Charge trapping studies on ultrathin ZrO 2 and HfO 2 high-k dielectrics grown by roomtemperature ultraviolet ozone oxidationShriram Ramanathan, Paul C. McIntyre, Supratik Guha, and Evgeni Gusev Citation: Applied Physics Letters 84, 389 (2004); doi: 10.1063/1.1636532 View online: http://dx.doi.org/10.1063/1.1636532 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/84/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Optical second harmonic generation studies of ultrathin high- k dielectric stacks J. Appl. Phys. 97, 083711 (2005); 10.1063/1.1861146 Characteristics of metal-insulator-semiconductor capacitors based on high- k HfAlO dielectric films obtained bylow-temperature electron-beam gun evaporation Appl. Phys. Lett. 85, 5950 (2004); 10.1063/1.1836875 Film and interface layer properties of ultraviolet-ozone oxidized hafnia and zirconia gate dielectrics on siliconsubstrates Appl. Phys. Lett. 85, 4699 (2004); 10.1063/1.1814799 Charge trapping and interface characteristics of thermally evaporated HfO 2 Appl. Phys. Lett. 85, 3289 (2004); 10.1063/1.1805708 Dielectric property and conduction mechanism of ultrathin zirconium oxide films Appl. Phys. Lett. 79, 3666 (2001); 10.1063/1.1418265

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Charge trapping studies on ultrathin ZrO 2 and HfO 2 high- k dielectricsgrown by room temperature ultraviolet ozone oxidation

Shriram Ramanathana) and Paul C. McIntyreb)

Department of Materials Science and Engineering, Stanford University, Stanford, California 94305

Supratik Guha and Evgeni GusevIBM Thomas J. Watson Research Center and Semiconductor Research and Development Center (SRDC),Yorktown Heights, New York 10598

~Received 8 August 2003; accepted 5 November 2003!

We report detailed charge trapping reliability characteristics of ultrathin ZrO2 and HfO2 dielectricsgrown by the ultraviolet ozone oxidation~UVO! method. We also discuss the dependence of chargetrapping on the top electrode deposition process, specificallyin situ versusex situprocessed gates.Electrical data show that the charge trapping characteristics depend on the deposition method for thetop electrode, likely due to exposure of the high-k material to ambient prior to deposition of theelectrode. It is also shown thatC–V hysteresis is not a complete measure of the charge trappingcharacteristics of the dielectrics. ©2004 American Institute of Physics.@DOI: 10.1063/1.1636532#

Several metal oxide dielectrics are currently being inves-tigated as potential candidates to replace SiO2 in futurecomplementary metal–oxide–semiconductor~CMOS! de-vices. Among these materials, oxides of Zr and Hf are con-sidered among the most promising due to their relativelyhigh dielectric constants, large conduction band offsets withSi, and solid state thermodynamic stability on Si and SiO2 .1

Besides research on high-dielectric constant materials, thereis also considerable research in the area of gate electrodes toreplace polysilicon due to~i! the incompatibility of somehigh-k materials with poly-Si gate electrodes,2 and ~ii ! nopolydepletion effects in metal gates.3 It is, therefore, impor-tant to understand the electrical properties of the high-k ma-terials in order to evaluate their application in devices andalso the correlation between the high-k material and the gateelectrode.

One of the important electrical properties which deter-mines the application of these metal oxides in CMOS de-vices is their charge trapping characteristics. Charge trapping~CT! can affect the reliability of the devices, alter the thresh-old voltage of transistors over time, and also affect the defectnature of the dielectrics. From this perspective, it is impor-tant to understand the electrical properties of high-dielectricconstant dielectrics, in particular charge trapping in thehigh-k layer itself and in combination with metal gates~bothas a function of the deposition method!. However, there arefew reports on the effects of the deposition method for thetop electrode on the electrical properties of the gate dielec-tric. While many studies have reported the electrical proper-ties of dielectrics,1 there is still not much understanding ofthe reliability of high-k dielectrics and especially their de-pendence on the gate electrode material.4–6

In this letter, we report detailed electrical properties of

oxides of Zr and Hf grown by the ultraviolet ozone~UVO!oxidation method. The focus of our studies is charge trappingon ZrO2 and HfO2, and we report also their dependence onthe top electrode. We show that the exposure of uncappedhigh-k oxide to ambient air and the method used to depositthe top electrode both play a significant role in determiningthe CT characteristics of MOS capacitors. We also discussreasons why commonly used capacitance–voltage (C–V)hysteresis results may not give an accurate picture of the CTproperties.

Thin ~;2 nm! films of Zr ~and Hf! were deposited onultrathin SiO2 films on ~001! Si substrates by sputtering atroom temperature. The metal films were subsequently com-pletely oxidized by room temperature ultraviolet ozone oxi-dation, described elsewhere.7 A 50 nm thick Pt film was de-positedin situ by magnetron sputtering at room temperature.In some cases, Pt top electrodes were also depositedex situby both sputtering and electron beam evaporation for com-parison purposes. For samples with Pt depositedin situ, ca-

a!Present address: Components Research, Intel Corporation, Hillsboro,OR 97124.

b!Corresponding author; electronic mail: pcmI@stanford.eduFIG. 1. Capacitance–voltage curves measured at 100 kHz for zirconia filmsoxidized for 30 and 60 min at 600 Torr.

APPLIED PHYSICS LETTERS VOLUME 84, NUMBER 3 19 JANUARY 2004

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pacitor structures were fabricated by photolithography meth-ods, while for ex situ samples, the Pt film was depositedthrough shadow masks. In all cases, the capacitors were sub-jected to a forming gas anneal~5% H2 in N2) at 400 °C for30 min. Detailed electrical studies were performed on theMOSCAP structures includingC–V measurements, andleakage current (I –V) measurements. Charge trapping stud-ies were performed by injecting varying levels of charge intothe dielectrics and measuring the shift in flatband as a func-tion of time, described in Ref. 8.

Figure 1 shows double sweepC–V characteristics ofZrO2 /SiO2 gate stacks within situ grown Pt top electrodesoxidized for 30 and 60 min. The equivalent oxide thickness~EOT!, after quantum mechanical correction,9 is ;1 and 0.85nm for the 60 and 30 min samples, respectively. The smalldifference in EOT is due to the fact that shorter oxidationtimes lead to less growth of the SiO2 underlayer, resulting inreduced EOT. There is negligible frequency dependence ofthe C–V curves. TheC–V hysteresis is,20 mV for theseUVO gate stacks. Figure 2 shows the charge trapping char-acteristics of the ZrO2 dielectric films. For stress voltage of21.5 V, the change in the flatband shift is less than 30 mVfor injected charge of nearly 100 C/cm2 for the 60 min oxi-dized sample. The sample oxidized for only 30 min displaysmuch higher trapping characteristics~almost 60 mV!, possi-

bly due to a higher concentration of oxygen vacancy relatedtraps due to the shorter oxidation time.

It is important to note that even thoughC–V curvesmeasured in both dielectrics show negligible hysteresis~Fig.1!, their CT behavior is quite different~Fig. 2!. Figure 3shows the CT behavior of the 60 min oxidized sample as afunction of the stress voltage. For bias stress of up to21.5 Vin the flatband, theC–V shift is less than 30 mV, indicatingthe good quality of the oxides grown by the UVO method.One should also note that even after normalization to in-jected charge, the flatband still depends on gate bias condi-tions, consistent with observations reported in Ref. 10.

Figures 4~a!–4~c! show, respectively,C–V curves mea-sured at 100 kHz for hafnia dielectrics grown on SiO2 . Oneof the goals of this experiment was to examine charge trap-ping for Pt top electrodes deposited byin situ sputteringversusex situsputtering versusex situelectron-beam evapo-ration. It is important to point out that theC–V hysteresis issimilar in all cases. There is a slight dependence on the EOTdue to interfacial layer growth upon exposure to ambientwhile breaking vacuum due to the shadow mask process.Figure 5 shows the CT behavior of the three hafnia samples.A remarkable difference is seen in their CT behavior, namelythe in situ Pt sputtered electrode shows the best result interms of charge trapping, while,ex situe-beam evaporated Pt

FIG. 2. Flatband shift as a function of the charge injected for~a! 30 and~b!60 min at 600 Torr.

FIG. 3. Flatband shift~for zirconia films oxidized for 60 min! as a functionof the charge injected for different stress voltages.

FIG. 4. Capacitance–voltage curves for identical hafnia films with~a! insitu sputtered Pt,~b! ex situsputtered Pt and~c! ex situevaporated Pt as topelectrodes.

FIG. 5. Flatband shift as a function of the charge injected for hafnia dielec-trics with Pt grown under different conditions.

390 Appl. Phys. Lett., Vol. 84, No. 3, 19 January 2004 Ramanathan et al.

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has the poorest CT characteristic. The substantial shift in theflatband voltage observed for the e-beam evaporated Ptsample is attributed to radiation damage of the dielectric byenergetic~.5 keV! x rays emitted from the e-beam filamentduring electrode deposition. This is a well-known problemwith electron-beam evaporation on SiO2 .11,12

In summary, we investigated electrical properties of ul-trathin HfO2 and ZrO2 gate dielectrics with an emphasis oncharge trapping behavior and its dependence on the gateelectrode deposition process. We found that both high-k ma-terials exhibited good electrical quality. One of the most im-portant observations of this work was that electrical proper-ties ~specifically charge trapping! do depend on gateelectrode processing, within situ sputtered Pt gate stacksshowing less charge trapping compared toex situprocessedgates.

This work was supported in part by the National ScienceFoundation through Award No. DMR 0072134. The authorsacknowledge David Chi~Stanford! for providing the hafniasamples and Baylor Triplett~Stanford! for several helpfuldiscussions.

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391Appl. Phys. Lett., Vol. 84, No. 3, 19 January 2004 Ramanathan et al.

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