I think the rationale may be that for the first few seconds or so of
deposition, the deposition parameters in the tube are not stabilized;
the gas flows, the total pressure, the precursors' partial pressures,
etc. have not reached steady-state. During this unstable period, if you
have a temporary excess of DCS vs NH3, the deposited film may be
silicon-rich, which may not be what you want. Same thing when the gas
flows are cut, at the end of the deposition. Making sure that the tube
is flushed with NH3 before and after deposition ensures that there is
never an excess of DCS vs NH3, so the film composition will be
homogeneous throughout its thickness.

Regards,
Pierre x286

-----Original Message-----
From: jedidi nader
Subject: [mems-talk] Silicon nitride process

Hello every body!

  I am about to make a DOE to optimise the deposit of silicon nitride by
LPCVD. I would like to know why it would be necessary to sweep the tube
with NH3 before and after the deposit of nitride ?

If someone can help me to find an answer for my question, I will be very
grateful.