Characterization of charge trapping in SiO2/Al2O3 dielectric stacks by pulsed C–V technique

In this work, charge trapping in SiO₂/Al₂O₃ dielectric stacks is characterized by means of pulsed capacitance–voltage measurements. The proposed technique strongly reduces the measurement time and, as a consequence, the impact of charge trapping on the measurement results. Flat band voltage shift and fast current transient during short stress pulses are systematically monitored and the centroid and the amount of the trapped charge are extracted using a first-order model.     

On the calculation of the quasi-bound-state energies and lifetimes in inverted MOS structures with ultrathin oxides and its application to the direct tunneling current

We discuss the inverted MOS structure with extremely thin oxides as a quasi-bound-state system. Its energy subbands can be regarded as resonances of a quantum mechanical system with a continuum of eigenstates. We derive an analytical calculation to extract the defining parameters of the Lorentzian peaks associated with the relative probability of localizing an electron in the confining potential region at the Si/dielectric interface and propose an algorithm that allows to estimate the energy levels and the lifetimes of these states, provided the solution of the approximate bound state system is known. This new method has the advantage of not being affected by computational errors, avoiding the time consuming scanning procedure usually employed in previously reported works. Furthermore, a comparison with semiclassical calculation of the lifetimes is performed and direct tunneling current is calculated, which allows it to fit experimental gate-leakage curves with high accuracy.     

Understanding the potential and limitations of HfAlO as interpoly dielectric in floating-gate Flash memory

Introduction of high-k dielectrics in Flash memory is seen as a must for the upcoming technology nodes. Hafnium aluminate (HfAlO) has been identified as a possible candidate for implementing the interpoly dielectric in floating gate memory. In this work, we establish a link between the material morphology and its electrical response, allowing to understand memory device behavior and to consequently assess the potential and limitations of HfAlO as IPD in a memory cell.     

Impact of PDA temperature on electron trap energy and spatial distributions in SiO2/Al2O3 stack as the IPD in Flash memory cells

SiO_xN_y/high-κ dielectric stack will soon replace the conventional SiO_xN_y-based dielectric stacks in the future generations of flash memory cells. Characterizing and reducing electron traps in the high-κ layer is an important task, as the large trap density may limit the memory retention via the trap-assisted tunneling. Since the Post-deposition Annealing (PDA) has great impact on the microstructure of high-κ dielectric, it is important to understand how PDA affects the properties of electron traps, such as the trap density, energy and spatial distributions. It is demonstrated in this paper that, by using a recently developed two-pulse C–V measurement technique, the energy and spatial distributions of electron traps throughout the SiO₂/high-κ stack can be characterized, and PDA temperatures have different impacts on traps at different energy levels and spatial locations.     

On the Roll-Off of the Activation Energy Plot in High-Temperature Flash Memory Retention Tests and its Impact on the Reliability Assessment

In this letter, we discuss the experimental behavior of high-kappa interpoly dielectrics in floating gate memory devices with respect to the activation energy plot. It is shown that the Arrhenius extrapolation may overestimate the ten-year lifetime predictions. However, a clear correlation between the activation energy plot and the trap levels in the interpoly dielectric can be established, based on theoretical grounds. It is shown that a single-trap level typically follows an 1/T law, while a roll-off of the activation energy plot suggests the existence of various trap levels in the interpoly dielectrics. These findings have practical implications on the high-temperature retention testing methodology of novel nonvolatile memory with high-kappa interpoly dielectrics.     

Distribution and generation of traps in SiO2/Al2O3 gate stacks

In this work we combine charge-pumping measurements with positive constant voltage stress to investigate trap generation in SiO₂/Al₂O₃ n-MOSFET. Trap density has been scanned either in energy or in position based on charge-pumping (CP) measurements performed under different operating conditions in terms of amplitude and frequency of the gate pulse. Our results have revealed that the traps are meanly localized shallow in energy level, deeper in spatial position and they are mostly generated near the Si/SiO₂ interface.     

Performance and reliability of HfAlOx-based interpoly dielectrics for floating-gate Flash memory

This paper discusses the performance and reliability of aggressively scaled HfAlO_x-based interpoly dielectric stacks in combination with high-workfunction metal gates for sub-45 nm non-volatile memory technologies. It is shown that a less than 5 nm EOT IPD stack can provide a large program/erase (P/E) window, while operating at moderate voltages and has very good retention, with an extrapolated 10-year retention window of about 3 V at 150 °C. The impact of the process sequence and metal gate material is discussed. The viability of the material is considered in view of the demands of various Flash memory technologies and direction for further improvements are discussed.     

VARIOT: a novel multilayer tunnel barrier concept for low-voltage nonvolatile memory devices

Low-voltage low-power nonvolatile floating-gate memory device operation can be achieved by using alternative tunnel barriers consisting of at least two dielectric layers with different dielectric constants k. Low-k/high-k (asymmetric) and low-k/high-k/low-k (symmetric) barriers enable steeper tunneling current-voltage characteristics. Their implementation is possible with high-k dielectric materials that are currently investigated for SiO/sub 2/ replacement in sub-100-nm CMOS technologies. We show that a reduction in programming voltages of up to 50% can be achieved. This would significantly reduce the circuitry required to generate the high voltages on a nonvolatile memory chip, while maintaining sufficient performance and reliability.     

A review of underlying fundamentals in a wet dispersion size analysis of powders

For the particle size distribution (PSD) characterization of dry powders various analytical technologies are commercially available. Most of these techniques require the sample to be dispersed, and for pharmaceutical powders liquid dispersion media are often preferred over dry dispersion. In practise, incomplete dispersion and/or dispersion instability are readily observed, potentially leading to a limited accuracy and precision of the method. It is reasonable to argue that it is not so much the size distribution measurement itself but the sample dispersion which is a far more critical aspect in the overall process. If one aims to develop a robust PSD test method, a basic knowledge of the underlying fundamentals is indispensable. For this purpose, and in order to acquaint the reader with a better conceptual understanding of this matter, the wet dispersion of a single component dry powder is discussed in this paper from a theoretical perspective in terms of its cohesion, wetting, rupture and stabilization. To allow a qualitative evaluation of the cohesiveness of a powder, the magnitude of the attractive Van der Waals force relative to liquid bridging and gravity force is discussed. Moreover, attention is paid to the impact of particle deformation, surface roughness and vapour adsorption. In the section on wetting attention is paid to (non-)aqueous dispersion media and the application of suitable wetting agents. Concerning particle disruption, this paper focuses on aspects which induce a lowering of the interparticulate forces of attraction. In addition, active disruption by means of agitation and sonication is discussed. In a further section the paper discusses the parameters which affect the stability of the dispersed particles including the use of enhancing surfactants. The paper concludes by giving some first-hand recommendations to assess the wet dispersion.     

Particle shape and orientation in laser diffraction and static image analysis size distribution analysis of micrometer sized rectangular particles

Laser diffraction (LD) and static image analysis (SIA) of rectangular particles [United States Pharmacopeia, USP30-NF25, General Chapter <776>, Optical Miroscopy.] have been systematically studied. To rule out sample dispersion and particle orientation as the root cause of differences in size distribution profiles, we immobilize powder samples on a glass plate by means of a dry disperser. For a defined region of the glass plate, we measure the diffraction pattern as induced by the dispersed particles, and the 2D dimensions of the individual particles using LD and optical microscopy, respectively. We demonstrate a correlation between LD and SIA, with the scattering intensity of the individual particles as the dominant factor. In theory, the scattering intensity is related to the square of the projected area of both spherical and rectangular particles. In traditional LD the size distribution profile is dominated by the maximum projected area of the particles (A). The diffraction diameters of a rectangular particle with length L and breadth B as measured by the LD instrument approximately correspond to spheres of diameter Ø_L and Ø_B respectively. Differences in the scattering intensity between spherical and rectangular particles suggest that the contribution made to the overall LD volume probability distribution by each rectangular particle is proportional to A²/L and A²/B. Accordingly, for rectangular particles the scattering intensity weighted diffraction diameter (SIWDD) explains an overestimation of their shortest dimension and an underestimation of their longest dimension. This study analyzes various samples of particles whose length ranges from approximately 10 to 1000 μm. The correlation we demonstrate between LD and SIA can be used to improve validation of LD methods based on SIA data for a variety of pharmaceutical powders all with a different rectangular particle size and shape.