High Density Non-Volatile Ferroelectric Memory Device in a Large Area via Anodic Aluminum Oxide Template

Abstract

Ferroelectric materials have gained much attention owing to their unique properties of spontaneous switchable polarization, piezoelectricity and pyroelectricity. During the past decades, the ferroelectrics of a thin film have been extensively studied both theoretically and experimentally. Although continuous ferroelectric thin film has shown the possibility for various applications such as information storage media and infrared cameras, individually switchable ferroelectric nanoislands have tremendous advantages due to storing ultrahigh-density memory, reducing cross-talk effect, and operating a data bit under low voltages. There are three-type approaches for preparing the array of ferroelectric nanoislands have been introduced in the literature: top-down, typical bottom-up, and modified bottom-up. Because top-down approach utilize a high energy beam to etch the ferroelectric films, the surface of ferroelectric films are damage, which result in changing the ferroelectric properties. However, typical bottom-up approaches have a difficulty in uniform size, array, and shape. To supplement both the limitations of top-down and typical bottom-up approaches, there has been much effort to develop novel modified bottom-up approaches, which combines advantages of both top-down and bottom-up approaches where ferroelectric nanoislands are grown selectively in predefined locations: soft electron beam lithography (soft-eBL), dip-pen lithography, block copolymer template, self-assembled growth, ion beam milling, and anodic aluminum oxide (AAO) template. Because soft-eBL requires that the electron beam must be scanned across patterned areas pixel by pixel, it needs long times and requires an expensive and highly sophisticated machine. Similarly, dip-pen lithography takes a long times to fabricate a high density array in a large area. Self-assembled growth is hard for controlling the size of the nanoisland and fabricating well-arranged nanoislands in a large area. Also, ion beam milling uses high energy irradiation during etching, which could damage the crystal structure of nanoislands. However, among these methods, AAO template is an ideal template for fabricating nanoislands array in a large area, because regular size (diameter and height) of AAO template is easily controlled by anodizing electrolyte (sulfuric acid, oxalic acid, and phosphoric acid), anodizing time, and wet etching time. In chapter 2 of this thesis, I supplemented limitation of an existing AAO membrane utilized method as a mask for fabricating high density inorganic multiferroic BiFeO3 (BFO) nanoislands array in a large area. However, downscaling the size is fundamentally limited due to the critical size for ferroelectricity and the limitation of array in lateral size. Also, ferroelectric metal oxide needs a high temperature to crystalize, which means it is hard to applicate flexible device. In the aspect of data writing process, reported papers shows only one data is written by applying voltage, referred to as single data writing (SDW). Thus, in chapter 3, I developed data-storing process using controlling architectural shape (multi floor cascading nanostructure) of polymer ferroelectric nanoislands for speedfully storing lots of data in one cell. Finally in chapter 4, I prepared AAO template on elastomeric polydimethylsiloxane (PDMS) plate, which enable to imprint polymer ferroelectric P(VDF-TrFE) not only on flat substrate, but also on curvature substrate with relatively low pressure compared with AAO template on aluminum plate. Also, I investigated two different ferroelectric piezoresponse by using piezoresponse force microscopy (PFM): (chapter 2) inorganic multiferroic BFO and (chapter 3) cross-linked polymer ferroelectric poly(vinylidene fluoride-ran-trifluoroethylene) copolymer [cP(VDF-TrFE)]. In chapter 2, I fabricated the high density array of BFO nanoislands in a large area, using nanoporous polystyrene (PS) template prepared by AAO membrane-assisted reactive-ion method. In the existing AAO membrane utilized method, due to very poor contact (or adhesion) between AAO membrane and the conducting substrate, pulsed laser deposition (PLD) method is only used to prepare array of multiferroic islands. In this situation, for depositing multiferroic materials, very thin (~ 200 nm thickness) AAO membrane should be used. However, very thin AAO membrane is not easy to obtain uniform contact to the substrate in a large area; thus the lateral area of nanoisland array is limited (a range of ~ mm2). To fabricate high density array in area (for example, a wafer size), I used nanoporous polystyrene (PS) template where the pores are prepared by reactive-ion etching with the aid of AAO membrane. Because of excellent adhesion between polymer film and the substrate, BFO precursors are easily incorporated into nanoholes in polymer template by spin-coating in a large area. Due to the existence of a PS matrix and the direct contact to substrate, each BFO nanoisland in each nanopore could not be agglomerated with each other even after thermal treatment for the removal of the organic moieties in the BFO precursor as well as PS matrix. Because a nanoisland array was formed through the entire area, the BFO crystals were characterized by synchrotron X-ray diffraction. The high density array of BFO nanoislands in a large area showed both ferroelectricity of individual nanoislands obtained by piezoresponse force microscopy (PFM) and macroscopic magnetism measured by superconducting quantum interference device (SQUID) based magnetic property measurement system (MPMS). A high density array of BFO nanoislands could be employed as next-generation memory device capable of electric writing and magnetic reading (or vice versa). In chapter 3, for developing high capacity storage, multi-level non-volatile memory (MLNVM) devices with fast writing speed is to prepare a high density array of nano-sized ferroelectric polymers that operate individually as a single memory cell and are capable of multiple data writing (MDW). Thus, I newly introduced a novel concept for preparing a high density array of multi floor cascading nanostructures along the thickness direction of cross-linked P(VDF-TrFE) [cP(VDF-TrFE)] by employing innovative AAO templates with pores having inverse multi floor cascading depth profiles. A high density array of multi floor cascading cP(VDF-TrFE) nanostructures was prepared by nanoimprinting method. I used atomic force microscopy (AFM) for accessing individually electrical writing and reading of the multi floor cascading nanostructures. After an external electric field was applied to each floor of cP(VDF-TrFE) nanostructures through a conductive AFM tip, the piezoresponse of the polarization states was monitored by piezoresponse force microscopy (PFM). Different switching behavior in each floor was observed by switching spectroscopy PFM (SS-PFM). In addition, first-order reversal curve-type (FORC)-PFM revealed that multi-level polarization states were successfully achieved even in a single floor due to having stable intermediate polarization states. I found that the piezoresponse of one floor was significantly influenced by applying an electric field to the other floor. This behavior was also confirmed by a simulation based on finite element modeling. Thus, the concept of the MDW is indeed feasible in multi floor ferroelectric nanostructures. Since this unique nanostructure is capable of storing information through MDW which allows one to have extremely a lot of information capacity in a single cell, this could be used as a next generation MLNVM device for high capacity data storage and fast writing (or operating) speed. In chapter 4, because of inflexibility of AAO template on aluminum plate, general nanoimprinting method by using AAO template is only used to imprint polymer film on flat substrate. Thus, I newly prepare AAO template on flexible polydimethylsiloxane (PDMS) plate to imprint ferroelectric P(VDF-TrFE) not only on flat substrate but also on curved substrate. To enhance the adhesion between AAO template and PDMS plate, PDMS was grafted to back-side of AAO template, which enables to easily peel off the AAO template from imprinted P(VDF-TrFE) nanoislands, followed by high temperature heating and pressure. Thus, the AAO template on PDMS plate was reusable. This flexible AAO template on PDMS plate could be used as roll-to-roll process for mass-production and large area imprinting of nanostructured polymer film.