Irradiation effects of poly(methyl methacrylate) (PMMA) induced by femtosecond-pulsed extreme ultraviolet (EUV) were investigated using Soft X-ray free electron laser (SXFEL) for realization of next generation extreme ultraviolet free electron laser (EUV-FEL) lithography. The sensitivity of PMMA upon exposure to femtosecond-pulsed SXFEL was much higher than that measured for conventional nanosecond-pulsed EUV source. The sensitivity enhancement upon exposure to femtosecond-pulsed SXFEL is similar to the result obtained using laser-induced-plasma based Soft X-ray laser (SXRL) (picosecond-pulsed EUV). This result speculates the reactions induced by femtosecond-pulsed SXFEL and picosecond-pulsed XRL were almost same, but it was different from those induced by nanosecond-pulsed EUV.
The short pulse laser ablation have been extensively studied for confirmation and discussion of damage formation including estimations of damage thresholds and probabilities of surface machining. Irradiation examines by the femtosecond soft xray laser reveals formations of smooth craters on silica glass surfaces, and the appropriate selection of the laser wavelength can make the nanometer size modification on silicon surface in the vicinity of the damage threshold. On the other hand, we also revealed that damage thresholds and modification structures obtained by the picosecond soft x-ray laser irradiation provide the same results as the femtosecond soft x-ray laser. This means that not only femtosecond but also picosecond soft x-ray laser irradiation experiments can be contribute to deep understandings of the ablation phenomena.
We evaluated laser-induced damage thresholds (LIDTs) on silica glasses by two kinds of ultra-short soft X-ray laser pulses (13.5 nm, 70 fs, and 13.9 nm, 7 ps). The comparison of our experimental results and the reported values observed by nanosecond soft X-ray pulse revealed a pulse width dependence of LIDTs on silica glasses in the soft X-ray region. The relationship between the pulse width and LIDTs provided valuable discussions of the laser-induced damage mechanism.
The interactions of short pulse lasers with matter are interesting subjects not only in applications such as surface fabrication but also in physical phenomena for study. Optical short pulse lasers have abilities to occur the ablation phenomena accompanying the creation of high temperature, high pressure, and excited states of electrons. The picosecond soft x-ray laser (SXRL) pulse also has ability to occur the ablation. The SXRL having the wavelength of 13.9 nm and duration of 7 ps is one of attractive x-ray source for ablation study, because the ablation threshold obtained with the focused SXRL pulse is much smaller than those obtained with other lasers having longer durations and/or longer wavelengths. The low ablation threshold of a material for the SXRL beam has a possibility of efficient nanometer scale surface machining by an ablation. The ablation study will lead to the physical research and the direct surface machining. In addition, the wavelength of the SXRL is very close to the wavelength of the extreme ultraviolet (EUV) lithography system (λ = 13.5 nm). In the presentation, we report on development of the soft x-ray laser irradiation system. The irradiation system has an intensity monitor based on the Mo/Si multilayer beam splitter. This intensity monitor provides the irradiation energy onto sample surface. The SXRL has an ability to confirm the ablation threshold and to examine the damage property of EUV optical elements, which have the same specifications of those in the EUV lithography. And more, it is possible to evaluate the doses for sensitivity of resists.
The effect of sub-ns duration and sub-mJ energy laser pulse on 13.5 nm extreme ultraviolet (EUV) source diameter and conversion efficiency has been investigated. It was demonstrated that an in-band EUV source diameter as low as 18 μm has been produced due to short scale length of the picoseconds duration laser plasma. Such EUV source is suitable for high brightness and high repetition rate metrology applications.
We evaluate the EUV emission and the spatial distributions of the plasma parameters by use of the two-dimensional (2-D) radiation hydrodynamic simulation in the microplasma high-brightness EUV source. The expected EUV source size, which is attributed to the expanding microplasma by the hydrodynamic motion, was evaluated to be 15 μm at the laser pulse duration of 150 ps [full width at half-maximum (FWHM)]. The numerical simulation suggests that the high brightness EUV source should be produced by use of a dot target based microplasma with the source diameter less than 20 μm. The emission at 13.5 nm was attributed to Sn charge states between Sn7+ and Sn12+ with the UTA spectral structure.
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