掃描探針顯微技術在材料表面奈米光電特性量測上的應用

Abstract

由於人類對微小化元件的殷切需求，已由原來的微米（10-6 m）範圍進入了奈米（10-9 m）範圍的時代，在面臨二十一世紀高科技發展的競爭中，奈米技術已是國家高科技發展政策中不可或缺的一環。奈米技術所涵蓋之領域非常廣泛，包括機械、材料、光學、微電子、量測、生化醫藥及原子物理等等，而掃描探針顯微技術(Scanning Probe Microscopy, SPM)的開發，亦是其相當重要的一部分。本研究論文是以自行研製之電弧式光纖探針拉引機為工具，藉由拉力大小、加熱時間和熔拉長度等參數的控制，可成功的製作出不同彎曲角度(介於30°~75°)與彎曲長度(介於600~900 mm)的光纖探針，尖端直徑約100至200 nm，此探針可直接應用於原子力顯微鏡(atomic force microscope, AFM) ，形成穿透式或接收式近場光學訊號偵測機制。此外，我們亦設計一側向力距離控制模組，配合連結電路之設計，將AFM改良為兼具量測穿透與反射光學影像之直立式光纖探針近場系統，目前已可穩定取得表面形貌及相對應之近場光學影像，其解析度分別約為80 nm和150 nm左右。 至於在奈米表面電性的研究方面，已成功架設完成導電性原子力顯微量測系統，選擇目前相當熱門的有機發光高分子(PLED)及壓電陶瓷(PZT)薄膜為量測樣品，分別針對其奈米尺度的電致發光和區域極化特性作一深入的探討，成果如下所述： I. 利用導電性原子力顯微術，對不同處理後之ITO薄膜表面的區域電性，作深入的研究與探討。在量測的樣品中，表面主要均為非導電區所構成，導電區呈不規則分布，大小約6~50 nm，尤其經紫外線-臭氧處理後的ITO樣品，其非導電的區域具有相當穩定的電流穿遂特性，且表層生成之氧化物，不會因高電壓(±8 V)的作用而造成破壞，驗證以此類ITO作為有機發光二極體(PLED)之導電基材，將有助於提升其發光品質與效率。再者，我們亦以探針加電壓的方式，成功的取得解析度約50 nm之PLED區域電致光學影像。由成果發現，除上述表面電性外，ITO薄膜微觀的表面形貌也是影響其發光性質之重大因素。 II. 選擇使用有機金屬鹽裂解法(metal-organic decomposition, MOD) 所鍍製具有良好鐵電性質的Pb (Zr0.52Ti0.48)O3 (PZT)薄膜為樣品，除觀察其表面形貌及殘留極化分布外，並嘗試利用導電探針製作表面單一極化區。由實驗結果發現，薄膜的表面晶粒的大小約為50-300 nm，且薄膜鍍製的相當均勻。當外加一AC電壓於樣品下電極與探針間，可產生反壓電效應而得到表面殘留極化影像，殘留極化區域的大小約為50-150 nm，強度約為2.4 pm/V。在製作單一極化區方面，本實驗使用了具上電極與不具上電極兩種不同型式之PZT樣品，發現其d33壓電係數(piezoelectric coefficient)分別為56.3 pm/V及51.2 pm/V，誤差值在10％以內，這結果較文獻記載更具可靠性，此乃因實驗中所用的為一支真正導電之探針所致。

As technology advances, producing lighter, thinner and smaller electro-optics components have becomes a vital requirement in manufacturing process. Similarly, the techniques of analyzing and testing of components must be upgraded to meet these changes. In this article, We take advantage of a combination of laser heating-pulling and electric arc bending to fabricate the bent tapered fiber probes. The bent angles can be varied from 300 to 700 and tip diameters fall within few tens of nanometers. These bent fiber probes can easily be adapted into any dynamic mode atomic force microscope. By proper manipulation of the bent angles, a spatial resolution of up to 60 nm is achievable. After coating the bent fiber probes with a thin layer of Pt/Pd film by ion sputtering, the transmission efficiency is measured to be around 10-5, which is applicable for near-field spectrum analysis experiment. Moreover, we present the modification of a commercial tapping mode atomic force microscope into a reflection and transmission dual mode scanning near-field optical microscope. In the configuration, the normal force detection unit is replaced by a shear force detection module and an interfacing circuit. The tip-sample distance control is therefore identical to the tapping mode operation. The detection of the near-field signals is based on photodiodes and lock-in technique, and the obtained resolutions for topography and near-field signal are around 80 and 150 nm respectively. On the other hand, we had also set up an atomic force microscope that was able to measure the conductivity on sample. PLED and PZT membrane, due to their popularity, were used in the experiment. In depth study on nano-range light emitting property by introducing current on the samples and the characteristics of sectional polarity were conducted. We state the investigations as follows: I. Nanoscale surface electrical properties of indium tin oxide films prepared by different cleaning methods for use as anode materials in organic light emitting diodes are studied by conducting atomic force microscopy. It is found that most of the surface area possesses a nonconducting feature, and an ultraviolet-ozone treatment produces the most nonconductive sample. The conducting regions, which distribute randomly and range from 6 to 50 nm in size, are attributed to the existence of Sn-rich oxide by a comparison with reported scanning electron microscopy images. After scanning the tip with a bias of -8 V on the nonconducting regions, oxide decomposition occurs on as-received and wet-cleaning processed samples, whereas no structure change appears on the ozone treated sample. The results indicate that the generation of stable oxide after ozone treatment is one of the origins for improved device performance. Then, we also report optical imaging with a resolution of around 50 nm on an electroluminescent polymer by conducting atomic force microscopy. The results indicate that brighter light emission occurs at topographically higher dot features on the polymer surface. By comparing surface morphologies of the polymer and the indium tin oxide substrate, it is found that similar dots exist on both surfaces and the polymer becomes thinner on these locations. Therefore, stronger luminescence intensity on the dot structures is caused by higher electric filed and consequent higher current density due to reduced polymer thickness. This observation suggests that substrate nonunifomity could be one of the major factors related to device failure. II. Pb (Zr0.52Ti0.48)O3 (PZT) film posses good ferroelectric property after electroplated by metal-organic decomposition (MOD) is used in this study. Besides analyzing its surface topography and residual polarization distribution, we have utilized a conductive scanning probe to create surface unitary polarization region. From the experiment result, we realized that an evenly electroplated PZT film with surface grain size between 50-300 nm when subjected to an external AC voltage applies between the electrode located underneath PZT film and the scanning probe will produce a reverse bias effect. Consequently, an image of surface residual polarization is attained. The region of residual polarization is about 50-150nm and its intensity is 2.4 pm/V. In constructing the unitary polarization district, the difference between the values obtained from two configurations, which are tip/PZT/electrode and tip/electrode/PZT/electrode, is less than 10%. In comparison with the reported variation from a similar experiment, which is as high as a factor of 3, the current consistency is believed to originate from the use of truly conductive probes.