Abstract:
A method and apparatus allowing for simultaneous direct viewing and electronic capture of images in an electron microscope (TEM). For this, the usual opaque direct viewing plate in the TEM is replaced in form and in function by a two-sided direct viewing plate including at least one scintillator. This plate produces light emissions from both its upper and lower surfaces, which allows an electronic camera below the plate to be used simultaneously with direct human viewing from above the plate. The method and apparatus are also compatible with traditional permanent image recording units that are often desired in such microscopes.

Description:
[0001]    This invention relates to an electron microscope. It relates especially to a method and apparatus allowing simultaneous direct observation and electronic capture of scintillation images produced in an electron microscope. 
       BACKGROUND OF THE INVENTION 
       [0002]    A transmission electron microscope (TEM) creates images by generating an electron beam that penetrates a very thin specimen. The projected image of electron intensity corresponds to the specimen structure. In order to reduce that intensity image to visible form, the image must be converted to a signal. In a TEM, that signal is generated by impact of the electron pattern onto a scintillator, a photographic emulsion or solid-state imaging plate, or onto the photosensitive element of an electronic camera. A scintillator is normally used for direct, live human viewing or as the input to an electronic camera, which can provide both real-time and recorded images. Photo-emulsions and imaging plates produce recorded images only. 
         [0003]      FIG. 1  shows a conventional TEM  10  supported on a table T. TEM  10  includes a housing  12  having an upper cylindrical column or section  12   a , which extends down to an enlarged lower section  12   b  that defines an interior observation chamber  14  at the foot of the column. An operator sitting in front of table T facing housing  12  can view chamber  14  through an x-ray shielding observation window  16  in housing section  12   b . Housing  12  is maintained under a relatively high vacuum and an electron gun  18  at the top of section  12   a  generates an electron beam e within the housing. 
         [0004]    Under the influence of electromagnetic lenses (not shown) in column  12   a , beam e is transmitted along an optical axis A through a specimen S removably positioned by a support  20  in column  12   a . The beam e, now including an electron pattern corresponding to the structure of a selected area of specimen S, is projected onto a scintillator  22   a  on the upper surface of an opaque direct viewing plate  22  centered on axis A within chamber  14 . Invariably the usual plate comprises a relatively thick flat metal substrate whose upper surface is coated with a scintillator such as phosphorus. Typically, plate  22  has a surface area that is large enough to be watched comfortably by an operator looking at the plate through window  16 . Such direct viewing plates typically range from 25 to 150 mm in diameter. 
         [0005]    In response to the impinging electrons, plate  22  produces a visible image at its upper surface corresponding to the electron pattern which image is visible to the operator looking through window  16 . Typically, he/she views plate  22  at an angle of about 45°. 
         [0006]    Usually control units  24  and  26  are positioned on table T on opposite sides of housing  12 . These units control the TEM, enabling the operator to shift the electron beam e relative to specimen S (or vice versa), so that various areas of the specimen may be viewed on plate  22 . Thus, while directly viewing the image on plate  22  and manipulating various controls C on units  24  and  26 , respectively, using both hands the operator may align the electron beam and survey the specimen S in a very ergonomic and efficient manner. 
         [0007]    Aside from ease of use, many TEM users prefer to operate the TEM while directly viewing the specimen image on plate  22  because TEMs have historically been optimized for this type of observation so that the visible image on the plate  22  is characterized by relatively high resolution, wide dynamic range of response and low apparent noise. 
         [0008]    Many TEMs, particularly those built before 2007, usually also include a permanent image recording unit  32  below the observation chamber  14 . Since the electron beam e is projected into the recording unit  32 , that unit must be maintained under the same vacuum as housing  12  when the TEM is in operation. The unit  32  may temporarily position a recording medium such as film F from the unit&#39;s supply tray  32   a  to a fixed location on axis A as shown before transferring that film to a storage tray  32   b . When the specimen S has been surveyed and the desired area thereof has been imaged on plate  22 , the plate may be swung up out of the way to the position shown at  22 ′ so that the electron beam e is projected onto and exposes the film F on axis A, thereby providing a permanent copy of that area of the specimen. 
         [0009]    Of course, the recording unit  32  may record images on other recording media such as conventional imaging plates instead of on film F. 
         [0010]    As alluded to above, some conventional TEMs also include means for producing real-time images corresponding to the specimen structure captured in the electron pattern in beam e. These images may be displayed on a monitor/recorder  28 , positioned on table T next to housing  12 . The input signal to monitor  28  may be provided by a conventional electronic camera having a dedicated photosensitive screen which should be perpendicular to the optical axis of the camera to avoid distortion and to maintain high optical performance. This requirement for perpendicularity arises because a high numerical aperture is needed to provide the required sensitivity and resolution for electronic recording. The usual optical couplers and lenses in the camera have a depth of field in the order of only 20 μm so that a deviation from such perpendicularity of only a few degrees would be detrimental to the image focus. This is the main reason why an electronic camera cannot simply acquire pictures suitable for recording on the opaque viewing plate  22  through the observation window  16 ; i.e. as noted above, that window is oriented at a large angle (45°) with respect to the plate. 
         [0011]    In practice, then, the electronic camera that provides the signal to monitor  28  is often mounted in the side of housing column  12   a , above chamber  14  as shown at  34  in  FIG. 1 . The housings of many TEMs include a port  36  with a window  36   a  in the wall of column  12   a  for this purpose. In this event, the camera&#39;s photosensitive screen  38  may be at the upper surface of a prism  42  located directly opposite port  36  so that screen  38  is perpendicular to axis A. Prism  42  may be mounted to the armature  44   a  of a linear actuator  44  secured to the side of housing section  12   a  opposite the camera  34 . Under the control of a controller  46 , the actuator  44  may move the prism  42  to an extended position shown in solid lines in  FIG. 1  wherein the screen  38  is centered on axis A and any visible image thereon is reflected by prism  42  to camera  34  such that the reflected image is also perpendicular to the optical axis of the camera. In response to the incoming image from screen  38 , camera  34  provides a signal to monitor  28 , causing the monitor to display and/or record that image. 
         [0012]    The prism  42  may also be moved by actuator  44  to a retracted position shown in phantom in  FIG. 1 . In this position, the phosphor screen  38  is not impinged by beam e. Rather, the beam carries on to form a visible image on the direct viewing plate  22  in observation chamber  14 . Obviously, the two members  22  and  38  cannot be imaged by beam e simultaneously. 
         [0013]    Instead of, or in addition to mounting the electronic camera above the direct viewing plate  22  as described above, some conventional TEMs provide for such mounting on axis A below that plate. In the event that the TEM includes a permanent imagine recording unit such as unit  32  in  FIG. 1 , the camera may be mounted to the underside of unit  32  as shown at  52  in  FIG. 1 . For this purpose, the bottom wall of unit  32  may include a port  54  covered by a photosensitive screen  58  optically coupled to the camera by a lens device  59 . In such a TEM, means are provided for swinging the direct viewing plate  22  up and away from axis A as shown in phantom at  22 ′ in  FIG. 1 . When plate  22  is in this out-of-the-way position and no film F is positioned by unit  32  on axis A, the electron beam e may be projected directly onto the screen  58 . The screen thereupon produces a visible image corresponding to the electron beam image of specimen S which visible image is viewed by camera  52  and displayed on monitor  28 . Here again, the opaque direct viewing plate  22  and the camera screen  58  cannot be imaged at the same time by the electron beam e. 
         [0014]    Conventional electron microscopes such as those outlined above are described, for example, in U.S. Pat. Nos. 4,206,349; 4,739,399 and 5,013,915. Such TEMs are disadvantaged in several respects. First, as noted above, each electronic camera requires its own dedicated photosensitive screen which, when operational, must be located within the evacuated housing  12  on axis A. This requires that the camera and its screen be spaced well above or below the direct viewing plate  22  so as not to interfere with the exposure of the direct viewing plate by electron beam e. When the camera screen is located above the direct viewing plate  22  as at  38  in  FIG. 1 , that screen must be movable or retractable from axis A so as not to interfere with the imaging of the direct viewing plate  22 . This necessitates the presence of the described mechanisms and x-ray-compatible shielding window which increase the overall cost and complexity of the microscope. 
         [0015]    On the other hand, if the photosensitive screen for the electronic camera is located below the direct viewing plate  22  as at  58  in  FIG. 1 , a mechanism must be provided in order to move that plate  22  away from axis A so that beam e can be projected onto the camera screen  58 , also adding to the cost and complexity of the instrument. This is especially true of TEMs which include a permanent recording unit such as unit  32  in  FIG. 1 . Because of the presence of the camera&#39;s screen  58 , each time a film F is retrieved from unit  32 , the entire housing  12  including unit  32  must be pumped down to the high vacuum necessary to subsequently operate the TEM. This also adds to the overall cost of the apparatus. 
         [0016]    Finally, it should be emphasized that the electromagnetic optics in a conventional TEM are designed specifically to optimize the image on the direct viewing plate  22  because this is the image that many operators prefer to view through window  16  while aligning the electron beam and surveying specimen S, even though such TEMs may include an electronic camera and associated display monitor. This is because, as noted above and as depicted in  FIG. 2 , when the camera&#39;s photosensitive screen is located above plate  22  as at  38  in  FIG. 2 , the visible image on that screen  38  produced by beam e is characterized by a lower resolution, e.g. 1-4 megapixels, and higher distortion as compared with the visible image produced on plate  22  which may have a resolution as high as 10-16 megapixels. On the other hand, when the electronic camera&#39;s scintillation screen is located below the direct viewing plate  22  as at  58  in  FIG. 2 , it is highly cropped as compared to the visible image on plate  22 . No wonder, then, that many operators prefer to directly view the image on plate  22  when surveying a specimen. 
       SUMMARY OF THE INVENTION 
       [0017]    Accordingly, the present invention aims to provide an electron microscope (TEM) including a direct viewing plate and an electronic camera which optimizes the performance of the camera while maintaining the ergonomic advantages of an operator being able to simultaneously observe a specimen image on the direct viewing plate of the TEM. 
         [0018]    Another object of the invention is to provide such a microscope which still allows for the operation of a permanent image recording unit found on many TEMs in use today. 
         [0019]    Another object of the invention is to provide a TEM of this type including an electronic camera that is able to capture an image that is located at the optimum design location in the microscope. 
         [0020]    A further object of the invention is to provide an electron microscope which allows an operator to observe an image on the TEM&#39;s direct viewing plate from above, while simultaneously allowing an electronic recording device to simultaneously capture the very same image from below that plate. 
         [0021]    A further object of the invention is to provide a method of converting a conventional electron microscope to a microscope having one or more of the above advantages. 
         [0022]    Other objects will, in part, be obvious and will, in part, appear hereinafter. The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others, and the apparatus embodying the features of construction, combination of elements and arrangement of parts which are adapted to effect such steps, all as exemplified in the following detailed description, and the scope of the invention will be indicated in the claims. 
         [0023]    Briefly, in accordance with our invention, a more or less conventional TEM is modified by substituting for the usual opaque direct one-sided viewing plate in the observation chamber of that TEM a two-sided direct viewing plate, meaning that it is observable from both sides. This allows the visible image formed on the upper surface of the plate by an electron beam to be viewed from above by an operator looking through the window of the TEM&#39;s observation chamber and simultaneously from below by an electronic recording device positioned below that plate. Resultantly, the performance of the electronic recording device may be optimized while still maintaining the ergonomic advantages of an operator being able to simultaneously observe the image on the direct viewing plate while surveying a specimen. As we shall see, the invention is even compatible with TEMs which include a permanent image recording unit located below the observation chamber. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    For a fuller understanding of the nature and objects of the invention, reference should be made to the following detailed description, taken in connection with the accompanying drawings, in which: 
           [0025]      FIG. 1 , already described, is an elevational view with parts in section showing a conventional electron microscope; 
           [0026]      FIG. 2  is a diagrammatic view showing the operation of the  FIG. 1  microscope; 
           [0027]      FIG. 3  is a diagrammatic view of an electron microscope incorporating the invention in a first configuration; 
           [0028]      FIG. 3A  is a scrap view of a modified direct viewing plate for use in our microscope; 
           [0029]      FIG. 4  is a view similar to  FIG. 3  of the microscope in a second configuration, and 
           [0030]      FIG. 5  is a similar view of the microscope in a third configuration. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0031]      FIG. 3  illustrates diagrammatically an electron microscope incorporating our invention. Since this microscope contains many of the same components described above in connection with  FIG. 1 , the same numeric identifiers are used to refer to those similar components in the description of  FIG. 3 . 
         [0032]    The  FIG. 3  TEM is similar to the  FIG. 1  instrument in that it includes a housing  12 , with a window  16  in a wall of the housing&#39;s observation chamber  14  that allows an operator O to view the interior of that chamber. An electron gun  18  at the top of housing  12  transmits an electron beam e along an optical axis A through a specimen S. However, the  FIG. 3  microscope differs markedly from conventional TEMs in that the electron beam e passing through specimen S is projected onto a direct viewing plate indicated at  60  which shows the TEM image from both sides of the plate. 
         [0033]    More particularly, plate  60  comprises a transparent substrate  60   a , e.g. of glass. The top surface of that substrate is covered by a transparent, electrically conductive coating, e.g. indium tin oxide, which constitutes a transparent electrode  60   b , which may be electrically grounded by way of the wall of housing  12 . Electrode  60   b  is, in turn, coated with a scintillator  60   c , such as phosphor P 20  or P 43 , depending on the desired wavelength of the light emissions from plate  60 . 
         [0034]      FIG. 3  shows another direct viewing plate embodiment  61  which may be used in lieu of plate  60 . 
         [0035]    Plate  61  is constructed as a composite of an upper later  61   a  and a lower layer  61   b  of phosphor separated by an electrically conductive, light-reflecting opaque film  61   c , which may be electrically grounded by way of the wall of housing  12 . This composite structure can be either self-supporting or it can be deposited on a glass substrate  61   d . The top phosphor layer  61   a  may be viewed from above while the bottom phosphor layer  61   b  is viewed from below, each layer being optimized for its individual purpose. 
         [0036]    The TEM embodiment depicted in  FIG. 3  happens not to include a permanent image recording unit such as unit  32  in  FIG. 1 . Rather, an electronic camera  62  is mounted to the bottom wall of housing  12  by way of a lens device  64  whose optical axis is coincident with axis A. A port  66  with a window  66   a  may be provided in the bottom wall of housing  12  for this purpose. The lens device  64  has a focal length that optically couples camera  62  to the scintillator  60   c  of plate  60 . 
         [0037]    When the  FIG. 3  TEM is in operation, the direct viewing plate  60  responds to the electron beam e by producing a visible image at its upper surface which corresponds to the structure of specimen S. Since plate  60  is transparent, the visible light emissions λ produced by the scintillator  60   c  may be seen from above the plate by an operator O looking through window  16 . Light emissions λ also pass through the transparent electrode  60   b  and the transparent substrate  60   a  and thus may be viewed from below the plate by camera  62 . 
         [0038]    In the case of plate  61 , both phosphor layers  61   a  and  61   b  will produce identical visible images of the specimen S in response to beam e so that the plate  61  as a whole may be viewed from above and below in the same way as plate  60 . 
         [0039]    The transparent direct viewing plate  60  or  61  is located at the “sweet spot” of the TEM. This is the location in the housing at which the field of view is relatively large, yet the magnification of the image projected onto plate  60  or  61  is high enough to operate the TEM at its highest resolution. Resultantly, the images captured by the camera and recorded by monitor  28  ( FIG. 1 ) have optimum resolution and sensitivity as compared with the camera images of prior TEMs developed on dedicated camera screens located above or below the direct viewing plate as in  FIG. 2 . Thus, the  FIG. 3  TEM offers the best of both worlds in that it allows the operator O to view the image on plate  60  or  61  while aligning beam e and surveying specimen S using the control units  24  and  26  shown in  FIG. 1 , while that very same image is being captured in real time by camera  62  for display and/or recording by monitor  28 . 
         [0040]      FIG. 4  shows a TEM embodiment in which an electronic camera indicated at  70  is fiberoptically coupled directly to the underside of a transparent direct viewing plate  72  which may be similar to plate  60  or  61 . In this case, a fiberoptic faceplate  74  is incorporated into the transparent substrate of viewing plate  72  and the camera comprises a CCD sensor  76  optically coupled to the faceplate. If necessary, sensor  76  may be cooled by a thermoelectric cooler (not shown) positioned flush against the underside of the sensor. When the electron beam e impinges on plate  72 , the scintillator  60   c  or  61   a  thereof produces a visible image which may be viewed from above through window  16 . That very same image or an identical one is coupled by the faceplate  74  to sensor  76  which thereupon produces a corresponding output signal. That output signal may be applied to a monitor such as monitor  28  in  FIG. 1  which will provide a real time display of that image for simultaneous viewing and/or recording by the operator. 
         [0041]    As described at the outset, many TEMs in use today include a permanent image recording unit such as the photographic recording unit  32  in  FIG. 1 . This is because many operators consider a photographic image to be the image “goal standard” in terms of the resolution and overall quality of the image, even surpassing the quality of the images captured by present-day electronic cameras. 
         [0042]      FIG. 5  illustrates a TEM incorporating our invention which also includes a permanent image recording unit  32  as in  FIG. 1  and, as in  FIG. 1 , has a direct viewing plate  80  which may be moved by a rotary actuator  82  from a position wherein the plate is centered on axis A as shown in  FIG. 1  to the position shown in  FIG. 5  wherein the plate is swung up away from axis A. Unlike the plate  22  in  FIG. 1 , however, the plate  80  in the  FIG. 5  TEM is transparent and may be substantially identical to plate  72  in  FIG. 4 , i.e. it has an electronic camera  70  fiberoptically coupled to the underside of the plate. When that plate is centered on axis A and exposed to the electron beam e, the operator may view the visible image on plate  80  from above while that very same image is captured by the electronic camera  70  at the underside of that plate for display on monitor  28  ( FIG. 1 ). On the other hand, when the direct viewing plate  80  is swung to the position shown in  FIG. 5 , the electron beam e may expose a film F on axis A in unit  32  to provide a permanent photographic quality copy of the image captured by the electron beam e. 
         [0043]    The  FIG. 5  TEM could also incorporate a direct viewing plate similar to plate  60  or  61  in  FIG. 3  and have an underlying electronic camera on axis A that is lens-coupled to that plate. A camera such as this is shown in phantom at  84  in  FIG. 5 , connected via a lens device  86  to a port  88  in the bottom wall of recording unit  32 . When the direct viewing plate is centered on axis A as shown in  FIG. 3  and no film F is present on axis A, camera  84  may capture the visible image produced on plate  84  by the electron beam e. 
         [0044]    In this case, since the camera does not have a dedicated photosensitive screen inside unit  32  as in  FIG. 1 , the interior of unit  32  may be partitioned off from housing  12  and a window provided in the partition so that camera  84  may view plate  60  through that window. Thus, a vacuum may be maintained in housing  12  when unit  32  is opened to remove or insert film F. 
         [0045]    Although our invention may be incorporated into newly manufactured TEMs, most existing TEMs may be modified to include the invention using a relatively simple conversion kit consisting of a transparent direct viewing plate, such as plate  60 , to replace the existing opaque viewing plate, e.g. plate  22  ( FIG. 1 ) and a lens device to replace the standard couplers, e.g. device  59  in  FIG. 1 , for coupling to the port  66  or  88  at the underside of housing  12 . That replacement device should be of the proper focal length to optically couple an electronic camera to the operative scintillator of the trans-parent plate  60 ,  61 ,  72  or  80 . 
         [0046]    It will thus be seen that the objects set forth above among those made apparent from the preceding description are efficiently attained and, since certain changes may be made in carrying out the above method and in the constructions set forth above without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. 
         [0047]    It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention described herein.