Low-energy ion emission from a xenon gas-puff laser-plasma X-ray source

We have measured low-energy ion emission from a gas-puff laser-plasma X-ray source. The ions may cause the degradation of the condenser mirror of the extreme ultra-violet projection lithography system. A 0.7 J in 8 ns Nd:YAG laser at 1.06 mum was focused onto the xenon gas-puff target with an intens ity of similar to 10(12) W/cm(2). The silicon (111) plates, placed at a dis tance of 32 mm from the laser-interaction region, were exposed with the xen on ions. The average ion energy was measured to be less than 50 eV with a F araday-cup detector placed close to the silicon plates. The xenon depositio n occurred in the silicon plates with a depth of less than 40 nm. The depos ition density was measured with a quadrupole secondary ion mass spectromete r to be 10(21)/cm(3) after 1500 laser shots. The energy-conversion efficien cy from the laser energy into the ions is similar to 0.1%/4 pi sr/shot. For the lithography system, if we can remove such ion bombardment completely u sing novel techniques such as electro-magnetic devices or gas flow curtain techniques, the lifetime of the condenser mirror will be extended significa ntly.