MONOLITHIC INTERFEROMETRIC ATOMIC FORCE MICROSCOPY DEVICE

Abstract

A fiber-facet AFM probe enabling high-resolution, high sensitivity measurement of a sample surface is presented. AFM probes in accordance with the present invention include an optically resonant cavity that is defined by two mirrors, at least one of which is a photonic-crystal mirror. One of the mirrors is movable and is mechanically coupled with an AFM tip such that a force imparted on the tip by an interaction with the sample surface induces a change in the cavity length of the optically resonant cavity and, therefore, its reflectivity.

Description

FIELD OF THE INVENTION

The present invention relates to imaging systems in general, and, more particularly, to atomic force microscopy.

BACKGROUND OF THE INVENTION

Atomic-force microscopy (AFM) is a technique for imaging a surface of a surface at the sub-nanometer (nm) scale. It has become commonly used for surface characterization, as well as mapping of certain material-specific surface properties, for materials such as polymers, ceramics, composites, glass, and biological tissue.

An AFM probe typically includes a very sharp tip that is mounted at the end of a cantilever. To image a surface, the tip is moved along the surface and its interaction with the surface is recorded. When the tip is in proximity with the surface, forces between the tip and the sample lead to a deflection of the cantilever, which can be measured with very high accuracy. By scanning the probe tip in two dimensions, a complete map of the surface structure and/or other physical properties of the surface can be developed.

Tapping-mode AFM (TM-AFM) is a particular type of AFM wherein the probe tip is brought into intermittent-contact with the surface so that it intermittently touches or “taps” the surface. TM-AFM is particularly attractive for measuring soft materials, since the tip is less likely to be “stuck” in the material. In addition, lateral forces on the tip, such as drag (which can reduce measurement accuracy), are virtually eliminated.

In TM-AFM, an actuator drives the cantilever such that it oscillates at its fundamental resonance frequency while the probe tip is scanned over the sample surface. The separation between the probe tip and the sample surface is adjusted via a feedback control loop to maintain constant oscillation amplitude at the probe tip. As the tip intermittently touches the sample during a scan, the tip experiences a contact force that induces dynamic effects on the mechanical behavior of the probe, such as oscillation amplitude changes, phase changes, and the development of harmonic components. These effects occur at frequencies much higher than the fundamental frequency of the probe and contain information about the physical properties of the surface over which the probe tip is scanned. Unfortunately, conventional AFM probes lack the capability to measure the higher signal frequency components with sufficient fidelity.

To overcome the drawbacks of conventional AFM probes, AFM probes having higher bandwidth capability were developed to enable direct measurement (typically via optical means) of the higher frequency components of the tip-sample interactions while preserving conventional operation in tapping mode. Examples of high-bandwidth AFM probes are disclosed in U.S. Pat. No. 8,082,593, which is incorporated herein by reference.

A typical high-bandwidth AFM probe has a cantilever body that extends from a reference structure to a first end, from which a sensor cantilevers to a second end that includes the probe tip. The sensor is characterized by higher resonance frequency than the cantilever portion; therefore, the sensor can respond to the dynamic effects that arise from tip-surface interactions. The mechanical behavior of the sensor versus that of the cantilever body is monitored, providing a signal that can be processed to yield more information about the properties of the surface than can be obtained with simpler cantilever-type probes.

Unfortunately, prior-art AFM probes and resulting systems have several drawbacks. First, they are typically relatively large. As a result, they are not well suited to many biological-imaging applications, such as in-vivo imaging.

Second, their relatively large size and cantilever design leads to a reduction in measurement resolution.

Third, prior-art AFM probes are typically too large and complex to use in array fashion. As a result, in order to image a two-dimensional region of a sample, an individual probe must be physically scanned over each object point in the region. Errors in the spatial positioning of the probe, as well as low-frequency signal components that arise from the scanning mechanism, can lead to a further degradation in measurement sensitivity.

Finally, optically interrogated prior-art AFM probes normally require a light source having an extremely stable wavelength output, which increases AFM system cost and complexity.

SUMMARY OF THE INVENTION

The present invention provides an ability to perform high-resolution AFM measurements in a confined space. Embodiments of the present invention are particularly well suited for use for in-vivo measurements of biological samples, wafer inspection, and arrayed measurement systems.

Embodiments of the present invention include AFM probes that comprise a pair of mirrors that define an optically resonant cavity. One of the mirrors is a movable mirror that is mechanically coupled with an AFM tip; therefore, the cavity length of the optically resonant cavity is based on force imparted on the AFM tip. In addition, including a photonic crystal in at least one of the mirrors (thereby defining a photonic crystal mirror) increases the finesse of the optically resonant cavity, which provides the AFM probe with improved measurement sensitivity. Further, the inclusion of a photonic crystal mirror affords some embodiments of the present invention high sensitivity over a broad range of wavelengths. As a result, the need for extreme wavelength stability in the light signal used to interrogate the AFM probe is obviated.

Embodiments of the present invention are suitable for attachment directly to a facet of an optical fiber. As a result, AFM probes in accordance with the present invention can have an extremely small form factor, thereby making them suitable for use for in-vivo measurements. The small form factor of these AFM probes can also be exploited to enable large, dense arrays of AFM probes that can rapidly scan a two-dimensional region of a surface with good spatial resolution.

An illustrative embodiment of the present invention is a fiber-facet AFM probe for measuring a surface of a sample. The AFM probe comprises a first membrane mirror and second membrane mirror that includes a photonic crystal, where the mirrors collectively define an optically resonant cavity for a light signal having a first wavelength. The first mirror is affixed to a facet of an optical fiber. The second mirror is movable and includes an AFM tip suitable for interacting with the surface of a sample. As a result, the cavity length of the cavity is based on force imparted on the tip as it is scanned over a surface under measurement.

In some embodiments, an AFM probe includes a first actuator for oscillating the AFM probe along the longitudinal axis of the optical fiber. In some embodiments, an AFM probe also includes a second actuator for scanning the AFM probe in a direction substantially orthogonal to the longitudinal axis.

In some embodiments, multiple AFM probes are arranged in an array that enables rapid measurement of large surface areas.

In some embodiments, only one of the first mirror and second mirror includes a photonic crystal. In some embodiments, at least one of the mirrors is not a membrane, but is disposed on the end of a mechanical element, such as a cantilever. In some embodiments, the first mirror is the facet of the optical fiber.

An embodiment of the present invention comprises an atomic-force microscope probe comprising: a first mirror that is partially transmissive for a first light signal having a first wavelength; a second mirror that is partially transmissive for the first light signal, wherein the second mirror is movable, and wherein the first mirror and the second mirror collectively define an optically resonant cavity; and a tip that is mechanically coupled with the second mirror; wherein at least one of the first mirror and second mirror includes a first arrangement of features that collectively define a first photonic crystal that is operative for partially reflecting the first light signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic diagram of a fiber-facet AFM probe in accordance with an illustrative embodiment of the present invention.

FIGS. 2A and 2B depict schematic drawings of a bottom view and a side view, respectively, of a sensor in accordance with the illustrative embodiment of the present invention.

FIG. 3 depicts operations of a method for fabricating an AFM probe in accordance with the illustrative embodiment of the present invention.

FIGS. 4A-H depict substrate 400 at different stages of the fabrication of probe 102.

DETAILED DESCRIPTION

FIG. 1 depicts a schematic diagram of a fiber-facet AFM probe in accordance with an illustrative embodiment of the present invention. Probe 100 comprises sensor 102, optical fiber 104, vertical actuator 106, and lateral actuator 108.

Sensor 102 is a Fabry-Perot cavity-based displacement sensor having a cavity length that is based on force arising from interactions between the sensor and surface 122. Sensor 102 includes a photonic crystal suitable for enhancing the finesse of the Fabry-Perot cavity for the wavelength of light signal 116. Sensor 102 is described in detail below and with respect to FIG. 2.

Optical fiber 104 is a conventional single-mode optical fiber having longitudinal axis 110, core 112 and facet 114. Optical fiber 104 has a diameter of approximately 125 microns. The use of a single-mode fiber for optical fiber 104 is preferable, since this mitigates measurement-sensitivity degradation from modal noise; however, optical fiber 104 can also be a multimode fiber without departing from the scope of the present invention.

Vertical actuator 106 is a conventional actuator operative for oscillating sensor 102 in the z-direction when AFM probe 100 is operated in tapping mode. Vertical actuator 106 is a conventional actuator, such as a piezoelectric actuator, which is mechanically coupled with sensor 102 and optical fiber 104.

Lateral actuator 108 is a conventional actuator operative for scanning sensor 102 along surface 122 of sample 120 (i.e., along a direction substantially orthogonal to longitudinal axis 110). In other words, lateral actuator 108 enables motion of sensor 102 in the x-y plane. Lateral actuator 108 is a conventional actuator, such as a piezoelectric actuator, which is mechanically coupled with sensor 102 and optical fiber 104.

Sensor 102 is affixed to optical fiber 104 at facet 114 in conventional fashion, using optically transparent epoxy, silicate solution, and the like.

FIGS. 2A and 2B depict schematic drawings of a bottom view and a side view, respectively, of a sensor in accordance with the illustrative embodiment of the present invention. FIG. 2B depicts a sectional view of sensor 102 through line a-a of FIG. 2A. Sensor 102 comprises optically resonant cavity 202, frame 208, and tip 210. It should be noted that sensor 102 is depicted in an idealized form. In practice, a typical sensor will have structure that deviates from this idealized form, as discussed below and with respect to FIG. 4G.

Optically resonant cavity 202 (hereinafter referred to as cavity 202) is a high-finesse cavity that comprises mirrors 204 and 206, which are separated by cavity length L. When cavity 202 is in its unexcited state (i.e., its quiescent state), cavity length L is equal to quiescent cavity length L₀. One skilled in the art will recognize that the instantaneous reflectivity of cavity 202 is strongly dependent upon the instantaneous cavity length L.

Mirror 204 is a photonic-crystal mirror that comprises a portion of a membrane of single-crystal silicon having a thickness of approximately 500 nanometers (nm). Mirror 204 has a square shape of approximately 70 microns per side and defines x-y plane 218. The size of mirror 204 is selected to accommodate the diameter of light signal 116, as well as allow for space around the membrane sufficient for the inclusion of tethers 212.

Mirror 204 includes a two-dimensional array of features 214, which are sized and arranged to collectively define photonic crystal 216, such that photonic crystal 216 is partially transmissive for light signal 116. Photonic crystal 216 is designed to support guided resonances that interfere with light signal 116 to create broadband reflections. As a result, the use of a photonic crystal mirror, such as mirror 204, affords some embodiments of the present invention high sensitivity over a broad range of wavelengths, thereby providing more tolerance for wavelength variation in light signal 116 than can be tolerated by prior-art optically interrogated AFM probes.

In the illustrative embodiment, features 214 are through-holes having a diameter of approximately 800 nm. Features 214 are arranged in a square array having a feature pitch of approximately 950 nm. One skilled in the art will recognize that the size, spacing, and depth of features 214 are based on the wavelength of light signal 116, as well as the desired reflectivity and finesse of optical cavity 202. As a result, one skilled in the art will recognize that the features of photonic crystal 216 can have any suitable dimensions and arrangement.

Mirror 206 is a continuous membrane of single-crystal silicon having a thickness of approximately 500 nm. Mirror 204 has a diameter larger than the diameter of light signal 116, but smaller than the diameter of optical fiber 104. A non-limiting exemplary value for the diameter of mirror 206 is within the range of approximately 80 microns to approximately 110 microns. In some embodiments, mirror 206 includes a photonic crystal that is analogous to photonic crystal 216. In some embodiments, each of mirrors 204 and 206 includes a photonic crystal, but the features of the two photonic crystals do not have the same dimensions.

Tethers 212 suspend mirror 204 from frame 208 such that mirrors 204 and 206 are separated by quiescent cavity length, L₀, which is approximately 1.5 microns, and typically within the range of approximately 1 micron to approximately 2 microns. Tethers 212 are designed such that they readily bend in a direction out of the x-y plane, but resist bending within the x-y plane. As a result, tethers 212 substantially enable mirror 204 to move along the z-direction but inhibit its motion within the x-y plane.

In some embodiments, tethers 212 and mirror 204 are collectively characterized by a mechanical resonance frequency that is higher than the oscillation frequency imparted on the probe by vertical actuator 106. This enables probe 100 to measure frequency components associated with tip-surface interactions that occur at frequencies higher than the oscillation frequency. As a result, in some embodiments, probe 100 is a high-bandwidth AFM probe. For the purposes of this Specification, including the appended claims, the term “high-bandwidth AFM probe” is defined as a probe that is designed for TM-AFM operation at a drive frequency, wherein the probe is operative for measuring frequency components associated with tip-surface interactions that occur at frequencies higher than the drive frequency. Examples of high-bandwidth AMF probes are found in U.S. Pat. Nos. 8,082,593, 7,302,833, 7,404,314, and 7,089,787, and U.S. Patent application Ser. No. 13/829,626, filed Mar. 14, 2013 (Attorney Docket: 146-035US1), each of which is incorporated herein by reference.

For clarity, each of tethers 212 is depicted as a simple beam; however, a typical tether 212 has a more complex design, such as a serpentine spring, a folded-beam spring, and the like. It will be clear to one skilled in the art, after reading this Specification, how to specify, make, and use tethers 212.

It should be noted that the thickness of mirrors 204 and 206, the materials they comprise (and their refractive indices), quiescent cavity length L₀, and the design of photonic crystal 216 are based on the wavelength of light signal 116 and the desired sensitivity of AFM probe 100.

Tip 210 is a column of suitable structural material having a height sufficient to enable it to scan surface 122 and a diameter that is typically is on the order of one period of photonic crystal 216.

In typical TM-AFM operation, AFM probe 100 is scanned over surface 122 while vertical actuator 106 oscillates sensor 102 along the z-direction, as shown. The separation distance between the probe and surface 122 is controlled such that tip 210 intermittently interacts with the surface as the probe is scanned in the x-y plane. As tip 210 taps surface 122, a tip-surface interaction force is imparted onto the tip. This force causes dynamic changes in cavity length L, thereby changing the reflectivity of optically resonant cavity 202. These dynamic changes give rise to high-frequency signal components in light signal 118, where these signal components are characteristic of the physical properties of surface 122.

FIG. 3 depicts operations of a method for fabricating an AFM probe in accordance with the illustrative embodiment of the present invention.

FIGS. 4A-H depict substrate 400 at different stages of the fabrication of probe 102.

Method 300 begins with optional operation 301, wherein substrate 400 is provided. Substrate 400 is a conventional silicon-on-insulator (SOI) substrate comprising handle substrate 402, buried oxide layer 404, and active layer 406. Handle substrate 402 is a conventional single-crystal silicon substrate, buried oxide layer 404 is a conventional silicon dioxide layer having a thickness of approximately 1 micron, and active layer 406 is a layer of single-crystal silicon having a thickness of approximately 2.5 microns. It will be clear to one skilled in the art, after reading this Specification, how to specify, make, and use probes formed from substrates comprising different materials and/or different layer thicknesses.

At operation 302, features 214 and tethers 212 are defined in active layer 406. Features 214 and tethers 212 are defined in active layer 406 via a suitable mask layer and a deep reactive-ion etch (DRIE) process. The DRIE process is timed so that features 214 extend only through the thickness of stratum 408, which has a thickness of approximately 500 nm. As a result, the depth of features 214 is approximately 500 nm. In some embodiments, a directional etch other than DRIE is used to define features 214 and tethers 212.

FIG. 4A depicts a schematic drawing of a cross-sectional side view of a portion of substrate 400 after the definition of mirror 204, features 214, and tethers 212 in stratum 408. For clarity, tethers 212 are not shown in FIG. 4A.

At operation 303, handling frame 414 and connecting tabs 416 are defined outside the region of sensor 102 via an appropriate mask and DRIE. Handling frame 414 and connecting tabs 416 are formed by etching completely through active layer 406 to buried oxide 404.

FIG. 4B depicts a schematic drawing of a top view of a portion of substrate 400 after the definition of mirror 204, features 214, and tethers 212 in stratum 408.

At operation 304, thermal oxide 420 is grown on all exposed silicon surfaces.

At operation 305, thermal oxide 420 is removed from the bottom surface 422 of each of features 214, as well as from regions 418 (as shown in FIG. 4B), which exposes the surface of silicon in stratum 410.

At operation 306, mirror 204 is partially released from substrate 400 in release etch 424. Release etch 424 is a substantially isotropic RIE etch, which attacks the exposed single-crystal silicon in stratum 410 through features 214. Since the etch attacks the silicon in a substantially isotropic manner, etch front 426 proceeds laterally at approximately the same rate as it proceeds vertically. As a result, the silicon material is removed from underneath the structural material of mirror 204 and tethers 212.

FIG. 4C depicts a schematic drawing of a sectional view of a portion of substrate 400 after the exposure of the bottom surfaces of features 214 and during release etch 424.

Release etch 424 is a timed to remove most, but not all, of active layer material from stratum 410. As a result, release etch 424 leaves pedestal 428 that provides support for the center of mirror 204 during subsequent operations. In some embodiments, all active layer material is removed from stratum 410 during release each 424.

FIG. 4D depicts a schematic drawing of a sectional view of a portion of substrate 400 after release etch 224.

At operation 307, all exposed silicon is again thermally oxidized. This oxidation accomplishes two results. First, it provides protection for the structural components of sensor 102 during subsequent processing. Second, it converts the remaining silicon in pedestal 428 into silicon dioxide.

In some embodiments, sensor 102 comprises two photonic-crystal mirrors. In these embodiments, a second directional etch is performed through features 214, using these features as a mask to pattern a matching arrangement of features into the silicon in stratum 412. This matching set of features collectively defines a second photonic crystal in mirror 206. In some embodiments, this matching arrangement of features extends completely through stratum 412. In some embodiments, it extends only partially through the thickness of stratum 412.

It should be noted that the second photonic crystal can be formed in mirror 206 at any of several points in method 300—for example, after either of operations 306 and 307.

In some embodiments, release etch 424 completely removes active layer material from both strata 410 and 412. In such embodiments, sensor 102 includes only one mirror within its structure. When attached to fiber 104, facet 114 acts as the second mirror of the optically resonant cavity.

At operation 308, well 430 is formed in handle substrate 402. Well 430 has a diameter that is slightly larger than that of optical fiber 104 so that well 430 can locate the fiber when sensor 102 is affixed to facet 114.

FIGS. 4E and 4F depict schematic drawings of a top and sectional view of substrate 400 after the formation of well 230. The sectional view shown in FIG. 4F is taken through line b-b of FIG. 4E.

At operation 309, the exposed silicon dioxide on substrate 400 is removed in a vapor-phase hydrofluoric acid release. At the end of operation 309, the single-crystal material of stratum 410 is substantially completely removed in region 432 of substrate 400; however, the single-crystal silicon in strata 408 and 412 in region 432 remains and forms the basis for the structural elements of sensor 202 (i.e., mirrors 204 and 206, and tethers 212), as well as handling frame 414 and connecting tabs 416.

At operation 310, tip 210 is provided at mirror 204 such that the tip and the mirror are mechanically coupled.

Tip 210 is fabricated separately using a single-crystal substrate other than substrate 400. To form tip 210, a conventional crystallographic-dependent etch (e.g., potassium hydroxide (KOH), ethylene diamine pyrocatechol (EDP), hydrazine, etc.) is used to create a pyramidal projection (i.e., nascent tip 210). Typically, this projection has a height within the range of approximately 2.3 microns to approximately 10 microns, although the projection can have any practical height. If desired, after its formation, nascent tip 210 can be further sharpened by thermal oxidization followed by oxide removal, via ion-beam milling, etc.

Once nascent tip 210 is fully formed on its native substrate, it is removed via ion-beam milling (or a similar process), transferred to mirror 204, and bonded to the mirror using a conventional bonding or welding technology.

In some embodiments, tip 210 is fabricated on substrate 400 at the beginning of the method 300; however, in such embodiments, the height of tip 210 must be carefully controlled to avoid hindering subsequent fabrication steps.

In some embodiments, tip 210 is grown directly on mirror 204 via a suitable deposition method, such as beam-assisted deposition.

FIG. 4G depicts a schematic drawing of a sectional side view of sensor 102 after it has been completely fabricated and is still attached to handling frame 414 by connecting tabs 416.

FIG. 4H depicts a scanning-electron microscope picture of a portion of a fully fabricated substrate 400 with tip 210 attached.

As depicted in FIG. 4G, each of mirrors 204 and 206 exhibit some surface features that are artifacts of the fabrication process used to form sensor 102. Although this surface structure may reduce the finesse of optically resonant cavity or shift the ideal operational wavelength slightly, it does not significantly degrade measurement sensitivity of AFM probe 100. In some embodiments, the design of sensor 102 compensates for such surface structure by choosing features sizes and mirror thicknesses that give rise to an acceptable finesse for cavity 202 (e.g., to achieve a reflectivity of greater than 90%).

At operation 311, sensor 102 is attached to facet 114. In some embodiments, sensor 102 is attached by first disposing a layer of adhesive (e.g., UV-curable epoxy, silicate solution, etc.) on facet 114. This layer of adhesive is then thinned by repeatedly contacting it with bare silicon or another suitable surface. Once the adhesive is suitably thin, optical fiber 104 is inserted into well 430 to align facet 114 with mirror 206. The adhesive is then fully cured.

At operation 312, handling frame 414 is removed by breaking connecting tabs 416, leaving the structure described above and with respect to FIG. 1 (without actuators 106 and 108).

It is to be understood that the disclosure teaches just one example of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.

Claims

1. An atomic-force microscope probe comprising: a first mirror that is partially transmissive for a first light signal having a first wavelength;a second mirror that is partially transmissive for the first light signal, wherein the second mirror is movable, and wherein the first mirror and the second mirror collectively define an optically resonant cavity; anda tip that is mechanically coupled with the second mirror;wherein at least one of the first mirror and second mirror includes a first arrangement of features that collectively define a first photonic crystal that is operative for partially reflecting the first light signal.

2. The probe of claim 1 further comprising an optical fiber having a first facet, wherein the first facet is the first mirror.

3. The probe of claim 1 further comprising an optical fiber having a first facet, wherein one of the first mirror and second mirror is mechanically coupled with the first facet.

4. The probe of claim 3 further comprising a first actuator that is operative for imparting a first motion on the first facet.

5. The probe of claim 4 further comprising a second actuator that is operative for imparting a second motion on the first facet, wherein the first motion is directed along a first direction and the second motion is directed along a second direction that is substantially orthogonal with the first direction.

6. The probe of claim 1, wherein the first mirror comprises the first arrangement of features, and wherein the second mirror comprises a second arrangement of features that collectively define a second photonic crystal that is operative for partially reflecting the first light signal.

7. The probe of claim 1 further comprising at least one tether that is mechanically coupled with the second mirror, the at least one tether being dimensioned and arranged to enable motion of the second mirror in a direction that is substantially orthogonal with a first plane, wherein the second mirror defines the first plane.

8. The probe of claim 1 further comprising a plurality of tethers that are mechanically coupled the second mirror, the plurality of tethers being dimensioned and arranged to enable motion of the second mirror in a direction that is substantially orthogonal with a first plane and inhibit motion of the second mirror in a direction substantially parallel with the first plane, wherein the second mirror defines the first plane.

9. An atomic-force microscope probe comprising: an optical fiber having a first facet;an optically resonant cavity comprising a first mirror that is movable with a first motion, the first mirror including a first photonic crystal that is partially transmissive for a first light signal, wherein the optically resonant cavity is located at the first facet, and wherein the optically resonant cavity has a cavity length that is based on the first motion; anda tip that is mechanically coupled with the first mirror.

10. The probe of claim 9, wherein the optically resonant cavity further comprises the first facet.

11. The probe of claim 9, wherein the optically resonant cavity further comprises a second mirror, the second mirror being mechanically coupled with the first facet.

12. The probe of claim 11 wherein the second mirror comprises a second photonic crystal that is partially transmissive for a first light signal.

13. The probe of claim 9 further comprising a first actuator that is operative for imparting a second motion on the first facet.

14. The probe of claim 13 further comprising a second actuator that is operative for imparting a third motion on the first facet, wherein the second motion is directed along a first direction and the third motion is directed along a second direction that is substantially orthogonal with the first direction.

15. The probe of claim 1 further comprising at least one tether that is mechanically coupled with the first mirror, the at least one tether being dimensioned and arranged to enable the first motion.

16-20. (canceled)