EP0283773B1 - Miniaturized secondary electron multiplier and its manufacturing procedure - Google Patents

Abstract

The invention relates to a secondary electron multiplier with discrete dynodes. The invention is based on the object, in comparison with the indicated prior art, of producing a miniaturised secondary electron multiplier, and arrays thereof, which require very little space, have good time resolution, good sensitivity and good flexibility for moulding. The object is achieved in that the dynodes are micro-structured and are fitted on an insulating substrate plate which is provided with electrical conductor tracks for the connection of the dynodes.

Description

The invention relates to a secondary electron multiplier according to the preamble of claim 1 and a method for producing such a secondary electron multiplier.

Such a secondary electron multiplier is known from the company publication SC-5 by Hamamatsu (catalog 1983) under the type designation R 1635. With eight steps, it has a diameter of 10 mm and a length of approx. 55 mm. These dimensions do not allow use in miniaturized measuring systems.

Micro-channel plates (Nuclear Instruments and Methods 162, 587-601 (1979)) are also known. Although they meet the requirement of small space requirements, they have a considerable dead time after a signal pulse, which limits their applicability to very weak radiation and particle signals.

Layered channel plates are also known (Advances in Electronics and Electron Physics 33A, 117-123 (1972)). Although they avoid the disadvantage of a long dead time, they have considerable electron losses from stage to stage, which in turn makes them unsuitable for applications with extremely small radiation or particle signals. Layered duct plates are also known (DE 24 14 658), in which such losses occur by forming the duct walls by means of etching should be reduced in size, but this type of design has narrow limits. Finally, arrays of secondary electron multipliers are known from high energy physics (F. Binon et al, Nuclear Instruments and Methods, A248 (1986), 86-102). Due to their large space requirement, they are completely unsuitable for the construction of miniaturized measuring systems.

The object of the invention is to create a micro-secondary electron multiplier and arrays thereof compared to the prior art shown, which have an extremely small space requirement, a high time resolution, a high sensitivity and a high flexibility in the shaping.

This object is achieved by means of the feature specified in the characterizing part of claim 1 and the method according to claim 13.

The remaining claims 2 to 12 and claims 14 to 16 indicate advantageous developments and embodiments of the subject matter or method according to the invention.

The micro-secondary electron multiplier and multiple arrangements (arrays) thereof as sensors in miniaturized measuring systems for radiation or particles are advantageously characterized by a small space requirement and high spatial and time resolution.

The use of deep X-ray lithography and micro-electroplating enables the construction of an extremely small system of discrete dynodes, the shape of which is selected in such a way that the electrons are focused from one dynode to the next, thus minimizing electron losses. The sensitivity is advantageously influenced. The voltage supply of the dynodes via discrete conductor tracks allows the external supply to be adapted to the signal amplitude, so that the dynamic range of the micro-secondary electron multiplier becomes very large. The greatly reduced length of the secondary electron multiplier shortens the electron transit time from cathode to anode, which has a favorable effect on the rise time of pulses and thus on the achievable time resolution.

The production of such fine structures by X-ray lithography-galvanoplastic (LIGA technology) or by means of the impression technique derived therefrom according to feature b) of claim 13 is described and illustrated, inter alia, in the KfK report 3995 of the Karlsruhe Nuclear Research Center (November 1985). Then, for example, an X-ray sensitive positive resist is applied to a metallic base plate and partially irradiated and developed with X-rays through a mask so that a negative shape of the webs to be produced is formed, the height of which corresponds to the layer thickness of the positive resist; it can be up to 2 mm, depending on the penetration depth of the X-rays. The negative mold is then galvanically filled with a metal using the base plate as an electrode, and the rest Resist material is removed with a solvent. In the impression technique, a positive of the bar structure to be produced using the LIGA technique is molded as a reusable tool with a plastic, whereupon the resulting negative shape is filled up by galvanic deposition of metal and the remaining plastic is removed. In both cases extremely precise and fine structures can be produced with lateral dimensions in the µm range with a freely selectable height of up to approx. 2mm. At slightly lower heights, minimal lateral dimensions in the submicron range can also be achieved. X-ray radiation from an electron synchrotron or storage ring (synchrotron radiation) is particularly suitable as a radiation source for this purpose.

The method described in claim 13 also makes it possible to build a large number of micro-secondary electron multipliers next to one another on the same base plate as a micro-secondary electron multiplier array. As a result, an extremely high packing density is achieved, which has a favorable effect on the spatial resolution that can be achieved, an aspect that is particularly important for tomography and for detectors in high-energy physics.

In the case of an array of micro secondary electron multipliers, the position of the signal inputs can be adapted to predetermined contours, for example to the Rowland circle, to an arched image area or to a cylinder jacket as in the scattered light radiometer described below as an exemplary embodiment.

Another advantage is that one of the substrate plates is provided with a translucent wall, which also carries photocathodes, so that the micro-secondary electron multiplier (array) can be made into a micro-photomultiplier (array).

If you give the translucent wall a lenticular cross-section and attach the photocathodes on a separate support made of translucent material, you can create an optical image between the light source and photocathode, which has an advantageous effect on the definition of the scattering volume and on the signal-to-noise ratio .

The invention is illustrated by the figures.

The structure of a micro-secondary electron multiplier is shown schematically in Figure 1. The dynodes 1, the conductor tracks 2 attached to their voltage supply and the anode 3 can be seen. These structures are applied to the base plate 4. A second plate, shown in dashed lines, carries a glass wall 6 on which the photocathode 7 is applied at a suitable point. Additional electrodes 8, 9 serve to focus the photoelectrons triggered on the photocathode onto the first dynode 1. The plates are connected to one another by glass soldering and, if necessary, form a vacuum-tight housing for the secondary electron multiplier. The multiplication requires electron energies of the order of magnitude 100 eV. With a typical safe operating value for the surface field strength of 1 kV / mm, there is a minimum interconnect spacing of 0.1 mm and for 9 dynodes with an edge length of 1 mm, a total length of approx. 10 mm. Surface charging and the resulting electrical flashovers are avoided by the, albeit weak, conductivity of the surface layer of the walls.

FIG. 2a schematically shows a multiple arrangement of micro-secondary electron multipliers. Here numerous micro-secondary electron multipliers are arranged side by side and the routing of the conductor tracks 2 has been adapted accordingly. 2b schematically shows a multiple arrangement with common dynodes 1.

3a to 3h show an example of the production of a micro-secondary electron multiplier or a multiple arrangement (arrays), X-ray depth lithography with synchrotron radiation and electroforming being used as the most important process steps. A detailed description of these processes can be found in EW Becker, W. Ehrfeld, P. Hagmann, A. Maner and D Münchmeyer "Fabrication of Microstructures with high aspect ratio and great structural heights by synchrotron radiation lithography, galvanoforming, and plastic molding (LIGA-process ) ", Microelectronic Engineering 4 (1986) 35-36. 3a shows a base plate 1 made of aluminum oxide ceramic. The thickness of the base plate 1 is about 1 mm, the area is about 10 cm x 10 cm. The base plate 1 is spun on with a thin layer 2 of photoresist (e.g. AZ 1350 Kalle, Wiesbaden) coated and pretreated according to the manufacturer's instructions (Fig. 3b). In a known manner, the photoresist is irradiated and developed lithographically through a mask, so that a photoresist structure 3 is formed on the base plate 1 (FIG. 3c). Subsequently, a 30 nm thick layer 4 made of titanium and then another 200 nm thick layer made of nickel is deposited over the entire surface by means of a sputtering process. The photoresist 3 is then removed with acetone in the immersion bath, the regions of the metal layers 4 and 5 which are located on the photoresist structure 3 also being removed. A metal layer structure 4, 5 remains on the base plate 1 (FIG. 3d). As described in the above-mentioned article, a layer 6 of a polymethyl methacrylate casting compound (PMMA) is now poured on in a thickness of 1 mm, polymerized and then structured by means of deep X-ray lithography with synchrotron radiation and subsequent development (FIG. 3f). Nickel, which represents the dynodes 8 of the micro-secondary electron multiplier, is electrodeposited into the PMMA shaped structure 7 thus produced. The remaining PMMA regions 7 are then removed in a solvent (FIG. 3g). In the same way, other elements of the micro-secondary electron multiplier such as anodes, shields and the like are produced in parallel with the dynodes 8 in the same working steps by specifying corresponding structures on the masks used in the lithography processes. Analogous to the process steps in FIGS. 3a to 3d, a cover plate 9 with metal structures 10, which is mirror-symmetrical to the base plate in FIG. 3d, is now produced. The metal structure 10 is soldered to the dynodes 8 by diffusion soldering with silver, whereby the micro-secondary electron multiplier, consisting of a base plate 1, a cover plate 9, discrete dynodes 8, conductor tracks 11 for contacting the dynodes and conductor tracks 12 for the vertical focusing of the electrons, is completed (FIG. 3h).

Another method for producing the microstructures is in the impression technique. X-ray lithography with synchrotron radiation is used to mold a positive of the dynode structure to be produced as a reusable tool with a plastic, whereupon the resulting negative shape is filled up by electrodeposition of metal and the remaining plastic is removed. The base plate required for fixing and contacting the dynodes is inserted into the mold during the molding process, so that the plastic forms a firm connection with the base plate. Both the direct production of the microstructures by means of deep x-ray lithography with synchrotron radiation and the impression technique enable extreme structural accuracy with lateral dimensions in the µm range with a freely selectable height of up to approx. 2mm.

A multi-channel scattered light radiometer (FIG. 4) is used as an application example. As is known, the scattering of light on small particles is an important aid in the investigation of size and shape parameters in particle systems (M. Kerker, The Scattering of Light, Academic Press, New York, 1969). One of the methods that provide the most information is measuring the angular distribution of the scattered light. Particularly cheap for the signal-to-noise ratio, the measurement time required and the time resolution is the simultaneous measurement of the scattered light at many different angles. The micro-secondary electron multiplier arrays according to the invention allow the construction of much smaller, more sensitive and robust electronic multichannel detectors than the state of the art (German Patent 23 38 481, US Patent 39 32762, German Utility Model G 8415886.7). The supply of the dynodes via conductor tracks allows the formation of groups of multi-channel micro-secondary electron multipliers, which can be connected to various voltage supplies. As a result, the sensitivity as a function of the scattering angle can be adapted to the scattered light angle distribution. This means, for example, that in the case of highly forward scattering particles, where the intensity difference between forward and backward can be several orders of magnitude, the rear detector area, about 90 ° -180 °, with the maximum gain, the middle area, about 20 ° -90 °, with a medium gain and the front area, 0 ° -20 °, can be driven just below the use of saturation effects.

Two sector-shaped regions are provided with multiple arrangements (arrays) of microsecond electron multipliers 2 on an annular base plate 1. The inputs of the microsecond multiplier 2 are each arranged on an arc and point to the center of the base plate 1. The sector areas are each surrounded by a glass wall 3, which carries photocathodes on its inner arc. which are each assigned to a microsecond electron multiplier.

The glass walls 3 are closed at the top with a cover plate 4, so that a vacuum-tight envelope of the multiple arrangements (arrays) is created. The signal outputs of the microsecond multiplier 2 are conducted with conductor tracks 5 to the outer edge of the base plate 1, where there are contacts 6 for external connection. The conductor tracks for supplying the multiple arrangements (arrays) are guided through metal-filled holes 7 to the underside of the base plate 1 and from there through conductor tracks 8 also to external connections 9 on the outer edge of the base plate 1.
A semiconductor laser 10, optical elements 11, diaphragms 12 and a wedge-shaped light sump 13 are arranged in the free sectors of the base plate 1 in such a way that a beam path which is suitable for the scattering of light due to density fluctuations of matter which is in the scattering volume 14 is produced.

The version shown in Fig. 4 makes it possible
to check the symmetry of the scattered radiation with respect to the direction of the incident primary beam. This can be of considerable importance, for example for systems of non-symmetrical particles, to which an orientation has been imparted by fluid dynamic or electromagnetic action.

The flat structure of such integrated measuring systems facilitates their use in several planes along a particle beam and thus the tracking of a temporal evolution of the particle parameters. It is also well suited for the application of a magnetic field for influencing the electron orbits. Although the application example used relates to light scattering, the scope also extends to scattering processes in which charged particles, such as electrons and ions, or excited neutrals are present, and also to radiation or particle sources which emit themselves.

Claims (16)

1. Secondary electron multiplier having discrete dynodes, characterised in that the dynodes have a miniature construction and are mounted on an insulating substrate plate, which is provided with electrical conductor paths for the connection of the dynodes.
2. Secondary electron multiplier according to claim 1, characterised in that the dynodes are produced on the substrate plate by a deep X-ray lithographic process, by a deep X-ray lithographic-galvanoplastic process or by a shaping technique or galvanoplastic process shaping technique derived therefrom.
3. Multiple arrangement (Array) of secondary electron multipliers according to claims 1 and 2, characterised in that a plurality of dynode arrangements are disposed on the substrate plate and are provided with separate inputs and outputs.
4. Secondary electron multiplier and multiple arrangement (array) according to claims 1 to 3, characterised in that the substrate plate, which carries the dynodes, is covered with a second insulating plate.
5. Secondary electron multiplier and multiple arrangement (array) according to claim 4, characterised in that one of two plates or both plates carry conductor paths, which serve to focus the electrons vertically.
6. Secondary electron multiplier and multiple arrangement (array) according to claim 4, characterised in that one portion of the dynodes is mounted on one substrate plate and the other portion is mounted on the other plate.
7. Multiple arrangement according to claim 3 to claim 6, characterised in that the imaginary connecting line between the signal inputs is a curve which is curved, as desired, within wide limits.
8. Secondary electron multiplier and multiple arrangement (array) according to claim 4 to claim 7, characterised in that a wall, which is light-permeable at suitable locations and is provided with photo-cathodes, is mounted between the plates, so that there is a vacuum-tight covering of the dynode arrangement.
9. Secondary electron multiplier and multiple arrangement (array) according to claim 4 to claim 8, characterised in that the light-permeable locations of the wall have a lens-shaped configuration, and in that the photo-cathodes are mounted on a separate, light-permeable carrier, so that there is an optical image relationship between the light source and the photo-cathode.
10. Multiple arrangement (Array) according to claim 3 to claim 9, characterised in that adjacent channels have common dynodes.
11. Secondary electron multiplier and multiplier arrangement (array) according to claim 1 to claim 10, characterised in that a magnetic field for the guidance of the electrons is applied from externally.
12. Multiple arrangement according to claim 3 to claim 11, characterised in that groups of dynodes are connected to different power supply means.
13. Method of producing miniature secondary electron multipliers and multiple arrangements (arrays) having discrete dynodes according to claim 1 to claim 12, characterised by the following manufacturing steps:

    a) applying conductor paths to an insulating substrate;

    b) producing dynodes on the conductor paths by a deep X-ray lithographic process, by a deep X-ray lithographic-galvanoplastic process or by a shaping technique or galvanoplastic process shaping technique derived therefrom;

    c) if necessary, connecting a cover plate to the dynodes, or providing a light-permeable wall with photo-cathodes and sealing it with a cover plate.
14. Method according to claim 13, characterised in that, in accordance with step b), an additional layer formed from a material having a high secondary electron coefficient is applied to the dynodes.
15. Method according to claim 14, characterised in that tin is galvanically applied to the dynodes and is subsequently oxidised in a wet-chemical manner.
16. Method according to claim 13, characterised in that the insulating regions between the conductor paths are made weakly conductive by the application of a suitable surface layer.

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