The demand for flexible electronic systems such as wearable computers, E-paper, and flexible displays has recently increased due to their advantages over present rigid electronic systems. Many researchers have studied various flexible electronic devices such as integrated circuits (ICs), organic light emitting diodes (OLEDs), sensors, and radio frequency identification (RFID) antennas. A flexible display for SoP is on the brink of commercialization, as flexible displays were demonstrated with several concept devices in the 2013 International Consumer Electronics Show (CES 2013). Although these works have demonstrated the feasibility of flexible electronic devices, their applications have been restricted to one or just a few components of the electronics and thus each electronic device must be integrated into a single device to perform its respective function within a flexible system. To realize such all-in-one flexible systems, the development of flexible memory is a key issue for data processing, information storage, and radio frequency (RF) communication.
A number of researchers have explored a variety of organic based flexible memories including flash memory, ferroelectric memory, and resistive memory, directly fabricated on flexible substrates at relatively low temperature by spin-coating, roll-to-roll, and other processes. Although these organic-based flexible memories have been well-established with advantages of achieving flexible electronics over large areas in a cost-effective manner, there are still big challenges to develop high density flexible memory with high-performance by resolving insufficient performance arising from inherent material properties, and non-compatibility with complementary metal？oxide？semiconductor (CMOS) processes.
Resistive random access memory (RRAM) is considered as a promising candidate for flexible memory in SoP due to its advantages of simple structure, high-density integration, low temperature process, high-speed switching property, and low power consumption. Several research groups have reported flexible resistive memory with a simple crossbar structure based on various materials such as GeO/HfON, TiO2, SiOx, Al2O3, Ag2Se. Nonvolatile memory devices with a cross-point structure, however, suffer unavoidable cell-to-cell interference during memory access operation. The cell-to-cell interference between neighboring memory cells occurs due to leakage current paths through adjacent low resistance state cells and induces not only unnecessary power consumption but also a misreading problem, a fatal obstacle in memory operation. To fabricate a fully functional flexible memory and prevent these unwanted effects, each memory cell must be integrated with a switching component such as a transistor. Unfortunately, most transistors built on plastic substrates (e.g. organic/oxide transistors) have insufficient effective mobility to drive conventional memory, compared to that of present silicon transistors. In this thesis, to demonstrate fully functional flexible memory, various structure of flexible resistive memory devices were demonstrated using several fabrication processes.
Chapter 2 describes a RRAM with a one transistor-one memristor (1T-1M) structure in a NOR type array on flexible substrates. High performance flexible single crystal silicon transistors was integrated with an amorphous titanium oxide (a-TiO2) based memristor to control the logic state of memory. The transistor had effective device mobility of 340 cm2/V？s in the linear regime, showing this flexible transistor using ultra-thin single crystal silicon as an active layer can be used as a switching element of memory. The 1T-1M RRAM device has asymmetric bipolar resistance switching (BRS) behaviour with an on/off ratio of 50 at -0.5 V (reading voltage) with good endurance and retention property. The 1T-1M RRAM unit cells were interconnected with each other through word, bit, and source lines in 8 x 8 NOR type array to control each memory unit cell independently. Finally, the first demonstration of random access memory operation of the RRAM on a flexible substrate was performed.
In chapter 3, High-performance flexible single crystal silicon diodes were integrated with copper oxide based resistive memory on a plastic substrate to prevent the cross-talk problem. . The integrated diodes exhibit high-performance electrical properties with a high rectifying ratio of 105 at ±1 V and a current density of 105 A/cm2 in the forward bias region. The resistance switching phenomenon of 1D-1R memory occurs stably and consistently in repeated DC weep and pulse mode. By integrating high-performance single crystal silicon diodes with plasma-oxidized resistive memory, cell-to-cell interference between adjacent memory cells is effectively prevented, and random access operation of a 1D-1R flexible memory device is thereby successfully performed on a plastic substrate.
In chapter 4, to address limitations of the above works, a conceptual strategy for the fabrication of flexible memory employing a one selector-one resistor (1S-1R) crossbar structure on a plastic substrate via an inorganic-based laser lift-off (ILLO) process was introduced. 32 × 32 1S-1R crossbar memory arrays for 1 kbit flexible memory were fabricated with an inorganic laser-reactive exfoliation layer on a rigid glass substrate using conventional CMOS process, and then subsequently transferred to a flexible substrate through the ILLO process. Structural design was optimized by finite element analysis simulations to predict and prevent possible thermal damage during laser irradiation. Based on the simulation, the dry ILLO process was applied for the large-area transfer of the memory device onto a flexible substrate without mechanical damage during the transfer process. Finally, the 1S-1R RRAM cells, formed on a plastic substrate, were successfully evaluated in a worst case scenario influenced by the suppression of sneak path of integrated selectors.
유연 전자기기는 뛰어난 휴대성, 사용자 친화적 인터페이스 등의 장점으로 차세대 전자기기로서 주목 받고 있다. 유연 전자기기의 구성요소 중 유연 메모리는 데이터 프로세싱, 저장, 외부와의 통신 등에 필수적인 구성요소로서 여겨지고 있다. 현재까지 많은 연구자들이 스핀 코팅(spin-coating), 롤-투-롤(roll-to-roll) 등의 공정을 이용해 플라스틱 기판 상에 유기물을 이용한 플레쉬 메모리, 강유전 메모리, 저항변화 메모리 등을 제시한 바가 있다. 이러한 방식은 낮은 비용으로 넓은 면적에 유연 소자를 제작할 수 있다는 장점을 가지고 있지만 낮은 성능과 반도체 공정과의 비호환성이 걸림돌이 되고 있다. 본 학위 논문은 이러한 점을 극복하기 위해 뛰어난 성능을 가지고 있는 무기물질을 이용하여 유연 메모리를 제작하는 방법을 제시하고 있다.
두 번째 장에서는 반도체 공정과 소프트 리소그라피를 이용한 one transistor-one memristor 구조의 유연 메모리를 개발하였다. 트랜지스터의 액티브 층으로 활용하기 위해 도핑 된 단결정 실리콘 박막을 SOI (silicon-on-insulator) 기판으로부터 분리해 유연 기판에 전사하고 트랜지스터를 제작한다. 그 후 PEALD를 (plasma-enhanced atomic layer deposition) 이용해 비정질 TiO2 저항변화 메모리를 집적하여 유연 저항변화 랜덤 액세스 메모리를(Resistive Random Access Memory, RRAM) 개발하였다. 제작된 소자는 유연기판상에서 신뢰적이고 재현성 있는 저항변화 특성을 가지고 있었으며 100번 이상의 쓰기/지우기 및 104 초 이상의 데이터 유지 특성을 보유하고 있었다. 또한 심한 굽힘 상태에서도 소자의 성능을 유지하였으며 1000번 이상의 반복 굽힘 에서도 소자가 파괴되지 않았다. 최종적으로 제작된 유연 메모리는 플라스틱 기판 상에서 주변 셀간의 간섭 없이 성공적인 랜덤 액세스 구동을 보여주었다.
세 번째 장에서는 완전구동 가능한 one diode-one resistor 구조의 RRAM을 개발하였다. 선택 소자로서 유연한 고성능 단결정 다이오드를 제작하기 위하여 SOI 기판에서 950 °C 이상의 온도에서 도핑을 한 실리콘 박막을 유연기판에 전사하고 이에 CuxO 저항변화 메모리를 스퍼터링과 산소 플라즈마를 이용해 집적하였다. 안정성과 신뢰성을 평가하기 위하여 쓰기/지우기 및 데이터 유지 측정을 진행하였으며 제작된 소자는 뛰어난 굽힘 성능을 보유하고 있었다.
네 번째 장에서는 기존의 전사공정을 탈피하여 무기 기반 레이저 리프트 오프 공정을 (inorganic-based laser lift-off, ILLO) 도입하여 one selector-one resistor 크로스바 구조의 메모리를 유연기판에 개발하였다. 유리기판에 레이저 감응 무기물질을 증착한 후 그 위에 크로스바 구조의 메모리를 제작을 하였으며 이를 ILLO 공정을 통해 변형 없이 유연기판에 성공적으로 전사하였다. 본 개발에 활용된 물질은 모두 무기물 기반으로 높은 공정온도가 가능하였다. 실험에 앞서 유한요소 시뮬레이션을 통해 열적 손상 및 변형 없는 최적의 구조를 선정하였다. 최종적으로 전사된 메모리는 주변 셀간의 간섭 없이 성공적으로 각 셀의 데이터를 어드레싱하였다. 본 실험에서 제시된 ILLO 공정은 차후 고집적 소자에 적용될 수 있을 것으로 기대된다.
