The soft breakdown (SBD) failure mode in 20 nm thick MgO dielectric layers grown on Si substrates was investigated. We show that during a constant voltage stress, charge trapping and progressive breakdown coexist, and that the degradation dynamics is captured by a power-law time dependence. We also show that the SBD current-voltage (I-V) characteristics follow the power-law model I = aVb typical of this conduction mechanism but in a wider voltage window than the one reported in the past for SiO2. The relationship between the magnitude of the current and the normalized differential conductance was analyzed

American Physical Society

Casey, P.

Cherkaoui, K.

Hughes, G.

Hurley, P. K.

Long, R.

Miranda, Enrique

Monaghan, S.

O'Connell, D.

O'Connor, E.

Applied Physics Letters

English

The soft breakdown (SBD) failure mode in 20 nm thick MgO dielectric layers grown on Si substrates was investigated. We show that during a constant voltage stress, charge trapping and progressive breakdown coexist, and that the degradation dynamics is captured by a power-law time dependence. We also show that the SBD current-voltage (I-V) characteristics follow the power-law model I = aV(b) typical of this conduction mechanism but in a wider voltage window than the one reported in the past for SiO(2). The relationship between the magnitude of the current and the normalized differential conductance was analyzed. (C) 2009 American Institute of Physics. (DOI: 10.1063/1.3167827

O'Connor, Éamon

Cherkaoui, Karim

Monaghan, Scott

Long, Rathnait D.

O'Connell, Dan

Hurley, Paul K.

Hughes, Gregory

Cork Open Research Archive

Title Electrical characterization of the soft breakdown failure mode in MgOlayersAuthor(s) Miranda, Enrique; O'Connor, Éamon; Cherkaoui, Karim; Monaghan,Scott; Long, Rathnait D.; O'Connell, Dan; Hurley, Paul K.; Hughes,Gregory; Casey, P.Publication date 2009Original citation Miranda, E., O’Connor, E., Cherkaoui, K., Monaghan, S., Long, R.,O’Connell, D., Hurley, P. K., Hughes, G. and Casey, P. (2009)'Electrical characterization of the soft breakdown failure mode in MgOlayers', Applied Physics Letters, 95(1), pp. 012901. doi:10.1063/1.3167827Type of publication Article (peer-reviewed)Link to publisher'sversionhttp://aip.scitation.org/doi/abs/10.1063/1.3167827http://dx.doi.org/10.1063/1.3167827Access to the full text of the published version may require asubscription.Rights © 2009 American Institute of Physics.This article may bedownloaded for personal use only. Any other use requires priorpermission of the author and AIP Publishing. The following articleappeared in Miranda, E., O’Connor, E., Cherkaoui, K., Monaghan,S., Long, R., O’Connell, D., Hurley, P. K., Hughes, G. and Casey, P.(2009) 'Electrical characterization of the soft breakdown failuremode in MgO layers', Applied Physics Letters, 95(1), pp. 012901 andmay be found at http://aip.scitation.org/doi/abs/10.1063/1.3167827Item downloadedfromhttp://hdl.handle.net/10468/4357Downloaded on 2018-08-23T18:53:46ZElectrical characterization of the soft breakdown failure mode in MgO layersE. Miranda, , E. O’Connor, K. Cherkaoui, S. Monaghan, R. Long, D. O’Connell, P. K. Hurley, G. Hughes, andP. CaseyCitation: Appl. Phys. Lett. 95, 012901 (2009); doi: 10.1063/1.3167827View online: http://dx.doi.org/10.1063/1.3167827View Table of Contents: http://aip.scitation.org/toc/apl/95/1Published by the American Institute of PhysicsElectrical characterization of the soft breakdown failure mode in MgOlayersE. Miranda,1,a E. O’Connor,2 K. Cherkaoui,2 S. Monaghan,2 R. Long,2 D. O’Connell,2P. K. Hurley,2 G. Hughes,3 and P. Casey31Escola Tècnica Superior d’Enginyeria, Universitat Autònoma de Barcelona, Campus UAB, 08193Bellaterra, Barcelona, Spain2Tyndall National Institute, University College Cork, Cork, Ireland3School of Physical Sciences, Dublin City University, Glasnevin, Dublin 9, IrelandReceived 21 April 2009; accepted 2 June 2009; published online 7 July 2009The soft breakdown SBD failure mode in 20 nm thick MgO dielectric layers grown on Sisubstrates was investigated. We show that during a constant voltage stress, charge trapping andprogressive breakdown coexist, and that the degradation dynamics is captured by a power-law timedependence. We also show that the SBD current-voltage I-V characteristics follow the power-lawmodel I=aVb typical of this conduction mechanism but in a wider voltage window than the onereported in the past for SiO2. The relationship between the magnitude of the current and thenormalized differential conductance was analyzed. © 2009 American Institute of Physics.DOI: 10.1063/1.3167827In recent papers, magnesium oxide MgO has beenpointed out as an appealing material for use as a gate insu-lator in metal-oxide-semiconductor MOS devices both forlow1 and high2 power applications. The main features of thisalkaline earth oxide are: a large band gap in the range 7.3 eVRef. 1–7.8 eV Ref. 3, ensuring sufficiently large bandoffsets with Si and therefore low leakage current, a dielectricpermittivity  ranging from 6.7 Ref. 4 to 10 Ref. 3 de-pending on the preparation method, high thermal conductiv-ity suitable for large power dissipation,2 and significantly, itschemical inertness, which minimizes the formation of an in-terfacial layer when deposited on Si substrates.1 In this letter,we have focused attention on a specific reliability aspect ofMgO-Si system, which is the conduction mechanism afterthe occurrence of soft breakdown SBD events.5 As is com-monly accepted, this failure mode arises as a consequence ofthe formation of a single or multiple nanoconstrictions acrossthe oxide layer after reaching a certain threshold density ofdefects caused by the electrical stress. This is a localizedconduction mode, independent of the device area, and hasbeen reported to occur in a number of high- thin films otherthan SiO2.6 However, contrary to the widespread opinion thatthis failure mechanism can only be observed in ultrathintox6 nm oxides, we have detected it in MgO films withnominal tox=20 nm, with tox the oxide thickness. Notably,our sample thickness almost doubles the maximum SiO2thickness tox=12 nm for which SBD has been reported.7 Inorder to demonstrate that we are strictly dealing with thesame charge transport mechanism, we have carried out athorough analysis of the conduction characteristics startingwith the widely accepted power-law model I=aVb, where aand b are constants.8 This allows us to examine the limits ofthe validity of the referred model, identify SBD events withleakage current levels several orders of magnitude lower thanthose detected in the past, which in thinner oxides wouldhave been overlooked and, finally, extend the relationshipexhibited by the fitting parameters a I at 1V, and b=d ln I /d ln V previously reported for the SiO2 /Si system.Hydrogen terminated Si surfaces were prepared by dip-ping n- and p-type 100 Si wafers 1015 cm−3 in a solutionof 5% hydrofluoric acid HF for 1 min, followed by rinsingin de-ionized water, and dried using nitrogen and were thenimmediately loaded into the deposition system. MgO films ofnominal tox=20 nm with 8.1 equivalent oxide thicknessEOT9.5 nm were deposited by electron beam evapora-tion from 99.9% MgO pellets at a rate of 0.2 Å/s, at 180 °C.The samples were capped in situ with 100 nm of amorphoussilicon -Si using a second e-beam source. For the NiSigate process, nickel was deposited by electron beam evapo-ration 80 nm through a patterned resist mask followedby a lift-off process. The rapid thermal annealing is a onestep process at 500 °C for 30 s in N2. The area of the devicestested is 1.610−5 cm2. Details about the SiO2 sampleswith tox4 nm considered in this study can be found in Ref.5.The dielectric breakdown of the MgO layer can be in-duced by either ramped or constant voltage stresses. Figure 1shows the effects of the latter type of stress on the leakageaAuthor to whom correspondence should be addressed. Tel.: 34-93-5813183. FAX: 34-93-5812600. Electronic mail: enrique.miranda@uab.es.1 10 10010-1010-9Progressive BDonsetExperimentalModelI=6.23x10-10 t-0.43n-type SiCurrent[A]Time [s]FIG. 1. Evolution of the gate leakage current under a constant voltage stressat 8 V. The solid and dashed lines correspond to experimental data andfitting results, respectively.APPLIED PHYSICS LETTERS 95, 012901 20090003-6951/2009/951/012901/3/$25.00 © 2009 American Institute of Physics95, 012901-1current. Notice that the current decreases according to theempirical law It= I0t−, where I0 and  are constants and tis the stress time, in agreement to what has been observed inother high-s such as Al2O3 and HfO2.9 The origin of thecurrent decay is still unclear but, since the process is partiallyreversible, it might be related to transient and permanentcharge trapping effects in pre-existing and induced traps. Theonset of dielectric breakdown is detected as a noise incre-ment in the I-t characteristic and this event is followed by aprogressive increase of the current progressive breakdownPBD. Alternatively, SBD can be achieved by successiveramped voltage stresses as shown in Fig. 2. The major dam-age to the sample is caused at the end of each sweep and, inmost of the cases, no current jump associated with SBD isobserved, an indication of the progressive opening of theleakage spots. The switching behavior, typical of digital SBDconduction,10 is clearly observed in the second and third I-Vcharacteristics. Contrary to what happens with ultrathin SiO2layers, in which SBD is only observable in a very limitedvoltage range because of the tunneling current that flowsthrough the nondamaged device area,11 in the present case ofa 20 nm thick sample the SBD curve can be detected in alarger voltage window. The SBD I-V curves cannot be asso-ciated with a single constriction and it is more likely thatmany leakage sites contribute to the total current. The insetin Fig. 2 supports this assertion and shows the occurrence ofseveral breakdown spots on the gate electrode after the stresssequence. According to our data the breakdown field forSBD in MgO is around 5–6 MV/cm which is comparable tothe breakdown field in HfO2 4–5 MV/cm but much lowerthan in SiO2 13–14 MV/cm.12 Regarding the voltage de-pendence of the SBD curves, notice that the linear relation-ship between logI and logV supports the use of thepower-law model. From the physical viewpoint, this modelhas been related to the conduction properties of nonlinearresistances networks with a distribution of percolationthresholds.13 However, it is worth pointing out that the modelis unable to capture the experimental fact that for larger dam-aged areas the slope of the I-V curves becomes shallower.This trend was first observed by Okada and Taniguchi14 inSiO2 and was thoroughly analyzed in Refs. 8 and 15 forsamples subjected to electrical stress and heavy ion irradia-tion, respectively. The change in slope is consistent with thefact that the hard breakdown HBD I-V characteristic hasslope close to unity,5 as expected for a simple resistor. Thisrelationship between the magnitude of the current a and itsnormalized differential conductance b which has not beenreported before for a high- with such a large thickness, hasbeen explained in terms of the mesoscopic transport proper-ties of the breakdown paths.16 Figure 3 shows the correlationbetween a and b obtained from a large number of I-V curvesand the inset shows a typical fitting to a SBD I-V character-istic in MgO using the power-law model. The points in Fig.3 cannot be considered as a statistically independent dataensemble since some of them were extracted from I-Vs mea-sured in the same sample after further degradation. On thecontrary, in Ref. 8 the parameters were extracted from inde-pendent measurements. Even though this parameter extrac-tion procedure can lead to uncertainty, what is remarkable isthat both data sets MgO and SiO2 follow very similar lineartrends. It is worth mentioning that thicker SiO2 films thanthose considered in this study only exhibit the HBD mode sothat the comparison between samples with similar largethicknesses is not possible. Figure 3 also shows that the cor-relation between a and b in MgO covers nearly ten orders ofmagnitude of the parameter a, whereas for SiO2 this correla-tion holds in a narrower range. A possible interpretation forthe observed correlation is that the size of the spots isstrongly linked to its conducting properties, in such a waythat the damaged area does not enter as a simple prefactor inthe current expression as commonly assumed for standardtransport mechanisms such as tunneling, Poole–Frenkel orSchottky emission. In addition, Fig. 3 shows that many of theSBD events occurring in MgO are softer than those detectedin SiO2. This can be a matter of the sample thickness, wherethe magnitude of tunneling currents permits the observationof softer events, but the possibility that it is related to par-ticular features of the conducting spots, such as the micro-scopic arrangement of the defect sites, cannot be ruled out. Inaddition, the fact that SBD can be observed in a thick oxidelayer is compatible with the idea that what fundamentallygoverns the conduction characteristics is the bottleneck ofthe constriction and not its total length.162 3 4 5 6 7 8 9 1010-1310-1210-1110-1010-910-810-710-6Fresh I-VSBDCurrent[A]Voltage [V]n-type SiFIG. 2. Experimental current-voltage characteristics. Symbols and lines cor-respond to the fresh and SBD I-V characteristics, respectively. Notice theuse of log-log axis in order to emphasize the power-law dependence. Themaximum voltage stress is 10 V. The inset shows several SBD spots distrib-uted over the device area.10-28 10-24 10-20 10-16 10-12 10-802468101214162 4 6 8 1010-1310-1210-1110-1010-910-810-7Current[A]Voltage [V]I=2.04x10-13 V5.62Experimental MgOLeast squares MgOLeast squares SiO2 [7]Linear extrapolation SiO2b=dln(I)/dln(V)aFIG. 3. Relationship between the parameters a and b in the power-lawmodel. The circles correspond to the region of SiO2 tox3.8–4.3 nmexperimental data given by the equation b=−0.78 loga−3.27 8. Thesquares correspond to the parameters extracted from the MgO SBD I-Vcurves. The solid line is given by the equation b=−0.90 loga−6.05.012901-2 Miranda et al. Appl. Phys. Lett. 95, 012901 2009To complete the electrical characterization of SBD inMgO, the roles played by the substrate type and gate voltageVG polarity were investigated. Figure 4a shows that simi-lar SBD currents can be found both for n VG0 and p-typeVG0 substrate samples in accumulation conditions. Iden-tical results arise for a n-type Si sample biased with bothinjection polarities as shown in Fig. 4b. Notice that this isnot the case for the fresh I-Vs. These plots indicate that theinjecting electrode does not play any major role after SBD aslong as there is sufficient charge available for conduction.The current seems to be limited by the breakdown path itself,so that its transmission properties become a key element forthe description of the phenomenon.5In summary, the postbreakdown electrical behavior ofMgO layers in MOS structures was investigated. It wasshown that SBD conduction can be detected in a relativelythick high- film and that the connection between the chargetransport mechanisms in MgO and SiO2 goes beyond the factthat they share the same power-law I-V model. The correla-tion found between the model parameters in both of the ma-terials investigated, and consequently, the apparent simplicityof the underlying physics, should perhaps be regarded as anindication of the low dimensionality of these conductingstructures.E.M. acknowledges the Generalitat de Catalunya BE-2007, the Ministerio de Ciencia y Tecnología MCyT,Spain Grant No. TEC2006-13731-C02-01 and the ScienceFoundation Ireland SFI under the Walton Awards schemeGrant No. 07/W.1/I1828 for their financial support. All au-thors acknowledge the SFI grant Grant No. 05/IN/1751 andthe SFI Ireland National Access Program at the Tyndall Na-tional Institute.1L. Yan, C. Lopez, R. Shrestha, E. Irene, A. Suvorova, and M. Saunders,Appl. Phys. Lett. 88, 142901 2006.2A. Posadas, F. Walker, C. Ahn, T. Goodrich, Z. Cai, and K. Ziemer, Appl.Phys. Lett. 92, 233511 2008.3II-VI and I-VII Compounds; Semimagnetic Compounds, Landolt-Börnstein, New Series, Group III Vol. 41B, edited by O. MadelungSpringer, Berlin, 1999.4B. Brennan, S. Mc Donnell, and G. Hughes, J. Phys.: Conf. Ser. 100,042047 2008.5E. Miranda and J. Suñé, Microelectron. Reliab. 44, 1 2004.6G. Ribes, J. Mitard, M. Denais, S. Bruyere, F. Monsieur, C. Parthasarathy,E. Vincent, and G. Ghibaudo, IEEE Trans. Device Mater. Reliab. 5, 52005.7S. Lombardo, A. La Magna, C. Gerardi, M. Alessandri, and F. Crupi,Appl. Phys. Lett. 75, 1161 1999.8E. Miranda, J. Suñé, R. Rodríguez, M. Nafría, and X. Aymerich, IEEEElectron Device Lett. 20, 265 1999.9S. Zafar, A. Callegari, E. Gusev, and M. Fischetti, J. Appl. Phys. 93, 92982003.10T. Tomita, H. Utsunomiya, T. Sakura, Y. Kamakura, and K. Taniguchi,IEEE Trans. Electron Devices 46, 159 1999.11E. Miranda, J. Suñé, R. Rodríguez, M. Nafría, and X. Aymerich, Solid-State Electron. 43, 1801 1999.12J. McPherson, J. Kim, A. Shanware, H. Mogul, and J. Rodríguez, Pro-ceedings of the IEEE International Electron Devices Meeting IEDM’02IEEE, New York, 2002, p. 633.13M. Houssa, T. Nigam, P. W. Mertens, and M. M. Heyns, J. Appl. Phys. 84,4351 1998.14K. Okada and K. Taniguchi, Appl. Phys. Lett. 70, 351 1997.15A. Cester, J. Suñé, and E. Miranda, Appl. Phys. Lett. 79, 1336 2001.16J. Suñé and E. Miranda, Tech. Dig. - Int. Electron Devices Meet. 2000,533.-8 -4 0 4 810-1310-1210-1110-1010-910-8-8 -4 0 4 8n-type(b)(a)Voltage [V]Current[A]Voltage [V]p-type n-typen-typeFIG. 4. a Effect of the substrate type and b effect of the injection polar-ity, on the SBD current. Open and filled symbols correspond to the fresh andSBD I-V characteristics, respectively.012901-3 Miranda et al. Appl. Phys. Lett. 95, 012901 2009

Electrical characterization of the soft breakdown failure mode in MgO layers

The soft breakdown (SBD) failure mode in 20 nm thick MgO dielectric layers grown on Si substrates was investigated. We show that during a constant voltage stress, charge trapping and progressive breakdown coexist, and that the degradation dynamics is captured by a power-law time dependence. We also show that the SBD current-voltage (I-V)characteristics follow the power-law model I=aVb typical of this conduction mechanism but in a wider voltage window than the one reported in the past for SiO2. The relationship between the magnitude of the current and the normalized differential conductance was analyzed

O'Connor, Eamon

Hughes, Greg

Casey, Patrick

O'Connell, Deborah

Diposit Digital de Documents de la UAB

Electrical characterization of the soft breakdown failure mode in MgO layersE. Miranda, E. O’Connor, K. Cherkaoui, S. Monaghan, R. Long, D. O’Connell, P. K. Hurley, G. Hughes, andP. Casey  Citation: Applied Physics Letters 95, 012901 (2009); doi: 10.1063/1.3167827 View online: http://dx.doi.org/10.1063/1.3167827 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/95/1?ver=pdfcov Published by the AIP Publishing                      This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:158.109.223.71 On: Tue, 18 Feb 2014 15:57:31Electrical characterization of the soft breakdown failure mode in MgOlayersE. Miranda,1,a E. O’Connor,2 K. Cherkaoui,2 S. Monaghan,2 R. Long,2 D. O’Connell,2P. K. Hurley,2 G. Hughes,3 and P. Casey31Escola Tècnica Superior d’Enginyeria, Universitat Autònoma de Barcelona, Campus UAB, 08193Bellaterra, Barcelona, Spain2Tyndall National Institute, University College Cork, Cork, Ireland3School of Physical Sciences, Dublin City University, Glasnevin, Dublin 9, IrelandReceived 21 April 2009; accepted 2 June 2009; published online 7 July 2009The soft breakdown SBD failure mode in 20 nm thick MgO dielectric layers grown on Sisubstrates was investigated. We show that during a constant voltage stress, charge trapping andprogressive breakdown coexist, and that the degradation dynamics is captured by a power-law timedependence. We also show that the SBD current-voltage I-V characteristics follow the power-lawmodel I=aVb typical of this conduction mechanism but in a wider voltage window than the onereported in the past for SiO2. The relationship between the magnitude of the current and thenormalized differential conductance was analyzed. © 2009 American Institute of Physics.DOI: 10.1063/1.3167827In recent papers, magnesium oxide MgO has beenpointed out as an appealing material for use as a gate insu-lator in metal-oxide-semiconductor MOS devices both forlow1 and high2 power applications. The main features of thisalkaline earth oxide are: a large band gap in the range 7.3 eVRef. 1–7.8 eV Ref. 3, ensuring sufficiently large bandoffsets with Si and therefore low leakage current, a dielectricpermittivity  ranging from 6.7 Ref. 4 to 10 Ref. 3 de-pending on the preparation method, high thermal conductiv-ity suitable for large power dissipation,2 and significantly, itschemical inertness, which minimizes the formation of an in-terfacial layer when deposited on Si substrates.1 In this letter,we have focused attention on a specific reliability aspect ofMgO-Si system, which is the conduction mechanism afterthe occurrence of soft breakdown SBD events.5 As is com-monly accepted, this failure mode arises as a consequence ofthe formation of a single or multiple nanoconstrictions acrossthe oxide layer after reaching a certain threshold density ofdefects caused by the electrical stress. This is a localizedconduction mode, independent of the device area, and hasbeen reported to occur in a number of high- thin films otherthan SiO2.6 However, contrary to the widespread opinion thatthis failure mechanism can only be observed in ultrathintox6 nm oxides, we have detected it in MgO films withnominal tox=20 nm, with tox the oxide thickness. Notably,our sample thickness almost doubles the maximum SiO2thickness tox=12 nm for which SBD has been reported.7 Inorder to demonstrate that we are strictly dealing with thesame charge transport mechanism, we have carried out athorough analysis of the conduction characteristics startingwith the widely accepted power-law model I=aVb, where aand b are constants.8 This allows us to examine the limits ofthe validity of the referred model, identify SBD events withleakage current levels several orders of magnitude lower thanthose detected in the past, which in thinner oxides wouldhave been overlooked and, finally, extend the relationshipexhibited by the fitting parameters a I at 1V, and b=d ln I /d ln V previously reported for the SiO2 /Si system.Hydrogen terminated Si surfaces were prepared by dip-ping n- and p-type 100 Si wafers 1015 cm−3 in a solutionof 5% hydrofluoric acid HF for 1 min, followed by rinsingin de-ionized water, and dried using nitrogen and were thenimmediately loaded into the deposition system. MgO films ofnominal tox=20 nm with 8.1 equivalent oxide thicknessEOT9.5 nm were deposited by electron beam evapora-tion from 99.9% MgO pellets at a rate of 0.2 Å/s, at 180 °C.The samples were capped in situ with 100 nm of amorphoussilicon -Si using a second e-beam source. For the NiSigate process, nickel was deposited by electron beam evapo-ration 80 nm through a patterned resist mask followedby a lift-off process. The rapid thermal annealing is a onestep process at 500 °C for 30 s in N2. The area of the devicestested is 1.610−5 cm2. Details about the SiO2 sampleswith tox4 nm considered in this study can be found in Ref.5.The dielectric breakdown of the MgO layer can be in-duced by either ramped or constant voltage stresses. Figure 1shows the effects of the latter type of stress on the leakageaAuthor to whom correspondence should be addressed. Tel.: 34-93-5813183. FAX: 34-93-5812600. Electronic mail: enrique.miranda@uab.es.1 10 10010-1010-9Progressive BDonsetExperimentalModelI=6.23x10-10 t-0.43n-type SiCurrent[A]Time [s]FIG. 1. Evolution of the gate leakage current under a constant voltage stressat 8 V. The solid and dashed lines correspond to experimental data andfitting results, respectively.APPLIED PHYSICS LETTERS 95, 012901 20090003-6951/2009/951/012901/3/$25.00 © 2009 American Institute of Physics95, 012901-1 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:158.109.223.71 On: Tue, 18 Feb 2014 15:57:31current. Notice that the current decreases according to theempirical law It= I0t−, where I0 and  are constants and tis the stress time, in agreement to what has been observed inother high-s such as Al2O3 and HfO2.9 The origin of thecurrent decay is still unclear but, since the process is partiallyreversible, it might be related to transient and permanentcharge trapping effects in pre-existing and induced traps. Theonset of dielectric breakdown is detected as a noise incre-ment in the I-t characteristic and this event is followed by aprogressive increase of the current progressive breakdownPBD. Alternatively, SBD can be achieved by successiveramped voltage stresses as shown in Fig. 2. The major dam-age to the sample is caused at the end of each sweep and, inmost of the cases, no current jump associated with SBD isobserved, an indication of the progressive opening of theleakage spots. The switching behavior, typical of digital SBDconduction,10 is clearly observed in the second and third I-Vcharacteristics. Contrary to what happens with ultrathin SiO2layers, in which SBD is only observable in a very limitedvoltage range because of the tunneling current that flowsthrough the nondamaged device area,11 in the present case ofa 20 nm thick sample the SBD curve can be detected in alarger voltage window. The SBD I-V curves cannot be asso-ciated with a single constriction and it is more likely thatmany leakage sites contribute to the total current. The insetin Fig. 2 supports this assertion and shows the occurrence ofseveral breakdown spots on the gate electrode after the stresssequence. According to our data the breakdown field forSBD in MgO is around 5–6 MV/cm which is comparable tothe breakdown field in HfO2 4–5 MV/cm but much lowerthan in SiO2 13–14 MV/cm.12 Regarding the voltage de-pendence of the SBD curves, notice that the linear relation-ship between logI and logV supports the use of thepower-law model. From the physical viewpoint, this modelhas been related to the conduction properties of nonlinearresistances networks with a distribution of percolationthresholds.13 However, it is worth pointing out that the modelis unable to capture the experimental fact that for larger dam-aged areas the slope of the I-V curves becomes shallower.This trend was first observed by Okada and Taniguchi14 inSiO2 and was thoroughly analyzed in Refs. 8 and 15 forsamples subjected to electrical stress and heavy ion irradia-tion, respectively. The change in slope is consistent with thefact that the hard breakdown HBD I-V characteristic hasslope close to unity,5 as expected for a simple resistor. Thisrelationship between the magnitude of the current a and itsnormalized differential conductance b which has not beenreported before for a high- with such a large thickness, hasbeen explained in terms of the mesoscopic transport proper-ties of the breakdown paths.16 Figure 3 shows the correlationbetween a and b obtained from a large number of I-V curvesand the inset shows a typical fitting to a SBD I-V character-istic in MgO using the power-law model. The points in Fig.3 cannot be considered as a statistically independent dataensemble since some of them were extracted from I-Vs mea-sured in the same sample after further degradation. On thecontrary, in Ref. 8 the parameters were extracted from inde-pendent measurements. Even though this parameter extrac-tion procedure can lead to uncertainty, what is remarkable isthat both data sets MgO and SiO2 follow very similar lineartrends. It is worth mentioning that thicker SiO2 films thanthose considered in this study only exhibit the HBD mode sothat the comparison between samples with similar largethicknesses is not possible. Figure 3 also shows that the cor-relation between a and b in MgO covers nearly ten orders ofmagnitude of the parameter a, whereas for SiO2 this correla-tion holds in a narrower range. A possible interpretation forthe observed correlation is that the size of the spots isstrongly linked to its conducting properties, in such a waythat the damaged area does not enter as a simple prefactor inthe current expression as commonly assumed for standardtransport mechanisms such as tunneling, Poole–Frenkel orSchottky emission. In addition, Fig. 3 shows that many of theSBD events occurring in MgO are softer than those detectedin SiO2. This can be a matter of the sample thickness, wherethe magnitude of tunneling currents permits the observationof softer events, but the possibility that it is related to par-ticular features of the conducting spots, such as the micro-scopic arrangement of the defect sites, cannot be ruled out. Inaddition, the fact that SBD can be observed in a thick oxidelayer is compatible with the idea that what fundamentallygoverns the conduction characteristics is the bottleneck ofthe constriction and not its total length.162 3 4 5 6 7 8 9 1010-1310-1210-1110-1010-910-810-710-6Fresh I-VSBDCurrent[A]Voltage [V]n-type SiFIG. 2. Experimental current-voltage characteristics. Symbols and lines cor-respond to the fresh and SBD I-V characteristics, respectively. Notice theuse of log-log axis in order to emphasize the power-law dependence. Themaximum voltage stress is 10 V. The inset shows several SBD spots distrib-uted over the device area.10-28 10-24 10-20 10-16 10-12 10-802468101214162 4 6 8 1010-1310-1210-1110-1010-910-810-7Current[A]Voltage [V]I=2.04x10-13 V5.62Experimental MgOLeast squares MgOLeast squares SiO2 [7]Linear extrapolation SiO2b=dln(I)/dln(V)aFIG. 3. Relationship between the parameters a and b in the power-lawmodel. The circles correspond to the region of SiO2 tox3.8–4.3 nmexperimental data given by the equation b=−0.78 loga−3.27 8. Thesquares correspond to the parameters extracted from the MgO SBD I-Vcurves. The solid line is given by the equation b=−0.90 loga−6.05.012901-2 Miranda et al. Appl. Phys. Lett. 95, 012901 2009 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:158.109.223.71 On: Tue, 18 Feb 2014 15:57:31To complete the electrical characterization of SBD inMgO, the roles played by the substrate type and gate voltageVG polarity were investigated. Figure 4a shows that simi-lar SBD currents can be found both for n VG0 and p-typeVG0 substrate samples in accumulation conditions. Iden-tical results arise for a n-type Si sample biased with bothinjection polarities as shown in Fig. 4b. Notice that this isnot the case for the fresh I-Vs. These plots indicate that theinjecting electrode does not play any major role after SBD aslong as there is sufficient charge available for conduction.The current seems to be limited by the breakdown path itself,so that its transmission properties become a key element forthe description of the phenomenon.5In summary, the postbreakdown electrical behavior ofMgO layers in MOS structures was investigated. It wasshown that SBD conduction can be detected in a relativelythick high- film and that the connection between the chargetransport mechanisms in MgO and SiO2 goes beyond the factthat they share the same power-law I-V model. The correla-tion found between the model parameters in both of the ma-terials investigated, and consequently, the apparent simplicityof the underlying physics, should perhaps be regarded as anindication of the low dimensionality of these conductingstructures.E.M. acknowledges the Generalitat de Catalunya BE-2007, the Ministerio de Ciencia y Tecnología MCyT,Spain Grant No. TEC2006-13731-C02-01 and the ScienceFoundation Ireland SFI under the Walton Awards schemeGrant No. 07/W.1/I1828 for their financial support. All au-thors acknowledge the SFI grant Grant No. 05/IN/1751 andthe SFI Ireland National Access Program at the Tyndall Na-tional Institute.1L. Yan, C. Lopez, R. Shrestha, E. Irene, A. Suvorova, and M. Saunders,Appl. Phys. Lett. 88, 142901 2006.2A. Posadas, F. Walker, C. Ahn, T. Goodrich, Z. Cai, and K. Ziemer, Appl.Phys. Lett. 92, 233511 2008.3II-VI and I-VII Compounds; Semimagnetic Compounds, Landolt-Börnstein, New Series, Group III Vol. 41B, edited by O. MadelungSpringer, Berlin, 1999.4B. Brennan, S. Mc Donnell, and G. Hughes, J. Phys.: Conf. Ser. 100,042047 2008.5E. Miranda and J. Suñé, Microelectron. Reliab. 44, 1 2004.6G. Ribes, J. Mitard, M. Denais, S. Bruyere, F. Monsieur, C. Parthasarathy,E. Vincent, and G. Ghibaudo, IEEE Trans. Device Mater. Reliab. 5, 52005.7S. Lombardo, A. La Magna, C. Gerardi, M. Alessandri, and F. Crupi,Appl. Phys. Lett. 75, 1161 1999.8E. Miranda, J. Suñé, R. Rodríguez, M. Nafría, and X. Aymerich, IEEEElectron Device Lett. 20, 265 1999.9S. Zafar, A. Callegari, E. Gusev, and M. Fischetti, J. Appl. Phys. 93, 92982003.10T. Tomita, H. Utsunomiya, T. Sakura, Y. Kamakura, and K. Taniguchi,IEEE Trans. Electron Devices 46, 159 1999.11E. Miranda, J. Suñé, R. Rodríguez, M. Nafría, and X. Aymerich, Solid-State Electron. 43, 1801 1999.12J. McPherson, J. Kim, A. Shanware, H. Mogul, and J. Rodríguez, Pro-ceedings of the IEEE International Electron Devices Meeting IEDM’02IEEE, New York, 2002, p. 633.13M. Houssa, T. Nigam, P. W. Mertens, and M. M. Heyns, J. Appl. Phys. 84,4351 1998.14K. Okada and K. Taniguchi, Appl. Phys. Lett. 70, 351 1997.15A. Cester, J. Suñé, and E. Miranda, Appl. Phys. Lett. 79, 1336 2001.16J. Suñé and E. Miranda, Tech. Dig. - Int. Electron Devices Meet. 2000,533.-8 -4 0 4 810-1310-1210-1110-1010-910-8-8 -4 0 4 8n-type(b)(a)Voltage [V]Current[A]Voltage [V]p-type n-typen-typeFIG. 4. a Effect of the substrate type and b effect of the injection polar-ity, on the SBD current. Open and filled symbols correspond to the fresh andSBD I-V characteristics, respectively.012901-3 Miranda et al. Appl. Phys. Lett. 95, 012901 2009 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:158.109.223.71 On: Tue, 18 Feb 2014 15:57:31

Miranda, E.

DCU Online Research Access Service

Electrical characterization of the soft breakdown failure mode in MgOlayersE. Miranda,1,a E. O’Connor,2 K. Cherkaoui,2 S. Monaghan,2 R. Long,2 D. O’Connell,2P. K. Hurley,2 G. Hughes,3 and P. Casey31Escola Tècnica Superior d’Enginyeria, Universitat Autònoma de Barcelona, Campus UAB, 08193Bellaterra, Barcelona, Spain2Tyndall National Institute, University College Cork, Cork, Ireland3School of Physical Sciences, Dublin City University, Glasnevin, Dublin 9, IrelandReceived 21 April 2009; accepted 2 June 2009; published online 7 July 2009The soft breakdown SBD failure mode in 20 nm thick MgO dielectric layers grown on Sisubstrates was investigated. We show that during a constant voltage stress, charge trapping andprogressive breakdown coexist, and that the degradation dynamics is captured by a power-law timedependence. We also show that the SBD current-voltage I-V characteristics follow the power-lawmodel I=aVb typical of this conduction mechanism but in a wider voltage window than the onereported in the past for SiO2. The relationship between the magnitude of the current and thenormalized differential conductance was analyzed. © 2009 American Institute of Physics.DOI: 10.1063/1.3167827In recent papers, magnesium oxide MgO has beenpointed out as an appealing material for use as a gate insu-lator in metal-oxide-semiconductor MOS devices both forlow1 and high2 power applications. The main features of thisalkaline earth oxide are: a large band gap in the range 7.3 eVRef. 1–7.8 eV Ref. 3, ensuring sufficiently large bandoffsets with Si and therefore low leakage current, a dielectricpermittivity  ranging from 6.7 Ref. 4 to 10 Ref. 3 de-pending on the preparation method, high thermal conductiv-ity suitable for large power dissipation,2 and significantly, itschemical inertness, which minimizes the formation of an in-terfacial layer when deposited on Si substrates.1 In this letter,we have focused attention on a specific reliability aspect ofMgO-Si system, which is the conduction mechanism afterthe occurrence of soft breakdown SBD events.5 As is com-monly accepted, this failure mode arises as a consequence ofthe formation of a single or multiple nanoconstrictions acrossthe oxide layer after reaching a certain threshold density ofdefects caused by the electrical stress. This is a localizedconduction mode, independent of the device area, and hasbeen reported to occur in a number of high- thin films otherthan SiO2.6 However, contrary to the widespread opinion thatthis failure mechanism can only be observed in ultrathintox6 nm oxides, we have detected it in MgO films withnominal tox=20 nm, with tox the oxide thickness. Notably,our sample thickness almost doubles the maximum SiO2thickness tox=12 nm for which SBD has been reported.7 Inorder to demonstrate that we are strictly dealing with thesame charge transport mechanism, we have carried out athorough analysis of the conduction characteristics startingwith the widely accepted power-law model I=aVb, where aand b are constants.8 This allows us to examine the limits ofthe validity of the referred model, identify SBD events withleakage current levels several orders of magnitude lower thanthose detected in the past, which in thinner oxides wouldhave been overlooked and, finally, extend the relationshipexhibited by the fitting parameters a I at 1V, and b=d ln I /d ln V previously reported for the SiO2 /Si system.Hydrogen terminated Si surfaces were prepared by dip-ping n- and p-type 100 Si wafers 1015 cm−3 in a solutionof 5% hydrofluoric acid HF for 1 min, followed by rinsingin de-ionized water, and dried using nitrogen and were thenimmediately loaded into the deposition system. MgO films ofnominal tox=20 nm with 8.1 equivalent oxide thicknessEOT9.5 nm were deposited by electron beam evapora-tion from 99.9% MgO pellets at a rate of 0.2 Å/s, at 180 °C.The samples were capped in situ with 100 nm of amorphoussilicon -Si using a second e-beam source. For the NiSigate process, nickel was deposited by electron beam evapo-ration 80 nm through a patterned resist mask followedby a lift-off process. The rapid thermal annealing is a onestep process at 500 °C for 30 s in N2. The area of the devicestested is 1.610−5 cm2. Details about the SiO2 sampleswith tox4 nm considered in this study can be found in Ref.5.The dielectric breakdown of the MgO layer can be in-duced by either ramped or constant voltage stresses. Figure 1shows the effects of the latter type of stress on the leakageaAuthor to whom correspondence should be addressed. Tel.: 34-93-5813183. FAX: 34-93-5812600. Electronic mail: enrique.miranda@uab.es.1 10 10010-1010-9Progressive BDonsetExperimentalModelI=6.23x10-10 t-0.43n-type SiCurrent[A]Time [s]FIG. 1. Evolution of the gate leakage current under a constant voltage stressat 8 V. The solid and dashed lines correspond to experimental data andfitting results, respectively.APPLIED PHYSICS LETTERS 95, 012901 20090003-6951/2009/951/012901/3/$25.00 © 2009 American Institute of Physics95, 012901-1Downloaded 03 Aug 2010 to 136.206.1.49. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspcurrent. Notice that the current decreases according to theempirical law It= I0t−, where I0 and  are constants and tis the stress time, in agreement to what has been observed inother high-s such as Al2O3 and HfO2.9 The origin of thecurrent decay is still unclear but, since the process is partiallyreversible, it might be related to transient and permanentcharge trapping effects in pre-existing and induced traps. Theonset of dielectric breakdown is detected as a noise incre-ment in the I-t characteristic and this event is followed by aprogressive increase of the current progressive breakdownPBD. Alternatively, SBD can be achieved by successiveramped voltage stresses as shown in Fig. 2. The major dam-age to the sample is caused at the end of each sweep and, inmost of the cases, no current jump associated with SBD isobserved, an indication of the progressive opening of theleakage spots. The switching behavior, typical of digital SBDconduction,10 is clearly observed in the second and third I-Vcharacteristics. Contrary to what happens with ultrathin SiO2layers, in which SBD is only observable in a very limitedvoltage range because of the tunneling current that flowsthrough the nondamaged device area,11 in the present case ofa 20 nm thick sample the SBD curve can be detected in alarger voltage window. The SBD I-V curves cannot be asso-ciated with a single constriction and it is more likely thatmany leakage sites contribute to the total current. The insetin Fig. 2 supports this assertion and shows the occurrence ofseveral breakdown spots on the gate electrode after the stresssequence. According to our data the breakdown field forSBD in MgO is around 5–6 MV/cm which is comparable tothe breakdown field in HfO2 4–5 MV/cm but much lowerthan in SiO2 13–14 MV/cm.12 Regarding the voltage de-pendence of the SBD curves, notice that the linear relation-ship between logI and logV supports the use of thepower-law model. From the physical viewpoint, this modelhas been related to the conduction properties of nonlinearresistances networks with a distribution of percolationthresholds.13 However, it is worth pointing out that the modelis unable to capture the experimental fact that for larger dam-aged areas the slope of the I-V curves becomes shallower.This trend was first observed by Okada and Taniguchi14 inSiO2 and was thoroughly analyzed in Refs. 8 and 15 forsamples subjected to electrical stress and heavy ion irradia-tion, respectively. The change in slope is consistent with thefact that the hard breakdown HBD I-V characteristic hasslope close to unity,5 as expected for a simple resistor. Thisrelationship between the magnitude of the current a and itsnormalized differential conductance b which has not beenreported before for a high- with such a large thickness, hasbeen explained in terms of the mesoscopic transport proper-ties of the breakdown paths.16 Figure 3 shows the correlationbetween a and b obtained from a large number of I-V curvesand the inset shows a typical fitting to a SBD I-V character-istic in MgO using the power-law model. The points in Fig.3 cannot be considered as a statistically independent dataensemble since some of them were extracted from I-Vs mea-sured in the same sample after further degradation. On thecontrary, in Ref. 8 the parameters were extracted from inde-pendent measurements. Even though this parameter extrac-tion procedure can lead to uncertainty, what is remarkable isthat both data sets MgO and SiO2 follow very similar lineartrends. It is worth mentioning that thicker SiO2 films thanthose considered in this study only exhibit the HBD mode sothat the comparison between samples with similar largethicknesses is not possible. Figure 3 also shows that the cor-relation between a and b in MgO covers nearly ten orders ofmagnitude of the parameter a, whereas for SiO2 this correla-tion holds in a narrower range. A possible interpretation forthe observed correlation is that the size of the spots isstrongly linked to its conducting properties, in such a waythat the damaged area does not enter as a simple prefactor inthe current expression as commonly assumed for standardtransport mechanisms such as tunneling, Poole–Frenkel orSchottky emission. In addition, Fig. 3 shows that many of theSBD events occurring in MgO are softer than those detectedin SiO2. This can be a matter of the sample thickness, wherethe magnitude of tunneling currents permits the observationof softer events, but the possibility that it is related to par-ticular features of the conducting spots, such as the micro-scopic arrangement of the defect sites, cannot be ruled out. Inaddition, the fact that SBD can be observed in a thick oxidelayer is compatible with the idea that what fundamentallygoverns the conduction characteristics is the bottleneck ofthe constriction and not its total length.162 3 4 5 6 7 8 9 1010-1310-1210-1110-1010-910-810-710-6Fresh I-VSBDCurrent[A]Voltage [V]n-type SiFIG. 2. Experimental current-voltage characteristics. Symbols and lines cor-respond to the fresh and SBD I-V characteristics, respectively. Notice theuse of log-log axis in order to emphasize the power-law dependence. Themaximum voltage stress is 10 V. The inset shows several SBD spots distrib-uted over the device area.10-28 10-24 10-20 10-16 10-12 10-802468101214162 4 6 8 1010-1310-1210-1110-1010-910-810-7Current[A]Voltage [V]I=2.04x10-13 V5.62Experimental MgOLeast squares MgOLeast squares SiO2 [7]Linear extrapolation SiO2b=dln(I)/dln(V)aFIG. 3. Relationship between the parameters a and b in the power-lawmodel. The circles correspond to the region of SiO2 tox3.8–4.3 nmexperimental data given by the equation b=−0.78 loga−3.27 8. Thesquares correspond to the parameters extracted from the MgO SBD I-Vcurves. The solid line is given by the equation b=−0.90 loga−6.05.012901-2 Miranda et al. Appl. Phys. Lett. 95, 012901 2009Downloaded 03 Aug 2010 to 136.206.1.49. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspTo complete the electrical characterization of SBD inMgO, the roles played by the substrate type and gate voltageVG polarity were investigated. Figure 4a shows that simi-lar SBD currents can be found both for n VG0 and p-typeVG0 substrate samples in accumulation conditions. Iden-tical results arise for a n-type Si sample biased with bothinjection polarities as shown in Fig. 4b. Notice that this isnot the case for the fresh I-Vs. These plots indicate that theinjecting electrode does not play any major role after SBD aslong as there is sufficient charge available for conduction.The current seems to be limited by the breakdown path itself,so that its transmission properties become a key element forthe description of the phenomenon.5In summary, the postbreakdown electrical behavior ofMgO layers in MOS structures was investigated. It wasshown that SBD conduction can be detected in a relativelythick high- film and that the connection between the chargetransport mechanisms in MgO and SiO2 goes beyond the factthat they share the same power-law I-V model. The correla-tion found between the model parameters in both of the ma-terials investigated, and consequently, the apparent simplicityof the underlying physics, should perhaps be regarded as anindication of the low dimensionality of these conductingstructures.E.M. acknowledges the Generalitat de Catalunya BE-2007, the Ministerio de Ciencia y Tecnología MCyT,Spain Grant No. TEC2006-13731-C02-01 and the ScienceFoundation Ireland SFI under the Walton Awards schemeGrant No. 07/W.1/I1828 for their financial support. All au-thors acknowledge the SFI grant Grant No. 05/IN/1751 andthe SFI Ireland National Access Program at the Tyndall Na-tional Institute.1L. Yan, C. Lopez, R. Shrestha, E. Irene, A. Suvorova, and M. Saunders,Appl. Phys. Lett. 88, 142901 2006.2A. Posadas, F. Walker, C. Ahn, T. Goodrich, Z. Cai, and K. Ziemer, Appl.Phys. Lett. 92, 233511 2008.3II-VI and I-VII Compounds; Semimagnetic Compounds, Landolt-Börnstein, New Series, Group III Vol. 41B, edited by O. MadelungSpringer, Berlin, 1999.4B. Brennan, S. Mc Donnell, and G. Hughes, J. Phys.: Conf. Ser. 100,042047 2008.5E. Miranda and J. Suñé, Microelectron. Reliab. 44, 1 2004.6G. Ribes, J. Mitard, M. Denais, S. Bruyere, F. Monsieur, C. Parthasarathy,E. Vincent, and G. Ghibaudo, IEEE Trans. Device Mater. Reliab. 5, 52005.7S. Lombardo, A. La Magna, C. Gerardi, M. Alessandri, and F. Crupi,Appl. Phys. Lett. 75, 1161 1999.8E. Miranda, J. Suñé, R. Rodríguez, M. Nafría, and X. Aymerich, IEEEElectron Device Lett. 20, 265 1999.9S. Zafar, A. Callegari, E. Gusev, and M. Fischetti, J. Appl. Phys. 93, 92982003.10T. Tomita, H. Utsunomiya, T. Sakura, Y. Kamakura, and K. Taniguchi,IEEE Trans. Electron Devices 46, 159 1999.11E. Miranda, J. Suñé, R. Rodríguez, M. Nafría, and X. Aymerich, Solid-State Electron. 43, 1801 1999.12J. McPherson, J. Kim, A. Shanware, H. Mogul, and J. Rodríguez, Pro-ceedings of the IEEE International Electron Devices Meeting IEDM’02IEEE, New York, 2002, p. 633.13M. Houssa, T. Nigam, P. W. Mertens, and M. M. Heyns, J. Appl. Phys. 84,4351 1998.14K. Okada and K. Taniguchi, Appl. Phys. Lett. 70, 351 1997.15A. Cester, J. Suñé, and E. Miranda, Appl. Phys. Lett. 79, 1336 2001.16J. Suñé and E. Miranda, Tech. Dig. - Int. Electron Devices Meet. 2000,533.-8 -4 0 4 810-1310-1210-1110-1010-910-8-8 -4 0 4 8n-type(b)(a)Voltage [V]Current[A]Voltage [V]p-type n-typen-typeFIG. 4. a Effect of the substrate type and b effect of the injection polar-ity, on the SBD current. Open and filled symbols correspond to the fresh andSBD I-V characteristics, respectively.012901-3 Miranda et al. Appl. Phys. Lett. 95, 012901 2009Downloaded 03 Aug 2010 to 136.206.1.49. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Electrical characterization of the soft breakdown failure mode in MgO layers

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