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download this shivaay 2 full movie hd 720p dvdrip and also watch it online on website1. Field of the Invention
The present invention relates to a semiconductor memory device and a fabrication method thereof and, more particularly, to a semiconductor memory device having a buried gate structure, and a fabrication method thereof.
2. Description of the Related Art
As the level of integration of semiconductor devices is increased, a semiconductor memory device such as a DRAM (Dynamic Random Access Memory) has a multi-layered structure including cell transistors and select transistors. The cell transistor includes a source, a drain, a floating gate, a control gate, a substrate, and the like, and the select transistor includes a source, a drain, a gate, a substrate, and the like.
Since these transistors have different depths of diffusion layers, the cell transistor and the select transistor require a separate process for fabrication. In particular, the cell transistor is formed by a shallow trench isolation (STI) process, and the select transistor is formed by a deep trench isolation (DTI) process.
FIGS. 1 and 2 are sectional views showing a conventional buried gate structure of a semiconductor memory device. Referring to FIGS. 1 and 2, a buried gate electrode 23 is formed in a deep trench isolation process. However, a buried gate electrode layer 23 does not exist in the STI process. As a result, a method of forming a buried gate electrode 23 in the STI process has been required.
A buried gate electrode 23 is formed by filling an oxide layer 25 with conductive material in the deep trench isolation process.
A method of forming a buried gate electrode layer 23 is illustrated in FIGS. 3 to 5. Referring to FIG. 3, an interlayer dielectric film (not shown) is formed on a semiconductor substrate 21. Then, a contact hole is formed in the interlayer dielectric film to expose a part of the semiconductor substrate 21.
Referring to FIG. 4, the buried gate electrode layer 23 is formed by filling the contact hole with conductive material to form a buried electrode 23. The buried electrode 23 may be formed of, for example, a polysilicon layer or a metal layer.
Referring to FIG. 5, a thin gate oxide film 24 is formed on the semiconductor substrate 21 and the buried electrode 23. Then, a gate electrode layer 22 is formed on the gate oxide film 24, and a select gate electrode pattern 23 is formed thereon by patterning the gate electrode layer 22 and the gate 0b46394aab