Abstract:
A charged particle filter such as a Wien filter in which components used as the pole pieces and electrodes are precisely and reliably secured to a supporting structure through which they extend and to which they are brazed. Electrical insulating gaps in the magnetic circuit are located very remotely from the pole faces of the pole pieces so as to minimize any adverse effect of the gaps on the produced magnetic field.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to a Wien filter used, for example, in a scanning electron microscope (“SEM”) and, in particular, to an improved design for minimizing aberrations that affect the primary electron beam to thereby improve instrument resolution. 
     BACKGROUND OF THE INVENTION 
     Various instruments are known which rely on emission of charged particles from a sample to derive characteristics of the sample. Examples of such instruments are electron microscopes (e.g., scanning electron microscopes), focused ion beam microscopes, and mass spectrometers which utilize various well known means to analyze charged particles emitted from the sample. 
     For facilitating the description of the present invention, it will be explained in connection with an SEM. However, it should be understood that the invention is not limited to an SEM and can be applied by one with ordinary skill in the art to other instruments such as those mentioned above. 
     An SEM operates by generating a primary scanning electron beam that impacts a sample a surface of which is being imaged. As a result, backscattered and secondary electrons are emitted from the sample surface and have respective trajectories backward along the original beam direction which is perpendicular to the sample surface (known as the on-axis direction) and at angles diverging therefrom. Emitted electrons are collected by a detector, which is arranged above the sample. The detector generates a signal from the electron emission collected from the sample surface as it is exposed to the electron beam. The signal from the detector is typically processed to create an image of the surface, which is then displayed on a video screen. 
     With structures such as high aspect-ratio trenches and contact holes, the only electrons able to escape are those emitted on-axis. However, standard SEMs do not readily detect on-axis emitted electrons. This is because the detector must be spaced from the axis so as not to impede the primary, or incident, beam. In that position, on-axis electrons do not impinge upon the detector. Also, the high voltage on the front surface of the detector, which is commonly used to attract the secondary electrons, may adversely affect the primary beam. 
     The term “filter” as used herein refers to devices used to in some way separate, or disperse, the particles of a charged particle beam through their inherent differences in either velocity (although often, in fact usually, this quantity is classified as “energy”), charge, or mass. This can be done through application of either electric or magnetic fields, or a combination of both. 
     Wien filters have long been known (see W. Wien,  Ann. Phys.  65 (1898), page 444). In such a filter, electrodes and magnetic poles are simultaneously utilized to create both an electric field and a magnetic field. The two fields are tuned, or adjusted, to apply equal and opposite forces to electrons in the incident beam, so that it is not deflected. However, an electron moving in the direction opposite to the incident beam will be oppositely affected by that same magnetic field force, which acts on such electron in the same direction as the electric field force. Thus, that electron will be deflected away from the beam axis. In that way, even the on-axis particles can be detected as they are deflected to a properly positioned detector. 
     U.S. Pat. No. 4,658,136 of Apr. 14, 1987, entitled “Secondary Electronic Detecting Apparatus” suggests the use of a Wien filter in a scanning electron microscope, but in practice Wien filters have not been used in that application because they have caused relatively small but significant disturbances in the electrostatic and magnetic fields through which the particles of the primary beam and the emitted particles pass, thereby disturbing the trajectories of such particles, which disturbances have been sufficient to undesirably degrade the primary beam and hence the resolution of the SEM. More specifically, the use of a Wien filter in an SEM requires that the magnetic and electrostatic fields be precisely matched and uniform in order to ensure that the primary electron beam is unperturbed and aberrations are kept to a minimum. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide a filter that generates electrical and magnetic fields which is constructed so as to significantly minimize aberrations that are due to mismatching of the electric and the magnetic fields, so that when the filter is used in combination with an SEM or the like, resolution of the instrument is not degraded to any appreciable degree. 
     Another object is to provide an external magnetic and electric field clamp, located adjacent to (above and below) the pole pieces to further improve the field shapes. 
     A further object of the present invention is to so construct a filter that its parts can be readily assembled to produce a precision structure which is physically and electrically stable under operating conditions. 
     Another object of the present invention is to provide a filter that has electric and magnetic fields which are uniform and precisely matched, thereby to effectively and accurately function in an SEM or the like. 
     Another object is to place the connectors to the poles and coils outside of the vacuum to facilitate manufacturing and eliminate outgassing from the coils. 
     These and other objects are attained by one aspect of the present invention directed to an improvement in an electromagnetic filter comprising a field-producing structure at least partly surrounding a passage. The improvement comprises a supporting structure around the passage having a plurality of inwardly extending openings therethrough. Each of a plurality of magnetically permeable field-producing structures extends continuously through one of the openings, terminating at one end in a pole face located radially inwardly from the supporting structure and constituted at its other end by a portion extending radially outwardly from the supporting structure. Each of the field-producing structures engages the supporting structure. Electromagnetic field-producing elements are operatively connected to the radially outwardly extending portions of the field-producing structures and are effective to act on the field-producing structures to produce magnetic and/or electric fields emanating therefrom and extending into the passage. The filter is adapted to function in combination with magnetic circuit means located outside the supporting structure for operatively engaging and coupling the radially outwardly extending portions of the field-producing structures with a gap between the field-producing structures and the magnetic circuit means, the gap being located radially outside the supporting structure. 
     In accordance with the present invention, a novel arrangement of the component parts of the filter is provided. In particular, the gap in the magnetic circuit which is required for electrical isolation is properly located so that the fringe magnetic fields associated with it, which otherwise tend to perturb the fields acting on the particles passing through the filter, do not create any deleterious effects on resolution. 
     We have discovered that locating the radial gap in the magnetic circuit near the central axis of the instrument produces undesirable disturbances in the magnetic field, which in turn adversely affect the resolution of the instrument. More specifically, a higher and more uniform and more stable magnetic field results when the radial gap is located laterally outside the structure which supports the pole pieces as far as possible from the axis of the beam and, even more preferably, outside the windings mounted on the pole pieces. These improvements in function are achieved by a structure which is comparatively simple, sturdier and more reliable than prior art Wien filter structures. 
     As indicated above, while the filter of the present invention is here specifically described for use with a scanning electron microscope, in connection with which its improved function is exceedingly important, it will be understood that the utility of the filter under discussion is not limited to that application, but is also advantageous in connection with most instruments where charged particles move through the same space in different directions and are to be differently acted upon depending upon the particular direction in which they are moving. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     To the accomplishment of the above, and to such other objects as may hereinafter appear, the present invention relates to the construction of a filter as defined in the appended claims and as described in this specification, taken together with the accompanying drawings, in which: 
     FIG. 1 is a perspective view of a Wien filter constructed according to the present invention; 
     FIG. 2 is a fragmentary view of FIG. 1 showing one of the pole pieces in an exploded fashion; 
     FIG. 3 is a cross-sectional view of the filter taken at a horizontal plane passing through the center of the arrangement shown in FIG. 1; and 
     FIG. 4 is a cross-sectional view taken at a vertical plane passing through the center of the arrangement shown in FIG. 1 showing pole pieces of the filter mounted in an external magnetic structure (not shown in FIG. 1) which, in part, defines the evacuated passage through which the particles to be detected pass. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The filter of the present invention, generally designated A, is, when used with an SEM, mounted in an external field clamp structure generally designated B (see FIG. 4) which defines a passage generally designated C through which the particles to be detected pass. 
     The filter A comprises a plurality of pole pieces generally designated D, here shown as eight in number, arranged around the passage C. In order to properly mount and locate the pole pieces D, a supporting structure generally designated E is provided in the form of a sturdy ring which surrounds the passage C. That ring E may be formed of non-magnetic structural material such as ceramic. An alumina material, 99% Al 2 O 3 , has been found to be effective for this purpose since it can be accurately formed and has the desired strength and mechanical stability. As can be seen from FIG. 3, the ceramic ring E is provided with a plurality of equally spaced and radially oriented through-openings  4  with, at the radially outer ends of the openings  4 , outwardly facing bearing surfaces  6  of appreciable area. 
     As depicted in FIGS. 2 and 3, each of the pole pieces D comprises a radially inwardly extending portion  8  which passes snugly through an opening  4  in the ceramic ring E and terminates in a pole face  10 . Radially outward of the portion  8  is an integral wider portion  12  adapted to engage a bearing surface  6  of the ceramic supporting structure E. Extending radially outwardly from the portion  12  is an integral narrower portion  14  on which a magnetic field-producing winding  16  is mounted. That winding is enclosed within a shielding spool  18  and surrounded by an electrical shield  20  and cover  21 , being held in place on its pole piece by the “0” ring  22  which engages a groove  23  formed in the circumference at the top and bottom of the outermost portion of the pole piece. An insulating shim  24  may be positioned between the shielded coil assembly and the pole piece portion  12 . Electrical connection to the winding  16  is made by means of the terminals  26  extending through the cover  21 . Each pole piece D is preferably of one-piece construction. 
     As may be seen from FIG. 4, each pole piece has extending laterally therefrom a terminal  28  in the form of a screw which, when appropriately energized, produces in the passage C the electrostatic field that, when functioning in conjunction with the magnetic field generated by the winding  16 , gives rise to the desired Wien filter effect. Electrical connections to the terminals  26  and  28  can be made through field clamp B in any well known manner. 
     FIG. 4 shows two diametrically opposing pole pieces D incorporated into an SEM and located inside a magnetic field clamp structure B which magnetically links the pole pieces D and which also at least partly defines the particle passage C. The gap  29  needed in the magnetic circuit for electrical isolation is formed between the outer tips  30  of the pole pieces and the mating inwardly facing surfaces  32  of the magnetic field clamp structure B. A thin insulator  33  about {fraction (2/1000)}th of an inch thick is designed to be mounted in that gap. 
     The gap  29  in the magnetic circuit is necessary but it presents a problem from a magnetic point of view. The gap distorts the magnetic field where the gap exists, and that effect extends through space so as to perturb the field that the electrons pass through, thus adversely affecting the resolution of the instrument. By locating the gap  29  at the outer tips  30  of the pole pieces D, and hence remote from the passage C through which the electrons pass, the field-disturbing effect of the gap  29  is greatly minimized, and hence the resolution of the instrument is significantly improved. 
     The surfaces  32  facing the radially outer tips  30  of the pole pieces are formed in one magnetic structural section  34  of the field clamp structure B which, in turn, is mounted on a cooperating supporting field clamp structure section  36 , those sections  34  and  36  as shown in FIG. 4 being portions of the overall microscope structure. The Wien filter ceramic supporting structure E rests on the field clamp structure  36  and engages the field clamp structure  34  at the radially outer pole piece tips  30 . “O” rings  38  and  40  are compressed between the ceramic structure E and the field clamp structures  34  and  36  so as to maintain a vacuum in passage C. 
     The pole pieces D are formed of a highly magnetically permeable material. The shim  24  is formed of any suitably electrically insulating material such as polyamide available from DuPont under the trademark Kapton. The spool  18  may be formed of brass, the shield  20  may be formed of aluminum and the cover  21  may be formed of copper-clad insulating board. 
     The pole pieces D are secured to the supporting ceramic ring E by brazing the surface  6  of the ceramic ring E, after being appropriately metallized, to the opposed face of portions  12  of the pole pieces D while the pole pieces D are uniformly pressed into position. A fixture and method to accomplish this assembly are disclosed in a co-pending patent application entitled “A Fixture for Assembling Parts of an Instrument such as a Wien Filter”, Ser. No. 10/011007 filed on Nov. 8, 2001, and assigned to the assignee of this application. A copper-silver-eutectic is very effective as the brazing material. In that regard a 1-mil thick brazing shim may be used. The brazing, it will be noted, not only reliably secures the pole pieces D in place on the supporting ring E but also produces the necessary hermetic seal between the ring E and the pole pieces D. The large areas of the brazed surfaces are important for sealing and for stability of the structural relationship. 
     Whereas, as pointed out above, wider portions  12  provide a substantial surface of engagement between each pole piece D and ceramic ring D, they serve another advantageous purpose as well. More specifically, the portions which extend like a flange around the main body of each pole piece provide some degree of shielding between the primary beam and the fringe field at gap  29  to even further reduce its perturbational effect on resolution of the instrument. 
     Although the detailed description provided above discusses specific embodiments of the present invention, various modifications thereto will be readily apparent to anyone with ordinary skill in the art. All such modifications are intended to fall within the scope of the present invention as defined by the following claims.