WO2013163538A1

WO2013163538A1 - Scanning probe

Info

Publication number: WO2013163538A1
Application number: PCT/US2013/038401
Authority: WO
Inventors: Ryan MURDICK
Original assignee: Rhk Technology, Inc.
Priority date: 2012-04-27
Filing date: 2013-04-26
Publication date: 2013-10-31

Abstract

An SPM probe, and a method of producing an SPM probe including annealing an electrically conductive wire under ultra-high vacuum, and immersing the wire in an etching solution contained in a first chamber of a container, which includes a second chamber that surrounds the first chamber, also contains the etching solution, and houses an electrically conductive ring surrounding the first chamber. The method also includes applying voltage across the wire and the ring, and retracting the wire in a direction out of the first chamber.

Description

SCANNING PROBE

TECHNICAL FIELD This disclosure relates generally to scanning probe microscopy and, more particularly, to scanning probes and probe manufacturing.

BACKGROUND

Scanning probe microscopy (SPM) is a technique that scientists use to analyze a sample material by monitoring interaction between a probe and the material. For example, atomic force microscopes (AFM) measure attractive and repulsive forces between a tip of a cantilevered probe and a material surface, both perpendicular or normal to the surface and parallel or lateral to the surface. Such forces can be displayed in image form as a function of the position of the tip as it scans across the surface of the material. Raman spectroscopy is a technique where a light beam is directed onto a sample material surface to induce an inelastic conformational change of underlying atoms, causing the atoms to emit photons that are of the same energy of that lost (or gained) from the irradiated beam. The photons can be dispersed according to wavelength onto a CCD screen, forming the Raman spectrum, which is has characteristic peaks based on what type of atomic bonds are present in the sample. The spatial resolution of Raman spectroscopy is approximately equal to the focal spot size, which is approximately half of the wavelength.

Tip-enhanced Raman Spectroscopy (TERS) is a technique where light is focused into a junction between an SPM probe tip and a sample material, where a strong plasmonic field enhancement amplifies a photon signal coming out of the junction. This results in greater spatial resolution as the limiting factor is the probe apex diameter rather than a confocal limit. The spatial resolution is approximately the size of the tip apex (20-30 nm). A TERS probe usually includes a metallic, for example, silver or gold, AFM or STM tip to enhance Raman signals of sample material molecules situated in its vicinity. But current TERS probe manufacturing techniques do not reliably yield probes with good quality tips on a reproducible basis. Typical yield is about 1 in 20.

BRIEF SUMMARY

One illustrative embodiment includes a method of producing an SPM probe including annealing an electrically conductive wire under ultra-high vacuum, and immersing the wire in an etching solution contained in a first chamber of a container, which includes a second chamber that surrounds the first chamber, also contains the etching solution, and houses an electrically conductive ring surrounding the first chamber. The method also includes applying voltage across the wire and the ring, and retracting the wire in a direction out of the first chamber.

Another embodiment includes a probe produced by the aforementioned method.

BRIEF DESCRIPTION OF THE DRAWINGS Features and advantages of one or more of the disclosed embodiments of this disclosure will be apparent to those of ordinary skill in the art from the following detailed description of illustrative embodiments and the claims, with reference to the accompanying drawings in which:

FIG. 1 is a schematic view of an illustrative embodiment of a probe etching apparatus that may be used in an illustrative embodiment of a process of producing a scanning probe;

FIG. 2 is a schematic view of an illustrative embodiment of a wire annealing apparatus that may be used in an illustrative embodiment of a process of producing a scanning probe; FIG. 3 is a schematic view of an illustrative embodiment of a scanning probe that may be produced using the presently disclosed method and the apparatus of FIGS. 1 and 2; FIG. 4 is a schematic view of a prior art scanning probe.

DETAILED DESCRIPTION

In general, a scanning probe and a method of producing the probe will be described using one or more illustrative embodiments. The example embodiments will be described with reference to their use with a particular manufacturing apparatus. However, it will be appreciated as the description proceeds that the inventions are useful regardless of the particular manufacturing apparatus and may be implemented in many embodiments.

Referring specifically to the drawings, FIG. 1 schematically illustrates an illustrative embodiment of an apparatus 10 that may be used to practice a method of producing a scanning probe. The apparatus 10 may include a granite block 12 that may be supported on a vibration isolation pad 14 of a floor 16 of a facility. The granite block 12 may be supported directly on the pad 14 or on a table 18 or the like resting on the pad 14. The apparatus 10 also may include a container 20 to hold an etching solution, a probe positioner 22 to advance and retract a probe wire 24 into and out of the container 20, and an electrical system 26 to provide electricity to the probe wire 24. As used herein the term "wire" includes a wire, a rod, or the like.

The container 20 may be carried directly on the granite block 12 or may be supported on the granite block 12 by an intermediate member 28, for example, a NYLON block or the like. The container 20 may include a closed bottom, sidewall, and an open top, and an open-ended conduit or bottomless test tube 30 may be disposed in the container 20. The tube 30 divides the container 20 into an inner or first chamber 32 to accept the probe wire 24, and an outer or second chamber 34 surrounding the first chamber 32 to accept an electrically conductive ring or annular cathode 36 around the tube 30. The container 20 and the tube 30 may be of any suitable shape(s). The tube 30 may be held and supported within the container 20 by the annular cathode 36. The bottom of the tube 30 may or may not be in contact with the bottom of the container 20. Accordingly, in one embodiment, the etching solution may be allowed to be communicated between the first and second chambers 32, 34 through the open bottom of the tube 20. In another embodiment, communication of the etching solution between the chambers 32, 34 may be reduced or prevented, for example, by sealing or attaching the bottom of the tube 20 to the bottom of the container 20. In a further embodiment, the tube 20 and container 30 may be a unitary product. The container 20 may include an electrical coupler 38, which may include a conductive metal carried on the sidewall and connected to the cathode 36.

The probe positioner 22 may be carried on the granite block 12 and may include a stationary base 40, a movable column 42, and a coarse positioner 44 therebetween for coarse positioning of the column 42. The coarse positioner 44 may include a manual rack and pinion type of mechanism or any other suitable device. The probe positioner 22 also may include a fine positioner 46 that may be carried by the column 42 and that may include a fixed portion 48 coupled to the column 42 and a movable portion 50 movably coupled to the fixed portion 48 and carrying a probe wire holder 52, for example, via an electrical isolator 54, for instance, a NYLON block or the like coupled to the movable portion 50. The fine positioner 46 may include a nano-positioner having piezo-electric elements, or any other suitable device(s), and may be powered and controlled in any suitable manner. The probe wire holder 52 may be composed of an electrically conductive metal and a conductive metal post 56 may extend through the isolator 54 and contact the holder 52.

The electrical system 26 may include an alternating current (AC) power regulator or supply 58 that may be powered by generator power, utility power, or any other suitable source, an electricity monitor 60 to monitor current through the probe wire 24, and wires 62, 64 that may include alligator clips or the like. For example, an anodic wire 62 with clip may be clipped to the metal post 56, and a cathodic wire 64 with clip may be clipped to the coupler 38 on the container 30. The electricity monitor 60 may include any suitable analog and/or digital equipment. For example, the electricity monitor 60 may continuously monitor current via an analog to digital converter card that may enable both controlling or driving of the voltage and monitoring of the current before a drop-off event occurs (e.g. wherein the probe tip is suitably etched and a short circuit results). At that point, the electricity monitor 60 implements a fast switch-off of the applied voltage. With reference to FIG. 2, a thermoelectric annealer 100 may be used to anneal the probe wire 24, before use in a probe etching process. The annealer 100 may include an electrical insulator 102 that may be of tubular shape and that carries the probe wire 24 therethough. The annealer 100 also may include an electrical heating wire 104 wrapped around the insulator 102 and coupled to any suitable electrical power supply (not shown) to heat the wire 104 and apply heat to the probe wire 24. The heating wire 104 may be composed of tungsten, or any other suitable material. The insulator 102 may be composed of sapphire, or any other suitable material.

Described below is an illustrative embodiment of a method of producing a scanning probe microscopy (SPM) probe. The method may include pre-annealing a probe wire and, thereafter, electrochemically etching the probe wire to produce a probe tip. The pre-annealing may be carried out using the apparatus shown and described with reference to FIG. 2, or any other suitable apparatus. The etching may be carried out using the apparatus shown and described with reference to FIG. 1 , or any other suitable apparatus.

The method includes annealing an electrically conductive wire under ultrahigh vacuum. The wire may be composed of gold, or of any other suitable metal. The wire may be about 100 microns in diameter. For example, the wire may be 80 to 120 microns in diameter. In any event, the wire may be annealed at about 600 degrees Celsius. For example, the wire may be annealed at 550 to 650 degrees Celsius. The annealing step includes disposing the wire in an insulator wrapped with a heating coil. For example, the insulator may include a sapphire tube through which the wire may be disposed. The heating coil may include a tungsten heating wire. The annealing step may include driving an electrical current through the heating coil to heat the wire. Any suitable equipment may be used to drive the electrical current. The annealing of the wire may occur over about twenty four hours. For example, the wire may be annealed for twelve to thirty six hours. The annealing time may include a warm up time. The warm up time may include activating the heating wire in any suitable manner to ramp up from room temperature to the annealing temperature over about five minutes. For example, the warm up time may be two and a half minutes to ten minutes. The cool down time may include deactivating the heating wire in any suitable manner to ramp down from the annealing temperature to any suitable post- annealing temperature over about twenty minutes. For example, the cool down time may be ten minutes to forty minutes. The annealing step may be carried out in an ultra-high vacuum chamber at about 10^~7 Torr 10^~8 Torr, for example, from 10^~7 Torr to 10^"10 Torr or any suitable vacuum level so that oxidation does not occur. Any suitable UHV chamber may be used.

The method also includes immersing the wire in an etching solution contained in a first chamber of a container, which container includes a second chamber that surrounds the first chamber, also contains the etching solution, and houses an electrically conductive ring surrounding the first chamber. The ring may be disposed about 20 mm below the surface of the etching solution. At the beginning of the etching process, the tip of the wire may be disposed about 1 mm below the surface of the etching solution. At the end of the etching process, the tip of the wire may be disposed about 300 to 800 microns from the surface of the etching solution. The aforementioned distances are merely illustrative, and any suitable distances may be used. The wire may be immersed in the etching solution in any suitable manner, for example, using the probe positioner and the container illustrated in FIG. 1 to advance the wire into the solution, or using any other suitable apparatus in any other suitable manner. The first chamber may be separated from the second chamber by an electrically insulative tube. The etching solution may include an acid and an alcohol. For example, the etching solution may include hydrochloric acid and ethanol in about a 1 : 1 ratio by volume, plus or minus about 5% to allow for production variation.

The method further includes applying voltage across the wire and the ring surrounding the wire. The voltage may be applied in accordance with alternating current at 1.4 to 1.9 mA with a period of 400 to 500 ms. The method additionally includes retracting the wire in a direction out of the first chamber. The wire may be retracted along its own longitudinal axis, for example, using the probe positioner illustrated in FIG. 1 , or using any other suitable apparatus in any other suitable manner. The wire may be retracted at a velocity of about 5 microns/second. For example, the wire may be retracted at two and a half to ten microns/second. The method also may include monitoring the current drawn during the applying voltage step. For example, the electricity monitor 60 of FIG. 1 may continuously monitor current.

The method further may include ceasing application of voltage across the wire and the ring. For example, the electricity monitor 60 of FIG. 1 may implement a fast switch-off of the applied voltage upon detection of a drop-off event. A drop-off event may include, for instance, a short circuit condition, or the like.

The method need not include flame annealing of the probe wire, or application of direct current power during etching. The method may be used to produce an SPM probe, for example, like a probe

200 shown in FIG. 3. It is believed that, as a result of using the presently disclosed method, the probe 200 may have a crystalline structure that includes larger single- crystalline domains from the UHV annealing. It is believed that the larger crystalline domains result in a stronger field enhancement as conceptually represented by the semi-spherical shape at the tip of the probe 200.

In contrast, as shown in prior art FIG. 4, it is believed that prior art probes, such as probe 300, have crystalline structures that are of relatively smaller domains. It is believed that the relatively smaller crystalline domains result in a weaker field enhancement as conceptually represented by the smaller semi-spherical shape at the tip of the probe 300.

Claims

1. A method of producing a scanning probe microscopy (SPM) probe comprising the steps of:

annealing an electrically conductive wire under ultra-high vacuum;

immersing the wire in an etching solution contained in a first chamber of a container, which includes a second chamber that surrounds the first chamber, also contains the etching solution, and houses an electrically conductive ring surrounding the first chamber;

applying voltage across the wire and the ring; and

retracting the wire in a direction out of the first chamber.

2. The method of claim 1 wherein the annealing step includes annealing the wire at 550 to 650 degrees Celsius.

3. The method of claim 1 wherein the annealing step includes disposing the wire in an insulator wrapped with a heating coil and driving an electrical current through the heating coil to heat the wire.

4. The method of claim 1 wherein the annealing step is carried out for about twenty four hours.

5. The method of claim 4 wherein the annealing step is carried out for twelve to thirty-six hours.

6. The method of claim 1 wherein the wire is composed of gold.

7. The method of claim 1 wherein the wire is about 100 microns in diameter.

8. The method of claim 7 wherein the wire is 80 to 120 microns in diameter.

9. The method of claim 1 wherein the annealing step includes a warm up time of about five minutes and a cool down time of about 20 minutes.

10. The method of claim 9 wherein the annealing step includes a warm up time of two and a half minutes to ten minutes and a cool down time of ten minutes to forty minutes.

11. The method of claim 1 wherein the immersing step includes the first chamber being separated from the second chamber by an electrically insulative tube.

12. The method of claim 1 wherein the immersing step includes the etching solution having an acid and an alcohol.

13. The method of claim 1 wherein the immersing step includes the etching solution having hydrochloric acid and ethanol in about a 1 : 1 ratio by volume.

14. The method of claim 1 wherein the applying step includes alternating current at 1.4 to 1.9 mA with a period of 400 to 500 ms.

15. The method of claim 1 wherein the retracting step is carried out at a velocity of about 5 microns/second.

16. The method of claim 15 wherein the retracting step is carried out at a velocity of 2.5 to 10 microns/second.

17. The method of claim 1 further comprising the steps of:

monitoring current drawn during the applying voltage step; and

ceasing application of voltage upon detection of a drop-off event.

18. The method of claim 1 wherein the immersing, applying, and retracting steps are carried out atop a granite block.

19. The method of claim 1 wherein the annealing step does not include flame annealing, and the applying step does not include direct current power.

20. An SPM probe produced by the method of claim 1.