# Patent

## Title
Quantitative measurements using multiple frequency atomic force microscopy

## Abstract

The imaging mode presented here combines the features and benefits of amplitude modulated (AM) atomic force microscopy (AFM), sometimes called AC mode AFM, with frequency modulated (FM) AFM. In AM-FM imaging, the topographic feedback from the first resonant drive frequency operates in AM mode while the second resonant drive frequency operates in FM mode and is adjusted to keep the phase at 90 degrees, on resonance. With this approach, frequency feedback on the second resonant mode and topographic feedback on the first are decoupled, allowing much more stable, robust operation.

## Background
This application is a continuation-in-part application of U.S. Ser. No. 13/241,689 filed Sep. 23, 2011, now U.S. Pat. No. 8,555,711 issued Sep. 27, 2011; which is a continuation application of U.S. Ser. No. 12/214,031 filed Jun. 16, 2008, now U.S. Pat. No. 8,024,963 issued Sep. 27, 2011, entitled “Quantitative Measurements Using Multiple Frequency Atomic Force Microscopy”. This application also claims priority from provisional application No. 61/628,323, filed Oct. 27, 2011. The disclosures of each of these parent applications are hereby incorporated by reference, in their entirety.


BACKGROUND OF THE INVENTION
For the sake of convenience, the current description focuses on systems and techniques that may be realized in a particular embodiment of cantilever-based instruments, the atomic force microscope (AFM). Cantilever-based instruments include such instruments as AFMs, molecular force probe instruments (1D or 3D), high-resolution profilometers (including mechanical stylus profilometers), surface modification instruments, chemical or biological sensing probes, and 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). As the deflection or oscillation of the cantilever changes, the position of the reflected spot on the PSD changes, causing a change in the output from the PSD. 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.
For the purposes of the specification, unless otherwise specified, 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 deflection detectors (including those referred to above as a PSD and those described in co-pending applications US20030209060A1 and US20040079142A1, Apparatus and Method for Isolating and Measuring Movement in Metrology Apparatus, which are hereby incorporated by reference in their entirety), strain gages, piezo sensors, magnetostrictive and electrostrictive sensors.
In both the contact and AC sample-characterization modes, the interaction between the probe 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. In this regard, 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.
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 periodic interactions between the tip and sample in AC modes induces cantilever flexural motion at higher frequencies. Measuring the motion allows interactions between the tip and sample to be explored. A variety of tip and sample mechanical properties including conservative and dissipative interactions may be explored. Stark, et al., have pioneered analyzing the flexural response of a cantilever at higher frequencies as nonlinear interactions between the tip and the sample. In their experiments, they explored the amplitude and phase at numerous higher oscillation frequencies and related these signals to the mechanical properties of the sample.
Unlike the plucked guitar strings of elementary physics classes, cantilevers normally do not have higher oscillation frequencies that fall on harmonics of the fundamental frequency. The first three modes of a simple diving board cantilever, for example, are at the fundamental resonant frequency (f0), 6.19f0 and 17.5 f0. An introductory text in cantilever mechanics such as Sarid has many more details. Through careful engineering of cantilever mass distributions, 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 A of the eigenmode of a cantilever oscillating in an AC mode:



  
    SHOAmp
    ⁢
    
      
    
    ⁢
    0
  




  
    
      A
      =
      
        
          
            F
            0
          
          ⁢
          Im
        
        
          
            
              
                (
                
                  
                    ω
                    0
                    2
                  
                  -
                  
                    ω
                    2
                  
                
                )
              
              2
            
            -
            
              
                (
                
                  
                    ωω
                    0
                  
                  /
                  Q
                
                )
              
              2
            
          
        
      
    
    ,
  



where F0 is the drive amplitude (typically at the base of the cantilever), m is the mass, ω is the drive frequency in units of rad/sec, ω0 is the resonant frequency and Q is the “quality” factor, a measure of the damping.

If, as is often the case, the cantilever is driven through excitations at its base, the expression becomes




  A
  =
  
    
      
        
          A
          drive
        
        ⁢
        
          ω
          0
          2
        
      
      
        
          
            
              (
              
                
                  ω
                  0
                  2
                
                -
                
                  ω
                  2
                
              
              )
            
            2
          
          -
          
            
              (
              
                
                  ω
                  0
                
                ⁢
                
                  ω
                  /
                  Q
                
              
              )
            
            2
          
        
      
    
    ⁢
    
      (
      
        SHO
        ⁢
        
          
        
        ⁢
        Amp
      
      )
    
  




where F0/m has been replaced with Adriveω02, where Adrive is the drive amplitude (at the oscillator).
The phase angle φ is described by an associated equation



  SHOPhase




  
    φ
    =
    
      
        
          tan
          
            -
            1
          
        
        [
        
          
            ωω
            0
          
          
            Q
            ⁡
            
              (
              
                
                  ω
                  0
                  2
                
                -
                
                  ω
                  2
                
              
              )
            
          
        
        ]
      
      .
    
  



When these equations are fulfilled, the amplitude and phase of the cantilever are completely determined by the user's choice of the drive frequency and three independent parameters: Adrive, ω0 and Q.
In some very early work, 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.
Recently, Rodriguez and Garcia published a theoretical simulation of a non-contact, attractive mode technique where the cantilever was driven at its two lowest eigenfrequencies. In their simulations, they observed that the phase of the second mode had a strong dependence on the Hamaker constant of the material being imaged, implying that this technique could be used to extract chemical information about the surfaces being imaged. Crittenden et al. have explored using higher harmonics for similar purposes.
There are a number of techniques where the instrument is operated in a hybrid mode where a contact mode feedback loop is maintained while some parameter is modulated. Examples include force modulation and piezo-response imaging.
Force modulation involves maintaining a contact mode feedback loop while also driving the cantilever at a frequency and then measuring its response. When 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. In any event, one may learn more about the surface properties because the elastic response of the sample surface is sensitive to force modulation. In particular, 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. In particular, the contact area of the tip with the sample, usually referred to as contact stiffness, may vary greatly depending on tip and sample properties. This in turn means that the change in resonance while maintaining a contact mode feedback loop, which may be called the contact resonance, is ill-defined. It varies depending on the contact stiffness. This problem has resulted in prior art techniques avoiding operation at or near resonance.
SUMMARY OF THE INVENTION
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, however, 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.



BRIEF DESCRIPTION OF THE DRAWINGS
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. 5A-5E Images of collagen fibers taken with the preferred embodiment.
FIGS. 6 and 7 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 (Dual Frequency Resonance Tracking Piezo Force Microscopy (DFRT PFM)).
FIG. 9 Resonance peaks in sweep of applied potential from dc to 2 MHz.
FIG. 10A-10D and 11 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 simultaneously measured at different frequencies.
FIG. 13 Amplitude and phase curves changing in response to varying tip-sample interactions being driven first at two different frequencies and then at a single frequency.
FIG. 14 Amplitude versus frequency sweeps around the second resonance made while feeding back on the first mode amplitude.
FIG. 15-16 Amplitude versus frequency and phase versus frequency curves simultaneous measured at different frequencies.
FIG. 17-19 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.
FIG. 20 Preferred embodiment of an apparatus for probing the first two flexural resonances of a cantilever and imaging in AM mode with phase is and FM mode in accordance with the present invention.
FIG. 21 Topography of a Si-epoxy (SU8) patterned wafer imaged using the Loss Tangent technique of the present invention.
FIG. 22 Steps in calculating the corrected Loss Tangent.
FIG. 23 Direct measurement of the second mode tip-sample interaction forces.
FIG. 24 EPDH/Epoxy cryo-microtomed boundary measured at 2 Hz and 20 Hz line scan rates.
FIG. 25 Simultaneous mapping of loss tangent and stiffness of an elastomer-epoxy sandwich.
FIG. 26 Simplified measurement of the phase of the second mode for small frequency shifts.
FIG. 27 Effects of choosing resonant modes that are softer, matched or stiffer than the tip-sample stiffness,
FIG. 28 Extension of thermal noise measurement method to higher modes.



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. As with other electronic components in this apparatus, the lockin amplifiers 1110 and 1120 can be made with analog circuitry or with digital circuitry or a hybrid of both. For 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.
There are a number of variations in the FIG. 1 apparatus that a person skilled in the art could use to extract information relative to the different eigenmodes employed in the present invention. Preferably, a direct digital synthesizer (DDS) 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. The amplitude and phase of 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. Instead of, or in addition to, the amplitude and phase of the cantilever motion, the quadrature pairs, usually designated x and y, can be calculated and used in a manner similar to the amplitude and phase.
In one method of using the FIG. 1 apparatus, the cantilever is driven at or near two or more resonances by the single “shake” piezo 1040. Operating in a manner similar to AC mode where 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. Although the work here will focus on using the amplitude of the fundamental (A0), we were able to successfully image using one of the higher mode amplitudes (A1) as a feedback signal as well as a sum of all the amplitudes A0+A1+ . . . . One can also choose to exclude one or more modes from such a sum. So for example, where three modes are employed, the sum of the first and second could be used to operate the feedback loop and the third could be used as a carry along signal.
Because higher eigenmodes have a significantly higher dynamic stiffness, the energy of these modes can be much larger that that of lower eigenmodes.
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. Using 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 A10 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.
Second, reduce the relative distance in the Z direction between the base of the cantilever and the sample surface so that the amplitude of the probe tip A1 is affected by the proximity of the sample surface without the probe tip making contact with the sample surface. The phase p1 will be greater than p10, 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.
Third, keeping the first eigenmode drive and surface controlling feedback loop with the same values, excite a second eigenmode of the cantilever at an amplitude A2. Increase A2 until the second eigenmode phase p2 shows that the cantilever eigenmode is interacting with predominantly repulsive forces; that is, that p2 is less than p20, the free second eigenmode phase. This second amplitude A2 is not included in the feedback loop and is allowed to freely roam over a large range of values. In fact, it is typically better if variations in A2 can be as large as possible, ranging from 0 to A20, the free second eigenmode amplitude.
Fourth, the feedback amplitude and phase, A1 and p1, respectively, as well as the carry along second eigenmode amplitude and phase, A2 and p2, respectively, should be measured and displayed.
Alternatively, 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. In this case, it is useful to display and record the drive amplitude and/or frequency required 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:
Third, keeping the first eigenmode drive and surface controlling feedback loop with the same values, excite a second eigenmode (or harmonic) of the cantilever at an amplitude A2. Increase A2 until the second eigenmode phase p2 shows that the cantilever eigenmode is interacting with predominantly repulsive forces; that is, that p2 is less than p20, the free second eigenmode phase. At this point, the second eigenmode amplitude A2 should be adjusted so that the first eigenmode phase p1 becomes predominantly less than p10, the free first eigenmode phase. In this case, the adjustment of the second eigenmode amplitude A2 has induced the first eigenmode of the cantilever to interact with the surface in a repulsive manner. As with the first preferred technique, the second eigenmode amplitude A2 is not used in the tip-surface distance feedback loop and should be allowed range widely over many values.
Fourth, the feedback amplitude and phase, A1 and p1, respectively, as well as the carry along second eigenmode amplitude and phase, A2 and p2, respectively, should be measured and displayed.
Either of the preferred techniques just described could be performed in a second method of using the FIG. 1 apparatus where the phase of the oscillating cantilever is used in a feedback loop and the oscillation frequency is varied to maintain phase essentially constant. In this case, it is preferable to use the oscillation frequency as an input into a z-feedback loop that controls the cantilever-sample separation.
Relative changes in various parameters such as the amplitude and phase or in-phase and quadrature components of the cantilever at these different frequencies could also be used to extract information about the sample properties.
A third preferred technique for using the first method of using the FIG. 1 apparatus 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. Using this third preferred technique, however, 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. As noted before, this operational mode can be identified by observing the phase of the cantilever oscillation. In the non-contact case, the phase shift is positive, implying that the resonant frequency has been lowered. With these conditions on the first eigenmode, the second eigenmode excitation can be introduced and the amplitude, drive frequency and, if applicable, set point chosen with the following considerations in mind:
1. 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.
2. 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.
3. The fundamental eigenmode is in an attractive mode and the second eiegenmode is in a repulsive mode.
4. 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.
5. The first eigenmode is in a repulsive mode and the second mode is in a repulsive mode.
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). Next, 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. There is a notable change in the fundamental eigenmode amplitude 4050 and more strikingly, the fundamental eigenmode phase 4060. The fundamental phase in fact 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. Furthermore, the phase curve of the second eigenmode indicates that it is also interacting overall repulsively with the sample surface.
More complicated feedback schemes can also be envisioned. For example, 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 A1 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 p2 at a predetermined value with or without the feedback loop on A2 being implemented.
As another example of yet another type of feedback that could be used, Q-control can also be used in connection with any of the techniques for using the first method of using the FIG. 1 apparatus. 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. This may enhance the result of mixed attractive/repulsive mode imaging because it is generally easier to keep one eignmode interacting with the sample in repulsive mode with a reduced Q-value or, conversely, in attractive mode with an enhanced Q-value. By reducing the Q-value of the lowest eigenmode and enhancing the Q-value of the next eigenmode, it is possible to encourage the mixed mode operation of the cantilever; the zeroth eigenmode will be in repulsive mode while the first eigenmode will more likely remain in attractive mode. 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.
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. To further enhance the contrast of this imaging technique it is useful to adjust the phase of the higher frequency drive relative to that of the lower. This method improves the contrast of both conventional cantilevers and the specially engineered “harmonic” cantilevers described by Sahin et al and other researchers.
On many samples, the results of imaging with the present invention are similar to, and in some cases superior to, the results of conventional phase imaging. However, while 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. Moreover, the method also works in fluid and vacuum, as well as air and the higher flexural modes show unexpected and intriguing contrast in those environments, even on samples such as DNA and cells that have been imaged numerous times before using more conventional techniques.
Although there is a wide range of operating parameters that yield interesting and useful data, there are situations where more careful tuning of the operational parameters will yield enhanced results. Some of these are discussed below. Of particular interest can be regions in set point and drive amplitude space where there is a transition from attractive to repulsive (or vice versa) interactions in one or more of the cantilever eigenmodes or harmonics.
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. For this example, 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. This is consistent with the interaction being more attractive at these regions, again to be expected from surface energy considerations (larger areas in proximity will have larger long-range attractive forces). Since the fundamental eignemode amplitude is being held relatively constant and there is a relationship between the amplitude and phase, the phase will be constrained, subject to energy balance and the feedback loop that is operating to keep the amplitude constant. 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.
When an AFM is operated in conventional amplitude modulated (AM) AC mode with phase detection, 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. This means that, even if there are mechanical properties of the sample that might lead to increased dissipation or a frequency shift of the cantilever, the z-feedback loop in part corrects for these changes and thus in this sense, avoids presenting them to the user.
If 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.
FIG. 6 demonstrates this idea more explicitly. 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. As expected, the amplitude values are constrained to a narrow range around  ̃0.6 Amax 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°. Thus, when the pixel counts are plotted, there is a bright spot 6020 with only small variations. Small variations in turn imply limited contrast. 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. These could include direct electric driving of the cantilever (“active cantilevers”), magnetic actuation schemes, ultrasonic excitations, scanning Kelvin probe and electrostatic actuation schemes.
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.
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. In the FIG. 2 apparatus, like the FIG. 1 apparatus, 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, However, in the FIG. 2 apparatus, 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 second frequency source 1090 in the FIG. 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.
In one method of using the FIG. 2 apparatus, 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. One example of 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 sample. In the FIG. 3 apparatus, the situation with the first frequency source 1080 is identical to the situation in the FIG. 2 apparatus. However, 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. Alternatively, 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). Instead of the modulation described in that paper, 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. In this embodiment, 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.
Another group of embodiments for the present invention has similarities to the conventional force modulation technique described in the Background to the Invention and conventional PFM where the cantilever is scanned over the sample in contact mode and a varying voltage is applied to the cantilever. In general this group may be described as contact resonance embodiments. However, these embodiments, like the other embodiments already described, make use of multiple excitation signals.
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). In the DFRT PFM apparatus of FIG. 8 the cantilever probe 1020 is positioned above a sample 1010 with piezoelectric properties and scanned relative to the sample 1010 by a scanning apparatus 1070 using contact mode. Unlike conventional contact mode however the chip 1030 of the cantilever probe 1020, or the cantilever probe 1020 itself (alternative not shown), 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 vertically 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. Instead of, or in addition to, the amplitude and phase of the cantilever motion, the quadrature pairs, usually designated x and y, can be calculated and used in a manner similar to the amplitude and phase.
In one method of using the FIG. 8 apparatus, 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 shows three examples of the changes in the native phase 12015 and amplitude 12010 of a cantilever with a resonant frequency f0 caused by interactions between the tip and the sample using DFRT PFM methods. These examples are a subset of changes that can be observed. In the first example, the resonant frequency is significantly lowered to f0′ but not damped. The phase 12085 and amplitude 12080 change but little relative to the native phase 12015 and amplitude 12010. In the second example the resonant frequency is again lowered to f0′, this time with damping of the amplitude. Here the phase 12095 is widened and the amplitude 12090 is appreciably flattened. Finally, in the third example, the resonant frequency is again dropped to f0′, 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. If there is an offset in the phase versus frequency curve as there is in this third 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 2 MHz using the DFRT PFM apparatus. Three resonance peaks are visible. Depending on the cantilever probe and the details of the tip-sample contact mechanics, 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. Because of the excellent separation between topography and PFM response possible with DFRT PFM, 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 and the amplitude 19040 during a measurement made at location 19020 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 about 1.5 hours. In these images, the written domains are clearly shrinking over time.
In AC mode atomic force microscopy, relatively tiny tip-sample interactions can cause the motion of a cantilever probe oscillating at resonance to change, and with it the resonant frequency, phase, amplitude and deflection of the probe. Those changes of course are the basis of the inferences that make AC mode so useful. With contact resonance techniques the contact between the tip and the sample also can cause the resonant frequency, phase and amplitude of the cantilever probe to change dramatically.
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.
It is advantageous to track more than two resonant frequencies as the probe scans over the surface when using contact resonance techniques. Increasing the number of frequencies tracked provides more information and makes it possible to over-constrain the determination of various physical properties. As is well known in the art, this is advantageous since multiple measurements will allow better determination of parameter values and provide an estimation of errors.
Since the phase of the cantilever response is not a well behaved quantity for feedback purposes in PFM, we have developed other methods for measuring and/or tracking shifts in the resonant frequency of the probe. One method is based on making amplitude measurements at more than one frequency, both of which are at or near a resonant frequency. FIG. 15 illustrates the idea. The original resonant frequency curve 14010 has amplitudes A1 14030 and A2 14020, respectively, at the two drive frequencies f1 and f2. However, if the resonant frequency shifted to a lower value, the curve shifts to 14050 and the amplitudes at the measurement frequencies change, A′1 14035 increasing and A′2 14025 decreasing. If the resonant frequency were higher, the situation would reverse, that is the amplitude A′1 at drive frequency f1 would decrease and A′2 at f2 would increase.
There are many methods to track the resonant frequency with information on the response at more than one frequency. One method with DFRT PFM is to define an error signal that is the difference between the amplitude at f1 and the amplitude at f2, that is A1 minus A2. A simpler example would be to run the feedback loop such that A1 minus A2=0, although other values could equally well be chosen. Alternatively both f1 and f2 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 resonant frequency and therefore the local contact stiffness. It is also possible to measure the damping and drive with the two values of amplitude. When the resonant frequency has been tracked properly, 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.
Finally, it is possible to modulate the drive amplitude and/or frequencies and/or phases of one or more of the frequencies. The response 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. In this experiment, 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. 2, an apparatus for using the present invention with a conductive (or active) 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. For this purpose the inventors refer to this apparatus and method as Dual Frequency Piezo Force Microscopy (DF PFM). In the DF PFM apparatus of FIG. 2 the response to driving the tip voltage of the cantilever probe, 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.
As noted, the user often does not have independent knowledge about the drive or damping in contact resonance. Furthermore, models may be of limited help because they too require information not readily available. In the simple harmonic oscillator model for example, the drive amplitude Adrive, drive phase φdrive, resonant frequency ω0 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. In 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). However, in dual or multiple frequency excitations, more measurements may be made, and this will allow additional parameters to be extracted. In the context of the SHO model, by measuring the response at two frequencies at or near a particular resonance, it is possible to extract four model parameters. When the two frequencies are on either side of resonance, as in the case of DFRT PFM for example, 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 and the sum of the phases provides a measure of the tip-sample drive phase.
Simply put, with measurements at two different frequencies, we measure four time averaged quantities, A1, A2, φ1, φ2 that allow us to solve for the four unknown parameters, Adrive, φdrive, f0 and Q.
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 Adrive. Curve 18010 shows the amplitude response of an oscillator with a resonance frequency of f0=320 kHz, a quality factor Q=110 and a drive amplitude Adrive=0.06 nm. Using DFRT PFM techniques, the amplitude A1 18012 is measured at a drive frequency f1 and the amplitude A2 18014 is measured at a drive frequency f2. Curve 18030 shows what happens when the Q value increases to 150. The first amplitude A1 18032 increases because of this increase in Q, as does the second amplitude A2 18034. Curve 18050 shows what happens when the quality factor Q, remains at 110 and the drive amplitude Adrive increases from 0.06 nm to 0.09 nm. Now, 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 Adrive.
This difficulty is surmounted by measuring the phase. 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, f1 and f2. The phase curve 18020 remains unchanged 18060 when the drive amplitude Adrive increases from 0.06 nm to 0.09 nm. Note that the phase measurements 18022 and 18062 at f1 for the curves reflecting an increase in drive amplitude but with the same quality factor are the same, as are the phase measurements 18024 and 18064 at f2. However, when the quality factor Q increases, the f1 phase 18042 decreases and the f2 phase 18044 increases. These changes clearly separate drive amplitude changes from Q value changes.
In the case where the phase baseline does not change, it is possible to obtain the Q value from one of the phase measurements. However, as in the case of PFM and thermal modulated microscopy, the phase baseline may well change. In this case, it is advantageous to look at the difference in the two phase values. When the Q increases, this difference 18080 will also increase. When the Q is unchanged, this difference 18070 is also unchanged.
If we increase the number of frequencies beyond two, other parameters can be evaluated such as the linearity of the response or the validity of the simple harmonic oscillator model
Once the amplitude, phase, quadrature or in-phase component is measured at more than one frequency, there are numerous deductions that can be made about the mechanical response of the cantilever to various forces. These deductions can be made based around a model, such as the simple harmonic oscillator model or extended, continuous models of the cantilever or other sensor. The deductions can also be made using a purely phenomenological approach. One simple example in measuring passive mechanical properties is that an overall change in the integrated amplitude of the cantilever response, the response of the relevant sensor, implies a change in the damping of the sensor. In contrast, a shift in the “center” of the amplitude in amplitude versus frequency measurements implies that the conservative interactions between the sensor and the sample have changed.
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.
In measuring the frequency response of a sensor, it is not required to excite the sensor with a constant, continuous signal. Other alternatives such as so-called band excitation, pulsed excitations and others could be used. The only requirement is that the appropriate reference signal be supplied to the detection means.
FIG. 16 shows one embodiment of a multi-frequency approach, with eight frequencies f1 through f8 being driven. As 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 non-linear 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. As an example of this sort of measurement, one can drive the cantilever at one or more frequencies near resonance and measure the response at nearby frequencies.
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, again 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. These and other techniques known in the art can benefit greatly from excitation at more than one frequency. Furthermore, it can also be beneficial if excitation of a mechanical parameter at one or more frequencies is combined with electrical excitation at the same or other frequencies. The responses due to these various excitations can also be used in feedback loops, as is the case with Kelvin force microscopy where there is typically a feedback loop operating between a mechanical parameter of the cantilever dynamics and the tip-sample potential.
Perhaps the most popular of the AC modes is amplitude-modulated (AM) Atomic Force Microscopy (AFM), sometimes called (by Bruker Instruments) tapping mode or intermittent contact mode. Under the name “tapping mode” this AC mode was first coined by Finlan, independently discovered by Gleyzes, and later commercialized by Digital Instruments.
AM AFM imaging combined with imaging of the phase, that is comparing the signal from the cantilever oscillation to the signal from the actuator driving the cantilever and using the difference to generate an image, is a proven, reliable and gentle imaging/measurement method with widespread applications. The first phase images (of a wood pulp sample) were presented at a meeting of Microscopy and Microanalysis. Since then, phase imaging has become a mainstay in a number of AFM application areas, most notably in polymers where the phase channel is often capable of resolving fine structural details.
The phase response has been interpreted in terms of the mechanical and chemical properties of the sample surface. Progress has been made in quantifying energy dissipation and storage between the tip and sample which can be linked to specific material properties. Even with these advances, obtaining quantitative material or chemical properties remains problematic. Furthermore, with the exception of relatively soft metals such as In—Tn solder, phase contrast imaging has been generally limited to softer polymeric materials, rubbers, fibrous natural materials. On the face of it this is somewhat puzzling since the elastic and loss moduli of harder materials can vary over many orders of magnitude.
The present invention adapts techniques used recently in research on polymers, referred to there as loss tangent imaging, to overcome some of these difficulties. Loss tangent imaging recasts our understanding of phase imaging Instead of understanding a phase image as depending independently on energy dissipation and energy storage, we understand it as depending on an inextricable linkage of energy dissipation and energy storage, one term that includes both the dissipated and the stored energy of the interaction between the tip and the sample. If one changes, for example if the dissipation increases it generally means that the other quantity does as well. This is similar to other dimensionless approaches to characterizing loss and storage in materials such as the coefficient of restitution. The impact method of estimating the loss tangent of a material has very early roots, dating back at least to the work of Zener in 1941.
In addition to loss tangent imaging, the present invention combines the quantitative and high sensitivity of simultaneous operation in a frequency modulated (FM) mode The microscope is set up for bimodal imaging with two feedback loops, the first using the first resonance of the cantilever and the second the second resonance. The first loop is an AM mode feedback loop that controls the tip-sample separation by keeping the amplitude of the cantilever constant (and produces a topographic image from the feedback signals) and at the same time compares the signal from the cantilever oscillation to the signal from the actuator driving the cantilever to measure changes in phase as the tip-sample separation is maintained constant. The second feedback loop is a FM mode feedback loop that controls the tip-sample separation by varying the resonant frequency of the cantilever. The frequency is varied in FM mode through a phase-locked loop (PLL) that keeps the phase (again a comparison of the signal from the cantilever oscillation to the signal from the actuator driving the cantilever at the second resonance) at 90 degrees by adjusting the frequency. Another feedback loop may be implemented to keep the amplitude of the cantilever constant through the use of automatic gain control (AGC). If the AGC is implemented, output amplitude is constant (CA). Otherwise, if the amplitude is allowed to vary, it is termed constant excitation (CE) mode.
Much of the initial work with FM mode was in air and it has a long tradition of being applied to vacuum AFM studies including UHV, routinely attaining atomic resolution and even atomic scale chemical identification. Recently there has been increasing interest in the application of this technique to various samples in liquid environments, including biological. Furthermore, FM AFM has demonstrated true atomic resolution imaging in liquid where the low Q results in a reduction in force sensitivity. One significant challenge of FM AFM has been with stabilizing the feedback loops.
Briefly, when AM mode imaging with phase is combined with FM imaging using bimodal imaging techniques, the topographic feedback operates in AM mode while the second resonant mode drive frequency is adjusted to keep the phase at 90 degrees. With this approach, frequency feedback on the second resonant mode and topographic feedback on the first are decoupled, allowing much more stable, robust operation. The FM image returns a quantitative value of the frequency shift that in turn depends on the sample stiffness and can be applied to a variety of physical models.
Bimodal imaging involves using more than one resonant vibrational mode of the cantilever simultaneously. A number of multifrequency AFM schemes have been proposed to improve high resolution imaging, contrast, and quantitative mapping of material properties, some of which have already been discussed above.
With bimodal imaging the resonant modes can be treated as independent “channels”, with each having separate observables, generally the amplitude and phase. The cantilever is driven at two flexural resonances, typically the first two, as has been described above. The response of the cantilever at the two resonances is measured and used in different ways as shown in FIG. 20. It will be noted that the FIG. 20 apparatus bear some resemblance to the apparatus shown in FIG. 1.
FIG. 20 is a block diagram of a preferred embodiment of an apparatus for probing two flexural resonances 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 1150 of the cantilever probe 1020 relative to the frame of the microscope (not shown) is measured with a detector (not shown), 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 (not shown), 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. 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. As with other electronic components in this apparatus, the lockin amplifiers 1110 and 1120 can be made with analog circuitry or with digital circuitry or a hybrid of both. For a digital lockin amplifier, one interesting and attractive feature is that the lockin analysis can be performed on the same data stream for both flexural resonances. This implies that the same position sensitive detector and analog to digital converter can be used to extract information at the two distinct resonances.
Resonance 1: As shown in the upper shaded area of FIG. 20, the flexural resonance signal from frequency synthesizer 1080 is compared to the signal from the cantilever deflection 1150 through lockin amplifier 1110. This feedback loop controls the z actuator (not shown) which moves the cantilever chip 1030 relative to the sample 1010 and thus controls the amplitude of the cantilever probe 1020 and the tip-sample separation. The amplitude signal resulting from this feedback is used to create a topographic image of the sample 1010. Simultaneously, the phase of the cantilever probe 1020 is calculated from this comparison and together with the amplitude signal is used to generate the tip-sample loss tangent image.
Resonance 2: As shown in the lower shaded area of FIG. 20, the second flexural resonance signal from frequency synthesizer 1090 is compared to the signal from the cantilever deflection 1150 through lockin amplifier 1120 and a second phase of the cantilever probe 1020 is calculated from this comparison. The Phase Locked Loop (PLL) device 1160 in turn maintains this phase at 90 degrees by making appropriate adjustments in the second flexural resonance signal. The required adjustment provides a FM based measure of tip-sample stiffness and dissipation. Tip-sample stiffness and dissipation can also be measured from the amplitude and phase of the second flexural resonance signal. FM mode may also employ an Automatic Gain Control (AGC) device to maintains the amplitude of the cantilever probe 1020 at a constant value.
The foregoing bimodal imaging approach to quantitative measurements with Loss Tangent and AM/FM imaging techniques has the great advantage of stability. With topographic feedback confined to the first resonant mode FM mode to the second resonant mode, even if the PLL or AGC control loops become unstable and oscillate, there is little or no effect on the ability of the first mode to stably track the surface topography.
In order to highlight some important limitations it is useful to take a mathematical approach to Loss Tangent imaging. As already noted in AM AFM operation, the amplitude of the first resonant mode is used to maintain the tip-sample distance. The control voltage, typically applied to a z-actuator results in a topographic image of the sample surface. At the same time, the phase of the first resonant mode will vary in response to the tip-sample interaction. This phase reflects both dissipative and conservative interactions. A tip which indents a surface will both dissipate viscous energy and store elastic energy—the two are inextricably linked. The loss tangent is a dimensionless parameter measures the ratio of energy dissipated to energy stored in one cycle of a periodic deformation. The loss tangent of the tip-sample interaction can be described by the following relation involving the measured cantilever amplitude V and phase φ:




  
    
      
        
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In this expression, Fis the tip-sample interaction force, z is the tip motion, ż is the tip velocity, ω is the angular frequency at which the cantilever is driven and < > represents a time-average. The parameter vfree is the “free” resonant amplitude of the first mode, measured at a reference position and is an important calibration parameter. Note that because the amplitudes appear as ratios in Equation (1), they can be either calibrated or uncalibrated in terms of the optical detector sensitivity. If we operate on resonance, the expression can be simplied to:




  
    
      
        
          tan
          ⁢
          
            
          
          ⁢
          δ
        
        ≈
        
          
            
              
                V
                /
                
                  V
                  free
                
              
              -
              
                sin
                ⁢
                
                  
                
                ⁢
                φ
              
            
            
              cos
              ⁢
              
                
              
              ⁢
              φ
            
          
          .
        
      
    
    
      
        (
        2
        )
      
    
  




There are a couple of important things to note in Equation (2):

    
    
        1. Attractive interactions between the tip and the sample will in general make the elastic denominator ωFis·z of Eq. (1) and (2) smaller. This will increase the cantilever loss tangent and therefore over-estimate the sample loss tangent.
        2. Tip-sample damping with origins other than the sample loss modulus, originating from interactions between, for example, a water layer on either the tip or the sample will increase the denominator in Eq. (1) and (2).
    
    


These factors point out an important limitation of loss tangent imaging. Equations (1) and (2) are really the loss tangent of the cantilever—but not necessarily the loss tangent originating from the sample mechanics: G′′ and G′. With proper choice of operating parameters, this effect can be minimized, improving the estimate of the loss tangent. For example, in the case of the mechanical loss tangent of a polymer surface, the scan should be in repulsive mode so that the cantilever is sampling the short range repulsive forces controlled by the sample elastic and loss moduli. It is important to take the steps required for this to be so.
To understand the importance of these parameters and to understand the practical limitations of loss tangent imaging, it is useful to perform an error analysis on the measurement. Errors in the measured loss tangent depend on phase errors and amplitude errors. Using standard error analysis, the fractional loss tangent error which is dependent on uncertainties in the amplitude and phase of the measurement is given by




  
    
      
        
          
            Δ
            ⁢
            
              
            
            ⁢
            tan
            ⁢
            
              
            
            ⁢
            δ
          
          
            tan
            ⁢
            
              
            
            ⁢
            δ
          
        
        =
        
          
            
              
              
                1
                
                  cos
                  ⁢
                  
                    
                  
                  ⁢
                  
                    φ
                    ·
                    tan
                  
                  ⁢
                  
                    
                  
                  ⁢
                  φ
                
              
              
            
            ⁢
            Δ
            ⁢
            
              
            
            ⁢
            
              V
              r
            
          
          +
          
            
              
              
                
                  tan
                  ⁢
                  
                    
                  
                  ⁢
                  φ
                
                -
                1
              
              
            
            ⁢
            Δ
            ⁢
            
              
            
            ⁢
            
              φ
              .
            
          
        
      
    
    
      
        (
        3
        )
      
    
  




For a simple harmonic oscillator cantilever model, there is a monotonic relationship between the phase and the drive frequency,




  
    tan
    ⁢
    
      
    
    ⁢
    φ
  
  =
  
    
      
        
          ωω
          free
        
        /
        Q
      
      
        
          ω
          free
          2
        
        -
        
          ω
          2
        
      
    
    .
  




This implies that measurements of the resonant frequency are equivalent to measuring a frequency-dependent phase shift φ(ω) subject to the condition φ(ω=ωfree)=90°. Equation (3) can be plotted for various experimental situations.

In the two decade history of phase imaging, there are very few examples of phase contrast over relatively hard materials. The above error analysis along with the Ashby (1987) provides some insight into this. There is a general trend that less elastic materials tend to be more lossy. However, there are many examples where a stiffer material might also exhibit higher dissipation. This underscores the danger in simply interpreting phase contrast in terms of only the sample elasticity.
Thermal noise limits the loss tangent resolution at small values and at high values. In particular, since the loss tangent diverges at a cantilever phase of 90 degrees, fluctuations near this point have a very strong effect on the estimated loss tangent.
This observation is remarkably consistent with a literature search for phase imaging. There are many examples of phase imaging of polymeric materials and very few of metals and ceramics with tan δ<10−2, consistent with the error analysis above. This insight is one benefit of the loss tangent point of view in that it provides some insight into a general contrast limitation the AFM community has been subject to for a long time.
It may be useful to mention here some practical experimental limitations on loss tangent imaging. Proper choice of the zero-dissipation point is critical for proper calibration of the tip-sample dissipation in the loss tangent mode. In particular, squeeze film damping can have a strong effect on the measured dissipation. Squeeze film damping causes the cantilever damping to increase as the body of the cantilever moves close to the sample surface. For rough surfaces, this can mean that the cantilever body height changes with respect to the average sample position enough to cause crosstalk artifacts in the measured dissipation and therefore the loss tangent. An example is shown in FIG. 3 below of a Silicon substrate patterned with SU8, an epoxy commonly used for photolithography. The silicon is expected to have a high modulus  ̃150 GPa while the literature puts the modulus of the SU8 at Lorenz et al. reported that SU-8 has a modulus of elasticity of 4.02 GPa. Since the SU8 is a polymer, we expect it to have some viscoelastic properties; in any event more than that of the silicon. As shown in the figure, the SU8 layer was >1.5 um thick. Surprisingly however, the loss modulus as calculated using Eq. (1) shown in FIG. 3(b) indicates that the tan δSi>tan δSU8. This non-physical result can be explained by air damping differences over the two materials. Since the Si is  ̃1.5 um lower than the SU8 epoxy, when the tip is measuring the Si, it experiences increased air damping. This is interpreted as a larger loss tangent over the Si.
FIG. 21 shows the topography 2101 of a Si-epoxy (SU8) patterned wafer. The loss tangent 2102 calculated using equation (1) shows surpising contrast inversion: the loss tangent of the cantilever over the Si is higher than over the relatively lossy Su8 polymer. The corrected loss tangent 2103 using Equation (1) and the schematic of the sample shows the  ̃1.5 um thick SU8 epoxy step and the amplitude curves measured over the Si (red) and SU8(black). The steps used to acquire the corrected loss tangent image 2103 are described in the text.
The reference amplitudes 2105 and 2106 for the cantilever are different over the two regions, because of the large difference in sample height. As discussed above, these reference values are critical for correctly estimating the loss tangent of the sample. Measuring the reference amplitude at the same height above the sample should mitigate this systematic error. Qualitatively, the results shown in 2103 are consistent with this conclusion—the dissipation over the lower Si features appears larger than over the SU8, consistent with air damping playing a role in miscalibrating the reference amplitude.
To correct for this topographic crosstalk, we have implemented a pixel-by-pixel referencing method for loss tangent imaging. In this method, illustrated in FIG. 22, where a normal AM mode imaging scan 2201 is made while the topography, amplitude and phase are recorded. The cantilever is then raised a height (Δz) above the surface and the same x-y scan is repeated. During this second pass 2202, the cantilever is operated in a phase-locked loop, allowing the resonant frequency and reference amplitude at that frequency to be measured. The drive frequency and quality factor 2200 are measured in the initial cantilever tune, typically performed before or after the scanning steps.
FIG. 22 shows steps to calculate the corrected loss tangent.
1. 2203 first the drive frequency is chosen and the quality factor are estimated in a tune far away from the surface.
2. 2202 the reference amplitude and resonant frequency are measured by scanning the cantilever a preprogrammed height above the surface.
3. 2201 the amplitude and phase during normal tapping mode are measured on the surface.
4. The parameters measured in steps 2203, 2202 and 2201 are used to calculate the loss tangent on a pixel by pixel basis. The reference height at each x-y pixel location is set by a single delta height (Δz) parameter—which in turn determines the reference amplitude and resonance frequency.
The results of this approach are shown in FIG. 21. After applying the correction 2103, the loss tangent over the Si is on the order of 0.01 and over the SU8 on the order of 0.05. The most important conclusion is that the order is now correct for the two materials—Si is smaller and SU8 is larger. Furthermore, the values over Si are consistent with the limits imposed by the noise analysis above (Eq. (3)).
Another improvement in loss tangent imaging is to include energy being transferred to higher harmonics of the cantilever. This can be a significant effect at low Q values. Energy losses to higher harmonics of the cantilever are more significant at lower Q than at higher Q. Tamayo has accounted for this energy dissipation by including the harmonic response of the cantilever. By extending this analysis to the storage power, we derived an expression for the loss tangent of Eq. (3) that now includes harmonic correction terms:




  
    
      
        
          tan
          ⁢
          
            
          
          ⁢
          δ
        
        =
        
          
            
              
                sin
                ⁢
                
                  
                
                ⁢
                
                  φ
                  1
                
              
              -
              
                
                  ∑
                  
                    n
                    ≥
                    1
                  
                  N
                
                ⁢
                
                  
                    n
                    2
                  
                  ⁢
                  
                    
                      A
                      n
                      2
                    
                    
                      
                        A
                        free
                      
                      ⁢
                      
                        A
                        1
                      
                    
                  
                
              
            
            
              
                cos
                ⁢
                
                  
                
                ⁢
                
                  φ
                  1
                
              
              +
              
                Q
                ⁢
                
                  
                    ∑
                    
                      n
                      ≥
                      1
                    
                    N
                  
                  ⁢
                  
                    
                      
                        A
                        n
                        2
                      
                      
                        
                          A
                          free
                        
                        ⁢
                        
                          A
                          1
                        
                      
                    
                    ⁢
                    
                      (
                      
                        
                          n
                          2
                        
                        -
                        1
                      
                      )
                    
                  
                
              
            
          
          .
        
      
    
    
      
        (
        4
        )
      
    
  




In Eq. (4), n is the order of the harmonic and An the amplitude at the nth harmonic, ranging from the fundamental at n=1 up to the limit N. In the case of the dissipation (the numerator), the harmonics behave as a “channel” for increased damping. Specifically, if energy goes into the harmonics, the fundamental mode, damping will appear to increase. In the case of the storage term (the denominator), energy going into the harmonics looks like a reduction in the kinetic energy of the cantilever. This has the effect of reducing the apparent storage power in Eq's (2) and (3). These two effects act in concert to increase the measured loss tangent.
In addition to measuring many of the harmonics of the loss tangent fundamental, the error associated with harmonic loss can be estimated and improved upon by simply measuring the response of the cantilever at one harmonic, for example the 6th or 4th harmonic, that is a harmonic close to the next highest resonant mode.
FM AFM has become a powerful technique for imaging surfaces at atomic resolution, and manipulating atomic surfaces. By measuring the frequency shift as the tip interacts with the surface, it is possible to quantify tip-sample interactions. In particular, the frequency shift of a cantilever in FM mode is given by




  
    Δ
    ⁢
    
      
    
    ⁢
    
      f
      2
    
  
  =
  
    
      
        f
        
          0
          ,
          2
        
      
      ⁢
      
        
          〈
          
            
              F
              ts
            
            ⁢
            z
          
          〉
        
        
          
            k
            2
          
          ⁢
          
            A
            2
            2
          
        
      
    
    ≈
    
      
        
          f
          
            0
            ,
            2
          
        
        2
      
      ⁢
      
        
          
            k
            ts
          
          
            k
            2
          
        
        .
      
    
  




In addition to the terms described for Equation (1) above, f0,2 is the second resonance frequency measured at a “free” or reference position, Δf2 is the shift of the second resonant mode as the tip interacts with the surface, k2 is the stiffness of the second mode and A2 is the amplitude of the second mode as it interacts with the surface. As with the expression for the loss tangent, it does not directly involve the optical lever sensitivity. Thus, we can relate the measured frequency shift to a tip-sample stiffness:





  
    
      
        
          k
          ts
        
        ≈
        
          
            
              2
              ⁢
              
                k
                2
              
              ⁢
              Δ
              ⁢
              
                
              
              ⁢
              
                f
                2
              
            
            
              f
              
                0
                ,
                2
              
            
          
          .
        
      
    
    
      
        (
        5
        )
      
    
  




The second mode resonant behavior provides a direct measure of the tip-sample interaction forces. FIG. 23 shows an example of this. In this measurement, the amplitude of the first resonance 2301 is plotted versus tip-sample distance. As the cantilever approaches the surface, it first experiences net attractive forces which reduce the amplitude 2302. At a certain point, there is a switch from net attractive to net repulsive interactions which results in a discontinuity in the amplitude-distance curve 2303. This effect is well-known in the literature. After that point, the interaction is dominated by repulsive forces and the curve is somewhat different 2304. The phase versus distance curve 2305 measured simultaneously gives additional information. The phase shift is positive during the net-attractive interaction 2306 portion of the approach curve. The attractive-repulsive transition 2307 shows the switch between the attractive dominated portion 2306 and the repulsive dominated 2308 portion of the curve. Note that each transition 2303 and 2307 have some small hysteresis between the approach and retract portions of the curves. The families of second resonant mode phase curves 2309, 2310 and second resonant mode amplitude curves 2311 and 2312 were measured while the first mode was held constant by a z-feedback loop. The feedback loop was enabled, keeping the first mode at an essentially constant value by adjusting the z-height of the cantilever while the drive frequency operating on the second mode 1090 was ramped. The frequency tunes were made by sweeping the second drive frequency 1090 but could be alternatively be chirped, band-excited, intermodulated or excited in some other manner that explores the frequency content of the interaction. The amplitude tune far from the surface 2315 and the phase far from the surface 2314 are plotted as dashed lines. During the attractive portion of the interaction, the phase curves 2309 move to the left, indicating that the resonance has shifted to a lower frequency, as expected from an attractive interaction. During the repulsive portion of the interaction, the phase curves move to the right, shifting towards higher frequencies 2310 as expected from a stiffer interaction. The amplitude curves show corresponding behavior, with the peak shifting to the left (lower frequencies) during the attractive portion of the interaction 2311 and towards higher frequencies during the repulsive portion of the interaction 2312. In addition, the peak values of the amplitude curves decreases, indicating an overall increase in tip-sample damping or dissipation 2311, 2312. By engaging a phase locked loop (PLL) 1160 and repeating the amplitude ad phase versus distance experiment, the output of the f2 2010 and A2 2013 results in the curve 2313. This is notable in that it tracks the peak values of the curves in the A2 vs frequency display in FIG. 23. This implies that the PLL can be used, at least in this case as a rapid means of interrogating the cantilever as to its second mode response. Note that other methods for measuring this could also be used as discussed above including the DART, Band Excitation, chirping, intermodulation and other techniques that provide information about the frequency response.
In some cases, it is advantageous to omit the phase-locked loop and to simply measure the phase of the second mode. Examples include when the transfer function of the cantilever actuation mechanism is subject to frequency-dependent amplitude and phase shifts. This is essentially bimodal or “DualAC” mode. This is often the case for operation in fluid, but can even be the case for higher quality factor situations where the cantilever actuator has a frequency-dependent transfer function. This can be used (as can the other modes described here) with a large variety of cantilever actuation means including acoustic, ultrasonic, magnetic, electric, photothermal, photo-pressure and other means known in the art.
To relate the phase shift to the stiffness of the sample, we can start with the relationship between the frequency and phase shifts for a simple harmonic oscillator;




  
    
      
        ∂
        φ
      
      
        ∂
        x
      
    
    =
    
      
        
          (
          
            
              x
              2
            
            +
            1
          
          )
        
        ⁢
        Q
      
      
        
          
            
              (
              
                
                  x
                  2
                
                -
                1
              
              )
            
            2
          
          ⁢
          
            Q
            2
          
        
        +
        
          x
          2
        
      
    
  
  ,




where x≡fdrive,2/f0,2 is the ratio of the drive frequency to the resonant frequency measured at the reference position. Using Equation (2) above, this can be manipulated to give the tip-sample interaction stiffness in terms of the phase shift measured at a fixed drive frequency:





  
    k
    ts
  
  ≈
  
    
      -
      
        
          f
          
            0
            ,
            2
          
        
        
          f
          
            drive
            ,
            2
          
        
      
    
    ⁢
    
      Δφ
      Q
    
    ⁢
    
      
        k
        2
      
      .
    
  




Note that this expression is only valid for small frequency shifts. While the complete, nonlinear expression is analytic, our experience is that the frequency shift of the higher modes are typically a few parts in a thousand or smaller, justifying the simple expression.
This mode of operation can be extremely sensitive, down to the level of repeating images of single atomic defects as shown in FIG. 26. In this Figure, two repeating images 2601, 2602 of the surface of Calcite in fluid are shown, each with the same defect 2603, 2604 visible. The corresponding stiffness images are shown on the right 2605, 2606 with the single atomic stiffness defect 2607, 2608 visible in each image. Line sections taken across the stiffness images show the sub-nanometer peak in the stiffness associated with the defect in each image 2609. This very high resolution mechanical property measurement in fluid represents a very significant advance in the materials properties measurements using AFM.
Since loss tangent can be measured using the first mode and FM is measured using the second resonance mode, both measurements can be made simultaneously. There are some practical experimental conditions to consider when applying this technique to nano-mechanical materials properties measurements:
The tip is sensitive to G′ and G′′ only in repulsive mode. Repulsive mode is favored for:

    
    
        1. larger cantilever amplitudes (>1 nm)
        2. stiffer cantilevers (>1N/m)
        3. sharp tips and
        4. lower setpoints (typically 50% of the free amplitude).
    
    


As a check, the first mode phase should always be <90° and typically <50° for the majority of materials. This assures you are sampling the repulsive tip-sample interactions. Good feedback tracking (avoid parachuting, make sure trace and retrace match) assures good sampling of the mechanical properties. Finally, the accuracy of both techniques depends strongly on careful tuning of the cantilever resonances. Specifically, the resonances should be <10 Hz error and the phase should be within 0.5 degrees. These are more stringent conditions than usual for AM mode but are well within the capabilities of commercial AFMs, with proper operation.
FIG. 25 shows an example of simultaneous loss tangent 2501 and stiffness 2502 mapping of an elastomer-epoxy sandwich. A natural rubber sheet was bonded to a latex rubber sheet with an epoxy. The sandwich was then microcryotomed and imaged. From macroscopic measurements, the elasticity E ̃40 MPa (natural rubber):4 GPa (epoxy):43 MPa (latex rubber) measured with a Shore durometer, while the macroscopic loss tangents were estimated to be 1.5, 0.1:2 and 2, respectively, measured with a simple drop test. An AC160 cantilever with a fundamental resonance of 310 kHz and a second mode resonance of 1.75 MHz was used. Histograms of the loss tangent 2503 and of the stiffness 2504 show clear separation of the three components. Note that the surface roughness was on the order of 500 nm and despite this, the materials are still clearly differentiated.
Since the second mode resonance depends on the interaction stiffness kis, the material modulus can be mapped by applying a particular mechanical model. One of the most simple models is a Hertz indenter in the shape of a punch. In this case, the elasticity of the sample is related to the tip-sample stiffness by the relation kis=2E′a, where a is a constant contact area. Combining this with equation (2) above results in the expression




  
    
      
        
          E
          ′
        
        =
        
          
            
              Δ
              ⁢
              
                
              
              ⁢
              
                f
                2
              
            
            
              f
              
                0
                ,
                2
              
            
          
          ⁢
          
            
              
                k
                2
              
              a
            
            .
          
        
      
    
    
      
        (
        6
        )
      
    
  




Thus if the contact radius and spring constant are known, the sample modulus can be calculated. Of course, other tip shapes could be used in the model. Calibration of the tip shape is a well-known problem. However, it is possible to use a calibration sample that circumvents this process. As a first step, we have used a NIST-traceable ultra high molecular weight high density polyethelene (UHMWPE) sample to first calibrate the response of the AC160 cantilever. Equation (6) can be rewritten as E′=C2Δf2, where C2 is a constant, measured over the UHMWPE reference that relates the frequency shift to the elastic modulus. This can then be applied to unknown samples.
Finally, this technique can be performed at high speeds using small cantilevers. The response bandwidth of the ith resonant mode of a cantilever is BWi=πfi,0/Qi where fi,0 is the resonant frequency of the ith mode and Qi is the quality factor. To increase the resonance frequency without changing the spring constant can be done by making cantilevers smaller. In contrast to normal AM imaging, the second resonant mode must still be accessible to the photodetector, requiring f2,0<10 MHz for the Cypher AFM. An example is shown in FIG. 24, where a EPDH/Epoxy cryo-microtomed boundary is measured at a 2 Hz 2401 and 20 Hz 2402 line scan rates. These images were 5 um on a side and were acquired with an AC55 cantilever from Olympus (f1,0≈1.3 MHz, f120≈5.3 MHz). The histograms of the 2 Hz 2403 and 20 Hz 2404 images show very little deviation between the peak elasticity measurements indicating the high speed image acquisition did not significantly affect the results.
Loss tangent and AM-FM provide two additional tools for quantifying nanoscale mechanical properties. These modes are compatible with conventional AM imaging, meaning that high resolution, high speed mechanical properties can be made on an enormous variety of samples:

    
    
        1. No invols calibration—instead relies on automated frequency tunes
        
            a. Sader k calibration
            b. Ratios of amplitudes
            c. Resonant frequency shifts
        
        
        2. Based on reliable, proven and well understood technology and physics—not a mysterious black box
        
            a. Tapping mode
        
        
        3. Simple optimization
        
            a. Decoupled loops
            
                i. Resonance 1: Topography and loss tangent
                ii. Resonance 2: Stiffness and Dissipation
            
            
        
        
        4. Quantitative results on a wide variety of samples
        5. High speed
        
            a. Demonstrated 10-20 Hz linescan rates on rough samples
        
        
        6. Extremely high resolution
        
            a. Repeating single atomic defect stiffness resolution
        
        
    
    


In general, one can choose to use any higher resonant mode for stiffness mapping. There are a couple of things to consider when making the choice.

    
    
        1. Avoid modes that are at or very close to integer multiples of the first resonance. If they are at integer values, one gets harmonic mixing between the modes which can cause instabilities. For example, it is quite common that the second resonance of an AC240 is  ̃6× the first. For that reason, we often use the third resonance mode instead, typically  ̃15.5× the first.
        2. The sensitivity is optimized when the stiffness of the mode is tuned to the tip-sample stiffness.
    
    


Note that the caveat 1 above applies to AFM measurement modes where the cantilever is not necessarily being driven at the second resonant mode. If a higher mode is too close to an integer multiple of the drive frequency, unwanted harmonic coupling can take place that leads to spurious, noisy and/or difficult to interpret results.
FIG. 27 shows the effects of choosing the resonant mode that was softer, matched or stiffer than the tip-sample stiffness, as mentioned in caveat 2 above. In this work, the fundamental resonance of an AC200 cantilever was at  ̃1.15 kHz. The second resonance 2701 was at  ̃500 kHz, the third 2702 was at  ̃1.4 MHz, the fourth 2703 at  ̃2.7 MHz, the fifth 2704 at  ̃4.3 MHz and the sixth 2705 was at  ̃6.4 MHz. The elasticity images, measured and calculated as discussed above are shown 2706, 2707, 2708, 2709 and 2710 in ascending order. Note that the contrast in the second mode (soft) 2706 is relatively low, as is the contrast in the 5th and 6th mode images (stiff) 2709, 2710. This is explicitly visible in the Elasticity histograms 2711, 2712, 2713, 2714 and 2715, arranged in ascending order.
Note that to optimize the response of the cantilever to mechanical stiffness contrast and accuracy, it may be advantageous to tune the amplitude of the second mode so that it is large enough to be above the detection noise floor of the instrument, but small enough to not significantly affect the trajectory and behavior of the fundamental mode motion as discussed above and in reference to FIG. 4. To this end, it may be useful to plot the higher mode amplitude and/or phase and/or frequency as a function of the second mode drive amplitude. This can be done for example, while the first mode is employed in a feedback loop controlling the tip-sample separation or in a pre-determined tip-sample position. By measuring A1 4010, 4050, P1 4020, 4060 and P2 4040 as a function of the higher mode drive amplitude 1090, the user can chose a drive amplitude that optimizes the signal to noise while minimizing the effect of nonlinearities on the measured signals.
Another issue with making the stiffness and other measurements quantitative is quantification of the higher mode stiffness. In general, this is a challenging measurement. One method is to extend the thermal noise measurement method to higher modes as shown in FIG. 28. The thermal measurement depends on accurately measuring the optical lever sensitivity. This can be done by driving each resonant mode separately and then measuring the slope of the amplitude-distance curve as the cantilever approaches a hard surface. This calibrated optical sensitivity can then be used in a thermal fit as is well known in the art to get the spring constant for that particular resonant mode. Typical fits for the fundamental, 2801 second resonance 2802 and third resonance 2803 yielded optical lever sensitivities that were then used to fit the thermal noise spectra to extract the spring constants for the first 2804 second 2805 and third mode of the AC240 cantilever.
The described embodiments of the invention are only considered to be preferred and illustrative of the inventive concept. The scope of the invention is not to be restricted to such embodiments. Various and numerous other arrangements may be devised by one skilled in the art without departing from the spirit and scope of the invention.

## Claims

1. A method of operating an atomic force microscope, comprising:
using a feedback loop to control a distance between the base of a probe tip of a cantilever of the atomic force microscope, and a surface of a sample being measured by the atomic force microscope, to maintain the probe tip of the cantilever in a pre-established relationship with respect to the surface of the sample in a Z direction while scanning the sample by creating relative movement between the probe tip of the cantilever and the sample;
exciting the a driving chip of the cantilever at a first and a second resonant frequencies of the cantilever which have been summed together by a circuit element;
providing said first and said second resonant frequency as a reference signal to a lock-in amplifier circuit;
controlling a distance between the base of the cantilever and the sample so that an amplitude of oscillation at the first resonant frequency of the probe tip is maintained essentially constant at an amplitude setpoint;
detecting changes in phase and/or amplitude in the oscillation at the first resonant frequency while the amplitude of oscillation at the first resonant frequency is maintained essentially constant at the amplitude setpoint;
detecting changes in phase and/or amplitude in the oscillation at the second resonant frequency while the amplitude of oscillation at the first resonant frequency is maintained essentially constant at the amplitude setpoint; and
measuring the amplitude and/or phase of the cantilever at the first and second frequencies.

2. The method according to claim 1, further comprising:
using the amplitude and/or phase to calculate a signal indicative of a loss tangent of the cantilever as it interacts with the sample surface.

3. The method according to claim 1, further comprising exciting the driving chip of the cantilever with a third resonant frequency which is also summed by the circuit element, and providing said third resonant frequency as a reference to the lock-in amplifier circuit.

4. The method according to claim 1, wherein the lock-in amplifier apparatus includes first and second lock-in amplifiers.

5. The method as in claim 4, wherein the first and second lock-in amplifiers are digital, and both receive the same data stream.

6. The method as in claim 1, where the circuit element includes an an analog adder.

7. The method as in claim 1, wherein the circuit element includes a digital circuit element.

8. An atomic force microscope, comprising:
an atomic force microscope cantilever, having a base and a probe tip;
a surface for holding a sample to be measured by the cantilever, so a surface of the sample is measured by the atomic force microscope;
circuits defining a feedback loop to maintain the probe tip of the cantilever in a pre-established relationship with respect to the surface of the sample in a Z axis direction defined between the probe tip and the sample, while scanning the sample,
a cantilever driving element, driven by said feedback loop to create relative movement between the probe tip of the cantilever and the sample;
the feedback loop operating to control a distance between the tip of the cantilever at both of first and a second resonant frequencies of the cantilever by controlling a distance between the base of the cantilever and the sample so that an amplitude of oscillation at the first resonant frequency of the probe tip is maintained essentially constant at an amplitude setpoint;
a controller, detecting changes in phase and/or amplitude in the oscillation at the first resonant frequency while the amplitude of oscillation at the first resonant frequency is maintained essentially constant at the amplitude setpoint, and detecting changes in phase and/or amplitude in the oscillation at the second resonant frequency while the amplitude of oscillation at the first resonant frequency is maintained essentially constant at the amplitude setpoint, and measuring the amplitude and/or phase of the cantilever at the first and second frequencies.

9. The atomic force microscope as in claim 8, further comprising a circuit that sums together said first and second resonant frequencies and uses a summed version as a reference signal.

10. The atomic force microscope as in claim 8, further comprising:
using the amplitude and/or phase to calculate a signal indicative of a loss tangent of the cantilever as it interacts with the sample surface.

11. The atomic force microscope as in claim 8, further comprising:
wherein said driving chip of the cantilever is excited with a third resonant frequency which is also summed by the circuit element, and providing said third resonant frequency as part of said reference.

12. The atomic force microscope as in claim 8, wherein the reference drives lock-in amplifiers that include first and second lock-in amplifiers.

13. The atomic force microscope as in claim 12, wherein the first and second lock-in amplifiers are digital, and both receive a same data stream.

14. The atomic force microscope as in claim 8, where the circuit element includes an analog adder.