Buffered oxide etch (BOE), also known as buffered HF or BHF, is a wet etchant used in microfabrication. Its primary use is in etching thin films of silicon dioxide (SiO2) or silicon nitride (Si3N4). It is a mixture of a buffering agent, such as ammonium fluoride (NH4F), and hydrofluoric acid (HF). Concentrated HF (typically 49% water) etches silicon dioxide too quickly for good process control. Buffered oxide etch is commonly used for more controllable etching. [1]
Some oxides produce insoluble products in HF solutions. Thus, HCl is often added to BHF solutions in order to dissolve these insoluble products and produce a higher quality etch.[2]
A common buffered oxide etch solution comprises a 6:1 volume ratio of 40% NH4F in water to 49% HF in water. This solution will etch thermally grown oxide at approximately 2 nanometres per second at 25 degrees Celsius.[1]