奈米碳管電晶體之高頻特性研究

Abstract

本論文以區域性背閘極奈米碳管電晶體作為設計架構，並且採用碳管旋塗法來塗佈碳管，源極/汲極區域則使用高功函數金屬並使用光阻掀離法來進行製作，以解決傳統蕭基位障之上閘極奈米碳管電晶體漏電流過大的缺點，並透過提升碳管濃度以及多次旋塗碳管的方法，將驅動電流大幅提升，搭配適當的光罩設計，將不必要的寄生效應降低，使奈米碳管電晶體能夠在高頻上有較好的操作性能。 本論文所製作之奈米碳管電晶體的元件結構可分為兩類，第一類為採用多指狀結構製作的碳管電晶體，第二類為採用單指狀結構製作的碳管電晶體。另外源極/汲極區域金屬又可分為兩類，其中一類以濺鍍鈀(Pd)/鋁(Al)作為源極/汲極區域金屬，另一類以濺鍍鈦(Ti)/鉑(Pt)作為源極/汲極區域金屬。不管何種元件，其下閘極/下閘極介電層皆為重摻雜磷多晶矽(50nm)/氧化鋁(10nm)。由量測結果得知使用鈀/鋁製作的碳管電晶體，無論是驅動電流抑或是電流開關比，相較於使用鈦/鉑製作的碳管電晶體都來得優越。所以本論文以鈀/鋁金屬製作的奈米碳管電晶體元件為分析重點。本論文也發現使用多指狀結構的碳管電晶體，雖具有較大的驅動電流，但電流開關比卻明顯小於單指狀結構的碳管電晶體，所以使用單指狀結構且搭配鈀/鋁金屬製作的碳管電晶體是較佳的選擇。 使用上述直流電性較好的元件進行高頻量測，且使用“開路測試元件加上短路測試元件”去內嵌法則將外部寄生效應去除後，可成功量測到電晶體的高頻特性。本論文元件的截止頻率(fcut-off)或是功率增益(MSG)都有超過GHz的表現，但仔細分析，會發現本論文元件的阻抗不匹配狀況相當嚴重，也就因此使得元件的高頻特性不如理論預期高達THz的性能，這是未來需要改進的方向，如此才能使碳管電晶體的高頻特性有所提升。 總言之，本論文使用的結構與製程方法確實可以製作出具有高頻操作性能的碳管電晶體，這提供將來從事碳管電晶體的高頻特性研究的一個參考基礎。

This thesis is based on the architecture of local bottom-gated carbon nanotube FETs (LBG-CNTFETs), which used spin-coating process to disperse CNTs. The source/drain metal contact was made by contact-metal lift-off process with high work-function metal to solve the disadvantage of high leakage current for conventional top-gate(TG) schottky barrier CNTFETs. The on-current of the CNTFETs can be substantially raised by increase the concentration of CNT solutions and using multiple spin-coating procedures. CNTFETs with good high frequency characteristics can be made by modifying the layout of the mask to avoid the parasitic effects. There are two kinds of device structures of CNTFETs in this thesis. One is multi-finger structure and the other one is single-finger structure. Two kinds of metals were used to form the source/drain contact of the CNTFETs. They are Pd/Al and Ti/Pt deposited by sputtering. All of the devices in this thesis have the bottom-gate/gate dielectric structure of in-situ doped N+ poly-Si(50nm) / Al2O3(10nm). Either on-current or on/off current ratio, CNTFETs with Pd/Al as source/drain contact metal are superior to the CNTFETs with Ti/Pt as source/drain contact metal. Therefore, this thesis focuses on the CNTFETs with Pd/Al as source/drain contact metal. Measurement results show that the CNTFETs with multi-finger structure have higher on-current but much lower on/off current ratio than the CNTFETs with single-finger structure. As a result, CNTFETs with single-finger structure and Pd/Al as source /drain contact metal are preferred for further analysis. The CNTFETs with better DC electrical properties were selected to perform high frequency analysis. The“open circuit test structure and short circuit test structure ” procedure was used to de-embed the external parasitic effects. The fcut-off and MSG of the CNTFETs in this thesis are over GHz. After taking a particular analysis, it is found that the impedance mismatch of CNTFETs in this thesis is very serious and hence the high frequency characteristics of CNTFETs are not as good as theoretical prediction of THz. The problem of impedance mismatch must be solved in the future in order to get the CNTFETs with better high frequency characteristics. To conclude, the CNTFETs suitable for high frequency characterization have been fabricated in this thesis. This provides a basis for the research on the high frequency characteristics of CNTFETs in the future.