Patent Publication Number: US-9837242-B2

Title: Charged particle filter

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of and claims priority from U.S. patent application Ser. No. 14/925,204, filed Oct. 28, 2015, entitled “Charged Particle Filter,” the contents of which is incorporated herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention is generally related to a charged particle filter, particularly for use in association with a particle detector in a microanalysis system. 
     BACKGROUND 
     As described in U.S. Pat. No. 8,049,182 issued to Bewick on Nov. 1, 2011, (hereinafter “Bewick”) hereby incorporated by reference in its entirety (however, where anything in the incorporated reference contradicts anything stated in the present application, the present application prevails) with reference to  FIG. 1 , for X-ray analysis in an electron microscope (EM)  100 , an X-ray spectrum is measured by sensing and measuring the energies of individual X-ray photons emitted by a sample  101  when it is hit by a focused electron beam  102 . Each X-ray photon is an energetic particle and the energy is typically converted into charge using an X-ray detector  105 . Electrons which are scattered back from the sample, so called “backscattered electrons” (BSE)  103 , may also travel towards the X-ray detector  105 . An electron with the same energy as an X-ray photon deposits the same amount of energy in the X-ray detector  105  and therefore gives a similar signal charge. Typically, the number of electrons travelling towards the detector is considerably larger than the number of X-ray photons, and therefore the signal due to electrons represents a large proportion of the measured X-ray spectrum and can overwhelm the contribution due to X-ray photons. The BSE contribution in the spectrum is typically a large background extending over all energies up to the primary beam energy. X-ray analysis may be conducted in principle in any apparatus using an incident beam of charged particles (e.g., electrons or ions) and although the description herein refers to electrons, the same principles apply in any charged particle apparatus. 
     The X-ray detector is usually isolated from the vacuum of the EM by a thin foil of typically polymer supported on a grid with high transparency. If BSE strike the foil or the grid, they generate X-ray signals characteristic of the materials in the foil or grid and these X-ray signals appear as a spurious contribution to the recorded X-ray spectrum. As a result of the incident beam, the X-ray photons are emitted in all directions and the X-ray detector only detects photons falling within a cone defined by the active area of the sensor within the X-ray detector. The higher the solid angle defined by this cone, the more signal is collected and this is highly desirable. The solid angle can be increased by increasing the active area of the X-ray detector, and so a large sensor and large aperture are beneficial. 
     The solid angle can also be increased by locating the sensor closer to the sample. However, when the sensor is placed closer to the sample  101 , the tube structure of the X-ray detector  105  containing the sensor can collide with the final pole piece  104  of the electron microscope  100 . This collision is shown by way of example at point A in  FIG. 1 . 
     Therefore, it is desirable to minimize the external diameter of the detector tube (shown at B in  FIG. 1 ) to minimize the distance between the detector face and the sample without colliding with the conical pole piece of the electron microscope. 
     SUMMARY 
     In one embodiment, a charged particle filter includes a magnetic deflector having a bore along an axis thereof passing through the magnetic deflector from a sample end to a detector end of the magnetic deflector, and through which bore charged particles pass when in use, the magnetic deflector being formed from two magnets positioned around the bore, with a gap between the two magnets, the two magnets each having a linear central section and two ends, each end forming a curved surface, the curved surface having an aspect ratio defined by a height in a range of between one tenth and ten times the gap between the two magnets, and a width in a range of between one tenth and ten times the gap. The gap between the two magnets can be in a range of between 1 mm and 20 mm, such as a gap in a range of between 4 mm and 10 mm. The curved surface of at least one end of the two magnets can be a conic section. In some embodiments, the curved surface of both ends of each of the two magnets can be a conic section. In certain embodiments, the conic section can have a radius of curvature in a range of between one tenth and ten times the gap between the two magnets, such as a radius of curvature in a range of between one half and two times the gap between the two magnets. In some other embodiments, the curved surface of at least one end of the two magnets can be a trigonometric function. In certain embodiments, the curved surface of both ends of each of the two magnets can be a trigonometric function. 
     In another embodiment, a charged particle filter includes a magnetic deflector having a bore along an axis thereof passing through the magnetic deflector from a sample end to a detector end of the magnetic deflector, and through which bore charged particles pass when in use, the magnetic deflector being formed from two magnets positioned around the bore, with a gap between the two magnets, the two magnets each having a linear central section and two ends, at least one end forming a slanted surface around the bore. In some embodiments, both ends form a slanted surface around the bore. The slanted surface can have a slope in a range of between 0.1 and 10.0, such as a slope in a range of between 0.5 and 2.0. 
     In yet another embodiment, a charged particle filter comprising a magnetic deflector having a bore along an axis thereof passing through the magnetic deflector from a sample end to a detector end of the magnetic deflector, and through which bore charged particles pass when in use, the magnetic deflector being formed from two magnets positioned around the bore, with a gap between the two magnets, the two magnets each having a linear central section and two ends, each end forming at least two curved surfaces. The at least two curved surfaces can include a conic section, a trigonometric function, or any combination thereof. 
     In still another embodiment, a particle analyzer includes a charged particle filter as described above and a particle detector positioned upon the axis facing the detector end of the magnetic deflector. 
     The invention has many advantages, such as enabling the detector to be located closer to the sample without colliding with the conical pole piece of the electron microscope. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a prior art arrangement of components in the vicinity of a sample in a scanning electron microscope. 
         FIG. 2A  is a schematic illustration of a side view of an exemplary charged particle filter. 
         FIG. 2B  is a schematic illustration of a perspective front view of an exemplary charged particle filter. 
         FIG. 3A  is a schematic illustration of a perspective side view of an end of a magnet including a cylindrical curved surface of an exemplary charged particle filter. 
         FIG. 3B  is a schematic illustration of a perspective side view of an end of a magnet including a elliptical curved surface of an exemplary charged particle filter. 
         FIG. 3C  is a schematic illustration of a perspective side view of an end of a magnet including a parabolic curved surface of an exemplary charged particle filter. 
         FIG. 3D  is a schematic illustration of a perspective side view of an end of a magnet including a hyperbolic curved surface of an exemplary charged particle filter. 
         FIG. 3E  is a schematic illustration of a perspective side view of a magnet including symmetrical cylindrical curved surfaces of an exemplary charged particle filter. 
         FIG. 3F  is a schematic illustration of a perspective side view of an end of a magnet including a curved surface described by a cosine function of an exemplary charged particle filter. 
         FIG. 3G  is a schematic illustration of a perspective side view of an end of a magnet including a curved surface described by a tangent function of an exemplary charged particle filter. 
         FIG. 3H  is a schematic illustration of a perspective side view of an end of a magnet including a curved surface described by a secant function of an exemplary charged particle filter. 
         FIG. 3I  is a schematic illustration of a perspective side view of an end of a magnet including a curved surface described by a cubic function of an exemplary charged particle filter. 
         FIG. 3J  is a schematic illustration of a perspective side view of an end of a magnet including a curved surface that is convex with respect to the bore of an exemplary charged particle filter. 
         FIG. 4  is a schematic illustration of a perspective side view of a magnet including asymmetrical curved surfaces of an exemplary charged particle filter. 
         FIG. 5  is a schematic illustration of a perspective side view of an end of a magnet including a slanted surface of an exemplary charged particle filter. 
         FIG. 6  is a schematic illustration of a perspective side view of an end of a magnet including two curved surfaces of an exemplary charged particle filter. 
         FIG. 7  is a schematic illustration of a side view of an exemplary particle analyzer. 
     
    
    
     Like reference numerals refer to corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In the description of the invention herein, it is understood that a word appearing in the singular encompasses its plural counterpart, and a word appearing in the plural encompasses its singular counterpart, unless implicitly or explicitly understood or stated otherwise. Furthermore, it is understood that for any given component or embodiment described herein, any of the possible candidates or alternatives listed for that component may generally be used individually or in combination with one another, unless implicitly or explicitly understood or stated otherwise. Moreover, it is to be appreciated that the figures, as shown herein, are not necessarily drawn to scale, wherein some of the elements may be drawn merely for clarity of the invention. Also, reference numerals may be repeated among the various figures to show corresponding or analogous elements. Additionally, it will be understood that any list of such candidates or alternatives is merely illustrative, not limiting, unless implicitly or explicitly understood or stated otherwise. In addition, unless otherwise indicated, numbers expressing quantities of ingredients, constituents, reaction conditions and so forth used in the specification and claims are to be understood as being modified by the term “about.” 
     Accordingly, unless indicated to the contrary, the numerical parameters set forth in the specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the subject matter presented herein. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the subject matter presented herein are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical values, however, inherently contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements. 
     In one embodiment shown in  FIG. 2A , a charged particle filter  200  includes a magnetic deflector  210  having a bore  220  along an axis thereof passing through the magnetic deflector  210  from a sample end  230  to a detector end  240  of the magnetic deflector  210 , and through which bore  220  charged particles pass when in use in an electron microscope  100 . A comparison to an X-ray detector  105  using rectangular magnets is also shown in  FIG. 2A . With the axis of the bore  220  at the same angle (30°) with respect to the plane of the sample  101  as the axis of the bore  106 , the distance C of detector end  240  of the magnetic deflector  210  to the sample  101  is less than the corresponding distance D of the detector  105  to the plane of the sample  101  at a distance E between the face of the detector  104  and the sample  101  without colliding with the final pole piece  104 , which is highly advantageous for the reasons described above. 
     Turning to  FIG. 2B , the magnetic deflector  210  of the charged particle filter  200  is formed from two magnets  250  and  260  positioned around the bore  220 . In principle, the magnets could be electromagnets, although permanent magnets are considered to be more practical for the present application. A variety of magnetic materials are suitable for the two magnets  250  and  260 , such as, for example, sintered neodymium-iron-boron (Nd—Fe—B) with a maximum energy product of 50 MGOe available from, for example, Dexter Magnetic Technology (Elk Grove Village, Ill.). 
     Turning back to  FIG. 2B , the linear sections  280  and  281  of the two magnets  250  and  260 , respectively, are located symmetrically with respect to the bore  220  and each have two ends  285 ,  295 , and  286 ,  296 , each end forming a curved surface around the bore  220 , with a gap  270  between the two magnets  250  and  260 . The linear central sections  280  and  281  are typically oriented parallel to the plane of the sample  101  (see  FIG. 2A ). The gap  270  between the two magnets  250  and  260  can be in a range of between 1 mm and 20 mm, such as a gap in a range of between 4 mm and 10 mm. In some embodiments, the gap  270  can be a gap of 1.5 mm, 2.0 mm, 2.5 mm, 3.0 mm, 3.5 mm, 4.0 mm, 4.5 mm, 5.0 mm, 5.5 mm, 6.0 mm, 6.5 mm, 7.0 mm, 7.5 mm, 8.0 mm, 8.5 mm, 9.0 mm, 9.5 mm, 10.0 mm, 10.5 mm, 11.0 mm, 11.5 mm, 12.0 mm, 12.5 mm, 13.0 mm, 13.5 mm, 14.0 mm, 14.5 mm, 15.0 mm, 15.5 mm, 16.0 mm, 16.5 mm, 17.0 mm, 17.5 mm, 18.0 mm, 18.5 mm, 19.0 mm, or 19.5 mm. In a specific embodiment, the gap  270  is 6.6 mm. 
     A plurality of shapes and combinations of shapes are suitable for the curved surfaces of the ends  285 ,  295 , and  286 ,  296  of the two magnets  250  and  260 , respectively. In the description of the curved surfaces and associated figures described below, when one end of a magnet is shown for clarity, the other end of the magnet can be formed from any one of the curved surfaces described herein, and the four ends  285 ,  295 , and  286 ,  296  can be formed from different curved surfaces from one another, unless otherwise specified. As shown in  FIG. 3A , the curved surface  310  has an aspect ratio defined by a height  315  in a range of between one tenth and ten times the gap  270  (shown in  FIG. 2B ) between the two magnets  250  and  260 , and a width  325  in a range of between one tenth and ten times the gap  270 . In some embodiments, the curved surface of one end  285 ,  295 , or  286 ,  296  of each of the two magnets  250  and  260 , respectively, is a conic section, such as a cylindrical curved surface  310 , as shown in  FIG. 3A , an elliptical curved surface  320 , as shown in  FIG. 3B , a parabolic curved surface  330 , as shown in  FIG. 3C , or a hyperbolic curved surface  340 , as shown in  FIG. 3D . The curved surfaces can be tangent to the linear central section  280 , as shown in  FIGS. 3A-3D , or offset from the linear central section (not shown). In some embodiments, the curved surface of both ends  285 ,  295 , and  286 ,  296  of each of the two magnets  250  and  260 , respectively, can be a conic section as described above. 
     In certain embodiments, the cylindrical curved surface  310  has a radius of curvature in a range of between one tenth and ten times the gap  270  between the two magnets  250  and  260 , such as a radius of curvature in a range of between one half and two times the gap  270  between the two magnets  250  and  260 . In the specific embodiment described above, the cylindrical curved surface  310  of both ends  285  and  295  has a radius of curvature of 12 mm, as shown in  FIG. 3E  (and a corresponding gap of 6.6 mm as described above). 
     In certain embodiments, the curved surface of one end  285 ,  295 , or  286 ,  296  of the two magnets  250  and  260 , respectively, is described by a trigonometric function, such as a curved surface  350  described by a cosine function, as shown in  FIG. 3F , a curved surface  360  described by a tangent function, as shown in  FIG. 3G , a curved surface  370  described by a secant function, as shown in  FIG. 3H , and their respective inverse functions (sine, cotangent, cosecant). In some embodiments, the curved surface of both ends  285 ,  295 , and  286 ,  296  of the two magnets  250  and  260 , respectively, is described by a trigonometric function as described above. In some other embodiments, as shown in  FIG. 3I , the curved surface  380  of one or both ends  285 ,  295 , and  286 ,  296  of the two magnets  250  and  260 , respectively, is described by a cubic function. In certain embodiments, the curved surface of one or both ends is described by a sum of two or more functions, such as a Fourier series of sine and cosine functions, or a power series function (ax+bx 2 +cx 3 + . . . ). In another embodiment, shown in  FIG. 3J , a curved surface  390  is convex with respect to the bore  220 . 
     In some embodiments, as shown in  FIG. 4 , the curved surface  410  of one end  285 ,  286  of each of the two magnets  250  and  260 , respectively, is described by a different function from the curved surface  420  of the other end  295 ,  296  of each of the two magnets  250  and  260 , respectively. The two curved surfaces  410  and  420  can be described by any combination of the curved surfaces described above. 
     The surfaces of the ends of the two magnets forming the magnetic deflector can be formed from a plurality of surfaces. In another embodiment, a charged particle filter includes a magnetic deflector as described above, except that each of the two magnets  250  and  260  have one end  285 ,  296  or  286 ,  296 , as shown in  FIG. 5 , that forms a slanted surface  510  around the bore. In some embodiments, both ends  285 ,  295 , and  286 ,  296  form a slanted surface  510  around the bore. The slanted surface  510  can have a slope in a range of between 0.1 and 10.0, such as a slope in a range of between 0.5 and 2.0. In some embodiments, the slanted surface  510  can have a slope of 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, or 10.0. 
     In yet another embodiment, a charged particle filter includes a magnetic deflector as described above, except that each of the two magnets  250  and  260  have one end  285 ,  295 , or  286 ,  296 , as shown in  FIG. 6 , that forms two curved surfaces  610  and  620  around the bore. The two curved surfaces  610  and  620  can include a conic section, a trigonometric function, or any combination of curved surfaces described above. Curved surfaces including more than two curved surface segments, and including slanted surface segments as described above, are also suitable. 
     In still another embodiment shown in  FIG. 7 , a particle analyzer  700  includes a charged particle filter  200  as described above and a particle detector  710  positioned upon the axis facing the detector end  240  of the magnetic deflector  210 . The particle analyzer  700  further includes a housing  720 , a flux return  730 , and a collimator  740 . A variety of X-ray detectors are suitable for the particle detector  710 , available from, for example, PN Detector (Munich, Germany). 
     Other Embodiments 
     A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other embodiments are within the scope of the following claims.