Radiant energy – Ionic separation or analysis – Ion beam pulsing means with detector synchronizing means
Reexamination Certificate
2002-06-07
2004-08-24
Wells, Nikita (Department: 2881)
Radiant energy
Ionic separation or analysis
Ion beam pulsing means with detector synchronizing means
C250S293000, C250S294000, C029S825000
Reexamination Certificate
active
06781120
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to a method for manufacturing a grid for gating a stream of charged particles.
Certain types of particle measurement instruments, such as ion mobility spectrometers, can require a gating device for turning on and off of a flowing stream of ions or other charged particles. This is accomplished by disposing a wire grid within the path of the ions; alternately energizing or de-energizing the grid then respectively traps the ions or allows them to flow.
Certain types of time of flight spectrometers, such as those described in the paper by Vlasak, P. R., et al., entitled “An interleaved comb ion deflection gate for m/z selection in time-of-flight mass spectrometery,” in
Review of Scientific Instruments
, Vol. 67, No. 1, January 1996, pp. 68-72, also utilize a gating device.
The most common methods for accomplishing this use an interleaved comb of wires also referred to as a Bradbury-Nielson Gate. Such a gate consists of two electrically isolated sets of equally spaced wires that lie in the same plane and alternate in potential. When a zero potential is applied to the wires relative to the energy of the charged particles, the trajectory of the charged particle beam is not deflected by the gate. To deflect the beam, bias potentials of equal magnitude and opposite polarity are applied to the two sets of wires. This deflection produces two separate beams, each of whose intensity maximum makes an angle alpha with respect to the path of the un-deflected beam.
One approach to manufacturing a gating grid is disclosed in U.S. Pat. No. 4,150,319 issued to Nowak, et al. In this technique, a ring-shaped frame is fabricated from a ceramic or other suitable high temperature material. The two sets of wires are wound or laced on the frame. Each set of wires is actually a single, continuous wire strand that is laced back and forth between two concentric series of through-holes that are accurately drilled around the periphery of the frame.
Another technique for manufacturing such a gate is described in U.S. Pat. No. 5,465,480 issued to Karl, et al. In this approach, the gating grid elements are produced from a thin metal foil by cutting or etching the foil to produce the grid structure. The gird elements are connected to side electrodes in a desired pattern to produce the two sets of wires. The foil grid structure is made mechanically stable by attaching it to an insulating support member. After the then-rigid grid structure is affixed to the insulating support member, the grid elements are selectively severed from the side electrodes to form the interdigitated grid.
Yet another approach for manufacturing such a grid is described in the paper by Kimmel, J. R., et al., entitled “Novel Method for the Production of Finely Spaced Bradbury-Nielson Gates,” in
Review of Scientific Instruments
, Vol. 72, No. 12, December 2001, pp. 4354-4357. In this method, a guide is first manufactured out of a polymer block. The guide has a series of evenly spaced parallel grooves. A hole is drilled through the center of the polymer block; this hole eventually carries the ion beam. The machined polymer block is mounted on an insulated face of an H-shaped portion of a single sided, copper clad circuit board, with the grooves running from top to bottom of the H. The polymer-to-copper clad contacts are then fixed using an epoxy. Two small portions of the single sided copper clad board are fixed on the bottom side of the polymer in the region where the block extends over the center bar of the H-shaped copper frame.
A hand cranked, rotating screw is then used as a weaving instrument. In particular, a gold-plated tungsten wire runs from a spool over a directing screw and is coupled to the hand cranked screw by a belt. The loose end of the wire is then fixed such as by using an epoxy. A weight is hung from the wire between the directing screw and the spool in order to provide a constant tension on the wire.
Beginning at one side of the center hole, the hand crank is turned, which rotates the frame, drawing the thread from the spool. While watching through a microscope, an assembler feeds a first wire set through alternating grooves in the surface of the polymer and around the frame, making sure to touch both contacts on each pass. After winding the wire across the entire width of the opening, the wire is bound to both copper contacts on either side of the hole using an epoxy. A razor blade is then used to remove the segment of the wire between the two contacts on the side of the frame opposite the polymer.
Using the same procedure as for the first wire set, a second wire set is then wound through the grooves located between the wires of the first set. The ends of the wires are then cut, leaving wire only on the polymer side of the frame.
SUMMARY OF THE INVENTION
There are deficiencies with each of the prior art approaches to fabricating such grid elements. For example, the technique described in the Nowak patent relies upon the precise placement of two sets of aligned holes on either side of a ring. Since it uses a single strand of wire which is hand woven through the holes, it does not take into consideration the need to assure a constant mechanical tension among wires in the assembled grid. Unless the mechanical tension is relatively uniform across all wires of the grid, undesirable artifacts are introduced by irregular tension. For example, at elevated operating temperatures, the larger coefficient of expansion of the metal as compared to the ceramic support could also cause the wires to sag, potentially shorting them out if they are not properly pre-tensioned. Likewise, the imprecise nature of tensioning the wire by hand often leads to wires that are not uniformly parallel. Therefore, the field normal to the grid does not decay as rapidly as theoretically possible.
Additionally, for high speed applications, the phase delays resulting from propagation of the bias current along the single continuous strands from the contact point may cause the ions to experience a deflection at different times, depending upon where they happen to be in the beam path.
Furthermore, because the frame in Nowak is circular, the individual wires are of different lengths. This means that each wire then presents a different characteristic impedance to current flowing through it. This likewise introduces different effects to different ions, depending upon where they happen to be in the beam path. Thus, ions traveling the center of the beam are subjected to a different electrical force than ions traveling in the outer portion of the beam where the grid wires are shorter.
Finally, the required thickness of the support structure in Nowak limits how closely two grids can be placed with respect to each other.
Kimmel's approach, similar to Nowak's, weaves a single thread around a frame. It also requires the assembler to carefully feed the wire through one of the alternately spaced grooves. The individual wires in the set are then bound to the copper contacts using epoxy. The method of machining a polymer block to small tolerances of 0.005 mm for each grid wire can require relatively expensive machine tools.
Furthermore, if the single wire breaks during winding or any part of the process one must start over again, from the beginning, to restring the wire. The assembly procedure envisioned is apparently so tedious that Kimmel himself estimates that it takes approximately three hours to manufacture a single gate.
The presence of large amounts of insulating polymer surfaces near the beam path may cause substantial charging effects which could be detrimental to the operation of the gate, particularly for gating low energy electrons. Furthermore, a device formed from a polymer with epoxy bindings may not survive the high expected operating temperatures of some applications such as ion mobility spectroscopy.
The process described in the Karl patent does provide a grid having wires with uniform tension. A separate support structure for the foil-like grid element is be fabricated from tu
Crothers C. Bronson H.
Frederick Brian G.
Jackson, III Robert H.
Kleban Peter H.
LeCursi Nicholas
Hamilton Brook Smith & Reynolds P.C.
Quash Anthony
University of Maine
Wells Nikita
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