掃描式探針顯微系統之研發

Abstract

本實驗主要的研究方向為自製掃描式穿隧電流顯微鏡(Scanning Tunneling Microscope，STM)，並發展穩定性更高的機械結構與電子電路系統。以實驗室已研發完成的教學用掃描式穿隧電流顯微鏡作為雛型加以改良，包含機械結構與電子電路兩部分，機械部分設法克服舊機台結構設計不良所造成的振動問題、掃描器存在微小漏電流的問題、以及進針時容易撞針的問題。在克服振動雜訊部分，測試並加裝更穩定的彈簧避震系統以及類似阻尼震盪系統，以達到更完善隔絕外來振動源的目的，並將外接線路部份做重新配置與組合，使機台與線路的連接點能達到最少化而避免由外接線路傳入振動雜訊；漏電流部分，將接受穿隧電流的探針與掃描器分開設計使得掃描器存在微小漏電流的問題得以解決，提升了縱向的解析度；在觀測進針部分，加裝攝影機監測進針狀況取代舊設計以肉眼直接觀察針尖與樣品距離，大幅提升了使用上的方便性與進針時避免撞針的安全性。電子電路方面則改善了溝通傳輸介面、置換效能更高的電流放大器與速度更快的微處理器，使得電子電路系統在操作時更加地快速與穩定，避免因為電子電路的設計缺陷而造成掃描圖形的雜訊與假訊號。 除此之外，以改良完成的掃描式穿隧電流顯微鏡為架構，進一步利用音叉探測的方式著手發展自製的原子力顯微鏡(Atomic Force Microscope，AFM)。利用蜂鳴片將音叉與黏於音叉上的探針振動至振動頻率，接收音叉操作在共振頻率下所送出的交流震盪訊號，將之轉換為單純的直流訊號並以振幅調變的方式來進行回饋，接著利用原有的電子電路系統進行掃描的動作，試著取得由探測原子力大小而成像的AFM圖形。

Based on the framework of our home-made scanning tunneling microscope (STM) system, this study is focused on the improvement of stability. There are two major parts, mechanical design and electronics, in this improvement. Regarding to the mechanical design, springs with more suitable stiffness and rubber feet were used to damp environmental vibration efficiently. The wiring connecting the STM stage and the electronic control-box was rearranged to decrease the number of joints, from which the external vibration is introduced into the STM stage. The bias for tunneling was modified to apply on the tip, which is on the opposite side to the scanner, to avoid the problem of leakage current and improve the resolution in the Z-direction. Instead of naked eye, we used a CCD camera to observe the distance between the tip and the sample for convenience and preventing the tip crashing in approaching process. As to the electronics, the parallel port was replaced by USB for communication interface. The IC with higher processing speed was used in preamplifier. This improvement on the electronics avoids the noise and artifacts resulted from the circuits. After improving the STM system, we added an atomic force microscope (AFM) mode by using a tuning fork as the feedback sensor. We used the AC signal, which was acquired as the tuning fork was driven to oscillate at the resonant frequency, for feedback with amplitude-modulation mode. Combining the tuning fork sensor with the scanner and electronics circuit of our improved STM system, we have obtained AFM images roughly. We have constructed a simple scanning probe microscope (SPM) system with the STM mode and the AFM mode.