Performance improvement of nanocrystal memory

Abstract

The rapid growth of the market for portable electronic appliances requires flash memory devices with high performance such as very large storage density, low power consumption and fast operation. Although Flash memories have been designed as a solution to the scaling problem of conventional EEPROM devices, aggressive scaling of the transistor dimensions and the dramatic increase in the memory array size demand a lower voltage memory cell design for the future. In the case of the tunneling oxide, it must be thin enough to allow a fast write/erase speed at reasonable voltage levels with negligible degradation after $10^5$ programming cycles and thick enough to avoid charge loss during read or normal operations. Thus, all scaling issues pertinent to flash memories are ultimately related to the reliability of the tunneling oxide. Theoretically, in order to ensure ten-year data retention time, the tunneling oxide could be scaled until electron flow through the full oxide thickness becomes significant. However, stress-induced leakage current (SILC) imposes a more stringent limitation on the tunneling oxide can discharge the conduction polysilicon memories was set as thin as about 10 nm from the beginning, and has scarcely been thinned over five successive generations to limit its thickness to 7-8 nm at the present states of nonvolatile memory technology. As a result, the dimension of the FG transistor have been scaled much more slowly than those of the logic transistor, and therefore, the Flash memory performance in terms of access time, write/erase speeds, and operation voltages has not been substantially improved with device scaling. In order to overcome the technological constraints imposed as the device size approach dimensions below the 100 nm range, new memory concepts are needed for ultrahigh density, low-voltage, low-power, and fast write/erase data storage. One of the most promising candidates is nanocrystal memory which has the two-dimensionally distribu...