Scanning probe microscope and alignment, focusing and measurement method

Abstract

A scanning probe microscope and method of operating a scanning probe microscope includes selecting a preferred focal position of a focused optical beam on a probe of the scanning probe microscope by adjusting a focal position of a focused optical beam on the probe relative to a tip of the probe and then measuring at least one of a response of the probe and optical radiation scattered from the probe as a function of the position of the focused optical beam. The preferred focal position of the focused optical beam on the probe is based on the measurement of at least one of the response of the probe and optical radiation scattered from the probe.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Patent Application Serial No. 63 / 306,450, filed February 3, 2022, which is incorporated by reference herein.

[0002] In scanning probe microscopy (SPM) techniques, which rely on illumination of the tip-sample interface with an optical beam to measure optical or other sample attributes in the near-field region around the tip, alignment of the optical beam to the tip is crucial. [Background technology]

[0003] Furthermore, in measurements involving tunable laser sources where multiple wavelengths are used in spectroscopic measurements, it is also very important to measure the local fluence incident on the tip. As used herein, this fluence is the radiant energy received by a surface per unit area. This fluence measurement can then be used to normalize the optical response of the sample as a function of wavelength by removing the effects of laser power changes, optical losses in the beam path and spot size. Existing methods of alignment and fluence measurement that rely on measuring the response when the tip is contacted to the surface of the sample are adversely affected by the local surface attributes of the sample (including topographical, mechanical and optical attributes). These local surface attributes of the sample may not be optimal or repeatable, resulting in beam alignment that makes the measurement less reliable. Local surface attributes are also included in any background spectrum acquired, which directly affects the spectral normalization in spectroscopic measurements involving tunable light sources. Summary of the Invention [Problem to be solved by the invention]

[0004] Moreover, bringing the tip into close proximity with the sample surface can lead to excessively large tip fluctuations due to optical interactions, posing a risk of tip damage if the tip comes into contact with this surface. In turn, such tip-surface interactions can reduce or limit the amplitude of the tip vibrations, thus complicating the alignment procedure and limiting the speed and signal-to-noise ratio of measurements of the tip response during alignment by limiting the amplitude to small values. [Means for solving the problem]

[0005] A scanning probe microscope and method of operating a scanning probe microscope includes selecting a preferred focal position of a focused optical beam on a probe of the scanning probe microscope by adjusting a focal position of a focused optical beam on the probe relative to a tip of the probe and then measuring at least one of a response of the probe and optical radiation scattered from the probe as a function of the position of the focused optical beam. The preferred focal position of the focused optical beam on the probe is based on the measurement of at least one of the response of the probe and optical radiation scattered from the probe.

[0006] A method of operating a scanning probe microscope with a focused optical beam on a probe of the scanning probe microscope according to an embodiment of the present invention includes illuminating the focused optical beam on the probe of the scanning probe microscope in the absence of a sample so as to interact with the focused optical beam on the probe, adjusting a focal position of the focused optical beam on the probe relative to a tip of the probe, measuring at least one of a response of the probe and optical radiation scattered from the probe as a function of the position of the focused optical beam, and selecting a preferred focal position of the focused optical beam on the probe based on the measurement of at least one of the response of the probe and the optical radiation scattered from the probe.

[0007] A method of operating a scanning probe microscope with a focused optical beam on a probe of the scanning probe microscope according to an embodiment of the present invention includes illuminating the focused optical beam on the probe of the scanning probe microscope in the absence of a sample so as to interact with the focused optical beam on the probe, adjusting a focal position of the focused optical beam on the probe relative to a tip of the probe, measuring a response of the probe as a function of the position of the focused optical beam, and selecting a preferred focal position of the focused optical beam on the probe based on the measurement of the response of the probe.

[0008] A scanning probe microscope according to an embodiment of the invention includes a cantilever having a probe, a light source for illuminating the probe with a focused optical beam, an optical system for adjusting a focal position of the focused optical beam on the probe, a dither device coupled to the cantilever for driving the cantilever to oscillate, an optical detection system for measuring a response of the probe to the focused optical beam, and a control unit coupled to the optical system and the optical detection system for selecting a preferred focal position of the focused optical beam on the probe based on the measured response of the cantilever to different adjusted focal positions of the focused optical beam on the probe.

[0009] Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. [Brief description of the drawings]

[0010] [Figure 1] 1 shows an example of a scanning probe microscope that relies on illumination of the tip-sample interface by an optical beam, to which embodiments of the present invention can be applied. [Diagram 2] 1 illustrates the beam waist of a focused light beam, the tip of a probe on a cantilever of a scanning probe microscope, and the direction of the resulting trapping force, according to an embodiment of the present invention. [Diagram 3]1 illustrates detection of light scattered by the tip of a probe in the presence of a focused incident beam, according to an embodiment of the present invention. [Figure 4] 2 shows the scanning probe microscope of FIG. 1 with a beam splitter and a scattered light detector according to an embodiment of the present invention. [Diagram 5] 5 illustrates dynamic correction of beam alignment using the position of focusing element 502 and the angle of the steering mirror, according to an embodiment of the present invention. [Figure 6] 1 is a process flow diagram of a method of operating a scanning probe microscope using a focused optical beam on a probe of the scanning probe microscope, in accordance with an embodiment of the present invention. In the detailed description, like reference numbers may be used to identify like elements. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] In the detailed description, like reference numbers may be used to identify like elements.

[0012] In the problem of aligning an optical beam to a probe tip in a scanning probe microscope that relies on illumination of the tip-sample interface, as discussed above, what is needed is an alignment method for aligning an optical beam and measuring the local fluence on the tip of the probe in "free space", i.e., without the presence of a sample, e.g., without bringing the tip of the probe into measurement proximity with the surface of the sample such that there is no interaction between the probe and the sample. As used herein, measurement proximity is when the surface of the sample is sufficiently close to the tip of the probe such that when the optical beam is focused on the probe, there is an interaction between the probe and the sample. An optical alignment method that does not require the presence of a sample is free of the adverse effects caused by local attributes of the sample surface.

[0013] An embodiment of the invention includes a method for optimized free-space alignment of an optical beam with respect to the probe tip, and a method for measuring the intensity at different wavelengths that are used for normalization. After alignment and intensity measurement, the surface of the sample is brought close to measurement proximity with the tip, the beam is aligned to the tip, and no additional adjustments to the alignment are required. This alignment method provides optimal and repeatable measurement results regardless of the attributes of the sample.

[0014] There are a variety of SPM techniques that rely on optical illumination of the tip-sample interface, including photothermal induced resonance (PTIR), photo-induced force microscopy (PiFM), scanning scattering near-field optical microscopy (sSNOM) and tip-enhanced Raman spectroscopy / microscopy. Procedures for aligning the focused optical beam at the tip-sample interface and determining its fluence are necessary for all such techniques. For some of these techniques, a tunable infrared (IR) laser source may be used to measure wavelength-dependent optical attributes of the sample with high spatial resolution, such as generating local IR absorption spectra for material identification purposes. When a tunable source is used, not only does the fluence of the focused beam at the tip-sample interface need to be statistically adjusted and determined; dynamic changes in steering and focus due to changes in the wavelength of the light source may require that the position and / or focus of the beam be adjusted frequently or even continuously for optimal performance as the wavelength changes.

[0015] One method for making such adjustments is described in U.S. Patent No. 8,242,448 to Prater et al., entitled "Dynamic Power Control, Beam Alignment and Focus for Nanoscale Spectroscopy." In this patent, as described in claim 1, a method for obtaining measurements of wavelength-dependent optical attributes with a probe microscope and an IR radiation source that is tunable over a range of wavelengths includes the steps of (a) interacting a probe of the probe microscope with a region of a sample, (b) adjusting a position of a radiation beam emitted from the radiation source with a beam steering mechanism, (c) directing a beam from the radiation source and the beam steering mechanism to a region of the sample under the probe, (d) measuring a response of the probe due to the interaction of the radiation with the sample at at least one wavelength of radiation, and (e) automatically adjusting the beam steering mechanism to substantially maximize the measured response of the probe.

[0016] Although this method is useful for aligning the optical beam to the tip-sample interface, the presence of an unknown sample surface introduces uncontrolled variables due to local sample topography, mechanical attributes such as hardness and attenuation, and optical attributes such as transmission, scattering, or absorption. These uncontrolled variables may ultimately result in alignment that is neither optimal nor repeatable. In turn, bringing the tip into close proximity with the sample's surface limits the available operating range of the tip's oscillation amplitude, which negatively impacts the measurement speed and signal-to-noise ratio of the oscillation amplitude. Embodiments of the present invention, as described herein, eliminate these problems.

[0017] An example of a scanning probe microscope 100 that relies on illumination of an optical beam at the tip-sample interface, to which embodiments of the present invention may be applied, is illustrated in Figure 1. As shown in Figure 1, the scanning probe microscope 100 uses an optical beam focused on its probe 102 to measure the optical or other sample attributes of the sample 104 in a near-field region around the tip of the probe. The focused optical beam is provided by a light source 106, which may be a tunable infrared (IR) laser.

[0018] The probe 102 is attached to a cantilever 108, which is coupled to a dither piezo device 110 to oscillate the cantilever 108. The dither piezo device 110 is controlled by a first frequency generator 112, which provides a drive signal to the dither piezo device 110 at a frequency f1.

[0019] The tunable laser 106 is controlled by a second frequency generator 114, which provides a drive or modulation signal at a frequency f2 to the tunable laser 106. The optical beam from the tunable laser 106 to the probe 102 can thereby be modulated at a frequency f2.

[0020] The sample 104 is placed on a sample z piezo device 116, which can move the sample in the z-direction, i.e., towards or away from the probe 102. The sample z piezo device 116 is controlled by a sample z controller 118, which can move the sample z piezo device as required. When the optical beam from the tunable laser 106 is aligned, as described herein, the sample 104 can be removed from the scanning probe microscope 100 or moved far enough away from the probe 102 that alignment can be performed in "free space".

[0021] As shown in FIG. 1, the scanning probe microscope 100 further includes a position feedback laser 120, an optical position detector 122, a first synchronous detector or amplifier 124, and a second synchronous detector or amplifier 126. The optical position detector 122 operates to optically measure the vibration of the cantilever 108 using light from the position feedback laser 120 that reflects off the cantilever. The signal from the optical position detector 122 is input to a first synchronous amplifier 124, which can provide the amplitude and phase of the cantilever vibration using the drive frequency f1 as a reference. The first synchronous amplifier 124 can also provide an amplitude error signal to the sample z controller 118 to maintain the spacing between the tip of the probe and the sample 104 at a desired spacing. The position feedback laser 120, the optical position detector 122 and the first synchronous detector 124 are part of an optical detection system that can measure one or more responses of the probe 102 to the focused optical beam from the tunable laser 106 during the alignment process, as described herein.

[0022] The signal from the optical position detector 122 is also input to a second synchronous amplifier 126, which can provide the amplitude and phase of the cantilever vibration to a computer control unit 128 using the modulation frequency f2 as a reference. The amplitude and phase signals from the first and second synchronous amplifiers 124 and 126 are sent to a computer control unit 128, which can process the received signals. The computer control unit 128 can also control the frequency generators 112 and 114 to adjust the generated frequencies f1 and f2, which can also be at least partially responsive to the received phase and / or amplitude signals. Furthermore, the computer control unit 128 can adjust the focal position of the focused optical beam from the tunable laser 106 using an optical system 130, which can include one or more focusing elements and one or more steering mirrors that can be controlled by the computer control unit. In an embodiment, the alignment of the optical beam from the laser 106 to the optical axis of the focusing optics or element is performed by a steering mirror. Once this is done, the focusing optics are moved in XYZ using piezos (fine movements) and stepper motors (hard movements) to position the focal spot. After the spot is found and located, the focal spot is moved so that this focal point is relative to the tip of the probe. In an alternative embodiment, the tip of the probe may move instead of the optical beam moving.

[0023] Optical alignment of a focused beam of light to a probe tip of a scanning probe microscope, such as the scanning probe microscope 100 shown in FIG. 1, is performed by positioning the focal point of the light beam near the probe tip (in "free space" with no nearby sample surface) and measuring the mechanical vibration of the cantilever probe or detecting the scattered light from the probe tip as a function of focal position. The optimal focal position may be the position where the maximum amplitude of the scattered light vibration or intensity is observed, or an empirically derived offset from such a position. Alternatively, the optimal focal position may be determined by measuring the phase of the vibration (relative to the driving source) or the resonant frequency of the cantilever probe. When aligning by measuring the amplitude or phase of the vibration, the focused light beam may be modulated, preferably at or near the resonant frequency of the cantilever probe. After the optical alignment procedure is completed, the local fluence profile can be measured by measuring the signal strength of the mechanical vibration of the cantilever probe as the wavelength of the light source is changed, or by detecting the scattered light from the probe tip. This local fluence profile provides the normalization information necessary to mathematically or dynamically correct the optical response of the sample in real time.

[0024] Focusing an optical beam on a cantilever probe affects the properties of the probe in at least three ways. First, absorption of light heats the cantilever probe, resulting in deflection of the probe due to thermal expansion or temperature / material gradients, due to temperature-dependent changes in the elasticity of the materials used in the cantilever probe structure or thermal expansion that changes the weight distribution of the cantilever probe structure and / or shifts the resonant frequency of the probe. Second, optical forces such as optical trapping forces that arise near the beam waist of the focused beam on the probe tip can deflect the cantilever probe. Optical trapping forces are described in detail in "Optical Trapping" by Keir C. Neuman and Steven M. Block (Reviews of Scientific Instruments, Volume 75, pages 2787-2809, September 2004) (doi:10.1063 / 1.1785844). FIG. 2 illustrates the beam waist 202 of a focused light beam 204, the tip 206 of a probe 208 on a cantilever 210 of a scanning probe microscope, and the resulting direction 212 of the trapping force (F0). As illustrated in FIG. 2, the tip 206 of the cantilever probe is positioned near the beam waist 202 of the focused light beam. The optical trapping force acts on the microscope body to push it towards a position near the center of the beam waist. The optical trapping force (F0) thereby acts on the tip to push the tip towards the central region of the beam waist. The vertical component of this force deflects the cantilever down. Third, a force gradient on the probe generated by the presence of the optical beam can shift the resonant frequency of the cantilever probe. Such a force gradient may occur near the beam waist of the focused beam in conjunction with the optical trapping force. If the intensity of the optical beam is modulated in frequency, the first two effects mentioned above can cause the cantilever probe to vibrate.

[0025] Using a focused optical beam directed at the probe tip in free space, a preferred position of the beam focus relative to the tip of the probe can be determined by observing the amplitude (relative to the modulation) and / or phase of the probe vibration. For example, a preferred focus position can be selected by selecting the position at which the amplitude of the vibration is maximized. Alternatively, a particular location of the focus relative to the tip may be established as the preferred focus position by thoroughly studying the position of the focus relative to the tip and how this affects the performance of the scanning probe microscope in a target mode of operation.

[0026] The third effect mentioned above (frequency shift caused by the force gradient arising from the optical beam) does not necessarily require modulation of the beam, although the beam may be modulated. A preferred focal position can be established by observing how the resonant frequency of the cantilever vibration mode shifts as a function of focal position due to the effect of the gradient arising from the optical beam. For example, if the focal position of the beam is adjusted relative to the tip of the probe, the preferred position can be selected as the point where the cantilever frequency shift (relative to the case without illumination) is maximum. Alternatively, the preferred position may be selected by selecting a position a certain distance away from the point of maximum frequency shift, or a position having a specific amount of frequency shift.

[0027] The resonant frequency of a cantilever probe can be measured in a variety of ways: 1. Observe the peak amplitude of the thermal excitation as a function of frequency and identify the peak frequency as the resonant frequency. 2. Drive the cantilever with a modulated light source or other light source such as a piezoelectric element and find the peak amplitude response as a function of frequency. 3. Drive the cantilever with a modulated light source or other light source such as a piezoelectric element and measure the phase shift (with respect to the driving excitation) as a function of frequency. 4. Drive the cantilever with a modulated or other light source and synchronize the drive frequency to the resonance frequency with a phase-locked loop or by using the resonance as a frequency-determining factor in a self-oscillating system. 5. Drive the cantilever probe simultaneously at multiple frequencies, e.g., excite the probe with a piezo driven by a white noise source, measure the frequency content of the resulting probe vibration (e.g., by performing a Fourier transform on the cantilever deflection signal), and identify the frequency of peak amplitude as the resonant frequency.

[0028] Instead of detecting mechanical movements or properties of the probe, an alternative is to detect light scattered by the probe tip in the presence of a focused incident beam. As illustrated in FIG. 3, when the probe tip 206 enters the light beam 204, some scattered light 300 is generated and may be detected by a sensitive photon detector 314, such as a photodiode or a mercury cadmium telluride (MCT) detector (for visible and near infrared light). Since the intensity of the scattered light is proportional to the intensity of the incident light, scattering is greatest at the beam waist. That is, scattering is greatest when the tip is in the most intense region of the incident beam (i.e., at the beam waist).

[0029] In a scanning probe microscope, a beam splitter may be used to detect light scattered from a probe without the presence of a sample. This is illustrated in Figure 4, which shows a scanning probe microscope 100 with a beam splitter 432 arranged to send scattered light from the probe 102 to a scattered light detector 434. The detected scattered light signal can be input to a second synchronous detector 126, and the amplitude and / or phase of the signal can be measured using the modulated signal from the frequency generator 114 as a reference. The sample z piezo device 116, sample z controller 118 and optical system 130 are not shown in Figure 4.

[0030] The sensitivity of the scattered light measurement can be improved by vibrating the cantilever (e.g., with a piezoelectric element) so that the position of the tip relative to the beam waist is oscillated. The use of synchronous detection can suppress the signal from background light and measure the scattered light intensity with maximized signal-to-noise ratio.

[0031] A further improvement to the optical detection system is to use an interferometer instead of the simple optical detector 314 shown in Figure 3. Such an interferometer is useful if a coherent light source such as a laser is used. A portion of the incident light beam interferes with the scattered light, allowing measurement of both the amplitude and phase of the scattered light, which can provide additional information for performing alignment of the incident beam to the probe tip.

[0032] For both mechanical and optical detection of the interaction of the focused light with the probe tip, the optimum alignment may not simply correspond to the position of the tip that provides the maximum vibration, frequency shift, or scattered light intensity. Since the tip is not merely a point, but has a body (the tip barrel) that extends upward from the tip, typically in a cone or pyramid shape, some response is expected when the tip is near the center of the beam waist, as well as when the barrel is illuminated. Due to the nature of the field enhancement that occurs near the pointed tip, there may be a local maximum in the magnitude of the effect when the tip is near the center of the waist, but the tip position that corresponds to the local maximum in response may be offset as a result of the barrel's interaction with the probe barrel, and thus the cantilever beam, with an additional local maximum in non-zero response. The ideal alignment position can be determined empirically by testing various tip positions relative to the position of maximum response and by checking the performance of the microscopy technique in use. For example, with PiFM, after alignment to a trial position, the surface of the sample can be brought close to the tip (while keeping the alignment of the incident focused light beam unchanged) and the magnitude of the PiFM signal, the fidelity of the absorption spectrum, or some similar measurement can be measured to evaluate a particular alignment position relative to other positions. After such empirical studies, the best alignment position (relative to the position of maximum response) can be determined and used routinely.

[0033] Depending on the type of light source in use, the position of the focused beam waist in space (relative to the light source) may or may not be fixed. In particular, with tunable sources such as quantum cascade lasers, which are typically used as tunable infrared sources for PiFM, small changes in the pointing or focus of the laser may move the beam waist position while sweeping across the wavelength tuning range of the light source during spectral acquisition. Deviations introduced by such dynamic changes in the beam waist position can be dynamically corrected. Measurements collected during the free-space tip alignment procedure can be used to perform such dynamic alignment.

[0034] An example of using free-space alignment measurements to perform dynamic correction of beam alignment is as follows: As a tunable light source, perform measurements of the probe vibration amplitude, vibration phase, resonant frequency or scattered light intensity / phase as a function of beam waist position at multiple different wavelengths by scanning the position of the focusing element, the angle of the steering mirror, or a combination of the two (exemplified in FIG. 5). As shown in FIG. 5, the position of the focusing element 502 can be scanned and the angle of the steering mirror 504 can be varied. The focusing element 502 and steering mirror 504 may be part of the optical system 130 in the scanning probe microscope 100 shown in FIG. 1. At each wavelength, record the position of optimal alignment versus the control signal for the focusing element and / or steering mirror. Adjust the position of the focusing element and / or the angle of the steering mirror to keep the beam waist position properly aligned to the tip while sweeping over part or all of the tuning range of the light source during spectral acquisition.

[0035] The number of wavelengths at which alignment measurements need to be made depends on the pointing and focus change behavior of the light source as it is tuned through its range of wavelengths. If very complex behavior is observed (i.e., behavior that cannot be easily approximated from measuring a small number of wavelengths), then a large number of alignment measurements must be made as a function of wavelength.

[0036] Most tunable light sources have a unique output as a function of wavelength. This unique output is specific to the individual light source module. In principle, one could measure the output as a function of wavelength using a conventional power meter, but such an approach may not be representative of the effective fluence at the tip. The alignment of the optical system and optical components between the light source and the imaging location may affect the optical fluence at the imaging probe and the spectral response on the sample, as may the physical attributes of the particular probe tip being used in the experiment. To be successful in mathematical or real-time normalization by use of an optical attenuation system, the optical response of the cantilever needs to be characterized as a function of wavelength. The recorded power profile as a function of wavelength can be used in a variety of ways, including independent methods or a combination of methods for normalization of the optical response of the sample. First, it is necessary to measure the optical response of the cantilever as a function of wavelength with the probe away from the surface of the sample, where the resulting spectrum is the power profile for the tunable light source. Additionally, for cases of high power sources that may cause damage to the probe, it may be advantageous to record the power profile with a fixed percentage attenuation.

[0037] Once the power profile has been recorded, any resulting spectrum taken at the surface of the sample can be normalized by a variety of methods, such as: 1. Actively adjusting the power at the tip-sample boundary using a rotatable polarizer or other controllable attenuating device (e.g.: neutral density filter, acousto-optical modulator or electro-optical modulator) or by controllable internal parameters of the light source (e.g.: pulse duration, power output), where the position / setting of the polarizer or other controllable attenuating device is mathematically predetermined by the power profile recorded far from the surface of the sample. 2. The spectral response at the tip-sample boundary is mathematically divided by the power profile as a function of wavelength. 3. For measurements where the power profile is lower in some wavelength region than the desired maximum polariscope or other controllable attenuation transmission, the resulting spectra can be mathematically post-processed with the position / setting of the known attenuating device as a function of wavelength and power profile.

[0038] In most SPM techniques that rely on optical illumination of the tip-sample interface, the light source is modulated or pulsed at a frequency to improve the signal-to-noise ratio. For many tunable laser sources, the intensity profile changes with different modulation or pulse frequencies and pulse durations, which in turn will generate a local fluence profile that varies with different modulation or pulse frequencies. Because different SPM measurement schemes require different modulation (or pulse) frequencies, to achieve the best normalization, it is important that measurements of the local fluence are collected at frequencies close to the pulse durations and modulation (or pulse) frequencies utilized in the actual measurements. For example, in one scheme, the probe may track the topography of the sample by using the second resonant mode of the cantilever while the laser is pulsed at the cantilever's first resonant frequency. However, in other schemes, the probe may track the topography of the sample by using the second resonant mode of the cantilever while the laser is pulsed at the difference between the second resonant frequency of the cantilever and the first resonant frequency, but the optical response is measured at the first resonant frequency of the cantilever; in this scheme, the nonlinear behavior of the optical response as a function of the tip-sample gap spacing is mixed with the optical response due to the periodic laser pulses to generate a measurable signal at the difference frequency, which is the first resonant frequency due to the choice of pulse frequency. In the latter scheme, where mixing occurs, the power profile cannot be effectively measured at the pulse frequency because the probe does not respond effectively, and the power profile is measured at the second resonant frequency, which is much closer to the actual pulse frequency than the first resonant frequency. In these examples, the use of the first and second resonances proves the concept, and other resonance pairs may be used in a similar manner.

[0039] When using a tunable light source, the required sweep speed must be considered. Since the probes used in most SPM techniques are based on mechanical resonators, it is important to consider their response time based on their quality factor. Therefore, for accurate recording of the probe's spectral response as a function of wavelength, the sweep speed of the light source must be controlled so that the resonator can respond to the changing power with changing wavelength. Similar considerations apply to any motor or electronically controlled attenuation system. Based on the rate of change of optical output power as a function of wavelength, the wavelength sweep speed must be considered so that there is enough time for the motor or electronic slew rate to properly track the changing output power.

[0040] Using the methods described above, a repeatable and reliable method can be achieved for static and dynamic alignment of the beam waist or focused beam to the tip of the cantilever probe, which records the power profile of the source at the probe as a function of wavelength, eliminating the effects of laser power fluctuations, optical losses in the beam path, and spot size as a function of wavelength.

[0041] 6 shows a flow chart of a method of operating a scanning probe microscope, such as scanning probe microscope 100, with a focused optical beam on the probe of the scanning probe microscope, according to an embodiment of the present invention. As used herein, a method of operating a scanning probe microscope includes optional calibration steps, such as aligning the focused beam on the probe of the microscope and measuring the local fluence on the tip, optional sample measurement steps, such as imaging the topography of the sample and measuring optical or other sample attributes in the near-field region around the tip, and optional post-sampling steps, such as processing the sample measurement data.

[0042] In step 602, a focused optical beam is projected onto a scanning probe microscope probe without the presence of a sample to interact with the focused optical beam on the probe. In an embodiment, the focused optical beam is projected onto the scanning probe microscope probe without a sample located on the scanning probe microscope or near the scanning probe microscope probe such that the focused optical beam interacts optically and / or thermally with the projected probe.

[0043] The focal position of the focused optical beam on the probe is adjusted relative to the tip of the probe, step 604. In an embodiment, this adjustment may involve changing the position of a focusing element located between the light source, i.e., laser, and the probe, and / or changing the angle of a steering mirror.

[0044] In step 606, at least one of a mechanical response of the probe and optical radiation scattered from the probe is measured as a function of the position of the focused optical beam. The measurement of the mechanical response of the probe may be an amplitude or phase of oscillation of the probe relative to a drive frequency on a cantilever to which the probe is attached. The measurement of the scattered optical radiation or light may be an intensity of the scattered optical radiation.

[0045] In step 608, a preferred focal position of the focused optical beam on the probe is selected based on at least one measurement of the mechanical response of the probe and the optical radiation scattered from the probe. This preferred focal position of the focused optical beam on the probe can then be used to measure an attribute of the sample using a scanning probe microscope.

[0046] It will be readily understood that the components of the embodiments, as generally described herein and illustrated in the accompanying drawings, could be arranged and designed in a wide variety of different configurations. As such, the detailed description of the various embodiments, as shown in the drawings, is merely representative of the various embodiments, rather than intended to limit the scope of the present disclosure. While various aspects of the embodiments are shown in the drawings, the drawings are not necessarily drawn to scale unless specifically indicated.

[0047] The present invention may be embodied in other specific forms without departing from its spirit and essential characteristics. The described embodiments are to be considered in all respects as illustrative and not restrictive. The scope of the present invention is, therefore, indicated not by this detailed description but by the appended claims. All changes which come within the meaning and range of equivalency of the claims are embraced within their scope.

[0048] References herein to features, advantages, or similar language do not imply that all features and advantages that may be realized in the present invention must be present in any single embodiment of the present invention, or that all features and advantages are present in any single embodiment of the present invention. Rather, these features and advantages are understood to mean that the particular feature, advantage, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, discussions of features and advantages and similar language herein may, but do not necessarily, refer to the same embodiment.

[0049] Indeed, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. Those skilled in the art will recognize, in light of the description herein, that the invention may be practiced without one or more of the particular features or advantages of a particular embodiment. In other cases, additional features and advantages may be recognized in a particular embodiment that may not be present in all embodiments of the invention.

[0050] Reference herein to "one embodiment," "an embodiment," or similar language means that a particular feature, structure, or characteristic described in connection with the illustrated embodiment is included in at least one embodiment of the invention, whereby the phrases "in one embodiment," "in an embodiment," and similar language used herein may, but do not necessarily, all refer to the same embodiment.

[0051] In the above description, specific details of various embodiments are provided. However, some embodiments may be practiced with less than all of these specific details. In other instances, specific methods, procedures, components, structures and / or functions are described in less detail than is necessary to enable various embodiments of the invention for simplicity and clarity.

[0052] Although the operations of the methods herein are illustrated and described in a particular order, the order of operations of each method may be changed such that certain operations may be performed in the opposite order, or such that certain operations may be performed at least in part concurrently with other operations. In other embodiments, differentiated operation orders or sub-operations may be implemented in an intermittent and / or alternating manner.

[0053] It should also be noted that at least some of the operations for the methods described herein may be implemented using software instructions stored on a computer usable storage medium for execution by a computer. By way of example, an embodiment of a computer program product includes a computer usable storage medium for storing a computer readable program.

[0054] A computer usable or computer readable storage medium may be an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system (or apparatus or device). Examples of non-transitory computer usable or computer readable storage media include semiconductor or solid state memory, magnetic tape, removable computer floppy disks, random access memory (RAM), read-only memory (ROM), rigid magnetic disks, and optical disks. Current examples of optical disks include compact disk with read-only memory (CD-ROM), compact disk with read / write (CD-R / W), and digital video disk (DVD).

[0055] Alternatively, embodiments of the invention may be implemented entirely in hardware, or in an implementation containing both hardware and software elements. In software embodiments, this software may include, but is not limited to, firmware, embedded software, microcode, etc.

[0056] Although specific embodiments of the present invention have been described and illustrated, the present invention is not limited to the specific forms or arrangements of parts so described and illustrated, and the scope of the present invention is defined by the accompanying claims and their equivalents.

Claims

1. 1. A method of operating a scanning probe microscope using a focused optical beam on a probe of the scanning probe microscope, comprising: illuminating the focused optical beam onto the probe of the scanning probe microscope without the presence of a sample so as to interact with the focused optical beam on the probe; adjusting a focal position of the focused optical beam on the probe relative to a tip of the probe; measuring at least one of a response of the probe and optical radiation scattered from the probe as a function of position of the focused optical beam; selecting a preferred focal position of the focused optical beam on the probe based on at least one measurement of a response of the probe and the optical radiation scattered from the probe.

2. The step of selecting a preferred focal position of the focused optical beam on the probe comprises: modulating the intensity of the optical beam to generate a modulated optical beam; measuring an amplitude of vibration of the probe as a function of the focal position in response to the modulated optical beam; and selecting the preferred focus position based on the measured amplitude.

3. The method of claim 2 , wherein the preferred focal position is the focal position at which the amplitude of oscillation of the probe is maximum.

4. The step of selecting a preferred focal position of the focused optical beam on the probe comprises: modulating the intensity of the optical beam to generate a modulated optical beam; measuring a phase of vibration of the probe in response to the modulated optical beam, the phase of the vibration corresponding to a phase of the modulation; and selecting a preferred focal position of the beam based on the measured phase relationship.

5. The step of selecting a preferred focal position of the focused optical beam on the probe comprises: vibrating the probe at a drive frequency of a drive source which is not a modulation frequency of the optical beam; measuring the amplitude of oscillation of the cantilever at said drive frequency; and selecting the preferred focus position based on a measurement of the amplitude of the vibration.

6. The step of selecting a preferred focal position of the focused optical beam on the probe comprises: vibrating the probe at a drive frequency of a drive source which is not a modulation frequency of the optical beam; measuring the phase of the cantilever oscillation at the drive frequency as a function of the focal position relative to the phase of the drive source; and selecting the preferred focal position based on a measured phase relationship between a phase of the cantilever oscillation and a phase of the drive source.

7. The step of selecting a preferred focal position of the focused optical beam on the probe comprises: measuring a resonant frequency of a vibration mode of the probe as a function of focus position; and selecting the preferred focus position based on the measured resonant frequency.

8. 8. The method of claim 7, wherein measuring a resonant frequency of a vibrational mode of the probe comprises measuring a frequency of a peak or maximum amplitude of a thermal excitation of the probe, or a Fourier transform of a thermal excitation of the probe.

9. The step of measuring a resonant frequency of a vibration mode of the probe comprises: driving vibration of the probe by means other than the optical beam; sweeping a drive frequency for the probe over a range expected to include the resonant frequency; and measuring the drive frequency at which a peak amplitude of excitation occurs.

10. 8. The method of claim 7, wherein measuring a resonant frequency of a vibrational mode of the probe comprises driving a vibration of the probe by means other than the optical beam, the resonant frequency of the vibration of the probe controlling a frequency of vibration.

11. 8. The method of claim 7, wherein measuring a resonant frequency of a vibration mode of the probe comprises driving a vibration of the probe by means other than the optical beam, performing a Fourier transform of a vibration signal, and selecting the resonant frequency based on a peak of the Fourier transform.

12. bringing the surface of the sample into measurement proximity of the tip of the probe in accordance with a focal position of the beam adjusted by the determined preferred focal position without the presence of a sample; The method of claim 1 , further comprising measuring a response of one or more of the probes to a surface of the sample.

13. 1. A method of operating a scanning probe microscope using a focused optical beam on a probe of the scanning probe microscope, comprising: illuminating the focused optical beam onto the probe of the scanning probe microscope without the presence of a sample so as to interact with the focused optical beam on the probe; adjusting a focal position of the focused optical beam on the probe relative to a tip of the probe; measuring a response of the probe as a function of position of the focused optical beam; selecting a preferred focal position of the focused optical beam on the probe based on a measurement of a response of the probe.

14. The step of selecting a preferred focal position of the focused optical beam on the probe comprises: modulating the intensity of the optical beam to generate a modulated optical beam; measuring an amplitude of vibration of the probe as a function of the focal position in response to the modulated optical beam; and selecting the preferred focus position based on the measured amplitude.

15. The step of selecting a preferred focal position of the focused optical beam on the probe comprises: modulating the intensity of the optical beam to generate a modulated optical beam; measuring a phase of vibration of the probe in response to the modulated optical beam, the phase of the vibration corresponding to a phase of the modulation; and selecting a preferred focal position of the beam based on the measured phase relationship.

16. The step of selecting a preferred focal position of the focused optical beam on the probe comprises: vibrating the probe at a drive frequency of a drive source which is not a modulation frequency of the optical beam; measuring the amplitude of oscillation of the cantilever at said drive frequency; and selecting the preferred focus position based on a measurement of the amplitude of the vibration.

17. The step of selecting a preferred focal position of the focused optical beam on the probe comprises: vibrating the probe at a drive frequency of a drive source which is not a modulation frequency of the optical beam; measuring the phase of the cantilever oscillation at the drive frequency as a function of the focal position relative to the phase of the drive source; and selecting the preferred focal position based on a measured phase relationship between a phase of the cantilever oscillation and a phase of the drive source.

18. The step of selecting a preferred focal position of the focused optical beam on the probe comprises: measuring a resonant frequency of a vibration mode of the probe as a function of focus position; and selecting the preferred focus position based on the measured resonant frequency.

19. bringing the surface of the sample into measurement proximity of the tip of the probe in accordance with a focal position of the beam adjusted by the determined preferred focal position without the presence of a sample; The method of claim 13 , further comprising measuring a response of one or more of the probes to a surface of the sample.

20. 1. A scanning probe microscope, comprising: a cantilever having a probe; a light source for illuminating said probe with a focused optical beam; an optical system for adjusting a focal position of the focused optical beam on the probe; a dither device coupled to the cantilever for driving the cantilever into oscillation; an optical detection system for measuring a response of the probe to the focused optical beam; and a control unit coupled to the optical system and the optical detection system for selecting a preferred focal position of the focused optical beam on the probe based on a measured response of the cantilever to different adjusted focal positions of the focused optical beam on the probe.

Images