tapping mode imaging applications and technology afm an004

8/4/2019 Tapping Mode Imaging Applications and Technology AFM AN004

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Figure 3. In contact AFM, electrostatic and/or surface tensionforces from the adsorbed fluid layer to destructive lateral shear tipsample forces.

Under ambient air conditions, mostsurfaces are covered by a layer offluid (condensed water vapor andother contaminants) which is typicallyseveral nanometers thick. When thescanning tip touches this layer,

capillary action causes a meniscusto form and surface tension pulls thecantilever down into the layer (Figure3). Trapped electrostatic charge onthe tip and sample can contributeadditional adhesive forces. Thesedownward forces increase the overallforce on the sample and, whencombined with lateral shear forcescaused by the scanning motion, candistort measurement data and causesevere damage to the sample, or justmove surface features.

Some researchers have overcome theproblems associated with the adhesiveforces by operating AFMs with thesample immersed in liquid. Whenscanning in liquids, the overall forcesin contact mode are lower than inambient air. However, becausehydrated samples are sometimessofter than dried samples, trackingforces can still cause reduced image

quality and sample damage due todeformation and/or movement of thesample by the scanning probe. Inaddition, many samples, such assemiconductor wafers, may not beimmersed in liquids.

An attempt to avoid this problem is thenon-contact mode in which the probeis held a small distance above thesample (Figure 2). Attractive Van derWaals forces acting between the tipand the sample are detected, and

topographic images are constructedby scanning the tip above the surface.Unfortunately, the attractive Van derWaals forces are substantially weakerthan the forces used by contact mode so weak in fact that the tip must begiven a small oscillation so that ACdetection methods can be used todetect the small forces between tipand sample. The attractive forces alsoextend only a small distance from thesurface, where the adsorbed fluid layermay occupy a large fraction of theiruseful range.

Hence, even when the sample-tipseparation is successfully maintained,non-contact mode providessubstantially lower resolution thaneither contact or TappingMode. Inpractice, the probe is frequently drawnto the sample surface by the adsorbedfluids surface tension, resulting inunusable data and sample damage

similar to that caused in contact mode.

TappingModeImaging in Air

TappingMode imaging overcomlimitations of the conventional scmodes by alternately placing thein contact with the surface to prhigh resolution and then lifting thoff the surface to avoid draggintip across the surface. TappingMimaging is implemented in ambby oscillating the cantilever asseat or near the cantilevers resonafrequency using a piezoelectric The piezo motion causes the cato oscillate with a high amplitudfree air amplitude, typically grthan 20nm) when the tip is not

contact with the surface. The ostip is then moved toward the suruntil it begins to lightly touch, orthe surface. During scanning, thvertically oscillating tip alternatecontacts the surface and lifts off,generally at a frequency of 50,to 500,000 cycles per second.the oscillating cantilever begins intermittently contact the surfacecantilever oscillation is necessarireduced (Figure 4) due to energ

loss caused by the tip contactingsurface. The reduction in oscillatamplitude is used to identify andmeasure surface features.

Figure 4. TappingMode cantilever oscillation amplitude in free aduring scanning.


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During TappingMode operation, thecantilever oscillation amplitude ismaintained constant by a feedbackloop (Figure 5). Selection of theoptimal oscillation frequency issoftware-assisted and the force on thesample is automatically set and can bemaintained at the lowest possible level(Table 1 and Figure 6). When the tippasses over a bump in the surface, thecantilever has less room to oscillateand the amplitude of oscillation

decreases. Conversely, when the tippasses over a depression, thecantilever has more room to oscillate

Figure 6. The cantilever tune screen assists the opeselecting the optimum TappingMode oscillation frequ

Figure 7. Comparison of large linear operating range forTappingMode vs. small operating range for non-contact mo

Figure 5. Block diagram for TappingMode operation.

and the amplitude increases(approaching the maximum free airamplitude). The oscillation amplitude ofthe tip is measured by the detector andinput to the SPM controller electronics.The feedback loop then adjusts thetip-sample separation to maintain aconstant amplitude.

TappingMode inherently prevents thetip from sticking to the surface andcausing damage during scanning.Unlike contact and non-contact modes,when the tip contacts the surface, ithas sufficient oscillation amplitude toovercome the tip-sample adhesion

forces. Also, the surface materiapulled sideways by shear forcesthe applied force is largely verti(see sidebar on page 6 for adddiscussion of tip-sample forces).

Another advantage of theTappingMode technique is its lalinear operating range (Figure 7makes the vertical feedback systhighly stable, allowing routinereproducible sample measuremeSeveral references which discussTappingMode imaging are listethe end of this application note.

Table 1. TappingMode Specifications.

Drive Frequency Range 10KHz to 1MHz

Drive Amplitude Software control and display ofand Frequency TappingMode parameters allows fast,Adjustment semi-automated on-screen optimization

Detector RMS-to-DC amplitude detector provides

phase-independent amplitude signal;Noise level > 0.5 RMS

Cantilevers Etched silicon cantilevers with orwithout coatings for specializedapplications; typically 50-500KHzresonant frequencies

Tip-Sample Motorized approach automaticallyApproach brings cantilever into TappingMode

operation at low tracking force


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TappingMode Imaging in Liquids

Similar advantages are realized withTappingMode operation in liquids. Inthis case, however, the liquid mediumtends to damp the cantileversresonance. When an appropriatefrequency is selected (usually in therange of 5,000 to 40,000 cyclesper second), the amplitude of thecantilever will decrease when the tipbegins to tap the sample, similar toTappingMode operation in air.

Once the cantilever is set intooscillation, the SPM feedbacksystem adjusts the position of the tipfor samples to maintain a constantoscillation amplitude. Again as

in air, the oscillating cantilevereliminates frictional and shear forceson the sample.

Figure 8. TappingMode image scanned inair of kinetoplast DNA from the trypanozome

of a malarial parasite. 2m scan courtesyOak Ridge National Labs, Oak Ridge,Tennessee.

Figure 9. Comparison of contact mode(top) and TappingMode (bottom) imagesof Bacteriorhosdopsin in liquid (buffer).100nm scan size.

Figure 10. Lambda Hind III DNA ion mica with TappingMode in wate

sample was scanned continuously fone hour without damage. Contact scanning of the same material causdamage in less than one minute the scan could be completed. 500courtesy M. Bezanilla, University ofCalifornia, Santa Barbara.

Examples

Figures 8 through 14 illustrate thecapabilities of TappingMode forimaging a variety of surfaces.Figures 8 through 10 show lifesciences imaged in both liquidand air, illustrating the dramaticimprovement in image quality forTappingMode relative to contactmode in both environments.

Figure 11 illustrates the capabilitiesof TappingMode relative to contactmodes for harder surfaces, such as insemiconductor and data storage usingside-by-side comparisons. Figures 12through 14 are TappingMode imagesfor a polymer and two thin films.


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1m scan. TappingMode. 2m scan. TappingMode.

1m scan. Contact mode. 2m scan. Contact mode.

Figure 11. Contact and TappingModeimages for the same (100) epitaxial wafer.In both cases, the left image was taken firstand the scan size was immediately doubledand re-scanned to include the area imagedin the first scan. The TappingMode imagesshow no surface alteration and better

resolution. Conversely, the damaged areaof the first scan can be easily seen in Figure11d. Contact mode imaging is extremelyinconsistent for silicon surfaces; in this casematerial has been removed by the scanningtip, while in other cases, additional oxidegrowth or more subtle changes may occur.This type of surface alteration often goesundetected since most researchers do notcheck for damage by rescanning theaffected area at lower magnification.

Figure 12. TappingMode image of highdensity polyethylene from a shopping bag.The structures in the image are the polymerlamellae which are approximately 30nmthick and all oriented in the same directionto increase the tensile strength. This structurecould not be seen with contact mode sincethe features were altered by the tip draggingacross the surface. 675nm scan.

Figure 13. Chemical vapor deposited (CVD)diamond film. During film formation, seedcrystals of diamond are placed on a siliconwafer which is then placed in the CVDdeposition chamber in which growth isinitiated to produce the thin film. This imageshows the film at early initiation of growth.The TappingMode technique was used tomore accurately profile the crystals and toavoid moving the seed crystals on thesurface. 1m scan. Sample courtesy ofStanford University.

Figure 14. Thermally evaporated g60 thick, deposited onto an oxidisilicon wafer. The films were used tostrain sensors with higher strain sensthan continuous films. 500nm scan L. ChunShien, P. Hesketh, and G. MUniversity of Illinois at Chicago.

a. b.

c. d.


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Summary

To obtain quality images, it is criticalthat the microscope tip not damagethe surface being scanned but that itcontact the surface to obtain high

resolution measurements. This is whereTappingMode imaging excels. Formany materials, this technique providesthe highest resolution possible withoutsample damage.

TappingMode imaging continues toexpand the ability of scanning probemicroscopy in both materials and lifesciences applications, and enables awide variety of imaging techniques formaterials characterization unattainable

with Contact Mode. Combined withPhase Imaging, using TappingModewith Veeco SPMs is the key toadvancing nanoscience research.For more information about thistechnique, and the advantages ofPhase Imaging, please visitwww.veeco.com.

More on Tip-Sample Forces in TappingMode

One of the key advantages of TappingMode imaging over contact A

is the low tip-sample shear forces generated during scanning. Becaus

the tip only contacts the surface briefly during each oscillation cycle,

lateral shear forces applied to the sample by the tip that can tear the

sample, distort data, or dull the tips are minimized.

The brief contact force is less than one might expect. In TappingMode

the cantilever is oscillated at or near its resonant frequency. Once the

cantilever amplitude is stabilized at the desired setpoint, the sample m

absorb only the small force due to the increased amplitude during a

single oscillation cycle; i.e., the time between two consecutive taps.

Because the cantilevers used in TappingMode have a high quality fac

(Q), the amplitude gained in one cycle is very small. The force due

this small amplitude increase can be absorbed by the vast majority of

samples with no damage to tip or sample. Because of these gentle

scanning forces, TappingMode has been used successfully to

reproducibly image such samples as polymers, unbaked photoresist

and DNA, as well as numerous other fragile samples. Also, we have

repetitively imaged the angstrom-level microroughness of the same 1m

region of a silicon wafer continuously over a 24-hour period without

degradation of the image or damage to the sample.

Finally, the cantilever is oscillated at frequencies from 50KHz to

500KHz. At these frequencies, many surfaces become stiff (viscoelast

and can more easily resist forces from the probe tip. This property furth

reduces the possibility of sample damage for extremely soft samples s

as polymers, biological specimens, and others and causes less distort

of the sample due to tip forces.


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References

1. Delain, E. et al, Comparitive observation of biological specimens,especially DNA and filamentous actin molecules in atomic force, tunneand electron microscopes, Microscopy, Microanalysis, MicrostructuresVol. 3, 457-70, 1992.

2. Hansma, H.G. and Hoh J.H., Biomolecular Imaging with the AtomicForce Microscope, Annual Reviews of Biophysics and BiomolecularStructure 1994.

3. Hansma, H.G. et al, Bending and straightening of DNA induced bythe same ligand: characterization with the atomic force microscope,Biochemistry, in press, 1994.

4. Hansma, H.G. et al, Recent advances in atomic force microscopy ofDNA, Scanning, Vol. 15, 296-99, 1993.

5. Hansma, P.K. et al, Tapping mode atomic force microscopy in liquids,Applied Physics Letters, Vol. 64, 1738-40, 1994.

6. Huber, C.A. et al, Nanowire array composites, Science, Vol. 263,800-802, 1994.

7. Putman, C.A.J. et al, Tapping mode atomic force microscopy in liquidApplied Physics Letters, Vol. 64, 2454-56, 1994.

8. Radmacher, M., Fritz M., Hansma H., and Hansma, P.K., Directobservation of enzyme activity with the atomic force microscope,Science, Vol. 265, 1577-79, 1994.

9. Schueler, P.A. et al, Physical Structure, Optical Resonance, and SurfacEnhanced Raman Scattering of Silver-Island Films on Suspended PolymeLatex Particles, Analytical Chemistry, Vol. 65, 3177-3186, 1993.

10. Umemura, K., Arakawa H. and Ikai A. et al, High resolution images a cell surface using a tapping mode atomic force microscope, Japane

Journal of Applied Physics, Vol. 32, L1711-14, 1993.

11. Vatel, O. et al, Atomic Force Microscopy and infrared spectroscopystudies of hydrogen baked Si surfaces, Japanese Journal of AppliedPhysics, Vol. 32, L1289-91, 1993.

12. Zhong, Q. et al, Fractured polymer/silica fiber surface studied bytapping mode atomic force microscopy, Surface Science Letters, Vol. L688-692, 1993.


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tapping mode imaging applications and technology afm an004

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