輻射對正反短通道效應之研究

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(3) The Effects of Irradiation on the NSCE and RSCE of MOSFETs 88-2215-E-009-029 87 8 01

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(8) LMNOPNQK %RS# ! T,KU67 89V;W+XTYZ[ \]^_`Uab-cd# !efg#h iejklmNn^ opqb%rst 9+. findings have important implications in designing MOS transistors for rad-hard applications. Keywords: rad-hard, MOS transistors, normal short-channel effects, reverse short-channel effects..

(9) : Recently reverse short channel effect (RSCE) has received many attentions. Unlike the normal short channel effect (NSCE) which is the threshold voltage (Vth) lowering with decreasing channel length (L). RSCE depicts instead an increase in Vth with decreasing L [1-6]. RSCE is believed to be due a non-uniform lateral surface channel dopant concentration (CS) as a result of enhanced boron diffusion near the source/drain (S/D) regions. It has been shown that by nitriding the gate oxide or after-gate re-ox, RSCE in nMOSFETs can be suppressed [7-8]. In this paper we report, for the first time, that RSCE in nMOSFETs can also be effectively suppressed by performing the nitridation on the sacrificial oxide (SO), which is stripped off before growing a pure SiO2 as the final gate oxide. The effects of irradiation on Vth-lowering (i.e., NSCE) were reported briefly before [9]. They found that NSCE was enhanced after irradiation for their nMOSFETs that depict minor before-irradiation NSCE. Neither their pre- nor after-irradiation devices depict RSCE. In this paper, we report for the first time the intriguing observations that while the NSCE is enhanced after irradiation on devices which. upvv !v# !+ Abstract In this project, we report, for the first time, the simultaneous enhancement of normal short-channel effect (NSCE) and reverse short-channel effect (RSCE) on MOS transistors. We have proposed a model to successfully explain this intriguing and seemingly contradictory observation. Medici simulation also confirms with our model. Our findings suggest that the conventional thinking of designing transistors with a slight RSCE before irradiation in hope that RSCE will be reduced so an ideal Vth-vs-L curve is achieved after irradiation is therefore impractical. In addition, we also discovered, for the first time, that the use of a nitrided sacrificial oxide is as effective in reducing the RSCE of the resultant transistors. Our 1.

(10) irradiation. The trend is even more clear by plotting delta Vth (Fig. 2). Similar trends are also observed in pMOSFETs (Fig. 3). Before irradiation the SiO2-devices depict NSCE, while the N2O-devices depict RSCE. Vth depicts an across-the-L increase (i.e., in absolute value) after irradiation for all pMOSFETs. Moreover, for the Si02-devices that depict minor NSCE, the NSCE is enhanced after irradiation. In contrast, for the N2O-devices that depict minor RSCE, the RSCE is also enhanced after irradiation. These trends are even more clear by plotting delta Vth (Fig. 4). We believe these seemingly contradicting observations can be explained as follows: taking nMOSFETs for example, a more uniform lateral CS (e.g., N2O-nMOSFETs), will depict NSCE before irradiation. A uniform hole trapping QF reduces CS uniformly, causing enhanced NSCE after irradiation. However, for nMOSFETs with a non-uniform lateral CS (e.g., Si02-"MOSFETs), boron CS is higher near the S/D regions, causing RSCE before irradiation. After irradiation, a uniform QF attracts uniform n-type electrons in the channel counterdoping the p-type CS, resulting in a more pronounced differential ratio in the boron CS between the middle channel and regions near S/D, causing a more pronounced RSCE. To confirm the above hypothesis, Medici simulations were performed. Simulation results show that for nMOSFETs with a uniform CS, NSCE is observed before irradiation (Fig. 5). By adding a uniform QF, an enhanced NSCE is indeed depicted (Fig. 5). While for nMOSFETs with a non-uniform lateral CS, RSCE is depicted before irradiation and an enhanced RSCE is indeed depicted after irradiation (Fig 6), agreeing with our experimental observations. Similarly, our Medici simulations also confirm that both. depict minor NSCE before irradiation, the RSCE is simultaneously enhanced after irradiation on devices which depict minor RSCE before irradiation. : Both n- and p-channel MOS transistors were fabricated. Our baseline devices (i.e., control) received a standard 25-nm SiO2 sacrificial oxide growth that was later stripped off before growing a 20-nm final gate oxide. Some wafers were deliberately split to receive an N2O-annealed 25-nm sacrificial oxide to study its effects. The N2O-sacrificial oxide was first grown by O2 oxidation, followed by an N2O anneal for 15 min at 925oC, to make up a final thickness of 25nm. For radiation study, devices were subjected to a radiation from a cobalt-60 source with 1Mrad(Si) dose, and their characteristics were re-measured. Threshold voltage: vs. L for all nMOSFETs are plotted in Fig. 1. Before irradiation, SiO2-devices depict RSCE, while N2O-devices depict NSCE, suggesting that performing SO in N2O is also effective in suppressing the enhanced boron diffusion, resulting in a more uniform CS, thereby suppressing RSCE in nMOSFETs. Our results suggest that, even though the N2O-SO was stripped off, the nitrogen incorporated at the interface remains, serving to suppress enhanced boron diffusion during later processing. Threshold voltage depicts an across-the-L reduction after irradiation for all nMOSFETs (Fig. 1), due to the well-known hole trapping (QF) after irradiation. More importantly, an interesting and seemingly contradicting phenomenon is observed. Specifically, for the SiO2-devices that depict minor RSCE, the RSCE is enhanced after irradiation Interestingly, for the N2O-devices that depict minor NSCE, the NSCE is also enhanced after 2.

(11) National Nano Device Labs for technical assistance during this study.. RSCE and NSCE are enhanced after irradiation (Figs. 7 and 8). The effects of substrate bias are also measured (Figs 9-12), confirming with previous reports that RSCE. is reduced with substrate bias [10]. The DIBL effects are also measured. for nMOSFET's, the N20-devices depict worse DIBL. DIBL also worsens for the irradiated devices (Fig. 13). For pMOSFETs, similar trends are observed (Fig. 14). The drive currents are plotted in Figs 15-16.. : [1] C. Mature et al., IEEE Elec. Dev. Lett., V. 10, p.556, 1989. [2] M. Orlowski et al, IEDM Tech. Dig. p. 632, 1987. [3] H. I. Hanafi et al., IEEE Elec. Dev. Lett. V. 12, p.575, 1993. [4] H. Jacobs et al., IEDM Tech. Dig., p.307, 1993. [5] C. Y. Lu et al., IEEE Elec. Dev. Lett., V. 10, p.JJ6448, 1989. [6] C. Y. Chang et al., IEEE Elec. Dev. Lett., V. 15. p437, 1994. [7] T. S. Chao et al., VLSI-TSA, 1997 [8] P. G. Y. Tsui et al., IEDM Tech. Dig., p.501, 1994. [9] J. S. T. Huang et al., IEEE Trans. Electron Dev. V. 36, p.1226, 1989. [10] N. D. Arora et al., IEEE Electron. Dev. Lett., V. 13, p.92, 1992. [11] F. Jong et al., IEE Elec. Lett., V. 34, p.404, 1998 [12] T. Huang et al., 1998 IEEE Nuc. and Rad. Effects Conf.. : In conclusion, we report, for the first time, that RSCE in MOSFETs can also be effectively suppressed by employing an N2O-treated sacrificial oxide [11]. More importantly, we also report, for the first time, the intriguing observations that for devices that depict NSCE, the NSCE is enhanced after irradiation. While for devices that depict RSCE, the RSCE is also enhanced after irradiation [12]. Our Medici simulations employing a uniform layer of trapped holes indeed agree with our experimental observations. Our findings suggest that short channel effects, be it NSCE or RSCE, are mostly likely to get worse after irradiation. The conventional thinking of designing devices with a slight RSCE before irradiation in hope that RSCE will be reduced and an ideal that Vth-vs-L curve would be achieved after irradiation is therefore impractical, if not impossible. Some works have been published [11-12]. A full paper of the account is current under preparation.. Fig. 1:Vth vs. Channel length for nMOSFET’s. Nitridation of sacrificial oxide is as effective in suppressing RSCE. The RSCE is enhanced after irradiation on. : The authors greatly acknowledge the support of National Science Council, and 3.

(12) SiO2-devices which depict minor RSCE prior to irradiation. While the NSCE is also enhanced after irradiation on N2O-devices which depict minor NSCE prior to irradiation.. Fig. 4: Delta Vth ( which respect to long L ) vs. Channel length for pMOSFET’s.. Fig. 2: Delta Vth ( which respect to long L ) vs. Channel length for nMOSFET’s.. Fig. 5: Simulated Vth vs. Channel length for nMOSFET’s with uniform lateral channel doping profile. The devices depict NSCE prior to irradiation, and the NSCE is further enhanced after irradiation, agreeing. with. experimental. observation. on. N2O-nMOSFET’s. Fig. 3: Vth vs. Channel length for pMOSFET’s. The NSCE is enhanced after irradiation on SiO2-devices which depict minor NSCE prior to irradiation. While the RSCE is also enhanced after irradiation on N2O-devices which depict minor RSCW prior to irradiation. Fig. 6: Simulated Vth vs. Channel length for nMOSFET’s with non-uniform lateral channel doping profile. The devices depict RSCE prior to irradiation, and the RSCE is futher enhanced after irradiation, agreeing 4. with. experimental. observation. on.

(13) SiO2-nMOSFET’s. - 0.7 5 -0.8. Vth (V). - 0.8 5 O2 O 2+ Radiatio n. -0.9. N 2O N 2O +Rad iation. - 0.9 5 -1 - 1.0 5. Fig. 10: Vth vs. Channel length with substrate bias as. -1.1 0. 5. 10. 15. 20. parameter for N2O-nMOSFET’s.. L ( m). Fig. 7: Simulated Vth vs. Channel length for pMOSFET’s. 0.1 O2 O2+Radiati on N2O N2O+Radiation. Vth(V ). 0.08 0.06 0.04. Fig. 11: Vth vs. Channel length with substrate bias as. 0.02. parameter for SiO2-pMOSFET’s.. 0 -0.02 0. 5. 10. 15. 20. L (m). Fig. 8: Simulated delta Vth ( which respect to long L ) vs. Channel length for pMOSFET’s.. Fig. 12: Vth vs. Channel length with substrate bias as parameter for N2O-pMOSFET’s.. Fig. 9: Vth vs. Channel length with substrate bias as parameter for SiO2-nMOSFET’s.. Fig. 13: DIBL effects for nMOSFET’s.. 5.

(14) Fig. 14: DIBL effects for pMOSFET’s.. Fig. 15: Drive current for nMOSFET’s.. Fig. 16: Drive current for pMOSFET’s.. 6.

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