Planar Retarding Grid Electron Spectrometer

Abstract

A retarding grid form of electron spectrometer utilizing a single grid structure, the retarding grid being planar and being positioned in a focusing structure that first defocuses the beam of electrons and then refocuses it onto an electron detector, the retarding grid being located in the focusing structure near the point of maximum diameter of the electron beam. The focusing structure produces very strong accelerating and decelerating fields near its beam entrance and exit regions, respectively, such that the retarding and accelerating fields at the retarding grid are very weak, resulting in sharp lines in the electron spectrum.

Description

BACKGROUND OF THE INVENTION

Chemical analysis spectrometers utilizing the technique of inducing electron emission from the sample under analysis and measuring the energies of the electrons emitted to thereby produce an electron energy spectrum of the sample are in common use. The more sophisticated forms of induced electron emission spectrometers employ a magnetic or electrostatic deflection type of energy analyzer for separation of the electrons into groupings in accordance with their energies, electrons being detected by a suitable electron detector such as an electron multiplier. The analyzer is swept so that at any given instant of time only electrons of a particular energy group are directed onto the electron detector. This type of analyzer is therefor very selective, rendering distinct electron energy peaks or lines separated along the energy spectrum.

A simplier form of electron spectrometer is the retarded field spectrometer wherein the emitted electrons are directed in a large solid angle cone of emission onto a spherical shaped detector plate, all points on the surface of the plate being equidistant from the point source of the electrons. A spherical shaped retarding grid is positioned between the source and the detector plate and a variable voltage on the grid serves to retard all those electrons with energy in electron volts less than the voltage on the grid and allowing all electrons with an energy greater than the retarding grid voltage to pass through to the detecting plate. The resulting current in the detector plate is a measure of the energy of the emitted electrons as established by the sweep voltage applied to the retarding grid. The spectrum obtained with this retarding grid spectrometer is a step or integral form of spectrum as opposed to individual peaks obtained with the deflection type spectrometer since all of the electrons in the separate energy groupings with a voltage above that of the retarding grid pass through the grid and are collected and recorded and these higher energy electron groups appear as one energy level in the recorder output. Separation is produced by increasing the retarding voltage to retard each successive group of electrons in the energy spectrum and noting the potential step down at which the particular electron energy group exclusion occurs. Since all of the electrons above the particular voltage level of the retarding grid pass to the detecting plate, the retarding grid spectrometer has a very high background current which creates a noise level high compared to the noise level of the more selective deflection type spectrometer. As a result, the advantage of the high current acceptance of the retarded grid analyzer is more than offset by the high background current and resultant noise.

One known proposed technique for reducing this background current comprises placing an electron focusing device, such as an electrostatic focusing condenser, after the retarding grid to separate the slower or lower energy electrons from the faster electrons, focusing the slower electrons onto a suitable detector such as an electron multiplier.

An additional disadvantage of the retarding field analyzer stems from the fact that, with a potential applied to the retarding grid, an electric field is set up between the detecting plate and the retarding grid; this electric field results in a space modulation of the potential across the grid as the retarding grid is voltage modulated for sweep purposes. This space modulation limits the resolution of the retarding field analyzer. This electric field E is given by the relationship

E = V/D

where V is the voltage on the retarding grid and D is the distance between the retarding grid and the detecting plate. The space modulation .DELTA. V is dependent upon the following relationship:

.DELTA.V .congruent. E .times. d/4

where d is the spacing between the wires in the mesh of the retarding grid. From the above relationships,

.DELTA.V = V .times. d/4D

thereby resulting in a resolution limit,

.DELTA.V/V = d/4D .

Also, capacitance between the retarding grid and collector plate results in a voltage induced in the collector plate as the voltage on the retarding grid is modulated for sweep purposes and this induced voltage in the collector serves to mask the current produced therein by the electrons of interest.

In order to control the capacity of coupling between the retarding grid and collector and to substantially reduce the effect of the electric field on the retarding grid, additional grids have been introduced between the retarding grid and the collector and/or at the front or the back sides of the retarding grid or both. Typical forms of such multiple grid devices are shown in articles entitled "Resolution and Sensitivity Considerations of an Auger Electron Spectrometer Based on Display LEED Optics" by N.J. Taylor in the Review of Scientific Instruments, Vol. 40, No. 6, June 1969, pages 792-803, "High-Sensitivity Electron Spectrometer" by D.A. Huchital et al in Applied Physics Letters, Vol. 16, No. 9, May 1, 1970, pages 348-351, and "Photoelectron Spectroscopy of the Rare Gases" by James A. Samson in The Physical Review, Vol. 173, pages 80 through 85.

The introduction of these additional grids, while improving the performance relative to the noted capacitance and electric field, results in a decrease in the electron transmission and the introduction of spurious signals into the spectrometer output due to secondary electrons. A typical spherical grid employs 1 mil wire with 100 wires per inch resulting in an electron transparency of approximately 80 percent for a single grid. Two or more grids in the electron path will substantially reduce the electron transparency, for example, to 50 percent or less. The additional intercepted electrons result in a substantial increase in emission of secondary electrons from the grids and these secondary electrons produce spurious signals in the electron detector output.

SUMMARY OF THE PRESENT INVENTION

The present invention provides an electron spectrometer of the retarding field type wherein a single planar retarding grid is provided for selection and control of the electrons of desired energies, the planar retarding grid being positioned within an electron focusing means in a plane normal to the central axis through the focusing means. In a preferred embodiment, the planar grid is positioned in the focusing means in the region where the electron beam is at maximum diameter. With a symmetrical form of focusing means as employed in the preferred embodiment, this grid is located at the mid-section of the focusing structure. The electrons passing through the planar retarding grid are focused onto a suitable detector such as an electron multiplier device.

Since only a single retarding grid is employed, electron transmission is improved over multiple grid forms of electron spectrometers. In addition, secondary electron emission from the retarding grid is reduced relative to the multiple grid devices, thus improving the spectrometer from the standpoint of spurious output signals.

Also, the planar grid is well spaced from other parallel surfaces within the focusing structure and, therefore, there is no problem with capacitive coupling to the electron detector region. The electric field at the grid is very weak and is not a resolution limiting factor as with prior retarding grid spectrometers.

The novel form of focusing means and planar retarding grid renders an output in the form of sharp peaks or lines, with trailing edges nearly as sharp as leading edges, much the same as the deflection type spectrometers, as contrasted with the integral or step type spectrum or the sharp leading edge-slow cut off sharp trace obtained with certain forms of retarding grid analyzers.

The manufacture of this new spectrometer is simple, with relatively few components; the grid, being planar, is easily fabricated.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal cross-section view of the novel retarding grid form of electron spectrometer.

FIG. 2 is a plot of the equipotential lines and the electron trajectories for an analyzer of the type shown in FIG. 1.

FIGS. 3(A) and 3(B) illustrate examples of expected and actual spectrums, respectively, obtained with this spectrometer.

FIGS. 4 and 5 are illustrations of the electron trajectories in this spectrometer under two different modes of operation.

FIG. 6 is a trace of the electron spectrum from argon obtained with the analyzer of the present invention.

FIG. 7 is a trace similar to that of FIG. 6 with an expanded scale.

FIG. 8 is a diagram illustrating two electron beam paths from a source plane to a retarding grid.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, the electron spectrometer comprises a sample chamber 11 which contains the sample to be analyzed, in this specific example argon gas fed into the chamber 11 through gas inlet 12. Means 13 is provided for irradiating the sample with a beam of ultraviolet from a helium discharge via an aperture 14 in the side of the chamber to ionize the gas and produce the photoelectrons to be analyzed. As with other known electron analyzers, other gases or solids may be analyzed, and other irradiation may be employed such as X-rays for producing the photoelectron or Auger electron emission desired.

The emitted electrons pass out from the sample chamber 11 via a small aperture 15 in an aperture plate 16 in the chamber and pass into a first Einzel lens comprising three spaced-apart annular copper electrodes 17, 18 and 19. The three electrodes are affixed together in axial alignment by three longitudinally extending support rods 21 and suitable insulators 22, the rods being positioned 120.degree. apart around the central axis of the structure. The three rods 21 are secured within the end electrode of the main focusing structure for support and in turn serve to mount the sample chamber 11. The two outermost electrodes 17 and 19 in the Einzel lens are grounded while the middle electrode 18 is coupled to a source 23 of negative voltage, for example, -8 volts as indicated in the drawings. The Einzel lens serves to adjust the launching angle of the electrons into the main focusing structure.

The main focusing structure comprises a plurality of annular copper electrodes affixed in axial alignment by support rods 24 spaced 90.degree. apart around the structure and suitable insulators 25. The first or entrance section of this focusing apparatus includes the four electrodes 26, 27, 28 and 29 while the second or exit section of the focusing apparatus includes the four electrodes 26', 27', 28' and 29', the latter four electrodes forming a mirror image of the first four electrodes. The electrodes 26 and 26' have inner diameters smaller than those of electrodes 27 and 27', with electrodes 28, 28', 29 and 29' having equal inner diameters and larger than those of electrodes 27 and 27'.

In one embodiment built and operated, the electrodes 26 and 26' had inner diameters of 1 inch and were 3/8 inch thick; electrodes 27 and 27' had 2 inch inner diameters and were 1 inch thick, and electrodes 28, 28', 29 and 29' had inner diameters of 4 inches and were 1 inch thick.

A planar 100 mesh circular grid 31 is positioned at the midpoint of the main focusing structure between the entrance or decelerating region and the exit or accelerating region and normal to the central axis through the structure. The grid is supported on the rods 24 and suitably insulated from the other electrodes in the structure.

Another Einzel lens including annular plates 32 and 33 is mounted at the exit of the main focusing section and serves to direct the electrons into an electron detector, in this case an electron multiplier device 34. A small opaque area 35 at the center of the grid 31 serves to prevent light from the source 15 from impinging upon the electron multiplier and giving a false registration of electrons.

The entire spectrometer device is enclosed in a stainless steel envelope structure 36 and evacuated by suitable vacuum pumps, not shown, in accordance with standard electron emission spectrometer techniques.

The voltage source 23 supplies the desired DC voltages to the various elements, and typical values of voltage are shown on the leads from the voltage source to the elements. The sweep voltage, e.g. -10 to +10 volts, for this spectrometer is applied to the sample chamber 11 so that, once the voltages for the focusing structure and retarding grid have been set up, no changes are made therein during the particular analysis. It should be noted that the voltages shown are only illustrative, and other values are utilized for different samples or different analyses.

In operation, as the electrons emitted from the sample enter the main focusing structure at source point 37, they are subjected to a very strong decelerating field such that, by the time they pass from the region within the focus electrode 27 and into the region defined by the two focus electrodes 28 and 29, the electrons have lost about 98 percent of their initial energy, and they approach the retarding grid 31 with very low energy. This can better be seen by reference to FIG. 2 which shows the equipotential lines in a focusing structure similar to that of FIG. 1 with an electron starting from a source with 10ev energy and moving to a retarding grid held at -10 volts. It is noted that both the -9.9 and -9.8 volt equipotential lines are spaced a considerable distance from the plane of the retarding grid, and that a substantial deceleration of the electron from 0v to -9.8v has occurred near the entrance region of the focusing structure.

Those electrons with an energy greater than zero volts will pass through the retarding grid 31 and those with less energy will be retarded and turned back. The electrons in the slightly higher energy grouping passing through the retarding grid will be accelerated in the second or exit region of the focusing means and will be focused into a focal point and then into the exit Einzel lens and into the electron multiplier.

As with the other forms of retarding grid analyzer, it should be expected that the recorded spectrum would be integral or step-like in form, or that the electron line would have a sharp leading edge and a slow trailing edge similar to the spectrum shown in FIG. 2 of the Huchital et al article cited above. However, the spectrum actually obtained with this new retarding grid analyzer has a trailing edge which is almost as sharp as the leading edge, giving a spectrum more closely resembling that obtained with the deflection type spectrometers. An example of the expected form of slow trailing edge spectrum is shown in FIG. 3(A) and a corresponding spectrum of the sharp line form actually obtained is illustrated in FIG. 3(B).

This sharp delineation of the energy groupings occurs because the deflection of the electrons leaving the grid 31 in the rear section of the focusing structure occurs at a very low energy. The change in kinetic energy of the electron relative to the energy at which the trajectory is bent determines how far off from its original trajectory it will move; the faster electrons will rapidly move off focus and will not pass to the electron multiplier; the slower electrons will deflect very rapidly and thus move off focus. Therefore, a sharp energy line is obtained for slow electrons at the focusing energy.

In order to more clearly understand the process taking place in this analyzer, reference is made to FIG. 4 which illustrates the electron beam flow through the analyzer from source to detector through the grid. The source is assumed to be at 10ev and the grid at -10v. The field in the focusing structure produces a radial deceleration to nearly zero energy in the initial decelerating region, during which period the electron trajectories are nearly straight paths. This is followed by a region of electron deflection at very low energy, i.e. 0.1ev, in which the electrons are brought to rest on the grid at normal incidence.

If now the source energy is increased by a very small amount, i.e. to 10.1ev, leaving the grid at -10v, then in the deflection region the electron energy will be increased from 0.1ev to 0.2ev, a 100 percent increase. The percent change in radius of curvature R goes as

.DELTA.R/R .about. .DELTA. E/E = 0.1ev/0.1ev = 1

in this example. Therefore, a change of source energy of only 1 percent will cause a major change in the electron trajectory such that the electron with increased energy goes substantially off the path to the detector as shown in the drawing, giving a very sharp trailing edge to the spectral line.

The preceding discussion assumed that the leading edge of the spectral line is produced by the retarding field grid cutoff. In general, this may not be the case but may only be one mode of operation. Another mode of operation is shown in FIG. 5 which illustrates the electron trajectory between source and detector, where the source and detector are at 10ev and the grid is at a -9.8v. In this case, the source electrons at 10ev pass the grid and are focused onto the detector by appropriate adjustment of the focusing voltage. In this example, it is quite possible to focus the beam at the detector under conditions such that the beam passes through the grid with a particular kinetic energy > 0. If the beam, under focused conditions, passes through the grid with a kinetic energy equal to or greater than the observed linewidth, then the line shape will be determined by the deflection characteristics of both sides, as indicated in the drawing, and the grid cutoff will not be observed. In the illustration, source electrons at both 10.1ev and 9.9ev pass the grid and are deflected from the path to the deflector, whereas those source electrons at 10ev impinge on the detector. The diameter of central disc 35 may be chosen to confine the paths of transmitted electrons to a region of high radius of curvature within the focusing structure.

Thus, this new analyzer may be viewed as a new type of deflection analyzer which in one limit of its operation becomes a new form of retarding field analyzer whose resolution is determined by the cutoff properties of the grid. In contrast with other types of deflection analyzers which require an inner or coaxial electrode to terminate the transverse field from an outer electrode, the present device uses a grid to establish a uniform electron potential over the cross-section of a cylindrical (hollow) lens. The grid corrects the non-uniformity of the lens potential across the transverse plane so that the entire electron beam may be brought to a predetermined low and uniform kinetic energy.

A typical line for argon radiated with helium ultraviolet light is shown in FIG. 6, and also in FIG. 7 but on an expanded scale, with typical values of grid and sweep voltages shown. The general formula is

-V.sub.g = (h.nu. - E.sub.b)/(e) - V.sub.s

where V.sub.g is the retarding grid voltage, e is the electron charge, h.nu. is the photon energy of the ultraviolet light in electron volts, E.sub.b is the binding energy of the photoelectrons, and V.sub.s is the sweep voltage applied to the sample.

The prior art suggested to utilize a planar grid in a retarding grid electron spectrometer and to position the grid slightly in front of the exit focal point before the electron detector region where the electron beam has been well converged. Such an analyzer is described in an article entitled "Electron Reflection Coefficient at Zero Energy. I. Experiments" by H. Heil et al. in The Physical Review, Vol. 164, No. 3, Dec. 15, 1967, pages 881-886. Referring to FIG. 8, there is shown two electron trajectories from a source 41 with radius y.sub.1 through a focusing means 42 to a retarding grid 43 with a radius y.sub.2. From Abbe's sine law,

.sqroot.E.sub.1 y.sub.1 sin .theta..sub.1 = .sqroot. E.sub.2 y.sub.2 sin .sigma..sub.2

where E.sub.1 and E.sub.2 are the kinetic energies of the electron at the source 41 and grid 43, respectively, .theta..sub.1 is the angle at which the electron leaves the source relative to the central axis 44, and .theta..sub.2 is the angle at which the electron approaches the grid sin .theta. 43 relative to the central axis. In operation of a retarding grid analyzer, the electrons with normal trajectories are brought to rest at the grid. The finite source radius y.sub.1 causes non-normal .theta..sub.2 paths and, for a retarding grid analyzer, we have

.theta..sub.2 = .pi. /2, E.sub.2 = .DELTA. E

where .DELTA.E is the limiting resolution of the analyzer due to y.sub.1. Therefore,

E.sub.1 y.sub.1.sup.2 sin.sup.2 .theta. = .DELTA. E y.sub.2.sup.2

or

.DELTA.E/E = (y.sub.1.sup.2 /y.sub.2.sup.2) sin.sup.2 .theta..sub.1.

Thus, the best resolution is obtained with the greatest value of y.sub.2 which means that the retarding grid should be placed near the maximum electron beam diameter as contrasted with a position nearer the focal point, and therefore grid 31 is so positioned in the spectrometer of FIG. 1.

It is noted that, although the decelerating section and accelerating section of the focusing structure on either side of the retarding grid 31 in FIG. 1 are symmetrical, this relationship is only preferred, and the two focusing sections may be non-symmetrical. Also, the retarding grid has been shown at the exact midpoint and, again, although this is preferred, the grid may be positioned near the midpoint and preferably near the region of maximum beam diameter for reasons given above.

The advantages of this form of planar retarding grid analyzer include the fact that the electric field at the grid is very weak and under the normal requirements of

.DELTA.E/E = 10.sup..sup.-2 to 10.sup..sup.-3

the mesh of the grid does not limit the resolution. Conversely, the ultimate resolution capability is higher than can be achieved with prior designs.

Since only a single grid is employed, scattering or interception is reduced as well as secondary emission from the grid, thus resulting in lower background noise. The sharp refocusing of the higher energy electrons passing the grid reduces the background, giving sharp peaks in the spectrum. The larger spacing between the retarding grid and the detector means permits modulation of the grid, if desired, without suffering from capacitive coupling to the detector.

Claims

What is claimed is:

1. An electron spectrometer comprising means for inducing electron emission from a sample under analysis, an electron detector means for measuring electrons impinging thereon, a focusing means of rotational symmetry about a central axis for focusing electrons emitted from said sample onto said electron detector means, a grid positioned in the focusing means in a region of maximum electron beam diameter and lying substantially normal to the central axis through said focusing means, and means for applying a variable voltage to said grid relative to said sample to cause electrons of selective energies to impinge onto said detector.

2. An electron spectrometer as claimed in claim 1 wherein said electron beam detector is an electron multiplier.

3. An electron beam spectrometer as claimed in claim 1 including a second electron beam focusing means positioned between said sample and said first focusing means for focusing the electrons from said sample into said first focusing means.

4. An electron spectrometer as claimed in claim 3 including a third electron beam focusing means positioned between said first focusing means and said electron detector means for focusing the electrons exiting from said first focusing means onto said electron detector.

5. An electron spectrometer as claimed in claim 1 wherein said focusing means is symmetrical in the central axial direction, said grid being planar.

6. An electron spectrometer as claimed in claim 5 wherein said grid is positioned at approximately the midpoint of the central axis through said focusing means.

7. An electron spectrometer as claimed in claim 5 wherein said focusing means comprises a plurality of annular electrodes spaced in axial alignment along said central axis, the inner diameters of the electrodes increasing in size from the electrodes on either end of the focusing means to the electrodes at the center of the focusing means.

8. An electron spectrometer as claimed in claim 1 including means for applying voltages to said focusing means to create a substantial electron decelerating field near the entrance end of said focusing means and a substantial accelerating field near the exit end of said focusing means, the decelerating and accelerating field at the grid being weak relative to the entrance decelerating and exit accelerating fields of said focusing means.

9. An electron spectrometer as claimed in claim 8 wherein said focusing means is symmetrical in the central axial direction, said grid being planar.

10. An electron spectrometer as claimed in claim 9 wherein said grid is positioned at approximately the midpoint of the central axis through said focusing means.

11. An electron spectrometer as claimed in claim 1 wherein said focusing means comprises a first section positioned between the grid and said sample and including an entrance region for the electrons and a second region near the grid, and a second section positioned between the grid and said electron detector and including an exit region for the electrons and a second region near the grid, said focusing means creating electron decelerating and accelerating fields in said entrance and exit regions, respectively, substantially greater than the electron decelerating and accelerating fields in said second regions of said first and second sections, respectively.

12. An electron spectrometer as claimed in claim 11 wherein the fields in the entrance region and the second region of said first section are symmetrical with respect to the fields in the exit region and the second region of said second section, respectively.

13. An electron spectrometer as claimed in claim 11 wherein the decelerating and accelerating fields in said first and second regions are at least ninety percent greater than the fields in said second regions.

14. An electron spectrometer as claimed in claim 11 wherein said focusing means is symmetrical in the central axial direction, said grid being planar and being positioned at approximately the midpoint of the central axis through said focusing means.

15. An electron spectrometer comprising means for inducing electron emission from a sample under analysis, an electron detector means for measuring electrons impinging thereon, an electrostatic focusing means of rotational symmetry about a central axis for focusing electrons emitted from said sample onto said electron detector means, a planar grid positioned in the focusing means and lying substantially normal to the central axis through said focusing means, and means for applying a variable voltage to said grid relative to said sample to cause electrons of selective energies to impinge onto said detector.

16. An electron spectrometer as claimed in claim 15 wherein said electrostatic focusing means comprises a plurality of annular electrodes spaced in axial alignment along said central axis, the inner diameters of the electrodes increasing in size from the electrodes on either end of the focusing means to the electrodes at the center of the focusing means.

17. An electron spectrometer as claimed in claim 15 including means for applying voltages to said focusing means to create a substantial electron decelerating field near the entrance end of said focusing means and a substantial accelerating field near the exit end of said focusing means, the decelerating and accelerating field at the grid being weak relative to the entrance decelerating and exit accelerating fields of said focusing means.

18. An electron spectrometer as claimed in claim 17 wherein said focusing means is symmetrical in the central axial direction and said grid is positioned at approximately the midpoint of the central axis through said focusing means.

19. An electron spectrometer as claimed in claim 15 wherein said focusing means comprises a first section positioned between the grid and said sample and including an entrance region for the electrons and a second region near the grid, and a second section positioned between the grid and said electron detector and including an exit region for the electrons and a second region near the grid, said focusing means creating electron decelerating and accelerating fields in said entrance and exit regions, respectively, substantially greater than the electron decelerating and accelerating fields in said second regions of said first and second sections, respectively.

20. An electron spectrometer as claimed in claim 19 wherein the fields in the entrance region and the second region of said first section are symmetrical with respect to the fields in the exit region and the second region of said second section, respectively.

21. An electron spectrometer as claimed in claim 19 wherein the decelerating and accelerating fields in said first and second regions are at least ninety percent greater than the fields in said second regions.

22. An electron spectrometer as claimed in claim 19 wherein said focusing means is symmetrical in the central axial direction, said grid being positioned at approximately the midpoint of the central axis through said focusing means.