I'm depositing a 1.2-1.3 micron  PECVD oxide layer with a
Plasma Therm  ETP-3       I am getting excellent uniformity and
rate of 200 angs/min OCCASIONALLY.  Then the rate and
uniformity gradually or abruptly goes to hell.

Question: what is the order of influence of each variable?
ie Temperature of pallet       (250C)
   Process pressure             (600mTorr)
   Power                               (20W)
   N2 flow                            (600sccm)
   N2O flow                          (600sccm)
   5% SiH4 flow                    (630sccm)
Any information or direction is gratefully appreciated.
Liz