BACKGROUND

The wavelength of radiation used for lithography in semiconductor manufacturing has decreased from ultraviolet to deep ultraviolet (DUV) and, more recently to extreme ultraviolet (EUV). Further decreases in component size require further improvements in resolution of lithography which are achievable using extreme ultraviolet lithography (EUVL). EUVL employs radiation having a wavelength of about 1-100 nm. One method for producing EUV radiation is laser-produced plasma (LPP). In an LPP-based EUV source, a high-power laser beam is focused on small droplet targets of metal, such as tin, to form a highly ionized plasma that emits EUV radiation with a peak maximum emission at 13.5 nm.

The intensity of the EUV radiation produced by LPP depends on the effectiveness with which the high-powered laser can produce the plasma from the droplet targets. Precise synchronization of the pulses of the high-powered laser with generation and movement of the droplet targets is desired to improve the efficiency of an LPP-based EUV radiation source. The laser-produced plasma may generate a shock wave in the LPP-based EUV source and the momentum carried by the shock wave may be transferred to the next droplet and cause droplet position deviation such that the next laser pulse may not efficiently hit the next droplet or may even miss the next droplet. A monitoring system to determine when a shock wave is generated and the extent of the shock wave and a control method to minimize the effect of the shock wave is desirable.

