Radiant energy – Electrically neutral molecular or atomic beam devices and...
Reexamination Certificate
2003-01-27
2004-03-23
Wells, Nikita (Department: 2881)
Radiant energy
Electrically neutral molecular or atomic beam devices and...
C250S492100, C250S492200, C250S374000, C315S111010, C315S111210
Reexamination Certificate
active
06710333
ABSTRACT:
FIELD OF INVENTION
The present invention relates to plasma processing technology. In particular the invention pertains to treatment of surfaces by a flow of plasma produced atomic or molecular medium excited to metastable level to clean the surfaces from undesirable contaminating residues, to improve surface adhesive bonding, to provide surface sterilization, or to modify selectively the chemical and crystalline structure of the surface treated for semiconductor, display, micromachining, and medical related technologies.
BACKGROUND OF THE INVENTION
There exists a plurality of methods of plasma treatment of solid surfaces for the purpose of cleaning, etching, and enhancing these surfaces. Within these methods, the plasma cleaning technologies form a separate group of treatments. Within this separate group there is a distinctive difference between passive plasma cleaning processes, when the surfaces treated are exposed passively to plasma or its products, and active ones, when the surfaces treated interact as electrically biased electrodes with charged components of plasma (electrons and ions). A description and a classification of plasma cleaning processes is presented in “Industrial Plasma Engineering”, Vol. 2, Applications to Nonthermal Plasma Processing, by J. Reece Roth, Institute of Physics Publishing, 2001, p.p. 341-359. The present invention relates closely to the passive plasma cleaning processes. More specifically, it relates to surface modification processes provided by a neutral long living metastable component excited in plasmas of a working gaseous medium
As it is well known from gas discharge laser technology, a significant concentration of excited atoms or molecules of the working gas can be achieved in gas discharge plasmas and used as an active medium for laser generation, see for example: “Kinetic Processes in Gases and Molecular Lasers” by B. F. F. Gordiets, A. I. Osipov, L. A. Shelepin, Gordon & Breach Publishing Group, 1988. The long living metastable states of the electrons in the atoms (or molecules) can exist due to high quantum symmetry of these metastable states provided by atomic (or molecular) electron configuration. Since interaction of the excited atoms (or molecules) with a surface violates this symmetry, the energy of excitation is liberated (with a final electron transition to ground level) and is absorbed in the surface of the solid with significant probability. This effect of originating an anomalous thermo-conductivity in gases of gas discharge lasers is described in a paper by E. V. Shun'ko: “Some features of power balance in gas discharge in CO
2
, N
2
, and their mixtures with He”, J. Appl. Phys., 70(12), 1991, p.p. 7273-7281.
1. Thus the atoms (or molecules) excited to metastable level can be utilized for modification of the surfaces of the solids.
2. The complete electron transition energy to ground level for certain metastable excited atoms can achieve the value ~15 eV which is enough to destruct structural bonds in the surface of the work piece and increase in this manner the surface energy above 70 erg/cm
2
(surface tension of water at the room temperature) improving surface adhesive bonding significantly.
3. The relatively high energy ~15 eV, which can be liberated from each electron transition in the process of metastable atom relaxation on the surface, enables one to form the free radicals in a biomaterial contaminating the surface that provides in this manner a surface sterilization.
4. Choosing necessary operating gas or gas composition within available gases and vapors, and choosing in this manner a necessary energy of the operating metastable level, one can selectively induce or disintegrate chemical bonds of the material coating the surface.
5
. The possible long life time of &tgr;
m
>0.001 sec (for example) realized for the metastable state of some kinds of atoms or molecules enables one to separate excited atoms from plasma by blowing operating gas through a discharge gap with the average flow velocity of v
g
>10 m/s, providing a physical disconnection of the operating gas saturated with the excited atoms from a plasma boundary at the distance d=v
g
×&tgr;
m
>1 cm. This disconnection provides the possibility for a soft surface cleaning avoiding destructive direct contact of plasma with patterns developed on the surfaces treated for semiconductor, display (Indium-Tin-Oxide), and micromachining technology.
The process of modification of the surface by its exposure to a flow of metastable atoms (molecules) mixed with a neutral gas and plasma can be realized with RF Capacitively Coupled Plasma (CCP) sources described in “Industrial Plasma Engineering”, Vol. 1, Principles, J. Reece Roth, Institute of Physics Publishing, 1995, p.p. 417-463. As seen in FIG. 12.12, page 443 of this book (see
FIG. 1
of the present disclosure), a plasma source with outer ring type electrodes
2
and
3
is mounted outside of a quartz wall (or ceramic) tube
1
serving as a plasma reactor is one of the possible RF plasma sources applicable for metastable excited atomic (molecular) medium generation. This arrangement of the electrodes enables one to avoid plasma contamination with an electrode material affecting in general the life time of the metastable medium. The operating gas (or gas composition) required for such a process is fed to one of the ends of the tube
1
under necessary pressure, whereby, at the supply of RF power to said electrodes
1
and
2
at a frequency of 1 to 100 MHz, plasma
4
mixed inside the tube
1
with neutral gas and neutral metastable atoms (molecules) spreads along this tube due to the electromagnetic field action and causes a gas flow through the inter-electrode space to contact with a work piece
5
and to deliver in this manner to the work piece
5
surface the excited metastable atomic (molecular) medium produced from the operating gas (or gas composition).
A process of surface modification can be provided by static exposure of the work piece
5
surface to a flow of a working mixture of plasma with a neutral gas and an excited atomic (molecular) medium during a certain exposure time &tgr;
ex
, or by moving the work piece with a constant velocity v
w
under the flow of this mixture that assumes the exposure time defined by the equation &tgr;
ex
=a/v
w
, where a is the diameter of the mentioned beam, see FIG.
1
.
However a practical application of the RF source type presented in
FIG. 1
for production of an excited to metastable state medium separated from plasma has problems. A combined plasma electrode configuration forms a typical dipole antenna propagating the electromagnetic field in surrounding space (as is seen in FIG.
1
). This propagation promotes in turn plasma spread along the tube
1
to distances of several inter-electrode lengths. As a result, the plasma has a natural slow decrease in density outward from the source electrodes
2
and
3
. Therefore at distance l
w
where this density is negligibly small and undesirable RF-arc breakdown from plasma to the surface of the workpiece becomes completely impossible, the density of the excited metastable medium (having a limited life time) produced by a weak plasma wing is negligible as well. To improve the output density of the excited metastable medium without jeopardizing the surface to be treated to RF-arc breakdown, one can increase the velocity of gas flow through the tube
1
. However increasing gas velocity is very costly for an industrial process provided with operating gases having a relatively high price.
It is clear that the best way to increase the output density of the metastable excited medium is to form a clear cut sharp boundary of high density plasma downstream. Then at a distance safe for RF breakdown between the plasma boundary and the workpiece surface, one can realize a maximum possible excited metastable medium density provided by this close distanced high density plasma. To form the sharp plasma boundary at the metastable excited medium outflow, a propagation of the RF electromagnetic field should
Carothers and Carothers
Wells Nikita
Wintek Corporation
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