Particle manipulation

Details Particle manipulation Particle manipulation

1999-11-29

2003-02-11

Padgett, Marianne (Department: 1762)

Coating processes

Direct application of electrical, magnetic, wave, or...

Plasma

C427S572000, C427S569000, C427S212000, C216S071000

Reexamination Certificate

active

06517912

ABSTRACT:

FIELD OF THE INVENTION
The invention concerns a method and a device for manipulating microscopic particles, especially for manipulating particles in a plasma-crystalline state.
BACKGROUND OF THE INVENTION
It is known that microscopic solid particles in a plasma may be oriented in a macroscopically regular arrangement as so-called plasma crystal. The properties of plasma crystals are for instance described by H. Thomas et al. in “Phys. Rev. Lett.”, Volume 73, 1994, page 652 ff., or by H. Thomas & G. E. Morfill in “Nature”, Volume 379, 1996, page 806 ff.
A quantitative description of plasma crystals on the basis of molecular-dynamic simulations of Yukawa systems and delimitation with respect to “liquid” states is described by S. Hamaguchi et al. in “Physical Review E”, Volume 56, 1997, p. 4671 ff. This publication was published after the priority date of the present application. The delimitation between a plasma-crystalline and a non-plasma-crystalline (for instance liquid) state is performed on the basis of a phase diagram, whose abscissa is formed by a dimensionless parameter &kgr; as quotient from the charge-dependant distance between particles and the so-called Debye length and its ordinate is formed by a parameter &Ggr;, which dimensionless describes the coulomb interaction of the particles. Because the abscissa and ordinate parameters depend on the operating parameters of the plasma, therefore changes in state of the plasma states of the particles may be achieved by changes in operating parameters.
Important aspects of plasma crystal formation will hereinafter be explained with reference to a conventional arrangement for formation of a plasma crystal according to FIG.
14
.
In a plasma state, which is for instance created by glow discharge or gas discharge, a gas includes differently charged particles, like positively or negatively charged ions, electrons and radicals, but also neutral atoms. If there are microscopic particles in the plasma (order of magnitude: &mgr;m), for instance dust particles, then these are electrically charged. The charge may be up to some hundred thousands of electron charges, depending on the particle size and the plasma conditions (gas type, plasma density, temperature, pressure, etc.). Under suitable particle and plasma conditions, coulomb forces are generated between the charged particles, under which effect the particles take a plasma crystalline state as a two or three dimensional arrangement. Besides the coulomb forces, an energy reduction at the particles by collision with neutral atoms within the plasma has an effect.
An arrangement for formation of plasma crystals is by example shown in
FIG. 14
(also see the above mentioned publication in Phys. Rev. Lett.). In a reactor (vessel walls not shown) with a carrying gas, two plane discharge electrodes are arranged one over the other. The lower circular or disc-shaped HF electrode
11
is fed with an alternating voltage, and the upper, ring-shaped counterelectrode
12
is for instance grounded. The distance between the electrodes amounts to about 2 cm. A control circuit
13
is installed for connecting the HF generator
14
to the HF electrode
11
and to feed the grounding and separating circuit
15
of the counterelectrode
12
. The high frequency energy may for instance be coupled in at a frequency of 13.56 MHz and a power of about 5 W. The carrying gas is formed by inert gases or reactive gases under a pressure of about 0.01-2 mbar. By means of a dust dispenser (not shown), dust particles are introduced into the reactor. The dust particles arrange themselves as a plasma crystal in a balanced condition, in which the gravitation force G effecting the particles are counterbalanced by the electrical field force E, which has an effect on the particles near the HF electrode
11
depending on their charge. If this is a mono dispersed dust grain distribution, then the plasma crystal arrangement is either performed as a mono layer in a plane, or as a multi-layer state when forming 3-dimensional plasma crystals. The plasma crystal is detectable by the naked eye under light up to a particle size of about 1 &mgr;m. The visibility of the plasma crystal is improved by a helium-neon laser
16
, arranged laterally, whose beam is fanned out to a diameter of about 150 &mgr;m to the size of the lateral crystal dimension using a cylinder lens combination
16
a
. Observation of the plasma crystal is performed using a CCD camera
17
, which is fitted with an enlarging macro optics
18
and controlled by image processing
19
, which is also connected to the laser
16
.
The behaviour of microscopic particles in plasma is of great theoretical and practical interest. The theoretical interest especially concerns the plasma crystals and their change of state. The practical interest is derived from the fact that plasma reactors employed for coating or processing procedures (especially in semiconductor technology) have an electrode structure according to FIG.
14
.
In prior arrangements for examination of plasma crystals, the means for influencing the plasma crystals were limited to the type of particles used and the plasma conditions realized. A means for deliberate and location-selective handling of plasma crystals is currently not available, so that up to now no practical use for plasma crystals was known.
OBJECT OF THE INVENTION
An object of the invention is to provide a method for manipulating particles in plasma, especially for influencing particles themselves or for modification of a substrate surface and a device for realizing the method.
SUMMARY OF THE INVENTION
The invention is based on the following basic findings. The properties of a plasma crystal, especially the geometric shape, does not only depend on the properties of the plasma more over or the particles. Moreover, it is possible to modify the shape of a plasma crystal, especially the shape of the outer edge or the cross sectional shape, by a location-selective effect on the above mentioned balance between gravitational forces and electrical forces. For this purpose, the external forces having an effect on the particles, for instance by a location-dependent change of a static, quasi-static or low frequency changing electrical field between the electrodes of a plasma reactor are varied by location-selective particle discharge or by location-selective particle irradiation (effect of adjusting forces). In this manner, particles in a plasma may be arranged on any curved plane with any edge in a plasma-crystalline state. The particles in the plasma may therefore be moved in a predetermined manner, whereby this movement is reversible, so that the plasma-crystalline state may even be switched between different shapes.
Another important aspect of the invention consists of the fact that by location-selective deformation of a plasma crystal, different parts of the plasma crystal are subject to different plasma conditions. This especially enables, in a plasma between two essentially plane electrodes, location-selective plasma treatment of parts of the plasma crystal (for instance coating or ablation). Such a location-selective particle treatment may be followed by deposition on a substrate.
Furthermore, an important aspect of the invention consists of the fact that formation of a plasma-crystalline state remains uninfluenced by the presence of a substrate in a plasma reactor, especially between reactor electrodes for creation of a glow discharge or gas discharge. It is especially possible to perform the above mentioned switching processes in the immediate vicinity of an areal, plane or curved substrate and subsequently reduce the distance between the particles in a plasma-crystalline state and the substrate surface in such a manner that at least a predetermined part of the particles is applied to the substrate surface. The reduction of the distance may be performed either by influencing the field forces holding the particles in position or by movement of the substrate surface. Therefore particles in a plasma-crystalline state may be deposited on substrate surfac

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