A substrate processing apparatus 27 comprises a chamber 35 capable of
processing a substrate 20, a radiation source 58 to provide a radiation, a
radiation polarizer 59 adapted to polarize the radiation to one or more
polarization angles that are selected in relation to an orientation 33 of
a feature 25 being processed on the substrate 20, a radiation detector 54
to detect radiation reflected from the substrate 20 during processing and
generate a signal, and a controller 100 to process the signal.
