The use of synthetic hydrocalcite as a chloride-ion getter for a barrier aluminum anodization process

Abstract

Chloride ion contamination at parts per billion concentrations plaques electrochemists studying barrier anodic aluminum oxide film growth and anodic aluminum oxide capacitor manufacturers. Chloride ion contamination slows film growth and reduces film quality. We have demonstrated that synthetic hydrocalcite substantially reduces the detrimental effects of chloride ion contamination in an aqueous electrolyte commonly used to grow barrier anodic aluminum oxide. We have determined that problems arise if precautions are not taken when using synthetic hydrocalcite as a chloride-ion getter in an aqueous electrolyte. Synthetic hydrocalcite is somewhat hydrophobic. If this powder is added directly to an aqueous electrolyte, some powder disperses; some floats to the top of the bath and forms scum that locally impedes anodic film formation. Commercially available powder contains a wide range of particle sizes including submicrometer-sized particles that can escape through filters into the electrolyte and cause processing problems. These problems can be over come if (1) the getter is placed in filter bags, (2) a piece of filter paper is used to skim trace amounts of getter floating on the top of the bath, (3) dummy runs are performed to scavenge chloride-ion loaded getter micelles dispersed in the bath, and (4) substrates are rinsed with a strong stream of deionized water to remove trace amounts of powder after anodization.