High-density Co/Al 2 O 3 core-shell nanocrystal memory
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High-density Co/Al2O3 core-shell nanocrystal memory Huimei Zhou, Zonglin Li, Jian Huang, and Jianlin Liu Department of Electrical Engineering, University of California, Riverside, California 92521 ABSTRACT Metal/high-k dielectric core-shell nanocrystal (NC) memory capacitors were demonstrated by e-beam evaporation process. This kind of metal oxide semiconductor (MOS) memory shows good performance in charge storage, programming and erasing speeds. Compared to Co NC memory, Co/Al2O3 core-shell NC memory shows improved retention performance since the additional Al2O3 shell layer acts as a barrier, which prevent the leakage. INTRODUCTION Nonvolatile memory devices with floating-gate structure are widely used in MP3 players, digital cameras, and memory cards1. NC floating gate memory devices have attracted much attention due to excellent memory performance and high scalability2. Different types of NCs such as Si NCs3, Ge NCs4-5, metal NCs6-8, silicide NCs9-11, and dielectric NCs12 have been proposed to achieve long retention performance. Among these NCs, metallic NCs have larger work function than electron affinity of Si NCs, and are advantageous to reduce the leakage current due to increased tunnel barrier height13. These NC memories also achieved high programming/erasing speeds14-16. As the memory technology node scales down further, thinner tunnel oxide layer is required for lower power operations. However, thinner oxide induces high leakage, leading to device degradation and deteriorating of retention performance. It is well known that leakage current of high-k materials is smaller than that of SiO2 for the same equivalent oxide thickness (EOT). To improve the retention performance of memory devices, recent studies have shown to use high-k dielectrics such as Al2O3 and HfO2 to replace SiO2 layers for the tunnel and control oxide layers17-18. In this work, we developed high-density Co NC memory capacitor using e-beam evaporation process. In addition, thin Al2O3 high-k layers were deposited by atomic layer deposition (ALD) as shell of the NCs to minimize the segregation of Co atoms in tunnel oxide during subsequent high-temperature device process and to reduce the leakage. Co/Al2O3 coreshell NC memory exhibited a higher retention performance compared to Co NC memory reference sample. EXPERIMENT The Co/Al2O3 core-shell NC MOS capacitors were fabricated by the following process. Firstly, a 5.0-nm-thick thermal oxide was grown in dry oxygen at 850oC. Then a thin Al2O3 layer of around 1nm was deposited on SiO2 layer by ALD. An ultra-thin (~1.5nm) blanket Co layer was deposited through e-beam evaporation followed by rapid thermal annealing (RTA) in N2 at 650oC to form NCs. Another Al2O3 layer of around 1nm was deposited by ALD again on the NCs to create Co/Al2O3 core-shell structure. Control oxide of about 15 nm thick was then
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deposited by low pressure chemical vapor deposition (LPCVD). After control gate pattern formation, the control gate and backside gate were formed to complete the fabrication of the MOS structure
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