Charged particle energy filter

An ion energy filter of the type useful in connection with secondary ion mass spectrometry is disclosed. The filter is composed of a stack of 20 thin metal plates, each plate being insulated from the others and having a centrally located hole with a unique radius. A metallic hemisphere is mounted on a base plate, and the 20 thin metal plates are attached to the base plate such that the plate with the smallest central hole is adjacent to the base plate and the radii of the holes in subsequent plates increase with increasing distance from the base plate. The relative potential of each plate is determined by a series string of 20 resistors with each plate being connected to a different junction in the series string. The radii of the centrally located holes are selected such that the voltage on each plate is inversely proportional to the radius of its centrally located hole.

This invention relates to charged particle energy filters, and more 
particularly to devices capable of selecting only charged particles having 
energies within a relatively narrow range of energy. 
BACKGROUND OF THE INVENTION 
Secondary Ion Mass Spectroscopy (SIMS) is a surface analysis technique that 
characterizes materials by determining the mass of the secondary ions that 
are made to leave the material. To achieve maximum mass resolution, only 
those secondary ions having energies within a relatively narrow range must 
be allowed to enter the mass analyzer. See the article entitled "New wide 
angle, high transmission energy analyzer for secondary ion mass 
spectrometry", by M. W. Siegel and M. J. Vasile, Rev. Sci. Instrum., 
52(11), November 1981, pp. 1603-1615. 
Several ion energy filters have been designed to accomplish this. In all of 
the designs, the ions are subjected to electrostatic or magnetostatic 
fields, combined with trajectory selecting apertures. Filter designs that 
produce an electrostatic field between two concentric hemispheres are 
popular. Unfortunately, as the distance between the hemisphere is 
increased to permit larger elliptical orbits, the performance is 
compromised by increasingly large fringe fields between the edges of the 
two hemispheres. 
One ion energy filter in the prior art establishes a force field E with 
spherical symmetry where E.alpha.(1/r.sup.2), like that which would be 
produced between two concentric spheres by using one hemisphere on an 
infinite plane with a potential distribution on the plane that follows the 
relationship V.alpha.(1/r) where r is the radial distance from the center 
of the plane. See U.S. Pat. No. 4,126,781 issued Nov. 21, 1978 to M. W. 
Siegel. Because of the boundary condition established on the plane, a 
second larger hemisphere is not required, and fringe fields are 
eliminated. 
In the Siegel patent as in the above-identified Siegel et al article, a 
shaped resistive disk is used to establish the potential distribution 
proportional to 1/r. This resistive disk is made of a ceramic material 
impregnated with metal particles. Unfortunately, this impregnated ceramic 
material is porous and hence incompatible with ultrahigh vacuum 
applications. It has a poor electrical performance attributable to its 
nonuniform resistivity and the random localized charging of its surface. 
SUMMARY OF THE INVENTION 
The present invention is based on the idea that the potential distribution 
can be segmented into a number of equipotential concentric rings, and 
those rings need not be coplanar, provided the potentials applied to them 
obey the relationship V.alpha.(1/r). The problem of providing an ion 
energy filter with an improved electrical performance in a SIMS chamber is 
solved in accordance with the present invention wherein a plurality of 
circular conductive plates, each one of which has a centrally positioned 
hole of a different size from all of the other plates, are assembled to 
each other and to a base plate so as to form a stack wherein each plate is 
electrically insulated from all of the other plates. The base plate has a 
conductive hemispherical structure mounted at its center and all of the 
plates, where needed, have two holes diametrically positioned a 
predetermined distance from the center through which the ions can pass. 
Each plate also has a tab which is connected to a different junction in a 
series of resistors. By choosing the radii of the central holes in the 
plates and the values of the resistors, application of a single potential 
to the entire series of resistors will establish a potential distribution 
that is proportional to the reciprocal of the radius.

DETAILED DESCRIPTION 
An ion energy filter can be constructed in accordance with the present 
invention by fabricating 19 thin stainless steel plates of the type shown 
in FIG. 5. Each plate has a central hole with a unique inside radius. Each 
plate 500 has a tab 520 located at a unique place on the circumference of 
the plate. As the central hole size is increased, the position of the tab 
is moved counterclockwise when the plates are viewed from the top. 
An ion energy filter employing this set of plates can be constructed by 
fabricating a stainless steel base plate 400 of the type shown in FIG. 4. 
A metallic hemisphere 230 is bolted to the center hole 430 of the base 
plate as shown in FIG. 2. A ceramic tube 271 is placed in each of the 
counterbored holes 403 through 410. A flat Teflon washer 311 is placed 
around each of the eight ceramic tubes and adjacent to the base plate 400. 
A plate 500 of the type shown in FIG. 5 having the smallest central hole 
is positioned above the eight ceramic tubes. The plate is oriented so that 
the holes 501 and 502 align with the base plate holes 401 and 402, and the 
tab 520 is adjacent to the base plate opening 420. The plate is then 
further positioned to align the eight holes 503 through 510 with the eight 
ceramic tubes. The plate is made to slide down the ceramic tubes until it 
contacts the eight Teflon washers. The installation of alternating layers 
of Teflon washers and plates is continued until all 19 flat plates are 
installed. The plates are installed in the order of increasing central 
hole size. 
A final set of Teflon washers is installed followed by the top ring 600 of 
the type shown in FIG. 6, which has counterbored holes 603 through 610 
which accept the ceramic tubes as shown in FIG. 2 for two of the tubes. 
A ceramic shoulder washer 272 is placed around each of eight 0-80 machine 
screws 261. The machine screws 261 are inserted into the base plate holes 
403 through 410 from the underside of the base plate. The screws are 
guided by the ceramic tubes to the threaded holes 603 through 610 in the 
Top ring 600. The screws are threaded into these holes and tightened until 
the Teflon washers are compressed to their nominal thickness. The 
assembled stack can be represented by the cross-sectional drawing in FIG. 
2. 
When assembled in the above manner and viewed from the top, the tabs 520 on 
the plates 500 form a counterclockwise spiral of evenly spaced tabs as 
shown in FIG. 1. A resistor is welded between each of the adjacent tabs. 
Resistors are also connected between the lowest flat plate 500 and the 
base plate, and between the highest flat plate 500 and top ring 600. All 
resistors have the same value of resistance. A wire is connected to base 
plate 400, and another wire is connected to top ring 600. When these wires 
are connected to a voltage source, the resulting current flowing through 
the chain of equivalent resistors produces potential steps of equal value 
of the set of plates. 
In order to support the filter on the end of a quadrupole mass filter, an 
insulating disc 241 is secured to the underside of the base plate 400. A 
metallic cap 242 is in turn secured to the insulating disc (refer to FIG. 
2). The disc and cap each have a hole on center of a diameter 
significantly larger than that of hole 401 in the base plate 400. The 
insulating disc 241 and metallic cap 242 are centered at the axis of the 
hole 401, and are of such diameters as to not interfere with the hole 402, 
the entrance aperture of the ion energy filter. 
If the energy filter is to be used in an environment having strong ambient 
electromagnetic fields, these fields may interfere with the fields 
produced by the filter. To prevent this, a large metallic outer hemisphere 
centered with the small metallic hemisphere 230 can be secured to the 
beveled rim of the top ring 600. A small hole must be placed in the outer 
hemisphere to allow the entrance of the primary ion beam. The center of 
this hole must be on the axis of the energy filter entrance aperture 
defined by holes 402 and 502. 
Each resistor 150 is fabricated by winding a resistance wire (having a 
composition of 73 percent Ni, 20 percent Cr, and 7 percent miscellaneous 
metals such as Al and Fe) onto a solid ceramic body to achieve a 
resistance of 503 ohms. For the 20 resistors made, the resistance ranged 
from 502 to 505 ohms with a .+-.20 ppm/degrees C. temperature coefficient. 
Copperweld leads having steel wires with a 40 percent conductive copper 
plating were secured to each end of the ceramic body to provide a means of 
external connection to the resistance wire. 
In summary, the ion energy filter is composed of a stack of 20 thin metal 
plates, each insulated from the others and each having a centrally located 
hole with a unique radius. The plate closest to the plane of the origin of 
the hemispherical field provided by metallic hemisphere 230 on the base 
plate 400 has the hole with smallest radius. The radii of the holes in the 
subsequent plates increase with increasing distance from the origin. The 
relative potential of each plate is determined by a chain of 20 resistors, 
with each junction connected to a plate. When a direct current is passed 
through the resistor chain, potentials are developed at each junction, and 
therefore on each plate. 
For simplicity of design, all the plates and insulating spacers have the 
same thickness, and all the resistors have the same value, resulting in 
equal potential steps along the series string of resistors. The radii of 
the holes in the plates were chosen to position these potential steps so 
as to satisfy the relationship V.alpha.(1/r) where V is the voltage on the 
plate and r is the radius of the hole. Each plate has a tab which extends 
beyond the outside diameter of the generally circular plate and is 
positioned such that when the plates are assembled to form a stack, the 
tabs occur at equal intervals on the circumference of the stack. This 
permits the resistor leads to be fastened from tab to tab, greatly 
simplifying the wiring. Only two wires (the ends of the resistor chain) 
are required to power the filter. 
The two diametrically opposed apertures (formed by holes 501 and 502 in 
each of the plates) are positioned in the stack of plates and in the base 
plate below them (by holes 401 and 402) to permit the entrance and exit of 
secondary ions. One of the apertures serves as an entrance aperture and 
the other serves as an exit aperture. In addition to providing access to 
the filter, these apertures act as lenses. When filtering positive ions 
for SIMS, the center hemisphere 230 is biased at the filters maximum 
negative potential. Because hemisphere 230 is mounted directly on base 
plate 400, and the material sample being analyzed is positioned just 
beneath the entrance aperture in this plate, the positive ions leaving the 
sample are accelerated into the entrance aperture of the base plate. This 
increases the secondary ion collection efficiency of the filter. As the 
ions continue on their paths to the interior of the filter, they must pass 
through the entrance aperture provided by the holes in the plates. Each 
plate that they pass is biased less negatively than the preceding plate. 
The ions, therefore, experience a deceleration. The effect of this 
deceleration is to launch the ions into the central force field of the 
filter at the energies required for near circular orbits. 
As the selected ions approach the exit aperture of the plates, they are 
accelerated out of the filter by the increasingly negative potentials on 
the plates and the base plate. When the ions travel between the base plate 
and the quadrupole mass analyzer, they experience a deceleration because 
of the large negative potential on the base plate relative to the virtual 
ground of the quadrupole axis. This deceleration is necessary for proper 
mass analysis. 
What has been described hereinabove is an illustrative embodiment of the 
present invention. Numerous departures may be made therefrom without 
departing from the spirit and scope of the present invention. For example, 
the resistors may be fabricated with unequal values and the radii of the 
central holes in the plates adjusted accordingly to continue to achieve a 
potential distribution which is proportional to the reciprocal of the 
radial distance from the center.