Electric lamp and discharge devices: systems – Discharge device load with fluent material supply to the... – Plasma generating
Reexamination Certificate
2000-08-08
2001-10-16
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Discharge device load with fluent material supply to the...
Plasma generating
C315S111210, C315S111410
Reexamination Certificate
active
06304036
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains generally to plasma generators. More particularly, the present invention pertains to systems and methods that use helicon waves to generate plasmas. The present invention is particularly, but not exclusively, useful as a system and method for varying the parameters of a helicon wave to conform with changes in the density of a plasma in a manner that facilitates the initiation and maintenance of the plasma.
BACKGROUND OF THE INVENTION
It is well known that a plasma containing free electrons and positive ions can be generated by heating vapor particles with RF energy. To do this, it is necessary to establish the proper conditions wherein the RF energy will effectively heat free electrons to ionize vapor particles, thus creating a plasma. One known way by which the transition from a vapor to a plasma can be effected is by radiating the vapor with a helicon wave in accordance with the so-called “helicon dispersion relation.” Typically, this is accomplished in a cylindrical shaped vacuum chamber wherein a uniform magnetic field has been established and oriented substantially parallel to the longitudinal axis of the chamber. The mathematical expression for the helicon dispersion relation in this case is:
k
∥
k=&ohgr;
pe
2
&ohgr;/&ohgr;
ce
c
2
=en
e
&mgr;
o
&ohgr;/B
wherein
&ohgr;
ci
<&ohgr;<&ohgr;
pe
, &ohgr;
ce
k is the magnitude of the wave vector for the helicon wave;
k
∥
is a component of the wave vector that is parallel to the magnetic field inside the chamber;
e is the electron charge;
&ohgr;
pe
is the electron plasma frequency;
&ohgr; is the angular frequency of the RF energy;
&ohgr;
ce
is the electron cyclotron frequency;
&ohgr;
ci
is the ion cyclotron frequency;
c is the speed of light;
n
e
is the electron density of the plasma in the chamber;
&mgr;
o
is the permeability of free space; and
B is the magnitude of the magnetic field in the chamber.
It is to be appreciated that, while the helicon dispersion relation set forth above may imply there is but a single k
∥
, in reality, an antenna will generate many k
81
components. Accordingly, the condition lends itself to a Fourier analysis. For purposes of this disclosure, however, it is deemed sufficient to consider only an optimal k
∥
component and thereby forgo an in-depth Fourier analysis.
Heretofore, in accordance with the helicon dispersion relation, plasma generators have been designed and manufactured using the specific structural parameters that are required for a particular operating condition. This has meant that the antenna for generating the RF energy, as well as other components of the system, have had to be specifically designed. In this design process, additional factors, such as the type of plasma to be produced, the size of the chamber, and the desired density of the plasma inside the chamber, have had to be considered. Further, it has often been necessary to pre-ionize a portion of the vapor in order to initiate the operation. It happens, however, that it may be desirable in some operations to be able to sequentially generate different types of plasmas using the same chamber. Also, for operational efficiencies, it may be desirable to have the ability to initiate a plasma generating operation from a zero plasma condition.
With the above in mind, it is helpful to review the helicon dispersion relation and identify effective operational variables that can be manipulated to satisfy the relation. Specifically, from the relation it is to be noted that:
k
∥
k=en
e
&mgr;
o
&ohgr;/B,
and that
&ohgr;
pe
2
&ohgr;/&ohgr;
ce
c
2
=en
e
&mgr;
o
&ohgr;/B.
Accordingly, as can be easily seen from the above expressions, k
∥
k is proportional to n
e
. Similarly, &ohgr; is also proportional to n
e
. It then follows that as the electron density of a plasma (n
e
) changes (e.g. during an initial start up operation) the helicon dispersion relation can be maintained to continue the production of plasma by making appropriate changes in either k
∥
k or &ohgr;.
In light of the above, it is an object of the present invention to provide a system and method for initiating and maintaining plasma production in a chamber which is operationally robust and effective over a range of electron densities. Another object of the present invention is to provide a system and method for initiating and maintaining plasma production in a chamber which is effective for producing different types of plasmas in successive operations using the same plasma chamber. Still another object of the present invention is to provide a system and method for initiating and maintaining plasma production in a chamber that is simple to operate, relatively easy to manufacture and comparatively cost effective.
SUMMARY OF THE PREFERRED EMBODIMENTS
A system for initiating and maintaining plasma production in accordance with the present invention includes a substantially cylindrical chamber defining a longitudinal axis. Also included are magnetic coils that are positioned around the chamber to establish a uniform magnetic field inside the chamber. For the present invention this uniform magnetic field is oriented generally parallel to the axis. Additionally, the system includes a source for introducing vapor particles into the chamber.
An important aspect of the present invention is an antenna that is mounted on the chamber. As intended for the present invention, this antenna is used to radiate RF energy into the chamber in a way that will ionize the vapor particles and thereby create a plasma. Specifically, the plasma is to be created according to the helicon dispersion relation, which can be expressed as:
k
∥
k=&ohgr;
pe
2
&ohgr;/&ohgr;
ce
c
2
=en
e
&mgr;
o
&ohgr;/B,
In addition to the above mentioned components, the system of the present invention also includes an interferometer, or some similar type device, for monitoring the electron density (i.e. plasma density level) inside the chamber. According to the monitored plasma density level, a controller is then used to vary the frequency phase, or power level of the RF energy that is radiated by the antenna. Stated differently, these selected parameters of the RF energy are controlled to maintain a match for the helicon dispersion relation. Specifically, with density level (n
e
) changes in the plasma, the RF energy also needs to somehow be changed in order to establish a match for the helicon dispersion relation. As mentioned above, it happens that this can be done by varying either k
∥
k or &ohgr;. Accordingly, the present invention alternatively contemplates two embodiments for the system.
In a first embodiment for the system of the present invention, the antenna is configured to vary the k
∥
component of the wave vector in the RF energy. As contemplated for the present invention, this is accomplished by using an antenna that comprises a plurality of individual antenna elements. Specifically, each of the antenna elements is a coil, or so-called “strap,” that is positioned around the chamber. More specifically, the antenna elements are positioned around the chamber in a side-by-side relationship, with each element being next to at least one other such antenna element. In this juxtaposed relationship, the antenna elements are collectively oriented in planes that are substantially perpendicular to the longitudinal axis of the chamber. Further, they are each separately connected directly to the controller. With this connection, the controller is able to establish a phase differential between the respective antenna elements, and the controller can selectively vary this phase differential to control k
∥
in the helicon dispersion relation. Thus, the match for the helicon dispersion relation is maintained. It is to be noted that in this embodiment of the present invention, the angular frequency of the RF energy, &ohgr;, will be held constant.
In a second embodiment for the system of the present invention, the antenna match is controlled by varying the angular frequency of the RF energ
Freeman Richard L.
Miller Robert L.
Archimedes Technology Group, Inc.
Nydegger & Associates
Tran Thuy Vinh
Wong Don
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