US 3665241 A
Description (Le texte OCR peut contenir des erreurs.)
United States Patent Spindt et al.
[ 51 May 23, 1972  FIELD IONIZER AND FIELD EMISSION CATHODE STRUCTURES AND METHODS OF PRODUCTION  Inventors: Charles A. Spindt, Menlo Park; Louis N.
l-leynick, Palo Alto, both of Calif.
 Assignee: Stanford Research Institute, Menlo Park,
 Filed: July 13, 1970  Appl.No.: 54,222
OTHER PUBLICATIONS C. A. Spindt, A Thin Film Field- Emission Cathode," J. of Applied Physics, Vol. 39, No. 7, 6- 1968, pp. 3504- 3505.
Primary Examiner-David Schonberg Assistant Examiner-Paul A. Sacher Attorney-Urban H. Faubion and James Todorovic  ABSTRACT Field-forming devices primarily useful as field ionizers and field emission cathodes and having as a basic element an array of closely spaced cones with sharp points supported on a substrate (in the most usual case conductive or semiconductive) are disclosed. Preferably, the field-forming structure is completed by a screen-like structure, e.g. as fine mesh screen, insulatively supported above the points with the center of apertures in the screen substantially aligned with the longitudinal axis of corresponding cones. A novel method of forming such structures includes placing a screen with a mesh corresponding to the desired number and packing density of sharp conical points in close proximity to, or in contact with, the substrate and projecting material through the screen onto the substrate whereby sharp cones of the material are formed on the substrates.
11 Claims, 9 Drawing Figures FIELD IONIZER AND FIELD EMISSION CATIIODE STRUCTURES AND METHODS OF PRODUCTION BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to field-forming structures such as field-ionizing and electron-emitting structures and particularly to such structures employing many cone-like emitters or ionizers on a single substrate.
Structures for forming electric fields are required for many practical applications. An electric field on the order of several megavolts per centimeter (cm) can be used to produce electron emission from materials. Electric fields on the order of 10' to volts per centimeter are useful in ionizing molecules by field extraction and collection of electrons therefrom (known as field ionization).
Electron emission is of course the heart of devices utilizing electron beams or clouds such as the many varieties of electron tubes upon which the electronics industry is built. The phenomenon of ionization plays a significant role in many scientific instruments and experiments; e.g., in ionization gauges and mass spectrometers. In mass spectrometry, an unknown material under investigation is ionized prior to injection into the analyzer or mass-separator section of the mass spectrometer. Ionization is usually produced by electron impact with the unknown material, utilizing a suitable electron source such as a thermionic emitter. However, electron impact with molecules not only ionizes them, but also tends to fragment them into two or more species, so that the mass spectrum, obtained by this ionization method, may show the presence of the daughter species but little or nothing of the parent species. Moreover, if any of the daughter species is the same as, or has a mass-to-charge ratio approximately equal to, another species originally present in the unknown material, then the mass spectrum obtained can be difficult or impossible to interpret correctly regarding the original constituents of the unknown material. In some applications where mass spectrometry is used to monitor or control other processes, e.g., the preparation of photoemissive surfaces, the use of a thermionic emitter for ionization is disadvantageous because the heat or light from the emitter tends to disturb the process. The use of a cold, non-luminous ionizer in such applications constitutes a significant improvement. Field ionization, a phenomenon in which molecules entering a region of very high electric field (10 to 10 V/cm) are ionized by extraction and collection of electrons by the field, causes substantially less fragmentation than electron-impact ionization. Also, this phenomenon does not require or involve the generation of light or heat.
In order'to reduce to a practical level the voltage required for producing the required high fields, sharp needles or points are used as emitters or field ionizing electrodes, a counter electrode is spaced from the needle-like structures and a voltage of appropriate polarity is applied therebetween. For field emission the counter electrode is made positive relative to the needle-like structures and for field ionization the reverse polarities are used (counterelectrode negative relative to the needle-like structures). However, even with the use of sharp points, if the counter electrode is spaced a macroscopic distance from the points, e.g., of the order of centimeters (usual in prior art devices), the voltages required for electron emission are of the order of kilovolts and for field ionization, approximately tenfold higher.
Despite the high field emission current density capability of a single needle-like emitter (on the order of 10 million amps per sq. cm), the total emission current from a single needle emitter is low, e.g., on the order of milliamperes, because of the minute size of its emitting area. Furthermore, the electrons are emitted over a large solid angle, and they obtain almost the total energy of the applied voltage, e.g., several thousand electron volts, within a short distance from the emitter tip. Therefore, the formation of narrow electron beams that are suitable, for example, for use in high-power, beam-type electron tubes, requires elaborate and expensive focusing apparatus.
Ionization efficiency of prior art field ionizers of the single needle-like structure is very low for reasons similar or analogous to the problems described above relative to the cathodes. That is, one reason ionization efiiciency is low is that the effective region where ionization takes place is confined to the small volume in the immediate vicinity of the apex of the sharp point so that the rate of ion production for a given pressure of material to be analyzed is much lower for field ionization than for electron-impact ionization. A second reason is that the field-produced ions attain velocities equivalent to the voltage applied between ionizer and counter electrode and the ions are impelled away from the ionizer over a very wide range of angles, so that only a small fraction of the ions are collimated into a beam suitable for injection into the analyzer of the mass spectrometer without employing complex ion-optical lenses.
Parallel operation of many needle-like members to increase the total current for a cathode and to provide a correspondingly large ionization volume in the case of the field ionizer is feasible, but the problems of formation of the parallel structures, focusing the electron beams (for the cathodes), and providing ion-optical collimation (in the field ionization structures) are formidable. For example, in the field ionizer case, ion-optical collimation is practical only if emission energies of the ions can be kept small, which necessitates spacings between the ionizer and counter-electrode of the order of microns with the ionizer point having a tip radius of a fraction of a micron, e.g., 0.1 micron. Also, it is desirable to space the needle-like structures as close together as possible without incurring significant decrease of the field at each point by the presence of its neighbors.
Many of the problems thought to be inherent in parallel operation of fine needle-like structures under consideration have been solved by a structure and the methods of producing that structure disclosed in U.S. Pat. Nos. 3,453,478 Needle- Type Electron Source, dated July 1, 1969, and 3,497,929, Method of Making a Needle-Type Electron Source, dated Mar. 3, 1970 in the names of Kenneth R. Shoulders and Louis N. Heynick, and assigned to Stanford Research Institute.
In the patents referred to above, the electric field-producing structure effectively includes two closely spaced surfaces. On the first, or emitting surface, a large number of sharp needlelike emitting sites are distributed with a packing density limited only by the fabrication technology used. The surface can be planar or curved and of a size to suit the intended application. The second surface, called an accelerator surface, is the electrode used to produce the field. It consists of a very thin foil or film of metal of the same contour as the surface with the emitter sites, and is suitably supported and electrically insulated therefrom in spacings ranging from fraction of a micron to several microns.
In the preferred embodiment, described in the patents, the accelerator surface is supported above the emitter surface by a dielectric layer therebetween, in the manner of a sandwich, and holes through the accelerator and dielectric layers are provided so as to expose the tips of several emitters at each hole location to the rim of the hole in the accelerator electrode. Because of the minimal separation range between the emitter surface and the accelerator surface, the voltage needed to produce field emission ranges from only a few volts to about volts, and the emitted electrons emerge from the holes in the accelerator with correspondingly low energies.
While the structure referred to above represents a considerable advance over any of the structures known to the prior art, the method of producing the structure can yield needle-like electrodes that are not necessarily uniform in numbers and shapes from emitter site to emitter site, thus introducing corresponding variations in performance. Many of the problems of the multiple-needle structure are overcome by providing a single, uniform needle-like electrode at each site with specific, essentially identical, configuration. A means of producing a single needle-like electrode at each site is described in an article by C. A. Spindt (one of the inventors of the present invention) entitled A Thin-Film Field-Emission Cathode" in the Journal of Applied Physics, Vol. 39, No. 7, 3,5043,505, June 1968. Further, a means providing a single uniform needle-like electrode at each site which represents an improvement over the method and structure described in the previous patents and the Spindt paper is described and claimed in a U.S. Pat. application Ser. No. 9,139, entitled Field Emission Cathode Structure, Devices Using Such Struc' ture, and Method of Producing Such Structure," filed Feb. 6, 1970, in the names of Louis N. Heynick, Kenneth R. Shoulders, and Charles A. Spindt and assigned to the assignee of the present invention.
Subsequent to conception of the structures and methods described in the above-referenced patents, application and paper, .use of similar structures operated in reverse polarity as a closely spaced parallel array of field ionizers, in which each sharp metal point produces positive ions was conceived. In the cathode structure sandwich, the dielectric film thickness is in the order of l 2 y. and the metal points are of about the same height above the emitter surface. However, field ionization in any such structure having specific values of tip sharpness and distance between counter electrode and points requires voltages approximately tenfold higher between electrodes than those required for field emission. Consequently, the dielectric layer between the emitter surface and counter-electrode must be capable of withstanding the higher fields without dielectric breakdown. This requirement can be met by making the dielectric thickness large relative to the distance between the counter-electrode and the tips, or by providing other means for insuring adequate insulation between the emitter surface and the counter-electrode.
In addition to providing a multi-point ionizer, the present invention provides uniform arrays of points, suitable electrode and counter-electrodes therefor, and improved means for producing such structures in which the ratio of dielectric thickness to distance between counter-electrode and tips and also geometries chosen optimally for field emitters or ionizers or both.
Particularly in view of the fact that the spacing between emitter tips and the counter-electrode may be different for field emitters and field ionizers, it is highly desirable to be able to produce the fine needle-like points of uniform shape and spacing on a substrate independent of a metal/dielectric/metal film sandwich. That is, it is important to be able to produce a precision, highly uniform bare point array on a substrate (electrode most commonly). With such a structure, one or more counter-electrodes may be added with the desired spacing, dielectric thickness or other adequate insulation, and the proper registry relative to the points of the bare point array. The present invention provides the capability of producing such results.
As described in greater detail below, in accordance with the teachings of the present invention a bare-point structure is provided in which a regular array of closely spaced metallic points of controlled geometry is formed by deposition through a fine mesh plate or screen uniformly over the surface of a metal substrate which represents an electrode.
Where the bare-point array is desired, the screen may be removed. Where a counter-electrode is desired, the screen may be left in place or removed and replaced by another counter-electrode of desired configuration. A field ionization structure is provided by making the counter-electrode of the arrangement just described negative relative to the substrate electrode and providing the proper electrode-counter-electrode spacing as well as ratio of such spacing to the distance between counter-electrode and electrode points. The field emitter is provided by applying the opposite polarity between electrodes and providing optimally different spacings. Additional electrodes can be added to the structure to provide multi-electrode control of the electron or ion optical characteristics as well as the current emerging from the holes. Multielement vacuum tubes can also be produced by adding appropriate electrodes and closing the device. Further, the field ionizer may be constructed by the same general method described in connection with the Heynick, Shoulders, and Spindt application referred to above with modifications described herein.
The novel features which are believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objectives and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which:
FIG. 1 is an enlarged fragmentary perspective view, showing a bare-point array (pyramidal embodiment) constructed in accordance with the principles of the present invention;
FIG. 2 is an enlarged fragmentary perspective view of a portion of a device utilizing the bare-point array of FIG. 1 and constructed in accordance with this invention;
FIGS. 3 through 5, inclusive, are cross-sectional views taken along the lines 33 of FIG. 2 for successive steps in the method of producing the structure of FIGS. 1 and 2;
FIG. 6 is a cross-sectional view similar to the device of FIG. 5, but illustrating another embodiment which is constructed in a different way;
FIG. 7 is an enlarged fragmentary perspective view of a field ionizer according to one embodiment of the present invention; and
FIG. 8 is a broken-away cross-sectional along lines 8-8 in FIG. 7.
FIG. 9 is a partially broken-away cross-sectional view of another embodiment of the invention.
A form of the basic bare-point array 10 useful for both field electron emitters and field ionization is illustrated in FIG. 1. The structure 10 includes a substrate 11 and an array of bare points 12 formed thereon. In the embodiment shown, the bare points 12 are pyramidal but may be of other conical shapes. The substrate 11 is preferably conductive in order to form one electrode. In the embodiment illustrated, substrate 11 is a sheet of molybdenum but it may be of other suitable metal, or a non-metal coated with a conductive film as, for example, a plate of aluminum oxide coated with a film of molybdenum. For some applications, it may be preferable to use a semi-conductive material or even an insulator for the substrate 11. As illustrated, the pyramids 12 are of molybdenum, have square bases, are 0.6 mil high, and are spaced apart by 1 mil (center to center). However, the pyramids 12 may be of resistive or insulating materials, or of composite materials, and the pyramid surfaces overcoated or otherwise treated to obtain the desired characteristics.
Bare-point arrays 10 require a field-producing electrode in order to produce the electric field required to cause electron emission or ionization in the region of the array of points or pyramids 12. The electrode is preferably but not necessarily analogous to the top conduction film in the sandwich configurations described in the previously cited patents and application. FIG. 2 illustrates a device incorporating the bare-point array of FIG. 1 and the additional electrode 12 (referred to as a counter-electrode) to provide the required electric field. As illustrated, the counter-electrode 13 comprises a screen (or plate) having a distribution of holes or apertures 14 therein, shown square in this embodiment, corresponding substantially to the distribution of points 12 on the substrate. The brokenaway cross-sectional view of FIG. 5 may help to visualize the device. Looking at the two FIGS. (2 and 5) it is seen that the device comprises a substrate 11 having points 12 formed thereon and a screen (counter-electrode 13) supported above the substrate by an insulating spacer 15 at the periphery of the screen. In a preferred version of this embodiment, registration between the screen 13 and the substrate 11 is maintained by spacer 15 so that the center of each screen hole 14 is substantially aligned with the axis of a difi'erent point 12 of the basepoint array 10, and so that the tips of the points 12 are substantially in the plane of the screen 13. Furthermore, the ratio of the height of points 12 above substrate 11 to the distance cal breakdown of insulator spacer 15. One means of assuring that the breakdown doesnt occur is seen in another version of this embodiment which is illustrated in FIG. 9.
For convenience and simplicity, the illustration of FIG. 9 has parts which correspond to those of FIGS. 2 through 6, inclusive, numbered correspondingly. Here, the perimeter of the substrate 11 (or counter-electrode 13, if preferred) is shaped so as to permit the use of a thicker insulator spacer 15, thereby permitting the application of higher 13 without causing electrical breakdown of insulator spacer 15.
The basic mode of operationof the field ionizer may be best explained in connection with FIGS. 2 and 5 wherein the ionizer points 12 of array 10 are shown connected by means of their common conductive substrate 11 to a positive terminal of a voltage power supply 16. The sharp tip points 12 are each located in or near a hole 14 of counter-electrode (screen) 13, which is connected to the negative terminal of the power supply 16. Application of a voltage from power supply 16 produces a high electric field in the region of the points 12. In such an arrangement, electrically neutral particles entering the holes 14 are positively ionized by the high electric field, the action of the field being to remove electrons from the particles, which electrons are collected by the points 12. The positive ions so created are impelled away from the ionizer points 12 through the holes 14 of the counter-electrode 13.
For producing field electron emission, the potential source is connected with its positive terminal to counter-electrode (accelerator electrode) 13, and its negative terminal connected to array (emitter electrode) 10. The potential source may be made variable for the purpose of controlling the electron emission current. Upon application of a potential between the electrodes 10 and 13 an electric field is established between the points (emitting protuberances) 12 and the counter-electrode 13, which of a polarity to cause electrons to be emitted from the points 12 through the holes 14in the screen 13.
Thus it is seen that efficiency limitations of one ionizer point or one field emitter cathode point as well as the limitations of prior attempts at parallel operation of such single point devices are largely overcome by providing a structure consisting of an array 10 of closely spaced points 12 with sharp tips in close proximity to the counter-electrode 13 which has a corresponding array of holes 14. In this structure the holes 14, the distance between points 12 or holes 14, as well as the spacing between point tips and the counter-electrode, are in the micron range, and most points 12 yield substantially equivalent performance. Insulator spacers 15 which separate the outer edges of the electrode 10 and counter-electrode 13 have the requisite thickness to withstand the electric field.
The ability to produce the bare-point array 10 with such uniformity of points 12 and hence the ability to produce field ionizers and field electron emitters of such precision and efficiency is highly dependent upon the methods of construction. The method of the present invention yields the precise results desired.
In order to understand the steps in one method used in the fabrication of the array 10 (of FIG. 1) and the completed device (FIGS. 2 and 5), reference may be had specifically to FIGS. 3 through 5, inclusive, which represent sections through one portion of the device illustrated in FIG. 2. FIG. 3 illustrates the substrate 11 of the bare-point array structure 10 before the field-forming points 12 are formed thereon. That is, FIG. 3 shows a starter structure consisting of only the substrate 1 l and a fine mesh screen (plate) 13 having a multiplicity of holes or apertures 14 therein supported above the substrate 11 by an insulating dielectric spacer 15. If the screen 13 is later to be removed to provide only the bare-point array 10 of (FIG. 1), the screen may be placed in direct contact with substrate 11 and the dielectric spacer 15 may be eliminated. Since this embodiment contemplates that the masking screen and the counter-electrode 13 will be one and the same, the spacer 15 is shown, and also a release layer 18 is provided on the screen 13 so that materials subsequently deposited thereon in the array-forming process may readily be removed.
In order to provide the sharp points 12 as shown in FIG. 4 and thereby complete the pyramidal array 10 of FIG. 1, a simultaneous deposition from two sources is performed. That is, simultaneously a closure material (e.g., a molybdenum-alumina composite) is deposited at a grazing incidence, and the material for the pyramid, e.g., molybdenum, is deposited straight on the substrate surface. In this step, the purpose of the deposition at grazing incidence is to add material on screen 13 so as to provide a mask with holes 14 of decreasing size for the deposition of material on substrate 11. As additional emitter material is deposited on substrate 11, the molybdenum-alumina composite masking material gradually closes the aperture at the upper lip of the holes 14, as shown in FIG. 4. The closure is indicated by theadditional film l9 deposited on the release layer 18. In FIG. 4 the apertures 14 are shown as being completely closed and pyramids 12 completely formed. Thus, cone-shaped (pyramidal here) points are formed on the conductive substrate 11. If the screen 13 used is provided with round apertures instead of the square ones shown, then the points 12 formed are right circular cones instead of the pyramids illustrated.
With the step just described, the array 10 of FIG. 1 is completed and'the screen 13 and spacer 15 may be removed, leaving the bare-point array 10. Another screen-like counterelectrode can then be added to form the structure of FIG. 2. If it is desired to use the screen 13 as counter-electrode of FIG. 2, Screen 13 and spacer 15 need not be removed. Instead, the materials deposited on the screen, viz., release layer 18 and the subsequently deposited closure layer 19 may be selectively etched or floated away, leaving the bare screen structure as illustrated in FIGS. 5 and 2.
If larger spacings between counter-electrode 13 and substrate 11 are desired, to accommodate thicker dielectric spacers 15, the points 12 may be formed on previously produced pedestals (not shown), which pedestals are produced by a prior deposition step, utilizing a source which deposits material along a direction perpendicular to the surface of the substrate, and which material is preferably the same material, e.g., molybdenum, as the metal electrode (substrate) 11 or a more resistive material, e.g., a molybdenumalumina composition. Such a deposition step would deposit a film on the release layer 18 without closing the screen holes 14 and, more importantly, pedestals with essentially vertical sides and bases of size and shape of apertures 14 in the screen 13 are deposited directly upon the substrate 11. The pedestal height is selected by controlling the amount of material deposited. Since the array of FIG. 1 does not have such pedestals, this step is not illustrated. However, a specific embodiment of a complete device incorporating such pedestals is shown in FIGS. 7 and 8, described later herein.
In the embodiment illustrated, the screen 13 has a uniform array of square holes 14, spaced on 1 mil centers. The pyramids formed then have square bases of corresponding size and spacing to the screen mesh and of a height controlled by the relative rates of deposition of the sources. In other embodiments, the holes 14 may have other configurations and/or the deposition rates may be varied during the formation process to provide a variety of shapes. Further, the formation process may be halted prior to hole closure so as to form truncated pyramids, cones, or suitable variants thereof.
An alternative deposition technique incorporates the use of a single deposition source which is broad enough to perform both the hole-closure and point-formation functions. This deposition source and technique may also be applied to the sandwich starter structure described in the previously cited patents and applications.
One embodiment specifically designed for field ionizer application and utilizing a metal/dielectric/metal sandwich structure with counter-electrode 30 which corresponds to the screen counter-electrode 13 of FIG. 2 is illustrated in FIGS. 7 and 8. In this embodiment the counter-electrode 30 is formed of the upper metal film of the sandwich structure which is provided with a plurality of holes or apertures 28 therethrough. Dielectric film 31, the center layer of the sandwich, is on top of a base metal film 32 which serves as a base electrode and which is connected to the positive terminal of the power supply 24. As illustrated, the base metal film 32 is shown on a dielectric support substrate 43 which only serves to support the base metal film 32. Films 30 and 32 are formed of metal such as molybdenum or tungsten, while film 31 which insulates films 30 and 32 from one another is formed of dielectric material, e.g., aluminum oxide.
The dielectric film 31 has holes corresponding to the holes in film 30 and each hole accommodates a point ionizer 40 with its base in contact with the base electrode 32 and tip 26 preferably aligned with the plane of the hole 28 in the top electrode30 to minimize the distance between tip and hole rim. FIG. 8 is a cross-sectional view along lines 88 in FIG. 7. In FIG. 8 the hole in dielectric film 31 is designated by numeral 36. The point ionizer 40 is shown in the form of a cone 41 on top of a pedestal 42, a configuration that permits independent selection of the height of the tips 26 above base 32, the sharpness of the tip 26, and the distance between tips 26 and hole rims 28 so as to provide optimum geometry for operation of the structure as a field ionizer.
Another highly practical way to utilize the bare-point arrays 10 is shown cross-sectionally in FIG. 6. In this embodiment a conductive screen 13 having substantially the same distribution of holes as the points 12 on the substrate 11 is supported above the substrate 11 by insulator spacers 23 of appropriate height and distribution. One method for producing such structures is to form an insulator 23 of the requisite thickness on the screen 13 by deposition or other means, which insulator thereby conforms substantially to the cellular structure of the screen, after which the screen-insulator combination is set and maintained on the substrate. An advantage of this method is that self-registry of holes and points is achieved.
Bare arrays of points can be made to yield very large emission currents by the use of an electrode to which appropriate positive potentials relative to the points are applied, e.g., in diode rectifiers, X-ray generating tubes, and Lenard-ray tubes. Therefore, it is contemplated that bare-point arrays 10 or individual members of such arrays be sealed off opposed to another electrode either with or without intermediate electrodes. Substrate-screen assemblies having the points in substantial or complete registration with the holes in the screen can be used as large-emission current cathodes by applying suitable positive potentials to the screen relative to the substrate. In contradistinction to the operation of the diode configurations cited above, the screen provides the fields required for electron emission from the points, but most of the emission drawn passes through the screen holes, so that the screen functions to control the current in the manner of a grid. Additional grids may also be employed to render the emission more uniform or otherwise control the emission.
The methods for producing an array of points in registry with holes in screens are adaptable to the production of cathodes subdivided into areas containing one or more points, from which areas emission can be drawn separately by the application of appropriate potentials thereto. Such methods can also be adapted to the production of arrays of individual but suitably interconnected field emission diodes, triodes, tetrodes, etc.
All the operational advantages of the multipoint field ionizer described above are achievable also with the field emitters of the same general structure with parameters optimized for such use. Again, such structures constitute field ionizers when operated with reverse polarities to those used for obtaining field emitted electrons, and such structures can be produced by the methods previously described so that the values of the geometric parameters are optimum for field ionization use. This is not to say, however, that the method of producing the ionizer from the metal/dielectric/metal sandwich is equivalent to, or can be made with, the same degree of accuracy as the improved screen-forming techniques herein described.
While particular embodiments of the invention are shown, it will be understood that the invention is not limited to these structures since many modifications may be made both in the material and arrangement of elements. It is contemplated that the appended claims will cover such modifications as fall within the true spirit and scope of this invention.
What is claimed is:
1. An electric field-forming device consisting of a plate-like substrate and a multiplicity of sharp needle-like elements located on one surface of said substrate, said needle-like elements being highly uniform in shape and uniformly spaced on said substrate said substrate and at least one portion of said needle-like elements being of conductive material and the other portion of said needle-like elements being of a higher resistivity material than said plate-like substrate.
2. An electric field-forming device as defined in claim 1 wherein said needle-like elements each have a substantially cylindrical pedestal on said substrate and an essentially conical portion on said pedestal.
3. An electric field-forming device as defined in claim 2 wherein said needle-like elements and said plate-like substrate are conductive and the said pedestal is of a higher resistivity material than either said second electrode or said conical portion.
4. An electric field-producing structure comprising first and second conductive electrodes and an insulator separating and insulating said first and second electrodes from each other, said first electrode comprising a conductive plate-like screen member having a plurality of apertures therethrough, said second electrode comprising a conductive plate-like member having a plurality of individual needle-like conical members projecting from one surface thereof and said insulator supporting said electrodes only at their outer periphery in such a manner that said needle-like conical members of said second electrode project toward said first electrode and at least some of said needle-like members are positioned with a projection of their longitudinal axes extending through apertures in said first electrode.
5. An electric field-producing structure as defined in claim 4 wherein said first and second electrodes are spaced further apart at their outer periphery than in the central portion where said conical members and apertures occur whereby said insulating member is thicker than the electrode spacing at said center portion of said electrodes.
6. An electric field-producing structure as defined in claim 4 wherein each of said individual needlelike conical members has its longitudinal axis in substantial alignment with the center of a corresponding aperture in said first electrode.
7. An electric field-producing structure as defined in claim 6 wherein said first and second electrodes are spaced further apart at their outer periphery than in the central portion where said conical members and apertures occur whereby said insulating member is thicker than the electrode spacing at said center portion of said electrodes.
8. A field-ionizing device including a structure as defined in claim 4 and means for applying a potential source between said first and second electrodes with said second electrode positive relative to said first electrode.
9. A field-ionizing device including the structure defined in claim 5 and means for applying a potential source between said first and second electrodes with said second electrode positive relative to said first electrode.
10. A field-ionizing device including the structure defined in claim 6 and means for applying a potential source between said first and second electrodes with said second electrode positive relative to said first electrode.