Shutdown drytek2 2001-02-13 13:04:43: reflected RF power jumps high during nitride etch

[zhang at snf.stanford.edu]

nitride etch
process    #3
SF6           50
CF3Br       33
O2             off
pressure  200
power       500
During etching, reflected power suddenly jumped high (from 
20 to 240 or so). This happened for 4-5 times, each time 
shortly after forward power came on. Whenever this happened,the plasma would turn from orignal light purple to yellowish 
and the plasma for the 4th tray from the top would go off.