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Methodology for improvement of data retention in floating gate flashmemory using leakage current estimation

Pyung Moon a, Jun Yeong Lim a, Tae-Un Youn b, Keum-Whan Noh b, Sung-Kye Park b, Ilgu Yun a,⇑a School of Electrical and Electronic Engineering, Yonsei University, Seoul, Republic of Koreab Research and Development Division, SK Hynix Semiconductor Inc., Icheon, Republic of Korea

a r t i c l e i n f o

Article history:Received 21 May 2013Received in revised form 10 June 2013Accepted 4 July 2013

a b s t r a c t

The importance of data retention characteristic is increased as the memory has been scaled down andmulti-level programming. The leakage current of the inter-poly dielectric (IPD) at low electric field isrelated with data retention and the charge of the threshold voltage distribution is increased when thenumber of storage charges in the floating gate is increased. In order to improve data retention character-istics, the minimization of leakage current variation with respect to the applied electric field on IPD isnecessary. In this paper, the effect of the electric potential of IPD on the leakage current is examinedand the leakage current at low electric field is predicted. Based on the results, the method for improvingthe data retention by reducing the leakage current is proposed.

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1. Introduction

As nonvolatile NAND flash memory has been scaled down toimprove the performance and power consumption, the oxide-ni-tride-oxide (ONO) thickness for the inter-poly dielectric (IPD) isalso reduced for the coupling ratio [1,2]. Reducing the ONO thick-ness increases the leakage current which causes the charge loss ofthe floating gate (FG) and the shift of the threshold voltage (Vth) ofthe cell [1]. In addition, the multi-level cell (MLC) which has sev-eral program states has been used for high programmable bits[3]. Since the small shift of Vth can generate the error due to nar-row Vth distribution and a small read margin in MLC, data reten-tion is one of the most important characteristics maintaining theperformance [3,4]. Therefore, the mechanism and various reduc-tion methods for the charge loss have been investigated [5–11].

The effects of ONO stack scaling on charge loss and data reten-tion were previously investigated [5,6] and the leakage mechanismof charge loss through ONO IPD was also demonstrated [7,8]. Inaddition, the charge loss under high temperature after cyclingand the tunnel oxide nitridation to reduce stress induce leakagecurrent (SILC) were investigated [9]. The correlation of retentioncharacteristic with the trap distribution of high-k based IPD andthe effect of interface trap generation on data retention were re-ported [10,11].

However, the leakage current at low electric field related withretention characteristic as varying the electric field of ONO stackhas not been demonstrated. Thus, in this paper, the effect of thelow electric field of IPD on the leakage current is examined and

predicted and the method for improving the retention characteris-tic is proposed by the reduction of leakage current.

2. Experiments and measurements

The ONO test structure was fabricated on 12-inch wafers usingthe 2x NAND flash process by SK hynix Semiconductor Inc. The teststructure of IPD was constructed with ONO stack sandwiched withtwo heavily doped poly-Si layers, which are the FG and the controlgate (CG), as schematically shown in Fig. 1. The silicon oxide (bot-tom), silicon nitride and silicon oxide (top) of ONO stack wassequentially grown by LPCVD [12].

To analyze the leakage current with varying the electric field,the current–voltage (I–V) were measured using an Agilent 4156Cparameter analyzer. The leakage current density–electric field (J–E) characteristics were then calculated from I–V curves, whichwere the applied bias divided by equivalent oxide thickness(EOT) of ONO stack. All electrical characteristic data were providedwith the current density (A/cm2) and electric field (V/cm). For theI–V measurement, the positive bias and ground were applied to CGand FG, respectively as shown in Fig. 1. Since the increment steppulse programming (ISPP) was used for the programming, whichis charging the electrons on FG, the positive bias was swept fromzero to a given sweep voltage (VS) and VS was varied. Each sweeprange was repeated twice to verify the effect of changing VS on theleakage current. At first, VS were increased from 4.22 MV/cm to12.66 MV/cm with a step of 0.84 MV/cm. Then, VS were decreasedfrom 12.66 MV/cm to 10.13 MV/cm with a step of 0.84 MV/cm.

From the J–E curve, we categorized the characteristic into threeregions according to the characteristic of leakage current to thechange of the leakage current as illustrated in Fig. 2. At first, the

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⇑ Corresponding author. Tel.: +82 (2) 2123 4619; fax: +82 (2) 2123 2879.E-mail address: iyun@yonsei.ac.kr (I. Yun).

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low electric field region (EL) related with the data retention charac-teristic is defined as a region where the leakage current is suffi-ciently low so that it was measured below the measurementresolution [4]. Secondly, the moderate field region (EM) is definedas a region where the leakage current is gradually increased. Final-ly, the high electric field region (EH) is defined as a region wherethe leakage current is exponentially increased.

3. Results and discussion

Fig. 3 shows the Vth distribution curves of the fresh cells for sev-eral programming states, such as PV1, PV2 and PV3 which werevaried with the number of charges in FG [12]. The number ofcharges was increased from PV1 (low state) to PV3 (high state)and the program voltage of PV3 was larger than that of PV1. Here,the retention characteristic of the cell was related with the changeof Vth distribution curves (DVth). The distortion of the Vth distribu-tion curves was increased as the number of charges in FG was in-creased (i.e. DVth-PV3 > DVth-PV1), as shown in Fig. 3. It wasindicated that the storage charges in FG were escaped from FG tosubstrate and/or CG via tunnel oxide and IPD, respectively [7–9].From the measured data, however, the leakage currents of ONOstack at EL, i.e., below the 4 MV/cm, were very low and almost un-changed as shown in Fig. 4. In other words, the leakage current wasprobably changed although the change of the leakage current at EL

cannot be observed and measured.In order to predict the leakage current of the ONO stack at EL

using the observable leakage current, the leakage currents ofvarious positive bias sweep ranges were analyzed. Fig. 4(a) shows

several J–E curves with respect to the different values of VS. J–Ecurves of EH were shifted toward positive direction due to the neg-

Fig. 1. Schematic of the test structure and measurement.

Fig. 2. Definition of three electric field regions based on the leakage current –voltage (I–V) characteristic.

Fig. 3. Data retention characteristics with varying the charges in floating gate underroom temperature.

Fig. 4. (a) The selected leakage current of ONO stack and total measured leakagecurrent (inset) with increasing the positive bias sweep ranges and (b) the predictedleakage current density at low electric field region.

P. Moon et al. / Microelectronics Reliability 53 (2013) 1338–1341 1339

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ative trapped charges, while the leakage currents at EL and EM wereincreased as VS was increased due to the detrapped charges fromthe ONO stack. However, the significant leakage current incrementat EM cannot be shown under VS of 8.44 MV/cm. To investigate theeffect of increasing VS on leakage current at EL, the leakage currentat EM was fitted using the exponential function as the followingequation for describing the trap-assisted tunneling (TAT) mecha-nism [13]:

lnðJÞ ¼ aEþ b ð1Þ

where J and E are current density and electric field of ONO stack,respectively, and a and b are fitting parameters, and the resultswere illustrated in Fig. 4(b). The predicted leakage currents atEL were increased when VS was increased from 8.44 to12.66 MV/cm. In other words, DVth of PV3 was larger than DVth

of PV1 because the applied potential on the ONO stack of PV3 islarger than that of PV1, which resulted in the increase of leakagecurrent [14].

Fig. 5(a) shows the leakage current of the ONO stack withdecreasing VS from 12.66 to 10.13 MV/cm with a step of0.84 MV/cm. The J–E curves of EH were shift toward the negativedirection but the shift was almost unchanged comparing withthe increasing VS case shown in Fig. 5(a). However, the leakage cur-rents at EM were drastically reduced with the decrease of VS. Thechanges of leakage current at EM were also fitted using the expo-nential function and the predicted leakage currents were shownin Fig. 5(b). The predicted leakage current at EL were decreased,especially the leakage current was reduced with three orders at3 MV/cm as shown in Table 1. In addition, the leakage currentwas also increased if VS was increased again (not shown in thispaper).

Based on the investigation, the methodology to improve theretention characteristic of memory cell is proposed. Since thedegradation of the retention characteristic of PV3 was due tothe large leakage current of the last applied large electric field,if the additional voltage pulse, which is at least above 80% ofthe amplitude of the last programming pulse, is applied to thecell after the end of PV3 programming like Fig. 6(b), then theleakage current at EL can be reduced significantly as shown inFig. 5. In other words, if the maximum electric field is appliedduring the ISPP, J–E curves can be fully reproducible so thatthe leakage currents of EL and EM are increased. Thus, the reten-

Fig. 5. (a) The measured leakage current density of ONO stack with decreasing thepositive bias sweep ranges and (b) the predicted leakage current density at lowelectric field region.

Table 1The predicted leakage current densities of ONO stack at low electric field region.

Positive bias sweep range(VS, MV/cm)

Predicted leakage current(A/cm) (@ 0 MV/cm)

Predicted leakage current(A/cm) (@ 1 MV/cm)

Predicted leakage current(A/cm) (@ 2 MV/cm)

Predicted leakage current(A/cm) (@ 3 MV/cm)

Increasing bias sweep range 8.44 7.114 � 10�18 1.074 � 10�16 1.621 � 10�15 2.447 � 10�14

9.28 2.487 � 10�13 8.204 � 10�13 2.707 � 10�12 8.929 � 10�12

10.13 4.227 � 10�13 1.339 � 10�12 4.242 � 10�12 1.344 � 10�11

10.97 7.171 � 10�13 2.158 � 10�12 6.495 � 10�12 1.955 � 10�11

11.81 7.469 � 10�12 1.682 � 10�11 3.786 � 10�11 8.525 � 10�11

12.66 1.376 � 10�11 2.920 � 10�11 6.197 � 10�11 1.315 � 10�10

Decreasing bias sweep range 11.81(�) 5.258 � 10�12 1.137 � 10�11 2.458 � 10�11 5.313 � 10�11

10.97(�) 6.703 � 10�14 2.343 � 10�13 8.190 � 10�13 2.862 � 10�12

10.13(�) 2.271 � 10�16 1.502 � 10�15 9.939 � 10�15 6.575 � 10�14

Fig. 6. Schematics of (a) general ISPP and (b) proposed ISPP to improve theretention characteristic.

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tion characteristic of the cell can be improved by the proposedadditional voltage pulse.

4. Conclusion

The leakage currents of ONO stack were measured and fittedusing exponential function as varying the bias sweep ranges to ver-ify the correlation the leakage current with applied electric field.When the bias sweep ranges were increased (i.e., last applied fieldwas increased), the estimated leakage current of the ONO stack atlow electric field was increased. However, as the bias sweep rangeswere then sequentially decreased, the predicted leakage current atlow electric field was significantly reduced. It is concluded that theleakage current at low electric field was determined by the last ap-plied electric field of the ONO stack. Based on the investigation, ifthe lower voltage pulse compared with the last applied voltage isadded and applied after the end of the cell programming, theretention characteristic of the cell can be improved and it is re-mained as a future work.

Acknowledgement

This work was supported as a research project of SK hynixSemiconductor, Inc.

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