Buffered oxide etch
Buffered oxide etch (BOE), also called buffered HF or BHF, is a wet chemical used in microfabrication to remove thin films of silicon dioxide (SiO2) and silicon nitride (Si3N4). It is a mixture of hydrofluoric acid (HF) with a buffering agent, usually ammonium fluoride (NH4F). The buffer keeps the pH steadier, which leads to a more predictable etch rate.
Chemistry (simplified): SiO2 + 4 HF + 2 NH4F → (NH4)2SiF6 + 2 H2O
Why use BOE: Concentrated HF etches too quickly and can damage photoresists, so buffering provides a slower, more controllable etch.
Common recipe and rate: A typical mix is about 6 parts 40% NH4F to 1 part 49% HF. At 25°C, thermally grown oxide etches at roughly 2 nanometers per second. Increasing temperature speeds up the etch. Stirring during etching helps keep the solution uniform.
Other uses: BOE can also etch some titanium alloys and newer Ti-based materials. It can help create larger pores in zeolites by adjusting how aluminum and silicon are removed, which influences catalytic properties.
Preparation: In the lab, dissolve NH4F in water and then add the HF solution.
This page was last edited on 3 February 2026, at 14:04 (CET).