1. Field of the Invention
This invention relates to a semiconductor device, more particularly to a trench capacitor structure of a DRAM (Dynamic Random Access Memory), and also to a method for manufacturing the structure.
2. Description of the Related Art
An example of a DRAM having a sheath-plate type trench capacitor is known from "Half-Vcc Sheath-Plate Capacitor DRAM Cell with Self-Aligned Buried Plate Wiring" published in IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 35, NO. 8, August 1988, and written by Toru KAGA, Yoshifumi KAWAMOTO, Tokuo KURE, Yoshinobu NAKAGOME, Masakazu AOKI, Hideo SUNAMI, Tohachi MAKINO, Nagatoshi OHKI and Kiyoo ITOH.
FIG. 1 is a cross sectional view, showing a conventional sheath-plate type trench capacitor. As is shown in FIG. 1, a trench is formed in a semiconductor substrate 21 in contact with a field oxide film 1. An area which includes a trench inner wall oxide film 4, a silicon oxide/silicon nitride (SiO.sub.2 /SiN) film 10, a polysilicon film 14, a diffusion layer 16 and part of the substrate 21 serves as a gate control diode. When a potential has been applied to the polysilicon film 14 (or to a polysilicon film 11 formed in a deep portion of the trench), a depletion layer grows in the vicinity of a peripheral portion of the trench inner wall oxide film 4, thereby forming an inversion layer such that it extends from the diffusion layer 16 along the outer periphery of the trench. As a result, the amount of a junction leak current between the substrate 21 and the diffusion layer 16 contacting the polysilicon film 14 increases.
To reduce the influence of the potential applied to the polysilicon films 11 and 14, it is necessary to make the oxide film 4 thick. However, increasing the thickness of the oxide film 4 inevitably reduces the area of a capacitor insulating film and hence the capacitance of the trench capacitor.
In the conventional structure, the potential applied to the polysilicon film filled in the trench increases the junction leak current between the substrate and the diffusion layer contacting the polysilicon film. To avoid this, it is necessary to reduce the capacitance of the capacitor to some extent. Actually, however, the capacitance is excessively reduced since an increase in junction leak current is prevented by increasing the thickness of the trench inner wall oxide film.