BOE (Buffered Oxide Etch)
Often referred to as both buffered HF or simply BHF, this wet etchant is critical to microfabrication when etching silicon nitride or silicon dioxide film. Typically BOE is a combined mixture of a buffering agent (such as ammonium fluoride) and hydrofluoric acid. Higher concentrations of hydrofluoric acid (49 percent in aqueous solutions) tend to etch silicon dioxide more rapidly than is desirable and has a tendency to peel the photoresist used in most lithographic patterning. BOE provides a more controllable etching process, and thus is preferred in most cases when working with delicate micro electronic circuits. Occasionally, certain oxides are known to produce insoluble byproducts in hydrofluoric acid solutions, and so hydrochloric acid is often added in order to dissolve these byproducts, resulting in a higher quality etch. A typical BOE solution consists of a 6:1 volume ratio of 40 percent ammonium fluoride to 49 percent hydrofluoric acid, both in aqueous solutions.