A low-energy (550 eV) argon beam is used to bombard the backsurfaces of 6 kinds of metal–oxide–semiconductor capacitors, and the resulting effects on their interface characteristics are then investigated. The gate oxide of these capacitors includes thermal oxide, trichloroethyene (TCE) oxide, NH3-nitrided oxide, reoxidized-nitrided oxide, rapid-thermal-nitrided oxide, and N2O-nitrided oxide. Measurements show that for bombardment times up to 45 min the interface-state density of all the devices, in general, decreases with increasing bombardment time/dose, and the midgap energy at the silicon surface tends to rise. Moreover, the bombardment is more effective in reducing acceptor-type than donor-type interface states. On the other hand, the change of fixed-charge density is more complex. For TCE, N2O-nitrided and reoxidized-nitrided oxides, fixed-charge density decreases initially with increasing bombardment time, but then increases, while the trend is reversed for the other gate oxides. A model with stress compensation and weak bond breaking is suggested to explain the results. ©1999 American Institute of Physics.published_or_final_versio

Cheng, YC

Huang, MQ

Lai, PT

Li, GQ

Zeng, SH

Journal of Applied Physics

Crossref

HKU Scholars Hub

Title A study of various oxide/silicon interfaces by Ar + backsurfacebombardmentAuthor(s) Lai, PT; Li, GQ; Huang, MQ; Zeng, SH; Cheng, YCCitation Journal of Applied Physics, 1999, v. 85 n. 4, p. 2253-2256Issued Date 1999URL http://hdl.handle.net/10722/42107Rights Journal of Applied Physics. Copyright © American Institute ofPhysics.JOURNAL OF APPLIED PHYSICS VOLUME 85, NUMBER 4 15 FEBRUARY 1999A study of various oxide/silicon interfaces by Ar1 backsurface bombardmentP. T. Laia)Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong KongG. Q. Li, M. Q. Huang, and S. H. ZengDepartment of Applied Physics, South China University of Technology, People’s Republic of ChinaY. C. ChengDepartment of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong~Received 8 October 1998; accepted for publication 5 November 1998!A low-energy ~550 eV! argon beam is used to bombard the backsurfaces of 6 kinds of metal–oxide–semiconductor capacitors, and the resulting effects on their interface characteristics are theninvestigated. The gate oxide of these capacitors includes thermal oxide, trichloroethyene ~TCE!oxide, NH3-nitrided oxide, reoxidized-nitrided oxide, rapid-thermal-nitrided oxide, andN2O-nitrided oxide. Measurements show that for bombardment times up to 45 min theinterface-state density of all the devices, in general, decreases with increasing bombardment time/dose, and the midgap energy at the silicon surface tends to rise. Moreover, the bombardment is moreeffective in reducing acceptor-type than donor-type interface states. On the other hand, the changeof fixed-charge density is more complex. For TCE, N2O-nitrided and reoxidized-nitrided oxides,fixed-charge density decreases initially with increasing bombardment time, but then increases, whilethe trend is reversed for the other gate oxides. A model with stress compensation and weak bondbreaking is suggested to explain the results. © 1999 American Institute of Physics.@S0021-8979~99!02004-6#I. INTRODUCTIONIn the development of ultra-large-scale integration, theeffects of interface characteristics of metal–oxide–semiconductor ~MOS! structure on the mobility of inversion-layer charge carriers are still important issues.1–3 It is wellknown that the lattice-damaged layer induced by high-energy~.10 keV! ion implantation with subsequent high-temperature ~.800 °C! and long-time ~.1 h! annealing caneffectively getter crystal defects, as well as metal impuritiesin silicon wafers,4–7 which can then be used to make betterdevices. Recently, low-energy ~550 eV! argon-ion bombard-ment on the device backsurface was used to improve thecharacteristics of n-MOS devices.8,9 In this work further in-vestigation was carried out for six kinds of MOS capacitorswith different gate oxides. The changes of their interface-state and fixed-charge densities, as well as the midgap energyat the silicon surface with bombardment time were charac-terized and a model including stress compensation and weak-bond breaking was proposed to explain the results.II. EXPERIMENTThe MOS capacitors (1003100 mm2) used in this workwere fabricated on p-type ~100!-oriented silicon wafers ~6–8V cm! using conventional n1-polysilicon-gate self-alignedMOS technology. A 21-nm-thick oxide was thermally grownby argon-diluted dry oxidation at 950 °C ~denoted as OXsample!. Some of the samples were nitrided in pure NH3either by rapid thermal annealing at 1200 °C for 1 min or bya!Electronic mail: laip@hkueee.hku.hk2250021-8979/99/85(4)/2253/4/$15.00Downloaded 13 Nov 2006 to 147.8.21.97. Redistribution subject to furnace annealing at 1000 °C for 60 min ~denoted as RTNand NO samples respectively!, with part of NO samples thenreoxidized in O2 for 30 min ~denoted as RON sample!.Moreover, gate oxides were also thermally grown at 1000 °Cin O2 with trichloroethyene ~denoted as TCE sample! forsuppressing oxide defects and at 1050 °C in pure N2O ~de-noted as N2O sample! to about the same thickness, 24 and 22nm, respectively. After completing the top electrodes of theMOS capacitors, the wafers were put into a closed chamberand a low-energy ~550 eV! Ar1 beam with 0.5 mA/cm2 in-tensity was used to bombard their backsurfaces under avacuum of 3.2 mPa at room temperature. Four different bom-bardment times ~0, 15, 30, 45 min! were performed and thecorresponding bombardment dose was 0, 2.79, 5.58, and8.3731018 cm22 respectively. Finally, a layer of aluminumwas evaporated on the backs of the wafers and then the de-vices were annealed in nitrogen at 450 °C for 20 min. Inter-face characteristics of the devices were determined by high-frequency and quasi-static capacitance–voltage (C – V)techniques.III. RESULTS AND DISCUSSIONSFigure 1 shows the interface-state density D it ~at mid-gap! for the six kinds of MOS capacitors versus bombard-ment time, with a typical curve of D it versus the energy inthe silicon bandgap for RTN sample illustrated in Fig. 2.Among all the nonbombarded oxides, the N2O-grown oxidehas the highest D it because it has an initial acceleratedgrowth phase before the linear growth region, which cangenerate lots of defects.10,11 Moreover, it is seen that, in gen-eral, the longer the bombardment time, the lower is D it for all3 © 1999 American Institute of PhysicsAIP license or copyright, see http://jap.aip.org/jap/copyright.jsp2254 J. Appl. Phys., Vol. 85, No. 4, 15 February 1999 Lai et al.the devices. The fixed-charge density Q f of all the samplesversus bombardment time is depicted in Fig. 3, together withtypical high-frequency and quasistatic C – V curves for RTNsample given in Fig. 4. Relatively high Q f in N2O, NO, andRON samples is related to the heavy nitridation of their gateoxides, which significantly roughens their oxide/Si inter-faces. Also, the results indicate that the change of Q f withbombardment time presents a turnaround behavior. For OX,NO, and RTN samples, Q f increases at the beginning andthen decreases with increasing bombardment time, while thechange of Q f for the other samples has an opposite trend.In order to explain these observations, a model of stresscompensation and weak-bond breaking is suggested as fol-lows. After the Ar1 bombardment, a lattice-damaged layer isproduced at the backsurface of the devices and gives rise to amechanical stress in the vicinity of the oxide/Si interface.This stress can partially compensate the stress created bythermal processing of the gate oxide, polysilicon deposition,aluminum evaporation, lattice mismatching, as well as theoriginal stress in the wafer. Therefore, defects associatedwith the stress decrease and hence, both D it and Q f decrease.Of course, if overcompensation of stress happens due toFIG. 1. Interface-state density vs bombardment time for MOS capacitorswith six different gate oxides.FIG. 2. Interface-state density vs the energy in band gap for a MOS capaci-tor with RTN gate oxide ~Ei is the intrinsic Fermi energy of bulk silicon!.The number after RTN indicates the bombardment time in minutes.Downloaded 13 Nov 2006 to 147.8.21.97. Redistribution subject to higher bombardment energy and/or longer bombardmenttime, new defects could be generated and hence, D it and Q fbecome larger. The changes of D it for RTN and OX samplesare in agreement with previous results.8,9 However, NOsample with long bombardment time displays oppositebehavior,8 which likely has higher original stress than theprevious sample grown at a higher temperature of1000 °C12,13 and hence, stress overcompensation does not oc-cur yet to increase D it again even after 45 min bombardment.On the other hand, weak Si–O and/or Si–N bonds not onlypresent at the oxide/Si interface,14,15 but could also occur inthe gate oxide near the interface due to the existence of somemicroscopic unoxidized silicon regions in the oxide.16 Thebreaking of these weak bonds induced by the bombardmentcould give rise to more dangling bonds at the interface and inthe oxide and hence, higher D it and Q f . On the contrary, theoxygen and/or nitrogen released from the weak bond break-ing can diffuse from the oxide to the interface and help toreduce D it by combining with the dangling bonds there.Therefore, if stress compensation plays a bigger role thanweak bond breaking, D it and Q f get smaller after the bom-bardment. But for the case with weak bond breaking as thedominant mechanism and/or the presence of stress overcom-FIG. 3. Fixed-charge density vs bombardment time for all MOS capacitors.FIG. 4. High-frequency and quasistatic C – V curves for two MOS capaci-tors with RTN gate oxide, one before and the other after 45 min bombard-ment.AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp2255J. Appl. Phys., Vol. 85, No. 4, 15 February 1999 Lai et al.pensation, D it and Q f could increase. Therefore, the changesof D it and Q f heavily depend on the bombardment energyand time, as well as the original stress, defect density, andbond energy in the devices.The D it decrease of all the samples here after the bom-bardment is likely due to the dominance of stress compensa-tion at the interface. Moreover, the oxygen and/or nitrogencreated by bombardment-induced weak bond breaking in theoxide can diffuse to the interface, reducing the danglingbonds there and hence, D it although a corresponding Q f in-crease is possible. For example, in OX, NO, and RTNsamples, the slight Q f increase and then decrease with bom-bardment time are likely due to weak bond breaking in theoxide and stress compensation, respectively. On the otherhand, for TCE, N2O, and RON samples, Q f decreases ini-tially with bombardment time, possibly because the strongerSi–Cl or Si–N bonds at their interfaces and in their oxidesFIG. 5. Midgap energy at silicon surface vs bombardment time for all MOScapacitors with VG512 V.FIG. 6. Midgap energy at silicon surface vs bombardment time for all MOScapacitors with VG522 V.Downloaded 13 Nov 2006 to 147.8.21.97. Redistribution subject to cannot be easily broken due to the special treatments on theoxides and hence, stress compensation should be the majorfactor. It is interesting to note that the nitrided oxides of NOand RTN samples are not so strong after all, possibly due tothe hydrogen species left in the oxide during NH3 nitridation,which can disrupt the Si–O bonds and thus create weakerSi–H bonds.For all the devices, their midgap energies (Em) at thesilicon surface deviated from the flat-band condition under agate voltage VG of 12 and 22 V versus bombardment timeare respectively presented in Fig. 5 and 6, and a typical Emvs VG for NO sample is given in Fig. 7. From Em2VG plots,Em of all 6 nonbombarded devices with VG50 are found tolie within 0.25–0.45 eV ~20.5–21 V for the flat-band volt-age!, which is due to the positive Q f giving rise to a bendingdown of the energy band. The Em increase after the bom-bardment should be related to the D it decrease, which weak-ens the shielding effect of interface-state charge against elec-tric field and gives rise to more severe band bending underboth 1VG and 2VG conditions. The changes of Em withbombardment time in Figs. 5 and 6 are not quite similar tothat of D it ~see Fig. 1!, possibly due to the fact that Em isalso related to the dopant concentration at the silicon surfacewhich remains unchanged after the bombardment. Further-more, the Em increase is larger under 1VG than 2VG for allthe devices. This implies that the bombardment is more ef-fective in eliminating acceptor-type than donor-type inter-face states, because the acceptor states are close to the top ofthe band gap and are charged with electrons due to the bend-ing down of the energy bands under 1VG .IV. CONCLUSIONIn conclusion, low-energy ~550 eV! argon bombardmentat the back of MOS devices with various gate oxides canlower their interface-state density and raise the midgap en-ergy of the silicon surface for bombardment times up to 45min. The corresponding change of fixed-charge density withbombardment time can first increase and then decrease, orvice versa. All these behaviors could be explained by thecompeting effects of stress compensation and weak bondbreaking.FIG. 7. Midgap energy at silicon surface vs gate voltage for a MOS capaci-tor with NO gate oxide.AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp2256 J. Appl. Phys., Vol. 85, No. 4, 15 February 1999 Lai et al.ACKNOWLEDGMENTSThe authors would like to thank C. L. Chan and H. H.Ng for their technical support in the microelectronics labo-ratory. This work was partially supported by RGC ResearchGrant, Hong Kong, and the CRCG Grant, The University ofHong Kong, Hong Kong.1 T. Ohmi, K. Kotani, A. Teramoto, and M. Miyashita, IEEE Electron De-vice Lett. 12, 652 ~1991!.2 T. E. Chang, C. Huang, and T. Wang, IEEE Trans. Electron Devices 42,738 ~1995!.3 X. J. Yuan, M. K. Kivi, S. Taylor, and P. H. Hurley, IEEE ElectronDevice Lett. 17, 239 ~1996!.4 J. S. Kang and D. K. Schroder, J. Appl. Phys. 65, 2974 ~1989!.5 T. Sakurai and T. Sugano, J. Appl. Phys. 52, 2889 ~1981!.6 K. D. Beyer and T. H. Yeh, J. Electrochem. Soc. 129, 2527 ~1982!.Downloaded 13 Nov 2006 to 147.8.21.97. Redistribution subject to 7 M. R. Ponponiak, T. Nagasaki, and T. H. Yeh, J. Electrochem. Soc. 124,1802 ~1977!.8 P. T. Lai, X. Zeng, G. Q. Li, and W. T. Ng, IEEE Electron Device Lett.16, 354 ~1995!.9 P. T. Lai, M. Q. Huang, X. Zeng, S. H. Zeng, and G. Q. Li, Appl. Phys.Lett. 68, 2687 ~1996!.10 H. R. Soleimani, A. Philipossian, and B. Doyle, Tech. Dig. Int. ElectronDevices Meet., 629 ~1992!.11 X. Zeng, P. T. Lai, and W. T. Ng, IEEE Electron Device Lett. 16, 436~1995!.12 A. Bhattacharya, C. Vorst, and A. H. Carim, J. Electrochem. Soc. 132,1900 ~1985!.13 P. Sutardja, W. G. Oldham, Tech. Dig. Int. Electron Devices Meet., 264~1987!.14 T. Sakurai and T. Sugano, J. Appl. Phys. 52, 2889 ~1981!.15 D. J. Chadi, R. B. Laughlin, and J. D. Jonnopoulos, The Physics of SiO2and its Interface ~Pergamon, New York, 1978!.16 J. S. Johannessen, W. E. Spicer, and Y. E. Styausser, J. Appl. Phys. 47,3028 ~1976!.AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

A study of various oxide/silicon interfaces by Ar + backsurface bombardment

http://hub.hku.hk/bitstream/10722/42107/1/44787.pdf

A study of various oxide/silicon interfaces by Ar + backsurface bombardment

Abstract

Similar works

Full text

Available Versions

Crossref

HKU Scholars Hub