Coating processes – Coating by vapor – gas – or smoke – Moving the base
Reexamination Certificate
1997-10-21
2003-01-21
Bueker, Richard (Department: 1763)
Coating processes
Coating by vapor, gas, or smoke
Moving the base
C118S726000, C118S729000, C118S730000
Reexamination Certificate
active
06509061
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general terms to the deposition of material by evaporation on a substrate having a large size. One application is the production of microtip or microdot electron sources using field effect electron emission, said sources being of large size. Said electron sources are e.g. used in field emission-excited cathodoluminescence display means and in particular in flat screens of large size (approximately 1 m
2
).
2. Discussion of the Background
A microtip emissive system and its production process are e.g. described in detail in FR-A-2 593 953 of Jan. 24, 1986 and in FR-A-2 663 462 of Jun. 13, 1990. Firstly a description will be given of the known procedure for producing such microtips in a structure of this type, such as can be gathered from the aforementioned document by referring to the enclosed
FIGS. 1
,
2
and
3
.
FIG. 1
shows an already produced structure having on a substrate
6
surmounted by an insulator
7
, a system of cathode conductors
8
, a resistive layer
9
, grids
10
a
superimposed in crossed manner with an intermediate insulator
12
and a layer
23
, e.g. of nickel, deposited on the surface to serve as a mask during the microtip production operations. This nickel layer
23
, the grids
10
a
and insulator
12
are perforated by holes
16
, in whose bottom is to be deposited the future microtips constituted by a conductive metal electrically connected to the cathode electrode
8
across the resistive layer
9
.
The following procedure is taken in the production of microtips and is illustrated in FIG.
2
. Firstly, e.g. deposition takes place of a molybdenum layer
18
a
on the complete structure. Said layer
18
a
has a thickness of approximately 1.8 &mgr;m. It is deposited under normal incidence with respect to the surface of the structure. This deposition procedure makes it possible to obtain molybdenum cones
18
housed in holes
16
having a height of 1.2 to 1.5 &mgr;m. This is followed by the selective dissolving of the nickel layer
23
by an electrochemical process so as to free, as shown in
FIG. 3
, perforated grids, e.g. of niobium
10
a,
so as to bring about the appearance of the electron emitting microtips
18
.
To within a few technological variants, the aforementioned known method described relative to
FIGS. 1
,
2
and
3
is still that which is used for producing the microtips of emissive cathode systems.
The stage of depositing the molybdenum layer
18
a
conventionally takes place by evaporation of molybdenum heated in a crucible and condensation of the molybdenum vapour on the substrate covered with the stacked structure
7
,
8
,
9
,
12
,
10
and
23
, perforated with holes
16
.
In order to obtain tips
18
in holes
16
, the vapour must arrive under a quasi-normal incidence on the surface of the aforementioned structure, e.g. under an angle between &thgr;=0 (normal incidence) and &thgr;
max
=9°. Otherwise the evaporated material may cover the walls of the holes
16
and create short-circuits between the electrodes
10
a
and the layers
8
and
9
, or may fill said holes without creating tips.
A conventional evaporator is illustrated in FIG.
4
. The material to be evaporated is heated in a source crucible
30
. The substrates
33
-
1
,
33
-
2
,
33
-
3
are positioned on a substrate holder
31
and are rotated on themselves about an axis
34
-
1
,
34
-
2
,
34
-
3
. The substrate holder
31
is positioned facing the crucible
30
and is itself rotated about the axis
34
-
2
. Evaporation takes place in vacuo at approximately 10
−5
to 10
−6
mbar in an enclosure
35
pumped by an adapted pumping system
36
.
In the case of the deposition of microtips, the substrate holder
31
has an appropriate shape to ensure that the substrates
33
receive the vapour in quasi-normal incidence with a maximum angle &thgr;
max
.
This apparatus is suitable for making deposits on small or average size substrates (max a few dozen cm). However, problems arise when the size of the substrates is increased.
Thus, on considering the case of
FIG. 4
, where the substrate
33
-
2
is positioned facing the source
30
at a distance h, we then obtain
h
=
AB
_
tg
θ
,
AB
_
being
the
radius
(
or
half
-
diagonal
)
of
the
substrate
.
For a substrate of 1 m diagonal and an angle &thgr;
max
of 9° (quasi-normal incidence), the height h is consequently
0.5
tg
9
°
≃
3.16
m
.
The expert also knows that the vacuum evaporation rate is proportional to 1/h
2
. It is therefore of interest to reduce the distance h between the source and the substrate and a distance of 3.16 m is unacceptable for maintaining a high deposition rate.
In summarizing, as the vapour incidence angle &thgr; is very small (a few degrees), it is virtually impossible to envisage the deposition on a substrate of 1 m diagonal, the deposition rate being much too low for an industrial process.
FR-A-2 701 601 describes a first apparatus for producing microtip sources, also known as the ICB apparatus. A crucible is heated by a heating element and the vaporized material is then partly ionized, which leads to the formation of clusters, which are accelerated. This type of apparatus is not suitable for producing deposits on substrates having a large surface.
A second apparatus is also described in said document and has a crucible provided with a plurality of nozzles through which a vapour is discharged into a vacuum tank, where it is ionized and then accelerated. In order to improve the directivity, said document proposes increasing the ratio L/r, where L is the thickness of the crucible and r the diameter of the nozzles.
However, this type of apparatus is incompatible with industrial production, particularly on large substrates. In particular, the nozzles can progressively become clogged by parasitic material deposits. If the ratio is L/r increased in order to increase the directivity, said problem becomes even more acute, because the vapours traverse the nozzles over a greater length, whilst the latter have a smaller diameter. This also leads to evaporated material flow rate variations.
Finally, the apparatuses described in the aforementioned documents involve ionization and acceleration sections and are consequently complex.
SUMMARY OF THE INVENTION
The object of the invention is to solve the aforementioned problem, i.e. to propose a simple apparatus making it possible to make deposits on large substrates and making it possible to maintain a relatively small vapour incidence angle and an adequate deposition rate for an industrial process.
To this end the invention relates to an apparatus for depositing a material by evaporation on a substrate, said apparatus being characterized in that it comprises an enclosure in which are placed n evaporation sources for the material to be deposited by evaporation and means for piping the vapours emitted by said sources towards the substrate.
The use of several sources makes it possible to decrease the distance between the evaporation source and the substrate and consequently the height of the system. Moreover, the means for channelling or piping to the substrate the vapours emitted by the sources make it possible to limit the incidence angle of the vapour which is condensed on the substrate. Thus, it is possible to simultaneously decrease the source-substrate distance and the angle of incidence of the vapour. The sources are placed directly facing the substrate and directly see the substrate or a portion thereof.
The means for piping the emitting vapours can be covers or walls separating the individual sources. The covers form compartments within the enclosure, each evaporation source being placed in a compartment. These covers or walls can be vertical.
According to a variant, the covers or walls can form cells at least partly having a truncat
Borel Michel
Charles Raymond
Ida Michel
Perrin Aime
Bueker Richard
Commissariat a l'Energe Atomique
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