Electric heating – Metal heating – By arc
Reexamination Certificate
1998-04-22
2001-02-06
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
C219S121570, C219S121410, C156S345420, C118S7230IR
Reexamination Certificate
active
06184488
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to a low inductance large area coil for an inductively coupled plasma source. More specifically, the present invention relates to a low inductance large area coil as a source for generating a plasma which can be used for treating semiconductor wafers in low pressure processing equipment.
Plasma generation is useful in a variety of semiconductor fabrication processes, for example enhanced etching, deposition, etc. Plasmas are generally produced from a low pressure gas by inducing an electron flow which ionizes individual gas molecules through the transfer of kinetic energy through individual electron-gas molecule collisions. The electrons are commonly accelerated in an electric field, typically a radio frequency (RF) electric field.
Numerous methods have been proposed to accelerate the electrons in the RF electric field. One method is to excite electrons between a pair of opposed electrodes which are oriented parallel to the wafer in a processing chamber. The use of an electric field normal to the wafer does not provide efficient conversion of the kinetic energy to ions, since a large portion of the electron energy is dissipated through electron collisions with the walls of the processing chamber or with the semiconductor wafer itself.
A more efficient technique for exciting electrons in the RF field is to use a single winding coil (SWC) parallel to the plane of the wafer and the plasma to excite the electrons. U.S. Pat. No. 4,948,458 discloses a device for employing such a technique, which is depicted in
FIGS. 1-3
. As shown in
FIG. 1
, a plasma generating device includes an enclosure
10
with an access port
12
formed in an upper wall
14
. A dielectric shield
16
is disposed below the upper wall
14
and extends across the access port
12
. The dielectric shield
16
is sealed to the wall
14
to define a vacuum-tight interior of the enclosure
10
. A planar single winding coil (SWC)
20
is disposed within the access port
12
adjacent the dielectric shield
16
and oriented parallel to the wafer W which is supported by a surface
22
. A process gas is introduced into the chamber
18
through a port
24
formed on side of the enclosure
10
.
FIG. 2
depicts schematically the plasma generating device illustrated in FIG.
1
. As shown in
FIGS. 1 and 2
, an RF source
30
is coupled via a coaxial cable
32
through an impedance matching circuit
35
to the SWC
20
. The impedance matching circuit
35
includes a primary coil
36
and a secondary loop
38
which may be positioned to adjust the effective coupling of the circuit and allow for loading of the circuit at the frequency of operation, thereby maximizing power transfer. The primary coil
36
is mounted on a disk
40
which may be rotated about a vertical axis
42
in order to adjust the coupling. A tuning capacitor
44
is provided in series with the secondary loop
38
to adjust the circuit resonant frequency to the RF driving frequency. Another capacitor
34
is provided to cancel part of the inductive reactance of the primary coil
36
in the circuit. By resonating an RF current at a resonant frequency tuned to typically 13.56 Mhz through the coil
20
, a planar magnetic field is induced, which penetrates the dielectric shield
16
. The magnetic field causes a circulating flow of electrons between the coil
20
and the wafer W. The circulating flow of electrons makes it less likely that the electrons will strike the enclosure wall
10
between the coil
20
and the wafer W, and since the electrons are confined to a plane parallel to the planar coil
20
, the transfer of kinetic energy in non-planar directions is minimized.
Shown in detail in
FIG. 3
, the SWC
20
comprises a singular conductive element formed into a planar spiral or a series of concentric rings. As shown in
FIGS. 1 and 3
, the SWC
20
also includes a center tap labeled (+) and an outer tap labeled (−), so that it can be connected to the circuitry of the plasma generating device.
In certain applications, such as the production of 400 mm wafers or large area flat panel displays, a large area plasma is needed. In order to produce a large area plasma, the area or diameter of the SWC
20
shown in
FIGS. 1-3
must be increased. For a fixed pitch between turns, the SWC
20
increases in inductance if turns are added to increase the diameter. At large diameters, the SWC
20
produces a high inductance, which reduces the self-resonating frequency of the SWC
20
. As the self-resonating frequency becomes nearer the radio frequency (RF) driving frequency, which is normally 13.56 MHz, impedance matching becomes more and more difficult. This is because it is hard to exactly match impedance within a small frequency range, due to the increased sensitivity of the match condition to changes in the settings of the impedance matching components. Therefore, a difficulty arises in maximizing power transfer when using an SWC to generate a larger area plasma.
SUMMARY OF THE INVENTION
The invention provides an apparatus for generating an inductively coupled plasma, the apparatus comprising an enclosure surrounding a plasma reaction chamber bounded by a dielectric shield, an inlet in the enclosure supplying a process gas into the chamber, a coil comprising at least two electrically conductive windings disposed outside the enclosure proximate the dielectric shield, and a radio frequency source coupled to the windings via impedance matching circuitry which matches the impedance of the radio frequency source to the windings and a frequency tuning mechanism which provides resonance, the radio frequency source being effective to resonate a radio frequency current in the coil and excite the process gas into a plasma within the chamber.
According to various aspects of the invention, the coil can have different configurations. For instance, the windings can be parallel and in the same plane, the coil can be non-planar, the windings can be connected together at opposite ends thereof, the windings can be unconnected together at opposite ends thereof, the windings can be interleaved, the windings can be non-interleaved but cover different surface areas and/or the turns of one of the windings are separated by the turns of the other one of the windings. The enclosure can comprise a multiple or single wafer etching apparatus wherein a wafer chuck supports one or more semiconductor wafers having a surface to be processed parallel to the plane of the coil.
The invention also provides a method for generating an inductively coupled plasma, the method comprising the steps of introducing a process gas into a plasma reaction chamber enclosed by an enclosure bounded by a dielectric shield, and resonating a radio frequency current in a coil comprising at least two electrically conductive windings disposed outside the enclosure proximate the dielectric shield, the radio frequency current being effective to excite the process gas into a plasma within the chamber.
The method can be carried out using the various coil configurations mentioned above. Further, the plasma can be used to process one or more substrates such as semiconductor wafers or flat panel displays. For instance, a semiconductor wafer can be located in the chamber and a layer on the wafer can be etched by the plasma. During processing, the chamber can be maintained at a wide range of pressures but in a preferred embodiment the pressure is maintained below 100 mTorr.
REFERENCES:
patent: 4340462 (1982-07-01), Koch
patent: 4612077 (1986-09-01), Tracy et al.
patent: 4617079 (1986-10-01), Tracy et al.
patent: 4948458 (1990-08-01), Ogle
patent: 5198718 (1993-03-01), Davis et al.
patent: 5234529 (1993-08-01), Johnson
patent: 5241245 (1993-08-01), Barnes et al.
patent: 5261962 (1993-11-01), Hamamoto et al.
patent: 5304279 (1994-04-01), Coultas et al.
patent: 5368710 (1994-11-01), Chen et al.
patent: 5401350 (1995-03-01), Patrick et al.
patent: 5405480 (1995-04-01), Benzing et al.
patent: 5688357 (1997-11-01), Hanawa
patent: 5795429 (1998-08-01), Ishii e
Lam Research Corporation
Lowe Hauptman Gopstein Gilman & Berner
Paschall Mark
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