Investigation of Threshold Voltage Distribution Temperature Dependence in 3D NAND Flash

摘要

The impact of temperature on array Vth distribution was investigated in 3D NAND flash. Cell Vth distributions were obtained under different program and read temperature splits. After the page is programmed under high temperature, it is found that the high tail of Vth distribution exhibits a larger shift than the low tail, during read at different temperatures (85 degrees C and -25 degrees C). On the contrary, the low tail of Vth distribution shows a larger shift than the high tail during cross temperature read, after the page programmed under low temperature. The temperature coefficient (Tco) of cell Vth shows cell to cell variations, which can be categorized into two types. For type (1), the Tco is correlated with the selected cell Vth due to polysilicon channel. For type (2), the Tco is independent of the selected cell Vth. The corresponding impacts on Vth distribution are studied via array Monte Carlo simulation. Based on the simulation results, the above temperature dependent observations can be well modeled by the combination of both Tco variation type (1) and (2). Furthermore, two optimization approaches are proposed to alleviate the Vth distribution broadening and are validated by experiments.