A low-energy (550 eV) argon-ion beam was used to directly bombard the backsurface of nitrided n-channel metal-oxide-semiconductor field-effect transistors (n-MOSFETs) after the completion of all conventional processing steps. The interface and oxide-charge trapping characteristics of the bombarded MOSFETs were investigated as compared to nonbombarded and reoxidized-nitrided n-MOSFETs. It was found that after bombardment, interface state density decreases and interface hardness against hot-carrier bombardment enhances, and oxide charge trapping properties were also improved. The improvements exhibit a turnaround behavior depending on bombardment conditions and could be attributed to stress compensation in the vicinity of the Si/SiO2 interface and an annealing effect. © 1997 American Institute of Physics.published_or_final_versio

Gheng, YC

Lai, PT

Lo, HB

Xu, JP

Journal of Applied Physics

Crossref

HKU Scholars Hub

TitleA study on interface and charge trapping properties of nitridedn-channel metal-oxide-semiconductor field-effect transistors bybacksurface argon bombardmentAuthor(s) Lai, PT; Xu, JP; Lo, HB; Gheng, YCCitation Journal Of Applied Physics, 1997, v. 82 n. 4, p. 1947-1950Issued Date 1997URL http://hdl.handle.net/10722/44844Rights Creative Commons: Attribution 3.0 Hong Kong LicenseA study on interface and charge trapping properties of nitrided n-channelmetal-oxide-semiconductor field-effect transistors by backsurfaceargon bombardmentP. T. Laia)Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong KongJ. P. XuDepartment of Solid State Electronics, Huazhong University of Science and Technology, 430074People’s Republic of ChinaH. B. Lo and Y. C. ChengDepartment of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong~Received 27 February 1997; accepted for publication 15 May 1997!A low-energy ~550 eV! argon-ion beam was used to directly bombard the backsurface of nitridedn-channel metal-oxide-semiconductor field-effect transistors ~n-MOSFETs! after the completion ofall conventional processing steps. The interface and oxide-charge trapping characteristics of thebombarded MOSFETs were investigated as compared to nonbombarded and reoxidized-nitridedn-MOSFETs. It was found that after bombardment, interface state density decreases and interfacehardness against hot-carrier bombardment enhances, and oxide charge trapping properties were alsoimproved. The improvements exhibit a turnaround behavior depending on bombardment conditionsand could be attributed to stress compensation in the vicinity of the Si/SiO2 interface and anannealing effect. © 1997 American Institute of Physics. @S0021-8979~97!05616-8#I. INTRODUCTIONWith the shrinking of transistors to submicrometer di-mensions in recent years, high quality thin gate dielectricsare required which possess excellent robustness against hot-carrier bombardment and radiation exposure, good blockingproperty against diffusion of dopants and impurities, etc.Thermal nitridation of SiO2 using NH3 has been reported tohave many advantages over thermal oxide.1–4 However, re-sidual tensile stress exists at the Si/SiO2 interface of the ni-trided oxide film and increases with higher interfacial nitro-gen concentration.5 In our previous work,6,7 the backsurfaceAr1 bombardment method was proposed to improve theelectrical characteristics of n-channel metal-oxide-semiconductor field-effect transistors ~n-MOSFETs! withthermally grown and NH3-nitrided oxides as gate dielectricsafter they were completely fabricated. It was shown that thechannel mobility of n-MOSFETs and the interface propertiesof MOS capacitors can be improved by this technique. In thiswork, studies on hot-carrier-induced degradation of interfaceproperties, and oxide charge trapping characteristics of bom-barded n-MOSFETs with nitrided gate oxides are performed,and results are expressed in terms of bombardment effects onhot-carrier-induced degradations of peak linear transconduc-tance (Gm), shift of threshold voltage (VT) and increase ofinterface state density (Dit). NH3 nitridation is chosen tointroduce electron traps in the gate oxide so as to facilitate acomparison between the effects of bombardment and reoxi-dation on the charge trapping behavior of the oxide.II. EXPERIMENTThe n-MOSFETs and MOS capacitances used in thisstudy were fabricated on p-type ~100! silicon wafers with aresistivity of 6–8 V cm by the self-aligned n1 polysiliconJ. Appl. Phys. 82 (4), 15 August 1997 0021-8979/97/82(4)/194Downloaded 07 May 2007 to 147.8.143.135. Redistribution subject togate process. Thermal gate oxide was grown at 100 °C inAr-diluted O2, and then nitrided at 1000 °C for 60 min inpure NH3. Finally, samples were annealed at 1000 °C for 30min either in N2 ~denoted as NO! or in dry O2 ~reoxidation,denoted as RNO!. The gate oxides had a thickness of 250 Åfor NO samples and 275 Å for RNO samples as determinedby capacitance-voltage (C-V) measurements. After complet-ing all conventional processing steps for the n-MOSFETs,the wafers of NO samples were put into a vacuum chamberand a low-energy ~550 eV! Ar1 beam with 0.5 mA/cm2 in-tensity was applied to directly bombard the backsurface ofthe wafers at room temperature under a vacuum of 3.2 mPa.Four different bombardment durations 0, 10, 20, and 40 min,were performed, with the corresponding samples denoted asNO0, NO10, NO20, and NO40, respectively. Subsequently,the wafers were annealed in N2 at 450 °C for 20 min. Hot-carrier stress with maximum substrate current (VD 5 2VG5 8 V! on the transistors and high-field injection with a con-stant current density ~21 mA/cm2! on the capacitances werethen performed to study their interface properties. After this,their Si/SiO2 interfaces were characterized by charge-pumping technique on the MOSFETs and high-frequency,quasi-static C-V measurements on the capacitors. Hole trap-ping was depicted by the changes of peak linear transcon-ductance (DGm) and threshold voltage (DVT) of thenMOSFETs after low-gate-voltage stress (VG 5 1 V andVD 5 7 V!, while electron trapping was characterized by thechange in gate voltage (DVG) during stressing with a con-stant current density ~21 mA/cm2! on the MOS capacitors.The channel length L and width W of the MOSFETs are 1and 24 mm, respectively, and the area of the capacitors are1024 cm2. All measurements were carried out under light-tight and electrically shielded condition.19477/4/$10.00 © 1997 American Institute of Physics AIP license or copyright, see http://jap.aip.org/jap/copyright.jspIII. RESULTS AND DISCUSSIONSA. Si/SiO2 interface propertiesTable I lists comparison between interface state densitiesDit before and after bombardment measured on the sametransistors for NO10, NO20, and NO40 samples. It can beclearly seen that Dit is decreased to a different extent afterbombardment for a different duration, with a minimum valuefor 20 min. The fact is further supported by C-V results onthe MOS capacitors adjacent to the transistors. Figure 1gives some typical quasi-static C-V characteristics beforeand after bombardment for different time. The extracted dis-tribution of interface state density in the energy band gap ispresented in Fig. 2, which is obviously in good agreementwith the results in Table I. Furthermore, results of hot-carrierstress on the bombarded MOSFETs in Fig. 3 and high-fieldstress on the MOS capacitors in Table II show that a corre-sponding improvement of interface hardness against hot-carrier bombardment is obtained as compared to the non-bombarded sample, even though it is not as large as that ofthe RNO sample. It can be observed that as bombardmenttime increases, interface state creation (DDit) decreases to aminimum value ~NO20 sample!, and then increases to avalue comparable with NO0 sample ~NO40 sample!, indicat-ing a turnaround behavior. Corresponding to DDit behaviors,DVTand DGm for all the samples also exhibit a similar varia-tion trend after hot-carrier stress, as shown in Fig. 4, suggest-ing that DVT and DGm should mainly rely on the change ofinterface states under maximum substrate current stress.TABLE I. Comparison between interface-state densities before and afterbombardment for different times ~10, 20, and 40 min!.NO10 NO20 NO40Pre-bombardmentDit(1011 cm22 eV21) 2.36–2.46 1.44–1.53 1.51–1.79Post-bombardmentDit(1010 cm22 eV21! 8.15–8.32 4.75–4.93 7.19–7.51DDit(%) 65.5–66.2 66.9–67.8 52.4–58.1FIG. 1. Typical quasi-static C-V characteristics for NO samples with dif-ferent bombardment time.1948 J. Appl. Phys., Vol. 82, No. 4, 15 August 1997Downloaded 07 May 2007 to 147.8.143.135. Redistribution subject toThe above facts might be attributed to relaxation of me-chanical stress, or recovery of distorted bonds at Si/SiO2 in-terface after bombardment. Measurement and simulation5showed that the higher residual tensile stress exists in thenitrided oxide film than in pure oxide film due to nitrogenincorporation near the Si/SiO2 interface. In other words,there is a distorted interfacial region because of the forma-tion of mismatched Si–N bonds in the Si–O network. Thebacksurface bombardment could generate a lattice-damagedlayer at the back of the wafer which is likely to give rise to acompressive stress at the surface of the wafer, thus compen-sating the original tensile stress. The resulting type andamount of net stress depend on bombardment duration undera given ion energy and density. If compressive stress justcancels tensile stress, a strainless Si/SiO2 interface, hence thelowest Dit and smallest DDit would be obtained. The resultsof NO20 sample seem to correspond to this case. Naturally,it can be deduced that if bombardment duration is too long,an overcompensation would result, and the initial tensilestress would become compressive after bombardment, thus aFIG. 2. Distributions of interface state density in the energy bandgap beforeand after bombardment for different times ~10, 20, and 40 min!.FIG. 3. Interface state creation as a function of stress time at VD 5 2VG5 8 V for all n-MOSFETs.Lai et al. AIP license or copyright, see http://jap.aip.org/jap/copyright.jspJ. Appl. Phys., Vol.Downloaded 07 MTABLE II. Increase in midgap interface-state densities of MOS capacitances after high-field injection stress~21 mA/ cm2) for 100 s.NO0 NO10 NO20 NO40 RNODDitm(1012 cm22 eV21) 2.93–3.43 1.35–1.57 1.12–1.33 2.69–3.25 1.00–1.15turnaround behavior occurs, as observed for the NO40sample. Therefore, a suitable bombardment time should bechosen for a given ion energy and density to obtain the bestinterface properties. For RNO samples, besides the relax-ation of interfacial stress, considerable elimination ofhydrogen-related species near the interface ~as described be-low! is mainly responsible for its least interface state genera-tion.B. Hole trapping propertiesFor all the smples, a low VG stress ~VD57 V and VG5 1 V! for 2000 s followed by 20 s of electron injection atVD 5 VG 5 8 V were employed to investigate the effects ofhole trapping on the change of peak linear transconductance(DGm) and shift of threshold voltage (DVT), and the corre-sponding results are given in Figs. 5~a! and 5~b!, respec-tively. A positive DGm and a negative DVT are observed inthe low VG stress phase, which can be attributed to the chan-nel shortening effect caused by hole trapping in the gateoxide localized near the drain junction.8,9 Subsequent shortelectron injection results in a large positive VT shift andGm degradation as a result of the compensation of thetrapped holes and the filling of generated neutral electrontraps by the injected electrons. As can be seen in Fig. 5,NO20 oxide has minimum hole trapping among the bom-barded samples based on the changes of Gm and VT , and aturnaround behavior similar to that in Figs. 1 and 2 is againfound for the hole-trapping characteristics. Therefore, it canbe concluded that bombardment with suitable duration canalso effectively reduce the hole traps in the oxide to a levelclose to that in RNO oxide, possibly by means of stresscompensation near the Si/SiO2 interface.FIG. 4. Degradation of peak linear transconductance and shift of thresholdvoltage after 5000 s stress at VD 5 2VG 5 8 V for all n-MOSFETs.82, No. 4, 15 August 1997ay 2007 to 147.8.143.135. Redistribution subject toC. Electron trapping propertiesElectron trapping properties of the gate oxides of thebombarded samples were also studied by monitoring thechange in gate voltage (DVG) to maintain a constant currentdensity ~21 mA/ cm2! through the MOS capacitors adjacentto corresponding transistors. As shown in Fig. 6, NO0 andRNO oxides exhibit largest and smallest DVG, respectively.The former results from significant trapping at electron trapsgenerated by hydrogen-related species decomposed fromNH3 during nitridation processing,10–12 which are effectivelyeliminated in the latter by the reoxidation step. While thebombarded samples show a gradually decreased DVG withbombardment time, suggesting suppressed electron trappingto some extent, this can be ascribed to an annealing effect onthe gate oxide during Ar1 bombardment,7 which can removeFIG. 5. ~a! Percentage change of peak linear transconductance and ~b!threshold voltage shift for all n-MOSFETs. Curve A immediately after lowVG stress at VG 5 1 V and VD 5 7V for 2000 s. Curve B after a subsequentshort electron injection phase at VG 5 VD 5 8 V for 20 s.1949Lai et al. AIP license or copyright, see http://jap.aip.org/jap/copyright.jsppart of the hydrogen-related species. As shown by the NO40device, the longer the bombardment time, the more obviousthe annealing effect is. So, it is expected that elimination ofmore hydrogen-related species, results in better suppressionof electron trapping and a harder interface can be achievedby optimizing the bombardment time, ion beam energy, andintensity. Further work is being carried out to verify thisconjecture.IV. SUMMARYIn summary, possible stress relief at the Si/SiO2 interfaceand in the gate oxide induced by backsurface Ar1 bombard-ment can reduce interface states and hole traps of nitridedn-MOSFETs, and also enhance interface resistance againsthot-carrier bombardment. Moreover, an annealing effect re-sulting from the bombardment can improve the electron-FIG. 6. Change in gate voltage vs time during constant current stressing~21 mA/cm2! for all capacitors adjacent to their corresponding transistors.1950 J. Appl. Phys., Vol. 82, No. 4, 15 August 1997Downloaded 07 May 2007 to 147.8.143.135. Redistribution subject totrapping properties of the oxides to some extent. Althoughthese improvements are not as large as those achieved byreoxidation, the bombardment does not decrease the nitrogencontent in the nitrided gate oxide, and thus its resistanceagainst dopant/impurity penetration into the conductionchannel of the devices. Therefore, backsurface bombardmentis a simple and promising technique for improving the reli-ability of MOS devices and is fully compatible with existingintegrated circuit ~IC! processing.ACKNOWLEDGMENTSThe authors would like to thank M. Q. Huang, G. Q. Li,and S. H. Zeng for their support of Ar1 bombardment. Thiswork is financially supported by Research Grant Council andCommittee on Research and Conference Grant grants.1 T. Ito, T. Nakamura, and H. Ishikawa IEEE Trans. Electron Devices ED-29, 498 ~1982!.2 S. K. Lai, J. Lee, and V. K. Dham, IEDM Tech. Dig. 190 ~1983!.3 F. L. Terry, Jr., R. J. Aucoin, M. L. Naiman, and S. D. Senturia IEEEElectron Device Lett. EDL-4, 191 ~1983!.4 W. Yang, R. Jayaraman, and C. G. Sodini IEEE Trans. Electron DevicesED-35, 935 ~1988!.5 H. S. Momose, T. Morimoto, K. Yamabe, and H. Iwai, IEDM Tech. Dig.65 ~1990!.6 P. T. Lai, Z. Xu, G. Q. Li, and W. T. Ng IEEE Electron Device Lett.EDL-16, 354 ~1995!.7 P. T. Lai, M. Q. Huang, X. Zeng, S. H. Zeng, and G. Q. Li Appl. Phys.Lett. 68, 2687 ~1996!.8 B. S. Doyle, M. Bourcerie, C. Bergonzoni, R. Benecchi, A. Bravis, K. R.Mistry, and A. Boudou IEEE Trans. Electron Devices ED-37, 1869~1990!.9 R. Bellens, P. Heremans, G. Groeseneken, and H. E. Maes IEEE ElectronDevice Lett. EDL-9, 232 ~1988!.10 T. Hori, H. Iwasaki, Y. Naito, and H. Esaki IEEE Trans. Electron DevicesED-34, 2238 ~1987!.11 S. T. Chang, N. M. Johnson, and S. A. Lyon Appl. Phys. Lett. 44, 316~1984!.12 S. K. Lai, D. W. Dong, and A. Hartsein J. Electrochem. Soc. 129, 2042~1982!.Lai et al. AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

A study on interface and charge trapping properties of nitrided n-channel metal-oxide-semiconductor field-effect transistors by backsurface argon bombardment

http://hub.hku.hk/bitstream/10722/44844/1/34427.pdf

A study on interface and charge trapping properties of nitrided n-channel metal-oxide-semiconductor field-effect transistors by backsurface argon bombardment

Abstract

Similar works

Full text

Available Versions

Crossref

HKU Scholars Hub