Synthesis and characterization of aerosol silicon nanocrystal nonvolatile floating-gate memory devices

Abstract

This letter describes the fabrication and structural and electrical characterization of an aerosol-nanocrystal-based floating-gate field-effect-transistor nonvolatile memory. Aerosol nanocrystal nonvolatile memory devices demonstrate program/erase characteristics comparable to conventional stacked-gate nonvolatile memory devices. Aerosol nanocrystal devices with 0.2 m channel lengths exhibit large threshold voltage shifts (Ͼ3 V), submicrosecond program times, millisecond erase times, excellent endurance ͑Ͼ10 5 program/erase cycles͒, and long-term nonvolatility (Ͼ10 6 s) despite thin tunnel oxides ͑55-60 Å͒. In addition, a simple aerosol fabrication and deposition process makes the aerosol nanocrystal memory device an attractive candidate for low-cost nonvolatile memory applications. © 2001 American Institute of Physics. ͓DOI: 10.1063/1.1385190͔We have fabricated floating-gate metal-oxidesemiconductor field-effect transistors ͑MOSFETs͒ in which the conventional stacked gate has been replaced with silicon nanocrystals formed and deposited as an aerosol. The memory operation of the aerosol nanocrystal floating-gate MOSFET depends on charge storage in the floating-gate, similar to conventional stacked-gate nonvolatile memory devices. 1 In a silicon nanocrystal nonvolatile memory device, however, charge is not stored on a continuous floating-gate polysilicon layer as is the case in conventional stacked-gate devices, but on a discontinuous floating-gate layer composed of discrete crystalline silicon nanocrystals. 2-4 Nanocrystal charge storage offers several potential advantages over conventional stacked-gate nonvolatile memory devices; ͑1͒ a simple low cost floating-gate fabrication process; ͑2͒ improved retention resulting from Coulomb blockade and quantum confinement effects 5 that enable the use of thinner tunnel oxides and lower operating voltages; ͑3͒ reduced punchthrough achieved by eliminating drain-to-floating-gate coupling, allowing higher drain voltages during readout, shorter channel lengths, and smaller cell area; and ͑4͒ excellent immunity to stress induced leakage current and defects within the floating-gate or insulating layers due to the distributed nature of the charge storage in the discontinuous nanocrystal layer.The potential for improved device performance and reliability strongly depends upon the ability to control particle core size, particle size distribution, crystallinity, areal particle density, oxide-passivation quality, and crystal-to-crystal insulation that prevents lateral charge conduction in the nanocrystal layer. In order to achieve the desired layer properties, we have developed a three-step nanocrystal fabrication process. Details of silicon nanocrystal synthesis and deposition onto 200 mm wafers can be found in a separate reference. 6 A continuous flow reactor generates silicon nanocrystal aerosol by silane pyrolysis at 950°C. Nanocrystals form by homogeneous nucleation and grow by chemical vapor deposition. Through the hot zone of the reactor, they are sintered to form dense sphe...

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Probing the size and density of silicon nanocrystals in nanocrystal memory device applications

“…To fully exploit their potential advantages over conventional floating gate memory, it is essential to control as accurately as possible Si nanocrystal size, depth distribution, and areal density, as well as nanocrystal surface passivation and oxide defect density in SiO 2 matrix, all in a process compatible with ultra-large-scale integration. Transmission electron microscopy ͑TEM͒ is the most widely used tool to characterize nanocrystal size and distribution with high resolution, [2][3][4] and sometimes electron diffraction is used to further substantiate the existence of crystallites. We have used a combination of contact-mode atomic force microscopy ͑AFM͒ and reflection high energy electron diffraction ͑RHEED͒ to identify the existence of nanocrystals, and used an ultrahigh vacuum scanning tunneling microscope ͑UHV STM͒ to estimate nanocrystal size and areal density.…”

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