WO1993016486A1

WO1993016486A1 - Method in the electron spectroscopy and an electron spectrometer

Info

Publication number: WO1993016486A1
Application number: PCT/FI1992/000043
Authority: WO
Inventors: Seppo Aksela
Original assignee: Dca Instruments Oy
Priority date: 1992-02-17
Filing date: 1992-02-17
Publication date: 1993-08-19
Also published as: AU1273192A

Abstract

A method and a two stage electrostatic cylindrical mirror analyzer (CMA) electron spectrometer (10) for gas phase and metal vapors. The spectrometer consists of the two stage analyzer (20, 30) where the source slit (14) and the image slit (15) are located on the symmetry axis (13). The central emission angle (Ζ) of the electron emission direction with respect to the symmetry axis (13) is preferably the magic angle 54.7°. The inner cylinders (21 and 31) of the analyzers (20 and 30) are equipped with apertures (21, 26, 35, 36) which enable the electron emission direction around the symmetry axis (13) at the angle that is between 0° and 360°, preferably 360°.

Description

METHOD IN THE ELECTRON SPECTROSCOPY AND AN ELECTRON SPECTROMETER

BACKGROUND OF THE INVENTION

This invention relates to a method in the gas phase electron spectroscopy to determine the intensity and/or the distribution of electrons and to a two stage electrostatic cylindrical mirror analyzer (CMA) electron spectrometer to be used as electron energy analyzer in the electron spectroscopy, specially for gas phase and metal vapors, which spectrometer consists of the first stage analyzer and the second stage analyzer where the source slit and the image slit are located on the symmetry axis or close to it.

The two common types of CMA-analyzers are the so called magic angle (54.7°) and the second order focusing (42.3°) types. The magic angle version is advantageous for determination of relative total intensities and partial cross sections which are then independent from the possible inherent angular distributions of the electrons. Therefore it is often used in the gas phase electron spectroscopy. Electron optical source and image slits are located on the surface of the inner cylinder in the previous models.

The main advantage of the second order focusing (42.3°) version is the very high transmission due to large useful angular acceptance. This also commercially available analyzer type is common both as single and double pass versions in the solid state spectrometers. In these models slits are on the axis of the cylinders making double pass analyzers possible.

The electron optical properties of the cylindrical mirror analyzer (CMA) have been studied by several authors. The total projection L along the cylinder axis of the flight path of the electron can be calculated from the expression L/r, = n cotΦ + 2(kπ)^1/z cosΦ exp(k sin'Φ) erf( k sinΦ)

where the following notations have been used:

r = the radius of the inner cylinder, n = the geometric parameter defined by the equation

1 S + l1 = nr1, where

1 = the distance of the source slit from the inner cylinder perpendicular to the symmetry axis,

1, = the distances of the image slit from the inner cylinder perpendicular to the symmetry axis,

Φ = central emission angle of the electron emission direction with respect to the symmetry axis, k = parameter that can be calculated from the expression

k = (E/Uq) In r-/r. where

E = the initial kinetic energy,

U = the reflecting voltage between the cylinders, q = the electronic charge, r- = the radius of the outer cylinder,

erf(x) = the error function

erf(x) = 2/ π e^"1 'dt

0

From the previous studies it is known that the condition for the second order focusing,

(δL/δΦ), . - (δ^JL/δΦ^z), . = 0,

can be obtained with different combinations of parameters n, k and Φ. Actually for all values of the geometrical parameter n between the most realistic values 0 and 4 unique combination of parameter k and central emission angle Φ can be found which satisfies the second order focusing condition.

SUMMARY OF THE INVENTION

For given value of geometric parameter n the first order focusing can be obtained by a set of k and Φ values. Thus if we select the geometric parameter n describing the location of the image slits and also fix the angle Φ it is still possible to find the value of k which satisfies the first order focusing condition

(δL/δΦ)_{ι k} = 0.

Especially interesting is the case, when the central emission angle Φ is the magic angle of 54.7°. It is well known that the first order focusing takes place when n = 0, k = 1 and Φ = 54.7°. The value n = 0 means that the source slit and the image slit are on the surface of the inner cylinder. It is a useful situation in a single pass analyzer without pre-retardation of the electrons or with the retardation before the electron optical source slit.

In this case the real source on the symmetry axis is extended and diffuse and the electron optical source is on the surface of the inner cylinder. In many case, however, it is advantageous to use an arrangement where also the source slit and the image slit are on the symmetry axis or close to it. Then also the effective source volume is of the same size as the slit widths.

A very important advantage of this arrangement is the possibility to use conveniently double pass analyzers and to locate retardation at the end of the first stage. Because in the case of retardation only the stage after retardation works as the effective energy analyzer, the location of the retardation to the front of the first stage in commercial analyzers does not have any physical advantage.

The case that the geometrical parameter n = 2 is very important because then the double pass analyzer can be easily realized. For the focusing with n = 2 at the magic angle of 54.7° the new value of k = 1.62925 has been found.

The corresponding distance between the source and image slits is L = 8.06686 x r.. The linear dispersion is very high 12.08 x r . Thus the base resolution of

Z--E/E = 1 o = s/D

from the combined slit width s = s, _ + s.- is obtained when

s = D/1000 = 0.01208 x r

(e.g. for r. = 32 mm, s = 0.39 mm) .

The base resolution of 1 %o from angular divergence corresponds the angular spread from 56.3° - 53.0° = 3.3°. If the base resolution of 5 %<_ is accepted angular divergence can be opened to 58.2° - 50.7° = 7.5°.

The purpose of this invention is to provide a new method in the gas phase electron spectroscopy and a new dual electrostatic cylindrical mirror analyzer (CMA) electron spectrometer for featuring a new set of operational parameters making possible for the first time the use of the magic angle in the double pass CMA analyzers.

The method of according to the invention is characterized by measuring the intensity and/or the distribution of electrons so that the central emission angle (Φ) of the electron emission direction with respect to the symmetry axis is between 50° and 60°, preferably the magic angle 54.7°, and measuring the electron detection around the symmetry axis at the angle that is between 0° and 360°, preferable 360°. The electron spectrometer according to the invention is characterized in that the central emission angle (Φ) of the electron emission direction with respect to the symmetry axis is between 50° and 60° or preferably the magic angle 54.7°, and the inner cylinders of the analyzers are equipped with apertures which enable the electron emission direction around the symmetry axis at the angle that is between 0° and 360°, preferable 360°.

The benefit of this kind of electron spectrometer is that the spectrometer can be used to measure intensities which are independent of the inherent angular distribution using synchrotron radiation, X-rays or electron exitation. The spectrometer can also be used in a constant pass-energy mode or in a constant fractional retardation mode.

The above and other features and advantages of this invention will become better understood by reference to the detailed description that follows, when considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a two stage cylindrical mirror analyzer (CMA) electron spectrometer. FIG. 2 presents the geometry of the flight path of the electron in a cylindrical mirror analyzer (CMA) . FIG. 3 presents a perspective view of a two stage cylindrical mirror analyzer (CMA) .

FIG. 4 is a schematic view of the cylindrical mirror analyzer (CMA) of prior art. FIG. 5 is a schematic view of the cylindrical mirror analyzer (CMA) according to the invention. FIG. 6 presents the flight distance of electron as a function of the central emission angle Φ. FIG. 7 is a sectional view of a retardation lens system. DETAILED DESCRIPTION OF THE DRAWINGS

In FIG. 1 is a dual electrostatic cylindrical mirror analyzer (CMA) electron spectrometer 10 including two analyzing stages, the first stage 20 and the second stage 30. The both analyzer stages 20, 30 are in a vacuum chamber 11 and they consist of inner cylinders 21, 31 and outer cylinders 22, 32. The sample 12 is located at the left end of the first stage analyzer 20. The opposite end of the first stage analyzer 20 is provided with a retardation lens system 40. The slit for the sample is arranged so that the spectrometer uses a magic angle Φ = 54.7° which makes the spectrometer ideal for angle distribution measurements. Because the analyzers 20, 30 are cylinder symmetric a full 360° angle can be used for electron detection. The spectrometer 10 uses a constant pass-energy mode or a constant fractional retardation mode.

The distortion of electric fields at the end of the cylinder electrodes 21, 22 and 31, 32 is reduced with cylinder end plates 23, 24 and 33, 34. The plates give accurate field densities especially at the sample end where the electrons are very sensitive to any forces acting on them.

The detector 50 is located at the right end of the second stage analyzer. The suspension of the detector is flexible and gives the freedom to use a microchannel-plate 51 in the detector. So the two-dimensional electron detection can be measured with no moving parts in the angle distribution measurements. Of course the detector 50 can be modified for special applications.

The spectrometer in FIG. 1 is specially designed for gas phase measurements and it can also be modified for experiments with metal vapors. For gaseous samples a special gas shell is used to increase the intensity and decrease the gas consumption. The spectrometer is bakeable and can be used in a ultra high vacuum (UHV) environment. Secondary electron emission from inner parts is minimized by the analyzer geometry design and a coating of the surfaces. The spectrometer is used to measure electron energies and intensities using synchrotron radiation or electron exitation in ESCA, XPS, UPS and AES instruments.

In FIG. 2 is schematically presented a sectional view of a cylindrical mirror analyzer (CMA) and the geometry of the flight path of the electron in the analyzer. The analyzer 20 is formed of two electrodes, the inner cylinder 21 and the outer cylinder 22, and there is a reflecting voltage U between those cylinder electrodes. The electron path goes from the source slit 14, curves under the influence of the electrostatic field between the cylinder electrodes and goes to the image slit 15. In the inner cylinder 21 there are two apertures 25 and 26 for the electron path.

The distance 1- between the source slit 14 and the inner cylinder 21 perpendicular to the symmetry axis 13 and the distance 1. between the image slit 15 and the inner cylinder 21 perpendicular to the symmetry axis 13 define the geometric parameter n. It has already been presented above^' that the values of the parameter n can be between 0 and 4. The value n is 0 when 1. = 0 and 1. = 0. In that case the source slit and the image slit are on the surface of the inner cylinder 21. In the cylinder symmetric case l_s = r. and 1, = r. which gives the value n = 2. Then the source slit 14 and the image slit 15 are on the symmetry axis 13.

In FIG. 3 is a perspective view of a two stage cylindrical mirror analyzer (CMA) . Both the first stage 20 and the second stage 30 consist of inner cylinders 21, 31 and outer cylinders 22, 32. The path of the electrons is referred by the reference number 16. Because the analyzers 20 and 30 are cylinder symmetric electrons can be detected at a full 360° angle between the cylinders.

In FIG. 4 is presented a schematic view of the cylindrical mirror analyzer (CMA) of prior art. The source slit 14 and the image slit 15 are small apertures located on the surface of the inner cylinder 21. The difference compared to the cylindrical mirror analyzer (CMA) according to the invention is clearly seen in FIG. 5.

In FIG. 5 is presented a schematic view of the cylindrical mirror analyzer (CMA) according to the invention. The source slit 14 and the image slit 15 are located on the axis 13 of the inner cylinder 21. The apertures for electrons in the inner cylinder 21 are the grooves 25 and 26 which go around the whole cylinder 21 so that the electron detection angle is 360°. When the central emission angle Φ of the electron emission direction with respect to the symmetry axis 13 is the magic angle = 54.7° the relative total intensities and partial cross sections are independent from the possible inherent angular distributions of the electrons.

In FIG. 6 is presented the flight distance L of electron as a function of the central emission angle Φ. It can be seen from the curve of FIG. 6 that derivate δL/δΦ = 0 when the angle Φ = 54.7°. That angle is called the magic angle. In the FIG.6 it can also be seen that the part of the curve is horizontal. Thus a small difference in the angle Φ does not cause differences in the flight distance L, which means the good quality that has been looked for.

FIG. 7 shows a sectional view of a retardation lens system in the dual electrostatic cylindrical mirror analyzer (CMA) electron spectrometer 10 of FIG.l. The view of FIG. 7 is an enlargement of the center of the spectrometer 10 where the first stage analyzer 20 and the second stage analyzer 30 have been joined together.

In FIG. 7 there is a retardation lens system 40 between the analyzer stages at the end of the first stage analyzer 20. The energy distribution of electrons is measured by using constant pass energy. Before the electrons come to the second stage analyzer 30 the retardation of electrons is done by the retardation lens 40 which is situated just before the second stage analyzer 30.

The retardation lens system 40 in FIG. 7 consists of three retardation elements 41, 42, and 43. The first retardation element 41 has the same potential with the inner cylinder 21 of the first stage analyzer 20 and the third retardation element 43 has the same potential with the inner cylinder 31 of the second stage analyzer 30. The potential of the second retardation element 42 in the middle can be set from outside. The retardation of electrons is done by regulating the potential of the second retardation element 42 so that the intensity of electrons in the detector is maximized.

The retardation lens system 40 at the end of the first stage analyzer 20 in essential to the two stage cylindrical mirror analyzer (CMA) electron spectrometer 10 in FIG. 1 according to the invention. However it is also possible to use an accelerating lens system 17 at the sample 12 end of the first stage analyzer 20 together with retardation lens systems. When there are lens systems 17, 40 at the both ends of the first stage analyzer 20 the sample 12 end lens system 17 is for accelerating the electrons and the other lens system 40 at the end of the first stage analyzer 20 is for retardating the electrons. The co-operation of the lens systems will result better intensity and resolution in the detector.

Claims

1. A method in the gas phase electron spectroscopy to determine the intensity and/or the distribution of electrons in the two stage electrostatic cylindrical mirror analyzer (CMA) electron spectrometer (10) where the source slit (14) and the image slit (15) are located on the symmetry axis (13) of the cylindrical mirror analyzer or close to it, characterized by

- measuring the intensity and/or the distribution of electrons so that the central emission angle (Φ) of the electron emission direction with respect to the symmetry axis (13) is between 50° and 60°, preferably the magic angle 54.7° , and

- measuring the electron detection around the symmetry axis at the angle that is between 0° and 360°, preferable 360°.

2. A method according to claim 1 characterized by measuring at the central emission angle (Φ) between 53.0° and 56.3° corresponding the base resolution of 1 %o .

3. A method according to claim 1 or 2 characterized by detecting the angular distribution of electrons at the angle that is between 0° and 360°, preferable 360°.

4. A method according to claim 1, 2 or 3 characterized by retardating the electrons before they pass to the second stage analyzer.

5. A two stage electrostatic cylindrical mirror analyzer (CMA) electron spectrometer (10) to be used as electron energy analyzer in the electron spectroscopy, specially for gas phase and metal vapors, which spectrometer consists of the first stage analyzer (20) and the second stage analyzer (30) where the source slit (14) and the image slit (15) are located on the symmetry axis (13) or close to it, characterized in that

- the central emission angle (Φ) of the electron emission direction with respect to the symmetry axis (13) is between 50° and 60°, preferably the magic angle 54.7°, and

- the inner cylinders (21 and 31) of the analyzers

(20 and 30) are equipped with apertures (21, 26, 35, 36) which enable the electron emission direction around the symmetry axis (13) at the angle that is between 0° and 360°, preferable 360°.

6. An electron spectrometer (10) according to claim 5 characterized in that the central emission angle (Φ) is between 53.0° and 56.3° corresponding the base resolution of 1 %o .

7. An electron spectrometer (10) according to claim 5 or 6 characterized in that the apertures (21, 26, 35, 36) in the inner cylinders (21 and 31) of the analyzers (20 and 30) are grooves forming openings of 360° angle or part of that angle around the cylinders .

8. An electron spectrometer (10) according to claim 5, 6 or 7 characterized in that at the end of the second stage analyzer (30) there is a microchannel-plate (51) to be used as a two-dimensional electron detector (50) that is without moving parts able to detect the angular distribution of electrons at the angle of 360° or at the angle that is a part of the 360° angle.

9. An electron spectrometer (10) according to any of claims 5-8 characterized in that the retardation lens system (40) is located between the first stage analyzer (20) and the second stage analyzer (30) or at the end of the first stage analyzer.

10. Electron spectrometer (10) according to claim 9 characterized in that the retardation lens system (40) consists of three retardation elements (41, 42, 43) , the first retardation element (41) that has the same potential with the inner cylinder (21) of the first stage analyzer (20) , the third retardation element (43) that has the same potential with the inner cylinder (31) of the second stage analyzer (30) , and between the said two elements (41, 43) the second retardation element (42) the potential of which can be set from outside.