EP0016415B1 - Method for measurement and regulation of the concentration of copper, formaldehyde and sodiumhydroxide in an electroless copper deposition bath and sampling apparatus for use in this method - Google Patents

Description

The invention relates to a method for measuring and regulating the concentration of copper, formaldehyde and sodium hydroxide solution in a bath for electroless deposition of copper, the copper ion concentration colorimetrically detecting the sodium hydroxide concentration by potentiometric titration and the formaldehyde concentration by amperometric titration and compared with adjustable target values, as well as on a drug-taking device for use in this method.

The main components of a chemical copper bath are to be analyzed and regulated with such a process so that the deposition conditions remain constant and perfect copper layers are achieved.

A method of the type mentioned at the outset has become known from US Pat. No. 4,096,301. In this process, a bath sample is continuously taken from the copper bath. A standardized acid of such a concentration and amount is also continuously added to this bath sample that a potentially predetermined final value can be achieved. After appropriate mixing, the bath sample passes through a pH value measuring station, in which the actual pH value is measured and compared with a predetermined target value. In the event of a deviation from the nominal value, sodium hydroxide solution is then metered into the copper bath in accordance with this deviation. The bath sample then passes through a colorimeter station, in which the copper ion concentration is checked and, in the event of a deviation from the nominal value, an amount of copper solution corresponding to this deviation is metered into the copper bath. After passing through the colorimeter station, sodium sulfide is continuously added to the bath sample and, after appropriate mixing, is fed to another pH value station, where the pH value of the bath sample is measured again and the difference is formed using the previously determined pH value. This difference is a measure of the formaldehyde concentration. If there is a deviation from the specified setpoint, a corresponding amount of formaldehyde is added to the bath.

A method of the same type is known from DE-A-2 751 104. In this known method, a bath sample is also continuously taken from the chemical copper bath and passed into a chamber where a deposition electrode is located. Adjacent to this chamber is a further chamber with a comparison electrode, which together with the deposition electrode serves to measure a mixed potential. After the so-called "mixed potential" has been recorded, the bath sample is fed to a pH value station and a colorimeter station via a heat exchanger. The individual bath components are then regulated as a function of this mixed potential.

GB-A-1 168370 has also disclosed a process of the type mentioned at the outset, in which the pH is kept at a constant value with the aid of alkali hydroxide in order to stabilize the bath, and the copper ions at the specified pH Concentration is measured colorimetrically. Depending on this measurement, a mixture of formaldehyde and copper salt is added in a certain molar ratio.

In the case of these known methods, the concentrations of the individual components are not displayed absolutely, but are only regulated to predetermined target values. The absolute content of the individual components is therefore never known. A further problem is the measurement of the pH value, since this value cannot be kept constant over a long period of time due to electrode drift. A temporary re-calibration is therefore absolutely necessary.

The invention has for its object to improve a method of the type mentioned so that the content of the main components can be analyzed, displayed and controlled more precisely.

This object is achieved in that a sample is taken discontinuously for each of the components mentioned and this is diluted with a certain amount of water, that the copper ion concentration by colorimetry, the sodium hydroxide solution concentration by potentiometric titration and the formaldehyde concentration are then independent of one another be determined by amperometric titration and that in the amperometric titration of the formaldehyde concentration as the titrant hydroxylammonium hydrochloride and as the working electrode a gold electrode operated with a constant polarization voltage of 0-200 mV against a silver-silver chloride reference electrode are used, the current between the working electrode and a counter electrode is measured.

Through the precise analysis of the individual components, absolute values of the concentration are obtained, so that these can then also be regulated very precisely. In this way, not only is the bathroom optimally utilized, but the copper layer is always the same. It should also be emphasized that carry-over reactions are avoided by the discontinuous sampling and thus the accuracy of the analysis is significantly improved.

In the potentiometric titration of the sodium hydroxide concentration, the titration end point is preferably made from the three by an approximation method known per se largest potential steps with constant addition of titrant are calculated, the supply of the titrant being added with the aid of a motor piston burette in constant volume units by appropriate step-by-step control of the buret motor and after each addition of titrant for stabilization a constant rest time is inserted before the measurement signal is detected. In this way an extremely precise determination of the sodium hydroxide concentration is achieved.

According to a further embodiment of the invention, it has also proven to be advantageous if a polarization voltage of +50 mV is used for the working electrode.

According to a further preferred embodiment of the invention, the end point of the amperometric titration is determined by the intersection of two straight lines, one of which runs parallel to the abscissa and the minimum of the titration curve and the other through several measuring points of the quasi-linear range of the rising part following the minimum the titration curve is determined. It has been found to be particularly favorable that the minimum of the titration curve is determined and stored in order to determine the one straight line, and that five measuring points of the quasi-linear region of the ascending part of the titration curve are used to determine the other straight line, and that this straight line is calculated using a regression method and determining the intersection of the two straight lines with the aid of a computer.

A particularly simply constructed sampling device for use in the method according to the invention is characterized in that the individual bath samples can be removed from the bath by means of slide-controlled sampling valves with the aid of measuring loops, the individual measuring loops being connected in series when the samples are taken and this series connection of the measuring loop being controlled by a controllable valve in parallel is switched. In a preferred embodiment of this sampling device, the content of the measuring loops can be transferred into the vessels with the aid of metering syringes which add distilled water.

The invention is explained in more detail with reference to the drawing, in which an exemplary embodiment is shown schematically.

• Figur 1 den chemischen Verfahrensablauf,
• Figur 2 den mechanischen Aufbau einer Proben nahmevorrichtung zur verwendung bei dem Verfahren im Prinzip und
• Figur 3 eine Titrationskurve der amperometrischen Titration.

Show it:

• FIG. 1 the chemical process sequence,
• Figure 2 shows the mechanical structure of a sampling device for use in the method in principle and
• Figure 3 is a titration curve of the amperometric titration.

In FIG. 1, 1 denotes a galvanic bath which is said to have a specific composition, the main components being copper, sodium hydroxide solution and formaldehyde. The concentrations of these components should be regulated to constant values. This chemical copper bath works, for example, at a temperature above 50 ° C. A certain proportion of the sample is taken from the chemical copper bath via a line 2. This portion passes through a cooling device 3 and is cooled there to at least 30 ° C. This portion is fed to the individual stations via a line 4. The upper part of FIG. 1 shows the process sequence for determining the copper ion concentration by colorimetry. We are looking for the concentration in grams of copper per liter, as indicated by a symbol 5. As symbols 6 and 7 show, a discontinuous sampling takes place, namely of 1 ml. This sample is diluted in a mixing vessel 8 with twice 20 ml of water, as indicated by an arrow 9. The two measuring cells 10 and 11 of a colorimeter 12 are filled from the mixing vessel 8, the measuring cell 11 having a thickness of 10 mm and the measuring cell 10 having a thickness of 20 mm. The measurement in the colorimeter 12 takes place at 690 nm. An alternating light colorimeter is preferably used, since this only requires a photo element for light measurement, onto which the measuring beam and the comparison beam alternately fall. Signals proportional to the light intensity can then be taken from an output line 13 and fed to a corresponding evaluation circuit 14, where the copper concentration C _{cu is} calculated from the product kA, k being a calibration factor and A being the measurement signal proportional to the copper concentration.

The process sequence for the titration of the sodium hydroxide solution is shown in the middle of FIG. The concentration of the sodium hydroxide solution in grams per liter is sought, as indicated by symbol 15. There is again a discontinuous sampling at 16, which is taken from line 4 with the aid of a metering device 17. A sample amount of 2 ml is preferably taken and mixed in a mixing vessel 18 with twice 20 ml of water, as indicated by an arrow 19. The sodium hydroxide solution is titrated with dilute hydrochloric acid (HCl) in the same mixing vessel 18. With the help of a motor piston burette 20, hydrochloric acid of 0.1 M in constant volume units AV = 0.2 ml is added by appropriate step-by-step control of the motor piston burette 20, as through a line 21 is indicated. After each individual addition of titrant, for example, a rest period of Δt = 1 ... .5s is inserted. This rest time can be shortened at the beginning of the titration and extended accordingly when the titration end point Ä is approached. A pH electrode is indicated at 22. The signals are fed to an evaluation circuit 24 via a line 23, the alkali concentration C _{NaoH being determined} from the product K'.Ä, where K 'is a calibration factor and Ä the calculated volumes in the titration end point.

In the lower part of FIG. 1, the chemical procedure for the amperometric titration of formaldehyde is shown. The concentration of formaldehyde in grams per liter should be determined, as symbol 25 shows. There is again a discontinuous sampling at 26 with the aid of a metering device 27, this sample being fed to a titration vessel 29 via a line 28. A symbol 30 illustrates that 15 ml of 1 M NaOH diluted with 45 ml of H ₂ O are fed to the titration vessel 29 before the actual titration. The two substances are mixed intimately with the aid of a stirring device 31. A gold electrode 32 as the working electrode, a platinum electrode 33 as the counter electrode and a silver / silver chloride electrode 34 as the reference electrode are immersed in the titration vessel 29. The working electrode 32 is polarized with a constant voltage of U = 0 to +200 mV with respect to the reference electrode 34. With the help of an engine piston burette 35, a titrant is added via a line 36, specifically hydroxylammonium hydrochloride (NH ₂ 0H. HCl).

With the aid of a corresponding circuit, the voltage between the working electrode 32 and the counter electrode 33 is regulated so that the voltage of the working electrode 32 always remains constant with respect to the reference electrode 34. If a silver / silver chloride electrode is used as the reference electrode, it is advantageous that a polarization voltage of +50 mV is selected. The current flowing between counter electrode 33 and working electrode 32 is measured and results in a specific titration curve depending on the amount of titrant added. The titration end point can then be determined by methods known per se using the titration curve according to FIG. Preferably, such a method is chosen that the titration end point can be determined fully automatically.

=2,16.K".Ä berechnet wird. Hierbei sind K" ein Eichfaktor und Ä des berechnete Volumen beim Titrationsendpunkt.The use of a gold electrode as the working electrode has the advantage that no copper can settle there because the gold electrode always has a positive potential. It has proven to be particularly favorable if the titrant NH _Z OH. HCl has a concentration of 0.5 g / l. The output signals of the amperometric titration are fed via line 37 to an evaluation device 38, where the end point of the titration and the formaldehyde concentration are determined with the aid of a computer:

= 2.16.K ".Ä is calculated. Here K" are a calibration factor and Ä the calculated volume at the titration end point.

The calculation of the titration end point is explained in more detail with reference to FIG. 3. Figure 3 shows the typical course of a titration curve K in an amperometric titration of formaldehyde under the aforementioned conditions. Here, depending on the amount V [ml] of the continuously added titrant, NH ₂ 0H. HCI the current I [mA] plotted.

The end point Ep of the amperometric titration is preferably determined by the intersection A of two straight lines G1, G2, one of which runs parallel to the abscissa axis and goes through the minimum of the titration curve and the other through several measuring points P1 ... P5 of the quasi-linear range of the the minimum following ascending part of the titration curve is determined.

To determine the one straight line, only the minimum of the titration curve needs to be recorded. Five measuring points P1 ... P5 of the linear range are used to determine the other straight line. This straight line can be calculated using a known regression method. The calculation itself is carried out using a computer. It has been shown that the deviation between the real and the calculated Ä-Ep is practically constant and as such a constant value can be taken into account by subtracting from the calculated value.

The concentration of copper, sodium hydroxide solution and formaldehyde are therefore determined completely independently of one another. The individual control processes and the measurement value processing are carried out with the aid of a control circuit 39 containing a microprocessor.

The concentration of the main components copper, sodium hydroxide solution and formaldehyde are therefore analyzed independently and the analysis results are recorded, as indicated at 40.

By comparing the measurement values obtained in this way with an adjustable target value, a signal that is proportional to the deviation is formed for each component. These signals can be used to control appropriate dosing groups to refresh the bath. The bath temperature can also be measured and logged.

FIG. 2 shows the mechanical construction of the bath guiding device in principle, with the same parts having the same reference numerals as in FIG. 1.

So that a measuring line 42 always receives the actual bath composition, part of the bath 1 is pumped in the circuit via a line 41 in the secondary flow. This flow can be controlled by means of a valve 43. If the valve 43 is closed, the liquid is pressed through the measuring line 42. The measuring line 42 can also be connected via a valve 44, which can be a slide valve, for example, to a line 46 which is connected to a container which contains a calibration solution for the purpose of calibrating the individual devices. With the help of a slide 45, the measurement can then Line 42 can either be connected to line 4 or to a line 46.

For example, a compressed air-controlled valve 7 is used for discontinuous sampling. The individual connection bores of the valve are designated by a to f and can be connected to one another or to one another by corresponding grooves 7a, 7b and 7c in the slide. A measuring loop 47 is connected between the connection bores b and c and is calibrated to 1 ml. In the slide position shown in FIG. 2, the line 4 is thus connected via the bores a and b of the valve 44 to the connection bore a in the valve 7. Through the longitudinal groove 7b in the slide of the valve 7, the sample passes through the connection bore b into the measuring loop 47 and from there via the connection bore c, the longitudinal groove 7c and the connection bore d into the measuring line 42, where it continues in a corresponding manner through the valves 17 and 27 and finally can flow back to the bathroom via the measuring line 4. In the slide position shown, the smaller diameter piston 7d of the valve 7 is constantly pressurized with compressed air from a compressed air supply system 48. If a valve 49 is activated for taking a sample, the compressed air of the compressed air system 48 now also reaches a piston 7a with a larger diameter than that of the piston 7d. Accordingly, the slide is moved to the right in relation to FIG. 2, so that the transverse groove 7a now connects the two connection bores a and b with one another, so that the sampling for the valves 17 and 27 can take place without hindrance. The longitudinal grooves 7b and 7c of the slide now connect the connection bores b and e or c and f; with the help of a dosing syringe 50, a precisely dosed amount of water, namely 20 ml, is taken from a line 51, which is fed via a line 52 to the connection bore f, so that the content of the measuring loop 47 is transferred to the line 9, which ends in the mixing vessel 8 . The mixing vessel 8 is provided with a drain valve 53 and a stirring motor 54. With the help of a further dosing syringe 55, part of the sample is removed from the mixing vessel via a line 56 and fed to the two measuring cuvettes 10 and 11 of the colorimeter 12. With the aid of an illumination device 57 and a photocell 58, the copper concentration can then be determined in a manner known per se. A valve 59 indicates that the sample raised in the measuring cuvettes 10 and 11 1 can either be returned to the vessel 8 or can be passed into a collecting container; the sample can also be lifted several times.

The valve 17 is constructed in the same way as the valve 7, except that a measuring loop 60 is inserted into the connection bores b and c, which is calibrated to 2 ml in the special case. With the aid of a dosing syringe 61, the content of this measuring loop can be transferred into the mixing vessel 18 by sucking in distilled water via a line 62, namely with a 20 ml stroke, as can be seen from FIG. The pH electrode 22 is immersed in the mixing vessel 18, so that it also serves as a titration vessel. 63 is a drain valve and 64 an agitator. The engine piston burette, designated 20, takes HCI via a line 65 and is supplied to the vessel 18 via line 21, the engine feeding the engine piston burette to the bath sample intermittently in each case 0.2 ml until the titration end point is recognized.

The valve 27 corresponds in structure and operation to the valves 7 and 17, except that a measuring loop 66 is connected between the connection bores b and c, which is calibrated to 0.1 ml. The valve 27 is activated via a valve 67 in the same way as the valve 49. With the aid of a metering syringe 68, the bath sample taken with the help of the measuring loop 66 is mixed with 45 ml of water and fed to the titration vessel 29 via a line 28. As has already been explained above, after the sampling and dilution with the aid of a metering device 69, a certain quantity of sodium hydroxide solution (NaOH) is added, namely 15 ml. The titration is then carried out with NH ₂ 0H. HCI via a line 36. The motor piston burette 35 is driven step by step until the titration end point is reached.

As has already been explained above, the amperometric titration is used for the determination of formaldehyde, since it is much more precise than the known other titration methods.

Claims (7)

1. A process for measuring and controlling the concentration of copper, formaldehyde, and caustic soda in a bath for the currentless deposition of copper, in which the copper ion concentration is determined colorimetrically and the caustic soda concentration is determined by potentiometric titration, and in each case a comparison is made with adjustable theoretical values, characterised in that, for each of the previously-mentioned components, a specimen is discontinuously taken and is diluted with a specified amount of water; that then, independently of one another, the copper ion concentration (C_cu) is determined by colorimetry, the caustic soda concentration (C_NaOH) is determined by potentiometric titration, and the formaldehyde concentration (C_CH20) is determined by amperometric titration; and that during the amperometric titration of the formaldehyde concentration, hydroxylamine hydrochloride (NH₂0H . HCI) is used as the titration agent, and the operating electrode (32) consists of a gold electrode operated at a constant polarisation voltage of 0-200 mV against a silver-silver chloride reference electrode (33), a measurement being made of the current between the operating electrode (32) and a counter-electrode (34).

2. A process according to Claim 1, characterised in that, during the potentiometric titration of the caustic soda, the end point of the titration is calculated by an approximation process, known per se, from the three largest potential steps with a constant supply of titration agent, the titration agent being supplied with the assistance of a motor-driven piston burette (20) in constant volume units (AV), by suitable stepwise control of the burette motor; and that, after each individual addition of titration agent (AV), for stabilisation purposes a constant rest time (At) is interposed before the measurement signal is determined.

3. A process according to Claim 1 or Claim 2, characterised in that a polarisation voltage of +50 mV is used for the operating electrode (32).

4. A process according to Claim 1 or Claim 3, characterised in that the end point (Ep) of the amperometric titration is determined by the intersection point (A) of the two straight lines (G1, G2), one of which runs parallel to the abscissa and passes through the minimum of the titration curve, and the other of which is determined by a plurality of measurement points (P1...P5) of the quasi-linear region of the rising part following the minimum of the titration curve.

5. A process according to Claim 4, characterised in that the first straight line is determined by determining and storing the minimum of the titration curve; that the other straight line is determined by taking five measurement points of the quasi-linear region of the rising part of the titration curve; and that the calculation of these straight lines is effected by a regression method and the intersection point of the two straight lines is determined with the aid of a computer.

6. A sampling device for use in the process as claimed in Claim 1, characterised in that the individual bath specimens can be obtained from the bath by way of slide-controlled sampling valves (7, 17, 27) with the aid of measuring loops (47, 60, 66), in which, during sampling, the individual measuring loops (47, 60, 66) are connected in series and a controllable valve (43) is connected in parallel to this series arrangement of the measuring loops.

7. A sampling device according to Claim 6, characterised in that the contents of the measuring loops (47, 60, 66) can be transferred to vessels (8, 18, 29) with the help of dispensing syringes (50, 61, 68) which add distilled water.

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