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.
References
- ^ a b Wolf, Stanley; Tauber, Richard (1986). Silicon Processing for the VLSI Era: Volume 1 - Process Technology. pp. 532–533. ISBN 978-0-9616721-3-3.
- ^ 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.
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