Physics Theses

The work presented in this thesis is primarily concerned with the optimisation of extreme ultraviolet (EUV) photoemission around 13.5 nm, from laser produced tin (Sn) plasmas. EUV lithography has been identified as the leading next generation technology to take over from the current optical lithography systems, due to its potential of printing smaller feature sizes on integrated circuits. Many of the problems hindering the implementation of EUV lithography for high volume manufacturing have been overcome during the past 20 years of development. However, the lack of source power is a major concern for realising EUV lithography and remains a major roadblock that must be overcome. Therefore in order to optimise and improve the EUV emission from Sn laser plasma sources, many parameters contributing to the make-up of an EUV source are investigated.Chapter 3 presents the results of varying several different experimental parameters on the EUV emission from Sn laser plasmas. Several of the laser parameters including the energy, gas mixture, focusing lens position and angle of incidence are changed, while their effect on the EUV emission is studied. Double laser pulse experiments are also carried out by creating plasma targets for the main laser pulse to interact with. The resulting emission is compared to that of a single laser pulse on solid Sn.Chapter 4 investigates tailoring the CO2 laser pulse duration to improve the efficiency of an EUV source set-up. In doing so a new technique for shortening the time duration of the pulse is described. The direct effects of shortening the CO2 laser pulse duration on the EUV emission from Sn are then studied and shown to improve the efficiency of the source.In Chapter 5 a new plasma target type is studied and compared to the previous dual laser experiments. Laser produced colliding plasma jet targets form a new plasma layer, with densities that can be optimised for re-heating with the main CO2 laser pulse.Chapter 6 will present some experiments carried out on laser produced gadolinium plasmas, with its photoemission around 6.7 nm seen as a potential beyond EUV source. Three different laser pulse durations and a range of laser intensities are utilised in experiments to try to optimise the in-band emission, while also observing the effect on ion emission from the plasma. Finally, the experiments presented in thesis and their results are summarised in Chapter 7, along with presenting possible future work.

This thesis presents work done on investigating colliding laser-produced plasmas with time-resolved, UV-visible spectroscopy and time-resolved visible imaging. A nanosecond Nd:YAG laser pulse was split with a wedge prism and the two laser pulses created were focused onto the target surface, with power densities of ϕ = 1:6 x 10^12 W/cm2. The separation between the two plasmas was 2.6 mm and in between them a stagnation layer was formed. Plasmas of silicon (Si, Z=14), tin (Sn, Z=50) and lead (Pb, Z=82) were investigated. Time-resolved spectroscopy was used to determine the expansion velocities for different ion stages of Si and Pb, for both single plasmas as well as in the case where two plasmas collided to yield stagnation layers. Time-resolved visible imaging was used to obtain the expansion velocities of both seed plasmas and stagnation layers plasma-plume front. Colliding plasmas of different elements, Pb and Si, were studied and expansion velocities of different ion stages were compared with those obtained for colliding two identical plasmas. Acceleration of ions due to an electric potential difference is observed in the stagnation layer. Obtaining information about expansion velocities of different ion species provides great insight into the dynamics of laser-produced plasma expansion. Charge and time resolved dynamics have not been used before to study stagnation layers.