A substrate is etched in a vacuum enclosure in a process which generates plasma
light emission. The process is monitored by passing emitted light via a window,
a thin film narrow band filter and a "Fabry-Perot" etalon to a detector. The output
signal from the detector is analyzed by shape recognition techniques to derive
a measure of the progress of the process. The shape recognition preferably makes
use of digital filtering and comparison with reference data derived from the theoretical
analysis or from a calibration run.
