Modeling of tunneling current and gate dielectric reliability for nonvolatile memory devices

We present a hierarchy of tunneling models suitable for the two- and three-dimensional simulation of logic and nonvolatile semiconductor memory devices. The crucial modeling topics are comprehensively discussed, namely, the modeling of the energy distribution function in the channel to account for hot-carrier tunneling, the calculation of the transmission coefficient of single and layered dielectrics, the influence of quasi-bound states in the inversion layer, the modeling of static and transient defect-assisted tunneling, and the modeling of dielectric degradation and breakdown. We propose a set of models to link the gate leakage to the creation of traps in the dielectric layer, the threshold voltage shift, and eventual dielectric breakdown. The simulation results are compared to commonly used compact models and measurements of logic and nonvolatile memory devices.     

Scattering from a system of parallel thin dielectric cylinders of finite length

Contents The problem of scattering from a system of cylinders is treated. The dielectric cylinders are considered to have radii small compared to the wavelength of the incident field, finite length and their axes are parallel to each other. The polarization of the incident fields is parallel to the axes of the cylinders. — For the determination of the field inside as well as outside the cylinders, the dyadic Green's function is used. — For the numerical treatment of the integral equations the method of moments is used and a few characteristic examples are studied. — Generalizations for more than two cylinders are outlined using the same procedure.

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Streuung eines Systems von dünnen parallelen dielektrischen Zylindern endlicher Länge

Übersicht Es wird das Problem der Streuung eines Zylinder-systems behandelt. Die dielektrischen Zylinder haben kleine Radien im Vergleich zur Wellenlänge des einfallenden Feldes sowie endliche Länge und zueinander parallele Achsen. —Für die Bestimmung des Feldes, sowohl innerhalb als auch außerhalb der Zylinder, wird die dyadische Greensche Funktion verwendet. — Für die numerische Behandlung der Integralgleichungen wird die Momentenmethode angewandt und anhand von vielen charakteristichen Beispielen untersucht. — Verallgemeinerungen für mehr als zwei Zylinder werden durch Anwendung der gleichen Methode umrissen.

     

Origin of giant dielectric response in ferroelectric thin film multilayers

Abstract

A thermodynamic theory of dielectric response in ferroelectric thin film multilayers is developed. The solution of Lame equation for static dielectric susceptibility has shown that susceptibility diverges at the transition temperature of the thickness induced ferroelectric phase. This divergence is shown to be the origin of the giant dielectric response observed in some multilayers. The theory gives an excellent fit to the temperature dependence of the giant susceptibility observed recently in multilayers of PbTiO₃-Pb_1-xLa_xTiO₃ (x = 0.28).     

Deposition and characterization of low-loss epitaxial non-linear dielectric thin films for microwave devices

Abstract

Oxide ferroelectric thin films for frequency-tunable microwave devices, in which the dielectric constant of the non-linear dielectric is varied by application of electric fields, have been deposited using PLD. We have fabricated single phase epitaxial Ba_0.6Sr_0.4TiO₃ and KTaO₃ thin film capacitors for applications at 300K and 77K, respectively. Single phase KTaO₃ films were obtained only with excess potassium source in the target along with KTaO₃ perovskite phase. The films have been characterized for structure and morphology by X-ray diffraction and AFM. The dielectric properties were measured in the low frequency range from 100 kHz to 10 MHz, using interdigitated capacitors. Low loss tangents (0.002 at 300K) were observed for highly oriented Ba_0.6Sr_0.4TiO₃ films. The importance of low losses for various devices is discussed and the dielectric constants, loss tangents and tunability of these films are reported in this paper.     

Iridium thin film as a bottom electrode for high dielectric (Ba,Sr)TiO3 capacitors

Abstract

We propose Ir thin films as new electrode materials for high dielectric BST capacitors. Ir was found to be superior to Pt in a number of aspects such as resistivity, adhesion and surface roughness. The Pt/BST/Ir/SiO₂/Si capacitors showed leakage currents as low as Pt/BST/Pt/SiO₂/Si ones, but higher capacitance resulted. For endurance properties with +5V unipolar pulse trains, the dielectric constant of BST films on Ir decreased by only 10% below its initial value after switching of 10⁹ cycles while that on Pt degraded by 30% after 10⁸ cycles. Ir bottom electrode effects on BST film properties were well explained by the formation of IrO₂ phases on the surface of Ir electrodes.     

The structure and dielectric properties of thin PZT-type ferroelectric films with a diffuse phase transition

Abstract

By means of r.f. sputtering of the ceramic targets or pressed powder targets with the chemical constitution of Pb (Zr_0.52Ti_0.46W_0.01Cd_0.01) O₃ the polycrystalline thin ferroelectric films with the perovskite type structure and of thickness d_f = (1–2, 5) × 10^?6 m on metal (stainless steel, platinum) or ceramic (polycor) substrates have been obtained. In case of thin film deposition on steel substrate and on platinum at low temperatures (T_x < 723 K) the nonferroelectric intermediate layer with the same chemical constitution but with the pyrochlore type structure have been created. Results of the X-ray analysis and dielectric investigations (low frequency dispersion) have proved existence of such a structure. The structural phase transition (P4mm ? Pm3m) takes place in the thin ferroelectric films. This is a diffuse type transition and the degree of diffuseness depends on the structural perfection of the thin films. The measure of the structural perfection of the thin films was taken to be the mean value of the lattice strains (microdeformations) ? Δd/d ? where “d”-interplane distance and the mean dimension (D) of the areas of coherent X-rays scattering (crystallites). With increase in ? Δd/d ? and decrease in D the degree of diffuseness increases. For quantitative and qualitative describing of this dependence in thin PZT films the phenomenological model of diffused phase transition developed earlier by the authors for thin BaTiO₃ films has been applied. In this case a good agreement between theory and experiment have been achieved for low values of ? Δd/d ?.     

Fast electrostatic discharges between a charged thin dielectricfilm and a spherical electrode with or without dielectric coating

Short-gap (less than 1 mm) electrostatic discharge (ESD) between a charged thin dielectric film on an earthed-plate electrode and a spherical electrode, connected to the plate, with or without an uncharged thin dielectric film coating was observed. Discharge-current waveform, discharge gap length, current rise time, discharging photograph, and current peak value were recorded by the authors' measuring system. The initial charge supply effects by irradiation of β-ray or ultraviolet light were found to be not highly apparent; that is, the reproducibility of ESD experiments was not greatly improved. Surface-charge-polarity dependence of surface ESD between charged and uncharged dielectric films decreases with increase of the film thickness. The discharge gap length of ESD with a 1.5 μm uncharged film coating on the spherical electrode decrease to 50 or 70% compared with that between a charged film and a bare spherical metal electrode. Peak current and transferred charge value of ESD also decrease to roughly one-third of that between the film and the spherical electrode     

Structure and dielectric properties of BaTi4O9 thin films for RF-MIM capacitor applications

BaTi₄O₉ thin films were grown on a Pt/Ti/SiO₂/Si substrate using RF magnetron sputtering. A homogeneous BaTi₄O₉ crystalline phase developed in the films deposited at 550^∘C and annealed above 850^∘C. When the thickness of the film was reduced, the capacitance density and leakage current density increased. Furthermore, the dielectric constant was observed to decrease with decreasing film thickness. The BaTi₄O₉ film with a thickness of 62 nm exhibited excellent dielectric and electrical properties, with a capacitance density of 4.612 fF/μm² and a dissipation factor of 0.26% at 100 kHz. Similar results were also obtained in the RF frequency range (1–6 GHz). A low leakage current density of 1.0 × 10⁻⁹ A/cm² was achieved at ± 2 V, as well as small voltage and temperature coefficients of capacitance of 40.05 ppm/V² and –92.157 ppm/^∘C, respectively, at 100 kHz.     

Low temperature metal-organic chemical vapor deposition of Ru thin films as electrode for high dielectric capacitors

Abstract

Low temperature metal-organic chemical vapor deposition (MOCVD) process of Ru films for use as electrode material was studied using a noble dome type reactor, liquid delivery technique and a new precursor. The films were grown at temperatures ranging from 275°C to 480°C in which film growth was controlled by a surface chemical reaction with a small activation energy of 0.21 eV. The root-mean-squared surface roughness was as low as 23 Å for a film grown at 290°C on a SiO₂ surface.     

Programmable mass-memory based on a voltage variable capacitor utilizing a ferroelectric thin film deposited on single crystal silicon

Abstract

An electronically programmable mass-memory is demonstrated with a projected density capability in excess of 4.5.10⁴ bits/mm², which possesses a nondestructive readout. It is based on the principle of a programmable voltage variable capacitor (VARICAP) and utilizes a ferroelectric thin film of (Ba,Sr)TiO₃ deposited by sputtering on a single crystal Si substrate.     

Observation of artificial nano-domains in ferroelectric thin films using nonlinear dielectric imaging and piezo imaging

Abstract

We have developed scanning nonlinear dielectric microscopy (SNDM) that is the first successful purely electrical method for observing polarization distributions of ferroelectric materials. Now the resolution of SNDM has been improved to sub-nanometer.

On the other hand, the piezoelectric response imaging (piezo-imaging) using scanning force microscopy (SFM) is well known as the method for observing the polarization distributions. In this study, we compare the resolution of SNDM with that of the piezo-imaging and confirm that the resolution of SNDM is much higher than that of piezo-imaging. Then, as the fundamental study to apply the SNDM system to a ferroelectric reading-recording system, we switched and observed the ferroelectric domains using the SNDM system.     

Magnetic resonance studies of trapping centers in high-/spl kappa/ dielectric films on silicon

The electrical properties of high dielectric constant materials being considered for replacements of SiO/sub 2/ in metal-oxide semiconductor (MOS) field effect transistors are dominated by point defects. These point defects play important roles in determining the response of these films in almost any imaginable reliability problem. A fundamental understanding of these defects may help to alleviate the problems which they can cause. The best known methods for determining the structure of electrically active defects in MOS materials and devices are conventional electron spin resonance (ESR) and electrically detected magnetic resonance (EDMR). In this paper, we review the limited ESR and EDMR work performed to date on high-/spl kappa/ materials. A discussion of magnetic resonance techniques as well as a brief overview of the extensively studied Si/SiO/sub 2/ system is also included.     

Thermography, a tool for reliability and safety

A thermography survey program, using appropriate equipment and properly trained personnel, can be an extremely effective cool to prevent equipment failures potentially resulting in lost production, equipment damage and personnel exposure to hazards. Basically, there are no limits to the application of infrared thermography in the hands of a certified, qualified inspector. Any equipment or object where temperature patterns or levels indicate equipment condition is a good application for infrared thermography. However, training and experience are essential in being able to safely collect useful data and to accurately interpret the data gathered. Without proper equipment operated by, and acquired data interpretation done by, well trained personnel with proper certification a thermography program can easily result in misinterpretation and lead to more harm than good     

Image theory for a point charge inside a layered dielectric sphere

Contents Following the approach of Neumann and Lindell [1] we derive an image line source that produces in a homogeneous medium the same electric field as a point charge inside a layered dielectric sphere. The image charge density function for the sphere with two interfaces is solved analytically, and approximations are derived in the case of a thin layer and a low-contrast layer. An image expression for the thin-layered sphere with three interfaces is developed. We suggest that point charge approximations of the image charge may be used to speed up computations of the electric potential on the scalp due to sources in the brain.

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Elektrostatische Spiegelungstheorie für Punktladungen in einer geschichteten dielektrischen Kugel

Übersicht Basierend auf dem Ansatz von Neumann und Lindell [1] wird eine scheinbare Linienladung abgeleitet, die in einem homogenen Medium das gleiche elektrische Feld hervorruft wie eine Punktladung innerhalb einer geschichteten dielektrischen Kugel. Die Dichtefunktion der Bildladung für eine Kugel mit zwei Trennflächen wird analytisch bestimmt, und Näherungen für den Fall einer dünnen Schicht und einer nur kleinen Änderung der Permittivität werden abgeleitet. Eine Abbildung für eine Kugel aus dünnen Schichten mit drei Trennflächen wird entwickelt. Die Punktladungsnäherungen der Bildladungen können verwendet werden, um die Berechnung des elektrischen Potentials auf der Kopfhaut aufgrund von Qellen im Gehirn zu beschleunigen.

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This study was supported in part by the Academy of Finland. We thank Ismo V. Lindell, Olli V. Lounasmaa and Seppo Ahlfors for criticsm and comments.     

Initiation from a point anode in a dielectric liquid

Many previous studies of electrical breakdowns in dielectric liquids in point-plane geometry have examined the relationships among the breakdown structure (or speed), the electrode geometry (point radius, gap length), and/or voltage. This paper explores the hypothesis that, for streamers initiating from a point anode, the critical volume model used for similar geometry in gaseous dielectrics is useful in liquids. The assumption of the critical volume is shown to be consistent with experimental data. Specifically, a critical volume of 0.4-1.0 mum ³ is consistent with the location of streamer initiation, with the independence of the initiation voltage for the 2nd anode mode from the tip radius for sharp tips, and the measured free paths of electrons in cyclohexane for the energies of interest     

Radiation from a dipole embedded in a dielectric slab

A unified analytical treatment of the radiation from an electric dipole of arbitrary orientation embedded at an arbitrary location within a symmetrically clad dielectric slab is presented. Both the emission into three-dimensional (3-D) radiation modes, corresponding to emission within the critical angle escape cone within the dielectric slab, and into the two-dimensional (2-D) waveguide modes are evaluated from a single calculation. The model is valid for arbitrary dielectric contrast between the slab and the cladding. The mathematical approach uses well-known complex analysis techniques: the 3-D radiation is described by a steepest descents integration around branch cuts while the 2-D waveguide modes correspond to simple poles. The division of the radiated power between the 3-D and 2-D modes is evaluated across the entire range from small dielectric contrast appropriate to diode lasers (≲1,1) to the very large dielectric contrast of free-standing semiconductor slabs (~12-19). Both enhancement and suppression, depending on position, slab width, dielectric contrast, and wavelength, of the total radiated power in comparison with that in an unbounded dielectric-medium are found for slab widths on the order of a wavelength with a maximum enhancement of ~30% for these one-dimensional Fabry-Perot structures. For thicker slabs the total radiation is almost constant and equal to that in the unbounded medium for low dielectric contrast while still exhibiting some modulation as increasing thickness allows additional waveguide modes     

Piezoelectric and dielectric aging in pb(zr,ti)o3 thin films and bulk ceramics

Abstract

Piezoelectric and dielectric aging was studied in Pb(Zr,Ti)O₃ (PZT) thin films and bulk ceramics. It was found that piezoelectric aging in thin films obeys the logarithmic time dependence with the relative aging rate much higher than that of the dielectric constant, while comparable aging rates of piezoelectric and dielectric constants were found in PZT ceramics. The origin of piezoelectric aging in PZT films was related to depolarization of the films rather than to suppression of the domain wall motion as was generally accepted for PZT ceramics.     

Optimal placement of switches in a radial distribution network for reliability improvement

This paper proposes differential search (DS) algorithm to solve distribution system reliability optimization problem. Optimum number and location of remote control switch (RCS) have been found, in order to enhance reliability of a radial distribution system. A multi-objective formulation has been considered with a view to enhance reliability at a compromised cost. DS algorithm utilizes the Brownian-like random-walk movement used by an organism to migrate. Simulation results obtained by DS algorithm have been compared with that of particle swarm optimization (PSO), differential evolutionary algorithm (DE), genetic algorithm (GA), ant colony optimization (ACO) and gravitational search algorithm (GSA). Results show that DS algorithm provides considerably superior performance, in terms of quality of solution obtained and computational efficiency.     

Controlling short-channel effects in deep-submicron SOI MOSFETs for improved reliability: a review

This paper examines the performance degradation of a MOS device fabricated on silicon-on-insulator (SOI) due to the undesirable short-channel effects (SCE) as the channel length is scaled to meet the increasing demand for high-speed high-performing ULSI applications. The review assesses recent proposals to circumvent the SCE in SOI MOSFETs and a short evaluation of strengths and weaknesses specific to each attempt is presented. A new device structure called the dual-material gate (DMG) SOI MOSFET is discussed and its efficacy in suppressing SCEs such as drain-induced barrier lowering (DIBL), channel length modulation and hot-carrier effects, all of which affect the reliability of ultra-small geometry MOSFETs, is assessed.     

Titanium as a substrate for thin film PZT deposition

Abstract

During high temperature processing an interface layer is formed between the PZT and Ti substrate by the oxidation of the substrate. The effects on the measured electrical properties of PZT are investigated using dielectric, IV and CV analysis.