Buffered oxide etch

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% HF in water) etches silicon dioxide too quickly for good process control and also peels photoresist used in lithographic patterning. 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] Temperature can be increased to raise the etching rate. Continuous stirring of the solution during the etching process helps to have a more homogeneous solution, which may etch more uniformly by removing etched material from the surface.