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This article provides an up-to-date overview of the problems associated with the detection of hot electrons escaping from laser-produced plasma and corresponding return current flowing from the ground to the target, which neutralises the positive charge occurring on the target due to the escaped electrons. In addition, the target holder system acts as an antenna emitting an electromagnetic pulse (EMP), which is powered by the return target. If the amount of positive charge generated on the target is equal to the amount of charge carried away from the plasma by the escaping electrons, the measurement of the return current makes it possible to determine this charge, and thus also the number of escaped electrons. Methods of return current detection in the mA–10 kA range is presented, and the corresponding charge is compared to the charge determined using calibrated magnetic electron energy analysers. The influence of grounded and insulated targets on the number of escaped electrons and EMP intensity is discussed. In addition to EMP detection, mapping of the electrical potential near the target is mentioned.

Over a wide and partly overlapping energy range, the single-electron capture cross-sections for collisions of metastable Sn 2 + (5 s 5 p   P o 3) ( Sn 2 + ∗ ) ions with H 2 molecules were measured (0.1–10 keV) and calculated (0.3–1000 keV). The semi-classical calculations use a close-coupling method on a basis of electronic wavefunctions of the (SnH2)2+ system. The experimental cross-sections were extracted from double collisions in a crossed-beam experiment of Sn 3 + with H 2 . The measured capture cross-sections for Sn 2 + ∗ show good agreement with the calculations between 2 and 10 keV, but increase toward lower energies, whereas the calculations decrease. Additional Landau–Zener calculations were performed and show that the inclusion of spin-orbit splitting cannot explain the large cross-sections at the lowest energies which we now assume to be likely due to vibrational effects in the molecular hydrogen target. [ABSTRACT FROM AUTHOR]

We consider monochromators designed for operation with a point-like soft X-ray laser-plasma radiation source and intended for characterizing mirrors and other X-ray optical elements, including multilayer X-ray optics. Three compact (~0.6 m) high-resolution monochromators are calculated: a Hettrick–Underwood scheme with a plane VLS grating for the range from 110 to 160 Å, a single-element monochromator with a translatable plane VLS grating (120–155 Å), and a single-element monochromator with a classical spherical Rowland grating (110–160 Å). The spectral resolution of the configurations is assessed using numerical ray tracing. The capabilities of these monochromators are compared with the capabilities of monochromators using the three-element Czerny–Turner scheme. The "throughput" factor is used as a comparative criterion (figure of merit), provided that the spectral resolving power is sufficiently high (conventionally λ/δλ ~ 500 or higher). [ABSTRACT FROM AUTHOR]

The concept of dense and hot plasmas can be used to build up powerful and brilliant radiation sources in the soft X-ray and extreme ultraviolet spectral range. Such sources are used for nanoscale imaging and structuring applications, such as EUV lithography in the semiconductor industry. An understanding of light-generating atomic processes and radiation transport within the plasma is mandatory for optimization. The basic principles and technical concepts using either a pulsed laser or a gas discharge for plasma generation are presented, and critical aspects in the ionization dynamics are outlined within the framework of a simplified atomic physics model. [ABSTRACT FROM AUTHOR]

We describe the experimental implementation of high-resolution soft X-ray spectrographs, which are stigmatic throughout their operating range. The optical configuration comprises a grazing-incidence plane VLS grating and a broadband normal-incidence focusing mirror with an aperiodic multilayer coating structure. The operating range is defined by the aperiodic multilayer mirror in use (Mo/Si: 12.5 – 25 nm; Mo/Be: 11 – 14 nm). The spectral resolution is determined by CCD detector resolution and is numerically equal to the product of the plate scale and the double pixel size. Vertically spaceresolved laser-produced plasma spectra of multiply charged ions are presented. The spatial resolution is equal to about 26 μm, the double pixel size. We discuss the prospect of extending high-resolution stigmatic spectral imaging below 11 nm and outline the data of numerical calculations of broadband normal-incidence mirrors based on aperiodic Ru/Sr and La/B4C multilayer structures. [ABSTRACT FROM AUTHOR]

Over a wide and partly overlapping energy range, the single-electron capture cross-sections for collisions of metastable Sn2+(5s5p Po3) (Sn2+∗) ions with H2 molecules were measured (0.1–10 keV) and calculated (0.3–1000 keV). The semi-classical calculations use a close-coupling method on a basis of electronic wavefunctions of the (SnH2)2+ system. The experimental cross-sections were extracted from double collisions in a crossed-beam experiment of Sn3+ with H2. The measured capture cross-sections for Sn2+∗ show good agreement with the calculations between 2 and 10 keV, but increase toward lower energies, whereas the calculations decrease. Additional Landau–Zener calculations were performed and show that the inclusion of spin-orbit splitting cannot explain the large cross-sections at the lowest energies which we now assume to be likely due to vibrational effects in the molecular hydrogen target.

Extreme ultraviolet lithography (EUVL) has been demonstrated to meet the industrial requirements of new-generation semiconductor fabrication. The development of high-power EUV sources is a long-term critical challenge to the implementation of EUVL in high-volume manufacturing (HVM), together with other technologies such as photoresist and mask. Historically, both theoretical studies and experiments have clearly indicated that the CO2 laser-produced plasma (LPP) system is a promising solution for EUVL source, able to realize high conversion efficiency (CE) and output power. Currently, ASML's NXE:3400B EUV scanner configuring CO2 LPP source system has been installed and operated at chipmaker customers. Meanwhile, other research teams have made different progresses in the development of LPP EUV sources. However, in their technologies, some critical areas need to be further improved to meet the requirements of 5 nm node and below. Critically needed improvements include higher laser power, stable droplet generation system and longer collector lifetime. In this paper, we describe the performance characteristics of the laser system, droplet generator and mirror collector for different EUV sources, and also the new development results.

We report a comparative study of a single plasma and a colliding laser produced plasma, investigated using a Faraday cup. An enhancement in ion emission and stagnation is observed in colliding plasma plume compared to single plasma plume. We observed that fast ion generation in laser ablated plasma can be achieved at large laser intensity on to the target. As laser intensity increases ionic yield increases for both colliding and single plume and at a fixed laser intensity ionic yield decreases with increase in ambient pressure. The double peak structure is observed in the ion signal at large fluence where the peaks correspond to fast and slow species. A Faraday cup composed of nine collectors is used to measure the spatial/angular distribution of ion of expanding plasma plume. Ionic yield is found to be larger in the colliding plasma plume than the single plasma plume at all spatial/angular positions.

The concept of dense and hot plasmas can be used to build up powerful and brilliant radiation sources in the soft X-ray and extreme ultraviolet spectral range. Such sources are used for nanoscale imaging and structuring applications, such as EUV lithography in the semiconductor industry. An understanding of light-generating atomic processes and radiation transport within the plasma is mandatory for optimization. The basic principles and technical concepts using either a pulsed laser or a gas discharge for plasma generation are presented, and critical aspects in the ionization dynamics are outlined within the framework of a simplified atomic physics model.

We have prepared various low-density Gd targets to investigate the effect of concentration on the extreme ultraviolet (EUV) radiation near 6.7 nm and out-of-band emission from laser-produced plasma source. Results show that an increased EUV radiation intensity and narrow-band spectra from the low-density target was obtained. Comparing with the original solid Gd target, the intensity and spectral purity of the 6.7 nm EUV radiation from the 40 % mass concentration Gd target are enhanced 45 % and 74.5 %, respectively. And the FWHM of the spectra are decreased to one-half of that from the solid target. The measurements of the plasma electron density for various concentration targets shown that the low-density target exhibited lower plasma electron density, which reduced the self-absorption effect and the yield of the low-charge ions in the plasma, resulting in the enhancement of EUV radiation intensity as well as narrower band spectra output. Finally, the out-of-band radiation in the range of 325–385 nm from the 40 % concentration Gd target is decreased to approximately 60 % of the solid target case, while the angular distribution profile was significantly uniform. Our findings are beneficial to the Gd target as a new promising source candidate for the EUV nanolithography in the future. • The relationship between target concentration and EUV characteristics was obtained, the optimal target was determined. • The mechanism of low-density was analyzed by the measurements of plasma electron density using interferometry. • The OOB radiation of low-density targets was significantly decreases, the angular distribution profile was uniform. [ABSTRACT FROM AUTHOR]

The interaction of a supersonic plasma jet outflow with a background laser-produced plasma plume is experimentally studied. The aim of these studies is to provide an experimental platform to study jet-ambient interactions in an environment relevant to high energy density physics scenarios. The jet outflow is produced by a conical wire array Z-pinch composed of 16 aluminum wires (40 μm diameter each) acting as load of the Llampudken generator (∼400kA, ∼350 ns). The laser plasma plume is produced by focusing a 2 × 1010 W/cm2 laser pulse onto an aluminum target. Our experimental results show that in the absence of laser plasma plume, there is no significant photoionized plasma from the target that interacts with the jet outflow. On the contrary, when combining both plasma sources, a new structure appears at the interaction region. The thickness of this structure is of the order of the inter-particle ion-ion mean free path, which was calculated for a wide range of parameters for each plasma. These results indicate the presence of a collisional shock layer created after the interaction. Further details and potential applications are shown and discussed.

We have studied the laser pulse shape dependence of the conversion efficiency of λ = 1.03 μm laser pulse energy into 13.5 nm extreme ultraviolet (EUV) emission from a Sn laser-produced plasma. Laser pulses of arbitrary temporal shape ranging from hundreds of picoseconds to several nanoseconds were generated using a programmable pulse synthesizer based on a diode-pumped chirped pulse amplification Yb: YAG laser. Measurements show that the conversion efficiency favors the use of nearly square pulses of duration longer than 2 ns, in agreement with hydrodynamic/atomic physics simulations. A 35% increase in conversion efficiency was obtained when Q-switched pulses were substituted by square pulses of a similar duration. Experiments conducted irradiating a Sn target with a sequence of two time-delayed 250 ps pulses showed a 30 percent increase in the EUV yield respect to a single pulse of the same total energy when the pulse separation was optimum at 2.1 ns. This suggests that re-heating of the plasma with delayed laser pulses could be used to improve the EUV yield. The spectroscopic characterization of EUV emission and in-band EUV images that characterize the source size are also presented.

In this study, we use the FLASH radiation hydrodynamic code and the FLYCHK atomic code to investigate the energy conversion and spectra associated with laserâ€"Sn target interactions with 1 and 2 µ m wavelength lasers. We found that the conversion efficiency (CE) reached as much as 3.38% with the 2 µ m laser, which is 1.48 percentage points higher than the 1 µ m laser (CE = 1.9%). In addition, we analyzed the contribution of dominant ionization states to the emission spectrum for both lasers. We observed that the growths of the out-of-band emission eventually led to a broadening of the spectrum, resulting in a reduction of spectral purity for the 1 µ m laser. By contrast, the emission main peaks were all centered near 13.5 nm for the 2 µ m laser, which is beneficial for efficient emission of light with a 13.5 nm wavelength (relevant for nanolithographic applications). [ABSTRACT FROM AUTHOR]

In this paper we present a high repetition rate experimental platform for examining the spatial structure and evolution of Biermann-generated magnetic fields in laser-produced plasmas. We have extended the work of prior experiments, which spanned over millimeter scales, by spatially measuring magnetic fields in multiple planes on centimeter scales over thousands of laser shots. Measurements with magnetic flux probes show azimuthally symmetric magnetic fields that range from 60 G at 0.7 cm from the target to 7 G at 4.2 cm from the target. The expansion rate of the magnetic fields and evolution of current density structures are also mapped and examined. Electron temperature and density of the laser-produced plasma are measured with optical Thomson scattering and used to directly calculate a magnetic Reynolds number of $1.4\times {10}^4$ , confirming that magnetic advection is dominant at $\ge 1.5$ cm from the target surface. The results are compared to FLASH simulations, which show qualitative agreement with the data. [ABSTRACT FROM AUTHOR]

A series of experiments is described which were conducted to measure the absolute spectral irradiances of laser plasmas created from metal targets over the wavelength region of 123–164 nm by two separate 1.0 μm lasers, i.e., using 100 Hz, 10 ns, 2–20 kHz, 60–100 ns full-width-at-half-maximum pulses. A maximum radiation conversion efficiency of ≈3%/2 π sr is measured over a wavelength region from ≈125 to 160 nm. A developed collisional-radiative solver and radiation-hydrodynamics simulations in comparison to the spectra detected by the Seya–Namioka-type monochromator reveal the strong broadband experimental radiations which mainly originate from bound–bound transitions of low-ionized charges superimposed on a strong continuum from a dense plasma with an electron temperature of less than 10 eV. [ABSTRACT FROM AUTHOR]

We present the experimental investigations of extreme ultraviolet (EUV) radiation of laser-produced plasma based on the micro-structured Gd target and obtain a strong EUV radiation intensity at the wavelength of 6.7 nm. The intensive extreme ultraviolet radiation and narrow-band spectra are achieved by laser irradiation in the groove position of the target. The spectral widths from the micro-structured target are approximately two-thirds width of that of the flat target, and the EUV intensity enhancement ratio is 1.48 comparing with that of the flat case. The measurements of the electron density of Gd plasma were performed using Nomarski interferometry. The results show that the plasma generated by the micro-structured Gd target is confined by the groove between the micro-columns. The plasma electron density distribution is mainly concentrated in the laser incidence direction and the density is higher than that of planar target. Finally, the parameters of the groove width of structured target and laser energy are varied to enhance the EUV radiation. This study shows that the confinement effect of the structured target can improve the extreme ultraviolet radiation. Those findings are expected to provide new approaches in developing the next-generation extreme ultraviolet light source. • A laser-produced plasma of 6.7 nm extreme ultraviolet radiation based on the micro-structured target is proposed. • A high intensity and narrowband spectrum is achieved. • The confinement mechanism of micro-structured target on the EUV radiation is revealed. • The characterization of plasma electron density distribution is obtained by using interferometry. [ABSTRACT FROM AUTHOR]

Irradiation of a thermonuclear capsule by a laser-induced X-ray pulse (indirect drive) is a way to maximally reduce the inhomogeneity of the capsule heating and, thereby, to minimize the negative influence of hydrodynamic instabilities on its compression. Nevertheless, the application of this method in the world's largest NIF facility with a laser pulse energy of about 1.8 MJ at the Lawrence Livermore National Laboratory (LLNL), USA has not yet provided the ignition of indirect-drive thermonuclear capsule – obtaining of the deuterium–tritium reaction energy equal to the spent laser energy. We devote this paper to the theoretical study of compression and burning of thermonuclear capsule with different degrees of violation of its heating uniformity by laser-induced X-ray pulse within the framework of further development of the indirect-drive approach. [ABSTRACT FROM AUTHOR]

We have studied the laser pulse shape dependence of the conversion efficiency of λ = 1.03 μm laser pulse energy into 13.5 nm extreme ultraviolet (EUV) emission from a Sn laser-produced plasma. Laser pulses of arbitrary temporal shape ranging from hundreds of picoseconds to several nanoseconds were generated using a programmable pulse synthesizer based on a diode-pumped chirped pulse amplification Yb: YAG laser. Measurements show that the conversion efficiency favors the use of nearly square pulses of duration longer than 2 ns, in agreement with hydrodynamic/atomic physics simulations. A 35% increase in conversion efficiency was obtained when Q-switched pulses were substituted by square pulses of a similar duration. Experiments conducted irradiating a Sn target with a sequence of two time-delayed 250 ps pulses showed a 30 percent increase in the EUV yield respect to a single pulse of the same total energy when the pulse separation was optimum at 2.1 ns. This suggests that re-heating of the plasma with delayed laser pulses could be used to improve the EUV yield. The spectroscopic characterization of EUV emission and in-band EUV images that characterize the source size are also presented. [ABSTRACT FROM AUTHOR]