Browsing by Author "Shirota, Riichiro"

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A New Disturb Free Programming Scheme in Scaled NAND Flash Memory
(2011)
Shirota, Riichiro
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Huang, Chen-Hao
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Arakawa, Hideki
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
New programming scheme is proposed to improve the program disturb characteristics in NAND Flash memory named Program Disturb Free Scheme (PDFS), which is executed by removing excess electrons from the channel and source/drain into bit line or source line using drift-diffusion mechanism, and also by recombining electrons in the surface states with accumulated holes before programming. Thus, no excess electron exists in the program inhibit cell string during programming, thereby program disturb can be suppressed drastically. By measuring 8Gbit NAND Flash memory with 50nm technology node, almost no Vt shift was observed even applying 30 times over programming (partial programming) in 2bit/cell operation. This universally applicable innovation is independent from generation of design rule. Therefore, new operation has broken new ground for the cell device engineering, especially for sub-30nm NAND which has seriously narrowed program operation margin.
A New Programming Scheme for the Improvement of Program Disturb Characteristics in Scaled NAND Flash Memory
(2012-10-01)
Shirota, Riichiro
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Huang, Chen-Hao
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Nagai, Shinji
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Sakamoto, Yoshinori
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Li, Fu-Hai
This paper investigates the new programming scheme to reduce the program disturb in the NAND Flash memory. Program disturb characteristics are determined by the unwilling electron injections in the floating gate of the unselected cells during programming. Thus, the key point to improve the program disturb characteristics is how to suppress the electron injection in the unselected cells. This requirement can be implemented by reducing the number of electrons in the unselected NAND strings prior to programming. By applying negative bias to all the word lines in the selected block, excess electrons can be removed from the channel and source/drain regions into the bit line or the source line using drift and diffusion mechanisms, and also electrons in the surface states can be recombined with accumulated holes before programming. After the pretreatment of electron reduction in the NAND string, a normal NAND program sequence follows. The advantage of the pretreatment before programming has been verified by measuring the 8-Gb NAND Flash memory with a 50-nm technology node. Significant reduction of the threshold voltage shift was observed even after the severe program disturb stress, which corresponds to around 30 times of the programming of the 2 bit/cell operation.
Analysis of the Correlation Between the Programmed Threshold-Voltage Distribution Spread of NAND Flash Memory Devices and Floating-Gate Impurity Concentration
(2011-11-01)
Shirota, Riichiro
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Sakamoto, Yoshinori
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Hsueh, Hung-Ming
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Jaw, Jian-Ming
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Chao, Wen-Chuan
The effect of the activated floating-gate (FG) impurity concentration on the programmed threshold-voltage (V(t)) distribution was newly investigated and analyzed. The lower FG impurity concentration leads to a wider threshold-voltage distribution, which is explained by the time-dependent tunnel-oxide electric-field enhancement effect induced by the reduction of the depletion region in the FG as the programming time is lengthened. Initially, the FG is deeply depleted at the interface of the tunnel oxide. However, as the programming time is prolonged, electrons by Fowler-Nordheim (FN) tunneling in the FG generate electron-hole pairs, and generated holes are gathered at the interface of the tunnel oxide, which reduces the depletion region, and enhance the oxide electric filed. The enhancement effect of the electric field for the tunnel oxide is coupled to the FN tunneling statistics and enlarges the distribution of the programmed Vt. This effect is more clearly observed at the lower FG impurity concentration, which gives the limitation of the minimum impurity concentration in FG. Monte Carlo simulations considering both the tunnel-oxide electric-field enhancement effect and FN tunneling statistics were carried out and showed good agreement with the experiments.
Characterization of the charge trapping properties in p-channel silicon-oxide-nitride-oxide-silicon memory devices including SiO2/Si3N4 interfacial transition layer
(2015-10-01)
Chiu, Yung-Yueh
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Yang, Bo-Jun
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Li, Fu-Hai
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Chang, Ru-Wei
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Sun, Wein-Town
The role of SiO2/Si3N4 interfacial transition (IFT) layer in the oxide-nitride-oxide (ONO) tri-layer is quantitatively analyzed for the first time by simulating the temperature and stress-accelerated retention characteristics of p-channel silicon-oxide-nitride-oxide-silicon (SONOS) devices. The ONO tri-layer is modeled as an alloy-dielectric by changing the atomic concentration of silicon, oxygen and nitrogen. It is revealed that simulated results including the IFT layer are more consistent with the experimental data than those neglecting the IFT layer. In addition, the results show that the trapped charge density in IFT layer is two times larger than in the bulk Si3N4 film, due to the oxygen atoms penetrated from SiO2 cause the extrinsic defects in the IFT layer. The energy levels of the trapped charge are continuously distributed, and the peak value is similar to 1.6 eV below the conduction band of the ONO tri-layer with a full width at half maximum of 0.45 eV. (c) 2015 The Japan Society of Applied Physics
Evaluation of the Role of Deep Trap State Using Analytical Model in the Program/Erase Cycling of NAND Flash Memory and Its Process Dependence
(2018-02-01)
Yang, Bo-Jun
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Wu, Yu-Ting
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Chiu, Yung-Yueh
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Kuo, Tse-Mien
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Chang, Jung-Ho
A method to analyze the kinetics of the charge accumulation in the tunnel oxide by the nand flash memory program and erase (P/E) cycling is proposed. Both electron trapping and detrapping processes are required to be considered owing to the oxide high electric field during P/E cycles. Consequently, the electron trapping in the deep trap state is concluded, whose trap energy (E-trap) is more than 3.5 eV. Furthermore, the as-grown trap state density (Ne), the trapping capture cross section (sigma), and the number of trapped positive charges can also be extracted to explain the tunneling current modulation and the V-T shift by oxide-trapped charges under the P/E stress. The trapped electrons are mainly distributed in the center of tunneloxide, and the distributed area extends as the P/E bias increases. In addition, the dependence of oxidation process is also shown. Both thermal dry and plasma oxidation have almost the same value of sigma (similar to 4x10(-17) cm(2)). However, 30% reduction of Ne is shown in plasma oxidation (similar to 1.25x10(19) cm(-3)) when compared with thermal dry oxidation (similar to 1.88 x 10(19) cm(-3)).
Impact of Source/Drain Junction and Cell Shape on Random Telegraph Noise in NAND Flash Memory
(2013-07-01)
Li, Fu-Hai
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Shirota, Riichiro
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傳播研究所
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電機資訊學士班
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Institute of Communication Studies
A comprehensive numerical study of threshold voltage fluctuation (Delta V-T) in scaled NAND flash memory caused by random telegraph noise (RTN) and discrete dopant fluctuation (RDF) in both the channel and the cell-to-cell space [source/drain (S/D)] region was carried out. Following a three-dimensional (3D) Monte Carlo (MC) procedure, the statistical distribution of Delta V-T is estimated, considering the effects of both the random placement of discrete doping atoms and a discrete single trap at the tunnel oxide/substrate interface. The result demonstrates the significant influence of the doping in the S/D regions. For the cells with and without an S/D junction, the electron concentration in the S/D region is determined by the pass voltage of the unselected cell (V-pass) and the neighboring cell V-T (V-T(n)), owing to the fringing fields of neighboring floating gates (FGs). As a result, Delta V-T increases in the S/D region as V-pass - V-T(n) decreases. The fluctuation amplitude strongly depends on the [single-trap RTN] position along the cell length (L) and width (W) directions. For the cell shape with rounding of the active area (AA) at the shallow trench isolation (STI) edge, the results indicate that the high Delta V-T area moves from the AA edge towards the center area along the W-direction. (C) 2013 The Japan Society of Applied Physics
Impact of String Pattern on the Threshold-Voltage Spread of Program-Inhibited Cell in NAND Flash
(2016-07-01)
Chiu, Yung-Yueh
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Aoki, Minoru
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Yano, Masaru
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Shirota, Riichiro
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電機工程學系
An anomalous threshold-voltage (V-t) spread of the program-inhibited cell is investigated for the first time in NAND flash memory. The program disturb characteristics are studied by applying the program-inhibited stress on the Nth cell of the unselected bitline with various string patterns for the 0th to (N - 1)th cell, using the global self-boosting method. Distinguishing features of the variance of the number of injected electrons (sigma(2)(n)) into the floating gate are observed. The variance is proportional to the mean value of injected electrons ((n) over bar) times 10. The other is proportional to (n) over bar times 20 and occurs only when the (N - 1) th cell is programmed in a high Vt level and the other cells are in the erased state. A 3-D TCAD simulation reveals that the former case is attributed to Fowler-Nordheim tunneling from the insufficiently boosting channel, and the latter is explained by hot-electron injection owing to the strong lateral electric field between the Nth and (N - 1) th cells.
New Method to Analyze the Shift of Floating Gate Charge and Generated Tunnel Oxide Trapped Charge Profile in NAND Flash Memory by Program/Erase Endurance
(2015-01-01)
Shirota, Riichiro
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Yang, Bo-Jun
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Chiu, Yung-Yueh
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Chen, Hsuan-Tse
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Ng, Seng-Fei
A new test system was devised and used to separate the amount of floating gate (FG) charge (Q(FG)) from the oxide trapped charge (Q(OX)) generated by program-and-erase (P/E) cycles. We also extracted the pure V-mid shift caused by the generation of Q(OX), which is separated from the part of V-t shift coming from Q(FG) deviation. The identification of Q(FG) and Vmid shift makes it possible to analyze the detailed the oxide trapped charge profile. The Q(FG) shift generated by P/E cycles displays asymmetry between the programmed and erased states: the absolute value of Q(FG) exhibits a maximum at similar to 100 cycles in the programmed states, while Q(FG) monotonically decreases in the erased one. Considering that the Fowler-Nordheim tunneling current is sensitive to the oxide trap near the cathode, itself the source of electron tunneling current, our results indicate that the hole trap is dominant near to Si, whereas the electron trap is dominant near FG.
Study Trapped Charge Distribution in P-Channel Silicon-Oxide-Nitride-Oxide-Silicon Memory Device Using Dynamic Programming Scheme
(2013-04-01)
Li, Fu-Hai
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Chiu, Yung-Yueh
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Lee, Yen-Hui
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Chang, Ru-Wei
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Yang, Bo-Jun
In this study, we precisely investigate the charge distribution in SiN layer by dynamic programming of channel hot hole induced hot electron injection (CHHIHE) in p-channel silicon-oxide-nitride-oxide-silicon (SONOS) memory device. In the dynamic programming scheme, gate voltage is increased as a staircase with fixed step amplitude, which can prohibits the injection of holes in SiN layer. Three-dimensional device simulation is calibrated and is compared with the measured programming characteristics. It is found, for the first time, that the hot electron injection point quickly traverses from drain to source side synchronizing to the expansion of charged area in SiN layer. As a result, the injected charges quickly spread over on the almost whole channel area uniformly during a short programming period, which will afford large tolerance against lateral trapped charge diffusion by baking. (C) 2013 The Japan Society of Applied Physics
The effect of hydrogen on programmed threshold-voltage distribution in NAND flash memories
(2019-08-01)
Chiu, Yung-Yueh
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Lin, Cheng-Han
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Yang, Jhih-Siang
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Yang, Bo-Jun
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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
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Lin, I-Chun
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Chang, Kai-Chieh
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Yang, Bo-Jun
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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.