Design and Fabrication of Sub-wavelength Annular Apertures for Femtosecond Laser Machining

近年來已經有許多人投入探討光學技術應用於微奈米加工的研究上，而關鍵就在於光學技術產生出的聚焦點大小會決定被加工物的尺寸規模，為了突破光學的繞射極限，一種典型的方式就是利用近場光學的方法，但是其焦深短以及近場高度控制上都有一些問題。本研究團隊曾提出以次波長圓環孔徑結構製作在金屬薄膜上，可獲得具有次波長尺度的貝索光束，成功克服繞射極限，並且可以應用在曝光微影製程以及雷射加工上，製作出高深寬比的結構。 但是在這個加工方法裡面仍有些缺點，像是雷射燒蝕時殘留在被加工物表面的殘渣導致雷射加工效率受到影響，因此本研究團隊提出使用飛秒雷射作為加工光源，飛秒雷射穿過次波長圓環孔徑金屬結構後，產生出來的貝索光束能維持長焦深的特徵，利用飛秒雷射本身的冷加工特性可以解決雷射燒蝕的問題。我們將先討論什麼尺寸的次波長圓環孔徑金屬結構適合飛秒雷射的780nm波段，同時利用時域有限差分法電磁模擬軟體，對次波長圓環孔徑的尺寸進行模擬，再利用自製縱向光學顯微鏡進行光強實驗，最後再將次波長圓環孔徑金屬結構與飛秒雷射光路結合進行加工實驗。The fundamental optical properties of traditional free-space light beam forced the users to make a compromise between the focal spot sizes the depth of focus when adopting optical technique to pursue micromachining. The focal spot size determines the minimum features can be fabricated. On the other hand, the depth of focus influences the ease of alignment in positioning the fabrication light beam. A typical approach to bypass the diffraction limit is to adopt the near-field approach. However, the depth of focus of the emitted light beam will be limited to tens of Nanometers in most cases, which posts a difficult challenge to control the distance between the optical emitted plane and the sample to be machined optically. More specifically, problems remained in this machining approach, which include issues such as residue induced by laser ablation tends to deposit near the optical emitted plane and leads to loss of coupling efficiency. We proposed a method based on illuminating femtosecond laser through a sub-wavelength annular aperture (SAA) on metallic film so as to produce Bessel light beam of sub-wavelength while maintaining large depth of focus first. To further advance the ease of use in one such system, producing sub-wavelength annular aperture with sub-wavelength focusing ability is detailed. It is shown that this method can be applied in material machining with an emphasis to produce high aspect ratio structure. More specifically, the ablation property of femtosecond laser was utilized to eliminate the laser melting induced residue deposition problem associated with traditional laser machining. Throughout the course of this research, we have optimized the parameters associated with the SAA structure for 780 nm light wavelength of the femtosecond laser by using finite difference time domain simulations method. A lateral microscope modified from traditional microscope was developed to facilitate the optical energy distribution of the emitted light beam to be used for optical machining. Finally, the SAA structure designed and the femtosecond laser were integrated to perform the intended optical micromachining.中文摘要 I Abstract II 致謝 IV 目錄 V 表目錄 VII 圖目錄 VIII 第一章 緒論 1 1.1 研究背景 1 1.2 文獻回顧 2 1.2.1 繞射極限 3 1.2.2 次波長圓環孔徑與貝索光束微影系統 5 1.3 研究動機 9 1.4 論文架構 10 第二章 原理 11 2.1 表面電漿 11 2.1.1 何謂表面電漿 11 2.1.2 激發表面電漿 16 2.2 貝索光束 19 2.3 狹縫理論 23 2.4 飛秒雷射 26 2.4.1 雷射脈衝能量吸收 26 2.4.2 瞬間吸收能量的轉移 28 2.4.3 材料移除與切削形成 29 2.5 時域有限差分法 32 第三章 實驗架構與模擬 34 3.1 系統架構 34 3.2 次波長圓環結構出光模擬 35 3.2.1 各狹縫參數討論 36 第四章 光強實驗與加工實驗 49 4.1次波長圓環孔徑結構製作 49 4.2光強實驗 51 4.2.1 光路系統 51 4.2.2 系統架設 54 4.3加工實驗 55 4.3.1光路系統 55 4.3.2系統架設 57 第五章 實驗結果分析與討論 59 5.1 光強實驗結果 59 5.2 加工實驗結果 70 第六章 結論與未來展望 77 6.1 結論 77 6.2 未來展望 78 參考文獻(References) 7