WO2008156722A1 - Mesures de propriétés d'un matériau en utilisant une microscopie à force atomique et à fréquences multiples - Google Patents

Mesures de propriétés d'un matériau en utilisant une microscopie à force atomique et à fréquences multiples Download PDF

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Publication number
WO2008156722A1
WO2008156722A1 PCT/US2008/007476 US2008007476W WO2008156722A1 WO 2008156722 A1 WO2008156722 A1 WO 2008156722A1 US 2008007476 W US2008007476 W US 2008007476W WO 2008156722 A1 WO2008156722 A1 WO 2008156722A1
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Prior art keywords
cantilever
amplitude
sample
phase
frequency
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PCT/US2008/007476
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English (en)
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Roger Proksch
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Roger Proksch
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Publication of WO2008156722A1 publication Critical patent/WO2008156722A1/fr

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01Q—SCANNING-PROBE TECHNIQUES OR APPARATUS; APPLICATIONS OF SCANNING-PROBE TECHNIQUES, e.g. SCANNING PROBE MICROSCOPY [SPM]
    • G01Q60/00—Particular types of SPM [Scanning Probe Microscopy] or microscopes; Essential components thereof
    • G01Q60/24—AFM [Atomic Force Microscopy] or apparatus therefor, e.g. AFM probes
    • G01Q60/32—AC mode

Definitions

  • Cantilever-based instruments include such instruments as AFMs, molecular force probe instruments (1 D or 3D), high-resolution profilometers (including mechanical stylus profilometers), surface modification instruments, chemical or biological sensing probes, and micro-actuated devices.
  • AFMs atomic force microscope
  • MFMs molecular force probe instruments
  • high-resolution profilometers including mechanical stylus profilometers
  • surface modification instruments including chemical or biological sensing probes
  • micro-actuated devices micro-actuated devices.
  • the systems and techniques described herein may be realized in such other cantilever-based instruments.
  • An AFM is a device used to produce images of surface topography (and/or other sample characteristics) based on information obtained from scanning (e.g., rastering) a sharp probe on the end of a cantilever relative to the surface of the sample. Topographical and/or other features of the surface are detected by detecting changes in deflection and/or oscillation characteristics of the cantilever (e.g., by detecting small changes in deflection, phase, frequency, etc., and using feedback to return the system to a reference state). By scanning the probe relative to the sample, a "map" of the sample topography or other sample characteristics may be obtained.
  • Changes in deflection or in oscillation of the cantilever are typically detected by an optical lever arrangement whereby a light beam is directed onto the cantilever in the same reference frame as the optical lever.
  • the beam reflected from the cantilever illuminates a position sensitive detector (PSD).
  • PSD position sensitive detector
  • Changes in the deflection or oscillation of the cantilever are typically made to trigger a change in the vertical position of the cantilever base relative to the sample (referred to herein as a change in the Z position, where Z is generally orthogonal to the XY plane defined by the sample), in order to maintain the deflection or oscillation at a constant pre-set value. It is this feedback that is typically used to generate an AFM image.
  • AFMs can be operated in a number of different sample characterization modes, including contact mode where the tip of the cantilever is in constant contact with the sample surface, and AC modes where the tip makes no contact or only intermittent contact with the surface.
  • Actuators are commonly used in AFMs, for example to raster the probe or to change the position of the cantilever base relative to the sample surface.
  • the purpose of actuators is to provide relative movement between different parts of the AFM; for example, between the probe and the sample. For different purposes and different results, it may be useful to actuate the sample, the cantilever or the tip or some combination of both.
  • Sensors are also commonly used in AFMs. They are used to detect movement, position, or other attributes of various components of the AFM, including movement created by actuators.
  • the term "actuator” refers to a broad array of devices that convert input signals into physical motion, including piezo activated flexures, piezo tubes, piezo stacks, blocks, bimorphs, unimorphs, linear motors, electrostrictive actuators, electrostatic motors, capacitive motors, voice coil actuators and magnetostrictive actuators, and the term “position sensor” or “sensor” refers to a device that converts a physical parameter such as displacement, velocity or acceleration into one or more signals such as an electrical signal, including capacitive sensors, inductive sensors (including eddy current sensors), differential transformers (such as described in co-pending applications US20020175677A1 and US20040075428A1 , Linear Variable Differential Transformers for High Precision Position Measurements, and US20040056653A1 , Linear Variable Differential Transformer with Digital Electronics, which are hereby incorporated by reference in their entirety), variable reluctance, optical interferometry, optical
  • the interaction between the stylus and the sample surface induces a discernable effect on a probe-based operational parameter, such as the cantilever deflection, the cantilever oscillation amplitude, the phase of the cantilever oscillation relative to the drive signal driving the oscillation or the frequency of the cantilever oscillation, all of which are detectable by a sensor.
  • a probe-based operational parameter such as the cantilever deflection, the cantilever oscillation amplitude, the phase of the cantilever oscillation relative to the drive signal driving the oscillation or the frequency of the cantilever oscillation, all of which are detectable by a sensor.
  • the resultant sensor-generated signal is used as a feedback control signal for the Z actuator to maintain a designated probe-based operational parameter constant.
  • the designated parameter In contact mode, the designated parameter may be cantilever deflection. In AC modes, the designated parameter may be oscillation amplitude, phase or frequency.
  • the feedback signal also provides a measurement of the sample characteristic of interest. For example, when the designated parameter in an AC mode is oscillation amplitude, the feedback signal may be used to maintain the amplitude of cantilever oscillation constant to measure changes in the height of the sample surface or other sample characteristics.
  • the first three modes of a simple diving board cantilever are at the fundamental resonant frequency (f 0 ), 6.19fo and 17.5 fo.
  • An introductory text in cantilever mechanics such as Sarid has many more details.
  • Sahin, et al. have developed a class of cantilevers whose higher modes do fall on higher harmonics of the fundamental resonant frequency. By doing this, they have observed that cantilevers driven at the fundamental exhibit enhanced contrast, based on their simulations on mechanical properties of the sample surface. This approach is has the disadvantage of requiring costly and difficult to manufacture special cantilevers.
  • the simple harmonic oscillator (SHO) model gives a convenient description at the limit of the steady state amplitude of the eigenmode A of a cantilever oscillating in an AC mode:
  • ⁇ is the drive frequency in units of rad/sec
  • #t ⁇ s the resonant frequency
  • Q is the "quality" factor, a measure of the damping.
  • phase angle ⁇ is described by an associated equation
  • the amplitude and phase of the cantilever are completely determined by the user's choice of the drive frequency and three independent parameters: A d ⁇ ve , ⁇ 0 and Q .
  • Martin, et al. drove the cantilever at two frequencies.
  • the cantilever response at the lower, non-resonant frequency was used as a feedback signal to control the surface tracking and produced a topographic image of the surface.
  • the response at the higher frequency was used to characterize what the authors interpreted as differences in the non-contact forces above the Si and photo-resist on a patterned sample.
  • Force modulation involves maintaining a contact mode feedback loop while also driving the cantilever at a frequency and then measuring its response.
  • the cantilever makes contact with the surface of the sample while being so driven, its resonant behavior changes significantly.
  • the resonant frequency typically increases, depending on the details of the contact mechanics.
  • dissipative interactions may be measured by measuring the phase of the cantilever response with respect to the drive.
  • a well-known shortcoming of force modulation and other contact mode techniques is that the while the contact forces may be controlled well, other factors affecting the measurement may render it ill-defined.
  • the contact area of the tip with the sample usually referred to as contact stiffness
  • Cantilevers are continuous flexural members with a continuum of vibrational modes.
  • the present invention describes different apparatus and methods for exciting the cantilever simultaneously at two or more different frequencies and the useful information revealed in the images and measurements resulting from such methods. Often, these frequencies will be at or near two or more of the cantilever vibrational eigenmodes
  • Past work with AC mode AFMs has been concerned with higher vibrational modes in the cantilever, with linear interactions between the tip and the sample.
  • the present invention is centered around non-linear interactions between the tip and sample that couple energy between two or more different cantilever vibrational modes, usually kept separate in the case of linear interactions. Observing the response of the cantilever at two or more different vibrational modes has some advantages in the case of even purely linear interactions however. For example, if the cantilever is interacting with a sample that has some frequency dependent property, this may show itself as a difference in the mechanical response of the cantilever at the different vibrational modes.
  • FIG. 1 Preferred embodiment for probing multiple eigenmodes of a cantilever.
  • FIG. 2 Preferred embodiment for exciting voltage-dependent motion in the cantilever probe.
  • FIG. 3 Preferred embodiment for probing an active device.
  • FIG. 4 Phase and amplitude shifts of the fundamental eigenmode with and without the second eigenmode being driven.
  • FIG. 5 Images of collagen fibers taken with the preferred embodiment.
  • FIG. 6 Two dimensional histogram plots of the amplitude and phase for the first and second eigenmodes.
  • FIG. 8 Preferred embodiment for probing an active sample in contact while measuring dynamic contact properties
  • FIG. 9 Resonance peaks in sweep of applied potential from dc to 2MHz.
  • FIG. 10 Images of a piezoelectric sample when the cantilever potential was driven at two different frequencies, one slightly below and the other slightly above the same contact resonance frequency.
  • FIG. 12 Amplitude versus frequency and phase versus frequency curves simultaneous measured at different frequencies.
  • FIG. 15 Amplitude and phase curves changing in response to varying tip-sample interactions being driven first at a single frequency and then at two different frequencies.
  • FIG. 16 Amplitude versus frequency sweeps around the second resonance made while feeding back on the first mode amplitude.
  • FIG. 18 Amplitude versus frequency and phase versus frequency curves simultaneous measured at different frequencies.
  • FIG. 19 Images of a piezoelectric sample when the cantilever potential was driven at two different frequencies, one slightly below and the other slightly above the same contact resonance frequency. DESCRIPTION OF THE PREFERRED EMBODIMENTS
  • FIG. 1 is a block diagram of a preferred embodiment of an apparatus for probing multiple eigenmodes of a cantilever in accordance with the present invention.
  • the sample 1010 is positioned below the cantilever probe 1020.
  • the chip of the cantilever probe 1030 is driven by a mechanical actuator 1040, preferably a piezoelectric actuator, but other methods to induce cantilever motion known to those versed in the art could also be used.
  • the motion of the cantilever probe 1020 relative to the frame of the microscope 1050 is measured with a detector 1060, which could be an optical lever or another method known to those versed in the art.
  • the cantilever chip 1030 is moved relative to the sample 1010 by a scanning apparatus 1070, preferably a piezo/flexure combination, but other methods known to those versed in the art could also be used.
  • the motion imparted to the cantilever chip 1030 by actuator 1040 is controlled by excitation electronics that include at least two frequency synthesizers 1080 and 1090. There could be additional synthesizers if more than two cantilever eigenmodes are to be employed.
  • the signals from these frequency synthesizers could be summed together by an analog circuit element 1100 or, preferably, a digital circuit element that performs the same function.
  • the two frequency synthesizers 1080 and 1090 provide reference signals to lockin amplifiers 1110 and 1120, respectively. In the case where more than two eigenmodes are to be employed, the number of lockin amplifiers will also be increased.
  • the lockin amplifiers 1110 and 1120 can be made with analog circuitry or with digital circuitry or a hybrid of both.
  • a digital lockin amplifier one interesting and attractive feature is that the lockin analysis can be performed on the same data stream for both eigenmodes. This implies that the same position sensitive detector and analog to digital converter can be used to extract information at the two distinct eigenmodes.
  • the lockin amplifiers could also be replaced with rms measurement circuitry where the rms amplitude of the cantilever oscillation is used as a feedback signal.
  • a direct digital synthesizer could be used to create sine and cosine quadrature pairs of oscillating voltages, each at a frequency matched to the eigenmodes of the cantilever probe 1030 that are of interest.
  • This implementation also allows dc voltages to be applied, allowing methods such as scanning Kelvin probing or simultaneous current measurements between the tip and the sample.
  • each eigenmode can be measured and used in a feedback loop calculated by the controller 1130 or simply reported to the user interface 1140 where it is displayed, stored and/or processed further in an off-line manner.
  • the quadrature pairs can be calculated and used in a manner similar to the amplitude and phase.
  • the cantilever is driven at or near two or more resonances by the single "shake" piezo 1040.
  • the cantilever amplitude is maintained constant and used as a feedback signal, but employing the teachings of the present invention, there are now a number of choices for the feedback loop.
  • the work here will focus on using the amplitude of the fundamental (Ao), we were able to successfully image using one of the higher mode amplitudes (Ai) as a feedback signal as well as a sum of all the amplitudes A 0 +A 1 +...
  • the method may be used to operate the apparatus with one.flexural mode experiencing a net attractive force and the other a net repulsive force, as well as operating with each mode experiencing the same net sign of force, attractive or repulsive.
  • This method with the cantilever experiencing attractive and repulsive interactions in different eigenmodes, may provide additional information about sample properties.
  • One preferred technique for using the aforesaid method is as follows. First, excite the probe tip at or near a resonant frequency of the cantilever keeping the tip sufficiently far from the sample surface that it oscillates at the free amplitude A 10 unaffected by the proximity of the cantilever to the sample surface and without making contact with the sample surface. At this stage, the cantilever is not touching the surface; it turns around before it interacts with significant repulsive forces.
  • phase pi will be greater than pio, the free first eigenmode phase. This amplitude is maintained at an essentially constant value during scanning without the probe tip making contact with the sample surface by setting up a feedback loop that controls the distance between the base of the cantilever and the sample surface.
  • the feedback amplitude and phase, Ai and pi, respectively, as well as the carry along second eigenmode amplitude and phase, A 2 and p 2 , respectively, should be measured and displayed.
  • the drive amplitude and/or phase of the second frequency can be continually adjusted to maintain the second amplitude and/or phase at an essentially constant value.
  • a second preferred technique for using the aforesaid method follows the first two steps of first preferred technique just described and then continues with the following two steps:
  • the second eigenmode amplitude A 2 should be adjusted so that the first eigenmode phase pi becomes predominantly less than pi 0 , the free first eigenmode phase.
  • the adjustment of the second eigenmode amplitude A 2 has induced the first eigenmode of the cantilever to interact with the surface in a repulsive manner.
  • the second eigenmode amplitude A 2 is not used in the tip-surface distance feedback loop and should be allowed range widely over many values.
  • the feedback amplitude and phase, Ai and P 1 , respectively, as well as the carry along second eigenmode amplitude and phase, A 2 and p 2 , respectively, should be measured and displayed.
  • a third preferred technique for using the aforesaid method provides an alternative for conventional operation in a repulsive mode, that is where the tip is experiencing a net repulsive force.
  • the conventional approach for so operating would be to use a large amplitude in combination with a lower setpoint, and a cantilever with a very sharp tip.
  • the operator begins, just as with the first two techniques, by choosing an amplitude and setpoint for the fundamental eigenmode that is small enough to guarantee that the cantilever is experiencing attractive forces, that is, that the cantilever is in non-contact mode.
  • this operational mode can be identified by observing the phase of the cantilever oscillation.
  • the phase shift is positive, implying that the resonant frequency has been lowered.
  • the second eigenmode excitation can be introduced and the amplitude, drive frequency and, if applicable, set point chosen with the following considerations in mind:
  • Both eigenmodes are in the attractive mode, that is to say that the phase shift of both modes is positive, implying both eigenmode frequencies have been shifted negatively by the tip-sample interactions. Generally, this requires a small amplitude for the second eigenmode.
  • the fundamental eigenmode remains attractive while the second eigenmode is in a state where the tip-sample interactions cause it to be in both the attractive and the repulsive modes as it is positioned relative to the surface.
  • the fundamental eigenmode is in an attractive mode and the second eiegenmode is in a repulsive mode.
  • the first eigenmode In the absence of any second mode excitation, the first eigenmode is interacting with the surface in the attractive mode. After the second eigenmode is excited, the first eigenmode is in a repulsive mode. This change is induced by the addition of the second eigenmode energy. The second eigenmode is in a state where the tip-sample interactions cause it to be attractive and/or repulsive.
  • the first eigenmode is in a repulsive mode and the second mode is in a repulsive mode.
  • FIG. 4 The transition from attractive to repulsive mode in the first eigenmode, as induced by the second eigenmode excitation, is illustrated in FIG. 4, where the amplitude and phase of the first and second eigenmodes are plotted as a function of the distance between the base of the cantilever and the surface of the sample. The point where the tip begins to interact significantly with the surface is indicated with a solid line 4000.
  • the fundamental amplitude 4010 of the cantilever decreases as the cantilever starts to interact with the surface, denoted by the solid line 4000.
  • the associated phase 4020 shows a positive shift, consistent with overall attractive interactions. For these curves, the second eigenmode amplitude is zero and therefore not plotted in the Figure (and neither is phase, for the same reason).
  • the second eigenmode is excited and the same curves are re-measured, together with the amplitude and phase of the second eigenmode, 4030 and 4040.
  • the fundamental eigenmode amplitude 4050 shows a brief positive excursion, but then transitions to a negative phase shift, indicating an overall repulsive interaction between the tip and sample.
  • the free amplitude of the first eigenmode is identical in both cases, the only difference in the measurement being the addition of energy exciting the higher oscillatory eigenmode. This excitation is sufficient to drive the fundamental eigenmode into repulsive interactions with the sample surface.
  • the phase curve of the second eigenmode indicates that it is also interacting overall repulsively with the sample surface.
  • one of the eigenmode signals can be used for topographical feedback while the other signals could be used in other feedback loops.
  • An example would be that Ai is used to control the tip-sample separation while a separate feedback loop was used to keep A2 at an essentially constant value rather than allowing it to range freely over many values.
  • a similar feedback loop could be used to keep the phase of the second frequency drive P 2 at a predetermined value with or without the feedback loop on A 2 being implemented.
  • Q- control can also be used in connection with any of the techniques for using the aforesaid method.
  • Using Q-control on any or all of the eigenmodes employed can enhance their sensitivity to the tip-sample forces and therefore mechanical or other properties of the sample. It can also be used to change the response time of the eigenmodes employed which may be advantageous for more rapidly imaging a sample. For example, the value of Q for one eigenmode could be increased and the value for another decreased.
  • Q-control can be implemented using analog, digital or hybrid analog-digital electronics. It can be accomplished using an integrated control system such as that in the Asylum Research Corporation MFP-3D Controller or by after-market modules such as the nanoAnalytics Q-box.
  • the cantilever In addition to driving the cantilever at or near more than one eigenmode, it is possible to also excite the cantilever at or near one or more harmonics and/or one or more eigenmodes. It has been known for some time that nonlinear interactions between the tip and the sample can transfer energy into cantilever harmonics. In some cases this energy transfer can be large but it is usually quite small, on the order of a percent of less of the energy in the eigenmode. Because of this, the amplitude of motion at a harmonic, even in the presence of significant nonlinear coupling is usually quite small. Using the methods described here, it is possible to enhance the contrast of these harmonics by directly driving the cantilever at the frequency of the harmonic.
  • the results of imaging with the present invention are similar to, and in some cases superior to, the results of conventional phase imaging.
  • phase imaging often requires a judicious choice of setpoint and drive amplitude to maximize the phase contrast
  • the method of the present invention exhibits high contrast over a much wider range of imaging parameters.
  • the method also works in fluid and vacuum, as well as air and the higher flexural modes show unexpected and interesting contrast in those environments, even on samples such as DNA and cells that have been imaged numerous times before using more conventional techniques.
  • the superior results of imaging with the present invention may be seen from an inspection of the images.
  • An example is shown in FIG. 5.
  • the FIG. 1 apparatus was operated using the fundamental eignemode amplitude as the error signal and the second eigenmode as a carry-along signal.
  • the topography image 5010 in FIG. 5 shows collagen fibers on a glass surface, an image typical of results with conventional AC mode from similar samples.
  • the fundamental eigenmode amplitude image 5020 is relatively similar, consistent with the fundamental eignemode amplitude being used in the feedback loop.
  • the fundamental eigenmode phase channel image 5030 shows some contrast corresponding to edges in the topography image.
  • the second eigenmode amplitude image 5040 shows contrast that is similar to the fundamental eigenmode phase image 5030. However, there are some differences, especially over regions thought to be contaminants 5041 and 5042. Finally, the second eigenmode phase image 5050 shows the most surprisingly large amount of contrast.
  • the background substrate 5053 shows a bright, positive phase contrast.
  • the putative contaminant patches, 5041, 5042 and 5051 show strikingly dark, negative phase shift contrast. Finally, regions where the collagen fibers are suspended 5052 show dark, negative phase contrast. In these last regions, the suspended collagen fibers are presumably absorbing some of the vibrational energy of the second eigenmode amplitude and thus, changing the response.
  • the cantilever amplitude is maintained constant and used as a feedback signal. Accordingly, the values of the signal used in the loop are constrained not only by energy balance but also by the feedback loop itself. Furthermore, if the amplitude of the cantilever is constrained, the phase will also be constrained, subject to conditions discussed below. In conventional AC mode it is not unusual for the amplitude to vary by a very small amount, depending on the gains of the loop.
  • AM amplitude modulated
  • the z-feedback loop in part corrects for these changes and thus in this sense, avoids presenting them to the user.
  • the technique for using the present invention involves a mode that is excited but not used in the feedback loop, there will be no explicit constraints on the behavior of this mode. Instead it will range freely over many values of the amplitude and phase, constrained only by energy balance. That is to say, the energy that is used to excite the cantilever motion must be balanced by the energy lost to the tip-sample interactions and the intrinsic dissipation of the cantilever. This may explain the enhanced contrast we observe in images generated with the techniques of the present invention.
  • the first image 6010 is an image of the number of pixels at different amplitudes (horizontal axis) and phases (vertical axis) in the fundamental eigenmode data for the collagen sample of FIG. 5.
  • the amplitude values are constrained to a narrow range around ⁇ 0.6Amax by the z-feedback loop. Constraining the amplitude values in turn, limits the values that the phase can take to the narrow range around 25°.
  • the second image 6030 plots the number of pixels at different amplitudes and phases in the second eigenmode data for the collagen sample. Since the amplitude of this eigenmode was not constrained by a feedback loop, it varies from ⁇ Amax to close to zero. Similarly, the phase ranges over many values. This freedom allows greatly increased contrast in the second eigenmode images.
  • the present invention may also be used in apparatus that induce motion in the cantilever other than through a piezoelectric actuator.
  • Direct electric driving of the cantilever (“active cantilevers”) using the present invention has several advantages over conventional piezo force microscopy (PFM) where the cantilever is generally scanned over the sample in contact mode and the cantilever voltage is modulated in a manner to excite motion in the sample which in turn causes the cantilever to oscillate.
  • PFM piezo force microscopy
  • FIG. 2 is a block diagram of a preferred embodiment of an apparatus for using the present invention with an active cantilever.
  • This apparatus has similarities to that shown in FIG.1 , as well as differences.
  • one of the frequency sources 1080 is used to excite motion of the cantilever probe 1020 through a mechanical actuator 1040, preferably a piezoelectric actuator, but other methods to induce cantilever motion known to those versed in the art could also be used, which drives the chip 1030 of the cantilever probe 1020,
  • the frequency source 1080 communicates directly 2010 with the actuator 1040 instead of being summed together with the second frequency source 1090, as in the FIG. 1 apparatus.
  • the 2 apparatus is used to vary the potential of the cantilever probe 1020 which in turn causes the sample 1010 to excite motion in the cantilever probe 1020 at a different eigenmode than that excited by the first frequency source 1080.
  • the resulting motion of the cantilever probe 1020 interacting with the sample 1010 will contain information on the sample topography and other properties at the eigenmode excited by the first frequency source 1080 and information regarding the voltage dependent properties of the sample at the eigenmode excited by the second frequency source 1090.
  • the sample holder 2030 can optionally be held at a potential, or at ground, to enhance the effect.
  • the amplitude of the cantilever at the frequency of the first source 1080 is used as the error signal.
  • the amplitude and phase (or in-phase and quadrature components) at the frequency of the second source 1090 or a harmonic thereof will contain information about the motion of the sample and therefore the voltage dependent properties of the sample.
  • these properties is the piezo-response of the sample.
  • Another is the electrical conductivity, charge or other properties that can result in long range electrostatic forces between the tip and the sample.
  • FIG. 3 is a block diagram of a preferred embodiment of an apparatus for using the present invention with the second frequency source modulating a magnetic field that changes a property of the surface.
  • the situation with the first frequency source 1080 is identical to the situation in the FIG. 2 apparatus.
  • the second frequency source 1090 instead of the second frequency source 1090 being used to vary the potential of the cantilever probe 1020, as with the FIG. 2 apparatus, in the FIG. 3 apparatus the second frequency source 1090 modulates the current through an excitation coil 3010 which in turn modulates the magnetic state of a magnetic circuit element 3020.
  • Magnetic circuit element 3020 could be used to modulate the field near an active sample or the excitation coil 3010.
  • magnetic circuit element 3020 could comprise the sample, as in the case of a magnetic recording head.
  • the FIG. 3 apparatus can be used with any other sort of 'active' sample where the interaction between the cantilever and the sample can be modulated at or near one or more of the cantilever flexural resonances by one of the frequency sources 1080 or 1090.
  • This could also be extended to high frequency measurements such as described in Proksch et al., Appl. Phys. Lett., vol. (1999).
  • the envelope of the high frequency carrier could be driven with a harmonic of one or more flexural resonances.
  • This method of measuring signals other than topographic has the advantage of requiring only one pass to complete as opposed to "LiftMode" or Nap mode that require temporally separated measurements of the topographic and other signals.
  • Another example of a preferred embodiment of an apparatus and method for using the present invention is from the field of ultrasonic force microscopy.
  • one or more eigenmodes are used for the z-feedback loop and one or more additional eigenmodes can be used to measure the high frequency properties of the sample.
  • the high frequency carrier is amplitude modulated and either used to drive the sample directly or to drive it using the cantilever as a waveguide.
  • the cantilever deflection provides a rectified measure of the sample response at the carrier frequency.
  • FIG. 8 is a block diagram of the first of these embodiments, which may be referred to as Dual Frequency Resonance Tracking Piezo Force Microscopy (DFRT PFM).
  • DFRT PFM Dual Frequency Resonance Tracking Piezo Force Microscopy
  • the chip 1030 of the cantilever probe 1020, or the cantilever probe 1020 itself is driven by excitation electronics that include at least two frequency synthesizers 1080 and 1090.
  • the cantilever probe 1020 responds to this excitation by buckling up and down much as a plucked guitar string.
  • the signals from these frequency synthesizers could be summed together by an analog circuit element 1100 or, preferably, a digital circuit element that performs the same function.
  • the two frequency synthesizers 1080 and 1090 provide reference signals to lockin amplifiers 1110 and 1120, respectively.
  • the motion of the cantilever probe 1020 relative to the frame of the microscope 1050 is measured with a detector 1060, which could be an optical lever or another method known to those versed in the art.
  • the cantilever chip 1030 is moved relative to the sample 1010 in order to maintain constant force by a scanning apparatus 1070, preferably a piezo/flexure combination, but other methods known to those versed in the art could also be used.
  • the amplitude and phase of each frequency at which the cantilever probe 1020 is excited can be measured and used in a feedback loop calculated by the controller 1130 or simply reported to the user interface 1140 where it is displayed, stored and/or processed further in an off-line manner.
  • the quadrature pairs can be calculated and used in a manner similar to the amplitude and phase.
  • the topography of the sample would be measured in contact mode while the amplitude and phase of the cantilever probe 1020 response to the applied potential at the lowest contact resonance and at the next highest contact resonance is simultaneously measured.
  • the responses can be analyzed to determine whether they originate from the actual piezoelectric response of the sample or from crosstalk between the topography and any electric forces between the tip of the cantilever probe 1020 and the sample. Even more information can be obtained if more frequencies are utilized.
  • FIG. 12 also shows three examples of the changes in the native phase 12015 and amplitude 12010 of a cantilever with a resonant frequency f 0 caused by interactions between the tip and the sample using DFRT PFM methods. These examples are a subset of changes that can be observed.
  • the resonant frequency is significantly lowered to fo' but not damped.
  • the phase 12085 and amplitude 12080 change but little relative to the native phase 12015 and amplitude 12010.
  • the resonant frequency is again lowered to fo', this time with damping of the amplitude.
  • the phase 12095 is widened and the amplitude 12090 is appreciably flattened.
  • the resonant frequency is again dropped to fo', this time with a reduction in the response amplitude.
  • This yields a phase curve with an offset 12105 but with the same width as the second case 12095 and a reduced amplitude curve 12100 with the damping equivalent to that of the second example.
  • prior art phase locked-loop electronics will not maintain stable operation. For example, if the phase set-point was made to be 90 degrees, it would never be possible to find a frequency in curve 12105 where this condition was met.
  • One example of these things occurring in a practical situation is in DRFT PFM when the tip crosses from an electric domain with one orientation to a second domain with another orientation. The response induced by the second domain will typically have a phase offset with respect to the first. This is, in fact where the large contrast in DFRT PFM phase signals originates.
  • FIG. 9 shows the cantilever response when the applied potential is swept from dc to 2MHz using the DFRT PFM apparatus.
  • Three resonance peaks are visible.
  • the number, magnitude, breadth and frequency of the peaks is subject to change. Sweeps such as these are useful in choosing the operating points for imaging and other measurements. In a practical experiment, any or all of these resonance peaks or the frequencies in between could be exploited by the methods suggested above.
  • FIG. 19 shows a measurement that can be made using DFRT PFM techniques.
  • a phase image 19010 shows ferroelectric domains written onto a sol-gel PZT surface.
  • the phase image shows only piezo response, there is no topographic roughness coupling into the phase.
  • the written domains appear as bright regions.
  • the writing was accomplished by locally performing and measuring hysteresis loops by applying a DC bias to the tip during normal DFRT PFM operation. This allows the local switching fields to be measured.
  • the piezo phase 19030 during a measurement made at location 19020 and the amplitude 19040 are plotted as a function of the applied DC bias voltage.
  • the loops were made following Stephen Jesse et al, Rev. Sci. Inst. 77, 073702 (2006). Other loops were taken at the bright locations in image 19010, but are not shown in the Figure.
  • DFRT PFM is very stable over time in contrast to single frequency techniques. This allows time dependent processes to be studied as is demonstrated by the sequence of images, 19010, 19050, 19060, 19070 and 19080 taken over the span of 1.5 hours. In these images, the written domains are clearly shrinking over time.
  • the resonant frequency of the cantilever probe using contact resonance techniques depends on the properties of the contact, particularly the contact stiffness. Contact stiffness in turn is a function of the local mechanical properties of the tip and sample and the contact area. In general, all other mechanical properties being equal, increasing the contact stiffness by increasing the contact area, will increase the resonant frequency of the oscillating cantilever probe. This interdependence of the resonant properties of the oscillating cantilever probe and the contact area represents a significant shortcoming of contact resonance techniques. It results in "topographical crosstalk" that leads to significant interpretational issues. For example, it is difficult to know whether or not a phase or amplitude change of the probe is due to some sample property of interest or simply to a change in the contact area.
  • the apparatus used in contact resonance techniques can also cause the resonant frequency, phase and amplitude of the cantilever probe to change unpredictably. Examples are discussed by Rabe et al., Rev. Sci. Instr. 67, 3281 (1996) and others since then.
  • One of the most difficult issues is that the means for holding the sample and the cantilever probe involve mechanical devices with complicated, frequency dependent responses. Since these devices have their own resonances and damping, which are only rarely associated with the sample and tip interaction, they may cause artifacts in the data produced by the apparatus. For example, phase and amplitude shifts caused by the spurious instrumental resonances may freely mix with the resonance and amplitude shifts that originate with tip-sample interactions.
  • FIG. 15 illustrates the idea.
  • the original resonant frequency curve 14010 has amplitudes A 1 14030 and A 2 14020, respectively, at the two drive frequencies fi and f 2 .
  • the curve shifts to 14050 and the amplitudes at the measurement frequencies change, A'i 14035 increasing and A' 2 14025 decreasing. If the resonant frequency were higher, the situation would reverse, that is the amplitude A ⁇ at drive frequency f ⁇ would decrease and A' 2 at f 2 would increase.
  • One method with DFRT PFM is to define an error signal that is the difference between the amplitude at fi and the amplitude at f 2 , that is Ai minus A 2 .
  • both fi and f 2 could be adjusted so that the error signal, the difference in the amplitudes, is maintained.
  • the average of these frequencies (or even simply one of them) provides the user with a measure of the contact resonance frequency and therefore the local contact stiffness. It is also possible to measure the damping and drive with the two values of amplitude.
  • the peak amplitude is directly related to the amplitude on either side of resonance.
  • One convenient way to monitor this is to simply look at the sum of the two amplitudes. This provides a better signal to noise measurement than does only one of the amplitude measurements.
  • Other, more complicated feedback loops could also be used to track the resonant frequency. Examples include more complex functions of the measured amplitudes, phases (or equivalently, the in-phase and quadrature components), cantilever deflection or lateral and/or torsional motion.
  • the values of the two amplitudes also allow conclusions to be drawn about damping and drive amplitudes. For example, in the case of constant damping, an increase in the sum of the two amplitudes indicates an increase in the drive amplitude while the difference indicates a shift in the resonant frequency.
  • the drive amplitude and/or frequencies and/or phases of one or more of the frequencies is used to decode the resonant frequency and, optionally, adjust it to follow changes induced by the tip- sample interactions.
  • FIG. 10 shows the results of a measurement of a piezo-electric material using DFRT PFM methods.
  • Contact mode is used to image the sample topography 10010 and contact resonance techniques used to image the first frequency amplitude 10020, the second frequency amplitude 10030, the first frequency phase 10040 and the second frequency phase 10050.
  • the two frequencies were chosen to be close to the first contact resonance, at roughly the half-maximum point, with the first frequency on the lower side of the resonance curve and the second on the upper side. This arrangement allowed some of the effects of crosstalk to be examined and potentially eliminated in subsequent imaging.
  • Another multiple frequency technique is depicted in FIG.
  • an apparatus for using the present invention with a conductive cantilever, and the methods for its use may also be advantageous in examining the effects of crosstalk with a view to potentially eliminating them in subsequent imaging.
  • the inventors refer which to this apparatus and method as Dual Frequency Piezo Force Microscopy (DF PFM).
  • DF PFM Dual Frequency Piezo Force Microscopy
  • the response to driving the tip voltage of the cantilever, due to the piezoelectric action acting through the contact mechanics will typically change as the probe is scanned over the surface.
  • the first signal will then be representative of changes in the contact mechanics between the tip and sample.
  • the second signal will depend both on contact mechanics and on the piezo electrical forces induced by the second excitation signal between the tip and sample. Differences between the response to the first excitation and the response to the second are thus indicative of piezoelectric properties of the sample and allow the contact mechanics to be separated from such properties.
  • ⁇ d rive, resonant frequency COQ and quality factor Q (representative of the damping) will all vary as a function of the lateral tip position over the sample and may also vary in time depending on cantilever mounting schemes or other instrumental factors.
  • conventional PFM only two time averaged quantities are measured, the amplitude and the phase of the cantilever (or equivalently, the in-phase and quadrature components).
  • more measurements may be made, and this will allow additional parameters to be extracted.
  • SHO model by measuring the response at two frequencies at or near a particular resonance, it is possible to extract four model parameters.
  • the difference in the amplitudes provides a measure of the resonant frequency
  • the sum of the amplitudes provides a measure of the drive amplitude and damping of the tip-sample interaction (or quality factor)
  • the difference in the phase values provides a measure of the quality factor
  • the sum of the phases provides a measure of the tip-sample drive phase.
  • FIG. 18 illustrates the usefulness of measuring the phase as a means of separating changes in the quality factor Q from changes in the drive amplitude A d ri ve .
  • Curve 18010 shows the amplitude response of an oscillator with a
  • the amplitude measurements made at f1 18052 and f2 18054 are exactly the same as in the case where the Q value increased to 150, 18032 and 18034, respectively.
  • the amplitude response does not separate the difference between increasing the Q value or increasing the drive amplitude Ad ⁇ ve-
  • Curves 18020, 18040 and 18060 are the phase curves corresponding to the amplitude curves 18010, 18030 and 18050 respectively. As with the amplitude measurements, the phase is measured at discrete frequency values, fi and f 2 . The phase curve 18020 remains unchanged 18060 when the drive amplitude A d rive increases from O.O ⁇ nm to 0.09nm. Note that the phase measurements 18022 and 18062 at fi for the curves with the same quality factor are the same, as are the phase measurements 18024 and 18064 at f 2 . When the quality factor Q increases, the fi phase 18042 decreases and the f 2 phase 18044 increases. These changes clearly separate drive amplitude changes from Q value changes.
  • phase baseline does not change
  • the Q value may well change.
  • This idea can be extended to more and more frequencies for a better estimate of the resonant behavior. It will be apparent to those skilled in the art that this represents one manner of providing a spectrum of the sensor response over a certain frequency range.
  • the spectral analysis can be either scalar or vector. This analysis has the advantage that the speed of these measurements is quite high with respect to other frequency dependent excitations.
  • FIG. 16 shows one embodiment of a multi-frequency approach, with eight frequencies being driven fi through f ⁇ .
  • the resonance curve changes in response to tip-surface interactions, a more complete map of the frequency response is traced out. This may be particularly useful when measuring nonlinear interactions between the tip and the sample because in that case the simple harmonic oscillator model no longer applies.
  • the amplitude and phase characteristics of the sensor may be significantly more complex.
  • Scanning ion conductance microscopy, scanning electrochemical microscopy, scanning tunneling microscopy, scanning spreading resistance microscopy and current sensitive atomic force microscopy are all examples of localized transport measurements that make use of alternating signals, sometimes with an applied dc bias.
  • Electrical force microscopy, Kelvin probe microscopy and scanning capacitance microscopy are other examples of measurement modes that make use of alternating signals, sometimes with an applied dc bias.

Abstract

L'invention concerne un appareil et des techniques d'extraction d'informations acheminées dans des modes propres ou des harmoniques plus élevés d'un cantilever oscillant ou de tout autre capteur oscillant dans le cadre d'une microscopie à force atomique et de travaux MEM connexes. L'invention concerne également un appareil et des techniques similaires d'extraction d'informations utilisant une résonance par contact avec plusieurs signaux d'excitation.
PCT/US2008/007476 2007-06-18 2008-06-16 Mesures de propriétés d'un matériau en utilisant une microscopie à force atomique et à fréquences multiples WO2008156722A1 (fr)

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US8726410B2 (en) 2010-07-30 2014-05-13 The United States Of America As Represented By The Secretary Of The Air Force Atomic force microscopy system and method for nanoscale measurement

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US20060272399A1 (en) * 2003-08-11 2006-12-07 Veeco Instruments, Inc. System for wide frequency dynamic nanomechanical analysis
US20060283240A1 (en) * 2001-12-06 2006-12-21 Jens Struckmeier Force scanning probe microscope

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US20040182140A1 (en) * 2003-03-17 2004-09-23 Weide Daniel Van Der Heterodyne feedback system for scanning force microscopy and the like
US20060272399A1 (en) * 2003-08-11 2006-12-07 Veeco Instruments, Inc. System for wide frequency dynamic nanomechanical analysis

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US8726410B2 (en) 2010-07-30 2014-05-13 The United States Of America As Represented By The Secretary Of The Air Force Atomic force microscopy system and method for nanoscale measurement
CN103347712A (zh) * 2010-12-30 2013-10-09 米其林集团总公司 用于确定轮胎负荷的基于压电的系统和方法

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