Method of monitoring major constituents in plating baths

A method of monitoring major constituents within a plating bath. The method involves applying an electrical signal to a working electrode positioned within the plating bath solution, varying signal parameters, and measuring the resultant response signal. The characteristics of the response signal indicate major constituent concentration levels. The method complements and is easily integrated with known voltammetric techniques for analysis of trace constituents, thus forming an integral part of an efficient overall plating bath analysis system. By adjusting major constituent concentration levels in accordance with measurements made using the method of the present invention, a high quality plating bath can be easily and inexpensively maintained.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to plating baths and methods for 
monitoring the major constituents contained therein. More particularly, 
the method of the present invention relates to a voltammetric analysis 
technique that provides signal spectra which accurately indicate 
concentrations of major constituents within the bath. The signal spectra 
can be used to maintain desired major constituent concentrations within 
limits in order to ensure optimal plating bath performance. 
2. Description of Related Art 
A typical plating bath solution is comprised of a combination of several 
different chemical constituents. The specific constituents vary depending 
upon the type of plating bath, but in general can be broadly divided into 
what are commonly known as major constituents and trace, or minor, 
constituents. The major constituents are defined as those chemical 
constituents which are in excess of 5 percent of the total bath volume. 
Trace or minor constituents, on the other hand, are defined as those 
present in smaller quantities, i.e. less than 5 percent of the total 
volume. For example, in an acid copper plating bath, a major constituent 
is sulfuric acid, which typically represents about 8 to 12 percent of the 
total volume. The acid copper plating bath might also contain trace 
constituents such as organic addition agents, degradation products and 
chemical contaminants, present in much smaller concentrations. 
The concentration levels of both major and trace constituents are important 
determinants of the quality of the resultant plating deposit. Trace 
constituent concentrations influence certain characteristics of the 
plating deposit, including tensile strength, ductility, solderability, 
uniformity, brightness and resistance to thermal shock. Monitoring and 
optimization of trace constituents assumes that the major constituent 
concentrations within the bath are already properly set and maintained. 
Should the major constituents fall outside of required concentration 
ranges, however, the bath may fail to satisfactorily perform its plating 
function. It is therefore important that major constituent concentrations 
be regularly monitored. 
Current techniques for monitoring the major constituents of plating baths 
typically involve removing a sample of the chemical solution from the 
plating tank for subsequent wet chemical analysis. Methods of measuring 
major constituent content in various types of plating baths are disclosed, 
for example, in K. E. Langford and J. E. Parker, "Analysis of 
Electroplating and Related Solutions", pp. 83-100, 65-68 and 174-180. In 
these analysis methods, for example, sulfuric acid content within an acid 
copper plating bath is determined using titration with sodium hydroxide; 
chromic acid content within a chromium plating bath is found using 
reduction titration with excess ferrous ammonium sulfate; free cyanide 
within a silver-cyanide plating bath is found by titration with silver 
nitrate; and carbonate within a silver-cyanide plating bath is analyzed by 
precipitation with barium chloride followed by titration with HCl. Major 
constituent concentrations in other types of plating baths are measured in 
a similar manner. 
Wet chemical analysis methods such as the above must be performed by highly 
skilled personnel. Specialized and costly chemical analysis equipment and 
supplies are required. Furthermore, the delay between drawing samples and 
receiving measurement results can be anywhere from several hours to 
several days. It is thus very tedious and expensive to monitor major 
constituent concentrations using currently available techniques. Moreover, 
the slow response time of wet chemical analysis limits the extent to which 
a high quality and high speed plating bath can be continuously maintained. 
The current major constituent monitoring techniques are quite different 
from real time trace constituent monitoring techniques such as those 
described in U.S. Pat. No. 4,631,116, assigned to the present assignee. 
The method disclosed therein uses voltammetric techniques to produce ac 
current spectra which vary as a result of changes in the concentration of 
various trace constituents. Voltammetric methods have been found to 
produce accurate results in real time for trace constituent analysis. 
However, voltammetric methods have not yet been considered for use in 
major constituent analysis. As a result, it is presently necessary to use 
voltammetric trace constituent measurement techniques in conjunction with 
the above-described major constituent wet chemical analysis in order to 
monitor the overall chemistry of the plating bath. The wet chemical 
analysis cannot be performed with the in-tank electrochemical sensors and 
other equipment typically used in trace constituent analysis. Two 
different sets of equipment must therefore be maintained in order to 
perform major and trace constituent analysis. No integrated measurement 
system is available which is capable of measuring both major and trace 
constituents. 
As is apparent from the above, there presently is a need for an accurate 
and inexpensive real time method for monitoring the concentration of major 
constituents within a plating bath. Furthermore, the method should 
complement and be easily integrated with known techniques and equipment 
suitable for measuring trace constituents, resulting in an efficient 
overall plating bath analysis system. 
SUMMARY OF THE INVENTION 
In accordance with the present invention a method for monitoring the 
concentration of major constituents within a plating bath is provided. The 
present invention is based upon the discovery that voltammetric techniques 
can be used to accurately monitor major constituent concentrations within 
a plating bath. Voltammetric techniques have been used for monitoring 
levels of very low concentration trace constituents, but have heretofore 
not been considered for measuring major constituents. The method of the 
present invention now makes possible the use of voltammetry to accurately 
determine major constituent concentrations. 
The method of the present invention involves the steps of applying an 
electrical signal to a working electrode in contact with the plating bath 
solution, varying the parameters of the electrical signal, and measuring 
the response signal. The characteristics of the response signal vary in 
accordance with the major constituent concentration within the solution, 
and thereby provide an accurate real time indication of major constituent 
concentration. 
In accordance with a preferred embodiment of the present invention, an ac 
signal superimposed on a dc sweep signal is applied to a working electrode 
which has been pretreated by a dc potential and is in contact with the 
plating bath solution. The dc sweep signal is varied at a selected sweep 
rate over a selected voltage range. An ac response current signal is 
thereby produced which includes peaks indicative of the concentration 
levels of major constituents within the plating bath. Various independent 
electrochemical parameters are varied to maximize the sensitivity of the 
ac current spectra peaks to particular major constituents. The method 
establishes a set of optimal electrochemical parameters for several 
exemplary plating baths and their respective major constituents. 
As a feature of the present invention, the method eliminates the delay, 
expense and complexity typically associated with current major constituent 
analysis methods requiring wet chemical analysis. Specialized chemical 
equipment and chemical analysis personnel are no longer required. The 
measurement results are available in real time, which facilitates 
continuous and efficient control of plating bath chemistry. 
As another feature of the present invention, the ac response current 
includes a readily identifiable peak, the amplitude of which varies with 
the concentration of the major constituent being measured. The constituent 
levels within the bath can then be adjusted until the desired optimal 
concentrations are present. 
As a further feature of the present invention, the method is easily 
integrated with known trace constituent measurement methods and equipment, 
thereby providing an efficient overall plating bath analysis system 
suitable for monitoring both trace and major constituents. For example, 
the method may be used in conjunction with an in-tank electrochemical 
sensor so as to eliminate the need to draw a sample of electrochemical 
solution from the plating bath. 
As an additional feature of present invention, optimal signal parameters 
for monitoring the concentrations of major constituents within commonly 
used acid copper, chromium and silver-cyanide plating baths are provided. 
Furthermore, the method provides an experimental framework for determining 
optimal measurement signal parameters for monitoring major constituents in 
other types of plating baths. 
The above-discussed features and attendant advantages of the present 
invention will become better understood by reference to the following 
detailed description of the preferred embodiment and the accompanying 
drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention is based upon the discovery that a known voltammetric 
technique, heretofore considered applicable only to the measurement of 
trace constituents, can be used to monitor major constituents within a 
plating bath. Although the following detailed description discusses only 
one exemplary voltammetric technique, it should be understood that the 
method of the present invention can be readily adapted for use with other 
voltammetric techniques. The exemplary voltammetric trace constituent 
monitoring technique to which the method of the present invention is 
related is described in U.S. Pat. No. 4,631,116. The contents of this 
patent are hereby expressly incorporated by reference. 
As discussed above, major constituents within a plating bath vary depending 
upon the type of bath, but in general are defined as those constituents 
which make up in excess of 5 percent of the total plating bath volume. The 
following detailed description will be directed to exemplary major 
constituents found within the commonly used acid copper, chromium and 
silver-cyanide plating baths. For example, in an acid copper plating bath, 
one exemplary major constituent is sulfuric acid, which has a 
concentration of about 8 to 12 percent of total volume. In a chromium 
plating bath, one exemplary major constituent is chromic acid, which has a 
concentration of about 225 g/l to 275 g/l CrO.sub.3. In a silver-cyanide 
plating bath, two exemplary major constituents are potassium cyanide and 
potassium carbonate, with typical concentrations of about 82 g/l to 113 
g/l and about 40 g/l to 180 g/l, respectively. Although the following 
description is directed to these three exemplary plating baths and four 
exemplary major constituents associated therewith, it should be understood 
that this is by way of illustration and not limitation. The method can be 
used to monitor other major constituents within acid copper, chromium and 
silver-cyanide plating baths. Furthermore, the method discloses general 
techniques which are useful for monitoring many other types of plating 
baths and the major constituents contained therein. 
The schematic diagram of FIG. 1 illustrates a preferred embodiment of a 
voltammetric system used to conduct the method of the present invention. 
The plating bath solution is located within an electrochemical cell 9. The 
electrochemical cell 9 is preferably part of an electrochemical sensor 
submerged within the plating bath. A pump can then be used to draw the 
solution through the cell. This eliminates the need to remove a sample of 
the solution from the tank, as is currently required by wet chemical 
analysis techniques. 
In the exemplary system of FIG. 1, a function or waveform generator 5 
provides an output 13 which is an ac signal of appropriate frequency and 
amplitude. The ac signal is applied to the external input 23 of a 
potentiostat 8 and to the reference input 16 of a lock-in amplifier 6. The 
potentiostat serves to superimpose the ac signal applied to its external 
input 23 upon an appropriate dc voltage sweep signal generated within the 
potentiostat. Alternatively, the dc sweep signal could be supplied by an 
external function generator. In some cases, the voltammetric signal is 
simply the one generated within potentiostat 8, upon which the AC is 
superimposed. 
The combined dc and ac signal output from potentiostat port 25 is then 
applied to the working electrode 10 in the electrochemical cell 9 via line 
28. The electrochemical cell 9 also contains a counter electrode 12 and a 
suitable reference electrode. The reference electrode 11 and counter 
electrode 12 are connected to the accompanying potentiostat ports 26, 27 
via lines 29, 30, respectively. The electrochemical cell 9 with electrodes 
10, 11 and 12 is a sensor design typically used in conjunction with 
voltammetric techniques. Other sensor designs could also be used. When the 
combined dc and ac signal is applied to the working electrode 10, a 
response current is generated between the working electrode 10 and the 
counter electrode 12. The response current is a combined dc and ac signal 
which varies depending upon the electrochemical processes occurring at the 
surface of the working electrode 10. The electrochemical processes are a 
function of the major constituent concentrations, and the response current 
is therefore responsive to these concentrations. 
From the potentiostat output 24 the response current is applied to the 
signal input 17 of lock-in amplifier 6 and to the external sweep input 33 
of strip chart recorder 7 or to a computerized data acquisition system. 
The lock-in amplifier filters out the DC component in the response signal 
and isolates out the desired harmonic component in the AC signal. It 
further resolves the AC harmonic signal into its in-phase and quadrature 
components. The ac harmonic which provides the best diagnostic information 
is the one which should be selected for measurement. In the exemplary 
spectra shown in FIGS. 2 through 9 the second harmonic of the ac portion 
of the response signal provided the best diagnostic information indicative 
of major constituent concentrations. For other plating baths or major 
constituents, different harmonics of the ac portion of the response signal 
may provide better results. 
The in-phase component of the ac portion of the response current is then 
passed from in-phase output 18 of lock-in amplifier 6 to a display signal 
input 31 of strip chart recorder 7. Similarly, the quadrature component is 
passed from quadrature output 19 of lock-in amplifier 6 to a second 
display signal input 32 of strip chart recorder 7. The strip chart 
recorder displays the in-phase and the quadrature components of the ac 
portion of the response current as a function of time as shown in FIGS. 2 
through 9. These displays represent unique AC response current spectra 
which indicate the major constituent composition within the plating bath 
solution. 
The specific equipment used in the exemplary system of FIG. 1 included a 
Wavetek Model 188 waveform generator, a 273 potentiostat, and a 
5208 (or 5210) lock-in amplifier. The Wavetek waveform generator is 
available from Wavetek San Diego, Inc., of San Diego, Calif. and the 
equipment is available from Princeton Applied Research, of Princeton, N.J. 
In order to optimize the accuracy of the response current spectra produced 
in accordance with the preferred voltammetric technique described above, 
it is necessary to vary a number of independent electrochemical 
parameters. These parameters include: 1) dc pretreatment voltage and time; 
2) type of ac waveform (i.e., sinusoidal, square, triangular, etc.); 3) ac 
signal amplitude and frequency; 4) dc sweep signal voltage range and sweep 
rate; 5) ac response signal harmonic measured (i.e., fundamental, second, 
etc.); 6) ac response signal phase angle measured; and 7) hydrodynamic 
conditions (i.e., degree of agitation). The above parameters were 
independently varied to determine the optimal parameter settings for 
monitoring several exemplary major constituents using the preferred 
voltammetric system of FIG. 1. It should be emphasized that while the 
parameter settings described in the examples below are optimal, the 
technique may produce useful results using parameters outside the 
specified optimal ranges. In applying other voltammetric techniques in 
accordance with the method of the present invention, a similar set of 
parameters applicable to that technique would have to be optimized. The 
set of applicable parameters may be estimated by reference to the manner 
in which the particular voltammetric technique has been applied to trace 
constituent detection. 
In general, certain settings of the above physical test parameters are 
particularly well-suited for monitoring major constituent concentrations 
in accordance with the preferred embodiment of FIG. 1. The working 
electrode is preferably pretreated to remove contaminants at an anodic 
potential of about 1.5 to 3 volts, for a period of about 5 to 30 seconds. 
A sinusoidal ac waveform with an amplitude of about 20 to 30 mv root mean 
square (rms) and a frequency of about 50 to 2,000 Hz is superimposed on 
adc signal which is swept between .+-.3 V which encompasses both stripping 
and plating electrode voltages. The DC sweep ranges depend on the specific 
baths. Optimal spectral peak resolution is obtained using the second 
harmonic of the ac response current, measured using a phase angle offset 
of about 0 to 45 degrees. Further improvements in monitoring accuracy are 
obtained by stirring the solution, and maintaining it at an optimum 
temperature range depending on each plating bath. 
Examples of the optimization of the exemplary voltammetric system of FIG. 1 
to the detection of specific major constituents are described below with 
reference to FIGS. 2-9. It should be noted that in FIGS. 2-9, the time 
scale and voltage settings were the same in all cases and are not 
specifically indicated in the figures. The important point is to compare 
the figures for the same plating bath solution with particular reference 
to the height of peak P, as described below. 
One major constituent within an acid copper plating bath is sulfuric acid. 
Optimal ac current spectra of the type shown in FIGS. 2 and 3 were 
obtained for sulfuric acid concentrations in an acid copper bath using the 
following system parameters. Prior to each measurement, the working 
electrode was pretreated at an anodic potential of about 1.5 to 3 volts 
for a period of about 10 to 30 seconds. An ac signal of about 20 to 30 mv 
rms amplitude and about 50 to 1,500 Hz frequency was superimposed on a dc 
sweep signal. The dc signal was swept from about 0.4 to -0.5 volts and 
reversed to about 0.5 volts at a rate of about 20 to 500 mv/sec. The most 
sensitive spectral peak was found on the quadrature component of the ac 
response signal second harmonic, measured using a phase angle offset of 
about 0 to 45 degrees. During the measurement, the solution within the 
electrochemical cell was stirred continuously. The solution was maintained 
at a temperature of about 25.degree. C. 
Referring now to FIG. 2, an acid copper plating solution containing 10 
oz/gal copper sulfate, 20 oz/gal acid, 5 ml/liter carrier, 30 ppm chloride 
and 5 ml/liter brightener was analyzed using the above system parameters. 
The height of peak P measures about 652 mv relative to a copper reference 
electrode and corresponds to a concentration of 20 oz/gal of sulfuric acid 
within the plating bath solution. In FIG. 3 the effect of increasing the 
concentration of sulfuric acid within the solution of FIG. 2 to 30 oz/gal 
is shown. The resulting ac response peak P measured 870 mv, thus 
reflecting the increase in the sulfuric acid content of the solution. The 
method was applied to several other concentrations of sulfuric acid to 
verify repeatability. When using the above identified optimal parameters, 
the sensitivity of the detection of sulfuric acid concentration was about 
22 to 30 mv/(oz/gal sulfuric acid). A one oz/gal change in the 
concentration of sulfuric acid in the solution would thus result in a 
change in the peak P voltage of about 22 to 30 mv. 
In another example, optimal system parameters have been determined for 
detecting the concentration of chromic acid in an ATOCHEM HCR-840 chromium 
plating bath. An ac signal of about 20 mv to 30 mv rms amplitude and about 
100 to 1,000 Hz frequency was superimposed on a dc sweep signal. The dc 
signal was swept from about 2.4 to -1.5 volts and reversed to about 2 
volts at a rate of about 50 to 100 mv/sec. The most sensitive spectral 
peak was found on the quadrature component of the ac response signal 
second harmonic, measured at a phase angle offset of about 10 degrees. 
During the measurement, the solution within the electrochemical cell was 
stirred continuously. The solution was maintained at a temperature of 
about 60.degree. C. 
The spectra of FIG. 4 result from the application of a signal within the 
above optimal range to a solution containing 225 g/liter chromium 
trioxide, 1 g/liter sulfate catalyst, and 4 g/l silicofluoride catalyst. 
The resulting peak P of the quadrature component of the ac portion of the 
response signal measured 135 mv. The resulting peak is well-defined and 
appears in a portion of the spectrum where there is little other activity. 
The effect of increasing the chromic acid content of the solution of FIG. 
4 to 275 g/liter of chromium trioxide, is shown in FIG. 5. The height of 
peak P increases to 180 mv. Applying the method to solutions with 
different concentrations of chromic acid produced consistent results. The 
sensitivity of this chromic acid detection method is thus about 0.9 to 1.0 
mv/(g/liter of chromium trioxide). 
In a further example, optimal system parameters have been determined for 
obtaining accurate ac current spectra indicating levels of the major 
constituent free potassium cyanide within a silver cyanide plating bath. 
An ac signal of about 20 to 30 mv rms amplitude and about 100 to 2,000 Hz 
frequency was superimposed on a dc sweep signal. The dc signal was swept 
from about 0 to -1.6 volts and reversed to about 0.5 volts at a rate of 
about 100 to 200 mv/sec. The most sensitive spectral peak was found on the 
in-phase component of the ac response signal second harmonic, measured at 
a phase angle offset of about 0 degrees. The solution was maintained at a 
temperature of about 24.degree. to 26.degree. C. 
A signal within the above-specified optimal range was applied to a silver 
cyanide plating solution to produce the spectra of FIGS. 6 and 7. 
Initially, the solution contained 55 g/liter silver cyanide, 82.5 g/liter 
total KCN, 55 g/liter free potassium cyanide (82.5 g/liter total potassium 
cyanide), 150 g/liter K.sub.2 CO.sub.3, 10 g/liter K.sub.2 SO.sub.4, 0.1 
ml/liter brightener, and had a pH of 12.3. The resultant spectra is shown 
in FIG. 6. The peak P of the in-phase component of the ac response signal 
appears at the onset of the plating portion of the dc swept signal and 
measures 75 mv. The effect of increasing the concentration of free 
potassium cyanide to 77.5 g/liter within the solution of FIG. 6 is seen in 
the spectra of FIG. 7. The height of peak P increased to 125 mv. Further 
measurements using other concentrations of free potassium cyanide yielded 
similar results. The sensitivity of this method as applied to the major 
constituent potassium cyanide within a silver cyanide plating bath is thus 
about 2 to 3 mv/(g/liter of free potassium cyanide). 
Finally, the method of the present invention was applied to determine the 
concentration of potassium carbonate, a major constituent in a silver 
cyanide plating bath. An ac signal of about 20 to 30 mv rms amplitude and 
about 200 to 1,000 Hz frequency was superimposed on a dc sweep signal. The 
dc signal was swept from about 0 to -1.5 volts where it was held for 10 
seconds and reversed to about 0.5 volts at a rate of about 100 to 200 
mv/sec. The most sensitive spectral peak was found on the quadrature 
component of the ac response signal second harmonic, measured at a phase 
angle offset of about 0 to 24 degrees. The solution was maintained at a 
temperature of about 24.degree. to 26.degree. C. 
A signal within the range described above was applied to a silver cyanide 
plating solution containing 40 g/liter potassium carbonate, 55 g/liter 
AgCN, 82.5 g/liter total KCN, 10 g/liter K.sub.2 S0.sub.4, and 0.1 ml/l 
brightener and having a pH of 12.5. The resulting spectra are shown in 
FIG. 8. The height of potassium carbonate diagnostic peak P measures 150 
mv. The effect of increasing the concentration of potassium cyanide within 
the same solution to 180 g/liter results in the spectra of FIG. 9, where 
the diagnostic peak P shown measured 320 mv. The sensitivity of the method 
as applied to potassium carbonate within a silver cyanide plating bath is 
thus about 11 to 15 mv/(g/liter of potassium cyanide). 
As can be seen in FIGS. 2 through 9, the method of the present invention 
produces reliable and repeatable spectra with easily distinguishable peaks 
corresponding to the concentration levels of various major constituents. 
These spectra can be used in conjunction with an overall plating bath 
analysis system which monitors and maintains proper levels of major 
constituents within various plating baths in real time without removing 
fluid from the plating tank. 
Although the above description has been limited to analysis of exemplary 
plating bath major constituents using an exemplary ac voltammetry 
technique, this is by way of illustration and not limitation. It will be 
understood by those skilled in the art that many alternate implementations 
of this method are possible without deviating from the spirit and scope of 
the invention, which is limited only by the appended claims.