Abstract:
A side-entry specimen holder for transmission electron microscopy is provided. The specimen holder is capable of rotating a specimen and tilting it in two axes. The specimen, when mounted in the holder, can be tilted in the plus/minus direction of the X-axis, the plus/minus direction of the Y-axis, and simultaneously have the ability of 360° rotation in the axis of the electron beam to permit alignment of microstructural features of the specimen for optimal viewing and analysis.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This patent application claims the benefit of U.S. Provisional Application Ser. No. 60/096,215, filed Aug. 12, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to specimen holders for transmission electron microscopes, and more particularly to side-entry specimen holders which are tiltable about two axes and rotatable about the electron beam axis. 
     In recent years, analytical specimen holders used in transmission electron microscopy (TEM) have become more widely used because of their simple and reliable mechanisms; however, as the technology of new materials advances so do requirements for TEM specimen viewing and specimen positioning. Conventional side entry TEM specimen holders have been developed which are capable of only a single axis tilt, which is the simplest type of specimen holder. In this form, the specimen is fixed to the specimen holder tip and has no means of independent motion. Tilting of the specimen in this axis (i.e., the longitudinal axis of the specimen holder) is accomplished by tilting the complete specimen holder about this axis in either direction. As technology has improved, the need for a second axis tilt (i.e., the axis perpendicular to the longitudinal axis of the specimen holder) of the specimen in addition to the first axis tilt became apparent and led to the development of double tilt specimen holders which are commercially available today. Double tilt specimen holders combine the two tilt axes as described above with the second axis tilt in either direction typically accomplished by movement of a drive rod mechanically coupled to a cradle supported by two pivot pins (axles). 
     TEM studies involving selected-area diffraction, stereo imaging, diffraction-contrast analysis, domain structure analysis and electron tomography all require precise of the specimen orientation. Single-tilt and double tilt holders are normally inadequate for these applications since the specimen orientation requires both tilting and rotation. Planar features such as interfaces and grain boundaries within a TEM specimen are most easily studied and analyzed if the specimen is first rotated until the planar feature is aligned parallel to the main tilt axis of the specimen holder. Such single tilt and rotate holders are also commercially available such as the Model 650 Single Tilt Rotate Specimen Holder available from Gatan, Inc. of Pleasanton, Calif. Stereological studies of crystalline materials have been hindered in that single tilt and rotate holders were limiting in use due to lack of a second axis tilt mechanism. Such stereological studies are complicated by the contrast change that occurs when a specimen is tilted. To minimize this effect, the specimen can first be rotated so that the g vector principally responsible for the contrast is aligned parallel to the tilt axis. The specimen can then be tilted without substantially altering the diffraction-contrast conditions. However, this involves manually changing the position of the specimen while outside of the transmission electron microscope (i.e., requires specimen unloading and re-loading) to attain proper alignment. The ability to tilt in two axes and rotate a specimen containing a feature (interface) to align the feature in the direction of various microscope detectors can facilitate feature analysis in the TEM. Moreover, the use of CCD cameras for TEM digital imaging makes it desirable to be able to rotate the specimen in a specific direction (visual image alignment) for a given crystallographic condition. 
     Thus, there remains a need in this art for a specimen holder which is able not only to rotate a specimen to obtain any desirable direction, but also to be able to tilt such a specimen in two tilt axes while viewing the specimen in a TEM. 
     SUMMARY OF THE INVENTION 
     The present invention meets that need by providing a side-entry specimen holder for transmission electron microscopy capable of rotating a specimen and tilting it in two axes. The specimen, when mounted in the holder, can be tilted in the plus/minus direction of the first (longitudinal) axis, the plus/minus direction of the second (normal to longitudinal) axis, and simultaneously have the ability of continuous 360° rotation in the axis of the electron beam to permit alignment while viewing the specimen in the TEM. Further, because of the provision for rotation of the specimen, the viewing axis of the specimen may be aligned to the first or second axis, permitting very high tilt angles. 
     In accordance with one aspect of the present invention, a side-entry specimen holder for a transmission electron microscope is provided and includes a specimen holder having a specimen cradle, a cradle frame, and a drive shaft connected to the specimen cradle and frame; a first tilt mechanism for tilting the specimen holder about a first longitudinal axis; a second tilt mechanism for tilting the specimen cradle about a second axis normal to the first axis; and a rotation mechanism for rotating the specimen cradle. In a preferred form, tilting in the first axis is accomplished by tilting the entire specimen holder. A preferred first tilt mechanism comprises a drive for rotating the support arm about its longitudinal axis. The second tilt mechanism comprises a drive for moving a drive shaft which is linked through a spherical bearing to a frame housing the specimen cradle. The frame for the specimen cradle also preferably includes a support arm (barrel). 
     The rotation mechanism may be any of a number of mechanisms which are described below and may be endless (full 360°) or limited in rotation. However, a preferred rotation mechanism comprises a pinion drive gear mating with a ring gear. The specimen cradle includes the ring gear about its periphery, which is engaged and driven by the pinion gear to provide for the rotation of the cradle. 
     In accordance with another aspect of the invention, a method for analyzing one or more physical features of a specimen in a transmission electron microscope is provided and includes the steps of mounting a specimen to be analyzed in a holder capable of tilting in two axes and rotation, inserting the holder and specimen into the path of an electron beam from the transmission electron microscope, rotating the specimen in the holder to align a physical feature of interest relative to a first tilt axis, and tilting the holder in both of the first and second axes to analyze said physical feature. The physical feature of interest may include microstructural features selected from the group consisting of grain boundaries, precipitates, and interfaces in crystalline materials. Optionally, the method includes the step of analyzing the specimen when tilted about the first axis, rotating the specimen in the holder while in the transmission electron microscope to align the physical feature of the specimen to the second axis, tilting the specimen about the second axis, and performing further analysis of the specimen. Another aspect of the invention provides a method for analyzing one or more physical features of a specimen in a transmission electron microscope and includes the steps of mounting a specimen to be analyzed in a holder capable of tilting in two axes and rotation, inserting the holder and specimen into the path of an electron beam from the transmission electron microscope (TEM), rotating the specimen in the holder to align a physical feature of interest with a TEM detector, and tilting the holder in both of the first and second axes to analyze the physical feature. 
     Accordingly, it is a feature of the present invention to provide a side-entry specimen holder for transmission electron microscopy capable of rotating a specimen and tilting it in two axes. This, and other features and advantages of the present invention, will become apparent from the following detailed description and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic perspective view of the distal end of the specimen holder illustrating the ability of the holder to tilt in two axes (longitudinal and normal to longitudinal) and rotate about the axis of the electron beam (Z-axis); 
     FIG. 2 is a side view, in section, of one embodiment of the specimen holder; 
     FIG. 3 is a top view, in section, of the distal end of one embodiment of the specimen holder; 
     FIG. 4 is a sectional view taken along line  4 — 4  in FIG. 2; 
     FIG. 5 is a side view of the specimen holder illustrating the operation of the tilt mechanism about the second axis; 
     FIG. 6 is a top view of the specimen cradle and a preferred gear drive mechanism; 
     FIG. 7 is a side view, in section, of the specimen cradle and the preferred gear drive mechanism of FIG. 6; 
     FIG. 8 is a top view of the specimen cradle and a friction drive mechanism; 
     FIG. 9 is a side view, in section, of the specimen cradle and the friction drive mechanism of FIG. 8; 
     FIG. 10 is a top view of the specimen cradle and a belt drive mechanism; 
     FIG. 11 is a partial side view, in section, of the belt drive mechanism of FIG. 10; 
     FIG. 12 is a side view, in section, of the specimen cradle and the belt drive mechanism of FIG. 10; 
     FIG. 13 is a top view of the specimen cradle and a spherical drive mechanism; 
     FIG. 14 is a side view, in section, of the specimen cradle and the spherical drive mechanism of FIG. 13; 
     FIG. 15 is a top view of the specimen cradle and a ribbon drive mechanism; and 
     FIG. 16 is a side view, in section, of another embodiment of the specimen holder. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be described in detail with respect to its preferred embodiment which is as a side-entry specimen holder for a transmission electron microscope. It will be apparent that the specimen holder may be adapted to operate in other types of electron and ion microscopes and related instruments. Further, as is conventional in this art, the specimen holder of the present invention may be modified to include either heating or cooling (e.g., cryogenic) capabilities. The specimen holder is designed to be inserted into the column of an electron microscope so that the specimen carried thereon may be aligned with an electron beam which traverses the column. 
     As shown in FIG. 1, a distal end or tip of specimen holder  10  includes a specimen cradle  12  contained in a frame  48 . As is conventional, the specimen  13 , which has been previously prepared, is mounted in the specimen holder tip  15  secured in cradle  12 . For purposes of this invention, and as illustrated in FIG. 1, the X-, Y-, and Z-axes of movement of the specimen holder are defined as follows. The X-axis refers to the longitudinal axis of specimen holder  10 , with movement toward the distal end or tip of specimen holder  10  being defined as the positive (+) direction and movement in the opposite direction being defined as the minus or negative (−) direction. Tilting of the specimen holder may occur in a first tilt axis  18  defined as the α-tilt (and also referred to as X-axis tilt) in either a clockwise (+) or counterclockwise (−) direction as shown. 
     The Y-axis is defined as the axis of the specimen holder which is perpendicular to the longitudinal axis, with minus and positive conventions as shown. Tilting of the specimen cradle may also occur in a second tilt axis  22 , defined as the Stilt (and also referred to as Y-axis tilt) in either a clockwise (+) or counterclockwise (−) direction. Finally, specimen cradle  12  may be rotated about the vertical (Z-) axis in either the positive (+) clockwise direction or minus (−) counterclockwise direction as shown. 
     Referring now to FIGS. 2-5, a first embodiment of the invention is illustrated wherein specimen holder  10  includes a specimen cradle  12  which is contained within frame  48  and which is connected, through appropriate linkages  30 ,  32 ,  34 , to a support arm  14 . As shown, a tilting of the entire specimen holder in either a clockwise (+) or counterclockwise (−) direction causes frame  48  and specimen cradle  12  to tilt about the X-axis. The linkages are such that any rotation, such as by rotating knob  28  or a drive mechanism (not shown), results in a direct and corresponding rotation of specimen cradle  12  in the direction of the rotation. 
     Tilting of frame  48  and cradle  12  in the Y-axis is accomplished by rotation of tilt screw knob  36 . As shown, linkage  30  comprises a spherical bearing and a bracket  37 . Linkage  30  is biased against drive shaft  26  by a spring  42 . Rotation of knob  36  in one direction causes the opposite end of the screw to bear against and move drive shaft  26  upwardly from the position shown in FIG.  2 . Conversely, rotation of the knob in the opposite direction causes the end of the screw to retract away from drive shaft  26 , permitting spring  42  to bias linkage  30  downward and move drive shaft  26  in the same downward direction. 
     Linkage  32  comprises a spherical bearing  44  and O-ring vacuum seal  46  which permit drive shaft  26  to pivot within a widened opening in support arm housing  14  as shown. Vertical movement of drive shaft  26 , either upwardly or downwardly, causes linkage  34  to act on frame  48  as shown in FIG.  5 . That is, linkage  34  includes a spherical bearing  50  having a pin  52  therein. The opposite ends of pin  52  are captured in slots  56 , 58  of clevis  54  which is carried on the distal end of drive shaft  26 . When the distal end of drive shaft  26  moves upwardly, frame  48  tilts downwardly (in the minus (−) direction), and vice versa as best shown in FIG.  5 . 
     As shown in FIG. 3, frame  48  is mounted in specimen tip  15  using axles  60  which are captured in corresponding holes. This permits frame  48  to tilt about the Y-axis. The rotation of knob  36  may be calibrated in a known manner to provide predictable and accurate angles of tilt of the specimen in the Y-axis. 
     Rotation of the specimen is accomplished by providing a motion exchange mechanism  62  which is linked to specimen cradle  12 . Motion exchange mechanism  62  may comprise a gear mechanism, a friction drive, a belt drive, a cam ribbon mechanism, a spherical drive, or other suitable device to cause cradle  12  to rotate about its vertical axis. A preferred embodiment of the motion exchange mechanism is shown in FIGS. 6 and 7. Specimen cradle  12  includes ring gear  64  which is engaged and driven by pinion gear  66  coupled through linkage  34 . A groove  68  and spring loaded balls serve to stabilize the rotation of cradle  12 . 
     An embodiment of the invention which uses a friction drive for the motion exchange mechanism is shown in FIGS. 8 and 9. Specimen cradle  12  includes a rim  53  which is engaged by a friction plate  61  to rotate the cradle. Guide rollers  70  support cradle  12 . The specimen cradle is tilted by linkage  34  pivoting the frame  48  on axles  60 . 
     An embodiment of the invention which uses a belt drive for the motion exchange mechanism is shown in FIGS. 10,  11 , and  12 . Specimen cradle  12  includes a groove that is engaged and rotated by belt  65  which may comprise a flexible material such as metal or synthetic materials. Belt  65  is in turn driven by pulley  63 . Pulley guides  67  change the direction of the belt coming off of pulley  63 . The specimen cradle  12  is tilted by linkage  34  pivoting the frame  48  on axles  60 . 
     An embodiment of the invention which uses a spherical drive for the motion exchange mechanism is shown in FIGS. 13 and 14. Specimen cradle  12  is tilted and rotated by linkage  34 . Pivoting the linkage  34  up and down provides the Y-axis tilt while pivoting side-to-side provides rotation for the cradle. Specimen cradle  12  is retained within a spherically-shaped guide bushing  69 . Coupling between cradle  12  and linkage  34  is by ball-and-socket joint  59 . Specimen cradle  12  pivots and rotates about two pins (axles)  60  fixed into specimen tip  15  and ride in a circular groove  57  in the circumference of cradle  12 . 
     An embodiment of the invention which uses a ribbon drive for the motion exchange mechanism is shown in FIG.  15 . Rotation of specimen cradle  12  is provided by a ribbon  53  passing around the cradle at the tip end of holder  15  and around a drive pulley at the opposite distal end of the holder (not shown). Rollers  70  retain the specimen cradle  12  in position. Rotating drive shaft  26  having an offset spherical ball contact  55  at its end tilts specimen cradle  12 . Ball contact  55  moves within a linear slot in frame  48  allowing the cradle  12  to tilt about axles  60  and provide Y-axis tilt. In this embodiment, drive shaft  26  does not provide rotation about the Z-axis to cradle  12 . 
     Another embodiment of the invention is shown in FIG. 16, where like elements are represented by like reference numerals. In this embodiment, rotation knob  28  is linked to a drive mechanism  72  to provide motorized rotation of the specimen cradle about the Z-axis. A universal joint and clevis coupling, generally indicated at  74  and  40 , respectively, is positioned between drive  72  and linkage  30 . A motor drive  73  operates to rotate knob  36  to provide tilt along the Y-axis. 
     In operation, after mounting the specimen in the holder, the holder is inserted into a column of an electron microscope where the position of the specimen is aligned with the electron beam. Where specific planar features of the specimen are desired to be observed and imaged, the specimen is first rotated, using motion exchange mechanism  62 , so that the interface orientation in the specimen, such as the g vector which is principally responsible for contrast, is aligned parallel to the tilt axis. The specimen may then be studied from numerous angles by utilizing the X- and Y-axis tilt mechanisms, and specimen rotation, respectively. 
     Typically, the X-axis tilt angle range may be greater than ±80°, while the Y-axis tilt angle range may be up to about ±60°. Thus, the specimen can be tilted in both the plus/minus direction of the X-axis as well as the plus/minus direction of the Y-axis. Further, because of the ability to rotate the specimen through 360°, the viewing axis of the specimen may be aligned to the X- or Y-axis, permitting very high tilt angles. 
     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.