In the printed circuit industry, copper is generally used as an interconnection medium on a substrate. In certain applications, the deposit is practially or completely formed by electroless copper deposition. When an electroless copper plating process is utilized, substantially uniform deposition is achieved regardless of the size and shape of the surface area involved. Very small holes (e.g., 0.15-0.25 mm) are difficult to electroplate because of the electric field distribution in the hole, but such holes are easily plated using an electroless plating process which does not depend on an applied electric field and its distribution. Fine line conductors which are placed near large surface conductor areas (e.g., heat sinks) are difficult to electroplate because of the electric field distortion caused by large conductive areas. Such fine line conductors next to large conductive areas can, however, be effectively plated with an electroless process.
Although there are many benefits to using an electroless plating process, crack formations in the plated copper can occur if the bath constituents are not maintained within precise limits. Typically, these cracks have been found in the electrolessly formed hole wall lining and at the junction with surface conductors. Such cracks on the circuit hole walls are usually not a serious functional problem because the circuit holes are later filled with solder at the time of component insertion. However, cracks can also occur in the fine line conductor traces. With increased component and circuit packaging density, conductor traces of 0.15 mm width are not uncommon and often can best be achieved with an electroless process. Since defects or cracks in the signal traces may not show up until subsequent manufacturing steps or while in use, and since the defects or cracks cannot easily be repaired, it is imperative to produce good quality, crack free copper to assure proper connectivity-and functioning of the circuit signal conductors in such high density circuit boards.
Originally, electroless plating baths were controlled by manual methods. A plating bath operator would take a sample of the solution out of the bath, do various tests on the sample to determine the state of the bath, and then manually adjust the bath by adding the chemical components necessary to bring the bath constituents back to a given bath formulation thought to be optimum. This process is time consuming and, because of manual intervention, not always accurate. Furthermore, becaus of the time lag between analysis and adjustment, the bath adjustments were often incorrect, either over-adjusting or under-adjusting the bath composition and often were not in time to maintain stable operation.
Many methods have been proposed in attempts to partially or totally automate the control of the electroless copper plating bath. Generally, the measurement step in these methods required that a sample be removed from the bath and put into a predetermined state. For example, the sample may have to be cooled or a reagent may have to be added before the actual measurement is taken. The adjustment made to the bath is determined from the prepared sample and measurement taken therefrom. Preparation of a sample can require as much as thirty minutes and, therefore, the adjustment based thereon is not proper for the bath's current state since the bath may have significantly changed state in the time between sample removal and bath adjustment.
Removal of a sample from the bath in order to measure various constituents is undesirable for an additional reason. When a sample is removed from the bath, the environment of the solution changes. Measurements taken off the sample, therefore, do not accurately reflect the plating solution in its natural plating environment.
In an electroless copper plating bath, an important component that must be controlled is the concentration of the reducing agent (e.g., formaldehyde). If the concentration of the reducing agent is too high, the bath decomposes causing uncontrolled plating and eventual destruction of the bath. If the concentration of the reducing agent is too low, the reaction is too slow and deposition of electrolessly formed copper stops or is inadequate. Also, plating often cannot be initiated on the catalyzed surfaces if the reducing agent concentration is too low.
One method of controlling formaldehyde used as a reducing agent is an electroless copper plating bath is illustrated by Slominski, U.S. Pat. No. 4,096,301, Oka et al, U.S. Pat. No. 4,276,323, and Oka, U.S. Pat. No. 4,310,563. This method requires that a sample be withdrawn from the tank, transported to another container, cooled down to a specific temperature and mixed with a sulfite solution in order to perform the actual measurement. The measurement steps and the cleaning of the receptacle to avoid contamination in the next cycle can take up to thirty minutes. By the time the bath adjustment is initiated, the bath may have substantially changed state and, therefore, is not correctly adjusted.
Tucker disclosed a method of control of electroless copper plating solutions in "Instrumentation and Control of Electroless Copper Plating Solutions"; Design and Finishing of Printed Wiring and Hybrid Circuits Symposium, American Electroplaters Society, 1976. In the process described, a sample is withdrawn from the bath and cooled down to a predetermined temperature. At the cooled down temperature, the cyanide concentration is measured using an ion specific electrode, the pH is measured, and then the formaldehyde concentration is measured by a titration of a sodium sulfite solution with a sample from the bath. The bath constituents are replenished according to the measurements. This process requires the time consuming steps of withdrawing a sample from the bath, cooling it down and mixing it with a reagent.
Polarography is another method that has been employed for measurement of electroless plating bath parameters. See Okinaka, Turner, Volowodiuk, and Graham, the Electrochemical Society Extended Abstracts, Volume 76-2, 1976, Abstract No. 275. This process requires a sample to be removed from the bath and diluted with a supporting electrolyte. A potential is applied to a dropping mercury electrode suspended in the sample, and the current is measured. From the current-potential curve, the concentration of formaldehyde is derived. This process, too, causes a significant time delay between sampling and adjustment.
Araki, U.S. Pat. No. 4,350,717, uses a colorimetric method for measurement. In the colorimetric method, a sample of the bath is drawn, diluted with reagent, heated to develop the color, and then measured with a colorimetric device to determine the concentrations. The heating step alone takes ten minutes. Together the sampling, mixing, heating and measuring steps cause a significant delay between measurement and adjustments in the bath.
Some in situ measurements in an electroless plating bath have been previously disclosed. In the article, "Determination of Electroless Copper Deposition Rate from Polarization Data in the Vicinity of the Mixed Potential", Journal of the Electrochemical Society, Vol 126, No. 12, December, 1979, Paunovic and Vitkavage describe in situ measurement of the plating rate of a bath. Suzuki, et al, U.S. Pat. No. 4,331,699, also describe a method for in situ measurement of the plating rate. A chrono potentiometric method for determining formaldehyde and copper is referred to in the Journal of the Electrochemical Society, Vol. 127, No. 2, February, 1980. However, these disclosures refer to measurement of specific variables and do not discuss real time methods for overall control of a plating bath, particularly when the electrolessly plated copper forms the conductive pattern of an interconnection board.
In the past, the typical procedure for checking the quality of copper plated in the bath was to place a test board in a plating bath and visually examine for the quality of he copper deposit. Unfortunately, the test board examined might not reflect the true copper quality of the actual work. Mistakes were made in visually examining the samples and often the visual inspections proved to be adequate. The copper quality could change after the test board was plated. A change in loading, i.e., the amount of surface area to be plated, could affect the quality. Frequently, the quality of the bath and, thus, the quality of the copper being plated at the time, would go bad while the actual boards were being plated. As a result, copper quality of the test board as such was not an effective process control parameter.
An object of this invention is to provide a controller for an electroless plating bath that provides for substantially real time control.
Another object of this invention is to provide a controller for an electroless copper plating bath which provides for in situ monitoring, digital measurement, and real-time control.
Still another object of the invention is to provide a contrller that can continuously determine the quality of deposited metal of the plating bath to consistently produce good quality, crack-free plating.
A still further object of this invention is to provide an in situ measurement and control of the stabilizer concentration in the bath.
Yet another object of the invention is to provide in situ measurement and control of the reducing agent concentration in the bath.
Another object of the invention is to provide a process and apparatus for in situ measurement of reducing agent concentration and other parameters that automatically regenerates the electrodes after the measurement.
Still another object is to provide an electrode which can be regenerated in situ to provide a reproducible surface on the electrode for use in making repetitive measurements in an electroless plating bath.