Estimates of EEPROM Device Lifetime

TSINGHUA SCIENCE AND TECHNOLOGY ISSNll1007-0214ll09/15llpp170-174 Volume 16, Number 2, April 2011

Estimates of EEPROM Device Lifetime* LI Leilei (李蕾蕾)1,2, YU Zongguang (于宗光)1,2,**, HAO Yue (郝 跃)1 1. School of Microelectronics, Xidian University, Xi’an, 710071, China; 2. 58th Research Institute, China Electronics Technology Group Corporation, Wuxi 214035, China Abstract: A method was developed to estimate EEPROM device life based on the consistency for breakdown charge, QBD, for constant voltage time dependent dielectric breakdown (TDDB) and constant current TDDB stress tests. Although an EEPROM works with a constant voltage, QBD for the tunnel oxide can be extracted using a constant current TDDB. Once the charge through the tunnel oxide, ΔQFG, is measured, the lower limit of the EEPROM life can be related to QBD/ΔQFG. The method is reached by erase/write cycle tests on an EEPROM. Key words: electrically erasable programmable read-only memory (EEPROM); time dependent dielectric breakdown (TDDB); breakdown charge

Introduction In recent years, the development of aerospace and military equipment has required higher reliability microelectronic devices and circuits. Many have studied common electrically erasable programmable readonly memory (EEPROM)[1-8]. Now, more studies are directed toward improving the reliability and estimating the life of EEPROM devices to enlarge their applications. Production lines often use high temperatures to accelerate lifetime studies with the Arrhenius model used to estimate the device life. However, this method requires many samples, and some have doubted the veracity of the model and have shown that the active energy, Ea , changes with temperature[9]. Since the Arrhenius model is based on the hypothesis that Ea does not change with temperature[10], then the Arrhenius model result is not accurate. And, the Arrhenius Received: 2011-01-26; revised: 2011-02-28

* Supported the State Important Sci-Tech Special Projects (2009ZX02306-04)

** To whom correspondence should be addressed. E-mail: [email protected]; Tel: 86-510-85866103

method needs many sample tests and information. An easier method to estimate the EEPROM lifetime would be useful. This paper presents a low cost method to estimate the EEPROM lifetimes based on estimates of the minimum write/erase (W/E) cycles for an EEPROM device.

1

Preliminaries

1.1

EEPROM

The EEPROM, electrically erasable programmable read-only memory[1], is a special type of programmable read-only memory (PROM) that can be erased by exposing it to an electrical charge. Like other types of PROM, EEPROM retains its contents even when the power is turned off. There are several kinds of EEPROMs, with the EEPROMs studied here being a floating gate tunnel oxide (FLOTOX) EEPROM, which has a control gate to control the device write and erase and a floating gate to store the signal charges[4]. The electrons tunnel from the floating gate towards the oxide layer separating the floating gate and the control gate to change a bit from 1 to 0. To erase the EEPROM programming, the electron barrier still has

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LI Leilei (李蕾蕾) et al.：Estimates of EEPROM Device Lifetime

to be overcome by a sufficiently charge programming voltage[1]. The FLOTOX EEPROM chip is physically similar to other EPROM chips. The chip is composed of cells with two transistors with the floating gate separated from the control gate by a thin oxide layer. However, the EEPROM chip’s oxide layer is much thinner than in an EPROM chip. In FLOTOX EEPROM chips, the insulating layer is only about 1 nm thick whereas in EPROM chips, the oxide layer is about 3 nm thick. The thinner oxide layer means a lower voltage is required to initiate changes in the cell value[5]. The FLOTOX EEPROM can be reprogrammed, but only a limited number of times. Thus FLOTOX EEPROM chips are popular for storing configuration data such as a computer’s BIOS code which does not require frequent reprogramming. Then, the oxide insulating layer can be damaged by frequent rewrites and re-erases[5]. Thus in the EEPROM lifetime the thin tunnel oxide is of interest. 1.2

TDDB

Oxide breakdown has always been a serious reliability concern in the semiconductor industry because of the continuous movement towards smaller and smaller devices. As other features of the device are scaled down, the oxide thickness has also been reduced. The oxide layers then become more vulnerable to the voltages fed into the device as they get thinner. The thinnest oxide layers today are already less than 50 angstroms. The oxide layer instantaneously breaks down at 8-11 mV per cm of thickness, or 0.08-0.11 V per angstrom of thickness[5]. Oxide breakdowns can be classified as: (1) EOS/ESD-induced dielectric breakdown, (2) early-life dielectric breakdown, (3) time-dependent dielectric breakdown (TDDB)[11]. TDDB involves breakdowns that occur after a very long time of use (mainly in the ‘worn-out’ stage), due to destruction of the oxide layer even with a normal bias and operation. TDDB is primarily due to the presence of weak spots within the oxide layer arising from poor processing or uneven growth. Previous studies have shown that SiO2 TDDB is a charge injection mechanism which may be divided into the build-up stage and the runaway stage.

(1) Build-up stage During the build-up stage, charges invariably get trapped in various parts of the oxide as current flows in the oxide. The trapped charges increase in number with time, forming strong electric fields (electric field = voltage/oxide thickness) and high current regions along the way. This process of electric field build-up continues until the runaway stage is reached. (2) Runaway stage During the runaway stage, the sum of the electric field built up by the charge injection and the electric fields applied to the device exceeds the dielectric breakdown threshold in some of the weakest points of the dielectric. These points start conducting large currents that further heat up the dielectric, which further increases the current flow. This positive feedback loop eventually results in electrical and thermal runaway, destroying the oxide. The runaway stage is very fast. The presence of defects in the dielectric greatly reduces the time needed to transition from the build-up to the runaway stage. Numerous studies have shown that oxide breakdown is accelerated by not just the voltage applied across the oxide, but also by elevated temperatures[5,11]. Thus, the tendency of a lot of devices to fail by oxide breakdown is usually assessed by burn-in tests, which subjects the samples to both electrical and thermal stresses. 1.3

QBD

QBD is the term applied to the charge-to-breakdown measurement of a semiconductor device. This is a standard destructive test method used to determine the quality of gate oxides in MOS devices[5]. QBD is equal to the total accumulated charge passing through the dielectric layer just before failure. Thus, QBD is a measure of the time-dependent gate oxide breakdown[8,11]. As a measure of oxide quality, QBD can also be a useful predictor of product reliability for specified electrical stress conditions. A voltage is applied to the MOS structure to force a controlled current through the oxide, i.e., to inject a controlled amount of charge into the dielectric layer. The charge needed to break down the gate oxide is determined by integrating the injected current over time until the measured voltage drops towards zero (when electrical breakdown occurs).

172

2

EEPROM Lifetime Theory

The TDDB test is one of the most important methods to evaluate a thin gate dielectric[11]. Tunneling electrons induce interface states and traps in the oxidation layer or near the interface until the trap density is higher than the critical value QBD , when breakdown occurs[11]. Thus, QBD represents the dielectric quality that is affected by the current density, electric field intensity and oxidation area[11]. Early inactivation tests used TDDB to evaluate the effects of oxidation, nitration, cleaning and etching on thin gate dielectrics as one of the most important methods to evaluate silicon die reliability and to estimate the number of write/erase cycles for an EEPROM to evaluate producter defects[12]. Common TDDB tests include constant voltage and constant current tests[11]. The constant current TDDB tests are most common, because the break-down times, tBD , are 101-102 s while the constant voltage TDDB tests have very long breakdown times of 103-105 s, and more than 107 s at low field strenths[13]. If the critical trap density, N BD , leading to destructive oxidation breakdown can be assumed to be independent of stress (for either constant voltage or constant current stress), then the formula below exits[14]. tBD J (t ) ∫ 0 QBD ( J (t )) dt = 1. QBD ( J = con) = QBD ( E = con) (1) Also, Here, tBD is the dielectric break-down time obtained in a constant voltage TDDB test, J(t) is the current density through the sample and QBD (J(t)) is QBD obtained with a constant current density J 0 = J (t ). Thus the constant voltage and constant current TDDB critical QBD are approximately the same and the critical trap densities causing device breakdown and the fault principles are similar for constant voltage and constant current tests. Although EEPROM devices work at constant voltage, the fewest number of write/erase cycles for an EEPROM and the shortest lifetime are obtained using constant current TDDB tests. Tunnel oxide as the weakness in an EEPROM device decides the shortest life of an EEPROM[15]. EEPROMs are written and erased at high voltages but read under low voltages, so the current during reading is much less than during

Tsinghua Science and Technology, April 2011, 16(2): 170-174

writing by a factor of about 10−9, so the effect of ‘read’ operations on the EEPROM lifetime can be neglected. The voltage on the floating gate (FG), VFG , is measured by estimates of the EEPROM lifetime using constant current TDDB tests. For the EEPROM schematic shown in Fig. 1 and the simplified equivalent circuit diagram shown in Fig. 2, VFG can be defined as[16] VFN = α DVD + α SVS + α BVB + α CGVCG + Q ΔQ α tunVtun + FG,initial + FG (2) CT CT where CT = CPP + CGS + COX + CGD + Ctun , α i = Ci / CT (i = D, S, B, CG , tun), Vtun = φS + VD and QFG,initial is the original electron charge in FG. φS is the barrier height and ΔQFG is the change in the stored electron charge in a write/erase cycle.

Fig. 1

EEPROM configuration

Fig. 2 Equivalent EEPROM circuit diagram

During a write cycle, VS = VD = VB = 0 and φS ≈ 0 and the charge accumulates in the drain. Equation (1) can be then written as Q ΔQFG VFN = α CGVCG + FG,initial + (3) CT CT If QFG,initial = 0 in Eq. (3), then

VFN = α CGVCG + and

α CG =

CPP + CGS

ΔQFG CT

CPP + Cox + CGD + Ctun

(4)

(5)

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LI Leilei (李蕾蕾) et al.：Estimates of EEPROM Device Lifetime

α CG can be obtained from the capacitor area, the dielectric thickness and the dielectric constant. The actual VFG for the EEPROM FG can then be determined along and with the mean electric charge, ΔQFG , in the tunnel oxide at VFG , the lower limit number of write/erase cycles, N we , is then (6) N we = QBD / ΔQFG The relationship between the breakdown charge and the capacitor area is then[17] QBD ( S ) = β S − b J − c (7) where QBD is the breakdown charge (C/cm2), S is the capacitor area, and J is the stress current density (A/cm2).

3

Tests and Results

The tunnel oxide layer thicknesses for the samples used in these tests were 8 nm. The dependence of QBD on S for J = 0.1 A/cm 2 is shown in Fig. 3. The relationship between QBD ( S , J ) and the current stress is shown in Fig. 4. The key parameters for both the positive and negative gate bias tests are listed in Table 1. β is strongly affected by the oxide characteristics, such as production conditions and gate material resulting in different b and c.

Table 1 Fit parameters from Eq. (6) for positive and negative gate biases

β

b

c

Vgate<0

2.46

0.09

0.26

Vgate>0

0.87

0.16

0.24

The VT characteristics of the tunnel oxide with an area of 20 000 μm2 were also measured in constant current stress tests. The current density ‘J ’ through the tunnel was kept constant at 0.1 A/cm2 by increasing the voltage. The tunnel oxide broke down at 86 s with QBD = 8.6 C/cm 2. From Eq. (7), QBD (2.56 μm 2 ) = 36.1 C/cm2. The main parameters of the samples are listed in Table 2. The write process was simulated by a voltage single rising edge pulse of 5 ms, VCG = 18 V and tW = 10 ms. Then α CG = 0.6, VCG = 18 V and VFG = 10.8 V. The current density variation shown in Fig. 5 gives QBD = 2.17 × 104 C/cm2. Thus, N we = 137 K from Eq. (6). Table 2

Device parameters

Parameter

Value 8 nm

Tunnel oxide thickness

2.56 μm2

Tunnel oxide area Superposition area of FG and CG

44 μm2

Distance from tunnel hole to active edge

0.8 μm

Channel length

3.2 μm

Channel width

3.0 μm

Fig. 3 Variation of QBD with capacitor area for constant current test with positive and negative gate biases

Fig. 5

Fig. 4 Variation of QBD for charge injection with positive and negative gate biases

Behavior of tunnel current

The reliability of this estimate was compared to write/erase tests on identical samples. The tests show that the threshold voltage window begins to narrow after about 104 cycles and closes at about 105 cycles as shown in Fig. 6. Thus, the estimate agrees well with the actual test results, which illustrates that the constant current TDDB test can effectively estimate the EEPROM devices lifetime.

Tsinghua Science and Technology, April 2011, 16(2): 170-174

174

of EEPROM with the FLOTOX structure. Journal of Xidian University, 2004, 31(2): 174-181. (in Chinese) [6] Dimitris E I, Franklin L D, Shankar P S, et al. Opposite-channel-based injection of hot-carriers in SOI MOSFET’s: Physics and applications. IEEE Transactions on Electron Devices,1998, 45(5): 1147-1154. [7] Yu Z G, Lu F, Xu Z, et al. The study for charge leakage on the floating-gate of FLOTOX EEPROM. ACTA Electronica Sinica, 2000, 28(5): 90-95. (in Chinese) [8] Li L L, Liu H X, Yu Z G, et al. Degradation of tunnel oxide Fig. 6

4

Endurance test EEPROM

Conclusions

A simple method was developed to estimate EEPROM lifetime based on the consistency of the breakdown charge QBD for constant voltage and constant current TDDB tests. The results show that QBD for the tunnel oxide can be extracted from constant current TDDB tests. Once the charge through the tunnel oxide, ΔQFG , is known, the lower limit of the EEPROM lifetime can be calculated as QBD / ΔQFG . The result agrees well with erase/write cycle tests. Acknowledgements

in E2PROM under constant current stress. ACTA Physica Sinica, 2006, 55(5): 2459-2463. (in Chinese) [9] De Salvo B, Ghibaudo G, Pananakakis G, et al. Experimental and theoretical investigation of nonvolatile memory data-retention. IEEE Transactions on Electron Devices, 1999, 46(7): 1518-1524. [10] Gao D Y, He B, He S W, et al. Discussion on limitations of the Arrhenius methodology. Chinese Journal of Energetic Materials, 2006, 14(2): 132-135. (in Chinese) [11] Liu H X, Hao Y. TDDB effect for thin gate dielectric. Journal of Semiconductors, 2001, 22(12): 1592-1595. (in Chinese) [12] Hu H S, Zhang M, Lin L J. Thin medium estimation by TDDB breakdown character. Acta Elctronica Sinica, 2000, 28(5): 80-83. (in Chinese)

The authors thank Dr. ZHOU Xinjie, Ms. LI Hongmei and Ms.

[13] Zhao Y, Wan X G, Xu X M. One method for fast gate oxide

ZHAO Ying for their comments and help to improve the paper.

TDDB lifetime prediction. Chinese Journal of Semicon-

References

ductors , 2005, 26(12): 2271-2274. [14] Koki E, Tukiko K. Correlation between two time-depend-

[1] Huang F H, Zhen G X, Wu R. Simulation and verification

ent dielectric breakdown measurements for the gate oxides

of characteristics for two-polysilicon flotox EEPROM cell.

damaged by plasma processing. IEEE Transactions on

Chinese Journal of Semiconductors, 2003, 24(6): 637-642.

Electron Devices, 1998, 45(1): 160.

(in Chinese) [2] Ren T, Liu Z H, Zhu J. Single poly EEPROM. Micronanoelectronic Technology, 2003, 8: 552-556. (in Chinese)

[15] Shi B H, Jia X Z, Zhang D S. Reliability of Micro-Electronic Devices. Xi’an: Xidian University Publishing Company, 1999. (in Chinese)

[3] Gu L, Shao B X. Threshold voltage of flotox MOSFET in

[16] Yu Z G, Hao Y. Electrically Re-Programmable Nonvolatile

EEPROM. Research and Progress of SSE, 1997, 17(2):

Memory. Beijing: National Defence Industry Publishing

121-126. (in Chinese)

Company, 2002. (in Chinese)

[4] Yu Z G, Xu Z, Ye S Y, et al. The research of trapped

[17] Liu H X, Hao Y. Experimental study of thin gate-oxide

charges in FLOTOX EEPROM tunnel oxide during

breakdown character. Chinese Journal of Semiconductors,

erase/write cycles. ACTA. Electronica Sinica, 2000, 28(5):

2000, 21(2): 146-150. (in Chinese)

68-71. (in Chinese) [5] Luo H W, Yang Y T, Zhu Z M, et al. The reliability study