More efficient plasmas

Appl. Phys. Express 7, 086202 (2014)

Credit: JSAP

Although laser-produced plasmas are promising as a new source technology for realizing extreme ultraviolet (EUV) radiation and soft X-rays, the generation process is not yet very efficient. Ideally, improvements are needed for the scheme to become practical for applications such as next generation EUV lithography, which is needed for the fabrication of ever smaller transistors in microelectronics. Kensuke Yoshida and co-workers from Japan, Ireland and China have now achieved one of the highest conversion efficiencies to date by using twelve laser beams to produce spherical plasmas. The team used the powerful GEKKO-XII system in Japan, a giant Nd:glass laser at Osaka University, which has twelve laser beams, each with a pulse energy of 1 joule at a wavelength of 1.053 μm. The total 12 joules of energy, delivered within a 1.3 ns Gaussian pulse, irradiated spherical targets (polystyrene balls coated with a 2-μm-thick metallic layer of Gd, Tb or Mo) almost uniformly by positioning the incident beams at twelve faces of a regular dodecahedron. By adjusting the size of the laser spot diameters from 100 to 1,000 μm, the laser intensity on the target surface could be varied from 3 × 10¹¹ W cm⁻² to 3 × 10¹³ W cm⁻². The international collaboration of researchers found that an optimum laser intensity of 1 × 10¹² W cm⁻² yielded an electron temperature of around 100 eV and an EUV conversion efficiency of 0.8%. Using plasmas with this spherical geometry is more efficient for light generation as there is significantly reduced kinetic energy loss compared with planar targets.