Coating processes – Direct application of electrical – magnetic – wave – or... – Plasma
Reexamination Certificate
2002-08-13
2003-11-18
Pianalto, Bernard (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Plasma
C118S7230VE, C427S237000, C427S248100, C427S294000, C427S598000
Reexamination Certificate
active
06649223
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and an apparatus for substrate treatments using plasma such as, for example, plasma assisted processing for antifouling coating depositions, cleaning, sterilization or the like. In particular, the present invention relates to plasma creation for treatment or deposition in hollow substrates having large aspect ratio such as flat boxes, small diameter tubes . . .
BACKGROUND OF THE INVENTION
Hollow substrates with a large aspect ratio are commonly used in various technological fields such as catheters or endoscopes for medical materials and packaging for food or pharmaceutical applications. The expression “large aspect ratio” means that the hollow substrate has at least one dimension that is much larger than another one, and more particularly that the depth of the substrate cavity is much larger than a dimension of the substrate aperture.
FIG. 7
shows two examples of hollow substrates which have a large aspect ratio L/a, where L is the depth of the substrate cavity and a is the smallest dimension of the substrate aperture. A tube
30
comprises an inner cavity having a large depth L with respect to the diameter a of the tube. Large aspect ratio substrates may also have a form of a flat box
31
which exhibits a little aperture height a in comparison with its cavity depth L.
In these above applications, plasma treatment or deposition is useful notably for combined cleaning, sterilization and deposition (PECVD) operations for medical materials or barrier coatings and sterilization treatments for food or pharmaceutical packaging. The plasma treatment methods are numerous and various, for example, the surface of a substrate can be modified by plasma treatment in order to increase or decrease its wettability.
In all the case, whatever the result expected (deposition, etching . . . ), the plasma processing is obtained from plasma creation.
A conventional method for creating plasma consists of submitting a process gas containing reactive species to an electric field which ionizes the process gas by accelerating electrons.
FIG. 8
shows a classical embodiment allowing capacitive plasma creation in a substrate. A substrate
50
contains a process gas. The substrate is flowed by an electric field
53
created between two electrodes
51
and
52
which are supplied by a power supply
54
. However, in the case of a small diameter tube, the plasma cannot be ignited with the electric field perpendicular to the diameter of the tube, because electrons cannot be accelerated up to the ionization potential (the trajectories are stopped by the tube walls). However, even if a suitable configuration can be found, the pressure needed for a capacitive discharge can lead to thermal damages of the substrate walls (case of polymers) and the gas dissociation is limited, which reduces the treatment possibilities. Moreover, the pressure range for a capacitive discharge is limited, which creates problems of control inside the tube itself.
Another traditional method used in plasma treatment inside a hollow substrate is the diffusion of active species created outside. As can be seen on
FIG. 9
, an external plasma source
40
creates active species diffused inside the hollow parts of a substrate
41
having a cavity length L and a thickness or diameter a. However, such a diffusion results in a non uniformity of the treatment or deposition process along the substrate because of consumption of active species during diffusion. In other words, in using an external plasma source for treatment inside the hollow parts of a substrate, the plasma diffusion density is decreased along the substrate length L.
Alternatively, a method for plasma treatment can be based on inductively coupled plasma. As illustrated in
FIG. 10
, the plasma is created inside a substrate
61
with a classical tangential electromagnetic flux generated by a current flowing in a coil
60
in the direction of an arrow
63
. In flowing in the coil
60
, the current produces a magnetic field
64
which generates an electric field
62
inside the substrate. The plasma production is realized from a process gas contained inside the substrate and by acceleration of the electrons along the electric field
62
tangential to the substrate.
FIG. 11
shows a matched embodiment for implementing the inductively coupled plasma method inside a tube
71
. In this embodiment, a coil
70
surrounds the tube
71
and carries a current in a direction of an arrow
72
so as to produce a magnetic field
74
which generate an electric field
73
. The plasma is thus created in the tube by accelerating the electrons according to the direction of the electric field
73
.
However, the efficiency of such configurations depends on a ratio R between the diameter or thickness a of the hollow substrate and the plasma skin depth
δ
:
R
=
a
δ
,
where the plasma skin depth &dgr; is determined by the process parameters, that is to say plasma density, gas nature and pressure, power, electron temperature, which are inherent to the required treatment or deposition. In this configuration, to obtain a suitable efficiency, the ratio R must be greater than 2 or 3. Accordingly, this criterion excludes such a process for low thickness or diameter hollow substrate, in particular when the thickness or diameter is lower than the plasma skin depth (a<&dgr;).
OBJECT AND SUMMARY OF THE INVENTION
In view of such aspects, an object of the present invention is to provide a method and an apparatus in which the above-mentioned problems can be solved. In other words, an object is to provide a method and an apparatus which allow a plasma treatment of hollow substrates in uniform way all-over the inside parts of the substrates.
To this end, there is provided a method for plasma treatment of a hollow substrate made of a non-conductive material in which a plasma is generated from a process gas inside the substrate, characterised in that the plasma generation comprises the step of producing through the substrate a magnetic field substantially perpendicular to a direction of a substrate depth L.
Thus according to the present invention, all the inside parts of a thin hollow substrate may be plasma treated uniformly with a great efficiency and with a high flexibility in pressure adjustment. Indeed, the simplest description of the inductive coupling between an induction source and a plasma is a circuit model in which the induction source is the primary winding of a transformer and the plasma the secondary winding. The plasma is represented by an inductance and a resistance, as a function of parameters such as power, gas pressure, plasma density . . . . In this model, the current induced in the plasma flows within a skin depth &dgr; given by the following equation (1):
δ
=
(
2
μωσ
)
1
2
,
where &mgr; is the permeability of the medium, &ohgr; the excitation frequency and &sgr; the plasma electrical conductivity. The electrical conductivity &sgr; is directly proportional to the plasma density and can be written according to the equation
σ
=
n
e
2
m
c
(
υ
+
jω
)
,
where n is the plasma density, e and m
e
the electronic charge and mass respectively and &ugr; the electron collision frequency.
According to Maxwell equation, the current in the plasma decreases in the skin depth &dgr; as
1
e
.
As the current decreases in proportion as it penetrates into the plasma, the skin depth &dgr; is thus an important parameter in plasma treatment or deposition because the value &dgr; determines the proportion according to which the current will decrease.
The efficiency of the plasma treatment versus a ratio
R
=
a
δ
for a classical tangential flux exhibits that for a given plasma skin depth &dgr; (plasma conductivity, density . . . ), the suitable efficiency is obtained with R=3. Accordingly, referring to equation (1), for a thin plasma thickness a, it is necessary to increase the excitation frequency &ohgr; to decrease &dgr; in order to have an optimum coupling efficiency.
Colpo Pascal
Rossi François
Bacon & Thomas PLLC
European Community (EC)
Pianalto Bernard
LandOfFree
Method and apparatus for inductively coupled plasma treatment does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for inductively coupled plasma treatment, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for inductively coupled plasma treatment will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3174867