IMAGING PLATFORM

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. Provisional Application Serial No.

61/538,654, filed September 23, 2011, which is incorporated herein by reference in its entirety and from which priority is claimed.

BACKGROUND

The disclosed subject matter relates to a platform for super-resolution imaging of a surface using a microsphere lens.

In certain conventional far-field optical microscopes, imaging resolution is limited to the diffraction limit, λ/2(« *sin(0)), where λ is the illuminating light wavelength, n is the refractive index, and Θ is the collection angle of the imaging optics. Generally speaking, the diffraction limit can be approximately half of the illuminating light's wavelength, or, e.g., approximately 200 nm in the visible spectrum.

In certain instances, it can be desirable to image at resolution below the diffraction limit. For example, as semiconductor device fabrication continues its trend toward increasingly smaller architecture, imaging techniques to resolve and inspect elements smaller than the diffraction limit can be useful for inspection or other purposes. Additionally, imaging for the biological sciences, such as imaging cell structures or certain proteins, can require imaging below the diffraction limit.

Certain techniques for imaging below the diffraction limit include scanning electron microscopy (SEM), stimulated emission depletion (STED) microscopy, near field optical microscopy (NSOM), and others. However, these techniques can include complex and/or cumbersome equipment and can require significant processing time. For example, some techniques can require the use of narrow spectrum light sources, fluorescent samples, expensive optical detection equipment, and intensive data processing techniques.

Accordingly, there is a need for improved techniques for superresolution imaging. S UMMARY

The disclosed subject matter provides systems for imaging a surface, in an exemplary embodiment, the system includes a nano-positioning device having a cantilever and an optically transparent microsphere lens coupled to the distal end of the cantilever. The system can also include an optical component to focus light on at least a portion of a surface to be imaged through the microsphere lens, and focus light reflected from the surface through the microsphere lens. A control unit can be communicatively coupled with the nano-positioning device and configured to position the microsphere lens at a predetermined distance above the surface.

In one embodiment, the nano-positioning device can be an atomic force microscopy apparatus. Alternatively, the nano-positioning device can be a near field scanning optical microscope apparatus. Alternatively, the nano-positioning device can be an apparatus capable of scanning a surface with a high position precision. The microsphere superiens can be attached to the tip of a cantilever associated with the nano-positioning device. The microsphere superiens can be a SiOa microsphere having a volume of between 3 to 5 μηι ² . The nano-positioning device can be configured to both translate the microsphere superiens about the surface and to position the microsphere superiens at a predetermined distance above the surface at each location, which can be, for example, between 2 nm and 20 nm.

In one embodiment, the optical component can include one or more objective lenses for focusing a virtual image created by the microsphere superiens. For example, conventional optical microscopy techniques can be employed to focus an image created by the microsphere superiens. The system can further include a camera adapted to receive light through the objective lens to generate an image.

The disclosed subject matter also provides methods for imaging a surface. In certain embodiments, a method can include positioning an optically transparent microsphere superiens at a predetermined distance above the surface. The surface can be illuminated, whereby light reflected from the surface passes through the microsphere lens and is focused through an optical component. The light can then be detected to form an image of the surface.

In one embodiment, the method can include identifying a surface defect of a substrate using the microsphere superiens. The microsphere superiens can be attached to the tip of a cantilever of an atomic force microscopy device. The microsphere superlens can then be positioned above the surface of the substrate at one or more substrate locations using the atomic force microscopy device. The substrate can be, for example, a heat assisted magnetic recording head. At each location, the distance of the microsphere superlens above the surface can be measured using the atomic force microscopy device. The device can be controlled to move the microsphere lens at a predetermined distance above the surface. The surface can then be imaged using an objective and the microsphere lens. The image of the surface can be processed to determine whether the substrate location includes a surface defect. BRIEF DESCRI PTION OF THE DRAWINGS

Fig. 1 is a schematic diagram of a system for superresolution imaging in accordance with an embodiment of the disclosed subject matter.

Fig. 2 is a flow diagram of a method for superresolution imaging in accordance with an embodiment of the disclosed subject matter.

Throughout the drawings, the same reference numerals and characters, unless otherwise stated, are used to denote like features, elements, components or portions of the illustrated embodiments. Moreover, while the disclosed subject matter will now be described in detail with reference to the Figs., it is done so in connection with the illustrative embodiments.

DETAILED DESCRIPTION

A platform for superresolution imaging of a surface using a

microsphere lens are disclosed herein. In accordance with the disclosed subject matter, microsphere lenses can be used for far-field superresolution imaging below the diffraction limit in the visible spectrum. The microsphere superlens can be placed within a nanometer scale distance to the surface of a sample so as to enhance evanescent waves in the near filed to compensate for their exponential decay into the far-field. That is, the microsphere superlens can project a near-field image including high spatial frequency information from evanescent waves into the far-field, which can then be imaged with conventional optics.

As used herein, the term "superresolution" can refer to optics in which resolution below the diffraction limit is attainable. The term "superlens" can refer to a lens which, with proper configuration, is capable of resolving an image below the diffraction limit.

Exemplary embodiments of the disclosed subject matter are described below, with reference to Fig. 1 and Fig. 2, for purposes of illustration, and not limitation.

In one embodiment, and with reference to Fig. 1 and Fig. 2, an optically transparent microsphere 110 can be positioned (210) at a predetermined distance above the surface of a sample 105. The microsphere 110 can be positioned several nanometers above the surface by precision control of the height of the microsphere 110, for purposes of example and not limitation, to facilitate smooth scanning, as described in more detail below, without colliding the surface of the sample 105. For example, the working distance between the surface and the microsphere 110 can be as small as possible to increase the near- field signal capturing before the evanescent decay. However, placing the microsphere 110 on the surface of the sample 105 can create scanning problems, for example because a surface topography, or other foreign bodies on the surface such as dust, can result in collisions which can result in scratches or other damage to the microsphere 110 or the surface of the sample 105. The distance of the microsphere 110 suspension above the surface can thus determined by considering the resolution, the scanning rate, and the response ti me in controlling the height change of the microsphere 110 with respect to the surface roughness. For example, a faster scanning rate can correspond to an increased distance above the surface of the sample 105.

The optically transparent microsphere lens 110 can be fabricated from a number of suitable materials and can have a number of suitable size and other characteristics to achieve superresolution foci. For example, the size and refractive index of the microsphere, as well as the medium in which the sphere is located, can impact the focal length, and therefore the resolution achieved by the microsphere lens. In an exemplary embodiment, the microsphere lens 110 can be formed from Si0 ₂ , with a refractive index of approximately 1.46 over a range of visible light, and a diameter of between approximately 2 pm to approximately 9 μτη.

For purposes of illustration, and not limitation, characteristics of microsphere lenses in accordance with the disclosed subject matter will now be described. Generally, assuming the medium in which the lens is to operate is air, a microsphere superlens can be defined by two parameters: index of refraction, n, and a size parameter, given by q = 2πα/λ. The "strength" of the lens can be arbitrarily given as the difference between the focus spot size of the lens and the diffraction limit divided by the radius, a. Thus, where the "strength" is negative (i.e., the focus spot size is less than the diffraction limit), the lens can be considered a superlens. The relationship between strength, size parameter, and index of refraction can be evaluated, for example, using Mie theory.

For a microsphere of n = 1.46, superresolution foci for the visible spectrum can exist for size parameters of below approximately 70, which can correspond to a diameter of approximately 2 μηι to approximately 9 μιη. For a microsphere of n = 1.8, super-resolution foci can exist for size parameters below approximately 250, which can correspond to particles as big as 30 μηι. Generally, increases index of refraction n, corresponds to an increased maximum size parameter to achieve superresolution; however, this does not hold true for n > 1.8. For example, where n = 2, superresolution decreases such that the maximum size parameter is less than the case in which n = 1.46.

The microsphere lens 110 can be positioned (210) at certain locations above the surface of a sample 105 with the use of a nano-positioning device 120. A suitable nano-positioning device can be employed. For purposes of illustration, the nano-positioning device 120 can include an atomic force microscopy (AFM) device. Such AFM devices are commercially available, and can include a cantilever 121 with a cantilever tip 122. The AFM device 120 can be operably connected to a control unit 129, which can be programmed or configured to cause the AFM device 120 to position the cantilever 121 relative to the sample. For example, with regard to position control in the plane of the sample surface, a piezostage with nanometer precision in both x- and y-axes can be used to direct the AFM tip to the point of interest. Line encoding can also be included to add the feedback control to the system. The control unit 129 can include one or more processors, one or more memories, which can be adapted to store executable code, which when executed can control the AFM device 120. Moreover, the AFM device 120 can measure the position of the cantilever 121 with the use of a light source 125 and detector 126. For example, the light source 125 can reflect light 127 off of the cantilever 121, and the detector 126 can detect the angle of the reflected light 128, and deduce the position of the cantilever in. three dimensional space with a high degree of accuracy and precision. The position of the cantilever can be fed back into the control unit 129 for further positioning by the AFM device 120.

The microsphere lens 110 can be physically coupled to a distal end of the cantilever 121. For example, the microsphere lens 110 can be attached to proximate that tip 122. The microsphere lens 110 can be attached, for example, with a drop of epoxy. Alternatively, the microsphere lens 110 can be attached with PDMS. For example, PDMS can be heated until it becomes viscous. The microsphere lens 110 and the cantilever tip 122 can be immersed in the viscous PDMS, contacted, and then cooled. Alternatively, SU-8, benzocyclobuten, and anodic bonding can be used to mount the microsphere lens to the cantilever.

Additionally or alternatively, the microsphere lens 110 can be attached to a near-field scanning optical microscope (NSOM) device. The microsphere can be attached as described above with reference to the cantilever of an AFM device.

Alternatively, the microsphere can be attached to the bottom of the probe tip of an NSOM device, where the light can still pass through the aperture of the probe tip.

An optical system 130 can be integrated with the AFM device 120 and the microsphere lens 120, The optical system 130 can be a conventional optical system. For example, the optical system 130 can include, for example, a light source 160 and an objective 135, along with additional optics for focusing, magnifying, or otherwise manipulating images. The optical system 130 can be arranged above the microsphere lens 110, such that light 137 from the light source 160 can be directed (220) through an objective 135, through the microsphere lens 110 and onto the surface of the sample 105. The light 137 can include white light (e.g., a combination of wavelengths in the visible spectrum), or a particular spectrum of light. For example, the light source can be a light bulb, a laser, an LED, or any other suitable light producing element.

Light 138 reflected off the surface of the sample 105 can pass through the microsphere lens 110 and through the objective 135. The optical system 130 can be configured, for example, such that the objective 135 is adapted to focus a virtual image produced via the microsphere lens 110. Where the microsphere lens 110 is positioned within a nm-scale distance above the sample 105, as described in more detail below, the virtual image created by the microsphere lens 110 can include enhanced evanescent wave information from the near-field.

In certain embodiments, a camera 150 or other optical detection device can be provided to detect (230) light focused through the optical system 130. For example, a camera 150 can be configured to generate an image of the surface of the sample 105 focused with the objective 135. The camera 150 can be any suitable camera, such as a CCD camera, electron multiplying CCD (emCCD) camera, CMOS camera, or other suitable imaging array.

In certain embodiments, imaging processing can be applied (230) to one or more images generated with the camera 150 to detect a surface characteristic. For example, a clustering or grouping algorithm can be applied to the image to determine if the surface contains a particular characteristic, such as a surface defect, fracture, dislocation, or the like. For example, a K-mean algorithm or Voronoi iteration algorithm can be used for clustering and grouping. Additionally or alternatively, an image processing algorithm can be applied to determine if there is a foreign object on the surface of the sample 105.

In an exemplary embodiment, the nano-positioning device, e.g., AFM device 120, can be configured to position the microsphere lens 110 between approximately 2 run and approximately 20 rrm above a first location on the surface of the sample 105. In certain embodiments, the microsphere lens 110 can be positioned between approximately 5 nm and 15 nm above the first location on the surface of the sample 105. For example, the microsphere 110 can be positioned at a first location above the sample 105 with the AFM device 120. The distance above the surface of the sample 105 at the first location can be measured, e.g., by reflecting light off of the cantilever 121 processing the position of the detected light 128. The AFM device 120 can then be controlled to move the cantilever tip and attached microsphere 110 to a desired distance above the surface. That is, a feedback loop can be established to position the microsphere 110 at a desired distance above the surface.

After the microsphere lens 110 has been positioned at a desired location, an image of the surface of the sample 105 can be generated with the camera 150. The AFM device 120 can then position (e.g., by translating the microsphere lens 110 above the surface) the microsphere lens 110 at a second location above the sample 105, and the feedback mechanism can again position the microsphere lens 110 at an appropriate distance above the surface at the second location. A second image of the surface of the sample 105 can be generated. This process can be repeated over a set of locations on the sample.

For purposes of illustration and not limitation, an exemplary embodiment of the platform and techniques disclosed herein will be described in connection with imaging of heat assisted magnetic recording (HAMR) heads. HAMR can be used, for example, in connection with magnetic data storage. Generally, a near-Field transducer (NFT) can efficiently couple light with surface Plasmon resonance and concentrate optical energy in a spot as small as 25 x 25 nm ² , thereby facilitating the magnetic switching of individual tracks within the magnetic storage media by temporarily reducing the anisotropy within the material However, the size of an NFT and surrounding components can be between approximately 20 to 300 nm, and therefore presents a challenge for inspection during the manufacturing process.

The platform and techniques disclosed herein can be used for inspection of HAMR head elements with resolution suitable to resolve and identify surface defects that are below the diffraction limit for the visible spectrum. For example, the techniques disclosed herein can enable imaging a HAMR head with sub- 50 nm resolution and above 1 μηι per snapshot, yielding rapid inspection. Moreover, the platform and techniques disclosed herein can be employed in serial fashion, thereby enabling high throughput, and can be integrated with low cost conventional optics.

In this exemplary embodiment, a HAMR head can be placed on a stage integrated with an AFM device, as demonstrated in Fig. 1. The AFM device can be, for example, a PSIO EX- 100 upright AFM device. A microsphere superlens can be attached to the tip of the cantilever of the AFM device, as described above. The AFM device can be programmed to position the microsphere superlens attached to the cantilever tip a predetermined distance above the HAMR head. The predetermined distance can correspond to a field of view (FOV) of the microsphere superlens. A set of scanning parameters, including scanning speed and scanning step size, can be determined based on the FOV and the AFM device can be configured to

translationally position the microsphere superlens across the HAMR head in accordance with the scanning parameters. At each scanning position, for example, a camera can take a snapshot of the HAMR head focused through the microsphere superlens and an objective (i.e., the objective can focus a virtual image created by the microsphere superlens). The scanning parameters can be determined such that the usable area of each snapshot does not substantially overlap, thereby allowing for superresolution imaging over the entire area of the HAMR heard. Moreover, at each scanning position, the

microsphere superlens can be repositioned to a predetermined distance above the surface of the HAMR head (i.e., along the axis normal to the surface of the HAMR head). In this manner, a HAMR head with a varying surface topography can be imaged.

As described above in connection with certain embodiments, a control unit 129 is provided to control the AFM device 120. In these embodiments, the control unit 129 plays a significant role in permitting precise control of the position of the cantilever tip and attached microsphere superlens. For example, the presence of the control unit 129 provides the ability to position the microsphere superlens with nanometer precision.

The presently disclosed subject matter is not to be limited in scope by the specific embodiments herein. Indeed, various modifications of the disclosed subject matter in addition to those described herein will become apparent to those skilled in the art from the foregoing description and the accompanying figures. Such modifications are intended to fall within the scope of the appended claims.