Electrostatic arrangement for influencing a particle beam

Abstract: The invention relates to an electrostatic arrangement (1) for influencing a particle beam, consisting of at least one first and one second electrode (2,3) which are disposed behind one another in the direction of the particle beam and can each be charged with a potential, wherein the two electrodes are in electrical contact with a high-resistance body (4) which has a channel (5) for the particle beam. The invention also relates to an optical unit consisting of such an electrostatic rearrangement as well as a further component for influencing the particle beam. Finally the electrostatic arrangement for use in a particle beam device is described.

Description: The invention relates to an optical unit for influencing a particle beam. The most varied electrostatic or combined electrostatic-magnetic arrangements are known in the art for influencing, particularly for focusing a particle beam. In the case of conventional electrostatic lenses arrangements are used with two or more electrodes which are charged with different potentials in order to cause in the space between the electrodes, a field distribution which acts as a lens for the particle beam. In this case the electrodes can be varied in shape and size. There are apertures, ring elements, electrodes of cylindrical or conical shape. The lens properties such as focus and aberration properties are fixed by the shape and size of the electrodes and the supplied potentials. However, with a limited number of electrodes it is very difficult to generate a complex field distribution. In the past, therefore, in order to solve this problem multi-electrode configurations have been used, wherein a further parameter is created by each electrode in order to influence the field distribution. The additional electrodes not only necessitate a substantially more difficult mechanical construction but also require additional voltage sources and are therefore very costly. US-A-4 126 781 relates to generating shaped electric fields for use as electrostatic lensesby employing surface currents on resistive materials to shape electric fields. In particular, this known electrostatic arrangement is applied to improved types of energy analyzers with specific application in the field of Secondary Ion Mass Spectrometry. US-A-2 313 018 discloses an electrostatic device for influencing a particle beam, which has two electrodes in respective contact with both ends of an insulating body coated with a high resistivity layer. The object of the invention is to improve the optical unit with an electrostatic arrangement for influencing a particle beam in such a way that a facility for influencing or detecting the electrical field distribution is created which is improved over the prior art and which is distinguished by particularly simple and compact production. According to the invention the object is achieved by the features of claims 1, 6 and 7. In the electrostatic arrangement the two electrodes are in electrical contact with a high-retsistance body, the body having a channel for the particle beam. In such an electrostatic arrangement the construction of the electrical field depends in particular upon the shape of the high-resistance body. In this case the body acts as a multiple voltage divider which replaces the plurality of electrodes. The electrostatic arrangement according to the invention is distinguished by particularly simple and compact production, with which almost any field distribution and thus almost any lens behaviour can be produced, a minimum of electrodes and components as well as voltage supplies being necessary. According to the invention the electrostatic arrangement is advantageously combined with further components for influencing the particle beam in the region of the electrostatic arrangement to form an optical unit. Thus the lens arrangement or optical unit is also of special Interest In particular in particle beam devices. Further variants of the invention are the subject matter of the subordinate claims and are explained below with the aid of the description of some embodiments and the drawings, in which: Figure 1 shows a three-dimensional representation of an electrostatic arrangement according to a first embodiment, Figure 2 shows a sectional representation of the embodiment according to Figure 1, Figures 3 to 12 show sectional representations of electrostatic arrangements according to further embodiments, Figure 13 shows a three-dimensional representation of an optical unit, consisting of an electrostatic arrangement as well as a deflector according to a first embodiment, Figure 14 shows a three-dimensional representation of an optical unit consisting of an electrostatic arrangement with a deflector according to the second embodiment, Figure 15 shows a three-dimensional representation of an optical unit consisting of an electrostatic arrangement with stigmator, Figure 16 shows a three-dimensional representation of an optical unit consisting of an electrostatic arrangement with a deflector according to a third embodiment, Figure 17 shows a representation of an optical unit consisting of an electrostatic arrangement as well as a superimposed magnetic lens, Figure 18 shows a sectional representation of an optical unit consisting of an electrostatic arrangement as well as a detector according to a first embodiment, Figure 19 shows a plan view of the optical unit according to Figure 18, Figure 20 shows a sectional representation of an optical unit consisting of an electrostatic arrangement as well as a detector according to a second embodiment. Fig. 21 shows a plan view of the optical unit according to Fig. 20 and Fig. 22 shows a schematic representation of a particle beam device with an optical unit according to the invention. According to the invention the optical unit is a combination of an electrostatic arrangement with a further component for influencing the particle beam in the region of the electrostatic arrangement. In Fig. 1 to 12 several embodiments of electrostatic arrangements are disclosed. Fig. 13 to 21 relate to the optical unit and Fig. 22 shows a particle beam device with such an optical unit. Figures 1 and 2 show an electrostatic arrangement for influencing a particle beam, consisting of a first electrode 2 and a second electrode 3 which are disposed one behind the other in the direction of the particle beam and can each be charged with a potential. The two electrodes 2, 3 are in electrical contact with a high-resistance body 4 which has a channel 5 for the particle beam. In the illustrated embodiment this channel also extends through the two electrodes 2 and 3. The material of the high-resistance body 4 has a specific resistance between 10 6 and 10 11 Ω cm, preferably between 10 7 and 10 9 Ω cm. A suitable material is for example Murflor Bronze (PTFE-CuSn). If the specific resistance were chosen too high there would be a danger that the body 4 would be charged with particles which strike the surface, i.e. the wall, of the channel 5. On the other hand, a resistance which is too low would give rise to very large currents and thus a very high power consumption. The two electrodes 2, 3 are attached, for example as planar electrodes, each on one end face 4a, 4b of the high-resistance body 4. The mounting can be achieved for example by pressing or vapour depositing. In addition to the two end faces 4a, 4b, the high-resistance body 4 is delimited by an inner wall 4c and an outer wall 4d. The inner wall 4c simultaneously forms the wall of the channel 5. Each of the two electrodes 2, 3 is charged with a potential V 2 , V 3 respectively, as shown schematically in Figure 2. The application of a voltages causes a current flowing through through the body 4, which in turn gives rise to a distribution of potential on the surface of the body 4. The electrostatic field resulting therefrom in the channel 5 is determined not only by the applied voltages V 2 , V 3 and the shape of the two electrodes 2, 3, but also by the shape and wall thickness of the high-resistance body 4. The high-resistance body can be made from electrically isotropic material in which the specific resistance has the same value in each direction. When anisotropic material is used for the high-resistance body use can be made of the fact that for example the specific resistance in the Z direction is different from the specific resistance in the X and Y direction. The use of anisotropic material can give rise to certain effects and properties of the electrostatic arrangement. Thus for example a high specific resistance in the Z direction causes a low current and thus a low power consumption within the high-resistance body. A low resistance in the X and Y direction causes a more uniform distribution of potential in this plane. However, with anisotropic material special asymmetries could also be achieved which produce new and as yet unknown properties. The high-resistance body 4 advantageously has a cladding 14 of electrically insulating material, as is shown schematically in Figure 2. In certain applications it is also advantageous to provide an electrical shielding. The high-resistance body shown in Figures 1 and 2 is constructed as a rotating body with a cross-section in the shape of a truncated cone, but within the scope of the invention other shapes are also conceivable in order to generate a desired field distribution. In the following description of Figures 3 to 5 various embodiments of the high-resistance body and the effects thereof on the construction of the electrical field are explained in greater detail. In Figure 3 an electrostatic arrangement 10 is shown which is delimited in the direction of the optical axis 8 by an upper and a lower end face 40a, 40b as well as an inner and an outer wall 40c, 40d. The inner wall 40c is substantially cylindrical, whilst the outer wall 40d bulges between the two end faces. In this embodiment the wall thickness of the high-resistance body 40 tapers continuously from the centre to the two end faces 40a, 40b. A first electrode 20 is in electrical contact with the end face 40a and a second electrode e30 is in electrical contact with the end face 40b. If the two electrodes 20, 30 are charged with different potentials the electrical field shown in Figure 3 by field lines 70 is produced in the region of the channel 50. The effect of this field on a particle beam 6 is represented in relation to three part-beams 6a, 6b, 6c. Although the electrostatic arrangement 10 contains, in addition to the high-resistance body 40, only two electrodes 20, 30 which can be charged with different potentials, a field distribution is produced which corresponds to the effect of two lenses each disposed in the region of the two end faces 40a, 40b. If it were wished to achieve such a field distribution in the conventional manner, a plurality of electrodes which can be charged with different potentials would have to be provided for each of these two "lenses". Figure 4 shows a further embodiment of an electrostatic arrangement 11, consisting of a high-resistance body 41, a first electrode 21 and a second electrode 22. The inner wall 41c delimits a channel 51 which tapers conically in the direction of the particle beam. Externally the high-resistance body 41 is again presented as a truncated cone, the external diameter increasing In the direction of the particle beam. Based on this, the wall thickness of the high-resistance body 41 also increases from its upper end face 41a to its lower end face 41b. Charging of the two electrodes 21, 22 with a different potential produces an electrical field which is represented schematically by field lines 71. The high-resistance body 42 illustrated In Figure 5 has an inner well 42c which has a convexity 42e approximately in the middle which forms a kind of "throttle point" of the channel 52. The outer well 42d is correspondingly constricted in this region. A further embodiment is illustrated in Figure 6, in which the high-realetence body 43 has a substantially cylindrical inner wall 43c and a substantially cylindrical outer wall 43d which has a convexity 43f approximately in the middle. Ultimately there are no limits to the shape of the high-resistance body. Thus, relatively complex shapes could certainly be produced for the formation of a certain electrical field, as is shown by the high-resistance body 44 in Figure 7. Whereas in the previous embodiments the high-resistance body is constructed as a rotating body, in Figure 8 a high-resistance body 49 is shown which does indeed have a cylindrical outer wall 49d but in which the inner wall forms a channel which is rectangular in cross-section. Naturally, constructions which are not rotationally symmetrical are also conceivable for the outer wall. Apart from an alteration in the shape of the high-resistance body, the electrical field can also be influenced by an alteration in the material from which it is made. Thus the high-resistance body 45 shown in Figure 9 consists of three part-bodies 45g, 45h and 45i which are adjacent to one another in the direction of the particle beam, at least those part-bodies which are adjacent to one another being made from different high-resistance materials. The use of material with different specific resistance suggests itself particularly when it is not already possible to achieve a desired distribution of potential by an alteration in the geometry of the high-resistance body. If for example a rapid change in the wall thickness of a multiple order of magnitude is necessary, the use of a material with another specific resistance might be more advantageous. However, it is also conceivable to provide for a change of material not only in the Z direction, i.e. in the direction of the particle beam, but also in the radial direction. Naturally a combination in both directions is also possible. A further possibility for shaping the electrical field which is formed is to provide additional electrodes. In Figure 10, therefore, an additional electrode 96 is applied as a metal ring on the outer wall 46d of the high-resistance body 46. By means of the additional electrode a stablilsatlon of the voltage distribution can be achieved with the generation of equipotentlel lines about the high-resistance body. A potential may also be applied to this electrode. However, such a ring electrode could also be provided on the inner wall 46c. In Figure 11 the additional electrode 97 to constructed as a planar electrode which extends from inner wall 47c of the high-resistance body to the outer wall 47d and is disposed between the two outer electrodes 27 and 37. This additional electrode 97 is likewise supplied with a potential, so that the electrostatic arrangement acts for example in the upper part as an accelerating lens and in the lower part as a retarding lens. Within the scope of the invention it is not essential for both electrodes to be provided on the end face of the high-reststance body. Thus at least one of the two electrodes can also be provided for example as a ring electrode on the outer wall. In the variant illustrated in Figure 12 one electrode 28 is provided on the end face of the high-resictance body 48 and the other electrode 38 is provided in the interior thereof. In this way a constant distribution of potential is achieved below the electrode 38 on the surface of the high-resistance body 48. The electrostatic arrangement as described by way of example in Figures 1 to 12 is combined with a further component for influencing a particle beam, thus forming an optical unit. Some embodiments of such optical units illustrated in Figures 13 to 21 are described below, where the configuration of the electrostatic arrangement 1 should be understood as merely by way of example and in particular can be replaced by an electrostatic arrangement according to one of Figures 1 to 12. Figure 13 shows a first optical unit 200, In which the electrostatic arrangement is combined with two deflecting electrodes 210, 211 which are for example disposed opposite one another on the outer surface of the high-resistance body 4. Each of the two deflecting electrodes 210, 211 is connected to a variable voltage source, a deflection of the particle beam being achieved by application of a suitable voltage. However, the combination of an electrostatic arrangement with a deflector can also be achieved in another way, as is shown by the optical unit 201 in Figure 14. Here electrodes 2 and 3 are again disposed on the end faces of the high-resistance body. Furthermore, two electrodes 212, 214 are provided on one end face and lie opposite two electrodes 213, 215 on the other end face. In the region of these further electrodes 212 to 215 the ring-shaped electrodes 2, 3 are correspondingly interrupted and isolated. In each case one electrode on one end face and the opposing electrode on the other end face are each connected to a voltage source so that a current I x1 flows through the high-resistance body 4 between the electrodes 212 and 213 and a current I x2 flows through the high-resistance body 4. The two currents I x1 and I x2 produce a voltage drop along the z axis which coincides with the optical axis 8. The potential drop produced in this way effects a deflection field over the entire length of the body. In the optical unit 202 shown in Figure 15 the electrostatic arrangement 1 is provided with a plurality of electrodes 216 which can be supplied with voltage and which are likewise disposed along an equipotential line on the surface of the high-resistance body 4. These electrodes can act as a stigmator by means of corresponding electrical circuitry. Instead of an electrostatic deflecting system the electrostatic arrangement 1 can also be combined with a magnetic deflecting system to form an optical unit 203, as is indicated schematically in Figure 16. If the electrostatic arrangement 1 is constructed as an electrostatic lens arrangement It can be assembled particularly advantageously with a magnetic lens 217 to form an electrostatic-magnetic objective lens 204, as shown in Figure 17. In Figures 18 and 19 an optical unit 205 is shown which comprises an electrostatic arrangement with a detector 103. This detector 103 is fixed on the Inner wall 4c of the high-resistance body 4 approximately in the middle of the channel 5 and is of ring-shaped construction. In a particle beam device such a detector 103 serves for example serves for detection of the secondary particles triggered on a specimen which strike this detector and there give rise to a current i Det . In Figures 20, 21 a second embodiment of an optical unit 206 is shown in which the detector 103' is divided into four segments 103'a, 103'b, 103'c and 103'd. On the segments the secondary particles give rise to currents i Det1 , i Det2 , i Det3 , etc., which can be separately processed and evaluated. Whereas in the embodiments illustrated in Figures 18 to 21 the detector protrudes into the channel 5 from the inner wall 4c of the high-resistance body, there are naturally also conceivable solutions in which the detector is inserted in a corresponding recess in the well 4c. The compact construction of the electrostatic arrangement 1 and the simple possibility of combining it with further componento for influencing the particle beam is particularly advantageously suited to use in particle beam devices where there is often only relatively little space available for installation. Figure 22 shows a particle beam device 100 with essentially a source 101 for generating a particle beam as well as means for Influencing the particle beam between the source 101 and a specimen 102 to be examined. These means include in particular an electrostatic arrangement, for example according to one of the previously described embodiments. A detector 103 is also provided in order to receive the particles triggered on the specimen 102. In this case the detector can be disposed according to choice before, behind or even in the electrostatic arrangement 1. In the illustrated embodiment the detector is placed before the electrostatic arrangement 1 in the direction of the particle beam. In addition to further apertures and diaphragms a blanking unit 104 is also shown. A deflecting system 105 serves to deflect the particle beam on the specimen 102. The electrostatic arrangement according to the invention is particularly well suited for use of the particle beam device in the low-voltage range because of its low chromatic and spherical aberration coefficients. Since the high-resistance body 4 is made from non-magnetic material, the electrostatic arrangement is particularly suitable for combination with superimposed magnetic fields. Moreover, due to the compact construction of the electrostatic arrangement this can be disposed very close on or in the gap of a magnetic lens, as is shown in Figures 17 and 22. Even better lens properties can be achieved by such an arrangement. Naturally the electrostatic arrangement can also be combined with a magnetic lens or another arrangement which generates a magnetic field without an iron circuit. In the case of electron beam applications and here in particular in the case of low-voltage applications the source 101 is preferably constructed as a field emission cathode or photocathode or as a thermal cathode. Due to its construction the electrostatic arrangement can be used particularly advantageously as a link between vacuum and ambient air. In this way the vacuum arrangement can be simplified considerably. In the electrostatic arrangement all supplies of voltage and potential can take place in a simple manner from outside, whilst the channel functions as a beam tube.

Claims: Optical unit (200 - 204) with an electrostatic arrangement (1) for influencing a particle beam (6), consisting of at least one first and one second electrode (2, 3) which are disposed behind one another in the direction of the particle beam and can each be charged with a potential and which are in electrical contact with a high-resistance body (4) which has a channel (5) for the particle beam (6) and a further component (210 - 217) for influencing the particle beam in the region of the electrostatic arrangement, characterised in that the further component (212 - 216) is in contact with the high-resistance body. Optical unit (201) as claimed in claim 1, characterised in that the further component is formed by a plurality of electrodes (216) which are disposed at the same height on an outer wall of the high-resistance body. Optical unit as claimed in claim 1, characterised in that the further component is formed by additional electrodes (212 - 215) on the end faces of the high-resistance body (4). Optical unit as claimed in claim 1, characterised in that the component is formed by a deflector (210-215) for the particle beam. Optical unit as claimed in claim 1, characterised in that the component is formed by a multipole arrangement Optical unit (200 - 204) with an electrostatic arrangement (1) for influencing a particle beam (6), consisting of at least one first and one second electrode (2, 3) which are disposed behind one another in the direction of the particle beam and can each be charged with a potential and which are in electrical contact with a high-resistance body (4) which has a channel (5) for the particle beam (6). and a further component in the region of the electrostatic arrangement, characterised in that the further component is formed by a detector (103, 103') which is in contact with the high-resistance body. Optical unit (200 - 204) with an electrostatic arrangement (1) for influencing a particle beam (6), consisting of at least one first and one second electrode (2, 3) which are disposed behind one another in the direction of the particle beam and can each be charged with a potential and which are in electrical contact with a high-resistance body (4) which has a channel (5) for the particle beam (6), and a further component (210 - 217) for influencing the particle beam in the region of the electrostatic arrangement, characterised in that the component is formed by a magnetic lens (17) which generates a magnetic field which is superimposed on the electrostatic arrangement.

Inventors: FROSIEN JUERGEN DR, LANIO STEFAN DR, SCHMITT REINHOLD, SCHOENECKER GERALD DR, FROSIEN, JUERGEN, DR., LANIO, STEFAN, DR., SCHMITT, REINHOLD, SCHOENECKER, GERALD, DR., Frosien, Jürgen, Dr., Schönecker, Gerald, Dr.
Applicants: ADVANTEST CORP, ADVANTEST CORPORATION
Application year: 1996
Main classification: H01J37
IPC codes: H01J37, G21K1