6000 Angstroms of oxide in 1.5 hours is not unreasonable at 80-deg C. For
better etch resistance you need to consider nitride.

The 45-deg sidewall angle is curious. My suspicion would be the silicon.
How certain are you that it is <100>? Is the etch rate (regardless of the
sidewall angle) consistent with a 100 surface? At 80C you should be getting
about 80um/hr.

I don't believe the purity of KOH pellets plays a significant role. We use
pellets from an ordinary chemistry store, and they work fine.











On Wed, Oct 21, 2015 at 3:48 PM, Dave  wrote:

> Hello,
>
> I am having some difficulty etching 100 wafers using 30% KOH at 80 °C.
>
> Rather than the expected 54.7° angle, I am experiencing a rough surface
> with a ~45° between the top of the wafer and my sidewall, with some
> rounding as well. Additionally, in a ~1.5 hour etch I lost all 6000
> angstroms of thermally grown SiO2.
>
> I am using a bit older KOH (unknown grade) and a slightly lower
> concentration than is typical, with no IPA. I am using few polypropylene
> holders to position my wafer in a beaker on a hotplate with stir bar.
>
> It seems like I am getting some sort of contamination that is effecting
> selectivity of the planes but it does not make much sense. Should I use a
> commercial etchant from Transene to be safe like PSE-200?
>
> Thanks,
> Dave C.
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