Study On Pulsed Laser Produced Droplet Plasma

With the development of the semiconductor industry,the degree of chip integration has been greatly improved,and the lithography technology,which is critical for chip preparation,also needs to be further improved.In the future,the most promising lithography technology is extreme ultraviolet lithography technology(EUVL),which can achieve the feature sizes of 7 nm and below.With its advantages of high conversion rate and low debris,laser produced droplet plasma has become the most potential way to produce 13.5 nm extreme ultraviolet source(EUV).However,in order to apply the EUV source to industrial production,it is necessary to further improve the EUV radiation efficiency and lower the generation of the plasma debris.Both of these aspects are closely related to the physical characteristics of the laser plasma,therefore,based on the water droplets target and the laser and droplet synchronous system,the study of the characteristics of pulsed laser produced droplet plasma was carried out.The main contents of this paper are as follows:(1)Firstly,the high frequency droplet was produced by the self-made droplet generator.Based on LED and CCD,a droplet diagnosis system was built to detect the movement and stability of the droplet.The results show that the droplet diameter is about187?m,the average droplet spacing is about 671?m,the injection velocity is about 19m/s,and the center position and droplet spacing of the droplet remain almost unchanged for a period of time.Then,based on the laser-photodiode and other devices,a space-time synchronization system for the interaction of the Nd:YAG laser and the droplet was built to ensure that the laser pulse can accurately act on the droplet target,Which laid a solid foundation for the subsequent research on the characteristics of laser plasma.(2)Based on the direct imaging method,the plume expansion characteristics of laser produce droplet plasma were studied.The evolution of droplet plasma plume intensity,plume expansion distance,plume center and plume morphology with time under different laser power densities were calculated and analyzed.The results show that as the laser energy increases,the radiation intensity of plasma and the distance of plume expansion gradually increases.The expansion of plasma within 100 ns is approximately linear expansion,the expansion velocity tends to zero after 100 ns and the plasma gradually dissipates.(3)Based on the shadow method,the evolution of the shock wave and neutral clusters generated by the laser-induced droplets with time was observed.The expansion distance and expansion speed of the shock wave with time were calculated and analyzed.The results show that laser produced droplets generate neutral clusters and shock waves,and the expansion radius of the shock waves increases linearly with time.When the laser power density is 2.41×10¹¹W/cm²,the expansion speed of the shock wave is about 90 m/s.(4)Based on the self-made Langmuir probe,the electron density and electron temperature of droplet plasma were diagnosed.The volt-ampere characteristic curves of the probe at different time were analyzed.The electron density of the droplet plasma was calculated by the probe current in the ion saturation region,and the electron temperature of the droplet plasma was obtained by linear fitting the transition region.The results show that as the laser energy increases,the electron density and electron temperature of the droplet plasma gradually increase.Within 2?s of plasma formation,the electron density of the plasma first increased and then decreased,reaching a peak at 0.75?s.The electron temperature of the plasma decreases slowly with time.When the laser power density is2.41×10¹¹W/cm²,the maximum electron density of the droplet plasma is about 6×10¹²cm^-3,and the maximum electron temperature is about 13 e V.