U.S. patent application number 10/544252 was filed with the patent office on 2006-07-13 for method of electroplating a workpiece having high-aspect ratio holes.
Invention is credited to Tafadzwa Magaya, Bert Reents.
Application Number | 20060151328 10/544252 |
Document ID | / |
Family ID | 32892263 |
Filed Date | 2006-07-13 |
United States Patent
Application |
20060151328 |
Kind Code |
A1 |
Reents; Bert ; et
al. |
July 13, 2006 |
Method of electroplating a workpiece having high-aspect ratio
holes
Abstract
In order to electroplate workpieces comprising high-aspect ratio
holes a method is disclosed comprising the steps bringing the
workpiece and at least one anode into contact with a metal plating
electrolyte, and applying a voltage between the workpiece and the
anodes, to the effect that a current flow is provided to the
workpiece. The current flow is a pulse reverse current flow having
a frequency of at most about 6 Hertz. According to the frequency
each cycle time comprises at least one forward current pulse and
least one reverse current pulse.
Inventors: |
Reents; Bert; (Berlin,
DE) ; Magaya; Tafadzwa; (Berlin, DE) |
Correspondence
Address: |
PAUL AND PAUL
2000 MARKET STREET
SUITE 2900
PHILADELPHIA
PA
19103
US
|
Family ID: |
32892263 |
Appl. No.: |
10/544252 |
Filed: |
February 4, 2004 |
PCT Filed: |
February 4, 2004 |
PCT NO: |
PCT/EP04/02208 |
371 Date: |
August 2, 2005 |
Current U.S.
Class: |
205/103 ;
257/E21.175 |
Current CPC
Class: |
H05K 2203/1572 20130101;
H05K 3/423 20130101; C25D 7/12 20130101; H05K 2203/1492 20130101;
C25D 5/08 20130101; C25D 5/18 20130101; H01L 21/2885 20130101 |
Class at
Publication: |
205/103 |
International
Class: |
C25D 5/18 20060101
C25D005/18 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 10, 2003 |
DE |
10311575.7 |
Claims
1. Method of electroplating a workpiece comprising high-aspect
ratio holes, the method comprising: a. bringing the workpiece and
at least one anode into contact with a metal plating electrolyte,
and b. applying a voltage between the workpiece and the anodes, to
the effect that a current flow is provided to the workpiece,
wherein the current flow is a pulse reverse current flow having a
frequency of at most about 6 Hertz with, in each cycle time, at
least one forward current pulse and at least one reverse current
pulse.
2. Method according to claim 1, comprising setting the ratio of the
duration of the forward current pulses to the duration of the
reverse current pulses of one cycle in a range from about 5 to
about 75.
3. Method according to any one of the preceding claims 1-2,
comprising setting the duration of the forward current pulses of
one cycle to at least about 100 ms.
4. Method according to any one of the preceding claims 1-2,
comprising setting the duration of the reverse current pulses of
one cycle to at least about 0.5 ms.
5. Method according to any one of the preceding claims 1-2,
comprising setting the peak current density of the forward current
pulses at the workpiece in a range from about 3 A/dm to about 15
A/dm.
6. Method according to any one of the preceding claims 1-2,
comprising setting the peak current density of the reverse current
pulses at the workpiece in a range from about 10 A/dm.sup.2 to
about 60 A/dm.sup.2.
7. Method according to any one of the preceding claims 1-2,
comprising a. applying a first voltage to between a first side of
the workpiece and at least one first anode, to the effect that a
first pulse reverse current flow is provided to the first side of
the workpiece, said first pulse reverse current flow having at
least one first forward current pulse and at least one first
reverse current pulse flowing in each cycle time, and b. applying a
second voltage to between a second side of the workpiece and at
least one second anode, to the effect that a second pulse reverse
current flow is provided to the second side of the workpiece, said
second pulse reverse current flow having at least one second
forward current pulse and at least one second reverse current pulse
flowing in each cycle time.
8. Method according to claim 7, comprising offsetting the first
forward and reverse current pulses relative to the second forward
and reverse current pulses, respectively.
9. Method according to claim 8, comprising offsetting the first
current pulses relative to the second current pulses by
approximately 180.degree..
10. Method according to any one of the preceding claims 1-2,
comprising providing the current flow, in each cycle time, with two
forward current pulses and one reverse current pulse with one zero
current break between the two forward current pulses.
11. Method according to any one of the preceding claims 1-2,
comprising varying, in the course of metal plating the workpiece,
at least one parameter of the pulse reverse current flow, selected
from the group comprising the ratio of the duration of the forward
current pulses to the duration of the reverse current pulses of one
cycle and the ratio of the peak current density of the forward
current pulses to the peak current density of the reverse current
pulses.
12. Method according to claim 11, comprising increasing, in the
course of metal plating the workpiece, the ratio of the peak
current density of the forward current pulses to the peak current
density of the reverse current pulses, and/or decreasing, in the
course of metal plating the workpiece, the ratio of the duration of
the forward current pulses to the duration of the reverse current
pulses of one cycle.
13. Method according to any one of the preceding claims 1-2,
comprising bringing the workpiece into contact with the metal
plating electrolyte by delivering the metal plating electrolyte
towards the surface of the workpiece at an electrolyte flow
velocity relative to the surface of the workpiece.
14. Method according to claim 13, comprising forcing the metal
plating-electrolyte under agitation towards the workpiece.
15. Method according to claim 13, wherein the electrolyte flow
velocity at the surface of the workpiece comprises a velocity
component normal to the surface of the workpiece being at least
about 1 m/sec.
16. Method according to any one of the preceding claims 1-2,
wherein the anodes are inert and dimensionally stable.
17. Method according to any one of the preceding claims 1-2,
wherein the metal plating electrolyte is a copper plating
electrolyte.
18. Method according to claim 17, wherein the copper plating
electrolyte contains at least one compound capable of oxidizing
copper metal to copper ions and wherein additional copper metal
pieces are brought into contact with the copper plating
electrolyte.
19. Method according to claim 18, wherein the compounds capable of
oxidizing copper metal to copper ions are ferric compounds.
Description
[0001] The production of high-aspect ratio printed circuit boards,
for example so-called back panels, poses well-known problems for
good quality electrolytic copper metallization. The panels can be
from 3 mm and up to 10 mm thick with aspect ratios of typically
10:1. However, there is a current trend requiring even thicker
panels and with an aspect ratio up to 15:1. Such panels typically
can be larger than "normal" production panels which gives added
problems in handling due to their weight. One of the limiting
factors in copper deposition is the mass transport of ions into the
high-aspect ratio holes. Achieving the required copper thickness in
the hole without over-plating the surface causing resist
over-plating with pattern plate or poor line definition with panel
plate are the main problems in the production of high-aspect ratio
panels. A further factor with back panels is the difficulty of
component mounting using the press-fit technique when the copper
deposit distribution is poor. To overcome throwing power problems
low electroplating current densities have been used which obviously
have a negative impact on productivity. As a solution to these
problems reverse pulse plating can allow the use of higher current
densities with improved surface distribution and throwing power in
the through-holes as described in DE 42 25 961 C2 and DE 27 39 427
A1.
[0002] In horizontal processing of printed circuit boards it has
emerged that the high-aspect ratio throwing power in Uniplate.RTM.
(Atotech Deutschland GmbH) systems has been a restriction to their
use for the production of thicker panels. Even for panels thicker
than 1.6 mm the copper throwing power has not been fully acceptable
depending on the aspect ratio. The reasons for this have been
because of the emphasis on the production of thinner material at
higher current densities with blind micro-vias. The high-current
density, in the order of 10 A/dm.sup.2 average and the requirement
to produce blind micro-vias under such conditions has required the
use of relatively high-copper concentrations at above 35 g/l. Both
of these factors have not enabled the best throwing power in
high-aspect ratio panels. Trials have been made to improve the
throwing power in standard Inpulse.RTM. (Atotech Deutschland GmbH)
equipment. But these have only given a marginal improvement. These
trials were limited by the pulse parameters which are available
with the standard Inpulse.RTM. system (copper plating system with
inert anodes, using the iron(II)/iron(III) redox system and reverse
pulse current technique in vertical and horizontal plating
devices).
[0003] Kruse describes in Galvanotechnik (3/2002, p. 680) a method
for reverse pulse plating of e.g. printed circuit boards, in which
the sum of the duration of two forward pulses intermitted by an off
pulse has been set to 5-250 ms, and the duration of reverse pulses
has been set to 0-5 ms.
[0004] In US 2003/0019755 A1 a method for pulsed electroplating a
metal on a substrate is described wherein an electrodeposition
(cathodic) pulse may range from about 500-3000 ms, while that for
an electrodissolution (anodic) pulse may range from about 1-300
ms.
[0005] In reverse pulse plating printed circuit boards, the
duration of forward pulses often has been set to 10-80 ms, and
duration of reverse pulses has been set to 0.5-6 ms. This has
resulted in a frequency range of about 12 to about 95 Hz. If
printed circuit boards have been to be produced which were 2 mm
thick and which contained through holes with an aspect ratio of
10:1 acceptable throwing power of copper deposition in the through
holes has been achieved at a current density in the range of 1-10
A/dm.sup.2 for the forward pulses and at a current density in the
range of 10-40 A/dm.sup.2 for the reverse pulses. If printed
circuit boards with a thickness of greater than 2 mm have been to
be produced, the current densities must be decreased in order to
achieve an acceptable result in throwing power.
[0006] In a joint project with the Kurt-Schwabe-Institut fur
Mess-und Sensortechnik e.V., Germany, the flow dynamics of copper
deposition was investigated. The results from this investigation
have been published by Reents, B., Thies, A., Langheinrich, P.:
"Online measurement of flow and mass transfer in micro-holes with
PIV and an electrochemical sensor array". Proc. ISE Symp., 2002,
Dusseldorf, Germany. The influences on copper deposition in blind
micro-vias have been documented as part of these experiments and
have been published by Reents, B., Kenny, S.: "The influence of
fluid dynamics on plating electrolyte for the successful production
of blind micro-vias". IPC Expo 2002 Proc. of the Techn. Conf. IPC,
Northbrook, Ill., USA (2002).
[0007] From the above it is apparent that a main problem in
electroplating printed circuit boards with high-aspect ratio
through holes is to achieve a sufficient metal plating thickness in
the hole. At the same time it is mandatory to run electroplating at
a minimum average current density at the printed circuit board in
order to ensure adequate efficiency of the process which may only
be garantueed if the through-put and hence plating current density
is high-enough. Finally also good surface quality must be ensured
which means that the metal deposit produced must be as even and
shiny as possible.
[0008] The object of the present invention is therefore to fulfill
the above requirements and more specifically to achieve sufficient
metal plating thickness in high-aspect ratio printed circuit
boards. Another object of the present invention is also to ensure
that electroplating efficiency is as high as possible which implies
that metal plating current density at the printed circuit boards
must be as high as possible. A suitable average plating current
density is held to be at least 1.7 A/dm.sup.2, more preferably at
least 2 A/dm.sup.2 and most preferably at least 3 A/dm.sup.2.
[0009] The solution to this object is achieved by the method of
electroplating a workpiece comprising high-aspect ratio holes
according to claim 1. Preferred embodiments of the invention are
outlined in the subordinate claims.
[0010] The method according to the present invention serves to
electroplate a workpiece which is preferably plate-shaped such as a
printed circuit board and which has high-aspect ratio holes. The
method comprises the following method steps: [0011] a. The
workpiece is brought into contact with a metal plating electrolyte
and at least one anode. [0012] b. A voltage is applied between the
workpiece and the anodes, to the effect that a current flow is
provided to the workpiece. The current flow generated is a pulse
reverse current flow. The pulse reverse current has a frequency of
at most about 6 Hertz, preferably at most 4 Hertz and more
preferably at most 2.5 Hertz. In each cycle time of the pulse
reverse current flow at least one forward current pulse and at
least one reverse current pulse are provided.
[0013] Preferably in one cycle time of the pulse reverse current
flow one forward current pulse and one reverse current pulse are
provided.
[0014] In a preferred embodiment the ratio of the duration of the
forward current pulses to the duration of the reverse current
pulses of one cycle is set to at least 5, more preferably to at
least 15 and still more preferably to at least 18. This ratio may
be set to at most 75 and more preferably to at most 50. The ratio
may most preferably be set to about 20.
[0015] The duration of the forward current pulses of one cycle may
preferably be set to at least 100 ms, more preferably to at least
160 ms and most preferably to at least 240 ms.
[0016] The duration of the reverse current pulses of one cycle may
preferably be set to at least 0.5 ms, more preferably to at least 8
ms and most preferably to at least 12 ms.
[0017] The peak current density at the workpiece of the forward
current pulses may be set to at least 3 A/dm.sup.2. It may be set
to at most 15 A/dm.sup.2. Most preferably the peak current density
at the workpiece of the forward current pulses may be about 5.5
A/dm.sup.2.
[0018] The peak current density at the workpiece of the reverse
current pulses may especially be set to at least 10 A/dm.sup.2. It
may be set to at most 60 A/dm.sup.2. Most preferably the peak
current density at the workpiece of the reverse current pulses may
be in the range of from about 16 to about 20 A/dm.sup.2.
[0019] In a preferred embodiment the ratio of the peak current
density of the forward current pulses to the peak current density
of the reverse current pulses may be set to at least 1, more
preferably to at least 2 and still more preferably to at least 3.
This ratio may be set to at most 15 and more preferably to at most
4. The ratio may most preferably be set to about 3.
[0020] In a preferred embodiment of the present invention the rise
times of the forward and reverse current pulses, respectively, may
be adjusted depending on the technical objective pursued.
[0021] The workpiece is preferably plate-shaped. It may more
preferably be a printed circuit board or any other plate-shaped
electrical circuit carrier, such as a semiconductor wafer
(integrated circuit) or any hybrid (IC-) chip carrier like a
multi-chip module.
[0022] In a preferred embodiment of the present invention the
method comprises the following method steps: [0023] a. A first
voltage is applied to between a first side of the workpiece and at
least one first anode, to the effect that a first pulse reverse
current flow is provided to the first side of the workpiece, said
first pulse reverse current flow having at least one first forward
current pulse and at least one first reverse current pulse flowing
in each cycle time. [0024] b. A second voltage is applied to
between a second side of the workpiece and at least one second
anode, to the effect that a second pulse reverse current flow is
provided to the second side of the workpiece, said second pulse
reverse current flow having at least one second forward current
pulse and at least one second reverse current pulse flowing in each
cycle time.
[0025] As to this last embodiment the first forward and reverse
current pulses of one cycle may be offset relative to the second
forward and reverse current pulses of one cycle, respectively. In a
more preferred embodiment of the present invention this offset
between the first current pulses and the second current pulses is
approximately 180.degree..
[0026] For further improving throwing power the current flow may
comprise, in each cycle time, two forward current pulses with one
zero current break between the two forward current pulses and one
reverse current pulse.
[0027] In another embodiment for improving throwing power the
current flow may comprise, in each cycle time, one forward current
pulse followed by one reverse current pulse and after that one zero
current break.
[0028] In another embodiment for improving throwing power the
current flow may comprise, in each cycle time, one forward current
pulse followed by one reverse current pulse without any zero
current break in this cycle.
[0029] In still another embodiment for improving throwing power the
current flow may comprise, in each cycle time, one forward current
pulse, followed by one zero current break and after that one
reverse current pulse.
[0030] Of course there are still more combinations possible
according to the order or the occurrence of the different pulses
and the zero current breaks. Furthermore it is possible to combine
different cycles.
[0031] Depending on the durations of the different current pulses
and where required of zero current breaks an average current
density I.sub.av can be specified in relation to the cycle time.
The average current density can be calculated by the following
equation: I av = i .times. I fw i * t fw i - j .times. I rv j * t
rv j t ct , ##EQU1## wherein further:
[0032] I.sub.fw=forward current density,
[0033] I.sub.rv=reverse current density,
[0034] t.sub.fw=forward pulse duration,
[0035] t.sub.rv=reverse pulse duration, wherein i,j=integers
.gtoreq.1, which denote individual forward and reverse pulses,
respectively, in each pulse cycle, and
[0036] t.sub.ct=cycle time, wherein a cycle time can additionally
comprise the time of zero current break pulses if such zero current
break pulses are used.
[0037] Preferably the average current density may be set in a range
of 1 to 10 A/dm.sup.2, more preferably 2 to 6 A/dm.sup.2 and most
preferably 3 to 5 A/dm.sup.2. Preferably the average current
density is set to a value of about 4 A/dm.sup.2.
[0038] Further in the course of metal plating the workpiece, at
least one parameter of the pulse reverse current flow, selected
from the group comprising the ratio of the duration of the forward
current pulses to the duration of the reverse current pulses of one
cycle and the ratio of the peak current density of the forward
current pulses to the peak current density of the reverse current
pulses of one cycle, may be varied. More specifically it turns out
to be advantageous to increase, in the course of metal plating the
workpiece, the ratio of the peak current density of the forward
current pulses to the peak current density of the reverse current
pulses and/or to decrease the ratio of the duration of the forward
current pulses to the duration of the reverse current pulses.
[0039] Another improvement of the invention comprises bringing the
workpiece into contact with the metal plating electrolyte by
delivering the metal plating electrolyte towards the surface of the
workpiece at an electrolyte flow velocity relative to the surface
of the workpiece. The metal plating electrolyte is preferably
forced under agitation towards the workpiece. More preferably the
electrolyte flow velocity at the surface of the workpiece comprises
a velocity component normal to the surface of the workpiece being
at least 1 m/sec.
[0040] Preferably the velocity may be set to at least about 1.4
m/sec, more preferably to at least about 7.2 m/sec. It may be set
to be at most about 11.5 m/sec.
[0041] In a further improvement of the present invention the method
comprises providing at least one anode being inert and
dimensionally stable.
[0042] Anodes are preferably used which contain titanium or
tantalum as the basic material, which is preferably coated with
noble metals or oxides of the noble metals. Platinum, iridium or
ruthenium, as well as the oxides or mixed oxides of these metals,
are used, for example, as the coating. Besides platinum, iridium
and ruthenium, rhodium, palladium, osmium, silver and gold, or
respectively the oxides and mixed oxides thereof, may also
basically be used for the coating. A particularly high resistance
to the electrolysis conditions could be observed, for example, on a
titanium anode having an iridium oxide surface, which was
irradiated with fine particles, spherical bodies for example, and
thereby compressed in a pore-free manner. Moreover, of course,
anodes may also be used, which are formed from noble metals, for
example platinum, gold or rhodium or alloys of these metals. Other
inert, electrically conductive materials, such as carbon
(graphite), may also basically be used. These anodes are provided
in order to reduce the excessive polarisation voltage and to keep
the anodes electrically conductive and also, at the same time, to
protect the anodes from electrolytic sputtering.
[0043] In practical usage the insoluble anode can be made from an
expanded metal sheet of titanium, which was activated with noble
metal (e.g. platinum).
[0044] In another usage a rod-like anode can extend into tubular
cathodes. To enlarge the effective surface of the anodes, the
cathodes may be formed from a tubular expanded metal which, at the
same time, renders possible a very good exchange of electrolyte as
a consequence of the lattice structure.
[0045] When using inert and dimensionally stable anodes for
performing conventional DC or pulse plating methods it has turned
out that the anodes got corroded after some time of usage and that
the organic additives added to the bath were increasingly consumed.
This has now been attributed to the evolution of gas generated both
at the workpiece and at the anodes. By using a low frequency pulse
plating method, e.g. at 6 Hertz or even lower, and especially by
setting either the forward or the reverse current pulse or both to
a time duration as long as possible such detrimental effects can be
avoided.
[0046] According to one specific embodiment of the present
invention the metal plating electrolyte may be a copper plating
electrolyte.
[0047] In this latter case, and especially if the at least one
anode is inert and dimensionally stable, copper ions may be
replenished to the electrolyte by dissolving copper metal. For this
purpose the copper plating electrolyte may contain at least one
compound capable of oxidizing copper metal to copper ions. Such
oxidizing compound may be for example a ferric compound, such as
ferric ion, ferric sulphate more specifically. After addition of
e.g. the iron-(II) sulphate heptahydrate to the electrolyte after a
short time the effective iron-(II)/iron-(II) redox system is
formed, wherein iron-(II) sulphate heptahydrate is excellently
suited for aqueous acid copper baths. The use of iron compounds
with anions which lead in the copper electrolyte to undesired
secondary reactions such as for example chloride or nitrate may
also not be used.
[0048] To regenerate the copper ions an ion generator is used,
which contains parts of copper. The generator is separated from the
electroplating chamber containing the anode. The electrolyte, which
is weakened by a consumption of copper ions, containing the
compounds as e.g. iron-(II) sulphate, is guided past the anodes,
whereby iron-(III) compounds are formed from the iron-(II)
compounds. The electrolyte is subsequently conducted through the
copper ion generator and thereby brought into contact with the
copper parts. The iron-(III) compounds thereby react with the
copper parts to form copper ions, i.e. the copper parts dissolve.
The iron-(III) compounds are simultaneously converted into the
iron-(II) compounds. Because of the formation of the copper ions,
the total concentration of the copper ions contained in the
electrolyte is kept constant. The electrolyte passes from the
copper ion generator back again into the electroplating chamber in
which it comes in contact with the workpiece and the anodes.
[0049] Preferably iron-(II) and iron-(III) compounds are used as
the electrochemically reversible redox system. Equally suitable are
the redox systems of the following elements: titanium, cerium,
vanadium, manganese and chromium. They can be added to the copper
deposition solution for example in the form of titanyl sulphuric
acid, cerium(IV) sulphate, sodium metavanadate, manganese(III)
sulphate or sodium chromate. Combined systems can be advantageous
for special applications.
[0050] The concentrations of the compounds of the redox system must
be set in such a way that, through the resolution of the metal
parts, a constant concentration of the metal ions in the deposition
solution can be maintained. This guarantees that the insoluble
anodes, coated with noble metals or oxides of the noble metals, are
not damaged.
[0051] As an alternative the oxidizing compound may also be oxygen.
Oxygen, contained in the air, is constantly introduced into the
electrolytic fluid by electrolyte movements so that oxygen
dissolves in the fluid. This oxygen is also capable of dissolving
copper by oxidizing the copper parts in the ion generator, wherein
oxygen ions are formed.
[0052] Now, referring to the investigations outlined above, further
experiments have been carried out to investigate the influences on
through-hole plating, particularly in high-aspect ratio holes.
Table 1 gives a summary of electrolyte exchange mechanism
considered and also the influencing factors.
[0053] The influencing parameters were held constant as far as
possible, and the artificial convection by means of forced flooding
was investigated.
[0054] A specially designed multilayer printed circuit board with
electrochemical flow sensor was used as part of these
investigations. A schematic of one test hole on the test board is
shown in FIG. 1. This test board comprises a micro electrode
array.
[0055] The test board was placed in a test chamber which allowed
the variation of key parameters as follows: [0056] Diameter of
nozzle [0057] Angle .alpha. between beam of fluid and workpiece
surface [0058] Distance between nozzle mouth and workpiece surface
[0059] Lateral flow along the surface of the workpiece [0060]
Pressure/Flow [0061] Density of the electrolyte [0062] Pulse
pumping
[0063] The test chamber is shown in FIG. 2. This test chamber is
being used for hydrodynamic studies. The test chamber comprises a
housing 1, which emcompasses an adjustable disc 2. On this disc 2
the test printed circuit board 3 is arranged in a vertical
arrangement. The item with numeral 4 is a stopper. The
electrochemical cell also comprises a counter electrode 5 and a
reference electrode 6, which both are also schematically displayed
in FIG. 1. A nozzle 7 serves to impinge metal plating electrolyte
to the surface of the printed circuit board 5 at an angle .alpha.
which is defined as the angle between the axis of the nozzle 7 and
the upper right hand part of the test printed circuit board 3 as
shown in this fig. Finally there is a lateral nozzle adjustment
means 8 which allows fine tuning of the point of impingement of the
metal plating electrolyte at the test printed circuit board.
[0064] FIG. 3 shows a microsection through one test coupon having a
hole with a diameter of 0.2 mm showing the inner layer electrode
connections, the results from experiments with this test coupon
being given in FIG. 4. This fig. illustrates the results of
investigation of fluid velocity and spray angle .alpha. as a
function of current I at the individual inner layer electrodes. The
experiments have been carried out under the following
conditions:
[0065] Ring electrodes being formed in the inner layer of the test
coupon circular to the hole with d=200 .mu.m;
[0066] The aspect ratios of the holes contained in the test coupons
being:
[0067] in FIG. 4.A: 1.3 (upstream);
[0068] in FIG. 4.B: 2.8 (middle);
[0069] in FIG. 4.C: 4.4 (downstream);
[0070] The aspect ratio was calculated in each individual case as
the ratio of the distance from the hole entry to the respective
inner layer, which was located upstream of the middle of the hole,
in the middle of the hole and downstream the middle of the hole,
respectively, to the hole diameter L.sub.x=-0.2 mm.
[0071] Fluid flow velocity v.sub.j(y) was as follows: [0072] 1)
0.66 m/sec [0073] 2) 1.46 m/sec [0074] 3) 3.7 m/sec [0075] 4) 7,2
m/sec [0076] 5) 11.5 m/sec
[0077] The curves in the diagrams in FIG. 4 are designated with
numerals 1, 2, 3, 4 and 5 to correspond to the above fluid flow
velocities v.sub.j(y). The results show that a maximum diffusion
current is achieved at a flow angle of 90.degree. and of course
with the highest impingement velocity.
[0078] In larger scale tests the technique of particle image
velocimetry (PIV) was used to image the flow of electrolyte through
a high-aspect ratio panel. FIG. 5 shows the experimental set-up
(particle image velocimetry apparatus) used to carry out the tests.
In this a dynamic system is illuminated by two laser beams, and the
resulting interference pattern information is recorded on a
camera.
[0079] The data obtained from one of the flow experiments through a
high-aspect ratio panel are shown in FIG. 6, which is an
illustration of vertical solution flow through a high-aspect ratio
panel. The individual arrows show the direction and size of
velocity vectors at the respective locations in the examined
area.
[0080] Therefore it can be concluded in general that the fluid flow
velocity of electrolyte solution shall be selected such that it has
a velocity component normal to the surface of the workpiece of at
least 1 m/sec, preferably of at least 5 m/sec and most preferred of
at least 10 m/sec.
[0081] The results from the experiments have enabled modifications
made to the Uniplate.RTM. Inpulse.RTM. system to improve the
production of blind micro-vias as reported.
Horizontal Application:
[0082] The standard Inpulse.RTM. module for horizontal processing
of printed circuit boards (in which boards are conveyed in a
horizontal path and in a horizontal plane of transport for
processing same) but may also be conveyed in a vertical or any
other plane of transport has a spray bar to cathode (workpiece)
separation of 95 mm and an anode to cathode separation of 75 mm. In
the Inpulse.RTM. 2 system both the spray bar and the anode are set
much closer to the cathode at 15 mm and 8 mm for the anode. This
enables a more intense electrolyte flow towards the panel and also
has an added advantage making the use of anode shielding
unnecessary whilst retaining excellent surface distribution. Also
the spray system itself has been modified to give a more directed
agitation towards the panel. These changes were made primarily to
enable the more efficient flooding of blind micro-vias. Using this
system experiments were made to investigate the optimal electrolyte
composition and pulse plating parameters to achieve best throwing
power in 3.2 mm thick panels with aspect ratio 10:1. The results
have shown that primarily the pulse wave form set-up and the
electrolyte adjustment may be individually important in giving
throwing power improvement. The best electrolyte composition was
found to be as follows: [0083] Copper: 20 g/l [0084] Sulphuric
acid: 270 g/l [0085] Chloride ions: 40 mg/l [0086] Iron(II): 7 g/l
[0087] Iron(III): 1 g/l [0088] Leveller Inpulse.RTM. H6: 1.7-2.0
ml/l [0089] Brightener Inpulse.RTM.: 4.0-5.5 ml/l
[0090] Of course the metal plating electrolyte may vary to some
extent. Throwing power may be efficiently improved if the
electrical conductivity of the metal plating electrolyte is
increased. This may be affected by increasing the acid
concentration for example. The additive concentrations are more
typical of electrolytes adjusted to produce high-aspect ratio
panels. In particular the copper concentration is 15-20 .mu.l lower
than in a standard Inpulse.RTM. electrolyte.
[0091] The pulse plating parameters were varied from DC plating
conditions at 4 A/dm.sup.2 to pulse plating with forwards 250 ms
and reverse 25 ms. A selection of the parameters used together with
the throwing power achieved is shown in Table 2.
[0092] Due to the weakness of corner flattening by means of
high-reverse conditions and surface roughness, the best throwing
power results were achieved with forwards 240 ms at a average
current density of 4 A/dm.sup.2 and reverse of 12 ms at a reverse
current density of 16 A/dm.sup.2 and not with 25 ms in reverse
time. A general tendency can be seen that with lower frequency the
throwing power is increased, as is clearly illustrated in Table
3.
[0093] In all tests a phase shift in pulse parameter of 180.degree.
was used. This means that the reverse pulse was applied to the
anodes on one side of the test panel at the same time that the
forward pulse was applied to the anodes on the other side. The
pulse wave form schematic in FIG. 7 (current as a function of time)
illustrates this setting showing phase shift between top and bottom
anodes (top curve: current at the top side of the cathode, bottom
curve: current at the bottom side of the cathode).
[0094] Microsection photographs of the panel produced in test 6
outlined in Table 2 are shown in FIG. 8. In this case a 10:1 aspect
ratio panel with thickness of 3.0 mm and hole diameter of 0.3 mm
was electroplated. As can be seen at the centre of the hole the
thickness achieved is very low, the panel plate with the
Inpulse.RTM. 2 system has a throwing power of approx. 70%.
[0095] As a comparison with similar panels, a throwing power of
only 30% would be achieved at 3 A/dm.sup.2 with horizontal DC. At 2
A/dm.sup.2 a throwing power of 55% is achieved under vertical
conditions in DC. Only with pulse plating under standard vertical
conditions with air agitation a throwing power of 90% is achieved,
but this is at an average current density of 2 A/dm.sup.2. Using
forced agitation improved throwing power is possible as discussed
hereinbelow. But even this is not at such a high-current
density.
Vertical Application:
[0096] In vertical plating of workpieces metal plating electrolytes
may be employed which have the same composition as the metal
plating electrolytes described above for horizontal processing.
Likewise in vertical plating pulse plating may be performed under
the same conditions as in horizontal processing. Therefore as to
these plating conditions in vertical plating reference is made to
the above description.
[0097] In vertical systems electrolyte agitation is usually made
with a combination of air agitation in the electrolyte itself and a
mechanical agitation of the circuit board being plated. This
mechanical agitation must ensure that the panels are moved evenly
and remain vertical in the electrolyte. Otherwise solution flow
will not be uniform through all the holes in the panel. To ensure
this cathode movement, systems are used which clamp the panel
securely and which are also used to supply current to the panel.
These agitation systems, air in the electrolyte and movement of the
panel, can lead to uneven fluid transport due to non-defined air
agitation and due to the movement of the panel through the
agitation bubbles.
[0098] To overcome these problems the use of Eductors (spray
nozzles which use the Venturi Principle, i.e. drawing of additional
liquid through the nozzle is affected by the spray created, so that
high-volume flow is achieved) is becoming more common. Eductors
using the Venturi Principle allow small pumps to circulate larger
volumes of liquid. The kinetic energy of one solution will cause
the flow of another. Typically the use of Eductors can give a 4-6
times increase in volume of solution movement when compared to the
volume pumped. This increased volume is however at a lower pressure
than the directly pumped solution. FIG. 9 shows two sizes of
commonly used Eductors in electrolytic copper plating systems. The
smaller Eductor shown will pump a lower volume, but will allow more
Eductors to be placed on one pipe, so giving a more even
electrolyte flow.
[0099] Currently the method of installation of the Eductors in a
vertical plating tank is on the floor underneath the cathode as
shown in FIG. 10, which shows the installation of Eductors in a
vertical Inpulse.RTM. line in a view from the top of the
installation to the bottom. At the bottom the Eductors 9 are
disposed on a feeding pipe 10.
[0100] This installation is with two pipes placed one on each side
below the cathode with the Eductors adjustable pointing upwards
towards or away from the cathode. There are similar installations
with the Eductors mounted on a single pipe running directly below
the cathode and substantially parallel to the cathode, the Eductors
mounted at a fixed angle pointing alternately away from the panel.
The disadvantages associated with this set-up are that the
electrolyte flow uniformity depends on the positioning of the
Eductors and also of the distance between the nozzle and the
panel.
[0101] To give more uniform flow the Eductors can be positioned
between the anodes in the plating cell pointing directly towards
the cathode. This set-up has the advantage of giving a more direct
flow of electrolyte towards the panel and is shown in FIG. 11 as a
view from the top of the installation to the side thereof. The
Eductors 9 are shown to be disposed at the sides of the tank in
front of the anodes 11. The disadvantage of all Eductor
installations is that the solution flow can never be completely
uniform over the panel surface. A compromise must be made between
the number of Eductors installed and flow uniformity.
[0102] To overcome the limitations of flow uniformity by the use of
Eductors a moving spray system has been developed and is being
tested in a trial tank in laboratory conditions. The system
consists of a spray head which moves regularly over the surface of
the cathode and produces an intensive forced flooding of the panel
and the through holes at the point of spraying. The head moves in a
plane between anode and cathode and delivers the electrolyte in the
direction towards the panel. It is so dimensioned that it does not
interfere with the electrodeposition process. Results with
high-aspect ratio panels have shown a significant improvement in
throwing power when compared to standard air agitation and a more
uniform deposit in comparison to Eductor agitated equipment on the
same scale. FIG. 12 shows plating results from a 3.0 mm panel with
a 0.3 mm hole (aspect ratio: 10:1) using the moveable spray system.
Average current density for electroplating was 2 A/dm.sup.2.
Throwing power was found to be 90-95%. Reinforcement plating was
performed by DC plating in horizontal plating equipment at a
current density of 5 A/dm.sup.2.
[0103] Investigations have been continued in the use of so-called
batch plating parameters to improve throwing power, particularly in
panels thicker than 5 mm. During the plating cycle the pulse
parameters are varied. Normally at the start of the cycle a strong
reverse charge is used to give a good throwing power followed by a
lower reverse charge at the end of the plating cycle to give good
surface finish. An example of such a plating sequence is given in
Table 4.
[0104] FIG. 13 shows plating results from a 5.0 mm panel with a 0.5
mm hole (aspect ratio 10:1) using a modified pulse plating sequence
together with the moveable spray system to give optimal electrolyte
exchange. The average current density applied is 1.7 A/dm.sup.2.
The throwing power was found to be 95-100%.
[0105] Use of both optimized pulse parameters as well as
electrolyte agitation gives significant improvements in throwing
power in trial line experiments.
[0106] Hence experiments in basic electrochemistry show a strong
influence of electrolyte agitation on copper electroplating
characteristics. Modifications to horizontal Inpulse.RTM. equipment
together with optimized plating parameters show improved throwing
power under experimental conditions.
[0107] In vertical equipment use of Eductors to improve agitation
is becoming a standard for new equipment. The use of a moving spray
flood system shows advantages in the trial line scale for vertical
systems.
[0108] Use of varying pulse parameters over the copper deposition
time offers the possibility to improve throwing power with
aggressive parameters whilst retaining optimal surface finish using
milder pulse parameters at the end of the processing time (cycle
time).
[0109] It is to be understood that various modifications and
substitutions by technical means may be applied to what has been
described by way of the above examples and of the drawings, without
departing from the scope of the invention as defined by the
appended claims. Further it is to be understood that various
combinations of features described in this application will be
suggested by the person skilled in the art and are to be included
within the purview of this invention and within the scope of the
appended claims. All publications, patents and patent applications
cited herein are hereby incorporated by reference. TABLE-US-00001
TABLE 1 Electrolyte exchange mechanisms and influencing factors
Electrolyte exchange by Influencing factors Diffusion Concentration
Migration Temperature Natural Convection Surface Tension Artificial
Convection Viscosity Density
[0110] TABLE-US-00002 TABLE 2 Test conditions for Inpulse .RTM. 2
trials for 3.2 mm thick panels Pulse Para- Pulse I average/ meters
in Phase Rise Throwing I reverse ms Forward/ Shift Time Surface
Power Test [A/dm.sup.2] Reverse [%] Factor Finish min. [%] 1 4 DC
DC -- -- Good 32 2 4/4 240/20 180 1.2 Good 53 3 4/16 80/4 180 1.2
Good 49 4 4/16 80/6 180 1.2 Rough 55 5 4/16 250/25 180 1.2 Rough 54
6 4/16 240/12 180 1.2 Good 71
[0111] TABLE-US-00003 TABLE 3 Test conditions for Inpulse .RTM. 2
trials for 3.2 mm thick panels, 0.3 mm holes, I average (I.sub.av)
= 4 A/dm.sup.2 Pulse Parameters Pulse I forward/ [ms] Phase Rise
Throwing I reverse Forward/ Shift Time Frequency Power Test
[A/dm.sup.2] Reverse [%] Factor [Hz] min. [%] 1 5.3/16 DC -- -- --
30 2 5.3/16 40/2 180 1.2 23.8 40 3 5.3/16 80/4 180 1.2 11.9 50 4
5.3/16 160/8 180 1.2 6.0 60 5 5.3/16 240/12 180 1.2 4.0 70
[0112] TABLE-US-00004 TABLE 4 Pulse plating bath sequence for thick
panel trials Exposure time A/dm.sup.2 Ratio Pulse timing Sequence
[min] Forward to Reverse [ms] 1 100 1:3 30:1.5 2 15 1:2.5 20:1 3 5
1:1.1 20:1
* * * * *