United States Patent [191
Lu
[54] ELECTROPLATING TEST CELL
[75] Inventor: Po-Yen Lu, West?eld, NJ.
[73] Assignee: AT&T Bell Laboratories, Murray
Hill, NJ.
[21] Appl. No.: 814,309
[22] Filed:
Dec. 23, 1991
Related US. Application Data
[63] Continuation of Ser. No. 549,855, Jul. 9, 1990, aban
cloned.
[51] Int. Cl.5 ........................................... .. G01N 27/42
[52]
US. Cl. ................... ..
. ............... .. 204/434
[58] Field of Search .................... .. 204/1531, 212, 434
[56]
References Cited
U.S. PATENT DOCUMENTS
2,149,344 3/1939 Hull ....................................... .. 204/1
2,760,928 8/1956 Ceresa
204/195
2,801,963 8/1957 Hull et al. ..... ..
204/195
3,121,053 2/1964 Hull, Jr. et a1. .
204/195
3,215,609 11/1965 Chapdelaine
204/434 X
3,223,598 12/1965 Jacky et al. ....... ..
204/434 X
4,102,770 7/1978 Moriarty et al.
I
204/434
4,252,027 2/1981 Ogden et al.
.... .. 73/826
4,487,681 12/1984 Cordes .............................. .. 204/434
OTHER PUBLICATIONS
Afshar, et al, ��Rotating Electrode Current Density
Cells to Simulate High Speed Electrodeposition��,
Transactions of the Institute of Metal Finishing, vol. 69,
part 1, pp. 37-44, Feb. 1991.
Transactions of the Institute of Metal Finishing, vol. 68,
part 1, p. 2.
Patent Abstracts of Japan, vol. 10, No. 271 (P-497)
[2327], Sep. 16, 1986 & JP-A-61 95 242 (Toyota Motor
Corp.) May 14, 1986.
R. 0. Hull, ��Current Density Range Characteristics—
lllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllll
US005268087A
[11] Patent Number:
[45] Date of Patent:
5,268,087
Dec. 7, 1993
Their Determination and Application��, Proc. Amer.
Electroplaters Society, 27 (1939), pp. 52-60.
J. S. Newman, Electrochemical Systems, Pren
tice-Hall, Inc., Englewood, Cliffs, NJ. (1973), pp. 2-9,
307-309. -
Chin-Sheug Wu, "The Three Dimensional incompres
sible Laminar Boundary Layer on a Spinning Cone��,
Appl. Sci. Res., Section A, vol. 8 (1959), pp. 140-146.
C. L. Tien, ��Heat Transfer By Laminar Flow From a
Rotating Cone��, Journal of Heat Transfer, C 82 (1960),
pp. 252-253.
J. Newman, ��Schmidt Number Correction For The
Rotating Disc��, The Journal of Physical Chemistry,
vol. 70, No. 7, Apr. 1966, pp. 1327-1328.
Dr. Hermann Schlichting, ��Boundary-Layer Theory��,
Sixth Ed., McGraw-Hill Book Company (1968), p. 93.
A. J. Arvia, et a1, ��Mass Transfer in the Electrolysis . .
. ��, Electrochimica Acta, 1962, vol. 7, pp. 65-78, Perga
mon Press, Ltd.
Primary Examiner-Nam Nguyen
Attorney. Agent, or Firm-Oleg E. Alber
[57]
ABSTRACT
Hull cell has been widely used in the plating industry
for many years to evaluate the plating chemistry as a
function of current densities. However, because of irre
producible mass transfer, Hull cells can only be used
qualitatively for process control. A new design of an
improved cell with extremely reproducible mass trans
fer performance utilizes a rotatable cathode, and per
mits quantitative analysis of the performance of the cell.
The improved cell can also be used to study the mass
transfer effect on deposit properties and throwing
power, which can not be provided by the traditional
Hull cell.
6 Claims, 3 Drawing Sheets
U S Patent
Dec. 7, 1993
Sheet 1 of 3
5,268,087
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US. Patent
Dec. 7, 1993
Sheet 2 of 3
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US. Patent
Dec. 7, 1993
Sheet 3 of 3
5,268,087
FIG. 5
THICKNES DISTRIBU N
(ARBITRARY SCALE)
POSITION 0F MEASUREMENT
5,268,087
1
ELECT ROPLATING TEST CELL
This application is a continuation of application Ser.
No. 07/549855, filed on Jul. 9, 1990, abandoned.
TECHNICAL FIELD
This invention concerns with an improved electro
plating test cell useful for evaluating and studying elec
trodeposition.
BACKGROUND OF THE INVENTION
Advances in analytical techniques make it feasible to
accurately determine the concentration of most metal
ions and salts in a plating solution. However, the analy
sis of organic plating additives (typically in the ppm
level), contaminants from drag-in, and plating reaction
by-products, is usually very difficult, if not impossible.
Even when ��complete�� analysis is possible for a plating
solution, there is always a concern about ��unknown
species�� that may affect the plated deposit. In practice,
in addition to the analytical technique, other methods
are used for routine checking of the overall perfor
mance of plating solutions.
Hull cell has been recognized as one of the most
important tools to monitor overall performance of plat
ing solutions. Hull cell was ?rst described by R. 0. Hull
in a paper entitled "Current Density Range Characteris
tics, Their Determination and Application��, Proc.
Amer. Electroplates�� Soc., 27 (1939) pp. 52-60. Also see
US. Pat. No. 2,149,344 issued on Mar. 7, 1939 to R. 0.
Hull. One of the principle advantages of Hull cell mea
surements is that it is possible to assess the deposit char
acteristics at various current densities on a single test
panel. It is also possible to carry out evaluations using
various temperatures, solution compositions, addition
agents, contaminants, etc.
Essentially, a Hull cell is an electro-plating cell with
a particular trapezoidal geometry (FIG. 6). A ?at cath
ode, 61, is ?xed at an angle to a flat anode, 62, within a
box-like container, 63, holding an electrolyte, 64. Both
the cathode and the anode occupy the full cross-section
of the cell. Several sizes of Hull cells are commercially
available with solution capacities of 250, 267, 320, 534
and 1000 cc. The principle behind the Hull cell is to
create a variation of solution resistance between the
electrodes and to use a current restricting angle be
tween the cathode and an insulating plane. The above
arrangement produces a large variation of current den
sity across the deposition on the test panel. Agitation
within the Hull cell is usually provided by an external
paddle, magnetic stirring bar or by forcing air through
the electrolyte in the vicinity of the cathode. However,
the results are usually less meaningful due to poorly
reproducible mass transfer characteristic between ex
perimental runs. Electrochemical reaction kinetics are
usually greatly in?uenced by trace amounts of organic
/ inorganic additives and the concentration of metal ions
and salts. Therefore, different degrees of agitation near
the plating object will result in large variations_in de
posit properties. Unfortunately, the traditional Hull cell
does not provide a reproducible mass transfer and,
therefore, can only be used for qualitative process con
trol.
The intention of a Hull cell is to create large varia
tions of current density over the test panel, typically
over one order of magnitude. It is conceivable that at
the high current density region, the plating reaction is
20
25
30
35
40
45
50
55
60
65
2
mass transfer limited or nearly mass transfer limited. A
deposit obtained from a mass transfer limited condition
has a different structure when compared to adeposit
plated at conditions that are not mass transfer limited.
Therefore, it is difficult to determine whether an irregu
lar deposit is the result of a change of plating variables
or is simply caused by irreproducible mass transfer.
Thus, there is a need for a new design of a cell which
permits assessment of deposit characteristics at various
current densities on a single test panel and which also
has reproducible mass transfer and could be useful for
quantitative measurements.
SUMMARY OF THE INVENTION
This invention is an electroplating test cell compris
ing a container for holding a desired volume of an elec
trolyte, an anode, a cathode positioned coaxially of the
container and of the anode, and a current density varia
tion creating (cdvc) means of an electrically insulating
material for creating, for a given total current, a current
density variation across the cathode, said cdvc means
being arranged at an angle of less than 90 degrees to that
surface of the cathode on which metal deposition is to
take place. The cathode is fully immersed in the electro
lyte and is capable of being rotated about its central axis
over a high range of RPMs. This design of the test cell
leads to reproducible mass transfer performance and
permits not only a qualitative but also a quantitative
analysis for the performance of the cell, which cannot
be effectively provided by the traditional Hull cell.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic representation of a cell with a
rotating cone cathode and a stationary disk anode.
FIG. 2 is a schematic representation of a cell with a
rotating cone cathode and a stationary cylinder anode.
FIG. 3 is a schematic representation of a cell with a
rotating disk cathode and a stationary cylinder anode.
FIG. 4 is a schematic representation of a cell with a
rotating cylinder cathode and a stationary washer-shape
anode.
FIG. 5 is a plot of deposit thickness distribution on a
rotating cone surface.
FIG. 6 is a schematic representation of the prior art,
original Hull
DETAILED DESCRIPTION
This invention is an improved electroplating test cell
useful for evaluating and studying electrodeposition.
The basis of an improved electroplating cell is the com
bination of the current density variation feature of a
Hull cell with the reproducible mass transfer feature of
a rotating electrode. The cell design consists of a cath
ode capable of being rotated about its central axis, an
anode (either stationary or rotatable), and current re
stricting shield (forming a less than 90 degree angle with
the cathode). The cell comprises a container of non
conducting, non-contaminating insulating material for
holding a predetermined amount of an electrolyte, an
anode and a cathode which are submersed in the elec
trolyte, and a current density variation creating (cdvc)
means.
The container is of an insulating material, such as
glass, glazed ceramic, plastic materials such as polyeth
ylene, polyprophylene, polyvinyl chloride, teilon and
others. The size of the container is selected such that
when a predetermined amount of electrolyte, e.g. 250,
500 or 1,000 ml, is in the container, the anode and the
5,268,087
3
cathode are covered completely by the electrolyte. The
electrode closest to the surface of the electrolyte, e.g. a
base of cone 11 in FIG. 1, should be covered completely
by a thin layer of electrolyte kept to a minimum, e.g.
less than 5 mm in thickness. Layer thickness which
could contribute to an unwanted uedge effect�� (which
takes primary current, e.g. 2A, to in?nity) due to cur
rent flow in the electrolyte layer, should be avoided.
Preferably, the container has a cylindrical shape, but
other con?gurations, e.g. polygonal shape, are possible
with an anode having a cylindrical shape or even with
an insulating cylinder or truncated cone insert placed
coaxially of the container between the walls of the con
tainer and the rotating cathode. Polygonal shape with
out a cylindrical insert is unwanted due to the unsym
metrical nature for current distribution in the angular
direction.
The anode may have various con?gurations includ
ing such con?gurations as a disc, an annulus, a cylinder.
The cathode also may have various con?gurations in
cluding such as a disk, a cone and a cylinder. The cath
ode which is rotatable about its central axis, is sus
pended in the container, preferably coaxially of the
longitudinal axis of the container. The cathode is sus
pended by means of a support arm (not shown) which
also includes variable drive means (not shown) to pro
vide for rotation of the cathode about its central axis.
The cathode has typically a polished metal surface, with
such metals as copper, brass or stainless steel being
preferred. A surface of the cathode facing toward the
surface of the electrolyte may be provided with an
insulating material to avoid the possibility of unwanted
deposition on that surface.
To permit repeated use of the cathode, a removable
test panel may be placed over the plating surface of the
cathode for each test. The panel should conform to the
plating surface, and be of a metal suitable for plating
thereon, with such metals as copper, brass or stainless
steel being preferred. The longitudinal dimensions are
preselected in a ratio which approximates that of a I-lull
cell with a corresponding volume of electrolyte.
Regarding the mass transfer aspect, rotating elec
trodes (including disk, cone, cylinder shape) are among
the few convective flow systems for which the hydro
dynamic equations and the convective-diffusion equa
tions have been rigorously solved and experimentally
con?rmed, for example, see C. L. Tien, ��Heat Transfer
by Laminar Flow From a Rotating Cone��, Journal of
Heat Transfer, August, 1960, pp. 252-253, and Dr. Her
mann Schlichting, ��Boundary-Layer Theory��,
McGraw-Hill Book Company, New York (1968), p. 93.
In a typical plating solution, because of the large
Schmidt number(v/D), the mass transfer boundary
layer is much thinner than the hydrodynamic boundary
layer, eg see, John Newman, ��Electrochemical Sys
tems��, Prentice-Hall, Inc., New Jersey (1973), p. 307.
Therefore, the mass transfer characteristics of a rotating
electrode are well de?ned. The superior mass transfer
reproducibility enables these rotating electrodes7 to be
used in measuring fundamental constants such as diffu
sion coef?cients and electrochemical reaction kinetic
constants.
Based on the above principle, many versions of the
improved cells may be designed. Below are described,
four different exemplary versions of this cell. Undoubt~
edly, some other versions of the improved cell using the
rotating symmetrical cathode principle can be easily
reconstructed on the basis of the within teachings.
25
40
45
50
55
60
65
4
One embodiment of the improved cell, 10, is schemat
ically represented in FIG. 1 in which a cathode, 11, and
an anode, 12, are positioned in a container, 13, of a
suitable non-conducting, non-contaminating material
holding a preselected volume of an electrolyte, 14. In
this embodiment, cathode 11 is a cone-shaped electrode
which is capable of being rotated about its central axis
and anode 12 is a stationary disk, with the wall 15 of the
container acting as a cdvc means. Current density at the
cathode varies from a high at the apex of the cone (near
est to the anode) to a low farthest from the anode. Vari
ation of the current density along the surface of the
cone can be further adjusted by adjusting at least one of
the gap (G) between the tip of the cone and the anode,
the diameter of the anode (L) and the angle (9) between
the cone surface and the central axis of the cone. Cur
rent density at the cathode varies from a high at the
apex of the cone (nearest to the anode to a low farthest
from the anode.
Another embodiment of the improved cell, denomi
nated as 20, is schematically represented in FIG. 2 in
which a cathode 21 and an anode 22 are positioned in a
container, 23, of suitable non-conducting, non-con
taminating material, holding a preselected volume of an
electrolyte, 24. Cathode 21 is a rotatable cone, while
anode 22 is a stationary cylinder encompassing the oath
ode, with the bottom, 25, of container 23 acting as a
cdvc means. Current density at the cathode varies from
a high at the base of the cone (nearest the anode) to a
low at the apex of the cone. Variation of the current
density can be further obtained by varying at least one
of a gap (G) between the edge of the cone and the
anode, the width of the anode (L) and the angle (6)
between the cone surface and the central axis of the
cone.
A further embodiment of the improved cell, denomi
nated 30, is schematically represented in FIG. 3 in
which a cathode, 31, and an anode, 32, are positioned in
a container, 33, of suitable non-conducting, non-con
taminating material, holding a preselected volume of an
electrolyte, 34. In this design, cathode 31 is a rotatable
disk and anode 32 is a stationary cylinder. The cathode
is positioned horizontally near the top of the level of
electrolyte in container 33 and the anode is placed verti
cally of the container near its wall. An insulating cone,
35, which acts as a cdvc means, is positioned on the
bottom of the container with a tip of the apex of the
cone being at a distance G from the cathode. Current
density across the cathode varies concentrically from a
high at the edge of the cathode nearest the anode to a
low at the center of the disc-cathode (nearest the apex
of the insulating cone). Variation of the current density
over the radius of the rotating disk can be obtained by
varying the gap (G) between the disk and the tip (apex)
of the cone-shaped insulator, the length of the anode (L)
and the angle (9) of the cone-shaped insulator.
A still further embodiment of the invention, denomi
nated 40, is schematically represented in FIG. 4 in
which a cathode 41 and an anode 42 are positioned in a
container 43 of non-conducting, non-contaminating
material, holding an electrolyte, 44. In this design, cath
ode 41 is an elongated, narrow cylinder capable of being
rotated about its longitudinal axis. Anode 42 is a station
ary washer-shaped (annular) electrode. The cathode
and the anode are positioned in the container with their
longitudinal axes being substantially coaxial. An in
verted, truncated insulating cone 45 positioned about
the cylinder between the cylinder and the side walls of
5,268,087
5
the container acts as a cdvc means. Current density
across the cathode varies from a high at an upper end of
the cylinder (nearest to the anode) to a low at a lower
end of the cylinder. Adjustments in variation of the
current density over the length of the cathode can be
obtained by varying the gap (G) between the anode and
the cathode, the radial length of the annular anode (L)
and the angle (6) between the surface of the rotatable
cylinder and the insulating object.
The feasibility of the improved cell was demonstrated
using a cell of the ?rst embodiment (FIG. 1) with a
rotating cone-shaped cathode. The rotating cone was
made of stainless steel. The base diameter of the cone
was 9.5 cm, and the angle (9) of the cone was 32.5
degrees. The anode was a ?at copper disk which had a
diameter of 9.5 cm and was placed at the bottom of a
1000 ml container ?lled with 600 cc of a copper plating
solution. The tip of the apex of the rotating cone was
positioned 2.5 cm away of the anode, and the rotation of
the cone was controlled at 100 rpm. The aqueous plat
ing solution contained 20 g/l of copper sulfate and l60
g/l of sulfuric acid. Trace amount of commercially
available copper plating additives were added. The
total current applied to the cell was 2 Amps and plating
time was approximately 80 minutes.
In this instance the metal was plated directly onto the
surface of the stainless steel cone, without the use of a
test panel. After the electroplating, the copper deposit
was peeled of the cone. A micrometer gage was used to
measure the thickness of the copper deposit along the
surface of the rotating cone. The thickness distribution
of the deposit is plotted in FIG. 5 indicating, as in a Hull
cell, a very wide achieved range of current densities.
To check the performance of reproducibility, three
more experimental runs were performed. The total
current applied to the cell was kept at 2 Amps, but the
plating time was shortened to only 5 minutes to reduce
additive concentration depletions during plating. Iden
tical deposit appearances (dull and bright regions) were
obtained for these three experimental runs. This demon
strates the reproducibility of the desired current varia
tion. In addition, unlike a traditional Hull cell which
usually showed some variations in appearance for the
top and the bottom region of the test panel due to differ
ent mass transfer conditions, the rotating cone cell did
not show any difference in appearance in the angular
direction. The observations were not surprising due to
the easiness of controlling the rotation speed, the well
defined hydrodynamics, and the complete cell geome
try symmetry. In addition, since the mass transfer con
ditions are extremely reproducible and can be easily
modi?ed by altering the rotation speed, the improved
cell also can be used to study the mass transfer effect on
deposit and throwing power.
I claim:
1. An improved electroplating cell comprising
a container of non-conducting, non-contaminating
material for holding electrolyte,
20
25
45
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65
6
an anode and a cathode positioned within the walls of
the container, and
a current density variation creating (cdvc) means of
an electrically insulating material and for creating
for a given total current a current density variation
across the cathode, said cdvc means being arranged
at an angle to that surface of the cathode on which
deposition is to take place, said angle being less
than 90 degrees,
the volume of the electrolyte, when present, being
sufficient for said anode, cathode and cdvc means
being immersed in the electrolyte,
wherein said anode and cathode are positioned coaxi
ally each of another, and said cathode is capable of
being rotated about its central axis.
2. The cell of claim 1, in which
said anode is a ?at disc positioned at the bottom of the
container, and
said cathode has a conical shape and is suspended
above the anode, an apex of the cone facing the
anode and being at a preselected distance from the
anode,
wherein an upright wall of the container acts as the
said cdvc means.
3. The cell of claim 1, in which
said anode has a cylindrical shape and is positioned
within the walls of the container, coaxially thereof,
said cathode has a conical shape and is suspended
within a volume encircled by the anode and with
an apex of the cone facing the bottom of the con
tainer, and
the bottom of the container acts as the said cdvc
means.
4. The cell of claim 1, in which
said anode has a cylindrical shape and is positioned
within the walls of the container coaxially thereof,
said cathode is in the form of a disk positioned near
the surface of the electrolyte in parallel to the bot
tom of the container, and
said cdvc means is a cone of electrically insulating
material protruding from the bottom of the con
tainer with an apex of the cone facing the cathode
and being at a preselected distance from the cath
ode.
5. The cell of claim 1, in which
said anode is in a form of an annulus positioned near
the surface of the electrolyte in parallel to the bot
tom of the container,
said cathode has a form of an elongated cylinder
positioned coaxially of the anode, diameter of the
cylinder being smaller than the central opening in
the anode, and
said cdvc means has a form of an inverted truncated
cone positioned coaxially about the cathode and
extending substantially the length of the cathode.
6. The cell of claim 1, in which
said anode is also capable of being rotated about the
central axis of the cathode.
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