Analytical method for determining formaldehyde in electroless copper plating bath

#The present invention relates to an analytical method for determining formaldehyde in an electroless copper plating bath.

#An electroless copper plating bath usually contains cupric ions, a complexing agent that forms a complex compound with cupric ions, and formaldehyde as a reducing agent. It has a comparatively high pH value. As the electroless copper plating proceeds, the concentrations of cupric ions and formaldehyde decrease and the pH goes down. The change in concentrations lowers the deposition rate of electroless copper plating and fluctuates the physical properties of the electroless copper deposit. Thus it is necessary to measure the concentrations of cupric ions and formaldehyde in the electroless copper plating bath and the pH of the bath continuously or intermittently, and to keep a prescribed level of concentrations by replenishing them and to keep the pH at a prescribed value.

#Heretofore, there have been proposed various methods for analyzing the components of the electroless copper plating bath. For example, the quantity of formaldehyde in the plating bath is determined by using a sulfite (sodium sulfite). This method utilizes the addition reaction of formaldehyde with sodium sulfite. According to this method, the plating solution is neutralized to a certain level of pH with a prescribed amount of acid aqueous solution of predetermined concentration, and then sodium sulfite is added to the neutralized plating solution. Formaldehyde reacts with sodium sulfite to give sodium hydroxide as shown in the following equation.

#FIGS. 1 to 3 are diagrammatic illustrations of apparatus suitable for carrying out several embodiments of the present invention.

#FIG. 4 (1) to (8) are graphs showing the results of potentiometric titration performed with different electrodes for the determination of formaldehyde in the electroless copper plating bath. (1) for Pt-Au electrodes, (2) for Pt-Ag/AgCl electrodes, (3) for Au-Ag/AgCl electrodes, (4) for Ag-Ag/AgCl electrodes, (5) Ag-Pt electrodes, (6) for Ag-Au electrodes, (7) for Ag-Ag electrodes, and (8) Ag-calomel electrodes.

#FIG. 5 (1) to (11) are also graphs showing the results of potentiometric titration performed with the Ag-Ag/AgCl electrodes for the determination of formaldehyde in the electroless copper plating bath. (1) to (5) show the results obtained when electroless copper plating baths each containing 0.02, 0.04, 0.06, 0.08, and 0.10 mol/liter of HCOH were analyzed by using a 0.01 mol/liter aqueous solution of hydroxylamine hydrochloride. (6) to (8) show the results obtained when electroless copper plating baths each containing 0.02, 0.06, and 0.10 mol/liter of HCOH were analyzed by using a 0.05 mol/liter aqueous solution of hydroxylamine hydrochloride. (9) to (11) show the results obtained when electroless copper plating baths each containing 0.02, 0.06, and 0.10 mol/liter of HCOH were analyzed by using a 0.10 mol/liter aqueous solution of hydroxylamine hydrochloride.

#In the analytical method for determining formaldehyde in an electroless plating bath according to the present invention, a potentiometric titration method is employed in which formaldehyde in the electroless plating solution sample is titrated with a hydroxylamine salt and the end point of the titration is detected by using an indicator electrode consisting of a silver electrode and a reference electrode. As the hydroxylamine salt, hydroxylamine hydrochloride, hydroxylamine sulfate, hydroxylamine oxalate and the like are used. Among them, hydroxylamine hydrochloride is most preferred.

#The reaction of formaldehyde with hydroxylamine hydrochloride is represented by the following equation.

#HCHO+NH.sub.2 OH.multidot.HCl.fwdarw.CH.sub.2 .dbd.NOH+HCl+H.sub.2 O