Buffered oxide etch

Buffered oxide etch (BOE), also known as buffered HF or BHF, is a wet etchant used in microfabrication. It is a mixture of a buffering agent, such as ammonium fluoride NH4F, and hydrofluoric acid (HF). Its primary use is in etching thin films of silicon nitride (Si3N4) or silicon dioxide (SiO2), by the reaction:

SiO2 + 4HF + 2NH4F → (NH4)2SiF6 + 2H2O

Concentrated HF (typically 49% HF in water) etches silicon dioxide too quickly for good process control and also peels photoresist used in photolithographic patterning. Buffered oxide etch is commonly used for more controllable etching.[1] Buffering HF with NH4F results in a solution with a more stable pH; thus, more stable concentrations of HF and HF−2, and a more stable etch rate.[2]

Some oxides produce insoluble products in HF solutions. Thus, HCl may be added to BHF solutions in order to dissolve these insoluble products and produce a higher quality etch.[3]

  1. ^ Wolf, Stanley; Tauber, Richard (1986). Silicon Processing for the VLSI Era: Volume 1 - Process Technology. pp. 532–533. ISBN 978-0-9616721-3-3.
  2. ^ Williams, K.R.; Muller, R.S. (Dec 1996). "Etch rates for micromachining processing". Journal of Microelectromechanical Systems. 5 (4): 256–269. doi:10.1109/84.546406.
  3. ^ Iliescua, Ciprian; Jing, Ji; Tay, Francis; Miao, Jianmin; Sun, Tietun (Aug 2005). "Characterization of masking layers for deep wet etching of glass in an improved HF/HCl solution". J. Surf. Coat. 198 (1–3): 314. doi:10.1016/j.surfcoat.2004.10.094.