PULSE ENERGY CONTROL FOR EXCIMER LASER

摘要

A process for controlling pulse energy in burst of pulses produced by an excimer laser. The energy of each pulse in each burst is measured. The rate of change of pulse energy with charging voltage is determined. A pulse energy error is determined for a previous pulse of the present burst. An integrated dose error is also determined for all previous pulses in the current burst. A charging voltage for the next pulse is determined using the pulse energy error, the rate of change of energy with charging voltage and a reference voltage. In a preferred embodiment, the rate of change of energy with voltage is determined by dithering the voltage during two pulses of each burst, once lower and once higher. The reference voltage is a voltage calculated using prior voltage and energy data. In this embodiment the method of determining the reference voltage during a first portion of the pulse is different from the method used during a latter portion of the burst. During the first set of pulses, for each pulse, a specified voltage calculated using voltage and energy data from a corresponding pulse in a previous burst is utilized as a prediction of the voltage needed to produce a pulse energy converging on a target pulse energy. For pulses (41) and thereafter the reference voltage for each pulse is the specified voltage for the previous pulse.