Total Dose Radiation Effects In Si Nanocrystal Non-Volatile Memory Transistors
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Total Dose Radiation Effects In Si Nanocrystal Non-Volatile Memory Transistors Mihail P. Petkov1, L. Douglas Bell1, Robert J. Walters2, and Harry A. Atwater2 Jet Propulsion Laboratory, California Institute of Technology Pasadena, CA 91109 USA 2 T. J. Watson Laboratory of Applied Physics, California Institute of Technology Pasadena, CA 91125 USA 1
ABSTRACT We report results pertinent to the high total dose tolerance of Si nanocrystal non-volatile memory cells. The nc-Si FETs made by ion implantation retained virtually unchanged write / erase characteristics, typical for the two-state devices, to cumulative doses exceeding 15 Mrad(Si). I. INTRODUCTION Flash memory is the leading non-volatile memory (NVM) technology in wireless communication devices, mass storage media, and many other applications. A number of other technologies (FRAM, MRAM and CRAM) utilizing unique properties of ferroelectric and magnetic materials, phase-change alloys, etc., are rapidly emerging as potential replacements. Nanocrystal silicon (nc-Si) NVM technology offers another appealing alternative. Recent work reported in the literature (e.g., see papers from Section D, 2004 MRS Fall Meeting) testifies to the enormous development in the field of nc-Si technology. A number of methods, spanning aerosol [1] or chemical vapor deposition growth [2], implantation [3-6], and others are being used to produce Si nanocrystals, which are later integrated into nc-Si devices. While these methods differ by their complexity, cost, and resulting device quality, they share common advantages with respect to flash memory: (1) thinner tunnel oxide helps to achieve faster write/erase times; (2) small nanocrystal size leads to low power consumption; and (3) multiple isolated nanocrystal floating gates enable greater redundancy. Most importantly, this is the only CMOS compatible fabrication process among all other competing NVM technologies. Thus, ncSi NVM can achieve fast implementation, high reliability, and fast product maturation. Furthermore, the distribution of the total stored charge over millions of isolated nodes facilitates a statistical approach to assessing bit status [7]. Thus, stored information can be preserved with incomplete crystal charging, enhanced leakage from a fraction of the crystals, or even with localized physical defects in the cell. This adds to the redundancy and reliability of nc-Si NVM. The use of any NVM technology in radiation environments brings out an entirely new set of requirements. Ionizing (electron and gamma) radiation causes cumulative changes (which scale with total ionization dose, TID) in device properties that can lead to permanent failure. Alternatively, proton and ion radiation can create transient events or single-event effects (SEE), which cause “soft errors” (e.g., bit-flip from “1” to “0”), or “hard errors” (e.g., latchup in a permanently fixed state), which may require reset or can lead to irreparable damage. For an NVM to fulfill the requirements of any given application in a radiation environment, preset T
