Development of High Performance EUV Light Source Using Condensed Cone-center-axis Plasma Generated by Laser with Nano-second Pulse-power

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To create a future system of extreme-ultraviolet (EUV) lithography for 45nmhp semiconductor production, an innovative light source with 13.5nm wavelength is demanded very high power over 115W at intermediate focus and long operation life at a high repetition rate over 7kHz. Recently, to generate EUV, two methods have been developed with laser produced plasma (LPP) and discharge produced plasma (DPP). Tin target has significant potential for high conversion efficiency at 13.5 nm of Sn spectrum peaks. In this study, we propose a new method to develop high performance EUV light source by using holed Sn target. When a pulse laser is irradiated to the conical hole, high density plasma can be generated to its center-axis. This condensed plasma is excited into high temperature by focused laser with nano-second pulse power and then emits the light with shorter wavelength including EUV. This light can be collected through the open hole at the backside. After this emission, this cone-center-axis plasma is cooling and becomes debris that is a serious problem in the EUV source. Using this holed target, the debris is remarkably decreased toward its backside. EUV emission through its hole is narrow directivity. For commercial EUV light source, repetitive use of the target is required. We irradiated holed targets repeatedly and measured EUV intensity through the holes. After the laser was irradiated several times, EUV intensity is approximately stable. This result suggests that this target can be used repeatedly. After the experiment, the irradiated cone was changed to cylindrical hole. Therefore, We measured the EUV intensity from new cylindrically holed target and obtained the maximum value of total EUV emission from the outlet of the hole. Our targets have other advantages for repetitive use and reducing debris from the outlet of the hole.