Browsing by Author "Takeshita, Toshiaki"

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The effect of hydrogen on programmed threshold-voltage distribution in NAND flash memories
(2019-08-01)
Chiu, Yung-Yueh
;
Lin, Cheng-Han
;
Yang, Jhih-Siang
;
Yang, Bo-Jun
;
Aoki, Minoru
This paper, for the first time, presents a comprehensive study into the effect of hydrogen concentration in the forming gas on the reliability of NAND Flash memories. It is newly observed that a higher hydrogen concentration leads to broaden the threshold-voltage (V-T) distribution after programming. This suggests that a higher hydrogen concentration makes a larger amount of ionized hydrogens (H+) exist in the shallow trench isolation (STI), which results in the fluctuation of the Fowler-Nordheim tunneling current during programming. It is also found that the V-T distribution width difference between higher and lower hydrogen concentration turns out to be smaller after the program/erase cycling. This could be explained by the neutralization of H+ due to the capture of the tunneling electrons at the STI edge. On the other hand, the higher hydrogen concentration proves to be able to provide better retention characteristics. Thus, the programming and retention characteristics are the tradeoff relationship. Therefore, optimization of the hydrogen concentration is crucial for the NAND Flash process integration. (C) 2019 The Japan Society of Applied Physics
Transconductance Distribution in Program/Erase Cycling of NAND Flash Memory Devices: a Statistical Investigation
(2019-03-01)
Chiu, Yung-Yueh
;
Lin, I-Chun
;
Chang, Kai-Chieh
;
Yang, Bo-Jun
;
Takeshita, Toshiaki
A new method for characterizing the evolution of oxide charge (Q(T)) generation during program/erase (P/E) cycles is developed. The concentration of Q(T) can be separated from the floating gate charge (Q(FG)) by statistically analyzing the distribution of transconductance reduction (Delta G(m,max)) after P/E cycles. The effect of the number and position of discrete Q(T) on the distribution of Delta G(m,max) is studied. The cycling dependence of the Q(T) generation is given by a fractional equation that includes a power of the number of P/E cycles in the denominator. For fewer than 30k cycles, the equation can be simplified as a power-law equation in the number of cycles with a power factor of 0.58. For more than 30k cycles, the Q(T) generation gradually goes to a plateau value Q(0) = 1.5 x 10(20) cm(-3). It is indicated that the Q(T) generation is limited by the concentration of inherent weak bonding points inside SiO2 or the Si/SiO2 interface. The temperature dependence of the Q(T) generation is also provided. The activation energy (EA) of the Q(T) generation is approximately 0.1 eV, which is compatible with the EA value of electron traps creation in SiO2.