Coating processes – Direct application of electrical – magnetic – wave – or... – Plasma
Reexamination Certificate
1999-09-22
2001-05-08
Padgett, Marianne (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Plasma
C427S535000, C427S534000, C216S071000, C118S7230ER
Reexamination Certificate
active
06228438
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The invention relates to a capacitively coupled radiofrequency (RF) plasma reactor and to a process for treating at least one substrate in such a reactor. Especially, the present invention applies to a large size capacitive capacitively coupled (RF) plasma reactor.
Often, such a reactor is known as a “capacitive” RF glow discharge reactor, or planar plasma capacitor or parallel plate RF plasma reactor, or as a combination of the above named. Capacitive RF plasma reactors are typically used for exposing a substrate to the processing action of a glow discharge. Various processes are used to modify the nature of the substrate surface. Depending on the process and in particular the nature of the gas injected in the glow discharge, the substrate properties can be modified (adhesion, wetting), a thin film added (chemical vapour deposition CVD, diode sputtering) or another thin film selectively removed (dry etching). The table shown below gives a simplified summary of the various processes possibly performed in a low pressure capacitive discharge.
Industry
Substrate type
Process
Inlet gas nature
Semiconductor
wafer
- Surface Cleaning
- Ar
up to 30 cm
- PECVD
- SiH
4
, . . .
diameter
- Dry Etching
- CF
4
, SF
6
, Cl
2
, . . .
- Ashing
- O
2
,
Disks for
Polymeror glass
- Diode sputtering
- Ar + others
memory
up to 30 cm
- PECVD
- Organometallics
diameter
- Surface activation
- O
2
, etc. . . .
Flat display
Glass
Same as for
Same as for
up to 1.4 m diagonal
semiconductors
semiconductors
Window pane
Glass up to 3 m
- Cleaning/
- Air, Argon
Web coaters
width, foil, plastic
activation,
Monomer, Nitrogen,
or metal
Nitriding, polymer
. . .
PECVD
The standard frequency of the radiofrequency generators mostly used in the industry is 13.56 MHz. Such a frequency is allowed for industrial use by international telecommunication regulations. However, lower and higher frequencies were discussed from the pioneering days of plasma capacitor applications. Nowadays, for example for PECVD applications, (plasma enhanced chemical vapour deposition) there is a trend to shift the RF frequency to values higher than 13.56 MHz, the favourite values being 27.12 MHz and 40.68 MHz (harmonics of 13.56 MHz). So, this invention applies to RF frequencies (1 to 100 MHz range), but it is mostly relevant to the case of higher frequencies (above 10 MHz). The invention can even be applied up to the microwave range (several GHz).
An important problem was noted especially if the RF frequency is higher than 13.56 MHz and a large size (surface) substrate is used, in such a way that the reactor size is no more negligible relative to the free space wave length of the RF electromagnetic wave. Then, the plasma intensity along the reactor can no longer be uniform. Physically, the origin of such a limitation should lie in the fact that the RF wave is distributed according to the beginning of a “standing wave” spacial oscillation within the reactor. Other non uniformities can also occur in a reactor, for example non uniformities induced by the reactive gas provided for the plasma process.
It is an object of the invention to propose a solution for eliminating, or at least notably reducing, an electromagnetic (or a process) non uniformity, in a reactor. Thus, according to an important feature of the invention, an improved capacitively coupled RF plasma reactor should comprise:
at least two electrically conductive electrodes spaced from each other, each electrode having an external surface,
an internal process space enclosed between the electrodes,
gas providing means for providing the internal process space with a reactive gas,
at least one radiofrequency generator connected to at least one of the electrodes, at a connection location, for generating a plasma discharge in the process space, and potentially an additional RF generator for increasing the ion bombardment on the substrate,
means for evacuating the reactive gas from the reactor, so that said gas circulates within the reactor, at least in the process space thereof,
at least one substrate defining one limit of the internal process space, to be exposed to the processing action of the plasma discharge, said at least one substrate extending along a general surface and being arranged between the electrodes,
characterized in that it further comprises at least one dielectric “corrective” layer extending outside the internal process space, as a capacitor electrically in series with said at least one substrate and the plasma, said at least one dielectric layer having capacitance per unit surface values which is not uniform along at least one direction of said general surface, for compensating a process non uniformity in the reactor or to generate a given distribution profile.
In other words, the proposed treating process in the reactor of the invention comprises the steps of:
locating the at least one substrate between at least two electrodes, the substrate extending along a general surface,
having a reactive gas (or gas mixture) in an internal process space arranged between the electrodes,
having a radiofrequency generator connected to at least one of the electrodes, at a connection location,
having a plasma discharge in at least a zone of the internal process space facing the substrate, in such a way that said substrate is exposed to the processing action of the plasma discharge,
creating an extra-capacitor electrically in series with the substrate and the plasma, said extra-capacitor having a profile, and
defining the profile of the extra-capacitor in such a way that it has location dependent capacitance per unit surface values along at least one direction of the general surface of the substrate.
It is to be noted that such a solution is general. It is valid for all plasma processes, but only for a determined RF frequency.
The “tailored extra-capacitor” corresponding to the above-mentioned said (substantially) “dielectric layer” acts as a component of a capacitive divider. Advantageously, the capacitive variations will be obtained through a non uniform thickness of the layer. Thus, the extra-capacitor will have a profile having a non planar-shape along a surface.
For compensating a non uniform voltage distribution across the process space of the reactor, said thickness will preferably be defined in such a way that:
the so-called “corrective layer” is the thickest in front of the location in the process space (where the plasma is generated) which is the farest away from the connection location where the radiofrequency generator is connected to said at least one electrode, the distance being measured by following the electrode external surface,
and said thickness preferably decreases from said process space location, as the distance between the process space location and the connection location on the corresponding electrode decreases.
Of course, it is to be understood that the above-mentioned “distance” is the shortest of all possible ways.
So, if the electromagnetic travelling waves induced in the process space combine each other near the center of the reactor to form a standing wave having a maximum of voltage in the vicinity of the reactor center, the thickness of the so-called “corrective layer” will be larger in the vicinity of the center thereof, than at its periphery.
One solution in the invention for tailoring said “corrective layer” is to shape at least one surface of the layer in such a way that the layer has a non planar-shaped external surface, preferably a curved concave surface facing the internal process space where the plasma is generated. Various ways can be followed for obtaining such a “non planar shaped” surface on the layer.
It is a priviledged way in the invention to shape at least one of the electrodes, in such a way that said electrode has a non planar-shaped surface facing the substrate, and especially a generally curved concave surface. It is another object of the invention to define the composition or constitution of the so-called “corrective layer”.
According to a preferred solution, said layer comprises at leas
Notaro & Michalos P.C.
Padgett Marianne
Unakis Balzers Aktiengesellschaft
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