Hello All,

   Hoping someone might be able to provide advice regarding some Si oxidation
furnace contamination issues we're having.  We have a new Tystar furnace that
we've been using pretty heavily for the past year or so.  Up until recently we
hadn't been all that rigorous about instituting a standard pre-furnace cleaning
procedure so it wasn't uncommon for wafers to be run that had come straight from
the Si DRIE (i.e. we didn't clean them in any way before putting them into the
furnace).  We assumed that the high temperature oxidizing environment would take
care of what little fluoropolymer remained on the etched wafers so there
wouldn't be any problems. However, we've recently noticed a buildup of a thin,
flaky, white, seemingly polymeric residue on the cool parts of our furnace which
is now making us reconsider that assumption.

    The film is easily scraped off in small sheet-like sections of up to a
couple mm long, but it crumbles easily under further handling.  The film looks a
lot like teflon and FTIR seems to indicate that it does bear at least some
resemblance to teflon. Based on these observations, we think that the
contamination was caused by volatilization and redeposition of the fluoropolymer
put down in the Bosch process. In light of this realization, we're now going to
most likely institute some sort of mandatory pre-furnace clean procedure (e.g.
piranha and/or RCA 1 & 2), but we just want to make sure that our hypothesis
regarding the contamination source and it's mitigation are valid.  Any and all
advise and suggestions would be greatly appreciated.


Thanks for the help,

Masa