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University of Toledo

1. Niraula, Dipesh. Physics and applications of conductive filaments in electronic structures: from metal whiskers to solid state memory.

Degree: PhD, Physics, 2019, University of Toledo

Two topics are explored in this dissertation: metal whiskers (MW) and resistive random access memory (RRAM). Since conductive filament lies at the heart of these distinct topics, they are combinedly presented here as two parts.The first part is dedicated to understanding MW growth and statistics. MW are hair-like conductive filaments that spontaneously grows on technologically significant metals such as Sn, Zn, Cd, and Ag. They can range from tens of nanometers to microns in diameter and hundreds of nanometers to millimeters in length. Longer whiskers can cause shorting in electrical devices posing a serious reliability concerns for virtually every industry.The electrostatic theory of MW explains the physics of whisker growth in terms of field induced nucleation. Imperfections on metals such as impurities, defects, grains, grain-boundaries etc. creates topographically inhomogeneous work function on metal surface. As a result, electrons redistribute themselves to minimize free energy creating superficial charged patches which produce strong near-surface electric field. When a local neighborhood of charged patches all possess like charge, the near-surface electric field amplifies providing necessary driving force for nucleation of metallic embryo protruding from the metal surface. The polarization energy is maximum along the vertical direction, thus embryo takes a needle shape and simultaneously grows.This dissertation includes a brief introduction to the electrostatic theory, whisker length distribution based on the uncorrelated random charge patches and central limit theorem, description of intermittent whisker growth that naturally follows the electrostatic theory, and data on field accelerated whisker growth testing on Zn samples where we found that applying electric field on metal supported whisker growth.The second part is dedicated to the RRAM device operation and various related observation. RRAM is a next-generation non volatile memory aimed at replacing the current flash-memory technology.RRAM operates by switching its resistance in response to external bias where high (OFF) and low (ON) resistive states are interpreted as `0' and `1' binary states. Resistive switching process can be described in terms of thermodynamics of phase transitions. Structurally, RRAM is a capacitor-like device with an extremely thin dielectric layer of about 10-70 nm. Bias voltage of merely ∼ 1 V across ∼ 10 nm dielectric layers results in field strength of ∼ 108 V/m. In response to that massive field, a conductive filament is formed through the insulator layer via phase transition. The formation process can be reversed by interchanging the bias polarity. In the reversal process, part of the filament dissolves by transforming back to insulating phase switching the device back to the original high resistant state.This dissertation includes essentials of the thermodynamic theory of resistive switching, details of scalable numerical model of RRAM device based on the theory that closely reproduces average device… Advisors/Committee Members: Karpov, Victor (Committee Chair).

Subjects/Keywords: Physics; Solid State Physics; Theoretical Physics; Electrical Engineering; Experiments; Materials Science; Nanoscience; Quantum Physics; Field Induced Nucleation, Metal Whiskers, Whisker Length Statistics, Intermittent Whisker Growth, Field Accelerated Growth, RRAM, OFF State Conduction, COMSOL Modeling, Ramp-Rate dependence, Cycle-to-Cycle Variations, Thermal Analysis, Scaling-down effect

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APA (6th Edition):

Niraula, D. (2019). Physics and applications of conductive filaments in electronic structures: from metal whiskers to solid state memory. (Doctoral Dissertation). University of Toledo. Retrieved from http://rave.ohiolink.edu/etdc/view?acc_num=toledo1561471348406944

Chicago Manual of Style (16th Edition):

Niraula, Dipesh. “Physics and applications of conductive filaments in electronic structures: from metal whiskers to solid state memory.” 2019. Doctoral Dissertation, University of Toledo. Accessed September 21, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1561471348406944.

MLA Handbook (7th Edition):

Niraula, Dipesh. “Physics and applications of conductive filaments in electronic structures: from metal whiskers to solid state memory.” 2019. Web. 21 Sep 2019.

Vancouver:

Niraula D. Physics and applications of conductive filaments in electronic structures: from metal whiskers to solid state memory. [Internet] [Doctoral dissertation]. University of Toledo; 2019. [cited 2019 Sep 21]. Available from: http://rave.ohiolink.edu/etdc/view?acc_num=toledo1561471348406944.

Council of Science Editors:

Niraula D. Physics and applications of conductive filaments in electronic structures: from metal whiskers to solid state memory. [Doctoral Dissertation]. University of Toledo; 2019. Available from: http://rave.ohiolink.edu/etdc/view?acc_num=toledo1561471348406944

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