Multistage cylindrical mirror analyzer incorporating a coaxial electron gun

A multi-stage cylindrical mirror analyzer incorporates a primary radiation source, such as an electron gun, disposed internally and along the axis of the multi-stage analyzer. The gun includes all of the optical elements for producing a well defined beam, correcting aberration thereof and scanning the beam on a sample. The components of the gun are distributed along the axial length of the analyzer. Aberration of the scanned beam due to traversal of a subsequent lense is minimized by placing the pivot point of the deflected beam trajectory substantially at the center of the lense. The greater dispersion of the multi-stage analyzer and the unit magnification thereof permit proportionately greater exit aperture dimensions, whereby a wider field of view may be realized.

FIELD OF THE INVENTION 
This invention relates to the field of surface analysis apparatus and in 
particular to the combination of a charged particle gun and a cylindrical 
mirror analyzer. 
BACKGROUND OF THE INVENTION 
A study of surfaces and near surface composition of a sample is 
accomplished with a well collimated ion or electron beam to impinge the 
sample and an efficient analyzer for the secondary radiations scattered or 
evolved from the surface. A well-known form of such apparatus is the 
cylindrical mirror analyzer (CMA) with internal axially aligned electron 
source. A representative example of such prior art apparatus is the Varian 
Model 981-2707 cylindrical mirror analyzer and integral gun, Model 
981-2773. This apparatus comprises coaxial cylinders with an electron gun 
disposed along the common axis and surrounded by the inner cylindrical 
wall of the analyzer. 
It has been known previously to employ multiple stages of cylindrical 
analyzers and the theoretical analyses of the optics thereof is well 
understood. 
SUMMARY OF THE INVENTION 
It is an object of the invention to achieve improved energy resolution and 
geometric resolution over a wide field of view for surface analytic 
apparatus such as an Auger microprobe incorporating electrostatic analysis 
by cylindrical mirrors. 
In one feature of the invention, an electron gun is disposed along the 
internal length of the common axis of a multi-stage CMA. 
In another feature of the invention, the field of view over which a nearly 
constant intensity excitation beam may be swept, for fixed range of 
variation in analyzer response, is increased approximately by a factor n 
where n is the number of stages of the analyzer. 
In yet another feature of the invention, aberration of the deflected beam 
due to traversal of a subsequent lense is minimized by pivoting the 
deflected beam about the center of the lense. 
This object and features are accomplished by disposing a charged particle 
gun including all of its attendant optical elements along the axis of a 
multi-stage CMA. The various elements of the gun are distributed 
internally within the several sections of a multi-stage CMA. The greater 
dispersion afforded by the multi-stage CMA permits a wider field of view 
for given energy resolution and geometric resolution.

DETAILED DESCRIPTION OF THE INVENTION 
A preferred embodiment of the invention comprises a two-stage CMA and 
axially disposed electron gun as illustrated in FIG. 1. For the purposes 
of this discussion "electron gun" refers to the entire beam forming and 
scanning apparatus. The two-stage CMA portion of the apparatus comprises a 
pair of spaced coaxial metal cylinders 10 and 12 with respective radii 
r.sub.10 and r.sub.12 arranged on axis 13. These cylinders form a 
cylindrical capacitor characterized by a radially directed electric field 
in the space therebetween. The inner cylinder has an intermediate aperture 
14 located at the midpoint of the axis which divides the stages of the 
CMA. Secondary electrons from the sample pass through this aperture 14 if 
their energies are within the energy band selected by the CMA. The 
principal purpose of aperture 14 is to prevent electrons which pass 
through the first stage from striking elements of the electron gun and 
scattering into the second stage. 
Nearly annular slots 15 and 15' are formed in the inner cylinder to permit 
entrance and exit respectively of the particle trajectories under analysis 
into the radial electric field space between cylinders 10 and 12. Similar 
slots 16 and 16' serve similar purposes for the second stage of the 
analyzer. These slots are each conventionally gridded by mesh 18 to 
preserve a generally equipotential cylindrical surface and prevent 
unwanted electric field distortion due to the discontinuities introduced 
by the presence of the slots. 
End effects introduce distortions of the electric field for finite length 
cylinders. These are relieved in a well known manner by a system of guard 
rings 19 for dividing the potential between cylinders with a resistive 
network (not shown). The extreme trajectories 17 and 17' are defined with 
respect to a focus 20. A sample surface 21 is positioned at focus 20. It 
will be appreciated that the sample is situated in a vacuum enclosure 
although such enclosure does not appear in FIG. 1. The focal distance 
determines the location of the focus and is a design parameter of the 
analyzer. This parameter and radii r.sub.10 and r.sub.12 geometrically 
determine .alpha., the mean angle of analyzer acceptance, as measured with 
respect to the analyzer axis. Optimum values for .alpha. may be found for 
given relative dimensions of the CMA according to well-known analytic 
treatments. In each stage of the CMA, the entrance and exit apertures are 
preferably symmetrically disposed on the axis with respect to the midplane 
from each of the respective stages and the stages are themselves 
symmetrically disposed in respect to intermediate aperture 14. In general, 
the two stages need not be identical (or symmetrically disposed with 
respect to the midplane). For example, a shorter second stage may be 
achieved if the electric field in the second stage is appropriately 
increased. It will readily occur to one skilled in the art to accomplish 
this end by employing the same potential difference between the cylinders 
while decreasing the inner-electrode space, as for example by increasing 
the inner radius. 
Final aperture 24 defines the image point which is preferably located 
symmetrically with the object point. Aperture 24 may be a simple circular 
hole as shown, or annular if displaced along the axis toward the 
intermediate aperture 14. The dimensions of aperture 24 are selected to 
accept a portion of the trajectories transmitted by the analyzer. In a 
preferred form, aperture 24 may be variable in its dimensions to permit 
selection of a particular narrow band of trajectories defined by the 
analyzer. This may be accomplished readily by providing a hermetically 
sealed rotary feedthrough not shown to position a desired aperture at the 
indicated position. Particle detector 25 such as, for example, an electron 
multiplier, or a scintillator and photomultiplier is provided for 
detection of the particles transmitted by the analyzer and aperture 24. 
A particle beam source, as for example, an electron gun, is disposed on the 
axis of the CMA as described below. Such a gun comprises an electron 
source 30, anode 32 for establishment of the longitudinal accelerating 
fields for the beam, 1st lense 34 alignment plates 36, anti-scattering 
aperture 38 and secondary electron suppression tube 39 with defining 
aperture 40 located therein, second electrostatic lens 42, a second set of 
alignment plates 44, objective aperture 46, stigmator assembly 48, 
deflector plates 50 and 50' and final lens 52. Other electron optical 
elements may be inserted in the space available, as may be desired. 
Because the primary beam passes through the same region as the analyzed 
beam, it is essential that the primary beam be carefully collimated to 
remove the possibility of scattering or secondary electron emission 
consequent to the primary beam striking intermediate aperture 14 or other 
structure in this region. Aperture 38 is carefully designed and positioned 
to prevent the entrance of such stray electrons into the second stage of 
the analyzer. Aperture 38 also serves a beam restrictive function. By 
minimizing the number of electrons passing through the front focal region 
of the second part of the analyzer, scattering from residual gas molecules 
in this region is minimized and can be reduced to a negligible level. Two 
sets of alignment plates 36 and 44 are provided to align the beam with 
respect to the respective apertures 40 and 46 whereas deflection plates 50 
and 50' provide transverse deflection for scanning the sample. The 
electrostatic lenses may be cylindrical, multiple aperture or quadrupole 
lenses as may be required for desired optical properties. 
The distribution of the elements of the electron gun along the axis of the 
two-stage CMA entails a division of components including all of the 
attendant electron optics, among the axial spaces of both stages of the 
CMA. Because the beam is often employed to scan a sample, certain benefits 
innure to the combination of an n-stage CMA with an internal axial gun. 
For example, a two-stage CMA possesses twice the dispersion, E 
(.DELTA.z/.DELTA.E), compared to a single stage CMA where E is the 
particle energy and z is the axial displacement of the intersection of 
trajectories. This remains true, although comparable single and two-stage 
instruments both possess magnification of unity and identical resolution. 
Because of the increased dispersion, the exit aperture 24 will be twice 
the diameter of the aperture of the comparable single stage analyzer for 
accepting the same energy band of trajectories. A magnification of unity 
for both instruments means that displacement of the beam on the object 
results in roughly equal displacement of the image thereof at the exit of 
the analyzer. Because of the greater dimension of this aperture, the beam 
may be scanned over a wider field of view, approximately twice that of the 
comparable single stage device for the same analyzer reduction and signal 
attenuation at the edges of the field of view. FIG. 2 illustrates the beam 
displacement dependence for response of the analyzer to an elastically 
scattered peak as the beam is swept across the sample. The response 
measurement is shown for each of three different values of resolution as 
determined by aperture dimension for exit aperture 24. Normailization of 
the curves permits comparison of the various resolutions for the extent of 
lateral sweep which incurs no more than 10 percent variation in analyzer 
response. 
While an n stage analyzer effectively widens the useful field of view by a 
factor approximately n, the effect is not without limit in angular width, 
nor for the number of stages. The angular width cannot be increased to the 
extent that the trajectories depart substantially from the acceptance 
angle .alpha. without incurring aberrations in the analyzer which degrade 
its resolution. For example, displacement of the object point from the 
axis will introduce a component in the electron trajectories which lay 
outside of a single radial plane. Greater displacements will produce 
trajectories, each of which to a greater degree contain a non-coplanar 
component. The non-coplanar component of motion ultimately degrades 
analyzer resolution and limits the performance of the instrument. 
Non-coplanar trajectories could be removed, for example by means of radial 
baffles, with consequent reduction in intensity of the detected signal. 
It will also be apparent that displacement of the trajectories 17 and 17' 
is also limited by components of the electron gun whereby large deflection 
of the incident beam results in trajectories which are not unobstructed 
over the entire annular acceptance region of the analyzer. 
Utility of the principle of plural stages of analysis is finally limited by 
the cumulative effect of aberrations in the several stages of such an 
analyzer. 
The electron gun of the preferred embodiment is arranged to place the final 
lense 52 close to the sample. Minimizing the distance to the sample from 
the final lense has the effect of minimizing the effect of spherical 
aberration, permitting greater beam concentration for a given beam 
diameter. Deflection plates 50 therefore precede lense 50. It has been 
found that aberration in the deflected (and thus non-paraxial) beam upon 
traversal of lense 52 is minimized by the artifice of arranging the 
deflection plates 50 and voltages applied thereto to pivot the beam 
substantially about the center 54 of lense 52. This is accomplished by 
dividing the deflectors into two units displaced by an intermediate drift 
space. Each unit comprises both x and y deflection plates. An "essing" 
technique is then utilized to direct the "essed" beam to cross the beam 
transport axis at a predetermined position. For example, y deflection is 
accomplished by first deflecting the beam away from the axis with the y 
plates of deflection plates 50 and the beam is then returned to the axis 
by the y plates of deflection plates 50'. The same potential difference 
(with polarity reversed) may be applied to both pairs of y deflection 
plates. The dimensions of the plates are chosen to cause the beam to cross 
the beam transport axis after the second deflection at center 54 of the 
lense 52. For a symmetrical lense, the center is understood to be the 
geometrical center. The technique is also applicable to an asymmetric 
lense wherein the center is understood to be the optical center of the 
asymmetric lense. 
Typical design parameters for the preferred embodiment include variable 
electron beam energy over the range from 100 ev to 10 Kev with optics 
sufficient to achieve a parallel beam of circular cross section with 
diameter ranging from 0.2 micron or less, to 10 microns. The voltages 
which are applied to the various optical elements, such as lenses, 
alignment plates, stigmators, deflection plates, etc., are arranged to 
track the beam energy in order to preserve the geometric properties of the 
beam over the beam energy range. The design for achieving these 
specifications is well known and beyond the scope of this work. 
Accordingly, the details of the optical elements are not further 
elaborated. 
The physical dimensions of the preferred embodiment include outer radius 
r.sub.10 =6 cm and inner radius r.sub.12 =2.5 cm. The preferred embodiment 
has a mean angle of acceptance (.alpha.) of 42.44.degree. with an angular 
spread of .+-.6.degree.. The length between object and image focii is 
13.091 inches. The intermediate aperture may assume dimensions ranging 
from 2 mm to 4 mm: where desired, a smaller diameter is used to function 
as a defining aperture thereby limiting the transmission of the analyzer. 
Although the invention has been shown and described with reference to 
preferred embodiments, it will be readily apparent to one of average skill 
in the art that various changes in the form and arrangement of the parts 
may be made to satisfy requirements without departing from the scope of 
the invention as defined by the dependent claims. It will be apparent, for 
example, that the invention is not limited to electron excitation and that 
the principals taught herein are equally applicable for similar studies 
wherein ion beams are employed. It will also be apparent that 
electromagnetic excitation of photoelectrons can utilize the principals of 
the invention especially where a spacially coherent radiation source, as 
for example a laser, is mounted in the interior of the mult stage CMA.