CF4 in the PQuest

Jim McVittie mcvittie at snf.stanford.edu
Wed Oct 5 10:37:56 PDT 2005


Hi Mike,


     Thanks for chairing this meeting, yesterday. I was chairing the
     Plasma/Thin Film NCCAVS Symposium in San Jose and could not make your
     mtg. Limiting the CF4 concentration is fine with me, however I think
     we should baseline or quantify the F level in the tool. I expect F
     questions will come up again, so we need some numbers. There are many
     measurements we can do but there are 2, which are easiest for us to
     implement. The first is to just measure the thermal oxide etch rate on
     a Si wafer in a pure O2 plasma. Any FC polymers or other sources of F
     will release free F in an O2 plasma and this will show up as an
     increase in oxide etch over the sputter rate for the conditions used.
     Using the endpoint  laser, we could measure the etch ratre over 30 min
     period. The second method is called actinometry, which is a  fancy
     name for an optical emission measurement, where you ratio you
     measurement to a known concentration of Ar. We can measure the F and
     Ar  emission for a baseline O2/F process with a few percent Ar added
     and then measure the F and Ar emission over a period after the F flow
     has been
     turned off until the F emission is in the noise.  Does anyone want to
     volunteer to help do these measurements.

    Thanks,  Jim


Michael Wiemer wrote:

> SpecMat,
>
> This afternoon we had a meeting between "users interested in using" and
> "users interested in forbidding" CF4 in the Pquest. Ed Myers was also in
> attendance.
> -----------------------------------
> The end result of the meeting was this rule:
>
> Etches using CF4 must contain high amounts of O2. As a guideline (this
> number is not perfectly strict, but is a good guideline) any etch with CF4
> should be ~20% CF4 and 80% O2. Small variations around this % are OK. No
> etches containing large amounts of CF4 and small amounts of O2 are allowed.
>
> This basically eliminates etching of Si3N4 and SiO2 with CF4.
> ------------------------------------
> Reasoning:
>
> The Pquest is a III-V etching tool and must maintain that priority/identity.
> CF4 etching is used mainly for etching of Si3N4 and SiO2 and not required
> for GaAs etching (GaAs etching is Cl2, BCl3, Ar). The problems with CF4 are
> as follows:
> 1.) High CF4 concentrations (particularly 100% CF4 0% Oxygen) can lead to
> polymer deposition on the chamber tooling (and thereby subsequent samples)
> under the right circumstances.
> 2.) Fluorine affects the etch rates of GaAs and, in particular, AlGaAs.
>
> GaAs users do not have quantitative data on these issues in this tool, but
> nevertheless, they exist. Variations in etch rate have been observed as well
> as suspected polymer deposition. These are the reasons for disallowing CF4.
> CF4 coupled with high amounts of O2 does not polymerize, though there may
> still be some residual Fluorine effect on the GaAs etch rate.
>
> Some GaAs substrate processes require the use of BCB (a polymer) for
> planarization and passivation. CF4 in small quantities mixed with large
> quantities of O2 is an etch for BCB. As there are not many etch tools which
> allow GaAs, and there is a large amount of O2 in this etch, large O2 small
> CF4 etches are deemed aceptable.
> --------------------------------------
> Conclusion:
>
> The GaAs users on the Pquest feel that the Pquest is a GaAs etcher, not a
> "etch anything" tool. The integrity of Cl based GaAs etching must be
> preserved by good, strong, policy. With so many other SiO2/Si3N4 tools in
> the lab, the Pquest does not also need to etch these materials with a gas
> incompatible with GaAs etching. At the same time, the Pquest has
> capabilities which, perhaps, some of the other SiO2 etchers in the lab lack.
> This is not a good reason to put an incompatible process in the Pquest.
> Instead, it is the start of a good argument to purchase yet another SiO2
> etching tool for SNF. Exceptions and workarounds to compatibility problems
> in the Pquest are not long term solutions. Given that Si/SiO2/Si3N4 users
> comprise 90% of the lab, one exception only leads to 100 exceptions, and
> then we won't have a GaAs etcher in the lab.
>
> Not all users at the meeting were particularly happy about the outcome of
> the meeting, however, none voiced an unwillingness to work with the new
> rules, once approved by SpecMat. There were 2 users interested in using CF4
> in the Pquest. One user would like to keep the option open for the future,
> the other is actively wanting to use CF4.
>
> If you have any questions, please contact me. I would like to resolve this
> as quickly as possible so that we can make this policy.
>
> Thank you,
>
> -Mike
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