WO2003087709A1

WO2003087709A1 - Nanowire microscope probe tips

Info

Publication number: WO2003087709A1
Application number: PCT/US2003/010304
Authority: WO
Inventors: Jie Han; Ramsey M. Stevens; Alan M. Cassell
Original assignee: Integrated Nanosystems, Inc.
Priority date: 2002-04-05
Filing date: 2003-04-02
Publication date: 2003-10-23
Also published as: AU2003223446A1

Abstract

Scanning microscope probe tips (10) having nanowires as tips (14) may be fabricated using well-controlled processes. The probe tips (10) may be fabricated in parallel on substrate wafers. Catalyst sites (44) for seeding nanowire growth may be formed on cantilevers (16) that are part of the probe tips (10). By controlling electric field orientation during growth, nanowire orientation may be controlled. The nanowires on the cantilevers (16) may be grown so that they are oriented at an angle that offsets the angle of orientation that the cantilevers (16) have when the probe tips (10) are installed in scanning probe microscopes.

Description

NANOWIRE MICROSCOPE PROBE TIPS

Background of the Invention

This invention relates to nanowire probe tips for scanning probe microscopes such as atomic force microscopes and scanning tunneling microscopes.

Scanning probe microscopes are non-optical microscopes that use small probe tips to make contact and non-contact measurements of surfaces. One type of scanning probe microscope is the atomic force microscope. The probe tips for atomic force microscopes are typically based on micromachined silicon. In a typical probe tip arrangement, a sharpened silicon tip structure is formed at the end of a cantilever beam. Very fine surface features on a sample may be resolved using this type of microscope. Surface measurements may be made by placing the probe tip in contact with the sample surface or by placing the probe tip close to the sample surface. As the probe tip is scanned across the surface of the sample, the position of the probe tip may be monitored by reflecting a laser beam off of the cantilever into a position detector. In other systems, the force exerted on the probe tip may be monitored by measuring change in resistance of a piezoelectric element that is compressed when the tip is deflected. By processing data such as position or force data, an image of the sample surface may be generated.

Probe tips are also required to make surface measurements using other types of scanning probe microscopy such as scanning tunneling microscopy, force modulation microscopy, magnetic force imaging microscopy, phase detection microscopy, scanning thermal microscopy, scanning capacitance microscopy, electrostatic force microscopy, etc. Probe tips may also be used in nanolithography instruments based on scanning probe microscopy arrangements.

With a scanning probe microscope it is possible to resolve atomic-sized features in surface topography and to make other high resolution surface measurements. Because resolution is affected by the sharpness of the probe tip, probe tips based on carbon nanotubes are being investigated for use in scanning probe microscopy.

Existing techniques for fabricating probe tips based on carbon nanotubes tend to suffer from poor control over nanotube length and orientation. These problems may make it necessary to trim the length of each carbon nanotube individually (i.e., by subjecting the end of the nanotube to a high electric field) . Such techniques may be cumbersome, particularly in a volume production environment. It is therefore an object of the present invention to provide improved probe tips for instruments such as scanning probe microscopes.

Summary of the Invention

This and other objects of the invention are accomplished in accordance with the present invention by providing nanowire probe tips that may be fabricated using micromachining techniques suitable for volume production. Each probe tip may be formed from a cantilever structure and a single associated nanowire.

The nanowire may be a nanotube structure such as a single-wall or multiple-wall carbon nanotube, a carbon nanofiber, or a tungsten sulfide multiple-wall nanotube. The nanowire may also be formed from a solid rod of material such as a solid rod of single-crystal semiconductor. The cantilever structures may be formed from silicon wafers using semiconductor microfabrication techniques . Catalyst sites may be patterned at the ends of each of the cantilever structures using e-beam lithography or other suitable catalyst patterning techniques. The catalyst sites may be formed from metals such as nickel, iron, cobalt, gold, or other suitable metals or compounds or suitable alloys or mixtures of such materials .

Chemical vapor deposition (CVD) techniques such as plasma chemical vapor deposition and thermal chemical vapor deposition or other suitable techniques may be used to grow the nanowires from the catalyst sites.

The nanowire catalyst sites may be formed on the cantilever structures. For example, e-beam lithography and lift-off techniques may be used to form a single catalyst site on each cantilever structure. The catalyst site may be placed near to the end of the cantilever structure to ensure that only the nanowire and not the cantilever structure will contact the sample surface when making measurements . By placing the catalyst site and the nanowire grown from the catalyst site at a location near the end of the cantilever, it may not be necessary to form protrusions on the cantilever to avoid contact between the cantilever and sample. Placement of the catalyst site and associated nanowire at the end of the cantilever may be particularly useful when it is desired to force the nanowire against the sample surface until it buckles, as is sometimes desired during microscope set-up procedures. If desired, the orientation of the nanowires that are grown on the probe tips may be controlled by impressing electric fields on the tips during nanowire growth. For example, it may be desirable to orient a nanowire at an angle of about 13° with respect to the normal of the cantilever surface (e.g., at an angle of between 5° to 20° to the surface normal or other suitable non-zero angle) . This type of orientation may be achieved by subjecting the nanowires to appropriately tilted electric fields during nanowire growth. Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description.

Brief Description of the Drawings FIG. 1 is a perspective view of an illustrative scanning probe microscope probe tip using a nanowire tip in accordance with the present invention. FIG. 2 is a side view of the cantilever portion of an illustrative probe tip for a scanning probe microscope in accordance with the present invention. FIG. 3 is a plan view of an illustrative scanning probe microscope probe tip before the probe tip has been removed from the wafer in which it was fabricated in accordance with the present invention.

FIG. 4a is a cross-sectional side view of an illustrative probe tip during fabrication after a silicon etch step has been performed in accordance with the present invention.

FIG. 4b is a cross-sectional side view of an illustrative probe tip during fabrication after an e-beam lithography step that defines a catalyst deposition location has been performed in accordance with the present invention.

FIG. 4c is a cross-sectional side view of an illustrative probe tip during fabrication after a catalyst site has been deposited on the cantilever of the tip in accordance with the present invention.

FIG. 4d is a cross-sectional side view of an illustrative probe tip during fabrication after an ultraviolet lithography step has been performed in accordance with the present invention. FIG. 4e is a cross-sectional side view of an illustrative probe tip during fabrication after a nitride patterning step has been performed in accordance with the present invention.

FIG. 4f is a cross-sectional side view of an illustrative probe tip during fabrication after another silicon etch step has been performed in accordance with the present invention. FIG. 4g is a cross-sectional side view of an illustrative probe tip during fabrication after still more silicon etching has been performed in accordance with the present invention. FIG. 4h is a cross-sectional side view of an illustrative probe tip during fabrication after an oxide etch step has been performed in accordance with the present invention.

FIG. 4i is a cross-sectional side view of an illustrative probe tip during fabrication after a nanowire growth step has been performed in accordance with the present invention.

FIG. 5 is a side view of an illustrative chemical vapor deposition chamber during nanowire growth showing how a tilted electric field may be used to orient the nanowires that are grown at an angle with respect to the substrate surface in accordance with the present invention.

FIG. 6 is a flow chart of illustrative steps involved in probe tip fabrication in accordance with the present invention.

FIG. 7 is a cross-sectional view of an illustrative probe tip having a nanowire formed on a micromachined protrusion on which a catalyst site is formed in accordance with the present invention.

Detailed Description

An illustrative scanning microscope probe tip 10 in accordance with the present invention is shown in FIG. 1. The scanning microscope equipment in which probe tip 10 is mounted is not shown in FIG. 1 to avoid overcomplicating the drawing. In general, probe tip 10 may be clamped or otherwise secured in a mount of the scanning microscope. The probe tip 10 is then scanned relative to sample 12 to make surface measurements. Either the probe tip 10 or the sample 12 or both may be moved to produce relative lateral movement between tip 10 and sample 12.

Sample 12 may be any suitable object for which it is desired to produce a surface scan. Sample 12 may be, for example, an integrated circuit at some stage in the fabrication process, a biological specimen, etc. Scanning probe tip arrangements of the type shown in FIG. 1 may be used in any suitable type of scanning probe microscope. For example, probe tips may be used in atomic force microscopes and scanning tunneling microscopes. Probe tips such as probe tip 10 may also be used to make surface measurements for other types of microscopy such as force modulation microscopy, phase detection microscopy, scanning thermal microscopy, scanning capacitance microscopy, electrostatic force microscopy, etc. Probe tips may also be used in nanolithography instruments based on scanning probe microscopy arrangements.

Probe tip 10 may have a nanowire tip 14. By using a nanowire tip, probe 10 may achieve extremely fine resolutions (e.g., nanometer scale). Tip 14 may also allow probe 10 to be used to examine surface structures having high aspect ratios, which might otherwise be difficult or impossible to probe using conventional probe tip arrangements .

Nanowire tip 14 may be a single-wall or multiple-wall carbon nanotube, a carbon nanofiber, or a tungsten sulfide multiple-wall nanotube. The nanowire may also be formed from a solid rod of material such as a solid rod of single-crystal semiconductor. Nanowire 14 may be attached to cantilever 16, which may in turn be part of a larger cantilever or probe tip structure 18. Structure 18 may be mounted in a scanning probe microscope mount. If tip 14 needs to be replaced, a new probe tip 10 may be swapped into place.

Tip 14 may have any suitable dimensions. For example, tip 14 may have a length in the range of 0.5 to 5 microns and a diameter in the range of 1 to 100 nanometers. The end of tip 14 (i.e., the endmost portion of tip 14 that may contact the sample surface) may typically be from 0.25 to 30 nanometers in diameter.

If desired, tip 14 may be physically altered or may be coated with a suitable substance prior to use of tip 14 in a system. For example, tip 14 may be coated with a dielectric film or dielectric nanoparticles may be attached to tip 14 (e.g., to facilitate probe tip measurements of capacitance) . Tip 14 may also be coated or have other suitable materials attached such as molecules, nanoparticles, or films that alter the chemical, electrical, magnetic, and optical properties of the probe tips. Nanoparticles may be formed by chemical and vapor deposition onto the ends of tip 14. These tip alterations may facilitate the use of tip 14 in connection with certain types of microscopy. In order to make surface measurements, the vertical position of cantilever 14 must generally be controlled or monitored. Various different techniques may be used. For example, tip 14 and cantilever 16 may be moved along the surface of sample 12 (e.g., by scanning structure 18 using positioning controls in the scanning probe microscope) . As tip 14 is scanned across the surface of sample 12, a laser beam 20 that is reflected from cantilever 16 may be used to monitor the vertical position of nanowire tip 14. The laser beam 20 may, for example, be monitored using a photodetector array or may form part of an interferometer. As another example, the position of cantilever 16 may be modulated and the resulting force or tunneling current experienced by tip 14 may be monitored.

These are merely illustrative techniques for using tip 10 in a scanning probe microscope system. Any suitable microscope arrangement may be used if desired. For example, measurement arrangements may be used such as arrangements based on measuring differences in piezoelectric potential, making capacitance measurements, or making measurements of other electrical or magnetic effects . Regardless of the specific type of microscope arrangement that is involved, the small dimensions afforded by using a nanowire for tip 14 generally result in excellent lateral resolution. Moreover, certain types of nanowire tips 14 (e.g., multiple-wall carbon nanotube tips) may be particularly robust, and may buckle resiliently rather than breaking when forced into contact with sample 12.

It may be desirable in some circumstances to force tip 14 to buckle, because the force and position information that is derived during such a buckling operation may have a unique signature. By deliberately buckling tip 14 it may be possible to confirm that tip 14 is properly located to begin operation in a surface analysis procedure. Buckling of tips 14 is not possible with conventional non-wire tips.

A side view of the cantilever and tip portion of an illustrative probe tip is shown in FIG. 2. In the example of FIG. 2, the probe tip has been installed in a scanning probe microscope such that the cantilever 16 forms an angle with respect to the sample surface (i.e., the longitudinal axis 22 of cantilever 16 forms an angle A with respect to the plane 24 that lies on the upper surface of sample 12) . In some microscopes the probe tips may be oriented at an angle A of 13°. Other angles A (greater or smaller) may be desired when the probe tip is to be used in other microscopes. In the example of FIG. 2, tip 14 has been tilted by the same angle (A) , so that tip 14 is orthogonal to surface 24. If desired, tip 14 may be tilted by some other angle, so that tip 14 is not orthogonal to surface 24. As another example, tip 14 may not be tilted at all, as shown by dotted line 26.

By controlling the orientation of the nanowire tip 14 during fabrication, probe tips 10 having well- defined characteristics may be formed. This is generally preferable to fabrication techniques that leave all or some of the details of the orientation and length of nanowire tip 14 to chance or that require extensive manual intervention to produce tips having suitable characteristics .

The end of cantilever 16 may protrude past the base of tip 14 by an amount X, as shown in FIG. 2. In order to prevent end portion 28 of cantilever 16 from contacting surface 24, tip 14 may be formed at a location that ensures that the length X is not too large. If tip 14 is formed close to end 28, length X will be relatively small (e.g., on the order of microns, 5-25 microns, 2-10 microns, 10-100 microns, or any other suitable distance) , and cantilever 16 will be able to form angles A that are relatively large (e.g., on the order of 13° or more) without causing end 28 to contact surface 24. As shown in FIG. 2, nanowire 14 may, if desired, be grown directly from cantilever 16 without using any mount or protrusion. In some probe tip arrangements it may be desirable to use such a protrusion or mount, but in situations in which a protrusion or mount is not used, it may be possible to simplify the probe tip fabrication process .

Probe tips 10 may be fabricated using well- controlled automated or semi-automated fabrication techniques. For example, probe tips 10 may be fabricated using silicon micro achining techniques that employ semiconductor industry processing tools. These techniques include the techniques used for forming microelectromechanical systems (MEMs) structures. Although MEMs structures such as the probe tips of the present invention may be readily formed by micromachining of silicon, silicon is merely an illustrative example of a suitable substrate material. Probe tips 10 may be formed by micromachining substrates formed from other semiconductors, glass, polymers, etc.

As an example, numerous probe tips 10 (e.g., hundreds of tips 10 or more) may be formed from a common silicon substrate (wafer) . After the probe tips have been fabricated in parallel, individual tips may be separated from the wafer.

A plan view of an illustrative probe structure that is suitable for micromachining from a silicon substrate is shown in FIG. 3. In the view of FIG. 3, probe tip 10 is shown as being attached to a portion of a wafer 30 by tabs 32. After probe tip 10 has been fabricated (along with the other probe tips 10 on the wafer) , tip 10 may be removed from the wafer by breaking tabs 32. Tip 10 may be roughly rectangular in shape (as shown in FIG. 3) or may have any other suitable shape. An advantage of using a generally rectangular shape having a length L of about 3.1 mm and a width D of about 1.3 mm is that this shape allows tip 10 to be mounted into the mounting fixtures of commonly used scanning probe microscopes.

The probe tip 10 may be fabricated using any suitable technique. One illustrative fabrication process is illustrated in connection with FIGS. 4a-4i. This process is, however, merely illustrative. Other suitable techniques may be used if desired. The cross-sectional views of FIGS. 4a-4i (and the other FIGS.) are not to scale . With the illustrative process of FIGS. 4a-4i, a silicon wafer 30 (a portion of which is shown in FIGS. 4a-4i) may initially be processed to form a silicon-on- insulator substrate having a buried insulator such as a buried oxide layer. One illustrative method of forming a suitable silicon-on-insulator structure is to grow a thermal oxide (silicon dioxide) layer 34 on top of silicon substrate 30 and to subsequently deposit a layer of polysilicon 36. If desired, polysilicon 36 may be transformed into crystalline silicon by lateral melting and regrowth. Silicon-on-insulator wafers may also be formed by ion implantation of oxygen. Hereinafter, upper layer 36 will sometimes be referred to simply as "silicon" layer 36.

If desired, alignment marks may be etched into the front and/or backside surfaces of wafer 30 prior to subsequent processing.

A patterned layer of any suitable masking material (e.g., patterned silicon nitride) may be used to form a backside mask for wafer 30. The wafer may then be etched on the backside (e.g., using an anisotropic KOH — potassium hydroxide -- wet etch) . The resulting patterned wafer is shown in cross-section in FIG. 4a. Cavities 38 and the associated etched surfaces on the bottom of the probe tip structure have been produced by the backside etch.

As shown in FIG. 4b, e-beam lithography may then be used to form a pattern of e-beam resist 40 on the frontside of wafer 30. E-beam resist 40 may be polymethylmethacrylate (PMMA) or any other suitable e- beam resist. A hole 42 may be formed in the e-beam resist as part of the patterning process. Hole 42 may be have a circular cross-section (as viewed from the surface of the resist) or may have any other suitable cross- section. The lateral dimensions of hole 42 are preferably comparable to the desired lateral dimensions for nanowire tip 14. For example, nanowire tips 14 may have lateral dimensions on the order of 10-100 nm, so a suitable lateral dimension for hole 42 is also on the order of 10-100 nm.

After the resist 40 has been patterned as shown in FIG. 4b (using e-beam lithography or any other suitable patterning technique) , a suitable catalyst layer may be deposited. The catalyst layer may be any material that is suitable for catalyzing the formation of nanowires. For example, carbon nanotube growth may be catalyzed using catalyst layers of nickel, iron, or cobalt. These are merely illustrative examples of suitable nanowire catalyst materials. Any other suitable material or combinations of such materials may be used if desired. After catalyst deposition, the e-beam or other resist 40 may be removed (e.g., using a solvent) . Using this type of lift-off process, the resist and unused catalyst may be removed to leave a small catalyst site 44 having the same dimensions as hole 42 (e.g., lateral dimensions on the order of 10-100 nm or other suitable size for catalyzing the growth of a desired type of nanowire) . Because the catalyst sites 44 can be formed directly on the surface of upper silicon layer 36 (i.e., on silicon or on any oxide or other layer on layer 36 without using a protrusion or other intervening substantially non-planar structure) , fabrication may be simplified.

The location of catalyst site 44 relative to cavities 38 and the angled side-wall surfaces of the probe tip structure is shown in FIG. 4c. Not shown in FIG. 4c (or in the other cross-sectional FIGS.) is the three-dimensional nature of cavities 38. As best shown in FIG. 3, these cavities are being used to define an undercut under cantilever 16 and the rest of a cavity 46 that almost completely surrounds tip 10 (when complete) with the exception of tabs 32 (shown as dotted tabs box 32 in FIG. 4c) .

As shown in FIG. 4d, after the catalyst site 44 has been formed, ultraviolet lithography or other suitable techniques may be used to pattern layer of photoresist 48 (e.g., ultraviolet photoresist) on the upper surface of the probe tip structure.

A silicon nitride or other suitable masking layer may be deposited on the photoresist 48. For example, a low-pressure chemical vapor deposition process may be used to deposit Si₃N₄. A lift-off process may be performed by removing the photoresist 48 (e.g., using a solvent) . The resulting pattern of nitride 50 is shown in FIG. 4e. Nitride mask 50 of FIG. 4e has regions 52 in which frontside silicon layer 36 is exposed.

Wafer 30 may then be etched in a silicon etchant (e.g., an anisotropic KOH wet etch) . This silicon etch step forms frontside cavities 54 and deepens and enlarges backside cavities 38, as shown in FIG. 4f. Catalyst site 44 is protected from the silicon etchant during this step by nitride mask 50. Additional silicon etching may then be performed to further enlarge cavities 38, until they reach the etch stop formed by oxide layer 34.

The oxide layer that separates upper cavities 54 from lower cavities 38 may then be removed by an oxide etch step. For example, an oxide etch may be performed that uses a combination of a SFε plasma etch and a buffered HF (hydrofluoric acid) wet etch or either of these etches alone. The nitride mask layer 50 may then be removed to form the nearly complete probe tip structure 10' that is shown in FIG. 4h. At the stage shown in FIG. 4h, the upper cavities 54 and the lower cavities 38 have joined, to form a cavity that has nearly completely severed probe tip structure 10' from wafer 10 (except for the tabs 32) . The upper surface of this cavity is shown as cavity 46 in the top view of probe tip 10 of FIG. 3. The catalyst site 44 is exposed for use in forming the base of a nanowire. The catalyst site may be located near to the end 28 of the cantilever 16, as described in connection with FIG. 2 (e.g., site 44 may be within a distance of end 28 such as 5-25 microns, 2-10 microns, 10-100 microns, or any other suitable distance).

After the completion of the silicon micromachining steps described in connection with FIGS. 4a-4h, nanowires 14 may be grown from the catalyst sites 44, as shown in FIG. 4i. The length of nanowire 14 may be controlled accurately by controlling the process conditions during growth. The growth process forms a finished probe tip device 10 that may be separated from wafer 10 by breaking tabs 32.

The nanowire tips 14 may be oriented at any desired angle with respect to the plane 24 that contains the upper surface of probe tip 10. For example, tip 14 may be oriented at an angle A of 13° with respect to the surface normal, as described in connection with FIG. 2 and as shown by the illustrative nanowire 14 of FIG. 4i. If desired, tip 14 may be oriented at another angle (e.g., an angle in the range of 0° to 45°) or may be oriented perpendicular to plane 24 (parallel to the surface normal) , as shown by the dotted outline 26 of FIG. 4i.

The orientation of the nanowires 14 may preferably be controlled by controlling the orientation of the electric field environment of the substrate 30 during nanowire growth. Nanowires tend to grow in an orientation that is aligned with the electric field in the vicinity of the growth region. An electric field strength in the range of 100-1000 V/cm may be sufficient to control the orientation of nanowires 14 during growth. With plasma chemical vapor deposition, an electric field may be created as part of the plasma deposition process. With thermal chemical vapor deposition, the electric field may be impressed upon the substrate separately.

An illustrative growth chamber arrangement for orienting nanotube tips 14 at an angle A (e.g., an angle A of 13°) is shown in FIG. 5. In the example of FIG. 5, electrodes 56 are used to impress an electric field E (preferably having a strength of about 100 V/cm to 1000 V/cm) on wafer 30 during fabrication. Wafer 30 is oriented so that the surface normal to wafer 30 (shown by dotted line 58) is tilted by an angle A with respect to the surface normal of lower electrode 56 (shown by dotted line 60). A wedge 62 (e.g., an electrically-insulated shim) or other suitable mount or controllable stage may be used to hold wafer 30 at an appropriate orientation with respect to electric field E during nanowire growth. With the arrangement of FIG. 5, nanowires such as nanowire 14 of FIG. 5 will tend to grow oriented with the electric field E (i.e., parallel to dotted line 60 and at an angle A with respect to the surface normal 58 of wafer 30) .

Plasma chemical vapor deposition (CVD) , thermal chemical vapor deposition, or any other suitable growth technique may be used to grow nanowire tips 14. The type of feedstock or precursor used during CVD nanowire growth is determined by the type of nanowire to be grown. For example, organometallic compounds or precursors such as silane or silicon tetrachloride or other vapor precursors may be used to grow single-crystal semiconductor nanowires (e.g., silicon nanowires, zinc oxide nanowires, germanium phosphate nanowires, indium phosphide nanowires, other II-VI semiconductor nanowires, III-V semiconductor nanowires, etc.). Feedstock such as methane, ethylene, acetylene, benzene, or other small hydrocarbon gasses or vapors may be used to grow single- wall and multiple-wall carbon nanotubes.

Dopants such as nitrogen, oxygen, or phosphorous may be incorporated into single-crystal semiconductor nanowires by introducing dopant gasses during nanowire growth or by using any other suitable doping technique. Doped nanowires tips 14 may be more conductive than undoped semiconducting nanowires, which may be advantageous when the nanowires are used as conductors (e.g., in scanning tunneling microscopes). Illustrative steps involved in forming probe tips 10 are shown in FIG. 6. At step 64, a substrate wafer (e.g., a silicon-on-insulator wafer in this example) or any other suitable substrate 30 may be patterned. For example, a backside etch process may be used to form cavities such as cavities 38 of FIG. 4a. Any suitable etching technique may be used, including wet and dry etching. Alignment marks may be formed to aid in subsequent lithography steps. Any suitable mask layers may be used during the etch step of FIG. 4e. For example, a nitride mask may be used.

At step 66, a patterning step such as a lithography step based on e-beam or ultraviolet light lithography may be used to pattern a suitable photoresist layer. A suitable metal catalyst may be deposited at step 68 and the resist removed using lift-off to leave an exposed pattern of deposited catalyst sites 44 (see FIGS. 4a-4i) . The process of steps 66 and 68 is merely illustrative. For example, the catalyst layer may be patterned using a resist pattern that is deposited after the catalyst layer is deposited followed by a catalyst etch step.

Catalyst may also be patterned by forming a hole or holes in appropriate portions of the substrate or in raised portions or structures (protrusions on the substrate) . The use of such protrusions as locations for nanowire growth may add complexity to the fabrication process, but may also provide structural support and may help to orient the nanowires 14. Accordingly, some tip configurations may benefit from such catalyst patterning and nanowire formation techniques .

During step 68, the catalyst sites 44 may be formed directly on the flat surfaces of cantilevers 16 (i.e., on the flat surfaces of layer 36) . Forming catalyst sites 44 directly on the cantilevers in this way may tend to make it easier to control the ultimate effective length of the nanowires 14, because there is no uncertainty in the height of the nanowire tips above the substrate surface beyond the (preferably small) uncertainty that is introduced during the growth phase. For example, there is no uncertainty in the ultimate reach or length of the nanowire tips that might otherwise be introduced by growing the nanowires from indeterminate side-wall locations on a protrusion on the cantilever. Each cantilever surface lies in the plane of the substrate surface .

After the catalyst site 44 (FIGS. 4a-4i) has been formed, a patterned nitride layer (such as nitride layer 50 of FIG. 4e) may be formed (e.g., using liftoff) . If desired, other suitable masking layers may be used instead of nitride.

At step 72, a silicon (substrate) etch may be used to enlarge cavities 38 and 54 (as shown in FIG. 4f) . Any suitable wet or dry etch may be used to etch the silicon in steps such as steps 72 and 64. Anisotropic etches such as wet KOH etches may be desirable when it is desired to form smooth well-defined etch planes, because such etches preferentially expose certain planes in the silicon substrate (e.g., 1-1-1 planes). Isotropic etches may be used when it is desired to undercut a particular region or when the angled planes of the anisotropic etch are not desired.

At step 74, an oxide etch may be used to etch through oxide layer 34 (as shown in FIGS. 4g and 4h) when such a layer is used. Any suitable wet or dry oxide etch may be used, such as SFε or buffered HF etches.

At step 76, the silicon nitride that was deposited at step 70 may be removed (e.g., using a wet or dry etch that has good selectivity relative to the silicon of layer 36 and the catalyst site 44) . Following step 76, the probe tips in the wafer have the cross- sectional appearance shown in FIG. 4h.

Nanowires may be grown from the catalyst sites 44 (if such sites are used) at step 78. The orientation of the nanowires need not be perpendicular to the surface normal of the substrate wafer 30. For example, a tilted growth chamber arrangement of the type shown in FIG. 6 may be used to grow nanowire tips 14 on cantilevers 16 that are tilted with respect to the surface normal by a desired amount. The desired tilt angle may, for example, be 13°. When probe tips 10 with tips 14 that are tilted in this way are installed in commonly-available scanning probe microscopes, the 13° tilt of the nanowire tip portion 14 offsets the 13° tilt in the orientation of cantilever 16 with respect to the surface of sample 12, as shown in FIG. 2. As a result, the tip 14 is oriented perpendicularly to the surface of the sample, which may be advantageous when measuring certain types of samples. This orientation is, however, merely illustrative. Different nanowire orientations may be used, depending on the desired application for probe tips 10.

Any suitable nanowire growth technique may be used to grow the nanowires at step 78, such as plasma or thermal CVD. Because the orientation and initial growth locations (e.g., catalyst site locations on the surface of the substrate) for the nanowires 14 on each wafer can be well controlled using electric fields during growth and lithographic catalyst patterning techniques, the lengths and orientations of the nanowires 14 may be well controlled, thereby reducing or obviating the need for substantial manual intervention in the probe tip fabrication process. In some circumstances, it may be desirable to use a protrusion on the cantilever surface to provide a raised platform for wire tip 14, as shown in FIG. 7. In the example of FIG. 7, nanowire 14 has been formed at an angle A of 13° or 0-45° (e.g., using the growth chamber technique described in connection with FIG. 5) and has been grown from a catalyst site 44 that has been deposited on a micromachined silicon protrusion 80. Protrusion 80 may be the shape of a sharp or blunt pyramid or any other suitable shape. Catalyst site 44 may be deposited using e-beam lithography and lift-off, electrochemical deposition, or any other suitable catalyst patterning technique. Catalyst site formation may be performed prior to or subsequent to the use of dry or wet etching to micromachine protrusion 80. It will be understood that the foregoing is only illustrative of the principles of the invention and that various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention. Many examples of such modifications have been given through the foregoing specification.

Claims

The Invention Claimed Is:

1. A method for fabricating scanning microscope probe tips from a substrate wafer having a flat surface, comprising: using lithography to form a plurality of catalyst sites on the flat surface of the substrate wafer; micromachining the substrate wafer to form a plurality of probe tip structures for the probe tips, each probe tip structure having a respective cantilever, wherein there is only a single catalyst site on each cantilever; and growing a plurality of corresponding nanowire tips for the probe tips from the catalyst sites, wherein there is only a single nanowire tip grown from each catalyst site on each cantilever.

2. The method defined in claim 1 further comprising protecting the catalyst sites before micromachining the substrate wafer.

3. The method defined in claim 2 wherein protecting the catalyst sites comprises protecting the catalyst sites from silicon etchant using a nitride mask.

4. The method defined in claim 1 wherein growing the nanowire tips comprises growing carbon nanotube tips .

5. The method defined in claim 1 wherein growing the nanowire tips comprises growing single- crystal solid rods.

6. The method defined in claim 1 further comprising coating the nanowire tips to alter the properties of the probe tips.

7. The method defined in claim 1 further comprising coating the nanowire tips with a dielectric to facilitate probe tip measurements of capacitance.

8. The method defined in claim 1 further comprising doping the nanowire tips.

9. The method defined in claim 1, further comprising micromachining a protrusion on each cantilever to provide a raised platform for the nanowire tip on that cantilever, wherein the catalyst sites are formed prior to micromachining the protrusion on each cantilever.

10. The method defined in claim 1, wherein each cantilever has a protrusion, the method further comprising forming the nanowire tip for each cantilever from the protrusion on that cantilever.

11. The method defined in claim 1 further comprising using lithography to form the catalyst sites on the substrate wafer before micromachining the substrate wafer to form the plurality of probe tip structures .

12. The method defined in claim 11 further comprising growing the nanowire tips after micromachining the substrate wafer to form the plurality of probe tip structures .

13. The method defined in claim 1 further comprising: using lithography to form the catalyst sites on the substrate wafer before micromachining the substrate wafer to form the plurality of probe tip structures; and growing the nanowire tips after micromachining the substrate wafer to form the plurality of probe tip structures .

14. The method defined in claim 1 wherein each cantilever has an associated surface normal and wherein growing the plurality of nanowire tips comprises growing the plurality of nanowire tips using an electric field to control the orientation of the nanowire tips so that the nanowire tips grow parallel to the surface normal.

15. The method defined in claim 1 wherein each cantilever has an associated surface normal and wherein growing the plurality of nanowire tips comprises growing the plurality of nanowire tips using an electric field to control the orientation of the nanowire tips so that the nanowire tips grow at a non-zero angle with respect to the surface normal.

16. The method defined in claim 1 wherein the substrate wafer is formed of silicon and wherein micromachining the substrate wafer includes etching the silicon using a potassium hydroxide wet etch.

17. The method defined in claim 1 wherein the substrate wafer is formed of silicon and wherein micromachining the substrate wafer includes etching the silicon using a dry etch.

18. The method defined in claim 1 wherein the substrate wafer is formed of silicon and wherein micromachining the substrate wafer includes etching a cavity in the silicon that undercuts each cantilever.

19. The method defined in claim 1 wherein the substrate wafer is a silicon-on-insulator substrate having a buried oxide layer that is used as an etch stop during subsequent silicon etching to undercut each cantilever.

20. The method defined in claim 1 wherein using lithography to form a catalyst site on each cantilever comprises using lift-off to form metal catalyst sites directly on each cantilever.

21. The method defined in claim 1 wherein using lithography to form the plurality of catalyst sites on the substrate wafer comprises forming catalyst sites that each have a lateral dimension of 10-100 nm.

22. A scanning microscope probe tip, comprising: a probe tip structure having a cantilever; a single catalyst site formed on the cantilever using lithography; and a single nanowire grown by itself from the catalyst site.

23. The probe tip defined in claim 22 wherein the nanowire is a carbon nanotube.

24. The probe tip defined in claim 22 wherein the nanowire is a solid rod of single-crystal semiconductor .

25. The probe tip defined in claim 22 wherein the cantilever has an associated surface normal and wherein the nanowire is parallel to the surface normal.

26. The probe tip defined in claim 22 wherein the cantilever has an associated surface normal and wherein the nanowire is oriented at an angle of between 5° to 20° to the surface normal.

27. The probe tip defined in claim 22 wherein the cantilever has an associated surface normal and wherein the nanowire is oriented at a non-zero angle with respect to the surface normal.

28. The probe tip defined in claim 22 further comprising a dielectric coating on the nanowire.

29. The probe tip defined in claim 22 further comprising a coating on the nanowire.

30. The probe tip defined in claim 22 wherein the nanowire is doped.

31. The probe tip defined in claim 22 wherein the catalyst site on the cantilever is formed by using lift-off to deposit the catalyst site directly on the cantilever.

32. The probe tip defined in claim 22^" wherein the catalyst site has lateral dimension of 10-100 nm.

33. The probe tip defined in claim 22 wherein the cantilever further comprises a protrusion from which the single nanowire is grown.