Plasma etching endpoint controller

An end point controller for an anisotropic plasma etching process comprising a series of alternating etching and polymerization steps, includes a monitor for monitoring the intensity of a characteristic process parameter, typically an optical emission, and producing a signal indicative of the intensity. A timer assembly determines the operation of the monitor during each etching step, the assembly comprising two sequential timers 100,105. The first timer 100 determines a time lag T1 (Fig 3) between the onset of the etching step and the operation of the second timer 105 which sets a time period T2 (Fig 3) within the etching step for monitoring the selected parameter. This enables the parameter to be monitored in the stable period T2 of each etching step. Other process parameters include the pressure of gas discharge, the plasma voltage and the electric current in the plasma.