DE1952168A1 - Electron diffraction spectrometer - Google Patents

Description

is differentiated, the energy distribution of the electrons leaving the target can be determined in a simple manner. Peaks in this energy distribution are characteristic of the composition of the target surface atoms - and of them The device can also be operated as a spectrometer. In one embodiment of the invention, the retardation grid electrode Lattice openings that define electron channels through the lattice, and those channels have one characteristic diameter, which is smaller than three times the characteristic length of the channels, in order to ensure uniformity of the retardation field and in this way to improve the resolution of the spectrometer. At a Another embodiment of the invention is a spherical screen grid electrode concentric to the retardation grid and has a radius in the range from 0.3 to 0.8 that of the retardation grating in order to increase the resolution of the To improve the spectrometer.

State of the art

Electron diffraction devices are already known. One such device is shown in U.S. Patent 3,313,936 Applicant described. Other similar devices operated as spectrometers are described in the Journal of Applied Physics, Vol. 39, No. 5, April 1968, pp. 2425-2432 and Journal of Applied Physics, Vol. 38, No. 8, July 196/7, pp. 3320-3322.

The known spectrometers have a target electrode, which is usually operated at ground potential and a surface, how has some crystallographic plane to be studied to determine its surface properties. An electron gun is provided to provide a To direct a stream of practically monoenergetic electrons onto the surface of the target electrode in order to thus scatter electrons produce. A spherical luminescent screen is arranged concentrically around the target electrode around certain of the scattered ones Collect electrons to thereby form a diffraction optical pattern on the screen. The one from the shield electrode

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recorded electron current is also dependent measured from the sampled electron beam potential to one To obtain energy distribution of the scattered electrons.

There is a spherical retardation grating structure between the target and the screen arranged to scatter electrons with a lower potential energy than a certain given one To prevent potential from reaching the screen. In this way, inelastically scattered electrons can be removed from elastically scattered ones Electrons are separated. It has also been observed that when the electron beam is at a glare angle of incidence the target surface to be examined is directed, the scattered electrons to be observed closer to the surface and therefore contain more information about the surface.

In these known spectrometers, the retardation field grid electrode structures are typically arranged between two shielding grid electrodes which are relatively close to the retardation field grid. With this arrangement it has become common to achieve an energy resolution of the electron energy distribution of about 2.4 i » , the resolution being defined as the full width at half maximum amplitude in electron volts of the observed electron tip divided by the electron volts in the center of the tip . A resolution of 2.4 ^ is relatively good, but it is desirable to improve this resolution by a factor of about 10 so that surface effects and characters can be more easily distinguished.

Summary of the invention

The object of the invention is to provide an electron diffraction spectrometer with delay field available that has an improved resolution.

According to the invention, in an electron diffraction spectrometer a retardation field grating structure with openings is provided,

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which define electron channels and have a characteristic diameter less than three times that characteristic length of the channels, so that the retardation field potential across the electron channels through the retardation grating is considerably more uniform in order to increase the resolution of the To improve the spectrometer.

Further according to the invention in an electron diffraction spectrometer a curved retardation field grid structure is provided around the target electrode, to which a screen grid structure is arranged concentrically between the retardation grid and the target electrode, this shielding fe is operated at practically target electrode potential and is arranged on a radius which is in the range from 0.3 to 0.8 of the radius of the retardation field grating, so that the resolution of the spectrometer is significantly improved.

According to a further development of the invention, there is the retardation field grid electrode from two concentrically arranged grids, the distance between the two grids being considerably larger is as the characteristic diameter of the openings in the grids.

According to a further embodiment of the invention, a screen grid electrode structure is provided on both sides of the Verzögerungsfeld- W gittere, the retarding field grid mn against the target and shield the phosphor screen.

According to another development of the invention, the electron gun has an elongated conductive tube which contains the actual electron beam system and so on is arranged to protrude radially through aligned openings in the screen and retardation grid, and this tube is in directed essentially at the target and ends essentially at the inner concave surface of the retardation grating, see above that the electron gun tube is the radial Electric fields do not interfere when the device for electron spectroscopy according to Auger is used.

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Further features and advantages of the invention emerge from the following description in conjunction with the drawing; show it:

Pig. 1 schematically, partly in the form of a block diagram, a known electron diffraction spectrometer with Delay field;

Fig. 2 shows the dependence of the voltage on the radial distance of the target electrode with the layers of the different electrodes of the spectrometer and the associated one Stress profile;

3 shows a representation corresponding to FIG. 1 of part of an electron diffraction spectrometer with features of FIG Invention;

4 schematically a part of the retardation field electrode structure known type and the associated electrostatic potential lines;

5 shows a diagram corresponding to FIG. 4 with potential lines of an improved retardation field grating structure according to the invention;

6 schematically shows the path of an electron which passes through an opening in an electrode, on both sides of which the electric fields E and E ¹ respectively prevail; and

7 shows the dependence of the shield current amplitude on the voltage at the delay field electrode for illustration the electron energy distribution in the known structure and corresponding signals with the two embodiments of the invention.

Fig. 1 shows a typical known retardation field electron diffraction spectrometer as shown in US Pat the above-mentioned articles in Journal of Applied Physics, Vol. 38 and 39, respectively. In short, the spectrometer has a target ^; electrode 1, which contains the material sample to be examined,

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for example a certain crystallographic level of a crystalline sample. The target electrode is usually operated at ground potential. An electron gun 2 or 2 · is arranged so that a stream of essentially monoenergetic electrons hits the surface of the target to be examined is directed. The spectrometer can be equipped with one or both beam generation systems be.

The electron beam generating system 2 is provided so that the primary beam on the surface of the target to be examined fc at glancing angles, i.e. grazing, relative to the one to be examined Surface. In this way penetrate the primary electrons hit the surface at a shallower depth than the electrons of a beam perpendicular to the surface 1 as it dated another electron gun 2 'would receive Accordingly, the electrons scattered on the surface of the target due to a beam from the system contain more electrons Information concerning the surface.

A spherical luminescent screen 3 is arranged concentrically to the target 1 in order to receive the scattered electrons and to generate a diffraction pattern on the luminescent screen 3. The screen can be observed through a vacuum-tight window 4 from the outside through the vacuum vessel, not shown, k of the spectrometer. A retardation field grating 5 is arranged concentrically to the target 1 and to the luminescent screen 3 in the space between the target and the screen 3. A delay voltage from a delay voltage source 6 is applied to the delay field grid 5 so that scattered electrons, which have a lower potential than that applied to the delay grid 5, are prevented from reaching the phosphor screen 3. When the retarding field potential is swept from the cathode potential to the potential of the retarding electrode 1, the electron energy spectrum of the scattered electrons is obtained. The wobble of the source 6 is derived from a wobble generator 7, which the output voltage

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the source 6, for example with a suitable mechanical Rod 8 changed.

Two grounded shield grids 9 and 11 comprise the retardation grid 5. When using electron diffraction only the elastically scattered electrons is carried out, the phosphor screen is kept in the range of +3 kV to +7 kV, see above that the phosphor is properly excited for visual observation of the diffraction pattern. In the case of sheet electron spectroscopy the shield electrode 3 is typically operated at about 300 V positive against the grounded shield electrode 11, so that the Most of the secondary electrons generated on the phosphor are sent back between the screen 3 and the screen electrode 11 to the phosphor and not in the delay field area between the gratings 11 and 5 enter where they could be further modulated. The 300 V voltage also ensures that the Electrons leaving the target with energies less than a few electron volts do not cause any charging problems on the phosphorus.

The potential is supplied by a voltage source 12. A Load resistor 13 lies between screen 3 and earth, so that the current collected by the screen has an output voltage this load resistance 13 causes. The delay voltage applied to the delay grid 5 is set with a relatively low frequency signal with a small amplitude, for example 1 V peak-peak and 100 to 1000 Hertz, with a Modulator 14 is amplitude-modulated. A part of the modulation signal supplied by the modulator is switched on via a switch 10 an input of a phase sensitive detector 16 is applied to be compared in phase with an output voltage, which is taken from the load resistor 13. The output of the phase detector is a DC voltage signal whose phase and the amplitude corresponds to the energy distribution function of the scattered electrons. The output of the phase sensitive Detector 16 is fed to an input of a recorder 17 and, as a function of the wobble signal, from wobble generator 7

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recorded to an energy distribution spectrum of the examined Sample as shown in curve A in Pig. 7 shown.

A more convenient mode of operation of the spectrometer .is in the mentioned article in Journal of Applied Physics, Vol. 39, April 1968. In this way of working, the Detector 16 tuned to the second harmonic of the modulation frequency from modulator 14. A diode multiplier 15 is switched via switch 10 in the line between the modulator 14 and the input of the Phase.ndetektors 16, so that the second Harmonics of the modulation frequency are compared with the second harmonic of the voltage drop across resistor 13 can. This allows the output voltage of the phase detector contain the first derivative of the energy distribution recorded as a function of the swept beam voltage becomes, and in this way the energy peaks of the energy distribution function easier to distinguish from the background. However, the resolution in the spectrum is independent of the Operating mode, and the resolution shown in Fig. 7 can also be achieved if the output voltage is the derivative this curves is.

In FIG. 2, the potential profile in the radial direction in the spectrometer according to FIG. 1 is shown in solid lines 19. In In the special case shown, the beam voltage V is -100 V with respect to the target electrode. The delay electrode 5 has a potential of about 98 V and the screen 3 has a positive one Potential of about 300 V. Electrons scattered by target 1 with Electron volts equal to or greater than 98 V run through the Deceleration electrode 5 through, are accelerated and bombard the luminescent screen 3 with an energy of about 400 Electron volts. A typical elastic tip electron energy profile is shown by line A in Figure 7; this shows a relatively broad energy peak with a peak at around 1025 eV when the primary beam has a beam voltage of 1000 V and a current of about 15 microamps, with the modulation of the delay field being about 1 V peak-to-peak.

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The resolution of curve A is about 2.4 # and it is desired to improve this resolution considerably.

One of the factors that has a detrimental effect on the resolution of the output spectrum of the known Pig spectrometer. 1 is due to the retardation field grid electrode 5 being relatively open, there are about thirty-nine square grid openings per centimeter (100 openings per linear inch), and the grid wires 5 are about 0.025 mm (0.001 inch) in diameter (see FIG. Pig. 4). Correspondingly, the potential lines V bend inwardly towards the grid in accordance with the delay potential which is fed to the delay grid 5, so that there is a considerable nonuniformity of the delay potential over the cross section of each of the openings in the delay grid 5. "This results in a relatively poor energy selection the retardation field electrode structure 5, which contributes significantly to the relatively poor resolution of only 2.4 i » .

In Pig. 3 and 5, a retardation grid electrode structure 5 'according to the invention is shown. In the improved grid structure 5 ¹ , two similar grids 5 ^{1 are} interconnected to operate at the same potential. These grids 5 'are concentrically arranged spherical sections with grid openings of, for example, 0.15 mm opening (100 mesh), the same as those in the known Pig grid. I and 4 can be used. The grids 5 'are expediently spaced apart in the radial direction, which is preferably considerably greater than a third of the characteristic cross-sectional dimension d of the openings in the grille 5'. In a typical embodiment, Js is about ten times as large as d. In such a case, the more positive potential lines penetrate only partially into the composite lattice structure 5 ', so that in the center of the lattice structure the potential surface V extends completely through the lattice to form a uniform potential barrier with which electrons are repelled try to pass through the grid with energies less than the retardation potential V

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1952 (68

to run through. Although a double mesh-like grating 5 'is shown, it is not necessary for the construction to consist of grids spaced apart from one another; a metallic cell structure can also be used, as is formed by the usual hexagonal grids used in reflex microwave tubes be used. The main requirement is that the electron channels leading through the lattice structure 5 'have a diameter d which is less than three times the length --o of the electron channel. Preferably these channels have a length - which is a multiple of the dimension d. If two grids 5 are used, the holes in the two grids need not be aligned.

The use of the improved retardation grating structure 5 * results in a significant improvement in the resolution of the energy spectrum of the scattered electrons, as shown by curve B in FIG. However, when the curve B in Fig. 7 is considered more closely, it shows that the resonance line characteristic shape is distorted on the lower energy side, and it is believed that this is caused by aberrations in the electron trajectories which result when the electrons pass through the first screen grid 9 when this grid is in the position shown in FIG. 1, ie has a radius of at least 95 ^° A of the radius of the delay grid 5.

In connection with FIG. 6 it can be shown that electrons which close to the edge of an opening 25 in a plate 26, on one side of which there is an electric field E and on the other side of which there is an electric field E ¹ , electrons passing through large-angle Experiencing deflections θ, approximately according to the following equation:

tan 0 «-

in which meanι

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V is the electron energy at opening 25;

r is the initial radial distance of an electron orbit

from the axis of the opening; and E * and E are the electric fields on both sides of the Plate 26.

In the case of the epectrometer according to FIGS. 1 and 2, the electrical one Field E on the target side of the grid 9 equals zero, pie Equation (1) is thus equivalent to:

tan θ - J Jf- (2)

With a finite V and a given opening size of the grating 9, δ can be reduced ^{by reducing E 1.} In the case of a spherical condenser, there is a minimum for the electric field when the radius of the inner spherical conductor is exactly equal to half the radius of the outer conductor. In the structure according to FIG. 3, the inner spherical screen grating 9 has therefore been shifted to a radius r ^ which is approximately half the radius Γρ of the inner concave surface of the retardation grating structure 5 '. Additional spherical electrodes 26 and 27 are provided at the edges of the spherical grid 9 between the grid 9 and the retardation grid 5 'in order to maintain the shape of the potential surfaces in the edge regions of the grid structures 9 and 5'. Suitable potentials between the ground potential of the grid 9 and the delay potential V are applied to the electrodes 26 and 27, respectively, in order to maintain the spherical shape of the equipotential surfaces in the area between the screen grid 9 and the delay grid 5 '. The improvement in resolution, which is obtained by shifting the inner screen grating 9 to about half the radius of the retardation grating structure 5 ¹ , can be seen from curve C in FIG. The resolution of the curve C is about 0.3 i » and represents an improvement in the resolution, inter alia, a factor of about 10, compared to the resolution that results from the known technology, namely the curve A.

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The expression for the electric field strength on the inner conductor of a spherical condenser is:

V T ₇

^E r - ' ^{(3) r} l ^r l ^ ^r 2 ¹ V

where mean:

r is the radius of the inner grid 9 »and Tp is the radius of the retardation grid 5 '·

In a typical embodiment, the retardation a-P grid has a radius r ₂ equal to 58 mm (2.3 inches) and the inner grid 9 is chosen to have a radius r- corresponding to about 38 mm (1.5 inches) 65 # of radius i * ₂ . When these values are substituted into equation (3), it can be seen that the electric field E is only about 10 i » greater than the minimum value obtained when r- ^ equals 0.5 r ₂ . However, this increase of 10 i » from the minimum still results in an electric field which is about one fifth of that which is achieved in the known spectrometer when the distance between the inner grating 9 and the retarding grating 5 is about 3 mm (0.120 inches). and τ _χ = 0.96 r ₂ · It can therefore be seen that the arrangement of the inner screen grid 9 precisely on the minimum electric field is not critical. However, a considerable improvement in the resolution of the spectrometer can be achieved if the radius of the inner screen grating 9 is in the range from 30 to 80 $> the radius of the retardation field grating 5 '.

In Fig. 3 an electron diffraction spectrometer is shown, with which both elastically and non-elastically scattered electrons can be observed. More precisely, that An obliquely arranged electron gun 2 can lead to Electron examinations of the surface of the target 1 according to Auger are used. When the spectrometer is this way used to observe the Auger electrons

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t> 8

the electron gun tube 28 · carrying the Contains cathode of the normally arranged electron gun 2 *, preferably at the inner end in the Cut off plane of the inner surface of the concave retardation field grating 5 '. In addition, the system pipe 28 'is electrical connected to the delay grid 5 'in order to disturb the electrical field in the area between the screen grid and the retardation grid 5 '.

On the other hand, when the apparatus of FIG. 3 is used for low-energy electron diffraction, that is, for examining electrons elastically scattered by the target 1, the normally arranged electron gun 2 * is used and the oblique system 2 is turned off. In this case, the system pipe is preferably operated 28 'at ground potential and thus the innermost grating of the double-retarding grid structure 5 ¹ insulated from the external grid 5 ¹ and connected to the system pipe 28 and the internal grille 9 to operate at ground potential. In this way, the beam is not degraded or deflected as it travels from the system tube 28 'to the sample 1. In the latter case, however, the outer grid of a retardation grid electrode 5 'is insulated from the inner grid and is operated at retardation field potential V. This reduces the resolution somewhat, but a resolution better than 0.5 # can be obtained if the grating 11 is connected to the delay grating 5 ¹ to obtain a double grating structure with the same properties as the double grating 5 '. Since no modulation voltages are applied to the grids 5 'in this operating mode, no AC voltage measurements are carried out and there is no need to provide an earth shield between the delay field grids 5 ¹ and 11 and the screen. A resolution of 0.5 # at 100 V corresponds to 0.5 eV, which is sufficient for electron diffraction purposes. The arrangement according to FIG. 3 allows a uniform electrode structure to be used both for the investigation of elastically scattered electrons and Auger electrons.

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Claims (6)

1952 ι 6 8 Patent application Electron diffraction spectrometer having means for generating a stream of practically monoenergetic electrons towards the surface of a target electrode to be examined to produce scattered electrons, a curved fluorescent screen electrode which is arranged concentrically around the target electrode, around certain of the target electrodes to collect scattered electrons, Jk to form a diffraction pattern on the screen, and a curved lattice structure concentric with the target electrode and the screen electrode placed in the space between these two to create a retardation field with which scattered electrons with a potential smaller than a certain predetermined potential can be prevented from reaching the phosphor screen, characterized in that the retardation field is substantially uniform and is designed so that at least one closed potential surface is formed around the target. 2. Spectrometer according to claim 1, characterized in that P the retardation grid electrode has grid openings which form electron channels through the grid and which have a characteristic diameter which is less than three times the characteristic length of the same, so that the retardation field potential is practically uniform across the electron channels through the retardation grating is to improve the resolution of the spectrometer. 3. Spectrometer according to claim 2, characterized in that the retardation field grid electrode consists of two concentrically arranged grids, the distance between which is considerably greater than the characteristic diameter of the openings in the grids * 009818/1261 ORIGINALIN _ ₁₅ - 1 9 ο ζ ι 6 8 4. Spectrometer according to claim 1, 2 or 3, with a curved screen grid between the retardation grid and the target electrode, which is operated essentially at target electrode potential, daduroh characterized, dafi the screen grid on a radius in the range of 30 to 80 »of the radius of the delay grating is arranged, so that the resolution of the spectrometer is further improved. 5. Spectrometer according to one of claims 1 to 4, characterized in that two curved screen grids are arranged concentrically to the delay field grating electrode, these screens are operated at the potential of the target, one of the screen grids between the target and the retardation grid is arranged and the other of the screen grids between the retardation grid and the Shield electrode is arranged. 6. Spectrometer according to one of claims 1 to 5, characterized in that the device for generating a monoenergetic electron flow to the surface of the target has an elongated conductive tube which contains an electron gun, and this Tube is arranged so that it protrudes radially through aligned openings in the screen and the retardation grid electrode structure towards the target and that inner end of the tube terminates substantially at the inner concave surface of the retardation grating. OHK “NAL 0 0 9818/1261