Scanning probe microscope and method for increasing a scanning speed of a scanning probe microscope in step-in scanning mode

Abstract

Scanning probe microscope (500) comprising: a scanning unit (555) configured to scan a measuring probe (400) in a step-in scanning mode over a sample surface (515); and b. a self-oscillation circuit (590, 790) configured to excite the measuring probe (400) to a natural oscillation (1050) during the step-in sampling mode; c. wherein the self-oscillation circuit (590, 790) comprises a phase shifter (630) configured to adjust a phase of the excitation (675) relative to the natural oscillation (1050) of the measuring probe (400); d. wherein the self-oscillation circuit (590, 790) comprises an automatic gain control (670) configured to adjust an amplitude of the natural oscillation (1050) of the measuring probe (400); and e. wherein the automatic gain control (670) comprises at least one amplifier (620), a sample-and-hold circuit (640, 740) and a control unit (610), and wherein the control unit (610) is designed to switch the sample-and-hold circuit (640, 740) between (630) a sample mode and a hold mode.

Description

1. Technical FieldThe present invention relates to a scanning probe microscope and a method for increasing a scanning speed of a scanning probe microscope operating in a step-in scanning mode.2. Prior ArtScanning probe microscopes scan a sample or its surface with a measuring probe and thus supply measurement data for generating a representation of the topography of the sample surface. Scanning probe microscopes are abbreviated below by SPM-English for Scanning Probe Microscope. Depending on the type of interaction between the measuring tip of a measuring probe and the sample surface, different SPM types are differentiated.In the atomic force microscope (AFM or SFM for scanning force microscope), a measuring tip of a measuring probe is deflected by atomic forces of the sample surface, typically attractive van der Waals forces and / or repulsive forces of the exchange interaction. The displacement of the probe tip is proportional to the force acting between the probe tip and the sample surface, and this force is used to determine the surface topography of the sample.In addition to AFM, there are a large number of other types of devices which are used for specific fields of application, such as, for example, scanning tunnel microscopes, magnetic force microscopes or optical and acoustic near-field scanning microscopes.Scanning probe microscopes can be used in various modes of operation. In a first contact mode, the measuring tip of a measuring probe is placed on the sample surface and, in this state, scanned over the sample surface. In this case, the deflection of a cantilever or a cantilever of the measuring probe which carries the measuring tip can be measured and used for imaging the sample surface. In a second contact mode, the deflection of the cantilever is kept constant in a closed control loop and the distance of the SPM is adjusted to the contour of the sample surface. In these two operating modes, on the one hand, the measuring tips are subject to severe wear and, on the other hand, sensitive samples can be damaged or even destroyed by contact with the measuring tip.In a third operating mode, the non-contact mode, the measuring tip is brought to a defined distance from the sample surface and the cantilever of the measuring probe is excited to oscillate, typically at or near the resonant frequency of the cantilever. The measuring probe is then scanned over the surface of the sample. In this mode, since the measuring tip does not come into contact with the sample, the wear thereof is small. However, the spatial resolution of the SPM is lower in this operating mode than in the contact operating modes and, in addition, the determination of the surface contour is difficult on account of the short range of the forces acting on the sample surface.In a fourth mode of operation, the intermittent mode (or tapping mode™), the cantilever is also set into a forced oscillation, but the distance between the SPM and the sample surface is chosen such that the measuring tip reaches the sample surface only during a small part of an oscillation period. From the change in the frequency, the amplitude or the phase of the forced oscillation caused by the interaction of the measuring probe with the sample surface, the contour of the surface of the sample is derived. The intermittent mode represents a compromise of the three above-mentioned modes.The authors L. Maning, B. Rogers, M. Jones, J. D. Adams, J. L. Fuste and S. C. Minne describe in the publication "Self-oscillating tapping mode atomic force microscopy", Rev. Science. Instr., Vol. 72, No. 9, pp. 4220-4222, a piezoelectric micro-cantilever probe used in an intermittent (or tapping mode™) mode of operation.Similar to the publication mentioned in the preceding section, the article "Improving tapping mode atomic force microscopy with piezoelectric cantilevers", authored by B. Rogers, L. Manning, T. Sulchek, J. D. Adams, in Ultramicroscopy 100 (2004), pp. 267-276, likewise describes a piezoelectric micro-cantilever sensor for use in an intermittent operating mode.In the article "All-digital PLL system for self-oscillation mode of micro-cantilevers with integrated bimorph actuator and piezoresistive readout", the authors N. Nikolov, N. Kenarov, P. Popov, T. Gotszalk and I. Rangelow, in Sensors & Transducers Journal, Vol. 98, Vol. 11, Nov. 2008, pp. 45-53, ISSN 1726-5479, describe a self-oscillation system for a micro-cantilever with an integrated bimorph actuator and piezo-resistive read-out unit.In a fifth operating mode, the scanning operating mode or simply the scanning mode (step-in mode, the English technical term step-in is used below), the movements perpendicular to the sample surface and parallel to the sample surface are carried out sequentially. For this purpose, the measuring tip of the measuring probe is lowered onto the sample surface and at the same time the interaction between the sample surface and the measuring tip is measured. The measuring tip is then brought substantially back into its initial position. The measuring tip is then displaced parallel to the sample surface by a defined section and the analysis process is continued with a further lowering process. These relationships are schematically illustrated in FIG. 1.In the article "In-line atomic force microscope for semiconductor process evaluation", Hitachi Review, Vol. 51 (2002), No. 4, pp. 130-135, the authors H. Koyabu, K. Murayama, Y. Kembo and S. Hosaka describe the step-in operating mode of an AFM. FIG. 1 is taken from the latter publication.U.S. Patent No. U.S. Pat. No. 7,129,486 B2 describes measuring and analyzing a time-force curve for a pulsed force mode (PFM) operating mode which is similar to the step-in scanning mode or the step-in operating mode.U.S. Patent No. U.S. Pat. No. 7,631,548 B2 concerns the step-in operating mode of a scanning probe microscope and describes how the detected time characteristics of a deflection signal can be used to analyze a sample surface.The last-mentioned patent application has taken from FIG. 2, which reproduces the time profiles of the lowering of a measuring tip of a cantilever of a measuring probe onto the sample surface and the retraction of the measuring tip from the sample. A characteristic feature of the step-in operating mode is the unintentional excitation of a vibration of the cantilever of a measuring probe, which is caused, for example, by the adhesion forces between the sample surface and the measuring tip. When the measuring probe is pulled back from the sample surface, these forces excite, when the measuring tip of the measuring probe jumps away from the sample surface, a vibration with the resonance frequency of the measuring probe or its intrinsic vibration. Depending on the magnitude of the spring constant of the measuring probe and the environmental conditions in which the measuring probe of the scanning probe microscope operates, the attenuation of this oscillation can be significantly lower than indicated in FIG. 2. FIG. 3 presents a second decay curve of an oscillation excited by the jumping away of the measuring probe from the sample surface. This figure is taken from the patent specification U.S. Pat. No. 8,650,660 B2.As long as the oscillation of the cantilever has a significant amplitude, a further scanning process cannot be started. A measurement on which a decaying oscillation of the measuring probe is superimposed would be very difficult to interpret. The decay time of the oscillation of the measuring probe excited when leaving the sample surface thus limits the scanning rate of a scanning probe microscope.<Neue Pages 4a and 4b> →The above-mentioned U.S. Pat. No. 8 650 660 B2 describes a peak force tapping (PFT) operating mode for an AFM. The PFT operating mode facilitates automated setting of the AFM on the one hand and the PFT operating mode prevents the decay of the oscillation of the cantilever, which is excited when the measuring tip is jumped off from the sample surface, from having to be waited before a new measurement cycle can be started. For this purpose, the measurement data of a measuring probe over the entire interaction cycles are recorded with a Pro-In the publication "Pico-Newton controlled step-in mode NC-AFM using a quadrature frequency demodulator and a slim probe in air for CD-AFM", by the authors S. Hosaka et al., Key Engineering Materials, Vol. 497, pages 95-100 (2012), on accurate measurements on a fine structure with steep slopes with a resolution in the nanometer range under ambient conditions with the aid of a contactless step-in operating mode of an atomic force microscope (AFM). If high aspect ratio structures are measured using an AFM in step-in mode operation with a sharpened and thin probe, it is necessary to perform AFM control with forces <1 nN, i.e., in the pico-Newton range, to prevent bending and sliding of the probe at the edge. Using a self-manufactured contactless step-in AFM and using a quadrature frequency demodulator to detect the shift of the resonant frequency of the cantilever, it could be shown that a silicon structure with steep fringes can be detected with a force of 2 pN in air faithfully.In the article "Step-in mode NC-AFM using a quadrature frequency demodulator for observing high-aspect ratio structures in air", the authors S. Hosaka et al., e-Journal Surf., Sci. Nanotech., Vol. 9, pp. 122-125 (2011) on studies of contactless step-in mode of an AFM (NC-AFM) for accurately measuring on a fine structure having steep slopes with a nanometer-scale resolution under environmental conditions. In order to avoid curving or sliding of the measuring probe on a steep franke, it is proposed to use a contactless step-in mode of operation of an AFM, wherein the force is controlled in the range of pico-newtons. A prototype contactless step-in AFM has been made using a quadrature frequency demodulator for detecting the resonant frequency shift of the cantilever.DE 103 32 451 B4 describes a method for determining a height profile on a semiconductor substrate surface, wherein the substrate surface runs with the aid of a scanning probe microscope (SPM) along a predefined scanning path (A), substantially parallel to the substrate surface, and wherein in each case the level (N1, N2) of the substrate surface is determined at the location of a probe (S) on the basis of a local interaction of the probe with the substrate surface, and wherein the scanning path on the substrate surface has a first section (A1) with a first level (N1), a second section (A2) adjoining the first section and a third section (A3) adjoining the second section with the first level, (a) scanning the substrate surface in the first section at a first operating mode (M1) at a first scanning speed (v1); (b) changing the scanning process to a second operating mode (M2) at a second scanning speed (v2) as soon as the second section of the substrate surface is reached; (c) scanning the substrate surface in the second section at the second scanning speed (v2); (d) changing the scanning process to the first operating mode as soon as the third section is reached; and (e) scanning the substrate surface in the third section at the first scanning speed.The authors M Watanabe et al. describe in the conference contribution "An advanced AFM sensor: its profile accuracy and low probe wear property for high aspect ratio patterns", Proc. Of SPIE Vol. 6518, Metrology, Inspection, and Process Control for Microlithography XXI, 65183L (2007), the development of an AFM sensor uses the step-in operating mode of an AFM in order to utilize the positive properties of this AFM operating mode even better.U.S. Patent No. U.S. Pat. No. 7,631,548 B2 concerns the step-in operating mode of a scanning probe microscope and describes how the detected time characteristics of a deflection signal can be used to analyze a sample surface.The last-mentioned patent application has taken from FIG. 2, which reproduces the time profiles of the lowering of a measuring tip of a cantilever of a measuring probe onto the sample surface and the retraction of the measuring tip from the sample. A characteristic feature of the step-in operating mode is the unintentional excitation of a vibration of the cantilever of a measuring probe, which is caused, for example, by the adhesion forces between the sample surface and the measuring tip. When the measuring probe is pulled back from the sample surface, these forces excite, when the measuring tip of the measuring probe jumps away from the sample surface, a vibration with the resonance frequency of the measuring probe or its intrinsic vibration. Depending on the magnitude of the spring constant of the measuring probe and the environmental conditions in which the measuring probe of the scanning probe microscope operates, the attenuation of this oscillation can be significantly lower than indicated in FIG. 2. FIG. 3 presents a second decay curve of an oscillation excited by the jumping away of the measuring probe from the sample surface. This figure is taken from the patent specification U.S. Pat. No. 8,650,660 B2.As long as the oscillation of the cantilever has a significant amplitude, a further scanning process cannot be started. A measurement on which a decaying oscillation of the measuring probe is superimposed would be very difficult to interpret. The decay time of the oscillation of the measuring probe excited when leaving the sample surface thus limits the scanning rate of a scanning probe microscope.<Neue Pages 4a and 4b> →The above-mentioned U.S. Patent No. U.S. Pat. No. 8,650,660 B2 describes a peak force tapping (PFT) operating mode for an AFM. The PFT operating mode facilitates automated setting of the AFM on the one hand and the PFT operating mode prevents the decay of the oscillation of the cantilever, which is excited when the measuring tip is jumped off from the sample surface, from having to be waited before a new measurement cycle can be started. For this purpose, the measurement data of a measuring probe are recorded over the entire interaction cycles with a sample or its surface. An interaction cycle is divided into an interaction section in which the measuring probe interacts with the sample and a part without interaction between the sample surface and the measuring probe. From the interaction section, the interaction force between the sample and the measuring probe is determined and from the part without interaction the zero point of the deflection of the measuring tip is calculated.This patent requires the precise recording of a large amount of data per step-in scan cycle, the automated or manual setting of markers for determining an interaction section or interaction zone within each step-in scan cycle, as well as a large computing capacity in order to determine from the measurement data the interaction between the measurement tip of the measurement probe and the sample surface.The present invention is therefore based on the problem of specifying a device and a method with the aid of which the above-described problem of step-in-scan mode of an SPM or the expenditure of previously known solutions can be at least partially avoided.3. SUMMARY OF THE INVENTIONAccording to an embodiment of the present invention, this problem is solved by an apparatus according to claim 1.Due to the fact that the measuring probe is jumped off from the sample surface, the measuring probe is excited in a very broad frequency spectrum and then carries out a relaxation oscillation at its intrinsic or resonant frequency. A self-oscillation circuit of a scanning probe microscope according to the invention excites the measuring probe during the step-in operating mode to oscillate exactly at the intrinsic or resonant frequency of the measuring probe. By operating the measuring probe at its natural frequency, the decaying oscillation of the measuring probe, which was caused by adhesion forces between the measuring probe and the sample surface, assists in promptly adjusting the natural oscillation with a predefined amplitude of the measuring probe after the measuring tip of the measuring probe has been released from the sample surface. After the measuring probe has jumped off from the sample surface, it is only necessary to wait until the amplitude of the oscillation induced by the jump-off reaches the predefined amplitude of the natural oscillation before a new scanning cycle or step-in cycle can be started. This enables a significant increase in the scanning speed of a scanning probe microscope in step-in scanning mode.The self-oscillation circuit may include a phase shifter configured to adjust a phase of excitation relative to the self-oscillation of the measurement probe.In order that the oscillation of the measuring probe at its natural frequency can be restored as quickly as possible after the measuring probe has jumped off from the sample surface, it is preferred that the excitation or the excitation signal and the natural oscillation of the measuring probe have a defined phase difference with respect to one another. The best possible excitation of the natural oscillation of the measuring probe is achieved if the excitation signal and the natural oscillation have a phase difference of substantially 90°.The term "substantially" as used herein means, as in other instances of this application, an indication of a measurement variable within its error tolerances when the measurement variable is measured with measurement devices according to the prior art.The phase shifter can be configured to set the excitation with a phase difference with respect to the best possible excitation of the natural oscillation of the measuring probe in the range of ± 30°, preferably ± 20°, more preferably ±10°, and most preferably ±5°.The self-oscillation circuit may include an automatic gain controller configured to adjust an amplitude of the self-oscillation of the measurement probe.The automatic gain control can comprise at least one amplifier, a sample-and-hold circuit and a control unit, wherein the control unit is configured to switch the sample-and-hold circuit between the sample mode and the hold mode.The self-oscillation circuit can be embodied as a digital circuit. The self-oscillation circuit may be implemented as a field programmable gate array (FPGA) or an application specific integrated circuit (ASIC).Furthermore, the scanning probe microscope can have a first actuator which is designed to transmit the excitation of the self-oscillation circuit to a cantilever of the measuring probe and can have a second actuator which is designed to transmit a signal from the control unit of the automatic gain control to the cantilever of the measuring probe.The scanning probe microscope can have a first laser system which is designed to transmit the excitation of the self-oscillation circuit to the first actuator and can have a second laser system which is designed to transmit the signal from the control unit to the second actuator.The first and the second actuator can be embodied as bimorph actuators. The cantilever of the measuring probe can comprise a bimorph actuator.The scanning probe microscope can furthermore have a detector which is designed to detect a deflection of the cantilever of the measuring probe and can have a detection unit which is designed to detect a vertical position of the free end of the cantilever of the measuring probe.The detector may comprise a photodetector and / or an interferometer and the detection unit may comprise a photodetector and / or an interferometer. The control unit of the self-oscillation circuit can be configured to determine an interaction between the measuring tip of the measuring probe and a sample surface from a measurement of a frequency change of the natural oscillation of the measuring probe.The scanning probe microscope can further have a control device which comprises the scanning unit and an excitation unit, wherein the excitation unit is designed to control the self-oscillation circuit.The natural oscillation of the measuring probe can comprise a frequency range from 1 kHz to 20 MHz, preferably 5 kHz to 10 MHz, more preferably 10 kHz to 5 MHz, and most preferably from 20 kHz to 2 MHz. The amplitude of the natural frequency of the measuring probe can comprise a range from 1 nm to 2000 nm, preferably 1 nm to 1000 nm, more preferably from 5 nm to 500 nm, and most preferably from 20 nm to 200 nm.The self-oscillation circuit may have a terminal configured to provide a control signal which is used by the self-oscillation circuit for controlling the amplitude of the self-oscillation of the measurement probe.According to a further embodiment of the present invention, the problem is solved by a method according to claim 12.Steps (a) to (e) may include: (f) in step (b): activating the first actuator configured to transmit the measurement probe to the measurement probe upon excitation of the self-oscillation circuit; (g) in step (a): activating a second actuator configured to change a distance between a measurement tip of the measurement probe and a sample surface; and (h) in step (e): detecting contact between the measurement tip of the measurement probe and the sample surface.Steps (a) to (e) may further comprise: (i) in step (b): activating the second actuator; (j) in step (e): deactivating amplitude control by switching a sample-and-hold circuit from a sample mode to a hold mode; (k) in step (e): deactivating the first actuator if an amplitude of the natural vibration falls below a predetermined threshold; (l) in step (e): determining a vertical position of the measuring tip of the measuring probe close to detecting contact of the measuring tip with the sample surface; (m) in step (e): deactivating the second actuator and waiting a predetermined period of time until loss of contact occurs between the measuring tip of the measuring probe and the sample surface; (n) in step (b): in-phase activating the first actuator, and (o) in step (d); activating the amplitude control by switching the sample-and-hold circuit from the hold mode to the sample mode.Detecting contact between the measuring probe and the sample surface may comprise determining a vertical position of the measuring tip of the measuring probe at this point.The method can further comprise the step of: determining a switch-on time for the in-phase activation of the first actuator from a decay curve of the natural oscillation of the measuring probe without activation of the first actuator.A computer program may comprise instructions which, when executed by a computer system, cause the computer system to carry out the method steps of the aspects specified above.4. DESCRIPTION OF THE DRAWINGSIn the following detailed description, presently preferred embodiments of the invention will be described with reference to the drawings, wherein FIG. 1 schematically illustrates a step-in scanning mode of a scanning probe microscope according to the prior art; FIG. 2 illustrates in the upper partial image the approach of a measuring probe to a sample surface and the deflection signal of a cantilever of the measuring probe, and in the lower partial image the approach of a measuring tip of the measuring probe to a sample surface, the contact of the measuring tip with the sample surface and the jumping of the measuring tip of the measuring probe from the sample surface; FIG. 3 shows two cycles of a step-in operating mode or a step-in scanning mode of a scanning probe microscope with associated relaxation oscillations according to the prior art, which were induced by the jump of the measuring probe from that of the sample surface; FIG. 4 presents a schematic representation of a measuring probe with a bimorph actuator; FIG. 5 shows a schematic representation of some components of a scanning probe microscope; FIG. 6 illustrates some components of a self-oscillation circuit; FIG. 7 shows some components of a self-oscillation circuit, which is embodied in the form of an FPGA; FIG. 8 is a diagram of a cycle of the step-in scan mode; FIG. 9 presents a cycle of a step-in scanning mode, in which the excitation frequency of the measuring probe does not match the resonance frequency of the measuring probe; FIG. 10 presents a cycle of a step-in scanning mode, in which the excitation frequency of the measuring probe corresponds to its resonant frequency; and FIG. 11 presents a flow chart of a method for increasing the scanning speed of a scanning probe microscope operating in step-in scanning mode.5. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTSReferring to Figs. 1 to 3, the difficulties of a scanning probe microscope operating in a scanning mode, a scanning mode or a step-in scanning mode will be briefly explained. Subsequently, presently preferred embodiments of an apparatus according to the invention and of a method according to the invention are explained in more detail.The diagram of FIG. 1 schematically shows the recording of measurement data of a scanning probe microscope which is operated in step-in scanning mode. In step (i), the measuring probe is placed on the surface of a sample. In the last phase of the approach between the measuring probe and the sample surface, the measuring tip of the measuring probe comes under the influence of the attracting forces of the sample surface, these are predominantly van der Waals forces and is attracted by the sample surface. In FIG. 3, this region is indicated by the letters A and B. At point B, the measuring probe is placed on the sample surface. In this state, the distance of the measuring probe is measured in the z-direction or in the vertical direction, i.e. perpendicular to the sample surface with respect to a reference point or a reference plane.Then, at step (ii) of FIG. 1, the measuring probe is retracted from the sample surface. In order that the adhesion forces between the measuring tip of the measuring probe and the sample surface can be overcome, the cantilever of the measuring probe has a significant stress before the contact between the measuring tip and the sample surface is released. At point D of FIG. 3, the contact between the measuring tip and the sample surface is released. The energy stored in the cantilever of the measuring probe discharges in an oscillation of the measuring probe at its resonant frequency with a maximum amplitude E.In step (iii), the measuring probe is laterally displaced by a predetermined distance in the state withdrawn from the sample surface. As soon as the oscillation induced by the jump of the measuring tip of the measuring probe from the sample surface has decayed, a new lowering step (i) can be started.FIG. 2 again shows in the upper partial image the lowering and the retraction of a measuring probe from a sample surface as well as the oscillation of the cantilever of the measuring probe caused by the detachment of the measuring probe from the sample surface. The lowering and the retraction of the measuring probe take place as a reaction of a control signal applied to the z actuator of the measuring probe, which control signal is illustrated in FIG. 2 under the curve of the deflection signal of the measuring tip of the measuring probe. The lower partial image of FIG. 2 presents on the left the distance range of the long-range attractive forces of the sample surface, in the middle the state in which the measuring tip of the measuring probe is in contact with the sample surface and on the right the moment of detachment of the measuring tip from the sample surface.The oscillation of the cantilever of the measuring probe caused by the jumping away of the measuring tip from the sample surface depends on several factors. The shape and material of the measuring tip as well as the material composition and the surface condition of the sample have an influence on this. The spring constant of the measuring probe is of great importance for the maximum magnitude of the amplitude and the decay behavior of the excited resonant oscillation. Finally, the environmental conditions under which the measuring probe operates have a decisive importance on the damping behavior of the relaxation oscillation of the measuring probe.FIG. 4 schematically shows a section through a measuring probe 400. The measuring probe 400 comprises a bending beam 410 which is referred to below as cantilever 410 as is customary in the art. In the example illustrated in FIG. 4, the cantilever 410 comprises a first layer 420, which can be manufactured, for example, from a semiconductor material such as silicon. Furthermore, the cantilever 410 has a second layer 430, which can comprise a metal, for example. The materials for the two-layer cantilever 410 are preferably selected according to their stiffness or elasticity, their thermal expansion and / or their mullibility.In the example of FIG. 4, the cantilever 410 performs the function of a bimorph actuator 440. By activating the bimorph actuator 440, the cantilever 410 of the measuring probe 400 can be excited, on the one hand, to oscillate at a predefined frequency and amplitude and, on the other hand, at a natural frequency of the measuring probe 400. Thus, the bimorph actuator 440 performs the function of a first actuator 480. In addition, the bimorph actuator 440 can be bent downward, i.e. in the direction of the measuring tip 460, by a corresponding excitation. The bimorph actuator 440 of the cantilever 410 of the measuring probe 400 can accordingly carry out the function of a second actuator 490.A measuring tip 460 is attached to the free end 450 of the cantilever 410. The measuring tip 460 of the measuring probe 400 interacts with a sample surface. The opposite end of the cantilever 410 or the base of the cantilever 410 is connected to a holding device 470. The holding device 470, which can be designed, for example, in the form of a holding plate, serves for attaching the measuring probe 400 in a scanning probe microscope, for example by clamping (not illustrated in FIG. 4 ).FIG. 5 schematically shows some components of a scanning probe microscope 500, the SPM measuring head of which has a holder (suppressed in FIG. 5 for reasons of clarity), which serves for installing a measuring probe 400 into the SPM 500. Scanning probe microscopes are differentiated according to the measured variable used for examining the sample 510. Scanning Tunneling Microscope (STM) uses the tunneling current between the sample 510 and the probe 460, which occurs when a voltage is applied between the sample 510 and the probe 460, to analyze the topography of the sample surface 515 of the sample 510. Atomic force microscope (AFM) determines the surface contour of the sample 510 from the deflection of the measuring tip 460 by the sample 510. Magnetic force microscopes (MFMs) measure the magnetic forces between the sample 510 and the measuring tip 460. Scanning near-field optical microscopes (SNOM) use evanescent electromagnetic waves as the interaction between the sample 510 and the measuring tip 460. Scanning near-field acoustic microscopes (SNAM) use near-field acoustic microscope interactions to scan the surface topography of the sample 510. This list of scanning probe microscopes is not complete.The principle disclosed in this application for exciting a natural oscillation of the cantilever 410 of the measuring probe 400 for accelerating the scanning rate or the scanning speed of a scanning probe microscope 500 in a step-in scanning mode can be applied to the measuring probes of all types of scanning probe microscopes which have a cantilever 410, i.e. an elastically flexible lever arm or, for short, a cantilever.Hereinafter, as an example of a scanning probe microscope 500, an atomic force microscope (AFM) 500 will be explained. The scanning force microscope 500 illustrated in FIG. 5 can be operated under ambient conditions or in a vacuum chamber (not illustrated in FIG. 5 ). The sample 510 to be analyzed is placed on a sample table 520. The sample table 520 can be positioned in three spatial directions by a positioning device 525. The positioning device 525 comprises, for example, one or more micro-displacement elements, for example in the form of spindle actuators and / or piezo actuators (not shown in FIG. 5 ).The measurement probe 400 is attached by a holder to an attachment device (not shown in FIG. 4 ) in the AFM measurement head of the scanning force microscope (AFM) 500. The holding device 470 of the measuring probe 400 can be connected to the measuring head of the AFM 500 via a piezoelectric actuator (not illustrated in FIG. 5 ). The piezo actuator connecting the measuring probe 400 to the AFM measuring head may perform the function of a scanning device. Alternatively or additionally, in a further embodiment, the relative movement between the sample surface 515 and the measuring tip 460 of the measuring probe 400 can be divided between the positioning device 525 and the piezo actuator which connects the holding device 470 to the AFM measuring head. For example, the positioning device 525 performs the movement of the sample 510 in the sample plane (xy plane) and the above-mentioned piezo actuator brings about the movement of the measuring tip 460 of the measuring probe 400 in the direction of the sample normal (z direction).Preferably, however, the sample table 520 is made stationary and the measuring tip 460 is brought to the region of the sample 510 to be analyzed by means of micro-displacement elements (not shown in FIG. 5 ).The measuring probe 400 can operate in a plurality of operating modes. On the one hand, the measuring probe 400 can be scanned at a constant height over the surface 515 of the sample 510. Alternatively, the measuring probe 400 can be guided over the sample surface 515 with constant force in a closed control loop. Furthermore, it is possible to set the cantilever 410 in a vibration perpendicular to the sample surface 515 with the aid of a modulation method and thereby scan the surface 515 of the sample 510 in a closed control loop.Preferably, however, the measuring probe 400 is operated in a scanning mode or a step-in operating mode. In this operating mode, the cantilever 410 of the measuring probe 400 is excited to oscillate at the natural or resonant frequency of the measuring probe 400. In the example illustrated in FIG. 5, a first laser system 530 is used to excite the cantilever 410 of the measuring probe 400 to oscillate at the natural frequency of the measuring probe 400. The laser system 530 generates a time-varying output power which is directed by means of the laser beam 532 onto the bimorph actuator 440 of the measuring probe 400 and which controls the function of the first actuator 480 of the bimorph actuator 440. In the example illustrated in FIG. 5, the first laser system 530 comprises a laser diode which emits light in the infrared range of the electromagnetic spectrum, in particular light in the range of 800 nm. Details of the circuitry employed to generate the varying output power or corresponding laser pulses in the laser system 530 are discussed below in the discussion of FIGS. 6 and 7.A second laser system 570 is used to bend the free end 450 of the cantilever 410 in the direction of the sample surface 515. For this purpose, in the example of FIG. 5, the beam 572 of the second laser system 570 is combined with the beam 532 of the first laser system 330 to form a common beam 537 of both laser systems 530 and 570 with the aid of the combination element 535. The combining element 535 may comprise a polarizing beam splitter cube. The combined beam 537 is preferably directed at the base point of the cantilever 410 of the measuring probe 400. The second laser system 570 emits a light beam 532 whose optical power is constant over time or changes only slowly over time. In the example of FIG. 5, the second laser system 570 also comprises a laser diode that emits in the infrared wavelength range. Thus, the second laser system 570 controls the operation of the second actuator 490 of the bimorph actuator 440. The first 530 and second 570 laser systems may have the same emission wavelength or may generate light of different wavelengths. The first 530 and the second laser system 570 can be focused onto the cantilever 410 of the measuring probe 400 by means of a common optical unit, which is suppressed in FIG. 5 for reasons of simplicity. It is also possible for the two laser systems 530 and 570 to each have their own optics and to be focused on different points of the cantilever 410.Furthermore, it is also possible to use a single laser system for carrying out both functions (not shown in FIG. 5 ). In this case, the laser beam of a single laser system supplies the bimorph actuator 440 with the energy in a timely manner, so that the latter excites the cantilever 410 to oscillate at its natural frequency as the first actuator 480 and bends the free end 450 of the cantilever 410 in a defined manner in the direction of the sample surface 515 as the second actuator 490. This configuration is favorable since, on the one hand, this reduces the complexity of the scanning probe microscope 500 and of a control device 550 and, on the other hand, enables working with only one light beam 537, which facilitates the adjustment of the SPM 500.No particular requirements are placed on the laser systems 530 and 570. The wavelength thereof can be selected as desired. However, wavelengths in the visible range of the electromagnetic spectrum facilitate the adjustment of the laser beam 535 or 575. However, it is advantageous to choose the wavelength of the laser radiation such that the proportion of absorbed radiation in the bimorph actuator 440 is as large as possible, i.e. the materials of the cantilever 410 and the laser system or systems 530 and 570 are matched to one another. An output power of a few mW is sufficient for heating the bimorph actuator 440 or the cantilever 410. To heat the cantilever 410 and to excite an intrinsic oscillation of the measuring probe 400, focusing on a focal spot <10 μm is necessary. In particular, the focal spot should be smaller than the width of the cantilever 410 so that only very little laser radiation 537 passes by the cantilever 410 onto the sample 510. The resonant frequency of measuring probes 400 lies in the frequency range of a few kHz to a few MHz. For modern laser systems 530, these requirements are not a problem.The deflection of the measuring tip 460 or its change due to the interaction of the measuring tip 460 of the measuring probe 400 with the surface 515 of the sample 510 can be detected with a light pointer system. A light pointer system is shown in FIG. 5. The light pointer system comprises a third laser system 540 and the detector 545 as essential components. The third laser system 540 passes through a detection unit 585 and directs the laser beam 542 onto the free end 450 of the cantilever 410. The laser beam 547 reflected by the cantilever 410 is recorded by a detector 545. In the example illustrated in FIG. 5, the third laser system 540 comprises a solid state laser emitting light in the visible region of the electromagnetic spectrum, in particular green light. The third laser system 540 is controlled by the control device 550 via the connection 541. The detector 545 of the light pointer system is often designed in the form of a four-quadrant photodiode. It is also possible to use a two-segment photodiode (not shown in FIGS. 5 and 6 ). The detector 545 can detect both the oscillation of the cantilever 410 at the natural frequency of the measuring probe 400 and the adjustable bending of the free end 440 of the cantilever 410 by the second actuator 490 for lowering the measuring tip 460 onto the sample surface 515.A detection unit 585 is installed in the scanning probe microscope 500 of FIG. 5. The detection unit 585 can comprise, for example, an optical interferometer, for example a laser interferometer and / or a photodiode. In the example illustrated in FIG. 5, the detection unit 585 comprises an interferometer whose light source forms the third laser system 540. In the example shown in FIG. 5, the detection unit 585 is used for measuring the z-position or the vertical position of the measuring tip 460 of the measuring probe 400 when the measuring tip 460 is placed on the surface 515 of the sample 510. For this purpose, with the measuring tip 460 placed on the sample surface 515, the interferometer 585 analyzes the light of the light beam 542 of the third laser system 540 reflected by the free end 450 of the cantilever. Furthermore, it is possible to use the detection unit 585 to determine the movement of the measuring tip 460 in the z-direction, i.e. perpendicular to the sample surface 515.In addition, a deflection of the free end 450 of the cantilever 410 can additionally or alternatively be detected with the aid of piezoresistive elements or sensors of the cantilever 410 (not illustrated in FIG. 5 ). In addition, it is also possible to determine the distance of the free end 450 of the cantilever 410 with respect to the sample surface 515 and a reference point or a reference plane from a combination of optical signals of the light pointer system and / or of the detection unit 585 and the measurement data of piezoresistive elements (likewise not shown in FIG. 5 ).Furthermore, the scanning force microscope 500 has a control device 550. The control device 550 comprises a scanning unit 555 and an excitation unit 560.The controller 550, the scanning unit 555, and the excitation unit 560 may be executed in hardware, software, firmware, or a combination thereof.The scanning unit 555 controls or regulates the movement of the positioning unit 525 and / or of the piezo actuator, which connects the measuring probe 400 to the AFM measuring head, via the connections 527 and 580. Furthermore, the scanning unit 555 of the control device 550 controls the third laser system 540 via the connection 541. The excitation unit 560 controls, via the connection 582, a self-oscillation circuit 590 which generates the excitation or the excitation signal with which the measuring probe 400 is excited to a self-oscillation.The self oscillation circuit 590 receives measurement data from the detector 545 via the connection 548. Further, the self-oscillation circuit 590 may also obtain measurement data from the detection unit 585 via the connection 587. The detection unit 585 supplies its measurement data to the scanning unit 555 of the control device 550 via the connection 586. Via the connection 531, the self-oscillation circuit 590 controls the first laser system 530, which in turn controls the first actuator 480 of the bimorph actuator 440 via the laser beam 532. Moreover, the self-oscillation circuit 590 controls the second laser system 570 via the connection 571. As already explained above, the laser beam 572 of the second laser system 570 controls the second actuator 490 of the bimorph actuator 440 and thus the bending of the measuring tip 460 of the cantilever 410 of the measuring probe 400 in the direction of the sample surface 515. For this purpose, as likewise already explained above, the combined laser beam 537 is directed onto the cantilever 410 of the measuring probe 400 in the vicinity of the base point of the cantilever 410 (i.e. the end of the cantilever 410 on which the holding device 470 is attached).In an alternative embodiment, the bimorph actuator 440 is heated using one or more resistive elements instead of the laser system 570. For example, a first resistive element may be substituted for the first laser system 530 and a second resistive element may be substituted for the second laser system 570 (not shown in FIG. 5 ). In another embodiment, the measuring probe 410 is excited to oscillate at its natural frequency with the aid of a piezoelectric element (not shown in FIG. 5 ). Mixed forms of the described embodiments can likewise be used.The interaction between the measuring tip 460 and the sample surface 515 can be detected by a change in the amplitude of the measurement probe 400 excited to oscillate. Alternatively, the interaction between the measuring tip 460 and the sample surface 515 can be determined from a change in the frequency of the natural oscillation when the measuring tip 460 approaches the sample surface 515.The diagram 600 of FIG. 6 shows essential components of the self-oscillation circuit 590 of FIG. 5, which are used for generating an excitation or an excitation signal for generating a natural oscillation of the measuring probe 400. In the example illustrated in FIG. 6, the first laser system 530 and the second laser system 570 are combined in the laser system 630. The control unit 610 of the self-oscillation circuit 590 is connected to the excitation unit 560 of the controller 550 of the SPM 500 (not shown in FIG. 6 ). The self-oscillation circuit 590 has an automatic gain control 670 in the form of an amplifier 620, a sample-and-hold circuit 640 and the control unit 610. Furthermore, the self-oscillation circuit 590 comprises a switch 660 by which the control unit 610 can switch between a sample and a hold mode of the sample-and-hold circuit 640. A phase shifter 630 adjusts the phase position of the excitation provided to the laser system 630 or the control signal provided to the laser system 630 of the phase of the natural oscillation of the measuring probe 400. Furthermore, the automatic gain control 670 has a combining unit 650 that combines the signals of the phase shifter 630, the sample-and-hold circuit 640, and the control unit 610. For example, the combining unit 650 may include a multiplying unit that multiplies the signals of the phase shifter 630 and the sample-and-hold circuit 640 (not shown in FIG. 6 ). Furthermore, the combining unit 650 may comprise a summation unit which adds the multiplied signal of the phase shifter 630 and the sample-and-hold circuit 640 and the signals of the control unit 610 (not shown in FIG. 6 ).Furthermore, the control unit 610 comprises a generator part for generating a voltage ramp. The generator portion generates a voltage signal that is part of the excitation signal 675 for the laser system 630. The voltage ramp of the generator part of the control unit 610 controls the second actuator 490 of the cantilever 410 and thus the distance of the measuring tip 410 of the measuring probe 400 from the sample surface 515 via the laser system 630.By the self-oscillation circuit 590 multiplying a part of the measurement signal 615 of the detector 545 by the amplified measurement signal 625 in the correct phase, the combination unit 650 generates an excitation 675 or an excitation signal 675 for a natural oscillation of the measurement probe 400 which has a positive feedback. For optimum excitation 675 of an eigenoscillation of the measuring probe 400, the excitation 675 has a phase difference of 90° with respect to the phase of the eigenoscillation of the measuring probe 400. The phase of the excitation signal 675 leads the phase of the natural oscillation of the measuring probe by π / 2. A deviation of the best possible phase difference from π / 2 to within the range of ± 30° is permissible without drastically restricting the operating range of the phase control of the natural oscillation of the measuring probe.The automatic gain control 670 controls the amplitude of the excitation signal 675 to a predefined value, i.e. the gain of the excitation is adjusted such that the losses of the natural vibration of the measuring probe 400 during a vibration period are just compensated. During the approach of the measuring tip 460 of the cantilever 410 to the sample surface 515, the control unit 610 actuates the switch 660 and switches the sample-and-hold circuit 640 from the sample mode, in which the amplitude of the natural oscillation of the measuring probe 400 is controlled, into the hold mode, in which the amplitude of the natural oscillation is no longer controlled, but is excited with a fixed excitation signal 675. The changeover from a control of the amplitude to a fixed excitation prevents the amplitude control of the natural oscillation from damaging a sensitive sample or the measuring probe 400 in the event of an interaction of the measuring tip 460 with the sample surface 515 and at the latest when the measuring tip contacts the sample surface 515, in that the control attempts to maintain a predefined oscillation amplitude of the natural oscillation of the measuring probe 400.After detecting the placement of the measuring tip 460 of the measuring probe 400 on the sample surface 515, the control unit 610 switches off the first laser system 530. In the combined laser system 630 shown in FIG. 6, the sinusoidal excitation of the first actuator 480 is interrupted, while the laser system 630 continues to activate the second actuator 490 of the cantilever 410 with the DC light component of its output power (not shown in FIG. 6 ). The control unit 610 determines the placement of the measuring tip 460 on the sample surface 515 from a decrease in the amplitude of the natural oscillation of the measuring probe 400 below a predefined threshold value. For this purpose, the control unit 610 continuously evaluates the signal of the detector 545. Immediately after detecting the placement of the measuring tip 460 of the measuring probe 400 on the sample surface 515, the scanning unit 555 determines the vertical position of the measuring tip 460 of the measuring probe 400 from the measurement data of the detection unit 585. Then, the control unit 610 of the self-oscillation circuit 590 turns off the second laser system 570. In the example shown in FIG. 6, the laser system 630 is turned off.After waiting a predefined time period which ensures that measuring tip 460 has lost contact with sample surface 515, control unit 610 switches first laser system 530 on again or controls laser system 630 with excitation signal 675. In parallel, the sample-and-hold circuit 640 is switched from the hold to the sample mode by operating the switch 660. As a result, the amplitude control of the natural oscillation of the measuring probe 400 is activated again. At the same time, by switching on the first laser system 530, the phase locked loop is closed again between the excitation signal 675 and the natural oscillation of the measuring probe 400.When the first laser system 530 is switched on again or when the laser system 630 is activated with the excitation signal 675, the time of switching on again is selected such that the phase difference between the excitation signal 675 and the natural vibration of the measuring probe 400, which is initiated by the jumping away of the measuring tip 460 from the sample surface 515, optimally matches one another.There are several ways to determine this time. Firstly, a fixed time period is maintained between the switch-off time of the second laser system 570 and the switching on of the first laser system 530 again (or between the switching-off of the laser system 630 and the activation thereof with the excitation signal 675). Since the time sequences within the different step-in measurement cycles are very similar, the switch-on time can be determined empirically.Secondly, the entire profile of the deflection of the measuring tip 460 during a step-in cycle can be measured once and the point D or E of FIG. 3 is determined. This knows the time interval between points B (turning off the first laser system 530) and D (jumping the measuring tip 460 back from the sample surface 515). It is then possible, for example, to wait for one or more periods of the relaxation oscillation of the measuring probe 400 until the first laser system 530 is switched on again. It is also possible to measure only the relaxation oscillation of the measuring probe 400, which is initiated by the jumping away of the measuring tip 460 from the sample surface, and to calculate the switch-on time for the first laser system 530 or for the laser system 630 from these measurement data.As a further alternative, the first laser system 530 may not be turned off during contact of the measurement tip 460 with the sample surface 515. Depending on the rigidity or the spring constant of the measuring probe 400, however, there may be a risk of over-controlling the electronics of the self-oscillation circuit 590 in this procedure.The self oscillation circuit 590 may be implemented in analog or digital form. Further, the self-oscillation circuit 590 may be realized in hardware, software, firmware, or a combination thereof.The diagram 700 of FIG. 7 shows an embodiment of a self-oscillation circuit 790 which is implemented as a digital circuit in the form of an FPGA (field programmable gate array). An analog-to-digital converter 710 (ADC) receives measurement signals 615 from the detector 545 in analog form and converts them into digital signals 715. The digital signals 715 are supplied to a transversal filter FIR (Finite Impulse Response) 720 of the self-oscillation circuit 790. The filter 720 removes the slow ramp portion of the measurement signal 615, which arises as a result of the lowering of the measurement tip 460 of the measurement probe 400 onto the sample surface 515 and which is superimposed on the natural oscillation of the measurement probe 400. The filtered digital data are fed to a second RMS (Route Mean Square) filter 725, which determines the effective value of the amplitude of the natural oscillation of the measuring probe 400. The filter 725 averages over several periods of the natural oscillation of the measuring probe 400. The output of the RMS filter 725 is fed to a PID (Proportional Integral Differential) controller 730. The controller 730 is the feedback element for amplitude control. The clock signal of the clock generator 760 is not applied to the PID controller 730 by means of the switch 732 during the period in which the amplitude control is not active. The output 735 of the PID controller 730 contains the amplitude portion A of the digital excitation signal 785 and is applied to an input of the sample and hold circuit S&H 740.A second portion of the output signal of the RMS filter 725 is applied to an input of a comparator CMP 745. Comparator 745 compares this signal to a threshold value Thrs present at a second input. If the amplitude of the natural oscillation of the measuring probe 400 falls below a predefined threshold value, the output of the comparator 745 becomes active, i.e. the latter implements the function A<B. The comparator 745 outputs its output signal to the FIFO (first-in-first-out) memory 775 and via the delay element DLY 750 to the sample-and-hold circuit 740 and, on the other hand, to the ramp generator RAMP 765.The ramp generator 765 can be realized, for example, in the form of a counter. The delay element 750 implements a waiting time until the amplitude control of the natural oscillation of the measuring probe 400 resumes. After the probe 460 is jumped off the sample surface 515, the filters 720 and 725 require some time to recover. The delay element 750 therefore delays the switching on of the amplitude control of the natural oscillation of the measuring probe 400 by a few periods of the relaxation oscillation. In the example shown in FIG. 7, a fixed waiting time is implemented in the form of a 1-bit signal.A second portion of the digitized input signal 715 is provided to the FIFO memory 775 of the self-oscillation circuit 790. The memory 775 realizes the function of a phase shifter. The delay of the FIFO memory 775 is determined by the quotient of the memory depth (e.g., 10 memory cells) and the clock frequency of the self-oscillation circuit 790. The output signal 780 of the FIFO memory 775 represents the phase component φ of the digital excitation signal 785 and is supplied to the multiplication unit 755.The clock rate generated by the clock generator CLK 760 is provided to the ramp generator 765, the FIFO memory 775 and the PID controller 730 during the period that the amplitude control is enabled. With the aid of the signal Sp (set point), the setpoint value of the oscillation amplitude of the natural oscillation of the cantilever 410 is set at the PID controller 730.The ramp generator 765 generates the signal 767 for bending the cantilever 410 towards the sample surface by activating the second actuator 490 of the cantilever 410. By means of the signal res for reset, the counter of the ramp generator 765 is stopped by the comparator 745 and the voltage ramp 767 at the output of the comparator 745 is reset to the initial value. As a result, the laser system 630 is switched off and the measuring tip 460 of the measuring probe 400 is pulled back from the sample surface 515.The multiplication unit 755 multiplies the signals of the outputs of the memory 775, i.e. the phase component φ and the sample-and-hold circuit 740, i.e. the amplitude component A of the excitation signal of the natural oscillation of the measuring probe 400. The summation element Σ 770 adds the output signals 767 of the ramp generator 765 and the multiplication unit 755.The output signal 785 of the self-oscillation circuit 790 is converted by the digital-to-analog converter DAC 795 into an analog excitation signal 665, which is supplied to the laser system 630. If, as shown in FIG. 5, a first laser system 530 for exciting the natural oscillation measuring probe 400 and a second laser system 570 for bending the cantilever 410 are used, the output signal of the multiplication unit 755 is supplied to the first laser system 530 after a digital-to-analog conversion. The output signal 767 of the ramp generator 765 controls the second laser system 570 after a corresponding digital-to-analog conversion. In this embodiment, the self-oscillation circuit 790 does not need the summation element 770.FIG. 8 presents a cycle of a scanning mode or a step-in scanning mode of the scanning probe microscope 500. The explanation of the diagram starts at the upper left. In the initial state, the measuring probe 400 is excited to oscillate at its natural frequency. In this state, the first laser system 530 is switched on in FIG. 5 and activates the first actuator 480 of the cantilever 410 of the measuring probe 400. In the first step, the continuous wave output power of a laser system, for example of the laser system 630, is increased or the second laser system 570 is switched on in order, by bending the cantilever 410 caused by the second actuator 490 of the cantilever 410, to lower the measuring tip 460 of the measuring probe 400 onto the sample surface 515.In the next block or step, the process waits until the amplitude of the natural oscillation of the measuring probe 400 falls below a predefined threshold. As explained in the context of FIG. 7, this is achieved by continuously measuring the amplitude of the natural oscillation of the measuring probe 400 with the aid of the detector 545 and comparing it with a predefined threshold value.If the measuring tip 460 of the measuring probe 400 is in contact with the sample surface 515, the excitation of the natural oscillation of the measuring probe 400 is ended in the third block by switching off the first laser system 530. In the laser system 630 of FIGS. 6 and 7, the sinusoidal excitation signal is stopped.In the next step, in the fourth block, the vertical position of the measuring tip 460 is measured with the aid of the third laser system 540 and the detection unit 585. The measurement of the vertical position of the measuring tip 460 with the second detection unit 585 takes place immediately after the measurement signal of the detector 545 enables the contact of the measuring tip 460 with the sample surface 515 to be established. As a result, the time interval for a step-in measurement cycle can be kept small. After the position of the measuring tip 460 has been determined, the bending of the cantilever 410 by the second actuator 490 is stopped by the switching off of the second laser system 570. In FIGS. 6 and 7, the laser system 630 is turned off.Then, in the fifth step or block, the test tip 460 is waited until it returns from the sample 510. In the context of the discussion of FIG. 6, three alternative methods are specified, on the basis of which it is possible to determine the point in time at which this occurs or how the point in time at which the first laser system 530 is reactivated or at which a sinusoidal excitation signal is applied to the laser system 630. Controlled by the excitation unit 560, the self-oscillation circuit 590, 790 generates an excitation 675 or an excitation signal 675 at the resonant frequency of the measuring probe 400 and superimposes the latter in the correct phase on the oscillation of the measuring probe 400 generated by the emission of the measuring tip 460. For this purpose, the laser system 530 or 630 activates the first actuator 480 of the cantilever 410.Then, in the sixth step, the sample-and-hold circuit 640, 740 is switched from the sample mode to the hold mode. As already explained above, this prevents the amplitude control of the natural oscillation of the measuring probe 400 from damaging the sample 520, the cantilever 410 and / or the measuring tip 460 of the measuring probe 400 when the measuring tip 460 approaches the sample surface 515. The SPM 500 is then prepared for another scan cycle.As already explained above, FIGS. 2 and 3 show the excitation of a vibration of the cantilever 410 of a measuring probe 400 at its resonant frequency by the jumping back of the measuring tip 460 of the cantilever 410 from the sample surface 515. FIG. 9 presents an example of a vibrational excitation of the cantilever 410 when the measuring tip 460 is lifted off the sample surface 515. Similar to FIG. 3, the oscillation of the cantilever 410 excited by the springback of the measuring tip 460 from the sample surface 515 is only weakly attenuated. This can occur, for example, if the spring constant of the cantilever 410 is small and the measuring probe 400 is operated in a vacuum environment, i.e. the ambient pressure experienced by the measuring probe 400 is in the region of a pascal or significantly below.In Figs. 9 and 10, the abscissa represents time in arbitrary units. The ordinate of the two figures shows the distance of the measuring tip 460 from the sample surface 515 in nanometers. In the initial situations, the measuring tip 460 has an average distance of 100 nm from the sample surface 515. The cantilever 410 of the measuring probe 400 oscillates with an amplitude of approximately 10 nm. At the time T 1 the second laser system 570 is switched on and the second actuator 490 of the cantilever 410 reduces the mean distance between the measuring tip 460 and the sample surface 515. At the time T 2 the control unit 610 detects that the amplitude of the oscillation of the measuring probe 400 has dropped below a predetermined threshold and switches off the first laser system 530. At time T 3 the measurement of the vertical position of the measurement tip 460 is performed. At time T 4 the control unit 610 turns off the second laser system 570.In FIG. 9, the excitation frequency of the cantilever 410 during the lowering of the measuring tip 460 of the measuring probe 400 onto the sample surface 515 is close to the resonance frequency of the cantilever 410, but is not identical thereto. After detaching the measuring tip 460 from the sample surface 515, the cantilever 410 carries out a suspension 950 which is caused by the superimposition with an excitation, the frequency of which is different from the natural frequency, and an excitation with the natural frequency of the measuring probe 400 which was induced by the jumping away of the measuring tip 460 from the sample surface. This beat 950, which continues until the natural oscillation of the measuring probe 400 has substantially decayed, prevents a new step-in cycle from being started before this point in time. The sampling rate of the step-in sampling mode is thus low, which results in long examination times of the sample surface 515.FIG. 10 presents the configuration of FIG. 9 with the difference that the excitation 675 of the measuring probe 400 takes place exactly at its resonance or natural frequency. In this case, the natural oscillation 1150 of the measuring probe 400 excited by the jumping back of the measuring tip 420 from the sample surface 515 helps to restore the natural oscillation 1050 present when the measuring tip 420 is lowered onto the sample surface 515 as quickly as possible. It is merely necessary to wait until the amplitude of the natural oscillation 1050 of the measuring probe 400 has dropped to the level during the approach of the measuring tip 420 to the sample surface 515.The beat 950 of FIG. 9 enters directly into the measurement error of the determination of the vertical position of the measurement tip 460. It is therefore necessary to wait with the next step-in measurement cycle until the amplitude of the natural oscillation has decayed below a tolerable measurement error. Typically, the tolerable measurement error is significantly less than 1 nm. In the case of the self-oscillation shown in FIG. 10, it is only necessary to wait until the amplitude of the self-oscillation reaches the setpoint value of the amplitude of the self-oscillation 1050. These differences are intended to be illustrated by way of a simple example.The measuring probe 400 has a quality factor Q=1000. The maximum amplitude of the natural oscillation 1050 amounts to 300 nm. In the case of the beat 950 explained in FIG. 9, it is necessary that the amplitude of the natural oscillation must have decayed to 0.5 nm before a new step-in measurement cycle can be started. The amplitude of the natural oscillation 1050 illustrated in FIG. 10 amounts to 50 nm. Without excitation, the amplitude of the natural oscillation of the measuring probe 400 decays exponentially. In order for the amplitude of the natural oscillation to drop to 50 nm, a period of about 150 oscillation periods is necessary, whereas a drop to an amplitude of 0.5 ns requires a period of about 500 oscillation periods. By exciting the measuring probe 400 at its natural frequency, the time duration of a step-in measuring cycle can thus be reduced by approximately a factor of three.Finally, FIG. 11 shows a flow diagram 1100 of a method which can be used to increase the scanning speed of a scanning probe microscope 500 during the step-in scanning mode. The method begins at 1110. At step 1120, a measurement probe 400 is scanned over a sample surface 515 in a step-in scan mode. In step 1130, the measuring probe 400 is excited to oscillate naturally during the step-in scanning mode using a self-oscillation circuit. The method ends at step 1140.

Claims

A scanning probe microscope (500) comprising: a. a scanning unit (555) configured to scan a measuring probe (400) over a sample surface (515) in a step-in scanning mode; and b. a self-oscillation circuit (590, 790) configured to excite the measuring probe (400) to a natural oscillation (1050) during the step-in scanning mode; c. wherein the self-oscillation circuit (590, 790) comprises a phase shifter (630) configured to adjust a phase of the excitation (675) relative to the natural oscillation (1050) of the measuring probe (400); d. wherein the self-oscillation circuit (590, 790) comprises an automatic gain controller (670) configured to adjust an amplitude of the natural oscillation (1050) of the measuring probe (400); and e. wherein the automatic gain control (670) comprises at least one amplifier (620), a sample-and-hold circuit (640, 740) and a control unit (610), and wherein the control unit (610) is configured to switch the sample-and-hold circuit (640, 740) between (630) a sample mode and a hold mode.Scanning probe microscope (500) according to Claim 1, wherein the phase shifter (630) is designed to set the excitation (675) with a phase difference with respect to the best possible excitation of the natural oscillation (1050) of the measuring probe (400) in the range of ±30°, preferably ±20°, more preferably ±10°, and most preferably ±5°.Scanning probe microscope (500) according to one of the preceding claims, wherein the self-oscillation circuit (590, 790) is designed as a digital circuit (700).The scanning probe microscope (500) according to any one of the preceding claims, wherein the self-oscillation circuit (590, 790) is embodied as a field programmable gate array, FPGA, (700) or as an application specific integrated circuit, ASIC.Scanning probe microscope (500) according to one of the preceding claims, further comprising a first actuator (480) which is designed to transmit the excitation of the self-oscillation circuit (590, 790) to a cantilever (410) of the measuring probe (400) and a second actuator (490) which is designed to transmit a signal of the control unit (610) of the automatic gain control (670) to the cantilever (410) of the measuring probe (400).Scanning probe microscope (500) according to the preceding claim, further comprising a first laser system (530) which is configured to transmit the excitation of the self-oscillation circuit (590, 700) to the first actuator (480) and a second laser system (570) which is configured to transmit the signal of the control unit (610) to the second actuator (490).Scanning probe microscope (500) according to Claim 5 or 6, wherein the first actuator (480) and the second actuator (490) are designed as a bimorph actuator (440).The scanning probe microscope (500) of claim 7, wherein the cantilever (410) of the measuring probe (400) comprises the bimorph actuator (440).Scanning probe microscope (500) according to one of Claims 5 - 8, further comprising a detector (545) which is designed to detect a deflection of the cantilever (410) of the measuring probe (400), and a detection unit (585) which is designed to detect a vertical position of a free end (450) of the cantilever (410) of the measuring probe (400).Scanning probe microscope (500) according to one of the preceding claims, further comprising a control device (550) comprising the scanning unit (555) and an excitation unit (560), wherein the excitation unit (560) is configured to control the self-oscillation circuit (590, 790).Scanning probe microscope (500) according to one of the preceding claims, wherein the amplitude of the natural oscillation (1050) of the measuring probe (400) comprises a range from 1 nm to 1000 nm, preferably 5 nm to 700 nm, more preferably from 10 nm to 500 nm, and most preferably from 20 nm to 200 nm.A method for increasing a scanning speed of a scanning probe microscope (500) operating in a step-in scanning mode, by the steps of: a. scanning a measuring probe (400) in a step-in scanning mode over a sample surface (515); b. exciting the measuring probe (400) to an eigenwave (1050) during the step-in scanning mode with a self-oscillation circuit (590, 790); c. adjusting a phase of the excitation (675) relative to the eigenwave (1050) of the measuring probe (400) with the aid of a phase shifter of the self-oscillation circuit (590, 790); d. adjusting an amplitude of the eigenwave (1050) of the measuring probe (400) with an automatic gain controller (670) of the self-oscillation circuit (590, 790); and e. switching a sample-and-hold circuit (640, 740) of the automatic gain control (670) from a sample mode to a hold mode when the measuring probe (400) approaches a sample surface (515).The method of claim 12, wherein steps a. to e. comprise: f. in step b.: activating a first actuator (480) configured to transmit the excitation of the self-oscillation circuit (590, 790) to the measurement probe (400); g. in step a.: activating a second actuator (490) configured to change a distance between a measurement tip (460) of the measurement probe (400) and a sample surface (515); and h. in step e.: detecting contact between the measurement tip (460) of the measurement probe (400) and the sample surface (515).The method according to claim 12 or 13, wherein steps a. to e. further comprise the sequence of steps: i. in step b.: activating the second actuator (490); j. in step e.: deactivating amplitude control by switching a sample-and-hold circuit (640, 740) from a sample mode to a hold mode; k. in step e.: deactivating the first actuator (480) if an amplitude of the natural oscillation (1050) of the measurement probe (400) falls below a predetermined threshold; l. in step e.: determining a vertical position of the measurement tip (460) of the measurement probe (400) after detecting contact of the measurement tip (460) with the sample surface (515); In step e.: Deactivation of the second actuator (490) and waiting for a predetermined period of time until a loss of contact occurs between the measuring tip (460) of the measuring probe (400) and the sample surface (515); n. In step b.: In-phase activation of the first actuator (480); and o. In step d.: Activation of the amplitude control by switching the sample-and-hold circuit (640, 740) from the hold mode to the sample mode.The method of claim 13, wherein detecting contact between the measurement tip (460) of the measurement probe (400) and the sample surface (515) comprises determining a vertical position of the measurement tip (460) of the measurement probe (400) at that point.Method according to one of Claims 12-15, further comprising the step of: determining a switch-on time for the in-phase activation of the first actuator (480) from a decay curve of the natural oscillation (1050) of the measuring probe (400) without activation of the first actuator (480).A computer program comprising instructions which, when executed by a computer system, cause the computer system to carry out the method steps of any of claims 12-16.

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