Probe tip with tip extension for scanning probe microscope

Abstract

A cantilever structure, a scanning probe microscope and a method of operating the scanning probe microscope use a cantilever having a probe tip with a tip extension to interface a sample. The tip extension can have a height less than 100 nm. The cantilever is driven to vibrate and a light source is used to transmit an optical beam onto the tip extension. Vibrations of the cantilever is optically measured to image the sample.

Description

PROBE TIP WITH TIP EXTENSION FOR SCANNING PROBE MICROSCOPECROSS REFERENCE TO RELATED APPLICATION

[0001] This application is entitled to the benefit of U.S. Provisional Patent Application Serial Number 63 / 650,492, filed on May 22, 2024, which is incorporated herein by reference.BACKGROUND OF THE INVENTION

[0002] There are several techniques that combine atomic force microscopy (AFM) with optical spectroscopy to break the diffraction limit associated with the optical technique. When the optical wavelength is in the infrared (IR) spectral range, such a technique is generally called AFM infrared spectroscopy (AFM IR). In AFM IR, a tunable infrared laser is focused onto a metal-coated AFM tip with the polarization predominantly aligned along the tip axis. The metal-coated tip is necessary to provide sufficient conductivity to have the tip act as a nano-antenna to create an enhanced optical field at the apex of the tip, which is used to probe the sample’s optical properties. Techniques such as photo-induced force microscopy (PiFM), photo-thermal induced resonance (PTIR), and scattering scanning nearfield microscopy (s-SNOM) are examples of AFM IR.

[0003] While the spatial resolution of the IR-related signal of the AFM IR tools varies slightly between the specific type of the technique, it generally is related to the radius of curvature of the apex of the metal-coated tip. A typical radius of curvature of metal-coated tips that is currently available ranges from about 20 nanometer (nm) to 30 nm. Unfortunately, it is difficult to obtain a smaller radius of curvature when a metal coating is applied to even the sharpest tips. When a smaller radius of curvature is targeted by applying a thinner coating layer, the coating becomes less uniform and incomplete in coverage, which leads to poor repeatability and reduced sensitivity. On the other hand, it is difficult to produce a sharp pure-metal tip at the end of a microfabricated cantilever.SUMMARY OF THE INVENTION

[0004] A cantilever structure, a scanning probe microscope and a method of operating the scanning probe microscope use a cantilever having a probe tip with a tip extension to interface a sample. The tip extension can have a height less than 100 nm. The cantilever is driven to vibrate and a light source is used to transmit an optical beam onto the tip extension. Vibrations of the cantilever is optically measured to image the sample.

[0005] In an embodiment, a cantilever structure comprises a cantilever, and a probe tip coupled to the cantilever. The probe tip has a tip extension, wherein the tip extension has a height less than 100 nm.

[0006] In an embodiment, a scanning probe microscope comprises a cantilever having a probe tip with a tip extension to interface a sample, a light source to transmit an optical beam onto the tip extension, a drive device coupled to the cantilever to drive the cantilever to vibrate, and an optical detection system to detect vibrations of the cantilever in response interactions between the probe tip with the tip extension and the sample.

[0007] In an embodiment, a method of operating a scanning probe microscope comprises driving a cantilever having a probe tip with a tip extension to vibrate so that the probe tip with the tip extension interfaces a sample, shining an optical beam on the tip extension that is interfacing with the sample, and optically measuring vibrations of the cantilever to image the sample.

[0008] Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrated by way of example of the principles of the invention.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Fig. 1 illustrate probe tips with tip extensions for use in scanning probe microscopes in accordance with an embodiment of the invention.

[0010] Fig. 2 shows an example of a scanning probe microscope in which a probe tip with a tip extension in accordance with embodiments of the invention can be implemented.

[0011] Fig. 3 shows how an enhanced probe tip with an extension tip is utilized in AFM-IR, in this case, for infrared photo-induced force microscopy in accordance with an embodiment of the invention.

[0012] Fig. 4 shows an actual carbonaceous tip extension on a metal coated tip with an aspect ratio of approximately 3 (approximately 45 nm tall and approximately 15 nm wide at the base) and a radius of curvature of approximately 6.5 nm in accordance with an embodiment of the invention.

[0013] Fig. 5 is a process flow diagram of a method of operating a scanning probe microscope using a probe tip with a tip extension in accordance with an embodiment of the invention.

[0014] Throughout the description, similar reference numbers may be used to identify similar elements.DETAILED DESCRIPTION

[0015] As noted above, for AFM IR techniques, a smaller radius of curvature of the apex of the metal-coated tip is desired to achieve greater spatial resolution of the IR-related signal of the AFM IR tools. However, it turns out that even for standard AFM measurements, sharp tips are desired to probe features that have high aspect ratios (depth versus width). One type of such a high aspect tip can be fabricated by creating an extension of amorphous carbonaceous material (for example, diamond-like carbon (DLC)) on top of a metal-coated tip via focused electron beam (e-beam) induced deposition (FEB ID) of carbonaceous material inside an electron microscope; the carbon species could be present on the tip surface or in the vacuum system. Because this “whisker” tip is not necessarily doped to conduct electricity, it is not recommended for electrical measurements but only for high resolution topography measurements. For example, a commercial tip called 160AC-SG, manufactured by Micromasch, has such a carbonaceous whisker extension at the end of a metal coated tip. The websitepoints out that the diamond-like spike is not metal coated and is recommended for high resolution tapping mode imaging.

[0016] For use as nano-antenna, the prevailing tips are silicon tips that are coated with a thin metallic film. Typically, the radius of curvature quoted for these types of tips are - 20 to 30 nm, with no easy way to reduce the tip radius.

[0017] For conductive AFM applications, solid metal tips (platinum and platinum-iridium) cantilever supported on standard AFM probe sized ceramic chip are available commercially from Rocky Mountain Nanotechnology. While the standard product specifies a tip radius of below 20 nm, special tips with tip radius below 10 nm are available upon request. While these tips may be good for conductive AFM measurements, they have too much variability in the cantilever properties such as spring constant (+ / - 40%), resonance frequency (+ / - 30%), and the tip shank length of 80 microns (+ / - 25%), which will introduce too much variability to be useful for many AFM-IR applications.

[0018] More recently, FEB ID has been applied to fabricate all-metal nanoprobes for AFM-based electrical operation modes (see Seewald, Lukas Matthias, Jurgen Sattelkow, Michele Brugger-Hatzl, Gerald Kothleitner, Hajo Frerichs, Christian Schwalb, Stefan Hummel, and Harald Plank. 2022. “3D Nanoprinting of All-Metal Nanoprobes for Electric AFMModes” Nanomaterials 12, no. 24: 4477). These tips grown directly on cantilevers are - 2 - 7 microns tall with tip radii in the 10 nm range on a regular basis. The tips are initially grown as a PtCx hollow cone, which is transferred into pure Pt materials by an e-beam-assisted purification process in low-pressure H2O environments. Depending on the beam currents, 7-micron tall hollow cone tips take 10 to 20 minutes to form. The authors did not report how much time it takes to purify the hollow cone into all-metal tip. While this type of tip may provide more repeatable structure compared to the solid metal tips from Rocky Mountain Nanotechnology, the fabrication time required for a tip length - 7 micron (or greater), suitable for nano-antenna purposes for IR spectral range, is almost prohibitive to produce cost effective tips.

[0019] Embodiments of the invention use whisker tips, which are considered incompatible for scanning probe applications involving electrical conductivity, such as conductive AFM where a conductive tip is required. However, forinstruments that combine optical spectroscopy with AFM, these whisker tips are found to provide enough optical conductivity to act as an effective nano-antenna. It is believed that these amorphous carbonaceous tips grown via e-beam induced deposition incorporate large number of impurities into the material, which act as localized states within the optical gap of the material. DLC films prepared via laser-assisted deposition have been reported to have a band gap of - 1.0 eV (corresponds to wavelength of - 1.2 micron), which would be too large of a gap to overcome for typical wavelength utilized for IR spectroscopy (2.5 - 17 microns) (see Wei, Q., Sankar, J., Sharma, A.K. el al. “Atomic structure, electrical properties, and infrared range optical properties of diamondlike carbon films containing foreign atoms prepared by pulsed laser deposition” Journal of Materials Research 15, 633-641 (2000)). However, amorphous semiconductors usually have all states localized. Conduction in such materials occurs through hopping, whose probability increases with concentration of impurities, resulting in increased conductivity. When enough impurities are present such that continuous channels might be formed for photon-assisted hopping conduction, the whisker tip could essentially behave like a “metallic” nano-antenna, providing the necessary field enhancement for near-field IR spectroscopy. While a standard SEM (scanning electron microscope) environment may have enough “impurity” elements to create sufficient localized states, inclusion of more localized states can be promoted by using a pre-cursor gas that contains metallic elements. Even such tips may be considered a poor conductor for conductive AFM (cAFM) measurements but should be adequate to act as nano-antenna for near-field optical applications.

[0020] Turning now to Fig. 1, enhanced probe tips 100-1, 100-2 and 100-3 that will provide the smaller tip radii for high resolution AFM IR measurements in accordance with an embodiment of the invention are shown. Each enhanced probe tip 100-1, 100-2 or 100-3 consists of a carbonaceous tip extension 102-1, 102-2 or 102-3, which is similar to the above-described whisker extension, that is deposited onto a standard metal-coated AFM tip 104 using FEB ID process with or without precursor material. Metal-coated tips are useful since, for effective antenna enhancement, the tip length should be roughly the same as the wavelength of the focused IR light, - 10 microns or 10,000 nm. As noted, it will take toomuch time to grow such long tips via FEB ID process. Many of the samples that require high resolution characterizations, such as high pitch line / space patterns utilized in extreme ultraviolet (EUV) lithography are typically less than 50 nm thick. Therefore, for most applications, the carbonaceous tip extensions, which may be DLC tip extensions, may have to be only about 50 nm tall, which will reduce the time required to grow these tip extensions individually.

[0021] As shown in Fig. 1, each metal-coated tip 104 of the enhanced probe tips 100-1, 100-2 and 100-3 has an approximately 25 nm radius of curvature 106. As used herein, “approximately” means plus or minus 20% of a given value. However, in other embodiments, the metal-coated tip 104 may be configured to have a different radius of curvature. The length of the metal-coated tip 104 may be approximately 10 microns. As an example, the metal-coated tip 104 may be silicon or other non-metallic tip coated with gold (Au), platinum-iridium (Ptlr) and platinum (Pt). In other embodiments, a solid metal tip may be used instead of a metal-coated tip for the enhanced probe tips 100-1, 100-2 and 100-3.

[0022] In the illustrated embodiment, the carbonaceous tip extensions 102-1, 102-2 and 102-3 are half-elliptical tip extensions that show tip radii less than - 3 nm. The exact shape of the carbonaceous tip extensions may be different from the half-elliptical, but generally, the tip extensions should have an aspect ratio of height to base (i.e., height versus base width of the tip extension) greater than 1 and terminating to a sharp tip. The carbonaceous tip extension 102-1 has a 10 nm base and a 10 mm height (i.e., aspect ratio of 1) with a radius of curvature 108-1 of 2.8 nm. The carbonaceous tip extension 102-2 has a 10 nm base and a 15 mm height (i.e., aspect ratio of 1.5) with a radius of curvature 108-2 of 1.8 nm. The carbonaceous tip extension 102-3 has a 10 nm base and a 40 mm height (i.e., aspect ratio of 4) with a radius of curvature 108-3 of 2.8 nm. In general, the carbonaceous tip extensions 102-1, 102-2 and 102-3 are less than 100 nm in height and have an aspect ratio of height to base between 1 and 5, inclusive, with a radius of curvature of less than 10 nm at the tip.

[0023] Turning now to Fig. 2, an example of a scanning probe microscope 200 in which an enhanced probe tip 202 with a tip extension 203 in accordance with an embodiment of the invention can be implemented is illustrated. As shown in Fig. 2, the scanning probe microscope 200 uses a focused optical beam on a tip-sample interface in order to measure optical or other sample properties of a sample 204 of interest in the near field region around the tip of the tip extension 203. The focus optical beam is provided by a light source 206, which may be a tunable infrared (IR) laser, e.g., Infrared Quantum Cascade Laser (QCL).

[0024] The enhanced probe tip 202 with the tip extension 203 is attached to a cantilever 208 (collectively, these can be considered a cantilever structure), which is coupled to a dither piezo device 210 to vibrate the cantilever 208. The dither piezo device 210 is controlled by a first frequency generator 212, which provides a driving signal at frequency fl to the dither piezo device 210. Thus, the dither piezo device 210 can be viewed as a drive device to drive the cantilever 208 to vibrate.

[0025] The tunable laser 206 is controlled by a second frequency generator 214, which provides a driving or modulation signal at frequency f2 to the tunable laser 206. Thus, the optical beam from the tunable laser 206 to the enhanced probe 202 can be modulated at frequency f2.

[0026] The sample 204 of interest is placed on a sample z piezo device 216, which can move the sample in the z-direction, i.e., towards or away from the enhanced probe 202. The sample z piezo device 216 is controlled by a sample z controller 218, which may move the sample z piezo device as needed.

[0027] As shown in Fig. 2, the scanning probe microscope 200 further includes a position feedback laser 220, a photo position detector 222, a first lock- in detector or amplifier 224 and a second lock-in detector or amplifier 226. The photo position detector 222 operates to optically measure the vibration of the cantilever 208 using light from the position feedback laser 220 reflected off the cantilever 208. The signal from the photo position detector 222 is input to the first lock-in amplifier 224, which can provide amplitude and phase of the cantilever vibration using the driving frequency fl as a reference. The first lock-in amplifier 224 may also provide an amplitude error signal to the sample z controller 218 so that the spacing between the enhanced probe tip and the sample 204 is maintained at a desired spacing. The position feedback laser 220, the photo position detector 222 and the first lock-in detector 224 are part of an optical detection system, which can measure one or more responses of the enhanced probe 202 with respect to the focused optical beam from the tunable laser 206.

[0028] The signal from the photo position detector 222 is also input to the second lock-in amplifier 226, which can provide amplitude and phase of the cantilever vibration to a computer control unit 228 using the modulation frequency f2 as a reference. The amplitude and phase signals from the first and second lock-in amplifiers 224 and 226 are transmitted to the computer control unit 228, which can process the received signals. The computer control unit 228 can also control the frequency generators 212 and 214 to adjust the frequencies fl and f2 that are produced, which may at least partly be in response to the received phase and / or amplitude signals.

[0029] Fig. 3 shows how an enhanced probe tip 302 is utilized in AFM-IR, in this case, for infrared photo-induced force microscopy in accordance with an embodiment of the invention. In the illustrated embodiment, the enhanced probe tip 302 is composed of a metal-coated tip 304 with a carbonaceous extension 306. An AFM cantilever (not shown in Fig. 3) is operated in a non-contact mode where the cantilever is driven at or near a mechanical resonance, in this case at the second flexural resonance fi, while the infrared light 308 is modulated or pulsed at fmwhere fmis the difference between the second and first flexural resonances. The enhanced probe tip 302 is used to image a sample 310 on a substrate 312. Since the photo-induced force varies non-linearly with gap spacing, the motion of the cantilever and the periodic modulation of the light at different frequency will “mix” to produce a signal at the sum and difference frequencies. The modulation frequency, fm, can be chosen such that it equals the difference frequency (fi - fo), in which case the photo-induced force can be detected by monitoring the amplitude of the cantilever at fo.

[0030] Fig. 4 shows an actual carbonaceous tip extension 400 on a metal coated tip 402 with an aspect ratio of approximately 3 (approximately 45 nm tall and approximately 15 nm wide at the base) and a radius of curvature of approximately 6.5 nm in accordance with an embodiment of the invention. This tip was used successfully to acquire high resolution PiFM image of lines with a width of about 12 nm and a spacing of about 10 nm. This demonstrates that short amorphous carbonaceous tips added onto a metal-coated tip can act as nanoantenna for near-field optical microscopy.

[0031] Turning now to Fig. 5, a flow diagram of a method of operating a scanning probe microscope, such as the scanning probe microscope 200, using an enhanced probe tip with a tip extension in accordance with an embodiment of the invention is shown. At block 502, a cantilever having a probe tip with a tip extension is driven to vibrate so that the probe tip with the tip extension interfaces a sample. At block 504, an optical beam is shined on the tip extension that is interfacing with the sample. At block 506, vibrations of the cantilever are optically measured to image the sample.

[0032] It will be readily understood that the components of the embodiments as generally described herein and illustrated in the appended figures could be arranged and designed in a wide variety of different configurations. Thus, the detailed description of various embodiments, as represented in the figures, is not intended to limit the scope of the present disclosure, but is merely representative of various embodiments. While the various aspects of the embodiments are presented in drawings, the drawings are not necessarily drawn to scale unless specifically indicated.

[0033] The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by this detailed description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

[0034] Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussions of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment.

[0035] Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize, in light of the description herein, that the invention can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention.

[0036] Reference throughout this specification to “one embodiment”, “an embodiment”, or similar language means that a particular feature, structure, or characteristic described in connection with the indicated embodiment is included in at least one embodiment of the present invention. Thus, the phrases “in one embodiment”, “in an embodiment”, and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.

[0037] In the above description, specific details of various embodiments are provided. However, some embodiments may be practiced with less than all of these specific details. In other instances, certain methods, procedures, components, structures, and / or functions are described in no more detail than to enable the various embodiments of the invention, for the sake of brevity and clarity.

[0038] Although the operations of the method(s) herein are shown and described in a particular order, the order of the operations of each method may be altered so that certain operations may be performed in an inverse order or so that certain operations may be performed, at least in part, concurrently with other operations. In another embodiment, instructions or sub-operations of distinct operations may be implemented in an intermittent and / or alternating manner.

[0039] It should also be noted that at least some of the operations for the methods described herein may be implemented using software instructions stored on a computer useable storage medium for execution by a computer. As an example, an embodiment of a computer program product includes a computer useable storage medium to store a computer readable program.

[0040] The computer-useable or computer-readable storage medium can be an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system (or apparatus or device). Examples of non-transitory computer-useableand computer-readable storage media include a semiconductor or solid state memory, magnetic tape, a removable computer diskette, a random access memory (RAM), a read-only memory (ROM), a rigid magnetic disk, and an optical disk. Current examples of optical disks include a compact disk with read only memory (CD-ROM), a compact disk with read / write (CD-R / W), and a digital video disk (DVD).

[0041] Alternatively, embodiments of the invention may be implemented entirely in hardware or in an implementation containing both hardware and software elements. In embodiments which use software, the software may include but is not limited to firmware, resident software, microcode, etc.

[0042] Although specific embodiments of the invention have been described and illustrated, the invention is not to be limited to the specific forms or arrangements of parts so described and illustrated. The scope of the invention is to be defined by the claims appended hereto and their equivalents.

Claims

WHAT IS CLAIMED IS:

1. A method of operating a scanning probe microscope comprising: driving a cantilever having a probe tip with a tip extension to vibrate so that the probe tip with the tip extension interfaces a sample; shining an optical beam on the tip extension that is interfacing with the sample; and optically measuring vibrations of the cantilever to image the sample.

2. The method of Claim 1, wherein the probe tip is a metal-coated probe tip.

3. The method of Claim 1, wherein the tip extension includes carbonaceous material.

4. The method of Claim 3, wherein the tip extension is a focused electron beam (e-beam) induced deposition (FEB ID) tip extension.

5. The method of Claim 4, wherein the tip extension includes a precursor material.

6. The method of Claim 1, wherein a height of the tip extension is less than 100 nm.

7. The method of Claim 1, wherein a base of the tip extension is approximately 20 nm and wherein the tip extension has a radius of curvature of less than 10 nm.

8. The method of Claim 1, wherein the tip extension has an aspect ratio of height to base between 1 and 5, inclusive.

9. The method of Claim 1, wherein the method uses am atomic force microscopy infrared spectroscopy (AFM IR) technique.

10. A scanning probe microscope comprising: a cantilever having a probe tip with a tip extension to interface a sample; a light source to transmit an optical beam onto the tip extension; a drive device coupled to the cantilever to drive the cantilever to vibrate; and an optical detection system to detect vibrations of the cantilever in response interactions between the probe tip with the tip extension and the sample.

11. The scanning probe microscope of Claim 10, wherein the probe tip is a metal-coated probe tip.

12. The scanning probe microscope of Claim 10, wherein the tip extension includes carbonaceous material.

13. The scanning probe microscope of Claim 12, wherein the tip extension includes diamond-like carbon (DLC).

14. The scanning probe microscope of Claim 12, wherein the tip extension is a focused electron beam (e-beam) induced deposition (FEB ID) tip extension.

15. The scanning probe microscope of Claim 14, wherein the tip extension includes a precursor material.

16. The scanning probe microscope of Claim 10, wherein a height of the tip extension is less than 100 nm.

17. The scanning probe microscope of Claim 10, wherein a base of the tip extension is approximately 20 nm and wherein the tip extension has a radius of curvature of less than 10 nm.

18. The scanning probe microscope of Claim 10, wherein the tip extension has an aspect ratio of height to base between 1 and 5, inclusive.

19. The scanning probe microscope of Claim 10, wherein the scanning probe microscope is an atomic force microscopy infrared spectroscopy (AFM IR) tool.

20. A cantilever structure comprising: a cantilever; and a probe tip coupled to the cantilever, the probe tip having a tip extension, wherein the tip extension has a height less than 100 nm.

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