Non-resonant microwave imaging probe

ABSTRACT

A non-resonant microwave imaging microscope and associated probe. The probe includes a sensor unit with two fixed electrodes, preferably a large outer electrode surrounding a small inner electrode which are approximately co-planar, thereby protecting the small inner electrode from an uneven topography. The outer electrode may be deposited on a conically shaped dielectric disk having a bore through which the inner electrode is placed. Non-resonant circuitry couples the inner electrode to the probe signal variably selected in the range of 10 MHz-50 GHz and to an amplifier whose output is coupled to a signal processor detector in-phase and out-of-phase components of the current or voltage across the two electrodes. A mechanical positioner moves the probe vertically towards the sample and scans it across the sample.

INFORMATION

DETAILED DESCRIPTION OF THE INVENTION

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A probe of the invention is illustrated in the cross-sectional view of FIG. . It includes a circularly symmetric center electrode that has a sharpened tip with a radius, for example, of between 0.1 to 10 μm. The center electrode is fit within a cylindrical bore of a insulating disk , which should be formed of dielectric material having a low dielectric constant, for example, alumina, sapphire, Teflon, etc. The bore has a diameter d, which may be within the range of 1 to 100 μm. The dielectric disk is also circularly symmetric and has a conically shaped bottom face except possibly for a flattened portion near the disk bore . An outer electrode is formed on the bottom of the dielectric disk , preferably by plating or sputtering, and is joined to a bulk annular electrode portion , which is typically electrically grounded and has an outer diameter B in the range of 0.1 to 10 mm. The electrode tip is preferably positioned within the plane of the lowest portion of the disk and its plated outer electrode or slightly behind it so that the tip does not protrude from the disk bore . The conical shape of the disk allows the probe to be laterally scanned within a short distance h above a sample being tested which may have a relatively rough upper surface. Since the tip does not protrude from the disk bore , it will not be damaged by the rough surface. The distance h is preferably held within the range of zero to the bore diameter d. With these parameters, the capacitance between the two electrodes , is typically between 0.001 to 0.5 pF. On the other hand, if the tip does protrude beyond the lowermost plane of the outer electrode , spatial resolution will be further increased. Even though the capacitance is being measured between the two electrodes , which are typically separated by distance substantially more than 1 μm, for those measurements dependent upon induced surface charge, the capacitance between the sample and the very small sharpened tip is much smaller than the serially connected capacitance between the sample and the much larger outer electrode . Hence, in a high-frequency measurement, most of the electromagnetic filed is near the high-impedance tip so the spatial resolution of the probe is on the order of the tip radius.

The probe is incorporated into a sensor unit schematically illustrated in FIG. and is mounted on a metal shielded enclosure . An XYZ positioning system is capable of moving the enclosure and attached probe independently in the x-, y-, and z-directions with sub-micron resolution. Such positioning systems are well known for use with atomic force microscopes and may be based on piezo electric tubes and other elements. Alternatively, the sample may be moved relative to a stationary probe to provide relative motion in three-dimensions between the probe and sample .

An electronic circuit included within the enclosure electronically couples an input drive signal from a microwave generator to the center electrode . A signal processor processes the signal output from the center electrode through the electronic circuit . Coaxial cables , couple microwave signals from the microwave generator to the electronic circuit and from the electronic circuit to the signal processor . The outer shields of the coaxial cables , , held nominally at ground, are connected to the shielded enclosure and thence to the outer electrode of the probe . It is understood that other types of transmission line, such as strip line particularly for higher frequencies, may replace at least part of the coaxial cables. The signal processor may be implemented as a lock-in amplifier at lower frequencies. At microwave frequencies, it may be implemented as a microwave mixer receiving the RF reference and sample signals, producing an output whose amplitude and phase define the real and imaginary components of the tip impedance. By measuirng the change in this impedance when the sample is brought near the tip , the dielectric constant and conductivity of the sample can be determined.

The system typically induces an RF or microwave signal across the electrodes , with a magnitude of 1 to 10V RMS at a frequency of between 10 MHz and 10 GHz. Generally, the higher frequencies provide better resolution, and gigahertz frequencies are of particular interest for semiconductor circuits. However, the lower RF frequencies may be interest in establishing characteristic energies in the sample material. The non-resonant design allows a single small probe to be used across the entire frequency range so that the RF or microwave generator , more generally referred to as a source of alternating electrical potential, is preferably tunable across all or part of this range.

The electronic circuit can incorporate any low-noise, preferably miniaturized design common in the art, such as strip line and/or discrete components or integrated circuits, which produce the required potential difference across the electrodes , and determines the current flowing between them. The electronic circuit is represented as an operational amplifier with a capacitor in a negative feedback loop, its positive terminal receiving the microwave drive signal, its negative terminal connected to the center electrode , and its output connected to the signal processor , thereby acting as a capacitance measuring system measuring the capacitance across the electrodes , . Elements needed to correct for phase shift and biasing are not shown. Although the feedback element is shown as a capacitance, which is preferred for high-frequency operation, other feedback impedances may be used depending upon the application. More complicated circuitry than that illustrated, commonly known in the art, can be used to cancel most of the output signal when no sample is present so the signal reaching the signal processor represents the difference in the tip impedance due to the presence of the sample.

The electrodes , and the electrical lines linking them to the electrical circuit are non-resonant with no strong peaks or nulls in their electrical characteristics. In particular, the electrical length of the connection between the electronic circuit and the tip of the center electrode is substantially less than one-quarter of the shortest electrical wavelength of the probing signal. By substantially less is meant no more than 75% of the quarter wavelength since even resonant probes are often operated somewhat off the resonance peak. For a 10 GHz microwave signal, such a length is less than about 7.5 mm. Such a short length has the further advantage of reduced capacitance contributed by the line and low noise pickup. The length may be determined between the probe tip and a coupling or transforming circuit that buffers the amplifying and biasing circuit from the non-resonant line.

The signal processor mixes the RF or microwave signal from the generator with the output of the electronic circuit to produce amplitude and phase signals A and φ representing the complex impedance across the electrodes , and hence of the surface area of the sample over which the tip of the probe is currently positioned. The phase φ may be referenced to the probing signal from the microwave or RF generator , which is directly input to the signal processor over an unillustrated transmission line. Alternatively, the signal processor may produce two output signals representing quadrature or parts of the signal from the probe, that is, the amplitude of two signal components that are 90° out of phase with each other. With proper design well known in RF and microwave circuitry, the bandwidth (sampling frequency or inverse time constant) of the electronic circuit and signal processor can be between 10 Hz and 1 MHz, which is quite adequate for fast, detailed imaging of the sample surface. On the other hand, there are some measurements which do not require a two-component or complex measurement, in which case an envelope detector or other simple one-output detecting circuitry may be used.

In the case of uneven topography of the sample, any combination of the amplitude and phase signals A and φ can be fed back to the positioning system to maintain the tip of the center electrode a fixed height in the z-direction above the sample surface while the probe is being scanned in the x- and y-dimensions.

Another embodiment, as schematically illustrated in FIG. 3, includes a modified probe and a modified electronic circuit , which minimize inter-electrode capacitance and decrease the common-mode output signal of the amplifier . A guard electrode is embedded in the dielectric disk between the center electrode and the grounded outer electrode . The input RF signal on the input coaxial cable is directly coupled to the guard electrode , and the capacitance between the guard electrode and the center electrode produces the desired RF signal between the center electrode and the grounded outer electrode . A voltage divider formed by an input capacitor between the input RF signal and the positive input of the operational amplifier and by a grounded varactor diode connected to the same positive input reduces the oscillating potential on the positive input of the amplifier to a value approximately equal to the that on the center electrode with no sample present, which condition produces a null output of the amplifier . Thereafter, as the probe approaches the sample, the output signal represents the current flowing through the sample rather than the total current between the electrodes , .

The embedded guard electrode can be fabricated by plating both it and the outer electrode on opposite sides of a thin conically shaped dielectric disk with its central aperture formed before or after plating. The dielectric in back of the guard electrode can be thereafter deposited or an apertured small bulk dielectric member can be affixed to provide mechanical support for the center electrode .

Another embodiment of circuitry ′ illustrated schematically in FIG. 4 includes substantially no feedback impedance around the amplifier so that it operates as a high-gain voltage detector. As in the circuitry of FIG. 3, with the probe withdrawn from the sample, the varactor is adjusted to produce a null output from the amplifier . Thereafter, when the probe is lowered to the sample, the amplifier output measures the voltage sensed between the electrodes , rather than the current. The circuit of FIG. 2 can be similarly modified.

The electronic circuit , as well as circuit of FIG. 2, ignore signal propagation times and are low-frequency idealizations of realistic designs. That is, the circuits , do not include elements for biasing and phase compensation that are included in standard and well known designs for microwave and RF circuits.

A sensor , illustrated in side cross-sectional view in FIG. and bottom plan view in FIG. 6, is suitable for fabrication by lithographic techniques well developed for integrated circuits. A via hole is etched in a beveled ceramic disk and is filled with a via metal . A center electrode and a surrounding outer electrode are sputtered and patterned on the via metal and the beveled face of the ceramic disk respectively . For increased spatial resolution, a nanotip, such as a carbon nanotube, with a radius of less than 100 nm can be attached to the center electrode. The ceramic disk is mounted to the bottom of a cylindrical grounded enclosure containing the miniaturized electronics, which may be a single microcircuit performing the functions described earlier. The grounded enclosure , which typical has a diameter and thickness in the range of 10 to 100 μm, is supported on a cantilever which together with the probe tip can be rapidly and accurately positioned and scanned over the sample surface.

The very light weight of the sensor allows more rapid scanning permitting acquisition of an image in fractions of a second rather than minutes. Because the sensor is non-resonant, its size is not limited to the wavelength of the probing RF or microwave signal.

The very small size of the two electrodes enhances the sensitivity of the measurement. The small spacing increases the spatial resolution, which is typically the larger of the dimension of the inner electrode and of the sample-electrode spacing. Further, the small spacing and dimension of the inner electrode decreases the probe capacitance so small changes in the electrical properties of the sample cause correspondingly larger percentage changes in the measured voltage or current.

The feedback and biasing made possible by the non-resonant electronics reduces the input impedance of the detection circuitry, giving it improved signal-to-noise performance relative to circuits employing small resonators.

The use of two electrodes in the probe, rather than a grounded sample, restricts the probing current to a small area of the sample, hence increasing the spatial resolution. The generally planar configuration of the two electrodes greatly enhances the mechanical ruggedness and reliability of the probe since the larger outer electrode surrounds and can be arranged to protect the fragile small center electrode.

Although the probe is most advantageously used with microwave probing signals, it may be used with lower frequency probe signals, for example, at RF frequencies extending down to 10 MHz and possibly below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an embodiment of a microwave or RF probe of the invention.

FIG. 2 is a schematic representation of electronics that may be used with the probe of FIG. .

FIG. 3 is a schematic representation of a second embodiment of a probe of the invention and electronics usable with it.

FIG. 4 is a schematic representation of a third embodiment using the probe of FIG. 3 but used with voltage sensitive electronics.

FIG. 5 is a cross-sectional view of a third embodiment of a probe of the invention taken along view line — of FIG. .

FIG. 6 is a bottom plan view of the probe of FIG. .

RELATED APPLICATION

This application claims benefit of U.S. Provisional Application Ser. No. 60/330,240, filed Oct. 17, 2001.

CLAIMS

1. An alternating potential microscope, comprising: a source of alternating electrical potential outputting an electrical signal within a wavelength range; a probe having a first electrode and a second electrode arranged to be non-resonant within said wavelength range, receiving said alternating potential, and positionable adjacent a surface of a sample to be characterized and scannable along said surface; and an electrical circuit coupling said source of alternating current to said probe; and a signal processor receiving a characterizing signal from an electrical signal across said electrodes and passing through said electrical circuit.

2. The microscope of claim 1, wherein said signal processor determines a current flowing between said electrodes.

3. The microscope of claim 1, wherein said signal processor determines quadrature components of said electrical signal relative to said alternating electric potential.

4. The microscope of claim 1, wherein said electrical circuit is separated from at least one of said electrodes by an electrical length substantially less than a quarter of a minimum wavelength within said wavelength range.

5. The microscope of claim 1, wherein said source of alternating electrical potential outputs a signal within a frequency range of 10 MHz to 50 GHz.

6. The microscope of claim 5, wherein said source is tunable over at least a portion of said frequency range.

7. The microscope of claim 1, wherein said first electrode surrounds said second electrode.

8. The microscope of claim 7, wherein said second electrode has a tip with a radius of less than 10 μm.

9. The microscope of claim 8, wherein said radius is less than 100 nm.

10. The microscope of claim 8, wherein said tip protrudes from a bore in a dielectric material separating said first and second electrodes no further than said first electrode.

11. The microscope of claim 7, wherein said first electrode is conically shaped.

12. The microscope of claim 7, further comprising a third electrode positioned between said first and second electrode.

13. The microscope of claim 12, wherein said first electrode is connected to a predetermined potential, said third electrode is connected to said source of alternating potential, and said circuit comprises an amplifier connected to said first electrode and connected through a capacitor to said source of alternating potential.

14. The microscope of claim 7, wherein said first electrode is connected to a predetermined potential and said circuit comprises an amplifier connected to said second electrode and to said source of alternating potential.

15. The microscope of claim 14, wherein said amplifier includes negative feedback.

16. The microscope of claim 1, further comprising a mechanical positioner for positioning said probe in three orthogonal dimensions with respect to said sample.

17. The microscope of claim 1, wherein said probe further comprises a dielectric disk having a face that is at least partially conically shaped; wherein said first electrode is coated on said face and has a central aperture and wherein said second electrode is positioned within said central aperture.

18. The microscope of claim 17, wherein said disk has a central bore with an end within said central aperture and wherein said second electrode has a sharpened tip disposed no further out of said bore than an outermost plane of said first electrode.

19. The microscope of claim 17, further comprising a guard electrode disposed between said first and second electrodes.

20. The microscope of claim 17, wherein said first electrode is held at a predetermined potential and further comprising an amplifying circuit connected to said second electrode and couplable to a source of alternating potential.

21. The microscope of claim 20, wherein said amplifying circuit includes negative feedback.

22. The microscope of claim 17, wherein said aperture has a diameter of no more than 100 μm.

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