NAND Flash technology represents currently one of the leading solution for highly performing non-volatile memories. The incessant development of its scaling processes brought the characteristic cell feature size (F) to current values around 15nm, allowing to obtain outstanding storage density, while keeping a low cost-per-bit ratio, and high read/write speeds, overcoming the performances offered by other technologies, such as magnetic hard-disk drives. Despite the clear advantages brought from the reduction of F, the scaling of the arrays led to multiple stability issues, becoming harder to control the more F was reduced, limiting the technological growth. In order to overcome these problems, a new approach was taken and the array structure was modified. By employing the third dimension, 3D NAND Flash arrays were developed, allowing greater storage densities while using a feature size big enough to avoid hard to manage issues. The main flaw of the processes involved for the manufacturing of 3D NAND Flash strings is related to the generation of the silicon channel. As a matter of fact, since it employs the deposition of the semiconductor material, the resulting channel is made out of polycrystalline silicon. Due to its nature, charge transport is strongly affected by the presence of interface states on grain boundaries. The random generation of grain profiles causes instability issues that have to be taken into account to properly design the device. In this thesis work a model was developed in order to simulate the instabilities arose by the polycrystalline nature of the channel of a 3D NAND Flash string. First, a space partitioning algorithm based on Voronoi diagrams was developed and it was applied to a 3D NAND Flash string implemented in a TCAD simulator. Then, a physical model involving the implementation of a thermionic boundary condition on the grain boundaries was employed. Starting from experimental results, the model was calibrated and a Monte Carlo statistic was implemented to analyze the temperature dependence of the variabilities introduced by different configurations of the grain profiles. Finally, random telegraph noise simulations were performed to analyze the impact of random trapping/detrapping of an electron in defect states. Chapter 1 gives an overview of NAND Flash technology, starting from a schematic layout of the memory array and by explaining how its main operations are performed. Then, the scaling trend is presented, along with the arising issues that led to the employment of the third dimension in order to increase the storage density. The basic structure of three different vertical-channel 3D NAND Flash strings is given. Finally, the instabilities caused by the polycrystalline nature of the channel are introduced. Chapter 2 describes the simulation setup used to perform the analyses presented in this work. First, the tools employed to perform the simulations are described, then the developed algorithm used to generate random grain profiles is explained. Moreover, the basic structure of the simulated string and its dimensions are given. Finally, the physical models used to properly simulate charge transport in the device and, more specifically, across the grain boundaries are presented. Chapter 3 first shows the procedure involved in the calibration of the model against experimental data. The calibrated model is then employed to perform Multi-WLs and Single-WL analyses. Finally, Monte Carlo statistics are done to analyze the temperature activation variability of the device. Finally, chapter 4 describes random telegraph noise statistics performed on the simulated 3D NAND Flash strings. First, experimental evidence of the instability of the threshold voltage in time is given. Then, TCAD simulations are performed in order to analyze the fluctuations given by the random charging/discharging of single defects, placed either on a grain boundary or on a polysilicon/oxide interface.
La tecnologia NAND Flash rappresenta, ad oggi, una delle soluzioni principali per memorie non volatile ad alte prestazioni. Lo sviluppo continuo dei processi di scaling hanno portato la caratteristica feature size (F) della singola cella a valori prossimi a 15nm, permettendo di ottenere una notevole densità di archiviazione mantenendo un basso costo e alte velocità di lettura e scrittura, superando le prestazioni offerte da altre tecnologie, come gli hard-disk magnetici. Nonostante i netti vantaggi portati dalla riduzione di F, lo scaling degli array ha indotto a molteplici problemi di stabilità, diventando sempre più difficili da controllare riducendo F, limitando la crescita tecnologica. Al fine di superare questi problemi, un nuovo approccio è stato preso e la struttura dell'array è stata modificata. Sfruttando la terza dimensione, array 3D NAND Flash sono stati sviluppati, permettendo maggiori capacità di memoria, ma utilizzando un feature size abbastanza grande da evitare problemi difficili da controllare. Il maggiore difetto dei processi coinvolti nella realizzazione di stringhe 3D NAND è collegato alla generazione del canale di silicio. Infatti, sfruttando la deposizione del materiale semiconduttore, il canale risultante è di silicio policristallino. A causa della sua natura, il trasporto di carica è fortemente influenzato dalla presenza di stati interfacciali sui bordi di grano. La generazione casuale di profili di grano causa problemi di instabilità che vanno presi in considerazione al fine di progettare opportunamente il dispositivo. In questo lavoro di tesi un modello è stato sviluppato al fine di simulare le instabilità nate dalla natura policristallino del canale di una stringa 3D NAND Flash. Inizialmente un algoritmo di partizionamento dello spazio basato sui diagrammi di Voronoi è stato sviluppato ed è stato applicato ad una stringa 3D NAND Flash implementata in un simulatore TCAD. Un modello fisico includente l'implementazione di una condizione termoionica al contorno applicata ai bordi di grano è stato poi impiegato. Il modello è stato calibrato partendo da dati sperimentali e una statistica Monte Carlo è stata implementata al fine di analizzare la dipendenza dalla temperatura delle variabilità introdotte dalle differenti configurazioni dei profili di grano. Infine, simulazioni del random telegraph noise sono state effettuate per analizzare l'impatto del trapping/detrapping di un elettrone in uno stato trappola. Nel capitolo 1 viene fornita una panoramica generale della tecnologia NAND Flash, partendo da una rappresentazione schematica dell'array di memoria e spiegando il funzionamento delle principali operazioni. I trend di scaling della tecnologia e i problemi scaturiti dalla riduzione del feature size che hanno portato all'utilizzo della terza dimensione, al fine di aumentare la densità di memoria, sono poi presentati. La struttura di base di tre differenti stringhe 3D NAND Flash a canale verticale è mostrata. Infine, le stabilità causate dalla natura policristallina del canale sono introdotte. Il capitolo 2 descrive il setup di simulazione utilizzato per effettuare le analisi presentate in questo lavoro. Inizialmente, gli strumenti sfruttati per effettuare le simulazioni sono descritti. L'algoritmo utilizzato per la generazione di profili casuali di grani è dunque spiegato. Inoltre, la struttura di base della stringa simulata e le sue dimensioni vengono date. Infine, vengono presentati i modelli fisici utilizzati per simulare correttamente il trasporto di carica nel dispositivo e, in particolar modo, attraverso i bordi di grano. Il capitolo 3 inizialmente mostra la procedura seguita per effettuare la calibrazione del modello fisico a partire da dati sperimentali. Il modello calibrato è poi utilizzato per effettuare analisi multi-cella e a singola cella. Infine, vengono realizzate statistiche Monte Carlo per analizzare l'attivazione in temperatura della variabilità del dispositivo. Infine, nel capitolo 4, vengono presentate le analisi statistiche effettuate per lo studio del random telegraph noise. Inizialmente viene mostrata l'evidenza sperimentale dell'instabilità temporale della tensione di soglia. Simulazioni TCAD sono quindi effettuate al fine di analizzare le fluttuazioni date da eventi casuali di carica/scarica di singoli difetti posizionati o in corrispondenza di un bordo di grano o su un'interfaccia polisilicio/ossido.
TCAD simulation of charge transport in the polysilicon channel of 3D NAND Flash strings
MANNARA, AURELIO
2016/2017
Abstract
NAND Flash technology represents currently one of the leading solution for highly performing non-volatile memories. The incessant development of its scaling processes brought the characteristic cell feature size (F) to current values around 15nm, allowing to obtain outstanding storage density, while keeping a low cost-per-bit ratio, and high read/write speeds, overcoming the performances offered by other technologies, such as magnetic hard-disk drives. Despite the clear advantages brought from the reduction of F, the scaling of the arrays led to multiple stability issues, becoming harder to control the more F was reduced, limiting the technological growth. In order to overcome these problems, a new approach was taken and the array structure was modified. By employing the third dimension, 3D NAND Flash arrays were developed, allowing greater storage densities while using a feature size big enough to avoid hard to manage issues. The main flaw of the processes involved for the manufacturing of 3D NAND Flash strings is related to the generation of the silicon channel. As a matter of fact, since it employs the deposition of the semiconductor material, the resulting channel is made out of polycrystalline silicon. Due to its nature, charge transport is strongly affected by the presence of interface states on grain boundaries. The random generation of grain profiles causes instability issues that have to be taken into account to properly design the device. In this thesis work a model was developed in order to simulate the instabilities arose by the polycrystalline nature of the channel of a 3D NAND Flash string. First, a space partitioning algorithm based on Voronoi diagrams was developed and it was applied to a 3D NAND Flash string implemented in a TCAD simulator. Then, a physical model involving the implementation of a thermionic boundary condition on the grain boundaries was employed. Starting from experimental results, the model was calibrated and a Monte Carlo statistic was implemented to analyze the temperature dependence of the variabilities introduced by different configurations of the grain profiles. Finally, random telegraph noise simulations were performed to analyze the impact of random trapping/detrapping of an electron in defect states. Chapter 1 gives an overview of NAND Flash technology, starting from a schematic layout of the memory array and by explaining how its main operations are performed. Then, the scaling trend is presented, along with the arising issues that led to the employment of the third dimension in order to increase the storage density. The basic structure of three different vertical-channel 3D NAND Flash strings is given. Finally, the instabilities caused by the polycrystalline nature of the channel are introduced. Chapter 2 describes the simulation setup used to perform the analyses presented in this work. First, the tools employed to perform the simulations are described, then the developed algorithm used to generate random grain profiles is explained. Moreover, the basic structure of the simulated string and its dimensions are given. Finally, the physical models used to properly simulate charge transport in the device and, more specifically, across the grain boundaries are presented. Chapter 3 first shows the procedure involved in the calibration of the model against experimental data. The calibrated model is then employed to perform Multi-WLs and Single-WL analyses. Finally, Monte Carlo statistics are done to analyze the temperature activation variability of the device. Finally, chapter 4 describes random telegraph noise statistics performed on the simulated 3D NAND Flash strings. First, experimental evidence of the instability of the threshold voltage in time is given. Then, TCAD simulations are performed in order to analyze the fluctuations given by the random charging/discharging of single defects, placed either on a grain boundary or on a polysilicon/oxide interface.| File | Dimensione | Formato | |
|---|---|---|---|
|
2017_10_Mannara.pdf
non accessibile
Descrizione: Thesis text
Dimensione
6.83 MB
Formato
Adobe PDF
|
6.83 MB | Adobe PDF | Visualizza/Apri |
I documenti in POLITesi sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
https://hdl.handle.net/10589/136670