A discharge-produced-plasma (DPP) source emitting in the extreme ultraviolet (EUV) spectral region is running at the ENEA Frascati Research Centre. The plasma is generated in low-pressure xenon gas and efficiently emits 100-ns duration radiation pulses in the 10–20-nm wavelength range, with an energy of $20~\text{mJ}/\text{shot}/\text{sr}$ at a 10-Hz repetition rate. The complex discharge evolution is constantly examined and controlled with electrical measurements, while a ns-gated CCD camera allowed observation of the discharge development in the visible, detection of time-resolved plasma-column pinching, and optimization of the pre-ionization timing. Accurately calibrated Zr-filtered PIN diodes are used to monitor the temporal behaviour and energy emission of the EUV pulses, while the calibration of a dosimetric film allows quantitative imaging of the emitted radiation. This comprehensive plasma diagnostics has demonstrated its effectiveness in suitably adjusting the source configuration for several applications, such as exposures of photonic materials and innovative photoresists.

The life time of the first multilayer mirror (the condenser) in optical systems for projection Extreme Ultra-Violet (EUV) lithography based on Laser Produced Plasmas (LPP) is limited both by the debris bombardment and by the EUV radiation photoablation. These problems can be dramatically reduced by using krypton as debris stopper, exploiting its high atomic weight (21 times the helium value) and its high transparency in the EUV region only between 60 and 90 eV. We demonstrate both theoretically and experimentally that by filling the laser-target interaction chamber with krypton at low pressure (a few mbar) it is possible to stop efficiently the small debris (having a size smaller than 1 micron, including ions, neutrals and clusters) while keeping a reasonable transmission of the EUV radiation in the 60-90 eV range. In particular, this is experimentally demonstrated for a solid target LPP based on a tantalum or copper tape target and pumped by a large energy (4 J - 120 ns) XeCl excimer laser. The poor transmission of krypton, out of the above mentioned spectral window, significantly reduces the exposition of the condenser mirror to useless radiation. This new technique for stopping the debris has recently been patented by our group. As far as the larger size debris is regarded, a dramatic reduction of their flux has been obtained by decreasing the laser intensity on the target, still keeping a laser to EUV energy conversion as high as 0.7 percent/eV/sr at 70 eV, from a Ta target, together with a shot to shot EUV pulse energy stability better than 1 percent.