Performance enhancement of next-generation nonvolatile memories using self-organized nanopatterning자가 나노패터닝을 이용한 차세대 비휘발성 메모리 성능 향상

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Next generation nonvolatile memories (NVMs) such as a ferroelectric random access memory (FRAM), a phase-change random access memory (PRAM), a resistive random access memory (RRAM), and a magnetic random access memory (MRAM) have attracted considerable attention because of their unique advantages when compared to the a dynamic random access memory (DRAM) and a flash memory. Although they are promising candidates for the NVM, there are unsolved problems for the practical applications. Among them, the PRAM is based on chalcogenide phase-change materials which have a large electrical contrast between amorphous and crystalline states. A critical issue for the commercialization is high power consumption for the memory operations. In order to achieve the low switching current, the contact area between a phase-change material and a heater should be shrunk through the further scaling-down. However, the conventional optical lithography has been restricted because of the limit of optical resolution. Many researches for the power reduction of PRAMs have been suggested via various methodologies without using high-cost nanoscale lithography. Nevertheless, practical solutions compatible with complementary metal-oxide semiconductor (CMOS) processes have not been reported. The RRAM is another option for alternating charge-based memories (DRAM and flash memory). The scaling down of the memory cells is very favorable because its principle comes from the formation/rupture of nanoscale conductive filaments (CFs) in the metal-insulator-metal (MIM) structure. However, the random filaments growth in the active metal-oxide layer causes large fluctuations of the resistive switching (RS) parameter such as operating voltages and resistance values. These unanticipated properties may result in the operation error. To resolve this uniformity issue, novel nanoscale approaches are required since the RS mechanism stems from nanosized-phenomenon. In addition, a flexible electronic system has been widely investigated as the next-generation technology in various areas, ranging from consumer electronics to bio-integrated devices. In particular, a flexible memory is an essential component for fully functional flexible electronics because of its important role in data processing, storage, and communications with external devices. However, there are still some challenges in developing high performance flexible memories compatible with conventional CMOS processes. Block copolymer (BCP) self-assembly has received a large amount of attention due to their potentials to overcome the challenges of traditional nanofabrication technologies. BCP film processing is scalable below sub-10 nm and low-cost efficiency, and is compatible with current semiconductor fabrication techniques. A solvent-assisted nanotransfer printing (S-nTP) technique based on self-assembled BCP templates was also proposed as a method to establish uniformly aligned nanoscale patterns in desired places such as rigid substrates, plastic films, glasses, curved surfaces, skins, and so forth. Compared to conventional nanotransfer printing (nTP) based on elastomeric mold, the s-nTP process has a number of advantages including high scalability, large-area uniformity, low cost, and versatile process. The s-nTP can easily produce well-defined two- and three-dimensional nanostructures in deep-nanoscale regime (sub-10 nm) with high transfer printing yield. These nanoscaling methods can be combined with various memory devices in order to achieve the performance enhancement and nanopatterning without high-cost optical lithography. In chapter 2, phase-change memory (PCM), which exploits the phase-change behavior of chalcogenide materials, affords tremendous advantages over conventional solid-state memory due to its non-volatility, high speed, and scalability. However, high power consumption of PCM poses a critical challenge and has been the most significant obstacle to its widespread commercialization. Here, we present a novel approach based on the self-assembly of a BCP to form a thin nanostructured $SiO_x$ layer that locally blocks the contact between a heater electrode and a phase-change material. The writing current is decreased five-fold (corresponding to a power reduction by 1/20) as the occupying area fraction of $SiO_x$ nanostructures is increased from a fill factor of 9.1% to 63.6%. Simulation results theoretically explain the current reduction mechanism by localized switching of BCP-blocked phase-change materials. In chapter 3, flexible memory is the fundamental component for data processing, storage, and radio frequency communication in flexible electronic systems. Among several emerging memory technologies, PRAM is one of the strongest candidate for next-generation NVMs due to its remarkable merits of large cycling endurance, high speed, and excellent scalability. Although there are a few approaches for flexible PCM, high reset current is the biggest obstacle for the practical operation of flexible PCM devices. In this paper, we report a flexible PCM realized by incorporating nano-insulators derived from a Si-containing BCP to significantly lower the operating current of the flexible memory formed on plastic substrate. The reduction of thermal stress by BCP nanostructures enables the reliable operation of flexible PCM devices integrated with ultrathin flexible diodes during more than 100 switching cycles and 1,000 bending cycles. In chapter 4, RRAM is a promising candidate for future NVM. RS in a MIM structure is generally assumed to be caused by the formation/rupture of nanoscale CFs under an applied electric field. The critical issue of RRAM for practical memory applications, however, is insufficient repeatability of the operating voltage and resistance ratio. Here, we present an innovative approach to reliably and reproducibly control the CF growth in unipolar NiO resistive memory by exploiting uniform formation of insulating $SiO_x$ nanostructures from the self-assembly of a Si-containing BCP. In this way, the standard deviation (SD) of set and reset voltages was markedly reduced by 76.9% and 59.4%, respectively. The SD of high resistance state also decreased significantly, from $6.3 x 10^7 \Omega$ to $5.4 x 10^4 \Omega$. Moreover, we report direct observations of localized metallic Ni CF formation and their controllable growth using electron microscopy and discuss electro-thermal simulation results based on the finite element method supporting our analysis results. In chapter 5, $Ge_2Sb_2Te_5-$ based PCMs, which undergo fast and reversible switching between amorphous and crystalline structural transformation, are being utilized for nonvolatile data storage. However, a critical obstacle is the high programming current of the PCM cell, resulting from the limited pattern-size of the optical lithography-based heater. Here, we suggest a facile and scalable strategy of utilizing self-structured CF nano-heaters for Joule heating of chalcogenide materials. This CF nanoheater can replace the lithographical-patterned conventional resistor-type heater. The sub-10 nm contact area between the CF and the phase-change material achieves significant reduction of the reset current. In particular, the PCM cell with a single Ni filament nanoheater can be operated at ultra-low writing current of $20 \mu A$. Finally, phase-transition behaviors through filament type nanoheaters were directly observed by using transmission electron microscopy. In chapter 6, memristor devices based on electrochemical metallization operate through electrochemical formation/dissolution of nanoscale metallic filaments, and they are considered a promising future NVM because of their outstanding characteristics over conventional charge-based memories. However, nanoscale conductive paths or filaments precipitated from the redox process of metallic elements are randomly formed inside oxides resulting in unexpected and stochastic memristive switching parameters including the operating voltage and the resistance state. Here, we present the guided formation of CFs in Ag nanocone/ $SiO_2$ nanomesh/Pt memristors fabricated by high-resolution S-nTP. Consequently, the uniformity of the memristive switching behavior is significantly improved by the existence of electric field-concentrator arrays consisting of Ag nanocones embedded in $SiO_2$ nanomesh structures. This selective and controlled filament growth was experimentally supported by analyzing simultaneously the surface morphology and current-mapping results using conductive atomic-force microscopy. Moreover, stable multi-level switching operations with four discrete conduction states were achieved by the nanopatterned memristor device demonstrating its potential in high-density nanoscale memory devices.
Advisors
Lee, Keon Jaeresearcher이건재researcher
Description
한국과학기술원 :신소재공학과,
Publisher
한국과학기술원
Issue Date
2017
Identifier
325007
Language
eng
Description

학위논문(박사) - 한국과학기술원 : 신소재공학과, 2017.2,[xxiii, 215 p. :]

Keywords

Nonvolatile Memory; Phase-Change Memory; Resistive Memory; Flexible Memory; Block Copolymer Self-Assembly; Solvent-Assisted Nanotransfer Printing; 비휘발성 메모리; 상변화 메모리; 저항 변화 메모리; 유연 메모리; 블록공중합체 자기조립; 용매 지원 나노전사 프린팅

URI
http://hdl.handle.net/10203/241932
Link
http://library.kaist.ac.kr/search/detail/view.do?bibCtrlNo=675768&flag=dissertation
Appears in Collection
MS-Theses_Ph.D.(박사논문)
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