US3731096A

US3731096A - High resolution, high etendue, retarding-potential electron concentrator

Info

Publication number: US3731096A
Application number: US00201699A
Authority: US
Inventors: F Carter
Original assignee: US Department of Navy
Current assignee: US Department of Navy
Priority date: 1971-11-24
Filing date: 1971-11-24
Publication date: 1973-05-01
Anticipated expiration: 1990-05-01

Abstract

The invention comprises improvements in an electronconcentractor device of the spherical-grid, retarding-potential, post-monochromator spectrometer type. A pair of fine-mesh, hightransparency, spaced, planar grids is added after the first curved coarse focusing grid thereby determining the lower-energy limit of the electron passband. A second oppositely curved coarse focusing grid is added after the planar grids to provide assistance in focusing the passband electrons on the collector lens.

Description

United States Paint 1 1 1 3,731,%

Carter May 11, R973 [54] HIGH RESOLUTION, HIGH ETENDUE, Electron Reflection Coefficient at Zero Energy, 1.

RETARDING-POTENTIAL ELECTRON CONCENTRATOR [75] Inventor: Forrest L. Carter, Bethesda, Md.

[73] Assignee: The United States of America as represented by the Secretary of the Navy [22] Filed: Nov. 24, 1971 [21] App1.No.: 201,699

[52] [1.8. CI ..250/49.5 AE

[51] Int. Cl ..G0lt 1/36 [58] Field of Search ..250/49.5 AE, 49.5 PE,

[56] References Cited OTHER PUBLICATIONS Photoelectron Spectroscopy of the Rare Gases" by J. A. R. Samson et al. from The Physical Review, Vol. 173, Sept, 1968, pages 80-85.

Experiments" by H. Heil et al. from The Physical Review, Vol. 164, Dec., 1967, pages 881-886. High-Sensitivity Electron Spectrometer by D. A. Huchital et al. from Applied Physics Letters, Vol. 16, May, 1970, pages 348-351.

Primary ExaminerWilliam F. Lindquist Attorney-R. S. Sciascia et al.

[57] ABSTRACT The invention comprises improvements in an electronconcentractor device of the spherical-grid, retardingpotential, post-monochromator spectrometer type. A pair of fine-mesh, high-transparency, spaced, planar grids is added after the first curved coarse focusing grid thereby determining the lower-energy limit of the electron passband. A second oppositely curved coarse focusing grid is added after the planar grids to provide assistance in focusing the passband electrons on the collector lens.

' 7 Claims, 1 Drawing Figure COLLECTOR 32 LENS G2 36 l ./AV t" 40 I,"

Patented May 1, 1973 COLLECTOR LENS 32\i SPECIMEN 1 INVENTOR FORREST L. CARTER 7%RNEYS IIIGII RESOLUTION, HIGH ETENDUE, RETARDING-POTENTIAL ELECTRON CONCENTRATOR STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION This invention relates to a lens system for focusing electrons of a specific energy range in electron spectrometers and like instruments.

The classic high-resolution electron spectrometer employed for material characterization by the ESCA technique (ESCA electron spectroscopy for chemical analysis) has a narrow entrance slit of dimensions 0.25mm by mm an acceptance solid angle of 0.8 percent. This results in an etendue of 0.00025 cm steradian and a low count rate (sensitivity).

on the other hand, a spherical-grid, retarding-potential, post-monochromator spectrometer has recently been reported to have an excellent etendue (0.5 cm steradian) and good resolution. (See Huchital and Rigden, Appl. Phys. Letters 16, 348 (1970). The desired spectra of electron intensity vs electron energy was obtained as a derivative of collector current vs. retarding voltage. The disadvantages of the reported device include:

1. a low pealbto-background ratio is obtained, nullifying in part the effect of the high etendue and suggesting that resolution aberration is present;

2. the requirement that high-mesh-number grids be sufficiently sturdy to maintain a spherical shape results in the use of a gridof low transparency.

SUMMARY OF THE INVENTION The invention involves a pair of fine-mesh, planar grids in combination with a pair of focusing grids, the combination being placed after the coarse-mesh spherical grid of a spherical-grid, retarding-potential, postmonochromator electro spectrometer (hereinafter called an I-I-R, or I-Iutchital-Rigden high-resolution spectrometer). The potential on the plamar grids is more negative than that on the preceding coarse-mesh grid but less negative than that on the monochromator grids.

OBJECTS OF THE INVENTION An object of the invention is to increase the number of electrons delivered to the slit (or collector) of an Ill- R spectrometer within a predetermined electron-ener gy range.

Another object is to increase the number of electrons delivered to the slit of an I-I-R spectrometer without introducing energy dispersion or resolution aberration.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

THE DRAWING The single FIGURE is a schematic representation of an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION The figure illustrates schematically an HR spectrometer to which a pair of spaced, fine-mesh,

planar grids

20 and 22 and a pair of oppositely curved, coarsemesh grids 18 and 241 have been added. The device has cylindrical symmetry about the line joining the specimen 14 and the collector lens so that this line may be called the axis of the device. (The dashed lines indicate grids; the solid lines are used to indicate connecting wires. Thus, grid G2 (I8) is connected by a wire 40 at the left to grid G6. (2%); the grids G5 and G7 (26 and 28) are connected by wires 42, 441 and 46.)

A conventional I-I-R spectrometer, as shown by Hutchital and Ridgen (cited above), has the spherical retarding grid 16, another spherical, fine-mesh, retarding grid closer to the collector lens and a postmonochromator grid extending along both sides from the fine-mesh grid to the collector lens.

Ultraviolet light, X-rays or electrons 12 are projected upon a specimen 14 which emits photoelectrons or Auger electrons in response thereto. The electrons leave the specimen 14 at ground potential and travel radially outward until they pass coarse grid G1 (16). The path of all electrons subject to collection then bends toward the cylinder axis under the influence of grid G2 (18) with the. retarding potential V -*V -AV, where A V is small but positive, V being negative. Electrons with energies less than V are reflected back toward the specimen 14 while those with energies slightly above V move toward the planar grids G3 (20) and G4 (22) which are at potential V The bending of the electron paths causes the electrons with energies slightly larger than V to pass through the planar grids G3 and G4 approximately perpendicular to the plane of the grids or approximately parallel to the axis of the cylinder. The planar grids, by repelling electrons with energies less than V (approximately), determine the lower velocity or energy limit of electrons which are allowed to pass through the planar grids.

The second coarse grid, G6 (24), then bends the electrons with energies slightly greater than V toward the axis so that they are concentrated at the slit the collector lens 32. These electrons are further assured of collection by the action of the'post-monochromator grids G7 and G8 (28 and 30) with the respective retarding potentials V +V and V It should be noted that grid Gl(16) is spherically curved, grid 62(18) has greater-than-spherical curvature, and grid G6(2d) also has greater-than-spherical curvature and is oppositely curved from grid 62(18). It is necessary that focusing grids l8 and 24 have greater-than-spherical curvature in order to bend electrons coming through the spherical grid 16 toward the axis. If the curvature of the focusing

grids

1,8 and 24 were less-than-spherical, the electrons would diverge (defocus) and if the curvature were spherical, there would be no electron-bending effect.

On the other hand, electrons which leave the specimen 14 with an energy significantly greater than the retarding potential V will, in general, miss the collector lens 32 and pass through the coarse grids G5 (26), G7 (28) and G8 (30) to be collected beyond. Accordingly, only a small percentage of electrons with an energy greater than V i-V will reach the collector lens 32. These will primarily be those on the line of sight from the specimen 14 to the collector lens 32 and they can be intercepted physically by a small stop (not shown) on the cylinder axis of symmetry at the spherical grid G1 (16). Thus, the electrons which reach the collector lens 32 have energies mostly lying in the range from V to V +V A single planar grid may be employed but the use of two improves the resolution. Using more than two provides a small gain in resolution but a loss in sensitivity so that the use of two planar grids provides optimum results.

Focusing is accomplished by the coarse grids G2 (18) and G6 (24) and the monochromator grids G (26), G7 (28) and G8 (30). The planar grids G3 (20) and G4 (22) provide the electron-energy passband lower limit. In this device, the lower section of grid 65(26) can be considered to be a pre-monochromator grid; the

planar grids

20 and 24 can be considered to be the monochromator grid; and the upper section of grid G5, as well as grids G7(28) and G8(30), can be considered to be post-monochromator grids.

Other alternatives are possible for the collector lens 32 shown in the FIGURE. The collector means may be a spectrometer slit, a Faraday cage, or a channeltron multiplier, with a simple electrostatic lens system, for example.

The resolution aberration due to the distortion of the electron paths by the presence of the magnetic field of the earth or nearly electrical equipment is precluded by the use of multiple layers of magnetic shielding around the device.

In the construction of the planar grids two points are of importance. First, the grids should have very high transparency to both reduce the number of inelastic collisions of the electrons with the grids enroute to the collector lens and to increase the effective etendue of the electron concentrator device. Second, the construction material of the grids is important. The formation of an oxide coating on the surface of the grid wires can permit an inhomogeneous buildup of charge on the grid such that appreciable aberration is introduced. in addition, the use of several metals in the construction of the entire device could result in a multitude of peaks in the spectrum which are spurious to the specimen. Thus, the concentrator grids and surfaces should be of the same non-oxidizing material such as gold, gold plating, platinum or palladium, for example. Gold has no stable oxides or nitrides at room temperature.

The resolution of the device is determined by the mesh number of the planar grids and by their separation. The higher the mesh number (i.e., the smaller the area of each space in the grid is), the higher the resolution. The wider the spacing between the grids, the better the resolution, but the smaller the sensitivity because more electrons are lost. Separation of the grids should be between 3 and 8 times the width of the openings in the grids.

Planar construction permits the use of decreased wire for the same mesh number and a significant increase in transparency (from 40 to 80 percent) over the H-R spectrometer because the wire mesh of the fine grids, G3 and G4, is not burdened with maintaining a spherical or otherwise curved shape. By inserting planar grids, finer mesh and thinner wire can be utilized thereby providing for higher resolution and higher transparency (higher efficiency).

his electron concentrator provides the advantage of greatly enhanced etendue over classical electron spectrometers, an enhancement of two to three orders of magnitude depending on constructional details, such as total acceptance angle, mesh number, number of fine planar grids and transparency of the grids. This magnitude of increase of etendue gives a corresponding increase in sensitivity or a corresponding reduction (by a factor of to 1,000) in the time required to collect data with the equivalent counting statistics. In an ESCA application, such an electron concentrator would then permit the detection of impurities at one hundredth the concentration of the former limit. Alternatively, in studies involving the location of peaks, such as required for chemical shifts information, the X-ray intensity level can be greatly reduced. This would permit the minimization of surface-charge problems associated with studies ofinsulating specimens.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. lt is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of the United States is:

1. In an electron-concentrator spectrometer of the high-resolution type having spherically curved retarding grid means between a specimen and collector means and having post-monochromator grid means, the improvementcomprising:

a first focusing grid located beyond said spherical grid means in the direction of said collector means;

a second focusing grid located beyond said first focusing grid,

both said grids having greater-than-spherical curvature but being oppositely curved, said first grid being curved in the same direction as said spherically curved grid means; and

high-transparency, fine-mesh, planar grid means lying perpendicular to a line drawn between the specimen and the collector means and positioned between said first and second focusing grids,

said planar grid means acting to determine the lower velocity limit of the electrons passing through said planar grid means.

2. A device as in claim 1, wherein said planar grid means has a higher mesh number and is constructed of finer wire than said first and second focusing grids.

3. A device as in claim 2, wherein said planar grid means comprises at least two planar grids.

4. A device as in claim 2, wherein all grid surfaces in said device are formed from non-oxidizing metal.

5. A device as in claim 2, further including means for applying equal potentials to said focusing grids and means for applying a more negative potential to said planar grid means than to said focusing grids.

6. A device as in claim 5, wherein said planar grid means comprises at least two planar grids.

7. A device as in claim 5, wherein all grid surfaces in said device are formed from non-oxidizing metal.

Claims

1. In an electron-concentrator spectrometer of the highresolution type having spherically curved retarding grid means between a specimen and collector means and having postmonochromator grid means, the improvement comprising: a first focusing grid located beyond said spherical grid means in the direction of said collector means; a second focusing grid located beyond said first focusing grid, both said grids having greater-than-spherical curvature but being oppositely curved, said first grid being curved in the same direction as said spherically curved grid means; and high-transparency, fine-mesh, planar grid means lying perpendicular to a line drawn between the specimen and the collector means and positioned between said first and second focusing grids, said planar grid means acting to determine the lower velocity limit of the electrons passing through said planar grid means.