Of course BOE etchant is also isotropic. Recall it is the HF in BOE that
is responsible for etching the oxide, the ammonium fluoride is just used
to control the pH.

My point was one should use a BOE or HF concentration that is suitable
for the application. In this case the topic creator wanted to wish the
native oxide layer which is only 20 Angstroms thick. If he used 5:1 BOE
etch for one minute then sure he will vertically etch his 20 angstroms
of SiO2 however he will also LATERALLY etch around 20 nm his SiO2.

I hope you now understand my point!

James

?? 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?
>

--
Dr. James Paul Grant
Postdoctoral Research Associate
Complex Systems Design Group
76 Oakfield Avenue Room 3
University of Glasgow
Glasgow
Scotland
G12 8LS

Telephone: +44(0)141 330 3374