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]
Production
editBuffered oxide etch can be produced in laboratory quantities by dissolving NH4F powder into water, and adding a solution of HF.[4]
Uses
editA buffered oxide etch solution of 6:1 volume ratio of 40% NH4F to 49% HF 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.
Buffered oxide etch can be used in the metallographic etching of titanium alloys. Newer Ti-alloys (such as Ti-Cu and Ti-Mo) are not etched as reliably by typical Ti-etching reagents, but can be etched by BOE and ammonium bifluoride.[5]
HF solution buffered with NH4F can be used to provide better etching of zeolite for the creation of larger pores, to improve the characteristically poor rate of diffusion in these microporous structures. Simple HF solutions (and other acidic methods of etching) show high selectivity in removing aluminum from zeolites, reducing the number of potential Brønsted acid sites, and subsequently reducing certain catalytic performance. However, the addition of a NH4F buffer results in a solution with more HF and HF−2 present, which removes Al and Si in more equal proportion.[6]
References
edit- ^ 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.
- ^ 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.
- ^ 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.
- ^ "Buffered Oxide Etch" (PDF). Integrated Nanosystems Research Facility. UC Irvine. Retrieved 18 September 2024.
- ^ Dumbre, Jayshri; Tong, Zherui; Dong, Dashen; Qiu, Dong; Easton, Mark (10 June 2024). "Buffered Oxide Etch: A Safer, More Effective Etchant for Additively Manufactured Ti-Alloys". Metallography, Microstructure, and Analysis. doi:10.1007/s13632-024-01094-x. Retrieved 18 September 2024.
- ^ Qin, Z.; et al. (23 July 2013). "Chemical Equilibrium Controlled Etching of MFI-Type Zeolite and Its Influence on Zeolite Structure, Acidity, and Catalytic Activity". Chemistry of Materials. 25 (14): 2759–2766. doi:10.1021/cm400719z. Retrieved 18 September 2024.
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