一維球對稱雷射激發電漿產生13.5奈米極紫外光源之模擬研究

Abstract

電射激發電漿產生13.5 nm極紫外光源之研究為一新興領域。相對於其它技術，雷射激發電漿有較高的轉換效率，且短波長的極紫外光有很高的應用價值。極紫外光可應用於半導體光微影技術，提供高解析度與高景深解析度；極紫外光亦可應用於光學同調斷層術，提供奈米級之解析度，彌補現今光學顯微鏡和電子顯微鏡解析範圍間的斷層。由於電射激發電漿的系統過於複雜、無法僅靠理論分析，故發展適當的模擬模型有其必要性。極紫外光產生的物理過程，包含流體力學、熱力學、原子物理、電磁輻射等。若要完整模擬此流程，需要極大的運算量。在此，發展一維球對稱的模型模型，並經過實驗之驗證，為完整且可靠的模擬模型。 此模型是根據MED103（已發展完成之流體程式）做修改及擴充，使其有更高的空間解析的、入射雷射波型有更高的自由度，且經由恰當的電漿平衡模型考慮離子價數隨時間的演變。最後，經由靶材離子（錫）的權重振子強度估算電漿在 13.5±2% nm 內的輻射量。最後，討論雷射強度（1010~1012 W/cm2）、脈衝長度（0~80 ns）、雷射波形（前三角波、後三角波與高斯波）、靶材大小（半徑10~100 μm）、靶材密度（1019~1022 cm-3）、靶材初始溫度（0~10 eV）對系統轉換效率之影響。並根據本實驗室之架構──雷射能量為1~2 mJ、雷射波形為前三角波、脈衝長度約5~15 ns；靶材為重量百分濃度為14%、半徑為20~40 μm之錫溶液小球，設計最佳的實驗參數。

The development of compact and high efficiency extreme ultraviolet (EUV) light sources have evolved from academic researches to industrial applications, e.g. metrology and lithography et al. The EUV light sources based on the laser-produced plasma attract researchers' attention due to its power scalability and spatial coherence. The plasma-based system is too complicated to be theoretically analyzed, so the computer simulation becomes indispensible. However, it is very complicated and computationally intensive to model the whole EUV generation process, which includes comprising seed ionization, plasma formation, and EUV emission. Here we present a simplified spherical-symmetric simulation model for the generation of 13.5 nm extreme ultraviolet source by laser-produced plasma with the experimental bench marks to make the simulation tool more accessible and easier. The model is achieved by revising the one-dimensional hydrodynamic code MED103 so that arbitrary laser pulse shape and more simulation cells can be considered and coupling the collision-radiative equilibrium equations to calculate the ion charge state distributions. In addition, in-band emission of 13.5±2% nm is estimated according to the contributions from the weighted oscillator strengths of Sn8+~Sn13+. The simulated temporal and spatial evolution of plasma density, electron temperature, EUV emission profile, and ion charge states for 1064-nm laser and tin-doped droplet target will be presented in the paper. The influences of laser duration, pulse shape, dopant density, and target diameter on EUV characteristics were examined discussed in the paper.