Study On Technology Of Short Pulse CO₂ Laser Amplification And Noise Isolation

The 13.5nm extreme ultraviolet（EUV）source based on laser produced plasma technology has higher lithography resolution than the last generation of 193nm deep UV excimer source,and is the core component of the new generation of large-scale integrated circuit manufacturing equipment.The EUV conversion efficiency of high power,high repetition rate and short pulse CO₂ laser interacting with droplet tin target is up to 6%,thus the high power,high repetition rate and short pulse CO₂ laser is the best main pump of LPP-EUV light source.Its power and stability are the key factors affecting the performance of LPP-EUV.At present,master oscillator power amplifier（MOPA）is used in the main pump CO₂ laser,that is,high repetition rate short pulse CO₂ seed light is generated by the oscillator,and the power is amplified by several CO₂ laser amplifiers.Among them,the amplification and isolation of short pulse CO₂laser is the key technology of high stability and high power MOPA system.For achieving high efficiency amplification of high repetition rate short pulse CO₂ laser,based on MOPA-CO₂ laser experimental platform,this paper systematically studies the amplification and isolation of short pulse CO₂ laser,and carries out a lot of laser dynamic physical modeling,numerical simulation and experimental analysis.The research content mainly includes the following five parts:（1）This paper introduces the development of EUV lithography technology,compares and analyzes three kinds of technology routes to produce EUV source,and focuses on the core components and key technologies of LPP-EUV source.Furthermore,the dynamic model of main pump short pulse CO₂ laser amplification in LPP-EUV source and the development status,technical difficulties and key technical problems of 10.6μm optical isolator at home and abroad are discussed.（2）The temporal characteristics of CO₂ seed light based on electro-optic cavity and continuous fast axial flow CO₂ laser amplifier are described.The relaxation rate coefficients in the amplification process are analyzed and calculated and the research method of dividing the periodic amplification process into three stages is determined.A dynamic model（temperature model）including vibrational and rotational energy levels is established,and the simulation method of gain and pulse evolution of short pulse CO2 laser amplification is given.（3）The influence of the parameters（discharge current,ratio,gas pressure,etc.）of the fast axial flow CO₂ laser amplifier on the small-signal gain and inversion density is simulated and analyzed,and the working parameters of the amplifier are optimized simulated.The pulse waveform evolution of the short pulse CO₂ laser amplifier is revealed,and the influence of the pedestal energy on the amplification efficiency and output waveform is analyzed quantitatively.The experimental research platform of MOPA-CO₂ laser was built,and the working parameters of the amplifier were optimized.The optimized gas pressure and ratio were 78 Torr and CO₂:N₂:He=8.3%:25%:66.7%,respectively.The small-signal gain was increased from 0.7%/cm to 1.1%/cm,which verified the correctness of the dynamic model.（4）The mechanism of various isolation methods at 10.6μm is compared and analyzed.According to the application requirements,the technical route of saturable absorption optical isolator is determined.The effects of gas diffusion effect,residual absorption effect,V-V and V-Tenergy transfer process on SF₆ saturable absorption and isolation performance were analyzed by using―Bath‖model and four-level model respectively.The results show that the gas diffusion makes SF₆ more difficult to saturate,showing the distribution law of low axis of normalized inversion density and high wall;SF₆ isolator has no obvious pulse chopping ability;V-V and V-T energy transfer process play a major role in saturated and unsaturated states respectively,and the correlation between SF₆ and seed light repetition frequency is weak when SF₆ is in saturation state.（5）The relationship between the damage threshold,saturable absorption parameters(α_ns,α_s,I_s)and the number of layers of graphene is measured.The damage mechanism is revealed.The experimental results show that the damage threshold is inversely proportional to the number of layers,and the small-signal absorption coefficient,saturation intensity and proportion of nonsaturable loss coefficient are proportional to the number of layers.By using the experimental parameters,the relationship between the transmittance and pulse waveform of graphene film and the number of times of passing through single-layer graphene is simulated.The effectiveness of graphene isolator is proved,which provides a new optical noise isolation method for 10.6μm laser.