新式奈米碳管電晶體製造與特性研究

Abstract

在本論文中，我們詳細描述了奈米碳管電晶體(CNT-FET)與生化感測器(bio-sensor)的光罩設計與其製程步驟。其中包括光罩中各個不同的功能區塊的描述以及主要元件的設計尺寸大小。同時我們也設計兩種不同的觸媒島(catalyst island)供在觸媒化學氣相沉積法(catalytic chemical vapor deposition)中，成長單壁奈米碳管(single-walled carbon nanotube) ，以提供奈米碳管電晶體中的半導體層，也就是通道(channel)使用。 首先，為了改善每一批次製造出來的CNT-FET，其電特性相差甚多(主要為P型與雙極性(ambipolar型)。我們提出一個有上下閘極(gate electrode)的雙閘極奈米碳管電晶體(DG CNT-FET)架構，且上下閘極的電壓可以獨立控制，藉以調變CNT-FET能帶(energy band) ，來獲得所需要的具特定電特性的CNT-FET。對一個Double-gated CNTFET來說，利用上閘極(top gate)之正負偏壓控制，可以調整CNT-FET中央能帶彎曲之方向，亦即在CNT中間製造了一個能障(energy barrier)，而依據上閘極偏壓方向的不同，所製造出的能障方向也不同。所以原本因為製程中不可控制因素而造成的雙向導通型的CNT-FET，可藉由上閘極之正負偏壓控制，即可調變使雙向導通型(ambipolar type)的奈米碳管電晶體轉換成p-type或n-type導通之CNT-FET。 其次，奈米碳管的導電性與其螺旋性(chirality)向量有關。而現存的CNT成長過程中，沒有方法可以控制其chirality，導致世界上所有的研究團隊所成長出來的CNT，其金屬性(metallic-type)/半導體性(semiconducting-type)的CNT往往都是混雜在一起，不僅比率很難控制，成長CNT之後也很難將半導體性的CNT分離出來。這是要將CNT有效運用在FET與sensor的研究上，一個很難突破的瓶頸，因為金屬性CNT的存在會破壞FET的運作，使得FET不具備良好的電晶體功能。而吾人提出以氬氣電漿(argon plasma)對成長完後的CNT做離子轟擊(ion bombardment)，可以使metallic-type CNT 因為以電漿處理時產生的eddy current而將其燒毀，或造成metallic-type CNT的point defect，使其等效/有效的對稱結構改變，因而顯示出semiconductin-type的特性。我們的實驗也顯示，原本p-type的CNT-FET在接受電漿處理後仍然為p-type。 為了使奈米碳管可以應用於微電子元件中，我們也發展一種定位成長SWNT的觸媒化學氣相沉積法。在此論文中，我們以奈米級鈷(Co)觸媒顆粒與四氧乙基矽(tetraethoxysilane, TEOS)的混合溶液來形成觸媒層，隨後會用黃光與蝕刻製程在特定的位置形成觸媒島(catalyst islands)，其結構可用於CVD方式成長SWNT。這個製程是與目前的積體電路製程相容，可以同時製造許多CNT-FET。藉由控制成長時間、還原時間與溫度，我們可以達到預期的成長效果。同時，經由覆蓋氮化矽的製程與否，我們可以得到N或是P型的CNT-FET，日後可以構成似互補式金氧半場效電晶體(CMOS)的架構。 最後，我們展示了一個新的CNT-FET元件架構。經由兩個互相垂直且覆蓋不同的介電層(inter layer dielectric, ILD)的奈米碳管，吾人可以得到一個channel length(即是bundle SWNT的直徑)小於50奈米的CNT-FET元件。同時這兩根奈米碳管可以分別扮演gate與channel的角色，搭配不同的passivation layer製程(ILD)，我們可以得到N或是P型的CNT-FET。這種特性可以增加日後CNT-FET電路的設計彈性，同時可以製造互補式的奈米碳管電晶體。

In this dissertation, we report the layout designs and the process recipe for fabricating carbon nanotube field effect transistors (CNT-FETs) and bio-sensors, including the definitions of cell blocks, characters of device structures in detail. For the purpose of aligned growth of carbon nanotubes, two kinds of layouts for catalyst islands are also designed for catalytic chemical vapor deposition (CCVD) method. To start with, we propose a conduction-type-tunable CNT-FETs with double-gated structure (DG CNT-FET). A specially designed narrow top-gate is created to modulate the energy band in the middle region of a single CNT. In the proposed DG device structure, the top-gate and bottom-gate biases exhibit independent modulation behaviors. Energy band diagram conducive to the physical mechanisms of the proposed DG CNT-FET device structure is proposed. Based on the proposed hypothesis, ambipolar CNT-FETs can indeed be converted to n- or p-type-like behaviors. Next, we also demonstrate a novel plasma treatment method that allows us to convert metallic-type carbon nanotubes to semiconducting-type CNT-FETs. This is important as the production of single-walled carbon nanotubes (SWNTs), irrespective of synthesis methods, still yields a mixture of both types thus far, with the metallic type being prevalent. However, semiconducting-type SWNTs are needed for CNT-FETs as well as many sensors. Judging from our experimental results, we believe that the ion bombardment during plasma treatment attacks both metallic- and semiconducting-type nanotubes; however, the metallic-type carbon nanotubes are more vulnerable to the attack than the semiconducting-type, and are subsequently transformed into the latter type. In order to apply CNTs to nanoelectronics, in this thesis we also demonstrate a precise growth of SWNTs on pre-assigned locations with only cobalt (Co) as catalyst. This is in contrast to the laborious and time-consuming physical manipulation of numerous nanotubes one at a time used in the conventional approach. Laterally-grown carbon nanotubes are accomplished in pre-assigned areas using an integrated-circuit (IC)-compatible process in this thesis. In order to synthesize SWNT to serve as the channel of a FET, the cobalt-mix-tetraethoxysilane (CMT) solution and catalytic chemical vapor deposition are used. Our results show that laterally-grown bundled-CNTs could be formed in CCVD with ethanol, by properly controlling the temperature of process, the process time, and the hydrogen reduction time. The use of pre-patterned catalyst islands, CCVD method and flexibility of silmutaneously manufacturing both n- and p-type CNT-FETs may open a new era for applications of CNT-based nanoelectronics. Finally, we introduce a complementary carbon-nanotube(CNT)-gated CNT thin-film field effect transistor. By using two perpendicularly-crossed SWNT bundles as the gate and the channel interchangeably, a sub-50 nm complementary CNT-FET is demonstrated. It is found that the new CNT-FET shows acceptable FET characteristics by interchanging the roles of the gate and the channel. The unique dual-functionality of the device will open up a new possibility and flexibility in the design of future complementary CNT electronic circuits.