CF4 in the PQuest

Michael Wiemer mwiemer at stanford.edu
Tue Oct 4 18:30:24 PDT 2005


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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