SONOS Memories: Advances in Materials and Devices

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SONOS Memories: Advances in Materials and Devices K. Ramkumar, V. Prabhakar, Ali Keshavarzi, Igor Kouznetsov, Sam Geha Cypress Semiconductor, 3833 North First St, San Jose, CA 95134, U.S.A. Abstract Silicon Nitride based charge trap devices have been studied since the 1980s for applications in non-volatile memories. Silicon-Oxide-Nitride-Oxide-Silicon (SONOS) stack as the non-volatile memory gate stack has been the focus since the 1990s. Several enhancements in SONOS layer materials have been invented to reduce the programming voltage and improve the reliability of the SONOS memory. SONOS memories are a widely used class of non-volatile memories today. This paper will review the history of SONOS and highlight the various innovations that have enhanced SONOS memory performance, reliability and low cost of manufacture. Topics covered include various improvements in the SONOS stack such as Band gap engineering, High Kā€“Metal Gate for SONOS, 3D SONOS, SONOS FinFETs (Field Effect Transistor) and embedded SONOS. Introduction Non-volatile memories for data and code storage have been growing rapidly in performance, reliability and capacity over the last three decades. With the advent of the many widely used consumer electronic gadgets such as cell phones, laptop computers, tablets and electronic games, the need for non-volatile memories has been ever increasing recently. While the need for larger and larger memory capacity is obvious, there has also been an increasing demand for more and more robust data retention performance over a ever widening range of temperature. This is because the non-volatile memories are now getting into gadgets used in industrial and automotive environment. This coupled with the need for low cost solutions is challenging the memory technology. SONOS seems to offer the non-volatile memory solutions in an ever widening range of applications. The key features of the SONOS memory device and their dependence on material characteristics are discussed in the following sections. SONOS History The SONOS transistor was conceived in the 1970s as a storage element with its structure being almost identical to that of a Metal-Oxide-Semiconductor (MOS) transistor with the gate dielectric being replaced by a Oxide-Nitride-Oxide (ONO) dielectric. This device has been studied extensively in the last three decades to understand the carrier transport, trapping and detrapping mechanisms in the nitride layer. When SONOS based memories were first introduced in the 1980s the prevailing solution for non-volatile memory was floating gate memories. There are different types of Floating gate transistors used in memories but in all cases, a conducting but electrically floating polysilicon layer is used for charge storage. The process flow required to realize the floating gate memory device is fairly complex. A SONOS memory device, on the other hand, greatly resembles a regular Metal-Oxide-Semiconductor Field Effect Transistor

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