I think James' point was not to imply that BOE is not-isotropic, but
that since BOE etches at a slower rate, it is easier to time and control
the undercut.

More importantly, if one wants to simply strip the oxide layer without
features and without caring about the surface smoothness, then obviously
a higher concentration would work quickly and very well. However, if one
wants to maintain a smooth Si surface without damaging the original
flatness of the wafer (such as for hydrophobic Si fusion bonding), one
should choose a much lower concentration of HF, such as diluted HF (DHF,
~1-6% HF in H2O) or 6:1, 10:1 or 50:1 BOE. High concentrations of HF
will damage the atomic flatness of the wafer.

Christopher Tan

汪飞 wrote:
> 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?
>