Patent Application: US-50351604-A

Abstract:
a method of fabricating miniature quadrupole electrostatic mass filter has been previously described . the electrodes are metallised cylinders , mounted in grooves etched in oxidised silicon substrates , which are held apart at the correct spacing by cylindrical spacer rods . this invention concerns an ion source mounted on extensions of the spacer rods , which project beyond the mass filter . the ion source consists of a cold - cathode electron emitter , which emits electrons with energies sufficient to cause impact ionisation , and electrostatic optics suitable for coupling the ion flux into the mass filter . methods of constructing a single self - aligned electron source and a similar dual source are described . arrangements for mounting the electron source and the ion coupling lens so that the electron and ion beams travel at right angles to one another for efficient separation are described . a method of fabricating a self - aligned one - dimensional einzel electrostatic lens from metallised cylinders mounted in the silicon substrates using etched grooves is described . a method of fabricating a - self - aligned two - dimensional einzel lens from metal plates is also described .

Description:
fig1 and 13 show the design of a cold - cathode field emission impact ionisation source that is specifically designed for use , with the quadrupole lens mass filter of fig7 . as previously described , this device consists of four cylindrical electrodes mounted in pairs on two oxidised silicon substrates , that are held apart by two cylindrical spacer rods . v - shaped grooves formed by anisotropic etching are used to locate both the electrodes and the spacers . this form of groove can be fabricated by etching ( 100 )- orientated silicon wafers down ( 111 )- orientated crystal planes in ( for example ) a mixture containing ethylene diamene , pyrocatechol and water , or in a mixture containing potassium hydroxide and water . the two halves of the structure are self - aligning , so that the correct relative spacing and orientation between the pairs of electrodes is automatically achieved to a high accuracy . the overall ion source assembly is illustrated in fig1 and is mounted on extensions of the two cylindrical spacer rods 9 , which are here lengthened to project beyond the filter . the electron source is a cold - cathode device , consisting of an array of field emission triodes , with each cathode 4 being controlled by a common gate 5 and a common anode 3 , so that the cathodes 4 operate in parallel and a high total emission current is obtained . the emitter is formed on two separate silicon substrates 1 , 2 . the cathodes 4 and the gate electrode 5 are formed in one substrate 2 , and the anode 3 on the other 4 . electrical connections 6 , 7 , 8 are provided for the anode 3 , gate 5 and cathodes 4 respectively . the cathodes 4 may be formed from an array of etched silicon tips , according ( for example , but not exclusively ) to fig9 . alternatively , a knife - edge metal emitter may be used , according ( again , for example , but not exclusively ) to fig1 . each silicon substrate again carries v - shaped alignment grooves 10 formed by anisotropic etching down crystal planes , with a dimension and spacing identical to the alignment grooves already existing on the quadrupole filter . the electron source may therefore be attached to the filter by placing the two substrates 1 , 2 on either side of the spacer rods 9 protruding from the filter . this arrangement is inherently compatible with the filter construction , and allows self - aligned addition of an electron source with an emission direction that is perpendicular to the intended ion flight path . if the substrates 1 , 2 used for the electron source are separate from those used for the filter , as shown in fig1 a , either half of the source may be removed and replaced as required . the filter electrodes 11 are as illustrated in fig7 . alternatively , the substrate 2 carrying the cathode array 4 and gate 5 may be combined with one of the two filter substrates as shown in fig1 b , and the substrate 1 carrying the anode 3 may be similarly combined with the other filter substrate , to form an integrated assembly . in the case when the electron source and filter substrates are combined , the cathode 4 must be insulated from its substrate , which extends beneath the filter and is held at ground potential . if the cathode material is itself silicon , the isolation may be obtained ( for example , but not exclusively ) by forming the cathodes 4 in a bonded silicon - on - insulator ( bsoi ) wafer instead of a conventional silicon wafer . a bsoi wafer consists of a layer of single - crystal silicon bonded to an oxidised silicon substrate . the bonded layer may be processed to form a silicon terrace carying the cathode array , which is isolated from the substrate by the silicon dioxide interlayer . alternatively , if the cathode 4 is not silicon , the desired isolation may be obtained by other methods involving deposited layers . a dual field - emission electron source may also be constructed . two identical substrates are used , each carying an array of cathodes 4 , 4 a and a gate electrode 5 , 3 , as shown in fig1 a . to operate the lower source , the cathode 4 and gate electrode 5 of the lower source are connected to appropriate voltages as usual . the cathode 4 a of the upper source is disconnected , and the gate electrode 3 of the upper source is connected to an appropriate voltage and used as a temporary anode for the lower source . by making simple changes to the electrical connections , the upper source may be operated as shown in fig1 b . in this way , a secondary electron source may be provided in the case of a failure in the primary source . the ion entrance optics may be constructed by several different methods . a form of einzel lens may be constructed as shown in fig1 . here the three electrodes required are three pairs of parallel cylinders 12 , 13 , rather than plates containing apertures . this arrangement functions as a lens that focuses the ions in one dimension only . the cylinders 12 , 13 may be mounted in grooves 14 in the silicon substrates as shown in fig1 , using the same mounting method as the filter electrodes 11 . the required ninety - degree relative orientation of the grooves can be achieved because the two possible lines of intersection between the ( 111 )- oriented planes with the surface of the ( 100 )- orientated silicon wafer lie at ninety degrees to one another . this process requires no significant modification to the process used to construct the mass filter . all that is required is the photo - lithographic definition of further locating grooves 14 , then etching , oxidation and metal coating of those grooves together with existing similar features , and finally soldering of additional electrodes 12 into those grooves . furthermore , the top surface of the electrodes 12 may be located at a significant height above each substrate , without the need for deep lithography and electroplating . that height may be controlled simply by appropriate choice of the width of the alignment groove 14 and the diameter of the cylindrical electrode 12 . the two halves of the lens are automatically located symmetrically on either side of the entrance pupil as shown in fig1 . alternative methods may also be used to form a lens that focuses in two directions , according to the general approach in fig1 and 5 . for example , deep photolithography and electroplating may be used to make a two - dimensional einzel lens . however , in the invention here , the lens is constructed in two halves on two separate silicon substrates , as shown in fig1 . each substrate carries a two - level metal structure 15 formed by successive applications of lithography , and electroplating . the structure formed in the level nearest each substrate is a set of three continuous bar electrodes 16 . the structure formed on the level furthest from each substrate is a set of three broken bar electrodes 17 , superimposed on the lower level structure . the two silicon substrates 1 , 2 carry etched alignment grooves 10 , so that they may be assembled on to a pair of cylindrical spacer rods 9 as shown in fig1 . in this case , the two halves of the electroplated metal structure 16 , 17 align together to form a lens with a split pupil . provided the central gap between the electrodes 16 , 17 is small , and appropriate and similar voltages are applied to each set of electrodes , this gap is of little significance . this process of construction requires only two levels of lithography and electroplating . furthermore , the heights of each level need not be accurately defined . provided the electrodes are all fabricated in a similar manner , the two halves of the lens are automatically located symmetrically on either side of the entrance pupil . an alternative method of forming a similar structure is to fabricate two stacked electrode assemblies 16 , 17 as entirely separate structures . the assemblies may then simply be soldered to the two substrates 1 , 2 in a perpendicular orientation at the entrance to the mass filter . the ion entrance optics may be combined with either the mass filter or the electron emitter , or all three elements may be combined . if any of the elements are separate , they may attach to the common pair of cylindrical spacer rods 9 . if all three elements are combined , one of the cylindrical spacer rods 9 a may be rotated through ninety degrees as shown in fig2 . alternatively , the cylindrical spacer rod 9 a may be replaced by a sphere , of the same diameter and located in pyramidal etched pits . alternatively , the other rod 9 may be replaced by two spheres , so that there are three spheres in total positioned at the apices of a triangle . either arrangement has the advantage of substantially eliminating axial motion , making the assembly truly self - 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