Flexible electronics is a spotlighted field in the approaching era of Internet-of-Things (IoT) based on its ultrathin, lightweight, and highly bendable nature, providing the strong potential to broaden the field of electronics ranging from soft appliances (e.g., wearable devices, e-papers, and rollable computers) to bio-implantable healthcare devices (e.g., artificial skins and in vivo chips). In particular, flexible memory is an essential element in achieving soft electronic systems, considering its vital functions in data processing, information storage, and communication operations. Flexible memories based on organic-, carbon-based (e.g., carbon nanotubes (CNTs) or graphene), and two-dimensional materials (e.g., $MoS_2$) have been widely studied to take advantage of their highly flexible properties and applicability of the cost-effective ambient process (e.g., spin coating, ink-jet printing, and roll-based process). Nevertheless, insufficient electrical performance (e.g., low carrier mobility), poor material properties (e.g., thermal instability and degradation), problematic issues on integration, and incompatibility with the conventional CMOS process have been an unsolved issue.
To address these problems, there have been efforts to realize flexible memories with inorganic materials directly on plastic substrates. However, various problems including limited thermal budget of the polymer substrate, imprecise multilayer alignment, and roughness issues occurred. Therefore, novel approaches have been researched to overcome the limitations in direct fabrication on plastic substrate through fabricating the memory on a conventional rigid substrate (e.g., silicon or glass wafer) with high-temperature CMOS process first, and then transferring the top thin device layer on a polymer substrates. The aim of this dissertation is to realize inorganic materials-based high-performance flexible memories ranging from mainstream charge-trap flash memory to next-generation data-storage devices (e.g., phase change memory and ferroelectric memory) by utilizing transfer technology.
In chapter 2, ACF-packaged ultrathin Si-based flexible NAND flash memory array was demonstrated on plastic substrate. Silicon-based flexible devices have received extensive attention owing to its outstanding performance, reliability and state-of-the-art CMOS compatibility, however, ultrathin Si-based device with flexible interconnections have not been demonstrated. There have been efforts to utilize anisotropic conductive film (ACF) as a packaging material which is highly elastic and resilient under various stress conditions, however, the required high pressure of the flip-chip bonding process made it difficult to apply the ACF to highly-vulnerable ultrathin silicon devices. Herein, highly-flexible and ultrathin ($1 \mum$ in thickness) Si-based NAND flash memory interconnected on plastic substrate was demonstrated by ACF-packaging the Si-based NAND device on plastic substrate first, and then delicately eliminating the sacrificial bottom Si of the wafer. Reliable interconnection and memory operation were achieved during the bending condition without physical degradation due to its mechanical stability. The work presented here suggests a useful methodology to realize various high-performance fully-packaged Si-based flexible large-scale integrated (LSI) devices.
In chapter 3, Si-based flexible NAND flash memory (f-NAND) was demonstrated on a plastic substrate through highly-productive roll-based transfer and simultaneous interconnection process. An ultrathin f-NAND chip was prepared on an intermediate transfer substrate by bonding the Si NAND flash on a rigid glass and subsequently removing the handle wafer. Roll-based flip-chip packaging technology with precise optical alignment allowed the f-NAND to be transferred and simultaneously interconnected on a flexible printed circuit board (FPCB) through thermo-compression ACF bonding. The ACF packaging materials exhibited outstanding bonding capability for continuous roll-based transfer and excellent flexibility of interconnection, because of its inherent elastic nature from the polymer-based thermo-setting resin. Non-linear elastic deformation of the dynamic roll-based bonding was investigated by a finite element analysis (FEA) to optimize the uniform pressure distribution on the FPCB with concentrated pressure on electrode bumps for reliable transfer and interconnection. Finally, the ACF-packaged Si f-NAND was successfully completed on the FPCB in a chip-on-flex structure, showing excellent flexibility and stable operations of the NAND memory. Unit flash memories with outstanding properties were connected in series to build a NAND flash string. Reliable operation of the flexible $16 \times 16$ NAND flash array was confirmed at the circuit level by programming and reading letters in ASCII codes. The results may open up new opportunities of integrating Si-based high-performance f-LSIs on plastics with the ACF packaging in highly-productive roll-based production.
In chapter 4, we demonstrate crossbar-structured flexible phase-change random access memory (f-PRAM) on plastics by highly-productive physical exfoliation technique. Phase change memory (PCM) has been considered one of the most attractive candidates in achieving next-generation flexible memories thanks to its outstanding merits such as fast switching speed, high-reliability, and excellent non-volatility. Nevertheless, critical challenges such as the effective integration of the low power-consuming PCM with the selection device and its efficient transfer to plastics remain to realize fully functional flexible PCM. In this work, an array of 1 schottky diode (SD)-1 PCM integrated cells was fabricated on rigid substrate by conventional semiconductor processes, and then physically lifted-off and transferred on flexible substrate, eliminating the problematic issues of the direct fabrication on plastics. The oxide-based SD (Pt/$TiO_2$/Ti) effectively suppressed the sneak current through the adjacent cells for the random access operation of the memory array. The chalcogenide $Ge_2Sb_2Te_5-based$ PCM operated in low switching power on plastics through the current confinement effect of the Ni filament-based nanoscale heater. The final flexible phase change random access memory demonstrated reliable memory properties on plastics during bending conditions.