1. Field
Embodiments described herein relate generally to a semiconductor storage device such as a NAND-type flash memory having a pump circuit.
2. Background Art
The miniaturization of a semiconductor memory is being progressed concomitantly with the increase in capacity thereof. Particularly, in a NAND-type flash memory, a write/erase count of about 100,000 is required as a reliability specification with respect to a write/erase count of an SD card or a solid state drive (SSD). Due to the miniaturization of the semiconductor memory, the reliability is deteriorated. As one of main factors of deterioration due to the write/erasure operation, electric field stress that is applied to a tunnel oxide film of a memory cell is considered.
In the NAND-type flash memory, the Fowler-Nordheim (FN) tunnel electric field is applied between a substrate and a floating gate of a memory cell to inject electrons into the floating gate (Program) or extract the electrons (Erase). For example, data of “0” or “1” is associated with a charge state (threshold voltage) of the floating gate and the data is stored in the memory cell.
When the write/erasure operation is repeated, electric field damage is gradually applied to a tunnel oxide film of the memory cell. The electrons that are accumulated in the floating gate come out through a defect of the tunnel oxide film, and a data storage characteristic is deteriorated.
In particular, if the size of the memory cell decreases, a ratio of the magnitude of the defect with respect to an area of the tunnel oxide film increases, and the probability of the data coming out may increase. If the probability of the data coming out may increase, the reliability of a non-volatile memory is deteriorated.
In the NAND-type flash memory according to the conventional art, an erase voltage (FN tunnel electric field) to be increased stepwisely is applied between a control gate and the substrate.
However, in the conventional art, the basic configuration to control switching of a pump circuit to generate the erase voltage is not mentioned.