CONTROL OF ELECTROLESS PLATING BATHS

Abstract: A method for analyzing and controlling an electroless plating bath solution is claimed, said method allowing analysis and control of the bath constituents without taking samples, directly within the working solution and without time delay. Analysis and control are per­ formed fully automatical.

Description: This invention relates to the control of plating bath solutions. Although electroless copper plating is primarily referred to in the specification, the invention is also applicable to other types of plating. In the printed circuit industry, copper is generally used as an interconnection medium on a substrate. In certain applications, the deposit is practically 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 formation 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 be best achieved with an electroless plating 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, because 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 electroless copper plating baths. Generally, the measurement step in these methods require 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 it may have significantly changed 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 in an electroless copper plating bath is described in US-A-4 096 301; US-A-4 276 332; and US-A-4 310 563. These methods require 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 actual measurement. The measurement steps and the cleaning of the receptacle to avoid contamination in the next cycle can take up to 30 minutes. By the time the bath adjustment is initiated, the bath may have substantially changed state and, therefore, is not correctly adjusted. JP-A-53009233 discloses a method for adjusting of an electroless plating bath by periodically altering an electric potential of the working electrode on the basis of the reference electrode to reduction-precipitate copper ion on the working electrode, and measuring an electric current passing between the working electrode and the opposite electrode to adjust a concentration of the non-electrolytic plating solution, the method comprising adjusting an electrical potential of the working electrode to be an electric potential for carrying out anodic dissolution of the reduction-precipitated copper, and adding a complexing agent. In "Instrumentation and Control of Electroless Copper Plating Solutions" by Tucker, Design & Finishing of Printed Wiring and Hybrid Circuits Symposium, American Electroplaters Soc., 1976, is described a method of control of electroless copper plating solutions. 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 a 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, as described by Okinaka, Turner, Volowodiuk and Graham in the Electrochemical Society Extended Abstracts, Vol. 76-2, 1976, Abstract No. 275. The therein described 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. US-A 4 350 717 describes a colorimetric method for measurement. A sample of the bath is drawn, diluted with reagent, heated to develop 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 Date in the Vicinity of the Mixed Potential", Journal of the Electrochemical Soc., Vol. 126, No. 12, 12/1979, the authors, Paunovic and Vitkavage, describe in situ measurement of the plating rate of a bath. In US-A 4,331,699 a similar process for in situ measurement of the plating rate is described. A chronopotentiometric method for determining formaldehyde and copper is referred to in the Journal of the Electrochemical Soc., Vol. 127, No. 2 2/1980. However, these disclosures refer to measurement of specific variables and do not disclose 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 the 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 inspection proved to be inadequate. 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 copper 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. It is an object of the invention to provide a method for controlling an electroless plating bath that provides for substantially real time control. Another object of the invention is to provide a method for controlling an electroless plating bath including in situ monitoring, digital measurement, and real time control. Still another object of the invention is to provide a method of control that can continuously determine the quality of deposited metal from the plating bath to consistently produce good quality, crack-free copper deposits. A still further object of the invention is to provide in situ measurements and control of the concentration of the stabilizer, the reducing agent and other parameters. Still another object is to provide a method for in situ regeneration of one or more electrodes to provide a reproducible surface on the electrode(s) for use in making repetitive measurements in an electroless plating bath. The method of the invention for analyzing an electroless plating bath solution comprising a metallic ion and a reducing agent for said metallic ion comprises the steps of (a) providing at least two electrodes in the plating bath solution; (b) performing an electrochemical analysis of at least one consituent of the plating bath solution using said electrodes; and (c) providing a reproducible surface on at least one of said electrodes after said analysis by electrochemical stripping and resurfacing in the plating bath solution to prepare for the next analysis cycle. The invention provides for a real time control of an electroless plating bath solution, in particular, an electroless copper plating bath solution wherein the main constituents are copper sulfate, complexing agent, formaldehyde, a hydroxide and a stabilizer. With the invention, all of the necessary constituen t concentrations and particularly the reducing agent, e.g., formaldehyde, concentration, are measured in situ and used to control the composition of the bath. A control cycle of less than one minute is required and, hence, real time control is achieved. The in situ measurements also provide quality indicia of the copper quality factors which are likewise used to control the composition of the bath. Data from the in situ measurements is fed to a computer which, in turn, controls additions to the bath to maintain a bath composition which provides good quality, electrolessly formed copper plating. In accordance with the invention, it has been discovered that the reducing agent, e.g., formaldehyde, concentration can be measured in situ in a matter of seconds by sweeping a potential across a pair of electrodes covering a predetermined range. The potential sweep drives the oxidation reaction of the reducing agent on the surface of the electrode. The oxidation current rises with the potential to peak current. The peak current measured over the range is a function of the reducing agent concentration. The potential that corresponds to the peak current also provides an indication of the stabilizer concentration. In accordance with the invention, it has also been discovered that application of a sweep potential across the electrodes covering a range going from cathodic to anodic produces data indicating the coper quality. If the intrinsic anodic reaction rate exceeds the intrinsic cathodic reaction rate, i.e., the ratio exceeds 1.0, copper plating quality is bordering on unacceptable. A ratio of 1.1 or greater for any significant length of time indicates unacceptable copper quality. In addition to measuring reducing agent concentration and intrinsic reaction rates, the sweep potential also can be used to measure copper concentration and other parameters. Other sensors also can be used to measure copper concentration, pH, temperature, and, where useful, specific gravity, cyanide concentration and other specific concentrations. The measured values are compared with set points for the particular bath formulation and additions to the bath are controlled in accordance with the extent of departure from the set points. For high quality copper plating, the quality index (ratio of intrinsic anodic reaction rate to intrinsic cathodic reaction rate) should be about 1.0 or less. If the quality index is only slightly above said range, i.e., 1.0 to 1.05, according to a preferred method of bath control according to the invention, the system adjusts the bath composition be altering certain set points. Normally, decreases in the formaldehyde concentration and/or increase in the copper concentration improves the intrinsic rate ratio and ensures adequate copper plating quality. If, after a number of iterations, the situation has not been corrected with the ratio returned to a value of or below 1.0, or if the quality index exceeds 1.05, water is added to the bath to overflow the system so the the dilution of reaction by-products and other contaminants, and replenishment of the bath constituents provides an improved bath composition. Alternatively, if the bath includes filtering equipment, the flow through the filter can be increased to purify the solution. If the quality index exceeds 1.1, any work in process should be removed and the bath shut down for treatment or discarded. Momentary excursions above 1.1 can be tolerated and good quality copper plating resumed if corrective action is taken to reduce the index to acceptable values. Although set point adjustment, water overflow and filtering control are used in combination in a preferred control method, they can be used individually to provide effective control. The electrodes used with the system according to the invention are periodically regenerated, preferably after each measuring cycle, in order to achieve a virginal reconstructed surface in situ, for real time, continuous measurement control. This is achieved by first applying a large stripping pulse capable of deplating the test electrode to remove all copper and other reaction by-products and then by permitting that electrode to replate in the bath to resurface the electrode with a clean copper coating. The electrode may be repeated either at the electroless plating potential or at an applied potential. The regenerated electrode is used as the test electrode in making measurements. The regenerated electrode eliminates problems associated with regeneration outside the bath and problems associated with the dropping mercury electrode regeneration technique. The method of the invention for analyzing an electroless plating bath solution comprising a metallic ion and a reducing agent for said metallic ion comprises (A) providing at least two electrodes in the plating solution; (B) performing an electrochemical analysis of at least one constituent of the plating solution comprising the steps of (a) applying a sweep potential to said electrodes, (b) monitoring current flow through said electrodes caused by said applied sweep potential, (c) determining the peak current flow caused by said applied sweep potential and calculating the concentration of at least one of said constituents of the plating bath solution as a function of said peak current, and (d) determining intrinsic cathodic reaction rate and intrinsic anodic reaction rate from the sweep potential and current flow data; (C) periodically providing a reproducible surface on at least one of said electrodes to prepare for the next analytical cycle by firstly applying a stripping pulse to the electrode, said pulse being capable or removing all deposited metal and other reaction products from the electrode and secondly resurfacing the electrode with a clean metal deposit by permitting the electrode to replate in the electroless plating solution. Fig .1 is a schematic illustration showing the overall process control including the various measurement sensors and the control of chemical additions to the plating bath solution. Fig, 2A is a set of voltage and current curves during a potential sweep from zero to 200 mV. Fig. 2B is a set of voltage and current curves during a potential sweep from -40 mV to +40 mV. Fig. 3A is a flow diagram for the overall computer program. Figs. 3B, 3C and 3D are flow diagrams for various program subroutines. Fig. 4 is a potential profile for a typical measurement cycle. Fig. 1 illustrates the invention used to control an electroless copper plating bath 4 where the principal constituents of the solution are copper sulfate (CuSO₄), complexing agent, formaldehyde (HCOH, a hydroxide such as sodium or potassium hydroxide (NaOH or KOH) and a stabilizer such as a sodium cyanide. A suitable electroless copper plating bath for the present invention includes one with a stabilizer system using both vanadium abd cyanide addition agents. The formulation is as follows: Copper Sulfate 0.028 moles/l EDTA 0.075 moles/l Formaldehyde 0.050 moles/l pH (at 25°C) 11.55 (HCHO)(OH⁻) 0.5 0.0030 Surfactant (nonylphenylpolyethoxyphosphate) 0.04 g/l Vanadium pentoxide 0.0015 g/l Sodium cyanide -105 mV vs. SCE Specific gravity (at 25°C) 1.090 Operating temperature 75°C For additional details concerning other suitable bath formulations, compare co-pending application EP-A-265 895. An electroless metal plating bath solution includes a source of metal ion and a reducing agent for the metal ions. The reducing agent oxidizes on a catalytic surface and provides electrons on the surface. These electrons, in turn, reduce the metal ion to form a metal plating on the surface. Thus, in electroless plating, there are two half reactions, one in which the reducing agent is oxidized to produce the electrons and the other in which the electrons reduce the metal ions to plate out the metal. In an electroless copper plating solution, such as indicated in Fig. 1, one of the half reactions is the reaction of a formaldehyde reducing agent (HCOH) in an alkaline solution (NaOH) to produce electrons on sites catalytic to the oxidation reaction. This reaction is referred to as the 'anodic reaction' and takes place on catalytic conductive surfaces such as copper and certain other metals. The other half reaction, reducing the copper ion to plate out copper metal, is referred to as the 'cathodic reaction'. At steady state, the anodic reaction rate is equal to and opposite to the cathodic reaction rate. The potential at which both, the anodic and the cathodic half reactions proceed without any external potential being applied is the 'mixed potential' of the plating solution, referred to herein as E mix . When an external potential is supplied, e.g., from a power supply to the surface of an electrode, the steady state is disturbed. If the electrode surface potential is positive relative to E mix , then the anodic reaction rate increases whereas, if the electrode surface potential is negative, the cathodic reaction rate increases. The intrinsic anodic reaction rate, R a , is measured on the surface of an electrode where the potential is slightly more positive than the mixed potential of the solution. Similarly, the intrinsic cathodic reaction rate, R c , is measured on an electrode surface slightly more negative than the mixed potential. In the Fig. 1 embodiment a sensor is placed in the bath. A counter electrode 10, a test electrode 11 and a reference electrode 8 are utilized to measure the formaldehyde, copper, stabilizer concentration, plating rate and the quality of the plated copper. A pH sensing electrode 14 is used to measure the pH, a cyanide sensing electrode 15 is used to measure the cyanide concentration, and the temperature of the bath is measured using a temperature sensing probe 16. The copper concentration also can be measured in situ utilizing a fiber optic spectrophotometric sensor 17. Specific gravity of the bath solution is measured by a probe 18. Preferably, these sensors are configured within a common bracket which is placed in the bath. The bracket allows for easy insertion and removal of the sensors and probes. The potential E mix is measured using a calomel or a silver/silver chloride electrode as reference electrode 8 in combination with a platinum test electrode 11 with an electroless copper coating developed in the bath. The electrodes develop the mixed potential of the solution in about 5 s. An analog to digital (A/D) converter 26 is connected to electrodes 8 and 11 to sense the potential E mix and to provide a corresponding digital indication thereof. The intrinsic anodic reaction rate and the intrinsic cathodic reaction rate and thus quality of copper, plating rate, formaldehyde, copper and stabilizer concentrations, or some selected group of these parameters, are measured using electrodes 8, 10 and 11. Electrodes 10 and 11 are platinum and, as previously mentioned, electrode 8 is a reference electrode such as a silver/silver chloride electrode (SCE). A variable power supply 20 is connected to apply a potential difference E between electrodes 10 and 11. A resistor 22 is connected in series with electrode 11 and is used to measure current I through the circuit. When the electrodes are placed in the bath, the plating bath solution completes the electrical circuit and the current flow I for the circuit passes through resistor 22. Power supply 20 is controlled to apply a potential sweep to the electrodes which drives the reaction on the surface of test electrode 11 anodic so as to measure the reducing agent concentration by driving the potential through the region of oxidation for that reducing agent. For accuracy, the potential sweep should begin at the mixed potential. At the start of the measurement sequence (after an initial equilibration period), the test electrode 11 is driven anodic by the power supply, i.e., the applied potential difference is positive at test electrode 11 and negative at counter electrode 10. The current I passing through resistor 22 is measured by measuring the potential drop across the resistor and converting to a digital value by means of an analog to digital (A/D) converter 24. The test electrode 11 is driven increasingly more anodic until a peak in the current response is reached. For formaldehyde, the sweep potential as measured by A/D converter 26 is increased at a 100 mV/s. rate for about two seconds, as shown in Fig. 2A. The current and potential data from converters 24 and 26 are recorded during application of the sweep potential. As shown in Fig. 2A, the current reaches a peak value, I peak , which is a function of formaldehyde concentration. The formaldehyde concentration is calculated utilizing the following equation: (HCHO) = I peak K₁/(T k (OH) 0.5 ) where I peak is the peak current T k is the temperature of the bath in degrees Kelvin, (OH) 0.5 is the square root of the hydroxide concentration value, and K₁ is a calibration constant. The temperature T k is provided by sensor 16 and the hydroxide concentration is derived from the measurement provided by pH sensor 14. The calibration constant is empirically determined based on comparison with known values of formaldehyde concentration. The circuit including test electrode 11 and counter electrode 10, resistor 22 and power supply 20, is used to measure the plating rate of the bath as well as the intrinsic reation rates. A potential is applied to electrodes 11 and 10 to initially lower the potential of test electrode 11 (relative to reference electrode 8) so that the potential V is negative 40 mV as measured by converter 26. The potential is then changed in the positive direction to provide a potential sweep at the rate of 10 mV/s for 8 s. Thus, as shown in Fig. 2B, the potential sweeps from -40mV to +40mV. During this period, the potential drop across the resistor is measured representing the current I. The values of V and I are recorded during the sweep. The copper plating rate can be calculated from this data using the equations explained by Paunovic and Vitkavage in their Journal of the Electrochemical Soc. article mentioned hereinbefore. The range of -40 mV to +40 mV is preferred, but other ranges can be used. Generally, larger ranges provide a larger error indication caused by deviations from linearity whereas smaller ranges permit more accurate determination of the zero cross over point at E mix . To determine the plating rate and intrinsic reaction rates, the incremental values for E m , relative to the bath potential E mix , are first converted to E j values according to the equation: wherein V j is the absolute value of the incremental voltage relative to E mix , where b a is the Tafel slope of the anodic reaction and b c is the Tafel slope of the cathodic reaction. For the bath composition described herein, the value of b a = 840 and the value of b c = 310. The deposition rate can then be calculated using the equation: The copper plating quality index 'Q' is determined by comparing the intrinsic reactions for the anodic potential values (positive potential region in Fig. 2B) and the cathodic potential values (negative potential region in Fig. 2B) and, thus, for the anodic and cathodic reactions. If the ratio 'Q' of the intrinsic anodic reaction rate to the intrinsic cathodic reaction rate is about 1.0, the quality of the deposited copper will be adequate to pass the thermal shock test of exposure to molten solder at 288°C for 10 s. The ratio can be as high as 1.1 and still satisfactory quality electroless copper plating is produced. In Fig. 2B are illustrated the current responses from the input of the -40 mV to +40mV potential sweep. For purposes of illustration, responses from three different solutions are shown. All three solutions are depicted with the same anodic response, but three different cathodic responses. The quality ratios of the three different cathodic responses to the anodic responses in Fig. 2B are 1.23, 1.02 and 0.85. As can be seen in the Figure, lower curve, the cathodic and anodic reaction rates may vary and result in poor copper quality. If the anodic reaction produces too many electrons, copper deposits too rapidly and the copper atoms have insufficient time to find their correct location in the crystal lattice. If the copper quality index Q is below 1.0, high quality copper crystals are formed. If Q is in the range of 1.0 to 1.05, good crystals are formed but moderate corrective action should be taken to reduce Q; if Q is in the range of 1.05 to 1.1, stronger corrective action should be taken; and if Q exceeds 1.1, the work in process should be removed and the plating process should be shut down. Thus, for adequate copper quality, Q must be below 1.1, preferably below 1.05, and most preferably below 1.0. For illustrative purposes, Q for the electrodes copper plating bath formulation described above has been measured as 0.89. The copper concentration can conveniently be determined by measuring optical absorption by copper in the solution. This may be accomplished using a pair of fiber optic light conductors 17 placed in the bath to measure copper concentration. The ends of the conductors are placed facing each other with a premeasured space between the ends. A light beam is transmitted through one of the fiber optic conductors, through the plating solution and then through the other fiber conductor. A spectrophotometer is used to measure the intensity of the beam emerging from the conductors at the copper absorbing wavelength. As the copper ion concentration in the solution increases, more light is absorbed. The copper concentration of the bath can therefore be established as a function of measured light absorption. In an alternative embodiment, copper is analyzed by a cyclic voltammetric method similar to that used to analyze the formaldehyde. A potential sweep moving in the negative direction from E mix is applied to the measuring electrode. The negative peak obtained is proportional to the copper concentration. Referring to Fig. 4, when this electrochemical copper analysis is used, the negative moving potential sweep for copper analysis takes place after measuring the plating ratio and before regenerating the electrode surface. Preferably, the electrode surface is regenerated before measuring the formaldehyde current and also is regenerated before measuring the copper peak current. A measure for the specific gravity of the bath also is desirable since an excessively high specific gravity is an indication that the bath is plating improperly. If the specific gravity is in excess of a desired set point, water is added to the plating bath soution to bring the specific gravity back to allowable limits. The specific gravity may be measured by various known techniques, for example, as a function of the light index of refraction. A probe 18 in the form of a triangular compartment with transparent sides may be placed in the bath such that the plating bath solution flows through the center of the compartment. A beam of light, other than red, is refracted by the bath solution. The specific gravity of the bath is proportional to the degree of refraction which can be measured by a series of detectors in a linear array located outside the transparent triangular compartment. If a cyanide stabilizer compound is used, a probe 15 for measuring cyanide concentration in the plating bath can usefully be included. This probe involves reading the potential difference between a selective ion electrode and a reference electrode (Ag/AgCl). This potential increases with temperature so that a correction is needed to compensate for temperature. The test electrode 11 is periodically regenerated in order to achieve a reproducible reference surface for continuous in situ measurements. After completion of each measurement cycle, the test electrode is preferably regenerated to prepare for the next cycle of measurements. A substantial potential, e.g., +500 mV above the mixed potential is supplied by power supply 20 for at least about 45 s, and preferably longer, to strip the electrode of copper and oxidation by-products generated by the previous measurements. In the stripped condition, electrode 11 is restored to a clean platinum surface. Since the electrode is in an electroless plating bath, copper plates onto the electrode surface after the stripping pulse ceases. About 5 s are adequate to resurface the electrode with copper in preparation for a new measurement cycle. This capability to regenerate the electrode in situ is important because it eliminates the need for time consuming removal of the electrodes from the bath in order to clean or regenerate their surfaces and is thus a prerequisite to real time control of the bath. Fig. 4 shows a voltage profile for a repetitive measurement cycle. For the first 5 s, no potential is applied to the electrodes. During this period, the electrodes are permitted to electrically float and equilibrate in the bath solution to assume the mixed potential E mix which is measured and recorded. Next, a positive sweep potential 120 is applied for 2 s (from t=5 to t=7) increasing the measured potential V to about 200 mV above E mix . This sweep provides data for determining the formaldehyde and stabilizer concentrations. Next, the electrodes again are permitted to electrically float or equilibrate for about 5 seconds (from t=7 to t=12) to again assume the mixed potential E mix . Next, a negative potential is applied (at t=12) followed by an 8 s positive sweep 122 (from t=12 to t=20) passing through E mix at about its midpoint. This sweep provides data for determining the intrinsic anodic and cathodic reaction rates, the copper quality index and the copper plating rate. To complete the cycle, a large positive stripping pulse 124 (500 mV above E mix for about 40 s) is applied to strip the platinum electrode of copper and other reaction by-products. During the initial 5 s of the next cycle, prior to application of the first potential sweep, the electroless plating solution resurfaces the test electrode with a clean copper coating. The overall cycle is about 1 min, but could be shorter if desired. The voltage profile can be tried. For example, the first and second voltage sweeps can be interchanged in time. Also, the potential sweeps may be combined into a single sweep going, for example, from -40 mV to +200 mV. Each cycle, however, should include a large stripping pulse followed by a period which permits resurfacing of the test electrode. In an alternative procedure using only the first voltage sweep, the intrinsic anodic reaction rate and the intrinsic cathodic reaction rate are calculated. The second voltage sweep is omitted. Instead of determining concentration of the reactants in order to replenish the solution, replenishments of the reducing agent, formaldehyde and/or the metal ion, copper, are made automatically, in order to maintain constant intrinsic reaction rates. When the second voltage cycle is omitted, the regenerated electrode surface can be reused for 10 to 15 sweep cycles before regenerating the electrode again. Another test voltage profile which can be used in analyzing an electroless copper test solution is a truncated triangular wave which starts at a cathodic voltage of approximately -735 mV vs. SCE. The voltage is increased at a rate of 25 mV/s for 2.3 s until it reaches -160 mV vs. SCE. The current recorded during this portion of the test voltage profile is used to calculate both the quality index and the formaldehyde concentration. The currents between -30 mV vs. E mix and E mix are used to calculate the intrinsic cathodic reaction rate. The currents from E mix to +30 mV vs. E mix are used to calculate the intrinsic anodic reaction rate. Formaldehyde concentration is determined from the peak current during the sweep. At -160 mV, copper is dissolved from the electrode. The voltage is held at -160 mV until the copper stripping current drops indicating that all the copper has been stripped from the electrode. The voltage is then swept in a negative direction at optionally -25 mV/s. until it reaches -735 mV vs. SCE. The voltage is held at -735 mV until the current rises indicating the electrode has been resurfaces with a fresh copper layer and is ready for a new cycle. The potential profile and the magnitudes of the applied potential depend on the type of plating solution. For example, in an electroless nickel plating solution comprising nickel ions and sodium hypophosphite (NaH₂PO₂) would use a similar voltage profile but corresponding to the reaction rates of the hypophosphite. Different constituents, particularly different reducing agents, in the bath require adjustments in the magnitudes of the applied potentials. Among the reducing agents that are suitable for the reduction of copper ions are formaldehyde and formaldehyde compounds such as formaldehyde bisulfite, paraformaldehyde, and trioxane, and boron hydrides such as boranes and borohydrides such as alkali metal borohydrides. Although a three electrode system including electrodes 8, 10 and 11 is shown in Fig. 1 and described hereinbefore, similar results can be achieved using two electrodes. The reference electrode 8 can be omitted if the remaining electrode 10 is made sufficiently large that current flow through the electrode does not significantly change the surface potential. The composition and operation of the plating solution is controlled by digital computer 30. The computer 30 receives information from sensors 8-18. The computer also controls power supply 20 in turn to control the potential supply to electrodes 10 and 11 so as to provide the required sweep potentials, stripping pulses and equilibration intervals. During a potential sweep, the values of I and V are measured via converters 24 and 26, and the incremental measured values are stored for later analysis. Computer 30 also controls valves 40 to 44 which control additions to the bath. In the example shown in Fig. 1, the valves respectively control the addition of copper sulfate, formaldehyde, sodium cyanide, sodium hydroxide, and water to the plating bath. Valves 40-44 are preferably of the open/shut type where the volume of chemical addition is controlled by controlling the duration of the interval during which the valve is open. In a typical operating cycle, the computer obtains information from the various sensors, analyzes the data and then opens the respective valves for predetermined time intervals to thereby provide correct quantity of chemical addition required in the bath. The computer can also provide various output indications such as a display 46 of the E mix value, a display 48 indicating the plating rate, and a display 49 indicating the copper quality. An indication of E mix is desirable since departure from the normal range indicates improper operation of the plating bath. An indication of the plating rate is desirable so the operator can determine the proper length of time required to achieve desired plating thickness. The copper quality indication is, of course, important to assure proper operation free from crack or other defects. The program for computer 30 is illustrated in flow diagram from in Figs. 3A-3D. Fig. 3A illustrates the overall computer program including a data acquisition sub-routine 50 followed by data analysis sub-routine 52 which, in turn, is followed by an addition control sub-routine 54. Preferably, the control system operates in regular cycles of approximately 1 min as indicated in Fig. 4. During one cycle, data is acquired an analyzed and the results used to control additions to the bath. A clock is used to time the cycle, and a clock reset 56 is used to initiate a new cycle after completion of the 1 min. cycle interval. The flow diagram for the data acquisition sub-routine 50 is shown in Fig. 3B. At the start of the sub-routine, a time delay 60 is provided for approximately 5 seconds so that 10 and 11 can equilibrate to the plating solution potential. After 5 s delay, the computer reads the potential E mix obtained via A/D converter 26 (Fig. 1) and stores this value in step 62. The computer next operates in a loop which provides the first voltage sweep (sweep 120, Fig. 4) for the C and T electrodes 10 and 11. Initially, the computer sets power supply 20 at a zero output voltage by seting E ps =0 in step 64. The power supply voltage is incremented in step 65. The value of V received from A/D converter 26 and the value of current I through resistor 22 obtained via A/D converter 24 are recorded in the computer memory in step 66. In decision 67, the computer next checks to determine if the value of V has reached 200 and, if not, returns to step 65 after a suitable time delay in step 68. The computer continues in loop 65-68, incrementally increasing the power supply output until such time as decision 67 determines that the value has reached E m = 200. The time delay in step 68 is adjusted so that the voltage sweep from zero to 200 mV takes approximately 2 s. After decision 67 has determined that the first sweep potential has reached its maximum value, the program progresses to step 70 during which the computer reads and stores values from pH probe 14, temperature T k probe 16, copper concentration probe 17, cyanide concentration probe 15 and specific gravity probe 18. The measured values all are stored at appropriate locations in the computer memory. In step 71, the program provides for a 5 seconds delay for the electrodes to equilibrate prior to the second voltage sweep. The program next progresses through another loop which provides the second potential sweep (sweep 122, Fig. 4) to electrodes 10 and 11 through suitable control of power supply 20. In step 72, the power supply is set so that the initial value of V = -40 mV. The first step in the loop is to increment the value of E ps and then to read and store the values of potential V and current I in steps 74 and 76. In decision 77, the program determines whether the end point value V = +40 mV has been reached and, if not, the program returns to step 74 after a time delay provided for in step 78. As the program progresses through the loop 74-78, the power supply voltage increases from the initial value of -40 mV to the final value V = +40 mV. The time delay in step 78 is adjusted so that the voltage sweep from -40 mV to +40 mV takes approximately 8 s. A determination that V is equal to +40 mV in decision 77 indicates completion of the data acquisition procedure. Before ending the subroutine, however, the program sets the power supply to 500 mV to start the stripping pulse (pulse 124, Fig. 4) which continues during the data analysis and addition control sub-routines. The flow diagram for the data analysis sub-routine 52 is shown in Fig. 3C. In step 80, the computer first analyzes the data in a first data array which is the data acquired during the first potential sweep applied to electrodes 10 and 11 (i.e., steps 65-68). The data is analyzed to determine the highest current value I peak and the corresponding voltage E m . The peak current value can be determined using a simple program whereby the initial value of current is placed in the accumulator and compared with each of the subsequent values. If the subsequent value is greater than the value in the accumulator, then the subsequent value is substituted for the accumulator value. At the completion of the comparisons, the value in the accumulator will be the largest value I peak of current in the data array. The corresponding voltage is E peak . In step 82, the computer next determines the formaldehyde concentration using the equation: FC = I peak K / (T k (OH) 0.5 ) I peak is the value determined in step 80, T k is the temperature value from probe 16 and (OH) is determined in the pH measurement from probe 14. The constant K is determined empirically from laboratory bench work. In step 84, the data is analyzed from the second data array which was acquired during the second voltage sweep from -40 mV to +40 mV (i.e., steps 74-78). The first step is to determine the E j values according to the equation: wherein V j is the absolute value of the incremental voltage relative to E mix , b a is the anodic reaction rate and b c is the cathodic reaction rate. The plating rate P can be determined in step 86 using the equation: For the overall plating rate for the process, the summations cover the entire range from -40 mV to +40 mV. For determining the copper quality index, Q, the intrinsic anodic reaction rate R a is determined over the range from zero to +40 mV in step 88 whereas the intrinsic cathodic reaction rate R c is determined over the range from -40 mV to zero in step 90. Thus, the equations for R a and R c are as follows: As shown in Fig. 2B, lower curve, the reaction rate in the anodic region remains fairly constant whereas the reaction rate in the cathodic region can vary. The copper quality index Q is calculated in step 92 and is the ratio of R a to R c . A copper quality index Q greater than 1.0 is undesirable and requires correction. A quality index Q greater than 1.1 normally requires shut down of the bath. The computer can also determine the stabilizer concentration which is a function of the voltage E peak . Thus, if compared with a standard reference peak value of E s , the stabilizer concentration SC can be determined in step 94 from the following equation: SC = (E s - E peak ) K Further analysis of the data is possible, but steps 80-94 provide the analysis found most useful in controlling the plating process and in displaying status indicators. The flow diagram for the additions control sub-routine 54 is shown in Fig. 3D. The additions control is achieved by comparing the various measured concentrations and quality indexes with corresponding set points. The valves 40-44 then are controlled to add chemicals to the bath in accordance with the departures from the set points. In step 100, the program first analyzes the copper quality index Q to determine if Q is in the range from 1.0 and 1.05. This is the range where mild bath adjustment is indicated which can normally be achieved by adjusting the set points for copper and formaldehyde. In step 100, if Q is in the range of 1.0 and 1.05, the copper concentration set point CC set is incremented or increased and the formaldehyde concentration set point FC set is decremented or decreased. It also may be desirable to keep track of the number of such adjustments since,if the quality index Q does not drop below 1.0 after three iterations, more drastic corrective action may be required. In step 102, the program determines if the copper quality index Q exceeds 1.05. If so, the system opens valve 44 to add water to the bath. The water addition dilutes the bath which then is replenished by the addition of new chemicals as the system re-establishes the concentration set point values. In step 104 the copper concentration CC is compared to to copper concentration set point CC set , and valve 40 is opened for a time period corresponding to the degree of departure from the set point. In step 106, the formaldehyde concentration FC is compared with the formaldehyde concentration set point FC set and valve 41 is opened for a time period corresponding to the degree of departure from the set point value. In step 108, the stabilizer concentration SC is compared with the stabilizer concentration set point SC set and valve 42 is opened for a period of time corresponding to the degree of departure from the set point value. Likewise, in step 110, the hydroxyl concentration OH is compared with the hydroxyl set point OH set to control the opened interval for valve 43. Thus, additions of the basic chemicals to the bath are controlled in steps 104-110 in accordance with the degree of departure of the actual concentrations from their respective set point values. After completion of the valve setting, the computer in step 112, awaits the clock reset in step 56 to set the power supply voltage to zero to thereby terminate the stripping pulse. The test electrode 11 is thereafter resurfaced during the five seconds interval provided by time delay 60. Although a specific computer program has been described according to the invention, there are numerous modifications that may be made without departing from the scope of the invention.

Claims: A method of analyzing a solution used for electrolessly depositing metal, said solution comprising a metallic ion and a reducing agent for said metallic ion, said method comprising: (A) providing at least two electrodes in the plating solution; (B) performing an electrochemical analysis of at least one constituent of the plating solution , said analysis comprising the steps of (a) applying a sweep potential to said electrodes, (b) monitoring current flow through said electrodes caused by said applied sweep potential, (c) determining the peak current flow caused by said applied sweep potential and calculating the concentration of at least one of said constituents of the plating bath solution as a function of said peak current, and (d) determining intrinsic cathodic reaction rate and intrinsic anodic reaction rate from the sweep potential and current flow data;. (C) periodically providing a reproducible surface on at least one of said electrodes to prepare for the next analytical cycle by firstly applying a stripping pulse to the electrode, said pulse being capable of removing all deposited metal and other reaction products from the electrode and secondly resurfacing the electrode with a clean metal deposit by permitting the electrode to replate in the electroless plating solution. The method of claims 1 wherein step (C) is performed after each analysis cycle. The method of claim 1 or 2 wherein the concentration of said reducing agent and/or said metallic ion is calculated from the respective peak current. The method of claim 1 to 3 wherein said metallic ion is copper ion, and said reducing agent is selected from formaldehyde, formaldehyde compounds and boron hydrides. The method of claim 4 wherein said analysis determines the concentrating of said formaldehyde, formaldehyde compound or boron hydride. The method of claim 4 wherein said analysis determines the concentration of the copper ion present in the bath solution. The method of claims 1 to 3 wherein the metallic ion is nickel ion and the reducing agent is hypophosphite. The method of Claims 1 to 7 used to control in real time the plating solution wherein the addition of one or more constituents of the plating solution is controlled according to said electrochemical analysis. The method of claims 1 to 8 wherein one of the electrodes is a test electrode and an other of the electrodes is the counter electrode and the test and the counter electrode are provided with a surface layer composed of the metal of said metallic ion present in the bath solution. The method of claims 1 to 9 comprising placing a counter electrode, a test electrode and a reference electrode in the plating solution; monitoring the solution potential between said reference electrode and said test electrode; applying a sweep potential to said test and counter electrodes; monitoring the current flow through said test and counter electrodes; and determining the peak current during application of said sweep potential. The method of claim 10 wherein the concentration of a stabilizer present in the bath solution is determined as a function of the bath potential at said peak current. The method of claim 11 further including controlling the addition of stabilizer to the bath solution in accordance with the departure is said stabilizer concentration from the desired concentration. The method of claims 1 to 12 wherein temperature and pH of the bath are included in the calculation of the concentration of one or more bath constituents. The method of claim 1 to 13 wherein a metal plating quality index as the ratio between the intrinsic anodic reaction rate and the intrinsic cathodic reaction rate is determined. The method of Claim 14 wherein the composition of the plating bath solution is controlled by periodic additions of copper ion and/or formaldehyde according to concentration departure from respective set points, and wherein the set point for the copper concentration is increased or the set point for the formaldehyde concentration is decreased as said ratio approaches 1.1. The method of claim 15 wherein said set points are adjusted so that the ratio of intrinsic anodic reaction rate and intrinsic cathodic reaction rate is below 1.05. The method of claim 16 wherein said set points are adjusted so that said ratio is below 1.00.

Inventors: CHRISTIAN, STEPHEN M., DUFFY, JOHN K., MCCORMACK, JOHN F., PAUNOVIC, MILAN
Applicants: AMP-AKZO CORPORATION
Application year: 1987
Main classification: C23C18
IPC codes: C23C18, G01N27