Method of monitoring acid concentration in plating baths

A method of monitoring the concentration of certain plating bath major constituents such as acid is provided which is insensitive to the effects of hydrogen produced during plating. The method involves applying an ac signal superimposed on a dc potential to a sensing electrode in contact with the solution, producing an ac response current. The steady state value of the ac response current is then measured and provides an accurate indication of the acid concentration within the solution. The method can be performed using a single sensing electrode. Furthermore, the method complements and is easily integrated with known voltammetric techniques and equipment suitable for analysis of other plating bath constituents.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to plating baths and methods for 
monitoring the constituents contained therein. More particularly, the 
method of the present invention relates to a voltammetric analysis 
technique for monitoring the concentration levels of certain plating bath 
major constituents such as acid. The method can be used to maintain 
desired constituent concentrations 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 
distinct electrochemical constituents which can be broadly divided into 
major constituents and trace constituents. The major constituents 
typically make up about 2 to 50 percent of the total bath weight or 
volume. Trace constituents are present in smaller quantities, usually less 
than 1 percent of the total weight or volume. Acid is an important major 
constituent in many plating baths. For example, in an acid cadmium plating 
bath, acid typically represents about 5 to 10 percent of the total bath 
weight. 
The concentration levels of both major and trace constituents will 
influence the quality of the resultant plating deposit, and should 
therefore be regularly monitored. Methods have been developed for in-tank 
monitoring of trace constituents as well as certain major constituents. 
For example, U.S. Pat. No. 4,631,116 discloses a method for monitoring 
trace constituents using an in-tank electrochemical sensor. Application 
Ser. No. 08/037,158 entitled "Method of Monitoring Major Constituents in 
Plating Baths" discloses a method for in-tank monitoring of major 
constituents such as sulfuric acid in an acid copper bath. The above 
patent and pending patent application are owned by the same assignee as 
the present invention. However, these techniques provide less than optimal 
accuracy for certain types of acid, particularly in plating baths in which 
large quantities of hydrogen are produced during plating. For example, the 
above voltammetric techniques are not well-suited to measurement of acid 
in an acid cadmium bath. As a result, alternative techniques are currently 
used to measure acid cadmium concentrations. 
One such technique involves the use of two sensors; one voltammetric, the 
other a conductivity sensor. The two sensors are required because often 
voltammetric sensors measure non-acid constituents, whereas conductivity 
relates to acid concentration. Other techniques currently used to measure 
relatively high acid concentrations in plating baths which produce large 
amounts of hydrogen during plating include pH sensors which are not very 
accurate at high acid concentrations. Use of the current measurement 
techniques is inconvenient, time-consuming and costly, since these 
techniques are not directly compatible with the voltammetric trace and 
major constituent measurement methods discussed above. Additional tests 
must be performed using a different set of equipment in order to properly 
monitor certain acid concentrations. No integrated measurement system is 
available which is capable of measuring these acid concentrations as well 
as most other major and trace constituent concentrations. 
As is apparent from the above, there presently is a need for an accurate 
and inexpensive real time method of monitoring acid concentrations in 
plating baths which produce large quantities of hydrogen. The method 
should require the use of only a single in-tank sensor. Furthermore, the 
method should complement and be easily integrated with known techniques 
and equipment suitable for measuring other plating bath constituents, 
resulting in an efficient overall plating bath analysis system. 
SUMMARY OF THE INVENTION 
The present invention provides a method for monitoring the concentration of 
acid as a plating bath major constituent. The present invention is based 
upon the discovery that the concentrations of certain major constituents 
such as acid can be accurately determined by measuring the steady state 
value of the ac response current produced when an ac signal superimposed 
on a dc signal is applied to a sensing electrode in contact with the 
solution. 
The method of the present invention involves the steps of providing at 
least one sensing electrode in contact with a plating bath solution 
containing a concentration of acid or other major constituent; applying a 
voltammetric signal comprising an ac signal superimposed on a dc signal to 
the sensing electrode such that an ac response current is produced having 
a steady state value proportional to the constituent concentration; and 
measuring the steady state value of the response current to determine the 
constituent concentration. 
As a feature of the present invention, the method is insensitive to the 
hydrogen produced during plating and is therefore well-suited to acid 
concentration measurements in plating baths which produce large amounts of 
hydrogen. The method can also be used to monitor acid concentrations in 
other types of plating baths. 
As another feature of the present invention, the measurements may be 
performed using a single in-tank electrochemical sensor. The measurement 
results are available in real time so that desired major constituent 
levels, and thereby the quality of the plating bath, can be continuously 
and efficiently maintained. 
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 and flexible overall plating bath analysis 
system suitable for accurately monitoring a wide variety of plating baths 
and their respective constituents. Since the present invention can be 
implemented using voltammetric equipment suitable for measuring most other 
plating bath constituents, only a single set of equipment need be 
maintained. The method of the present invention thus serves to complement 
and extend the capabilities of existing voltammetric analysis techniques. 
As an additional feature of the present invention, optimal signal 
parameters for monitoring acid concentration in an exemplary acid cadmium 
bath are disclosed. Furthermore, the method provides an experimental 
framework for determining optimal measurement signal parameters for 
monitoring acid or other major constituent concentrations in a wide 
variety of different plating baths.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention has wide application to many different plating baths 
and their respective constituents. The following description is directed 
towards measurement of acid concentration, and applies the method to 
measuring acid concentration in an exemplary acid cadmium bath. It should 
be understood, however, that this is by way of example and not limitation. 
The method can be used to measure acid concentrations in most plating 
baths, including, for example, acid copper. Furthermore, although the 
method is particularly well-suited to the detection of acid concentrations 
as described herein, it could also be used to monitor a variety of other 
plating bath constituents. Other constituents which could be measured 
using this method include hydroxide ion. 
The schematic diagram of FIG. 1 illustrates a preferred exemplary system 
for conducting the method of the present invention. It should be noted 
that the equipment of this system is readily compatible with the equipment 
used in conjunction with other voltammetric plating bath monitoring 
techniques. The present method therefore serves to extend the capability 
of existing voltammetric techniques without the need for additional 
equipment. 
In the exemplary system of FIG. 1, the plating bath solution is located 
within an electrochemical cell 9. The electrochemical cell 9 is preferably 
part of an in-tank electrochemical sensor submerged within the plating 
bath. A pump (not shown) can be used to draw the solution through cell 9. 
Waveform generator 5 provides an output 13 which is an ac signal of 
suitable waveform, amplitude and frequency. The ac signal is preferably a 
sinusoidal signal, but other ac waveforms could also be used, such as 
square waves. 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 8 forms the desired voltammetric signal by superimposing the 
ac signal applied to its external input 23 upon an appropriate dc signal 
generated within the potentiostat. Alternatively, the dc signal could be 
supplied by an external signal source. The ac and dc signal 
characteristics will be discussed in greater detail below. Potentiostat 8 
also ensures that signal amplitudes are not affected by variations in 
current flow through electrochemical cell 9. An exemplary potentiostat 
suitable for use in the system of FIG. 1 is the model 273 available 
from Princeton Applied Research, of Princeton, N.J. 
The voltammetric signal consisting of combined ac and dc signals is applied 
to the sensing electrode 10 in the electrochemical cell 9 via line 28. The 
sensing electrode is preferably constructed of an inert material such as 
platinum. The electrochemical cell 9 also contains a counter electrode 12 
and a reference electrode 11. All system measurements are taken relative 
to the reference electrode 11. The reference electrode can be a standard 
calomel reference electrode or any other suitable reference electrode. The 
reference electrode 11 and counter electrode 12 are connected to the 
potentiostat 8 via lines 29, 30 respectively. This three-electrode 
electrochemical sensor design is suitable for use with many different 
voltammetric techniques. Further detail on this sensor can be found in 
U.S. application Ser. No. 07/945,751 entitled "In-tank Electrochemical 
Sensor," assigned to the present assignee. It should be understood, 
however, that alternative electrode arrangements may also be used. 
When the combined dc and ac signal is applied to sensing electrode 10, a 
response current is generated between sensing electrode 10 and counter 
electrode 12. The response current has an ac component and a dc component. 
The response current is measured in the following manner. The response 
current passes back through potentiostat 8 from output 24 to the signal 
input 17 of lock-in amplifier 6. The lock-in amplifier separates the ac 
component of the response current from the dc component. A reference 
signal is supplied from the voltammetric ac signal source, waveform 
generator 5, to the reference input 16 of lock-in amplifier 6. The 
reference is coherent with the ac component of the response current signal 
and lock-in amplifier 6 can then be used to measure the ac response 
current. Alternatively, the output 18 of lock-in amplifier 6 can be 
applied to input 19 of a digital voltmeter 7 set to measure ac millivolts. 
In another possible embodiment, the lock-in amplifier could be eliminated 
altogether and input 19 of voltmeter 7 could be connected directly to 
output 24 of potentiostat 8. When the voltmeter is set to measure ac 
millivolts it will be unaffected by the dc component of the response 
current. Other suitable methods of measuring ac voltage could be used in 
place of voltmeter 7. 
In order to optimize the response current accuracy as an indicator of a 
particular acid concentration, the ac signal waveform, amplitude and 
frequency and the dc signal amplitude and duration can be varied. These 
parameters were independently varied to determine the preferred system 
parameters for monitoring acid concentration using the preferred 
voltammetric system of FIG. 1. It should be noted, however, that 
alternative combinations of ac and dc signal parameters may also produce 
similar measurement results. 
In general, certain system parameters are particularly well-suited for 
selectively monitoring particular acid concentrations. The preferred 
signal characteristics for the ac and dc components of the voltammetric 
signal are as follows. All potentials and voltages are given with respect 
to a saturated calomel electrode. The ac signal preferably has an 
amplitude of about 10 to 200 mv rms and a frequency of about 5 to 60 kH. 
The high frequency eliminates the effect of the electrode reactions and 
responds mainly to solution conductivity. The method of the present 
invention is thus able to isolate the effect of the acid from that of the 
other plating bath constituents due to the high conductivity of acid. For 
example, in the case of an acid cadmium plating bath, the cadmium ions are 
much less conductive. However, the high proton conduction in most acids 
permits the acid to respond to the ac component of the voltammetric 
signal. The ac response current is therefore primarily a function of the 
acid concentration within the bath. 
The dc signal is set at an anodic potential of about 2.0 to 3.5 volts 
applied for a period of about 5 to 15 seconds. This dc signal is similar 
to the anodic pretreatment signals described in U.S. Pat. No. 4,631,116 
and application Ser. No. 08/037,158. The dc component of the preferred 
voltammetric signal of the present invention thus also provides the 
cleaning and activation functions of a pretreatment signal. The dc signal 
removes any absorbed organics or other contaminants from the sensing 
electrode surface and otherwise prepares it for measurements of ac 
response current. The sensitivity of the method is insensitive to the 
degree of stirring or agitation of the plating bath solution. 
The steady state magnitude of the ac component of the response current 
generated and measured as described above provides an accurate indication 
of acid concentration. The ac current should be given sufficient time to 
reach a steady state value before it is measured. In general, the ac 
response current will reach steady state in about 3 to 10 seconds. For 
purposes of this specification, the response current is considered to have 
reached steady state when it consistently maintains about .+-. one percent 
of its final value. Other ac wave forms could also be used to indicate 
acid concentration, including triangular or square wave. 
The voltammetric system of FIG. 1 has been applied to the detection of acid 
concentration in an exemplary acid cadmium plating bath available from 
LeaRoanal of Freeport, N.Y. The acid was a major constituent within this 
exemplary acid cadmium bath, at a concentration level of about 5-10 
percent of total bath weight. Cadmium ions comprised the other major 
constituent within the bath, present in a concentration of about 3-5 
percent of total bath weight. The acid cadmium bath typically produces 
large amounts of hydrogen during plating. 
The ac component of the voltammetric signal applied to this exemplary 
solution was a sinusoidal signal having an amplitude of about 10 to 100 mv 
rms and a frequency of about 30 to 55 kHz. The ac component was 
superimposed on a dc signal set at an anodic potential of 3.0 volts. The 
magnitude of the ac component of the response current reached its steady 
state value after this voltammetric signal had been applied for 5 seconds. 
The steady state ac response current was then measured for various acid 
concentrations. The results of these measurements are summarized in Table 
1 below. The sensing electrode was a 1 mm diameter platinum wire sheathed 
at both ends, so that only a 1/8 inch long cylindrical surface was exposed 
to the plating solution. 
TABLE I 
______________________________________ 
Steady state ac response current at various 
normalized acid concentrations 
Steady State Current 
Acid Concentration 
(ma) 
______________________________________ 
1.0 12.1 
0.8 10.6 
1.2 13.3 
______________________________________ 
The above measurement results are normalized to a value of 1.0, which 
corresponds to an acid concentration of about 75 grams/liter. The steady 
state ac response current of 12.1 ma corresponding to this acid 
concentration is plotted as point P1 in FIG. 2. Decreasing the acid 
concentration to a normalized value of about 0.8 results in an ac response 
current of about 10.6 ma. This measurement is plotted as point P2 in FIG. 
2. Increasing the acid concentration to a normalized value of 1.2 results 
in an ac response current of 13.3 ma, as shown by point P3 in FIG. 2. 
It can be seen from FIG. 2 that the steady state ac response current is a 
linear function of acid concentration. The maximum total error or 
deviation from linear for a given measurement point is about 3 percent. 
The above measurements are also relatively insensitive to the 
concentration levels of other major constituents within the plating bath. 
For example, increasing the cadmium ion content by 20% had no effect on 
the acid concentration measurements. 
Although the above detailed description is directed to detecting acid 
concentrations in plating baths which generate large amounts of hydrogen 
during plating, this is by way of example and not limitation. The method 
can also be used to monitor major constituents other than acid, such as 
hydroxide ion. Furthermore, the method can be applied to plating baths 
which do not generate large quantities of hydrogen during plating. It will 
be understood by those skilled in the art that these and many other 
alternate implementations are possible without deviating from the scope of 
the invention, which is limited only by the appended claims. 
The contents of the patents and copending patent applications set forth 
above are hereby incorporated by reference.