Quality of low-pressure chemical-vapor-deposited (LPCVD) oxide and N2O-nitrided LPCVD (LN2ON) oxide is investigated under high-field stress conditions as compared to thermal oxide. It is found that LPCVD oxide has lower midgap interface-state density Dit-m and smaller stress-induced Dit-m increase than thermal oxide, but exhibits enhanced electron trapping rate and degraded charge-to-breakdown characteristics, which, however, are significantly suppressed in LN2ON oxide, suggesting effective elimination of hydrogen-related species. Moreover, LN2ON oxide shows further improved Si/SiO2 interface due to interfacial nitrogen incorporation. © 1997 American Institute of Physics.published_or_final_versio

Cheng, YC

Lai, PT

Lo, HB

Xu, J

Applied Physics Letters

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Title Quality improvement of low-pressure chemical-vapor-depositedoxide by N2O nitridationAuthor(s) Lai, PT; Xu, J; Lo, HB; Cheng, YCCitation Applied Physics Letters, 1997, v. 70 n. 8, p. 996-998Issued Date 1997URL http://hdl.handle.net/10722/42105Rights Applied Physics Letters. Copyright © American Institute ofPhysics.Quality improvement of low-pressure chemical-vapor-deposited oxideby N2O nitridationP. T. Laia)Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong KongXu JingpingDepartment of Solid State Electronics, Huazhong University of Science and Technology,People’s Republic of ChinaH. B. Lo and Y. C. ChengDepartment of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong~Received 23 September 1996; accepted for publication 16 December 1996!Quality of low-pressure chemical-vapor-deposited ~LPCVD! oxide and N2O-nitrided LPCVD~LN2ON! oxide is investigated under high-field stress conditions as compared to thermal oxide. Itis found that LPCVD oxide has lower midgap interface-state density D it-m and smallerstress-induced D it-m increase than thermal oxide, but exhibits enhanced electron trapping rate anddegraded charge-to-breakdown characteristics, which, however, are significantly suppressed inLN2ON oxide, suggesting effective elimination of hydrogen-related species. Moreover, LN2ONoxide shows further improved Si/SiO2 interface due to interfacial nitrogen incorporation. © 1997American Institute of Physics. @S0003-6951~97!02808-8#Low-pressure chemical-vapor-deposition ~LPCVD! ox-ide is finding increasingly widespread applications in thefield of microelectronics, such as gate dielectric for thin-filmtransistors in high-density static random-access memories1and stacked gate dielectric for metal–oxide–semiconductorfield-effect transistors2 ~MOSFETs!. Two primary problemsencountered in using LPCVD oxide were significant bulktrappings3–5 and high defect density.6 To reduce defect den-sity, a stacked thermal LPCVD gate oxide technology wasdeveloped,2,6 however, bulk trapping still has no satisfactorysolution. In addition, improvement in the quality of Si/SiO2interface is also an inevitable concern. For this reason, theN2O-nitridation technique was applied to LPCVD oxide7 andsome improvements were achieved in terms of midgapinterface-state generation DD it-m and shift of flatband voltageDV fb . In this letter, besides further descriptions of DD it-mand DV fb , improvements on charge-to-breakdown Qbd , andchange in gate voltage during constant-current stress DVg ,electron-trap generation and trapping rates are also reportedand the involved mechanisms are analyzed.MOS capacitors used in this study were fabricated onp-type ~100! silicon wafers with a resistivity of 6–8 V cm bya self-aligned n1 polysilicon-gate process. All oxidation andanneal processes were performed in a conventional horizon-tal furnace. LPCVD oxide ~LOX! was deposited at 450 °C,33.8 Pa using silane and oxygen. The flow rate of SiH4and O2 were 0.75 and 100 sccm, respectively, which resultedin a low and controllable deposition rate of about 3 Å/minand good thickness uniformity. After deposition somesamples were subjected to a 1000 °C anneal in N2 ambientfor 35 min, which makes the LPCVD oxide densified andH2-related by products diffuse out of the oxide. Othersamples received in situ nitridation at the same temperatureand for the same time in pure N2O ambient ~LN2ON!. Ther-mal oxide ~OX! was grown at 850 °C for 70 min in dry O2 toa!Electronic mail: laip@hkueee.hku.hk996 Appl. Phys. Lett. 70 (8), 24 February 1997 0003-6951Downloaded¬10¬Nov¬2006¬to¬147.8.21.97.¬Redistribution¬subjecserve as control sample and then annealed at 1000 °C for 35min, or 900 °C for 35 min in N2 ambient ~denoted as OX1and OX2, respectively! to improve the quality of theSi/SiO2 interface and oxide bulk to a different extent. Con-nection to gate electrode was formed by Al metallization andsintering in forming gas at 430 °C for 30 min following n1polysilicon-gate preparation. The area of the capacitors was1024 cm2. Designed oxide thicknesses are 140 and 110 Åfor OX and LOX oxides, respectively, which is mainly basedon the consideration that LOX oxide would be thickened by;25% after N2O nitridation at 1000 °C for 35 min.8 As aresult, final oxide thicknesses measured by the capacitance–voltage ~C–V) technique are 145, 145, 115, and 150 Å forOX1, OX2, LOX, and LN2ON oxides, respectively, with thethickness of LOX oxide increased by 30% after N2O nitrida-tion. All constant-current stresses were performed with elec-tron injection from the gate, i.e., a negative gate bias. Mid-gap interface-state density D it-m, flatband voltage V fb, andfixed charge Qf were determined by high-frequency and qua-sistatic C–V measurements.The initial Qf and D it-m of the four MOS capacitorsextracted from C–V data have comparable values~Qf51.2–2.131011 cm22, D it-m53.35–5.9431010 cm22eV21!; however, different behaviors occur among the fourdielectrics under high-field stress. Presented in Fig. 1 isDD it-m under constant-current injection of 21 mA/cm2 forthe four dielectrics. Surprisingly, a smaller DD it-m for LOXsample than OX sample is observed. This might imply thatsmoother SiO2 /Si interface for LOX oxide than OX oxide isformed9 and the interfacial strain of LOX oxide is relieved toa larger extent than OX oxide during the high-temperatureanneal in N2.10 To show the advantages of the annealing step,DD it-m of OX1 and OX2 samples are compared in Fig. 1. Ascan be seen, OX1 sample exhibits slightly improved inter-face hardness than OX2 sample, which obviously resultsfrom the formation of stronger Si—O bonds at the interface11and a larger reduction of interfacial stress12 at a higher an-nealing temperature. For N2O-nitrided LPCVD oxide,/97/70(8)/996/3/$10.00 © 1997 American Institute of Physicst¬to¬AIP¬license¬or¬copyright,¬see¬http://apl.aip.org/apl/copyright.jspDD it-m is more greatly reduced than LOX oxide, demonstrat-ing the distinctive advantage of oxynitrides—an excellentinterface hardness against hot-carrier bombardment as a re-sult of interfacial nitrogen incorporation.Shown in Fig. 2 is the corresponding DV fb under thesame stress conditions as those used in Fig. 1. The four di-electrics all display a negative-going shift in V fb after stress,indicating hole trapping. Like DD it-m in Fig. 1, DV fb for OX1is also slightly smaller than the one for OX2 because ofslightly better interface qualities achieved at higher anneal-ing temperature. Moreover, DV fb for LOX sample is lessthan for OX samples, which is similar to that reported in Ref.13. It is well known that DV fb is the combined effect ofcharge trapping at the substrate interface and in the bulkoxide, and mainly reflects the charges near the Si/SiO2interface.14 Hole trapping is likely due to generated donor-like interface states15 and bulk trapping, which are positivelycharged when they are unoccupied under C–V measurementFIG. 1. Midgap interface-state creation DD it-m of MOS capacitors with dif-ferent gate dielectrics under constant-current injection stress of 21mA/cm2.FIG. 2. Flatband voltage shift of MOS capacitors with injected charge underthe same stress conditions as those in Fig. 1.Appl. Phys. Lett., Vol. 70, No. 8, 24 February 1997Downloaded¬10¬Nov¬2006¬to¬147.8.21.97.¬Redistribution¬subjecconditions. So, considering the results in Fig. 1, it can bededuced that LOX sample should have a lower donorlikeinterface-state creation than OX samples in the entire charge-injection range. However, it could not be concluded that bulkhole trapping in LOX oxide is less than OX oxide becausethe influence of hole trapping might be partly canceled bysignificant electron trapping in the bulk of LOX oxide asanalyzed in the next paragraph; but, undoubtedly, both areconsiderably improved by the N2O nitridation as shown inFig. 2. Lastly, the DV fb saturation of LN2ON sample withinjected charge in Fig. 2 depicts again its better interfaceresistance against stress-induced degeneration and less bulktrapping than LOX and OX oxides.Figure 3 shows the change in gate voltage DVg duringconstant-current stressing ~210 mA/cm2! for the oxides. Be-tween the two thermal-oxide samples, OX1 sample is chosenfor comparison purpose because it has better qualities and,more importantly, has about the same thermal budget asLOX and LN2ON samples. Unlike DV fb , DVg is very sen-sitive to charges located near the cathode. So, to eliminatethe effect of oxide thickness tox , DVg is normalized by re-spective oxide thickness. The increase of gate voltage duringstressing implies electron trapping. Obviously, LPCVD ox-ide suffers from the highest electron-trap generation andtrapping rates among the three oxides ~see Fig. 4 below!,which is similar to that reported in Ref. 13, and probably dueto hydrogen-related species such as 2H and 2OH producedby the thermal decomposition of silane during deposition.However, LN2ON oxide shows only a small DVg , suggest-ing greatly suppressed electron trapping. This indicates thatN2O nitridation for LOX oxide is very effective in annealingout the hydrogen species, hence reducing electron-trap gen-eration and trapping rates.Depicted in Fig. 4 are charge-to-breakdown Qbd and cor-responding electron-trap generation and trapping rates for thethree oxides under different injection current densities2Jg . Qbd of LOX sample is much lower than that of OX1sample. This can be associated with two possible causes: alarge defect density and enhanced electron trapping rate inthe LPCVD oxide film.16 Since Qbd of LOX dielectrics isFIG. 3. Normalized change in gate voltage vs time during constant-currentstressing ~210 mA/cm2!.997Lai et al.t¬to¬AIP¬license¬or¬copyright,¬see¬http://apl.aip.org/apl/copyright.jspsignificantly improved after N2O nitridation in our experi-ments and the charge trapping rate at localized defect sites inLPCVD oxide is lower than that in thermal oxide,5 it can besuggested that Qbd degradation is likely due to enhancedelectron-trap generation and trapping rates induced byhydrogen-related species in our LOX oxide, which is propor-tional to the slope of the gate-voltage shift versus time dur-ing constant-current injection, i.e., dVg /dt of the linear partof the curves in Fig. 3. It can clearly be seen that the higherthe dVg /dt , the lower the Qbd.16 The Qbd and dVg /dt valuesare averaged over 20 capacitors, and dVg /dt is similarly nor-malized by respective oxide thickness. It can be evaluatedthat N2O nitridation makes Qbd of our LOX dielectrics in-crease by a factor of 1.6–1.8, and the corresponding dVg/dt decrease by approximately the same factor.In summary, N2O-nitrided LPCVD oxide exhibits con-siderably suppressed electron-trap creation and trappingFIG. 4. Charge-to-breakdown Qbd and electron trap generation/trapping ratedVg/dt as a function of injection current density.998 Appl. Phys. Lett., Vol. 70, No. 8, 24 February 1997Downloaded¬10¬Nov¬2006¬to¬147.8.21.97.¬Redistribution¬subjecrates, excellent charge-to-breakdown characteristics and in-terface immunity against interface-state generation due to in-terfacial nitrogen incorporation, and reduction of H2-relatedbyproducts resulting from silane decomposition. Therefore,to obtain high-quality LPCVD gate dielectrics and enhanceddevice reliability, N2O nitridation of deposited oxide is avery promising way. Even for stacked thermal/LPCVD gateoxide, the same treatment would probably result in furtherimprovement of interface and bulk qualities of the compositeoxide.This work was financially supported by the RGC andCRCG Research Grants, the University of Hong Kong.1S. Ikeda, S. Hashiba, I. Kuramoto, H. Katoh, S. Ariga, T. Yamanaka, T.Hashimoto, N. Hashimoto, and S. Meguro, in IEDM Technical Digest,1990, p. 469.2Y. Lee, L. D. Yao, E. Hansen, R. Chau, B. Sabi, S. Hossaini, and B.Asakawa, IEEE Trans. Electron Devices ED-40, 163 ~1993!.3G. Kawamato, G. Magyer, and L. Yau, IEEE Trans. Electron DevicesED-34, 2450 ~1987!.4H. H. Tseng, P. Tobin, J. Hayden, and K. M. Chang, in IEDM TechnicalDigest, 1991, p. 75.5 J. Lee, I. C. Chen, and C. Hu, IEEE Electron Device Lett. EDL-7, 506~1986!.6R. Moazzami and C. Hu, IEEE Electron Device Lett. EDL-14, 72 ~1993!.7 J. Ahn, W. Ting, T. Chu, S. Lin, and D. L. Kwong, Applied Phys. Lett. 59,283 ~1991!.8Z. H. Liu, J. T. Krick, H. J. Wann, P. K. Ko, C. Hu, and Y. C. Cheng, inIEDM Technical Digest, 1992, p. 625.9S. Ang and S. Wilson, J. Electrochem. Soc. 134, 1254 ~1987!.10 J. Ahn, W. Ting, and D.-L. Kwong, IEEE Electron Device Lett. EDL-13,186 ~1992!,11N. Bhat and K. C. Saraswat, in ECD Extended Abstract, 1994, p. 179.12S. P. Tay, A. Kalnitsky, G. Kelly, J. P. Ellul, P. DeLalio, and E. A. Irene,J. Electrochem. Soc. 137, 3579 ~1990!.13N. Bhat, P. P. Apte, and K. C. Saraswat, IEEE Trans. Electron DevicesED-43, 554 ~1996!.14P. Fazan, M. Dutoit, C. Martin, and M. Ilegems, Solid State Electronics30, 829 ~1987!.15T. Hori and H. Iwasaki, IEEE Electron Device Lett. EDL-9, 168 ~1988!.16C.-H. Lin, J. Cable, and J. C. S. Woo, IEEE Trans. Electron DevicesED-42, 1329 ~1995!.Lai et al.t¬to¬AIP¬license¬or¬copyright,¬see¬http://apl.aip.org/apl/copyright.jsp

Quality improvement of low-pressure chemical-vapor-deposited oxide by N2O nitridation

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Quality improvement of low-pressure chemical-vapor-deposited oxide by N2O nitridation

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