S. Yuspeh

Experimental scaling law for mass ablation rate from a Sn plasma generated by a 1064 nm laser

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.

Laser wavelength effects on the charge state resolved ion energy distributions from laser-produced Sn plasma

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 μm and intensities of 1.8×1010 and 3.4×1011 W/cm2, 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 rec...

Efficient 13.5nm extreme ultraviolet emission from Sn plasma irradiated by a long CO2 laser pulse

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.

Dynamics of laser-produced Sn microplasma for a high-brightness extreme ultraviolet light source

The effect of laser focal spot diameters of 26 and 150 μm on 13.5 nm extreme ultraviolet (EUV) 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.

Optimization of the size ratio of Sn sphere and laser focal spot for an extreme ultraviolet light source

The effect of the ratio of Sn sphere diameter to laser focal spot size (SD/FSS) on conversion efficiency (CE) from laser to in-band (2%) 13.5nm extreme ultraviolet (EUV) light was investigated by fixing the laser spot size and irradiating variable diameter spheres. It was found that a minimum SD/FSS, i.e., 2.5, is necessary to produce high in-band CE, which is 15% higher than planar targets. Two-dimensional plasma density profile maps showed that the density of the dominant in-band EUV emission region and the size of the surrounding absorbing plasma can be manipulated by geometric effects of the SD/FSS ratio.

Two dimensional expansion effects on angular distribution of 13.5nm in-band extreme ultraviolet emission from laser-produced Sn plasma

The angular distribution of extreme ultraviolet emission at 13.5nm within 2% bandwidth was characterized for laser irradiated, planar, Sn targets at prototypic conditions for a lithography system. We have found that two dimensional plasma expansion plays a key role in the distribution of in-band 13.5nm emission under these conditions. The angular distribution was found to have two peaks at 45° and 15°. This complex angular distribution arises from the shape of both the emitting plasma and the surrounding absorbing plasma. This research reveals that the detailed angular distribution can be important to the deduction of conversion efficiency.

Heating dynamics and extreme ultraviolet radiation emission of laser-produced Sn plasmas

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.

Recombination effects during expansion into vacuum in laser produced Sn plasma

The distance over which the charge state distribution evolves during the expansion of laser produced Sn plasma in vacuum is investigated experimentally. This distance is found to be less than 6 cm with a planar target irradiated by a 1.064 μm laser at 8.3×1011 W/cm2 but greater than 60 cm when a 10.6 μm laser at 2.5×1010 W/cm2 is used. The difference is attributed to the laser wavelength dependence of the coronal electron density and the subsequent recombination processes during expansion. Important implications to the extreme ultraviolet x-ray source application are discussed specifically.

Spectroscopic studies of tin plasma using laser induced breakdown spectroscopy

Laser-induced Sn plasma generated at different laser intensities has been characterized using visible emission spectroscopy. A CO2 laser pulse 85 ns in duration is used to generate plasma from a planar Sn sample in a vacuum of 10?5 torr. The plasma electron temperature is inferred by the Boltzmann plot method from singly ionized Sn emission lines, and plasma electron density is inferred using Stark broadened profiles. Electron temperature is measured in the range of (0.53 ? 1.28) eV, and electron density is measured in the range of (9.19?1015 ? 7.45?1016) cm?3, as the laser intensity is varied from (1?1010 to 2.5?1010) W/cm2. The plasma shielding effect has been observed within the laser intensities of (2?1010 ? 2.5?1010) W/cm2.

Interaction of a

The interaction of a CO2 laser pulse with Sn-based plasma for a 13.5-nm extreme ultraviolet (EUV) lithography source was investigated. It was noted that a CO2 laser with wavelength of 10.6 ¿m is more sensitive to surface impurities as compared with a Nd:YAG laser with wavelength of 1.06 ¿m . This reveals that a CO2 laser is more likely absorbed in a thinner layer near the target surface. Compared with a Nd:YAG laser, a CO2 laser shows higher in-band (2% bandwidth) conversion efficiency (CE) with a solid Sn target due to less reabsorption of the EUV emission induced by the plasma. However, with foam targets containing low concentrations of Sn, the in-band CE is lower than that with solid Sn. The CE can be enhanced with plasma confinement. These results suggest that a driving laser with wavelength between 1.06 and 10.6 ¿m may be an even better choice to generate higher CE from laser to 13.5-nm EUV emission.