The influence of interfacial nitrogen on edge charge trapping at the interface of gate oxide/drain extension in metal-oxide-semiconductor transistors was investigated. Positive edge charge trapping was observed for both pure and nitrided oxides with an oxide thickness of 6.5 nm. Results showed that nitrogen at the interface enhance the edge charge trapping.published_or_final_versio

Ang, CH

Chen, TP

Fung, S

Huang, JY

Tan, SS

Tse, MS

Applied Physics Letters

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TitleInfluence of interfacial nitrogen on edge charge trapping at theinterface of gate oxide/drain extension in metal-oxide-semiconductor transistorsAuthor(s) Chen, TP; Huang, JY; Tse, MS; Tan, SS; Ang, CH; Fung, SCitation Applied Physics Letters, 2003, v. 82 n. 18, p. 3113-3115Issued Date 2003URL http://hdl.handle.net/10722/42476Rights Creative Commons: Attribution 3.0 Hong Kong LicenseAPPLIED PHYSICS LETTERS VOLUME 82, NUMBER 18 5 MAY 2003Influence of interfacial nitrogen on edge charge trapping at the interface ofgate oxideÕdrain extension in metal–oxide–semiconductor transistorsT. P. Chen,a) J. Y. Huang, M. S. Tse, and S. S. TanSchool of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798C. H. AngChartered Semiconductor Manufacturing Limited, Singapore 738406S. FungDepartment of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong~Received 1 November 2002; accepted 12 March 2003!In this work, the edge charge trapping at the interface of gate oxide/drain extension caused byFowler–Nordheim injection is determined quantitatively by using a simple approach to analyze thechange of the drain band-to-band tunneling current. For both pure and nitrided oxides with an oxidethickness of 6.5 nm, positive edge charge trapping is observed while the net charge trapping in theoxide above the channel is negative. It is found that the nitrogen at the interface can enhance theedge charge trapping. The results could be explained in terms of the creation of positive fixed oxidecharges at the interface as a result of the electrochemical reactions involving holes and hydrogenions. © 2003 American Institute of Physics. @DOI: 10.1063/1.1572471#Nitrogen incorporation in gate dielectric films has beenwidely employed in deep submicron metal–oxide–semiconductor ~MOS! devices.1 There have been many stud-ies on the charge trapping in the nitrided oxides, and bothpositive and negative charge trapping have been observed.2–4However, these studies were mainly concerned with thecharge trapping in the oxide above the channel, and lessstudies have been reported on the charge trapping in the ox-ide edge overlapping the drain extension. The edge chargetrapping at the interface of this overlap region has a stronginfluence on the gate-induced drain leakage current resultingfrom the drain band-to-band tunneling because the band-to-band tunneling current has an exponential dependence on thesilicon surface electric field in the overlap region.5–8 As asmall change in the electric field due to the edge chargetrapping could cause a significant change in the band-to-bandtunneling leakage current, the edge charge trapping could bea reliability concern for MOS transistors and flash memorydevices. Obviously, a study on the influence of the nitridationof the gate oxide on the edge charge trapping and thus on theband-to-band tunneling current is necessary. In this work, weused a simple approach to quantitatively examine the influ-ence of interfacial nitrogen concentration on the edge chargetrapping.The devices used in this study weren1-polycrystalline-silicon ~polysilicon!-gate n-channel MOSfield-effect transistors ~MOSFETs! with a channel length/width of 0.5/20 mm and a gate oxide thickness of 6.5 nm.Either pure SiO2 or nirided oxides were used as the gatedielectrics. The pure SiO2 was grown using partial-wet pro-cess, and further, in situ NO nitridation and N2 /O2 annealingwere carried out for nitrided oxides. The secondary ion massspectroscopy measurement showed the nitrogen piling up atthe SiO2 /Si interface for the nitrided oxides, and two nitro-a!Electronic mail: echentp@ntu.edu.sg3110003-6951/2003/82(18)/3113/3/$20.00Downloaded 07 Nov 2006 to 147.8.21.97. Redistribution subject to gen concentrations ~1.8 and 4.0 at. % at the interface! wereused in this study. The devices were stressed under a con-stant gate current ~10 mA/cm2) with either substrate or gateFN injections. The drain band-to-band tunneling current wasmeasured with a gate-controlled-diode configuration9 inwhich the drain was kept at a constant positive voltage whilethe gate voltage was varied in a sufficiently negative rangecorresponding to the band-to-band tunneling. The gate cur-rent was also monitored to ensure that the contribution of thegate oxide leakage was insignificant.When the surface electric field (Es) in the silicon of thedrain extension under the gate is strong enough, a band-to-band tunneling will occur in this silicon region, giving rise toa drain current.5 The band-to-band tunneling current can beexpressed as6,10J5AEs2 exp~2B/Es!, ~1!where A and B are two constants6 which can be determinedexperimentally from the plot of ln(J/Es2) versus (1/Es). Ifthere is an electric charge trapped at or very close to theSiO2 /Si interface in the drain extension overlap region ~i.e.,the edge charge trapping mentioned herein! as a result ofelectrical stress, the silicon surface electric field will change,and the electric field is now Es5Es01DE where Es0 is thesurface field without charge trapping ~i.e., the electric fieldbefore stress, which can be calculated with the formulasgiven in Ref. 5! and DE is the surface field change. Thus, theinfluence of the edge charge trapping on the band-to-bandtunneling current can be described in terms of DE byJ5A~Es01DE !2 expS 2 BEs01DE D . ~2!DE can be calculated easily by using113 © 2003 American Institute of PhysicsAIP license or copyright, see http://apl.aip.org/apl/copyright.jspDE5( i51n F lnS Jm~ i !Es0~ i !2D 2ln A1B 1Es0~ i !G F S 2 1Es0~ i ! 1BS 1Es0~ i ! D2G1 1 2 2 , ~3!3114 Appl. Phys. Lett., Vol. 82, No. 18, 5 May 2003 Chen et al.( i51n F S 2 Es0~ i ! 1BS Es0~ i ! D Gwhere Jm(i) is the poststress band-to-band tunneling currentmeasured at a different applied surface field Es0(i) (i51,2,...n , n is the number of the data points!.Figure 1 shows that the electrical stress leads to a reduc-tion of the silicon surface electric field ~i.e., DE,0! and thusa reduction of the band-to-band tunneling current. On theother hand, the reduction of the electric field ~and thus theband-to-band tunneling current! is enhanced with a highernitrogen concentration at the interface, as shown in Fig. 2.The reduction of the electric field means a net positive edgecharge trapping in the drain extension region. The edgecharge trapping can be determined with QT5eSiDE whereeSi is the Si permittivity and QT is the areal density of theedge charge trapping at the interface in the drain extensionregion. Figure 3 shows the edge charge trapping as a func-tion of stress time for different interfacial nitrogen concen-trations. For the gate oxide thickness of 6.5 nm used in thisstudy, a net positive edge charge trapping is always ob-served. As shown in Fig. 3, the stress time dependence of theedge charge trapping follows a power law, i.e., QT}tn,where t is the stress time and n is an exponent. In addition, alarger interfacial nitrogen concentration leads to a largeredge charge trapping (n is also larger!, indicating that theinterfacial nitrogen can enhance the positive charge trapping.In contrast to the positive edge charge trapping shown inFig. 3, a net negative charge trapping above the channel re-gion is observed. Figure 4 shows the threshold voltage shift(DVT) as a function of stress time for different interfacialnitrogen concentrations. The positive DVT indicates a netnegative charge trapping in the gate oxide above the channelregion. It has been clear that electron trapping throughout theoxide is the dominant charge trapping in a thick gate oxideFIG. 1. An example showing the stress effect on the drain band-to-bandtunneling current as a result of the positive edge charge trapping. The curvesin ~b! represent the calculated band-to-band tunneling current with Eq. ~2!using the DE values determined with Eq. ~3!.Downloaded 07 Nov 2006 to 147.8.21.97. Redistribution subject to for sufficient long stress time. Therefore, the net negativecharge trapping shown in Fig. 4 is understandable.The positive edge charge trapping could be explained asdescribed next. In the gate oxide overlapping the drain ex-tension region, only those charges trapped at the interface orvery close to the silicon surface will affect the silicon surfaceelectric field and thus affect the band-to-band tunneling. It iswell accepted that high field stress leads to both electron andhole trapping in the gate oxide and the release of hydrogenions (H1). In addition, holes are induced at the interface ofthe drain extension as the gate is strongly biased with nega-tive voltage during the tunneling current measurement. Atthe interface, the following electrochemical reactions maytake place12,13O3wSiuH1h1→O3wSi11H0, ~4!wSiuNi1H1 → w Siu NiH1. ~5!A positive fixed oxide charge is thus created from Eqs. ~4!and ~5!. On the other hand, high field stress leads to electrontrapping throughout the gate oxide. In the gate oxide over-lapping the drain extension region, only the positive fixedoxide charge and the electron trapping at the interface orvery close to the silicon surface can affect the silicon surfaceelectric field. For the 6.5 nm gate oxides used in this study,the electron trapping that can affect the silicon surface elec-tric field is less significant than the positive fixed oxidecharge, and thus the net charge trapping leading to theFIG. 2. Comparison of the band-to-band tunneling current reduction after a100 s stress between the two nitrided oxides with 1.8 and 4.0 at. % N.DIBB5IBB~after stress)2IBB~before stress) where IBB is the band-to-bandtunneling current.AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp3115Appl. Phys. Lett., Vol. 82, No. 18, 5 May 2003 Chen et al.change of the electric field is positive. However, in the gateoxide above the channel, electron trapping throughout theoxide contributes to the threshold voltage (VT), and thus thenet charge trapping is negative leading to a VT shift to amore positive voltage. This explains why the net edge chargetrapping that affects the band-to-band tunneling is positivewhile the net charge trapping above the channel that affectsthe threshold voltage is negative. In addition, the aforemen-tioned electrochemical reactions @Eqs. ~4! and ~5!# show thatthe edge charge trapping due to the positive fixed oxidecharges depends on the nitrogen in the oxides, which ex-plains the results of Fig. 3. However, in contrast to the posi-tive edge charge trapping, the electron trapping in the oxidesover the channel is insensitive or less sensitive to the nitro-gen in the oxides. Therefore, there should be no obviousdifference in the VT shift between the pure and nitrided ox-ides as the VT shift is mainly due to the electron trappingover the channel. This is the reason for the negligible differ-ence in the VT shift between the pure and nitrided oxidesshown in Fig. 4.Equations ~4! and ~5! have been used to explain thenitrogen-enhanced negative bias temperature instability~NBTI! effect.13 We have also observed the nitrogen-enhanced positive fixed charge trapping above the channelfrom the NBTI experiments. It has been clear that for thenitrided oxides the nitrogen piles up at the interface and,therefore, a higher interfacial nitrogen concentration leads toa larger amount of positive charge trapping as a result of theH1 trapping by nitrogen in the reaction of Eq. ~5!. Thenitrogen-enhanced positive fixed charge trapping may bealso related to the fact that the amount of interfacial Si—Hbonds, which are involved in the reaction of Eq. ~4!, in-creases with nitrogen incorporation near the interface.13 Onthe other hand, from first-principle calculations, we have alsoFIG. 3. Edge charge trapping as a function of stress time for differentinterfacial nitrogen concentrations.Downloaded 07 Nov 2006 to 147.8.21.97. Redistribution subject to found that the reaction energy for the electrochemical reac-tion involving H1 trapping is reduced in nitrided oxides.Therefore, the nitrogen-enhanced edge charge trapping canbe explained in terms of the nitrogen-enhanced H1 trappingand/or the increase of the amount of interfacial Si—H bondswith the interfacial nitrogen concentration.In summary, we have conducted a study of the edgecharge trapping in the gate oxide overlapping the drain ex-tension caused by FN injection by using a simple approachto analyze the change of the drain band-to-band tunnelingcurrent. For both pure and nitrided oxides with an oxidethickness of 6.5 nm, positive edge charge trapping is ob-served while the net charge trapping in the oxide above thechannel is negative. A higher interfacial nitrogen concentra-tion leads to a larger edge charge trapping. The results can beexplained in terms of the creation of positive fixed oxidecharges at the interface as a result of the electrochemicalreactions involving holes and hydrogen ions.1 M. L. Green, E. P. Gusev, R. Degraeve, and E. L. Garfunkel, J. Appl.Phys. 90, 2057 ~2001!.2 Z. H. Liu, P. T. Lai, and Y. C. Cheng, IEEE Trans. Electron Devices 38,344 ~1991!.3 M. K. Mazumder, A. Teramoto, J. Komori, M. Sekine, S. Kawazu, and Y.Mashiko, IEEE Trans. Electron Devices 46, 1121 ~1999!.4 M. Severi, L. Dori, M. Impronta, and S. Guerri, IEEE Trans. ElectronDevices 36, 2447 ~1989!.5 H.-J. Wann, P. K. Ko, and C. Hu, Tech. Dig. - Int. Electron Devices Meet.1992, 147 ~1992!.6 S. A. Parke, J. E. Moon, H.-J. Wann, P. K. Ko, and C. Hu, IEEE Trans.Electron Devices 39, 1694 ~1992!.7 J. Chen, T. Y. Chan, I. C. Chen, P. K. Ko, and C. Hu, IEEE ElectronDevice Lett. 8, 515 ~1987!.8 J. H. Chen, S. C. Wong, and Y. H. Wang, IEEE Trans. Electron Devices48, 1400 ~2001!.9 T. P. Chen, D. S. H. Chan, and W. K. Chim, Semicond. Sci. Technol. 11,672 ~1996!.10 T. Wang, L. P. Chiang, N. K. Zous, C. F. Hsu, L. Y. Huang, and T. S. Chao,IEEE Trans. Electron Devices 46, 1877 ~1999!.11 T. P. Chen, J. Huang, M. S. Tse, and X. Zeng, Solid-State Electron. 46,2013 ~2002!.12 S. Ogawa, M. Shimaya, and N. Shiono, J. Appl. Phys. 77, 1137 ~1995!.13 C.-H. Aang, C.-M. Lek, S. S. Tan, B. J. Cho, T. P. Chen, W. H. Lin, and J.Z. Zhen, Jpn. J. Appl. Phys., Part 2 41, L314 ~2002!.FIG. 4. Threshold voltage shift as a function of stress time for differentinterfacial nitrogen concentrations.AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

Influence of interfacial nitrogen on edge charge trapping at the interface of gate oxide/drain extension in metal-oxide-semiconductor transistors

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Influence of interfacial nitrogen on edge charge trapping at the interface of gate oxide/drain extension in metal-oxide-semiconductor transistors

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