MONOLITHIC INTERFEROMETRIC ATOMIC FORCE MICROSCOPY DEVICE

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.

DETAILED DESCRIPTION

FIG. 1depicts a schematic diagram of a fiber-facet AFM probe in accordance with an illustrative embodiment of the present invention. Probe100comprises sensor102, optical fiber104, vertical actuator106, and lateral actuator108.

Sensor102is a Fabry-Perot cavity-based displacement sensor having a cavity length that is based on force arising from interactions between the sensor and surface122. Sensor102includes a photonic crystal suitable for enhancing the finesse of the Fabry-Perot cavity for the wavelength of light signal116. Sensor102is described in detail below and with respect toFIG. 2.

Optical fiber104is a conventional single-mode optical fiber having longitudinal axis110, core112and facet114. Optical fiber104has a diameter of approximately 125 microns. The use of a single-mode fiber for optical fiber104is preferable, since this mitigates measurement-sensitivity degradation from modal noise; however, optical fiber104can also be a multimode fiber without departing from the scope of the present invention.

Vertical actuator106is a conventional actuator operative for oscillating sensor102in the z-direction when AFM probe100is operated in tapping mode. Vertical actuator106is a conventional actuator, such as a piezoelectric actuator, which is mechanically coupled with sensor102and optical fiber104.

Lateral actuator108is a conventional actuator operative for scanning sensor102along surface122of sample120(i.e., along a direction substantially orthogonal to longitudinal axis110). In other words, lateral actuator108enables motion of sensor102in the x-y plane. Lateral actuator108is a conventional actuator, such as a piezoelectric actuator, which is mechanically coupled with sensor102and optical fiber104.

FIGS. 2A and 2Bdepict 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. 2Bdepicts a sectional view of sensor102through line a-a ofFIG. 2A. Sensor102comprises optically resonant cavity202, frame208, and tip210. It should be noted that sensor102is 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 toFIG. 4G.

Optically resonant cavity202(hereinafter referred to as cavity202) is a high-finesse cavity that comprises mirrors204and206, which are separated by cavity length L. When cavity202is 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 cavity202is strongly dependent upon the instantaneous cavity length L.

Mirror204is a photonic-crystal mirror that comprises a portion of a membrane of single-crystal silicon having a thickness of approximately 500 nanometers (nm). Mirror204has a square shape of approximately 70 microns per side and defines x-y plane218. The size of mirror204is selected to accommodate the diameter of light signal116, as well as allow for space around the membrane sufficient for the inclusion of tethers212.

Mirror204includes a two-dimensional array of features214, which are sized and arranged to collectively define photonic crystal216, such that photonic crystal216is partially transmissive for light signal116. Photonic crystal216is designed to support guided resonances that interfere with light signal116to create broadband reflections. As a result, the use of a photonic crystal mirror, such as mirror204, affords some embodiments of the present invention high sensitivity over a broad range of wavelengths, thereby providing more tolerance for wavelength variation in light signal116than can be tolerated by prior-art optically interrogated AFM probes.

In the illustrative embodiment, features214are through-holes having a diameter of approximately 800 nm. Features214are 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 features214are based on the wavelength of light signal116, as well as the desired reflectivity and finesse of optical cavity202. As a result, one skilled in the art will recognize that the features of photonic crystal216can have any suitable dimensions and arrangement.

Mirror206is a continuous membrane of single-crystal silicon having a thickness of approximately 500 nm. Mirror204has a diameter larger than the diameter of light signal116, but smaller than the diameter of optical fiber104. A non-limiting exemplary value for the diameter of mirror206is within the range of approximately 80 microns to approximately 110 microns. In some embodiments, mirror206includes a photonic crystal that is analogous to photonic crystal216. In some embodiments, each of mirrors204and206includes a photonic crystal, but the features of the two photonic crystals do not have the same dimensions.

Tethers212suspend mirror204from frame208such that mirrors204and206are 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. Tethers212are 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, tethers212substantially enable mirror204to move along the z-direction but inhibit its motion within the x-y plane.

In some embodiments, tethers212and mirror204are collectively characterized by a mechanical resonance frequency that is higher than the oscillation frequency imparted on the probe by vertical actuator106. This enables probe100to measure frequency components associated with tip-surface interactions that occur at frequencies higher than the oscillation frequency. As a result, in some embodiments, probe100is 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 tethers212is depicted as a simple beam; however, a typical tether212has 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 tethers212.

It should be noted that the thickness of mirrors204and206, the materials they comprise (and their refractive indices), quiescent cavity length L0, and the design of photonic crystal216are based on the wavelength of light signal116and the desired sensitivity of AFM probe100.

Tip210is a column of suitable structural material having a height sufficient to enable it to scan surface122and a diameter that is typically is on the order of one period of photonic crystal216.

In typical TM-AFM operation, AFM probe100is scanned over surface122while vertical actuator106oscillates sensor102along the z-direction, as shown. The separation distance between the probe and surface122is controlled such that tip210intermittently interacts with the surface as the probe is scanned in the x-y plane. As tip210taps surface122, 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 cavity202. These dynamic changes give rise to high-frequency signal components in light signal118, where these signal components are characteristic of the physical properties of surface122.

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

FIGS. 4A-Hdepict substrate400at different stages of the fabrication of probe102.

Method300begins with optional operation301, wherein substrate400is provided. Substrate400is a conventional silicon-on-insulator (SOI) substrate comprising handle substrate402, buried oxide layer404, and active layer406. Handle substrate402is a conventional single-crystal silicon substrate, buried oxide layer404is a conventional silicon dioxide layer having a thickness of approximately 1 micron, and active layer406is 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 operation302, features214and tethers212are defined in active layer406. Features214and tethers212are defined in active layer406via a suitable mask layer and a deep reactive-ion etch (DRIE) process. The DRIE process is timed so that features214extend only through the thickness of stratum408, which has a thickness of approximately 500 nm. As a result, the depth of features214is approximately 500 nm. In some embodiments, a directional etch other than DRIE is used to define features214and tethers212.

FIG. 4Adepicts a schematic drawing of a cross-sectional side view of a portion of substrate400after the definition of mirror204, features214, and tethers212in stratum408. For clarity, tethers212are not shown inFIG. 4A.

At operation303, handling frame414and connecting tabs416are defined outside the region of sensor102via an appropriate mask and DRIE. Handling frame414and connecting tabs416are formed by etching completely through active layer406to buried oxide404.

FIG. 4Bdepicts a schematic drawing of a top view of a portion of substrate400after the definition of mirror204, features214, and tethers212in stratum408.

At operation304, thermal oxide420is grown on all exposed silicon surfaces.

At operation305, thermal oxide420is removed from the bottom surface422of each of features214, as well as from regions418(as shown inFIG. 4B), which exposes the surface of silicon in stratum410.

At operation306, mirror204is partially released from substrate400in release etch424. Release etch424is a substantially isotropic RIE etch, which attacks the exposed single-crystal silicon in stratum410through features214. Since the etch attacks the silicon in a substantially isotropic manner, etch front426proceeds laterally at approximately the same rate as it proceeds vertically. As a result, the silicon material is removed from underneath the structural material of mirror204and tethers212.

FIG. 4Cdepicts a schematic drawing of a sectional view of a portion of substrate400after the exposure of the bottom surfaces of features214and during release etch424.

Release etch424is a timed to remove most, but not all, of active layer material from stratum410. As a result, release etch424leaves pedestal428that provides support for the center of mirror204during subsequent operations. In some embodiments, all active layer material is removed from stratum410during release each424.

FIG. 4Ddepicts a schematic drawing of a sectional view of a portion of substrate400after release etch224.

At operation307, all exposed silicon is again thermally oxidized. This oxidation accomplishes two results. First, it provides protection for the structural components of sensor102during subsequent processing. Second, it converts the remaining silicon in pedestal428into silicon dioxide.

In some embodiments, sensor102comprises two photonic-crystal mirrors. In these embodiments, a second directional etch is performed through features214, using these features as a mask to pattern a matching arrangement of features into the silicon in stratum412. This matching set of features collectively defines a second photonic crystal in mirror206. In some embodiments, this matching arrangement of features extends completely through stratum412. In some embodiments, it extends only partially through the thickness of stratum412.

It should be noted that the second photonic crystal can be formed in mirror206at any of several points in method300—for example, after either of operations306and307.

In some embodiments, release etch424completely removes active layer material from both strata410and412. In such embodiments, sensor102includes only one mirror within its structure. When attached to fiber104, facet114acts as the second mirror of the optically resonant cavity.

At operation308, well430is formed in handle substrate402. Well430has a diameter that is slightly larger than that of optical fiber104so that well430can locate the fiber when sensor102is affixed to facet114.

FIGS. 4E and 4Fdepict schematic drawings of a top and sectional view of substrate400after the formation of well230. The sectional view shown inFIG. 4Fis taken through line b-b ofFIG. 4E.

At operation309, the exposed silicon dioxide on substrate400is removed in a vapor-phase hydrofluoric acid release. At the end of operation309, the single-crystal material of stratum410is substantially completely removed in region432of substrate400; however, the single-crystal silicon in strata408and412in region432remains and forms the basis for the structural elements of sensor202(i.e., mirrors204and206, and tethers212), as well as handling frame414and connecting tabs416.

At operation310, tip210is provided at mirror204such that the tip and the mirror are mechanically coupled.

Tip210is fabricated separately using a single-crystal substrate other than substrate400. To form tip210, 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 tip210). 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 tip210can be further sharpened by thermal oxidization followed by oxide removal, via ion-beam milling, etc.

Once nascent tip210is fully formed on its native substrate, it is removed via ion-beam milling (or a similar process), transferred to mirror204, and bonded to the mirror using a conventional bonding or welding technology.

In some embodiments, tip210is fabricated on substrate400at the beginning of the method300; however, in such embodiments, the height of tip210must be carefully controlled to avoid hindering subsequent fabrication steps.

In some embodiments, tip210is grown directly on mirror204via a suitable deposition method, such as beam-assisted deposition.

FIG. 4Gdepicts a schematic drawing of a sectional side view of sensor102after it has been completely fabricated and is still attached to handling frame414by connecting tabs416.

FIG. 4Hdepicts a scanning-electron microscope picture of a portion of a fully fabricated substrate400with tip210attached.

As depicted inFIG. 4G, each of mirrors204and206exhibit some surface features that are artifacts of the fabrication process used to form sensor102. 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 probe100. In some embodiments, the design of sensor102compensates for such surface structure by choosing features sizes and mirror thicknesses that give rise to an acceptable finesse for cavity202(e.g., to achieve a reflectivity of greater than 90%).

At operation311, sensor102is attached to facet114. In some embodiments, sensor102is attached by first disposing a layer of adhesive (e.g., UV-curable epoxy, silicate solution, etc.) on facet114. 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 fiber104is inserted into well430to align facet114with mirror206. The adhesive is then fully cured.

At operation312, handling frame414is removed by breaking connecting tabs416, leaving the structure described above and with respect toFIG. 1(without actuators106and108).