GB1577193A

GB1577193A - Energy analyser for charged particles

Info

Publication number: GB1577193A
Application number: GB2322277A
Authority: GB
Original assignee: Leybold Heraeus GmbH
Current assignee: Balzers und Leybold Deutschland Holding AG
Priority date: 1976-06-02
Filing date: 1977-06-01
Publication date: 1980-10-22
Also published as: DE2624730A1

Description

(54) ENERGY ANALYSER FOR CHARGED PARTICLES (71) We, LEYsoLD-HERAEus GmbH & BR< Co. KoMMANDITcEsELLsCHAFT, a German Company of Bonner Strasse 504, 5, Koln- Bayental, Federal Republic of Germany, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to apparatus for analysing the energies of charged particles.

Energy analysers for charged particles as hitherto known comprise means for gen exiting an electric or magnetic analyser field through which the particles pass and ar guided after leaving the analyser field thy tough an outlet aperture to a detector system Known forms of energy analyser include a spherical condenser, cylindrical condenser, magnetic sector field spectro ml,ter, or the like.

When carrying out experiments on samples of different sizes using the hitherto ki town types of apparatus, there is the dange;r that, in the case of small samples, the substrate on which the sample is supported will interfere in the spectrum of the sample.

The resolution of a spherical con do user for example is determined by the ratio of the inlet and outlet aperture width to the radius of the spherical condenser and the opening of the beam in the radial direc ton. On the other hand, the surface area of the inlet aperture (width x length) and the opening of the beam (total solid angle) is a decisive factor for the luminosity. The length of the aperture as well as the opening of the beam in the tangential direction do not, to a first approximation, affect the resolution, and this istrictly correct for the case of the ideal 180 spherical condenser. In order to obtain as high a sensitivity as possible, it has been suggested to make the inlet and outlet aperture as long as possible.However, this is limited by the following factors: 1. the effective surface area of the detector system used to detect the particles leaving the outlet aperture, and 2. by limitations in the ilumination of the inlet aperture of the spherical condenser.

If for example in photo-electron spectroscopy an image of the sample is formed at the inlet aperture of the spherical condenser by means of a lens system, the maximum useful length of the inlet aperture is given by the length of the sample multiplied by the magnification factor of the lens. In order to achieve maximum sensitivity, the aperture length is adapted to the diameter of the detector system that is available, generally a secondary electron multiplier.

The magnification factor of the lens is then adjusted to the maximum significant sample size. If only fairly small samples are to be investigated, which are applied to a base or substrate, then the latter will necessarily interfere in the spectrum of the sample.

It is the object of the invention to design and construct improved apparatus for analysing the energies of charged particles which is not subject to the above limitations of the prior art.

According to the present invention there is provided apparatus for analysing the energies of charged particles, comprising analyser means for generating an electric or magnetic field through which the particles pass, a rectangular outlet aperture in the analyser means for exit of the charged particles as a beam of rectangular crosssection, a detector system arranged in the path of the beam leaving the analyser field through the outlet aperture, and an electrode system provided between the energy anaylser and the detector system for adjusting the length of the cross-section of the particle beam as determined by the size of the said outlet aperture.

Depending on whether positive or negative particles are analysed, a positive or negative voltage can be applied to the electrode system of the invention which, depending on the value of the applied voltage, more or less sharply "trims" the cross-section of the particle beam leaving the outlet aperture. In changing from a larger to a smaller sample, particles from the sample substrate can be prevented from being recorded simply by reducing the cross-section of the particle beam leaving the outlet aperture and without altering the optimally adjusted sensitivity.

Preferred embodiments of energy analyser apparatus according to the invention are illustrated diagrammatically in the drawings, of which: Figure 1 shows in elevation a first embodiment of the invention comprising an annular electrode system; Figure 2 is a cross-sectional plan view of the electrode system of Figure 1; Figure 3 shows in elevation a second embodiment of apparatus according to the invention comprising an electrode system consisting of two rod electrodes; Figure 4 is a cross-sectional plan view of the electrode system of Figure 3; Figure 5 shows in elevation yet another embodiment of the invention comprising an electrode system formed of a plurality of pairs of rod electrodes; and Figure 6 is a cross-sectional plan view of the electrode system of Figure 5 and further. shows switching means for the electrodes.

In all the Figures of the drawings a housing of the energy analyser apparatus is denoted by reference 1, a boundary field border by reference 2, and a rectangular outlet aperture arranged in the boundary field border by reference 3. In front of the outlet aperture 3 is located a grid 4, intended to prevent the inverse amplification factor of a post-acceleration field between the energy analyser and a detector system 5.

In the embodiments illustrated, the detector system 5 is a secondary electron multiplier.

The electrode system of the invention is denoted by the reference 6 in all the Figures. It is biased with respect to the cathode 7 (or the first dynode) of the secondary electron multiplier 5 by means of a voltage source 8, preferably a stepwise or continuously variable source. When analysing positive particles the electrode system 6 is positively biased, and when analysing negative particles it is negatively biased.

In the embodiment shown in Figures 1 and 2 the electrode system 6 is formed by an annular electrode 9 whose internal diameter is somewhat greater than the length of the aperture 3 (see Figure 2). If, for example, a negative voltage relative to the cathode 7 of the secondary electron multiplier 5 is applied to this electrode via a lead 10, the opposing field acting in particular at the edge of the electrode limits the length of the rectangular cross-section of the particle beam leaving the outlet aperture.

The size of the cross-section of the particle beam and thus the size of the sample region detected can be regulated by altering the magnitude of the applied voltage.

In the embodiment shown in Figures 3 and 4 the electrode system 6 consists of two rod electrodes 11 and 12 arranged perpendicularly to the outlet aperture 3 and spaced apart by a distance which is somewhat greater than the length of the aperture 3.

The electrodes 11 and 12 are electrically connected to one another. The pass-band width is regulated by the magnitude of the voltage applied between the cathode 7 of the secondary electron multiplier 5 and the electrodes 11 and 12.

In the further embodiment shown in Figures 5 and 6 the electrode system 6 consists of several pairs (in the present case three pairs) of electrodes 13, 14 and 15 which are all likewise perpendicular to the aperture 3 and are parallel to one another. The pairing of the electrodes is chosen such that, with reference to the mid-point of the aperture 3, the outermost electrodes are connected together to form the pair 13, the next outermost electrodes are connected together to form the pair 14, (and so on if more than three pairs of electrodes are provided), and finally the inner electrodes are connected together to form the pair 15. The voltage supply for the electrode pairs is provided via a switching means 16 that has at least as many switching positions as there are electrode pairs.Switching means 16 shown has four switching positions, denoted by 17, 18, 19 and 20, in one of which, 17, no voltage is applied to any of the electrode pairs. In the switching position 18 a voltage is applied only to the outer electrode pair 13. In the switching position 19 voltage is applied to the two outer electrode pairs 13 and 14. In the switching position 20 a voltage is applied to all electrode pairs. The electric field developed prevents the penetration of positive or negative particles depending on whether there is a positive or negative biasing with respect to the cathode of the secondary electron multiplier. In this way the cross-section of the electron or ion beam in question can be constricted by steps without the voltage source 8 having to be of a variable form.

WHAT WE CLAIM IS: 1. Apparatus for analysing the energies of charged particles, comprising analyser

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. the said outlet aperture. Depending on whether positive or negative particles are analysed, a positive or negative voltage can be applied to the electrode system of the invention which, depending on the value of the applied voltage, more or less sharply "trims" the cross-section of the particle beam leaving the outlet aperture. In changing from a larger to a smaller sample, particles from the sample substrate can be prevented from being recorded simply by reducing the cross-section of the particle beam leaving the outlet aperture and without altering the optimally adjusted sensitivity. Preferred embodiments of energy analyser apparatus according to the invention are illustrated diagrammatically in the drawings, of which: Figure 1 shows in elevation a first embodiment of the invention comprising an annular electrode system; Figure 2 is a cross-sectional plan view of the electrode system of Figure 1; Figure 3 shows in elevation a second embodiment of apparatus according to the invention comprising an electrode system consisting of two rod electrodes; Figure 4 is a cross-sectional plan view of the electrode system of Figure 3; Figure 5 shows in elevation yet another embodiment of the invention comprising an electrode system formed of a plurality of pairs of rod electrodes; and Figure 6 is a cross-sectional plan view of the electrode system of Figure 5 and further. shows switching means for the electrodes. In all the Figures of the drawings a housing of the energy analyser apparatus is denoted by reference 1, a boundary field border by reference 2, and a rectangular outlet aperture arranged in the boundary field border by reference 3. In front of the outlet aperture 3 is located a grid 4, intended to prevent the inverse amplification factor of a post-acceleration field between the energy analyser and a detector system 5. In the embodiments illustrated, the detector system 5 is a secondary electron multiplier. The electrode system of the invention is denoted by the reference 6 in all the Figures. It is biased with respect to the cathode 7 (or the first dynode) of the secondary electron multiplier 5 by means of a voltage source 8, preferably a stepwise or continuously variable source. When analysing positive particles the electrode system 6 is positively biased, and when analysing negative particles it is negatively biased. In the embodiment shown in Figures 1 and 2 the electrode system 6 is formed by an annular electrode 9 whose internal diameter is somewhat greater than the length of the aperture 3 (see Figure 2). If, for example, a negative voltage relative to the cathode 7 of the secondary electron multiplier 5 is applied to this electrode via a lead 10, the opposing field acting in particular at the edge of the electrode limits the length of the rectangular cross-section of the particle beam leaving the outlet aperture. The size of the cross-section of the particle beam and thus the size of the sample region detected can be regulated by altering the magnitude of the applied voltage. In the embodiment shown in Figures 3 and 4 the electrode system 6 consists of two rod electrodes 11 and 12 arranged perpendicularly to the outlet aperture 3 and spaced apart by a distance which is somewhat greater than the length of the aperture 3. The electrodes 11 and 12 are electrically connected to one another. The pass-band width is regulated by the magnitude of the voltage applied between the cathode 7 of the secondary electron multiplier 5 and the electrodes 11 and 12. In the further embodiment shown in Figures 5 and 6 the electrode system 6 consists of several pairs (in the present case three pairs) of electrodes 13, 14 and 15 which are all likewise perpendicular to the aperture 3 and are parallel to one another. The pairing of the electrodes is chosen such that, with reference to the mid-point of the aperture 3, the outermost electrodes are connected together to form the pair 13, the next outermost electrodes are connected together to form the pair 14, (and so on if more than three pairs of electrodes are provided), and finally the inner electrodes are connected together to form the pair 15. The voltage supply for the electrode pairs is provided via a switching means 16 that has at least as many switching positions as there are electrode pairs.Switching means 16 shown has four switching positions, denoted by 17, 18, 19 and 20, in one of which, 17, no voltage is applied to any of the electrode pairs. In the switching position 18 a voltage is applied only to the outer electrode pair 13. In the switching position 19 voltage is applied to the two outer electrode pairs 13 and 14. In the switching position 20 a voltage is applied to all electrode pairs. The electric field developed prevents the penetration of positive or negative particles depending on whether there is a positive or negative biasing with respect to the cathode of the secondary electron multiplier. In this way the cross-section of the electron or ion beam in question can be constricted by steps without the voltage source 8 having to be of a variable form. WHAT WE CLAIM IS:

1. Apparatus for analysing the energies of charged particles, comprising analyser

means for generating an electric or magnetic field through which the particles pass, a rectangular outlet aperture in the analyser means for exit of the charged particles as a beam of rectangular cross-section, a detector system arranged in the path of the beam leaving the analyser field through the outlet aperture, and an electrode system provided between the energy analyser and the detector system for adjusting the length of the cross-section of the particle beam as determined by the size of the said outlet aperture.

2. Apparatus according to Claim 1, wherein the electrode system is connected to a voltage source that is stepwise or continuously variable.

3. Apparatus according to Claim 1 or Claim 2, wherein the electrode system comprises an annular electrode.

4. Apparatus according to Claim 1 or Claim 2, wherein the lectrode system comprises two parallel rod electrodes arranged perpendicularly to the outlet aperture.

5. Apparatus according to Claim 1 or Claim 2, wherein the electrode system comprises a plurality of pairs of rod electrodes arranged parallel to one another and perpendicularly to the outlet aperture, and means for connecting a voltage source at choice to the outermost pair, with reference to the mid-point of the aperture, of electrodes alone, to the two outer pairs to the three outer pairs or to all pairs of rod electrodes.

6. Apparatus according to Claim 5, wherein the connecting means comprises switching means having at least as many switching positions as there are electrode pairs.

7. Apparatus for analysing the energies of charged particles, substantially as hereinbefore described with reference to Figures 1 and 2, Figures 3 and 4, Figure 5 or Figure 6 of the accompanying drawings.