Amit Shiwalkar wrote:
>
> Hello,
>         I have done some etching ( isotropic ) on Si with HNO3 (63%)&
> HF(49.3%) mixture and using DI H20 as diluent. On certain occasions it was
> observed that there were blackish-brown marks ( like charring ) on the
> wafer. This wafer was P-type. Also if the etching was done using glacial
> ascetic acid as diluent then on no ocassion were these brown marks
> observed. Also these marks were not consistently observed even in the H2O
> diluent recipe.
>         I would appreciate if anybody could give me any references or
> suggestions about the cause of this phenomenon. Classic recipe handbooks
> like Ghandhi, Donovan & Burger etc. do not mention any such phenomenon.
>         Note that even in the case of glacial ascetic acid as the diluent,
> some places(where water droplets were retained)  were charred in the step
> where etching was stopped by a dip in DI water.
>
> Thanking you all in advance
>
> Regards
> Amit Shiwalkar
>
Probably, what you observe is a thin layer of porous silicon. Porous
silicon can be formed like that, without an
electrochemical etch cell, and the etching it is called a dash-etch
then. You will find this phenomenon in this type
of literature (porous silicon). For example, Dr. Steckl's group at our
university has investigated this. I would
recommend to use a mixture with low HF-percentage, such that the silicon
surface is oxidized, and the etching
mechanism is diffusion limited, giving you true isotropy and a smooth
surface(called polish etch). Good luck.

Alexander Hoelke
Center for Microelectronic Sensors and MEMS (CMSM)
University of Cincinnati
Phone: 513-556-1997
email: holkead@email.uc.edu