Method of manufacturing a pointed electrode, and device for using said method

The manufacture of pointed electrodes (28) (field emitters) is considerably simplified by a directed sputtering deposition instead of by vapor deposition. Directed deposition of the material (12) to be sputtered can be effected by means of a collimating filter (4) and, if necessary, a cover plate (8).

BACKGROUND OF THE INVENTION 
The invention relates to a method of manufacturing, on a substrate, at 
least one electrode whose cross-section parallel to a main surface of the 
substrate is larger at the base than at the top in which method material 
for forming the electrode is deposited on the substrate via an apertured 
mask parallel to a main surface of the substrate, the material also 
growing on the mask in a lateral direction within the aperture during 
deposition. 
Such an electrode may be conical or pyramid-shaped, but also, for example 
wedge-shaped. At this main surface the substrate may already be provided 
with, for example, metal tracks on which the material is deposited. 
The invention also relates to an electrode manufactured by such a method, 
which electrode may function, for example, as an electron source (field 
emitter) or ionizer. 
Such electron sources are particularly useful in flat display devices. The 
ionizers are notably used in scientific apparatus. 
A method of the type mentioned in the opening paragraph is described in 
U.S. Pat. No. 5,007,873. In this method, the electrodes (field emitters) 
are obtained by depositing the electrode material through a mask on to the 
substrate by vapour deposition in which the material for manufacturing the 
electrodes is released from a source by electron beam vapour deposition. 
The greater part of the electrode material is deposited on the surface of 
the substrate perpendicularly or at a small angle with respect to the 
normal. Due to growth of electrode material in the lateral direction at 
the location of the apertures in the mask, a part of this material causes 
these apertures to become gradually smaller, and the cross-sections of the 
growing electrodes become smaller so that the electrodes acquire a 
pointed, for example, a conical shape. 
If the electrode is used as a (field) emitter, it is necessary for a 
satisfactory emission that said point or cone has an acute apex angle. 
Moreover, to obtain a uniform setting throughout the image when using 
larger flat display devices having a plurality of emitters per pixel, 
there should not be too much spread in the shapes of the emitters. This 
means that the source from which the material is released should be placed 
far remote from the substrate. The vacuum installation to be used then 
becomes disproportionately large and expensive, while evacuation also 
takes a long time. Moreover, the material choice is limited when these 
types of methods (vapour deposition, electron beam deposition) are used, 
because measures should generally be taken to achieve relaxation of 
mechanical stress in the layer growing on the mask. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is, inter alia an object of the invention to provide a method of 
manufacturing such pointed electrodes which can be used on an industrial 
scale and with simple equipment. 
It is another object of the invention to provide such a method which a 
large number of material may be used. 
To this end the method according to the invention is characterized in that 
it comprises at least one step in which material for manufacturing the 
electrode is deposited by sputtering in a direction substantially 
perpendicular to the main surface of the substrate. 
In this respect, substantially perpendicular is understood to mean that a 
part of material to be sputtered may be deposited on the main surface at a 
small angle (up to approximately 15 degrees) with respect to the normal. 
The use of sputtering not only has the advantage that a smaller vacuum than 
with vapour deposition is sufficient, but also a large number of materials 
can be used for deposition without the necessity of using a second 
material for realising relaxation in the deposited closing layer, as is 
the case when using the method of vapour deposition. Moreover, materials 
can be deposited in a simple manner from a plurality of components (such 
as, for example zirconium carbide, which has a very low work function) by 
means of sputtering. Even when the constituents are not released in the 
correct proportions due to unequal sputtering rates at the source, there 
is still such a state or balance at the source that material of the 
desired composition is deposited on the substrate. 
However, in the case of vapour deposition, two sources which must be 
arranged at slightly different angles (with respect to the substrate) have 
to be used already at slightly different vapour pressures of the 
constituents. To obtain a substantially perpendicular incidence of the 
material, these sources have to be arranged far away from the substrate, 
which renders the manufacture much more complicated (hence more 
expensive), while evacuation of such a device takes a longer time and is 
more expensive. 
More generally, sputtering requires less space than vapour deposition, so 
that less evacuation capacity is sufficient. Other advantages of 
sputtering as compared with vapour deposition are: 
1) more efficient use of material 
2) a better (larger) deposited, which adds to the lifetime of the emitters 
3) wider processing tolerances; in sputtering a larger number of parameters 
is adjustable, such as, for example, sputtering pressure (pressure of an 
inert gas such as argon present in the sputtering space) and the voltage 
on the substrate. 
For example, sputtering in a direction substantially perpendicular to the 
main surface can be achieved in that the material to be sputtered or a 
constituent thereof is obtained in a particle-generating portion having an 
exit aperture, while at least a particle-collimating filter is present 
between the exit aperture and the mask. The stream of particles which has 
a considerable spatial spread in directional components after it has left 
the particle-generating portion is directed, as it were, by the filter and 
acquires such a distribution in directional components that the particles 
are deposited substantially perpendicularly to the main surface of the 
substrate. The exit aperture is preferably larger than the main surface of 
the substrate so as to prevent peripheral effects. 
The particles can be generated in a plasma in a magnetron accommodating a 
source with the material to be deposited (or a constituent thereof), as is 
further described in EP-A-0 440 377. 
In another embodiment, the particles are generated in a magnetron with a 
hollow cathode as described in greater detail in U.S. Pat. No. 4,824,544, 
whose contents are herein incorporated by reference. 
The filters generally comprise a plurality of parallel ducts having, for 
example a square, hexagonal or circular cross-section. 
As has been stated hereinbefore, it is important that field emission 
cathodes have a structure which is as uniform as possible when 
manufacturing display devices in which such field emission cathodes are 
used. To achieve this, the substrate, the filter and the sputtering source 
may be moved parallel with respect to each other to the main surface, for 
example during deposition. This may involve a rotation but also, for 
example a translation in which the substrate is led along different 
sources and the type of deposition (material, deposition rate) is varied 
dependent on the source. 
The uniformity can be improved by providing a cover plate between the 
filter and the mask, said cover plate, in a cross-section of the filter 
perpendicular to the axis of the filter, having apertures of substantially 
the same size as the particle-passing apertures. Substantially the same 
size is herein understood to mean that the diameters of the apertures in 
the cover plate differ by at most 20% from those of the apertures in the 
filter. 
A further preferred embodiment of a method according to the invention is 
characterized in that the particle-collimating filter is present on the 
mask. Little material is then lost. In this case the filter may comprise a 
plate having apertures at the location of the areas where electrode 
material is to be deposited; notably if a pixel is made which comprises a 
plurality of emitters per pixel and if the apertures coincide with the 
area of a pixel, the emitters within one pixel are equal in shape due to 
this measure and consequently have a uniform emission behaviour; if 
necessary, the filter may comprise a plurality of such plates (or it may 
even be temporarily provided on the substrate by use of deposition 
techniques). 
The method may also be used for manufacturing a structure which is further 
provided with, for example a metal layer and an extra layer of insulating 
material. An electron source (field emitter) provided with a grid 
electrode is obtained by means of such a method. 
A further method according to the invention is characterized in that the 
electrodes are manufactured in a plurality of steps, with 
particle-collimating filters being used in consecutive steps, whose 
apertures passing the particles, viewed in a cross-section, have smaller 
sizes in consecutive steps. 
In this method a coarse structure may initially be provided while using a 
small quantity of material and a filter having large apertures; the 
ultimate point, which is necessary for, for example, field emission, is 
then obtained via a filter having smaller apertures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows diagrammatically a cylindrical sputtering device 1 with walls 
2. A particle-generating portion 3 in which, for example, molybdenum 
particles 11 are generated, is present within the cylinder. The 
particle-generating portion 3 is secured to the walls 2 by securing means 
10 which are not further described. The particles pass a collimating 
filter 4, which is composed, for example, of a plurality of densely packed 
tubular parts, and then impinge on a substrate 7 which is present on a 
mounting table 6. A plurality of substrates 7 may be present on the 
mounting table 6. The particles 11 acquire a direction substantially 
perpendicular to the substrate 7 (particles 12) in the filter 4. In this 
embodiment a cover plate 8 having apertures, whose function will 
hereinafter be described, is present between the filter 4 and the 
substrate 7. The device 1 also has means (not shown) for generating a 
plasma, means for setting pressure and temperature, etc. 
A portion of the particles 12 has a small deviation with respect to this 
perpendicular direction (up to, for example 15 degrees). The maximum 
deviation is determined by the ratio between the length and the diameter 
of the tubular parts, which is between 1:2 and 1:10. 
A substrate 7 (FIG. 2) comprises, for example a layer of glass having a 
main surface 20 on which molybdenum strips 21 having a thickness of 
approximately 0.3 .mu.m are provided, for example, by first providing a 
layer of molybdenum 21 and subsequently etching in accordance with a 
pattern in a suitable etchant. Subsequently a layer of silicon oxide 22 is 
provided which is coated with a layer 23 of molybdenum. 
If necessary, the layer 23 is first divided into strip-shaped bands which 
together with the strips 21 define a matrix structure. The strip-shaped 
bands also have apertures 25; the silicon oxide is removed at the location 
of these apertures, for example, by means of dry etching. Subsequently a 
lift-off layer 24 of aluminum oxide is provided by means of an oblique 
vapour deposition process. By vapour depositing obliquely, aluminum oxide 
is prevented from being deposited on the bottom (and the walls) of the 
aperture 25. The device of FIG. 2a is thereby obtained. The double layer 
23, 24 functions as a mask in the next step. 
One or more of these substrates is subsequently placed on the table 6 in 
the sputtering device 1. Molybdenum particles generated in the portion 3 
and subsequently passed through the filter 4 are deposited (in this 
example) by means of sputtering on the surface 26 of the double layer 23, 
24 and strips 21 located on the substrate 7. Since a certain spread in the 
direction of the particles occurs (for example, up to approximately 10 
degrees around the perpendicular to the surface 26 the growing layer 27 of 
molybdenum above the apertures 26 also grows laterally at the location of 
these apertures and gradually closes until the situation of FIG. 2b is 
achieved. 
As growth as the layer 27 proceeds the aperture through which material to 
be deposited is passed becomes smaller, resulting in deposition of a 
pointed body 28 within the apertures are 25. In this embodiment, the body 
28 is cone-shaped because the apertures 25 are chosen to be round and 
sputtering is continued until the aperture is completely closed. 
Subsequently the lift-off layer 24, and consequently the layer 27, are 
removed, and the device according to FIG. 2c is obtained. 
The (molybdenum) particles 11 may be generated in the portion 3 by a 
magnetron in which a source is present on the cathode plate with the 
material from which the particles are formed, as described in EP-A-0 440 
377. Another possibility is described in U.S. Pat. No. 4,824,544, in which 
the source is placed on a magnetron assembly within a chamber whose 
(vacuum) wall also functions as an anode; the cathode is a hollow cathode 
which is isolated from the vacuum wall. The ions of the combined plasma 
thus obtained bombard a particle source which supplies the particles 11 or 
composite particles to be sputtered. These particles leave the 
particle-generating portion 3 via an exit aperture 13. 
The particles 11 thus obtained subsequently reach the filter 4 where they 
acquire a direction with a component substantially perpendicular to the 
mounting table 6 (and hence to the main surface 20 of the substrate 7). At 
the location of the exit apertures of the filter, a plurality of exiting 
particles 12 has a deviating directional component. Closure of the layer 
27 by means of growth as described with reference to FIG. 2b is then 
realised. 
As described in the opening paragraph, it is important for a uniform 
electron emission that the electrode bodies 28 are mutually equal as much 
as possible when they are used as field emitters. This is not very well 
possible without extra measures, notably under the walls 14 of the ducts. 
The duct walls 14 of the composite parts of the filter may have a large 
thickness as compared with the mutual distance of juxtaposed apertures 25 
in the layer 22. This may give rise to a non-uniform growth, notably of 
the pointed emitters 28 located under these duct walls. This will be 
further explained with reference to FIG. 3. 
The emitter 28a halfway between the apertures 15a and 15b, receives an 
approximately equal quantity of material from both apertures 
(approximately along the lines 16a and 16b). However, this material is 
deposited with a larger horizontal component than that for an emitter 
located directly underneath one of the apertures 15, so that this emitter 
will have a blunter (and lower) point than the emitters underneath the 
apertures 15. The emitter 28b receives material from the aperture 15a at a 
different horizontal rate component (line 17a) than from the opening 15b 
(line 17b) while this is conversely true for the emitter 28c. This leads 
to a non-uniform growth of these emitters, which becomes manifest in a 
non-uniform emission in the ultimate display device. 
This non-uniform growth can be obviated by giving the substrate a 
continuous movement such as a rotation about a central axis or a 
translation. If necessary, a cover plate 8 may be arranged between the 
filter 4 and the substrate 7, provided with 23, corresponding in this 
embodiment to those of the filter 4. The lateral growth of the layer 27 is 
now only caused by particles 12 having a small horizontal directional 
component. Emitters 28 having an extra sharp point are then obtained. 
As the apertures 15 in the filter have a smaller diameter, the horizontal 
component of the rate of the particles 12 decreases so that the points of 
the emitters 28 become sharper, hence improving the efficiency of the 
emitters to be manufactured. On the other hand, the closing layer 27 will 
also become considerably thicker, while a larger quantity of material is 
left on a possibly present cover plate 8. This may be largely prevented by 
using a device as shown in FIG. 4. It comprises, for example, two 
component devices 1a, 1b, each as described with reference to FIG. 1. In 
the first device 1a the filter 4a (and a possible cover plate 8a) has, for 
example a coarse structure with large hexagonal apertures 15a (see FIG. 
5). The particles 12 have a larger rate component perpendicular to the 
normal so that the layer 25 grows and closes more rapidly. Initially, the 
emitters 28 grow with a less sharp slope, but less material is lost and 
the process is faster. However, before the layer 25 has fully grown and 
closed, the substrate is moved via a vacuum lock 18 or another means to 
the device 1b which is substantially analogous to the device 1a, except 
that the apertures 15b in the filter 4b have a smaller diameter (see FIG. 
6). The last part of the emitters is then formed in the device 1b. FIG. 7 
shows the completed emitter 28 comprising a part 28a formed in device 1a 
and a sharp point 28b formed in device 1b. After mounting in a display 
device, the actual emission takes place in points 28b which are very sharp 
and uniform in their emission behaviour due to a suitable choice of the 
sputter parameters and the filter dimensions in device 1b. These steps may 
also be performed within one one device by replacing the filter 4a (and 
PG,10 possibly an associated cover plate 8a) after some time by the filter 
4b (and the associated cover plate 8b) having smaller apertures. The 
filters 4a and 4b may also be accommodated in one device 1, while 
intermediate steps with filters of a different diameter are also possible. 
FIG. 8 shows diagrammatically a device in which the filters 4 are provided 
on different substrates 7 which already have a structure as shown in FIG. 
1a. This may be effected, for example by providing plates 9 of, for 
example glass on these structures, which plates have apertures at the 
location of those areas where electrode material must be deposited on the 
lift-off layer 24 and in the apertures 25. When using apertures which 
coincide with a pixel having a plurality of electrodes per pixel, the 
electrodes within one pixel hardly vary in shape and consequently have a 
uniform emission behavior. In order to deposit the molybdenum particles on 
the surfaces of the substrates 7 at a smaller angle with respect to the 
normal, extra plates 9' may be used. Also, a temporary pattern of the 
desired shape of, for example polyimide which is removed after completion 
may be used. Otherwise, the reference numerals denote the same parts as in 
FIG. 1. Here again, the maximum deviation of the angle with respect to the 
normal is determined by the ratio of the length and the diameter of the 
apertures in the plates 9' and the assembly of plates. 
FIG. 9 is a diagrammatic elevational view of a flat display device 50 
having a large number of (field) emitters 28 per pixel. This display 
device comprises a crossbar or matrix system of row conductors 31 and 
column conductors 32 on a substrate 30, which conductors are mutually 
insulated by a layer 34 of silicon oxide under the column conductors 32. 
For the sake of simplicity, the column conductors 32 and the layer 34 have 
been shown as a whole in the greater part of FIG. 8. At the location of 
the crossings, the double layer 32, 34 has apertures 33 (approximately 200 
per crossing) within which the (field) emitters 28 are located by, for 
example, the described hereinbefore. Due to the manner of manufacture, 
these emitters have a very uniform emission behaviour throughout the 
surface of the display device. 
The face plate 35 has a conducting layer 36 and phosphors (not shown). A 
voltage may be applied between the layer 36 and the row conductors 31 by 
means of a voltage source 37 so as to create an accelerating field between 
the plates 30 and 35, so that electrons generated by the (field) emitters 
28 are accelerated towards the face plate 35 where they cause pixels 38 to 
luminesce. To this end, a control unit 39 connects the row conductors 31 
successively to a voltage line 42 via switches 41 by means of a control 
line 40. Incoming information 43 is applied to the column conductors 32 
(for example, via shift registers 44) and connection lines 45. Mutual 
synchronization is effected via the line 46. 
The invention is of course not limited to the embodiments described, but 
several variations are possible within the scope of the invention. For 
example, before the first lift-off mask 24 is provided, a metal layer 19 
and a second layer 29 of insulating material can be provided. After using 
a method according to the invention, a device as shown in FIG. 10 having 
an extra grid 19 is obtained. 
Instead of passing a single collimating filter 4, the particles may 
alternatively pass a plurality of such filters which may have different 
sizes of the apertures 15. The same applies to the cover plate in the 
device of FIG. 1.