The ablation depth in planar Sn targets irradiated with a pulsed 1064 nm laser was investigated over laser intensities from 3×1011 to 2×1012 W/cm2. The ablation depth was measured by irradiating a thin layer of Sn evaporated onto a Si wafer, and looking for signatures of Si ions in the expanding plasma with spectroscopic and particle diagnostics. It was found that ablation depth scales with laser intensity to the (5/9)th power, which is consistent with analytical models of steady-state laser ablation, as well as empirical formulae from previous studies of mass ablation rate in overlapping parameter space. In addition, the scaling of mass ablation rate with atomic number of the target as given by empirical formulae in previous studies using targets such as C and Al, are shown to remain valid for the higher atomic number of the target (Z=50) used in these experiments.

The effect of pulse duration on in-band (2% bandwidth) conversion efficiency (CE) from a CO2 laser to 13.5nm extreme ultraviolet (EUV) light was investigated for Sn plasma. It was found that high in-band CE, 2.6%, is consistently obtained using a CO2 laser with pulse durations from 25to110ns. Employing a long pulse, for example, 110ns, in a CO2 laser system used in an EUV lithography source could make the system significantly more efficient, simpler, and cheaper as compared to that using a short pulse of 25ns or shorter.

The effects of laser wavelength on the charge state resolved ion energy distributions from laser-produced Sn plasma freely expanding into vacuum are investigated. Planar Sn targets are irradiated at laser wavelengths of 10.6 and 1.064 m and intensities of 1.8ϫ 10 10 and 3.4 ϫ 10 11 W / cm 2 , respectively. These parameters are relevant to the extreme ultraviolet x-ray source application. An electrostatic deflection probe and single channel electron multiplier are used to record the charge state resolved ion energy distributions 100 cm from the laser plasma source. At the longer laser wavelength, higher charge state ions are observed. At both laser wavelengths, the peak ion energies increase approximately linearly as a function of charge state, and all ion energies greatly exceed the initial thermal electron temperature. The differences in the ion energy distributions are attributed to the laser wavelength dependence of the laser energy absorption, the resulting plasma density in the corona, and the subsequent recombination after the laser pulse. Numerical simulations of the plasma expansion from a collisional-radiative steady state model support the experimental results.

The effect of laser focal spot diameters of 26 and 150 m on 13.5 nm extreme ultraviolet ͑EUV͒ radiation is investigated. Simulations show that the smaller spot size has a shorter electron plasma density scale length and deeper and denser laser energy deposition region. This results in additional time required for plasma expansion and radiation transport to efficiently emit EUV light. This is experimentally observed as an increase in the delay between the EUV emission and the laser pulse. The shorter scale length plasma reabsorbs less EUV light, resulting in a higher conversion efficiency, smaller and slightly brighter light source.

The impact of 1.064 μm laser absorption depth on the heating and in-band (2% bandwidth) 13.5 nm extreme ultraviolet emissions in Sn plasmas is investigated experimentally and numerically. In-band emission lasting longer than the laser pulse and separation between the laser absorption and in-band emission region are observed. Maximum efficiency is achieved by additional heating of the core of the plasma to allow the optimal temperature to expand to a lower and more optically thin density. This leads to higher temperature plasma that emits less in-band light as compared to CO2 produced plasma sources for the same application.