開發節能型研究用之掃描式穿隧電流顯微鏡

Abstract

我們根據發展教學用掃描式穿隧電流顯微鏡的經驗，設計一台研究用的掃描式穿隧電流顯微鏡，希望能改善過去機台容易受外部振動雜訊與降低熱飄移對掃描的影響，並加入自動步進器使操作便利與可靠；而電路部分則欲將Z軸獨立來解決大範圍掃描時運算放大器飽和失真的問題，以及修改回饋電路PI controller提升P Gain的靈敏度。在機械結構部分，為解決教學版STM存在的震動雜訊和熱飄移現象，放棄舊有長方形的架構，改成圓形對稱的設計使各方向的熱膨脹現象均勻，降低環境溫度對機台影響。而在教學版STM因為影像不時會觀察到震動雜訊，為了加強避震系統對震動的抑制，研究型STM加入了磁鐵阻尼陣列，利用導體在磁場中運動會在導體內產生Eddy current抵抗磁通量變化來衰減STM系統對震動的響應改善掃圖的穩定性。以前使用傳統的手調步進，雖然機構簡單，但對使用者而言每次使用都須格外小心，否則很容易造成探針和樣品的碰撞，所以在機台設計上應設法避免此種情形。因此我們利用低電壓的壓電材料做出了機構簡單容易驅動的微型步進器，使研究型STM的操作部分得到簡化，而且拿掉許多手動步進的機構設計可以讓機構更對稱與增加剛性。 由於現有的掃描器最大掃描範圍約為4.2um×4.2um，在部分樣品的掃描上稍有不足，首先我們利用三個低電壓bimorph 壓電片組裝成大範圍的掃描器，成功將掃描範圍加至100um×100um，有助於未來發展原子力顯微鏡所需的大範圍掃描器。另一方面我們也發展了鎢針蝕刻自動斷電系統，解決了傳統做針時因人工斷電容易使針尖品質參差不齊的問題，不僅節省時間也提高針尖的品質。

A new design of the green and research type scanning tunneling microscope (STM) is attempted in this thesis. To approach the green style, a low voltage, which reduces much energy consumption in comparison with that driven by a high voltage, is applied to drive the whole STM system, including the piezo scanner and the inertial movement of the step motor. In addition, the vibration isolation spring system is equipped with an eddy current damping system. With this damping system, even a considerable vibration fluctuation can be damped in couple seconds. More surprisingly, comparing with our educational STM, the new STM system has an ultra-low thermal drift probably owing to its symmetrical geometry centering around the tip-sample module. Finally, the design principles of integral and proportional gains, and the independent Z-axis scanner are discussed. In the thesis, a large area scanner, driven by a low voltage, is developed as well. The scanning area achieves 100 □m by 100 □m which can be applied for applications in atomic force microscopy. In addition, an automatic tip maker which can make much sharper STM tips, is also mentioned.