WO2015099561A1

WO2015099561A1 - Arrangement and method for field emission

Info

Publication number: WO2015099561A1
Application number: PCT/RU2013/001165
Authority: WO
Inventors: Stepan Alexandrovich Polikhov; Reiner Franz Schulz; Georgy Borisovich SHARKOV
Original assignee: Siemens Research Center Limited Liability Company
Priority date: 2013-12-24
Filing date: 2013-12-24
Publication date: 2015-07-02

Abstract

The present invention relates to an arrangement (1) for field emission with at least one extraction grid (1) comprising elements (6) with high electrical and thermal conductivity. The space in between elements (6) is transparent for electrons (5). The present invention further relates to a method to provide field emission electrons (5) from a source, emitted by a cathode (2) and accelerated in an electric field (4) between cathode (2) and at least one extraction grid (1), passing the at least one extraction grid (1) in a direction away from the cathode (2). A high amount of electrons (5) pass through the grid (1) and electrons absorbed by the grid (1) are transferred in high amount as current through the grid (1) with small amount of heat production within the grid (1).

Description

ARRANGEMENT AND METHOD FOR FIELD EMISSION

DESCRIPTION The present invention relates to an arrangement for field emission with at least one extraction grid comprising elements with high electrical and thermal conductivity. The present invention further relates to a method to provide field emission electrons from a source, emitted by a cathode and accelerated in an electric field between cathode and at least one extraction grid, passing the at least one extraction grid in a direction away from the cathode .

For many devices using vacuum tubes, for example X-ray tubes an electron source is required. A type of electron source is using the field emission effect. In field emission units a critical component in terms of stability and functionality is the so called extraction grid. The term extraction grid in the following is used in the meaning of a structure, applicable as electrode in a field emission unit. A proper extraction grid is constructed to apply an appropriate electrical field for the extraction of electrons from a cathode, particularly to form a high intensity electron beam.

The life time of extraction grids has to be high, to be useable in devices like X-ray tubes. The grid should have a high transparency for electrons. Electrons absorbed in the grid reduce the beam intensity and so the efficiency of the field emission unit. They have to be removed from the grid, generating a current which introduces heat to the grid due to resistive losses within the grid material. The heat has to be removed, and excessive heat can damage the grid or in worst case destroy the grid. Electrons hitting the grid material with high energy can damage the material too by detaching atoms from the grid material. The grid can get dissolved with time.

Electrons hitting the grid material introduce heat by their kinetic energy. Electron beams with high electron density and kinetic energy can lead to a high amount of heat. Electrons being absorbed by the grid material introduce a current. Current flow within a material with Ohmic resistance introduces thermal energy to the material due to electrical losses. To keep the grid below a critical temperature, above which the grid is damaged or destroyed, the grid material should have a high thermal conductivity. To reduce electrical losses within the grid at high currents and related heat production, the grid material should have high electrical conductivity.

The dimension of the grid depending on the application can be small. Dimensions, particularly thickness, are often in the range of some Millimeter or even down to Micrometer. It is important to construct the grid in a mechanically stable way even with low dimensions. A destruction of the grid by effects like arcing and others has to be avoided by the choice of shape and material .

The design of field emission units, also called as field emission cathodes is known from the state of the art, see for example US6042900A. The main feature of these devices is the ability to generate a high density electron beam at room temperature or at temperatures lower than required for thermionic emission. Main components are a surface with a morphology which reduces the work function and/or a required extraction voltage, and means to create an electric field at the cathode surface, high enough to assure a significant electron flux. The electric field strength at the cathode surface is in general reached by using an extraction grid made of metal wires in a woven/meshed way, i.e. with wires crossing each other. Extraction grids known from the state of the art absorb a significant part of electrons extracted from the cathode, in general up to 60%. These electrons clash into the meshed grid material. The extracting mesh is exposed to a significant thermal load due to resistive losses caused by electric current passing through it as well as kinetic energy of electrons, which is proportional to the voltage between the mesh and cathode, typically in the range of kV. The object of the present invention is to present an arrangement for field emission and a method to provide field emission electrons from a source, which solve the above described problems. Particularly is an object of the present invention to present an arrangement and method with increased efficiency of electron production and/or a reduced amount of electrons absorbed by the extraction grid, a reduced thermal load of the grid, high mechanical stability and/or long life in operation. A high amount of electrons from field emission should be provided for applications, for example to investigate a probe/sample .

The above objects are achieved by an arrangement for field emission according to claim 1 and a method to provide field emission electrons from a source according to claim 10. Advantageous embodiments of the present invention are given in dependent claims. Features of the main claims can be combined with each other and with features of dependent claims, and features of dependent claims can be combined together.

The arrangement for field emission comprises at least one extraction grid with elements of high electrical and thermal conductivity. The space in between the elements is transparent for electrons. This enables a high amount of electrons coming from the cathode to pass through the grid by passing by the elements without collision with the grid material. Fewer electrons are absorbed by the grid and the amount of thermal load is reduced. The grid is more long time stable and a destruction or damage of the grid by high temperatures, especially a temperature exceeding a critical temperature of the grid material, is prevented .

The elements can comprise or can be made of a metal and/or diamond, particularly single crystalline or polycrystalline diamond material. The use of a metal with high thermal and electrical conductivity increases the amount and rate of heat removal, and less heat is produced by absorbed electrons due to a low electrical resistance. Metals are mechanically stable. Single or polycrystalline diamond material has a high thermal conductivity. Heat generated in the grid can be easily conducted out of it, i.e. removed to the environment. Diamond, i.e. carbon in diamond crystal structure has a high mechanical stability and a high resistance to dissolving by atom removal due to collisions with high energetic electrons . Diamond material can withstand destruction within an electron beam well, resulting in a long life time of the grid. Diamond is mechanically stable at high temperatures .

The elements can comprise a doped material, particularly a doped bulk material or a doped surface region, and/or the surface, particularly on one side of the extraction grid, can be coated with a conducting material, particularly a metal. Examples for such metals are platinum, gold, silver, copper, tungsten or aluminum. The doping and/or plating of metals increase the electrical conductivity. Electrons adsorbed by the elements can be removed from the grid without a high amount of heat produced due to resistance, even at high current densities. Plating of the grid with metal on one side, particularly the side opposite the side facing the cathode, increases the life time of the arrangement. Less metal is removed by electrons crashing with the grid. A diamond grid with plated metal on one side shows a long life time, and high thermal and electrical conductivity. Heat and current introduced by electrons from the electron beam crashing with the grid can be removed fast and easy to the environment, without increasing the temperature of the grid above a critical temperature where the grid is damaged or destroyed.

The elements of the at least one extraction grid can be arranged in a honeycomb like structure. This structure increases significantly the electronic transparency of the grid compared with a net like structure, enables a good electric field distribution for electrons to pass the grid without collision, and thus reduces the thermal load of the grid and increases its life time. A combination of diamond material, particularly coated with metal on one side of the elements of the grid, in honeycomb structure can give a high mechanical stability with good electrical and thermal properties, and so a long life time.

The elements of the at least one extraction grid can alternatively be arranged in parallel, particularly all in parallel in essentially a plane, in a bridge like structure without crossing elements. Instead of a woven structure with crossing elements, this structure reduces the surface of the grid material within the electron beam. A reduced surface reduces the amount of electrons crashing into the grid material producing thermal load. A reduction in thermal load and number of crashes increases the life time of the grid and the yield of electrons from the arrangement is increased.

The at least one extraction grid can comprise a frame, particularly a ring-shaped frame, with the elements mechanically, thermally and/or electrically attached to the frame. An arrangement with grid comprising a frame, with elements mounted to the frame gives a high mechanical stability. The grid can be mounted via the frame, for example clamping the frame in a clamp. Current and heat can be transferred via the frame from the elements to the environment, reducing the thermal load of the grid. The at least one extraction grid can be formed flat on one side and/or concave on the opposite side. Particularly the grid can be formed concave on the side opposite to the cathode. This structure can result in an electrical field focusing the electron beam in direction of the anode. A focusing can lead to a high electron density at the anode surface and for a small anode area to a high electron density all over the area, for example advantageous to analyze anode surfaces as probes .

A cathode electrode can be comprised, particularly a plate like electrode arranged in parallel to the at least one extraction grid. With a plate like grid opposite the cathode, particularly in parallel to the cathode plate, an electrical field can be introduced between cathode and grid, particularly a substantially homogenous electrical field, with high electron output coming from the cathode surface . Means for applying a voltage between the cathode electrode and the at least one extraction grid can be comprised, particularly a circuit electrically connected to the cathode electrode and the at least one extraction grid comprising a voltage supply.

A method according to the present invention to provide field emission electrons from a source, particularly using an arrangement as described above, comprises the steps :

- electrons are emitted by a cathode and accelerated in an electric field between cathode and at least one extraction grid,

- the electrons are passing the at least one extraction grid in a direction away from the cathode, and

- a high amount of electrons pass through the grid and electrons absorbed by the grid are transferred in high amount as current through the grid with only a small amount of heat production within the grid.

The absorption rate of electrons within the grid can be smaller than for a metal grid with the same distance between elements in-between woven elements, and/or the absorption rate of electrons within the grid is less than 60 percent, particularly less than 10 percent of the total amount of incoming electrons from the cathode. This leads to a reduced amount of thermal load at the grid compared to grids known from the state of the art, increased life time of the grid in operation and a high electron exploitation rate of the arrangement.

A high conductivity of the grid can enable a high current density within the grid by absorbed electrons without substantial heating of the grid.

Electrons passing through the at least one extraction grid can be focused in the direction away from the cathode by a concave surface shape of the grid, particularly formed on the far site of the grid relative to the cathode .

The method and/or arrangement can be used in an X-ray tube as electron source.

The advantages in connection with the described method to provide field emission electrons from a source, particularly a cathode, according to the present invention are similar to the previously, in connection with the arrangement for field emission described advantages and vice versa. The present invention is further described hereinafter with reference to illustrated embodiments shown in the accompanying drawings, in which: illustrates a schematic view of an arrangement for field emission with a cathode 2 and an extraction grid 1 in side view, and shows a top view of an extraction grid 1 with only parallel elements 2 according an embodiment of the present invention, and

FIG 3 shows a top view of an alternative embodiment of the present invention with an extraction grid 1 in form of a honeycomb structure, and

FIG 4 shows a top view of the extraction grid 1 of FIG

3 with enlarged view to the honeycomb structure, and FIG 5 shows a cross-section of the extraction grid 1 with a flat and with a concave side, and

FIG 6 shows a schematic view of electron focusing with an arrangement comprising an extraction grid 1 with concave side.

In FIG 1 a schematic view of an arrangement for field emission with a cathode 2 and an extraction grid 1 in side view is shown. An electric circuit 3 is schematically depicted, applying a negative voltage to the cathode and a positive voltage to the grid or grounding the extraction grid. A power supply and means for control and/or regulation of current and/or voltage are not shown in the FIG for reasons of simplicity. A voltage applied between cathode electrode 2 and extraction grid 1 is in the range of a kV to some MV for example . The voltage depends among others on the cathode 2 materials and surface form, the distance between cathode 2 and grid 1, the way of application of voltage and the required electron beam energy and intensity. The advantages of the present invention are especially distinct for high voltages, but even for low voltage like for example in the range of some volt V it can be advantageous to use the arrangement .

The applied voltage between cathode electrode 2 and extraction grid 1 induces an electrical field, in FIG 1 indicated with direction to the cathode 2 by solid thin arrows with high density within a space between cathode 2 and grid 1 indicated by a circle 4. Electrons are pulled out from the cathode 2 surface by the field and accelerated within the electric field in direction to the grid 1. The amount of electrons generated and accelerated depends among others from the work function of the cathode material and the electrical potential at the surface generated by the applied voltage. An example for a grid 1 structure, i.e. the grid set-up is shown in FIG 2. In the depicted embodiment elements 6 are arranged in parallel. The elements 6 are comprised and mounted to a holder 7 with ring shape. The holder 7, also called frame 7 can be used to fix the grid 1 within an arrangement. For example the frame 7 can be clamped, glued or screwed to a housing comprised by an X-Ray tube, not shown in the FIG for simplicity. The frame can consists of or comprise a metal, for example copper or steel, for a high mechanical stability and to conduct thermal and electrical energy from the grid 1 to the environment. A voltage to the grid 1, which acts as electrode, can be applied via the frame 7 using means to apply a voltage not shown in FIG 2 for simplicity.

The grid 1 is assembled by the elements 6 with empty space in-between elements 6. In the embodiment of FIG 2 elements 6 are of bridge, i.e. thin bar form. The elements 6 are assembled all in parallel to each other, with equidistant gap in-between. The gap can be for example in the range of Millimeter or Micrometer. The shape of the elements 6 can depend among others on the method the grid is produced. For example metal fibers used as elements 6, particularly with circular cross- section, can be braced or welded to the frame 7 formed from a ring shaped metallic shim. Alternatively the structure of FIG 2 can be formed for example from a metal sheet by stamping out the empty space in-between the elements 6. In this case elements 6 can have a shape with a rectangular cross -section. The same shape can be obtained by etching out the empty space in-between elements, for example after a photolithographic process, from a flat diamond wafer. A metal layer can be deposited, particularly only on one side of the wafer before or after etching.

A thickness of the grid 1 is for example in the range of Millimeter, or in the range of Micrometer, or down to some Nanometer. A diameter of a flat ring shaped or wafer shaped grid 1, comprising the frame 7 and elements 6, is for example in the range of some Centimeter down to some Millimeter.

As seen in FIG 1, electrons accelerated towards the grid 1 within the electric field 4 pass through the grid 1 at free/empty space in-between elements 6 comprised by the grid 1. An electron beam, indicated by thick arrows starting from the cathode 2 surface, is generated. The electrode beam passes through the grid 1, in the area respectively with a diameter of circled 5 space as shown in FIG 1. The electron beam can be used for example to be focused on a sample, which can be analyzed with different methods . The arrangement can be used among others in electron microscopy or X-Ray tubes.

In FIG 3 and in more detail as cutaway view in FIG 4, a grid 1 with elements 6 in honeycomb structure/form respectively shape are shown in top view. The elements 6 are surrounded like in the embodiment of FIG 2 by a ring shaped frame 7. The grid 1 is of wafer form, with hexagonal formed gaps respectively free space in-between elements 6. The hexagonal gaps for example have a diameter 8 in the range of some Millimeter to some Micrometer or Nanometer. The elements 6 have for example a width 9 in the same dimension, particularly in the range of some Millimeter to some Micrometer or Nanometer.

With a diamond wafer as grid 1 material, the gaps can be produced for example by chemical etching. Other methods are possible too, for example lithographic methods with etching with laser or electrons. One side can be plated, for example electroplated or with a CVD method, using a metal to increase the electrical conductivity of the grid. Alternatively or additionally the grid 1 material can be doped with dopants like boron atoms. In case of for example a metal sheet as grid 1 particularly with circle circumference, gaps can be cut, punched out or etched out. In FIG 5, and in use in FIG 6, a cross-section of the extraction grid 1 according to an embodiment of the present invention is shown, with a flat side and with a concave side. Both sides are opposite sides of the grid 1, resulting in a small thickness 11 in the middle of the diameter of the grid 1 and a thicker thickness 10 at the outer circumference of the grid. The thickness 10 ensures a mechanically stable grid 1, which can be mounted easy and reliable. The ring shaped frame 7 has for example a width 12 in the range of Millimeter or Centimeter. The increase of thickness from the middle to the outer circumference of the grid 1 results in a good thermal and electrical conductivity, since heat and current have to be transferred to the frame 7 with increasing values particularly summing up from the middle of the grid to the circumference.

The grid 1 with concave side can alternatively be formed by a deformed flat plate with the same thickness all over the grid, particularly except the ring shaped frame 7. This layout is denoted in FIG 6, in case the grid 1 inside the ring 7 is not built up from a solid material filling up all space within the ring 7 but for example from a bent sheet 13, particularly made of a metal. The side facing the cathode 2 is convex and the opposite side concave. The electric field distribution resulting from this grid 1 structure focuses the electron beam 5 towards for example a sample, not shown in FIG 6 for simplicity, which can be arranged opposite to the side of the grid facing the cathode 1.

The above described features of embodiments according to the present invention can be combined with each other and/or can be combined with embodiments known from the state of the art. For example more than one extraction grid 1 or other electrodes can be comprised by the arrangement. The shape of the extraction grid 1 can be different from a round plate, for example rectangular, particularly in the shape of plate with square surface. The cathode 2 can have plate, tip or other forms. Electrodes can be arranged in parallel or for example tilted to each other. The empty space between elements can have rectangular, honeycomb or other forms, like for example circular. Various materials and different sizes of structures can be used for the grid 1 and other components . Various distances between components of the assembly can be used according to the application of the assembly. Semiconductor materials like silicon can be used among others instead of diamond material for the grid. The grid 1 can be plated with for example metal on both sides. The shape of the grid 1 can be convex on one side and plane on the opposite side, for example to defocus respectively widening the electron beam.

Further components, not shown in the FIG or described above can be comprised by the arrangement, for example a housing, holders for the electrodes and probes. The arrangement can be arranged within vacuum or an inert gas as well as in the atmosphere. The arrangement can be used in a vacuum tube, for example X-ray tube. Among others it can be used in raster electron microscopes, devices for medical applications, or scientific devices to analyze samples .

The arrangement according to the present invention for field emission is characterized by a high mechanical stability and long life time in use. The structure and material of components like the extraction grid ensure a generation of electrons with high efficiency, a long life time of components by keeping the temperature below a critical value by high transparency to electron beams, by high thermal and/or by high electrical conductivity.

Claims

1. Arrangement for field emission with at least one extraction grid (1) comprising elements (6) with high electrical and thermal conductivity,

characterized in that space in between elements (6) is transparent for electrons (5) .

2. Arrangement according to claim 1, characterized in that the elements (6) comprise or are made of a metal and/or diamond, particularly single crystalline or polycrystalline diamond material.

3. Arrangement according to claim 1 or 2, characterized in that the elements (6) comprise a doped material and/or the surface, particularly on one side of the extraction grid (1) , is coated with a conducting material, particularly a metal, particularly platinum, gold, silver, copper, tungsten or aluminum.

4. Arrangement according to any one of claims 1 to 3, characterized in that the elements (6) of the at least one extraction grid (1) are arranged in a honeycomb like structure .

5. Arrangement according to any one of claims 1 or 3 , characterized in that the elements (6) of the at least one extraction grid (1) are arranged in parallel, particularly all in parallel in essentially a plane, in a bridge like structure without crossing elements (6) .

6. Arrangement according to any one of claims 1 to 5, characterized in that the at least one extraction grid (1) comprises a frame (7) , particularly a ring-shaped frame (7) , with the elements (6) mechanically and/or electrically attached to the frame (7) .

7. Arrangement according to any one of claims 1 to 6, characterized in that the at least one extraction grid (1) is formed flat on one side and/or concave on the opposite side.

8. Arrangement according to any one of claims 1 to 7, characterized in that a cathode electrode (2) is comprised, particularly a plate like electrode arranged in parallel to the at least one extraction grid (1) .

9. Arrangement according to claims 8, characterized in that means for applying a voltage between the cathode electrode (2) and the at least one extraction grid (1) are comprised, particularly a circuit (3) electrically connected to the cathode electrode (2) and the at least one extraction grid (1) comprising a voltage supply.

10. Method to provide field emission electrons from a source, particularly using an arrangement according to any one of claims 1 to 9, with electrons (5) emitted by a cathode (2) and accelerated in an electric field (4) between cathode (2) and at least one extraction grid (1) , passing the at least one extraction grid (1) in a direction away from the cathode (2) ,

characterized in that a high amount of electrons (5) pass through the grid (1) and electrons absorbed by the grid

(1) are transferred in high amount as current through the grid (1) with small amount of heat production within the grid (1) .

11. Method according to claim 10, characterized in that the absorption rate of electrons within the grid (1) is smaller than for a metal grid (1) with the same distance between elements in-between woven elements (6) , and/or the absorption rate of electrons within the grid (1) is less than 60 percent, particularly less than 10 percent of the total amount of incoming electrons from the cathode (2) .

12. Method according to any one of claims 10 or 11, characterized in that a high conductivity of the grid (1) enables a high current density within the grid (1) by absorbed electrons without substantial heating of the grid (1) .

13. Method according to any one of claims 10 to 12, characterized in that electrons (5) passing through the at least one extraction grid (1) are focused in the direction away from the cathode (2) by a concave surface shape of the grid (1) , particularly formed on the far site of the grid (1) relative to the cathode (2) .

14. Method according to any one of claims 1 to 13, characterized in that the method and/or arrangement is used in a X-ray tube as electron source.