Abstract:
A Bradbury-Nielson gate (BNG) includes a set of evenly spaced, co-planar, and parallel wires. The wires alternate in a repeating ABAB pattern, where all of the A wires are electrically connected to each other, all of the B wires are electrically connected to each other, and the set of A wires is electrically isolated from the set of B wires. Improved fabrication of Bradbury-Nielson gates is provided based on two key ideas. The first key idea is the use of wire positioning template surfaces having wire insertion features with enhanced spacing. Wire insertion features having enhanced spacing allow for non-microscopic assembly of finely spaced wire arrays. The second key idea is the use of two template surfaces, each having wires spaced by twice the eventual gate wire spacing. The use of two template surfaces facilitates making the alternating electrical contact required for a BNG.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional patent application 60/771,235, filed on Feb. 7, 2006, entitled “Template-Based Fabrication of Bradbury-Nielson Gates”, and hereby incorporated by reference in its entirety. 
     
    
     GOVERNMENT SPONSORSHIP 
       [0002]    This invention was made with Government support under contract number FA9550-04-1-0076 from the AFOSR. The Government has certain rights in this invention. 
     
    
     FIELD OF THE INVENTION 
       [0003]    This invention relates to wire gates for controlling charged particle motion. 
       BACKGROUND 
       [0004]    Motion of charged particles is often controlled by wire gates, as employed in applications such as electron microscopy, mass spectrometry, and ion mobility spectrometry. Electric fields can be generated by applying electric potentials to the wires, and these electric fields can act on charged particles to alter their motion. Many different kinds of wire gates have been considered in the art for controlling charged particle motion. One kind of gate commonly known as a Bradbury-Nielson gate (BNG) can provide excellent performance, especially in demanding applications requiring precise timing control, such as Hadamard transform time-of-flight mass spectrometry. 
         [0005]    A BNG includes a set of evenly spaced, co-planar, and parallel wires. The wires alternate in a repeating ABAB pattern, where all of the A wires are electrically connected to each other, all of the B wires are electrically connected to each other, and the set of A wires is electrically isolated from the set of B wires. The main advantage of the BNG is that its electric field decays very rapidly as distance increases away from the plane of the wires. The deflection region, where electric fields are non-negligible, extends out to about one wire spacing from the plane of the BNG. Thus decreasing the BNG wire spacing decreases the size of the deflection region, which in turn improves the time resolution of the BNG. 
         [0006]    Although fabrication of BNGs having large wire spacing tends to be straightforward, BNG fabrication difficulty increases significantly as the wire spacing decreases. The main difficulties encountered are precisely placing the wires of the BNG (i.e., so they are parallel, co-planar and evenly spaced with the desired spacing), and providing alternating electrical contact to the BNG wires as described above. These fabrication difficulties are further increased by the common requirement in practice that the BNG have a large active area (i.e., on the order of 5 cm×5 cm). 
         [0007]    Several methods have been considered in the art to address some of these issues. In an article by Vlasek et al. (Rev. Sci. Instrum., 67(1), pp. 68-72, January 1996) a wire spacing of 1 mm was achieved by weaving a wire through holes drilled through two frames separated by two threaded rods. An article by Stoermer et al. (Rev. Sci. Instrum., 69(4), pp. 1661-1664, April 1998) demonstrated a wire spacing of 0.5 mm by winding wire on the threads of two nylon screws. A wire spacing of about 0.16 mm is reported by Brock et al. (Rev. Sci. Instrum., 71(3), pp. 1306-1318, March 2000), where wire segments are individually soldered to electrode pads on the BNG frame. The wire positioning and soldering in this case entailed time-consuming manual assembly under a microscope. 
         [0008]    A template based approach for BNG fabrication was considered by Kimmel et al. (Rev. Sci. Instrum., 72(12), pp. 4354-4357, December 2001, and in U.S. Pat. No. 6,664,545). In this work, 0.1 mm spaced V-grooves are machined into a plastic mount, and then two sets of wires are wrapped into the grooves under a microscope. Although this approach reduces fabrication time compared to the approach of Brock et al., it still entails lengthy microscope assembly work. 
         [0009]    Microfabrication methods have also been employed for BNG fabrication. Examples in the art of such methods include U.S. Pat. No. 6,977,381, US Patent Application 2005/0258514, and US Patent Application 2006/0231751. Although microfabrication methods can provide BNGs having very small wire spacing (e.g., as low as 0.015 mm), it is difficult for microfabrication methods to provide BNGs having a large active area. For example, in one report of a microfabricated BNG, the maximum active area was on the order of 5 mm by 5 mm. 
         [0010]    Accordingly, it would be an advance in the art to provide a BNG fabrication method for large-area BNGs having small wire spacing that does not require laborious assembly under a microscope. 
       SUMMARY 
       [0011]    Improved BNG fabrication is provided according to embodiments of the invention based on two key ideas. The first key idea is the use of wire positioning template surfaces having wire insertion features with enhanced spacing. Wire insertion features having enhanced spacing allow for non-microscopic assembly of finely spaced wire arrays. For example, insertion features having a spacing of about 1 mm (microscope not necessary) can correspond to a set of grooves spaced by as little as 0.025 mm (microscope required for conventional assembly methods). 
         [0012]    The second key idea is the use of two template surfaces, each having wires spaced by twice the eventual gate wire spacing. The use of two template surfaces facilitates making the alternating electrical contact required for a BNG. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  shows part of a template having wire insertion features according to an embodiment of the invention. 
           [0014]      FIG. 2  shows two templates according to an embodiment of the invention in a separated configuration. 
           [0015]      FIG. 3  shows the two templates of  FIG. 2  in a mated configuration. 
           [0016]      FIGS. 4   a - c  show top, end and bottom views, respectively, of a wire-wound template according to an embodiment of the invention. 
           [0017]      FIG. 5   a - d  show BNG and template configurations at various points in an exemplary assembly process according to an embodiment of the invention. 
           [0018]      FIGS. 6   a - b  show top and end views, respectively, of a wire-wound template according to an alternate embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1  shows part of a template having wire insertion features according to an embodiment of the invention. Here a template  100  has grooves (e.g.,  108 ,  110 , and  112 ) and corresponding wire insertion features (e.g.,  102 ,  104 , and  106 ). The spacing between adjacent grooves is shown as d, and the spacing between adjacent wire insertion features is shown as h. The insertion feature spacing h is substantially greater than the groove spacing d. This can be accomplished, as shown on  FIG. 1 , by providing an end face of the array of grooves that intersects the set of grooves at an angle substantially less than 90 degrees (preferably less than 10 degrees). Preferably h is greater than about 1 mm, since a 1 mm separation is about the smallest separation that can readily be wire-wound without using a microscope. The groove spacing d, which is twice the BNG gate spacing, can be selected according to the desired BNG gate spacing. A gate spacing as low as 0.025 mm can be provided, corresponding to a d of 0.05 mm. 
         [0020]      FIG. 2  shows two templates according to an embodiment of the invention in a separated configuration. A first template  210  has a first template surface  202  with grooves in which wires  206  are disposed. A second template  212  has a second template surface  204  in which wires  208  are disposed.  FIG. 3  shows the two templates of  FIG. 2  in a mated configuration. The surfaces of templates  210  and  212  mate such that the two sets of wires segments on each surface alternate. 
         [0021]      FIGS. 1-3  schematically show some key aspects of embodiments of the invention in isolation to facilitate comprehension of key points of the more detailed example of  FIGS. 4   a - c  and  5   a - d.    
         [0022]      FIGS. 4   a - c  show top, end and bottom views, respectively, of a wire-wound template according to an embodiment of the invention.  FIG. 4   b  shows a view along direction  402  on  FIG. 4   a.    FIG. 4   a  shows a view along direction  404  on  FIG. 4   b.    FIG. 4   c  shows a view along direction  406  on  FIG. 4   b.  A template  412  has a template surface  416  including an array of parallel grooves. Template  412  also has a set of wire insertion features  422 , one for each groove, where the separation between adjacent wire insertion features is substantially larger than the groove spacing (i.e., as described in connection with  FIG. 1 ). The enhanced separation of wire insertion features in this example is provided by the acute end face angle θ. Template  412  includes an aperture  414 , across which wires  410  extend in a pattern determined by the grooves of surface  416 . 
         [0023]    Wire is preferably wound onto template  412  by winding a continuous length of wire into the grooves of template surface  416 . More specifically, wire can be wrapped repeatedly around the template, alternating between passing through a groove of surface  416  as shown on the top view of  FIG. 4   a,  and moving from one groove to the next as shown on the bottom view of  FIG. 4   c.  This winding can be guided by wire insertion features  422 . Preferably, wire winding is performed with a mechanical jig for keeping the wire under constant tension as it is wound onto the template. Once the winding is complete, the wires can be affixed to the template by members  418  and  420 , and then the wires on the bottom surface of template  412  extending across aperture  414  can be removed, providing the wire configuration shown on  FIGS. 4   a - c.  The resulting arrangement of wires is a single co-planar and parallel array of wire segments having a spacing defined by the grooves of surface  416  and extending across aperture  414 . The details of how the wires are held in position on template  412  are not critical in practicing the invention. Similarly, details relating to how the wires on the bottom surface of template  412  are removed are also not critical in practicing the invention. 
         [0024]    Templates for practicing the invention can be formed by any suitable technology, such as machining or lithography. For example, aluminum can be machined to form a suitable template, or silicon can be lithographically patterned (e.g., by deep reactive ion etching (DRIE)) to form a suitable template. In cases where the template is machined from a metal, it is preferred that a black surface finish be applied to the template (e.g., by anodization) to improve visual contrast between the wire (typically gold coated tungsten having a diameter of 0.01 mm to 0.02 mm) and the template. 
         [0025]      FIG. 5   a - d  show BNG and template configurations at various points in an exemplary assembly process according to an embodiment of the invention. On  FIG. 5   a,  a BNG frame  502  includes an aperture  508  and first electrical contacts  504  and  506 . Frame  502  should be electrically insulating and have sufficient mechanical strength to maintain structural integrity. Electrical contacts  504  and  506  can be fabricated of any electrically conductive material (e.g., conductive paint, a metal strip, etc.) such that any two wires that make contact to electrical contact  504  are thereby in electrical contact with each other (and similarly for electrical contact  506 ). Frame  502  provides mechanical support for the wires of the BNG, which will extend across aperture  508  once fabrication is completed. The composition and dimensions of BNG frame  502  are not critical in practicing the invention. However, one of the advantages of the invention is that BNGs having a large active area (e.g., on the order of 5 cm by 5 cm) can be provided. Optionally, BNG frame  502  can include grooves spaced by the BNG wire spacing d. Wire spacing uniformity can be improved when wires are transferred to a grooved frame as compared to a frame without grooves. 
         [0026]      FIG. 5   b  shows an arrangement of two wire-wound templates as in  FIGS. 4   a - c  mated as shown in  FIG. 3 . Although it may be necessary to view the mating of the templates under a microscope to verify proper alternation, such verification is not time consuming. Insulating strips  514  and  516  (e.g., of Teflon® or any other electrically insulating material) are sandwiched between the sets of wires corresponding to the two templates. More specifically, a set of wires  512  includes alternating wires from a first template (one wire being labeled as  512   a ) and a second template (one wire being labeled  512   b ). It is convenient to refer to wire sets  512   a  and  512   b  as including all wires having the same relation to strips  514  and  516  as wires  512   a  and  512   b  respectively. Since insulating strips  514  and  516  are sandwiched between alternating sets of wires, the wires of set  512  pass over and under the insulating strips in an alternating manner, as shown. 
         [0027]      FIG. 5   c  shows the configuration when the wire wound template sandwich of  FIG. 5   b  is disposed on top of the BNG frame of  FIG. 5   a.  Insulating strips  514  and  516  are aligned with and disposed on top of first contacts  504  and  506 , and are disposed away from aperture  508 . As a result, the wires of set  512   b  are electrically connected by contacts  504  and  506 , while the wires of set  512   a  are isolated from the wires of set  512   b.  At this stage of assembly, wires of set  512  are affixed to frame  502  (e.g., by being secured with metal plates and glued down). 
         [0028]      FIG. 5   d  shows the finished BNG resulting from cutting the wires connecting the BNG frame to the template and adding final electrical connections. More specifically, second electrical contacts  518  and  520  are disposed on top of insulating strips  514  and  516 . Electrical contact of all the wires of set  512   a  to each other is thereby provided, while electrical isolation between wire sets  512   a  and  512   b  is preserved. 
         [0029]    The preceding description of  FIGS. 4   a - c  and  5   a - d  amounts to an illustrative example of the following method of fabricating a Bradbury-Nielson gate for controlling charged particle motion. 
         [0030]    First, a gate spacing d is selected for the wires of the gate to be fabricated. Second, a first template surface having a first set of parallel grooves separated by 2d is provided, where the first template surface includes a first set of wire insertion features in one to one correspondence with the first set of grooves, and where the spacing of the wire insertion features is substantially larger than 2d. Third, a first set of wire segments is disposed in the first set of grooves by inserting the first set of wire segments into the first set of wire insertion features. These wire segments are preferably sections of a single continuous length of wire at the time wire winding is done. 
         [0031]    Fourth, a second template surface having a second set of parallel grooves separated by 2d is provided, where the second template surface includes a second set of wire insertion features in one to one correspondence with the second set of grooves, and where the spacing of the wire insertion features is substantially larger than 2d. Fifth, a second set of wire segments is disposed in the second set of grooves by inserting the second set of wire segments into the second set of wire insertion features. These wire segments are also preferably sections of a single continuous length of wire at the time wire winding is done. 
         [0032]    Sixth, attaching the first and second sets of wire segments to a frame such that wire segments from the first and second sets of wire segments are co-planar, parallel and alternating between the first and second sets of wire segments. 
         [0033]    Seventh, making a first electrical connection such that all of the first set of wire segments are in electrical contact. 
         [0034]    Eighth, making a second electrical connection such that all of the second set of wire segments are in electrical contact and such that the first and second sets of wire segments are electrically isolated from each other. 
         [0035]    According to the invention, wire weaving for BNGs having wire spacing as small as 0.025 mm can be done in about 1-2 hours without using a microscope. Such BNGs can also have large active areas (e.g., on the order of 5 cm by 5 cm). Fabrication of BNGs according to methods of the invention with wire spacing of 0.05 mm, 0.1 mm, 0.2 mm and 0.5 mm has been performed. In these tests, wire weaving time for a 10 mm by 15 mm active area BNG with 0.1 mm wire spacing was about one hour, and wire weaving time for a 8 mm by 15 mm active area BNG with 0.05 mm wire spacing was about two hours. The performance of the resulting gates was characterized experimentally and compared with theoretical calculations based on the idealized BNG geometry. Close agreement between experiment and theory was obtained, indicating close agreement between ideal BNG geometry and the actual as-fabricated BNG geometry. The uniformity of the wire spacing was also directly measured. The BNGs having wire spacing of 0.5 mm, 0.2 mm, 0.1 mm and 0.05 mm had a spacing standard deviation of 0.03 mm (6%), 0.014 mm (7%), 0.0065 mm (6.5%), and 0.009 mm (18%), respectively. 
         [0036]    Although template fabrication time can be significant, once fabricated, templates can be reused to fabricate multiple BNGs. 
         [0037]    The preceding example shows BNG fabrication using two separate templates. It is also possible to employ a single template for BNG fabrication according to another embodiment of the invention. 
         [0038]      FIGS. 6   a - b  show top and end views, respectively, of a wire-wound template according to an alternate embodiment of the invention using a single template.  FIG. 6   b  shows a view along direction  602  of  FIG. 6   a.  A template  604  has a first template surface  606  and a second template surface  608 , each template surface having an array of parallel grooves. Each groove has a corresponding wire insertion feature, and the spacing of the wire insertion features is substantially greater than the groove spacing. The two arrays of grooves have the same spacing, and are offset from each other by half their groove spacing, as shown on  FIG. 6   b.  Wire can be wound into the grooves of template  604  as described above, resulting in an array of wire segments  610  extending across an aperture of template  604 . Preferably, the groove depths of the two template surfaces are selected so that the wires are in or nearly in the same plane. It is also preferable to dispose one or more insulating strips (not shown on  FIG. 6   a ) in the aperture of template  604  prior to wire winding, to provide an arrangement of wires and insulators that facilitates making the alternating electrical connections of a BNG (e.g., as described above in connection with  FIGS. 5   a - d ). 
         [0039]    As indicated above, machining and microlithography are both suitable techniques for fabricating templates. Machining is suitable for BNG wire spacing of about 0.05 mm or greater, since machining precision tends to be sufficiently accurate for such spacing. For reduced wire spacing (e.g., about 0.025 mm to about 0.05 mm), a lithographic single-template approach is preferred. 
         [0040]    For example, a  4 ″ silicon wafer can have 0.1 mm deep channels etched into its front and back surfaces by DRIE forming two sets of parallel grooves offset by half the groove spacing (as on  FIG. 6   b ). Wire insertion features for each groove as described above can be patterned into the surfaces of the silicon wafer. A transparency mask process at 3600 dpi can provide sufficient photolithographic resolution. Although a chrome mask process can provide the resolution required for smaller wire spacing (i.e., less than 0.025 mm), wires having a sufficiently small diameter for such spacing tend to be too mechanically fragile to withstand the weaving procedure. Accordingly, BNGs having such small wire spacing are preferably fabricated by other methods (e.g., microfabrication) that do not entail winding wire on a template. 
         [0041]    The preceding description has been by way of example as opposed to limitation, and numerous variations of the given examples can be made in practicing the invention. For example, the material composition of the templates, BNG frames, and wires is not critical in practicing the invention, and any suitable material may be chosen for these elements. Similarly, the details of how wires are affixed to the BNG frame are not critical for practicing the invention.