Investigation on Process Dependence of Self-Assembled Metal Nanocrystals
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Investigation on Process Dependence of Self-Assembled Metal Nanocrystals Chungho Lee, Zengtao Liu and Edwin C. Kan, School of Electrical and Computer Engineering, Cornell University, Ithaca, NY 14853. ABSTRACT We report the systematic characterization of metal nanocrystal formation on ultra-thin tunnel gate oxide (2~3nm) for memory applications. To get a high density and small average size of nanocrystals, the process parameters including annealing temperature, initial film thickness, and substrate doping are investigated for Au, Ag, and Pt nanocrystal formation with Si nanocrystal structure as control samples. The observation of nanocrystal formation by scanning electron microscope (SEM) shows that annealing below melting temperature of deposited film contributes to the reshaping of nanocrystals, while the initial film thickness to actual nanocrystal growth. In addition, the Schottky charge effect from substrate doping is not negligible if the tunnel oxide is thin. Controlling the process parameters, Au, Ag, and Pt nanocrystals of 4.0×1011cm-2, 2.8×1011cm-2, and 2.4×1011cm-2 can be formed with mean size of 6.2nm, 6.6nm, and 8.0nm, respectively. The observation of nanocrystals by scanning transmission electron microscope (STEM) shows that nanocrystals are spherical and crystalline. Metal contamination to the Si/SiO2 interface is also closely monitored with many process recipes of metal nanocrystal formation on 2~3nm oxide showing atomically clean interface. Electrical evaluation of nanocrystal formation is carried out by C-V measurements of metal-oxide-semiconductor (MOS) capacitors with embedded metal nanocrystals. INTRODUCTION The general methods of metal nanocrystal formation includes colloidal, aerosol, and selfassembly. The colloidal method is a chemical method by precipitation from solution, but the contamination associated with chemicals is a major concern [1]. The aerosol method is a physical method by gas phase condensation that gives a good size selection, but it has complications from fabrication apparatus on particle delivery and size selection [2]. Selfassembly is a simple method that involves the spontaneous formation of nanocrystals to achieve a local minimum energy state. We choose self-assembly method and establish the stable nanocrystal formation in conventional CMOS process. For nonvolatile memory applications, high density, small mean size, and tight size distribution of nanocrystals are preferred. Small and uniform size of nanocrystals enhances the size effect such as Coulomb blockade. High density of floating nanocrystals gives replete coverage over the active area to screen the gate field from the channel at Si/SiO2 interface. We demonstrate that both size and density of self-assembled nanocrystals can be controlled by various process parameters such as annealing temperatures, initial layer thickness, and substrate doping. Different metals have their specific sets of process parameters to achieve the desirable size and density, and only a limited selection of process control will be p
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