Abstract: Disclosed is an apparatus and a method in which off-droplet measurements instead of on-droplet measurements of prepulse energy are used for pulse energy control. Prepulse energy is measured during an off-droplet nonexposure period and controlled to a prepulse energy setpoint. The prepulse energy can then be controlled open-loop to the prepulse energy setpoint during on-droplet periods. This effectively decouples the EUV dose control loop from the prepulse energy control loop and avoids negative side effects of coupling such loops, for example, loss of the part of the dose adjustment range available to the dose controller.

Abstract: A method and apparatus for control of a dose of extreme ultraviolet (EUV) radiation generated by a laser produced plasma (LPP) EUV light source that combines pulse control mode and pulse modulation. The EUV energy created by each pulse is measured and total EUV energy created by the fired pulses determined, a desired energy for the next pulse is determined based upon whether the total EUV energy is greater or less than a desired average EUV energy times the number of pulses. If the desired pulse energy for the next droplet is within the range of one or more pulse modulation actuators, the pulse is modulated; otherwise, the pulse is fired to miss the droplet. This provides greater control of the accumulated dose as well as uniformity of the EUV energy over time, greater ability to compensate for pulses that generate EUV energy that is higher or lower than nominal expected values, and ability to provide an average EUV energy per pulse that is less than the nominal minimum EUV energy per pulse of the system.