UC San Diego

Laser produced Sn plasma, in its role as an efficient extreme ultraviolet (EUV) x-ray source, is being studied extensively in support of next generation manufacturing of integrated circuits by nanolithography. The ability to diagnose and manipulate the properties of ions emitted from the laser produced plasma (LPP) must be achieved in order for the technology to meet stringent performance requirements. Here we study the emission and expansion of ions from Sn LPP, in parameter space relevant to the EUV x -ray source application. Several particle and radiation plasma diagnostics, in addition to analytical and numerical analysis, are all used to elucidate the complex relationships between the target properties, irradiation conditions, and resultant plasma and ion properties. Two specific laser systems of current interest to the application, at wavelengths of 1.064 micrometers and 10.6 micrometers are both utilized, which allows for direct comparisons of the effects of laser wavelength on ion properties. Details of the available experimental apparatus, including the Nd:YAG and CO₂ laser systems, are discussed first. Following, the design and realization of a custom charged particle plasma diagnostic, hereafter referred to as the ion probe, is described. The successful development of the ion probe enabled measurements of the energy distribution for each charge state of each ion species in expanding plasma, which is a new diagnostic capability. Measurements of mass ablation from Sn plasma produced by a 1.064 micrometer laser are discussed next, specifically the scaling of mass ablation rate with laser intensity. These measurements are useful in the design of mass-limited targets, and also are used to infer mechanisms of laser energy absorption and heat conduction within the plasma. In addition to the ion probe, an EUV spectrometer and a calibrated EUV calorimeter were both utilized as diagnostics to measure the mass ablation rate by complementary methods. Laser intensity was scanned from 3x10¹¹W/cm² to 2x10¹² W/cm², encompassing parameter space of the EUV x-ray source application to the low end of parameter space of the laser fusion application. Accordingly, previous theoretical results relevant to the laser fusion application can be applied in the data analysis. Experiments at two different laser wavelengths to extensively study the dynamics of ion expansion into vacuum are discussed next. In one set of experiments, the ion probe was used to measure energy distributions for all charge states of Sn ions at laser intensities of 3x10¹¹W/ cm² to 2x10¹², from 1.064 micrometer and 10.6 micrometer lasers, respectively. 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. In a second set of experiments, the distance from the target surface over which the charge state distribution evolves in vacuum is investigated. A Faraday cup translated along the path of plasma expansion is utilized in these measurements. It was found that at the longer laser wavelength, the charge state distribution is decaying to lower charge states over distances up to hundreds of millimeters from the target surface, whereas at the shorter laser wavelength the charge state distribution reaches a frozen-in state within a few tens of millimeters from the target surface. These experimental results are used to infer mechanisms of ion acceleration and recombination in the expanding plasma. In the last section, analytical models of ion expansion into vacuum relevant to the experimental results are first discussed. More detailed analyses are then carried out through numerical simulations. First, a zero-dimensional code following the time evolution of plasma temperature and the population of each ion species is discussed and used to verify some qualitative features observed in the experiments. A more complex one-dimensional Lagrangian hydrodynamics code utilizing the zero-dimensional code as one step in its algorithm is then used to simulate ion properties throughout the spatial expansion into vacuum. Due to the significant number of assumptions made in both codes, only qualitative features of the experiments are reproduced