Vibration amplitude of a tip-loaded quartz tuning fork during shear force microscopy scanning

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Vibration amplitude of a tip-loaded quartz tuning fork during shear force microscopy scanning

P. Sandoz,^1,a兲 É. Carry,¹and J.-M. Friedt²

1FEMTO-ST/LOPMD, Université de Franche-Comté, UMR CNRS 6174, 16 route de Gray, 25030 Besançon, France

2Association Projet Aurore, UFR-ST La Bouloie, 16 route de Gray, Besançon, France

共Received 31 March 2008; accepted 7 July 2008兲

This Note reports on experimental results obtained with a recently published vision method for in-plane vibration measurement关Sandozet al., Rev. Sci. Instrum.78, 023706共2007兲兴. The latter is applied to a tip-loaded quartz tuning fork frequently used in scanning probe microscopy for shear-force monitoring of the tip-sample distance. The vibration amplitude of the tip-loaded prong is compared to that of the free one and the damping induced by tip-surface interactions is measured.

The tuning-fork behavior is characterized during approaches from free space to surface contact.

Tip-surface contact is clearly identified by a drastic reduction in the prong vibration amplitude.

However, no differences were observed between hydrophilic and hydrophobic surfaces.

Experiments reported here show that the vibration amplitude of the quartz tuning fork in free space is a good estimate of the vibration amplitude of the tip interacting with the sample surface during shear force sample-tip feedback. The experimental setup for measuring the amplitude is easily integrated in an inverted microscope setup on which the shear force microscope is installed for simultaneous scanning probe and optical microscopy analysis of the sample. © 2008 American Institute of Physics. 关DOI:10.1063/1.2965137兴

Shear force microscopy 共SFM兲 is the only scanning probe microscopy technique in which the tip-sample distance is controlled independently of the physical quantity mapped by the probe. However, the vibration of the tip induces spatial averaging and the lateral resolution is no longer solely limited by the probe tip radius but also by the tip vibration amplitude. Among the various methods presented previously for detecting the tuning-fork motion over the surface and measuring tip-sample interactions, quantitative analysis was performed on piezoelectric excitation of the vibration through a tuning fork using a voltage to vibration amplitude constant,^1,2 interferometric methods,³ capacitive measurement,^4,5 two or four quadrant-photodiode shadow measurement,^6,7 and injection of the reflected laser beam in the laser cavity.⁸None of these techniques provide full two- dimensional 共2D兲 description of the tip motion, since they are always based on the projection of the motion field in a given direction.

We proposed recently a technique based on image processing for in-plane vibration amplitude measurement.⁹ We implemented this method on a quartz tuning fork loaded with a sharp tungsten tip as represented in Fig.1. The prong end faces were polished and a pattern of dots was etched by focused ion beam following a 2D periodical grid. The periodical grid forms complementary spatial frequencies in the spectral domain and any grid displacement is evaluated through the spectral phase shift induced. A subpixel resolution is thus obtained in the prong displacement measurement, in the range of 10⁻²– 10⁻³pixel by using a standard charge couples device 共CCD兲 camera. The usual 2␲ ^phase

ambiguities associated with phase measurements are re- moved by choosing a finite number of periods in the dot pattern and by identifying the central dot by correlation.

Digital dot-pattern position measurements are thus performed on image sequences recorded during tuning-fork vibration. In-plane displacements of the prong end face are retrieved, allowing vibration amplitude measurement.

The tip-loaded tuning fork forms a high-Q device 共Q

⬃1600兲 that we used as shear force probe in a scanning microscope by detecting the vibration damping due to tip- surface interactions.^1,10The experimental setup is depicted in Fig. 2. The tip-loaded tuning fork is approached coarsely from the inspected sample by means of aZ-axis piezoelectric transducer 共PZT兲. The sample is mounted on a three-axis closed loop PZT共PI P517兲fixed on the stage of an inverted optical microscope. The latter is illuminated by means of a strobed light emitting diode共LED兲. The stroboscopic illumination is required since the tuning-fork frequency 共⬃33 kHz兲 is outside the bandwidth of the video camera 共25 frames/s兲. In practice the illumination frequency is shifted by 2 Hz with respect to the tuning-fork dither frequency in order to explore the prong position excursion with an apparent frequency of 2 Hz, i.e., compatible with the video rate of the CCD camera. Since we choose a transparent glass slide as inspected sample, images can be recorded from either the sample surface or the shear force probe end face simply by shifting the focus position along theZdirection.

A dual frequency synthesizer共Tektronix AFG320兲deliv- ers the driving signals for both the tuning fork and the LED.

A lock-in amplifier provides the magnitude of the current output of the tuning fork as well as its phase with respect to the excitation sinewave. The lock-in outputs allow the detec-

a兲Electronic mail: [email protected].

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tion of the tuning-fork resonances as well as the determina- tion of suitable setting points for shear force experiments.

These outputs are also fed into a computer used for the servocontrolling of the 共x,y,z兲 position of the three-axis PZT with nanometer resolution in order to perform lateral scanning at constant tip-sample distance.

Figure 3 shows the resonance curves obtained for the tip-loaded and unloaded prongs. As could be expected, a smaller vibration amplitude is observed for the tip-loaded one, about 85% of the free one’s. The figure presents also the magnitude of the lock-in signal as well as the phase difference between the excitation signal and the current due to direct piezoelectric effect in the tuning fork. The latter signal is of practical interest since it exhibits an extremum that can be convenient for the servocontrol of the tip position.

Figure 4 presents the calibration curve of the prong vibration amplitude versus the excitation voltage. We observe a linear behavior and a constant ratio between the loaded and unloaded prongs. This means that such calibration allows the deduction in the vibration amplitude of the tip-loaded prong from the unloaded one’s. Practically, this allows the observation of the unloaded prong a few millimeters aside from the shear force interaction.

Figure 5 presents the tip-loaded prong vibration amplitude observed while the shear force probe is approached to the surface until contact共approximately image 280兲and then retracted共approximately image 540兲. The vibration damping is clearly observed, about 50% in the case of the figure. We observed that the damping level depends on the experimental conditions. Our interpretation is that because of the tip contact with the surface, the prong behaves as a double encas- tred beam. Then the vibration amplitude at the prong end level depends on the length and stiffness of the tip glued to the prong. Indeed we observed a damping level increase after a large number of surface approaches. Our interpretation is that tip-end damaging due to successive approaches is re- sponsible for a harder contact with the surface.

Near field microscopy has been widely used for investi- gating the sensitivity of tip-substrate interactions with respect to the chemical properties of both the tip and sample surfaces.¹¹Attraction and friction forces between the tip and

FIG. 1. Scanning electron microscopy image of the tip-loaded tuning fork.

lock in amplifier I−>V

closed loop position controller open loop

high voltage DC power supply

piezoelectrictube

coarsepositioning

quartz tuning fork probe

magnitude phase

electrical

LED strobe pulse glass slide

dual frequency synthesizer f0

f0+2Hz

W tip

recording25fps

Olympus IX71 Physik Instrumente

P517

microscope inverted nanopositioner

camera

FIG. 2. 共Color online兲Experimental setup.

160 180 200 220 240 260 280 300 320 340 360

0 50 100 150 200 250 300

Excitation frequency (Hz + 33kHz)

Vibrationamplitude(nm)

-120 -100 -80 -60 -40 -20 0

Lock-inphase(degrees)

Excit. volt. amp.: 1V

FIG. 3. Vibration and electrical char- acterization of a tip-loaded tuning fork around resonance. Dotted line: free prong vibration amplitude; solid line plus crosses: tip-loaded prong vibration amplitude; solid: lock-in phase 共deg兲; circle: lock-in magnitude.

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sample can be mapped at the nanometer scale and allow the local identification of surface properties.^12–14 For instance, hydrophobic and hydrophilic surfaces were clearly distin- guished by lateral force microscopy共LFM兲.¹⁵We checked if our SFM was capable of such surface property discrimina- tion. For that purpose, the surface approach-retraction ex- periment described above was carried out successively on glass plates with and without hydrophobic surface silaniza- tion. We did not notice any difference between the two surface types, neither in the damping level nor in the transitory regime between far field and near field. We alternated several times hydrophilic and hydrophobic plates in order to exclude any tip damage disturbance. One may point out that cantile- vers used in LFM exhibit a smaller force constant 关about 0.1– 100 N/m 共Ref. 15兲兴 than tuning forks 关tens of kN/m 共Ref. 16兲兴, which makes them much more sensitive to the magnitude of the surface-probe interaction forces. This difference is a sufficient reason for not discriminating the hydrophobic/hydrophilic surface types in our case.

Finally the prong vibration amplitude was measured during SEM scanning of a grating with a period of 280 nm. The tip distance was maintained by servocontrolling the phase difference between the tuning-fork excitation signal and the current induced by the direct piezoelectric effect. Figure 6 presents the results obtained in a case where the servocontrol fails in some points, as described by the phase variation dia- gram. In normal conditions, i.e., when the phase keeps close to the setpoint value of −150°, the prong vibration is as large as in free space. This indicates that tip-surface contact is

avoided, as deduced from Fig. 5. However, for positions where the servocontrol fails to maintain the phase difference, the vibration amplitude is damped, indicating tip-surface contact. An excellent agreement can be noticed between both diagrams. However, the surface damage observed in the grating profile around pixel 55th does not produce a vibration damping nor a drastic phase variation. Then a tip-surface contact can be excluded in this zone.

Results presented in this Note were obtained with a homemade SFM that is not optimized at this stage. Image processing was performed on images of a dot pattern carved on each prong end with period of 3␮m. The latter is observed with a 20⫻objective共numercial aperture of 0.5兲and a standard CCD camera. Subnanometer amplitudes can be addressed with this approach in optimized conditions, i.e., highly stable device and environment, use of a scientific grade camera, with a high magnification 共40⫻兲 objective with a wider numeric aperture, and a dot-pattern period close to the diffraction limit共⬍2 ␮^m兲. This method is restricted to the observation of共at least partially兲transparent samples but provides a quite simple and unique way of following the tuning-fork vibration amplitude during SFM scanning. Im- ages sequences were processed a posteriori using MATLAB

routines but the algorithms are compatible with real-time implementation.

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0 40 80 120 160 200

0 10 20 30 40

Vibrationamplitude(nm)

Tuning fork excitation voltage (mV) circle: Free prong

cross: Tip loaded prong dotted: Resonance frequency solid: Phase extremum frequency

FIG. 4. Calibration of the vibration amplitude of the tip-loaded and unloaded prongs versus excitation voltage for two servocontrol configura- tions.

0 100 200 300 400 500 600 700 800 900 1000

0 50 100 150 200 250

Image number

Prongdisplacement(nm)

Exc.: 400mV

FIG. 5. Modulation of the vibration amplitude of the tip-loaded prong during a tip-surface approach and back- ward.

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1 2 3 4 5 6 x 105

−0.5 0 0.5

frame number (25 fps) vibrationampl (pixel)

0 1000 2000 3000 4000 5000 6000 7000

−150

−100

−50 0

phase (degrees)

time (a.u.)

50 100 150 200 250

0 100 200 300

spatial pixel

Z(nm)

FIG. 6. Vibration control during SFM scanning of a surface grating; up:

prong vibration amplitude; middle: phase variation between tuning-fork excitation voltage and induced current; down: reconstructed surface profile.

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