Base plate for charged particle energy analyser

Abstract

 A base plate (4) for mounting one or more electrodes (2,3) in a charged particle energy analyzer (1) comprises a pair of sheets (29,30) overlying each other in spaced apart parallel relation and secured to each other at discrete points across their common area. Preferably the sheets (29,30) are held at a fixed spacing from each other by means of annular spacers (32) and are secured to each other by means of bolts (31) received through said annular spacers (32), which may comprise rivet nuts (32).

Description

[0001] This invention relates to a base plate for mounting one or more electrodes in a charged particle energy analyser of a type where a charged particle is deflected while travelling along a path in a gap between two electrodes across which an electric field is applied. More particularly, the invention relates to a base plate for such analysers having electrodes with a curved or part-curved surface, such as part-spherical, toroidal or cylindrical electrodes.

[0002] This application is a divisional of EP-A-92923029.0.

[0003] Conventionally, electrodes for such analysers have been machined from a solid block of metal. For example part-spherical electrodes machined from solid stainless steel are described in a paper by Simpson in Review of Scientific Instruments Volume 35 Number 12 (December 1964) pages 1698 to 1704 and part-spherical electrodes machined from solid aluminium are illustrated in a paper by Pullen et al in Journal of Chemical Physics Volume 53 Number 2 pages 768 to 782. The machining of electrodes from a solid block has enabled electrodes of a desired shape to be manufactured to high tolerances with little distortion, which has allowed analysers incorporating such electrodes to have a high resolution and transmission efficiency. However, the machining of e.g. hemispherical electrodes from a solid block of metal is relatively expensive and difficult to repeat on a commercial scale, and the electrodes so formed have tended to be heavy and bulky which is disadvantageous under some conditions.

[0004] It has also been suggested for example in a paper by Brunt, Read and King in the Journal of Physics E 1977 Volume 10 pages 134 to 139, to use hemispherical electrodes formed by a spinning technique. Electrodes can be formed more cheaply by a spinning technique but tend to be less accurately shaped which can affect the resolution and/or transmission efficiency of the analyser. This is particularly disadvantageous in high performance analysers. In the arrangement proposed in the paper by Brunt et al the spun electrodes require cleaning by mechanical abrasion and solvent action or acid etching prior to use and the problem of achieving a high transmission efficiency is not addressed.

[0005] The applicant addresses this problem in EP-A-92923029.0 by providing a charged particle energy analyser comprising a pair of electrodes having opposed curved surfaces and arranged so as, on application of an electric field, to deflect a charged particle following a path therebetween, wherein at least one said electrode is formed from a sheet of metal by a pressing technique.

[0006] The applicant also provides a charged particle energy analyser electrode having a curved surface formed by application of a pressing technique to a solid sheet of metal.

[0007] Electrodes can be formed by a pressing technique more cheaply than conventional electrodes machined from a solid block of metal and at least preferably more accurately than by a spinning technique. There is thus provided a charged particle energy analyser and an electrode for such an analyser which can be manufactured inexpensively and accurately so as to allow the possibility of a relatively inexpensive high performance analyser combining both high resolution and high transmission efficiency.

[0008] Any appropriate pressing technique can be applied to a sheet of metal to form a said electrode and a number of possible techniques are discussed in the Sheet Metal Industries Year Book 1984.

[0009] It has previously been known to manufacture part spherical retarding grids by means of a pressing technique from a fine wire mesh made of metal. The grids are placed directly in the path of a stream of charged particles to apply a retarding field as the charged particles pass directly through the mesh. Such grids differ from the present electrodes in that they are more easily formed into a desired shape than an electrode formed from a solid sheet of metal, and furthermore accuracy is less important since the grids merely retard the charged particles rather than setting up a precise field therebetween to deflect them. The grids have a tendency to distort under heating between uses to remove water vapour and in practice a precise shape could not in any case be maintained.

[0010] The applicant also provides a method of forming a charged particle energy analyser electrode having a curved surface, comprising pressing a solid sheet of metal through an aperture in a die by means of a tool having a shape conforming to said curved surface. Preferably a deep drawing technique is used to shape the electrode with a minimum of stretching of the sheet and means are provided for applying a normal force to the unbent peripheral portion of the sheet during the drawing process to suppress wrinkling of the peripheral portion under the resulting compressive stress.

[0011] In order to form the electrode to a high accuracy it is necessary to select a sheet material having suitable properties and to carefully control the pressing operation. It is particularly important to minimize the variation in thickness of an electrode where the face which the charged particles see is not the face which contacts the tool, i.e. is the convex face. Factors which can affect the accuracy of the electrode include the size of the sheet metal blank, the dimensions of the punch tool and die, lubrication, tool pressure and normal pressure on the blank.

[0012] A preferred material for forming the electrode is sheet aluminium and one material which has been found to be particularly suitable is 18 SWG aluminium to British Standard 1470 S1C 'O' condition. 'O' condition materials to this standard are preferred to '00' or 'H' type materials which have a greater tendency to thin unduly during pressing or to split. 18 SWG thickness has been found to be generally preferable.

[0013] A preferred shape of the electrode is hemispherical or part-hemispherical, preferably with an integral annular flange. The integral annular flange can be formed from the unbent peripheral portion remaining after the pressing operation.

[0014] Preferably, the curved surface of the electrode is formed to a tolerance which is at least as high and preferably higher than can be readily achieved in a spinning technique and is preferably approaching that obtainable in conventional machining processes. This may be for example to within about 0.15mm or better. More preferably the tolerance over a substantial portion and preferably the whole of the curved surface may be to within about 0.13mm (0.005 inches).

[0015] In a part-hemispherical electrode there may be little effect on the performance of the electrode if the tolerances in the region of the inlet and outlet are less than in a central region, for example to within a tolerance of 0.13mm (0.005 inches) in a central region and to a lower tolerance in a region up to 15° from a diametral plane. This is advantageous when a pressing technique is used since the regions of the curved surface closest to the flange are more difficult to form accurately.

[0016] Preferably in such an analyser having a pair of part hemispherical electrodes the radial gap between the electrodes is wider than is conventional relative to the mean radius of the electrodes, for example about 50% of the mean radius, to minimise the effect of any geometrical imperfections in the electrodes.

[0017] As mentioned above, these features are particularly well suited to high performance analysers, and can achieve both high resolution and transmission efficiency. The performance of such analysers are often compared based on a standard set of conditions, using Ag3d⁵/₂ electrons emitted from a silver sample irradiated with unmonochromated Mg K_α characteristic x-rays from a source having an anode power of 300 W. Under these conditions an analyser in accordance with the above features with a typical mean radius of the electrodes of about 50mm can achieve a transmission of more than 100,000 counts per second at a resolution of better than 1 eV.

[0018] The applicant provides a charged particle energy analyser having electrodes formed by pressing, which under the standard conditions set out above has a transmission of more than 100,000 counts per second at a resolution of better than 1 eV. Such a performance is surprisingly high. Preliminary tests have indicated that a transmission of at least 130k counts per second at a resolution of 0.85 eV is possible.

[0019] In preferred analysers which incorporate an inner and an outer hemispherical or part hemispherical electrode, the electrodes are preferably arranged so that their flanges lie in the same plane, and more preferably in the diametral plane or a plane lying parallel to this plane. In a particularly preferred arrangement an inner part hemispherical electrode has an angular extent of substantially 8° to 172° and an outer part hemispherical electrode has an angular extent of substantially 5° to 175°.

[0020] The electrodes are preferably provided with a plurality of apertures, for example four or six apertures, spaced around the annular flange for mounting to a base plate. Conveniently the integral annular flange formed in the pressing operation is sufficiently thin to allow the apertures to be formed by punching. The electrodes may be then mounted to the base plate by means of bolts and isolated electrically therefrom by means of insulating bushes and washers.

[0021] Viewed from one aspect the present invention provides a base plate for mounting one or more electrodes in a charged particle energy analyser, comprising a pair of sheets overlying each other in spaced apart parallel relation and secured to each other at discrete points across their common area.

[0022] A particularly convenient and stable means of accurately mounting electrodes, e.g. in a part hemispherical analyser, is to mount them to a base plate comprising two spaced apart parallel sheets secured to each other such as by means of rivet nuts and bolts at a plurality of discrete points across their overlying areas. This provides a particularly lightweight mounting means which maintains the rigidity of thicker, solid slabs of metal as used hitherto.

[0023] Any suitable securing means may be used, but preferably the parallel sheets, e.g. of metal, which form the base plate are secured together by means of nuts and bolts and are held at a fixed distance from each other by means of spacers. The spacers may be of annular construction adapted to receive the bolts through a central aperture therein. Preferably the plates are secured together by rivet nuts and bolts, the rivet nuts being swaged into one plate and each receiving a bolt which is passed through an aperture in the other plate. Each rivet nut protrudes from the first plate so as to space the second plate therefrom and is preferably provided with a 'knife-edge' for biting into the second plate.

[0024] The aforementioned structure of the base plate enables the parallel sheets to be relatively thin whilst the structure maintains the necessary rigidity to accurately locate the electrodes. Advantageously, therefore, apertures for mounting the electrodes can be punched through each sheet accurately in a simple operation, rather than requiring a more time-consuming and complex jig boring operation as would be necessary with a conventional, solid base plate.

[0025] There is thus provided a means of mounting electrodes in the form of a relatively inexpensive base plate having a structure which is both rigid and relatively lightweight in comparison to known base plates. Such a base plate is applicable to electrodes formed by any technique including a pressing technique, a spinning technique and a conventional solid block machining technique.

[0026] An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein:-

Fig. 1 is a sectional view of an analyser according to one embodiment of the invention, with a portion of the outer casing omitted;

Fig. 2 is a side view of the analyser of Fig. 1 with a portion of the outer casing broken away;

Fig. 3 is a view of the analyser of Fig. 1 from one end showing input and output arrangements;

Fig. 4 is a view of the analyser of Fig. 1 from the other end with one section of the outer casing omitted and the outer electrode partly broken away;

Fig. 5 is a sectional view of the inner electrode taken through the line A-A of Fig. 6;

Fig. 6 is an end view of the inner electrode of the illustrated embodiment;

Fig. 7 is a sectional view of the outer electrode taken through the line B-B of Fig. 8; and

Fig. 8 is an end view of the outer electrode of the illustrated embodiment.

[0027] An embodiment of a charged particle energy analyser 1 in accordance with the invention is shown in Figs. 1 to 4. The analyser 1 is suitable for analysing the energies of charged particles such as electrons or ions in surface analysis techniques. It includes an inner part-hemispherical electrode 2 and an outer part-hemispherical electrode 3 defining therebetween a gap along which charged particles travel in use and are deflected by an electric field applied between the electrodes.

[0028] The part-hemispherical electrodes 2,3 are mounted to a base plate 4 as will be described in more detail below. The electrodes 2,3 and the base plate 4 are housed within a magnetically screening mumetal outer casing 5 comprising two sections joined by welding along their engaging peripheral flanges. Only one section of the outer casing 5 is shown in Figs. 1 and 4 and the whole casing is best seen in Fig. 2. A magnetically screening shield 6 is located around the join in the outer casing to ensure adequate screening, and a vacuum is maintained within the casing.

[0029] A lens arrangement 7 housed in a high permeability casing is mounted at the inlet to the analyser 1. The lens arrangement 7 acts to focus charged particles from a source (not shown) into a beam which then enters the analyser through an entrance slit (not shown), and also optionally acts to accelerate or retard the charged particles. The entrance slit to the analyser is surrounded by a screen 8 having a Herzog slit 9 which reduces fringe effects (and which hides the entrance slit from view in the drawings).

[0030] At the output of the analyser, an exit slit (not shown) surrounded by a shield 10 having a Herzog slit 11 selectively permits the charged particles to be detected by one or more electron multipliers 12, also housed within the mumetal outer casing 5 and mounted on a flange for easy removal. Figs 2 and 3 show high voltage feeds 13 into the multiplier 12, which can be clipped into or out of connection therewith. Fig. 3 also shows a removable glass or ceramic service plug 14 which includes electrical inputs to the electrodes.

[0031] The part-hemispherical electrodes 2,3 can be seen in more detail in Figs. 5 to 8. The inner electrode 2 shown in Figs. 5 and 6 comprises a part-hemispherical segment of external radius R_IE 76.2mm and the outer electrode 3 shown in Figs. 7 and 8 comprises a part hemispherical segment of internal radius R_OE 127mm. The electrodes 2,3 are arranged concentrically such that there is a constant gap of diameter 50.8mm therebetween. The width of the gap is substantially 50% of the mean value of the external radius of the inner hemisphere and the internal radius of the outer hemisphere, which is wider than in conventional analysers and minimises the effects of geometrical imperfections. In this embodiment, the spherically curved section of the outer casing has a radius of substantially 160mm.

[0032] Neither of the electrodes 2,3 forms a complete hemisphere. The spherically curved portion of the inner electrode 2 extends through an angle of approximately 8° to 172° and the spherically curved portion of the outer electrode 3 extends through an angle of approximately 5° to 175°. In addition to the spherically curved portions of the electrodes 2,3, each has an integrally formed peripheral flange portion 20,21 joined to the spherically curved portion via a short radiused section of maximum radius 5mm.

[0033] In the analyser the part hemispherical electrodes 2,3 are arranged with their peripheral annular flanges 20,21 lying in the same plane, which plane is parallel to and spaced from the diametral plane 22 by 6.5mm.

[0034] The part-hemispherical electrodes 2,3 are formed from 18 SWG sheet aluminium to British Standard BS 1470 S1C 'O' condition by deep drawing. In one of several known pressing processes, the relatively thin sheet of aluminium is placed on a flat surface of a die having a circular aperture therein. The hemispheres are each pressed through the circular aperture by a male hemispherically shaped tool which has a radius slightly less than that of the circular aperture. A blank holder is located above the flat surface of the die and presses the unbent peripheral portion of the sheet therebelow to prevent wrinkling as material is drawn into the aperture as compressive stresses arise in the peripheral portion.

[0035] The part-hemispherical electrodes 2,3 so formed are manufactured to a tolerance of 0.13mm (0.005 inches) across the spherically curved area, as compared with the shape of a perfect sphere.

[0036] Although in this embodiment substantially the entire surface area is manufactured to within the same tolerance, in practice, the accuracy of the region of the electrodes adjacent the entrance and exit slits which is largely shielded by shields 8,10 is not so critical to performance. In other embodiments regions (marked on the drawings) extending through an angle of substantially 8° to 15° on the inner electrode 2 and substantially 5° to 15° on the outer electrode 3, can be manufactured to a lower tolerance. This is advantageous in that these are the regions which are most difficult to manufacture to accurate tolerances in a pressing technique.

[0037] As mentioned above, both electrodes 2,3 are formed by pressing with a male, spherically curved tool and it is therefore very important with the inner electrode in particular which presents a convex face to the charged particles, that variations in the thickness of the spherically curved portions are minimized. This is achieved by a precise choice of the radial dimensions of the die and pressing tool, adequate lubrication, tool pressure, blank holder pressure and dimensions and properties of the pressed metal sheet.

[0038] The annular flange 20 of the inner electrode 2 is provided with four apertures 23 punched therethrough, spaced around the flange, for mounting to the base plate 4. Similarly, the annular flange 21 of the outer electrode 3 is provided with six apertures 24 punched therearound. The flange 20 of the inner electrode is based on an annulus of outer radius substantially 98mm and is shaped with peripheral recesses and two opposing flat sides to facilitate mounting without interfering e.g. with the entrance and exit arrangements, as best shown in Fig. 4. The flange 21 of the outer electrode is an annulus of outer radius substantially 150mm.

[0039] The electrodes 2,3 are mounted to the base plate 4 via ceramic insulating bushes 25 and washers 26 by bolts 27,28. The base plate 4 is formed of two relatively thin metal plates 29,30 which are joined at a plurality of points by means of bolts 31 extending through rivet nuts 32 which are swaged into and protrude from one of the plates 29 and receive the bolts 31 so as to hold the sheets 29,30 at a fixed separation. The recesses in the peripheral flange 20 of the inner electrode are provided to avoid interference of the flange 20 with the bolts 31. The base plate 4 is mounted to the outer casing 5 by means of an electrically isolated bolt 33.

[0040] The structure of the base plate 4 with two relatively thin, lightweight sheets joined in such a way as to provide sufficient rigidity to accurately locate the electrodes is particularly inexpensive and convenient as compared to conventional solid base plates.

[0041] Furthermore, the aforementioned structure of the base plate facilitates mounting of the electrodes since apertures can easily and accurately be punched in the two thin sheets 29,30 conforming to the mounting apertures 23,24 in the peripheral flanges 20,21 of the electrodes 2,3. The punching can be computer numerically controlled so that it is performed to a very high precision, is repeatable and suitable for mass production at low cost.

[0042] In tests on Ag3d⁵/₂ electrons from a silver sample irradiated with unmonochromated Mg K_α characteristic x-rays with 300 W anode power, a transmission of 130,000 counts per second has been achieved at a resolution of 0.85 eV and a transmission of 1,300,000 counts per second has been achieved at a resolution of 1.50 eV, with a substantially linear interpolation therebetween, within a 10% band around the count rate.

[0043] Thus, the analyser illustrated in this embodiment can be manufactured inexpensively and accurately to high tolerances so as to allow high performance in terms of both energy resolution and transmission efficiency.

Claims

1. A base plate for mounting one or more electrodes in a charged particle energy analyser, comprising a pair of sheets (29,30) overlying each other in spaced apart parallel relation and secured to each other at discrete points across their common area.

2. A base plate as claimed in claim 1, wherein said sheets (29,30) are held at a fixed spacing from each other by means of annular spacers (32) and are secured to each other by means of bolts (31) received through said annular spacers (32).

3. A base plate as claimed in claim 2, wherein each said bolt (31) is received by a rivet nut (32) swaged into a first of said plates (29).

4. A base plate as claimed in claim 3, wherein each said rivet nut (32) protrudes from said first plate (29) so as to space the second plate (30) therefrom.

5. A base plate as claimed in claim 4, wherein each rivet nut (32) is provided with a knife edge for biting into the second plate (30).

6. A base plate as claimed in any preceding claim, wherein apertures in said parallel sheets (29,30) for mounting one or more electrodes can be formed by punching.

7. A charged particle energy analyzer comprising one or more electrodes (2,3) mounted to a base plate (4), characterised in that said base plate (4) comprises a pair of sheets (29,30) overlying each other in spaced apart parallel relation and secured to each other at discrete points across their common area.

8. A charged particle energy analyzer as claimed in claim 7, wherein said base plate (4) further includes the features of any of claims 1 to 6.

Drawing