Dear James,

"Obviously HF etching is isotropic (etches in all directions) so to reduce
the undercut of patterned features one should choose an BOE concentration
and etch time accordingly."

I think BOE etching is also isotropic. So, why do you think it can help with
better undercut profile?

2008/11/13 James Paul Grant 

> Hello Moshe,
>
> Perhaps this is a simplistic view however the major difference between BOE
> and 49% HF is their etch rate.
>
> 49% HF will etch much faster than 5:1 BOE which in turn will etch much
> faster than 10:1 BOE. Might I suggest you have a look at the (in my view)
> excellent paper, "Etch Rates for Micromachining Processing -Part II" by Kirt
> Williams (Journal od Micromechanical systems Vol. 12, No. 6 December 2003).
>
> The native oxide on silicon is only 20 A thick and 10: 1 BOE will remove
> native oxide at around 20 nm/minute so I would use this BOE for around 30
> seconds.
>
> Obviously HF etching is isotropic (etches in all directions) so to reduce
> the undercut of patterned features one should choose an BOE concentration
> and etch time accordingly.
>
> James

--
Best regards,
Yours sincerely
Fei Wang
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