ObjectiveSolid-state lasers are expected to replace CO2 lasers as the driving source of a new generation of laser-produced plasma (LPP) light sources for extreme ultraviolet (EUV) lithography, owing to their compact size, high wall-plug efficiency, and high power output potential. However, the plasma density and optical depth of 1-μm laser-produced plasma are larger than those of 10-μm CO2 laser, leading to low energy conversion efficiency (CE) from driving laser to in-band radiation with a center wavelength of 13.5 nm and bandwidth of 2%. Recent research illustrates that the CE of 1-μm laser-produced Sn plasma is expected to meet the engineering requirement of the LPP-EUV source under systematic optimization. In this work, an experimental platform of the LPP-EUV source driven by a 1-μm solid-state laser is established and the EUV radiation characteristics of 1-μm laser-induced solid Sn target plasma are experimentally studied. Under the laser peak power density of 8.24×1010 W/cm2, a maximum CE of 3.42% is achieved, which reaches an advanced level in the world. We hope that the established platform and related research results can provide technical support for domestic research and development of solid-state laser-driven plasma light sources for EUV lithography and inspection.MethodsAn experimental platform of the LPP-EUV source driven by a 1-μm solid-state laser is established first, consisting of a vacuum chamber, a 1-μm Nd∶YAG nanosecond pulsed laser, a digital delay generator, an in-band energy meter, and a flat-field spectrometer. An energy meter and a spectrometer are used for in-band EUV radiation energy measurements and 7?23 nm EUV spectra, respectively. To accurately determine the in-band energy, the calibrated sensitivity of the energy meter is calculated by using the EUV spectrum of Sn plasma and the responsivity at 12.5?14.5 nm calibrated by the metrology beamline in the National Synchrotron Radiation Laboratory (NSRL). The wavelength of the spectrometer was calibrated using the absorption edge of Si and the spectral lines of the Sn ion. The total efficiency of the spectrometer system is calculated using the reflectance of the Au mirror, diffraction efficiency of the grating, and quantum efficiency of the X-ray charge coupled device (CCD). Then, the EUV radiation characteristics of 1-μm laser-induced solid Sn target plasma are studied by the measurement of CEs and EUV spectra of Sn plasma under different laser peak intensities, and values of spectral purity (SP) calculated from EUV spectra.Results and DiscussionsThe EUV radiation at 7-23 nm is unresolved transition arrays (UTAs) centered in around 13.5 nm, emitting from multiply excited states of multiply charged Sn ions in the plasma. As the laser peak power density increases, the EUV radiation intensity below 15 nm gradually increases, and that above 15 nm gradually weakens (Fig. 4). The spectral peak position of the UTA moves from the left to the right of the operating center wavelength of the 13.5 nm light source for EUV lithography. Besides, the UTA appears to sag around 13.5 nm, which is self-absorption phenomenon, because 1-μm laser-induced solid Sn target plasma has a large optical depth. As the laser peak power density increases gradually, the SP and CE first increase and then decrease (Fig. 5). The CE reaches a maximum value of 3.42% at the laser peak power density of 8.24×1010 W/cm2, and the corresponding SP is 7.04%, which is close to the CE value of twice. Before the CE reaches its maximum, the increase in the laser peak power density has a significant effect on the SP and CE. When the peak power density is 8.24×1010 W/cm2, the SP is three times higher than that of 9.97×109 W/cm2, and the CE is five times higher. When the CE reaches its maximum, the SP and CE decrease gradually with an increase in the peak power density. The above results illustrate that the SP and CE are sensitive to variations in the laser peak power density, and it is necessary to optimize the laser peak power density on the Sn target during the development of the LPP-EUV source. The study of the Sn plasma EUV spectrum SP is also very important, as it can further help estimate the upper limit of the CE and provide relevant information about the out-of-band radiation.ConclusionsIn this work, the 1-μm solid-state laser Sn plasma EUV source is studied. An experimental platform of the LPP-EUV source driven by a 1-μm solid-state laser is established. The responsivity of the in-band energy meter and efficiency curve of the flat-field spectrometer are calibrated. Subsequently, the 7?23 nm EUV spectra of the laser-induced solid Sn target plasma are measured under different laser peak power densities. The dependence of the EUV spectrum of the Sn plasma on the laser peak power density and its mechanism are analyzed. The dependency of conversion efficiency and spectral purity on laser peak power density is studied. The results illustrate that the SP and CE are sensitive to the laser peak power density, and it is necessary to finely optimize the laser peak power density on the Sn target during the development of the LPP-EUV source. Under the laser peak power density of 8.24×1010 W/cm2, a maximum CE of 3.42% is achieved, which reaches an advanced level in the world. The established platform and related research results are important for the domestic independent development of EUV lithography and its key devices and technologies.