Field emission devices

A klystron device comprises an array of cold-cathode field emission elements arranged to form a distributed amplifier which further comprises a modulation strip line and a catcher strip line. A collector electrode is spaced from the catcher strip line. The device may include a deflector for returning electrons emitted by the elements back to the modulation strip line so that the device acts as an oscillator.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to field emission devices, and particularly to 
amplifier and oscillator devices which rely on field emission. 
2. Description of Related Art 
Although high-power microwave and millimeter-wave circuits having 
invariably involved the use of thermionic vacuum devices, most low-power 
high-frequency devices are now formed by conventional solid state 
techniques. 
Transit time induced limitation of high frequency performance in vacuum 
electronic devices can usually be made negligibly small because of the 
ballistic electron motion in a vacuum. However, just as in solid state 
devices, the ultimate speed of operation of a vacuum device is likely to 
be capacitance limited. In conventional large-scale vacuum electronic 
devices, a number of particular designs have been developed to overcome 
this limitation. These designs involve some combination of velocity 
modulation and distributed amplification. 
The combination of velocity modulation and a relatively long drift space 
can result in a spatial separation of fast and slow electrons. The 
bunching of electrons occurring as faster electrons overtake slower 
electrons emitted earlier can produce an approximately 50% modulation of 
the current at the frequency of a small modulating signal applied thereto. 
This forms the operational basis of the klystron. The main limitations to 
the gain available from such device are the energy spread of the electron 
beam prior to modulation and control of the momentum of the electrons both 
before and after modulation. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a small microwave or 
millimeter-wave device which is fabricated by semiconductor fabrication 
techniques, but which produces an electron beam in vacuum to allow 
high-frequency amplification or oscillation analogous to that of a 
klystron vacuum tube. 
According to the invention there is provided a device of the klystron type, 
comprising an array of cold-cathode field emission elements arranged to 
form a distributed amplifier. 
The distributed amplifier may be of a travelling wave type or of a standing 
wave (cavity) type. 
The distributed amplifier preferably comprises a modulation strip line to 
which an input modulation signal is applied, and a catcher strip line from 
which an amplified output signal is obtained. Alternatively, a modulation 
strip line may be provided, and electron flow in the elements may be fed 
back to the modulation strip line whereby the device acts as an 
oscillator. The feedback may be caused by bending of the electron beams in 
the elements under the influence of an electric field and/or a magnetic 
field. In the case of travelling wave amplification, the catcher strip 
line is preferably made of uniform impedance to minimise reflection and to 
allow the continuous build-up of an amplified travelling wave. 
Alternatively, the catcher strip line may have specific impedance 
discontinuities to induce reflections and to allow the build-up of an 
amplified standing wave with the output being provided by the residual 
transmission at at least one of the impedance discontinuities.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In a device in accordance with the invention a field emission electron 
source preferably comprises an array of low-voltage field emitters in the 
form of sharp-tipped cathodes. Field emission provides an electron energy 
spread of about 0.25 eV, which is considerably lower than that of 
thermionic cathodes. A single field emitter may also tend to have a very 
small angular spread of emission, which is considered to result from the 
strong anisotropy of the work function of the emitter material. For an 
array comprising multiple emitter tips, unless all of the tips have 
identical crystallographic orientation, and therefore identical work 
function anisotropy, the array will probably give a large statistical 
spread of emission angles. In order to minimise the resulting spread of 
longitudinal electron velocities, a cathode/grid structure used in the 
present invention preferably contains an integrated lens which produces 
collimation. 
FIG. 1 of the drawings shows, schematically, such a cathode/grid structure 
1. The structure comprises a substrate 2 on which is formed a cathode tip 
3 of, say, 2 .mu.m height, an extraction grid 4, a lens grid 5 and an 
energy boosting grid 6. The grid spacings may be, for example, 1 .mu.m. In 
use, the grids 4,5 and 6 might typically be biased at +200 volts, +1 volt 
and +100 volts, respectively, relative to the cathode tip 3, and the 
resulting electron trajectories 7 are indicated schematically. It will be 
seen that the electron beam leaving the structure is substantially 
collimated. 
The substrate 2 may be formed of silicon, which may be coated with a metal, 
such as niobium, molybdenum, platinum, tungsten or gold. Many of the 
cathode tips are formed simultaneously in an array by masking and etching 
the substrate material. The cathode tips are then covered with a layer 8 
of dielectric material, such as silicon dioxide, which is then planarised 
by etching. Alternatively, the layer 8 may be formed of other insulating 
material and may be of multilayer construction which may be chosen 
specifically to minimise problems of thermal expansion mismatch. Such 
layers might be, for example, of phophorus or boron-doped silicon dioxide 
or of silicon nitride. A conductive layer or multilayer is then formed 
over the dielectric layer. The layer may be of, for example, niobium, 
molybdenum, heavily-doped silicon or a silicon aluminium alloy. The 
conductive layer is then selectively masked and the unmasked areas are 
removed by etching, leaving a hole in the layer immediately above each 
tip. The remainder of the conductive layer forms the extraction grid 4. 
Similarly, alternate dielectric and conductive layers are deposited, and 
the masking and etching processes are repeated, to form the lens grid 5 
and the energy boosting (accelerator) grid 6. The underlying dielectric 
layers are then etched by a dry., e.g. plasma, etching process, using the 
conductive layer as a mask, until the cathode tips are reached. Any oxide 
remaining immediately adjacent to each tip is then removed by a wet 
etching process, in order to avoid damaging the tips. Hence, the cathode 
tips are revealed through apertures in the dielectric and conductive 
layers. 
FIG. 2 shows, schematically, a cross-section through a distributed 
amplifier device 9 in accordance with the invention. The device preferably 
includes a cathode/grid structure 1 comprising an array of cathode tips 
with associated grids, mounted on a substrate 2, as just described. A 
modulation microstrip transmission line structure 10, formed as described 
below, is spaced from the structure 1 by an annular dielectric spacer 11. 
A drift space 12 is formed within an annular dielectric spacer 13 which is 
bonded to the structure 10. A catcher microstrip transmission line 
structure 14, of similar construction to the structure 10, is mounted on 
the spacer 13. A collector anode 20 is spaced from the catcher line by an 
annular dielectric spacer 15. 
A modulation input signal is fed into one end of the modulation strip line 
via input leads 16 and 17, and an amplified output signal is taken from 
the catcher stripline via leads 18 and 19. 
For a given modulation frequency f, beam velocity v and velocity modulation 
.delta.v produced by a signal on the modulation stripline 10, the length s 
of the drift space 12 for optimum beam current modulation is given 
approximately by 
##EQU1## 
Hence, the required length of the device decreases with increasing 
frequency. For 100 GHz operation with a 200 volt electron beam amplifying 
a 1 mW signal on a 50 .OMEGA. modulation strip line, s is about 4 mm. For 
such parameters the gap between the modulation strip line and the ground 
plane (described below) must also be small, for example about 10 .mu.m or 
a few tens of .mu.m, so that the transit time is negligibly small compared 
with the signal period. This in turn requires that the 50 .OMEGA. line 
width shall be similarly small, for example about 100 .mu.m or a few 
hundred .mu.m. These dimensions allow monolithic integrated fabrication, 
but to provide sufficient current for power amplification this implies the 
use of a long transmission line with the cathode, modulation, drift and 
current pick-up distributed along it. 
For this reason the catcher and modulation strip lines are matched to allow 
coherent distributed amplification. Due to this symmetry, it may be 
convenient to replace half of the drift space, the catcher and the 
collector anode by a retarding reflection anode to return the beam to the 
modulation grid, thereby producing a "reflex klystron" oscillator, as will 
be described below, or with an electro-static mirror or magnetic mirror to 
return the beam to a matched catcher strip line running parallel to the 
modulation stripline and on the same substrate. 
FIG. 3 shows a more detailed cross-sectional view of the distributed 
amplifier configuration of FIG. 2. The collector anode 20 preferably has 
tapered cavities 21 in its surface facing the cathode tips, in order to 
suppress the production of secondary electrons and ions, and to allow 
dissipation of any residual beam energy over a larger area. Referring to 
FIG. 4, the modulator 10 comprises a disc 22 of insulating material, which 
is preferably insulating (intrinsic or compensated) silicon for ease of 
fabrication, but which may be, for example, sapphire or quartz. A layer 23 
of high-conductivity metal, such as gold possibly with a layer of chromium 
thereunder as an adhesion layer, is deposited to a thickness of, say, 0.5 
.mu.m over the whole of one surface of the disc 22 to act as a ground 
plane. A microstrip line 24 of approximately 50 .OMEGA. impedance is 
formed on the opposite surface of the disc. The line 24 is similarly 
formed of gold on chromium. Aligned apertures 25,26 are forced through the 
metal layers 23,24, respectively, by masking and etching. The major part 
of the area of the disc 20 beneath the microstrip line is then etched 
away, leaving an aperture 27 in the disc, with the stripline just 
supported around its edges. The spacing of the modulator 10 from the 
cathode tips is not critical, and although the grid 6 might be in contact 
with the modulator 10, in practice it may be spaced up to, say, a 
millimetre from that grid. Since the gap between the modulator strip line 
and the ground plane is about 10 .mu.m or a few tens of .mu.m to minimise 
transit time delay, the apertures can be, say, 10 .mu.m square and can be 
aligned over several tips. FIG. 5 shows an alternative configuration for 
the microstrip line 24 which has tapered regions to obtain an 
approximately uniform 50 .OMEGA. impedance. The aperture 30 through the 
disc 20 also has tapered ends, but the subtended angles between the 
aperture ends are larger than those of the strip line, so that greater 
support is provided for the broadening strip line. 
The spacer 13 (and possibly the spacers 11,15) preferably comprises a 
sodium glass ring which is bonded by an electrostatic bonding technique to 
the modulator 10 to form a vacuum-tight seal therebetween. 
The catcher microstrip line 14 may be of similar construction to the 
modulator 10, and may be inverted so that its ground plane is adjacent the 
collector anode 20. This structure is also bonded to the spacer 13. 
An alternative catcher line configuration is shown in FIG. 6. Because the 
current modulation produced at the plane of the catcher transmission line 
is highly non-sinusoidal, this amplifier or oscillator will produce a 
range of harmonics of the input frequency. It may therefore be convenient 
to tune the output using a tuned cavity with a sufficiently high Q value 
to suppress higher harmonics i.e. to use a standing wave geometry rather 
than a travelling wave geometry. Typically, such a cavity could be forced 
by including partially reflecting local deviations in the catcher line 
impedance. For example, the catcher line 28 could be terminated at one end 
29 by an open circuit and could include a partially-transmitting 
discontinuity 30 spaced from the end 29 by such a distance as to obtain a 
standing wave mode between the discontinuity 30 and the end 29. The 
modulator strip line is preferably of the same configuration as the 
catcher line. Separate patches of active cathode area are addressed by 
patches 31,32 of modulator/catcher strip line. These patches are spaced by 
approximately 1/2 wavelength because no net amplification would be 
achieved by electron beam coupling at the intervening nodes. 
Preferably all of the components of the described devices are bonded 
together in such a manner as to form a vacuum-tight enclosure in which 
electrons from the cathode tips 3 travel to the collector anode 20. 
Alternatively, the device may be mounted in a further enclosure (not 
shown) which is itself vacuum-tight. 
FIG. 7 shows, schematically, a klystron-type oscillator device. In this 
case, as mentioned previously, the catcher line 14 and the collector anode 
20 of FIG. 3 are omitted, and a reflector electrode 33 is bonded to the 
spacer 13. In use of the device, the electrode 33 is biased negatively 
with respect to the cathode potential, the reflector electrode to cathode 
voltage being, for example, -10 volts. This electrode electron beams such 
as those schematically represented by arrows 34, to turn back towards the 
modulator 10, thereby producing feedback which causes the device to 
oscillate. Variation of the voltage on the reflector electrode will alter 
the transit times of the electrons, and can therefore enable tuning of the 
oscillation frequency of the device. 
Alternatively, or additionally, a magnetic field may be applied 
transversely to the general direction of electron flow to cause reversal 
of the electron beams. Again, the magnitudes of the electric and/or 
magnetic fields will determine the oscillation frequency. 
In an alternative arrangement in FIG. 8, the catcher strip line 14 is 
mounted alongside the modulator 10, and the electron beams are bent, by an 
electric field applied by a deflector 35 and/or a magnetic field applied 
by an electromagnetic source 36 above, so that they reach the catcher line 
via curved paths. The catcher and modulator lines may be coupled together 
so that feedback occurs, causing oscillation of the device. Again, 
adjustment of the electric and/or magnetic field strength will vary the 
tuning of the device. 
Although the cathode/grid structure in each embodiment described above 
includes three grid electrodes, this number may be reduced to two or one 
if additional collimation of the electron beams is not required. 
The catcher and modulator strip lines 10 and 14 may be identical in 
configuration and construction. 
Whereas the embodiments described above include a silicon substrate with or 
without a metallic coating, alternatively a substrate of metal, 
particularly but not exclusively a single crystal metal, may be used.