Method for obtaining self-aligned openings, in particular...

Details Method for obtaining self-aligned openings, in particular... Method for obtaining self-aligned openings, in particular...

2000-04-04

2001-08-21

Ramsey, Kenneth J. (Department: 2879)

Electric lamp or space discharge component or device manufacturi

Process

With assembly or disassembly

Reexamination Certificate

active

06276981

ABSTRACT:

FIELD OF THE INVENTION
This invention involves a process for making at least one group of apertures spaced in a precise manner on a structure by photolithography, this group of apertures including a first aperture or apertures made in a first layer of material and a second aperture made in a second layer of material which covers the first layer of material, the first aperture or apertures being located within the second aperture. It involves in particular the making of a auto-aligned focusing grid for a flat micropoint screen.
STATE OF THE ART
Documents FR-A-2 593 953 and FR-A-2 623 013 disclose devices for visualisation by cathodoluminescence excited by field emission. These devices include a micropoint emitting cathode electron source.
By way of illustration,
FIG. 1
is a cross section view of such a micropoint viewing screen. The screen is composed of a cathode
1
, which is a plane structure, oriented with respect to another plane structure which forms the anode
2
. The cathode
1
and the anode
2
are separated by a space in which a vacuum has been created. The cathode
1
includes a glass substrate
11
on which the conductor level
12
has been applied in contact with the electron emitting points
13
. The conductor level
12
is covered with a layer of insulation
14
, made of silica for example, which is itself covered by a conducting layer
15
. Holes
18
of about 1.3 &mgr;m in diameter were made through the layers
14
and
15
up to the conducting level
12
to apply the points
13
on this conductor level. The conducting level
15
acts as an extraction grid for the electrons which will be emitted by the points
13
. The anode
2
. includes a transparent substrate
21
covered by a transparent electrode
22
on which luminescent phosphors or luminophores
23
have been deposited.
The operation of this screen will now be described. The anode
2
is brought to a positive voltage of several hundred volts with respect to the points
13
(typically 200 to 500 V). A positive voltage of several dozens of volts (typically 60 to 100 V) with respect to the points
13
is applied to the extraction grid
15
. Electrons are then drawn from the points
13
and are attracted by the anode
2
. The trajectories of the electrons are within a half-angle cone at the peak ⊖, depending on various factors such as the shape of the points
13
. This angle causes a defocusing of the electron beam
31
which increases as the distance between the anode and the cathode is increased. One way to increase the yield of the phosphors, and thus the luminosity of the screens, is to work with higher anode-cathode voltages (between 1,000 and 10,000 V), which implies separating the anode and the cathode further in order to avoid the formation of an electric arc between these two electrodes.
If good resolution on the anode is desired, the electron beam must be refocused. This refocusing is classically obtained with a grid which can either be placed between the anode and the cathode or placed on the cathode.
FIG. 2
illustrates the case where the focusing grid is placed on the cathode.
FIG. 2
repeats the example of
FIG. 1
, but limited to a single micropoint for greater clarity in the drawing. An insulating layer
16
was applied to the extraction grid
15
and bears a metallic layer
17
which acts as a focusing grid. Holes
19
of an appropriate diameter (typically between
8
and 10 ·m) and concentric to holes
18
, were etched in layers
16
and
17
. The insulating layer
16
electrically insulates the extraction grid
15
and the focusing grid
17
. The focusing grid is polarised with respect to the cathode in order to give the electron beam the shape shown in FIG.
2
.
Simulation calculations show that centering of the holes
19
of the focusing grid with respect to the holes
18
of the extraction grid is extremely important. This structure is generally made using the classic photoetching techniques used in microelectronics. For example, with a first level of photoetching, the holes
19
of the focusing grid are defined, then a second level of photoetching is used to make holes
18
in which the points will be placed. To ensure proper functioning, the second level must be positioned in an extremely precise manner with respect to the first level. This can only be done with very high-quality, expensive equipment, a serious drawback if large areas are treated.
SUMMARY OF THE INVENTION
The invention solves the problem of precision alignment of holes located on different levels. This is achieved by a process which requires only a single photolithography step and only one template for two types of patterns: those for the holes on the lower level of lesser diameter and those for the upper level holes of greater diameter. The precision of the relative positioning of the holes is therefore that of the template pattern.
The purpose of the invention is thus the making by photolithography of at least one group of apertures spaced in a precise manner on a structure, this group of apertures including a first aperture or apertures made in a first layer of material and a second aperture made in a second layer of material which covers the first layer of material, the first aperture or apertures being located within the second aperture, characterised in that it includes:
the depositing on the free side of the first layer of material of a photosensitive resin layer of a determined thickness,
the etching of this resin layer by photolithography, by means of a single template, to leave on the aforesaid first layer of the material one spot of resin per group of apertures, the exterior limit of the spot of resin corresponding to the second aperture, the spot of resin including an aperture or apertures corresponding to the first aperture or apertures,
vacuum application, on the first layer and on the remaining resin, of material to form the second layer, this deposit being done so that the part of the first layer located at the bottom of the aperture or apertures of the spot of resin is not covered by this deposit,
the etching of the first layer of material from the aperture or apertures of the spot to obtain the first aperture or apertures in the aforesaid first layer,
elimination of the remaining resin and material from the second layer covering the aforesaid remaining resin to obtain the second aperture in the aforesaid second layer.
The aforesaid group of apertures can include a first aperture which is a circular hole centred in the second aperture which is also a circular hole. It may also include first apertures which are circular holes oriented along the main axis of the second aperture which is a slit.
This process is advantageously applied to manufacturing of a micropoint electron source with an extraction grid and a focusing grid. According to the invention, a manufacturing process for such a source involves:
a step during which are successively applied to one side of an electrically-insulated support: means for cathodic connection, a first layer of electrical insulation of a thickness adapted to the height of the future micropoints, a first conducting layer to form the extraction grid, a second electrically insulating layer of thickness corresponding to the distance which must separate the extraction grid from the focussing grid, and a layer of photosensitive resin of a given thickness,
a step for etching the resin layer by photolithography, by means of a single template, to leave on the aforesaid second insulating layer one spot of resin per aperture of the focussing grid, the exterior limit of the aforesaid spot of resin corresponding to the aforesaid aperture of the focussing grid, the spot of resin including one aperture per aperture of the extraction grid contained in the aforesaid aperture of the focussing grid,
a step for vacuum application on the second insulating layer and on the remaining resin of a material to form the focussing grid, this deposit being made so that the part of the second insulating layer located at the bottom of each aperture of the spot of resin is not covered by th

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