The present invention relates to a semiconductor device having a trench capacitor in which at least a portion thereof is formed in a semiconductor substrate. More specifically, the invention relates to a semiconductor device that can be suitably adapted to a dynamic memory cell of a large scale.
In the conventional STC's (stacked capacitor cells, Koyanagi et al., IEEE, J. Solid-State Circuits, SC-15, No. 4, p. 661, 1980), the capacitors are all formed on a semiconductor substrate irrespective of whether they have a planar structure or a three-dimensional structure (Takemae et al., IEEE, ISSCC, p. 254, 1985). As the size of the element is reduced, therefore, it becomes difficult to maintain a sufficient capacity against soft error caused by alpha particles. With the three-dimensional structure, furthermore, if the surface irregularity is increased to increase the capacity, the word lines tend to be easily broken down.
A device in which a trench sidewall is formed in the semiconductor substrate to increase the capacity has been disclosed in "Sunami et al., A Corrugated Capacitor Cell (CCC) for Megabit Dynamic MOS Memories, IEEE, IEDM, Technical Digest, pp. 806-808, 1982".
However, this cell has problems in that (1) the capacity increases owing to the formation of a trench permiting, however, the soft error to develop due to alpha particles. This is due to the fact that formation of the trench causes the area of the depletion layer to increase, whereby the electric charge produced by alpha particles is easily collected. Furthermore, (2) formation of a deep trench permits a punch through current to flow across the cells. To prevent this current from flowing, the cells must be formed being separated by more than a predetermined distance, making it difficult to form the elements in small sizes. Moreover, (3) an oxide film formed in the trench has a plane index that is deviated from (100). Therefore, if the reliable wet-type oxide film-forming method is employed, the SiO.sub.2 film grows to have a thickness about twice that of the oxide film formed on the flat portion, and the capacity is not increased as expected.