Impact of moisture on charge trapping and flatband voltage in Al 2 O 3 gate dielectricfilmsSufi Zafar, A. Callegari, Vijay Narayanan, and Supratik Guha Citation: Applied Physics Letters 81, 2608 (2002); doi: 10.1063/1.1506788 View online: http://dx.doi.org/10.1063/1.1506788 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/81/14?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Al 2 O 3 / NbAlO / Al 2 O 3 sandwich gate dielectric film on InP Appl. Phys. Lett. 96, 022904 (2010); 10.1063/1.3292217 Electrical performance of Al 2 O 3 gate dielectric films deposited by atomic layer deposition on 4 H - Si C Appl. Phys. Lett. 91, 203510 (2007); 10.1063/1.2805742 Interfacial reaction depending on the stack structure of Al 2 O 3 and Hf O 2 during film growth and postannealing Appl. Phys. Lett. 85, 4115 (2004); 10.1063/1.1807968 Characterization of nonstoichiometric TiO 2 and ZrO 2 thin films stabilized by Al 2 O 3 and SiO 2 additions J. Vac. Sci. Technol. A 21, 1996 (2003); 10.1116/1.1622675 Effects of plasma nitridation of Al 2 O 3 interlayer on thermal stability, fixed charge density, and interfacial trapstates of HfO 2 gate dielectric films grown by atomic layer deposition J. Appl. Phys. 94, 1898 (2003); 10.1063/1.1590418

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Impact of moisture on charge trapping and flatband voltage in Al 2O3 gatedielectric films

Sufi Zafar,a) A. Callegari, Vijay Narayanan, and Supratik GuhaIBM Research, Thomas J. Watson Research Center, Yorktown Heights, New York 10598

~Received 18 February 2002; accepted for publication 22 July 2002!

High permittivity gate dielectric films exhibit significant charge trapping during electrical stressing.In this letter, the impact of low temperature postmetallization anneals on charge trapping in Al2O3

films is reported; anneals were performed in vacuum and forming gas ambient. These anneals notonly reduce charge trapping but also move the flatband voltage towards an ideal value thatcorresponds to the difference in work function. Based on these experiments, it is concluded thatabsorbed moisture in the films provides hydrogen that concomitantly reduces charge trapping andfixed charge densities in Al2O3 films. © 2002 American Institute of Physics.@DOI: 10.1063/1.1506788#

As SiO2 gate dielectric films are made thinner in ad-vanced complementary metal–oxide–semiconductor~CMOS! devices, the leakage current increasesexponentially.1 To continue the scaling of CMOS devices, areplacement gate dielectric is needed. High permittivitymetal–oxide films are possible replacement candidates.2–4

To successfully replace the SiO2 gate dielectric by high per-mittivity thin films, electrical properties such as the conduc-tion mechanism, reliability, and charge trapping need to bebetter understood. In this letter, we focus on the charge trap-ping that gives rise to flatband voltage shifts. As the dielec-tric film is electrically stressed, charges are injected into thedielectric film. Some of the charge injected get trapped in thefilm, thereby causing the flatband voltage (Vfb) to shift ac-cording to the stress time. Charge trapping in high dielectricfilms is poorly understood. It is not known whether the trapsare an intrinsic property of the material or related to extrinsicimpurities. In this letter, we focus on charge trapping inAl2O3 gate dielectric films. Experiments were performed thatshow low temperature postmetallization anneals reducecharge trapping and fixed charge densities in Al2O3 gate di-electric films.

Charge trapping measurements were performed on MOScapacitor structures with Al2O3 gate dielectric films. Al2O3

films were deposited at 650 °C by ultrahigh vacuum deposi-tion in a molecular beam epitaxy system equipped with stan-dard thermally heated effusion sources; details of the depo-sition are given elsewhere.5 Al2O3 films were deposited onhydrofluoric treatedn-type silicon substrates with dopingdensity of ;131016 cm23. After deposition, the sampleswere transferred in air and subjected to a forming gas annealat 600 °C for about 30 min. Transmission electron micros-copy ~TEM! images show the presence of an 8–10 Å thickinterfacial layer at the silicon interface and a 40 Å thickAl2O3 layer. The equivalent oxide thickness of the stack isestimated to be 20 Å from high frequency~100 kHz! C–Vcurves using the Maserjian method.6 A gate was formed byevaporation of aluminum through a shadow mask. The gate

area is in the range of 6.931025– 1.331024 cm2. All theelectrical measurements were performed at room tempera-ture.

Before discussing the charge trapping results, the meth-odology for measuring charge trapping as a function of stresstime and injected charge density is briefly described. A seriesof voltage pulses are applied at the gate. During each pulse,the current density is measured to obtain an estimate of theinjected charge density (Qinj). Between each stress pulse, acapacitance versus voltage curve is measured to estimate theshift in flatband voltage (DVfb). To ensure thatDVfb occursonly due to the stress voltage pulse,C–V measurement wasperformed over a limited voltage range as shown in Fig. 1.Figure 1 shows theC–V curves measured between stressingpulses for a MOS capacitor with an Al2O3 film. The C–Vcurves are measured between20.6 and 0.0 V at 100 kHz andthe stress voltage pulse applied at the gate has a height of 2.0V. TheC–V curve is observed to shift towards positive volt-age with stressing time; the shift in flatband voltage (DVfb)is measured with respect to theC–V curve obtained beforestressing att50 s. SinceDVfb is attributed to trapping ofcharges by the gate dielectric film, the trapped charged den-sity (Nt) can be calculated by assuming that the trappedcharge centroid is located at the interface of the Al2O3 andthe interfacial silicon oxide. Therefore,Nt5(DVfb* d)/k«0 ,

a!Electronic mail: szafar@us.ibm.com

FIG. 1. C–V curves for increasing stress time at a stress voltage (Vs) of 2.0V; the C–V curves measured at 100 kHz; the curve labeledt50 s corre-sponds to theC–V curve measured before stressing; theC–V curve movestowards the right with an increase in stress time.

APPLIED PHYSICS LETTERS VOLUME 81, NUMBER 14 30 SEPTEMBER 2002

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where«0 is the permittivity of free space,d is the physicalthickness, andk510 is the dielectric constant of the Al2O3

film. In order to further quantify the charge trapping charac-teristics, we define a parameter, referred to as the trappingprobability (Pn), Pn5(DNt)/(DQinj) whereDNt andDQinj

are the changes in the trapped charge and injected chargedensities, respectively. Using this methodology, charge trap-ping in Al2O3 gate dielectric film was investigated as a func-tion of postmetallization anneals.

Charge trapping characteristics for Al2O3 films weremeasured before and after postmetallization anneals. Twotypes of anneals were performed: vacuum and forming gas~95% N2 and 5% H2). The motivation for choosing these twotypes of anneals is as follows. Bulk Al2O3 films are reportedto be hygroscopic and are used as moisture sensors.7 If thinAl2O3 gate dielectric films show hygroscopic propertiessimilar to those of bulk films, then vacuum annealing shouldbe effective in removing the absorbed moisture and therebycausing changes in the films. Another unrelated experimentshows that Al gates react with absorbed moisture in SiO2

films and form hydrogen and the hydrogen then passivates Siinterfacial states.8,9 If a similar Al gate related reaction alsooccurs in Al2O3 films, then absorbed moisture in the Al2O3

film would be converted to hydrogen that would then bringabout changes in the electrical properties of the gate dielec-tric stack. In order to determine whether absorbed moisturein Al2O3 is removed or reduced to hydrogen, vacuum andforming gas~fg! anneals were performed. Figure 2 shows thedependence ofDVfb on stressing time before and aftervacuum and fg anneals. The vacuum anneal was performedat a pressure of 1027 Torr whereas the forming gas annealwas done at 760 Torr; both anneals were performed at 20565 °C for 1.5 h. Dashed line curves denote the data beforeannealing and open and closed symbols denote the data aftervacuum and fg annealing, respectively. As shown in Fig. 2,DVfb decreases after both the vacuum and fg anneals and thedecrease is approximately the same for both types of anneals.For example, at stress time of 10 s,DVfb582 mV beforeannealing andDVfb53065 and 2465 mV after vacuum andforming gas annealing, respectively. Using theDVfb datashown in Fig. 2, the charge trapping probability (Pn) is es-timated. Figure 3 shows the dependence ofPn on injectedcharge density before and after anneals. The trapping prob-

ability decreases by about a factor of;3 after both vacuumand forming gas anneals;Pn curves corresponding tovacuum and forming gas anneals are approximately thesame. In summary, Figs. 2 and 3 show that charge trapping isreduced by low temperature postmetallization anneals. Twopossible reasons for the reduction in charge trapping are re-moval of absorbed moisture from the Al2O3 film or the ab-sorbed moisture undergoes a chemical reaction similar to thatobserved in postmetallization studies of SiO2 films8,9 andthereby provides hydrogen which then passivates defects. Ifthe removal of absorbed moisture is the dominant mecha-nism then one would expect charge trapping to be larger forthe vacuum anneal because the pressure during the vacuumanneal is about 10 orders of magnitude smaller than thatduring the forming gas anneal. Since the charge trapping isapproximately the same during both anneals, it is proposedthat the charge trapping is reduced due to the absorbed mois-ture undergoing chemical reaction and thereby providing hy-drogen to passivate defects in the Al2O3 gate dielectric stack.

In addition to investigating the impact of postmetalliza-tion anneals on charge trapping, the dependence of currentdensity versus voltage (J–V) and C–V were also investi-gated. Both vacuum and forming gas anneals were per-formed at 205 °C for 1.5 h. The anneals produced no mea-surable change in theJ–V curves. Figure 4 showsC–Vcurves measured before and after anneals. As is shown, both

FIG. 2. Dependence of the shift in flatband voltage (DVfb) on stressing timebefore and after annealing of MOS capacitors with an Al2O3 gate dielectricand Al gates; annealing was done in vacuum and forming gas ambient at205 °C for 1.5 h and the electrical measurements were done at room tem-perature.

FIG. 3. Dependence of the trapping probability (Pn) on the injected chargedensity (Qinj) before and after annealing of Al2O3 MOS capacitors; anneal-ing was done in vacuum and forming gas ambient at 205 °C for 1.5 h andthe electrical measurements were performed at room temperature.

FIG. 4. Capacitancevs. voltage curves measured at 100 kHz before andafter vacuum and forming gas annealing of Al2O3 capacitors; annealing wasdone at 205 °C for 1.5 h and theC–V measurements were done at roomtemperature.

2609Appl. Phys. Lett., Vol. 81, No. 14, 30 September 2002 Zafar et al.

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vacuum and forming gas anneals cause theC–V curves toshift towards the right with no measurable change in shape.The flatband voltage (Vfb) is 20.55, 20.49, and20.42 Vbefore annealing, after vacuum annealing, and after forminggas annealing, respectively. Since the idealVfb520.23 V~5difference in work function!, both anneals causeVfb tomove closer to the ideal value, thereby indicating a decreasein fixed charge density. It may be noted that no measurablechange in the gate dielectric stack is observed. These annealinduced shifts in the flatband voltage are attributed to passi-vation of defects by hydrogen. As discussed earlier, hydro-gen is supplied by absorbed moisture in Al2O3 films for thecase of the vacuum anneal.

In conclusion, low temperature postmetallizationvacuum and forming gas anneals reduce charge trapping inAl2O3 gate dielectric films. In addition, anneals also causethe flatband value to move toward its ideal value. These im-provements in charge trapping and flatband voltage are at-tributed to the passivation of defects by hydrogen.

The authors would like to thank C. Cabral and D. Laceyfor their help in the forming gas anneal and D. Buchanan, E.

Gusev, and S. Cohen for helpful discussions. They wouldalso like to thank B. Linder and G. Singco for their help withthe computer programs.

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2610 Appl. Phys. Lett., Vol. 81, No. 14, 30 September 2002 Zafar et al.

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