Abstract:
Apparatus having a magnetic lens configured to diverge an electron beam are useful in three-dimensional imaging using an electron microscope. The magnetic lens includes a body member having a core and defining a gap, and a winding surrounding a portion of the core. The body member and winding are configured such that an electrical current through the winding produces a magnetic field proximate to the gap.

Description:
RELATED APPLICATION 
       [0001]    This Application is a Divisional of U.S. application Ser. No. 11/926,791, filed Oct. 29, 2007 (allowed), which is commonly assigned and incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    Various embodiments are directed to performing imaging of a specimen using an electron microscope. 
       BACKGROUND 
       [0003]    Electron microscopes are used in a variety of applications that require high resolution imaging and analysis. In particular, electron microscopes are used in applications such as metallurgy, crystallography, biological sciences, and the semiconductor industry. Any technology which increases the resolution offered by electron microscopes would be desirable. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  shows a scanning transmission electron microscope (STEM) system in accordance with at least some embodiments; 
           [0005]      FIG. 2A  shows an electron probe; 
           [0006]      FIG. 2B  shows an annular aperture in accordance with at least some embodiments; 
           [0007]      FIG. 2C  shows an electron probe in accordance with at least some embodiments; 
           [0008]      FIG. 3A  shows a an overhead elevation view of a diverging lens in accordance with at least some embodiments; 
           [0009]      FIG. 3B  shows a cross-section elevation view of a diverging lens in accordance with at least some embodiments; 
           [0010]      FIG. 4  shows a partial STEM system in accordance with at least some embodiments; and 
           [0011]      FIG. 5  shows a method in accordance with at least some embodiments. 
       
    
    
     NOTATION AND NOMENCLATURE 
       [0012]    Certain terms are used throughout the following description and claims to refer to particular system components. As one skilled in the art will appreciate, design and manufacturing companies may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. 
         [0013]    In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection or through an indirect connection via other intermediate devices and connections. Moreover, the term “system” means “one or more components” combined together. Thus, a system can comprise an “entire system,” “subsystems” within the system, an electron microscope, a magnetic lens, or any other device comprising one or more components. 
         [0014]    “Electron probe” refers to a volume enclosed by a substantially equal-intensity contour surface around a focus of an electron beam. One example of the intensity of the contour surface is an electron intensity substantially equal to half the electron intensity at the focus center. 
         [0015]    “Vertical direction” refers to the direction parallel to the optical axis of an electron beam. “Horizontal direction” refers to the direction perpendicular to the optical axis of an electron beam. 
       DETAILED DESCRIPTION 
       [0016]    The following discussion is directed to various embodiments of the invention. Although one or more of these embodiments may be preferred, the embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims. In addition, one skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to intimate that the scope of the disclosure, including the claims, is limited to that embodiment. 
         [0017]      FIG. 1  shows a system  100  in accordance with at least some embodiments. In particular, system  100  is an electron microscope (e.g., scanning transmission electron microscope (STEM)). An electron beam generated by the electron source  102  is focused onto specimen  108  using a series of converging magnetic lenses  104 A- 104 B and  105 . In the illustrative figures optical analogs of the magnetic lenses are shown for schematic purposes only. The electron source  102  accelerates the electron beam to a predetermined acceleration voltage from an electron gun (e.g. cold field emission gun or a Schottky thermally assisted field emission gun). The electron beam is focused (equivalently referred to as converged or demagnified) by passing the electron beam through a magnetic field of converging lens  104 A and  104 B. The electron beam is further focused by the objective lens  105  to form an electron probe. Finally, the electron probe is horizontally positioned on the specimen  108  by the beam deflection coils  106  and/or by physical positioning of the specimen  108 . 
         [0018]    The electron beam transmitted through and scattered by the specimen  108  is passed through projection lenses  110  and  112 , and then detected by the detector  114 . The detector  114  comprises a bright field (BF) detector and an annular dark field (ADF) detector. The BF detector detects the direct transmitted electron beam through the specimen  108 , while the ADF detector detects the electron beam scattered by the specimen  108 . As discussed in greater detail below, the image is acquired by scanning the specimen  108  with an electron probe using the beam deflection coils  106  and changing the strength of the condenser lens  105 . A computer system (not shown in  FIG. 1  so as to not to unduly complicate the figure) synchronized with the scanning of the specimen  108  forms a scanning transmitted image. 
         [0019]    The resolution (i.e. spatial) of the system  100  is limited by the electron probe size, and the electron probe size is affected by aberration of the lenses  104 A,  104 B and  105 . Due to aberration, the electron probe may be diffused along the longitudinal axis and the latitudinal axis of the electron beam.  FIG. 2A  illustrates an electron probe  203  diffused at least along the longitudinal axis of the electron beam  202 . The diffusion of the electron probe  203  is caused at least in part by chromatic and/or spherical aberration of the lenses  104 A,  104 B and  105 . In accordance with the various embodiments, the aberration of the magnetic lenses is reduced, at least in part, by annular aperture  118 , resulting in an electron probe smaller than previously achievable. 
         [0020]      FIG. 2B  illustrates an overhead elevation view of annular aperture  118  in accordance with at least some embodiments. In particular, annular aperture  118  comprises an outer portion  204  and an inner portion  205 . Outer portion  204  is shown in  FIG. 2B  as a ring; however, other shapes may be equivalently used. Likewise, inner portion  205  is shown to be circularly symmetric, but other shapes may be equivalently used. The outer portion  204  and the inner portion  205  are separated by uniform width  207  to create an opening  208  which, in this particular example, is also circularly symmetric. The outer portion  204  and the inner portion  205  are bridged (e.g., at locations  206 ) to prevent movement between the inner and outer portions. The annular aperture  118  is made of a heavy metal (e.g. Molybdenum, Platinum, or Tungsten) capable of blocking the electron beam, and the annular aperture  118  is grounded to prevent charge accumulation. 
         [0021]    Referring simultaneously to  FIG. 1  (which shows the annular aperture in cross-section) and  2 B (which shows an overhead elevation view), in accordance with at least some embodiments annular aperture  118  is situated co-axially with the electron beam. The annular aperture  118  may be moved (i.e. by mechanical control) in the horizontal direction to align the center of the inner portion  205  of the annular aperture  118  with the optical axis of the electron beam. Annular aperture  118  enables only the portion of the electron beam incident on the opening  208  to pass, and the remaining portion of the electron beam is blocked by the outer portion  204  and inner portion  205 . The result is an electron beam confined to a hollow-core or hollow-cone geometry. When focused to create an electron probe, the hollow-cone electron beam creates an electron probe with less diffusion along the longitudinal axis. 
         [0022]      FIG. 2C  illustrates a cross-sectional elevation view of a hollow-cone electron beam  210  focused to create electron probe  250 . As illustrated by reference briefly to  FIG. 2A , the electron probe  250  is significantly smaller than the electron probe  203  formed by a solid-cone electron beam. The smaller size of the electron probe  250  is due in part to the aberration reduction provided by the annular aperture  118 , and also due in part to use of a hollow-cone electron beam enabling the electron beam to converge at larger angles (measured from the axis of the electron beam). 
         [0023]    While in some embodiments use of the annular aperture  118  alone may significantly reduce the electron probe  250  size by reducing aberration and resultant diffusion, some aberration (and thus diffusion) may still be present. The aberration that remains may be equivalently thought of as the portion of the aberration from lenses  104 A,  104 B and  105  that remains after the aberration reduction by the annular aperture  118 . The amount of remaining aberration is proportional to the angular range of the electron beam that passes through opening  208  of the annular aperture. Stated otherwise, the amount of remaining aberration is related to width  207  of the opening  208 . 
         [0024]    In accordance with some of the embodiments, a diverging lens  120  ( FIG. 1 ) is used to correct at least some of the remaining aberration (and resulting diffusion). The diverging lens  120  is a magnetic lens situated co-axially with the hollow-cone electron beam at a location after the annular aperture  118  and before the specimen  108 . The diverging lens  120  diverges a hollow-cone electron beam, and in the embodiments of  FIG. 1  the diverging lens  120  diverges the hollow-cone electron beam prior to the beam passing through the deflection coils  106 .  FIG. 3A  shows an overhead view of the diverging lens  120 . In particular,  FIG. 3A  shows that the diverging lens  120  is, in some embodiments, rotationally symmetric (i.e. circular), with the portion is dashed lines illustrating internal features not actually visible in the overhead view.  FIG. 3B  shows a cross-sectional elevation view of the diverging lens  120  taken along line  3 B- 3 B of  FIG. 3A . In particular, the diverging lens  120  comprises a body member  301 , made of soft iron (or any other ferromagnetic substance). In order to avoid charge accumulation on the metallic members of the diverging lens  120 , the body member  301  may be electrically grounded. The diverging lens  120  further comprises a solid core  302  within the body member  301  (e.g., at the center of the body member  301 ) and with air gaps  304  defining edges. In order to create magnetic flux through the body member  301 , an electrical coil  303  wound around the core  302  is excited by way of an electrical current. The electric current generates a magnetic field across the air gaps  304 . Because of fringing of the magnetic field outside the of the air gaps  304 , a hollow-cone electron beam  300  passing through the magnetic field  306  diverges to the outside of the lens  120 , as illustrated in  FIG. 3B . 
         [0025]    The diverging lens  120 , like magnetic lenses  104 A- 104 B and  105 , has inherent spherical and chromatic aberrations. As illustrated in  FIG. 3B , the portion of the electron beam closest to the gaps  304  diverges more than portions of the electron beam that pass further away from the gaps  304 . The spherical aberration of the diverging lens  120  is substantially opposite that of the converging magnetic lenses  104 A- 104 B and  105 , and by adjusting the strength of the magnetic field of the diverging lens  120  (i.e. by the current applied to the electrical coil  303 ), the spherical aberration(s) caused by lenses  104 A,  104 B and  105  may be substantially cancelled by the diverging lens  120 . Likewise, chromatic aberration in the diverging lens  120  cancels with the chromatic aberration of the magnetic lenses  104 A- 104 B and  105 . 
         [0026]    Returning to  FIG. 1 , the resolution of the system  100  is controlled, at least in part, by the electron probe size, and the electron probe size is limited by the aberrations of the magnetic lens. The electron probe formed by system  100  has small horizontal and vertical dimensions, as the aberrations are limited and/or corrected by the annular aperture  118  and the diverging lens  120 . As a result, the system  100  is capable of performing high resolution three-dimensional imaging by scanning the specimen  108  with the electron probe in both horizontal and vertical directions. 
         [0027]    The scanning of the specimen  108  with the electron probe is controlled by the objective lens  105  and the beam deflector coils  106 . In particular, changing the strength of the deflector coils  106  enables scanning of the specimen  108  in the horizontal direction, and changing the strength of the objective lens  105  enables scanning of the specimen  108  in the vertical direction (i.e. through its thickness). The detector  114  detects the electron beams transmitted through and/or scattered by scanning the specimen  108  with the electron probe. 
         [0028]    In  FIG. 1 , the electron beam passes through specimen  108 , through the projection lenses  110  and  112 , and then to the detector  114 . In other embodiments, the electron beam may pass through further annular apertures before being incident upon the detector.  FIG. 4  shows such other embodiments for detecting the direct transmitted electron beam and the scattered electron beams by the detector  114 . In particular,  FIG. 4  shows an annular aperture  401  situated substantially at the back focal plane  402  between the post-field objective lens  110  and projection lens  112 . The annular aperture  401  blocks the scattered electron to provide a filtered image at the bright field detector  404 . In yet another embodiment, the annular aperture  401  blocks the direct transmitted electron beams to provide a cleaner image at the annular dark field detector  406 . 
         [0029]      FIG. 5  shows a method in accordance with at least some embodiments. In particular, the method starts (block  500 ) and moves to generating an electron beam (block  504 ). Next, a hollow-cone electron beam is created (block  508 ) by passing the electron beam through a annular aperture. Thereafter, the hollow-cone electron beam is focused to form a probe (block  512 ). Next, a specimen is scanned using the probe (block  516 ). Finally, three-dimensional imaging is performed based on the scanning (block  520 ), and the method ends (block  524 ). 
         [0030]    The above discussion is meant to be illustrative of the principles and various embodiments of the present invention. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. For example, the three-dimensional imaging may be performed using a scanning transmission electron microscope, or the three-dimensional imaging maybe performed with any other types of electron microscope (e.g. scanning electron microscope (SEM)). Moreover, the various embodiments are discussed with both the annular aperture and diverging lens; however, in other embodiments the annular aperture may be used without the diverging lens. Finally, while in the various embodiments discussed the three-dimensional imaging of a specimen, in other embodiments three-dimensional electron energy loss spectroscopy (EELS) analysis of a specimen may be performed. Finally, the figures illustrate only one annular aperture situated in the electron microscope prior to specimen; however, in other embodiments additional annular apertures may be situated in the electron beam prior to the specimen as well. It is intended that the following claims be interpreted to embrace all such variations and modifications. 
       CONCLUSION 
       [0031]    Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. Many adaptations of the disclosure will be apparent to those of ordinary skill in the art. Accordingly, this application is intended to cover any adaptations or variations of the disclosure.