Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – With means applying electromagnetic wave energy or...
Reexamination Certificate
2000-04-21
2003-06-10
Seidleck, James J. (Department: 1711)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
With means applying electromagnetic wave energy or...
C422S186000, C422S186040
Reexamination Certificate
active
06576202
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to gas ionization apparatus and methods generally and more specifically to capacitance coupled gas plasma reactors and methods.
BACKGROUND OF THE INVENTION
Capacitance coupled plasma reactors are usually constructed with a pair of parallel plate electrodes facing each other, spaced apart in parallel, and placed inside a vacuum chamber. An external electric field, either DC or AC, is applied to the opposite electrodes. Under low pressure and with proper spacing between the electrodes, a stable plasma can be generated and maintained by first ionizing and then creating a glow discharge in gas flowing between the electrodes. Multiple pairs of alternating polarity parallel plates can be spaced apart and/or stacked together to form multiple regions where plasma discharge may occur. Such capacitance coupled plasma reactors have been widely used in a variety of industries for applications such as substrate etching, substrate cleaning, substrate film deposition, gas treatment, ion beam source and for various chemical reactions.
As the term “capacitance coupled plasma” implies, the electrodes form a capacitor, typically of the parallel plate type. The most fundamental type is simply two flat plates of opposite electrical polarity and is often referred to as a “planar diode.” The electrodes may be arranged in a variety of geometric configurations, including configurations having curved surfaces, such as concentric parallel cylinders or concentric spheres with parallel tangents. Typically the surfaces of the alternate polarity electrodes will be equally spaced throughout the structure to maintain the parallel plate relationship. The geometric regularity and symmetry between the surfaces of the electrodes in such structures are thought to be desirable for the production of a uniform electric field and hence a more uniform plasma. Concave or convex pairs of flat plate electrodes have also been used to focus or defocus the intensity of the plasma concentration in specific regions for special applications such as focus sputtering, focus etching or to provide a focused ion source. A number of prior art capacitively coupled parallel plate electrode designs having different geometric configurations are taught in U.S. Pat. No. 4,735,633, entitled Method and System for Vapor Extraction From Gases, which issued to the inventor of the present invention and is assigned to the assignee of the present invention. The electrode configurations taught in the '633 patent provide large surface area to volume ratio for compact plasma reactors. Reactors employing electrode configurations as taught in the '633 patent have been successfully used in industry to provide greater than 99% reaction efficiency.
In addition to electrode spacing, another critical parameter for plasma generation and maintenance in capacitively coupled plasma reactors is the operating pressure. A stable glow discharge plasma can be more efficiently and easily maintained at lower pressures. This is because the production and maintenance of the plasma depends on the ionization of gas molecules in the reactor volume to produce sufficient secondary electrons to participate in the cascade collisional ionization process to offset and balance the loss of electrons (and ions) to the electrode surfaces. The mean free path, i.e., the average distance a primary electron will travel in the reactor volume before colliding with a molecule to create secondary electrons, depends upon the operating pressure. Generally, the higher the pressure, the smaller the value of the mean free path. The value of the mean free path places a limitation on the distance in which primary electrons within the electric field potential between the electrodes can accelerate to acquire the ionization potential energy needed to facilitate the ionization process. Thus, the smaller the value of the mean free path, the less ionization potential energy an electron will acquire for a given operating potential before colliding with a gas molecule, and the less secondary ionization is likely to occur.
For a given operating pressure, the electrode spacing determines the number of mean free path ionization collisions an electron will be involved in before it reaches and is lost to the electrode surface. For very short electrode spacing, no glow discharge can be generated and maintained. This space is known as dark space. Once a plasma is ignited in the reactor volume, it becomes a conducting sheet itself equivalent to an electrode. Between the plasma and the electrodes, there is always a space gap in which glow discharge ionization does not occur. Only ions and electrons are accelerated in this gap without further glow ionization discharge, and such space is the known as the “dark space shield.” The thickness of the dark space shield is also pressure dependent.
Thus, the point at which the gas molecules will break down and a stable glow discharge plasma can be generated and maintained depends on the relationship of the applied external electric field potential, the breakdown voltage, the electrode spacing and the operating pressure. Paschen experimentally found that the breakdown potential voltage (V) varies with the product of pressure P (in units of Torr) and the electrode spacing d (in units of cm). The relationships Pashen identified are known as the law of glow discharge and are reflected in the “Paschen curves” shown in FIG.
1
.
FIG. 1
shows Paschen curves
10
for several different gases. The electrode design for a capacitively coupled, parallel plate plasma reactor must adhere to the physical requirements shown by the Paschen curves.
The Paschen curves
10
of
FIG. 1
show there is a minimum breakdown voltage (V) for every gas for the product of Pd at about 1 Torr-cm, i.e., at about point
15
. Thus, in practical terms, if the spacing between parallel plate electrodes is fixed at about 1 cm, the lowest external voltage necessary to apply to the electrodes to initiate ionization and breakdown of a gas under vacuum is obtained at a pressure of about 1 Torr. As can be seen from the Paschen curves
10
, for a given electrode spacing d, as the pressure P increases, the minimum external voltage necessary to satisfy the 1 Torr-cm breakdown parameter slowly increases. However, as pressure is reduced, the minimum necessary voltage sharply increases (in linear scale of Pd). Thus, for example, given a power supply that can provide a maximum voltage of 1000 V, a reactor with fixed electrode spacing of about 1 cm can be operated at pressures up to about 300 Torr for neon gas, for example for neon light applications. But the same 1000V power supply will not be capable of generating and maintaining a plasma in Neon gas at pressures below about 0.1 Torr unless the electrode spacing is increased several times, such that the breakdown voltage 15 of the Paschen curve
10
occurs at a Pd value below the 1000V maximum supply limit.
Thus, in practical application, the relationships shown in the Paschen curves
10
determine the minimum electrode spacing and hence the minimum size for a reactor for a given power supply rating and operating pressure range. In most applications, it is desirable to use a low voltage power supply, either AC or DC, rather than a high voltage power supply because of the intrinsically lower cost of lower voltage supplies. It is also desirable to use smaller spacing between electrodes so that the reactor will be smaller and more compact. However, when operating at pressures below about 0.5 Torr, which may be required in certain applications such as in many semiconductor processing applications, it is a must to increase the electrode spacing to a few centimeters or more, thus increasing the reactor size, or alternatively to employ considerably more expensive high voltage power supplies. Though additional magnetic field sources could be used to confine the plasma in very low pressure operation application, this solution is very costly, further complicates and upsets the capacitive coupling of the applied and di
Seidleck James J.
Townsend & Townsend & Crew LLP
Tran Thao
LandOfFree
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