Thanks for the response Robert, its very thorough. The current problem with
my sidewalls is that they are generally bowed in in the middle and I would
like them to be straighter. I also get undercutting when I'm etching with an
anisotropy of about .9, which isn't terrible but I would like to improve.
The most successful chemistry so far actually being SF6/Ar.
I have tried a variety of pressures and powers, the most successful of which
have been low pressures and medium high powers so I generally stick to
20-30mTorr and 200W. I usually cannot get the pressure below 20mT when the
gases are flowing. All the etching I have done has been at 25C, I have the
ability to change the temperature some but I don't think I can cool it
significantly below that.
I am processing small pieces of wafer, I has tried fixing them to the chuck,
which I believe is not electrostatic, with tape to prevent them from moving
in the chamber but have abandoned this as they seem to stay put as long as
the chuck isn't wiped with IPA right before the chamber is pumped. I have
thought about using paste fix them and increase the thermal contact between
the chuck and sample but since the chuck isn't cooled that much I didn't
think this would make that big a difference.
All the recipes I have tried with SF6/O2 have been SF6 rich rather than O2
rich, I will have to give a more O2 rich recipe a try, thanks!
When reading about using SF6 as an etchant I have seen mixed reports, some
claiming that it is only suited for isotropic etching and others claiming
that with the addition of O2 it can etch anisotropically, though from the
responses I've gotten here it seems to be a common chemistry for anisotropic
etching. Do you think exploring this combo further would be better than
looking other chemistries to improve the etching?
Thanks Again,
Nate
On Mon, Aug 23, 2010 at 6:14 PM, Robert Ditizio wrote:
> Nate:
>
> I have a few suggestions that might help. You don't mention why the
> profile is not meeting your requirements but based on the information
> you provided, you are probably seeing an undercut profile. Also, you
> don't provide any parameters for the conditions that you have tried but
> presumably you are running at a few hundred millitorr and at or near
> room temperature, or higher, for the system that you described.
>
> In order to get a straight vertical profile, with the combination of
> SF6/O2, you might consider trying to operate at the lowest pressure and
> lowest temperature that you can safely achieve in your system. The safe
> operating ranges are typically provided by the manufacturer. The use of
> backside helium cooling will also help to keep the wafers cooler during
> processing and should be set to 5-10 Torr is possible. If an
> electrostatic clamp is used, you might find that you need to optimize
> the clamping voltage setpoint to get optimal clamping and cooling. If
> you are processing small pieces of wafers, then you could investigate
> bonding of the pieces to a larger substrate using eutectic alloys or
> vacuum grease, if allowable in your system.
>
> Once you have determined the operating limits in temperature and
> pressure, you might consider starting with an oxygen rich process, say
> 10:1/O2:SF6 to determine if you can obtain sufficient passivation of
> your sidewall to produce a non-undercut profile. If you can achieve a
> non-undercut, angled profile at 10:1, then with further reduction in the
> O2/SF6 ratio you should be able to find a mix that produces the vertical
> profile angle. If you do see an undercut at 10:1, then you may want to
> try increasing the O2/SF6 ratio further. As you introduce more O2 to
> the mix, your silicon and SiN etch rates will drop but this is a
> tradeoff that you may be forced to make. If you are unable to get a
> positive profile, then you could try reducing the bias power level.
> This will also result in further reduction in etch rate but may provide
> the profile performance that you are targeting.
>
> If you cannot find a set of conditions that produces the non-undercut
> profile by reducing the pressure, temperature, and power, then it is
> unlikely that this chemistry will work for you.
>
> Regards,
> Robert Ditizio
--
Nate Lawrence
PhD Candidate
Department of Electrical and Computer Engineering,
Boston University