GB1588758A

GB1588758A - Method and apparatus for control of electroless plating solutions

Info

Publication number: GB1588758A
Application number: GB48323/77A
Authority: GB
Original assignee: Kollmorgen Technologies Corp
Current assignee: Kollmorgen Technologies Corp
Priority date: 1976-11-22
Filing date: 1977-11-21
Publication date: 1981-04-29
Also published as: IL53298A0; CH637995A5; AU3076077A; JPS5365226A; CA1112523A; ZA775495B; DE2751104A1; AU512805B2; NL7712683A; FR2371522A1; FR2371522B1; ATA827777A; SE442410B; JPS5753857B2; ES464266A1; SE7713192L; DE2759952C2; AT354213B; IT1116376B; IL53298A

Abstract

A copper-plating bath operating without an external power supply can be controlled automatically by measuring, with the aid of two electrodes, of which one is positioned in bath fluid and serves as the deposition electrode and the other serves as the reference electrode, the mixed potential arising between the electrodes and employing this measurement for controlling and/or regulating one or more bath parameters.

Description

(54) METHOD AND APPARATUS FOR CONTROL OF ELECTROLESS PLATING SOLUTIONS (71) We, KOLLMORGEN TECHNOLOGIES CORPORATION of Republic National Bank Building at Dallas, Texas, United States of America, a corporation duly organized and existing under the laws of the State of Texas, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a method and apparatus for operating an electroless copper plating bath solution and specifically to a method and apparatus for operating an electroless plating bath solution by utilizing the mixed potential of the bath.

Electroless copper plating baths are basically comprised of a reducible copper salt and a reducing agent. These plating baths are designated to deposit useful copper coatings only on parts to be plated and to avoid simultaneous deposition on the sides and bottom of the plating tank and/or on areas of the parts where plating is not desired. Because electroless copper plating baths comprise a homogeneous solution of reducing agent and metal salts, it is difficult to maintain a heterogeneous reduction to the metal only on desired surfaces and to avoid homogeneous reduction which decomposes the plating solution.

Among the reducing agents useful in electroless copper plating solutions are formaldehyde, borohydrides and aminoboranes; Substances called stabilizers are added to an electroless copper plating bath solution to prevent solution decomposition and to control unwanted indiscriminated metal deposition. Many compounds have been proposed as stabilizers, including cyanide compounds, divalent sulphur compounds, polyalkylene oxides, selenium compounds and mercury compounds and oxygen.

Electroless copper plating baths also contain other additives such as sequestrants or complexing agents, ductility agents, or surfactants.

Reference is made to William Goldie "Metallic Coating of Plastics", Vol. I, Electromechanical Publications, Ltd., Middlesex, England, 1968, the disclosure of which is incorporated herein by reference.

The ductility of electrolessly deposited copper can be enhanced by having present in the bath solution an extraneous ion such as an element selected from vanadium, arsenic, bismuth, or mixtures thereof. (Patents Nos. 1.153.758 and 1.304.624) Other ductility agents such as sodium cyanide and the polyalkylene oxides are also useful and described, e.g., in patents Nos. 910.709 and 1.263.445.

The performance of an electroless copper plating bath can be improved by the addition thereto of small amounts, e.g., less than 5 g/l, of certain surfactants. Such surfactants may include organic phosphate esters and oxyethylated sodium salts, and mixtures thereof.

(Such a surfactant may be obtained under the tradename Gafac RE-610 (RTM)). (Anionic complex organic phosphate esters. Broadly speaking, in actual practice an electroless copper plating bath will contain a copper salt in solution; chemicals which act as reducing agents, freeing electrons to convert copperions to copper at sites where metal surfaces or clumps of metal atoms are present as catalysts; one or more sequestrants which hold the metal in solution by inhibiting the formation of insoluble metallic salts and for the reduction in bulk by the reducing agent present; and other components which influence the plating rate and the crystal structure of the depositing metal and hence its properties. A typical electroless copper plating bath may be comprised of a copper salt, a base, a reducing agent, a sequestrant, a ductility agent, a stabilizer, and a surfactant.A preferred solution for electroless plating of copper may be comprised as follows: (1) about 0.02 - 0.08 molar CuS04 or CuCl2; (2) NaOH or KOH in an amount such that the pH of the solution is in the range of about 11 to 14; (3) about 0.01 - 3.0 molar reducing agent such as formaldehyde or one of the hydrides, borohydries, or amine boranes; (4) about 0.02 - 0.4 molar sequestrant, or complexing agent, such as Rochelle salt, Na4EDTA, Quadrol (RTM)* (*NNN'N' tetrakis-2-hydroxypropylethylenediamine), a polyalkanol amine, or an amino acid 5 about 10-3 to 10-' g/l of a ductility agent such as NaCN; (6) about 10-') to 10-3 g/l of a modifier or stabilizer; and (7) up to about 5 g/l of a surfactant such as an alkylphenoxy polyethoxy phosphate ester.

The basic operating conditions of an electroless metal plating bath are maintained by chemical analysis and appropriate additions of chemical components. The plating bath has several variables, or parameters, that may be measured and controlled, such as pH, metal ion concentration, reducing agent concentration, ductility agent concentration, sequestering agent concentration, stabilizer concentration, temperature, plating rate, loading (total area of metal being plated as compared to the solution volume), type and degree of agitation, the rate of removal of H20 (generated by the reduction reaction), accumulation of by-products, and trace amounts of contaminants which affect the rate and quality of plating.Chemical analysis and manual additions of replenishers are possible; however, manual maintenance of optimum conditions is difficult to achieve as loading and other conditions change and, furthermore, because of the time-parallax between analysis and manual maintenance. Automatic control of some or all of the measurable parameters of an electroless metal plating bath solution is highly desirable. Many bath parameters can now be measured and controlled automatically and respective reference is made to Dr. Günther Herrmann, Galvanotechnik, 65, 950 (1974).

An electrical potential of a bath solution termed the "mixed potential" uniquely describes the condition of an electroless plating solution.

(Milan Paunovic, Plating, 68,1165 (1968).

The control of stabilizer activity or active stabilizer concentration itself is, in particular, inherently difficult because the quantities of certain stabilizers employed are too small for convenient analysis. In general, attempts to utilize the mixed potential to monitor and control bath solution stabilization have been uniformly unsuccessful because all bath parameters, not just the stabilizer concentration, influence to some degree the mixed potential of a bath. Small changes in the mixed potential may be due to variations in one or more bath parameters that would not be sufficient to affect the overall bath operation.

As the mixed potential (which is the ox-redox potential of the electroless plating bath) is influenced by a wide variety of parameters, it can, therefore, not be used alone as the controlling means for such electroless plating bath. If, however, the parameters which influence the mixed potential are controlled otherwise and kept in specified brackets, the mixed potential has been found to be well suited in controling certain parameters. This is most significantly true for such parameters which otherwise cannot or not reliably be controlled in an economical fashion. Stabilizers of the divalent sulphur group as, e.g., potassium sulfide and 2-mercaptobenzothiazole have been found by applicant to be reliably controlled by using the mixed potential.This way. the serious problems which occurred in the past by using the mixed potential in controlling electroless plating baths have been solved by using the measurements of the mixed potential in accordance with the present invention.

The difficulties which occurred by using the measurement of the mixed potential for controlling electroless plating baths in the past are demonstrated by Example A.

On the other hand, minute additions of stabilizers may completely inhibit plating and darken and discolor previously deposited metal, or may fail to prevent spontaneous decomposition of the plating solution by uninhibited, wild plating. This effect would be accompanied by wild fluctuations in mixed potential. In sum, use of the mixed potential as a control parameter for stabilizer activity has been heretofore rejected because of inability to establish a responsive relationship between the mixed potential and stabilizer activity.

Applicants have surprisingly discovered a novel method and apparatus for operating an electroless copper plating bath and avoiding decomposition thereof.

According to this invention the performance of an electroless metal plating bath solution can be efficiently controlled by the method comprising measuring the resulting potential caused by the ox-redox reactions of the electroless metal deposition process, hereinafter referred to as mixed potential, E,,i,. and adjusting one or more bath parameters to maintain the mixed potential within a predetermined range. comprising establishing the predeter mined range by measuring and adjusting a certain first set of bath parameters effectively measurable by known methods; and determining the Emix of the thus adjusted bath solution; and employing the Emix of the thus adjusted bath solution; and employing the measured Emix for determining and adjusting one or more bath parameters different from the ones of the first set of bath parameters and hereinafter referred to as "second set of bath parameters" thus operating the bath solution within predetermined and defined conditions, e.g., concentrations(s) of one or more stabilizers or other bath constituents.

The mixed potential of the bath is measured by two electrodes, a plating electrode and a reference electrode, and the stabilizer activity and/or one or more other parameters affecting the mixed potential are measured and/or adjusted to maintain the mixed potential within a predetermined range. The predetermined range will depend upon several factors such as the particular plating solution or the particular reference electrode.

The mixed potential can be measured by-using both electrodes in the bath itself. If the bath is operating at high temperatures, the reference electrode will preferably be located in a contiguous chamber.

The apparatus of this invention will be comprised of means for measuring the mixed potential as well as means for monitoring and/or controlling other factors primarily affecting the mixed potential such as pH, stabilizer activity, temperature, reducing agent activity, and copper ion concentration.

In the preferred embodiment of this invention, the temperature of an electroless copper plating bath solution is readily measured and controlled with conventional instrumentation.

Commercially available glass electrodes may be employed to measure pH. The pH sensing means, which must be suitable for high pH solutions, may be either a combination pH/reference electrode or a pH electrode -used in conjunction with a reference electrode.

Many reference electrodes are useful, particularly a standard calomel electrode (SCE) or a silver/silver chloride electrode.

It has been found preferable to reduce the temperature of the bath solution adjacent to the reference electrode below the normal plating temperature or by interposing a salt bridge, or both.

The Cu ++ ion concentration can be determined with a colorimeter which measures the absorption of light of suitable wavelength (red length). This measurement may be affected by temperature and, with some sequestrants, by pH, and is, therefore, best made under standardized conditions or with suitable corrections based on temperature or pH,-or both.

The cyanide ion activity may be measured with a commercial specific ion (cyanide ion) electrode, provided that the solution activity is reduced so that plating does not occur on the sensing electrode or the reference electrode. Since this measurement is also dependent upon the pH and temperature, measurement under standard conditions or with electrical corrections, or both, is preferred.

The formaldehyde concentration or activity may also be measured by automated chemical analysis. One method of mesurement involves (i) dilution with a known volume of liquid, (ii) addition of an excess of reagent which forms a colored species in the presence of formaldehyde, and (iii)-measurement of the color density by photometric or colorimetric means.

A second method involves addition of an amount of acid (typically sulfuric acid) proportion to a sample stream for the purpose of lowering the pH of the sample stream by a known amount. Sodium sulfite is then added in moderate excess and fixed volume and concentration. The sodium sulfite reacts with the formaldehyde causing an increase in pH proportional to the amount of formaldehyde present in the sample. The final pH is measured as the determinant of the amount of formaldehyde reacted. The amount of acid initially is set so that the final pH after reaction is at a level convenient for accurate measurement, taking into account the buffer points resulting from various chemical species in the plating solution.Since the final measurement is a pH value, it will be measured against a datum point such as the initial pH of the plating solution, the pH of the plating solution after the acid addition, or some other pH value, as would be appreciated by those skilkled in the art. Control of solution pH and electrical correction for deviation from a standard are important to formaldehyde determination by this second method.

As would also be appreciated by those skilled in the art, other bath solution reagents can be similarly measured and controlled.

The accumulation of by-products may be detected by measurement of the specific gravity of the plating solution.

The mixed potential of the bath solution is basically the electrical sum of half cell reactions, the principal ones being the oxidation of a reducing agents and the reduction of copper ions to copper. When two or more oxidation reduction reactions occur simultaneously on the same conductive surface, such as an electrode. each oxidationreduction system will strive to set up its own equilibrium with its own characteristic electrode potential. Since only one potential can exist on a condutive surface, a steady state will be reached where the electrode reaction occurs. This compromise potential is the mixed potential.

As a specific example, the equilibrium potential for 0.1M copper in a 0.175M EDTA solution at pH 12.5 was reported as -0.47V vs. SCE [Paunovic , M., Plating 55, 1167 (1968)j.

The equilibrium potential for formaldehyde was reported as -1.01V vs. SCE in the same medium.

The mixed potential of these two reactions occurring on a copper surface was the compromise potential -0.65V vs. SCE.

The mixed potential of an electroless copper plating bath is measured by use of two electrodes, a plating electrode in active plating solution and a reference electrode located in proximity to the plating electrode. The mixed potential of the solution is developed by the plating reaction, and this potential may be measured between the plating electrode and the reference electrode.

The plating electrode is a metallic electrode with a copper surface, which is continuously plated with copper by the electroless copper solution. This electrode may be made of a substrate metal such as platinum, overplated with copper. Nearly any metallic electrode immersed in an electroless copper plating solution is soon covered by copper. Many reference electrodes are useful for measuring the mixed potential, a calomel or silver/silver chloride electrode being preferred.

If a silver/silver chloride reference electrode is employed, it may be either a single or double juntion type. It would be advantageous to employ a double junction electrode to reduce the incidence of contamination of either the plating solution or the electrode solution.

There are certain practical problems associated with the measurement of mixed potential. The measurement is carried out in active solution, and in the case of some types of electroless copper plating solutions, the active solution is at an elevated temperature such as about 40 to 95"C. If under these conditions the reference electrode is adjacent to the plating electrode, metal will usually deposit on the reference electrode, rendering it inactive; if the reference electrode is placed at a distance, the measurement may be unstable. A satisfactory measurement is achieved by using a reference electrode, preferably a double junction electrode, in a separate chamber containing cooler, less agitated solution and connected by a short continuous liquid path to active solution adjacent to the plating electrode.The liquid path may vary and may typically be 5-10 cm. The length of the path is not particularly critical; however, excessive length should be avoided.

The reference electrode chamber may be thermally isolated from the plating electrode by a thermal barrier such as an air gap or a foam spacer. The double junction electrode may contain a silver/silver chloride junction in a chamber communicating via a porous plug with a second chamber containing a solution of salt which is non-reactive with the plating solution, which second chamber communicates via a second porous plug with plating solution. The plating solution in the reference electrode chamber will preferably be of reduced plating activity due to cooling and restricted circulation.

The mixed potential of a bath does not usually respond in a linear manner to changes in concentration of solution components. For example, formaldehyde may have greater effect in Quadrol sequestered solutions than in EDTA sequestered solutions. In either solution, the mixed potential increases, i.e., becomes more negative, initially as the formaldehyde concentration increases; however, the mixed potential becomes insensitive to further changes if formaldehyde is present "in excess" so diffusion rates predominate or if Cut+, for example. is not increased to sustain a faster reaction rate.

In accordance with the present invention. the mixed potential of an electroless copper plating bath is to be maintained within a predetermined rage by monitoring and/or adjusting certain parameters. The predetermined range can vary greatly and will depend upon the particular bath solution and reference electrode utilized. It has been found that for commercial operations the desired mixed potential for an electroless copper plating bath will be a value from about -9()()mV to --1.5V vs. SCE and from about -i55mV to -1.455V vs. a silver/silver chloride electrode. The mixed potential is advantageously maintained at a value from about -6()()mV to -85()mV vs. SCE and from about -555mV to 805V vs. a silver/silver chloride electrode.

Preferably, the mixed potential is maintained at a value from about -630mV to -760mV vs. SCE and from about -585mV to -715mV vs. a silver/silver chloride electrode.

As one skilled in the art would appreciate, a given value versus- a calomel electrode translates to respective specific values versus other reference electrodes. Values versus calomel and silver/silver chloride electrodes differ by 45mV, and, for example, - the value -650mV vs. SCE corresponds to -605mV vs. a silver/silver chloride electrode.

In actual practice the mixed potential will be maintained with a range centered Qn the desired value, determined for the specific solution employed, preferably from about + 5mV to + 50mV of that value, more preferably within + 10mV or + 25mV. For example, a specific EDTA-based electroless copper plating bath having a calomel reference electrode may be maintained at -675mV, + 10mV or + 25mV.

The mixed potential is the determinative or controlling variable of this invention. Other bath parameters are monitored and/or adjusted, and some of the other parameters are adjusted primarily to affect the mixed potential. For example, it has been found that in a bath where the mixed potential has become more positive then desired, e.g., has gone to about -600mV from the range of about -680mV + 20mV, the mixed potential can be brought back by raising the pH or adding reducing agent such as formaldehyde. Similarly, where the mixed potential has become more negative than desired, the incremental addition of stabilizer has returned the mixed potential to the desired range.

The apparatus of this invention monitors and/or adjusts several parameters of an electroless copper plating bath to maintain the mixed potential within a certain range. As mentioned above, virtually all bath parameters affect the mixed potential. - It is desirous from a practical point of view to monitor and/or control only those parameters having the greatest effect upon the mixed potential, parameters such as temperature, copper ion concentration, reducing agent activity, stabilizer activity, pH, etc. Advantageously the apparatus will measure the bath mixed potential and will monitor and control within predetermined ranges the temperature, pH, copper ion concentration, and reducing agent concentration. Also, the apparatus may measure the mixed potential and monitor and automatically control within predetermined rages the temperature, pH, and copper ion concentration.Preferably the apparatus will measure the mixed potential, will monitor and automatically control the temperature, copper ion concentration, and pH, and will monitor and/or adjust the stabilizer or cyanide ion ctivity and reducing agent activity to maintain the mixed potential within a certain range. In one embodiment, the apparatus will measure the mixed potential and adjust the reducing agent or the stabilizer activity, or both, to maintain the mixed potential within a predetermined range, and will monitor and automatically control the temperature, copper ion concentration, pH, and cyanide activity.

The apparatus can sense or monitor the various bath parameters in the bath itself.

However, preferably a sample stream of -active solution will be drawn from the bath and that stream will be analysed to determined the values of the desired parameters.

The parameters affecting the mixed potential can be measured by the instrumentation described above. The apparatus of the invention will be comprised of respective means according to which selected parameters are monitored and/or controlled. A device for monitoring and automatically controlling temperature, copper ion concentration, and pH will be comprised of means for measuring and controlling each of these parameters as well as means for measuring the mixed potential. As shown in Figure 1, such an apparatus is comprised of pump 1 to draw a sample stream from plating tank 2. The stream passes through valve 3 to chamber 4 containing mixed potential plating electrode 5. Adjacent to chamber 4 is another chamber, chamber 6, containing reference electrode 7, which in conjunction with the plating electrode 5 measures the mixed potential of the plating solution.Chamber 4 may also contain thermister 8. which thermistor may alternately be situated at position 9 on standpipe or degasser 10. The standpipe 10 functions to permit gas bubbles to escape from the sample stream before the stream passes to analytical sensing means.

The sample stream then passes from chamber 4 through hear exchanger 11 to a second heat exchanger 12 and then to chamber 13 having colorimeter or copper cell 14 for measuring copper ion concentration. The sample stream passes from chamber 13 to chamber 15 which has another thermister 16 and combination pH/reference electrode 17.

The sample stream returns to the first heat exchanger 11 and then to the plating tank 2. The second heat exchanger 12 is cooled by cooling water from an external source.

Another embodiment of this invention comprised of means for measuring temperature, copper uon concentration, pH. cyanide ion (or stabilizer) activity, formaldehyde activity, and accumulation of by-products. is shown in Figure 2. Here. a sample stream of plating solution is drawn from plating tank 18 by means of pump 19. The sample stream passes through valve 20 to chamber 21 containing mixed potential plating electrode 22. Adjacent to chamber 21 is chamber 23 which contains reference electrode 24. Chamber 21 ma also contain thermister 25, which may alternately be inserted at position 26 on standpipe or degasser 27.

From chamber 21 the sample stream passes successively to heat exchangers 28 and 29 and then to chamber 30, which contains copper cell 31 and cyanide ion electrode 32. The sample stream then goes to chamber 33 which contains a second thermister 34 and pH/reference electrode 35. Part of the sample stream then passes to chamber 36. Chamber 36 contains density sensor 37 for determining the accumulation of by-products. The remainder of the sample stream passes to reagent pump 38 which pumps the plating solution to a point where first sodium sulfite and then air are added, and then to mixing coil 39 where the stream is contacted with an acid. The sodium sulfite and acid are supplied from containers 40 41, respectively. The partial sample stream containing acid and sulfite passes from the mixing coil 39 to chamber 42 where its pH is measured by pH/reference electrode 43.An increase in pH will be proportional to the amount of formaldehyde present.

The partial sample stream in chamber 42 will normally pass to waste through pipe 44.

However, when the stream is compatible with the bath solution, the stream may be returned to the plating tank 18 either directly or in admisture with the stream from chamber 36.

The plating solution sample stream passes from chamber 36 to heat exchanger 28 and then back to the plating tank 18. The second heat exchanger 29 is cooled by externally supplied water.

The apparatuses in Figures 1 and 2 are also comprised by means of automatically registering and utilizing the signals generated by the sensors. Each of the sensors produces a millivolt signal which is received by a potentiometer-type device such as a digital adjustment potentiometer. This latter device will receive the signal produced, ascertain.

whether the signal falls within certain limits, and will cause an appropriate response, if warranted.

Similarly, the apparatus of this invention will in general comprise means for measuring values and means for reacting to the values measured. Typically. as above, the parameters will be sensed by an apparatus which will generate an electrical signal, preferably in millivolts. Each signal generated will in turn be sensed by a potentiometer-type device into which certain limits have been incorporated. The device will in turn cause an appropriate response if the sensed signal is outside a predetermined range to cause the parameter to return to a proper value. Such a response may, for example, be the additon of a chemical solution to the bath, the particular chemical soluton having a surplus or lack of the particular parameter. For example, if the apparatus detected a low pH, the apparatus could cause the addition of a soluton consisting essentially of alkali solution.

Also, in one embodiment of this invention the mixed potential signal may cause (i) the addition of stabilizer, preferably sulfur containing stabilizer, if the mixed potential becomes more negative than desired and/or (ii) the addition of a reducing agent such as formaldehyde if the mixed potential becomes more positive than desired.

In another embodiment of this invention where the formaldehyde is otherwise analyzed and controlled. such as by the method and apparatus in Figure 2. variations in the mixed potential may result in other responses. While a negative tending mixed potential may be used as a signal to add stabilizer. a positive tending mixed potential signal may be used as a signal to add a copper deposition accelerator or a formaldehyde oxidation catalyst.

Comparative experiment A An electroless copper plating bath comprised as follows was employed: Copper sulfate 10 g/l Tetrakis (2-hydroxypropyl) 17 g/l ethylene diamine Formaldehyde (aq. 37% soln.) 15 ml/l pH 12.8 Temperature 28"C Sodium cyanide 25 mg/l Potassium sulfide 0.4 mg/l 2-Mercaptobenzothiazole 0.04 mg/l This plating bath was operated with copper sulfate concentration controlled by a flow-through colorimeter and copper additions added by a proportional control based on the colorimeter measurements. The tetrakis (2-hydroxypropyl)-ethylenediamine was not consumed in the plating reaction. and thus a daily or weeklv analysis and addition was sufficient for control. Formaldehyde in a standard aqueous 37Cjc formaldehyde solution was added in proportion to the copper sulfate. and additional adjustments in formaldehyde were made by daily analysis. The pH was measured continuously using a glass electrode and a saturated calomel reference electrode. Sodium hydroxide in an aqueous solution was added proportionally based on the pH signal. Sodium cyanide in an aqueous solution was added in proportion to the sodium hydroxide.

Potassium sulfide and 2-mercaptobenzothiazole is aqueous solution were added as sulfur stabilizers in proportion to the elapsed tiffle and the copper area plated. The mixed potential was monitored between a grounding panel maintained in the plating bath and the saturated calomel reference electrode used for pH mesurement. The mixed potential measurement was a very unsteady signal, subject to spurious transient voltages which varied from -520 to -720mV.

The plating bath was dificult to control. Either it plated copper on the walls of the tank and sometimes spantaneously decomposed or, if enough sulfur stabilizer was added to avoid plating the tank, the deposited copper tended to become dark and passivated, preventing further deposition on the work piece.

The following examples illustrate, but do not limit, the present invention: Example 1.

The mixed potential measurement of Experiment A was modified substituting a silver/silver chloride reference electrode adjacent to the plating mixed potential electrode.

However, it was seen that when the reference electrode was immediately adjacent to the plating electrode, metal deposited on the reference electrode rendering it inactive.

The set-up was modified by using a double junction silver/silver chloride reference electrode in a separate chamber, and satisfactory measurement was achieved. The double junction reference electrode contained a cool solution and was connected by a continuous liquid path with active solution adjacent to the plating electrode. The electrodes were connected to a differential amplifier, and the signal from the amplifier was used to monitor and control the addition of a mixture of an aqueous solution containing potassium sulfide and 2-mercapto-benzothiazole in a ratio of 10:1. The mixed solution was controlled at -635+ l0mV by the additions. The plating bath operated unexpectedly well, that is, plating bright, smooth copper without passivation or spontaneous decomposition.

Example II The bath of Experiment A was operated in a tank equipped with a circulation pump. One wall of the tank was lower than the others, at which point the plating solution spilled over.

The pump circulated the solution from the bottom of the spillway back into the plating tank. The level of solution at the bottom of the spillway was 1/3 the height of the tank, allowing oxygen to be absorbed from the air and allowing hydrogen, which is a reaction by-product, to escape as the bath poured over the spillway. The mixed potential was -600mV, and the copper deposited was dark. The bath passivated after a few hours operation.

The level of the solution at the bottom of the spillway was raised, reducing the amount of oxygen absorbed or hydrogen released, until the mixed potential reached -640mV.

The level of solution at the bottom of the spillway was approximately 3/4 of the height of the tank. The mixed potential of the bath solution was then maintained at -635mV l 10mV by controlling both the height of the spillway and the sulfide additions, and the bath was operated continuously for two weeks without passivation or spontaneous decomposition.

This experiment showed that the mixed potential can be used to control oxygen and hydrogen in the bath.

Example 111 An electroless copper plating bath was prepared with the following formulation: Copper sulfate 12.5 g/l Tetrakis (2-hydroxypropyl) 15.6 g/l ethylenediamine Formaldehyde (aq. 37% soln.) 3.5 ml/l Sodium hydroxide to pH 12.95 Sodium cyanide 10 mg/l Potassium sulfide 1.0 mg/l 2-Mercaptobenzothiazole 0.1 mg/l Gafac RE-610 (RTM) 0.14 g/l Temperature 53 C Work load 2.5 sq. decimeters of plating area/liter This plating bath was operated by continuously monitoring the copper concentration with a flow-through colorimeter. Copper was added automatically in an aqueous solution of copper sulfate based on the colorimeter measurement. The pH was measured by a pH electrode and was controlled by the automatic addition of an aqueous solution of sodium hydroxide.Cyanide was replenished by adding sodium cyanide in an aqueous solution in proportion to the pH controlled sodium hydroxide addition, and formaldehyde was replenished by adding a standard aqueous 37% formaldehyde solution in proportion to the colorimetrically controlled copper addition.

The mixed potential was measured using a double juntion silver/silver chloride reference electrode as in Example II, and an aqueous solution haveing potassium sulfide and 2-mercaptobenzothiazole in a ratio of 10:1 was added automatically by a proportionally controlled pump that was to control the mixed potential at -650 to -655 mV.

This particular type of plating bath operates at elevated temperatures and thus cannot tolerate a formaldehyde concentration in excess of 5 or 6 ml/l without spontaneous decomposition. In spontaneous decomposition, the bath homogeneously reduces all the copper ion to copper powder and turns from blue (ionic copper) to a red slurry (copper dust) and finally to water white (as all the copper powder settles to the bottom of the tank).

Spontaneous decomposition is accompanied by heavy foam buildup and hydrogen evolution.

Due to an electrical failure, the formaldehyde solution valve failed to shut down, and the formaldehyde concentration inadvertently rose to 22 ml per liter. Contrary to all previous experience, the bath did not decompose but continued to operate normally because the pump for the solution of potassium sulfide and 2-mercaptobenzothiazole, in response to the mixed potential signal, added large amounts of the potassium sulfide and 2mercaptobenzothiazole stabilizers. After thus operating normally for a total of about sixteen hours, because of the subtantially increased consumption the reservoir of the solution of potassium sulfide and 2-mercaptobenzothialzole ran dry, and in consequence the bath decomposed within a half hour thus showing the usefulness of the present invention even under extreme conditions.

Example IV The bath of Experiment A was operated with automatic additions and the plating rate was monitored using chronopotentiometry. The bath was allowed to plate for a fixed period of time, twenty minutes, on a palladium electrode. After twenty minutes, the electrode was transferred to a copper sulfate solution and- anodically stripped. The time rquired to electrolytically strip the copper from the electrode at a fixed potential is a direct measure of the plating rate. The mixed potential of th bath was used to control the addition of stabilizers as in Example II. The plating rate as measured chronopotentiometrically varied directly with the fluctuation of the mixed potential.

Example V An electroless copper plating bath was comprised as follows: Copper sulfate 0.036 moles/l pH 12.9 Quadrol (RTM) 0.08 moles/l Temperature 53o C Formaldehyde 0.05 moles/l (initially) Sodium cyanide 30 mg/l (initially) Gafac RE-610 (RTM) 0.1 g/l The copper ion concentration, pH, temperature, and formaldehyde concentration were constantly monitored and appropriate additions to the bath were made automatically when so indicated. The cyanide ion concentration was measured and controlled at - 150mV using a calomel elecrode and an Orion cyanide ion electrode.

Amounts of an aqueous solution of a sulfur stabilizer, potassium polysulfide, were added whenever the mixed potential. as measured using a double junction silver/silver chloride reference electrode. became more negative than -670mV. Additional formaldehyde in a standard aqueous 37cos formaldehyde solution was added whenever the mixed potential became more positive than -640mV. The bath oprated well for about a week, plating bright, smooth uniform copper without plating the walls of the container.

Continuing chemical analysis of the bath solution showed that the formaldhyde concentration was maintained between 0.01 and 0.04 moles/l, which is sufficient for normal operation of this bath.

Example Vl An electroless copper plating bath was comprised as follows: Copper sulfate 0.036 moles/l pH 12.9 Quadrol (RTM) 0.08 moles/l Temperature 53 C Formaldehyde 0.05 moles/l-(initially) Gafac RE-610 (RTM) 0.1 g/l The copper ion concentration, pH, temperature, and formaldehyde concentration were constantly monitored and appropriate additions to the bath were made automatically when so indicated.

The mixed potential was measured by means of a silver/silver chloride reference electrode. Amounts of an aqueous solution of potassium polysulfide were added whenever the mixed potential became more negative than -670mV. Additional formaldehyde in a standard aqueous 37% formaldehyde solution was added whenever the mixed potential became more positive than -640mV. The bath operated well, plating bright, smooth, uniform copper without plating the walls of the container.

Continuing analysis of the bath solution showed that the formaldehyde concentration was maintained between 0.01 and 0.04 moles/l, which is sufficient for normal operation of this bath.

Example VII In order to show the control of formaldehyde alone, an electroless copper plating bath comprised as follows was employed: Copper sulfate 0.036 moles/l pH 12.9 Quadrol (RTM) 0.08 moles/l Temperature 53"C Formaldehyde 0.05 moles/l Gafac RE-- 610 (RTM) 0.1 g/l The copper ion concentration, pH, and temperature were constantly monitored, and appropriate additions to the bath were made automatically when so indicated. Additional formaldehyde in a standard aqueous 37% formaldehyde solution was added whenever the mixed potential became more positive than -640mV.

The bath operated well, plating bright, smooth, uniform copper without plating the walls of the container.

Example VIII In order to demonstrate the control of bubbling air using the mixed potential, an electroless copper plating bath was comprised as follows: Copper sulfate 0.04 moles/l Quadrol (RTM) 0.06 moles/l Formaldehyde 0.26 moles/l Sodium hydroxide 0.36 moles/l Sodium cyanide 0.0002 moles/l Temperature 24"C The mixed potential was controlled by bubbling air through the bath solution. When air was bubbled through the solution, the mixed potential as measured against a calomel electrode increased to -703mV. When the air bubbling was stopped the mixed potential slowly decreased to -720mV.

Example IX An electroless copper plating employing the complexing Chel DM 41,(RTM) was run. The bath solution was comprised as follows: Copper sulfate 7.5 g/l Che DM 41 (RTM) 22 ml/l Formaldehyde (aq. 37% soln.) 15 ml/l Sodium hydroxide 6 g/l Temperature 27"C Gafac RE-610 (RTM) 0.1 g/l Initially the mixed potential was -720mV vs. SCE and the deposit was dark. Sodium cyanide was added incrementally until the mixed potential increased to -680mV vs. SCE, at which point the plated copper became bright and smooth.

Example X The mixed potential is useful for controlling agitation. When the solution is vigorously agitated, air is mixed into the bath and the mixed potential increases; when the agitation: ceases, the mixed potential decreases. A plating bath was comprised as follows: Copper sulfate 14.6 g/l Rochelle salt 7.5 g/l Sodium hydroxide 7.5 g/l Formaldehyde (aq. 37% soln. 38 ml/l Temperature 26"C The on- and off-periods of the agitator are contolled by the measured value of the mixed potential so that its value is kept in the range of -640 to -680 mV vs. SCE. The copper plated was bright and smooth.

WHAT WE CLAIM IS: 1. A method of operating an electroless metal plating bath which comprises measuring the resulting potential caused by the ox-redox reactions of the electroless metal deposition process, hereinafter referred to as the mixed potential, E,i,, and adjusting one or more bath parameters to maintain the mixed potential within a predetermined range, comprising establishing the predetermined range by measuring and adjusting a certain first set of bath parameters effectively measurable by known methods; and determining the Emix of the thus adjusted bath solution; and employing the measured Emix for determining and adjusting one or more bath parameters different from the ones of the first set of bath parameters and hereinafter referred to as "second set of bath parameters" thus operating the bath solution within predetermined and defined conditions, e.g., concentration(s) of one or more stabilizers or other bath constituents.

2. The method of claim 1 wherein the mixed potential is measured by means of a plating electrode and a reference electrode.

3. The method of claim 2 wherein the reference electrode is a calomel or silver/silver chloride electrode.

4. The method of claim 2 wherein the reference electrode is a single or double junction silver/silver chloride electrode.

5. The method of one of claims 2 to 4 wherein the bath is operated at an elevated temperature and the reference electrode is located in cooler, less agitated solution in an adjacent chamber. said chamber being connected to the bath by a solution bridge preferably of 5 to 10 cm.

6. The method of one or more of claims 1 to 5 wherein the second set of bath parameters is adjusted as a function of the mixed potential to maintain said mixed potential from + 5 mV to 1 50 mV of a predetermined value from -200 mV to - 1.5 mV vs. SCE.

7. The method of claim 6 wherein the bath solution is an electroless copper plating bath solution and the mixed potential is maintained + 25 mV, and preferably t 10 mV of a value from -600 mV to -850 mV vs. SCE.

8. The method of claims l to 7 which comprises measuring the mixed potential and incrementally adding reducing agent when the En,iX becomes more positive than a predetermined value.

9. The method of claims 1 to 7 comprising measuring the mixed potential and incrementally adding stabilizer when the mixed potential becomes more negative than a pre-determined value.

10. The method of claim 9 wherein the stabilizer is air or oxygen in admixture.

Il. The method of claim l() wherein the amount of air or oxygen is adjusted by controlling agitation of the bath in contact with said air or oxygen.

12. The method of claim 11 wherein the recirculating bath solution passes a weir and the agitation is controlled bv adjusting the height of the weir.

13. A method of operating an electroless metal plating bath comprising adjusting a first

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Copper sulfate 7.5 g/l Che DM 41 (RTM) 22 ml/l Formaldehyde (aq. 37% soln.) 15 ml/l Sodium hydroxide 6 g/l Temperature 27"C Gafac RE-610 (RTM) 0.1 g/l Initially the mixed potential was -720mV vs. SCE and the deposit was dark. Sodium cyanide was added incrementally until the mixed potential increased to -680mV vs. SCE, at which point the plated copper became bright and smooth. Example X The mixed potential is useful for controlling agitation. When the solution is vigorously agitated, air is mixed into the bath and the mixed potential increases; when the agitation: ceases, the mixed potential decreases. A plating bath was comprised as follows: Copper sulfate 14.6 g/l Rochelle salt 7.5 g/l Sodium hydroxide 7.5 g/l Formaldehyde (aq. 37% soln. 38 ml/l Temperature 26"C The on- and off-periods of the agitator are contolled by the measured value of the mixed potential so that its value is kept in the range of -640 to -680 mV vs. SCE. The copper plated was bright and smooth. WHAT WE CLAIM IS:

1. A method of operating an electroless metal plating bath which comprises measuring the resulting potential caused by the ox-redox reactions of the electroless metal deposition process, hereinafter referred to as the mixed potential, E,i,, and adjusting one or more bath parameters to maintain the mixed potential within a predetermined range, comprising establishing the predetermined range by measuring and adjusting a certain first set of bath parameters effectively measurable by known methods; and determining the Emix of the thus adjusted bath solution; and employing the measured Emix for determining and adjusting one or more bath parameters different from the ones of the first set of bath parameters and hereinafter referred to as "second set of bath parameters" thus operating the bath solution within predetermined and defined conditions, e.g., concentration(s) of one or more stabilizers or other bath constituents.

10. The method of claim 9 wherein the stabilizer is air or oxygen in admixture.

set of bath parameters to maintain the mixed potential within a predetermined range, measuring the mixed potential and causing its deviation from a first preset value by sequentially adding one bath constituent belonging to a second set of bath constituents until a second preset value is reached, said second value being different from the first preset value; and subsequently adding a second constituent belonging to said second set until the first preset value is achieved.

14. A method of operating an electroless metal plating bath comprising adjusting a first set of bath parameters to maintain the mixed potential within a predetermined range, measuring the mixed potential and causing its deviation from a first predetermined set point to start a sequence comprising adding one bath constituent until a second predetermined set point for Emix is reached; and adding a second predetermined constituent until a third predetermined set point is reached; and repeating said procedure as many times as desired; and finally adding a bath constituent to adjust the Emix to the first predetermined set point.

15. An apparatus for operating an electroless metal plating bath comprised of means for measuring the mixed potential and means for automatically adjusting one or more selected bath parameters to maintain the mixed potential within a predetermined range.

16. An apparatus for operating an electroless metal plating bath comprised of means for measuring and adjusting the concentration of one set of bath constituents; and means for measuring the mixed potential; and means for automatically adjusting a second set of bath parameters to maintain the mixed potential within a predetermined range or to adjust the mixed potential to one or more preselected set points.

17. The apparatus as claimed in claim 15 or 16 comprised of means for automatically adding one component, e.g., the reducing agent when the mixed potential becomes more positive than a predetermined value; and/or means for automatically adding stabilizing agent(s) when the mixed potential becomes more negative than a predetermined value.