US 5,955,849 AGrant

Cold Field Emitters With Thick Focusing Grids

Issue Date:1999-09-21

•12 Claims

•10 Drawing Sheets

Abstract

A field emitter for producing an electron beam includes at least one cold cathode unit. Each of the cold cathode units includes an emitter cone having an emitter tip and a gate spaced apart from the emitter tip for extracting electrons from the emitter tip in a propagation direction upon application of a positive dc voltage on the gate with respect to the emitter tip. The gate forms a gate cavity for propagation of the extracted electrons therethrough. Each of the cold cathode units further includes at least one lens electrode disposed further in the propagation direction from the emitter tip than the gate, the at least one lens electrode forming at least one lens cavity for propagation of the extracted electrons therethrough. The at least one lens electrode is for focusing the extracted electrons in part of the gate cavity, part of the at least one lens cavity, and part of the region therebetween.

Metadata

Assignee

The United States of America as represented by the Secretary of the Navy

Inventors

Cha-Mei Tang
Antonio C. Ting
Thomas A. Swyden

Application Information

Application Number:US 2019639

Filing Date:1994-02-25

Priority Date:1993-11-15

Art Unit:362

Classifications

IPC:

G09G 104H01J 2970

Field of Search:

315313445250366;382336;309;310;447;41444-46396 R

Patent Drawings (10 sheets)

Description

Field of the Invention

The present invention relates to vacuum cathodes, and more particularly to cold field emitters with thick focusing grids.

Description of the Related Art

Deflection control or modulation of electron beams produced by thermionic emission was suggested almost half a century ago. See, Karl R. Spangenberg, "Vacuum Tubes," McGraw-Hill, 1948, .sctn.20.11, pages 727-728. However, it was recognized then that such a design had severe shortcomings. Thermionic cathodes do not provide sufficiently high current. Thermionic cathodes have a large velocity spread, thus distorting the response. Other vacuum tube deflector designs which have been proposed are large, precluding efficient operation at high frequency. Some designs require the use of high anode potentials, thereby reducing the device efficiency.

Cold field-emission cathode units now produce electron beams with higher current density and with lower energy spread do than thermionic cathodes. Previously proposed designs for cold cathodes fail to provide sufficiently high collimation. Furthermore, previously proposed deflection amplifiers using cold cathodes are inefficient at high frequencies because of their large size, high capacitance, large emittance, high anode voltage, or physical beam interception. Previously proposed designs also fail to provide for high gain over a wide bandwidth or else they are not suitable for use in integrated circuits.

Summary of the Invention

It is an object of the invention to provide a cold field emitter for producing an electron beam.

It is a further object of the invention to provide a cold field emitter for producing an electron beam suitable for use in deflection amplifiers, flat panel displays, electron beam lithography, and small spot x-ray sources.

Another object of the invention is to provide a cold emitter array for producing a collimated, high current density, low voltage electron beam ribbon suitable for use in a vacuum deflection amplifier.

A further object of the invention is to provide a cold emitter array for producing a collimated, high current density, low voltage electron beam ribbon suitable for use in a vacuum deflection amplifier, which deflection amplifier has high gain over a wide bandwidth at microwave frequencies.

A still further object of the invention is to provide a cold emitter array for producing a collimated, high current density, low voltage electron beam ribbon suitable for use in a vacuum deflection amplifier, which deflection amplifier is an integrated device suitable for use in integrated circuits.

It is also an object of the invention to provide a cold emitter array for producing a collimated, high current density, low voltage electron beam ribbon suitable for use in a vacuum deflection modulator, which modulator produces a prebunched electron beam suitable for input to a high power amplifier, such as a klystron, traveling wave tube, or distributed amplifier for use at microwave frequencies.

These and other objectives are achieved by a field emitter for producing an electron beam, which includes at least one cold cathode unit. Each of the cold cathode units includes an emitter cone having an emitter tip and a gate spaced apart from the emitter tip for extracting electrons from the emitter tip in a propagation direction upon application of a positive dc voltage on the gate with respect to the emitter tip. The gate forms a gate cavity for propagation of the extracted electrons therethrough. Each of the cold cathode units further includes at least one lens electrode disposed further in the propagation direction from the emitter tip than the gate, the at least one lens electrode forming at least one lens cavity for propagation of the extracted electrons therethrough. The at least one lens electrode is for focusing the extracted electrons in part of the gate cavity, part of the at least one lens cavity, and part of the region therebetween.

The electron beam produced by the field emitter is suitable for use in deflection amplifiers at microwave frequencies, in flat panel displays, in electron-beam lithography, and in the production of small-spot x-ray sources.

Brief Description of the Drawings

These and other objects, features and advantages of the invention, as well as the invention itself, will become better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein like reference numerals designate identical or corresponding parts throughout the several views, and wherein:

FIG. 1 is a side perspective view of an electron deflection device.

FIG. 2 is a diagram of the x-direction normalized emittance associated with emission from a cold cathode unit of FIG. 1.

FIG. 3 is a side view through the z-y plane of the field emitter of FIG. 1.

FIG. 4 demonstrates electron trajectories and equipotential lines for a cold cathode unit, sidewall lens design of FIGS. 1 and 3.

FIG. 5 demonstrates electron trajectories and equipotential lines for a cold cathode unit of FIG. 1 having two lens electrodes of thick focusing grid design.

FIG. 6 is a side view of the deflection and anode sections of the deflection device of FIG. 1 suitable for use as a deflection amplifier.

FIG. 7 is a side view of the deflection and anode sections of the deflection device of FIG. 1 suitable for use as a frequency doubling deflection amplifier.

FIG. 8 is a side view of the anode section of the deflection device of FIG. 1 suitable for use as a deflection amplifier, the anode section being of the push-pull configuration.

FIG. 9 is a side view of the deflection and anode sections of the deflection device of FIG. 1 suitable for use as an electron beam source.

FIG. 10 is a side view of the deflection and anode sections of the deflection device of FIG. 1 suitable for use as an electron beam source.

FIG. 11 is a side view of the deflection and anode sections of a multiple beam deflection device suitable for use as a deflection amplifier.

Detailed Description of the Preferred Embodiment

Referring now to the drawings, FIG. 1. shows a field emitter 200 for producing a collimated electron beam input ribbon R.sub.i (not shown). The device 100 further includes a deflection section 300 integrated with the field emitter 200, the deflection section 300 being responsive to the input signal I.sub.s for deflecting the input ribbon R.sub.i to produce a continuously modulated output ribbon R.sub.cm (not shown) having a time-varying path. The device 100 further includes an anode section 400 responsive to the continuously modulated output ribbon R.sub.cm for producing the output signal U.sub.s. The device 100 is preferably encapsulated in a vacuum on the order of 10.sup.-6 -10.sup.-9 Torr.

The field emitter 200 is a linear array of a plurality of cold cathode units 210 disposed along a y-axis. The field emitter 200 produces an input ribbon R.sub.i propagating in the z-direction, that is, upwards as shown in FIG. 1. The x and y axes are orthogonal with respect to the z-direction. The input ribbon R.sub.i is elongated along the y-axis and, as discussed below, is relatively thin in the x-direction.

The field emitter 200 produces a highly collimated input ribbon R.sub.i. The trajectories of all electrons in a perfectly collimated beam are parallel to each other. A highly collimated beam has low normalized emittance. As defined in Tang, C. M. et al., "Field-Emission Arrays--a Potentially Bright Source," in "Nuclear Instruments and Methods in Physics Research" A318 (1992) 353-357, North Holland, which is incorporated herein by reference, and as discussed further below, normalized emittance .di-elect cons..sub.n is determined with reference to a phase space emittance diagram, such as the diagram shown in FIG. 2. The abscissa of FIG. 2 is displacement parallel to the x-axis, and the ordinate is (.beta...gamma...theta.) which is the product of the relativistic factors .beta., .gamma. and .theta., where ##EQU1## c is the speed of light in vacuo, and the vector v having components v.sub.x, v.sub.y, and v.sub.z is the velocity of electrons emitted from the field emitter 200.

The emittance diagram shown in FIG. 2 is determined for a cold cathode unit 210 with respect to a fixed z-displacement (FIG. 1). As the y-displacement (FIG. 1) is varied for a fixed x-displacement (FIG. 1), the product .beta...gamma...theta. for each electron falls within a range as shown by the line segment r. The product .beta...gamma...theta. stays within the range r as the y-displacement is increased, until it reaches a sharp boundary past which there is no further electron emission. The area occupied by the electrons in (x, .beta...gamma...theta.) phase space on this diagram, that is, the area of the shaded propeller-type shape in FIG. 2, is the normalized emittance .di-elect cons..sub.n in the x-direction. The normalized emittance .di-elect cons..sub.n (the area) does not vary in the paraxial limit as the z-displacement is varied.

For nonrelativistic propagation of electrons, .beta...gamma...theta..apprxeq.v.sub.x /c. For a given beam x-thickness, the more collimated the input ribbon R.sub.i, the smaller the maximum value for .beta...gamma...theta., and therefore, the more horizontal the shaded shape. A perfectly collimated beam has zero normalized emittance. For purposes of this invention, the normalized emittance in the x-direction associated with the field emitter 200 is on the order of a few 10.sup.-3 .pi. millimeter-milliradians (mm-mrad). Normalized emittance of at most 10*10.sup.-3 .pi. mm-mrad is considered sufficiently low, but normalized emittance of at most 2*10.sup.-3 .pi. mm-mrad is preferable, and normalized emittance of 10.sup.-3 .pi. mm-mrad is most preferable. Cold cathode units 210 with normalized emittance as high as 1000*10.sup.-3 .pi. mm-mrad (which is less than that obtained from most thermionic cathodes) could also be used, but yield lower gain-bandwidth product and are not as effective at frequencies above 1 gigaHertz (GHz).

Collimation is a measure of the extent to which the input ribbon R.sub.i approaches a perfectly collimated beam, that is, an electron ribbon in which the electron trajectories are parallel to each other. As discussed earlier, a highly collimated beam has low normalized emittance. An input ribbon R.sub.i with normalized emittance .di-elect cons..sub.n of 10.sup.-3 .pi. mm-mrad and an x-thickness of 4 micrometers (.mu.m) would be considered highly collimated if it had a collimation angle=arctan(v.sub.x /v.sub.z) of 5.degree.. Highly collimated beams are capable of being focused to a very thin sheet within this device, thereby enhancing the performance of this deflection device. In fact, the electron deflection device of the present invention serves to keep the electron beam focused to a very thin sheet within this device.

Further desirable specifications of the input ribbon R.sub.i produced by the field emitter 200 are that it have energy on the order of tens of electron volts (eV), preferably below 200 eV and most preferably below 50 eV, with energy spread under 0.2 eV, and that the input ribbon have thickness in the x-direction of under about 200 .mu.m and most preferably under about 10 .mu.m. Furthermore, the input ribbon R.sub.i has current density J between 20 and 1000 Ampere/cm.sup.2 (A/cm.sup.2), preferably on the higher end of this range. In other words, the field emitter 200 preferably produces a high current density input ribbon. Although not absolutely required for the practice of this invention, the total current emitted from the field emitter 200 may be on the order of 1 .ANG.but may be more or less depending on the application desired. For example, a 100 eV input ribbon R.sub.i. with current of 40 milliAmps (mA) and 25% rf efficiency will produce 1 Watt rf power. The cold cathode units 210 may be spaced apart in the y-direction by as little as 2 .mu.m. Each cold cathode unit 210 produces a beamlet with up to about 100 micro Amperes (.mu.A) current.

Referring now to FIG. 3, an exemplary field emitter 200 is a vacuum cone-type field emitter. This invention may also be practiced using other cold emitter designs known to persons of ordinary skill in the art, such as one dimensional field emitter wedges (with or without lens electrodes) and one dimensional field emitter edges (with or without lens electrodes).

The field emitter 200 includes a plurality of cold cathode units 210 regularly spaced along the y-axis. Each cold cathode unit 210 emits an electron beamlet (not shown) from the tip 215 of an emitter cone 220, the propagation direction of the electron beamlet being generally parallel to the z-axis and in the direction from the base of the emitter cone 220 to the emitter tip 215. The emitter cone 220 is mounted on a conducting substrate 230 and is therefore at the same voltage as the substrate 230. The substrate 230 is common to all the cold cathode units 210. The plurality of beamlets emitted by the plurality of cold cathode units 210 constitutes the input ribbon R.sub.i.

A gate or grid 240 for extracting the electron beamlet is positioned apart from the emitter cone 220. A first lens electrode 250 for focusing the electron beamlet is positioned above the gate 240, that is, further in the propagation direction from the emitter tip 215 than the gate 240. The first lens electrode 250 provides beam collimation, i.e. the voltage gradient between the gate 240 and the first lens electrode 250 results in focusing of the beamlet. The gate 240 and first lens electrode 250 can be circular lenses, linear lenses, or other appropriate designs. The gate 240 and first lens electrode 250 each form a cavity 245 and 255, respectively, for propagation of the electron beamlet therethrough. The beam electrode focusing occurs in part of the lens electrode cavity 255 and in part of the space between the lens electrode cavity 255 and the gate cavity 245. The gate 240 need not be a lens. The gate 240 is mounted on an insulating spacer 260 which spacer 260 is thus between the substrate 230 and the gate 240, and the first lens electrode 250 is mounted on an insulating spacer 270 on the gate 240, which spacer 270 is thus between the gate 240 and the first lens electrode 250. The insulating spacers 260 and 270 are typically silicon dioxide (SiO.sub.2). Optionally, one or more additional lens electrodes 280 (FIG. 5) may be positioned above the first lens electrode 250 to further focus the beamlet. An example of a double lens electrode design is discussed further below.

The emitter tip 215 has a very small radius of curvature, for example, on the order of 185 .ANG. or less. Upon application of a relatively low electric potential drop between the gate 240 and the emitter cone 220, a high electric field is produced at the emitter tip 215, causing electron emission from the emitter tip 215. The voltage of the emitter tip 215, which is the same as the voltage of the substrate 230, is considered the reference voltage herein. The emitter cone 220 need not be heated as thermionic cathodes must be, and in fact it may be operated at room temperature. Electrons are continuously extracted from the emitter cone 220 by application of a constant positive dc voltage to the gate 240, which is positioned near the emitter tip 215. The field emitter 200 is designed so that the emitted electrons do not intercept either of the insulator spacers 260 and 270.

Referring now to FIG. 4, an example of a single lens electrode design for the field emitter 200 of FIG. 1 and 3 is shown. In this design, the gate 240 and the first lens electrode 250 are self-aligned. This example is referred to as having a sidewall lens. The generally horizontal and vertical lines of FIG. 4 depict electron trajectories and equipotentials, respectively, as determined by computer simulation. These trajectories and potentials are shown in cross-section through the x-z plane with the y-displacement selected so the x-z plane passes through the emitter tip 215. The origin of the x-z plane is selected so as to coincide with the emitter tip 215.

In this example, the surface of the emitter cone 220 forms an angle of 35.degree. with respect to the vertical. Gate voltage V.sub.g =100 V is applied to the gate 240, and the emitter cone 220 is considered ground, i.e., the reference voltage. V.sub.g is optimally between 90 V and 130 V. A first voltage, V.sub.1 =3 V, optimally 3-9 V, is applied to the first lens electrode 250, and so the input ribbon R.sub.i has 3 eV energy. The gate 240 has thickness t.sub.g of 0.4 .mu.m and the emitter tip 215 is t.sub.tip =0.15 .mu.m above the bottom of the gate 240. The bottom of the first lens electrode 250 is t.sub.g,1 =0.5 .mu.m above the top of the gate 240. The minimum diameters d.sub.g and d.sub.1 of the gate cavity 245 and the first lens electrode cavity 255 perpendicular to the z-axis are 1.3 .mu.m and 1.8 .mu.m, respectively. The thickness of the first lens electrode 250 is t.sub.1 =2.6 .mu.m, which is related to d.sub.1 as t.sub.1 =1.44*d.sub.1. As shown in FIG. 4, the inside edge of the gate 240 and the first lens electrode 250 are each at an opening angle .PHI.=15.degree. with respect to the z-axis, and these inside edges fall along the same conic section. As further shown in FIG. 4, the inside opening of the lens electrode 250 is tapered and becomes larger as the distance from the emitter tip 215 increases. The distance between the gate 240 and the substrate 230 is t.sub.s,g =0.7 .mu.m. The thickness t.sub.1 of the first lens electrode 250 (2.6 .mu.m) is considerably greater than the gate thickness t.sub.g (0.4 .mu.m) and than the distance between the gate 240 and the first lens electrode 250 (0.5 .mu.m). As described further below, an accelerating electric field of 2 V/.mu.m, optimally 2-10 V/.mu.m, is applied in the z-direction beyond the first lens electrode 250.

Further design considerations of this sidewall lens field emitter 200 shown in FIG. 3 are set forth in the following

TABLE 1.

A sidewall lens field emitter 200 can be non-aligned as well as self-aligned, although the latter is preferable in terms of production. In a self-aligned cold cathode unit 210, the gate cavity 245 and first lens cavity 255 are axially symmetric with respect to the axis of the emitter 220. Manufacture of a self-aligned field emitter 200 generally takes fewer fabrication masks than manufacture of a non-aligned field emitter 200.

Referring now to FIG. 5, an example of a double lens electrode design for the field emitter 200 is shown. This example is referred to as having a thick focusing grid. In addition to having a first lens electrode 250, this structure also has a second lens electrode 280 positioned above the first lens electrode 250, that is, further from the substrate 230. The second lens electrode 280, just as the gate 240 and the first lens electrode 250, preferably forms a cavity 285 for propagation of the electron beamlet therethrough. The thick focusing grid cold cathode unit 210 is preferably self-aligned. The generally horizontal and vertical lines of FIG. 5 depict electron trajectories and equipotentials, respectively, as determined by computer simulation. These trajectories and potentials are shown in cross-section through the x-z plane with the y-displacement selected so that the x-z plane passes through the emitter tip 215. The origin of the x-z plane is selected so as to coincide with the emitter tip 215. The optimal and preferred specification ranges for this design are set forth in the following

TABLE 2.

A feature of this thick-grid design is that the thickness t.sub.g of the gate 240 is slightly less than its maximum inside diameter d.sub.gtop. In the most preferred embodiment as stated on TABLE 2, the gate opening angle is 22.5.degree.. In other words, the gate cavity 245 formed by the gate 240 is a conic frustrum bounded on opposite ends by surfaces with smaller and larger diameter. The gate cavity 245 opens out in the direction of propagation of the electron beamlet so the bottom surface is the one with smaller diameter d.sub.gbot and the top surface is the one with larger diameter d.sub.gtop. The emitter tip 215 is preferably inside the gate cavity 245, and most preferably near the bottom surface of the gate cavity 245, i.e. within 0.02 .mu.m of the bottom surface. The emitter tip can even be slightly below the bottom surface. As specified on TABLE 2, the emitter tip 215 is preferably near the bottom surface of the conic frustrum 245 or inside the gate cavity 245. The gate thickness t.sub.g is in the range of approximately 0.45.multidot.d.sub.gbot .ltoreq.t.sub.g .ltoreq.d.sub.gtop and most preferably t.sub.g =0.7 .mu.m=0.63.multidot.d.sub.gbot. As the emitter tip 215 is moved further up inside the gate 240, that part of the gate 240 which is above the emitter tip 215 has height t.sub.g -t.sub.tip, and is in the range 0.45.multidot.d.sub.gbot .ltoreq.t.sub.g -t.sub.tip .ltoreq.d.sub.gtop. These specifications are to be compared with the sidewall lens specification. As set forth in the earlier discussion for the most preferred sidewall lens embodiment, d.sub.g =1.3 .mu.m and t.sub.g =0.4 .mu.m, so t.sub.g =0.31.multidot.d.sub.g.

It is intended that the gate electrode 240 provide not only extraction of the electron beam, but also focusing. Comparison of electron trajectories in FIGS. 4 and 5 depicting the side-wall lens design and the two lens electrode thick-grid design, respectively, shows that there is considerably more focusing inside the gate cavity 245 and near the emitter tip 215 in the thick-grid design than in the sidewall lens design. With proper choice of bottom and top diameters d.sub.gbot and d.sub.gtop, respectively, grid thickness t.sub.g, and voltage settings V.sub.g, V.sub.1, v.sub.2 and accelerating voltage, the grid 240 will act as a lens. Too thin a grid electrode produces no focusing effect near the emitting tip, while too thick a grid electrode produces focusing fields too far downstream from the emitting tip 215. As stated above, an approximate parameter with which to start design is for the grid thickness above the emitter tip 215 t.sub.g -t.sub.tip to be slightly less than the top grid diameter d.sub.gtop.

As further indicated on TABLE 2, a higher voltage gradient (V.sub.1 -V.sub.g)/t.sub.g,1 between the grid 240 and the high voltage or first lens electrode 250 provides better collimation. This voltage gradient (V.sub.1 -V.sub.g)/t.sub.g,1 is limited by breakdown voltage V.sub.270 of the insulating spacer 270.

Referring back to FIG. 1, the field emitter 200 can be used in an electron deflection device 100 responsive to an electrical input signal I.sub.s (not shown) for producing an output signal U.sub.s (not shown). The electron deflection device 100 is designed for optimal use at microwave frequencies, i.e., in the range of about 10.sup.9 Hz (1 GHz) to about 3*10.sup.11 Hz (300 GHz), but it can also be used at lower frequencies.

Referring now to FIGS. 6 and 1, the deflection section 300 of an electron deflection device 100 includes first and second electrically conductive deflector faces 310 and 320, respectively disposed symmetrically with respect to the y-axis so as to face inward towards the y-axis. The first and second deflector faces 310 and 320 are supported on first and second deflector plates 330 and 340, respectively. An input electrical circuit (not shown) responsive to the electrical input signal I.sub.s is connected to the deflector plates 330 and 340 for applying electric potentials .PHI..sub.A and .PHI..sub.B to the first and second deflector faces 310 and 320, respectively.

The deflector faces 310 and 320 form a vacuum deflection region 350 therebetween, and are disposed so that the z-axis passes through the deflection region. As a result of the input electrical circuit's application of potentials .PHI..sub.A and .PHI..sub.B to the deflector faces 310 and 320, respectively, the deflection section 300 produces an electric field E in the deflection region 350.

The electrical input circuit provides time varying potentials .PHI..sub.A and .PHI..sub.B so that the potential difference .PHI..sub.A -.PHI..sub.B, i.e. the modulation voltage, and therefore the x-component E.sub.x of the electric field E in the deflection region 350, is proportional to the ac component of the input signal I.sub.s. The maximum amplitude of the modulation voltage, i.e. the variation .DELTA.V.sub.m of the modulation voltage, is typically on the order of a tenth of a Volt to a few Volts, and preferably less than 4 Volts. The peak of the electric field component E.sub.x is typically in the range 5-500 mV/.mu.m. This small modulation voltage makes higher frequency operation and higher gain feasible.

The dc values of the applied potentials .PHI..sub.A and .PHI..sub.B can provide an appropriate field between the field emitter 200 and the deflector sector, such as the accelerating field referred to earlier.

It is preferable that the potentials .PHI..sub.A and .PHI..sub.B be applied to the deflector faces 330 and 340, respectively, so that the potentials propagate in the x-direction, but they can also be applied so as to propagate in the y-direction.

The modulated electric field E transversely deflects the input ribbon R.sub.i in the x-direction to produce a continuously modulated output ribbon R.sub.cm. Although the input ribbon R.sub.i is propagated with constant velocity in the z-direction, the deflection section 300 transversely deflects the input ribbon R.sub.i to produce the continuously modulated output ribbon R.sub.cm propagating in time-varying directions. The deflection section 300 provides a modulated x-component of the beam velocity of ribbon R.sub.cm.

The specifications of the deflection section 300 are selected so as to satisfy the above conditions. In order to effectively deflect the input ribbon R.sub.i in the x-direction, the deflector faces 310 and 320 should be at least as long in the y-direction (perpendicular to the x-z plane) as the input ribbon R.sub.i is. This y-dimension is approximately the size of the field emitter 200 in the y-direction.

In order to effectively modulate the input ribbon R.sub.i at high frequencies, the transit time L.sub.z /v.sub.z of electrons passing through the deflection region 350 should be short with respect to 1/f, where L.sub.z is the z-dimension of the deflector faces 320 and 340, v.sub.z as stated earlier, is the z-component of the electron velocity, and f is the frequency of the ac component of the input signal I.sub.s. The z-dimension L.sub.z of the deflector faces 310 and 320 is typically on the order of tens of .mu.m. The factor .theta.=.omega.L.sub.z /v.sub.z is a normalized measure of the z-dimension of the deflector faces 320 and 340, where .omega.=2.pi.f is the angular frequency of the input signal I.sub.s. The factor .theta. is preferably in the range 0<.theta..ltoreq..pi..

The gap in the x-direction between deflector faces 310 and 320, i.e., the x-size of the deflection region 350, should be as small as possible and yet large enough to accommodate the width of the input ribbon R.sub.i (which is preferably under 200 .mu.m and most preferably under 10 .mu.m) and provide room for deflection so that the input ribbon R.sub.i does not physically intercept any structural part of the deflection section 300. A small gap between deflector faces 310 and 320 maximizes the electric field component E.sub.x in the deflection region 350 so as to maximize deflection of the input ribbon R.sub.i and thereby maximize the amplifier gain.

Space charge effects become significant as the line current density .DELTA.I/.DELTA.y of the input ribbon R.sub.i increases, based on the following relation: ##EQU2## where e is the absolute value of the elementary electron charge, m.sub.e is the electron mass, .di-elect cons..sub.0 is the free space permeability, eV.sub.i is the electron energy in units of electron Volts (eV), D is the gap between deflectors 330 and 340, and the electron beam is located in the center between the deflectors 330 and 340. For eV.sub.i =10 eV, D=8 .mu.m, and the average potential of deflectors 330 and 340 is 10 V, then the space charge effect becomes important as the line current density exceeds 10 .mu.A/.mu.m, and becomes detrimental as the line current density approaches 30 .mu.A/.mu.m. Therefore, the field emitter 200 produces an input ribbon having line current density as large as possible but less than (.DELTA.I/.DELTA.y).sub.max as specified in the above relation.

A feature of the electron deflection device 100 is that the input ribbon R.sub.i produced by the field emitter 200 is focused to a very thin sheet, and the continuously modulated output ribbon R.sub.cm produced by the deflection section 300 remains focused to a very thin sheet as far as the anode section 400.

The anode section 400 responsive to the continuously modulated output ribbon R.sub.cm for producing the output signal U.sub.s includes a first anode 410 and a second anode 420 positioned and designed so that the electron beam R.sub.cm intercepts the first anode 410 during part of the deflection cycle and the second anode 420 during part of the deflection cycle. The first anode 410 is loaded with potential .PHI..sub.1 and impedance Z.sub.1, and the second anode 420 is loaded with potential .PHI..sub.2 and impedance Z.sub.2. As a result of the electron beam R.sub.cm intercepting the anodes 410 and 420, the anode section 400 produces first and second electrical output signals U.sub.1 and U.sub.2. respectively. These signals U.sub.1 and U.sub.2 can be selectively combined to constitute the output signal U.sub.s.

The first and second anodes 410 and 420 are each disposed in the path of the continuously modulated output ribbon R.sub.cm. The first anode 410 is disposed closer to the deflection section 300 than the second anode 420.

The first anode 410 is loaded with potential .PHI..sub.1 selected so that it is on an equipotential surface with respect to the second anode 420. For example, if the second anode 420 were loaded at .PHI..sub.2 =150 V and were disposed 85 .mu.m from a deflection section 300 having a dc bias of 10 V, then a first anode 410 disposed 45 .mu.m from the deflection section 300 would be loaded at ##EQU3## The applied potentials .PHI..sub.1 and .PHI..sub.2 are further selected so that .PHI..sub.A, .PHI..sub.B <<.PHI..sub.1 -I.sub.1 .multidot.Z.sub.1 and .PHI..sub.1 .apprxeq.<.PHI..sub.2 -I.sub.2 .multidot.Z.sub.2, where I.sub.1 and I.sub.2 are the peak total current to the first and second anodes 410 and 420, respectively. These conditions minimize anode loading problems caused by electron repulsion from the anodes due to voltage depression resulting from output current I.sub.1 or I.sub.2.

The second anode 420 can be a single plate. The first anode 410 provides a slit 430 for passage of the continuously modulated output ribbon R.sub.cm to pass therethrough during part of the deflection cycle. As shown in FIG. 6, the first anode 410 includes sections 440 and 450 with the slit 430 therebetween, asymmetrically positioned and designed so that the continuously modulated output ribbon R.sub.cm intercepts section 440 during part of the deflection cycle and does not intercept section 450 during any part of the deflection cycle. Sections 440 and 450 can be separate plates or different sections of a single plate containing a slit 430. Section 450 serves to balance the electric field near the anode section 400 so that equipotential surfaces are generally perpendicular to the path of the continuously modulated output ribbon R.sub.cm.

Referring now to FIG. 7, a configuration of anode section 400 is shown in which the continuously modulated output ribbon R.sub.cm strikes the first anode 410 during at least two separate parts of the deflection cycle and passes through the slit 430 during another part of the deflection cycle. Sections 440 and 450 can be symmetrically disposed about the path of the continuously modulated output ribbon R.sub.cm. Just as in the configuration shown in FIG. 6, sections 440 and 450 can be separate plates or different sections of a single plate containing a slit 430. The output signal U.sub.s for such a configuration has a frequency twice that of the input signal I.sub.s frequency. The load parameters .PHI..sub.1, .PHI..sub.2, Z.sub.1 and Z.sub.2 satisfy the same conditions as the device shown in FIG. 6.

It is not necessary that the first anode 410 be asymmetric as shown in FIG. 6 in order to produce an output signal U.sub.s of the same frequency as the input signal I.sub.s, or that the first anode 410 be symmetric as shown in FIG. 7 in order to produce an output signal U.sub.s of twice the input signal I.sub.s frequency. The same results could be achieved by applying a bias between the first and second deflector faces 310 and 320 and shifting the slit 430, or by using some combination of bias and slit 430 position.

A person of ordinary skill in the art may readily design an anode section 400 for use in an electron deflection device 100 of the present invention so as to be responsive to the continuously modulated output ribbon R.sub.cm for producing an output signal U.sub.s. For example, rather than loading the first and second anodes 410 and 420 with potentials .PHI..sub.1 and .PHI..sub.1 and impedances Z.sub.1, and Z.sub.2, respectively, inductive bias chokes (not used) could be used. As another example, anode sections 440 and 450 could be loaded with different impedances.

As a further example, the push-pull configuration of U.S. Pat. No. 2,553,753 to Adler, designed for use with a thermionic emitter, may be readily adapted for use with a cold cathode unit 200 (FIGS. 1, 3). Referring now to FIG. 8, an acceleration voltage, for example 90 V, is applied to an accelerating electrode 460, which includes slots 462 and 466 for passage of the continuously modulated output ribbon R.sub.cm therethrough. A beam splitting element 470 serves to divide the projected beam R.sub.cm into two portions so that the ribbon R.sub.cm intercepts the first anode 410 during part of the deflection cycle and the second anode 420 during another part of the deflection cycle. The first anode 410 and the second anode 420 are connected across a primary winding 482 of a center-tap transformer 480, the center tap being held at the accelerating voltage. A secondary winding 484 of the transformer 480 produces the output signal U.sub.s.

The above designed electron deflection device 100 may be used as in amplifier in the microwave range. Other uses may be readily determined by a person of ordinary skill in the art. For example, it may also be used as an oscillator in the microwave range. When so used, the factor .theta.=.omega.L.sub.z /v.sub.z is preferably in the range 2.pi.<.theta..ltoreq.2.86.pi., and most preferably .theta.=2.57.pi..

Referring now to FIGS. 9 and 1, an electron deflection device 100 is shown which is suitable for use as a modulator or beam source to produce a chopped output ribbon U.sub.s. The output ribbon is suitable for input to a high power amplifier, such as a klystron, klystrode, traveling wave tube, distributed amplifier or a gigatron, or to a free electron laser.

The anode section 400 of the device shown in FIG. 9 comprises a first anode 410 comparable to the first anode 410 of the deflection amplifier shown in FIG. 6 and described earlier. The first anode 410 is positioned and designed so that the electron beam R.sub.cm intercepts the first anode 410 during part of the deflection cycle. The first anode 410 provides a slit 430 for passage of the continuously modulated output ribbon R.sub.cm therethrough during another part of the deflection cycle, thereby producing the output ribbon U.sub.s. The first anode 410 includes sections 440 and 450 with the slit 430 therebetween, asymmetrically positioned and designed so that the continuously modulated output ribbon R.sub.cm intercepts section 440 during part of the deflection cycle and does not intercept section 450 during any part of the deflection cycle. Section 450 serves to balance the electric field near the anode section 400 so that equipotential surfaces are generally perpendicular to the path of the continuously modulated output ribbon R.sub.cm.

Referring now to FIGS. 10 and 1, an electron deflection device 100 is shown which also is suitable for use as a modulator or beam source to produce a chopped output ribbon U.sub.s. The anode section 400 comprises a first anode 410 comparable to the first anode 410 of the deflection amplifier shown in FIG. 7 and described earlier. The anode section 400 is designed so that the continuously modulated output ribbon R.sub.cm strikes the first anode 410 during at least two separate parts of the deflection cycle. Just as in the configuration shown in FIG. 9, the first anode 410 provides a slit 430 for passage of the continuously modulated output ribbon R.sub.cm therethrough during part of the deflection cycle, thereby producing the output ribbon U.sub.s. The output ribbon U.sub.s for such a configuration has a frequency double the frequency of the input signal I.sub.s. Sections 440 and 450 can be symmetrically disposed about the path of the continuously modulated output ribbon R.sub.cm.

A simulated example of this configuration uses a drive frequency of 40 GHz to produce an 80 GHz prebunched chopped output ribbon U.sub.s. In this example, the gap 430 of the first anode 410 is 10 .mu.m. The first anode 410 is 45 .mu.m from the deflector section 430 and is at potential .PHI..sub.1 =90 V. A modulation voltage variation of .DELTA. V.sub.m =4 V is applied to an input ribbon R.sub.i having current of 7 mA.

Referring now to FIG. 11, part of a multiple beam device is shown which is suitable for use as a deflection amplifier. This multiple beam device includes at least 2 units 100 comparable to the devices 100 as shown in FIGS. 1, 3 and 6. The at least two devices 100 are spaced apart in the x-direction and integrated as appropriate so that adjacent continuously modulated output ribbons R.sub.cm are deflected in opposite directions. For example, adjacent field emitters 200 may share a common substrate 230, insulating spacers 260 and 270, and parts of the lens electrode 250. Adjacent deflection sections 300 may have a common deflector plate 340. Adjacent anode sections 400 may have a common second anode 420. The first anode 410 shown in FIG. 11 differs from the first anode 410 shown in FIG. 6 in that adjacent first anodes 410 (FIG. 6) will have a total of two slits 430 and four sections 440 and 450 (FIG. 6), and can be combined to constitute a first anode 410 (FIG. 11) having one slit 430 and two sections 440 (FIG. 11).

Another configuration for a multiple beam device which is suitable for use as a deflection amplifier also includes at least 2 units 100 comparable to the devices 100 as shown in FIGS. 1 and 3 and spaced apart in the x-direction. Unlike the configuration shown in FIG. 11, the adjacent devices 100 are integrated as appropriate so that adjacent continuously modulate output ribbons R.sub.cm are deflected in the same direction. Such a multiple beam device is essentially a combination of devices as shown in FIGS. 1, 3, and 6, 7 or 8. If it includes devices 100 comparable to those shown in FIGS. 1, 3 and 7, the output frequency can be double the input frequency.

Multiple deflection devices suitable for use as a modulator or beam source for producing at least two chopped output ribbons U.sub.s include at least 2 units 100 comparable to the devices 100 as shown in FIGS. 1, 3, 9 and 10. The at least two devices 100 are spaced apart in the x-direction and integrated as appropriate so that adjacent continuously modulate output ribbons R.sub.cm are deflected in opposite directions. Just as shown in FIG. 11, adjacent field emitters 200 may share a common substrate 230, insulating spacers 260 and 270, and parts of the lens electrode 250. Adjacent deflection sections 300 may have a common deflector plate 340 and adjacent anode sections 400 may have common first anodes 410. If it includes devices 100 comparable to those shown in FIGS. 1, 3 and 10, the output frequency can be double the input frequency.

Another design for multiple deflection devices suitable for use as a modulator or beam source for producing at least two chopped output ribbons U.sub.s includes at least 2 units 100 comparable to the devices 100 as shown in FIGS. 1, 3 and 9 or 10. The at least two devices 100 are spaced apart in the x-direction and integrated as appropriate so that adjacent continuously modulate output ribbons R.sub.cm are deflected in the same direction.

The following results have been obtained from computer simulations of this invention using software MAGIC and EGUN2. MAGIC is available from Mission Research Corp., Newington, Va. 22122; EGUN is discussed in W. B. Herrmannsfeldt, "EGUN--An Electron Optics and Gun Design Program," SLAC Report 331 (1988).

Using the configuration of FIG. 6 with an input ribbon having a current of 14 mA and energy 10 eV, the required input power for each deflector is 3.2*10.sup.-4 W while the output power is 2.45*10.sup.-3 W, yielding a power gain of 7.66. The gain bandwidth product is G.DELTA.f=2.2 GHz.

For input ribbon having 2A current, and multiple units being connected together as discussed above as a multiple beam deflection amplifier, the gain bandwidth product is G.DELTA.f=300 GHz, and the power gain is 819. The device operates at an rf power efficiency of 15%.

Further computer simulated examples also use the configuration shown in FIG. 6 and frequency 10 GHz, but have longer deflector faces 310 and 320 in the z-direction as measured by the factor .theta.=.omega.L.sub.z /v.sub.z. In these examples, the line current density is 20 .mu.A/.mu.m, the input ribbon R.sub.i has width .delta.x=10 .mu.m and energy 34 eV, .DELTA. V.sub.m =0.24 V, short circuit current gain bandwidth product f.sub.T =60 GHz, and the gap between deflector faces 310 and 320 is D=20 .mu.m. For .theta.=.pi., L.sub.z =170 .mu.m, .DELTA. V.sub.m =0.24 V, and f.sub.T =60 GHz. For .theta.=.pi./2, L.sub.z =85 .mu.m, .DELTA. V.sub.m =0.34 V, and f.sub.T =155 GHz. As .theta. approaches 0, f.sub.T approaches 260 GHz. This result is further described in C. M. Tang, Y. Y. Lau, T. A. Swyden, "Deflection Microwave Amplifier with Field Emission Arrays," submitted to International Electronics Device Meeting '93, which paper is incorporated herein by reference.

A simulation of 40 GHz amplification uses anode voltages of .PHI..sub.1 =90 V and .PHI..sub.2 =120 V, modulation voltage variation .DELTA. V.sub.m =2 V, input ribbon current 375 mA, and loads Z.sub.1 =Z.sub.2 =200.OMEGA.. The first and second anodes 410 and 420, are 30 .mu.m and 55 .mu.m, respectively, from the deflection section 300. The simulation shows that using multiple units of this configuration to obtain total input beam R.sub.i current of 375 mA provides gain bandwidth product of G.DELTA.f=165 GHz and power gain of 24. The device operates at an rf power efficiency of 24%.

Referring back to FIG. 3, the single lens electrode embodiment for the cold cathode unit 210 may be a thick-grid design. As set forth more fully in TABLE 3, this single lens electrode thick-grid embodiment has the same features as the double lens electrode thick-grid embodiment described above except that it does not have a second lens electrode 280 and that it utilizes an accelerating electric field of at least 10 V/.mu.m.

Because the single lens electrode thick-grid cold cathode unit 210 does not include a second lens electrode 280 to slow down the electron beamlet, the cold cathode unit 210 may produce an electron beamlet with energy on the order of hundreds of electron volts. The input ribbon R.sub.i produced by the field emitter 200 may not be suitable for use in the electron deflection device discussed earlier. However, the input ribbon R.sub.i produced by such a field emitter 200 is collimated and suitable for use in flat panel displays and electron beam lithography

The above described embodiment of a field emitter 200 as including a linear array of a plurality of cold cathode units 210 disposed along a y-axis is for illustration only and is not meant to limit the scope of the invention. The field emitter 200 is for production of an electron beam, not necessarily a ribbon, and it includes at least one cold cathode unit 210 as described above, preferably a thick focusing grid design with at least one lens electrode 240.

A field emitter 200 which includes a single cold cathode unit 210 produces a collimated electron beam suitable for use in small spot x-ray sources, electron-beam lithography, and flat panel displays. A field emitter 200 which includes a 2-dimensional array of cold cathode units 210 is suitable for use in flat panel displays.

It is understood that many other changes and additional modifications of the invention are possible in view of the teachings herein without departing from the scope of the invention as defined in the appended claims.

Claims

What is claimed is:

1. A field emitter for producing an electron beam, said field emitter comprising at least one cold cathode unit, each of said at least one cold cathode units comprising: (a) an emitter cone having an emitter tip; (b) a gate spaced apart from said emitter tip for extracting electrons from said emitter tip in a propagation direction upon application of a positive dc voltage on said gate with respect to said emitter tip, wherein said gate forms a gate cavity for propagation of the extracted electrons therethrough; (c) at least one lens electrode disposed further in the propagation direction from said emitter tip than said gate, said at least one lens electrode forming at least one lens cavity for propagation of the extracted electrons therethrough, said at least one lens electrode being for focusing the extracted electrons in the region comprising at least one part of the gate cavity, at least part of the gate cavity, at least part of the at least one lens cavity, and at least part of the region therebetween; and (d) a substrate; wherein said gate forms a gate cavity in the shape of a conic frustrum disposed rotationally symmetric about the rotation axis of said emitter cone, and oriented so that the radius of the conic frustrum increases along the propagation direction, wherein the conic frustrum is bounded on opposite ends by a bottom surface having a bottom gate diameter, and by a top surface having a top gate diameter; wherein an opening through both the lens and the gate electrode forms a conic frustrum; wherein said emitter tip is disposed in a position at or below the top surface.

2. The field emitter of claim 1 wherein said emitter tip is disposed near the bottom surface.

3. The field emitter of claim 1 wherein said bottom diameter is between at least 0.05 .mu.m and at most 2.5 .mu.m.

4. The field emitter of claim 1 wherein the distance in the propagation direction between the bottom surface and the emitter tip is between at least 0.45 times the bottom gate diameter and at most the top gate diameter.

5. The field emitter of claim 1 wherein the minimum diameter of the at least first lens cavity is between 0.05 .mu.m and 10.0 .mu.m perpendicular to the propagation direction.

6. A field emitter for producing an electron beam, said field emitter comprising at least one cold cathode unit, each of said at least one cold cathode units comprising: (a) an emitter cone having an emitter tip; (b) a gate spaced apart from said emitter tip for extracting electrons from said emitter tip in a propagation direction upon application of a positive dc voltage on said gate with respect to said emitter tip, wherein said gate forms a gate cavity for propagation on the extracted electrons therethrough; and (c) at least one lens electrode disposed further in the propagation direction from said emitter tip than said gate, said at least one lens electrode forming at least one lens cavity for propagation of the extracted electrons therethrough, said at least one lens electrode being for focusing the extracted electrons in the region comprising at least one part of the gate cavity, at least part of the gate cavity, at least part of the at east one lens cavity, and at least part of the region therebetween; (d) a substrate; wherein said gate forms a gate cavity in the shape of a conic frustrum disposed rotationally symmetric about the rotation axis of said emitter cone, and oriented so that the radius of the conic frustrum increases along the propagation direction, wherein the conic frustrum is bounded on opposite ends by a bottom surface having a bottom gate diameter, and by a top surface having a top diameter; wherein said emitter tip is disposed in a position at or below the top surface; wherein said emitter is disposed near the bottom surface; wherein said bottom diameter is at least 0.05 .mu.m and at most 2.5 .mu.m; wherein an opening through both the lens and the gate electrode forms a conic frustrum; wherein the thickness of said gate in the propagation direction is between at least 0.45 times the bottom gate diameter and at most the top gate diameter.

7. A field emitter for producing an electron beam, said field emitter comprising at least one cold cathode unit, each of said at least one cold cathode units comprising: (a) an emitter cone having an emitter tip; (b) a gate spaced apart from said emitter tip for extracting electrons from said emitter tip in a propagation direction upon application of a positive dc voltage on said gate with respect to said emitter tip, wherein said gate forms a gate cavity for propagation of the extracted electrons therethrough; and (c) at least one first lens electrode disposed further in the propagation direction from said emitter tip than said gate, said at least one lens electrode forming at least one lens cavity for propagation of the extracted electrons therethrough, said at least one lens electrode being for focusing the extracted electrons in the region comprising at least part of the gate cavity, at least part of the at least one lens cavity, and at least part of the region therebetween; wherein each of said cold cathode units comprises at least the first and a second lens electrode for focusing the extracted electrons upon application of a voltage on each of said at least first and second lens electrodes with respect to said emitter tip and with respect to said gate, said first lens electrode being disposed closer to said gate than said second lens electrode, wherein said gate is disposed closer to said emitter tip than both said first and second lens electrodes, said first and second lens electrodes forming a first and a second lens electrode cavity, respectively for propagation of the extracted electron lens therethrough; wherein said gate forms a gate cavity in the shape of a conic frustrum disposed rotationally symmetric about the rotation axis of said emitter cone, and oriented so that the radius of the conic frustrum increases along the propagation direction, wherein the conic frustrum is bounded on opposite ends by a bottom surface having a bottom gate diameter and by a top surface having a top gate diameter, the bottom gate diameter being between at least 0.08 .mu.m and at most 2.5 .mu.m; and the thickness of said gate in the propagation direction is between at least 0.45 times the bottom gate diameter and at most the top gate diameter.

8. The field emitter of claim 7 wherein: the minimum diameter of the first lens cavity is between at least 0.08 .mu.m and at most 10.0 .mu.m perpendicular to the propagation direction; the minimum diameter of the second lens cavity is between at least 0.08 .mu.m and at most 10.0 .mu.m perpendicular to the propagation direction; and the ratio of the thickness of said second lens electrode to the minimum diameter of the second lens electrode cavity is less than 2.0.

9. A cold field emitter system for producing a collimated electron beam, said field emitter system comprising: a substrate; a field emitter on said substrate; a gate spaced apart from said emitter substantially in the direction of beamflow, said gate having a gap disposed to permit passage of said beam therethrough; a lens section spaced apart from said emitter and said gate, substantially in the direction of beamflow, said lens having a gap, said gap disposed to permit passage of said beam therethrough; wherein the thickness of said lens in the direction of beamflow is about that of a diameter of said gap of said lens; wherein said emitter, responsive to said gate, emits said beam, said beam propagating to said gate and through gap in said gate, to said lens and through gap in said lens; wherein an opening through both the lens and the gate electrode forms a conic frustrum.

10. The field emitter system in claim 9 wherein the tip of said emitter protrudes from said substrate in the form of a conic shape.

11. A cold field emitter system for producing a collimated electron beam, said field emitter system comprising: a substrate; a field emitter wherein said emitter protrudes from the substrate and forms a conic shape; a gate spaced apart from said emitter displaced substantially in the direction of beamflow; a lens section spaced apart from said emitter and said gate, substantially in the direction of beamflow, said gate having a gap, said lens section having a gap; wherein the thickness of said lens section is about that of a diameter of said gap in said lens such that said lens gap and said gate gap form a conic frustrum disposed in a rotationally symmetric relationship to said emitter tip and oriented such that the smaller end of said chamber is proximate to the emitter tip; wherein an opening through both the lens and the gate electrode forms a conic frustrum.

12. A cold emitter system for producing a collimated electron beam, said field emitter system comprising: a field emitter; a gate spaced apart from said emitter substantially in the direction of beamflow, said gate having a gap disposed to permit passage of said beam therethrough; at least one lens section spaced apart from said emitter and said gate substantially in the direction of beam flow, said at least one lens having a gap, said gap disposed to permit passage of said beam therethrough; wherein the thickness of said at least one lens in the direction of beamflow is about that of a diameter of said gap of said lens; wherein said emitter, responsive to said gate, emits said electron beam, said electron beam propagating to said gate and through said gap in said gate, to said at least one lens and through said gap in said at least one lens; wherein an opening through both the lens and the gate electrode forms a conic frustrum.

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Abstract

Description