高性能奈米碳管電晶體之製程與特性研究

Abstract

由於奈米碳管具有優異的電性，故奈米碳管電晶體具有相當大的潛力應用於未來的奈米電子元件。採用旋塗法、源極/汲極光阻掀離法製作傳統蕭基位障之上閘極奈米碳管電晶體的缺點為奈米碳管跨接於源極/汲極的或然率低、上閘極介電層漏電流過大等問題，故本論文設計區域性背閘極奈米碳管電晶體，以解決上述傳統蕭基位障之上閘極奈米碳管電晶體的缺點。 本論文所製作之高性能奈米碳管電晶體的元件結構及製程材料可分為三類，第一類為下閘極與鈀(Pd)源極/汲極有重疊區域之長閘極式區域性背閘極奈米碳管電晶體，其下閘極/下閘極介電層為氮化鈦/30nm氮化矽，且其電流開關比達106、次臨界斜率為263.5(mV/dec)、良率達54.3%；第二類為鈀源極/汲極、下閘極/下閘極介電層為重摻雜多晶矽/10nm氧化鋁之長閘極式區域性背閘極奈米碳管電晶體，其電流開關比達106、次臨界斜率為139.1(mV/dec)、互導最大值為1.27(μS)、良率達74.3%；第三類為下閘極與鈀源極/汲極無重疊區域之短閘極式區域性背閘極奈米碳管電晶體，其下閘極/下閘極介電層為重摻雜多晶矽/10nm氧化鋁，且其電流開關比達106、次臨界斜率為263.5(mV/dec)、良率達65.2%，經由直流分析得鈀與半導體性奈米碳管之接觸達到了歐姆接觸。 本論文分別採用鈀、鎳(Ni)、鉻(Cr)來製作區域性背閘極奈米碳管電晶體的源極/汲極，研究真空高溫退火製程前後對於不同金屬間的直流電性差異。由於鈀、鎳與奈米碳管間有較強的黏著與濕潤作用力，真空高溫退火製程後，鈀源極/汲極的導通電流提高十倍；鎳源極/汲極的導通電流提高100倍；然鉻與奈米碳管間的黏著與濕潤作用力較弱，故鉻源極/汲極的導通電流並無太大的變化。 本論文也探討了水氣極性分子對於奈米碳管電晶體遲滯效應的影響、電漿製程及電性應力法燒除通道上之金屬性奈米碳管對於奈米碳管電晶體直流電性的影響。 總而言之，本論文透過上述之製程結構、製程材料及真空高溫退火製程，已成功地製作出鈀源極/汲極與半導體性奈米碳管為歐姆接觸的高性能區域性背閘極奈米碳管電晶體，對於未來奈米碳管電晶體之高頻特性研究立下了良好的基礎。

Carbon nanotube field-effect transistors (CNTFETs) have been considered as one of the candidates for nano-electronics applications in the future due to the excellent electrical characteristics. Conventional top-gate (TG) schottky barrier CNTFETs are fabricated by a spin-coating process and a source/drain contact-metal lift-off process. The disadvantages of these processes are as follows: (1) The probability for carbon nanotubes to cross between source and drain is low and (2) There exists drastically high leakage current through top-gate dielectric. Since these two disadvantages have significant influence on the electrical performance of the TG-CNTFETs, we propose a novel device concept and the relative process technologies which is called the local bottom-gate carbon nanotube FETs (LBG-CNTFETs) in this thesis. Two kinds of device structures, gate materials, and gate dielectric materials are designed to form the three kinds of LBG-CNTFETs with Pd source/drain. The first device structure is long-gated LBG-CNTFET with TiN gate/30nm PE-SiN gate dielectric with gate to source/drain overlap. The on/off current ratio achieves 106, the subthreshold swing is lower than 264 mV/dec, and the yield is higher than 54%. The second device structure is long-gated LBG-CNTFET with in-situ doped poly-Si gate/10nm Al2O3 gate dielectric with gate to source/drain overlap. The on/off current ratio achieves 106, the subthreshold swing is lower than 140 mV/dec, the maximum transconductance achieves 1.27μS, and the yield is higher than 74%. The third device structure is short-gated LBG-CNTFET with in-situ doped poly-Si gate/10nm Al2O3 gate dielectric without gate to source/drain overlap. The on/off current ratio achieves 106, the subthreshold swing is lower than 264 mV/dec, and the yield is higher than 65%. According to these results, it is concluded that the contact property between Pd and CNTs should be ohmic contact. The effect of high vacuum annealing process on the electrical performance of LBG-CNTFETs is evaluated. Three kinds of metal include Pd, Ni, and Cr are selected to act as source/drain contact of the LBG-CNTFETs. Due to the strong sticking and wetting effect between metal and CNTs, the turn-on drain current has been improved by tenfold for Pd and by hundredfold for Ni. On the contrary, for the metals with weak sticking and wetting effect between metals and CNTs such as Cr, the turn-on drain current shows less improvement after annealing. The hysteresis effect of LBG-CNTFETs arising from atmospheric water molecules adsorption, the damage of plasma radicals on CNTs, and the electrical stress condition to remove metallic CNTs from channel are also discussed in this thesis. To conclude, high-performance local bottom-gated CNTFETs with ohmic contact between source/drain metal and CNTs of have been realized by the novel device structures and process soptimization. It is believed that this thesis provides a solid foundation for the radio-frequency study in the future.