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Description  |
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BACKGROUND OF THE INVENTION
Field Of The Invention
The present invention relates to a system for generating a uniform field of
energized gas and, more particularly, to a system having segmented
electrodes for use in plasma processing.
In the manufacture of many electronic components, such as integrated
circuits and semiconductor devices, there is a need to deposit material
onto, and etch material from, substrates. In particular, metal such as
copper is often deposited and material such as epoxy glass is often
removed.
In the field of plasma deposition, an atom may be displaced from the
surface of the target connected to a cathode by a process called
sputtering or sputter deposition. In this process, the target may be
constructed of copper or of another material. The cathode to which the
target is attached is subjected to a high voltage in an inert atmosphere
such as argon. The inert gas is ionized, forming a plasma from which
positive ions escape to bombard the exposed surface of the target and to
dislodge by momentum transfer the atoms or clusters of atoms of the target
material. It is this dislodging of the target atoms that is known as
sputtering. By repeating this process, a number of these primarily neutral
atoms move through the space in front of the target, in a relatively high
vacuum, until they strike and condense on the surface of a receiver, known
as a sample, substrate or workpiece, which is generally in close proximity
to the target. A coating of atomic or molecular layers of target material
can thus be built up on the substrate. The coating, which is generally
less than 10 .mu.m, is generally sufficient for the metallization of
integrated circuits.
The most commonly used plasma reactors have one or more targets oriented
such that the surface of the target is parallel to the surface of the
sample on which atoms are to be deposited. Atoms emitted from the target
tend to be emitted in a non-uniform distribution. In particular, atoms
around the edges of the target are not emitted at the same rate as atoms
in the center of the target. Accordingly, the deposition of material on
the substrate proximate the target is not uniform. Atoms that are ejected
from the target perpendicularly and received at the sample surface
perpendicularly provide optimum atom deposition thereof. A greater amount
of material may be built up in the center of the substrate than at its
edges.
Similarly, in etching processes, it has been found that material is etched
to a greater extent along the periphery of a substrate than in the
interior portions thereof. Substrates used to make printed circuit boards
and cards, because they are large, are especially susceptible to
non-uniformity of a plasma field, resulting in non-uniform etching,
non-uniform deposition and/or non-uniform cleaning of thru holes or
viaduct holes (commonly called vias). Vias are paths for electrical
interconnections between a first-level conductive pattern and a second- or
higher-level conductive pattern. Cleaning of thru holes is referred to as
desmearing. In the etching process, for example, a more intense plasma
field along the periphery of a workpiece results in a higher etching rate
for that portion of the workpiece, whereas a relatively sparse plasma
density in the center of the workpiece results in a predictably low etch
rate for those sections. It has been found that non-uniformity of an
electric field in proximity to a printed circuit board results in
non-uniform plasma treatment thereof.
Decreasing the surface area of an electrode results in a more uniform
electric field and corresponding plasma field around it. Unfortunately,
for larger workpieces such as printed circuit boards and relatively large
semiconductor devices, a small electrode is not economically feasible. A
series of smaller electrodes, or a segmented electrode, however, in
accordance with the present invention can solve the problem of non-uniform
plasma field.
U.S. Pat. Nos. 4,361,441 and 4,394,162 issued to Tylko disclose a plasma
reactor having spaced stationary electrode structures of which at least
one is annular. Between the electrodes an arc is established by a power
supply. The arc is pulsated and caused to orbit or circulate around the
annular electrode structure to form a conical plasma zone between the
electrodes while particulate solids are introduced to the plasma zone.
Such apparatus is a possible alternative to industrial processes such as
the manufacture of steel, ferro-alloys and cermets. The use of a conical
plasma zone, however, is not especially useful for plasma processing of
electronic components such as generally planar printed circuit boards and
semiconductor devices.
U.S. Pat. No. 4,282,077 issued to Reavill discloses a generally planar
electrode plasma reaction chamber, each of a plurality of electrodes being
individually connected to a radio frequency (rf) power generator. The
electrodes are arranged for defining separate part cells with means for
individually and separately adjusting each of the cells.
U.S. co-pending patent application Ser. No. 894,240, filed Aug. 4, 1986 for
"Side Source Center Sink Plasma Reactor" assigned to the present assignee,
teaches the use of a plurality of planar electrodes in a continuous plasma
etching system in which the plasma reactor chamber has provision for
continuously introducing a gas and for forcing the gas through one or more
thru holes in the substrates in order to remove material therefrom.
It would be advantageous to provide a plasma reactor system for generating
uniform plasma fields.
It would further be advantageous to provide a system having two or more
independently controlled electrodes for creating a uniform plasma field.
It would further be advantageous to provide a plasma system in which two or
more electrodes were movable with respect to a substrate so that the
plasma field created by the electrodes could be adjusted and controlled
with specificity.
It would further be advantageous to provide a plasma system for uniformly
desmearing or etching semiconductor devices.
It would further be advantageous to etch material from relatively large
substrates uniformly.
It would further be advantageous to provide a system for depositing
material onto a substrate in a uniform manner.
SUMMARY OF THE INVENTION
A reactor for generating a uniform field of energized gas for plasma
processing. The reactor chamber is capable of sustaining a vacuum. A
mechanism for mounting a workpiece is disposed within the reactor chamber
so that a workpiece can be exposed to energized gas. A first electrode in
the chamber is positioned in operative relationship to the workpiece
mounting mechanism and a second electrode within the reactor is positioned
to surround the first electrode at least partially.
BRIEF DESCRIPTION OF THE DRAWINGS
A complete understanding of the present invention may be obtained by
reference to the accompanying drawings, when taken in conjunction with the
detailed description thereof and in which:
FIG. 1 is an isometric view of a plasma reactor in accordance with the
present invention;
FIG. 2 is a plan view of an electrode taken along section 2--2 of FIG. 1;
FIG. 3 is a plan view of an alternate embodiment of an electrode in a
concentric configuration;
FIG. 4 is a plan view of an alternate embodiment of an electrode having
outer segments partially surrounding a central segment;
FIG. 5 is a cross-sectional view of the present invention showing electrode
segments in ever-increasingly radiating positions relative to one another;
and
FIG. 6 is a cross-sectional view of electrode segments shown in an
alternate spatial relationship with respect to one another.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, there is shown a plasma reactor chamber 10 capable
of sustaining a vacuum. Such a chamber is available from Leybold-Heraeus
as model number Z-600.
Mounted in the chamber 10 by suitable means, not shown, is an electrode
structure 12. The electrode structure 12 has a radio frequency power
supply 14 attached thereto. Such a power supply 14 is available from
Huttinger, Inc. as model number IS2.5/13560.
The electrode structure 12 has a number of electrode subunits or segments
attached to it. The electrode segments are shown as reference numerals
16-20 and are described in greater detail hereinbelow.
Disposed beneath the electrode structure 12 is a platform or sample holder
24. Two electrodes are normally required for sustaining a plasma reaction.
One of the electrodes can be either the platform 24 or the vacuum chamber
wall 10. The platform 24 is a conventional sample holder or mounting
mechanism on which is mounted a workpiece 26. The workpiece 26 can be a
printed circuit board, a semiconductor device or any structure to be
etched or on which material is to be deposited. The workpiece 26 is
electrically grounded in the preferred embodiment, but need not be when a
fixed or floating potential is desired.
Referring now also to FIG. 2, there is shown a view along section lines
2--2 of FIG. 1. The segmented electrode structure is shown at reference
numeral 12. An outer electrode 16 in the shape of a rectangle is connected
to a first power supply, to shown. A second electrode 18 is disposed
within the first electrode 16 and is shaped identically to the first
electrode 16, but smaller in both dimensions. A space 16A between the
first electrode 16 and the second electrode 18 prevents an unwanted
electrical connection or short circuit between the two electrodes 16 and
18 and allows each of them to be energized independently. Any dielectric
or electrically non-conductive material, not shown, can fulfill the
function of the air gap 16A wherever such gaps are shown in the
embodiments hereinbelow depicted. Such material can add stability to the
electrode structure and may facilitate the manufacture thereof.
A second power supply, not shown, is attached to the second electrode 18.
Such a second power supply can be actuated independently of the first
power supply, not shown, to energize the second electrode 18 independently
of the first electrode 16.
In a like manner, a third electrode 20 is nested within the second
electrode 18 and separated therefrom by a gap 18A to provide electrical
insulation therebetween. A third power supply, not shown, is attached to
the third electrode 20.
Referring now also to FIG. 3 there is shown a plan view of an alternate
embodiment of the electrode structure 12 shown in a concentric
configuration. An outer electrode segment 22 forms a circle in which is
disposed a smaller circular segment 24 separated by a gap or space 22A.
Within the second segment 24 is a third segment 26 separated therefrom by
a gap or space 24A. Connected to the outer segment 22 is a first source of
power 28. Similarly, a second source of power 30 is connected to the
second electrode segment 24 and a third source of power 32 is connected to
the central electrode segment 26. Thus, each of the segments 22-26 can be
powered independently with respect to one another.
The advantage of using concentric electrode segments, is that circular
semiconductor wafers can be processed uniformly and expeditiously. It
should be understood that, while a concentric configuration is shown in
FIG. 3 and a rectangular configuration in FIGS. 1 and 2, any other
polygonal configuration using concentric polygons can be used. For example
triangles, hexagons, septagons, octagons, and the like are also
contemplated and should be considered to be within the scope of the
present invention.
Referring now also to FIG. 4 there is shown a plan view of an alternate
embodiment of an electrode having outer segments partially surrounding a
central segment. In particular, the outer electrode segment 34 of this
embodiment has three sides. Within the outer electrode 34 is disposed a
second electrode segment 36 also having three sides but being smaller in
both dimensions. Finally, within the second electrode segment 36 is the
central electrode segment 38. Separating the outer electrode 34 and second
electrode 36 is a space 34A. Similarly a space 36A separates the second
electrode segment 36 from the central segment 38.
In plasma processing prepatterned samples or workpieces, it is often
necessary or advantageous to adjust the electrodes in a manner similar to
that shown in FIG. 4. This occurs specifically when the workpiece, circuit
lines, vias and through-holes or drilled holes are not symmetrical in both
X and Y dimensions.
Referring now also to FIG. 5 there is shown an exploded cross-sectional
view of the segmented electrode structure 12 shown in FIGS. 1 and 2 in
which the electrodes 16-20 are spaced from one another so that no two of
them are co-planar. The workpiece 26 is shown without support in this
FIGURE for purposes of diagrammatic simplicity. Suitable and conventional
mounting mechanisms would normally be used. The electrode segments 16, 18
and 20 are disposed above the workpiece 26. Another set of electrodes 17,
19 and 21 is disposed symmetrically below the workpiece 26, forming a
mirror image of the upper electrodes and corresponding to the upper
electrode segments. In the preferred embodiment, one set of segments 16,
18, 20 is grounded and one set 17, 19, 21 is powered. In this
configuration, both major surfaces of the workpiece 26 can be processed
simultaneously.
Separately controlled segments 17, 19 and 21 can be energized at different
times to achieve uniformity of etching or deposition. In this way, the
central portion, needing greater processing time, could be initiated first
and outer portions could be sequentially powered up; or the entire area
could be powered up initially and power could be removed from the outer
segments sequentially. Other powering up schemes or strategies could be
used depending on workpiece patterns and requirements. In the preferred
embodiment, in order to etch material from the center of a workpiece 26 to
the same extent as the edges thereof are etched, electrode 20 is
positioned closest to the workpiece 26. Electrode 18, which surrounds
electrode 20, is spaced at a slightly greater distance from the workpiece
26. Similarly, electrode 16 surrounds the central electrodes 18 and 20 and
is positioned at a successively greater distance from the workpiece 26.
Referring now also to FIG. 6, there is shown an alternate embodiment of the
present invention. Electrode 130 is positioned closest to a workpiece 138.
Electrode 132 is placed slightly closer to the workpiece 138. Electrodes
134 and 136 are both placed at greater distances from electrode 130, but
not at progressively greater distances from the workpiece 138. All of
these electrodes 130, 132, 134 and 136 are grounded. Disposed on the lower
side of the workpiece 138 are four powered electrodes 131, 133, 135 and
137.
The powered, lower electrode segments 131-137 are located in a
substantially symmetrical panel to the grounded, upper electrode segments
130-136 with respect to the workpiece 138 in this embodiment. It should be
understood, however, that symmetry is not necessarily required for
operability. In fact, it can be seen that any configuration of electrodes
relative to one another and relative to a workpiece can be devised in
accordance with the present invention depending upon the purpose for which
the reactor chamber is used.
Since other modifications and changes varied to fit particular operating
requirements and environments will be apparent to those skilled in the
art, the invention is not considered limited to the example chosen for
purposes of disclosure, and covers all changes and modifications which do
not constitute departures from the true spirit and scope of this invention
.
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