不同浮動閘極材料P通道快閃記憶體性能與可靠性之改進

Abstract

近幾年來，快閃式記憶體(Flash Memory)已廣泛地應用於大量資料儲存上。過去，快閃式記憶體產品的設計多採用N通道快閃式記憶體元件。然而，為達到低功率消耗的快閃式記憶體，P通道快閃式記憶體元件將是另一種最佳的選擇。又因為元件的性能與可靠性是快閃式記憶體設計上的主要課題，然而以往的研究大多著重於發展新的元件結構，源極與汲極工程（Source/Drain Engineering），通道工程（Channel Engineering），以及改變元件的操作方式等。 本研究論文旨在提出一個新的快閃式記憶體元件設計方向，即是以浮動閘極工程來改善元件特性。其中，吾人針對不同浮動閘極P通道的快閃記憶體來進行特性的研究。根據實驗結果，並得出以下幾點結論。首先，我們比較N 和P型的浮動閘極之快閃式記憶體的性能。由於P型浮動閘極在寫入（抹除）操作時形成較高（低）的穿遂氧化層（Tunnel Oxide）電場，使其擁有較快的寫入速度卻較慢的抹除速度。其次，在耐久（Endurance）測試上，N型與P型浮動閘極元件所得之特性幾乎一致。其次，吾人發現P型浮動閘極有較好的閘極擾動特性、較佳的讀取擾動特性與較大的半衰期容許電壓。尤其在汲極擾動的特性下，利用P型浮動閘極之P通道快閃記憶體有3個數量級的改善。而且，P型浮動閘極之快閃記憶體具也有較佳的資料流失（Intrinsic Charge Loss）特性。簡言之，從本研究結果可知，P通道P型浮動閘極之快閃記憶元件的結構有較多的優點，更適合應用於未來高可靠性（Reliability）快閃記憶體的產品設計。

Recently, the flash memory has been wildly used for mass data storage. In the past, n-channel flash memories were used in the design of flash memory products. However, low power operation has become the major trend of a flash memory. P-channel flash cell is one of the best candidate. On the other hand, the performance and reliability are another major concern for designing low power flash memories. However, previous studies are mainly focused on the development of novel cell structure, the source/drain engineering, the channel engineering, and the operation methods. In this thesis, we proposed a different design concept based on the floating-gate engineering. The p-channel flash memory cell is developed with various types of floating-gate materials. From the experimental results, several studies have been made in this work. First, the basic performance of n- and p-type floating-gate flash memories has been compared. Since the tunnel oxide electric field in the p-type floating-gate cell is larger (smaller) during programming (erasing), it has faster program speed but slower erase speed. Second, for both n- and p-type floating-gate cells, the programming/erase cycling endurance characteristics are almost identical. Third, p-type floating-gate cell has better gate disturb characteristic, better read disturb characteristic, and a larger lifetime tolerance voltage.In particular, a 3-order improvement of the drain disturb can be achieved by using a p-type doped floating-gate in p-channel cell. Fourth, the p-type floating-gate cell also exhibits better data retention characteristics. In short, p-channel flash memories with p-type floating-gate structure are most advantageous for highly reliable cell design and in particular a good candidate for future memory applications.