening angle α on the other. The contact surface 11 between object 2 and tip 4 influences, on the one hand, the spatial resolution of the measuring probe 1 and, on the other hand, the propagation of the wave packets of coherent sound quanta 8 in the object 2, so that with a measuring probe 1, depending on the ratio of contact surface 11 and wavelength of the wave packets of coherent sound quanta 8, a detection of deep-lying or near-surface structures of the object 2 can be carried out.

[0036] In Fig. 3 A device 9 for examining surface areas of an object 2, in particular by scanning surface areas of the object 2, is shown in perspective. The substrate 3 of the measuring probe 1 is fixed to a holder at an end opposite the tip 4 and an excitation source 10 is directed onto the transducer 5, which in this embodiment is arranged on the substrate 3. In contrast to a measuring probe 1 from Fig. 1 The layer structure of the transducer 5 here contains two layers 6, which can be excited to emit and detect wave packets of coherent sound quanta 8. For this purpose, an excitation source 10 is provided on or in the device 9, which is directed directly onto the top side of the transducer 5. The excitation source 10 comprises a femtosecond laser, which is designed to emit pump and probe laser pulses, wherein the pump laser pulse is shown as a central line with double the line width and the probe laser pulse encloses a specific angle to it and, as can be seen from the arrows of the beams, is reflected by the measuring probe 1. However, the course of the laser pulses serves only as a schematic representation, and their actual course can sometimes deviate significantly from this.Alternatively, a second radiation source can be used to generate probe laser pulses, so that the excitation source 10 emits only the pump laser pulse. The reflected probe laser pulse is detected by a detector unit (not shown in this figure), allowing the analysis of the object 2 to be performed by inferring the vibrations and the position of the measuring probe 1.

[0037] Fig. 4 shows a sectional view of a device 9 according to Fig. 3 along line IV-IV. It can be seen that the probe laser pulse is partially reflected by the surface of the transducer 5, and the other part, like the pump laser pulse, penetrates into the transducer 5 up to the first layer 6. The pump laser pulse excites the multiple quantum wells 7 contained in layer 6, but not shown, to emit wave packets of coherent sound quanta 8, which are transmitted through the substrate 3 and the tip 4 to the object 2. Furthermore, the two layers 6 can be designed such that they can be excited by different pump laser pulses and / or interact with each other, so that different wave packets of coherent sound quanta 8 can be emitted. In addition, the emitted wave packets of coherent sound quanta 8 are focused by the tip 4, so that the spatial resolution during the analysis of the object 2 is improved.The circular inset depicts the contact surface 11 between the tip 4 and the object 2. This illustrates how the wave packets of coherent sound quanta 8 from the tip 4 enter the object 2 via the contact surface 11, which is defined by the aperture angle α, and are at least partially reflected by structures beneath the surface of the object 2. These are subsequently transmitted back to the tip 4 and thus to the measuring probe 1 and are detectable by the probe laser pulse due to the influence of the wave packet of coherent sound quanta 8 on the reflection of the layer 6.By adjusting the contact area 11 between tip 4 and object 2, the period of the multiple quantum wells 7 and the total thickness of the superlattices forming the multiple quantum wells 7 in the layer 6, the wavelength and spatial localization of the wave packets of coherent sound quanta 8 and thus the spatial resolution in the vertical direction can be determined in a range from several nanometers to a few micrometers.

[0038] Fig. 5 shows a perspective view of the device 9 with a further embodiment of the measuring probe 1. Here, the converter 5 is integrated directly into the tip 4 of the measuring probe 1. This offers the advantage that the pump laser pulse is additionally focused from the tip 4 onto the layer 6. Accordingly, both the substrate 3 and the tip 4 must be made of a material that is permeable to the laser pulse, with silicon, Si 3 N 4 , AlN, GaN, and / or SiC being preferred.

[0039] This additional focusing of the pump laser pulse at the tip 4, based on total internal reflection, eliminates the need for particularly precise alignment of the excitation source 10. Furthermore, the penetration depth of the wave packets of coherent sound quanta 8 is increased by the gained proximity to the object 2, enabling the analysis of even deeper structures.

[0040] Fig. 6 shows a sectional view of a device 9 according to Fig. 5 along the line VI-VI. It can be seen that the pump laser pulse of the excitation source 10 penetrates the substrate 3 and a part of the tip 4 before reaching the converter 5 for the excitation of the layer 6. The excitation process of the multiple quantum wells 7 contained in the layer 6 by the femtosecond pump laser pulse proceeds analogously to the representation in Fig. 4 .

[0041] In this figure, only one layer 6 is shown, but alternatively, multiple layers 6 can be arranged in the converter 5. The pump laser pulse is focused onto the converter 5 by the tip 4.

[0042] Fig. 7 and Fig. 8 show typical measurement curves for tips 4 with different opening angles and contact surfaces 11 of the tips 4. For Fig. 7 the opening angle α is 20° with a diameter of the contact surface 11 of 350 nm and in Fig. 8 30° with a diameter of the contact surface 11 of 200 nm. As can be seen, these parameters can be used to influence the propagation of the acoustic waves into the object 2 and their reflection and thus to adapt or optimize them to the application.

Claims

1. A measuring probe (1) for the examination of surface areas of an object (2), in particular by scanning surface areas of the object (2), comprising a substrate (3), a tip (4) for interaction with the object (2) and a transducer (5) for sending acoustic signals into the object (2), wherein the substrate (3) carries the tip (4) and the transducer (5), wherein the transducer (5) is built up by a plurality of layers, whereof at least one layer (6) is constituted as a structure that is periodic in the propagation direction, in particular as a multi-quantum well (7), and can be excited to send acoustic signals and wherein the tip (4) is designed to transmit acoustic signals onto the object (2).

2. The measuring probe (1) according to claim 1, characterised in that the substrate (3) has a thickness between 0.2 µm and 3 µm.

3. The measuring probe (1) according to claim 1 or 2, characterised in that the tip (4) has a height between 1.0 µm and 15 µm, preferably between 1.5 µm and 10 µm, in particular between 2.0 µm and 8.0 µm.

4. The measuring probe (1) according to any one of claims 1 to 3, characterised in that the tip (4) is designed levelled off at the tapering end with an opening angle (α) between 5° and 40°, preferably between 15° and 35°.

5. The measuring probe (1) according to any one of claims 1 to 4, characterised in that the multi-quantum well (7) is formed from a superstructure of at least two materials, preferably GaN and InN, so that when excited it sends a wave packet of coherent sound quanta (8).

6. A device (9) for the examination of surface areas of an object (2), in particular by scanning surface areas of the object (2), comprising at least one excitation source (10) and a measuring probe (1) according to any one of claims 1 to 5, wherein at least one layer (6) can be excited by an excitation source (10) in order to transmit an acoustic signal.

7. The device (9) according to claim 6, characterised in that the excitation source (10) is designed to send, preferably femtosecond-long, laser pulses.

8. The device (9) according to claim 6 or 7, characterised in that the excitation source (10) is designed as a pulsed solid-state laser, preferably a titanium-sapphire laser, particularly preferably as an Er-, Yb- or Nd-doped fibre laser.

9. The device (9) according to any one of claims 6 to 8, characterised in that the excitation source (10) is created by carrying out a pump-probe experiment.

10. A scanning force microscopy process for examining surface areas of an object (2), in particular with a device (9) according to any one of claims 1 to 9, wherein a measuring probe (1), which comprises a substrate (3), a multilayer constituted transducer (5) and a tip (4), enters into an interaction with the surface of the object (2) through the substrate (3) fastened on one side and the tip (4) arranged thereon, while an excitation source (10) is directed onto the transducer (5), wherein at least one layer (6) of the multilayer constituted transducer (5), which is formed as a structure that is periodic in the propagation direction, in particular as a multi-quantum well (7), is excited by the excitation source (10) to send acoustic signals, in order to transmit the latter via the tip (4) onto the object (2).

11. The scanning force microscopy process according to claim 10, characterised in that laser pulses delivered by the excitation source (10) are divided into test and pump laser pulses.

12. The scanning force microscopy process according to claim 10 or 11, characterised in that wave packets of coherent sound quanta (8) are sent by an excitation of a multi-quantum well (7) contained in the layer (6).

13. The scanning force microscopy process according to claim 12, characterised in that the wave packet of coherent sound quanta (8) are sent with a frequency between 0.1 THz and 2 THz, preferably between 0.1 THz and 1.5 THz, particularly preferably between 0.1 THz and 0.5 THz.

14. The scanning force microscopy process according to claim 12 or 13, characterised in that a penetration depth of the wave packets of coherent sound quanta (8) into the object (2) is determined by a contact area (11) of the tip (4), which results in particular from its opening angle (α).