Stabilization and Control of Powderless Copper Etching Baths.

An effective additive to ferric chloride for the powderless etching of copper is formamidine disulfide, an S-S dimer of thiourea. Effectiveness of the additive is reduced by decomposition, as the bath stands at room temperature. Decomposition is greatly retarded by passage of electric current, at low voltage, between pairs of inert electrodes immersed in the bath. Monomeric film formers are not stabilized by the current, but may be preserved by reducing temperature when the bath is not in use. In deep-etch baths, a copper concentration of 15-35 grams per liter is necessary to give optimum etching performance. A method of maintaining the copper content of the bath in this range is described. A simple method of determining copper in ferric chloride, using a spectrophotometer operating at 850 millimicrons in the infrared spectrum, is also described.