Abstract:
A resolution enhancement device is provided which utilizes either high extra-mural absorbent optical fibers in the transfer optic, and/or which uses a transfer optic which is bonded to the scintillator without the use of any glues or adhesives. The device provides improved resolution of electron images from electron microscopes while not reducing the sensitivity of the apparatus.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 09/675,787, filed Sep. 29, 2000 now U.S. Pat. No. 6,455,860. The entire disclosure of that application is hereby incorporated by reference. This application claims the benefit of U.S. provisional application Serial No. 60/156,799, filed Sep. 30, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a resolution enhancement device that may be used, for example, in an electron microscope, and more particularly to the detection of electron images by converting them into light images and transferring them onto an electronic light-imaging device to enhance the resolution of such images while not sacrificing sensitivity. 
     Electron microscopes use a beam of accelerated electrons that pass through or are deflected by a sample to provide an electron image and/or diffraction pattern of the sample. To provide a record of these images and/or diffraction patterns, the electrons have been converted into light images using scintillator materials (e.g., single crystal YAG and phosphors), and an imaging sensor captures the light images and/or patterns. A transfer optic, typically one or more optical lenses or a fiber optic plate, transfers the light image to the imaging sensor. While photographic film and cameras have long been used to capture such light images and/or diffraction patterns, charge-coupled devices (CCD) of the type originally developed for astronomy to read light images into a computer have found increasing use in this field. Such CCD cameras offer excellent resolution, sensitivity, linearity, up to 2,048×2,048 pixels, are reusable, and make the image available for viewing within seconds of recording. 
     A conductive medium is typically coated onto the entrance surface of the scintillator to prevent the buildup of electrical charges and also to prevent the entry of light from external sources. When the transfer optic is a fiber optic plate, the scintillator is typically glued onto the fiber optic plate, and the plate is then coupled with optical coupling oil or glue to the imaging sensor. 
     The resolution of prior art devices is limited by a number of factors including the extent to which light generated at a particular spot on the scintillator is imaged onto a single pixel at the imaging sensor. Current image coupling devices lose resolution due to leakage (scattering) of light sideways, either in the scintillator, the transfer optic, or both. Such light scattering increases background noise and creates a “haze,” making it difficult to image objects generating only weak intensity light which are located near objects which generate stronger intensity light. 
     One solution to the problem of image resolution is suggested by Mooney et al, U.S. Pat. No. 5,635,720. There, a light absorptive layer is positioned on a scintillator to absorb reflected, scattered light from the scintillator and prevent that scattered light from reaching the imaging device. However, while improving resolution of the image, the sensitivity of the device is reduced because of the absorption of light. 
     Extra mural absorption materials such as glasses have been used to absorb stray light from optical fibers. However, typically, extra mural absorption glass is introduced as separate fibers into a fiber bundle (for example, every n th  fiber is a substituted EMA fiber, where n is a number &gt;&gt;1). Such technique provides a statistical level of absorbed light, typically less than 10%, but does not provide the type of selective light absorption required in electron microscopy. 
     Accordingly, the need still exists in the art for an apparatus that will improve the ultimate resolution of image sensors used to record images while not reducing the sensitivity of the apparatus. 
     SUMMARY OF THE INVENTION 
     The present invention meets that need by providing a resolution enhancement device which utilizes either high extra-mural absorbent optical fibers in the transfer optic, and/or which uses a transfer optic which is bonded to the scintillator without the use of any external glues or adhesives. The device provides improved resolution of electron images, such as for example from electron microscopes, while not reducing the sensitivity of the apparatus. This enables the device to be used to observe and image objects that generate only weak light intensity but which are positioned near other objects which generate stronger light intensity. 
     In accordance with one embodiment of the invention, a resolution enhancement device is provided and includes an imaging sensor adapted to receive and record a light image, a scintillator adapted to convert an electron image into a light image, and a transfer optic associated with the scintillator and the imaging sensor for transferring the light image from the scintillator to the imaging sensor. The transfer optic comprises at least one optical fiber including a layer of cladding material, the at least one optical fiber being oriented lengthwise with respect to an optical axis of the device. The at least one optical fiber includes a layer of light absorptive material on the layer of cladding material which attenuates at least a portion of off-axis light entering the transfer optic. By “off-axis” light, it is meant light that enters the transfer optic at an angle greater than the critical angle. Preferably, the transfer optic comprises multiple optical fibers packed in an array, typically a hexagonal array. 
     In a preferred embodiment of the invention, the scintillator comprises a layer that includes a first surface for receiving an electron image and a second surface adjacent the transfer optic. The resolution enhancement device includes a light reflective layer positioned on the first surface of the scintillator. The imaging sensor is preferably a charge-coupled device and the transfer optic is a fiber optic plate. The scintillator may be any scintillator material that has found use in this art including single crystal yttrium-aluminum-garnet as well as coatings of particulate phosphors. Preferably, the cladding material and the light absorptive layer have a difference in refractive indices of less than about 0.1. 
     A preferred environment for the present invention is in an electron microscope having a projection chamber through which an electron beam forming an electron image and/or diffraction pattern traverse. Such an apparatus includes a scintillator located in the path of the electron beam for converting the electron image into a light image, and an imaging sensor positioned to receive and record the light image. The apparatus further includes the transfer optic associated with the scintillator for transferring the optical image to the imaging sensor as described previously. 
     In a preferred embodiment, the transfer optic and the scintillator are bonded to one another in the absence of a bonding agent such as a glue or other adhesive. Such a glue layer increases light scattering due to refractive index mismatches at the scintillator/glue and glue/transfer optic interfaces. The present invention eliminates such a glue layer and instead directly bonds the scintillator and transfer optic to one another. In a preferred form, the transfer optic and the scintillator are bonded using optical contacting of the respective surfaces followed by heat treatment to form a virtually defect-free bond interface without the need for glues or other bonding agents. Also, preferably, the transfer optic and the scintillator have refractive indices that differ by less than about 0.1. This embodiment also finds use in an electron microscope. 
     In a further embodiment of the invention, a resolution enhancement device is provided and includes an imaging sensor adapted to receive and record a light image, a scintillator adapted to convert an electron image into a light image, and a transfer optic associated with the scintillator and the imaging sensor for transferring the light image from the scintillator to the imaging sensor. The transfer optic and the scintillator are bonded together by optical contacting and subsequent heat treatment. The transfer optic comprises at least one optical fiber including a layer of cladding material, with the at least one optical fiber being oriented lengthwise with respect to an optical axis of the device. The at least one optical fiber includes a layer of light absorptive material on the layer of cladding material which attenuates at least a portion of off-axis light entering the transfer optic. 
     In yet another embodiment, an apparatus for improving the resolution of electron images is provided and includes an electron beam forming an electron image and a scintillator located in the path of the electron beam for converting the electron image into a light image. A light reflective layer is positioned between the scintillator and the source of the electron beam, and an imaging sensor is positioned to receive and record the light image. The apparatus further includes a transfer optic associated with the scintillator for transferring the optical image to the imaging sensor, the transfer optic and the scintillator being bonded to one another in the absence of a bonding agent. The transfer optic comprises at least one optical fiber including a layer of cladding material, the at least one optical fiber is oriented lengthwise with respect to an optical axis of the apparatus. The at least one optical fiber includes a layer of light absorptive material on the layer of cladding material which attenuates at least a portion of off-axis light entering the transfer optic. Preferably, the light reflective layer comprises an electrically conductive material such as, for example, aluminum. 
     In yet a further embodiment, an electron microscope is provided and includes a projection chamber through which an electron beam forming an electron image and/or diffraction pattern traverses. The microscope includes an apparatus for improving the resolution of images produced by the electron microscope and is positioned in the projection chamber to intercept the electron beam. The resolution enhancement apparatus includes a scintillator located in the path of the electron beam for converting the electron image into a light image, a light reflective layer positioned between the scintillator and the source of the electron beam, and an imaging sensor positioned to receive and record the light image. The apparatus further includes a transfer optic associated with the scintillator for transferring the optical image to the imaging sensor. The transfer optic comprises at least one optical fiber including a layer of cladding material, the at least one optical fiber is oriented lengthwise with respect to an optical axis of the apparatus and includes a layer of light absorptive material on the layer of cladding material which attenuates at least a portion of off-axis light entering the transfer optic. 
     Accordingly, it is a feature of the present invention to provide resolution enhancement for image sensors that are used to record images from electron microscopes while not compromising or degrading the sensitivity of the apparatus. This, and other features and advantages of the present invention, will become apparent from the following detailed description, the accompanying drawings, and the appended claims. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an enlarged schematic cross-sectional view of the apparatus of the present invention; 
     FIG. 2 is a schematic cross-sectional view of the apparatus of the present invention positioned in the projection chamber of an electron microscope; 
     FIG. 3 is a cross-sectional view of an exemplary optical fiber, including cladding and EMA layer; 
     FIG. 4 is a cross-sectional view of a portion of a hexagonally stacked bundle of optical fibers; and 
     FIG. 5 is an enlarged cross-sectional view of a portion of a hexagonally packed bundle of optical fibers that have been stacked, pressed, and drawn. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, the resolution enhancement apparatus of the present invention is illustrated. The resolution enhancement device, generally indicated at  10 , includes scintillator  12  supported by a transfer optic such as fiber optic plate  14 . In a preferred form scintillator  12  comprises a single crystal such as a yttrium-aluminum-garnet crystal. Such crystals can be manufactured to a thickness of from about 5 to 50 μm. However, it should be recognized that other scintillators may be used including polycrystalline materials and particulate materials. For example, a powdered phosphor may be coated onto the fiber optic plate to form the scintillator. Generally, particle sizes for the phosphor should be 1 μm or less, and the coating applied to a thickness of between about 1 to 25 μm to minimize random electron and light scatter from the particles. 
     As the transfer optic, a fiber optic plate is provided which includes stacked glass fibers which are clad on their surfaces with a cladding glass or other material. Additionally, in the present invention, a layer of light absorptive material overlies the layer of cladding. FIGS. 3-5 (which are not drawn to scale) schematically illustrate, in cross-section, the general construction of such optical fibers. There, an optical fiber core  100  has a layer of cladding material  102  thereon. Preferably, optical fiber  100  is a glass having a relatively high refractive index (typically at least about 1.8). This provides a relatively close match with the refractive index of single crystal YAG scintillators (RI approx. 1.83). Preferably, the difference in refractive indices between the core glass and the scintillator is less than about 0.1. This reduces light refraction and scattering at the optical fiber/scintillator interface. Moreover, the relatively high refractive index of the core glass provides the refractive index mismatch needed with the cladding layer to provide a high numerical aperture fiber (≧0.8). Numerical aperture is defined as the sine of the half angle at which light (in air) enters a fiber, propagates along its length, and exits at the other end. Numerical aperture is a function of the refractive indices of the core glass and cladding material. Cladding layer  102  is also preferably a glass such as a boro-silicate glass. Typically, cladding glass  102  has a refractive index of about 1.5. 
     Overlying cladding material  102  is a layer of a light (optically) absorptive material  104 . Preferably, light absorptive material  104  is a high extra mural material such as a dark colored glass (most preferably black) that is designed to substantially attenuate any high angle, off-axis light which may be scattered from adjacent optical fibers. Such glasses are also typically boro-silicate glasses having refractive indices of approximately 1.5. The glasses may be colored using techniques which are known, such as, for example, adding small amounts of certain metals to the glass composition. 
     Generally, the cladding material  102  and the light absorptive material  104  have slightly lower melting points than the core glass fiber so that a fiber bundle can be formed and then heated to soften the cladding and light absorptive layers to produce a fused fiber bundle. As shown schematically in FIG. 4, groups of optical fibers are gathered together to form, typically, hexagonally stacked bundles. Upon heating, the bundles of fibers may be drawn and pressed so that the bundles assume a more hexagonal shape as shown in FIG.  5 . The pressed, drawn, and stacked fiber bundles are then consolidated into a fiber optic plate  14 . The fiber bundles may also be cut one or more times during fabrication. As shown, the fibers in the fiber optic plates are oriented lengthwise with respect to an optical axis of the device. Typically, such fiber optic plates have a thickness on the order of a few millimeters and diameters up to about 40 mm. Individual fibers have a diameter of typically less than about 10 μm. The fiber optic plate thus has parallel oriented fibers that transmit light in an ordered fashion such that an image at one end is transferred fiber by fiber (pixel by pixel) to the other end. 
     Referring back again to FIG. 2, scintillator  12  optionally includes thereon a light reflective layer  16  which acts to reflect scattered light back through the transfer optic to improve the sensitivity of the apparatus. Reflective layer  16  may comprise a thin layer of a conductive metal such as aluminum that is transparent to electrons. The reflective layer thus may also perform the function of preventing the buildup of electrical charges on the apparatus which could discharge and cause arcing. Additionally, light reflective layer  16  is opaque to external light sources and prevents such light from entering the apparatus. 
     To further improve the resolution capabilities and sensitivity of the apparatus, scintillator  12  and fiber optic plate  14  are secured together without the use of a conventional bonding agent such as an adhesive or glue. Rather, scintillator  12  and fiber optic plate  14  are bonded through the use of an optical contacting technique and subsequent heat treatment as taught by Meissner, U.S. Pat. No. 5,846,638, the subject matter of which is incorporated by reference herein. 
     It is desirable that the transfer optic transfers the maximum amount of light from the scintillator to the imaging sensor. With a glue or other bonding agent, there will be some portion of the light which is reflected at each interface (scintillator/glue and glue/transfer optic) resulting in an increase in background “noise” (i.e., stray light). Providing a bonding agent-free interface increases the sensitivity of the apparatus while also improving resolution because there is less reflection and light scatter. 
     When combined with the light absorptive extra mural layer, high angle light entering each optical fiber and light not hitting the center of the fiber (and which gets scattered as high angle light) are substantially attenuated (absorbed), thereby also improving image resolution. High angle light is light that enters the transfer optic at an angle greater than the critical angle. 
     Referring back to FIG. 1, fiber optic plate  14  may be optically coupled to an imaging sensor  20  using a fluid oil  18 . Such an oil, selected to have an index of refraction the same as or very close to that of the glass in fiber optic plate  14 , improves the transmission of light between fiber optic plate  14  and imaging sensor  20 . Further, as imaging sensor  20  will be operated at relatively low temperatures, as described in greater detail below, the coupling oil should have a low freezing point so that it will remain fluid at temperatures down to about −40° C. 
     Referring now to FIG. 2, a schematic view of a typical use of the present invention is shown in which an imaging device such as a charge-coupled device (CCD) camera  40  is mounted on the projection chamber  42  of a transmission electron microscope (TEM). As will be appreciated, the apparatus of the present invention may also find use in a scanning electron microscope (SEM), or a scanning, transmission electron microscope (STEM) as well. Typically, the projection chamber is attached to the end of an optical column of a TEM and houses a viewing screen  44  which is either lowered into an observation position or raised into a position in which it does not intercept electron beam  46  that is projected into the chamber. The projection chamber may also house a film magazine comprising a transport mechanism (not shown) which inserts a sheet of photographic film  48  into an exposure position and returns the sheet into the magazine after exposure. 
     The typical projection chamber further has several ports suitable for attaching an imaging device such as a camera, one of which is usually situated at the bottom of the chamber. The chamber is normally evacuated via a vacuum pipe  50  leading to a gate valve  52  which can either open or close the chamber to a high vacuum (e.g., 10 −6  torr) pump  54 . The gate valve in most modern TEMs is controlled pneumatically via two inlets  56  and  58  such that introduction of pressurized air into one inlet causes the valve to open, and the introduction of pressurized air into the other inlet causes the valve to close. 
     An electron beam  46  forming an electron image or diffraction pattern from a specimen in the microscope traverses the projection chamber  42 . Camera  40  includes resolution enhancement device  10  (shown in enlarged cross-section in FIG.  1 ). Device  10  includes a scintillator  12  that converts the electron image into a light image. Scintillator  12  is supported on a transfer optic such as fiber optic plate  14 . By light image, it is generally meant light in the visible spectrum, although there are some scintillation materials that can produce light outside of the visible spectrum in either the near infrared or in the ultraviolet regions of the spectrum. It is within the scope of the present invention to use scintillator materials that produce images in the infrared, visible, and/or ultraviolet portion of the spectrum. 
     Fiber optic plate  14  is optically coupled to an imaging sensor such as a two-dimensional charge-coupled device (CCD) sensor  20  with an optically-coupling oil layer  18 . Such CCD sensors are commercially available from several manufacturers including Kodak, Ford, Scientific Imaging Technologies (SITe), Hamamatsu, Thomson CSF, and English Electric Valve Ltd. Preferred solid-state imaging devices are scientific grade CCDs whose imaging areas comprise 1024×1024 or more pixels. However, it should be appreciated that any imaging device that is capable of capturing a light image and producing an electronic signal may be utilized including a cathode ray television tube. 
     The preferred CCD should be operated cold to keep its dark current small enough so that the noise in the dark current accumulated during a typical exposure does not limit the performance of the camera. The typical exposure in an electron microscope is from about 1 to 20 seconds. Maintaining the CCD at a temperature of about −25° to about −40° C. is typically sufficiently low for the accumulated dark current to be acceptably small at exposure times of up to about 1 minute. Such a temperature is conventionally achieved using a thermoelectric cooling device (not shown), whose cold side may be in contact with the imaging sensor  20 . 
     The CCD is connected to an external electronics unit  60  through a vacuum feed-through  62  that transfers the captured images to the memory of a digital computer  64 . The images may be displayed on a view screen  66 , such as a CRT, attached to the computer. For the present invention, the images may be digitized and then displayed using Digital/Micrograph software commercially available from Gatan, Inc., Pleasanton, Calif. Other details of operation of the apparatus are set forth in commonly-owned U.S. Pat. Nos. 5,635,720 and 5,065,029, the disclosures of which are incorporated by reference. 
     Referring again to FIG. 2, in operation, an electron image or diffraction pattern  46  from a sample (not shown) traverses projection chamber  42  and impinges on scintillator  12 . Electrons in the beam that collide with the scintillation material produce corresponding light photons that travel towards fiber optic plate  14 . Light which is randomly scattered, reflected internally within scintillator  12 , or deflected laterally and reflected is substantially attenuated by light absorptive layers  104  on the optical fibers. 
     The light image is then directed to imaging sensor  20  through the transfer optic, fiber optic plate  14 . Once the image impinges on sensor  20 , it is detected and then displayed on view screen  66  of digital computer  64 . Because substantially all of the laterally scattered light has been absorbed previously, the image resolution is enhanced. 
     While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes in the methods and apparatus disclosed herein may be made without departing from the scope of the invention, which is defined in the appended claims.