(i) Technical Field
The present disclosure relates to a semiconductor memory device and to a method of fabricating the same, and more particularly, to a semiconductor memory device including a cylinder type storage node and to a method of fabricating the semiconductor memory device.
(ii) Description of the Related Art
A memory cell of a direct random access memory (DRAM) among semiconductor memory devices may include a field effect transistor as a switching device for controlling read/write operations, and a capacitor for storing information. As the scale of the DRAM has been reduced, the area occupied by the capacitor of the memory cell has been continuously decreased. To ensure a sufficient effective capacity despite the reduction of cell area, a concave type or a cylinder type storage node having a three-dimensional capacitor shape has been suggested. The cylinder type storage node is widely used in DRAM in association with a capacitor over bit line (COB) structure, in which the capacitor is formed on a bit line.
The cylinder type storage node may be required to have a height of about 2 μm or higher to obtain an effective capacity of about 25 fF or greater per unit memory cell in the case of one cylindrical storage node (OCS) structure with a design rule of about 70 nm. As the cylinder type storage node has a very large aspect ratio, the cylinder type storage node may be structurally unstable. The unstable structure of the cylinder type storage node may cause defects such as, for example, slanting or lift off of the cylinder type storage node, which may cause a twin bit error or a multi-bit error during the operation of the DRAM.
In general, to ensure the mechanical stability of the cylinder type storage node which has a large aspect ratio, an etch stop layer may be formed under a lower electrode of the cylinder type storage node for supporting the cylinder type storage node. However, as the aspect ratio of the cylinder type storage node gradually increases, there may be a limit in providing the cylinder type storage node with sufficient mechanical stability by using the etch stop layer. In addition, when an etching process is performed to form a contact hole which exposes a storage node contact plug in the etch stop layer, it may be difficult to control critical dimensions (CD) on a lower portion of the cylinder type storage node due to skew of an etched surface of the etch stop layer.