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Localized picosecond resolution with a near-fieldmicrowave/scanning-force microscope

D. W. van der Weidea)Department of Electrical and Computer Engineering, University of Delaware, Newark,Delaware 19716-3130

~Received 12 November 1996; accepted for publication 4 December 1996!

We modify a scanning-force microscope tip/cantilever with a coaxial metal shield for use as anultra-small field probe, observing 30 ps waveforms on an integrated circuit at a spatial resolution setby the tip radius, the first direct measurements of electric field at these levels of temporal and spatialresolution. This new ability to acquire topography while maintaining constant tip–sample distancefor calibrated measurement or excitation of near-zone picosecond time-resolved electric fields isuseful not only for probing advanced circuits but also for localized broadband spectroscopy ofcondensed matter and biological samples. ©1997 American Institute of Physics.@S0003-6951~97!00206-4#

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Current integrated circuits~IC’s! with sub-micrometerfeatures and switching rates in the hundreds of megahpresent serious challenges to internal-node measuretechniques. While established electron-beam probes aredressing industry needs for circuit diagnostics today, threquire vacuum, measure only conductive surfaces, andfer from sample charging effects. Future computatiostructures will have nanometer dimensions and gigahclocks, outstripping the capabilities of contemporary probmethods. As the density of these structures increases tolevel of a complex ‘‘artificial material’’~perhaps comprisedof quantum dots, ‘‘artificial atoms’’!,1 characterizing themwith scanning probes, as one might a real material, becoan increasingly attractive solution.

In response to this need, several groups have developroximal probing techniques which use nonlinearities intip–sample interaction to downconvert picosecond waforms on a sample via coherent~i.e., pump-probe! measure-ment schemes. For example, frequency mixing by meanthe nonlinear force-voltage response of the scanning-fomicroscope~SFM! cantilever2,3 allows.100 GHz signals tobe measured,4 but is usually subject to drift since the SFMheight feedback loop is opened to make the voltage measment. Scanning tunnelling microscopes have also allowfast waveform measurements,5,6 but require conductive surfaces, preventing probing through dielectric layers. Becafrequency conversion occurs in the tip-sample interactiboth techniques require the tip to carry a mixing signalsource of interference, especially since neither techniscreens long-range Coulomb interactions. Furthermore,herence between the tip and sample signals is requirethat an output can be measured within the,100 kHz usablebandwidth of ‘‘bare-tip’’ scanning probes.

By contrast, we extend this bandwidth beyond 10 Gand, more importantly, add a screen to localize the elecfield measurement, by modifying a SFM tip/cantilever withshielded transmission line allowingdirect non-contact prob-ing of local electric fields, eliminating the need to injectsignal onto the tip. The SFM now becomes useful for locized broadband spectroscopy not only on active samamenable to coherent measurements, such as circuits

a!Electronic mail: dan@ee.udel.edu

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also on passive, even nonconducting samples of intesuch as surfaces and membranes.

Such samples are already being explored in the visregime: near-field scanning optical microscopy~NSOM! hasattracted much attention for localized probing,7 even at themolecular level.8 Sub-wavelength confinement of visibllight, however, was preceded by microwave demonstratio9

and a wide range of contrast mechanisms in semiconducsuperconducting and biological materials and systems awexploration at the level of nanometer spatial resolution inspectrum lying between dc and visible light.10

This probe is an early step toward generalizing tNSOM to these longer wavelengths while maintaining ttopographical resolution of the SFM: By modifying a commercially available silicon tip/cantilever with a dielectricmetal shield connected to a coaxial cable and samplingcilloscope, we have measured 30 ps waveforms, along wthe ,10 nm level topographical resolution expected ofnon-contact SFM~Fig. 1!. For these measurements we choa nonlinear transmission line~NLTL !,11 a sample exhibitingboth spatial nonlinearity and waveforms similar to futuretrafast computing circuits. The temporal resolution wachieve with this shielded probe is limited by the capacitaof the output cabling. The spatial electric field resolutiafforded by the coaxial geometry of the probe is primardetermined by the;10 nm tip radius of the centeconductor12,13 since the otherwise-large ‘‘antenna’’ effect othe tip/cantilever is now shielded. Hence we now distingubetween the spatial resolution of topography and thatfield, and because of the shield, these two quantitiesproach the same value. Direct measurements of electriccombined at such levels of temporal and spatial resoluhave not been reported until now.

While much work has been reported on the use of opended coaxial probes for dielectric characterization of marials and biological systems,14 considerations of high spatiaresolution are rarely addressed. Seminal work in micromelevel scanning localization of microwave fields in coaxgeometries by Fee, Chu and Ha¨nsch15 has been followed byrecent work demonstrating that electric field resolutionprimarily set by the tip radius in a shielded system, due toquasistatic field distribution at the aperture.12,13 None of theliterature suggests the combination of the SFM cantile

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FIG. 1. 1003100mm topographical scan of NLTL output section with the time-domain waveform of one 1.400 GHz cycle measured at the diode inby the arrow; the 10%–90% falltime of the steepest edge is 30 ps; the calibrated voltage scale is on the right. Incomplete contact metal liftoff can alsthis is otherwise difficult to detect under optical illumination.

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with a coaxial tip in this way, even though it offers not onacquisition of sample topography and field together, but acontrol of the tip-sample distance by the surface force graent. Enabling both non-contact imaging and calibrated fimeasurements are two distinct advantages of our approa

As with NSOM, this technique can work in both emision and detection modes, but as the extension of a miwave waveguide, it can also work in reflection15 andtransmission,13made especially convenient by high-precisinetwork analyzers available in the microwave regime, anvantage of our broadband approach over resonant coatechniques.12 We note, however, that a microwave bridge16

or resonant enhancement using a filter could be employeincrease sensitivity. While we report its use in detection,have also observed dielectric contrast in the magnitudephase of~microwave! reflections in larger-scale experimentwhich when scaled down should be especially useful for stially resolved measurements of dielectric contrast in tfilms, such as gate oxides.

To make these measurements, we first prepared a cmercially available conductive silicon tip/cantilever withmetal shield connected to a coaxial output cable. This pcess involves a 90 °C 90-min bake-out to drive off wafrom the tip/cantilever, followed by a drop of HMDS appliewith a pipette to the cantilever to prepare the surfacephotoresist, a convenient, low-stress, low-permittivity dieletric which can form a thick coating for low-capacitancshielding. After a 2 min air dry, a single brush fiber is usedapply a drop of AZ-5214 photoresist to the cantilever, flowed by a 120 °C 30 min ‘‘hard bake.’’ The tip/cantileverthen mounted in an electron-beam evaporator for deposof 300 Å of titanium ~at 8 Å/s! for adhesion followed by2000 Å of gold~at 4–6 Å/s! for shielding. Scanning electromicroscopy of the resultant structure shows complete coage of the tip and cantilever with the expected half-cylindof metal ~peak height.16 mm! running the length of thecantilever~inset, Fig. 2!.

Using a micromanipulator with a sharp-tipped tool, ecess gold at the base of the cantilever can be removed. Wthis reduces the area for attaching the output wire, mak

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attachment more difficult, it substantially reduces the capatance between the metal and the conducting silicon bodythis case to;0.03 pF, the capacitance due to the hacylinder of metal on the cantilever alone. Using silver epo0.86 mm ~outer diameter! semi-rigid coaxial cable17 is an-chored at the side of the probe body, minimizing its profiAn 18mm diam gold bond wire is epoxied to the coax cenconductor, then to the gold-coated base of the cantilethen looped back on itself to the coax center conductor aghalving its inductance~Fig. 2!.

The tip assembly is mounted into a Topometrix ExploSFM and arranged to have a;15° tilt between the cantileveand sample planes to allow clearance for the output c~Fig. 2!. Although this limits the imaging area, this limit cabe readily overcome with lower-profile cable17 and a slightlymodified scanner; in this first demonstration, however,instead made a large-area topographical scan~Fig. 1! with anidentical tip minus the coax.

The initial resistance measured at the output coax winfinite, as expected. Then by scanning the tip over a gocoated sample and gradually lowering it just to the pointcontact, the metal shell over the tip was removed, exposthe underlying silicon tip and forming a coaxial shield wi;5 mm diameter~Fig. 2!, as verified electrically by observation of both a short circuit and of ac signal extinctionthe tip was brought into hard contact with a conductor. Whthis is already the smallest reported coaxial structure toused for field measurements, we are developing a microchined version with a;100 nm diameter for even greatefield confinement, ease of assembly to a cable, and the adant advantages of batch fabrication.18

The sample, a NLTL on GaAs, is a high-impedance cplanar waveguide transmission line periodically loaded wreverse-biased Schottky diodes~inset, Fig. 3! to compress aninput sinusoid into a sawtooth waveform having a pico-sub-picosecond edge, here a falling edge due to the polaof the diodes.11 To approximate the switching waveformexpected from advanced computational structures in theGHz regime, we drove the NLTL sample with 1.400–2.0GHz signals from a Hewlett–Packard 83620A microwa

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synthesizer, amplified by an HP83020A power amplifiThe waveform was measured on an HP54750A 50 GHz spling oscilloscope, triggered by another 83620A synthesi

As shown in Fig. 3, we measured the expected spanonlinearity along the NLTL, with a progressively steepfalling edge moving along the line; while three sample traare shown, the spatial resolution of the field could beserved through measurable variations in the waveform,0.1 mm steps of the probe moving along the line. A neobservation afforded by this measurement was that, althothe diodes are approximately equidistant from one anotthe first one~Diode A! had a much slower waveform than isuccessors, probably due to a signal reflection from thebend in the transmission line just before it. We also obserthat the metal liftoff process was incomplete on this sam~Fig. 1!. This is difficult to see under optical illuminationsince the gold contacts give nearly uniform reflection. Sudirect local diagnostics of very fast, very dense IC’s are npossible with this new probing technique.

From previous measurements, we expected;5 ps~10%–90%!, 3 V falling edges under these drive conditionmeasurements with the new probe gave 30 ps edges503 lower amplitudes, consistent with our estimates.18 Thisnot only enables a calibration of the voltage scale but poout the bandwidth limitation of the 95 pF/m output coaxcable. We estimate values for Rcantilever '10 V, Ccantilever'0.03 pF, and Lbondwire'0.5 nH, and the tip–sample capactance Ctip at ;0.4 pF, a value corresponding to the areathe shield opening at a;2 nm tip–sample working distanceBy adjusting this distance, a tradeoff can be made betwprobe invasiveness and the low-frequency edge of the msurement band (;200 MHz here!. Further advances can bmade with localized instrumentation near the tip, for eample a buffer amplifier to minimize capacitive loading

FIG. 2. Schematic arrangement of shielded SFM tip/cantilever connectea coaxial output cable~above!, and electron microscope image of actushielded SFM tip surrounded by coaxial shield~below!. Inset of lower im-age is the same tip before removing the shield.

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the output cable or a high-speed sampling bridge fortended measurement bandwidth.

Because of the coaxial structure of the tip, it exhibitscutoff wavelength, unlike the pulled-fiber NSOM, enablinoperation at much lower power due to dramatically lowlosses. Indeed, it is a complement to localized visible-ligspectroscopy, extending localized spectroscopy into thecrowave regime. For shorter-wavelength probing with extation from a far-infrared source, we can integrate micromchined planar antennas19 with the coaxial tip to extendlocalized spectroscopy to the far-infrared ‘‘molecular fingeprint’’ regime,10 and hence probe biological samples wi‘‘softer,’’ lower-energy photons than those of visible light

Thanks are due to F. Keilmann and P. Neuzil for usediscussions, and to T. Bork and J. Bergey for assistance.work was supported by ONR/DARPA Grant N00014-96-0862.

1R. C. Ashoori, Nature~London! 379, 413 ~1996!.2A. S. Hou, F. Ho, and D. M. Bloom, Electron. Lett.28, 2302~1992!.3F. Ho, A. S. Hou, and D. M. Bloom, Electron. Lett.30, 560 ~1994!.4A. Leyk, C. Bohm, D. W. van der Weide, and E. Kubalek, Electron. Le31, 1046~1995!.

5G. J. Nunes and M. R. Freeman, Science262, 1029~1993!.6D. Botkin, J. Glass, D. S. Chemla, D. F. Ogletree, M. Salmeron, andWeiss, Appl. Phys. Lett.69, 1321~1996!.

7E. Betzig and J. K. Trautman, Science257, 189 ~1992!.8X. S. Xie and R. C. Dunn, Science265, 361 ~1994!.9E. A. Ash and G. Nicholls, Nature~London! 237, 510 ~1972!.10F. Keilmann, Infrared Phys. Technol.36, 217 ~1995!.11D. W. van der Weide, Appl. Phys. Lett.65, 881 ~1994!.12T. Wei, X.-D. Xiang, W. G. Wallace-Freedman, and P. G. Schultz, ApPhys. Lett.68, 3506~1996!.

13F. Keilmann, D. W. van der Weide, T. Eickelkamp, R. Merz, andStockle, Opt. Commun.129, 15 ~1996!.

14D. Berube, F. M. Ghannouchi, and P. Savard, IEEE Trans. MicrowaTheory Tech.44, 1928~1996!.

15M. Fee, S. Chu, and T. W. Ha¨nsch, Opt. Commun.69, 219 ~1989!.16M. Golosovsky and D. Davidov, Appl. Phys. Lett.68, 1579~1996!.17Obtainable from Micro-Coax, Collegeville, PA 19426.18D. W. van der Weide and P. Neuzil, J. Vac. Sci. Tech. B14, 4144~1996!.19D. W. van der Weide, J. Opt. Soc. Am. B11, 2553~1994!.

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FIG. 3. Excerpts from a linear scan along the NLTL diodes showing waform compression at a 2.000 GHz drive. Distortion of the signal at diodprobably results from the waveguide bend just before it.

679Daniel W. van der WeideAIP¬license¬or¬copyright,¬see¬http://ojps.aip.org/aplo/aplcr.jsp