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Emerging non volatile memories reliability
No full textThis work presents the results of the research activity performed during the XXIX-th cycle of the Ph.D. school in Engineering Science of Università degli Studi di Ferrara. In particular the thesis focuses on the electrical characterization, physics, modeling and reliability of innovative non-volatile memories, addressing three of the most promising candidates for the floating gate based memories replacement which are currently facing a technology dead end. The manuscript is organized as follows.
In Chapter 1 planar CT-NAND memory arrays are considered, showing that the main reliability issues affecting such technology are endurance and retention. Enhanced program and read algorithm able to reduce such limitations will be presented and characterized, highlighting the advantages obtained in terms of reliability. After that, the performances of Solid State Drives (SSD) integrating CT-based memories and using the proposed algorithms will be simulated and evaluated.
In Chapter 2 the results obtained on RRAM will be presented and discussed focusing on the variability, which is the main reliability issue of these nonvolatile memories. The impact of Forming, Set and Reset operations on variability will be evaluated, starting from single pulse operations up to program and verify algorithms. The quantum point contact model will be used to give a physical explanation of the results obtained in characterization. After that, the process parameters impact on variability and reliability will be discussed. Finally, the fundamental variability limits of such technologies will be defined through an extensive array characterization and radiation hard application perspectives will be provided.
In Chapter 3, the results obtained on TAS-MRAM technology will be reported. The reliability and the cell-to-cell variability will be evaluated during endurance tests by extracting a set of characteristic parameters from measurements performed on 1kbits arrays. After a preliminary optimization of the writing parameters on fresh devices, the effectiveness of the optimized parameters will be verified during cycling by evaluating their advantages in terms of cell-to-cell variability and breakdown reduction. After that, a novel TAS-MRAM array with optimized read procedure (Self Referenced) will be tested and compared with the previous one to highlight its advantages in terms of reliability.
The conclusions of this work will be reported at the end of the manuscript outlining what has been accomplished, proposing possible applications for the technologies studied in this thesis and suggesting future works that could extend and improve the understanding of the reliability issues on such memory technologies.Questa tesi presenta i risultati dell’attività di ricerca svolta durante il ciclo XXIX di dottorato in Scienze dell’Ingegneria presso l’Università degli Studi di Ferrara. In particolare la tesi tratta la caratterizzazione elettrica, la analisi statistica ed il modeling dei meccanismi fisici e dell’affidabilità di memorie non volatili innovative. Nel manoscritto vengono considerate tre delle tecnologie più promettenti per la futura sostituzione delle memorie flash, che stanno attualmente raggiungendo i loro limiti di scalabilità tecnologici. Il manoscritto è organizzato come segue.
Nel capitolo 1 vengono studiate matrici di celle di memoria CT-NAND planari, mostrando che l’affidabilità di tale tecnologia è principalmente limitata dalla bassa capacità di ritenzione dei dati e dalla breve durata in termini di cicli di scrittura e cancellazione. Nella prima parte del capitolo sono stati caratterizzati algoritmi di programmazione e lettura innovativi in grado di ridurre tali limitazioni, analizzandone i vantaggi ottenuti in termini di affidabilità. In seguito sono state simulate e valutate le prestazioni che sarebbero ottenute in dischi a stato solido (SSD) nel caso in cui tali memorie ed algoritmi fossero utilizzati.
Nel capitolo 2 i risultati ottenuti sulla tecnologia RRAM sono presentati e discussi prestando particolare attenzione alla variabilità, che è il problema principale di questa tecnologia di memorizzazione. L’impatto delle operazioni di Forming, Set e Reset sulla variabilità viene valutato sia per operazioni a singolo impulso che per complessi algoritmi di programmazione che prevedono operazioni di verifica della corretta memorizzazione dei dati. Il modello quantum point contact è stato utilizzato per fornire una spiegazione fisica dei risultati ottenuti durante la caratterizzazione elettrica. Successivamente è stato discusso l’impatto dei parametri di processo su variabilità ed affidabilità delle celle di memoria. Infine, nella parte finale del capitolo i limiti intrinseci di variabilità di tale tecnologia sono stati definiti attraverso una caratterizzazione estensiva di matrici di celle di memoria e sono state fornite alcune prospettive di utilizzo per applicazioni spaziali.
Nel capitolo 3 vengono riportati i risultati ottenuti sulla tecnologia TAS-MRAM. L’affidabilità e la variabilità tra celle di memoria sono state valutate durante test di durata in cui sono state svolte milioni di operazioni di programmazione e cancellazione, estraendo dalle misure su matrici di celle i parametri caratteristici in grado di descriverne il funzionamento. Dopo un’ottimizzazione preliminare delle operazioni di scrittura, l’efficacia delle condizioni di programmazione ottimizzate è stata verificata con test di durata valutandone i vantaggi nella riduzione sia della variabilità tra celle che del numero di celle degradate durante il test. In seguito è stata testata una matrice di celle TAS-MRAM in cui la procedura di lettura è stata ottimizzata (Self Referenced) ed i risultati ottenuti sono stati confrontati con i precedenti per evidenziarne i vantaggi in termini di affidabilità.
Nelle conclusioni di questo lavoro riportate alla fine del manoscritto sono riassunti i risultati ottenuti, proposte possibili applicazioni per le tecnologie analizzate e suggerite eventuali attività future che potrebbero estendere e migliorare la comprensione delle problematiche di affidabilità sulle tecnologie di memoria considerate
Quality of Service implications of Enhanced Program Algorithms for Charge Trapping NAND in future Solid State Drives
No full text3D-NAND memories based on Charge Trapping (CT) technology represent the most promising solution for hyperscaled Solid State Drives (SSD). However, the intrinsic low reliability offered by that storage medium leads to a high number of errors requiring an extensive use of complex Error Correction Codes (ECC) and advanced read algorithms such as Read Retry. This materializes in an overall SSD’s Quality of Service (QoS) reduction. In order to limit the errors number, enhanced program algorithms able to improve the reliability figures of CT memories have been introduced. In this work, the impact of such program algorithms combined with Read Retry and ECC is experimentally characterized on CT-NAND arrays. The results are then exploited for co-simulations at system level, assessing reliability, performance and QoS of future SSD integrating CT-based memories
Impact of the Incremental Programming Algorithm on the Filament Conduction in HfO2-Based RRAM Arrays
No full textIn this paper, the set operation of HfO2 based 1T-1R arrays is studied by applying incremental step pulse with verify algorithm. To evaluate the impact of the voltage step increment on the conduction mechanism of filaments, the voltage increments between consecutive pulses are varied between 0.05 and 0.4 V. The extracted leakage values after the set operation were discussed in the framework of the quantum point contact model. In the so called low resistive state, the conductive filaments demonstrate a defined signature of conductance quantization
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Bit error rate analysis in Charge Trapping memories for SSD applications
No full textDifferent program algorithms are experimentally characterized on CT-NAND Flash Arrays at 4X technology node. The advantages in terms of endurance, retention and read disturb reduction obtained with the proposed algorithms are shown. The positive effects of these algorithms can be increased using an error-reduction procedure called Read Retry, making feasible the usage of CT-NAND memories in MLC-SSD applications
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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