The present invention presents a method for detecting an endpoint of an
etch process for etching a substrate in plasma processing system (1)
comprising: etching the substrate; measuring at least one endpoint
signal; generating at least one filtered endpoint signal by filtering the
at least one endpoint signal, wherein the filtering comprises applying a
Savitsky Golay filter (12) to the at least one endpoint signal; and
determining (14) an endpoint of the etch process from the at least one
filtered endpoint signal.