Solid-state memory is an essential component of the digital age. With advancements in healthcare technology and the Internet of Things (IoT), the demand for ultra-dense, ultra-low-power memory is increasing. In this review, we present a comprehensive perspective on the most notable approaches to the fabrication of physically flexible memory devices. With the future goal of replacing traditional mechanical hard disks with solid-state storage devices, a fully flexible electronic system will need two basic devices: transistors and nonvolatile memory. Transistors are used for logic operations and gating memory arrays, while nonvolatile memory (NVM) devices are required for storing information in the main memory and cache storage. Since the highest density of transistors and storage structures is manifested in memories, the focus of this review is flexible NVM. Flexible NVM components are discussed in terms of their functionality, performance metrics, and reliability aspects, all of which are critical components for NVM technology to be part of mainstream consumer electronics, IoT, and advanced healthcare devices. Finally, flexible NVMs are benchmarked and future prospects are provided.

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Review on Physically Flexible Nonvolatile Memory for Internet of Everything Electronics

A flexible version of traditional thin lead zirconium titanate ((Pb1.1Zr0.48Ti0.52O3)-(PZT)) based ferroelectric random access memory (FeRAM) on silicon shows record performance in flexible arena. The thin PZT layer requires lower operational voltages to achieve coercive electric fields, reduces the sol-gel coating cycles required (i.e., more cost-effective), and, fabrication wise, is more suitable for further scaling of lateral dimensions to the nano-scale due to the larger feature size-to-depth aspect ratio (critical for ultra-high density non-volatile memory applications). Utilizing the inverse proportionality between substrate's thickness and its flexibility, traditional PZT based FeRAM on silicon is transformed through a transfer-less manufacturable process into a flexible form that matches organic electronics' flexibility while preserving the superior performance of silicon CMOS electronics. Each memory cell in a FeRAM array consists of two main elements; a select/access transistor, and a storage ferroelectric capacitor. Flexible transistors on silicon have already been reported. In this work, we focus on the storage ferroelectric capacitors, and report, for the first time, its performance after transformation into a flexible version, and assess its key memory parameters while bent at 0.5 cm minimum bending radius.

Thin PZT‐Based Ferroelectric Capacitors on Flexible Silicon for Nonvolatile Memory Applications

Epitaxy of BaTiO3 thin film on Si(001) using a SrTiO3 buffer layer for non-volatile memory application

Engineering of molecular architectures of β-diketonate precursors toward new advanced materials

Poly(ethylene glycol) (PEG) additives with different molecular weights were used to modify sol-gel precursor solutions for preparing lead zirconate titanate (PZT) thin films. The morphology, crystalline structure, and mechanical and electrical properties of the films were characterized. The relationship between the characteristics of the films and the molecular weight of PEG was investigated. It was observed that the PEG eliminated cracking of the films during multiple pyrolysis treatments. However, with the increase of the PEG molecular weight, the films became less dense, which led to decreased Young's modulus and dielectric constant and increased coercive field. Our experiments showed that films prepared from sols modified by PEG with a molecular weight of 200 exhibited a dense morphology and excellent mechanical and electric properties without cracking.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400062543

Effects of poly(ethylene glycol) additive molecular weight on the microstructure and properties of sol-gel-derived lead zirconate titanate thin films

The development of lead zirconate titanate (PZT)-based capacitors has been a long time goal of ferroelectric random access memories (FRAM). However, PZT-based perovskites with common platinum (Pt) electrodes have suffered from a significant reduction of the remanent polarization (Pr) after a certain number of read/write cycles (electrical fatigue). We now report the development of fatigue-free lanthanum-modified PZT capacitors using common Pt electrodes. The capacitors fabricated at 580 °C by applying a PZT seed layer exhibited fatigue-free behavior up to 6.5×1010 switching cycles, a quite stable charge retention profile with time, and comparatively high Pr values, all of which assure their suitability for practical FRAM applications.

Fatigue-free lead zirconate titanate-based capacitors for nonvolatile memories

The development of lead zirconate titanate (PZT)-based capacitors using common Pt electrodes has been a long-time goal of ferroelectric random access memories (FRAM).In this work, a series of Pb1−xLax(Zr0.55Ti0.45)O3 capacitors (for 0.01 < x < 0.05) having fatigue-free characteristics have been grown on Pt/ Ti/SiO2/Si substrates. Typically 2–3 mol% La-modified PZT capacitors fabricated at 580 °C by applying a PZT seed layer exhibited fatigue-free behavior up to 6.5 × 1010 switching cycles, a low coercive field of 50–55 kV/cm, and a stable charge retention profile with time, all of which assure their suitability for the future nonvolatile FRAM.

Fatigue-free La-modified Pb(Zr, Ti)O3 capacitors using aseed layer

Pb0.98(La1−xLix)0.02(Zr0.55Ti0.45)O3(PLLZT with 0.1 ≤x≤ 0.7) thin films were sol-gel-grown on Pt(111)/Ti/SiO2/Si substrates, employing a thin lead zirconate titanate (PZT) template layer. Films annealed at >550°C showed a highly (111)-oriented preferential growth. Typical values of the switchable remanent polarization (2Pr) and the coercive field (Ec) of the PLLZT/PZT/Pt film capacitor for x= 0.3 were 50 μC/cm2 and 39 kV/cm, respectively, at 5 V. All the PLLZT/PZT/Pt capacitors (for 0.1 ≤x≤ 0.7) exhibited fatigue-free behavior up to 6.5 × 1010 switching cycles, a quite stable charge retention profile with time, and high 2Prvalues, all which assure their suitability for nonvolatile ferroelectric memories.

Santiranjan R. Shannigrahi

Papers

Fatigue-free lead zirconate titanate-based capacitors for nonvolatile memories

Fatigue-free La-modified Pb(Zr, Ti)O3 capacitors using aseed layer

Fatigue‐Free Lead Zirconate Titanate (Pb(Zr,Ti)O3)‐Based Capacitors Co‐modified by Lanthanum and Lithium for Nonvolatile Memories