Also, you might wand to consider gas-phase HF etching. It's quite a bit
slower (etch rates reported around 200nm / min) but it has the
potentially huge advantage of not immersing structures in liquids, which
could be very important for fragile cantilevers and objects with
sticking issues. Basically just put your wafer upside down on top of a
PTFE/Teflon cylinder of proper diameter in a beaker with just the bottom
1-2 cm filled with 40% HF solution - some people prefer to heat the HF
to 40 degrees centigrade, but I do fine just using room temperature (of
course, HF vapors are nasty, so remember to do this in a fumehood with
4H gloves etc...). Afterwards you can heat the whole thing to 120
degrees centigrade to further reduce the probability of sticking.

See:
Phuc Hong Pham: J. Micromech. Microeng. 17 (2007) 2125–2131

// Morten

SEBESTA Edward wrote:
> I see some problems with this.
>
> 1. CF4 will have difficulty in etching SiO2. The stoichiometry is unbalanced.
>
> 2. The anistropic or isotropic nature of a plasma etch is driven by the
substrate to plasma bias and not the chemistry.
>
> 3. When SiO2 is etched with CHF3 or something with a lower F:C ratio, it needs
an Reactive Ion Etch where there is a carbon residue and bombarment of this
residue by use of a bias to drive a Carbon oxygen reaction to facilitate the
etching of SiO2. This is inherently anisotropic.
>
> 4. If you need to etch isotropically 5 microns, I would use a wet etch
solution which is inherently isotropic.
>
> Ed