Search

The focusing characteristics of a ruthenium-coated cylindrical mirror were investigated on the basis of its ability to collect and focus broadband 5-17-nm soft X-rays emitted from a laser-produced plasma. Based on the plasmas spectral intensity distribution and the reflectivity function of the mirror, we defined the optimum position of the integrated cylindrical mirror at which the X-ray energy flux transported and focused through the mirror was maximum. A minimum spot diameter of 22 mm at a distance of approximately 200 mm from a soft X-ray source was confirmed. The maximum intensity of the collected soft X-rays was 1.3 mJ/cm2 at the center of the irradiation zone. Thus, the irradiation intensity was improved by approximately 27 times when compared to that of 47 μJ/cm2 without the mirror. The debris sputtering rate on the reflection surface of the mirror can be reduced to 1/110 by argon gas at 11 Pa, while the attenuation rate of the soft X-rays due to absorption by the buffer gas can be suppressed to less than 10% at the focal point. The focusing property of the mirror is expected to be maintained for 3000 h or longer without significant degradation for a 100 W/320 pps laser shot if the ruthenium layer is thicker than 10 μm. These results suggest that a stand-alone broadband soft X-ray processing system can be realized by using laser-produced plasma soft X-rays. [ABSTRACT FROM AUTHOR]

We investigate the characteristics of laser plasma debris that is responsible for damaging optics. The debris is composed of fast ions, neutral particles, and fragments, and originates from a solid Xe target on a rotating drum that we developed as an extreme ultraviolet (EUV) source. The ice fragments appear to be a problem most notably with solid Xe targets; however, we find that the damage induced by Xe ice fragments can be avoided by simply reducing the laser pulse energy. We find the number of fast neutral particles to be an order of magnitude less than the number of ions, and we clarify that the plasma debris is primarily composed of fast ions. In addition, we find that the number of fast ions having a few dozen keV of energy decreases when using the rotating target compared with the rest target. We attribute this to a gas curtain effect from the Xe gas localized at the rotating target surface. We estimate the sputtering rate of the Mo/Si mirror, which is caused primarily by the fast ions, to be 104 nm/1×106 shots at 190 mm from the source plasma and at an 11.25° angle from the incident laser beam. Up to the 1×106 shots exposure, remarkable degradation of the mirror reflectivity is not observed though the sputtering damages the mirror. Mitigation of the ions by using gas and/or magnetic fields will further improve the mirror lifetime. By comparing with a liquid jet Xe target, we conclude that the sputtering rate per conversion efficiency when using the solid Xe targets on the rotating drum is the same as that when using the liquid Xe targets. The high conversion efficiency of 0.9% in the rotating drum solid Xe target makes this technique useful for developing laser plasma EUV sources. [ABSTRACT FROM AUTHOR]

A cryogenic Xe jet system with an annular nozzle has been developed in order to continuously fast supply a Xe capillary target for generating a laser-plasma extreme ultraviolet (EUV) source. The cooling power of the system was evaluated to be 54 W, and the temperature stability was ±0.5 K at a cooling temperature of about 180 K. We investigated experimentally the influence of pressure loss inside an annular nozzle on target formation by shortening the nozzle length. Spraying caused by cavitation was mostly suppressed by mitigating the pressure loss, and a focused jet was formed. Around a liquid-solid boundary, a solid-Xe capillary target (100/70 μm [lowercase_phi_synonym]) was formed with a velocity of <=0.01 m/s. Laser-plasma EUV generation was tested by focusing a Nd:YAG laser beam on the target. The results suggested that an even thinner-walled capillary target is required to realize the inertial confinement effect. [ABSTRACT FROM AUTHOR]

In order to generate high-average-power extreme ultraviolet radiation around 13.5 nm from laser-produced plasma, a fast rotating cryogenic drum system which can continuously supply a solid Xe target for high-repetition laser pulses has been developed. It was successfully operated at a rotating speed of 1000 rpm and an up-down speed of 3 mm/s. The growth rate of the solid Xe layer is kept at a constant speed (for example, 18 μm/s) with a constant rate of Xe input flow, independent of the rotating speed. It has been demonstrated that wipers play an important role in recovering the laser craters with a recovery rate of 150 μm/s. These performances allow laser illuminations on the solid Xe target with a pulse repetition rate of 10 kHz without overlapping laser craters. [ABSTRACT FROM AUTHOR]

A new cross-beam technique has been developed to measure the charge transfer cross sections of metallic atoms which have not been experimentally investigated previously. Two high-density atomic beams were produced by the vaporization of a metal heated with an electron beam gun. A pulsed ion beam produced from one atomic beam by laser photoionization was collided with another atomic beam. By colliding the pulsed ion beam produced in this manner with the second atomic beam, the apparatus was simplified in comparison with conventional one; additionally, only a single pulsed laser shot is required to obtain the data necessary to determine a cross section. By using repetitive laser pulses it is possible to acquire in a short time a more accurate cross section as a function of impact energy. The apparatus was successfully applied to the measurement of the charge transfer cross section of gadolinium, a metallic atom with a relatively high ionization potential and high melting point. [ABSTRACT FROM AUTHOR]

Maximum extreme ultraviolet (EUV) conversion efficiencies (CEs) of 2.3% and 1.8% were achieved in planar Li targets by using pulsed 2ω and 1ω Nd:YAG laser irradiation, respectively. In a forced recombination scheme, the total CE can be expected to be about 4%. The maximum kinetic energy of the lithium ion debris was found to be less than 1 keV, indicating that mirror damage caused by lithium ion debris is more easily mitigated by using a magnetic field than for tin ions. These results suggest that a Li target is a reasonable candidate for an EUV lithography source. [ABSTRACT FROM AUTHOR]

An extreme ultraviolet source having a tamper has been studied. This target scheme recombines forcedly lithium ions by low temperature electrons from the tamper, converting Li3+ rapidly to excited Li2+ which emit intense 1s-2p Lyman α emissions at 13.5 nm. A strong 13.5 nm emission appeared at 20–30 ns after the time of laser peak within a small space volume near the tamper. The authors obtained an enhancement of extreme ultraviolet conversion efficiency by a factor of about 2 with the tamper against that of a target without the tamper at the same laser irradiation condition. [ABSTRACT FROM AUTHOR]

The extreme ultraviolet emission characteristics of laser-produced lithium plasma are experimentally and theoretically investigated. Taking into account the target oxidation, the ion fractional populations are analyzed and the averaged plasma parameters are obtained. Theoretical results show an insignificant influence of oxygen on the temperature dependence of the lithium ion fractional number. A comparison between the theoretical spectrum for stationary and homogenous plasma and experiments shows a reasonable agreement. [ABSTRACT FROM AUTHOR]

Conversion efficiency and spectra of extreme ultraviolet radiations from a cryogenic planar solid xenon target were investigated as a function of laser wavelength (ω, 2ω, and 3ω Nd:YAG) and the laser focus spot size (50–700 μm) at the intensity 1010–5×1012 W/cm2. The conversion efficiency increased with laser intensity and reached the maximum value at about 1011 W/cm2 for all colors. It was found that an edge effect appears more strongly at the ω-laser case, indicating more lateral energy loss, while it appears only weakly for higher harmonics. Shorter-wavelength lasers generated significant conversion efficiencies even at lower laser energies; that is, with smaller laser spots. As the wavelength decreased from ω, 2ω, and 3ω, a spectral hump appeared in the extreme ultraviolet band around 13.5 nm region, while the spectral intensity at 10.8 nm drastically decreased. High-energy photon generation in the tail of 10.8 nm peak was found to be strongly suppressed at shorter-wavelength laser (3ω), while the conversion efficiency at 13.5 nm was as large as that at ω. This indicates that a Xe[XI] ion-rich plasma have been efficiently produced in the ablation plasma by using 3ω laser without overheating the underdense plasma responsible for extreme ultraviolet emission. [ABSTRACT FROM AUTHOR]

Studies the soft x-ray emissions from laser plasma of cryogenic mixture targets. Uses of a laser plasma x-ray source (LPXS); Calculation of the x-ray spectral intensity per unit solid angle using the count number; Laser intensity dependence of x-ray conversion efficiencies.

Soft x-ray spectral radiations from Xe, H[sub 2]O, and CO[sub 2] cryogenic targets irradiated by a 1 μm neodymium doped YAG-slab laser at pulse widths of 12–20 ns and at laser intensities of 5×10[sup 10]–10[sup 12] W/cm[sup 2] have been observed. These targets radiate soft x-rays in a wavelength range of 10–13 nm which is useful for projection microlithography. We have found a strong x-ray spectral peak at λ=10.8 nm with a Xe cryogenic target. The measured x-ray conversion efficiency with the Xe target was 0.8%/sr(λ=10.8±0.27 nm) at a laser intensity of 1×10[sup 12] W/cm[sup 2]. This was ten times or more efficient than that with H[sub 2]O and CO[sub 2] targets. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]