Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02693039 2010-01-12
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Ralf Stein
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Method for plasma-assisted chemical vapour deposition
on the inner wall of a hollow body
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The invention relates to a method for plasma-assisted
chemical vapour deposition for coating or material
removal on the inner wall of a hollow body.
Such methods are known by the generic terms plasma
coating (PECVD, "Plasma Enhanced Chemical Vapour
Deposition") or ion etching and plasma etching.
In this context, a workpiece is introduced into a
vacuum chamber and fixed there. The chamber is
evacuated to a residual gas pressure in the high-vacuum
or ultra high-vacuum range and an inert working gas is
admitted. A low-pressure plasma is subsequently ignited
by feeding in an RF field via an RF electrode arranged
in the vacuum chamber. An ionized gas is produced in
this case, said gas containing an appreciable
proportion of rapidly moving free charge carriers such
as ions or electrons.
In PECVD, besides the working gas further reaction
gases, as they are called, are fed into the chamber,
which gases can be, in particular, carbon-containing or
silicon-containing gases. In the low-pressure plasma,
the electrons have such high energies that chemical
reactions between the gas constituents and constituents
of the surface of the workpiece are possible which are
not possible at thermal equilibrium. Layers form on the
surface of the workpiece in this way, which layers can
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comprise carbon or silicon oxide, for example,
depending on the reaction gas. It is thus possible for
example to produce high-strength, low-friction and
biocompatible diamond-like carbon (DLC) coatings which
are used e.g. in implants, gearwheels and the like.
Ion etching and plasma etching, by contrast, involve
removing material from the surface of the workpiece in
order e.g. to clean the latter. For this purpose, the
ions of the low-pressure plasma generated have to have
a certain minimum energy. The acceleration of argon
ions in the high vacuum in the direction of the
substrate to be processed has the effect that, upon
impingement, momentum is transferred from the high-
energy ions to the substrate and the surface of the
latter is sputtered and removed uniformly.
In plasma etching, moreover, the etching is effected by
a chemical reaction. In this case, instead of pure
argon a reactive gas such as oxygen, for example, is
fed to the plasma.
Both PECVD and also ion etching and plasma etching have
proved to be extremely worthwhile in the surface
treatment of workpieces. However, at least when a
plasma is generated by radio-frequency excitation,
neither of these methods is suitable for coating or for
etching the inner surfaces of hollow bodies such as
e.g. containers, bottles, tubes, cannulas, bores and
the like.
This is due to the fact that conductive hollow bodies
form a Faraday cage in the electric field. The ions
produced are oriented according to the field lines of
said electric field. Since these run along and around
the outer wall of the hollow body but not through its
inner volume, an inner coating is not physically
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possible in a straightforward manner. In order to
circumvent this effect, the plasma has to be brought
into the inner volume of the hollow body. In this case,
a negative area replacing the role of the inner wall of
the chamber as negative area would have to be
introduced into the inner volume. In this case, the
size of the negative area must in principle be at least
twice as large as the surface to be coated, in order to
ensure a deposition sufficient for the layer
construction.
Accordingly, it is practically not possible to comply
with this principle within a hollow body.
In the case of a cylindrical hollow body, by way of
example, the inner surface area of the cylinder wall is
A = 2 nrh. A planar electrode, set up perpendicularly
in the hollow body, could have at most a surface area
of 2rh, however, that is to say would be smaller than
the surface to be coated by a factor of 3.14 rather
than being twice as large according to the technical
requirement.
Similar relationships hold true for other hollow bodies
such as e.g. cones and truncated cones or complexly
shaped hollow bodies.
DE 197 26 443 describes a method for surface refinement
of inner surfaces of hollow bodies in which the plasma
is ignited by a hollow cathode corona discharge. What
is disadvantageous here is that only relatively short
hollow bodies in which the depth does not exceed the
opening diameter can be coated from the inside. A
variant that enables longer hollow bodies to be coated
on the inside provides for the hollow cathode to be
inserted into the hollow body and to run along the
inner side. Thus, although longer hollow bodies can be
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coated on the inside, they nevertheless have to have a
rectilinear wall course.
EP 1 548 149 describes a method for forming a thin
oxide coating on the inner side of a hollow body. In
this case, a hollow body to be coated on the inner side
is introduced into a cylindrical chamber that functions
as an RF electrode. A gas tube, which simultaneously
functions as an earth electrode, is introduced into the
interior of the hollow body.
The disadvantage of this method resides in the
formation of the layer properties. The gas tube
functions as an earth electrode in the device described
in EP 1 548 149. For this reason, the layer properties
(hardness, thickness, lattice structure of the
deposition, purity of the layer, doping with functional
elements, water-repelling or -absorbing) cannot be
established as desired.
It is not possible to establish and control these
properties in the case of an introduced earth electrode
which, with respect to its area, is smaller by a factor
of 1 than the area to be coated.
DE3821815 discloses a device for coating an inner wall
of a hollow body with a diamond-like hard carbon
coating with the aid of a plasma-assisted CVD method.
In this case, a process gas containing at least one
hydrocarbon gas is conducted through the interior of
the unheated hollow body, in which a plasma excites the
process gas, wherein it is dissociated and ionized and
the resulting ions, for forming the coating, are
accelerated onto the inner wall to be coated. The
device has an RF generator connected to the hollow
body, with an earthing arrangement for forming the
plasma between the hollow body and the earthing
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arrangement and with a feed line opening into the
interior of the hollow body for the controlled
introduction of the process gas into the interior of
the hollow body. The earthing arrangement is connected
to a vacuum housing into which the interior of the
hollow body leads and which surrounds the hollow body
at a distance from the lead-in junction.
This device has proved to be unsuitable in practice,
for various reasons. Thus, in the method carried out
with this device, not only the inner wall of said
hollow body but also the outer wall thereof is coated.
Furthermore, this device is only suitable for the
coating of hollow bodies having a rectilinear inner
course (so-called "blind holes"), that is to say e.g.
not for container-like vessels having a narrowed neck.
An additional factor is that in this device the hollow
body itself functions as an electrode since it is
conductively connected to the radio-frequency
electrode. This is necessary in this device since
otherwise the field strength of the electromagnetic
alternating field generated would not suffice to ensure
an inner coating. The penetration depth of an
electromagnetic alternating field generated only by the
radio-frequency electrode in the base region of the
vacuum chamber (that is to say the maximum thickness of
an if appropriate metallic material through which the
alternating field penetrates with sufficient strength
still to initiate a coating reaction) is in the region
of 2 cm in this device. Therefore, in this device,
hollow bodies having a larger wall thickness have to
rely on the fact that they themselves function as an
electrode; therefore, they must necessarily be composed
of metal.
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Furthermore, it has been found that the geometries of
the hollow bodies to be coated are very limited. Thus,
despite describing that besides workpieces having a
ratio of tube diameter to tube length in the range of
between 20 mm to 60 mm and 2 mm to 20 mm, tube
diameters of greater than 20 mm and less than 2 mm,
respectively, can also be coated with this device, this
has been found to be problematic in practice.
Therefore, the abovementioned method is not suitable
for a large number of applications in which hollow
bodies having a larger internal diameter are intended
to be coated.
It is an object of the present invention to provide a
method for plasma-assisted chemical vapour deposition
for coating or material removal on the inner wall of a
hollow body which does not have the disadvantages
mentioned.
This object is achieved by means of a method having the
features of newly presented Claim 1. The dependent
claims specify preferred embodiments.
It should be taken into account here that value ranges
delimited by numerical values should always be
understood as inclusive of the relevant numerical
values.
Accordingly, provision is made of a method for plasma-
assisted chemical vapour deposition for coating or
material removal on the inner wall of a hollow body, in
particular composed of a non-metallic material, having
a cross-sectional area, a longitudinal extent and at
least one opening. The method has the following steps:
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1. introducing the hollow body to be coated on its
inner side into a vacuum chamber with an earthed inner
side, a large-area radio-frequency electrode being
arranged in the interior of the vacuum chamber,
2. positioning the hollow body in the centre of the
vacuum chamber, it being necessary to comply with a
minimum distance of 15 cm on all sides between the
outer wall of the hollow body and the inner wall of the
vacuum chamber,
3. introducing a gas lance comprising a tube having
an internal diameter of 0.001 - 10 mm, a maximum
external diameter of 12 mm and a terminal nozzle having
a terminal opening diameter of 0.002 - 6 mm through the
opening into the hollow body, the gas lance being
connected to a gas feed unit via a non-electrically
conductive line and, in particular, not being earthed
or being in electrically conductive contact with the
radio-frequency electrode,
4. positioning the gas lance in the hollow body in
such a way that the gas lance is positioned centrally
relative to the cross section of the hollow body and
the nozzle of the gas lance, relative to the
longitudinal extent of the hollow vessel, is arranged
in the region of the transition from the second
longitudinal third to the third longitudinal third,
measured from the opening of the hollow body,
5. closing the vacuum chamber and evacuating the
latter to a residual pressure of 0.001 - 5 pascals,
6. introducing an inert working gas and also one or a
plurality of reaction gases via the gas feed unit and
the gas lance into the hollow body, and
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7. igniting a cavity plasma to form a plasma cloud
arranged at the tip of the gas lance, by applying an
electric radio-frequency field to the RF electrode.
It is relevant here that the hollow body to be coated
is not earthed. It is preferably provided here that the
vacuum chamber is evacuated to a residual pressure of
0.01 - 2 pascals. Particularly preferably, the vacuum
chamber is evacuated to a residual pressure of 0.1 -
1 pascal.
What is important in this method is that the gas lance
is not grounded or earthed, but rather is electrically
insulated. For this purpose, it is preferably provided
that said gas lance is insulated with the aid of a PTFE
ring (polytetrafluoroethylene) and the gas supply line
within the chamber interior is produced from PTFE.
Suitable hollow bodies to be coated include, in
principle, all possible hollow bodies, that is to say
not only hollow bodies closed on one side (such as e.g.
vessels, containers, etc.) but also tubular hollow
bodies without a base, such as e.g. cannulas, bodies
having a through hole or tubes. The latter hollow
bodies have to be closed off with a cover or stopper on
one side prior to coating.
In both cases care must be taken to ensure that the gas
lance is arranged in the hollow body in such a way that
the gas lance is positioned centrally relative to the
cross section of the hollow body and the nozzle of the
gas lance, relative to the longitudinal extent of the
hollow vessel, is arranged in the region of the
transition from the second longitudinal third to the
third longitudinal third, measured from the opening of
the hollow body. This means that the gas lance has to
be advanced to relatively just before the bottom of the
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vessel (or before the second opening of the hollow body
closed off with a cover or stopper). A minimum distance
of 10 cm has to be complied with here. In the case of
substrate objects having a depth of 10 cm or less, the
tip of the gas lance is positioned directly above the
opening of the hollow body.
In principle, low-pressure plasmas as in the present
invention ensure a larger mean free path length \ of
the gas molecules and therefore delay the formation of
a plasma. The arrangement of the gas lance according to
the invention"has the effect, by contrast, that the gas
molecules emerging from the gas lance collide with the
bottom of the vessel or the abovementioned cover or
stopper as a result of their acceleration. This
promotes the gas dissociation process and the formation
of a plasma. For this reason, a comparatively lower
strength of the electromagnetic alternating fieldis
sufficient, that is to say that it is not necessary for
the hollow body that is to be coated itself to function
as an electrode.
Preferably, the minimum distance between the outer wall
of the hollow body and the inner wall of the vacuum
chamber is 15 cm. The maximum distance, by contrast, is
given by the dimensioning of the vacuum chamber used.
The gas lance has preferably an internal diameter of
0.005 - 6 mm and particularly preferably an internal
diameter of 0.01 - 6 mm or 0.1 - 6 mm and a maximum
external diameter of 10 or 8 mm. The terminal nozzle
preferably has a terminal opening diameter of 0.01 - 3
or 0.1 to 2 mm.
The arrangement and dimensioning of the gas lance
ensure that the plasma forms only at the tip of the gas
lance, i.e. only in the interior of the hollow body to
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be coated. Since the gas molecules experience their
greatest acceleration at the instant of the plasma-
induced dissociation, this acceleration fully benefits
the treatment of the inner surface of the hollow body.
Therefore, it is also possible to dispense with an
electrode in the interior of the hollow body.
In this way, a plasma is enabled to be ignited and
maintained only in the interior of the hollow body.
This type of plasma is referred to as "cavity plasma"
hereinafter. This ensures that said hollow body is
coated only on the inner side and not on its outer
side.
The plasma-induced molecular dissociation takes place
at the instant at which the gas mixture leaves the
lance nozzle. It takes place with the formation of very
short-wave light.
The splitting energy released during dissociation
accelerates the what is now truly "plasma matter" to
approximately 250,000 km/h. On account of this
acceleration, the carbon impinges on the inner surface
to be coated and deposits as a hard material layer. The
type of deposition varies depending on the gas used and
the purity and composition thereof.
The dissociation ratio is 1:12 e.g. in the case of H2C2.
This means that the H atoms are 12 times lighter than
the C atoms. The acceleration of the dissociation plus
the acceleration of the individual atoms and the
impingement on the substrate are therefore in the ratio
of 1:12.
Therefore, twelve times more C atoms than H atoms at
the same velocity impinge on an identical area in the
same period of time. Since the H atoms are undesirable
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in a hard material layer, the quantity of reaction gas
fed in has to be calculated with regard to the inner
area to be coated.
The following relationship determined empirically by
the inventor can be used for calculating the quantity
of reaction gas to be fed in:
V = A / 12 * E
In this case, A is the surface area to be coated [cm2],
E is the dissociation energy supplied, and V is the
volume of the reaction gas per minute [cm3/min].
On account of the mass inertia and the dissociation
energy released, the carbon therefore requires less
area per acceleration free space in order to arrive at
the required 250,000 km/h.
If the H2C2 is introduced in a three-dimensional hollow
body by means of a gas lance, then it must be ensured
that the C atoms impinge directly on the substrate at
maximum acceleration and are not deflected, decelerated
or even stopped by equivalently accelerated H atoms.
This is ensured by virtue of the fact that the nozzle
of the gas lance, relative to the longitudinal extent
of the hollow vessel, is arranged in the region of the
transition from the second longitudinal third to the
third longitudinal third, measured from the opening of
the hollow body. As a result, the atoms are accelerated
up to their maximum and impinge on the substrate
directly at the end of this phase without being impeded
by other atoms.
Investigations by the inventor have revealed moreover,
that in order to ensure the abovementioned deposition
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on the inner surface of a hollow body, the dissociation
energy (EA) in watts must be higher than the opening
diameter (Do) of the hollow body in cm at least by the
factor 65.5.
This means, therefore, that given an opening diameter
(Do) of the hollow body of 15 cm, the dissociation
energy (EA) on the basis of the relationship
EA = Do * 65.5
must be at least 15 * 65.5 = 982 watts.
By introducing this minimum dissociation energy, which
can be correspondingly established at the RF generator,
the atoms of the reaction gas that has undergone
transition to the plasma are accelerated in such a way
that their oscillation amplitude is larger than the
opening diameter of the hollow body. This ensures that
only transversely accelerated atoms can leave the
hollow body.
In this way, contrary to the statements in the
introduction, the inner surface of a hollow body can
also be coated by means of the method according to the
invention.
In this case, the dimensioning of the nozzle of the gas
lance prevents the plasma from flashing back into the
gas lance as would be feared with nozzles having larger
dimensions.
It is also important that the diameter of the gas lance
does not widen in the direction of the nozzle. As a
result of this, on account of the Bernoulli effect, the
pressure of the incoming gas would decrease in the flow
direction in the region of the cross-sectional
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widening, which would promote a flashback of the plasma
into the gas lance and thus destruction of the gas
lance. The formation of the plasma cloud at the tip of
the gas lance would be prevented in this way.
In one preferred configuration of the method according
to the invention, it is provided that the radio-
frequency electrode in the interior of the vacuum
chamber has at least two leads via which radio-
frequency voltages can be fed into the radio-frequency
electrode.
In this way, an alternating field having very high
field strengths such as is required for forming the
cavity plasma can be generated in the chamber. An
alternating field generated in this way has a
sufficiently high penetration depth, such that even
hollow bodies having large wall thicknesses can be
penetrated and coated on the inner side. The hollow
body itself therefore does not have to function as an
electrode and can therefore also be composed of a non-
metallic material. It is therefore irrelevant whether
the hollow body is in electrically conductive contact
with the radio-frequency electrode or whether it is
completely electrically insulated.
This feature is advantageous particularly with the
property that in the method according to the invention
the temperatures in the interior of the coating chamber
generally do not exceed 2002C. On account of these low
temperatures, therefore, even plastic hollow bodies can
be provided with an extremely durable inner coating.
This is advantageous particularly because, on account
of the unrequired electrically conductive connection
between the hollow body and the radio-frequency
electrode, non-metallic hollow bodies can indeed also
be coated by the method according to the invention.
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In this case, three or more leads are preferably
provided since an even more homogeneous alternating
field can be established in this way.
In this case, it is preferably provided that the
individual leads to the radio-frequency electrode are
adjusted separately in such a way that a homogeneous
alternating field having uniformly high field strengths
can be generated in the entire chamber. This feature
benefits the coating quality.
This can be effected by means of a so-called matchbox,
for example, which is connected between a radio-
frequency generator and the radio-frequency electrode.
This has e.g. trimming potentiometers for the
individual leads to the radio-frequency electrode which
are adjusted separately. In this case, the same bias
voltage is set at all the regulators, which indicates
identical field strengths and thus a homogeneous
alternating field.
In a further preferred configuration of the method
according to the invention it is provided that said
hollow body merely has an opening whose narrowest
diameter is narrower than the narrowest diameter of the
inner space of the hollow body. Such a hollow body can
be e.g. a bulk container, a bottle or the like. Hollow
bodies having such geometries cannot be coated in
particular by the method known from DE3821815.
Furthermore, it is preferably provided that said hollow
body to be coated has an inner volume in the range of
between a few cm3 and 1,000,000 cm3. For technical
reasons, a limit is imposed on the size of the hollow
body to be coated only because the size of the vacuum
chambers currently available is limited.
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Thus, e. g. a bulk container has an inner volume in the
range of 10,000 - 100,000 cm3. An engine block having
four cylinders has e.g. four inner volumes in the range
of between 250 and 700 cm3 . A gas cylinder has e.g. an
inner volume in the range of 20,000 - 100,000 cm3.
Here, too, it holds true that hollow bodies having such
volumes cannot be coated with sufficient quality in
particular by the method known from DE3821815.
It is preferably provided that the working gas is a gas
selected from the group containing argon, helium,
hydrogen, oxygen or some other noble gas.
Furthermore, it is particularly preferably provided
that the reaction gas is a gas selected from the group
containing oxygen.
Such a method for plasma-assisted chemical vapour
deposition for material removal is also referred to as
plasma etching. Oxygen is particularly suitable as a
reaction gas for this method since the oxygen ions
produced in the plasma are particularly heavy and
therefore bring about surface removal particularly
effectively in the accelerated state.
Investigations by the applicants have revealed that
e.g. the inner surface of a used bulk container such as
is used e.g. for the production of vaccines and which
is extremely contaminated after use by dried-in and/or
chemical blood constituents can be cleaned extremely
thoroughly by this method.
Pursuant to applicable regulations, e.g. a high-grade
steel for medical use must be absolutely free of
residues of previous substances in contact with it.
This has been achieved hitherto in the case of bulk
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containers, for example, by means of a very costly
cleaning process using acids and alkaline solutions.
The method according to the invention, in which a
plasma is ignited after oxygen has been fed with supply
of high energy, makes it possible to clean ("etch") the
surface of the substrate in a manner absolutely free of
residues. This can be attributed in particular to the
high atomic weight of the oxygen atoms, which reliably
remove contaminants upon sufficient acceleration.
In a further preferred configuration of the method
according to the invention, it is provided that the
reaction gas is a gas selected from the group
containing hydrocarbon gases such as methane, ethane,
ethene, ethyne, propane or silane gases such as
tetramethylsilane or hexamethyldisiloxane.
The former reaction gases are suitable for forming a
DLC layer and the latter e.g. suitable for forming an
Si02 layer.
The term DLC ("Diamond-Like Carbons") is understood to
mean layers of molecular carbon which have a net or
lattice of spZ- and sp3-hybridized carbon atoms. The
ratio of the two variants to one another depends on the
coating conditions. If, the former predominate, the
coating has graphite-like properties (low coefficient
of friction), and if the latter predominate, the
hardness and the transparency of the coating increase.
Mixed coatings containing both variants frequently
combine both advantages.
Investigations by the applicants have revealed that the
inner surfaces of bulk containers and other hollow
bodies can be effectively coated with a DLC layer by
this method.
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Preferably, in the method according to the invention,
the plasma is ignited by applying a DC voltage radio-
frequency field with the following parameters:
1. frequency: 10 kHz - 100 GHz
2. electrical power: 500 - 5000 W
3. gas feed: 0- 90 scm3.
The frequency preferably lies in the range of 10-
MHz. The frequency is particularly preferably
13.56 MHz (RF, radio-frequency).
10 The electrical power to be introduced is calculated
according to the following formula: power (watts) =
area to be coated (m2) x 1750. In this case, the factor
mentioned last can lie between 1500 and 2200 and is
determined empirically in practice. A hollow body
15 having an inner surface area to be coated of 0.85 m2
would accordingly have to be coated with a power of
approximately 1500 watts.
Surprisingly, the bias voltage established under these
conditions is in the region of 0 V, to be precise on
all the leads. Moreover, this value is independent of
whether or not the hollow body to be coated is in
electrically conductive contact with the radio-
frequency electrode.
The gas feed is regulated in a gas-specific manner and
adjusted in a range of 0-90 sccm depending on the
object and desired layer properties. It is preferably
provided here that the quantity of reaction gas to be
introduced for the coating is 0.1 - 10 sccm of reaction
gas per 10 cm2 of inner surface area to be coated.
The unit sccm denotes standard cubic centimetre, i.e.
the volume of the gas to be introduced in cubic
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centimetres per minute (volume per minute). A valve
with a mass flow controller is used for the adjustment.
At a given pressure of the gas supply line, therefore,
the opening state of the valve governs the inflowing
volume per minute.
In the case of hydrocarbon gases it holds true that the
layer becomes all the harder, the more gas is used
since the proportion of available carbon atoms
increases.
In the case of silane gases, by contrast, it holds true
that the ratio of the silane gas to oxygen determines
the hardness of the layer. For hard coatings, the ratio
is e.g. 100 sccm HMDSO (hexamethyldisiloxane) to
400 sccm oxygen. By contrast, a reduction of the oxygen
proportion leads to softer layers.
Particularly preferably, the quantity of the reaction
gas to be introduced is 0.5 - 5 sccm of reaction gas
per 10 cm2 of inner surface area to be coated.
Furthermore, it is preferably provided that the
reaction gas is doped with one or more gases containing
Si, N, F, B, 0, Ag, Cu, V or Ti. These dopants can
contribute to having a targeted influence on the
properties of the coating applied. Thus, e.g. the
doping of the reaction gas with a gas containing Si
(e.g. hexamethyldisiloxane) leads to a reduction of
friction even under moist conditions and also to a
higher thermal stability. A doping with N, F, B or 0
influences the surface tension, the wettability and the
hardness of the coating. A doping with metals
contributes to influencing the conductivity of the
coating, while a doping with Ag, Cu, V or Ti influences
the biological behaviour of the coating, in particular
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the biocompatibility which is hugely important e.g. for
implants.
Layer growth rates of up to 4pm/h and layer
thicknesses of up to 7pm are achieved with the method
according to the invention.
The invention furthermore provides a hollow body having
an inner surface, characterized in that the latter was
treated by a method according to any of the preceding
claims in such a way that a material removal was
performed on the inner surface and/or the latter was
provided with a coating. The coating can be, as
mentioned above, e.g. a DLC, TiOx or Si02 coating.
Particularly preferably, said hollow body is a hollow
body selected from the group containing vessels,
bottles, containers, cannulas, hollow needles,
syringes, inner walls of cylinder or piston bores in
internal combustion engines, inner sides of bearings,
in particular ball or rolling bearings.
The hollow bodies mentioned can comprise non-metallic
materials, in particular, since the hollow body - in
contrast to the description in DE 3821815 - does not
function as an electrode. This opens up new
possibilities in lightweight construction. Thus, it is
possible, for example, to produce a highly loaded
metallic workpiece - thus e.g. an engine block of an
internal combustion engine - from a plastic and to coat
the inner walls of the cylinder bores with a surface
having a high loading capability in a manner according
to the invention.
The following advantages, inter alia, can be achieved
with the method according to the invention:
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a) improved cleaning of three-dimensional hollow
bodies, in particular bulk containers, in conjunction
with a reduced outlay;
b) improved corrosion protection of the coated
surfaces;
c) no diffusion of a substrate situated in the hollow
body into the inner surface layer of the hollow body;
d) reduction of the coefficient of friction of the
inner surface; and
e) improved heat dissipation.
The invention furthermore provides a device for
carrying out a method according to any of the preceding
claims.
The present invention is elucidated in greater detail
by the figures shown and discussed below. It should be
taken into account here that the figures are only
descriptive in nature and not intended to restrict the
invention in any form.
Figure 1 shows a section through a vacuum chamber 10
according to the invention in frontal view with a
radio-frequency electrode 11 arranged at the bottom of
the chamber, a hollow body 12 to be coated on the inner
side and having an opening 13, said hollow body being
arranged by way of a mount 14 on the radio-frequency
electrode.
The radio-frequency electrode 11 in the interior of the
vacuum chamber 10 has three leads 15 via which radio-
frequency voltages generated by a radio-frequency
generator (RF generator) 16 are fed into the radio-
frequency electrode 11. By means of a regulable
matchbox 17, as it is called, connected between the RF
generator 16 and the radio-frequency electrode 11, the
individual leads to the radio-frequency electrode 11
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can be adjusted separately with the aid of trimming
potentiometers in order to generate a homogeneous
alternating field having uniformly high field strengths
in the entire chamber.
Figure 2 shows the same vacuum chamber 20 in a lateral
sectional view, with the radio-frequency electrode 21,
the hollow body 22 to be coated on the inner side in
plan view with an opening 23, and also the mount 24,
which is not electrically conductive. The hollow body
is a bulk container in the example shown. A gas lance
25 is inserted into the hollow body through the opening
23 of the hollow body, said gas lance having a terminal
nozzle 26 having a diameter of 0.6 mm at its distal
end. The gas lance is connected to a gas supply (not
illustrated) via a hose and is guided via a height-
adjustable mount 27, by means of which it can be
ensured that the gas lance can be positioned in the
hollow body 22 in accordance with the dimensioning in
the main claim. For this purpose, the mount is arranged
on a carrier 28 in height-adjustable fashion.
The radio-frequency electrode 21 in the interior of the
vacuum chamber 20 has three leads 29 via which radio-
frequency voltages generated by a radio-frequency
generator (RF generator) 30 are fed into the radio-
frequency electrode 21. The individual leads to the
radio-frequency electrode 21 can be adjusted separately
via a regulable matchbox (not illustrated) connected
between the RF generator 16 and the radio-frequency
electrode 11.
Figure 3 again shows a vacuum chamber 30 in a lateral
sectional view, with a radio-frequency electrode 31, a
hollow body 32 to be coated on the inner side and
arranged upright in plan view with an opening 33,
through which a gas lance 34 is introduced into the
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hollow body. In the example shown, the hollow body is a
bulk container composed of high-grade steel. Thus, in
contrast to the embodiment shown in Figure 2, the
hollow body is electrically conductively connected to
the radio-frequency electrode 31, and therefore
likewise functions as an electrode.
Figure 4 shows the same vacuum chamber 40 as Figure 2,
with the radio-frequency electrode 41, the hollow body
42 to be coated on the inner side in plan view with an
opening 43, through which a gas lance 25 is introduced
into the hollow body. An electromagnetic alternating
field is set [values, three leads, very homogeneous
field] at the radio-frequency electrode and gas flows
into the hollow body through the gas lance. On account
of the electromagnetic interactions, the gas molecules
that emerge are accelerated and a spherical plasma 45
is formed, which is also referred to as a cavity plasma
since it essentially remains within the hollow body and
does not pass into the actual vacuum chamber 40. The
coating effects described above are established here on
account of the plasma. On account of the via the
suction-extraction connector 46, the outflowing gas or
plasma is sucked in the direction of the opening 43.
Figure 5 shows a coated bulk container 50 in cross
section with a wall 51 and the coating 52. The bulk
container has a depression 53 in the region of its
bottom. Moreover, the gas lance shown in the previous
figures and the spherical plasma that is formed are
illustrated schematically. It can be discerned that the
coating applied on account of the effects of the
spherical plasma has a greater thickness particularly
in the region of the exit opening of the gas lance than
in the edge regions of the container bottom or on the
inner walls of the container. The thickness of the
coating is greatly exaggerated in the illustration; in
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practice, it varies in the range of between 50 nm and
20 um.
When the bottom of the container is viewed directly by
an observer, this thickness gradient is discernible by
virtue of the iridescent colour caused by interferences
with the waves of the visible light spectrum (350 -
800 nm).
Figure 6 shows the coating process under way on a
horizontally arranged bulk container. For this purpose
a photograph was taken through the porthole of the
chamber in the direction of the opening of the bulk
container. It can be discerned that the plasma formed
burns only in the interior of the container, and not
for instance in the entire chamber as known in devices
from the prior art. The cavity plasma discussed above
is involved here.
Figure 7 shows the coating process under way on a
vertically arranged bulk container. For this purpose a
photograph was taken through the porthole of the
chamber in the direction of the opening of the bulk
container. Here, too, it can be discerned that the
plasma formed burns only in the interior of the
container, and not for instance in the entire chamber
as known in devices from the prior art. The cavity
plasma discussed above is involved here.
Figure 8 shows a bulk container coated by the method
according to the invention in frontal view. The
container is still arranged in the coating chamber; the
non-electrically conductive mount can be discerned in
the lower region. In particular, the depression already
discussed can be discerned in the region of the bottom
of the container. It is furthermore readily discernible
that the container has been coated with a DLC coating
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. , .,=
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in the inner region, while the outer side of the
container has not been coated (evident from the
metallically lustrous surface composed of high-grade
steel).
Figure 9 shows the bottom of a bulk container coated by
the method according to the invention in frontal view.
Here, too, the depression already discussed can be
discerned in the region of the bottom of the container.
Here, too, on the basis of the different brightnesses
it is once again readily discernible that the container
has been coated with a DLC coating in the inner region,
while the outer side of the container has not been
coated (evident from the metallically lustrous surface
composed of high-grade steel).
Figure 10 shows the transition region between the
bottom and the inner wall of a coated bulk container.
In this case, it is in particular readily discernible
that a welding seam arranged in the transition region
has likewise been coated well.
