Long Retention Performance of a MFIS Device Achieved by Introducing High-k Al 2 O 3 /Si 3 N 4 /Si Buffer Layer
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Long Retention Performance of a MFIS Device Achieved by Introducing High-k Al2O3/Si3N4/Si Buffer Layer Yoshihisa Fujisaki1, 2, Kunie Iseki2 and Hiroshi Ishiwara2 1 R&D Association of Future Electron Devices, 1-10-14 Kitaueno, Taitoh-ku, Tokyo 110-0014, Japan 2
Frontier Collaborative Research Center, Tokyo Institute of Technology 4259 Nagatsuda, Midori-ku, Yokohama 226-8503, Japan ABSTRACT We introduced high-k Al2O3/Si3N4 buffer layer in MFIS (Metal-Ferroelectric-InsulatorSemiconductor) devices to reduce the leakage current though the buffer (I) layer. We prepared the buffer layer by nitridizing Si substrate by atomic nitrogen radicals and then deposited Al2O3 film using ALD (Atomic Layer Deposition) technique. The interface state density between the ALD-Al2O3/Radical-Si3N4 stacked insulator and a Si substrate is as low as 1011 cm-2eV-1. The current density less than 10-9 A/cm2 is realized under the 1V bias application using films with the capacitance density of 12fF/mm2. The c-axis oriented Bi3.45La0.75Ti3O12 (BLT) ferroelectric films were deposited to make MFIS structure. With this structure, we obtained the retention time as long as 1.5x106 sec (about 17 days). This excellent retention character is attributable to the high insulating property of the ALD-Al2O3/Radical-Si3N4 stacked insulator and also attributable to the perfect elimination of defects at the interfaces in the MFIS structure.
INTRODUCTION The ferroelectric gate field-effect transistor (FET) is a promising device to realize nonvolatile highly integrated memories with low power consumption. Compared to the most widely used nonvolatile rewritable memory, i.e. Flash memory, ferroelectric transistors have great advantages of low power consumption and high re-writing speed. Among the structures that have been proposed till now, MFIS structure is the simplest one to use and a much easier one to fabricate compared to its alternatives. The most important thing in developing an MFIS structure is to find a good insulator that acts as a buffer between the Si substrate and the ferroelectric material. We developed perfectly hydrogen-free and high density Si3N4 film produced by atomic nitrogen radicals [1]. This Si3N4 film shows high oxidation resistance even under highly
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oxidating ambience. We adopted it as a buffer layer in a MFIS structure and obtained better ferroelectric performances compared to previous reports having a Si3N4 buffer layer in the MFIS structure[2-4]. This improvement is attributable to the difference in hydrogen content in the films. Conventional Si3N4 films have high density of hydrogen, which terminates Si dangling bonds. However, the hydrogen in the Si3N4 dissociates from the film if the film is annealed at temperatures higher than 550°C [5, 6]. This means that crystallization annealing of a ferroelectric film at a high temperature degrades the ferroelectric characteristics of the MFIS structures [7]. To the contrary, the MFIS structure with our Si3N4 made by atomic nitrogen radicals is not degraded by the
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