利用光激發鎳奈米薄膜產生兆赫超音波脈衝

THz Photoacoustic Pulse Generation Using Ultra-thin Nickel Nano-films

周冠妤(Kuan-Yu Chou)

指導教授 ： 孫啟光

國立臺灣大學/電機資訊學院/光電工程學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202100132

摘要

皮秒及飛秒聲學作為非侵入式的結構影像工具之一，在應用於量測電子元件的多層結構上時，相較於傳統的穿透式電子顯微鏡或橢圓儀，具有非破壞性及更佳的穿透深度等優點，因此，如何產生更高解析度的超音波脈衝一直是個重要的議題。依據彈性理論，音波產生的範圍及機制為影響超音波脈衝頻寬的主要因素，由於電子元件中幾乎皆含有金屬層的結構，因此我們選擇利用鎳奈米薄膜作為產生兆赫超音波脈衝的超薄金屬材料，並研究其音波產生的機制。

我們設計了一個簡單的音波產生模型，透過飛秒雷射激發不同厚度的金屬膜後，能量在熱電子及晶格之間傳遞而產生應力及應變，來模擬鎳奈米膜產生音波的機制。根據模擬的結果，熱電子在空間分布上產生的膨脹壓力是影響超音波脈衝波形的主要因素，而實驗上我們也成功利用二奈米的鎳薄膜產生了解析度10奈米、頻寬1.4兆赫茲的超音波脈衝。

我們的研究不僅推導出鎳奈米薄膜產生兆赫超音波脈衝的機制，更成功的產生出目前單層金屬膜相關研究中最高解析度的超音波脈衝。

關鍵字

奈米金屬膜 ； 熱電子膨脹壓力 ； 超音波脈衝 ； 空間解析度 ； 飛秒雷射

並列摘要

Picosecond Ultrasonic Laser Sonar (PULSE), one of the noninvasive technologies with a high thickness sensitivity, is capable of probing multilayer films directly on the production wafers. Different from transmission electron microscopy (TEM) or ellipsometry, PULSE system takes advantages as a nondestructive tool with deeper measuring depth. To reach high spatial resolution in the current PULSE system, generation of terahertz (THz) photoacoustic pulses through ultra-thin metal film has been focused in recent studies. In this study, we demonstrate the high potential of ultra-thin nickel (Ni) films for THz photoacoustic generation. After the excitation of femtosecond laser, the photoacoustic frequency launched by the ultra-thin Ni nano-films could be up to 1.4 THz, corresponding to a record-breaking spatial resolution in the single metallic layer system. The detailed thickness dependent mechanism was discussed through the two-temperature model, which indicates that the THz acoustic signals were generated due to the significant contribution of hot electron pressure in Ni. By looking into the ultrafast hot electron relaxation mechanism, our results provide a guideline to further break the technical limit of the metallic THz photoacoustic frequency generation in the near future.

並列關鍵字

nano film ； hypersound ； photoacoustic ； femtosecond laser ； terahertz ； resolution

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