Conventional wet etching techniques for selectively removing silicon nitride (Si3N4) have utilized hot (approximately 145-180°C) aqueous phosphoric acid (H3PO4) solutions (often referred to as hot phos). The typical Si3N4:SiO2 selectivity is about 40:1 when using 85% fresh hot phosphoric acid. Advantageously, as the nitride layer is removed, hydrated silicon oxide forms and dissolves in the etchant. Consistent with Le Chatelier principle, this inhibits the additional removal of silicon oxide from the device surface; thus selectivity gradually increases with use [.
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