정전기수력학 인쇄를 활용한 단일벽 탄소나노튜브 기반 트랜지스터 및 응용

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 전기·컴퓨터공학부, 2020. 8. 홍용택.As the demand and research for electronic devices on flexible and stretchable substrates gradually continues comparable to the conventional rigid silicon-based electronic devices, interest in new semiconducting materials capable of low-temperature processes and large-area processes is increasing. Single-walled carbon nanotube (SWCNT) is one of the representative materials satisfying the new interests thanks to its excellent electrical and mechanical properties. SWCNT can be advantageous for non-vacuum, low-temperature, and large-area processes in response to various solution processes such as dipping, inkjet printing, and gravure printing. For high-performance devices with low power consumption based on next-generation electronics, the demand for ultra-fine patterning technology based on the solution process is also increasing. In this thesis, SWCNT-based all electrohydrodynamic-jet (E-jet) printing system was established, a SWCNT-based thin-film transistor (SWCNT-TFT) with a channel length of 5 microns was implemented through the system. In addition, by developing and grafting technology to control the threshold voltage of SWCNT-TFTs based on the solution process, we have demonstrated highly integrated and high-resolution SWCNT-based applications including logic gate, pixel circuits for image detector and display. In addition to the micrometer scale fine pattern technology by the E-jet printing system, a new solution process-based vertical stacking technology is also introduced to further improve the transistor density, enabling high-resolution, highly integrated electronic applications in a continuous environment without any vacuum or high temperature process. The technology introduced in this thesis for high performance, high resolution, and high integration of SWCNT-based devices makes it possible to fabricate a 250 pixel per inch active matrix backplane utilizing only the solution process.유연 기판 및 신축성 기판상의 전자 소자에 대한 수요 및 연구가 종래의 단단한 실리콘 기반의 전자 기술만큼이나 많은 관심을 받고 있어, 이를 위한 저온 공정 및 대면적 공정이 가능한 새로운 반도체 물질 연구에 대한 관심이 증가하고 있다. 단일벽 탄소나노튜브는 뛰어난 전기적 및 기계적 특성 뿐만 아니라 비 진공, 저온, 그리고 대면적 공정이 가능한 담금 공정, 잉크젯 프린팅, 그리고 그라비아 인쇄법과 같은 용액공정에 대응하기에 이러한 요구를 충분히 충족시킨다. 마찬가지로 용액 공정 기반 소자의 고성능 및 저전력화를 위한 용액 공정기반의 초 미세 패터닝 기술에 대한 필요성도 증가하고 있다. 본 학위 논문에서는 단일벽 탄소나노튜브 기반의 전 정전기수력학 인쇄 시스템을 구축하여 5마이크론의 채널 길이를 갖는 단일벽 탄소나노튜브 기반 박막트랜지스터를 구현하였다. 또한 용액 공정기반의 단일벽 탄소나노튜브 기반 박막트랜지스터의 문턱 전압을 조절하는 기술을 개발하고 이를 접목시켜 논리소자와 영상센서 및 디스플레이를 위한 픽셀 회로를 포함한 단일벽 탄소나노튜브 기반의 고해상도, 고집적화된 응용소자를 개발하였다. 정전기수력학 인쇄 시스템을 통한 마이크론 수준의 미세 패터닝 기술 뿐만 아니라 집적도를 더욱 향상시키기 위한 용액 공정기반의 새로운 수직 적층형 기술을 도입하여 고해상도 및 고집적화된 단일벽 탄소나노튜브 기반의 전자 소자를 어떠한 진공 공정이나 고온공정 없이 연속된 환경에서 구현하였다. 본 학위논문에서 제시한 단일벽 탄소나노튜브 기반 소자의 고성능, 고해상도, 고집적화를 위한 기술은 250 ppi급의 능동형 매트릭스 백플레인의 제작을 순수 용액공정만으로 실현 가능하게 한다.1 Introduction 1 1.1 Single-Walled Carbon Nanotubes 1 1.2 Band structure of SWCNTs 8 1.2.1 Energy bandgap of SWCNTs 8 1.2.2 Density of states for SWCNTs 11 1.2.3 Detection for classifying species of SWCNTs 13 1.3 Sorting out semiconducting SWCNTs 16 1.3.1 Pre-deposition of the nanotubes and sorting later 16 1.3.2 First sorting out SWCNTs and deposition later 18 1.4 Operation of SWCNT-TFTs 21 1.4.1 SWCNT-TFTs as Schottky-barrier FETs 22 1.4.2 Random network of SWCNTs 26 1.5 Reported SWCNT-TFTs and applications 28 1.6 Technical points for microelectronics based on SWCNT-TFTs 32 1.7 Organization 34 2 Tunable threshold voltage in single-walled carbon nanotube thin-film transistors 35 2.1 Introduction 35 2.2 Experimental details 37 2.2.1 Fabrication process for solution-processed SWCNT-TFTs 37 2.2.2 Post-treatments for tunable threshold voltage in solution-processed SWCNT-TFTs and measurement of their electrical properties 38 2.3 Results and discussion 39 2.3.1 Post-chemical encapsulation for tunable threshold voltage 39 2.3.2 Contact resistance analysis by the Y-function method in SWCNT-TFTs employing chemical encapsulation 41 2.3.3 Shift of energy band in SWCNT-TFTs 42 2.3.4 Cycling tests for post-treatments 45 2.3.5 SWCNTs-based p-type only inverter 46 2.4 Conclusion 49 3 All electrohydrodynamic-jet printing system for single-walled carbon nanotube thin-film transistors 50 3.1 Introduction 50 3.2 Experimental details 55 3.2.1 Ink manufacturing for E-jet printed metal, dielectric, and active layers 55 3.2.2 Optimized E-jet printing conditions and fabrication process for all E-jet printed SWCNT-TFTs 57 3.3 Results and discussion 60 3.3.1 Constituting of all E-jet printing system 60 3.3.2 Optimized E-jet printed metal electrode 63 3.3.3 Optimized E-jet printed polymer dielectric 67 3.3.4 E-jet printing of S/D electrodes with short channel length 74 3.3.5 Formation of SWCNT networks in E-jet printing system 76 3.3.6 Overall process for all E-jet printing and electrical characteristics of all E-jet printed SWCNT-TFTs 78 3.4 Conclusion 83 4 All electrohydrodynamic-jet printing system based circuit design for high-resolution and highly integrated applications 85 4.1 Introduction 85 4.2 Experimental details 89 4.2.1 In-situ fabrication of via-hole and diode-connected SWCNTs-TFTs in all E-jet printing system 89 4.2.2 Fabrication process of all E-jet printed inverter with vertically stacked SWCNT-TFTs 90 4.2.3 Fabrication process of all E-jet printed active pixel sensor for image sensor with vertical stacking structure 92 4.2.4 Fabrication process of all E-jet printed pixel circuit for active matrix polymer light-emitting diode with vertical stacking structure 95 4.3 Results and discussion 98 4.3.1 In-situ via-hole formation technology based on all E-jet printing system 98 4.3.2 Additional E-jet printing of PVP layer on the SWCNT-TFTs 99 4.3.3 Electrical characteristics for all E-jet printed diode-connected SWCNT-TFTs 101 4.3.4 Electrical characteristics for all E-jet printed inverter with vertically stacked SWCNT-TFTs 103 4.3.5 Structure design for active pixel sensor based on vertically stacked E-jet printed SWCNT-TFTs 107 4.3.6 All E-jet printed pixel circuit for active matrix polymer light-emitting diode with vertical stacking structure 110 4.4 Conclusion 118 5 Conclusion 119 Appendix 121 A.1 Post-treatment with DI-water on SWCNT-TFT 121 A.2 Variation of characteristics of SWCNT-TFTs by post-treatment time with NH4OH 123 A.3 Surface energy variation by a ratio between cross-liking agent and PVP 124 A.4 Analysis for surface roughness parameters 125 A.5 Electrical characteristics of E-jet printed SWCNT-TFTs according to channel structure 128 Bibliography 130 Abstract in Korean 149Docto