RADIATION SOURCE AND LITHOGRAPHIC APPARATUS

摘要

The present invention provides a method of monitoring the operation of a radiation source fuel droplet stream generator comprising a fuel-containing capillary and a piezo-electric actuator (500). The method comprises analysing the resonance frequency spectrum of a system comprising the fuel-containing capillary and the piezo-electric actuator in particular to look for changes in the resonance frequencies of the acoustic system which may be indicative of a change in the properties of the system requiring investigation.