The NAND Flash technology currently represents the main solution for data storage on non-volatile memory devices. The constant demand for superior performance and reduced area occupancy have led the semiconductor industry to invest in the refinement of miniaturization techniques and the research of new technologies. For the past thirty years, the process node has been constantly reduced and has reached the value of approximately 15 nm, ensuring NAND Flash technology a high storage density and reduced cost per bit. However, scaling has brought to light new stability issues that make the process more difficult and high costly demanding. For this reason, the solution was to shift from a two-dimensional approach to a more complex three-dimensional structure that also utilizes the vertical one to add additional memory cells. 3D NAND Flash memories have such a high integration density that extreme scaling becomes unnecessary, avoiding all the issues that arise from it. The main innovation of this 3D structure is the presence of a cylindrical channel in which the cells are formed by surrounding control gates that control the charge inversion. Due to the verticality of the structure, actual production processes can only generate channel in polycrystalline silicon, with a random configuration of crystals that makes it less efficient. indeed, polysilicon is characterized by percolative conduction, a phenomenon which limits the maximum current of the individual cell. Moreover, the presence of traps at the grain boundaries that capture and emit charge in a bias-dependent way generates transient phenomena in the current which limit cells performance. To promote the development of new technologies aimed at solving these main issues, it beacomes necessary to study the behavior of 3D NAND Flash memories, to better understand the physics behind these phenomena. Moreover, the application of electronic devices in aerospace rather than in quantum fields has required an investigation of their performance even at cryogenic temperatures. In this regime, the current can flow due to conductive mechanisms which are different from those occurring at room temperature. This thesis work focuses on the measurement and following physical description of an instability which occurs in the string current during the Read phase. The presence of a transient limits the speed of the Read phase of the single cell and the maximum performance memory can achieve. The study proposed below shows a dependence of this phenomenon both on the bias history and on the temperature. Precisely for this reason, the analysis was conducted by selecting a wide temperature range to cover the entire spectrum from room temperature (300K) to the deep cryogenics (4.2K). For this purpose, a custom-made measurement setup was implemented to achieve current sampling over a temporal scale of more than six decades, from tens of microseconds to tens of seconds. The sets of measurements conducted on different types of devices show a monotonic dependence on the pre-bias time, which had already been studied previously, even if at higher temperatures only, and a non-monotonic dependence on temperature with a peak of instability at 150K. In addition, the possibility of conducting measurements on both polycrystalline channel devices and crystalline channel devices has allowed to associate the cause of the instability to grain boundaries traps. In fact, in this latter category of memory cells, we observe a reduced transient amplitude that is independent of the pre-bias phase. All the measurement results and their corresponding physical explanations will be presented in Chapter 3. Thanks to this study, we have opened new perspectives for a more detailed evaluation of the polysilicon channel impact on the reliability of 3D NAND Flash technology and for the study of new string designs which could lead to an improvement in read/write operations speed and in overall performances of these memories.
La tecnologia NAND Flash rappresenta ad oggi la principale soluzione per l'archiviazione dei dati su supporti di memoria non volatile. La costante richiesta di prestazioni superiori e di una minor occupazione di area hanno portato l'industria del semiconduttore ad investire sul perfezionamento delle tecniche di miniaturizzazione e sulla ricerca di nuove tecnologie. Da trent'anni a questa parte il nodo di processo è stato costantemente ridotto e ad oggi ha raggiunto circa i 15 nm, garantendo alla tecnologia NAND Flash un'alta densità di archiviazione e ridotto costo per bit. Tuttavia, lo scaling ha fatto emergere nuovi problemi di stabilità che rendono il processo più difficile e oneroso. Per questo motivo la soluzione è stata passare da un approccio bidimensionale a una struttura più complessa e tridimensionale che sfrutta anche la dimensione verticale per aggiungere ulteriori celle di memoria. Le memorie 3D NAND Flash hanno una densità d'integrazione così alta che rendono superfluo lo scaling estremo, evitando tutti i problemi che da esso derivano. La principale novità di questa struttura 3D è la presenza di un canale cilindrico in cui le celle sono formate da control gate che, circondando il canale, controllano l'inversione di carica. A causa della verticalità della struttura, l'attuale processo di produzione può generare solamente canali in silicio policristallino, con una configurazione casuale dei cristalli che lo rendono meno efficiente dal punto di vista elettronico. Il polisilicio infatti è caratterizzato da un fenomeno detto conduzione percolativa che limita la corrente massima della singola cella. Inoltre, la presenza di trappole ai bordi di grano che catturano ed emettono carica in maniera che dipende dal bias generano fenomeni transitori nella corrente che limitano le prestazioni delle celle. Per favorire lo sviluppo di nuove tecnologie atte a risolvere questi (ed altri) problemi si rende necessario studiare il comportamento delle memorie 3D NAND Flash, per comprendere meglio la fisica che si cela dietro questi fenomeni. Inoltre, l'applicazione dei dispositivi elettronici in ambito aerospaziale piuttosto che in ambito quantistico ha richiesto un'indagine delle loro prestazioni anche a temperature criogeniche. In questo regime la corrente può scorrere grazie a meccanismi conduttivi diversi rispetto a quelli che si verificano a temperatura ambiente. Tra le varie possibilità, questo lavoro di tesi si concentra sulla misura e sulla conseguente descrizione fisica di un'instabilità che si verifica nella corrente di stringa durante la fase di Read. La presenza di un transitorio limita la velocità di lettura della singola cella e quindi le prestazioni massime che la memoria può raggiungere. Lo studio proposto di seguito mostra una dipendenza di questo fenomeno sia dalla storia del bias, sia dalla temperatura. Proprio per questo motivo l'analisi si è svolta scegliendo un ampio range di temperatura tale da coprire l'intero spettro da RT (300K) fino al regime criogenico profondo (4.2K). A tal fine è stato implementato un setup di misura ad hoc che permettesse di raggiungere un campionamento della corrente su una scala temporale di oltre sei decadi, dalle decine di microsecondi alle decine di secondi. I vari set di misure condotte su varie tipologie di dispositivi mostrano una dipendenza monotona dal tempo di prebias che però era già stata studiata in precedenza, seppur solo fino a temperature maggiori, e una dipendenza non monotona dalla temperatura con un picco di instabilità a 150K. In aggiunta, la possibilità di effettuare misure sia su dispositivi classici a canale policristallino, sia su dispositivi a canale cristallino ha permesso di associare la causa dell'instabilità alle trappole ai bordi di grano. Infatti, in quest'ultima categoria di celle memorie abbiamo un'ampiezza del transitorio visibilmente ridotta e indipendente dalla fase di pre-bias. Tutti i risultati delle misure e le relative spiegazioni fisiche verranno presentati nel Capitolo 3. Grazie a questo studio, abbiamo aperto nuove prospettive per una valutazione più dettagliata dell'impatto del canale in polisilicio sull'affidabilità delle memorie Flash 3D NAND e per lo studio di nuovi design di stringa che potrebbero portare a un miglioramento della velocità di lettura e delle prestazioni generali di queste memorie.
Cryogenic investigation of read current instability in 3D NAND flash memory
Burattini, Michelangelo
2023/2024
Abstract
The NAND Flash technology currently represents the main solution for data storage on non-volatile memory devices. The constant demand for superior performance and reduced area occupancy have led the semiconductor industry to invest in the refinement of miniaturization techniques and the research of new technologies. For the past thirty years, the process node has been constantly reduced and has reached the value of approximately 15 nm, ensuring NAND Flash technology a high storage density and reduced cost per bit. However, scaling has brought to light new stability issues that make the process more difficult and high costly demanding. For this reason, the solution was to shift from a two-dimensional approach to a more complex three-dimensional structure that also utilizes the vertical one to add additional memory cells. 3D NAND Flash memories have such a high integration density that extreme scaling becomes unnecessary, avoiding all the issues that arise from it. The main innovation of this 3D structure is the presence of a cylindrical channel in which the cells are formed by surrounding control gates that control the charge inversion. Due to the verticality of the structure, actual production processes can only generate channel in polycrystalline silicon, with a random configuration of crystals that makes it less efficient. indeed, polysilicon is characterized by percolative conduction, a phenomenon which limits the maximum current of the individual cell. Moreover, the presence of traps at the grain boundaries that capture and emit charge in a bias-dependent way generates transient phenomena in the current which limit cells performance. To promote the development of new technologies aimed at solving these main issues, it beacomes necessary to study the behavior of 3D NAND Flash memories, to better understand the physics behind these phenomena. Moreover, the application of electronic devices in aerospace rather than in quantum fields has required an investigation of their performance even at cryogenic temperatures. In this regime, the current can flow due to conductive mechanisms which are different from those occurring at room temperature. This thesis work focuses on the measurement and following physical description of an instability which occurs in the string current during the Read phase. The presence of a transient limits the speed of the Read phase of the single cell and the maximum performance memory can achieve. The study proposed below shows a dependence of this phenomenon both on the bias history and on the temperature. Precisely for this reason, the analysis was conducted by selecting a wide temperature range to cover the entire spectrum from room temperature (300K) to the deep cryogenics (4.2K). For this purpose, a custom-made measurement setup was implemented to achieve current sampling over a temporal scale of more than six decades, from tens of microseconds to tens of seconds. The sets of measurements conducted on different types of devices show a monotonic dependence on the pre-bias time, which had already been studied previously, even if at higher temperatures only, and a non-monotonic dependence on temperature with a peak of instability at 150K. In addition, the possibility of conducting measurements on both polycrystalline channel devices and crystalline channel devices has allowed to associate the cause of the instability to grain boundaries traps. In fact, in this latter category of memory cells, we observe a reduced transient amplitude that is independent of the pre-bias phase. All the measurement results and their corresponding physical explanations will be presented in Chapter 3. Thanks to this study, we have opened new perspectives for a more detailed evaluation of the polysilicon channel impact on the reliability of 3D NAND Flash technology and for the study of new string designs which could lead to an improvement in read/write operations speed and in overall performances of these memories.File | Dimensione | Formato | |
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Thesis_Burattini_Michelangelo.pdf
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Descrizione: Tesi
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executive_summary_burattini.pdf
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Descrizione: Executive summary
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3.21 MB | Adobe PDF | Visualizza/Apri |
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https://hdl.handle.net/10589/234766