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Description  |
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BACKGROUND
The present invention pertains to a plasma reactor which utilizes
electromagnets to enhance plasma.
Plasma etching is a well known dry etching technique which physically and
chemically etches a workpiece. The basic plasma reactor consists of an
evacuated chamber with electrodes. An etching gas, such as CF.sub.4, is
introduced into the chamber and is disassociated by the flow of electrons
between electrodes, to form a plasma. One of the electrodes, acts as a
stage for the workpiece. The workpiece, is then physically and chemically
etched by the plasma. The disassociation of the etching gas produces ions
with high kinetic energy which bombard the workpiece thereby physically
etching it. The disassociation of the etching gas also produces reactive
neutrals which react with the workpiece thus chemically etching it.
It has been found that relatively high pressure plasma etchers achieve high
etch rates from increased reactive radical densities due to the decreased
collisional mean free path of electrons in the discharge. This however,
results in an increased degree of isotrophy.
Anisotropic etching can be obtained by decreasing the pressure in the
reaction chamber. In this case anisotropy is increased but etch rate is
lowered. The lowering of the etch rate is due to the decreased production
of reactive radicals due to the increased collisional mean free path of
electrons in the discharge.
A low pressure plasma etcher with high etch rates can be obtained by
utilizing magnets. A magnetic field perpendicular to the electric field
causes electrons to execute a cycloidal motion which continues until the
electron suffers an exciting or ionizing collision with a gas molecule
producing reactive radicals. While anisotropic etching and increased etch
rates are achieved uniformity suffers due to the concentration of reactive
radicals along magnetic field lines.
In U.S. Pat. No. 4,526,643 titled "Dry Etching Apparatus Using Reactive
Ions", issued July 2, 1985, a dry etching apparatus employing permanent
magnets is disclosed. These magnets are attached to a belt which is
rotated underneath the workpiece. This causes a scanning motion of the
magnetic field across the surface of the workpiece. In bar magnets the
magnetic lines connecting north and south poles are curved. The bigger the
bar magnet, the less the magnetic lines will be curved. In order to
approximate a linear magnetic line, the magnet must be very large.
However, even the magnetic lines of large magnets will not become
completely linear. Thus, in this prior art with the magnets being
necessarily limited in size due to the number, space and mechanics
involved, the field is non-linear and results in reduction in uniformity
of the etch. Further, the mechanical movement of the magnets themselves is
very difficult when located inside a vacuum and can cause numerous
problems.
SUMMARY OF THE INVENTION
The present invention pertains to a plasma system with magnetically
enhanced plasma. Magnetic fields are produced in the reaction chamber of a
plasma reactor device by electromagnetic oils. The magnetic field is then
set into motion electrically by exciting the electromagnets in a series
fashion.
It is an object of the present invention to provide a new and improved
plasma reactor with magnetically enhanced plasma.
It is further object of the present invention to provide a plasma reactor
with improved uniformity of plasma etch processes.
It is further object of the present invention to provide a plasma reactor
with increased magnetic field control.
These and other objects of this invention will become apparent to those
skilled in the art upon consideration of the accompanying specification,
claims and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring to the drawings, wherein like characters indicate like parts
throughout the figures:
FIG. 1 is a top view of a magnetically enhanced plasma etch system
embodying the present invention;
FIG. 2 is a simplified cut-away side view of the device in FIG. 1;
FIG. 3 is a simplified schematic of an electrical circuit for exciting the
device of FIG. 1;
FIG. 4a is a graphical representation of the input to FIG. 3; and
FIG. 4b is a graphical representation of switching signals at various times
in FIG. 3.
DESCRIPTION OF THE PREFERRED EMBODIMENT
A preferred embodiment of the apparatus in accordance with the invention is
illustrated in FIG. 1. A magnetic core 12, which has a plurality of
projections 14, encircles a reaction chamber 18. While in this embodiment
a solid core is used, it should be understood by those skilled in the art
that an air core is possible. Projections 14, in this embodiment, occur in
pairs with each member directly opposite the other. Projections 14 are
each encircled by a magnetic oil 16. When electric current is passed
through these coils, they generate a magnetic field (B). In this
embodiment, each member in the pair of projections acts as a magnetic
pole, with one being north and the opposing member being south. This acts
to produce a magnetic field with magnetic lines that connect the two poles
and pass directly through reaction chamber 18 in a straight line.
FIG. 2 is a simplified cut-away side view of device 10. Magnetic core 12
and coil 16 can be seen surrounding the reaction chamber 18. The reaction
chamber 18 volume has a top electrode 20 and a bottom electrode 22 in this
embodiment. Electrode 22 acts as a pallet for a workpiece 24. In FIG. 2,
the magnetic core 14 and coil 16 shown are a pair with one being north and
the other being south. When an electric current runs through coil 16 a
magnetic field is produced between the two poles. This field goes directly
across reaction chamber 18 and is perpendicular to the electric field
produced by electrodes 20 and 22. This causes the increased ionization of
the etching gas due to the increased path of the ionizing electrons.
In order to form a uniform plasma and thereby uniformly etch a workpiece
the magnetic field must be perpendicular to the electric field and evenly
distributed throughout the reaction chamber 18. This is accomplished by
using electromagnets which produce straight magnetic field lines thereby
being perpendicular to the electric field at all points, and by the
electrical rotation of the magnetic field which evenly distribute the
magnetic field lines throughout the reaction chamber.
The rotation of the magnetic field can be accomplished in many ways. A
convenient embodiment for the rotation of the magnetic field is
illustrated as a simplified schematic in FIG. 3. An AC signal such as 60
Hz is inputted to a switching circuit 28 and clock device 29. In this
embodiment four pairs of electromagnetic poles 31A and 31B through 34A and
34B are utilized. Each pair of poles is energized singly. Poles 31A, 32a,
33A and 34A each have coils which are wound in the same direction while
their respective pairs are wound in the opposite direction. This causes,
when a pair is energized, one pole such as 31A to be north and its pair
31B to be south. The field is rotated by energizing first one pair then
energizing the next pair and so on around the reactor with the proceeding
pair being denergized and the adjacent pair being energized. For the
magnetic field to accomplish a complete rotation, when 180 degrees of
rotation has been reached, poles 31B through 34B which had been south must
now be north with the opposing member of the pairs changing to south.
FIG. 4A shows a graphical representation of the input to FIG. 3. It shows
the positive half and the negative half of an alternating current. FIG. 4B
shows a graphical representation of the switching signals with relation to
FIG. 4A. As shown by FIG. 4B the current is switched four times in the
positive half and four times in negative half of the signal. This causes
the full rotation of the magnetic field. A first pair of poles is
energized by the first switch with the positive half of the signal and
then the current is switched to the second half of poles and so on until
the field has rotated 180 degrees and each pair of poles has received a
positive signal. The current then alternates to negative and the poles are
again energized consecutively with opposite effect. With this negative
signal, the poles that are south now become north and the poles that were
north now become south. As the current once more switches from air 31 to
pair 32 and so on, the magnetic filed completes its 360 degree rotation in
the reaction chamber.
The circuitry of FIG. 3 produces a complete revolution of the magnetic
field for each cycle of current applied to the input. Thus, the magnetic
field is rotating at 60 cycles per second. It will of course, be
understood by the skilled in the art that higher or lower frequencies can
be utilized to obtain faster or slower rotation of the magnetic field.
There is thus provided by the present invention a substantially improved
plasma reactor which, due to the linear magnetic lines and the rotation of
said lines, causes a uniform magnetic field throughout the reaction
chamber and produces a substantially more uniform etch. Also, due to the
electrical control of the magnetic fields, the intensity as well as the
speed of rotation of the magnetic field can be controlled with greater
precision.
A current intensity control 30 is connected between the current input and
switching circuit 28 and is adjustable to control the amount of current
being applied to the coils of pole pairs 31-34. Control 30 may be, for
example, a simple reostate or potentiometer. By controlling the amount of
current being applied to the coils, the intensity of the magnetic field
can be controlled and, hence, the density of the plasma.
Having thus described the invention, it will be apparent to those skilled
in the art that various modifications can be made within the spirit and
scope of the present invention. For example, while in the preferred
embodiment a rotational magnetic field is described, a translation
magnetic field utilizing electromagnets to sweep a magnetic field over a
workpiece or a series of workpieces is possible. Further, while substrate
etching has been used in describing this invention, it can also be used
for sputtering and plasma enhanced chemical vapor deposition (PECVD) of
substrates.
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