Note: Descriptions are shown in the official language in which they were submitted.
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CERAMIC EVAPORATION 8OAT8 HAVING IMPROVED INITIAL WETTING
PERFORMANCE AND PROPERTIES
BACKGROUND OF THE INVENTION
The invention relates to ceramic evaporation boats
having improved initial wetting performance and properties and
processes for producing such products.
The most widely used method of coating flexible
substrates with metals, in particular with aluminum, is high
vacuum tape coating. The substrate to be coated is passed over
a cooled roller while being exposed to aluminum vapor which
deposits on the substrate surface as a thin metal layer.
To generate the constant vapor stream required,
ceramic evaporators known as evaporation boats, are heated to
about 1450°C by direct passage of electric current through the
evaporation boat. Aluminum wire is continuously fed to the
boat, liquefied on the ceramic surface and evaporated in a
vacuum of about 10-° mbar. In metallization units, a series of
evaporation boats are arranged in such a way that a uniformly
thick aluminum layer is deposited across the entire width of
the substrate.
Evaporation boats are generally made of hot-pressed
titanium diboride (TiB2) and boron nitride (BN) and/or
aluminum nitride (A1N). In such evaporation boats, TiB2 is the
electrically conductive component which allows the evaporator
to be heated like an ohmic resistance.
One of the main problems in the operation of web
coating units is the initial wetting of the evaporation boats
with the metal to be vapor-deposited. In practice, the
operator has to have a great deal of experience in order to be
able to carry out the initial wetting of the evaporation boats
in an optimal way. Thus, the term "break-in procedure" has
CA 02271933 1999-OS-14
become established to describe this initial wetting of the
evaporation boat which illustrates the complexity of this
step. Thus, during the running-up phase, the cavity of the
evaporator can be incompletely wetted. This results in
increased deposits on the side opposite that where the wire is
fed which forces the operator to run the evaporation boat
"hot" for a given evaporation rate, i.e., to heat it to a very
high temperature. This leads to a drastic decrease in the life
of the evaporation boats.
In addition, incomplete wetting corresponds to non-
uniform wetting of the cavity of the evaporation boat. As a
result, uniform, continuous evaporation of the metal to be
evaporated is not possible later. This forces the operator to
adjust the evaporator heating continually. As a result, the
evaporation boat is, on average, run too hot. This greatly
reduces the life of the evaporation boats, as already
mentioned.
In the case of evaporation boats having a very high
electric resistance, the voltage of the vapor deposition unit
is generally insufficient tv heat it to the wetting
temperature. If such an evaporation boat were to be wetted
more readily than conventional evaporation boats, some wetting
would take place even before the full wetting temperature is
reached. As a result, the system evaporation boat-aluminum
bath has a lower electric resistance. This immediately
produces a higher current which in turn leads to better
heating of the evaporation boat and consequently also to even
better wetting.
This also makes the problem of the difference in the
resistance from evaporation boat to evaporation boat
significantly less critical and stoppage of the vapor
deposition units due to high-resistance evaporation boats does
not occur.
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The better the wetting of the evaporator material by
the metal to be evaporated, the lower the risk of the
evaporation boat being overheated and thus the life of the
evaporation boat being drastically reduced. In addition, good
wetting leads to optimum metal bath formation in the cavity of
the evaporation boat and thus to improved evaporation
conditions and more uniform stressing of the evaporation boat
which, in turn, increases the life of the evaporation boat.
It is an object of the invention to provide an
evaporation boat of ceramic material for the evaporation of
metal comprising a conductive component and a nonconductive
component, which boat is initially wetted more readily by the
metal to be evaporated.
BRIEF SiJ~RY OF THE INVENTION
The object is achieved by an evaporation boat in which
the conductive component of the ceramic material is
concentrated at the surface of the evaporation boat at which
evaporation of the metal occurs.
Preferably, the content of conductive component at the
surface of the evaporation boat at which evaporation of the
metal occurs is at least two ~ (2$) (relative) higher than in
the remaining material of the evaporation boat.
Preferably, a layer of the conductive component of the
ceramic material is located on the surface of the evaporation
boat at which the evaporation of the metal occurs.
The layer of conductive component should preferably be
in electrical contact with the remaining evaporator boat
30. material. As a result, this layer becomes self-conducting and,
owing to the low resistivity of the material, becomes hotter
than the remaining evaporator boat material. This again leads
to an improvement in the wettability of the surface of the
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evaporator boat.
The concentration of the conductive component at the
surface of the evaporation boat is preferably achieved
starting from an evaporation boat known from the prior art by
means of one of the three methods described below:
ly The surface of the evaporation boat from which the
evaporation of the metal is to take place in normal
operation is heated by means of a high-energy beam so
that the nonconductive components [in general BN (melting
point: 2300°C) and A1N (melting point: 2300°C)] evaporate
and at the same time the conductive component [in general
TiB2 (melting point: 2900°C) ] is only melted. The energy
content of the high-energy beam is therefore preferably
selected such that it heats the surface of the
evaporation boat to more than 2900°C but not less than
2700°C. This results, after cooling, in a layer enriched
in the conductive ceramic component (in general TiB2) on
the surface of the evaporation boat. Briefer heating
gives a layer which is less enriched in the conductive
ceramic component on the surface of the evaporation boat.
2) Powder comprising the conductive ceramic component can be
applied to the surface of the evaporation boat and welded
on by means of a high-energy beam so as to form an
electrically conductive layer of the conductive ceramic
material. This can be achieved, for example, in the case
of TiB2 as a conductive ceramic component, using a method
analogous to a TiBz powder coating process known per se.
3) Powder comprising the conductive ceramic component can be
processed with an organic or inorganic binder to form a
paste and the paste applied to the surface of the
evaporation boat. The binder is selected so that it
evaporates during the heating of the evaporation boat.
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Thus, when the evaporation boat is heated, the binder
evaporates and the desired electrically conductive layer
which can subseguently be wetted by aluminum is formed.
The binder used can be, for example, glycerol. This layer
can be additionally treated by means of a high-energy
beam as described in 2) to obtain better contact between
the electrically conductive layer and the remaining
evaporator material.
DETAILED DESCRIPTION OF TgE INVENTION
As a rule, the electrically conductive component of
the ceramic material is TiB2.
For this reason, a TiBZ-containing powder is generally
used as a powder comprising the conductive ceramic component.
Preference is given to using TiB2 powder.
As the high-energy beam, it is possible to use, for
example, a laser beam. The laser used can be, for example, a
gas, solid-state or semi-conductor laser.
The heating of the surface of the ceramic evaporation
boat by means of a high-energy beam is preferably carried out
under inert gas conditions. Examples of inert gases are helium
and argon.
The evaporation boats of the present invention have
the following advantages over known evaporation boats:
1) From the commencement of use, they display good uniform
wetting which leads to a constant (over time) and uniform
(in space) evaporation rate without scattering.
2) They run in a steady manner from the beginning and the
power does not have to be continually adjusted.
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3) The scattering in the resistance of the evaporation boats
which results from the scarcely avoidable scatter in the
resistance of the sintered body from which the
evaporation boats are produced does not have the adverse
effect of the higher-resistance evaporation boats no
longer being wettahle and the vapor deposition units
therefore having to be stopped.
The following examples illustrate the invention.
Example 1: Production of an evaporation boat according to the
invention
The surface from which metal evaporation is to take
place of an evaporation boat having the dimensions 1Ox20x120
mm and produced from a ceramic material consisting of 97.58 by
weight of TiBz and 52.5 by weight of BN was irradiated by
means of a YAG laser (wavelength = 1.06 ~m/beam diameter = 6
mm/power = 100 W) in a plurality of traces (80 mm long) in a
stream of argon.
Esample 2: Production of an evaporation boat accordinq to the
invention
Grooves having a depth of about 0.5 mm were scratched
mechanically into the surface of an evaporation boat from
which metal evaporation is to take place (1Ox20x120 mm/47.5$
by weight of TiBz and 52.5 by weight of BN). TiB2 powder was
sprinkled into these grooves. This powder was irradiated by
means of a YAG laser as described in Example 1. Unbound powder
residues were subsequently blown off using compressed air.
Fxample 3: Production of an evaporation boat according to the
invention
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A paste of Ti82 powder and glycerol as binder was
applied by means of a spatula to the surface of an evaporation
boat from which metal evaporation is to take place (1Ox20x120
mm/47.5~ by weight of TiB2 and 52.5 by weight of Bt3).
Example 4: Comparison of the evaporation boats of the
invention with a known evaporation boat
As a comparison evaporation boat, a further
evaporation boat having the same dimensions as described above
was produced from the same batch of sintered material (47.5
by weight of TiBz and 52.5 by weight of Bt3) from which the
evaporation boats described in Examples 1 to 3 had been
produced.
This comparison evaporation boat and the evaporation
boats of the invention produced in Examples 1 to 3 were
compared under defined conditions.
The evaporators were clamped in place at the end faces
and, before heating, 2 g of A1 wire was laid on the middle of
the surface of the evaporation boat from which metal
evaporation is to occur. A vacuum of < 1x10-4 mbar was applied.
In this high vacuum, the evaporation boats were heated
linearly over a period of 10 minutes to a power of 3.96 KW by
means of a power ramping software program.
The evaporator voltage was measured directly at the
end faces of the evaporators. At the same time, the current
flowing through the evaporation boat was measured at a
different point in the circuit. The measured values for
voltage and current were recorded at one second intervals.
They were available as a data file after the experiment.
The wetting of the evaporator surface by the aluminum
was generally indicated by a disproportionate increase in the
current at a given voltage compared to the increase in the
current through an evaporator without aluminum placed on it,
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since the wetting aluminum makes possible an additional flow
of current compared to an evaporator without aluminum.
If the heating power at which this increase in current
takes place is calculated for the glowing surface, a wetting
power per unit area of glowing surface of 44W/cm2 is obtained
for the comparison evaporation boat while that for the
evaporation boats from Examples 1 to 3 is < 90W/cm2.
This result demonstrates that the evaporation boats of
the invention were wetted at lower temperatures than the
comparison evaporation boat and thus have a better initial
wetting performance than known evaporators.
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