Electron field emission devices

An electron field emission device includes a field emissive cathode which may include a plurality of sharp field emitter tips 2. First and second grids 4 and 7 are located between the cathode and an anode. The grids comprise parallel strips of electrically conductive material, those of the first grid being arranged orthogonal to those of the second. Applications of suitable voltages to the grids enable selected regions of the cathode to be made electron emissive and produce collimated electron beams. The anode may be phosphor coated to produce a visible display or an infra-red image may be produced. Relatively low voltages may be used to switch the regions on and off. As the beams are collimated, the anode may be located a relatively large distance from the cathode, enabling high voltage phosphors to be used.

BACKGROUND OF THE INVENTION 
This invention relates to electron field emission devices and more 
particularly, but not exclusively, to display arrangements including such 
devices. 
Much cathode ray tube display technology is now being superseded by both 
passive and more recently active matrix liquid crystal display technology. 
This flat panel technology has many advantages over cathode ray tube (CRT) 
including lighter weight compact shape and lower cost. However, such 
displays have a number of obvious disadvantages, notably the poor viewing 
angle and their intrinsically non emissive nature. Although active matrix 
liquid crystal technology has been developed to overcome the latter 
deficiency it is at the expense of device complexity and the resulting 
reduced manufacturing yield leads to high cost, particularly as the 
technology is scaled up to larger screen sizes. In addition, because such 
devices involve the shuttering of a back light they are intrinsically 
inefficient. 
A competitive technology based on arrays of field emitter cold cathodes has 
been developed which provides all the advantages of the flat, liquid 
crystal display but with the brightness and viewing angle of the cathode 
ray tube. This novel technology is, however, yet to come into production 
because the lack of low voltage phosphors and the limited lifetimes of 
field emitter tips. 
SUMMARY OF THE INVENTION 
The present invention arose from consideration of an improved field emitter 
display (FED) but is envisaged that it may also be advantageously employed 
in non-display applications. 
The main drawback with conventional FED devices is that they require close 
anode screens for proximity focusing of the electron beams emitted from 
small patches of matrix addressed cathode tips behind each pixel. This 
close spacing (approximately 100 .mu.m) requires correspondingly low 
phosphor voltages (less than 1000 V) relative to the cathode in order to 
minimise electrical breakdown problems. As a result conventional phosphors 
technologies used in conventional CRTs cannot be used. 
According to the invention, there is provided an electron field emission 
device comprising: a field emissive cathode and an anode with first and 
second grids located between them and means for controlling emission from 
selective areas of the cathode by applying appropriate voltages to the 
grids, at least one of the grids comprising a plurality of discrete 
regions which are independently addressable by said means. 
Advantageously, the voltages applied by the means for controlling emission 
are arranged to produce substantially collimated electron emission from a 
selected area or areas. 
The invention concerned is particularly advantageously used in a display 
device in which the two grid cathode design provides both beam collimation 
and matrix addressing of the cathode elements behind individual anode 
regions. Because of the collimated nature of the beam, close spacing of 
the anode, which may be phosphor coated, is not required and relatively 
high voltages are possible. As low voltage phosphors are not required, the 
display offers the possibility of better colour, longer life and 
ultra-high resolution and brightness greater than that of previously known 
field emitter displays. It is possible that the characteristics of such a 
device could be superior to those possible even with CRT technology.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A display device in accordance with the invention include a cathode, 
illustrated in cross-section in FIG. 1, comprising a conductive substrate 
1 supporting conductive substantially conical field emission tips 2 
typically between 1 and 2 microns in height and with bases about 1 micron 
across. This structure is overlain by an insulating layer 3, typically 
between 1 and 2 microns thick, itself overlain by a thin electrically 
conductive layer, typically 0.3 microns thick, which acts as a first grid 
4 and has circular apertures 5 exposing the underlying tips placed 
concentric with those tips. The grid 4 is further covered by an additional 
insulator layer 6 and a second metal layer which acts as a second grid 7 
and also has aperture 8 centered about the axis of the tips perpendicular 
to the substrate 1. The aperture in the second grid 7 are typically larger 
that those in the first and for a first grid aperture of 1.5 microns 
diameter the second grid typically has a 3 micron diameter. It should be 
noted that the particular dimensions are only given by way of example and 
that they could differ substantially depending on the fabrication methods 
employed and on the particular application requirements. 
Each of the grids 4 and 7 comprises parallel strips of conductive material, 
those of the first being arranged orthogonal to those of the second as 
shown in FIG. 2 to provide matrix addressing of patches tips at their 
crossing points. Each patch contains one or more tips aligned with grid 
apertures in one column of the first grid 4 and in one row of the second 
grid 7. A method for fabricating an array of tips is described in our 
co-pending patent application published under serial number GB 2254958A. 
Although the tips must be electrically conducting and may be composed of 
the same material as that of the substrate, typically silicon, they could 
be formed of a different material (molybdenum for example) possibly on an 
insulating substrate such as a silica glass. Whether the tips are of 
silicon, molybdenum or another material they may be connected to a common 
electrical supply (not shown) but may also consist of electrically 
isolated patches, for example on an insulating substrate, and be 
separately addressable. 
Expressing all voltages relative to that of the tips, the cathode is 
operated by typically applying +60 V to the first grid 4 and +4 V to the 
second grid 7 to provide an average emission current greater than 10 nA 
from each tip. Emission is switched off either by reducing the first grid 
voltage to about 35 V or by preventing emissions passing the second grid 7 
with a second grid bias close to zero. Such a bias reflects the emission 
back to the first grid 4. Biasing the second grid 7 at about -20 V not 
only reflects the emission but also suppresses the magnitude of the 
current, typically to less than a tenth of its "on" value. Thus a 
modulation of 25 V in the biases on the two grids provide a means of 
turning on a particular patch within a large array of patches. Sequential 
addressing of the rows and columns of the grids 4 and 7 results in a time 
multiplexed emission pattern from the array of cathode patches. 
The performance of this cathode may be further enhanced by having the first 
grid 4 composed of a very resistive material such as polysilicon or 
amorphous silicon. In this case emission is further suppressed by the 
charging of the first grid 4 when emission is switched back to it by 
negative bias on the second grid 7. This enables the complete emission to 
be switched off by as little as minus one volt applied to the second grid 
7 as a consequence of the low energy spread of field emitted electrons. 
Moreover, such a resistive grid tends to suppress emission from the most 
emissive tips of the array thus providing a mechanism for reducing 
emissive current variations across the array of emitter patches. Such a 
resistive grid also provide a soft failure mechanism whereby the current 
resulting from an electrical short between the first and the second grid 
or between the first grid and the tips will not lead to destructing high 
leakage current. 
To optimise the effectiveness of this current suppression the first grid 4 
preferably include a coarse mesh of conductive material 9 beneath or 
overlying the high resistivity grid film. Examples of two such structures 
are shown in FIGS. 3a and 3b. 
When the second grid 7 is at approximately 4 volts positive with respect to 
the emitter tips 2 any emission current, which depends on the voltage of 
the first grid, will form an approximately collimated beam emerging 
perpendicular from the cathode surface. Provided there is a sufficiently 
high electric field, typically several hundreds of volts per millimeter, 
in the region above the cathode surface, the resulting angular spread of 
the beam may have full angular spreads of about one degree or less. The 
optimal angular spread and the second grid voltage required depends on 
this external field and on the potential of the first grid. The actual 
second grid voltage required to switch off the emission is close to the 
tip voltage plus the work function of the second grid material but is 
suppressed by both high field above the cathode and by high first grid 
voltages. Thus emission currents reflected back to the first grid by a 
sufficiently higher second grid bias may also be transmitted by slight 
increase of the first grid voltage. Thus it is possible for suitable 
cathode geometries to switch the emission from cathode patches by 
modulation of only a few volts on both first and second grids, provided 
the first grid is sufficiently resistive to suppress the emission current 
from the tips whenever it is reflected back to the first grid. In the 
absence of such a mechanism the cathode would tend to be subjected to 
excessive heating and reduced efficiency. 
The collimated nature of the emission from a cathode in accordance with the 
invention results in a number of devices incorporating such a cathode 
becoming viable. In particular it is possible to space an anode a few 
millimeters from the cathode substrate and still maintain a better than 
100 micron narrow electron irradiated patch at the anode. If the anode 
consists of a phosphor coated conducting glass sheet then the matrix 
addressed cathode is capable of producing light emission from regions of 
the glass anode 12 having diameters of less than one hundred microns by 
irradiation with electron emission from patches of the underlying cathodes 
switched on by application of suitable voltages to a row and column via 
contact pads 10, as shown in FIG. 4. 
The relatively large gap between the cathode and glass anode permits a 
relatively high voltage, typically between +1000 V and +10,000 to be 
applied to the anode relative to the cathode and high brightness to be 
obtained, at about 50% total power efficiency, with only a few tens of 
nanoamps of current from each cathode patch. 
In one embodiment of a display device employing the invention, the anode 
typically consists of a glass substrate 13 with a coating of Indium Tin 
Oxide (ITO) 14 and a coating 15 (which may be continuous, as shown, or 
segmented) of phosphor material overlaying the ITO layer (FIG. 5a). 
Suitable phosphors are conventional CRT phosphors, with or without metal 
coating, or if lower voltage operations is required fluorescent polymer 
materials, and particularly those based on polyphenylene vinylene, or any 
other low voltage phosphor. In a 3-color version, this phosphor layer may 
consist of an array of three different phosphor "dots" 16A, 16B and 16C as 
shown in FIG. 5b. Each dot is aligned with a corresponding one the cathode 
patches. Alternatively the ITO may be patterned into parallel tracks 17 
with the three different phosphors 16A, 16B and 16C applied sequentially 
to each in turn as in FIG. 5c so that the phosphor to be irradiated is 
selected by applying the anode bias sequentially to each anode ITO track 
in turn. 
The relatively large gap between the anode and cathode and the presence of 
electron beams within the intervening gap makes it difficult to support 
the anode by pillars within the area of the cathode. Support may be 
achieved by using insulating spacers 18 around the periphery of the 
cathode, positioned between the cathode substrate and anode plate 13 as 
shown in FIG. 6. The whole sandwich structure is then sealed into an 
external vacuum tight envelope (not shown) capable of maintaining a high 
vacuum, typically about 10.sup.-6 mbar. The envelope may be transparent to 
visible radiation on in other parts of the electromagnetic spectrum. 
The eternal envelope, which may have curved walls, results in no structural 
support being required within the area of the cathode between it and the 
anode since in this implementation vacuum is maintained within and outside 
the cathode-anode gap. In a preferred embodiment, the drive electronics 
for the time multiplexed addressing of individual cathode patches are also 
accommodated within the eternal envelope so that only a small number of 
electrical connections need to be made through the wall of the vacuum 
envelope. The drive circuitry might also advantageously be fabricated on 
the substrate of the cathode structure where this is of silicon or some 
other appropriate material. 
Although it is envisaged that the cathode is fabricated on a single 
substrate it might also be built up from a number of separate cathode 
tiles. Similarly, although it is intended that in most arrangements the 
anode consists of a single glass sheet, in another embodiment of the 
invention, a number of separate cathode and anode modules could be 
included within the one envelope. 
With a phosphor coated anode the device provide a visibly display. If the 
anode comprises an infra-red transparent material such as quartz or 
sapphire and the conducting layer is a thin metal sheet then the device 
can provide an infra-red display. In this case, higher electrical currents 
are requires such that the combination of high current and high anode 
voltage gives significant anode heating. In further modification, the 
performance of such a device may be improved by having an anode structure 
which includes a thin metal sheet 21 in the form of multiple bridge 
structures 19, as shown in FIG. 7a, in which there are unsupported regions 
of metal with a vacuum (or gas) gap between the metal and the anode 
substrate 20. Similar constructions are disclosed in our previous 
application published under serial number GB 2209432A. Such structures may 
be formed by firstly coating the IR transparent material 20 with a 
sacrificial material, such as a CVD deposited silica glass on an organic 
resist, patterning it to leave only pillars 22 of the material (as shown 
in FIG. 7b); coating the whole surface with a thin metal layer 21 (FIG. 
7c); patterning this layer with a resist mask 23 with aperture 24 over the 
edges of the metal domes (FIG. 7d) and etching away the exposed metal. The 
resist is then removed and the sacrificial layer dissolved via the 
apertures formed in the metal layer. Thus regions of thin metal are formed 
supported only by their edges (as shown in FIG. 7e). The thinness of the 
material gives low heat capacity and the low thermally conductive path to 
the substrate combines with the high thermal conductivity of the substrate 
20, such that relatively little electron beam power is required to produce 
a hot IR emissive metal film and yet the film rapidly returns to ambient 
temperature once the electron beam is turned off. As with the high 
emission device, these the bridge structures might advantageously be 
aligned directly above corresponding cathode patches on the cathode 
substrate and by sequentially addressing the rows and columns of the 
cathode array a dynamic (in this case infra-red) emission scene may be 
displayed on the anode surface.