High-density Ferroelectric Random Access Memories Research Articles

Role of Defects in the Breakdown Phenomenon of Al1-xScxN: From Ferroelectric to Filamentary Resistive Switching.

Aluminum scandium nitride (Al1-xScxN), with its large remanent polarization, is an attractive material for high-density ferroelectric random-access memories. However, the cycling endurance of Al1-xScxN ferroelectric capacitors is far below what can be achieved in other ferroelectric materials. Understanding the nature and dynamics of the breakdown mechanism is of the utmost importance for improving memory reliability. The breakdown phenomenon in ferroelectric Al1-xScxN is proposed to be an impulse thermal filamentary-driven process along preferential defective pathways. For the first time, stable and robust bipolar filamentary resistive switching in ferroelectric Al1-xScxN is reported. A hot atom damage defect generation model illustrates how filament formation and ferroelectric switching are connected. The model reveals the tendency of the ferroelectric wurtzite-type Al1-xScxN system to reach internal symmetry with bipolar electric field cycling. Defects generated from bipolar electric field cycling influence both the energy barrier between the polarization states and that required for the filament formation.

Boosting Polarization Switching-Induced Current Injection by Mechanical Force in Ferroelectric Thin Films.

When scaling the lateral size of a ferroelectric random access memory (FeRAM) device down to the nanometer range, the polarization switching-induced displacement current becomes small and challenging to detect, which greatly limits the storage density of FeRAM. Here, we report the observation of significantly enhanced injection currents, much larger than typical switching currents, induced by polarization switching in BiFeO3 thin films via conductive atomic force microscopy. Interestingly, this injected current can be effectively modulated by applying mechanical force. As the loading force increases from ∼50 to ∼750 nN, the magnitude of the injected current increases and the critical voltage to trigger the current injection decreases. Notably, changing the loading force by an order of magnitude increases the peak current by 2–3 orders of magnitude. The mechanically boosted injected current could be useful for the development of high-density FeRAM devices. The mechanical modulation of the injected current may be attributed to the mechanical force-induced changes in the barrier height and interfacial layer width.

Flexible Robust and High-Density FeRAM from Array of Organic Ferroelectric Nano-Lamellae by Self-Assembly.

Ferroelectric memories are endowed with high data storage density by nanostructure designing, while the robustness is also impaired. For organic ferroelectrics favored by flexible memories, low Curie transition temperature limits their thermal stability. Herein, a ferroelectric random access memory (FeRAM) is demonstrated based on an array of P(VDF‐TrFE) lamellae by self‐assembly. Written data shows enhanced thermal endurance up to 90 °C and undergoes 12 thermal cycles between 30 and 80 °C with little volatilization. The promoted thermal stability is attributed to pinning effect at interfaces between grain boundaries and lamellae, where charged domain walls and charged defects are coupled. These results provide a strategy for improving robustness of organic flexible FeRAMs, and reveal an attracting coupling effect between different phases of ferroelectric polymer.

Ferroelectric HfO2-based materials for next-generation ferroelectric memories

Ferroelectric random access memory (FeRAM) based on conventional ferroelectric perovskites, such as Pb(Zr,Ti)O3 and SrBi2Ta2O9, has encountered bottlenecks on memory density and cost, because those conventional perovskites suffer from various issues mainly including poor complementary metal-oxide-semiconductor (CMOS)-compatibility and limited scalability. Next-generation cost-efficient, high-density FeRAM shall therefore rely on a material revolution. Since the discovery of ferroelectricity in Si:HfO2 thin films in 2011, HfO2-based materials have aroused widespread interest in the field of FeRAM, because they are CMOS-compatible and can exhibit robust ferroelectricity even when the film thickness is scaled down to below 10 nm. A review on this new class of ferroelectric materials is therefore of great interest. In this paper, the most appealing topics about ferroelectric HfO2-based materials including origins of ferroelectricity, advantageous material properties, and current and potential applications in FeRAM, are briefly reviewed.

Ferroelectric properties of niobium-doped strontium bismuth tantalate films

The characteristics of ferroelectric thin films of strontium bismuth tantalate (SBT) and niobium-doped strontium bismuth tantalate (SBTN) deposited by radio-frequency (RF) magnetron sputtering on Pt/TiO2/SiO2/Si substrates were investigated. For the formation of the structure of the ferroelectric material, the deposited films were subjected to a subsequent annealing at temperatures of 970–1070 K in an O2 atmosphere. The results of the X-ray diffraction analysis demonstrated that, in contrast to SBT films, in which the Aurivillius phase is formed only at annealing temperatures of 1050–1070 K, the formation of this phase in SBTN films is observed already at a temperature of 970 K. The dependences of the dielectric permittivity, remanent polarization, and coercive force of the SBT and SBTN films on the subsequent annealing conditions were determined. It was found that, upon doping of the SBT films with niobium, the remanent polarization increases by a factor of approximately three, the Curie temperature increases by 50 K, and the dielectric permittivity also increases. It was revealed that, in contrast to the SBT films, the polarization of the SBTN films is observed already at an annealing temperature of approximately 970 K. It was shown that the replacement of SBT films by SBTN films in the manufacture of high-density nonvolatile ferroelectric randomaccess memory (FeRAM) capacitor modules makes it possible to decrease the synthesis temperature from 1070 to 990–1000 K, which improves the compatibility with the planar technology of semiconductor devices. However, it turned out that an increase in the coercive field makes niobium-doped SBT films less attractive for the use in FeRAM.

4.0-nm-thick amorphous Nb–Ni film as a conducting diffusion barrier layer for integrating ferroelectric capacitor on Si

We have successfully integrated La0.5Sr0.5CoO3/PbZr0.4Ti0.6O3/La0.5Sr0.5CoO3 (LSCO/PZT/LSCO) capacitors on silicon substrate using a ∼4.0-nm-thick amorphous Nb–Ni film as the conducting diffusion barrier layer. Transmission electron microscopy technique confirms that the Nb–Ni film is still amorphous after fabrication of the capacitors, and the interfaces related to Nb–Ni are clean and sharp without any findable interdiffusion/reaction. The LSCO/PZT/LSCO capacitor, measured at 5V, possesses very good properties, such as large remanent polarization of ∼22.1μC/cm2, small coercive voltage of ∼1.27V, good fatigue-resistance, and small pulse width dependence, implying that ultrathin amorphous Nb–Ni film is ideal as the conducting diffusion barrier layer to fabricate high-density silicon-based ferroelectric random access memories.

Random-Oriented (Bi,La)4Ti3O12Thin Film Deposited by Pulsed-DC Sputtering Method on Ferroelectric Random Access Memory Device

A ferroelectric (BLT) thin film fabricated by the pulsed-DC sputtering method was evaluated on a cell structure to check its compatibility to high density ferroelectric random access memory (FeRAM) devices. The BLT composition in the sputtering target was . Firstly, a BLT film was deposited on a buried Pt//Ir bottom electrode stack with W-plug connected to the transistor in a lower place. Then, the film was finally crystallized at for 30 seconds in oxygen ambient. The annealed BLT layer was found to have randomly oriented and small ellipsoidal-shaped grains (long direction: ~100 nm, short direction: ~20 nm). The small and uniform-sized grains with random orientations were considered to be suitable for high density FeRAM devices.

Tristate Memory Using Ferroelectric-Insulator-Semiconductor Heterojunctions for 50% Increased Data Storage

Ferroelectric random-access memory (FeRAM) is considered to be one of the best candidates for universal memory. However, difficult scaling of the memory cell size has hindered the realization of high density FeRAM. Given that size scaling is inherently limited by the complicated crystal structure and processing of ferroelectric materials, the highly stable and step-wise three memory state of one cell can be another pathway to high-density FeRAM. A feasible structure and actual operation of a tristate memory function for high-density FeRAM is presented that uses stacked ferroelectric Pb(Zr,Ti)O3/insulating Al2O3/semiconducting ZnO layers with Pt top and bottom electrodes. The complicated electrical responses of the stacked structure to external stimuli are well understood based on the separated trapping of the compensating charges at the Pb(Zr,Ti)O3/Al2O3 and Al2O3/ZnO interfaces and the discrete dissipation of the trapped charges during polarization switching in one direction. This unique function of the structure induces three discrete charge states that can be used to increase the memory density by 50% compared to conventional FeRAM at a given cell size.

A VISION OF FRAM AS A FUSION MEMORY

ABSTRACT The purpose of this paper is to attempt to find a plausible solution for a high density FRAM (Ferroelectric Random Access Memory). To complete this, firstly, we give a very brief but an important review of how FRAM technologies have been evolved during the past many years. Secondly, not only are a few examples demonstrated in terms of how a FRAM-embedded system is advantageous when adopted in a system but it is also put forward how FRAM can play an essential role in a fusion memory. Finally, several process technologies for integrating a reliable high-density FRAM are mentioned, accordingly resulting in very reliable cell-charge populations after full integration, along with wafer-level reliabilities.

Electron Backscatter Diffraction Analysis for Polarization of SrBi2(Ta,Nb)2O9 Ferroelectric Capacitors in Submicron Small Area

We investigate the size and crystal orientation of each small grain of ferroelectric SrBi2(Ta,Nb)2O9 (SBTN) films within very small areas by an electron backscatter diffraction (EBSD) analysis technique. The obtained map of the grains reveals that the size of c-axis-oriented grains increases as the average grain size of the films increases. On the other hand, the size of a-axis-oriented grains, each of which has a finite remnant polarization normal to the films, is almost unchanged. The area fraction of the a-axis-oriented grains is in good agreement with the measured polarizations of ferroelectric capacitors with different average grain sizes. This result implies that an increase in the number of relatively small a-axis-oriented grains is effective for increasing the total polarization of the ferroelectric films. The demonstrated analysis technique is very useful for the precise design of future high-density ferroelectric random access memories (FeRAMs) with very small capacitor structures, each of which consists of only a few grains.

Micropatterning of Ferroelectric Thin Films by Electron-Beam-Induced Reaction Process

The micropatterning of ferroelectric thin films is a valuable technology for realizing high-density ferroelectric random access memories, sensors and actuators. We fabricated 500-nm- to 20-µm-sized circular dots of (Pb,Sr)TiO3 micropatterns by an electron-beam-induced reaction process using a scanning-electron-microscope-based electron beam lithography system. The piezoelectric response was observed in 10-µm-sized dots by piezoresponse scanning force microscopy. Using an atomic force microscope, every fabricated dot pattern was found to have a mesa shape. To obtain 100-nm-sized dots with high density, further optimization of EB irradiation and crystallization conditions is necessary.

Barrier performance of ultrathin Ni–Ti film for integrating ferroelectric capacitors on Si

Ultrathin amorphous Ni–Ti film is investigated as conductive diffusion barrier layer to integrate La0.5Sr0.5CoO3∕PbZr0.4Ti0.6O3∕La0.5Sr0.5CoO3 (LSCO/PZT/LSCO) capacitors on silicon. X-ray photoelectron spectroscopy results demonstrate that Ni in LSCO∕Ni–Ti∕Si heterostructure is not oxidized after 550°C annealing in oxygen. The structural properties of LSCO∕PZT∕LSCO∕Ni–Ti∕Si are characterized by x-ray diffraction and transmission electron microscopy. It is found that Ni–Ti film is still amorphous and that there are no discernible reactions at the interfaces of the sample. LSCO/PZT/LSCO capacitor, measured at 5V, possesses very good ferroelectric properties, such as low coercive field (∼1.28V), high remnant polarization (∼27.9μC∕cm2), and good fatigue-free characteristic, implying that ultrathin amorphous Ni–Ti film can be used as barrier layer for fabricating high-density ferroelectric random access memories.

Impact of (111)-Oriented SrRuO3/Pt Tailored Electrode for Highly Reproducible Preparation of Metal Organic Chemical Vapour Deposited Pb(Zr,Ti)O3 Films for High Density Ferroelectric Random Access Memory Applications

In the present paper we report a comparative study of the structural, morphologic, and electric properties of Pb(Zr,Ti)O3 films, deposited by pulsed metal-organic chemical vapour deposition (MOCVD), on different (111)-oriented bottom-electrode stacks (SrRuO3/Pt, Pt, and Ir substrates). The Pb input ratio in the MOCVD chamber was systematically modified for each deposition run to obtain PZT films with various compositions within the Pb process window. Our results clearly demonstrate that only PZT films deposited on SrRuO3 show high quality and reproducible properties throughout the process window, i.e., high (111) texture, low roughness (<5 nm), high Pr (∼40 µC/cm2), low Vc (<1 V), as well as a relatively low leakage current (∼10-5 A/cm2 at 1.5 V). This study provides further evidence that SrRuO3/Pt substrates are good candidates for integration in next-generation high-density ferroelectric random access memories (FeRAM).

METAL-ORGANIC CHEMICAL VAPOR DEPOSITION OF PbZrxTi1 –xO3 THIN FILMS ON Ir–Ru ALLOY ELECTRODES

ABSTRACT Surface smoothness and ferroelectric properties of PbZrxTi1-xO3 (PZT) thin films, grown by metal-organic chemical vapor deposition (MOCVD), was improved by applying Ir–Ru alloy electrode. It was found that the specific composition ratio of Ir and Ru could guarantee the best quality of the alloyed films. That is 35∼55% in Ru portion called as the ‘multi-phase' because two different crystallographic structures exist. This multi-phase electrode has high thermal stability under oxygen annealing atmosphere. Moreover, PZT thin films grown on the multi-phase electrodes, have smoother surface roughness, larger remnant polarization and lower current leakage than PZT thin films on Ir electrode. It is thought think that the grain boundaries between different phases of the alloyed electrode could supply high nucleation sites of PZT to restrict large grain growth and regulate grain shape. Therefore, the multi-phase alloyed electrode can be utilized for growing ultra thin PZT film, which is necessary for high-density ferroelectric random access memory.

Integration of SrBi2Ta2O9 thin films for high density ferroelectric random access memory

Ferroelectric random access memory (FeRAM) is an attractive candidate technology for embedded nonvolatile memory, especially in applications where low power and high program speed are important. Market introduction of high-density FeRAM is, however, lagging behind standard complementary metal-oxide semiconductor (CMOS) because of the difficult integration technology. This paper discusses the major integration issues for high-density FeRAM, based on SrBi2Ta2O9 (strontium bismuth tantalate or SBT), in relation to the fabrication of our stacked cell structure. We have worked in the previous years on the development of SBT-FeRAM integration technology, based on a so-called pseudo-three-dimensional (3D) cell, with a capacitor that can be scaled from quasi two-dimensional towards a true three-dimensional capacitor where the sidewalls will importantly contribute to the signal. In the first phase of our integration development, we integrated our FeRAM cell in a 0.35μm CMOS technology. In a second phase, then, possibility of scaling of our cell is demonstrated in 0.18μm technology. The excellent electrical and reliability properties of the small integrated ferroelectric capacitors prove the feasibility of the technology, while the verification of the potential 3D effect confirms the basic scaling potential of our concept beyond that of the single-mask capacitor. The paper outlines the different material and technological challenges, and working solutions are demonstrated. While some issues are specific to our own cell, many are applicable to different stacked FeRAM cell concepts, or will become more general concerns when more developments are moving into 3D structures.

Ni–Al diffusion barrier layer for integrating ferroelectric capacitors on Si

We report on the use of amorphous Ni–Al film (a-Ni–Al) as conductive diffusion barrier layer to integrate La0.5Sr0.5CoO3∕PbZr0.4Ti0.6O3∕La0.5Sr0.5CoO3 capacitors on silicon. Cross-sectional observation by transmission electron microscope reveals clean and sharp interfaces without any discernible interdiffusion/reaction in the sample. The physical properties of the capacitors are vertically characterized as the parameters of memory elements. Excellent ferroelectric properties, e.g., large remnant polarization of ∼22μC∕cm2, small coercive voltage of ∼1.15V, being fatigue-free, good retention characteristic, imply that amorphous Ni–Al is an ideal candidate for diffusion barrier for the high-density ferroelectric random access memories integrated with silicon transistor technology.

Bit Distribution and Reliability of High Density 1.5 V Ferroelectric Random Access Memory Embedded with 130 nm, 5 lm Copper Complementary Metal Oxide Semiconductor Logic

High density embedded ferroelectric random access memory (FRAM), operable at 1.5 V, has been fabricated within a 130 nm, 5 lm Cu/fluorosilicate glass (FSG) logic process. To evaluate FRAM extendability to future process nodes, we have measured the bit distribution and reliability properties of arrays with varying individual capacitor areas ranging from 0.40 µm2 (130 nm node) to 0.15 µm2 (∼65 nm node). Wide signal margins, stable retention (≫10 years at 85 °C), and high endurance read/write cycling (≫1012 cycles) have been demonstrated, suggesting that reliable, high density FRAM can be realized.

Micropatterning of SrBi2Ta2O9 Ferroelectric Thin Films Using a Selective Deposition Technique Combined with Patterned Self-Assembled Monolayers and Liquid-Source Misted Chemical Deposition

We demonstrated a novel patterning method using a selective deposition technique of ferroelectric thin films without etching. Self-assembled monolayers (SAMs) of 1H,1H,2H,2H-perfluorodecyltriethoxysilane [CF3(CF2)7CH2CH2Si(OC2H5)3; FAS] were patterned on Pt/SiO2/Si substrates. The patterned SAMs on these surfaces defined the selective area on which the liquid-source misted chemical deposition (LSMCD) of SrBi2Ta2O9 (SBT) was performed. Pt top electrodes were then sputter-deposited on the patterned SBT films by the liftoff method. The remanent polarization (Pr) of the obtained SBT films was 7.5 µC/cm2 with good squareness of the hysteresis loops for 50 µm×50 µm square capacitors. By this method, we succeeded in fabricating uniform lines of ferroelectric thin films of 0.5 µm width. These results show that a selective deposition technique is promising for realizing high-density ferroelectric random access memories (FeRAMs).

Mechanism of polarization enhancement in La-doped Bi4Ti3O12 films

Bi 4 − x La x Ti 3 O 12 (La-doped BIT, x=0–0.6) thin films prepared by metalorganic decomposition method, were studied using x-ray diffraction (XRD), polarization hysteresis and Raman scattering techniques. The XRD and polarization studies showed that La substitution reduces structural distortion, relaxes internal strain and enhances polarization, while the Raman data indicate that the added La occupies mainly at A sites in the BIT films. Raman modes also indicate an increase in TiO6 symmetry, internal strain relaxation, lower Ti–O hybridization, and decrease in polarization along the a axis. Further, the polarization in the highly c-oriented BIT films significantly increases with the added La, hence a polarization tilt towards the c axis is implied. The higher polarization observed can be attributed, therefore, to microscopic changes involving decreased distortion in the TiO6 octahedra and a polarization tilt towards the c axis, caused by doping with the electronically less active ion (e.g., La, Y) onto the A sites in the BIT structure. This understanding is important to the commercialization of high-density ferroelectric random access memories.

Novel Si Codoped Pb(Zr,Ti,Nb)O3 Thin Film for High-Density Ferroelectric Random Access Memory

We have succeeded in fabricating Pb(Zr,Ti,Nb)O3 (PZTN) thin films with more than 10 at.% Nb at the B site in the ABO3 structure, which are suitable for application to high-density and reliable ferroelectric random access memory (FeRAM). To prepare the PZTN thin films, we used a sol-gel spin-coating method in which we codoped 1–3 mol% Si to promote the solvation of Nb atoms into the original Pb(Zr,Ti)O3 (PZT) films as a solid solution. X-ray diffraction (XRD) reciprocal space mapping and Raman scattering revealed that our PZTN film was a single ferroelectric phase of the ABO3 proposed perovskite-type structure with Nb substituted for B-site atoms. Additionally, we suggested that, through this Nb substitution, the formation of oxygen vacancies in PZTN was effectively suppressed; the number of vacancies was much lower than in conventional PZT. The resultant 150-nm-thick PbZr0.2Ti0.6Nb0.2O3 film had a very low leakage current and demonstrated excellent imprint and data retention properties. We have also succeeded in obtaining excellent electric properties in 0.8×0.8 µm2 capacitors fabricated with PZTN. In addition, we confirmed the high reliability and absence of data degradation of our PZTN material during operation as a 1 kbit FeRAM chip. The results of a first-principles calculation indicated that the reduction of the number of oxygen vacancies led to a sufficiently wide band gap to lower the leakage current in our PZTN films.