Precision electrostatic lens system and method of manufacture

A precision electrostatic lens system is formed from lens electrodes having high precision inner bores by aligning said bores on a ceramic rod, inserting segmented glass spacers between adjacent electrodes, and brazing the combination together to provide a monolithic structure capable of maintaining accurate tolerances.

BACKGROUND OF THE INVENTION 
The present invention relates to electrostatic lens systems and 
particularly to a system for accurately focusing a charged particle beam 
to a very small spot. 
Apparatus for generating charged particle microbeams for use in laboratory 
instruments such as scanning electron microscopes and the like require 
highly accurate beam focusing. The electrostatic focusing lens system 
employed should be of high precision, exhibiting a high degree of 
concentricity and circularity, to generate a beam that can be concentrated 
to a very small spot on a target or specimen. 
The lens electrodes in such a focusing lens system need to be provided with 
precisely aligned apertures through which the charged particle beam 
passes, while the electrodes must also be spaced at precise distances in 
parallel relation to one another. Stacked ceramic cylinders have been 
employed for spacing and supporting focusing electrodes, wherein the 
cylinders provide an outer reference. An accurately manufactured cylinder 
peripherally supports an electrode or electrodes therewithin such that the 
electrodes are then centered. However, not only must the ceramic cylinders 
be machined to high tolerances, but also the lens electrodes must have 
accurately machined circumferential surfaces where they are received by 
the cylinders. Since the tolerances at the interfaces are added together, 
it can be very difficult to hold accurate concentricity and parallelism. 
More commonly, a glassed structure is employed wherein focusing electrodes 
are supported on metal wires that extend inwardly from glass members. This 
method may employ an aluminum centering rod or a mandrel that is later 
removed chemically after the glassed supporting structure has been formed. 
However, focusing columns manufactured in this way tend to vibrate and in 
particular resonate at certain frequencies. The result is undesired beam 
vibration. Furthermore, these structures are difficult to manufacture 
since considerable manufacturing skill is required. 
SUMMARY OF THE INVENTION 
According to the present invention, a precision electrostatic lens 
structure is manufactured by forming plural metal electrodes with side 
surfaces adapted for parallel juxtaposition and having accurate bores 
extending centrally therethrough in a direction substantially 
perpendicular to the side surfaces. A ceramic aligning rod is positioned 
through the bores of adjacent electrodes where the rod is closely received 
by the bores for precisely aligning the electrode bores. Dielectric spacer 
members of accurately predetermined thickness are inserted between 
adjacent electrodes and a brazing material is inserted at the interfaces 
between the adjacent electrodes and the spacer members. The electrodes 
with spacer members therebetween are then urged together at an elevated 
temperature for a predetermined time period for accomplishing brazing 
between the adjacent electrodes and the spacer members. The aligning rod 
is removed from the resulting monolithic focusing structure which is then 
capable of accurately maintaining the desired configuration. 
In accordance with the preferred form of the present invention, the spacer 
members are segmented and the segments are spaced from one another around 
the axis of aligned bores to minimize differential expansion and 
contraction between the electrode surfaces and spacer members during the 
heating step or during gun usage. Improved access to the electrodes is 
enabled in a compact structure. The assembly produced is characterized by 
enhanced concentricity and parallelism between elements on the order of 
about ten microns. 
It is accordingly an object of the present invention to provide an improved 
precision electrostatic lens structure capable of accurately concentrating 
charged particle beams. 
It is another object of the present invention to provide an improved 
precision electrostatic lens structure which is capable of maintaining 
dimensional accuracy. 
It is another object of the present invention to provide an improved 
precision electrostatic lens structure exhibiting very high precision 
concentricity, circularity and electrode parallelism. 
The subject matter of the present invention is particularly pointed out and 
distinctly claimed in the concluding portion of this specification. 
However, both the organization and method of operation, together with 
further advantages and objects thereof, may best be understood by 
reference to the following description taken in connection with 
accompanying drawings wherein like reference characters refer to like 
elements.

DETAILED DESCRIPTION 
Referring to FIG. 1 illustrating a charged particle gun structure, the 
interrelationship of the elements will be discussed with respect to 
electron emission although it will be realized the same focusing system 
can be employed with appropriate changes in potential for similarly 
controlling an ion beam. In the present example electron beam 10 is 
derived from Schottky emission source 12, the tapered point of which 
extends through a small aperture in the lower or forward end of the 
suppresser electrode 14. The Schottky emitter is supported from conducting 
rods 16 extending through ceramic member 18 as received within the upper 
cylindrical part of the suppresser electrode. Shield 20, maintained at the 
potential of the suppresser electrode by means not shown, extends 
downwardly in surrounding relation to the suppresser electrode and 
emitter, as well as the upper portion of the lens structure. 
The lens structure comprises an extractor electrode 22 having an upper 
cylindrical part 24 covered at its upper extremity by extractor cap 26 
except for a small (e.g. 15 mil) central aperture in the cap aligned with 
a similar sized aperture in suppresser electrode 14 and through which 
electron beam 10 passes. The central aperture in cap 26 is located in a 
thin section of the cap surface with the underside of the cap tapered away 
from this aperture. Lower cylindrical portion 28 of extractor electrode 22 
is substantially solid except for central bore 30 suitably having a 
diameter of 120 mils. A beam defining piece 32 is positioned within a 
recess at the upper end of bore 30 being provided with a small beam 
defining central aperture on the order of 2 mils in diameter. 
The electrode 22 includes upwardly open cylindrical section 24 with a 
relatively large inside diameter, and the lower, larger outside diameter 
cylindrical section 28 is provided with precision inner bore 30 intended 
to be substantially coaxial with the path of electron beam 10. A radial 
flange 27 extends outwardly from the lower part of electrode section 24 
for mounting and electrical connection purposes, receiving a post or lead 
29 suitably connected to an appropriate voltage source. 
The electrostatic lens system further comprises a second or focusing 
electrode 34 spaced precisely below extractor electrode 22. Upper 
cylindrical section 36 of electrode 34 is substantially greater in inside 
diameter than the exterior diameter of electrode section 28 thereabove and 
is disposed in surrounding or overlapping relation with respect to 
approximately the lower one-third of electrode section 28. The lower wall 
37 of electrode section 36 is provided with a central bore 38 precisely 
aligned with bore 30, i.e., coaxial with the path of electron beam 10, and 
suitably of the same diameter, e.g. 120 mils. A lower cylindrical skirt 
portion 40 of electrode 34 extends downwardly from lower wall 37, the 
inside diameter of the skirt portion being substantially larger than that 
of bore 38 while its outside diameter is suitably less than that of 
electrode section 36. The inside cylindrical cavity of skirt portion 40 is 
precisely coaxially aligned with the path of electron beam 10 and bore 38. 
Electrical post or lead 42 connects electrode 34 to a suitable operating 
voltage. 
The third or lower cylindrical electrode of the electrostatic lens 
comprises a precisely positioned anode or ground electrode 44 having an 
upper inside diameter greater than the exterior diameter of the 
aforementioned skirt 40, and including a central transverse wall 46 
located below skirt 40, the wall 46 having surfaces parallel with 
juxtaposed surfaces of the other electrodes and provided with a precisely 
located center bore 48 in coaxial alignment with the aforementioned bores 
30 and 38 as well as with the path of electron beam 10. The diameter of 
bore 48 is suitably the same as that of bores 30 and 38, e.g. 120 mils in 
the case of a specific embodiment. The upper open portion of electrode 44 
overlaps approximately the lower quarter of skirt 40. Electrode 44 further 
includes a lower cylindrical flange 48 which raises wall 46 above the 
surface of support plate 50 separating the electron gun from a chamber 
therebelow suitably housing deflection apparatus and lens means (not 
shown) for a scanning electron microscope or the like. An aperture 52 in 
plate 40 passes electron beam 10. 
The exterior diameters of electrode 44, electrode section 36, and flange 27 
are at least approximately the same, and the differences between inside 
and outside diameters of electrode 44 and electrode section 36 are 
approximately the same so as to engage dielectric spacer elements 54 and 
56 disposed respectively between flange 27 and electrode section 36 and 
between electrode section 36 and electrode 44. Source 12 is precisely 
aligned with respect to the centerline of bores 30, 38 and 48 with a 
movable centering means (not shown) ultimately supported from plate 50. 
In a specific example, the following voltages were applied to the various 
electrodes: 
______________________________________ 
Emmiter 12 -25 KV 
Suppresser 14 -25.5 KV 
Extractor 24 -20 KV 
Focus 34 -24.3 KV 
Anode 44 0 
______________________________________ 
The focusing properties of this general type of lensing system are as 
described in U.S. Pat. No. 4,629,898 to Orloff and Swanson. 
According to the process of the present invention, the above-described 
electrodes and spacer elements are brazed together in a precision manner 
to provide a monolithic structure wherein close tolerances are held for 
accurately focusing an electron beam or other charged particle beam. The 
materials selected are suitable for comparatively high temperature brazing 
and are chosen not to outgas at the extremely low pressures encountered 
during normal operation of the FIG. 1 apparatus. 
A high degree of parallelism is desired between the various juxtaposed 
electrode surfaces as well as between the electrodes and the insulating 
spacers 54 and 56 disposed therebetween. A refractory material for the 
insulating spacers 54, 56 may be employed, with alumina or a similar 
ceramic being suitable examples. However, a machinable glass of the type 
manufactured under the name Macor by Corning Glass Company is preferred 
because of its electrical breakdown avoidance properties and because its 
machinability facilitates the preparation of spacers 54, 56 having exactly 
parallel side faces spaced apart by a predetermined thickness such that 
the electrodes spaced therewith can be maintained in precise parallelism 
and at exact positions. A high temperature compatible conductive material 
for the electrodes, substantially matching the thermal expansion 
coefficient of the spacers, is employed and in a specific embodiment 
comprised a titanium alloy composed of approximately 90% titanium, 6% 
aluminum and 4% vanadium. 
FIG. 2 illustrates the process of assembling the electrode and spacer 
elements together for brazing. The components are suitably assembled for 
positioning on an oven platform 58. A refractory rod 60 is inserted 
through the central bores in electrodes 22, 34 and 44 for precisely 
aligning the same, the juxtaposed electrode surfaces 62, 64, 66, 67 and 68 
being perpendicular to the aforesaid bores. The rod 60 is closely received 
in the bores for rendering the same nearly perfectly coaxial, and may also 
extend into a mating hole in platform 58 to insure perpendicularity. Rod 
60 is suitably a ceramic material and in a preferred embodiment was formed 
of alumina, a material that does not adhere to the titanium electrodes. 
Assuming the diameter of each bore is 0.1202+0.0002-0 inches, the outside 
diameter of the rod should be 0.1200+0-0.0002 inches, with the spacing 
between the rod and the apertures it centers desirably being about 2/10 of 
a thousandth of an inch. In this example, the central bores in each of the 
electrodes 22, 34 and 44 are of the same diameter, but bore diameters of 
progressively changing sizes can be accommodated with a stepped refractory 
rod, i.e., assuming the bores proceed in order of ascending or descending 
diameter. 
Opposing juxtaposed surfaces 62, 64, 66, 67 and 68 of the electrodes are 
machined to be precisely parallel with each other as well as with 
juxtaposed surfaces 70, 72, 74 and 76 where the latter meet parallel faces 
or pads 78, 80, 82 and 84 of the spacer elements 54, 56. The spacers are 
also machined to a high tolerance in terms of thickness and parallelism, 
and therefore the parallelism of electrode surfaces 62, 64, 66, 67 and 68 
is assured. 
In accordance with the preferred embodiment, the spacers 54, 56 inserted 
between adjacent electrodes are segmented and preferably separated as 
illustrated in the cross sections of FIGS. 3 and 4. Thus, spacer 54 
includes three sections 54A, 54B and 54C, comprising 60-degree segments or 
portions of a circular configuration disposed at 120-degree intervals 
about the axis of the electrode bores and rod 60, and having 60-degree 
spacing therebetween. In manufacture, the spacers 54, 56 are formed as 
completely solid figures of revolution and are severed into the segments 
as shown, wherein, for purposes of the present discussion, a segment is 
defined as a solid having a transverse cross section (perpendicular to the 
beam) of a truncated sector of a circle. This segmentation reduces the 
possibility of cracking after the subsequent brazing step because of the 
thus limited differential expansion or contraction between the ceramic and 
metal elements. The segmentation further provides better access to the 
electrodes, e.g. for connecting the same, and provides a smaller and more 
compact structure. The segments can have other cross sectional shapes if 
desired. 
It will be noted that the parallel pads 78, 80, 82 and 84 on insulating 
spacers 54, 56 providing spacer faces which meet the electrodes are 
limited in radial extent so as to abut electrode surfaces 70, 72, 74 and 
76 proximate the outer periphery of the electrodes and are relieved away 
from the electrodes except for inner circular pads 86 and 88 which abut 
the outer periphery of elongated electrode sections 28 and 40. Moreover, 
the spacers 54, 56 are provided with circumferential grooves at 90 and 92, 
whereby additionally to provide a more lengthy surface path between 
adjacent electrodes to help prevent high voltage breakdown during 
operation. 
The insulating spacer segments, suitably having brazing material applied 
thereto, are slideably inserted between adjacent electrodes, along the 
parallel surfaces of the electrodes they separate, until the pads 86, 88 
respectively abut elongated cylindrical section 28 of electrode 22 and 
skirt 40 of electrode 34. The insulating spacer segments are then drawn 
toward the electrodes by means of tensioning wires 94 and 96 threaded 
around grooves 90 and 92 respectively in the various spacer segments and 
drawn tight. 
Brazing material applied between the electrodes and spacer elements, i.e., 
at the interfaces defined at 70, 78; 72, 80; 74, 82; and 76, 84, 
respectively, suitably comprises CuSil A.B.A. (active braze alloy) 
manufactured by GTE Products Corp. of Belmont, Calif. This material is 
provided in paste form for application. The brazing is desirably a 
one-step process avoiding metalizing of the glass or ceramic parts. Other 
brazing materials such as foils or wires or other preparations can 
alternatively be utilized. 
After insertion of rod 60 and the segmented spacer elements 54, 56, with 
the spacer elements drawn toward the electrodes with wires 94, 96, the 
assembly with the brazing material applied is located in a vacuum oven and 
a weight 98, having a central aperture for passing rod 60, is placed on 
top of electrode 22. The weight 98, which is suitably a one pound 
molybdenum slug, insures the stack of elements is compressed vertically 
during the braze whereby the elements settle to a flat or parallel 
condition. The combination is raised in temperature to approximately 
810.degree. C. over a six-hour period, gradually increasing the 
temperature over approximately 2.75 hours, holding the temperature at 
810.degree. C. for one-half hour, and gradually reducing the temperature 
during a following period of 2.75 hours. The slow cycling up and down in 
temperature avoids introducing too much thermal stress along the brazed 
surfaces. After cooling, the weight 98 and rod 60 are removed, with the 
resulting structure, taken from the oven, comprising a monolithic 
electrode column characterized by very close tolerances and capable of 
maintaining those tolerances for an extended time under adverse 
conditions. It has been found that the resulting assembly is characterized 
by enhanced concentricity and parallelism on the order of about 10 
microns. For use in an electron gun or the like, beam defining piece 32, 
not present during previous assembly and brazing, is received within an 
accurately countersunk hole in electrode 22. The gun may be subsequently 
baked for an extended period for outgassing in a conventional manner at 
approximately 180.degree. C. 
While a preferred embodiment of the present invention has been shown and 
described, it will be apparent to those skilled in the art that many 
changes and modifications may be made without departing from the invention 
in its broader aspects. The appended claims are therefore intended to 
cover all such changes and modifications as fall within the true spirit 
and scope of the invention.