Emissive display including field emitters on a transparent substrate

An emissive display comprising: a first substrate including a plurality of controllable anodes; a layer of phosphor on each of the controllable anodes, wherein each phosphor emits light when the anode on which the phosphor is located is activated and electrons bombard the phosphor; a second substrate comprising a light transmissive lens having a display area through which a display is viewed, first and second electrically conductive layers and an insulating layer, wherein the first and second electrically conductive layers and the insulating layer are light transmissive, wherein the insulating layer is between the first and second electrically conductive layers and wherein the first and second electrically conductive layers and the insulating layer are on a side of the lens facing the controllable anodes and phosphor layers, wherein the second electrically conductive layer is located closer to the first substrate than the first electrically conductive layer and comprises a first plurality of holes corresponding to a second plurality of holes in the insulating layer; a plurality of opaque field emitter cones mounted to the first electrically conductive layer in the first and second plurality of holes, emitting electrons to selectively bombard the phosphors layers, wherein the plurality of opaque field emitter cones covers less than ten percent of the display area of the second substrate, wherein the emitted electrons travel through a space between the second electrically conductive layer and the phosphor layer, wherein light emitted from the phosphor layer travels back through said space and through the second substrate to be viewed by a viewer of the display.

BACKGROUND OF THE INVENTION 
Many known vacuum fluorescent displays use a filament and grid combination 
to source electrons for the vacuum fluorescent display. To improve the 
efficiency and brightness of displays, it is desirable to move the 
electron source closer to the phosphors that are the light source. 
However, because the filaments and grids are typically suspended, 
mechanical shock to the display can give rise to vibrations in the grid 
and the filament. This potential for vibrations limits the practical 
minimum distance between the electron source comprising the filament and 
grid and the phosphors because it is undesirable to allow the filament 
and/or grid to short circuit to the substrate carrying the phosphors. 
A known type of display that eliminates the filament and grid includes an 
array of field emitters as the electron source. The field emitter array is 
fabricated directly to a substrate, such as silicon, and therefore does 
not have the suspension and vibration characteristics of the filament and 
grid electron sources. In known vacuum fluorescent displays using field 
emitter arrays, electrons travel from the field emitters across a short 
distance to phosphors mounted on transparent anodes, which are mounted on 
a transparent substrate. For reconfigurable displays, the substrates may 
include transparent thin film transistors to control the voltage levels of 
the anodes. The electrons impinging on the phosphors excite the phosphors 
so that the phosphors emit light. The emitted light travels through the 
transparent conductor, transparent thin film devices, if any, and the 
transparent substrate on which the phosphors and thin film devices are 
mounted to be viewed by a viewer of the display. 
Descriptions of field emitters and their construction are included in the 
articles, C. A. Spindt, I. Brodie, L. Humphrey and E. R. Westerberg, 
"Physical properties of thin-film field emission cathodes with molybdenum 
cones," Journal of Applied Physics, Vol. 47 , No. 12, December 1996, Pages 
5248-5263, and C. A. Spindt, C. E. Holland, A. Rosengreen, I. Brodie, 
"Field-Emitter Arrays for Vacuum Microelectronics," IEEE, Transactions on 
Electronic Devices, Vol. 38, No. 10, October 1991, Pages 2355-2363. The 
disclosures of the above two articles are incorporated herein by 
reference. 
These known field emitter displays are unsuitable for high brightness 
display applications because they have a limited brightness achievable for 
a given amount of power supplied to the display. As much as 40% of the 
light emitted by the phosphors is reabsorbed into the phosphor and 
substrate before it reaches the eye of the viewer of the display. Thus for 
high brightness display applications, the art has been typically limited 
to filament and grid vacuum fluorescent displays. 
Use of filament and grid vacuum fluorescent displays places limitations on 
the size and structure of the display. Typically, a display with a 
filament and a grid requires the grid to be approximately one millimeter 
from the emissive phosphors. This has a variety of impacts on display 
performance, one is display efficiency, which is reduced because the 
electrons must travel over a fairly large distance, i.e., over 1 mm, to 
reach the phosphors. 
A second impact is that the spacing of display elements is limited, which 
has an extremely noticeable impact on reconfigurable vacuum fluorescent 
displays. For example, when pixels are spaced as close as 500 .mu.m, the 
operation of one pixel may have an undesirable coupling effect on the 
operation of a neighboring pixel. Pending U.S. patent application, Ser. 
No. 08/205,462, assigned to the assignee of this invention, recognizes 
this coupling effect and sets forth a structure including an isolation 
grid on the substrate surrounding each pixel that eliminates the coupling 
effect and also the necessity for a suspended acceleration grid. This 
structure allows movement of the pixels closer together without any 
resulting undesirable coupling between the pixels. For example, displays 
using the isolation grid have achieved a pixel pitch of about 370 .mu.m 
while retaining a 50% fill factor, where the fill factor is defined as the 
ratio of the phosphor covered area of the pixel (the light emitting area) 
to the total pixel area. Further reducing the pixel pitch while 
maintaining the fill factor is a challenge, though, because the isolation 
grid limits the spacing between neighboring pixels to no less than about 
100 .mu.m and more typically in the range of 120-150 .mu.m. This thus 
limits the pixel density of the reconfigurable display. 
One prior field emitter display proposal places the field emitters on the 
same substrate with the phosphors. This configuration eliminates the 
necessity to mount the phosphors on a transparent substrate but the pixel 
fill factor is seriously reduced due to the space required for the large 
multiplicity of field emission structures fabricated on the common 
substrate with the phosphor. Thus such displays will have inadequate 
brightness, fill factor and/or pixel density for many applications. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide an apparatus in accordance 
with claim 1. 
Advantageously this invention overcomes prior existing limitations in the 
size, structure, brightness and efficiency of vacuum fluorescent displays 
and especially of reconfigurable vacuum fluorescent displays. 
Advantageously, this invention makes use of a field emitter array for a 
vacuum fluorescent display with a new structure. 
Advantageously, this invention provides a field emitter array for use in a 
vacuum fluorescent display that eliminates the necessity of a transparent 
substrate from carrying the emissive phosphors. This invention 
advantageously allows the use of a silicon wafer or other opaque thermally 
conductive substrate for carrying the excitable phosphors of a vacuum 
fluorescent display and allows implementation of addressing transistors on 
to the thermally conductive substrate, eliminating the necessity of 
transparent thin film transistors in a display with a field emitter array. 
Advantageously, this invention provides a vacuum fluorescent display with a 
field emitter array allowing close proximity between the field emitter 
array and the light emissive phosphors, increased brightness, increased 
efficiency and lower field voltages of the display. This invention allows 
the field emitter array to be placed in a range of 100 .mu.m from the 
emitting phosphor surface of the pixel of a vacuum fluorescent display 
viewed by a viewer of the display, this compares to the spacing between 
the filament and the pixel of 500 .mu.m to 2 mm in the above mentioned 
patent application Ser. No. 08/205462 and in known prior art, and compares 
to the placing of the field emitter structure closest to the surface of 
the phosphor furthest from the viewer of the display in known prior art 
field emitter displays. Further, by achieving a field emitter structure so 
close to the viewing side of the display pixels, the coupling effect 
previously mentioned is eliminated without the necessity of an isolation 
grid structure on the substrate surrounding each pixel. Thus, this 
invention enables a decrease in the spacing between neighboring pixels in 
a display and a corresponding increase in pixel density. The distance 
between neighboring pixels is reduced from a minimum of about 100 .mu.m to 
distances in the range of 50 .mu.m and possibly even less. The result is a 
decrease in pixel pitch to 170 .mu.m or less and an increase in pixel 
density of five times that previously available for front lit vacuum 
fluorescent displays. Additionally, the increase in pixel density allows a 
50 to 80% reduction in the size of the display when used as an image 
source to be projected or for other small size applications, providing a 
corresponding significant cost savings. 
Further advantages arising out of this structure include the use of a 
silicon wafer to provide addressing electronics such as CMOS transistors, 
providing performance advantages and improved heat dissipation over thin 
film transistors fabricated on transparent substrates. Alternatively, the 
silicon wafer may serve as a substrate for thin film transistors, offering 
improved display brightness because of the improved heat transfer of the 
silicon as compared to that of known transparent substrates. 
In a preferred example, this invention provides an emissive display 
comprising: a first substrate including a plurality of controllable 
anodes; a layer of phosphor on each of the controllable anodes, wherein 
each phosphor emits light when the anode on which the phosphor is located 
is activated and electrons bombard the phosphor; a second substrate 
comprising a light transmissive lens having a display area through which a 
display is viewed, first and second electrically conductive layers and an 
insulating layer, wherein the first and second electrically conductive 
layers and the insulating layer are light transmissive, wherein the 
insulating layer is between the first and second electrically conductive 
layers and wherein the first and second electrically conductive layers and 
the insulating layer are on a side of the lens facing the controllable 
anodes and phosphor layers, wherein the second electrically conductive 
layer is located closer to the first substrate than the first electrically 
conductive layer and comprises a first plurality of holes corresponding to 
a second plurality of holes in the insulating layer; a plurality of opaque 
field emitter cones mounted to the first electrically conductive layer in 
the first and second plurality of holes, emitting electrons to selectively 
bombard the phosphors layers, wherein the plurality of opaque field 
emitter cones covers less than ten percent of the display area of the 
second substrate, wherein the emitted electrons travel through a space 
between the second electrically conductive layer and the phosphor layer, 
wherein light emitted from the phosphor layer travels back through said 
space and through the second substrate to be viewed by a viewer of the 
display. 
In one example, the field emitters are placed a distance in a range of 
approximately 100 to 400 .mu.m from a substrate with emissive material 
thereon that emits light when bombarded by electrons emitted from the 
field emitters. In another example the emissive material is located on 
each of a plurality of individually addressable pixels, wherein each pixel 
is within a distance of 90 .mu.m or less of a neighboring pixel. In yet 
another example, the individually addressable pixels having a fill factor 
of 50% achieve a pixel density of 35 per square mm or a pixel pitch of 170 
.mu.m.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIGS. 1-3, an emissive display according to this invention is 
shown as display 11 including a cover lens 28 and base 13. Base 13 
includes support 10, comprising glass on which is bonded silicon substrate 
14, held in place by adhesive 12. The substrate 14 includes an array of 
addressing transistors (not shown) selectively biasing conductive pixel 
pads 18, separated from the array of addressing transistors by insulating 
silicon dioxide layer 16. Each conductive pixel pad 18 includes an 
emissive material 20, such as a phosphor of the type known to those 
skilled in the art of vacuum fluorescent displays, on the top surface of 
the conductive pad 18. The emissive material 20 is of a type that emits 
visible light when bombarded by electrons. 
Details for fabrication of the addressing electronics and/or driver circuit 
for the vacuum fluorescent display are known to those skilled in the art 
and need not be set forth herein in more detail. In a known manner, the 
conductive pads 18 (anodes) are electrically coupled to the desired 
conductor or semi-conductor elements on the silicon wafer 14 vis-a-vis 
holes selectively provided in the silicon dioxide layer 16 and deposited 
with metal. 
The cover lens 28 includes an optically transparent insulating layer 26, 
transparent conductor 24 and field emitter array 22. According to this 
invention, all components of the field emitter array 22 are optically 
transparent except for the cone shaped emitter structures 36 themselves 
(described below with reference to FIG. 2). However, the cone-shaped 
emitter structures 36 cover approximately 4 percent or less of the light 
transmissive area of the array and thus do not impede viewing of the 
display through the lens 28. 
Referring now also to FIG. 2, the structure of the field emitter array 22 
according to this invention and its fabrication are now described in more 
detail. The lens 28 acts as a transparent second substrate base and 
comprises, for example, glass or fused quartz. The lens 28 is cleaned and 
an optically transparent insulating layer 26 such as silicon dioxide is 
deposited thereon to a thickness of approximately 1.0 .mu.m. The use of 
silicon dioxide insulating layer 26 is discretionary but preferred because 
(a) it provides a well defined substrate upon which the transparent 
conductor 24 can be deposited, (b) it suppresses the out diffusion of 
ionic contaminants such as sodium from the glass substrate lens 28, which 
contaminants could adversely affect phosphor and/or field emitter 
performance; and (c) it improves the adhesion of the transparent 
conductive layer 24 to the substrate lens 28. 
After the silicon dioxide layer 26 is deposited, a transparent conductor, 
for example, indium tin oxide, is then deposited to form layer 24 having a 
thickness of approximately 400 .mu.m. While 400 nm is a preferred 
thickness given for this example, the thickness of the conducting layer 24 
and the other conducting layer 34 discussed below may be varied from 
implementation to implementation. However, the thickness of each 
conducting layer is restricted in part by the transparency of the film (as 
in the case when titanium nitride is used in place of indium tin oxide) 
and the resistivity of the resulting deposited layer. It is desirable to 
choose a resistivity of the transparent conducting layer 26 that is low 
enough to support an adequate emission current from each field emitter. 
However, it is also desirable to insure that a current significantly in 
excess of the adequate emission current cannot be supplied to a given 
emitter. This insures that in case of failure of a given emitter, the 
remaining emitters serving a given pixel are not also deactivated. That 
is, the resistivity of the transparent conducting films is selected such 
that a short circuit between the conducting layers 24 and 34 at a 
particular emitter is isolated in terms of its influence on adjacent 
emitters. This approach, which essentially amounts to placing resistors in 
series with each emitter structure, has been employed in the case of 
non-transparent conductors, as described in the above-mentioned Spindt et 
al. article, "Field-Emitter Arrays for Vacuum Microelectronics." 
The film 24 may be photolithographically patterned into strips to define 
the width of the ultimate pixels. Patterning of the conductor layer 24 can 
be done by wet chemical means, by a plasma assist process, by reactive ion 
etching using silicon tetrachloride or by ion milling as is known to those 
skilled in the art. 
After the first transparent conductive layer 24 is deposited and patterned, 
a second insulator layer 32, preferably silicon dioxide and thicker than 
the original layer 26, is deposited on the first transparent conductor 
layer 24. The thickness of the second insulating layer 32 is set by the 
desired height of the field emission sources which typically range from 
0.5-2.0 .mu.m. After the transparent silicon dioxide layer 32 is 
deposited, a second transparent conducting layer 34, for example indium 
tin oxide, is then deposited, typically to a thickness of approximately 
200 nm. The transparent conducting layer 34 in the completed display 
serves as the extraction grid for the field emitters. Once the second 
transparent conducting layer 34 is applied, a photolithographic process is 
used to define the locations of the field emitters by patterning the 
transparent conductive layer 34 using, for example, reactive ion etching 
and a silicon tetrachloride etching gas in combination with a photoresist 
masking layer. 
After the transparent conductor film is patterned, the photoresist masking 
layer used in the patterning is removed. The patterned transparent 
conductive layer 34 is then itself employed as a masking layer to define 
the etching of the underlying silicon dioxide layer 32 using, for example, 
buffered hydrofluoric acid. At this point, the substrate includes the two 
transparent conductor layers 24 and 34 separated by the silicon dioxide 
layer 32 with an array of holes 33, 35 that penetrate both the transparent 
conductor layer 34 and the silicon dioxide layer 32, exposing portions 44 
of the transparent conductive layer 24. 
The field emitter structures themselves are then fabricated into the 
exposed portions 44 of the transparent conducting layer 24. A typical 
material used for the emitter structures is molybdenum, which is an opaque 
material, and the resultant structures are cone shaped, as illustrated by 
structures 36 in FIG. 2. The fabrication process for the field emitter 
structures 36 is performed in a manner known to those skilled in the art 
and an example of a typical desirable fabrication process for field 
emitter structures 36 is described by Spindt et al. in the above-mentioned 
article, "Physical properties of thin-film field emission cathodes with 
molybdenum cones." Because such fabrication processes are well known to 
those skilled in the art and well described in the literature, the details 
of the fabrication of the emitter structures 36 need not be set forth 
herein. 
When the second substrate, including the electron source comprising the 
field emitter array 22, is assembled to the remainder of the display 
including the first substrate comprising glass base substrate 10 and the 
phosphor 20, and when the space between the substrates is vacated of air, 
the display structure is complete and can be operated as described herein. 
In general, electrons are emitted from the field emitters 36 without 
requiring the application of heat. The conductive layer 24 is biased to a 
negative voltage reference or ground thereby providing the negative 
voltage reference or ground reference to emitters 36. The conductive layer 
34 is biased to a higher voltage reference, for example, 10-100 volts 
higher than the voltage reference applied to the field emitters 36, 
creating an emission field around the tips 37 of the emitters 36. During 
display operation, the voltage references can be applied to the conductive 
layers 26 and 24 around the entire array of field emitters 36 and need not 
be selectively switched on and off. This causes electrons to emit from the 
emitters 36. 
The display 11 is caused to emit light in the desired display patterns by 
selectively activating the anodes 18 to high voltage references. When a 
particular anode is so activated, it creates an acceleration field between 
the anode 18 and the nearby emitters 36, causing electrons 19 freed due to 
the emission field around the emitters 36 to drive toward the anode 18, 
striking the phosphors 20 in the path and causing emission of light from 
the phosphors 20. The circuitry and control required to selectively 
activate the particular anodes 18 is well known to those skilled in the 
art of reconfigurable vacuum fluorescent displays and need not be set 
forth herein. 
In contrast to prior art field emitter displays, the light emitted by 
phosphors 20 does not need to travel through substrate 13 to be viewed. 
Such a path of light travel significantly reduces the magnitude of 
luminance created by the display due in part because the most light 
emitter from the phosphors is on the side of the phosphors furthest from 
the viewer of the display and increasing current to the display to 
overcome this handicap is both inefficient and may cause damage to the 
display, including premature degradation of the phosphors. By forcing the 
light to be viewed from the side of the phosphors furthest from that first 
struck by the electrons, 40% of the light emitted is reabsorbed by the 
phosphors and substrate before it reaches the eye. Further, substrate and 
circuitry carrying the phosphors must be transparent and therefore has a 
limited ability to include heat-dissipating structures, limiting the 
current capacity of the prior art display. 
According to this invention, the emitted light is viewed (by an observer's 
eye 31) through lens 28, thus providing a direct path for the brightest 
emitted light from the surface 33 of the phosphors 20 through the 
transparent layers 24, 26, 32 and 43 and through transparent lens 28. The 
only optical restrictions to the light transmission are the emitter cones, 
which are opaque. But as will now be explained, this restriction has 
minimum impact. The result is an increase in display brightness according 
to this invention of over 60% as compared to field emitter displays 
according to the prior art using the same amount of power. 
Typically, each field emitter generates 0.1 mA of emission current. A 
vacuum fluorescent display that typically requires an electron current 
density of 20 mA/cm.sup.2 and comprises a series of addressable pixels, 
each having an area of 1.2.times.10.sup.-3 cm.sup.2, requires on the order 
of 24 .mu.A for each pixel. Such a display requires 240 field emitters at 
each pixel, which is easily achieved. For example, assuming each emitter 
base has a diameter of 5 .mu.m, then 240 emitters requires a total area of 
approximately 5 .times.10.sup.-5 cm.sup.2, which is approximately only 4 
percent of the total pixel area. Because the molybdenum emitters are 
opaque and the remainder of the structure of the substrate is transparent, 
the opaque emitters block out only 4 percent of the total light 
transmitting area corresponding to the pixel, resulting in minimal 
interference with the emission of light from the display. 
In another example, assume it is desirable to increase electron current 
density available to each pixel so that the field emitters are packed as 
closely as possible in the proximity of each pixel. Using field emitters 
having a height of 1.5-2.0 .mu.m and base diameters of approximately 1.0 
.mu.m, this allows a spacing between the emitters of approximately 10 
.mu.m. For the above-described display with a pixel area of 
1.2.times.10.sup.-3 cm.sup.2, pixel dimensions are approximately 350 .mu.m 
on the sides. This allows an array of 35.times.35 field emitters for each 
pixel for a total of 1225 emitters for each pixel. Summing up the total 
area covered by the field emitters, which is the total opaque area, 
results in only 9.6.times.10.sup.-6 cm.sup.2 of opaque area yielding less 
than a 1 percent opaque coverage of the total pixel area. Again, the 
opaque emitters have a negligible adverse affect on display luminance. 
For the first time, then, this invention provides a vacuum fluorescent 
display using field emitters that allow viewing of the display through the 
field emitter array. By achieving this advantageous structure, this 
invention increases the light emitting efficiency of front-viewed displays 
and for the first time allows a vacuum fluorescent display using field 
emitters suitable for high brightness display implementations such as in a 
vehicle projected head up display. 
Before further describing the structural improvements according to this 
invention, an example of a high brightness display for which this 
invention is suitable, a projected display such as a vehicle head-up 
display, is explained as follows. In a projected head-up display, any 
light traveling in the direction, other than the direction of light 
projection path, is wasted. Further, the vehicle windshield, when used as 
the combiner or projection screen for the display, typically reflects only 
about 10 percent of the light from the display to the vehicle driver's eye 
60. Thus the display 50 must be capable of emitting light in the range of 
6000 ft L. (foot Lamberts), or at least 1750 candela/m.sup.2, to achieve a 
suitably bright display for the vehicle operator. According to this 
invention, a field emitter display structure is provided capable of 
achieving the required illumination intensity. 
While it is desirable to achieve a display with increased brightness, it is 
also desirable to increase pixel density on the display, or, put another 
way, reduce the necessary spacing between neighboring display elements. As 
explained above, for front-viewed vacuum fluorescent displays, or displays 
that are viewed from the same side from which the phosphors are bombarded, 
there has existed until now certain restrictions on the pixels spacing and 
density and pixel to electron source spacing. The structure of this 
invention allows the spacing of the electron source to the phosphor for 
front-viewed displays to be reduce from approximately 500 .mu.m (or even 
more typically 1.2-2 mm) to the range of 100 .mu.m. Thus the distance 21 
in FIG. 1 can be as small as in the range of 100 .mu.m to achieve the 
greatest display efficiencies and, in other examples, will typically fall 
within the range of 100-400 .mu.m. By allowing the electron source to be 
in the range of 100-400 .mu.m from the display elements or pixels, the 
undesirable coupling effect addressed in the U.S. Pat. No. 5,541,478, is 
eliminated. 
Prior to this invention, neighboring pixels in a front-viewed vacuum 
fluorescent display were required to be in the range of at least 100-150 
.mu.m from each other to avoid an undesirable coupling effect, and that 
was achieved using an isolation grid on the substrate with the pixels. 
According to this invention, neighboring pixels can be in the range as 
small as approximately 50 .mu.m from each other and will in various 
examples be in the range of 50-90 .mu.m. This distance is represented by 
distance 23 in FIG. 1. Further improvements may be achievable with 
advances in pixel fabrication techniques. 
Prior to this invention, to achieve a minimum practical pixel pitch of 200 
.mu.m, the maximum actual phosphor size on each pixel was only 
approximately 90.times.90 .mu.m using known phosphor deposition techniques 
while achieving a display without undesirable coupling. Thus, the 
resulting "fill factor" or ratio of the light emissive area to the total 
area of the individual pixel was very small, approximately 20%. An example 
of this is shown in FIG. 4, where reference 50 indicates the space 
allocated for each of two neighboring pixels and distance 54 represents 
the pixel pitch. The portions 52 represent those areas filled with 
phosphor 20, and therefore the only area of each pixel capable of emitting 
light. The reconfigurable display with such pixels has a grainy luminous 
appearance, and the area average averaged luminance of the display is 
small because only 20% of each pixel area is capable of emitting light. 
In contrast to the prior art, according to this invention the space between 
phosphors can now be reduced to approximately 50 .mu.m and may be reduced 
further as lithography techniques for depositing the phosphor improve. The 
result is that even if the pixel pitch is reduced to 150 .mu.m, the fill 
factor of each pixel is still at least 44% and if the pixel pitch is kept 
at 200 .mu.m, the fill factor increases to 56%, an increase of 180%. This 
improvement is shown in FIG. 5, where each pixel 56 has a pitch 60 equal 
to 200 .mu.m, the 56% fill area of phosphor, represented by references 58, 
shows a substantial reduction in the amount of space between light 
emitting phosphors and a corresponding substantial increase in light 
emitting area in each pixel. This will eliminate the graininess mentioned 
above as appearing in the prior art. 
As a practical example of the advantages according to this invention, 
assume a display is required to have an array of pixels with a minimum 50% 
fill factor and a pixel pitch as small as possible. Prior to this 
invention, the minimum pixel pitch capable of meeting this standard and 
not having undesirable pixel-to-pixel coupling was 375 .mu.m. According to 
this invention, the pixel pitch can be reduced to 170 .mu.m while still 
achieving the 50% fill factor, reducing the necessary pixel area by a 
factor of nearly five. Thus, on the same size substrate that only one 
display was made, now almost five displays can be made still having a 50% 
fill factor. This enables a cost reduction in display manufacture and the 
availability of reconfigurable vacuum fluorescent displays for new 
applications requiring very small display devices. 
The sum result of all of these advantages is that this invention enables an 
increase in pixel density of five times over that previously available. 
Prior pixel densities limited to seven pixels per square millimeter are 
now replaced by pixel densities of 35 pixels per square millimeter when 
using this invention. Further improvement in pixel density will be 
realized with further improvements in the ability to pattern the 
phosphors. 
While this invention enables a range of advantageous improvements in 
display design, in one example, the improvements can be described as 
including a display with an array of pixels having a pixel pitch of less 
than 350 .mu.m while each pixel has a fill factor of at least 50%. 
Materials suitable for the transparent conductor material may include 
indium tin oxide, as described above, titanium nitride or other 
transparent electrically conducting materials known to those skilled in 
the art, with the requirements that the materials are able to survive the 
process used to fabricate the emitter structures 36, cannot adversely 
affect performance of the resulting display, for example, do not out-gas 
under the operating conditions and can be precisely patterned using 
lithographic processes familiar to those skilled in the art. 
This invention, as described above, is ideal for use in high brightness 
reconfigurable vacuum fluorescent displays. By allowing light to transmit 
through the field emitter array, as opposed to the substrate upon which 
the phosphor is deposited, CMOS circuits can be fabricated in a silicon 
wafer applied as part of the substrate carrying the phosphor. The CMOS 
circuits are more efficient and the silicon wafer is more heat conductive 
than typical thin film transistor layers in their base substrates, 
allowing for a more efficient and brighter emissive phosphor. 
The advantages of the improved heat transfer available to the display using 
the field emitters according to this invention may be understood with 
reference to the following example. Assuming a high brightness vacuum 
fluorescent display including a conventional filament and grid structure 
capable of luminance intensity of 7000 ftL having a total display area of 
1 cm.sup.2. An example such display operates at 50 volts with pixel 
current densities of approximately 20 mA cm.sup.-2, with a comparable 
current flow lost to the grid structure. The total power dissipated by the 
electron flow from the electron source to the phosphor is approximately 2 
Watts, which is the amount of energy that must be dissipated through the 
substrate on which the phosphor elements are deposited. A display 
constructed on a glass substrate, having a thermal conductivity of 
approximately 1 W/(mK), and 0.25 cm thick, has a thermal resistivity of 
approximately 25 K/W. Thus for a 2 Watt load, the expected temperature 
rise of the phosphor in the display with respect to ambient temperature is 
50 K. 
A prior art field emission display using the same pixel current density has 
less power lost to the phosphor substrate, in the range of 1 Watt. 
Assuming a glass substrate for the phosphor (remembering that in this 
prior art example the substrate carrying the phosphor must be 
transparent), the thermal resistivity is again 25 K/W and the 
corresponding temperature rise in the phosphor is 25 K above ambient 
temperature. 
Assume an example display according to this invention with the same pixel 
power density as the prior art that must dissipate 1 Watt of power in the 
substrate carrying the phosphor. However, the substrate can be opaque, for 
example silicon, since the display is not viewed through the substrate. 
The thermal conductivity of silicon is in the range of 84 W/(mK), much 
higher than that of glass and the resulting substrate had a thermal 
resistivity of approximately 0.3 K/W. As a result, the temperature rise 
across the silicon substrate is less than 1 K. Thus, for the same power 
dissipation from the prior art field emission display, the phosphor 
temperature in the display according to this invention is approximately 24 
K lower than that of the prior art. Since phosphor temperature is 
inversely proportional to phosphor brightness, the display according to 
this invention is brighter. In the example in which 1 Watt of power is 
dissipated per square cm, the brightness of the display according to this 
invention due to improved heat dissipation alone is 67% greater than the 
prior art field emitter display in which the phosphor must be located on 
the transparent substrate. 
The above-described examples use molybdenum emitter cones 36 in the emitter 
array 22. In another example, the field emission cones can be fabricated 
from the same Indium Tin Oxide used to form the conductive layer 24. In 
this example, the display fabrication is as described above, up to the 
point at which the opaque molybdenum field structures are fabricated. At 
that point, instead of fabricating the molybdenum field emitter 
structures, the deposition of indium tin oxide is substituted with a short 
preliminary step of sputter etching the exposed portions 44 of layer 24 to 
assure adhesion of the indium tin oxide film field emitter material to the 
substrate conductive layer 24. 
The advantages of U.S. Pat. No. 5,151,632, assigned to the assignee of this 
invention, may be used with this invention if desired. 
This invention need not be limited to displays including driver electronics 
such as the matrix-addressable display, but may be used for emissive 
displays that do not need the matrix-addressing transistor mentioned above 
to obtain the same benefits previously described herein. 
In the above examples of this invention, the term "lens" is used to 
describe the transparent glass or substrate through which the display is 
viewed and does not imply a necessity to focus light or the display. 
Referring again to FIGS. 1 and 2, in another example according to this 
invention, the field emitter array is fabricated in a known manner in 
which groups of field emitters are selectively addressed in row and column 
fashion. One known technique for this is to pattern the conductive layer 
24 as a series of row electrodes and the conductive layer 34 as a series 
of column electrodes or vise verse. Particular groups of field emitters 
are activated by selectively addressing the corresponding row and column 
electrodes comprising conductive layers 24 and 34. 
In this example display, it is not necessary to physically or electrically 
define the phosphor or anode elements of the display device into pixels 
because the pixels are defined by the configuration of the sets of field 
emitters that are biased in the conventional matrix addressing fashion. 
Thus in this example, the phosphor layer 20 and, possibly, the conductive 
pads 18 are formed as one large anode (as opposed to the space separated 
anodes shown in FIG. 1). 
Because in this example it is unnecessary to pattern the phosphor layer, 
the fill factor for the pixels of the display is as high as almost 100%. 
The advantage of this example of the invention is a brighter display 
(approximately 40% brighter) than prior art displays of similar type 
because the phosphors are viewed through the lens 11 including the emitter 
array 22. Additional brightness gains are realized because the substrate 
on which the anode is fabricated need not be transparent, and can 
therefore be fabricated with improved heat dissipation. 
While the above described examples refer to vacuum fluorescent displays of 
a type using relatively low emitter voltages, this invention may be used 
in displays in which high emitter voltages are used, i.e., 1000-2000 volts 
and higher, in which case the array of field emitters is spaced further 
from the display phosphors.