In recent years, non-volatile memory semiconductor devices, in which data is rewritable, have been widely used. In a flash memory of a typical non-volatile memory, a transistor that composes a memory cell includes a floating gate known as a charge storage layer or an Oxide Nitride Oxide (ONO) film. Then, data is stored by storing the charge in the charge storage layer.
In addition, for higher memory capacity, there have been developed flash memories having various memory cell structures. In U.S. Pat. No. 6,011,725, there is disclosed a NOR flash memory in which two charge storage regions can be formed in the ONO film of one memory cell (conventional example 1). In Japanese Patent Application Publication No. 7-45797, there is disclosed a NAND flash memory in which bit lines made of diffusion layers are respectively formed on the side surfaces of the trenches provided in the semiconductor substrate and floating gates are formed on the side surfaces (conventional example 2). In Japanese Patent Application Publication No. 2003-508914, there is disclosed a flash memory having the bit lines and word lines, the bit lines made of the diffusion layers being respectively provided at corners of projections between the trenches formed in the semiconductor substrate and running in a length direction of the trenches, the word lines running in a width direction of the trenches (conventional example 3).
In the conventional example 1, the memory cells are formed on a plane of the semiconductor substrate, and the memory capacity is insufficient. In the conventional examples 2 and 3, higher memory capacity is achieved by providing the trenches formed in the semiconductor substrate so that the floating gates or the ONO films on the side surfaces of the trenches serve as the charge storage layers. However, the fabrication methods become complicated. For example, the bit lines are separately formed in a width direction of the trenches, so the fabrication methods are complicated.