Note: Descriptions are shown in the official language in which they were submitted.
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Description
Plasma Reactor and Method Therefor
Technical Field
The present invention relates to plasma
reactors, and more particu:Larly, to plasma reactors
which uniformly condition workpieces treated by a gas
discharge plasma within ~aid reactor.
In the manufacture of multilayer printed
circuit boards (either of the rigid or flexible
variety), interconnecting holes are drilled through
the boards and interconnec~ing me~allic layers are
plated within the drilled holes to provide an
electrical connection between exposed edge portions of
the conducting layers of the printed circuit boards.
Ty~ically, the printed circuit board's conducting
layers are defined patterns of copper, separated by
layers of insulating plastic.
A problem which has been encountered in
forming interconnecting holes is commonly known as
"drill smear" in the printed circuit board art. The
drill smear problem is the result of resin from the
board, as well as bonding agents that hold the boards
together, coating the inside surface of the
interconnecting holes. The resulting smeared layers
tend to insulate the edge portions of the conducting
layers exposed within the drilled holes, and if no~
removed prior to plating of the apertures, individual
circuits will be shielded from the plating and,
therefore, not function pro~erly.
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Background Art
In the past the problem of drill smear was
treated by either wet (acid) chemistry or dry (plasma)
chemistry; however, each of these methods was plagued with
numerous problems. A typical example of using the wet
chemistry method to attack drill smear can be found in
U.S. Patent No. 4,155,775 to Alpaugh et al issued May 22,
1979. Likewise, an example oE utilizing dry chemis~ry to
solve this problem can be found in U.S. Patent No.
~,012,307 to Phillips issued March 15, 1977.
Problems and Objects
In utilizing wet chemistry, corrosive chemicals
are used to attack the smear and tranæform it into a
residue that is then washed away with water, whereas in
dry chemistry, a plasma is used to chemically convert the
drill smear into gaseous by-products that are carried away
by a mechanical pump.
In general, wet chemistry is considered the
less desirable method since it creates undue hazards for
personnel and excess pollutants both in the form of vapor
and waste materials that are difficult to dispose of
properly. Moreover, plasma de-smsarillg is a one-step
operation as compared ~o the wet de-smearing operation
which is multistep. Also, the dry chemistry method etches
back the non-metallic portion of the multilayer printed
cirucit board adjacent to the conducting layers in the
region of the drilled holes, thereby providin~ an
increased exposed surface area of the conducting layers to
which the interconnecting metallic layer is subsequently
plated. Accordingly, improved mechanical adhesion of the
interconnecting metallic layer results from the etching
back operation.
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While it is advantageous to use dry
chemistry, certain difficulties and deficiencies have
been encountered in prior art plasma reactors. In
particular, the electrode and chamber design utilized
in such plasma reactors are not compatible to large
scale production system&. Furthermore, non-uniformity
in workpiece conditioning has been encountered when
utilizing large scale plasma reactor systems having
annular-shaped elec~rodes.
Therefore, it is an object of the
apparatus and method of the present invention to
overcome the heretofore de~;cribed deiciencies of the
prior art.
A particular objective of the present
invention is to provide uniformly conditioned
workpieces treated in a plasma reactor apparatus
suitable for large scale production operations.
These and other features and attendant
advantages of the present invention will be more fully
appreciated as ~he same become better understood from
the following detailed description thereof.
Disclosure of the Inven~ion
The plasma reactor of the present
invention is ideally sui~ed for conditioning
workpieces, such as multilayer printed circuit boards,
in a gas discharge plasma. In particular, the
conditioning of the printing circuit boards may
include the de-smearing and etching back of the
interconnecting holes formed therein.
A vacuum vessel ha~ing a chamber ~herein
is the elasma reactor housing. Within ~he chamber is
supported a series of parallel disposed electrodes
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adapted to have alternate polarities. In addi~lon,
workpiece supporting ~eans are provided for position-
ing each workpiece between adjacen~ electrodes of
alternate polarities. The workpiece supporting means
are disposed within the vacuum vessel chamber and are
electrically isolated from the electrode&.
Means are further provided for directing a
uniform flow of gas across the wor~piece while the
workpiece is conditioned in the gas discharge plasma.
In particular, the uniform gas directiny means include
a vacuum vessel door defining a parabolic surface
within the vacuum vessel chamber, vertically disposed
baffle plates, and at least three radial gas inlets
disposed equidistantly about the chamber. The radial
gas inlets each have a discharge end which is directed
at the parabolic surface of ~he vacuum vessel door and
located between the baffle plates and the parabolic
surface.
Parallel arrangement of ~he plasma reactor
electrodes, as well as the uniform flo~ of the plasma
gas across surfaces of the workpieces, provide for the
uniform conditioning of workpieces disposed within ~he
plasma reactor of the present invention.
In the method of the present invention
utilizing the above-described plasma reactor, each
workpiece is positioned parallel to and between a pair
of adjacent electrodes of the series of electrodes.
The adjacent electrodes are caused to have alternate
polarities, and a gas discharge plasma is generated
therebetween for conditioning of the workpiece.
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~rief Description of the Drawings
Figure 1 is a side elevational view of the
plasma reac~or of the present invention.
Figure Z is a partially cross-sectional,
elevational view of the pl,asma reactor of Figure 1.
Figure 3 iB a partially cross-sectional
view along lines 3-3 of Figure 2.
Figure 4 i6 a partially cross-sectional,
exploded view o~` a top channel member used in the pre-
sent invention, as shown along lines 4-4 of Figure 2.
Figure 5 is a partially cross-sectional,
exploded view of means used in the present invention
for carrying workpieces therein, as shown alony lines
5-5 of Figure 3.
Figure 6 is a partially cross-sectional,
exploded view of a bottom channel member used in the
present invention, as shown along lines 6-6 of Figure
2.
Figure 7 is a partially cross--sectional,
exploded view of an end electrode retainer member used
in the present invention, as shown along lines 7-7 of
Figure Z.
Best Mode for Carrying Out the In~ention
Referring now to the drawings, wherein
like reference numerals represent iden~ical or
corresponding parts throughout the several views, and
: more particularly to Figures 1, 2 and 3 thereof, the
plasma reactor of the present invention is indicated
generally by reference numeral lO. Plasma reactor lO
includes a vacuurn vessel 12 having a generally
cylindrically-shaped section 14 with a parabolically
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shaped door 16 hingedly mounted thereto at brackets
18 by means of nuts and bolts 20 and 22, respectively.
Vacuum vessel 12 is supported on a work surface by
legs 15 which are fixed to section 14. A chamber 24
is defined within vacuum vessel 12. Typicall~,
vacuum vessel 12 is constructed of internally welded
aluminum, and door 16 and cylindrical section 14
sealingly mate at flange members 14(a) and 16(a),
respectively, so that when required an evacuated
condition is maintained within the chamber 24.
Door 16 is provided with a viewing
window 26 and further defines a parabolic s~lrface
28 within the chamber 24.
In utili~ing the present invention for
lS production type operations, the vacuum vessel 12
may be, for example, 38 inches in diameter and 48
- inches deep.
A box-like cage frame 30, constructed
from angle rails, is carried by vacuum vessel 12
along support members 32 which extend longitudinally
through cylindrical section 14 and are welded
thereto. Support meJ~erS 32 are themselves formed
of angle rails. Teflon*pads, not shown, are
positioned between support members 32 and cage
frame 30 to electrically isolate cage frame 30
from vacuum vessel 12.
Secured to cage frame 30 is a series of
parallel, vertically disposed electrodes 34, includin~
end electroaes 34(a). Each electrode is planar in
structure and approximately 2 feet hy 3 feet in
dimensions. While electrodes 34 and 34(a) are
depicted as being imperforate, it is nevertheless
anticipated by the p~esent invention that the
electrodes may be perforated.
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Whereas ~he rear wall of the vacuum vessel
12 is shown flat, it may appear parabolic in practice
for purposes of structural integrity and improved
pumping speed uniformity at the rear of the electrode
cage 30.
Referring to F-igures 2 and 7, end
electrodes 34(a) abut cage frame 30 along their
periphery and are held thereto by means of end
electrode retainer members 36. Retainer members 36
are L-like in shape and are themselves secured to cage
frame 30 by means of bolts 38, washers ~O and nuts
42. End electrodes 34~a) are electrically isolated by
conventional insulation means (not ~hown) from
retainer members 36 and cage frame 30.
Referring to Figures ~, 4 and 6, opposed
pairs of top and bottom electrode support channel
members 44(a) and 44(b), respectively, are used for
positioning electrodes 34 along the cage frame 30.
Each of the channel members 44(a) and ~4(b) is U-like
in configuration so that the electrodes 34 may be dis-
: posed within the groove, or channel, formed therein.
With particular attention ~o Figure 4, top c~annel
members 44(a) are secured to cage frame 30 by means of
screws 46; however, members 44(a) are electrically
isolated from cage frame 30 by a Teflon insulation
strip 48 disposed between frame 30 and channel member
44(a), and further by means of threading screw 46
through an electrically insulated shoulder bushing 50
as it extends through frame 30. Drawing attention to
Figure 6, bottom channel members 44(b) are secured ~o
cage frame 30 by means of screws 52 and are
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electrically isolated from cage frame 30 by means of
Teflon insulation strips 54 and electrically
insulating shoulder bushings 56. It is significant to
note that screws 52 are longer than screws 46, thereby
extending into groove 58 of electrodes 34. Accord-
ingly, screws 52 in cooperation with grooves 58 assist
in the proper alignment of electrodes 34.
End electrode retainer members 36 are
positioned on cage frame 30 so as to allow for
expansion and contraction of end electrodes 3~ta) and
thereby eliminate warping of the electrodes which
could bring them out of parallel alignment with
adjacent electrodes. Likewise, the grooves formed in
top and bottom electrode channel members 44(a) and
44(b) are constructed to appropriate tolerances so
that the electrodes retained therein do not warp out
of parallel alignment because of electrode expansion
and contraction.
The series of electrodes, which includes
electrodes 34 and 34(a), is arranged in the present
invention so that adjacent electrodes are of opposite
polarities. For the purpose of describing the present
invention, it is assumed that end electrodes 34(a), as
well as electrodes 34 which are alternately positioned
between electrodes 34(a) and referenced by the letter
G, are maintained at ground potential by means of
common grounding strap 60 and auxiliary grounding
strap 62. Common strap 60 extends between and is
bolted to the uppermost support members 32.
Accordingly, common strap 60 is grounded by its
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electrical contact wi~h vacuum vessel 12 via support
members 32. FuLthermore, common strap 69 extends
downwardly at its en~s and is bolted to each of the
end electrodes 3~(a). Auxiliary straps 62, extending
in a generally vertical manner, have one end thereof
bolted to common strap 60 and an opposi~e end thereof
bolted to one oE the electrodes 34 which is to be
maintained at ground potential.
The remaining elec~rodes 34, tho~e elec-
trodes not at ground potent:ial. are designated by the
letter H and are maintained at a predetermined R.F.
potential by electrical communication with R.F. gener-
ator 64, as shown in Figure 1. Two R.F. feed-throughs
66, Figure 3, are in electrical contact with R.F'.
generator 64, extend through the bottom of vaccum
vessel 12 and are electrically isolated therefrom by
insulation sleeve6 68. A common conducting strap 67
extends between and is bolted to each R. F . feed-
through. Strap members 70 are secured by bolts to
common conducting strap 67 and extend therefrom to the
alternately disposed elec~rodes 34 which are being
maintained at a predetermined R.F. potential. Conduc-
ting straps 70 are fixed to their respective electro-
des 34 by conventional bolting means. Therefore, the
present invention provides parallel disposed pairs of
adjacent electrodes of alternate polarity; i.e.,
ground potential and a predetermined R.F. potential.
While two R.F. feed-throughs 66 are shown
in Figure 3, it is nevertheless understood that the
present invention can operate with only one R.F.
feed-through.
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Each workpiece to be conditioned in the
plasma reactor 10, typically printed circuit boards
designa~ed PC in the Figures, are disposed parallel to
and between adjacent pairs of electrodes of alternate
polarities. The means for so positioning the printed
circuit boards include hollow-slotted bracket members
74, each of which is fixed to cage ~rame 30 between
adjacent top channel members 44(a) as it extends
longitudinally along vacuum vessel 12. Bracket
members 74 are secured to cage frame 30 by means of
mlts and bolts 76 and 7B, respectively, as shown in
Figure S. To assure that the printed circuit boards
are not part of the electrical circuitry of the
present invention, Teflon insulating strips 80 are
disposed between bracket members 74 and cage frame
30. Moreover, nuts 78 and bolts 76 are electrically
isolated from brac~et members 74 and cage frame 30 by
electrical insulating shoulder bushings 82.
T-rails 84 are supported within bracket
members 74 and extend through slots 86 thereof.
Apertures 88, as shown in Figure 3, are formed in
T-rails a4 to receive one hooked end of suspension
members 90. The opposite hooked ends of suspension
members 90 are received within openings in the prin~ed
circuit board, whereby two or more suspension members
gO support the board in a suhstantially parallel
relationship between two adjacent electrodes.
It is understood that the suspension
member 90 can take over conventional forms than that
described above, such as alligator clips or the like.
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A rear wall of vacuum vesGel 12 is
designated by the numeral 92 and includes two exit
ports 94 in communicaticn with a vacuum pump 96 by
means of conduits 98. The atmosphere in the vacuum
vessel chamber 24 is evacuated through ports 94 by
activation of vacuum pump 96. Whi:Le two evacuation
ports 94 are described, it is nevertheless anticipated
that only one exit port 94 is needed in the operation
of the present invention.
The gas or gas mixture which is ignited
into a gas discharge plasma in chamber 24 is provided
by an external gas source 99. Gas from source 99
flows through conduit pipe~; 100, which are secured to
vacuum vessel 12 by conventional conduit fixtures 102,
and exits into chamber 24 from three radial gas inlets
104 equidistantly po6itioned abou~ vacuum vessel 12.
Radial gas inlets 104 are elbowed and positionad for-
wardly in cylindrical section 1~ so that the gas exit-
ing from their discharge ends 106 is directed towards
parabolic surface 28 of closed door 16. Moreover,
vertically-disposed baffle plates 108 are fastened by
conventional means to cage frame 30 in a forward
position in cylindrical section 14 and extend from
caye frame 30 to the interior surface of vacuum vessel
12. At least the discharge ends 106 of each radial
gas inlet 104 extend through openings 110 in baffle
pla~es 108 so that discharge ends 106 are positioned
between baffle plates 108 and parabolic surface 28.
The equidis~ant positioning of inlets 104, the direct-
ing of the discharge gas to~ard parabolic surface ~8,
and the baffle plate~ 108 provide for a uniform flow
of the gas across the surfaces of the circuit boards
upon the generation of a gas discharge plasma.
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In a typical operation of the present in-
vention, the printed circuit boards to be conditioned.
i.e., de-smeared and etched back, are disposed within
chamber 24 between adjacent elec~rodes. Door 16 is
secured in its closed position and chamber 24 i8
evacuated to a pressure of approximately 50 millitorr
by means of vacuum pump '~6. Vacuum pump 96 is
preferably of a type which is capable of at least lZ0
c~m operation. Upon evacuation of chamber 34, a gas
mixture, typically oxygen and freon in ratios of 7 to
3 or 8 to 2, is pumped into chamber 24 to bring the
relative chamber pressure up to approximately 250
millitorr. The R.F. generator 64, capable of
operating in a ~requency range of 30 to fiO KHz with a
power potential of 4800 watts, is activated to
establish a predetermined ~.F. potential at
alternately positioned electrodes 34 having been
designated by the letter ~ in Figure 2: end electrodes
34(a) and those electrodes 3~ designated by the letter
G are, of course, maintained at ground potential.
Once the electrodes in communication with ~.F.
generator 64 have reached their appropriate polarity
potential, the gas mixture is ignited into a discharge
plasma liberating free atomic oxygen and free fluorine
for the purpose of removing drill smear and the
etching back of non-metallic material of the
multilayer printed circuit boards.
The parallel arrangement and close
proximity of the electrodes to ~he prin~ed circuit
boards have resulted in a shortening oE the condition-
ing cycle time, since the gases which are ignited in
the plasma do not have to diffuse grea~ distances in
order to condition the printed circuit boards.
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Moreover, since each printed circuit board is
positioned between a pair of adjacent electrodes and
the plasma is struck along a surface approximately
parallel to that of the printed circuit board itself,
improved batch uniformity across each circuit board
and from circuit board to circuit board is realized.
It has been found that the operation of
the subject inven~ion is a:Lso useful in the
pretreatment of laminate panels for the purpose of
improving their bondability with laminating
adhesives. In pretreating applications, the apparatus
and method of the present invention are similar to
that discussed above, although the process time and
power requirements are substantially less.
Accordingly, the present invention
provides a much sought after improvement in the plasma
art, whereby large scale production and uniform
conditioning of workpieces, such as multilayer circuit
boards, are readily obtainable.
While the invention has been described
with respect to a specific embodiment, it is not
limited thereto. The appended claims therefore are
intended to be construed to encompass all forms and
embodiments of the invention, within its true and ~ull
scope, whether or not such forms and embodiments are
expressed therein.
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