金屬與奈米碳管之接觸性質研究

Abstract

奈米碳管在未來有可能取代傳統矽半導體成為新一代的奈米元件，但由於奈米碳管與金屬之接觸阻抗會對於奈米元件的電性有極大影響，本論文遂針對各種金屬與奈米碳管之接觸阻抗進行研究。本論文開發成功可定義0.2um金屬線的的掀離法(lift-off)製程，並建立隨機式與對準式兩種製作多端點接觸結構的方法。在量測方面則採用以兩端點、三端點以及四端點的量測方式量測總阻抗以及接觸阻抗。我們挑選鉑、金、鋁、鉻、銅、鉭、鎢、鈦、鎳等九種金屬做為與奈米碳管之接觸金屬，以了解何種金屬可得到最低的接觸阻抗，並由此九種金屬的特性分析影響接觸阻抗的因素。結果發現鉑的接觸阻抗可得到最低值為數百Ω，最主要的因素為鉑的化學活性低而不易在與奈米碳管之介面和氧起化學反應使得接觸阻抗上升，另外藉由退火處理可將鈦與碳原子形成Ti-C鍵結而破壞原有的氧化層，接觸阻抗也由數百kΩ降至數kΩ。金屬功函數不是主要因素，金屬化學活性以及接觸面反應才是決定接觸阻抗的關鍵。經由低溫量測，可發現碳管與金屬界面的蕭基位障，而且碳管本身缺陷也可能造成位障，但此接面位障不大使得在室溫下因熱激發影響不易顯現。本論文除了建立奈米碳管接觸阻抗分析之外，也將奈米碳管之場效應、電流承載能力及崩潰現象做概略上之分析及說明。

Carbon nanotube (CNT) is one of the most promising materials for nano-electronics devices in the future. Since the contact resistance between metal and CNT has significant influence on the electrical performance of CNT devices, we focus on the metal/CNT contact properties in the thesis. At first, we develop a lift-off process to define metal lines as narrow as 0.2um. Random process and aligned process are developed to fabricate multiple contacts test structure. The CNTs were dispersed on SiO2/Si substrate and form ropes with diameter of around 20nm. In the measurement aspect, two terminals, three terminals, and four terminal methods are employed to measure the total resistance, CNT resistance, and contact resistance. Nine kinds of metals include Pt, Au, Ti, Cu, W, Ta, Cr, Al, Ni, are selected to form contact structure to find out which metal on the tubes could achieve the lowest contact resistance, and to investigate the main factors affecting the value of contact resistance. From the results, we find that the sputtered platinum on CNT exhibits the lowest contact resistances. The resistance value is about several hundred ohms. The extreme low chemical activity of platinum is the most dominant factor. Chemical reaction causes the oxidized interface, and it may be the insulating material resulting in high contact resistance. Because Titanium and carbon formed the Ti-C bond, and this carbide breaks the interfacial oxide between titanium and CNT, if we choose titanium as the contact metal, the contact resistance can be greatly reduced after high temperature annealing. At low temperature, we find the effect of Schottky barrier at the interface between metal and CNT ropes. The structural defects in CNT may also cause another barrier. However, the I-V characteristics show almost ohmic property at room temperature. This observation implies that the barrier height is too low to influence the carrier transport at room temperature. The conductivity of CNT ropes modulated by electric field, the current carrying capability, as well as the breakdown phenomenon are also discussed briefly in this thesis.