Maria Mitkova

Nanoscale memory elements based on solid-state electrolytes

We report on the fabrication and characterization of nanoscale memory elements based on solid electrolytes. When combined with silver, chalcogenide glasses such as Se-rich Ge-Se are good solid electrolytes, exhibiting high Ag ion mobility and availability. By placing an anode that has oxidizable Ag and an inert cathode (e.g., Ni) in contact with a thin layer of such a material, a device is formed that has an intrinsically high resistance, but which can be switched to a low-resistance state at small voltage via reduction of the silver ions. An opposite bias will return the device to a high-resistance state, and this reversible switching effect is the basis of programmable metallization cell technology. In this paper, electron beam lithography was used to make sub-100-nm openings in polymethylmethacrylate layers used as the dielectric between the device electrodes. The solid electrolyte film was formed in these via-holes so that their small diameter defined the active switching area between the electrodes. The Ag-Ge-Se electrolyte was created by the photodiffusion, with or without thermal assistance, of an Ag layer into the Ge-Se base glass. Combined thermal and photodiffusion leads to a nanophase separated material with a dispersed Ag ion-rich material with an average crystallite size of 7.5 nm in a glassy insulating Ge-rich continuous phase. The nanoscale devices write at an applied bias as low as 0.2 V, erase by -0.5 V, and fall from over 10/sup 7/ /spl Omega/ to a low-resistance state (e.g., 10/sup 4/ /spl Omega/ for a 10-/spl mu/A programming current) in less than 100 ns. Cycling appears excellent with projected endurance well beyond 10/sup 11/ cycles.

A Low-Power Nonvolatile Switching Element Based on Copper-Tungsten Oxide Solid Electrolyte

We describe the materials aspects and electrical characteristics of W-(Cu/WO3)-Cu switching elements. These materials are compatible with back-end-of-line processing in CMOS integrated circuits where both tungsten and copper already play a significant role. Devices based on Cu/WO3 solid electrolytes formed by photodiffusion of copper into tungsten oxide switch via the electrochemical formation of a conducting filament within the high resistance electrolyte film. They are able to switch reversibly between widely spaced nonvolatile resistance states at low voltage (<1 V) and current (<10 muA). Electrical characterization revealed that devices consisting of plasma-grown oxides have a variable initial threshold voltage and poor retention, whereas devices based on deposited oxide exhibit a stable switching threshold and good retention, even at elevated operating temperature (>125 degC). This difference in behavior was attributed to the observation that the copper tends to oxidize in the plasma-grown oxide whereas the copper in the deposited oxide exists in an unbound state and is, therefore, more able to participate in the switching process

Silver incorporation in Ge-Se glasses used in programmable metallization cell devices

Abstract We investigate the nature of thin films formed by the photodissolution of Ag into Se-rich Ge–Se glasses for use in programmable metallization cell devices. These devices rely on ion transport in the film so produced to create electrically programmable resistance states. The way in which Ag incorporates into the chalcogenide film during photodiffusion is examined using Rutherford backscattering spectroscopy analysis and Raman spectroscopy. The results suggest that an Ag-rich phase separates due to the reaction of Ag with free Se from the chalcogenide glass leaving a Ge-rich chalcogenide matrix.

Nonvolatile memory based on solid electrolytes

Programmable metallization cell (PMC) memory utilizes electrochemical control of nanoscale quantities of metal in thin films of solid electrolyte. A silver or copper layer and an inert electrode formed in contact with a Ag/sup +/or Cu/sup 2+/-containing electrolyte film creates a device in which information is stored using large nonvolatile resistance change caused by the reduction of the metal ions. Key attributes are low voltage, low current, rapid write and erase, good retention and endurance, and the ability for the storage cells to be physically scaled to a few tens of nm. This work describes the principle of operation of PMC devices and presents representative results from cells with diameters ranging from micron scale to nanoscale dimensions based on Ag-Ge-Se, Ag-Ge-S, and Cu-WO/sub 3/ solid electrolytes.

Nanoscale phase separation in Ag-Ge-Se glasses

We have investigated the materials formed by Ag photodiffusion in Se-rich Ge-Se thin glass films. The amount of Ag that can be incorporated by this method saturates at a level dependent on the quantity of Se in the starting glass. The photodiffused Ag reacts with the Se to form distinct Ag2Se and Ge-rich backbone phases as determined by Raman and XRD analyses. The conducting Ag2Se phases exist as nano-crystals with a size in the order of 2 nm in the backbone structure.

Amorphous insulators and semiconductors

Preface. 1: Glass Formation. Strong and Fragile Liquids Glass Transitions and Polyamorphic Transitions in Covalently Bonded Glassformers C.A. Angell. Kinetics of Glass Formation I. Gutzov, A. Dobreva. Chalcogenide Glasses M.T. Mora. Synthesis, Structure and Some Modes of Application of New Chalcogenide and Chalco-Halide Glasses of Silver M. Mitkova. 2: Structure. X-Ray and Neutron Diffraction: Experimental Techniques and Data Analysis A.C. Wright. Computer Modelling of Glasses and Glassy Alloys N. Mousseau. Molecular Dynamics Methods and Large-Scale Simulations of Amorphous Materials P. Vashishta, et al. Reverse Monte Carlo Simulations for Determining Disordered Structures: Basics and Application for Amorphous Semiconductors L. Pusztai. Reverse Monte Carlo Simulations of Glasses - Practical Aspects N. Zotov. A Thermodynamic Model for the Calculation of the Physical Properties and Structural Characteristics of Glasses and Melts N.M. Vedishcheva, et al. High Resolution and Multidimensional Nuclear Magnetic Resonance Probes of Glass Structure J.W. Zwanziger, et al. Porous Silica: Model Fractal Materials, their Structures and their Vibrations E. Courtens, R. Vacher. 3: Vibrational States and Elasticity. The Structure and Mechanical Properties of Networks M.F. Thorpe, et al. The Elastic Moduli of Random Networks: Calculations A.R. Day. One-Fold Coordinated Atoms, Constraint Theory and Nano-Indentation Hardness P. Boolchand, et al. Silicates and Soft Modes M.T. Dove. Vibrational Dynamics in Glasses C. Levelut, et al. Optical Studies of Nanophase Titania R.J. Gonzalez, R. Zallen. 4: Electronic Properties and Defects. Electronic Structure Methods with Applications to Amorphous Semiconductors D.A. Drabold. Amorphous Silicon: Defects and Disorder G.J. Andriaenssens. Tunneling States S. Hunklinger. Electronic Structure of Amorphous Insulators and Semiconductors by X-Ray Photoelectron and Soft X-Ray Spectroscopies C. Senemaud. Participants. Index.

Programmable metallization cell memory based on Ag-Ge-S and Cu-Ge-S solid electrolytes

Programmable metallization cell (PMC) memory is based on the electrochemical growth and removal of nanoscale metallic pathways in thin films of solid electrolyte. Our previous studies concentrated on electrolytes formed from silver-doped germanium selenide glasses but these materials are not able to withstand the temperatures used in standard back-end-of-line processing for the fabrication of CMOS integrated circuits. This paper concerns our more recent work on silver-doped germanium sulfide electrolytes and describes the electrical characteristics of PMC devices made from these materials following annealing at 300 degC and 430 degC. We also present results from devices that use copper in place of silver as this metal is currently used in integrated circuit interconnect

Sheet resistance measurement of non-standard cleanroom materials using suspended Greek cross test structures

This paper presents work on the development, fabrication and characterization of a suspended Greek cross measurement platform that can be used to determine the sheet resistance of materials that would contaminate Complementary Metal Oxide Semiconductor (CMOS) processing lines. The arms of the test structures are made of polysilicon/silicon nitride (Si/sub 3/N/sub 4/) to provide a carrier for the film to be evaluated and thick aluminum (Al) probe pads for multiple probing. The film to be evaluated is simply blanket deposited onto the structures and because of its design automatically forms a Greek cross structure with (Al) probe pads. To demonstrate its use, 1) gold (Au), 2) copper (Cu), and 3) silver(Ag) loaded chalcogenide glass Ag/sub y/(Ge/sub 30/Se/sub 70/)/sub 1-y/ have been blanket evaporated in various thicknesses onto the platform in the last processing step and autopatterned by the predefined shape of the Greek crosses. The suspension of the platform ensured electrical isolation between the test structure and the surrounding silicon (Si) substrate. The extracted effective resistivity for Au (5.1/spl times/10/sup -8/ /spl Omega//spl middot/m), Cu (1.8- 2.5/spl times/10/sup -8//spl bsol/ /spl Omega//spl middot/m) and Ag/sub y/(Ge/sub 30/Se/sub 70/)/sub 1-y/ (2.27/spl times/10/sup -5/ /spl Omega//spl middot/m-1.88 /spl Omega//spl middot/m) agree with values found in articles in the Journal of Applied Physics (1963), the Journalof Physics D: Applied Physics (1976), and the Journalof Non-Crystalline Solids (2003). These results demonstrate that the proposed Greek cross platform is fully capable to measure the sheet resistance of low (Au, Cu) and high Ag/sub y/(Ge/sub 30/Se/sub 70/)/sub 1-y/ resistive materials.

POLARIZATION-DEPENDENT, LASER-INDUCED ANISOTROPIC PHOTOCRYSTALLIZATION OF SOME AMORPHOUS CHALCOGENIDE FILMS

We report the observation of the influence of light polarization on the photocrystallization process and on the properties of crystallized films. Irradiation with linearly polarized He–Ne laser light results in the preparation of polycrystalline films with strong optical anisotropy (dichroism), the sign of which is determined by the direction of the electrical vector of light. The results obtained allow one to select from previously proposed mechanisms of photocrystallization. Large values of photoinduced dichroism in the films studied can be interesting for different applications of photoinduced anisotropy.

Total-Ionizing-Dose Effects on the Resistance Switching Characteristics of Chalcogenide Programmable Metallization Cells

Programmable metallization cells (PMCs) are emerging ReRAM devices exhibiting resistance switching due to cation transport in a solid-state electrolyte and redox reactions at the electrodes. Their non-volatility and low power requirements have led to increased interest in their development for non-volatile memory applications. Investigation of the total dose response of PMCs will contribute to our understanding of radiation induced effects in these novel memory devices as well as assess their suitability for use in ionizing radiation environments. This work investigates the impact of total ionizing dose on the switching characteristic of silver doped Ge30Se70 PMC memory devices. The results obtained show that the resistance switching characteristic of these cells which use a solid state electrolyte based on Ge30Se70 is not affected by a total dose exposure of up to 10 Mrad( Ge30Se70).