Spherical electrode type charged particle analyzer

Abstract: A charged particle analyzer comprises a spherical grid (1), a spherical electrode (2) concentric therewith, an emission portion (W) for emitting charged particles, a screen plate (3) having an exit opening (A), a plurality of second spherical grids (8,81,91,9) forming a high pass filter for the charged particles, a detector (4) for detecting the charged particles, and an obstacle ring (10) being in parallel with an equatorial plane normal to a line connecting the emission portion and the center of the exit opening with being within the spherical electrode (2).

Description: BACKGROUND OF THE INVENTION The present invention relates to an analyzer for analyzing the composition, structure, or electronic condition of a sample by measuring the kinetic energy of charged particles emitted from the sample and the angular distribution of the particles, and more particularly, to such an analyzer suitable for measuring the energy distribution of the charged particles emitted from the sample or the angular distribution of charged particles of particular energy to be noted from the surface of the sample in a two-dimensional manner. Conventionally, to analyze the energy of charged particles from a sample, the energy is measured for the charged particles emitted within a particular small solid angle, so that the energy distribution of the charged particles in the direction is examined. The present inventors proposed an improved charged particle analyzer for measuring the angular distribution of the charged paraticles of a given kinetic energy emitted from a sample into a large solid angle, which was issued on July 18, 1989, as a US Patent No. 4,849,629, entitled "CHARGED PARTICLE ANALYZER" . That invention is described with reference to FIG. 1 showing a preferred embodiment of the present invention. A part-spherical grid 1 has a center O. Concentric with the part-spherical grid 1, a part-spherical electrode 2 is disposed. A sample S is positioned within the part-spherical grid 1 and far from the spherical center of the part-spherical grid 1. A screen plate 3 is provided having an opening A symmetric with the sample S, with respect to the center of the part-spherical grid 1. A two-dimensional detector 4 is positioned within a space opposed to the part-spherical grid 1 as to the screen plate 3 and faced to the opening A. A suitable voltage is applied between the part-spherical grid 1 and the part-spherical electrode 2, then no electric field appears within the bottom space of the part-spherical grid 1. When an excitation beam is incident on the sample S, the sample S emits charged particles. The emitted charged particles linearly travel at the bottom space of the part-spherical grid 1 from the incident portion of the exciting beam. They fly an elliptical orbit having one focus at 0 within the space between the part-spherical grid 1 and the part-spherical electrode 2. Some charged particles with too high energy are eliminated by being struck with the part-spherical electrode 2. Some charged particles with lower energy can return to the bottom space of the part-spherical grid 1. Some charged particles having particular energy defined by a voltage applied between the part-spherical grid 1 and the part-spherical electrode 2 can transit through the opening A in the direction parallel with the direction of the emission of the charged particles from the sample S. This means that the charged particles transiting through the opening A all have the same energy, and the angular distribution of the charged particles is the same as the angular distribution of them at the time when they are emitted from the surface of the sample S. The output pictures of the two-dimensional detector 4 represent the angular distribution of the charged particles with the particular energy. The particular energy can be selected by the voltage applied between the part-spherical grid 1 and the part-spherical electrode 2. Thus, the above-described analyzer can measure the angular distribution of the charged particles having the particular energy among the charged particles emitted from the excited portion on the surface of the sample S. As apparent from the above description, the part-spherical grid 1 and the part-spherical electrode 2 do not conform any low pass filter. According to that invention, no energy filter is provided by which all of the charged particles having energy more than particular energy Ec are introduced into the part-spherical electrode 2 and all of the charged particles having enery less than Ec are reflected. On the other hand, the charged particles with the particular energy can be gathered into the opening A and transit it while the charged particles with the other energy are scattered over the screen plate 3 not to transit it. The selection of the charged particles in terms of the energy is done in this manner. The resolution of the energy depends upon the position of the sample, from which the charged particles are emitted, the position and the largeness of the opening A. With a simulation using a computer, the transmittance of the charged particles with various kinetic energies through the opening A is calculated as a function of a deviation of energy as to the particular energy Eo (ΔE/Eo), which results are shown in FIG. 6. The numerals in the graph of FIG. 6 represent 10 times a ratio of a distance s from the center O of the sample S and the opening A as compared to the radius a of the part-spherical grid 1, namely, 10 × s/a. When the ratio s/a is small, the transmittance shows that the analyzer is a low-pass filter. When the ratio s/a exceeds 0.5, the transmittance curve is symmetric. When the sample S and the opening A are far from the center O, the half value width of the transmittance is narrow. When the largeness of the opening A is 1 % of the radius a of the part-spherical grid 1 and s/a =0.9, the resolution is about 1 %, which is approximate to the resolution of the conventional spectrometer. By the way, the disadvantage of this spectrometer is that the resolution is not uniform about the emission angle of the charged particles from the sample. Although the total transmittance over all of the emission angles is as illustrated in the graph of FIG. 6, the resolution at some angular region is better than the average value, and at some region it is worse than the average value. In particular, around the condition of emission angle  = 90° , the resolution is the worst as illustrated by the broken line of FIG. 4. In FIG. 4, the abscissa is the emission angle  and the ordinate is a deviation of energy (%) from the energy (Eo) to be analyzed. In FIG. 4, assuming that the transmittance of the charged particles of the energy Eo is 1, the plotted data represent the energy of the charged particle having about one half of the transmittance. At the range of  = 90° ∼ 180° , the data are about symmetric with respect to  = 90° , so that the transmittance of only  = 0° ∼ 90° is illustrated. The broken line data of FIG. 4 are given by the previous invention, indicating that around  = 90° , the resolution is very poor at the high energy side and the low energy side. In practice, around  = 90° , the S/N ratio is too bad to observe the pictures. In the graph of FIG. 6(b), both sides cannot reach zero promptly due to the poor resolution at this angular range. At the lower angle specified by arrow A in the graph of FIG. 4, the orbit of the charged particles collides with the outer spherical electrode 2, so that the resolution at the high energy side becomes good. In Soviet Jnl. Technical Physics, Vol. 33, No. 2, February 1988, pages 135-144, a charged particle analyser has been described, in which an electrode and a grid are arranged to form an ellipsoidal mirror which at the same time serves as a low pass energy filter. The charged particles emitted from the sample are focussed by the ellipsoidal mirror onto the exit opening, and a plurality of part-spherical grids are arranged between the exit opening and the detector in order to form a high pass filter for the charged particles which have passed through the exit opening. SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved charged particle analyser of the type indicated in claim 1, in which the non-uniformity of the energy resolution dependent on the direction of the emission of charged particles from the surface of the sample is eliminated so that a high energy resolution over all the directions of emission of charged particles from the surface of the sample can be obtained. According to the invention, this object is achieved with the features indicated in the characterizing part of claim 1. Useful details of the invention are specified in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more fully understood from the detailed desription given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein: FIG. 1 shows a cross-sectional view of a part-spherical electrode type charged particle analyzer according to a preferred embodiment of the present invention; FIG. 2 shows a cross-sectional view of another example of an obstacle plate used for the analyzer of the present inention; FIG. 3 shows a cross-sectional view of still another example of the obstacle plate; FIG. 4 shows a graph of a relation between the emission direction and the energy resolution when the obstacle plate as shown in FIG. 2 is applied to the analyzer of the present invention; FIG. 5 shows a graph of a relation between the emission direction and the energy resolution when the obstacle plate as shown in FIG. 3 is applied to the analyzer of the present invention; and FIG. 6 is a graph of the overall transmittance. DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a preferred embodiment of a part-spherical electrode type charged particle analyzer of the present invention. A part-spherical grid 1 and a part-spherical electrode 2 are concentric having a common center of O. In this preferred embodiment, the radius of the part-spherical electrode 2 is twice the radius of the part-spherical grid 1. As far as the radius of the electrode 2 is twice the radius of the grid 1, in theory, the solid angle of detection is 2π steradian, which is the whole emission angle from sample surface. Guard rings 5 are provided between the edges of the grid 1 and the electrode 2. They are arranged in the concentric manner and connected to the resistor 6 as illustrated. One side of the resistor 6 is connected to the grid 1 and is also grounded. The other side of the resistor 6 is connected to the eletrode 2 and is also connected to the power supply 7. The guard rings 5 are provided for preventing the electric field between the grid 1 and the electrode 2 from being disturbed at the edges of the grid 1 and the electrode 2. A screen plate 3 is positioned at the bottom of the grid 1, being made of an electrically conductive material and grounded. With the above-described structure, the energy of charged particles to be detected can be scanned by changing the output voltage of the power supply 7. The screen plate 3 has an entry window W, separated by a distance slightly smaller than the radius of the grid 1 from the center O, for setting a sample S. The screen plate 3 has an exit opening symmetrical with the position of the entry window W with respect to the center O. Apertures h 1 and h 2 are bored in the grid 1 and the electrode 2, respectively, through which exciting beams such as X-rays for exciting the sample S are incident on the sample S. An electron gun 11 is positioned at a space between the grid 1 and the screen plate 3 for emitting electron beams to the sample S. The sample S can be excited with the X-rays or the electron beams. A two-dimensional detector 4 is positioned below the screen plate 3 and faced against the exit opening A. The two-dimensional detector 4 composed of microchannel plates and a fluorescent plate. The light emitted from the fluorescent plate is detected with a two-dimensional photomultiplier Ph, so that the output is processed with a computer Cp to calculate the emission points on the fluorescent plate. By recording the emission points two-dimensionally, the angular distribution of the charged particles emitted from the sample S and having the characteristic energy as defined by the voltage applied between the electrode 2 and the grid 1 is represented. Four-fold part-spherical grids 8, 81, 91, and 9 are provided between the exit opening A and the two-dimensional detector 4. They are concentric with the center of the exit opening A. The potential of the innermost grid 8 is the same as that of the screen plate 3. The grids 81 and 91 are set to a slightly lower voltage than the energy of the charged particles to be detected, so that the charged particle having energy lower than the specific energy to be detected are blocked. The voltage of the outermost grid 9 is set, so that the charged particles being incident upon the two-dimensional detector 4 are accelerated. An obstacle plate 10 is a ring-segment-like plate which is axially symmetric around the axis OA, and positioned in an equatorial plane passing through the spherical center O of the spherical electrode 2 and normal to the axis OA. The obstacle plate 10 is isolated from the electrode 2 and is set to a voltage identical with the normal potential between the grid 1 and the electrode 2 at the internal peripheral edges. Here, the potential adjacent the portion where the obstacle plate 10 penetrates the electrode 2 is disturbed, but the potentials within the electrode 2 as a whole are not disturbed. The orbits of the charged particles are not changed as compared to the case without the obstacle plate 10. The charged particles having the energy higher than the particular energy among the charged particles emitted from the sample S in the normal direction are obstructed by the obstacle plate 10. The orbits of the charged particles finally going into the exit opening A are the same regardless of setting the particular energy, as shown in FIG. 1. Then the inner radius of the obstacle plate 10 can be fixed while the applied voltage should be changed in proportional to the voltage applied to the electrode 2. FIG. 2 shows a more efficient shape of the obstacle plate 10, in which the obstacle plate 10 is shaped a partial sphere having an angle of 2α around the center O of the electrode 2. FIG. 4 shows a graph of the energy resolution according to the emission direction of the charged particles when the obstacle plate of FIG. 2 is applied. The radius a of the grid 1 is set 1.5 times the distance s from the center O to the exit opening A (s/a=2/3) and the radius r and the half angle α of the obstacle plate 10 is set to r=1.7613a, α = 10.92° in the case of FIG. 4. The broken line of FIG. 4 indicates the resolution data without the obstacle plate 10. In this example, the maximum point of the resolution appears around 80° (10° inclination from the normal) and there is a small problem relating to the uniformity of resolution. This maximum point of the resolution is due to the effect at the edges of the obstacle plate 10. To resolve this problem, as shown in FIG. 3, a plurality of ring-segments having slightly different radii r are provided. FIG. 5 shows a graph of using six obstacle rings-segments to make the resolution approximately uniform. In this example, to obtain the resolution of ± 0.5%, s/a=0.79 and (r, α)=(1.6222a,2° ), (1.63026a,6° ), (1.6438 6a,10° ), (1.66255a,14° ), (1.68173a,18° ), and (1.70389a,22° ). Then, the large dip B of FIG. 4 can be eliminated and 6 small dips are present, so that approximately uniform resolution can be given. As described above, in the charged particle analyzer as illustrated in FIG. 1, the envelope plane of the orbits of the charged particles with the specific energy emitted from the emission point of the sample into the exit opening A is not a sphere having the center O of the electrode 2, but a shape of which the center is vertically displaced downward compared to the sphere. Therefore, the orbits of the charged particles having more energy than the specific energy are out of that envelope plane. When attention is directed to the particular charged particle emitted from the sample in the vertical direction and down to the exit opening A in a direction normal to the screen plate 3, the energy resolution of such a charged particle is the lowest as shown in the broken line of FIG. 4. The charged particles having the energy more than the particular energy are obstructed by colliding with the obstacle plate 10, so that they cannot pass through the exit opening A. When the interruption energy of high-pass filter of the part-spherical grids positioned below the exit opening A is set slightly lower than the above particular energy, the charged particles having the energy lower than the particular energy, which cannot be removed by the obstacle plate, can be removed. Hence, the energy resolution of the charged particles in this direction can be improved, so that the energy resolution is uniform in all directions. Although the above description is exemplified about the angular distribution of the charged particles having the particular energy emitted from the sample, the present invention is evidently applied to measure the energy distribution of the charged particles emitted to a particular direction from the sample or measure the energy distribution of all the charged particles emitted. While only certain embodiments of the present invention have been described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the present invention as claimed.

Claims: A charged particle analyser comprising: a part-spherical grid (1), a part-spherical electrode (2) disposed outside of and concentric with said part-spherical grid; an entry window (W) for the charged particles, said entry window being provided in a plate (3) and provided in a plane passing through the spherical center (O) of said part-spherical grid; an exit opening (A) which is provided in said plate and is positioned to be symmetric with the position of said entry window with respect to the spherical center of said part-spherical grid; and a detector (4) positioned on the side of the exit opening (A) opposite to said part-spherical grid, said detector facing said exit opening; characterized by a plurality of second part-spherical grids (8, 81, 91, 9) disposed around the center of said exit opening (A) in a position between said exit opening and said detector (4), said plurality of part-spherical grids forming a high pass filter for said charged particles; and a curved obstacle plate (10) which is provided within said part-spherical electrode (2) and is axially symmetric with respect to an axis (OA) passing through the spherical center (O) of the part-spherical electrode and through said exit opening (A), said obstacle plate being arranged in parallel with an equatorial plane passing through said spherical center (O), wherein said obstacle plate (10) is arranged for preventing the charged particles with an energy higher than a particular energy (E 0 ) from entering into said exit opening (A), and said plurality of second part-spherical grids have an interruption energy slightly lower than said particular energy and serve to to remove the charged particles having lower energy than the particular energy. The charged particle analyser as set forth in claim 1, wherein said obstacle plate is shaped as a section of a sphere which is concentric with said part-spherical electrode (2). The charged particle analyser as set forth in claim 1, wherein said obstacle plate (10) is divided into a plurality of ring segments disposed at different radii (r 1-r 6) about the spherical center (O) of the part-spherical electrode and being offset from said equatorial plane by different angles (α 1 - α 6 ).

Inventors: DAIMON HIROSHI, INO SHOZO, DAIMON, HIROSHI, INO, SHOZO
Applicants: SHIMADZU CORP, SHIMADZU CORPORATION
Application year: 1990
Main classification: H01J49
IPC codes: H01J49