Hi T.

You may try vapor HF after covering the front side with photoresist.
One of my past colleagues has been using it very effectively.
Please take a look:

Phuc Hong Pham et al 2007 J. Micromech. Microeng. 17 2125-2131   doi:
10.1088/0960-1317/17/10/026
Phuc Hong Pham et al 2006 J. Micromech. Microeng. 16 2532-2538   doi:
10.1088/0960-1317/16/12/003

Hope this helps.

Greetings
Nam Le
Ritsumeikan University


On Fri, Aug 15, 2008 at 2:10 AM, Taekyung Kim  wrote:
> Hi folks,
>
> I have a problem with buried oxide layer etching after SI deep RIE
> ..
>
> The pattern on the back side is 120 um x 120 um square and 400 um depth.
>
> DRIE stopped at the BOX layer.
>
> Since the wafer was thermally oxidized and the oxide on the front side must
> be protected, only the oxide on the back side and the BOX layer must be
> etched.
>
> However, due to high aspect ratio, I couldn't etch the box layer.
>
> Because the device layer must be intact, SiO2 RIE is not suitable for this.
>
> Anyone has any idea?