Experimental research on laser-produced Gd target plasma source for 6.7 nm lithography

Extreme ultraviolet (EUV) lithography at λ =6.7 nm is a challenging subject for next generation semiconductor lithography beyond 13.5 nm. The availability of strong radiation at the operating wavelength and low-debris of the plasma source are the two most important aspects for the development of laser-produced Gd plasma source at 6.7 nm. In this paper, experimental research on the extreme ultraviolet source based on the laser-produced Gd plasma is performed. Strong radiation near 6.7 nm from the source has been obtained, which is attributed to the n=4-n=4 transitions in Gd ions that overlap to yield an intense unresolved transition array (UTA). Dependence of spectral variation near the strong emission region of Gd plasma on the incident laser power density and detection angles is given. It is found that the intensity of EUV radiation around 6.7 nm is increased with increasing laser power density, and the emission peak around 7.1 nm increases faster than that of emission peak around 6.7 nm after the laser intensity reaching 6.4×1011 W/cm2, which is ascribed to the unique spectroscopic behavior of Gd ions. In addition, the energy of the ion debris from laser-produced Gd plasma source as well as the angular distribution of the ion yield off the target normal are measured with Faraday cup. Results show that the ion energy corresponding to the peak position of Gd ion energy distribution is about 2.6 keV at 10° off the target normal, and the yield of Gd ions decreases with the increase of the angle from the target normal. Furthermore, the stopping ability of an ambient magnetic field for ion debris from laser Gd plasma source is evaluated, and the result shows that the energetic Gd ion can be effectively mitigated by applying a 0.9 T magnetic field.