FLASH memories are the most important of nowadays nonvolatile

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1 2912 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 5, OCTOBER 2004 Charge Loss After 60 Co Irradiation of Flash Arrays G. Cellere, Member, IEEE, A. Paccagnella, Member, IEEE, S. Lora, A. Pozza, G. Tao, Member, IEEE, and A. Scarpa Abstract Flash memories are the most important among modern nonvolatile memory technologies. We are showing new results on the threshold voltage shifts in Flash memory arrays after 60 Co irradiation. A (relatively) high total dose, exceeding 100 krad (SiO 2 ), is needed to induce errors in the array, but threshold voltage shifts are all but negligible even at lower doses. These shifts can be accurately described by using a model which considers the charges generated by irradiation in all the oxides surrounding the floating gate. We are also showing that cycling (endurance) and total ionizing dose effects mutually add. Index Terms Flash memories, radiation effects, semiconductor devices radiation effects, total ionizing dose. I. INTRODUCTION FLASH memories are the most important of nowadays nonvolatile memory technologies [1], [2], featuring extremely high density, good retention properties, and letting the user to electrically write or erase the single bit inside the array [1], [3]. Finally, read and write operations are quite fast, even if not as good as in competing but less mature technologies [1]. All these reasons make Flash memories of great interest for the space and satellite industry. Unfortunately, Flash memories are also a very complex technology, featuring complicated control circuitry [1]. The latter has been demonstrated to be the most vulnerable part of commercial devices during both total ionizing dose (TID) [4] [7] and single event effect (SEE) [7] [10] experiments. In particular, the most radiation sensitive part of commercial Flash memories are the finite state machine used to control the circuit, the output buffers, and the charge pumps used to obtain the high voltages needed for programming and erasing. Because of these results and of the device complexity, the memory array itself has seldom received the attention of the scientific community. In particular, a model was developed by Snyder, et al. [11] to account for charge loss from charged floating gates (FGs) after Co irradiation. The model considers three contributions. The first one is the charge escaping form the FG because of the photoelectric effect (electrons gain enough energy from impinging radiation to jump over the oxide barrier). The other contributions derive from the charge generated in the oxides and surviving recombination. While electrons are quickly swept away, holes will drift toward the FG: part of them will reach it, thus recombining stored electrons (second contribution), while part Manuscript received September 15, 2003; revised February 20, G. Cellere, A. Paccagnella, and A. Pozza are with the Department of Information Engineering (DEI), Padova University, Italy ( giorgio. cellere@ieee.org, paccag@dei.unipd.it). G. Cellere and A. Paccagnella are also with Istituto Nazionale di Fisica Nucleare (INFN), Padova, Italy. S. Lora is with ISOF-CNR, Legnaro, Padova, Italy ( lora@lnl.infn.it). G. Tao and A. Scarpa are with Philips Semiconductor, Nijmegen, The Netherlands ( gioqiao.tao@philips.com; andrea.scarpa@philips.com). Digital Object Identifier /TNS will be trapped in the oxide, resulting in a reduction of threshold voltage (third contribution). Being developed to describe Co irradiation effects, this model did not work to describe the effect of heavy ion irradiation on charged FGs [9], [10]. Its usefulness to describe TID effects in modern technologies is not straightforward. For example, border effects negligible in 47.5-nm-thick gate oxides, such as those used in [11], can be of primary importance in modern technologies, featuring 10 nm (or less) thick tunnel oxides. Further, lateral dimensions of the FG cannot be neglected for a modern technology (but this approximation was acceptable in [11]), and newer devices feature complex sandwiches of SiO and nitride and interpoly (floating gate-to-control gate, FG-to-CG) dielectric. In this paper, we are showing for the first time results of Co irradiation on a modern Flash technology, featuring 8.3 nm tunnel oxide. We are showing that the basic ides beyond Snyder s model are still good for these devices, and we are discussing the mutual effect of irradiation and electrical stress on memories. Thanks to the large statistical set (millions of cells are comprised in a Flash device), we are also giving new insights on the generation and recombination kinetics in oxide-nitride-oxide (ONO) multilayers. II. EXPERIMENTAL AND DEVICES Used devices are 10 Flash memory arrays (2.7 Mbit) manufactured by Philips (Nijmegen, NL), each divided into six blocks. These test (e.g., noncommercial) devices have on-chip decoders, but no sense amplifiers. Therefore, the address selection is on the chip, but no 0/1 output is available. The analog measurements of current and voltages (as well as programming and erasing) are done off-chip with a 1-mV error by using proprietary external equipment at the Philips site. In detail, the of a cell is determined by keeping constant the drain voltage, and sweeping the control gate voltage, until the bit line current reaches a chosen value. In this technology, each Flash memory cell consists of two transistors; one is used to access the cell while the other is a FG transistor, used to store the data. The tunnel oxide thickness is 8.3 nm, while the oxide used as interpoly (that is, separating the FG from the Control Gate, CG) is a sandwich of SiO /Si N /SiO, each 6 nm thick. The total cell area is about 0.6 m and the gate area is about 0.04 m. The cell can be programmed and erased by forcing electrons or holes into the FG, by means of Fowler-Nordheim tunneling through the tunnel oxide. In the 0 state there is an excess of electrons in the FG, so the transistor is high, above 1.7 V. In the 1 state there are holes in the FG, therefore, is lower than 1 V. Irradiations were performed at the ISOF-CNR Co source at LNL-INFN, Padova. Dose rate was kept constant at about /04$ IEEE

2 CELLERE et al.: CHARGE LOSS AFTER Co IRRADIATION 2913 Fig. 1. V cumulative probability plots for 0 and 1 programmed sectors after TID [fresh, 9 krad (SiO ), 27 krad (SiO ), 90 krad (SiO ), 270 krad (SiO ), 900 krad (SiO )]. Fig. 2. Average value of V distribution as a function of total dose, for 0 and 1 programmed sectors. Lines are exponential fits. 12 rad (Si)/s for all irradiations. Devices were kept unbiased during irradiation; nevertheless, a non null electric field was applied across the tunnel oxide because of the charged FG. They were measured a couple of days after irradiation. III. CHARGE LOSS FROM FG: DESCRIPTION AND MODELING In Fig. 1, we show the cumulative probability distributions of before and after irradiation for sectors programmed in the 0 and 1 state after different total ionizing dose (TID) levels (fresh, 9 krad (SiO ), 27 krad (SiO ), 90 krad (SiO ), 270 krad (SiO ), 900 krad (SiO )). Let us consider at first data for devices in the 0 state (high ). As expected, with increasing dose the distributions shift to the left. In fact, electrons and holes are generated in the tunnel and in the interpoly oxide because of irradiation. Part of them will recombine quickly [12], [13]. The remaining electrons will be swept away toward the substrate or the CG, while surviving holes will either reach the FG, where they will recombine part of the stored charge, or will be trapped in the oxide [11]. The net result is the reduction of FG, moving toward the intrinsic (that is, of the FG transistor with no charge trapped in either the tunnel oxide or the FG). A further reduction can be determined by electrons escaping the FG because of energy loss by photons (as happens for UV irradiation used to erase EPROM) [11], but this mechanism has been neglected in the current version of the model. This same description holds true for the devices in the 1 state, apart from reversed oxide field and carrier role (electrons reach the FG, not holes), since in this technology the two logical states are almost symmetrical with respect to the intrinsic FG condition. This is confirmed by Fig. 1, since after 900 krad (SiO ) charge accumulated in the FG is almost null for both logical states. Note that the lower part of the 0 distribution approaches the 1.7 V limit after 90 krad (SiO ), that is, failures in the array are possible (even if not sure) after this dose has been exceeded. For the upper part of the 1 distribution, a larger dose [exceeding 1 Mrad (SiO )] is needed to reach the upper limit (1 V). However, the reduction in the size of the window separating the 1 from the 0 cells leads to reduced noise margin, enhancing the probability of errors during operation, the effect of anomalous SILC [14], and so on. Also note that the slope of the distribution increases for irradiated samples (and with TID). This was Fig. 3. Calculated (lines) and experimental (point) number of electrons (holes) lost from FG as a function of dose. Closed symbols: 0, open symbols = 1, error bars = distributions 0.1 and 99.9% values. See text for details on calculations. expected since in FGs with larger (either positive or negative) the electric field in the oxide is larger, leading to enhanced charge yield, hence to enhanced charge loss [12], [15]. In Fig. 2, we report the average (50%) value of the distributions as a function of total dose. For both 1 and 0 programmed devices, the shift is fitted very well by an exponential law. This fact is important since it is in full agreement with what expected after the model by Snyder [11]. This model can be verified by comparing the number of generated carriers to the charge loss from experimental data, as a function of total dose. This is the subject of Fig. 3, where points are the average values of distributions reported in Fig. 1, and error bars corresponds to the 0.1% and 99.9% extent of distributions. Closed symbols are for 1 programmed sectors, and open symbols are for 0 programmed sectors. All lines reported in this figure are from rather complex calculations, which will be briefly described in the following. A schematic view of a basic FG transistor is reported in Fig. 4 to help the reader. The first characteristic to focus on is the thin continuous line, corresponding to charge loss from electrons or holes generated in the tunnel oxide only.asafirst step, we calculated the energy deposed by radiation in the tunnel oxide. An electron/hole pair is then generated for each ev deposited in the oxide [21]. At our oxide field (evaluated around 1.5 MV/cm in the tunnel oxide, for both 0 and 1 ) about 85% of these carriers will survive and

3 2914 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 5, OCTOBER 2004 Fig. 4. Schematic view of a Flash cell (out of scale). STI = Shallow Trench Isolation. See text for details. reach the FG [13], as depicted by the characteristic (we will discuss later the charge trapped in the oxide). It is evident from Fig. 3 that the charge loss derived from this calculation is by far lower than that found experimentally. Consider also that this plot is by itself an overestimation of the actual number of holes surviving recombination, since, as soon as the first holes reach the FG, the oxide field becomes lower (the oxide field was kept constant in our calculations). The remaining charge loss should derive from electron/hole pairs generated in the ONO multilayer. Charge generation happens only in the two SiO layers, since in the nitride layer the yield is almost zero, thanks to a very high recombination rate [16], [17]. Therefore, we should account for two contributions. The first one is the charge generated in the bottom SiO layer. About 75% of carriers generated in his region will survive and will be injected in the FG. This contribution is depicted by the dotted line in Fig. 3, and is larger than that from the tunnel oxide. The second contribution is the charge generated in the top SiO layer. Thanks to the reduced barrier height of the nitride in comparison to SiO, almost all carriers surviving the recombination (either holes in the 1 state or electrons in the 0 state), will be injected in the nitride and trapped at the SiO /Si N interface ( in Figs. 3 4) [17], [18]. However, if a given number of holes (electrons) are injected from top SiO, that same number of electrons (holes) will be injected from the bottom SiO (in fact, the two layers feature the same thickness and electric field). These carriers can either recombine or be trapped. In both cases, their effect will obviously be negligible. We can evaluate a worst-case scenario: all holes (electrons) generated in the top oxide are injected in the nitride and trapped at the bottom SiO /Si N interface, while no hole (electron) is injected from the bottom oxide. The effect of this sheet charge distribution on apparent charge loss from FG is depicted by the characteristic in Fig. 3, and is negligible in comparison to and (the Y-axis scale in logarithmic). The fourth and most important characteristic reported in Fig. 3 is the thick continuous line, corresponding to the sum of and contributions described earlier. This curve is in extremely good accordance with experimental data for low doses, while it slightly overestimates the charge loss at high doses. This can be the effect of several phenomena, very difficult to accurately model. Among them: Fig. 5. cumulative probability distribution of threshold voltage after 2.7 Mrad (SiO ) as a function of the number of W/E cycles done before irradiation. i) The oxide field decreases during irradiation, thanks to the reduced stored charge. Therefore, charge yield will decrease too: by considering it constant, we are overestimating charge yield (especially at high doses). ii) Border effects, negligible in thick oxides [11], [16] can play an important role in thinner oxides, such as the ones we are dealing with [19]. iii) Some charge loss may locally happen in the high fringing field regions in the shallow trench isolation (STI). iv) Holes and electrons behavior in the oxide is all but equal: both mobility and trapping properties strongly differ. However, no evidence of any asymmetry between the 0 state (holes trapped in the FG, electrons across the oxide) and the 1 (electrons trapped in the FG, holes across the oxide) case can be seen from our data. v) Charge loss because of interaction between stored carriers and photons (which is probably a secondary effect in these conditions) has been neglected. IV. MUTUAL EFFECTS OF CYCLING AND IRRADIATION Among these problems, the effect of holes and electron trapping in the oxide can be discussed looking at the mutual effect of endurance test, that is, performance during write/erase (W/E) cycling, and irradiation. In Fig. 5, we report the distributions of six different sectors from one device. Three couples of sectors were subjected to 20, , and W/E cycles before irradiation. After this step and before irradiation, a sector from each couple was programmed at 0, and the other was programmed at 1. Prerad distributions are shown for two sectors only for clarity (distributions were almost identical among sectors programmed in the same state). The distributions after 2.7 Mrad (SiO ) are also reported. Total dose is in this case quite high; this allows us to fully discharge the FG. As expected, the distributions for the erased and programmed sectors of each couple are very similar (practically superimposed for two couples, and very close for the third). Interestingly, after irradiation the distributions shift toward right when increasing the number of W/E cycles before irradiation. This is related to buildup of defects in the oxide because of the electrical stress. These defects, likely E centers,

4 CELLERE et al.: CHARGE LOSS AFTER Co IRRADIATION 2915 for the 0 state overlap is once again around cycles. This seemingly allows saying that endurance performances are not strongly degraded by irradiation. However, the effect of negative charge trapping is evident in the growing (which rises more steeply for the irradiated sample). This fact may result in more probable error when reading 1 programmed cells, which can exceed the 1-V threshold after a lower W/E cycle count. As previously discussed, this effect can be explained by considering E centers generated by the radiation, which adds to those generated by the electrical stress during W/E cycles, resulting in increased negative charge trapping after cycling in irradiated samples. Fig. 6. Endurance performance for 0 and 1 programmed sectors, nonirradiated device. Values corresponding to the 0.01, 50, 99.99% are reported for both the 1 and 0 programmed sectors. Fig. 7. Endurance performance for 0 and 1 programmed sectors, after 2.7 Mrad (SiO ). Values corresponding to the 0.01, 50, 99.99% are reported for both the 1 and 0 programmed sectors. are then occupied by electrons generated during irradiation [20]. The larger the number of W/E cycles, the larger the number of electrically active defects, and finally, the larger the distribution shift. Also note that from these data the effect of charge trapping is by far lower that than of charge generation. In Figs. 6 and 7, we face the same problem from a different perspective, that is, what happens during endurance experiments in irradiated devices. Fig. 6 reports the typical endurance performance for an unirradiated sample. The evolution of the average (50%) value of the threshold voltage distribution is reported as a function of the number of cycles, for a sector programmed at 0 and for a sector programmed to 1. Further, we also show the values corresponding to the and 0.01% of the distribution; these extreme values represent respectively the low- and high- tails of the distribution, and are of primary importance when applied to a large array (0.01% of cells means more than 3000 cells in a 32 Mbit commercial device!). The 99.99% curve for the 1 state overlap the 0.01% curve of the 0 state after about cycles, that is, almost 100 times more than required for typical reliability projection ( W/E cycles are usually guaranteed for Flash memories). The same graph is reported after 270 krad (SiO ) irradiation in Fig. 7. By comparing the two figures, it is evident that the distributions are not broader for the irradiated sample and that the number of cycles after which the 99.99% curve for the 1 state and the 0.01% curve V. DISCUSSION AND CONCLUSION We have shown, for the very first time, results on the threshold voltage shifts in modern Flash memory arrays after Co irradiation. A total dose in excess of 100 krad (SiO ) is needed to induce errors in the array. Note that this appears somewhat larger than doses typically found for malfunctioning or errors in the control circuitry of similar devices [4], [5] thus explaining why the control circuitry is usually considered the most radiation sensitive part of nonvolatile memories. While this is for sure true, our data show that it is only a partial view of the picture. In fact, corruption of the stored information may endanger the actual reliability of the device under operating conditions well before an error in actually detected. For example, the reduction of the 0 -to- 1 window leads to a reduction of the noise margin. Further, we have shown that cycling (endurance) and TID effects mutually add: irradiation on cycled devices result in enhanced charge trapping, and irradiation leads to a latent damage which is put in evidence by cycling (that is, by electrical stress). In other words, a device subjected to ionizing radiation will be more prone to effects such as the erratic SILC [14]. All these considerations prove that hardening the control circuitry alone might not be enough to ensure proper operation of Flash devices in a radiation environment. The shifts can be accurately described by using an improved model based on that proposed in [11]. The validity of the previous model was not to be taken for granted, due to the technological evolutions of last 20 yr. Among the differences between modern technologies and that used in [11], we underline the reduced FG size, which leads to a lateral dimension of the FG comparable to the planar one. Vertical FG dimension was neglected in [11], where the FGs were some micrometer wide. Further, we have proved that the use of the oxide-nitride-oxide as interpoly dielectric has important implications on the array reliability under ionizing radiation. In fact, only charge generated in the bottom SiO layer of the ONO, that is, in the one closer to the FG, leads to actual charge loss from FG. This could turn into a design issue: once established a good retention performance, a designer could keep as thin as possible the bottom ONO SiO layer (increasing the thickness of the top SiO ), thus increasing the radiation hardness of the device. Two effects considered of primary importance in [11] were neglected in present paper. The first one is charge trapping in tunnel oxide. Given the small thickness of our oxides, this effect appears negligible, and we do not expect it to play a major role

5 2916 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 5, OCTOBER 2004 in future FG technologies. The second and more important one is the direct interactions between charge stored in the FG and impinging radiation. From our data, this phenomenon appears negligible in comparison to phenomena taking place in the oxides. However, further experiments involving other technologies and a larger set of experimental data are needed to definitely assess this point. The electron emission mechanism could, therefore, be an important issue to be added to our model in future. REFERENCES [1] P. Cappelletti, C. Golla, P. Olivo, and E. Zanoni, Flash Memories. Boston, MA: Kluver Academic, [2] S. Lai, Flash memories: Where we were and where we are going, in IEDM Tech. Dig., 1998, p [3] P. Pavan, R. Bez, P. Olivo, and E. Zanoni, Flash memory cells An overview, Proc. IEEE, vol. 85, pp , Aug [4] D. N. Nguyen, C. I. Lee, and A. H. Johnston, Total ionizing dose effects on flash memories, in 1998 IEEE Radiation Effect Data Workshop, pp [5] D. R. Roth, J. D. Kinninson, B. G. Karlhuff, L. R. Lander, G. S. Bognaski, K. Chao, and G. M. Swift, SEU and TID testing of the Samsung 128 Mbit and the Toshiba 256 Mbit flash memory, in 2000 IEEE Radiation Effect Data Workshop, pp [6] S. Bertazzoni, G. C. Cardarilli, M. Salmeri, A. Salsano, G. Bacis, C. Golla, M. L. Longo, F. Desideri, D. Di Giovenale, G. C. Grande, S. Sperandei, M. Ricci, and P. G. Picozza, TID on 16 Mb flash memories in radiation environment, in Proc. RADECS2000, pp [7] D. N. Nguyen, S. M. Guertin, G. M. Swift, and A. H. Johnston, Radiation effects on advanced flash memories, IEEE Trans. Nucl. Sci., vol. 46, pp , Dec [8] H. R. Schwartz, D. K. Nichols, and A. H. Johnston, Single-event upset in flash memories, IEEE Trans. Nucl. Sci., vol. 44, pp , Dec [9] G. Cellere, P. Pellati, A. Chimenton, A. Modelli, L. Larcher, J. Wyss, and A. Paccagnella, Radiation effects on floating-gate memory cells, IEEE Trans. Nucl. Sci., vol. 48, pp , Dec [10] G. Cellere, A. Paccagnella, L. Larcher, A. Chimenton, J. Wyss, A. Candelori, and A. Modelli, Anomalous charge loss from floating-gate memory cells due to heavy ions irradiation, IEEE Trans. Nucl. Sci., pp , Dec [11] E. S. Snyder, P. J. McWhirter, T. A. Dellin, and J. D. Sweetman, Radiation response of floating gate EEPROM memory cells, IEEE Trans. Nucl. Sci., vol. 36, pp , Dec [12] T. P. Ma and P. V. Dressendorfer, Ionizing Radiation Effects in MOS Devices and Circuits. New York: Wiley, [13] G. A. Ausman, Field Dependence of Geminate Recombination in a Dielectric Medium, Adelphi, Harry Diamond Lab. Rep. No. 2097, [14] D. Ielmini, A. S. Spinelli, A. L. Lacaita, and A. Modelli, Statistical modeling of reliability and scaling projections for flash memories, in IEDM Tech. Dig., [15] L. Osanger, Initial recombination of ions, Phys. Rev., vol. 54, p. 554, [16] N. S. Saks, Response of MNOS capacitors to ionizing radiation at 80 K, IEEE Trans. Nucl. Sci., pp , Dec [17] V. A. K. Raparla, S. C. Lee, R. D. Scrimpf, D. M. Fleetwood, and K. F. Galloway, A model of radiation effects on nitride-oxide films for power MOSFET applications, Solid State Electr., vol. 47, pp , [18] H. Aozasa, I. Fujiwara, A. Nakamura, and Y. Komatsu, Analysis of carrier traps in Si N in oxide/nitride/oxide for metal/oxide/nitride/oxide/silicon nonvolatile memories, Jpn. J. Appl. Phys., vol. 38, pp , [19] N. S. Saks, M. G. Ancona, and J. A. Modolo, Radiation effects in MOS capacitors with very thin oxides at 80 K, IEEE Trans. Nucl. Sci., vol. 31, pp , Dec [20] A. J. Lelis, T. R. Oldham, H. E. Boesch Jr., and F. B. McLean, The nature of the trapped hole annealing process, IEEE Trans. Nucl. Sci., vol. 36, pp , Dec [21] J. M. Benedetto and H. E. Boesch, The relationship between Co and 10 kev x-ray damage in MOS devices, IEEE Trans. Nucl. Sci., vol. 33, 1986.